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Spectroscopic and magnetic studies of some octahedral and square planar ferrous complexes Tsin, Tsang Bik 1975

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SPECTROSCOPIC AND MAGNETIC STUDIES ,OF SOME OCTAHEDRAL AND SQUARE PLANAR FERROUS COMPLEXES By TSANG BIK TSIN B . S c . / U n i v e r s i t y of Hong Kong, 1970 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in the Department of CHEMISTRY We accept t h i s t h j ^ i s as conforming t o the requ i red standard. THE UNIVERSITY OF BRITISH COLUMBIA February, 1975 In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced degree at the U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by the Head o f my Department o r by h i s r e p r e s e n t a t i v e s . It i s u n d e r s t o o d tha t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department o f The U n i v e r s i t y o f B r i t i s h Co lumbia V a n c o u v e r 8, Canada Date i ABSTRACT Three s e r i e s of f e r r o u s complexes have been prepared, and 57 c h a r a c t e r i z e d with the a i d of Fe Mossbauer spectroscopy, magnetic s u s c e p t i b i l i t y and e l e c t r i c a l c o n d u c t i v i t y measurements, and v a r i o u s other s p e c t r o s c o p i c techniques where a p p r o p r i a t e . The f i r s t s e r i e s c o n s i s t s of f o u r h i g h - s p i n octahedral s o l v a t e s of f e r r o u s perch I o r a t e , namely FeCCCH^^SCQgtCIO^^, F e [ ( C 6 H 5 ) 2 S O l l 6 ( C I 0 4 ) 2 , F e C C C H ^ S O l l g t C I O ^ and F e C C ^ N O J g C C I O ^ . Magnetic s u s c e p t i b i l i t i e s have been measured over the temperature range 80-320° K, and Mossbauer s p e c t r a over the range 4.2-330° K. Signs of 2 the quadrupole c o u p l i n g constants e qQ and values of the asymmetry parameters n were obtained from magnetic p e r t u r b a t i o n Mossbauer s p e c t r a . 2 A n a l y s i s of the e qQ vs T data i n terms of c r y s t a l f i e l d theory y i e l d s values f o r the a x i a l and rhombic f i e l d s p l i t t i n g s , and s p i n - o r b i t and s p i n - s p i n c o u p l i n g c o n s t a n t s . The values found are c o n s i s t e n t w i t h the s u s c e p t i b i l i t y data. The (CH^^SO and (.C^H^)^S0 complexes are t e t r a g o n a l l y d i s t o r t e d , w i t h s i n g l e t |xy> ground s t a t e s , w h i l e the other two are t r i g o n a l l y d i s t o r t e d , w i t h ground s t a t e s which are o r b i t a l d oublets. The C 5H 5N0 d e r i v a t i v e shows slow s p i n - l a t t i c e r e l a x a t i o n below <v 30° K, and i s the f i r s t example of such behaviour f o r an 2+ o c t a h e d r a l l y coordinated Fe i o n . The Mossbauer spectrum of t h i s complex at 4.2° K in a 10 kG a p p l i e d f i e l d i s t r e a t e d in the s p i n Hamiltonian approximation and confirms slow r e l a x a t i o n . i i Eleven new complexes of the c h e l a t i n g Iigand 2-(2'-pyridyI)benzimidazole (pyben) have been s y n t h e s i z e d . These are Fe(pyben) 2(NCS> 2 and Fe(pyben) 3A 2'xH 20, where A = CI0~, N0~, NCS~, Br~, I ~ , BF~, B ( C 6 H 5 ) ^ , C C r(NH 3) 2(NCS ) 4 3 " and x = 0, 1,2 (but not a l l combin-a t i o n s ) . These compounds have been c h a r a c t e r i z e d by Mossbauer s p e c t r a (4.2-300° K), s u s c e p t i b i l i t y measurements (80-320° K), s o l i d s t a t e v i s i b l e s p e c t r a (80-300° K), and conductance measurements (295° K). Most 5 I of the complexes are shown t o e x h i b i t T 2~ A| s p i n c r o s s o v e r , the d e t a i l s of which are s e n s i t i v e both t o the nature of the anion and t o the number of waters of c r y s t a l l i z a t i o n . The l a t t e r e f f e c t i s explained i n terms of hydrogen bonding between water molecules and the imino hydrogen on the pyben Iigand. From magnetic p e r t u r b a t i o n Mossbauer s p e c t r a i t Is deduced 2+ t h a t the Fe(pyben)^ c a t i o n has a mer-octahedraI s t r u c t u r e i n both s p i n s t a t e s , i n d i c a t i n g s u b s t a n t i a l inequivalence of the p y r i d i n e and imidazole n i t r o g e n s in pyben. The f i n a l s e c t i o n of the t h e s i s describes the s y n t h e s i s and p r o p e r t i e s of two new f e r r o u s p o r p h y r i n s , Fe(OEP) and Fe(OTBP) (H 20EP = octaethyI p o r p h y r i n , H 20TBP = octamethyItetrabenzporphyrin), and several of t h e i r adducts with amine bases and in one case t e t r a h y d r o f u r a n (THF). The complexes have been s t u d i e d using Mossbauer, n.m.r., e l e c t r o n i c and mass s p e c t r o s c o p i c techniques as w e l l as magnetic measurements. The square p l a n a r Fe(OEP) and Fe(OTBP) are h i g h - s p i n compounds. Mossbauer parameters of these and r e l a t e d complexes are s e n s i t i v e t o the nature of the p e r i p h e r a l groups on the p o r p h y r i n , and can be c o r r e l a t e d with changes in o and ir bonding p r o p e r t i e s of the l i g a n d s . Except f o r Fe(OTBP)(THF) 2, which has a B 2 g ground s t a t e , a l l other adducts obtained are diamagnetic. For Fe(OEP) a t 4.2° K i n a p p l i e d magnetic f i e l d s , the s p i n r e l a x a t i o n r a t e appears t o be comparable t o 57 the Larmor prece s s i o n frequency of the Fe nucleus, whereas Fe(OTBP) a f a s t - r e l a x i n g paramagnet under s i m i l a r c o n d i t i o n s . An apparently polymeric species CFe(OTBP)] n can be obtained and i s an e l e c t r i c a l semiconductor. I t i s suggested t h a t the polymeric s t r u c t u r e i n v o l v e s Fe-Fe a bonds. i v ACKNOWLEDGEMENTS I am extremely g r a t e f u l t o Dr. J.R. Sams f o r h i s i n v a l u a b l e guidance and help during the course of t h i s i n v e s t i g a t i o n . I would l i k e t o thank Dr. D. Dolphin f o r the o c t a e t h y I p o r p h y r i n compound, Dr. J.B. Farmer f o r the use of the atomic a b s o r p t i o n s p e c t r o -photometer, Dr. G.B. P o r t e r f o r the use of the low temperature v i s i b l e s p e c t r a l c e l l , Dr. R.C. Thompson f o r the use of the Guoy balance and the A.C. conductance b r i d g e . I would l i k e t o thank Mr. Mark Vagg f o r a l l the m o d i f i c a t i o n work done on the magnetic p e r t u r b a t i o n Mossbauer apparatus. I-would a l s o l i k e t o thank Mrs. L i a S a l l o s f o r processing most of the Mossbauer s p e c t r a , and Miss J a c q u e l i n e Garnett f o r the t y p i n g of t h i s t h e s i s . F i n a n c i a l support from the National Research Council i n the form of a postgraduate s c h o l a r s h i p was g r e a t l y a p p r e c i a t e d . V TABLE OF CONTENTS Page ABSTRACT I ACKNOWLEDGEMENTS iv TABLE OF CONTENTS v LIST OF TABLES v i i LIST OF FIGURES i x INTRODUCTION 1 CHAPTER I THE MOSSBAUER EFFECT 4 Isomer S h i f t 9 Quadrupole S p l i t t i n g 11 Combined Quadrupole and Magnetic I n t e r a c t i o n 20 CHAPTER I I EXPERIMENTAL TECHNIQUES 25 CHAPTER I I I ELECTRONIC GROUND STATES OF FOUR HIGH-SPIN FERROUS COMPLEXES 33 In t r o d u c t i o n 33 Pr e p a r a t i o n of Complexes 35 Res u l t s and D i s c u s s i o n 35 a. O r b i t a l Ground States of the Complexes 42 b. C r y s t a l F i e l d , S p i n - O r b i t and Spin-Spin S p l i t t i n g Parameters 47 c. Slow S p i n - L a t t i c e R e l a x a t i o n and Paramagnetic Hyper f i n e Spl i t t i n g 62 v i Page CHAPTER IV COMPOUNDS SHOWING HIGH-SPIN - LOW-SPIN CROSSOVER 78 Int r o d u c t i o n 78 Prep a r a t i o n of the Complexes 85 General Observations 92 Conductance Measurements 94 Infrared Data 94 Magnetic Data 99 E l e c t r o n i c Spectra 106 Mossbauer Data I l l Dis c u s s i o n of the Cation S t r u c t u r e 123 CHAPTER V FERROUS PORPHYRINS AND THEIR DERIVATIVES 130 Int r o d u c t i o n .... 130 Pr e p a r a t i o n of the Complexes 135 Weight Loss Experiments 138 General D i s c u s s i o n 138 Dis c u s s i o n of the Mossbauer Data 145 Magnetic P e r t u r b a t i o n Measurements on the High-Spin Ferrous Porphyrins 159 PolyCoctamethyltetrabenzporphyriniron( I I ) J 161 Summary 167 BIBLIOGRAPHY 169 APPENDIX I 177 APPENDIX II 180 v i i LIST OF TABLES Table Page I Values of QyALENCE a n c l 11 ^ a r ' o u s A + o m i c O r b i t a l s . . . . i g II A n a l y t i c a l Data and Important I.R. Bands f o r F e L 6 ( C I 0 4 ) 2 Complexes 37 III E f f e c t i v e Magnetic Moments y f f of the F e L g C C I O ^ Complexes 38 IV 5 7 F e Mossbauer Parameters f o r the F e L g ( C I 0 4 ) 2 Comp I exes 40 V C r y s t a l F i e l d Parameters Derived from Quadrupole Spl i t t i n g Data 55 VI A n a l y t i c a l Data f o r the Ferrous Complexes of 2-(2'-Pyridy I )benzimidazole 90 VII Molar Conductances of the Pyben Complexes in Methanol a t 25° 95 VIII Molar S u s c e p t i b i l i t i e s and E f f e c t i v e Magnetic Moments of the Pyben Complexes as a Function of Temperature. 100 IX E l e c t r o n i c Spectra of the Pyben Complexes i n Methanol a t 25° ...107 X S o l i d S t a t e V i s i b l e Bands of the Fe(pyben> 3A 2*xH 20 Complexes as a Function of Temperature 109 57 XI Fe Mossbauer Parameters f o r the Pyben Complexes 112 v i Table P A 9 E XII Comparison of Observed Room Temperature Magnetic Moments with those C a l c u l a t e d by the Simple Model Described i n the Text 119 XIII Signs of V z z and Magnitudes of n Deduced from Magnetic P e r t u r b a t i o n Mossbauer Measurements 1.24 XIV A n a l y t i c a l and Magnetic Data f o r the Ferrous P o r p h y r i n Complexes 139 XV Fe Mossbauer Parameters f o r the Ferrous Porphyrin Complexes. • ..146 XVI 5 7 F e Mossbauer Parameters f o r Fe(OTBP) and r_Fe(OTBP)Dn. .165 i x LIST OF FIGURES Figure Page 57 1 A T y p i c a l Fe Mossbauer Spectrum i n the Absence of a Magnetic F i e l d 5 57 2 Approximate Energy Level Diagram f o r an Fe Nucleus, Showing the E f f e c t s of an A x i a l l y Symmetric efg and an A p p l i e d Magnetic F i e l d a t an Angle 0 t o the z A x i s of the efg 16 3 Schematic Diagram of a T y p i c a l Mossbauer Spectrometer 27 4. Schematic Diagram of the Apparatus Employed f o r Obtaining V a r i a b l e Temperature Mossbauer Spectra 29 5 Schematic Diagram of the Magnetic P e r t u r b a t i o n Apparatus 30 6 Q u a n t i z a t i o n Axes f o r an Octahedral C r y s t a l F i e l d 43 5 2+ 7 S p l i t t i n g of the D Term of Fe by the C r y s t a l F i e l d . . 49 8 Comparison of Observed and C a l c u l a t e d Quadrupole S p l i t t i n g s as a Function of Temperature f o r the F e L 6 ( C I 0 4 > 2 Complexes 56 9 Comparison of Observed and C a l c u l a t e d E f f e c t i v e Magnetic Moments as a Function of Temperature f o r the FeL ( C I 0 . ) 9 Complexes 59 X Figure Page 10 Mossbauer Spectra in Longit u d i n a l Applied Magnetic F i e l d s : (a) Fe(DPS0) c(C10.) n at 220° K and O 4 2 H q x + = 50 kG; (b) F e ( P y N O ) 6 ( C I 0 4 ) 2 a t 230° K and 2 H e x + = 35 kG. In Both Cases e qQ>0 and n = 0 61 11 Z e r o - F i e l d Mossbauer Spectra of F e ( D M S 0 ) 6 ( C I 0 4 ) 2 and F e ( D P S O ) 6 ( C I 0 4 ) 2 , Showing the Absence of Line Broadening a t Low Temperatures 63 12 Mossbauer Spectra of Fe(PyN0> 6(CI0 4) 2 Between 30.1 and 8.2° K , Showing the Asymmetric Line Broadening Observed a t Low Temperatures 64 13 Mossbauer Spectra of Fe(DMS0K(CI0 ) 0 a t 4.2° K i n o 4 Ap p l i e d Magnetic F i e l d s . From top t o bottom the F i e l d s are 3.4, 10, 30 and 50 kG, r e s p e c t i v e l y 68 14 Mossbauer Spectra of F e ( D P S O ) 6 ( C I 0 4 ) 2 a t 4.2° K i n App l i e d Magnetic F i e l d s . From top t o bottom the f i e l d s are 5.6, 10, 35 and 50 kG, r e s p e c t i v e l y 69 15 Mossbauer Spectra of F e ( P y N O ) 5 ( C I 0 4 ) 2 a t 4.2° K in App l i e d Magnetic F i e l d s . From top t o bottom the f i e l d s are I . I , 2.3, 5.0 and 30 kG, r e s p e c t i v e l y . . . 70 16 Energy Level Diagrams f o r F e ( D M S 0 ) 6 ( C I 0 4 ) 2 and Fe ( P y N 0 ) 6 ( C I 0 4 > 2 Derived from the C r y s t a l F i e l d Model, Showing the E f f e c t s of the A x i a l F i e l d s and Sp i n - O r b i t Coupling 72 x i F i g u r e Page 17 The 10 kG Magnetic P e r t u r b a t i o n Spectrum of F e ( P y N 0 ) 6 ( C I 0 4 ) 2 at 4.2° K. The S o l i d Line i s the T h e o r e t i c a l Spectrum C a l c u l a t e d in the Spin Ham i I ton i an Approximation..... 75 18 The Tanabe-Sugano Diagram f o r a d 6 E l e c t r o n System 79 19 S t r u c t u r e s of the Ligands Discussed i n Chapter IV 81 20 Temperature Dependence of the Molar S u s c e p t i b i l i t i e s of the Pyben Complexes 101 21 Temperature Dependence of the E f f e c t i v e Magnetic Moments of the Pyben Complexes 102 22 Mossbauer Spectra of FeCpyben^CIO^^H,^") between 200 and 295° K H 4 23 Mossbauer Spectra of FeCpyben^CCIO^^'H.^O between 8.7 and I90°K H 5 24 Temperature Dependence of the Mossbauer Area F r a c t i o n s of the Pyben Complexes 117 25 Mossbauer Spectrum of Fe(pyben> 3(BF 4) 2'2H 20 a t 80° K in a L o n g i t u d i n a l Magnetic F i e l d of 50 kG. Computed Spectra f o r V z z>0 and n = 0.7 and 0.9 are Shown f o r Comparison 125 26 E l e c t r o n i c Spectrum of Fe(OTBP)(py) 2 in P y r i d i n e a t 25°...142 27 E l e c t r o n i c Spectrum of F e ( 0 E P ) ( p y ) 2 in P y r i d i n e a t 25° 144 28 P o s s i b l e Ground States f o r Ferrous Porphyrins Mossbauer Spectrum of Fe(OTBP)(py> 2 at 84° K in an a p p l i e d magnetic f i e l d of 50 kG. The f u l l curve i s the t h e o r e t i c a l spectrum c a l c u l a t e d f o r the parameters 6 = 0.77, A Eg = +0.68,. r = 0.29 ( a l l in mm s" 1) and n = 0 156 Mossbauer Spectrum of Fe(OTBP) a t 4.2° K i n an a p p l i e d magnetic f i e l d of 50 kG. V z z i s p o s i t i v e 57 and the e f f e c t i v e f i e l d a t the Fe nucleus i s estimated t o be ^ 80 kG 160 Mossbauer Spectrum of Fe(OEP) a t 4.2° K in an Applied Magnetic F i e l d of 25 kG. 162 1 INTRODUCTION This t h e s i s i s concerned with the p r e p a r a t i o n and c h a r a c t e r i z a -t i o n of three s e r i e s of ferrous complexes. The p r i n c i p a l techniques used f o r studying these complexes have been MSssbauer spectroscopy and magnetic s u s c e p t i b i l i t y measurements, although e x t e n s i v e use has a l s o been made of i n f r a r e d and e l e c t r o n i c s p e c t r a of the complexes, and i n some cases of e l e c t r i c a l conductance measurements and nuclear magnetic resonance s p e c t r a . From the Mossbauer s p e c t r a one can o b t a i n two c h e m i c a l l y important parameters, namely the isomer s h i f t and quadrupole s p l i t t i n g . (A more complete d i s c u s s i o n i s given in Chapter I ) . The former i s r e l a t e d t o the t o t a l s e l e c t r o n d e n s i t y a t the nucleus, and i s very 4 2 fi 0 useful f o r d i f f e r e n t i a t i n g h i g h - s p i n ( t _ e ) and low-spin (t° e ) 2g g 2g g 2+ f e r r o u s complexes. High-spin Fe systems t y p i c a l l y have isomer s h i f t values of about 1.5 mm s~' (with respect t o sodium n i t r o p r u s s i d e ) ' , w h i l e f o r low-spin Fe'', isomer s h i f t s are c h a r a c t e r i s t i c a l l y about 0.6 mm s '. Since one can u s u a l l y measure t h i s parameter with an accuracy of ± 0.02 mm s ' o r b e t t e r , these two s p i n s t a t e s are r e a d i l y d i s t i n g u i s h a b l e . The quadrupole s p l i t t i n g i s r e l a t e d t o the e l e c t r i c f i e l d g r a d i e n t (efg) at the nucleus, e s t a b l i s h e d when the d i s t r i b u t i o n of the surrounding e l e c t r o n s has lower than c u b i c symmetry. For low-spin f e r r o u s compounds, quadrupole s p l i t t i n g s are u s u a l l y small and nearly independent of temperature. High-spin f e r r o u s complexes on the other hand, tend t o show s p l i t t i n g s which are much l a r g e r in magnitude and 2 s t r o n g l y temperature dependent.' In the l a t t e r case, d e t a i l e d measurements of the quadrupole s p l i t t i n g over a wide range of temperatures (e.g., 4.2 - 300°K) enable one t o estimate c r y s t a l f i e l d 2 s p l i t t i n g parameters as well as s p i n - o r b i t and s p i n - s p i n c o u p l i n g constants. The r e c e n t l y e s t a b l i s h e d technique of magnetic p e r t u r b a t i o n MSssbauer spectroscopy^, in which the sample i s subjected t o a f a i r l y large a p p l i e d magnetic f i e l d , provides two f u r t h e r pieces of information about the e f g . As discussed below, the efg i s a 3 x 3 t e n s o r which i s symmetric and t r a c e l e s s , and which can be d i a g o n a l i z e d by a proper choice of axes. The diagonal elements i n the p r i n c i p a l a x i s system are chosen such t h a t |V z z| |Vy Vl - l v x x l * a n c' ^ h e ^ w o '"dependent parameters are taken t o be V and the asymmetry parameter n = (V -V )/V . A zz xx yy zz m a g n e t i c a l l y perturbed MSssbauer spectrum enables one t o determine both the s i g n of V z z and magnitude of n(O^n-l).^ The former i s determined by the shape of the charge d i s t r i b u t i o n about the nucleus. A p o s i t i v e V z z corresponds t o an o b l a t e charge d i s t r i b u t i o n , and a negative V"zz t o a p r o l a t e d i s t r i b u t i o n . Moreover, i f the asymmetry parameter i s zero, then the e fg i s a x i a l I y symmetric, i . e . , V = V = - %V , whereas a non-zero a 7 i > ' xx yy z z ' n i n d i c a t e s t h a t a l l t h r ee p r i n c i p a l a x i s d i r e c t i o n s are i n e q u i v a l e n t . In the f i r s t two chapters we d i s c u s s in d e t a i l the chemical information t h a t can be obtained v i a Mossbauer spectroscopy, and describe the apparatus and experimental procedures employed in t h i s study. In the f i n a l t h r e e chapters r e s u l t s of the s p e c t r o s c o p i c and magnetic measurements on the t h r e e types of complexes s t u d i e d are presented and dis c u s s e d . 3 The f i r s t s e r i e s of complexes, described i n Chapter I I I , was 4 f i r s t prepared by Reedijk and van der Kraan. This c o n s i s t s of f o u r high-spin complexes formed from f e r r o u s p e r c h l o r a t e and the oxygen-donor Lewis bases dimethyl s u l p h o x i d e , diphenyl sulphoxide, tetramethylene sulphoxide, and p y r i d i n e - N - o x i d e . Mossbauer and magnetic s u s c e p t i b i l i t y data obtained over a wide range of temperature are used t o estimate the ground s t a t e s p l i t t i n g parameters f o r these compounds. At low temperatures, one of the complexes shows unusual behaviour, which can be a t t r i b u t e d t o slow s p i n - l a t t i c e r e l a x a t i o n . Chapter IV describes a s e r i e s of eleven new complexes. Ten of these can be w r i t t e n in the form Fe(pyben) 3A 2•xH 20, where pyben i s the bidentate c h e l a t i n g Iigand 2 - ( 2 ^ - p y r i d y I ) b e n z i m i d a z o l e , A = CI0~, NO^, NCS", Br", l " , BF~, B C C ^ ) " , [ C r t N H ^ ^ N C S ) ^ " , and x=0, I o r 2. The remaining compound i s Fe(pyben) 2(NCS> 2. The t r i s ( p y b e n ) d e r i v a t i v e s a l l e x h i b i t h i g h - s p i n - low-spin crossover behaviour, which i s t r a c e d w i t h the a i d of Mossbauer spectroscopy and other p h y s i c a l techniques. The 2+ bonding and s t r u c t u r e of the FeCpyben)^ system i n both h i g h - s p i n and low-spin s t a t e s i s a l s o discussed. The f i n a l s e r i e s of compounds, described i n Chapter V, c o n s i s t s of two square planar f e r r o u s porphyrins and some of t h e i r adducts with Lewis bases. The two porphyrins are octamethyItetrabenzporphyrin (OTBP) and o c t a e t h y I p o r p h y r i n (OEP). The complexes in t h i s s e r i e s which have been prepared and c h a r a c t e r i z e d are: Fe(OTBP), Fe(OTBP)(py> 2, Fe(0TBP)(THF) 2, Fe(OTBP)(3-pic> 2, Fe(OTBP)(py)^, Fe(OTBP)(4-pic) 4, Fe(OTBP)(IQ) 4, Fe(OEP) and Fe(OEP)(py) 2, where py = p y r i d i n e , THF = t e t r a h y d r o f u r a n , 3-pic = 3 - p i c o l i n e , 4-pic = 4 - p i c o l i n e , and IQ = i s o q u i n o l i n e . 4 CHAPTER I THE MOSSBAUER EFFECT The Mossbauer e f f e c t , o r nuclear gamma resonance, a r i s e s from the r e c o i l - f r e e emission and a b s o r p t i o n of y r a y s by s u i t a b l e n u c l e i . There are many isotopes f o r which the e f f e c t has been observed, but only a l i m i t e d number of these have s u i t a b l e nuclear parameters f o r the e f f e c t t o be of p r a c t i c a l i n t e r e s t . The most commonly st u d i e d of 57 119 the Mossbauer n u c l e i are Fe and Sn. Since the work w i t h i n t h i s t h e s i s i s concerned e n t i r e l y with i r o n systems, the Mossbauer process 57 i s i l l u s t r a t e d here only f o r Fe. In the b a s i c Mossbauer experiment, the energy of the Y ~ r a y s emitted by a r a d i o a c t i v e source i s modulated by ap p l y i n g a Doppler v e l o c i t y t o the source, and those Y~i"ays having the c o r r e c t energies can be resonantly absorbed by absorber n u c l e i . The spectrum c o n s i s t s of a p l o t of the number of t r a n s m i t t e d photons versus the photon energy (or Doppler v e l o c i t y ) , and one or more peaks are observed where resonance o c c u r s . From a Mossbauer spectrum, two parameters can be obtained which are of s p e c i a l i n t e r e s t t o chemists. These are the isomer s h i f t (6) and the quadrupole s p l i t t i n g (AEg). The former i s r e l a t e d t o the e f f e c t i v e s e l e c t r o n d e n s i t y a t the nucleus and the l a t t e r t o the p o i n t group symmetry of the e l e c t r o n i c environment around the nucleus a r i s i n g from bonding o r i o n i c e f f e c t s . A t y p i c a l spectrum f o r i r o n i s shown i n Figure I. The isomer s h i f t denotes the p o s i t i o n of the c e n t r o i d of the 5a FIGURE I 57 A T y p i c a l Fe Mossbauer Spectrum in the Absence of a Magnetic F i e l d 6 spectrum with respect t o a standard absorber, and the quadrupole s p l i t t i n g i s the magnitude of the s e p a r a t i o n of the two l i n e s . In order t o understand the mechanisms by which the e l e c t r o n i c environment a f f e c t s the nucleus, i t i s p r o f i t a b l e t o look at the i n t e r -a c t i o n s which are r e s p o n s i b l e f o r the isomer s h i f t and quadrupole s p l i t t i n g i n a s e m i - c l a s s i c a l p i c t u r e . ^ ' ^ From c l a s s i c a l e l e c t r o s t a t i c s , a charge aggregate l i k e the nucleus i n t e r a c t s with the e l e c t r i c p o t e n t i a l f i e l d s e t up by e l e c t r o n s around i t according t o Coulomb's law. Energy = H nucI ear voIume p(x,y,z)V(x,y,z)dv (I) where p(x,y,z) i s the charge d e n s i t y f u n c t i o n d e s c r i b i n g the nucleus, V(x,y,z) the p o t e n t i a l f i e l d s e t up by the surrounding e l e c t r o n s , and dv the d i f f e r e n t i a l volume element. By the T a y l o r expansion of V(x,y,z) around the nuclear centre of mass, V(x,y,z) = V(0 ,0,0) + T (^-\ x + h I — \ x x Q ' ' \ 3x o a a i V 3 x 9x„ ) o a g a=l \ a/ a,B=l \ a 8/ + higher terms (2) L e t t i ng = 00° (3) 7 and s u b s t i t u t i n g eqn.(2) i n t o e q n . ( l ) we o b t a i n H = V(0,0,0) (p(x,y,z)dv + £ V a [ p ( x , y , z ) x a d v I V o,6=l a g p(x,y,z)x aXgdv + higher terms (4) The f i r s t i n t e g r a l can be evaluated as p(x,y,z)dv = Ze where Ze i s the t o t a l nuclear charge. The next t h r e e terms d e s c r i b e the i n t e r a c t i o n of the nuclear d i p o l e with the e l e c t r i c f i e l d having 5 components V a ( a = l , 2, 3 ) , and can be shown t o v a n i s h . The next nine terms d e s c r i b e the i n t e r a c t i o n of the nuclear quadrupole moment with the e l e c t r i c f i e l d g r a d i e n t (efg) tensor V where otp Q a g p ( x , y , z ) x a x p d v (5) With these s u b s t i t u t i o n s , equation (4) becomes H = V(0,0,0) Ze + h I V Q g Q^g + higher terms. a, 3 = I ( 6 ) The leading term represents the e l e c t r o s t a t i c energy of a p o i n t nucleus, and i s independent of whether the nucleus i s i n i t s e x c i t e d s t a t e o r 8 ground s t a t e . Since the Mossbauer t r a n s i t i o n measures energy d i f f e r e n c e s between e x c i t e d and ground nuclear s t a t e s , t h i s term can be neglected when c o n s i d e r i n g the Mossbauer process. The higher order terms d e s c r i b e o c t a p o l e , hexadecapoIe, e t c . i n t e r a c t i o n s , which can be shown t o be 5 n e g l i g i b l e compared t o the quadrupole i n t e r a c t i o n . With these s i m p l i f i c a t i o n s , the only terms of i n t e r e s t are 3 the quadrupolar terms % Y V „ Q' . Since Q ' i s symmetric, i t i s ^ r % , aB aB aB expedient t o de f i n e a new te n s o r t h a t i s both symmetric and t r a c e I ess >y 5 e— i where <5 „ = I i f a = 3 aB 0 i f o / B whence (8) The f i r s t nine terms are r e s p o n s i b l e f o r the quadrupole s p l i t t i n g w h i l e the second group of terms i s r e s p o n s i b l e f o r the isomer s h i f t and w i l l be discussed f i r s t . 9 Isomer S h i f t By d e f i n i t i o n , 3 y f S 9ee = P(x,y,z)r2 :dv (9) e - l w j where r 2 = x 2 + y 2 + z 2 , and from e l e c t r o s t a t i c s 3 I V , , = - 4 i r o (10) j = l J J where a i s the e l e c t r o n i c charge d e n s i t y a t the nuc l e a r centre of mass. Since only s e l e c t r o n s have a f i n i t e charge d e n s i t y a t t h a t p o i n t 6 3 I j = l J J ' ( I I ) V j j = 4 * 6 l * ( o ) l 2 where iJ/(o) i s the value of the wavefunction of s e l e c t r o n s a t t h a t p o i n t . Hence, H...S. = 6-Q ^ j , Vjj)=f * e | * ( o ) | 2 j p ( x , y , z ) r 2 d v For a source nucleus i n the presence of s e l e c t r o n s both the ground s t a t e and e x c i t e d s t a t e energies (E and E ) wiI I be s h i f t e d e g upward by the term H ( s . Thus the energy of the y photon w i l l be given by (12) 10 e = (E + H ^ I T E D ) - (E + H f ° U N D ) e + (|-ire 2Z)|^ ( o ) | 2 ( < r 2 > - <r 2> ) (13) where r , z ) r 2 d v ,z)dv jp(x,y,; p(x,y,; • and E = E - E . S i m i l a r l y , f o r an absorber nucleus o e g 2 E = E + M e 2Z ) U ( o ) | 2 ( < r 2 > - <r 2> ) ' (14) Y' o J a 1 e g The energy d i f f e r e n c e between source and absorber i s the isomer s h i f t : E - E =6 = | T T e 2Z[<r 2> - <r 2> (o)|2-U ( o ) | 2 : (15) "a e g a s According t o t h i s formula, i t i s observed t h a t 6 a r i s e s from two 2 f a c t o r s . The f i r s t i s a nuclear f a c t o r , -TTI e2ZL~<r2> - <r 2> H, and ' 3 e g - " 57 i s f i x e d f o r Fe. The magnitude of t h i s q u a n t i t y i s not a c c u r a t e l y known, however, i t s s i g n f o l l o w s from the s i g n of C<r 2> g - <rz>^}, I which i s known t o be negative f o r the 14.4 keV Mossbauer t r a n s i t i o n 57 in Fe. The second f a c t o r a r i s e s from the s e l e c t r o n charge d e n s i t y a t the nucleus. If i n a s e r i e s of experiments, | ^ ( o ) | 2 i s kept constant ( i . e . the same source i s used f o r a l l experiments), o r i f a f i x e d 11 standard i s used t o c a l i b r a t e the isomer s h i f t , then 6 wiI I be p r o p o r t i o n a l t o the s e l e c t r o n d e n s i t y at the absorber nucleus. In 57 the case of Fe, s i n c e rj<r2> - <r 2> ] i s negative, an increase i n s e g e l e c t r o n d e n s i t y at the absorber nucleus leads t o a decrease in isomer s h i f t . I t i s w e l l known t h a t p and d e l e c t r o n s can e x e r t screening e f f e c t s on the s e l e c t r o n s , so t h a t changes in p and d o r b i t a l occupancy w i l l i n f l u e n c e the isomer s h i f t by a l t e r i n g the e f f e c t i v e s e l e c t r o n d e n s i t y at the nucleus. This screening e f f e c t can best be 3+ 5 i l l u s t r a t e d i n the case of h i g h - s p i n iron compounds. Fe (d ) s a l t s have isomer s h i f t s ( r e l a t i v e t o sodium n i t r o p r u s s i d e ) i n the range — I 2+ 6 0.3 t o 0.7 mm s , while Fe (d ) s a l t s have isomer s h i f t s in the range 1.4 t o 1.7 mm s '. The presence of the e x t r a d e l e c t r o n screens the s e l e c t r o n d e n s i t y at the nucleus and thus increases the isomer s h i f t . Quadrupole S p l i t t i n g The quantum mechanical e q u i v a l e n t # n „ of the term HQ.S. 6 ^ 0_. Q a 3 VaB ex,p- I can be co n s t r u c t e d using the usual correspondence^ 6ae =Jp(x,y,Z) C3x ax 6 - 5 a gr 2 3dv (16) (17) 12 where 1^, 1^ are nuclear s p i n angular momentum op e r a t o r s . C i s a constant w i t h i n an angular momentum ma n i f o l d ^ . Thus Q.S. 6 I L~ p ( x , y , z ) ( 3 x a x e - 6 a g r 2 ) d v : v a,3=1 J o3 I < a, 3= I p(x.y,z)Cdv ) C 5 . ( I a I 3 + I BI o)-« o 3I(I + l ) : V o B a3 (18) If an " e l e c t r i c quadrupoIe moment" Q.is defined such t h a t eQ = <II|Q 2 2|II> (19) then eQ = c r ( < I l | 3 l | - I ( I + l ) | l I > ) = C*D5I 2 - 1(1+1)1 (20) whence eQ K 2 I - I ) (21) 1 3 and M = 22 y r-(i i + i i ) -6 ^KI+DDV „ (22) ^Q.S. 6K2 I -D £ _ . L 2'aB B V aB J cxB a, p-1 5 Using the no t a t i o n L = I ± i i , ± x y V = V o zz V = V ± iV +1 x z y z V = J*(V - V ) ± IV (23) ±i xx yy xy and B = ^ 41(21-1) we have *QS. = B [ I ( 3*z V o + ( IVz + W V - I + ( I _ I z + I z I _ ) V ! + I + 2 V_ 2 + I 2 V 23 (24) Since V^ i s symmetric and has van i s h i n g t r a c e , i t i s p o s s i b l e t o choose a p r i n c i p a l a x i s system whereby V „ = 0 f o r a ^ B. Furthe r , one can ap choose the a x i s system such t h a t V I * |V I 2 |V | (25) zz yy 1 xx 1 14 and eqn.(24) can be w r i t t e n as _ = B<f[3l2 - K I + D 3 V + hCll + I 2 ) (V Q.S. I z _J zz + - V ) xx yy ) = B V Z Z { L ~ 3 I Z 2 - I C I + I>H + J ( i 2 + i 2 )} (26) V - V where n = — — ^ V zz (0 4 T\ $ I) n i s c a l l e d the asymmetry parameter. The above Hamiltonian i s v a l i d in general n the Fe case, the Mossbauer t r a n s i t i o n i n v o l v e s a nuclear s p i n I = — e x c i t e d s t a t e and = ground s t a t e . Since n u c l e a r s t a t e s with I < I do not have a quadrupole moment, only the e x c i t e d s t a t e i s a f f e c t e d by t h i s Hamiltonian. The eignvalue m a t r i x can then be set up f o r the I = — 3 case : n 5_ 2 3 2 3 J_ 2 2 _ 3 0 0 -3 /3~~n 0 /3~n ~2 /3~n -3 3_ '2 /3~T (BV ) zz (27) 15 On d i a g o n a I i z i n g t h i s matrix a p a i r of doubly degenerate e i g e n f u n c t i o n s i s obtained with e n e r g i e s 3 E = ± 3 B V ri + n 2/,] h (28) zz J • The energy d i f f e r e n c e between these two l e v e l s i s the quadrupole s p l i t t i n g : AEg = 6 B V z zD + n 2 / 3 ] % = %e2QqD + n 2 / 3 D ^ (29) where eq = V z z» the z component of e f g . Again, eqn.(28), l i k e e q n . ( l 5 ) , contains two f a c t o r s , one nuclear and the other e l e c t r o n i c . The nuclear f a c t o r i n v o l v e s the 57 8 quadrupole moment Q which i s estimated f o r Fe t o be about 0.2 barn . The e l e c t r o n i c f a c t o r i n v o l v e s efg components t h a t a r i s e from e l e c t r o n s surrounding the nucleus. For a pure quadrupole i n t e r a c t i o n the Mossbauer t r a n s i t i o n w i l l g i v e r i s e t o a spectrum c o n s i s t i n g of two l i n e s separated by AEg. In the usual case of a randomly o r i e n t e d p o l y c r y s t a I I i n e absorber, the l i n e i n t e n s i t i e s are e q u a l , as shown in Figure 2 ( a ) . Under these c o n d i t i o n s one cannot determine which of the two l i n e s a r i s e s from the l±~ > -*• l±~ > nuclear s p i n t r a n s i t i o n and which from the 1 g 2 e , 1 . 3 * 2 g ± 2 " > e t r a n s i t i o n . (The r e l a t i v e energies of the e x c i t e d FIGURE 2 Approximate Energy Level Diagram f o r an 57 Fe Nucleus, Showing the E f f e c t s of an A x i a l I y Symmetric efg and an A p p l i e d Magnetic F i e l d a t an Angle 0 t o the z A x i s of the e f g . 2 n A - e qQ A - - T J L -a = g.B H , 31 n ext 3 = g 3 H . M o n ext FIGURE 2 T y p i c a l Spectrum of (Fe AEp = + 2.0 mm s" n = 0 -2.0 0.0 +2.0 T y p i c a l Spectrum of + 2.0 mm s 30 kG 57 Fe V ext l o n g i t u d i n a l n = 0 17 s t a t e sub-1 eve Is depend on the s i g n of V : i f V > 0 the |± > r • zz zz 1 2 e s t a t e l i e s higher in energy, and the |± ^  > e s t a t e i s higher i f \/ <0.) Thus, n e i t h e r the s i g n of V nor the value of n can be zz zz deduced i n t h i s case, but only the magnitude of AEg, so t h a t one does not obtai n the f u l l information p o t e n t i a l l y a v a i l a b l e from the quadrupole i n t e r a c t i o n . In order t o r e l a t e the efg t o the chemical environment of the nucleus, the source of the efg i s now examined. The c o n t r i b u t i o n t o the efg a t the nucleus can be separated i n t o two terms: (I) the l a t t i c e c o n t r i b u t i o n (^LATTICE^ °'UE ^° c n a r 9 e s o n ^ n e ''9 a n c's and neighbouring ions in the c r y s t a l ; (2) the valence c o n t r i b u t i o n ( e clyALENCE^ a r ' s ' n 9 from an asymmetric d i s t r i b u t i o n of e l e c t r o n s in bonding and non-bonding o r b i t a l s . The e l e c t r o n s in the valence s h e l l w i l l u s u a l l y make the major c o n t r i b u t i o n t o the second term, although inner s h e l l e l e c t r o n s which acquire a non-spherical d i s t r i b u t i o n due t o induced p o l a r i z a t i o n e f f e c t s g can a l s o c o n t r i b u t e . Thus one can w r i t e eq as eq = (I - Y a > ) e q L A T T | C E + (I - R ) e q V A L E N C E (30) where R(0.2>R>-0.2) and y (~7>y >-IOO) are Sternheimer f a c t o r s 1 oo ' oo accounting f o r t h e induced p o l a r i z a t i o n of inner core e l e c t r o n s , 2 (3cos 0.-1) ^LATTICE = l e q i 3" 1 ( 3 I ) i . r , 1 8 and ( 3 c o s 2 0 . - l ) "VALENCE = " I e p j K 3 ^ — > ( 3 2 ) J r j Here q. i s the charge on the i t h ion having p o l a r coordinates 0., r. (approximating the ions as poi n t charges), p. the population of the j t h J valence s h e l l o r b i t a l , and <(3cos 20 . - I ) / r 3 > the e x p e c t a t i o n value of J J t h i s p o p u l a t i o n over the e l e c t r o n coordinates 0., r., the summations J J being taken over a l l ions i and a l l valence o r b i t a l s j . The numerical values of o^tENCE ^ O I~ v a r ' o u s a"f°mic o r b i t a l s can be obtained from the 9 angular p a r t s of the wavefunctions , and are given in Table I. 9 It i s g e n e r a l l y assumed t h a t the valence term makes the major c o n t i b u t i o n t o the efg f o r f e r r o u s complexes, owing t o the r 3 dependence. From Table I i t can a l s o be seen t h a t i f e i t h e r a p o r d s h e l l i s empty, h a l f - f i l l e d o r completely f i l l e d i t wiI I not c o n t r i b u t e 2+ to the e f g . In the case of hig h - s p i n Fe ions with l i t t l e covalency, the efg a r i s e s p r i m a r i l y because there i s one d e l e c t r o n i n a d d i t i o n t o the s p h e r i c a l l y symmetric h a l f - f i l l e d 3d s h e l l . Since both V z z and n w i l l depend on which d o r b i t a l t h i s e x t r a e l e c t r o n o c c u p i e s , i t i s important t o know both these parameters. 19 TABLE Values of g^LENCE a n C* n ^ o r v a r ' o u s atomic o r b i t a l s — b_ O r b i t a l qVALENCE n 2 -3 p x + 5 < r > -3 2 -3 Py + | < r > + 3 4 - 3 P z " F < r > 0 d 2 2 + 4- < r " 3 > x - y 7 4 _ _-3 ' z z 7 d , - ^ < r > 4 -3 d + Z . < r - > 0 x y 7 d - \ < r - 3 > +3 x z 7 d y z " T < r * 3 > " 3 — Taken from Ref. I, p.59. — <r 3> i s the e x p e c t a t i o n value of l / r 3 f o r the a p p r o p r i a t e r a d i a l f u n c t i o n . 20 Combined quadrupole and magnetic i n t e r a c t i o n In order t o determine the value of n and the s i g n of Vzz> a magnetic p e r t u r b a t i o n experiment may be c a r r i e d out in which an ex t e r n a l magnetic f i e l d ( u s u a l l y 10-50 kG in magnitude) i s a p p l i e d t o the absorber. This a p p l i e d f i e l d l i f t s the remaining degeneracy of the m^. nuclear s p i n substates v i a the nuclear Zeeman e f f e c t . A general treatment has been developed f o r the case of diamagnetic compounds 3 and w i l l be discussed f i r s t . The Hamiltonian d e s c r i b i n g the i n t e r a c t i o n of the nuclear magnetic moment y_ with an e f f e c t i v e magnetic f i e l d H. at the nucleus (which f o r a diamagnet w i l l equal the a p p l i e d f i e l d ) i s 3 K M = - y • H = -gB I'M (33) Mag — — s n where g i s the gyromagnetic r a t i o and B^ the nuclear magneton. The t o t a l Hamiltonian w i l l be a combination of magnetic and quadrupole terms. In the e fg p r i n c i p a l a x i s system, t h i s can be w r i t t e n as _ ^ = % . S . + ^ M a g = 4 I T 2 ? ^ jy D l2 - KI+I) + J d 2 + I2_)J - gB HQ sin0cosd>-+ I sin©sin<f> + I cosG) 3 n x y z (34) where H i s the magnitude of H_, and 0, <}> the p o l a r angles of H. with respect t o the z a x i s of the e f g . 57 The I = h ground s t a t e of Fe has zero quadrupole moment so th a t f o r t h i s s t a t e <K=$Z,. and one h a s 3 Mag km > = hg B HcosO o n -%g oB nHsin0e I* -hg 3 HsinOe a o n hg 3 HcosO a o n - I * (35) 21 This can be diagonaIized t o g i v e the eigenvalues E = ±hg 6 H 3 o n (36) where g Q i s the ground s t a t e gyromagnetic r a t i o , and e i g e n f u n c t i o n s I - > = - s i n | e " ^ l % , % > + c o s | Us, ~h> I + > = cos | \h, h> + s i n | e'*|3s, -\ 2 , —2- (37) For the e x c i t e d s t a t e the matrix elements are < 3 r m » | -^C-lIm > = 3 2 3 2 3 2 2 2 2 3A + -|a cose V ^ a sin6e >/3nA \/3 . . I* -^ a s i n6e %/3nA -3A + ^  cos6 a si n 9 e vTnA a sin8e VJnA I* -3A - cos9 -^ a sinOe 2>a sinGe 3A - ^ cos9 22 Where A = — r ^ — , <* = —g|3 nH and i s the gyromagnetic r a t i o of the e x c i t e d s t a t e . S o l u t i o n s t o t h i s matrix can be obtained f o r any given e2qQ> 1, H, 0 and <|> by computer diagonal i z a t i o n . In order t o compute a t h e o r e t i c a l spectrum, t r a n s i t i o n p r o b a i l i t i e s connecting the v a r i o u s substates have t o be c a l c u l a t e d . To do t h i s one must know the eigenvectors in terms of the b a s i s kets f o r both ground and e x c i t e d s t a t e s , and the Clebsch-Gordan coup Iing c o e f f i -c i e n t s ' ^ f o r the a p p r o p r i a t e t r a n s t i o n s . Then one must sum over the r e l a t i v e amounts of l e f t - and right-handed c i r c u l a r l y p o l a r i z e d y-photons a v a i l a b l e . T h i s i n t e n s i t y problem has been t r e a t e d by C o l l i n s and T r a v i s 3 , and the c a l c u l a t i o n s are discussed i n Appendix I. 57 The r e s u l t f o r an Fe compound with a x i a l I y symmetric efg ( i . e . , n=0)is the appearance of a c h a r a c t e r i s t i c t r i p l e t - d o u b l e t p a t t e r n as in Figure 2 ( b ) . For a p o s i t i v e V z z the t r i p l e t l i e s a t lower energy, whereas the doublet l i e s at lower energy f o r a negative V z z» As n increases from zero the spectrum becomes more symmetrical, and when n = I a symmetric t r i p l e t - t r i p l e t p a t t e r n i s observed. Thus, by generating t h e o r e t i c a l s p e c t r a and comparing them with experimental ones, the s i g n of V z z and an estimate of the n value can be o b t a i n e d . For paramagnetic systems the p i c t u r e i s f a r more complicated, and several d i f f e r e n t s i t u a t i o n s may a r i s e . Johnson'' has examined the problem in d e t a i l f o r a case where the r e l a x a t i o n r a t e of the e l e c t r o n i c s t a t e s i s very f a s t compared t o the Larmor prece s s i o n frequency of the nucleus. The e f f e c t i v e f i e l d a t the nucleus, H e ^ , can be expressed i n t h i s case by'' 23 H e f f = H e x t + H ° ( 3 9 ) where <S> i s the average of the t o t a l e l e c t r o n i c s p i n S and H° the ' n s a t u r a t i o n value of the i n t e r n a l f i e l d H . The magnetization can be r e l a t e d t o the s u s c e p t i b i l i t y x by <S>/S = XH e x +/Ny (40) where R i s the magnetic moment and N Avogadro's number. ( P r o p e r l y , both H and x are t e n s o r q u a n t i t i e s , but t h i s does not a l t e r the q u a l i t a t i v e p i c t u r e given here.) Since the s u s c e p t i b i l i t y i s small a t high tempera-t u r e s , the magnetization <S> induced by a moderate a p p l i e d f i e l d 50 kG) under these circumstances w i l l be s m a l l , and H ^ ^  2; ^ e x+. Thus in t h i s case the paramagnetic system behaves in e s s e n t i a l l y the same way as a diamagnetic one, and the treatment given above w i l l s t i l l be v a I i d. On the other hand, a t 4.2°K (the temperature a t which magnetic p e r t u r b a t i o n measurements are normally made) x w i l l be large and H ^ can be very much l a r g e r than the a p p l i e d f i e l d . In a d d i t i o n , s i n c e i s an a n i s o t r o p i c t e n s o r , ^Qff and w i l l i n general not be col l i n e a r . Thus, Mossbauer s p e c t r a obtained under these c o n d i t i o n s can have very d i v e r s e appearances depending on the d e t a i l e d e l e c t r o n i c s t r u c t u r e of the complex, and i t i s o f t e n impossible t o determine the si g n of V z z from such a measurement. Further d i f f i c u l t i e s can a r i s e i f the s p i n r e l a x a t i o n i s not f a s t compared t o the nuclear Larmor frequency, s i n c e i n t h i s case <S> may be non-zero even in the absence of an a p p l i e d f i e l d . 24 For the s i m p l e r h i g h - s p i n f e r r i c case, s i n c e Fe 3 1* i s an S-state i o n , the ground s t a t e i s j u s t a Kramers' doublet and the problem of v a r y i n g r e l a x a t i o n r a t e s has been given an adequate t h e o r e t i c a l 12-14 treatment . For h i g h - s p i n f e r r o u s systems the s i t u a t i o n i s f u r t h e r complicated by the e x i s t e n c e of several types of o r b i t a l ground s t a t e s . There has been a very recent attempt t o t r e a t an e i g h t - c o o r d i n a t e h i g h -s p i n f e r r o u s complex having an o r b i t a l s i n g l e t ground s t a t e , i n both 15 the f a s t and slow r e l a x a t i o n l i m i t s . However, the theory f o r the case of intermediate r e l a x a t i o n r a t e s has not been extended t o inc l u d e f e r r o u s systems thus f a r . 25 CHAPTER I I EXPERIMENTAL TECHNIQUES Analyses Microanalyses of C, H, N were performed by Mr. P. Borda of t h i s Department and by Drs. F. and E. Pascher, M i c r o a n a l y t i c a l Laboratory, Bonn, Germany. The i r o n analyses were c a r r i e d out with the a i d of a Perkin-Elmer 305A atomic a b s o r p t i o n spectrophotometer. Infrared Spectra Infrared s p e c t r a of s o l i d samples were recorded e i t h e r with nujol mulls between caesium i o d i d e p l a t e s o r with KBr p e l l e t s , using a Perkin-Elmer 457 g r a t i n g spectrometer over the range 4000-250 cm Molar Conductances Molar conductances i n methanol were measured using an A.C. conductance b r i d g e . The measurements were made i n a 25° o i l bath, and the A.C. bridge frequency was s e t at 1000 Hz. E l e c t r o n i c Spectra The e l e c t r o n i c s p e c t r a were recorded on a Cary Model 14 spectrophotometer. KBr p e l l e t s were used f o r s o l i d s w h i l e s p e c t r a of s o l u t i o n s were obtained in I cm standard quartz o p t i c a l c e l l s . In order t o o b t a i n v a r i a b l e temperature data, a brass l i q u i d n i t r o g e n dewar f i t t e d with g l a s s windows was used. The temperature v a r i a t i o n was achieved by i 26 passing c o n t r o l l e d amounts of c o l d (79°K) ni t r o g e n vapour i n t o the inner chamber of the dewar.- Temperatures were measured with a copper-constantan thermocouple attached t o the copper sample holder i n s i d e the dewar. Magnetic Measurements Magnetic s u s c e p t i b i l i t i e s of powder samples were measured with a v a r i a b l e temperature Gouy balance over the temperature range 80-320°K. A l l measurements were made at two f i e l d s t r e n g t h s . C a l i b r a t i o n of the apparatus was achieved using mercury t e t r a t h i o c y a n a t o c o b a I f a t e ( 1 1 ) . Pascal's constants were used t o c o r r e c t f o r diamagnetism'^. MOssbauer Spectra The Mossbauer spectrometer c o n s i s t e d of an A u s t i n Science A s s o c i a t e s S-3 d r i v e u n i t and K3-K l i n e a r motor, a Reuter-Stokes RSG-61 pr o p o r t i o n a l counter (Xe - CC^ f i l l gas a t two atmospheres p r e s s u r e ) , and a Nuclear-Chicago model 24-2 400-word multichannel a n a l y s e r operating in m u l t i s c a l e r mode. A l s o included (Nuclear-Chicago modules) were a model 40-9B high v o l t a g e power supply, a model 23805 p r e a m p l i f i e r , a model 33-15 a m p l i f i e r - s i n g l e - c h a n n e l a n a l y s e r , a model 23-4 a n a l o g - t o - d i g i t a l c o n v e r t e r , and a model 021308 time-base generator. A schematic diagram of a t y p i c a l Mossbauer spectrometer i s shown in Figure 3. A l l s p e c t r a i n zero a p p l i e d f i e l d were obtained i n t r a n s m i s s i o n 57 geometry using a 25 mCi Co(Cu) source which was maintained at ambient temperature. C a r e f u l l y powdered samples, contained in a copper c e l l 2 with Mylar windows of c r o s s - s e c t i o n a l area 2.5 cm , were mounted i n a J a n i s Model DT-6 v a r i a b l e temperature c r y o s t a t . The c r y o s t a t was f i t t e d with a Cryogenic Research Model TC-IOI automatic temperature c o n t r o l l e r , FIGURE 3 Schematic Diagram of a T y p i c a l Mossbauer Spectrometer FIGURE 3 TRANSDUCER ABSORBER Y SOURCE DETECTOR PREAMPLIFIER AMPLIFIER PULSE HEIGHT ANALYZER MOSSBAUER DRIVE UNIT' COLD FINGER HIGH VOLTAGE POWER TIME BASE GENERATOR 400 WORD MULTICHANNEL ANALYZER DATA OUTPUT ITT~ DI SPLAY OSCILLOSCOPE 2 8 by means of which the temperature could be s e t and maintained constant t o w i t h i n ± 0.02° throughout the data a c q u i s i t i o n time. Temperatures were measured with c a l i b r a t e d Ge and Pt r e s i s t a n c e thermometers. The design of the c r y o s t a t (see Figure 4) i s such t h a t the sample i s cooled by exchange gas and i s not in d i r e c t contact with the l i q u i d helium bath. Thus the lowest sample temperature which could be achieved i n t h i s system was approximately 8°K. For s p e c t r a recorded above room temperature, the samples were mounted a t the top of a s o l i d copper rod, which was wrapped with heating tape powered through a v a r i a b l e transformer. Temperatures were monitored with a copper-constantan thermocouple and were found t o vary by less than ± 0.5° during a run, Mossbauer measurements in a p p l i e d l o n g i t u d i n a l magnetic f i e l d s of up t o 50 kG were c a r r i e d out in a J a n i s Model IIMDT helium c r y o s t a t f i t t e d with a Westinghouse superconducting s o l e n o i d . The v e r t i c a l l y 57 mounted Co(Cu) source was d r i v e n , v i a a long t h i n - w a l l e d s t a i n l e s s s t e e l d r i v e rod, by an A u s t i n Science A s s o c i a t e s K-3 l i n e a r motor (Figure 5 ) . The copper sample c e l l was f i t t e d i n t o a copper r i n g located at t h e centre of the a p p l i e d f i e l d . This r i n g was equipped w i t h a heater and a copper-constantan thermocouple. To o b t a i n s p e c t r a with the absorber a t 4.2°K, thermal contact with the l i q u i d helium r e s e r v o i r was provided by f i l l i n g the inner vacuum space shown in Figure 5 with helium exchange gas at a pressure s l i g h t l y less than one atmosphere. For para-magnetic samples i t was necessary i n many cases t o record magnetic p e r t u r -ba t i o n s p e c t r a with the absorber a t a temperature of 200°K o r hi g h e r , as discussed i n Chapter I. In order t o heat the samples t o high temperatures while m a i n t a i n i n g the superconducting s o l e n o i d a t 4.2°K, the sample FIGURE 4 Schematic Diagram of the Apparatus Employed f o r Obtaining V a r i a b l e Temperature Mossbauer Spectra 29b FIGURE 4 HELIUM INLET COMMON VACUUM TUBE FOR HELIUM EXCHANGE GAS SOURCE TO VACUUM LINE MYLAR WINDOW ALUMINIUM FOIL TO VACUUM LINE LIQUID NITROGEN RESERVOIR LIQUID HELIUM RESERVOIR DETECTOR COPPER ABSORBER CHAMBER FIGURE 5 Schematic Diagram of the Magnetic P e r t u r b a t i o n Apparatus 30 t> FIGURE 5 TO DRIVE UNIT TRANSDUCER COMPARTMENT TRANSDUCER TO VACUUM LINE 31 chamber was evacuated t o a pressure of less than 10 ^ t o r r . A f t e r t r a n s f e r of l i q u i d helium t o the system, cryopumping was s u f f i c i e n t t o prevent excessive heat t r a n s f e r from the sample t o the s o l e n o i d helium bath. The temperature of the sample was then monitored throughout the experiment with the thermocouple and was found t o vary by less than ± 2° d u r i n g a run. C a l i b r a t i o n of the Doppler v e l o c i t y s c a l e was a f f e c t e d a f t e r each experiment with e i t h e r a disodium pentacyanonitrosyI f e r r a t e ( I I ) absorber ( f o r low v e l o c i t i e s ) , o r a m e t a l l i c i r o n f o i l absorber ( f o r high v e l o c i t i e s ) . Isomer s h i f t s are quoted r e l a t i v e t o the c e n t r o i d of the disodium pentacyanonitrosyI f e r r a t e (I I ) (sodium n i t r o p r u s s i d e ) spectrum. For s p e c t r a obtained in the absence of an a p p l i e d f i e l d , the data p o i n t s were least-squares f i t t e d t o L o r e n t z i a n components using a programme based on one o r i g i n a l l y s u p p l i e d by the National Bureau of Standards. In t h i s programme the p o s i t i o n s , widths and i n t e n s i t i e s of the Mossbauer l i n e s are t r e a t e d as unconstrained f i t t i n g parameters. For some of the s p e c t r a discussed in Chapter IV which c o n s i s t e d of two strong and two weak a b s o r p t i o n s , i t was not p o s s i b l e t o f i t the weak l i n e s using unconstrained parameters. In these cases f o u r - l i n e f i t s could be obtained by c o n s t r a i n i n g the widths of the two weak l i n e s . T h e o r e t i c a l magnetic p e r t u r b a t i o n s p e c t r a used f o r comparison with the experimental s p e c t r a were generated by a programme s u p p l i e d by Dr. G. Lang'^. The magnitude and s i g n of the quadrupole c o u p l i n g constant 2 e qQ, and the magnitudes of the asymmetry parameter, l i n e w i d t h and ext e r n a l magnetic f i e l d are used as a d j u s t a b l e parameters in t h i s programme. 32 N.M.R. Spectra Proton nuclear magnetic resonance sp e c t r a were run on a Varian T-60 spectrometer w i t h chemical s h i f t s given in ppm downfield from i n t e r n a l TMS standard. Mass Spectra Mass s p e c t r a were measured with an AEI MS-9 spectrometer. 33 CHAPTER I I I ELECTRONIC GROUND STATES OF FOUR HIGH-SPIN FERROUS COMPLEXES Introdu c t i o n In g e n e r a l , quadrupo le s p I i t t i n g s ] A E Q | in octahedral h i g h - s p i n I 18 f e r r o u s complexes show a continuous v a r i a t i o n with temperature ' However, Fe(H20)g(CI0 4)2 was reported t o show anomalous behaviour i n i t s Mossbauer s p e c t r u m 1 9 . At II0°K | A E Q | = 3.4 mm s " 1 , a t 295°K the s p l i t t i n g was only 1.4 mm s ', and between 220-250°K fou r l i n e s were v i s i b l e i n the 19 spectrum. These r e s u l t s were i n t e r p r e t e d i n terms of a t e t r a g o n a l d i s -t o r t i o n with a x i a l compression and an lxy> o r b i t a l ground s t a t e a t low temperature, with a phase t r a n s i t i o n leading t o a x i a l e l o n g a t i o n and a doubly degenerate lxz>, lyz> ground s t a t e at high temperature. A sub-20 sequent study of t h i s compound using magnetic p e r t u r b a t i o n techniques i n d i c a t e d t h a t the quadrupole c o u p l i n g constant e 2qQ was negative at 5°K, so t h a t the low-temperature ground s t a t e i s l z 2 > r a t h e r than lxy>, and the d i s t o r t i o n i s t r i g o n a l r a t h e r than t e t r a g o n a l . However, the p r e d i c t e d ' 9 s i g n r e v e r s a l was c o n f i r m e d ^ , e 2qQ being p o s i t i v e a t 296°K. 21 Reedijk and van der Kraan have published Mossbauer data (at room temperature only) f o r f i v e s o l v a t e s of f e r r o u s p e r c h l o r a t e of the type F e L ^ C C ^ ^ , where L was e i t h e r a sulphoxide I igand o r p y r i d i n e -N-oxide. These data were i n t e r e s t i n g in t h a t f o r f o u r of the f i v e complexes | A E Q | * was about 1.5 mm s ', s i m i l a r t o the value found f o r 22 *The values given f o r A E Q in Table II of r e f . 21 are in f a c t ^ | A E N | . A c c o r d i n g l y , these values have been m u l t i p l i e d by two in t h i s t h e s i s . 34 the hexahydrate at the same temperature. On the other hand, f o r the dimethylsulphoxide d e r i v a t i v e |AE„| was reported t o be 2.56 mm s '. V 21 Reedijk and van der Kraan suggested t h a t the small s p l i t t i n g of about 1.5 mm s ' i n d i c a t e d 'hardly d i s t o r t e d ' octahedral c a t i o n s , w h i l s t the lar g e r value f o r the (CH^^SO complex showed a g r e a t e r d i s t o r t i o n in t h i s case. However, s i n c e d i s t o r t i o n s of comparable magnitude w i l l produce a q u a d r u p o l e s p l i t t i n g f o r an o r b i t a l s i n g l e t ground s t a t e which i s roughly t w i c e t h a t f o r a doublet s t a t e , the a l t e r n a t i v e e x p l a n a t i o n of a d i f f e r e n t o r b i t a l ground s t a t e in the (CH^^SO d e r i v a t i v e seemed t o be e q u a l l y p l a u s i b l e . A more e x t e n s i v e i n v e s t i g a t i o n of such f e r r o u s s o l v a t e s was thus of i n t e r e s t f o r several reasons. F i r s t l y , t h e re was the p o s s i b i l i t y t h a t a t low temperatures one might observe phase t r a n s i t i o n s of the type 19 reported f o r F e ( H 2 0 ) g ( C I 0 4 ) 2 . Secondly, a d e t a i l e d study of the temperature dependence of |AEQ| in these d e r i v a t i v e s , together with determinations of the si g n s of the efg's at i r o n , would enable one t o deduce the o r b i t a l ground s t a t e s and t o estimate the c r y s t a l f i e l d s p l i t t i n g parameters. T h i r d l y , s i n c e c r y s t a l f i e l d parameters can a l s o be estimated from the temperature dependence of the magnetic s u s c e p t i b i l i t y , i t was of i n t e r e s t t o compare the r e s u l t s of two independent e v a l u a t i o n s of these parameters. This Chapter describes the r e s u l t s of d e t a i l e d magnetic 57 s u s c e p t i b i l i t y and Fe Mossbauer measurements (the former between 80-300°K, the l a t t e r between 4.2-340°K) on four octahedral complexes of the type F e L 6 ( C I 0 4 ) 2 , where L = ( C H ^ S O (DMSO), (CgH^SO (DPSO), (CH 2) 4S0 (TMSO), and C ^ N O (PyNO). 35 P r e p a r a t i o n of the Complexes A l l chemicals used in the preparation of the complexes were of reagent grade and were used without f u r t h e r p u r i f i c a t i o n . The commercial sources of these chemicals are: Ferrous perch I o r a t e hexahydrate Dimethyl sulphoxide Di phenyl sulphoxide Tetramethylene sulphoxide Pyridine-N-oxide E t h y l orthoformate - A l f a Inorganics Inc. - F i s h e r S c i e n t i f i c Co, - Eastman Kodak Co. - A l d r i c h Chemical Co. 21 A l l f our compounds were prepared by the same procedure and a l l operations were c a r r i e d out under an atmosphere of dry n i t r o g e n : 1.81 gms (5.0 mmol) of f e r r o u s perch I o r a t e hexahydrate was d i s s o l v e d i n 15 ml of 100$ e t h a n o l , and 20 ml of e t h y l orthoformate was added. The s o l u t i o n turned brown immediately. A s o l u t i o n of the Iigand (35 mmol) i n 100$ ethanol (25 ml) was added s l o w l y with s t i r r i n g . The p r e c i p i t a t e which formed was f i l t e r e d through a s i n t e r e d g l a s s f u n n e l , washed with dry d i e t h y l e t h er se v e r a l times, and d r i e d i n vacuo. Results and D i s c u s s i o n A n a l y t i c a l and I.R. data f o r the complexes are given i n Table II Only t h e s t r u c t u r a l l y r e l e v a n t I.R. bands have been l i s t e d , and agreement 23—26 with p r e v i o u s l y published data i s g e n e r a l l y good. In a l l four compounds only the and bands of the CIO^ ion are observed, showing 36 t h a t the anions r e t a i n t e t r a h e d r a l symmetry and r u l i n g out the p o s s i b i l i t y of iron-perchI o r a t e c o o r d i n a t i o n . The E-0 s t r e t c h i n g frequencies (E = S, N) are some 30-60 cm ' lower in the complexes than in the f r e e I igands, which i n d i c a t e s t h a t the ligand molecules are coordinated t o i r o n 27 -1 through the oxygen atoms . For the PyNO complex v(Fe-O) i s seen a t 307 cm , and t h i s band appears a t about 400 cm ' in the sulphoxide d e r i v a t i v e s . 2+ There i s some u n c e r t a i n t y about the p o s i t i o n of v(Fe-0) in Fe(DPS0)g . 26 Prabhakaran and Pate I have assigned t h i s s t r e t c h t o a weak band a t 430 cm ', whereas f o r the other three complexes s t u d i e d here v(Fe-0) appears as a strong a b s o r p t i o n . However, the only other band i n the -I 2+ 600-250 cm region in Fe(DPS0)g not a t t r i b u t a b l e t o a ligand mode i s a strong band a t 260 cm ', which seems too low in comparison with v(Fe-0) f o r the other sulphoxide complexes. The assignment of v(Sn-0) i n DPSO 28 adducts with d i o r g a n o t i n h a l i d e s a l s o appears t o be u n c e r t a i n Data f o r the e f f e c t i v e magnetic moments V e f f appear i n Table I I I , and c l e a r l y i n d i c a t e t h a t the four complexes are hi g h - s p i n (S=2). A l l values f a l l in the r a t h e r narrow range 5.30-5.54 B.M., and there i s l i t t l e v a r i a t i o n amongst the fou r s o l v a t e s . This suggests, c o n t r a r y t o 21 the c o n c l u s i o n s of Reedijk and van der Kraan , t h a t the magnitudes of the a x i a l d i s t o r t i o n s in a l l the complexes are q u i t e s i m i l a r . Although the V e f f values do not have a pronounced temperature dependence, i t can be seen t h a t f o r the DMS0 and DPSO d e r i v a t i v e s u rr decreases smoothly e f t with decreasing temperature, w h i l s t the values f o r the TMSO and PyNO complexes increase i n i t i a l l y before showing a s l i g h t d e c l i n e a t low tem-perature. As w i l l be seen below, t h i s d i f f e r e n c e in the behaviour of as a f u n c t i o n of temperature presumably a r i s e s from the f a c t t h a t the two p a i r s of complexes (DMSO and DPSO on the one hand, TMSO and PyNO on TABLE I I A n a l y t i c a l Data and Important Ir Bands f o r FeL f i(CIO^^ Complexes % Calcd. % found v(E-O)-'- v(Fe-O)- CI0~ bands-^ i C H N Cl C H N Cl (cm - 1) (cm" 1) (cm"') 431s 1085 vs, br (CH 3) 2S0 19.90 4.98 0 9.83 19.79 4.94 0 9.76 990 vs, br 410s 616 vs 1082 vs, br ( C 6 H s ) 2 S 0 5 8 , 8 5 4 , 0 9 0 4 , 8 4 5 8 , 6 8 3 , 8 8 0 " 9 8 1 v s ' b r 4 1 9 w ( ? ) 614 vs 1084 vs, br (CH 2) 4S0 32.79 5.46 0 8.07 32.55 5.48 0 - 964 vs, br 393s 619 vs 1090 vs, br C 5H 5N0 43.67 3.64 10.20 8.60 43.45 3.45 10.34 8.40 1217s 307s 617 vs a E = S o r N b_ s = str o n g , w = weak, v = very, br = broad -^ 1 TABLE I I I E f f e c t i v e Magnetic Moments of the F e L 6 ( C I 0 4 ) 2 Complexes T(°K) . weff(B.M.) T(°K) y e f f (B.I FE(DMS0)6 (CL04)2 FE(TMS0)6(CL04)2 80.4 5.30 80 .7 5.39 97.0 5.33 103.8 5.42 112.0 5.31 123.9 5.43 125.1 5.34 143.9 5.45 140.1 5.32 165.2 5.45 153.5 5.35 186.6 5.44 165.5 5.35 205.7 5.45 180.4 5.36 224.0 5.47 196.5 5.36 244.1 5.47 213.0 5.38 263.3 5.45 229.9 5.38 284.8 5.45 246.6 , 5.38 307.2 5.41 262.3 5. 38 278 .1 5.39 FE(PYN0)6(CL04)2 295.0 5.40 81.2 5.51 309.5 5.42 90.2 5. 55 109.6 5.54 FE(DPS0)6(CL04)2 129.1 5.52 81.0 5.32 151.9 5. 53 92.2 5.33 173.7 5. 52 111.3 5.34 192.0 5.51 132 .0 5.32 214.2 5.50 153.6 5.33 237.1 5.46 174.0 5.36 256.7 5.44 193.0 5.34 280.5 5.42 209.7 5.37 302.6 5.41 227.7 5.38 249.8 5.38 272.1 5.38 300.4 5.41 39 the other) have d i f f e r e n t o r b i t a l ground s t a t e s . Mossbauer isomer s h i f t s and quadrupole s p l i t t i n g s , together with s p e c t r a l l i n e w i d t h s , are l i s t e d in Table IV. Results f o r the DMSO, TMSO, and PyNO complexes are in only moderately good agreement with those 21 of Reedijk and van der Kraan . (The DPSO d e r i v a t i v e was not reported in r e f . 21.) In p a r t i c u l a r , the 'room temperature' | A E Q | values reported 21 -I by these authors are between 0.08 and 0.16 mm sec s m a l l e r than values at 295°K l i s t e d in Table IV. The present measurements were repeated on several d i f f e r e n t samples of each compound and the r e s u l t s were a c c u r a t e l y r e p r o d u c i b l e . The 6 values in Table IV l i e w i t h i n the range normally observed f o r octahedral S=2 f e r r o u s complexes' and show l i t t l e v a r i a t i o n amongst the f o u r s o l v a t e s . This implies t h a t the extent of covalency of the Fe-0 bonds i s probably very s i m i l a r in a l l these d e r i v a t i v e s . The tem-perature dependence of the 6 values can be a t t r i b u t e d t o a second-order 29 Doppler s h i f t and w i l l not be considered f u r t h e r . The quadrupole s p l i t t i n g data show marked d i f f e r e n c e s , both in temperature dependence and in the magnitude at a given temperature. At 295°K the TMSO and PyNO complexes have s p l i t t i n g s of 1.64 mm s~', whereas f o r the other two d e r i v a t i v e s the s p l i t t i n g s are about 2.7 mm s '. The l a t t e r complexes a l s o show a more pronounced temperature dependence of | A E Q | , and i t i s c l e a r t h a t on the b a s i s of Mossbauer data the four compounds d i v i d e i n t o the same two p a i r s as noted above in connection with the magnetic moments. In no case was a c l e a r f o u r - I i n e spectrum observed, and there i s no i n d i c a t i o n in any of these complexes of the type of phase t r a n s i t i o n 19 reported f o r Fe(H 0 ) A ( C I 0 . ) 9 . However, at about I00°K, the l i n e s of 40 TABLE IV 57 rFe Mossbauer Parameters f o r the FeLgCCIO^^ Complexes _,0l.. - I . AE_(mm s ) r.(mm s ) r„(mm s ) T( K) 5(mm s ) Q I 2 FE(DMSO>6(CL04)2 7.9 1.64 3.22 .28 .28 15.0 1.64 3. 19 .27 .26 40.0 1.63 3.20 .26 .26 60.0 1.63 3. 19 .25 .25 81.8 1.63 3. 19 .27 -27 82.5 1.63 3. 18 .28 .28 100.0 1.63 3. 17 .27 .27 105.0 1.63 3. 17 .27 .27 115.1 1.63 3.17 .26 .26 131.0 1.61 3.15 .26 .26 160.0 1.60 3.12 .26 .26 190.1 1.58 3.08 .25 .25 220.0 1.57 3.02 .27 .25 250.0 1. 55 2.92 .25 .24 273.0 1.54 2.86 .26 .24 295.5 1.52 2.71 .23 .23 DPSQ)6(CL04)2 8.8 1.63 3.37 .27 .28 30.0 1.63 3.36 .26 .28 60.0 1.63 3.36 .28 .29 82.7 1.62 3.38 .28 .29 85.0 1.63 3.37 .28 .29 95.1 1.62 3.37 .28 .29 114.7 1.61 3.34 .27 .29 145.0 1.60 3.28 .28 .28 175.0 1.59 3.19 .27 .28 235.0 1.58 3. 10 .25 .26 235.0 1.56 2.99 .25 .25 265.0 1.55 2.87 .23 .24 294.8 1.51 2.68 .23 .24 315.5 1.50 2.58 .29 .30 331.0 1.49 2. 50 .32 .33 41 TABLE IV (Continued) -rrox/s r , - Ix AE.(mm s -') • r.(mm s ') r„(mm s ') T(°K) 6(mm s ) Q I 2 F E ( T M S 0 ) 6 ( C L 0 4 J 2 8 . 6 1 . 6 1 2 . 3 4 . 4 8 . 5 1 1 5 . 0 1 . 6 2 2 . 3 6 . 52 . 5 4 2 5 . 0 1 . 6 1 2 . 3 3 . 4 9 . 5 0 4 0 . 0 1 . 6 1 2 . 3 1 . 5 3 . 5 0 6 0 . 0 1 . 6 1 2 . 2 8 . 4 7 . 4 9 8 0 . 8 1 . 5 9 2 . 15 . 4 3 . 3 9 8 7 . 1 1 . 6 0 2 . 0 5 . 2 9 . 2 7 1 0 0 . 1 1 . 6 0 1 . 9 7 . 2 7 . 2 7 1 0 7 . 0 1 . 6 0 1 . 9 3 . 2 7 . 2 7 1 3 0 . 0 1 . 5 8 1 . 9 1 . 2 5 . 2 3 1 4 0 . 2 1 . 5 8 1 . 8 7 . 2 5 . 2 3 1 7 0 . 0 1 . 5 7 1 . 8 4 . 2 8 . 2 6 2 1 0 . 0 1 . 5 6 1 . 8 0 . 3 0 . 2 7 2 5 0 . 0 1 . 5 2 1 . 7 4 . 2 7 . 2 5 2 9 5 . 1 1 . 5 0 1 . 6 5 . 2 5 . 2 5 P Y N 0 ) 6 ( C L 0 4 ) 2 8 . 2 1 . 5 6 1 . 9 2 . 4 0 . 5 5 9 . 0 1 . 5 6 1 . 9 1 . 3 8 . 5 7 1 1 . 0 1 . 5 7 1 . 9 3 . 3 8 . 5 5 1 5 . 0 1 . 5 7 1 . 9 1 . 3 7 . 5 3 2 0 . 0 1 . 5 6 1 . 9 0 . 3 3 . 4 3 2 5 . 0 1 . 5 6 1 . 9 0 . 3 1 . 3 5 3 0 . 1 1 . 5 6 1 . 8 9 . 2 9 . 3 1 4 0 . 0 1 . 5 5 1 . 8 7 . 2 8 . 2 8 6 5 . 0 1 . 5 5 1 . 8 5 . 2 8 . 2 3 8 1 . 0 1 . 5 5 1 . 8 2 . 2 7 . 2 7 8 3 . 7 1 . 5 5 1 . 8 1 . 2 7 . 2 5 1 0 0 . 0 1 . 5 4 1 . 7 9 . 2 6 . 2 5 1 1 5 . 0 1 . 5 4 1 . 7 8 . 2 6 . 2 5 1 3 0 . 2 1 . 5 3 1 . 7 6 . 2 8 . 2 7 1 5 9 . 9 1 . 5 2 1 . 7 4 . 2 4 . 2 4 1 9 0 . 0 1 . 5 0 1 . 7 1 . 2 4 . 2 4 2 2 0 . 0 1 . 4 8 1 . 6 8 . 2 5 . 2 4 2 5 0 . 0 1 . 4 7 1 . 6 7 . 2 4 . 2 4 2 7 3 . 1 1 . 4 6 1 . 6 6 . 2 3 . 2 3 2 9 4 . 9 1 . 4 4 1 . 6 4 . 2 3 . 2 3 3 1 8 . 5 1 . 4 3 1 . 6 3 . 3 2 . 3 0 3 3 3 . 6 1 . 4 1 1 . 6 2 . 3 2 . 3 0 42 the TMSO spectrum s t a r t t o broaden, and at the same time | A E Q | increases from 1.97 mm s~' a t I00°K t o about 2.3 mm s"' a t 60°K, s t a y i n g a t t h a t value on f u r t h e r c o o l i n g . The reason f o r t h i s phenomenon i s not c l e a r . It i s p o s s i b l e t h a t below I00°K the TMSO spectrum i s in f a c t a four l i n e spectrum. This could a r i s e from two non-equivalent i r o n s i t e s having s l i g h t l y d i f f e r e n t e l e c t r i c f i e l d g r a d i e n t s . U n l i k e F e d ^ O ) ^ ( C 1 0 ^ ) 2 , however, the broadened spectrum does not r e s o l v e i n t o two narrow l i n e s agai even at temperatures as low as 8°K, which might suggest an incomplete phase change. The PyNO compound a l s o shows l i n e broadening, but only below about 30°K, and in t h i s case the broadened spectrum becomes h i g h l y asymmetric. T h i s e f f e c t i s a t t r i b u t e d t o slow spin-1 a f t i c e r e l a x a t i o n and w i l l be discussed i n d e t a i l i n the l a s t s e c t i o n of t h i s Chapter. O r b i t a l Ground States of the Complexes An octahedral c r y s t a l f i e l d s p l i t s the f e r r o u s 3d o r b i t a l s i n t o t r i p l y degenerate t ^ and double degenerate e^ subsets, with the t r i p l e t l y i n g lower by an energy lODq.. If the f o u r f o l d a x i s C^ i s taken as the q u a n t i z a t i o n a x i s (see Figure 6), the d o r b i t a l s transform as f o l l o w s : e g = l x 2 - y 2 > , l z 2 > + 2 g = l x y > ' l x z > ' l y z > On the other hand, i f one of the C^ axes ( l y i n g along the P , I , G d i r e c t i o n s of the octahedron) i s taken t o be the a x i s of q u a n t i z a t i o n , then in terms of the r e a l d o r b i t a l s one has i n s t e a d 3 ^ : 43a FIGURE 6 Q u a n t i z a t i o n Axes f o r an Octahedral C r y s t a l F i e l d 44 J 1 l x 2 - y 2 > + 2 3 lxz> /T"ixy> - /¥ lyz> I z 2> lxz> / ~ f " "xy> + fY "yz> An a x i a l f i e l d p a r t i a l l y l i f t s the degeneracy of the -t o r b i t a l s " 5 0 , s p l i t t i n g them i n t o a doublet and a s i n g l e t separated by 3Ds. In the t e t r a g o n a l (C^) case the s i n g l e t i s lxy>, and in the t r i g o n a l case i t i s l z 2 > . * In both i n s t a n c e s , i f the s i n g l e t l i e s lower the d i s t o r t i o n corresponds t o a compression along the q u a n t i z a t i o n a x i s , whereas a doublet ground term corresponds t o an e l o n g a t i o n along t h i s a x i s . If there i s a l s o a rhombic f i e l d , the remaining s p a t i a l degeneracy of the t 2 o r b i t a l s i s removed, the doublet being s p l i t by l2Dr. Each of the 3d wavefunctions a l s o has a f i v e - f o l d s p i n degeneracy which w i l l be s p l i t by the s p i n - o r b i t c o u p l i n g . For the moment t h i s l a s t f e a t u r e w i l l be ignored, and only the o r b i t a l p a r t s of the s t a t e s considered. * For a tet r a g o n a l f i e l d the e o r b i t a l s are a l s o s p l i t , but they remain degenerate in the case of a t r i g o n a l f i e l d . 45 The c o n t r i b u t i o n t o the efg from a s i n g l e e l e c t r o n in each of the d o r b i t a l s has been given in Table I. From t h i s t a b l e one can see t h a t f o r s i m i l a r values of Ds an o r b i t a l l y nondegenerate ground s t a t e ( e i t h e r lxy> o r lz 2>) should produce an a p p r e c i a b l y l a r g e r quadrupole s p l i t t i n g than t h a t f o r a doublet s t a t e . I t i s p o s s i b l e , of course, f o r an o r b i t a l s i n g l e t ground term t o produce only a small | A E Q | a t room temperature i f |3Ds|/k i s not much l a r g e r than 300°K, s i n c e t h i s would provide s i g n i f i c a n t thermal population of the doublet. However,|A E Q | would then be expected t o show a very pronounced increase on lowering the temperature, as the e l e c t r o n i s p r o g r e s s i v e l y l o c a l i z e d i n the 4 -3 s i n g l e t . F i n a l l y , s i n c e an efg of j e <r > i s expected t o y i e l d a quadrupole s p l i t t i n g of about 4 mm s~' ( d e t a i l s are given in the next s e c t i o n ) , experimental | A E Q | values s u b s t a n t i a l l y g r e a t e r than 2 mm s ' are only c o n s i s t e n t with a s i n g l e t ground s t a t e . From t h i s d i s c u s s i o n and the data in Table IV i t i s c l e a r t h a t 2+ 2+ f o r both the Fe(DMSO)fi and Fe(DPSO) f i complexes the ground s t a t e must 2+ 2+ be an o r b i t a l s i n g l e t , whereas Fe(TMSO)g and Fe(PyNO)g have doublet ground s t a t e s . To i d e n t i f y these s t a t e s more p r e c i s e l y we must know the signs of V z z , the p r i n c i p a l component of the efg t e n s o r . For a para-magnetic complex, however, there are c e r t a i n d i f f i c u l t i e s a s s o c i a t e d with 31 32 the usual magnetic p e r t u r b a t i o n method ' f o r determining the s i g n of V z z« As discussed in Chapter I, the e f f e c t i v e magnetic f i e l d H ^ at the nucleus, measured by the Mossbauer spectrum, can be very d i f f e r e n t from H b x ^ . i f the measurement i s made a t low temperature. The s i t u a t i o n i s f u r t h e r complicated by the f a c t s t h a t (a) both the magnetization and 2+ h y p e r f i n e f i e l d t ensors are a n i s o t r o p i c f o r h i g h - s p i n Fe , and (b) f o r a 46 p o l y c r y s t a I Iine sample the magnitude of the s p l i t t i n g s i s an average o v e r a l l p o s s i b l e o r i e n t a t i o n s of H ^ r e l a t i v e t o the z a x i s of the efg . These d i f f i c u l t i e s can be overcome by m a i n t a i n i n g the specimen at high temperature so t h a t the magnetization <S> produced by the a p p l i e d f i e l d i s n e g l i g i b l e and H ^ ^ e x+* ^he s i t u a t i o n i s then s i m i l a r t o 32 t h a t f o r a diamagnetic complex , where the l i n e of the quadrupole doublet which a r i s e s from the \±H> -+\±h> nuclear s p i n t r a n s i t i o n s s p l i t s i n t o an g e y apparent t r i p l e t , and t h a t from the \±h>^•\±^/'2>e t r a n s i t i o n s s p l i t s i n t o a doublet. To ensure t h a t the r e s u l t s would be unambiguous, determinations of the s i g n of V z z in a l l four complexes were made with the samples at 2I0°K or higher in a p p l i e d f i e l d s of 35-50 kG. In each case i t was found t h a t V z z>0. This i s an important and s u r p r i s i n g r e s u l t because i t shows there are two fundamentally d i f f e r e n t types of d i s t o r t i o n s from octahedral symmetry in these s o l v a t e s . For the DMSO and DPSO complexes the ground s t a t e i s lxy> and the d i s t o r t i o n i s a compression along the t e t r a g o n a l a x i s . For the TMSO and PyNO d e r i v a t i v e s the ground s t a t e i s e s s e n t i a l l y the doublet ( / | l x 2 - y 2 > -/I lxz», ( / | lxy> + / I lyz», and the d i s t o r t i o n corresponds t o an e l o n g a t i o n along the t r i g o n a l a x i s . The reasons f o r the occurrence of two d i s t i n c t types of d i s t o r t i o n s i n these complexes are not completely c l e a r , but may w e l l a r i s e from d i f f e r e n t s t e r i c requirements of the li g a n d s . The most obvious d i f f e r e n c e i s t h a t both TMSO and PyNO co n t a i n h e t e r o c y c l i c r i n g s which are 2+ only two bonds removed from the Fe ion in the complexes, whereas DMSO 47 and DPSO do not. Since Fe-O-N and Fe-O-S bond angles are both expected 33-35 to be roughly 120°, the presence of s i x r i n g s of s u b s t a n t i a l s i z e in c l o s e proximity t o the c e n t r a l ion should produce c o n s i d e r a b l e s t e r i c crowding. M o l e c u l a r models suggest t h a t t h i s i s indeed so. DMSO i s of course the l e a s t bulky of the four l i g a n d s , but even f o r the DPSO complex, s i n c e the phenyl r i n g s are one bond (about 1.8 °0 f a r t h e r away from the f e r r o u s ion than are the h e t e r o c y c l e s i n the PyNO and TMSO d e r i v a t i v e s , the s t r u c t u r e appears t o be less crowded. I t i s worth noting here t h a t the X-ray c r y s t a l s t r u c t u r e of ( C H ^ S n C I ^ P y N O " 5 3 shows the PyNO groups t o be t r a n s , whereas the DMSO ligands are c i s in (CH^J^SnCI.^DMSO" 5 4. C r y s t a l F i e l d , S p i n - O r b i t and Spin-Spin S p l i t t i n g Parameters 18 36 Both I n g a l l s and Gibb have t r e a t e d the e f f e c t s of c r y s t a l l i n e f i e l d s and s p i n - o r b i t coup Iing on the quadrupole s p l i t t i n g i n octahedral 2+ Fe systems, and we have followed these authors in general o u t l i n e . The e f f e c t of the non-cubic p a r t of the c r y s t a l f i e l d i s t r e a t e d i n terms of the p e r t u r b a t i o n Hamiltonian VT + VR + VS0 + VSS ( 4 I ) where V-j. i s the a x i a l ( t e t r a g o n a l o r t r i g o n a l ) f i e l d term, the rhombic term, V<,Q the s p i n - o r b i t c o u p l i n g , and V ^ g the i n t r a i o n i c s p i n -18 36 spin c o u p l i n g . The l a s t i e r m was omitted by both I n g a l l s and Gibb Its i n c l u s i o n s i g n i f i c a n t l y improves agreement between c a l c u l a t e d AE n values and the low-temperature data i n the 8-40°K r e g i o n . In f a c t , 4 8 f o r the range of a x i a l d i s t o r t i o n s encountered here, omission of V^^ from the Hamiltonian causes |A E Q | t o decrease r a t h e r than increase as the temperature i s lowered below ^80°K, contrary t o what i s observed experimentalIy. In o perator n o t a t i o n , eqn. (41) can be w r i t t e n a s 3 0 H = Ds(L 2 -2) + Dr(L? + L 2 ) - AL~L S + Z + 2 Z H(L+S_ + L_S+>3 - D a ( S 2 - 2) (42) where the L + and S + are r e s p e c t i v e l y o r b i t a l and s p i n angular momentum s h i f t o p e r a t o r s , Ds and Dr the a x i a l and rhombic f i e l d parameters, X and Da the s p i n - o r b i t and s p i n - s p i n c o u p l i n g c o n s t a n t s . In order t o choose t h e basis f u n c t i o n s on which t h i s 5 Hamiltonian a c t s , one considers f i r s t the D ground s t a t e term of the 2+ 5 f r e e Fe i o n . Due t o the f a c t t h a t the angular dependence of a D term i s e x a c t l y the same as t h a t of a s i n g l e d e l e c t r o n , the response t o an e x t e r n a l c r y s t a l f i e l d w i l l be the same, and thus the q u a l i t a t i v e arguements presented in the previous s e c t i o n based on a s i n g l e d 5 e l e c t r o n are completely v a l i d . S i m i l a r t o a d e l e c t r o n , t h i s D term 5 5 i s s p l i t by the c r y s t a l f i e l d i n t o two terms ( T 2g and E^ ) separated by lODq as shown i n Figure 7. 5 The term 1^ corresponds t o the d- e l e c t r o n arrangement 4 2 5 3 3 t„ e , w h i l e the E term corresponds t o t„ e . 2g g g r 2g g 21 O p t i c a l s p e c t r a of the present compounds show t h a t the c u b i c f i e l d s p l i t t i n g s lODq are in the range 9300-10,000 cm -'. Thus there w i l l 4 9 a FIGURE 7 S p l i t t i n g of the D Term of Fe by the C r y s t a l F i e l d 4 9 b FIGURE 7 50 5 5 be no app r e c i a b l e admixture of T„ and E terms under our 2g g experimental c o n d i t i o n s . Since only the ground s t a t e and the th e r m a l l y a c c e s s i b l e e x c i t e d s t a t e s (^200 cm ' higher) are of i n t e r e s t in the c a l c u l a t i o n of Mossbauer and magnetic parameters, i t i s q u i t e s u f f i c i e n t 5 t o i n v o l v e only the T~ term. Thus, in order t o lessen the computation 5 times r e q u i r e d , the E g term has been neglected (hence the absence of the cub i c f i e l d term V i n ^ C ) . This allows us t o t r u n c a t e the 25 x 25 o 5 matrix t o a 15 x 15 matrix which contains only the T„ s e t of o r b i t a l s . 2g For the t r i g o n a l l y d i s t o r t e d complexes, the b a s i s s e t of 5 15 T 0 wavefunctions used i s 2g |2,0>IMs> , (43) ( >/||2, ±2>+ / I|2,±l» |Ms> ( 4 4 ) where i n the |L,ML> | Mg> n o t a t i o n , M L i s the z component of the t o t a l o r b i t a l angular momentum L, and M g = 0 , ±1, ±2 i s the z component of s p i n angular momentum. For t e t r a g o n a l d i s t o r t i o n s the corresponding b a s i s s e t i s -L ( 2,2> - |2,-2> ) M > , ^2 S (45) 1 (|2,l> ± |2,-l> )|M > /2 S (46) 51 With these two b a s i s s e t s , the 15 x 15 Hamiltonian matrices can be set up as shown in Appendix I I . The q u a n t i t i e s D s A , Dr/X and Dcr/X were t r e a t e d as independent parameters which were read i n t o .the computer. The matrix was then d i a g o n a l i z e d t o o b t a i n the eigenvalues e./X and corresponding eigenvectors |i> such t h a t + cM s (V!i 2' 2 >- Vji^-'^iv] (47) in the t r i g o n a l case, and (48) in the t e t r a g o n a l case,where A^ e t c . are constants. These were subse-s quently used t o c a l c u l a t e the quadrupole s p l i t t i n g s and magnetic moments. The c o n t r i b u t i o n s t o the nine components V . (a,b = x,y,z) of 52 I 8 the efg tensor were c a l c u l a t e d by means of the formula _ 2 -3 [ 3 2T<r ^2 A A A V = - 9 T<r > r (L L. + L. L ) - « uL(L+l) (49) ab z > a b b a ab where l _ , L, are angular momentum op e r a t o r s . For each e i g e n s t a t e | i> a D the ensemble averages 15 Z I <!|V . / e l i>exp(-e /kT) (50) . u . 1 ab 1 i i = l were formed, where Z = \ exp(-£./kT) i s the p a r t i t i o n f u n c t i o n , and the i 1 efg matrix was d i a g o n a I i z e d . In the p r i n c i p a l a x i s system the quadrupole s p l i t t i n g can be w r i t t e n as L~see eqn.(30)3 AE Q = W 1+ C( l " R ) e q v A L E N C E + ( l - T j e q ^ , ^ (51) As discussed i n Chapter I, the major c o n t r i b u t i o n t o the efg at i r o n i n 2+ h i g h - s p i n Fe complexes comes from the asymmetric d i s t r i b u t i o n of 3d e l e c t r o n s . Thus t o a f i r s t approximation the small l a t t i c e c o n t r i b u t i o n 18 can be neglected . Secondly, s i n c e a l l s i x ligands are i d e n t i c a l the 2 3 e l e c t r o n d i s t r i b u t i o n in the d sp hy b r i d bonding o r b i t a l s should not depart s i g n i f i c a n t l y from s p h e r i c a l symmetry. Thus, we have approximated AE Q as AE Q = % e 2 q Q ( l - R ) ( l + ( 5 2 ) 53 where i t i s understood t h a t q now contains c o n t r i b u t i o n s from the t 0 o r b i t a l s o n l y . Rearranging, AE Q = Jse 2Q(l-R)Cq2+ i ^ q ) 2 ^ = J2e 2Q(l-R)4<r" 3>)(F 2 + ^ f 2 ) H (53) / q 3 nq where F q and F^ q are given in terms of the p r i n c i p a l components of the efg b y 1 8 F q = ( y <r~ 3>Z)~' I < i | V z z / e | i> exp(-e./kT) (54) F n q = 4<r" 3>Z)"' I < i | ( V x x - V y y ) / e | i > exp(-e./kT) (55) 2 — 3 18 37 The q u a n t i t y j e 2Q(l-R)<r~ > has been estimated * t o have a numerical value of about 4.1 mm s whence AE Q = 4 . K F 2 + i F 2 q ) ^ mm s" 1 (56) For v a r i o u s choices of the s p l i t t i n g parameters Ds, Dr, Da and X , eqn.(56) can be used t o generate curves of AEg as a f u n c t i o n of temperature, which can then be compared with experimental data. The parameter values obtained w i l l of course depend on the numerical f a c t o r in eqn. (56); the value of 4.1 mm s ' i s the one which has u s u a l l y been employed by other w o r k e r s . ' 8 , 3 6 ' 3 7 54 In order t o deduce c r y s t a l f i e l d parameters from the Mossbauer data v i a eqn. (56), one can proceed as f o l l o w s . For three of the compounds the s p e c t r a obtained i n a p p l i e d magnetic f i e l d s i n d i c a t e d t h a t n =0, so th a t the efgs in these cases have e f f e c t i v e l y a x i a l symmetry. For these, Dr was s e t t o zer o . (The exception was the TMSO complex which i s discussed below.) A l s o , the r e s t r i c t i o n 70 - X ^ 103 cm"' was imposed. T h e o r e t i c a l curves were p l o t t e d i n the form AE^ vs kT/X. Comparison of such curves, c a l c u l a t e d f o r several d i f f e r e n t ranges o f parameter v a l u e s , w i t h the experimental data then provided reasonable f i r s t estimates of Ds, Da and X. Since changes i n these three parameters a f f e c t the computed curves in r a t h e r d i f f e r e n t ways (see below and r e f . 3 6 ) , i t i s r e l a t i v e l y s t r a i g h t f o r w a r d t o decide the d i r e c t i o n i n which these f i r s t estimates should be v a r i e d , and a t t h i s p o i n t , l e a s t squares techniques were adopted t o r e f i n e the parameter v a l u e s . X was allowed t o vary i n steps of ±5 cm ', 3Ds i n steps o f ±10 cm"' and Da i n steps of ±2 cm Within these step l i m i t a t i o n s "best f i t " s e t s of Ds, Da, X values (as judged by standard d e v i a t i o n s , ) were obt a i n e d , and these appear in Table V . Figure 8 compares the experimental quadrupole s p l i t -t i n g data with t h e o r e t i c a l curves computed from these parameter values. For Fe(TMSO)g(CI0 4)2 the magnetic p e r t u r b a t i o n spectrum i n d i c a t e d t h a t n was q u i t e l a r g e , about 0.7, showing c l e a r l y the presence of a rhombic d i s t o r t i o n , so t h a t in t h i s case Dr i 0. Th i s might seem t o complicate matters somewhat, s i n c e as Gibb" 5 6pointed out, changes in Dr have r a t h e r s i m i l a r e f f e c t s on the c a l c u l a t e d curves as do changes in Ds. Indeed, . G i b b 3 6 has questioned whether one can make meaningful estimates of Dr from AEg data. What he f a i l e d t o c o n s i d e r , however, i s t h a t one now has an a d d i t i o n a l experimental q u a n t i t y t o hand, namely the value of n, 55 TABLE V C r y s t a l F i e l d Parameters Derived from Quadrupole S p l i t t i n g Data Compound 3Ds(cm~ ) l2Dr(cm ) A ( c n f 1 ) Da(cm~') b K— Fe(DMS0) c(CI0.) o o 4 2 -500 103 28 0.89 Fe(DPS0) c(CI0„) o o 4 2 -475 90 23 0.90 F e ( P y N O ) 6 ( C I 0 4 ) 2 -455 80 24 0.94 Fe(TMS0) c(CI0.) o 6 4 2 -440 -250 80 28 0.88 — O r b i t a l r e d u c t i o n f a c t o r d erived by f i t t i n g magnetic moment data, using Ds, A and Da values obtained from Mossbauer data. FIGURE 8 Comparison of Observed and C a l c u l a t e d Quadrupo S p l i t t i n g s as a Function of Temperature f o r th FeL^CIO^),, Complexes X CD Observed CaIcuIated 56 h Fe(DMSO)c(CI0.)„ D 4 z FIGURE 8 8 3 x 8 » 8 » 8 g F e ( D P S 0 ) c ( C I 0 / t ) o o 4 z 8 8 8 6 2 xx x Fe(TMSO)6(.CI04>2 8 » 8 F e ( P y N 0 ) 6 ( C I 0 4 > 2 8 • » • 8 8 " 8 I —I 1 1 1 1 1 1 1 1 1 35.0 70.0 10S.0 143.0 17S.0 2)0.0 24S.0 280.0 315.0 350 TEMP. (°K) 57 so t h a t one need only c o n s i d e r p a i r s of Ds and Dr values c o n s i s t e n t with both A E Q ( T ) and n. The c r y s t a l f i e l d parameters l i s t e d i n Table V f o r the TMSO d e r i v a t i v e were t h e r e f o r e obtained w i t h the a d d i t i o n a l c o n s t r a i n t 0.6 - l c a | c - 0.8. It should be noted t h a t only the high temperature p o r t i o n of the A E ^ vs T curve was f i t t e d , s i n c e as mentioned above there appears t o be anomalous behaviour of A E f o r t h i s compound at low temperature. The e f f e c t i v e magnetic moment can be c a l c u l a t e d from the eigenvectors and eigenvalues obtained above, using second-order p e r t u r -bation t h e o r y 3 ^ ' 3 8 . The s u s c e p t i b i l i t y x k i n a p a r t i c u l a r d i r e c t i o n (k = x,y,z) i s given by X | < = N Z"1 I ([W^jVkT - 2 w [ ^ e x p ( - e . / k T ) (57) where Wi|k =<»K|1> t (58) w!,k a sJ , l < M» ' k l J> l 2 /V ej ) ( 5 9 ) y k = - B(<L k + 2S k ) (60) N i s Avogadro's number,$ the Bohr magneton and K the o r b i t a l r e d u c t i o n 39 f a c t o r . (In h i g h l y covalent compounds K may be as small as 0.7, but in i o n i c complexes i s u s u a l l y c l o s e t o u n i t y . ) The corresponding magnetic moment in the k d i r e c t i o n i s 58 in u n i t s of Bohr magnetons, and the e f f e c t i v e moment i s then obtained as *eff = T K + tf + »l ) H ( 6 2 ) Owing t o the small temperature dependence of M ^ f o r a l l four complexes and t o the f a c t t h a t the measurements do not extend below 80°K, i t was c l e a r l y i m p r a c t i c a l t o t r y t o estimate a l I the q u a n t i t i e s Ds, Da,A a'nd.icfrom these data. Thus the values of Ds, Da and A obtained from f i t t i n g the Mossbauer r e s u l t s were employed t o f i t the P e f f vs T. data by a d j u s t i n g K . In each case the value of < s6 found i s about 0.9, which 39 seems very reasonable f o r complexes of t h i s type . I t can be seen from Figure 9 t h a t the f i t of the data i s c e r t a i n l y adequate, so t h a t the Veff values are f u l l y c o n s i s t e n t with the parameters l i s t e d i n Table V. Since the c r y s t a l f i e l d treatment t h a t has been employed here i s o n ly approximate, the derived parameters should be viewed a c c o r d i n g l y . However, several comments on the r e s u l t s are appropriate here. F i r s t l y , one sees t h a t the magnitudes of the a x i a l f i e l d s are very s i m i l a r f o r a l l f our s o l v a t e s d e s p i t e the f a c t t h a t the AEg values show marked d i f f e r e n c e s . This i s , of course, a consequence of the d i f f e r e n t o r b i t a l ground s t a t e s , but c l e a r l y i l l u s t r a t e s t h a t i t i s q u i t e i n a p p r o p r i a t e t o argue about the magnitude of the c r y s t a l f i e l d s p l i t t i n g i n a compound on the b a s i s of a s i n g l e measurement of | A E Q | . Secondly, with the exception of the DMSO complex, the A values are about 80-90$ of the f r e e ion value (A = 103 cm ' ) , suggesting a 39 s l i g h t d e r e a l i z a t i o n of the f e r r o u s 3d e l e c t r o n s onto the I igands A s a t i s f a c t o r y f i t of the data f o r Fe(DMS0) 6(CI0 4> 2 could not be obtained FIGURE 9 Comparison of Observed and C a l c u l a t e d E f f e c t i v e Magnetic Moments as a Function of Temperature f o r the Fel _ , ( C I 0 . ) 9 Complexes X Observed CD C a l c u l a t e d B B B B B B R R R B B R R B. B 8 Fe(DMS0)6(CI04>2 $ 8 B $ B B B B B B B Fe(DPSO)6(CI04)2 g B B R R B R 8 8 B 8 Fe(TMSO)6(CI04)2 g g B R 8 - ft ft R B 8 $ © Fe(PyN0)6(CI04>2 • i i i 1 1 1 1 1 r 0-0 35.0 70.0 105.0 140.0 175.0 210.0 245.0 280.0 315.0 TEMPERATURE (°K) C D c m VO 60 using a X value less than 100 cm ', which implies very l i t t l e i f any metal ligand back ir donation in t h i s case. This may be due t o an absence of low-lying ir-acceptor o r b i t a l s in DMSO. The s p i n - s p i n coupling term Da(S 2 -2) was introduced here t o account f o r the low temperature behaviour of AEg. An examination of Gibb's r e s u l t s 3 6 in which t h i s term was omitted, shows t h a t f o r the parameter ranges 100 cm -' * Ds * 300 cm ' and 60 cm -' * X * 100 cm"', which are appr o p r i a t e here, the AEg vs T curves e x h i b i t maxima, and a d e c l i n e in AEg a t low temperature i s p r e d i c t e d . This behaviour i s not observed f o r any of our four complexes. Rather, i t i s found t h a t below 80°K AEg i s e f f e c t i v e l y constant f o r DMSO and DPSO d e r i v a t i v e s and continues t o increase f o r the TMSO and PyNO complexes. This temperature dependence cannot be d u p l i c a t e d in the t h e o r e t i c a l curves unless the Da term i s inc l u d e d . The f a c t t h a t the magnetic p e r t u r b a t i o n spectrum of Fe(PyN0)g-( C I O ^ ^ g i v e s no i n d i c a t i o n of a non-zero n (see Figure 10) r a i s e s an i n t e r e s t i n g q u e s t i o n . Since Kramer's theorem does not apply t o even-electron systems, a non-degenerate o r b i t a l ground s t a t e i s demanded by the J a h n - T e l l e r 40 p r i n c i p l e . Furthermore, s i n c e n e i t h e r s p i n - o r b i t nor s p i n - s p i n c o u p l i n g l i f t s the o r b i t a l degeneracy, i t i s t h e r e f o r e deduced t h a t unless the ground s t a t e i s e i t h e r lxy> o r lz 2> t here should be a Jahn- T e l l e r - i n d u c e d rhombic d i s t o r t i o n t o produce a s i n g l e t ground s t a t e . T h i s should lead t o a no n - a x i a l I y symmetric efg and a non-zero n. Since n values of 0.2 or less have almost no observable e f f e c t on the t r i p l e t - d o u b l e t Mossbauer spectrum 3, i t seems more l i k e l y t h a t Dr i s non-zero but j u s t too small t o be detected, than t h a t a v i o l a t i o n of the J a h n - T e l l e r p r i n c i p l e has been observed. Another p o s s i b i l i t y i s t h a t although n i s apparently zero a t 61a FIGURE 10 Mossbauer Spectra in Lo n g i t u d i n a l A p p l i e d Magnetic F i e l d s : (a) Fe(DPS0),(CI0.) o at 220° K and O 4 2 H e x t = 5 0 k G ; ( b ) p e ( P y N O ) 6 ( C I 0 4 ) 2 a t 230° K 2 and H g x t = 35 kG. In Both cases e qQ>0 and n - 0. FIGURE 10 1 0 0 4 9 9 4 X 98H o C O C O C O < I— L U . ^ 1 0 0 4 L U 95i 9 0 H 0 0 ' o Fe(DPS0)&(a04)2> o 09 o e> o o-* <$> o OpO oo o o o O Q O O O O T= H= 50KG-© v 'o a o © o. o o -2. o +2 o a o o o o o o Fe(PyN0)6(Cl04)2 T=230K H= 35 kG: S» co-ot. -2 i— 0 +2 VELOCITY (mms-') 62 230°K where the magnetic p e r t u r b a t i o n spectrum was obtained, i t may be temperature dependent and non-zero at 4.2°K. (Temperature dependent n 41 42 values have been observed before. ' ) However, r e s u l t s presented below show t h a t n cannot be as large as 0.1 even a t 4.2°K, which allows us t o set an upper l i m i t of about 15 cm ' f o r the J a h n - T e l l e r s p l i t t i n g of the o r b i t a l doublet ground s t a t e i n F e ( P y N 0 ) g ( C I 0 4 ) 2 . Slow S p i n - L a t t i c e R e l a x a t i o n and Paramagnetic Hyperfine S p l i t t i n g In the absence of coo p e r a t i v e e f f e c t s , s p i n - l a t t i c e r e l a x a t i o n 2+ 18 of Fe i s u s u a l l y extremely r a p i d . I n g a l l s has estimated the 2+ r e l a x a t i o n time f o r Fe i n an approximately octahedral environment as -9 -1 I about 10 - 10 sec, s i g n i f i c a n t l y s h o r t e r than the nuclear Larmor precession time (MO ^ sec.) Thus, i f a fer r o u s complex i s s t i l l para-magnetic down t o very low temperatures, i t s Mossbauer spectrum i s expected t o remain a sharp doublet, and in the absence of an a p p l i e d f i e l d , no magnetic h y p e r f i n e s t r u c t u r e w i l l be o b s e r v e d . T y p i c a l of such f a s t - r e l a x i n g paramagnets•are the Fe(D M S 0 ) 6 ( C I 0 4 ) 2 and F e ( D P S 0 ) 6 ( C I 0 4 ) 2 complexes. As shown in Figure II the s p e c t r a of both compounds are sharp quadrupole doublets down t o about 8°K (they are s t i l l sharp a t 4.2°K), and there i s no evidence of 20 paramagnetic h y p e r f i n e s p l i t t i n g . S i m i l a r r e s u l t s were reported f o r F e ( H 2 0 ) 6 ( C I 0 4 ) 2 at 5°K. However, very d i f f e r e n t behaviour i s observed f o r F e ( P y N 0 ) 6 ( C I 0 4 ) 2 , where below 30°K the l i n e s broaden asymmetrically (see Figure 12). There are several p o s s i b l e mechanisms f o r asymmetric l i n e broadening i n Mossbauer s p e c t r a , a l l but one of which can be r e j e c t e d i n 63a FIGURE Z e r o - F i e l d Mossbauer Spectra of FeCDMSCOgCCI0^)2 and F e ( D P S 0 ) 6 ( C I 0 4 ) 2 , Showing the Absence of Line Broadening a t Low Temperatures. FIGURE I I o C O C O CO -z. < CC 0 o ° ° ° ° » O ^ x ^ o * ° ° ° o o # ^ ° o » o o « oo °> °° ° «°° e T = 8 1 . 8 K 0 o o o 0 0 • * o o o o • o ADO .« o oSj, o o. > „° ° j r ^ o o # * * • . -aOa . 0° °» < * o o o T = 7 . 9 K < b ° o » o « 0 0 o ° ° * o ° o e o o • • • • 0 o « o Fe(DMS0) 6(Cl0 4) 2 e ° e » e ? o C O C O C O z: < cr r— T=82.7K °. f % ° o o o „ o e • o OO a . & O T=8.8K \ . K > o °J 0 0 0 o o o o > Fe(DPS0) 6(Cl0 4) 2 o o o OB o o o o o •A r -2 0 2 4 VELOCITY (mms-i) FIGURE 12 Mossbauer Spectra of F e ( P y N 0 ) 6 ( C I 0 ^ 2 Between 30.1 and 8.2° K, Showing the Asymmetric Line Broadening Observed a t Low Temperatures. FIGURE 12 0 +2 VELOCITY (mms-') 65 the present case. 43 (1) P r e f e r e n t i a l o r i e n t a t i o n of the c r y s t a l l i t e s : This p o s s i b i l i t y can be e l i m i n a t e d a t once, s i n c e the r e s u l t i n g asymmetry should appear at higher temperatures as w e l l . Moreover, the samples were thoroughly ground before o b t a i n i n g the s p e c t r a , and d i f f e r e n t samples of the compound showed i d e n t i c a l behaviour. 44 45 (2) G o l ' d a n s k i i - K a r y a g i n asymmetry ' : This e f f e c t a r i s e s from an anisotropy of the r e c o i l - f r e e f r a c t i o n , and has a temperature dependence opposite t o t h a t observed here. That i s , the spectrum i s symmetric o r nearly so a t low temperature, but becomes asymmetric as the temperature increases. Moreover, i n t h i s case although the i n t e n s i t y r a t i o v a r i e s with temperature, the widths of the two l i n e s are not a f f e c t e d . 46-48 3+ (3) A n t i f e r r o m a g n e t i c exchange c o u p l i n g : With c e r t a i n Fe magnetic dimers the exchange i n t e r a c t i o n i s s u f f i c i e n t l y strong so t h a t only the diamagnetic ground s t a t e i s populated a t 4.2°K, but weak enough t h a t higher s t a t e s can be populated on r a i s i n g the temperature. In such cases the Mossbauer spectrum i s symmetric a t 4.2°K and becomes asymmetric with i n c r e a s i n g temperature as f l u c t u a t i o n s of the e l e c t r o n s p i n s become p o s s i b l e . Not only is the temperature dependence of t h i s asymmetry con t r a r y t o our o b s e r v a t i o n s , but such exchange c o u p l i n g would be h i g h l y 2+ u n l i k e l y with s i x bulky ligands surrounding the Fe i o n . 49-51 (4) Slow s p i n - s p i n r e l a x a t i o n between Kramers doublets : T h i s e f f e c t i s q u a l i t a t i v e l y s i m i l a r to the magnetic dimer case, and again t h e asymmetry should increase as the temperature i s r a i s e d from 4.2°K. 2+ Moreover, Fe i s not a Kramers i o n , so t h i s e x p l a n a t i o n can be di s c a r d e d . 66 (5) Two i n e q u i v a l e n t s i t e s f o r the F e Z + ions: The p o s s i b i l i t y of having two s i t e s with s l i g h t l y d i f f e r e n t isomer s h i f t s and quadrupole s p l i t t i n g s below 30°K cannot be excluded r i g o r o u s l y a t t h i s p o i n t . Such a s i t u a t i o n might a r i s e e i t h e r from an isomeric conversion o r incomplete phase t r a n s i t i o n . However, t o o b t a i n the smooth v a r i a t i o n in s p e c t r a l shape seen i n Fi g u r e 12 would r e q u i r e t h a t the s i t e p o p u l a t i o n s , 6 and | A E Q | a l l change c o n t i n o u s l y w i t h temperature. Magnetic p e r t u r b a t i o n s p e c t r a obtained a t 4.2°K (see below) a l l o w us t o r e j e c t t h i s p o s s i b i l i t y , 2+ s i n c e they correspond t o a system in which there i s only one Fe s i t e . (6) Slow s p i n - l a t t i c e r e l a x a t i o n ' 2 ' " ^ : The s p i n - l a t t i c e r e l a x a t i o n time i s temperature dependent, and increases with decreasing temperature. If the f l u c t u a t i o n s of the e l e c t r o n i c s p i n s are not f a s t compared t o the nuclear precession frequency, asymmetric l i n e broadening w i l l occur, with the asymmetry i n c r e a s i n g as the temperature i s lowered. Thus, the only e x p l a n a t i o n c o n s i s t e n t w i t h the s p e c t r a shown in Figure 12 i s an increase in the s p i n - l a t t i c e r e l a x a t i o n time a t low temperature, leading t o an onset of paramagnetic hyp e r f i n e s p l i t t i n g . T h is appears t o be the f i r s t example of t h i s e f f e c t i n an approximately 2+ octahedral f e r r o u s complex. The only other cases of s l o w - r e l a x i n g Fe 52 53 reported thus f a r are i n the mineral g i l l e s p i t e (BaFeSi^OjQ) ' and 15 the t e t r a k i s ( 1,8-naphthyr i di ne) complex FeCCgHgN^^tClO^^. In the former, the f e r r o u s ions are i n a square planer environment of oxygens, w h i l s t i n the l a t t e r they are o c t a c o o r d i n a t e , both arrangements being 2+ q u i t e unusual f o r Fe . 54 55 From G r i f f i t h ' s r e s u l t s ' f o r even-electron systems, one expects t h a t in the PyNO complex the l i n e of the quadrupole doublet 67 a r i s i n g from the \±h> -*• \±-k> nuclear s p i n t r a n s i t i o n s w i l l broaden • g 2 e v before the l±%> -*- \±h> l i n e as the r e l a x a t i o n r a t e decreases. This i s g e because the lowest s p i n - o r b i t - s p l i t s t a t e i s a doublet which has an 55 56 e f f e c t i v e h y p e r f i n e f i e l d p a r a l l e l t o the t r i g o n a l z a x i s ' . As can be seen from Figure 2 f o r 0 = 0 , i f the f l u c t u a t i o n s are s u f f i c i e n t l y slow so t h a t t h i s e f f e c t i v e f i e l d i s not time-averaged t o z e r o , the \±?r > s t a t e w i l l be s p l i t by 3a whereas the \±h, > s t a t e i s s p l i t only by an amounta (a= g ( 8 H e ^ ) . The magnetic p e r t u r b a t i o n spectrum shown in Figure 10 confirms t h a t the broad h i g h - v e l o c i t y l i n e corresponds t o the \±h > -»• l±5- > t r a n s i t i o n s , g 2 e When the compounds Fe(DMS0) g(CI0^ 2, Fe(DPS0)g(CI0 4) 2 and Fe(PyNO)g(CI0 4) 2 are placed i n a p p l i e d magnetic f i e l d s a t 4.2°K, the Mossbauer s p e c t r a obtained are completely d i f f e r e n t from the magnetic p e r t u r b a t i o n s p e c t r a a t high temperatures. (No attempt was made t o study the TMSO complex i n a p p l i e d f i e l d s a t low temperature because of the added c o m p l i c a t i o n s of a large n value and the p o s s i b i l i t y of more 2+ than one s i t e f o r the Fe ion i n t h i s case.) Some of the r e s u l t s are shown in Figures 13-15. These s p e c t r a are very complex, c o n s i s t i n g sometimes of seven o r more l i n e s , and i t i s o b v i o u s l y not p o s s i b l e t o f i t them t o a 'normal' t r i p l e t - d o u b l e t p a t t e r n . Furthermore, the s p e c t r a are s t r o n g l y dependent on the magnetic f i e l d s t r e n g t h , and f o r a given f i e l d show marked d i f f e r e n c e s from compound t o compound. Due t o the complexity and s e n s i t i v i t y of these s p e c t r a , t h e o r e t i c a l i n t e r p r e t a t i o n could p o t e n t i a l l y provide a wealth of information concerning minute d e t a i l s of the e l e c t r o n i c s t a t e s i n these paramagnetic systems. For the same reason, the i n t e r p r e t a t i o n of these spectra i s not l i k e l y t o be easy, and i t i s a l s o l i k e l y t h a t each FIGURE 13 Mossbauer Spectra of Fe(DMSO) 6(CI0 4) 2 a t 4.2° K in A p p l i e d Magnetic F i e l d s . From top t o bottom the F i e l d s are 3.4, 10, 30 and 50 kG, r e s p e c t i v e l y . FIGURE 13 c Q CO CO C O c E % o .* • * • • •-: v • * # a . tf ~ I 1 1 1 1 1 1 -6.0 -4.0 -2.0 0.0 2.0 4.0 6.0 8.0 68 b Velocity (mm/sec) FIGURE 14 Mossbauer Spectra of F e C D P S O y c i O ^ a t 4.2° K in A p p l l e d Magnetic F i e l d s . From top t o bottom the f i e l d s are 5.6, 10, 35 and 50 kG, r e s p e c t i v e l y . FIGURE 14 • • • • • . o • • • > • • — . CO £ v CO • m • * • • • • 9 • O i 1 1 1 1 1 r 1 -6.0 -4.0 -2.0 0.0 2.0 4.0 6.0 8.0 69b Velocity (mm/sec) 70a FIGURE 15 Mossbauer Spectra of F e ( P y N 0 ) 5 ( C I 0 4 ) 2 a t 4.2° K in A p p l i e d Magnetic F i e l d s . From top t o bottom the f i e l d s are I . I , 2.3, 5.0 and 30 kG, r e s p e c t i v e l y . 70b FIGURE 15 > • # • c o CO CO '£• CO c E «5* V V . - . ^ V A * —i 1 1 1 1 1 1 1 -6.0 -4.0 -2.0 0.0 2.0 4.0 6.0 8.0 Velocity (mm/sec) 71 d i f f e r e n t case w i l l have t o be t r e a t e d i n d i v i d u a l l y . Energy l e v e l schemes obtained from the c r y s t a l f i e l d c a l c u -l a t i o n s above are shown in Figure 16 f o r the DMSO and PyNO complexes. (The scheme f o r L = DPSO i s q u i t e s i m i l a r t o the DMSO case.) One of the 2+ most i n t e r e s t i n g f e a t u r e s of t h i s diagram i s t h a t f o r FetDMSO)^ the ground s p i n - o r b i t m u l t i p l e t i s spread over only 14 cm ' with the two -1 2+ lowest s t a t e s s p l i t by only 2 cm , w h i l e f o r FeCPyNO)^ the ground doublet l i e s 110 cm ' below the next higher s p i n - o r b i t - s p l i t s t a t e . Thus i n the former case there w i l l be s i g n i f i c a n t p o p u l a t i o n of both the two lowest l e v e l s a t 4.2°K, whereas in the l a t t e r only the ground s t a t e w i l l be occupied. The f a c t t h a t in the PyNO complex the lowest s p i n - o r b i t doublet i s well separated from any other s t a t e suggested t h a t i t might be p o s s i b l e t o t r e a t t h i s system approximately as a Kramers doublet, using the s p i n 57 Hamiltonian formalism . The Hamiltonian used i n t h i s approximation i s Si= B H - g . S + (^r^ 2-) C l 2 - £ + . ! < I 2 - I 2 ) ] r K — = — 4 z 4 3 x y + I . A - S - g B I • H (63) — = — 3 n n — — where the f i r s t term i s the e l e c t r o n i c Zeeman i n t e r a c t i o n , the second the nuclear quadrupole i n t e r a c t i o n , the t h i r d the magnetic i n t e r a c t i o n between the e l e c t r o n s p i n and the nucleus, and the f o u r t h the d i r e c t nuclear Zeeman i n t e r a c t i o n with the e x t e r n a l f i e l d . The g, A and efg tensors are a l l assumed t o have the same p r i n c i p a l axes. We have attempted t o f i t the 10 kG a p p l i e d f i e l d spectrum of F e ( P y N 0 ) 6 ( C I 0 4 ) 2 in both the slow and f a s t r e l a x a t i o n l i m i t s , and the procedure i s FIGURE 16 Energy Level Diagrams f o r Fe(DMS0)g(CI0 4) 2 and Fe(PyNO) 6(CI0 4> 2 Derived from the C r y s t a l F i e l d Model, Showing the E f f e c t s of the A x i a l F i e l d and Sp i n - O r b i t Coupling. Fe(PyNO) 6(CI0 4) 2 E x c i t e d S t a t e | z 2> Ground St a t e ^ | \ x 2 - y 2 > -Vl | x y > + $ Fe(DMS0K(CI0.) o D 4 2 E x c i t e d S t a t e |xz> |yz> Ground State ]xy> FIGURE 16 73 b r i e f l y o u t l i n e d . Since the e l e c t r o n i c Zeeman term in eqn.(63) i s l a r g e r than the remaining terms, the f i r s t step i s t o use the Hamiltonian oK= 8 H_.g_.Si (64) to f i n d the " e f f e c t i v e s p i n s " <S_> and f o r both members of the Kramers doublet, when the e x t e r n a l f i e l d H makes angles G u, <J>U t o the — n H z a x i s of the e f g . In the slow r e l a x a t i o n l i m i t , the i n t e r n a l f i e l d due t o the lower member of the doublet i s given by Hint = -4'^- > / 9n 3n <65> so t h a t the l a s t two terms in eqn.(63) become I.A'S - g B I-H = - g B I'(H. + H) — = — n n • n n — —i nt — " 9 n e n I- iJeff (66) An i d e n t i c a l procedure i s used t o o b t a i n H ,, f o r the upper member of C T T the doublet. Spectra are then computed f o r each s t a t e by Lang's programme'^ as described i n Chapter I I , but using the H^^ values instead of the a p p l i e d f i e l d . The thermal average of these two s p e c t r a ( i . e . , weighted by the a p p r o p r i a t e Boltzmann f a c t o r s ) then g i v e s the composite spectrum in the slow r e l a x a t i o n l i m i t , f o r a given d i r e c t i o n of __ r e l a t i v e t o the z a x i s of the e f g . The powder averaged spectrum i s obtained by. i n t e g r a t i n g over a l l p o s s i b l e o r i e n t a t i o n s (0^, <j>^) of H_ 74 as discussed i n Appendix I. In the f a s t r e l a x a t i o n l i m i t , once the " e f f e c t i v e s p i n s " have been obtained one computes the th e r m a l l y averaged s p i n of the Kramers doublet, given by _ <S_> exp(-g3H/2kT) + <S+> exp(+g3H/2kT) _s = : ; exp(-g3H/2kT) + exp(+g3H/2kT) (67) H_.n^ . i s now c a l c u l a t e d as in eqn.(65), but using S[ instead of <S+>, and the r e s u l t i n g i s employed i n Lang's programme'^ t o compute the spectrum f o r a p a r t i c u l a r d i r e c t i o n of jH and then the powder averaged spectrum. Due t o the f a c t t h a t n = 0 f o r t h i s complex i t was assumed in the pa r a m e t e r i z a t i o n t h a t g = g = q and A = A = A,. The r • x • s y •. a j_ . . x y -»• components of the £ and A^  t e n s o r s , g , g ( ( (= g^) and A ±, A(, (=Az> were then v a r i e d i n order t o f i t the measured 10 kG magnetic p e r t u r b a t i o n spectrum. A s a t i s f a c t o r y f i t was achieved i n the slow r e l a x a t i o n I i m i t with the parameters: g x = 1.0, g ( | = 7.0, A x = I .4 mm s ' and A|( = -1.9 mm s~'. The f i t t e d spectrum i s shown in Figure 17. The agreement between the t h e o r e t i c a l and experimental s p e c t r a i s q u i t e good c o n s i d e r i n g the approximate nature of the model used i n the c a l c u l a t i o n s . In the process of comparing experimental and t h e o r e t i c a l s p e c t r a , several i n t e r e s t i n g observations were made. F i r s t l y , i t was found t h a t the shape of the computed spectrum i s very s e n s i t i v e t o the FIGURE 17 The 10 kG Magnetic P e r t u r b a t i o n Spectrum of F e ( P y N 0 ) 6 ( C I 0 4 ) 2 at 4.2° K. The S o l i d Line i s the T h e o r e t i c a l Spectrum C a l c u l a t e d in the Spin Hamiltonian Approximation. 75 b FIGURE 17 76 values chosen f o r Aj_ and A(| , p a r t i c u l a r l y the l a t t e r , although not very s e n s i t i v e t o a change of s i g n of A|, . (This parameter i s t e n t a t i v e l y assigned t o be negative s i n c e a s l i g h t l y b e t t e r f i t was obtained i n t h i s case.) On the other hand the sp e c t r a were f a i r l y i n s e n s i t i v e t o the g values assumed, as long as they were h i g h l y a n i s o t r o p i c . Secondly, the very important f a c t emerged t h a t s p e c t r a c a l c u l a t e d on the assumption of f a s t s p i n r e l a x a t i o n between the two members of the doublet ( i . e . , using a time-averaged H ^ ) do not resemble the experimental spectrum f o r any combination of the'parameter values. Slow e l e c t r o n i c r e l a x a t i o n i s t h e r e f o r e confirmed f o r t h i s system. T h i r d l y , an attempt was made t o f i t the spectrum with a non-zero asymmetry parameter by s e t t i n g n = 0.1 in eqn.(63). Even t h i s small n value (which corresponds t o a rhombic d i s t o r t i o n of 15 cm ') caused s i g n i f i c a n t changes in the computed spectrum, and agreement with the measured spectrum was worse than with n = 0. Th i s s e n s i t i v i t y of the sp e c t r a t o small values of n i s q u i t e d i f f e r e n t from the usual diamagnetic c a s e 3 . F o u r t h l y , i t was not p o s s i b l e t o produce s p e c t r a t h a t resemble the experimental r e s u l t s f o r e i t h e r the DMSO o r DPSO complexes using t h i s type o f ground s t a t e , in e i t h e r the slow o r f a s t r e l a x a t i o n l i m i t s . Because of the small s p l i t t i n g s of the ground s p i n - o r b i t m u l t i p l e t s in these complexes, a more e l a b o r a t e treatment w i l l be needed t o e x p l a i n the observed s p e c t r a . From the g and A values obtained i t i s seen t h a t the z a x i s of the efg t e n s o r i s the easy a x i s of magnetization, with |A„| > |AjJ. The h i g h l y a n i s o t r o p i c g values a l s o r e f l e c t the f a c t t h a t the i n t e r n a l f i e l d i s strong along the e f g z a x i s . An a n i s o t r o p i c g_ t e n s o r i s 2+ expected f o r high-spin Fe systems because the ground s t a t e doublet i s not of pure s p i n c h a r a c t e r . The ex t e n s i v e mixing of o r b i t a l c h a r a c t e r i n t o the doublet imposes s p a t i a l dependences on the g and A values as observed. 78 CHAPTER IV Compounds Showing High-Spin - Low-Spin Crossover  I n t r o d u c t i o n It has long been p r e d i c t e d t h e o r e t i c a l l y , t h a t depending on the s t r e n g t h of the c r y s t a l f i e l d some t r a n s i t i o n metal ions can e x i s t in e i t h e r of two ground s t a t e s , commonly known as the " h i g h - s p i n " and 58 " l o w - s p i n " s t a t e s . The Tanabe-Sugano diagram (Figure 18) shows term s p l i t t i n g s f o r a d 6 e l e c t r o n system, such as a f e r r o u s i r o n , i n an octahedral c r y s t a l f i e l d . When the c r y s t a l f i e l d i s weak, i . e . lODq/B i s small (where B i s the i n t e r e l e c t r o n i c r e p u l s i o n parameter), i t i s 5 seen t h a t the ground s t a t e i s T^ which corresponds t o the arrangement 4 2 ^2g eg> with four unpaired e l e c t r o n s per f e r r o u s i o n . For the strong f i e l d case, i t i s seen t h a t the ground s t a t e i s 'A| with no unpaired • . +6 0 e l e c t r o n s , i . e . , t 0 e . 2g g For intermediate c r y s t a l f i e l d s where the energies of these two s t a t e s are s i m i l a r , i t should be p o s s i b l e f o r both forms t o c o e x i s t , and one might expect a crossover from one s p i n s t a t e t o another at a -1 2+ p a r t i c u l a r lODq/B v a l u e . Since B = 1058 cm f o r Fe , the value of lODq app r o p r i a t e t o a crossover s i t u a t i o n in t h i s case i s approximately 20,000 cm" 1. This crossover phenomenon i s by no means r e s t r i c t e d t o a d 6 c o n f i g u r a t i o n . In f a c t , f o r octahedral symmetry i t i s t h e o r e t i c a l l y 4 5 7 p o s s i b l e f o r d , d and d systems t o e x h i b i t crossover behaviour as w e l l . In p r a c t i c e , however, only F e 5 + ( d ^ ) , F e 2 + ( d 6 ) and C o 2 + ( d ^ ) 79a FIGURE 18 The Tanabe-Sugano Diagram f o r a d E l e c t r o n System (Taken from Ref. 39) FIGURE 18 80 octahedral complexes have been found t o show ground s t a t e c r o s s o v e r , 2+ 8 although the phenomenon has been observed f o r Ni (d ) i n a square 59 planar environment A number of i r o n (I I) compounds are known which e x h i b i t s p i n c r o s s o v e r , and in every case i r o n i s bonded t o s i x n i t r o g e n atoms. Since the d e t a i l s d i f f e r s i g n i f i c a n t l y from one system t o another, we s h a l l review b r i e f l y the e a r l i e r work. 5 I The f i r s t f e r r o u s compounds f o r which a - Aj crossover was observed were Fe(phen) 2(NCS) 2 and Fe(phen) 2(NCSe) 2 6 (^» 6' (phen = I ,10-phenanthroline; see Figure 19 f o r the s t r u c t u r e s of t h i s and other ligands discussed h e r e ) . Many other Fe(phen) 2X 2 complexes are known, but a l I are e i t h e r f u l l y h i g h - s p i n o r f u l l y low-spin depending on the Iigand f i e l d s t r e n g t h of X. Fe(phen) 2(NCS) 2 has a room-temperature 2+ magnetic moment of 5.2 B.M., as expected f o r h i g h - s p i n Fe , w h i l e a t l i q u i d n i t r o g e n temperature the moment i s about 0.65 B.M. A Mossbauer study was c a r r i e d out over the temperature range 80-300°K by D e z s i , e t 62 a l . In the t r a n s i t i o n r e g i o n , which occurs a t about I75°K, t h e r e are four l i n e s in the Mossbauer s p e c t r a , the outer p a i r being c h a r a c t e r i s t i c of the S = 2 s t a t e and the inner p a i r corresponding t o the S = 0 c o n f i g -u r a t i o n . The t r a n s i t i o n i s q u i t e abrupt, and occurs over a temperature range of less than 20°K. Thus, i t i s not p o s s i b l e t o i n t e r p r e t the crossover simply i n terms of changes in thermal po p u l a t i o n of c l o s e -5 I l y i n g T 2 and Aj m a n i f o l d s . The o b s e r v a t i o n of l i n e s due t o both S = 0 and S = 2 s t a t e s in the t r a n s i t i o n region i n d i c a t e s t h a t the e l e c t r o n i c r e l a x a t i o n between the two s p i n s t a t e s i s slow compared t o the Larmor 57 precession frequency of the Fe nucleus. This f e a t u r e i s common t o F I G U R E 1 9 S t r u c t u r e s of the Ligands Discussed in Chapter IV FIGURE 19 81b I,I0-PHENANTHROLINE (phen) 2,2'-BIPYRIDYL (bipy) 2-METHYL-I,I0-PHENANTHROLINE (mephen) 2-(2'-PYRIDYL)IMIDAZOLINE (pylH) 2-(2»-PYR IDYL)IMIDAZOLE (pyIm) 2-(2'-PYRIDYL)BENZIMIDAZOLE (pyben) 82 5 I a l l cases of - A| t r a n s i t i o n s observed t o date. F i s h e r and D r i c k a m e r 6 3 s t u d i e d the response t o high pressure of several i r o n ( l l ) phenanthroline complexes and found a complicated dependence of high-low s p i n e q u i l i b r i u m on pressure. 64 65 Konig and co-workers ' have c a r r i e d out an X-ray s t r u c t u r e a n a l y s i s of F e ( b i p y ) 2 ( N C S ) 2 (bipy = 2 , 2 ' - b i p y r i d y I , Figure 19), which shows behaviour s i m i l a r t o Fe(phen) 2(NCS) 2, and found t h a t the low-spin s p e c i e s i n f a c t has a s h o r t e r Fe-N (mean) bond d i s t a n c e than does the h i g h - s p i n s p e c i e s . Spin crossover has a l s o been observed in a f e r r o u s complex 66 67 with the c h e l a t i n g ligand h y d r o - t r i s ( l - p y r a z o l y I ) b o r a t e ' . By changing or g a n i c s u b s t i t u e n t s on the ligand i t was p o s s i b l e t o prepare compounds which were e i t h e r pure h i g h - s p i n , pure low-spin, o r which showed s p i n e q u i l i b r i u m . For the l a t t e r complex i t was found t h a t a s i n g l e c r y s t a l was completely p u l v e r i z e d by slow thermal c y c l i n g through the t r a n s i t i o n r e g i o n , and i t was suggested t h a t the two s p i n forms have d i f f e r e n t c r y s t a l s t r u c t u r e s 6 ^ . The four s a l t s Fe(mephen)- 5X 2 (mephen = 2-methy I-1,10-phenanthroline, F i g . 19; X = CIO^, BF~, I , BPh^) have been s t u d i e d in d e t a i l 6 8 ' 6 9 . In each case there i s an incomplete change in ground s t a t e , with l i n e s due t o h i g h - s p i n i r o n ( l l ) s t i l l present in the Mossbauer sp e c t r a at 4.2°K. These r e s u l t s were i n t e r p r e t e d as i n d i c a t i n g t h a t 5 I not a l l of the molecules are involved in the T 2 - A ^ e q u i I i b r i u m , and t h a t there i s a permanently paramagnetic f r a c t i o n of molecules. I t has a l s o been shown in these and o t h e r c a s e s 6 8 t h a t the energy s e p a r a t i o n 5 I between the T 2 and A j terms i s not c o n s t a n t , but has a pronounced 83 temperature dependence. C e r t a i n f e r r o u s complexes of both 2 - ( 2 ' - p y r i d y I ) i m i d a z o l e (pyim, Figure 19) and" 2 - ( 2 ' - p y r i d y I ) i m i d a z o l i n e (pyiH, Figure 19) a l s o 5 » show behaviour i n d i c a t i v e of the - A( c r o s s o v e r , but there are important d i f f e r e n c e s in d e t a i l . For example, Fe(pyil-D^vCIO^) ,^ e x i s t s as two magnetic isomers. One i s diamagnetic a t room temperature and below, whereas the o t h e r shows an abrupt change i n magnetic moment at about I20°K (y = 5.25 B.M. a t 295°K and 2.7 B.M. a t 83°K) 7 1. The e t f Mossbauer spectrum of the l a t t e r isomer a t 80°K shows l i n e s due t o both high- and low-spin iron(||), with o n ly the hi g h - s p i n species present a t 294°K. On the other hand, F e ( p y i m ) 3 ( C I 0 4 ) 2 - H 2 0 and several o t h e r 2+ 73 Fe(pyim)^ s a l t s show very gradual changes in magnetic moment with temperature. The V values do not exceed about 4.0 B.M. a t room temperature and are i n the range 0.6 - 2.9 B.M. a t about 90°K. Mossbauer s p e c t r a of a l l the pyim complexes s t u d i e d thus f a r i n d i c a t e a g r e a t e r p o p u l a t i o n of the S = 0 s t a t e at 295°K. 5 I In summary, none of the T 2 - A| crossovers i n i r o n (I/) complexes observed t o date can be explained by a simple thermal e q u i -l i b r i u m between two s t a t e s . The e l e c t r o n i c r e l a x a t i o n time i s long (K 10 7 s) and the t r a n s i t i o n o f t e n seems t o i n v o l v e a change i n dimensions or c o n f i g u r a t i o n of the complex. The t r a n s i t i o n may occur over a temperature range of only a few degrees, o r may be spread over more than 100 degrees. Konig and Kremer 7 0 have c l a s s i f i e d compounds of the former type where one can d e f i n e a s p e c i f i c t r a n s i t i o n temperature as "Group I" compounds, and those of the l a t t e r type where the moment changes g r a d u a l l y with temperature as "Group I I " compounds. 84 The ligands pyim and pyiH are of p a r t i c u l a r i n t e r e s t because the ^NH group on the imidazole r i n g shows changes in a c i d i t y when the 74 ligands are chelated t o v a r i o u s metals . Moreover, the marked d i f f e r e n c e s 2+ 2+ in behaviour of the Fe(pyim)^ and FeCpyiH)^ complexes shows t h a t the crossover phenomenon i s extremely s e n s i t i v e t o minor changes i n the Iigand s t r u c t u r e . I t was thought t h a t f e r r o u s complexes of the c l o s e l y r e l a t e d compound 2 - ( 2 ' - p y r i d y I ) b e n z i m i d a z o l e (pyben, Figure 19) might a l s o show h i g h - s p i n - low-spin c r o s s o v e r . If so, they might provide more information on the e f f e c t of the imino hydrogen on the t r a n s i t i o n c h a r a c t e r i s t i c s . We have t h e r e f o r e prepared and s t u d i e d a number of s a l t s of formula F e ( p y b e n ) ^ 2 • xH20 (A = CI0~, N0~, NCS~, Br~, I ~ , BF~, BPh~, ECr(NH^) 2(NCS) 4J ; x = 0,1,2, but not a l l combinations). These complexes 5 I do indeed show T 2 - A| s p i n e q u i l i b r i a , and the d e t a i l s are s t r o n g l y a f f e c t e d by the nature of the anion A and the number of waters of c r y s t a l I i z a t i o n . M a t e r i a l s The chemicals used were obtained commercially and used without f u r t h e r p u r i f i c a t i o n . The sources of these compounds were as f o l l o w s : Ferrous c h l o r i d e , anhydrous: A l f a Inorganics; Ferrous p e r c h l o r a t e hexahydrate: Matheson, Coleman and B e l l ; Ferrous bromide (99$) and ammonium t e t r a f I u o r o b o r a t e : ROC/RIC; Ferrous ammonium s u l p h a t e , potassium i o d i d e , potassium t h i o c y a n a t e , sodium tetraphenyI borate and ammonium tetrathiocyanatodiamminechromate(111) (Reinecke s a l t ) : F i s h e r S c i e n t i f i c ; 85 Ammonium n i t r a t e : BDH 2-( 2 ' - p y r i d y I ) b e n z i m i d a z o l e : A l d r i c h Chemicals. P r e p a r a t i o n of the Complexes Some of the complexes synthesized were found t o be s l i g h t l y moisture s e n s i t i v e , w h i le the f e r r o u s s t a r t i n g m a t e r i a l s were very s e n s i t i v e t o oxygen. Thus the i n i t i a l mixing of reagents in the prep a r a t i o n of these compounds was c a r r i e d out under a dry nitrogen atmosphere. A n a l y t i c a l data are I i s t e d i n Table V I . T r i sL~2- (2' -py r i dy I) benz i m i dazo I e ] i ron (11) perch I o r a t e monohyd r a t e , F e ( p y b e n ) 3 ( C I 0 4 ) 2 - H 2 0 4 g of f e r r o u s p e r c h l o r a t e hexahydrate i n 30 ml of 100$ ethanol was added t o 6.5 g of Iigand in 200 ml of 100$ ethanol a t room temperature. The complex p r e c i p i t a t e d a f t e r a few minutes and was f i l t e r e d and washed with 100$ e t h a n o l . The orange coloured product was d r i e d i n vacuo. Tr isL"2- ( 2 , - p y r i dy I) benz? mi dazo I eD 1 ron (I I) perch I o r a t e d i hydrate, Fe(pyben) 3(C!0 4) 2-2H 20 This was obtained by leaving the above monohydrate in a i r f o r one-half hour. The brown c r y s t a l s were then c o l l e c t e d and analysed. 86 T r i s C 2 - ( 2 ' - p y r i d y I ) b e n z i m i d a z o l e U i r o n ( | | ) b r o m i d e , F e ( p y b e n ) 3 B r 2 A t y p i c a l p r e p a r a t i o n was as f o l l o w s : 0.8 g of anhydrous fe r r o u s bromide in 30 ml of 100$ ethanol was added t o 2.4 g of Iigand i n 100 mlof 100$ ethanol a t room temperature. The deep red s o l u t i o n was f i l t e r e d , concentrated under reduced pressure t o h a l f volume and l e f t t o stand o v e r n i g h t . The orange c r y s t a l s were c o l l e c t e d , washed with 100$ e t h a n o l , and d r i e d in vacuo. T r i s C 2 - ( 2 ' - p y r i d y I ) b e n z i m i d a z o l e 3 i r o n ( I I) i o d i d e , F e t p y b e n ) ^ ^ 0.2 g of anhydrous f e r r o u s c h l o r i d e i n 30 ml methanol was added t o I.I g of Iigand i n 80 ml methanol. The deep red s o l u t i o n was f i l t e r e d through a f i n e s i n t e r e d g l a s s f i I t e r i n t o 50 ml of an aqueous s o l u t i o n c o n t a i n i n g 5 g of potassium i o d i d e . The volume was reduced t o about 80 ml, and the s o l u t i o n was cooled in an ice bath f o r 4 h. The red-orange c r y s t a l s were c o l l e c t e d by f i l t r a t i o n and washed s e v e r a l times with c o l d water. The product was d r i e d i n vacuo. T r i s C 2 - ( 2 t - p y r i dyI)benz imi d a z o I e ] i r o n ( I I ) n ? t r a t e monohydrate, Fe(pyben) 3(N0 3) 2-H 20 0.5 g of F e ( p y b e n ) 3 ( C I 0 4 ) 2 - H 2 0 in 200 ml of methanol was added to a large excess of ammonium n i t r a t e in a minimum volume 30 ml) of water. The s o l u t i o n was concentrated under reduced pressure t o a volume of about 60 ml and l e f t t o stand o v e r n i g h t . The red-orange c r y s t a l s were c o l l e c t e d and washed with water. The product was r e c r y s t a I I i z e d from c o l d methanol and d r i e d in vacuo. 87 T r i sL~2- (2'-pyridy I )benz imi dazol eHi ron (| | ) thiocyanate monohydrate, Fe(pyben) 3(NCS) 2«H 20 1.2 g of 2-(2'-pyridyI)benzimidazole was d i s s o l v e d i n 70 ml of 95% ethanol and 55 ml of water added. 0.7 g of f e r r o u s ammonium sulphate was d i s s o l v e d in 70 ml of water t o which was added 50 ml of 95% e t h a n o l . The ice c o l d f e r r o u s ammonium sulphate s o l u t i o n was added with s t i r r i n g t o the ligand s o l u t i o n in an ice bath, and I.6 g of potassium t h i o c y a n a t e d i s s o l v e d i n 30 ml of ice c o l d water was immediately added t o the above mixture. The r e s u l t i n g red s o l u t i o n was l e f t in the ice bath o v e r n i g h t . The large orange c r y s t a l s were f i l t e r e d , washed with water several times, and d r i e d in vacuo. Dith?ocyanatobisL~2-(2'-pyr?dyl )benzim?dazole!]i r o n d I ), Fe(pyben) 2(NCS) 2 0.5 g of f e r r o u s c h l o r i d e in 50 ml of methanol was added t o 2.3 g of ligand in 100 ml of methanol. The deep red s o l u t i o n formed was added t o 5 g. ( l a r g e excess) of potassium t h i o c y a n a t e d i s s o l v e d i n a minimum volume of water. The s o l u t i o n was b o i l e d under r e f l u x f o r 15 min. The red p r e c i p i t a t e which formed was c o l l e c t e d , r e c r y s t a l l i z e d from methanol, and d r i e d in vacuo. TrisC2-(2'-pyridyI)benzim?dazole3iron(M ) t e t r a f I u o r o b o r a t e monohydrate, Fe( pyben ^ ( B F ^ ^ I - ^ O 1.3 g of F e ( p y b e n ) 3 B r 2 in 230 ml of methanol was added t o 4 g of ammonium t e t r a f I u o r o b o r a t e in 150 ml of water, and the red s o l u t i o n 88 was concentrated t o 180 ml volume. The puple c r y s t a l s which formed were c o l l e c t e d on a f i l t e r and washed with water several times t o remove excess ammonium t e t r a f I u o r o b o r a t e . The crude compound was r e c r y s t a l l i z e d by d i s s o l v i n g i t i n 150 ml of methanol and c o n c e n t r a t i n g the s o l u t i o n t o about 30 ml volume; p r e c i p i t a t i o n was then a f f e c t e d by the a d d i t i o n of 5 ml of water. The purple c r y s t a l s were c o l l e c t e d and washed with water. The monohydrate was obtained as an orange product a f t e r the purple c r y s t a l were d r i e d i n vacuo. TrisL"2-(2'-pyridy I )benzimidazol eHi ron( I I ) t e t r a f I uoroborate d i h y d r a t e , Fe(pyben) 3(BF 4> 2-2H 20 The procedure was i d e n t i c a l t o t h a t used f o r the monohydrate d e r i v a t i v e above, except t h a t the purple c r y s t a l s were d r i e d i n a i r t o give a s t a b l e brown compound. T r i s C 2 - ( 2 ' - p y r i d y I ) b e n z i m i d a z o l e j i r o n ( I I) tetraphenyI borate monohydrate, Fe(pyben) 3(BPh 4) 2'H 20 I .0 g of Fe(pyben> 3Br 2 d i s s o l v e d i n 400 ml 95% ethanol was added t o 2.0 g of sodium tetraphenyI borate in 150 ml of 95% e t h a n o l . The s o l u t i o n was concentrated t o about 175 ml and l e f t t o stand o v e r n i g h t at 0°. The red p r e c i p i t a t e which formed was f i l t e r e d and d r i e d i n vacuo. 89 Tr i sL"2-(2 '-pyridy I) benz imi dazo I ej]i ron(| I ) t e t rath iocyanatod iammi nechromate -(III), Fe(pyben) 3CCr(NH 3) 2(NCS) 4] I.I g o f F e ( p y b e n ) 3 B r 2 in 175 ml of methanol was added t o I.0 g of Reinecke s a l t in 160 ml of methanol. 20 ml of water was added and the volume of the s o l u t i o n reduced t o about 100 ml. The p r e c i p i t a t e which formed was c o l l e c t e d by f i l t r a t i o n , washed with a 1:1 methanol/water mixture and d r i e d in vacuo. V 90 TABLE VI A n a l y t i c a l Data f o r Tne Ferrous Complexes o f  2-(2 ' - P y r i d y l ) b e n z i m ? d a z o l e Compounds C% Colour F e ( p y b e n ) 3 ( C I 0 4 ) 2 - H 2 0 Found (C a l c . F e ( p y b e n ) 3 ( C I 0 4 ) 2 ' 2 H 2 0 Found ( C a l c . F e ( p y b e n ) 3 B r 2 F e ( p y b e n ) 3 I 2 Found ( C a l c . Found ( C a l c . F e ( p y ben) 3(N0 3) 2-H 20 Foun^ ( C a l c . Fe(pyben) 3(NCS) 2«H 20 Found ( C a l c . Fe(pyben) 2(NCS) 2 Found (Calc. F e ( p y b e n ) 3 ( B F 4 ) 2 - H 2 0 Found ( C a l c . 50.42 50.34 49.00 49.30 53.42 53.90 47.97 48.27 54.60 55.20 58.55 58.83 55.59 55.50 52. 14 51 .90 3.25 3.37 3.49 3.54 3.40 3.37 3.00 3.02 3.66 3.70 3.86 3.74 3.38 3.20 3.37 3.40 14.75 14.67 14.38 15.81 15.70 14.00 14.10 19.70 19.70 19.88 19.87 19.74 19.90 15.42 15.15 6.50 6.50) 6.35 6.39) 6.93 6.97) 6.23 6.24) 7.14 7.15) 7.20 7.20) 9.88 9.93) 6.70) Orange (RT) Purple (LN) Brown (RT) Purple (LN) Orange (RT) Purple (LN) Orange (RT) Purple (LN) Orange (RT) Purple^ (LN) Orange (RT) Brown (LN) Red (RT) Red (LN) Orange (RT) Purple (LN) TABLE VI - Continued/-F e ( p y b e n ) 3 ( B F 4 ) 2 - 2 H 2 0 Found 50.31 3.30 14.61 (Calc. 50.80 3.64 14.80 6.50) Fe(pyben) 3(BPh 4) 2*H 20 Found 77.86 4.95 9.68 (Ca l c . 77.80 5.30 9.71 4.30) Fe(pyben) 3CCr(NH 3) 2(NCS) 4] 2 Found 41.41 3.02 22.68 ( C a l c . 41.40 3.05 23.00 4.37) RT = room temperature, LN = l i q u i d nitrogen temperature 92 General Observations The Iigand 2-(2'-pyridyI)benzimidazole forms two types of compounds with f e r r o u s s a l t s , depending on the s t r e n g t h of the anion as a c o o r d i n a t i n g Iigand and the c o n d i t i o n s of the r e a c t i o n . The f i r s t type i s Fe(pyben) 5A 2*xH 20. These compounds are not oxygen s e n s i t i v e as f i r s t suspected. This might be due t o the b u l k i n e s s of the ligands surrounding the i r o n ( M ) c e n t r e , making i t i n a c c e s s i b l e t o o x i d a t i v e a t t a c k . The f e r r o u s nature of these compounds was confirmed b y f e r r o - a n d f e r r i - c y a n i d e t e s t s . The monohydrates are moisture s e n s i t i v e , while those which could be i s o l a t e d as anhydrous complexes or dihydrates are not a f f e c t e d by atmospheric moisture. Although the dihydrates are r e a d i l y converted t o the monohydrates at room temperature on the vacuum l i n e , attempts t o d r i v e o f f the l a s t water of c r y s t a l l i z a t i o n In the monohydrates were u n s u c c e s s f u l . A t y p i c a l experiment involved heating a sample t o 150° i n vacuo f o r f o u r t o s i x hours, and measuring the weight l o s s . In no case was t h e r e an a p p r e c i a b l e loss of weight. The presence of water a f t e r heating was a l s o confirmed by the appearance of a 3300 cm ' peak in the I.R. s p e c t r a . In the case of the t h i o c y a n a t e anion a second type of compound, Fe(pyben) 2(NCS) 2, i s a l s o formed, in which the anion i s coordinated d i r e c t l y t o the c e n t r a l metal i o n . The formation of the t r l s ( p y b e n ) complex,Fe(pyben) 3(NCS)^H 20, c a n be a f f e c t e d by keeping the r e a c t i o n tem-perature at 0° and using excess Iigand in a mixed water/methanol s o l v e n t . The b i s adduct i s by f a r the more s t a b l e complex in t h i s case. I t can be prepared e i t h e r by b o i l i n g the t r i s complex in methanol o r d i r e c t l y at room temperature using a ligandrmetal mole r a t i o of 2:1 i n methanol. 93 Even at room temperature, Fe(pyben).j(NCS) 2 decomposes q u i c k l y in methanol l o s i n g one mole of the I igand t o form Fe(pyben> 2(NCS) 2. This was observed by means of I.R. and e l e c t r o n i c s p e c t r a , conductance mea-surements and m i c r o a n a l y s i s of the f i n a l product. The behaviour of the th i o c y a n a t e d e r i v a t i v e s |s q u i t e unique in the s e r i e s s i n c e the other s a l t s appear t o be very s t a b l e i n s o l u t i o n . For example, the bromide s a l t can be l e f t in methanol f o r several weeks at room temperature and the o r i g i n a l t r i s ( p y b e n ) complex can be r e t r i e v e d , as confirmed by m i c r o a n a l y s i s , conductance, and e l e c t r o n i c s p e c t r a . Several attempts were made t o prepare the c h l o r i d e s a l t , s i n c e a compound of the formula F e ( p y b e n ) 1 2 * 6 H 2 0 was reported by C h i s w e l l e t 75 a I. . However, a l l attempts t o ob t a i n t h i s hexahydrate were u n s u c c e s s f u l . An impure s a l t thought t o be FeCpybenJ^Cl 2 contaminated w i t h f r e e Iigand was obtained instead (no band due t o H 20 was found i n the I.R. spectrum). The d i f f i c u l t y here i s t h a t the c h l o r i d e s a l t i s very s o l u b l e in methanol and moderately s o l u b l e in water as w e l l , so t h a t the p r e c i p i t a t i o n process used f o r the other s a l t s i s i n e f f e c t i v e in t h i s case. When the water/methanol r a t i o i s r a i s e d s u f f i c i e n t l y t o b r i n g down the s a l t , f r e e I igand c o p r e c i p i t a t e s . Attempts t o make the c h l o r i d e from acetone/water or ethanol/water were e q u a l l y u n s u c c e s s f u l . E f f o r t s t o prepare s a l t s of the type FefpybenJ^B, where B i s a 2- 2-di n e g a t i v e i o n , y i e l d e d very i n t r a c t a b l e products f o r B = CO^ , SO^ 2-and S^ O-j, , and were not pursued f u r t h e r . 94 Conductance Measurements The molar conductances of the complexes are given i n Table V I I . A l l the complexes except FeCpyben^CNCS^ have s i m i l a r values of conduc-tances and these a l l l i e w i t h i n the range expected f o r 10 ^ M s o l u t i o n s of 2:1 e l e c t r o l y t e s in methanol (160 - 220 fi ' mol ' c m 4 2 ) . 7 6 The values are near the lower end of the range however, presumably because 2+ of the large mass of the c a t i o n Fetpyben)^ . A d d i t i o n a l l a t t i c e water seems t o enhance the conductance in those cases where more than one hydrate could be i s o l a t e d . The complex F e t p y b e n ^ t N C S ^ shows a much lower conductance than the r e s t , i n d i c a t i n g an e s s e n t i a l d i f f e r e n c e in s t r u c t u r e f o r t h i s compound. However the value 78 l i e s barely i n the range expected f o r 1:1 e l e c t r o l y t e s (80 - 115 fi ' mol ' cm + 2 f o r 10 3 M s o l u t i o n s in m e t h a n o l ) 7 6 . This could be due t o the so I v o l y s i s of the covalent complex in methanol into species such as CFetpyben^tCH^OH) (NCS)U 2 ( C H 3 0 H ) 2 : and CFe(pyben) 9(CH,0H) 9l 2 +. 7 7 Infrared Data Infrared s p e c t r a between 4000 and 250 cm ' were obtained f o r a l l the compounds. These s p e c t r a are very complex due t o the large number of ligand bands present. Thus, instead of t a b u l a t i n g a l l the data, we s h a l l only d i s c u s s those bands which provide s t r u c t u r a l and bonding information about the compounds. The I.R. s p e c t r a of pyben and 78 some pyben-metal complexes have been s t u d i e d p r e v i o u s l y by Lane et aJL The s p e c t r a of our i r o n complexes are very s i m i l a r t o those reported f o r other t r a n s i t i o n metal complexes with t h i s l i g a n d , thus i n d i c a t i n g t h a t i n t e r a c t i o n between the c e n t r a l ion and the ligand i s s i m i l a r . 95 TABLE VI I Molar Conductances of the Pyben Complexes In  Methanol a t 25° CompI exes A M ^ 'moT'cm2) F e ( p y b e n ) 3 ( C I 0 4 ) 2 * H 2 0 F e ( p y b e n ) 3 ( C I 0 4 ) 2 ' 2 H 2 0 F e ( p y b e n ) 3 B r 2 . Fe(pyben) 3(N0 3)^H 20 Fe(pyben) 3(NCS) 2'H 20 F e ( p y b e n ) 3 I 2 F e ( p y b e n ) 3 ( B F 4 ) 2 - H 2 0 F e ( p y b e n ) 3 ( B F 4 ) 2 * 2 H 2 0 F e ( p y b e n ) 3 C C r ( N H 3 ) 2 ( N C S ) 4 D 2 Fe(pyben) 3(BPh 4> 2'H 20 Fe(pyben) 2(NCS) 2 176.0 188.0 160.0 164.5 decomposed 169.0 177.0 192.0 Not s o l u b l e Not s o l u b l e 78.8 -3. Concentration(10 ^M) 1.158 0.861 0.817 0.938 I .130 I .1 18 0.985 0.96 96 79 -I -I The p y r i d i n e r i n g band at 996 cm i s s h i f t e d by ^ 10 cm t o 1007 cm '. A l s o , other p y r i d i n e bands at 1279 cm ', I 154 cm ', -I 79 1046 cm , are a l l s h i f t e d upwards s i m i l a r l y . These upward s h i f t s i n d i c a t e u n e quivocally the involvement of the p y r i d i n e n itrogen i n bonding. The two bands which could be assigned t o the benzene r i n g , namely those at 1119, 1012 cm ', do not show any apparent upward s h i f t . 78 Examination of the s p e c t r a of pyben and i t s metal complexes reveals t h a t t h e r e i s a strong band at 1314 cm ' in the f r e e Iigand which s p l i t s i n t o t w o sharp bands on c h e l a t i o n . These occur a t 1324 and 1302 cm"' in the case of the i r o n complexes. This band does not belong t o e i t h e r o r t h o - s u b s t i t u t e d benzene or p y r i d i n e , and can t e n t a t i v e l y be assigned t o the imidazole fragment. The s p l i t t i n g of t h i s band on c h e l a t i o n can thus be regarded as an i n d i c a t i o n of the involvement of the imidazole n i t r o g e n in bonding. FeCpyben^CCIO^^'xh^O (x = 1,2) g i v e i d e n t i c a l i n f r a r e d s p e c t r a in the range 3200-250 cm ', and both show a band due t o water of hydra-t i o n a t 3180 cm '. The dihydrate shows an a d d i t i o n a l band a t 3500 cm -' which can be e l i m i n a t e d by evacuating the sample at room temperature. 80 — I In both compounds the perch I orate band centred a t ^ 1083 cm i s s p l i t by 60 cm '. (This can be compared t o HCIO^, where the s p l i t t i n g — I 81 i s 280 cm ). The moderate s p l i t t i n g seen here i s probably due t o a small d i s t o r t i o n of the CIO. anion by l a t t i c e e f f e c t s . The v„CI0T 4 7 4 4 band i s u n s p l i t a t 624 cm ', while Vj and are assigned t o very weak bands at 968 and 458 cm ', r e s p e c t i v e l y . In the case of Fe(pyben^(NO-^'h^O, the n i t r a t e band at 1350 cm ' i s spI i t by 59 cm '. This i s small compared t o coordinated — — I 80 N0^ groups, which t y p i c a l l y show s p l i t t i n g s of ^ 200 cm , and can 97 be a t t r i b u t e d t o a l a t t i c e d i s t o r t i o n . The V| and anion modes appear as very weak bands at 1038 and 825 cm r e s p e c t i v e l y , w h i le i s very l i k e l y masked by a broad ligand absorption at 744 cm The l a t t i c e water in the compound F e ( p y b e n ) 3 ( N C S ^ " ^ ^ a b s o r b s at 3350 cm ' and t h i s H 20 molecule cannot be removed by pumping. The CN s t r e t c h ( V j ) of the NCS anion occurs at a s l i g h t l y lower frequency (2030 cm ') than in potassium t h i o c y a n a t e (2050 cm ' ) , but the l i n e remains u n s p l i t . The v 2 band appears as a weak absorption a t e x a c t l y the same p o s i t i o n as i n KNCS (471 cm ') w h i l e i s not observed, probably masked by a strong ligand band at 744 cm -'. There i s no a p p r e c i a b l e d i f f e r e n c e between the spectrum of Fe(pyben) 2(NCS) 2 and t h a t of Fe(pyben)-j(NCS) 2'H 20 as f a r as ligand bands are concerned. However, Fe(pyben) 2(CNS) 2 does not have a l a t t i c e water band in the region above 3000 cm ', and the t h i o c y a n a t e CN s t r e t c h appears as a strong doublet at 2080 and 2022 cm '. The reported s p l i t t i n g of t h i s band i s about 10 cm ' f o r the analogous phen and bipy c o m p l e x e s 6 4 , 8 2 . Assuming a c i s-conf i gu r a t ion i s adopted, the l a r g e r s p l i t t i n g here (58 cm ') may be due t o the f a c t t h a t the pyben ligand i s asymmetric while phen and bipy are symmetric Iigands. The N-C-S bending mode (v 2> i s found at 474 cm ' as a weak band. Further evidence t h a t the t h i o -cyanate group i s coordinated t o ir o n in Fe(pyben) 2(NCS) 2 i s the — I 80 appearance of a strong 788 cm band which can be assigned t o the C-S s t r e t c h (v^) of the NCS l i g a n d . The p o s i t i o n of t h i s band i s i n d i c a t i v e 80 of N-bonded t h i o c y a n a t e I igands For the p a i r Fe(pyben) 3(BF 4> 2'H 20 and Fe(pyben> 3(BF 4) 2*2H 20, the spectra are i d e n t i c a l except t h a t t h e r e i s one broad band at 9 8 3278 cm 1 f o r the monohydrate w h i l e the di h y d r a t e has an e x t r a band with a double maximum at 3528 and 3598 cm The l a t t e r can be e l i m -inated by pumping the di h y d r a t e at room temperature. The BF^ bands 83 — | appear a t the expected p o s i t i o n s : . a t 1053 cm i s strong and broad, i s found at 518 cm ' with a s p l i t t i n g of less than 5 cm ' (due presumably t o c r y s t a l d i s t o r t i o n e f f e c t s ) , and Vj appears as a strong band at 759 cm '. The s p e c t r a l bands due t o the CrCNH^^NCS)^ ion i n 84 FeCpyben^rc^NH-^vNCS^IL^ are s i m i l a r t o those of other r e i n e c k a t e s , i n d i c a t i n g a s i m i l a r anion environment. The CN s t r e t c h appears at 2063 cm ', and the NCS bending mode occurs a t 494 cm The Cr-NH^ s t r e t c h appears at 466 cm ' as a weak shoulder. The NH^ deformation 84 absorptions are at the same p o s i t i o n s as in ammonium r e i n e c k a t e : the symmetric deformation a t 1257 cm ', and the rocking mode a t 708 cm '. A strong band at 350 cm ' can be assigned t o the Cr- NCS s t r e t c h w h i l e the C-S s t r e t c h appears a t 848 cm '. The l a t t i c e water abs o r p t i o n of the compound F e t p y b e n O ^ B P I - ^ ^ ^ O produces a broad band at 3304 cm -'. The i n f r a r e d spectrum due t o the anion fragment BPh^ i s very complicated and a complete a n a l y s i s of i t could not be found i n the l i t e r a t u r e . Comparison of the bands due t o the BPh^ ion in the complex with the spectrum of Na BPh^ i n the range 2000-250 cm ' revealed t h a t both the p o s i t i o n s and i n t e n s i t i e s of the bands are s i m i l a r in the two compounds. T h i s suggests a s i m i l a r i t y of anion environment. The s t r o n g e s t bands are due t o phenyl C-H out-of-plane 85 _ I _ | deformations , and appear at 741 cm and 714 cm with i n t e n s i t i e s s i m i l a r t o the corresponding bands in Na BPh^. 99 The major f e a t u r e s of the I.R. s p e c t r a of these complexes can be summarized as f o l l o w s : F i r s t l y , i t appears t h a t both the p y r i d i n e and imidazole nitrogens are involved i n bonding t o the f e r r o u s i o n , suggesting t h a t pyben i s a c t i n g as a b i d e n t a t e c h e l a t i n g l i g a n d . Secondly, f o r most of the polyatomic anions t h e r e i s evidence f o r some symmetry lowering. This could be caused by d i s t o r t i o n s a r i s i n g from c r y s t a l packing f o r c e s , although the asymmetric nature of the. pyben ligand i s another p o s s i b l e cause. T h i r d l y , f o r the perch I o r a t e and t e t r a f I u o r o b o r a t e complexes there are c l e a r d i f f e r e n c e s between the monohydrates and dihydrates above 3000 cm F i n a l l y , the data provide good evidence t h a t in F e t p y b e n ^ N C S ^ the NCS groups are coordinated t o i r o n v i a the nitrogen atoms. Magnetic Data The r e s u l t s of magnetic s u s c e p t i b i l i t y measurements on the complexes between 80°K-300°K are l i s t e d i n Table V I M . The molar suscep-t i b i l i t i e s and magnetic moments are p l o t t e d as f u n c t i o n s of temperature in Figures 20 and 21, r e s p e c t i v e l y . As seen from F i g . 21, the temperature dependence of M f f v a r i e s g r e a t l y from compound t o compound. The complexes can be separated roughly i n t o four groups, depending on the anion. The n i t r a t e and the two t h i o c y a n a t e complexes have U e f f values between about 4.8 and 5.4 B.M. throughout the temperature range. The bromide, i o d i d e , and t e t r a f I u o r o b o r a t e monohydrate d e r i v a t i v e s show moments of ^ 5.4 B.M. at room temperature, decreasing t o about 3.5 - 4.2 B.M. at l i q u i d n i t r o g e n temperature. The moments f o r the p e r c h l o r a t e monohydrate and d i h y d r a t e , t e t r a f I u o r o b o r a t e d i h y d r a t e and TABLE VIII Molar S u s c e p t i b i l i t i e s and E f f e c t i v e Magnetic Moments of the pyben Complexes as a Function of Temperature T (°K) (c.g.s. "eff ) (B.M.) ( c - 9-s.) FEtPYBEN)31CL04)2.H20 79.8 90.7 106.2 124.0 139.7 157.0 181.4 202.9 221.0 2*5.4 264.5 264.4 306.4 0.006268 0.004 795 0.00S549 0.005812 0.006373 0.007441 0.010856 0.015673 0.015865 0.014867 0.013956 0.013028 0.012030 2.00 2.05 2.17 2.40 2.67 3.06 3.97 5.04 5.30 5.40 5.43 5.44 5.43 FEtPYBEN)3IC10412.2H20 80.5 95.0 113.4 134.0 153.4 170.0 190.5 208.3 229.5 251.4 267.9 290.3 310. 5 0.002581 0.002334 0.002391 0.002562 0.003018 0.003535 0.004443 0.005626 0.007377 0.009855 0.011204 0.011760 0.011527 FE(PYBEN)3(N03)2.H2O 79.0 98.6 125.1 152.5 179.7 204.7 231.4 264.5 278.8 296.0 037373 031276 025658 021904 019278 017087 015315 013517 012B28 012089 F£(PYBEN)3<NCS)2.H20 81.1 99.1 120.0 141.0 162.0 180.2 200.0 219.9 241.1 259.2 283.7 310.2 0.037909 0.032794 0.028067 0.024632 0.021739 0.019709 0.018048 0.016585 0.015156 0.014107 0.012999 0.011943 FEIPYBENI3BR2 80.4 0.020657 94.0 104.5 116.4 128.3 138.8 149.3 163.1 174.6 1B5.0 194.4 205.6 219.7 241.1 263.0 297.1 0.019689 0.019336 0.018955 0.018720 0.018573 0.018529 0.018339 0.018016 0.017664 0.017282 0.016592 0.015932 0.014905 0.013936 0.012880 1.29 1.33 1.47 1.66 1.92 2.19 2.60 3.06 3.68 4.45 4.90 5.23 5.35 4.86 4.97 5.07 5.17 5.26 5.29 5.32 5.35 5.3.5 5.35 4.96 5.10 5.19 5.27 5.31 5.33 5.37 5.40 5.41 5.41 5.43 5.44 3.65 3.85 4.02 4.20 4.38 4.54 4.70 4.89 5.02 5.11 5.18 5.22 5.29 5.36 5.41 5.44 0.006234 0.005 756 0.005492 0.005782 0.006 331 0.007398 0.010831 0.015647 0.015865 0.014850 0.0139*0 0.013037 0.012038 0.002512 0.002265 0.002361 0.002557 0.003001 0.003512 0.004438 0.005618 0.007398 0.009829 0.011211 0.011716 0.011525 0.037249 0.031269 0.025536 0.021714 0.019255 0.017142 0.015286 0.013540 0.012863 0.012086 0.037612 0.032812 0.028018 0.024648 0.021805 0.019744 0.018075 0.016555 0.015204 0.014196 0.013025 0.011924 0.020584 0.019639 0.019318 0.018996 0.018710 0.018580 0.018398 0.018233 0.018033 0.017651 0.017365 0.016610 0.015916 0.014909 0.013998 0.012940 " e f f (B.M.) 1.99 2.04 2.16 2.40 2.66 3.05 3.96 5.04 5.30 5.40 5.44 5.45 5.43 1.27 1.31 1.46 1.65 1.92 2.19 2.60 3.06 3.69 4.45 4.90 5.22 5.35 4.85 4.97 5.06 5.15 5.26 5.30 5.32 5.35 S.3S 5.35 4.94 5.10 5.19 5.27 5.31 5.33 5.38 5.40 5.41 5.43 5.44 5.44 3.64 3.84 4.02 4.20 4.38 4.54 4.69 4.88 5.02 5.11 5.20 5.23 5.29 5.36 5.43 5.45 T X, (°K) (c.g.s.) FE t PYBEN)312 81.1 0.019791 96.0 0.019451 112.8 0.019569 131.2 0.014821 150.5 0.019392 169.9 0.018903 188.1 0.018075 206.8 0.016935 225.8 0.016018 247.6 0.014745 264.0 0.013913 287.1 0.012865 3D9.5 0.012022 FEtPYBEN)3IBF4l2.H20 M e f f (B.M.) 3.58 3.86 4.20 4.56 4.83 5.07 5.21 5.29 5.38 5.40 5.42 5.44 5.45 82.0 102.2 120.6 137.6 156.2 171.5 187.6 204.8 220.1 240.0 256.1 281.4 305.5 0.026723 0.023501 0.021078 0.019834 0.018567 0.017922 0.017189 0.016382 0.015667 0.014633 0.013989 0.013033 0.012233 4.19 4.38 4.51 4.67 4.82 4.96 5.08 5.18 5.25 5.30 5.35, 5.42 5.47 FE(PYBEN)3(BF4)2.2H20 81.1 101.1 119.0 139.4 159.9 178.8 198.8 218.5 238.0 256.3 279.0 304.0 0.002918 0.002451 0.002298 0.002279 0.002339 0.002552 0.003075 0.003822 C.005009 0.006694 0.008815 0.010111 1.38 1.41 1.48 1.59 1.73 1.91 2.21 2.58 3.09 3.70 4.44 4.96 FEtPYBEN)3(BPH40 2.H20 81.6 96.0 111.4 131.7 152.8 173.5 193.6 216.2 234.0 254.7 281.4 303.5 0.017512 0.016230 0.014659 0.013588 0.012652 0.012156 0.011647 0.011256 0.011065 0.010874 0.010499 0.010272 FE(PY8EN)2(NCS)2 78.6 92.9 113.0 136.9 166.8 191.0 216.8 236.3 253.1 275.6 306.0 0.041233 0.036161 0.030599 0.025758 0.021524 0.019002 0.016762 0.015504 0.014395 0.013189 0.011897 3.38 3.53 3.61 3.78 3.93 4.11 4.2S 4.41 4.55 4.71 4.86 4.99 5.09 5.18 5.26 5.31 5.36 5.39 S.39 5.41 5.40 5.39 5.40 FE(PYBEN)3ICR(NH3)2<UCS)4I2 Xj measured In f i e l d strength of * 4.0 kG X, " " " • " <N. 8.0 kG 81.2 103.3 122.4 144.6 165.0 184. 2 203.8 220.7 240.4 260.3 282.7 308.9 0.004319 0.003749 0.003313 0.002864 0.002701 0.002574 0.002689 0.002909 0.003123 0.00JH40 0.005158 0.096511 1.68 1.76 1 .80 1.82 1 .89 1.95 2.09 2.27 2.45 2.83 3.41 4.01 x2 (c.g.s. 0.019531 0.019474 0.019623 0.019620 0.01942 I 0.018829 0.017917 0.016888 0.015863 0.014747 0.013881 0.012923 0.011973 0.026578 0. 023173 0.021034 0.019809 0.018576 0.017738 0.017011 0.016247 0.015507 0.014686 0.013979 0.012959 0.012088 0.002808 0.002445 0.002287 0.002321 0.002369 0.002609 0.003065 0.003882 0.005092 0.006756 0.008884 0.-010145 0.017374 0.015811 0.014424 0.013331 0.012680 0.012170 0.011752 0.011432 0.011223 0.010937 0.010592 0.010331 0.041201 0.036253 0.030708 0.025861 0.021633 0.019026 0.016A22 0.015501 0.014435 0.013231 0.011925 0.004228 0.003600 0.003357 0.002988 0.002825 0.002616 0.002 709 0.002912, 0.003307 0.004067 0.00522S 0.006520 100 w e f f (B.M.) 3.56 3.87 4.21 4.54 4.84 5.06 5.19 5.28 5.35 5.40 5.41 5.45 5.44 4.18 4.35 4.50 4.67 4.82 4.93 5.05 5.16 5.23 5.31 5.35 5.40 5.44 1.35 1.41 1.48 1.61 1.74 1.93 2.21 2.60 3.U 3.72 4.45 4.97 3.37 3.48 3.59 3.75 3.94 4.11 4.27 4.45 4.58 4.72 4.88 5.01 5.09 5.19 5.27 5.32 5.37 5.39 5.40 5.41 5.40 5.40 5.40 1.66 1.73 1.81 1.86 1.93 1.96 2.10 2.27 2.52 2.91 1.44 4.01 101a FIGURE 20 Temperature Dependence of the Molar S u s c e p t i b i l i t i e s of the Pyben Complexes o F e ( p y b e n ) 3 ( C I 0 4 ) 2 - H 2 0 F e ( p y b e n ) 3 ( C I 0 4 ) 2 - 2 H 2 0 + F e ( p y b e n ) 3 ( N 0 3 ) 2 . H 2 0 X Fe(pyben) 3(NCS) 2-H 20 Fe(pyben> 3Br 2 Fe(pyben.) 3I 2 F e ( p y b e n ) 3 ( B F 4 ) 2 - H 2 0 z F e ( p y b e n ) 3 ( B F 4 ) 2 - 2 H 2 0 Y F e ( p y b e n ) 3 ( B P h 4 ) 2 - H 2 0 X Fe(pyben) 2(NCS) 2 * Fe(pyben) 3[Cr(NH 3) 2(NCS ) 4 D FIGURE 21 Temperature Dependence of the E f f e c t i v e Magnet Moments of the Pyben Complexes o F e ( p y b e n ) 3 ( C I 0 4 ) 2 ' H 2 0 F e ( p y b e n ) 3 ( C I 0 4 ) 2 ' 2 H 2 0 + F e ( p y b e n ) 3 ( N 0 3 ) 2 - H 2 0 X Fe(pyben) 3(NCS) 2.H 20 Fe(pyben> 3Br 2 F e ( p y b e n ^ 3 I 2 X F e ( p y b e n ) 3 ( B F 4 ) 2 ' H 2 0 z Fe(pyben) 3(BF 4) 2«2H 20 Y Fe(pyben) 3(BPh 4) 2«H 20 X Fe(pyben) 2(NCS) 2 • Fe(pyben) 3[Cr(NH 3) 2(NCS) 43, 1 0 3 r e i n e c k a t e s a l t s have the most pronounced temperature dependence, being £ 2.0 B.M. at about 80°K, and in the range 4.0 - 5.4 B.M. at room temperature. For the tetraphenyl borate complex the moment decreases almost l i n e a r l y from 5.0 t o 3.4 B.M. between 300 and 80°K. In the previous Chapter i t was seen t h a t f o r t y p i c a l h i g h - s p i n f e r r o u s complexes y ^ i s roughly 5.4 B.M. and v a r i e s by only about 0.2 B.M. between 80 and 300°K. It was a l s o found t h a t changing the s i x ligands of the inner c o o r d i n a t i o n sphere of i r o n from, say DMSO t o PyNO, caused only s l i g h t changes i n e i t h e r the magnitude or temperature dependence of V e f f The s i t u a t i o n i s c l e a r l y very d i f f e r e n t here, where changes o u t s i d e the inner c o o r d i n a t i o n sphere ( i . e . , a change of anion and/or number of waters of c r y s t a l l i z a t i o n ) can cause dramatic changes in the magnitude and temperature dependence of V e f f However, there does not appear t o be any obvious c o r r e l a t i o n between V Q f f and +he nature of the anion, as w i l l be discussed in more d e t a i l below. The magnetic data by themselves do not e s t a b l i s h the e x i s t e n c e of high-spin - low-spin e q u i l i b r i a , although a n t i f e r r o m a g n e t i c i r o n - i r o n i n t e r a c t i o n s (which could a l s o cause such anomalous magnetic behaviour) seem very u n l i k e l y i n these systems. However, Mossbauer data presented below d e f i n i t e l y show the presence of f e r r o u s ions in 5 I both T 2 and Aj s t a t e s , and t h a t the r e l a t i v e populations of the two s t a t e s depend on temperature, nature of the anion and degree of h y d r a t i o n . Thus, we w i l l d i s c u s s the magnetic moment data in terms of the e f f e c t s of these three v a r i a b l e s on the composition of the h i g h - s p i n -low-spin "mixture". 104 The temperature dependence of f o r Fe (pyben) ^ CNO-^'I^O i s not unusually large f o r a pure high-spin f e r r o u s complex, although I the Mossbauer s p e c t r a show there are some molecules in the Aj ground s t a t e below about I80°K. S i m i l a r l y , f o r the two th i o c y a n a t e complexes Fe(pyben) 3(NCS) 2"H 20 and Fe(pyben) 2(NCS) 2, the moments have small temperature dependence and are nearly i d e n t i c a l throughout the a c c e s s i b l e 5 I temperature range, yet the former e x h i b i t s T 2 - A| crossover below ^ I70°K while the l a t t e r shows no 'Aj component down t o II5°K. I t i s c l e a r from these examples t h a t the magnetic data do not guarantee the p u r i t y of the s p i n system. Since the use of V e f f values t o e x t r a c t such parameters as c r y s t a l f i e l d s p l i t t i n g s s p i n - o r b i t couplings and o r b i t a l r e d u c t i o n f a c t o r s i s not v a l i d i f the compound e x i s t s as a mixture of s p i n s t a t e s , i t i s e s s e n t i a l i n such cases t o have other evidence (such as Mossbauer spectra) t o ensure t h a t the s p i n system i s pure before a n a l y s i n g v Q f f values t h e o r e t i c a l l y . For the Br and I complexes the moments a t 80°K are «\» 3.6 B.M., i n d i c a t i n g t h a t t h ere i s s t i l l a s u b s t a n t i a l f r a c t i o n of h i g h - s p i n species present even a t t h i s temperature. The f a c t t h a t these two s a l t s g i v e almost i d e n t i c a l vs T curves suggests t h a t the s i z e of the anion alone i s probably not the major f a c t o r c o n t r o l l i n g the r a t i o of o _ h i g h - s p i n t o low-spin species ( i o n i c r a d i i are 1.96 A f o r Br and ° — 86 2.19 A f o r I ) . On the other hand i t appears t h a t very large anions such as BPh^ and C C r t N H - ^ t N C S ) ^ lead t o s m a l l e r room-temperature moments and thus favour the formation of complexes with 'Aj ground s t a t e s . If we assume t h a t these ions are q u a s i - s p h e r i c a l (BPh^ i s t e t r a h e d r a l 84 and the r e i n e c k a t e anion trans-octahedraI ) the e f f e c t i v e r a d i i are 105 estimated t o be about 4.4 and 4.9 A, r e s p e c t i v e l y . Some of the most i n t e r e s t i n g r e s u l t s are those f o r the C10^ and BF^ s a l t s , where both mono- and dihydrates were obtained. F e ( p y b e n ) 3 ( C I 0 4 ) 2 ' H 2 0 has a moment of 5.44 B.M. at 300°K, which drops t o 2.00 B.M. at 80°K. On i n t r o d u c t i o n of a second water molecule i n t o the l a t t i c e , the room-temperature moment decreases s l i g h t l y t o 5.35 B.M., while a t 80°K u f f i s only 1.29 B.M. I t thus appears t h a t the e x t r a H 20 molecule increases the f r a c t i o n of low-spin molecules a t a l l temperatures. An even more dramatic example of t h i s e f f e c t i s seen with the BF^ d e r i v a t i v e s , where the second l a t t i c e water molecule lowers the room-temperature moment by ^  0.5 B.M. and t h a t a t 80°K from 4.19 t o 1.38 B.M. These r e s u l t s are e s p e c i a l l y s t r i k i n g in view of the ease with which the dihydrates can be converted t o the monohydrates on a vacuum l i n e , a f t e r which the o r i g i n a l M e f f vs T curves f o r the mono-hydrates are e x a c t l y r e s t o r e d . Except f o r one previous case, these are 5 I the f i r s t examples of complexes showing T 2 - Aj crossover where two d i f f e r e n t hydrates of a given s a l t have been obtained, and the e f f e c t of the hydration s t a t e on the magnetic p r o p e r t i e s i s much more pronounced than we had expected. More w i l l be s a i d about t h i s below. 87 During the course of t h i s work, Sasaki and Shigematsu reported magnetic s u s c e p t i b i l i t y data f o r F e C p y b e n ^ C I O ^ ^ ' f ^ O . 5 I Although these authors suggest the occurrence of T 2 - Aj s p i n e q u i -l i b r i u m in the complex, t h e i r magnetic moment data are very d i f f e r e n t from o u r s . In p a r t i c u l a r , they r e p o r t u values of 5.25 and 3.33 B.M. ef f at 298 and 77.2°K r e s p e c t i v e l y , compared t o our values of 5.43 B.M. 1 0 6 (306.4°K) and 2.00 B.M. (79.8°K). We suggest t h a t the discrepancy i s most probably due t o the e x i s t e n c e of more than one magnetic isomer of F e ( p y b e n ) 3 ( C I 0 4 ) 2 - H 2 0 ( s i m i l a r t o the s i t u a t i o n in F e ( P y i H > 3 ( C I 0 4 ) 2 71 87 mentioned above ) and t h a t we and Sasaki and Shigematsu have obtained and s t u d i e d d i f f e r e n t isomers. T h e i r p r e p a r a t i o n procedure involved mixing e t h a n o l i c s o l u t i o n s of pyben and FeCI 2«4H 20, followed by a d d i t i o n of p e r c h l o r i c a c i d and then water. Three separate prepara-t i o n s of t h i s complex by our route described above gave c o n s i s t e n t and r e p r o d u c i b l e magnetic and Mossbauer data, but we have observed t h a t a d d i t i o n of small amounts of a c i d t o a r e a c t i o n mixture can change the room-temperature moments of these s a l t s by 20% or more. E l e c t r o n i c Spectra Molar e x t i n c t i o n c o e f f i c i e n t s (e ), and wavelengths of max ' ° maximum absor p t i o n (X ) at 25° in methanol s o l u t i o n are l i s t e d i n max Table IX f o r most of the complexes. Both the peak p o s i t i o n s and i n t e n s i t i e s are very s i m i l a r f o r a l l the compounds except Fe(pyben)0(NCS)„ which has both a lower £ and X , i n d i c a t i n g the ' 2 2 max max' 3 e s s e n t i a l d i f f e r e n c e between t h i s system and the r e s t of the complexes. The assignment of the 490 nm a b o s r p t i o n t o the d-d t r a n s i t i o n 5 5 -1 T ^ ->• Eg g i v e s a lODq value of about 20,000 cm , as expected f o r intermediate ligand f i e l d s near the h i g h - s p i n - low-spin crossover p o i n t . However, the i n t e n s i t y of the band i s abnormally high f o r a d-d t r a n s i t i o n . This could be due t o the f a c t t h a t there i s a strong ligand band near 330 nm with enhancement of the d-d band being a r e s u l t of 88 " i n t e n s i t y s t e a l i n g " . An a l t e r n a t i v e e x p l a n a t i o n i s t h a t t h i s i s a 107 TABLE IX E l e c t r o n i c Spectra of the Pyben Complexes i n Methanol a t 25° COMPLEXES F e ( p y b e n ) 3 ( C I 0 4 ) 2 * H 2 0 F e ( p y b e n ) 3 ( C I 0 4 ) 2 * 2 H 2 0 Fe(pyben) 3(N0 3) 2-H 20 Fe(pyben) 3(NCS) 2-H 20 Fe(pyben)3Br 2 F e ( p y b e n ) 3 I 2 F e ( p y b e n ) 3 ( B F 4 ) 2 - H 2 0 F e ( p y b e n ) 3 ( B F 4 ) 2 - 2 H 2 0 Fe(pyben) 3(BPh 4) 2-H 20 Fe (pyben ) 3 [ C r ( N H 3 ) 4 (NCS ) J Fe(pyben) 2(NCS) 2 max x 10 10.3 9.55 I I .0 10.2 9.80 10.2 9.80 X nm 490 490 490 decomposed 490 490 490 490 2"J2 not s u f f i c i e n t l y s o l u b l e not s u f f i c i e n t l y s o l u b l e 5.72 474 108 charge t r a n s f e r band of the t - TT* type, but i f t h i s i s the case, the '.2g i n t e n s i t y i s an order of magnitude s m a l l e r than normally observed f o r 88 such t r a n s i t i o n s An i n t e r e s t i n g f e a t u r e of the s o l u t i o n s p e c t r a l data in Table IX i s t h a t the 490 nm bands in Fe(pyben) 3(CIO^j^'xhLjO a " d FeCpyben^tBF^^'xH^O have very s i m i l a r e x t i n c t i o n c o e f f i c i e n t s f o r x = I o r 2, d e s p i t e the f a c t t h a t the mono- and d i h y d r a t e s show very d i f f e r e n t magnetic behaviour in the s o l i d s t a t e . As i n d i c a t e d in Table V I I , most of the s o l i d complexes in t h i s s e r i e s undergo very dramatic c o l o u r changes when cooled t o about 80°K. However, these c o l o u r changes are not observed when methanol s o l u t i o n s of the complexes are cooled. These observations s t r o n g l y suggest t h a t the h i g h - s p i n -low-spin crossover i s e x c l u s i v e l y a s o l i d s t a t e e f f e c t , and i t was t h e r e f o r e of i n t e r e s t t o study the temperature dependence of the e l e c t r o n i c s p e c t r a of some of these complexes as s o l i d s . No such mea-surements appear t o have been reported p r e v i o u s l y f o r other compounds showing s p i n e q u i l i b r i u m . The t h r e e complexes F e ( p y b e n ) 3 ( C I 0 4 ) 2 * H 2 0 , Fe(pyben)-j(N0 3)2*H20 and FeCpybenJ^B^ were chosen t o study in the form of KBr p e l l e t s . A l l t h r e e are b r i g h t orange in c o l o u r a t room temperature, changing t o dark purple when cooled i n l i q u i d n i t r o g e n , and each e x h i b i t s a d i f f e r e n t temperature dependence of (see F i g . 21). The s o l i d s t a t e s p e c t r a l data f o r these three d e r i v a t i v e s are given in Table X. At room temperature, i n a d d i t i o n t o the strong ligand band a t 330 nm, each spectrum c o n t a i n s a weak fe a t u r e a t about 490 nm, which s h i f t s t o s l i g h t l y longer wavelength and increases in i n t e n s i t y as the 109 TABLE X S o l i d State V i s i b l e Bands of the Fe(pyben) 3A 2 - x H ^ Q Complexes as a. Function of Temperature  ^max. I ntens i t T(°K) Peak I Peak 2 Peak I Peak F E ( P Y B E N ) 3 ( C L 0 4 ) 2 . H 2 0 8 8 . 0 5 4 4 . 2 2 . 4 1 0 7 . 0 5 4 4 . 2 1 . 0 1 2 8 . 0 5 4 4 . 1 8 . 8 1 7 2 . 0 5 3 8 . 1 4 . 0 1 8 5 . 0 5 3 8 . 1 2 . 4 2 0 8 . 0 5 3 4 . 1 0 . 1 2 2 3 . 0 5 3 0 . 8 . 2 2 3 5 . 0 5 3 0 . 6 . 4 2 4 6 . 0 5 1 0 . 3 . 2 2 5 7 . 0 5 1 0 . 3 . 2 2 6 3 . 0 5 1 0 . 3 . 0 2 7 3 . 0 5 1 0 . 2 . 4 2 9 3 . 0 5 0 5 . 1 . 3 3 0 3 . 0 4 9 5 . 0 . 8 3 1 3 . 0 4 9 0 . 0 . 2 3 2 3 . 0 4 9 0 . 0 . 2 C P Y B E N ) 3 ( N 0 3 ) 2 . H 2 Q 7 9 . 0 5 0 0 . 5 4 5 . 4 . 8 1 0 3 . 0 5 0 0 . 5 4 5 . 4 . 8 1 3 4 . 0 5 0 0 . 5 4 5 . 4 . 9 1 6 7 . 0 5 0 5 . 5 4 0 . 4 . 8 1 7 1 . 0 5 0 5 . 5 4 0 . 4 . 9 1 9 4 . 0 5 0 0 . 5 4 0 . 4 . 8 2 1 3 . 0 5 0 0 . 5 3 5 . 4 . 2 2 2 4 . 0 5 0 0 . 1 . 8 2 4 1 . 0 5 0 0 . 1 . 8 2 5 4 . 0 4 9 0 . 1 . 5 2 6 8 . 0 4 9 5 . 1 . 4 2 9 8 . 0 4 9 0 . 1 . 3 3 2 0 . 0 4 8 5 . 0 . 9 3 2 7 . 0 4 8 5 . 0 . 8 F E ( P Y B E N ) 3 8 R 2 1 0 3 . 0 5 0 5 . 5 4 4 . 9 . 6 1 4 7 . 0 5 0 5 . 5 4 3 . 9 . 8 1 5 7 . 0 5 0 5 . 5 4 0 . 9 . 5 1 6 8 . 0 5 0 5 . 5 3 8 . 9 . 2 1 7 9 . 0 5 0 5 . 5 3 5 . 9 . 0 1 8 5 . 0 5 1 5 . 8 . 8 1 9 3 . 0 5 1 5 . 8 . 6 2 0 4 . 0 5 1 5 . 8 . 4 2 1 7 . 0 5 1 0 . 8 . 0 2 2 7 . 0 5 0 5 . 7 . 2 2 3 4 . 0 5 0 5 . 7 . 0 2 4 7 . 0 5 0 0 . 6 . 8 2 5 5 . 0 4 9 8 . 1 . 8 2 6 1 . 0 4 9 0 . 1 . 6 2 7 3 . 0 4 9 0 . 1 . 7 2 9 3 . 0 4 9 0 . 1 . 3 110 temperature i s lowered. For the n i t r a t e d e r i v a t i v e the i n t e n s i t y of the l a t t e r band shows a sudden jump below ^ 220°K, c o i n c i d e n t with the appearance of another band of equal i n t e n s i t y a t ^  540 nm. The i n t e n s i t i e s of both bands remain e s s e n t i a l l y constant on f u r t h e r c o o l i n g . In the bromide complex t h e r e i s a sharp increase in i n t e n s i t y of the 490 nm band a t ^  250°K, but there is no c l e a r s e p a r a t i o n i n t o two bands u n t i l the temperature i s lowered t o ^  I80°K. For the p e r c h l o r a t e monohydrate-, only the 540 nm band i s seen c l e a r l y below, ^ 240°K, and i t s i n t e n s i t y continues t o r i s e with decreasing temperature.. The 540 nm ab s o r p t i o n a t low temperature i s o b v i o u s l y respon-s i b l e f o r the c o l o u r changes observed- i n these complexes. Moreover> as we s h a l l see below, the temperature a t which t h i s band appears i s approximately the same as t h a t a t which the presence of a low-spin-s p e c i e s can be detected in the Mossbauer s p e c t r a , and the i n t e n s i t i e s show a qua I i t a t i v e c o r r e l a t i o n w i t h the f r a c t i o n of 'Aj molecules deduced from Mossbauer- area rati>6s£ T h i s 540 nm band i s almost c e r t a i n l y due t o a charge t r a n s f e r t r a n s i t i o n i n v o l v i n g the I igand i r * o r b i t a l s . The i n t e n s i t y and p o s i t i o n ' of the band are c h a r a c t e r i s t i c of a unique c l a s s of compounds c o n t a i n i n g 89 "methine chromaphores" . The appearance of t h i s band i n d i c a t e s the formation of a five-membered aromatic r i n g system in which the i r o n t e l e c t r o n s c o n t r i b u t e s u b s t a n t i a l l y t o the i r - e l e c t r o n resonance 2g system of the methine chromaphore: c—c / \ N |\ c—c N V W N I l l This chromaphore i s known t o a r i s e only f o r a low-spin ground s t a t e in 88 f e r r o u s complexes , which confirms t h a t the 540 nm band i s a s s o c i a t e d with the 'A| s p e c i e s . Mossbauer Data 57 The Fe Mossbauer parameters f o r a l l the complexes are l i s t e d in Table X I . Most of the s p e c t r a obtained f o r t h i s s e r i e s of compounds c o n s i s t e d of four l i n e s , which c o u l d r e a d i l y be i d e n t i f i e d as two quad-rupole d o u b l e t s , and the r e l a t i v e i n t e n s i t i e s of these doublets changed with temperature. This behaviour i s i l l u s t r a t e d i n Figures 22 and 23 where s p e c t r a obtained f o r Fe(pyben) 3(CIO^^'H^Q between 295 and 8°K are shown. At 250°K and above only one p a i r of l i n e s i s seen, with <S and | A E _ | values in the ranges commonly observed f o r high-spin f e r r o u s s a l t s ' . At 230°K two weak shoulders have appeared on the l o w - v e l o c i t y l i n e of the h i g h - s p i n doublet. With f u r t h e r lowering of the temperature, the inner p a i r of l i n e s gains i n t e n s i t y r e l a t i v e t o the outer p a i r . The parameters (6 and | A E Q | ) of the inner doublet are t y p i c a l of those I expected f o r low-spin f e r r o u s d e r i v a t i v e s . Thus, the Mossbauer s p e c t r a 5 I unequivocally show the e x i s t e n c e of temperature-dependent - A| s p i n e q u i l i b r i a in these complexes. I t can a l s o be seen from Figures 22 and 23 t h a t the t r a n s i t i o n in Fe(pyben) 3(CI0 4)2*H20 i s spread over a t l e a s t 100°, and thus conforms t o Konig's "Type I I " c l a s s i f i c a t i o n 7 0 , and moreover, t h a t there i s an incomplete change in s p i n s t a t e s i n c e the high-spin component i s s t i l l present a t 8.7°K. For a l l the complexes 6 and | A E Q | values f o r the h i g h - s p i n species are very s i m i l a r both in magnitude and temperature dependence, 112 TABLE XI 57 Fe Mossbauer Parameters f o r the pyben Complexes S=o _) (mm s ]i(iTW) s~ ) S=2 I (mm s ) (mm s ) (mm s~ )(mm s ) F e ( P Y B E N ) 3 < C L 0 4 ) 2 . H 2 0 AREA FRACTION ( 5T 2/TOTAL) 8 . 7 . 7 5 . 4 8 . 2 3 . 23 1 .37 2 . 4 6 . 3 9 . 3 9 .23 8 4 . 0 . 7 5 . 4 8 .25 .25 1.36 2 . 5 2 . 3 3 . 3 3 .16 1 0 0 . 0 . 7 5 . 4 8 .25 .25 1.36 2 . 5 6 . 3 3 . 3 7 . 2 0 1 3 0 . 0 . 7 5 . 4 7 . 2 7 . 2 7 1.34 2 . 5 6 . 32 . 3 7 . 2 2 1 6 0 . 0 . 7 4 . 4 6 . 2 7 . 3 0 1 .33 2 .51 .40 . 3 8 .31 1 8 0 . 0 . 7 3 . 4 6 . 2 9 .25 1.31 2 . 5 3 .31 . 3 6 .38 1 9 0 . 0 . 7 3 . 4 5 . 2 9 . 2 7 1 .30 2 . 5 1 . 3 2 . 3 5 . 4 5 2 0 0 . 0 . 7 3 . 4 4 . 2 9 . 2 8 1 .30 2 . 4 9 . 3 3 . 3 6 . 6 3 2 1 0 . 0 . 7 0 . 4 1 . 2 4 . 2 4 1 . 30 2 . 4 7 . 32 . 3 6 . 7 7 220.0 .72 . 3 8 *24 .24 1.30 2 . 4 4 . 3 3 .35 .87 2 3 0 . 0 . 6 6 . 3 8 . 2 4 . 2 4 . 1 . 2 9 2 . 4 2 . 32 . 3 5 . 9 0 2 5 0 . 0 1 .29 2.30 . 3 6 . 3 3 1.00 295.0 1.25 2.17 . 3 8 .33 1.00 F E ( P r B E N ) 3 ( C L 0 4 ) 2 . 2 H 2 0 • 8 . 5 . 7 5 . 46 . 2 5 .25 .0 8 5 . 0 . 7 4 . 4 6 .25 . 2 5 . 0 1 2 0 . 0 . 7 5 . 4 7 .23 .23 1.36 2 . 5 6 . 3 6 .36 .18 1 6 0 . 0 . 7 4 . 4 5 . 2 5 . 2 4 1 .34 2 . 4 8 . 3 6 . 3 6 .27 1 8 0 . 0 . 7 3 . 4 6 . 2 5 . 2 9 1 .33 2 . 5 6 .38 .38 . 3 4 2 0 0 . 0 . 7 3 . 4 4 .25 . 2 5 1 .31 2 . 5 2 . 3 8 . 3 8 . 4 8 220V0 . 7 0 . 4 8 . 2 9 . 2 9 1 .29 2.51 . 3 3 .33 .57 2 4 0 . 0 . 7 8 . 4 3 . 2 9 . 2 9 1.28 2 . 4 2 . 3 4 .33 . 6 9 29 3 . 0 . 7 5 . 4 2 . 2 5 . 2 5 1 .25 2 . 2 1 . 3 6 . 3 3 . 8 4 FE(PYBeN13«N03)2 .H20 8 . 0 . 7 8 . 4 5 . 2 8 .28 1.36 2 . 5 2 . 3 6 . 4 0 .80 4 0 . 0 . 7 8 . 4 4 . 2 7 . 2 5 1 . 3 6 2 . 6 2 . 3 4 . 3 6 . 81 8 5 . 0 . 7 4 . 4 2 . 2 9 . 2 9 1 .35 2 . 6 5 . 3 4 . 3 6 . 8 2 1 0 0 . 0 . 7 4 . 3 7 . 2 9 . 2 9 1 .36 2 . 6 3 . 3 4 . 3 5 . 8 6 1 2 0 . 0 . 7 2 . 3 2 . 2 9 . 2 9 1 . 3 6 2 . 6 0 . 3 4 . 3 5 . 9 0 1 5 0 . 0 . 6 7 . 2 9 . 2 6 . 2 6 1 .34 2 . 5 6 . 3 2 . 3 4 . 9 2 1 8 0 . 0 1 .34 2 . 4 5 . 3 6 . 3 4 1 .00 210.0 - 1.32 2 . 3 5 .31 .32 1 .00 2 4 0 . 0 1.31 2 . 2 7 . 3 3 . 3 4 1 . 0 0 270.0 1.30 2 . 1 7 . 3 3 . 3 0 1 . 00 2 9 5 . 0 1 .24 1 . 8 9 . 3 3 . 2 9 1 .00 FECPYBENi3(NCS)2.H20 8 . 3 . 7 6 . 4 6 . 2 7 . 2 7 1.41 2 . 5 6 .32 . 3 7 . 8 9 4 0 . 0 . 7 6 . 4 6 . 2 7 . 2 7 1.41 2 . 6 5 . 3 0 .35 . 8 9 8 4 . 0 . 7 7 . 4 4 . 2 7 . 2 7 1 .40 2 . 6 5 . 2 9 . 3 2 . 8 9 1 0 5 . 0 . 7 4 . 4 9 . 2 7 . 2 7 1 .39 2 . 6 2 .28 . 3 0 . 8 9 1 1 5 . 0 . 7 6 . 4 7 . 2 7 .27 1 . 3 7 2 . 6 1 . 2 8 . 3 0 . 8 9 1 4 0 . 0 . 7 5 . 4 6 . 2 7 . 2 7 1.38 2 . 5 4 . 2 9 . 2 9 . 9 0 170.0 1.38 2 . 4 2 . 3 3 . 2 8 1 .00 2 0 0 . 0 1. 36 2 . 3 3 . 3 3 . 2 8 1 .00 2 3 0 . 0 1 .35 2 . 2 3 . 3 4 . 3 0 1 . 0 0 260.0 1 .33 2 . 1 3 . 3 3 . 3 0 1 . 0 0 295.0 1 .27 2 . 0 6 . 33 .30 1 .00 FE(PYBEN)3BRZ 8 .4 . 7 7 . 4 5 . 2 7 . 2 5 1.38 2 . 5 6 .28 . 31 . 41 4 0 . 0 . 7 6 . 4 4 . 2 6 . 2 4 1. 38 2 . 6 4 . 2 7 . 2 8 .41 8 4 . 0 . 7 6 . 4 4 . 2 5 . 2 4 1.38 2 . 6 8 . 2 8 . 2 8 . 41 1 1 0 . 0 . 7 4 . 4 3 . 2 5 . 2 4 1 .36 2 . 6 6 . 2 8 . 2 8 .50 1 4 0 . 0 . 7 3 .41 . 2 4 . 2 4 1 .35 2 . 6 3 . 2 6 . 2 9 . 61 1 7 0 . 0 . 7 3 . 3 7 .25 .25 1 .33 2 . 5 7 . 3 0 . 2 9 .72 2 0 0 . 0 . 7 2 .38 . 2 5 .25 1.32 2 . 4 9 . 3 0 . 2 9 . 8 3 2 3 0 . 0 . 6 8 . 3 4 . 2 4 . 2 5 1. 30 2 . 4 6 . 3 0 . 2 9 . 8 7 260.0 1.30 2 . 37 . 3 5 . 2 8 1 . 0 0 2 9 3 . 0 1 .25 2 . 2 3 .32 . 2 8 1 .00 113 TABLE XI (Conti nued/-) s=o S=2 AE F E ( P Y 8 E N ) 3 I 2 ) (mn s ') (mm s )(mm s ) Q . - I v - I ' - I * - I (mm s ) (nm s ) (mm s Hmm s ) AREA FRACTION ( 5T 2/TOTAL) 8 . 3 . 7 5 . 4 3 . 2 5 . 2 5 1 .37 2 . 5 3 . 2 8 . 2 6 . 4 0 8 0 . 0 . 7 4 . 41 . 2 6 . 2 6 1 .37 2 . 6 5 . 27 . 2 7 . 4 3 1 1 5 . 0 . 7 5 . 4 2 . 2 6 . 2 6 1. 36 2 . 6 5 . 2 7 . 2 7 . 5 9 1 3 0 . 0 . 7 5 . 3 9 .26 . 2 6 1 .36 2 . 6 4 . 2 7 . 2 7 . 6 4 1 6 0 . 0 . 7 2 . 41 . 2 6 . 2 6 1 . 3 5 2 . 6 0 . 2 8 . 2 9 . 7 4 1 9 0 . 0 . 7 0 . 3 7 . 2 6 . 2 6 1 .34 2 . 5 3 . 3 0 . 2 8 . 8 4 2 2 0 . 0 . 7 6 . 3 4 .26 . 2 6 1.31 2 . 4 6 . 2 8 . 2 9 . 8 8 2 9 5 . 0 1 .27 2 . 2 0 . 32 . 3 2 1 .00 F E ( P Y B E N ) 3 < B P 4 ) 2 . H 2 0 8 . 5 . 7 5 . 4 4 . 2 3 . 2 4 1 .36 2 . 5 6 . 2 8 . 3 2 . 5 2 8 6 . 0 . 7 5 . 4 2 . 2 3 . 2 4 1 . 3 6 2 . 6 7 .30 . 2 9 . 5 3 1 0 0 . 0 . 7 4 . 4 0 . 2 3 . 2 4 • 1 . 3 6 2 . 6 5 . 3 0 . 2 9 . 6 1 1 3 0 . 0 . 7 4 . 4 0 . 2 4 . 2 3 1 .35 2 . 6 2 . 31 . 3 2 . 6 7 1 6 0 . 0 . 7 3 . 4 0 . 2 7 . 2 5 1 .34 2 . 5 9 . 3 2 . 3 4 . 7 1 1 9 0 . 0 . 7 0 . 3 9 . 2 6 . 2 6 1.31 2 . 5 4 . 3 2 . 3 5 . 7 7 2 2 0 . 0 . 6 7 . 3 7 .28 . 31 1 .30 2 . 4 4 . 3 2 . 3 5 . 8 3 2 5 0 . 0 . 5 8 . 2 9 . 2 4 . 2 5 1 .28 2 . 3 4 . 3 4 . 3 8 . 8 9 2 9 5 . 0 . 5 9 . 2 6 . 2 5 . 2 5 1 . 2 4 2 . 1 7 . 3 8 . 3 8 . 8 8 F E ( P Y B E N ) 3 ( 8 P 4 ) 2 . 2 H 2 0 1 1 5 . 0 . 7 3 . . 5 0 . 2 9 . 2 5 . 0 130 .0 . 7 3 . 4 8 . 2 9 . 2 5 . 0 1 6 0 . 0 . 7 2 . 4 7 . 2 8 . 2 5 1 .29 2 . 3 4 . 3 0 . 3 0 . 1 1 • 1 9 0 . 0 . 7 1 . 4 6 . 2 8 . 2 5 1 . 2 9 2 . 3 4 .31 .31 . 1 1 2 2 0 . 0 . 71 . 4 6 . 2 8 . 2 5 1 .31 2 . 3 6 . 3 0 . 3 0 . 1 3 2 5 0 . 0 . 6 8 . 4 3 . 3 0 . 2 5 1 .27 2 . 1 7 . 3 0 . 3 0 . 2 1 2 7 3 . 0 . 6 8 . 3 8 . 2 9 . 2 8 1 . 2 4 2 . 1 0 . 3 0 . 3 0 . 3 5 2 9 5 . 0 . 7 7 . 4 2 . 2 9 . 2 9 1 .21 1 . 8 3 . 3 0 . 3 0 . 6 1 F E ( P Y B E N ) 3 C B P H 4 ) 2 . H 2 0 8 . 4 . 7 5 . 3 6 . 2 9 . 2 4 1 .36 2 . 5 5 . 3 9 . 4 0 .33 8 7 . 0 . 7 4 . 3 4 .30 - . 2 7 1 .35 2 . 6 7 . . 3 9 . 3 9 . 3 4 1 0 0 . 0 . 7 2 .33 . 3 0 . 3 0 1 .37 2 . 7 1 . 3 9 . 3 9 . 3 8 1 2 0 . 0 . 7 4 . 3 4 . 2 8 . 2 5 1 . 3 6 2 . 6 6 . 3 6 . 3 5 . 4 0 1 5 0 . 0 . 7 1 . 3 4 . 2 8 . 2 5 1 .34 2 . 6 0 . 3 6 . 3 5 . 4 5 1 8 0 . 0 . 7 2 . 3 4 . 2 8 . 2 5 1 .33 2 . 5 4 . 3 6 . 3 5 . 4 8 2 1 0 . 0 . 6 7 . 3 3 . . 2 8 . 2 5 1 .32 2 . 4 2 . 3 6 . 3 5 . 5 4 2 4 0 . 0 . 6 6 . 3 4 . 2 8 . 2 5 1 .29 2 . 3 6 . 3 6 . 3 5 . 5 7 2 7 0 . 0 . 6 4 . 3 4 . 2 8 . 2 5 1 .27 2 . 2 2 . 3 6 . 3 5 . 5 9 2 9 5 . 0 . 5 7 . 3 2 . 2 8 . 2 5 1 .22 2 . 1 2 . 3 6 . 3 5 . 6 3 F E ( P V B E N ) 2 ( N C S ) 2 1 1 5 . 0 1 .42 2 . 5 8 . 3 0 . 2 8 1 .00 2 9 S . 0 1 .32 1 . 8 9 . 3 0 . 2 8 1 .00 F E « P Y B E N ) 3 ( C R « N H 3 ) 2 « N C S ) 4 ) 2 1 1 5 . 0 . 7 2 . 3 7 . 2 7 . 2 7 2 9 5 . 0 . 6 7 . 2 9 . 2 8 .26 .0 .0 114a FIGURE 22 Mossbauer Spectra of F e ( p y b e n ) 3 ( C I 0 4 ) 2 ' H 2 0 between 200 and 295 8K 114 b FIGURE 22 VELOCITY ( M M SEC"') 115a FIGURE 23 Mossbauer Spectra of FeCpyben^CCIO^-l-^O between 8.7 and 190 °K FIGURE 23 1 1 6 and the nature of the anion appears t o have l i t t l e o r no e f f e c t on these parameters. The same i s t r u e of the parameters f o r the low-spin s p e c i e s . However, the r e l a t i v e i n t e n s i t i e s of the two doublets are s t r o n g l y dependent on the nature of the anion and the number of waters of c r y s t a I I i z a t i o n . In order t o estimate the r e l a t i v e amounts of h i g h - s p i n and low-spin species present at a given temperature, one can d e f i n e an "area f r a c t i o n " as 5 A F = Area under the T 2 p a i r of l i n e s T o tal s p e c t r a l area These values are l i s t e d i n the f i n a l column of Table X I . I t i s important t o r e a l i z e t h a t in equating A.F. t o the ac t u a l f r a c t i o n of h i g h - s p i n ions present one i s t a c i t l y making the assumption t h a t the r e c o i l - f r e e f r a c t i o n s are i d e n t i c a l f o r both h i g h - s p i n and low-spin species in a given sample. T h i s assumption may not be s t r i c t l y v a l i d , but i t seems improbable t h a t the r e c o i l - f r e e f r a c t i o n s f o r the two-spin s t a t e s w i l l d i f f e r a p p r e c i a b l y . In p r a c t i c e , when A.F. i s very c l o s e t o zero o r u n i t y , s t a t i s t i c a l e r r o r introduced from f i t t i n g of the Mossbauer spectrum makes i t s e s t i m a t i o n very d i f f i c u l t . However, w i t h i n the range 0.1 - A.F. - 0.9 the u n c e r t a i n t y in t h i s parameter i s judged t o be < 0.05, based on d i f f e r e n t runs of the same compound. The A.F. data in Table XI i n d i c a t e t h a t FeCpyben^CNCS^ i s e s s e n t i a l l y a pure h i g h - s p i n complex in the temperature range s t u d i e d , whereas Fe(pyben) 3CCr(NH 3) 7(NCS) 4I] 7 i s purely low-spin. For a l l the FIGURE 24 Temperature Dependence of the Mossbauer Area F r a c t i o n s of the Pyben Complexes 0 Fe(pyben ) 3 ( C I 0 4> 2'H 2 0 A F e ( p y b e n ) 3 ( C I 0 4 ) 2 . 2 H 2 0 + Fe(pyben ) 3 ( N 0 3> 2.H 2 0 X Fe(pyben) 3(NCS) 2.H 2 0 Fe(pyben) 3Br 2 F e ( p y b e n ^ 3 I 2 X F e ( p y b e n ) 3 ( B F 4 ) 2 ' H 2 0 z F e ( p y b e n ) 3 ( B F 4 ) 2 - 2 H 2 0 Y F e ( p y b e n ) 3 ( B P h 4 ) 2 * H 2 0 117b FIGURE 24 o 118 other complexes s p i n crossover i s observed, but in no case i s there a complete change in ground s t a t e between 8 and 300°K(see F i g . 24). As mentioned above, the temperature a t which the s o l i d s t a t e v i s i b l e band at 540 nm appears i s roughly the same as t h a t a t which A.F. begins t o depart from u n i t y in the three complexes f o r which s o l i d s t a t e v i s i b l e s p e c t r a were obtained. It was a l s o seen in Table X t h a t the i n t e n s i t y of t h i s band increased very s l i g h t l y with decreasing T f o r the n i t r a t e d e r i v a t i v e , somewhat more f o r the bromide complex, and q u i t e s t r o n g l y f o r the p e r c h l o r a t e monohydrate. The A.F. values i n d i c a t e t h a t these i n t e n s i t y changes are a t l e a s t q u a l i t a t i v e l y r e l a t e d t o the f r a c t i o n of low-spin species present (I - A.F.). Although t h e r e i s a c l e a r q u a l i t a t i v e c o r r e l a t i o n between V Q f f and A.F., there i s no q u a n t i t a t i v e c o r r e l a t i o n as a few examples w i l l i l l u s t r a t e . In the s i m p l e s t p o s s i b l e approach one might a s s i g n a zero I 5 moment t o the Aj s t a t e , a moment of about 5.4 B.M. t o the T^ s t a t e , and then compute an "average moment" at a given temperature from the area f r a c t i o n . This i n v a r i a b l y leads t o c a l c u l a t e d moments much s m a l l e r than t h e observed v a l u e s , as shown in Table X I I . Further i n d i c a t i o n s t h a t t h i s approach i s i n c o r r e c t are seen in the low temperature data f o r the p e r c h l o r a t e and t e t r a f I u o r o b o r a t e d i h y d r a t e s , and the re i n e c k a t e s a l t . A l l t h r ee complexes appear t o be f u l l y in the 'Aj s t a t e a t l i q u i d n i t r o g e n temperature, yet have moments of ^  1.3 - 1.7 B.M. at 80°K. Even i f we assume t h a t we cannot d e t e c t an A.F. value s m a l l e r than O.I., the maximum moments obtained f o r these s a l t s would be only ^ 0.5 B.M. on the basis of t h i s procedure. 90 Konig and Kremer have discussed t h i s problem i n some d e t a i l . 119 TABLE XI I Comparison of Observed Room Temperature Magnetic Moments with those C a l c u l a t e d by the Simple Model Described in the Text S a l t y ( C a l c . from A.F.) y(expt) (B.M.) (B.M.) Fe(pyben) 3(CI0 4) 2«2H 20 Fe(p y b e n ) 3 ( B F 4 ) 2 - 2 H 2 0 Fe(pyben) 3(BPh 4) 2-H 20 4.5 5.35 3.29 4.96 3.40 4.99 F e ( p y b e n ) 3 C C r ( N H 3 ) 2 ( N C S ) 4 ] 2 < 0.54 4.01 120 They poi n t out t h a t i t i s i n a p p r o p r i a t e t o assign a moment value of about 5.4 B.M. t o the high-spin s t a t e due t o the e f f e c t s of s p i n - o r b i t c o u p l i n g on ground and e x c i t e d s t a t e s near the s p i n crossover energy. There i s a l s o e x t e n s i v e mixing of e i g e n s t a t e s in t h i s range of e n e r g i e s , which they suggest could lead t o a non-zero moment f o r the low-spin s t a t e . F u r t h e r -more, the moments c a r r i e d by both species vary as the energy d i f f e r e n c e 70 90 Ae between the two s t a t e s changes. Konig and Kremer ' have po s t u l a t e d t h a t in general Ae changes with temperature in complexes showing s p i n crossover, implying t h a t both the h i g h - s p i n and low-spin f r a c t i o n s have temperature-dependent moments. There are two aspects of the data presented so f a r which are r a t h e r p u z z l i n g and r e q u i r e f u r t h e r comment. The f i r s t i s t h a t FeCpybenJ-jCNCS^'f^O shows s p i n crossover (although t o only a l i m i t e d extent) whereas Fe(pyben) 2(NCS) 2 remains f u l l y h i g h - s p i n down t o II5°K. Of the anions used in t h i s study NCS l i e s highest in the spectrochemicaI s e r i e s , i . e . , i t i s the s t r o n g e s t f i e l d l i g a n d , and when bonded d i r e c t l y t o i r o n as i n Fe(pyben) 2(NCS) 2 might be expected t o enhance the low-spin c h a r a c t e r of the f e r r o u s ion by i n c r e a s i n g lODq A t e n t a t i v e e x p l a n a t i o n of why t h i s does not happen w i l l be o f f e r e d below. The second strange f e a t u r e i s t h a t f o r FeCpyben^CCIO^^'xh^O and Fe(pyben) 3.(BF 4) 2*xH 20, both y .^(T) and A.F.(T) change d r a s t i c a l l y when x goes from I t o 2. I t i s c l e a r from the ease with which the second l a t t i c e water molecule can be pumped o f f at room temperature t h a t i t i s only l o o s e l y bound, whereas f u r t h e r dehydration of the complexes cannot be achieved in vacuo even at 150°. There seem t o be only two p o s s i b l e 5 I mechanisms by which the T ?/ A, r a t i o s could be a f f e c t e d by t h i s second 121 water mo I e c u l e : 65 (1) C r y s t a l packing e f f e c t s . Konig and Watson have shown t h a t the Fe-N bonds in F e C b i p y ^ C N C S ^ are s l i g h t l y s h o r t e r in the low-spin than i n the hi g h - s p i n s t a t e . If the pyben complexes are s i m i l a r in t h i s respect i t might be supposed t h a t the a d d i t i o n a l water molecule produces a l a t t i c e compression e f f e c t which increases the low-spin f r a c t i o n . However, F i s h e r and Drickamer 6^ have s t u d i e d the e f f e c t of pressure on a number of phen and bipy F e ( I I ) complexes, and i t appears from t h i s work t h a t pressures of the order of 10 kbar o r more are required t o produce a net hi g h - s p i n -> low-spin c o n v e r s i o n . I t i s almost inconceivable t h a t t h i s l o o s e l y held water molecule could cause an e f f e c t of t h i s magnitude. (2) Hydrogen bonding e f f e c t s . As was mentioned above the imino hydrogen on the benzimidazole p o r t i o n o f the pyben molecule i s s l i g h t l y a c i d i c . The a c i d i t y increases on c h e l a t i o n 7 ^ , and seems t o bear a d i r e c t r e l a t i o n t o the s t r e n g t h of the N-metal bond. This e f f e c t was a t t r i b u t e d t o a resonance mechanism of the type: H M According t o t h i s scheme, the formation of a m u l t i p l e N-metal bond w i l l enhance the a c i d i t y of the imino hydrogen. To t u r n t h i s argument around, a weakening of the N-H bond by hydrogen bonding t o a water molecule should cause a strengthening of the N-metal o bond. In order t o prevent 1 2 2 too much charge accumulation on the metal, such an increase in N -*• metal a-donation would probably be accompanied by an increase i n back r r-donation from the metal t _ o r b i t a l s t o a s s i s t formation of the 2g 89 methine chromophore c h a r a c t e r i s t i c of the low-spin s t a t e . These e f f e c t s would be s y n e r g i c a l l y r e l a t e d , s i n c e strengthening of the N-metal a bond and of the methine chromophore w i l l both enhance the low-spin f r a c t i o n . The energy of such an N-H...0 hydrogen bond would presumably l i e -I 91 between those of an N-H...N bond (1.3 kcaI mole in NH^) and an -I 91 N-H...F bond (5 kcaI mole i n NH^F) . These energies are t y p i c a l of those encountered in p h y s i c a l a d s o r p t i o n systems such as the rare gases 92 on g r a p h i t e , where the adsorbed species can be r e a d i l y pumped o f f , and would account f o r the ease with which the d i h y d r a t e s are converted t o the monohydrates. Hydrogen bonding may a l s o account f o r the behaviour of the thio c y a n a t e complexes. If we assume t h a t NCS and pyben are approximately equal in the spectrochemicaI s e r i e s , and chemical evidence suggests t h i s i s l i k e l y t o be t r u e , then i t i s perhaps not too s u r p r i s i n g t o f i n d t h a t F e t p y b e n ^ N C S ^ i s a high - s p i n complex. In F e ( p y b e n ) 3 ( N C S ^ ^ O , on the o t h e r hand, s i n c e the NCS ions are not d i r e c t l y bonded t o i r o n , i t would be p o s s i b l e f o r them t o form weak hydrogen bonds with the N-H groups on the Iigands. Note t h a t t h i s i s the only anion s t u d i e d here which would even remotely be expected t o hydrogen bond. 123 D i s c u s s i o n of the Cation S t r u c t u r e In order t o o b t a i n more information on the e l e c t r o n i c and 2+ geometrical s t r u c t u r e of the Fe(pyben)^ c a t i o n i n these complexes, i t was decided t o use magnetic p e r t u r b a t i o n Mossbauer spectroscopy t o deter-mine the n values and sign s of V"zz in some of the compounds. As pointed out above, f o r both the low-spin and hi g h - s p i n s p e c i e s , isomer s h i f t and quadrupole s p l i t t i n g values show only a very s l i g h t dependence on the nature of the ani o n , suggesting t h a t the b a s i c s t r u c t u r e of the c a t i o n in a given s p i n s t a t e i s probably very s i m i l a r in a l l the complexes. T h i s suggestion i s f u r t h e r supported by the f a c t t h a t the I.R. s p e c t r a of the c a t i o n s are i d e n t i c a l throughout the s e r i e s . For these reasons, and because of the d i f f i c u l t i e s encountered in o b t a i n i n g magnetic p e r t u r b a t i o n s p e c t r a a t high temperatures, only Fe(pyben) 3'(CI 0 4) 2«H 20 was studi e d by t h i s technique a t 295°K, w h i l e such s p e c t r a were obtained f o r t h r e e complexes which are f u l l y low-spin at 80°K: F e ( p y b e n ) 3 ( C I 0 4 ) 2 * 2 H 2 0 , F e ( p y b e n ) 3 ( B F 4 ) 2 - 2 H 2 0 and Fe<pyben) 3CCr<NH 3) 2(NCS ) 4 I ] . The r e s u l t s of these measurements are presented in Table X I I I , and the spectrum of BF~ s a l t i s shown i n Figure 25. For the high-spin p e r c h l o r a t e mono-hydrate d e r i v a t i v e , we f i n d Vz_>0 and n = 0.5. The three complexes with 'Aj ground s t a t e s a l l show n values very c l o s e t o u n i t y and the signs of the efg's cannot be determined unambiguously. The two nitrogens of the pyben l i g a n d which are bonded t o i r o n are not e q u i v a l e n t . If we denote the p y r i d i n e n i t r o g e n by N and the imidazole n i t r o g e n by n, then t he ligands can arrange themselves around the i r o n atom t o g i v e e i t h e r a mer- or fac-octahedraI s t r u c t u r e : TABLE XII I Signs o f V z z and Magnitudes o f n Deduced f r o m Magnetic P e r t u r b a t i o n Mossbauer Measurements -SIGN OF V n zz F e ( p y b e n ) 3 ( C I 0 4 ) 2 - H 2 0 (high-spin) + * 0.5 F e ( p y b e n ) 3 ( C I 0 4 ) 2 - 2 H 2 0 (low-spin) ? ^ 0 . 9 F e ( p y b e n ) 3 ( B F 4 ) 2 - 2 H 2 0 (low-spin) ? ^ 0 . 9 Fe(pyben) 3L~Cr(NH 3) 2(NCS) 43 2 •( low-spin) ? ^ 0 . 9 FIGURE 25 Mossbauer Spectrum of FeCpyberO^CBF^^^H at 80° K in a L o n g i t u d i n a l Magnetic F i e l d 50 kG. Computed Spectra f o r V z z>0 and n and 0.9 are Shown f o r Comparison. FIGURE 25 126 mer f a c In the mer-octahedraI form, the symmetry about i r o n w i l l be C 2 y along the N-Fe-n d i r e c t i o n , w h i l e in the f a c - form, i t i s C 3 v along the t r i g o n a l C l , I , I ] a x i s of the octahedron. Due t o the asymmetry of the l i g a n d , s t e r i c requirements w i l l favour the mer-octahedraI form s i n c e in t h i s case the threebenzimidazole groups can be f u r t h e r apart than in the l a c - o c t a h e d r a l case. 5 The T 2 ground s t a t e can be t r e a t e d i n the same way as the complexes discussed in Chapter I I I . The mer-octahedraI s t r u c t u r e corresponds t o a t e t r a g o n a l d i s t o r t i o n with the z a x i s along the N-Fe-n d i r e c t i o n , and sinae the N-Fe-N and n-Fe-n axes are not e q u i v a l e n t there w i l l be a s u b s t a n t i a l rhombic f i e l d as w e l l . On the o t h e r hand, the fac-octahedral s t r u c t u r e corresponds t o a t r i g o n a l d i s t o r t i o n and the rhombic term i s expected t o be s m a l l . The magnitude and temperature dependence of A E Q f o r F e ( p y b e n ) 3 ( C I O ^ ' h ^ O (see Table XI) s t r o n g l y 127 suggest t h a t the ground s t a t e i s an o r b i t a l s i n g l e t . in the te t r a g o n a l case the s i n g l e t i s lxy>, which w i l l g i v e V >0 as observed, in c o n t r a s t 2 t o the t r i g o n a l case where the Iz > s i n g l e t w i l l produce a negative V z z» A f i t of the A E Q V S T data using the c r y s t a l f i e l d model o u t l i n e d in Chapter III y i e l d s a te t r a g o n a l d i s t o r t i o n of -385 cm ' and a rhombic s p l i t t i n g of -270 cm '. With these c r y s t a l f i e l d parameters the c a l c u -lated n value i s 0.45 in good agreement with the observed value. Thus, the Mossbauer data show c o n c l u s i v e l y t h a t the c a t i o n in FeCpyben^CCIO^^'r^O adopts a mer-octahedraI c o n f i g u r a t i o n , and there seems l i t t l e reason t o doubt t h a t the other pyben complexes are s i m i l a r . From the c r y s t a l f i e l d treatment the s p i n - o r b i t c o u p l i n g constant X i s estimated t o be ^  100 cm This i n d i c a t e s t h a t there i s no e x t e n s i v e del oca I i z a t i o n of the t 2 e l e c t r o n s onto the Iigands, 5 c o n s i s t e n t with an " i o n i c " f e r r o u s system. The low-spin ground s t a t e i s more d i f f i c u l t t o t r e a t i n a reasonably exact way. Both the a and TT bonding systems w i l l c o n t r i b u t e t o 93 94 the efg, ' and the e f f e c t s of these two c o n t r i b u t i o n s cannot be se p a r a t e l y c a l c u l a t e d . However, low-spin f e r r o u s complexes are expected 95 96 97 t o f o l l o w the p o i n t charge model f a i r l y c l o s e l y . This model p r e d i c t s ' V = V = V = 0 f o r a fac-octahedraI s t r u c t u r e , and hence zero qiiad-xx yy zz : — ' ^ rupole s p l i t t i n g (although d i s t o r t i o n s from r e g u l a r geometry could make |AEp|^ 0 ) . For the mer-octahedraI case the p r e d i c t i o n s are V^ x = 0 and V = -V z z, t o g i v e n = I and an indeterminate s i g n of the e f g . Thus, our r e s u l t s f o r the low-spin complexes are a l s o f u l l y c o n s i s t e n t with mer-octahedraI s t r u c t u r e s . 128 I t i s i n t e r e s t i n g t o compare these r e s u l t s with those obtained f o r Fe(mephen) 3(BF 4) 2 and F e ( m e p h e n ) 3 ( C L 0 4 ) 2 . 6 8 In the 5 T 2 s t a t e the temperature dependence of | A E Q | f o r these compounds suggests t h a t the ferr o u s ion has an o r b i t a l s i n g l e t ground s t a t e , although in magnitude the s p l i t t i n g s are con s i d e r a b l y s m a l l e r than those of the pyben d e r i -v a t i v e s . This i m p l i e s a s m a l l e r c r y s t a l f i e l d d i s t o r t i o n in the mephen complexes, which i s very l i k e l y due t o the f a c t t h a t the two nitrogens in mephen (see F i g . 19) are e s s e n t i a l l y e q u i v a l e n t , in c o n t r a s t t o the s i t u a t i o n in pyben. T h i s d i f f e r e n c e between the two types of ligands i s even more c l e a r l y seen in the 'Aj s t a t e s . In the mephen complexes i s a l s o p o s i t i v e , but n i s c l o s e t o zero showing a q u i t e symmetric environment about the f e r r o u s i o n . 2+ Our s t u d i e s on the Fe(pyben) 3 complexes have brought t o l i g h t 5 1 some new fe a t u r e s of the T 2 - Aj s p i n crossover phenomenon. From s o l u t i o n and s o l i d s t a t e v i s i b l e s p e c t r a we have been able t o show t h a t the crossover very probably occurs only i n the s o l i d s t a t e . This implies t h a t the phenomenon depends not only on the f i e l d s t r e n g t h s of the ligands bonded t o i r o n , but i s u l t i m a t e l y c o n t r o l l e d by c r y s t a l l i n e forces in the l a t t i c e . That these fo r c e s are e i t h e r very weak o r r e l a t i v e l y remote from the f e r r o u s ion i s demonstrated by.the f a c t . t h a t large changes in area f r a c t i o n from one compound to another are not accompanied by any s i g n i f i c a n t changes in c r y s t a l f i e l d s p l i t t i n g parameters, (as i n d i c a t e d by A E Q values f o r the hi g h - s p i n f r a c t i o n s ) . As seen in Chapter III these 2+ parameters are normally q u i t e s e n s i t i v e t o small changes in the Fe envi ronment. 129 Accompanying the - 'A j crossover in these complexes i s the appearance of a strong v i s i b l e a b sorption band, producing a dramatic c o l o u r change. This band has been i n t e r p r e t e d as a r i s i n g from the formation of a "methine chromaphore" t y p i c a l o f covalent systems, and i n d i c a t e s t h a t r e d i s t r i b u t i o n s of both the f e r r o u s 3d e l e c t r o n s and the ligand IT e l e c t r o n s are involved in the crossover process. The i n f l u e n c e of the hydration s t a t e of the c r y s t a l on the s p i n e q u i l i b r i u m has been a t t r i b u t e d t o hydrogen bonding between the water molecule and the a c i d i c hydrogen on the benzimidazole fragment of the l i g a n d . Dosser, e t a l . 7 ^ have a l s o observed t h a t the magnetic moments of Fe(py irrO-jSO^'Xr^O (x = 2,3) were somewhat d i f f e r e n t (the t r i hydrate g i v i n g lower U Q f f v a l u e s ) , but o f f e r e d no e x p l a n a t i o n f o r t h i s behaviour. Hydrogen bonding seems l i k e l y in t h i s case as w e l l . The asymmetry of the pyben l i g a n d a l l o w s i n p r i n c i p l e the 2+ e x i s t e n c e of two geometrical isomers of the Fe(pyben)^ c a t i o n . Magnetic p e r t u r b a t i o n Mossbauer s p e c t r a f o r both s p i n s t a t e s i n d i c a t e t h a t i t i s the meridionaI isomer which occurs i n these complexes. 130 CHAPTER V FERROUS PORPHYRINS AND THEIR DERIVATIVES Intr o d u c t i o n The porphyrins c o n s t i t u t e one of the most important c l a s s e s of compounds in b i o l o g i c a l systems. The b a s i c s t r u c t u r e c o n s i s t s of four p y r r o l e u n i t s l i n k e d together t o form a large planar aromatic r i n g system (the porphin n u c l e u s ) , w i t h s u b s t i t u e n t s a t the e i g h t 8 p o s i t i o n s of the p y r r o l e r i n g s . A number of n a t u r a l l y o c c u r r i n g pigments are metal c h e l a t e complexes of the p o r h y r i n s . Thus the haeme p r o s t h e t i c group found in haemeproteins cont a i n s a f e r r o u s ion bonded t o the four p y r r o l e n i t r o g e n s , and i n haemoglobin f o r example, i s attached t o the p r o t e i n v i a an imidazole n i t r o g e n which occupies the f i f t h c o o r d i n a t i o n s i t e of the metal. In deoxyhaemoglobin the f e r r o u s ion i s in a hi g h - s p i n (S = 2) s t a t e , whereas oxygenation induces a switch t o the low-spin (S = 0) s t a t e . The a b i l i t y of the haeme u n i t t o undergo r e v e r s i b l e oxygenation must be s t r o n g l y i n f l u e n c e d by the d e t a i l e d e l e c t r o n i c s t r u c t u r e of the fer r o u s ion. Since in most circumstances e l e c t r o n s p i n resonance cannot be observed in fer r o u s complexes, Mossbauer measurements may o f f e r the only method of studying such s t r u c t u r a l d e t a i l s of the i r o n atom. Thus, con s i d e r a b l e e f f o r t has been expended i n the study of haemeproteins by 98 t h i s technique, and an e x c e l l e n t review has been pub Iished by Lang 131 I t seemed of i n t e r e s t t o i n v e s t i g a t e s y n t h e t i c i r o n p o r p h y r i n s , both from the p r e p a r a t i v e and s t r u c t u r a l p o i n t s of view. For example, i t should be p o s s i b l e in p r i n c i p l e t o prepare a s e r i e s of ferrous porphyrins in which system a t i c changes are made in the porphyrin s t r u c t u r e , and t o study the e f f e c t s of these changes on the e l e c t r o n i c ground s t a t e of i r o n . At the same time the i n f l u e n c e of various a x i a l ligands could be i n v e s t i g a t e d . The present chapter describes some attempts in these d i r e c t i o n s -It i s d i f f i c u l t t o i s o l a t e e i t h e r natural o r s y n t h e t i c f e r r o u s porphyrins as s o l i d complexes, and only two a p p l i c a t i o n s of Mossbauer spectroscopy t o such systems had been reported when t h i s work began. 99 E p s t e i n , e t a l . s t u d i e d some hexacoordinate adducts of the type FeD<2> where L was e i t h e r protoporphyrin IX (PP) o r meso-tetraphenyIporphin (TPP), and X = p y r i d i n e ( p y ) , p i p e r i d i n e (pip) and imidazole (im). Only F e ( T P P ) ( p i p ) 2 was i s o l a t e d and c h a r a c t e r i z e d , the other complexes only being s t u d i e d as fr o z e n s o l u t i o n s . Kobayashi and co-workers' 1^ have s t u d i e d Fe(TPP) and i t s bis-adducts with p y r i d i n e and t e t r a h y d r o f u r a n (THF) in the s o l i d s t a t e , using Mossbauer and magnetic s u s c e p t i b i l i t y measurements. Fe(TPP) ( s t r u c t u r e I) was the f i r s t example of a f e r r o u s 132 porphin t o be i s o l a t e d in the s o l i d s t a t e without a d d i t i o n a l Iigands in the a x i a l c o o r d i n a t i o n s i t e s , and i t s p r e p a r a t i o n was reported inde-A + 1 ^ 4. 101,102 . . , pendentIy by two groups. This square planar complex has a high- s p i n ground s t a t e f o r the fe r r o u s i o n , whereas FeCTPPMpy^ i s A - 4 . - 100-102 ,- -pj.T— . . . . . . . • , , 100 . diamagnetic . For the THF adduct, Kobayash i, et_ a_l_. have assigned an inter m e d i a t e - s p i n (S = I) ground s t a t e t o the fe r r o u s ion on the ba s i s of a magnetic moment value of 2.75 B.M, at room temperature, 102 while Col I man and Reed have reported a value of 5.1 B.M., correspon-ding t o an S = 2 ground s t a t e . Fe(TPP)(THF^ i s r e l a t i v e l y unstable with respect t o loss of THF,' 0' and t h i s and other Fe(TPP) d e r i v a t i v e s are q u i t e r e a d i l y o x i d i z e d t o y-0L"Fe(TPPl| 2 . 1 0 1 ' 1 0 3 E p s t e i n ' s 1 0 3 recent Mossbauer data f o r several s i m i l a r oxo-bridged f e r r i c porphin dimers are v i r t u a l l y i d e n t i c a l t o those r e p o r t e d ' 0 0 f o r "Fe(TPP) (THF) 2", so there i s some doubt t h a t t h i s was the actu a l compound s t u d i e d by Kobayashi, e t a I .' 0 0 (elemental analyses quoted i n r e f . 101 were a l s o poor). I t should a l s o be noted here t h a t one of the important r e s u l t s of c r y s t a l f i e l d theory i s t h a t a f e r r o u s ion can have only S = 0 o r S = 2 s p i n s t a t e s i n a re g u l a r octahedral environment, and t h a t t h e r e must be a s u b s t a n t i a l lowering of the symmetry t o s t a b i l i z e an S = I s t a t e . Although these are the only Mossbauer s t u d i e s t o have been reported f o r s y n t h e t i c f e r r o u s p o r p h y r i n s , the s t r u c t u r a l l y r e l a t e d ferrous phthalocyanine L"re(Pc)l system has been e x t e n s i v e l y i n v e s t i g a t e d , In Fe(Pc) the p y r r o l e s of the b a s i c porphin nucleus are replaced by i s o i n d o l e groups and the methine bridges by nit r o g e n atoms (see s t r u c t u r e 133 2 • • , . 104-106 . , , , . ,. , Johnson and co-workers have shown t h a t in the square planar Fe(Pc) complex the f e r r o u s ion does in f a c t have an S = I ground s t a t e , although in hexacoordinate adducts with v a r i o u s amine bases i t i s f u l l y low-spin Magnetic p e r t u r b a t i o n Mossbauer s p e c t r a of Fe(Pc) and F e ( P c ) ( p y ) 2 show t h a t V z z>0 and n - 0 in both c a s e s ' ' T h i s s i g n 109 of V z z i s unexpected f o r the s p i n t r i p l e t ground s t a t e of Fe ( P c ) , as discussed below, but f o r the diamagnetic F e ( P c ) ( p y ) 2 the p o s i t i v e V z z i n d i c a t e s t h a t the bonding t o i r o n i s str o n g e r in the Pc plane than i n 4 -U • | . • 104 the a x i a I d i r e c t i o n In some ways n e i t h e r Fe(TPP) nor Fe(Pc) represents a very good model system f o r haeme. The n a t u r a l l y o c c u r r i n g f e r r o u s porphyrins have f u l l 8 - s u b s t i t u t i o n of the p y r r o l e r i n g s , but no m e s o - s u b s t i t u t i o n . On the other hand, Fe(TPP) has no $ - s u b s t i t u t i o n but complete meso-subst i t u t i o n . Although Fe(Pc) can i n one sense be regarded as having f u l l 8 - s u b s t i t u t i o n , the fused benzene r i n g s are hardly e q u i v a l e n t t o the 8 - s u b s t i t u e n t s of heme, and there i s a l s o the problem of the =N-ra t h e r than =CH- b r i d g e s . 134 Bonnett and co-workers ' have reported the pre p a r a t i o n of octamethyItetrabenzporphyrin (H 20TBP) and i t s b i s p y r i d i n e Mg(I I) c h e l a t e complex Mg(OTBP)(py) 2. The corresponding f e r r o u s sytem Fe(OTBP) seemed an a t t r a c t i v e one t o i n v e s t i g a t e because of i t s s t r u c t u r a l s i m i l a r i t y t o the very s t a b l e Fe(Pc) system (see s t r u c t u r e 3 ) . We have a l s o prepared 3 and s t u d i e d o c t a e t h y I p o r p h y r i n i r o n ( I I ) , which i s perhaps an even b e t t e r model compound f o r haeme (see s t r u c t u r e 4 ) . I t was hoped t h a t Mossbauer 4 135 and other data on these porphyrin complexes would help c l a r i f y the in f l u e n c e of the d i f f e r e n t s t r u c t u r a l f eatures on the e l e c t r o n i c s t a t e of i ron. P r e p a r a t i o n of the Complexes A l l chemicals were of reagent grade and were obtained e i t h e r from F i s h e r S c i e n t i f i c Company o r A l d r i c h Chemical Company. The procedures described below were a l l c a r r i e d out in a dry nit r o g e n atmos-phere. The Soxhlet e x t r a c t o r used had a volume of 75 ml. 1, 3,4,7-tetramethyIisoindole was prepared from 2,5-I 12 hexanedione and ammonium sulphate as described by F l e t c h e r . The crude product was r e c r y s t a l I i z e d from d i e t h y l e t h er t o y i e l d the pure i s o i n d o l e . Crude octamethyItetrabenzporphyri ni r o n ( I I ) . 3.6 g (21 mmol) of I,3,4,7-tetramethyIisoindoIe and 13 g (230 mmol) of reagent grade i r o n powder were sealed in a t h i c k - w a l l e d g l a s s tube of 180 ml volume, and allowed t o react a t 350° f o r 4 hr. The tube was opened and the s o l i d residue was washed with petroleum ether (60 - 110°) and then benzene t o a f f o r d the crude product I_. Bi s ( p y r i d ? n e ) o c t a m e t h y l t e t r a b e n z p o r p h y r i n i r o n ( I I) 30 g of J_ was e x t r a c t e d f o r 3 hr with 600 ml of a 10/1 pyridine/petroleum ether mixture. The green s o l u t i o n was f i l t e r e d and the f i l t r a t e evaporated t o 50 ml volume. The purple m i c r o c r y s t a I s which formed were c o l l e c t e d by f i l t r a t i o n and d r i e d in vacuo a t room temperature. When neat p y r i d i n e i s used f o r the e x t r a c t i o n , one obt a i n s t e t r a k i s ( p y r i d i n e ) o c t a m e t h y I t e t r a b e n z p o r p h y r i n i r o n ( I I ) . 136 B i s ( t e t r h y d r o f u r a n ) o c t a m e t h y I t e t r a b e n z p o r p h y r i ni ron(I I) 800 ml of THF, which had been f r e s h l y d i s t i l l e d over calcium hydride, were used t o e x t r a c t 30 g of I_ f o r 0.75 hr. The green e x t r a c t was f i l t e r e d and evaported t o 50 ml volume producing purple micro-c r y s t a l s of product. T h i s was d r i e d i n a stream of dry n i t r o g e n gas f o r 4 hr a t room temperature. Bis(3-p i c o l i ne)octamethyItetrabenzporphyri ni ron(I I) 3- P i c o l i n e was d i s t i l l e d over calcium h y d r i d e . 200 ml of the f r a c t i o n c o l l e c t e d at 144 ± 0.5° was used t o e x t r a c t I_ in a s i m i l a r manner t o the p y r i d i n e e x t r a c t i o n above. T e t r a k i s ( 4 - p i c o l i n e ) o c t a m e t h y I t e t r a b e n z p o r p h y r i n i r o n ( I I) 4- P i c o l i n e was d i s t i l l e d over calcium hydride and the f r a c t i o n b o i l i n g a t 145 ± 0.5° c o l l e c t e d . 200 ml of t h i s d i s t i l l a t e was used t o e x t r a c t I_ as above. The corresponding i s o q u i n o l i n e adduct was prepared s i m i l a r l y using the i s o q u i n o l i n e f r a c t i o n c o l l e c t e d a t 242°, t o y i e l d t e t r a k i s ( i s o q u i no Iine)octamethyItetrabenzporphyrini ron(I I) OctamethyItetrabenzporphyriniron(11) The b i s p y r i d i n e adduct was heated i n vacuo a t 180° f o r I hr t o a f f o r d the blue square planar complex in pure form. PolyCoctamethyltetrabenzporphyriniron(I I ) J 2 - P i c o l i n e , f r e s h l y d i s t i l l e d over calcium h y d r i d e , was used t o e x t r a c t I_. The greenish brown e x t r a c t was l e f t t o stand o v e r n i g h t and f i l t e r e d t o g i v e a black product, thought t o be Cre(OTBP)]^. 137 B i s ( p y r i di ne)octaethyIporphyri ni ron(I I) OctaethyIporphyrin (H^OEP) was k i n d l y s u p p l i e d by Dr. David Dolphin of t h i s Department, who a l s o suggested the f o l l o w i n g s y n t h e t i c r o u t e ' I g of h^OEP was d i s s o l v e d in 150 ml of r e f l u x i n g DMF, followed by the a d d i t i o n of 2 g of F e ( C I 0 4 ) 2 ' 6 H 2 0 . The s o l u t i o n was b o i l e d f o r 0.25 hr, cooled t o room temperature, and added t o 500 ml of s a t u r a t e d aqueous sodium c h l o r i d e s o l u t i o n , the mixture being l e f t t o stand in a i r o v e r n i g h t . The c o l l o i d a l p r e c i p i t a t e which had formed was c o l l e c t e d on an F grade s i n t e r e d f i l t e r and washed with hot H 20 t o remove excess NaCl. The residue was d i s s o l v e d in 100 ml of CHCI^ and the s o l u t i o n was t r e a t e d with several 50 ml p o r t i o n s of 5M HCl i n a separatory f u n n e l . The chloroform layer was then washed with water, d r i e d with anhydrous CaCI 2, and f i l t e r e d . The volume of the f i l t r a t e was reduced t o 100 ml on a hot p l a t e , and then kept constant by a d d i t i o n of e t h a n o l i c HCL (100:1) w h i l e the s o l u t i o n was b o i l e d . This procedure p r e c i p i t a t e s o c t a e t h y I p o r p h y r i n i r o n ( I I I) c h l o r i d e , Fe(0EP)CI, which was washed with ethanol and d r i e d in a i r . I g of Fe(0EP)CI was d i s s o l v e d in 170 ml of p y r i d i n e i n a 500 ml f l a s k equipped with condenser and dropping f u n n e l . The s o l u t i o n was heated t o 50° under a n i t r o g e n atmosphere, and 3.3 ml of hydrazine hydrate was added through the dropping f u n n e l . The s o l u t i o n immediately turned from brown t o red. The temperature was maintained at 50° f o r 0.25 hr, and the s o l u t i o n was then cooled in an i c e bath while n i t r o g e n was bubbled through. 7 ml of deoxygenated a c e t i c a c i d was added, and a f t e r a few minutes deoxygenated water was added t o p r e c i p i t a t e the product. The p r e c i p i t a t e was washed with deoxygenated i c e cold.water and d r i e d in vacuo t o g i v e the orange Fe(OEP)(py) 7. 138 O c t a e t h y I p o r p h y r i n i ron(I I) I g of F e ( 0 E P ) ( p y ) 2 was heated in vacuo at 150° f o r 2.5 hr t o y i e l d the pure f e r r o u s p o r p h y r i n . A n a l y t i c a l data f o r the complexes are given i n Table XIV. Weight Loss Experiments Since i t was found t h a t these f e r r o u s porphyrin adducts lose s o l v e n t when heated in vacuo, weight loss experiments were c a r r i e d out t o determine the number of s o l v e n t molecules attached. The r e s u l t s are l i s t e d in Table XIV along with the m i c r o a n a l y t i c a l data. A t y p i c a l experiment i s as f o l l o w s . A thoroughly ground sample ( u s u a l l y about 0.5 g) of the s o l v a t e d compound was weighed in a small weighing b o t t l e . T his b o t t l e was placed in a tube ( f i t t e d with a B45 cone and socket and a stopcock) which was then attached t o a vacuum l i n e equipped w i t h a c o l d t r a p . The sample was then heated in vacuo t o 150° -180°, and weighed a t h a l f - h o u r l y i n t e r v a l s u n t i l constant weight was obtained. The Mossbauer spectrum was recorded, and the sample was r e d i s s o l v e d in the a p p r o p r i a t e s o l v e n t t o o b t a i n the o r i g i n a l adduct. The Mossbauer spectrum was recorded again t o confirm the r e v e r s i b i l i t y of the process. General D i s c u s s i o n OctamethyItetrabenzporphyri n CompI exes The method used t o prepare the Fe(OTBP) compounds f o l l o w s c l o s e l y Bonnett's route t o the corresponding magnesium compound'' 0''''. However, the 20$ y i e l d obtained i s lower than t h a t of the magnesium 139 TABLE XIV A n a l y t i c a l and Magnetic Data f o r the Ferrous Porphyrin Complexes MICROANALYSIS COLOUR & MAGNETIC COMPOUND C H_ H PROPERTY WT. LOSS Fe(OTBPHpy) (C a l c f 77.50 77.70 5.41 5.52 10.30 10.10) Purple (dia) 1 hr I80°C 19.0$ (Calc. 18.9$) Fe(OTBP)(py) (Calc. 77.3 77.4 5.75 5.65 1 1.00 1 1.30) PurpIe (dia) 1 hr I80°C 31.4$ ( C a l c . 31.8$) Fe(OTBP)(THF)„ ( C a l c . 76.23 76. 10 6.24 6.34 6.71 6.81) PurpIe ( y e f f=5.5 B.M.) 1 hr I30°C 17.7$ (Ca l c . 17.6$) F e ( 0 T B P ) ( 3 - p i c ) 7 ( C a l c . 1 78.30 77.80 5.58 5.80 9.63 9.30) PurpIe (dia) <• 1 hr I80°C 21.8$ (Calc. 21.6$) Fe(0TBP)(4-pic). ( C a l c . 77.60 77.70 6.42 6.13 10. 14 10.34 PurpIe (dia) 1 hr I80°C 35.0$ (Ca l c . 35.4$) Fe(OTBP)(IQ) ( C a l c . 81.00 80.50 5.32 5.37 9.1 1 9.40) PurpIe (dia) 1 hr 200°C 43.5$ (Ca l c . 43.3$ Fe(OTBP) (Ca l c . 78.40 78.10 5.57 5.35 8.50 8.28 Blue (p f f=5.95 B.M.) [Fe(OTBP)] ( C a l c . 78.24 78.10 5.40 5.35 8.13 8.28) Black ( v a r i a b I e ) 0$ Fe(OEPMpy) ( C a l c f 74.00 73.70 7.24 7.19 1 1 .25 11.19) Orange (dia) 2%-hr I50°C 21 .3$ (21.2$) Fe(OEP) (Ca l c . 73.58 73.46 . 7.59 7.49 9.44 9.53) Brown (v e f f=4.7 B.M.) 140 p o r p h y r i n , reported t o be 81$.' 1 1 Our lower y i e l d may be due t o the presence of s i d e r e a c t i o n s which are absent in the magnesium case. Secondly, due t o the high s o l u b i l i t y of the Fe(OTBP)(py) 2 in p y r i d i n e , p r e c i p i t a t i o n from t h i s s o l v e n t i s f a r from q u a n t i t a t i v e . Attempts t o increase the y i e l d by a l t e r n a t i v e routes such as r e f l u x i n g i r o n powder and the i s o i n d o l e in chloronaphthalene showed t h a t only a very small amount of the porphyrin was formed. An attempt t o convert the magnesium compound i n t o the f r e e porphyrin with t r i f I u o r o a c e t i c a c i d , followed by i n s e r t i o n of i r o n was e q u a l l y u n s u c c e s s f u l . In order t o c h a r a c t e r i z e t h i s f e r r o u s porphyrin system more completely, n.m.r., e l e c t r o n i c and mass s p e c t r a l measurements were made. The I.R. s p e c t r a of these compounds are very d i f f u s e and cannot be i nte rp reted eas i I y . The ' H n.m.r. spectrum of Fe(OTBP)(py) 2 was obtained in both p y r i d i n e and DMSO-d^ s o l u t i o n s , and data are given in ppm downfield from i n t e r n a l TMS. In both s o l v e n t s the s p e c t r a a r i s i n g from the methyl and methine (bridge) protons are i d e n t i c a l , c o n s i s t i n g of two s i n g l e t s a t 3.8 (area 6) and 11.8 (area 1). The OTBP r i n g protons are masked i n p y r i d i n e s o l u t i o n , but in DMSO-d^ there i s a poorly defined m u l t i p l e t centred a t 7.9 D n Fe(Pc) t h i s m u l t i p l e t i s seen at 7.6 i n the same s o l v e n t . ] The coordinated p y r i d i n e s give r i s e t o two doublets a t 8.6 and 7.8, and a t r i p l e t at 7.5 (area r a t i o 2:2:1), only s l i g h t l y s h i f t e d from t h e i r p o s i t i o n s in neat p y r i d i n e (8.5, 7.6, 7.2). The n.m.r. spectrum leaves no doubt as t o the e x i s t e n c e of Fe(OTBP). The bridge proton resonance at 11.8 ppm i s very s t r o n g l y s h i f t e d downfield from the usual aromatic absorption region of about 141 7 ppm, i n d i c a t i n g a very large r i n g c u r r e n t . The d e s h i e l d i n g of the methyl protons i s somewhat l e s s , although even here there i s a down-f i e l d s h i f t of nearly 2.5 ppm from the usual p o s i t i o n of about 1.5 ppm. The mass spectrum of Fe(OTBP) shows a prominent parent peak (P +) at m/e = 676, and an even stronger peak at m/e = 338 which can be assigned t o the doubly-charged P + + s p e c i e s . This type of mass spectrum i s c h a r a c t e r i s t i c of porphyrin systems''^. Due t o the very high m e l t i n g p o i n t of Fe(OTBP), i t was necessary t o heat the sample t o about 450° in order t o o b t a i n a s u f f i c i e n t l y high vapour pressure f o r the mass spec-trometer, and a t lower m/e values there i s evidence of some decomposition. The s o l u t i o n e l e c t r o n i c spectrum of Fe(OTBP)(py) 2 i n s p e c t r o -grade p y r i d i n e provides d e f i n i t i v e evidence of a porphyrin chromaphore 4 (see Figure 2 6 ) . There i s a s p l i t band a t 395 and 433 nm (e = 8 x 10 , a Y max * 5 115 I.7 x 10 ), which i s the unique Soret band of the porphyrins and has the l a r g e s t e x t i n c t i o n c o e f f i c i e n t known. Two f u r t h e r absorptions are 4 4 seen at 560 nm (e = 2 x 10 ) and 603 nm (e = 9 x 10 ). These max max bands are in good agreement, both i n p o s i t i o n s and i n t e n s i t i e s , with those I 15 observed by Linstead f o r the t e t r a b e n z p o r p h y r i n (TBP) compound Fe(TBP)(py) 2. A l l t h e adducts are purple i n c o l o u r and (exoept f o r the THF complex) diamagnetic. T h e square planar Fe(OTBP) compound, on the other hand, i s blue and has a room temperature magnetic moment of 5.95 B.M. i n d i c a t i n g a high s p i n ground s t a t e . The large V value suggests a large o r b i t a l c o n t r i b u t i o n , as in the case of the phthalo-105 cyanine i r o n compound . The THF adduct has a room temperature moment of 5.5 B.M., q u i t e t y p i c a l of high s p i n octahedral f e r r o u s complexes. 142a FIGURE 26 E l e c t r o n i c Spectrum of Fe(OTBP)(py) ? i n P y r i d i n e a t 25° FIGURE 26 142 b 143 The Fe(OTBP) adducts with amine bases are remarkably s t a b l e in a i r . For example, the b i s p y r i d i n e complex showed no s i g n of decomposition a f t e r standing on the bench f o r two weeks. However on very long exposure t o the atmosphere i t g r a d u a l l y changes c o l o u r due t o loss of p y r i d i n e . The THF adduct shows much lower a i r s t a b i l i t y . Even when kept under ni t r o g e n t h i s adduct s l o w l y loses THF t o form a black compound which analyses f o r Fe(OTBP). A d i s c u s s i o n of the p r o p e r t i e s of t h i s black m a t e r i a l i s given at the end of t h i s chapter. The square planar Fe(OTBP) compound obtained by p y r o l y s i s of the adducts a l s o appears t o be q u i t e s t a b l e i n a i r . I d e n t i c a l Mossbauer spectra and a n a l y t i c a l data were obtained before and a f t e r exposure t o the atmosphere f o r more than a week. However, i t was n o t i c e d t h a t over long periods of time in a i r , the blue c o l o u r of t h i s compound gradualIy darkens. OctaethyI porphyrin Complexes Since o c t a e t h y I p o r p h y r i n has been c h a r a c t e r i z e d p r e v i o u s l y , ' ' 3 only n.m.r. and e l e c t r o n i c s p e c t r a ( i n p y r i d i n e ) of the F e ( 0 E P ) ( p y ) 2 compound were obtained f o r i d e n t i f i c a t i o n purposes. The n.m.r. spectrum d i f f e r s l i t t l e from t h a t of the profonated l i g a n d H 20EP. The methyl protons absorb at 1.9 ppm and the methylene protons at 4.0 ppm, e x a c t l y the same p o s i t i o n s as i n H 20EP. However, the methine bridge hydrogen resonance i s s h i f t e d s l i g h t l y from 10.2 ppm i n H 20EP t o 10.0 ppm i n the ir o n complex. The e l e c t r o n i c spectrum ( F i g . 27) shows a Soret band a t 5 409 nm (e = 1.2 x 10 ) and two more bands at 520 nm and 549 nm. max 4 4 ( e m a x = 1.5 x 10 , 2.5 x 10 ). The diamagnetic orange F e ( 0 E P ) ( p y ) 7 144a FIGURE 27 E l e c t r o n i c Spectrum of Fe(OEP)(py) 7 in P y r i d i n e a t 25° FIGURE 27 1 1 I I -1 400 500 600 WAVE LENGTH (nm) 145 adduct i s c o n s i d e r a b l y more a i r s e n s i t i v e than the corresponding OTBP complex. When Fe(OEP)(py) 2 was l e f t in a i r f o r one day, i t s Mossbauer spectrum showed t h a t i t had s u f f e r e d decomposition. The dark brown square planar Fe(OEP) complex i s extremely a i r s e n s i t i v e and most operations on t h i s compound were c a r r i e d out in vacuo t o prevent decomposition. A weighed sample of Fe(OEP)(py> 2 was loaded i n t o a c a l i b r a t e d Gouy tube under a dry nitrogen atmosphere, and placed on the vacuum l i n e . A f t e r heating the sample at 150° t o constant weight (2.5 hr) the weight loss corresponded t o the removal of 2 moles p y r i d i n e per mole Fe(OEP) (py),,. The Gouy tube was then sealed in vacuo and room temperature s u s c e p t i b i l i t y measurements Indicated a magnetic moment of 4.7 B.M. Th i s i s s l i g h t l y lower than the sp i n - o n l y value of 4.9 B.M. expected f o r a h i g h - s p i n f e r r o u s complex, but s i m i l a r t o the value 4.75 B.M. found f o r F e ( T P P ) 1 0 1 ' 1 l 6 . . Attempts were made t o o b t a i n Fe(OEP)(THF) 2 by two rou t e s . In the f i r s t , Fe(OEP) was d i s s o l v e d i n deoxygenated THF and the s o l u t i o n was evaporated t o dryness in vacuo a t room temperature. This procedure aff o r d e d o n l y Fe(OEP). The second route involved t h e attempted re d u c t i o n of Fe(0EP)CI i n THF r a t h e r than p y r i d i n e , but y i e l d e d an o i l y product which could not be c r y s t a l l i z e d . D i s c u s s i o n of the Mossbauer Data Mossbauer parameters f o r the OTBP and OEP complexes are given in Table XV along with r e l e v a n t data f o r some r e l a t e d compounds. A few general comments on the data should be made. 146 TABLE XV 5 7 F e Compound Fe(OTBP) Fe(TPP) Fe(OEP) Fe(Pc) Fe(OTBP)py 2 Fe(TPP)Py 2 Fe(OEP)Py 2 Fe(PP)py 2 Fe(Pc)py 2 Fe(OTBP)py 4 Fe(0TBP(4-p!c) Fe(OTBP)(IQ) 4 Fe(OTBP)(THF)« sbauer Parameters f o r the Ferrous Porphyr! n Come 1 exes -1 r|_ . F2 , T(°K) &(mm s ') AE (mm s ) (mm s w - 1 ) (mm s ) n. Ref. 295 0.94 0.56 .30 .23 115 1.04 +0.61 .27 .28 <v. 0 83 1.05 0.62 .27 .28 8.6 1.06 +0.63 .27 .28 <\. 0 77 0.78 1.32 (101) 295 0.81 1.49 .25 .20 115 0.88 + 1.60 .31 .26 i. 0 4.2 0.86 1.60 .28 .27 293 0.66 2.62 (104^ 77 0.77 2.69 108) 4 0.75 +2.70 <\. 0 295 0.69 0.73 .25 .26 -115 0.73 0.70 .28 .31 84 0.77 +0.68 .28 .31 i< 0 8.8 0,77 0.67 .28 .32 300 0.62 1.22 (101) 77 0,67 1.15 295 0.65 1.21 .27 .26 115 0.72 1.17 .31 .30 85 0.73 + 1.14 .31 .30 <x, 0 8.25 0,73 1.13 .36 .36 77 0.72 1.21 (99) Frozen S o l u t i o n 293 0.52 2.02 (I04e 77 0.59 ) .97 108) 4 0.58 +1.96 , 295 0.69 0.81 .29 .27 ? 115 0.78 +0.76 • ?8 .27 0 295 0.67 0.85 .29 .26 115 0.72 +0.82 .29 .28 % 0 295 0.70 0.79 .27 .25 115 0.76 +0.74 .28 .25 <\« 0 295 0.65 0.92 .27 .27 115 0.72 +0.87 .31 .29 <v. 0 295 1.18 2.21 .34 .29 250 1.22 2.29 .26 .26 220 1.22 2.47 .24 .25 190 1.25 2.54 .26 .24 160 1.26 2.61 .24 .23 130 1.28 2.64 .24 .23 115 1.27 2.66 .24 .23 105 1.29 2.66 .24 .23 .83 1.29 +2.67 .24 .23 t 0 60 1.30 2.69 .24 .21 30 1.30 2.72 .24 .22 7.75 1.30 2.74 .25 .22 147 F i r s t l y , f o r the square planar complexes i t i s seen t h a t each one has a d i s t i n c t i v e s e t of <S and | A E Q | parameters. Fe(OTBP) and Fe(Pc) l i e a t opposite ends of the s c a l e w i t h Fe(OEP) and Fe(TPP) having intermediate v a l u e s . S i m i l a r behaviour i s found f o r the b i s p y r i d i n e adducts except t h a t the intermediate OEP, TPP and PP complexes have nearly i d e n t i c a l Mossbauer parameters. An attempt w i l l be made below t o r e l a t e these d i f f e r e n c e s t o the a and ir bonding strengths of the tetra-^ dentate l i g a n d s . Secondly, the isomer s h i f t s f o r Fe(OTBP) and i t s THF adduct are f a i r l y t y p i c a l of hig h - s p i n (s = 2) f e r r o u s systems, whereas the other complexes a l l have 6 values c l o s e t o 0.7 mm s ', the value u s u a l l y considered to be the e m p i r i c a l d i v i d i n g l i n e between h i g h - s p i n and low-spin f e r r o u s d e r i v a t i v e s . Indeed, f o r most of these compounds a p o s i t i v e assignment of s p i n s t a t e cannot be made on the b a s i s of 6 values alone, and one must r e l y on magnetic moment data. Moreover, one sees t h a t in going from a t e t r a c o o r d i n a t e h i g h - s p i n compound t o a hexacoordinate low-spin one t h e r e i s only a small change in 6, i n c o n t r a s t to the very large changes found in Chapter IV. T h i r d l y , the | A E Q | values vary w i d e l y , ranging from about 0.6 mm s" 1 f o r Fe(OTBP) t o 2.7 mm s" 1 f o r Fe(Pc) and Fe(OTBP)(THF> 2. For t h i s l a s t complex the temperature dependence of | A E Q | i s c h a r a c t e r i s t i c of octahedral high-spin f e r r o u s compounds (see Chapter I I I ) , but f o r a l l the other d e r i v a t i v e s | A E Q J i s ne a r l y independent of temperature. F i n a l l y , magnetic p e r t u r b a t i o n Mossbauer measurements show t h a t f o r a l l t h e OTBP and OEP d e r i v a t i v e s f a s well as f o r F e ( P c ) , 1 0 6 and Fe(Pc) ( p y ) 2 , "^3, V z z i s p o s i t i v e and n i s e s s e n t i a l l y zero. This i n d i c a t e s 148 a c o n c e n t r a t i o n of charge i n the porphyrin (or phthalocyanine) plane and shows the equivalence of the x and y d i r e c t i o n s w i t h i n t h a t plane. We s h a l l assume V z z>0 a l s o f o r Fe(TPP), although t h i s has y e t t o be confirmed e x p e r i m e n t a l l y . Before d i s c u s s i n g the Mossbauer parameters i n r e l a t i o n t o the bonding in these complexes, i t i s important t o c o n s i d e r in general the s t r u c t u r a l c h a r a c t e r i s t i c s of the t e t r a d e n t a t e Iigands and the d i f f e r e n c e s expected i n t h e i r a and TT bonding p r o p e r t i e s . The four porphyrins OTBP, OEP, TPP and PP are a l l expected t o have very s i m i l a r r i n g s i z e s , and i n the f e r r o u s complexes the Fe-N bond d i s t a n c e s should be nearly i d e n t i c a l . Thus, one does not expect any s i g n i f i c a n t d i f f e r e n c e s i n N-*-Fe a-donor s t r e n g t h amongst the porphyrins. On the other hand, phthalocyanine has a s u b s t a n t i a l l y I 17-122 sm a l l e r r i n g s i z e due t o the =N- r a t h e r than =CH- b r i d g e , and the Fe-N d i s t a n c e should be s i g n i f i c a n t l y s h o r t e r i n Fe(Pc) than in the porphyrins. This should have the e f f e c t of making Pc the s t r o n g e s t a donor of the Iigands considered here. The n i t r o g e n b r i d g i n g atoms in Pc are a l s o expected t o i n f l u e n c e the ir bonding s t r e n g t h of t h i s l i g a n d . Simple Huckel TT 123 e l e c t r o n c a l c u l a t i o n s on such r i n g systems have shown t h a t the t o t a l e l e c t r o n i c charge a t the bridge atoms i s lower than a t the other r i n g atoms. That i s , the formation of an aromatic TT e l e c t r o n system favours removal of a c e r t a i n amount of e l e c t r o n d e n s i t y from the bridge atoms. Since n i t r o g e n i s more e l e c t r o n e g a t i v e than carbon, the r i n g c u r r e n t system with a methine bridge w i l l be i n h e r e n t l y s t r o n g e r than w i t h a n i t r o g e n b r i d g e . On the o t h e r hand, the fused benzene r i n g s i n Pc 149 should c o n t r i b u t e a d d i t i o n a l resonance energy which w i l l probably more than compensate f o r any weakening of the TT system induced by the b r i d g i n g n i t r o g e n s . I t t h e r e f o r e seems reasonable t o suggest t h a t OTBP w i l l have the g r e a t e s t TT bonding strength of the Iigands under d i s c u s s i o n , with Pc probably second. Of the remaining p o r p h y r i n s , TPP should have the next highest IT bonding s t r e n g t h because of the e l e c t r o n r e l e a s i n g phenyl groups in the four meso p o s i t i o n s . However, these phenyl groups are o r i e n t e d perpen-122 d i c u l a r t o the plane of the porphyrin so t h a t the phenyl IT system w i l l not c o n t r i b u t e d i r e c t l y t o the r i n g c u r r e n t . Protoporphyrin (PP) i s expected t o be the weakest TT bonding l i g a n d because of the e l e c t r o n withdrawing v i n y l and a c i d i c s i d e chains on the p y r r o l e r i n g s . Thus, the suggested order of TT bonding stre n g t h s i s 0TBP>Pc>TPP>0EP>PP. Th i s order has been p a r t i a l l y v e r i f i e d by the n.m.r. sp e c t r a of Fe(OTBP)(py) 2 and Fe(OEP)(py) 2, s i n c e the p o s i t i o n of the methine proton resonance i s d i r e c t l y r e l a t e d t o and i s a good i n d i c a t i o n of the stren g t h . o f the ^  e l e c t r o n system. As we saw above t h i s resonance occurs 1.8 ppm f u r t h e r downfield in the OTBP compound, i n d i c a t i n g a stronger d e s h i e l d i n g of t h i s proton. With these q u a l i t a t i v e c o n s i d e r a t i o n s i n mind we t u r n t o a more d e t a i l e d examination of the Mossbauer data. For a paramagnetic f e r r o u s ion in a square planar environment there are fou r p o s s i b l e e l e c t r o n i c 5 5 3 3 ground s t a t e s , namely B_ and E f o r the S = 2 case, and E and B„ 2g g g 2g f o r the S = I case. These ground s t a t e s are i11ustrated s c h e m a t i c a l l y in Figure 28. From Table I and the estimate of ^  4 mm s ' f o r the q u a n t i t y 4 -3 ^e ( l - R ) < r > as discussed i n Chapter I I I , simple c r y s t a l f i e l d model 150a FIGURE 28 P o s s i b l e Ground States f o r Ferrous Porphyrins under D«, b ! g ( dx2-y2> a l g ( dz2> e (d , d ) g x z ' yz b„ (d ) 2g xy ISO b FIGURE 28 A— -f-. g--H- b2g B 2g V 2 2/e - f r " 3 > Hg —f Hg D2g •g -rr t T T V z z / e = - 7 < r > Hg - f - °ig f i r - e g V z z / 9 = . |< r - \ Hg 5zg - f -—f— 44- -H-B 2g V /e zz 151 estimates f o r the s i g n and magnitude of AEg f o r each of these ground s t a t e s can be o b t a i n e d , and these are included i n Figure 28. The extended 124 125 Huckel MO c a l c u l a t i o n s of Gouterman and co-workers ' p r e d i c t a 3 E g ground s t a t e i f the f e r r o u s ion l i e s in the plane of the c y c l i c o l i g a n d , whereas i f i t l i e s 0.492 A out of the porphyrin plane the p r e d i c t e d 5 ground s t a t e i s ^2g' ^or a'' T n r e e f e r r o u s porphyrins which have been i s o l a t e d , s u s c e p t i b i l i t y data i n d i c a t e s p i n q u i n t e t ground s t a t e s . In 124 view of Gouterman's c a l c u l a t i o n s and the observed p o s i t i v e values of V z z in these compounds i t might seem reasonable t o assume t h a t the ground 5 s t a t e s are in f a c t B_ . 2 9 However, r e s u l t s f o r Fe(Pc) i n d i c a t e t h a t one should be very cautious about a s s i g n i n g the ground s t a t e s in systems such as these on the b a s i s of c r y s t a l f i e l d p r e d i c t i o n s of the s i g n of the e f g . Low temperature s u s c e p t i b i l i t y measurements show t h a t Fe(Pc) i s an i n t e r -mediate-spin complex, but t h e r e appears t o be some u n c e r t a i n t y about the exact nature of the ground s t a t e . Johnson and c o -workers' 0" 5'' 0 6 have assigned the ground s t a t e as 3 E ^ , but Barraclough, et a I.'°^ suggest i t i s ^ B2g* ' n e i t h e r case AEg i s p r e d i c t e d t o be n e g a t i v e , whereas the measured value i s large and p o s i t i v e . J o h n s o n ' 0 6 has suggested t h a t the most l i k e l y e x p l a n a t i o n of t h i s s i g n r e v e r s a l i s t h a t the e f f e c t s of covalency are so great as t o completely swamp the negative c o n t r i b u t i o n from the i r o n ' s own valence e l e c t r o n s , thus rendering the c r y s t a l f i e l d approximation completely inadequate. Note t h a t the in-^plane N->-Fe a donation w i l l be i n t o the i r o n 3d 2 2> 4„ and 4„ o r b i t a l s (assuming x — v P P 2 ^ y i r o n uses dsp hybrids f o r a bonding), a l l of which make p o s i t i v e c o n t r i b u t i o n s t o V z z« 152 The e f f e c t of strong a bonding i s a l s o r e f l e c t e d t o some extent 124 3 in the o r b i t a l occupancies c a l c u l a t e d by Gouterman f o r the ground s t a t e of a f e r r o u s p o r p h y r i n . These c a l c u l a t i o n s i n d i c a t e the presence of about 0.9 e l e c t r o n in the i r o n b. (d ? ?) o r b i t a l instead of zero. Ig x^-y^ Using these o r b i t a l occupancies the c a l c u l a t e d A E N becomes +1.13 mm s~'. It i s c l e a r t h a t t h i s approximate MO c a l c u l a t i o n i s an improvement over the c r y s t a l f i e l d approach, s i n c e the p r e d i c t e d s i g n of i s at l e a s t c o r r e c t . However, the magnitude of A E Q i s s t i l l very much s m a l l e r than the measured v a l u e , i n d i c a t i n g t h a t at l e a s t f o r Fe(Pc) the e f f e c t s of covalency are s t i l l g r o s s l y underestimated. T h i s i s not too s u r p r i s i n g , s i n c e as we have suggested above phthalocyanine should be a much stronger a donor than the porphyrins. In view of these r e s u l t s we t h i n k i t would be unwise t o make \ 5 5 a d e f i n i t e choice between EU and E as the ground s t a t e of i r o n i n 2g g 3 the f e r r o u s porphyrins. As we s h a l l see below, Fe(OTBP) (THF),, does i n f a c t have an o r b i t a l s i n g l e t ( 5B ) ground s t a t e , and i f one assumes '2 \ J — - i 1-- ' * r t h a t the only p e r t u r b a t i o n introduced by these weak a x i a l ligands i s an increase i n the energy of the i r o n a| ( d ^ ) o r b i t a l , t h i s would imply a 5 B 2 g 9 r o u n c l s+ a"l" e ' n 'the neat f e r r o u s porphyrins as w e l l . Although we consider t h i s the more l i k e l y ground state> the choice must s t i l l be regarded as t e n t a t i v e . Despite the u n c e r t a i n t i e s concerning the exact nature of the ground s t a t e s i n these complexes, Gouterman's c a l c u l a t i o n s ' ^ ' ' ^ suggest an important d i f f e r e n c e between Fe(Pc) and the f e r r o u s porphyrins: only i f the i r o n atom l i e s in the plane of the m a c r o c y c l i c ligand i s a s p i n t r i p l e t ground s t a t e p r e d i c t e d . This may i n d i c a t e t h a t the very strong a bonding i n Fe(Pc) i s able t o keep the i r o n atom i n the 153 plane, w h i l e in the porphyrins a more s t a b l e c o n f i g u r a t i o n i s achieved w i t h the i r o n atom s l i g h t l y out of the plane. A p o s s i b l e mechanism f o r s t a b i l i z i n g an out-of-plane c o n f i g u r a t i o n i s the increase in TT bonding which could be achieved. Under the TT o r b i t a l s of the 126 planar porphyrin span e , a„ and b„ , so t h a t only the e„(d ,d ) ^ ^ r ' r g' 2u 2u' ' g x z ' yz o r b i t a l s on i r o n have a p p r o p r i a t e symmetry t o overlap with the porphyrin ir system in the in-plane c o n f i g u r a t i o n . In the out-of-plane c o n f i g u r a t i o n the t r a n s f o r m a t i o n p r o p e r t i e s of d^ 2 are a l t e r e d and t h i s o r b i t a l i s then able t o mix with l i g a n d IT o r b i t a l s transforming as 124 a 2 u ' a n ' n T e r a C T ' o n which i s symmetry forbidden f o r the in-plane case. 5 124 For the B^ .^  ground s t a t e of f e r r o u s p o r p h y r i n , Gouterman's o r b i t a l occupancy numbers lead t o a p r e d i c t e d E Q of + 3.81 mm s ', which does not d i f f e r a p p r e c i a b l y from the simple c r y s t a l f i e l d estimates but i s much l a r g e r than the observed s p l i t t i n g s (see Table XV). The most probable e x p l a n a t i o n of the small p o s i t i v e AEg values found f o r a l l three porphyrins i s the occurrence of strong forward I igand->metaI ir bonding. If the i r o n atom i s in an out-of-plane c o n f i g u r a t i o n as suggested above, some degree of a bonding with the d 2 .2 o r b i t a l w i l l be l o s t , and t h i s w i l l tend t o make A E „ less p o s i t i v e , x —y V At the same time however, the three d o r b i t a l s , d , d and d o, can * ' xz' yz zz' p a r t i c i p a t e i n TT bonding with f i l l e d e^ and a2u o r b i t a l s on the porphyrin r i n g s . The r e s u l t i n g increase in e l e c t r o n d e n s i t y in these o r b i t a l s w i l l g i v e a negative c o n t r i b u t i o n t o Vzz> leading t o a red u c t i o n in the magnitude of AEp. Since we expect the a bonding in the three porphyrins t o be very s i m i l a r , the extent t o which ] A E Q | i s reduced 154 should be a d i r e c t measure of the strength of the TT bonding in the complex. On t h i s b a s i s we would conclude t h a t of the t h r e e porphyrins stu d i e d OTBP i s the s t r o n g e s t TT donor and OEP the weakest. The order ob-t a i n e d i s e x a c t l y the same as t h a t suggested on s t r u c t u r a l grounds above. The isomer s h i f t data are in at l e a s t q u a l i t a t i v e agreement with these ideas of strong a bonding and strong (but v a r i a b l e ) forward TT bonding. The very low 6 values f o r the Fe(TPP) and Fe(OEP) complexes probably r e f l e c t a sub-s t a n t i a l augmentation of 4s e l e c t r o n density of i r o n a r i s i n g from the a donation. The AEg values suggest t h a t the d i f f e r e n c e in TT bonding str e n g t h s of TPP and OEP i s not l a r g e , and the 6 values are a l s o s i m i l a r . For Fe(OTBP) on the other hand, t h e r e i s a large decrease in AEg i n d i c a t i n g an increase in the donor s t r e n g t h of the l i g a n d . The e x t r a e l e c t r o n density introduced i n t o the i r o n d o r b i t a l s would increase the s h i e l d i n g of the 4s e l e c t r o n s and r a i s e the isomer s h i f t as observed. One f u r t h e r p o i n t of i n t e r e s t concerning the AEg values of the f e r r o u s porphyrins i s the very small temperature dependence observed. In the usual octahedral h i g h - s p i n f e r r o u s complexes there w i l l always be at l e a s t one l o w - l y i n g e x c i t e d s t a t e which can be t h e r m a l l y populated at room temperature, and the |AEg| value observed i s a thermal average of the values f o r the ground and e x c i t e d s t a t e s . Lowering the temperature depletes the e x c i t e d s t a t e (s) and produces the usual temperature-dependent |AEg|. The lack of temperature dependence found here i n d i c a t e s t h a t there are no t h e r m a l l y a c c e s s i b l e e x c i t e d s t a t e s in these complexes, and t h a t |AEp| measured at any temperature r e f l e c t s the t r u e value of |AEg| f o r the ground s t a t e . We estimate t h a t the b 2 g ~ e g s e P a r a - r ' o n ' n these complexes i s g r e a t e r than 1000 cm '. With the i n t r o d u c t i o n of two THF molecules in the a x i a l p o s i t i o n s of Fe(OTBP), there i s a very large increase in |AE n|. The 155 magnitude and temperature dependence of |AEQ| and the p o s i t i v e V Z Z found from a magnetic p e r t u r b a t i o n measurement i n d i c a t e t h a t the ground s t a t e i s the lxy> o r b i t a l s i n g l e t . The p r i n c i p a l e f f e c t of the a x i a l f i e l d introduced by the THF Iigands i s t o r a i s e s u b s t a n t i a l l y the energy of the i r o n d z 2 o r b i t a l ' 0 ' ' 1 2 4 and t h i s must be l a r g e l y r e s p o n s i b l e f o r the sharp ( p o s i t i v e ) increase in ]A EQ| compared t o Fe(OTBP) i t s e l f . However, the strong temperature dependence of |AEQ| shows t h a t the b 2g - e^ s e p a r a t i o n has a l s o been narrowed. A s a t i s f a c t o r y f i t of the Eg vs T data with a c r y s t a l f i e l d model as described i n Chapter I I I could not be obtained, probably because of the strong covalency in the porphyrin plane. However, a very rough estimate g i v e s a b 2 g - e g s p l i t t i n g of * 400 cm"'. The f a c t t h a t TJ = 0 i n d i c a t e s t h a t d and d remain degenerate (or very nearly s o ) . This degeneracy can be maintained i n the hexacoordinate case o n l y i f the i r o n atom l i e s 124 i n the porphyrin plane , so t h a t our r e s u l t s appear t o favour an in-plane c o n f i g u r a t i o n in Fe(OTBP)(THF) 2. The s i g n i f i c a n t l y g r e a t e r 6 value f o r t h i s compound i s a l s o c o n s i s t e n t with the lack of. involvement of the d 9 o r b i t a l in TT bonding with the porphyrin. Turning now t o the diamagnetic hexacoordinate b i s p y r i d i n e adducts, one sees from Table XV t h a t Fe(OTBP)(py) 2 has the s m a l l e s t |AEQ| , F e ( P c ) ( p y ) 2 the l a r g e s t , and t h a t the other three porphyrin complexes have intermediate and nearly i d e n t i c a l v a l u e s . The Mossbauer spectrum of Fe(OTBP)(py) 2 at 84°K in a 50 kG a p p l i e d magnetic f i e l d i s shown i n Figure 29, and V Z Z i s c l e a r l y p o s i t i v e . The only p o s s i b l e ground s t a t e f o r an octahedral low-spin f e r r o u s system i s ' A| g» a n d in the "pure" c r y s t a l f i e l d l i m i t t h i s s t a t e has zero quadrupole s p l i t t i n g . However, d i f f e r e n c e s in bonding i n t e r a c t i o n s with a x i a l and e q u a t o r i a l Iigands can produce a non-zero e f g . FIGURE 29 Mossbauer Spectrum of Fe(OTBP)(py) 2 at 84° K in an a p p l i e d magnetic f i e l d of 50 kG. The f u l curve Is the t h e o r e t i c a l spectrum c a l c u l a t e d f o the parameters 6 = 0.77, AEg = +0.68,.r = 0.29 ( a l l in mm s ') and n = 0. 157 I 24 Gouterman and co-workers have a l s o c a r r i e d out MO c a l c u -l a t i o n s f o r the f e r r o u s porphyrin bisaquo adduct. Using t h e i r d o r b i t a l p o p u l a t i o n s the p r e d i c t e d value of A E Q i s +1.10 mm s~',. remarkably c l o s e t o the values found f o r the b i s p y r i d i n e adducts of 124 f e r r o u s TPP, OEP and PP. T h e i r c a l c u l a t i o n s suggest t h a t the major c o n t r i b u t i o n t o V comes from the imbalance i n e l e c t r o n d e n s i t i e s in zz the d x2_ y2 a n d o r b i t a l s . That i s , the covalent bonding t o the pla n a r porphyrin i s stronger than t h a t t o the a x i a l l i g a n d s . The f a c t t h a t the phthalocyanine d e r i v a t i v e shows the l a r g e s t A E Q presumably r e f l e c t s very strong a donation i n t o the i r o n 3 d ^ 2 y 2 o r b i t a l i n t h i s case (4 and 4 may a l s o c o n t r i b u t e ) . Since the p x p y porphyrins should be poorer a donors than Pc there w i l l be s m a l l e r imbalances in d 2 2 a n c l d 2 charge d e n s i t i e s , and s m a l l e r A E N v a l u e s , x -y z y The very small s p l i t t i n g f o r the OTBP complex can be a t t r i b u t e d t o the great ir donor s t r e n g t h of t h i s l i g a n d , which W i l l increase the d , xz dy z p o p u l a t i o n s . For the other t h r e e porphyrins the data suggest t h e r e are only modest d i f f e r e n c e s in t h e i r o v e r a l l a and ir bonding c h a r a c t e r i s t i c s . The isomers s h i f t s f o r the b i s p y r i d i n e adducts behave q u i t e s i m i l a r l y t o those of the t e t r a c o o r d i n a t e compounds, w i t h the OTBP complex having the highest 6 and the Pc one the lowest, and the exp l a n a t i o n of t h i s t r e n d i s presumably s i m i l a r t o the one given above. As we have already mentioned, these 6 values l i e a t the upper end of the usual range f o r low-spin f e r r o u s compounds, i n d i c a t i n g t h a t the 3d (and p o s s i b l y 4p) o r b i t a l charge d e n s i t i e s and the r e s u l t a n t s h i e l d i n g o f the s e l e c t r o n d e n s i t y are g r e a t e r than normally encountered 158 in such systems. Since the major c o n t r i b u t i o n t o the quadrupole s p l i t t i n g in the b i s p y r i d i n e complexes comes from d i f f e r e n t e l e c t r o n d e n s i t i e s in the dx2_y2 a r ) d c l z2 o r b i t a l s , i t seemed of i n t e r e s t t o study the e f f e c t of o t h e r a x i a l Iigands on the Mossbauer parameters f o r the Fe(OTBP) system. Therefore attempts were made t o prepare the 2:1 adducts of t h i s complex with 2-, 3- and 4 - p i c o l i n e and i s o q u i n o l i n e . Only with 3 - p i c o l i n e were we able t o i s o l a t e the des i r e d 2:1 adduct. As seen from Table XV t h i s complex has a AEg value s i m i l a r t o t h a t of the b i s p y r i d i n e d e r i v a t i v e , suggesting only a s l i g h t e f f e c t on the efg at i ron. The i n a b i l i t y t o o b t a i n an adduct with 2 - p i c o l i n e i s probably the r e s u l t of s t e r i c hindrance due t o the ortho-methyI group. However, with 4 - p i c o l i n e and i s o q u i n o l i n e , instead of the expected 2:1 adducts we obtained 4:1 adducts. A 4:1 adduct with p y r i d i n e could a l s o be prepared when the crude Fe(OTBP) was e x t r a c t e d with neat p y r i d i n e r e p l a c i n g the pyridine/petroleum e t her mixture used t o prepare Fe(OTBP)(py> 2. There have been no previous r e p o r t s of such 4:1 adduct formation with any meta I I oporphyr i n o r phthalocyanine complex, and attempts t o prepare a 4:1 adduct of 4 - p i c o l i n e with Fe(Pc) were unsuccessfuI. I n t e r e s t i n g l y , the 4:1 complexes appear t o be a t l e a s t as s t a b l e as the 2:1 adducts with respect t o loss of base molecules, e i t h e r at room temperature o r on he a t i n g , suggesting t h a t the two " e x t r a " Iigands are reasonably s t r o n g l y bound. The f a c t t h a t Fe(OTBP)(py)^ shows a l a r g e r quadrupole s p l i t t i n g than Fe(OTBP)(py)^ could i n d i c a t e a weakening of the porphyr i n->i ron TT bonding due t o involvement of the 1 5 9 t h i r d and f o u r t h p y r i d i n e groups with the r i c h ir system of OTBP. The a l t e r n a t i v e e x p l a n a t i o n of a weakening of the a x i a l Iigand a donation due t o s t e r i c i n t e r f e r e n c e seems less l i k e l y in view of the s t a b i l i t y of the 4:1 adducts. Because of the u n c e r t a i n t y concerning the nature of the bonding in the 4:1 complexes i t i s d i f f i c u l t t o know how t o i n t e r p r e t the small d i f f e r e n c e s in t h e i r Mossbauer parameters. Magnetic P e r t u r b a t i o n Measurements on the High-Spin Ferrous Porphyrins In view of the unusual e f f e c t s which can be observed when 2+ paramagnetic Fe complexes are subjected t o a p p l i e d magnetic f i e l d s a t low temperatures, we should comment b r i e f l y on the behaviour of the high-spin f e r r o u s porphyrins under these c o n d i t i o n s . In zero f i e l d , the t h r e e compounds Fe(OEP), Fe(OTBP) and Fe(OTBP)(THF) 2 a I I show simple t w o - l i n e Mossbauer s p e c t r a . a t l i q u i d helium temperature, w i t h no i n d i c a t i o n of l i n e broadening. Thus, under these c o n d i t i o n s , s p i n r e l a x a t i o n i s f a s t compared t o the nuclear precession frequency... The THF adduct was not s t u d i e d i n an a p p l i e d f i e l d a t 4.2° K, but at 83° K in a l o n g i t u d i n a l magnetic f i e l d of 50 kG, the Mossbauer spectrum c o n s i s t e d of a simple t r i p l e t - d o u b l e t p a t t e r n w i + h H e f f * H e x f For Fe(OTBP) a t 4.2° K, an a p p l i e d f i e l d of 50 kG induces a smal I m a g n e t i z a t i o n , and H i s estimated t o be <\» 80 kG under these c o n d i t i o n s . However, as shown in Figure 30,. d e s p i t e t h i s augmentation of the a p p l i e d f i e l d the spectrum resembles much more c l o s e l y t h a t of a diamagnetic compound than those of the F e L ^ C d O ^ ^ s o l v a t e s discussed in Chapter III (see Figures 13-15), and the s i g n of V z z i s c l e a r l y p o s i t i v e . At 115° K with a 50 kG a p p l i e d f i e l d the spectrum Is FIGURE 30 Mossbauer Spectrum of Fe(OTBP) a t 4.2° K in an ap p l i e d magnetic f i e l d of 50 kG. V z z i s p o s i t i 57 and the e f f e c t i v e f i e l d a t the Fe nucleus i s estimated t o be ^ 8 0 kG. FIGURE 30 160b 8 a 'a s s 4 a 3 a Q B 0 B a a sB 1 B % a 'a 8 Q + Q 5— • + Q O Q O O CO E E o _o 2 161 "normal" and t here i s no apparent i n t e r n a l f i e l d . These r e s u l t s imply very f a s t s p i n r e l a x a t i o n a t a l l temperatures. T h i s i s p a r t i c u l a r l y 2+ i n t e r e s t i n g s i n c e one of the few examples of s l o w - r e l a x i n g Fe ions i s g i l l e s p i t e , in which ir o n i s a l s o in a square planar environment. Fe(OEP) behaves somewhat d i f f e r e n t l y in a p p l i e d f i e l d s . At 115° K with Hex_j. = 50 kG we estimate H = 67 kG from the observed Zeeman s p l i t t i n g . At 4.2° K the behaviour of t h i s complex i n a magnetic f i e l d i s much more complicated, the s p e c t r a l shape depending on the magnitude of the a p p l i e d f i e l d , but being q u i t e d i f f e r e n t from those shown in F i g s . 13-15. The l i n e s are broad and the s p e c t r a d i f f u s e and i l l - d e f i n e d (see Figure 31), which w i l l render any t h e o r e t i c a l t r e a t -ment extremely d i f f i c u l t . However, from the large number of s p e c t r a c a l c u l a t e d in d i f f e r e n t r e l a x a t i o n l i m i t s f o r Fe(PyNO)g(CI04)2» i t seems l i k e l y t h a t Fe(OEP) represents an example of intermediate r e l a x a t i o n , i n which the s p i n r e l a x a t i o n r a t e and the nuclear Larmor frequency have comparable time s c a l e s . PoIyCoctamethyItetrabenzporphyri ni r o n ( I I ) ] When the crude Fe(OTBP) from the sealed tube r e a c t i o n i s e x t r a c t e d e i t h e r with 2 - p i c o l i n e , a n i l i n e o r qui no I i n e and the product d r i e d in vacuo at room temperature, a black m a t e r i a l i n v a r i a b l y r e s u l t s which analyses c o r r e c t l y f o r Fe(OTBP). This product can a l s o be obtained by leaving e i t h e r a s o l u t i o n of the THF adduct in THF, o r a s o l u t i o n of the p y r i d i n e adduct in 1:50 p y r i d i n e / p e t r o l e u m ether t o stand o v e r n i g h t in a n i t r o g e n atmosphere. Since oxygen was excluded FIGURE 31 Mossbauer Spectrum of Fe(OEP) a t 4.2° K in an Ap p l i e d Magnetic F i e l d of 25kG FIGURE 31 163 from these systems, i t i s u n l i k e l y t h a t Fe(OTBP) has been converted t o an oxo-bridged f e r r i c dimer, as i s common with f e r r o u s porphyrins. We s h a l l designate t h i s black compound as r_Fe(0TBP)3n t o d i s t i n g u i s h i t from the blue Fe(OTBP), and because there i s c o n s i d e r a b l e evidence p o i n t i n g t o a polymeric form f o r t h i s compound. Evidence a g a i n s t L~Fe(0TBP)Hn being an o x i d a t i o n product i s q u i t e s u b s t a n t i a l . F i r s t l y , a n a l y t i c a l data f o r a l l the black L~Fe(0TBP)I] ' ' n compounds obtained agree a c c u r a t e l y w i t h an Fe(OTBP) f o r m u l a t i o n . (Data f o r the compound obtained from 2 - p i c o l i n e s o l u t i o n appear in Table XIV.) The c a l c u l a t e d carbon content f o r p-0CFe(OTBP)] 2 i s 77.1$, and in no case d i d the microanalyses show less than 78.1$ carbon. Secondly, the blue Fe(OTBP) can be l e f t in a i r f o r several weeks without showing any conversion t o e i t h e r the black compound or an oxo-bridged dimer. A n a l y t i c a l data, Mossbauer and e l e c t r o n i c s p e c t r a (the l a t t e r in p y r i d i n e s o l u t i o n ) of Fe(OTBP) are unchanged a f t e r t h r e e weeks' exposure of the compound t o the atmosphere. We f e e l t h i s evidence i s very c o n v i n c i n g t h a t the bl a c k m a t e r i a l i s not y-0L~Fe(OTBP)D 2. When L"Fe(OTBP)l n j s d i s s o l v e d i n p y r i d i n e i t f i r s t forms a dark o l i v e green s o l u t i o n . The e l e c t r o n i c spectrum of t h i s s o l u t i o n 5 contains a Soret band a t 417 nm (e = 1 x 1 0 ) and two other bands max 4 4 at 485 nm (e = I x 10 ) and 660 nm (e = 5 x 10 ). This spectrum max max K i s very d i f f e r e n t from t h a t of Fe(OTBP)(py)^ shown i n Figure 26 above. However, a f t e r two hours the s o l u t i o n has become deep apple green in c o l o u r . In a d d i t i o n t o the 417, 485 and 660 nm bands, the e l e c t r o n i c spectrum now contains bands due t o Fe(OTBP)(py) 2 which strengthen with time a t the expense of the o r i g i n a l spectrum. Complete conversion t o 164 the Fe(OTBP) ( p y ) 2 spectrum r e q u i r e s one or two days, depending on the p a r t i c u l a r sample, but a f t e r t h i s time evaporation of the s o l u t i o n t o dryness a t room temperature a f f o r d s pure Fe(OTBP)(py) 2. T h i s behaviour i s suggestive of a polymeric s t r u c t u r e which is g r a d u a l l y broken down in s o l u t i o n . Further i n d i c a t i o n of the polymeric nature of r_Fe(0TBP)3n i s provided by magnetic s u s c e p t i b i l i t y measurements at room temperature. The s u s c e p t i b i l i t y v a r i e s from sample t o sample and i s f i e l d dependent, suggesting the presence of exchange i n t e r a c t i o n s t y p i c a l of molecular 127 aggregates . The e f f e c t i v e magnetic moments range from 6 t o 14 B.M., values which are abnormally high f o r monomeric systems. Althoughthe blue Fe(OTBP) i s q u i t e s t a b l e and shows no conversion t o the black compound under normal c o n d i t i o n s , samples of Fe(OTBP) subjected t o pressures of 10-13 kbar on an h y d r a u l i c press do show p a r t i a l conversion t o L~Fe(0TBP)H n. The Mossbauer parameters of Fe(OTBP) and L~Fe(0TBP)!]n are q u i t e d i f f e r e n t (see below), and s p e c t r a of the pressed products showed l i n e s corresponding t o both s p e c i e s . However, complete conversion could not be a f f e c t e d a t these pressures, even when the sample was l e f t under pressure f o r several days. In view of the unusual magnetic moments observed, i t was decided t o study the s o l i d s t a t e e l e c t r i c a l c o n d u c t i v i t i e s of these compounds. P e l l e t s made on the h y d r a u l i c press were coated with s i l v e r p a i n t and connected t o a vacuum-tube voltmeter. The dimensions of the p e l l e t s were determined with a micrometer gauge. The s e n s i t i v i t y of the voltmeter was such t h a t c o n d u c t i v i t i e s of about 10 ' cm ' or g r e a t e r could be detected. Several samples of f_Fe(OTBP)2n obtained TABLE XVI 165 5 7 Fe Mossbauer Parameters f o r Fe (OTBP) and L"Fe(0TBP)1 Compound Fe(OTBP) CFe(0TBP)1 r. To i i • 1 2 T(°K) 6 (mm s 1 j AEp(mm s~') (mm s ) (mm s~' ) n 295 0.94 0.56 .30 .23 115 1.04 +0.61 .27 .28 ^ 0 83 1.05 0.62 .27 .28 8.6 1.06 +0.63 .27 .28 ^ 0 295 0.58 0.48 .26 .24 84 0.68 -0.49 .30 .31 ^ 0 8.25 0.69 0.50 .29 .32 166 from d i f f e r e n t s o l v e n t s were s t u d i e d at room temperature, and a l l showed c o n d u c t i v i t i e s of % 10 6 fi ' cm '. T h i s value i s q u i t e t y p i c a l of those 127 found f o r polymeric o r g a n i c semiconductors , and can be taken as an i n d i c a t i o n t h a t bonds are formed between neighbouring molecules which are c o n s i d e r a b l y s t r o n g e r than the c r y s t a l packing f o r c e s t h a t e x i s t i n normal covalent c r y s t a l s . A sample of the p a r t i a l l y converted blue —8 -1 -1 Fe(OTBP) gave a c o n d u c t i v i t y of ^ 10 fi cm , whereas no c o n d u c t i v i t y could be detected with p e l l e t s of Fe(OTBP)(py> 2 and y - 0 L F e(0EP ) H 2 . (The l a s t compound was k i n d l y provided by Dr. D. Dolphin.) These r e s u l t s provide a d d i t i o n a l evidence t h a t L~Fe(0TBP)I] i s not an n oxo-bridged f e r r i c dimer. The Mossbauer parameters f o r samples of CFe(0TBP)H n are independent of the s o l v e n t from which the m a t e r i a l was i s o l a t e d . These parameters are compared with those of the blue Fe(OTBP) i n Table XVI. The isomer s h i f t of CFe(0TBP)3 n i s c o n s i d e r a b l y lower than t h a t of Fe(OTBP), i n d i c a t i n g a g r e a t e r e f f e c t i v e s e l e c t r o n d e n s i t y at the iro n nucleus in the polymer. Although the magnitudes of AEg are q u i t e s i m i l a r in the two compounds, V z z i s found t o be negative f o r CFe(0TBP)] [ rather than p o s i t i v e . For every other f e r r o u s porphyrin and phthalocyanine d e r i v a t i v e s t u d i e d t o date, V z z>0. This s i g n r e v e r s a l i n d i c a t e s t h a t t here i s now an excess of e l e c t r o n d e n s i t y a t i r o n in the a x i a l d i r e c t i o n over t h a t in the porphyrin plane. In view of the evidence in favour of a polymeric s t r u c t u r e , a l i k e l y e x p l a n a t i o n of the negative V z z i s the involvement of the d z 2 o r b i t a l i n the formation of an i r o n - i r o n a bond. 167 Summary The data we have obtained f o r the f e r r o u s OTBP and OEP complexes, together with previous r e s u l t s on other f e r r o u s porphyrins and f e r r o u s phthalocyanine, show t h a t the e l e c t r o n i c s t r u c t u r e of i r o n i s very s e n s i t i v e t o changes in a and ir bonding p r o p e r t i e s of the planar r i n g systems. T h i s Ls p a r t i c u l a r l y t r u e i n the h i g h - s p i n t e t r a c o o r d i n a t e s p e c i e s , and i n d i c a t e s t h a t i t i s advantageous t o study the neat f e r r o u s porphyrins without a x i a l l i g a n d s . Furthermore, although t h e o r e t i c a l a n a l y s i s of the a p p l i e d f i e l d Mossbauer s p e c t r a obtained a t 4.2° K f o r Fe(OTBP) and Fe(OEP) has not yet been attempted (and i t i s expected t h a t such an a n a l y s i s w i l l be very d i f f i c u l t ) , there i s p o t e n t i a l l y a great deal more informa t i o n t o be obtained on these complexes. The development of a s a t i s f a c t o r y t h e o r e t i c a l framework t o e x p l a i n these r e s u l t s i s an obvious extension of the present study. Because of the very small temperature dependence of | A E Q | f o r these d e r i v a t i v e s , i t may be p o s s i b l e t o base such a theory on the s p i n Hamiltonian formalism, but some account w i l l have t o be taken of the strong covalency e f f e c t s which are present. I t would a l s o be very i n t e r e s t i n g t o study a d d i t i o n a l f e r r o u s porphyrins in which there are less d r a s t i c changes i n the porphyrin s t r u c t u r e than the ones examined here. Again, t h i s w i l l not be easy. Although Fe(OTBP) has remarkable s t a b i l i t y , Fe(OEP) i s an extremely s e n s i t i v e compound and r e q u i r e d very c a r e f u l handling i n o b t a i n i n g p h y s i c a l measurements. Several attempts have been made t o i s o l a t e Fe(PP) in the s o l i d s t a t e without success, and other f e r r o u s porphyrins c l o s e l y r e l a t e d t o haeme are l i k e l y t o be e q u a l l y d i f f i c u l t t o o b t a i n . 168 The i n t r o d u c t i o n of Iigands in the a x i a l c o o r d i n a t i o n s i t e s almost i n v a r i a b l y leads t o a ' A . ground s t a t e f o r i r o n , and tends t o 1 i g mask the d i f f e r e n c e s between the v a r i o u s complexes s t u d i e d . However, the p o s i t i v e V z z observed in a l l cases i s in agreement with the i n t u i t i v e concept t h a t bonding t o the porphyrin nitrogens i s stronger than t h a t t o the a x i a l Iigands, and the d i f f e r e n c e s in Mossbauer parameters which do remain are c o n s i s t e n t with the bonding p r o p e r t i e s deduced from the square planar complexes. The p r o p e r t i e s of CFe(0TBP)U n are unusual and i n t e r e s t i n g , p a r t i c u l a r l y i t s semiconducting behaviour and the negative V z z found. We have suggested t h a t the polymeric nature of t h i s m a t e r i a l may be due t o Fe-Fe bonding between adjacent planar molecules, but the mechanism of formation remains u n c l e a r . 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Phys., ±8, I 174 (1950). 124. M. Zerner, M. Gouterman and H. Kobayashi, Theor. Chim. A c t a . , 6, 363 (1966). 125. A.M. S c h a f f e r , M. Gouterman and E.R. Davidson, Theor. Chim. A c t a . , 30, 9 (1973). 126. A.B.P. Lever, J . Chem. S o c , 1821 (1965). 127. Ya. M. Paushkin, T.P. Vis h n y a i c o v a , A.J. Lunin and S.A. Nizova, "Organic Polymeric Semiconductors", Wiley Inc., New York, 1974, Chap. 4. 177 APPENDIX I In order t o compute the t h e o r e t i c a l Mossbauer spectrum, t r a n s i t i o n p r o b a b i l i t i e s have t o be c a l c u l a t e d . The procedure used i s out I i ned here. The u n p o l a r i z e d y r a d i a t i o n contains equal numbers of r i g h t -handed and left-handed c i r c u l a r l y p o l a r i z e d quanta. These can be represented by S=l angular momentum f u n c t i o n s | l , l > ' and | l , - l > ' , r e s p e c t i v e l y , along the a x i s of propagation. The |l , 0> s t a t e i s not used due t o the absence of l o n g i t u d i n a l l y p o l a r i z e d y ^ a y s . Since i t i s convenient t o c a l c u l a t e the y a b s o r p t i o n process along the p r i n c i p a l a x i s of the e f g which l i e s a t an angle (9,<f>) with respect t o the y propagation a x i s , i t i s necessary t o transform i n t o the efg a x i s rep-r e s e n t a t i o n v i a the equations^ |l,±l>'= t i u t c o s e j e 1 * ! I, l> ; i ( i i c o s e ^ " ' * 1 1 , - i > 1 /2sin0|1,0* (A-In order t o s i m p l i f y f u r t h e r equations, the c o e f f i c i e n t s w i l l be denoted by A + ( l ) = i i d i c o s e J e 1 * A +(0) = - sin e A +(-|)= T i ( | + 0 0 5 6 ) 6 " ' * ( A 178 The i n t e n s i t i e s w i l l be r e l a t e d t o overlap i n t e g r a l s of the type <i,m|<l ,M||-,m«> (A-3) where l - ^ i ™ * are the nuclear ground s t a t e basis k e t s , | I ,M> the photon ba s i s k e t s , and ||m'> the nuclear e x c i t e d s t a t e basis ket v e c t o r s . The i n t e g r a l s (A-3) are in f a c t the Clebsh-Gordan C o e f f i c i e n t s connecting the I = ^ a n c' 1 = ' j nuclear s t a t e s and are t a b u l a t e d ' ^ . The i n t e n s i t y of the spectrum f o r the unp o l a r i z e d beam w i l l be given by the equation"^. 1 ( 8 , o > ) = j\ + ll (A-4) where T + i s the i n t e n s i t y c o n t r i b u t e d by the right-handed c i r c u l a r l y 2 p o l a r i z e d l i g h t and T_ the c o n t r i b u t i o n from the left-handed component. r + = [b* b*] C + ( i , | ) c + ( i , i ) C + ( i - i ) 0 c + ( - ^ } c + ( - W > c + { - h~9 I I I 3, (A-5) r _ = [ b * b * ] C_(l>|) c _ ( i , i ) c _ ( i , - i ) c-(-^ I> cJ-?rj> c - ( - i " | > a i a. (A-6) 179 Where C^Cm^nr^) = <2~, I ,rri|, (m2-m|) | j,m2> A+(m2-m|) <lr, I ,m., (m„-m.) | :=-,m„> are CI ebsch-Gordan c o e f f i c i e n t s , are e i g e n f u n c t i o n s f o r the I = ground s t a t e , are e i g e n f u n c t i o n s f o r the I = ^ e x c i t e d s t a t e . The above r e s u l t i s f o r s p e c i f i c 6 and angles. The powder averaged spectrum should then be an int e g r a t e d spectrum over a l l values of 0 and <{>. Due t o the f a c t t h a t the efg has m i r r o r symmetry about i t s p r i n c i p a l planes, only values of 0 and t}> l y i n g in one oc t a n t need be i n t e g r a t e d ' 7 . The computer simulates the i n t e g r a t i o n process by a sum of elements d(cos0)d<|> over the f i r s t o c t a n t where ten increments of 0 and cj) are normally computed. APPENDIX I I Hamiltonian Matrix of Tetragonal D i s t o r t i o n |A> = •!•-<|2,2>-|2,-2» |B> -m |2,l> |C> - |2,-l> |A>|2> |A>||> |A>|0> |A>|-I> |A>|-2> |B>|2> |B>||> |B>|0> • |B>|-I> |B>|-2> |0|2> |C>|l> |C>|0> |C>|-I> |C>|-2> <2|<A| 2DS -2Do + Jl X <l|<A| 2D3+DO - A + S5 X <0|<A| 2Ds+2Da - fix + ^3 X <-l|<A| 2Ds+Da - /3X + /2 X <-2|<A| 2D3-2D0 - /2\ <2|<B| - Jz\ •0S-2X-2DJ 6Dr <l|<B| - S?X -D>X+D0 6Dr <0|<B| - /3 X -Ds+2D0 6Dr <-l|<B| - ^ X <-2|<B| -D=*2X-2Do 6Dr <2|<C| 6Dr -DSf2X-0a 6Dr <l |<C| + Jl X 6Dr -Qs+X+CB <0|<C| + /5 X 6Dr -Ds+2Da <-l|<C| + S5 x 6Dr -CB-X+Do <-2|<C| + Jl X 6Dr -Ds-2X-2Do APPENDIX II /Continued Hamiltonian Matrix of Trigonal D i s t o r t i o n |A> = |2,0> |B> = ^ f |2,-2> + ^  |2.l> |C>=y||2,2> - ^ | 2 , - l > |A>|2> |A>||> |A>|0> |A>|-I> |A>|-2> |B>|2> |B>||> |B>|0> |B>|-I> |B>|-2> |C>|2> JO|l> |C>|0> |C>|-I> |C>|-2> <2|<A| -2Ds-2Da 4Dr - fix 4Dr <||<A| -2Ds+Do 4Dr - fix fix 4Dr <0|<A| -2DS+2Do 4Dr - ^ X /3 X 4Dr <H|<A| -2Ds+D<J • 4Dr - /2 X A X 4 Dr <-2|<A| -2Ds-2Do 4Dr /2 X 4Dr <2|<B| 4Dr Ds+2X-2Do -2Dr <l |<B| - ^ X 4Dr Ds+X+Do -2Dr <0|<B| - /3 X 4Dr Ds+2Dc -2Dr <-l|<B| - /3 X 4Dr Ds-X+Da -2Dr <-2|<B| - ^ X 4Dr ' Ds-2X-2Da -2Dr <2|<C| 4Dr /2\ -2Dr Ds-2X-2Do -<I|<C| 4Dr /3 X -2Dr Ds-X+Da <0|<C| 4Dr /3X -2Dr Ds+2Da <-||<C| 4Dr -2Dr Ds+X+Do <-2|<C| 4Dr -2Dr Ds+2X-2D° 

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