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Interactions of axial ligands and dioxygen with cobalt (II) macrocycle complexes Smith, David W. 1980

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INTERACTIONS OF AXIAL LIGANDS AND DIOXYGEN WITH COBALT(II) MACROCYCLE COMPLEXES by DAVID W. SMITH B.Sc, Carleton U n i v e r s i t y , 1974 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n THE FACULTY OF GRADUATE STUDIES Department of Chemistry We accept t h i s t hesis as conforming to the required standard The Uni v e r s i t y of B r i t i s h Columbia August, 1980 © David W. Smith, 1980 In presenting this thesis in partial fulf i lment of the requirements for an advanced degree at the University of Brit ish Columbia, I agree that the Library shall make i t freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. I t is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. . CHEMISTRY Department of The University of Brit ish Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Date Sept. 19, 1980 - i i -ABSTRACT Studies on the int e r a c t i o n s of a x i a l ligands and molecular oxygen with Co(II) porphyrin and S c h i f f base complexes have revealed a number of in t e r e s t i n g and s i g n i f i c a n t features regarding the nature of such interactions. The binding of a f i r s t a x i a l ligand to cobalt(II) octamethyltetra-benzoporphyrin (CoOMBP), K l -L + CoOMBP -, LCoOMBP, (I) occurs with greater ligand a f f i n i t y than i s observed f o r other Co(II) porphyrin (CoP) complexes. Also, LCoOMBP i s shown by v i s i b l e spectroscopy to completely bind a second a x i a l ligand, K2 , L + LCoOMBP ^  LCoOMBP, (II) at high ligand concentrations i n several cases, while other LCoP complexes do not under s i m i l a r conditions. These binding properties are at t r i b u t e d to the very weak a - b a s i c i t y of OMBP causing approaching a x i a l ligands to see a metal centre with more p o s i t i v e charge. In contrast, molecular oxygen binds more strongly to LCoOMBP complexes, K. LCoOMBP + 0 „ ^ °2 __±rLCoOMBP (0 J , (III) , _ 2 , than would be predicted from the a - b a s i c i t y properties of OMBP. This observation i s explained i n terms of the strong Tr-donating a b i l i t i e s of OMBP enhancing the a f f i n i t y of the Co(II) complex for dioxygen; thus good evidence i s obtained for coordinated dioxygen being able to accept T r-electron density. The ir-donating a b i l i t i e s of OMBP are also invoked to - i i i -explain the observed similarities in AH values associated with K, and K ; 1 2 i t i s suggested that the 7 r-interactions inhibit metal centre displacement from the porphyrin plane upon the formation of a five-coordinate adduct. Evidence for u-interactions between porphyrin and axial ligand is provided by an observed enhanced AH value for the u-accepting ligand PPh^. The formation of dioxygen adducts of LCoOMBP is shown to be enhanced in more polar solvents, and a linear correlation between K and dielectric °2 constant is observed in varying mixtures of the binary toluene-DMF solvent system. Ligand binding is demonstrated to be slightly inhibited in more polar solvent systems. These effects are considered to be due to differences in relative polarity of the products and the reactants of the systems studied. Studies on ligand binding to a variety of CoP complexes show that the compounds with stronger a-base porphyrins bind ligands less strongly than those with weaker a-base porphyrins. This trend does not appear to hold when DMF is the axial ligand being studied; the strong IT-donor a b i l i t y of this ligand i s proposed as a possible explanation of the apparent anomalous behavior of DMF. The TT-donor effects of DMF are also invoked to explain the enhanced AH associated with dioxygen binding to DMFCoOMBP, compared to when predominantly a-donating-ligand-complexes of CoOMBP are oxygenated. A wide variety of LCoP complexes were monitored for oxygenation, and a good correlation i s found between the a-basicity of the porphyrin and the oxygen aff i n i t y of the LCoP complex when L is kept constant; complexes of stronger porphyrin a-basicity are found to bind dioxygen more strongly than those of weaker a-basic porphyrins. This i s explained in terms of the stronger a-base putting more electron density on the metal centre, so that transfer of electron density to dioxygen can occur more readily and the cobalt-dioxygen bond (Co(III)-0 o ) can be more easily formed. - i v -I r r e v e r s i b l e o x i d a t i o n o f LCoP c o m p l e x e s , 2 LCoP + 0 2 - L C o P(0 2 ) C o P L , (TV) i s a l s o e x a m i n e d . I n t h e c o u r s e o f t h e s e e x p e r i m e n t s , s t r o n g e v i d e n c e i s o b t a i n e d f o r t h e f o r m a t i o n o f a g g r e g a t e s o f CoOMBP c o m p l e x e s i n n o n -a q u e o u s m e d i a a t c o n c e n t r a t i o n s a s l o w as 10 M , t h e f i r s t s u c h CoP a g g r e g a t e s t o be o b s e r v e d u n d e r t h e s e c o n d i t i o n s . O x i d a t i o n o f a v a r i e t y o f LCoP c o m p l e x e s i s shown t o p r o c e e d b y a f i r s t o r d e r p r o c e s s i n c o b a l t , s u g g e s t i v e o f a n " a c t i v a t e d i n t e r m e d i a t e " i n w h i c h c o o r d i n a t e d d i o x y g e n may more c l o s e l y r e s e m b l e t h e p e r o x i d e - l i k e d i o x y g e n i n t h e p r o d u c t t h a n t h e s u p e r o x i d e - l i k e m o i e t y p r e s e n t i n L C o P ( C > 2 ) . O x i d a t i o n o f CoOMBP c o m p l e x e s i s s u b s t a n t i a l l y e n h a n c e d when Im i s t h e a x i a l l i g a n d , so much s o t h a t f o r m a t i o n o f t h e 1:1 C o : d i o x y g e n a d d u c t c o u l d n o t b e i n d e p e n d e n t l y s t u d i e d , e v e n a t l o w e r t e m p e r a t u r e s w h e r e o x i d a t i o n i s n o r m a l l y i n h i b i t e d . T h i s e n h a n c e m e n t i s t h o u g h t t o b e due t o t h e a b i l i t y o f Im t o h y d r o g e n b o n d t o t h e 1:1 C o : d i o x y g e n a d d u c t , a n d t o t h u s e n h a n c e t h e f o r m a t i o n o f t h e " a c t i v a t e d i n t e r m e d i a t e " a n d t o p e r m i t a more f a c i l e o x i d a t i o n o f t h e c o b a l t p o r p h y r i n s y s t e m . I f o t h e r n o n - h y d r o g e n b o n d i n g compounds a r e u s e d as l i g a n d s , t h e LCoOMBP c o m p l e x e s s o m e t i m e s t a k e d a y s t o o x i d i z e i n t o l u e n e . I f a more p o l a r s o l v e n t s y s t e m i s u s e d , t h e n t h e r a t e o f o x i d a t i o n i s i n c r e a s e d . T h i s e f f e c t i s e s p e c i a l l y m a n i f e s t e d when t h e o x i d a t i o n o f (CH^-Im)^CoOMBP i s s t u d i e d i n m e d i a c o n t a i n i n g n e c e s s a r i l y s u b s t a n t i a l amounts o f p o l a r C H ^ - I m ; t h e r e a c -t i o n i s now m e a s u r a b l e o n a s t o p p e d ^ f l o w k i n e t i c s t i m e s c a l e , i n s p i t e o f t h e s i x - c o o r d i n a t e n a t u r e o f t h e CoOMBP s p e c i e s p r e s e n t u n d e r t h e s e c o n d i t i o n s . I n t h e S c h i f f b a s e s y s t e m , N , N - b i s ( s a l i c y l a l d e h y d e ) e t h y l e n e d i i m i n a t o -c o b a l t ( I I ) ( C o ( s a l e n ) ) , l i g a n d a d d u c t f o r m a t i o n i s f o u n d t o be much w e a k e r and - v -d i o x y g e n a d d u c t f o r m a t i o n s t r o n g e r t h a n i n t h e CoP s y s t e m s , and t h i s i s t h o u g h t t o be due t o t h e g r e a t e r e l e c t r o n d e n s i t y p r e s e n t on t h e m e t a l c e n t r e o n t h e C o ( s a l e n ) s y s t e m . The C o ( s a l e n ) s y s t e m i a shown t o be so d i o x y g e n s e n s i t i v e t h a t f o u r - c o o r d i n a t e C o ( s a l e n ) i s e s s e n t i a l l y a b l e t o c o m p l e t e l y o x y g e n a t e a t l o w t e m p e r a t u r e s , a l t h o u g h t h e r a t e o f o x y g e n a t i o n o f t h e f o u r - c o o r d i n a t e c o m p l e x i s many o r d e r s o f m a g n i t u d e l e s s t h a n t h a t o f t h e f i v e - c o o r d i n a t e L C o ( s a l e n ) s y s t e m s . Compounds w i t h u n s a t u r a t e d c a r b o n - c a r b o n b o n d s a r e shown t o i n t e r a c t w i t h t h e c o b a l t m a c r o c y c l e c o m p l e x e s . S t y r e n e and 3,4 - d i c h l o r o b u t e n e u n d e r one a t m o s p h e r e o f o x y g e n c a u s e r e v e r s i b l e v i s i b l e s p e c t r a l c h a n g e s t o o c c u r w i t h C o ( s a l e n ) o v e r a p e r i o d o f s e v e r a l d a y s . M a l e i c a n h y d r i d e and t e t r a c y a n o e t h y l e n e b i n d t o CoOMBP, b u t n o t t o o t h e r CoP c o m p l e x e s , t h u s a g a i n s h o w i n g t h e i m p o r t a n c e o f t h e T T - d o n a t i n g a b i l i t i e s o f OMBP. - v i -TABLE OF CONTENTS Page ABSTRACT i i TABLE OF CONTENTS LIST OF TABLES x x i LIST OF FIGURES XXVri ABBREVIATIONS AND COMMON NAMES xxxiv ACKNOWLEDGEMENTS x l i i CHAPTER I. INTRODUCTION 1 1.1 B i o l o g i c a l Oxygen Carrying Systems 1 1.1.1 General 1 1.1.2 Myoglobin and Hemoglobin 5 1.2 Synthetic Oxygen C a r r i e r s 8 1.2.1 Rationale f o r Synthetic Oxygen C a r r i e r s 8 1.2.2 General Synthetic Oxygen C a r r i e r s 9 1.2.3 Porphyrins 10 1.2.4 S c h i f f Bases 12 1.2.5 T r a n s i t i o n Metal Complexes 18 1.2.5.1 Iron Complexes 18 1.2.5.2 Copper Complexes 20 1.2.5.3 Chromium Complexes 20 1.2.5.4 Manganese Complexes 20 1.2.5.5 Titanium Complexes 21 1.2.5.6 Ruthenium Complexes 21 1.2.5.7 Rhodium Complexes 21 - v i i -Page 1.3 Cobalt Complexes 22 1.3.1 General 22 1.3.2 Early Cobalt Dioxygen Work .' 2 3 1.3.3 Cobalt S c h i f f Base Compounds. 23 1.3.4 Cobalt Porphyrin Compounds 24 1.3.5 Other Cobalt Compounds 27 1.4 C a t a l y t i c Properties 27 CHAPTER I I . APPARATUS AND EXPERIMENTAL PROCEDURE 29 I I . 1 Instrumentation 29 11.2 Spectroscopic C e l l s 31 II.2.1 U l t r a v i o l e t - v i s i b l e Spectroscopic C e l l s .... 31 I I . 2.2 ESR C e l l s 36 11.3 Constant Temperature Baths 38 II.3.1 Above 0°C 38 I I . 3.2 At or Below 0°C 38 11.4 Equilibrium Constant Measurements 38 11.4.1 Ligand Binding 38 11.4.2 Oxygen Binding 39 11.5 Spectrophotometry K i n e t i c Measurements 40 11.6 Gas-Uptake Measurements 40 11.6.1 Description 40 11.6.2 Procedure f o r a T y p i c a l Gas-Uptake Experiment 42 11.6.3 Gas S o l u b i l i t y Measurements 45 11.7 Materials 45 II.7.1 Gases 45 - v i i i -Page II.7.2 Liquids 46 11.7.2.1 General 46 11.7.2.2 Solvents 46 11.7.2.3 Ligands 46 II. 7.3 Solids 47 II. 7.3.1 Ligands 47 11.7.3.2 Macrocycles and Co(II) Complexes .... 47 CHAPTER III. THERMODYNAMICS OF REVERSIBLE BINDING OF SELECTED LIGANDS TO CoOMBP 51 111.1 Qualitative Spectral Observations 51 111.1.1 L + CoOMBP - * LCoOMBP, 51 III. 1.2 L + LCoOMBP V ' L2CoOMBP, K2 55 111.2 Treatment of Data • 59 111.3 Sources of Error 65 111.4 Ligand Binding Constants 69 III. 4.1 L + CcOMBP =^—=^ LCoOMBP, 69 111.4.2 L + LCoOMBP, - LCoOMBP, K 2 74 111.5 Ligand Binding in 1,2-Dichloroethane 76 CHAPTER IV. THERMODYNAMICS OF REVERSIBLE DIOXYGEN BINDING TO TO CoOMBP COMPLEXES 79 IV. 1 Qualitative Spectral Observations 79 IV.1.1 LCoOMBP + 02Z f LCoOMBP (02) , K Q 79 IV. 1.2 L CoOMBP + 0 - LCoOMBP (O) + L, K 81 2 2 2 2/L - i x -Page IV.2 Species Present When the CoOMBP-PBu3 System i s Oxygenated 85 IV. 3 Treatment of Data 87 IV.4 Sources of Error 91 IV.5 Dioxygen Binding Constants 93 IV. 5.1 LCoOMBP + 0 2 - ' LCoOMBP(02) , K Q 93 IV. 5.2 L CoOMBP + 0 . ~LCoOMBP(0) + L, K 97 2 2 2 2/L IV. 6 Oxygenation Reaction i n Solvent Systems Other Than Neat Toluene 9 8 CHAPTER V. THERMODYNAMICS OF REVERSIBLE LIGAND BINDING TO CoDADIXDME AND CoDBrDIXDME 105 V. l Q u a l i t a t i v e Spectral Observations 105 V. l . l L + CoDADIXDME- T LCoDADIXDME, K 105 -L cl V . l . 2 L + CoDBrDIXDME ~ ~ ± LCoDBrDIXDME, K .. 108 V.2 Treatment o f Data 108 V.3 Sources of Error 110 V.4 Ligand Binding Constants 113 V. 5 Comparison of Ligand Binding Among D i f f e r e n t Cobalt Porphyrins 113 CHAPTER VI. THERMODYNAMICS OF REVERSIBLE DIOXYGEN BINDING TO LCoDADIXDME AND LCoDBrDIXDME 128 VI. 1 Q u a l i t a t i v e S p e c t r a l Observations 128 VI. 2 Treatment of Data 132 VI.3 Sources of Error 132 VI.4 Dioxygen Binding Constants 136 -x-Page VI. 5 Comparison of Dioxygen Binding Among D i f f e r e n t Cobalt Porphyrins 139 CHAPTER VII. KINETICS OF THE OXIDATION OF COBALT(II) PORPHYRIN COMPLEXES 154 VII. l Introduction 154 VII.2 Q u a l i t a t i v e Spectral Observations 155 VII. 3 Treatment of Data 164 VII. 4 Aggregation , 165 VII.5 K i n e t i c s of the Oxidation of LCoOMBP 173 VII.5.1 K i n e t i c s of the Oxidation of ImCoOMBP i n Toluene ... 173 VII. 5.2 Ki n e t i c s of the Oxidation of LCoOMBP i n Toluene, L = CH3-Im, Pip, or Py 189 VII.5.3 K i n e t i c s of the Oxidation of LCoOMBP i n Solvents Other Than Toluene , 195 VII. 5.3.1 Ki n e t i c s of the Oxidation of CH^Im i n a Variety of Solvents .195 VII.5.3.2 Kin e t i c s of the Oxidation of LCoOMBP i n DCE 197 VII.6 K i n e t i c s of the Oxidation of LCoOMBP 200 VII. 7 K i n e t i c s of the Oxidation of LCoP, P j- OMBP 212 VII. 8 Concluding Comments 228 CHAPTER VIII. THERMODYNAMICS AND KINETICS OF REVERSIBLE LIGAND AND DIOXYGEN BINDING TO Co(SALEN) COMPLEXES 232 VIII. 1 Q u a l i t a t i v e Spectral Observations 232 VIII. 2 Treatment of Data 236 VIII.3 Sources of Error 249 - x i -Page VIII.4 Studies i n Dichloromethane 249 VIII.4.1 L + Co(salen) . " LCo(salen), K 249 VIII.4.2 LCo(salen) + 0 tLC o ( s a l e n ) ( 0 ), K 254 2. 0 2 VIII.4.3 Co(salen) + 0^ ": Co (salen) (.0^1 256 VIII. 5 Studies i n Other Solvents 265 VIII.5.1 Rationale 265 VIII.5.2 L + Co (salen) '-, " LCo (salen) 265 VIII. 5.3 LCo (salen) + 0 2 ~ *T LCo (salen) (0 ) 265 CHAPTER IX. THERMODYNAMICS AND KINETICS OF REACTIONS OF COBALT(II) MACROCYCLIC SYSTEMS WITH SUBSTRATES CONTAINING UNSATURATED CARBON-CARBON BONDS 269 IX. 1 Reactions of Co(II) Porphyrin Systems with Ethylene and Acetylene 269 IX.2 Reactions of Co(II) Porphyrin Systems with Electron Withdrawing Olefins •' 276 IX.3 Reactions of Co(salen) Complexes with Unsaturated Substrates 285 CHAPTER X. GENERAL CONCLUSIONS AND SOME RECOMMENDATIONS FOR FUTURE WORK 291 REFERENCES 297 APPENDIX. RAW DATA USED FOR DETERMINING THERMODYNAMIC AND KINETIC PARAMETERS REPORTED IN THIS THESIS '. . - 321 APPENDIX I. RAW DATA USED FOR DETERMINING THE THERMODYNAMIC PARAMETERS OF REVERSIBLE LIGAND BINDING TO CoOMBP 322 - x i i -Page APPENDIX l a . Im + CoOMBP — ImCoOMBP 323 APPENDIX l b . CH3-Im + CoOMBP CH3-ImCoOMBP 325 APPENDIX Ic. Pip + CoOMBP zz, "PipCoOMBP 326 APPENDIX Id. Py + CoOMBP ±r PyCoOMBP 327 APPENDIX l e . ET^N + CoOMBP — ' ET3NCoOMBP 328 APPENDIX I f . DMF + CoOMBP — = t rDMFCoOMBP 329 APPENDIX l g . THF + CoOMBP ^ THFCoOMBP 330 APPENDIX Ih. PPh 3 + CoOMBP — = ± : PPh3CoOMBP ' 332 APPENDIX I i . PBu 3 + CoOMBP ^  I P B U CoOMBP 333 APPENDIX I j . CH -Im CH3-ImCoOMBP=^=-=-=:(CH3-Im) 2CoOMBP 334 APPENDIX Ik. Pip + PipCoOMBP ^  '(Pip) 2CoOMBP 335 APPENDIX I I . Py + PyCoOMBP .. "(Py) 2CoOMBP 337 APPENDIX Im. PBu 3 + PBu CoOMBP., :^(PBu3) 2CoOMBP 339 APPENDIX In. CH -Im + CnDMRP — - CH^-TmCnnMRP i n 1,2-dichloroethane 340 APPENDIX Io. Pip + CoOMBP -. PipCoOMBP i n 1,2-dichloroethane 341 APPENDIX Ip. Py + CoOMBP - PyCoOMBP i n 1,2-dichloroethane 342 APPENDIX Iq. THF + CoOMBP ^ = - = i THFCoOMBP i n 1,2-dichloroethane 343 APPENDIX I I . RAW DATA USED FOR DETERMINING THE THERMODYNAMIC PARAMETERS OF REVERSIBLE DIOXYGEN BINDING TO L CoOMBP. (n = 1 or 2) 344 n APPENDIX H a . CH3-ImCoOMBP + Oy.' - CI^-ImCoOMBP (0 2) 345 APPENDIX l i b . PipCoOMBP + O^. *" PipCoOMBP (Q2) 3 4 6 APPENDIX l i e . PyCoOMBP + O - TPyCoOMBP(0) 347 - x i i i -Page APPENDIX I Id. Et3NCoOMBP + C>2 . * Et3NCoOMBP (C^) 348 APPENDIX l i e . DMFCoOMBP + O ^ *T DMFCoOMBP (O^) 350 APPENDIX I l f . PPh3CoOMBP + C»2 - ': PPh3CoOMBP (C^) 352 APPENDIX I l g . PBu3CoOMBP + ~ *: PBu3CoOMBP(02) 353 APPENDIX I l h . PBu3CoOMBP + *: PBu3CoOMBP (O^) as a function of CPBu^ 354 APPENDIX H i . L2CoOMBP + *= LCoOMBP (O^ + L @ -45°C . 356 APPENDIX I I j . CH3-ImCoOMBP + 0 2 » -CH3-ImCoOMBP(02) i n a solvent system of 4:1 by volume toluene: 1,2-dichloroethane ..• 357 APPENDIX I l k . L2CoOMBP + 02-. -LCoOMBP (Q2) + L @ -45°C In a solvent system of 4:1 by volume toluene: 1,2-dichloroethane 359 APPENDIX I I I . CH3-ImCoOMBP + ° 2 " *' CH^-ImCoOMBP In a solvent system of 1:1 by volume toluene:DMF 360 APPENDIX Hm. PipCoOMBP + 0 2 * t: PipCoOMBP (0 2) In a solvent system of 1:1 by volume toluene :DMF 362 APPENDIX Hn. PyCoOMBP + 0 , *TPyCoOMBP (O,^ ) In a solvent system of 1:1 by volume toluene :DMF 364 APPENDIX Ho. PipCoOMBP + 0 ^ ^PipCoOMBP (0 2) With varying amounts of DMF i n the solvent system 366 APPENDIX I I I . RAW DATA USED FOR DETERMINING THE THERMODYNAMIC PARAMETERS OF REVERSIBLE LIGAND BINDING TO CoDADIXDME AND CoDBrDIXDME APPENDIX I l i a . CH^-Im + CoDADIXDME^==±TCH^-ImCoDADIXDME .... - x i v -Page APPENDIX I H b . P i p + CoDADIXDME PipCoDADIXDME 370 APPENDIX I I I c . Py + CoDADIXDME rPyCoDADIXDME 371 APPENDIX H i d . DMF + CoDADIXDME ~T - DMFCoDADIXDME 372 APPENDIX H i e . CH -Im + CoDBrDIXDME ~ — CH -ImCoDBrDIXDME .. 373 3 3 APPENDIX I H f . P i p + CoDBrDIXDME ' PipCoDBrDIXDME 374 APPENDIX I H g . Py + CoDBrDIXDME — PyCoDBrDIXDME 375 APPENDIX I H h . DMF + CoDBrDIXDME — DMFCoDBrDIXDME 376 APPENDIX IV. RAW DATA USED FOR DETERMINING THE THERMODYNAMIC PARAMETERS OF REVERSIBLE DIOXYGEN BINDING TO LCoDADIXDME AND LCoDBrDIXDME '. 377 APPENDIX I V a . CH -ImCoDADIXDME + 0^z=^: CH^-ImCoDADIXDME (O^) . 378 APPENDIX IVb. PipCoDADIXDME + 0^ „ *:PipCoDADIXDME(0 2) 380 APPENDIX I V c . PyCoDADIXDME + 0 2 ~ PyCoDADIXDME (0 ) 382 APPENDIX TVd. DMFCoDADIXDME + 0 2 ^ = ^ r DMFCoD AD IXDME ( 0 2 ) 384 APPENDIX IVe. CH - ImCoDBrD IXDME + 0^=±z O^-ImCoDBrDIXDME ( 0 2 ) 385 APPENDIX I V f . PipCoDBrDIXDME + 0 ,^ *:PipCoDBrDIXDME(0 2) ... 387 APPENDIX I V g . PyCoDBrDIXDME + 0 2 . ': PyCoDBrDIXDME(0 2) 389 APPENDIX IVh. DMFCoDBrDIXDME + 0 „ T DMFCoDBrDIXDME ( 0 2 ) ... 390 APPENDIX V. RAW DATA USED FOR DETERMINING THE KINETICS OF OXIDATION OF COBALT PORPHYRIN COMPLEXES 391 APPENDIX Va. CoOMBP A g g r e g a t i o n D a t a w i t h T oluene as a S o l v e n t . 392 APPENDIX Vb. CoOMBP A g g r e g a t i o n D a t a w i t h DCE as a S o l v e n t 394 APPENDIX Vc. ImCoOMBP + 0 2 fcImCoOMBP(O )CoOMBP•Im As a f u n c t i o n o f oxygen p r e s s u r e 396 - X V -Page APPENDIX Vd. ImCoOMBP + *• ImCoOMBP (O,,) CoOMBP • Im As a function of ligand concentration 398 APPENDIX Ve . ImCoOMBP + *• ImCoOMBP (O^) CoOMBP • Im As a function of ligand concentration 401 APPENDIX Vf. ImCoOMBP + 0 2 »*ImCoOMBP (0 2) CoOMBP • Im As a function of ligand concentration 403 APPENDIX Vg. ImCoOMBP + 0 2 *-ImCoOMBP ( 0 ^ CoOMBP • Im As a function of cobalt concentration 405 APPENDIX Vh. ImCoOMBP + 0 2 *• ImCoOMBP(02)CoOMBP•Im As a function of cobalt concentration 408 -APPENDIX V i . ImCoOMBP + 0 2 »-ImCoOMBP (0 2) CoOMBP • Im As a function of temperature 409 APPENDIX V j . CH -ImCoOMBP + 0 2 > CH^-ImCoOMBP(02)CoOMBP'CH^-Im As a function of cobalt concentration 411 APPENDIX Vk. CH3-ImCoOMBP + 0 2 *~CH^-ImCoOMBP(02)CoOMBP-CH^-Im As a function of CH^-Im concentration 414 APPENDIX V l . CH3-ImCoOMBP + 0 2 »- O^-ImCoOMBP (02> CoOMBP • CH3"Im As a function of oxygen pressure 416 APPENDIX Vm, PipCoOMBP + 0 2 »- PipCoOMBP (0 2) CoOMBP-Pip 418 APPENDIX Vn. PyCoOMBP + O *• PyCoOMBP (O ) CoOMBP-Py 419 APPENDIX Vo. CH -ImCoOMBP + O *• CH -ImCoOMBP ( 0 ) CoOMBP • CH-Im As a function of solvent , 420 APPENDIX Vp. CH3-ImCoOMBP + ~0 >• CH3~ImCoOMBP (0^) CoOMBP •CH3_Im i n DCE 424 APPENDIX Vq. PyCoOMBP + O *• PyCoOMBP (O ) CoOMBP-Py i n DCE 426 -xvi-Page APPENDIX Vr. PipCoOMBP + 0 2 ^PipCoOMBP(02)CoOMBP-Pip i n DCE 428 APPENDIX Vs. (CH^-Im) 2CoOMBP + O^ *-CH -ImCoOMBP(02)CoOMBP•CH -Im As a function of oxygen pressure 430 APPENDIX Vt. (CH-Im) CoOMBP + 0 2 *~ CH3-ImCoOMBP(02)CoOMBP-CH -Im As a function of temperature 434 APPENDIX Vu. Py2CoOMBP + 0 2 >PyCoOMBP(O )CoOMBP•Py In 1:1 v/v toluene :Py 437 APPENDIX Vv. Py2CoOMBP + 0 2 !~ PyCoOMBP (0 2) CoOMBP-Py In neat Py as a function of temperature 438 APPENDIX Vw. Py CoOMBP + 0 2 a- PyCoOMBP (0 2) CoOMBP • Py In neat Py as a function of cobalt concentration .. 440 APPENDIX Vx. (CH -Im) 2CoOMBP + 0^ >• CH3-ImCoOMBP(02)CoOMBP-CH^-Im As a function of solvent 441 APPENDIX Vy. Py2CoOMBP + 0 2 *• PyCoOMBP(02)CoOMBP•Py In 1:1 v/v DCE:Py as a function of temperature .... 444 APPENDIX Vz. Py2CoOMBP + 0 2 >• PyCoOMBP (0,^ ) CoOMBP - Py In 1:1 v/v DCE:Py as a function of cobalt concentration .. 446 APPENDIX Vaa. PipCoOMBP + 0^ »-PipCoOMBP(0 2)CoOMBP-Pip In 1:1 v/v DCE:Pip • 448 APPENDIX Vbb. CH3~ImCoTPP + 0 2 *• CH3-ImCoTPP(02)CoTPP-CT^-Im 449 APPENDIX Vcc. CH3-ImCoTPP + 0 2 *- CH 3~ImCoTPP(0 2)CoTPP •CH.j-Im In DCE as a function of cobalt concentration 450 - x v i i -Page APPENDIX Vdd. CH3-ImCoEpI + C>2 * CH3-ImCoEpI (O^CoEpI -CH3-Im In DCE asa function of cobalt concentration 451 APPENDIX Vee. C^-ImCoEpI + 0^ ^ CH^TjnCoEpI (0 2) CoEpI-Cl^-Im In DCE as a function of temperature 454 APPENDIX V f f . CH3-ImCoOEP + C>2 »» CH^ImCoOEP (C^) CoOEP-C^-Im In DCE as a function of cobalt concentration 456 APPENDIX Vgg. CH3-ImCoOEP + C>2 CH3~ImCoOEP (0 2) CoOEP *CH3-Im In DCE as a function of temperature 459 APPENDIX Vhh. CH3~ImCo0EP + 0 2 CH^ImCoOEP CoOEP -CH3-Im In DCE as a function of oxygen pressure 461 APPENDIX V i i . CH3-ImCoOEP + 0 2 >CH^-ImCoOEP (O.^ ) CoOEP • Clr^-Im As a function of solvent 465 APPENDIX V j j CH3-ImCoPpIXDME + 0 2 *~ CH -ImCoPpIXDME(02)CoPpIXDME-CH3-Im As a function of cobalt concentration 467 APPENDIX Vkk CH3-ImCoOEP + 0 2 »*CH3-ImCo0EP (0 2) CoOEP-O^-Im In DCE as a function of CH3~Im concentration 469 APPENDIX VI. RAW DATA USED FOR DETERMINING THE THERMODYNAMIC AND KINETIC PARAMETERS OF LIGAND BINDING AND OXYGENATION OF Co(SALEN) COMPLEXES 471 APPENDIX V i a . CH3~Im + Co(salen). " CH^ImCo (salen) In dichloromethane 472 APPENDIX Vib. Pip + Co(salen)^ J PipCo(salen) In dichloromethane . . . 474 - x v i i i -Page APPENDIX V i c . Py + Co (salen) ' :PyCo (salen) In dichloromethane 476 APPENDIX VId. CH -ImCo(salen) + O "r CH-ImCo (salen) ( 0 ) In dichloromethane . 478 APPENDIX Vie. PipCo(salen) + — P i p C o ( s a l e n ) ( 0 2 ) In dichloromethane 480 APPENDIX V l f . PyCo(salen) + 0 2-. ^ PyCo (salen) (0 2) In dichloromethane • 481 APPENDIX VIg. Co(salen) + 0 _ Co(salen)(0 2) In dichloromethane 483 APPENDIX Vlh . Co(salen) + 0 ^ Co (salen) (O ) 2 2 Kin e t i c s i n dichloromethane as a function of oxygen pressure @ -83.5°C • 484 APPENDIX V i i . Co (salen) + 0 2 „ **r Co (salen) (0 2) Ki n e t i c s of dichloromethane as a function of oxygen pressure @ -78°C 486 APPENDIX VI j . Co(salen) + 0 Co (salen) (0 2) Kin e t i c s i n dichloromethane as a function of temperature 488 APPENDIX VIk. CH -Im + Co (salen) ^ _==±: C^-ImCo (salen) In DMF 490 APPENDIX V l l . Pip + Co(salen) , *:PipCo(salen) In DMF..... 492 APPENDIX Vim. Py + Co (salen),.. PyCo(salen) In DMF 494 APPENDIX VIn. CH -Im + Co (salen)^ »• C^-ImCo (salen) In toluene 496 APPENDIX VIo. Pip + Co (salen) „ •• PipCo (salen) In toluene . 499 APPENDIX VIp. Py + Co (salen) ._. - PyCo (salen) In toluene ... 500 -xix-Page APPENDIX V l q . Cl^-ImCo (salen) + 0^ .. 'r CB^-ImCo (salen) (0 2) In DMF .- 504 APPENDIX VIr. PipCo(salen) + . PipCo (salen) (p 2) In DMF 506 APPENDIX V i s . PyCo(salen) + 0 2 , PyCo(salen) (0 2) In DMF 507 APPENDIX V l t . CH 3-ImCo(salen) + 0 2 , CH^ImCo(salen) (0 2) In toluene 509 APPENDIX VIu. PipCo(salen) + O ~ PipCo(salen)(0,) 2 2 In toluene 510 APPENDIX VIv. PyCo(salen) + 02... '=PyCo(salen)(0^) In toluene 511 APPENDIX VII. RAW DATA USED FOR DETERMINING THE THERMODYNAMIC AND KINETIC PARAMETERS OF REACTING OLEFINIC SUBSTRATES WITH COBALT MACROCYCLIC SYSTEMS 512 APPENDIX V i l a . MA + CoOMBP— * MACoOMBP In toluene 513 APPENDIX VHb. MA + PnOMRP — - MAPnOMRP In DCE 515 APPENDIX V H c . MA + PipCoOMBP ^ Pip + MACoOMBP MA + PipCoOMBP ^  ^ PipCoOMBP (MA) PipCoOMBP (MA) *• PipCoOMBP + MA 516 APPENDIX VHd. MA + DMFCoOMBP^ ' - DMFCoOMBP (MA) In DMF .. 518 APPENDIX V i l e . MACoOMBP + Pip ^PipCoOMBP(MA) MA + CoOMBP > MACoOMBP 520 - X X -APPENDIX V l l f . PipCoOMBP + C H »»PipCoOMBP ( C ^ ) In DCE Page In DCE PipCoOMBP + C2H_ > PipCoOMBP (C H_) In DCE PipCoOMBP (C 2H 2) »-PipCoOMBP + C 2H 2 In DCE • 521 APPENDIX V l l g . Olefins Reacting with Co(salen) Complexes 523 APPENDIX VIII. DATA USED IN DETERMINING DIELECTRIC CONSTANTS ....... 525 APPENDIX IX. ESR PARAMETERS DETERMINED IN THIS THESIS 529 - x x i -LIST OF TABLES Table Page 1.1 Properties of Oxygen C a r r i e r s 4 1.2 T r i v i a l Names and Side Chain Substituents of Porphyrins 11 1.3a Mnemonic Names and Substituents of Type A S c h i f f Bases 17 I.3b Mnemonic Names and Substituents of Type B S c h i f f Bases 17 111.1 Spectral Data From 350 to 750 nm f o r the Reaction: L + CoOMBP - LCoOMBP 54 111.2 Spectral Data From 350 to 750 nm f o r the Reaction: L + LCoOMBP X I m C o O M B P 57 111.3 Thermodynamic Data for the Binding of the F i r s t A x i a l Ligand t o CoOMBP i n a Toluene Solution 70 111.4 Thermodynamic Data f or the Binding of the Second A x i a l Ligand to LCoOMBP i n a Toluene Solution 74 111.5 Thermodynamic Data f o r the Binding of the F i r s t A x i a l Ligand to CoOMBP i n a DCE Solution 77 IV.la Spectral Data From 350 to 750 nm for the Reaction: LCoOMBP + 0 ~ LCoOMBP (0 ) 82 IV.lb Spectral Data From 350 t o 750 nm f o r the Reaction: L2CoOMBP + 0 2 „ LCoOMBP (O^ + L 85 IV.2 Oxygen S e n s i t i v i t y as a Function of [PBu^ at -63.5°C 86 IV.3 Thermodynamic Data f o r the Binding of Dioxygen to LCoOMBP i n a Toluene Solution 94 IV.4 Expected P. 0„ Values (Torr) f o r L„CoOMBP Complexes at H 2 2 -45°C , 98 - x x i i -Table Page IV.5 Thermodynamic Data f o r the Binding of Dioxygen t o LCoOMBP i n a 1:1 by Volume Ratio of Toluene to DMF 100 IV. 6 Oxygen A f f i n i t i e s at -56.5°C of PipCoOMBP i n a Var i e t y of Mixed Toluene-DMF Solvent Systems 102 V. l a Spectral Data From 350 to 750 nm for the Reaction: L + CoDADIXDME . - LCoDADIXDME 107 V.lb S p e c t r a l Data From 350 to 750 nm for the Reaction: L + CoDBrDIXDME 1 ^ = ^ : LCoDBrDIXDME 107 V.2a Thermodynamic Data f o r the Binding of an A x i a l Ligand to CoDADIXDME i n a Toluene Solution 114 V.2b Thermodynamic Data f o r the Binding of an A x i a l Ligand to CoDBrDIXDME i n a Toluene Solution 114 V.3a Thermodynamic Data f o r the Binding of CH^-Im t o Several Cobalt Porphyrin Complexes i n Toluene 117 V.3b Thermodynamic Data f o r the Binding of Pip to Several Cobalt Porphyrin Complexes i n Toluene 118 V.3c Thermodynamic Data f o r the Binding of Py to Several Cobalt Porphyrin Complexes i n Toluene 119 V. 3d Thermodynamic Data f o r the Binding of DMF to Several Cobalt Porphyrin Complexes i n Toluene 120 VI. l a Spectral Data From 350 to 750 nm for the Reaction: LCoDADIXDME + 0 , LCoDADIXDME (0 2) 131 VI.lb Spectral Data From 350 to 750 nm for the Reaction: LCoDBrDIXDME + 0 „ - LCoDBrDIXDME (O^) 131 VI.2a Thermodynamic Data f o r the Binding of Dioxygen to LCoDADIXDME i n a Toluene Solution 137 - x x i i i -Table Page VI.2b Thermodynamic Data f or the Binding of Dioxygen to LCoDBrDIXDME i n a Toluene Solution 138 VI.3a Thermodynamic Data f o r the Binding of Dioxygen to CH^-Im Complexes of Several Cobalt Porphyrins 140 VI.3b Thermodynamic Data f or the Binding of Dioxygen to Pip Complexes of Several Cobalt Porphyrins 141 VI.3c Thermodynamic Data f o r the Binding of Dioxygen to Py Complexes of Several Cobalt Porphyrins 142 VI. 3d Thermodynamic Data for the Binding of Dioxygen to DMF Complexes of Several Cobalt Porphyrins 143 VII. l Spectral Data From 350 to 750 nm for the Reaction: 2L CoP + 0 : — • LCoP (OJ CoPL + 2(n-l)L 162 n 2 2 VII.2a K i n e t i c Data f or the Oxidation of ImCoOMBP as a Function of Oxygen Pressure 177 VII.2b K i n e t i c Data f or the Oxidation of ImCoOMBP as a Function of Ligand Concentration; Oxygen Pressure - 100 Torr 177 VII.2c K i n e t i c Data for the Oxidation of ImCoOMBP as a Function of Ligand Concentration; Oxygen Pressure ~ 400 Torr 178 VII.2d K i n e t i c Data for the Oxidation of ImCoOMBP as a Function of Ligand Concentration; Oxygen Pressure - 800 Torr 178 VII.2e K i n e t i c Data f or the Oxidation of ImCoOMBP as a Function of Cobalt Concentration; Oxygen Pressure - 160 Torr, Imidazole Concentration - 1.8 x 10~ 3 M 179 VII.2f K i n e t i c Data f o r the Oxidation of ImCoOMBP as a Function of Cobalt Concentration; Oxygen Pressure - 100 Torr, Imidazole -3 Concentration ~ 1.8 x 10 M 179 -xxiv-Table Page VII.2g K i n e t i c Data for the Oxidation of ImCoOMBP as a Function of Cobalt Concentration; Oxygen Pressure - 100 Torr, Imidazole Concentration - 1.5 x 10 3 M 180 VII.2h K i n e t i c Data for the Oxidation of ImCoOMBP as a Function of Temperature 180 VII.3 Oxidation Rate as a Function of Solvent P o l a r i t y when 1.8 x 10~ 5 M CH3-ImCoOMBP i s Exposed to 800 Torr 0 2 at 20°C 196 VII.4 K i n e t i c Data for the Oxidation of PipCoOMBP i n DCE 199 VII.5a K i n e t i c Data f o r the Oxidation of (CH^Im)2CoOMBP i n 1:1 v/v tolueneiCH^-Im as a Function of Oxygen Pressure 199 VII.5b K i n e t i c Data for the Oxidation of (CH^-Im)2CoOMBP i n 1:1 v/v toluene:CH 3~Im as a Function of Temperature 199 VII.5c K i n e t i c Data for the Oxidation of (CH -Im) CoOMBP as a 3 2 Function of Solvent 206 VII.6a K i n e t i c Data f o r the Oxidation of PyCoOMBP as a Function of Cobalt Concentration i n 1:1 v/v DCE:py 206 VII.6b K i n e t i c Data f o r the Oxidation of PyCoOMBP as a Function of Temperature i n 1:1 v/v DCE:Py 206 VII.7 K i n e t i c Data f o r the Oxidation of PipCoOMBP as a Function of Cobalt Concentration i n 1:1 v/v DCE:Pip 211 VII.8a K i n e t i c Data f o r the Oxidation of CH^ImCoOEP i n DCE as a Function of Cobalt Concentration 211 VII.8b K i n e t i c Data f o r the Oxidation of CH^ImCoOEP i n DCE as a Function of Oxygen Pressure 211 - X X V -Table Page VII. 8c Kinetic Data for the Oxidation of CH^ImCoOEP in DCE as a Function of CH^-Im Concentration 217 VII.8d Kinetic Data for the Oxidation of CH^ImCoOEP in DCE as a Function of Temperature 217 VII.8e Kinetic Data for the Oxidation of CH^ImCoOEP as a Function of Solvent 217 VII. 9a Kinetic Data for the Oxidation of CH^ImCoEpI in DCE as a Function of Cobalt Concentration 224 VII. 9b Kinetic Data for the Oxidation of CH^ImCoEpI in DCE as a Function of Temperature 224 VII. 10 Kinetic Data for the Oxidation of CH^ImPpIXDME in DCE as a Function of Cobalt Concentration 224 VII. 11 Kinetic Data for the Oxidation of CI^-ImCoTPP in DCE as a Function of Cobalt Concentration 224 VIII. 1 Spectral Data From 350 to 750 nm for the Reactions Involving Co(salen) 237 VIII.2 Thermodynamic Data for the Binding of an Axial Ligand to Co(salen) in a Dichloromethane Solution 250 VIII.3 Thermodynamic Data for the Binding of Dioxygen to LCo(salen) in a Dichloromethane Solution 255 VIII. 4 Kinetic Data for the Oxygenation of Co (salen) 257 VIII.5 Solubility of Dioxygen in CHjCl at Low Temperature 257 VIII.6a Thermodynamic Data for the Binding of an Axial Ligand to Co (salen) in a DMF Solution 266 VIII.6b Thermodynamic Data for the Binding of an Axial Ligand to Co(salen) in a Toluene Solution 266 - x x v i -T a b l e Page V I I I . 7 a Thermodynamic Data f o r t h e B i n d i n g o f D i o x y g e n t o LCo ( s a l e n ) i n a DMF S o l u t i o n 267 V I I I . 7b Thermodynamic D a t a f o r t h e B i n d i n g o f D i o x y g e n t o L C o ( s a l e n ) i n a Toluene S o l u t i o n 267 IX. 1 S p e c t r a l D a t a From 350 t o 750 nm f o r t h e R e a c t i o n s o f U n s a t u r a t e d S u b s t r a t e s w i t h C o b a l t M a c r o c y c l e s 271 IX.2 K i n e t i c s o f 780 T o r r o f E t h y l e n e o r A c e t y l e n e R e a c t i n g w i t h PipCoOMBP i n DCE a t 20°C 274 IX.3 Thermodynamic Data f o r t h e B i n d i n g o f M a l e i c A n h y d r i d e t o CoOMBP i n a V a r i e t y o f S o l v e n t s 279 - x x v i i -LIST OF FIGURES Figure Page 1.1 Examples of d i f f e r e n t porphyrins 2 1.2 The structure of the beta chain of hemoglobin 7 1.3 A v a r i e t y of cobalt macrocycle complexes 13 1.4 V i s i b l e spectra of a v a r i e t y of cobalt macrocycle complexes 14 1.5 A v a r i e t y of cobalt S c h i f f base complexes 16 I I . 1 C e l l used for d i e l e c t r i c constant measurements 30 II.2 Anaerobic s p e c t r a l c e l l 32 I I . 3 10 cm path-length c e l l 33 11.4 6 cm path-length c e l l 34 11.5 Short path-length c e l l used f o r low temperature work .... 35 11.6 Anaerobic ESR c e l l 3 7 11.7 Gas-Uptake Apparatus 41 11.8 Gas-Uptake Ampoule 43 111.1 CoOMBP + CH 3-Im==rCH 3-ImCoOMBP @ 23°C 52 111.2 PBu + CoOMBP :^=^ PBu CoOMBP @ 22°C 53 3 3 I I I . 3 CH3-ImCoOMBP + CH3~Im ^ =^(CH 3~Im) 2CoOMBP @ -23°C ... 56 111.4 PBu 3 + PBu3CoOMBP (PBu 3) 2CoOMBP @ 22°C 58 111.5 Equilibrium p l o t s f or CH^Im + CoOMBPz^zzTO^-ImCoOMBP .. 61 111.6 Van't Hoff p l o t s f o r L + CoOMBP ^ = ± r LCoOMBP L = Im, CH3"Im, Pip, and Py 62 I I I . 7 Van't Hoff p l o t s f o r L + CoOMBP --==±: LCoOMBP L = Et N, THF, and DMF 63 I I I . 8 Van't Hoff p l o t f or PPh^ + CoOMBP PPh^CoOMBP 64 - x x v i i i -Figure Page III.9 Equilibrium p l o t s f o r CF^-Im + CH^-Im CoOMBP - z — z ± (CH^Im) CoOMBP 66 I I I . 10 Van't Hoff p l o t s f o r L + LCoOMBP -~ L2CoOMBP L = CH -Im, Pip, and Py 67 IV. 1 Spectra of oxygen binding to CH -ImCoOMBP @ -56.5°C ••• 80 IV.2 ESR s i g n a l of CH -ImCoOMBP i n frozen toluene @ 77 K .. 83 IV.3 Spectra of oxygen binding to (CH -Im) CoOMBP & -45°C . 84 O IV. 4 Equilibrium p l o t s f o r CH -ImCoOMBP + 0 2 -. CH -ImCoOMBP ( 0 ^ 88 IV.5 Van't Hoff p l o t s f o r LCoOMBP + O - — LCoOMBP(02) L = CH -Im, Pip, Py, Et^N, and DMF 89 IV.6 Equilibrium p l o t f o r (CH -Im)2CoOMBP + 0 2 - =r-CH3-ImCoOMBP (O ) @ - 4 5°C 92 IV. 7 Oxygen A f f i n i t y vs. D i e l e c t r i c Constant f o r PipCoOMBP i n toluene-DMF systems 104 V. l CH3~Im + CoDADIXDME CH^ImCoDADIXDME @ 22°C .. 106 V.2 CH -Im + CoDBrDIXDME - CH -ImCoDBrDIXDME @ 22°C 109 3 3 V.3 Equilibrium p l o t s f o r CH^Im + CoDADIXDME ~ CH -ImDADIXDME I l l 3 V.4 Van't Hoff p l o t s f o r L + CoDADIXDME LCODADIXDME 112 V.5 Porphyrin band p o s i t i o n as a function of porphyrin b a s i c i t y 116 V.6a CH3~Im a f f i n i t y vs. porphyrin b a s i c i t y f o r a v a r i e t y of cobalt porphyrin complexes 122 V.6b Pip a f f i n i t y vs. porphyrin b a s i c i t y f o r a v a r i e t y of cobalt porphyrin complexes 123 -xxix-Figure Page V.6c Py a f f i n i t y vs. porphyrin b a s i c i t y f o r a v a r i e t y of cobalt porphyrin complexes 124 V. 6d DMF a f f i n i t y vs. porphyrin b a s i c i t y f o r a v a r i e t y of cobalt porphyrin complexes 125 VI. 1 CH -ImCoDADIXDME + C> r = ± - CH -ImCoDADIXDME (0 ) .. 129 3 2 3 2 VI.2 CH^-ImCoDBrDIXDME + 0 2 ,^ ^ CH^-ImCoDBrDIXDME (0^) 130 VI.3 Equilibrium p l o t s f o r CH.^-ImCoDADIXDME + 0^ ^  — CH3-ImCoDADIXDME(0 ) 133 VI.4 Van't Hoff p l o t s f o r LCoDADIXDME + C>2 . — LCoDADIXDME(0 ); L = CH -Im, Pip, Py, and DMF 134 VI. 5 Van't Hoff p l o t s f o r LCoDBrDIXDME + C>2 ~ LCoDBrDIXDME; L = CH -Im, Pip, Py, and DMF 135 VI.6a Dioxygen a f f i n i t y vs. porphyrin b a s i c i t y f o r a v a r i e t y of CH3~Im cobalt porphyrin complexes 145 VI.6b Dioxygen a f f i n i t y v s . porphyrin b a s i c i t y f o r a v a r i e t y of Pip cobalt porphyrin complexes 147 VI.6c Dioxygen a f f i n i t y v s . porphyrin b a s i c i t y f o r a v a r i e t y of Py cobalt porphyrin complexes 148 VI. 6d Dioxygen a f f i n i t y v s . porphyrin b a s i c i t y f o r a v a r i e t y of DMF cobalt porphyrin complexes 152 VII. l Spectra showing the oxidation of PipCoOMBP i n DCE @ 30°C 156 VII.2 Spectra showing the oxidation of CH3-ImCoEpI by 800 Torr 0 2 i n DCE @ 20°C 158 VII.3 Spectra showing the oxidation of CH -ImCoOEP by 800 Torr 0 2 i n DCE @. 30°C 1 5 9 - X X X -Figure Page VII.4 Spectra showing the oxidation of CH^-ImCoTPP by 800 Torr 0 2 i n DCE @ 20°C 160 VII.5 ESR signals showing the oxidation of CH^-ImEpI by 800 Torr 0 2 i n DCE @ 22°C 161 VII.6 V i s i b l e spectra of CoOMBP as a function of concentration @ 22°C i n toluene 167 VII. 7 Aggregation p l o t of CoOMBP i n toluene @ 22°C 170 VII.8 Beer's Law p l o t s of CH -ImCoOMBP i n toluene @ 22°C 171 VII. 9 Beer's Law p l o t s of (CH^Im) 2Co0MBP i n toluene @ 22°C 172 VII. 10 ESR spectrum of CH3-ImCoPpIXDME i n toluene @ 77 K 174 -5 VII.11 Spectra showing the oxidation of 2 x 10 M ImCoOMBP by 395 Torr O @ 22°C 176 2 VII.12 Oxidation rate of ImCoOMBP as a function of oxygen pressure 181 VII.13 Oxidation rate of ImCoOMBP as a function of Im concentration 182 VII.14 Analysis of Im dependence on the oxidation of ImCoOMBP 185 VII.15 Arrhenius p l o t f o r the oxidation of ImCoOMBP 188 VII.16 Oxidation of CH -ImCoOMBP analyzed f o r f i r s t order i n cobalt 191 at several cobalt concentrations -5 VII.17 Temperature dependence on the oxidation of 2 x 10 M PipCoOMBP i n DCE 198 -5 VII.18 F i r s t order rate p l o t s f o r the oxidation of 2 x 10 M (CH3~Im)2CoOMBP i n 1:1 toluene:CH 3~Im 202 -5 VII.19 Temperature dependence of the oxidation of 2 x 10 M (CH -Im) 2CoOMBP 203 VII.20 F i r s t order rate p l o t s f o r the oxidation of 2 x 10 ^ M (CH -Im) 2Co0MBP 205 -xxxi-Figure Page VII.21 F i r s t order rate p l o t s f o r the oxidation of Py^CoOMBP i n 1:1 DCE:Py as a function of [Co] 207 -5 VII.22 Temperature dependence of the oxidation of 2 x 10 M PyCoOMBP 208 VII. 23 F i r s t order rate p l o t s f o r the oxidation of PipCoOMBP i n 1:1 DCE:Pip 210 VII.24 F i r s t order rate p l o t s f o r the oxidation of CH^-ImCoOEP by 800 Torr 0 2 i n DCE @ 30°C as a function of [Co3 213 VII.25 Oxygen pressure dependence of the oxidation of ~7 x 10 ~* M CH -ImCoOEP @ 30°C i n DCE 214 VII.26 F i r s t order rate p l o t s f o r the oxidation of CH^-ImCoOEP i n DCE as a function of CH^-Im 216 VII. 27 Temperature dependence of the oxidation of -8 x 10 M CH -ImCoOEP i n DCE 218 VII.28 Analysis of oxygen dependence on the oxidation of CH3-ImCoOEP at 30°C 219 -4 VII.29 F i r s t order rate p l o t s f o r the oxidation of 6 x 10 M CH3"ImCoOEP i n DCE @ 30°C i n a v a r i e t y of solvents 221 VII.30 F i r s t order rate p l o t s for the oxidation of CH3-ImCoEpI i n DCE @ 30°C as a function of [Co] 223 -5 VII.31 Temperature dependence of the oxidation of -1 x 10 M CH3-ImCoEpI i n DCE 225 VII.32 F i r s t order rate p l o t s for the oxidation of CH^ImCoPpIXDME i n DCE as a function of Lcol 226 VII.33 F i r s t order rate p l o t s f o r the oxidation of CH3-ImCoTPP i n DCE as a function of [j2o] 227 - x x x i i -Figure Page VIII. 1 CH3-Im + Co (salen) —2_rCH 3-ImCo (salen) @ 15°C i n dichloromethane 233 VIII. 2 CH3-ImCo (salen) + C>2. * CH3-ImCo (salen) @ -78°C i n dichloromethane 234 VIII.3 Spectra showing the oxygenation of Co(salen) by 205 Torr 0 2 @ -83.5°C i n dichloromethane 235 VIII.4 ESR s i g n a l of CH^ImCo (salen) @ 77 K i n dichloromethane .... 240 VIII.5 A. ESR si g n a l of CH^-ImCo(salen)(0 2) @ 77 K i n dichloromethane 241 B. ESR s i g n a l of Co(salen)(0 2) @ 77 K i n dichloromethane 241 VIII. 6 Equilibrium p l o t s f or CH^Im + Co (salen) ^  CH 3~ImCo(salen) 242 VIII.7 Equilibrium p l o t s f o r CH -ImCo(salen) + 0 2 - — CH3~ImCo (salen) (0 2) 243 VIII.8 Equilibrium p l o t s for Co(salen) + 0 2 - " Co(salen)(0_) . 244 VIII.9 Van't Hoff p l o t s f o r L + Co (salen) -_^____±LCo (salen) L = CH -Im, Pip, and Py 246 VIII.10 Van't Hoff p l o t s f o r LCo(salen) + 0 1^Z____^ LCo(salen) (0 2) L = CH -Im, Pip, and Py 247 VIII.11 Van't Hoff p l o t f o r Co(salen) + 0 v C o ( s a l e n ) ( 0 ) .. 248 VIII.12 Ligand binding constants vs. pK for substituted p y r i d i n e s i n dichloromethane @ 25°C 252 VIII.13 Hammett p l o t f o r substituted pyrindines binding to Co (salen) i n dichloromethane @ 25°C 253 VIII.14 Oxygenation rate constants of Co(salen) as a function of oxygen pressure 258 - x x x i i i -Figure Page VIII.15 Oxygenation rate constants of Co(salen) as a function of oxygen pressure 259 VIII.16 Arrhenius p l o t of the forward rate constants of the oxygenation of Co(salen) at one atmosphere of oxygen 261 VIII. 17 Arrhenius p l o t f or the deoxygenation of Co(salen)(O^) 262 IX. 1 Spectra showing the reaction of PipCoOMBP with 780 Torr acetylene @ 20°C 270 IX.2 Spectra showing PipCoOMBP regeneration @ 20°C a f t e r removal of acetylene 273 IX.3 F i r s t order rate pl o t s of PipCoOMBP reacting with ethylene and acetylene 275 IX.4 Spectral changes occurring upon the addition of MA to CoOMBP i n toluene @ 22°C 277 IX. 5 Equilibrium p l o t s f o r MA + CoOMBP MACoOMBP 278 IX.6 Van't Hoff p l o t s f or MA + CoOMBP zz * MACoOMBP i n toluene and DMF 280 IX.7 Spectral changes to MACoOMBP upon addition of Pip @ 22°C ... 281 IX.8 Spectra showing the generation of PipCoOMBP from PipCoOMBP (MA) 283 IX.9 Spectra showing the regeneration of the Co(salen) peak a f t e r oxygen i s removed from the Co(salen)-3,4-dichlorobutene system 287 IX.10 ESR signal of Co(salen)-3,4-dichlorobutene system at 77 K .. 289 IX.11 ESR si g n a l of Co(salen)-3,4-dichlorobutene system at 77 K af t e r exposure to 0 o f o r 42 h at ambient temperature 290 -xxxiv-ABBREVIATIONS AND COMMON NAMES The following l i s t of abbreviations and common names, many of which are commonly adopted i n chemical research l i t e r a t u r e , w i l l be employed i n t h i s t h e s i s . A eq A O Ac acacen atm B benacen Br CH -Im cm Co(acacen) Co(benacen) absorbance acetate, -CH COOH or i n ESR, hyperfine coupling observed absorbance at the s t a r t of an experiment observed absorbance when equilibrium has been established observed absorbance when a reaction has been completed a c e t y l , -COCH3 bis(acetylacetone)ethylenediimine, see Figure I.5a and Table I.3a atmosphere(s) a x i a l base or 2-hyroxyethyl, -CHOHCH3 bis(benzoylacetylacetone)ethylenediimine, see Figure I.5a and Table I.3a bromo, -Br N-methylimidazole, c 4 H g N 2 centimetre(s) Co(II) bis(acetylacetone)ethylenediimine, see Figure 1.5a and Table I.3a Co(II) bis(benzoylacetylacetone)ethylenediimine, see Figure I.5a and Table I.3a CoDADIXDME CoDBrDIXDME CoDpIXDME CoEpI CoMc CoMpIXDME CoOEP CoOMBP CoP CoPc CoPFP CoPpIXDME Co SB CoTPP CoT(p-0Me)PP CoT(p-X)PP cot DADIXDME DBrDIXDME DCB -xxxv-Co(II) diacetyldeuteroporphyrin IX dimethyl ester, see Figure I . l a and Table 1.2 Co (II) dibromodeuteroporphyrin IX dimethyl ester, see Figure I . l a and Table 1.2 Co(II) deuteroporphyrin IX dimethyl ester, see Figure I . l a and Table 1.2 Co (II) etioporphyrin I, see Figure I . l a and Table 1.2 Co(II) complex of a tetradentate macrocycle Co (II) mesoporphyrin IX dimethyl ester, see Figure I . l a and Table 1.2 Co(II) octaethylporphyrin, see Figure I.Id and Table 1.2 Co(II) octamethyltetrabenzoporphyrin, see Figure I . l c and Table 1.2 a Co(II) porphyrin complex a Co(II) phthalocyanine complex, see Figure I.3b Co(II) picket fence porphyrin Co (II) protoporphyrin IX dimethyl ester, see Figure I . lb and Table 1.2 a Co(II) S c h i f f base complex Co(II) a,B,y,6-tetraphenylporphyrin, see Figure I . l c and Table 1.2 Co(II) a,B,y,6-(p-methoxy)-tetraphenylporphyrin, see Figure I . l c and Table 1.2 a Co(II) complex of a para substituted a , 8, y, <5-tetra-phenylporphyrin, see Figure I.e and Table 1.2 cyclooctene diacetyldeuterporphyrin IX dimethyl ester, see Figure I . l a and Table 1.2 dibromodeuteroporphyrin IX dimethyl e s t e r , see Figure I . l a and Table 1.2 3,4-dichlorobutene, CH2=CHCC1CC1H2 -xxxvi-DCE 1,2-dichloroethane, CH2C1CH2C1 DCM dichloromethane, CH Cl 2 2 DEF diethylfumarate, trans-C^O CCH=CHCC>2C Hg DEM diethylmaleate, cis-C2H502CCH=CHCC>2C2H5 DMA N,N-dimethylacetamide, CH^ONCCH.^ DMF N,N-dimethylformamide, HCON(CH3)2 DpIXDME deuteroporphyrin IX dimethyl ester, see Figure I.la and Table 1.2 E or Et ethyl, -CH2CH3 Epl etioporphyrin I, see Figure I.la and Table 1.2 Eq'm equilibrium ESR electron spin resonance Et.N triethylamine, (C H )_N 3 2 5 3 e.u. entropy unit(s), cal K mole FeTPP Fe(II) a,g,y,6-tetraphenylporphyrin, see Figure I.lc and Table 1.2 g gram or an ESR g value, q_ , 2.0023193 free electron G gauss h hour(s) H ESR applied magnetic f i e l d strength or hydrogen Hb hemoglobin I nuclear spin number Im imidazole, C 3 H 4 N 2 k kinetic rate constant k , observed rate constant obs k, forward rate constant - X X X V 1 1 -reverse rate constant or i n the case of the ImCoPpIXDME + 0^ system, the anti c i p a t e d bimolecular rate constant of oxidation bimolecular oxidation rate constant i n the ImCoOMBP constant t o t a l rate constant observed t o t a l rate constant forward rate constant reverse rate constant rate constant for the formation of the "activated intermediate" species i n the oxidation of CoP cmplexes degree(s) Kelvin or equilibrium constant equilibrium constant of aggregation (4-coordinate species) equilibrium constant of as s o c i a t i o n (5-coordinate species) equilibrium constant of d i s s o c i a t i o n equilibrium constant f o r Im s t a b i l i z e d intermediate equilibrium constant of maleic anhydride binding equilibrium constant of dioxygen binding equilibrium constant of ligand displacement by dioxygen equilibrium constant of the binding of one a x i a l ligand equilibrium constant of the second a x i a l ligand binding equilibrium constant for the Co(salen) intermediate a x i a l ligand or l i t r e five-coordinate Co(II) macrocycle complex six-coordinate Co(II) macrocycle complex dioxygen adduct of a five-coordinate macrocycle complex five-coordinate complex of a Co(II) porphyrin - x x x v i i i -L2CoP six-coordinate complex of a Co(II) porphyrin LCoP(C>2) 'dioxygen adduct of a five-coordinate Co (II) porphyrin LCoP(02) "activated" dioxygen adduct intermediate LCoSB five-coordinate adduct of a Co(II) Schiff base LCoSB(C>2) dioxygen adduct of a five-coordinate Co(II) Schiff base ln natural logarithm log common logarithm M molar or metal MA maleic anhydride, cis-butenedioic acid anhydride Mb myoglobin Mc a tetradentate macrocycle meso refers to the positions of the bridging methines of a porphyrin ring mL m i l l i l i t r e ( s ) mm millimetre(s) MPh para-methoxyphenyl -C_H -OCH. , also referred to as p-OMe o 4 3 MpIXDME mesoporphyrin IX dimethyl ester, see Figure I.la and Table 1.2 MnTPP Mn(II) a,B,y,6-tetraphenylporphyrin N normal .nm nanometre NMR nuclear magnetic resonance P porphyrin or propionate, -CH2CH2COOH PBu, tri-n-butylphosphine, (n-C HQ)_P 3 4 " 3 Pc phthalocyanine PFP picket fence porphyrin, meso-tetra- (a,ct,a,a-o-pival-amidephenyl)porphyrin Ph phenyl, -C.H. o 5 Pip piperidine, Ct.H11N -xxxix-PpIXDME PPh. Py P, \°2 RhOEP RhTPP RuOEP RuTPP s or sec salen SB sty t T TCNE THF TiTPP T(p-OCH 3)PP T(p-X)PP pH at which a base i s 50% i n the protonated form protoporphyrin IX dimethyl ester, see Figure I . l a and Table 1.2 triphenylphosphine, P (CgH^ .) ^  pyridine, C _ H _ N pressure at which 50% of the substrate i s converted pressure at which 50% of the substrate i s oxygenated s t a t i s t i c a l c o r r e l a t i o n c o e f f i c i e n t Rh(II) octaethylporphyrin, see Figure I.d and Table I. Rh(II) ct, 8,y,S-tetraphenylporphyrin, see Figure I . l c and Table 1.2 Ru(II) octaethylporphyrin, see Figure I.Id and Table I Ru(II)- a, S,y, 5-tetraphenylporphyrin, see Figure I . l c and Table 1.2 second (s) N,N-bis(salicylaldehyde)ethylenediimine, see Figure I. and Table I.3b S c h i f f base styrene, C.HCCH=CH 6 5 2 time temperature tetracyanoethylene, (CN)2C=C(CN) tetrahydrofuran , C 4 H 8 ° T i ( I I ) a,8 ,y,6-tetraphenylporphyrin, see Figure I . l c and Table 1.2 a,8,y,6-(p-methoxy)phenylporphyrin, see Figure I . l c and Table I.2 a para-substituted a,&,y,6-tetraphenyporphyrin, see Figure I . l c and Table 1.2 - x l -UV V a chain B chain Y 6 X AH ultraviolet vinyl, -CH=CH2, or unit displacement on the vertical scale of the oscilloscope used in stopped-flow determinations fraction of product formed during a particular experiment f i r s t meso position on a porphyrin ring one of the two types of polypeptide chains present in a Hb tetramer second meso position on a porphyrin ring one of the two types of polypeptide chains present in a Hb tetramer third meso position on a porphyrin ring fourth meso position on a porphyrin ring dielectric constant or molar extinction coefficient observed molar extinction coefficient at the start of an experiment observed molar extinction coefficient when a reaction has been completed wavelength micro frequency bonding where electron density i s on either side of internuclear plane antibonding T f-orbital bonding where electron density i s maximum on an internuclear axis or s t a t i s t i c a l standard deviation enthalpy of reaction - x l i -AH* enthalpy of a c t i v a t i o n AS entropy of reaction AS^ entropy of a c t i v a t i o n lhx 1-hexene, CH =CH CH CH CH„CH„ 2 2 2 2 2 3 2ot 2-octene, CH3CH=CHCH2CH2CH2CH2CH3 4-CN-Py 4-cyanopyridine 4-NH2~Py 4-aminopyridine [Co] t o t a l cobalt reactant concentration [eo 1 t o t a l cobalt concentration °C degree(s) centigrade approximately > greater than >> much greater than < le s s than << much less than dimer the term dimer i s used to apply to the peroxo-bridged binuclear oxidation products formed inf.this work; t h i s compares to the oxo-bridged products formed i n the oxidation of ir o n systems which are also termed "dimers" dioxygen molecular oxygen, C>2, should be ref e r r e d to as dioxygen, but i s commonly ref e r r e d to as simply oxygen. In t h i s t hesis the term dioxygen i s used when an C>2 moiety i s bound to a substrate; gaseous C>2 or 0 i n solu t i o n i s r eferred to as oxygen. the oxidation state of most metals i n t h i s thesis i s M(II); any other oxidation state i s s p e c i f i e d i n the th e s i s . M - x l i i -ACKNOWLEDGEMENTS I wish to thank the help and useful suggestions of the many colleagues I had the p r i v i l e g e of working with at UBC. I would l i k e to e s p e c i a l l y acknowledge Professor Brian R. James for h i s guidance and encouragement throughout the course of t h i s work, p a r t i c u l a r l y f o r h i s many comments during the preparation of t h i s t h e s i s . F i n a n c i a l support of t h i s research from the Natural Science and Engineering Research Council of Canada i s g r a t e f u l l y appreciated. F i n a l l y , I would l i k e thank my family, e s p e c i a l l y my wife Joy, whose constant support and encouragement helped to make t h i s t hesis p o s s i b l e . -1-CHAPTER I  INTRODUCTION I.1 Biological Oxygen Carrying Systems 1.1.1 General The subject of molecular oxygen (C^) interacting with metal complexes of macrocyclic ligands has fascinated workers in many scien t i f i c disciplines, ever since such a process has been recognized as that which occurs in oxygen storage and oxygen transport in biological systems. The moiety of major importance in vertebrates i s the so-called heme unit, which consists of an iron(II) porphyrin (usually protoporphyrin IX, Figure I.lb) complex. Myoglobin (Mb) \ which stores oxygen in body tissues, consists of one polypeptide chain of 153 amino acids and one heme group, and has a molecular weight of 17,800. Hemoglobin (Hb)\ which transports oxygen from the lungs to the body tissues, consists of four Mb type subunits arranged as a tetrahedron, and has a molecular weight of 64,500. Heme units are also the 2 prosthetic group present in several oxygenases (enzymes that incorporate one or two o atoms from 0^ into a substrate) and in cytochrome c oxidase^, the terminal enzyme in the respiratory redox chain that reduces O^  to 4 water. The enzymes catalase and peroxidase , which u t i l i z e H2°2' ^oth contain heme centres and are related to the oxygen systems. -2-I . l c . Tetraphenylporphyrin I.Id. Octaethylporphyrin Figure 1.1. Examples of d i f f e r e n t porphyrins. Other oxygen-carrying metal complexes i n b i o l o g i c a l systems include the hemerythrins, the hemocyanins, and hemovanadin. Hemerythrins^ are iron-containing oxygen carrying proteins found i n many invertebrate species. However, the pro s t h e t i c group does not contain a porphyrin moiety; rather the i r o n i s bound d i r e c t l y to the p r o t e i n side chains. In general, hemerythrin occurs as an octamer of a molecular weight of about 108,000, with each subunit containing two iron atoms that bind one molecule of 0„. 2 6 Hemocyanins are nonheme copper-containing proteins found i n some of the arthropods and mollusks. Hemocyanins consist of a number of subunits which have a molecular weight of from 25,000 per copper atom to 37,000 per copper atom, depending on t h e i r source. Like hemerythrins, the manner i n which the metal centre i s bound to the p r o t e i n i s not c l e a r . Hemocyanins absorb one molecule of 0^ for every two copper atoms i n the prote i n , but the structure of the metal-dioxygen complex has not been well established. 7 L i t t l e i s known about the nature of hemovanadin . I t i s found i n the 1.5 to 2.0 N H SO. s o l u t i o n i n the vacuoles of the blood c e l l s of c e r t a i n 2 4 tunicates. The p a r t i a l pressure necessary to oxygenate one ha l f of the available hemovanadin s i t e s , the p ^ ° 2 ' ^ o r fc^e c e l l s has been found to be about 2 Torr. A summary of some of the c h a r a c t e r i s t i c s of these b i o l o g i c a l oxygen c a r r i e r s i s given i n Table 1.1. Table I.1 Properties of Oxygen C a r r i e r s . Data taken from Basolo et a l , Chem. Rev., 79, 139 (1979). OXYGEN CARRIER HEMOCYANIN HEMERYTHRIN HEMOGLOBIN HEMOGLOBIN (a CHAIN) HEMOGLOBIN (/3 CHAIN) MYOGLOBIN COBOGLOBIN METAL Cu Fe Fe Fe Fe Fe Co OXIDATION STATE OF THE DEOXYGENATED FORM +1 +2 +2 +2 +2 +2 +2 METAL:DIOXYGEN RATIO 2Cu:10 2 2Fe:K>2 lFe:10 lFe:10 2 lFe:10 lFe:K> 2 lCo:10 2 MAGNETIC-ELECTRONIC PROPERTIES OF: THE DEOXYGENATED FORM zero spin zero spin high spin high spin high spin high spin low spin THE OXYGENATED FORM zero spin zero spin zero spin zero spin zero spin zero spin low spin P^0 2 @ 20°C, TORR 4.3 a 4 b 2.5 C transport 0.46 d 0.40 d e 0.65 storage 57 f BIOLOGICAL FUNCTION transport storage NUMBER OF SUBUNITS var i a b l e 8 4 1 1 1 1 COOPERATIVITY yes s l i g h t yes no no no * no a. Z. E r - e l , N. Shaklai, and E. Daniel, J . Moi. B i o l . , 64, 341 (1972). b. G. Bates, M. Brunori, G. Amiconi, E. Antonini, and J . Wyman, Biochem., 1_, 3016 (1968). c. E. Antonini, J . Wyman, M. Brunori, C. F r o n t i c e l l i , E. Bucci, and A. R o s s i - F a n e l l i , J . B i o l . Chem., 240, 1096 (1965) . d. M. Brunori,R. W. Noble, E. Antonini, and J . Wyman, J . B i o l . Chem., 241, 5328 (1966). e. A. R o s s i - F a n e l l i , and E. Antonini, Arch. Biochem. Biophys., 7_7, 478 (1958). f. B. M. Hoffman, C. A. Spilburg, and D. H. Petering, J . B i o l . Chem., 247, 4219 (1972). *. The term coboglobin can r e f e r to e i t h e r apomyoglobin or apohemoglobin reconstituted with cobalt. Hoffman et a l studied reconstituted sperm whale and horse myoglobins, and so no cooperativity was observed. Cooperativity i s observed for cobalt reconstituted hemoglobin. -5-1.1.2. Myoglobin and Hemoglobin The heme group i n myoglobin i s completely buried within the folded polypeptide chain, except f o r one edge which contains the two hydro p h i l i c propionic acid groups. The hydr o p h i l i c groups of the protein are spread uniformly over the outside surface of the molecule, and the strongly hydrophobic side chains l i e inside the molecule. Some of the hydrophobic groups l i n e the inner surfaces of the alpha h e l i c e s , while the Q others form a hydrophobic environment for the heme. The function of myoglobin, to store and release dioxygen, takes place at the i r o n centre of the heme group. The purpose of the polypeptide chain around the heme i s to keep the ferrous iron from being oxidized (metmyoglobin with a f e r r i c i r o n does not bind dioxygen), and to enhance the dioxygen binding a b i l i t y of the heme. The heme group i s joined to the polypeptide chain by binding i n one of i t s a x i a l coordination s i t e s to an imidazole nitrogen of a h i s t i d i n e residue. This s o - c a l l e d proximal h i s t i d i n e side chain i s locked i n t o p o s i t i o n by a hydrogen bond from i t s other r i n g nitrogen to the carbonyl oxygen of a nearby leucine. The s i x t h coordination s i t e , trans to the coordinated imidazole, i s vacant i n deoxymyoglobin, and i s the s i t e f o r the dioxygen i n oxymyoglobin. In metmyoglobin the oxygen s i t e i s occupied 9 by a coordinated water molecule. On the f a r side of the dioxygen moiety of oxymyoglobin l i e s a d i s t a l h i s t i d i n e , which i s too f a r from the ir o n atom to coordinate d i r e c t l y with i t , but i n d i r e c t i n t e r a c t i o n s ; e.g. hydrogen bonding with a coordinated 8,9 oxygen or water molecule can pos s i b l y occur -6-Upon oxygenation, the iron in myoglobin transforms from a high spin S=2 state to a spin paired S=0 state* 0. The change in spin state upon oxygenation i s accompanied by the iron going from out of the plane toward the coordinated imidazole by several tenths of an Angstrom in deoxymyoglobin**, to occupying the mean plane of the porphyrin ring in . , . 11a,12 oxymyoglobm As well as being able to bind dioxygen, the sixth coordination site of the protein iron can also coordinate other small molecules or ions, such as CO10*, NO13, NO"14, OH"14, F" 1 4, N ^ 1 5 , CN" 1 6, and H^ 1 7. Of the four myoglobin type subunits in hemoglobin, there are two alpha chains and two beta (shown in Figure 1.2) chains. They form a 18 compact spheroidal molecule with approximate dimensions of 64 x 55 x 50 A The four heme groups are arranged in separate pockets on the surface of the molecule, and have environments similar to that of the myoglobin heme. Upon oxygenation, the two alpha hemes are brought closer together by 1 A, while the beta hemes are separated by 6.5 ^2b,19^ rj<___s heme-heme interaction upon oxygenation gives rise to the "cooperativity effect," by which the subsequent dioxygen binding is more facile than i f no heme-heme interaction were present^'12a,20^ cooperativity effect i s of biological use, as i t means that hemoglobin can release more of i t s oxygen to myoglobin in a low oxygen environment, than i f there were no cooperativity effect. Another way in which oxygen transfer from hemoglobin to myoglobin is 21 aided i s the Bohr effect . Since oxyhemoglobin behaves as a stronger acid than deoxyhemoglobin, the oxygenated form is favored in the basic lung tissues, and the release of oxygen to myoglobin is aided by the presence Figure 1.2. The structure of the beta chain of hemoglobin. Taken from M. F. Perutz and L. F. TenEyck, Cold Spring Harbor Symp. Quant. B i o l . , 36, 295 (1971). -8-of l a c t i c acid and carbonic acid i n the body t i s s u e s . Like the coopera-t i v i t y e f f e c t , the Bohr e f f e c t i s not present i n myoglobin or i n fragmented hemoglobin u n i t s . I.2 Synthetic Oxygen C a r r i e r s 1.2.1 Rationale f o r Synthetic Oxygen C a r r i e r s K i n e t i c , thermodynamic, and spectroscopic studies, on the reaction of dioxygen with any natural protein systems,-whether heme or non-heme systems, are r e l a t i v e l y complex, and conclusions regarding the nature of the 22 in t e r a c t i o n are sometimes d i f f i c u l t to draw Since the active s i t e of these proteins often consists of a metal quadridentate macrocyclic complex, i t i s i n s t r u c t i v e to study the i n t e r -actions of such complexes with O^, both to better appreciate the nature of the metal-dioxygen i n t e r a c t i o n s , and the role of the protein i n the natural system. This model system approach c l e a r l y has l i m i t a t i o n s , and has been 22 23 reviewed extensively ' . For example, the heme-heme i n t e r a c t i o n s of the hemoglobin protein chains, which generate the cooperativity and the Bohr e f f e c t s , cannot be elucidated by studies using single metal s i t e s . However, knowledge gained from model systems can be used to help assign the r o l e of the metal i n the system, and from that, the ro l e of the pr o t e i n i n the system can be better understood. Hemes, as well as promoting oxygen storage and transport, are also 2-4 responsible f o r several enzymatic oxygenation reactions . I f s i m i l a r model complexes can aid s i m i l a r reactions i n v i t r o , then there may be important implications for i n d u s t r i a l c a t a l y s i s . -9-1.2.2 General Synthetic Oxygen C a r r i e r s Although the study of t r a n s i t i o n metal complexes that r e v e r s i b l y bind dioxygen i s a r e l a t i v e l y new f i e l d , many such complexes have been reported. 24 Several of these, such as Vaska's Ir(PPh^) (CO)(Cl)(O^) complex , incorporate a "side-on" r-dioxygen metal linkage. While these compounds 25 have been studied extensively , they are probably poor models f o r b i o l o g i c a l systems. These compounds consist of metals i n zero or low oxidation states with ligands such as phosphines, which have l i t t l e or no b i o l o g i c a l r o l e . The metals of these compounds tend to be " s o f t " , and thus favor binding dioxygen in a soft ir -linkage, while metals of b i o l o g i c a l systems, such as i r o n , are usually i n higher oxidation states and tend to be "hard". Thus a hard a-bonding mode i s favored. 26 I t has been noted that for the b i o l o g i c a l metals iron(II) and copper(I), a second order Jahn-Teller e f f e c t would lead to a bent metal-dioxygen bond, even i f a T-bond were i n i t i a l l y formed. 7r-Bonding i s also thought to be favored by complexes susceptible to a two electron oxidative addition (e.g. Rh(I) and I r ( I ) ) , while one electron reductants would favor "end-on" binding and r e s u l t i n a one electron reduction of dioxygen to . . 25b superoxide Among the f i r s t row t r a n s i t i o n metals, often those capable of undergoing a one electron oxidation tend to i n t e r a c t with dioxygen Thus Mn(II), F e ( I I ) , Co(II), Cu(I), and Cr(II) a l l complex dioxygen, while F e ( I I I ) , Co(III), Cu(II), as well as Ni(II) and Zn(II), which have no r e a d i l y accessible higher oxidation states, do not. -10-1.2.3. Porphyrins Since a porphyrin ligand i s present i n the p r o s t h e t i c group of the most important b i o l o g i c a l oxygen c a r r i e r s , and such ligands are used i n numerous model study systems, i t i s important to understand the nature of porphyrins. Porphyrins are macrocyclic compounds that contain four pyrrole or 2 substituted pyrrole rings joined by sp hybridized carbon atoms. Formally, they can be viewed as being derived from the parent compound, porphine (Figure I . l a ) , by s u b s t i t u t i o n of the hydrogen atoms at the p e r i p h e r a l posi t i o n s of the macrocyclic r i n g . Since the porphyrin system i s a highly d e l o c a l i z e d one, s u b s t i t u t i o n by e i t h e r electron withdrawing or electron donating groups at the peripheral p o s i t i o n s can have profound e f f e c t s on the molecule's complexing properties with metals, even though the substituents may be separated by many atoms from the metal. Also the f a c t that many substitutions may be made enhances the v e r s a t i l i t y of these compounds. A formal nomenclature system, l a b e l i n g the eight "pyrrole" p o s i t i o n s 27 from 1 to 8 and the four "meso" po s i t i o n s a,/3,7, and 5 , has been devised However, t r i v i a l names have been adopted for most porphyrins, and w i l l be used throughout t h i s t h e s i s . Table 1.2 l i s t s the s u b s t i t u t i o n patterns and t r i v i a l names of the compounds discussed. Porphyrins can f a l l i n t o either the "natural" porphyrins, a c l a s s of compounds that are derived e i t h e r from, or by modification of, metallo-porphyrin complexes obtained from nature, or a class of compounds that are obtained from synthesis. Protoporphyrin IX, mesoporphyrin IX, and the deuteroporphyrin IX compounds are examples of natural porphyrins, as they can be prepared -11-Ta±>le 1.2 T r i v i a l Names and Side Chain Substituents of Porphyrins.  See Figure I . l a . PORPHYRIN 1 2 3 SUBSTITUENTS 4 5 6 ON 7 POSITION 8 a 0 7 5 Protoporphyrin IX M V M V M P P M H H H H Deuteroporphyrin IX M H M H M P P M H H H H Diacetyldeutero-porphyrin IX M Ac M Ac M P P M H H H H Dibromodeutero-porphyrin IX M Br M Br M P P M H H H H Etioporphyrin I M E M E M E M E H H H H Octaethylporphyrin E E E E E E E E H H H H Mesoporphyrin IX M E M E M P P M H H H H Coproporphyrin III M P M P M P P M H H H H Uroporphyrin I I I A P A P A P P A H H H H Hematoporphyrin IX M B M B M P P M H H H H a,/3,7, 6 — tetraphenyl-porphyrin H H H H H H H H Ph Ph Ph Ph a,/3,7,6 — (p-methoxy)-tetraphenylporphyrin H H H H H H H H MPh MPh MPh MPh M- -CH B- -CHOHCH 3 3 E- "CH 2CH 3 P- -CH2CH2COOH V- -CH=CH2 H- -H A- -CH2C0OH Ph- -Phenyl Ac- -C0CH3 MPh -p-0CH 3-Phenyl , Br- -Br also referred to as p-OMe -12-from chloroprotoporphyrin IX i r o n ( I I I ) , known as hemin, which i s obtained from blood hemoglobin. The propionic acid side chains are often converted to dimethyl or d i e t h y l esters i n order to prevent side reactions r e s u l t i n g from the proton of the propionic acid side chains, or to aid i n s o l u b i l i t y . n p Of the synthetic porphyrins, a, /3, y,6-tetraphenylporphyrin (TPP) and 29 octethylporphyrin IOEP) are the most commonly studied.. OEP resembles more c l o s e l y the natural porphyrins, but TPP i s more widely studied because of i t s r e l a t i v e ease of synthesis, and because s u b s t i t u t i o n i n t o the phenyl rings can be r e a d i l y achieved 3 0. 31 Phthalocyanine i s a compound that i s s i m i l a r to porphine except that the porphine methine bridges are replaced by aza bridges, and the pyrrole rings are further d e l o c a l i z e d by the presence of benzo groups (Figures I.3a and I.3b). This macrocycle gives r i s e to s p e c t r a l properties d i f f e r e n t from those of porphyrins; for example the r e l a t i v e i n t e n s i t i e s of the v i s i b l e and the Soret bands of the c o b a l t ( H ) complexes are reversed (Figures I.4a and I.4b). An e s p e c i a l l y i n t e r e s t i n g compound i s octamethyltetrabenzoporphyrin (OMBP), since i t i s s t r u c t u r a l l y intermediate between a porphyrin and a phthalocyanine (Figure I.3c). The intermediate nature of t h i s macrocycle i s further suggested by the spectrum of CoOMBP, i n which the i n t e n s i t i e s of the Soret and v i s i b l e bands are approximately the same (Figure I.4c). 1.2.4 S c h i f f Bases Many metal-Schiff base complexes have been found to r e v e r s i b l y bind dioxygen. These complexes generally include tetradentate ligands formed by the S c h i f f base condensation reaction (Eq. 1.1). For example, ethylene--13-CoOMBP Figure I.3c Figure 1.3. A v a r i e t y of cobalt macrocycle complexes. 10 U J i.oH o z < CO tr O <j> m < 0.5-1 — r - , 1 1 — r — 400 450 500 550 600 WAVELENGTH, NM 650 I 700 F i g . 1.4a. VIS spectrum of cobalt etioporphyrin I. From work i n t h i s t h e s i s . HI o 2 < m tr O 05-| m < T 1 I 1 1 1 400 500 600 WAVELENGTH NM 700 Fig. Fron Spectrosc. I.4b. VIS spectrum of cobalt phthalocyanine. £ m L. Edwards and M. Gouterman, J . Moi. 1 33, 292 (1970). 7 too T ISO S00 SMI (00 WAVELENGTH N H F i g . I.4c. VIS spectrum of CoOMBP, from t h i s thesis. Figure 1.4. V i s i b l e spectra of a v a r i e t y of cobalt macrocycle complexes. -15-R R C = N' (I . D R diamine can be reacted with a dicarbonyl compound to give a tetradentate compound with two nitrogen and two oxygen donors (Eq. 1.2). Pentadentate S c h i f f base compounds can be u t i l i z e d , and s u l f u r or 32 nitrogen atoms may replace one or both of the oxygen atoms as donors S c h i f f bases are often r e f e r r e d to by t h e i r mnemonic abbreviations. For example, bis(acetylacetone)ethylenediimine becomes" acacen, and N ,N-bis(salicylaldehyde)ethylenediimine becomes salen. I f the S c h i f f base compound has substituents, then the mnemonic simply adds the group symbol; e.g. N,N-bis(3-methoxysalicylaldehyde)ethylenediimine i s 3-MeOsalen. Examples of S c h i f f base compounds are given i n Figures 1.5a and 1.5b, and i n Table 1.3. A tetradentate S c h i f f base ligand i s generally arranged i n a plane around the chelated metal. Some chelating ligands, such as acacen, allow some d i s t o r t i o n from p l a n a r i t y , but the i n c l u s i o n of an aromatic r i n g , as i n salen, causes the ligand to be more r i g i d , and adds to i t s tendency for p l a n a r i t y . I f the number of carbon atoms i n the alkylene bridge i s increased 33 above two, then tetrahedral d i s t o r t i o n s can occur , and i f the number of 34 separating carbons exceeds s i x , the complex can become tetrahedral 35 Pentadentate complexes can adopt e i t h e r square pyramidal or t r i g o n a l 36 bipyramidal geometries. -16-Figure 1.5c. Structure of PyCo(benacen) (C>2) . From G. A. Rodley and W. T. Robinson, Nature(London), 235, 438 (1972). Figure 1.5. A v a r i e t y of cobalt S c h i f f base complexes. -17-Table I.3a. Mnemonic Names and Substituents of Type A S c h i f f Bases. Compound V A B R Co(acacen) - ( C H 2 ) 2 " CH 3- H- 0 Co(Meacacen) - ( C H 2 ) 2 - CH 3- CH 3- 0 Co(Phacacen) - ( C H 2 ) 2 - CH 3- C 6 H 5 " 0 Co(benacacen) - ( C H 2 ) 2 - C 6 H 5 " H- 0 Co(Clbenacacen) -<CH_)_- p-ClC^H -* 6 4 H- 0 Co(sacacen) - ( C H 2 ) 2 - CH 3- H- S Co(acacpn) -CH -CH-2 1 CH 3 CH 3- H- 0 Table I.3b. Mnemonic Names and Substituents of Type B S c h i f f Bases. Compound W Co(salen) Co(3-MeOsalen) Co(4,5-Me 2salen) Co(saloph) Co(sal(t) or (m)chxn) Co(o-Mesalpt) Co(amben) Co(tsalen) - ( C H 2 ) 2 -- ( C H 2 ) 2 -- ( C H 2 ) 2 -" C 6 H 4 --CH CH--(CH 2) 3-NH-(CH 2) 3-- ( C H 2 ) 2 -- ( C H 2 ) 2 -CH 0 H 3 H H H H H H H H H H H H H H H H H H H H CH 3 CH 3 H H H H H 0 0 0 0 0 CH, 0 N S -18-As with the porphyrins, a wide v a r i e t y of substitutions can be made i n S c h i f f base compounds to a l t e r t h e i r e l e c t r o n i c and s o l u b i l i t y properties. Unlike the porphyrins, the S c h i f f base compounds do not have extensive conjugated systems available for 7r-bonding to the metal, and i t i s generally thought that porphyrins are better able to de l o c a l i z e electron density from the metal centre, causing metalloporphyrin complexes to be less 32 37 able to bind dioxygen than metal-Schiff bases ' 1.2.5 T r a n s i t i o n Metal Complexes 1.2.5.1 Iron Complexes Since Fe(II) i s the metal used i n most b i o l o g i c a l systems, i t would be a l o g i c a l choice for model system studies. Recently, important advances have been made with iron systems, but these have not been accomplished without considerable d i f f i c u l t y . Under ordinary conditions i r o n complexes tend to 3 8 rapi d l y and i r r e v e r s i b l y oxidize . Further, ferrous porphyrin species i n 39 solution tend to form six-coordinate complexes with any ligand present , while i n b i o l o g i c a l systems a five-coordinate moiety may be present; e.g. i n deoxymyoglobin. Iron(II) porphyrins have been designed so that the formation of the bridged M-oxo-dimer (Fe(III)-O-Fe(III)), the usual oxidation product, i s 40 s t e r i c a l l y hindered. Such complexes include the picket fence porphyrins 41 and the capped porphyrins . The l a t t e r type of compound also prevents the bonding of an a x i a l ligand on one side of the porphyrin r i n g , while the picket fence type does not. Five-coordinate iron(II) porphyrin complexes can be obtained by using s t e r i c a l l y hindered ligands, such as 1,2-dimethyl-42 imidazole and 2-methylimidazole , and such species are capable of binding 43 dioxygen to form six-coordinate LFeP*(0 ?) complexes . C r y s t a l structures -19-of two such p i c k e t fence systems that show the bent geometry of the end-on iron-dioxygen l i n k (Fe-0'°) have been reported 4 0-^ Attaching an oxygen carrying complex to the surface of a s o l i d or to a polymer can prevent the approach of two metal atoms to each other, and thus 39c can impede "dimerization," such as formation of Fe(III)-O-0-Fe(III) , a l i k e l y step i n i r r e v e r s i b l e oxidation. This approach was f i r s t taken by 44 Wang , who showed that dioxygen binds r e v e r s i b l y to 1-(2-phenylethyl)-imidazole heme d i e t h y l ester embedded i n a matrix of polystyrene and 1-(2-phenylethyl)imidazole. Attachment of Fe(II) tetraphenylporphyrin to 45 a s i l i c a g e l support has been shown to produce an e f f i c i e n t oxygen c a r r i e r Fe(II) protoporphyrin attached to several poly-4-vinylpyridine and poly-L-l y s i n e polymers, has also been shown to r e v e r s i b l y bind dioxygen to a r a t i o 46 of l F e r l O ^ i n the s o l i d state . Similar behavior has been observed i n solut i o n f or iron(II) porphyrin compounds covalently bonded to a highly 47 cross-linked polystyrene . The binding of dioxygen to an iron(II) protoporphyrin poly-L-lysine polymer i n solu t i o n has exhibited cooperativity, the i n s e r t i o n of dioxygen i n t o the system promoting the reestablishment 48 of the o-helix conformation of the polymer . Recently water soluble poly-aminophosphazenes have been proposed as " c a r r i e r " ligands for iron(II) 49 porphyrins , but these polymers have not been able to prevent i r r e v e r s i b l e oxidation. At low temperatures the oxidation reaction ( l i k e l y v i a a bimolecular F e ( I I I ) ( 0 2 ) + Fe(II) process) i s often s u f f i c i e n t l y i n h i b i t e d so that r e v e r s i b l e oxygenation can be studied. This was f i r s t s u c c e s s f u l l y observed with a five-coordinate pyrroheme-N-(3-(1-imidazoyl)-propylamide system at -45°C i n dichloromethane^ 0. Several workers subsequently examined the -20-r e v e r s i b l e binding to six-coordinate iron(II) porphyrin complexes at low temperatures^ 1, and d e t a i l e d k i n e t i c and thermodynamic studies have been accomplished i n some c a s e s ^ . 1.2.5.2 Copper Complexes Model systems of copper are of i n t r i n s i c i n t e r e s t , as copper can transport oxygen i n b i o l o g i c a l media, although porphyrin complexes are not involved. Two synthetic complexes that bind one mole of dioxygen r e v e r s i b l y per two moles of copper are a copper(I) complex of the penta-dentate ligand derived from 2,6-diacetylpyridine and h i s t i m i n e ^ 3 , and a binuclear complex of two copper(I) atoms l i g a t e d to 1,4-bis(1-oxo-54 4,10-dithio-7-azacyclododecan-7-ylmethyl)benzene 1.2.5.3 Chromium Complexes Stomberg^ has described numerous dioxygen complexes of chromium of the types (Cr(0 ) X ), (CrO(0 ) X), or (Cr(0 ) ) i n which the metal centre i s formally Cr(IV) or Cr(V) and X i s a mono- or bidentate ligand. The dioxygen moieties are bound side-on and are proposed as peroxides that are 26 bound with a w-linkage . However, i n the chromium dioxygen adduct, 56 PyCrTPP(0 ), the dioxygen appears to be bound end-on as a superoxide 2 Redox p o t e n t i a l s p r e d i c t that dioxygen w i l l l i k e l y bind more strongly to chromium than to i r o n , cobalt, or manganese systems^ 7, and t h i s i s consistent with the i r r e v e r s i b i l i t y of the formation of the chromium-dioxygen porphyrin complexes. 1.2.5.4 Manganese Complexes I n i t i a l attempts to study the i n t e r a c t i o n of dioxygen with manganese tetradentate chelates resulted i n i r r e v e r s i b l e oxidation of Mn(II) to Mn(III) 58 or to Mn(IV) . However, re v e r s i b l e oxygenation has been observed with meso--21-o 59 tetraphenylporphinatopyridine manganese(II), PyMnTPP, i n toluene at -78 C The dioxygen appears to be *-bonded to the metal and the system i s formulated 59b c as a manganese(IV) peroxide ' . Similar behavior i s reported for para-60 substituted TPP complexes of Mn . The r e v e r s i b l e formation of a manganese phthalocyanine dioxygen adduct, formulated as a superoxide, has been observed 61 at room temperature i n dimethylformamide and dimethylacetamide 1.2.5.5 Titanium Complexes 62 Recent reports describe r e v e r s i b l e dioxygen binding to FTi(III)TPP. The FTi(III)TPP was embedded i n a matrix of 0Ti(IV)TPP i n order to prevent the i r r e v e r s i b l e formation of Ti(IV)TPP(C^), where dioxygen i s bound as a 6 3 peroxide . Reversible oxygenation could be c a r r i e d f o r several cycles on t h i s system i n the s o l i d state, but when FTi(III)TPP was exposed to dioxygen at -80°C i n toluene, i r r e v e r s i b l e oxidation occurred. ESR evidence suggested that the dioxygen adduct was best formulated as a Ti(IV)-superoxide. 1.2.5.6 Ruthenium Complexes Solutions of (CH 3CN) 2Ru(II)0EP have been shown to r e v e r s i b l y absorb 0^ i n 64 . . . a r a t i o of 10 2:lRu at ambient conditions . At the present time i t i s uncertain whether Ru binds dioxygen i n a a-end-on or a T-side-on manner. 65 However, THF 2Ru(II)OEP reacts with ""-bonding C 2 H 4 r e v e r s i b l y which suggests that a side-on Ru(IV)(C^) configuration i s l i k e l y . 1.2.5.7 Rhodium Complexes In 1972 i t was reported that an i s o l a t e d Rh(II)TPP complex was unreactive to ° 2 6 6 , k u t i t h a s recently been suggested that the i s o l a t e d complex was i n a f a c t a dioxygen adduct 6 7. Oxygenation of Rh(II)TPP i s thought to be e s s e n t i a l l y i r r e v e r s i b l e , as i s the oxygenation of (Rh(II)OEP) . ESR evidence suggests that these adducts are formally Rh(III)-superoxide species. -22-1.3 Cobalt Complexes 1.3.1 General The most widely studied t r a n s i t i o n metal centre f o r dioxygen binding i s Co(II) and much of the current knowledge of metal^dioxygen moieties has come from in v e s t i g a t i o n s on such systems. Because cobalt has one more electron than i r o n , the eighteen electron rule implies that five-coordinate complexes of Co(II) are l i k e l y to be much more stable than those of F e ( I I ) , and so the oxygenation reaction can be studied as a simple addition to a vacant coordination s i t e i n Co(II) complexes, rather than as a s u b s t i t u t i o n reaction that occurs with most simple six-coordinate Fe(II) complexes. In t h i s respect, cobalt model systems resemble some natural systems more c l o s e l y than do i r o n systems. The presence of the unpaired electron i n cobalt allows the use of ESR as a valuable t o o l to probe the electron d i s t r i b u t i o n between the metal and the dioxygen, and much information can be obtained on the nature of the metal-dioxygen bond. Indeed, t h i s technique has been dominant i n suggesting that dioxygen may be bonded to cobalt as a superoxide i n a bent manner, with 6 8 a formal one electron transfer from the cobalt to dioxygen , s i m i l a r to that . . . . 69 i n i t i a l l y proposed by Pauling f o r oxyhemoglobin . I t has recently been suggested that the amount of r e a l electron t r a n s f e r may vary s u b s t a n t i a l l y from complex to complex, and i n some cases there may be l i t t l e e lectron t r a n s f e r , thus making the formal assignment of oxidation states somewhat 70 misleading . A l t e r n a t i v e T-bonded formulations have also been proposed 71 for natural oxygen c a r r i e r s v Although dioxygen appears to bind to the natural systems and to cobalt p r i m a r i l y as a a-bonded superoxide, some TT — i n t e r a c t i o n s between the cobalt and the dioxygen are possi b l e . The use of cobalt macrocyclic complexes as models for b i o l o g i c a l i r o n -23-systems has some drawbacks. For example, cobalt does not bind dioxygen as strongly as i r o n (e.g. i n compounds where only the metal i s d i f f e r e n t , the P^C>2 value for the cobalt complex i s generally at l e a s t an order of magnitude 72 greater than for the i r o n complex ). To some extent t h i s i s an advantage since i t r e s u l t s i n less tendency of the cobalt to undergo i r r e v e r s i b l e 23c oxidation . Also, e s p e c i a l l y at low temperatures, the dioxygen a f f i n i t y of Fe(II) complexes i s so great that i t i s often d i f f i c u l t to get solutions s u f f i c i e n t l y oxygen-free to observe only the deoxy species. 1.3.2 E a r l y Cobalt Dioxygen Work In 1852 i t was reported that the exposure of ammoniacal solutions of 73 Co(II) s a l t s to the atmosphere res u l t e d i n the formation of brown compounds 74 These were characterized i n 1898 as containing the diamagnetic cation 4+ ((H N) Co(0 )Co(NH ) ) . In 1933 i t was observed that c r y s t a l s of the 3 5 2 3 5 75 cobalt S c h i f f base complex Co(salen) darkened when exposed to a i r and i n 76 1938 Tsumaki showed that t h i s was due to r e v e r s i b l e oxygen binding. This 77 78 work was then followed up by that of C a l v i n et a l and Diehl et a l Interest i n t h i s behavior was intense, and for a short period during World War II the US Navy used cobalt S c h i f f bases as a means of producing pure 79 oxygen , and more recently a cobalt S c h i f f base complex was proposed as a 80 source of pure oxygen for the crew of the B - l bomber 1.3.3 Cobalt S c h i f f Base Compounds 68a 81 By 1969 ' i t had been established that under c e r t a i n conditions, some cobalt S c h i f f base complexes would r e v e r s i b l y bind dioxygen i n non-aqueous so l u t i o n s . Although four-coordinate cobalt(II) S c h i f f base chelates bind dioxygen very poorly, the presence of one a x i a l ligand g r e a t l y enhances 82 oxygenation . There i s no c o r r e l a t i o n between the dioxygen a f f i n i t y and the -24-pK of the a x i a l ligand, although a l i n e a r c o r r e l a t i o n has been established cl between the dioxygen a f f i n i t y and the ease of oxidation of the cobalt(II) 82 83 centre, measured as E, for Co(II) —*-Co(III) ' H Generally, the greater the electron density on the cobalt centre, the easier i t i s for that centre to bind dioxygen. This i s evidenced by the decreased dioxygen a f f i n i t y of the cobalt S c h i f f base complex when the oxygen atoms of the macrocyclic ligand are replaced by »-electron-withdrawing 84 s u l f u r atoms , or when an a x i a l nitrogen-donor i s replaced by a phosphorus -32 or sulfur-donating one The nature of the bound dioxygen i s well established i n cobalt(II) S c h i f f base-dioxygen complexes. As well as the previously mentioned ESR 81 85 work, there have been i n f r a r e d , resonance Raman spectroscopy , and X-ray 86 PES (photoelectron spectroscopy) studies that have a l l given r e s u l t s consistent with what i s e s s e n t i a l l y a Co(III)-superoxide system. Theoretical c a l c u l a t i o n s also p r e d i c t Co (III) (O.,) ' , and f i n a l l y numerous cr y s t a l l o g r a p h i c studies have shown that the dioxygen i s bound i n the 87 Pauling bent end-on manner with 0-0 bond lengths comparable to that of superoxide (Figure I.5c). Cobalt S c h i f f base work i n t h i s thesis w i l l be concerned with Co(I I ) ( s a l e n ) , i t s r e a c t i v i t y toward a x i a l ligands and dioxygen, as well as solvent dependencies of the reactions. 1.3.4 Cobalt Porphyrin Compounds Hoffman and other workers have suc c e s s f u l l y substituted the i r o n of hemoglobins and myoglobins with cobalt, and noted the r e s u l t i n g properties 88 of r e v e r s i b l e oxygen binding ; the cobalt(II) protoporphyrin IX group i s thought to be situated i n the same place as the o r i g i n a l heme group, with -25-the metal centre s l i g h t l y displaced from the plane of the porphyrin toward the proximal h i s t i d i n e . Of i n t e r e s t , cobalt-substituted hemoglobin e x h i b i t s c o o p e r a t i v i t y , while cobalt-substituted myoglobin binds dioxygen about 1% as 72 strongly as natural myoglobin The P,0 value f o r the 1-methylimidazole complex of CoPpIXDME i n H 2 o 4 89 toluene at 20 C i s -2.5 x 10 Torr , while the corresponding value for 88a coboglobin i s 57 Torr . The presence of the protein d e f i n i t e l y enhances the oxygen a f f i n i t y of the cobalt complex centre. Differences i n thermodynamic parameters have been discussed i n terms of several e f f e c t s , including transformation of the p r o t e i n to a more stable conformation upon 27c oxygenation , and the presence of the protein preventing solvation of the 90 metal centre Extensive thermodynamic work on oxygenation and binding of nitrogen-donor ligands has taken place using various complexes of CoPpIXDME i n several solvents. Judging by discrepancies i n the data from the work of several 89 91 groups ' , there appear to be d i f f i c u l t i e s i n obtaining good thermodynamic values on these systems. A comparison of the equilibrium constants, the AH, and the AS values for a x i a l N-donor ligand binding shows that no simple c o r r e l a t i o n e x i s t s between any of these parameters and the pK of the cl 89 protonated ligand . However, for ligands of a s t r u c t u r a l l y s i m i l a r s e r i e s , e.g. substituted p y r i d i n e s , a c o r r e l a t i o n does e x i s t between the ligand 89/92 _. binding constant (K ) and the pK of the protonated ligand . Similar observations have been noted f o r the a f f i n i t y of dioxygen and the pK of cl 92 93 the a x i a l ligand ' . Work done on various para substituted PyCo(II)Tp-XPP complexes revealed a Hammett r e l a t i o n s h i p between the a-value of the para-94 substituents and log . Limited oxygenation studies on cobalt 91d 93 95 mesopophyrin, deuterporphyrin ' and octaethylporphyrin complexes, -26-89 91 together with the work on CoPpIXDME systems ' , suggest that oxygen a f f i n i t y i s p o s s i b l y r e l a t e d to the ease of tra n s f e r of electron density to dioxygen, since the more basic a x i a l ligands tended to enhance the cobalt's a f f i n i t y for dioxygen. Work with the above cobalt porphyrin systems yi e l d e d a range of porphyrin b a s i c i t i e s too narrow to generate any conclusion about a re l a t i o n s h i p between oxygen a f f i n i t y and electron donating a b i l i t i e s of the planar porphyrin ligands^"*". Anomalously high oxygen a f f i n i t y has been noted for a cobalt p i c k e t 90a fence porphyrin system . Thus, CH 3-ImCo(II)PFP has a P,^ value of 140 Torr at 25°C i n t o l u e n e 9 0 b , which compares to an estimated value of 15,500 92 Torr for CH^-ImCo(II)T(p-OCH^)PP ; the presence of the picket fence i s thought to break up adverse sol v a t i o n e f f e c t s which hinder oxygenation i n the TPP system. It has also been shown that the presence of a more polar solvent 91a enhances the formation of the more polar C o ( I I I ) ' ( 0 2 ) product Although a few of the cobalt porphyrin complexes studied form s i x -89 92 coordinate species under c e r t a i n circumstances ' , almost no quan t i t a t i v e work has been done with these species. Cobalt(II) octamethyltetrabenzoporphyrin (CoOMBP) i s unusual i n that i t was found to r e a d i l y form six-coordinate species with some a x i a l ligands. 96 Further, OMBP i s a very weakly basic porphyrin . Therefore, extensive ligand and dioxygen binding studies on t h i s p a r t i c u l a r metalloporphyrin were a considered l i k e l y to give a better i n s i g h t i n t o the ro l e s that the porphyrin and a x i a l ligands play i n dioxygen binding i n natural systems. 96c Since OMBP i s also considered to be a r e l a t i v e l y strong TT-donor , i t was also of i n t e r e s t to examine the e f f e c t of such a porphyrin on 0 2 a f f i n i t y . More extensive studies i n a v a r i e t y of solvents were also considered l i k e l y to lead to a better understanding of the solvent r o l e . -27-1.3.5 Other Cobalt Compounds In the late 1940s, i t was discovered that cobalt amino acid complexes 97 were capable of binding dioxygen reversibly , and recently stable 1:1 dioxygen complexes of Co(II) polyamines have been observed in large zeolite . . 68g,98 cavities where oxidation via "dimerization" i s sterically hindered The structure of ((CN)^Co(0^)) 3 shows a cobalt-dioxygen bond that is 99 o i n the bent end-on configuration , with a Co-0-0 bond angle of 153.4 . This angle i s somewhat larger than that reported for neutral cobalt-dioxygen complexes, and i s thought to arise in part from the repulsive interactions between CN anions and the superoxide. The structure of (cis-(C^H ) -PCH=CHP(C H ) ) Co(0 ) BF contains a 6 5 2 6 5 2 2 2 4 ir-bonded dioxygen^" 0 0 3, but this probably results from the cobalt in the deoxygenated species being in the softer +1 oxidation state, rather than being in the more common +2 oxidation state. However, the dioxygen adduct of the high spin Co (II) ((CF 3) (0) C (0) (-CF2^2 c o m P l e x h a s been formulated as , .100b TT—bonded 0^ to Co (II) 1.4 Catalytic Properties Cobalt(II) phthalocyanine has been used to catalyze a net insertion of oxygen into cumene or acrolein**^ . The same system has been used to 102 oxidize aldehydes , and this behavior has been noted also with Co{II)(T(p-CH 3)PP) 1 0 3 and Co(II)TPP 1 0 4. Co(II)(salen) 1 0 5 and Co(II)TPP 1 0 6 a l s o promote insertion of oxygen into indoles, giving products that are analogous to those resulting from the oxidation of tryptophan by tryptophan 2,3-dioxygenase10^. These cobalt complexes are thought to i n i t i a t e the reaction by forming a dioxygen complex with subsequent abstraction of hydrogen from the substrate. Reaction of dioxygen with a substrate radical then leads to peroxy species and oxidation by a free radical process. Cobalt - 2 8 -S c h i f f base-dioxygen adducts have also been shown to i n i t i a t e free radical oxidations of various organic substrates, such as flavones, phenols, and ascorbic a c i d and again i n i t i a t i o n i s thought to involve abstraction of hydrogen from the substrate. Similarly Co(IH) and Fe(III) porphyrins 109 have been found to catalyze the peroxidation of several olefins 25a Analogous mechanisms have been postulated for enzymic oxygenase systems , but such processes are l i k e l y less well understood than the "simple" protein-free model systems. Although workers have shown that some olefins, such as tetracyano-ethylene (TCNE), do interact with cobalt tetradentate macrocycle complexes^ 1 0, formation of a cobalt-olefin bond has yet to be established. During the present studies, a report of a TT-bonded interaction between TCNE and a Mn(II)TPP complex appeared 1 1 1. If cobalt complexes could form such a bond, then these complexes may be able to act as catalysts for reactions of o l e f i n s . This thesis describes some exploratory work in this area. -29-CHAPTER II  APPARATUS AND EXPERIMENTAL PROCEDURE I I . 1 Ins trumentati on U l t r a v i o l e t (UV) and v i s i b l e (VIS) absorption spectra were recorded on a Perkin Elmer 202, a Carv 14 or a Cary 17 spectrophotometer, f i t t e d when necessary with a thermostated c e l l compartment. Electron spin resonance (ESR) spectra were recorded on a Varian Associates model E3 spectrometer. A Wissenschaftlichen und Techniksh Werkstatten model DM01 Diplometer f i t t e d with a DFL1 c e l l was kindl y loaned by N. L. Paddock to measure d i e l e c t r i c constants from e = 1.0 to t = 3.6. For materials with a higher d i e l e c t r i c constant, measurements were obtained with a Bronton E l e c t r o n i c s Corporation model 74C-58 Capacitance Bridge which was provided by W. N. Hardy. The material to be studied was placed i n a c e l l designed by W. N. Hardy and b u i l t i n the chemistry department mechanical shop (Figure II .1) . Stopped-flow k i n e t i c s measurements were made on a Durrum-Gibson model D 115 Stopped-Flow Spectrophotometer equipped with a 2 cm path-length cuvette and a thermostated compartment. Absorbance versus time traces were observed on a Tektronix Inc. R5103N os c i l l o s c o p e f i t t e d with a D l l s i n g l e beam storage u n i t , a 5A15N a m p l i f i e r , and a 5B10N time base. Flash photolysis measurements were made with a Beckman model 2400 DU spectrophotometer, the photolyzing l i g h t being provided by a Braun 2000 F27 -30-( I v 1 A m m m m m ^ • » • • • 1 A. OUTER S T A I N L E S S S T E E L BARREL B . INNER S T A I N L E S S S T E E L BARREL (SEPARATED FROM OUTER BY . 0 1 5 I N . ) C. NYLON SCREW CAP 0. T E F L O N PLUG E. S T A I N L E S S S T E E L TUBE F . TEFLON PLU6 6 . ALUMINUM ELECTRODE Figure II.1. C e l l used f o r d i e l e c t r i c constant measurements. Sample was placed between the two s t a i n l e s s s t e e l b a r r e l s . Electrodes from the capacitance bridge were clipped to G and E. -31-f l a s h e r placed next to the cuvette, perpendicular to the path of the l i g h t from the spectrophotometer. The fl a s h e r was equipped with a f i b r e -o p t i c feedback system which cut the pulse time to 100 jusec. By using an appropriate Corion o p t i c a l f i l t e r , l i g h t of a selected wavelength could be flashed on the sample without interference from other l i g h t wavelengths generated by the f l a s h e r . Temperature readings (when a mercury thermometer was not appropriate to use) were made with a copper-constantan thermocouple. Voltages generated were measured on a Honeywell model 2476 potentiometer. Weights were measured on a Mettler H20 balance. II.2 Spectroscopic C e l l s II.2.1 U l t r a v i o l e t - V i s i b l e Spectroscopic C e l l s Anaerobic s p e c t r a l quartz c e l l s with path-lengths of 1,2, or 10 mm (Figure II.2) were used f o r spectroscopic measurements above 0°C. A Kontes-Martin Ltd. 10 cm path-length quartz c e l l (Figure II.3) was used to study ligand binding i n d i l u t e solutions (~10 6 M) of cobalt complexes. The c e l l had a so l u t i o n capacity of approximately 60 mL and was constructed so that a constant temperature bath surrounded the so l u t i o n without i n t e r f e r i n g with the l i g h t beam. A 6 cm path-length quartz c e l l (Figure II.4) was used for measuring in t e r a c t i o n s of solutions of cobalt complexes with gases. The c e l l had a sol u t i o n capacity of approximately 15 mL and was again constructed so that constant temperature could be maintained. For low temperature spectroscopic work on solutions i n shorter path-length c e l l s , a device (Figure II.5) constructed by the chemistry department mechanical shop was used. O p t i c a l c e l l s with a path-length of -32-Figure II.2. Anaerobic Spectral C e l l . -33-SERUM CAP To VACUUM LINE THERMOCOUPLE OPTICAL CELL DEWAR FLASK CONSTANT TEMPERATURE BATH 4 SEALED-IN QUARTZ WINDOWS Figure II.3. 10 cm path-length c e l l . -34-Figure II.4. 6 cm path-length quartz c e l l . -35-SERUM CAP To VACUUM LINE SERUM CAP CENTRAL TUBE RUBBER STOPPER E V A C U A T E D S T A I N L E S S S T E E L J A C K E T E V A C U A T I O N V A L V E C O N S T A N T T E M P E R A T U R E B A T H C O P P E R B L O C K O P T I C A L C E L L - Q U A R T Z W I N D O W C O P P E R B L O C K Figure II.5 Short path-length spectroscopic c e l l for low temperature work. -36-1 mm or 10 mm were inserted i n t o the c e n t r a l tube of the c e l l holder. A serum cap sealed the volume between the c e l l and the c e n t r a l tube to prevent condensation on the c e l l . The c e l l holder was then f i l l e d with a constant temperature bath which thermally e q u i l i b r a t e d with the copper block i n the c e l l holder, and then with the c e l l . The constant temperature bath was insulated by the evacuated s t a i n l e s s s t e e l jacket and the rubber stopper. When using the Figure II.2 type c e l l s , the solvent was degassed by repeatedly freezing i t i n the f l a s k of the c e l l , pumping o f f any uncondensed gas, and then thawing the solvent. The degassed solvent could then be added to any s o l i d sample placed i n the quartz c e l l . For the c e l l s i l l u s t r a t e d i n Figures II.3, II.4, or II.5 the solvents were degassed i n a separate f l a s k , and then transferred through a s t a i n l e s s s t e e l tube under argon to the spectroscopic c e l l , where the s o l i d sample had already been placed. II.2.2 ESR C e l l s Samples prepared for electron spin resonance measurements were contained i n 1 mm in s i d e diameter quartz c e l l s f i t t e d with a high vacuum stopcock and a E-7 j o i n t (Figure I I . 6 ) . Solvents were i n i t i a l l y degassed i n a Figure II.2 type c e l l . The degassed solvent was mixed with the s o l i d sample i n the quartz c e l l , and then the sample was transferred to the ESR c e l l through the B7 j o i n t . -37-Figure II.6. Anaerobic ESR c e l l . -SS-I I .3 Constant Temperature Baths 11.3.1 Above 0°C The UV-VIS spectroscopic c e l l s shown i n Figures I I . 3 , I I . 4 , and I I . 5 were maintained at a constant temperature by f i l l i n g the constant temperature bath container with water and immersing a copper c o i l i n i t . The copper c o i l was attached to a Haake model FK c i r c u l a t i n g thermostating bath. By appropriately adjusting the thermostating bath, the desired temperature could be held i n the spectroscopic c e l l . Constant temperature of a Figure I I . 2 type c e l l could be maintained by using the thermostated c e l l compartment of the spectrophotometer. 11.3.2 At or Below 0°C Low temperatures were attained by surrounding the s o l u t i o n with a 112 c r y o s t a t i c bath, prepared as described by Shriver . Slushes of the following materials were used: i c e - d i s t i l l e d water (0°C); carbon t e t r a c h l o r i d e (-23°C); bromobenzene (-30.8°C); anisole (-37.4°C); chlorobenzene (-45°C); octane (-56.5°C); chloroform (-63.5°C); dry i c e -acetone (-78°C) ; and et h y l acetate (-83.5°C). ESR samples were studied at -196°C (77K) by pl a c i n g the ESR c e l l i n a finger Dewar f i l l e d with l i q u i d nitrogen. I I . 4 E quilibrium Constant Measurements II.4.1 Ligand Binding Equilibrium constant measurements f o r the reaction: L + CoMc where CoMc represents a four-coordinate cobalt macrocycle complex, and L i s -39-any a x i a l l y coordinating ligand, were made by following VIS and near UV (e.g. the Soret band) sp e c t r a l changes with changes i n added ligand concentration. Aliquots of degassed ligand, or degassed ligand i n toluene s o l u t i o n , were added from a c a l i b r a t e d syringe to a s o l u t i o n of cobalt complex under argon. Complete spectra were recorded from 750 to 350 nm. Usually, the t o t a l volume of ligand s o l u t i o n added to CoMc to obtain f u l l y formed LCoMc was r e l a t i v e l y small, and any d i l u t i o n corrections were minor. In c e r t a i n cases, s u b s t a n t i a l amounts of ligand were needed to completely generate LCoMc, and sometimes LCoMc could not be f u l l y formed. In most cases, however, good analyses for were achieved by allowing f o r the d i l u t i o n e f f e c t , and/or by taking i n t o consideration estimated s p e c t r a l c h a r a c t e r i s t i c s of the LCoMc species; e.g. by using data from a lower temperature where the complex was completely formed. Ligand binding equilibrium constants were generally determined at o temperatures from -23 to 40 C. Equilibrium constant measurements f o r the binding of a second ligand to a cobalt macrocycle complex were c a r r i e d out i n a manner s i m i l a r to that described above. II.4.2 Oxygen Binding Equilibrium constants for the reaction: LCoMc + 0 ( g ) ... •• - LCoMc (0^ (II. 2) where LCoMc[Q ) represents the cobalt macrocycle dioxygen adduct, were measured i n a manner s i m i l a r to that used for ligand binding. Known pressures of oxygen were admitted to the evacuated sample s o l u t i o n , the -40-pressure being measured e i t h e r with a mercury manometer (for pressures > 10 Torr) or a di-n-butylphthalate manometer (for pressures <40 T o r r ) . Before the spectrum was taken, the c e l l (as i n Figure 11.3, 11.4, or II.5) containing the sample was c a r e f u l l y shaken i n order to ensure complete mixing of the gas i n the s o l u t i o n without any s p i l l i n g of the constant temperature bath. These measurements were generally determined from -83.5 to -23°C. Sometimes, spectra of the f u l l y formed oxygenated species could not be obtained at a p a r t i c u l a r temperature, but such spectra could generally be achieved at some lower temperature, and good analyses were effected by making use of the lower temperature s p e c t r a l data. 11.5 Spectrophotometric K i n e t i c Measurements Samples for spectrophotometric k i n e t i c studies were prepared i n a manner s i m i l a r to that described f o r the equilibrium studies. Since the concentrations of the cobalt macrocycle species were generally very low (^ 1 x 10 5 M), and the s o l u b i l i t i e s of the gases were such that t h e i r -3 concentrations were of the order of 1 x 10 M, the gas concentrations i n s o l u t i o n (and the p a r t i a l pressure of the gas), remained e s s e n t i a l l y constant throughout the course of any experiment. 11.6 Gas-Uptake Measurements II.6.1 Description A constant pressure gas-uptake apparatus (Figure II.7) was used for gas-uptake and gas s o l u b i l i t y measurements., A f l e x i b l e glass s p i r a l tube connected a c a p i l l a r y manometer (D) at tap (C) to a pyrex two-neck reaction f l a s k (A), which was equipped with Figure II.7. Gas-Uptake Apparatus. -42-a dropping side-arm bucket. The reaction flask was clipped to a piston-rod which was driven by an offset wheel connected to a Welch variable speed electric motor (I) for shaking purposes, and the flask was thermostated in a silicone o i l bath. The capillary manometer contained di-n-butyl-phthalate (a liquid of negligible vapor pressure at ambient temperatures) and was connected to the gas-measuring burette, which consisted of a mercury resevoir (E) and a precision bored tube (N) of known diameter. The gas burette was in turn connected by an Edwards high vacuum needle valve (M) to the gas handling part of the apparatus. This consisted of a mercury manometer ( F ) , gas inlet ( Y ) , and connections to the Welch Duo-Seal rotary vacuum pump ( G ) . The capillary manometer and gas burette were thermostated at 25°C in a perspex water bath. The silicone o i l bath consisted of a four-litre glass beaker insulated by polystyrene foam on a l l sides and enclosed by a wooden box, with the top also covered by polystyrene. Both thermostat units were controlled by Jumo thermo-regulators and Merc to Merc relay control c i r c u i t s , with heating provided by a 40 watt elongated light bulb. With mechanical sti r r i n g and good insulation, the temperature could be maintained within * 0.05°C. A vertic a l l y mounted cathetometer was used to follow the gas-uptake, and time was recorded during kinetic experiments using a Lab-Chron 1400 timer. II.6.2 Procedure For A Typical Gas-Uptake Experiment For each experiment, the required amount of solvent (e.g. 5 mL.) was pipetted into the reaction flask . Generally the cobalt complex could be weighed into a small glass bucket, which was then suspended from the side arm (S). In some cases, - 4 3 -^— Hook formed during seal-off Figure II.8 however, the solvent tended to wet the sample in the bucket, and i t was necessary to employ sealed ampoules instead of open buckets in the uptake procedure. A bulb was blown on a glass tube," a constriction was placed in the stem, and a glass rod was welded to the bottom of the bulb so that breakage of the ampoule could be guaranteed when i t was dropped into the solvent. After being tested for leaks, the bulb was f i l l e d with the sample, then sand, and sealed off under argon, with the pressure being regulated with the aid of a balloon as shown in Figure I I . 8 . These precautions served to minimize gas-uptake when the ampoule was broken. The tube was sealed off in the shape of a hook (Figure II.8) so that the ampoule could be suspended from the side arm. - 4 4 -The solvent was degassed by the freeze-pump-thaw method while the reaction flask was connected by the spiral and tap (C) to the gas handling part of the gas-uptake apparatus at (0). The reactant gas was admitted at a pressure somewhat less than that required for the experiment, and then taps (C) and (P) were closed. The whole system up to tap (H) was pumped down with taps (K), (L), (J), and (M) open. The flask and spiral arrangement were disconnected from (0) and transferred into the thermostated o i l bath with the spiral connected to the di-n-butylphthalate manometer through tap (H) . Tap (H) was opened and after the air between tap (H) and (C) was pumped out, tap (Q) was closed. Gas was then admitted to the rest of the gas-uptake apparatus up to tap (C), which was then opened. The solution was shaken to achieve equilibrium at the desired temperature and pressure. Tap (J) and needle valve (M) were closed while the i n i t i a l reading of the mercury level in (N) was taken. Taps (K) and (L) were closed, the bucket dropped by rotating the side arm, and the timer and the shaker were started simultaneously. As a result of any gas uptake, the di-n-butylphthalate level on the l e f t hand side of the manometer rose, and to maintain no difference in the heights of the levels, gas was admitted into the gas measuring burette through tap (J) and needle valve (M) to give a resulting rise of mercury in (N). The change in height of mercury was noted as a function of time. Since the diameter of (N) was known, the corresponding volume of gas was determined, and the uptake of gas as a function of time could be followed. The use of a small volume of solution (~^ 5 mL.), a relatively large indented vessel (~'30 mL.), and a high shaking rate ensured the absence of diffusion control in the rate of gas consumption. -45-I I . 6 . 3 G a s S o l u b i l t y M e a s u r e m e n t s Some e x p e r i m e n t s w e r e c a r r i e d o u t t o d e t e r m i n e g a s s o l u b i l i t i e s i n t h e s o l v e n t s u s e d . T h e s e d e t e r m i n a t i o n s c o u l d b e made u n d e r s p e c i f i c t e m p e r a t u r e a n d p r e s s u r e c o n d i t i o n s u s i n g t h e g a s - u p t a k e a p p a r a t u s and a r e a c t i o n f l a s k c o n t a i n i n g a s t o p c o c k a t i t s n e c k . The e n t i r e s y s t e m , i n c l u d i n g t h e r e a c t i o n f l a s k c o n t a i n i n g a m e a s u r e d amount o f s o l v e n t , was d e g a s s e d . The t a p a t t h e f l a s k was t h e n c l o s e d , a n d t h e f l a s k was p l a c e d i n t h e b a t h a t t h e d e s i r e d t e m p e r a t u r e . The s y s t e m was t h e n e v a c u a t e d t o t h e f l a s k t a p a n d f i l l e d w i t h g a s t o t h e a p p r o x i m a t e p r e s s u r e d e s i r e d . The f l a s k t a p was t h e n o p e n e d , and t h e p r e s s u r e was a d j u s t e d i m m e d i a t e l y t o t h a t r e g u i r e d . T a p s (K) a n d (L) w e r e c l o s e d , t h e s h a k e r s t a r t e d , a n d t h e i m m e d i a t e u p t a k e m e a s u r e d a s d e s c r i b e d i n t h e p r e v i o u s s e c t i o n . T h i s a f f o r d e d a means o f c a l c u l a t i n g g a s s o l u b i l i t y . F o r g a s s o l u b i l i t y m e a s u r e m e n t s a t low t e m p e r a t u r e s , t h e s i l i c o n e o i l b a t h was r e p l a c e d w i t h a l a r g e b a t h c o n t a i n i n g t h e a p p r o p r i a t e s l u s h . I I . 7 M a t e r i a l s I I .7.1 G a s e s C P . g r a d e c a r b o n m o n o x i d e was o b t a i n e d f r o m t h e M a t h e s o n Gas C o . P u r i f i e d a r g o n , n i t r o g e n , a n d o x y g e n w e r e s u p p l i e d b y C a n a d i a n L i q u i d A i r L i m i t e d . The a r g o n , n i t r o g e n , a n d o x y g e n w e r e p a s s e d t h r o u g h a c o l u m n o f f r e s h p h o s p h o r u s p e n t o x i d e a n d t h e n t h r o u g h m o l e c u l a r s i e v e s (BDH, t y p e 5A) a t -78 C t o remove any t r a c e s o f w a t e r . E t h y l e n e a n d a c e t y l e n e w e r e o b t a i n e d f r o m t h e M a t h e s o n Gas C o . I n o r d e r t o remove any o x y g e n p r e s e n t , t h e s e g a s e s w e r e f r o z e n i n l i q u i d n i t r o g e n a n d a n y u n c o n d e n s e d m a t e r i a l was pumped o f f . The f r o z e n g a s e s -46-were then warmed to room temperature, and this freeze-pump-warm process was repeated for several cycles u n t i l a l l uricondensed materials were eliminated. II.7.2 Liquids 11.7.2.1 General Generally a l l liquids and solutions were handled and stored under an 112 argon atmosphere using Schlenk techniques 11.7.2.2 Solvents Toluene (Fisher Scientific Co., spectral grade) was refluxed over and d i s t i l l e d from sodium-benzophenone ketyl under nitrogen immediately prior to use. Dichloromethane (DCM) and 1,2-dichloroethane (DCE) (Fisher Scientific Co., spectral grade) were refluxed over and d i s t i l l e d from phosphorus pentoxide immediately prior to use. Chlorobenzene (Mallinckrodt Co., analytical grade) was dried over phosphorus pentoxide and d i s t i l l e d under a reduced pressure of argon. 1-Chlorobutane (Eastman Kodak Co.) and chloroform (Fisher Scientific Co., spectral grade) were dried over phosphorus pentoxide and d i s t i l l e d under vacuum prior to use. 11.7.2.3 Ligands 1-Methylimidazole (CH^-Im) (Aldrich Chemical Co., analytical grade), piperidine (Pip) (ICN Pharmaceuticals Inc., reagent grade), pyridine (py) (American Chemical and Scientific Co., reagent grade), and triethylamine (Et3N) (Eastman Kodak Co., reagent grade) were dried over and d i s t i l l e d from KOH under reduced argon pressure prior to use. N,N-Dimethylformamide (DMF) and N,N-dimethylacetamide (DMA) (Fisher Scientific Co., analytical grade) were dried over and d i s t i l l e d from calcium oxide under reduced argon pressure prior to use. Tetrahydrofuran (THF) (Fisher Scientific Co., -47-a n a l y t i c a l grade) was dri e d over sodium-benzophenone k e t y l and d i s t i l l e d from l i t h i u m aluminum hydride under reduced argon pressure p r i o r to use. Tri-n-butylphosphine (PBu^) (Aldrich Chemical Co., a n a l y t i c a l grade) was p u r i f i e d by vacuum d i s t i l l a t i o n . The o l e f i n s : diethylmaleate (DEM) (Aldrich Chemical Co.), d i e t h y l -fumarate (DEF) (K & K Laboratories Co.), styrene (sty) (MCB Co.) , 1-hexene (lhx) (ICN Pharmaceuticals Inc.), 2-octene (2oc) (K & K Laboratories Co.), cyclooctene (cot) (K & K Laboratories Co.), and 3,4-dichlorobutene (DCB) (Petro-Tex Chemical Corp.) were p u r i f i e d by passage through an activated alumina (Fisher S c i e n t i f i c Co.) column and subsequent vacuum d i s t i l l a t i o n . 2.7.3 Solids 2.7.3.1 Ligands Imidazole (Im) (Aldrich Chemical Co.), acrylamide (Acr) (K & K Laboratories Co.), and maleic anhydride (MA) (Fisher S c i e n t i f i c Co.) were r e c r y s t a l l i z e d from toluene and dri e d under vacuo for 24 hours. Triphenylphosphine (PPh^) (MCB Co.) was r e c r y s t a l l i z e d from ethanol and d r i e d under vacuo f o r 24 hours. Tetracyanoethylene (TCNE) (Eastman Kodak Co.) was p u r i f i e d by sublimation. 2.7.3.2 Macrocycles and Co(II) Complexes Bis(salicylaldehyde)ethylenediimine (salen) was prepared by the method 77q of Bailes and Cal v i n . 28.6 g of salicylaldehyde (Eastman Kodak Co.) and 7g of 98% ethylenediamine (Mallinckrodt Co.) were dissolved i n 300 mL of 95% ethanol. The r e s u l t i n g p r e c i p i t a t e was f i l t e r e d and washed with col d water and r e c r y s t a l l i z e d from ethanol. Analysis: Calculated for C, N ,N 0 : C, 71.6%; H, 6.0%; N, 10.5%. 16 16 2 2 Found: C, 71.4%; H, 6.2%; N, 10.4%. -48-N,N-bis(salicylaldehyde)ethylenediiminato cobalt(II) (Co(salen)) was 77q prepared by a method s i m i l a r to that of B a i l e s and C a l v i n . 2.0 g of salen was dissolved i n 20 mL of 95% ethanol at 60°C. A 25 mL aqueous s o l u t i o n containing 2.0 g of cobalt acetate, which had been r e c r y s t a l l i z e d from a c e t i c a c i d , was added dropwise. The r e s u l t i n g mixture was heated at 65°C for two hours, and then l e f t at room temperature f o r twelve hours before f i l t r a t i o n . The s o l i d was d r i e d i n vacuo f o r 48 hours. This preparation was done under nitrogen. A n a l y s i s : Calculated for C H, N O Co: C, 59.1%; H 4.3%; N, 8.6%. 16 14 2 2 Found: C, 59.0%; H, 4.2%; N, 8.7%. Cobalt(II) protoporphyrin IX dimethyl ester (CoPpIXDME) and cobalt(II) 2 7b etioporphyrin I (CoEpI), prepared by the method of Falk , were k i n d l y donated by K. J . Reimer and B. Halko, r e s p e c t i v e l y . Cobalt(II) 113 ai&,y,5 -tetraphenylporphyrin (CoTPP), prepared by the method of Adler , was also donated by B. Halko. Diacetyldeuteroporphyrin IX dimethyl ester (DADIXDME) and dibromo-deuteroporphyrin IX dimethyl ester (DBrDIXDME) were generously provided by H. C. Welborn. Cobalt(II) octaethylporphyrin (CoOEP) was prepared by the method of 27b Falk . 100 mg of octaethylporphyrin (OEP), provided by J . B. Paine I I I , was added to 15 mL of r e f l u x i n g CHC1 3 under nitrogen. One mL of a b o i l i n g s o l u t i o n of cobalt(II) acetate i n a c e t i c a c i d was then added, and r e f l u x i n g continued u n t i l the v i s i b l e spectrum ind i c a t e d the disappearance of free porphyrin. The chloroform s o l u t i o n was then washed repeatedly with d i s t i l l e d water u n t i l a l l traces of cobalt acetate and a c e t i c a c i d were removed. The chloroform was then stripped o f f , and the product r e c r y s t a l l i z e d from benzene -49-and dried in vacuo. Analysis: Calculated for C H, N Co: C, 73.1%; H, 7.4%; N, 9.5%. 36 44 4 Found: C, 73.3%; H, 7.3%; N, 9.3%. Cobalt(II) diacetyldeuteroporphyrin IX dimethyl ester (CoDADIXDME) and cobalt(II) dibromodeuteroporphyrin IX dimethyl ester were prepared in a manner similar to that used for CoOEP, but with scaled down proportions due to the very small amount of free porphyrins available. Elemental analyses were not done on the very small amounts of cobalt porphyrin complexes obtained. Cobalt(II) octamethyltetrabenzoporphyrin (CoOMBP) was prepared by the 114 method of Dolphin and coworkers with some modifications. 5.4 g of 1,3,4,7-tetramethylisoindole was loaded into a 450 mL Carius tube with 27 g of cobalt metal powder (Eastman Kodak Co.). The Carius tube was evacuated, o sealed, and then heated at 390 C for four hours. The resulting solid was extracted with pentane for 24 hours, toluene for 8 hours, and then pyridine for 10 hours. The resulting pyridine solution was stripped to about 20 mL, when 100 mL of hexane was then added. The precipitate was f i l t e r e d and then o extracted with hexane for 48 hours. The remaining solid was dried at 80 C in vacuo for 36 hours. Analysis: Calculated for C„ H N Co: C, 77.8%; H, 5.3%; N, 8.2%. 44 36 4 Found: C, 77.8%; H, 5.4%; N, 8.4%. The molar extinction coefficients at the absorption maxima for CoOMBP in 4 - 1 - 1 4 -1 -1 toluene were determined to be 6.4 x 10 M cm (412 nm) and 6.2 x 10 M cm ( 6 1 8 nm) (Chapter VII.4). In pyridine the corresponding values were -4 -1 -1 4 -1 -1 8 . 2 x 10 M cm (428 nm) and 4.3 x 10 M cm (617 nm) which compare with 4 -1 -1 the reported data of Dolphin et a l (e = 7.15 x 10 M cm for the Soret 4 -1 -1 114 band and e = 3 . 6 7 x 1 0 M cm for the visible band) . Under these -50-conditions the b i s pyridine adduct would have been formed. 1,3,4,7-Tetramethylisoindole, a precursor of CoOMBP, was prepared by a modified method of F l e t c h e r 1 " ^ . 11.5 g of 2,5-hexanedione (Eastman Kodak Co.) was added to a 200 mL aqueous sol u t i o n of 22 g of ammonium su l f a t e The r e s u l t i n g s o l u t i o n was refluxed under argon for 20 hours, cooled, and 25 mL of 20% NaOH s o l u t i o n was added. The r e s u l t i n g p r e c i p i t a t e was then f i l t e r e d , d r i e d i n vacuo for 18 hours, and then p u r i f i e d by sublimation at 80°C. An a l y s i s : Calculated f o r C 1 2 H 1 5 N : c> 83.2%; H, 8.7%; N, 8.1%. Found: C, 83.0%; H, 8.8%; N, 8.0%. -51-CHAPTER III THERMODYNAMICS OF REVERSIBLE BINDING OF SELECTED LIGANDS TO CoOMBP I I I . l Q u a l i t a t i v e S p e c t r a l Observations I I I . 1.1 L + CoOMBP ^ - " LCoOMBP, . The v i s i b l e spectrum of a so l u t i o n of CoOMBP i n toluene (Figure I.3c) 4 -1 -1 had a Soret band at 412 nm (e = 6.4 x 10 M cm ) and a v i s i b l e band at 4 -1 -1 618 nm (e = 6.2 x 10 M cm ) (Chapter VII.4). Addition of a nitrogen-coordinating ligand to the s o l u t i o n , i n the absence of a i r (Figure I I I . l ) , r e sulted i n a blue s h i f t of the Soret band to approximately 408 nm, with l i t t l e change i n the peak molar e x t i n c t i o n c o e f f i c i e n t ; the v i s i b l e band decreased to about two t h i r d s of i t s o r i g i n a l i n t e n s i t y , with l i t t l e or no change i n i t s peak p o s i t i o n . Addition of an oxygen-coordinating ligand to the CoOMBP so l u t i o n resulted i n a red Soret s h i f t of about 5 nm and minor v i s i b l e changes. While the changes that occurred upon addition of DMF or DMA were very s i m i l a r , changes upon THF addition were somewhat d i f f e r e n t , p a r t i c u l a r l y with respect to the Soret i n t e n s i t y . Addition of a phosphorus-coordinating ligand (Figure III.2) caused l i t t l e change i n the peak pos i t i o n s and varying decreases i n t h e i r i n t e n s i t i e s . The sp e c t r a l changes associated with the coordination of one ligand are shown i n Table I I I . l . -52-WAVELENGTH, N M . Figure I I I . l . CoOMBP + CH -Im „ '-CH -ImCoOMBP @ 23°C. For raw data see Appendix l b . -54-Table I I I . l . Spectral Data From 350 to 750 nm for the Reaction:  L + CoOMBP — LCoOMBP Absorbing Species Absorption Maxima (nm)^ _^ Isosbestic Points (nm) (Extinction Coefficients, M cm )  CoOMBP 412 (6.4 x 618 - (6.2 x io 4) + Im 407 (6.5 x 104> 385 397 410 618 (4.6 x io 4) 428 612 628 + CH -Im 408 (6.5 x io:> 385 398 411 3 618 (4.7 x io 4) 427 610 628 + Pip 408 (6.5 x 384 398 410 617 (4 .1 x 104) 440 610 630 + Py 407 (5.6 x 382 394 407 618 (4.6 x io 4) 432 609 630 + Et_N 407 (6 .9 x 380 399 410 3 618 (4.7 x io 4) 434 612 628 + DMF 418 (6.0 x io:> 381 416 618 (5.4 x io 4) 610 630 + THF 417 (8.2 x 382 413 616 (4.6 x io 4) 607 634 + PPh, 412 (6.3 x «:> 382 438 3 619 (5.3 x io 4) 610 625 + PBu 412 (-3.5 x 387 446 3 618 (-2.8 x 104) 608 629 Note: As w i l l be explained i n Chapter VII, CoOMBP does not obey Beer's Law as i t undergoes aggregation i n so l u t i o n . From considering t h i s , e are estimated to be 6.4 x 10 M cm and 6.2 x 10 M cm , re s p e c t i v e l y . 618nm -55-Spectral changes to some four-coordinate cobalt porphyrin complexes (e.g. CoPpIXDME), occurring a f t e r DMF i s added, have recently been at t r i b u t e d to small impurities of dimethylamine 1 1^. In the present work, sp e c t r a l changes r e s u l t i n g from DMF addition are d i f f e r e n t from those observed with nitrogen-donors, and are s i m i l a r to those shown f o r the binding of THF. Also the spe c t r a l changes with Et^N were found to be very s i m i l a r to those observed the c y c l i c amines Im, CH^-Im, Pip, and Py. Thus, i t was concluded that changes r e s u l t i n g from DMF addition were due to the binding of that ligand, and not to the coordination of an amine impurity, such as dimethylamine. Oxygen-donors generally bound les s strongly to CoOMBP than nitrogen-donors. DMA bound so weakly that DMACoOMBP could not be f u l l y formed at ambient temperatures, and so no quantitative equilibrium measurements were made for t h i s ligand. I f the ligand additions were c a r e f u l l y done, d i s t i n c t i s o s b e s t i c points could be achieved i n a l l cases, a f t e r allowing, i f necessary, f o r changes from solvent d i l u t i o n . Generally the addition of an a x i a l ligand caused a growth i n s p e c t r a l i n t e n s i t y from 440 to 460 nm, and sometimes the equilibrium analyses were c a r r i e d out using data i n t h i s region. I I I . 1.2 L + LCoOMBP ImCoOMBP, K . Of the ligands studied, CH -Im, Pip, Py, and PBu 3 were found to f u l l y coordinate two molecules to the cobalt centre at ambient temperatures. S o l u b i l i t y reasons prevented the determination of the values f o r the Im system, although i t s values were s i m i l a r to those of the above ligands. The addition of a second nitrogen-coordinating ligand (Figure III.3) -56-a D N V B H O S a v Figure III.3. CH3-ImCoOMBP + P H ^ - T T n (PH^-Tni) ^ PnOMRP @ -23°C For raw data see Appendix I j , f i n a l trace not shown as i s o s b e s t i c points are l o s t due to d i l u t i o n e f f e c t s . -57-caused a Soret band of increased i n t e n s i t y to form i n the 430 nm region. There was l i t t l e change i n the size or p o s i t i o n of the v i s i b l e band. The bi s CH^-Im and Pip complexes had sharp side bands at about 455 nm, while there was no such d i s t i n c t peak f o r the bi s Py system. When the second PBu^ coordinated to the cobalt (Figure I I I . 4 ) , the decreases i n the molar e x t i n c t i o n c o e f f i c i e n t of the sp e c t r a l bands r e s u l t i n g from the coordination of the f i r s t PBu 3 were reversed, and the Soret i n t e n s i t y became greater than that of the four-coordinate complexes. The spe c t r a l changes associated with the coordination of a second ligand are summarized i n Table III.2. Table III.2. Spectral Data From 350 to 750 nm for the Reaction: L + LCoOMBP LCoOMBP Ligand Absorption Maxima (nm)^ ^ Isosbestic Points (nm) (Extinction C o e f f i c i e n t s , M cm ) CH -Im 433 (9.2 x 10 ) 375 422 457 (7.6 x 10p 618 (4.5 x 10 ) Pip 430 (9.4 x 10 4) 355 377 419 456 (4.8 x 10p 602 636 616 (3.8 x 10 ) Py 428 (8.2 x 10*) 372 417 617 (4.3 x 10 ) 606 645 PBu 412 (~8.0 x 10 4) 366 382 403 614 (~5.5 x 10 ) . 424 618 For the three amine ligands, there was a d i s t i n c t i n t e r v a l i n the range of added ligand concentration between complete formation of LCoOMBP and the i n i t i a l formation of LCoOMBP, and so accurate determinations of and K 2 could be achieved. Formation of the b i s PBu^ adduct overlapped -58-33NV8bOS8Y Figure I I I . 4 . PBu 3 + PBu^oOMBP ~ " (PBu 3) 2CoOMBP @ 2 2 C. For raw data see Appendix Im; apparent loss of i s o s b e s t i c points i s due to d i l u t i o n e f f e c t s . -59-with the formation of PBu^CoOMBP, and thus the determination of and was more d i f f i c u l t than when both the i n i t i a l and f i n a l s p e c t r a l values are known, and the equilibrium values are considered approximate. The s p e c t r a l changes for a l l K and determinations were instantaneous upon mixing (even on a stopped-flow time s c a l e ) . In a l l cases the and values were found to decrease with temperature, and since the spectra indicated that less products were present as the solutions were warmed, the r e v e r s i b l e nature of these reactions was demonstrated. III.2 Treatment of Data The equilibrium constant values were obtained by following e i t h e r the increase i n s p e c t r a l i n t e n s i t y i n the 440 to 460 nm region, or by following the changes i n i n t e n s i t y at a peak p o s i t i o n , as aliquots of ligand or ligand s o l u t i o n were added. Generally, the absorbances were read where the absorbance changes were the greatest, and where the s p e c t r a l curves were not too steep. As mentioned, the ligand binding data could sometimes be analyzed over two separate ranges of ligand concentration, corresponding to the binding of the f i r s t and second a x i a l ligands. The formation of a ligand adduct obeys the equation: K l CoOMBP + nL ^ 'ImCoOMBP (III.l) The f r a c t i o n , Y, of L CoOMBP present at a known ligand concentration n was c a l c u l a t e d from the following expression: -60-where A i s the absorbance at a known ligand concentration, corrected for any d i l u t i o n due to ligand addition A^ i s the absorbance observed when no ligand i s present A^ i s the absorbance of the f u l l y formed coordinated complex, again corrected for any d i l u t u i o n . The equilibrium constant, , was determined from a p l o t of A - A log - — _ - ~ vs. l o g f L ] . CO [ L CoOMBP] ^ (A - A ) K = -S = 1 = — 2 ( I I I . 3 ) 1 LCoOMBPlLj" (1 - Y ) [ L ] n (A - A) [ L ] D A - A and log K = log °- - nlog[L] (III.4) J . J\ — A oo where [L] i s the molar concentration of the free ligand studied. From such a p l o t , the slope of the l i n e i s n, the number of ligands binding to the complex. The experimental slopes ranged from 0.8 to 1.2, thus showing that only one ligand did coordinate i n a l l cases. Experimental scatter often occurred at e i t h e r extreme of the p l o t , and where possibl e , A - A the range of — values used was kept from 0.2 to 5.0. The p l o t s of CO A - A log — vs. logf_CH -IrrfJ are shown i n Figure III.5, the intercepts on the A» " A 3 abscissa giving -log K values. The AH and AS values can then be obtained from van't Hoff l n K vs. 1/T p l o t s , as shown in Figures III.6-III.8. The formation of a b i s ligand adduct from a mono ligand adduct obeys the equation: K2 LCoOMBP + nL L CoOMBP (III . 5) n+1 -61-log[CH 3-Im] Figure III.5. Equilibrium p l o t s f o r CH -Im + CoOMBP-— CH -ImCoOMBP Figure I I I . 6 . Van't Hoff p l o t s for L + CoOMBP = = i LCoOMBP L = Im, CH -Im, Pip, and Py Figure III.7. Van't Hoff p l o t s for L + CoOMBP L = Et N, THF, and DMF LCoOMBP -64-Figure III.8. Van't Hoff p l o t for PPh + CoOMBP ^  - PPh CoOMBP. -65-Thus the analysis of the data f o r the formation of a six-coordinate complex from LCoOMBP i s analogous to that of the formation of a f i v e -coordinate complex from CoOMBP. Again, the experimental slopes of the l i n e s A - A of the appropriate log °- vs. l o g [ L j p l o t s ranged from 0.8 to 1.2, Aco — A consistent with the addition of one further ligand . The p l o t s of A - A log vs. logCCH -Im] are shown i n Figure III.9, the intercepts on the Aco —* A 3 abscissa giving -log K,, values. The AH and As values then can be obtained from a van't Hoff p l o t as shown i n Figure III.10. III.3 Sources of Error When performing ligand binding experiments, care must be taken that trace amounts of oxygen do not enter the system. This requirement was s a t i s f i e d f a i r l y e a s i l y when studying the equilibrium, although the Im system was very a i r s e n s i t i v e . Normally, the binding of the second a x i a l ligand was studied using the same solution that had just been used to measure K^. However, the CH^-Im - CoOMBP system became very a i r sensitve upon coordination of the second of the second CH^-Im (see Chapter V I I ) , and b i s adduct formation had to be studied on f r e s h l y made solutions of CoOMBP. S u f f i c i e n t trace oxygen apparently leaked i n t o the system during studies on the mono adduct formation, such that substantial amounts of oxidized complex were formed i f b i s adduct formation were studied subsequently. When ligand binding was investigated i n the more polar solvent, 1,2-dichloroethane, a l l the complexes studied became more oxygen s e n s i t i v e . Ligand binding sometimes had to be studied using several fresh CoOMBP -66--3.0 -2U -1.8 -1.2 -0.6 0 Q6 l og [CH 3 - Im] Figure III.9. Equilibrium p l o t s f or CH -Im + CH^-ImCoOMBP^ (CH^-Im) ^ CoOMBP -67-Figure I I I . 10. Van't Hoff p l o t s for L + LCoOMBP^=^L CoOMBP 2 L = CH -Im, Pip, and Py -68-solutions containing f i x e d concentrations of ligand, i n order to avoid complications a r i s i n g from the oxidation process. Usually the concentration of adduct formed was i n s i g n i f i c a n t compared to the concentration of added free ligand. However, i n a few instances (notably when studying binding of CH^-Im or Pip at lower temperatures) values were so high that a substantial f r a c t i o n of the added ligand coordinated to CoOMBP. In these cases the amount of ligand bound to the metal centre had to be subtracted from the t o t a l amount added i n order to to c a l c u l a t e the concentration of free ligand. For some systems studied, s u b s t a n t i a l amounts of ligand had to be added to f u l l y form the adduct. The r e s u l t i n g high ligand concentration can cause changes i n the nature of the solvent with respect to solvation c h a r a c t e r i s t i c s and d i e l e c t r i c constant. Changes i n d i e l e c t r i c constants became important only when ligand concentration became s u b s t a n t i a l . For example a 0.65 M toluene s o l u t i o n of DMF (the most polar ligand studied) had a d i e l e c t r i c constant of 3.63 compared with f= 2.38 f o r neat toluene. Generally formation of at l e a s t 80% of the l i g a t e d species had occurred at lower ligand concentrations. Almost a l l of the s p e c t r a l data needed to ascertain equilibrium constants were obtained before s u f f i c i e n t ligand was present i n s o l u t i o n to cause s i g n i f i c a n t changes i n the d i e l e c t r i c constant, and since a good analysis of the s p e c t r a l data was generally achieved, any changes i n the d i e l e c t r i c constant and s o l v a t i o n properties did not appear to present a problem f o r the ligand binding studies. Also, as w i l l be shown i n Chapter III.5, changing the solvent from toluene (< = 2.38) to 1,2-dichloro-ethane (« = 10.36) caused only s l i g h t d ifferences i n CoOMBP-ligand binding constants. -69-III.4 Ligand Binding Constants I I I . 4.1 L + CoOMBP ^  1 LCoOMBP , . The equilibrium constants of ligand binding to CoOMBP are given i n Table I I I . l ; the raw data f o r which are given i n Appendices I a - I i . Also l i s t e d i n Table III.3 are the values of AH and AS obtained, and the pK^ of the protonated ligand studied. o The order i n ligand binding a b i l i t i e s observed at 25 C i s : Pip>CH -Im>Py>PBu > Im » PPh » E t N > DMF > THF . ^ 3 ^ 3 3 3 As mentioned i n the introduction, no simple c o r r e l a t i o n between K , A H and A S , and the pK of the protonated ligand was observed i n some 3. CoPpIXDME or CoTPP systems. In the present work, a wider v a r i e t y of ligands has been used, and again no o v e r a l l c o r r e l a t i o n i s observed. For example, Pip, which has a pK^ s i m i l a r to that of Et^N, binds more strongly to 4 CoOMBP by a factor of approximately 10 . Furthermore, Et^N binds about as strongly as DMF, although the pK of the l a t t e r ligand i s 13 units lower cl than that of the former. In general, the nitrogen-donors bound more strongly to CoOMBP, with corresponding higher reaction exothermicities, than the oxygen-and the phosphorus-donors. Within the N-donors, the r e l a t i v e l y weak binding of Et^N i s seen to r e s u l t from a more unfavourable entropy change compared to the c y c l i c amines; t h i s could r e s u l t from a greater loss of freedom of motion upon coordination, although differences i n sol v a t i o n i n the systems could also be important. The s u r p r i s i n g l y high a f f i n i t y of PBu 3 compared to Et N appears to be r e f l e c t e d i n a much less unfavourable entropy change (in f a c t AS ~ 0 f o r the phosphine system), since the exothermicity i s r e l a t i v e l y small. This i s -70-Table III.3. Thermodynamic Data f o r the Binding of the F i r s t A x i a l Ligand to CoOMBP i n a Toluene Solution. LIGAND TEMP. °C. K , M LU1 kcal/mole aSj e .u. PK. Im CH3-Im Pip Py E t 3 N DMF THF PPh. PBu. 4 5 1.6x10* -7.1 -6 22 9.1x10^ 40 3.7x10 -23 8.1x10^ -8.5 -7 0 2.5x10 23 5.5x10 -23 1.1x10^ -7.9 -4 0 3.8x10 26 8.9x10 -23 4.8x10^ -8.0 -6 0 1.3x10 27 2.1x10 5 15.5 -6.8 -19 22 7.4 40 3.8 5 3.6 -3.7 -11 22 2.5 40 1.7 -23 8.7 -5.0 -16 5 4.2 22 2.3 40 1.6 5 2.8x10^ -7.8 -17 22 1.1x10 40 5.6x10 5 3.5x10^ C J -4 rJ 0 22 2.3x10 40 1.5x10 6.95 7.25 11.30 5.27 -2.0 -2.08 2.73" 8.43" -71-Notes to Table I I I . 3. a. Since the number of data points on a p a r t i c u l a r van't Hoff p l o t i s small, error estimates are not very meaningful. Standard deviations were estimated on the basis of le a s t squares analysis for a l l the above data sets. I t was concluded that a and cr were ± 0 . 4 kcal/mole and i 1.5 e.u., re s p e c t i v e l y . As the sc a t t e r associated with the van't Hoff p l o t s f o r CH^-Im and PBu^ were greater than normal, the and the values f o r CH^-Im binding were estimated to be i 0.8 kcal/mole and ± 3 e.u., re s p e c t i v e l y , while the respective errors associated with PBu^ binding were set at i 1.2 kcal/mole and i 6 e.u. b. K. Hoffman, Chem. Heterocycl. Compounds, 6_, 3 (1953). c. K. Sch o f i e l d , "Hetero-Aromatic Nitrogen Compounds," Plenum Press, New York, N.Y.,1967, p. 146. d. A. J . Gordon, R. A. Ford, "The Chemist's Companion, A Handbook of P r a c t i c a l Data, Techniques, and References," J . Wiley, New York, N.Y. , 1972, p. 60. e. J . T. Edwards, H. S. Chang, K. Yates, and R. Stewart, Can. J . Chem., 38, 1518 (1960). f. E. M. Arnett and C. Y. Wu, J . Amer. Chem. S o c , 84, 1684 (1962). g. C. A. S t r e u l i , Anal. Chem., 32^ 985 (1960). consistent with a Co-P bond that i s weaker and longer than a Co-N bond, which again seems consistent with a lower loss of r o t a t i o n a l freedom on coordination of PBu^ compared with Et^N, e s p e c i a l l y i n view of the bul k i e r a l k y l groups i n the phosphorus donor. In the case of PPh^, the Co-P bond appears to be as strong as a Co-N bond, but the r e l a t i v e l y unfavourable entropy change r e s u l t s i n the lower equilibrium constant. This may be due to 93 PPh^ being a strong r-acceptor which could cause the Co-P bond order to be greater than one. This would r e s u l t i n a greater enthalpy of bond formation and a greater loss of r o t a t i o n a l freedom than i f no ir - ir i n t e r a c t i o n s had been present. The oxygen-coordinating ligands are the least e f f e c t i v e i n binding to CoOMBP, and t h i s i s mainly r e f l e c t e d i n lower exothermicities suggesting, for example, that a Co-0 bond i s weaker than a Co-N bond. There a l s o appears to be a somewhat greater loss of entropy associated with binding an oxygen donating ligand. Upon formation of a five-coordinate adduct, the cobalt centre i s , . 88c,88e,90,117 ^ . usually displaced from the centre of the rin g , and t h i s probably r e s u l t s i n the f u l l degree of exothermicity of the reaction • 96c being r e a l i z e d . Since OMBP i s an e s p e c i a l l y strong i r-donating porphyrin , w - r porphyrin-cobalt i n t e r a c t i o n s could serve to "lock" the cobalt centre more f i r m l y i n the porphyrin plane, thus preventing the f u l l release of energy when an a x i a l ligand i s bound. Also the ligand, not being so f i r m l y bound to CoOMBP, would be more free to rotate, r e s u l t i n g i n less loss of entropy. Indeed exothermicities for binding a f i r s t c y c l i c amine to CoOMBP are somewhat less than those reported i n other cobalt(II) porphyrin systems. For example, A H values of -9.7, -9.1,and -9.2 kcal/mole were reported for -73-9 Id Py binding to CoPpIXDME, CoDpIXDME, and CoMpIXDME, r e s p e c t i v e l y CoPpIXDME was reported to bind Im, CH^-Im, Pip, and DMF with respective 89 AH vales of -7.9, -10.7, -10.4 and -7.9 kcal/mole . The AS values associated with Py binding to CoPpIXDME, CoDpIXDME, and CoMpIXDME have 91d been reported to be -18, -16, and -17 eu, r e s p e c t i v e l y , a l l of which are more negative than the AS value for Py binding to CoOMBP. CoPpIXDME has had respective AS values of -10, -19,-17, and -21 eu reported for the binding of Im, CH^-Im, Pip, and DMF. Again these values i n d i c a t e a greater loss of entropy than i s observed when the corresponding ligands bind to CoOMBP. 93 In examining the binding of PBu^ and PPh^ to CoMpIXDME , respective values at 16°C were found to be 1.8 x 10 4 M 1 and 41.8 M 1 , somewhat smaller than i n the CoOMBP system, but r e l a t i v e binding strengths of the ligands to the two porphyrin complexes were comparable. However, the AH value associated with PBu^ binding to CoMpIXDME was reported as -13.3 kcal/mole and the corresponding AS value at -27 eu, while the respective values associated with PPh^ binding were reported to be -7.0 kcal/mole and -17 eu. The thermodynamic parameters for PBu^ binding to CoMpIXDME are s u b s t a n t i a l l y more negative than those for CoOMBP, which i s consistent with the pic t u r e of cobalt being "locked-in" the OMBP plane. However, the reported AH value of PPh^ binding to CoMpIXDME i s si m i l a r to that observed for binding to CoOMBP. This could be due to PPh^ being better able to accept ir-electron density from CoOMBP, thus strengthening the Co-P bond i n PPh^CoOMBP with respect to PPh^CoMpIXDME, even though the cobalt centre may remain "locked i n " the OMBP plane. Ligand binding to CoOMBP w i l l be further compared with ligand binding to other cobalt(II) porphyrin complexes i n Chapter V. -74-III.4.2 L + LCoOMBP: ImCoOMBP, K 2 . The equilibrium constants for a second a x i a l ligand binding to LCoOMBP are given i n Table III.4, together with the Ltt^ a n d A s 2 v a l u e s and the pK values. The raw data are l i s t e d i n Appendices Ij-Im. c l Table I I I . 4 Thermodynamic Data for the Binding of the Second A x i a l Ligand to LCoOMBP i n a Toluene Solution. LIGAND TEMP. °C. V M -1 AH 2  kcal/mole *S2 p K a e .u. CH -Im -23 26 .3 3 0 7 .8 25 1 .7 Pip -23 40 0 10 27 3 .4 Py -23 10 0 2 .2 27 0 .69 PBu 5 1 .4 x io' 3 22 1 .2 x io' 40 0 .9 x 10' For references see Table III .3. -8.5 -7.4 -8.0 rJ -2 -28 7.25 -25 11.30 -28 rJ 0 5.27 8.43 The formation of a six-coordinate cobalt porphyrin complex at ambient temperatures i s of i n t e r e s t as l i t t l e information on the binding of a 89 92b ligand i n the s i x t h coordination s i t e i s available ' . Previous work suggests that the formation of the second Co-N bond i s less exothermic than the formation of the f i r s t . For example, the AH 2 value for Pip binding to 92b CoT(p-MeO)PP was reported to be -1.7 kcal/mole , while the AH^ value was -6.8 kcal/mole. - 7 5 -The present work suggests that for the CoOMBP system the AH values for the formation of both a x i a l ligand bonds are s i m i l a r , and that i t i s a more unfavourable entropy change that leads to the much weaker binding. The entropy d i f f e r e n c e s are much greater than what would r e s u l t from the usual s t a t i s t i c a l f a c t o r ; i . e . the difference r e s u l t i n g from a ligand being able to bind to two equivalent s i t e s rather than one. 92b I t should be noted that Walker determined thermodynamic parameters of the second l i g a t i o n by the i n d i r e c t means of studying the dioxygen binding a b i l i t i e s of LCoT(p-MeO)PP and L^CoT(p-MeO)PP separately at low temperatures, and assigning the differences i n those values to the value of the binding of the second ligand, as the second a x i a l ligand bound so weakly that f u l l formation of the six-coordinate adduct could not be observed at ambient temperatures. Since the K 2 value for the Pip-CoT(p-MeO)PP system was reported to be 4.0 M 1 at - 6 5 ° C 9 2 b , a high concentration of ligand would be required f o r complete binding, and the toluene solvent would then take on some of the c h a r a c t e r i s t i c s the more polar ligand. Thus, when dioxygen binding i s studied on the six-coordinate species, the oxygen a f f i n i t y i s . 80b l i k e l y to be increased r e l a t i v e to that of the five-coordinate species , which i s maintained using low ligand concentrations (^10 3 M), meaning that the solvent i s e s s e n t i a l l y pure toluene. This could mean exothermicities of second ligand binding estimated by the simple Hess' Law treatment are too low. The observed s i m i l a r i t y of the AH values for the binding of the f i f t h and s i x t h ligands i n the present work i s i n t e r e s t i n g . Normally when the f i r s t a x i a l ligand coordinates, the metal i s l i f t e d out of the plane of the porphyrin r i n g towards the ligand, while addition of a second i d e n t i c a l 118 ligand brings the metal back i n t o the porphyrin plane . Presumably, t h i s -76-action could weaken, to some extent, the f i r s t c obalt-ligand bond, and the observed would be that of the second ligand binding minus the bond weakening of the f i r s t ligand. Although placing the cobalt i n t o the plane of the porphyrin should increase the bond strength of the porphyrin N-Co bonds, t h i s e f f e c t should not be as great as the change i n bond strength to the a x i a l ligand, or the metal would not have moved from the plane upon i n i t i a l l i g a t i o n . Strong Tr -donating properties of OMBP could mean that the metal i s very near the plane of the porphyrin r i n g i n CoOMBP so that the second ligand binding could release as much enthalpy as the f i r s t . However, some displacement from the porphyrin r i n g i s probably necessary to explain the greater loss of entropy upon the second l i g a t i o n . III.5 Ligand Binding i n 1,2-Dichloroethane Large solvent e f f e c t s f o r dioxygen binding, and, to some extent, ligand 91b 119 binding have been reported f o r systems inv o l v i n g metal porphyrins ' Since i t has been established that the nature of the solvent plays a major 91b ro l e i n determining the oxygen a f f i n i t y of a cobalt porphyrin complex , and since on numerous occasions large concentrations of ligand were required to f u l l y form an adduct, a study on ligand binding i n an alternate solvent was c a r r i e d out. A solvent sometimes used i n metalloporphyrin 52b studies i s dichloromethane . However, clean spectra of CoOMBP i n dichloromethane could not be r e l i a b l y achieved, regardless of the method used for p u r i f i c a t i o n , and so 1,2-dichloroethane (f = 10.36) was used as an a l t e r n a t i v e to toluene (« = 2.38). -77-The ligand binding reactions that took place i n toluene also occurred i n DCE, and the binding of the f i r s t ligand r e s u l t s are summarized i n Table I I I . 5 , with the raw data l i s t e d i n Appendices I.n-I.q. Table III.5 Thermodynamic Data for the Binding of the F i r s t A x i a l  Ligand to CoOMBP i n a DCE Solution. LIGAND Temp., °C K # M -1 A H 1 kcal/mole A S ] e.u. CH -Im 0 7.7 x 10 3 5 7.4 x 10 20 3.4 x 10 40 1.5 x 10 * Pip 0 1.0 x 10 5 9.8 x 10 20 5.3 x 10 40 2.7 x 10 * Py 0 5.3 x 10 5 3.4 x 10 20 1.6 x 10 40 7.5 x 10 THF 5 1.5 20 1.2 40 0.71 -7.5 -7.0 -7.5 -4.7 -5 -16 7.25 -3 11.30 5.27 -2 .08 * Values for 0 C are calculated from the data i n the table for ease of comparison with the binding data i n toluene. a. For references see Table III.3. or was +0.8 kcal/mole and o^g was i 3 e.u. On the whole, the eguilibrium constants i n DCE were s l i g h t l y lower than i n toluene. Generally, more polar solvents w i l l tend to s t a b i l i z e more polar species. In the l i g a t i o n reaction, CoOMBP could be pictured as an e s s e n t i a l l y non polar complex that binds with ligands of varying p o l a r i t y . When the adduct i s formed, any ligand p o l a r i t y i s l i k e l y to be dispersed over the -78-whole of the adduct, and thus a polar solvent i s more l i k e l y to s t a b i l i z e the free ligand and should tend to d e s t a b i l i z e the adduct. Q u a l i t a t i v e l y such a p i c t u r e accounts for the above trend. Although quantitative studies on the reaction, L + LCoOMBP ~—** L^CoOMBP i n DCE, could not be c a r r i e d out due to a i r s e n s i t i v i t y , the r e l a t i v e binding a b i l i t i e s were observed to be Pip (K 2 @ 20°C ^4 M 1) >CH3~Im (K 2 @ 20°C ^1.7 M 1 ) > P y (K 2 @ 20°C ^0.8 M "S , the same as for the mono adduct formation, and the same as observed for K and K i n toluene. In a l l cases, 1 2 the binding of the second ligand was weaker than measured i n toluene, and again t h i s could be r a t i o n a l i z e d i n terms of differences i n s o l v a t i o n of the free ligand. -79-CHAPTER IV THERMODYNAMICS OF REVERSIBLE DIOXYGEN BINDING TO CoOMBP COMPLEXES IV.1 Q u a l i t a t i v e S p e c t r a l Observations IV. 1.1 LCoOMBP + O LCoOMBP (O ), K 2 2 O^ Upon exposure to oxygen gas at low temperatures, solutions of LCoOMBP complexes exhibited a change i n v i s i b l e spectrum which was a function of the oxygen pressure and which was r e v e r s i b l e ; i . e . pumping o f f the oxygen gas and/or warming the so l u t i o n restored the spectrum of LCoOMBP. The sp e c t r a l changes that occurred when a s o l u t i o n of CH^-ImCoOMBP was exposed to oxygen are shown i n Figure IV.1. This type of s p e c t r a l change had been 89 previously assigned to the formation of a 1:1 cobalt:dioxygen adduct . Of the complexes studied, only the THF adduct showed no signs of dioxygen binding at low temperature, although the PPh 3 adduct exhibited a very weak tendency to bind dioxygen. No quantitative work was undertaken on the oxygenation of ImCoOMBP, as the complex i r r e v e r s i b l y oxidized very r e a d i l y , even at low temperatures. In the case of the nitrogen-donating ligands, the Soret band s h i f t e d 4 -1 -1 to about 438 ran (f ^ 6 x 10 M cm ) and the v i s i b l e band s h i f t e d to 4 624 nm (( ~ 4 x 10 ). When the DMF adduct bound dioxygen, a new Soret band 4 -1 -1 was observed at 434 nm (6 = 5.8 x 10 M cm ), and the v i s i b l e band 4 -1 -1 s h i f t e d to 619 nm (6 = 4.5 x 10 M cm ). When oxygen was added to the -80--81-CoOMBP-PBu3 system, the v i s i b l e band s h i f t e d to 632 nm (t ^ 3 x 10 M cm ) and the Soret band to 443 nm (« rJ 2 x 10 M cm ) . As the PPh^ adduct bound dioxygen so weakly, no q u a n t i t a t i v e work could be undertaken. However, the v i s i b l e band did diminish and red s h i f t s l i g h t l y and a decrease i n the i n i t i a l Soret band was accompanied by a s p e c t r a l growth i n the 450 nm region, but peak p o s i t i o n s of the dioxygen adduct could not be assigned, since i t was estimated that no more than 20% of the adduct was formed under any of the conditions used. The s p e c t r a l changes associated with the dioxygen adduct formation are l i s t e d i n Table IV.la. Evidence f o r formation of the 1:1 cobalt:dioxygen complex was corroborated by ESR measurements i n toluene at 77 K (Figure IV.2), which recorded spectra t y p i c a l of such adducts^ 2 a' 1 2 <" >. Although some 1:1 cobalt:dioxygen complex could be formed at ambient temperatures (e.g. i n the c y c l i c amine systems), the rate of formation was too great to be measured by stopped-flow techniques. IV.1.2 LCoOMBP + 0^ * LCoOMBP ( 0 ^ + L, KQ The six-coordinate complexes (CH^-Im)2CoOMBP, PipCoOMBP, and PyCoOMBP a l l reacted with dioxygen to some extent under c e r t a i n conditions. The spectra generated from the b i s c y c l i c amine complexes were those of the LCoOMBP(02) complexes, as shown i n Fgure IV.3. When oxygen was pumped o f f the system, the v i s i b l e s p e c t r a l changes were reversed i n a l l cases. Upon addition of oxygen to LCoOMBP, ESR signals of LCoOMBP (02> were generated. *For a discussion of the species present i n the Co0MBP-PBu3 system when oxygenation occurs, see Chapter IV.2. Table I V . l a . Spectral Data From 350 to 750 nm for the Reaction: LCoOMBP + 0 - =- LCoOMBP (0 ) 7 2 Absorbing Species Absorption Maxima (nm) Isosbestic Points (nm) -1 -1, (Extinction C o e f f i c i e n t , M cm ) CH -ImCoOMBP 408 (6.5 4 x 10 ) 618 (4.7 4 x 10 ) 3 4 4 (5 -,,-A 422 620 CH -ImCoOMBP (OJ 438 (7 x 10 ) 624 x 10 ) 3 2 4 (4.1 PipCoOMBP 1 408 (6.5 x 10 ) 618 x 10 ) A 421 619 * PipCoOMBP (OJ 438 (7 x 104) 624 (5 x 10 ) PyCoOMBP 1 407 (5.6 4 x 10 ) 618 (4.6 4 x 10 ) A 421 619 * PyCoOMBP (OJ 438 (7 x 104) 624 (5 x 10 ) Et NCoOMBP 407 (6.9 4 x 10 ) 618 (4.7 4 x 10 ) 3 I Et NCoOMBP(0) 3 2 A 4 419 620 437 (6.5 x 10 ) 624 (5.4 x 10 ) DMFCoOMBP 418 (6.0 4 x 10 ) 618 (5.4 4 x 10 ) I 4 425 622 DMFCoOMBP(0) 434 (7.0 x 10 ) 619 (6.5 x 10 ) PPh CoOMBP 412 (6.3 4 x 10 ) 619 (5.3 4 x 10 ) 3 I A 422 626 r EPh3CoOMBP(02) 450 (~6 x 10 4) 620 (-5 x 10 ) PBu CoOMBP 412 (3.5 x 10 4) 618 ( 2. 4 8 x 10 ) 3 1 424 626 1 PBu CoOMBP(0 ) 443 (3.5 x 104) 632 (3.5 4 x 10 ) 3 2 • -84-30NVQc)0S9V Figure IV.3. Spectra of oxygen binding to (CH3-Im)2CoOMBP @ -45° c . For raw data see Appendix H i . -85-Once the (PBu^)2CoOMBP complex was f u l l y formed i n toluene, no oxygen a f f i n i t y was observed. The s p e c t r a l changes associated with oxygenation of ImCoOMBP are l i s t e d i n Table IV.lb_. Table IV.lb. Spectral Data From 350 to 750 nm for the Reaction: LCoOMBP + 0„ LCoOMBP(0 ) + L Absorbing Species Absorption Maxima (nm) . . . -1 -1, (Extinction C o e f f i c i e n t , M cm ) Isosbestic Points (nm) (CH -Im) 2CoOMBP CH3-ImCoOMBP(02) Pip2CoOMBP PipCoOMBP(0 ) Py CoOMBP I PyCoOMBP(0 ) 435 (9.2 x 10 4) 457 (7.6 x 10 4) 618 (4.5 x 10 4) 4 4 438 (7 x 10 ) 624 (5 x 10 ) 430 (9.4 x 10 4) 456 (4.8 x 1Q4) 616 (3.8 x 10 ) 4 4 428 (8.2 x 10 ) 617 (4.3 x 10 ) 4 4 438 (7 x 10 ) 624 (5 x 10 ) 448 442 IV.2 Species Present When the CoOMBP-PBu^ System i s Oxygenated At ambient temperatures there was a considerable degree of overlap i n forming the mono and b i s PBu^ adducts of CoOMBP, and the band posi t i o n s of the two species were quite s i m i l a r . In preparation of samples f o r oxygenation, some of the CoOMBP often d i d not go i n t o s o l u t i o n since i t adhered to the walls of the quartz c e l l used f o r low temperature work, so the observed e x t i n c t i o n c o e f f i c i e n t s could not determine which of the two species was present i n s o l u t i o n . -86-Since AH f o r the five-coordinate complex was estimated to be somewhat larger than that f o r formation of the six-coordinate complex (see Chapter I I I . 4 ) , the increase i n the value, as lower temperatures are used, would be greater than the increase i n the value. Thus e s s e n t i a l l y f u l l formation of the five-coordinate complex with l i t t l e formation of the six-coordinate complex should be observed when an appropriate concentration o -6 of PBu^ i s used. As shown i n Table IV.2, at -63.5 C a 2.4 x 10 M solution of CoOMBP was observed to be more oxygen s e n s i t i v e as the concentration of PBu^ was increased to 1 x 10 5M. The CoOMBP-PBu^ system was e s s e n t i a l l y -5 -4 constant with respect to oxygen s e n s i t i v i t y f o r 1 x 10 -^PBu^<10 M, and then the system became less oxygen s e n s i t i v e a f t e r the concentration of PBu^ -4 exceeded 10 M.  Table IV.2 Oxygen S e n s i t i v i t y of CoOMBP as a Function of PBu^ at -63.5°C. PBU 3 M P^0 2, Torr 1.01 X io"6 589 4.04 X io"6 184 1.01 X io"5 123 4.04 X io"5 111 1.01 X io" 4 118 4.04 X io" 4 158 4.04 X io"3 1' co II For raw data see Appendix H h . The observed behavior i s consistent with f u l l formation of the f i v e -coordinate complex at lower PBu^ concentrations before a s i g n i f i c a n t amount of the six-coordinate complex occurs. When extra ligand i s added, some s i x --87-coordinate complex i s formed, and t h i s explains the observed decrease i n oxygen a f f i n i t y at elevated ligand concentrations. IV.3 Treatment of Data The equilibrium constants f o r binding were obtained by following ei t h e r the increase i n s p e c t r a l i n t e n s i t y of the Soret band of the 1:1 cobalt:dioxygen complex, or by following the decrease i n s p e c t r a l i n t e n s i t y of the Soret band of the non-oxygenated complex. The formation of a 1:1 cobalt:dioxygen complex from a mono ligand adduct obeys the equation, LCoOMBP + nO, . —1 LCoOMBP(0J (IV.1) 2 2 n and thus occurs by a process of addition that i s analogous to the formation of a ligand adduct complex. The data, therefore, can be treated i n a manner s i m i l a r to that described i n Chapter I I I . A - A The experimental slopes of the li n e s of the appropriate log — Aoo — A vs. log P pl o t s ranged from 0.8 to 1.2 (Figure IV.4), consistent with the °2 addition of one dioxygen molecule t o a five-coordinate complex. Also, where A - A possible , ^ values were kept i n the 0.2 to 5.0 range. Van't Hoff p l o t s A - A for the K values obtained from the pl o t s of log vs. log P are 0„ _ " 2 o values obtained from the pl o t s of log 2^ " D O *~ shown i n Figure IV.5. The formation of a 1:1 cobalt:dioxygen complex from a six-coordinate complex b i s ligand complex obeys the o v e r a l l s u b s t i t u t i o n reaction, \„,. L CoOMBP + nO ' - LCoOMBP(Oj + L (IV.2) 2 2 2 n -89--90-and, therefore, a d i f f e r e n t treatment of data i s necessary. The f r a c t i o n , Y, of LCoOMBP(0 ) present at a known p a r t i a l pressure 2 n was calculated from the following expression, A - A Y = 2. (IV.3a) A - A oo 0 while the corresponding f r a c t i o n , 1-Y, of L^CoOMBP present was given by A m - A 1 - Y = - — — - (IV. 3b) A oo ~~ &Q where A i s the absorbance measured at a known p a r t i a l pressure of oxygen A i s the absorbance measured when no- oxygen i s present o and AM i s the absorbance calculated f or the f u l l y oxygenated complex, based on data f o r oxygenation of LCoOMBP. The equilibrium constant for such a reaction has the expression: [LCoOMBP ( 0 ) ] [ L J Y[CoJ [L] K = — = (IV.4) °2/L [LCoOMBP] ( P Q ) " (1-Y) [ C o ] T ( P 0 ) n where [Co] i s the t o t a l concentration of cobalt present. The value for LLJ w i l l be equal to the amount r e s u l t i n g from the d i s s o c i a t i o n of LCoOMBP, or Y[Co] T, plus the i n i t i a l concentration of free L; i . e . , Y[Co] (Y[Co] + [ L ] ) K = ^ (IV.5) °2/L (1 - Y ) [ C o ] T (P Q ) n 2 where [L]. i s the i n i t i a l concentration of free L present i n s o l u t i o n . In the case of the b i s c y c l i c amine complexes, the amount of added ligand present was so great that the Y [ C O ] t term was i n s i g n i f i c a n t , and so -91-Y[L], K = (IV.6) U 2 / L (1-Y)(P ) 2 and Y A - A l o g V / T / T i = l o g r ^ r - n l o g V = l o g ^ ^ i - l o g V (IV-7) */wm i 2 2 As the oxygen a f f i n i t i e s of these complexes were r e l a t i v e l y low, few quantitative experiments could be done. However, i n the case of the (CH^-Im)2CoOMBP system at -45°C, close to 50% oxygenation was achieved and A - A the p l o t of log — vs. log P had a slope of 1.1 (Figure IV.6). A^ - A 0 2 As the f i n a l spectrum of LCoOMBP (o^) was not achieved i n any LCoOMBP system, quantitative thermodynamic studies were not undertaken. IV.4 Sources of Erro r When performing oxygenation experiments on LCoOMBP systems, i t i s important that the s t a r t i n g concentration of LCoOMBP be as high as po s s i b l e . From the measured thermodynamic parameters l i s t e d i n Table I I I . l , i t was possible to adjust the concentration of ligand so that at l e a s t 95% of the cobalt was i n i t i a l l y present as LCoOMBP, with n e g l i g i b l e amounts of CoOMBP and L^CoOMBP. In the case of the c y c l i c amines, the amount of ligand required to f u l l y form LCoOMBP was so small that s o l v a t i o n e f f e c t s due to changes i n the medium (toluene/ligand) were considered to be n e g l i g i b l e . In the case of ligands, such as DMF and THF, which bind weakly to CoOMBP, the amount of ligand required to f u l l y form the five-coordinate complex i s r e l a t i v e l y large (ligand concentration several molar i n toluene), and solvation e f f e c t s i n these cases must be considered. For these systems, the concentration of ligand was kept constant f o r oxygenation experiments at a l l temperatures, such that the five-coordinate complex was f u l l y formed under a l l conditions used. There was no evidence of formation of b i s ligand adduct i n these systems. S i m i l a r l y , i n studying the L^CoOMBP complexes, the ligand-solvent composition was kept constant with s u f f i c i e n t ligand present to f u l l y form ImCoOMBP. A minimum of ligand was added so that sol v a t i o n e f f e c t s would be minimized, and to prevent ligand p r e c i p i t a t i o n on lowering the temperature. E r r o r i n oxygenation measurements occurred i f any binuclear peroxo-bridged oxidation products were formed. Spectral evidence (an increase of ex t i n c t i o n c o e f f i c i e n t i n the 440 nm region at constant oxygen pressure) indicated that t h i s problem was s u f f i c i e n t i n the ImCoOMBP system to prevent oxygenation measurements. Similar observations were made during some of the higher temperature (-23°C) oxygenation experiments, e s p e c i a l l y i f 1,2-dichloroethane were part of the solvent system. In most cases, however, formation of the oxidation product was not a A - A problem. This was evidenced by: the appropriate p l o t s of log - — _ ~ v s -log P which had slopes i n d i c a t i v e of 1:1 cobalt.-dioxygen; the good 2 i s o s b e s t i c points that were present when oxygenation was monitored; the t o t a l l y r e v e r s i b l e nature of the sp e c t r a l changes; and by a strong accompanying ESR si g n a l that was t y p i c a l of 1:1 cobalt:dioxygen complexes. IV.5 Dioxygen Binding Constants IV. 5.1 LCoOMBP + 0 2 ~ LCoOMBP (0^) , K Q . The P, O values of the five-coordinate LCoOMBP complexes are given i n h 2 Table IV.3. Also l i s t e d are the estimated values of AH and AS, and the pK a -94-Table IV. 3. Thermodynaniic Data for the Binding of Dioxygen to LCoOMBP in a Toluene Solution. LIGAND TEMP., °C. Torr A H , kcal/mole A S , e.u. PK_ Im CH3-Im irreversible oxidation Pip Py Et,N 3 DMF THF PPh. PBu, -56.5 -45 -31 -63.5 -56.5 -45 -63.5 -56.5 -45 -78 -63.5 -56.5 -45 -23 -78 -63.5 -56.5 -45 -23 -63.5 -63.5 -45 -23 62 98 198 104 184 269 91 141 240 54 129 200 295 1371 8.5 25 91 389 2700 -5.8 -4.9 -5.1 -5.1 -12.2 no reaction 4000 123 204 398 -3.0 -32 -32 -33 -34 -65 6.95 7.25 11.30 5.27 11.01 -2.0 -2.08 2.73 -24 8.43 # and g- values were estimated to be ± 0.5 kcal/mole and * 2 e.u, respectively. b. For references see Table III.3. c. For raw data see Appendix Ila - Appendix Ilg. -95-of the ligand studied. The order of the oxygen a f f i n i t i e s f o r the LCoOMBP complexes as a function of a x i a l ligand at -63.5°C with toluene as a solvent i s : DMF^ CH3-Im>Py~Pip>Et3N>PBu3 S>VPh^ THF. No simple c o r r e l a t i o n e x i s t s between , AHQ , , and the pK & of the protonated ligand. Indeed the A H q and A S Q values associated with a l l of the nitrogen-donor systems are very s i m i l a r , while the DMF complex binds dioxygen more exothermically and with a le s s favourable entropy change. DMF and THF, which have s i m i l a r pK and K values (Table I I I . 3 ) , behave very d i f f e r e n t l y with respect to promoting oxygen a f f i n i t y of CoOMBP. The THF complex does not bind dioxygen at a l l , while the DMF complex binds dioxygen the most exothermically of a l l the systems studied, in c l u d i n g the much more basic c y c l i c amine complexes, which also have values several orders of magnitude greater than the DMF complex (Table III.3) . I f oxygenation enhancement of LCoOMBP occurred s t r i c t l y by ff-electron donation from the a x i a l ligand to the metal centre, which i n turn could more r e a d i l y t r a n s f e r e l e c t r o n density to dioxygen, and s t a b i l i z e a Co(III)-0 species, then the DMF complex, with i t s low K and pK values, 2 l a would be expected to have a very low oxygen a f f i n i t y . Possibly oxygenation enhancement can also occur through T r-electron density t r a n s f e r from the ligand. (The p o s s i b i l i t y of oxygenation enhancement from increased solvent p o l a r i t y i s discussed i n Section 6.) An i n f r a r e d study of LMo(CO) 5 complexes has suggested that, compared 121 to Py and Pip, DMF i s a very strong r-donating ligand . Since OMBP i s 96c also a strong ir-donor , i t i s possible that the ir-donatmg properties of DMF, coupled with those of OMBP, could be responsible f o r the observed -96-oxygen binding r e s u l t s . Based on the above considerations, s i m i l a r behavior was noted i n the LCoPpIXDME systems, as the DMF adduct bound dioxygen better than the Py adduct, but not as w e l l as the CH3~Im adduct. However, the observed thermodynamic parameters i n the DMF system were s i m i l a r to the 89 other ligand adducts of CoPpIXDME . In contrast to t h i s evidence of TT-electron e f f e c t s , other work 7 0' 9 1* 3 suggests that ligand r-donor a b i l i t y i s not important i n dioxygen binding, and that other ligand c h a r a c t e r i s t i c s , such as hydrogen bonding a b i l i t y or stereochemistry are factors a f f e c t i n g oxygen a f f i n i t y . From work i n t h i s t h e s i s , i n a manner al s o observed i n the LCoPpIXDME 89 systems , the CH^-Im complex i s seen to bind dioxygen more strongly than the Pip (or Py) complex, although CH3-Im i s a weaker base than Pip. Mossbauer studies on i r o n porphyrins ind i c a t e that CH3~Im i s a stronger 12 2 r-donor than e i t h e r Pip or Py , and so again the importance of the i r-donating e f f e c t s i s evident. That the PPh 3 complex binds dioxygen very weakly i s consistent with i t s low b a s i c i t y . Since PPh 3 i s a T-acceptor, oxygenation could be s u b s t a n t i a l l y i n h i b i t e d , even though the £H^ value i n the PPh 3 system approached those of the nitrogen-donor systems (Table III.3) . THF i s the weakest base of any of the systems studied, with a very low value, and the i n a b i l i t y of t h i s complex to bind dioxygen can be explained by t h i s property. S i m i l a r l y , ir-bonding e f f e c t s do not need to be invoked to understand the oxygen a f f i n i t y of the PBu 3 system. Studies with other phosphine ligand systems should probably be undertaken before more precise assignments of the importance of ir-donor and a-donor properties of these ligands are made. -97-IV.5.2 LCoOMBP + 0 ^ LCoOMBP (O ) + L, K _ _ °2/L The P 3 5 ° 2 values for LCoOMBP at -45°C, i n a solu t i o n of 95% by volume of toluene and 5% by volume of ligand, were as follows; 950 Torr f o r L = CH^-Im and about 4000 Torr for L = Py. No oxygen a f f i n i t y was detected for Pip CoOMBP under these conditions I. 0 2 K2/L The reaction being studied, LCoOMBP + 0 2 . LCoOMBP ( O j + L, can be thought of as a combination of two other reactions that have been — v "2 studied separately, LCoOMBP + L , L CoOMBP and K ° 2 . LCoOMBP + 0 2 •• LCoOMBP (0 ) . The equilibrium constants can be combined to give: Since P. 0 values for the f i v e - and six-coordinate complexes are H 2 given by 1/K and L/K , re s p e c t i v e l y , Equation IV.8 y i e l d s 2 2/L P O = P 0 -K "L (IV.9) h 2(six-coordinate complex) h 2(five-coordinate complex) 2 The p^°2 values f o r the oxygenation of the six-coordinate complexes, as calculated from Eq. IV.9, are l i s t e d i n Table IV.4. From these considerations, the six-coordinate complexes are expected to ex h i b i t a very weak oxygen a f f i n i t y i n the order Py>CH3~Im>Pip. The experimental data agree well f o r the Py and Pip systems, but with the CH^-Im complex the oxygen a f f i n i t y i s an order of magnitude greater than expected, and the observed oxygen a f f i n i t i e s are i n the order CH3~Im>Py>Pip. Solvent e f f e c t s may play a r o l e . In order to ensure f u l l -98-Table IV.4. Expected P^° 2 Values (Torr) for ImCoOMBP Complexes at -45~C. LIGAND P, 0„... J X K , M _ 1 Expected P. 0_ . _ h 2(five-coord) 2 * h 2(six-coord) (cone. , M )  CH3~Im (0.628) 98 150 11,000 Pip (0.505) 270 170 24,000 Py (0.671) 141 48 4,200 formation of the six-coordinate adduct for oxygenation studies, the ligand constituted 5% of the total solution volume. Under these circumstances the solvent takes on some of the characteristics of the ligand used. For example dielectric constants at 22°C were measured to be 2.54, 2.89, and 3.71 for the Pip, Py, and CH^-Im systems, respectively, while for neat toluene the value was 2.38. The CH^Im system i s the most polar, and i t is possible that the enhanced oxygenation results from stabilization of the Co(III)-O species. Enhanced oxygen a f f i n i t y has been previously reported as the medium becomes more p o l a r ^ l a , C . These observations suggest that the nature of the solvent might play a major role in determining oxygen a f f i n i t y , and care must be taken in the interpretation of data from studies where large concentrations of ligand are required. IV.6 Oxygenation Reaction in Solvent Systems Other Than Neat Toluene. I n i t i a l studies in more polar solvent systems were done using a 4:1 by volume ratio of toluene ( t = 2.38) and 1,2-dichloroethane ( <= 10.36). Studies in neat DCE were not feasible, as this solvent freezes at -35°C. The 4:1 toluene:DCE solvent system has a dielectric constant of 3.41. -99-A l t h o u g h t h e o b s e r v e d t h e r m o d y n a m i c p a r a m e t e r s i n t h e m i x e d s o l v e n t s y s t e m w e r e n o t s u b s t a n t i a l l y d i f f e r e n t f r o m t h o s e o b s e r v e d i n n e a t t o l u e n e ( i . e . f o r t h e CH^ImCoOMBP + 0^ . C H ^ I m C o O M B P ( O J r e a c t i o n i n t h e m i x e d s o l v e n t s y s t e m t h e AH a n d AS v a l u e s w e r e -5.3 k c a l / m o l e a n d -31 e . u . , r e s p e c t i v e l y , c f . T a b l e I V . 3 ) , t h e P.O v a l u e s w e r e r e d u c e d b y a f a c t o r A 2 o f a b o u t t h r e e ( e . g . t h e P^° 2 o f CH^-ImCoOMBP a t -45°C was 98 T o r r i n n e a t t o l u e n e a n d was 36 T o r r i n t h e m i x e d s o l v e n t s y s t e m ; see A p p e n d i x I I j ) . A l t h o u g h a n i n c r e a s e i n d i e l e c t r i c c o n s t a n t u p o n DCE a d d i t i o n c a u s e d a n o t i c e a b l e i n c r e a s e i n o x y g e n a f f i n i t y i n t h e s e f a i r l y n o n p o l a r m e d i a , q u a n t i t a t i v e w o r k was n o t done w i t h h i g h e r c o n c e n t r a t i o n s o f D C E , as DCE t e n d e d t o " f r e e z e o u t " a n d i r r e v e r s i b l e o x i d a t i o n was j u d g e d t o be o c c u r r i n g , b a s e d on o b s e r v e d i n c r e a s i n g s p e c t r a l i n t e n s i t y a t 440 nm u n d e r c o n s t a n t o x y g e n p r e s s u r e . I n t h e c a s e o f t h e s i x - c o o r d i n a t e C o O M B P - c y c l i c amine c o m p l e x e s , when t h e s o l v e n t s y s t e m was c h a n g e d f r o m 5% l i g a n d a n d 95% t o l u e n e b y v o l u m e t o 5% l i g a n d , 20% D C E , a n d 75% t o l u e n e b y v o l u m e , some c h a n g e s i n o x y g e n a f f i n i t y w e r e o b s e r v e d . The P,0 v a l u e a t -45°C f o r t h e CH - I m s y s t e m "5 Z 3 (t = 4.68) was r e d u c e d f r o m 950 T o r r t o 500 T o r r ; f o r t h e P y s y s t e m (t = 3.86) t h e o x y g e n a f f i n i t y was i n c r e a s e d (P^0 2 - 2500 T o r r ) , b u t s t i l l no a c c u r a t e q u a n t i t a t i v e r e s u l t s c o u l d be a c h i e v e d ; f o r t h e P i p s y s t e m (• = 3.53) t h e r e was s t i l l no o x y g e n a t i o n e v i d e n t . H o w e v e r , when t h e s o l v e n t s y s t e m was c h a n g e d t o a r a t i o o f 1:1 b y v o l u m e o f D C E : P i p (« = 7.4) , t h e n t h e six c o o r d i n a t e PipCoOMBP e x h i b i t e d some o x y g e n a f f i n i t y . A g a i n , due t o t h e l o w d e g r e e o f o x y g e n a f f i n i t y , a n d a t e n d e n c y f o r i r r e v e r s i b l e o x i d a t i o n t o o c c u r , no q u a n t i t a t i v e m e a s u r e m e n t s w e r e made a t h i g h e r DCE c o n c e n t r a t i o n s . Raw d a t a a r e p r e s e n t e d i n A p p e n d i x I l k . -100-In studying the oxygenation reaction of DMFCoOMBP, i t was necessary to use substantial amounts of DMF to obtain the f u l l y formed DMFCoOMBP adduct. The 1:1 volume to volume ratio of toluene to DMF used had a measured dielectric constant of about 18. To check how much of the enhanced oxygen af f i n i t y , reflected in the greater exothermicity (Table IV.3) of the DMFCoOMBP system, was due to a solvent effect, and how much was due to the effect of DMF as a ligand, studies were carried out on the cyclic amine complexes in the 1:1 toluene:DMF system. The predominant species in solution was the CoOMBP-cyclic amine complex, as verified by the Soret peak position of the visible spectrum. The results of these studies are shown in Table IV.5.  Table IV.5. Thermodynamic Data for the Binding of Dioxygen to LCoOMBP in a 1:1 by Volume Ratio of Toluene to DMF. LIGAND TEMP., P J 5 ° 2 ' AH, AS, °C. Torr kcal/mole e.u. CH -Im -56.5 6.8 -7.1 -37 3 -45 12 -31 44 -23 63 Pip -56.5 15 -6.9 -37 -45 29 -31 93 -23 145 Py -56.5 42 -6.7 -38 -45 66 -31 151 -23 457 For raw data see Appendix III - Appendix Iln. -101-In comparison to neat toluene as solvent (Table IV.3), the mixed toluene-DMF system enhances the oxygen a f f i n i t y of the CoOMBP-cyclic amine complexes by about an order of magnitude and this i s seen to be due to a more favourable AH term, which seems reasonable i f the polar LCo(III)-0 2 species i s being stabilized. The AS values in the two media are not very different, suggesting any greater solvation of the Co(III)-C>2 product i s balanced by a similar increased solvation of the reactants. The above trend in observed thermodynamic parameters is similar to that reported for 91c oxygenation of CoPpIXDME, CoDpIXDME, and CoMpIXDME systems From these results i t can be seen that the oxygen sensitivity of the DMFCoOMBP complex was in part due to the effect of DMF as ligand and also due to the solvent properties of DMF. In the mixed solvent system, a l l the cyclic amine complexes became more oxygen sensitive than the DMF adduct, at the temperatures studied. However, oxygenation enhancement due to DMF as ligand was shown by the observation that the DMF system had the highest degree of exothermicity of a l l the complexes studied, even in the mixed solvent system. Oxygenation studies were then carried out at -56.6°C on the PipCoOMBP system containing varying amounts of toluene and DMF as the solvent system (Table IV.6). The results show that the oxygenation enhancement is not as great as observed in the mixed toluene-DCE solvent system, for a corresponding change in dielectric constant. Also, a l l the reactions in the toluene-DMF solvent systems were completely reversible. Thus, the dielectric constant i s not the only solvent characteristic to play a role in adjusting oxygen sensitivity of the various CoOMBP complexes. It should be noted that DMF i s a weak base, while DCE could, to some small extent, yield acidic protons . This was confirmed when PipHCl crystals were observed -102-Table IV.6. Oxygen A f f i n i t i e s at -56.6°C of PipCoOMBP i n a V a r i e t y of Mixed Toluene-DMF Solvent Systems. % BY VOLUME OF DMF AS SOLVENT DIELECTRIC CONSTANT P ^ , TORR 0 2.38 184 5 3.63 136 10 4.92 79 20 7.80 54 33 11.2 20 50 ~ 18 14 For raw data see Appendix IIo. p r e c i p i t a t i n g from a 1:1 Pip:DCE system a f t e r some time at ambient temperature (Chapter VII.7). The presence of an a c i d i c proton i n the ImCoPpIXDME system has been invoked to explain the enhanced i r r e v e r s i b l e oxygenation rate of that 123 system . In the present studies the ImCoOMBP system was s i m i l a r l y i r r e v e r s i b l y oxidized (Chapter VII.5). C l e a r l y s i m i l a r oxidation e f f e c t s would be observed i f protons from the solvent are a v a i l a b l e t o the system. Many attempts have been made to c o r r e l a t e the r e a c t i v i t y of a system 124 with the d i e l e c t r i c constant of a solvent . However, since a solvent cannot be merely regarded as a d i e l e c t r i c continuum and s p e c i f i c solute-solvent i n t e r a c t i o n s must be accounted f o r , few of these c o r r e l a t i o n s have 125 been successful . Such c o r r e l a t i o n s have succeeded when reactions have 126 been studied i n very s i m i l a r solvents or i n varying concentrations of 127 binary solvent systems , presumably because the solute-solvent i n t e r a c t i o n changes p a r a l l e l those of the d i e l e c t r i c constant. -103-Several r e l a t i o n s h i p s between the observed reaction rate constant, k, and solvent dielectric constant have been proposed. Among the more 128 notable are ones predicting an inverse relationship between k and « , a 129 < - l direct relationship between log k and + j- , or a d i r e c t r e l a t i o n s h i p between log k and Y, an empirical measurement of solvent ionizing power 130 related to t . Since K can be related to k (K = Kf/^ r)> such relationships between e and K should also hold. However, for the PipCoOMBP + O^ reaction i n toluene-DMF systems, the best correlation obtained was a direct one between « and K (P,0 )^ with a correlation coefficient, r, of 0.980 (Figure IV.7). -104-Figure IV.7. Oxygen A f f i n i t y vs. D i e l e c t r i c Constant for PipCoOMBP i n toluene-DMF systems. -105-CHAPTER V THERMODYNAMICS OF REVERSIBLE LIGAND BINDING TO CoDADIXDME AND CoDBrDIXDME. V.l Qualitative Spectral Observations V . l . l L + CoDADIXDME LCoDADIXDME, K . l a The visible spectrum of solutions of CoDADIXDME in toluene had a 412 nm 4 -1 -1 Soret band (« = 8 x 10 M cm ) and a visible band at 574 nm ( t = 1 x 10 4 M ''"cm ^ ) with a shoulder at 546 nm (c = 7 x 10 3 M c^m )^ . Addition of a nitrogen-coordinating ligand caused the position of the Soret band to 4 -1 -1 shift to 420 nm (« ^ 6 x 10 M cm ) , while the visible band and shoulder 3 - 1 - 1 coalesced to a broad band at 562 nm It ^8 x 10 M cm )(Figure V . l . The lack of good isosbestic points i s due to dilution effects from the addition of the ligand solution; correcting for this shows that isosbestic points occurred at approximately 422 nm, 568 nm, and 602 nm). Addition of DMF caused the position of the Soret band to red shift by about 7 nm (« = 4 -1 -1 7 x 10 M cm ) and the visible absorption to coalesce to a very broad band 3 -1 -1 with a maximum intensity at 568 nm (t = 8 x 10 M cm ). The spectral changes associated with the formation of ligand adducts of CoDADIXDME are listed in Table V.la. There was no evidence for formation of any six-coordinate complex, 6ince further addition of substantial amounts of ligand caused no changes in the spectra of the five-coordinate complexes. -106-3 0 N V 8 a o s a v Figure V . l . CH3-Im + CoDADIXDME , CH3-ImCoDADIXDME @ 22°C Apparent loss of i s o s b e s t i c points due to d i l u t i o n e f f e c t from ligand s o l u t i o n addition. For raw data see Appendix I l i a . -107-Table V.la. Spectral Data From 350 to 750 nm fear the Reaction: L + CoDADIXDME - LCoDADIXDME Absorbing Species Absorption Maxima (nm) Isosbestic Points (nm) (Extinction Coefficients, M "*"cm "S  CoDADIXDME 412 (8 X l c v 546 (7 X 574 (1 X 1 04 ) + CH -Im 420 (6 X io 4) 422 570 596 3 562 (8 X io3) + Pip 420 (6 X 422 568 602 562 (8 X io4) + Py 420 (6 X io 4) 422 570 598 562 (8 X io3) +DMF 419 (7 X io 4) 425 568 612 568 (8 X io3) Table V.lb. Spectral Data From 350 to 750 nm for the Reaction: L + CoDBrDIXDME ~ — LCoDBrDIXDME Absobing Species Absorption Maxima (nm) Isosbestic Points (nm) (Extinction Coefficient, M ^ cm ^ )  CoDBrDIXDME 398 (9 X ™3> 522 (7 X 104> 555 (1 X 104) + CH -Im 400 (8 X 368 402 548 574 3 549 (9 X io3) + Pip 400 (8 X 10J, 370 402 550 576 547 (9 X 103> + Py 400 (8 X io4> 368 402 548 574 548 (9 X 103) + DMF 402 (8 X i o 4 , 372 447 542 552 (9 X 103) -108-V.1.2 L + CoDBrDIXDME LCoDBrDIXDME, K,,. l b The visible spectrum of solutions of CoDBrDIXDME in toluene had a 4 -1 -1 398 nm Soret band (« = 9 x 10 M cm ) and a visible band at 555 nm ( t = 1 x 10 4 M 1cm 1) with a shoulder at 522 nm (« = 7 x 10 3 M 1cm 1 ) . Addition of a nitrogen-coordinating ligand caused the position of the 4 -1 -1 Soret band to sh i f t to 400 nm (« 8 x 10 M cm ), while the visible 4 -1 -1 band broadened and the peak position became 547 nm (• ^ 9 x 10 M cm ) . The 522 nm shoulder became less intense and less pronounced (Figure V.2). Once ligand solution dilution effects had been corrected for, isosbestic points were found at approximately 378 nm, 549 nm, and 574 nm. When the concentration of a cyclic amine ligand was increased to several molar, there was a 10% decrease i n the molar extinction coefficient of the Soret band of the five-coordinate adduct, accompanied by the formation of a shoulder at 413 nm, while the visible band shifted to 550 nm with a 10% decrease in extinction coefficient. Also a band began to appear in the 525 nm region. These spectral changes were probably due to the formation of some six-coordinate adduct, but a limiting spectrum was not reached. Addition of DMF to CoDBrDIXDME caused the Soret band to shift to 402 nm with e decreasing by about 5%. The visible band shifted to 552 nm and decreased by about 10% in intensity. The shoulder at 522 nm shifted to 525 nm and became less distinct. V.2 Treatment of Data The equilibrium constants, K, and K were obtained by following the la lb decrease in the intensity of the visible band. The spectral changes of the Soret bands were not analyzed because the curve of those bands (in the case -109-30NV9H0S8V Figure V. 2. CH3-Im + CoDBrDIXDME CH^-ImCoDBrDIXDME @ 22°C Apparent loss of i s o s b e s t i c points due to d i l u t i o n e f f e c t s from ligand s o l u t i o n addition. For raw data see Appendix H i e . -110-of CoDADIXDME), or because there was l i t t l e change i n the peak p o s i t i o n (in the case of CoDBrDIXDME). The formation of the ligand adduct obeys the equation: K l CoP + nL . L CoP (V.l) n where CoP represents a cobalt porphyrin complex. The analysis presented i n Chapter III for ligand binding to CoOMBP was used. The experimental data analyzed s a t i s f a c t o r i l y , and Figure V.3 A - A shows the log ~ vs. log [L] p l o t s for the CH -Im - CoDADIXDME 00 ~ 3 system, while Figure V.4 shows the van't Hoff p l o t s that are derived from A - A the various log — vs. log [L] p l o t s . A co — A V. 3 Sources of Error In studying ligand binding to CoDADIXDME and CoDBrDIXDME, the sources of error are s i m i l a r to those considered f o r studying l i g a n d binding to CoOMBP (Chapter I I I . 3 ) . The problem of oxygen entering the system d i d not appear to be important, as there was no evidence of i r r e v e r s i b l e oxidation. For the three nitrogen-donors examined, the concentrations of the ligands used were always s u f f i c i e n t l y small that changes i n the solvent were n e g l i g i b l e . -2 I t was necessary to add s u b s t a n t i a l volumes of d i l u t e (^10 M) ligand s o l u t i o n to achieve complete sp e c t r a l changes. Presumably i f more concentrated stock solutions were used, then l e s s e r volumes would be required, and good i s o s b e s t i c points would be achieved. In monitoring c y c l i c amine ligand binding to CoDADIXDME and CoDBrDIXDME, the s p e c t r a l changes were somewhat les s marked than those observed with the -111-" 5 0 " 4 . 5 - 4 . 0 - 3 . 5 - 3 0 Log [CH3-Im] Figure V.3. Eguilibrium p l o t s for CH -Im + CoDADIXDME - — - CH -ImCoDADIXDME -112-Figure V.4. Van't Hoff p l o t s f o r L + CoDADIXDME L = CH -Im, Pip, and Py LCoDADIXDME -113-CoOMBP system, but satisfactory analysis of the results was s t i l l readily achieved. Substantial amounts of DMF were necessary to approach f u l l formation of the five-coordinate adducts, and therefore some solvent effects probably occurred. Further, the dilution effects were such that any determination of equilibrium constants was rendered approximate. Also, f u l l formation of the CoDADIXDME adduct was not realized, and this made K A. a. determinations even more d i f f i c u l t . V.4 Ligand Binding Constants The equilibrium constants for ligand binding to CoDADIXDME and CoDBrDIXDME, together with the values of AH and AS obtained, and the pK a of the protonated ligand studied, are given in Tables V . 2 a and V . 2 b . The order in ligand binding abi l i t y for both cobalt porphyrin complexes at the temperatures studied i s CH3"Im>Pip>Py >> DMF. There are no obvious correlations between the K^, A H , or AS^ values and the pKa of the protonated ligand. V.5 Comparison of Ligand Binding Among Different Cobalt Porphyrins If the binding a b i l i t y of an axial ligand to a metal porphyrin complex were solely dependent on simple electrostatic considerations, then the donor electrons of the ligand would be most strongly attracted to the metal centre with the highest positive charge. Thus, in a series of cobalt porphyrin complexes, the poorest porphyrin donor (i.e. the weakest base) should form complexes with the most strongly bound axial ligand(s). Such considerations have been invoked to account for trends in axial ligand binding in both the six-coordinate iron porphyrin^"*1 and five-coordinate 22 cobalt porphyrin systems. -114-Table V.2a. Thermodynamic Data for the Binding of an Axial Ligand to CoDADIXDME in a Toluene Solution. LIGAND TEMP., °C. la a A H , kcal/mole a AS, e .u. b P Ka CH-Im 5 2.6 x 10* -7.0 -5 7.25 3 22 1.4 x 10* 40 6.3 x 10 Pip 5 2.0 x 10 4 -7.6 -8 11.30 22 1.0 x 10 40 4.8 x 10 py 5 1.1 x lot -8.5 -12 5.27 22 5.0 x 10 3 40 2.0 x 10 DMF 5 nJM 4 «J -3 A/-7 -2.0 22 CJ. 3 40 .^2 Table V.2b. Thermodynamic Data for the Binding of an Axial Ligand to CoDBrDIXDME in a Toluene Solution. LIGAND TEMP., o„ K l h ' M -1 A H , A S , e.u. PK. CH-Im 5 2.5 X -7.1 -6 7.25 3 22 8.9 X 1 0 3 40 6.0 X i o 3 Pip 5 1.6 X -3 -6.9 -6 11.30 22 7.8 X 1 03 40 4.5 X i o 3 py 5 7.1 X 1 0 3 -8.2 -10 5.27 22 3.7 X 1 03 40 1.4 X i o J DMF 5 1.4 -5 -16 -2.0 22 1.1 40 .7 o for cyclic amine binding was estimated to be ± °- 7 kcal/mole and a AH J estimated to be ± 3 e.u. b. For references see Table I I I . l . c. For raw data see Appendix I l i a - Appendix I l l h . -115-The method most commonly used to determine the basicity of a porphyrin is to measure the pK value of the third nitrogen of the cl porphyrin ring (K^P +z^=^E+ + H 2 P ' P K 3 ) 2 7 t > ' C - When the above basicity measurements were i n i t i a l l y carried out, an increase in absorption band peak wavelength was noted as the pK^ value decreased . However, subsequent studies on an extensive series of 2 ,4-substituted deutero-porphyrins indicated that while a qualitative trend between band position and pK^ may exist, other properties such as resonance and steric effects and molecular symmetry are also important, and an estimate of porphyrin basicity based solely on spectroscopic data i s probably unreliable 132 (Figure V.5) . For example, the approximately 20 nm difference in peak positions of DAD and DBrD, i n spite of their similar basicities has been 133 explained in terms of DAD having larger resonance effects than DBrD Tables V.3a-3d l i s t series of cobalt porphyrin complexes, in decreasing order of porphyrin basicity, on which ligand binding studies have been done. The Tables summarize the ligand binding constants, the observed A H and A S values, and the basicity of the porphyrins. Tables V3a, V3b, V3c, and V3d deal with binding of a single CH^Im, Pip, Py, and DMF ligand, respectively. 91c Previous work on pyridine binding to a series of 2 ,4-substituted deuteroporphyrins has shown that the binding a b i l i t y of the amine does increase as the porphyrin basicity decreases. The present work extends the range of porphyrin basicities used. Figures V.6a-6d show that there i s a reasonable correlation between ligand a f f i n i t y at 23°C and porphyrin basicity for the binding of CH^Im, Pip, and Py. Discrepencies, considered 89 91c to be within experimental error, between the data of different groups ' for Py binding to CoPpIXDME at 23°C are included in Table V.3c and -116-630H 6 0 0 H x f -o m 5 7 0 - J U J A 2 0 H 4 0 0 O i o 0 8 Og 04 Q8 ° 9 04 o i ° OIO 09 08 04 T 3 . 0 011 0 7 O 5 06 03 BAND I 01 02 O n o 7 0 5 O 6 0 3 BAND II 01 02 o n 0 7 05 O 6 O 3 SORET BAND 01 0 2 4.0 PK 3 7 5 . 0 "T 6.0 Nature of the 2,4-substituents: 1. Hydrogen 2. Ethyl 3. 2'Ethoxycarbonylcyclopropyl 4. Bromo 5. Acetyloxime 6. Vinyl 7. Oximino 8. Methoxycarbonyl 9. Propionyl 10. Acetyl 11. Formyl Figure V.5. Porphyrin band p o s i t i o n as a function of porphyrin b a s i c i t y , taken from r e f . 132. -117-Table V.Ja. Thermodynamic Data for the Binding of CH-Im to Several Cobalt Porphyrin Complexes in Toluene. PORPHYRIN TEMP., K , M " •1 AH AS PK3 REFERENCE °C kcal/mole e.u. OEP -45 1.7 X 1 04 -10 -20 6* 95 -31 2.8 X 1 02 23 9.1 X i o 2 PpIXDME -23 1.4 X -4 -10.7 -19 4.8b 89 0 2.5 X 1 0 3 23 5.0 X i o 3 T(p-OMe)PP 25 2.4 X i o 3 -11.4 -23 >3C 92 DADIXDME 5 2.6 X -7.0 -5 3.3* Table V.2a 22 1.4 X 1 0 3 40 6.3 X i o 3 DBrDIXDME 5 2.5 X -7.1 -6 3.0b Table V.2b 22 8.9 X 1 0 3 40 6.0 X i o 3 OMBP -23 5.2 X -9.2 -10 ~2.0 d Table III.3 0 2.3 X 1 04 23 3.2 X i o 4 For an explanation of the footnotes see p. 121. -118-Table V.3b. Thermodynamic Data for the Binding of Pip to Several  Cobalt Porphyrin Complexes in Toluene. PORPHYRIN TEMP., o K, M -1 AH kcal/mole AS pK3 REFERENCE e.u. 3 OEP 23 1.9 X 10 PpIXDME -23 1.8 X "4 -10.4 0 4.1 X 1 0 3 23 6.8 X i o 3 T(p-OMe)PP 25 2.4 X i o 3 -6.8 DADIXDME 5 2.0 X -7.6 22 1.0 X 1 0 3 40 4.8 X i o 3 DBrDIXDME 5 1.6 X 1 03 -6.9 22 7.8 X 1 03 40 4.5 X i o 3 OMBP -23 9.4 X 1 0 5 -7.3 0 3.4 X 1 04 23 8.9 X i o 4 -17 4.8 -7 >3^  95 89 92 -8 3.3 Table V.2a -6 3.0 Table V.2b -4 ^2.0 Table III. 3 For an explanation of the footnotes see p. 121. -119-Table V.3c. Thermodynamic Data for the Binding of Py to Several  Cobalt Porphyrin Complexes in Toluene. PORPHYRIN TEMP., K, M _ 1. AH AS pK3 REFERENCE °C kcal/mole e.u.  OEP 23 7.0 X MpIXDME 22.9 1.1 X 27.5 8.9 X 33.7 5.9 X 38.3 5.0 X DpIXDME 22.7 1.6 X 28.0 1.2 X 31.8 1.0 X 36.9 7.6 X PpIXDME 23.0 2.0 X 27.6 1.7 X 33.6 1.1 X 37.8 9.9 X PpIXDME -23 5.0 X 0 1.0. X 23 6.0 X 10' i o ; iot lot i o ' i o : io: ID; 10' i o ; io: io: 10' i o ; 10" -9.2 -9.1 -9.7 -6.9 T(p-OMe)PP 25 7.0 X i o 2 -8.5 DADIXDME 5 1.1 X 1 03 -8.5 22 5.0 X 1 03 40 2.0 X i o 3 DBrDIXDME 5 7.1 X 1 03 -8.2 22 3.7 X 1 03 40 1.4 X i o 3 OMBP -23 4.8 X 1 0 5 -8.0 0 1.3 X 1 04 23 2.1 X i o 4 -17 -16 -18 -6 -16 -12 -10 -6 6 a b 5.8 5.5 4.8 4.8 >3^  95 91c 91c 91c 89 92 3.3 Table V.2a 3.0 Table V.2b 2.0 Table III.3 For an explanation of the footnotes see p.121. -120-Table V.3d. Thermodynamic Data for the Binding of DMF to Several Cobalt Porphyrin Complexes in Toluene. PpIXDME -23 0 23 1.8 x 10 4.3 x 101 1.5 x 101 -7.9 -21 4.8' 8 9 DADIXDME 5 22 40 .4 .3 .2 ~-3 3.3 Table V.2a DBrDIXDME 5 22 40 1.4 1.1 .7 -4.5 _16 3.0 Table V.2b OMBP -23 0 23 3.6 2.5 1.7 -3.7 -11 2.0 Table III.3 For an explanation of the footnotes see p.121. -121-Notes to Tables V.3a - V.3d. a. D. J . Kent, BSc. Thesis, UBC, Vancouver, B.C., (1973). The method of determining the pK^ value was to note the positions of the v i s i b l e bands of the porphyrin complex and to compare them with the spectroscopic data f o r other porphyrins of known b a s i c i t y . For reasons mentioned i n the text of t h i s chapter, t h i s method should probably not be r e l i e d upon f o r a precise determination. b. W. S. Caughey, W. J . Fujimoto, and B. P. Johnson, Biochem., 5_, 3830 (1966) . c. B. R. James, i n "The Porphyrins," V o l . 5, D. Dolphin,'Ed., Academic Press, New York, N.Y., 1978, p. 265. d. No determination of the pK^ value of OMBP has been reported. Since the method of measuring pK^ values i s not an absolute one, and a series of several porphyrins should be used (ref 27b), no determination of the pK^ value of OMBP was attempted i n t h i s work. However, work with FeOMBP complexes (ref 89), and with ZnOMBP complexes (K. J . Reimer, M. Reimer, and C. Spindle-Sprague, 62nd Canadian Chemical Conference, Vancouver, B.C. (1979)), indicates that the b a s i c i t y of OMBP i s much less than the other porphyrins examined. For purposes of placi n g CoOMBP on Figures V.6a-6d, OMBP was a r b i t r a r i l y assigned a pK^ value of 2.0 ± 0.5, which seems reasonable, from the p l o t s of amine binding. -122--123-Figure V.6b. Pip A f f i n i t y vs. Porphyrin B a s i c i t y f or a V a r i e t y of Cobalt Porphyrin Complexes. -124 Log Ki Figure V.6c. Py A f f i n i t y vs. Porphyrin Bascity for a V a r i e t y of Cobalt Porphyrin Complexes. - 1 2 5 -L o g K-j Q 5 0.5-0 H Q 8 5 . PpIX (ref 8 9 ) 6. DADIX 7. DBrDIX 8 . OMBP - 0 . 5 H O* T 3 T 2 PK3 Figure V.6d. DMF A f f i n i t y vs. Porphyrin B a s i c i t y for a Va r i e t y of Cobalt Porphyrin Complexes. -126-Figure V.6c. The amine ligand binding constants of CoDBrDIXDME are c o n s i s t e n t l y s l i g h t l y lower than those measured f o r CoDADIXDME, although the reported pK 3 value for DBrDIXDME i s s l i g h t l y lower than that of DADIXDME. When the 132 -pK^ values were measured , some d i f f i c u l t y i n determining the value of DBrDIXDME was noted as that p a r t i c u l a r porphyrin was only sparingly soluble. Also the pK^ measurements were c a r r i e d out i n 2.5% aqueous sodium dodecylsulfate at 25°C. Since the ligand binding studies were c a r r i e d out in toluene, i t i s possible that the r e l a t i v e b a s i c i t i e s of the porphyrins could be changed somewhat as the solvent i s changed. CH3-Im binds more strongly to CoDADIXDME and to CoDBrDIXDME than does Pip; however, Pip binds more strongly than CH^-Im to CoOMBP and to 89 CoPpIXDME . This l a t t e r behavior i s reasonable i n view of the stronger b a s i c i t y of Pip than CH^-Im. Also, since CH3-Im i s a stronger TT-donor than 122 i s Pip , the strongly TT-donating OMBP may be expected to hinder the coordination of Cl^-Im, r e l a t i v e to Pip, to the metal centre of CoOMBP. Since other porphyrins, such as DADIXDME and DBrDIXDME, are generally not as 96c strongly ir-donating as OMBP , any repulsive u-effects on CH3~Im are probably not so important i n the other porphyrin systems. For a l l the cobalt porphyrin systems studied, Py was the weakest binding c y c l i c amine, consistent with Py being the weakest a-base of these compounds. Figure V.6d shows the binding constants of DMF to the d i f f e r e n t cobalt porphyrins; no obvious c o r r e l a t i o n with porphyrin b a s i c i t y i s apparent. As mentioned i n Chapter I I I , problems with impurities of dimethylamine i n DMF 116 have been proposed , and such impurities could explain the apparent high DMF a f f i n i t y of CoPpIXDME. Dimethylamine-free DMF would thus r e s u l t i n the lower DMF a f f i n i t i e s observed i n the other cobalt porphyrins. Ignoring the -127-DMF binding to CoPpIXDME, increased DMF a f f i n i t y i s noted as the porphyrin b a s i c i t y i s decreased. A l t e r n a t i v e l y , the scattered DMF a f f i n i t i e s could 121 r e s u l t from i t s strong TT-donor nature i f IT-effects dominate over a-e f f e c t s ( i . e . b a s i c i t y ) . -128-CHAPTER VI THERMODYNAMICS OF REVERSIBLE DIOXYGEN BINDING TO LCoDADIXDME AND LCoDBrDIXDME VI.1 Q u a l i t a t i v e Spectral Observations -5 Addition of dioxygen to 2 x 10 M toluene solutions of f i v e -coordinate CoDADIXDME-amine adduct (Figure VI.1) caused the v i s i b l e band at 3 -1 -1 562 nm to s p l i t i n t o two bands at about 555 nm (e ~ 8 x 10 M cm ) and 3 -1 -1 590 nm (e ~ 7 x 10 M cm ). The Soret band s h i f t e d from 420 nm (G -4 -1 -1 4 -1 -1 6 x 10 M cm ) to about 438 nm (e - 7 x 10 M cm ). Numerous i s o s b e s t i c points occurred around both the Soret and v i s i b l e bands. Spectral changes of a s i m i l a r pattern were observed f o r oxygenation of 2 x 10 5 M toluene solutions of five-coordinate CoDBrDIXDME-amine adducts (Figure VI.2). These changes were reversed when oxygen was pumped o f f the system. Bands at 552 nm (e = 8 x 10 3 M ''"cm 1) and 588 nm (e = 6 x 10 3 M 1cm 1) appeared when dioxygen was reacted with toluene solutions of DMFCoDADIXDME; 4 -1 -1 the Soret band s h i f t e d to 436 nm (e = 8 x 10 M cm ) with i s o s b e s t i c points occurring at 422, 495, 543, 561, and 578 nm." Similar s p e c t r a l changes were observed when oxygen was added to the DMFCoDBrDIXDME system. As with the amine systems, the s p e c t r a l changes were r e v e r s i b l e . The s p e c t r a l changes associated with oxygenation of the CoDADIXDME and CoDBrDIXDME are l i s t e d i n Table VI.1. -129-O O o 30NV9dOS8V Figure VI. 1. CH3-ImCoDADIXDME + 0 2 •< := CH3-ImCoDADIXDME (C^) @ -56.5°C For raw data see Appendix TVa. -130--131-Table V I . l a . Spectral Data From 350 to 750 nm for the Reacti on: LCoDADIXDME + 0N - TLCoDADIXDME(Q ) 2 2 Absorbing Species Absorption Maxima (nm) Isosbestic Points (nm) (Extinction C o e f f i c i e n t s , M "*"cm ^) 4 CH.-ImCoDADIXDME 438 (7 x 10.) .„ ... _ 3 i r r r ,„ ,„3, 426 486 548 i 5 5 5 ( 8 X 1 ° 3 ) 561 581 CH3-ImCoDADIXDME(Oj 591 (7 x 10 ) PipCoDADIXDME 438 (7 x loj) 4 2 6 ^ ^ I 555 (8 x 10 3) 5 7 9 PipCoDADIXDME(OJ 589 (7 X 10 ) PyCoDADIXDME 437 (7 X 10*) 4 2 6 ^ ^ Q i 5 5 5 ( 8 X 1 0 3 } 562 580 PyCoDADIXDME(02) 590 (7 x 10 ) DMFCoDADIXDME 436 (8 x 10*) 4 G 5 5 4 3 i 5 5 2 <8 x 1 0 3 } 561 578 DMFCoDADIXDME(0 ) 588 (6 x 10 ) Table V I . l b . Spectral Data From 350 to 750 nm for the Reaction: LCoDBrDIXDME + C>2„ .t LCoDADIXDME(02) Absorbing Species Absorption Maxima (nm) Isosbestic Points (nm) _ ~ -1 -1. CH -ImCoDBrDIXDME ^ 1 412 (8 X ioJ, 363 410 484 I 536 (8 X io 3 529 542 560 CH3-ImCoDBrDIXDME(02) 569 (6 X 10 ) PipCoDBrDIXDME 412 (8 X 4 io3) 363 410 494 I 536 (7 X io3) 530 540 559 PipCoDBrDIXDME(C^) 568 (6 X 10 ) PyCoDBrDIXDME 412 (8 X ioJ, 363 410 490 1 537 (7 X 10 ) 528 540 559 PyCoDBrDIXDME(02) 569 (6 X 10 ) DMFCoDBrDIXDME 415 (9 X io4) 407 1 541 3V 496 529 \ (8 X 10 J DMFCoDBrDIXDME (C>2) 571 (6 X 3 ioJ) 544 ! 562 -132-VI.2 Treatment of Data The equilibrium constants were obtained by following the decrease i n the s p e c t r a l i n t e n s i t y of the v i s i b l e band of the five-coordinate complex, or the increase i n the s p e c t r a l i n t e n s i t y of the higher wavelength v i s i b l e band of the oxygenated complex. In addition to the reasons mentioned i n Chapter V, the Soret band was not monitored because of d i f f i c u l t i e s a r i s i n g from working with the n e c e s s a r i l y d i l u t e s olutions. The formation of the oxygenated complex obeys the equation: \ ' LCoP + nn ^ — - T r n P j n l (VI.1) 2 2 n and the data were analyzed by the same method described f o r dioxygen binding A - A t o LCoOMBP (Chapter IV). Figure VI.3 shows the log j- vs. log P 0 0 2 p l o t s obtained from data on the oxygenation of CH^-lmCoDADIXDME, and Figures VI.4 and VI.5 show the van't Hoff p l o t s that are derived from the A - A various p l o t s of log ——--—-^ vs. log P Q . ro 2 VI. 3 Sources of Error From the thermodynamic parameters l i s t e d i n Table V . l , i t was possible to adjust the ligand concentration so that at l e a s t 95% of the cobalt complex was i n i t i a l l y present i n the form of LCoP f o r the oxygenation measurements. Although there was some evidence f o r six-coordinate c y c l i c amine complexes of LCoDBrDIXDME at high ligand concentrations (Chapter V), the formation of such complexes did not occur to any great extent at ligand concentrations used i n the oxygenation measurements. There was never s u f f i c i e n t formation of the six-coordinate complexes, even at low Figure -134--135--136-temperatures, to do oxygenation studies on such species. The concentrations of the amine ligands were very small during oxygenation of the LCoP systems, and the solvent could be considered as "pure" toluene. In the case of the DMF adduct, i t was necessary to study dioxygen binding i n a 1:1 volume to volume r a t i o of toluene to DMF, so that the DMF adduct would be f u l l y formed and a constant solvent would be maintained. The formation of an i r r e v e r s i b l y oxidized species was not a problem, as judged by the completely r e v e r s i b l e and clean s p e c t r a l changes, and by the consistent analysis of the s p e c t r a l data f o r formation of a 1:1 cobalt:dioxygen adduct. VI.4 Dioxygen Binding Constants The PjJ°2 v a l u e s °f t n e five-coordinate LCoDADIXDME complexes i n toluene are given i n Table VI.2a, together with the A H and A s values, and the pK of the ligand studied. Table VI.2b l i s t s the comparable values of the f i v e -coordinate LCoDBrDIXDME systems. The order of the oxygen a f f i n i t i e s f o r both porphyrin systems as a function of a x i a l ligand over the temperature range studied i s CH3~Im~DMF>Pip>Py. Although the order of dioxygen a f f i n i t y of the five-coordinate c y c l i c amine systems i s the same as that of the amine binding to the four-coordinate complex, there i s again no obvious c o r r e l a t i o n between K , AH , AS , and the pK of the protonated amine ligand. The u-donor 0 ' 0 ' 0 ' a 2 2 2 122 properties of CH^-Im within the amine s e r i e s i s thought to enhance the oxygenation of cobalt porphyrin complexes, as suggested f o r the 89 CoPpIXDME system and observed i n the CoOMBP system (Chapter IV). The c y c l i c amines bind to CoDADIXDME and CoDBrDIXDME more strongly than does -137-Table VI.2a. Thermodynamic Data f o r the Binding of Dioxygen to LCoDADIXDME i n a Toluene S o l u t i o n . LIGAND TEMP., o_ Torr. AH kcal/mole A S e.u. pK CH3-Im -63.5 -56.5 -45 -23 45 74 186 447 -6.7 -40 7.25 Pip -63.5 -56.5 -45 -23 17.2 263 513 1288 -5.7 -37 11.30 Py -78 -63. -56. -45 120 422 537 962 -6.0 -40 5.27 DMF -78 -63. -45 20 48 135 -5.2 -33 -2.0 for dioxygen binding was estimated to be t 0.6 kcal/mole. AH r for dioxygen binding was estimated to be * 2 e.u. A S a. For references see Table I I I . l , -138-Table VI.2b. Thermodynamic Data f o r the Binding of Dioxygen to LCoDBrDIXDME i n a Toluene Solution. LIGAND TEMP., P 3 50 2, Torr AH AS pK a a °C. kcal/mole e.u. CH-Im -78 10 -6.1 -37 7.25 3 -63.5 48 -56.5 78 -45 127 -23 537 Pip -78 61 -6.9 -43 11.30 -63.5 219 -56.5 359 -45 813 -23 ^3000 Py -78 151 -5.7 -39 5.27 -63.5 288 -56.5 479 -45 851 -23 ~2200 DMF -78 36 -3.7 -26 -2.0 -63.5 63 -45 145 <T'H f o r dioxygen binding was estimated to be ± 0.6 kcal/mole. g. f o r dioxygen binding was estimated to be 1 2 e.u. a. For references see Table I I I . l . -139-4 DMF by a f a c t o r of 10 , while the DMF adduct binds dioxygen as well as the CH^-Im adduct. Even a f t e r consideration of enhanced dioxygen binding i n 1:1 toluene:DMF mixtures due to solvent e f f e c t s (Chapter TV), the dioxygen a f f i n i t y of the DMF adduct i s much greater than expected from any a-ligand 121 b a s i c i t y c o r r e l a t i o n , and again the strong ir-donor a b i l i t y of DMF i s thought to be responsible. VI.5 Comparison of Dioxygen Binding Among D i f f e r e n t Cobalt Porphyrins Considerations from Chapter V in d i c a t e that the tendency of the f i r s t a x i a l ligand to bind with a cobalt porphyrin complex i s enhanced as the b a s i c i t y of the porphyrin i s decreased. Since the cobalt-dioxygen bond i s generally thought to be formed by the t r a n s f e r of some electron density from the cobalt to the dioxygen, the dioxygen binding a b i l i t y of a cobalt porphyrin complex should increase as electron density at the cobalt centre i s increased. Therefore, the complexes with the most basic porphyrins should bind dioxygen t o the greatest degree. Tables VI.3a-d l i s t , i n decreasing order of porphyrin b a s i c i t y , a se r i e s of cobalt porphyrin complexes on which binding studies have been done. Values l i s t e d i n these tables include dioxygen binding constants (as pJ 5° 2^ » the corresponding AH and AS values, and the b a s i c i t y of the porphyrin. Tables VI.3a, VI.3b, VI.3c, and VI.3d deal with dioxygen binding to the CH^-Im, Pip, Py, and DMF cobalt porphyrin complexes, r e s p e c t i v e l y . Some work has been done previously on binding of dioxygen to a se r i e s 89,91c of amine-coordinated cobalt 2,4-substituted deuteroporphyrins . No d e f i n i t e conclusions concerning a - c o r r e l a t i o n between dioxygen binding and porphyrin b a s i c i t y could be drawn, since an i n s u f f i c i e n t range of porphyrin b a s i c i t i e s were used within one p a r t i c u l a r amine system. Experiments -140-Table VI.3a. Thermodynamic Data f or the Binding of Dioxygen to CH-j-Im Complexes of Several Cobalt Porphyrins. PORPHYRIN TEMP., P, O , Torr *i 2 AH kcal/mole AS e.u. pK 3 REFERENCE OEP MpIXDME PpIXDME -63.5 -45 -31 -45 -45 -37.4 -31 5 43 200 31 50 110 229 -11.0 -56 95 -11.8 5.8^ -59 4.8 95 89 T(p-0Me)PP -65 -45 29 154" -8.9 -49 - 3 92b TPP -45 633 - 3 95 DADIXDME -63.5 -56.5 -45 -23 46 74 186 447 -6.7 -40 3.3 Table VI .2 a DBrDIXDME -78 -63.5 -56.5 -45 -23 10 48 78 127 537 -6.1 -37 3.0 Table VI.2b OMBP -56.5 -45 -31 62 98 198 -5.8 -32 "2.0" Table IV.4 -141-Table VI.3b. Thermodynamic Data f o r the Binding of Dioxygen to  Pip Complexes of Several Cobalt Porphyrins. PORPHYRIN TEMP., P 0 2, Torr AH AS pK 3 REFERENCE kcal/mole e.u. OEP -45 562 95 PpIXDME -63.5 -45 -31 45 224 832 -9.0 -50 4.8 89 T(p-0Me)PP -65 -45 147 714' -8.2 -49 92b DADIXDME -63.5 -56.5 -45 -23 112 263 513 1288 -5.7 -37 3.3 Table VI.2a DBrDIXDME -78 -63.5 -56.5 -45 -23 61 219 359 813 3000 -6.9 -43 3.0 Table VI.2b OMBP -63.5 -56.5 -45 104 184 269 -4.9 -32 "2.0" Table IV.4 -142-Table VI. 3c. Thermodynamic Data for the Binding of Dioxygen to  Py Complexes of Several Cobalt Porphyrins. PORPHYRIN TEMP., V2' Torr AH A S pK 3 REFERENCE o C kcal/mole e.u. OEP -45 708 6 b 95 MpIXDME -58.0 -54.5 -49.5 -44.5 50 78 141 192 -9.0 -50 5.8 b 91c DpIXDME -57.5 -51.0 -49.0 -46 104 189 250 313 -9.9 -55 5.5 b 91c PpIXDME -59.0 -53.5 -49.0 -43.5 78 125 208 400 -10.3 -57 4.8 91c PpIXDME -63.5 -57.5 -45 123 178 691 -9.2 -53 4.8 b 89 PpIXDME -52.7 -45 -41.6 -29.5 -5.9 1009 1595t 1958 5507 22700 -7.8 -48.7 4.8 91b T(p-OMe)PP -65 1 5 7 t 1168 T -9.3 -55 ~3 C 92b -45 DADIXDME -78 -63. -56. -45 120 432 537 962 -6.0 -40 3.3 Table VI.2a DBrDIXDME OMBP -78 -63.5 -56.5 -45 -23 -63.5 -56.5 -45 151 288 479 851 2200 91 141 240 -5.7 -5.1 -39 3.0 Table VI.2b -33 "2.0" Table IV.4 -143-Table VI.3d. Thermodynamic Data f o r the Binding of Dioxygen to  DMF Complexes of Several Cobalt Porphyrins. PORPHYRIN TEMP., P. 0„, Torr AH AS pK 3 REFERENCE *i 2 J o C kcal/mole e.u.  PpIXDME -63.5 -45 -31 22 186 759 -11.0 -59 4.8 89 DADIXDME -78 -63.5 -45 20 48 135 -5.2 -33 3.3 Table VI.2a DBrDIXDME -78 -63.5 -45 36 63 145 -3.7 -26 3.0 Table VI.2b OMBP -78 -63.5 -56.5 -45 -23 8.5 25 91 389 2700 -12.2 -65 "2.0" Table IV.4 Notes f or Tables VI.3a-d. For references a-d, see Table V.3. * Values taken frcm data where Im i s the a x i a l l i g a n d . t P, O^ values at -45°C calculated from thermodynamic data presented. -144-described in this thesis on CoDADIXDME, CoDBrDIXDME and CoOMBP systems extend the range of porphyrin basicities studied. Figure VI.6a indicates that a reasonable dioxygen binding-porphyrin basicity correlation (log P,0 at -45°C vs. pK ) exists for the three CH -Im coordinated 2,4-substituted deuteroporphyrins studied. Oxygen binding to ImCoMpIXDME appears to f a l l within the CH^-Im cobalt porphyrin binding correlation. The assumption that ImCoMpIXDME binds dioxygen in a similar manner to CH^-ImCoMpIXDME i s supported by observations in the CoPpIXDME 89 system that ImCoPpIXDME has a P 0 2 value of 69 Torr while CH^ImCoPpIXDME has a pJ5°2 value of 50 Torr. Also the two ligands have similar basicities (Table III.3). CH^-ImCoOEP appears to f a l l within this correlation, but CH3~ImCoOMBP, although i t binds dioxygen less strongly than CH^ImCoOEP and CH^-ImCoPpIXDME, binds dioxygen more strongly than the corresponding DADIXDME and DBrDIXDME complexes, despite these being more basic porphyrins than OMBP (Chapter V). The CH^-ImCoT(p-OMe)PP system binds dioxygen in line with the correlation of Figure VI.6a. On the other hand, ImCoTPP binds dioxygen much less strongly than the above relationship would predict. However, the pK3 value reported for TPP can only be considered approximate, since i t was determined from spectroscopic peak positions (Chapter V.5). Other work on 2-methylimidazole complexes of cobalt porphyrins in DMF found that at -10°C the order of oxygen a f f i n i t y was MpIXDME>DpIXDME~ 91c PpIXDME ; in a series of coboglobins where the axial ligand i s the imidazole moiety of a histidine residue, ambient temperature studies revealed an oxygen a f f i n i t y sequence of DpIXDME>MpIXDME-PpIXDME for -145-Figure VI.6a. Dioxygen A f f i n i t y vs. Porphyrin B a s i c i t y f o r a Variety of CH-Im Cobalt Porphyrin Complexes. -146-sperm whale myoglobin, and MpIXDME>DpIXDME>PpIXDME for substituted human 134 hemoglobin . Since the nature of these systems i s d i f f e r e n t from the ones p l o t t e d on Figure VI.6a (e.g. d i f f e r e n t solvent, presence of p r o t e i n ) , the precise dioxygen binding constants do not f a l l on that p l o t , but i n each case the l e a s t basic CoPpIX system bound dioxygen the least strongly i n the s e r i e s . That the CoDpIX system sometimes bound dioxygen more strongly than the CoMpIX system and sometimes the r e l a t i v e binding a b i l i t i e s were reversed, i s not s u r p r i s i n g as the b a s i c i t i e s of the two systems are so s i m i l a r . Figure VI.6b shows that a good dioxygen binding-porphyrin b a s i c i t y c o r r e l a t i o n e x i s t s f o r the Pip coordinated 2,4-deuteroporphyrins. Although the dioxygen a f f i n i t y of PipCoT(p-OMe)PP f a l l s within t h i s c o r r e l a t i o n , the dioxygen binding by PipCoOEP i s much weaker, and that by PipCoOMBP much stronger, than those an t i c i p a t e d by the c - b a s i c i t y c o r r e l a t i o n . Figure VI.6c shows dioxygen binding data to the Py cobalt porphyrin systems. There appears to be a reasonably good dioxygen binding-porphyrin b a s i c i t y c o r r e l a t i o n f o r the Py coordinated 2,4-substituted deuteroporphyrins, and oxygenation of PyCoT(p-OMe)PP f a l l s reasonably within t h i s r e l a t i o n s h i p . As with the Pip cobalt porphyrin complexes, PyCoOEP binds dioxygen less strongly than would be expected, while PyCoOMBP again binds dioxygen more strongly. In Chapter V i t was noted that the Py binding data to CoPpIXDME varied 89 91c somewhat between those of two d i f f e r e n t groups ' . Again there i s a s i m i l a r discrepency f o r the oxygenation data of PyCoPpIXDME. In addition 91c o to the values reported by Yamamoto et a l ( p i . 0 o @ ~ 4 5 c = 3 0 0 Torr) and * 2 89 o Stynes et a l (P,0 @ -45 C = 691 T o r r ) , there are the data of Drago et * 2. a l 9 ^ which state that PyCoPpIXDME binds dioxygen more weakly ( P L 0 o @ -45°C -147-Log Figure VI.6b. Dioxygen A f f i n i t y vs. Porphyrin B a s i c i t y f o r a Variety of Pip Cobalt Porphyrin Complexes. -148-Figure VI.6c. Dioxygen A f f i n i t y vs. Porphyrin B a s i c i t y for a Variety of Py Cobalt Porphyrin Complexes. -149-= 1595 Torr). The data of the f i r s t two groups are considered to agree reasonably well since the experiments were indeed performed by different groups and the spectrophotometric t i t r a t i o n data were analyzed somewhat differently. Stynes et a l treated their data by a method similar to the one outlined in Chapter III, while Yamamoto et a l examined theirs by the Ketalaar^"3^ method where (VI.2) A - A P K (e - e )[CoJ " (e - e ) [Com] o 2 2 o T J oo o L T J where A q i s the absorbance when no oxygen i s present A is the absorbance at a set oxygen pressure e is the molar extinction coefficient of LCoP o e i s the molar extinction coefficient of LCoP(0_) oo 2 and [ c o T l i s the total molar concentration of cobalt present. Drago's group also chose a different method of analyzing their spectro-photometric data. For each A - A q and P Q data point of an individual run, plots were calculated based on the equation: dK P 0„ A - A 0„ [LCoP] 2 ° + —I 7- • (VI.3) d(e - e ) _ .2 A - A oo o ( eoo _ E ) O O Where the lines resulting from those data points intersected, the "true" K ^ and - E q could be determined. However, Drago's group worked with a 0.13 cm path-length c e l l which required cobalt porphyrin concentrations of -4 -4 x 10 M. Under these conditions of relatively high concentration and low temperature, the po s s i b i l i t y of porphyrin aggregation (see Chapter VII) -150-cannot be ignored, and i f any aggregate were present, t h i s would r e s u l t i n an observed lower oxygen a f f i n i t y . Also the authors appeared to have r e l i e d on oxygen gas d i f f u s i n g into the s o l u t i o n f o r e q u i l i b r a t i o n , and i t i s possible that f u l l e q u i l i b r a t i o n did not take place, which would again r e s u l t i n a lower observed oxygen a f f i n i t y . The observed P O values at -45°C for the CH-Im and Py adducts of * Z 3 CoDADIXDME are s l i g h t l y higher than those f o r the corresponding CoDBrDIXDME complexes, i n s p i t e of the higher b a s i c i t y of the d i a c e t y l porphyrin, although the p ^ 0 2 values of the Pip complexes follow the b a s i c i t y c o r r e l a t i o n . However, p^O values of a LCoDADIXDME complex are s i m i l a r (within 50%) to the corresponding value of a LCoDBrDIXDME complex, consistent with the s i m i l a r i t y i n porphyrin b a s i c i t y . Since the experimental error associated with a measured P,0 value i s -±10%, and the measurement i 2 132 of the pK 3 value of DBrDIXDME was complicated by s o l u b i l i t y problems , the oxygen binding values of LCoDADIXDME and LCoDBrDIXDME are probably not inconsistent with the oxygen a f f i n i t y - p o r p h y r i n b a s i c i t y c o r r e l a t i o n . In a l l cases, the oxygen a f f i n i t y of LCoOMBP complexes were s u b s t a n t i a l l y greater than a n t i c i p a t e d from any a - b a s i c i t y c o r r e l a t i o n , and i t seems reasonable to a t t r i b u t e t h i s to the extra T;-electron density on the cobalt centre, r e s u l t i n g from the unusually strong Tr-donating properties of OMBP. This extra IT-electron density could then enhance dioxygen binding * by donating to the empty TT - o r b i t a l s of dioxygen. The dioxygen a f f i n i t y of CH^-ImCoOEP f a l l s within the range expected from consideration of the reported a - b a s i c i t y of OEP, but the dioxygen a f f i n i t i e s of PipCoOEP and PyCoOEP are much less than would have been expected. -151-In a l l cases the dioxygen a f f i n i t y of a CH^-Im adduct of a p a r t i c u l a r cobalt porphyrin complex was the strongest of a l l the nitrogen donor ligands studied, and t h i s could again be due to the a b i l i t y of CH^-Im to donate T T -122 electron density to the cobalt centre . With the exception of CoOMBP, where the Pip and Py adducts bind dioxygen s i m i l a r l y , the Py adduct of a given cobalt porphyrin binds dioxygen l e s s strongly than a Pip adduct. This 122 could be r a t i o n a l i z e d by Py being a IT-acceptor and Pip not TT-interacting , but the trend i s also consistent with the cr-basicity of Py and Pip. Figure VI.6d shows dioxygen binding constants at -45°C f or DMF adducts of d i f f e r e n t cobalt porphryins. There i s no obvious c o r r e l a t i o n with 121 porphyrin b a s i c i t y . The strong TT-donor e f f e c t s of DMF complexes could possibly override simple porphyrin a - b a s i c i t y e f f e c t s i n determining the order of dioxygen binding a b i l i t y . For example, the exothermicity associated with oxygenation of DMFCoOMBP i s much greater than those associated with oxygenation of other CoOMBP ligand adducts, presumably because the TT-e f f e c t s from both OMBP and DMF markedly enhance oxygenation. However, the DMF adducts of CoDADIXDME, CoDBrDIXDME, and CoPpIXDME bind dioxygen with exothermicities s i m i l a r to those of other adducts of these'systems, so oxygenation enhancement from TT-interactions f o r these three porphyrins does not appear to be as important as i n the CoOMBP system. Ligand binding constants (K^) of the cobalt porphyrins increase by a factor of about 6 as the range of porphyrin b a s i c i t i e s goes from MpIXDME to DBrDIXDME. A s i m i l a r decrease i n K i s noted f o r the same porphyrins. °2 Presumably as a x i a l ligand binding (K^) i s enhanced with the more weakly basic porphyrin complexes, there should be more electron t r a n s f e r from the ligand to the metal centre i n the case of the weakly basic -152-2.75 Log P1/202 O ' 2.50 5. PpIX (ref 89) 7. DADIX 8. DBrDIX 9. OMBP 2.25H O = O 8 0 7 2.00 T 6 i r 5 A PK3 T 3 Figure VI.6d Dioxygen A f f i n i t y vs. Porphyrin B a s i c i t y f o r a Variety DMF Cobalt Porphyrin Complexes. -153-porphyrins. This extra electron density from the a x i a l ligand would then be expected to compensate f o r the decreased electron density of the weakly basic porphyrins, and temper the r e l a t i o n s h i p between dioxygen binding and porphyrin b a s i c i t y , e s p e c i a l l y since the o-electron density from the ligand goes to the cobalt d 2 o r b i t a l , which i s thought to be the cobalt o r b i t a l z which tr a n s f e r s o-electron density to dioxygen. However, porphyrin b a s i c i t y seems to a f f e c t both ligand and dioxygen a f f i n i t i e s by s i m i l a r magnitudes. -154-CHAPTER VII KINETICS OF THE OXIDATION OF COBALT(II) PORPHYRIN COMPLEXES V I I . l Introduction Work i n the preceding chapters has dealt with the formation of f i v e - o r six-coordinate cobalt porphyrin complexes and t h e i r dioxygen adducts, which are a l l formed r e v e r s i b l y at rates too great to measure on a stopped-flow scale. Observations i n t h i s work have suggested that at higher temperatures (e.g. ambient) an i r r e v e r s i b l e oxidation reaction of a cobalt(II) porphyrin complex can occur with dioxygen. The loss of the f a m i l i a r ESR signals of the 1:1 Co:0 2 systems at the completion of the oxidation reaction i s consistent with the formation of a bridged peroxo dimer, f o r example, LCoOMBP(02)CoOMBP, s i m i l a r to a corresponding reaction proposed f o r the 123 CoPpIXDME system . In contrast to an expected second order dependence on cobalt ( r e s u l t i n g from a l i k e l y mechanism invo l v i n g a bimolecular rate determining step: k LCoP(0 2) + LCoP — * LCoP(0 2)CoPL (VII.l) 123 where P represents porphyrin), a f i r s t order dependence was observed —6 —5 over a cobalt concentration range from 5 x 10 to 5 x 10 M, and i t was proposed that the reaction proceeded by a rate determining rearrangement or d i s t o r t i o n of LCoP(0„): -155-* k * (VII.2) A change i n geometry r e s u l t i n g i n the 0 moiety being bound more as a peroxide than a superoxide was suggested, the LCoP(0 ) intermediate 2 then r a p i d l y reacting with LCoP t o give the f i n a l product. The more obvious alternate explanation f o r the observed cobalt dependence invokes formation of LCoPfO^) as the rate determining step followed by rapid reaction with LCoP; however, t h i s p i c t u r e i s not consistent with the 89 observed rapid formation of LCoPfO^) at lower temperatures 123 In the above studies , the reaction rate was greatly enhanced when using the ligands imidazole and benzimidazole, both of which have a c i d i c protons, and hydrogen bonding was thus considered to help form the intermediate, and accelerate the o v e r a l l reaction. The maximum concentration of CoPpIXDME used was 10 M^, and i t was confirmed that cobalt porphyrin aggregation was not a factor at these concentrations""""'"'. With the above r e s u l t s i n mind, a s i m i l a r k i n e t i c i n v e s t i g a t i o n was undertaken on complexes of CoOMBP. Unexpected observations caused work to be done on other porphyrin systems including CoEpI, CoOEP, CoPpIXDME, and CoTPP. VII.2 Q u a l i t a t i v e Spectral Observations Upon oxidation of L CoOMBP complexes (L = Im, CH -Im, Pip, and Py; n 3 n = 1 or 2) a new Soret band occurred i n the 440 nm region (Figure VII.l) with molar e x t i n c t i o n c o e f f i c i e n t s s i m i l a r to those of the LCoOMBP(0^) dioxygen adducts (Table IV.I). The v i s i b l e band red s h i f t e d to about 630 nm with an approximate increase of 10% i n e x t i n c t i o n c o e f f i c i e n t . Often, * 123 -156-o ui a 3 0 N V 8 y O S 8 V Figure V I I . l . Spectra showing the oxidation of PipCoOMBP i n DCE @ 30°C-For raw data see Appendix Vr. -157-e s p e c i a l l y f o r the r e l a t i v e l y f a s t reactions, good i s o s b e s t i c points were achieved. On some of the reactions which took days to complete the i s o s b e s t i c points were l o s t . Oxidation of the CoEpI, CoOEP, CoPpIXDME, and CoTPP complexes was accompanied by a red Soret s h i f t of about 20 nm and an increase i n molar e x t i n c t i o n c o e f f i c i e n t (Figure VII.2). In the CoEpI, CoOEP, and CoPpIXDME systems the v i s i b l e band around 550 nm s p l i t i n t o two somewhat smaller bands on e i t h e r side of the o r i g i n a l band (Figure VII.3) . In the CoTPP system the v i s i b l e band at 540 nm s h i f t e d to 556 nm with a 20% increase i n molar e x t i n c t i o n c o e f f i c i e n t (Figure VII.4). Generally good i s o s b e s t i c points were achieved i n these reactions, a f t e r sometimes allowing f o r i n i t i a l rapid formation of some 1:1 Co:0 2 adducts which could occur under the conditions used. Although the CoPpIXDME systems i n toluene were reported not to form 123 a dioxygen adduct at ambient temperatures , many of the other porphyrin systems described i n t h i s thesis exhibited a s u b s t a n t i a l rapid i n i t i a l s p e c tral change when oxygen was added, e s p e c i a l l y when the solvent was DCE. This change, the magnitude of which was dependent on the pressure of 0^ i n the system, was at l e a s t p a r t i a l l y r e v e r s i b l e i f oxygen were immediately removed. As a r e s u l t of these observations, and from the f a c t that a room temperature ESR s i g n a l was detected upon admission to the CoEpI and CoOEP systems (Figure VII.5), i t was concluded that the i n i t i a l s p e c t r a l changes were due to the formation of a 1:1 C o : 0 2 a d d u c t a t ambient temperatures. The f i n a l " i r r e v e r s i b l e " spectrum i n each case i s a t t r i b u t e d to the bridged • 123 peroxide species A summary of the v i s i b l e s p e c t r a l changes i s presented i n Table V I I . l . -158-LU O Z < CD CL O (/) CD < 350 i r 400 450 WAVELENGTH, NM 500 Figure VII.2. Spectra showing the oxidation of CH -ImCoEpI by 800 Torr O i n DCE @ 2 0 C. 2 For raw data see Appendix Vee. -159-450 500 550 600 WAVELENGTH, NM Figure VII.3. Spectra showing the oxidation of CH^-ImCoOEP by 800 Torr 0 2 i n DCE @ 30°C. For raw data see Appendix V f f . -160-450 500 550 600 650 WAVELENGTH, NM Figure VII.4. Spectra showing the oxidation of CH3-ImCoTPP by 800 Torr 0 2 i n DCE @ 20°C. For raw data see Appendix Vcc. -161-H -(G) Figure VII.5. ESR sig n a l s showing the oxidation of CH^ImEpI by 800 Torr 0 2 i n DCE @ 22°C. For raw data see Appendix Vdd. Table V I I . l Spectral Data From 350 to 750 nm for the Reaction: 2L CoP + 0 -n 2 LCoP(0 2)CoPL + 2(n-l)L (P represents a porphyrin) Absorbing Specicies Absorption Maxima (nm) (Extinction C o e f f i c i e n t , M '''cm "S Isosbestic Points (nm) ImCoOMBP 1 (ImCoOMBP)2(02) CH -ImCoOMBP 3 J (CH3-ImCoOMBP)2(02) PipCoOMBP \ (PipCoOMBP) (0 2) PyCoOMBP (PyCoOMBP) 2(0 2) CH -ImCoEpI (CH 3-ImCoEp) 2(0 2) CH -ImCoOEP 3 J (CH 3-ImCoOEP) 2(0 2) 4 10 ) 618 (4.6 X io 4 ) io 4 ) 628 (4.2 X io 4 ) io 4 ) 618 (4.7 X io 4 ) . ' io 4 ) 629 (4.7 X io 4 ) 4 : 10 ) 618 (4.7 X io4 ) 4 10 ) 629 (4.7 X io4 ) 4 ; 10 ) 618 (4.6 X io 4 ) : 104) 634 (4.8 X io 4 ) : 105) 546 (1.6 X io 4 ) 539 (1 .2 x io 4 ) 562 (1. 1 X io 4 ) : 105) 547 (1.8 X io 4 ) 539 (1 .4 x io 4 ) 562 (1. 3 x io 4 ) 425 510 630 428 630 396 427 624 361 408 438 558 568 358 408 428 544 557 565 Table V I I . l contd. Spectral Data From 350 to 750 nm f o r the Reaction: 2L CoP + 0 *-LCoP(0 )CoPL + 2(n-l)L (P represents porphyrin) £ , .. • Absorbing Species Absorption Maxima (nm) Isosbestic Points (nm) (Extinction C o e f f i c i e n t , M "*"cm "S CH -ImCoPpIXDME _._ __ . J _ _ , 365 416 440 572 (CH 3-ImCoPpIXDME) 2(0 2) CH -ImCoTPP | 372 424 454 547 (CH -ImCoTPP) (O ) 434 (3.0 x 10 ) 556 (1.8 x 10 ) 3 2 2 I J (CH-Im) CoOMBP -__ . . \ • 360 430 454 623 (CH3-ImCoOMBP) 2 (O^ (Pip) CoOMBP — . J 2 . 418 435 458 (PipCoOMBP) 2(0 2) Py CoOMBP I . 4 3 6 458 629 i XPyCoOMBP)2(02) 404 (1.0 x io5) 556 (7.2 x 103) 426 (1.1 x 105) 540 (5.5 x i o 3 , 574 (5.3 x 103) 412 (2.4 x io 5 ) 540 (1.5 x 104) 434 (3.0 x i o 5 , 556 (1.8 4 x 10 ) 435 (9.2 x 105) 457 (7.6 x io4) 618 (4.5 x 104) 442 (9.7 x io4) 632 (5.0 x 104) 430 (9.4 4 x 10 ) 456 (4.8 x io4) 618 (3.8 x 104) 440 (~9 x : io4) 630 (~5 4 x 10 ) 428 (8.2 x io4) 618 (4.3 4 x 10 ) 442 (9.7 x io4) 635 (4.8 4 x 10 ) A l l the above s p e c t r a l c h a r a c t e r i s t i c s were measured i n DCE except for the ImCoOMBP and (CH^Im)2CoOMBP systems, which were studied i n toluene. . -164-VII.3 Treatment of Data The oxidation reactions were followed by observing the decrease i n the i n t e n s i t y of the Soret band of the non oxidized species or the increase i n the Soret band of the oxidized species for the CoOMBP systems. In the other cobalt porphyrin systems, the Soret band was usually so sharp that accurate absorbance readings were often d i f f i c u l t to achieve at the various stages of oxidation, and so Soret changes were usually monitored only when cobalt dependence studies were done at low concentration. On the other hand, changes i n the v i s i b l e band could be r e a d i l y followed. The observed reaction rate was often f i r s t order i n cobalt and the observed rate constant could r e a d i l y be determined under pseudo f i r s t order conditions by the standard r e l a t i o n s h i p : A - Aro l n ^- = -k , t (VII. 3) A - A obs o 0 0 where A i s the absorbance at time, t A i s the absorbance at the completion of the reaction 00 A i s the absorbance at the s t a r t of the reaction o and k , i s the observed rate constant, obs Dependence on other parameters, such as oxygen pressure and ligand concentration, was obtained by noting how the pseudo f i r s t order rate constant changed as a p a r t i c u l a r parameter was varied. The reaction rate was sometimes not f i r s t order i n cobalt, and attempts analyze for other reaction orders (e.g. second order, three halves order, and h a l f order) were not generally successful. In such cases the reaction rate i s usually reported i n terms of the f i r s t h a l f l i f e at a p a r t i c u l a r cobalt concentration. -165-VII.4 Aggregation For the LCoPpIXDME + system i n toluene, a f i r s t order reaction rate 123 dependence had been established f or cobalt . However, the dependence on cobalt i n the LCoOMBP system i n toluene, when the ligand was CH^-Im, Pip or Py, was found i n t h i s work to be le s s than f i r s t order. Because of t h i s , the p o s s i b i l i t y of CoOMBP complexes forming aggregates at higher cobalt concentrations was investigated. Porphyrins have been long known to aggregate, and the subject has been recently reviewed"*"3^. As e a r l y as 1937^""^ a v a r i e t y of free porphrins was shown to form films on water surfaces, i n which the polar side chains were i n the water, while the more hydrophobic parts of the molecule were stacked face to face. The a b i l i t y of these porphyrins to form face to face stacks diminishes as the solvent i s changed from water to ones of lower d i e l e c t r i c 138 constant . Porphyrin aggregates tend to break up as temperature 139 increases , or i n the case of metalloporphyrins, i f a coordinating 136a solvent i s added , although t h i s may only change the form of . 140 aggregation In metalloporphyrins the nature of the metal i s important i n determining to what extent the complex w i l l aggregate. Weaker solvation of the metal w i l l r e s u l t i n a greater tendency of the metalloporphyrin complexes to aggregate. In non aqueous s o l u t i o n s , Cu(II), N i ( I I ) , Zn(II), and Pd(II) porphyrin 141 complexes aggregate more r e a d i l y than the corresponding free porphyrins , although there i s l i t t l e Zn(II) porphyrin aggregation i n aqueous 142 solutions . The higher oxidation state of the metal, the le s s a metallo-porphyrin w i l l tend to aggregate. For example Mn(II) hematoporphyrin i n 0.1 M KOH was shown to be aggregated, while the Mn(IV) complex d i d not •,. • 143 aggregate under the same conditions -166--3 -4 ESR and NMR have been used to study aggregation i n 10 to 10 M 144 porphyrin and metalloporphyrin systems where spin p a i r i n g i s monitored 145 i n the former, and chemical s h i f t m the l a t t e r . Porphyrin concentrations i n UV-VIS observations are usually a couple of orders of magnitude lower than i n resonance studies. I f the porphyrin complex does not obey Beer's Law and an i s o s b e s t i c point i s found i n d i l u t i o n studies where the amount of porphyrin i n the l i g h t path i s kept constant, t h i s i s taken as strong evidence f o r porphyrin dimerization or further aggregation^""^ '-*-^ 2 »146 ^  To fu r t h e r corroborate evidence of aggregation, temperature jump k i n e t i c 147 behavior of the porphyrin system may be noted . In the case of Mn(III) . 148 hematoporphyrin aggregation has been demonstrated by polarographic and 149 143,149 k i n e t i c studies , while Beer's Law f o r t h i s system was observed An observed aggregation of CoOMBP i n toluene at spectroscopic -6 -4 . . concentrations (10 - 10 M) during the present studies i s e s p e c i a l l y i n t e r e s t i n g as neither free nor metallo-porphyrins have been reported to 136a,150 aggregate i n non aqueous solvents under these conditions , although 144 they have been shown to aggregate at concentrations used i n ESR arid 151 NMR studies. OMBP, with i t s r i g i d planar structure and highly aromatic nature, might be expected to be e s p e c i a l l y prone to aggregation i n non 152 aqueous solvents. In f a c t NiOMBP, when p a r t i a l l y oxidized by I , has been shown to e x h i b i t u n i d i r e c t i o n a l conduction properties as NiOMBP i s able to form " i n f i n i t e " c l o s e l y packed stacks. The change i n e x t i n c t i o n c o e f f i c i e n t of CoOMBP as a function of concentration i s shown i n Figure VII.6. A gradual decrease i n e x t i n c t i o n c o e f f i c i e n t of the CoOMBP i n s o l u t i o n of both the Soret and the v i s i b l e band was observed with increasing concentration i n both toluene and DCE, with the Soret band blue s h i f t i n g by about 10 nm. -167-For raw data see Appendix Va. -168-Th e aggregation reaction i s : nCoOMBP~ * (CoOMBP) (VII.4) n From considerations i n Chapter III.2 i t can be shown that e - e [CoOMBP] = [ C o T J (VII.5) o 0 0 1 e ~ E and [(CoOMBP) ] = - • — [Co 1 (VII.6) L n n e - e T o 0 0 where [Co T] i s the concentration of t o t a l cobalt present e i s the molar e x t i n c t i o n c o e f f i c i e n t of CoOMBP o and e i s the cobalt molar e x t i n c t i o n c o e f f i c i e n t of (CoOMBP) . 00 n e - e r -. fCo 1 L(CoOMBP) J n e - E ^ L T J Therefore K = — = 2 ; (VII.7) • » [COOMBPJ" J e - e e - E and log n + log K + nlog — Ico 1 = log — Tco l^ (VII.8a) agg E - E L T J E - E L T J O 00 0 00 or log n + log K + nlogfCoOMBP] = log n f(CoOMBP) 1 (VII.8b) agg J n where K a g g i s t n e aggregation equilibrium constant. Therefore, a p l o t of log n [(CoOMBP) J vs. log [CoOMBP], values which are n r e a d i l y determined from the spectroscopic data, should give a l i n e with the slope n and the i n t e r c e p t log n + log K agg A d i f f i c u l t y i n analyzing these data arose i n not being able to experimentally f i n d l i m i t i n g e x t i n c t i o n c o e f f i c i e n t s . At low cobalt concentration (-10 M) the observed absorbance of solutions with ti measured cobalt concentration fluctuated by 10 - 15%, compared with -169--5 absorbance variance at ~10 M of about 3%. At the higher cobalt concentrations the spe c t r a l peaks moved o f f scale, and so e x t i n c t i o n c o e f f i c i e n t determinations could not be made. However, by assigning to the 4 -1 -1 v i s i b l e band an e value of 6.2 x 10 M cm and an e value of o 0 0 4 - 1 - 1 2.4 x 10 M cm , a st r a i g h t l i n e with a slope of 1.98 was obtained 4 -1 (Figure VII.7). K was determined to be 1.3 x 10 M i n toluene and agg 4 -1 -1.5 x 10 M i n DCE. Although these r e s u l t s could not be checked by temperature jump k i n e t i c s due to the nature of the solvent, they are i n d i c a t i v e of the presence of a dimer of CoOMBP i n the above solvents. In CHCl^ solutions, aggregates formed from the TT—TT i n t e r a c t i o n s of the 148 porphyrin tend to be dimers at concentrations where NMR can be used The presence of a CoOMBP dimer could have e f f e c t s on measuring ligand binding constants. However, i n these studies the CoOMBP concentrations were —6 of the order of 2 x 10 M, and under these conditions the monomer —6 constitutes more than 95% of the cobalt concentration (-1.92 x 10 M). Even at the concentrations where most of the oxidation k i n e t i c s were c a r r i e d out -5 (-2 x 10 M), the amount of CoOMBP monomer present i n the absence of - 5 ligand would be -1.7 x 10 M. When oxidation i s studied, at l e a s t 95% of the CoOMBP i s i n i t i a l l y converted to LCoOMBP, and t h i s would furt h e r reduce -7 the concentration of any dimer (~3 x 10 M). Beer's Law studies were also c a r r i e d out on five-and six-coordinate CoOMBP complexes i n both toluene and DCE. However, no deviations were detected (Figures VII.8,9). From these studies, formation of a CoOMBP dimer i s c l e a r l y important, although the concentration of that dimer i s not s i g n i f i c a n t under the conditions used to measure the ligand binding constants. No v i s i b l e spectral evidence i s found f o r aggregation of the five-and six-coordinate complexes, -170-log[CoOMBPl Figure VII.7 Aggregation p l o t of CoOMBP i n toluene @ 22°C. For raw data see Appendix Va. -171-6.0-6x10^ -A 5.0 H 4.5 x x - X 408 nm O - O o - ° o o 618nm _ ! ]  "5.0 -4.0 tog[CH3-ImCoOMBP] O Figure VII.8 Beer's Law p l o t s of CH3-ImCoOMBP i n toluene @ 22 C. For raw data see Appendix Va. -172-9.0H 8.0 H 6x10' 70-6.0 H 5.0 H LO-O O O 435 nm ' 6 " " ' 457 nm X 618 nm ~ n 1— -5.0 -4.0 log [(CH3-Im)2CoOM BP] Figure VII.9 Beer's Law p l o t s of (CH3"Im) CoOMBP i n toluene @ 22 C For raw data see Appendix Va. -173-although t h i s cannot be e n t i r e l y ruled out, as aggregation has been noted 148 149 i n six-coordinate Mn(III) hematoporphyrin systems ' , although Beer's 143 149 Law appeared to be obeyed ' ESR studies have been extensively used to investigate cobalt(II) porphyrin systems^"3^3'"*"^3. While a signal i s r e a d i l y detected f o r -3 CH^-ImCoPpIXDME at concentrations of 10 M i n toluene (Figure VII.10), signals f o r CH^-ImCoOMBP solutions under s i m i l a r conditions are weak and i l l defined, as are signals f or comparable solutions of (CH^-Im)2CoOMBP. This suggests that some spin p a i r i n g mechanism l i k e l y e x i s t s i n CoOMBP systems that does not e x i s t i n CoPpIXDME systems. One possible way i s f o r the CoOMBP moieties, even i n f i v e - o r six-coordinate systems, to dimerize. 140 NMR studies on dicyanohemin have shown that that six-coordinate complex i n methanol e x i s t s as a dimer, which i s formed by a TT-TT linkage of a pyrrole r i n g of each of the component molecules. A s i m i l a r mechanism of aggregation could operate for the five-and six-coordinate CoOMBP systems. VII.5 K i n e t i c s of the Oxidation of LCoOMBP VII.5.1 K i n e t i c s of the Oxidation of ImCoOMBP i n Toluene When a solution.of ImCoOMBP i n toluene was exposed to one atmosphere of oxygen, rapid oxidation of the system was observed and the reaction rates were thus monitored p r i m a r i l y at lower oxygen pressures where more r e a d i l y measurable rates p r e v a i l e d . Although there was no detectable amount of the ImCoPpIXDME(0^) complex i n the previously reported ambient temperature 123 k i n e t i c studies , observation of a rapid i n i t i a l s pectral change i n the present work led to the conclusion that there was some i n i t i a l ImCo0MBP(02) formation during the k i n e t i c studies. Calculations based on the thermodynamic -174-Figure VII.10 ESR spectrum of CH -ImCoPpIXDME i n toluene @ 77 K. -175-data of oxygen binding to CoOMBP-amine systems (Chapter IV) al s o p r e d i c t about 10 to 20% formation of the 1:1 cobalt:dioxygen adduct at 20°C under one atmosphere of oxygen pressure, and the sp e c t r a l evidence was compatible with t h i s . A set of spectra r e s u l t i n g from the oxidation of ImCoOMBP i s shown i n Figure VII.11, and a summary of the k i n e t i c data i s presented i n Tables VII.2a-h. At lower concentrations of ImCoOMBP (up to -10 ^  M) the pseudo f i r s t order rate constants remained e f f e c t i v e l y constant (Tables VII.2e,f), -4 but at higher cobalt concentrations (up to -2.5 x 10 M), k ^ g values increased with increasing cobalt concentration (Tables VII.2f,g). The oxygen 123 -3 -1 concentrations i n s o l u t i o n (-10 M atm ) were such that they remained e f f e c t i v e l y constant during a k i n e t i c run, and a p l o t of ^ 0 ^ s v s - oxygen pressure (Figure VII.12) indicates a reasonable f i r s t order dependence on oxygen pressure (but the order may be decreasing at higher pressures). The dependence on ligand concentration (Figure VII.13) approached f i r s t order at low concentrations, but decreased toward zero order at higher concentrations. These dependences are s i m i l a r to those observed f o r the oxidation of the 123 ImPpIXDME system . (However, no decrease i n oxygen dependence was observed i n that system). There i s some question about the exact nature of the f i n a l product (e.g. ImCo(III)OMBP(OH) could be formed by abstraction of a ligand or even a solvent hydrogen). The presence of i s o s b e s t i c points probably rules out slow formation of the hydroxide product through a pos s i b l e slowly formed peroxide bridged intermediate. Also the spectrum of the f i n a l product i s s i m i l a r to that of the 1:1 cobalt:dioxygen adduct, and other r e l a t e d systems have been 23c shown to give a bridged dimer . Therefore, the product i s thought to be the bridged dimer ImCoOMBP(02)CoOMBP-Im. -176-Q m o *- o 30NV8cJ0S9V Figure VII.11 Spectra showing the oxidation of 2 x 10 5M ImCoOMBP by 395 Torr 0^ @ 22°C? for raw data see Appendix Ve. (CImj = 0.55 x 10'4 M) -177-Table VII.2a. K i n e t i c Data f o r the Oxidation of ImCoOMBP as a  Function of Oxygen Pressure. LCo] , M P , Torr °2 o Temperature, C Lim] 9 M ^obs -1 , sec 1.9 x i o " 5 54 22 14.6 X i o ' 4 1.66 x 10" 3 1.8 x i o " 5 97 22 14.6 X i o " 4 2.92 -3 x 10 1.8 x i o " 5 147 22 14.6 X i o " 4 4.03 x 10~ 3 1.7 x i o " 5 190 22 14.6 X io" 4 4.75 x 10~ 3 1.8 x i o " 5 278 22 14.6 X i o " 4 7.15 x I O - 3 Table VII.2b. K i n e t i c Data f o r the Oxidation of ImCoOMBP as a Function of Ligand Concentration; Oxygen Pressure ~ 100 Torr. [Co] , M P , Torr °2 o Temperature, C [Im] F M kobs' -1 sec -5 -4 0.82 1.6 x 10 95 22 2.2 X 10 X 10 -5 -4 1.39 -3 1.7 x 10 3 95 22 4.4 X 10 X 10 -5 ,~-4 2.03 1.8 x 10 96 22 7.3 X 10 X 10 -5 -4 2.08 , -3 1.7 x 10 95 22 9.2 X 10 X 10 -5 -4 -3 1.8 x 10 S 97 22 14.6 X 10 2.92 X 10 -5 -4 3.59 1.6 x 10 95 22 18.4 X 10 X 10 -5 -4 4.77 , -3 1.7 x 10 D 95 22 29.2 X 10 X 10 -178-Table VII.2c. K i n e t i c Data f o r the Oxidation of ImCoOMBP as a Function of Ligand Concentration; Oxygen Pressure ~ 400 Torr. [Co] , M P , Torr 2 c Temperature, C [im] , M k obs -1 , sec -5 -4 0.80 ,„-3 2.0 x 10 395 22 0.55 x 10 x 10 1.7 x i o " 5 405 22 1.1 -4 x 10 1.26 x 10~ 3 1.9 x i o " 5 395 22 2.2 -4 x 10 2.43 x 10~ 3 -5 -4 3.73 -3 1.7 x 10 b 397 22 4.4 x 10 x 10 Tab le VII.2d. K i n e t i c Data f or the Oxidation of ImCoOMBP as a Function of Ligand Concentration; Oxygen Pressure ^ 800 Torr. L C C D , M P , Torr °2 o„ Temperature, C C l m ] , M obs -1 sec 1.8 x i o " 5 825 22 0.15 X i o " 4 0.20 x 10~ 3 1.9 x i o " 5 825 22 0.23 X i o " 4 0.51 x 10~ 3 1.7 x i o " 5 825 22 0.30 X i o " 4 1.00 x 10" 3 1.6 x i o " 5 825 22 0.60 X i o " 4 1.49 x 10~ 3 1.7 x i o " 5 825 22 1.2 X i o ' 4 2.11 x 10" 3 -179-Table VII.2e. K i n e t i c Data for the Oxidation of ImCoOMBP as a Function of Cobalt Concentration; Oxygen Pressure ^160 Torr, -3 Imidazole Concentration ^ l . B x 10 M. QCo] , M P , Torr 2 Temperature, C [Im], M k , , sec obs -1 0.06 x 10 -5 0.1 x 10 -5 0.2 x 10 -5 160 160 160 22 22 22 18.4 x 10 -4 18.4 x 10 18.4 x 10 -4 4.6 x 10 -3 4.4 x 10 -3 4.5 x 10 -3 Table VII.2 f. K i n e t i c Data f or the Oxidation of ImCoOMBP as a Function of Cobalt Concentration; Oxygen Pressure -"100 Torr, -3 Imidazole Concentration «1.8 x 10 M. CCo] , M P Q , Torr Temperature, Llml, M k , , sec obs -1 0.8 x 10 1.6 x 10 2.4 x 10 4.5 x 10 7.0 x 10 8.0 x 10 -5 100 95 100 100 100 100 22 22 22 22 22 22 18.4 x 10 18.4 x 10 -4 18.4 x 10 18.4 x 10 18.4 x 10 18.4 x 10 -4 2.56 x 10 -3 3.48 x 10 3.52 x 10 4.18 x 10 -3+ -3+ 5.68 x 10 -3t 5.92 x 10 t "k , " i n these cases determined from i n i t i a l l i n e a r regions obtained obs from f i r s t order i n cobalt analyses. In these experiments reasonable f i r s t order p l o t s could be achieved over 75% of the reaction before the deviations became marked. -180-Table VII.2g. Kinetic Data for the Oxidation of ImCoOMBP as a Function of Cobalt Concentration; Oxygen Pressure ^100 Torr, Imidazole Concentration -'I.S x 10 3 M. -5 --5 .-5 £cd] , M P , Torr Temperature, C [im] , M kobs' S S C 2 -1 4 . „ . -3* 1.8 x 10 97 22 14.6 x 10 2.96 x 10 -4 -3* 13 x 10 100 22 14.6 x 10 4.34 x 10 -4 -3* 23 x 10 95 22 14.6 x 10 7.02 x 10 Table VII. 2h. Kinetic Data for the Oxidation of ImCoOMBP as a  Function of Temperature. Leo] , M P Q , Torr 2 o Temperature, C [Im] , M kobs' -1 sec 2.3 x io" 5 97 10 14.6 x io" 4 3.78 x 10~3 1.8 x io" 5 97 22 14.6 x io" 4 2.92 x 10~3 2.6 x io" 5 98 30 14.6 x io" 4 2.46 x 10"3 1.6 x io" 5 100 35 14.6 x io" 4 1.87 x 10"3 * "k , " in these cases determined from i n i t i a l regions obtained from f i r s t obs order in cobalt analyses. In these experiments deviation from f i r s t order occurred very quickly (e.g. at less than 25% of extent of reaction). The reactions could not be analyzed for second order in cobalt, and attempts to graphically determine the reaction order did not produce linear plots, presumably because the reaction order in cobalt changes as the cobalt concentration changes. Better f i r s t order approximations occurred at lower cobalt concentrations. -181-0 100 200 300 OXYGEN PRESSURE, TORR Figure VII.12 Oxidation rate of ImCoOMBP as a function of oxygen pressure. -182-Figure VII.13 Oxidation rate of ImCoOMBP as a function of Im concentration. -183-Since a rapid i n i t i a l equilibrium i s observed f o r the reaction ImCoOMBP + 0 2 1 ImCoOMBP(02) (VII.9) t h i s cannot be the rate determining step although i t would be consistent with f i r s t order rate dependence on both Co and 0 2-123 The mechanism proposed previously f o r the oxidation of the ImCoPpIXDME system was: K l , Im + CoPpIXDME ^ r = = i ImCoPpIXDME (VII. 10) \ ImCoPpIXDME + 02~, =± ImCoPpIXDME ( 0 ^ (VII. 11) ImCoPpIXDME (0 2) - »- ImCoPpIXDME (0 2) * (VII. 12) * f a s t ImCoPpIXDME ( 0 ) + ImCoPpIXDME ImCoPpIXDME (0 2) CoPpIXDME• Im (VII. 13) This led to the rate law: k * K l K 0 L 0 2 ^ L ^ C ° J T ^ [ p r o d u c t ] = 1 I K i [ L ] (VII. 14) where [CoJ^ i s the t o t a l concentration of reactant cobalt. 123 In the CoPpIXDME systems the Im complex was found to oxidize f a s t e r than other five-coordinate amine complexes, and t h i s was r a t i o n a l i z e d i n terms of Im forming a hydrogen bond with the 0 2 moiety of both species i n reaction VII.12, p a r t i c u l a r l y i n the t r a n s i t i o n state where charge separation would be more important. 123 In the CoPpIXDME system the f i r s t to zero order dependence i n Im was explained i n terms of formation of ImCoPpIXDME under the conditions of oxidation. This was supported by p l o t t i n g k o b s 1 vs. [ L ] from the intercept/slope a value s i m i l a r to that obtained spectrophotometrically was obtained. However, i n the studies on the CoOMBP system, the Im -184-concentration was such that the ImCoOMBP species was f u l l y formed (Chapter I I I ) , and the p l o t of ^ vs [im] 1 (Figure VII.14) yielded an intercept/slope 2 -1 value of 7 x 10 M , s u b s t a n t i a l l y lower than the spectrophotometrically 3 -1 determined value of 9 x 10 M . Therefore, an explanation of the observed Im dependence i n terms of formation of a hydrogen bonded Im to the O^ moiety of ImCoOMBP(0^) seems reasonable. 123 Although the oxidation of the CoPpIXDME system*" was f i r s t order i n cobalt at a l l concentrations studied, the cobalt dependence of the CoOMBP system became greater than f i r s t order at higher cobalt concentrations. Oxidation through a bimolecular pathway at higher cobalt concentrations would explain t h i s observation. Considering that CoOMBP dimerizes, the o v e r a l l mechanism becomes: K h(CoOMBP) . CoOMBP K Im + CoOMBP ImCoOMBP (VII.15) (VII.16) K ImCoOMBP + O 2 ^ ImCoOMBP(0 ) + ImCoOMBP (VII.18) k. ImCoOMBP(02)CoOMBP•Im i n t (VII.19) ImCoOMBP(02)••-Im (VII.20) ImCoOMBP(0 )'''Im + ImCoOMBP (VII.17) k^ _ (VII.21) f a s t ImCoOMBP(02)CoOMBP•Im + Im The o v e r a l l rate law would then be: -185-Figure VII.14 Analysis of Im dependence on the oxidation of ImCoOMBP -186-CHn ] [ 0 2][CcG T -^•[product] = - 2 " _ . , ( d + KjClm] + [ l m j [ 0 2 3 + Lo^) + 2 [Co"3 T V 2 2 ~ D r -1 r I ( V I I . 2 2 ) k K M Led] x (K. k £lm] + : , \ ((1 + K flm] + K K [im]TO J + K K K i n t ^ I m 3 &> 3) + 2[Co] V Under the experimental conditions used, the amount of (CoOMBP) present was n e g l i g i b l e and the ImCoOMBP species was e s s e n t i a l l y f u l l y formed. When t h i s i s taken into consideration the o v e r a l l rate law s i m p l i f i e s t o: K 0 [ 0 2 L R C ° ] T ci 2 dlfproduct] = ( i + K [ 0 J + K . K [ l m ] [ 0 J ) 0 2 2 i n t 0 2 2 ( V I I . 2 3 ) x (K. k [im] + i n t " L i " u (1 + K COJ + K. K [ l m ] [ 0 1) 0 2 2 i n t 0 2 2 From t h i s rate law i t can be seen that second order dependence on cobalt i s only observed i f k [ C o ] T i s large. I f k^Tco^ i s n e g l i g i b l e and Clm] i s r e l a t i v e l y small, the reaction i s f i r s t order i n Im. At higher Im concentrations the reaction would become zero order i n Im. Experiments were not c a r r i e d out 154 at higher Im concentrations since Im aggregation becomes important . F i r s t order dependence i n oxygen i s observed i f very l i t t l e of the 1:1 cobalt: dioxygen adduct i s i n i t i a l l y formed. At s u f f i c i e n t l y low concentrations of cobalt, where most of the studies were c a r r i e d out, the reaction becomes f i r s t order i n cobalt and obeys the rate law: -187-K K. k*[oJClm] [ C o J ] 0 2 mt 2 T ^[product] = ( 1 + ^ + K . n t K 0 2 C l m : C 0 2 ] , < V I I - 2 4 > According to Equation VII.24, under conditions where the reaction is predominantly f i r s t order in cobalt, a intercept/slope value obtained from -1 -1 - K i n t K 0 £°2^ a plot of k vs ClnQ (as in Figure VII.14) represents 2 2 Since the amount of dioxygen adduct formed under the conditions of this work has been estimated to be on the order of 10% (see p 175), an estimate of 4 -1 K. being about 10 M can be made. The a b i l i t y of ImCoOMBP(0) to form a mt 2. hydrogen bonded adduct Im must be less than that of ImCoPpIXDME(02) , where the corresponding species was thought to be completely formed when any 123 dioxygen adduct was available to interact with any hydrogen bonding Im This appears consistent with the weaker basicity of OMBP96, since the weakly basic porphyrin would be less able to transfer electron density to the dioxygen moiety, and thus make i t less susceptible to hydrogen bonding. * - l The intercept value in Figure VII.14 i s equivalent to k and gives a * -3 - l * k value of approximately 6 x 10 sec . This compares to a k value of 1.0 x 10 ^  sec * in the ImCoPpIXDME system"*"^3; the difference between the two systems can possibly be accounted for by the relative basicities of the porphyrins used in the two systems. The pseudo f i r s t order rate constant increased as the temperature was lowered (Figure VII.15, Table VII.2h, AH = -4.2 kcal/mole, and AS = -84 eu) This observation i s consistent with an increase in K and K. . a t low Q>2 m t -188-Figure VII.15 Arrhenius p l o t for the oxidation of ImCoOMBP. -189-temperatures; both of theses values would be expected to be associated with exothermic processes. Although a decrease i n the rate constant k* must occur, t h i s i s not s u f f i c i e n t to counteract the above enhancements to the pseudo f i r s t order constant. VII.5.2 K i n e t i c s of the Oxidation of LCoOMBP i n Toluene, L = CH^-Im, Pip, or Py When toluene solutions of five-coordinate LCoOMBP (L = CJ^-Im, Pip, or Py) were exposed to about an atmosphere of oxygen at 20°C, an i n i t i a l rapid decrease i n Soret i n t e n s i t y was noted. This was followed by a slow further decrease i n Soret i n t e n s i t y with an absorbance increase i n the 440 nm region. Isosbestic points were maintained f o r several hours, but then l o s t as the reaction proceeded. The reaction approached 95% completion a f t e r times up to one week. The reaction was monitored by v i s i b l e spectroscopy, ESR, and gas-uptake. As the concentration of LCoOMBP was increased, the time required to complete the reaction increased u n t i l at some of the highest concentrations -2 used (~2 x 10 M), no oxygen-uptake was observed. ESR signals of the 1:1 -3 cobalt:dioxygen adduct (@ -10 M Co) were detected throughout the course of the r e a c t i o n . The observation of the i n i t i a l decrease i n Soret i n t e n s i t y i s consistent with the formation of a 1:1 cobalt:dioxygen adduct at room temperature i n toluene. This conclusion i s supported by the observation that the amount of i n i t i a l Soret change i s dependent on the oxygen pressure (Appendix V.j) and by c a l c u l a t i o n s based on the thermodynamic data of CoOMBP-amine oxygenation (Chapter IV) which p r e d i c t that about 20% of the dioxygen adduct would be present at 20°C. This ambient temperature detection of the oxygenation adduct contrasts with observations for the -190-89 91c 91c 91c CoPpIXDME , CoMpIXDME , and CoDpIXDME systems in which no observable amount of reversible oxygenation occurred i n toluene. In experiments with varying ligand concentrations, essentially the same -4 -2 oxidation rates were observed. The ligand concentrations (3 x 10 - 10 M) were such that f u l l formation of the five-coordinate adduct was accomplished. This implies that, in contrast to the ImCoOMBP system, the ligand plays no role in the oxidation of the complex other than forming the oxygen-sensitive five-coordinate adduct. Figure VII.16 shows oxidation vs. time curves for CH^ -ImCoOMBP as a function of cobalt concentration. If the oxidation mechanism present in the ImCoOMBP system (Eq. VII.15-VII.21) were applicable in the CH^ -ImCoOMBP system, then a f i r s t order dependence in cobalt should be observed. Although a reasonable f i r s t order dependence was observed at —6 lower concentrations (-10 M), the reaction took longer to complete at higher cobalt concentrations, contrary to expectation i f "dimerization" occurred by a rate determining direct reaction of CH^-ImCoOMBP(0^) and CH^ -ImCoOMBP. Similar kinetic dependences were observed for the PipCoOMBP and PyCoOMBP systems. The oxidation rates for the latter two systems were somewhat less than for the CH^ -ImCoOMBP system and were not extensively investigated. The faster rate for the CH3~ImCoOMBP system i s consistent with larger i n i t i a l oxygenation, as observed and calculated from the low temperature binding data, i f the reaction occurs by a mechanism including steps similar to Eq. VII.15-VII.21. ESR experiments were carried out on the LCoOMBP systems at 77 K. When these solutions (-10 3 M) were i n i t i a l l y exposed to oxygen an intense 1:1 cobalt:dioxygen adduct signal was detected.If the solution were l e f t for 12 h at ambient temperature, the intense cobalt:dioxygen signal at 77 K remained. Over a period of ten days at ambient temperature the dioxygen -191-0 10,000 20,000 t i m e ( sec ) Figure VII.16 Oxidation of CH3-ImCoOMBP analyzed f o r f i r s t order i n cobalt at several cobalt concentrations. -192-adduct signal at 77 K was seen to decrease to about one tenth of i t s original intensity. While the visible spectroscopy experiments were done on LCoOMBP -6 -4 solutions of 10 - 10 M, and the E S R measurements were carried out at - 3 LCoOMBP concentrations of 10 M, the gas-uptake determinations were made -2 when the LCoOMBP concentrations approached 10 M. An experiment carried out -2 on a 2 x 10 M LCoOMBP solution indicated no reaction whatsoever. At a concentration of 5 x 10 3 M the reaction approached only 10% completion after two days. When the solutions from the gas-uptake experiments were appropriately diluted, the visible spectra verified that very l i t t l e oxidation had taken place. If a gas-uptake solution were taken to 7 7 K under oxygen, then a sharp E S R signal formed, indicating the presence of dioxygen" adduct. The above observations are consistent with the slow formation of oxidation products after an i n i t i a l formation of a 1:1 cobalt:dioxygen adduct. That the reaction time i s increased so markedly at higher cobalt concentrations implies that aggregation of the five-coordinate LCoOMBP probably plays a major role. As the dioxygen adduct i s formed at low temperatures at a cobalt concentration where the oxidation reaction i s effectively stopped at ambient temperature, i t i s concluded that oxygenation i s more exothermic than the proposed aggregation process. In spite of the above kinetic observations suggesting aggregation, no significant visible spectroscopic changes were detected as the concentrations of five-and six-coordinate CoOMBP adducts were varied from 5 x 10 5 x 10 M (Ch. VII.4), Therefore, the nature and degree of the aggregation could not be determined, A possible mode of aggregation could be similar to 140 that which occurs in the dicyanohemin system , where a pyrrole ring of one porphyrin i s weakly TT-bonded to a pyrrole ring of another porphyrin. Since -193-the four-coordinate CoOMBP system, which could be expected to more r e a d i l y 136a form higher order aggregates , tended to dimerize at concentrations where v i s i b l e spectroscopic studies were c a r r i e d out, dimerization of the f i v e -coordinate CoOMBP systems appears to be the most l i k e l y extent of aggregation at concentrations used i n v i s i b l e spectroscopic determinations. Assuming that the five-coordinate complex does dimerize, the o v e r a l l mechanism could be: K_ h(CoOMBP), L + •CoOMBP LCoOMBP as (VII.26) / >5 (LCoOMBP) K (VII.27) 2 LCoOMBP(02) k (VII. 2 8) LCoOMBP(02) + CoOMBP-L fa s t LCoOMBP(02)CoOMBP-L (VII. 29) (VII.15) (VII. 2 5) and the r e s u l t i n g rate law would be: — [ p r o d u c t ] = k K^Co^CCo^ (VII. 30) / ( l + K 1" 1[Lr J' + K Q C0 2 3 ) ' + 2[Co: r r(K T ass From t h i s rate law i t can be seen that as the concentration of cobalt increases, the dependence on cobalt decreases from f i r s t order to h a l f order. This i s consistent with the observed increase i n oxidation time, although the order of the reaction could not be determined at higher concentrations, presumably because the order i s increasing as the concentration -194-of reacting cobalt i s decreasing during the course of a particular reaction. At higher cobalt concentrations aggregates greater than dimeric of the four-or five-coordinate species could be possible, and i f such a higher order aggregate becomes the dominant form of cobalt, then the order in cobalt consumption would further decrease with increasing cobalt concentration. The oxygen dependence would decrease from f i r s t to zero order with increasing oxygen pressure as the 1:1 cobalt:dioxygen adduct becomes f u l l y formed. Similarly the ligand dependence decreases from f i r s t to zero order as the f i v e -coordinate adduct becomes f u l l y formed. However, i f the four-coordinate dimer i s dominant the ligand dependence would be f i r s t order; i f the five-coordinate dimer i s dominant the reaction rate i s independent of ligand concentration. Under the experimental conditions used in these studies, there i s almost no four-coordinate dimer present and the five-coordinate LCoOMBP species i s essentially f u l l y formed. The overall rate law then simplifies to The main contrast in the oxidation rates of the CH^-Im, Pip, and Py CoOMBP systems to the ImCoOMBP system i s the marked relative slowness of the former systems. As has been noted when Im is present, the reaction rate is dependent on the concentration of Im, even when the five-coordinate adduct i s f u l l y formed, presumably because the Im helps form the intermediate oxygenation species, possibly by a hydrogen bond from the acidic proton of Im. CH^-Im, Pip, and Py have no acidic protons and no ligand concentration enhancement of oxidation i s observed. Also in the ImCoOMBP system there i s no kinetic evidence for the role of ImCoOMBP aggregates. Perhaps the acidic kV to l[Co] —[product] (VII. 31) 2 -195-proton of Im hinders aggregate formation; pseudo f i r s t order reactions can be observed when CCoTJ^ i s monitored. Support of t h i s supposition i s obtained from studies on the oxidation of the ImCoPpIXDME system where the k i n e t i c s of that system were explained by proposing that Im i s associated with the 123 porphyrin complex before oxygenation occurs . Similar association of Im with CoOMBP could reasonably be expected to hinder aggregation. Although CH^-Im, Pip, and Py do not enhance the oxidation rate the same way Im does, oxidation of LCoOMBP s t i l l appears to take place through an analogous LCoOMBP(02) activated intermediate, since the oxidation reaction approaches f i r s t order i n cobalt at lower concentrations of CoOMBP. VII.5.3 K i n e t i c s of the Oxidation of LCoOMBP i n Solvents Other Than Toluene VII.5.3.1 K i n e t i c s of the Oxidation of CH^-ImCoOMBP i n a Var i e t y of Solvents Upon the observation that the CH^Im, Pip, and Py CoOMBP systems oxidized so slowly, with r e s u l t i n g d i f f i c u l t i e s i n performing k i n e t i c analyses, the oxidation rate was monitored i n a series of somewhat more polar solvents. In order to avoid complicating side reactions, the solvent had to be e s s e n t i a l l y non coordinating and non p r o t i c . I t was also desirable that the solvents have low freezing points so that oxygenation studies could be attempted. With the above requirements i n mind, a s e r i e s of chlorinated hydrocarbons; chlorobenzene (e = 5.6), 1-chlorobutane (e = 7.39), dichloromethane (e = 8.93), and DCE (e = 10.36)^"^; and a se r i e s of mixed toluene-DCE solvents; toluene:DCE 5:1 v/v (e = 3.22), toluene:DCE 4:1 v/v (e = 3.43), and toluene:DCE 1:1 (e = 5.32) were used. Since the CH -ImCoOMBP system was found to have the highest and most convenient oxidation rates for study, t h i s species was studied i n the above solvents. Trends observed i n the more polar solvents were s i m i l a r to those observed -196-in toluene. Although good isosbestic points occurred throughout the course of the reaction, the reactions usually did not analyze for f i r s t order in cobalt. When the cobalt concentration in dichloromethane was increased, the reaction took longer to proceed to completion. A more substantial spectral change was observed as the solvent became more polar, consistent with greater oxygenation in more polar media (Chapter IV). The reaction rate tended to increase with solvent polarity, although there was no general correlation between the reaction half l i f e and e (Table VII.3). However, a reasonable correlation existed within the toluene-DCE mixtures. Table VII.3 Oxidation Rate as a Function of Solvent Polarity when -5 o 1.8 x 10 M CH -ImCoOMBP i s Exposed to 800 Torr O at 20 C Solvent E t, (sec) Toluene 2.38 8400 5:1 toluene:DCE 3.22 7200 4:1 toluene:DCE 3.43 1900 1:1 toluene:DCE 5.32 250 Chlorobenzene 5.6 5200 1-Chlorobutane 7.39 8500 Dichloromethane 8.93 450 1,2-Dichloroethane 10.36 130 t, i s used since linear f i r s t order rate plots could not always be H obtained, t^ in these measurements does not include the i n i t i a l spectral change due to oxygenation. -197-VII.5.3.2 Kinetics of the Oxidation of LCoOMBP in DCE As the oxidation of CH^ -ImCoOMBP in DCE proceeded f a i r l y rapidly and yielded good isosbestic points, further studies were carried out in DCE with the ligands CH^-Im, Pip, and Py. In the CH"3-ImCoOMBP system, the observed reaction rates were rather large for conventional kinetic studies. The reaction times remained essentially the same as the cobalt concentration was increased from -6 -4 1.6 x 10 to 1.4 x 10 M; the i n i t i a l spectral change, along with the oxidation rate, increased somewhat as the oxygen pressure was changed from 200 to 800 Torr (Appendix Vp). This behavior i s similar to that of CH^ -ImCoOMBP in toluene, but the consistent f i r s t order dependence on cobalt suggests that aggregation of porphyrin complexes is unimportant. In the Py system, clean spectra were obtained but the reaction time increased as the cobalt concentration was increased from 1.8 x 10 ^  to -4 1.6 x 10 M (Appendix V q ) . Therefore, some aggregate could be present in this system. For the Pip system, reasonable f i r s t order dependence on cobalt was observed, the rates being very similar at cobalt concentrations of -5 -4 1.9 x 10 M and 1.9 x 10 M (Table VII.4), indicating that aggregation was not important at these concentrations. A linear Arrhenius plot of pseudo f i r s t order rate constants was obtained from 20°C to 40°C (AH* = 9.7 kcal/mole, AS*= -39 eu, Figure VII.17, Table VII.4), The increase of the rate constant with temperature in this system contrasts with the oxidation of ImCoOMBP in toluene. When Pip i s the axial ligand, there i s presumably no stabilization of LCoOMBP(O ) b y ligand hydrogen bonding, and so there i s no exothermic K. . step which could lower the observed AH int value. Figure V I I , 1 7 Temperature dependence on the oxidation of 2 x 1 0 M PipCoOMBP i n DCE A, Pseudo f i r s t order rate p l o t s B, Arrhenius p l o t For raw data see Appendix Vr. -199-Table VII.4 Kinetic Data for the Oxidation of PipCoOMBP in DCE Cobalt concentration, M P„ , Torr T, °C [Pip] , M k , sec 1 0 obs 2 -5 -4 -3 1.8 x 10 800 20 7.6 x 10 1.12 x 10 1.9 x 10~5 800 30 7.6 x 10~ 4 2.05 x 10~3 -5 -4 -3 1.9 x 10 800 40 7.6 x 10 3.66 x 10 1.9 x 10" 4 800 40 7.6 x 10~ 4 3.35 x i o " 3 Table VII.5a Kinetic Data for the Oxidation of (CH -Im) CoOMBP in 1:1 v/v toluene:CH -Im as a Function of Oxygen Pressure Cobalt concentration, M P , Torr °2 o T, C [CH3-Im] , M k , obs -1 sec 2 x 10~5 100 20 6.28 1.7 x i o ' 3 2 x 10~5 200 20 6.28 3.7 x i o " 3 2 x 10"5 300 20 6.28 6.4 x i o " 3 2 x IO - 5 400 20 6.28 1.1 x i o " 2 Table VII.5b Kinetic Data for the Oxidation of (CH -Im) CoOMBP in 1:1 v/v toluene:CH -Im as a Function of Temperature Cobalt concentration, M P Q , Torr o T, C [CH -Im] , M k , , obs -1 sec 2 x 10~5 400 10 6.28 6.0 x i o " 3 2 x 10~5 400 20 6.28 1.1 X i o " 2 2 x 10~5 400 30 6.28 2.0 x i o " 2 -200-Although the reaction did proceed faster and aggregation appeared to -4 be less important in DCE than in toluene at concentrations up to 10 M, ESR experiments indicated the persistence of the LCoOMBP species (detected as the dioxygen adduct) for 48 h at [Co] - 10 M, and the expected amount of gas-uptake (02:Co = 1:2) s t i l l had not taken place after 96 h at LCo} -2 - 10 M. Therefore, aggregation probably dccurs at higher concentrations, even in the more polar solvents. VII.6 Kinetics of the Oxidation of LCoOMBP The presence of a second axial ligand on CoOMBP should block the path to oxygenation and any subseguent oxidation via an inner sphere mechanism. Oxygenation studies (Chapter IV) showed that a l l LCoOMBP complexes were less oxygen sensitive than the corresponding LCoOMBP complexes. However, the bis CH^-Im complex was more oxygen sensitive than expected and solvent polarity contributions from CH3~Im were thought to be a factor. In studying ambient temperature oxidation, the solvent system consisted of 50% by volume of the coordinating amine, and so again contributions of the ligand to solvent polarity were considered. In studying PipCoOMBP (-2 x 10 ^ M) in the least polar of the solvent systems used, 1:1 v/v toluene:Pip (£ = 3.22), there was no evidence of any reaction after the system was exposed to 800 Torr of 0 2 at 20°C. For PyCoOMBP (-2 x 10~5 M) in 1:1 v/v toluene :Py (e = 7.37) under similar conditions a pseudo f i r s t order rate constant (k ~ 1.4 x 10~5 sec" 1) was measured and the reaction proceeded somewhat more quickly to completion than did 2 x 10 5 M PyCoOMBP under 800 Torr 0 2 in toluene (Ch. VII.5.2), suggesting a possible role of solvent polarity in enhancing oxidation. The oxidation of (CH^-Im)^CoOMBP in 1:1 v/v toluene:CH3-Im (c = 18.5) -201-proceeded so quickly that stopped-flow kinetic techniques had to be used to -5 monitor the reaction at cobalt concentrations of 2 x 10 M. As well as the reaction being f i r s t order in oxygen (Figure VII.18, Table VII.5a), a good f i r s t order dependence on cobalt was obtained at concentrations of 2 x 10 ^ M, but the reaction time increased somewhat as the concentration was increased -4 -3 to 2 x 10 M. A 5.6 x 10 M solution of (CH^-Im)2CoOMBP in the mixed toluene-CH^-Im solvent showed a Co.-O^  uptake in the ratio of 1:0.7 after -2 42 h of monitoring (see p 204). No uptake was observed in a 1.2 x 10 M -5 (CH3~Im)2CoOMBP solution. If the oxidation reaction of 2 x 10 M solutions of (CH^-Im)2CoOMBP were monitored as a function of temperature, a reasonably linear Arrhenius plot of the pseudo f i r s t order rate constants was obtained (AH = 12.8 kcal/mole, AS = -24 eu, Figure VII.19, Table VII.5b). Compared to the LCoOMBP systems that were studied (Ch. VII.5), the AH^ value of the (CH3~Im)2CoOMBP system is large, probably as a result of an endothermic dissociation of (CH3-Im)2CoOMBP being a necessary step for the oxidation of this system. The three bis amine complexes of CoOMBP were further studied in solutions of neat ligand. Pip2CoOMBP in neat Pip (e = 5.8) showed no signs of oxidation after 4h under 800 Torr of oxygen at 40°C, nor at 20°C after 1 day. PyCoOMBP in neat Py (e = 12.4) oxidized very slowly (t^ @ 40°C - 15 h) via a non f i r s t order in cobalt process. During the course of some of these reactions exposure to light was seen to cause an increase in reaction rate, possibly due to photolysis of a coordinated Py. However, in attempts to study this by flash photolysis, the recombination of the Py could not be observed, possibly because i t was so fast or only a small amount of - 2 0 2 -(CH3-Im)2CoOMBP i n 1:1 toluene:CH 3-Iin. For raw data see Appenix Vs. I 3.4 1/T x 10 3 (socf 1 ) Figure VII.19 Temperature dependence of the oxidation of 2 x 10 M (CH3~Im)2Co0MBP. A. Pseudo f i r s t order rate p l o t s . B. Arrhenius p l o t . For raw data see Appendix Vt. -204-dissociation was required to cause the observed enhancement of reaction. 156 In neat CH^-Im (e - 35) , (CH^-Im)^CoOMBP was observed to oxidize at a rapid f i r s t order rate (Figure VII.20, Table VII.5c). Gas-uptake results also showed an immediate uptake of oxygen to a 0.5:1 02:Co ratio for the oxidation reaction. This was followed by a very slow steady uptake over the course of days which i s not related to the oxidation (CH^-Im)2CoOMBP. The data suggest that in neat CH^-Im there i s insufficient aggregation occurring to impede the oxidation reaction. The very slow 0^ uptake li k e l y refers to oxidation of the amine solvent (this may or may not be catalyzed by the complex). The previous uptakes of 0.7:1 02:Co measured in the toluene solutions (see p 201) could similarly refer to amine oxidation. Comparable behavior has been noted for the oxidation of phosphines to phosphine oxide in the presence of metal 157 complexes In solvent systems consisting of 1:1 v/v ligand:DCE the oxidation rates were even greater than those in the neat ligand solutions. (CH3-Im)2CoOMBP was oxidized with an observed pseudo f i r s t order rate constant of 2.3 x 10 _ 1 sec" 1 at 30°C under 400 Torr in 1:1 CH3~Im:DCE (e - 25, Figure VII.20, Table VII.5c). This system also produced an immediate gas uptake in a 0.5:1 O^ '-Co ratio. The oxidation of PyCoOMBP in 1:1 Py:DCE (e = 12.7), (Figures VII.21,22, Tables VII.6a,6b) gave linear f i r s t order in cobalt plots and yielded a * =f= + linear Arrhenius plot (AH = 3.6 kcal/mole, AS = -64 eu). The low AH value, compared to that of the (CH^Im) 2CoOMBP system, i s consistent with DCE enhancing the formation of the LCoOMPB(02) intermediate compared to the toluene-CH -Im solvent used in the (CH-Im) CoOMBP system; enhanced formation of LCoOMBP(02> could occur from any protons present in DCE hydrogen bonding -205-T i m e (sec) Figure VII.20 F i r s t order rate p l o t s f o r the oxidation of 2 x 10 (CH3-Im)2CoOMBP. For raw data see Appendix Vx. • In neat Cf^-Im (lower time axis) O In 1:1 DCE:CH,-Im (upper time axis) -206-Table VII.5c K i n e t i c Data f o r the Oxidation of (CH,-Im) CoOMBP as a Function of Solvent Cobalt concentration, M P , Torr T, °C Solvent k , sec 2 X i o " 5 400 20 1 :1 tol:CH-Im 1.1 x 10 2 2 X i o " 5 400 20 3 neat CH-Im 1,5 x 10~ 2 -5 2.0 2 X 10 -5 400 30 1 :1 tol:CH 3-Im x 10 2 X 10 400 30 1 :1 DCE:CH-Im 2.4 x 10 Table VII.6a K i n e t i c Data for the Oxidation of PyCoOMBP as a Function of Cobalt Concentration i n 1:1 v/v DCE:Py Cobalt concentration, M P , Torr T, C [Py) , M k o b s ' S e C 2 x 10~ 6 800 20 6.21 1.55 x 10 1.8 x 10" 5 800 20 6.21 1.43 x 10 1.9 x 10~ 4 800 20 6.21 1.74 x 10 Table VII.6b K i n e t i c Data f o r the Oxidation of PyCoOMBP as a Function of Temperature i n 1:1 v/v DCE:Py Cobalt concentration, M P^ , Torr T, °C [Py] , M k 0 b s ' S 6 C 1 1.8 x 10~ 5 800 20 6.21 1.43 x 1 0 _ 4 1.8 x 10~ 5 800 30 6.21 1.88 x 10~ 4 1.9 x 10" 5 800 40 6.21 2.32 x 10~ 4 -207-0 10000 20000 T i m e (sec) Figure VII. 21 F i r s t order rate p l o t s for the oxidation of Py^CoOMBP in 1:1 DCE:Py as a function of fCoT . For raw data see Appendix Vz. O CCoT = 2 x i o - 6 M © CCo] = 1.8 x 10~ 5 M • CCoJ = 1.9 x 10~ 4 M 10000 20000 -0032 .0033 .003^ Time (sec) Vf IK"1) -5 Figure VII.22 Temperature dependence of the oxidation of 2 x 10 M Py^CoOMBP. A. Pseudo f i r s t order rate p l o t s . B. Arrhenius p l o t . For raw data see Appendix Vy. -209-to LCoOMBP(O^) • In contrast to the studies in toluene, f i r s t order in cobalt behavior persisted as the cobalt concentration was increased from -6 -4 2 x 10 to 1.9 x 10 M, again showing that the nature of the solvent is important in determining the degree of aggregation of porphyrin systems. The oxidation of PipCoOMBP solutions in 1:1 Pip:DCE (e = 7.4) proceeded in a f i r s t order manner when the cobalt concentration was -5 -4 2 x 10 M, but at 2 x 10 M the observed rate was much less and non f i r s t order (Figure VII.23, Table VII.7). However, after about an hour, crystals of Pip'HCl precipitated. This also occurred in the absence of complex. The mixed Pip-DCE solvent system was the only one where the ligand hydrochloride was observed, probably in part due to Pip being the strongest base of a l l the ligands studied, so that there i s sufficient formation of the salt to cause precipitation. Formation of amine hydrochloride has been 51e noted previously in chlorinated hydrocarbon solvents . Oxidation of PipCoOMBP i s enhanced in the more polar DCE solvent system; protons are present in this system, and the presence of protons has been shown to enhance 123 oxidation rates and has been suggested to prevent porphyrin aggregation (Ch. VII.5.2). Since there i s a large concentration of basic Pip required to f u l l y form Pip^CoOMBP, and since amine hydrochlorides were not observed when similar concentrations of the other strongly basic ligands, CH3~Im and Py, were used, the concentration of free protons present must be very small. However, the formation constant of the hydrogen bonded Im adduct of the 4 -1 ImCoOMBP system < K £ n t ~ 10 M ) suggests that any available protons could form hydrogen bonded adducts of LCoOMBP(0^) complexes. The presence of even trace amounts of such complexes could account for the observed enhanced oxidation, i f the oxidation rate of these complexes were sufficiently high. The mechanism of oxidation of L^CoOMBP systems appears to be similar -210-o--0.2H OAH i < -0.6H -0.8-i o 4000 8000 Time (sec) Figure VII.23 F i r s t order rate p l o t s f o r the oxidation of PipCoOMBP i n 1:1 DCE:Pip. For raw data see Appendix Vaa • LCo] = 2.0 x I O - 5 M -4 O CCoD = 2.1 x 10 M -211-Table VII.7 K i n e t i c Data f o r the Oxidation of PipCoOMBP as a Function of Cobalt Concentration i n 1:1 v/v DCE:Pip o _ -. -1 Cobalt concentration, M P^ , Torr T, C [Pipl, M k , , sec 0 2 obs 2.0 x 10~5 800 20 5.05 1.37 x 10~4 2.1 x 10~4 800 20 5.05 slower, non f i r s t order Table VII.8a K i n e t i c Data f o r the Oxidation of CH -ImCoOEP i n DCE as a Function of Cobalt Concentration Cobalt concentration, M P , Torr °2 T, °C [CH -InG , M , -1 k , , sec obs 7.5 x 10~ 6 800 30 0.0625 1.32 x 10~ 3 3.1 x 10~ 5 800 30 0.0625 1.53 x 10" 3 7.9 x 10~ 5 800 30 0.0625 1.60 x 10" 3 6.6 x 10~ 4 800 30 0.0625 1.22 x 10~ 3 2 x 10" 3 800 23 0.0625 -3.2 x 10~ 4 non f i r s t order Table VII. 8b K i n e t i c Data f o r the Oxidation of CH -ImCoOEP i n DCE as a Function of Oxygen Pressure Cobalt concentration, M P„ , Torr °2 T, °C [CH -Im] , M , -1 k . , sec obs 6.7 X io" 5 100 30 0.0625 2.62 x 10~4 7.9 X io" 5 150 30 0.0625 3.55 x 10~4 7.2 X 10"5 200 30 0.0625 5.72 x 10~4 8.2 X io" 5 300 30 0.0625 7.60 x I O - 4 7.9 X io" 5 400 30 0.0625 1.00 x 10~3 5.6 X io" 5 530 30 0.0625 1.29 x 10~3 6.5 X io" 5 670 30 0.0625 1.49 x 10"3 7.9 X io" 5 800 30 0.0625 1.60 x 1 0 -3 -212-to that outlined in equations VII.15, 25-29 with the additional formation of LCoOMBP and possibly (LCoOMBP) 2 > VII.7 Kinetics of the Oxidation of LCoP, P \* OMBP In order to determine whether the observed aggregation phenomena were peculiar to CoOMBP, oxidation of several other cobalt(II) porphyrin complexes 89 was investigated. Previous equilibrium studies have shown that CoPpIXDME , 89 95 CoMpIXDME , and CoOEP and the five-coordinate analogues do not form aggregates at concentrations from 10 to 10 M, although studies were not carried out at higher concentrations. When 8 x 10 ^ M solutions of CH^-ImCoOEP in DCE were exposed to oxygen, a large, rapid, i n i t i a l spectral change was observed. This was followed by spectral changes with good isosbestic points. The reaction rates analyzed for f i r s t order in cobalt and the pseudo f i r s t order constants were —6 essentially independent of cobalt concentration from 7.5 x 10 to -4 6.6 x 10 M (Figure VII.24, Table VII.8a). A strong ESR signal of the cobalt-dioxygen adduct, observed in a 2 x 10 3 M CH^-ImCoOEP solution under 800 Torr 0 2 at 22°C, gradually disappeared, but in a non f i r s t order manner and over a period of time longer than observed in the more dilute systems. The amount of i n i t i a l spectral change was dependent on oxygen pressure (Figure VII.25), but was too fast to be followed, even by using stopped-flow techniques. Gas-uptake studies at 15°C under 800 Torr of 0 2 on a 6 x 10 3 M CH^-ImCoOEP solution showed an i n i t i a l 1:3 O^Co rapid uptake, followed by uptake over the next five hours to the 1:2 O^Co ratio expected for formation of CH3-ImCo0EP(02)Co0EP'CH3-Im. Oxygen dependence studies (Figure VII.25, -5 o Table VII.8b) on 8 x 10 M CH^-ImCoOEP at 30 C showed that the reaction was f i r s t order in oxygen to 400 Torr, but decreased towards a zero order -213-1000 2000 Time (sec) Figure VII.24 F i r s t order rate p l o t s f o r the oxidation of CH^ImCoOEP by 800 Torr 0 2 i n DCE @ 30°C as a function of [Co]. See Appendix v f f . X [Co] = 7.5 x 10~ 6 M © [Co] = 6.6 x 10~ 4 M • [Co] = 3.1 X 10~ 5 M 0 [Co] = 2 x 10" 3 N O [Co] = 7,9 x 10~ 5 M Figure VII.25 Oxygen pressure dependence of the oxidation of ~7 x 10 M CH3-ImCoOEP @ 30°C i n DCE. A. Pseudo f i r s t order rate p l o t s . B. Pressure dependence p l o t . For raw data see Appendix Vhh. -215-dependence at higher pressures. The pseudo f i r s t order rate constant was essentially invariant as the CH^Im concentration was changed from 0.0251 to 0.248 M (Figure VII.26, Table VII.8c). The amount of i n i t i a l spectral change upon exposure to 800 Torr 0^ increased as the temperature at which the reaction o o was studied was lowered from 30 C to 10 C (Figure VII.27). A linear Arrhenius relationship was found for the pseudo f i r s t order rate constants (AH* = 18.5 kcal/mole, AS* = -10 eu, Figure VII.27, Table VII.8d). The f a i r l y large observed AH* value i s consistent with much of the reacting system being present as enthalpically favored CH^-ImCoOEP(0^). The observed f i r s t order dependence in cobalt at concentrations less than 10 3 M is consistent with the mechanism involving a rate determining formation of CH^ImCoOEP {O^) postulated for the CoOMBP systems (Chapter VII. 5) 123 and for the ImCoPpIXDME system in toluene . Aggregation of CH^ImCoOEP does not appear to be important at these concentrations. Although the increase in reaction time at a CH^-ImCoOEP concentration of 2 x 10 3 M in the ESR experiment i s consistent with aggregation, the oxygen concentration i s not -3 -1 greatly in excess (0^ solubility 7.2 x 10 Matm ), and this could also contribute to the apparent "slow down" in reaction. However, the further extended reaction time in the 6 x 10 3 M CH^-ImCoOEP system used in the gas-uptake experiment is indicative of aggregation as the oxygen concentration i s kept constant throughout the experiment. The observed oxygen dependence i s consistent with increasing formation of CH^-ImCoOEP(O^) as the pressure i s increased. That the oxygen dependence decreases at higher pressures lends support to this picture and i s interesting 123 as studies on other systems showed s t r i c t l y f i r s t order dependence, because l i t t l e dioxygen adduct had been formed. The intercept/slope value of the plot of k " 1 vs P. _ 1 (Figure VII.28) yields a P, 0„ value of obs 0^ *s 2 -216-O - i 0 1000 2000 T i m e (sec) Figure VII.2 6 F i r s t order rate p l o t s for the oxidation of CH -ImCoOEP i n DCE as a function of [CH^-Iml. For raw data see Appendix Vk. O [CH -Im] = 0.0251 M © CCH 3-Im] = 0.0827 M • rCH3-Im] = 0.248 M -217-Table VII.8c K i n e t i c Data f o r the Oxidation of CH3-ImCo0EP i n DCE as a Function of CH_,-Im Concentration Cobalt concentration, M P o , Torr T, °C [CH -Im] , M k , obs sec 7.8 x 10~ 5 800 24 0.0251 1.12 x 10~" 8.2 x 10~ 5 800 24 0.0827 1.25 x 10~" 7.6 x 10~ 5 800 24 0.248 1.60 x 10~' Table VII.8d K i n e t i c Data for the Oxidation of CH3~ImCoOEP i n DCE as a Function of Temperature Cobalt concentration, M P , Torr 2 o T, C [CH -Im] , M k , obs -1 sec 7.0 x I O - 5 800 10 0.0625 1.8 x i o "4 7.9 x 10~ 5 800 20 0.0625 6.4 x i o- 4 7.9 x 10~ 5 800 30 0.0625 1.60 : x 10~ Table VII. 8e K i n e t i c Data f o r the Oxidation of CH -ImCoOEP as a Function of Solvent Cobalt concentration, M P , Torr °2 o T, C Solvent K K obs -1 , sec -4 6.6 x 10 6.0 x 10~ 4 6.1 x I O - 4 800 800 800 30 30 30 DCE CH3~Im DMA 1.22 1.09 . 1.03. x 10~ 3 x 10~ 3 -4 x 10 o-i Figure VII.27 Temperature dependence of the oxidation of ~8 x 10 M CH3-ImCoOEP i n DCE. A. Pseudo f i r s t order rate p l o t s . B. Arrhenius p l o t . For raw data see Appendix Vgg. -219-0 .002 .004 .006 .008 .01 V (Torr"1) M 0 2 Figure VII.28 Analysis of oxygen dependence on the oxidation of CH^-ImCoOEP o at 30 C. -220-3 o 2.9 x 10 Torr at 30 C. Although the experimental error involved m this determination must be considered large as the intercept occurs very close to 3 the origin, the P^C^ does agree reasonably with the value of 2.4 x 10 Torr estimated from the i n i t i a l spectral change observed when 800 Torr of i s added to CH3-ImCoOEP. The gas-uptake results imply a p ^ ° 2 v a i u e of -1.6 x 10 3 Torr at 15°C, and the i n i t i a l spectral changes suggest a P,0 T 2 value of -700 Torr at this temperature. While the discrepency between these two values i s not unreasonable, i t i s also consistent with aggregated CH-Im at the higher concentration. Direct P measurements of the CH -ImCoOEP 3 0 2 3 system in DCE are not feasible as oxidation proceeds rapidly at ambient temperature and the solvent freezes at lower temperatures. The independence of the rate on CH^-Im is consistent with the five-coordinate CH^-Im species being f u l l y formed at a l l concentrations studied, with the CH3-Im taking no further part in the reaction mechanism. This view is supported by independent CH3~Im binding studies where was found to be 9.1 x 10 2 M 1 at 23°c in toluene 9 5. Solvent variation studies suggest that values do not vary much from solvent to solvent (Chapter III). The linear Arrhenius plot and observed pseudo f i r s t order nature of the reaction throughout the temperature range studied are more indications that no aggregation occurs at CH^ImCoOEP concentrations of 8 x 10 5 M. -4 In DMA the oxidation rate of 6 x 10 M O^-ImCoOEP was an order of magnitude slower than in the less polar DCE (Figure VII.29, Table VII.8e). However, DMA binding to CoOEP was not measured, so i t cannot be certain that the reacting species i s CH3~ImCoOEP, although the CH3~Im adducts of 89 CoPpIXDME and CoOMBP (Chapter IV) appear to remain as such in the similar DMF solvent system. In neat CH3~Im the oxidation rate was comparable to that observed in DCE (Figure VII.29, Table VII.8e). -221-0 1000 2000 Time (sec) Figure VII.29 F i r s t order rate p l o t s f o r the oxidation of 6 x 10 M CH -ImCoOEP i n DCE @ 30°C i n a v a r i e t y of solvents. Upper time axis r e f e r s to DMA, lower to DCE and CH^-Im. For raw data see Appendix V i i . -222-Similar observations were also noted in the other porphyrin systems. With CH3-ImCoEpI in DCE a f i r s t order dependence in cobalt was observed at concentrations used in visible spectroscopy (Figure VII.30, Table VII.9a). Large i n i t i a l spectral changes were observed and an ambient temperature ESR signal of CI^-ImCoEpI (C>2) was detected which took longer to go to products than was the case when visible studies were done at lower concentrations. Also a linear Arrhenius relationship for the pseudo f i r s t order rate constants was observed (AH* = 11.8 kcal/mole, AS* = -32 eu. Figure VII.31, Table VII.9b). Again the AH* value i s f a i r l y high and this is consistent with much of the reacting system being present as LCoP(0 2). -6 -4 For CH3-ImCoPpIXDME in concentrations from 7.6 x 10 to 7.9 x 10 M in DCE pseudo f i r s t order behavior was observed at 20°C (Figure VII.32, Table VII.10). In DCE reasonable pseudo f i r s t order plots were achieved for the oxidation of CH^ImCoTPP (Figure VII.33, Table VII. 11), and the rate constants remained reasonably constant over a 200 fold change in cobalt -6 -4 concentration from 4.0 x 10 to 7.6 x 10 M. In contrast to the other porphyrin systems studied in DCE, there was very l i t t l e i n i t i a l oxygenation in this system. Oxidation of CH3-ImCoTPP in toluene proceeded very slowly (t - 4 d at 30°C under 800 Torr 0 2). The observed behavior was similar -6 -5 at cobalt concentrations of 3.6 x 10 and 5.1 x 10 M (Appendix Vbb). -223-0 600 1200 1800 Time (sec) Figure VII.30 F i r s t order rate p l o t s f o r the oxidation of CH^-ImCoEpI i n DCE @ 30°C as a function of L"Co3. See Appendix Vdd. • [CoJ = 6.0 x 10~ 6 M O LCoJ = 4.7 x 1 0 _ 4 M <B [Co3 = 1.1 x 10~ 5 M X CCoJ = 2 x 10" 3 M © [Co] = 5.2 x 10~ 5 M -224-Table VII.9a K i n e t i c Data f o r the Oxidation of CH,-ImCoEpI i n DCE as a Function of Cobalt Concentration Cobalt concentration, M P , Torr 2 T, °C CCH3-InO , M . -1 k , , sec obs 6.0 x I O - 6 800 30 0.0625 1.75 x I O - 3 1.1 x I O - 5 800 30 0.0625 1.51 x 1 0 " 3 5.2 x 1 0 ~ 5 800 30 0.0625 1.60 x 10" 3 4.7 x I O - 4 800 30 0.0625 1.54 x 10~ 3 2 x 1 0 " 3 800 22 0.0625 -3.7 x 1 0 ~ 4 non f i r s t order Table VII.9b K i n e t i c Data f o r the Oxidation of CH -ImCoEpI j . n DCE as a Function of Temperature Cobalt cocentration, M P Q , Torr T, °C [CH 3-Im], M . -1 k , , sec obs 1.2 x I O - 5 800 10 0.0625 3.6 x 1 0 ~ 4 1.2 x I O - 5 800 20 0.0625 8.1 x 1 0 " 4 1.1 x 1 0 ~ 5 800 30 0.0625 1.51 x 1 0 ~ 3 Table VII. 10 K i n e t i c Data f o r the Oxidation of CH -ImPpIXDME i n DCE as a Function of Cobalt Concentration Cobalt concentration, M P Q , Torr T, °C [CH 3-Im], M k , , obs -1 sec 7.6 x 1 0 ~ 6 800 20 0.0625 7.7 x i o ' 4 6.0 x 1 0 ~ 5 800 20 0.0625 8.0. x i o " 4 7.9 x 1 0 ~ 4 800 20 0.0625 6.2 x i o " 4 Table VII.11 K i n e t i c Data f o r the Oxidation of CH -ImCoTPP i n DCE as a Function of Cobalt Concentration Cobalt concentration, M P , Torr 2 T, °C [CH 3-Infl, M k . obs -1 , sec 4.0 x 10" 6 800 30 0.0625 6.36 x 10~ 4 5.2 x 10" 5 600 30 0.0625 7.04 x I O - 4 7.6 x 10~ 4 800 30 0.0625 7.23- x I O - 4 Figure VII.31 Temperature dependence of the oxidation of ~1 x 10 M CI^-ImCoEpI i n DCE. A. Pseudo f i r s t order rate p l o t s . B. Arrhenius p l o t . For raw data see Appendix Vee. -226-O-i 0 900 1800 2700 3600 T i m e ( s e c ) Figure VII.3 2 F i r s t order rate p l o t s f o r the oxidation of CH -ImCoPpIXDME i n DCE as a function of [Co]. See Appendix V j j . O [Co] = 7.6 x 10~ 6 M • [Co] = 6.0 x 10~ 5 M -4 X [Co] = 7.9 x 10 M -227-Time (sec) Figure VII.33 F i r s t order rate p l o t s f o r the oxidation of CH^-ImCoTPP i n DCE as a function of [Co]. See Appendix Vcc O [CoJ = 4.0 x 10 M -5 © [Co] = 5.2 x 10 M • Ceo] = 7.6 x 10~ 4 M -228-VII.8 Concluding Comments From these studies i t can be seen that numerous species may be involved in the irreversible oxidation of cobalt porphyrin systems. An overall scheme for this process could be: "a (CoP) MLCoP) + LCoP, k LCoP (OJ CoPL < - fl LCoP (OJ LCoP(Oj • • -L LCoP(Oj' LCoPfOj fast. f ast^ + LCoP LCoP (OJ CoPL + L +LCoP LCoP (OJ CoPL Jj(L 2CoP) 2 LCoP(02)•.-H LCoP(02) ---H "fast, +LCoP LCoP(02)CoPL + H This scheme could be further complicated by the presence of higher order aggregates of cobalt porphyrins, especially at higher concentrations. Although studies in DCE suggest the reaction i s enhanced by free protons, the precise pathway has not been elucidated, and evidence has been presented in other 158 studies for outer sphere 0 2 oxidation of metalloporphyrins with protons being involved. Deriving an overall rate law for the above scheme, entailing a l l the steps indicated, yields the following somewhat intractable expression: -229-d K o / ° 2 ] [ C O J T — [ p r o d u c t ] = , — — ; / ( l + K . CL] + K CL] + ( i + K . C U + K + [ H 3 ) K CO 1)' 1 2 i n t H 2 \ + 2 C C o J T ( K A S 2 + K 2 K 2 W + h 2^'\~ 2) (VII.32) x k rco] d L J T / ( l + K 1 " 1 C L ] " 1 + K 2CLJ + ( l + K . n t C L j + K H + C H + J ) K Q CO2J)' + 2[co] m(K 2 + K 2 K _ 2 C L ] 2 + K " 2 [ L D " 2 K ~ 2) T ass^ ass 2 I D + k * i K i n t [ L l + k*2 + k * 3 V [ H l In s p i t e of the very complex o v e r a l l nature of the oxidation of cobalt porphyrins, several of the parameters involved i n these systems are sometimes eliminated by s e l e c t i n g appropriate r e a c t i o n conditions, and s i m p l i f i e d treatments can be used (e.g. Eq. VII.24). From d e t a i l e d studies of the cobalt porphyrins, the r e l a t i v e energy l e v e l s of the species involved i n oxidation can be estimated, and are represented i n the energy l e v e l diagram below: 0 1 (LCOP) 0 2 -230-The r e l a t i v e energy l e v e l s of CoP, LCoP, L^CoP, and LCoPfOj are determined by independent thermodynamic studies on one species being converted to another (Ch. I l l - Ch. V I ) . The energy l e v e l s of the dimeric species are estimated from reports that aggregation i s an exothermic 139 process and an observation that oxygenation of LCoOMBP i s more exothermic than aggregation of that species (Ch. VII.5). L i g a n d - s t a b i l i z e d or proton-s t a b i l i z e d dioxygen adducts should l i e at lower energy than non- s t a b i l i z e d adducts. The slowness i n formation of any "activated intermediate" demonstrates a su b s t a n t i a l energy b a r r i e r to the formation of these species. The energy l e v e l of the "activated intermediate" could l i e anywhere between that of the t r a n s i t i o n state of i t s formation and that of the f i n a l unreactive product. For a negative A H * value to be observed i n the oxidation of the ImCoOMBP system, the energy l e v e l of ImCoOMBP must be higher than that of the t r a n s i t i o n state i n that process. The above diagram shows that t h i s seems reasonable. As mentioned previously, the pathway involved i n proton-enhanced oxidation i s not yet c l e a r , and what i s represented on the energy l e v e l diagram r e s u l t s from a p l a u s i b l e process. One s t r i k i n g aspect of the foregoing work i s the observation of a f i r s t order dependence on cobalt at 10 6 - 10 5 M f o r o x i d a t i o n , throughout a l l the cobalt porphyrin complexes studied. Some form of act i v a t e d intermediate * 123 (LCoPfOj ) has already been postulated f o r these systems . As the oxidation r e a c t i o n i s enhanced i n more polar media, t h i s suggests that the activated intermediate could be even more polar than the superoxide-like Co(III)-0'^ moiety generally accepted to be present i n the 1:1 Co-.O^ 22 23 159 adduct ' . A recent report suggests that an e s s e n t i a l step f o r the rea c t i o n of Pt(PPh (where the dioxygen i s present as a peroxide) with HX (X = N0 3 , C l , of B F 4 ) or RCOR' i s the rearrangement of the peroxide to a superoxide-like intermediate: -231-Ph„P 0—0 (VII.33) Ph,P ((PPh 3) 2Pt(I)0 2 ) An observation of the above study was a decrease in reaction rate with an increase i n solvent polarity. In the LCoP-dioxygen systems considerable 2 3 change in geometry and bonding characteristics (sp — * s p ) of coordinated dioxygen i s required for formation of the bridged dimer species. Kinetic findings in this chapter suggest that a large amount of this change to the more peroxide-like dioxygen occurs before a rapid attack by another cobalt complex. A peroxide-like species, having more charge density than a superoxide-like species, would be expected to be formed more readily in more polar media, thus accounting for the enhanced oxidation rate in the more polar solvents. From these studies i t can be seen that the nature of the solvent plays a major role i n aiding the oxygenation or oxidation of a cobalt(II) porphyrin complex. As would be anticipated, the solvent polarity i s important in stabilizing the polar 1:1 cobalt:dioxygen adduct. However, solvents such as DCE appear to f a c i l i t a t e oxidation by another important process, that i s by preventing aggregation of porphyrin complexes, which appears to occur generally for cobalt porphyrins i f their concentration becomes sufficiently high. Of the cobalt porphyrins studied, the OMBP systems appear to aggregate the most - 6 readily i n non aqueous solvents. At vi s i b l e spectroscopic concentrations (10 -10 5M) the other cobalt porphyrins do not appear to aggregate, as deduced from the kinetic results presented in this chapter and observations that 89 89 95 CoPpIXDME , CoMpIXDME , and CoOEP have constant K.^ and K Q values throughout this range. Similar observations for CoDADIXDME and CoDBrDIXDME systems indicate that aggregation of these porphyrins i s not important in this concentration range where Kn and K n are determined. -232-CHAPTER VIII THERMODYNAMICS AND KINETICS OF REVERSIBLE LIGAND AND DIOXYGEN BINDING TO Co(SALEN) COMPLEXES VIII.1 Q u a l i t a t i v e S p e c t r a l Observations The v i s i b l e spectrum of a so l u t i o n of Co(salen) i n dichloromethane 4 -1 -1 has a prominent band at 410 nm (E = 1.15 x 10 M cm ) which i s somewhat broader than cobalt porphyrin Soret bands. A s i m i l a r v i s i b l e spectrum i s observed i n DMF or toluene, with the band p o s i t i o n s h i f t e d to 408 nm i n the former solvent and 418 nm i n the l a t t e r . Addition of a nitrogen-coordinating ligand (L) caused a general decrease of absorption i n the band region with an i s o s b e s t i c point occurring between 530 and 555 nm (Figure VIII.1). Exposure of a five-coordinate LCo(salen) adduct t o dioxygen at low temperature caused a broad band to develop i n the 390 nm region with i s o s b e s t i c points occurring at e i t h e r side of the band (Figure VIII.2). Addition of dioxygen at low temperature t o Co(salen) resulted i n spe c t r a l changes somewhat s i m i l a r to those occurring when amine ligands were added (Figure VIII.3); at higher concentrations, the orange Co(salen) so l u t i o n became very dark (black) upon oxygenation. A l l of the above reactions were reversed when the s o l u t i o n was warmed and/or when dioxygen was pumped o f f . Amine l i g a t i o n , and oxygenation of an amine adduct, were too f a s t to be measured on a stopped-flow time scale; For raw data see Appendix V i a . 400 450 500 550 600 650 700 WAVELENGTH, NM Figure VIII.2 CH -ImCo(salen) + 0„„ CH -ImCo(salen) (OJ @ -78 C i n dichloromethane. 3 3 2 3 2 Dotted trace shows the spectrum of CH 3-ImCo(salen) (OJ Co (salen) CH^Im. For raw data see Appendix VId. -235-Figure VIII.3 Spectra showing the oxygenation of Co(salen) by 205 Torr O @ -83.5°c i n dichloromethane. See Appendix Vlh. -236-oxygenation of Co(salen) proceeded at a rate r e a d i l y measured by conventional methods. A band more d i s t i n c t than that of the 1:1 dioxygen adducts frequently occurred i n the 390 nm region (Figure VIII.2) , e s p e c i a l l y i n the l a t e r stages of an experiment and at higher temperatures; t h i s spectrum was a t t r i b u t e d to a bridged dioxygen LCo(salen)(O^)Co(salen)L species. A summary of the s p e c t r a l observations i s presented i n Table V I I I . l . VIII.2 Treatment of Data The ligand and dioxygen binding reactions of Co(salen) are analogous to the corresponding reactions of CoOMBP, and are treated i n a manner s i m i l a r t o that discussed for those systems (Chapters I I I , IV). A 1:1 binding behavior was co n s i s t e n t l y observed i n these systems. In the case of CH^-Im and Pip reacting with Co(salen), the 1:1 observed binding behavior was corroborated by a three l i n e superhyperfine s p l i t t i n g i n ESR signals at 77 K i n dichloromethane (Figure VIII.4) r e s u l t i n g from the coordinated nitrogen ( 1 = 1 ) . However, i n the case of the Co(salen)-Py system a f i v e l i n e superhyperfine ESR s p l i t t i n g was observed at 77 K i n frozen dichloromethane-Py, i n d i c a t i v e of a b i s Py adduct under these conditions. Confirmation that 1:1 00:0^ adduct occurred upon low temperature addition of dioxygen to four-and five-coordinate Co(salen) systems was obtained from the presence of a sharp ESR s i g n a l (Figure VIII.5), i n d i c a t i v e of some unpaired electron density on the oxygen atoms (I = 0). A - A Figure VIII.6 shows the log — vs log [CH -Im] p l o t s r e s u l t i n g from j~A rA j CO the addition of CH3-Im to Co(salen) i n dichloromethane. Figures VIII.7 and 8 A - A show the log ^ — — vs log P Q p l o t s f or the addition of dioxygen to oo 2 Table V I I I . 1 Spectral Data From 350 to 750 nm for the Reactions Involving Co(salen). a. L + Co(salen) LCo (salen) Absorbing Species Solvent Absorption Maximum, nm (Extinction C o e f f i c i e n t , M "'"cm 1) Isosbestic Points (nm) Co(salen) CH 2C1 2 410 (1.15 x 10 4) + CH3~Im CH 2C1 2 544 + Pip CH 2C1 2 530 + Py CH C l 2 2 479 475 519 + °2 CH C l 2 2 434 Co(salen) DMF 408 (1.13 x 10 4) + CH3~Im DMF DMF 554 + Pip 552 + Py DMF 446 456 534 Co(salen) Toluene 418 (1.18 x 10 4) + CH3-Im Toluene 449 + Pip Toluene 445 + Py Toluene 450 464 532 Table V I I I . l cont. Spectral Data From 350 to 750 nm for the Reactions Involving Co(salen). b. LCo(salen) + O 1 LCo(salen) (0 ) Oxygenated Species Solvent Absorption Maximum, nm Isosbestic Points (nm) -1 - l x (Axial Ligand) (Extinction C o e f f i c i e n t , M cm ) CH3-Im CH 2C1 2 385 (9 X io3) 362 444 Pip CH 2C1 2 386 (9 X io3) 443 489 Py CH 2C1 2 384 (9 X io3) 445 CH3-Im DMF 385 (9 X io3) 364 456 Pip DMF 389 (9 X io3) 452 Py DMF 386 (9 X io3) 447 515 CH3-Im Toluene 382 (8 X io3) 366 446 Pip Toluene 382 (8 X io3) 372 457 Py Toluene 382 (8 X io3) 368 Table VIII.1 cone. Spectral Data From 350 to 750 nm for the Reactions Involving Co(salen). c. LCo(salen) (0_) Co (salen) I, Absorbing Species Solvent Absorption Maximum (Axial Ligand) (Extinction C o e f f i c i e n t , M ^cm "S CH3-Im CH Cl„ 2 2 387 (7 X i o3 ) Pip C H c i 2 2 392 (7 X i o 3 ) Py C H <:i 2 2 390 (7 X i o 3 ) CH -Im 3 DMF 390 (7 X i o 3 ) Pip DMF 392 (7 X i o 3 ) Py DMF 390 (7 X i o 3 ) CH3~Im Toluene 390 (7 X i o 3 ) -240-3100 3200 H (Gauss) 3300 3 1 0 0 1 3 2 0 0 H (Gauss) 3 3 0 0 3 4 0 0 Figure VIII.5 A. ESR si g n a l of CH^ImCo (salen) (0^) @ 77 K i n dichloromethane. B. ESR si g n a l of Co (salen) (C>2) @ 77 K i n dichloromethane. I -242-Figure VIII.6 Eguilibrium p l o t s f o r CH^-Im + Co(salen) For raw data see Appendix V i a . i-CH -ImCo(salen) -243--244-Figure VIII.8 Equilibrium p l o t s f or Co(salen) + 0 For raw data see Appendix VIg. Co(salen) (OJ -245-CH^-ImCo(salen) i n dichloromethane and Co(salen) i n dichloromethane, re s p e c t i v e l y . Figure VIII.9 shows the van't Hoff p l o t s a r i s i n g from ligand addition to Co(salen) i n dichloromethane. Figures VIII. 10 and 11 show the van't Hoff p l o t s r e s u l t i n g from dioxygen addition t o Co(salen) amine adducts and t o Co(salen) i n dichloromethane, r e s p e c t i v e l y . Since the oxygenation of Co(salen) approached an equilibrium p o s i t i o n at lower oxygen pressures or higher temperatures, the k i n e t i c s of that reaction were analyzed by the following expression: A - A l n A o eq = _k obs-t = -(k-obs + k ,obs)t (VIII. 1) A t i - l where A i s the absorbance of Co(salen) i n the absence of oxygen o A i s the absorbance reading at time = t A i s the absorbance reading at equilibrium eq kt o b s ^ s t-ke t o t a l observed pseudo f i r s t order rate constant k^obs i s the pseudo f i r s t order forward rate constant and k ^obs i s the f i r s t order reverse rate constant Since the equilibrium constant, K, could a l s o be determined, and K i s r e l a t e d to the rate constants by the expression: K = k A _ x (VIII.2) the forward and back rate constants could be determined. Dir e c t determination of the off rate was hampered by slow d i f f u s i o n of oxygen from the dichloromethane solution on pumping. I f the temperature were raised to cause the equilibrium to s h i f t s u f f i c i e n t l y to the l e f t so that the r e s i d u a l oxygen i n the dichloromethane did not oxygenate the complex, the reaction was completed before thermal equilibrium was reached. -246-Figure VIII.9 Van't Hoff p l o t s for L + C o ( s a l e n ) ^ fcLCoisalen) L = CH -Im, Pip, and Py -247--248--249-VIII.3 Sources of Erro r Errors s i m i l a r to those noted i n the cobalt porphyrin studies are also present during the studies on ligand and dioxygen binding to Co(salen) complexes. These complexes, e s p e c i a l l y i n the more polar dichloromethane and DMF solvent systems, bound dioxygen more r e a d i l y than corresponding cobalt porphyrin complexes, and the problem of i r r e v e r s i b l e oxidation was more Important; the presence of the bridged peroxide species was frequently noted i n ligand and dioxygen binding experiments. Ligand binding studies could s t i l l be c a r r i e d out, although several attempts were sometimes necessary to complete a p a r t i c u l a r experiment s u c c e s s f u l l y . Lowering the temperature f o r dioxygen binding studies u s u a l l y prevented i r r e v e r s i b l e oxidation, but oxygenation studies could not be completed on the PipCo(salen) system i n dichloromethane at -23°C and -31°C due to oxidation. Compared to the formation of cobalt porphyrin adducts, addition of higher concentrations of ligand were necessary to complete the formation of LCo(salen) adducts. However, from considerations s i m i l a r to those for binding a second ligand to CoOMBP (Chapter I I I ) , solvent v a r i a t i o n e f f e c t s were not thought to be important. Tne concentration of ligand (up to -0.5 M) required to perform oxygenation studies was not considered t o s u b s t a n t i a l l y a f f e c t the nature of the solvent under the conditions used. VIII.4 Studies i n Dichloromethane VIII.4.1 L + Co(salen) ^  * LCo(salen), The equilibrium constants of CH^Im, Pip, and Py binding to Co (salen) are given i n Table VIII.2, the raw data being given i n Appendices V i a - V i c . Table VIII.2 also gives the values obtained for A H and A S , and the pK of the cl protonated ligands. -250-Table VIII.2 Thermodynamic Data f o r the Binding of an A x i a l Ligand to Co(salen) i n a Dichloromethane Solution. -1 LIGAND TEMP., o C K l f M CH3-Im 3 99 15 73 25 39 35 22 Pip 3 123 15 69 25 51 35 26 Py 3 27.5 15 14.1 25 9.5 * 35 5.2 + 4-CN-Py' 15 4.57 25 2.14 35 1.62 + 4-NH2~Py' 3 275 15 105 25 82 35 46 t PPh 3 15 0.16 25 0.10 AH 1  kcal/mole As, e.u. PK. -8.5 -21 -7.8 -18 -8.3 -24 -8.7 -27 -11.8 -31 7.25 11.30 5.27 1.86 9.30 2.73 a. For references see Table III.3; see Py r e f f o r values of substituted Py's. t. D. Hegedic, B. R. James, unpublished r e s u l t s . *. Data obtained from t ; K values for CH3~Im, Pip, and Py binding from t very s i m i l a r to those presented above; thermodynamic parameters analyze f o r : AH = -9.2 kcal/mole, AS = -24 e.u. (CH -Im); AH = -9.5 kcal/mole, AS = -24 e.u. (Pip); and AH = -8.9 kcal/mole, As = -25 e.u. (Py). These parameters are considered to agree with those presented above. 4-CN-Py = 4-cyanopyridine, 4-NH2~Py = 4-aminopyridine -251-The order f o r ligand binding a b i l i t i e s observed at 25°C i s : 4-NH2~Py> Pip>CH3-Im>Py>4-CN-Py>PPh3. As with the cobalt porphyrin complexes studied, no general c o r r e l a t i o n e x i s t s between K , AH , AS and the pK of the i l l a protonated ligand, although the three pyridine ligands e x h i b i t a reasonable c o r r e l a t i o n between log K and pK (Figure VIII.12), s i m i l a r to that observed x a 89 i n the CoPpIXDME systems , and a good Hammett r e l a t i o n s h i p i s a l s o obtained (Figure VIII.13, p = +1.17) . The above general ligand binding order i s 89 s i m i l a r t o that observed i n the CoPpIXDME , CoDADIXDME (Chapter V), CoDBrDIXDME (Chapter V), and the CoOMBP (Chapter III) systems, although the r e l a t i v e p o s i t i o n s of two adjacent ligands are sometimes reversed. The a f f i n i t y of a p a r t i c u l a r ligand f o r Co(salen) i s two to three orders of magnitude les s than for cobalt porphyrin systems. In contrast to the cobalt porphyrin studies i n toluene, Co(salen) complexes were i n i t i a l l y examined i n dichloromethane. However, studies on CoOMBP (Chapter III) , and subsequent studies on Co(salen) (Chapter VIII.5), show that the solvent has some e f f e c t on ligand binding a b i l i t y , but not s u f f i c i e n t to account f o r the large observed differences between Co(salen) and cobalt porphyrin complexes. Attempts to study quantitative ligand binding to cobalt complexes of substituted acacen S c h i f f bases were f r u s t r a t e d by very small v i s i b l e s p e c t r a l changes, but the approximate ligand a f f i n i t y f o r a serie s of these 82 complexes has been reported to be about one to two orders of magnitude less than f o r cobalt porphyrins. The weaker ligand binding to the S c h i f f base complexes compared to the porphyrins i s a t t r i b u t e d to the more 32 ,37 electron r i c h metal centre i n S c h i f f base systems -252--253--254-VIII.4.2 LCo(salen) + 0 ^ * LCo(salen)(0 ), K . £ 1 2 2 _ The P,0 values of the five-coordinate LCo(salen) complexes are T 2 given i n Table VIII.3, the raw data being given i n Appendices VId- VIg. Also l i s t e d are the estimated AH and AS values and the pK of the a protonated ligand studied. The order of oxygen a f f i n i t i e s f o r the LCo(salen) complexes as a function of a x i a l ligand at -45°C i s Pip>CH3-Im >Py, which i s generally the same as the order of ligand binding to Co(salen) 89 and of dioxygen binding to LCoPpIXDME , LCoDADIXDME (Chapter V I ), LCoDBrDIXDME (Chapter VI), and t o LCoOMBP (Chapter IV). Sometimes within the porphyrins the CH^-Im adduct binds dioxygen more strongly than the Pip adduct and t h i s trend i s apparent i n the LCo1salen) systems at -63.5°C. A 82 s i m i l a r dioxygen binding sequence has also been reported f o r LCo(benacen) with the CH3~Im adduct binding dioxygen more strongly than the Pip adduct. LCo(salen) complexes bind dioxygen by one or two orders of magnitude more strongly than the corresponding LCoP complexes, again consistent with the more electron r i c h nature of the S c h i f f base complex 3 2' 3 7. LCo(benacen)^ 2 complexes bind dioxygen somewhat more strongly than the corresponding LCo(salen) complexes (e.g. P,0 @ -23°C f o r CH-ImCo(benacen) ~8 Torr vs. T 2 3 43 Torr f o r CH^-ImCofsalen), implying that salen withdraws more electron density from the metal centre than benacen. Despite the Co(benacen) studies being c a r r i e d out i n toluene and Co(Salen) being examined i n dichloromethane, t h i s conclusion seems reasonable since Co(salen) has a more extensive network of tr-electrons around the metal centre than does Co(benacen). The more polar dichloromethane would be expected to enhance oxygenation (Chapter TV), but the oxygen a f f i n i t y of the Co(salen) system i s s t i l l less than that of the Co(benacen) system. -255-Table VIII.3 Thermodynamic Data f o r the Binding of Dioxygen to LCo(salen) i n a Dichloromethane Solution. LIGAND TEMP . , o C P^0 2, Torr AH kcal/mole AS e .u. PK_ CH3-Im CH3-Im Pip Py Py no ligand -78 -63.5 -45 -31 -23 -45 -31 -23 -78 -63 .5 -45 -78 -63.5 -45 -31 -45 -31 -23 -83.5 -78 -63.5 -45 0.89 2.1 8.6 25.6 42.9 10.2 26.3 44.7 0.87 2.3 5.4 2.0 4.0 13 .2 27.5 12.6 26.3 50.1 34.7 71 224 1900 -7.5 -7.5 -4.8 -6.5 -6.7 -8.2 a. For references see Table III.3. *. D. Hegedic, B. R. James, unpublished r e s u l t s -37 -37 -25 -33 -35 -51 7.25 7.25 11.30 5.27 5.27 ±. -23 C data from other source (*y c o n s i d e r e d i n c a l c u l a t i n g thermodynamic parameters -256-Inspite of LCo(salen) complexes having higher dioxygen a f f i n i t i e s than the cobalt porphyrins, the enthalpy of oxygenation of the Co(salen) systems i s s i m i l a r to that of the CoOMBP (Chapter IV) and the CoDADIXDME and qq CoDBrDIXDME (Chapter Vl) systems, and i s somewhat les s than the CoPpIXDME systems. As the r e l a t i v e l y favorable entropy term i n the Co(salen) system appears to be important i n oxygenation, perhaps the amount of i n t e r n a l rearrangement of the Co(salen) complexes i s d i f f e r e n t from the cobalt porphyrins upon oxygenation. In the more strongly dioxygen binding 82 substituted acacen complexes of cobalt , the enthalpy of oxygenation i s much greater than i n the Co(salen) systems (AH Q of PyCo(benacen) = -16.6 kcal/mole). VIII.4.3 Co(salen) + 0 - '-Co(salen) (0 ), K 2 2 0 2 The four-coordinate Co(benacen) system was found to oxygenate i n o o 82 toluene only very s l i g h t l y at -83.5 C and not at a l l at -63.5 C , and t h i s system was considered a i r st a b l e . In contrast, exposure of four-coordinate Co(salen) to dioxygen i n dichloromethane resulted i n v i s i b l e spectroscopic changes which were r e v e r s i b l e i n nature. The thermodynamic parameters f o r the oxygenation are presented i n Table VIII.3. The dioxygen a f f i n i t y of the four-coordinate species i s s u b s t a n t i a l l y l e s s than those of the five-coordinate complexes; the AH value i s close to that for 0 2 binding to CH^-ImCo(salen), but the AS value i s much less favorable, and the weaker binding i s generally r e f l e c t e d i n the unfavourable entropy. K i n e t i c studies were c a r r i e d out on t h i s system (Table VIII.4), and the o v e r a l l reaction to equilibrium analyzed for f i r s t order i n cobalt (Figure VIII.14). However, the observed forward rate constant, which increased with oxygen pressure (Figure VIII.15), decreased with temperature -257-Table VIII.4. K i n e t i c Data f o r the Oxygenation of Co(salen). TEMP \ Eq'm % k^obs, -1 sec k^obs r -1 sec k ^ obs, -1 sec °C Torr Oxygenated -83.5 205 85.5 0.88 X io" 3 0.75 X io" 3 0.13 X io" 3 -83.5 415 92 .3 1.58 X io" 3 1.46 X io" 3 0.12 X io" 3 -83.5 795 95.8 3.2 X io" 3 3.1 X io" 3 0.1 X io" 3 -78 197 73.5 0.62 X io" 3 0.46 X io" 3 0.16 X io" 3 -78 407 85.1 1.19 X io" 3 1.01 X io" 3 0.18 X io" 3 -78 790 91.7 1.88 X io" 3 1.72 X io" 3 0.16 X io" 3 -63.5 743 77.6 1.33 X io" 3 1.03 X io" 3 0.30 X io" 3 -45 750 28.1 1.65 X io" 3 0.47 X io" 3 1.18 X io" 3 Table VIII.5. S o l u b i l i t y of Dioxygen i n CH C l at Low Temperature. TEMP., °C P , Torr O S o l u b i l i t y (M) 0 So l u b i l i t y / a t m (M atm ) -78 190 1.55 X io" 3 6.20 X io" 3 -78 380 3.15 X io" 3 6.30 X io" 3 -78 670 4.45 X io" 3 5.93 X io' 3 -78 760 6.25 X io" 3 6.25 X io" 3 -63.5 190 1.45 X io" 3 5.80 X io" 3 -63.5 380 2.90 X io" 3 5.80 X io" 3 -63.5 670 4.60 X io" 3 6.13 X IO" 3 -63.5 760 5.63 X io" 3 5.63 X io" 3 -45 380 2.98 X io" 3 5.96 X io" 3 -45 760 6.33 X io" 3 6.33 X io" 3 -258-i < < -3.0 H 1 0 0 0 t i m e (sec ) 2 0 0 0 Figure VIII.14 F i r s t order p l o t s of Co(salen) oxygenation i n CH^Cl^ @ -78 C as a function of oxygen pressure. -259-Figure VIII.15 Oxygenation rate constants of Co(salen) as a function of oxygen pressure. -260-(Figure VIII.16, AH = -4.7 kcal/mole and AS = -94 e.u.), while the observed reverse rate constant increased with temperature, as i s usual (Figure VIII.17, AH = 3.9 kcal/mole and AS = -55 e.u.). Generally, gas solubility decreases as temperature increases, and this effect could explain enhanced oxygenation rates at lower temperatures. However, measured oxygen so l u b i l i t i e s showed l i t t l e variation from (6.0 ± 0.4) x 10 3 M atm 1 on going from -78°C to -45°C (Table VIII.5). The data agree reasonably well with a literature value of (4.9 ± 0.3) x 10 3 M atm 1 _00 52b at -78 C The increasing forward rate constant at lower temperatures could possibly result from an exothermic coordination process i n which Co(salen) obtains the preferred, but only partially formed, five-coordinate geometry for dioxygen binding. One possibility i s a dimerization of Co(salen), possibly by linkage of a Co of one complex to a salen-N of another, which could then react with dioxygen. However, this i s inconsistent with analysis of thermodynamic data that indicates a 1:1 stoichiometry, and the kinetic data which shows a f i r s t order dependence on cobalt. Trace water could also coordinate to Co(salen) at lower temperatures to give a five-coordinate dioxygen-binding geometry. However, no change from the characteristic four-68 coordinate Co(salen) ESR signals was observed after addition of water -2 (-10 M) to dichloromethane solutions of Co(salen), and water-saturated dichloromethane solutions of Co(salen) yielded visible spectra no different from that of the four-coordinate species. Co(salen) similarly gives a four-coordinate visible spectrum in neat acetone, and in acetone with water added. Also oxygenation proceeds at similar rates with and without the presence of added water in the system. Therefore, water coordination does not appear to be important. A third source of coordination could be from -261-Figure VIII.16 Arrhenius p l o t of the forward rate constants of the oxygenation of Co(salen) at one atmosphere of oxygen. -262--263-the dichloromethane solvent. Although ESR spectra of Co(salen) in dichloromethane at 77 K indicate that no such coordination takes place, i t is very d i f f i c u l t to completely rule out the possibility of some trace solvent adduct being responsible for oxygenation of Co(salen) with no added ligand present. 23c ESR studies indicate that, in four-coordinate cobalt Schiff base or porphyrin complexes, the unpaired electron i s in an orbital oriented in the plane of the macrocycle, the axial d o orbital being f u l l y occupied. z The presence of an axial ligand usually serves to raise the energy level of the d_2 orbital above that of the planar orbital, and under these conditions the axial d 2 orbital i s now h a l f - f i l l e d , and i t i s thought z 23c that these conditions are necessary for oxygenation to proceed . Thus the presence of an exothermic process that places the unpaired electron in the d_2 orbital can explain the observed greater oxygenation rate at lower temperatures. A possible mechanism would thus be: k f * * Co(salen) • '• Co(salen) , K (VIII.3) r k l Co(salen) + 0 •. - Co(salen)(0 ) (VIII.4) where Co(salen) is "inactive" Co(salen) with paired d 2 electrons z * and Co(salen) i s active Co(salen) with an unpaired d 2 electron. z * Assuming that Co(salen) exists in a steady state yields the following rate law: d[Co (salen) ( O j ] / k f[co(salen)] + k_ 1[Co(salen) ( O j ] \ dt = k i k + k,CcO C°2^ r r 2' (VIII.5) -k^ECo (salen) ( O j ] - 2 6 4 -I f k f and are very large, compared to k^ and k , the rate law becomes: dTco(salen) ( 0 2 ) ] ^ — = k xK CCo (salen)] [ o 2 3 - k^ C C o (salen) (02)] (VIII. 6) and the k i n e t i c treatment i s that described e a r l i e r (Chapter VIII.2) with * k ^ b s = k 1K . * The spectroscopic data require that K be very small, as no intermediate i s detected; t h i s i s consistent with the steady state treatment. The A H * of - 4.7 kcal/mole associated with k^obs suggests that the AH value associated with K i s exothermic to at l e a s t that amount. This degree of exothermicity i s about h a l f that associated with the binding of nitrogen-donating ligands. The exothermicity of coordinating water i s expected to be l e s s than that of a nitrogen-donating ligand (Chapter I I I ) , and that of an e s s e n t i a l l y non-coordinating dichloromethane solvent molecule would be even l e s s . Oxygenation and deoxygenation rates of cobalt S c n i f f base or porphyrin complexes that have an a x i a l amine coordinated are too great to be measured by conventional means. For the Co(salen) i n dichloromethane system, the deoxygenation rate constant i s r e l a t i v e l y small, being of the order of -3 -1 10 sec . This suggests that the nature of the reacting complex i s quite d i f f e r e n t from the usual six-coordinate L C o P ( 0 2 ) or LCo(salen)(O^) species. However, i n the above observations there i s nothing that r u l e s out the p o s s i b i l i t y of weakly coordinating dichloromethane being responsible for * the K step, rather than some i n t e r n a l rearrangement of the four-coordinate Co(salen) complex leading to the a c t i v a t i o n of the complex to oxygenation. -265-VIII.5 Studies i n Other Solvents VIII.5.1 Rationale Since the above work with Co(salen) systems was done i n dichloromethane and other systems, such as the substituted acacen complexes of cobalt and the cobalt porphyrin complexes, were done i n toluene, and solvent e f f e c t s , e s p e c i a l l y with dioxygen binding, can play a major r o l e , Co(salen) studies were also c a r r i e d out i n toluene and i n DMF. VIII.5.2 L + Co(salen) - LCo(salen) The equilibrium constants of CJ^-Im, Pip, and Py binding to Co(salen) i n DMF and i n toluene are given i n Tables VIII.6a and VIII.6b, r e s p e c t i v e l y , the raw data f o r which are given i n Appendices VIk - VIp. Also l i s t e d are AH and AS values obtained, and the pK& of the protonated lig a n d studied. In DMF, the order of lig a n d binding a b i l i t i e s at 25°C (Pip>CH3"Im>Py) i s the same as i n dichloromethane, while i n toluene CH^-Im binds more strongly than Pip. Py always binds to Co (salen) l e s s strongly than CH^Im or Pip. The general trend f o r ligand a f f i n i t y as a function of solvent i s toluene>CH 2Cl 2>DMF, consistent with s l i g h t l y greater a f f i n i t y i n less polar solvent systems, as observed i n the CoOMBP systems, and presumably s i m i l a r reasoning applies (Chapter I I I ) . VIII.5.3 LCo(salen) + 0 2 LCo (salen) (0 2) The P.O values of five-coordinate Co(salen) complexes i n DMF and i n « 2 toluene are given i n Tables VIII.7a and VIII.7b, r e s p e c t i v e l y , the raw data being given i n Appendices V l q - VIv. In DMF, determinations of AH and AS values were d i f f i c u l t , as the solvent freezes at -60°C, and i r r e v e r s i b l e oxidation tended to occur above -45°C. Since dioxygen tended to bind so -266-Table VIII.6a. Thermodynamic Data f o r the Binding of an A x i a l  Ligand to Co(salen) i n a DMF Solution. LIGAND TEMP. °C K l f M _ 1 AH X kcal/mole A S 1 e .u. K CH -Im 5 47.2 -3.6 -6 • 7.25 3 15 31.1 25 27.1 35 23.4 45 18.8 Pip 5 88 -5.0 -9 11.30 25 48.2 35 38.9 55 21.9 Py 5 23.7 -6.9 -19 5.27 15 14.1 25 7.9 35 5.9 65 2.4 Table VIII .6b. Thermodynamic Data f o r the Binding of an A x i a l Ligand to CoCsalen) i n a Toluene Solution. LIGAND TEMP., °C K , M -1 A H 1 kcal/mole AS, e.u. CH3~Im Pip Py 5 15 25 35 45 55 5 15 25 35 45 65 5 15 25 35 45 305 236 171 130 99 88 531 291 81.3 47.9 37.6 16.2 56.2 26.9 20.4 12.4 7.4 -5.2 7.25 -9.8 -7.8 -23 -20 11.30 5.27 -267-Table VIII. ,7a. Thermodynamic Data f o r the Binding of Dioxygen to LCo(salen) i n a DMF Solution. LIGAND TEMP. w a p K a °C Torr kcal/mole e .u. CH -Im -45 1.1 ~ -6.2 ~ -27 7.25 3 -23 3.0 0 10, oxidation a problem Pip -45 2.5 - -5.4 ~ -26 11.30 -23 7, oxidation a problem Py -45 4.7 - -5.0 ~ -25 5.27 -23 7.0 0 25 Table VIII .7b. Thermodynamic Data f o r the Binding of Dioxygen to LCo(salen) i n a Toluene Solution. a LIGAND TEMP. , P ^ , AHQ AS Q pK^ 2 2 o. Torr kcal/mole e.u. CH -Im -45 24 7.25 -45 30 Pip -78 132 11.30 Py --45 154 5.27 a. For references see Table III.3. -268-weakly to Co(salen) complexes i n toluene, AH and AS values were not determined i n t h i s solvent e i t h e r . LCo(salen) complexes are seen to have considerably higher dioxygen a f f i n i t i e s i n the more polar solvents, consistent with observations i n the 91a CoPpIXDME and CoOMBP (Chapter IV) systems. 91a For CH^ImPpIXDME the P ^ ° 2 value decreased from 388 to 12.6 Torr (a f a c t o r of -30) on going from toluene to DMF. The change f o r CH -ImCo(salen) was by a s i m i l a r f a c t o r (P,0 - 27 Torr i n toluene; 3 *i z P^0 2 - 1.1 Torr i n DMF). With PipCo (salen) strong dioxygen binding was observed i n DMF (PjJ°2 @ -45°C = 2.5 T o r r ) , but the dioxygen a f f i n i t y i n toluene was very weak, and the binding studies had to be c a r r i e d out at -78°C to observe a Pj^>2 of 132 Torr, which was even higher than that of most cobalt porphyrin complexes (Chapter VI). For both PyCo(salen) and CH -ImCo(salen) the P,0 values at -45°C were l e s s than those for the corresponding complexes of a l l the porphyrins studied under s i m i l a r conditions (Chapter VI). The enhanced oxygen a f f i n i t y of Co(salen) complexes i n more polar solvents can also be shown by gas-uptake experiments which indi c a t e an immediate 1:1 Co:0 2 uptake f o r CH^ImCo (salen) i n neat CH^Im or DMF at 15°C. The immediate uptake i n CH 2C1 2 was 1:0.6 Co:0 2, which compares to an expected 74% oxygenation c a l c u l a t e d from the thermodynamic data i n Table VIII.3. No oxygen-uptake was seen i n toluene, but since Co(salen) has a very l i m i t e d s o l u b i l i t y i n t h i s solvent, no conclusion could be made about CH,-ImCo(salen) oxygen a f f i n i t y , based on gas-uptake data alone. -269-CHAPTER IX THERMODYNAMICS AND KINETICS OF REACTIONS OF COBALT(II) MACROCYCLIC  SYSTEMS WITH SUBSTRATES CONTAINING UNSATURATED CARBON-CARBON BONDS IX.1 Reactions of Co(II) Porphyrin Systems with Ethylene and Acetylene Evidence that unsaturated hydrocarbon substrates react with CoOMBP systems was obtained when a DCE so l u t i o n of PipCoOMBP was exposed to an atmosphere of ethylene or acetylene (Figure IX. 1) at 20°C; sp e c t r a l changes (Table IX.1), s i m i l a r to those r e s u l t i n g from oxidation (Table V I I . l ) , occurred over a period of several hours (Appendix V l l f ) . The s p e c t r a l changes, which suggested that the reaction had proceeded almost to completion, were reversed s u b s t a n t i a l l y when the system was evacuated (Figure IX.2). In s p i t e of the resemblance of the s p e c t r a l changes to those of i r r e v e r s i b l e oxidation, t h e i r r e v e r s i b l e nature discounts the p o s s i b i l t y that they are due to i r r e v e r s i b l e oxidation rather than a reaction with the substrate. The rate of the forward reaction with the substrates (Table IX.2) i s about one fourth of that of i r r e v e r s i b l e oxidation (Table VII.4); i t i s highly u n l i k e l y that s u f f i c i e n t oxygen could enter the system to cause an oxidation reaction at that rate. Oxidation did appear to play a r o l e , however, as an increase i n s p e c t r a l i n t e n s i t y i n the 440 nm region, i n d i c a t i v e of increasing ( P i p C o O M B P ) , occurred before complete r e v e r s a l of the reaction with e i t h e r substrate could be achieved. £00 ^50 500 550 600 650 700 WAVELENGTH, NM Figure IX.1 Spectra showing the rea c t i o n of PipCoOMBP with 780 Torr acetylene @ 20°C. For raw data see Appendix V l l f . Table IX. 1 Spectral Data From 350 to 750 nm for the Reactions of Unsaturated Substrates with Cobalt Macrocycles Absorbing Species Absorption Maxima (nm) Isosbestic Points (nm) (Extinction Coefficient, M "^cm • PipCoOMBP 408 (6.3 x 104) 618 (3.9 x 104) + C2 H4* 439 (7.7 x 104) 629 (4.3 x 104) 427 457 624 + C2 H2* CoOMBP 439 412 (8.8 x 104) (6.4 x 104) 633 618 (4.6 x 104) i (6.2 x 104) 426 457 623 + acrylamide 415 (6.1 x 104) 618 (5.9 x 104) 435 606 634 + MA 356 (6.2 x 104) 629 (5 444 x 1C (1.0 x 104) »3) 374 MACoOMBP as above + Pip 444 (6.2 x 104) 631 (3.0 x 104) 405 472 595 DMFCoOMBP 418 (5.9 x 104) 618 (5.3 x 104) + MA 444 (7.3 x 104) 632 (4.3 x 104) 362 429 457 CoOMBP 412 (6.4 x 104) 618 (6.2 x 104) + TCNE 440 (~1 x id 4) 630 (~5 x 103) * Extinction coefficients based on estimated spectral characteristics of ful l y formed species. Table IX. 1 cont. Spectral Data From 350 to 750 nm for the Reactions of Unsaturated Substrates with Gobalt Macrocycles Absorbing Species Absorption Maxima ( n m ) I s o s b e s t i c Points (nm) -1 -1. Co(salen) 410 (1.15 x 104) + cot, 0 2 410 (1.12 x 104) , i.e. no evidence of any reaction + 1-hx, 0 2 410 4 (1.11 x 10 ), i.e. no evidence of any reaction + sty, 0 2 410 (6.6 x 103), Co(salen) peak substantially reduced + DCB, 0 2 410 (8.0 x 103), Co(salen) peak somewhat reduced + DEF, 0 2 , Co(salen peak completely eliminated I NJ NJ I 1J0 WAVELENGTH, NM Figure IX.2 Spectra showing PipCoOMBP regeneration 0 20 C a f t e r removal of acetylene. For raw data see Appendix V l l f . -274-Table IX.2 Kinetics of 780 Torr of Ethylene or Acetylene Reacting with PipCoOMBP in DCE at 20°C, Co , M Reaction k , sec 1 obs' -5 -4 1.5 x 10 PipCoOMBP + C_ H 4~—^PipCoOMBP(C^) 2.25 x 10 1.3 x 10~5 PipCoOMBP + C 2H 2^=^ PipCoOMBP (C 2H 2) 2.75 x 10~4 1.3 X 10~5 PipCoOMBP (C 2H 2)T^=irPipCoOMBP + C 2H 2 4.6 x 10~5 Although the reversal of the acetylene reaction analyzed for f i r s t order in cobalt (Figure IX.3), the reversal of the ethylene reaction was slower and sufficient data could not be obtained before substantial formation of the oxidation product occurred, presumably as a result of air leakage into the evacuated system. PipCoOMBP reacted at ambient temperature with ethylene and acetylene in a manner that approximated f i r s t order in cobalt (Figure IX.3, Table IX.2). When other axial ligands, CH3-Im and Py, were added and the reactions of LCoOMBP with ethylene or acetylene were studied, spectral changes similar to those described above were observed. However, no reversal was observed, and as these systems tended to be more oxygen sensitive than the PipCoOMBP systems/ the possibility of irreversible oxidation due to trace 0 2 cannot be completely ruled out in these systems. A l l the above systems were studied at lower temperatures (-78°C, -63.5°C, and -45°C) in toluene and in the 4:1 toluene:DCE by volume mixed solvent system, but there was l i t t l e evidence for the reaction with ethylene or acetylene under these conditions. Other cobalt(II) porphyrin systems (LCoPpIXDME, LCoOEP; L = CH3-Im, Pip, or Py) and four-or six-coordinate CoOMBP complexes did not appear to -275-- 0 . 5 H < i < < - 1 . 0 H 1 0 0 0 2 0 0 0 3 0 0 0 t i m e ( s e c ) 4 0 0 0 Figure IX.3 F i r s t order rate p l o t s of PipCoOMBP reacting with ethylene and acetylene. O PipCoOMBP + C 2H 4: • PipCoOMBP + C 2H 2; X PipCoOMBP (C 2H 2)^=: = T PiDCoOMBP (C-H J 2 4 =1 PipCoOMBP (C 2H 2) "PipCoOMBP + C 2H 2 -276-r e a c t w i t h e i t h e r o f t h e s u b s t r a t e s i n DCE a t a m b i e n t t e m p e r a t u r e s . LCoOMBP d i d n o t r e a c t w i t h e t h y l e n e o r a c e t y l e n e a t a m b i e n t t e m p e r a t u r e s i n t o l u e n e . P r e s u m a b l y , a n y r e a c t i o n b e t w e e n a c o b a l t ( I I ) p o r p h y r i n a n d a n u n s a t u r a t e d s u b s t r a t e w o u l d i n v o l v e t h e t r a n s f e r o f some e l e c t r o n d e n s i t y b e t w e e n t h e c o b a l t ( I I ) p o r p h y r i n and t h e i r * - o r b i t a l o f t h e s u b s t r a t e . B e c a u s e o f t h i s , s t u d i e s w e r e c a r r i e d o u t o n a c t i v a t e d o l e f i n s c o n t a i n i n g e l e c t r o n - w i t h d r a w i n g g r o u p s . I X . 2 R e a c t i o n s o f C o ( I I ) P o r p h y r i n S y s t e m s w i t h E l e c t r o n W i t h d r a w i n g O l e f i n s I n i t i a l s t u d i e s w i t h CoOMBP and a c r y l a m i d e r e s u l t e d i n s p e c t r a l c h a n g e s ( T a b l e I X . 1 ) s i m i l a r t o t h o s e o b s e r v e d u p o n a d d i t i o n o f DMA o r DMF t o CoOMBP ( T a b l e I I I . l ) . T h e r e f o r e , i t was c o n c l u d e d t h a t t h e b i n d i n g o f t h i s l i g a n d o c c u r r e d t h r o u g h t h e a m i d e g r o u p ; t h e b i n d i n g t o CoOMBP o f a c r y l a m i d e , l i k e DMA, was so weak t h a t e g u i l i b r i u m c o n s t a n t s c o u l d n o t b e d e t e r m i n e d . D r a m a t i c s p e c t r a l c h a n g e s o c c u r r e d when m a l e i c a n h y d r i d e (MA) was added t o CoOMBP ( T a b l e I X . 1 , F i g u r e I X . 4 ) , a s t h e S o r e t b a n d u n d e r w e n t a m a r k e d b l u e s h i f t a n d t h e i n t e n s i t y o f t h e v i s i b l e b a n d was g r e a t l y d i m i n i s h e d . The b i n d i n g o f MA t o CoOMBP i n t o l u e n e was s u c h t h a t e q u i l i b r i u m c o n s t a n t s c o u l d be r e a d i l y d e t e r m i n e d ( T a b l e I X . 3 ) . The a d d i t i o n o f MA t o CoOMBP was t r e a t e d a s a r e a c t i o n s i m i l a r t o t h e a d d i t i o n o f o t h e r l i g a n d s , a n d t h e b i n d i n g d a t a w e r e a n a l y z e d i n a l i k e manner ( C h a p t e r I I I ) , t h e a n a l y s i s b e i n g c o n s i s t e n t w i t h t h e f o r m a t i o n o f a 1:1 a d d u c t ( F i g u r e I X . 5 ) . U n l i k e t h e b i n d i n g o f p r e d o m i n a n t l y o - d o n a t i n g l i g a n d s ( C h a p t e r I I I ) , t h e r e a c t i o n w i t h MA was n o t v i r t u a l l y i n s t a n t a n e o u s . S e v e r a l m i n u t e s were r e q u i r e d t o e s t a b l i s h e q u i l i b r i u m a t a p a r t i c u l a r c o n c e n t r a t i o n o f MA, b u t t h e k i n e t i c s o f t h e r e a c t i o n c o u l d n o t be s t u d i e d c o n v e n i e n t l y s i n c e 5 5 0 WAVELENGTH, NM Figure IX.4 Spectral changes occurring upon the addition of MA to CoOMBP in toluene @ 22 C. For raw data see Appendix V i l a . -278-- 3 . 0 - 2 . 0 - 1 0 log[MA] Figure IX.5 Equilibrium p l o t s f o r MA + CoOMBP For raw data see Appendix V i l a . MACoOMBP -279-Table IX. 3. Thermodynamic Data for the Binding of Maleic Anhydride  to CoOMBP in a Variety of Solvents. SOLVENT TEMP., °C AHMA kcal/mole AS MA e .u. Toluene 5 82.2 -12.2 -35 22 20.9 40 6.6 DCE 20 -5 DMF 5 31.7 -9.7 -28 22 11.7 40 3.8 by the time a particular addition was made and the system was properly mixed, the reaction was almost complete. By studying the MA system at several temperatures in toluene (Table IX.3, Figure IX.6), CoOMBP i s seen to bind MA with a somewhat more favourable AH and less favorable AS than the primarily o-bonding N-and O-donor ligands considered in Chapter III. The above observations suggest substantial rearrangement of the electronic structure of CoOMBP upon formation of the MA-CoOMBP adduct, possibly by the transfer of some n-electron density from OMBP to MA through the Co centre, upon formation of the metal-olefin Tr-bond, Co—1| Reversibilty of the formation of MACoOMBP i s demonstrated by a partial regeneration of the spectrum of CoOMBP as the temperature of the system i s raised. Also, when Pip i s added to the MA-CoOMBP system, a general growth in spectral intensity i s noted (Figure IX.7), and a new species with -280-.0032 .0033 .0034 .0035 .0036 Vf (K~1) Figure IX.6 Van't Hoff p l o t s f o r MA + CoOMBP MACoOMBP i n toluene and DMF. 400 450 500 550 600 650 WAVELENGTH, NM Figure IX.7 Spectral changes to MACoOMBP upon addition of Pip @ 22°C. For raw data see Appendix V i l e . -282-an intense band at 444 nm is formed. This suggests that PipCoOMBP(MA) results from this addition, as CoOMBP complexes with two axial lgands so far studied generally have a strong band in the 440 nm region. The vis i b l e band position at 631 nm confirms that this adduct i s not PipCoOMBP as the position of the visible band of that complex i s at 616 nm; also insufficient Pip i s present to form Pip2CoOMBP. At lower MA concentrations (<0.1 M), addition of Pip -3 (-5 x 10 M) results in i n i t i a l formation of PipCoOMBP(MA) followed by the gradual generation of the five-coordinate PipCoOMBP complex (Figure IX.8). -3 Addition of MA to a 2 x 10 M Pip in toluene solution of PipCoOMBP results in i n i t i a l formation of some PipCoOMBP(MA) followed by formation of MACoOMBP, although this adduct i s not f u l l y formed at concentrations of MA where -2 MACoOMBP i s f u l l y formed in the absence of Pip. Addition of MA to a 4 x 10 M Pip in toluene solution of PipCoOMBP (-85%) and PipCoOMBP (-15%) resulted in the spectrum of PipCoOMBP(MA). Analysis of the equilibria i s made d i f f i c u l t due to the occurrence of more than one reaction at a particular time. Upon addition of MA to CoOMBP in DCE (Table IX.3), the observed value was less than that observed in toluene, consistent with observations for the binding of other ligands (Chapter III). That ethylene and acetylene bind to LCoOMBP in DCE and not in the less polar toluene could be rationalized in terms of stabilization of a polar adduct in the more polar DCE, analogous to stabilization of dioxygen adducts as Co(III)-0 2 species. The larger and already polar olefin, MA, i s lik e l y to delocalize any transferred electron density, thus solvent polarity would be expected to stabilize the reactants rather than the products. Enhanced formation of ethylene and acetylene adducts of CoOMBP with the presence of an axial Pip i s consistent with the axial ligand contributing greater electron density to the metal centre, and thus enhancing transfer of 1.0 111 o z < CD m o L O C D < 0.5H ro co oo 1 600 450 1 I 500 550 WAVELENGTH NM 500 650 700 Figure IX.8 Spectra showing the generation of PipCoOMBP from PipCoOMBP(MA). For raw data see Appendix VIIc. -284-electron density to the substrate. The electron withdrawing MA is able to form an adduct with CoOMBP without the aid of an axial ligand, but i t appears that substantial changes occur i n the electronic structure of OMBP. Appearance of a PipCoOMBP spectrum upon the addition of Pip to MACoOMBP in a dilute MA in toluene solution suggests that axial Pip weakens the Co-MA bond. The formation of a spectrum similar to that of other six coordinate species, upon addition of Pip to MACoOMBP in stronger 'MA solutions, suggests that electronic changes to OMBP upon MA coordination are somewhat reversed upon coordination of a second axial ligand. When the MA addition to CoOMBP i s carried out in DMF, the reacting 121 species i s DMFCoOMBP (Chapter III) . DMF, a strong -rr-donor , might be expected to stabilize the formation of the MA adduct. However, the K MA values in DMF and the AHw„ value in DMF (Table IX.3) are somewhat less in MA magnitude than in toluene, although MA binds more strongly to DMFCoOMBP in DMF than to CoOMBP in DCE. Weaker binding in DMF than in toluene would be consistent with destabilizing the product in a more polar solvent. Stronger MA binding in DMF than in DCE could be due to u-stabilization from the DMF ligand. The spectrum of DMFCoOMBP(MA) is similar to that of other s i x -coordinate OMBP systems which suggests that a bond weakening similar to that occurring when Pip i s the axial ligand; this would also explain the weakened binding in DMF compared to toluene. However, this effect appears to be discounted by the stronger binding in DMF than in DCE; perhaps the IT-electron density helps compensate for the electronic changes occurring to OMBP upon the binding of MA to CoOMBP. The formation of the DMFCoOMBP adduct with MA appears to be complete, while when MA i s added to PipCoOMBP incomplete product formation i s apparent at a comparable concentration of MA. This also suggests that the IT-donor nature of DMF i s important in maintaining a MA adduct of CoOMBP. -285-Studies similar to those for MA binding were carried out with tetracyanoethylene (TCNE). This reaction was hampered by the formation of TCNE charge transfer complexes with toluene and the previously studied 160 amine ligands . However, when DCE was the solvent, there were spectral changes similar to those observed for the MA reaction with CoOMBP in DCE (Table IX.1). TCNE concentrations required to complete the reaction were similar to those of MA to f u l l y form the adduct with CoOMBP. This i s evidence that the ligand binding takes place through the olefinic bond, since this i s the only functional group that MA and TCNE have in common. Attempts to detect a well resolved ESR signal for the olefin complexes of CoOMBP were unsuccessful; weak and i l l defined spectra similar to those for LCoOMBP (L = CH^-Im, Pip, or Py, Chapter VII) were observed. Presumably similar spin coupling phenomena that l i k e l y occur for the LCoOMBP systems also occur for the MA complex. No reaction with MA or TCNE was observed when other cobalt(II) porphyrin complexes (LCoPpIXDME, LCoDADIXDME, or LCoDBrDIXDME; L = CH3-Im, Pip, Py, or no ligand) were involved. Presumably this results from the strong 96c ir-donor a b i l i t y of OMBP , allowing greater s t a b i l i t y of a cobalt-olefin * interaction by greater n-back donation to the olefin u -orbitals. IX.3 Reactions of Co(salen) Complexes with Unsaturated Substrates Four-coordinate Co(salen) showed no sign of reaction when exposed to ethylene, acetylene, or TCNE. Five-coordinate CH^-ImCo(salen), upon exposure to ethylene or acetylene, showed some slight spectral changes at -63.5°c (an absorbance increase from 0.95 to 0.99 at 400 nm under one atmosphere of gas). The spectral changes, which were reversible upon evacuation, resembled those which would be expected from the presence of a small amount of oxygen (Figure VIII.2) , but an ESR investigation of the ethylene or acetylene systems at 77 K revealed a decrease (by 40%) of the CH,-ImCo(salen) signal, with no new -286-s i g n a l apparent. The formation of an adduct of LCo(salen) with an unsaturated substrate appears to involve the loss of unpaired electron density, which could occur by the coupling of spins, possibly i n a manner s i m i l a r to that which i s present i n the CoOMBP complexes with unsaturated substrates. Attempts to study i n t e r a c t i o n s of LCo(salen) with the activated, electron-withdrawing o l e f i n , TCNE, were f r u s t r a t e d by the formation of TCNE-amine charge t r a n s f e r complexes Ambient temperature v i s i b l e spectroscopic studies were c a r r i e d out with a v a r i e t y of l i q u i d o l e f i n s . The presence of peroxides, which reacted very r e a d i l y with Co(salen), was a problem f o r a l l these o l e f i n s and the peroxides had to be thoroughly removed (Chapter I I ) . Neither five-coordinate LCo(salen), nor four-coordinate Co(salen), complexes would react with 1-hexene, cyclooctene, styrene, 3,4-dichlorobutene, diethylmaleate, or diethylfumarate i n dichloromethane at room tempreature. Also there was no v i s i b l e spectroscopic evidence f o r binding of the above o l e f i n s at lower temperatures (-78°c, -63.5°C, -45°C, -31°C, or -23°C). Upon exposure to 800 Torr oxygen at 22°c, the v i s i b l e spectra of the Co(salen)-cyclooctene and Co(salen)-1-hexene systems d i d not change over a period of 10 h; t h i s i s the same behavior as when no o l e f i n i s present i n the system. The 410 nm Co(salen) band diminished quite considerably over 12 h when styrene or 3,4-dichlorobutene were present, while the same band disappeared completely when diethylfumarate was present. A f t e r the systems were evacuated, the 410 nm peak i n both the styrene and 3,4-dichlorobutene systems (Figure IX.9) were l a r g e l y regenerated a f t e r 40 h, although there was no peak regeneration i n the diethylfumarate system. -287-400 450 500 550 WAVELENGTH, NM Figure IX.9 Spectra showing the regeneration of the Co(salen) peak a f t e r oxygen i s removed from the Co(salen) - 3,4-dichlorobutene system. For raw data see Appendix V l l g . Spectrum before oxygen i s admitted to the system -288-ESR spectra at 77K of Co(salen) i n the presence of styrene or 3,4-dichlorobutene exhibited a very broad s i g n a l with no f i n e structure (Figure IX.10); t h i s contrasts markedly with the c h a r a c t e r i s t i c , well-defined s i g n a l obtained from the amine complexes of Co(salen) (Figure VIII.4). If an atmosphere of oxygen were added to the above systems at ambient temperature, and the system then taken to 77 K, the same si g n a l r e s u l t s . If the Co(salen)-o l e f i n (styrene or 3,4-dichlorobutene) system were exposed to oxygen for 42 h at ambient temperature, a sharp, well-resolved ESR signal (Figure IX.11) could be obtained. This s i g n a l i s s i m i l a r to that observed for dioxygen adducts of Co(salen)-amine systems (Figure VIII.5), but the A_ values associated with the o l e f i n systems were s u b s t a n t i a l l y greater than those associated with the amine systems (Appendix IX). These preliminary r e s u l t s suggest that o l e f i n i c compounds can bind, a l b e i t very weakly, with Co(salen), which can then activ a t e Co(salen) to bind dioxygen. I n i t i a l attempts to i s o l a t e a product from some of these systems yielded a dark brown powder a f t e r dichloromethane was evaporated from the Co(salen)-styrene system under a stream of oxygen. However, the product has yet to be characterized. Further studies of these systems should complement work that has already been performed i n studying the use of metallomacrocycles i n s e l e c t i v e l y o x i d i z i n g unsaturated substrates, 161 such as 3-substituted indoles and flavonols I 1 1 1 1 1 1 1 1 — 3 1 0 0 3 3 0 0 3 5 0 0 3 7 0 0 3 9 0 0 H (Gauss) Figure IX.10 ESR s i g n a l of Co(salen) - 3,4-dichlorobutene system at 7 7 K. 1 1 1 1 1 1 i i ~r 3000 3100 3200 3300 3400 H (Gauss) Figure IX. 11 ESR si g n a l of Co(salen)-3,4-dichlorobutene system at 77K af t e r exposure to for 42 h at ambient temperature. -291-CHAPTER X GENERAL CONCLUSIONS AND SOME RECOMMENDATIONS FOR FUTURE WORK Work i n t h i s thesis i s an important extension of previous work done on 22 23 ligand and dioxygen binding to cobalt(II) porphyrin complexes ' . Work was done p r i m a r i l y on CoOMBP, CoDADIXDME, and CoDBrDIXDME complexes i n which the porphyrin a - b a s i c i t y i s weaker than i n the cobalt(II) porphyrin complexes previously studied. I t was d e f i n i t e l y established that a x i a l ligands bind to cobalt(II) more strongly with decreasing b a s i c i t y of the porphyrin. One of the porphyrins, OMBP, was s u f f i c i e n t l y weakly basic that d i r e c t quantitative studies of binding both a f i r s t and second a x i a l ligand to a cobalt(II) porphyrin were accomplished f o r the f i r s t time: K l , L + CoOMBP ^  "~ LCoOMBP (X.l) K2 . L + LCoOMBP ,^ ImCoOMBP (X.2) The AH of the second ligand binding was found to be very s i m i l a r to that of the f i r s t ligand binding, and the order of a f f i n i t y f o r the f i r s t a x i a l ligand binding (Pip>CH3~Im>Py) was the same as for the second ligand binding. 22 23 Trends previously reported ' f o r ligand a f f i n i t y with the more strongly basic porphyrin complexes of cobalt(II) were s i m i l a r to those observed i n the present work with the weakly basic porphyrin complexes. For -292-a l l cobalt(II) complexes studied there i s no overall correlation between pK of the protonated binding axial ligand and the thermodynamics of the ligand binding, although a Hammett relationship has been noted for substituted 8 9 pyridines on binding to CoPpIXDME As a result of CoOMBP binding ligands so strongly, the quantitative thermodynamic parameters for binding the weakly basic ligand THF could be determined at ambient temperature. This ligand could not be studied with the more strongly basic porphyrin complexes of cobalt(II). For dioxygen binding to LCoP complexes i t was established that dioxygen tends to bind more strongly with complexes of more strongly basic porphyrins. Trends in dioxygen binding to cobalt(II) complexes of weakly basic porphyrin complexes as a function of axial ligand are similar to those reported previously for strongly basic porphyrin complexes. No overall correlation between the thermodynamic parameters of dioxygen binding and the pK of cl protonated axial ligands was apparent for the cobalt(II) porphyrin complexes, except for a limited Hammett relationship found earlier for dioxygen binding 8 9 with CoPpIXDME complexes of substituted pyridines Evidence was also obtained for the a b i l i t y of a cobalt(II) porphyrin complex to stabilize a dioxygen adduct with ir-interactions, as well as a-interactions. Thus, CoOMBP complexes bind dioxygen more strongly than would be expected from the cr-basicity of OMBP, and this i s rationalized in terms of the exceptional IT-donor a b i l i t y of OMBP. Also, compared with other ligand adducts, the DMF complex of CoOMBP binds dioxygen with a greatly enhanced exothermicity. Dioxygen binds more strongly to DMF complexes of cobalt(II) porphyrins than would be expected from the pK value of DMF, and this i s attributed to DMF being a strong ir-donor compared with the other axial -293-ligands investigated. . A more polar solvent has been found to play a major r o l e i n s t a b i l i z i n g the Co(III)-0 adduct, and a l i n e a r c o r r e l a t i o n between solvent p o l a r i t y and dioxygen binding to PipCoOMBP i n the binary toluene-DMF solvent mixtures has been found. Evidence f o r solvent p o l a r i t y playing a r o l e i n s t a b i l i z i n g C o ( I I I ) - 0 2 complexes was also obtained from studying the re a c t i o n : K °2/L ImCoOMBP + 0 2 1 °* T,t-r ,nMRP(n ^ + L (X.3) When CH3~Im was the a x i a l ligand, dioxygen bound more strongly than was expected from considerations of the K and K values of that system. This °2 a r i s e s since such a high concentration of CH^-Im i s required f o r f u l l formation of the b i s amine adduct that the p o l a r i t y of the solvent i s s u b s t a n t i a l l y greater than that of the toluene i n which K and K were °2 measured. In contrast, no evidence f o r oxygenation of PipCoOMBP was obtained; Pip i s e s s e n t i a l l y non polar compared with CH^-Im, and thus the dioxygen adduct of t h i s system i s presumably not s u f f i c i e n t l y s t a b i l i z e d . With a solvent system of 4:1 by volume toluene:DCE, the oxygen a f f i n i t y of LCoOMBP was found to be increased to a greater extent than occurred i n a toluene-DMF mixture of s i m i l a r p o l a r i t y ; the tendency to i r r e v e r s i b l y oxidize was also greater than i n the toluene-DMF solvent mixture. This can be explained by the possible presence of more protons i n DCE than i n basic DMF enhancing the formation of the C o ( I I I ) - 0 2 adduct and the presumed peroxide-like intermediate present during oxidation. Binding of the ligands CH3~Im, Pip, Py, and THF was found to be somewhat weaker i n a more polar solvent. However, the decrease i n ligand a f f i n i t y was not as great as the amount of enhancement observed i n dioxygen binding, -294-presumably because ligand adduct formation does not involve as much change in overall polarity as does dioxygen adduct formation. In the course of this work, the f i r s t evidence of metalloporphyrin aggregation in non polar solvents at concentrations as low as 10 ^ M was obtained for some CoOMBP systems. This phenomenon has recently been observed in systems involving another metal (ZnOMBP150). Aggregation of CoOMBP complexes i s thought to hinder oxygenation and subsequent oxidation of CoOMBP complexes, since the reaction time becomes progressively larger at higher CoOMBP concentrations to the point where CoOMBP complexes become essentially inert to oxidation at concentrations where gas-uptake -2 determinations are made (10 M). 123 As i n CoPpIXDME complexes , i t was found that having a protic axial ligand, such as Im, enhanced oxidation rates, and hydrogen bonding from the free ligand i s thought to stabilize an activated oxygenated intermediate and lower a kinetic barrier for the oxidation process. When Im was the axial ligand on CoOMBP, no evidence of aggregation was noted, and so i t i s concluded that the availabilty of protons can help break up aggregates. If porphyrin aggregation could be avoided, the cobalt(II) complexes studied in this thesis were oxidized via a process that was f i r s t order in 89 cobalt, as was observed i n the CoPpIXDME complexes . This i s explained by invoking a slow rearrangement of the superoxide-like Co(III)-0 2 to an activated intermediate in which the coordinated dioxygen i s more peroxide-like in character. Oxidation rates of the LCoP and L^CoOMBP systems studied in this thesis were somewhat greater in DCE than in toluene. Two factors could help account for this: the more polar solvent could enhance the formation of the _* intermediate Co(III)-O,, species that according to the oxidation mechanism -295-reacts subsequently with the remaining LCoP complex; and any protons present could also aid in formation of the dioxygen adduct intermediate, as well as break up any relatively unreactive porphyrin aggregates present. The importance of solvent polarity in the oxidation process was shown further by the very fast oxidation of CoOMBP in substantial concentrations of polar CH3-Im, even though in these solvent systems there are two axial CH^-Im ligands coordinated to the CoOMBP. The more CH^-Im present, the faster the reaction proceeded, and the less evidence there was of porphyrin aggregation, -2 even at concentrations as high as 10 M (where gas-uptake determinations were made). Studies with Co(salen) show that although ligands bind substantially less strongly to Co(salen) than to cobalt(II) porphyrin complexes, which i s rationalized in terms of greater electron density on the metal centre in the Schiff base complex, similar trends in ligand binding exist for the Schiff base and porphyrin system. Also, a Hammett relationship occurs for the binding of substituted pyridines to Co(salen). When dioxygen binding to LCo(salen) systems was studied, the dioxygen a f f i n i t y as a function of axial ligand was found to be similar to that found for the cobalt(II) porphyrins. Variation of solvent plays a similar role in ligand and dioxygen binding to Co(salen) as i t does in cobalt(II) porphyrin systems. Enhanced oxygen af f i n i t y for the more electron-rich metal centre in Schiff base systems was further demonstrated when dioxygen was found to bind reversibly to four-coordinate Co(salen) at low temperatures (e.g. -78°C) 22 23 This behavior i s unusual since four-coordinate cobalt porphyrins ' and 81 82 four-coordinate cobalt(II) complexes of other Schiff base systems ' are essentially inert to oxygenation at comparable temperatures. -296-Some preliminary studies strongly indicate that CoOMBP forms complexes with electron withdrawing olefins. Visible spectroscopic data reveal interactions of MA and TCNE with CoOMBP which seem amenable to detailed kinetic and mechanistic studies, and these should be carried out before firm conclusions are made. 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