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Synthetic utilization of the redox properties of some group 6 organometallic nitrosyl complexes Richter-Addo, George Bannerman 1988

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SYNTHETIC UTILIZATION OF THE REDOX PROPERTIES OF SOME GROUP 6 ORGANOMETALLIC NITROSYL COMPLEXES By GEORGE BANNERMAN RICHTER-ADDO B . S c . (Hons), U n i v e r s i t y of Cape Coast , 1982 D i p . E d . , U n i v e r s i t y of Cape Coast , 1982 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 thes i s as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA September 1^88 © George Bannerman Richter -Addo, 1988 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of C rf 6>> (STfLty The University of British Columbia Vancouver, Canada Date / l ™ QCT08eZ} l^j . DE-6 (2/88) ( i i ) Abstract The redox behavior of a s e r i e s of organometal l ic complexes conta in ing Cp'M(NO) groups (Cp' = n , 5 -C 5 H 5 (Cp) or q 5 - C 5 M e 5 ( C p * ) ; M = Mo or W) has been i n v e s t i g a t e d both by c y c l i c voltammetry and by chemical means. The neu t ra l 16 -e lect ron C p ' M o ( N 0 ) X 2 compounds (X = C l , Br or I) undergo a s i n g l e , e s s e n t i a l l y r e v e r s i b l e , one -e lec t ron reduct ion i n C I^C^ /O . IM [n-Bu^N]PFg at r e l a t i v e l y low p o t e n t i a l s ( < - 0 . 1 V vs SCE). The e l e c t r o c h e m i c a l l y observed reduct ions can be e f f e c t e d on a prepara t ive s c a l e by employing C P 2 C 0 as the chemical reductant . The i s o l a b l e 17-e lect ron [Cp'Mo ( N O X 2 ] r a d i c a l anions are c l e a n l y reconverted to t h e i r 16 -e lect ron neut ra l precursors by treatment with [Cp^FejBF^. In c o n t r a s t , the Cp'WtNO)^ compounds undergo r a p i d decomposit ion to t h e i r [Cp'VKNCOI^ monohalo dimers upon e lec t rochemica l r e d u c t i o n . E l e c t r o p h i l e s NE.+ (E = 0 or p-O^NCgH^N) undergo unprecedented i n s e r t i o n s in to the Cr -C o-bonds of CpCr(NO) 2R complexes (R = Me, CH 2 SiMe 3 or Ph) to a f f o r d [ C p C r ( N 0 ) 2 { N ( E ) R } ] * c a t i o n i c complexes. Present evidence i s cons is ten t with these i n s e r t i o n s occur r ing v i a c h a r g e - c o n t r o l l e d , in termolecular at tacks by NE + at the Cr-R groups i n c l a s s i c a l S„2 p rocesses . The newly-formed N(E)R l igands func t ion as Lewis bases through n i t rogen atoms toward the formal ly 16 -e lect ron [ C p C r ( N O ) 2] + ca t ions and may be d i s p l a c e d from the chromium's coord ina t ion sphere by the more s t r o n g l y coord inat ing C l " an ion . The r e s u l t i n g CpCr tNO^Cl can be reconverted to CpCrtNO^R. thereby completing a c y c l e by regenerat ing the i n i t i a l organometal l ic reac tan t . The e n t i r e sequence of s t o i c h i o m e t r i c reac t ions forming the c y c l e thus c o n s t i t u t e s a s e l e c t i v e method ( i i i ) for the formation of new carbon-n i t rogen bonds, the net organic conversions mediated by the CpCr(NO) 2 group being NE + + R" -• N(E)R. * The e l e c t r o p h i l i c [Cp'M(NO) 2 ] + ca t ions (Cp'=Cp or Cp ; M = C r , Mo or W) condense with methyl p r o p i o l a t e and 2 ,3-d imethyl -2-butene to a f f o r d c a t i o n i c organometa l l ic lactone complexes. These complexes undergo f a c i l e O - d e a l k y l a t i o n to y i e l d the neut ra l Cp'M(NO)^{.r\^-lactone) d e r i v a t i v e s . Furthermore, the neut ra l Cp'W(NO)^(r\^"-lactone) compounds decompose i n a i r to t h e i r Cp'W(O)_(n^-lactone) dioxo products . ( i v ) TABLE OF CONTENTS A b s t r a c t i i L i s t o f T a b l e s v i i L i s t o f F i g u r e s i x Acknowledgement . . . x i i L i s t o f A b b r e v i a t i o n s x i i i C h a p t e r 1 G e n e r a l I n t r o d u c t i o n 1 C h a p t e r 2 E l e c t r o c h e m i c a l S t u d i e s o f t h e Complexes [ C p ' M ( N O ) X 2 ] n M = Mo o r W ; Cp ' = n 5 ~ C 5 H 5 ( C p ) o r r | 5 - C - 5 M e 5 (Cp") ; X = C l , B r o r I ; n = 1 o r 2 ] : S y n t h e s i s and C h a r a c t e r i z a t i o n o f t h e [ C p ' M o ( N O ) X 2 ] R a d i c a l A n i o n s and [Cp " w (N0 ) I ] 2 5 - I n t r o d u c t i o n 6 - E x p e r i m e n t a l S e c t i o n 8 E l e c t r o c h e m i c a l Measurements 9 ESR Measurements 11 - R e s u l t s and D i s c u s s i o n 17 Molybdenum Complexes 17 C y c l i c V o l t a m m e t r y S t u d i e s 17 P r e p a r a t i o n o f t h e R a d i c a l A n i o n Complexes 24 D i r e c t E v i d e n c e f o r t h e I n v o l v e m e n t o f S i n g l e E l e c t r o n T r a n s f e r (SET) D u r i n g t h e R e a c t i o n s o f G r i g n a r d R e a g e n t s w i t h t h e C p ' M o ( N O ) X 2 C o m p l e x e s . I m p l i c a t i o n s f o r t h e Improved S y n t h e s e s o f t h e C p ' M o ( N O ) R 2 Compounds 31 T u n g s t e n Comp lexes 35 (v) The Cp'W(NO)X2 Complexes 35 The Tungsten Cp'W(NO)(CH2SiMe3)2 Dialkyl Complexes 39 - Summary A3 - References and Notes 45 Chapter 3 Insertions of Electrophiles into Metal-Carbon Bonds: Formation of New Carbon-Nitrogen Linkages Mediated by the CpCr(NO)2 Group 52 - Introduction 53 - Experimental Section 54 - Results and Discussion 66 Insertions of the Nitrosonium Ion into Chromium-Carbon Bonds 66 Possible Mechanisms for the Nitrosonium Insertion Reactions 73 A Stoichiometric Cycle for the Formation of New Carbon-Nitrogen Bonds 83 Deprotonation of the Formaldoxime Ligand in [CpCr(NO)2{N(OH)CH2)]PF6 89 - Summary 96 - References and Notes 98 Chapter 4 Some Characteristic Chemistry of the Electrophi l ic [Cp'M(NO)2]BF4 Complexes (Cp* = Cp or Cp"; M = Cr, Mo or W) 102 - Introduction 103 - Experimental Section 105 - Results and Discussion 112 (vi) Generation of [Cp'M(NO)2]BF^ (Cp* = Cp or Cp ; M = Cr, Mo or W) 112 Reactions of [Cp'M(NO)^]BF^ with 2,3-Diraethyl-2-butene and Methyl Propiolate 115 Preparation of the Neutral I 1 Cp*M(NO)2-C=C(H)C(Me)2C(Me)2OC(=0; Lactone Complexes (7-12) 124 I 1 Decomposition of the CpW(NO)2-C=C(H)C(Me)2C(Me)2OC(=0) Complexes in Air 132 - Summary 133 - References and Notes 136 Epilogue 140 Appendix 141 ( v i i ) LIST OF TABLES Table 2.1 Data for the Reduction of Dihalo Complexes of Molybdenum . . . . 18 Table 2.2 A Comparison of the N i t r o s y l - S t r e t c h i n g Frequencies of the New 17-e lect ron Rad ica l Anion Complexes with t h e i r Neutra l Precursors 26 Table 2.3 E l e c t r o n Spin Resonance Data for the [Cp 2Co][Cp'Mo(NO)X 2] Rad ica l Anion Complexes 28 Table 2.A E lec t rochemica l Data for the E l e c t r o r e d u c t i o n of Cp W(NO)I 2 i n C H 2 C 1 2 fo r a V a r i e t y of Scan Rates 41 Table 3.1 C y c l i c Voltammetry Data for the Oxidat ions of Some CpCr(N0) 2R Complexes 76 Table 3.2 Se lected Bond Lengths (A) and Angles (deg) for [CpCr(NO) 2 (N(NC 6 H A N0 2 )Me}] + BF 4 ~ 85 Table 3.3 Se lected Bond Lengths (A) and Angles (deg) for the 2 [{CpCr(N0) 2} 2{u,n, -N(CH 2 )0 } ] + c a t i o n as i t e x i s t s i n i t s BPh, " s a l t 91 4 Table 4.1 IR Data for Var ious Cp'M(N0) 2X Complexes i n C H 2 C 1 2 113 Table 4.2 Numbering Scheme for the C a t i o n i c and Neutra l Lactone Complexes 116 Table 4.3 P h y s i c a l Data for the C a t i o n i c Lactone Complexes 1-6 . . . . 119 Table 4.4 *H NMR Chemical S h i f t s of the C a t i o n i c Lactone Complexes 1-6 120 13 Table 4.5 C NMR Chemical S h i f t s of Some Lactone Complexes 121 Table 4.6 P h y s i c a l and Mass Spec t ra l Data for the Neutra l Lactone Complexes 126 ( v i i i ) Table 4.7 H NMR Chemical S h i f t s of the Neutral Lactone Complexes 127 Table 4.8 Selected Bond Lengths (A) for the CpMo (NO) (n.1 -lactone) compound 9 130 Table 4.9 Selected Bond Angles (deg) for the CpMo (NO) 2 (n.'-lactone) compound 9 131 (ix) LIST OF FIGURES F igure 2.1 The low-temperature c y c l i c voltammetry c e l l : (A) Pt-bead working e l e c t r o d e ; (B) Pt -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 h o l d e r ; (E) Luggin probe; (F) g lass jacket for coolant 10 F igure 2.2 Ambient temperature c y c l i c voltammogram of CpMoCNO)^ i n C l ^ C ^ conta in ing 0.1 M [n-Bu^N]PF^ measured at a p la t inum-bead e lect rode at a scan rate of 0.24 V s ^ 20 F igure 2.3 C y c l i c voltammogram of CpMotNO)^ i n C H ^ C ^ at a scan ra te of 0.44 V s " 1 22 F igure 2.4 C y c l i c voltammogram of [CpMo(N0)I]2 i n C I ^ C ^ at a scan rate of 0.14 V s _ 1 23 F igure 2.5 The ESR spectrum of [Cp 2Co] [CpMo(N0)Cl 2] i n DMF 29 A F igure 2.6 The ESR spectrum of [Cp 2Co] [Cp Mo(N0)Br 2] i n DMF 30 F igure 2.7 The ESR spectrum of a DMF s o l u t i o n of an equimolar mixture of Me 3 SiCH 2 MgCl and Cp*Mo(N0)Br 2 at 20°C 33 F igure 2.8 C y c l i c voltammograms of CpWtNO)^ i n C ^ C ^ at a scan rate of 0.32 V s (a) scanning p o s i t i v e p o t e n t i a l s f i r s t , and (b) scanning negat ive p o t e n t i a l s f i r s t 36 * Figure 2.9 C y c l i c voltammograms of Cp W(N0)I 2 i n C H 2 C l 2 at d i f f e r e n t scan rates (a) 2.64 V s - 1 (b) at 0.53 V s _ 1 and (c) at 0.13 V s " 1 38 F igure 2.10 P lo t of i / i vs scan rate for the f i r s t r e v e r s i b l e 6 - p , a - p , c * redox couple of Cp W(N0)I i n CH C l •. 40 ( X ) Figure 2.11 Figure 3.1 Figure 3.2 Figure 3.3 Figure 3.A Figure 3.5 Figure 3.6 Figure 3.7 Temperature dependence of i / i values as a function -p ,a -p ,c of scan rate for the f i r s t reversible redox couple of Cp"w(NO)I2 in THF 42 The solid-state molecular structure of the [CpCr(NO)2(N(OH)CH2)]• cation 68 -4 Ambient temperature cycl ic voltammogram of 5 x 10 M CpCr(N0)2Me in CH 2 C1 2 containing 0.1 M [n-Bu 4N]PF 6 measured at a platinum-bead electrode at a scan rate of 0.30 V s _ 1 75 The 300 MHz 1 H NMR spectrum of [CpCr(NO)2{N(NC^H^N02)Me)]BF^ in CT>3N02(*) 82 Views of the molecular structure of the [CpCr(NO)2{N(NC6H4N02)Me]]+ cation (a) approximately paral le l to the Cp ligand and (b) approximately perpendicular to the Cp ligand. Only non-hydrogen atoms are shown 84 Solid-state molecular structure of the [{CpCr(NO)} 2{u, 2 H -N(CH 2)0}] + cation as i t exists in i t s BPh^" salt . Hydrogen atoms have been omitted for c lar i ty 90 The 300 MHz 1 H NMR spectrum of [{CpCr(NO) 2) 2{u,n 2-N(CH 2)0}]PF 6 in CD 2C1 2 93 The 75 MHz 1 3 C NMR spectrum of [{CpCr(NO) 2) 2{u,n 2-N(CH 2)0)]PF 6 in CD 3N0 2 . The inset contains an expansion of the signals due to the Cp carbons at 6 105.94 and 105.06 ppm 94 ( x i ) Figure A . l The 300 MHz 1 H NMR spectrum o f complex 3 i n acetone-dg 118 Figure A.2 The 300 MHz *H NMR spectrum o f complex 9 i n acetone-d_ 6 128 Figure A.3 S o l i d - s t a t e molecu la r s t r u c t u r e o f the C p M o C N O ^ C n / - ! 3 ^ 0 1 1 6 ) complex 9 129 (xii) ACKNOWLEDGMENTS My sincere gratitude goes to ray research supervisor, Professor Peter Legzdins, for being a constant source of support to me (both academically and personally) over the years. Without question, whatever I have achieved academically over the last 4 years has been largely due to his high standards and energetic enthusiasm for hard work. I also want to thank the occupants of 319/325 (Neil , Jeff, Be, Teen, Nancy and Everett) for providing the friendly research environment, and Neil and Nancy for proofreading this thesis. My gratitude goes to Drs. F .G. Herring, L. Weiler and F. Aubke for reading portions of this thesis and making suggestions for improvement. I would also l ike to thank Ms. Lani Coll ins „and Ms. Bev Gray for typing this thesis. I also acknowledge the financial support of the University of Br i t i sh Columbia in the form of a Graduate Fellowship (1985-87). F ina l ly , I thank a very special friend, Ms. Lethika Raveendran, for her loving friendship and support. ( x i i i ) L I S T OF A B B R E V I A T I O N S Cp n^-cyc lopentadienyl * 5 Cp rj -pentamethylcyclopentadienyl Me C H 3 , methyl Ph C 5 H 5 , phenyl E t 2 0 - ( C H 3 C H 2 ) 2 0 , d i e t h y l ether THF C^HgO, te t rahydrofuran C C D , benzene-d. 6 6 - 6 CDC1, ch loro form-d . 3 -1 C D 2 C 1 2 - dichloromethane-d_ 2 CD.NO„ n i t r o m e t h a n e - d „ 3 2 -3 (CD 3 ) 2 CO - ace tone -d , - 0 IR i n f r a r e d NMR nuclear magnetic resonance MS mass spectrometry m/z mass-to-charge r a t i o i n the mass spectrum P + molecular ion ( in the mass spectrum) CV c y c l i c voltammetry SCE saturated calomel e lec t rode V v o l t s 1 3 c ih) - proton decoupled carbon-13 (xiv) ESSE QUAM VIDERI: 10 BE IS BETTER THAN TO SEEM TO BE. -Motto of the Accra Academy, Ghana 1 C H A P T E R 1 General Introduction 2 Organometallic nitrosyl complexes are compounds that contain both nitric oxide and organic groups coordinated in some fashion to one or more metal centers. In monometallic nitrosyl complexes, the nitrosyl ligands may participate in two principal bonding modes: a) terminal, linear M-NO b) terminal, bent M-NO The terminal, linear bonding mode of the NO ligand is very common in organometallic nitrosyl chemistry, and four simple resonance forms for this bonding mode can be envisaged, + - - + - + MEN-0 « • M*=N=0 < • M=N=0 « • M-NEO When linearly bonded, the nitrosyl ligand is considered to be a formal three-electron donor. Furthermore, the MNO bonding involves a synergic * combination of o-donation from the nitrosyl ligand to the metal and MTT •* NOTT back-bonding. As expected, the extent of metal n-donation is largely dependent on the degree of electron-richness of the metal center. In principle, therefore, an increase in the electron-richness of the metal center should result in increased back-bonding to the nitrosyl ligand and thus decrease the NO bond order and its infrared (IR) stretching frequency (V^Q). Indeed, this shift in is one of the most sensitive probes to changes in the electron-richness of the metal center in an organometallic nitrosyl complex. The terminal, bent bonding mode of the nitrosyl ligand is by far less common than its linear form. Formally, the bent NO ligand is an analogue of an organic nitroso group or the NO group in C1N0. The MNO linkage consists of a 3 doubly bonded NO group, a single o-bond between the nitrogen and the metal, and a lone pair of electrons on the nitrogen atom, i.e. M-Nx V 2 The nitrogen atom in this case is sp hybridized and the resulting M-N-0 is bent. When bonded in this way, the nitrosyl ligand is a formal one-electron donor. The work presented in this thesis deals primarily with the determination and synthetic utilization of the redox properties of some Group 6 organometallic nitrosyl complexes containing the Cp'M(NO) or Cp'MCNO^  5 5 * fragments (Cp' = n -C^CCp) or n, -C5Me5(Cp ) ; M = Cr, Mo or W). (M = Cr, Mo or W; R = H or Me) 4 A major impetus for this research is embodied in the bel ief that a knowledge of the redox properties of a class of compounds w i l l provide a clearer understanding of their known chemistry as well as fac i l i ta te the development of new chemistry. Chapter 2 of this thesis deals with the results of cycl ic voltammetry studies of various mononitrosyl complexes of molybdenum and tungsten. In part icular , the [Cp'M(NO) ^ = ^° 0 r ^ ' n = ^ ° r ^ compounds were studied in detai l since these compounds were then the sole precursors used to obtain a variety of a l k y l , hydride and diene derivatives containing the Cp'M(NO) group. Chapter 3 centers on the oxidation electrochemistry of various dini trosyl complexes of chromium. Results of the reactions of these CpCrtNO^R complexes (R = alkyl or aryl) with nitrogen-containing electrophiles are presented in the l ight of a stoichiometric cycle for carbon-nitrogen bond formation. F ina l ly , Chapter 4 describes the synthetic u t i l i t y of the electrophil ic [Cp'M(N0)2]+ cations (M = Cr, Mo or W) in organic synthesis. Of particular interest is the condensation reaction involving these cations, methyl propiolate and 2,3-dimethyl-2-butene to form organometallic lactone complexes of the form Cp'M(NO)^(r^-lactone). Since the bonding in organometallic n i trosyl complexes is largely dominated by covalent interaction with the metal atom, assignment of oxidation states to the metal atom and the NO is undesirable. Throughout this thesis, therefore, rationales involving formal oxidation states are avoided, and a l l ligands are considered to be neutral (for example, the Cp ligand is formally a neutral five-electron donor whereas the halide ligand is considered a neutral one-electron donor). 5 CHAPTER 2 Electrochemical Studies of the Complexes [Cp'MCNCOX,,^ [M = Mo or W; • 5 5 * Cp* = rj _ C 5 H 5 (Cp) or n -C 5Me 5(Cp ); X = C l , Br or I; n = 1 or 2 ] : Synthesis ._ * and Characterization of the [Cp'Mo(NO)X2] Radical Anions and [Cp W(NO)I]2. 6 Introduction 1 2 During previous work in our laboratories, several alkyl, hydride and 3 diene derivatives of complexes containing the CpM(NO) group [M = Mo or W] have been synthesized by u t i l i z i n g the diiodo precursors [CpMCNO)^] (n = 1 or 2) as shown in Scheme 2.1 (for n = 1). C p M ( N O ) I : Na/Hg; xs diene (M=Mo) V 2RMgX (M=Mo,W; R=alkyl) 1 / 2 [ C p W ( N O ) ( H ) 2 ] 2 C p M o ( N O)(77 4 - d i e n e ) C p M ( N 0 ) R 2 Scheme 2.1 Curiously, i t had not been possible to extend these reactions to prepare the analogous hydride complexes of molybdenum, the diene complexes of tungsten, and the aryl analogues of the formally 16-electron dialkyl complexes. Most * disturbingly, the Cp MotNO)!^ (R = alkyl) complexes could not be obtained in more than marginal yields starting from the diiodo precursor. In an effort to try to understand the factors that govern the outcome of these halide substitution reactions, I undertook an electrochemical study of the 7 [Cp'MCNCOl^J precursor complexes ( M = Mo or W ; Cp' = Cp or Cp ) and later extended i t to encompass the analogous dichloro and dibromo complexes of molybdenum. This Chapter contains (1) the results of the electrochemical studies (2) the description of the isolat ion of a series of molybdenum dihalo radical anions, and (3) spectroscopic evidence for the occurrence of single electron transfer (SET) during the reactions of Grignard reagents with the [Cp'Mo(N0)X2] n complexes. To avoid the controversy of the value of n in the [Cp'Mo(N0)X2] n complexes in the sol id s t a t e , I shall simply refer to these complexes as monomers (n = 1) since evidence presented in this Chapter is most consistent with their formulation as monomers in solution. 8 Experimental Section A l l reactions and subsequent manipulations involving organometallic reagents were performed under anhydrous conditions and under an atomosphere of prepurified dinitrogen. Conventional techniques^ were employed unless otherwise noted. A l l solvents were purif ied according to published procedures/ Tetrahydrofuran (THF) and diethylether were d i s t i l l e d from Na/benzophenone; toluene from Na; and C H 2 C I 2 from ^ 2 ^ 5 * ^ e s ° l v e n t s were freshly d i s t i l l e d and purged with N 2 for ^10 min prior to use. A l l chemicals were of reagent grade or comparable purity, and were either purchased from commercial suppliers or prepared by published procedures. Standard analytical and spectroscopic techniques were used to ascertain their purity . The [Cp'W(NO) c o m P l e x e s (Cp' = Cp or Cp ) were prepared by known * 11 procedures. The [Cp Mo(NO)X2]n compounds (for X = Br, I ) were prepared from Cp Mo(NO)(CO)2 and elemental X2 in a manner similar to the preparation of their perhydro analogues, and their purity was checked by elemental analyses. Infrared spectra were obtained with a Nicolet Model 5DX FT-IR instrument 1 13 (internally calibrated with a He/Ne laser) . H NMR and C NMR spectra were obtained with a Varian Associates XL-300 NMR spectrometer or a Bruker WP-80 NMR spectrometer. Low-resolution mass spectra were recorded at 70 eV on an Atlas CH4B or a Kratos MS50 spectrometer using the direct- insert ion method by the staff of the Mass Spectrometry Laboratory headed by Dr. G.K. Eigendorf. Elemental analyses were performed by Mr. P. Borda of this Department. [CpMo(N0)X2]n complexes (X = C l , Br, I; n 10a 9 Electrochemical Measurements Electrochemical measurements were accomplished with a PAR Model 173 potentiostat equipped with a Model 176 current-to-voltage converter and a Model 178 electrometry probe. The triangular waveform potential required during cyclic voltammetry studies was obtained with a Wavetek Model 1A3 function generator in conjunction with a unity-gain inverter (±15 V, 50 mA) constructed in the Electronics Shop under the direction of Mr. Joe Sallos. Cyclic voltammograms were recorded on a Hewlett-Packard Model 7090A X-Y recorder. A Fisher Model 5000 strip-chart recorder was used for the current-time plot during bulk electrolyses measurements. Each electrochemical study employed cyclic voltammetry (CV) measurements, 12 and the three-electrode c e l l employed for CV has been previously described. A similar c e l l was employed for low-temperature CV and is shown in Figure 2.1. The c e l l was constructed by Mr. Steve Takacs of the Departmental glassblowing shop. A l l potentials are reported versus the aqueous saturated calomel electrode o' (SCE) , and E values were determined as the average of cathodic and anodic peak potentials, i.e. (E + E )/2. Compensation for iR drop in potential -p,c ~~p,a measurements was not employed in this study. The [n-Bu^N]PFg support electrolyte was prepared by metathesis of [n-Bu^N]I with NH^PFg in hot acetone 12 and was recrystallized thrice from ethanol. Solvents (THF, CH 2Cl 2, CH3CN) were obtained from BDH Chemicals (spectral grade) and were stirred over alumina (Woelm neutral, activity 1) whilst simultaneously being purged with N 2 for 15 min just prior to use. The solutions employed during CV were typically -A (5-7) x 10 M in the organometallic complex and 0.1 M in [n-Bu^N]PFg and were Figure 2.1. The low-temperature cyclic voltammetry c e l l : (A) Pt-bead working electrode; (B) Pt-wire auxiliary electrode; (C) aqueous saturated calomel reference electrode; (D) fine-fritted reference c e l l holder; (E) Luggin probe; (F) glass jacket for coolant. 11 maintained under an atmosphere of N 2« Under these conditions, the Cp 2Fe/Cp 2Fe + o' couple i s measured at E = +0.46 V versus SCE in CH2C12. The ratio of the 13 cathodic peak current to anodic peak current ( i / i ) is 1. Furthermore, - P i C —p,a 1/2 1 4 the cathodic peak current ( i ) increases linearly with v . The p, c separation of the cathodic and anodic peak potentials (AE) increases somewhat with a rise in scan rate, from 67 mV at 0.14 V s * to 78 mV at 0.28 V s Consequently, redox couples exhibiting similar behavior to that of the Cp 2Fe/Cp 2Fe + couple (which i s known to be highly r e v e r s i b l e ) ^ are considered to be reversible. In general, reversible processes observed in cyclic voltammetry experiments were studied in detail f i r s t before irreversible waves. E S R Measurements Dimethylformamide (DMF) was dried over BaO overnight, f i l t e r e d through Celite and then deaerated with N 2 prior to use. Samples for ESR measurements were prepared in a Vacuum Atmospheres Model HE-43 Dry Box as follows: a weighed amount of the solid radical anion complex was dissolved in enough DMF to make up a 5 x 10 M solution. A portion of this solution was then transferred by disposable syringe into a sealed melting-point capillary tube to a height of about 0.5 cm. The capillary tube was then sealed at the open end with silicone grease (Dow Corning, High Vacuum). The X-band ESR spectra were then recorded using the spectrometer and interfaced computer system described by Phillips and Herring.^ The spectra were recorded by Dr. F.G. Herring. Preparation of Cp Mo(NO)Cl2. This compound was obtained as a byproduct * from the preparation of Cp Mo(N0)2Cl by the stoichiometric reaction of 12 * 17 Cp Mo(N0)(C0)2 with N0C1 in CH2C12 at room temperature. The red precipitate formed i n the reaction mixture was collected by f i l t r a t i o n and recrystallized from large quantities of CH 2C1 2 to obtain Cp Mo(NO)Cl2 as a red-brown powder in -^ 20% yield. Anal. Calcd for C^H^NOC^Mo: C, 36.14; H, 4.52; N, 4.22. Found: C, 36.03; H, 4.65; N, 4.31. IR (Nujol mull) M 1645 (s) cm"1. Reactions of Cp'Mo(NO)X2 (Cp' = Cp or Cp ; X = C l , Br or I) with Cobaltocene. A l l of these reactions were carried out in THF in a similar manner. However, the procedures described below vary slightly, and enable the desired complexes to be obtained in optimum yields. Preparation of [Cp2Co][CpMo(NO)Cl2]. To a rapidly stirred, green solution of CpMo(N0)Cl2 (0.22 g, 0.84 mmol) in THF (30 mL) was added solid 18 Cp2Co (0.16 g, 0.83 mmol). A green solid precipitated over a 2 min period. This' solid was collected by f i l t r a t i o n , washed with THF ( 2 x 5 mL) and dried in -3 vacuo (5 x 10 mm) for 1 h to obtain 0.31 g (83% yield) of [Cp2Co][CpMo(N0)Cl2] Anal. Calcd for C^H^NOCl^oCo: C, 39.91; H, 3.33; N, 3.10. Found: C, 40.08; H, 3.41; N, 2.91. IR (Nujol mull) \> 1552 (s) cm"1; also 3076 (m), 1640 (m) , 1527 (sh) , 1413 (m) , 1063 (w) , 1010 (m) , 1004 (sh) , 864 (m) , 805 (m) -1 cm Preparation of [Cp2Co][Cp Mo(HO)Cl 2l. To a stirred, green solution of Cp*Mo(N0)Cl2 (0.33 g, 1.0 mmol) in THF (70 mL) was added solid Cp2Co (0.188 g, 1.0 mmol). A green solid precipitated over a 5 min period. Diethylether (80 mL) was added to the reaction mixture to complete the precipitation, and the 13 green microcrystalline solid was collected by f i l t r a t i o n , washed with Et 20 (20 mL) and then dried in vacuo for 1 h to obtain 0.42 g (80% yield) of [Cp2Co][Cp*Mo(N0)Cl2]. Anal. Calcd for C2()H25NOCl2MoCo: C, 46.07; H, 4.80; N, 2.69. Found: C, 46.13; H, 4.92, N, 2.67. IR (Nujol mull) v 1523 (s) cm"1; also 3091 (m), 1610 (w), 1415 (m), 1063 (w), 1009 (w), 864 (m) cm"1. Preparation of [Cp2Co][CpMo(NO)Br2]. To a stirred, red-brown solution of CpMo(N0)Br2 (0.351 g, 1.0 mmol) in THF (30 mL) was added solid Cp2Co (0.186 g, 0.98 mmol). A dark green, powdery precipitate formed after ^2 min, and the reaction mixture was stirred for an additional 3 min to ensure complete reaction. Et^O (50 mL) was added to complete the precipitation, and the solid was collected by f i l t r a t i o n and dried in vacuo for 1 h to obtain 0.47 g (87% yield) of [Cp2Co][CpMo(NO)Br2]. Anal. Calcd for C^H^NOBr^oCo: C, 33.33; H, 2.78; N, 2.59. Found: C, 33.44; H, 2.95; N, 2.45. IR (Nujol mull) \j 1557 (s) cm"1; also 3090 (m), 1652 (w), 1414 (m), 1063 (w), 1009 (m), 1004 (sh), 862 (m), 804 (m) cm"1. * Preparation of [Cp2Co][Cp Mo(NO)Br2]. To a stirred, green solution of Cp*Mo(N0)Br2 (0.357 g, 0.848 mmol) in THF (50 mL) was added solid Cp2Co (0.153 g, 0.810 mmol). The reaction mixture was stirred for 3 min whereupon a green microcrystalline solid precipitated, leaving a light brown supernatant solution. This green solid was collected by f i l t r a t i o n , washed with THF (20 mL) and dried in vacuo for 1.5 h to obtain 0.43 g (82% yield) of [Cp2Co][Cp*Mo(N0)Br2]. Anal. Calcd for C„nH-_N0Br,MoCo: C, 39.34; H, 4.10; N, 2.30. Found: C, 14 39.70; H, 4.09; N, 2.20. IR (Nujol mull) \> 1534 (s) cm ; also 3086 (m), 1638 (w), 1414 (m), 1065 (w), 1009 (w), 864 (m) cm"1. Preparation of [CP2C0][CpMo(N0)I2]. To a stirred, red solution of CpMo(N0)I2 (0.45 g, 1.0 mmol) in THF (30 mL) was added solid Cp2Co (0.19 g, 1.0 mmol). The solution turned brown instantly, and a dark brown solid percipitated. Diethylether (30 mL) was added to complete precipitation of the solid, which was collected by f i l t r a t i o n , washed with Et 20 (2 x 10 mL) and dried in vacuo for 2 h to otain 0.54 g (84% yield) of brown [Cp2Co][CpMo(NO)I2]. Anal. Calcd for C15H15NOI2MoCo: C, 28.39; H, 2.37; N, 2.21. Found: C, 28.30; H, 2.36; N, 2.38. IR (Nujol mull) v 1557 (s) cm"1; also 3065 (w), 1653 (w), 1412 (m), 1059 (m), 1007 (m), 999 (sh), 862 (m), 804 (m) cm"1. Attempts to obtain crystals of this complex from THF/Et20 at -20°C only resulted in the formation of black micro-crystals of [CpMo(NO)I]2 readily g identifiable by i t s characteristic IR and mass spectra, and by elemental analyses. Preparation of [Cp2Co][Cp Mo(NO)I2]. To a stirred, red solution of Cp*Mo(N0)I2 (0.515 g, 1.0 mmol) in THF (30 mL) was added solid Cp2Co (0.19 g, 1.0 mmol). The color changed from red to green in *\.2 min, and a green microcrystalline solid precipitated. The green solid was collected by f i l t r a t i o n , washed with THF (2 x 10 mL) and then Et 20 (3 x 10 mL), and dried in vacuo for 2 h to obtain 0.49 g (70% yield) of [Cp2Co][Cp*Mo(N0)I2]. Anal. Calcd for C2()H20NOI2MoCo: C, 34.09; H, 2.84; N, 1.99. Found: C, 33.76; H, 3.05; N, 1.89. IR (Nujol mull) \j 1541 (s) cm"1; also 3083 (w), 15 1536 (w), 1412 (ra), 1113 (w), 1007 (w), 862 (m) cm 1. Oxidation of the Radical Anion Complexes of Molybdenum. Oxidation of the [CP2C0][Cp'Mo(N0)X2] radical anion complexes (obtained above) by equimolar amounts of [Cp2Fe]BF^ in acetonitrile resulted in a l l cases in the clean conversion of the radical anions to their neutral dihalo precursor complexes. The reactions were monitored (and nitrosyl-containing products identified) by IR spectroscopy. Attempted Synthesis of [CpFe(n6-C5Me6)] [CpW(NO) ( C ^ S i l f e ^ ] . To a vigorously stirred Et 20 solution (20 mL) of CpW(NO)(CK^SiMe^2 (0.15 g, 6 1 9 0.33 mmol) was added a fi l t e r e d Et 20 solution (20 mL) of CpFe(n -C6Me&) (0.093 g, 0.33 mmol). The i n i t i a l violet color of the reaction mixture (due to the dialkyl complex) became red-brown after *v.l min, and the reaction mixture was stirred for 0.5 h to ensure complete reaction. Removal of the solvent in vacuo resulted only in the deposition of an intractable red tar. * Preparation of [Cp WCNO)!^. To an orange toluene solution (50 mL) of Cp*W(N0)(C0)2 (1.04 g, 2.57 mmol) was added solid Cp*W(N0)I2 (1.55 g, 2.57 mmol). The resulting green solution was then refluxed at *\<110oC for 2 h during which time the color turned a darker green. The fi n a l reaction mixture was allowed to cool to room temperature, filter-cannulated into a separate flask, and then kept at -20°C for 3 days. A dark green crystalline solid precipitated during this time, and i t was collected by f i l t r a t i o n , washed with cold toluene ( 2 x 5 mL) [Caution: this solid i s soluble in toluene to an appreciable extent] and dried i n vacuo overnight to obtain 0.89 g (36% yield) * of [Cp W(N0)I]„ as air-sensitive green crystals. 16 Anal. Calcd for C ^ H ^ N ^ I ^ : C, 25.21; H, 3.15; N, 2.94. Found: C, 25.29; H, 3.21; N, 2.86. IR (Nujol mull) v.„ 1597 (s) cm"1. lU NMR (C,D,) 6 1.98 (s). 1 3C{ 1H} NMR (CCDC) 6 118.68, 114.84, 12.48, 11.57. Low resolution o o mass spectrum (probe temperature 120°C) m/z 952 (P + ) . 17 Results and Discussion Molybdenum Complexes Cyclic Voltammetry Studies. The electrochemical behavior of a l l the * Cp'Mo(N0)X2 compounds (Cp' = Cp or Cp ) are s imilar. [Data for the reductions 20 of these compounds are summarized in Table 2.1.] A l l of these complexes undergo very faci le one-electron reductions in C I ^ C ^ , representable by the general equation shown below, Cp'Mo(N0)X2 — [Cp'Mo(N0)X2] (2.1) -e * (Cp' = Cp or Cp ; X = halide) * In general, the pentamethylcyclopentadienyl (Cp ) compounds are more d i f f i c u l t to reduce (by "v-0.2 V) than their perhydro analogues, consistent with the 19 expected increase in electron density at the metal center. The electrochemical behavior of a representative example is now discussed in deta i l . A cyc l ic voltammogram of CpMo(N0)I2 in CH 2 C1 2 i s shown in Figure 2.2. o 1 This compound exhibits a reversible one-electron reduction at E = -0.04 V vs SCE. At a scan rate (v) of 0.12 V s the separation in peak potentials (AE) 13 i s 65 mV, and the anodic to cathodic peak current rat io ( i / i ) i s 0.93. - p . a -p , c The i / i value increases with a r ise in scan rate from 0.93 at 0.12 V s 1 - p . a -p , c _ 1 1/2 to 0.99 at 0.24 V s . Furthermore, a l inear plot of i vs v i s obtained p t c 14 over the scan-rate range available. In addition, the peak potentials do 18 Table 2.1. Data for the Reduction of Dihalo Complexes of Molybdenum.— Compound Scan Rate (\>, V s"1) , b (V) c AE (mV) i / i - p , a -p , c Comments CpMo(NO)Cl2 0.06 0.13 -0.10 -0.10 65 71 0.85 0.92 Cp*Mo(NO)Cl2 0.06 0.25 -0.35 -0.35 73 89 1.00 1.00 CpMo(NO)Br2 0.09 0.21 -0.05 -0.05 67 89 0.86 0.91 second weak wave at E = -1.73 V -P .c (0.51 V s ) CpMo(NO)Br2~ 0.1A -0.12 72 0.94 second weak wave at E = -1.72 V (0.51 V s ) Cp*Mo(NO)Br2 0.06 0.24 -0.29 -0.29 69 80 0.90 1.00 CpMo(NO)I 0.12 0.24 -0.04 -0.04 65 68 0.93 0.99 second weak wave at E = -1.60 V -P .c (0.44 V s ) CpMo(NO)I2~ 0.24 -0.13 80 0.98 second weak wave at E = -1.61 V CpMo(NO)I2~ 0.07 -0.21 64 0.90 smaller waves at -1.01 and -1.78 V (0.13 V s"1) continued. Table 2.1 (continued) 19 Cp*Mo(NO)I2 0.05 -0.25 57 0.98 second wave at E = -1.79 V -p ,c (0.55 V s _ 1 ) CpMo(NO)I2(PMePh2) 0.12 -0.98 60 0.90 3. - In CH 2 C1 2 containing 0.1 M [n-Bu^N]PFg, at a Pt-bead electrode, unless otherwise stated. Potentials are measured vs SCE. — Defined as the average of cathodic and anodic peak potentials. - Defined as the difference between anodic and cathodic peak potentials, i . e . |E - E I in mV. — Ratio of anodic peak —p ,a —p, c 13 e f current to cathodic peak current. - in THF. - in CH„CN. 20 • Q3 -a3 Volts V S SCE F i g u r e 2.2. Ambient temperature cyc l ic voltammogram of C p M o ( N O ) i n C T ^ C ^ containing 0.1 M [n-Bu^N]PF^ measured at a platinum-bead electrode at a scan rate of 0.24 V s"1. 21 separate somewhat with increasing scan rate, being 65 mV at 0.12 V s and 68 mV at 0.24 V s \ Bulk electrolysis of a THF solution of this compound consumes 1 faraday per mole of monomeric CpMo(NO)I^- A l l the above data are consistent with the one-electron stoichiometry of the reduction of the 16-electron C p M o ( N O ) t o i t s 17-electron radical anion, i . e . e-CpMo(N0)I2 „ . [CpMo(NO)I2]* (2.2) -e As shown in Figure 2.3, i f the scan i s extended to the solvent l imit (^  -2 V) , a second wave is evident at E = -1.60 V (0.44 V s )^ which gives r ise to -p ,c corresponding anodic peaks (at +0.46 V and +0.69 V) due to the release of I in 21 solution. This second reduction wave is assigned to the formation of [CpMo(NO)I]2» which arises from the decomposition of the [CpMo(NO)I^l radical 22 anion. This assignment is confirmed by the recording of a cycl ic voltammogram of an authentic sample of [CpMo(NO)I]^. The latter compound exhibits a single (irreversible) reduction peak at -1.6 V. Interestingly, oxidation of the [CpMo(NO)I]2 compound results in the heterolytic cleavage of the Motu-I^Mo unit to regenerate CpMo (NO) ^ as shown in Equation 2.3. C p ^ ^ ^Cp -^ " M o * C ^ M o < : d ~ " CpMo(N0)I + "CpMo(NO) " (2.3) ON ^ I - ^ NO The cyc l ic voltammogram indicating this particular transformation i s shown in Figure 2.4. Thus, when the anodic scan is reversed after the oxidation peak at +1.05 V, a reversible redox couple is observed at -0.04 V ( i / i = 0.90, - p , a -p , c 22 • 1 O -1 - 2 Volts vs S C E Figure 2.3. Cycl ic voltammogram of CpMoCNO)^ in C I ^ C ^ at a scan rate of O.AA V s _ 1 . 23 + 1.0 I 0.0 Volts vs SCE Figure 2.4. Cycl ic voltammogram of [CpMo(NO)I]2 i n CK^C^ at a scan rate of 0.14 V s"1. 24 AE = 70 mV) which is due to the formation of CpMo(NO)(v ide supra). The formally 13-electron "CpMo(NO)+" byproduct of the reaction probably decomposes 23 in the absence of any trapping ligands. The lower i / i ratios of the redox couples of the CpMo(N0)Xo —p, a —p, c z complexes upon reduction at low scan rates, and the observation (for some of these complexes, Table 2.1) of second reduction waves at more negative • — potentials, i s consistent with the slow decomposition of the [CpMo(N0)X2] 24 radical anions to their respective [CpMotNOX^ monohalo dimers. Preparation of the Radical Anion Complexes. Cobaltocene may be used to 25 effect the reductions outlined in Equation 2.1 on a preparative scale. When sol id CP2C0 is added to THF solutions of the Cp'Mo(N0)X2 compounds, the 17-electron [Cp'Mo(N0)X2] radical anion complexes precipitate as green to brown sol ids . The general equation for this reaction is outlined in Equation 2.4. Cp2Co + Cp'Mo(N0)X2 • [Cp2Co] + [Cp'Mo(N0)X2]* (2.4) Isolated in this manner, these radical anion salts are obtained in high yields as analyt ical ly pure solids which are very soluble in DMF, s l ight ly soluble in THF, and insoluble in Et20. These radical anion salts are a i r - and moisture-sensitive, especially in solutions. However, as sol ids , they are stable at -10°C under an atmosphere of for at least three weeks (for X = Cl or Br) . The iodo complexes decompose in less than two days even at - 2 0 ° C . Not surprisingly, the chemical oxidation of the [Cp'Mo(N0)X„] complexes 25 26 by [Cp2Fe]BF^ in acetonitri le cleanly regenerates the neutral dihalo , 27 . precursor complexes, i . e . [Cp'Mo(NO)X2]' + Cp 2 Fe + • Cp*Mo(NO)X2 + Cp 2Fe (2.5) The IR data for the [Cp'Mo(NO)X2] radical anion complexes are l i s ted in Table 2.2, and are compared with those of their dihalo precursors. As may be noted, a decrease in the n i trosy l stretching frequency (vJJQ) of %120 cm 1 i s observed 28 upon reduction of the netural dihalo compounds. The values for the radical anions are, however, indicative of the NO ligands being linear and terminal ( i .e . not adopting bent geometries upon reduction of the compounds). A str iking observation is that the ^ Q ' s of the anionic 17-electron • — [Cp'Mo(NO)X2] complexes are in the same range as those of the neutral 30 18-electron CpMo(NO)L2 compounds (L = phosphine or phosphite). For instance, + • - -1 [Cp„Co] [CpMo(NO)I-] has a v of 1570 cm in THF, compared to the \> of -1 30 1568 cm for the CpMo(NO)(Ph2PCH2CH2PPh2) compound in the same solvent. The ni trosy l ligand in the latter compound is also d i s t inct ly l inear. Consequently, i t appears that not enough electron density is backdonated to the NO ligands in these [Cp'Mo(N0)X2] complexes to cause them to bend. Interestingly, i t has also been computed that the lowest unoccupied molecular orbi ta l (LUMO) of the related 16-electron CpMo(N0)Me2 model compound is localized on the metal center and contains no NO 2n character. 1 Therefore, the reduction of the 16-electron CpMo(N0)Me2 compound to i t s 17-electron [CpMo(N0)Me2] radical anion i s not expected to result in a significant Table 2.2. A Comparison of the Nitrosyl-Stretching Frequencies of the New 17-electron Radical Anion Complexes with Their Neutral Precursors.— ^NO' C n f l Complex X = Cl X = Br X = = I Cp' = Cp Cp' = Cp* Cp1 = Cp Cp' = Cp* Cp' = Cp Cp' = Cp* Cp'Mo(NO)X2 1665 1647 1670^ 1649 1670- 1663 [Cp2Co][Cp'Mo(NO)X2] 1552 1523 1557 1534 1557 1541 - as Nujol mulls, unless otherwise stated. b 8 as KBr pellets . 27 bending of the n i trosy l ligand in the radical anion complex. In contrast, the • — added electron density in [CpMo(NO)(CO)2] [obtained by electroreduction of the 18-electron CpMo(NO)(CO)2 precursor] is localized largely in the Mo-NO group, a feature which is believed to result in the concomitant bending of the 31 • -n i trosy l ligand. That the added electron density in the [Cp'Mo(N0)X2] complexes is not localized on their Mo-NO groups is c learly evident in their 14 ESR spectra which exhibit no N hyperfine coupling. The ESR data for a l l the molybdenum radical anion complexes synthesized in this study are l i s ted in Table 2.3. The ESR spectra obtained for the dichloro and diiodo radical anion complexes are s imilar, each consisting of a single, strong central resonance flanked by molybdenum sate l l i t e signals. The ESR spectrum of [CP2C0][CpMo(N0)Cl2] is shown as a representative example in Figure 2.5. A strong central resonance (g = 1.9819) with five broad sa te l l i t e signals due to 95 97 Mo and Mo hyperfine coupling (I = 5/2, nat. abund. 15.9% and 9.5% respectively) is observed.^ No ^ N or '''H hyperfine coupling is evident in this spectrum, and this is so for the spectra of a l l the complexes. However, in sharp contrast to the ESR spectra for the dichloro (and diiodo) radical anion complexes, those for the dibromo analogues exhibit halide hyperfine A interactions. For example, the ESR spectrum of [CP2C0] [Cp MoCNOJB^], shown in Figure 2.6, consists of a 7-line central pattern due to coupling of the 79 81 unpaired electron to two Br nuclei (I = 3/2; Br, 50.54% nat. abund.; Br, 33 49.46% nat. abund.) ( a B r = 10.4 G). The spectrum of the perhydro analogue, [Cp_Co][CpMo(N0)Br„], also displays a pattern similar to that shown in Figure 28 Table 2.3. Electron Spin Resonance Data for the [Cp2Co] [Cp'Mo(NO)X2] Radical Anion Complexes.— Complex [CpMo(NO)Cl2]*~ [Cp*Mo(N0)Cl2]"" [CpMo(N0)Br2] g-value 1.9819 1.9840 2.0103 Temp(°C) 14.5 22.6 22.7 Complex [Cp*Mo(NO)Br2] [CpMo(NO)I ]"" [Cp*Mo(N0)I2]*" g-value 2.0092 2.0596 2.0582 Temp(°C) 22.8 18.5 19.9 - As DMF solutions, except for the iodo complexes which were examined as THF solutions. 29 30 31 2.6. Like the dichloro radical anion complexes, these dibromo analogues are stable in DMF solution for at least 3 h at room temperature. In contrast, the diiodo radical anions are relatively unstable in DMF. For example, the i n i t i a l spectrum of [Cp Mo (NO) I,,] as the cobalticinium salt i n DMF displays a strong central resonance (g = 2.0142) and an identical Mo isotope pattern as in Figure 2.5. This central signal gradually gives way to a new broad signal (at g = * 2.064) attributed to the neutral Cp Mo(NO)I• radical, formed by I loss from 34 the radical anion. This latter feature is in agreement with the cyclic voltammetric results discussed earlier. Therefore, the processes outlined in Scheme 2.2 plausibly account for the formation of [Cp'Mo(N0)I]2 from their diiodo precursors upon reduction. Cp'Mo(N0)I2 J 3 *- [Cp'Mo(N0)I2] -e -I 1/2 tCp'Mo(NO)!]- •dimerization C p , M o ( N 0 ) I < Scheme 2.2 Consistent with Scheme 2.2 is the observation that attempts to grow crystals of [Cp2Co][CpMo(NO)I2] only result i n the formation of [CpMo(NO)I]2> It is probable, therefore, that the analogous dichloro and dibromo radical anion complexes eventually undergo similar decomposition to their monohalo dimers, 35 albeit at much slower rates. Direct Evidence for the Involvement of Single Electron Transfer (SET) 32 During the Reactions of Grignard Reagents with the Cp*Mo(N0)X2 Complexes. Implications for the Improved Syntheses of the Cp'Mo(N0)R2 Compounds. The * ESR spectrum of an equimolar mixture of MegSiC^MgCl and Cp Mo(NO)Br2 in DMF i s shown in Figure 2.7. This spectrum is qualitatively identical to that of [CP2C0] [Cp MoCNCOB^] (Figure 2.6) and provides evidence for the occurrence of 36 39 SET from the Grignard reagent to the dibromo complex, ' i . e . RMgCl + Cp*Mo(NO)Br2 • [RMgCl] + [Cp*Mo(NO)B^] * (2.6) (R = CH 2SiMe 3) Although Grignard reagents are known to function as SET agents to organic AO Al compounds such as ketones and to organic groups bound to transit ion metals, very l i t t l e is presently known about the nature of their reactions with organometallic halo compounds. Thus, the formation of the [Cp MotNOjB^] radical anion in reaction 2.6 is of fundamental significance. Presumably, the very low potentials required for the reductions of the Cp'Mo(N0)X2 compounds fac i l i ta te SET from the Grignard reagents to these complexes. It is reasonable, therefore, to assume that i f the dihalo radical anions are indeed intermediates in the reactions of the Cp'Mo(N0)X2 compounds with Grignard reagents to generate ultimately the Cp'Mo(N0)R2 complexes, then a sufficient lifetime for these radical anions is necessary to allow hal ide-alkyl metathesis ( i . e . R for X) to occur to y i e ld the f ina l alkyl complexes. If this i s not so, then rapid decomposition of these [Cp'Mo(N0)X2] radical anions may then lead 33 34 only to the formation of the monohalo dimers (vide supra). Since the order of s t a b i l i t y of the radical anions [both in solutions (by ESR) and in the so l id state (by IR)] is in the order Cl > Br >> I, i t seems log ica l , then, that the highest yields (and perhaps the 'cleanest' formation reactions) of the Cp'Mo(N0)R2 complexes from this synthetic route should be obtained when the dichloro precursors are used. Indeed, many previously unobtainable Cp'Mo(N0)R2 compounds (R = alkyl or aryl) are now synthesizeable cleanly by this route. For example, CpMo(NO)I + 2 PhMgCl CpMo(NO)Ph2 (2.7) CpMo(NO)Cl2 + 2 PhMgCl • CpMo(NO)Ph2 (2.8) Also, i t i s now understandable why attempts to synthesize the Cp Mo(N0)R2 complexes from Cp Mo(N0)l2 and RMgCl (R = alkyl) only lead to the formation of * 4 [Cp Mo(NO)I]2 in high y ie lds , i . e . Cp Mo(NO)I2 + 2 RMgCl • [Cp Mo(NO)I] (2.9) Cp*Mo(NO)Cl2 + 2 RMgCl • Cp*Mo(NO)R2 (2.10) To summarize, i t therefore appears that a suff ic ient ly stable [Cp'Mo(N0)X2] complex i s desirable for the overall hal ide-alkyl (or aryl) 35 metathesis reaction. Nevertheless, a more detailed systematic study needs to be done to unravel the individual steps that lead f ina l ly to dialkylation (or arylat ion) . It should be noted, however, that although we have not been able to achieve monoalkylation from the dihalo precursors with the RMgX (or RLi) 42 reagents, the monoalkylated complex [CpMo(NO)Br(Me)]^ i s obtainable by employing the less nucleophilic trimethylaluminum as the alkylating agent on 43 the dibromo precursor. This procedure may well circumvent the i n i t i a l formation of the molybdenum containing radical anion as outlined in equations 44 2.1 and 2.6. Tungsten Complexes The Cp'W(N0)X2 Complexes. In the previous section, the electrochemsitry of the Cp'Mo(N0)l2 complexes was discussed. As noted ear l i er , both complexes (Cp1 = Cp or Cp ) exhibit similar reduction behavior in C F ^ C ^ solution, undergoing faci le reversible reductions. Unlike the molybdenum analogues, however, the Cp'VKNO)^ complexes exhibit somewhat different electrochemical behavior when Cp is replaced by Cp . The cyc l ic voltammograms of these complexes w i l l now be discussed in some deta i l . Cycl ic voltammograms of CpVKNO)^ in CH^C^ are shown in Figure 2.8. This compound undergoes an irreversible reduction at E = -0.35 V and a somewhat P t c o 1 reversible reduction at E = -0.64 V (AE = 88 mV). The f i r s t reduction, however, remains irreversible even at higher scan rates up to 15 V s The observed anodic peaks at positive potentials (in Figure 2.8b) are due to the 21 release of I in solution (vide supra) and are notably absent when the anodic scan is performed f i r s t (Figure 2.8a). These peaks due to the I / I „ system 36 +o!i o a^s Volts vs SCE Figure 2.8. Cyclic voltammograms of CpW(NO)I2 in CH 2 C1 2 at a scan rate of 0.32 V s . (a) scanning positive potentials f i r s t , and (b) scanning negative potentials f i r s t . 37 are s imi larly observed i f only the f i r s t reduction wave is passed. * Cyclic voltammograms of the permethylated analogue, Cp W(NO)I2> in CH 2 C1 2 are shown in Figure 2.9. In contrast to the CpW(N0)I2 compound, the f i r s t o' reduction of this complex is largely reversible and occurs at E = -0.52 V. The magnitude of the return anodic current ( i ) for this couple i s enhanced - p , a at higher scan rates (Figure 2.9a). Furthermore, the intensity of the wave for o1 the second redox couple at E = -1.06 V is more pronounced at lower scan rates (Figure 2.9c) and i s enhanced by the addition of an authentic sample of [Cp W(N0)I] 2. This latter compound can be synthesized independently as 45 outlined in Equation 2.11. Cp W(NO)(CO)2 + Cp W(N0)I2 • [Cp W(N0)I]2 (2.11) toluene It is thus concluded that [Cp W(N0)I]2 i s being generated electrochemically by the reduction of the diiodo precursor. This paral le ls the behavior of the molybdenum complexes presented ear l i er , although in the tungsten cases the monoiodo dimers are reversibly reduced, presumably to their [Cp W(N0)I]2 bimetallic radical anions. * Electrochemically, therefore, the overall process to generate [Cp W(N0)I]2 may be c lass i f ied as a reversible electron transfer (E^) followed by an 46 irreversible chemical reaction (C^). E r Cp*W(N0)I2 ^ [Cp*W(N0)I2]* (2.12) -e 38 -1 Volts vs SCE * Figure 2.9. Cycl ic voltammograms of Cp W(NO)I2 in C K ^ C ^ at different scan rates (a) 2.64 V s _ 1 (b) at 0.53 V s _ 1 and (c) at 0.13 V s"1. 39 k f 1 C± [Cp W(NO)I2] —• i— | [Cp W(NO)I]2 + I (2.13) The i / i dependence on scan rate (\0 for the E step (Equation 2.12) i s ~~P > a — p , c r shown in Figure 2.10, and the observed trend of higher i / i values ~~P i a ~P»c (increased chemical revers ibi l i ty) at higher scan rates is consistent with the 46,47 J . U X S v a i i a L i u n u i _ , _ - p , a -p , c electroreduction (E ) as a function of scan rate (v) may then be used to proposed E^C^ mechanism.~T",~" This ri t o  of i _ _ for the estimate the rate constant (k^ in s )^ for the coupled chemical reaction (C^). 47 by employing the method of Nicholson and Shain. The results are collected in Table 2.4. As might have been expected, the s tab i l i t y of the electrogenerated * ._ [Cp W(N0)I2] radical anion is markedly enhanced by lower temperatures (Figure 48 2.11). For instance, the redox couple (for the f i r s t reduction) in THF at o1 -1 E = -0.49 V (at 0.07 V s ) has an i / i value of 0.74 at - 3 0 ° C , whereas "P.a -p , c * . -at 0°C i t is only 0.46. It is thus conceivable that the [Cp W(N0)I2] complex may well be chemically accessible at lower temperatures. This hypothesis has not, however, been tested experimentally. The Tungsten Cp'W(NO) (CH^SiMe^ Dialkyl Complexes. Cycl ic voltammetry of the tungsten dia lkyl compounds reveals that they are reduced reversibly, i . e . Cp'W(NO)(CH 2SiMe3)2 -e [Cp'W(NO)(CH2SiMe3)2] (2.14) Figure 2.10. Plot of i / i vs scan rate for the f i r s t reversible redox couple f -p ,a -p ,c Cp W(N0)I2 in CH 2 C1 2 . Al Table 2.A. Electrochemical Data for the Electroreduction of Cp W(N0)I2 i n CH_Cl„for a Variety of Scan Rates.— Scan Rate E°' (V) Current ( i ) -p , c AE i / i - p , a -p , c K f (\>, V s"1) (uA) (mV) 0.1A -0.52 5.52 58 0.52 0.9A 0.21 -0.52 6.57 59 0.57 1.13 0.28 -0.52 7.58 6A 0.63 1.16 0.A2 -0.52 9.A2 66 0.67 1.A6 0.83 -0.52 12.8 68 0.82 1.25 1.38 -0.52 16.7 78 0.89 1.32 — Various symbols used are defined in the Experimental Section. — Rate of irreversible chemical reaction (C_^ ) following the reversible electron transfer (E ). Estimated from i / i using the method of Nicholson and - r - p , a -p , c 6 A7 Shain, assuming a simple f irst-order reaction. 3 0 > o a o a 0.07 0.14 0.28 Scan Rate (Volta/soc) 0.56 Figure 2.11. Temperature dependence of i / i values as a function of scan rate for -A - - P » a — P » c the f i r s t reversible redox couple of Cp WCNO)^ in THF. 43 For Cp1 = Cp, the reversible redox couple occurs (in CH 2C1 2) at E = -1.51 V -1 * (0.15 V s , AE = 90 mV, i / i 0.97) and for Cp1 = Cp this wave occurs -p .a -p , c r at E° ' = -1.64 V (0.19 V s"1, i / i = 0.90). The cathodic shift in the - p , a -p , c o' * E value in moving from Cp to Cp is consistent with the increased electron density on the metal center due to the greater electron donating capabil ity of the permethylated r ing . These d ia lkyl complexes are s ignif icant ly more d i f f i c u l t to reduce than their diahlo precursors, and this is not expected 49 since alkyl ligands are suff ic ient ly better donor ligands than are halides. Unfortunately, attempts to isolate these [Cp'W(NO)R2] radical anions have so far been unsuccessful.^^'^^ For example, no reaction occurs between CpW(NO) (C^SiMe^) 2 a n ( * Cp2Co in Et 2 0 at room temperature over a 3 h period. o' This is not too surprising since Cp2Co (E = -0.82 V, vide supra) may not be a o' suitable reducing agent for the reduction of the d ia lkyl compounds (E -1.61 V) . Regrettably, employing the more potent reducing agent, 6 19a CpFe(n -C^Meg) (E-jy 2 = -1.78 V) for the reduction of the d ia lkyl compound only results in the p'roduction of an intractable tar . Summary A l l the CpMo(NO)X2 complexes undergo one-electron reversible reductions at o 1 * 0 > E S -0.1 V vs SCE in dichloromethane solution. Their Cp analogues are more d i f f i c u l t to reduce (by *\-0.2 V) , reflecting the greater electron-donating capabil ity of the Cp ligand. Furthermore, the W complexes are more d i f f i c u l t to reduce (by ^0.3 V) than their Mo congeners. The radical anion complexes of the form [Cp'Mo(N0)X_] can be isolated as their cobalticinium salts . ESR 44 measurements on solutions of these radical anions show the unpaired electron to be coupled only to the molybdenum center, or also to the halide (for X = Br) . In the case of tungsten, the [Cp WtNO)^] complex, generated electrochemically, is unstable and rapidly decomposes to [Cp WtNCOl^, which has also been synthesized independently. This study has shown that the successful accomplishment of the Cp'Mo(N0)X2 • Cp'Mo(NO)R2 metathesis reaction can be predicted by understanding the electrochemical behavior of the precursor Cp'Mo(N0)X2 compounds. As a result of this work, several new Cp1Mo(NO)(alkyl)2 compounds have been made. Also, the previously unknown Cp'M(NO)(aryl^ compounds have now been successfully synthesized in our laboratories by employing the dichloro precursor complexes (and not the diiodo compounds) for the metathesis reaction. The work presented in this Chapter (and the predictions derived from i t ) is invaluable to our research group since a large portion of our research deals with studies involving the Cp'M(NO)R„ compounds or complexes derived from them. 45 References and Notes 1. Legzdins, P . ; Rettig, S. J . ; Sanchez, L . ; Bursten, B. E . ; Gatter, M. G. J . Am. Chem. Soc. 1985, 107, 1411. 2. (a) Legzdins, P . ; Martin, J . T . ; Oxley, J . C. Organometallics 1985, 4, 1263. (b) Legzdins, P . ; Martin, J . T . ; Einstein, F. W. B . ; Jones, R. H. Organometallics 1987, 6, 1826. 3. (a) Hunter, A. D.; Legzdins, P . ; Nurse, C. R.; Einstein, F. W. B; W i l l i s , A. C. J . Am. Chem. Soc. 1985, 107, 1791. (b) Hunter, A. D. ; Legzdins, P . ; Einstein, F. W. B . ; W i l l i s , A. C . ; Bursten, B. E . ; Gatter, M. G. J . Am. Chem. Soc. 1986, 108, 3843. 4. P h i l l i p s , E . C. unpublished observations. 5. (a) Hunter, A. D. Ph.D. Dissertation, The University of Br i t i sh Columbia, 1985. (b) Martin, J . T. Ph.D. Dissertation, The University of Br i t i sh Columbia, 1987. 6. Shriver, D. F . ; Drezdon, M. A. The Manipulation of Air-Sensitive Compounds; 2nd. Ed. Wiley-Interscience: New York, N.Y. 1986. 7. Perrin, D. D. ; Armarego, W. L. F . ; Perrin, D. R. Purification of Laboratory Chemicals; 2nd E d . ; Pergamon: Oxford, 1980. 8. Seddon, D . ; K i t a , W. G . ; Bray, J . ; McCleverty, J . A. Inorg. Synth. 1976, 16, 24. 9. James, T. A . ; McCleverty, J . A. J. Chem. Soc. A., 1971, 1068. 10. (a) Legzdins, P . ; Martin, D. T. ; Nurse, C R . Inorg. Chem. 1980, 19, 1560. (b) Dryden, N. H . ; Legzdins, P . ; Einstein, F. W. B . ; Jones, R. H. Can. J. Chem. in press. 46 11. (a) Malito, J . T. ; Shakir, R. ; Atwood, J . L. J. Chem. Soc, Dalton Trans. 1980, 1253. (b) Nurse, C. R. Ph.D. Dissertation, The University of Br i t i sh Columbia, 1983. 12. Legzdins, P . ; Wassink, B. Organometallics, 1984, 3, 1811. 13. Nicholson, R. S. Anal. Chem. 1966, 38, 1406. 14. Diffusion control was tested for in a l l redox processes during cycl ic voltammetry experiments by observing the behavior of peak currents (i^) as a function of scan rate (\J) over at least one order of magnitude of scan rate. For a l l reversible couples in this study, i ^ varied l inearly with 1/2 \j and not with \). Representative examples are displayed in the Appendix. 15. Holloway, J . D. L . ; Geiger, W. E . J . Am. Chem. Soc. 1979, 101, 2038. 16. P h i l l i p s , P. S.; Herring, F. G. J. Magn. Reson. 1984, 57, 43. 17. The [CpMo(NO)Cl^]2 analogue can also be formed in low yields by this route: Legzdins, P . ; Malito, J . T. Inorg. Chem. 1975, 14, 1875. 18. King, R. B. Organometallic Syntheses; Academic: New York, 1965, Vol . 1, pp 70-71. 19. (a) Hamon, J . - R . ; Astruc, D . ; Michaud, P. J . Am. Chem. Soc. 1981, 103, 758. (b) Robbins, J . L . ; Edelstein, N . ; Spencer, B . ; Smart, J . C. J . Am. Chem. Soc, 1982, 104, 1882. 20. The data for the electrochemical oxidations of a l l the complexes studied are summarized in the Appendix. A l l of these complexes undergo irreversible oxidations. With the exception of the oxidation of 47 [CpMo(NO)I]2, these oxidations were not investigated further. 21. These anodic peaks are enhanced by the addition of Bu^NI and are assigned to the oxidation of I and : Samuel, E . ; Guery, D . ; Vedel, J . J . Organomet. Chem. 1984, 263, C43. 22. It is unlikely that this peak is due to the reduction of the radical anion to give the dianion, since i t s current value is so low. However, • — [Cp2Zrl2] is reported to be reduced to the dianion: E l Murr, N; Chaloyard, A . ; Tirouf le t , J . J . Chem. Soc, Chem. Commun. 1980, 446. 23. In this connection, i t has recently been discovered that [CpCr(NO)I]2 i s oxidized by 2 equiv. of AgPF& to [CpCr(NO)(CH^CN)^\ * +[PF^]~ in acetonitri le: Chin, T. T. unpublished observation. 24. Other examples of this kind of halide loss can be found, see: Connelly, N. G . ; Geiger, W. E . Adv. Organomet. Chem. 1984, 23, 1. 25. Under identical experimental conditions, the Cp2Co/C2Co+ couple occurs at o 1 -0.82 V vs SCE, in C H ^ ^ . This E value is suff ic ient ly negative of those of the dihalo complexes and yet suff ic ient ly positive of the second reduction peaks; a situation that is ideal for effecting the desired single-electron transfer. 26. The Cp 2 Fe + cation is a well-known one-electron oxidant in organometallic chemistry: Schumann, H. J. Organomet. Chem. 1986, 304, 341. 27. Although Cp 2 Fe + does not react with C p M o ( N O ) i n CH3CN, N0+ does to give a flocculent yellow powder of elemental composition [CpMo(NO) (CH3CN)3] (I) (PF g ): IR (Nujol mull) \) N Q 1707 (s) cm"1; also 48 2328 (m) and 2299 (ra) cm (\>_„ region) , 841 (s) cm . H NMR LN I I , O (CD3N02) 6 6.68 (s, 5H, C ^ ) , 2.65 (s, 9H, CH_3CN). 1 3 C f^H) NMR (CD3N02) 6 141.10 (s, CH3CN), 113.18 (s, C ^ ) , 5.04 (s, C^CN). 28. In contrast, a decrease of ^200-180 cm 1 i s observed after reduction of 29 various CpM(N0)2X complexes (M = Mo or W). 29. Legzdins, P . ; Wassink, B. Organometallics 1988, 7, 482. 30. Hunter, A. D. ; Legzdins, P. Organometallics 1986, 5, 1001. 31. Geiger, W. E. ; Rieger, P. H . ; Tulyathan, B . ; Rausch, M. J . Am. Chem. Soc. 1984, 106, 7000. 95 97 32. The individual contributions of the Mo and Mo isotopes are unresolved, and this is not unusual: (a) Atherton, N. M . ; Denti, G . ; Ghedini, M . ; Ol iva , C. J. Magn. Reson. 1981, 43, 167. (b) Hanson, G. R.; Wilson, G. L . ; Bailey, T. D.; Pibrow, J . R.; Wedd, A. G. J . Am. Chem. Soc. 1987, 109, 2609. 33. The extent of hyperfine coupling is of similar magnitude to that found for the [Cp„MoBr_] + cation, where a D = 15.8 G: Cooper, R. L . ; Green, z z or M. L. H. J. Chem. Soc. A. 1967, 1155. 34. A similar argument has been used regarding the electrogeneration of HB(Me 2pz)Mo(N0)I» (Me2pz = 3,5-dimethylpyrazolyl): Briggs, T. N . ; Jones, C. J . ; McCleverty, J . A . ; Neaves, B. D . ; E l Murr, N . ; Colquhoun, H. M. J. Chem. Soc, Dalton Trans. 1985, 1249. 35. In this connection, the very weak bands at ^1620 cm 1 evident in the IR spectra of the [Cp„Co][Cp'Mo(N0)X 9] complexes may be due to some 49 Cp'Mo(NO)X», resulting from decomposition of the radical anion in the so l id state. 36. Integration of this signal to ascertain the concentration has not been performed. As a matter of caution, however, i t must be noted that in related organic systems, integrations (concentrations) have been found to 37 be dependent on the nature of R, or even on the purity of the Mg used in 38 making the Grignard reagent. 37. Ashby, E . C . ; Goel, A. B. J. Am. Chem. Soc. 1981, 103, 4983. 38. Ashby, E. C ; Wiesemann, T. L. J. Am. Chem. Soc. 1978, 100, 189. 39. In general, i t i s believed that organometallic cation radicals such as R^ M (M = Sn or Pb) are short-l ived due to their tendency to cleave the * + M-R bond. It is entirely possible that the [RMgX] radical cation is stabi l ized by some means such as solvation or aggregation: Maruyama, K . ; Katagir i , T. J . Am. Chem. Soc. 1986, 108, 6263. 40. For leading references on Grignard reagents as SET agents to organic compounds, see (a) Kochi, J .K . Organometallic Mechanisms and Catalysis; Academic: New York, N . Y . , 1978, Chapter 17. (b) Jones, P.R. Adv. Organomet. Chem. 1977, 15, 274. (c) Ashby, E . C . ; Bowers, J . ; Depriest, R. Tetrahedron Lett. 1980, 21, 3541. (d) Ashby, E . C ; Goel, A. B. J . Am. Chem. Soc. 1981, 103, 4983. (e) Ashby, E . C. Pure Appl. Chem. 1980, 52, 545. 41. In a few cases, Grignard reagents have been known to function as SET agents to organic groups bound to transit ion metals. (a) Top, S.; Jaouen, G. J. Organomet. Chem. 1987, 336, 143. (b) Astruc, 50 D. In The Chemistry of the Metal-Carbon Bond; Hartley, F. R . , E d . ; Wiley-Interscience: Chichester, 1987; Vol . 4, p. 630. 42. Alegre, B . ; de Jesus, E . ; de Miguel, A. V . ; Royo, P . ; Lanfredi, A. M. M. ; T ir ip i cch io , A. J. Chem. Soc, Dalton Trans. 1988, 819. 43. Trialkylaluminum reagents (R^Al) are capable of abstracting halide to form [R^AIX] . For general discussions, see (a) Yamamoto, A. J . Organomet. Chem. 1986, 300, 347. (b) Mole, T . ; Jeffrey, E . A. Organoaluminum compounds; Elsevier: New York, N .Y. ; 1972, p. 374. 44. (p_-tol)Li is reported to react with HB(Me2Pz)Mo(NO)l2 to form a complex formulated as HB(Me2pz)3Mo(NO) I y L K O E t j ) . 3 4 9 45. A similar method has been used to synthesize [CpMo(NO)I]^» hut the W analogues were unknown prior to this work. 46. For simplified reviews concerning the characterization of electrode processes, see (a) Mabbott, G. A. J. Chem. Ed. 1983, 60, 697. (b) Heinze, J . Angew. Chem. Int. Ed. Engl. 1984, 23, 831. 47 More rigorous treatments are also available. 47. Nicholson, R. S.; Shain, I. Anal. Chem. 1964, 36, 706. 48. For excellent references on low-temperature electrochemistry, see (a) Van Duyne, R. P . ; Rei l ley , C. N. Anal. Chem. 1972, 44, 142. (b) ibid, p. 153. (c) ibid, p. 158. 49. Kochi, J . K. Pure Appl. Chem. 1980, 52, p. 578. 50. A few related Cp2TiR2 dia lkyl complexes also undergo one-electron reversible reductions. (a) K i r a , M . ; Bock, H . ; Umino, H . ; Sakurai, H. J . Organomet. Chem. 1979, 173, 39. (b) Koch, L . ; Fakhr, A . ; 51 Mugnier, Y. ; R o u l l i e r , L. ; Moise, C ; Laviron, E. J. Organomet. Chem. 1986, 314, C17. + •-51. Compounds of the type Na [Cp 2ZrR 2] have been reported i n the 52a l i t e r a t u r e , although t h i s i s at variance with c y c l i c voltammetry r e s u l t s which show the irreversible nature of the reduction of the d i a l k y l percursors. 52. (a) Lappert, M. F. ; Ri l e y , P. I.; Yarrow, P. I. W. J. Chem. Soc, Chem. Commun. 1979, 305. (b) Lappert, M. F.; Pi c k e t t , C. J . ; Ril e y , P. I.; Yarrow, P. I. W. J. Chem. Soc, Dalton Trans. 1981, 805. 52 CHAPTER 3 Insertions of Electrophiles into Metal-Carbon Bonds: Formation of New Carbon-Nitrogen Linkages Mediated by the CpCr(NO)_ Group. 53 Introduction Complexes containing o-bonded organic ligands play a central role in transition-metal organometallic chemistry.''' Of the various chemical properties exhibited by these complexes, probably none is more important than their a b i l i t y to undergo insertion reactions of the type M-C + XY - M-(XY)-C (3.1) 2 where XY can be a variety of ent i t ies . The particular examples of reactions 3.1 having XY = CO have been the most studied and are the best understood mech-2 an i s t i ca l ly . In contrast, v i r tua l l y nothing is presently known about the cases when XY = N0+ which is isoelectronic with CO. Indeed, i t was only recently that the f i r s t examples of the insertion of N0+ into transition-metal-3 carbon bonds were i n i t i a l l y communicated by this research group and then by 4 other investigators, i . e . C H 2 C 1 2 3 CpCr(N0) 2CH 3 + NOPFg — • [CpCr(NO) (N(OH)CH2)]PFg (3.2) and 1 1 C H 3 N 0 2" C H 2 C 1 2 4 [{CpCo}3{u3,n -CR}{u3,n "CR'}] + N0BF4 ^—^- (3 .3T R'O I I t {CpCo} 3 {u3 , n^CR} {p3 , n2-C=N) ] BF^ where Cp = q^-C^H^ and R or R1 = a lkyl or ary l . Yet, reactions such as 3.2 and 54 3.3 are of fundamental significance since they lead to the formation of new carbon-nitrogen bonds, an intriguing goal in i t s own right from the viewpoint of organic synthesis. Thus, I was interested in studying further the chem-i s t r y presented in eqn 3.2 with a view to determining the origin of the formaldoxime ligand in the product. Furthermore, the product complex had only been obtained in somewhat low y ie ld (46% based on Cr) and no intermediates had been detected during the course of the reaction. In this Chapter, I present the results of the extension of the chemistry outlined in reaction 3.2 to encompass the insertions of not only the n i t ro -sonium ion but also the aryldiazonium cation into the Cr-C o-bonds of several CpCr(NO)2R (R = alkyl or aryl) complexes. Furthermore, I also describe some derivative chemistry of the products resulting from insertion that has led to the development of a stoichiometric cycle for the formation of new C-N linkages, each step of the cycl ic process being mediated by the CpCr(NO)2 group. Experimental Section A l l reactions and subsequent manipulations were performed under anaerobic and anhydrous conditions. General procedures routinely employed in this study have been described in the previous Chapter. The commercially available reagents AgPFfe (Strem), NOPFg (Alfa), [PPN]C1 (bis(triphenylphosphine)iminium chloride, A l f a ) , CD^Mgl (Aldrich), and Proton Sponge (l,8-bis(dimethylaraino)-napthalene, Aldrich) were used as received without further puri f icat ion . Reaction of CpCr(NO)2Me with NOPFg. A green solution of CpCr(NO)2Me7 (0.576 g, 3.00 mmol) in CH-Cl- (45 mL) was treated with f inely ground NOPF, 55 (0.525 g, 3.00 mmol), and the mixture was s t irred vigorously at room tempera-ture. After 1 h, a green-brown precipitate began to form, and an IR spectrum of the supernatant solution revealed new bands at 1842 (s), 1834 (w), 1742 (s), 1734 (w) and 1710 (w) cm 1 in addition to the v's characteristic of CpCr(N0)2Me in this solvent at 1776 and 1669 cm After 1.5 h, s t i rr ing was stopped, and the green-brown precipitate was permitted to sett le . The green supernatant solution was removed by f i l t e r cannulation. The remaining prec ip i --3 tate was washed with CH 2 C1 2 (5 mL) and dried in vacuo (5 x 10 mm) at 20°C for 1 h to obtain 0.806 g (73% yield) of a brown so l id . This so l id contained a mixture of the isomers [CpCr(NO)2(N(0)Me)]PF&, and [CpCr(N0)2{N(0H)CH2}]PF6, in an approximate rat io of 2:3 as judged by i t s *H NMR spectrum. Anal. Calcd for C c H o N.0 o PF,Cr: C, 19.63; H, 2.20; N, 11.45. Found: C, o o i i o 19.71; H, 2.30; N, 11.33. IR (Nujol mull) vQJ{ 3482 (s) cm"1; \ ) m 1852 (s) , 1759 (vs), 1559 (w) cm"1. J H NMR (CD_C1_, -20°C) 6 8.92 (br s, 1H, H.-ONCH.H^), Z Z A A D 7.66 (d, 1H, 231 = 5.2 Hz, H^ONCH^Hg), 7.14 (d, 1H, H^ONCH^ ), 5.98 (s, H A" h B 7H, C 5 H 5 ) , 1.81 (br s, 2H, N0CH_3) . Recrystal l ization of the brown so l id from CH 2 C1 2 afforded dark green microcrystals of [CpCr(NO)2(N(0H)CH2)]PF^ as the sole product. Its spectro-scopic properties are the same as those exhibited by the mixture (vide supra) except for the absence of the 1559 cm 1 band in i t s IR spectrum and the methyl proton signal at 5 1.81 in i t s 1 H NMR spectrum. Single crystals of this salt suitable for X-ray crystallographic analysis were grown by maintaining a saturated C H 2 C l 2 solution of the complex under N 2 at -20°C for 2 days. Preparation of CpCr(NO)2CD^. To a s t i rred , green s lurry of 56 CpCr(NO) 2Cl (3.83 g, 18.0 mmol) i n E t 2 0 (160 mL) at room temperature was added dropwise over 10 min a clear s o l u t i o n of CD3MgI (1.0 M i n E t 2 0 , 18.0 mL, 18.0 mmol) i n E t 2 0 (40 mL t o t a l volume). The supernatant s o l u t i o n became darker green i n color as the n i t r o s y l reagent was consumed and a green-brown p r e c i p i -tate formed. After the addition of the Grignard reagent had been completed, the reaction mixture was further s t i r r e d for 15 min and was then f i l t e r e d through a short ( 3 x 5 cm) column of alumina (Woelm neutral, a c t i v i t y 1) supported on a medium porosity f r i t to obtain a green f i l t r a t e . Solvent removal from the f i l t r a t e under reduced pressure and sublimation of the remain--3 ing residue at 80-90°C and 5 x 10 mm onto a water-cooled probe afforded .1.97 g (57% y i e l d ) of CpCr(NO) 2CD 3 as a dark green, m i c r o c r y s t a l l i n e s o l i d . IR (Et 20) \JNQ 1779 (s), 1676 (vs) cm"1. 2H NMR (CH 3N0 2) 6 0.12 (s, C D 3 ) . Low-r e s o l u t i o n mass spectrum (probe temperature 150°C) m/z 194 ( P + ) . Prepara t ion of [CpCr(NO) 2{N(OD)CD 2}]PFg. This complex was prepared by t r e a t i n g CpCr(NO) 2CD 3 with NOPF^ i n CH 2C1 2 at room temperature for 4 h. It was i s o l a t e d i n a manner i d e n t i c a l to that employed for i t s perhydro analogue (vide supra) i n 61% y i e l d . IR (Nujol mull) \j 1852 (s) , 1759 (vs) cm"1; \Jqd 2581 (m) cm"1. 2H NMR (CH.N0-) 6 8.89 (br s, ID, OD), 7.61 (br s, ID, ONCD.DJ, 7.25 (br s, ID, ONCD.D^). In the presence of even trace amounts of H„0, the A - D z complex undergoes f a c i l e deuterium-hydrogen exchange at the OD p o s i t i o n . Prepara t ion of CpCr(NO) 2 CH 2 SiMe 3 . A s t i r r e d , green s l u r r y of CpCr(N0) 2Cl (4.25 g, 20.0 mmol) i n E t 2 0 (200 mL) was treated dropwise over 5 9 min with a s o l u t i o n of Me_SiCH„MgCl (1.25 M i n Et„0, 25 mL, 31 mmol) at room 57 temperature. The reaction mixture became red-brown in color and a brown prec i -pitate deposited. After an additional 10 min of s t i r r i n g , an IR spectrum of the f inal red-brown supernatant solution was devoid of the n i trosy l absorptions at 1811 and 1705 cm 1 characteristic of CpCr(N0) 2Cl, but did exhibit new bands at 1775 and 1674 cm Solvent was removed from the f inal reaction mixture in vacuo, and the residue remaining was extracted with hexanes (3 x 100 mL) unt i l the extracts were colorless. The volume of the combined extracts was reduced to 30 mL under reduced pressure, and the resulting green solution was transfer-red to the top of an alumina column (Woelm neutral, ac t iv i ty 1, 3 x 15 cm) made up in E t 2 0 . Elution of the column with Et 2 0 developed a single green band which was eluted from the column and collected. Solvent removal from the -3 eluate in vacuo afforded a green o i l which upon cooling to -195°C at 5 x 10 mm so l id i f i ed into needle-like crystals of analyt ical ly pure CpCr(NO) 2CH 2SiMe 3 (3.03 g, 57% y ie ld ) . Anal., Calcd for C . H . c N o 0 „ S i C r : C, 40.91; H, 6.06; N, 10.61. Found: C, 40.87; H, 6.06; N, 10.53. IR (CH 2C1 2) \) 1772 (s), 1669 (vs) cm"1. 1 H NMR (CD2C12) 6 5.49 (s, 5H, C ^ ) , 0.29 (s, 2H, CH2) , 0.08 (s, 9H, CHg) . Low-resolution mass spectrum (probe tempera'ture 150°C) m/z 249 ( [P-CH 3 ] + ) . Reaction of CpCr(NO)2CH2SiMe3 with NOPF&. To a mixture of CpCr.(N0)2CH2SiMe3 (0.528 g, 2.00 mmol) and finely ground N0PF& (0.350 g, 2.00 mmol) was added CH 2 C1 2 (25 mL), and the mixture was s t i rred rapidly at room temperature for 25 min. The reaction mixture became darker green in color as the NOPFg was consumed. An IR spectrum of the f ina l green solution displayed new \>xT's at 1840, 1740 and 1613 cm 1 in addition to weak bands at 1772 and 58 1669 cm due to the ni trosyl ligands of the CpCr (NO) ^ H ^ i M e ^ reactant. The addition of hexanes (120 mL) to the f inal solution induced the separation of a green o i l . The supernatant solution was removed by cannulation, and the o i l -3 was reprecipitated from CH 2 C1 2 (10 mL)/hexanes (100 mL) and dried at 5 x 10 mm and 20°C for 2 h to obtain 0.71 g (81% yield) of [CpCr(N0)2{N(0)CH2SiMe3}]PF6 as a s l ight ly impure, viscous green o i l . Anal. Calcd for C.H. ,N_0,SiPF c Cr: C, 24.60; H, 3.64; N, 9.57. Found: C, 9 16 3 3 6 23.16; H, 3.06; N, 10.00. IR (neat) \)VT_ 1838 (s) , 1736 (vs) , 1628 (w) cm"1. NU : H NMR (CD2C12) 6 7.80 (s, 2H, CH2) , 5.89 (s, 5H, C ^ ) , 0.30 (s, 9H, S i -(CH 3 ) 3 ) . Reaction of [CpCr(NO)2{N(0)CH2SiMe3}]PF6 with H 20. A s t i rred , green solution of [CpCr(N0)2{N(0)CH2SiMe3}]PF& (0.680 g, 1.55 mmol) in C H 2 C l 2 (15 mL) was treated with 3 drops of d i s t i l l e d H 20 whereupon a green so l id began to precipitate after ^2 min. After 45 min, the almost colorless supernatant solu-tion was removed by f i l t e r cannulation. The remaining green so l id was washed -3 with CH 2 C1 2 (5 mL) and was dried at 20°C and 5 x 10 mm to obtain 0.31 g (55% yield) of [CpCr(NO)2(N(0H)CH2)]PF& which was identif ied by i t s characteristic spectroscopic properties (vide supra). Similar treatment of [CpCr(NO) 2(N(0)CH 2SiMe 3)]PF& with D 20 afforded a comparable y i e ld of [CpCr(N0)2{N(0D)CH2)]PF6: IR (Nujol mull) \>QD 2577 (m) cm"1; 1852 (s), 1759 (vs), 1559 (w) cm" 1. 2 H NMR (CH.N0-) 6 8.77 (s). Reaction of CpCr(NO)2CH2Sine3 with I 2 - A vio let solution of I 2 (0.127 g, 0.50 mmol) in Et 2 0 (20 mL) was added dropwise by cannula to a s t irred green solution of CpCr(N0) 2CH 2SiMe 3 (0.264 g, 1.00 mmol) in Et 2 0 (20 mL) at 59 room temperature. The reaction mixture darkened in color, and some dark so l id precipitated. After 10 min, the reaction mixture was quickly f i l t ered through a short ( 3 x 5 cm) column of alumina (Woelm neutral, ac t iv i ty 1) supported on a medium porosity f r i t . The brown-black f i l t ra t e was taken to dryness in vacuo, and the remaining black residue was recrystal l ized from Et^O at -5°C to obtain 0.06 g (20% yie ld based on Cr) of CpCr(NO)2I as a black, crystal l ine so l id . Anal. Calcd for C ^ N ^ C r l : C, 19.74; H, 1.64; N, 9.21. Found: C, 19.90;' H, 1.66; N, 9.19. IR (Nujol mull) \> 1812 (s), 1695 (vs) cm"1. Low resolution mass spectrum (probe temperature 120°C) m/z 304 (P + ) . Treatment of CpCr(NO)2CH2SiMe3 with MeOH. A s t irred CH 2 C1 2 (20 mL) solution of green CpCr(NO) 2CH2SiMe3 (0.264 g, 1.00 mmol) was treated with an excess ('v-l mL) of deaerated methanol. After being s t irred at ambient tempera-ture for 5 days, the reaction mixture had not changed in color, and i t s IR spectrum exhibited only ^ Q ' S a u e to the organometallic reactant at 1774 and 1669 cm"1. Reaction of CpCr(NO)2Ph with NOPF&. A green solution of CpCr(NO)2Ph7 (0.220 g, 0.866 mmol) in C H 2 C l 2 (10 mL) was treated with so l id , f inely ground NOPFg (0.152 g, 0.866 mmol), and the mixture was s t irred rapidly at ambient temperature. The supernatant solution became red as the nitrosonium salt d i s -solved. An IR spectrum of this solution displayed new ni trosy l absorptions at 1848 and 1744 cm 1 in addition to those due to the phenyl reactant. Dark red microcrystals began to precipitate from this solution after ^10 min. Hexanes (60 mL) were added to complete the precipitation of this so l id which was then -3 collected by f i l t r a t i o n and dried at 20°C and 5 x 10 mm for 1 h to obtain 0.28 g (76% yield) of [CpCr(NO)_{N(0)Ph}]PF, as a red, microcrystalline so l id . 60 Anal. Calcd for C ^ ^ N ^ P F g C r : C, 30.77; H, 2.33; N, 9.79. Found: C, 31.02; H, 2.29; N, 9.59. IR (Nujol mull) v N Q 1850 (s), 1766 (vs), 1489 (ra) cm" 1. NMR (CD2C12) 6 8.21-7.60 (ra, 5H, CgHj) , 6.01 (s, 5H, C ^ ) . Reaction of CpCr(NO)2Me with NSPF^. Over a 20-min period, an orange solution of NSPFg i n C H 3 N 0 2 (2 mmol in 30 mL, generated in s i tu from 1/3 (NSCl) 3 and AgPF^)^ was fi lter-cannulated onto a s t i rred , olive-green solution of CpCr(N0)2Me (0.384 g, 2.00 mmol) in CH 2 C1 2 (30 mL) held at 0°C by means of an ice bath. As the reaction progressed, the reaction mixture darkened in color. The ice bath was then removed, and the mixture was allowed to warm slowly to room temperature as s t i rr ing was continued for an additional 2.5 h. Hexanes (100 mL) were then added to the f inal reaction mixture whereupon two immiscible l iqu id phases separated. The upper hexanes layer containing unreacted CpCr(N0)2Me was removed by cannulation and discarded. Solvent was removed from the lower nitromethane layer in vacuo to obtain a viscous green o i l . Triturat ion of this o i l with CH 2 C1 2 (10 mL) resulted in the formation of 0.31 g of a green powder. IR (Nujol mull) 1829 (s), 1723 (vs) cm also 3331 (s) and 1659 (w) cm Unfortunately, this green powder was unstable, decomposing even when maintained at -20°C overnight. Consequently, a consistent elemental analysis of this material could not be obtained. Reaction of CpCr(NO)2Me with [p-O^NCgH^lBF^. The paranitrophenyl-diazonium sa l t , [£-0 2NC^H^N 2]BF^ (0.474 g, 2.00 mmol)''"1 was dissolved in CH 3N0 2 (50 mL), and the resulting l ight orange solution was transferred dropwise by cannula onto a s t i rred CH„C19 (50 mL) solution of CpCr(N0)9Me 61 (0.384 g, 2.00 mmol) at ambient temperature. The reaction mixture was s t irred for a total of 2 h, during which time the or ig inal ly green solution became red, and IR monitoring revealed replacement of the original U J J Q ' S a t 1777 and 1669 cm 1 by new bands at 1846 and 1746 cm Solvent was removed from the f inal mixture in vacuo, and the remaining o i l y residue was reprecipitated from Cl^Cl^hexanes. Triturat ion of the o i l thus obtained with C H 2 C l 2 afforded a green-brown microcrystalline so l id and a red supernatant solution. The crystals were collected by f i l t r a t i o n , washed quickly with C H 2 C l 2 ( 2 x 5 mL), and dried at 20°C and 5 x 10~3 mm for 1 h to obtain 0.212 g (25% yield) of analyt ical ly pure [CpCr(NO)2{N(NC6HAN02)Me}]BF^. Single crystals of this salt were grown by maintaining a concentrated solution of this complex in CH 2 C1 2 at -20°C for 3 days. Anal. Calcd for C.~H._0.N c BF,Cr: C, 33.58; H, 2.80; N, 16.32. Found: 12 12 4 5 4 C, 33.44; H, 2.87; N, 16.20. IR (Nujol mull) v 1833 (s), 1726 (vs) cm"1; also 1605 (m), 1590 (m), 1560 (w) cm"1. ^ NMR (CD3N02) 6 8.43 (d, 2H, meta H, 3 J „ „ = 9.3 Hz), 7.29 (d, 2H, ortho H), 6.12 (s, 5H, C L H e ) , 4.26 (s, 3H, CH,). A X 5 5 3 Definitive assignment of the of the diazene ligand in the IR spectrum requires a proper 1^N labeling study. Reaction of [CpCr(NO)2{N(0)Ph}]PF6 with [PPN]C1. To a s t i rred , red solution of [CpCr(N0)2{N(0)Ph}]PF6 (0.268 g, 0.625 mmol) in CH 2 C1 2 (25 mL) at ambient temperature was added a s l ight excess of [PPN]C1 (0.40 g, 0.70 mmol) whereupon the solution immediately became green. After 1 min, an IR spectrum of this solution was devoid of absorptions due to the nitrosobenzene-containing complex, but did exhibit strong bands attributable to CpCr(N0) 2Cl (V^Q 1817 62 (s), 1711 (vs) cm ) and uncomplexed PhNO (1506, 1439 cm ). Removal of solvent from the f inal reaction mixture in vacuo and sublimation of the residue -3 at 20°C and 5 x 10 mm onto a Dry-Ice-cooled probe produced 0.02 g (34% yield) of PhNO which was identif ied by comparison of i t s spectroscopic properties with those exhibited by an authentic sample. The re lat ive ly low isolated y ie ld of PhNO from this clean (by IR) conversion was undoubtedly a ref lect ion of the nitroso compound's considerable v o l a t i l i t y . Sequential Treatment of CpCr(NO)2Ph with [g-C^NCgH^lBF^ and [PPN]C1. A s t i rred , green solution of CpCr(N0)2Ph (0.31 g, 1.2 mmol) in CH 3N0 2 (5 mL) at room temperature was treated with so l id [p-O^C^H^N^BF^ (0.24 g, 1.0 mmol) whereupon the reaction mixture became red-brown within 10 min. After 5 h, the reaction mixture was f i l tered through a medium-porosity glass f r i t , and solvent was removed from the f i l t r a t e in vacuo. The remaining brown residue was dissolved in CH 2 C1 2 (40 mL), [PPN]Cl (0.63 g, 1.1 mmol) was added, and the resulting solution was s t irred for 15 min whereupon i t became green-orange in color. The f inal reaction mixture was taken to dryness under reduced pressure, and the residue thus obtained was extracted with hexanes (2 x 70 mL) to leave behind a green-brown powder. Solvent was removed from the combined orange extracts in vacuo, the resulting residue was dissolved in Et 2 0 (10 mL), and this solution was transferred to the top of an alumina column (Woelm neutral, act iv i ty 1, 2 x 10 cm) made up in E t 2 0 . Elution of the column in a ir with Et 2 0 developed a single orange band which was eluted from the column and collected. Solvent removal from the eluate in vacuo afforded an orange so l id which was recrystal l ized from hexanes to obtain 0.08 g (35% yield) of p_-02NCgH^N2Ph as an orange microcrystalline so l id . Anal. Calcd for C ^ H g N ^ : C, 63.43; H, 3.99; N, 18.49. Found: C, 63 63.50; H, A.01, N, 18.AA. Low-resolution mass spectrum (probe temperature 150°C) m/z 227 (P + ) . The green-brown powder remaining after the hexanes extraction was recrystal l ized from Cl^C^-hexanes to obtain 0.16 g (75% yield) of golden CpCr(N0) 2Cl which was readily identif iable by i t s characteristic IR and mass 8 spectra. Reaction of [CpCr(NO)2{N(OH)CH2)PF5 with CpCr(NO)2Me. To a s t i rred , green CH 2 C1 2 solution (25 mL) of CpCr(N0)2Me (1.92 g, 1.00 mmol) was added sol id [CpCr(N0)2{N(0H)CH2)]PF6 (0.367 g, 1.00 mmol). The mixture was s t irred vigorously for 10 days at ambient temperature [Caution: a gas is evolved during this reaction, and appropriate steps must be taken to avoid excessive pressure build-up in the reaction vessel] . After this period the reaction mixture was f i l t ered . Addition of hexanes (100 mL) to the f i l t r a t e resulted in the precipitation of a green flocculent powder. This powder was collected by f i l t r a t i o n and dried in vacuo for 3 h to obtain 0.A6 g (85% yie ld based on Cr) of green [{CpCr(NO) 2} 2{u,n 2-N(CH 2)0)]PF 6. Anal. Calcd for C - ^ H ^ N ^ P F g C r ^ C, 24,31; H, 2.21; N, 12.89. Found: C, 24.22; H, 2.2A; N, 12.71. IR (Nujol) v^Q 1832(s), 1809(s), 1716(s) , 1700(s) cm"1; \) N C 1567(w) cm"1. IR (CH2C12) \> m 1833(s), 1811(s), 1731(s), 1709(s) cm"1' 1 H NMR (CD Cl ) 6 7.25 (d, 1H, -ONCH H , 2 J . = 6.3 Hz), 7.13 (d, Z Z A D !„ Irj H A H B 1H, -ONCH^Hg) , 5.81 (s, 5H, C ^ ) , 5.7A (s, 5H, C ^ ) . 1 3C{'H} NMR (CD 2 Cl 2 ) 6 151.Al (s, CH 2 ) , 10A.59 (s, C ^ ) , 103.76 (s, C ^ ) . 1 3 C NMR (CDgNO^ 6 153.33 (dd, CH A H B , 1 J 1 3 1 = 185 Hz, 173. A Hz), 105.9A (dq, C ^ , X J 1 3 =182.2 64 Hz, n J = 6.5 Hz), 105.06 (dq, C H . 1J.~ = 182.6 Hz, n J . - = 6.5 C - H C - H C - H Hz) . This compound was also obtainable in comparable y ie ld by the direct reaction of CpCr(N0)2Me with NOPF^ in a 2:1 rat io . The evolution of CH^ was 12 established by GC-MS analysis of the atmosphere in the reaction flask. Reaction of [CpCr(NO)2{N(OH)CH2}]PF6 with Proton Sponge. To a s t i rred , green s lurry of [CpCr(N0)2{N(0H)CH2)]PF6 (0.07 g, 0.20 mmol) in CH 2 C1 2 (10 mL) was added sol id Proton Sponge (0.04 g, 0.19 mmol). The reaction mixture imemdiately turned red-brown. An IR spectrum of the red-brown solution displayed new strong bands at 1800 and 1690 cm 1 attributable to v ^ . However, these bands were gradually replaced over a 1.5 h period by new bands at 1833, 1809, 1731 and 1701 cm ^. A yellow so l id also precipitated during this time. Removal of the so l id by f i l t r a t i o n , and addition of hexanes (30 mL) to the green-brown f i l t r a t e resulted in the precipitat ion of a green so l id . This so l id was collected by f i l t r a t i o n , washed with hexanes (5 mL) and dried in vacuo for 1 h to obtain 0.03 g (55% yie ld based on Cr) of [{CpCr(N0) 2) 2-2 {u,n, -N(CH2)0}]PFg which was readily identif iable by i t s characteristic spectroscopic properties (vide supra). 2 Preparation of [{CpCr(NO)2>2{u,n -N(CH 2)0}]BPh 4. To a s t i rred , green methanol solution (100 mL) of [{CpCr(NO) 2) 2 {u,n 2 -N(CH 2 )0}]PF & (0.46 g, 0.85 mmol) in a ir was added sol id NaBPh^ (0.29 g, 0.85 mmol). A shiny, green micro-crystal l ine so l id precipitated after 5^ min. This so l id was collected by f i l t r a t i o n , washed with methanol (10 mL) and then dried in vacuo for 6 h to obtain 0.40 g (66% yie ld based on Cr) of golden-brown 65 [{CpCr(NO) 2) 2{}i,n 2-N(CH 2)0}]BPh 4. Suitable crystals of this salt for X-ray crystallographic analysis were grown by slow evaporation of a saturated solu-tion of the complex in a 1:1 mixture of CH 2Cl 2/hexanes at room temperature. Anal. Calcd for C ^ H ^ C r ^ C ^ B : C, 58.58; H, 4.46; N, 9.76. Found: C, 58.31; H, 4.68; N, 9.67. IR (Nujol mull) \> 1817(s), 1786(s), 1722(s) and 1696(s) cm"1. 1 H NMR (d,-acetone) 6 7.48-6.73 (m, 22H, CH 0 and 4 x C ,H C ) , 6.07 (s, 5H, C 5 H 5 ) , 6.02 (s, 5H, C ^ ) . 66 Results and Discussion Insertions of the Nitrosonium Ion into Chromium-Carbon Sigma Bonds. Treatment of various CpCr(NO)2R complexes (R = alkyl or aryl) with N0+ results in the clean insertion of the nitrosonium ion into the Cr-C o bonds, i.e. (3.A) the cationic products resulting from transformations 3.4 generally being isolable in good yields as their PF^ sal ts . In the special case when R = Me, the insertion f i r s t generates a brown complex whose spectroscopic properties are consistent with i t containing the [CpCr(NO)2{N(0)Me}]+ cation. Speci f ical ly , i t s IR spectrum (Nujol mull) exhibits a band at 1559 cm 1 and i t s 1 H NMR spectrum ( C D 2 C l 2 , -20°C) displays a singlet at 6 1.81, both features 13 being attributable to the nitrosomethane ligand. In solutions, however, this brown complex isomerizes irrevers ibly to the novel formaldoxime complex, i . e . The isomerization step depicted in eqn 3.5 is rapid in poorly coordinating 67 14 solvents such as C r ^ C ^ and CH^C^,, and the f inal green formaldoxime complex, [CpCr(NO)2{N(OH)CH2)]PF6, i s best isolated by crysta l l i zat ion from C H 2 C l 2 . In the so l id state, the latter salt may be handled in a ir for short periods of time, but i t is best stored under dry N 2 > A s ingle-crystal X-ray crystallographic ana lys i s^ of 3 [CpCr(NO)2{N(OH)CH2)]PF6 establishes that the organometallic cation possesses a normal "three-legged piano stool" molecular structure. A thermal e l l ipso id plot of this structure is shown in Figure 3.1, and pertinent intramolecular dimensions are tabulated in the caption. Within the cation, the CpCr(NO)2 fragment is normal, closely resembling that found in C p C r ( N 0 ) 2 C l . ^ The CrN(0H)CH2 portion of the cation is essentially planar, and the intramolecular dimensions of the formaldoxime ligand resemble those of free formaldoxime.1'' Consequently, in valence-bond terms, the bonding within this grouping is representable as •OH Cr — < %H2 with the formaldoxime ligand functioning as a formal two-electron donor to the metal center. The spectroscopic properties of [CpCr(N0)2{N(0H)CH2)]PFg can be readily understood in terms of i t s solid-state molecular structure, thereby indicating that the basic structural units persist in solutions. For instance, the *H NMR spectrum of the salt in CD 2C1 2 exhibits a broad singlet at 6 8 .92 and an AB pattern at 6 7.66 and 7.14 attributable to the hydroxyl and methylene protons, respectively, of the formaldoxime ligand. 68 Figure 3 . 1 . The solid-state molecular structure of the [CpCr(NO)2{N(OH)CH2)]+ cation. 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 . 0 3 4 ( 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) , C r - C ^ (centroid) = 1.843, Cr-N(l)-CU) = 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) = 9 3 . 5 ( 3 ) , 0(2)-N(2)-Cr = 1 7 4 . 5 ( 6 ) , 0(3)-N(3)-Cr = 172 .2 (5 ) . 69 The reaction between the perdeutero methyl complex, CpCr (NO) 2 C D 3 ' a n < ^ ^ apparently proceeds in an analogous manner, i . e . In this instance, the f inal green sa l t , [CpCr(NO)2{N(OD)CD2)]PF6> i s isolable in 61% y ie ld . The occurrence of this conversion thus verif ies that the hydroxyl H atom of the formaldoxime ligand in reaction 3.5 does indeed o r i g i -nate from the methyl group in the organometallic reactant and not from some other source such as the solvent. The presence of the perdeuterated formald-oxime ligand in the f inal complex is c learly evident in i t s IR [Nujol mull; 2581 cm"1; \> 0 H /v 0 D calcd: 1.37; found: 1.35] and 2 H NMR [CH 3N0 2; three sing-lets of equal intensity at 6 8.89, 7.61 and 7.25] spectra. However, this ligand undergoes faci le deuterium-hydrogen exchange at the OD position in the presence of even trace amounts of H»0, i . e . 70 Thus, as reaction 3.7 progresses, there appears and grows a \) band at 3480 -1 2 cm in the Nujol mull IR spectrum, and the H NMR signal (CH3N02) at 6 8.89 diminishes. When R = CH^SiMe^ in the general transformation 3.4, the N0+ inserted product can be precipitated as i t s PF^ salt by the addition of hexanes to the f inal reaction mixture. Isolated in this manner, the complex is a s l ight ly impure, dark green tar which exhibits IR and NMR spectra (see Experimental Section for details) that are completely consistent with i t s cation possessing the molecular structure shown in the middle of eqn 3.8 below. Also as i n d i -cated in eqn 3.8, this cation i s readily hydrolyzed to the ubiquitous formald-oxime-containing complex. Interestingly, i f D 20 rather than H 20 is employed to effect the second step of reaction 3.8, the f ina l product displays deuterium 71 I , ' C \ ON' ^ ' C H z S i M e j N0+ R.T. 15min O N ' * NO CH 2 S iMe 3 H 2 0 R.T. 2m in -A-72 incorporation solely at the hydroxyl posit ion, i . e . I A ON'* NO CH2SiMe3 + D 2 0 CH 2 CI 2 1 ••-7" ° N O N — N OD CH 2 This latter observation is indicative of the hydrolysis reaction proceeding so as to afford the formaldoxime complex d irect ly , e.g. I C r -C H 2 — S i M e 3 - H H O S i M e 3 " I / O H C r - ^ N ^ NO (3.10) "CH 2 Had the cleavage of the C-Si l ink by D^O proceeded so as to generate i n i t i a l l y the CH2DN0-containing cation which would then isomerize to the f ina l product (eqn 3.5), a distribution of the deuterium label amongst the hydroxyl and methylene sites of the formaldoxime ligand would have been observed. For comparison, i t may be noted that the analogous carbon-silicon bond in the neutral CpCr(NO)2CH2SiMe3 reactant i s much less prone to cleavage. Thus, the complex is inert to H»0 and MeOH, and I» cleanly breaks the Cr-C bond instead 73 to produce CpCr(NO) 2I. F ina l ly , the N0+ insertion reaction (eqn 3.4) can also be extended to encompass chromium-aryl linkages as well . For instance, A + NO+  0N'NVPh I ON* * ON Ph (3.11) and the [CpCr(NO)2(N(0)Ph)]PF& product is isolable in 76% yie ld as a red, microcrystalline so l id . This nitrosobenzene complex is very soluble under ambient conditions in noncoordinating solvents such as CH 2 C1 2 and CH^NO^ but can be crysta l l ized from them at - 2 0 ° C . The physical and spectroscopic properties of the complex are fu l ly in accord with i t s cation having the molecular structure depicted in eqn 3.11. In part icular , the fact that the \J^Q exhibited by the PhNO ligand is 17 cm 1 lower than in the free state is indicative of i t being coordinated in a monodentate fashion through N . 1 ^ ' 1 ^ Possible Mechanisms for the Nitrosonium Insertion Reactions. Although there are many mechanistic pathways that can be envisaged for the unprecedented reactions 3.4, the most l i k e l y ones involve (a) oxidatively induced, intramolecular insertion of bound NO into the Cr-C o bonds, (b) direct attack of N0+ at the metal center followed by migration into the Cr-C o bonds, or (c) charge-controlled, intermolecular attacks by N0+ at the Cr-R groups. Each pathway i s next considered in some deta i l . (a) Oxidatively Induced Migratory Insertion of Bound NO. Ample ev i -dence exists in the chemical l i terature that during i t s reactions withorgano-74 metall ic substrates the nitrosonium cation may, on occasion, simply function as a one-electron oxidant 20 Hence, i t i s conceivable that the insertion reactions 3.4 may well proceed in the manner summarized in eqn 3.12. As indicated, the N0+ I ° N O 4 N ' R + N0 + l ..Cr.. ON'oVR • + NO i ON * ON R (3.12) cation could oxidize the CpCr(NO)reagent to i t s 17-electron radical cation while being i t s e l f reduced to NO*. The organometallic cation thus formed could then insert a bound NO into i t s Cr-R bond, presumably via intramolecular nucleophilic attack of R onto the nitrogen atom of a n i trosy l ligand (under-21 • + lined in eqn 3.12). F ina l ly , the resulting 15-electron [CpCr(NO)(N(0)R)] species would be trapped by the s t i l l present NO* radical to afford the f ina l d ini trosyl cation shown in eqn 3.12. Note that i f this mechanism is indeed operative, then i t would be a ni trosyl ligand or ig inal ly in the chromium's coordination sphere (as opposed to the external NO+) that would f ina l ly consti-tute a part of the bound RNO group. Any discussion concerning the v i a b i l i t y of this mechanism must f i r s t involve a consideration of the relative ease of oxidation of the various CpCr(NO)2R complexes. Consequently, I have investigated the oxidation behavior of these compounds in CH 2 C1 2 by employing cycl ic voltammetry at a platinum-bead electrode with [n-Bu^N]PFg as the support electrolyte. A typical cyc l ic vo l t -ammogram, that of CpCr(N0)2Me, is shown in Figure 3.2, and data for the oxida-22 tions of a l l three compounds are collected in Table 3.1. The pertinent 75 Volts vs SCE Figure 3.2. Ambient temperature cyc l ic voltammogram of 5 x 10_ Z f M CpCr(NO)2Me in CH 2 C1 2 containing 0.1 M [n-Bu4N]PF f e measured at a platinum-bead electrode at a scan rate of 0.30 V s 76 Tab le 3 . 1 . C y c l i c Voltammetry Data f o r the O x i d a t i o n s o f Some CpCr(NO) Complexes. a b -1 compd- E — Scan rate (V s ) - p , a CpCr(NO)2Me +1.33, +1.55 0.30 CpCr(NO) 2CH 2SiMe 3 +1.42 0.39 CpCr(N0)2Ph +1.32 0.19 - In C H o C l „ / 0 . 1 M [n-Bu,N]PF,. 2 2 — 4 0 - V vs. SCE, for which the Cp,Fe/Cp„Fe + couple occurs at +0.46 V ( i / i = 1) , ^ /L p f Si p 9 c AE = 70 mV). 77 feature that is immediately evident is that the CpCr(NO)2R complexes a l l undergo irreversible oxidations at f a i r l y high potentials > 1.3 V vs SCE. (For comparison, isoelectronic CpFe(CO)2Me undergoes a similar irreversible oxidation at +1.10 V under identical experimental conditions.) Since the reduction potential of N0 + in CH 2 C1 2 has been estimated as being in the range 23 + -0.22 to -0.11 V, i t is unlikely that NO would be able to oxidize the CpCr(NO)2R species cleanly and completely. To do just that, I chose instead 3+ the more potent one-electron oxidant [Fe(phen)3] whose standard reduction 24 25 potential is 1.13 V at 25°C. ' Indeed, at room temperature in C I ^ C ^ , CpCr(NO)2Me ( V N Q 1777 (s), 1669 (vs) cm *) i s completely converted by 1 equiv 3+ + of [Fe(phen)3] into a CpCr(N0) 2 -containing product (\>N0 1846 (s) , 1745 (vs) cm ^). However, when this oxidation product i s treated with NO gas, none of the formaldoxime complex i s produced. This state of affairs persists even i f the sequential transformations are effected at - 7 8 ° C , i . e . A 4 ™ ON ' C H * [Fe(phen)3XPF6)j C H 2 C l 2 . - 7 8 " C I ON" * ON —N / O H (3.13) CH2 Furthermore, there is no spectroscopic evidence for the formation of the NO-inserted products when CpCr(NO)2CH2SiMe3 and CpCr(N0)2Ph are treated in the manner depicted in eqn 3.13. While the exact natures of the organometallic products formed in these latter conversions remain to be ascertained, the 78 evidence presently at hand thus argues against reactions 3.A proceeding via the mechanistic steps outlined in eqn 3.12. (b) Attack of N0 + at the Metal Center. Another possible mechanistic pathway for the insertion reactions 3.A involves the direct attack of the N0+ electrophile at the chromium center in C p C r ( N O ) t o i n i t i a l l y generate the 20-electron, four-legged "piano-stool" [CpCr(NO).jR]+ intermediate 3 complex (which may attain an 18-electron configuration by adopting either q -Cp or bent NO geometries). Subsequent insertion of one of the NO ligands into the Cr-R l ink may then occur to afford the f inal [CpCr(NO)2(N(0)R}]+ complex. In this mechanistic pathway, i t can be envisaged that either one of the following two poss ib i l i t i e s could arise , namely (i) any one of the three NO ligands in [CpCr(NO)^R]+ could insert into the Cr-R bond to y ie ld the insertion product, or ( i i ) only the incoming N0+ inserts , a process that necessitates the "preferential activation" of the now-bonded N0+ electrophile. Clearly, the crucial experiment to perform to unambiguously determine the mode of reaction is the one that involves the use of the ^N-label led nitrosonium sal t , ^NOPFg, in the reactions 3,A. Regrettably, this salt is not readily available, nor are any of i t s convenient precursors such as ^NOCl . It has not been possi le , therefore, to demonstrate whether i t is the "external" N0+ or the bound NO that eventually ends up in the Cr-R group. (c) Direct Electrophilic Attack at the Chromium-Carbon Bonds. The third plausible mechanistic pathway for the formation of the NO +-inserted products of reactions 3.A involves charge-controlled, intermolecular attacks by N0+ at the chromium-carbon o bonds of the C p C r ( N O ) r e a c t a n t s . Such a pathway is i l lus trated for CpCrtNO^Me in Scheme 3.1 in which the attack by N0 + i s 79 Not-, . C r v , N O -ON'O'N - C H 3 I ..Cr.. I ,-CA 0 N ' 0 N C H j N O * -r\-,Cr ON*" A N C H ON i I / O H ,Cr * - N ON" » ON C H 2 Scheme 3.1 80 26 portrayed as being a c lass ica l S^ 2 process. The isomerization of the CH N^O ligand to bound CH2=N0H shown in the last step of Scheme 3.1 has already been demonstrated to occur for the perdeuteromethyl analogue (eqn 3.6) and i s 27 probably fac i l i ta ted by the acidic species present in the reaction mixture. Note that i f this mechanism is indeed operative, then i t i s the external N0 + (as opposed to a n i trosy l ligand in the organometallic reactant) that f ina l ly is a constituent part of the formaldoxime ligand. In other words, treatment of the various CpCr(NO)2R complexes with electrophiles NE + which are formally valence isolelectronic with N0+ should result in the insertion of NE + (and not N0+) into the Cr-C o bonds i f mechanisms analogous to that portrayed in Scheme 3.1 hold. Unfortunately, the reaction of CpCr(N0)2Me and "NS+" (generated in situ) affords a green so l id of unknown composition. An IR spectrum of this so l id as CpCr(N0)2Me + "NS+" ^ green so l id (3.14) a Nujol mull (see Appendix) displays absorptions attributable to at 1829 and 1723 cm and also to (possibly) MyTL! at 3331 cm Unfortunately, this green so l id is thermally unstable, decomposing to a brown species slowly in the so l id state and rapidly in solutions. This proc l iv i ty of the impure complex to decompose has prevented the determination of consistent elemental analyses or acquisition of NMR spectra for i t . Its true identity, therefore, remains unknown. Fortunately, when an aryldiazonium cation (also valence isoelectronic with the nitrosonium cation) is used in place of NS+ i n reaction 3.14, a tractable and thermally stable product is obtained. Thus, treatment of a green 81 CH 2 C1 2 solution of CpCr(NO)2Me at room temperature with a l ight orange CH 3N0 2 solution of [p-0_NC,H.N_]BF, results in the mixture becoming red-brown in color Z o 4 Z 4 as the following conversion occurs: I ..Cr.. + O N ' O N ' C H * + 0) BF 4 " N0 2 C H 2 C I 2 / R.T.. CH3N<V 2hr (3.15) The f inal product salt is isolable as a green-brown crystal l ine so l id whose spectroscopic properties are consistent with i t possessing the molecular struc-ture shown in eqn 3.15. For instance, a Nujol mull IR spectrum of this moder-ately air-stable so l id displays bands at 1605, 1590 and 1560 cm 1 that are assignable to the newly-formed diazene ligand. Furthermore, a NMR spectrum of the complex in CD,jN02 (Figure 3.3) exhibits an AX pattern in the 6 8.5-7.2 region due to the protons of a para-substituted phenyl r ing , and singlets at 6 6.12 and 4.26 due to the cyclopentadienyl and methyl protons, respectively. To confirm the mode of linkage of the paranitrophenylmethyldiazene ligand to the CpCr(NO)_ fragment, the salt was subjected to a s ingle-crystal X-ray crysta l lo-82 _+ .Cr — N ° N ON N. N02 1 - T — T - _ T — I — * — I — r ~ i l l —I—r—i—»—i—|—i—i—i—r -|- * T » 1 1 1 f Figure 3.3. The 300 MHz H NMR spectrum of [CpCr(NO)2{N(NC6H4N02)Me}]BF^ in CD 3N0 2(*). 83 graphic analysis. Two views of the solid-state molecular structure of the organometallic cation are shown in Figure 3.4, and selected intramolecular dimensions of the cation are presented in Table 3.2. As for the formaldoxime-containing cation (vide supra), the CpCr(NO)^ fragment in the [CpCrCNO^iNtNCgH^K^Me}]* cation is normal, and the internal dimensions of the diazene ligand are fu l ly in accord with i t functioning as a Lewis base 28 towards the chromium center. Interestingly, the diazene ligand adopts a c i s -configuration, probably for steric reasons, and there is no evidence for the existence of the trans-diazene-containing isomer either in the so l id state or in solutions. Certainly, the atomic connectivity of the diazene-containing cation in the so l id state is consistent with i t being formed by the electro-p h i l i c attack of external [p^NCgH^NN] + at the Cr-Me bond CpCr(NO)2Me. The successful incorporation of the aryldiazonium cation into the Cr-Me links thus suggests that the formal N0+ insertion into this and related CpCr(NO)2^ compounds (eqn 3.4) also probably proceeds by direct e lectrophil ic attack as depicted in Scheme 3.1 for R = Me. A S t o i c h i o m e t r i c C y c l e f o r the Format ion o f New Carbon-Ni t rogen Bonds. As pointed out in the preceding sections, the newly-formed nitrosoalkane and diazene ligands in the cationic products of transformations 3.4 and 3.15 may be simply viewed as Lewis bases coordinated to the cationic metal center. Conse-quently, they should, in pr inc ip le , be displaceable from the chromium's coord-ination sphere by other, more strongly coordinating two-electron donors. For pract ical reasons, I have found that the chloride anion i s the Lewis base of choice for effecting these displacement reactions. For example, treatment of a red dichloromethane solution of [CpCr(NO),{N(0)Ph}]+ at 20°C with one equiva-84 Figure 3.4. Views of the molecular structure of the [CpCr(NO) 2{N(NC 6H4N0 2)Me}]+ cation (a) approximately para l le l to the Cp ligand and (b) approximately perpendicular to the Cp ligand. Only non-hydrogen atoms are shown. 85 Tab le 3 . 2 . S e l e c t e d Bond Lengths (A) and Angles (deg) f o r [CpCr(NO) 2 {N(NC 6 H A N0 2 )Me}] + BF 4 " • Cr - N(l) 1.709(5) Cr - N(2) 1.699(5) Cr - N(3) 2.057(4) Cr - C(l) 2.191(5) Cr - C(2) 2.201(5) Cr - C(3) 2.212(5) Cr - C(4) 2.181(5) Cr - C(5) 2.189(5) N(l) - 0(1) 1.152(7) N(2) - 0(2) 1.167(7) N(3) - N(4) 1.225(6) N(3) - C(6) 1.478(7) N(4) - C(7) 1.450(6) C(7) - C(12) 1.375(8) C(7) - C(8) 1.374(8) C(12) - C( l l ) 1.397(8) C( l l ) - C(10) 1.368(9) C(8) - C(9) 1.371(8) C(9) - C(10) 1.368(10) C(10) - N(5) 1.490(7) N(5) - 0(3) 1.218(11) N(5) - 0(4) 1.213(11) C(l) - C(2) 1.383(9) C(2) - C(3) 1.389(9) C(3) - C(4) 1.413(10) C(4) - C(5) 1.409(9) C(5) - C(l) 1.396(9) N(l) - Cr - N(2) 9 5 . 6 ( 3 ) ° N(l) - Cr - N(3) 9 5 . 3 ° ( 2 ) N(2) - Cr - N(3) 97.8(2) Cr - N(l) - 0(1) 173.2(5) Cr - N(2) - 0(2) 172.4(5) Cr - N(3) - C(6) 120.4(3) Cr - N(3) - N(4) 118.1(3) N(4) - N(3) - C(6) 121.5(4) N(3) - N(4) - C(7) 121.3(4) N(4) - C(7) - C(12) 117.8(5) N(4) - C(7) - C(8) 119.7(5) C(12) - C(7) - C(8) 121.9(5) C(7) - C(12) - C( l l ) 118.9(5) C(12) - C( l l ) - C(10) 117.7(5) C(l l ] 1 - C(10) - C(9) 123.7(5) C(10) - C(9) - C(8) 118.3(6) continued.. . 86 Table 3.2. (continued) C(7) - C(8) - C(9) 119.6(5) C(l l ) - C(10) - N(5) 117.9(6) C(9) - C(10) - N(5) 118.4(6) C(10) - N(5) • - 0(3) 117.7(7) C(10) -- N(5) - 0(4) 118.3(7) 0(3) - N(5) - 0(4) 124.0(6) C(l) - C(2) - C(3) 109.2(5) C(2) - C(3) - C(4) 107.3(5) C(3) - C(4) - C(5) 107.6(5) C(4) - C(5) - C(l) 107.6(5) C(5) - C(l) - C(2) 108.3(5) 87 lent of bis(triphenylphosphine)iminium chloride ([PPN]C1) results in the reac-tion mixture rapidly becoming green. Monitoring the progress of this trans-formation by IR spectroscopy reveals the clean conversion of the organometallic reactant to CpCr(NO)2Cl at 1817 and 1711 cm and the formation of free nitrosobenzene (diagnostic bands at 1506 and 1439 cm . The balanced chemical equation for this process is thus the nitrosobenzene being isolable from the f inal reaction mixture by sublima-tion after removal of the CH 2 C1 2 solvent in vacuo. The success of reaction 3.16 thus permits the construction of a cycle of stoichiometric reactions for the formation of new carbon-nitrogen bonds mediated by the CpCr(NO)2 group. Such a cycle is shown below for the particular case of N0+ insertion, Crp representing the CpCr(NO)„ group: (3.16) 88 Thus, when R = Ph, reaction 3.16 is shown at the bottom of the cycle. The CpCrCNO^Cl by-product of this reaction may be reconverted to the original 7 CpCr(N0)2Ph reactant simply by treatment with Ph^Al, and the insertion of NO may then be effected again. In pr inc iple , cycles similar to that shown above should also hold for R = Me or CH 2SiMe 3 and for [p/^NCgH^]"*" as the elec-trophile in place of -N0+. Preliminary investigations indicate that this is indeed true and that i t usually is not necessary to isolate the intermediate insertion products. Thus, sequential treatment of CpCr(NO)2Ph with [p_-02NCgH^N2] BF^ and then [PPN]C1 affords good yields of the unsymmetrical diazene, p_-02NC6H4N=NPh, and CpCr(NO) 2 Cl. The net organic transformations mediated by the CpCr(NO)2R groups in cycles such as that shown above are thus NE + + R - N(E)R (3.17) 89 where NE is the external nitrogen-containing electrophile and R is the organic group i n i t i a l l y o-bonded to chromium. The f inal N(E)R product is formed selectively, the new C-N linkage being generated exclusively at the 29 carbon atom or ig inal ly attached to the metal center. Depro tona t ion o f the Formaldoxime L i g a n d i n [CpCr (NO) 2 {N(OH)CH 2 ) ]PF 6 . Interestingly, the [CpCr(N0)2{N(0H)CH2)]PF6 compound reacts with CpCr(NO)2Me to generate an unprecedented type of an oximato-bridged bimetall ic complex. The specif ic transformation being considered i s presented in eqn 3.18: I .Cr ON' * ON II CH 2 OH l .Cr ON >N II CH 2 I CHj 4 'NO ON CH2CI2 + I NO ON + CH. (3.18) This bimetall ic complex is air-stable and i s most soluble in good solvating solvents such as nitromethane. Single-crystal X-ray crystallographic analys i s^ of the dichromium cation (as i t s BPh^ salt) revealed a new type of an oximato-bridged molecular struc-ture (Figure 3.5). The intramolecular dimensions of the bimetall ic cation (Table 3.3) indicate two structural ly normal CpCr(NO)2 groups*^ held together 90 Figure 3.5. Solid-state molecular structure of the [{CpCr(NO) 2} 2{u,n 2-N(CH 2)0}] + cation as i t exists in i t s BPh^" sa l t . Hydrogen atoms have been omitted for c l a r i t y . 91 Table 3.3. Selected Bond Lengths ( A ) and Angles (deg) for the [{CpCr(NO)2}2{u,n2-N(CH2)0}]+ cation as i t exits i n i t s BPh^~ salt . Cr(l) - N(l) = 1.725(4) N(l) - 0(1) = 1.169(5) Cr(l) - N(2) = 1.718(4) N(2) - 0(2) = 1.174(5) Cr(l) - 0(3) = 1.956(3) N(3) - 0(3) = 1.348(5) Cr(2) - N(3) = 2.206(4) N(3) - C(l) = 1.282(4) Cr(2) - N(4) = 1.714(4) N(4) - 0(4) = 1.160(5) Cr(2) - N5) = 1.706(5) N(5) - 0(5) = 1.170(6) N(l) - Cr(l) - N(2) = 94.5(2) Cr(l) - N(l) - 0(1) = 165.7(4) N(l) - Cr(2) - 0(3) = 103.6(2) Cr(l) - 0(3) -N(3) = 128.5(5) N(3) - Cr(2) - N(4) = 99.9(2) Cr(2) - N(3) - 0(3) = 109.3(3) N(4) - Cr(2) - N(5) = 94.2(3) Cr(2) - N(4) - 0(4) = 171.7(4) 92 only by single bonds to the N and 0 atoms of the formaldoximato ligand, respectively. A l l other structural ly characterized bimetall ic complexes having an oximato ligand spanning the two metal centers possess a metal-metal 30 31 2 + bond. ' The spectroscopic properties of the [{CpCr(N0)2}2{u,n -N(CH2)0}] cation (as i t s more soluble PF^ salt) are readily interpretable in terms of i t s solid-state molecular structure, a fact which indicates that the basic dichromium structural units persist in solutions. For example, the *H NMR spectrum of the compound in CD 2 C1 2 i s shown in Figure 3.6 and contains signals due to the methylene and cyclopentadienyl protons (see Experimental Section for detai ls ) . 13 Part icularly interesting is the C NMR spectrum of the compound xn CD.jN02 (Figure 3.7) which exhibits two Cp carbon signals at 6 105.9A and 105.06. Each 13 1 appears as a doublet of quintets because of short-range C- H coupling of 182 Hz and long-range coupling of 6.5 Hz to the four other protons of the Cp r ing. Furthermore, the signal due to the methylene carbon of the bridging formaldoximato group appears as a doublet of doublets at 6 153.33 (^ J-io i C - H 185, 173 Hz), the coupling constants being fu l ly consistent with this carbon 2 32 33 retaining i t s sp hybridization. * The sequence of reactions leading to the production of the oximato-bridged bimetall ic complex may thus be represented as 93 5 Figure 3.6. CD 2 C1 2 . The 300 MHz *H NMR spectrum of [{CpCr(NO) 2} 2{u,n 2-N(CH 2)0}]PF 6 in 94 6 ppm Figure 3.7. The 75 HMz 1 3 C NMR spectrum of [{CpCr(NO) 2} 2{u,n 2-N(CH 2)0}]PF 6 in CD^NO^ The inset contains an expansion of the signals due to the Cp carbons at 6 105.94 and 105.06 ppm. 95 + + I N 0 + ,0H 'CH2 CpCr(NO) 2CH 3 - C H 4 (3.19) Certainly, the occurrence of reaction 3.18 explains why low yields of the 3 formaldoxime complex are obtained (during i t s synthesis from CpCr(NO)2Me and NOPFg, vide supra) i f (a) an excess of CpCr(NO)2Me is used or (b) the insoluble NOPFg is not f inely ground (in effect causing (a) l oca l ly ) . Also, the occurrence of reaction 3.18 is of fundamental significance since 34 free oximes such as diacetyl monoxime (pK =9.3) or even carboxylic acids SL 35 such as p_-fluorobenzoic acid (pK = 4.14) do not react with CpCr(NO)»Me under identical experimental conditions. The role of the e lectrophil ic [CpCr(NO) 2] + cation in activating formaldoxime by coordination i s thus to increase substan-t i a l l y i t s Bronsted acidity above that which i t possesses in i t s free state. Once activated, the bound formaldoxime can then undergo deprotonation by 36 CpCr(NO)2Me to afford the observed products. Surprisingly, the reaction of [CpCr(N0)2{N(0H)CH2)]PF6 with Proton Sponge also results in the production of the oximato-bridged bimetallic complex (in 55% yie ld based on Cr) . This trans-formation occurs presumably via the i n i t i a l formation of CpCr(N0)2-0N=CH2 (IR 1800 and 1690 cm "") which then readily displaces the formaldoxime ligand in unreacted [CpCr(NO)~{N(0H)CH_}]+ as shown in Scheme 3.2. 96 C + _ N ' 0 H bose t \ H 2 " H B A S E + C r p - O - N , + yOH TJH2 -HONCH," C r p - 0 ^ N ^ C r p II C H 2 Scheme 3.2 Regrettably, attempts to isolate the intermediate CpCr(NO)2_0N=CH2 compound have not been f r u i t f u l . Summary The NE + electrophiles (E = 0 or p_-02NCgH N^) undergo unprecedented insertions into the Cr-C o bonds of various CpCr(NO)2R compounds to afford [CpCr(NO)2{N(E)R)]+ cationic complexes. When R = Me and E = 0, the i n i t i a l l y formed product isomerizes intramolecularly to [CpCr(N0)2{N(0H)CH2)]+, which then reacts further with CpCrtNO^Me to give the oximato-bridged complex, 2 + [{CpCr (NO) 2}2lu,r\ -N(CH2)0}] . At present, i t appears that the CpCr (NO) 2 R compounds undergo the requisite insertion of NE + into the Cr-R bonds because these bonds are prone to non-oxidative attack by electrophiles and the compounds themselves are re lat ive ly d i f f i c u l t to oxidize. Nevertheless, the newly-formed N(E)R ligands may be displaced from the chromium's coordination sphere by the more strongly coordinating Cl anion. The resulting CpCrtNO^Cl can be reconverted to CpCr(NO)_R by treatment with the appropriate Grignard or 97 organoaluminum reagent, thereby completing a cycle by regenerating the i n i t i a l organometallic reactant. The entire sequence of stoichiometric reactions form-ing the cycle thus constitutes a selective method for the formation of new carbon-nitrogen bonds, the net organic conversions mediated by the CpCr(NO)^ group being NE + + R N(E)R. In pr inc iple , therefore i t should be possible to broaden the scope of this synthetic methodology by extending this chemistry to encompass a wide range of CpCr(NO)2_containing organometallic complexes and other electrophiles. 98 References and Notes (1) Collman, J . P . ; Hegedus, L . S.; Norton, J . R.; Finke, R. G. Principles and Applications of Organotransition Metal Chemistry; University Science Books: M i l l Valley, CA, 1987. (2) Alexander, J . J . In The Chemistry of the Metal-Carbon Bond; Hartley, F. R.; Patai , S., Eds.; Wiley: Toronto, 1985; Vol . 2, Chapter 5. (3) Legzdins, P . ; Wassink, B . ; Einstein, F, W. B . ; W i l l i s , A. C. J. Am. Chem. Soc. 1986, 108, 317. (4) Goldhaber, A . ; Vollhardt, K. P. C ; Walborsky, E. C ; Wolfgruber, M. J. Am. Chem. Soc. 1986, 108, 516. (5) Migratory insertions of neutral n i t r i c oxide into transition-metal-carbon bonds also result in the formation of new C-N linkages.^ (6) Seidler, M. D. ; Bergman, R. G. J. Am. Chem. Soc. 1984, 106, 6110 and references therein. (7) Hoyano, J . K. ; Legzdins, P . ; Malito, J . T. J . Chem. Soc, Dalton Trans. 1975, 1022. (8) Hoyano, J . K . ; Legzdins, P . ; Malito, J . T. Inorg. Synth. 1978, 18, 126. (9) Whitmore, F. C . ; Sommer, L . H. J. Am. Chem. Soc 1946, 68, 481. (10) Herberhold, M . ; Haumaier, L . Organomet. Synth. 1986, 3, 281. (11) Balz, G . ; Schiemann, G. Chem. Ber. 1927, 60, 1186. (12) Using a DBI-30W (5u) column at 5° C and a Kratos MS-80 mass spectro-meter. 99 (13) For comparison, free MeNO exhibits a XJ^ Q of 1564 cm ; see Feuer, H. The Chemistry of the Nitroso and Nitro Groups; Wiley-Interscience: Toronto, 1969; Vol . 1, p. 140. (14) Drago, R. S. Pure Appl. Chem. 1980, 52, 2261. (15) The X-ray structural analysis was performed by Drs. F . W. B. Einstein and R. H. Jones of Simon Fraser University. (16) Greenhough, T. J . ; Kolthammer, B. W. S.; Legzdins, P . ; Trotter, J . Acta Crystallogr., Sect. B 1980, B36, 795. (17) Levine, I. N. J. Chem. Phys. 1963, 38, 2326. (18) Boyd, A. S. F . ; Browne, G . ; Gowenlock , B. G . ; McKenna, P. J . Organomet. Chem. 1988, 345, 217 and references therein. (19) For leading references to other RNO complexes of transit ion metals, see (a) S te l la , S.; F l o r i a n i , C . ; C h i e s i - V i l l a , A . ; Guastini, C. J . Chem. Soc, Dalton Trans. 1988, 545. (b) P izzot t i , M . ; Porta, F. ; Cenini, S.; Demartin, F . ; Masciocchi, N. J . Organomet. Chem. 1987, 330, 265 and references therein. (20) Caulton, K. G. Coord. Chem. Rev. 1975, 14, 317. (21) Related oxidatively promoted alkyl to acyl migratory insertions involving CpFe(CO) (L)Me complexes which are isoelectronic with CpCr(N0)2Me have been documented: see Magnuson, R. H . ; Meirowitz, R.; Zulu, S.; Giering, W. P. J . Am. Chem. Soc 1982, 104, 5790 and references therein. (22) A l l three complexes undergo reversible 1-electron reductions in C I ^ C ^ . o' For CpCr(N0) 2CH 2SiMe 3 this redox couple occurs at E = -1.03 V, and for o' CpCr(N0)2Ph this occurs at E = -1.07 V. The reduction behaviour of the CpCr(NO)»Me compound has been analyzed in some detai l previously, 100 see: Legzdins, P . ; Wassink, B. Organometallics 1988, 7, 482. (23) Connelly, N. G . ; Deraidowicz, Z . ; Kel ly , R. L . J. Chem. Soc, Dalton Trans. 1975, 2335. (24) Rieger, P. H. Electrochemistry; Prentice-Hall: Englewood C l i f f s , NJ, 1987; p. 455. 3+ 2+ (25) The Fetphen)^ /Fe(phen) 3 redox couple is also reported to be at +1.83 V in CH3CN, see: Schraid, R.; Kirchner, K . ; Dickert, F. L . Inorg. Chem. 1988, 27, 1530. (26) Rogers, W. N . ; Page, J . A . ; Baird, M. C. Inorg. Chem. 1981, 20, 3521 and references therein. (27) Boyer, J . H. In The Chemistry of the Nitro and Nitroso Groups; Feuer, H . , E d . ; Wiley-Interscience: Toronto, 1969; Part 1. (28) Einstein, F. W. B . ; Sutton, D . ; Tyers, K. G. Inorg. Chem. 1987, 26, 111 and references therein. (29) For comparison,' PhMgCl reacts with NOC1 to produce diphenylamine (Pl^NH) 29a and not the expected PhNO. Also, the reaction of PhCH2MgCl (or t-BuMgCl) with [p-R-C 6 H 4 N 2 ]BF A (R = H, Me, OMe, N0 2 > COMe) results in the elimination of N 2 and the production of coupled products such as 29b p_RC6H4CH2Ph and 2-RC^-CgH^R, see: (a) Marsh, P. G. Diss. Abstr. Int. B. 1975, 35(8), 3838. (b) Singh, P. R.; Khanna, R. K . ; Jayaraman, B. Tetrahedron Lett. 1982, 23, 5475. 101 (30) (a) Khare, G. P. ; Doedens, R. J . Inorg. Chem. 1976, 15, 86. (b) Airae, S.; Gervasio, G . ; Milone, L . ; Rossetti , R.; Stanghellini , P. L . J. Chem. Soc, Chem. Commun. 1976, 370. (31) For an example of a monometallic complex containing a bidentate oximato ligand, see: Khare, G. P. ; Doedens, R. J . Inorg. Chem. 1977, 16, 907. (32) (a) Levy, G. C . ; Lichter, R. L . ; Nelson, G. L . Carbon-13 Nuclear Magnetic Resonance Spectroscopy 2nd ed.; John Wiley and Sons: New York, 1980. (b) Mann, B. E . ; Taylor, B. F. 13C NMR Data for Organometallic Compounds; Academic: London, England, 1981. (c) Becker, E . B. High Resolution NMR: Theory and Chemical Applications, 2nd ed.; Academic: New York, 1980. (33) The related Cp Ru(PMe3)2~0N=CH(Me) compound has been assigned i t s 13 structure on the basis of the magnitude of this C-H coupling 4- * 6 constant. (34) Krueger, P. J . In The Chemistry of the Hydrazo, Azo and Azoxy Groups, Part 1; Patai , S., Ed; Wiley-Interscience: New York, 1975, p. 167. (35) Dean, J . A. Handbook of Organic Chemistry; McGraw-Hill: New York, 1987, Section 8. (36) Cp 2Zr(Cl)Me has also been employed recently to deprotonate other organic 37 groups bound to organotranstion-metal centers. (37) Tso, C. T . ; Cutler, A. R. J. Am. Chem. Soc 1986, 108, 6069. 102 CHAPTER A Some Characteristic Chemistry of the Electrophi l ic [Cp'M(NO)„]BF Complexes (Cp' = Cp or Cp ; M = Cr, Mo or W) 103 Introduction The [CpW(N0)2]+ cation i s a versati le organometallic electrophile.* It is generated by the reaction of the chloro precursor with AgBF^ as shown in equation A.1 CH Cl CpW(N0)2Cl + AgBF 4 — [ C p W ( N 0 ) 2 ] B F 4 + AgCli (A.l) and is best formulated as a coordinatively unsaturated 16-electron compound, although i t may incorporate a solvent molecule into the tungsten's coordination sphere.^" Not surprisingly, i t readily forms 1:1 adducts with various 1 2 2-electron donor ligands such as phosphines and phosphites. ' Curiously, the only isolable o lef in adduct of this cationic compound i s 2 1 3 [CpW(N0)2(n - C g H 1 4 ) ] B F 4 (CgH^ = cyclooctene). ' Other unsaturated hydrocarbons simply do not react with [CpW(N0)2]+, or undergo dimerization or isomerization probably v ia the electrophile-induced formation of carbocations. Futhermore, 2,3-dimethyl-2-butene and phenylacetylene undergo a rapid [2 + 2] cycloaddition reaction in the presence of [CpW(N0)2]+ to form the cyclobutene 4 shown in equation A.2. Wp = CpW(N0)2 104 Several years ago, Rosenblum and coworkers reported that some olefins and acetylenic esters condense in the presence of the [CpFe(C0) 2] + cation (commonly referred to as the Fp + cation) to y ie ld cyclobutenes, 1,3-dienes and some Fp(n^-lactone) products, as shown in the general equation 4.3."''^ Fp = CpFe(CO)2 As mentioned in Chapter 3 of this thesis, the Fp + cation is valence + * isoelectronic with [Cp'MCNO)^ (Cp' = Cp or Cp ; M = Cr, Mo or W). Prompted by Rosenblum's report, I decided to attempt transformations analogous to reactions 4.3 by employing the isoelectronic [Cp'M(N0) 2] + complexes. In this Chapter, I present the results of this study, and also describe the chemistry of some of the [Cp'M(N0) 2] + cationic complexes that further delineates their e lectrophil ic character. 105 Experimental Section A l l reactions and subsequent manipulations were performed under anaerobic and anhydrous conditions unless specified otherwise. General experimental procedures employed in this study were the same as those described in the preceding two chapters. Methyl propiolate (99%), 2,3-dimethyl-2-butene (98%), AgBF^ and NaBPh^ (Gold label) were purchased from Aldrich and were used without further puri f icat ion. Isobutylene (C.P. grade) was purchased from Matheson Gas 7 * 1 Company. The CpM(N0)2Cl compounds (M = Cr, Mo or W) and Cp W(N0)2C1 were prepared by published procedures. The Cp M(N0)2C1 compounds for Cr and Mo were synthesized in a manner similar to that employed for the W analogue, and their purity was checked by elemental analyses. Preparatory gas chromatography was performed on a Varian Model 90-P gas chromatograph, and GC-MS samples were run on a Varian Vista 6000 gas chromatograph interfaced with a Nermag R10-10 quadrupole mass spectrometer with the assistance of Ms. C M . Moxham. * Preparation of [Cp*M(NO)2]BFA (Cp' = Cp or Cp ; B = Cr, Mo or W). These complexes were prepared by treating C H 2 C l 2 solutions of their Cp'M(N0) 2Cl precursors with AgBF^ in the manner described previously for M = W.^ The reactions were monitored by IR spectroscopy and took 15 to A5 min to go to completion. (The IR data for these complexes are l i s ted in Table A . l . ) In each case, the conversion was clean (by IR) and assumed to be quantitative. The AgCl precipitate was removed by f i l t r a t i o n through a medium-porosity f r i t , and the f i l t ra tes were then allowed to react with the desired organic substrates. Reaction of [CpW(lK»2]BF4 with Isobutylene. To a cold, green dichloromethane solution ( -10°C, AO mL) of [CpW(NO)_]BF, (A mmol) was added an 106 excess of isobutylene (25 mL, ^60 fold excess, previously condensed and maintained at - 7 8 ° C ) . The reaction mixture was s t irred for 2.5 h while being maintained at % - 1 0 ° C . After this period, the reaction mixture was f i l tered through alumina (3 x 4 cm, Woelm neutral, act iv i ty 1) supported on a medium-porosity f r i t . The clear, colorless f i l t ra t e was concentrated under reduced pressure to ^20 mL, and a gas chromatographic analysis of this mixture showed i t to contain four products. Analyses by GC-MS and NMR spectroscopy of the fractions separated by preparatory GC revealed these products to be: 8 1 (i) 2,4,4,6,6-pentamethyl-2-heptene (28% of product mixture ): H NMR (CDC13) 6 5.16 (m, 1H, =CH) , 1.68 (m, 6H, 2 x CHg) , 1.43 (s, 2H, CH_2) , 1.13 (s, 6H, 2 x CHg), 0.93 (s, 9H, 3 x CH^). Low resolution mass spectrum (probe temperature 80°C) m/z 168 (P + ) . ( i i ) 2,4,4,6,6-pentamethyl-l-heptene (27% of product mixture): NMR (CDC1,) 6 4.86 (m, 1H, =CH.HJ , 4.65 (m, 1H, =CH.H_), 2.00 (s, 2H, CH„), 1.78 (s, 3H, CH 3 ) , 1.30 (s, 2H, CH_2) , 1.00 (s, 15H, 5 x CH 3 ) . Low resolution mass spectrum (probe temperature 80°C) m/z 168 (P + ) . ( i i i ) 2,4,4,6,6,8,8-heptamethyl-2-nonene (26% of product mixture): NMR (CDC13) 6 5.16 (m, 1H, =CH) , 1.70 (m, 6H, 2 x CH_3) , 1.55 (s, 2H, CH_2) , 1.32 (s, 2H, CH 2 ) , 1.15 (s, 6H, 2 x CHj) , 1.03 (s, 6H, 2 x CH ) , 0.97 (s, 9H, 3 x CH 3 ) . Low resolution mass spectrum (probe temperature 80°C) m/z 224 (P + ) . (iv) 2,4,4,6,6,8,8-heptamethyl-l-nonene (19% of product mixture): NMR (CDC13) 6 4.87 (m, 1H, =CH^Hg) , 4.66 (m, 1H, =CH^Hg) , 2.00 (s, 2H, CH_2) , 1.78 (s, 3H, CH 3 ) , 1.37 (s, 2H, CH2> , 1.33 (s, 2H, CH_2) , 1.10 (s, 6H, 2 x CH_3) , 1.03 (s, 6H, 2 x CH_3) , 1.00 (s, 9H, 3 x CH_3). Low resolution mass spectrum (probe temperature 80°C) m/z 224 (P + ) . 107 Reactions of [CpM(N0)2]BFA with NaBPh^ (M = Cr or Mo). These reactions proceeded s imilarly for both Cr and Mo, and that for Cr is outlined below. To a s t i rred , green CH 2 C1 2 solution (40 mL) of [CpCr(NO)^BF^ (2.0 mmol) was added sol id NaBPh^ (0.68 g, 2.0 mmol). An IR spectrum of the reaction mixture after 2 min showed a decrease in intensity of the v. 's at 1844 and NO 1740 cm due to the starting dini trosyl reagent, and the appearance of new, strong \> 's at 1819 and 1712 cm"1 attributable to [CpCr(NO)jBPh.. These NO z 4 latter bands decreased in intensity with time, and after a period of "vl h, the only v X J O ' s e v i d e n t in the IR spectrum of the reaction mixture were at 1790 and 1685 cm The reaction mixture was then f i l tered through alumina ( 2 x 5 cm, Woelm neutral, act iv i ty 1). The alumina was washed with CH 2 C1 2 (20 mL) and the combined f i l t rates taken to dryness in vacuo to obtain 0.19 g (74% yield) of 9 CpCr(N0)2Ph, which was identif ied by i t s characteristic spectroscopic properties: IR (hexanes) \j„_ 1794 (s) and 1694 (s) cm"1. J H NMR (C,D,) 6 7.30 NO O O (m, 5H, CgH,.) , 4.73 (s, 5H, C^H )^ . Low resolution mass spectrum (probe temperature 120°C) m/z 254 (P + ) . g The known CpMo(N0)2Ph compound was s imilarly obtained as a green o i l (75% yield) presumably via the i n i t i a l formation of [CpMo(N0)2]BPh4 in C H 2 C l 2 (IR u „ _ 1769 and 1670 cm" 1). NO Reactions of [Cp,M(NO)2]BF4 with 2,3-Dimethyl-2-butene and Methyl Propiolate. These reactions were performed using a four- to f ive-fold excess of the organic reagents. (a) M = Cr. To a s t i rred , green dichloromethane solution (40 mL) of [CpCr(N0) 2]BF 4 (3.2 mmol) was added a dichloromethane solution (about 10 mL 108 total volume) of methyl propiolate (1.0 mL, 13 mmol) and 2,3-dimethyl-2-butene (1.0 mL, 17 mmol). The reaction mixture was then refluxed for 1 h. After this period, the reaction mixture was cooled to room temperature, and Et 2 0 (200 mL) was added to precipitate an olive-green so l id . The supernatant solution was decanted, and the remaining so l id was washed with Et 2 0 (40 mL) and then dried in vacuo for 1 h to afford the lactone sa l t , l 1 [CpCr(N0)2-C=C(H)C(Me)2C(Me)20C(0Me)]BF4 (compound 1 ) , as an analyt ical ly pure olive-green so l id in 73% yie ld based on CpCr(N0) 2Cl. The pentamethylcyclopentadienyl analogue, 2, was s imi larly generated from [Cp Cr(N0) 2 ]BF 4 , but this reaction required refluxing the reaction mixture for 2 h. The numbering scheme for these lactone complexes is presented in Table 1 13 4.2, and their analyt ica l , IR, H NMR and C NMR data are collected in Tables 4.3 - 4.5. (b) M = Mo. These reactions were performed in a manner similar to that described for the chromium analogues, but the reaction mixtures were simply s t irred at room temperature for 45 min, and the products were isolated as outlined above. ( c ) M = W. Unlike the coversions involving Cr and Mo, the W reaction mixtures had to be f i l t ered after 45 min of s t i rr ing at room temperature, and the f i l t ra tes then treated with Et 2 0 to obtain the lactone salts in pure form. The cyclopentadienyl case is presented as a representative example. To a s t i rred , green dichloromethane solution of [CpW(NO)BF, (3.36 mmol 109 in 70 mL) was added a CH 2 C1 2 solution (%10 mL total volume) of methyl propiolate (1.0 mL, 13 mmol) and 2,3-dimethyl-2-butene (1.0 mL, 17 mmol). The reaction mixture immediately became very dark green, and after 2 min, a bright green so l id precipitated. After A5 min, this precipitate was collected by f i l t r a t i o n , washed with CH 2 C1 2 (5 mL) and dried in vacuo to obtain 0.A8 g (30% yield) of bright green [CpW(N0)2«-O=C(0Me)CECH]BF4. This so l id was formulated as such based on i t s characteristic IR spectrum: IR (Nujol mull) 1757 (s) and 1661 (s) cm S also VQ=Q 2122 (m) cm ^; 1603 (m) cm Unfortunately, this compound decomposed (turned brown) in < 15 min at room temperature under an atmosphere of N 2 , and immediately upon exposure to a i r . Nevertheless, when the f i l t ra t e obtained from the above procedure was treated with Et 2 0 (120 mL), the desired lactone salt precipitated and was isolated in the usual manner (vide supra) in 25% y ie ld . Reaction of [CpW(NO)2«-0=C(OMe)CECH]BF4 with P(OPh) 3. To a s t irred slurry of [CpW(NO)2"K)=C(OMe)CECH]BF4 (0.2A g, 0.5 mmol) in dichloromethane (30 mL) was added excess-P(OPh)3 (1.0 mL, 3.8 mmol). The reaction mixture became homogeneous in < 2 min, and then Et 2 0 (80 mL) was added to precipitate a bright green so l id . This sol id was collected by f i l t r a t i o n , washed with Et 2 0 (10 mL) and then dried in vacuo for 1 h to obtain 0.26 g (77% yield) of [CpW(NO)2{P(OPh)3}]BF^, which was identif ied by i t s characteristic 1 2 -1 spectroscopic properties: ' IR (CH 2C1 2) v N Q 1788 (s) and 1711 (s) cm *H NMR (acetone-d,) 6 7.70-7.AO (m, 15H, 3 x C , H , ) , 6.A3 (d, 5H, C C H C , J_„ = —O O J D—D PH 1 Hz). 110 Prepara t ion o f the Neut ra l Lactone Complexes 7-12. The preparations of these six compounds were similar; the procedure employed for the cyclopentadienylchromium complex is outlined below. To a s t i rred , green solution of I 1 [CpCr(NO)2-C=C(H)C(Me)2C(Me)2OC(=OMe)]BFA (0.910 g, 2.11 mmol) in acetone (50 mL) was added sol id Nal (0.33 g, 2.2 mmol). The reaction mixture was s t irred for A5 min and then taken to dryness in vacuo to obtain a dark green so l id . This so l id was then redissolved in Et 2 0 (70 mL) and f i l t ered through alumina ( 2 x 3 cm, Woelm neutral, act iv i ty 1). The alumina was then washed with THF (30 mL), and the combined f i l trates were taken to dryness to y ie ld I 1 CpCr(N0)2-C=C(H)C(Me)2C(Me)20C(=0), compound 7 , as an analyt ical ly pure olive-green so l id in 74% yie ld based on Cr. The physical , analyt ical , mass spectral , IR and ^H NMR data for these 13 complexes are presented in Tables A.5 and A.7. The C NMR data for the cyclopentadienyl compounds are also l i s ted in Table A.5. Decomposition o f the Tungsten Lactone Complexes i n A i r . When a green i I dichloromethane solution (30 mL) of CpW(N0)2-C=C(H)C(Me)2C(Me)20C(=0) (0.A1 g, 0.89 mmol) was exposed to a i r , i t became orange after 1^ h and then pale yellow after 2^ h. Drying the pale yellow solution with anhydrous Na^O^, f i l t e r i n g off the used drying agent, and removal of the solvent from the f i l t r a t e under reduced pressure, led to the isolat ion of 0.28 g (73% yield) of pale yellow CpW(0)2-C=C(H)C(Me)2C(Me)20C(=0): IR (Nujol mull) \> 953 (m) and 910 (m) cm"1; also 1686 (s) and 1578 (w). 1 H NMR (acetone-dg) 5 7.08 (s, 1H, =CH), 6.66 (s, 5H, C 5 H 5 ) , 1.38 (s, 6H, 2 x CH 3 ) , 1.23 (s, 6H, 2 x CH 3 ) . Low resolution mass spectrum (probe temperature 200°C) m/z A3A (P + ) . I l l The pentamethylcyclopentadienyl analogue, * » 1 Cp W(0)2-C=C(H)C(Me)2C(Me)2OC(=0), was prepared in a manner similar to that outlined above in 78% y ie ld : IR (Nujol mull) v 945 (m) and 899 (m) cm"1. 2 H NMR (acetone-d6) 6 6.87 (s, 1H, =CH), 2.18 (s, 15H, C 5 ( C H 3 ) 5 ) , 1.37 (s, 6H, 2 x CH^), 1.18 (s, 6H, 2 x CH^). Low resolution mass spectrum (probe temperature 200°C) m/z 504 (P + ) . 112 Results and Discussion Generation of [Cp*M(NO)2]BF4 (Cp' = Cp or Cp*; M - Cr, Mo or W). A l l the [Cp'M(NO) 2] + cations are generated by chloride abstraction from their Cp'M(NO)2Cl precursors by reaction with AgBF^, i . e . Cp'M(NO)2Cl + AgBF^ • [Cp'M(NO)2]BFA + AgCli (4.4) These reactions 4.4 (called 4.1 for Cp' = Cp and M = W, vide supra) are conveniently monitored by IR spectroscopy and take 15 - 45 min to go to completion. As may be noted from Table 4.1, the u X J Q ' S shift to higher wavenumbers (by ^30 cm as a result of the replacement of the coordinating Cl ligand by the weakly coordinating BF^ anion. [The IR data for the PF^ analogues (obtained by employing AgPF^ instead of AgBF^ in reactions 4.4) are also l i s ted in the Table, and in general the physical properties of the BF^ salts are indistinguishable from those of their PFg analogues.] These complexes are best formulated as 16-electron, coordinatively unsaturated [Cp'M(N0) 2] + cations which may be stabi l ized by solvation. " * ^ Nevertheless, the chemistry of these cations i s dominated by their e lectrophil ic character. For example, a l l the [CpM(N0)2]+ cations are suff ic ient ly e lectrophi l ic to abstract a Ph group from the BPh^ anion to form the known CpM(NO)2Ph compounds, i . e . CH Cl [CpM(N0)2]BF4 + NaBPhA — - — C p M ( N 0 ) 2 P h + NaBF^ + BPh3 (4.5) (M = Cr, Mo or W1) 113 Table 4.1. IR Data for Various Cp'MCNO^ X Complexes in CB^Cl Compound v N 0 ( s ) , cm"1 X = Cl X = BF. 4 X = PF, 0 CpCr(NO)2X 1817 1844 1840 1711 1740 1736 Cp*Cr(NO)2X 1782 1809 1806 1680 1709 1709 CpMo(NO)2X 1760 1790 1784 1669 1698 1695 Cp*Mo(NO)2X 1726 1757 1750 1642 1671 1667 CpW(NO)2X 1733 1765 1748 1650 1682 1663 Cp*W(NO)2X 1705 1734 1723 1624 1654 1644 114 presumably v ia the i n i t i a l formation of [CpM(NO)^\BPh^ (see Experimental Section). Certainly, the clean production of the CpM(NO)2Ph compounds provides a more convenient (and higher yield) route to these phenyl derivatives than 9 obtaining them by treatment of their chloro precursors with Ph^Al. The e lectrophil ic nature of the [Cp'MXNO),,]* cations is further exemplified by the reaction of the cyclopentadienyltungsten compound with isobutylene (Scheme 4.1). x x y + > o o = w + 28% 27% ^ = WP » ) 0 0 0 + ) 0 0 0 26% 19% Wp = CpW(NO)2 Scheme 4.1 The fact that oligomerization of the isobutylene occurs in this reaction is indicative of the a b i l i t y of the e lectrophi l ic [CpW(NO)2]+ cation to generate incipient carbocations. It is thus proposed that adduct formation between [CpW(NO)»] + and isobutylene occurs to y ie ld a carbocation as shown below. 115 Such an organometallic carbocation could then be attacked by another molecule 12-14 of the o le f in , a process that may account for the linear oligomerization 15-17 observed. Regrettably, i t has not been possible to obtain a suitable 2 + crystal of any [Cp'MCNO^Cn -olefin)] compound for X-ray crystallographic studies in order to ascertain the nature of the bonding in these complexes. Reactions of [Cp,Mo(N0)2]BF4 with 2,3-Dimethyl-2-butene and Methyl Propiolate. When exposed to dichloromethane solutions of [Cp'MCNO^JBF^, 2,3-dimethyl-2-butene" and methyl propiolate condense to y i e ld the methylated lactone salts 1 - 6 (equation 4 . 6 ) . M ON NO Mev° V CH2CI2, 20* H I OMe (4.6) 1 - 6 (M = Cr, Mo or W; R = H or Me) Table 4.2. Numbering Scheme for the Cationic and Neutral Lactone Complexes. 1 OMe LON /LV-J -TO Compound Number Compound Number [CpCr(NO)2-C=C(H)C(Me)2C(Me)2OC(OMe)]BF4 1 • I CpCr(NO)2-C=C(H)C(Me)2C(Me)20C(=0) 7 [Cp Cr(NO)2-C=C(H)C(Me)2C(Me)2OC(OMe)]BF4 2 * • * Cp Cr(N0)2-C=C(H)C(Me)2C(Me)20C(=O) 8 1 i [CpMo(NO)2-C=C(H)C(Me)2C(Me)20C(OMe)]BF^ 3 i 1 CpMo(NO)2-C=C(H)C(Me)2C(Me)20C(=0) 9 [Cp Mo(NO)2-C=C(H)C(Me)2C(Me)2OC(OMe)]BF4 4 Cp*Mo(NO)2-C=C(H)C(Me)2C(Me)20C(=0) 10 [CpW(NO)2-C=C(H)C(Me)2C(Me)20C(OMe)]BF^ 5 CpW(N0)2-C=C(H)C(Me)2C(Me)20C(=0) 11 [Cp W(NO)2-C=C(H)C(Me)2C(Me)2OC(OMe)]BF4 6 Cp*W(NO)2-C=C(H)C(Me)2C(Me)2OC(=0) 12 117 These salts are isolated as analyt ical ly pure sol ids , and are air-stable as sol ids . In solution, they may be handled in a ir for at least 4 h without any noticeable decomposition. Their spectroscopic properties are fu l ly consistent with their formulations. For example, the *H NMR spectrum of the cyclopentadienylmolybdenum compound, 3, is shown in Figure 4.1. [The physical , analytical and spectral data for complexes 1-6 are collected in Tables 4.3 - 4.5.] As i l lus trated by this example, the assignments of the signals in the NMR spectra for a l l the complexes are quite straightforward, and are as indicated. However, an apparently inconsistent feature of this spectrum is the low intensity of the resonance due to the Cp protons. Indeed, 18 a measurement of the T^ relaxation time for these protons indicates i t to be 19 1 very long, 21 s. Thus, in recording the H NMR spectra for these and other compounds synthesized in this work, pulse delays of >50 s must be used in order to obtain meaningful integrations of the signals. It is proposed that reactions 4.6 occur via coordination of the acetylenic ester to the metal center (through the acetylene link) followed by nucleophilic attack at the bound acetylenic ester by the incoming o lef in , as shown below in equation 4.7. 2Me OMe Cp H 2Me 1111111111111111111111II11111II111\1111111111I111111111111111111111111 III 11II1111 7.0 6.0 5.0 4.0 3.0 2.0 1.0 PPM 0.0 Figure 4.1. The 300 MHz H NMR spectrum of complex 3 in acetone-d Table A.3. Physical Data for the Cationic Lactone Complexes 1 - 6 . Compound Color Yield % Analytical Data IR (Nujol)-(cm ) C (calcd) H (calcd) N (calcd) [CpCr(NO)2-C=C(H)C(Me)2C(Me)20C(OMe)]BFA 1 olive green 73 41.90 (A1.67) 4.93 (4.86) 6.48 (6.48) 1800(s), 1721(s), 1665(s), 1564(w), 1518(m) [Cp*Cr(NO)2-C=C(H)C(Me)2C(Me)20C(OMe)]BF4 2 olive green 68 47.68 (47.81) 6.09 (6.18) 5.57 (5.58) 1763(s), 1682(s), 1659(s), 1561(w), 1512(m) [CpMo (NO) 2-4=C (H) C (Me) 2 C (Me) 20c!(0Me) ] BF 4 3 bright green 66 38.00 (37.82) 4.35 (4.41) 5.91 (5.88) 1752(s), 1680(s), 1630(s), 1564(m), 1518(m) [Cp*Mo (NO) 2-C=C (H) C (Me) 2 C (Me) 20c"(0Me) ] BF 4 A pale brown 82 44.03 (43.96) 5.63 (5.68) 4.96 (5.13) 1709(s), 1624(s), 1561(m), 1520(m) [CpW(NO)2-4=C(H)C(Me)2C(Me)2OC(OMe)]BF4 5 bright green 25 31.69 (31.91) 3.78 (3.72) 4.90 (4.96) 1732(s), 1669(s), 1618(s) , 1564(m), 1522(m) [Cp*W(NO)2-C=C(H)C(Me)2C(Me)2OC(OMe)]BF4 6 pale green 63 37.84 (37.85) 4.93 (4.89) 4.29 (4.42) 1692(s), 1613(s), 1560(m), 1522(m) 2 in the 1900 - 1500 cm 1 region. Table 4 . 4 . H NMR Chemical Shifts of the Cationic Lactone Complexes 1 - 6 . Compound Chemical Shifts [(CD 3) 2C=0, 6 in ppm] Cp H C(CH 3 ) 2 C(CH 3 ) 2 -0 OCH3 1 1 [CpCr(NO)2-C=C(H)C(Me)2C(Me)20C(OMe)]BF^ 1 5.87 (s) 7.33 (s) 1.24 (s) 1.68 (s) 4.41 (s) * 1 1 [Cp Cr(NO) -C=C(H)C(Me)2C(Me)20C(OMe)]BF^ 2 1.87 (s)- 7.19 (s) 1.29 (s) 1.72 (s) 4.48 (s) l l [CpMo(NO)_-C=C(H)C(Me)„C(Me)„0C(OMe)]BF, 2 2 2 4 3 6.30 (s) 7.50 (s) 1.28 (s) 1.72 (s) 4.44 (s) * i " i [Cp Mo(NO)2-C=C(H)C(Me)2C(Me)20C(OMe)]BF^ 4 2.01 (s)- 7.42 (s) 1.32 (s) 1.74 (s) 4.50 (s) 1 1 [CpW(NO)2"C=C(H)C(Me)2C(Me)20C(OMe)]BF^ 5 6.41 (s) 7.59 (s) 1.28 (s) 1.72 (s) 4.44 (s) 1 1 [Cp W(NO)2-C=C(H)C(Me)2C(Me)2OC(OMe)]BF^ 6 2.10 (s)- 7.49 (s) 1.33 (s) 1.75 (s) 4.52 (s) - this is r] -C^Me T a b l e 4 . 5 . C NMR C h e m i c a l S h i f t s o f Some L a c t o n e Complexes . Compound Chemical Shif ts [ ( C D ^ C O , 6 i n ppm]-Cp M-C= =CH CMe2 C(Me) 2 C(Me) 20 C(Me) 20 C=0 O-Me [CpCr(NO) 2-C=C(H)C(Me) 2C(Me) 2OC(OMe)]BF^ 1 102.01 137.70 178.76 42.66 22.46 23.05 99.91 n .o . 60.72 , ._, CpCr(NO) 2-C=C(H)C(Me) 2C(Me) 20C(=0) 7 101.23 n .o . 161.18 41.20 23.99 24.09 83.75 n.o . n . a . 1 1 tCpMo(NO)2-C=C(H)C(Me)2C(Me)20C(OMe)]BF^ 3 104.18 134.76 182.59 43.22 22.66 23.23 101.74 n .o . 61.31 1 1 CpMo(NO)2-C=C(H)C(Me)2C(Me)20C(=0) 9 103.74 144.94 166.29 41.56 24.07 24.07 85.48 170.44 n . a . 1 1 [CpW(NO)2-C=C(H)C(Me)2C(Me)2OC(OMe)]BF4 5 102.78 130.01 182.13 42.89 22.66 22.95 100.80 n.o . 61.32 CpW(NO)2-C=C(H)C(Me)2C(Me)20C(=0) 11 102.24 n.o . 166.05 41.00 23.74 23.89 84.17 n.o . n . a . — a l l peaks appear as s inglets n.o. not observed n .a . not appl icable 122 Indeed, in the case where M = W and R = H, the overall reaction A.6 is hampered by the precipitat ion of the O-bound methyl propiolate adduct, This compound is f a i r l y insoluble in dichloromethane, and i t decomposes in the solvents in which i t dissolves. Consequently, no consistent NMR data have been obtained for this complex. However, an IR spectrum of i t (as a Nujol mull) contains bands attributable to a coordinated methyl propiolate ligand. Thus, although the CEC stretching frequency does not change s ignif icant ly upon the coordination of the acetylenic ester (y(-=(-. 2122 cm * bound vs 2128 cm * free), the C=0 stretching frequency does by 118 cm ^(^Q_Q 1603 cm * bound vs 1721 cm ^free) consistent with this ligand binding through the carbonyl 21 oxygen. Nevertheless, the methyl propiolate is readily displaced by P(0Ph) 3 to give the known [CpW(NO)2{P(OPh)3)]BF^ compound in high y i e l d . Therefore i t appears that in addition to the observed reaction leading to lactone salt formation, there i s a competing reaction pathway which involves the formation 123 of the insoluble [CpW(NO) 2*-0=C (OMe) CECH] BF^ complex. The precipitation of this latter product effectively reduces the ava i lab i l i ty of the [CpW(NO)2]+ cation for reaction 4.7, and accounts for the low y ie ld (25%) of the lactone salt obtained in this instance. In a sense, the attachment of the [CpW(NO)2]+ cation to the carbonyl oxygen of the acetylenic ester is reminescent of the AlCl^ catalyzed condensation reactions 4.8 and 4.9 in which the Lewis acid (in this case, 0. .OMe v» Y AICI3 v ^ C O O M e H X + 1 — ^ X\a + ™ C l 76% 4% AlClj) is believed to coordinate to the carbonyl oxygen of the acetylenic 22 esters in the transit ion states of these transformations. However, no [2 + 2] cycloaddition or ene products are observed in any of the [Cp'M(N0„] 124 reactions with 2,3-dimethyl-2-butene and methyl propiolate (compare with equation 4.2), and thus i t i s probable that in the [Cp'M(NO) 2] + reactions, ent irely different transit ion states (such as in equation 4.7) are involved during the production of the organometallic lactone sal ts . Thus, i t can be concluded from the results of this study that the reactions of [Cp'M(NO) 2] + (equation 4.6) resemble those of the valence isoelectronic [CpFe(CO) 2] + compound (equation 4.3) rather than those of other Lewis acids such as AlCl^ (equation 4.8). Also, the reactions of a l l the [Cp'M(NO) 2] + complexes with 2,3-dimethyl-2-butene and methyl propiolate lead to the selective formation of 6-membered-ring lactones in high yie lds . No 5-membered-ring lactones are observed in any of the reactions involving the dini trosyl complexes. I 1 P r e p a r a t i o n o f the N e u t r a l Cp , M(N0) 2 -C=C(H)C(Me) 2 C(Me) 2 OC(=O) Lactone Complexes (7 - 12) . The organometallic complexes 7 - 1 2 are obtainable by exposure of acetone solutions of their precursor methylated lactone salts ( 1 - 6 ) to equimolar .amounts of Nal as shown in equation 4.10. + OMe 0 N AoS<r (4.10) (M = Cr, Mo or W; R = H or Me) 1 - 6 7 - 1 2 125 Compounds 7 - 1 2 are green to brown sol ids , and their physical , analyt ical , mass spectral and IR data are collected in Table A.6. The *H NMR data for these complexes are presented in Table A.7, and the *H NMR spectrum of the cyclopentadienyl compound 9 i s shown in Figure A.2. This spectrum qual i tat ively resembles that exhibited by i t s precursor lactone salt (Figure A . l ) , and the measured T^ value for the Cp proton resonance is also very large (38 s) . Furthermore, the notable absence of the resonance around A ppm is 23 consistent with the successful accomplishment of the O-dealkylation reaction outlined in equation A.10. The low resolution mass spectra for these complexes a l l show the parent ions as the highest m/z peaks. The connectivity of the atoms in these monomeric complexes has been confirmed by a s ingle-crystal X-ray crystallographic analysis of the cyclo-2A pentadienylmolybdenum compound ( 9 ). The molecular structure of this compound in the so l id state is depicted in Figure A.3, and selected bond lengths and angles are l i s ted in Tables A.8 and A.9 respectively. Although there are no other structural ly characterized neutral compounds containing the CpMo(NO)2 fragment, the intramolecular dimensions of this unit appear normal in comparison to the intramolecular dimensions of the related CpMtNO^Cl compounds 25 (M = Cr or W) already discussed in Chapter 3 of this thesis. Thus the N(l)-Mo-N(2) bond angle of 9 2 . 3 ( 1 ) ° i s close to that found for the CpM(N0)2Cl compounds ( 9 3 . 9 ° for Cr, and 92° for W ) . 2 5 Also, the O(l)-N(l)-Mo and 0(2)-N(2)-Mo bond angles of 1 7 4 . 4 ( 2 ) ° and 1 7 4 . 3 ( 2 ) ° indicate that the n i trosy l ligands are essential ly l inear . Interestingly, the C(15)-0(4) bond i s short [1.354(3)A] i n comparison to the C(14)-0(A) bond [l.A77(3) A] and may be o-I + indicative of some participation of the -C=0- resonance form in the complex, Table 4.6. Physical and Mass Spectral Data for the Neutral Lactone Complexes. Compound Color Yield % Analytical Data Mass Spectrum m/z IR (Nujol) C (calcd) H (calcd) N (calcd) y N0' C n f l vc=cr c m _ 1 CpCr(NO)2-C=C(H)C(Me)2C(Me)20C(=0) 7 olive green 74 50.67 (50.91) 5.60 (5.45) 8.39 (8.48) 330 (P+) 1786 1661 1678 * 1 l Cp Cr(NO)2-C=C(H)C(Me)2C(Me)20C(=0) 8 olive green 72 57.02 (57.00) 7.11 (7.00) 7.00 (7.00) 400 (P+) 1757 1649 1684 1 "I CpMo(NO)2-C=C(H)C(Me)2C(Me)20C(=0) 9 bright green 51 44.49 (44.92) 4.87 (4.81) 7.13 (7.49) 376 (P+) 1737 1630 1656 * i — 1 Cp Mo(NO)2-C=C(H)C(Me)2C(Me)20C(=0) 10 green brown 57 51.60 (51.35) 6.35 (6.31) 6.19 (6.13) 446 (P+) 1709 1610 1680 CpW(N0)2-C=C(H)C(Me)2C(Me)20C(=0) 11 bright green 71 36.60 (36.36) 4.02 (3.90) 6.02 (6.06) 462 (P+) 1717 1617 1672 Cp.W(N0)2-C=C(H)C(Me)2C(Me)20C(=0) 12 bright green 63 42.95 (42.86) 5.35 (5.26) 5.19 (5.26) 532 (P+) 1692 1599 1680 Table 4.7. H NMR Chemical S h i f t s of the Neut ra l Lactone Complexes. Compound Chemical Shifts [ ( C D ^ C O , fi in ppm] Cp H C(CH 3 ) 2 C(CH 3 ) 2 -0 1 1 CpCr(NO)2-C=C(H)C(Me),C(Me)20C(=0) 7 5.66 (s) 6.42 (s) 1.06 (s) 1.32 (s) * 1 1 Cp Cr(NO)2-C=C(H)C(Me)2C(Me)20C(=0) 8 1.81 (s)- 6.20 (s) 1.08 (s) 1.32 (s) f" 1 CpMo(NO)2-C=C(H)C (Me)2C(Me)20C(=0) 9 6.16 (s) 6.63 (s) 1.12 (s) 1.38 (s) Cp Mo(NO)2-C=C(H)C(Me)2C(Me)20C(=0) 10 1.94 (s)- 6.43 (s) 1.10 (s) 1.33 (s) 1 1 CpW(NO)2-C=C(H)C(Me)2C(Me)20C(=0) 11 6.23 (s) 6.77 (s) 1.07 (s) 1.33 (s) Cp W(N0)2-C=C(H)C(Me)2C(Me)20C(=0) 12 2.02 (s)- 6.54 (s) 1.11 (s) 1.33 (s) - this i s p -Ct - M e ON - i — o Mo *\ NO 0 2Me 2Me Cp H I I I I I I I I I 1 I I I I | I I I I I I I I I 1 I I I I I < I ' ' I • ' 1 1 | 1 1 1 1 1 ' ' J _! 1 f-i nnu 10 2 PPM Figure A.2. The 300 MHz *H NMR spectrum of complex 9 in acetone-d&. 129 C(5) C C D CU9> C(16) Figure 4.3. Solid-state molecular structure of the CpMo(NO)2(n -lactone) complex 9. 130 Table 4.8. Selected Bond Lengths (A) for the CpMo(NO)2(n -lactone) Compound 9. Bond Length-Mo - N(l) 1.813(2) Mo - N(2) 1.823(2) Mo - C(l) 2.360(3) Mo - C( l l ) 2.170(2) N(l) - 0(1) 1.181(3) N(2) - 0(2) 1.180(3) 0(3) - C(15) 1.212(3) 0(4) - C(15) 1.354(3) C( l l ) - C(12) 1.331(3) C( l l ) - C(15) 1.478(3) C(12) - C(13) 1.506(3) C(12) - H(12) 0.93(3) 0(4) - C(14) 1.477(3) - E . s . d . ' s are in parentheses. 131 Table A.9. Selected Bond Angles (deg) for the CpMo(NO)_(n -lactone) Compound 9.— N(2) - M o - N(l) 92.3(1) C( l l ) - Mo - N(l) 92.8(1) 0(1) - N(l) - Mo 174.4(2) 0(2) - N(2) - Mo 174.3(2) C(12) - C( l l ) - Mo 123.8(2) C(13) - C(12) - C( l l ) 125.7(2) C(14) - C(13) - C(12) 107.3(2) C(15) - 0(4) - C(14) 120.5(2) 0(4) - C(15) - 0(3) 116.5(2) C( l l ) - C(15) - 0(3) 124.2(2) C( l l ) - C(15) - 0(4) 119.3(2) H(12) - C(12) - C( l l ) 121 (2) - E . s . d . ' s are in parentheses. 132 which would then impose some part ia l double-bond character onto the C(15)-0(4) bond. Furthermore, the C(ll)-C(15) bond distance of 1.478(3) A i s shorter 27 than that expected for an unconjugated C-C bond of this type (1.497 A) and is close to the value expected for the (conjugated) a,8-unsaturated ketone form 28 (1.47 A), a feature thus suggesting some derea l i za t ion of electron density over the whole C(12)-*0(4) fragment. I 1 Decomposition of the Cp'W(N0)2-C=C(H)C(Me)2C(Me)20C(=O) Complexes i n A i r . The i n i t i a l l y green dichloromethane solutions of the neutral tungsten lactone complexes (11 and 12) turn orange and then yellow over the course of hours (2 h for the Cp complex, and 6 h for the Cp complex) when exposed to a i r , cleanly producing the corresponding dioxo compounds in high yields (70-80%). + M 2 N 0 2 W (4.1D (R = H.Me) These dioxo derivatives have been characterized fu l ly by spectroscopic 29 methods. Thus, their IR spectra are devoid of n i trosy l absorptions and now contain bands assignable to ^=o (see Experimental Section). Also, the NMR spectra for these dioxo products are very similar to those of their dini trosyl precursors, indicating that the {Cp'W(n^-lactone)} group remains intact during 133 reaction 4.11. Furthermore, the low resolution mass spectra of these complexes display the expected parent ion peaks as their highest m/z peaks. The success of the reactions 4.11 therefore implies that a possible extension of this chemistry to the chromium analogues may indeed be the preferred route to the unknown Cp'CrfO^R type compounds. Regrettably, i n i t i a l attempts to effect this extension have not been f r u i t f u l . Summary This work has shown that the [Cp'MCNO^]"1" organometallic cations (Cp1 = Cp or Cp ; M = Cr, Mo or W) are potent electrophiles. For instance, 2,3-dimethyl-2-butene and methyl propiolate condense in their presence to form the cationic lactone salts 1 - 6 . These salts are converted to their neutral lactone compounds (7 - 12) upon 134 treatment with Nal in acetone. Furthermore, the tungsten complexes 11 and 12 decompose to their dioxo derivatives, Cp'W(0)2 (n/ - lactone)» by exposure of their dichloromethane solutions to a i r . Most interestingly, the new Cp'M (NO) 2(11 ^ "-lactone) complexes ( 7 - 12 ) now jo in a rare class of Cp'MCNO^R" compounds (R" = heteroatomic organic ligand) which cannot be synthesized in the more common manner used for the carbonyl analogues. This pathway is unavailable to the dini trosyl complexes since the [Cp'MCNO^J anions are presently unknown. Following the chemistry outlined in Chapter 3 of this thesis, the question arises as to whether the transformations outlined in equation 4.12 may be cleanly carried out. [Cp'M(NO)2] + R"X Cp'M(NO)2R" + X + -CpM(NO)2CI Cl (4.12) (M = Cr, Mo or W; E = 0 or NAr; R - H or Me) If they can, this would then lead to the formation of new types of a-substituted lactones, the formation of which (to the best of my knowledge) does not appear to have any precedent. Preliminary experiments in this regard 135 do suggest that these transformations are indeed feasible, although the proper reaction conditions to isolate the products in pure form have yet to be determined. It is possible that the NE + electrophiles may not attack the metal-carbon bonds of the organometallic complexes alone, but may also be involved in e lectrophil ic attack at the carbonyl oxygen of the lactone ligands (to obtain the [C=ONE]+ analogues of the cationic lactone salts 1 - 6 ). Nevertheless, the reactions outlined in equation 4.12 above constitute a promising area of research and, hopefully, w i l l have an impact in the development of synthetic methods for the production of new organic compounds. 136 References and Notes 1. Legzdins, P . ; Martin, D. T. Organometallics 1983, 2, 1785. 2. Stewart, R. P . , J r . ; Moore, G. T. Inorg. Chem. 1975, 14, 2699. 3. The molybdenum analogue of this compound has been br ie f ly described, see: Hames, B. W.; Legzdins, P. Organometallics 1982, 1, 118. 4. Martin, D. T. Ph.D. Dissertation, the University of Br i t i sh Columbia, 1984. 5. (a) Rosenblum, M . ; Scheck, D. Organometallics 1982, 1, 397. (b) Samuels, S . -B . ; B e r r y h i l l , S. R.; Rosenblum, M. J . Organomet. Chem. 1979, 166, C9. 6. The course of this reaction is strongly dependent on the structure of the olef in reactant. For example, 1,2-disubstituted olefins y ie ld cyclobutenes and 1,3-dienes in addition to the lactone products, whereas 1,1-disubstituted or trisubstituted olefins y ie ld only the lactone products. "*a 7. Legzdins, P . ; Malito, J . T. Inorg. Chem. 1975, 14, 1875. 8. The yields were determined by GC. 9. Hoyano, J . K . ; Legzdins, P . ; Malito, J . T. J . Chem. Soc, Dalton Trans. 1975, 1022. 10. (a) Beck, W.; Sunkel, K. Chem. Rev. in press. (b) Regina, F. J . ; Wojcicki, A . Inorg. Chem. 1980, 19, 3803. 11. Thus, coordinating solvents such as acetonitri le coordinate to the metal center to give the isolable [Cp'M(NO) 9(solvent)]* compounds. 137 Richter-Addo, G. B . ; Legzdins, P. Chem. Rev. in press. 12. It has been calculated that indeed, i t i s such linkages that render bound 13 olefins susceptible to nucleophilic attack. However, X-ray crystallographic analyses of some [CpFe(CO^(olefin)] + complexes reveal that i t i s the complexes having the symmetrically bound olef in that are reactive. Those having the unsymmetrically bound olef in are unreactive 14 towards nucleophiles. 13. Eisenstein, 0.; Hoffmann, R. J . Am. Chem. Soc. 1980, 102, 6148. 14. Chang, T. C. T . ; Foxman, B. M . ; Rosenblum, M . ; Stockman, C. J . Am. Chem. Soc. 1981, 103, 7361. 15. These oligomers were identif ied by their characteristic *H NMR s p e c t r a . ^ 16. Francis, S. A . ; Archer, E . D. Anal. Chem. 1963, 35, 1363. 17. Procedures for the isolat ion and identi f icat ion of complex hydrocarbon mixtures have been outlined. (a) Stehling, F. C ; Bartz, K. W. Anal. Chem. 1966, 38, 1467. (b) Archer, E . D. ; Shively, J . H . ; Francis, S. A. Anal. Chem. 1963, 35, 1369 and references therein. 18. The T^ measurements were performed with the assistance of Dr. S. 0. Chan of the Departmental NMR laboratory. 19. Long relaxation times (of ^15 s) are frequently observed for the cyclopentadienyl proton resonances for some CpM(NO)-containing compounds 20 (M = Mo or W). A pulse delay of 0 s was used to obtain the spectra shown in Figures 4.1 and 4.2 20. (a) Hunter, A. D. ; Legzdins, P. Organometallics 1986, 5, 1001. (b) Legzdins, P . ; Martin, J . T . ; Oxley, J . C. Organometallics 138 1985, 4, 1263. (c) Martin, J . T. Ph.D. Dissertation, The University of Br i t i sh Columbia, 1987. 21. For a general discussion of the bonding of aldehyde and ketone ligands in organometallic chemistry, see: Huang, Y . - H . ; Gladysz, J . A. J . Chem. Ed. 1988, 65, 298. 22. (a) Snider, B. B. Acc. Chem. Res. 1980, 13, 426. (b) Snider, B. B . ; Roush, D. M . ; Rodini, D. J . ; Gonzalez, D . ; Spindell , D. J. Org. Chem. 1980, 45, 2773. (c) Snider, B. B . ; Rodini, D. J . ; Conn, R. S. E . ; Sealfon, S. J. Am. Chem. Soc. 1979, 101, 5283. (d) Snider, B. B . ; Roush, D. M. J. Am. Chem. Soc. 1979, 101, 1906. 23. For other examples of 0-dealkylation in organometallic chemistry, see (a) Davison, A . ; Reger, D. L. J. Am. Chem. Soc. 1972, 94, 9237. (b) Bodner, G. S.; Smith, D. E. ; Hatton, W. G. ; Heah, P. C ; Georgiou, S.; Rheingold, A. L . ; Geib, S. J . ; Hutchinson, J . P . ; Gladysz, J . A. J . Am. Chem. Soc. 1987, 109, 7688. 24. The structural analysis was performed by Drs. R. H. Jones and F. W. B. Einstein of Simon Fraser University. 25. Greenhough, T. J . ; Kolthammer, B. W. S.; Legzdins, P . ; Trotter, J . Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1980, B36, 795. 26. Similar situations have been well documented for various structural ly characterized carboxylic esters. Schweizer, W. B . ; Dunitz, J . D. Helv. Chim. Acta. 1982, 65, 1547. 27. Handbook of Chemistry and Physics; Weast, R. C . ; Ast le , M. J . , Eds.; CRC: Flor ida , 1980; 60th E d . , F216. 139 28. Al len , F. H. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1981, B37, 890. 29. The CpWCNO^Fc compound (Fc = ferrocenyl) is reported to undergo a similar conversion to CpW(0)2Fc. Herberhold, M . ; Kniesel, H . ; Haumaier, L . ; Gieren, A . ; Ruiz-Perez, C. Z. Naturforsch., B: Anorg. Chem, Org. Chem. 1986, 41b, 1431. 140 Epilogue The work presented in this thesis shows that a knowledge of the redox behavior of a class of compounds may help to explain some of the chemistry observed for these complexes. As a result of cyc l ic voltammetry (and ESR) studies on a series of organometallic nitrosyl-dihalo complexes, new routes to previously unknown Cp'Mo(N0)R2 complexes (R = alkyl or aryl) have been found. Clearly, the extensions of such studies (described in Chapter 2) to other organometallic-halide systems for which no alkyl (or aryl) derivatives are known, are warranted. It is not assumed, however, that trends similar to those observed for the nitrosyl-dihalo complexes studied w i l l be seen in such extensions. The use of organometallic n i trosyl complexes in organic synthesis has only received l i t t l e attention, although the potential applications are enormous. Detailed studies of the reactions of nitrogen-containing electrophiles with various CpCr(NO)compounds have ultimately led to the creation of a stoichiometric cycle for the formation of carbon-nitrogen bonds. Furthermore, the application of the Group 6 organometallic n i trosy l compounds for the formation of carbon-carbon and carbon-oxygen bonds has been developed in the last chapter of this thesis. It i s my hope that this work w i l l draw more attention to the potential applications of organometallic n i trosy l complexes as catalysts and specific reagents for organic transformations. 141 APPENDIX 142 143 Plot of i vs square root of scan rate for the electrochemical oxidation - p , a Cp 2Fe in CH 2 C1 2 . Ferrocene in CH2CI2 ipa vs square root of scan rate square root of s c a n rate 1AA Plot of i vs scan rate for the electrochemical reduction of Cp Mo(N0)Cl2 -p , c CH 2 C1 2 . Cp*Mo(N0)CI2 in CH2CI2 ipc vs scan rate 60 - i 10 -\ 1 1 | 1 1 1 — I — I — I — I — I — I — I — I I I I I I ' 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 scan rote (V/s) 145 Plot of i vs square root of scan rate for the electrochemical reduction of -p , c Cp Mo(NO)Cl2 in CH 2 C1 2 . Cp*Mo(N0)CI2 in CH2CI2 ipc vs square root of scan rate 60 T 10 -\ 1 1 1 1 1 1 1 1 1 1 1 r-0.2 0.4 0.6 0.8 1 1.2 .1.4 square root of scan rate Compound- Scan Rate (v, Vs" 1) E 1 - p.a E 2 - P,a E 3 - p.a E 1 - p,c E 2 - p.c E 3 - P,c Comments CpMo(NO)Cl2 0.21 1.67 1.91 Broad Cp*Mo(NO)Cl2 0.A3 1.A7 1.68 Broad CpMo(NO)Br2 0.41 1.93 0.A1 E* broad and weak - p.c Cp*Mo(NO)Br2 0.A1 1.A9 1.7A 1.19 0.36 E 's broad and weak - p . c CpMo(NO)I, CpMo (NO) 0.17 O.AA 1.05 0.33 0.96 1.79 0.A6 1.69 -0.04 0.58 0.19 2 E i s E, (see text) — 3 P . C -1/2 E associated with - P.c broad and weak Cp*Mo(NO)I2 0.36 1.6A 0.58 0.33 E . - P.a CpW(NO)I2 0.A0 1.5A Cp*W(NO)I2 0.A0 1.35 1.75 1.A6 0.61 E 's broad - P . c CpMo(NO)I2PMePh2 0.20 0.98 1.A7 1.70 1.A1 0.53 2 E weak and broad - p,a CpW(NO)(CH2SiMe3)2 Cp*W(NO)(CH2SiMe3)2 0.15 0.20 1.15 0.97 -0.10 -0.45 E 1 broad E 2 - P.c - p.c broad - in CH-Cl . (0.1 M [n-Bu,N]PF, - in CH.CN (0.1 M [n-Bu.N]PF,) 2 2 — 4 0 i _ 4 O 147 IR spectrum of Cp Mo(NO)Cl_ as a Nujol mull. • Q E S 9 "OS f B T ' 2 f EOZ. " E E 9 2 2 "S2 Z . rZ ."eT 0 8 9 2 ' 8 T I T S ' Q -3DNVXXI WSNVaJ.X 148 IR spectrum of Cp Mo(NO)Br. in THF. a o S t O - 6 9 * 9 f r 'AS f I B " E f E 9 E "f E E I S "22 E 9 Z " T X 9 / . 8 S " O -.. 3 3 N V i J . I W S N V d l X 149 150 151 IR spectrum of [CpMo(NO)(CH3CN)3](I)(PF fe) as a Nujol mull. o o 9 Q i ' B 2 E B B " E E B S D "6 T f E 2 "frT E B O f "6 T S B S 'V 2 B E S " 0 -3 3 N V 1 1 I W S N V W 1 X 152 IR spectrum of [Cp W(NO)I], as a Nujol mull . • o d 0 9 8 ••ZL f I S ' 0 9 8 9 1 " 8 * 2 2 8 ' S E S i t - E 2 D E I ' l l 6 S I 2 "I-3 D N V J . X I W S N V a i Z 153 IR spectrum of CpCr(NO)2CH2SiMe3 i n CH 2Cl D • ri • 172 "BS E f 2 •BV S fr2 "6E O S 2 ' 6 2 E S 2 - 6 I 2 9 S S "6 S O f i - 0 -3 3 N V 1 1 I W S N V y i X 155 BSZ. - 8 9 O S 2 Y S B.V/.-SV S E E * « E B 2 Z ' 2 2 T 2 2 ' I X S 8 B 2 "O— 3 3 N V J . J . I H 5 N V U 1 X 156 IR spectrum of [CpCr(NO)5{N(0)Ph}]PF, as a Nujol mull. 157 158 159 160 IR spectrum of [{CpCr(N0)2}2{u,n -N(CH2)0}]BPh^ as a Nujol mull . o o e S S - 9 S Z I E S O B Y E S 6 2 "82 EBZ. "S I 6 E Z . 2 "8 B 9 E 2 ' O -3 3 N V 1 1 I W S N V M 1 Z 161 T Z 9 ' 9 9 SStr *SS B 2 2 " V t E E O " E E LEB'XZ I P 9 'OT 2 5 S S - 0 -3 3 N V J . J . I W 5 N V U 1 X 162 163 IR spectrum of [Cp Cr(NO) 2]PF 6 i n CH 2 C1 2 . o 0 ri E.VB 'JL9 9 6 2 - 9 £ S f Z . 'VP 0 0 2 ' E E E S S "12 SDX - O X EZVV ' X -a a N v n i H S N v y i x 164 165 r i IR spectrum of [CpCr(NO) -C=C(H)C(Me) C(Me) OC(OMe)]BF as a Nujol mull . o o oes -eg e9E - S S Z .OE svo -ee E B B "12 ze/. -ox eesv -o-3DNVJ.± IwsNvaxx 166 B V Z "D9 E 2 T "OS 8 6 6 * 6 E EZ.8 "62 B f Z . "61 6 2 2 3 - 6 0 2 D S 'O-3 3 N V 1 J. I M S N V d l X 167 168 IR spectrum of Cp Cr(NO) -C=C(H)C(Me) C(Me) 0C(=0) as a Nujol mull. Q O £ 1 6 ' E E 9 1 2 "82 9 1 S *22 9 T B " 9 : Z. T T " T T O i T t ' £ 9 2 B 2 3 3 N V l X I W S N V a x % 171 IR spectrum of [CpW(NO) K)=C(OMe)CECH]BF as a Nujol mull. o • • B O i Y Z IOZ, "fr9 9 6 9 " IS 1 6 9 " 8 E 9 8 9 " S S 1 8 9 "SI 8 E 2 E " O -3 3 N V X X I H S N V M X X 172 173 174 175 O S E - 2 E S E 6 ' S S ZZS'XZ Z O I '91 E 6 9 E B Z 2 "5 8 V E I " O -H D N V l l I H S N V a i X 

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