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The redox chemistry of a variety of organometallic dinitrosyl complexes of Cr, Mo and W Wassink, Berend 1985

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THE REDOX CHEMISTRY OF A VARIETY OF ORGANOMETALLIC DINITROSYL COMPLEXES OF Cr, Mo AND W By BEREND WASSINK B . S c , UNIVERSITY OF BRITISH COLUMBIA, 1980 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n THE FACULTY OF GRADUATE STUDIES DEPARTMENT OF CHEMISTRY We accept t h i s t h e s i s as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA JULY 1985 © BEREND WASSINK, 1985 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. I t i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of Chemistry  The University of B r i t i s h Columbia 1956 Main Mall Van couve r, Canada V6T 1Y3 Date August 20, 19 85 D E - 6 n / « n i i ABSTRACT An understanding of the electrochemical properties of organometal-l i c n i t r o s y l compounds provides a good understanding of their chemistry and a more r a t i o n a l way to approach synthetic i n v e s t i g a t i o n s . C y c l i c voltam-metry studies of [(T) 5-C 5H 5)Cr(NO) 2] 2 in CH 2C1 2 and CH3CN reveal that the dimer undergoes a single two-electron oxidation to form [ ( T i 5 - C 5 H 5 ) C r ( N O ) 2 ] + which i s reduced to [(n 5-C 5H 5)Cr(NO) 2]• (or [(n 5-C 5H 5)Cr(NO) 2(CH 3CN)]• i n the presence of CH3CN) i n a subsequent reduction step. The r a d i c a l couples to form [(Ti 5-C 5H 5)Cr(NO) 2 ] 2 or decomposes. The dimer also i s r e v e r s i b l y reduced i n a one-electron step i n CH 2C1 2 and quasi-reversibly i n CH3CN. These inferences are supported by c y c l i c voltammograms of (T) 5-C 5H 5)Cr(NO) 2BF 1 + and [ ( Ti 5-C 5H 5)Cr (NO) 2(CH 3CN) ]PF 6. In contrast, the is o e l e c t r o n i c dimer [(T) 5-C 5H 5)Fe(CO) 2] 2 oxidizes i n two one-electron steps (the f i r s t of which i s re v e r s i b l e and negative of the oxidation of [(t| 5-C 5H 5)Cr(NO) 2] 2) and reduces to form [(ri 5-C 5H 5)Fe(CO) 2 ] ~ . The d i f f e r i n g oxidation behaviours of these dimers suggest that t h e i r reactions with HBF 4»OMe 2 ( [(Ti 5-C 5H 5)Cr(NO) 2 ] 2 cleaves into [( n 5-C 5H 5)Cr(NO) 2 ] + and [ ( r i 5 - C 5 H 5 ) F e ( C O ) 2 ] 2 forms [ {(n 5-C 5H 5)Fe(CO) 2 } 2 H ] + ) do not occur by i n i t i a l electron t r a n s f e r . The new r a d i c a l anion complex [(n 5-C 5H 5)Fe(n 6-C 5Me 6)] [ {(Ti 5-C 5H 5)Cr(NO) 2 } 2 ] can be i s o l a t e d by reaction of the neutral dimer with (T} 5-C 5H 5)Fe(Ti 6-C 6Me 6) i n E t 2 0 . Its spectroscopic properties are consistent with d e r e a l i z a t i o n of the extra electron onto i i i the NO ligands, p a r t i c u l a r l y the bridging n i t r o s y l groups. These observa-tions provide a better understanding of the r e a c t i v i t y of t ( T i 5-C 5H 5)Cr(NO) 2] 2 w i t h - nucleophiles. A comparative electrochemical study of the oxidations of ( T I 5 - C 5 H 5 ) M ( N O ) 2R (M = Cr, R = CH3; M = Mo, W, R = CH 3, C 2H 5), ( T i 5-C 5H 5)Fe(CO) 2CH 3 and (T) 5-C 5H 5)M(CO) 3R (M = Cr, R = CH 3; M = Mo, W, R = CH 3, C 2H 5) i n CH 2C1 2 reveals that the d i n i t r o s y l complexes are harder to oxidize than th e i r related carbonyl compounds. E l e c t r o p h i l i c cleavage reactions of M-R bonds i n these complexes, which proceed d i f f e r e n t l y for the n i t r o s y l and carbonyl complexes are proposed to involve d i f f e r e n t mech-anisms, with the n i t r o s y l - a l k y l complexes reacting with e l e c t r o p h i l e s by d i r e c t attack at the metal-alkyl bonds, rather than by p r i o r oxidation. Interestingly, ( T i 5-C 5H 5)Cr ( N O ) 2CH 3 reacts with N O P F G to form the N O - i n s e r -t i o n product [ ( i i 5 - C 5 H 5 ) C r ( N O ) 2 ( C H 2 N O H ) ] P F 6 which has been s t r u c t u r a l l y and spectroscopically characterized. The reactions of ( T I 5 - C 5 H 5 ) M ( N 0 ) 2 C H 3 (M = Mo, W) with e l e c t r o p h i l e s and oxidants result i n cleavage of the M-CH3 bonds• The complexes ( T I 5 - C 5 H 5 ) M ( N O ) 2Y (M = Cr, Y = CH 3; M = Mo, Y = CH 3, C 2H 5, C l ; M = W, Y = CH3, C 2H 5, H, C l ) , [ ( r) 5-C 5H 5)M (NO) 2L] BF^ (M = Mo, L = PPh 3; M = W, L = PPh 3, P(OMe) 3, n 2-C 8H 1 4) and W(N0) 2C1 2L 2 (L = P(OMe) 3, PMePh2) exhibit quite r e v e r s i b l e , one-electron reductions i n CH 2C1 2 and the new r a d i c a l complexes [( T I 5 - C 5 H 5 ) 2Co] [ ( T I 5 - C 5 H 5 ) M ( N O ) 2Y] (M = Mo, Y = CH 3, C 2H 5, C l ; M = W, Y = CH3, H , Cl) are i s o l a b l e by reactions of ( T I 5 - C 5 H 5 ) 2Co with the neutral precursor. Spectroscopic characterization of these and an X-ray cr y s t a l l o g r p a h i c analysis of [(n5-C5H5) 2Co] [ ( r) 5-C 5H 5)Mo ( N O ) 2C 2H 5] i v suggest that the anions possess monomeric, "three-legged piano s t o o l " geometries with d e l o c a l i z a t i o n of the extra electron onto the NO ligands. In l i g h t of these observations the chemistry of ( T I 5 - C 5 H5)M ( N0) 2 Y complexes becomes more understandable. V Table of Contents ABSTRACT i i Table of Contents v L i s t of Figures i x L i s t of Tables x i i i Table of Abbreviations xiv Acknowledgements xv Chapter One - GENERAL INTRODUCTION 1 Chapter Two - THE ELECTROCHEMICAL METHODS AND EQUIPMENT 9 I) Introduction 9 (a) C y c l i c Voltammetry 9 (b) Instrumentation 15 II) Experimental Section 17 III) Results and Discussion 28 (a) The C y c l i c Voltammetry C e l l 28 (b) The Bulk E l e c t r o l y s i s C e l l 32 (c) The Scope and Limitations of the Electrochemical Methodology 35 Chapter Three - REDOX S T U D I E S OF B I S [ ( T I 5 - C Y C L O P E N T A D I E N Y L ) -DINITROSYLCHROMIUM] AND RELATED COMPLEXES 36 I) Introduction 36 II) Experimental Section 38 III) Results and Discussion 49 Cy c l i c Voltammetry Studies 49 v i (a) [ ( r i 5 - C 5 H 5 ) C r ( N O ) 2 ] 2 49 (b) ( T 1 5-C 5H 5)Cr ( N O) 2BF l t 59 (c) [(ri 5-C 5H 5)Cr(NO) 2(CH 3 C N)]PF 6 62 (d) ( T i 5 - C 5 H 5 ) C r ( N O ) 2 C l 64 (e) ( T i 5-C 5H 5) 2Cr 2(NO ) 3 N H 2 64 Bulk E l e c t r o l y s e s 67 Preparative Work 72 (a) Protonation of [ ( T i 5 - C 5 H 5 ) C r ( N O ) 2 ] 2 72 (b) Protonation vs. Oxidative Cleavage of [ ( T I 5 - C 5 H 5 ) M ( L O ) 2 ] 2 (M = Cr, L = N ; M = Fe, L = C) 76 (c) I s o l a t i o n of [ ( n 5 - C 5 H 5 ) F e ( i i 6 - C 6 M e 6 ) ] -[{(ri 5-C 5H 5)Cr(N0) 2} 2] and i t s Properties 77 IV) Summary 84 Chapter Four - THE OXIDATION ELECTROCHEMISTRY OF SOME GROUP 6 (T15-CYCL0PENTADIENYL)DINITR0SYLALKYLMETAL COMPLEXES AND RELATED CARBONYL COMPOUNDS: IMPLICATIONS FOR ELECTROPHILIC CLEAVAGE OF METAL-ALKYL BONDS 86 I) Introduction 86 II) Experimental Section 88 III) Results and Discussion 101 Cy c l i c Voltammetry Studies 101 Implications for E l e c t r o p h i l i c Cleavage Reactions of M-R Bonds i n (r| 5-C 5H 5)M(NO) 2R (M = Cr, R = CH3; M = Mo, W, R = CH 3, C 2H 5) 103 v i i (a) (n 5-C 5H 5)Cr(NO) 2CH 3 108 Reaction of (n 5-C 5H 5)Cr(NO) 2CH 3 with [ F e ( P h e n ) 3 ] ( P F 6 ) 3 114 Reaction of ( r i 5-C 5H 5)Cr(NO) 2CH 3 with AgBF^ 117 Reaction of ( r i 5-C 5H 5)Cr(NO) 2CH 3 with N0PF 6 119 (b) ( T I 5 - C 5 H 5 ) M ( N 0 ) 2R (M = Mo, W; R = CH 3, C 2H 5) 134 Reaction of (n 5-C 5H 5)Mo(NO) 2CH 3 with N0PF 6 137 Reactions of ( T I 5 - C 5 H 5 ) W ( N O ) 2CH 3 with Oxidants and Ele c t r o p h i l e s 138 Attempted reaction of (n 5-C 5H 5 ) W(NO) 2PF 6 and ( T I 5 - C 5 H 5 ) W ( N O ) 2 C H 3 140 (c) ( r i 5 - C 5 H 5 ) C r ( C 0 ) 3 C H 3 140 (d) ( T I 5 - C 5 H 5 ) M ( C O ) 3 R (M = Mo, R = CH 3, C 2H 5; M = W, R = H, CH 3, C 2H 5) 147 Reaction of ( r i 5-C 5H 5 ) W(CO) 3CH 3 with AgBF 4 151 IV) Summary 152 Chapter Five - THE REDUCTION CHEMISTRY OF COMPLEXES CONTAINING THE { ( T I 5 - C 5 H 5 ) M ( N 0 ) 2} (M = Cr, Mo, W) GROUP 154 I) Introduction 154 II) Experimental Section 156 III) Results and Discussion 167 Cy c l i c Voltammetry Studies 167 (a) (T! 5-C 5H 5)Cr(NO) 2CH 3 167 (b) (n 5-C 5H 5)M(NO) 2R (M = Mo, W; R = CH 3, C 2H 5) 169 (c) (t) 5-C 5H 5 ) W(NO) 2H 171 v i i i (d) ( T I 5 - C 5 H 5 ) M ( N 0 ) 2 C 1 (M = MO, W) 172 (e) [ (T} 5-C 5H 5)M(NO) 2L]BF i + [M = Mo, L = PPh 3; M = W, L = PPh 3, P(OMe) 3] 178 (f) [(n 5-C 5H 5)W(NO) 2(n 2-C 8H l l t)]BF l t 181 (g) W(N0) 2C1 2L 2 [L = PMePh2, P(OMe) 3] 184 Overview of the Electrochemical Results 184 Preparative Work 190 (a) [ ( T I 5 - C 5 H 5 ) 2 C O ] [ ( T I 5 - C 5 H 5 ) M ( N 0 ) 2 R ] (M = Mo, R = CH 3, C 2H 5; M = W, R = CH 3) 190 (b) Reactions of (n 5-C 5H 5)Cr(NO) 2CH 3 with Reducing Agents 204 (c) [(n 5-C 5H 5) 2Co ] [ ( T i 5-C 5H 5)W(NO) 2Y] (Y = H, D) 206 (d) t ( T 1 5-C 5H 5) 2Co] [(n 5-C 5H 5)M(NO) 2Cl] (M = Mo, W) 211 (e) [ ( T) 5-C 5H 5)W(NO) 2L] (L = P(0Me) 3, PPh 3) 218 IV) Summary 222 EPILOGUE 226 REFERENCES AND NOTES 227 APPENDIX 240 LIST OF FIGURES ix Figure 1. A triangular voltage waveform and the resultant c y c l i c voltammogram for the oxidation of ferrocene ... 11 Figure 2. Schematic representation of the apparatus for c y c l i c voltammetry 16 Figure 3. The c y c l i c voltammetry c e l l 19 Figure 4. The bulk e l e c t r o l y s i s c e l l 21 Figure 5. F i l l i n g of the reference electrode housing with an aqueous KCl/Agar s a l t bridge 23 Figure 6. Background CV scans obtained at: (a) a normal electrode, and ; (b) a "cracked" electrode 30 Figure 7. C y c l i c voltammograms of [ ( r i 5-C 5H 5)Cr(NO) 2] 2 i n CH 2C1 2. 50 Figure 8. C y c l i c voltammograms of [ ( T i 5-C 5H 5)Cr(NO) 2] 2 i n CH3CN.. 53 Figure 9. C y c l i c voltammograms of [ ( T i 5-C 5H 5)Fe(CO) 2] 2 i n CH 2C1 2. 56 Figure 10. Possible geometries for ( T) 5-C 5H 5)Cr(NO) 2Y where Y i s a weakly coordinating anion 61 Figure 11. C y c l i c voltammogram of [(Ti5-C5H5)Cr(NO) 2(CH 3CN) ] PF 6 i n CH 2C1 2 63 Figure 12. C y c l i c voltammogram of ( n 5 - C 5 H 5 ) 2 C r 2 ( N 0 ) 3 N H 2 i n CH 2C1 2 65 Figure 13. S t a t i c molecular structure of [ { ( T i 5-C 5H 5)Cr(NO) 2} 2OH]BF 1 + 75 Figure 14. X-band ESR spectrum of [ ( r i 5 - C 5 H 5 ) F e ( T i 6 - C 6 M e 6 ) ] [ { ( T i 5 - C 5 H 5 ) C r ( N O ) 2 } 2 ] i n DMF. (a) Experimentally observed, and (b) simulated 80 X Figure 15. A non-centrosymmetric s t r u c t u r e f o r [ { ( T i 5 - C 5 H 5 ) C r ( N O ) 2 } 2 ] ~ 81 Figure 16. P o s s i b l e s t r u c t u r a l m o d i f i c a t i o n of [ { ( n 5 - C 5 H 5 ) C r ( N O ) 2 } 2 ] ~ by i n t e r a c t i o n with DMF 81 Figure 17. C y c l i c voltammograms of ( T i 5-C 5H 5)Cr(NO) 2CH 3 i n CH 2C1 2 ( o x i d a t i o n ) 110 Figure 18. C y c l i c voltammogram ( o x i d a t i o n ) of (r| 5-C 5H 5)Cr(NO) 2CH 3 i n CH3CN 112 Figure 19. Molecular s t r u c t u r e of [(n 5-C 5H 5)Cr(NO) 2(CH 2N0H)]PF 6 . 121 Figure 20. 80 MHz lH NMR spectrum of . [ ( r i 5-C 5H 5)Cr(NO) 2(CH 2NOH)]PF 6 i n CD 3N0 2 125 Figure 21. A s t a t i c molecular s t r u c t u r e f o r [ ( T i 5-C 5H 5)Cr(NO) 2(CH 2NOH)]PF 6 i n CD 3N0 2 126 Figure 22. C y c l i c voltammograms of (a) ( T i 5-C 5H 5)Mo(NO) 2CH 3 and (b) ( T I 5 - C 5 H 5 ) W ( N O ) 2 C H 3 , both i n CH 2C1 2 ( o x i d a t i o n ) ... 136 Figure 23. C y c l i c voltammograms of (ii 5-C 5H 5)Cr(CO) 3CH 3 i n CH 2C1 2. 141 Figure 24. C y c l i c voltammograms of (a) ( T I 5 - C 5 H 6 ) M O ( C O ) 3CH 3, (b) ( T I 5 - C 5 H 5 ) W ( C O ) 3 C H 3 and (c) ( T I 5 - C 5 H 5 ) W ( C O ) 3 H a l l i n CH 2C1 2 149 Figure 25. C y c l i c voltammogram of ( T I 5 - C 5 H 5 ) W ( C O ) 3 C 2 H 5 i n CH3CN .. 150 Figure 26. C y c l i c voltammogram of ( T i 5-C 5H 5)Cr(NO) 2CH 3 i n CH 2C1 2 (reduction) 168 Figure 27. C y c l i c voltammogram of (r) 5-C 5H 5)Mo(NO) 2 C 2 H 5 i n CH 2C1 2 (reduction) 170 Figure 28. C y c l i c voltammogram of (r) 5-C 5H 5)Mo(NO) 2C1 i n CH 2C1 2 .. 173 x i Figure 29. C y c l i c voltammogram of [ ( T I 5 - C 5 H 5 ) M O ( N O ) 2 P P h 3 ] B F 4 i n CH 2C1 2 179 Figure 30. C y c l i c voltammograms of [(n 5-C 5H 5 ) W(NO ) 2 ( T i 2-C 8H l l f)]BF l t i n CH 2C1 2: (a) i n i t i a l scan; (b) aft e r ~ 0.5 h; (c) a f t e r ~1.5 h 182 Figure 31. Plots of E 1 / 2 vs. v N Q for various { ( T I 5 - C 5 H 5 ) M ( N O ) 2 > -containing (M = Cr, Mo, W) complexes i n CH 2C1 2 189 Figure 32. Molecular structure of [ ( T ) 5 - C 5 H 5 ) M O ( N O ) 2 C 2 H 5 ] ~ , including selected bond lengths and angles 195 Figure 33. (a) Structure of the [(r) 5-C 5H 5) 2 C O]+ counterion and (b) the arrangement of the cations and anions i n c r y s t a l l i n e [ ( n 5-C 5H 5) 2Co] [ ( n 5-C 5H 5)Mo(NO) 2C 2H 5] (projection along the a axis) 196 Figure 34. ESR spectra of (a) [(n 5-C 5H 5) 2Co] [ ( n 5-C 5H 5)Mo(NO) 2CH 3] i n DMF at 35°C and (b) [ ( T I 5 - C 5 H 5 ) 2 C O ] [ ( T I 5 - C 5 H 5 ) M O ( N O ) 2 C 2 H 5 ] i n CHgCN at 20°C 201 Figure 35. ESR spectrum of [(n 5-C 5H 5) 2Co][(n 5-C 5H 5 ) W(N0) 2CH 3] i n DMF at ~ -20°C 202 Figure 36. ESR spectra of (a) [(n 5-C 5H 5) 2Co] [ ( n 5-C 5H 5 ) W(NO) 2H] i n CH3CN at -22°C and (b) [ ( T I 5 - C 5 H 5 ) 2 C O ] [ ( T I 5 - C 5 H 5 ) W ( N O ) 2 D ] i n DMF at -22°C 209 Figure 37. ESR spectra of (a) [(n 5-C 5H 5) 2Co][(n 5-C 5H 5)Mo(NO) 2Cl] i n DMF at 25°C and (b) [ ( T I 5 - C 5 H 5 ) 2 C O ] [ ( T I 5 - C 5 H 5 ) W ( N O ) 2 C 1 ] i n DMF at ~ -35°C ... 214 x i i Figure 38. Schematic r e p r e s e n t a t i o n of an ESR spectrum derived from coupling of an unpaired e l e c t r o n to two equivalent 1 1 +N n u c l e i , a 9 5Mo or a 9 7Mo nucleus: a9b^D = a97jio = 1/3 a M 215 N Figure 39. ESR spectrum of (n 5-C 5H 5)W(NO) 2P(OMe) 3 i n CH3CN at at ~ -25°C 221 Figure 40. P l o t s of E 1 / 2 f o r the couple [ (n 5-C 5H 5)M(NO) 2 Y] N ~ - e" ^ = = t ( T i 5-C 5H 5)M(N0 ) 2 Y ] ( 1 ~ N ' > + (n = 0,1) i n CH 2C1 2 v s . v N Q ( N u j o l mull) of [ ( T I 5 - C 5 H 5 ) 2 C O ] [ ( T I 5 - C 5 H 5 ) M ( N O ) 2 Y ] and [ ( T I 5 - C 5 H 5 ) M ( N O ) 2 Y ] « complexes (M - Mo, W; Y = CH 3, C 2H 5, H, C l , PPh 3, P(0Me) 3 as appropriate) 223 x i i i LIST OF TABLES Table I . V a r i a t i o n i n CO and NO IR St r e t c h i n g Frequencies of Some Co(C0)(N0)L 2 Complexes 2 Table I I . Oxidation Peak P o t e n t i a l s and Associated Redox Data of Some Organometallic N i t r o s y l - and Carbonyl-Alky1 Complexes 102 Table I I I . Data f o r the Reductions of Several D i n i t r o s y l Complexes 185 Table IV. I n f r a r e d S t r e t c h i n g Frequencies and Reduction P o t e n t i a l s of Various D i n i t r o s y l Complexes 188 Table V. In f r a r e d S t r e t c h i n g Frequencies of Ra d i c a l Anion and Neutral D i n i t r o s y l - A l k y l Complexes 199 Table VI. N i t r o s y l IR S t r e t c h i n g Frequencies of the I s o l a t e d R a d i c a l Species and Their Oxidation P o t e n t i a l s 222 x i v TABLE OF ABBREVIATIONS In general the abbreviations and symbols used in this thesis are as recommended i n the "Handbook for Authors of Papers in American Chemical Society Publications". In addition, the following are also employed: °C degrees Celsius E anodic peak p o t e n t i a l p,a E cathodic peak p o t e n t i a l p,c E 1 / 2 (E + E )/2 1 / 2 P.a p,c i anodic peak current p,a i cathodic peak current n number of electrons involved i n a redox couple v scan rate ( V s - 1 ) CV c y c l i c voltammogram r) 5 pentahapto HOMO highest occupied molecular o r b i t a l LUMO lowest unoccupied molecular o r b i t a l FT Fourier transform SCE saturated calomel electrode Phen 1,10-phenanthroline THF tetrahydrofuran DME 1,2-dimethoxyethane DMF N,N'-dimethylformamide v„„ Infrared n i t r o s y l stretching frequency NO XV ACKNOWLEDGEMENTS I wish to express my a p p r e c i a t i o n to the f a c u l t y and t e c h n i c a l s t a f f of the chemistry department f o r t h e i r expert a s s i s t a n c e and guidance during the course of t h i s study, and i n p a r t i c u l a r Mr. S. Rak and Mr. S. Takacs of the glass shop f o r t h e i r e x p e r t i s e i n producing the s p e c i a l glassware I r e q u i r e d . I am al s o g r a t e f u l to my co-workers, Dr. D.T. Mar t i n , Dr. CR. Nurse, Dr. J.C. Oxley, A.D. Hunter, J.T. M a r t i n , Dr. L u i s Sanchez and George Richter-Addo whose diverse backgrounds and i n s i g h t s proved i n v a l u a b l e to me during t h i s work. I would a l s o g r a t e f u l l y make mention of Professor F. Aubke whose f o r e s i g h t was instrumental i n my embarking on a graduate career. Also I would l i k e to thank Dr. W. E. Geiger and h i s a s s o c i a t e s f o r t h e i r i n v a l u a b l e a s s i s t a n c e w i t h the e l e c t r o c h e m i c a l methods. I would l i k e most of a l l to express my sinc e r e a p p r e c i a t i o n to Professor Peter Legzdins f o r h i s guidance and high standards and without whose support, f r i e n d s h i p and confidence t h i s work would have been impossible. I am al s o g r a t e f u l to the Natural Sciences and Engineering Research C o u n c i l f o r t h e i r support i n the form of postgraduate s c h o l a r s h i p s . L a s t l y , I wish to thank my parents and f r i e n d s f o r t h e i r support and bearing with me throughout the various t r i a l s of my graduate work. x v i The Lord's lovingkindnesses indeed never cease, for His compassions never f a i l . They are new every morning; great i s Thy f a i t h f u l n e s s . Lamentations 3:22-23 1 Chapter One GENERAL INTRODUCTION H i s t o r i c a l l y , t r a n s i t i o n m e t a l n i t r o s y l and c a r b o n y l complexes have been c o n s i d e r e d to p o s s e s s v e r y s i m i l a r b o n d i n g , s t r u c t u r e s and r e a c t i v i t y . ^ However, w i t h t he renewed i n t e r e s t i n n i t r o s y l complexes over 2 t h e l a s t two decades i t has become c l e a r t h a t they d i f f e r m a r k e d l y from c a r b o n y l compounds. The n i t r o s y l l i g a n d i s known to p a r t i c i p a t e i n t h r e e 3 p r i n c i p a l bonding f o r m s : a) t e r m i n a l , l i n e a r M-N-O; b) t e r m i n a l , bent M-N^ Q; c ) b r i d g i n g . The t e r m i n a l , l i n e a r b o n d i n g mode of n i t r o s y l compounds i s the most common. The NO l i g a n d f u n c t i o n s as a net t h r e e - e l e c t r o n donor i n the 4 5 t e r m i n a l , l i n e a r form. ' The i d e a l i z e d MNO a n g l e i s 180°, but i n r e a l i t y 4 6 t h i s a n g l e can d e v i a t e c o n s i d e r a b l y from p e r f e c t l i n e a r i t y . ' The bo n d i n g i n v o l v e s a s y n e r g i s t i c c o m b i n a t i o n of a - d o n a t i o n from the n i t r o s y l l i g a n d * 4 to the m e t a l and MTC-* NOTC b a c k - b o n d i n g . The e x t e n t of m e t a l u - d o n a t i o n i s dependent on the degree of e l e c t r o n d e n s i t y a t the m e t a l c e n t r e , w h i c h i n t u r n i s i n f l u e n c e d by the a n c i l l a r y l i g a n d s and the charge on the complex. The l i n e a r NO l i g a n d i s c o n s i d e r e d t o be a v e r y p o w e r f u l rc-acid. To i l l u s t r a t e t h i s , the v a r i a t i o n s i n CO and NO IR s t r e t c h i n g f r e q u e n c i e s o f C o ( C 0 ) 2 ( N 0 ) L and C o ( C 0 ) ( N 0 ) L 2 ( L = n e u t r a l two - e l e c t r o n donor l i g a n d s ) complexes w i t h L has been used to propose a " s p e c t r o c h e m i c a l s e r i e s " f o r 2 it-bonding l i g a n d s . ^ The n-acceptor a b i l i t i e s of the most s t r o n g l y accepting ligands included i n the study increase i n the order PC1 3 < A s C l 3 < S b C l 3 < PF 3 < CO < NO. In s i t u a t i o n s where NO and CO have to compete f o r r e l a t i v e l y low e l e c t r o n d e n s i t y at the metal (as i n Co(CO)(NO)L 2 where L i s also a strong it-acid) they have been considered to be s i m i l a r i n t h e i r it-acceptor a b i l i t i e s . However, when the metal i s more e l e c t r o n - r i c h ( f o r Co(CO)(NO)L 2 where L now i s a good net donor) the NO group i s thought to be the stronger i t - a c i d . This i s i l l u s t r a t e d by the data i n Table I , where A represents the f r a c t i o n a l change i n or v ^ Q . For a s e r i e s of r e l a t e d complexes NitCO)^, Co(CO) 3NO, Fe(CO) 2(N0) 2 > Mn(CO)(NO) 3, Cr(N0) L ) and Fe(CO) 5 an X-ray photoelectron spectroscopy study has concluded that the g it-acceptor a b i l i t y of NO dominates over CO n - a c i d i t y . Back-bonding to CO was found to be inf l u e n c e d by dominant, competitive n-back-bonding to NO. Table 1.^ V a r i a t i o n i n CO and NO IR s t r e t c h i n g frequencies of some Co(C0)(N0)L 9 complexes, Compound v c o ( c m - 1 ) A v N 0 ( c m - 1 ) Co(CO)(NO)(PCl 3) 2 2044.5 1793.2 4.29% 4.24% Co(CO)(NO)(PPh 3) 2 1956.7 1717.0 2.40% 3.43% Co(C0)(N0)(Phen) 1909.8 1658.1 3 The o b s e r v e d t r e n d s i n C I s , N I s and 0 I s b i n d i n g e n e r g i e s have been found t o r o u g h l y c o r r e l a t e w i t h the CO and NO s t r e t c h i n g f o r c e c o n s t a n t s , l e n d i n g some s u p p o r t t o the IR s t u d y of the Co(CO)„_ (NO)L (x = 1,2) complexes d e s c r i b e d above. F i n a l l y , the o x i d a t i o n p o t e n t i a l s of a s e r i e s o f complexes [ C r ( C O ) 5 L ] ( L = NO, z = +1; L = n e u t r a l t w o - e l e c t r o n donor l i g a n d s , z = 0; L = h a l i d e s , p s e u d o - h a l i d e s , z = -1) have been used t o 9 a s s e s s the e l e c t r o n - r i c h n e s s of the Cr c e n t r e r e l a t i v e t o Cr(C0) 6. A l i g a n d c o n s t a n t , P , has been d e f i n e d such t h a t P L = E l / 2 [ C r ( C O ) 5 L ] Z " E l / 2 [ C r ( C O ) 6 ] * A r e l a t i v e l y h i g h , p o s i t i v e v a l u e of P L i n d i c a t e s t h a t L i s a good i t -a c c e p t o r , w h i l e a l a r g e , n e g a t i v e P v a l u e i s r e f l e c t i v e o f a good n e t Li donor. On t h i s b a s i s n - a c i d i t y i s proposed t o i n c r e a s e i n the o r d e r CO < P F 3 < NO. Thus i t i s g e n e r a l l y a c c e p t e d t h a t NO i s a b e t t e r n - a c i d t h a n CO. I n the e n s u i n g c h a p t e r s t h i s a c c e p t o r a b i l i t y of NO w i l l be seen t o e x e r t a major i n f l u e n c e on the redox c h e m i s t r y of o r g a n o m e t a l l i c n i t r o s y l c o mplexes. Four s i m p l e v a l e n c e bond resona n c e h y b r i d s can be e n v i s a g e d f o r the t e r m i n a l , l i n e a r m e t a l - n i t r o s y l l i n k a g e , i . e . + - - + - + M=N-0: <-> M5=N=0 <-> M=N=0 <-> M 4r- N^O: I t h a s , however, t r a d i t i o n a l l y been c o n s i d e r e d to be bound f o r m a l l y as N0+, 4 which i s i s o e l e c t r o n i c with CO. In valence bond terms both the N and the 0 are sp hybridized. The N0 + formalism breaks the metal-nitrosyl i n t e r -actions down into three components: a) transfer of one electron from NO to a metal, M, giving M~ and N0+; b) a-donation of the lone pair from the nitrogen atom of N0 + to M and; c) back-bonding from the metal to N0 +, i . e . du -> pit . The N0 + formalism spawns from a desire to treat t r a n s i t i o n metal 1 4 n i t r o s y l and carbonyl chemistry as being e s s e n t i a l l y s i m i l a r . ' However, i t can lead to some i n t u i t i v e l y and p h y s i c a l l y unreasonable i n t e r -pretations. Secondly the formalism appears to be cumbersome. The electron density that i s f i r s t transferred from NO to M to give M~ and N0 + i s * subsequently involved i n dit •* pn back-donation, a l l of which would seem to be unduly awkward. Another area of d i f f i c u l t y associated with the NO ligand centres on the assignment of formal oxidation states. While t h i s can be useful for the purposes of electron-bookkeeping, i t can also lead to 2b unreasonable physical p i c t u r e s . For example, the formal oxidation numbers of the metals i n Co(CO)3NO, Fe(CO) 2(N0) 2, Mn(CO)(NO) 3 and Cr(NO) 4 are, respectively, -1,-2, -3 and -4, i f the NO ligand i s treated as N0 +. For organometallic n i t r o s y l complexes, where the bonding interactions are 2b l a r g e l y of a covalent nature , i t i s best to avoid rationales involving the N0 + formalism and formal oxidation states. Indeed t h i s w i l l be seen to be the case i n Chapter 4 i n attempting to r a t i o n a l i z e the redox chemistry and e l e c t r o p h i l i c cleavage reactions of some n i t r o s y l - a l k y l complexes. Perhaps an i n t u i t i v e l y less taxing simple model can be had by tre a t i n g the NO ligand as a neutral, three-electron donor. However, the best 2b des c r i p t i o n of the bonding involves molecular o r b i t a l treatments. 5 The t e r m i n a l , bent bonding mode of NO has no analogue i n metal-carbonyl chemistry and r e s u l t s i n NO a c t i n g as a formal one-electron 1 9 donor. In terms of Valence Bond Theory, the N and 0 atoms are sp^ hy b r i d i z e d and the MNO linkage i s f o r m a l l y analogous to an organic n i t r o s o compound, i . e . M-N\ The i d e a l i z e d s p 2 MNO angle of 120° i s not n e c e s s a r i l y adopted. Indeed t h i s angle i s observed to vary between the two extremes of 120° and 180° ( p e r f e c t l y l i n e a r ) , depending on the extent of i n t e r a c t i o n of the n i t r o g e n lone p a i r w i t h the metal."*'^ The bent form i s not as common as the te r m i n a l , l i n e a r mode and i s ge n e r a l l y found i n n i t r o s y l complexes of l a t e t r a n s i t i o n metals, at the a p i c a l p o s i t i o n of a square-base pyramid or d i s t o r t e d octahedron."\ The b r i d g i n g n i t r o s y l l i g a n d i s a l s o q u i t e r a r e . An example of both the doubly- and t r i p l y - b r i d g i n g forms i s found i n (r|5-C5H5) 3Mn 3(NO) 1+ which contains three doubly-bridging NO groups around the perimeter of an Mn 3 t r i a n g l e and a t r i p l y - b r i d g i n g NO capping the Mn 3 t r i a n g l e ^ Both forms are considered to be fo r m a l l y t h r e e - e l e c t r o n donors.^ The development of organometallic n i t r o s y l chemistry has lagged 1 2a 5 behind that of carbonyl chemistry. ' ' This i s due i n part to the d i f f i c u l t i e s encountered i n s y n t h e s i z i n g n i t r o s y l complexes. Most carbonyl complexes are prepared using CO gas d i r e c t l y or from precursors synthesized i n t h i s manner.''" While some n i t r o s y l complexes are a c c e s s i b l e from 6 r e a c t i o n s using NO gas, t h i s approach i s not g e n e r a l l y a p p l i c a b l e . Since n i t r i c oxide i s very r e a c t i v e and can f u n c t i o n as an oxidant, decomposition side r e a c t i o n s are a greater problem than i n preparations of metal carbonyl compounds.^" I n t e r e s t i n g l y , however, some of the f i r s t n i t r o s y l complexes were made from NO gas and metal carbonyl complexes i n which the metal r e q u i r e s an odd number of e l e c t r o n s , i n a d d i t i o n to i t s valence e l e c t r o n s , 12 13 to a t t a i n the favoured 18-electron, noble gas c o n f i g u r a t i o n , ' f o r example, Co 2(CO) 8 + 2N0 > 2 Co(CO)3NO + 2C0. 1 4 (1.1) Unlike metal carbonyls, binary metal n i t r o s y l compounds are very r a r e , d i f f i c u l t to synthesize and u n s t a b l e . ^ Perhaps another deterrent to a more thorough e x p l o r a t i o n of n i t r o s y l chemistry derives from the miscon-ception that the chemistry of m e t a l - n i t r o s y l complexes i s e s s e n t i a l l y s i m i l a r to that of metal carbonyl compounds. As i n d i c a t e d by recent reviews of organometallic e l e c t r o c h e m i s t r y , n i t r o s y l complexes are f a r l e s s studied e l e c t r o c h e m i c a l l y than t h e i r carbonyl c o u n t e r p a r t s , ^ probably f o r much the same reasons as desribed above. Yet, with the growing i n t e r e s t i n organometallic n i t r o s y l chemistry has come a much increased use of e l e c t r o c h e m i c a l techniques to t r y to understand some of the unique chemistry displayed by these compounds. At the outset of t h i s work, approximately ten years of research i n t o organometallic n i t r o s y l chemistry i n these and other l a b o r a t o r i e s had r a i s e d some i n t r i g u i n g questions: a) why are simple, low-valent a n i o n i c 7 n i t r o s y l complexes so e l u s i v e ? ; b) why have the dimers [(TI -C 5H 5)M(NO) 2] 2 (M = Mo,W) not been able to be prepared from the r e a c t i o n s of ( T I 5 - C 5 H 5 ) M ( N O ) 2 C 1 (M = Mo.W) with reducing agents while [( T) 5-C 5H 5)Cr(NO) 2 ] 2 can be made i n t h i s way?; c) what i s at the heart of some of the unique o x i d a t i o n and reduction chemistry of [ ( T) 5-C 5H 5)Cr(NO) 2] 2? J and d) why do the a l k y l complexes ( T ) 5 - C 5 H 5 ) M ( N O ) 2 R (M = Cr,Mo,W) react d i f f e r e n t l y toward e l e c t r o p h i l e s than do r e l a t e d carbonyl complexes? Accordingly, the o b j e c t i v e s of t h i s study are to implement workable el e c t r o c h e m i c a l methodology, determine the redox behaviour of a v a r i e t y of Group 6 organometallic n i t r o s y l compounds, apply the r e s u l t s s y n t h e t i c a l l y , and where p o s s i b l e use the redox behaviour to e x p l a i n some of the chemistry observed for these complexes. A major impetus f o r t h i s research i s embodied i n the b e l i e f that a knowledge of the redox p r o p e r t i e s of a c l a s s of compounds w i l l provide a c l e a r e r understanding of t h e i r known chemistry as w e l l as f a c i l i t a t i n g the development of new chemistry. Chapter 2 describes the equipment and procedures employed for c y c l i c voltammetry and bulk e l e c t r o l y s i s work. With t h i s methodology i n hand the redox p r o p e r t i e s of [ ( T) 5-C 5H 5)Cr(NO) 2 ] 2, the i s o e l e c t r o n i c [ (T) 5-C 5H 5)Fe(CO) 2 ] 2 and some r e l a t e d complexes are d e t a i l e d i n Chapter 3. The major species a s s o c i a t e d w i t h the redox processes are i n v e s t i g a t e d . As a r e s u l t the new r a d i c a l anion [ {(Ti 5-C(jH 5)Cr(NO) 2 ) 2 ] ~ has been i s o l a t e d and c h a r a c t e r i z e d s p e c t r o s c o p i c a l l y . The combined s y n t h e t i c and e l e c t r o c h e m i c a l work provides some i n s i g h t s i n t o the chemistry of [ ( n 5 - C 5 H 5 ) C r ( N O ) 2 ] 2 . Chapter 4 centres on the o x i d a t i o n e l e c t r o c h e m i s t r y of a r e l a t e d s e r i e s of carbonyl-and n i t r o s y l - a l k y l compounds, (n 5-C 5H 5)M(CO) 3R, ( n 5-C 5H 5)Fe(CO) 2CH 3 and 8 ( T I 5 - C 5 H 5 ) M ( N O ) 2 R (M = Cr,Mo,W; R = methyl and/or e t h y l ) , with a view toward probing the d i f f e r e n c e s e x h i b i t e d by n i t r o s y l - a l k y l and car b o n y l -a l k y l complexes i n r e a c t i o n s with e l e c t r o p h i l e s . The re a c t i o n s of some of these compounds with oxidants under various c o n d i t i o n s are reported. Of p a r t i c u l a r i n t e r e s t i s the new i n s e r t i o n r e a c t i o n of NO i n t o the metal-methyl bond of ( T i 5-C 5H. 5)Cr(NO) 2CH 3, brought about by r e a c t i o n w i t h NOPF6. F i n a l l y , Chapter 5 deals w i t h the reduction chemistry of n e u t r a l and c a t i o n i c complexes c o n t a i n i n g the { ( - n 5-C 5H 5)M(NO) 2} group. R e v e r s i b l e or q u a s i - r e v e r s i b l e reductions are observed f o r a l l the species studied and se v e r a l new r a d i c a l anion complexes have been synthesized and c h a r a c t e r i z e d . Again some valuable i n s i g h t s i n t o the chemistry of some of these complexes are gained from t h i s work. 9 Chapter Two THE ELECTROCHEMICAL METHODS AND EQUIPMENT I) Introduction The f i e l d of el e c t r o c h e m i s t r y i s a d i s c i p l i n e a l l i t s own. I t has grown markedly i n the l a s t few decades and i s comprised of a great v a r i e t y of techniques. This growth i s due l a r g e l y to advances i n instrumentation and mathematical treatments of ele c t r o d e and r e l a t e d phenomena.^ In t h i s study c y c l i c voltammetry i s used to c h a r a c t e r i z e redox behaviour of some organometallic n i t r o s y l complexes, and so an o u t l i n e of the p r i n c i p l e s involved i s presented, and the design and implementation of c e l l s f or c y c l i c voltammetry and bulk e l e c t r o l y s i s are d e t a i l e d . A v a r i e t y of t e c h n i c a l problems encountered i n the design and use of the c e l l s are discussed. (a) C y c l i c Voltammetry. C y c l i c voltammetry has become i n c r e a s -i n g l y popular. I t b a s i c a l l y i n v o l v e s the a p p l i c a t i o n of a t r i a n g u l a r waveform p o t e n t i a l to an ele c t r o d e immersed i n a s o l u t i o n and recording the current response as a f u n c t i o n of the ap p l i e d v o l t a g e . C y c l i c voltammetry can be used to a s c e r t a i n the redox p r o p e r t i e s of a compound and can be di a g n o s t i c of the kind of mechanistic processes involved with the e l e c t r o n t r a n s f e r ( s ) . Due to t h i s s e n s i t i v i t y to the k i n e t i c s associated with e l e c t r o n t r a n s f e r s and accurate p o t e n t i a l measurement, c y c l i c voltammetry has been termed " e l e c t r o c h e m i c a l s p e c t r o s c o p y " . ^ 10 C y c l i c voltammetry has been considered i n d e t a i l . 17,18 An overview of some of the basic aspects i s given here. A t y p i c a l CV curve i s shown i n Figure 1 for the oxidation of ferrocene, for example. As E°, the thermo-dynamic oxidation p o t e n t i a l , i s approached a current flows due to oxidation of the depolarizer (ferrocene) in order to maintain equilibrium concentra-tions of the oxidized and reduced forms i n the v i c i n i t y of the electrode as governed by the Nernst equation. The current peaks as a l l of the depolar-i z e r i n the immediate area of the electrode i s consumed and then drops off to a plateau being maintained by d i f f u s i o n of the depolarizer i n toward the electrode. P r e c i s e l y the same process occurs once the d i r e c t i o n of p o t e n t i a l scan i s reversed at E (Figure 1). Now the species present at the electrode i s v i r t u a l l y e n t i r e l y the oxidized form, [ ( T } 5 - C 5 H 5 ) 2Fe] + which i s reduced as E° again Is approached. The current peaks i n the opposite d i r e c t i o n and drops off toward zero as the oxidized form i s depleted. Note that t h i s applies to a s i t u a t i o n where only one form of the redox couple i s i n i t i a l l y present; and for a reduction E^ ^ i s approached f i r s t . Some key quantitative relationships characterize this type of 18 r e v e r s i b l e electron transfer: = (E + E )/2 p,a p,c electrons involved). E -E = AE = 59/n mV at 298° K p,a p,c p 11 Figure 1 . A t r i a n g u l a r voltage waveform and the r e s u l t a n t c y c l i c voltam-mogram f o r the o x i d a t i o n of ferrocene: (Ep a ) anodic peak p o t e n t i a l ; (E c ) cathodic peak p o t e n t i a l ; ( E g p ) switching p o t e n t i a l ; ( i ' a ) anodic pelft°current from zero-current l i n e ; ( i ' ) cathodic peak current from zero-current l i n e ; ( i S p ) current at switching point; ( t S p ) switching point where scan d i r e c t i o n i s reversed. 12 E -E. ,„ = 28/n mV at 298°K p,a 1/2 E.. occurs where the current i s about 85% of i 1/2 p,a i / i = 1 ( f o r t o t a l chemical r e v e r s i b i l i t y ) p,c p,a In a d d i t i o n : E i s independent of scan ra t e p,a i i s a l i n e a r f u n c t i o n of v * / 2 (v = scan r a t e i n V s - 1 ) p,a (The same kind of r e l a t i o n s apply of course f o r reductions) The p r o p o r t i o n a l i t y of i to v 1 / 2 i s a consequence of d i f f u s i o n p,a c o n t r o l of the current where the current i s supported by d i f f u s i o n of the d e p o l a r i z e r toward the e l e c t r o d e . I f the current i s dependent on adsorp-t i o n of the d e p o l a r i z e r onto the e l e c t r o d e then i becomes p r o p o r t i o n a l p ,a to v. This s i t u a t i o n i s more complex to analyze and u n d e s i r a b l e . There are two kinds of r e v e r s i b i l i t y which must be d i s t i n g u i s h e d : e l e c t r o c h e m i c a l r e v e r s i b i l i t y and chemical r e v e r s i b i l i t y . A redox process i s s a i d to be e l e c t r o c h e m i c a l l y r e v e r s i b l e i f the rate of e l e c t r o n t r a n s f e r i s f a s t compared with the scan r a t e . I f t h i s i s so, e q u i l i b r i u m c o n d i t i o n s are very n e a r l y maintained and the Nernst equation at any time i s a p p l i c -a b l e . I f the process i s chemically r e v e r s i b l e then the i n i t i a l l y produced e l e c t r o l y s i s product has a s u f f i c i e n t l y long l i f e t i m e to regenerate the s t a r t i n g m a t e r i a l i n the r e t u r n scan and thus be detected. I f the e l e c t r o -13 lyzed species i s r e l a t i v e l y s h o r t - l i v e d then a r a p i d scan rate may be able to outrun the decomposition and give the system the appearance of chemical r e v e r s i b i l i t y . In such a case i / i ( f o r an o x i d a t i o n ) would approach p,c p,a u n i t y as the scan ra t e becomes s u f f i c i e n t l y f a s t . Thus both types of r e v e r s i b i l i t y may be dependent on scan r a t e . In a v a r i e t y of solvents the f e r r o c e n e / f e r r i c i n l u m couple i s a completely r e v e r s i b l e , d i f f u s i o n -c o n t r o l l e d process over a wide range of scan r a t e s . ^ The measurement of i / i i s made somewhat d i f f i c u l t i n that the p,a p,c current b a s e l i n e cannot be e a s i l y l o c a t e d . However, an e m p i r i c a l method has been developed which g e n e r a l l y works w e l l provided E , the switching sp 20 p o t e n t i a l , i s w i t h i n 600 mV of the peak p o t e n t i a l : i i ' 0.4851 = + i t SP + 0.086 p,a p,a p,a (The q u a n t i t i e s i 1 , i ' and i are as shown i n Figure 1 and p,c' p,a sp measured from the zero-current l i n e ) An i r r e v e r s i b l e redox process occurs under co n d i t i o n s of slow e l e c t r o n t r a n s f e r and the ra t e constant f o r the opposite process i n the r e t u r n scan i s very slow. Thus f o r an o x i d a t i o n wave there w i l l be no cathodic wave i n the r e t u r n scan. E q u i l i b r i u m c o n d i t i o n s are not main-tained and the Nernst equation does not apply. In a d d i t i o n , cannot e a s i l y be determined. The peak p o t e n t i a l E s h i f t s to more p o s i t i v e p,a p o t e n t i a l s with i n c r e a s i n g scan r a t e . (E^ c f o r an i r r e v e r s i b l e r e d u c t i o n grows more negative w i t h i n c r e a s i n g scan r a t e ) . The peak c u r r e n t , i , i s 14 a g a i n a l i n e a r f u n c t i o n of v 1 / 2 i f the c u r r e n t i s d i f f u s i o n l i m i t e d . When the scan r a t e and r a t e of e l e c t r o n t r a n s f e r a r e comparable a q u a s i -r e v e r s i b l e wave i s o b t a i n e d . The s e p a r a t i o n i n peak p o t e n t i a l s grows w i t h i n c r e a s e d scan r a t e i n t h i s c a s e . The t h r e e c a s e s mentioned above i n v o l v e s i m p l e e l e c t r o n t r a n s f e r . Coupled c h e m i c a l r e a c t i o n s and m u l t i p l e - e l e c t r o n t r a n s f e r s c o m p l i c a t e m a t t e r s ; however, each u n i q u e c o m b i n a t i o n of e l e c t r o n t r a n s f e r s and a t t e n d a n t r e a c t i o n s r e s u l t s i n a unique type of c y c l i c voltammogram and a g r e a t many of t h e s e have been m a t h e m a t i c a l l y t r e a t e d . ^ C y c l i c v o l t a m -m e t r y , t h e r e f o r e , can o f f e r a p o w e r f u l m e c h a n i s t i c p r o b e . A note of c l a r i f i c a t i o n must be made on the measured p o t e n t i a l , E .„ The thermodynamic p o t e n t i a l i s denoted E° and i s measured f o r a 1/ Z. h a l f - c e l l w i t h r e s p e c t t o the s t a n d a r d hydrogen e l e c t r o d e (SHE) i n aqueous s o l u t i o n s . T h i s r i g o r o u s l y r e q u i r e s the e v a l u a t i o n of a c t i v i t i e s of the s p e c i e s i n v o l v e d and i s u s u a l l y d i f f i c u l t . A d e v i c e f o r a v o i d i n g t h i s i s c a l l e d the f o r m a l e l e c t r o d e p o t e n t i a l , E°', w h i c h i s d e f i n e d as the measured e q u i l i b r i u m p o t e n t i a l of a h a l f - c e l l w i t h r e s p e c t t o SHE ( t h a t i s , the measured p o t e n t i a l when the l o g a r i t h u m term of the N e r n s t e q u a t i o n i s z e r o ) , a g a i n i n aqueous s o l u t i o n s . I t i s r e l a t e d t o E°. R i g o r o u s l y , f o r the redox p r o c e s s Ox + n e ^ ^ Red; v D 1 / 2 2 1  E Eo _ R T ^ Y * e d Ox 1 / 2 n F W W ' 2 where y Q x and y R e d a r e a c t i v i t y c o e f f i c i e n t s , and and D R e d a r e d i f f u -s i o n c o e f f i c i e n t s of Ox and Red, r e s p e c t i v e l y . The v a l u e t h e n i s 15 r e l a t e d to E° by a c o r r e c t i o n t e r m . T h i s term may be n e g l i g i b l e f o r s i m p l e e l e c t r o n t r a n s f e r s i f a n d Y R e ( j a r e assumed t o be n e a r l y e q u a l and l i k e -w i s e Drt and D„ ,. F o r more complex waves t h e s e f a c t o r s can be v e r y Ox Red d i f f e r e n t i f the mass t r a n s p o r t c h a r a c t e r i s t i c s of Ox and Red a r e d i f f e r e n t . Where aqueous r e f e r e n c e e l e c t r o d e s are used w i t h o r g a n i c s o l v e n t s the problems caused by j u n c t i o n p o t e n t i a l s are complex and can have a l a r g e e f f e c t on the measured p o t e n t i a l . Under t h e s e c o n d i t i o n s E^/2 c a n n o t ^ e r e l a t e d t o E°'. I n t h i s s t u d y I s t a k e n t o be a c h a r a c t e r i s t i c p o t e n t i a l f o r a c o u p l e under the c o n d i t i o n s s p e c i f i e d , and as s u c h , i s r e p r o d u c i b l e . (b) Instrumentation. The equipment r e q u i r e d f o r c y c l i c v o l t a m -m e t r y , d e p i c t e d s c h e m a t i c a l l y i n F i g u r e 2, i n c l u d e s the e l e c t r o c h e m i c a l c e l l e quipped w i t h a t h r e e - e l e c t r o d e a r r a y , the p o t e n t i o s t a t f o r m e a s u r i n g and m a i n t a i n i n g the e l e c t r o d e p o t e n t i a l and m e a s u r i n g the c u r r e n t , a v o l t a g e s o u r c e c a p a b l e of g e n e r a t i n g a t r i a n g u l a r waveform and a r e c o r d i n g d e v i c e . T e c h n i c a l a s p e c t s of the equipment i n v o l v e d i n v a r i o u s e l e c t r o -22 c h e m i c a l t e c h n i q u e s have been d e s c r i b e d . The t h r e e - e l e c t r o d e geometry a l l o w s f o r measurement of the w o r k i n g e l e c t r o d e p o t e n t i a l ( d e s i g n a t e d by E i n F i g u r e 2) w i t h r e s p e c t t o the r e f e r e n c e e l e c t r o d e w i t h o u t a l a r g e c u r r e n t p a s s i n g between them. L a r g e c u r r e n t s between the w o r k i n g and r e f e r e n c e e l e c t r o d e s would add a s u b s t a n t i a l IR term (Ohmic d r o p ) to the measured p o t e n t i a l . T h i s problem becomes a c u t e i n non-aqueous s o l v e n t s w i t h o r g a n i c s a l t s as the s u p p o r t i n g e l e c t r o l y t e . L a r g e c u r r e n t s can a l s o d e s t a b i l i z e the r e f e r e n c e e l e c t r o d e 16 s o u r c e p o t e n t i o s t a t r e c o r d e r Figure 2. Schematic representation of the apparatus for c y c l i c voltammetry: (WE) working electrode; (RE) reference electrode; (AE) a u x i l i a r y electrode. 17 by consuming one component of i t s redox c o u p l e c a u s i n g a s h i f t away from i t s e q u i l i b r i u m p o t e n t i a l . The r e f e r e n c e e l e c t r o d e has a h i g h impedance and s h o u l d be h i g h l y n o n - p o l a r i z a b l e , t h a t i s , the passage of s m a l l c u r r e n t s t h r o u g h i t s h o u l d have no e f f e c t on i t s p o t e n t i a l . The measured c u r r e n t , i , f l o w s between the a u x i l i a r y and w o r k i n g , e l e c t r o d e s . The c u r r e n t i s l a r g e l y c a r r i e d by the s u p p o r t i n g e l e c t r o l y t e , a g e n e r a l l y n o n - i n t e r a c t i v e s a l t w h i c h i s p r e s e n t i n a c o n c e n t r a t i o n of g r e a t e r t h a n one h u n d r e d f o l d e x c e s s compared to the d e p o l a r i z e r . T h i s s u p p r e s s e s c o n t r i b u t i o n s t o the c u r r e n t from m i g r a t i o n of the d e p o l a r i z e r under the i n f l u e n c e of an e l e c t r i c f i e l d . C y c l i c voltammograms a r e r e c o r d e d i n q u i e s c e n t s o l u t i o n s t o a v o i d c o n v e c t i o n which a l s o would c o n t r i b u t e t o the c u r r e n t . The c u r r e n t thus o b t a i n e d i s l a r g e l y due t o d i f f u s i o n of the d e p o l a r i z e r under the i n f l u e n c e of a c o n c e n t r a t i o n 18c g r a d i e n t , a s i t u a t i o n which i s r e a d i l y a n a l y z e d m a t h e m a t i c a l l y . The t r i a n g u l a r waveform s o u r c e can be a f u n c t i o n g e n e r a t o r or a more s o p h i s t i c a t e d programmable waveform g e n e r a t o r . The r e c o r d i n g s y s t e m can employ e i t h e r an X-Y r e c o r d e r w h i c h i s s u i t a b l e f o r s l o w s c a n r a t e s (< 1 V s - 1 ) , or an o s c i l l o s c o p e f o r w h i c h much h i g h e r scan r a t e s may be u s e d . S t a t e - o f - t h e - a r t systems use c o m p u t e r - a s s i s t e d d a t a a c q u i s i t i o n . II) Experimental Section A l l o p e r a t i o n s were e f f e c t e d under an atmosphere of p r e p u r i f i e d 23 n i t r o g e n u s i n g c o n v e n t i o n a l t e c h n i q u e s u n l e s s o t h e r w i s e n o t e d . C H 2 C 1 2 ( F i s h e r s p e c t r a n a l y z e d ) was not p u r i f i e d f u r t h e r and was s i m p l y d e a e r a t e d 18 by sparging w i t h p r e p u r i f i e d n i t r o g e n p r i o r to use. CH3CN (Burdick and Jackson, UV-grade) was d i s t i l l e d from CaH 2 and then deaerated with p r e p u r i f i e d n i t r o g e n . Both solvents were stored under N 2 over alumina (Woelm n e u t r a l , a c t i v i t y 1). A l l other chemicals used were of reagent grade or comparable p u r i t y . Reagents were e i t h e r obtained from commerical s u p p l i e r s or prepared according to published procedures and t h e i r p u r i t y was ascertained by elemental analyses and i n the case of the supporting e l e c t r o l y t e , by the c y c l i c voltammetric background. The supporting e l e c t r o l y t e employed i n these s t u d i e s was [n-Bu^N]PF g. The Cyclic Voltammetry C e l l . The c e l l used f o r c y c l i c voltammetry i s shown i n Figure 3 and was constructed by Mr. S. Rak and Mr. S. Takacs of t h i s department. The design was derived from a c e l l by van 24 Duyne and R e i l l e y . The working electrode (designated by A i n Figure 3) was a platinum-bead of approximately 1 mm diameter sealed i n s o f t b o r o s i l i c a t e glass having a s i m i l a r c o e f f i c i e n t of thermal expansion as that of platinum. E l e c t r i c a l contact was e f f e c t e d with a copper wire dipped i n t o a mercury p o o l . The electrode was held i n place w i t h an a i r t i g h t screw-cap adapter. The a u x i l i a r y e l e ctrode (B) was a platinum-wire c o i l of ~1.5 cm diameter, p o s i t i o n e d roughly c o n c e n t r i c a l l y about the working e l e c t r o d e . An aqueous saturated calomel reference electrode w i t h a platinum contact (C) was housed i n a f i n e - f r i t t e d reference c e l l holder (D) to minimize water leakage. Contact between the 25 reference and working compartments was maintained by a Luggin probe ( E ) . The working electode was kept at a distance of approximately one bead-diameter away from the Luggin probe t i p . 19 Figure 3 . The c y c l i c voltammetry c e l l : (A) platinum-bead working el e c t r o d e ; (B) platinum-wire a u x i l i a r y e l e c t r o d e ; (C) aqueous saturated calomel reference e l e c t r o d e ; (D) f i n e - f r i t t e d reference c e l l holder; (E) Luggin probe. 20 The Bulk E l e c t r o l y s i s C e l l . The apparatus ( u l t i m a t e l y a r r i v e d at) for bulk e l e c t r o l y s e s i s shown i n Figure 4. The c e l l was designed using a 500-mL r e a c t i o n k e t t l e ( F i s h e r S c i e n t i f i c ) and a four-necked l i d equipped with ground-glass f l a n g e s . The r e q u i s i t e a d d i t i o n s and attachments were constructed by Mr. S. Rak, Mr. S. Takacs, and the mechanical shop headed by Mr. B. Powell (P.Eng.). An inner glass compartment (designated by D i n Figure 4) that reduced the c e l l volume to ~150 mL could be introduced i f needed. A platinum-gauze sheet and c y l i n d e r served as the working (A) and a u x i l i a r y (B) e l e c t r o d e s , r e s p e c t i v e l y , and a s i l v e r - w i r e placed between them served as a quasi-reference electrode (C). The working and a u x i l i a r y electrodes were separated by a glass compartment having a f i n e - f r i t t e d base. The various components of the c e l l such as the a u x i l i a r y c e l l compartment, working and reference electrodes (housed i n glass tubes) were held i n place by a T e f l o n p l a t e imbedded i n the l i d of the c e l l . The a u x i l i a r y e l e c t r o d e was housed i n a glass rod and kept i n place with a Teflon plug placed i n the mouth of the a u x i l i a r y compartment. Leads f o r the electrodes were run through a three-necked adapter, each neck being sealed by a 5-mm rubber septum. Any two of the leads on the i n s i d e of the c e l l , were i n s u l a t e d w i t h lengths of polyethylene tubing. Instrumentation. E l e c t r o c h e m i c a l measurements were accomplished with a Princeton Applied Research Model 173 p o t e n t i o s t a t equipped with a Model 176 c u r r e n t - t o - v o l t a g e converter and a Model 178 electrometer probe. The probe was mounted e x t e r n a l to the p o t e n t i o s t a t , the connection being made by a minimum length of high impedance wir e . C y c l i c voltammograms were recorded on a Hewlett-Packard Model 7035B X-Y recorder. The same recorder 21 Figure 4. The bulk e l e c t r o l y s i s c e l l : (A) platinum-gauze working electrode; (B) c y l i n d r i c a l platinum-gauze a u x i l i a r y electrode; (C) si l v e r - w i r e quasi-reference electrode; (D) inner glass compartment. 22 was used to record current-time behaviour during bulk e l e c t r o l y s e s . A l t e r -n a t i v e l y a F i s h e r Series 5000 R e c o r d a l l s t r i p chart recorder was employed for the current-time behaviour. An I n t e r s t a t e E l e c t r o n i c s Corp. Model F52A f u n c t i o n generator or a Wavetek Model 143 f u n c t i o n generator were used to generate the t r i a n g u l a r waveforms needed f o r c y c l i c voltammetry. The l a t t e r instrument was used i n conjunction with a u n i t y - g a i n i n v e r t e r (±15 V, 50 mA; constructed by the e l e c t r o n i c s shop, headed by Mr. Joe S a l l o s of t h i s department) since the Wavetek instrument could produce only an i n i t i a l l y p o s i t i v e - g o i n g ramp. 26 Preparation of the Aqueous Saturated Calomel Electrode . The s a l t bridge was prepared by gently heating a s t i r r e d mixture of Agar (0.5 g) and KC1 (1.5 g) i n 20 mL of d i s t i l l e d water to b o i l i n g u n t i l the s o l i d s d i s s o l v e d . This s o l u t i o n was removed from the heat and l e f t to stand f o r about 5 min to f a c i l i t a t e d i s s i p a t i o n of bubbles. The e l e c t r o d e housing was dipped i n t o the warm g e l and a b a l l - v a l v e d p i p e t t i n g bulb ( F i s h e r S c i e n t i f i c ) was used to c a r e f u l l y draw the gel up to the T-junction as shown i n Figure 5. This was l e f t undisturbed f o r ~10 min to allow the ge l to set a f t e r which time the housing was removed from the beaker and the ge l was cut f l u s h with the glass tube. Mercury was then added followed by a saturated aqueous KC1 s o l u t i o n and l a s t l y H g 2 C l 2 which could be conven-i e n t l y s e t t l e d on top of the mercury by running a f i l e over the g l a s s . F i n a l l y the electrode was capped with a small septum and stored with the t i p of the s a l t bridge immersed i n an aqueous saturated KC1 s o l u t i o n . I t s p o t e n t i a l was checked from time to time against a commercial SCE (Brinkmann Instruments). 23 Figure 5 . F i l l i n g of the reference electrode housing with an aqueous KCl/Agar s a l t bridge. 24 Preparation of [n_-Bu^N]PF6. A s o l u t i o n of [n-Bu HN]I (50 g, 0.14 mole) was prepared i n hot acetone (200 mL). A second s o l u t i o n of NH 4PF 6 ( A l d r i c h , 99.5%) was made by heating 30 g of the s a l t i n 90 mL of b o i l i n g acetone. Not a l l the s o l i d d i s s o l v e d and the s o l u t i o n was f i l t e r e d hot through a coarse f r i t . The f i l t r a t e , c o n t a i n i n g >28 g (>0.17 mole) of NH^PFg was added slowly to the hot s o l u t i o n of [n-Bu^NJI with s t i r r i n g and was heated f o r 10 min near b o i l i n g . The s o l u t i o n was f i l t e r e d hot and 600 mL of d i s t i l l e d water was added to the f i l t r a t e , which r e s u l t e d i n the p r e c i p i t a t i o n of a white s o l i d . The mixture was cooled, f i l t e r e d and b r i e f l y a i r - d r i e d . The p r e c i p i t a t e was d i s s o l v e d i n 220 mL of hot ethanol (95%) and f i l t e r e d hot. The f i l t r a t e was cooled to room temperature and the s o l i d was c o l l e c t e d by f i l t r a t i o n and a i r - d r i e d . This s o l i d was r e c r y s t a l l i z e d twice as above (except f o r the f i l t r a t i o n ) and a i r - d r i e d . The r e s u l t a n t white c r y s t a l s were powdered and d r i e d at 50-70°C i n vacuo ( 5 * 1 0 - 3 mm) for 24 h to o b t a i n 57 g (86% y i e l d based on [n-Bu^NJI) of [n-Bu^NJPFg, which was stored under N 2. General Procedure for Obtaining a C y c l i c Voltammogram. A l l components of the CV c e l l were heated i n an oven at ~160°C overnight p r i o r to use except f o r the e l e c t r o d e s . In order to ob t a i n a reproducible surface, the platinum working electrode was pretreated by suspending the t i p of the electrode j u s t above the surface of r e f l u x i n g n i t r i c a c i d f o r ~15 min, followed by a thorough r i n s e with d i s t i l l e d water, then immersing the electrode t i p i n a saturated ferrous ammonium s u l f a t e s o l u t i o n (made up i n 1 M H 2S0 4) f o r s e v e r a l minutes, and f i n a l l y r i n s i n g the electrode w i t h copious q u a n t i t i e s of d i s t i l l e d water. 25 The c e l l was assembled as shown i n Figure 3 w i t h the a d d i t i o n of [n-Bu^NJPFg (1.6 g, 4 mmol) and a small magnetic s t i r r i n g bar, but w i t h stoppers i n place of the reference electrode and working e l e c t r o d e . The working electrode was heated with a h o t - a i r gun to dry i t and then placed i n the c e l l with the platinum-bead kept l e v e l w i t h , and i n f r o n t of, the Luggin probe t i p . Approximately 40 mL of the appropriate solvent was t r a n s f e r r e d to the working compartment of the c e l l by cannulation and the s o l u t i o n was thoroughly s t i r r e d before a l l o w i n g i t to enter the reference compartment to give a 0.1 M [n_-Bu 4N]PF 6 s o l u t i o n . The f i n e - f r i t t e d holder (D i n Figure 3) was f i l l e d by c l o s i n g o f f the working compartment, the increased solvent vapour pressure f o r c i n g s o l u t i o n i n t o the reference c e l l . The reference electrode was r i n s e d with d i s t i l l e d water and wiped dry w i t h a paper t i s s u e and introduced to i t s holder. The c u r r e n t - v o l t a g e background was obtained i n two portions by applying s u i t a b l e amplitude triangular—waveform voltages going from 0.0 V to the p o s i t i v e l i m i t where a sharp current r i s e begins due to solvent/ e l e c t r o l y t e o x i d a t i o n , and l i k e w i s e from 0.0 V to the negative l i m i t . The background g e n e r a l l y was < 0.2 LIA wide. For CH 2C1 2 the a v a i l a b l e p o t e n t i a l range under these c o n d i t i o n s was +2.0 - -2.0 V and f o r CH3CN a range of +2.5 - -2.3 V could be obtained. The compound of i n t e r e s t was e i t h e r added as a s o l i d to the working compartment or a s o l u t i o n c o n t a i n i n g the compound was added to the c e l l by c a n n u l a t i o n . The d e p o l a r i z e r concentration was t y p i c a l l y 5-7 x 10" 4 M. C y c l i c voltammograms were g e n e r a l l y obtained at ambient temperature. Scans from 0.0 V to the p o s i t i v e and negative l i m i t s were performed and 26 then i n d i v i d u a l waves were s t u d i e d . I f low temperature studies became necessary the SCE was replaced by a length of s i l v e r w i r e , since the aqueous SCE would f r e e z e , and the c e l l was cooled with a cold bath. Enough ferrocene was added to the c e l l at the end of the experiment to make a 5-7 x I0~h M s o l u t i o n as an i n t e r n a l s t a n d a r d , 1 9 for which E 1 / n = +0.47 V, AE 1/2 p = 70 mV, i / i =1.0 and E i s independent of the scan ra t e i n CH 0C1 0. pc pa pa 1 1 The X-Y recorder l i m i t e d scan rates to <1 V s - 1 . A f t e r a c y c l i c voltammetry experiment was completed, the c e l l was cleaned by immersion i n a KOH-ethanol bath, followed by a d i l u t e aqueous HCI wash, a wash with detergent s o l u t i o n and a very thorough d i s t i l l e d water r i n s e . The f r i t t e d reference holder was cleaned with a mixture of aqueous hydrogen peroxide (30%) and concentrated s u l f u r i c a c i d . This then was washed with detergent s o l u t i o n and r i n s e d w e l l with d i s t i l l e d water. Bulk Electrolyses. The bulk e l e c t r o l y s i s c e l l was assembled under N 2 i n a Vacuum Atmospheres Corp. Dri-Lab Model HE-43-2 drybox. The s i l v e r - w i r e reference was placed c e n t r a l l y between the working and a u x i l i a r y e l e c t r o d e s , and as c l o s e to the former as p o s s i b l e without a c t u a l l y c o n t a c t i n g i t . The septa used to s e a l the electrode leads i n t o the three-necked adapter (Figure 4) were pre-punctured with a small needle and f i t t e d i n t o short glass tubes (<2 cm). Then the septa were ge n t l y s l i d onto the leads w i t h the a i d of a t h i n , sturdy p l a s t i c tube, while r e s t r a i n i n g the leads w i t h tweezers, and pressed down i n t o the l i g h t l y - g r e a s e d necks of the adapter. Enough [ii-Bu^NlPFg was added to give a 0.1 M s o l u t i o n . P r i o r to each experiment, the working electrode was 27 pretreated i n the same manner as described above f o r the c y c l i c voltammetry platinum-bead e l e c t r o d e . E l e c t r o l y s e s were performed at p o t e n t i a l s f a r enough past the peak p o t e n t i a l of the redox couple to produce high i n i t i a l currents except i n cases where m u l t i p l e and closely-spaced voltammetric waves occurred. Maximum currents a t t a i n a b l e were ~150-200 mA. The bulk e l e c t r o l y s i s c e l l and i t s f r i t t e d a u x i l i a r y compartment were c a r e f u l l y cleaned as o u t l i n e d above f o r the CV c e l l a f t e r an experiment was f i n i s h e d . 28 III) Results and Discussion The designs of c e l l s f o r various e l e c t r o c h e m i c a l techniques have 22 been discussed. Several f a c t o r s need to be considered i n the construc-t i o n and use of c e l l s f o r c y c l i c voltammetry and bulk e l e c t r o l y s i s . These are addressed below to the extent that i s appropriate to the requirements of t h i s study. (a) The C y c l i c Voltammetry C e l l . The electrode geometry i s of prime importance and can serve to s i g n i f i c a n t l y reduce the Ohmic-drop e r r o r s i n the measured p o t e n t i a l . The c e l l used i n t h i s study employs a concentr i c working and a u x i l i a r y electrode arrangement f o r convenience and moderately uniform current d i s t r i b u t i o n . The working electrode i s e f f e c -t i v e l y very c l o s e to the reference electrode by v i r t u e of the Luggin probe. The p r i n c i p l e behind t h i s component of the c e l l i s that the l a r g e l y enclosed glass surface s u b s t a n t i a l l y s h i e l d s the reference e l e c t r o d e and thereby g r e a t l y reduces the e l e c t r i c f i e l d gradient between the working and reference e l e c t r o d e s . The e f f e c t of t h i s i s to g r e a t l y minimize the i R l o s s , since the current flowing between the working and reference e l e c -25 trodes i s very low. This circumvents the problem of p h y s i c a l l y p l a c i n g the reference electrode r i g h t near the working electrode and a l s o reduces water leakage i n t o the e l e c t r o l y t e . Platinum i s used as the m a t e r i a l f or the working and a u x i l i a r y e lectrodes since i t i s h i g h l y conducting and r e l a t i v e l y chemically 18a i n e r t . The s i z e of the working electrode i s kept small (<1 mm) to avoid 29 l a r g e c u r r e n t s , a g a i n m i n i m i z i n g the Ohmic-drop e r r o r . A l t h o u g h a d r o p p i n g mercury e l e c t r o d e o f f e r s the advantage of a r e a d i l y r e p r o d u c i b l e , c l e a n s u r f a c e , i t has a f a i r l y h i g h r e s i s t a n c e (due t o the v e r y narrow mercury c a p i l l a r y ) and o f t e n becomes c h e m i c a l l y i n v o l v e d w i t h e l e c t r o g e n e r a t e d f r e e 18b r a d i c a l s . I t i s a l s o u n s u i t a b l e f o r o x i d a t i o n s owing t o i t s easy o x i d i z a b i l i t y . The major d i s a d v a n t a g e of a p l a t i n u m w o r k i n g e l e c t r o d e i s t h a t i f the d e p o l a r i z e r o r e l e c t r o l y s i s p r o d u c t s c h e m i c a l l y i n t e r a c t w i t h , or a d s o r b o n t o , the e l e c t r o d e i r r e v e r s i b l y , t he v o l t a m m e t r i c r e s p o n s e can be s e v e r e l y d i s t o r t e d . The p r e t r e a t m e n t p r o c e d u r e c l e a n s the e l e c t r o d e by o x i d i z i n g i m p u r i t i e s and the m e t a l s u r f a c e w i t h HN0 3, and s u b s e q u e n t l y r e d u c i n g the c l e a n e d s u r f a c e w i t h Fe** i n the form of f e r r o u s ammonium s u l f a t e . Care must be t a k e n t o r i n s e the e l e c t r o d e v e r y t h o r o u g h l y w i t h d i s t i l l e d water a f t e r t h i s t r e a t m e n t . The use of c h r o m i c a c i d t o p r e t r e a t the e l e c t r o d e e v e n t u a l l y r e s u l t s i n the p l a t i n u m s u r f a c e becoming d u l l 18a g r e y , p r o b a b l y due t o a d s o r p t i o n of d i c h r o m a t e i o n s , and t h i s p r a c t i c e has not been c o n t i n u e d . The p r e t r e a t m e n t p r o c e d u r e o u t l i n e d i n the E x p e r i -m e n t a l S e c t i o n p r o v i d e s r e p r o d u c i b l e r e s u l t s i n o b t a i n i n g c y c l i c v o l t a m -mograms and the p l a t i n u m - b e a d e l e c t r o d e appears t o be b o t h c o n v e n i e n t and s u i t a b l e . P r e l i m i n a r y w o r k i n g e l e c t r o d e s employed p l a t i n u m - b e a d s s e a l e d i n a uraniu m g l a s s , w h i c h proved t o be h i g h l y u n r e l i a b l e . W h i l e s u i t a b l e c y c l i c voltammograms can be o b t a i n e d on o c c a s i o n when t h e s e e l e c t r o d e s a r e new, d r a s t i c and u n d e s i r a b l e changes i n background o c c u r a f t e r a few e x p e r i m e n t s as shown i n F i g u r e 6. When the p l a t i n u m - b e a d i s s e a l e d i n a g l a s s h a v i n g a s i m i l a r c o e f f i c i e n t of t h e r m a l e x p a n s i o n to p l a t i n u m , t h i s p roblem i s not 30 ( a ) p o t e n i a l c u r r e n t Figure 6 . Background CV scans obtained at a platinum-bead electrode in 0.1 M [n-Bu 4N]PF 6/CH 2Cl 2' ( a) f ° r a normal electrode, and; (b) a "cracked" electrode. 31 encountered. Apparently, the platinum-glass seals of the f i r s t - g e n e r a t i o n electrodes r e a d i l y deteriorate with heating due to differences i n the glass and platinum thermal expansion c o e f f i c i e n t s , causing microscopic cracks, 26 which i n turn r e s u l t i n large capacitances. A high capacitance, of course, generates a large charging-current, hence the thick background. A mercury contact on the inside of the working electrode i s used i n place of soldered wire contacts. The l a t t e r arrangement i s thought to s t r a i n the platinum-glass s e a l , due to frequent attachment and detachment of the wire leads from the potentiostat. Secondly, i f the soldered wire connection breaks i t i s awkward to r e p a i r . The reference electrode, an aqueous SCE, i s chosen simply for convenience. It can be r e a d i l y prepared and maintained for a long period of time (depending on frequency of use). Eventually i t s p o t e n t i a l begins to d r i f t somewhat to negative p o t e n t i a l s . This i s e a s i l y detected by the ferrocene-ferricinium couple. I n i t i a l c e l l designs incorporating a r e f e r e n c e - c e l l holder s i m i l a r to the one shown in Figure 3 (but with much smaller sintered-glass f r i t s ) produced severe problems. In the process of sealing i t into the reference c e l l housing the f r i t becomes la r g e l y fused. C y c l i c voltammograms can be obtained with this kind of arrangement; but suffer from wild, e r r a t i c f l u c t u a t i o n s i n current and p o t e n t i a l . Even movements of the hand i n the v i c i n i t y of the reference electrode connection cause these sudden changes. The f r i t did not appear to be completely fused, since the potentiostat could apply a controlled p o t e n t i a l without overloading, however, the shi e l d i n g caused by the defective f r i t must be 32 extremely h i g h , making the electrometer probe h y p e r s e n s i t i v e to minute v a r i a t i o n s i n l o c a l e l e c t r i c f i e l d s . The f i n a l c e l l design e v e n t u a l l y a r r i v e d at has proven to be q u i t e s u i t a b l e . Ferrocene, which i s always added near the end of a c y c l i c voltammetry experiment, serves as an i n t e r n a l standard to check the perfor mance of the reference electrode and the c e l l i t s e l f . I t s peak o x i d a t i o n p o t e n t i a l , E , i s i n v a r i a n t with scan r a t e over the range a v a i l a b l e ; pa i / i = 1 at a l l scan r a t e s ; and AE = 70 mV. Why the AE value i s p,c p,a p p ~10 mV greater than the t h e o r e t i c a l 59 mV i s not c l e a r , though r e s i s t a n c e e f f e c t s may be i n v o l v e d . The o x i d a t i o n p o t e n t i a l of ferrocene i n CH 2C1 2 i +0.47 V and i n CH3CN i s +0.37 V with AE p = 60 mV. The thermodynamic oxida t i o n p o t e n t i a l of ferrocene i s thought to be f a i r l y constant i n a v a r i e t y 19 of organic solvents and the observed d i f f e r e n c e here may be due to a combination of r e s i s t a n c e and j u n c t i o n p o t e n t i a l e f f e c t s . I f a redox couple gives s i m i l a r behaviour to ferrocene, i t i s taken to be e l e c t r o -chemically and chemically r e v e r s i b l e . The use of ferrocene as an i n t e r n a l standard provides some recourse f o r other workers who might wish to compar E l / 2 v a ^ u e s obtained from a d i f f e r e n t experimental arrangement to those quoted here, by r e f e r e n c i n g them to ferrocene. C y c l i c voltammograms obtained under the con d i t i o n s o u t l i n e d above are ge n e r a l l y r e p r o d u c i b l e w i t h i n ±20 mV. (b) The Bulk E l e c t r o l y s i s C e l l . The co n d i t i o n s f o r bulk e l e c -t r o l y s i s and c y c l i c voltammetry are very d i f f e r e n t . In a bulk e l e c t r o l y s i experiment, high i n i t i a l currents are d e s i r a b l e i f the r e a c t i o n i s to be completed i n a reasonable length of time. This i s f a c i l i t a t e d by a l a r g e 33 working electrode surface area, or more a p p r o p r i a t e l y , a large s u r f a c e -area-to-volume r a t i o ; r a p i d , e f f i c i e n t s t i r r i n g ; and c a r e f u l placement of 22 the reference e l e c t r o d e . The working electrode employed i n t h i s study i s a platinum-gauze sheet (again chosen because of i t s c o n d u c t i v i t y and r e l a -t i v e chemical i n e r t n e s s ) of about 4x7 cm, folded i n h a l f . The a u x i l i a r y and working electrodes are separated by a distance of ~1 cm and the former i s i s o l a t e d from the working electrode compartment by a glass tube with a f i n e f r i t (~3 cm diameter) at i t s base. This i s intended to minimize mixing of the contents of the anode and cathode compartments. The choice of such a separator i s always a compromise between two opposing f a c t o r s : the maximization of i o n i c m i g r a t i o n of the support e l e c t r o l y t e , and the minimization of migration by the reactants and products. A f i n e , s i n t e r e d -g l a s s d i s k hinders mixing of the s o l u t i o n s i t separates moderately w e l l f o r short periods of time (<2 h ) , but a l s o adds s u b s t a n t i a l r e s i s t a n c e to i o n i c m i g r a t i o n . I t i s , however, a convenient compromise. Again, i f the pro-ducts or reactants are i o n i c species the support e l e c t r o l y t e should be i n excess. The f i r s t a u x i l i a r y electrode used had been a s i l v e r - w i r e c o i l , which works w e l l f o r o x i d a t i o n r e a c t i o n s . For reductions, though the s i l v e r a u x i l i a r y e l e ctrode r e a d i l y o x i d i z e s to form AgPFg which migrates i n t o the working compartment under the i n f l u e n c e of the applied e l e c t r i c f i e l d . The a u x i l i a r y e l e c t r o d e , t h e r e f o r e , has been changed to a platinum-gauze c y l i n d e r . Reduction r e a c t i o n s c a r r i e d out at the working electrode r e s u l t i n the production of a gas at the anode, as evidenced by vigorous bubbling i n the a u x i l i a r y compartment, which fumes p r o f u s e l y upon exposure to a i r . This gas probably r e s u l t s from the o x i d a t i o n of the e l e c t r o l y t e and 34 may i n t e r f e r e with the process occurring in the working compartment. An attempt to f l u s h the c e l l continuously with N 2, with the aid of gas i n l e t s as shown in Figure 6 i s moderately successful. The process occurring i n the a u x i l i a r y compartment during oxidation reactions at the working electrode i s probably reduction of the solvent, or perhaps of [n-Bu^N]"1". The l a s t factor needed for e f f i c i e n t e l e c t r o l y s e s (placement of the reference electrode) i s probably the most c r i t i c a l . The closer to the working electrode t h i s electrode can be placed, the less severe i s the Ohmic drop. A Luggin-type probe i s used on occasion to keep the electrode i n place. Better performance i s obtained i f the electrode i s housed i n a glass tube which does not cover the whole length of the wire, the l a s t segment of i t being covered with a c l o s e - f i t t i n g polyethylene tube to within ~0.5 cm of the t i p . This helps to prevent contact between the working and reference electrodes, but allows for minimum separation. The electrode i s placed on, or near to, a l i n e of minimum separation between the working and a u x i l i a r y electrodes. Displacement of the reference wire toward the edges of the working electrode produces higher currents, l i k e l y due to a non-uniform e l e c t r i c f i e l d d i s t r i b u t i o n . Under such conditions i t i s recommended that the reference be kept at the centre of the working and 22 a u x i l i a r y electrodes arrangement. A s i l v e r - w i r e i s chosen since an aqueous SCE would be far more awkward to use. The p o t e n t i a l control obtained with a wire quasi-reference electrode i s not as dependable as that given by a true reference electrode; but i t i s found to be adequate. In addition, a s i l v e r - w i r e and an aqueous SCE provide s i m i l a r reference p o t e n t i a l s . 35 The b e s t performance of the c e l l i s o b t a i n e d when r e v e r s i b l e , c l e a n e l e c t r o n t r a n s f e r r e a c t i o n s a r e s t u d i e d . For example, the o x i d a t i o n of f e r r o c e n e (0.19 g, 1.0 mmol) i n C H 2 C 1 2 a t ~ +0.9 V proceeds to c o m p l e t i o n w i t h i n 30 min at room t e m p e r a t u r e w i t h an i n i t i a l c u r r e n t of ~150 mA to produce [ ( T I 5 - C 5 H 5 ) 2 F e ] P F 6 . (c) The Scope and Limitations of the Electrochemical Methodology. The p r o c e d u r e s and methods d e t a i l e d above c l e a r l y do not approach the r i g o r o u s and " s u p e r - d r y " c o n d i t i o n s employed i n s t a t e - o f - t h e - a r t e l e c t r o -c h e m i c a l r e s e a r c h . * ^ The s o l v e n t and s u p p o r t i n g e l e c t r o l y t e systems used h e r e i n must s t i l l c o n t a i n s m a l l r e s i d u a l q u a n t i t i e s of water and t r a c e s o f o t h e r i m p u r i t i e s . (However, the a d d i t i o n of d r y a l u m i n a t o the s o l v e n t s does improve the background m a r k e d l y . A l u m i n a has been used d i r e c t l y 27 i n s i d e e l e c t r o c h e m i c a l c e l l s by some workers but t h i s c o u l d g r e a t l y reduce the d e p o l a r i z e r c o n c e n t r a t i o n due t o a d s o r p t i o n onto the a l u m i n a . ) T r a c e s of water or oxygen can t u r n i n h e r e n t l y c h e m i c a l l y r e v e r s i b l e c o u p l e s i n t o c h e m i c a l l y i r r e v e r s i b l e ones i n some c a s e s . Y e t , the o b t a i n i n g of " s u p e r - d r y " and r i g o r o u s l y o x y g e n - f r e e c o n d i t i o n s r e q u i r e s c o n s i d e r a b l e e x p e n d i t u r e of t i m e , e f f o r t and r e s o u r c e s . I f a p a r t i c u l a r r e d o x c o u p l e i s so e x c e e d i n g l y s e n s i t i v e to t r a c e i m p u r i t i e s i t w i l l be v e r y d i f f i c u l t t o i n v e s t i g a t e on a s y n t h e t i c b a s i s , and t h u s , f o r the purposes of a s y n t h e t i c o r g a n o m e t a l l i c c h e m i s t , the methodology employed i n t h i s s t u d y i s a d e q u a t e . I n d e e d , as w i l l be demonstrated i n the f o l l o w i n g c h a p t e r s , some v e r y chemi c a l l y s e n s i t i v e redox p r o c e s s e s can be s t u d i e d w i t h the c y c l i c v o l t a m m e t r i c methodology o u t l i n e d above. 36 Chapter Three REDOX STUDIES OF BIS[(T) 5-CYCLOPENTADIENYL)DINITROSYLCHROMIUM] AND RELATED COMPLEXES I ) Introduction At the commencement of t h i s study the preparation and c h a r a c t e r i z a -t i o n of the b i m e t a l l i c c a t i o n s [ ( T) 5-C 5H 5) 2M 2(NO) L . H ] + (M = Mo,W) had been 28 accomplished. S a l t s of these ca t i o n s are obtained r e a d i l y by the re a c t i o n of the monomeric compounds (r)5-C5H5)M(NO) 2H (M = Mo or W) with one h a l f equivalent of h y d r i d e - a b s t r a c t i n g carbocation-containing s a l t s such as [Ph 3C]BF( +. Unlike r e l a t e d carbonyl-hydride complexes, the b i m e t a l l i c c ations cannot be deprotonated by various bases, i n c l u d i n g t y p i c a l l y non-coo r d i n a t i n g bases (however, [ ( T I 5-C 5 H 5 ) M (CO ) 3 ] 2 (M = Mo,W) can be r e v e r -29 s i b l y protonated by H 2SOi t ) and consequently are not s u i t a b l e precursors to the as yet unknown [( T} 5-C 5H5)M(NO) 2 ] 2 dimers. Rather, the b i m e t a l l i c c a t i o n s are cleaved by bases to form the monomeric species ( TI 5-C5H5)M(NO) 2H and [(r)5-C5H5)M(NO) 2(B) ] + (B = base). The question a r i s e s , t h e r e f o r e , as to whether [ (r) 5-C 5H 5)Cr(NO) 2 ] 2 can be protonated to form the analogous hydride complex or not. I t has been found, though, that the dimeric chromium compound undergoes f a c i l e cleavage to form [ (r) 5-C 5H 5)Cr(NO) 2 ] + 30 upon treatment w i t h HBF 1 +»0Me 2 i n CH 2C1 2 or CH 3N0 2. This stands i n marked contrast to the behaviour of the i s o e l e c t r o n i c [ (T) 5-C 5H 5)Fe(CO) 2 ] 2 29 30 analogue, which can be c l e a n l y and r e v e r s i b l y protonated ' to form 37 [ { ( T l 5-C 5H 5)Fe(CO) 2} 2H] + . During the r e a c t i o n of [(ri 5-C 5H 5)Cr(NO) 2] 2 w i t h HBF l 4»OMe 2 i n CH 2C1 2 a product i s oft e n obtained, i n low y i e l d s , which at f i r s t had been thought to be the protonated dimer. This, however, proves not to be the case. In an attempt to probe the d i f f e r e n c e s i n r e a c t i v i t y of the two i s o e l e c t r o n i c dimers with HBFlt«OMe2, the el e c t r o c h e m i c a l o x i d a t i o n s of [( n 5-C 5H 5)Cr(NO) 2 ] 2 and [ ( T i 5-C 5H 5)Fe(CO) 2 ] 2 have been i n v e s t i g a t e d to a s c e r t a i n i f the r e l a t i v e ease of o x i d a t i o n of the two dimers can e x p l a i n these d i f f e r e n c e s . As w i l l be described, however, the i r o n - c o n t a i n i n g complex i s more r e a d i l y o x i d i z e d , i n d i c a t i n g that a process other than i n i t i a l e l e c t r o n t r a n s f e r occurs. As the e l e c t r o c h e m i c a l behaviour of [ ( r i 5-C 5H 5)Cr (NO) 2 ] 2 was studi e d i t became c l e a r that i t s redox chemistry i s of i n t e r e s t i n i t s own r i g h t , being s u b s t a n t i a l l y d i f f e r e n t from that of [(r| 5-C 5H 5)Fe(CO) 2 ] 2. This i s i n 31-34 keeping with the divergent chemical r e a c t i v i t i e s of these two 30 molecules, one example of which has been mentioned above. In t h i s chapter the redox behaviour of [ ( r i 5 - C 5 H 5 ) C r ( N O ) 2 ] 2 i s presented and contrasted w i t h that of the ir o n - c a r b o n y l analogue. Some of the c h a r a c t e r i s t i c chemistry of the chromium dimer becomes more r e a d i l y understood as a r e s u l t of these i n v e s t i g a t i o n s . F i n a l l y , the sagacious choice of a reducing agent and r e a c t i o n c o n d i t i o n s f a c i l i t a t e s the synthesis of a nov e l , h i g h l y r e a c t i v e , r a d i c a l anion [ {(r) 5-C 5H 5)Cr(NO) 2 } 2 ] T as i t s [ ( n 5 - C 5 H 6 ) F e ( r i 6 - C 6 M e 6 ) ] + s a l t . 3 5 38 II) Experimental Section The methodology employed for electrochemical work was outlined i n Chapter 2. A l l synthetic work was performed under anaerobic conditions 23 u t i l i z i n g p r e p u r i f i e d nitrogen and conventional techniques unless other-wise noted. A l l solvents were p u r i f i e d according to published pro-36 cedures. CH 2C1 2, benzene CH3CN, n-heptane and THF were d i s t i l l e d from CaH 2; hexanes were d i s t i l l e d from CaH 2 or LiAlH^, and; 1,2-dimethoxyethane and toluene were d i s t i l l e d from Na. The solvents were purged with N 2 just before use. A l l chemicals were of reagent grade or comparable p u r i t y , and were either purchased from commerical suppliers or prepared by published procedures. Standard a n a l y t i c a l and spectroscopic techniques were used to ascertain t h e i r p u r i t y . Unless otherwise stated, reactions were ca r r i e d out at ambient temperature. Infrared spectra were obtained with a Perkin-Elmer model 598 spectrophotometer, c a l i b r a t e d with the 1601 cm - 1 band of a polystyrene f i l m or with a Nicolet model 5DX FT-IR (Fourier Transform-Infrared) instrument. The ESR spectra were obtained using a spectrometer and interfaced computer 37 system operated by Dr. F.G. Herring. Proton magnetic resonance spectra were obtained with a Varian Associates EM-360 spectrometer or a Bruker WP-80 spectrometer. Spectra recorded with the Bruker 80 MHz instrument were provided by the NMR lab s t a f f headed by Dr. S.O. Chan, or with the aid of D.T. Martin and J.T. Martin. Low-resolution mass spectra were recorded with an Atlas CH4B spectrometer or a Kratos MS50 instrument, both at 70 eV, by the s t a f f of the mass-spectrum lab headed by Dr. G.K. Eigendorf. The 39 l a t t e r instrument was also used for high-resolution mass spectra. Elemental analyses were performed by Mr. P. Borda. Electrochemical Oxidation of [ ( t i 5-C 5H 5)Cr(NO ) 2 ]2' T n e e l e c -t r o l y s i s c e l l (Figure 4) was charged with [( T) 5-C 5H 5)Cr(NO) 2] 2 (0.20 g, 0.56 mmol), [n-Bu^NJPFg (8.0 g, 21 mmol), and CH 2C1 2 (200 mL). The s t i r r e d s olution was electrolyzed at +1.00 V for ~0.5 h whereupon i t gradually changed i n color from intense red to dark green. Integration of the current vs. time graph showed that a t o t a l charge of 89 C ( i . e . 0.8 electrons per Cr) had been passed. The f i n a l green solution was trans-ferred by cannulation into a 300-mL f l a s k , and i t s volume was reduced to ~20 mL under reduced pressure. An IR spectrum of this solution was devoid of absorptions c h a r a c t e r i s t i c of the organometallic reactant but did display two new bands at 1838 (s) and 1731 (s,br) cm - 1 i n the n i t r o s y l -stretching region. This s t i r r e d s o l u t i o n was then treated dropwise with a solution of [(Ph 3P) 2N]Cl i n CH 2C1 2 u n t i l the l a t t e r absorptions were replaced completely by bands at 1816 (s) and 1711 (vs) cm - 1 in i t s IR spectrum. The r e s u l t i n g solution was taken to dryness i n vacuo, and the residue was extracted with benzene (2 x 25 mL). The combined extracts were concentrated under reduced pressure to ~5 mL and were transferred by syringe to the top of a short ( 3 x 4 cm) column of F l o r i s i l made up i n CH 2C1 2. E l u t i o n of the column with CH 2C1 2 resulted i n the development of a single green-brown band which was c o l l e c t e d . Removal of solvent from the eluate i n vacuo and r e c r y s t a l l i z a t i o n of the residue from CH 2Cl 2-hexanes afforded ~0.1 g of golden CpCr(N0) 2Cl s t i l l contaminated with a small amount of [n-Bui.N]PF6. The organometallic component was i d e n t i f i e d by i t s 40 c h a r a c t e r i s t i c s p e c t r a l p r o p e r t i e s : IR (CH 2C1 2) 1816 ( s ) , 1711 (vs) cm - 1; *H NMR (C 6D 6) 6 4.78 ( s ) ; l o w - r e s o l u t i o n mass spectrum (probe temperature 50° C), m/z_ 212 ( P + , most intense parent i o n ) . Oxidation of [ ( T ) 5 - C 5 H 5 ) C r ( N 0 ) 2 ] 2 with AgBF^. A mixture of [ ( T ] 5 - C 5 H 5 ) C r ( N O ) 2 ] 2 (0.177 g, 0.500 mmol) and AgBF^ (0.195 g, 1.00 mmol) was treated with CH 2C1 2 (20 mL), and s t i r r e d . A r a p i d r e a c t i o n occurred, the i n i t i a l l y red-brown s o l u t i o n t u r n i n g green. W i t h i n 15 min a l l the s t a r t i n g m a t e r i a l had been consumed, and s i l v e r - m e t a l had deposited, to on generate a s o l u t i o n of ( r i 5-C 5H 5)Cr(NO) 2BF 1 +: IR(CH 2C1 2) v N Q 1845 ( s ) , 1739 (s) cm - 1, a l s o v__ _ 1098 (m.br), 1061 (m), 1020 (m) cm - 1, a l s o 4 3113 cm - 1. No other n i t r o s y l species appeared to be present by IR s p e c t r o -scopy. In a second experiment, (t| 5-C 5H 5)Cr(NO) ^ F ^ was generated i n the same manner and then treated w i t h a s t o i c h i o m e t r i c q u a n t i t y of [(Ph 3P) 2N]C1. An IR spectrum of the r e s u l t a n t s o l u t i o n showed only bands i c 39 f o r NO-stretching at 1817 and 1711 cm - 1 due to ( T] 5-C 5H 5)Cr(NO) 2C1. The same i n f r a r e d spectrum was recorded i n the absorbance mode and the peak absorbances of the two NO bands were determined and compared with l i n e a r Beer's Law p l o t s ( f o r the 1816 and 1711 cm - 1 bands) obtained f o r a u t h e n t i c s o l u t i o n s of ( T) 5-C 5H 5)Cr(NO) 2C1 i n CH 2C1 2 of known concentrations, the same thickness IR c e l l being used throughout. This a n a l y s i s revealed e s s e n t i a l l y q u a n t i t a t i v e conversion of the dimer to ( r| 5-C 5H 5)Cr(NO) 2C1. Oxidation of [ T ) 5 - C 5 H 5 ) C r ( N O ) 2 ] 2 with AgPF 6. A s o l u t i o n of ( • n 5-C 5H 5)Cr(NO) 2PF 6 was generated i n the same manner as described above f o r the B F u ~ - c o n t a i n i n g analogue. A CH 2C1 2 s o l u t i o n (30 mL) c o n t a i n i n g 41 [ r | 5 - C 5 H 5 ) C r ( N O ) 2 ] 2 (0.177 g, 0.500 mmol) was tr e a t e d w i t h s o l i d AgPFg (0.253 g, 1.00 mmol), and over the course of ~30 min s i l v e r - m e t a l p r e c i p i t a t e d to give a green-brown s o l u t i o n : IR(CH 2C1 2) v N Q 1835 ( s ) , 1731 (vs) cm - 1, a l s o v _ 848 (m) cm - 1, a l s o 3112 (m) cm - 1. Reaction of [ ( T j 5 - C 5 H 5 ) C r ( N O ) 2 ] 2 with HBF 1 | »0Me 2 . A s t i r r e d s o l u t i o n of [ ( r ) 5 - C 5 H 5 ) C r ( N O ) 2 ] 2 (0.21 g, 0.59 mmol) i n CH 2C1 2 (25 mL) was tre a t e d w i t h 13.6 M HBF 1 +»0Me 2 (0.090 mL, 1.2 mmol) r e s u l t i n g i n an immed-i a t e colour change from red-brown to green-brown and the p r e c i p i t a t i o n of a small amount of a dark s o l i d . An IR spectrum of the supernatant s o l u t i o n e x h i b i t e d two strong, sharp bands at 1838 and 1728 cm - 1, c o n s i s t e n t w i t h 30 the formation of [ ( T i 5-C 5H 5)Cr(NO) 2 ] + . The r e a c t i o n mixture was f i l t e r e d through a column of C e l i t e ( 2 x 3 cm), and the volume of the f i l t r a t e was reduced to ~10 mL i n vacuo. The f i l t r a t e was tr e a t e d dropwise with E t 2 0 u n t i l the mixture became t u r b i d , and i t was then cooled to -10°C f o r 48 h. F i l t r a t i o n of the f i n a l , cooled mixture afforded v a r i a b l e amounts ( t y p i c a l l y 0.04-0.10 g, 15-36% y i e l d based on Cr) of a n a l y t i c a l l y pure [ {(n 5-C 5H 5)Cr(NO) 2> 2OH]BF 1 + as a m i c r o c r y s t a l l i n e , dark green s o l i d : IR (Nujol mull) v 1806 ( s , b r ) , 1677 (s,br) cm - 1, a l s o v„,T 3505 (w,br) cm - 1, a l s o 3115 (w) cm - 1, a l s o v _ on at, 1088 (m,br), 1054 (m.br), 1014 (m) cm - 1; IR (CH 2C1 2) v N Q 1820 ( s ) , 1807 ( s h ) , 1719 ( s . b r ) , 1698 (sh) cm - 1, a l s o v„ u 3484 (w,br) cm - 1, a l s o 3109 Uri (w) cm - 1, a l s o v__ - 1076 cm - 1; LH NMR ( CDClo) 6 5.78 ( s , 10H, CgJJg), 0.11 BF, ( s , IH, OH). Anal. Calcd f o r C 1 0H 1 1Cr 2N 4O 5BF l t: C, 26.22; H, 2.42; N, 12.23. Found: C, 25.87,; H, 2.51; N, 12.26. 42 The same product was isolated in a similar manner, and in similar yields when 0.01 mL of water was added deliberately to the reaction mixture. Alternate Preparation of [{(Ti 5-C 5H 5)Cr(N0) 2} 20H]BF^. A stirred golden solution of (T)5-C5H5)Cr(NO) 2C1 (0.47 g, 2.2 mmol) in CH2C12 (25 mL) was treated with solid AgBF^ (0.43 g, 2.2 mmol). A white precipitate formed immediately, and the supernatant solution became dark green. After stirring for ~15 min to ensure completion of the reaction, the mixture was filtered to obtain a dark green solution whose IR spectrum displayed absorptions at 1838 and 1728 cm - 1. Treatment of the f i l t r a t e with H20 (0.020 mL, 1.1 mmol) and stirring the resultant mixture for 18 h produced no change in these bands. The mixture was then treated with ~5.4 M aqueous KOH (0.20 mL, ~1.1 mmol), and became darker green. A dark green o i l deposited as well. After 0.5 h the f i n a l reaction mixture was taken to dryness in vacuo, and the residue was extracted with CH2C12 (3 x 10 mL). The combined extracts were concentrated in vacuo to ~7 mL, and Et 20 was added dropwise to induce the formation of a dark green microcrystalline solid. This solid was collected by f i l t r a t i o n to afford 0.21 g (41% yield based on Cr) of [ { ( T i 5-C 5H 5)Cr(N0) 2} 20H]BFi+ which was identified by i t s characteristic spectroscopic properties (see above). The use of a stoichiometric amount of Et 3N in place of KOH in the procedure above produced the identical product, but in only 12% yield. Metathesis of [ { ( T i 5 - C 5 H 5 ) C r ( N 0 ) 2 } 2 0 H ] B F 1 | with Na[BPh H]. A solution of Na[BPhH] (0.31 g, 0.90 mmol) in H20 (70 mL) was added dropwise to a stirred suspension of [ { ( r i 5-C 5H 5)Cr(NO) 2)20R]oF^ (0.41 g, 0.90 mmol) 43 i n H 20 (40 mL), r e s u l t i n g i n the formation of a f l o c c u l e n t yellow-green p r e c i p i t a t e . The f i n a l mixture was s t i r r e d f o r 0.5 h and f i l t e r e d . The c o l l e c t e d s o l i d was d r i e d i n vacuo and then r e c r y s t a l l i z e d from CH 2Cl2 -Et20 to o b t a i n 0.30 g (48% y i e l d ) of a n a l y t i c a l l y pure [{(Ti 5-C 5H 5)Cr(NO) 2} 2OH][BPh 1 +] as a yellow-green s o l i d : IR (CH 2C1 2) v 1822 ( s ) , 1812 ( s h ) , 1721 ( s ) , 1703 (sh) cm - 1, al s o v._ 3505 (w.br) cm - 1; UH *H NMR ((CD 3) 2CO) 6 7.47-6.77 (m, 20H, B ( C 6 H 5 ) 4 ) , 5.92 ( s , 10H, CgHg), 0.83 ( s , IH, OH). Anal. Calcd f o r C 3 l + H 3 ] B C r 2 N 4 0 5 : C, 59.15; H, 4.53; N, 8.12; 0, 11.59. Found: C, 59.13; H, 4.64; N, 8.02; 0, 11.22. Reduction of [ (n 5-C 5H 5)Cr(N0) 2(CH 3CN)]PF g. A d d i t i o n of c 40 [ ( r i 5 - C 5 H 5 ) C r ( N O ) 2 ( C H 3 C N ) ] P F 6 (0.132 g, 0.364 mmol), [n-B U i +N]PF 6 (6.0 g, 15 mmol), and CH 2C1 2 (150 mL) to the e l e c t r o l y s i s c e l l produced a green s o l u t i o n . This s t i r r e d s o l u t i o n was e l e c t r o l y z e d at -0.40 to -0.47 V f o r —1 h u n t i l the current had decreased to < 1% of i t s i n i t i a l v alue, a t o t a l charge of 31 C ( i . e . 0.9 e l e c t r o n s per Cr) having been passed. The f i n a l red s o l u t i o n was t r a n f e r r e d by c a n n u l a t i o n , i n t o a 500-mL f l a s k and concentrated i n vacuo to —15 mL. An IR spectrum of t h i s s o l u t i o n e x h i b i t e d v N 0 ' s at -1830 (w), -1720 (w) and -1670 (s) cm - 1. A f t e r removal of the CH 2C1 2 under reduced pressure, the remaining residue was extracted with benzene (4 x 25 mL), and the combined e x t r a c t s were reduced i n volume to -10 mL i n vacuo. Chromatography of t h i s concentrate on a F l o r i s i l column (3 x 10 cm) with benzene as eluant produced a red-brown band which was el u t e d and c o l l e c t e d . Removal of solvent from the eluate under reduced pressure afforded 0.03 g (20% y i e l d based on Cr) of s o l i d 44 [ ( n 5-C 5H 5)Cr(NO) 2] 2 which was i d e n t i f i e d by i t s d i s t i n c t i v e s p e c t r a : ' IR (CH 2C1 2) v N Q 1667 ( s ) , 1512 (m) cm - 1; lYL NMR (CDC1 3) 6 5.17 ( s , t r a n s ) , 4.95 ( s , c i s ) ; l o w - r e s o l u t i o n mass spectrum (probe temperature 120°C), m/z_ 354 ( P + , most intense parent i o n ) . Reduction of [ ( T i 5 - C 5 H 5 ) C r ( N O ) 2 l 2 . A s t i r r e d 0.1 M s o l u t i o n of [n-Bu 4N]PF 6 i n CH 2C1 2 (100 mL) a l s o c o n t a i n i n g [ ( n 5 - C 5 H 5 ) C r ( N O ) 2 ] 2 (0.374 g, 1.06 mmol) was e l e c t r o l y z e d at -1.00 V. The current decayed very slowly as the c o l o r of the s o l u t i o n changed from intense red to brown. The e l e c t r o l y s i s was stopped a f t e r 470 C ( i . e . 4.8 e l e c t r o n s per C r 2 ) had been passed and the current had dropped to ~2% of i t s i n i t i a l v alue. An IR spectrum of the f i n a l s o l u t i o n revealed that the n i t r o s y l bands c h a r a c t e r i s t i c of the organometallic reactant (see above) had disappeared completely and had been replaced by a broad, weak absorption at -1610 cm - 1. In a subsequent experiment, an i d e n t i c a l i n i t i a l mixture was reduced at -1.10 V, but the e l e c t r o l y s i s was stopped a f t e r 192 C ( i . e . 1.9 e l e c t r o n s per C r 2 ) had been passed. The r e s u l t i n g brown s o l u t i o n was t r a n s f e r r e d by cannulation i n t o a 500-mL f l a s k and was taken to dryness i n vacuo. The r e s i d u a l s o l i d was extracted with benzene u n t i l the e x t r a c t s were only f a i n t l y coloured ( t o t a l volume ~200 mL), and the volume of the combined e x t r a c t s was reduced to ~5 mL under reduced pressure. The concentrate was syringed onto a F l o r i s i l column (2 * 11 cm) made up i n benzene. E l u t i o n of the column with benzene f i r s t removed a red-brown band which contained 0.013 g (3%) of [ ( n 5 - C 5 H 5 ) C r ( N O ) 2 ] 2 , then a green band which upon c o l l e c t i o n and solvent removal afforded 0.029 g (8% y i e l d ) of 45 (n 5-C 5H 5) 2Cr 2(NO) 3(NH 2) which was i d e n t i f i e d by i t s spectroscopic p r o p e r t i e s ' : IR (CH 2C1 2) v N Q 1642 ( s ) , 1511 ( s h ) , 1503 (m) cm"1; l o w - r e s o l u t i o n mass spectrum (probe temperature 120°C), m/z 340 ( P + , most intense parent i o n ) . Further e l u t i o n of the column with CH 2C1 2 produced an orange band which was c o l l e c t e d and taken to dryness i n vacuo to obtain an orange s o l i d (~0.05 g) composed of [n-Bu 1 +N]PF 6 and [ ( n 5 - C 5 H 5 ) C r ( N 0 ) ( N H 2 ) ] 2 . 33 The l a t t e r compound was a l s o i d e n t i f i e d s p e c t r o s c o p i c a l l y : IR (THF) v N Q 1625 (m) cm - 1; l o w - r e s o l u t i o n mass spectrum (probe temperature 120°C), m/z_ 340 ( P + , most intense parent i o n ) . F i n a l l y , e l u t i o n of the column with THF r e s u l t e d i n the development of a brown band which was eluted and taken to dryness under reduced pressure to o b t a i n a small amount of a brown s o l i d : IR (THF) v N Q 1656 ( s ) , 1641 (sh) cm - 1. Reduction of (T| 5-C 5H 5)Cr(NO) 2Cl. A s t i r r e d , golden-green c o l o u r -c 39 ed, CH 2C1 2 s o l u t i o n (100 mL) c o n t a i n i n g (n 5-C 5H 5)Cr(NO) 2C1 (0.096 g, 0.45 mmol) and [n-Bu^NJPFg (4.0 g, 10 mmol) was reduced i n the e l e c t r o l y s i s c e l l at -1.00 V u n t i l the current had diminished from a peak of 140 mA to 20 mA (6 min). At t h i s p o i n t , the current began to increase s l i g h t l y . Consequently, the p o t e n t i a l was reduced to -0.69 V, and e l e c t r o l y s i s was continued u n t i l the current had decayed to < 4 mA. D u r i n g . t h i s time a t o t a l charge of 44 C ( i .e. 1.0 e l e c t r o n s per Cr) had been passed, and the c o l o r of the s o l u t i o n had changed to dark brown. The f i n a l s o l u t i o n was then cannulated i n t o a 500-mL f l a s k and was concentrated under reduced pressure to ~15 mL. An IR spectrum of t h i s s o l u t i o n displayed only a s i n g l e band at ~1640 cm - 1 a t t r i b u t a b l e to NO s t r e t c h i n g . Various attempts 46 to i s o l a t e a t r a c t a b l e n i t r o s y l - c o n t a i n i n g product from t h i s s o l u t i o n were unsu c c e s s f u l . Oxidation of [ ( n 5 - C 5 H 5 ) F e ( C O ) 2 ] 2 . The e l e c t r o l y s i s c e l l was charged w i t h 6 g of [n-Bu^NjPFg and CH 2C1 2 (150 mL) to make a 0.1 M s o l u t i o n . To the working compartment was added [(n 5-C 5H 5)Fe(C0) 2 ] 2 (0.253 g, 0.713 mmol) and the s o l u t i o n was e l e c t r o l y z e d at +0.80 V. The current r a p i d l y dropped o f f toward zero and a t o t a l of 27 C was passed ( i . e . 0.2 e l e c t r o n s per Fe). An IR spectrum of the red s o l u t i o n showed mostly unreacted [ ( n 5 - C 5 H 5 ) F e ( C O ) 2 ] 2 but a l s o two bands at 1076 and 2030 cm - 1. The s o l u t i o n was then tr e a t e d with excess [ ( P h 3 P ) 2 N ] C l and the higher bands disappeared, while a new band grew i n at 2054 cm - 1 due to (r| 5-C 5H 5)Fe(CO) 2C1 ( i d e n t i f i e d by comparison w i t h an aut h e n t i c sample i n CH 2C1 2), the lower band of which was obscured by the presence of the more intense bands of the dimer. c c 44 45 Preparation of [ ( n 5 - C 5 H 5 ) F e ( T t 6 - C 6 M e 6 ] P F 6 . To a 2-L, t h r e e -necked f l a s k , equipped with a condenser and gas i n l e t , was added A1C1 3 (111 g, 0.83 mol), aluminum-powder (5.5 g, 0.20 mol), ferrocene (38.8 g, 0.21 mol) and C 6 ( C H 3 ) 6 (45 g, 0.45 mol), followed by a d d i t i o n of n-heptane (~700 mL) by c a n n u l a t i o n . The mixture was s t i r r e d w i t h a mechanical s t i r r e r f o r a few minutes and then H 20 (3.5 mL, 0.19 mol) was added. The mixture was then s t i r r e d f o r 12 h at 73°C, using a thermostatted water bath. At the end of t h i s time the f l a s k was removed from the warm water bath and cooled to 0°C. I c e - c o l d water (~600 mL, purged with N 2) was slowly added to the cooled r e a c t i o n mixture with s t i r r i n g . A f t e r ~15 min the r e s u l t a n t two-phase mixture was t r a n s f e r r e d to a large separation 47 funnel and the aqueous layer removed into a large Erlenmeyer f l a s k . The organic layer was washed in several portions with a t o t a l of ~700 mL of H 20 and the washings and o r i g i n a l aqueous layer were combined. Dilute aqueous ammonia was added to the s t i r r e d , yellow-brown so l u t i o n u n t i l the pH was nearly 7, as indicated by Litmus paper. As pH 7 was approached a thick, gelatinous p r e c i p i t a t e formed. This mixture was then f i l t e r e d through a large Btfchner funnel, and washed with three portions of ~150 mL of water, a l l of which required several hours. The f i l t r a t e was s t i r r e d and treated with aqueous HPF 6 (60-65%) to p r e c i p i t a t e a flocculent yellow s o l i d , which was c o l l e c t e d by f i l t r a t i o n and washed with H 20. This s o l i d was r e c r y s t a l l i z e d from acetone - E t 2 0 to y i e l d 25.1 g of [ ( n 5-C 5H 5)Fe ( T i 6-C 6Me 6)]PF 6 (28% y i e l d based on ferrocene) as a yellow powder: lE NMR ((CD 3) 2C0) 6 4.74 (s, 5H, C 5H 5), 2.54 (s, 18H, C 6 ( C H 3 ) 6 ) . c (• 45 Preparation of ( t i : ) - C 5 H 5 ) F e ( T i 0 - C g M e 6 ) . A suspension of [ ( r i 5 - C 5 H 5 ) F e ( T i 6 - C 6 M e 6 ) ] P F 6 (9.3 g, 22 mmol) i n DME (~100 mL) was s t i r r e d over sodium amalgam (1 . 5 g, 65 mmol of Na i n 163 g, 12 mL of Hg) for 75 min to give a very intense, dark green solu t i o n , which was f i l t e r e d away from the amalgam. The f i l t r a t e was taken to dryness i n vacuo, and the s o l i d extracted with hexanes (4 x 50 mL). The combined extracts were f i l t e r e d through a medium-porosity f r i t and taken to dryness under reduced pressure to give a green s o l i d (3.23 g, -50% y i e l d ) . The ( r i 5-C 5H 5)Fe ( T i 6-C 6Me 6) r a d i c a l i s extremely a i r - s e n s i t i v e and d i f f i c u l t to obtain a n a l y t i c a l l y pure. Preparation of [ ( T ) 5 - C 5 H 5 ) F e ( T i 6 - C 6 M e 6 ) ] [ { ( T i 5 - C 5 H 5 ) C r ( N O ) 2 } 2 ] . This reaction was performed under argon. To a s t i r r e d , red solution of 48 [ ( T i 5-C 5H 5)Cr(NO) 2 ] 2 (0.46 g, 1.3 mmol) i n E t 2 0 (250 mL) at room temperature was added by f i l t r a t i o n a dark green s o l u t i o n of ( T i 5-C 5H 5)Fe(n 6-C 6Me 6) (0.36 g, 1.3 mmol) i n the same solvent (50 mL). A gray p r e c i p i t a t e formed i n s t a n t l y , and the f i n a l mixture was s t i r r e d f o r 15 min to ensure comple-t i o n of the p r e c i p i t a t i o n . The p r e c i p i t a t e was then c o l l e c t e d by f i l t r a -t i o n through a medium-porosity f r i t , washed with E t 2 0 (150 mL), and d r i e d i n vacuo (5 * 1 0 - 3 mm) f o r 30 min at 20°C to obtain 0.56 g (69% y i e l d ) of a n a l y t i c a l l y pure [( n 5-C 5H 5)Fe ( T i 6-C 6Me 6)] [ { ( T i 5-C 5H 5)Cr(NO) 2 } 2 ] as an o l i v e -green, extremely a i r - s e n s i t i v e powder: IR (Nujol mull) v 1580 ( s ) , 1331 (s,br) cm - 1; a l s o 3177 (w), 3091 (m), 1417 (m), 1392 ( s h ) , 1073 (m), 1011 (m), 796 (s) cm - 1. Anal, c a l c d f o r C 2 7H 3 3CrgFeN^O,: C, 50.88; H, 5.22; N, 8.79; 0, 10.04. Found: C, 50.76; H, 5.21; N, 8.50; 0, 9.95 (using V 2 0 5 as a combustion a d d i t i v e ) . 49 III) Results and Discussion C y c l i c Voltammetry Studies, (a) [(T) 5-C 5H 5)Cr ( N 0 ) 2 ] 2 * Ambient-temperature c y c l i c voltammograms of [ ( n 5 - C 5 H 5 ) C r ( N O ) 2 ] 2 i n CH 2C1 2 are disp l a y e d i n Figure 7. From these and the r e s u l t s of bulk e l e c t r o l y s e s , i t i s i n f e r r e d that the p r i n c i p a l transformations undergone by the dimer when el e c t r o n s are e i t h e r removed from or added to i t are as summarized i n the f i r s t part of Scheme I . When p o s i t i v e p o t e n t i a l s are scanned f i r s t ( Figure 7a), an i r r e v e r s i b l e , two-electron o x i d a t i o n corresponding to step (a) i n Scheme I having E = +0.85 V vs. SCE at a scan r a t e of 0.11 V s - 1 i s p,a observed. The peak c u r r e n t , i v a r i e s l i n e a r l y w i t h v 1 / 2 over a range of p ,a scan rates from 0.08 - 0.23 V s - 1 (The p l o t of i vs. v i s d i s t i n c t l y p ,a non-linear.) Reversal of t h i s scan past the peak p o t e n t i a l r e s u l t s i n the se q u e n t i a l d e t e c t i o n of the f o l l o w i n g f e a t u r e s : (1) a chemically i r r e v e r -s i b l e r eduction w i t h E = -0.21 V corresponding to step ( b ) , (2) two p, c smaller reductions at E = -0.47 and -0.69 V due to the a d d i t i o n of e l e c -p,c trons to other products l a b e l e d (c) i n Scheme I and (3) a r e v e r s i b l e one-e l e c t r o n r e d u c t i o n at E 1 / 2 = ~I»00 V having AE^ = 80 mV and a r a t i o of peak cu r r e n t s , i / i = 0.97 which corresponds to step ( d ) . Consistent w i t h p,a p,c these assignments i s the f a c t that i f a CV i s recorded i n i t i a l l y between 0 and -1.4 V, only the r e v e r s i b l e reduction at -1.00 V i s evident (Figure 7b). However, a f t e r s e v e r a l complete scans of the type shown i n Figure 7a, a CV beginning i n the red u c t i o n region e x h i b i t s the peaks at -0.47 and -0.69 V i n a d d i t i o n to the dominant feature at -1.00 V (Figure 7c). 50 Scheme I + 2e' (b) * 2 CpCr(NO);- -> minor products (c) 2 C p C r ( N O ) 2 + ^ - |CpCr(NO) 2] rCpCr(N0) 2l 7 (a) L J 2 - e * L -«2 (d) 2 C p F e ( C O ) 2 + ^ [ c p F e ( C O ^ CpFe(CO) e (e) 52 Analogous CVs of [ (T) 5-C 5H 5)Cr(NO) 2 ] 2 i n CHgCN are q u a l i t a t i v e l y s i m i l a r to those observed i n CH 2C1 2, two r e p r e s e n t a t i v e traces being d i s -played i n Figure 8. The peak p o t e n t i a l s s h i f t s l i g h t l y to more p o s i t i v e values f o r the o x i d a t i o n processes and to more negative values f o r the reduction processes w i t h each successive scan. In CH3CN, the i n i t i a l two-e l e c t r o n i r r e v e r s i b l e o x i d a t i o n peak occurs at E = +0.68 V at a scan p,a rate of 0.19 V s - 1 (Figure 8a), or +0.31 V with respect to the ferrocene/ f e r r i c i n i u m couple. ( In CH-Cl, E = +0.32 V vs. ferrocene at 0.11 V s - 1 . ) 2 2 p > a Again, i v a r i e s l i n e a r l y with v 1 / 2 over the range 0.03 - 0.29 V s - 1 . p,a The removal of e l e c t r o n s i n t h i s case, i . e . [ ( n 5 - C 5 H 5 ) C r ( N O ) 2 ] 2 C H ^ C N > 2 [ (n 5-C 5H 5)Cr(NO) 2(CH 3CN)]+ (3.1) 40 may be f a c i l i t a t e d by the formation of the well-known, 18-electron, CH 3CN-containing c a t i o n which i s more s t a b l e than the f o r m a l l y 16-electron [(r| 5-C 5H 5)Cr(NO) 2 ] + formed by the analogous o x i d a t i o n i n CH 2C1 2. The [ ( T) 5-C 5H 5)Cr(NO) 2(CH 3CN)] + c a t i o n produced i n the o x i d a t i o n reduces at E P»c = -0.37 V (Figure 8a), or with reference to ferrocene o x i d a t i o n , E 6 p,c -0.68 V. Likewise [ ( T) 5-C 5H 5)Cr(NO) 2] + i n CH 2C1 2 reduces w i t h E p c = -0.68 V vs ( n 5 - C 5 H 5 ) 2 F e / [ ( T i 5 - C 5 H 5 ) 2 F e ] + at 0.11 V s - 1 . Unlike the analogous process i n CH 2C1 2, t h i s r eduction i s p a r t i a l l y chemically r e v e r s i b l e , i . e . [ ( T i 5-C 5H 5)Cr(NO) 2(CH 3CN ) ] + ^ ^ [ ( T i 5-C 5H 5)Cr(NO) 2(CH 3CN)]» (3.2) 53 Figure 8. C y c l i c voltammograms of [( r) b-C 5H 5)Cr(NO) 2] 2 i n CH 3CN at scan rates of (a) 0.19 V s - 1 and (b) 0.29 V s - 1 , r e s p e c t i v e l y . 54 I f the CV scan i s reversed at -0.50 V (Figure 8b), the corresponding anodic peak i s detectable at E = -0.30 V. Hence t h i s redox couple i s c o r r e c t l y p ,a represented by the parameters E ^ / 2 = -0.34 V with AE^ = 70 mV and i J^-p c = 0.40 at a scan ra t e of 0.10 V s - 1 . Consistent with the p a r t i a l chemical r e v e r s i b i l i t y of r e a c t i o n 3.2, i / i does increase with i n c r e a s i n g scan P.a p,c r a t e . Therefore the r a d i c a l species formed i n r e a c t i o n 3.2 appears to be more st a b l e than i t s analogue formed In CH 2C1 2 from re d u c t i o n of [(r) 5-C 5H 5)Cr(NO) 2 ] + , and only a very weak s i g n a l due to reduction of a minor product [ l a b e l l e d (c) i n Scheme I] i s observable i n the CV i n Figure 8a. This inference i s i n accord with the recent observation that the congeneric r a d i c a l s , [(n 5-C 5H 5)W(NO) 2L]• (L = PPh 3, P(0Ph) 3 e t c . ) , are 46 s u f f i c i e n t l y thermally s t a b l e to be i s o l a b l e . The chromium-containing r a d i c a l s e v i d e n t l y undergo d i m e r i z a t i o n with concomitant expulsion of CH3CN from each metal's c o o r d i n a t i o n sphere, i . e . 2 [(n 5-C 5H 5)Cr(NO) 2(CH 3CN)]» > [ ( n 5 - C 5 H 5 ) C r ( N O ) 2 ] 2 + 2 CH3CN (3.3) However, t h i s process seems to be s u b s t a n t i a l l y l e s s e f f i c i e n t than the d i m e r i z a t i o n of the r a d i c a l i n CH 2C1 2 (see the s e c t i o n on the CV of [ ( r i 5-C 5H 5)Cr(NO) 2(CH 3CN)]PF 6, below). The f i n a l features of the CV shown i n Figure 8a r e s u l t from the one-electron reduction of the parent n i t r o s y l dimer. In CH3CN, t h i s r eduction i s l e s s r e v e r s i b l e than i n CH 2C1 2 and occurs at E n / 0 = "0.95 V with AE = 120 mV and i / i =0.83. Con-1 1 1 P P>a p,c 55 s i s t e n t l y , i f negative p o t e n t i a l s are scanned f i r s t , the CV of [ ( T l 5-C 5H 5)Cr(NO) 2 ] 2 i n CH3CN e x h i b i t s only the s i g n a l s due to t h i s redox couple [step (d) of Scheme I ] . The e l e c t r o n - t r a n s f e r processes engaged i n by [ ( T) 5-C 5H 5)Cr(NO) 2 ] 2 contrast with those i n which i t s i s o e l e c t r o n i c carbonyl analogue, [ ( T i 5 - C 5 H 5 ) F e ( C O ) 2 ] 2 , i s a p a r t i c i p a n t . The l a t t e r transformations are summarized f o r comparison i n the second part of Scheme I , and t h e i r occur-rence can be i n f e r r e d from the CVs presented i n Figure 9. Thus, a CV of 47 the carbonyl dimer i n CH 2 C1 2 between 0 and +0.91 V (Figure 9a) shows only a r e v e r s i b l e , one-electron o x i d a t i o n at E 1 / 2 = +0*67 V having AE^ = 70 mV and i / i =0.93. This corresponds to step (e) of Scheme I . I f , how-p,c p,a ever, the o x i d a t i o n scan i s extended to +1.6 V (Figure 9b), a second oxida-t i o n corresponding to step ( f ) occurs i r r e v e r s i b l y at E = +1.21 V, and p,a the i n i t i a l s i g n a l at E 1 / 2 = +0.67 V becomes l e s s r e v e r s i b l e . Successive o x i d a t i o n scans r e s u l t i n broadening of the observed s i g n a l s probably because of contamination of the electrode surface by decomposition 49 c products. The o x i d a t i o n of [ ( r| 0-C 5H 5)Fe(CO) 2 ] 2 proceeds i n two one-e l e c t r o n steps through the r e a d i l y detectable [ {( T) 5-C 5H 5)Fe (C0) 2 } 2 ] • i n t e r -mediate, e v e n t u a l l y a f f o r d i n g [ (T) 5-C 5H 5)Fe(CO) 2 ] + as i n d i c a t e d i n Scheme I, but no analogous intermediate i s detected i n the o x i d a t i o n of [ ( T ) 5 - C 5 H 5 ) C r ( N O ) 2 ] 2 ; i t simply s p l i t s apart i n t o [( T] 5-C 5H 5)Cr(NO) 2 ] + d i r e c t l y . In contrast to the chromium dimer, [ (T) 5-C 5H 5)Co(NO) ] 2 does not cleave so r e a d i l y upon o x i d a t i o n . 5 0 In f a c t [ { ( T) 5-C 5H 5)Co(NO)} 2]PF g can be i s o l a t e d from the r e a c t i o n of N0PF6 w i t h ( T I 5 - C 5 H 5 ) C O ( C O ) 2, and the c a t i o n -r a d i c a l undergoes a f u r t h e r one-electron o x i d a t i o n e l e c t r o c h e m i c a l l y to 56 Figure 9. C y c l i c voltammograms of [ ( T I 5 - C 5H 5)Fe(CO) 2] 2 i n C H 2 C l 2 a t scan rates of (a) 0.07 V s - 1 and (b) 0.14 V s " 1 . 57 [ { ( T I 5 - C 5 H 5 ) C O ( N O ) } 2 ] + 2 . Likewise [(n 5-C 5Me 5)Co(NO)] 2 e x h i b i t s the same e l e c t r o n t r a n s f e r r e a c t i o n s , and [ {(r| 5-C 5Me 5)Rh(NO) } 2] 2* 5 1 a l s o o x i d i z e s to the d i c a t i o n . Thus [ ( T i 5-C 5H 5)Cr(NO) 2 ] 2 appears to be unusual i n that i t s p l i t s apart so r e a d i l y upon e l e c t r o n removal (see a l s o below), which may 52 be due to the s t a b i l i t y of [ ( T ) 5 - C 5 H 5 ) C r ( N O ) 2 ] + . At negative p o t e n t i a l s , the i r o n - c a r b o n y l dimer i n CH 2C1 2 d i s p l a y s only an i r r e v e r s i b l e two- e l e c -53 54 tron r e d u c t i o n with E = -1.84 V which corresponds u l t i m a t e l y ' to the p,c formation of [ ( n 5 - C 5 H 5 ) F e ( C O ) 2 ] ~ ( i . e . step ( g ) ) , a feature that has been p r e v i o u s l y observed by other i n v e s t i g a t o r s . Recently, [ ( n 5 - C 5 H 5 ) F e ( C O ) 2 ] 2 has been shown to reduce i n two one-electron s t e p s 5 5 to form the s h o r t -l i v e d [ { ( T i 5-C 5H 5)Fe(CO) 2 } 2 ] ~ r a d i c a l anion which very r a p i d l y decays i n t o [ ( T i 5 - C 5 H 5 ) F e ( C O ) 2 ] ~ and [ ( T) 5-C 5H 5)Fe(CO) 2] The r a d i c a l anion does not p e r s i s t . Each successive scan of the complete CV of [ ( n 5 - C 5 H 5 ) F e ( C O ) 2 ] 2 i n CH 2C1 2 r e s u l t s i n the i r r e v e r s i b l e o x i d a t i o n p o t e n t i a l becoming more p o s i -t i v e and the reduction p o t e n t i a l becoming more negative. A fundamental d i f f e r e n c e between the two dimeric complexes c o n s i -dered i n Scheme I i s thus t h e i r behaviour upon e l e c t r o n a d d i t i o n . The i r o n carbonyl dimer undergoes s c i s s i o n w i t h a net two-electron r e d u c t i o n , i . e . [ ( n 5 - C 5 H 5 ) F e ( C O ) 2 ] 2 + 2 & > 2 [ ( n 5 - C 5 H 5 ) F e ( C O ) 2 ] " (3.4) whereas the chromium-nitrosyl dimer r e t a i n s i t s b i m e t a l l i c nature upon one-electron r e d u c t i o n , i . e . [ ( n 5 - C 5 H 5 ) C r ( N O ) 2 ] 2 "*"£ > [ {(n 5-C 5H 5)Cr(NO) 2 ) 2 ] ~ (3.5) 58 A p o s s i b l e explanation of these observations i s that the greater re-acidity of the NO ligands r e s u l t s i n a greater d e r e a l i z a t i o n of the e x t r a e l e c t r o n density i n [ { (r| 5-C 5 H 5)Cr(NO) 2 > 2 F and consequently, a diminished tendency f o r the b i m e t a l l i c anion to undergo cleavage. In a complementary f a s h i o n , the occurrence of reac t i o n s 3.4 and 3.5 may be viewed as r e f l e c t i n g the i n h e r e n t l y d i f f e r i n g natures of the lowest unoccupied molecular o r b i t a l s of the two dimers, that f o r [ ( T i 5 - C 5 H 5 ) C r ( N O ) 2 ] 2 e v i d e n t l y being a r e l a t i v e l y low-energy nonbonding or bonding o r b i t a l . Obviously, the con f i r m a t i o n of t h i s l a t t e r inference must await a d e t a i l e d t h e o r e t i c a l a n a l y s i s . I t may also be p o s s i b l e that the process by which the i n i t i a l l y formed [ { ( T i 5 - C 5 H 5 ) F e ( C O ) 2 } 2 ] ~ anion r a d i c a l decays i n t o the thermodynamically s t a b l e [(r) 5-C 5 H 5)Fe(CO) 2 ] ~ anion, i s not a v a i l a b l e to [ { ( T l 5 - C 5 H 5 ) C r ( N O ) 2 } 2 ] ~ , i f the [ ( r i 5 - C 5 H 5 ) C r ( N O ) 2 ] ~ species i s as unstable 52 as p r e d i c t e d by a recent t h e o r e t i c a l study. The r e l a t e d n i t r o s y l - b r i d g e d dimer [ ( T I 5 - C 5 H 5 ) C O ( N O ) ] 2 has been reported to undergo one-electron r e d u c t i o n 5 ^ to the r a d i c a l anion [ { ( T I 5 - C 5 H 5 ) C O ( N O ) } 2 ] ~ ; however since t h i s r eduction process i s i r r e v e r -s i b l e , 5 0 and i n l i g h t of the recent synthesis of Na [ ( T ) 5 - C 5 H 5 ) C O ( N O ) ] 5 7 ' 5 8 by sodium reduction of the n e u t r a l dimer i t may be best to formulate t h i s r e duction as a net two-electron process. The analogous compounds [(r) 5-C 5Me 5)Co(NO)] 2 5 ° and [(r) 5-C 5Me 5)Rh(NO)] 2 5 1 do undergo one-electron r e d u c t i o n e l e c t r o c h e m i c a l l y to the corresponding r a d i c a l anions. Why the l a t t e r two dimers, which contain b u l k i e r and r e l a t i v e l y b e t t e r e l e c t r o n -donor C 5Me 5 l i g a n d s , reduce r e v e r s i b l y and [ ( T) 5-C 5 H 5)Co(NO)] 2 does not, i s not c l e a r , although t h i s could perhaps be a ma n i f e s t a t i o n of s t e r i c 59 f a c t o r s , the bulkier C^Me^ ligands k i n e t i c a l l y s t a b i l i z i n g the r a d i c a l s against intermolecular reactions. The dimer [ ( r i 5-C 5H 5)Fe(NO) ] 2 has been found to reduce r e v e r s i b l y in a one-electron step also to give a r a d i c a l a n i o n . ^ Including [ ( T) 5-C 5H 5)Cr (NO) 2] 2» these are the only organometallic compounds containing a {M 2 ( L I - N O ) 2} core for which r e v e r s i b l e one-electron reductions have been electrochemically characterized to date. I t seems reasonable that the a c c e s s i b i l i t y of such r a d i c a l anions r e f l e c t s the strong u - a c i d i t y of NO. (The case of [ { (T) 5-C 5H 5)Fe(NO) } 2 ] T may be an exception owing to the multiple Fe=Fe int e r a c t i o n . ) (b) (T)5-C5H5)Cr(NO)2BF,,. Treatment of a 4 x 10 - 1 + M s o l u t i o n of [ ( r i 5 - C 5 H 5 ) C r ( N O ) 2 ] 2 i n CH 2C1 2 with 2 equiv. of HBF l t»OMe 2 r e s u l t s i n the O A generation of an 8 x 10 - l + M solution of ( T] 5-C 5H 5)Cr(NO) 2BF 1 +. A CV of this solution recorded at a scan rate of 0.15 V s - 1 displays the f a m i l i a r signals due to an i r r e v e r s i b l e reduction at E = -0.24 V, two reduction P»c waves at -0.45 and -0.71 V with lesser i and f i n a l l y a reduction at E 1 / 2 p, c = -0.99 V which exhibits chemical r e v e r s i b i l i t y . These features correspond reasonably well to the CV presented i n Figure 7a at negative potentials and confirm the sequential reduction processes that originate with [ ( r i 5 - C 5 H 5 ) C r ( N 0 ) 2 ] + as outlined i n Sheme I. There may, however, be subtle e f f e c t s on these processes due to the presence of a BF^ - ion, or perhaps even l i g a t e d Me20 since the second and t h i r d waves have greater r e l a t i v e peak currents than they do for the CV of [ ( r i 5-C 5H 5)Cr(NO) 2] 2 i n CH 2C1 2. The BF^ - and PFg - anions exert somewhat d i f f e r e n t influences on the cation and t h i s i s demonstrated by the IR spectra of the cation with BF 4~ and PF 6~ counter ions. An i n f r a r e d spectrum of a solution of 60 (r| b-C 5H 5)Cr(NO) 2 P F 6 generated from the parent dimer and AgPFg i n CH 2C1 2 d i s p l a y s v bands at 1835 (s) and 1731 (vs) cm - 1 and a strong band at 848 cm - 1 due to PFg - s t r e t c h i n g . Considering that the P F 6 - band does not i n d i c a t e s u b s t a n t i a l d i s t o r t i o n from octahedral symmetry i t would appear 40 that the anion i s only very weakly bound to the c a t i o n . However, the 60 BF, - i o n i s thought to be a be t t e r e l e c t r o n donor, and the IR spectrum of ( r i 5 - C 5 H 5 ) C r ( N 0 ) 2 B F , i n CH 2C1 2 confirms t h i s . The BF, - bands appear at 1098 ( E ) , 1061 ( A x ) , and 1020 (Aj) cm - 1, co n s i s t e n t with d i s t o r t i o n of the BF, -ion to l o c a l C 3v symmetry.^* C u r i o u s l y , however, the NO absorptions are ~10 cm - 1 higher than f o r ( T i 5 - C 5 H 5 ) C r ( N O ) 2 P F 6 . I t may be p o s s i b l e that the cations (r|5-C5Hc-)Cr(NO) 2Y (Y = weakly c o o r d i n a t i n g anions) may possess s t r u c t u r e s that depend on Y, varying f o r example, between the two extremes of "three-legged p i a n o - s t o o l " and "two-legged p i a n o - s t o o l " geometries as depicted i n Figure 10. A l t e r n a t i v e l y , ( n 5 - C 5 H 5 ) C r ( N O ) 2 P F 6 i n d i c h l o r o -methane may e x i s t as a d i s c r e t e CH 2C1 2 s o l v a t e as i n the case of [ ( T I 5 - C 5 H 5 ) W ( C O ) 3 ( C H 2 C 1 2 ) ] P F 6 . 6 1 In any event, whatever the d i f f e r e n c e s , they may w e l l i n f l u e n c e the redox p r o p e r t i e s of the c a t i o n s . With BF, -being a s u b s t a n t i a l l y b e t t e r c o o r d i n a t i n g anion than P F g - , ^ i t would not be unreasonable to expect that i t remains bound i n the presence of a large excess of [n-Bu , N]PF 6 i n CH 2C1 2. The E P c v a l u e for ( r) 5-C 5H 5)Cr ( N O ) 2BF, generated from the parent dimer and HBF,»0Me2 i n CH 2C1 2 i s very s l i g h t l y more negative than E f o r ( r) 5-C 5H 5)Cr ( N O ) 2 P F 6 i n the CV of P»c [ ( n 5 - C 5 H 5 ) C r ( N O ) 2 ] 2 (Figure 7) by ~20-30 mV which may be r e f l e c t i v e of t h i s f e a t u r e . 61 Figure 1 0 . Possible geometries for (r\5-C 5H 5)Cr(N0)Y where Y i s a weakly coordinating anion. 62 (c) [(Ti 5-C 5H 5)Cr(NO) 2(CH 3CN)]PF 6. A CV of t h i s i s o l a b l e 40 complex i s shown i n Figure 11. The f i r s t r e duction of the complex occurs at E = -0.27 V at a scan ra t e of 0.10 V s - 1 and i s chemically i r r e v e r -s i b l e at t h i s scan r a t e . The r e s u l t a n t [ ( r | 5 - C 5 H 5 ) C r ( N O ) 2(CH 3CN) ] • r a d i c a l presumably i s not as l o n g - l i v e d i n CH 2C1 2 as i n CH3CN since no anodic wave i s detected i n the r e t u r n scan when the scan i s extended to only j u s t past the f i r s t r e d u c t i o n peak. Furthermore a wave occurs at E = -0.71 V w i t h p,c a r e l a t i v e l y greater i than i n CHqCN (Figure 8) probably due to the p,c 0 r e d u c t i o n of products derived from the r a d i c a l . F i n a l l y a wave at E 1 / 2 = -0.99 V with AE p = 80 mV occurs due to red u c t i o n of [ ( T i 5 - C 5 H 5 ) C r ( N O ) 2 ] 2. These observations concur w i t h the transformations o u t l i n e d i n Scheme I. A CV of [ ( T ) 5 - C 5 H 5 ) C r ( N O ) 2 ( C H 3 C N ) ] P F 6 i n CH3CN e x h i b i t s the f a m i l i a r wave f o r reduction of [ ( r i 5 - C 5 H 5 ) C r ( N O ) 2(CH 3CN) ] + at E = -0.37 V vs. SCE, which, P» c again shows some degree of chemical r e v e r s i b i l i t y . A second wave appears at E = -0.77 V and a t h i r d at E = -1.57 V at a scan rate of 0.16 V s - 1 . p,c p,c Both of these l a s t waves have f a i r l y low peak currents compared with the i n i t i a l peak at E = -0.95 V. A reduction wave f o r [ ( r i 5 - C 5 H 5 ) C r ( N O ) 2 ] 2 p, c at E 1 / 2 = -0.95 V (as i n Figure 8a) does not appear to be present as such i n the CV of [ ( T ) 5 - C 5 H 5 ) C r ( N O ) 2(CH 3CN) ] P F 6 i n CH3CN. Thus, while [ ( r i 5 - C 5 H 5 ) C r ( N O ) 2 ] • and [ ( T i 5 - C 5 H 5 ) C r ( N O ) 2(CH 3CN) ] • y i e l d to d i m e r i z a t i o n r e a d i l y i n CH 2C1 2, t h i s process would appear to be more i n h i b i t e d i n CH3CN. I t may be that the expulsion of CH3CN i s much l e s s favoured i n the presence of a vast excess of a c e t o n i t r i l e , h i n d e r i n g d i m e r i z a t i o n . Since the odd e l e c t r o n i s l i k e l y housed i n a l a r g e l y NO-based molecular o r b i t a l as i n the [ ( T ) 5 - C 5 H 5 ) W ( N O ) 2 L ] • (L = phosphines, phosphites) r a d i c a l s , ^  the NO ligands 63 0 Volts vs SCE Figure 1 1 . C y c l i c voltammogram of [(n 5-C 5H 5)Cr(NO) 2(CH 3CN)]PF 6 i n CH 2C1 2 at a scan rate of 0.10 V s - 1 . 64 of the solvated [(n b-C 5H 5)Cr(NO) 2(CH 3CN)]• could become the pr e f e r r e d s i t e of r e a c t i v i t y i f the d i m e r i z a t i o n pathway i s blocked, and lead to decom-p o s i t i o n of the complex. Path (c) of Scheme I may therefore be a more s i g n i f i c a n t mode of r e a c t i v i t y f o r [(n 5-C 5H 5)Cr(NO) 2(CH 3CN)]• i n a c e t o n i -t r i l e than i n CH 2C1 2. (d) ( T i 5 - C 5 H 5 ) C r ( N O ) 2 C l . The redox p r o p e r t i e s of t h i s compound are of i n t e r e s t i n comparison to those displayed by the complexes f o r m a l l y containing the [ ( n 5 - C 5 H 5 ) C r ( N 0 ) 2 ] + c a t i o n . The CV of (n 5-C 5H 5)Cr(NO) 2C1 i n CH 2C1 2 i s devoid of any features corresponding to o x i d a t i o n of the compound out to the solvent l i m i t . When negative p o t e n t i a l s are scanned, the only observable feature i s i r r e v e r s i b l e r eduction of the compound at E = -0.68 V at a scan ra t e of 0.07 V s - 1 . Hence, (n 5-C 5H 5)Cr(NO) 2C1 i s somewhat more d i f f i c u l t to reduce than ( n 5 - C 5 H 5 ) C r ( N 0 ) 2 ] + i n CH 2C1 2, and no el e c t r o c h e m i c a l l y detectable amounts of [ ( i i 5 - C 5 H 5 ) C r ( N O ) 2 ] 2 or i t s precursors appear to be produced i n the process. (e) ( T i 5-C 5H 5) 2Cr 2(NO) 3(NH 2). The molecular s t r u c t u r e of t h i s b i m e t a l l i c n i t r o s y l complex i n the s o l i d s t a t e resembles that of [ ( T i 5-C 5H 5)Cr(NO) 2 ] 2, a b r i d g i n g NO l i g a n d i n the l a t t e r simply having been 43 replaced by a b r i d g i n g NH 2 group. I t s reduction behaviour i n s o l u t i o n i s al s o q u a l i t a t i v e l y s i m i l a r . I f a CV of the amido complex i n CH 2C1 2 i s recorded i n i t i a l l y between 0 and -1.6 V at a scan rate of 0.10 V s - 1 , only a q u a s i - r e v e r s i b l e , one-electron reduction at E 1 / 2 = -1.27 V having AE^ = 110 mV and i / i 0.94 i s evident. [The E value (-1.33 V at p,a p,c p,c 0.10 V s - 1 ) s h i f t s to more negative p o t e n t i a l s as the scan rate i s increased.] The e f f e c t s of the ^ -NH2 l i g a n d , a non-rc-acid, are thus to 65 Figure 1 2 . C y c l i c voltammogram of ( ry'-C 5H 5) 2Cr 2(N0) 3(NH 2) i n CH 2C1 2 at a scan rate of 0.10 V s - 1 . 66 make the amido complex r e l a t i v e l y more d i f f i c u l t to reduce and the process +e~^ ( n 5 - C 5 H 5 ) 2 C r 2 ( N O ) 3 ( N H 2 ) ^ [ ( n 5 - C 5 H 5 ) 2 C r 2 ( N O ) 3 ( N H 2 ) ] - (3.6) -e~ l e s s r e v e r s i b l e . In contrast to [ ( n 5 - C 5 H 5 ) C r ( N O ) 2 ] 2 (Figure 7), however, the b i m e t a l l i c amido complex undergoes two successive, q u a s i - r e v e r s i b l e one-electron o x i d a t i o n s i n CH 2C1 2 when p o s i t i v e p o t e n t i a l s are scanned f i r s t (Figure 12). The transformations i n v o l v e d , i . e . - e ^ + ( T i 5 - C 5 H 5 ) 2 C r 2 ( N O ) 3 N H 2 ^ _ [(n 5-C 5H 5) 2Cr 2(NO) 3(NH 2) ] • +e~ -e~ ^ [ ( n 5 - C 5 H 5 ) 2 C r 2 ( N O ) 3 ( N H 2 ) ] 2 + (3.7) +e~ occur at E , / 9 = +0.45 V with AE = 120 mV and i / i 0.60 and E , / 0 = l / z p p,c p,a +0.98 V with AEp = 140 mV at a scan rate of 0.18 V s - 1 , r e s p e c t i v e l y . The f i r s t o x i d a t i o n occurs more r e a d i l y than f o r [ ( n 5 - C 5 H 5 ) C r ( N O ) 2 ] 2 and i s again a consequence of the r e l a t i v e l y b e t t e r electron-donating p r o p e r t i e s of the NH 2 group. Indeed, the presence of the b r i d g i n g amido l i g a n d apparently s t a b i l i z e s (T)5-C5H5) 2 C r 2 ( N 0 ) 3(NH 2) considerably against undergoing s c i s s i o n upon e l e c t r o n removal. Many r e l a t e d n i t r o s y l -c o n t a i n i n g b i m e t a l l i c compounds appear to maintain t h e i r b i m e t a l l i c nature upon o x i d a t i o n . For example, the cat i o n s generated i n the oxi d a t i o n s i n equations 3.8 and 3.9, i . e . 67 -e , -e [ ( T I 5 -C 5H 5)Mn(NO) SR] 2 ^ = ^ : [ { ( n 5 - C 5 H 5 )Mn(NO) SR} 2 ] t ; +e" +e" [{(n 5-C 5H 5)Mn(NO)SR} 2]+ 2 6 2 ( 3 . 8 ) -e~ 63 [ ( n 5 - C 5 H 5 ) M o ( N O ) S R ] 2 ^ ^ [ { ( T I 5 - C 5 H 5 ) M O ( N O ) S R } 2 ] . ( 3 . 9 ) +e" are a l l s t a b i l i z e d , by the presence of b r i d g i n g t h i o l a t e l i g a n d s , w i t h respect to s c i s s i o n . While the r a d i c a l - c a t i o n s [ { ( n 5 - C 5 H 5 ) M n ( N O ) S R } 2 ] P F 6 are i s o l a b l e , [ ( T ) 5 - C 5 H 5 ) 2Cr 2 ( N 0 ) 3 ( N H 2 ) ]"• does not appear to have long-term s t a b i l i t y i n s o l u t i o n , since the r a t i o of peak c u r r e n t s , i / i i s p,c p,a considerably l e s s than u n i t y . The peak current r a t i o f o r the second o x i d a t i o n a l s o appears to be q u i t e low. Reversal of the CV scan of the amido-bridged compound a f t e r the o x i d a t i v e processes a l s o r e v e a l s the presence of a small and broad set of reduction peaks i n the region between - 0 . 3 9 and - 0 . 5 7 V (Figure 1 2 ) which are probably associated w i t h s m a l l amounts of byproducts derived from the o x i d i z e d complexes i n r e a c t i o n s 3 . 7 . I f the CV scan i s reversed before the second o x i d a t i o n occurs, a s i m i l a r y shaped envelope of reduction peaks i s observable i n t h i s r e g i o n . Bulk E l e c t r o l y s e s . The i n t e r p r e t a t i o n s of the CVs discussed i n the preceding paragraphs are sub s t a n t i a t e d by the f o l l o w i n g independent 68 experiments, complete d e t a i l s of which are presented i n the Experimental S e c t i o n . (A) C o n t r o l l e d p o t e n t i a l e l e c t r o l y s i s of an i n t e n s e l y red s o l u t i o n of [CpCr(NO) 2] 2 i n CH 2C1 2 (with [n-Bu^NJPFg as support e l e c t r o l y t e ) at +1.00 V r e s u l t s i n the removal of 0.8 el e c t r o n s per chromium atom and the ne a r l y K 40 q u a n t i t a t i v e formation of green ( r i b-C 5H 5)Cr(NO) 2PFg [IR(CH 2C1 2) v^Q 1838 ( s ) , 1731 (s,br) c m - 1 ] . The o x i d a t i o n process i s s u b s t a n t i a l l y slower than that f o r ferrocene under analogous experimental c o n d i t i o n s . The u n i s o l a b l e organometallic product can be converted i n s i t u to t r a c t a b l e CpCr(N0) 2Cl by metathesis with [(Ph 3P) 2N]C1. (B) Bulk r e d u c t i o n of green [( T] 5-C 5H 5)Cr(NO) 2(CH 3CN) ] P F 6 i n CH 2C1 2 at -0.45 V i s q u i t e slow and consumes 0.9 e l e c t r o n s per chromium atom. IR monitoring of the progress of the conversion v e r i f i e s that the p r i n c i p a l n i t r o s y l - c o n t a i n i n g product formed i s [(r) 5-C 5H 5)Cr(NO) 2 ] 2 which i s i s o l a b l e by chromatography on F l o r i s i l . The IR spectrum of the f i n a l e l e c t r o l y z e d s o l u t i o n a l s o d i s p l a y s two weak n i t r o s y l absorptions at ~1830 and ~1720 cm - 1 reminiscent of those e x h i b i t e d by the d i n i t r o s y l complexes, c 64 ( T) 3-C 5H 5)Cr(N0) 2X where X = a h a l i d e or a pseudohalide. While the minor product could not be i s o l a t e d , i t i s l i k e l y that t r a n s f e r of e l e c t r o n s to t h i s complex produces the small r e d u c t i o n peak at E^ ^ = -0.71 V i n the CV of [ ( T i 5-(C 5H 5)Cr(NO) 2(CH 3CN)]PF 6 i n CH 2C1 2 (see above). In t h i s context i t i s of i n t e r e s t to note that E f o r reduction of ( r i 5-C 5H 5)Cr(NO) 2C1 i n p ,c CH 2C1 2 i s -0.68 V and i t i s l i k e l y that many ( T i 5-C 5H 5)Cr(NO) 2X complexes would reduce over a f a i r l y narrow p o t e n t i a l r e g i o n . 69 (C) Exhaustive reduction of [(n 5-C 5H 5)Cr(NO) 2 ] 2 i n CH 2C1 2 at -1.00 V requires the t r a n s f e r of 4.8 e l e c t r o n s per dimer before the current drops to near zero. E v i d e n t l y the i n i t i a l l y formed b i m e t a l l i c r a d i c a l anion decomposes to other e l e c t r o a c t i v e species. I f the c o n t r o l l e d p o t e n t i a l e l e c t r o l y s i s i s e f f e c t e d at -1.10 V and i s stopped a f t e r 1.9 e l e c t r o n s per dimer have been t r a n s f e r r e d , the f i n a l s o l u t i o n s t i l l contains a small amount of [ ( r i 5 - C 5 H 5 ) C r ( N O ) 2 ] 2 . The n i t r o s y l - c o n t a i n i n g products formed i n low y i e l d s are the b r i d g i n g amido compounds, green (t| 5-C 5H 5) 2 C r 2 ( N 0 ) 3 ( N H 2 ) and orange [(ri 5-C 5H 5)Cr(NO)(NH 2)] 2, and a brown complex as yet u n i d e n t i -f i e d , a l l of which are separable by column chromatography on F l o r i s i l . A s i m i l a r d i s t r i b u t i o n of products i s known to r e s u l t when the n e u t r a l d i n i t r o s y l dimer i s t r e a t e d w i t h reducing agents such as Na/Hg or 33 Na[H 2Al(OCH 2CH 2OCH 3) 2]. However, the dominant mode of r e a c t i v i t y when el e c t r o n s are added to the b i m e t a l l i c r a d i c a l anion or i t s d e r i v a t i v e s i s the formation of n o n - n i t r o s y l products. The [ ( T i 5 - C 5 H 5 ) C r ( N O ) 2 ] ~ anion i s not formed i n s p e c t r o s c o p i c a l l y detectable amounts, or i f i t i s at a l l , i t r a p i d l y decomposes. (D) Complete e l e c t r o c h e m i c a l r e d u c t i o n of ( T i 5-C 5H 5)Cr (NO) 2C1 i n CH 2C1 2 r e s u l t s i n the t r a n s f e r of 1.0 e l e c t r o n s per chromium atom and i n the formation of a brown product complex [IR (CH 2C1 2) v ~1640 (br) cm"1] whose i d e n t i t y could not be a s c e r t a i n e d . The f a c t that no [(r|5-C5H5Cr(NO) 2 ] 2 Is formed as a product i n d i c a t e s that during i t s chemical s y n t h e s i s , i . e . 70 ( n 5 - C 5 H 5 ) C r ( N O ) 2 C l r ^ ^ n S > [ ( n 5 - C 5 H 5 ) C r ( N O ) 2 ] 2 (3.10) where reducing agent = Zn/Hg i n THF, 3 8 or Na/Hg i n benzene,^ the i n i t i a l step does not i n v o l v e the simple e l e c t r o n t r a n s f e r , i . e . ( T) 5-C 5H 5)Cr(NO) 2Cl + e" > [ ( T i 5-C 5H 5)Cr(NO) 2C1 ] • (3.11) Consequently, transformation 3.10 could proceed v i a the r e a c t i o n s ( r i 5 - C 5 H 5 ) C r ( N O ) 2 C l + M > (T] 5-C 5H 5)(N0)Cr:' >M (3.12) "^ NO"' 6+ , C l J 6+ . C l 6 " (n 5-C 5H 5)(NO)Cr:' ^M > l/2[(T, 5-C 5H 5)Cr(NO) 2] 2 + MCI (3.13) V-N0'' where M = Na or 1/2 Zn. 65 The h e t e r o l y t i c cleavage of the polar Cr-Cl linkage involved i n these r e a c t i o n s would be f a c i l i t a t e d by the e l e c t r o p o s i t i v e reductant and/or a polar s o l v e n t , and the e l e c t r o n t r a n s f e r involved i n the second step would proceed with concomitant MCI e l i m i n a t i o n to generate [ ( r ) 5 - C 5 H 5 ) C r ( N O ) 2 ] • . (E) E l e c t r o c h e m i c a l o x i d a t i o n of [ (r| 5-C 5H 5)Fe(CO) 2] 2 does not go to completion, probably due to p a s s i v a t i o n of the electrode by products of the 49 o x i d a t i o n . This behaviour i s apparent i n the CV of the dimer i n CH 2C1 2 a l s o (see above). An IR spectrum of the r e s u l t a n t r e a c t i o n mixture does 71 show high energy v bands (2076, 2030 cm - 1) due to the presence of 66 [ ( T l 5 - C 5 H 5 ) F e ( C O ) 2 ] + . Treatment of the s o l u t i o n with [ ( P h 3 P ) 2 N ] C l r e s u l t s i n the formation of (n 5-C 5 H 5)Fe(C0) 2C1 as evidenced by an IR spectrum of the r e s u l t a n t s o l u t i o n . These r e s u l t s are c o n s i s t e n t w i t h the transforma-t i o n s shown i n Scheme I for the i r o n - c o n t a i n i n g dimer. The p o t e n t i a l a p p l i e d to o x i d i z e the dimer (+0.80 V vs. Ag-wire) would not seem to be «- + s u f f i c i e n t to o x i d i z e [ {(TT-Cc-Hc-)Fe(CO) 2 } 2 ] • to the mono-metallic c a t i o n although i t i s d i f f i c u l t to t e l l with a quasi-reference e l e c t r o d e , and a large surface-area working electrode which may experience an e l e c t r i c f i e l d gradient over i t s s u r f a c e . I t i s a l s o q u i t e p o s s i b l e that the r a d i c a l c a t i o n slowly cleaves as i n r e a c t i o n 3.14, i . e . [ { ( n 5-C 5 H 5)Fe(C0) 2} 2]t > [ (n 5-C 5 H 5)Fe(CO) 2]+ + [ ( i , 5 - C 5 H 5 ) F e ( C O ) 2 ] * (3.14) At the outset of t h i s work i t had been hoped t h a t , having acquired information about the redox p r o p e r t i e s of [ ( T i 5-C 5H 5)Cr(N0) 2 ] 2 from the c y c l i c voltammetry s t u d i e s , i t would be p o s s i b l e to e f f e c t s p e c i f i c o x i d a t i o n s and reductions on a preparative s c a l e by employing appropriate bulk e l e c t r o l y t i c methods. However, the r e a l i z a t i o n of t h i s e x p e c t a t i o n has been thwarted by two f a c t o r s , namely (1) the d i f f i c u l t y of separating the de s i r e d organometallic products, p a r t i c u l a r l y i o n i c s p e c i e s , from the excess of the [n-Bu^NlPFg support e l e c t r o l y t e always present, and 72 (2) the inherent d i f f i c u l t i e s a s s o ciated with c a r r y i n g out e l e c t r o l y s e s of long d u r a t i o n i n C H 2 C 1 2 . ^ [For example, during bulk reductions i n the e l e c t r o l y s i s c e l l (Figure 4 ) , the o x i d a t i o n product formed i n the a u x i l i a r y compartment i s a gas whose e v o l u t i o n enhances the v a p o r i z a t i o n of CH 2C1 2 and consequently diminishes the a u x i l i a r y c e l l volume.] As a r e s u l t , i t i s necessary to r e s o r t to chemical means to e f f e c t the redox conversions of [ ( r) 5-C 5H 5)Cr(NO) 2 ] 2 on a s y n t h e t i c a l l y u s e f u l s c a l e . Preparative Work, (a) Protonation of [(Ti 5-C 5H 5)Cr(NO) 2]2' T n e net two-electron o x i d a t i o n of [ ( r| 5-C 5H 5)Cr(NO) 2] 2 t o [ ( T] 5-C 5H 5)Cr(NO) 2 ] + i n CH 2C1 2 may be conveniently c a r r i e d out on a preparative s c a l e by employing two equivalents of a p r o t o n i c a c i d as i n r e a c t i o n 3.15. The product c a t i o n 30 i s q u i t e thermally s t a b l e but i s u n i s o l a b l e . Monitoring of t h i s CH 2 C 1 2 [ ( n 5 - C 5 H 5 ) C r ( N O ) 2 ] 2 + 2HBFl+«OMe2 > 2(T] 5-C 5H 5)Cr(NO) 2BF, + H 2 + 20Me 2 (3.15) r e a c t i o n by *H NMR spectroscoy i n d i c a t e s that a 2:1 stoichiometry of a c i d to dimer, r e s p e c t i v e l y , i s required to t o t a l l y consume the dimer. A d d i t i o n of one equivalent produces only (n 5-C 5H 5)Cr(NO) 2 B F 4 and one-half equivalent 30 of the n e u t r a l dimer. No evidence f o r an intermediate i s de t e c t a b l e . On the other hand, treatment of [ ( n 5 - C 5 H 5 ) F e ( C O ) 2 ] 2 i n CH 2C1 2 w i t h HBF 4»0Me 2 c l e a n l y r e s u l t s i n the formation of [ {(ri5-C5H5)Fe(CO) 2} 2H]BF 1 +. Unlike i t s 68 tungsten congener, (t) 5-C 5H 5)Cr (NO) 2C1 reacts w i t h h a l i d e - a b s t r a c t i n g 73 s i l v e r - s a l t s i n a stepwise manner, f i r s t generating { ( T i 5 - C 5 H 5 ) C r ( N O ) 2 } 2 c l ] + « 6 9 T n i s complicates the formation of (Ti 5-C 5H 5)Cr(NO) 2 B F 4 from the s t a r t i n g h a l i d e complex and AgBF^ or AgPF 6; however both of these s i l v e r - s a l t s react c l e a n l y w i t h [(T] 5-C 5H 5)Cr(NO) 2 ] 2 to generate the c a t i o n . This may prove to be a superior route to [(T) 5-C 5H 5)Cr(NO) 2]+. The r e s u l t s of the b u l k - e l e c t r o l y t i c o x i d a t i o n , the o x i d a t i o n w i t h AgBFt^ and AgPFg, and the protonation r e a c t i o n s taken together i n d i c a t e that [ ( T i 5 - C 5 H 5 ) C r ( N O ) 2 ] 2 v i r t u a l l y q u a n t i t a t i v e l y cleaves i n t o the c a t i o n i n these r e a c t i o n s . During the study of the r e a c t i o n s of HBF^OMej with [ (r| 5-C 5H 5)Cr(NO) 2 ] 2 a small amount (< 36% maximum) of a c a t i o n i c species i s produced, which at f i r s t had been thought to be the protonated c a t i o n [{(T) 5-C 5H 5)Cr(NO) 2} 2H]BF [ +. I t soon became apparent, however, that a hydroxo-bridged species had been formed. C a r e f u l a d d i t i o n of E t 2 0 to a s o l u t i o n c o n t a i n i n g mostly (n 5-C 5H 5)Cr(NO) 2BF t + a f t e r treatment of the dimer with HBF L t»OMe 2 r e s u l t s i n the formation of low y i e l d s of [{(ri 5-C 5H 5)Cr(NO) 2} 2OH]BF 1 + as a m i c r o c r y s t a l l i n e s o l i d . This complex most l i k e l y a r i s e s from a subsequent r e a c t i o n of (ri 5-C 5H 5)Cr(NO) 2BF t 4 w i t h a base (perhaps a d v e n t i t i o u s OH- remaining on the glassware a f t e r c l e a n i n g w i t h KOH/ethanol) as i n r e a c t i o n 3.16: CH 2 C12 2(ri 5-C 5H 5)Cr(NO) 2BF l 4 + H 20 + base > [ {(r)5-C5H5)Cr(NO) 2> 2OH]BF 4 + [Hbase] + + BF^ - (3.16) 74 The hydroxo-containing c a t i o n can be independently synthesized by treatment of ( n 5 - C 5 H 5 ) C r ( N O ) 2 C l with AgBF, i n CH 2C1 2 followed by the a d d i t i o n of aqueous KOH or aqueous E t 3 N . This hydroxo complex i s a dark, o l i v e - g r e e n , diamagnetic s o l i d which can be handled i n a i r f o r s e v e r a l hours without n o t i c e a b l e decomposi-t i o n . I t i s s o l u b l e i n CH 2C1 2, CH 3N0 2, and acetone, s p a r i n g l y s o l u b l e i n H 20 and v i r t u a l l y i n s o l u b l e i n E t 2 0 . I t s Nujol-mull IR spectrum d i s p l a y s two strong absorptions at 1806 and 1677 cm - 1, c h a r a c t e r i s t i c of t e r m i n a l n i t r o s y l l i g a n d s , and a weak band at 3505 cm - i due to the OH l i g a n d . I t s H^ NMR spectrum ( i n CDC1 3) e x h i b i t s two sharp resonances at 6 5.78 and o'. 11 of r e l a t i v e i n t e n s i t y 10:1 a t t r i b u t a b l e to c y c l o p e n t a d i e n y l and hydroxo protons, r e s p e c t i v e l y . A s t a t i c molecular s t r u c t u r e (Figure 13) of t h i s c a t i o n which i s c o n s i s t e n t w i t h these data i s shown below. The p o s s i b l i t y of c i s and trans rotamers, or an asymmetrically bridged OH l i g a n d i s i n d i c a t e d by the presence of four v ^ Q ' S of t h i s complex i n CH 2C1 2 [1820 ( s ) , 1807 ( s h ) , 1719 ( s ) , 1698 (sh) c m - 1 ] . A l t e r n a t i v e l y , c oupling of the NO stetches may generate the shoulders. A r e a c t i o n between (r| 5-C 5H 5)Cr(NO) 2BF [ + (generated from the dimer and HBFit»0Me2 i n CH 2C1 2 followed by removal of the solvent i n vacuo) and aqueous Na[BPh 4] r e s u l t s i n the formation of (n 5-C 5H 5)Cr(NO) 2(H0BPh 3) i n moderate y i e l d s . When a suspension of [{(T i 5-C 5H 5)Cr(NO) 2) 2OH]BF 4 and NalBPh,] are mixed together i n water a metathesis r e a c t i o n occurs and [{(n 5-C 5H 5)Cr(NO) 2} 20H]BPh 4 r e s u l t s , i n d i c a t i n g that the new complex (r) 5-C 5H 5)Cr(NO) 2(HOBPh 3) probably does not a r i s e from the c a t i o n [{(n 5-C 5H 5)Cr(NO) 2} 2OH]+. 75 Figure 13. S t a t i c molecular structure of [ {( T} 5-C 5H 5)Cr(NO) 2} 2OH]BF i+. 7 6 (b) Protonation vs. Oxidative Cleavage of [ ( T I 5-C 5H 5)M(L0 ) 2 ] 2 (M • Cr, L - N; M • Fe, L • C). The comparative e l e c t r o c h e m i c a l behaviour of [ ( r i 5 - C 5 H 5 ) F e ( C O ) 2 ] 2 i s of i n t e r e s t to a s c e r t a i n i f the d i f f e r e n c e s i n o x i d a t i o n p o t e n t i a l s can account f o r the r e a c t i v i t y of these dimers toward HBF^. As i n d i c a t e d above, however, the ir o n - c a r b o n y l dimer i s somewhat ea s i e r to o x i d i z e than the i s o e l e c t r o n i c chromium-nitrosyl analogue. (While a comparison of E 1 / 2 p o t e n t i a l s cannot be made, ^ f o r the chromium-nitrosyl dimer i s ~170 mV p o s i t i v e of E ^ / 2 f o r the i r o n - c o n t a i n i n g dimer). I f HBF^ i s capable of o x i d i z i n g [ ( T) 5-C 5H 5)Cr(NO) 2 ] 2, i t should al s o be able to o x i d i z e the i r o n dimer. Another p o s s i b i l i t y , t h e r e f o r e , i s that the f i r s t step i n these r e a c t i o n s i s indeed protonation of the dimers, which r e s u l t s i n the re a c t i o n s shown below, i . e . [ ( r ) 5 - C 5 H 5 ) F e ( C O ) 2 ] 2 + HBF l +»OMe 2 » [ { ( r i 5-C 5H 5)Fe(CO) 2} 2H]BF 1 + + OMe2 (3.17) [ ( T i 5 - C 5 H 5 ) C r ( N O ) 2 ] 2 + HBF 4«OMe 2 CH 2CI2 » [{(ri 5-C 5H 5)Cr(NO) 2} 2H]BF 1 + + OMe2 (3.18) [ { ( T 1 5-C 5H 5)Cr(NO) 2} 2H]BF £ t CH2CI 2 » [ ( T ) 5 - C 5 H 5 ) C r ( N O ) 2 ] + + (n 5-C 5H 5)Cr(NO) 2H (3.19) (r) 5-C 5H 5)Cr(NO) 2H + HBF l t«OMe 2 CH 2CI2 * [ ( r i 5 - C 5 H 5 ) C r ( N O ) 2 ] + + H 2 + OMe2 (3.20) 77 While no evidence f o r a r a d i c a l ca t i o n [ { ( r p - C ^ H ^ F e C C O ) ^ ] • i s found i n the r e a c t i o n s of the parent dimer with HBF1+*OMe2, the n i t r o s y l - b r i d g e d 30 complex [(r| 5-C 5H 5)Co(N0) ] 2 i s r e a d i l y o x i d i z e d by t h i s a c i d , i .e . CH 2CI2 [ ( T I 5 - C 5 H 5 ) C O ( N O ) ] 2 + HBF l 4»OMe 2 > [ { ( T 1 5 - C 5 H 5 ) C O ( N O ) l ^ B F , + 1/2 H 9 + 0Me 9 (3.21) The c o b a l t - c o n t a i n i n g dimer would appear to be e a s i e r to o x i d i z e ( E 1 / 2 = -0.09 V vs. ( n 5 - C 5 H 5 ) 2 F e / [ ( T i 5 - C 5 H 5 ) 2 F e ] + i n CH 2C1 2/0.1 M [n-Bu^N]PF65°) than [ ( T i 5 - C 5 H 5 ) F e ( C O ) 2 ] 2 ( E 1 / 2 = +0.20 V vs. (T I 5 -C 5 H 5 ) 2Fe/[ ( n 5-C 5H 5) 2Fe] + i n CH 2C1 2/0.1 M [ n - B u ] P F 6 ) , and considerably e a s i e r to o x i d i z e than [ ( n 5-C 5H 5)Cr(N0) 2] 2. (c) I s o l a t i o n of [ ( T l 5 - C 5 H 5 ) F e ( T i 6 - C 6 M e 6 ) ] [ { ( t i 5 - C 5 H 5 ) C r ( N 0 ) 2 } 2 ] and i t s Properties. The one-electron reduction of the n e u t r a l n i t r o s y l c. c 45 dimer i s best e f f e c t e d with (n : : ,-C 5H 5)Fe ( T i t >-C 6Me 6) i n E t 2 0 . This provides f o r a homogeneous and potent, one-electron reducing system. The prepara-45 44 t i o n s of the green reductant and i t s c a t i o n i c precursor have been reported but some d e t a i l s e s s e n t i a l to s u c c e s s f u l syntheses have not been c l e a r l y o u t l i n e d . The preparations described i n the Experimental Section have been s u c c e s s f u l l y c a r r i e d out on s e v e r a l occasions. The r e a c t i o n between [ ( n 5 - C 5 H 5 ) C r ( N 0 ) 2 ] 2 and (n 5-C 5H 5)Fe(n 6-C 6Me 6) can be c a r r i e d out as i n r e a c t i o n 3.22, i . e . 78 E t 2 0 [ ( n 5-C 5H 5)Cr(NO) 2] 2 + ( n 5-C 5H 5)Fe ( T i 6-C 6Me 6) > [ (n 5-C 5H 5)Fe (n 6-C 6Me 6)] + [ { ( T i 5 - c 5H 5)Cr(NO ) 2 } 2 ] T (3.22) Under these c o n d i t i o n s , the de s i r e d b i m e t a l l i c r a d i c a l anion p r e c i p i t a t e s as i t s [ ( r i 5 - C 5 H 5 ) F e ( T ] 6 - C 6 M e 6 ) ] + s a l t and i s thus precluded from being reduced f u r t h e r . More i m p o r t a n t l y , the problem of the i n s t a b i l i t y of the r a d i c a l anion i n s o l u t i o n i s circumvented by immediate p r e c i p i t a t i o n . This strategy of i s o l a t i n g a h i g h l y r e a c t i v e anion by mixing two reagents i n a homogeneous s o l u t i o n i s very e f f e c t i v e and has been a p p l i e d s u c c e s s f u l l y to subsequent syntheses (see Chapter 5 ) . The product s a l t from r e a c t i o n 3.22, i s o l a b l e i n 69% y i e l d , i s a paramagnetic, o l i v e - g r e e n s o l i d which i s a i r -s e n s i t i v e both i n s o l u t i o n and i n the s o l i d s t a t e . Small samples of the s o l i d r a p i d l y become red-brown on exposure to a i r ( l a r g e r samples burn i n a i r ) and an IR spectrum ( N u j o l mull) reveals that a major product i s the n e u t r a l dimer, [(T) 5-C 5H 5)Cr(NO) 2] 2. Thus, as expected the r a d i c a l anion i s extremely o x i d a t i v e l y s e n s i t i v e . I t d i s s o l v e s r e a d i l y i n polar organic solvents but r e a c t s with most of them to give brown s o l u t i o n s . An IR spectrum of a CH 2C1 2 s o l u t i o n of the r a d i c a l anion complex, for example, shows s e v e r a l n i t r o s y l bands, i n c l u d i n g bands for the n e u t r a l dimer. These observations are c o n s i s t e n t with the e l e c t r o l y t i c r eduction r e s u l t s f o r 79 [(T) 5-C 5H 5)Cr(NO) 2] 2 i n CH 2C1 2 (see above). A Nujol-mull IR spectrum of the complex d i s p l a y s two strong n i t r o s y l absorptions at 1580 and 1331 cm - 1 a t t r i b u t a b l e to t e r m i n a l and b r i d g i n g NO groups, r e s p e c t i v e l y . These absorptions occur ~^0 and ~175 cm - 1, r e s p e c t i v e l y , lower i n energy than those e x h i b i t e d by [(r) 5-C 5H 5)Cr(NO) 2] 2 [IR ( N u j o l mull) v N Q 1669 ( s , b r ) , 1505 (s) cm"1] and i n d i c a t e s u b s t a n t i a l d e r e a l i z a t i o n of the e x t r a e l e c t r o n density i n the r a d i c a l anion onto the NO l i g a n d s , p a r t i c u l a r l y the b r i d g i n g n i t r o s y l groups. Consistent with t h i s i n t e r p r e t a t i o n i s the f a c t that the ESR spectrum of the s a l t i n DMF (Figure 14a) i n d i c a t e s strong coupling of the e l e c t r o n to one n i t r o g e n atom ( a N = 5.89 G) and weaker 1 couplings to a second n i t r o g e n atom (a = 0.89 G) and ten equivalent 2 c y c l o p e n t a d i e n y l protons ( a u = 1.05 G) (Figure 14b). This spectrum a l s o £1 i n d i c a t e s that i n DMF the b i m e t a l l i c r a d i c a l anion does not possess a centrosymmetric molecular s t r u c t u r e analogous to that found f o r the parent n i t r o s y l dimer i n the s o l i d s t a t e . 7 0 One p o s s i b i l i t y i s shown i n Figure 15. A l t e r n a t i v e l y , DMF might become chemically involved with the anion, perhaps as depicted i n Figure 16, which would also r e s u l t i n the inequivalence of the two n i t r o s y l l igands as demonstrated by the ESR spectrum of Figure 14a. U n f o r t u n a t e l y , repeated attempts to grow s i n g l e c r y s t a l s of t h i s complex f o r an X-ray c r y s t a l l o g r a p h i c study f a i l e d . Thus f a r only one other organometallic r a d i c a l anion c o n t a i n i n g 59 b r i d g i n g n i t r o s y l s has been i s o l a t e d , name l y N a [ { ( r i b - C 5 H 5 ) F e ( N 0 ) } 2 ] . The 80 ( a ) Figure 14. The X-Band ESR spectrum ( i n dimethylformamide) of [ ( r i 5-C 5H 5)Fe ( n 6-C 6Me 6)][ {(n 5-C 5H 5)Cr(NO) 2 } 2 ] . (a) Experimentally observed; g-value = 1.9975. (b) Simulated employing the f o l l o w i n g spin Hamiltonian parameters: a N = 5.89 G, a N = 0.89 G, a R = 1.05 G, p-p l i n e w i d t h = 0.56. Figure 16 . Possible s t r u c t u r a l modification of [ {( T)5-C 5H 5)Cr(NO) 2} 2] * D V i n t e r a c t i o n with DMF. 82 n e u t r a l parent dimer contains a formal Fe=Fe double bond, and the metal-metal i n t e r a c t i o n may c o n t r i b u t e to the s t a b i l i t y of the r a d i c a l anion. An IR spectrum of t h i s r a d i c a l anion complex i n CH3CN e x h i b i t s one at 1404 cm - 1; ~100 cm - 1 lower i n energy than the n e u t r a l dimer, c o n s i s t e n t with increased rt-back-bonding to N O . The b r i d g i n g NO's i n the chromium-con t a i n i n g anion s h i f t to a greater degree (~175 c m - 1 ) . An ESR spectrum of Na [ { (T] 5-C 5H 5)Fe(NO) } 2] shows only a s i n g l e , broad resonance, and the p o s s i b l i t y of the a d d i t i o n a l e l e c t r o n r e s i d i n g i n an o r b i t a l that i s not 59 mainly NO-based has been suggested. An analogous c a r b o n y l - c o n t a i n i n g r a d i c a l anion, Na[{(n 5-C 5H 5)Co(CO)} 2] has als o been prepared and charac-t e r i z e d . ^ * In general however, most homo-bimetallic c a r b o n y l - c o n t a i n i n g dimers undergo net two-electron e l e c t r o c h e m i c a l reduction to form the c o r -responding monometallic anions.*^ This has a l s o been demonstrated f o r react i o n s of M 2 ( C O ) 1 0 (M = Mn, Re), Co 2(CO) 8, [ ( T I 5 - C 5 H 5 ) M ( C O ) 3 ] 2 (M = Cr, 72 Mo, W), and [ ( T I 5 - C 5 H 5 )M ( C O ) 2 ] 2 (M = Fe, Ru) with chemical reductants, as 7 3 w e l l as fo r [ ( T i 5 - C 5H 5) N i ( C O ) ] 2- The i s o l a t i o n of the complex [ ( T i 5 - C 5 H 5 ) F e ( T i 6 - C 6 M e 6 ) ] [ { ( T i 5 - C 5 H 5 ) C r ( N O ) 2 } 2 ] therefore d i s t i n g u i s h e s the n e u t r a l n i t r o s y l precursor from i t s r e l a t e d [(ri 5-C 5H 5)M(CO) n] 2 analogues. Some of the chemistry of [ ( r | 5 - C 5 H 5 ) C r ( N 0 ) 2 ] 2 can now be understood i n terms of the f a c i l e r e d u c t i o n of the dimer. Reduction of ( T ) 5 - C 5 H 5 ) C r ( N 0 ) 2 C l to generate the chromium-dimer i s always accompanied by 33 38 the formation of small amounts of ( r ) 5 - C 5 H 5 ) 2Cr 2(N0) 3(NH 2) , ' p a r t i c u -l a r l y i n THF. E v i d e n t l y [ ( T i 5 - C 5 H 5 ) C r ( N O ) 2 ] 2 a l s o begins to reduce and the r a d i c a l anion reacts i n part with the solvent to convert a b r i d g i n g -n i t r o s y l group i n a bridging-amldo l i g a n d . This i s supported by the 83 bul k - e l e c t r o c h e m i c a l reduction of the parent dimer (see above). The 33 34 re a c t i o n of the dimer with Na[H 2Al(OCH 2CH 2OCH 3) 2] or L i [ E t 3 B H ] produces ( t ) 5 - C 5 H 5 ) 2 C r 2 ( N O ) 3 N H 2 , [(n 5-C 5H 5)Cr(NO)(NH 2)] 2 and (T) 5-C 5H 5) 2Cr 2(NO) 2(NH 2)(OH), i n a l l of which b r i d g i n g n i t r o s y l s have been reduced. (The hydroxo-bridged complex may r e s u l t from attack of OH" produced during the r e a c t i o n , e i t h e r free i n s o l u t i o n , or bound to a b r i d g i n g , n i t r o g e n - c o n t a i n i n g l i g a n d . ) The nuc l e o p h i l e could f i r s t reduce the dimer or H~ might a t t a c k the b r i d g i n g - n i t r o s y l d i r e c t l y , a l s o c o n s i s t e n t with the b r i d g i n g - n i t r o s y l groups f u n c t i o n i n g as e l e c t r o n acceptors toward reducing agents. S i m i l a r l y , formation of K 34 ( T i b - C 5 H 5 ) 2 C r 2 ( N O ) 3 ( E t N B E t 2 ) i n the r e a c t i o n of the dimer with L i [ E t 3 B H ] might r e s u l t from B E t 3 a t t a c k on the i n i t i a l l y formed r a d i c a l - a n i o n , or by attack of BEt 3 at a f u n c t i o n a l i z e d NO, i . e . N N C r ^ C r + BEt-K .OBEt-N / \ C r — C r Et BEt< N ' C r — C r + OH (3.23) The fa c t that the y i e l d s of a l l these products are quite low (15% f o r the mono-amido complex and < 5% for the remaining complexes) probably r e f l e c t s 84 the i n s t a b i l i t y of [ { ( T i 5-C 5H 5)Cr(NO) 2 } 2 ] ~ , or f u n c t i o n a l i z e d a n i o n i c d e r i v a t i v e s of the dimer i n s o l u t i o n which r e s u l t s i n decomposition i n t o n o n - n i t r o s y l - c o n t a i n i n g products as i n the b u l k - e l e c t r o l y t i c r eduction of the n e u t r a l dimer. This s i t u a t i o n could w e l l be aggravated by cati o n s such as L i + or Na + c o o r d i n a t i n g to the b r i d g i n g NO l i g a n d s , 7 ^ f u r t h e r p o l a r i z i n g the complex. Secondly, the r e a c t i o n of the dimer with a l k y l l i t h i u m reagents ( C H 3 L i , n-BuLi) occurs at the b r i d g i n g n i t r o s y l l i g a n d s to form b r i d g i n g methylene-amido-type ligands f o l l o w i n g h y d r o l y s i s , 7 5 i . e . 1) CH 3 L i [ ( n 5-C 5H 5)Cr(NO) 2] 2 > ( n 5-C 5H 5) 2Cr 2(NO) 3(N=CH 2) (3.24) 2) H 20 I f t-BuLi i s used then ( n 5-C 5H 5) 2Cr 2(NO) 3(t-BuNOH) forms. These r e s u l t s taken together, and the ele c t r o c h e m i c a l r e d u c t i o n s , suggest that the LUMO of the n e u t r a l dimer i s l a r g e l y {(p.-NO)2}-based. The a b i l i t y of the n e u t r a l dimer to remain i n t a c t upon reduction i s undoubtedly c e n t r a l to i t s observed r e a c t i v i t y w i t h n u c l e o p h i l e s . IV Summary. In a d d i t i o n to e s t a b l i s h i n g the fundamental redox p r o p e r t i e s of [ ( T ) 5 - C 5 H 5 ) C r ( N O ) 2 ] 2 , t h i s work has shown that because of the e l e c t r o n i c perturbations which occur when M-CO linkages are replaced by f o r m a l l y i s o e l e c t r o n i c M'-NO bonds, 7^ 7 7 the redox behaviour of organometallic n i t r o s y l complexes cannot n e c e s s a r i l y be i n f e r r e d from that e x h i b i t e d by t h e i r i s o e l e c t r o n i c and i s o s t r u c t u r a l carbonyl analogues. Furthermore, the 85 redox properties of [ (r) b-C 5H 5)Cr(NO) 2 ] 2 have shed some l i g h t on the reactions of t h i s dimer, and have been successfully exploited to synthesize the new r a d i a l anion complex [ ( r i 5-C 5H 5)Fe(n 6-C 6Me 6) ] [ { ( T i 5-C 5H 5)Cr(NO) 2 > 2 ] . 86 Chapter Four THE OXIDATION ELECTROCHEMISTRY OF SOME GROUP 6 (Ti5-CYCLOPENTADIENYL)DINITROSYLALKYL-METAL COMPLEXES AND RELATED CARBONYL COMPOUNDS: IMPLICATIONS FOR ELECTROPHILIC CLEAVAGE OF METAL-ALKYL BONDS I) Introduction E l e c t r o p h i l i c cleavage reactions of metal-alkyl bonds are the 78 subjects of extensive and ongoing i n v e s t i g a t i o n s . These reactions are of importance since they are employed to remove the a l k y l group from a metal 78a centre or to fun c t i o n a l i z e i t . Two general types of cleavage reactions 79 may be distinguished, i n s e r t i o n , i . e . M-R + E > M-E-R M-R + L > M-E-R I I E L and elimination, i . e . M-R + EY > M-Y + ER . In the former types of processes the e l e c t r o p h i l e may ar r i v e from outside the metal's coordination sphere, or, as i n the case of CO i n s e r t i o n , i t may be bound to the metal p r i o r to i n s e r t i o n . The elimination reactions occur 87 by a vari e t y of mechanisms, depending on the e l e c t r o p h i l e and the a l k y l complex. Previous work i n these laboratories has shown that the n i t r o s y l -containing a l k y l complexes (T) 5-C 5H 5)M(NO) 2R (M = Cr, R = CH 3; M = Mo, R = CH 3, C 2H 5) react d i f f e r e n t l y toward ce r t a i n e l e c t r o p h i l e s than do related 80 carbonyl-containing a l k y l compounds. Thus, for example, t r i p h e n y l c a r -benium sa l t s can be reacted with suitable carbonyl-alkyl compounds to 81 generate carbene complexes v i a a-hydrogen abstraction, whereas t h e i r reactions with related n i t r o s y l - a l k y l compounds r e s u l t i n cleavage of the metal-alkyl bond. In addition, while ( T I 5 - C 5 H 5 ) M O ( N O ) 2CH 3 reacts r e a d i l y with HgCl 2 to produce ( T I 5 - C 5 H 5 ) M O ( N O ) 2C1, the carbonyl complex 82 (n 5-C 5H 5)Mo(CO) 3CH 3 i s unreactive toward HgCl 2. One of the mechanisms that has been proposed for some e l e c t r o p h i l i c cleavage reactions of metal-alkyl bonds involves prior oxidation of the 78 organometallic complex by the e l e c t r o p h i l e . Accordingly, the comparative electrochemistry of ( T I 5 - C 5 H 5 ) M ( N O ) 2 a l k y l (M = Cr, Mo, W) compounds and related carbonyl-containing a l k y l complexes becomes of i n t e r e s t . Thus, the oxidation electrochemistry of various n i t r o s y l - a l k y l complexes (T}5-C5H5)M(NO) 2R (M = Cr, R = CH3; M = Mo, W, R = CH3, C 2H 5) and the carbonyl-alkyl compounds (T) 5-C 5H 5)M(CO) 3R (M = Cr, R = CH 3; M = Mo, W, R = CH3, C 2H 5) and (T) 5-C 5H 5)Fe(CO) 2CH 3 i s described i n this chapter, and i t s relevance to e l e c t r o p h i l i c cleavage of the M-R bonds i n (r)5-C5H5)M(NO) 2 R complexes i s discussed. Reactions of some of the ni t r o s y l - c o n t a i n i n g a l k y l compounds with e l e c t r o p h i l e s and/or oxidants such as [F e ( P h e n ) 3 ] ( P F 6 ) 3 , N0PF6 and AgBFn are presented and of p a r t i c u l a r i n t e r e s t is the reaction of 88 ( T i 5-C 5H 5)Cr(NO) 2CH 3 with NOPF6 which r e s u l t s i n NO i n s e r t i n g i n t o the Cr-CH 3 bond. II) Experimental Section The c y l i c voltammetric and s y n t h e t i c work was c a r r i e d out w i t h the techniques and procedures d e t a i l e d i n Chapters 2 and 3. Nitromethane was d i s t i l l e d from CaH 2 under reduced pressure at 40-60°C. The a l k y l compounds were prepared according to published procedures, or by m o d i f i c a t i o n s thereof as o u t l i n e d below. Their p u r i t y was ascertained by elemental a n a l y s i s and/or spectroscopic methods. C y c l i c voltammograms were recorded f o r ( T I 5 - C 5 H 5 ) M ( N O ) 2 R (M = Cr, R = CH 3; M = Mo, R = CH 3, C 2H 5; M = W, R = H, CH 3, C 2 H 5 ) , ( T i 5-C 5H 5)Fe(CO) 2CH 3 and ( T I 5 - C 5 H 5 ) M ( C O ) 3R (M = Cr, R = CH 3, M = Mo, R = CH 3 > C 2H 5; M = W, R = H, CH 3, C 2H 5) i n CH 2C1 2 and, i n s e l e c t e d cases, i n CH3CN a l s o . The X-ray c r y s t a l l o g r a p h i c work was c a r r i e d out by Dr. F.W.B. E i n s t e i n and Dr. A.C. W i l l i s . Reactions of (ti 5-C 5H 5)Cr(NO) 2CH 3 with [ F e ( P h e n ) 3 ] ( P F 6 ) 3 . A f l a s k c o n t a i n i n g ( r i 5-C 5H 5)Cr(NO) 2CH 3 (0.192 g, 1.00 mmol) and 83 [ F e ( P h e n ) 3 ] ( P F 6 ) 3 (1.03 g, 1.00 mmol) was cooled w i t h a l i q u i d - n i t r o g e n bath and CH 2C1 2 (~25 mL) was added slo w l y . The frozen mixture was then l i q u e f i e d by warming to -78°C with a Dry Ice/acetone bath and s t i r r e d . A red s o l u t i o n and red s o l i d r e s u l t e d . An IR spectrum recorded on an a l i q u o t of the supernatant s o l u t i o n at room temperature showed that v i r t u a l l y a l l of the s t a r t i n g methyl compound had been consumed, new NO absorptions 89 having appeared at 1845 (m) and 1743 (s) cm . The r e a c t i o n mixture was c 84 then t r e a t e d with ( T] 5-C 5H 5) 2 C O (0.189 g, 1.00 mmol), i n CH 2C1 2 (~10 mL) . An IR spectrum of the supernatant s o l u t i o n revealed only two, strong NO bands at 1777 and 1669 cm - 1. The solvent was removed i n vacuo and a green s o l i d was sublimed from the s o l i d residue (5 * 1 0 - 3 mm) at 40-60°C onto a water-cooled probe. This gave 0.06 g of ( n 5-C 5H 5)Cr(NO) 2CH 3 (30% y i e l d ) : IR (hexanes) v 1784 ( s ) , 1682 (vs) cm - 1; l o w - r e s o l u t i o n mass spectrum (probe temperature 120°C), m/z_ 192 ( P + , most intense parent i o n ) . I f t h i s same procedure was c a r r i e d out i n CH 2C1 2 at room tempera-ture an IR spectrum of the supernatant s o l u t i o n [ p r i o r to (r^-C^H^^Co a d d i t i o n ] d i s p l a y e d NO bands at 1846 and 1745 cm - 1. Upon a d d i t i o n of cobaltocene, no ( T) 5-C 5H 5)Cr(NO) 2CH 3 could be detected by IR spectroscopy u n l i k e p r e v i o u s l y . Cooling a r e a c t i o n mixture generated from ( T) 5-C 5H 5)Cr(NO) 2CH 3 and [ F e ( P h e n ) 3 ] ( P F 6 ) 3 at room temperature, before adding the s o l u t i o n of c o l b a l t o c e n e , l i k e w i s e d i d not regenerate any ( n 5-C 5H 5)Cr(N0) 2CH 3. In another experiment, the o x i d a t i o n r e a c t i o n was conducted as above (on a 0.5 mmol sc a l e ) i n the presence of a s t o i c h i o m e t r i c q u a n t i t y of P(0Ph) 3 (0.13 mL, 0.51 mmol). The i n i t i a l l y frozen mixture was warmed to -78°C and s t i r r e d . An IR spectrum of the r e s u l t a n t mixture showed absorp-t i o n s i n the n i t r o s y l s t r e t c h i n g region at 1848 (w), 1755 (m) and 1735 (sh) cm - 1. The a d d i t i o n of ( T ) 5 - C 5 H 5 ) 2Co r e s u l t e d i n the formation of ( T i 5-C 5H 5)Cr(NO) 2CH 3 which was i s o l a t e d from the mixture as described above 90 i n 30% y i e l d and i d e n t i f i e d by i t s c h a r a c t e r i s t i c IR spectrum i n hexanes (see above). F i n a l l y , ( T i 5-C 5H 5)Cr(NO) 2CH 3 (0.10 g, 0.52 mmol) was weighed i n t o a small v i a l (~1*1 cm) and the v i a l was placed i n a f l a s k c o n t a i n i n g [ F e ( P h e n ) 3 ] ( P F 6 ) 3 (0.51 g, 0.49 mmol). To another f l a s k , appended to the f i r s t by means of an adapter was added CH 2Cl2 (~20 mL). Both f l a s k s were cooled to -78°C with a Dry Ice/acetone bath and then the CH 2C1 2 was added to the two reagents and the mixture was s t i r r e d . A f t e r ~30 min almost a l l of the s t a r t i n g methyl compound had been consumed as evidenced by an IR spectrum recorded f o r a sample of the supernatant s o l u t i o n . The major n i t r o s y l absorptions observed i n t h i s spectrum were, again, at 1845 and 1743 cm - 1. The mixture was a dark red-brown c o l o u r . At t h i s point h a l f an equivalent of E t 3 N (0.035 mL, 0.25 mmol) was added to the r e a c t i o n mixture. An IR spectrum of the supernatant s o l u t i o n showed that the bands at 1845 and 1743 cm - 1 had diminished i n i n t e n s i t y and new absorptions appeared at 1777 and 1669 cm - 1. A second 0.5 equivalent of E t 3 N was added. An IR spectrum of the mixture showed that the bands at 1777 and 1669 cm - 1 had grown i n i n t e n s i t y . In a d d i t i o n some very weak absorptions remained at 1833, 1817, 1734 and 1726 cm - 1. The solvent was removed under reduced pressure and from the residue was sublimed (5 * 10~ 3 mm) at 40-60°C, (r| 5-C 5H 5)Cr(NO) 2CH 3 (0.06 g) i n ~60% y i e l d , which was i d e n t i f i e d by i t s c h a r a c t e r i s t i c IR and mass spectra (see above). Reaction of (Ti5-C5H5)Cr(NO)2CH3 with AgBF,. A s o l u t i o n of ( T i 5-C 5H 5)Cr(NO) 2CH 3 (0.384 g, 2.00 mmol) i n CH 2C1 2 (30 mL) was s t i r r e d over s o l i d AgBF, (0.389 g, 2.00 mmol). An IR spectrum of the green mixture 91 recorded a f t e r ~5 min displayed v N 0 ' s at 1847 (m), 1758 (sh) and 1739 (m) cm - 1, i n a d d i t i o n to the predominant s t a r t i n g m a t e r i a l bands at 1777 and 1669 cm - 1. A f t e r another 40 min had elapsed, the bands at 1847, 1756 and 1740 cm - 1 i n t e n s i f i e d as the s t a r t i n g m a t e r i a l was consumed. An IR spectrum recorded f o r the same sample about 10 min l a t e r showed that the band at 1756 cm - 1 decreased s u b s t a n t i a l l y i n i n t e n s i t y with concomitant i n t e n s i f i c a t i o n of the bands at 1847 and 1740 cm - 1. A f t e r a t o t a l r e a c t i o n time of ~2 h most, but not a l l , of the s t a r t i n g m a t e r i a l had reacted and a small amount of a blue substance had deposited. An IR spectrum of the f i n a l , green supernatant s o l u t i o n showed two strong bands at 1844 and 1740 cm - 1. The r e a c t i o n mixture was f i l t e r e d through a medium-porosity f r i t and the f i l t r a t e was treated with [ ( P h 3 P ) 2 N ] C l (1.15 g, 2.00 mmol). (A Nujol-mull IR spectrum of the blue s o l i d revealed i t to be n o n - n i t r o s y l containing.) The volume of the s o l u t i o n was reduced to ~15 mL i n vacuo. A d d i t i o n of E t 2 0 (160 mL) r e s u l t e d i n the p r e c i p i t a t i o n of a white s o l i d . The mixture was f i l t e r e d , and the f i l t r a t e was taken to dryness i n vacuo. This residue was d i s s o l v e d i n CH 2C1 2 (~10 mL) and chromatographed on a F l o r i s i l column made up i n CH 2C1 2- A brown-green band developed and was el u t e d w i t h CH 2C1 2. A blue-green band remained atop the column. Concen-t r a t i o n of the eluate i n vacuo and a d d i t i o n of hexanes r e s u l t e d i n the p r e c i p i t a t i o n of s l i g h t l y impure ( n 5 - C 5 H 5 ) C r ( N O ) 2 C l 3 9 (0.15 g, -36% y i e l d ) : IR (CH 2C1 2) v N Q 1817 ( s ) , 1711 (s) cm - 1; l o w - r e s o l u t i o n mass spectrum (Probe temperature 120°C), m/z 212 ( P + , most intense parent i o n ) . A n a l , c a l c d f o r C 5 H 5 C l C r N 2 0 2 : C, 28.25; H, 2.37; N, 13.18. Found: C, 29.17; H, 2.33; N, 12.88. 92 I f two equivalents of AgBF, were employed, the r e a c t i o n proceeded s i m i l a r l y , though a l l of the s t a r t i n g methyl complex was consumed i n ~30 min. Not a l l of the AgEF, appeared to have reacted, however. ( A d d i -t i o n of aqueous KC1 to the blue s o l i d obtained as above generated a milky white suspension, presumably of AgCl.) On three separate occasions the react i o n s of (n 5-C 5H 5)Cr(NO) 2CH 3 and AgBF, were performed as o u t l i n e d above i n CH 2C1 2, with u l t i m a t e a d d i t i o n of [(Ph 3P) 2N]C1. The concentrations of ( r i 5 - C 5 H 5 ) C r ( N O ) 2 C l were measured by comparison with Beer's Law p l o t s of the IR absorbances (of the n i t r o s y l s t r e t c h e s ) v s . concentration derived from authentic (r) 5-C 5H 5)Cr(NO) 2C1 i n CH 2C1 2. when e i t h e r one or two equivalents of AgBF 4 were used, the amount of (n 5-C 5H 5)Cr(NO) 2C1 f i n a l l y formed was 0.45+0.03 mole per mole of (n 5-C 5H 5)Cr(NO) 2CH 3. Attempted reaction of (Ti 5-C 5H 5)Cr(NO) 2CH 3 with ( n 5-C 5H 5)Cr(NO) 2BF^. A mixture of [(r| 5-C 5H 5)Cr(NO) 2 ] 2 (0.177 g, 0.500 mmol) and AgBF, (0.195 g, 1.00 mmol) i n CH 2C1 2 (20 mL) generated (T|^—C^Hg)Cr(NO)2BF1+ (1 mmol) as described i n the Experimental Section of Chapter 3. The r e s u l -tant s o l u t i o n was f i l t e r e d i n t o a f l a s k c o n t a i n i n g ( n 5 - C 5 H 5 ) C r (NO) 2CH 3 (0.19 g, 1.0 mmol) and s t i r r e d . An IR spectrum of the green s o l u t i o n displayed NO bands at 1845 ( s ) , 1776 ( s ) , 1740 ( s ) , 1670 ( s ) . Hexanes (10 mL) were c a r e f u l l y layered onto the CH 2C1 2 s o l u t i o n and t h i s mixture was l e f t standing for s e v e r a l days. A l l that occurred was e x t r a c t i o n of (n 5-C 5H 5)Cr(NO) 2CH 3 i n t o the hexanes l a y e r as evidenced by an IR spectrum of a sample taken from i t (v 1784, 1682 c m - 1 ) . Reaction of ( T i 5-C 5H 5)Cr(NO) 2CH 3 with N0PFfi. A s o l u t i o n of (n 5-C 5H 5)Cr(NO) 2CH 3 (0.384 g, 2.00 mmol) i n CH 2C1 2 (20 mL) was s t i r r e d over 93 s o l i d NOPF6 (0.280 g, 1.60 mmol). An excess of ( T) b-C 5H 5)Cr(NO) 2CH 3 was used to ensure complete consumption of the NOPFg. A f t e r the r e a c t i o n had proceeded f o r ~15 min, an IR spectrum of the supernatant s o l u t i o n revealed NO absorptions at 1847 and 1745 cm - 1 as w e l l as the bands f o r the s t a r t i n g m a t e r i a l at 1777 and 1669 cm - 1. A f t e r ~1 h a dark green, m i c r o c r y s t a l l i n e s o l i d became apparent. A f t e r a t o t a l of 3.5 h t h i s s o l i d was c o l l e c t e d by f i l t r a t i o n and washed r a p i d l y with CH 2C1 2 ( 2 x 5 m L ) . The s o l i d was r e d i s -solved i n CH 2C1 2 (~350 mL) and f i l t e r e d i n t o a clean f l a s k . The green f i l t r a t e was then slowly concentrated i n vacuo to produce a dark green, m i c r o c r y s t a l l i n e s o l i d which was c o l l e c t e d by f i l t r a t i o n , washed with a l i t t l e CH 2C1 2 (~2 x 5 m L ) and d r i e d i n vacuo (5 x io~ 3 mm). I t was i d e n t i -f i e d as [ ( T i 5-C 5H 5)Cr(NO) 2(CH 2NOH)]PF 6 (0.260 g, 44% y i e l d based on N0PF 6): IR (Nujol m ull) v N Q 1854 ( s ) , 1761 (s) cm - 1, also 3480 (m), 3130 (m), 3000 ( s h ) , 1646 (w), 1433 (m), 1349 (m), 1015 (w), 996 (m), 945 (m), 882 (m), 849 ( s ) , 831 (s) cm - 1, IR (CH 2C1 2) v N Q 1847 (m), 1746 (m) cm - 1, al s o 851 (w) cm - 1; IR (CH 3N0 2) v 1847 ( s ) , 1748 (s) cm - 1, also 849 (s) cm - 1; lB. NMR (CD 3N0 2) 6 8.84 (d, IH, H J L ] { _ x = 0.9 Hz, H^ONCH^Hg), 7.67 (d, IH, A X 2 j l H _ 1 H = 5 , 1 H z » "x^^A^-B^ 7 , 3 0 (DD' 1H' V^-A^V' 6 , 0 8 (S' 5H' A D CgHjj); XH NMR (CD 2C1 2) 6 8.92 ( s , b r , IH, KL^ONCH^), 7.66 (d, IH, 2 j l H _ 1 H = 5 * 2 H Z » H X ° N C H A V ' 7 ' 1 4 ( D ' 1 H " X ^ ^ A V ' 5 , 9 8 ( S ' 5 H ' C 5 ^ ) ; A a 1 3C{ 1H} NMR (CD 3N0 2) 6 159.1 ( s , H0NCH 2), 105.4 ( s , £ 5 % ) ; 1 3C NMR (gated lR decoupled) (CD 3N0 2) 6 159.0 (dd, ^ U ^ I H = 1 7 8 - 7 186.9 Hz, HONCH^), 105.3 (dq, ^ 1 3 ^ ^ = £ 5 % ) ' 94 Anal. Calcd f o r C 6H 8CrF 6N 30 3P: C, 19.63; H, 2.20; N, 11.45. Found: C, 19.39; H, 2.15; N, 11.11 ( V 2 0 5 used as a combustion a d d i t i v e ) . S i n g l e c r y s t a l s of t h i s compound s u i t a b l e f o r an X-ray c r y s t a l l o -graphic study were obtained by preparing a saturated s o l u t i o n of i t i n CH 2Cl2 (~30 mL) and c o o l i n g to -25°C f o r ~24 h. This produced long, green needles of the s o l i d . I f the r e a c t i o n was conducted with equal s t o i c h i o m e t r i e s of the two reagents, and l e f t to s t i r f o r ~24 h, again a dark green m i c r o c r y s t a l l i n e p r e c i p i t a t e formed as above, and the f i n a l r e a c t i o n mixture e x h i b i t e d an IR spectrum with absorptions of 1844 (m), 1837 ( s h ) , 1817 (w), 1744 (m), 1735 (sh) and 1711 (w) cm - 1. 38 Preparation of (TJ 5-C 5H 5)MO(N0) 2CH 3. A s t i r r e d s o l u t i o n of c 39 (n 5-C 5H 5)Mo(NO) 2Cl (5.20 g, 20.3 mmol) i n CH 2C1 2 (40 mL) was cooled to ~ -65°C with a Dry Ice/acetone bath and then treated with a 2 M s o l u t i o n of Me 3Al i n toluene (11 mL, 22 mmol). The cold-bath was then removed and the green s o l u t i o n was allowed to warm to room temperature. F i l t r a t i o n of the mixture through an alumina (Woelm, n e u t r a l a c t i v i t y 1) column ( 3 x 7 Cm) supported on a medium-porosity f r i t followed by washing the column with a s u f f i c i e n t q uantity of CH 2C1 2 to remove a l l traces of green m a t e r i a l y i e l d e d a deep green s o l u t i o n . This was taken to dryness i n vacuo, and the residue was sublimed (5 x 1 0 - 3 mm) at 40-60°C onto a water-cooled probe to produce 2.21 g (46% y i e l d ) of ( T I 5 - C 5 H 5 ) M O ( N O ) 2CH 3 as a green s o l i d : IR (hexanes) 1739 ( s ) , 1650 (s) cm - 1. Ana l . Calcd f o r C 6H 8MoN 20 2: C, 30.53; H, 3.43; N, 11.87. Found: C, 30.82; H, 3.45; N, 11.77. 95 Reaction of (tl 5-C 5H 5)Mo(NO) 2CH 3 with NOPFfi. A mixture of NOPFg (0.175 g, 1.00 mmol) and (n 5-C 5H 5)Mo(NO) 2CH 3 (0.236 g, 1.00 mmol) was s t i r r e d i n CH 2C1 2 (20 mL). The supernatant s o l u t i o n r a p i d l y darkened to brown-green and gas e v o l u t i o n was apparent. An IR spectrum of the super-natant s o l u t i o n a f t e r 30 min showed two strong, broad bands at 1773 and 1686 cm - 1. A d d i t i o n of [Et^NJCl (0.17 g, 1.00 mmol) i n s t a n t l y caused the s o l u t i o n to become green again, and an IR spectrum of the supernatant s o l u t i o n d isplayed v„„ bands at 1760 and 1669 cm - 1 c o n s i s t e n t with the NO c 39 formation of ( T I O - C 5 H 5 ) M O ( N 0 ) 2 C 1 . A s o l u t i o n generated s i m i l a r l y was l i k e w i s e treated w i t h one equivalent of [Et 1 +N]Br. Attempts to p u r i f y these 39 h a l i d e complexes by conventional methods produced only low y i e l d s of somewhat impure (n 5-C 5H 5)Mo(NO) 2X (X = C l , Br) owing to s u b s t a n t i a l decom-p o s i t i o n . The bromide complex was i s o l a t e d i n ~10% y i e l d : l o w - r e s o l u t i o n mass spectrum (probe temperature 100°C), m/z 302 ( P + , most intense parent i o n ) . A n a l . Calcd f o r C 5H 5BrMoN 20 2: C, 19.96; H, 1.67; N, 9.31. Found: C, 20.74; H, 1.60; N, 9.02. Preparation of (TI 5-C 5H 5)W(NO) 2CH 3. A s o l u t i o n of (t) 5-C 5H 5)W(NO) 2BF, i n CH 2C1 2 (160 mL) was generated by the r e a c t i o n of ( T I 5 - C 5 H 5 ) W ( N O ) 2 C 1 (5.00 g, 14.5 mmol) and AgBF, (2.82 g, 14.5 mmol). 6 8 A b r i l l i a n t green s o l u t i o n formed w i t h concomitant p r e c i p i t a t i o n of AgCl. The mixture was cooled w i t h a Dry Ice/acetone bath f o r ~10 min and a roughly s t o i c h i o m e t r i c amount of a 2 M Me 3Al s o l u t i o n i n toluene (8 mL, 16 mmol) was added by s y r i n g e . An instantaneous r e a c t i o n occurred as evidenced by the formation of a large q u a n t i t y of a very f i n e , brown p r e c i 96 p i t a t e . The supernatant solution remained green. The mixture was trans-ferred by cannulation into tubes equipped with septa and centrifuged for several minutes u n t i l the brown s o l i d had s e t t l e d . The green so l u t i o n was then cannulated onto a short column ( 3 x 7 cm) of alumina (Woelm, neutral, a c t i v i t y 1) supported on a medium porosity f r i t and f i l t e r e d into a f l a s k below. The column was washed with CH 2C1 2 u n t i l the washings were colour-l e s s . (Direct f i l t r a t i o n of the reaction mixture through alumina i n v a r i a b l y rendered the column v i r t u a l l y impervious.) The solvent was removed i n vacuo and the s o l i d residue was then sublimed (5 x 10" 3 mm) onto a water-cooled probe at 40-60°C to give 1.95 g (40% y i e l d ) of (n 5-C 5H 5)W(NO) 2CH 3 as a bright green s o l i d : IR (hexanes) v N Q 1718 ( s ) , 1638 (s) cm - 1; low-resolution mass spectrum (probe temperature 50°C), m/^ z 324 ( P +, most intense parent i o n ) . Anal. Calcd for C 6H 8N 20 2W: C, 22.24; H, 2.49; N, 8.65. Found: C, 22.25; H, 2.48; N, 8.55. Reaction of (ti 5-C 5H 5)W(N0) 2CH3 with AgBF,. A mixture of (n 5-C 5H 5)W(NO) 2CH 3 (0.18 g, 0.56 mmol) and AgBF, (0.11 g, 0.56 mmol) was s t i r r e d i n CH 2C1 2 (20 mL). Over the course of 4.5 h bands i n the IR spectrum of the supernatant s o l u t i o n due to the s t a r t i n g material (1709, 1620 cm - 1) diminished slowly to approximately half of their o r i g i n a l i n t e n s i t y . No new n i t r o s y l absorptions were detected. The solution became orange-brown and an amorphous p r e c i p i t a t e formed along with a s i l v e r -mirror. The solution was taken to dryness under reduced pressure. A Nujol-mull IR spectrum of the s o l i d residue revealed a new n i t r o s y l band at 97 1612 cm - 1. The s o l i d residue e x h i b i t e d s o l u b i l i t y only i n polar organic solvents such as CH3CN and DMF. Reaction of (TI 5-C 5H 5)W(N0) 2CH 3 with NOPFg. A s o l u t i o n of ( T I 5 - C 5 H 5 ) W ( N O ) 2 C H 3 (0.16 g, 0.49 mmol) i n CH 2C1 2 (15 mL) was treated w i t h N0PF 6 (0.18 g, 1.0 mmol) and s t i r r e d . The i n i t i a l l y white N0PF g became dark brown immediately but remained i n s o l u b l e . The s o l u t i o n g r a d u a l l y changed from a b r i g h t green colour to dark green-brown. An I R spectrum of the mixture a f t e r ~30 min revealed that some of the s t a r t i n g m a t e r i a l ( v „ „ NO 1709, 1620 cm - 1) had been consumed and new broad bands had appeared at 1744 and 1658 cm - 1. At the end of 3.5 h almost a l l the s t a r t i n g m a t e r i a l had been consumed and the new, broad n i t r o s y l absorptions remained i n the supernatant's I R spectrum. These were considerably weaker than the o r i g i -n a l s t a r t i n g m a t e r i a l bands. The a d d i t i o n of [ ( P h 3 P ) 2 N ] C l (0.29 g, 0.51 -mmol) to the s o l u t i o n caused i t s I R bands at 1744 and 1658 cm - 1 to s h i f t to i c 39 1734 and 1651 cm - 1, c o n s i s t e n t w i t h the formation of ( T i b-C 5H 5 )W(NO) 2C1. A r e a c t i o n mixture generated i n the same way from ( T I 5 - C 5 H 5 ) W ( N O ) 2 C H 3 and two equivalents of N0PF 6 was treated w i t h one equivalent of PPh 3 (0.13 g, 0.50 mmol). An I R spectrum of the mixture displayed v ^ Q ' S a t ~1768 and ~1693 cm - 1, most l i k e l y due to 68 ( T i 5-C 5H 5 ) W(NO) 2PPh 3] +, however, t h i s c a t i o n could not be separated from the attendant by-products. Reaction of (TI 5-C 5H 5)W(N0 ) 2CH 3 with [ F e ( P h e n ) 3 ] ( P F 6 ) 3 . A mixture of (n 5-C 5H 5 ) W(NO) 2CH 3 (0.162 g, 0.500 mmol) and blue [ F e ( P h e n ) 3 ] ( P F g ) 3 (0.516 g, 0.500 mmol) was s t i r r e d i n CH 3N0 2 (10 mL). The s o l u t i o n i n s t a n t l y became intense dark red. An I R spectrum of the supernatant 98 s o l u t i o n showed only weak absorptions due to n i t r o s y l s t r e t c h i n g at 1831 and 1771 cm - 1. The same r e a c t i o n performed i n CH2CI2 d i d not consume a l l of the methyl complex; weak bands at 1709 and 1620 cm - 1 i n the IR spectrum of the mixture remained. A d d i t i o n of [ ( P h 3 P ) 2 N ] C l d i d not form any ( T I 5 - C 5 H 5 ) W ( N O ) 2C1 i n e i t h e r case as i n d i c a t e d by IR spectra of the r e s u l t a n t s o l u t i o n s . Attempted re a c t i o n of (T| 5-C 5H 5)W(N0) 2CH 3 and (T ) 5-C 5H 5)W(N0) 2PF 6. A green s o l u t i o n of ( T I 5 - C 5 H 5 ) W ( N O ) 2C1 (0.27 g, 0.78 mmol) i n CH 2C1 2 (15 mL) was s t i r r e d with s o l i d AgPF 6 (0.25 g, 0.99 mmol) f o r ~10 min to form ( T I 5 - C 5 H 5 ) W ( N O ) 2 P F 6 6 8 [ V n q 1775 ( s ) , 1672 ( s ) ] . The s o l u t i o n was f i l t e r e d away from the AgCl p r e c i p i t a t e through a medium-porosity f r i t i n t o a f l a s k c o n t aining ( T I 5 - C 5 H 5 ) W ( N O ) 2CH 3 (0.26 g, 0.80 mmol). An IR of the green s o l u t i o n showed only NO bands for the two reagents, i . e . 1755, 1709, 1672 and 1620 cm - 1. Attempts to i s o l a t e an adduct by the a d d i t i o n of hexanes f a i l e d . Reaction of (n 5-C 5H 5)W(C0) 3CH 3 with AgBF^. A s o l u t i o n of Q C ( T I 5 - C 5 H 5 ) W ( C O ) 3CH 3 (0.348 g, 1.00 mmol) i n CH 2C1 2 (20 mL) was tr e a t e d w i t h AgBF^ (0.195 g, 1.00 mmol) and s t i r r e d . The progress of the r e a c t i o n was monitored by IR spectroscopy. A f t e r ~40 min much of the s t a r t i n g methyl complex (v = 2021, 1918 cm - 1) had been consumed. Once a l l of the s t a r t i n g m a t e r i a l had disappeared the s o l u t i o n became pink and a s u b s t a n t i a l q u a n t i t y of a grey p r e c i p i t a t e formed. A s i l v e r m i r r o r was not evident. New bands became apparent i n the IR spectrum of the supernatant s o l u t i o n at 2072 and ~1960 (br) cm - 1. The lower energy band g r a d u a l l y 99 s h i f t e d to higher frequency, f i n a l l y remaining at 1973 cm - 1, and bands f o r ( T ) 5 - C 5 H 5 ) W ( C O ) 3CH 3 began to reappear. To t h i s point ~1.5 h had elapsed. During the next 4.5 h the four bands at 2072, 2021, 1973 and 1918 s t e a d i l y grew i n i n t e n s i t y . No other CO bands were apparent. At the end of t h i s time the two lower v 's were of approximately equal i n t e n s i t y , as were the two higher energy bands and the s o l u t i o n was deep red i n c o l o u r . The r e a c t i o n mixture was f i l t e r e d i n t o a f l a s k c o n t a i n i n g [ ( P h 3 P ) 2 N ] C l (0.57 g, 1.0 mmol) i n CH 2C1 2 (5 mL), to give an orange s o l u t i o n (v 2054, 2021, 1959, 1918 c m - 1 ) . A f t e r c oncentrating the s o l u t i o n to ~5 mL i n vacuo, E t 2 0 (60 mL) was added to induce p r e c i p i t a t i o n of a white s o l i d , presumably c o n t a i n i n g [ ( P h 3 P ) 2 N ] C l and [ ( P h 3 P ) ^ J B F , . This mixture was f i l t e r e d and the f i l t r a t e was taken to dryness under reduced pressure. D i s s o l u t i o n of the s o l i d i n CH 2C1 2 (~5 mL) followed by c a r e f u l a d d i t i o n of hexanes u n t i l the s o l u t i o n became t u r b i d , and c o o l i n g at -25°C overnight r e s u l t e d i n the formation of long, red-brown needles. These were c o l l e c t e d by f i l t r a t i o n to give a n a l y t i c a l l y pure ( n 5 - C 5 H 5 ) W ( C O ) 3 C l 8 5 i n 5% y i e l d : IR (CC1,) v C Q 2059 (m), 1969 ( s ) , 1951 (m) cm - 1; l o w - r e s o l u t i o n mass spectrum (probe temperature 100°C), m/z_ 368 ( P + , most intense parent i o n ) . Anal. Calcd for C 8H 5C10 3 W : C, 26.08; H, 1.37. Found: C, 26.36; H, 1.45. The mother l i q u o r s t i l l c o n t a i n i n g some of the c h l o r i d e complex, was taken to dryness i n vacuo. Sublimation of the s o l i d residue (5 x 1 0 - 3 mm) onto a water-cooled probe at 60°C produced 0.015 g (4% y i e l d ) 100 of (T)b-C5H5)W(CO)3CH3 which was i d e n t i f i e d by i t s c h a r a c t e r i s t i c s p e c t r a l p r o p e r t i e s : IR (hexanes) v 2028 (m), 1933 ( s ) , 1901 (w) cm - 1; l o w - r e s o l u t i o n mass spectrum (probe temperature 120°C), m/z 348 ( P + , most intense parent i o n ) . Preparation of (T) 5-C 5H5)Cr(C0) 3CH 3. This compound was made by metathesis of N a [ ( n 5 - C 5 H 5 ) C r ( C O ) 3 ] 3 9 w i t h CH3I i n THF at -20°C, according 86 to the method of A l t . . The solvent was removed i n vacuo at ~5°C and the s o l i d residue was sublimed (5 * 10~3 mm) at ~40°C onto a water-cooled probe, y i e l d i n g (r| 5-C 5H 5)Cr(C0) 3CH3 as an a n a l y t i c a l l y pure s o l i d . A nal. Calcd f o r C 9H 8Cr0 3: C, 50.01; H, 3.73. Found: C, 49.78; H, 3.71. The spectroscopic p r o p e r t i e s of t h i s compound and t h e i r relevance to i t s c y c l i c voltammograms are discussed i n the f o l l o w i n g s e c t i o n . 101 III) Results and Discussion C y c l i c Voltammetry Studies. The compounds ( T ) 5 - C 5 H 5 ) M ( N 0 ) 2 R (M = Cr, R = CH3; M = Mo, W, R = CH 3, C 2H 5), ( T ) 5 - C 5 H 5 ) M ( C 0 ) 3 R (M = Cr, R = CH3; M = Mo, R = CH 3, C 2H 5; M = W, R = H, CH 3, C 2 H 5 ) 8 5 and ( T i 5-C 5H 5)Fe(CO) 2 C H 3 8 a l l exhibit i r r e v e r s i b l e f i r s t oxidation waves due to fast follow-up reac-tions upon electron removal, and/or very slow electron t r a n s f e r . The former, however, w i l l be seen to be the more l i k e l y . Oddly, ( T ) 5 - C 5 H 5 ) W ( N 0 ) 2 H does not exhibit a well defined oxidation wave i n CH 2C1 2/ 0.1 M [n-Bu 4N]PF 6 at platinum. The oxidation electrochemistry of (•ri 5-C 5H 5)Cr(NO) 2CH 3 appears to be d i f f e r e n t from that observed for i t s molybdenum and tungsten analogues. The f i r s t oxidation waves of most of the compounds i n this study appear to display d i f f u s i o n - l i m i t e d peak currents, i being l i n e a r with v 1 / 2 (v = scan rate i n V s - 1 ) . Only for p ,a ( T I 5 - C 5 H 5 ) M O ( C O ) 3 C H 3 i s a plot of i a vs. v 1 / 2 d i s t i n c t l y non-linear, and a plot of i vs. v l i n e a r . It i s , however, d i f f i c u l t to be conclusive p,a about these assessments with the small range of scan rates a v a i l a b l e . A l l of the a l k y l complexes and (r| 5-C 5H 5)W(C0) 3H also, display follow-up waves a f t e r the f i r s t oxidation wave i s passed. For ( T) 5-C 5H 5)Cr(NO) 2CH 3 and ( T i 5-C 5H 5)Cr(CO) 3CH 3 these follow-up processes may perhaps be simpler. The behaviour of the remainder of the compounds aft e r the f i r s t oxidation appears to be more complex. Table II summarizes the c y c l i c voltammetric data associated with the primary oxidation waves of the a l k y l compounds. The s p e c i f i c s of each c y c l i c voltammogram are presented i n more d e t a i l below. Most of the discussion centres on the q u a l i t a t i v e information 102 which can be from the f i r s t o x i d a t i o n waves and i t s relevance to re a c t i o n s of some of these compounds with oxidants and e l e c t r o p h i l e s . Table I I . Oxidation Peak P o t e n t i a l s and Associated Redox Data of Some Organometallic N i t r o s y l - and Carb o n y l - A l k y l Complexes. Compound ( T) 5-C 5H 5)Cr(NO) 2CH 3 (Ti 5-C 5H 5)Cr(NO) 2CH 3 c ( T I 5 - C 5 H 5 ) M O ( N O ) 2 C H 3 ( T I 5 - C 5 H 5 ) M O ( N 0 ) 2 C 2 H 5 ( T 1 5 - C 5 H 5 ) W ( N O ) 2 C H 3 ( T) 5-C 5H 5 ) W(NO) 2CH 3 c ( n 5 - c 5H 5 ) w(N0 ) 2 c 2H 5 (n 5-C 5H 5 )W(NO) 2H ( T) 5-C 5H 5)Fe(CO) 2CH 3 ( n 5-C 5H 5)Cr(CO) 3CH 3 Scan Rate 0.07 0.30 0.25 0.07 0.30 0.07 0.29 0.04 0.19 0.38 0.07 0.30 0.07 0.29 ( T ) 5 - C 5 H 5 ) M O ( C O ) 3 C H 3 ( T ) 5 - C 5 H 5 ) M O ( C O ) 3 C 2 H 5 E l E2 E p,a p,a P.c 1.26 1.50 0.44 1.33 1.55 0.40 1.28 -0.83 1.31 1.36 1.21 1.25 1.34 1.38 1.40 1.34 1.38 1.27 1.31 1.62 1.66 -1.6 -1.6 Comments E i s assoc i -ated with the f i r s t wave Broad Broad Second wave has lower i „ p,a Second wave not w e l l resolved Second wave not w e l l resolved no c l e a r o x i d a t i o n wave at l e s s than +1.8 V 0.18 1.10 0.07 0.22 0.86 0.88 1.31 0.37 The peak at E 1.35 0.37 = +0.37 V i s independent of scan rate and coupled with a peak at E = +0.43 V H ' pc 0.05 0.26 0.05 0.26 1.08 1.11 0.99 1.02 103 Table I I continued (n 5-C 5H 5)w(C0) 3CH 3 0.05 0.36 (Ty>-C 5H 5)W(C0) 3C 2H 5 0.04 0.25 (n 5-C 5H 5 ) W(C0) 3C 2H 5 c 0.08 0.23 ( T I 5 - C 5 H 5 ) W ( C O ) 3 H 0.04 1.23 0.29 1.26 (a) i n CH 2C1 2/0.1 M [n-Bu 4N]PF g unless otherwise noted, (b) V s - 1 . (c) i n CH3CN/0.1 M [n-B U l +N]PF g. Implications f o r E l e c t r o p h i l i c Cleavage Reactions of M-R Bonds i n (T) 5-C 5H 5 )M(N0 ) 2R (M = Cr, R - CH 3; M = Mo, W, R - CH 3, C 2 H 5 ) . I t i s c l e a r from Table I I that the n i t r o s y l - c o n t a i n i n g a l k y l complexes are more d i f f i c u l t to o x i d i z e than the carbonyl-containing a l k y l compounds (n 5-C 5H 5)M(CO) 3R (M = Cr, R = CH 3; M = Mo, W, R = CH 3, C 2H 5) and (r| 5-C 5H 5)Fe(CQ) 2CH 3.. The f i r s t E^ & values for the n i t r o s y l complexes are higher than f o r the carbonyl complexes. These observations are c o n s i s t e n t with the greater i t - a c i d i t y of N O . While a comparison of thermodynamic o x i d a t i o n p o t e n t i a l s cannot be made, since E 1 / 2 values are not a v a i l a b l e , k i n e t i c a l l y the n i t r o s y l complexes are somewhat more d i f f i c u l t to o x i d i z e than t h e i r corresponding carbonyl analogues. I t i s relevant to note a l s o that the d i f f e r e n c e i n f i r s t i o n i z a t i o n p o t e n t i a l s of (n 5-C 5H 5)Cr(NO) 2CH 3 and (n 5-C 5H 5)Fe(CO) 2CH 3 7 7 (~21 kJ/mole) corresponds f a i r l y w e l l to the observed d i f f e r e n c e i n E values of ~200 mV (~19 kJ/mole). 1.06 1.57 Second wave has 1.11 1.64 lower i a p,a 1.08 Very complex 1.10 follow-up reduc-t i o n behaviour 0.94 —0.42 The wave at 0.96 E p = -0.42 i s followed by a wave at E = +0.12 V 104 E l e c t r o p h i l i c cleavage r e a c t i o n s of ( T) 5-C 5H 5)Fe(CO)(L ) R (L = C O , t e r t i a r y phosphines; R = a l k y l groups) with halogens, mercuric h a l i d e s , and c u p r i c h a l i d e s are b e l i e v e d to proceed v i a o x i d a t i o n of the organometallic complexes to f o r m a l l y Fe**^ or F e ^ i n t e r m e d i a t e s . 8 7 L i k e w i s e , the r e a c t i o n s of ( T I 5 - C 5 H 5 ) R U ( C O ) 2 _ X L x r (x = 0 - 2; L = PPh 3; R = CH 3, CH 2Ph) wi t h halogens, HCl, mercuric h a l i d e s and c u p r i c h a l i d e s are b e l i e v e d to 88 f o l l o w the same o x i d a t i v e mechanism. This i n v o l v e s a t t a c k of the e l e c t r o p h i l e at the metal c e n t r e , or more s p e c i f i c a l l y , at the HOMO of the complex, which most l i k e l y i n v o l v e s non-bonding d - e l e c t r o n s . 7 7 A study of the k i n e t i c s of the r e a c t i o n s of ( T i 5-C 5H 5)Fe(CO) 2 R ( R = various a l k y l , a r y l 89 groups), with mercuric h a l i d e s by W o j c i c k i and D i z i k e s i s c o n s i s t e n t w i t h the o x i d a t i v e mechanism. However, these workers a l s o found that ( T i 5-C 5H 5)Cr(NO) 2CH 3 reacts much more r a p i d l y w i t h mercuric h a l i d e s ( r e a c t i o n 4.1) than does the i s o e l e c t r o n i c compound (r| 5-C 5H 5)Fe(CO) 2CH 3 ( r e a c t i o n 4.2). This d i f f e r e n c e has been a t t r i b u t e d to a lower formal ( T) 5-C 5H 5)Cr(NO) 2CH 3 + H g C l 2 > (n 5-C 5H 5)Cr(NO) 2C1 + CH 3HgCl (4.1) (n 5-C 5H 5)Fe(CO) 2CH 3 + H g C l 2 > (n 5-C 5H 5)Fe(CO) 2C1 + CH 3HgCl (4.2) o x i d a t i o n s t a t e of chromium ( I f NO i s considered to be N0 +, the o x i d a t i o n s t a t e of the metal i n ( i i 5-C 5H 5)Cr(NO) 2CH 3 i s zero, whereas i n (n 5-C 5H 5)Fe(CO) 2CH 3 i t i s +2), enhancing the r e a c t i v i t y toward mercuric h a l i d e s , due to e a s i e r o x i d a t i o n . C l e a r l y , however, t h i s cannot be the case, since the n i t r o s y l complexes are more d i f f i c u l t to o x i d i z e . (This 105 also i l l u s t r a t e s the dangers of invoking r a t i o n a l e s based on formal o x i d a -t i o n s t a t e s . ) I t seems odd at f i r s t that ( T) 5-C 5H 5)Cr (NO) 2CH 3 i s more r e a c t i v e towards mercuric h a l i d e s than i s the i s o e l e c t r o n i c i r o n analogue when one considers the greater n - a c i d i t y of NO, and the higher o x i d a t i o n p o t e n t i a l of the n i t r o s y l complex. This apparent anomaly suggests the p o s s i b i l i t y of two d i f f e r e n t mechanisms for these r e a c t i o n s of (r) 5-C 5H 5)Cr(N0) 2CH 3 and ( T i 5-C 5H 5)Fe(CO) 2CH 3. Hubbard, 7 7 i n a comparative study of these methyl complexes using u l t r a v i o l e t photoelectron spectroscopy, has concluded that the C r - d T t e l e c t r o n density i s more s u b s t a n t i a l l y s t a b i l i z e d by n-donation to the n i t r o s y l ligands than the corresponding F e - d i t e l e c t r o n s are by back-bonding to CO. The s t a b i l i z a t i o n of the metal e l e c t r o n density i n the n i t r o s y l complex i s accompanied by a d e s t a b i l i z a t i o n of the a-electron density i n the Cr-CH 3 bond, again r e l a t i v e to the s i t u a t i o n i n (n 5 - C 5 H 5 ) F e ( C 0 ) 2 C H 3 . (The metal-carbon cJ-bond i s 140 kJ/mole below the HOMO of the i r o n complex, while f o r the chromium complex the corresponding d i f f e r e n c e i s only 94 kJ/mole.) For (n 5-C 6H 5)Cr(NO) 2CH 3 the Cr-CH 3 a-electron density i s s a i d to remain l a r g e l y In the v i c i n i t y of the metal-These combined observations and conclusions suggest an a l t e r n a t e , perhaps c h a r g e - c o n t r o l l e d , mechanism f o r the e l e c t r o p h i l i c cleavage r e a c t i o n s of (n 5-C 5H 5)Cr(NO) 2CH 3 w i t h mercuric h a l i d e s . This would i n v o l v e d i r e c t a t t a c k of the e l e c t r o p h i l e at the Cr-CH 3 bond, analogous to an S^2 process observed f o r some re a c t i o n s of m e t a l - a l k y l complexes with 7 8a e l e c t r o p h i l e s . These S 2-type pathways are l e s s common than the oxi d a -106 t i v e cleavages. 78a Scheme I I depicts the proposed pathways f o r the r e a c t i o n of ( n b - C 5 H 5 ) C r ( N O ) 2 C H 3 with H g C l 2 . This type of mechanism might be expected, not unreasonably, to extend to the (r| 5-C 5H 5)M(NO) 2R (M = Mo,W) congeners a l s o . Indeed, ( T I 5 - C 5 H 5 ) M O ( N O ) 2 C H 3 reacts r e a d i l y with HgCl 2, 8° i . e . ( T I 5 - C 5 H 5 ) M O ( N O ) 2 C H 3 + HgCl 2 > ( T I 5 - C 5 H 5 ) M O ( N O ) 2 C 1 + CH 3HgCl (4.3) While the analogous r e a c t i o n w i t h ( T | 5 - C 5 H 5 ) W ( N O ) 2CH 3 has not been reported, on the basis of the e l e c t r o c h e m i c a l r e s u l t s obtained i n t h i s work, i t i s a n t i c i p a t e d to react s i m i l a r l y . In contrast to r e a c t i o n 4.3, the compounds (T) 5-C 5H 5)M(CO) 3CH 3 (M = Mo, W) are unreactive toward H g C l 2 . This would appear to be an e l e c t r o n i c , rather than a s t e r i c e f f e c t since ( T ) 5 - C 5 H 5 ) M O ( C O ) 2 ( L ) C H 3 [ L = PPh 3, P (n-Bu) 3] does react w i t h mercuric U TA 8 2 4 h a l i d e s , i . e . Again, on the basis of the e l e c t r o c h e m i c a l s t u d i e s , ( T i b-C 5H 5)Mo(N0) 2CH 3 would be expected to be unreactive toward H g C l 2 i f only an o x i d a t i v e mechanism were to be operative since ( T ) 5 - C 5 H 5 ) M O ( C O ) 3CH 3 i s ea s i e r to o x i d i z e than ( T ) 5 - C 5 H 5 ) M O ( N O ) 2CH 3 . Further support f o r a d i f f e r e n t mechanism f u n c t i o n i n g i n the rea c t i o n s of these n i t r o s y l - a l k y l complexes with e l e c t r o p h i l e s has been 80 provided by t h e i r r e a c t i v i t y toward [ P h 3 C ] + , i . e . ( T I 5 - C 5 H 5 ) M O ( C O ) 2 ( L ) C H 3 + HgX 2 » ( T ) 5 - C 5 H 5 ) M O ( C 0 ) 2 ( L ) X + CH3HgX (4.4) 107 S c h e m e II Cr 0 N ^ / X C H 3 HgCI 2 N O HgCI 2 Cr HgCI 2 O Cr N O HgCl Cr ° N O + CH 3HgCI 108 ( T I 5 - C 5 H 5 ) M ( N O ) 2 C H 3 + [ P h 3 C ] P F 6 > ( n 5 - C 5 H 5 ) M ( N O ) 2 P F 6 + P h 3 C C H 3 (M = Cr, Mo) (4.5) However, triphenylcarbenium s a l t s are known to react with r e l a t e d c a r b o n y l -c o n t a i n i n g a l k y l complexes (which lack 8-hydrogen atoms) to form carbene . 81 complexes, e.g. (n 5-C 5H 5)Fe(CO)' [Ph 3C]+ (T) 5-C 5H 5)Fe(C0) sag + 90 + Ph 3CH (4.6) [ P h 3 C ] + s 4.91 ( n 5-C 5H 5)W(CO) 2(PPh 3)CH 3 > [ ( n 5-C 5H 5)W(CO) 2(PPh 3)CH 2] + y i (4.7) These r e a c t i o n s have been proposed to occur v i a i n i t i a l e l e c t r o n 92 t r a n s f e r , a process which, again, i s not as favourable f o r the n i t r o s y l - c o n t a i n i n g methyl complexes, and not co n s i s t e n t w i t h the observed 80 M-CH3 cleavage. F i n a l l y , the r e a c t i o n s , A1C1 3 (n 5-C 5H 5)M(NO) 2CH 3 > (n 5-C 5H 5)M(NO) 2C1 (M = Cr, Mo, W) (4.8) also are best explained by d i r e c t a t t a c k of A1C1 3 at the M-CH3 bond. (a) (ii 5-C 5H 5)Cr(NO) 2CH 3. Figure 17 shows c y c l i c voltammograms of (n 5-C 5H 5)Cr(NO) 2CH 3 i n CH 2C1 2 i n which the scan i s i n i t i a t e d toward p o s i t i v e p o t e n t i a l s . This complex behaves somewhat d i f f e r e n t l y toward o x i d a t i o n than do i t s molybdenum and tungsten analogues. The f i r s t E 109 value of +1.26 V at 0.07 Vs 1 occurs at s l i g h t l y lower p o t e n t i a l s so that E increases i n the order Cr < Mo < W, though not s u b s t a n t i a l l y . For the p ,a chromium-containing methyl compound, however, the f i r s t and second o x i d a -t i o n waves are moderately w e l l separated. The f i r s t wave appears to have a d i f f u s i o n - l i m i t e d peak c u r r e n t , i being l i n e a r with v 1 / 2 . Since a p,a s a t i s f a c t o r y current b a s e l i n e f o r the second wave at E = +1.50 V at p,a 0.07 V s - 1 could not be e s t a b l i s h e d , the i dependence on scan ra t e could p,a not be analyzed. This second o x i d a t i o n would appear to a r i s e from a product formed i n the f i r s t e l e c t r o n removal step. I f the scan i s reversed past the second peak (Figure 17a), s e v e r a l cathodic values with very small peak currents appear between +0.5 and -0.7 V. I f , however, the scan i s reversed j u s t past the f i r s t peak as shown i n Figure 17b, the r e t u r n p o r t i o n of the scan becomes much cleaner and a new cathodic wave appears at E = +0.44 V at a scan rate of 0.07 V s - 1 . The p o s i t i o n of the f i r s t p,c o x i d a t i o n peak i s somewhat v a r i a b l e and h i g h l y dependent on scan r a t e and temperature. Thus, as the temperature i s v a r i e d between ~40°C and 30°C (A s i l v e r - w i r e quasi-reference electrode was used and the c e l l was Immersed i n a c o o l i n g / h e a t i n g bath), E for the f i r s t o x i d a t i o n peak s h i f t s by p,a +180 mV, the same scan rate being employed throughout. This probably r e f l e c t s a slower rate of e l e c t r o n t r a n s f e r . ^ 8 ^ , c At a c o o l i n g bath temperature of ~ -40°C the f i r s t and second waves appear to coalesce. This argues against a s h i f t i n E being caused only by increased r e s i s t a n c e . p,a No doubt the r e s i s t a n c e of the [n-Bu L tN]PF 6/CH 2Cl2 e l e c t r o l y t e increases w i t h decreasing temperature and c o n t r i b u t e s to the s h i f t of E but i f p ,a r e s i s t a n c e e f f e c t s were the only cause of t h i s , E f o r the second peak p,a 110 Volts vs S C E Figure 17. C y c l i c voltammograms of (ri 5-C 5H 5)Cr(NO) 2CH 3 i n CH 2C1 2 ( o x i d a t i o n ) (a) between 0 and +2.0 V at 0.12 V s - 1 and (b) between 0 and +1.4 V at a scan rate of 0.17 V s - 1 . I l l s h o u l d s h i f t s i m i l a r l y . A l s o , as the te m p e r a t u r e d e c r e a s e s from ~40°C t o ~ -30°C the r e l a t i v e peak c u r r e n t f o r the c a t h o d i c wave i n c r e a s e s s u b s t a n -t i a l l y . I f the c u r r e n t base l i n e f o r the c a t h o d i c wave i s e x t r a p o l a t e d from the f l a t p o r t i o n i n the r e t u r n s c a n , then i / i i s e s t i m a t e d t o be p,c p,a ~0.05 a t ~40°C and ~0.2 a t 30°C. At ambient t e m p e r a t u r e s the a n o d i c and c a t h o d i c waves shown i n F i g u r e 17b s p r e a d a p a r t as s c a n r a t e i s i n c r e a s e d , from a s e p a r a t i o n of 0.82 V a t 0.06 V s - 1 t o 0.93 V a t 0.41 V s - 1 . The s e p a r a t i o n i n c a t h o d i c and a n o d i c peak p o t e n t i a l s i s known t o be a l i n e a r f u n c t i o n of v - 1 ^ 2 f o r a q u a s i - r e v e r s i b l e p r o c e s s w i t h l a r g e d i f f e r e n c e s 93 between E and E . A p l o t o f AE v s . v - 1 / 2 i s l i n e a r o v er the p,a p,c v p a c c e s s i b l e s c a n r a t e range of 0.06 to 0.41 V s - 1 . I t w o u l d , however, be unwise to c o n c l u d e t h a t t h i s c o n s t i t u t e s c o n c l u s i v e e v i d e n c e f o r a q u a s i -r e v e r s i b l e p r o c e s s , c o n s i d e r i n g the l i m i t e d d a t a a v a i l a b l e . I t i s e n t i r e l y p o s s i b l e t h a t t h e s e a r e c o u p l e d , c h e m i c a l l y i r r e v e r s i b l e p r o c e s s e s , g i v e n what i s known a t t h i s j u n c t u r e . I n any e v e n t , i t appears t h a t l i t t l e , i f any, [ ( r i 5 - C 5 H 5 ) C r ( N O ) 2 ] + i s i n i t i a l l y formed i n the f i r s t o x i d a t i o n s i n c e o n l y a v e r y weak r i s e around -0.21 V i s ob s e r v e d i n the r e t u r n s c a n , i f i t i s extended p a s t 0 V (compare w i t h F i g u r e 7 ) . A CV of ( n 5 - C 5 H 5 ) C r ( N O ) 2 C H 3 i n CH3CN ( F i g u r e 18) d i s p l a y s an o x i d a -t i o n at E = +1.28 V and a c o u p l e d c a t h o d i c wave a t E = -0.83 V a t a p,a r p,a scan r a t e o f 0.25 V s - 1 . The s e p a r a t i o n i n peak p o t e n t i a l s i s f a r g r e a t e r i n CH 3CN (2.11 V) t h a n i n C H 2 C 1 2 (0.90 V) a t s i m i l a r s c a n r a t e s . The b e t t e r donor CH3CN p r o b a b l y c o o r d i n a t e s to the o x i d i z e d form, s t a b i l i z i n g i t c o n s i d e r a b l y w i t h r e s p e c t t o r e d u c t i o n . C o m p e l l i n g e v i d e n c e f o r s o l v a -112 _ J I I + 1 0 -1 Volts vs SCE Figure 18. C y c l i c voltammogram (oxidation) of ( T I 5 -C 5H 5)Cr(NO) 2CH 3 i n CH,CN at 0 . 2 5 V s - 1 . (The sharp r i s e past - 0 . 8 3 V i s due to reduction of ( T i b-C 5H 5)Cr(NO) 2CH 3; for d e t a i l s see Chapter 5 . ) 113 t l o n upon e l e c t r o n removal has been reported f o r the e l e c t r o c h e m i c a l o x i d a -c 94 t i o n of (Ti 6-C 5Me 6)W(CO) 3 i n CH3CN and DMF, i . e . -e~ + S (n 6-C 6Me 6)W(CO) 3 ^, " [(T) 6-C 6Me 6)W(CO) 3S]« +e" - S (S = CH3CN, DMF) (4.9) A CV of the i s o e l e c t r o n i c (n ; >-C 5H 5)Fe(CO) 2CH 3 i n CH 2C1 2 at room temperature d i s p l a y s a chem i c a l l y i r r e v e r s i b l e o x i d a t i o n at E = +1.10 V p,a at 0.18 V s - 1 . This i s somewhat lower than the f i r s t o x i d a t i o n peak of (Ti 5-C 5H 5)Cr(NO) 2CH 3 by ~200 mV at a s i m i l a r scan r a t e . The c y c l i c voltam-mogram of (n 5-C 5H 5)Fe(CO) 2CH 3 i n other solvents has been p r e v i o u s l y reported and r e s u l t s i n the formation of an a c e t y l - c o n t a i n i n g r a d i c a l 95 c a t i o n upon o x i d a t i o n . Put more simply, (Tr-C 5H 5)Fe(CO) 2CH 3 undergoes o x i d a t i v e l y induced i n s e r t i o n of CO i n t o the Fe-CH 3 bond. In acetone, a f t e r the o x i d a t i o n wave i s passed a cathodic wave occurs i n the r e t u r n scan due to the red u c t i o n of a solvated [(n : >-C 5H 5)Fe(CO)(C(0)CH 3)S n] • (S = solvent) c a t i o n . In the presence of CH3CN, the cathodic wave s h i f t s toward more negative p o t e n t i a l s i n accord with the bett e r donor a b i l i t y of CH3CN compared with acetone. This i n t e r p r e t a t i o n has been sub s t a n t i a t e d by chemical o x i d a t i o n of ( r ) 5 - C 5 H 5 ) F e ( C 0 ) 2 C H 3 i n CH3CN u s i n g , f o r example, C e ^ s a l t s . 9 ^ This produces a r a d i c a l c a t i o n which d i s p l a y s \ IR stretches f o r one te r m i n a l CO l i g a n d and a coordinated a c e t y l l i g a n d . This o x i d a t i v e l y induced i n s e r t i o n i s very r a p i d , even at -78°C and r e s u l t s i n the i n c o r p o r a t i o n of CH3CN i n t o the metal's c o o r d i n a t i o n sphere. I t i s 114 tempting to suggest that an analogous process a p p l i e s f o r the o x i d a t i o n of (T| 5-C 5H 5)Cr(NO) 2CH 3. At t h i s point though, such a p r o p o s i t i o n i s specu-l a t i v e . Reaction of (n 5-C 5H 5)Cr(NO) 2CH 3 with [ F e ( P h e n ) 3 ] ( P F 6 ) 3 . In order to explore the o x i d a t i o n of (ri 5-C 5H 5)Cr(NO) 2CH 3 f u r t h e r i t s r e a c t i o n s w i t h some chemical oxidants have been i n v e s t i g a t e d . The deep blue compound 83 [ F e ( P h e n ) 3 ] ( P F 6 ) 3 i s a potent, one-electron oxidant and when i t i s mixed with an equimolar q u a n t i t y of (n 5-C 5H 5)Cr(NO) 2CH 3 i n CH 2C1 2 a t , or near, -78°C the mixture becomes intense red or red-brown i n c o l o u r . I f the r e a c t i o n i s c a r e f u l l y maintained at -78°C i t requires ~30 min to proceed to completion as evidenced by IR monitoring of i t s progress. At s l i g h t l y warmer temperatures i t i s r a p i d . The slower r e a c t i o n at -78°C may be r e f l e c t i v e of a higher E of the n i t r o s y l complex at lower temperature p,a and/or decreased s o l u b i l i t y of the oxidant. The reddish c o l o u r a t i o n i s probably due to formation of [ F e ( P h e n ) 3 ] ( P F 6 ) 2 which i s i n t e n s e l y blood-red. In e i t h e r case, an IR spectrum of the f i n a l r e a c t i o n mixture recorded at room temperature shows V^Q absorptions at 1845 and 1743 cm - 1, suggestive of the { ( n 5 - C 5 H 5 ) C r ( N O ) 2 } + moiety (see Chapter 3 ) . Since the IR spectrum i s recorded at room temperature i t i s not n e c e s s a r i l y i n d i c a t i v e of what i s present i n s o l u t i o n at -78°C. I f the r e a c t i o n i s c a r r i e d out at room temp-erature a s i m i l a r IR spectrum of the r e s u l t a n t mixture i s obtained with v 's at 1846 and 1745 cm - 1. This co n t r a s t s w i t h the el e c t r o c h e m i c a l NO o x i d a t i o n of (ri 5-C 5H 5)Cr(NO) 2CH 3 i n which very l i t t l e i f any [ ( n 5 - C 5 H 5 ) C r ( N O ) 2 ] + i s formed, at l e a s t i n i t i a l l y . This could be a f u n c t i o n of the d i f f e r i n g time scales of these experiments and/or superior 115 conditions of the e l e c t r o c h e m i c a l system, more amenable to preserving unstable species. When the chemical o x i d a t i o n i s c a r r i e d out at room temp erature or the cold o x i d i z e d mixture i s warmed to room temperature, one of the processes occuring appears to be expulsion of the methyl group, i . e . [ F e ( P h e n ) 3 ] ( P F 6 ) 3 (n 5-C 5H 5)Cr(NO) 2CH 3 : > [(ri 5-C 5H 5)Cr(NO) 2 ] + + CH3» (4.10) At low temperatures the o x i d a t i o n of (•n 5-C 5H 5)Cr(NO) 2CH 3 i s at l e a s t p a r t i a l l y c hemically r e v e r s i b l e as evidenced by the reformation of the s t a r t i n g methyl compound upon a d d i t i o n of cobaltocene to the o x i d i z e d mixture. That the s t a r t i n g m a t e r i a l i s recovered i n only 30% y i e l d i n t h i r e a c t i o n i n d i c a t e s that the o x i d a t i o n r e a c t i o n i s accompanied by attendant decomposition r e a c t i o n s . The o x i d a t i o n conducted at room temperature cannot be reversed by adding cobaltocene. No (Ti 5-C 5H 5)Cr(NO) 2CH 3 r e s u l t s as i n d i c a t e d by an IR spectrum of the r e s u l t a n t supernatant s o l u t i o n , the bands f o r (n 5-C 5H 5)Cr(NO) 2CH 3 (1777, 1669 cm - 1) being absent. Thus the intermediate generated at low temperature i s thermally unstable under thes c o n d i t i o n s . In an attempt to trap t h i s intermediate the o x i d a t i o n r e a c t i o n has been conducted at —78°C i n the presence of an equimolar amount of P(0Ph) 3 (which was chosen because i t reacts only very slowly with [ F e ( P h e n ) 3 ] ( P F 6 ) 3 ) . The idea behind t h i s r e a c t i o n i s that i f o x i d a t i v e l y induced NO i n s e r t i o n occurs then P(0Ph) 3 could be used to s t a b i l i z e the product, i . e . 116 ( n 5-C 5H 5)Cr(NO) 2CH 3 + P(OPh) 3 [ (n 5-C 5H 5)Cr(NO)(CH 3NO)P(OPh) 3]• (4-11) which could then be reduced with cobaltocene to a n e u t r a l product, i . e . [(n 5-C 5H 5)Cr(NO)(CH 3NO)P(OPh) 3 ] t ±£-> (n 5-C 5H 5)Cr(NO)(CH 3NO)P(OPh) 3 (4.12) However, a d d i t i o n of cobaltocene to the o x i d i z e d mixture again regenerates ( -n 5-C 5H 5)Cr(NO) 2CH 3, i n 30% y i e l d . I f P(0Ph) 3 i s incorporated i n t o the metal's c o o r d i n a t i o n sphere during o x i d a t i o n , subsequent reduction causes i t s e x p u l s i o n . F i n a l l y , when the o x i d a t i o n r e a c t i o n i s c a r r i e d out at -78°C and E t 3 N i s added, again (t| 5-C 5H 5)Cr(N0) 2CH 3 i s regenerated; t h i s time i n about 60% y i e l d . The increased recovery of the s t a r t i n g m a t e r i a l can be a t t r i -buted, at l e a s t i n p a r t , to b e t t e r c o n t r o l of the temperature than i n the p r e v i o u s l y described r e a c t i o n s . E v i d e n t l y , rather than c o o r d i n a t i n g to the ox i d i z e d s p e c i e s , E t 3 N simply reduces i t . Triethylamine undergoes chemi-c a l l y i r r e v e r s i b l e o x i d a t i o n i n a p r o t i c s o l v e n t s . In CH3CN at a platinum 97 electrode i t o x i d i z e s at E = +0.66 V vs. Ag/AgNO.,. What the o x i d a t i o n p,a J p o t e n t i a l i s i n CH 2C1 2 under the c o n d i t i o n s employed i n t h i s study, i s not p r e c i s e l y known. However, a crude c y c l i c voltammogram of E t 3 N i n CH 2C1 2 with 0.1 M [n-Bu HN]PF 6 as the supporting e l e c t r o l y t e at platinum i n a i r shows an i r r e v e r s i b l e wave at E = +1.0 V. I t would appear to be l i k e l y , p,a t h e r e f o r e , that E t 3 N o x i d i z e s at a more p o s i t i v e p o t e n t i a l than the cathodic wave (E = +0.44 V) seen i n the r e t u r n scan of a CV of 117 (n 5-C 5H 5)Cr(NO) 2CH 3 in CH 2C1 2 (Figure 17b). In other words, the species reduced by reaction with E t 3 N would not seem to be the same one which i s reduced at E = + 0.44 V, generated from the f i r s t oxidation wave of ( T) 5-C 5H 5)Cr(NO) 2CH 3 i n CH 2C1 2. Perhaps the simplest, though not the only i n t e r p r e t a t i o n of these r e s u l t s i s the i n i t i a l formation of a r a d i c a l cation upon oxidation, i . e . [Fe ( P h e n ) 3 ] ( P F 6 ) 3 + (n 5-C 5H 5)Cr(NO) 2CH 3 > [(n 5-C 5H 5)Cr(NO) 2CH 3]• (4.13) CH 2C1 2, -78°C which has short term s t a b i l i t y at -78°C i n CH 2C1 2. However, p o s s i b i l i t i e s such as coupling of two r a d i c a l cations to form a b i m e t a l l i c d i c a t i o n , o x i d a t i v e l y induced i n s e r t i o n of NO into the Cr-CH 3 bond and others cannot be discounted at t h i s point. The fate of the r a d i c a l cation at room temperature could involve a v a r i e t y of reaction pathways. Reaction of (ti 5-C 5H 5)Cr(NO) 2CH 3 w i t h AgBF,. The reaction of the methyl compound with AgBF, i s very complex. Ultimately the {(t| 5-C 5H 5)Cr(NO) 2 } + moiety i s produced in some form as indicated by an IR spectrum of the f i n a l reaction mixture ( v N Q 1844, 1740 cm - 1), i n s l i g h t l y under 50% y i e l d . This was v e r i f i e d by d e r i v a t i z i n g the product cation with [(Ph 3P) 2N]Cl to form (n 5-C 5H 5)Cr(NO) 2Cl. While the chloride complex could be i s o l a t e d , the y i e l d was s u b s t a n t i a l l y lowered by losses incurred during chromatography on F l o r i s i l . Analyzing the concentration of ( T) 5-C 5H 5)Cr(NO) 2C1 formed using a Beer's Law c a l i b r a t i o n plot obtained from the absorbances of the n i t r o s y l stretching bands of ( T i 5-C 5H 5)Cr(NO) 2C1 118 gives a more accurate i n d i c a t i o n of the extent of { ( T) 5-C 5H 5)Cr(NO) 2 } + form-a t i o n , assuming q u a n t i t a t i v e conversion of the c a t i o n i n t o the n e u t r a l c h l o r i d e complex and that the c a t i o n i s the o n l y , or the major species that produces the chloro complex. Monitoring of the progress of the r e a c t i o n by IR spectroscopy reveals a band at ~1756 cm - 1 which p e r s i s t s u n t i l the s t a r t i n g m a t e r i a l i s consumed, and then decays. I t appears to be an i n t e r -mediate i n the formation of the c a t i o n since i t s i n t e n s i t y decreases with concomitant i n t e n s i f i c a t i o n of the bands around 1844 and 1740 cm - 1. Several p o s s i b l i t i e s f o r t h i s intermediate can be envisaged, such as an adduct formed between A g + and (n 5-C 5H 5)Cr(NO) 2CH 3. The A g + c a t i o n could coordinate to a v a r i e t y of s i t e s , i n c l u d i n g the C 5H 5 r i n g , the metal 98 centre, an oxygen atom of an NO l i g a n d , or perhaps the Cr-CH 3 bond (see above). Considering the o x i d a t i o n r e a c t i o n with [ F e ( P h e n ) 3 ] ( P F g ) 3 described above, the r e l a t i v e l y long l i f e t i m e of the intermediate and a somewhat low frequency (1756 c m - 1 ) , i t seems u n l i k e l y that o x i d a t i o n to form [ ( n 5 - C 5 H 5 ) C r ( N O ) 2 C H 3 ] ^ and Ag° i s o c c u r r i n g i n the r e a c t i o n w i t h AgBF^. The p r e c i s e nature of the intermediate i s not evident at t h i s p o i n t , though a species such as [(n 5-C 5H 5)Cr(NO)(CH 3NO)]^ would not be i n c o n s i s t e n t with the s i n g l e t e r m i n a l n i t r o s y l ( a l b e i t low frequency) a b s o r p t i o n . The formation of an intermediate such as [{(ri 5-C 5H 5)Cr(NO) 2 } 2 C H 3 ] + i s discounted by the f a c t that (n 5-C 5H 5)Cr(NO) 2BF 1 + and ( n 5 - C 5 H 5 ) C r ( N 0 ) 2 C H 3 do not appear to i n t e r a c t . Monitoring of the r e a c t i o n between (n 5-C 5H 5)Cr(NO) 2CH 3 and AgBF^ by NMR spectroscopy i n CD 2C1 2 r e v e a l s a p l e t h o r a of resonances, i n c l u d i n g s e v e r a l i n the c y c l o p e n t a d i e n y l r e g i o n . I t appears that s e v e r a l products 119 are formed i n t h i s r e a c t i o n , a major product being the { ( T ) 5 - C 5 H 5 ) C r ( N O ) 2 } + 79 group. I t seems t h e r e f o r e , that simple net o x i d a t i v e cleavage of the Cr-CH 3 bond by A g + i s not the only mode of r e a c t i v i t y . On a s y n t h e t i c s c a l e , the f i n a l r e a c t i o n mixture always contains small amounts of an i n s o l u b l e blue m a t e r i a l , the i d e n t i t y of which has not been a s c e r t a i n e d . I t may a r i s e from one or more of the products formed i n i t i a l l y , i n which connection i t i s noted that s l i g h t l y more than one equivalent of AgBF, i s required to completely consume (T) 5-C 5H 5)Cr(NO) 2CH 3. Reaction of ( T i 5 - C 5 H 5 ) C r ( N O ) 2 C H 3 w i t h NOPFg. This r e a c t i o n i n CH 2C1 2 proceeds slowly over a period of ~3.5 h and a dark green, micro-c r y s t a l l i n e s o l i d forms, which has been i d e n t i f i e d as [ ( T i 5-C 5H 5)Cr(NO) 2(CH 2NOH)]PF 6, i . e . CH2CI2 ( T i 5-C 5H 5)Cr(NO) 2CH 3 + NOPF6 > [(n 5-C 5H 5)Cr(NO) 2(CH 2NOH) ]PF 6 (4.14) The f i n a l green r e a c t i o n mixture contains s e v e r a l s p e c i e s . The r e s u l t a n t s o l u t i o n ' s IR spectrum d i s p l a y s n i t r o s y l absorptions for c a t i o n i c species at 1844 and 1744 cm - 1, s u b s t a n t i a l l y diminished i n i n t e n s i t y compared to the s t a r t i n g m a t e r i a l bands at the outset of the r e a c t i o n . These bands are at s l i g h t l y lower frequency than those of the i s o l a t e d product (1847, 1746 cm - 1) and could be due to a mixture of species. In a d d i t i o n to the V ^ Q ' S of the s t a r t i n g methyl complex, t h i s IR spectrum contains two weak bands at 1816 and 1710 cm" 1, reminiscent of (n 5-C 5H 5)Cr(NO) 2C1 (1817, i 39 1711 cm" 1). I f t h i s compound i s present, i t could conceivably a r i s e from 120 a side r e a c t i o n of a r a d i c a l - c a t i o n with CH 2C1 2. ( I t i s noted at t h i s point that the r e a c t i o n of (n 5-C 5H 5)Cr(NO) 2CH 3 and AgPFg i n CH 2C1 2 proceeds very slowly and does not produce [ ( T i 5-C 5H 5)Cr(NO) 2(CH 2NOH)]PF 6 to any appreciable extent.) The new, c a t i o n i c n i t r o s y l complex i s i s o l a b l e i n moderate y i e l d s as a green, m i c r o - c r y s t a l l i n e , diamagnetic s o l i d and appears to be st a b l e i n a i r f o r s e v e r a l days, not e x h i b i t i n g any apparent decomposition during t h i s time. I t i s sol u b l e i n good s o l v a t i n g solvents l i k e nitromethane but only s l i g h t l y s o l u b l e i n CH 2C1 2. The molecular s t r u c t u r e of [(n 5-C 5H 5)Cr(NO) 2(CH 2N0H)]PF 6, d e t e r -99 mined by an X-ray c r y s t a l l o g r a p h i c a n a l y s i s , i s shown i n Figure 19. The geometry about the c e n t r a l chromium atom i s that of a normal "three-legged piano s t o o l " . The arrangement of the { ( T) 5-C 5H 5)Cr(NO) 2 } + fragment i s almost i d e n t i c a l to that found i n ( T) 5-C 5H 5)Cr(NO) 2C1. 6 5 The Cr-N(2) and Cr-N(3) distances [1.702(6) and 1.709(5) A, r e s p e c t i v e l y ] and the Cr-N distances of ( T i 5-C 5H 5)Cr(NO) 2C1 are the same w i t h i n experimental e r r o r . Likewise the N(2)-0(2) and N(3)-0(3) distances [1.163(6) and 1.152(6) A, r e s p e c t i v e l y ] are the same as those f o r ( T) 5-C 5H 5)Cr(NO) 2C1. The N(2)-Cr-N(3) angle of 93.5(3)° and the N-Cr-N angle found f o r the c h l o r i d e complex are a l s o i d e n t i c a l w i t h i n experimental e r r o r . The Cr-N(2)-0(2) and Cr-N(3)-0(3) angles [174.5(6)° and 172.2(5)°, r e s p e c t i v e l y ] are somewhat c l o s e r to p e r f e c t l i n e a r i t y than those observed f o r ( T i 5 - C 5 r l 5 ) C r ( N 0 ) 2C1 [170.0(3)° and 168.8(3)°]. The {CrN(CH 2)0} moiety i s e s s e n t i a l l y planar, c o n s i s t e n t w i t h the C ( l ) and N ( l ) atoms being s p 2 h y d r i d i z e d , i n valence bond terms. The angles about C ( l ) [ H ( 2 ) - C ( l ) - H ( l ) = 118.0(77)°, H ( l ) - C ( l ) - N ( l ) = 124.6(53)° and N ( l ) - C ( l ) - H ( 2 ) = 116.8(53)°] are not 121 H 1 3 H t Figure 19. Molecular structure of [ ( T i 5-C 5H 5)Cr(NO) 2(CH 2NOH)]PF 6 . y Selected bond distances (A) and angles (deg) are C ( l ) - N ( l ) = 1.253(9), N(l)-0(1) = 1.392(7), Cr-N(l) = 2.034(5), Cr-N(2) = 1.702(6), Cr-N(3) = 1.709(5), N(2)-0(2) = 1.163(6), N(3)-0(3) = 1.152(6), Cr-C 5H 5 (centroid) = 1.843, H(10)-F(2) - 2.138, Cr-N(l)-C(l) = 128.9(5), C ( l ) - N ( l ) - 0 ( 1 ) = 111.5(6), 0(1)-N(l)-Cr = 119.6(4), N(2)-Cr-N(3) = 93.5(3), 0(2)-N(2)-Cr = 174.5(6), 0(3)-N(3)-Cr - 172.2(5). 122 i n c o n s i s t e n t with an spz h y d r i d i z e d carbon atoms, C ( l ) , although the determination of the angles i s rather imprecise. While t h i s would appear to c o n s t i t u t e the f i r s t s t r u c t u r a l chrac-t e r i z a t i o n of a t r a n s i t i o n metal formaldoxime complex ( s t r u c t u r a l i n v e s t i -gations of simple oxime complexes are r a r e 1 0 0 ) , the s t r u c t u r e of an acetaldoxime complex (CH 3CHNOH) 1 +NiCl 2 has been d e s c r i b e d . 1 0 1 The Ni-N-C, Ni-N-0 and O-N-C angles of the nickel-oxime compound are a l l very s i m i l a r to those observed f o r the chromium formaldoxime complex of t h i s work. S i m i l a r l y the non-hydrogen atoms of the acetaldoxime ligands of the n i c k e l complex are al s o planar. The C ( l ) - N ( l ) distance of 1.253(9) A and the N(l) - 0 ( 1 ) distance of 1.392(7) A (Figure 19) both are very s i m i l a r to those found for (CH3CHNOH)^NiClj [C-N = 1.249(5), 1.253(3) A and N-0 = 1.382(5), 102 1.378(5) A]. A microwave study of formaldoxime puts the C-N distance at 1.276 A and the N-0 distance at 1.408 A, which are ne a r l y the same as the C ( l ) - N ( l ) and N(l ) - 0 ( 1 ) d i s t a n c e s , r e s p e c t i v e l y , observed f o r the coordinated formaldoxime i n Figure 19. The C(1)-N(l)-0(1) angle of 102 111.5(6)° and the C-N-0 angle for free formaldoxime (110°) are als o very s i m i l a r . The CH2N0H molecule, t h e r e f o r e , does not appear to s u f f e r s u b s t a n t i a l d i s t o r t i o n upon c o o r d i n a t i o n . The s t r u c t u r e of some N-bonded palladiura-oxime complexes have been described. These include complexes of cyclohexanone oxime, acetone oxime 103 and (+)-camphor oxime. The N-C and N-0 distances (1.27-1.29 A and 1.39-1.42 A, r e s p e c t i v e l y ) are very close to those observed f o r the chromium-formaldoxime complex. Likewise the geometries of the oxime ligands are s i m i l a r to that of the CH 2N0H li g a n d i n Figure 19. 123 The hydroxyl H-atom of the formaldoxime complex i s l i n k e d to the PF 6~ counterion by hydrogen bonding [H(10)-F(2) = 2.14 A ] , although t h i s does not appreciably d i s t o r t the octahedral geometry of the PFg - i o n . A s i m i l a r feature i s observed for [ ( T I 5 - C 5 H 5 ) 3Mn 3([i 2-NO) 3(u 3-NOH) ]PF 6 i n which the NOH l i g a n d and the PFg - anion d i s p l a y a hydrogen bonding i n t e r a c t i o n 104 [H-F = 2.24(4) A]. The IR spectrum of the new n i t r o s y l complex i n the s o l i d s t a t e (Nujol mull) i s co n s i s t e n t with the X-ray c r y s t a l l o g r a p h i c s t r u c t u r e . Two strong, terminal-NO absorptions occur at 1854 and 1761 cm - 1. The OH s t r e t c h gives a sharp band at 3480 cm - 1. A weak absorption at 1646 cm - 1 i s best assigned to the N=CH2 s t r e t c h of the oxime l i g a n d . Two medium i n t e n -s i t y bands at 996 and 945 cm - 1 are l i k e l y due to the oxime-CH out-of-plane deformation and the NO s t r e t c h . An overtone associated with CH deformation may be assignable to a weak band at 1990 cm - 1, and i f i t i s associated w i t h the fundamental at 945 cm - 1, then the band at 996 cm - 1 would be due to the oxime-NO s t r e t c h . * ^ 5 The vapour-phase IR spectrum of formaldoxime*^ 6 has been reported and a CH 2 out-of-plane deformation has been assigned to an absorption at 950 cm - 1, while the NO s t r e t c h has been associated w i t h a band at 888 cm - 1. The N-0 s t r e t c h would be a n t i c i p a t e d to be more suscep-t i b l e to s h i f t i n g upon c o o r d i n a t i o n than the oxime-CH deformations. The oxime-CH in-plane deformation can be assigned to a medium-intensity absorp-t i o n at 1349 cm - 1. F i n a l l y , the bands at 3130(m), 1433(m), 1015(w) and 882(m) may be due to absorptions o r i g i n a t i n g with the c y c l o p e n t a d i e n y l r i n g . The s o l u t i o n IR spectra are i n agreement with the complex possessing a c a t i o n i c { C r ( N 0 ) 2 } + moiety ( i n CH 2C1 2, v = 1847, 1746 cm - 1 and i n 124 CH 3N0 2, v N Q = 1847, 1748 cm - 1). The % NMR spectra of t h i s compound are very informative, p a r t i c u -l a r l y i n CD3NO2 (Figure 20). A s t a t i c molecular structure consistent with the observed spectrum i s shown in Figure 21. The protons H and H are A B c l e a r l y inequivalent and coupled to each other, 2 J i „ , being 5.1 Hz. V "B That H^ and H^ are coupled whereas H^ and H^ are not i s probably a manifes-tation of a planar "W"-conformation being required for a four-bond coupling to be observable, as i n the case of planar, three-carbon fragments, 1 0 7  i .e. H \ H C C The value for ^ J u , i u of 0.9 Hz i s not a t y p i c a l . There i s probably rapid r o t a t i o n about the N-0 bond of the oxime ligand, which would make the observed coupling constant an average value. In CD 2Cl2 the hydroxyl proton resonance is broadened, probably due to hydrogen-bonding, and thus exhibits no observable coupling. Resonances due to the methylene protons occur at simi l a r chemical s h i f t s to those i n CDoNO, and 2 J i „ i „ i s v i r t u a l l y the V H B same i n both solvents. As expected, the ^C^H} spectrum i n CD 3N0 2 shows two s i n g l e t s , one for a single methylene carbon, and one for a 1 3 C i n the C 5H 5 r i n g . In the gated-decoupled 1 3C NMR spectrum the methylene carbon resonance becomes s p l i t into a doublet of doublets due to coupling to two inequivalent protons, H^ and H g. The other 1 3C signals are normal for an T) 5-C 5H 5 r i n g , a doublet of raultiplets a r i s i n g from a p a r t i a l l y resolved 125 J Figure 20. 80 MHz XH NMR spectrum of [(n 5-C 5H 5)Cr(NO) 2(CH 2NOH)]PF 6 i n CD 3N0 2 . 126 ABB'CC'X spin system where 1 3C = X and 1 J i 3 f , _i„ = 183.1 Hz. Figure 2 1 . A s t a t i c molecular s t r u c t u r e f or [(n 5-C 5H 6)Cr(NO) 2(CH 2NOH)]PF 6 i n CD 3N0 2. Complexes of dioximes and oxime-containing c h e l a t i n g ligands are commonplace, while those of simple aldoximes and ketoximes are r a r e . * ^ ^ Generally they are synthesized from d i r e c t i n t e r a c t i o n of the ligands and a metal complex. Perhaps the best known oxime complexes are those of v i c i n a l dioximes such as dimethylglyoxime (DMGH,), of which the N i 1 1 complex, 127 109 Ni(DMGH) 2, i s the f i r s t to have been reported. While the new complex described i n t h i s study i s a novel example of an organometallic f o r m a l -doxime compound, the more i n t e r e s t i n g aspect of i t s synthesis i s that i t i n v o l v e s , e i t h e r d i r e c t l y or i n a formal sense, i n s e r t i o n of NO i n t o the Cr-C bond. This i s , of course, r e l e v a n t to the important area of C-N bond 57a formation. Furthermore, while metal-mediated n i t r o s a t i o n s of organic molecules are well-known, they are not c l e a r l y understood.***^ Migratory i n s e r t i o n of coordinated n i t r i c oxide i n t o m e t a l - a l k y l bonds has r e c e n t l y been d e f i n i t i v e l y demonstrated by Bergman and co-workers, i . e . ( T ] 5 - C 5 H 5 ) C O ( N O ) C H 3 + P E t 3 ~ 7 8 S (Ti 5-C 5H 5)Co(CH 3NO)PEt 3 5 7 (4.15) (n 5-C 5H 5)Fe(NO)(CH 3) 2 + PMe 3 4 5" C> (n 5-C 5H 5)Fe(CH 3NO)(CH 3)PMe 3 5 9 (4.16) The mechanism b e l i e v e d to be operative i n these r e a c t i o n s i s spontaneous i n s e r t i o n followed by trapping of the n i t r o s o a l k y l intermediate by a donor ligand."' 7* 5 As Bergman notes, while spontaneous i n s e r t i o n i s observed f o r ( T I 5 - C 5 H 5 ) C o ( N 0 ) a l k y l complexes, i t i s curious that some other n i t r o s y l - a l k y l compounds, most notably (n 5-C 5H 5)M(NO) 2R (M = Cr, Mo, W; R = a l k y l groups), do not undergo the analogous i n s e r t i o n . Conditions amenable to the very well-known c a r b o n y l - i n s e r t i o n , such as the presence of phosphines or Lewis a c i d s , * * * do not f a c i l i t a t e n i t r o s y l i n s e r t i o n for the (•n 5-C 5H 5)M(NO) 2R compounds. 8^' 5 7^ A few other more complex r e a c t i o n s i n which NO i n s e r t i o n i s thought to occur have been described, i . e . 128 O - N ( ( n 5 - C 5 H 5 ) 2 Z r ( C H 3 ) 2 + NO > ( n 5 - C 5 H 5 ) Z r ' XN=0 (4.17) W ( C H 3 ) 6 + 4N0 > ( C H 3 ) l + W ^ " r I (4>ig) —78°P 114 [ ( n 3 - C 3 H 5 ) N i B r ] 2 + 4N0 > 2(CH2=CH-CH=N0H)Ni(N0)Br (4.19) [(ri 3-C 3H 5)M(NO)(PPh 3) 2] + 3C0 > [M(C0) 3(PPh 3) 2]+ + CH2=CH-CH=NOH (M - Rh, I r ) 1 1 5 (4.20) The l a t t e r two re a c t i o n s have been proposed to i n v o l v e n u c l e o p h i l i c attack, of a t e r m i n a l , b e n t - n i t r o s y l l i g a n d on a coordinated a l l y l group to form 3-nitrosopropene which then isomerizes to the more stabl e 3-oximinopropene tautomer. Two r e l a t e d r e a c t i o n s that form oximes are depicted i n r e a c t i o n s 4.21 and 4.22. Ru(NO) 2(PPh 3) 2 + C 6H 5CH 2Br t Q ^ e n e > C 6H 5CH=NOH 1 1 6 (4.21) heat [Co(N0)(C0) 2X]- + C 6H 5CH 2X 1 ) a c e t o n e » 4 h > C6H5CH=NOH 2) H+ (X= h a l i d e s ) 1 1 7 (4.22) F i n a l l y , r e a c t i o n 4.23 in v o l v e s the formation of an oximate complex, 118 presumably v i a NO i n s e r t i o n , i . e . 129 C 6H 6, 85°C (T) 5-C 5Me 5)Ru(NO)(C 2H 5) 2 > (Ti 5-C 5Me 5)Ru(PMe 3) 2(ONCHCH 3) (4.23) PMe 3 This l a t t e r r e a c t i o n i s q u i t e complex but does demonstrate the f e a s a b i l i t y of oxime formation v i a NO i n s e r t i o n The two most l i k e l y pathways f o r the formation of the new formaldoxime complex [(n 5-C 5H 5)Cr(NO) 2(CH 2NOH)] + i n v o l v e o x i d a t i v e l y induced i n s e r t i o n and d i r e c t , charged-controlled a t t a c k of N0 + at the Cr-CH 3 bond. With the preceding d i s c u s s i o n on e l e c t r o p h i l i c cleavage r e a c t i o n s of ( T ) 5 - C 5 H 5 ) M ( N 0 ) 2 a l k y l (M = Cr, Mo) complexes i n mind, the l a t t e r mode of r e a c t i v i t y seems most l i k e l y . This i s depicted below i n Scheme I I I , analogous to the processes shown i n Scheme I I . The iso m e r i z a -t i o n of CH3NO to CH2=NOH i n the l a s t step of Scheme I I I i s not unusual. The conversion of a l k y l n i t r o s o compounds i n t o oximes i s ca t a l y z e d by polar 119 s o l v e n t s , strong acids and n i t r i c oxide, and rea c t i o n s 4.19-4.23 provide ample precedent for t h i s rearrangement at t r a n s i t i o n metal centres. The i n s e r t i o n of S0 2 i n t o m e t a l - a l k y l b o n d s 7 9 , 1 1 1 has some s i m i l a r i t i e s to the i n s e r t i o n r e a c t i o n proposed i n Scheme I I I . For example, the re a c t i o n s 79 between (T) 5-C 5H 5)Fe(CO)(L)R (L = CO, PPh 3; R = a l k y l groups) and S0 2, i . e . (n 5-C 5H 5)Fe(CO)(L)R + S0 2 > ( r i 5 - C 5 H 5 ) F e ( C O ) ( L ) S ( 0 2 ) R (4.24) are b e l i e v e d to proceed without p r i o r c o o r d i n a t i o n of the incoming S0 2 group. In contrast though, these r e a c t i o n s are thought to inv o l v e i n i t i a l Scheme I I I C H . N< 131 79 o x i d a t i o n of the o r g a n o m e t a l l i c complex. O x i d a t i v e l y - i n d u c e d i n s e r t i o n , however, remains a p o s s i b i l i t y at t h i s p o i n t , i . e . ( n 5 - C 5 H 5 ) C r ( N O ) 2 C H 3 + N 0 + > [ ( n 5 - C 5 H 5 ) C r ( N O ) ( C H 3 N O ) ] * + NO (4.25) [ ( T ) 6 - C 5 H 5 ) C r ( N O ) ( C H 3 N O ) ] ^ + NO > [ ( n 5 - C 5 H 5 ) C r ( N O ) 2 ( C H 2 N O H ) ] + (4.26) T r a n s f o r m a t i o n s 4.25 and 4.26 i n v o l v e i n t r a m o l e c u l a r n i t r o s y l i n s e r t i o n , as i n r e a c t i o n s 4.15 and 4.16. A g a i n , t h e rearrangement of CH 3NO t o f o r m a l -doxime i s not u n a n t i c i p a t e d i n such a p r o c e s s . As has been d e s c r i b e d above, CO can be a c t i v a t e d toward i n s e r t i o n i n t o m e t a l - a l k y l bonds by 120 o x i d a t i o n . Other examples of t h i s p r o c e s s have been r e p o r t e d , e.g. IV ( T ) 5 - C 5 H 5 ) M ( C O ) 3 C H 2 - £ - C 6 H 4 F C e C H ' Q 1 C 1 > £ - F C 6 H 4 C H 2 C 0 2 C H 3 + o t h e r p r o d u c t s (M = Mo, W) (4.27) i n a d d i t i o n to the o x i d a t i v e i n s e r t i o n s of ( T ) 5 - C 5 H 5 ) F e ( C O ) ( L ) C H 3 [L = CO, P Ph„ P ( P - i - P r ) q ] , 9 5 ' 9 6 e.g. ( T i 5 - C 5 H 5 ) F e ( C O ) 2 C H 3 C N > [ ( r i 5 - C 5 H 5 ) F e ( C O ) ( C ( 0 ) C H 3 ) ( C H 3 C N ) n ] • (4.28) The l o s s of an e l e c t r o n d e c r e a s e s the amount of e l e c t r o n d e n s i t y a v a i l a b l e f o r b a c k - b o n d i n g t o CO, a c t i v a t i n g i t toward n u c l e o p h i l i c a t t a c k . 132 Likewise, spontaneous i n s e r t i o n of coordinated NO i n t o the Cr-CH 3 bond upon o x i d a t i o n of (T^-C^H^CrCNO) 2CH 3 could be a n t i c i p a t e d as a consequence of enhanced e l e c t r o p h i l i c i t y of the NO l i g a n d s . An u l t r a v i o l e t photoelectron spectroscopic study i n d i c a t e s that the f i r s t i o n i z a t i o n s of both (T) 5-C 5H 5)Cr(NO) 2CH 3 and (r) 5-C 5H 5)Fe(CO) 2CH 3 i n v o l v e e l e c t r o n s i n l a r g e l y metal-based o r b i t a l s engaged i n back-bonding to the NO and CO l i g a n d s , r e s p e c t i v e l y . 7 7 Thus removal of an e l e c t r o n from the HOMO of (n 5-C 5H 5)Cr(NO) 2CH 3 by o x i d a t i o n would indeed be expected to lessen back-bonding to NO. (The s t a b i l i s a t i o n of Cr-dit e l e c t r o n density by back-bond-ing to NO i s very considerable i n the n e u t r a l complex, which may e x p l a i n why the (r) 5-C 5H 5)M(NO) 2 a l k y l (M = Cr, Mo, W) complexes do not undergo spontaneous NO i n s e r t i o n . ) C e r t a i n l y , N0 + would be a n t i c i p a t e d to be a strong enough oxidant to oxidze (Ti 5-C 5H 6)Cr(NO) 2CH 3. Another oxidation-based mode of r e a c t i v i t y i s more s t r i c t l y analogous to the S0 2 i n s e r t i o n r e a c t i o n s , i . e . (n 5-C 5H 5)Cr(NO) 2CH 3 + N0 + •> [(T) 5-C 5H 5)Cr(NO) 2CH 3^ N0»] (4.29) [(T) 5-C 5H 5)Cr(NO) 2CH 3 "i ' N0«] » [(n 5-C 5 H 5)Cr(NO) 2(CH 3NO)] + (4.30) [(n 5-C 5H 5)Cr(NO) 2(CH 3NO)] + * [(Ti 5-C 5H 5)Cr(NO) 2(CH 2NOH)]+ (4.31) An e l e c t r o n t r a n s f e r intermediate forms i n transformation 4.29 with the r a d i c a l c a t i o n and NO remaining i n the solvent cage and c o l l a p s i n g i n t o the i n i t i a l " i n s e r t i o n " product ( r e a c t i o n 4.30). This set of transformations 133 87 i s more r e f l e c t i v e of simple o x i d a t i v e cleavage of the Cr-CH, bond than i n s e r t i o n . Given the d i s c u s s i o n above on e l e c t r o p h i l i c cleavage r e a c t i o n s of n i t r o s y l - a l k y l complexes, t h i s may perhaps be l e s s l i k e l y than the two pr e v i o u s l y discussed pathways. An example i n which N0 + i s known to f u n c t i o n as an oxidant, i . e . [ ( T) 5-C 5H 5)M(CO) 3CH 2C 5H l tNH] + ^ > [HNC 5H 4CH 2C0 2H] + (M = Mo, W) (4.32) 78b leads to a CO-insertion product. A f o u r t h p o s s i b l e r e a c t i o n pathway could proceed v i a i n i t i a l 81 a-hydrogen a b s t r a c t i o n from the coordinated CH 3 group, i . e . ( T) 5-C 5H 5)Cr ( N O) 2CH 3 + N0 + > [ ( r i 5 - C 5 H 5 ) C r ( N O ) 2(CH 2) ]+ + HNO (4.33) [ ( n 5 - C 5 H 5 ) C r ( N O ) 2 ( C H 2 ) ] + + HNO > [ ( T] 5-C 5H 5)Cr(NO) 2(CH 2NOH) ] + (4.34) This again, however, seems u n l i k e l y at t h i s time since the more common mode of r e a c t i v i t y of the known h y d r i d e - a b s t r a c t i n g reagent, [ P h 3 C ] + , toward ( T ) 5 - C 5 H 5 ) M ( N O ) 2CH 3 (M = Cr, Mo) complexes i s cleavage of the M-CH3 bonds. Which, i f any, of the above described r e a c t i o n modes a p p l i e s to the r e a c t i o n of ( - n 5-C 5H 5)Cr(NO) 2CH 3 w i t h N0 + i s not known. Given the preceding d i s c u s s i o n concerning e l e c t r o p h i l i c m e t a l - a l k y l bond cleavage, d i r e c t 134 att a c k of N0 + at the Cr-CH 3 bond seems most l i k e l y at t h i s time. However, competitive r e a c t i o n paths could be operative (and probably are, considering that [(n 5-C 5H 5)Cr(NO) 2(CH 2NOH)] + i s not the only product of t h i s r e a c t i o n ) . I t may yet come about that o x i d a t i o n of (n 5-C 5H 5)Cr(NO) 2CH 3, f o r example by [ F e ( P h e n ) 3 ] ( P F 6 ) 3 , followed by a d d i t i o n of an appropriate trapping agent w i l l y i e l d an NO-insertion product. (b) (TI 5-C 5H 5)M(NO) 2R (M - MO, W; R= CH 3, C 2 H 5 ) . The prepara-t i o n s of (n 5-C 5H 5)Mo(NO) 2CH 3 and (n 5-C 5H 5 ) W(NO) 2CH 3 d e t a i l e d i n the E x p e r i -mental Section represent the most convenient syntheses of these compounds. In the case of the molybdenum-containing complex the y i e l d of the r e a c t i o n 38 i s s i m i l a r to the p r e v i o u s l y reported preparation but the use of CH 2C1 2 makes the r e a c t i o n q u i c k e r . Synthesis of (T) 5-C 5H 5 )W(NO) 2CH 3 from 68 ( T I 5 - C 5 H 5 ) W ( N O ) 2BF 1 + and Me 3Al i s f a r superior to the r e a c t i o n performed i n benzene with ( n 5 - C 5 H 5 ) W ( N O ) 2 C l , which does not proceed to completion even 38 a f t e r 48 h. The methyl compound i s formed i n s t a n t l y i n the r e a c t i o n s t a r t i n g from the c a t i o n and i n twice the y i e l d (~40%) compared to the previous method. The r e a c t i o n i s accompanied by the p r e c i p i t a t i o n of a f i n e , u n i d e n t i f i e d brown s o l i d due to a competing r e a c t i o n , which may i n v o l v e reduction of ( T I 5 - C 5 H 5 ) W ( N O ) 2BF, (see Chapter 5). The CVs of these four compounds i n CH 2C1 2 are q u a l i t a t i v e l y q u i t e s i m i l a r . Figure 22a shows a CV of (n 5-C 5H 5)Mo(NO) 2CH 3 and Figure 22b a CV of (n 5-C 5H 5 ) W(NO) 2CH 3, both i n CH 2C1 2. A l l four complexes give r i s e to broad o x i d a t i o n waves. The peak p o t e n t i a l s for ( T I 5 - C 5 H 5 ) M ( N O ) 2 R are : 1.31 V (M = Mo; R = CH 3); 1.21 V (M = Mo; R = C 2 H & ) ; 1.35 V (M = W; R = CH 3); and 1.27 V (M = W; R = C 2 H 5 ) , a l l at 0.07 V s - 1 . The peak currents 135 appear to be largely diffusion controlled. In the case of ( T I 5 - C 5 H 5 ) M O (N O ) 2 C H 3 a plot of i vs. v 1 / 2 is nearly linear, curving only p ,a slightly, while a plot of i vs. v deviates more severely from linearity. p ,a Not surprisingly, E values for the ethyl-containing compounds are lower p ,a than for the corresponding methyl analogues (AE = 100 mV for M = Mo and p,a AE = 80 mV for M = W ). The tungsten-methyl and -ethyl compounds have p,a E values only slightly more positive than their respective molybdenum p,a counterparts. At slower scan rates a second oxidation wave is clearly visible for ( T I 5 - C C H C)W ( N O) ,CH, and its ethyl analogue at E = +1.6 V and i t appears to have some degree of chemical reversibility (E - +1.4 V); however E p,c p,a shifts to more positive potentials with increased scan rates. The second wave becomes obscured by broadening of the f i r s t wave at sweep rates greater than 0.2 Vs - 1. In CHoCN the f i r s t oxidation wave appears at E = p,a +1.34 V vs. SCE, or +0.97 V vs. ( T I 5 - C 5 H 5 ) 2 F e / [ ( T I 5 - C 5 H 5 ) 2 F e ] + at a scan rate of 0.07 V s - 1 . In CH 0C1 9 this peak occurs at E = +0.88 V vs. the 2 2 P.a ferrocene/ferrocinium couple at a similar scan rate. Given the d i f f i c u l -22 ties of trying to compare redox potentials in differing solvents, i t i s best not to try to draw any conclusions about the relative ease of oxida-tion in these solvents. The second oxidation wave which is observed in CH 2C1 2 becomes a barely discernable hump in CH3CN at E^ ^  = +1.6 V vs. SCE. If the scan is extended beyond +2 V a very large oxidation current is detected, possibly due to multiple-electron transfers. The molybdenum-containing alkyl complexes do not exhibit the second oxidation peak but their oxidation waves are very broad. The second wave could arise from a 136 (a) + 1 0 Volts vs SCE Figure 22. C y c l i c voltamograms of (a) ( T I 5 - C 5 H 5 ) M O ( N O ) 2 C H 3 at a scan r a t e of 0.15 v s - 1 and (b) (n 5-C 5H 5)W ( N O ) 2CH 3 at a scan rate of 0.19 V s - 1 , both i n CH 2C1 2 ( o x i d a t i o n ) . 137 v a r i e t y of sources. Adsorption of the s t a r t i n g a l k y l compounds onto the electrode or o x i d a t i o n of the products of the f i r s t wave may be i n v o l v e d . A l l four compounds d i s p l a y s e v e r a l low-current cathodic waves i n the r e t u r n scans between +0.6 and -0.7 V. This suggests that the o x i d a t i o n processes give r i s e to a v a r i e t y of products and the nature of these products i s as yet unclear. I t i s p e c u l i a r that ( T I 5 - C 5 H 5 ) W ( N O ) 2H does not e x h i b i t an o x i d a t i o n wave i n CH 2C1 2. The molybdenum- and t u n g s t e n - a l k y l compounds e x h i b i t only s l i g h t l y d i f f e r e n t CVs and the products of these o x i d a t i o n waves are not known. A v a r i e t y of r e a c t i o n pathways encompassing o x i d a t i v e i n s e r t i o n and cleavage of the m e t a l - a l k y l bond can be envisaged, as w e l l as other modes of r e a c t i v i t y not n e c e s s a r i l y i n v o l v i n g the a l k y l group d i r e c t l y . Reaction of (TI 5-C 5H 5)MO(NO) 2CH 3 w i t h NOPFg. This r e a c t i o n proceeds quite r a p i d l y i n CH 2C1 2, i n s p i t e of the i n s o l u b i l i t y of NOPFg. Gas e v o l u t i o n occurs and could be due to generation of NO and/or other gaseous products. C e r t a i n l y , the pungent odor of N0 2 i s pervasive during these experiments. New bands appear In the IR spectrum of the r e s u l t a n t green-brown s o l u t i o n at 1773 and 1686 cm - 1 as the s t a r t i n g m a t e r i a l (V^Q 1730, 1635 cm - 1) i s consumed. I f a product analogous to [(t) 5-C 5H 5)Cr(NO) 2(CH 2NOH) ] + i s formed during t h i s r e a c t i o n i t has not as yet been i s o l a t e d . A d d i t i o n of h a l i d e s a l t s to the r e a c t i o n mixture generate (n 5-C 5H 5)Mo(NO) 2X (X = C l , Br) which can be i s o l a t e d i n only low y i e l d , due i n part to losses during chromatography. The elemental analyses of these products i n d i c a t e the presence of small amounts of i m p u r i t i e s , p o s s i b l y [(Ph 3P) 2N]+ s a l t s . The ready formation of (n 5-C 5H 5)Mo(NO) 2X (X = 138 C l , Br) suggests that the M-CH3 bond i s cleaved by NOPFg and that a c a t i o n i c d i n i t r o s y l complex i s produced. Reactions of (n 5-C 5H 5)W(NO) 2CH3. The r e a c t i o n of t h i s methyl compound with AgBF^ proceeds very slowly and i s not complete even a f t e r 4 . 5 h. This could be due i n part to o c c l u s i o n of the i n s o l u b l e r e a c t a n t . No so l u b l e n i t r o s y l - c o n t a i n i n g species are generated. The s o l i d that does form (V^JQ 1612 cm - 1) can only be d i s s o l v e d i n very polar organic solvents which also d i s s o l v e AgBF^, a f a c t o r which encumbers i s o l a t i o n of the prod u c t ( s ) . Unlike the analogous r e a c t i o n with ( T i 5-C 5H 5)Cr(NO) 2CH 3 which produced a v a r i e t y of species, i n c l u d i n g the { ( T] 5-C 5H 5)Cr(NO) 2 } + fragment, the c a t i o n i c { ( n 5 - C 5 H 5 ) W ( N O ) 2 } + moiety i s not formed. The s i n g l e v N Q of the brown s o l i d i n d i c a t e s that l o s s of NO ligands occurs. Reaction of the methyl complex with NOPFg re q u i r e s greater than one equivalent of NOPFg to consume completely the reactant i n a period of ~ 3 . 5 h. Again t h i s may be the r e s u l t of o c c l u s i o n of s o l i d NOPFg, which i n s t a n t l y d i s c o l o u r s when the r e a c t i o n i s commenced. The higher absorptions which appear i n the IR spectrum of the r e a c t i o n mixture ( 1 7 4 4 , 1658 c m - 1 ) , are p o s s i b l y due to a c a t i o n i c species, but not simple ( T I 5 - C 5 H 5 ) W ( N O ) 2 P F 6 6 8 ( V N Q 1755, 1672 c m - 1 ) . 1 2 1 The lowered i n t e n s i t y of these bands and the bands of (n 5-C 5H 5 ) W(NO) 2C1 a f t e r [ ( P h 3 P ) 2 N ] C l a d d i t i o n , w i t h respect to the s t a r t i n g m a t e r i a l , i n d i c a t e s u b s t a n t i a l decompositon during the course of the r e a c t i o n . Since ( T) 5 - C 5 H 5 ) W ( N O ) ^ F ^ i s known to be 68 s o l u t i o n unstable t h i s i s not s u r p r i s i n g . As i n the case of the 139 molybdenum-analogue, again the W-CH3 bond of (n b-C 5H 5 ) W(NO) 2CH 3 i s cleaved by NOPF6 to produce a d i n i t r o s y l s p e c ies. A d d i t i o n of PPh 3 to the r e s u l t a n t r e a c t i o n mixture probably generates [(ri 5-C 5H 5 ) W(NO) 2(PPh 3) ] + , as i n d i c a t e d by the r e s u l t a n t s o l u t i o n ' s IR spectrum, again suggesting the formation of the { ( n 5 - C 5 H 5 ) W ( N O ) 2 } + moiety. Judging by the IR spectra of the f i n a l r e a c t i o n mixtures, the extent of {( T I 5 - C 5 H 5 ) W ( N O ) 2 } + formation i s l e s s than that of {(r| 5-C5H 5)Mo(N0) 2} + i n these re a c t i o n s of the methyl compounds with NOPFg. The r e a c t i o n of ( T I 5 - C 5 H 5 ) W ( N O ) 2CH 3 with [ F e ( P h e n ) 3 ] ( P F g ) 3 i n e i t h e r CH 3N0 2 or CH 2C1 2 l a r g e l y destroys the methyl complex. C e r t a i n l y no form of { ( T I 5 - C 5 H 5 ) W ( N O ) 2 } + r e s u l t s . That not a l l of the s t a r t i n g methyl compound i s consumed may be r e f l e c t i v e of the second o x i d a t i o n wave i n the CV of (n 5-C 5H 5 ) W(NO) 2CH 3 (Figure 22b). Thus simple o x i d a t i o n of t h i s a l k y l complex appears to r e s u l t i n d i s i n t e g r a t i o n . However, N0 + does generate small amounts of the { ( T I 5 - C 5 H 5 ) W ( N O ) 2 } + moiety, i n d i c a t i n g that i t does not act purely as an oxidant towards ( T I 5 - C 5 H 5 ) W ( N O ) 2CH 3. A v a r i e t y of a l k y l e l i m i n a t i o n pathways are known to e x i s t f o r e l e c t r o p h i l i c cleavage r e a c t i o n s . At l e a s t three are observed f o r r e a c t i o n s of (Ti 5-C5H 5)Fe(CO) 2R 89 (R = various a l k y l , a r y l groups) with mercuric h a l i d e s , i . e . (ri 5-C 5H 5)Fe(CO) 2R •SS^2—> (T) 5-C 5H 5)Fe(CO) 2X + RHgX (4.35) (Ti 5-C 5H 5)Fe(CO) 2R ^2—> (n 5-C 5H 5)Fe(CO) 2HgX + RX (4.36) 140 ( T) 5-C 5H 5)Fe(CO) 2R - 2 8^2—> 1/2 Hg 2X 2 + RX + 2CO (4.37) + other products For a given R group, o f t e n the r e a c t i o n occurs by more than one pathway, though one i s g e n e r a l l y g r e a t l y favoured. S i m i l a r l y a v a r i e t y of r e a c t i o n modes may be a v a i l a l b e to ( T ) 5 - C 5 H 5 ) M ( N O ) 2 R (M = Cr, Mo, W; R = a l k y l groups) complexes i n t h e i r r e a c t i o n s with e l e c t r o p h i l e s and/or oxidants. Attempted Reaction of (TJ 5-C 5H 5)W(NO) 2PF 6 and ( t i 5 - C 5 H 5 ) W ( N 0 ) 2 C H 3 . The r e a c t i o n between ( T) 5-C 5H 5)W(NO) 2 P F 6 and ( T) 5-C 5H 5)W(NO) 2CH 3 has been attempted to see i f [ { ( T I 5 - C 5 H 5 ) W ( N O ) 2> 2CH 3 ]PF 6 could be formed. The analogous r e a c t i o n between ( T) 5-C 5H 5)W(NO) 2 P F 6 and ( T I 5-C 5H 5)W(NO) 2H r a p i d l y 28 generates [ {(T I 5-C5H 5)W(N0) 2> 2H]PF 6 i n CH 2C1 2. However, an analogous r e a c t i o n between ( T ] 5-C 5H 5)W(NO) 2 P F 6 and ( T I 5-C 5H 5)W(NO) 2CH 3 does not occur. 1 In l i g h t of the h y d r i d i c nature of the hydride l i g a n d of (n 5-C 5H 5)W(NO) 2H i t seems reasonable that the W-H bond could act as a Lewis base toward [ ( T l 5 - C 5 H 5 ) W ( N O ) 2 ] + . The b i m e t a l l i c hydride complex i s b e l i e v e d to be f l u x i o n a l i n s o l u t i o n , possessing a b r i d g i n g W W core at l e a s t part of the t i m e . 1 2 3 In the case of (n 5-C 5H 5)W(N0) 2CH 3 and [(T) 5-C 5H 5)W(NO) 2]+, CH. however, s t e r i c f a c t o r s and a p o t e n t i a l l y l e s s favourable W W i n t e r -a c t i o n may prevent formation of a b i m e t a l l i c complex. (c) (•n5-C5H5)Cr(CO)3CH3. Of the s i x carbonyl compounds s t u d i e d , (T| 5-C 5H 5)Cr(CO) 3CH 3 e x h i b i t s the most i n t r i g u i n g and unique o x i d a t i o n behaviour. C y l i c voltammograms of t h i s compound i n CH 2C1 2 are shown i n Figure 23. The o x i d a t i o n behaviour c l e a r l y i s q u i t e complex. I f a Figure 23. C y c l i c voltammograms of (r| b-C 5H 5)Cr(CO) 3CH 3 i n CH 2C1 2 at scan rat e of 0.11 Vs" 1: (a) between 0 and +2 V; (b) between 0 and +0.7 and; (c) between 0 and +1 V. 142 complete scan i s run out to +1.9 V four waves are observed as depicted i n Figure 23a. I f the scan i s extended to encompass only the f i r s t wave (Figure 23b), the couple i s seen to be r e v e r s i b l e with E + E /2 = p,a p,c +0.40 V and AE = 60 mV. Both E and E are independent of scan ra t e P P,a p,c over the range 0.04-0.21 V s - 1 . The wave has very low peak c u r r e n t s , making i / i d i f f i c u l t to measure. The peak current r a t i o i s estimated to be p,c p,a greater than, or equal to one, being between 1.0 and 1.2 at 0.06 V s - 1 . I f the scan i s extended to ~ +1 V a second wave i s observed with E = p,a +0.85 V at 0.04 V s - 1 to +0.88 V at 0.22 V s - 1 . In the r e t u r n scan a cathodic peak i s observed at E = +0.37 V, and the peak p o t e n t i a l i s p,c independent of scan r a t e . In a d d i t i o n , i i s much greater at t h i s point p ,c than i n the case when only the f i r s t wave i s scanned (compare Figures 23b and 23c). Two successive scans, one immediately a f t e r the other, s t a r t i n g from 0 V and extended to ~ +1 V are shown i n Figure 23c. As the second scan begins from 0 V, the peak current at E = +0.43 V becomes substan-p ,a t i a l l y greater than i n the f i r s t scan. The peak current r a t i o , i / I p,a p,c measured f o r the cathodic r i s e at E = +0.37 V and the second anodic r i s e P»c (Figure 23c, scan 2) at E = +0.43 V i s estimated to be ~0.6. The second p, a time the o x i d a t i o n at E = +0.87 V (at 0.11 V s - 1 ) i s scanned, i i s P,a p,a somewhat l e s s than when i t i s scanned the f i r s t time. F i n a l l y , the peak currents at E = +0.87 V and E = +0.37 V are l i n e a r with v 1 / 2 , p,a p,c i n d i c a t i n g d i f f u s i o n l i m i t e d c u r r e n t s . I t i s c l e a r that the waves at E p ,a « +0.87 and E = +0.37 V are interconnected. That E f o r the second p,c p,a o x i d a t i o n i s scan rate dependant whereas E and E f o r the f i r s t , p,c p,a 143 r e v e r s i b l e wave, are not argues against q u a s i - r e v e r s i b l e behaviour. One p o s s i b l e i n t e r p r e t a t i o n of these r e s u l t s i s depicted i n Scheme IV, namely a c a r b o n y l - a l k y l (A) r ) 2 - a c e t y l (B) e q u i l i b r i u m . Some support f o r t h i s n o t i o n may be obtained with the a i d of IR and *H NMR spectroscopy. Thus an IR spectrum of a concentrated s o l u t i o n of f r e s h l y sublimed, (Ti 5-C 5H 5)Cr(CO)3CH 3 i n CH 2C1 2 r e v e a l s very strong absorptions f o r the t e r m i n a l CO s t r e t c h e s of the a l k y l complex (2008, 1927 cm - 1) and a weak band at 1605 cm -*, a t t r i b u t a b l e to the presence of an a c y l c a r b o n y l . o i 124 ( G e n e r a l l y , r p - a c y l CO s t r e t c h e s appear between 1620 and 1450 cm - 1, though t h i s i s not n e c e s s a r i l y d i a g n o s t i c ) . The *H NMR spectrum of a concentrated s o l u t i o n of t h i s compound i n CD 2C1 2 at ~25°C d i s p l a y s four s i n g l e t s . Two are assignable to the major component, (ri 5-C 5H 5)Cr(CO)3CH3, at 6 4.83 (C 5H 5) and 0.70 (CH 3) and two f o r a minor species at 6 4.96 ( C 5H 5) and 3.20 (CH 3). These l a t t e r two resonances are t e n t a t i v e l y assigned to ( r i 5 - C 5 H 5 ) C r ( C O ) 2 ( n 2 - C ( 0 ) C H 3 ) . The s i n g l e t at 6 3.20 i s not o 125 a t y p i c a l f o r an r r - a c e t y l l i g a n d . Unfortunately the i n t e g r a t i o n s of the c y c l o p e n t a d i e n y l proton resonances are erroneously low i n t h i s p a r t i c u l a r 123 spectrum, probably due to a long T^ r e l a x a t i o n time. On the basis of the methyl resonances, however, the r a t i o of a l k y l to a c y l components i s estimated to be ~13:1. The proposed c a r b o n y l - a l k y l *"~ r| 2-acyl e q u i l i b r i u m has been observed i n other systems. For example, Ru(PPh 3) 2(CO) 2(X)(£-C 6H,CH 3) (X = C l , Br, I) i n CH 2C1 2 e x i s t as an e q u i l i b r i u m mixture of o-aryl and n 2 - a c y l 126 , complexes, i . e . 144 S c h e m e IV 145 Ru(PPh3)2(C0)2(X)(£_C6H^CH3) _ 2^2 Ru(PPh3) 2(CO)(X ) (n 2-C(0)-p-C 6H i +CH 3) (4.38) In the case of X = I , the e q u i l i b r i u m s u b s t a n t i a l l y favours the T) 2-acyl form. I t i s a l s o p o s s i b l e that heating of (Ti 5-C 5H 5)Cr(CO) 3CH 3 during sublimation generates the minor component. From the observed CVs i n Figure 23 both K = k f / k ^ and k^ (Scheme IV) would be expected to be s m a l l . S p e c i -f i c a l l y , k^ must be small compared with the scan r a t e . I f k^ i s large with respect to the scan r a t e , then as species B i s consumed at E = +0.43 V, P.a more of i t would be produced as a r e s u l t of what would be a r a p i d A ^ B e q u i l i b r i u m , and i would be q u i t e a b i t grea t e r . Also E would s h i f t P>a M 6 P,a 18b to more p o s i t i v e p o t e n t i a l s with increased scan r a t e . With reference to Scheme IV, the processes o c c u r r i n g might be r e v e r s i b l e o x i d a t i o n of B at E = +0.43 V to generate a r a d i c a l c a t i o n , C, which i s al s o produced by p,a o x i d a t i o n of A at E = +0.87 V followed by a r a p i d rearrangement, i n t h i s p,a case i n s e r t i o n . 9 5 , 9 * ' ' 1 2 0 The conc e n t r a t i o n of C at the electrode would be greater than what i s generated by o x i d a t i o n at E = +0.43 V and so when C p ,a i s reduced at E = +0.37 V, i i s s u b s t a n t i a l l y i n c r e a s e d . Again i f k,. p,c p,c ' f and K are qui t e small then the reduced species B, should give a r e l a t i v e l y greater anodic current at E = +0.43 V (Figure 23c, scan 2 s t a r t i n g from p ,a 146 0 V) than i f only the wave at E = +0.43 V i s i n i t i a l l y scanned (as i n p,a Figure 23b), and t h i s feature i s indeed observed. This i s also r e f l e c t e d i n that i at E = +0.87 V i s s l i g h t l y l e s s i n the second scan compared p,a p,a with the f i r s t , since more of A i s present as C a f t e r the second scan. The r a d i c a l c a t i o n , C, may a l s o undergo subsequent decomposition r e a c t i o n s . S t r u c t u r a l r e o r g a n i z a t i o n upon e l e c t r o n t r a n s f e r i s a well-known 127 phenomenon. I t i s a l s o c l e a r that the waves at E = +0.43 V, E = v p,a p,c +0.37 V and E = +0.87 V are coupled. Furthermore, i t seems l i k e l y that p,a (Ti 5-C 5H 5)Cr(CO) 3CH 3 l a r g e l y maintains i t s e m p i r i c a l composition i n a CV scan s t a r t i n g from 0 V, extending to ~ +1 V and back again, i . e . i t i s regenerated upon red u c t i o n of the o x i d i z e d form. In t h i s respect (Ti 5-C 5H 5)Cr(CO) 3CH 3 may p a r a l l e l (T) 5-C 5H 5)Fe(CO) 2 C H 3 . 9 5 ' 9 6 Given the propensity of the r e l a t e d compounds (-n5-C5H5)M(CO) 3 A r (M = Mo, Ar = 4- f l u o r o b e n z y l , 3-pyridylmethyl; M = W, Ar = 4-fluorobenzyl) to undergo 120 i n s e r t i o n upon o x i d a t i o n , i t i s p l a u s i b l e that such a process could be o c c u r r i n g f o r (T) 5-C 5H 5)Cr(CO) 3CH 3. While the nature of the redox processes described above f o r the chromium complex i s not d e f i n i t i v e l y known, Scheme IV o f f e r s a p o s s i b l e r a t i o n a l e . As the CV scan i s extended to ~ +1.9 V two other waves appear. The t h i r d wave (Figure 23a) also has a scan rate dependent E , being +1.31 V at 0.07 V s - 1 and +1.35 V at 0.22 V s - 1 . The p,a peak currents of the second and t h i r d waves are s i m i l a r at the same scan rates and i f o r the t h i r d wave i s l i n e a r with v 1 / 2 , i n d i c a t i n g d i f f u s i o n p,a c o n t r o l l e d c u r r e n t s , ( i i s estimated from a baseline determined by the p,a decaying anodic current of the second wave.) This wave probably i s due to 147 o x i d a t i o n of the product generated at E = +0.87 , which, i n terms of p ,a Scheme IV would be o x i d a t i o n of [(Ti 5 - C 5H 5)Cr(CO) 2(Ti 2-OCCH3)] This wave appears to have no counterpart i n the re t u r n scan, and i ^ f o r the wave at E = +0.37 V i s diminished, i n d i c a t i n g a net d e s t r u c t i v e consumption of p, c the species responsible f o r the wave at E^ ^  = +0.87 V. The fo u r t h wave (Figure 23a) with E = +1.47 V appears as a low-current shoulder on the p ,a t h i r d wave. I t s o r i g i n and nature are unknown. (d) (T1 5-C 5H 5)M(CO) 3R (M - Mo, R - CH 3, C 2H 5; M - W, R - H, CH 3, C 2 H 5 ) . Q u a l i t a t i v e l y , the CVs of these f i v e compounds i n C H 2 C 1 2 are qui t e s i m i l a r . Figure 24a d i s p l a y s a CV of ( T ) 5 - C 5 H 5 ) M O ( C O ) 3CH3, Figure 24b a CV of (ti 5 - C 5H 5 ) W(CO) 3CH3 and Figure 24c shows a CV of ( n 5 - C 5 H 5 ) W ( C O ) 3 H . Each e x h i b i t s a s i n g l e , sharp wave with E values between +1 and +1.3 V, p,a which s h i f t to more p o s i t i v e p o t e n t i a l s with i n c r e a s i n g scan r a t e s . Only f o r ( T ) 5 - C 5 H 5 ) M O ( C O ) 3CH3 i s a p l o t of i vs. v l i n e a r and a p l o t of i p,a p, a vs. v 1 / 2 not l i n e a r . In the case of ( T I 5 - C 5 H C ) W ( C O )3CH3, i appears to be p,a l i n e a r with v 1 / 2 , but i s nearly l i n e a r w i t h v a l s o . (This i s l i k e l y due to the very l i m i t e d a c c e s s i b l e scan range.) Thus, weak adsorption may be involved i n some of these o x i d a t i o n s but t h i s only s l i g h t l y a f f e c t s E p,a 18b and l e s s so at slow scan r a t e s . The E values f o r the methyl and p,a e t h y l compounds are a l l very s i m i l a r . Only ( T I 5 - C 5 H 5 ) M O ( C O ) 3 C 2 H 5 i s mar g i n a l l y e a s i e r to o x i d i z e than the other three complexes, and then only by ~10 mV (see Table I I ) . As expected ( T I 5 - C 5 H 5 ) W ( C O ) 3H i s s l i g h t l y more d i f f i c u l t to o x i d i z e than the tungsten-containing a l k y l s , by ~150 mV, again a small d i f f e r e n c e . S i m i l a r trends are extant f o r the ( T ) 5 - C 5 H 5 ) M ( N O ) 2R 148 complexes (see Table I I ) , except that ( T I 5 - C 5 H 5 ) W ( N O ) , H does not e x h i b i t a w e l l - d e f i n e d o x i d a t i o n wave i n CH 2C1 2, As can be seen from Figure 24, a f t e r the f i r s t o x i d a t i o n s , s e v e r a l low-current peaks f o l l o w i n the r e t u r n scans, i n d i c a t i n g that complex chemical r e a c t i o n s f o l l o w e l e c t r o n removal. A second, low-current wave i n the o x i d a t i o n scan i s most n o t i c e a b l e f o r the CV of ( T I 5 - C 5 H 5 ) W ( C O ) 3CH 3 (Figure 24b). The CV of ( T I 5 - C 5 H 5 ) W ( C O ) 3 C 2 H 5 i n CH 2C1 2 i s somewhat unusual i n i t s reduction behaviour f o l l o w i n g o x i d a t i o n . A f t e r the o x i d a t i o n peak at E = +1.09 V i s passed, s e v e r a l reduction peaks appear around 0 V. As p ,a the scan rate v a r i e s from 0.04 V s - 1 to 0.25 V s - 1 , the f i r s t r e duction wave moves from E = +0.29 V to ~0.0 V. In CH0CN the s i t u a t i o n changes p,c 3 (Figure 25). A f t e r the o x i d a t i o n wave at E = +0.95 V i s passed, a p ,a reduction peak i s seen at E^ ^ = -0.42 V. An anodic wave al s o appears at E = +0.12 V, perhaps due to o x i d a t i o n of the species generated at E ~ p,a * t - 6 p ) C -0.42 V. The follow-up waves i n CHgCN do not show the d r a s t i c scan r a t e dependance observed f o r t h i s compound i n CH 2C1 2 (see above). 94 A c e t o n i t r i l e may w e l l become coordinated to a c a t i o n produced by o x i d a t i o n at ~ +0.95 V. In a d d i t i o n , i n CH 3 CN a second wave occurs as p o t e n t i a l s are scanned to +2.5 V at E = +2.37 V (at 0.16 V s - 1 ) , on the p,a edge of the solvent l i m i t . Organometallic c a r b o n y l - a l k y l complexes of t e n undergo m e t a l - a l k y l 87 bond cleavage upon o x i d a t i o n . In some cases t h i s r e s u l t s i n i n s e r t i o n and i n some cases the a l k y l group i s e l i m i n a t e d . An example i n which both , « i 1 2 0 , types of r e a c t i v i t y occur i s shown below, l . e . 149 + 1 0 -1 Volts vs SCE Figure 24. C y c l i c voltammograms of (a) ( T ) 5 - C 5 H 5 ) M O (C O ) 3CH 3 at a scan rat e of 0.35 V s - 1 , (b) (n 5-C 5H 5 ) W(CO) 3CH 3 at a scan rate of 0.15 V s - 1 and (c) ( T I 5 - C 5 H 5 ) W ( C O ) 3 H at a scan rate of 0.24 V s - 1 , a l l i n CH 2C1 2. Figure 25. A c y c l i c voltammogram of ( T I 5 - C 5 H 5 ) W ( C O ) 3 C 2 H 5 i n CH3CN at a scan rate of 0.47 V s - 1 . 151 C e I V (n 5-C 5H 5)W(CO) 3(p-CH 2C 6H 1 +F) ~ c l > p-FCgH^CHjCOjCHg Me OH + p-FCgH^Cl^Cl + p-C6H1+CH2OCH3 (4.39) One or both of these types of processes could be occ u r r i n g i n the oxi d a -t i o n s of ( T) 5-C 5H 5)M(CO) 3R (M = Mo, R = CH 3, C 2H 5; M = W, R = H, CH 3, C 2 H 5 ) . Reaction of (TI 5-C 5H 5)W(CO) 3CH 3 w i t h AgBF^. This r e a c t i o n proceeds slowly i n CH 2C1 2. Once a l l of the s t a r t i n g m a t e r i a l i s consumed, a grey p r e c i p i t a t e r e s u l t s , and the supernatant s o l u t i o n i s red and i t s IR spectrum d i s p l a y s weak, absorptions at 2072 and 1973 cm - 1. Gradually these bands increase i n i n t e n s i t y and absorptions c h a r a c t e r i s t i c of ( T) 5-C 5H 5 ) W(C0) 3CH 3 reappear and grow i n i n t e n s i t y . A f t e r f i l t e r i n g the mixture i n t o a s o l u t i o n c o n t a i n i n g [(Ph 3P) 2N]C1, (n 5-C 5H 5 ) W(C0) 3C1 and ( T I 5 - C 5 H 5 ) W ( C O ) 3 C H 3 can be i s o l a t e d i n low y i e l d s . These observations suggest that AgBF 4 and (T) 5-C 5H 5 )W(CO) 3CH 3 react i n i t i a l l y to form a l a r g e l y i n s o l u b l e adduct which then slowly r eleases [ ( T I 5 - C 5 H 5 ) W ( C O ) 3 ] + and ( T I 5 - C 5 H 5 ) W ( C O ) 3CH 3 back i n t o s o l u t i o n . Beck and co-workers have prepared (n 5-C 5H 5 ) W(CO) 3BF 4 and i t s IR spectrum shows two v bonds i n CH 2C1 2 at 2067 and 1975 c m - 1 . 6 1 One of the probable o v e r a l l processes o c c u r r i n g i s shown i n r e a c t i o n 4.40, i . e . (n 5-C 5H 5)W(CO) 3CH 3 + AgBF 4 > (n 5-C 5H 5)W(CO) 3BF U + CH 3' + Ag° (4.40) 152 The accompanying reappearance of ("n b-C 5H 5)W(CO) 3CH 3 i s s u r p r i s i n g . I t i s evident, however, from the IR monitoring of the progress of the r e a c t i o n , that the s t a r t i n g m a t e r i a l ' s IR absorptions reappear only a f t e r [ ( T ) 5 - C 5 H 5 ) W ( C O ) 3 ] + i s generated. I t may be that the adduct i n i t i a l l y formed i n t e r a c t s with the c a t i o n to l i b e r a t e (T) 5-C 5H 5 )W(CO) 3CH 3. Thus AgBF^ does not appear to be a c t i n g as a simple oxidant, although u l t i m a t e l y e l e c t r o n t r a n s f e r does occur. I t seems f i r s t to f u n c t i o n as an e l e c t r o -p h i l e to form an adduct, and then e l e c t r o n t r a n s f e r occurs afterwards. A s i m i l a r r e a c t i o n attempted w i t h ( T I 5 - C 5 H 5 ) M O ( C O ) 3 C H 3 proceeds only very s l o w l y , the m a j o r i t y of the s t a r t i n g m a t e r i a l being unconsumed a f t e r 3.5 h. Considering the s i m i l a r i t i e s i n the CVs and o x i d a t i o n p o t e n t i a l s of ( T ) 5 - C 5 H 5 ) W ( C O ) 3 C H 3 and (n 5-C 5H 5)Mo(CO) 3CH 3, they ought to react s i m i l a r l y to simple o x i d a n t s . The much greater r e s i s t a n c e of ( T ) 5 - C 5 H 5 ) M O ( C O ) 3 C H 3 to AgBF^ al s o argues against simple e l e c t r o n removal. I t also appears that AgBF^ Is a somewhat b e t t e r e l e c t r o p h i l e toward ( T I 5 - C 5 H 5 ) W ( C O ) 3CH 3 than H g C l 2 , which r e p o r t e d l y does not react with e i t h e r the molybdenum- or tungsten-methyl compounds. IV) Summary In general the n i t r o s y l - a l k y l complexes included i n t h i s study are harder to o x i d i z e than t h e i r corresponding carbonyl analogues. I t has been proposed that t h i s feature and the divergent chemical r e a c t i v i t y of the n i t r o s y l - and c a r b o n y l - a l k y l compounds towards e l e c t r o p h i l e s p r e v i o u s l y reported by other workers are i n d i c a t i v e of d i f f e r i n g mechanisms f o r these 153 reactions. The electrochemical data are not consistent with an oxidative process believed to be dominant for e l e c t r o p h i l i c cleavage reactions of 7 8 87 many carbonyl-alkyl compounds. ' This leaves d i r e c t attack of an e l e c t r o p h i l e at the metal-alkyl group of these n i t r o s y l - a l k y l species as 7 8 a the most l i k e l y a l t e r n a t i v e mechanism. This proposal i s not i n c o n s i s -tent with the observations found i n this study. Many of the reactions of the n i t r o s y l - a l k y l compounds with e l e c t r o -philes and oxidants are complex, and no doubt w i l l require considerable e f f o r t to c l a r i f y . The most i n t e r e s t i n g reaction encountered in t h i s work i s the formation of [(T) 5-C 5H 5)Cr(NO) 2(CH2NOH) ] + from (n 5-C 5H 5)Cr(NO) 2CH 3 and NOPFg. This new formaldoxime complex arises from i n s e r t i o n of NO into the Cr-CH 3 bond. Whether this i n s e r t i o n i s intramolecular or r e s u l t s from d i r e c t attack of N0+ at the Cr-CH 3 bond remains to be seen. In any event this type of reaction i s i n t r i g u i n g both from a mechanistic viewpoint and the p r a c t i c a l aspect of C-N bond formation. 154 Chapter Five THE REDUCTION CHEMISTRY OF COMPLEXES CONTAINING THE {(TI 5-C 5H 5)M(NO) 2} (M - Cr, Mo, W) GROUP I) Introduction As has been demonstrated i n Chapters 3 and 4, organometallic n i t r o s y l complexes e x h i b i t unique redox p r o p e r t i e s which can have profound e f f e c t s on t h e i r chemistry. A wide v a r i e t y of d i n i t r o s y l complexes of the types (n 5-C 5 H 5)M(N0) 2Y (M = Cr, Mo, W, Y = h a l i d e s 3 9 , a l k y l g r o u p s ; 3 8 , 4 1 M = W, Y = H ) 6 8 ' 1 2 2 and [ ( T I 5 - C 5 H 5 ) M ( N O ) 2 L ] + (M = Cr, L = a m i n e s ; 4 0 M = Mo, W, 68 128 68 L = phosphines, phosphites; ' M = W, L = n 2 - C 8 H L L T ) have become a v a i l -able i n the l a s t s e v e r a l years. Some of these compounds e x h i b i t p r e v i o u s l y u n a n t i c i p a t e d r e a c t i v i t y toward n u c l e o p h i l e s . Thus, f o r example, while many organometallic c a r b o n y l - h a l i d e complexes can be coverted to a l k y l or 85 a r y l d e r i v a t i v e s using Grignard reagents, s i m i l a r r e a c t i o n s w i t h ( T ) 5 - C 5 H 5 ) M ( N O ) 2 X (M = MO, W; X = h a l i d e s ) lead only to decomposition of the 38 n i t r o s y l compounds i n t o i n t r a c t a b l e m a t e r i a l s . Only f o r ( T ) 5 - C 5 H 5 ) C r ( N O ) 2 X ( X = h a l i d e s ) does t h i s r e a c t i o n y i e l d corresponding a l k y l or a r y l compounds. 8 5 The ( T I 5 - C 5 H 5 ) M ( N O ) 2 R (M = Mo, W; R = a l k y l , a r y l groups) compounds can be prepared from ( T ) 5 - C 5 H 5 ) M ( N O ) 2C1 (M = Mo, W) and (r | 5 - C 5 H 5)M(NO) 2 B F i + precursors only be using r e l a t i v e l y m i l d a l k y l a t i n g -. , 38 .,68 68 agents such as organoaluminum, organotin or organoboron compounds, as a p p r o r i a t e . In a d d i t i o n the ( T I 5 - C 5 H 5 ) M ( N O ) 2 C 1 (M = Mo, W) compounds 38 t o t a l l y decompose upon treatment w i t h reducing metals such as Na or Zn 155 and others, while ( n 5 - C 5 H 5 ) C r ( N O ) 2 C l can be converted c l e a n l y to the dimer [(r | 5 - C 5 H 5 ) C r ( N O ) 2 ] 2 under s i m i l a r c o n d i t i o n s . 3 8 ' ^ The analogous dimers [ ( T ) 5 - C 5 H B ) M ( N O ) 2 ] 2 ( M = M°y w ) h a v e v e t t o b e i s o l a t e d , although they have 28 been the object of much s y n t h e t i c e f f o r t . Recently, r e a c t i o n s of the c a t i o n i c complexes [ ( T I 5 - C 5 H 5 ) M ( N O ) 2 L ] P F 6 [L = PPh 3, P(OPh) 3, P(OMe) 3] with 46 a l k o x i d e s have been reported. The a n t i c i p a t e d outcome of these r e a c t i o n s had been the synthesis of a l k y l n i t r i t e complexes, i .e. [ ( r i 5 - C 5 H 5 ) M ( N O ) 2 L ] + + R O " > ( T ) 5 - C 5 H 5 ) M ( N O ) ( L ) ( N ( 0 ) O R ) (5.1) Instead, the r a d i c a l s [ ( r | 5 - C 5 H 5 ) M ( N O ) 2 L ] • r e s u l t i n these r e a c t i o n s . This i n i t i a l l y s u r p r i s i n g transformation i s beli e v e d to be due to a very f a c i l e , r e v e r s i b l e reduction of the c a t i o n i c s t a r t i n g m a t e r i a l s , a feature which has been revealed by c y c l i c voltammetry. In view of these s o r t s of react i o n s and the v a r i e t y of d i n i t r o s y l compounds a v a i l a b l e , i t becomes of i n t e r e s t to i n v e s t i g a t e the reduction behaviour of these species. In general most carbonyl-containing h a l i d e , hydride, a l k y l and a r y l compounds that have been i n v e s t i g a t e d e l e c t r o c h e m i c a l l y , undergo i r r e v e r s i b l e reduc-t i o n with s c i s s i o n of the m e t a l - h a l i d e , -hydride, - a l k y l or - a r y l bonds. This w i l l be seen not to be so prevalent f o r complexes of the { ( T I 5 - C 5 H 5 ) M ( N O ) 2 } ( M = Group 6 metal) fragments. Indeed, once again an i n v e s t i g a t i o n of the redox p r o p e r t i e s of these n i t r o s y l complexes by c y c l i c voltammetry leads to a c l e a r e r understanding of t h e i r r e a c t i v i t y . F u r t h e r -more, by much the same means as demonstrated i n Chapter 3, a v a r i e t y of r a d i c a l anion complexes have been i s o l a t e d and c h a r a c t e r i z e d . 156 II) Experimental Section The c y c l i c voltammetric and s y n t h e t i c work was c a r r i e d out as described i n the preceding chapters. D i e t h y l ether was d i s t i l l e d from Na[Ph 2CO] and hexanes from L i A l H ^ . A l l s t a r t i n g m a t e r i a l s were prepared e i t h e r according to procedures described i n Chapter 4, or by published methods, and t h e i r p u r i t y was determined by elemental analyses and/or spectroscopic techniques. C y c l i c voltammograms were recorded f o r the reductions of (n 5-C 5H 5)M(NO) 2R (M = Cr, R = CH 3; M = Mo, R = CH 3, C 2H 5; M = W, R = CH 3, C 2H 5, H), ( T I 5 - C 5 H 5 ) M (N O ) 2 C 1 (M = M O , W ) , [(n 5-C 5H 5)M(NO) 2L]BF 1 + [M = Mo, L = PPh 3; M = W, L = P(OMe) 3, PPh 3, T i 2-CgH 1 4 J and W ( N 0 ) 2 L 2 C 1 2 [L = P(OMe) 3, PPh 2Me]. The ESR spectra were obtained with e i t h e r a Varian E3 37 spectrometer or a spectrometer and i n t e r f a c e d computer system operated by Dr. F.G. Herr i n g . The 1H NMR spectra were recorded with a Varian EM360 instrument. The X-ray c y r s t a l l o g r a p h i c work was c a r r i e d out by Dr. F .W.B. E i n s t e i n and Dr. R. Jones. Preparation of [ ( n 5 - C 5 H 5 ) 2 C o ] [(TI 5-C 5H 5)W(NO )2CH 3] . A medium-q 84 po r o s i t y f r i t was charged with s o l i d ( n 3-C 5H 5) 2Co (0.465 g, 2.46 mmol), topped with a septum and attached to a three-necked f l a s k . To t h i s f l a s k was added (n 5-C 5H 5 ) W(NO) 2CH 3 (0.800 g, 2.47 mmol). D i e t h y l ether (-20 mL) was added to the f l a s k by cannulation and l i k e w i s e —50 mL of E t 2 0 was added to the f r i t . The cobaltocene s o l u t i o n was f i l t e r e d i n t o the r a p i d l y s t i r r e d ( T I 5 - C5H5)W(N O) 2CH 3 s o l u t i o n . Immediately the r e a c t i o n mixture became green-brown and a dark, m i c r o c r y s t a l l i n e s o l i d p r e c i p i t a t e d which 157 was c o l l e c t e d by f i l t r a t i o n and washed with s e v e r a l portions of E t 2 0 (~100 mL i n t o t a l ) u n t i l the washings were c o l o u r l e s s . The s o l i d was d r i e d i n vacuo (5 * 1 0 - 3 mm) at room temperature for a few hours. This procedure gave 0.814 g (65% y i e l d ) of [ ( r i 5 - C 5 H 5 ) 2Co] [ (T] 5-C 5H 5)W (NO) 2CH 3] as a purple-black, m i c r o c r y s t a l l i n e s o l i d which was both a i r - and thermally s e n s i t i v e . IR (Nujol mull) v N Q 1511(s), 1427(s) cm - 1, also 3100(m), 3087(m), 3069(sh), 1414(sh), 1361(m), 1263(w), 1172(w), 1112(w), 1061(w), 1013(m), 946(w), 906(w), 865(m), 847(w), 816(m), 803(m) cm - 1. Anal, c a l c d f o r C 1 6H 1 8CoN 20 2W: C, 37.45; H, 3.54; N , 5.46. Found: C, 37.15; H, 3.46; N , 5.25. The ESR spectra of t h i s and the f o l l o w i n g compounds displayed basic f i v e - l i n e patterns i n the approximate i n t e n s i t y r a t i o s 1:2:3:2:1. (These are discussed i n the Results and Discussi o n Section.) P r e p a r a t i o n of [ ( T I 5 - C 5 H 5 ) 2 C O ] [ ( n 5 - C 5 H 5 ) M o ( N 0 ) 2 C H 3 ] . This r e a c t i o n was c a r r i e d out using ( T I 5 - C 5 H 5 ) M O (N O ) 2CH 3 (0.450 g, 1.91 mmol) and ( T) 5-C 5H 5) 2Co (0.360 g, 1.90 mmol) i n a manner i d e n t i c a l to that described above f o r the tungsten-containing analogue. Dark m i c r o c r y s t a l s p r e c i p i -tated from a green-brown r e a c t i o n mixture, which were c o l l e c t e d by f i l t r a -t i o n , washed with E t 2 0 (~200 mL) u n t i l the washings were c o l o u r l e s s , and d r i e d i n vacuo (5 * 1 0 - 3 mm). This produced 0.510 g (63% y i e l d ) of [ ( T ) 5 - C 5 H 5 ) 2Co] [ ( T ] 5 - C 5 H 5 )M O (N O ) 2CH 3] as a green-black, m i c r o c r y s t a l l i n e s o l i d which e x h i b i t e d thermal and a i r - s e n s i t i v i t y : IR (Nujol mull) v 1526(s), 1445(s,br), [1443(s,br) with Nujol spectrum subtracted] cm - 1, a l s o 3101(m), 3094(sh), 1415(m), 1363(m), 1263(w), 1141(w), 1112(w), 1059(w), 1011(m), 945(w), 904(w), 866(m), 847(w), 807(m), 794(m) cm - 1. 158 Anal, c a l c d for C 1 6H 1 8CoMoN 20 2: C, 45.20; H, 4.27; N, 6.59. Found: C, 45.22; H, 4.33; N, 6.71. Attempted Reaction of ( T i 5 - C 5 H 5 ) C r ( N O ) 2 C H 3 with ( T I 5 - C 5 H 5 ) 2 C O . The same procedure as that used to prepare [ ( T | 5 - C 5 H 5 ) 2Co] [ ( T I 5 - C 5 H 5 ) W ( N O ) 2CH 3 ] 38 was employed i n t h i s experiment, using (n 5-C 5H 5)Cr(NO) 2CH 3 (0.192 g, 1.00 mmol) and ( T ] 5 - C 5 H 5 ) 2 C o (0.189 g, 1.00 mmol). A brown-green s o l u t i o n formed upon mixing E t 2 0 s o l u t i o n s (~40 mL t o t a l ) of these two reagents but no s o l i d p r e c i p i t a t e d . An IR spectrum of the s o l u t i o n d isplayed NO absorp-t i o n s only f o r (n 5-C 5H 5)Cr(NO) 2CH 3 (1777, 1669 c m - 1 ) . The mixture was s t i r r e d overnight with no f u r t h e r change. Reaction of (n 5-C 5H 5)Cr(NO) 2CH 3 with (Ti 5-C 5H 5)Fe ( n 6-C 6Me 6). In the same way as described above, an E t 2 0 (~50 mL) s o l u t i o n of ( n 5 - C 5 H 5 ) F e ( T i 6 - C 6 M e 6 ) 4 5 (0.280 g, 0.989 mmol) was f i l t e r e d i n t o a r a p i d l y s t i r r e d s o l u t i o n of (Ti 5-C 5H 5)Cr(NO) 2CH 3 (0.190 g, 0.990 mmol) i n E t 2 0 (~20 mL). An instantaneous r e a c t i o n occurred and a grey, f l o c c u l e n t p r e c i p i t a t e formed, l e a v i n g a l i g h t green s o l u t i o n . The s o l i d was c o l l e c t e d by f i l t r a t i o n , washed i n s e v e r a l portions with E t 2 0 (~150 mL) and d r i e d i n vacuo (5 x 10" 3 mm) f o r a few hours to y i e l d 0.27 g of a grey powder, which was extremely pyrophoric i n a i r . IR (Nujol mull) 1555(vs), 1509(s) cm"1, also 3084(m), 3074(sh), 1416(m), 1261(w), 1162(w), 1121(w), 1110(w), 1072(m), 1010(m), 969(m), 857(m), 785(m) cm - 1. Elemental analyses of t h i s powder, prepared on two separate occasions, showed i t to be q u i t e v a r i a b l e i n ni t r o g e n content: C, 56.74; H, 6.73; N, 3.42 and C, 56.57; H, 6.44; N, 4.70. 159 Pre p a r a t i o n of [ ( T) 5-C 5H 5) 2Co] [ ( T ) 5 - C 5 H 5 ) MO ( N O ) 2 C 2 H 5 ] . A s o l u t i o n of ( T I 5 - C 5 H 5 ) 2 C O (0.163 g, 0.862 mmol) i n E t 2 0 (~50 mL) was f i l t e r e d i n t o a r a p i d l y s t i r r e d s o l u t i o n of ( T I 5 - C 5 H 5 ) M O ( N O ) 2 C 2 H 5 (0.216 g, 0.864 mmol) i n E t 2 0 (~20 mL). A green-brown s o l u t i o n r e s u l t e d but no p r e c i p i t a t i o n occurred, u n l i k e p r e v i o u s l y . The s o l u t i o n was cooled wi t h a Dry I c e / acetone bath and a f t e r ~10 min a m i c r o c r y s t a l l i n e s o l i d formed. This was allowed to s e t t l e , the supernatant s o l u t i o n was removed by cannulation and the s o l i d was washed with E t 2 0 (2 x ~15 mL) at -78?C, the washings being removed again by ca n n u l a t i o n . The green-black m i c r o c r y s t a l l i n e s o l i d was dr i e d i n vacuo (5 x 10" 3 mm) f o r a few hours to give 0.195 g (52% y i e l d ) of [ ( T I 5 - C 5 H 5 ) 2 C O ] [ ( T I 5 - C 5 H 5 ) M O ( N O ) 2 C 2 H 5 ] : IR (Nujol mull) 1527(s), 1458(s), [1458(s) with Nujol spectrum subtracted] cm - 1, also 3104(m,br), 1415(m), 1361(w), 1262(w), 1145(w), 1113(w), 1059(w), 1013(m), 893(w), 874(w), 861(m), 847(w), 801(m), 788(m) cm - 1. Anal, c a l c d f or C 1 7 H 2 0CoMoN 20 2: C, 46.49; H , 4.59; N, 6.38. Found: C, 46.40; H , 4.57; N, 6.47. I f the r e a c t i o n was c a r r i e d out with more concentrated s o l u t i o n s (0.42 mmol of each reactant i n a t o t a l of 20 mL of E t 2 0 ) at room tempera-ture a green-black m i c r o c r y s t a l l i n e s o l i d d i d p r e c i p i t a t e upon mixing as observed f o r the above described p r e p a r a t i o n s . S u i t a b l e s i n g l e c r y s t a l s f o r an X-ray c r y s t a l l o g r a p h i c a n a l y s i s were obtained by f i l t e r i n g a s o l u t i o n of ( n 5 - C 5 H 5 ) 2 C o (0.110 g, 0.582 mmol) i n E t 2 0 (~40 mL) i n t o an E t 2 0 s o l u t i o n (~30 mL) of ( n 5 - C 5 H 5 ) M o ( N O ) 2 C 2 H 5 (0.150 g, 0.600 mmol). The thoroughly mixed green-brown s o l u t i o n was slowly cooled to -25°C. Over the course of ~6h large diamond-shaped, green-black pl a t e s formed. The mother 160 l i q u o r was removed by syringe and the c r y s t a l s were d r i e d i n vacuo (5 x 1 0 - 3 mm) f o r a few hours. Preparation of [ ( T ) 5 - C 5 H 5 ) 2 C O ] [ ( t i 5 - C 5 H 5 ) W ( N 0 ) 2 H ] . A s o l u t i o n of ( T I 5 - C 5 H 5 ) 2 C O (0.095 g, 0.503 mmol) i n hexanes (~40 mL) was f i l t e r e d i n t o a s t i r r e d s o l u t i o n of ( T ] 5 - C 5 H 5 ) W ( N O ) 2 H 6 8 ' 1 2 2 (0.155 g, 0.500 mmol), al s o i n hexanes (~30 mL). A f i n e , dark brown s o l i d p r e c i p i t a t e d from s o l u t i o n immediately. This was c o l l e c t e d by f i l t r a t i o n , washed with hexanes (~100 mL) u n t i l the washings were only f a i n t l y coloured, and d r i e d i n vacuo (5 x 1 0 - 3 mm) to give 0.163 g (65% y i e l d ) of [ ( T I 5 - C 5 H 5 ) 2 C O ] [(r) 5-C 5 H 5)W(NO ) 2 H ] as a dark brown, thermally s e n s i t i v e powder: IR (Nujol mull) v N 0 1521(s), 1445(s,br), [1438(s) with Nujol spectrum substracted] cm - 1, als o 3073(m,br), 1848(m), 1411(m), 1354(w), 1262(w), 1110(w), 1057(w), 1009(m), 863(w), 851(m), 812(m) cm - 1. Anal, c a l c d f o r C 1 5 H 1 6CoN 20 2W: C, 36.10; H , 3.23; N, 5.61. Found: C, 35.09; H , 3.00; N, 5.60 and C, 35.20; H , 3.17; N, 5.50 ( V 2 0 5 used as a combustion a d d i t i v e f o r the second a n a l y s i s ) . I f E t 2 0 was used as the solvent instead of hexanes, the r e a c t i o n appeared to proceed s i m i l a r l y , a dark brown, m i c r o c r y s t a l l i n e s o l i d p r e c i -p i t a t i n g from s o l u t i o n as above. When t h i s s o l i d was washed with E t 2 0 though, the washings r e t a i n e d a more intense c o l o u r a t i o n than when hexanes were used and the C, H and N analyses of the product e x h i b i t e d much lower ni t r o g e n content. Preparation of [(n5-C5H5)2Co] [( T J 5-C 5H 5)W(N0) 2D] . This compound was prepared i n the same manner as the hydrido analogue from (n 5-C 5 H 5)W(NO) 2D 6 8 (0.150 g, 0.482 mmol) and ( n 5 - C 5 H 5 ) 2 C o (0.091 g, 0.481 161 mmol), and was i s o l a t e d as a s l i g h t l y impure, dark brown powder (0.165 g, -69% y i e l d ) : IR (Nujol mull) 1513(s), 1447(s,br) [1439(s) with Nujol spectrum substracted] cm - 1, als o 3086(m), 3075(m), 1525(m), 1411(m), 1337(w), 1316(w), 1262(w), 1112(w), 1060(w), 1008(m), 862(m), 811(m) cm - 1. Anal, c a l c d f o r C 1 5 H 1 5 C o D N 2 0 2 W : C, 36.03; H , 3.43; N, 5.60. Found: C, 35.29; H , 3.18; N, 5.09. Preparation of [(n 5-C 5H 5) 2Co][(n 5-C 5H 5 )W (N0) 2 C1]. A s o l u t i o n of <; 39 ( T i b-C 5 H 5 ) W(N0) 2C1 (0.344 g, 0.999 mmol) i n E t 2 0 (-40 mL) was treated with a f i l t e r e d s o l u t i o n of ( n 5 - C 5 H 5 ) 2 C o (0.189 g, 1.00 mmol) i n E t 2 0 (-50 mL). I n s t a n t l y a grey, f l o c c u l e n t p r e c i p i t a t e formed which was c o l l e c t e d by f i l t r a t i o n , washed with E t 2 0 (3 * 20 mL) and d r i e d i n vacuo f o r ~2h. This produced 0.471 g (88% y i e l d ) of [ ( T I 5 - C 5 H 5 ) 2Co ] [ ( T ) 5 - C 5 H 5 ) W (N 0 ) 2C1] as a grey, h i g h l y pyrophoric and quite thermally s e n s i t i v e powder: IR (Nujol mull) v N Q 1708(w), 1585(sh), 1534(s), 1456(s) cm - 1, also 3090(m,br), 1413(s), 1262(w), 1112(w), 1059(w), 1009(m), 863(m), 814(m) cm - 1. Anal, c a l c d f o r C 1 5 H 1 5 C l C o N 2 0 2 W : C, 33.77; H , 2.83; N, 5.25. Found: C, 33.45; H , 2.80; N, 4.97. Preparation of [(n 5-C 5H 5) 2Co] [(ti 5-C 5H 5)Mo(NO) 2Cl]. The same procedure as described above to prepare the tungsten analogue was employed i n t h i s r e a c t i o n s t a r t i n g from ( t ) 5 - C 5 H 5 ) 2 C o (0.189 g, 1.00 mmol) and c 39 ( T ] 5 - C 5 H 5 ) M O ( N O ) 2 C 1 (0.256 g, 0.998 mmol). This generated 0.347 g (78% y i e l d ) of grey-green, pyrophoric and thermally s e n s i t i v e [ ( T ) 5 - C 5 H 5 ) 2 C o ] [ ( T ) 5 - C 5 H 5 ) M o ( N O ) 2 C l ] : IR (Nujol m ull) v 1558(s,br), 162 1485(s,br), [1490(s,br) with N u j o l spectrum substracted] cm - 1, al s o 3092(m,br), 1413(m), 1355(sh), 1261(w), 1112(w), 1059(m), 1012(m), 863(m), 807(m) cm - 1. Anal , c a l c d for C 1 5H 1 5ClCoMoN 20 2: C, 40.43; H, 3.39; N, 6.29. Found: C, 40.61; H, 3.50; N, 6.08. Preparation of (Ti 5-C 5H 5)W(N0) 2P(0Me)3. A s o l u t i o n of [(Ti 5-C 5H 5)W(NO) 2P(OMe) 3]BF 1 + (0.541 g, 1.04 mmol) [This compound was Aft prepared from (n 5-C 5H 5)W(NO) 2 a ¥ k and P(0Me) 3. ] i n CH 2C1 2 (~10 mL) was trea t e d with s o l i d ( n 5 - C 5 H 5 ) 2 C o (0.197 g, 1.04 mmol) and s t i r r e d . The s o l u t i o n i n s t a n t l y became an intense red-purple c o l o u r . An IR spectrum of t h i s s o l u t i o n d isplayed v absorptions at 1608(s) and 1542(s) cm - 1. Addi-t i o n of E t 2 0 (~80 mL) to the r e a c t i o n mixture r e s u l t e d i n the p r e c i p i t a t i o n of a pale y e l l o w s o l i d . The mixture was f i l t e r e d through a medium-porosity f r i t and the f i l t r a t e was taken to dryness under reduced pressure. The s o l i d , purple residue was d i s s o l v e d i n CH 2C1 2 (~5 mL) and E t 2 0 (~100 mL) was added. The mixture was cooled to -78°C with a Dry Ice/acetone bath and a purple, m i c r o c r y s t a l l i n e s o l i d p r e c i p i t a t e d over the course of 1.5 h. The supernatant s o l u t i o n was removed by cannulation, the s o l i d was washed with hexanes (~10 mL) and the washings were s i m i l a r l y removed. The s o l i d was d r i e d i n vacuo (5 * 1 0 - 3 mm) f o r a few hours to give 0.211 g (47% y i e l d ) of (n 5-C 5H 5)W(NO) 2P(0Me) 3 as a purple, m i c r o c r y s t a l l i n e s o l i d : IR (CH 2C1 2) v N Q 1608(s), 1542(s) cm"1; IR (Nujol mull) v N Q 1593(s), 1533(s) cm"1, al s o 3105(m), 1353(w), 1263(w), 1177(m), 1049(m), 1021(s), 931(w), 844(m), 808(m), 794(m), 759(m) cm - 1. 163 Anal, calcd f or CgH 1 1 +N 20 5PW: C, 22.19; H, 3.26; N, 6.47. Found: C, 21.89; H, 3.28; N, 6.32. Preparation of (Ti 5-C 5H 5)W(NO) 2PPh 3. A mixture of (n 5-C 5H 5) 2Co (0.178 g, 0.942 mmol) and [ ( T I 5 - C 5 H 5 ) W ( N O ) 2 P P h 3 ] B F 1 + 6 8 (0.620 g, 0.942 mmol) was s t i r r e d i n CH 2C1 2 (~25 mL) f o r ~10 min. An intense red-purple c o l o u r a -t i o n developed r a p i d l y i n the s o l u t i o n . Hexanes (~10 mL) were c a r e f u l l y added u n t i l a pale yellow s o l i d had p r e c i p i t a t e d , accompanied by a small amount of purple s o l i d . The mixture was f i l t e r e d through a medium p o r o s i t y f r i t and the f i l t r a t e was taken to dryness i n vacuo to give 0.290 g (54% y i e l d ) of (n 5-C 5H 5)W(NO) 2PPh 3 as a purple powder: I R (CH 2C1 2) v N Q 1598(s), 1532(s) cm - 1; I R ( N u j o l mull) v N Q 1594(s), 1527(s) cm - 1, al s o 1583(m), 1570(w), 1479(w), 1436(m), 1261(w), 1185(w), 1158(w), 1095(m), 1071(w), 1028(w), 1014(w), 851(w), 815(m), 749(m), 742(w), 706(w), 695(m) cm - 1. Anal, c a l c d f o r C 2 3H 2 0N 2O 2PW: C, 48.35; H, 3.50; N, 4.90. Found: C, 48.64; H, 3.69; N, 5.00. Reaction of [ ( T I 5 - C 5 H 5 ) 2 C O ] [ (n 5 -C 5H 5)Mo(N0) 2CH 3] with [ ( T i 5 - C 5 H 5 ) 2 F e ] B F H . A mixture of [( n 5-C 5H 5) 2Co ] [ ( n 5-C 5H 5)Mo(NO) 2CH 3 ] (0.190 g, 0.447 mmol) and [ ( n 5 - C 5 H 5 ) 2 F e ] B F 1 + (0.120 g, 0.440 mmol) was s t i r r e d i n CH 2C1 2 (~25 mL) at -78°C for ~5 min. The mixture was then allowed to warm to room temperature w i t h s t i r r i n g to produce a green s o l u t i o n , an I R spectrum of which e x h i b i t e d n i t r o s y l absorptions at 1730(s) and 1636(s) cm - 1. The volume of the s o l u t i o n was reduced to ~10 mL i n vacuo whereupon a yellow s o l i d p r e c i p i t a t e d . A d d i t i o n of E t 2 0 (~20 mL) p r e c i p i t a t e d more of the yellow powder which was c o l l e c t e d by f i l t r a t i o n to give 0.09 g (73% y i e l d ) of [ ( T I 5 - C 5 H 5 ) J C O J B F^ which was i d e n t i f i e d by 164 comparison of i t s Nujol-mull IR, and 1H NMR [(CD 3) 2CO] spectra w i t h those of an authentic sample. The f i l t r a t e from which [ ( T ) 5 - C 5 H 5 ) 2Co]BFi + was c o l l e c t e d was taken to dryness i n vacuo. The s o l i d residue was d i s s o l v e d i n E t 2 0 (~3 mL) and the r e s u l t a n t green s o l u t i o n was t r a n s f e r r e d by syringe onto an alumina column (Woelm, n e u t r a l , a c t i v i t y 1) (2 x 10 cm) made up i n E t 2 0 . A yellow band developed which was elu t e d from the column with E t 2 0 . The solvent was removed i n vacuo to give 0.03 g (36% y i e l d ) of ( T ) 5 - C 5 H 5 ) 2Fe which was i d e n t i f i e d by comparison of i t s NMR (CDC1 3) spectrum with that of an authentic sample and i t s c h a r a c t e r i s t i c l o w - r e s o l u t i o n mass spectrum (probe temperature 120°C), m/z_ 186 (P+, most intense parent i o n ) . F i n a l l y , a green band was eluted from the column with E t 2 0 . The solvent was removed i n vacuo and from the s o l i d residue was sublimed, (5 x 1 0 - 3 mm) at 40-60°C onto a water-cooled probe, 0.05 g (47% y i e l d ) of ( T I 5 - C 5 H 5 ) M O ( N O ) 2CH 3: IR (hexanes) v 1739(s), 1651(s) cm - 1; low-r e s o l u t i o n mass spectrum (probe temperature 120°C), m/z_ 236 ( P + , most intense parent i o n ) . Reaction of [ ( T I 5 - C 5 H 5 ) 2 C O ] [ ( T I 5-C 5H 5)W(NO) 2CH 3] w i t h AgBF^. A mixture of AgBF 4 (0.048 g, 0.25 mmol) and purple-black [ ( T ) 5 - C 5 H 5 ) 2 C O ] [ ( T ) 5 - C 5 H 5 ) W ( N O ) 2 C H 3 ] (0.128 g, 0.250 mmol) was s t i r r e d i n CH3CN (~10 mL) at -25°C f o r ~5 min. A black p r e c i p i t a t e formed and the mixture was warmed to room temperature. An IR spectrum of the green super-natant s o l u t i o n e x h i b i t e d v X T^ absorptions at 1706(s) and 1620(s) cm - 1. No NO other NO bonds were apparent. The solvent was removed i n vacuo and 0.04 g (49% y i e l d ) of (T) 5-C 5H 5)W(NO) 2CH 3 was sublimed (5 x 1 0 - 3 mm) from 165 the residue at 40-60°C: IR (hexanes) v N Q 1720(s), 1639(s) cm - 1; low-r e s o l u t i o n mass spectrum (probe temperature 120° C ) , m/z_ 324 ( P + , most intense parent i o n ) . The s o l i d residue was extracted with CH 2C1 2 and the supernatant s o l u t i o n was f i l t e r e d away from the remaining s o l i d m a t e r i a l . The solvent was removed i n vacuo and the r e s u l t a n t yellow s o l i d was i d e n t i f i e d as [ ( • r i 5 - C 5 H 5 ) 2 C o ] B F l t by i t s c h a r a c t e r i s t i c Nujol-mull IR spectrum i n comparison with that of an authentic sample. Reaction of [ ( T 1 5 - C 5 H 5 ) 2 C O ] [ ( T J 5 - C 5 H 5 ) W ( N O ) 2 H ] with AgBF^. This r e a c t i o n was e f f e c t e d i n the same manner as the r e a c t i o n f o r the methyl analogue described above, with AgBF^ (0.020 g, 0.10 mmol) and [ ( T I 5 - C 5 H 5 ) 2 C O ] [ ( T I 5 - C 5 H 5 ) W ( N O ) 2 H ] (0.050 g, 0.10 mmol) i n CH 2C1 2 (-10 mL) i n i t i a l l y at -78°C . The mixture was allowed to warm to room temperature. An IR spectrum of the supernatant s o l u t i o n d i s p l a y e d v bands only at 1721(s) and 1635(s) cm - 1. The solvent was removed i n vacuo and from the residue was sublimed (5 x 1 0 - 3 mm) at 40-60°C onto a water-cooled probe, ( T I 5 - C 5 H 5 )W (N O ) 2 H . ( y i e l d i nestimable due to small r e a c t i o n s c a l e ) : low-r e s o l u t i o n mass spectrum (probe temperature 120° C ) , m/z_ 310 ( P + , most intense parent i o n ) . Reaction of [ ( n 5 - C 5 H 5 ) 2 C o ] [ ( n 5 - C 5 H 5 ) M o ( N 0 ) 2 C l ] with AgBF^. S i m i l a r l y to the two p r e v i o u s l y described o x i d a t i o n s , AgBF^ (0.146 g, 0.749 mmol) and [ ( T I 5 - C 5 H 5 ) 2Co ] [ ( T ] 5 - C 5 H 5 ) M O(N O) 2C1] (0.334 g, 0.748 mmol) were s t i r r e d together i n CH 3 CN (-10 mL) at -25° C f o r -5 min and then the mixture was warmed to room temperature. Once again only two NO absorptions were observed i n the IR spectrum of the supernatant s o l u t i o n , at 1757(s) and 166 1668(s) cm . The green s o l u t i o n was taken to dryness i n vacuo and the s o l i d residue was treated with CH 2C1 2 (~20 mL). This s o l u t i o n was f i l t e r e d through a 1x3 cm F l o r i s i l column made up i n CH 2C1 2. The column was washed with CH 2C1 2 u n t i l the washings were only f a i n t l y coloured. The combined f i l t r a t e s were taken to dryness i n vacuo and the s o l i d was r e d i s s o l v e d i n CH 2C1 2 (~5 mL). Hexanes (~5 mL) were added to induce p r e c i p i t a t i o n of [(n 5-C 5H 5) 2Co]BF 1 + which was c o l l e c t e d by f i l t r a t i o n and i d e n t i f i e d by i t s c h a r a c t e r i s t i c IR spectrum. The f i l t r a t e was concentrated i n vacuo and a green, m i c r o c r y s t a l l i n e s o l i d p r e c i p i t a t e d which was c o l l e c t e d onto a s i n t e r e d glass f i l t e r to give 0.09 g (47% y i e l d ) of ( T ) 5 - C 5 H 5 ) M O ( N O ) 2 C 1 : IR (CH 2C1 2) v N Q 1760(s), 1669(s) cm - 1; l o w - r e s o l u t i o n mass spectrum (Probe temperature 120°C), m/z_ 258 ( P + , most intense parent i o n ) . Reaction of (Tj 5-C 5H 5)W(NO) 2P(OMe) 3 with AgBF^. A s o l u t i o n of (Ti 5-C 5H 5)W(NO) 2P(OMe) 3 (0.108 g, 0.249 mmol) i n CH 2C1 2 (~10 mL) was tr e a t e d with s o l i d AgBF^ (0.049 g, 0.25 mmol). The i n i t i a l l y red-purple s o l u t i o n r a p i d l y became b r i g h t green, and a dark, m e t a l l i c p r e c i p i t a t e formed. The mixture was f i l t e r e d through a medium p o r o s i t y f r i t and the f i l t r a t e was concentrated to ~5 mL under reduced pressure. This s o l u t i o n was tr e a t e d with E t 2 0 (~20 mL) which r e s u l t e d i n the p r e c i p i t a t i o n of a green s o l i d . The s o l i d was c o l l e c t e d by f i l t r a t i o n , washed with E t 2 0 (2x~10 mL) and d r i e d i n vacuo (5 x 10" 3 mm) for a few hours to give b r i g h t green [(n 5-C 5H 5)W(NO) 2P(OMe) 3]BF l i: IR (Nujol mull) v 1767(s), 1689(s) cm"1; XH NMR (CDC1 3) 6 6.40 (d, 5H, 3 J 3 1 p _ l H = °' 9 H z . C 5 i s ) > 3 ' 8 8 <d» 9 H » 3 j 3 1 p - l H = 12.0 Hz, P ( 0 C H 3 ) 3 ) . 167 III) Results and Discussion C y c l i c Voltammetry Studies, a) (T) 5-C 5H 5)Cr(NO) 2CH 3. A c y c l i c voltammogram of (Ti 5-C 5H 5)Cr(NO) 2CH 3 i n CH 2C1 2 i s shown i n Figure 26. (Data f o r the reductions of the compounds involved i n t h i s Chapter are summarized i n Table I I I at the end of the CV s t u d i e s s e c t i o n . ) Under these c o n d i t i o n s t h i s compound e x h i b i t s a l a r g e l y r e v e r s i b l e one-electron reduction at E ^ / 2 = -1.01 V vs. SCE. The peak p o t e n t i a l s do separate somewhat with i n c r e a s i n g scan r a t e , being 70 mV at 0.06 V s - 1 and 90 mV at 0.28 V s - 1 . In 20 a d d i t i o n i / i increases with a r i s e i n scan ra t e from 0.87 at p,a p,c 0.06 V s - 1 to 0.99 at 0.28 V s - 1 . I f the scan i s extended to the solvent l i m i t as i n Figure 26, a second, low-current wave i s evident at E = p,c -1.87 V (0.12 V s - 1 ) and may r e s u l t from a product derived from the i n i t i a l l y formed r a d i c a l anion, i . e (n 5-C 5H 5)Cr(NO) 2CH 3=||^[(r| 5-C 5H 5)Cr(NO) 2CH 3] • > other products (5.2) I t seems u n l i k e l y that the second wave i s due to reduction of the r a d i c a l anion since i t s peak current i s so low. Secondly the reduction wave of the [(n 5-C 5H 5)M(NO) 2L]* (M = Mo, W; L = phosphines) n e u t r a l r a d i c a l s i s over 1.4 V negative of the r e d u c t i o n of the corresponding [ ( n 5 - C 5 H 5 ) M ( N O ) 2 L ] + c a t i o n s (see below), whereas the two cathodic waves of (r| 5-C 5H 5)Cr(NO) 2CH 3 are only separated by ~0.85 V. Figure 26. C y c l i c voltammogram of (n 5-C 5H 5)Cr(NO) 2CH 3 i n CH 2C1 2 (reduction) at a scan rate of 0.12 V s - 1 . 169 Of the compounds Inv e s t i g a t e d i n t h i s study, ( n b - C 5 H 5 ) C r ( N O ) 2 C H 3 i s the most d i f f i c u l t to reduce. I t also contrasts sharply with (T) 5-C 5H 5)Cr ( N O ) 2C1, the reduction of the l a t t e r being chemically i r r e v e r -s i b l e at the maximum a c c e s s i b l e scan rates (see Chapter 3). The methyl compound i s a l s o the only n e u t r a l {(T) 5-C 5H 5)M ( N O ) 2} - con t a i n i n g complex to e x h i b i t a second re d u c t i o n peak, and the only a l k y l compound with scan ra t e dependent peak p o t e n t i a l s . In c o n t r a s t , none of the carbon y l - c o n t a i n i n g a l k y l compounds discussed i n Chapter 4 [ i . e . ( T I 5 - C 5 H 5 ) M ( C O ) 3R (M = Cr, Mo, W); (T] 5-C 5H 5)Fe(CO) 2CH 3] e x h i b i t cathodic waves i n CH 2C1 2 under these c o n d i t i o n s . In CH3CN or DMF, however, (Ti 5-C 5H 5)Fe(CO) 2_ x(PPh3) xR (x = ^129a ^129b. R _ Q J C 6H 5 and other a l k y l or a r y l groups) do e x h i b i t a two-electron, i r r e v e r s i b l e r e d u c t i o n , or, as i n the case of some of the phosphine s u b s t i t u t e d compounds, two one-electron, i r r e v e r s i b l e r e d u c t i o n s , e.g. (T) 5-C 5H 5)Fe(CO) 2CH 3 + 2e - > [(T)5-C5H5)Fe(CO)2]~ + CH 3" (5.3) The i n i t i a l l y formed carbanion presumably a b s t r a c t s a proton from the so l v e n t . In any event, reduction of these compounds r e s u l t s i n s c i s s i o n of the Fe-R bonds. b) (T) 5-C 5H 5)M(NO) 2R (M » Mo, W; R - CH 3, C 2 H 5 ) . These four compounds are very s i m i l a r i n t h e i r reduction behaviour. The two methyl compounds reduce r e v e r s i b l y at E 1 / 2 = -0.83 V and the two e t h y l compounds undergo reduction at a s l i g h t l y more negative p o t e n t i a l of E 1 / 2 = -0.86 V, not s u r p r i s i n g l y . Figure 27 d i s p l a y s a c y c l i c voltammogram of 170 171 (r| b-C 5H 5)Mo(NO) 2 C 2 H 5 . In a l l four cases the peak p o t e n t i a l s are indepen-dent of scan rate up to ~0.2 V s - 1 , the maximum scan-rate region employed. The peak separations are 60-65 mV under these c o n d i t i o n s . The extent of chemical r e v e r s i b i l i t y i s s u b s t a n t i a l i n these systems, with the lowest i / i r a t i o being 0.95 f o r ( T I 5 - C 5 H 5 ) W ( N O ) 2 C 2 H 5 at 0.04 V s - 1 . For p,a p,c (Ti 5-CrHr)Mo(NO)oC 0Hc i / i i s u n i t y at even 0.05 V s - 1 . These f a c t s * * •» p,a p,c i n d i c a t e considerable s t a b i l i t y f o r the [(t| 5-C 5H 5)M(N0) 2R] • r a d i c a l anions. In CH3CN, (n 5-C 5H 5 ) W(NO) 2CH 3 e x h i b i t s only one reduction wave at E 1 / 2 = -0.85 V w i t h AE = 60 mV even up to 0.22 V s - 1 . The couple i s q u i t e P chemically r e v e r s i b l e , i / i ranging from 0.83 at 0.02 V s - 1 to 0.97 at p,a p,c 0.22 V s - 1 . No other cathodic wave i s present i n CH3CN out to -2.35 V. The polarographic r e d u c t i o n of the r e l a t e d complex (n 5-C 5H 5 ) W(CO) 3CH 3 i n CH3CN r e p o r t e d l y proceeds v i a a s i n g l e , i r r e v e r s i b l e 130 , two-electron step, i . e . (T) 5-C 5H 5 ) W(CO) 3CH 3 + 2e" > [(n 5-C 5H 5 ) W(CO) 3 ] " + CH 3 _ (5.4) The r e d u c t i o n r e s u l t s i n cleavage of the m e t a l - a l k y l bond. This does not appear to be happening with the n i t r o s y l - a l k y l compounds. E v i d e n t l y , as p r e v i o u s l y observed f o r [ { ( r i 5 - C 5 H 5 ) C r ( N O ) 2 } 2 ] • (Chapter 3 ) , the n i t r o s y l l i g a n d s s t a b i l i z e the i n i t i a l l y formed r a d i c a l anions against bond ruptures. c) (T) 5-C 5H 5)W(N0)2H. This compound i s s l i g h t l y e a s i e r to reduce than the a l k y l complexes described above. A c y c l i c voltammogram of ("n 5-CcH 5)W(NO)2 H i n CH 2C1 2 d i s p l a y s a s i n g l e , q u i t e r e v e r s i b l e one-electron 172 reduction at E | / 2 = " 0 . 7 9 V. The peak separation increases w i t h i n c r e a s i n g scan rate over a range of 60 mV at 0 . 0 2 V s - 1 to 9 0 mV at 0 . 1 8 V s - 1 , s i m i l a r to the red u c t i o n wave of (Ti 5 - C 5H 5)Cr(NO) 2 C H 3 . The extent of chemical r e v e r s i b i l i t y increases with increased scan ra t e a l s o . Thus the i / i p,a p,c r a t i o i s 0 . 8 5 at 0 . 0 2 V s - 1 and 0 . 9 5 at 0 . 1 6 V s - 1 . No other cathodic waves are evident a f t e r t h i s process i s scanned i n C H 2 C 1 2 . In c o n t r a s t , the polarographic r e d u c t i o n of ( T I 5 - C 5 H 5 ) W ( C O ) 3 H i n CH3CN i s al s o reported to occur i n a one e l e c t r o n , but i r r e v e r s i b l e step and r e s u l t s i n cleavage of the W-H bond. The scan rate dependence of A E ^ f o r ( T I 5 - C 5 H 5 ) W ( N 0 ) 2 H reduction and the somewhat lowered chemical r e v e r s i b i l i t y of t h i s process suggest a diminished s t a b i l i t y f o r [ ( T ) 5 - C 5H 5 ) W(NO) 2H] • r e l a t i v e to the molybdenum- and tungsten-containing a l k y l compounds, (see above) although i t would appear to be chemically a c c e s s i b l e nonetheless. d) (T| 5-C 5H 5)M (N0) 2 C1 (M - Mo, W). A c y c l i c voltammogram of ( T I 5 - C 5 H 5 ) M O ( N O ) 2 C 1 i n C H 2 C 1 2 i s shown i n Figure 2 8 . Both ( T ) 5 - C 5 H 5 ) M O ( N O ) 2 C 1 and ( r ) 5 - C 5 H 5 ) W ( N 0 ) 2 C 1 e x h i b i t one-electron r e d u c t i o n waves at E 1 / 2 = - 0 . 5 9 and - 0 . 6 2 V, r e s p e c t i v e l y w i t h high degrees of chemical r e v e r s i b i l i t y . For ( T ) 5 - C 5 H 5 ) M O ( N O ) 2 C 1 , i / i ranges from 0 . 9 2 p ,a p >c at 0 . 0 3 V s - 1 to 0 . 9 7 at 0 . 2 3 V s - 1 , and f o r ( T ) 5 - C 5 H 5 ) W ( N O ) 2 C 1 t h i s r a t i o i s 0 . 8 5 at 0 . 0 2 V s - 1 and 0 . 9 5 at 0 . 1 6 V s - 1 . In both cases AE i s independent P of scan ra t e over the ranges i n v e s t i g a t e d . On these time scales both compounds are r e l a t i v e l y s t a b l e toward follow-up chemical r e a c t i o n s upon e l e c t r o n t r a n s f e r , no other r e d u c t i o n processes being apparent i n C H 2 C 1 2 , 173 Volts vs SCE Figure 28. C y c l i c voltammogram of ( T I 5 - C 5 H 5 ) M O (N O ) 2C1 in CH 2C1 2 at a scan rate of 0.08 V s - 1 . 174 ( T I 5 - C 5 H 5 ) M ( N O ) 2C1 +e" [ ( n 5 - C 5 H 5 ) M ( N O ) 2 C l ] ~ M = Mo, W (5.5) The at l e a s t t r a n s i e n t s o l u t i o n s t a b i l i t y of [(n 5-C 5 H 5)M(NO) 2C1]^ (M = Mo, W) formed by r e v e r s i b l e reduction of the n e u t r a l precursors i s p a r t i c u l a r l y novel i n l i g h t of the cathodic processes i n which various c a r b o n y l -c o n t a i n i n g h a l i d e complexes p a r t i c i p a t e . In general a complex I^MX reduces with involvement of the M-X bond i n one of two types of processes, i . e . L M-X + e~ > [L Ml + X -, n n or L M-X + 2e~ > [L M l - + X -, n n 16a while i t - and dati v e bonds are e l e c t r o c h e m i c a l l y i n a c t i v e . Thus, f o r example (n 5-C 5H 5)W(CO) 3X + e~ > [(n 5-C 5H 5)W(CO) 3]" + X- (X = C l , B r , I ) (5.6) (n 5-C 5H 5)Fe(CO) 2X + e~ ^  [ ( n 5 - C 5 H 5 ) F e ( C O ) 2 ] ' + X~ (X = Cl.Br, i ) 5 4 . 1 2 9 b THF (5.7) 175 (Ti 5-C 5H 5)Mo(CO) 2(PPh 3)I + 2e - ^ > [(n5-C5H5)Mo(CO)2PPh3)]" + i - 1 2 9 b (5.8) CH3CN M(CO)5X + 2e" p r > [M(CO)5]" + X" DME 131 132 (M = Mn, X = Cl.Br; M = Re, X = Cl.Br.I) ' (5.9) The dinitrosyl complex [Fe(NO) 2C1] 2 also undergoes chloride loss upon reduction in THF in a complicated series of reductions and chemical 133 reactions. The f i r s t steps believed to be occurring are depicted in reactions 5.10 and 5.11, i.e. THF [Fe(NO) 2Cl] 2 > 2 Fe(N0) 2Cl(THF) n Fe(N0) 2Cl(THF) n + e~ > Fe(NO) 2(THF) n + C l " (5.11) Clearly, however, the NO ligands of (n5-C5H5)M(NO)2C1 (M = Mo, W) exert considerable influence on their reductions, rendering them rever-sible. The LUMO of (n 5-C 5H 5)W(NO) 2Cl is known to be an orbital possessing a large degree of NO 2n c h a r a c t e r 7 6 , 7 7 and is antibonding between the N and 46 0 atoms of each NO group, as noted previously (see above). Thus the i n i t i a l l y formed radical anions [ ( T I 5 - C 5 H 5 )M (N O ) 2 C 1 ] • (M = Mo, W) are stabilised by derealization of the extra electron density onto the NO ligands. This effect no doubt is at the heart of the reversibility of the 176 reductions displayed by ( T I 5 - C 5 H 5 ) M ( N O ) 2R (M = Cr, R = CH3; M = Mo, R = CH 3, C 2H 5; M = W, R = H, CH 3, C 2H 5) as w e l l . This same acceptor a b i l i t y of the NO ligands has been used to explain the r e v e r s i b l e reduction of cis_-M(R2CNCS2) 2(N0) 2 (M = Mo, W; R = a l k y l groups) as well as the solution and frozen-solution ESR spectra of the resultant [cis-M(R ?CNS ?)(N0) 0]~» A , 1 . 134 r a d i c a l anions. The f a c i l e reductions of (T)5-C5H5)M(NO) 2C1 (M = Mo, W) may also explain why the corresponding a l k y l compounds ( T I 5 - C 5 H 5 ) M ( N O ) 2 R cannot be synthesized from the halide complexes and a l k y l a t i n g agents such as Grignard reagents. (Such reactions produce only i n t r a c t a b l e decomposition 38 products. ) The nucleophiles may simply attack the NO ligands instead, perhaps v i a i n i t i a l reduction. Only less strongly-reducing a l k y l a t i n g reagents such as trialkyl-aluminium compounds react with the ( T I 5 - C 5 H 5 ) M ( N 0 2 ) C 1 (M = Mo, W) complexes to produce the a l k y l compounds. Another complicating factor may be that once formed, the d i n i t r o s y l - a l k y l complexes themselves are susceptible to reduction as described above. A strongly reducing nucleophile such as a Grignard reagent could p o t e n t i a l l y reduce the a l k y l complex to a r a d i c a l anion or attack the coordinated NO groups d i r e c t l y , which, i n either case would lead to rapid decomposition of the n i t r o s y l complex. The r e v e r s i b l e reductions of ( T ) 5 - C 5 H 5 ) M ( N O ) 2 C 1 (M = Mo, W) are s u b s t a n t i a l l y d i f f e r e n t from the behaviour of (r) 5-C 5H 5)Cr(NO) 2Cl (see Chapter 3) which undergoes i r r e v e r s i b l e reduction, r e s u l t i n g i n substantial d i s i n t e g r a t i o n of the complex. While chloride ion loss upon electrochemical reduction i s possible, i t also seems l i k e l y that the metal-n i t r o s y l interactions i n (T) 5-C 5H 5)Cr(NO) 2C1 are not s u f f i c i e n t l y strong to 177 s t a b i l i z e a r a d i c a l anion. U l t r a v i o l e t photoelectron spectra of (n 5 - C 5 H 5 ) M ( N O ) 2 C l (M = Cr, W) complexes i n d i c a t e that the W-Cl bond i s stronger than the Cr-Cl bond by ~0.9 e V . 7 6 This s i g n i f i c a n t d i f f e r e n c e has been suggested to be the cause of the d i f f e r i n g r e a c t i v i t i e s which the M-Cl bonds of these complexes d i s p l a y . This f a c t o r may als o be invo l v e d i n the d i f f e r i n g behaviours of (r ) 5 - C 5 H 5 ) C r ( N O ) 2C1 and ( T ) 5 - C 5 H 5 ) M ( N O ) 2C1 (M = Mo, W) toward e l e c t r o c h e m i c a l r e d u c t i o n , though i t i s not n e c e s s a r i l y the only one, e s p e c i a l l y since [ ( r i 5 - C 5 H 5 ) C r ( N O ) 2 ] 2 which would be a n t i c i p a t e d as a r e s u l t of C l - expulsion does not appear to be formed (see Chapter 3). Two fa c t o r s seem to be associated with the formation of [ ( T] 5-C 5H 5)Cr ( N O ) 2 ] 2 from (Ti 5-C5H 5)Cr ( N O ) 2C1 and metal reducing agents. F i r s t , (•n5-C5Hcj)Cr(NO)2C1 undergoes i r r e v e r s i b l e r e d u c t i o n , i n d i c a t i n g a ra p i d follow-up chemical r e a c t i o n (or slow e l e c t r o n t r a n s f e r ) and second, the metal reducing agent probably i n t e r a c t s s t r o n g l y with the h a l i d e complex, and does not simply t r a n s f e r an e l e c t r o n . However, the (T) 5-C 5H 5)M ( N O ) 2C1 (M = Mo, W) complexes are subject to r e v e r s i b l e reduction and are probably r e a d i l y reduced to h i g h l y r e a c t i v e r a d i c a l anions by reducing metals and subsequently decompose i n s o l u t i o n . This would e x p l a i n why the [ ( T I 5 - C 5 H 5 ) M ( N O ) 2 ] 2 (M = Mo, W) dimers have not been a c c e s s i b l e from reac-38 t i o n s of the corresponding d i n i t r o s y l - h a l i d e s w i t h reductants. Indeed the propensity of {(T) 5-C 5H 5)M (NO) 2 } - c o n t a i n i n g (M = Mo, W) complexes to undergo r e v e r s i b l e r e d u c t i o n to r e a c t i v e anions may ne c e s s i t a t e the e x p l o r -a t i o n of other routes to the dimers that do not i n v o l v e reducing c o n d i t i o n s . 178 e) [(TI 5-C 5H 5)M(NO) 2L]BF^ [M - Mo, L - PPh 3; M - W, L - PPh 3, P(OMe) 3]. The f i r s t , r e v e r s i b l e , reductions of these c a t i o n i c complexes are very f a c i l e as exemplified by a c y c l i c voltammogram of [(Ti 5-C 5H 5)Mo ( N O) 2PPh 3]BF 1 + (Figure 29), and as noted by A n g e l i c i and 46 c co-workers (though the e l e c t r o c h e m i s t r y of [ ( T T - C 5 H 5 ) M O ( N O ) jPPh^BF^ has not been p r e v i o u s l y r e p o r t e d ) • The reduction p o t e n t i a l s f o r [(T] 5-C 5H 5)M ( N O) 2L]BF l t i n CH 2C1 2 are -0.12 V (M = Mo, L = PPh 3 ) , -0.14 V (M = W, L = PPh 3) and -0.10 V [M = W, L = P(0Me) 3]. In a l l three cases the peak p o t e n t i a l s are scan rate independent with AE being 60-65 mV, depend-P ing on L. The peak current r a t i o , i / i , i s lowest f o r p,a p,c [(n 5-C 5H 5)Mo ( N O) 2PPh 3]BF 1 + (0.81 at 0.02 V s - 1 to 0.96 at 0.09 V s - 1 ) . Again, 46 the LUMOs of the c a t i o n s are b e l i e v e d to be l a r g e l y of NO 2 it c h a r a c t e r . A weak r i s e around E = -0.65 V (Figure 29) a f t e r the f i r s t wave i s p ,c passed i s apparent i n the CVs of a l l three compounds, and may be due i n part to a minor species derived from the [ ( T ) 5 - C 5 H 5 ) M ( N O ) 2 L ] • r a d i c a l s . One p o s s i b i l i t y could i n v o l v e a slow phosphine or phosphite l o s s , i . e . [(n 5-C 5H 5)M(NO) 2L]+ + e" = ^ ^ [ ( n 5 - C 5 H 5 ) M ( N O ) 2 L ] . ^ - [ ( T I 5 - C 5 H 5 ) M ( N O ) 2 ] * + L (5.12) Unfortunately, however, a very weak cathodic wave i n t h i s region a l s o appears i n the background scans of the supporting e l e c t r o l y t e (CH 2C1 2/0.1 M [n-Bu^NJPFg) employed during these s t u d i e s , complicating t h i s i n t e r p r e t a t i o n . A second cathodic wave occurs i n the CVs of a l l three compounds at qui t e negative p o t e n t i a l s as shown i n Figure 29. The three 179 Volts vs SCE Figure 29. C y c l i c voltammogram of [(Ti 5-C 5H 5)Mo(N0) 2PPh 3]BF 4 i n CH 2C1 2 at a scan rate of 0.21 V s - 1 . 180 waves occur a t : E 1 / 2 = -1.49 V [M = W, L = P(0Me) 3] with A E P = 200 mV at 0.17 V s - 1 ; E 1 / 2 = -1.53 V (M = W, L = PPh 3) with A E P = 190 mV at 0.16 V s - 1 ; and E 1 / 2 = -1.65 V (M = Mo, L = PPh 3) with A E P = 420 mV at 0.21 V s - 1 . For the complex [ ( n 5 - C 5 H 5 ) W ( N O ) 2 P ( 0 M e ) 3 ] B F 4 i n p a r t i c u l a r , the peak separation f o r t h i s wave v a r i e s from 180 mV at 0.09 V s - 1 to 250 mV at 0.35 V s - 1 , while E 1 / 2 i s i n v a r i a n t With scan r a t e . These waves thus appear to be q u a s i - r e v e r s i b l e r e d u c t i o n s . The extent of chemical r e v e r s i b i l i t y appears to increase w i t h scan ra t e but the trend i s d i f f i c u l t to q u a n t i f y . I t has been suggested by A n g e l i c i and co-workers that t h i s process may i n v o l v e reduction of the 19-electron [ ( n 5 - C 5 H 5 ) W ( N O ) 2 L ] • r a d i c a l s to the 46 corresponding 20-electron anions, i . e . [ ( T I 5 - C 5 H 5 ) W ( N 0 ) 2 L ] « + e~ ^ [ ( T i 5 - C 5 H 5 ) W ( N 0 ) 2 L ] - (5.13) (Presumably t h i s process, i f a p p l i c a b l e , would be a n t i c i p a t e d f o r [ ( T I 5 - C 5 H 5 ) M O ( N O ) 2PPh 3] • a l s o . ) As can be seen from Figure 29 an anodic wave also occurs at E = -0.58 V f o r the molybdenum complex (and at E p,a * p,a = -0.61 V f o r the two tungsten complexes). For [ ( n 5 - C 5 H 5 ) W ( N O ) 2 P ( 0 M e ) 3 ] B F 4 t h i s anodic peak p o t e n t i a l does not s h i f t with scan r a t e . Furthermore, the peak current of t h i s wave does not increase with increased scan r a t e . One p o s s i b l e r a t i o n a l e to e x p l a i n these observations i n v o l v e s l o s s of phosphine or phosphite l i g a n d s a f t e r r e d u c t i o n , i . e . 181 [ ( n 5 - C 5 H 5 ) M ( N O ) 2 L ] - = = = ^ [ ( T ) 5 - C 5 H 5 ) W ( N O ) 2 ] - + L (5.14) [ ( n 5 - C 5 H 5 ) M ( N O ) 2 r -e" ^ = ^ [ ( T ) 5 - C 5 H 5 ) W ( N O ) 2 ] . (5.15) The f a t e of the p u t a t i v e [ ( T I 5 - C 5 H 5 ) W ( N O ) 2 ] • r a d i c a l i s not known. f ) t ( T l 5 - C 5 H 5 ) W ( N O ) 2 ( T i 2 - C 8 H l l 4 ) ] B F H . To date, t h i s i s the only 68 i s o l a b l e o l e f i n complex of the {(n 5-C 5 H 5)W(NO) 2} + c a t i o n . The re d u c t i o n behaviour of t h i s compound i s h i g h l y time dependent as demonstrated by the c y c l i c voltammograms shown i n Figure 30, recorded over a period of a few hours. The f i r s t scan (Figure 30a) i s not u n l i k e the CVs of [(T) 5-C 5 H 5)M(NO) 2L]BF 4 (M = Mo, L = PPh 3; M = W, L = P(0Me) 3, PPh 3) j u s t considered (Figure 29). A r e v e r s i b l e r eduction occurs at E 1 / 2 = -0.08 V wi t h AE = 60 mV at 0.16 V s - 1 . This complex i s the e a s i e s t of the P c a t i o n i c - n i t r o s y l compounds to reduce, as would be expected on the b a s i s of the a n t i c i p a t e d donor a b i l i t i e s of phosphines or phosphites compared with 135 o l e f i n s . A second wave occurs at E = -0.76 V, which again i s a P»c f a i r l y weak r i s e but t h i s time i t i s at a somewhat more negative p o t e n t i a l than f o r [(r) 5-C 5 H 5)W(NO) 2L]BF^ [L = PPh 3, P(0Me) 3]. F i n a l l y a t h i r d wave occurs at E = -1.54 V. This wave appears to have no chemical r e v e r s i -P»c b i l i t y at these sweep rates (0.16 V s - 1 ) , u n l i k e the behaviour of the other c a t i o n i c n i t r o s y l compounds discussed above. In the r e t u r n scan of Figure 30a an anodic peak i s seen at E = -0.55 V. With each successive scan, p ,a however, the waves at E, , 0 = -0.08 V and E = -1.54 V di m i n i s h w i t h ' 1' p,c concomitant i n t e n s i f i c a t i o n of the wave at E = -0.74 V. This phenomenon p,c 182 Figure 30. C y c l i c voltammograms of [ ( T I 5 - C 5 H 5 ) W ( N O ) 2 (n 2-C 8H 1 1 +) ] B F 4 i n CH 2C1 2 at scan rates of 0.16 V s - 1 : (a) i n i t i a l scan; (b) a f t e r ~ 0.5 h; (c) a f t e r ~1.5 h. 183 i s i l l u s t r a t e d i n Figure 30b,c. U l t i m a t e l y the processes r e s p o n s i b l e f o r the reductions at E 1 / 2 = -0.08 V and ^ = -1.54 V are l a r g e l y suppressed and replaced by the wave at E 1 / 2 ~ "0.69 V w i t h AE^= 80 mV. As can be seen from Figure 30c the CV does remain somewhat "messy" i n t h i s r e g i o n , making i t d i f f i c u l t to d e f i n i t i v e l y e s t a b l i s h E 1 / 2 a n a AEp« Nevertheless, the process does appear to have s u b s t a n t i a l chemical r e v e r s i b i l i t y . The E^ ^ value does not correspond to that of ( T ) 5 - C 5 H 5 ) W ( N O ) 2 H o r , a simple a l k y l analogue l i k e the methyl or e t h y l d e r i v a t i v e s (see above). The process i s , however, temptingly reminiscent of a n e u t r a l ( T I 5 - C 5 H 5 ) M ( N O ) 2 Y s p e c i e s . One p o s s i b i l i t y may i n v o l v e gradual l o s s of H+ , i . e . (5.16) (although i t i s d i f f i c u l t to imagine how (n 5-C 5H 5)W(NO) 2C 8H 1 3 could be s t a b l e toward cleavage by H + ) . The ease of reduction of [(n 5-C 5H 5)W(NO) 2(T) 2-CgH 1 1 +) ( E 1 / 2 = -0.08 V, i n i t i a l l y ) suggests that (r| 5-C 5H 5)W(NO) 2BF 1 + would a l s o be r e a d i l y reduced. This may w e l l be behind the s u b s t a n t i a l q u a n t i t y of brown u n i d e n t i f i e d m a t e r i a l which accompanies the syntheses of (n 5-C 5H 5)W(NO) 2CH 3 and (T) 5-C 5H 5)W(NO) 2C 2H 5 from (T) 5-C 5H 5)W(N0) 2 B F 4 and the corresponding t r i a l k y l a l u m i n u m reagent (see Chapter 4). Reduction of the c a t i o n to a n e u t r a l r a d i c a l , [ (T) 5-C 5H 5)W(NO) 2] •, by the a l k y l a t i n g agents followed by a r a p i d r e a c t i o n of t h i s r a d i c a l complex to form the brown m a t e r i a l could be a s i g n i f i c i a n t competing process with (T) 5-C 5H 5)W(NO) 2alkyl formation. In t h i s context, i t i s noted that the e t h y l complex i s formed i n much lower 184 y i e l d s (< 16%) than the methyl analogue (~ 40%). g) W(N0) 2C1 2L 2 [L - PMePh2, P(0Me) 3l. These two compounds 136 possess c i s - n l t r o s y l l igands and trans-phosphorous donor groups. Their c y c l i c voltammograms e x h i b i t a s i n g l e , f a i r l y r e v e r s i b l e r e d u c t i o n , i . e . W(N0) 2L 2C1 2 + e~ W[W(N0) 2L 2C1 2]« [L = P(0Me 3), PMePh 2] (5.17) For L = P(OMe),, E, = -0.67 V w i t h AE = 80 mV and i / i 0.93 at 0.08 V s - 1 . For L = PMePh 2, E 1 / 2 = -0.80 V with AE p = 60 mV and i / i 0.87 at 0.08 V s - 1 . Again i t appears that the {M(N0) 2} fragment p,a p,c dominates the r e d u c t i o n behaviour, f a c i l i t a t i n g r e v e r s i b l e e l e c t r o n t r a n s f e r . A r e l a t e d s e r i e s of complexes Fe(NO) 2L 2 and Co(N0)(C0)L 2 (L = 1,10-phenanthroline, 2,2*-bipyridine and d i - 2 - p y r i d y l ketone) a l s o undergo completely r e v e r s i b l e one-electron reductions e l e c t r o c h e m i c a l l y to form the 137 corresponding r a d i c a l anions. These processes occur at q u i t e negative p o t e n t i a l s i n DME and are followed by a second, a l s o r e v e r s i b l e , one-e l e c t r o n reduction to form the d i a n i o n s . Although i t i s d i f f i c u l t to q u a n t i f y , the W(N0) 2L 2C1 2 complexes j u s t described are c e r t a i n l y much ea s i e r to reduce, and a second reduction wave i s not present i n the CVs of these compounds i n CH 2C1 2 under these c o n d i t i o n s . Overview of the Electrochemical Results. Table I I I summarizes the r e d u c t i o n behaviour of the various d i n i t r o s y l complexes examined i n 185 Table I I I . Data f o r the Reductions of Several D i n i t r o s y l Complexes. Compound3 E b  E l / 2 AE p c i / i 2 0 P»a/J-p,c Comments [(n 5-C 5H 5)Cr(NO) 2CH 3CN]PF 6 E a b = -0.27 ( 0 ! l 0 V s - 1 ) (n 5-C 5H 5)Cr(NO) 2Cl E = -0.68 (S'.07 Vs" 1) ( T1 5- C5 H5) C r( N°)2 C H3 -1.01 -1.01 70 90 0.87 0.99 at 0.06 V s - 1 at 0.28 V s - 1 Second weak wave at E c= -1.87 ( o . i r v s - 1 ) (n 5-C 5H 5)Cr(NO) 2CH 3 -0.99 d 60 0.99 at 0.21 V s - 1 [(r 1 5-C 5H 5)Mo(NO) 2PPh 3]BF l t -0.12 65 65 0.81 0.96 at 0.02 Vs" 1 at 0.09 V s - 1 Also E 1 / 2 = -1.65 w i t h AEp = 420 mV and E a = -0.58 (0.21* Vs" 1) ( T } 5 - C 5 H 5 ) M O ( N O ) 2 C 1 -0.59 75 75 0.92 0.97 at 0.03 V s - 1 at 0.23 V s - 1 (n 5-C 5H 5)Mo ( N O) 2CH 3 -0.83 65 65 0.96 0.97 at 0.04 V s - 1 at 0.22 V s - 1 (n 5-C 5H 5)Mo ( N O) 2C 2H 5 -0.86 65 1.00 at > 0.05 Vs" 1 [(n 5-C 5H 5)W(NO) 2PPh 3]BF 4 -0.14 65 65 0.90 0.95 at 0.02 V s - 1 at 0.11 Vs" 1 Also E 1 / 2 = -1.53 with AEp = 190 mV and E a = -0.61 (0.16'Vs" 1) 186 Table I I I . continued [(T) 5-C 5H 5)W(NO) 2P(OMe) 3]BF l t -0.10 60 0.85 60 0.99 [(T) 5-C 5H 5)W(N0) 2(n 2-C 8H 1 1 +)]BF u -0.08 60 >0.46 at 0.01 V s - 1 at 0.21 V s - 1 Also E 1 / 2 = -1.49 w i t h AE = 200 mV and E & = -0.61 (0.17 Vs" 1) at 0.16 Vs " 1 and E = -1.54. With time these d i m i n i s h and a new wave grows i n at E 1 / 2 = -0 wit h AEp = 80mV (T) 5-C 5H 5 ) W(N0) 2C1 -0.62 60 0.85 at 0.02 V s - 1 60 0.95 at 0.16 V s - 1 (n 5-C 5H 5 ) W(NO) 2H -0.79 60 0.85 at 0.02 V s - 1 90 0.95 at 0.18 Vs" 1 (T,5-C5H5)W(NO)2CH3 -0.83 60 0.96 at 0.05 V s - 1 60 0.98 at 0.18 Vs" 1 (TI5-C5H5)W(NO)2CH3 -0.85 d 60 0.83 at 0.02 Vs" 1 60 0.97 at 0.22 Vs" 1 (TI5-C5H5)W(NO)2C2H5 -0.86 65 0.95 at 0.04 Vs" 1 65 0.96 at 0.18 V s - 1 W ( N O ) 2 ( P ( O M e ) 3 ) 2 C l 2 -0.67 80 0.93 at 0.08 Vs" 1 W(NO) 2(PMePh 2) 2Cl 2 -0.80 60 0.87 at 0.08 V s - 1 (a) In CH 2C1 2/0.1 M [n-Bu uN]PF 6 unless otherwise noted. (b) V vs. SCE. (c) mV. (d) i n CH3CN/0.1 M [n-Bu^NJPF 6 * 187 t h i s study. From the previous d i s c u s s i o n i t seems reasonable to i m p l i c a t e the strong rc-acid nature of the NO lig a n d s as being l a r g e l y r e s p o n s i b l e f o r the r e v e r s i b l e character of the reductions described above. Only some chromium-containing d i n i t r o s y l compounds e x h i b i t i r r e v e r s i b l e r e d u c t i o n , perhaps c o n s i s t e n t with the weaker f i r s t row t r a n s i t i o n m e t a l - n i t r o s y l 12 bonds. I t would be expected that i f the n i t r o s y l l igands dominate the r e v e r s i b l e r e d u c t i o n behaviour observed f o r the so many and v a r i e d d i n i t r o s y l compounds of the Group 6 metals, then there should be a c o r r e l a t i o n between E 1 / 2 a n c* p h y s i c a l p r o p e r t i e s associated with the NO liga n d s i n these complexes. The i n f r a r e d s t r e t c h i n g frequency of the n i t r o s y l group i s one such property. A lower frequency i s i n d i c a t i v e of greater back-bonding to the NO l i g a n d compared with a higher ( a l l other f a c t o r s being e q u a l ) . The greater the extent of t h i s d e r e a l i z a t i o n onto the NO ligands i s , the more d i f f i c u l t i t should be for the n i t r o s y l groups to accept more e l e c t r o n d e n s i t y . Thus, the lower the the more negative E 1 / 2 should be. Table IV l i s t s the n i t r o s y l s t r e t c h i n g frequencies of the various {(r| 5-C 5H 5)M(NO) 2}-containing compounds i n CH 2C1 2 used In t h i s study and t h e i r E 1 / 2 values, also i n CH 2C1 2. P l o t s of E 1 / 2 vs. V ^ J Q are shown i n Figure 31. This i l l u s t r a t e s that f o r a given metal, |E 1 / f 2| does increase roughly l i n e a r l y w i t h decreasing v N Q . Furthermore a given chromium complex i s somewhat harder to reduce than i t s molybdenum analogue, which d i f f e r s l i t t l e i n i t s reduction p o t e n t i a l compared to the tungsten analogue. These r e s u l t s are c o n s i s t e n t w i t h (a) the n-acceptor e f f e c t of the NO l i g a n d and the LUMO of the [ ( n 5 - C 5 H 5 ) M ( N O ) 2 Y ] n + (M = Cr, Mo, W; Y = h a l i d e s , H, a l k y l 188 Table IV. I n f r a r e d S t r e t c h i n g Frequencies and Reduction P o t e n t i a l s of Various D i n i t r o s y l Complexes.  Compound v N O a ( c m " -1) E l / 2 [(n 5-C 5H 5)Cr(NO) 2CH 3CN]PF 6 1844 1744 ~ -0.23° ( n 5 - C 5 H 5 ) C r ( N O ) 2 C l 1817 1711 ~ -0.64 c ( n 5-C 5H 5)Cr(NO) 2CH 3 1777 1669 -1.01 [(Ti 5-C 5H 5)Mo(NO) 2(PPh) 3]BF 1 + 1792 1710 -0.12 ( n 5-C 5H 5)Mo ( N O) 2Cl 1759 1665 -0.59 (n 5-C 5H 5)Mo ( N O) 2CH 3 1730 1635 -0.83 (n 5-C 5H 5)Mo(NO) 2C 2H 5 1727 1633 -0.86 [ ( i 1 5 - C 5 H 5 ) W ( N O ) 2 ( T i 2 - C 8 H l l t ) ] B F 1 + 1785 1704 6 8 -0.08 t ( n 5-C 5H 5 ) W(NO) 2PPh 3]BF l t 1770 1694 -0.14 [(Ti 5-C 5H 5 ) W(NO) 2P(OMe) 3]BF 1 + 1767 1689 -0.10 (n 5 -c 5H 5)w(NO) 2ci 1733 1650 -0.62 ( T I 5 - C 5 H 5 ) W ( N O ) 2 H 1718 1632 -0.79 ( T I 5 - C 5 H 5 ) W ( N O ) 2 C H 3 1720 1609 -0.83 ( T I 5 - C 5 H 5 ) W ( N O ) 2 C 2 H 5 1707 1619 6 8 -0.86 (a) CH 9C1 9 s o l u t i o n , (b) V o l t s vs. SCE. (c) Estimated from E + 0.04 V at slow scan r a t e s . ' 1600 1650 1700 1750 1800 c m -1 u NO (CH2CI2) Figure 31. Plots of E 1 / 2 vs. v N Q for various { ( T ) 5 - C 5 H 5 )M(N0) 2}-containing (M = Cr, Mo complexes i n CH ?C1 2. 190 groups, n = 0; Y = phosphines, phosphites, o l e f i n s , amines, n = 1) complexes i n v o l v i n g a l a r g e l y N0-2n o r b i t a l , and (b) the i n c r e a s i n g strength of the M - NO i n t e r a c t i o n i n the order M = C r < M = Mo< M = W . At the outset of t h i s work the ele c t r o c h e m i c a l r e d u c t i o n behaviour of organometallic n i t r o s y l complexes had been l i t t l e s t u d i e d . A report on the e l e c t r o c h e m i s t r y of ( T i 5-C 5H 5)NiNO i n DMF at a mercury el e c t r o d e i n d i c a t e s that t h i s compound reduces with concomitant r a p i d r e a c t i o n to 138 form ( T I 5 - C 5 H 5 ) 2 N i and other products. The re d u c t i o n e l e c t r o c h e m i s t r y of ( T ) 5 - C 5 H 5 ) W ( C O ) 2 N 0 has been b r i e f l y examined by Dessy and co-workers, and a s h o r t - l i v e d r a d i c a l anion (presumably [ ( T ) 5 - C 5 H 5 ) W ( C O ) 2N0]"») r e p o r t e d l y i s f o r m e d . 1 3 1 A d e t a i l e d i n v e s t i g a t i o n of (•n5-C5H1+R)M(CO) 2N0 (M = Cr, R = H, C(0)CH 3; M = Mo, R = H) has shown that these compounds undergo q u a s i -r e v e r s i b l e reduction In CH3CN, DMF or THF to give the corresponding r a d i c a l 139 anions. An ESR study of frozen s o l u t i o n s of these a n i o n i c r a d i c a l s suggest that the ext r a e l e c t r o n d e n s i t y i s l o c a l i z e d i n the MNO group and that the n i t r o s y l l i g a n d becomes s u b s t a n t i a l l y bent upon red u c t i o n to accommodate t h i s . However, no evidence f o r such bending of the NO groups i s suggested by the el e c t r o c h e m i s t r y of the d i n i t r o s y l complexes considered i n t h i s study. (This i s confirmed by an X-ray c r y s t a l l o g r a p h i c s t r u c t u r e of [ ( T I 5 - C 5 H 5 ) 2 C O ] [ ( T I 5 - C 5 H 5 ) M O ( N O ) 2 C 2 H 5 ] described below). Preparative Work, (a) [ ( T) 5-C 5H 5) 2Co] [(n 5-C 5H 5)M(NO) 2R] (M - Mo, R = CH3, C 2H 5; M • W, R • CH 3). The new r a d i c a l anion complexes [ (n 5-C 5H 5) 2Co ] [ (n 5-C 5H 5)M(NO) 2Y] ( M = Mo, Y = CH 3, C 2H 5, C l ; M - W, Y -CH 3, H, D, Cl) may be r e a d i l y synthesized i n moderate to high y i e l d s from t h e i r n e u t r a l precursors and cobaltocene, a moderately potent, one-electron 191 reductant, i . e . ( T I 5 - C 5 H 5 ) M ( N O ) 2 Y + (n 5-C 5H 5) 2Co > [ ( T I 5 - C 5 H 5 ) 2 C O ] [ ( T I 5 - C 5 H 5 ) M ( N O ) 2 Y ] (5.18) A l l but the hydrido and deutero derivatives are best prepared using E t 2 0 as the solvent. The two methyl compounds [(n 5-C 5H 5) 2Co][(n 5-C 5H 5)M(NO) 2CH 3] (M = Mo, W) p r e c i p i t a t e i n s t a n t l y when solutions of the two reagents are mixed and are obtained as a n a l y t i c a l l y pure, purple-black (M = W) and green-black (M = Mo) m i c r o c r y s t a l l i n e s o l i d s i n moderate y i e l d s . The moly-bdenum compound i n p a r t i c u l a r must be prepared with rapid addition of the two reactants and e f f i c i e n t s t i r r i n g . If the reaction mixture i s not s t i r -red, or i s s t i r r e d slowly, a brown, amorphous and insoluble material accom-panies the formation of the desired product. On occasion traces of t h i s brown s o l i d can be detected even when rapid s t i r r i n g i s employed. This may possibly hinder the preparation of [ ( T I 5 - C 5 H 5 ) 2 C O ] [ ( n 5 - C 5 H 5 ) M o ( N 0 ) 2 C H 3 ] on a large scale. Simply adding E t 2 0 to an equimolar mixture of the two s o l i d reagents also r e s u l t s i n formation of the desired complex but the resultant s o l i d i s of poor q u a l i t y , being contaminated with other insoluble by-products. Not s u r p r i s i n g l y , attempts to grow single c r y s t a l s of this moly-bdenum complex by allowing two separate solutions of (n 5-C 5H 5) 2Co and (r) 5-C 5H 5)Mo(NO) 2CH 3 to slowly d i f f u s e together f a i l e d , r e s u l t i n g only i n the formation of a brown amorphous s o l i d . It may be that the i n i t i a l l y formed r a d i c a l anion i s susceptible to further attack by the 19-electron r a d i c a l , cobaltocene. This kind of problem does not appear to be as severe 192 for the tungsten complex. These methyl complexes are somewhat a i r - and thermally s e n s i t i v e . The molybdenum complex, for example, decomposes slowly i n a i r overnight i n t o an amorphous s o l i d of unknown composition. Under N 2 at ambient temperatures i t a l s o g r a d u a l l y decomposes over the course of s e v e r a l days. At -25°C, however, these complexes appear to be s t a b l e under N 2 f o r at l e a s t s e v e r a l months. These r a d i c a l anion compounds are s o l u b l e i n good s o l v a t i n g solvents such as CHjCN and DMF at 25°C and only s l i g h t l y s o l u b l e i n CH 2C1 2 at ~ -78°C. The molybdenum complex gives greenish s o l u t i o n s while the tungsten complex forms red-purple s o l u t i o n s . They very r a p i d l y decompose i n a l l the above solvents at room temperature to give brown s o l u t i o n s . The tungsten complex i s s t a b l e i n CHjCN for a few hours at -25°C a f t e r which i t s t a r t s to c l e a r l y show signs of decom-p o s i t i o n . The complex [ ( T I 5 - C 5 H 5 ) 2 C o ] [ ( T ) 5 - C 5 H 5 ) M O ( N O ) 2 C 2 H 5 ] p r e c i p i t a t e s from E t 2 0 s o l u t i o n at room temperature only i f the r e a c t i o n mixture c o n t a i n i n g cobaltocene and ( T I 5 - C 5 H 5 ) M O ( N O ) 2 C 2 H 5 i s concentrated enough. I f the mixture i s too d i l u t e f o r t h i s to occur, the d e s i r e d complex can be induced to p r e c i p i t a t e by c o o l i n g the s o l u t i o n . In e i t h e r case, green-black micro-c r y s t a l s of t h i s r a d i c a l anion complex are obtained i n moderate y i e l d . The r e a c t i o n appears to be very c l e a n , no other i n s o l u b l e by-products being apparent. The o x i d a t i o n p o t e n t i a l of ( T I 5 - C 5 H 5 ) 2Co has been reported to be 140 -0.94 V i n CH3CN and -0.80 V i n DME (vs. SCE). E x a c t l y what the r e l a t i v e r eduction and o x i d a t i o n p o t e n t i a l s of ( T I 5 - C 5 H 5 ) M O ( N O ) 2 C 2 H 5 and cobaltocene, r e s p e c t i v e l y , are i n E t 2 0 i s not known but i t does seem l i k e l y 193 that the difference i s not very large. Indeed, this would seem to be the case for a l l three ( T ) 5 - C 5 H 5 ) M ( N O ) 2R (M = M O, R = CH 3, C 2H 5; M = W, R = CH 3) compounds since t h e i r reduction potentials f a l l between - 0 . 8 3 to - 0 . 8 6 V (vs. SCE) i n C H 2 C 1 2 . It seems l i k e l y therefore that an equilibrium between the reductant and the oxidant i s set up i n E t 2 0 which i s driven toward the r a d i c a l anion complex by p r e c i p i t a t i o n from solu t i o n , i . e . (n 5-C 5H 5)M ( N O) 2R + ( T ) 5 - C 5 H 5 ) 2Co [ ( T I 5 - C 5 H 5 ) 2 C O ] [ ( T ) 5 - C 5 H 5 ) M ( N O ) 2R] (solv.) V [ (T) 5-C 5H 5) 2Co ] [ ( n 5-C 5H 5)M(N O ) 2R] (s ) (5.19) In the case of M = Mo and R = C 2Hg the equilibrium would not be anticipated to be much further to the l e f t than for M = Mo and R = CH 3 since the difference i n reduction potentials of these two compounds i s only 30 mV. The ethyl compound, however, may form a more soluble r a d i c a l anion complex and not p r e c i p i t a t e out of s o l u t i o n except at a s u f f i c i e n t l y high concen-t r a t i o n or upon cooling. (The i s o l a t e d complex i s s l i g h t l y soluble i n E t 2 0 to give pale green solutions.) A l t e r n a t i v e l y , a supersaturated solution of [(n 5-C 5H 5)Co][(n 5-C 5H 5)Mo ( N 0) 2C 2H 5] may be forming, again which either would need to be concentrated or cooled to draw the complex out of s o l u t i o n . This not too d r a s t i c difference i n oxidation and reduction potentials of the reagents i s probably what f a c i l i t a t e s the i s o l a t i o n of these complexes as m i c r o c r y s t a l l i n e or c r y s t a l l i n e s o l i d s . 194 Another d i s t i n g u i s h i n g feature of c r y s t a l l i n e [ ( n 5 - C 5 H 5 ) 2 C o ] [ ( T ) 5 - C 5 H 5 ) M O ( N O ) 2 C 2 H 5 ] i s i t s remarkable a i r - and thermal s t a b i l i t y . A f t e r exposure of t h i s compound to a i r at room temperature for s e v e r a l days i t shows no apparent decomposition. In f a c t , such a sample exposed to a i r i n t h i s manner can be o x i d i s e d i n CH 2C1 2 back to ( T I 5 - C 5 H 5 ) M O ( N O ) 2 C 2 H 5 by AgPFg as evidenced by an IR spectrum of the r e s u l -tant supernatant s o l u t i o n which d i s p l a y s absorptions only f o r the n e u t r a l e t h y l compound. However, f o r long term storage t h i s complex i s best kept under N 2 at -25°C. I t i s s o l u b l e i n CH3CN and DMF at 25°C to give greenish s o l u t i o n s . The molecular s t r u c t u r e of [(n 3-C 5H 5)Mo(NO) 2C 2H 5]• i s depicted i n Figure 32, while Figure 33 shows the s t r u c t u r e of the c o b a l t i c i n i u m counterion, and the arrangement of the c a t i o n s and anions i n the c r y s t a l . The [ ( T ) 5 - C 5 H 5 ) 2 C O ] + c a t i o n appears to be normal, with the exception of c 142 e c l i p s e d T] 3-C 5H 5 r i n g s . The complex i s composed of d i s c r e t e c a t i o n and anion p a i r s and the [ ( T I 5 - C 5 H 5 ) M O ( N O ) 2 C 2 H 5 ] ~ anion possesses a "three-legged piano s t o o l " geometry. Disorder i n the p o s i t i o n s of the NO and e t h y l ligands l i m i t s the conclusiveness of some aspects of the s t r u c t u r e . A comparative s t r u c t u r e of the n e u t r a l (-n5-C5H5)Mo(NO) 2 C 2 H 5 precursor i s not a v a i l a b l e since t h i s compound i s a l i q u i d at room temperature, making the handling of c r y s t a l s of t h i s m a t e r i a l very awkward. P r i o r s t r u c t u r a l i n v e s t i g a t i o n s of complexes c o n t a i n i n g the {(n 5-C 5H 5)Mo(NO) 2} group a l s o are l a c k i n g . Nevertheless, some i n t e r e s t i n g s t r u c t u r a l features are apparent, p a r t i c u l a r l y for the {M(NO)2} fragment. Both n i t r o s y l groups are e f f e c t i v e l y l i n e a r [O(l,av)-N(l,av)-Mo = 172.8(22)° and 0(2,av)-N(2,av)-Mo 195 CIS C14 Figure 32. 1 4 1 Molecular s t r u c t u r e of [(n 5-C 5H 5)Mo(NO) 2C 2H 5 ] ~ . Selected bond lengths (A) and angles (deg) are Mo-N(l.av) = 1.802(12), Mo-N(2,av) = 1.799(12), Mo-C(l,av) = 2.230(15), N(1,av)-0(l,av) = 1.233(12), N(2,av) -0(2,av) = 1.232(12), Mo-C(C 5H 5,av) = 2.404, C(l,av)-C(2,av) = 1.525(15), N(l,av)-Mo-N(2,av) = 100.6(13), 0(1,av)-N(l,av)-Mo = 172.8(22), 0(2,av)-N(2,av)-Mo = 176.3(24), C(2,av)-C(l,av)-Mo = 110.7(15). 196 Figure 3 3 . 1 * 1 , 1 4 2 ( a ) structure of the [ ( n 5 - C 5 H 5 ) 2 C O ] + counterion, and (b) the arrangement of the cations and anions i n c r y s t a l l i n e [ ( T I 5 - C 5 H 5 ) 2 C O ] [ ( n 5 - C 5 H 5 ) M o ( N O ) 2 C 2 H 5 ] (projection along the a a x i s ) . 197 = 176.3(24)°] and are not s u b s t a n t i a l l y d i f f e r e n t from the W-N-0 and Cr-N-0 angles of ( T I 5 - C 5 H 5 ) M ( N O ) 2 C 1 (M = Cr, W). 6 5 In c o n t r a s t , the n i t r o s y l l i g a n d of (n 5-C 5 H 5)Mo(CO) 2N0 becomes s u b s t a n t i a l l y bent upon one-electron 139 r e d u c t i o n . A n g e l i c i and co-workers have reported no s i g n i f i c a n t bending of the n i t r o s y l groups i n the fo r m a l l y 19-electron r a d i c a l c; 46 ( T i 3-C 5 H 5)W(NO) 2P(OPh) 3. The r a t i o n a l e of the s t r u c t u r e of t h i s n e u t r a l r a d i c a l complex o f f e r e d by these workers i s probably a p p l i c a b l e to the c l o s e l y r e l a t e d [ ( n 5 - C 5 H 5 ) M o ( N O ) 2 C 2 H 5 ] ~ r a d i c a l anion a l s o . The N ( l , a v ) -Mo-N(2,av) angle of 100.6(13)° i s comparable to the N-W-N angle of 102.7(6)° found i n (n 5-C 5 H 5)W(NO) 2P(0Ph) 3. The LUMO of ( n 5 - C 5 H 5 ) C r ( N 0 ) 2 C 1 i s thought to be l a r g e l y NO based and i s antibonding between the two NO l i g a n d s . 7 6 ' 7 7 Thus population of t h i s molecular o r b i t a l by an ex t r a e l e c t r o n would be expected to cause a widening of the N-Cr-N angle to minimize t h i s antibonding i n t e r a c t i o n . ' A s i m i l a r e f f e c t would be a n t i c i p a t e d f o r ( n 5 - C 5 H 5 ) M o ( N O ) 2 C 2 H 5 . The N-Cr-N and N-W-N angles of (n 5 - C 5 H 5 ) M ( N O ) 2 C l (M = Cr, W) are 93.9(1)° and 92.0(4)°, r e s p e c t i v e l y , and are s i g n i f i c a n t l y smaller than the N(1,av)-Mo-N(2,av) angle. Secondly, the LUMO of (T) 5-C 5 H 5)Cr(NO) 2Cl i s antibonding between the N and 0 atoms of each 46 c n i t r o s y l l i g a n d , which would be expected a l s o f o r ( T T - C 5 H 5 ) M O ( N O ) 2 C 2 H 5 . Thus population of t h i s o r b i t a l should r e s u l t i n a lengthening of the N-0 di s t a n c e s . The N-0 distances of [ ( T I 5 - C 5 H 5 ) M O ( N O ) 2 C 2 H 5 ] ~ [1.232(12) and a 46 1.233(12) A] are comparable to those found f o r (n b-C 6 H 5)W(NO) 2P(OPh) 3 [1.20(2) and 1.22(2) A] and longer than those of ( T I 5 - C 5 H 5 ) M ( N O ) 2C1 [M = Cr, N-0 = 1.157(4), 1.159(4) A; M = W, N-0 = 1.15(1), 1.17(1) A]. Both of these f e a t u r e s , the N(l,av)-Mo-N(2,av) angle and the N-0 di s t a n c e s , are 198 consistent with the extra electron density residing i n a l a r g e l y NO-based % o r b i t a l . It i s d i f f i c u l t to assess the s i g n i f i c a n c e of the Mo-N d i s -tances [Mo-N(l,av) = 1.802(12) and Mo-N(2,av) = 1.799(12) A] since other {(t) 5-C 5 H 5)Mo(NO) 2) complexes have not been s t r u c t u r a l l y characterized, although these distances would be anticipated to be s l i g h t l y shorter than for neutral ( T ) 5 - C 5 H 5 ) M O ( N O ) 2 C 2 H 5 due to a stronger Mo-NO n-bonding i n t e r 4 12 a c t i o n . ' The large standard deviations of some of the distances and angles associated with the {Mo(N0)2} core l i m i t the d e f i n i t i v e n e s s of the analysis, but the anticipated trends are c l e a r l y evident. The Nujol-mull IR spectra of the new r a d i c a l anion complexes are consistent with s u b s t a n t i a l l y increased back-bonding to the n i t r o s y l ligands. The v absorptions of these complexes are among the lowest observed yet for terminal n i t r o s y l groups and l i e well below the range generally anticipated for terminal n i t r o s y l l i g a n d s , 5 ' 1 5 i n d i c a t i n g that the mode of attachment of a n i t r o s y l ligand to a t r a n s i t i o n metal i s best not i n f e r r e d s o l e l y from the n i t r o s y l stretching frequency. The v N 0 ' s of the r a d i c a l anion complexes occur at lower energies by ~190-210 cm - 1 as shown i n Table V. A si m i l a r e f f e c t i s observed for v M r i absorptions of NO [ ( n 5 - C 5 H 5 ) F e ( T i 6 - C 6 M e 6 ) ] [ { ( T i 5 - C 5 H 5 ) C r ( N O ) 2 } 2 ] (see Chapter 3) which drop by ~175 cm - 1 for the bridging NO ligands. The neutral r a d i c a l s [(n 5-C 5 H 5)W(N0 2)L]• [L = PPh 3, P(0Ph) 3, P(0Me) 3] also display v *s ~160-175 cm - 1 lower than their precursor cations, [(n 5-C 5 H 5)W(NO) 2L]BF^. 4 6 For some of the new a l k y l complexes the lower energy n i t r o s y l absorption overlaps with the strong 1462 cm - 1 band of Nujol. U t i l i z i n g a computer 199 Table V. I n f r a r e d S t r e t c h i n g Frequencies of R a d i c a l Anion and N e t u r a l D i n i t r o s y l - A l k y l Complexes. Compound VN0 (cm- 1) [ ( T , 5 - C 5 H 5 ) 2 C O ] [ ( T ) 5 - C 5 H 5 ) W ( N O ) 2CH 3] 1511 1427 ( T I 5 - C 5 H 5 ) W ( N O ) 2 C H 3 1718 1638 [ ( T I 5 - C 5 H 5 ) 2 C O ] [ ( T I 5 - C 5 H 5 ) M O ( N O ) 2 C H 3 ] 1526 1445 (1443)° ( T) 5-C 5H 5)Mo(NO) 2CH 3 1739 1650 [ ( n 5 - C 5 H 5 ) 2 C o ] [ ( n 5 - C 5 H 5 ) M o ( N 0 ) 2 C 2 H 5 ] 1527 1458 (1458)° ( T I 5 - C 5 H 5 ) M O ( N O ) 2 C 2 H 5 1734 1645 (a) Nujol m u l l , (b) Hexanes. (c) With the Nujol spectrum subtracted from the experimental spectrum. a s s i s t e d s u b t r a c t i o n process, the Nujol spectrum can be subtracted from the experimental spectrum. The s u b t r a c t i o n i s deemed to be complete f o r these spectra when the more w e l l - r e s o l v e d 1377 cm - 1 band of Nujol i s j u s t removed. For the three r a d i c a l anions considered here the e f f e c t i s q u i t e small (see Table V). Each of the r a d i c a l anion complexes show medium i n t e n s i t y C-H s t r e t c h e s a t t r i b u t a b l e to the c y c l o p e n t a d i e n y l r i n g s between 3069 and 3104 cm - 1. For [(n 5-C 5H 5) 2Co][(n 5-C 5H 5)W(NO) 2CH 3] these are moderately w e l l resolved i n t o two sharp bands (3100, 3087 cm - 1) and a 200 shoulder (3069 c m - 1 ) . For [ ( T I 5 - C 5 H 5 ) 2 C O ] B F 1 + t h i s band occurs at 3119 cm - 1. Medium i n t e n s i t y bands around 1415 and 1360 cm - 1 seen f o r a l l three compounds are l i k e l y due to C-H v i b r a t i o n s of the C 5H 5 r i n g and C-H symmetric d e f o r m a t i o n s * 0 5 of the CH 3 or C 2H 5 groups, r e s p e c t i v e l y . The ESR spectra of the three r a d i c a l anion a l k y l complexes (Figures 34, 35) i n CH3CN or DMF at 20 40°C e x h i b i t f i v e - l i n e patterns of approximate i n t e n s i t y r a t i o s 1:2:3:2:1, c h a r a c t e r i s t i c of coupling of the odd e l e c t r o n to two equivalent 1 1 +N n u c l e i . (In a l l three cases a^ i s of the order of 7 G). The a N values are estimated from i n f l e c t i o n - p o i n t - t o -i n f l e c t i o n - p o i n t separations as shown i n Figures 34 and 35. While g values have not been determined, the point of greatest i n t e r e s t i s that these patterns are c o n s i s t e n t with d e r e a l i z a t i o n of the extra e l e c t r o n d e n s i t y onto the t e r m i n a l , l i n e a r n i t r o s y l l i g a n d s . A s i m i l a r pattern i s seen f o r the n e u t r a l r a d i c a l s [(n 5-C 5H 5)W(NO) 2L]• [L = PPh 3, P(0Ph) 3] with the a d d i t i o n a l feature of coupling to a 3 1 P n u c l e u s . 4 6 On the basis of t h e i r IR and ESR sp e c t r a , a l l three a l k y l - c o n t a i n i n g r a d i c a l anion complexes are a n t i c i p a t e d to possess q u a l i t a t i v e l y s i m i l a r molecular s t r u c t u r e s both i n the s o l i d s t a t e and i n s o l u t i o n , namely "three legged piano s t o o l " geometries analogous to [ ( T I 5 - C 5 H 5 ) 2 C O ] [(n 5-C 5H 5)Mo(N0) 2C 2H 5] (Figure 32). The s t r u c t u r a l features discussed above and the spectroscopic data suggest a s i m p l i f i e d representa-t i o n of these complexes as depicted below, and i n simple valence bond terms the resonance hybrids a l s o shown below would be expected to make major c o n t r i b u t i o n s to the o v e r a l l bonding. Two i n t r i g u i n g features come out of t h i s p i c t u r e . F i r s t l y the metal centre, i n t h i s extreme, f o r m a l l y (b) Figure 3 4 . ESR spectra of (a) [ (T ) 5 - C 5 H 5 ) 2 C O ] [ ( n 5 - C 5 H 5 ) M o ( N O ) 2 C H 3 ] i n DMF at ~ -35°C: a N = 7.1 G, and (b) [ ( n 5 - C 5 H 5 ) 2 C o ] [ ( T I 5 - C 5 H 5 )M O (N O ) 2C 2H 5] i n CH.CN at 20°C: a„ = 7.0 G. 202 203 maintains an 18-electron configuration as a consequence of the powerful u-acceptor a b i l i t y of the NO ligands. Secondly, the N-0 bond order must be * s u b s t a n t i a l l y reduced since antibonding it o r b i t a l s of NO are being populated. While the precise bonding d e t a i l s must await a t h e o r e t i c a l analysis, i t i s l i k e l y that a representation s i m i l a r to those suggested for c 46 [ (n 5-C 5H 5)W(N0) 2L]• [L = P(0Me) 3, P(0Ph) 3, PPh 3] and 134 cis-M(R 2CNCS 2) ?(N0) ? (M = Mo, W, R = various a l k y l groups) applies, namely that the HOMOs of the [(n 5-C 5H 5)M(NO) 2R]~ (M = Mo, R = CH 3, C 2H 5; M = W, R = CH 3) anions i s anticipated to be la r g e l y an NO-based, 2it o r b i t a l . The anions [(n 5-C 5H 5)Mo(NO) 2CH 3]~ and [ (n 5-C 5H 5)W(NO) 2CH 3]~ can be oxidized r e a d i l y to the neutral a l k y l complexes by [(r| 5-C 5H 5) ^ eJBF^ and 204 AgBF^, r e s p e c t i v e l y , e.g. CH-2CI2 [ ( T I 5 - C 5 H 5 ) 2 C O ] [ ( T 1 5 - C 5 H 5 ) M O ( N O ) 2 C H 3 ] + [ ( T) 5-C 5H 5) 2Fe ]BF„ > (T) 5-C 5H 5)Mo(N0) 2CH 3 + [(n 5-C 5H 5) 2Co]BF 1 + + ( T I 5 - C 5 H 5 ) 2Fe (5.20) These r e a c t i o n s appear to be clean as i n d i c a t e d by IR monitoring of t h e i r progress which shows n i t r o s y l absorptions only f o r the appropriate ( T I 5 - C 5 H 5 ) M ( N O ) 2CH 3 species (M = Mo, W) i n each r e a c t i o n . Recovery of the n e u t r a l a l k y l complexes lends f u r t h e r support to the formulation of these compounds as [ ( T I 5 - C 5 H 5 ) 2Co ] [ ( T I 5 - C 5 H 5 ) M ( N O ) 2CH 3] (M = Mo, W), that i s cobaltocinium s a l t s of r a d i c a l anion n i t r o s y l complexes. R a d i c a l anion complexes such as these would appear to have l i t t l e precedent i n t r a n s i t i o n metal carbonyl chemistry. As mentioned e a r l i e r , most a l k y l complexes of the type (T] 5-C 5H 5)M(CO) nR and M(C0) nR (M = t r a n s i -t i o n metal, R = a l k y l group) reduce with concomitant expulsion of the a l k y l 16a group. The a b i l i t y of the NO ligands to accept and s t a b i l i z e excess e l e c t r o n d e n s i t y i s no doubt key to the i s o l a t i o n of these new complexes. (Their s o l u t i o n and thermal s e n s i t i v i t y may, however, in v o l v e a l k y l l o s s . ) A l k y l - c o n t a i n i n g t r a n s i t i o n metal r a d i c a l complexes are not unknown though. For e x a m p l e 1 4 6 ( n 5 - C 5 H 5 ) 2 T i C H 2 ( C H 3 ) 3 , ( n 5 - C 5 H 5 ) 2 N b ( C H 3 ) 2 , V(CH 2Ph) 4 and Re(CH 3) 6 are a l l f a i r l y w e l l - c h a r a c t e r i z e d , paramagnetic compounds (although each of these possess a l e s s than 18-electron valence s h e l l ) . (b) Reactions of ( T) 5-C 5H 5)Cr(NO) 2CH 3 w i t h Reducing Agents. Unlike the a l k y l complexes j u s t described, (Ti 5-C 5H 5)Cr(NO) 2CH 3 does not react w i t h ( T ) 5 - C 5 H 5 ) 2 C O to any appreciable extent. This would not appear 205 to be a function of the s o l u b i l i t y of an i n i t i a l l y formed low concentration of the reduced species as i n the case of [ ( T I 5 - C 5 H 5 ) M O ( N O ) 2 C 2 H 5 ] ~ . The reduction p o t e n t i a l of the chromium complex i s 180 mV negative of ( T I 5 - C 5 H 5 ) M O ( N O ) 2 C H 3 ( i n C H 2 C 1 2 ; see Table III) and this probably renders (n 5-C 5H 5)Cr(NO) 2CH 3 too d i f f i c u l t to reduce by cobaltocene i n E t 2 0 . This i s supported by an IR spectrum of the reaction mixture which displays bands only for ( T i 5 - C 5 H 5 ) C r ( N 0 ) 2CH 3 . c c 45 When a f i l t e r e d E t 2 0 s o l u t i o n of (Ti b-C 5H 5)Fe(Ti 6-C 6Me 6) i s mixed with (n 5-C 5H 5)Cr(NO) 2CH 3 i n E t 2 0 an instantaneous reaction occurs and a grey p r e c i p i t a t e forms. This s o l i d i s very pyrophoric, bursting into flame upon contact with a i r . On the basis of the CVs of ( T i 5 - C 5 H 5 ) C r ( N 0 ) 2CH 3 and i t s molybdenum and tungsten analogues and the previously described r a d i c a l anion complexes, i t seems l i k e l y that a r a d i c a l anion i s formed, but i t i s too unstable to be i s o l a t e d pure in this manner. This d i f f i c u l t y i s not s u r p r i s i n g when one considers the CV of (n 5-C 5H 5)Cr (NO) 2CH 3 (Figure 2 6 ) . The lower i / i r a t i o s compared with the Mo and W analogues (Table p,a p,c I I I ) , the second reduction at E = - 1 . 8 7 V and the v a r i a t i o n of peak potentials with scan rate are a l l consistent with the behaviour of t h i s compound toward the chemical reductant (t| 5-C 5H 5)Fe(Ti 6-C 6Meg). Elemental analyses of the grey s o l i d are quite variable and low in nitrogen content. The Nujol-mull IR spectrum of this material exhibits a strong NO absorption at 1555 cm - 1 and a weaker but s t i l l quite strong band at 1509 cm - 1. This i n t e n s i t y pattern does not correspond well to a {M(N0) 2} moiety with l i n e a r n i t r o s y l ligands. A l l the same, the r a d i c a l anion [(n 5-C 5H 5)Cr(NO) 2CH 3]• may perhaps yet prove to be obtainable at low temperature. 206 (c ) [ ( T) 5-C 5H 5) 2CoH(ti 5-C 5H 5)W(NO) 2Y] (Y • H, D ) . These two compounds are best prepared u t i l i z i n g hexanes as the s o l v e n t . The reac-t i o n s can be c a r r i e d out i n the same manner as f o r the a l k y l analogues described above and generate the r a d i c a l anion complexes i n moderate y i e l d s as brown s o l i d s which are s o l u b l e i n CH3CN and DMF, to give r e d - v i o l e t s o l u t i o n s at 2 5 °C. They r a p i d l y decompose i n these solvents at room temperature. Unfortunately, these complexes could not be obtained as a n a l y t i c a l l y pure m a t e r i a l s . The carbon analyses are g e n e r a l l y s l i g h t l y low and i n the case of the deutero analogue, the n i t r o g e n content i s low a l s o . A combustion a d d i t i v e does not improve the analyses. These are o f t e n employed when incomplete combustion i s suspected. They do, however, introduce a p o t e n t i a l problem i n that these a d d i t i v e s are a l s o good o x i d i z i n g agents and can sometimes react with the compound of i n t e r e s t before the a n a l y s i s can be performed. The IR and ESR spectra of these compounds provide good evidence f o r t h e i r formulations as r a d i c a l anion species analogous to the a l k y l -c o n t a i n i n g r a d i c a l anions. The Nujol-mull IR spectrum of [ ( T I 5 - C 5 H 5 ) 2 C O ] [ ( T I 5 - C 5 H 5 ) W ( N O ) 2 H ] d i s p l a y s two strong NO absorptions at 1521 and 1445 cm - 1, c o n s i s t e n t with a h i g h l y e l e c t r o n - r i c h d i n i t r o s y l complex. The lower band overlaps with the strong 1462 cm - 1 absorption of N u j o l . The Nujol spectrum can be subtracted from the experimental spectrum as described above l e a v i n g the lower energy NO band at ~ 1 4 3 8 cm - 1. The W-H s t r e t c h appears as a medium i n t e n s i t y band at 1848 cm - 1, somewhat lower 122 than found for (n 5-C 5H 5 ) W(NO) 2H ( 1 8 9 7 c m - 1 ) . A broad, medium i n t e n s i t y 207 band at 3073 cm _ i i s due to C - H s t r e t c h i n g of the c y c l o p e n t a d i e n y l r i n g s . L i k e w i s e , a medium i n t e n s i t y band at 1411 cm - 1 can be assigned to C - H v i b r a t i o n s of the C5H5 r i n g s . The V ^ Q absorptions of the deutero analogue occur at 1513 and 1447 cm - 1, s i m i l a r , but not i n d e n t i c a l t o , the NO bands of the hydrido complex. Again, the lower appears at ~1439 cm - 1 i f the Nujol spectrum i s subtracted out. A medium i n t e n s i t y band shouldering the 1513 cm - 1 band occurs at 1525 cm - 1. I t would appear to be too weak to be a n i t r o s y l band of [ ( T I 5 - C 5 H 5 ) 2 C O ] [ ( T I 5 - C 5 H 5 ) W ( N O ) 2 D ] and i s at too high an energy to be the W-D s t r e t c h . I t may be due to a small amount of [ ( T I 5 - C 5 H 5 ) 2 C O ] [ ( T ) 5 - C 5 H 5 ) W ( N 0 ) 2 H ] . The precursor deuteride complex (r) 5-C 5 H 5 )W(NO) 2 D has been prepared with an aged L i [ E t 3 B D ] s o l u t i o n and such s o l u t i o n s are known to react with ( T ) 5 - C 5 H 5 ) W (N O ) 2C1 to produce mixtures of 68 122 (r) 5-C 5 H 5 ) W(N0) 2 D and i t s hydrido analogue. ' A l t e r n a t i v e l y , s m all amounts of the hydrido complex may a r i s e from H / D exchange from a v a r i e t y of sources during the r e a c t i o n between cobaltocene and ( T ) 5 - C 5 H 5 ) W ( N O ) 2 D . Of course, the band at 1525 cm - 1 could be due to an u n i d e n t i f i e d impurity as w e l l . The W-D s t r e t c h of [ ( T I 5 - C 5 H 5 ) 2 C O ] [ ( T I 5 - C 5 H 5 ) W ( N O ) 2 D ] may give r i s e to one of the weak bands at 1337 cm - 1 or 1316 cm - 1, n e i t h e r of which are seen i n the IR spectrum the hydrido complex. The band at 1337 cm - 1 i s s h i f t e d down from the W - H band of the hydrido analogue (1848 cm - 1) by a f a c t o r of 1.38 which i s nearly i d e n t i c a l to the s h i f t observed f o r the W - H and W-D s t r e t c h e s of the n e u t r a l complexes (T) 5-C 5 H 5 )W(NO) 2 H and ( T ) 5 - C 5 H 5 ) W ( N O ) 2 D . The band at 1316 cm - 1 i s s h i f t e d down from the W - H s t r e t c h of the r a d i c a l anion by a f a c t o r of 1.40. In any event, the band at 1848 cm - 1 observed i n the IR spectrum of the hydrido-containing anion i s 208 not present i n the IR spectrum of the deutero analogue, f u r t h e r supporting the assignment of the W-H s t r e t c h . 37 The ESR spectra of both of these complexes are q u a l i t a t i v e l y s i m i l a r (Figure 36) and supportive of the formulation of these complexes as r a d i c a l anions analogous to the a l k y l complexes discussed p r e v i o u s l y (see above). The f a m i l i a r 1:2:3:2:1 f i v e - l i n e p a t t e r n i s obvious f o r these species. No coupling to e i t h e r the *H or 2H l i g a n d n u c l e i i s r e s o l v a b l e i n these s p e c t r a . The l i n e widths of the spectrum a r i s i n g from the hydrido complex are somewhat broader, perhaps r e f l e c t i n g a l a r g e r , though unresolved coupling to a *H nucleus compared wi th a 2H nucleus. Thus these complexes again are best thought of as possessing monomeric, "three-legged piano s t o o l " geometries, w i t h extensive d e r e a l i z a t i o n of the a d d i t i o n a l e l e c t r o n d e n s i t y onto the n i t r o s y l l i g a n d s , i . e . 209 Figure 36. ESR spectra of (a) [ ( n 5 - C 5 H 5 ) 2 C o ] [ ( n 5 - C 5 H 5 ) W ( N O ) 2 H ] i n CH3CN at -22°C: g = 2.01186; a N = 7.3 G , and (b) [(r| 5-C 5 H 5) 2Co] [ ( T I 5 - C 5 H 5 ) W ( N 0 ) 2 D ] i n DMF at -22°C: g = 2.00025; a N =7.2 G . 210 Unlike for the syntheses of the a l k y l r a d i c a l complexes, E t 2 0 i s not a suitable solvent for preparing the hydrido analogue. Although a more a t t r a c t i v e m i c r o c r y s t a l l i n e s o l i d i s obtained when the reaction i s c a r r i e d out i n Et 2 0 , s a t i s f a c t o r y C, H and N analyses cannot be obtained, the nitrogen content being e s p e c i a l l y low and v a r i a b l e . An IR spectrum (Nujol mull) of th i s s o l i d reveals that i t does l a r g e l y contain [ (n 5-C 5H 5) 2Co ] [ (n 5-C 5H 5 )W(NO) 2H] as evidenced by v ^ , v w _ R and C 5H 5 absorptions i d e n t i c a l to those of the complex prepared i n hexanes. In E t 2 0 , however, i t may more r e a d i l y decompose, perhaps due to s l i g h t s o l u b i l i t y i n th i s solvent. Certainly when the complex i s obtained from E t 2 0 and washed with this solvent the washings r e t a i n some colouration even after much ether has been used. In accord with the previous discussion on the electrochemistry of ( T ) 5 - C 5 H 5 ) W ( N O ) 2H, the i s o l a t i o n and characterization of [(n 5-C 5H 5) 2Co][(n 5-C 5H 5 ) W(NO) 2H] appears to be quite d i s t i n c t from the chemistry of t r a n s i t i o n metal carbonyl-hydrides, which generally undergo 16a metal-hydride bond s c i s s i o n upon reduction. The long-term thermal i n s t a b i l i t y and the solu t i o n i n s t a b i l i t y (at room temperature) of t h i s r a d i c a l anion may perhaps involve W-H bond cleavage as one pathway of decomposition. Other examples of hydride-containing r a d i c a l complexes include, f o r example, ( r| 5-C 5H 5) 2NbH 2 and [ ( T ) 5 - C 5 H 5 ) 2 T i H 2 ] ~ (also a r a d i c a l . . 146 anion). The oxidation of [ ( r i 5-C 5H 5 ) W(NO) 2 H ] T i s r e a d i l y accomplished by AgBF 4 i n CH 2C1 2. The reaction i s quite rapid and regenerates ( T I 5 - C 5 H 5 ) W ( N O ) 2 H which i s e a s i l y i d e n t i f i e d by i t s c h a r a c t e r i s t i c IR and 211 mass s p e c t r a . The chemical r e v e r s i b i l i t y of the reduction of ( T ] 5 - C 5 H 5 ) W ( N O ) 2 H i s thus f u r t h e r supported. (d) [ ( n 5 - C 5 H 5 ) 2 C o ] [ ( n 5 - C 5 H 5 ) M ( N O ) 2 C l ] (M - Mo, W). The syntheses of these compounds are a l s o r e a d i l y accomplished i n good y i e l d s by mixing E t 2 0 s o l u t i o n s of cobaltocene and the appropriate h a l i d e complex. The s o l i d s thus obtained are h i g h l y pyrophoric, amorphous, pale grey-green (M = Mo) and grey (M = W) powders. Both are quite thermally s e n s i t i v e , the tungsten complex being more so. Thus s o l i d [ ( T l 5 - C 5 H 5 ) 2 C o ] [ ( T } 5 - C 5 H 5 ) W ( N O ) 2 C 1 ] takes on a brown c o l o u r a t i o n overnight under N 2. Both are sol u b l e i n CH3CN and DMF to give greenish s o l u t i o n s which are s t a b l e only at low temperatures ( 25°C). S a t i s f a c t o r y elemen-t a l analyses are obtainable f o r these complexes i f they are kept c o l d (~ -20°C) u n t i l a few minutes p r i o r to the a n a l y s i s being c a r r i e d out. The f a c t that these complexes are acquired as amorphous powders, while the a l k y l - c o n t a i n i n g r a d i c a l anions are c r y s t a l l i n e m a t e r i a l s may be r e f l e c t i v e of two d i f f e r e n c e s . F i r s t l y , the reduction p o t e n t i a l s of (n 5-C 5H 5)M(NO) 2Cl (M = Mo, W) are ~230 mV l e s s negative than the a l k y l complexes, making them more s u s c e p t i b l e to reduction by cobaltocene. Secondly, the i n i t i a l l y formed [(n 5-C 5H 5)M(NO) 2C1]~ anion may be much l e s s s o l u b l e i n E t 2 0 than an a l k y l - c o n t a i n i n g analogue. These f a c t o r s may also be behind the greater i s o l a t e d y i e l d s of the c h l o r o - c o n t a i n i n g r a d i c a l anion complexes. The Nujol-mull IR spectrum of the molybdenum complex d i s p l a y s V NQ bands at very low frequencies (1558, 1485 cm - 1; 1490 cm - 1 with the Nujol spectrum subtracted out as described p r e v i o u s l y ) , as would be expected for 212 [ ( T 1 b - C 5 H 5 ) M o ( N O ) 2 C l ] ~ . These absorptions are 198 and ~165 cm-1 (respec-t i v e l y ) lower than those of (n 5-C 5 H 5)Mo(NO) 2Cl (1756, 1655 cm"1) i n a Nujol m u l l . A broad, medium i n t e n s i t y absorption at 3092 cm - 1 can be a t t r i b u t e d to C - H s t r e t c h i n g of the c y c l o p e n t a d i e n y l r i n g s . The band at 1413 cm - 1 can a l s o be assigned to C - H v i b r a t i o n s a r i s i n g from the C 5 H 5 r i n g s . For [ ( r | 5 - C 5 H 5 ) 2 C o ] [(r) 5-C 5 H 5 ) W(NO) 2C1] the IR spectrum i s somewhat more complex. Two strong NO bands are observed at 1534 and 1456 cm - 1. The l a t t e r a bsorption overlaps n e a r l y c o i n c i d e n t a l l y with the 1462 cm - 1 band of N u j o l . The Nujol spectrum could not be s a t i s f a c t o r i l y subtracted from the experimental spectrum due to other bands al s o overlapped w i t h the 1377 cm - 1 band of N u j o l . The band at 1456 cm - 1, however, obtained from a concen-t r a t e d sample, i s l i k e l y a good estimate of the lower frequency since bands that overlap t h i s c l o s e l y w i t h the Nujol absorption at 1462 cm - 1 do not s h i f t much upon s u b t r a c t i o n of the Nujol spectrum. (Compare w i t h , f o r example, [ ( n 5 - C 5 H 5 ) 2 C o ] [ ( n 5 - C 5 H 5 ) M o ( N O ) 2 C 2 H 5 ] : v (Nujol mull) 1458 cm - 1, with or without the Nujol spectrum s u b t r a c t e d ) . These bands are 192 and ~174 cm - 1, r e s p e c t i v e l y , lower i n frequency than those of n e u t r a l ( n 5 - C 5 H 5 ) W ( N O ) 2 C l (1726, 1630 cm - 1) under the same c o n d i t i o n s . Two other broad bands, both s u b s t a n t i a l l y weaker, also appear i n the IR spectrum of [ ( T I 5 - C 5 H 5 ) 2 C O ] [ ( T ) 5 - C 5 H 5 ) W ( N 0 ) 2 C 1 ] , at 1705 and 1588 cm - 1. S i m i l a r absorptions are not apparent i n the IR spectrum of the molybdenum analogue. I t may be that these new bands r e s u l t from thermal decomposition of the [ ( n 5 - C 5 H 5 ) W ( N O ) 2 C 1 ] V r a d i c a l anion, perhaps even v i a C l ~ l o s s , i . e . [ ( T I 5 - C 5 H 5 ) W ( N O ) 2 C 1 ] ~ > [ ( T l 5 - C 5 H 5 ) W ( N O ) 2 ] • + C l " (5.21) 213 The fate of the putative [ (T) b-C5H5)W(NO) 2 ] • r a d i c a l i s not c l e a r , other than that i t i s proposed to generate the species with the IR bands at 1708 and 1585 cm - 1. This p o s s i b i l i t y i s suggested on the basis of a preliminary i n v e s t i g a t i o n of the reaction of (T) 5-C5H5)W(NO) 2B¥^ with ( T I 5 - C 5 H 5 ) 2Co, which i n CH 2C1 2 produces a brown, insoluble, amorphous s o l i d of as yet undetermined composition with v J J Q ' s a t 1708 and 1585 cm - 1 i n i t s Nujol-mull IR spectrum. This material probably r e s u l t s from a reaction of the i n i t i a l l y formed [ (T) 5-C 5H 5)W(NO) 2]•, i . e . CH 2C1 2 (n 5-C 5H 5)W(N0) 2BF 1 + + (n 5-C 5H 5) 2Co > [ ( T ) 5 - C 5 H 5 ) W ( N O ) 2 ] • + [( Tl 5-C 5H 5) 2Co]BF 1 + (5.22) [ ( T ) 5 - C 5 H 5 ) W ( N O ) 2 ] • > brown s o l i d (5.23) The proposed loss of C l ~ from [(r| 5-C 5H 5)W(NO) 2C1]~ would appear to be slow on the CV time scale i n CH 2C1 2 (Table III) since the electrochemical reduc-ti o n of (r) 5-C 5H 5)W(NO) 2C1 i s quite r e v e r s i b l e . Whether, or not, t h i s proposed C l - loss accompanies the p r e c i p i t a t i o n of the r a d i c a l anion complex from E t 2 0 , occurs during drying of the s o l i d i n vacuo at room temp-erature or while the IR sample i s prepared, or even r e s u l t s from i n t e r -action with the NaCl plates, i s not known at this time. Both [ (T| 5-C 5H 5)M(N0) 2 C 1 ] ~ (M = Mo, W) complexes exhibit ESR spectra in CH.3CN or DMF s i m i l a r to those previously discussed (Figure 37). Inter-e s t i n g l y , however, the ESR spectrum of [(n 5-C 5H 5) 2Co][(n 5-C 5H 5)Mo(NO) 2C1] 214 Figure 3 7 . E S R spectra of (a) [ ( T I 5 - C 5 H 5 ) 2 C O ] [(n 5-C 5 H 5)Mo(NO) 2C1] i n DMF at 25°C: a N * 7.4 G; a M o « 2.5 G, and (b) [ ( n 5-C 5 H 5) 2Co][(n 5-C 5 H 5)W(NO) 2Cl] i n DMF at ~ -35°C: a N = 7.2 G. 215 a(9 5Mo) = a(97Mo) =1 a( 1 4N) Figure 38. Schematic r e p r e s e n t a t i o n of an ESR spectrum derived from coupling of an unpaired e l e c t r o n to two equivalent l 4 N n u c l e i , a 9 5Mo or a 9 7Mo nucleus: a 9 5 M o (15.72%; I = 5/2) = a 9 7 M o (9.46%; I = 5/2) = 1/3 a N -216 e x h i b i t s more hyperfine s t r u c t u r e than j u s t the expected 5 - l i n e p a t t e r n (Figure 37a). I t appears that a second, more h i g h l y coupled spectrum i s superimposed onto the f a m i l i a r 1:2:3:2:1 5 - l i n e p a t t e r n . I t therefore seems u n l i k e l y that t h i s spectrum can be i n t e r p r e t e d as a r i s i n g from coupling to two equivalent 1 1 +N atoms and to the 3 5C1 (75.53%; I = 3/2) and 3 7 C 1 (24.47%; I = 3/2) n u c l e i . A more s a t i s f a c t o r y i n t e r p r e t a t i o n i n v o l v e s coupling to the 9 5Mo (15.72%; I = 5/2) and 9 7Mo (9.46%; I = 5/2) n u c l e i . I f age - ag?,, , and a w = 1/3 a„, an 18-line p a t t e r n accounting f o r ~25% 7 JMo "Mo Mo N of the t o t a l i n t e n s i t y would r e s u l t , superimposed onto a 5 - l i n e p a t t e r n accounting f o r the remaining 75% of the t o t a l i n t e n s i t y . This i s depicted s c h e m a t i c a l l y i n Figure 38 and would give a spectrum s i m i l a r to the one observed In Figure 37a. The r e s o l u t i o n of t h i s spectrum i s not high enough to unambiguously evaluate the coupling constants. The value of a^ (~7.4 G) i s estimated from the i n f l e c t i o n - p o i n t - t o - i n f l e c t i o n - p o i n t separations as shown i n Figure 37 while the value of a ^ i s estimated to be of the order of the peak-to-peak separations of the two outermost l i n e s on e i t h e r s i d e of the spectrum (~2.5 G) as shown i n Figure 37a. Coupling to the 9 5Mo and 9 7Mo n u c l e i i s not resolved f o r e i t h e r of the molybdenum complexes [(n 5-C 5H 5) 2Co][(n 5-C 5H 5)Mo(NO) 2R] (R = CH 3, C 2H 5) under s i m i l a r c o n d i t i o n s . Neither i s coupling to the 1 8 3 W nucleus (14.40%; I = 1/2) re s o l v a b l e f o r any of the r a d i c a l complexes c o n t a i n i n g tungsten, i n c l u d i n g [ ( n 5 - C 5 H 5 ) 2 C o ] [ ( T I 5 - C 5 H 5 ) W ( N O ) 2 C 1 ] (see Figure 37b). (An unperturbed, a l b e i t somewhat b r o a d - l i n e d , f i v e - l i n e p a t t e r n i s observed (a„ = 7.2 G).) 217 Both the ESR spectra and the IR spectra are consistent with the same type of formulation for these complexes as described above for the hydrido and a l k y l species, i . e . _ :o The observation of n i t r o s y l absorptions i n the IR spectrum of the tungsten complex other than those a t t r i b u t a b l e to [ ( T ) 5 - C 5 H 5 ) W ( N O ) 2 C 1 ] • i s t e n t a t i v e l y r a t i o n a l i z e d as being due to a species a r i s i n g from slow C l ~ loss from the r a d i c a l anion (see above). The expulsion of halide ion from t r a n s i t i o n metal carbonyl halide complexes upon reduction i s well 163. c known, * and indeed, as described previously, this i s a useful Q Q synthetic method for forming [(T) 5-C 5H 5)Cr(NO) 2] 2 from (Ti 5-C 5H 5)Cr(NO) 2C1 and Na/Hg or Zn/Hg. However, (n 5-C 5H 5)M(NO) 2C1 (M = Mo, W) do not form analogous dimers upon treatment with reducing metals. The contention that this i s due to formation of the [ ( T ) 5 - C 5 H 5 ) M ( N O ) 2 C 1 ] ~ anions i s supported by 218 th e i r i s o l a t i o n as [ ( T T - C 5 H 5 ) 2 ^ ° I s a l t s and the fact that they r a p i d l y decompose at room temperature i n s o l u t i o n . The d e r e a l i z a t i o n of the extra electron density onto the n i t r o s y l ligands while s t a b i l i z i n g the r a d i c a l anion complex as a whole, also would highly activate the NO ligands toward subsequent chemical reactions. The possible loss of C l ~ from [ ( T I 5 - C 5 H 5 ) 2 C O ] [ ( T ) 5 - C 5 H 5 ) W ( N 0 ) 2 C 1 ] i n the s o l i d state would be a r e l a t i v e l y slow process compared with the instantaneous cleavage of the W-Cl bond of, 130 131 for example, (r) 5-C 5H 5 )W(CO) 3C1 ' upon reduction. The i s o l a t i o n and ch a r a c t e r i z a t i o n of these new chloro-containing r a d i c a l anion complexes i s therefore an i n t e r e s t i n g and unique facet of organometallic n i t r o s y l chemistry. It i s also i n t r i g u i n g that the alkyl-containing r a d i c a l anions are s u b s t a n t i a l l y more robust than the chloro analogues. Generally carbonyl-containing a l k y l compounds are less thermally and a i r - s t a b l e than their related halide complexes. Some organometallic halo-containing 146 r a d i c a l species are known. For example, Cr(C0) 5I i s a well known r a d i c a l complex and another Group 6 metal r a d i c a l i s ( t | 3 - C 5 H 5 ) C r C l 2 . However, few anionic, electron r i c h organometallic r a d i c a l complexes containing halide ligands appear to be known. Chemical oxidation of the [ ( T ) 5 - C 5 H 5 ) M O ( N 0 ) 2 C 1 ] ~ species can be accomplished with AgBF 4 i n CH3CN. The neutral ( T I 5 - C 5 H 5 ) M O ( N 0 ) 2C1 compound i s the only i s o l a b l e n i t r o s y l complex obtained during this r e a c t i o n . This reaction i s consistent with the { ( T ) 5 - C 5 H 5 ) M O ( N O ) 2C1} s t r u c t u r a l e n t i t y maintaining i t s i n t e g r i t y upon reduction. e ) [ (T} 5-C 5H 5)W(N0) 2L] (L - P(0Me) 3 > PPhg). The syntheses of 46 these compounds have been reported by Angelic! and co-workers. The 2 1 9 precursor c a t i o n s [(n 5-C 5H 5)W(NO) 2 L ]PF 6 [ L = PPh 3, P(OPh) 3, P(OMe) 3] can be reduced by a v a r i e t y of reductants, i . e . [ (n 5-C 5H 5)W(NO ) 2 L ] + + reducing agent > [(n 5-C 5H 5)W(NO) 2 L ] • (reducing agent = Zn, N 2H 4«H 2 0 , a l k o x i d e s , 0 H ~ ) ( 5 . 2 4 ) The r a d i c a l complexes can be i s o l a t e d i n low to moderate y i e l d s , depending on the reductant. Cobaltocene a l s o can be used to generate these complexes, i . e . CH 2C12 [(n 5-C 5H 5)W(NO ) 2 L]BF 4 + (n 5-C 5H 5) 2Co > [ ( T I 5 - C 5 H 5 ) W ( N O ) 2 L ] • + [ ( T ) 5 - C 5 H 5 ) 2 C o ] B F t + [ L = PPh 3, P(OMe) 3] ( 5 . 2 5 ) When green s o l u t i o n s of the c a t i o n i c phosphine or phosphite compounds are tre a t e d with an equimolar q u a n t i t y of cobaltocene an immediate r e a c t i o n occurs and the r e a c t i o n mixture becomes red-purple. An IR spectrum of t h i s s o l u t i o n reveals that a l l of the s t a r t i n g m a t e r i a l i s consumed. The r a d i c a l complexes can be i s o l a t e d from the r e a c t i o n mixtures as purple s o l i d s i n the manner described i n the Experimental S e c t i o n . These two 46 complexes have been described as being very a i r - and thermally s e n s i t i v e . When prepared using ( n 5 - C 5 H 5 ) 2 C o , however, they are found to be quite thermally s t a b l e , decomposing only s l i g h t l y overnight i n CH 2C1 2 s o l u t i o n at room temperature. In a d d i t i o n [(r) 5-C 5H 5)W(NO) 2PPh 3] • appears to be more a i r - s t a b l e than f i r s t r eported, e x h i b i t i n g no obvious decomposition as a 220 s o l i d overnight as evidence by an IR spectrum (Nujol mull) of a sample of t h i s complex handled i n t h i s way. I t may be that trace i m p u r i t i e s present i n the m a t e r i a l s obtained v i a r e a c t i o n 5.24 c a t a l y z e the decompositions of the (n 5-C 5H 5)W(N0) 2L compounds prepared i n t h i s manner. I t would appear t h e r e f o r e , that the use of cobaltocene as a reducing agent i s somewhat more pr e f e r a b l e i n these r e a c t i o n s . The s o l u t i o n IR spectrum of (n 5-C 5H 5)W(NO) 2PPh 3 ( i n CH 2C1 2) prepared by r e a c t i o n 5.25 using ( n 5 - C 5 H 5 ) 2 C o ( v N 0 = 1598, 1532 cm - 1) corresponds f a i r l y w e l l to that reported p r e v i o u s l y (1595, 1526 c m - 1 ) . The d i f f e r e n c e f o r the t r i m e t h y l p h o s p h i t e complex i s somewhat greater (1608, 1542 cm - 1 f o r the product from r e a c t i o n 5.25 and 1605, 1533 cm - 1 f o r the 46 species obtained by r e a c t i o n 5.24). The trimethylphosphite complex has 46 been reported as being too unstable to o b t a i n a s o l u t i o n ESR spectrum. When prepared according to r e a c t i o n 5.25, however, (r) 5-C 5H 5)W(NO) 2P(0Me) 3 can be i s o l a t e d and does e x h i b i t a 1 0 - l i n e ESR spectrum i n CH3CN at 25°C with a N = 6.8 G and a p - 4.5 G (Figure 39), q u i t e s i m i l a r to that reported c 46 f o r (n 0-C 5H 5)W(NO) 2PPh 3 i n acetone. This i s c o n s i s t e n t w i t h coupling of the odd e l e c t r o n to two equivalent 1 1 +N n u c l e i (as described above f o r the r a d i c a l anion complexes) and to a 3 1 P nucleus of P(0Me) 3. Oxidation of (T|5-C5H5)W(NO) 2P(0Me) 3 by AgBF 4 i n CH 2C1 2 r a p i d l y regenerates [ (T i 5-C 5H 5)W(NO) 2P(OMe) 3]BF l t as i n d i c a t e d by IR monitoring of the r e a c t i o n ' s progress. A *H NMR spectrum of the i s o l a t e d green s o l i d i n CDC13 reveals the presence of a coordinated P(0Me) 3 l i g a n d 221 Figure 39. ESR spectrum of (n 5-C 5H 5)W(NO) 2P(OMe) 3 in CH3CN at 25°C: a„ = 6.8 G; a = 4.5 G. N p [6 3.88 (d, 3 J 3 i p _ i H = 12.0 Hz)]. Uncomplexed P(0Me) 3 i s known to 147 isomerize to OP(Me)(OMe)2> however, t h i s does not appear to be a problem in the preparation of (t) 5-C 5H 5)W(NO) 2P(0Me) 3 described herein. 222 IV) Summary Table VI l i s t s the n i t r o s y l s t r e t c h i n g frequency data of the i s o l a t e d paramagnetic complexes described i n t h i s study and the E 1 / 2 data of the various n e u t r a l precursors i n CH 2C1 2. P l o t s of E ^ / 2 V S * V^Q are shown i n Figure 40. I t seems l i k e l y that the o x i d a t i o n p o t e n t i a l s of the [ ( T I 5 - C 5 H 5 ) M ( N O ) 2 Y ] ~ (M = Mo, W; Y = C l , H, CH 3, C 2H 5 as appropriate) anions generated during a re d u c t i o n scan f o r a given ( T ) 5 - C 5 H 5 ) M ( N O ) 2 Y compound i n CH 2C1 2 would be the same as those of the [(T) 5-C 5H 5) 2Co] [(T) 5-C 5H 5)M(N0) 2 Y ] i s o l a t e d compounds, o r , at l e a s t they would be s i m i l a r . In other words, E 1 / 2 measured f o r the reduction of a (n 5-C 5H 5)M(NO) 2 Y complex i n CH 2C1 2 should be very s i m i l a r to E 1 / 2 f o r the o x i d a t i o n of the r a d i c a l anion obtained as i t s [ ( n 5 - C 5 H 5 ) 2 C o ] + s a l t (minor e f f e c t s due to the c o b a l t o -Table VI. N i t r o s y l IR S t r e t c h i n g Frequencies of the I s o l a t e d R a d i c a l Species and Their Oxidation P o t e n t i a l s . Complex 3 v N 0 b ( c m - l ) E C [ ( T I 5 - C 5 H 5 ) M O ( N O ) 2 C 2 H 5 ] ~ 1527 1458 -0.86 [(n 5-C 5H 5)Mo(NO) 2CH3]~ 1526 1443 -0.83 [ ( T I 5 - C 5 H 5 ) M O ( N O ) 2 C 1 ] ~ 1558 1490 -0.59 [(n 5-C 5H 5)W(N0) 2CH 3]" 1511 1427 -0.83 [(n 5-C 5H 5)W(N0) 2 H F 1521 1438 -0.79 [(Ti 5-C 5H 5)W(N0) 2C1]~ 1534 1456 -0.62 [(n 5-C 5H 5)W(N0) 2PPh 3]. 1594 1527 -0.14 [(r,5-C5H5)W(NO)2P(OMe)3]« 1593 1533 -0.10 (a) A n i o n i c complexes as [ ( T I 5 - C 5 H 5 ) 2 C O ] + s a l t s , (b) N u i o l m u l l (c) For the couple [ ( n 5 - C 5 H 5 ) M ( N O ) 2 Y ] n T -e~ [ ( T ) 5 - C 5 H 5 ) M ( N O ) 2Y] ^ 1 " n ) + (n = 0,1) i n C H 2 C 1 2 (V vs. S C E ) . 223 O O ° 1400 1450 1500 1550 C M-1 u NO (Nujol mull) Figure 40. Plots of E 1 / 2 for the couple [ ( r i 5 - C 5 H 5 ) M ( N O ) 2 Y ] N * -e" ^  *" [ ( n 5 - C 5 H 5 ) M ( N O ) 2 Y ] ( 1 - n ) + (n = 0,1) in CH-Clj vs. v N Q (Nujol mull) of [ ( n 5 - C 5 H 5 ) 2 C o ] [ ( T ) 5 - C 5 H 5 ) M ( N O ) 2 Y ] and [(n=-C 5H 5)M(NO) 2Y]• complexes (M = Mo, W; Y = CH 3, C 2H 5, H, C l , PPh 3, P(0Me) 3 as appropriate). 224 cinium counterion not w i t h s t a n d i n g ) . Thus the c o r r e l a t i o n observed between E 1 / 2 a n d VNO ^ n ^ l S u r e *0 I s a n t i c i p a t e d to be sound. As would be expected, the lower the v N Q ' s °f t n e i s o l a t e d complex are, the e a s i e r i t i s to o x i d i z e . Such trends are w e l l known f o r the o x i d a t i o n of n i t r o s y l complexes. For example, the o x i d a t i o n p o t e n t i a l s of [Cr(NO)(CNR) 5]PF 6 (R = 148 a l k y l , a r y l groups) have been found to be l i n e a r w i t h v ^ Q « L i k e w i s e , E l / 2 ( o x i d a t i o n ) and are l i n e a r l y r e l a t e d f o r the complexes [(•n 5-C 5H 5)Mn(NO)(L)L'] n + (L, L' = n i t r o g e n - and/or phosphorus-containing 2-electron donor li g a n d s and various bidentate s u l f u r - c o n t a i n i n g l i g a n d s ) . The reduction p o t e n t i a l s of [(bipy) 2Ru(N0)X] (X = h a l i d e s , pseudo-halides and 2-electron nitrogen donors) a l s o vary l i n e a r l y w i t h 149 V ^ Q . The f a c t that t h i s dependence of E 1 / 2 on v N Q o f t e n a r i s e s may r e f l e c t the n - a c i d i t y of NO. C e r t a i n l y the NO ligands i n the new complexes described i n t h i s work appear to have a profound i n f l u e n c e , markedly s t a b i l i z i n g these reduced s p e c i e s . As can be seen from the preceding d i s c u s s i o n , for a wide range of compounds, the NO l i g a n d often dominates the reduction behaviour of these complexes. The s y n t h e t i c s t r a t e g y employed i n preparing the new organometallic n i t r o s y l r a d i c a l anions described herein i s a simple but e f f e c t i v e one. A solvent i s chosen f o r which the reductant and oxidant ( i . e . the n i t r o s y l complexes) are both s o l u b l e but the e l e c t r o n - t r a n s f e r product i s not. This f a c i l i t a t e s the i s o l a t i o n of the desired i o n i c compounds as f a i r l y pure s o l i d s while avoiding a c c e l e r a t e d decomposition rates that they might experience i f the products remained s o l u b l e . Secondly, the reducing agent becomes a bulky counterion. Other reductants, such as Na or Zn, r e s u l t i n 225 the formation of small counterions which can strongly i n t e r a c t with the 74 n i t r o s y l ligands and thus d e s t a b i l i z e an anionic complex by p o l a r i z i n g the metal-ligand linkage. The synthesis and characterization of the new r a d i c a l complexes are of i n t e r e s t i n th e i r own r i g h t s , representing a l i t t l e explored area of the chemistry of Group 6 organometallic n i t r o s y l compounds. The anionic complexes described i n this study j o i n a small family of simple, n i t r o s y l - c o n t a i n i n g anions which only i n the l a s t f i v e years has begun to grow more s t e a d i l y . 226 EPILOGUE The u t i l i z a t i o n of c y c l i c voltammetry i n t h i s study has led to new insights into the chemistry of organometallic n i t r o s y l complexes. At the outset of this work i t had not occurred to most workers in the f i e l d to consider the p o s s i b i l i t y of encountering f a c i l e , chemically r e v e r s i b l e reductions for complexes of the {( T I 5 - C 5 H 5 ) M ( N O ) 2} moieties. Yet, as has been seen i n the preceding chapters such redox behaviour sheds l i g h t on the reactions of these compounds with nucleophiles. An electrochemical inves-t i g a t i o n of the oxidations of ( T I 5 - C 5 H 5 )M ( N O ) 2 a l k y l complexes i n comparison with related carbonyl-alkyl compounds, while not solving the question of what mechanisms are operative i n t h e i r respective reactions with e l e c t r o -p h i l e s , does suggest that there i s a marked difference i n t h e i r reaction pathways as has been outlined i n Chapter 4. An understanding of the redox properties of organometallic n i t r o s y l complexes thus has been very f r u i t f u l . 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Naturforsch. B. 1970, 25B, 786. (139) Geiger, W.E.; Rieger, P.H.; Tulyathan, B.; Rausch, M.D. J . Am.  Chem. Soc. 1984, 106, 7000. (140) Geiger, J r . , W.E. J . Am. Chem. Soc. 1974, 96, 2632. (141) The X-ray d i f f r a c t i o n data f o r [ ( T I 5 - C 5 H 5 ) 2 C o ] [ ( n 5 - C 5 H 5 ) M o ( N 0 ) 2 C 2 H 5 ] was c o l l e c t e d and analyzed by Dr. R. Jones and Dr. F.W.B. E i n s t e i n : monoclinic; space group P2 1/n; a = 8.714(4) A, b = 15.768(4) A, c= 12.874(3) A; 6 = 105.29(4)°; V = 1706.3 A 3; Z = 4; absorption c o e f f i c i e n t = 16.98 cm - 1; d i f f r a c t o m e t e r , Enraf-Nonius CAD4F; r a d i a t i o n , Mo Ka, graphite monochromator (\(Ka,) = 0.70930 A); scan 238 range = 0 < 29 < 50°; r e f l e c t i o n s = 1828 with Io > 3aIo; R = 0.034, R = 0.034; e r r o r i n observation of u n i t weight = 1.04. A l l w non-hydrogen atoms were r e f i n e d . The hydrogen atoms were included i n t h e i r t h e o r e t i c a l p o s i t i o n s . In the course of the refinement, s u b s t a n t i a l d i s o r d e r i n the anion became apparent. Two o r i e n t a t i o n s were discerned f o r the e t h y l group and as a r e s u l t of t h i s i t became p o s s i b l e to f i n d the p o s i t i o n s of the n i t r o s y l l i g a n d s . The two o r i e n t a t i o n s were r e f i n e d with r e s t r a i n t s being a p p l i e d to the MoNO and MoC 2H 5 fragments. Only one o r i e n t a t i o n i s shown i n Figure 32. (142) The Co-C distances of [ ( n 5 - C 5 H 5 ) 2 C o ] [ ( n 5 - C 5 H 5 ) M o ( N O ) 2 C 2 H 5 ] , which range from 2.002(7) to 2.023(7) A, are comparable to those of c 143 144 [(n 5-C 5H 5) 2Co]C10 i t [Co-C(av) = 2.028(7) A] ' and shorter than c 145 those of n e t u r a l ( n 5 - C 5 H 5 ) 2 C o [Co-C(av) = 2.096(8) A]. The only p o s s i b l y unusual feature about the cobaltocinium counterion i n [ ( T ) 5 - C 5 H 5 ) 2 C O ] [ ( T } 5 - C 5 H 5 ) M O ( N O ) 2 C 2 H 5 ] i s that the C 5H 5 rings are i n an e c l i p s e d conformation, whereas f o r [(T) 5-C5H 5) -JCOJCIOI^ they are 144 r e p o r t e d l y staggered. (143) Frasson, E.; Bombieri, G.; Panattoni, C. Acta C r y s t a l l o g r . 1963, 16_, A68. (144) Kemmitt, R.D.W.; R u s s e l l , D.R. i n "Comprehensive Organometallic Chemistry"; W i l k i n s o n , G. Ed.; Pergamon Press: Willowdale, Ontario, 1982, Volume 5, pp. 244-245. (145) BUnder, W.; Weiss, E. J . Organomet. Chem. 1975, 92, 65. (146) See reference 79, Chapter 3. (147) Marck, V. Mech. Mol. Migr. 1969, 2, 319. 239 (148) L l o y d , M.K.; McCleverty, J.A. J . Organomet. Chem. 1973_, 61_, (149) Callahan, R.W.; Meyer, T.J. Inorg. Chem. 1977, 16, 574. 240 APPENDIX Selected Infrared Spectra of Compounds Described i n this Thesis. 243 244 245 246 [ ( r i 5 - C 5 H 5 ) C r ( N O ) 2 ] 2 as a N u j o l m u l l . u • o eoz-ss /.as *it z.B6 •oe et>e'oz SBZI. *B O T B S - O -3 3 N V J . J . I W S N V a j . X 247 [ ( n 5-C 5H 5)Fe (T i 6-C 6Me 6)]PF 6 as a Nujol mull. u a a eee -ee /LBB *OB OQO -OS eee *ee Laa -ox oooo •• 3 3 N V 1 1 I W S N V f c U X 248 t(ri 5-C 5H 5)Cr(NO) 2(CH 2NOH)]PF 6 as a Nujol mull. 249 250 3 3 N V J . X I W S N V a i X 251 [ ( T I 5 - C 5 H 5 ) 2 C O ] [ ( n 5 - C 5 H 5 ) M o ( N O ) 2 C H 3 ] as a Nujol mull. 252 (TT - CCHC)Mo(NO)oCHo as a hexanes solution u a • a ooo -os ooo "as ooo ooo ~az aaaa -a 3 O N v i ± i W S N v y i x 254 255 256 • I i n r a xofnN e SB [ e H 0 3 ( O N ) M ( S H S 0 - G U ) ] [ O Q z ( 5 H S 0 - s u ) 1 258 •Ct-Hc )W(NO) ,CH, as a hexanes s o l u t i o n . u o aoa 'as ooa *Q9 oao 'av ooo *a2 oooa *o 33HVXXI WSNVH1X 259 [ ( T I 5 - C 5 H 5 ) 2 C O ] [ ( T I 5 - C 5 H 5 ) W ( N O ) 2 H ] as a Nujol mull. 260 [ ( T I 5 - C 5 H 5 ) 2 C O ] [(n 5 - C 5 H 5)W(N0 ) 2 D ] as a Nujol mull. 261 (ri b-C 5H 5)W(NO) 2H as a hexanes s o l u t i o n . 262 [ ( T I 5 - C 5 H 5 ) 2 C O ] [ ( T I 5 - C 5 H 5 ) W ( N O ) 2 C 1 ] as a Nujol mull. a o d T N Q O Q in 2 I E - D S t ? 2 I - 2 f 9 E 6 " E E Bf L '92 6 S S I I "S 3 3 N V l l I H S N V a i % 263 264 265 [(T 1 5-C 5H 5)W(NO) 2P(OMe) 3]BF l + as a Nujol mull. 2 6 7 ( T i 5-C 5H 5)W(NO) 2PPh 3 as a CH 2C1 2 s o l u t i o n . a • • _ o o a • in 851 "SS O f ! "2Z. E2I "65 SOX "Sf BSD "EE O/LO "OS 3 3 N V 1 1 I W S N V y j . % 

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