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Mechanistic studies on activation of dioxygen using iridium and platinum triphenylphosphine complexes… Sue, Chen-youn 1988

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M E C H A N I S T I C S T U D I E S O N A C T I V A T I O N O F D I O X Y G E N U S I N G I R I D I U M A N D P L A T I N U M T R I P H E N Y L P H O S P H I N E C O M P L E X E S : P R O T O N A T I O N O F C O O R D I N A T E D P E R O X I D E , A N D O X Y G E N A T I O N O F C O O R D I N A T E D H Y D R I D E B y C H E N - Y O U N S U E M . Sc. , U n i v e r s i t y o f W a t e r l o o , 1984 B . E n g . , T s i n g h u a U n i v e r s i t y , B e i j i n g , 1968 A T H E S I S S U B M I T T E D I N P A R T I A L F U L F I L M E N T O F T H E R E Q U I R E M E N T S F O R T H E D E G R E E O F D O C T O R O F P H I L O S O P H Y i n T H E F A C U L T Y O F G R A D U A T E S T U D I E S D e p a r t m e n t o f C h e m i s t r y W e accept t h i s thes is as c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d T H E U N I V E R S I T Y O F B R I T I S H C O L U M B I A D e c e m b e r 1988 © C h e n - y o u n Sue , 1988 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department The University of British Columbia Vancouver, Canada DE-6 (2/88) ABSTRACT Kinetic and mechanistic studies on the activation of dioxygen using iridium and platinum triphenylphosphine complexes via oxygenation-protonation (i.e. protonation of coordinated peroxide) and protonation-oxygenation (i.e. oxygenation of coordinated hydride) processes are described. Dioxygen is activated in the form of a dioxygen (peroxide) metal complex, which upon protonation produces H2O2 stoichiometricaUy. Oxidation of the oxidizable ligands or substrates sometimes occurs when free H2O2 is present in the reaction systems. The reversible oxygenation of trans-IrCl(CO)(PPh3)2 (eq. 1) has been re-examined at 25 °C to obtain data in benzene (ka = (3.4 ± 0.3) x 10 - 2 M _ 1 s - 1 , k_i = ~1.5 x 10 - 5 s _ 1) and in dichloromethane (ki = (6.6 ± 0.3) x IO"7 s-'-mm H g - 1 , k_j = (1.6 ± 0.1) x IO"5 s _ 1). IrCl{CO)(PPh3)2 + 02P=IrCl(02)(CO){PPh3)2 (1) (I) (II) Complex II reacts stoichiometrically with trifluoroacetic acid to produce H2O2 which further oxidizes the CO and PPI13 ligands, but with HCl the oxidation of CO or PPI13 ligand by H2O2 does not take place; II reacts with free PPh3 to give I and O P P I 1 3 . The reaction of IrHCl2(CO)(PPh3)2, III, with O2 follows the sequence of steps outlined in eqs. 2, 1, 3 and 4. JrHCl2(CO){PPh3)2^=IrCl(CO)(PPh3)2 + HCl (2) K-2 IrCl(02)(CO){PPh3)2 + 2 HCI—*H202 + IrCl3{CO)(PPh3)2 (3) IrCl{CO)(PPh3)2 H-^2 C02,OPPh3,H20, (4) IrCl(02)(CO)(PPh3)2, and other "Jr(J1If species This reaction sequence is proposed based on kinetic studies and product analysis. A t 25 °C i n dichloromethane, the rate constant (k2) of the reductive-elimination reaction is (3.1 ± 0.1) x 10" 5 s _ 1 and the rate constant (k-2) of the oxidative-addition reaction is (4.4 ± 0.2) x 10 2 M _ 1 s _ 1 . W i t h i n the reversible reactions (2) and (1), HC1 and O2 compete with the intermediate I. Reaction (1) is eventually favoured because of the rapid protonation reaction (3) that scavenges the free HC1 and prevents the reverse of reaction (2). Reaction (3) is therefore the driving force of the overall reaction between I I I and O2. However, the forward step of reaction (2) is rate-determining i n the complex process. Removal of one chloride ligand from I I I leads to the eUmination of HC1 from I I I and the formation of a cationic iridium(I) species, which reacts with O2 slowly at room temperature. Reaction between I r H ( C O ) ( P P h 3 ) 3 and 0 2 gives I r H ( 0 2 ) ( C O ) ( P P h 3 ) 2 that decomposes to form the final products, outlined i n eqs. 5 to 7. IrH(CO)(PPh3)3 ^ lTH(CO)(PPh3)2 + PPh3 JrH(CO)(PPh3)2 + 0 2 — > IrH{02){CO){PPh3)2 IrH{02)(CO){PPh3)2 —>OPPh3, C02, H20, andnIr{III)" species (5) (6) (7) - i v -T h e c o m p l e x Pt(PPh3)4 d issoc ia tes i n benzene a n d t o l u e n e as d e s c r i b e d i n e q . 8. Pt(PPh3)4^Pt(PPh3)3 + PPh3 ( 8 ) w i t h 3 1 P { 1 H } N M R d a t a g i v i n g K = (2 .0 ± 0.2) x 1 0 " 1 M a t 2 2 ° C i n t o l u e n e ( A H 0 = 2.7 ± 0.3 k c a l m o l e - 1 ; A S 0 = 5.9 ± 0.6 e . u . ) . T h e P t ( P P h 3 ) 3 species does n o t m e a s u r a b l y d i ssoc ia te f u r t h e r t o p r o d u c e P t ( P P h 3 ) 2 i n c o n t r a s t t o some l i t e r a t u r e r e p o r t s . T h e exchange o f P P h 3 b e t w e e n P t ( P P h 3 ) 4 a n d P t ( P P h 3 ) 3 s t u d i e d v i a 3 1 P { 1 H } a n d 1 9 5 P t { x H } N M R s p e c t r o s c o p y , is f r o z e n o u t a t -90 ° C . T h e r e a c t i o n s o f O2 w i t h P t ( P P h 3 ) n ( n = 3, 4 ) are i r r e v e r s i b l e a n d t a k e p lace t h r o u g h t h e r e a c t i o n s teps s h o w n i n eqs. 8, 9 , a n d 10. Pt{PPh3)3 + 0 2 — ^ Pt(02){PPh3)2 + PPh3 ( 9 ) PPh3 +Pt(02)(PPh3)22^3Pt(PPh3)3 + 2 0PPh3 ( 1 0 ) O n e stage i n v o l v e s t h e f o r m a t i o n o f P t ( 0 2 ) ( P P h 3 ) 2 a n d P P h 3 a n d a l a t e r s tage , t h e n e t o x y g e n t r a n s f e r f r o m P t ( 0 2 ) ( P P h 3 ) 2 t o f ree P P h 3 , p r e s u m a b l y v i a l i b e r a t e d p e r o x i d e , as de-s c r i b e d p r e v i o u s l y i n t h e l i t e r a t u r e . K i n e t i c s tud ies i n d i c a t e a f i r s t o r d e r d e p e n d e n c e o n b o t h P t ( 0 2 ) ( P P h 3 ) 2 a n d P P h 3 f o r r e a c t i o n 10 , t h e r a t e c o n s t a n t s b e i n g (0 .9 ± 0 .2) M " 1 B ~ 1 f o r r a c t i o n 9 a n d (1 .38 ± 0 .01) x 1 0 - 1 M _ 1 s ~ ] f o r r e a c t i o n 10 a t 25 °C i n b e n z e n e . T h e f i n a l p r o d u c t s o f t h e r e a c t i o n o f P t ( P P h 3 ) „ w i t h 0 2 a r e P t ( 0 2 ) ( P P h 3 ) 2 a n d O P P h 3 . T h e P t ( 0 2 ) ( P P h 3 ) 2 c o m p l e x reac ts w i t h H C l i n s o l u t i o n t o p r o d u c e s t o i c h i o m e t r i c a l l y H 2 0 2 a n d c i s - P t C l 2 ( P P h 3 ) 2 , t h e H 2 0 2 b e i n g u n a b l e t o o x i d i z e t h e P P h 3 i n c i s - P t C l 2 ( P P h 3 ) 2 . I n c o n t r a s t t o a l i t e r a t u r e r e p o r t , t h e r e is n o fac i l e r e a c t i o n b e t w e e n trans-PtHCl(PPh3)2, or [PtH(S)(PPh 3) 2]PF 6 (S = solvent) and 0 2 , at ambient temperatures in dichloromethane, acetone, or tetrahydrofuran. However, over several days, oxidation reactions result from a slow loss of HC1 from PtHCl(PPh 3) 2 , as outlined in eqs. 11 to 14. PtHCl(PPh3)2^"Pt(PPh3)2" + HCl (11) nPt{PPh3)2" + 02 — * Pt{02){PPh3)2 (12) Pt{02){PPh3)2 +2HCl-^ PtCl2{PPh3)2 + H202 (13) Pt(02)(PPh3)2 —> OPPh3, and other decomposition products (14) - vi -TABLE OF CONTENTS PAGE ABSTRACT ii T A B L E OF CONTENTS vi LIST OF TABLES x LIST OF FIGURES xi ABBREVIATIONS AND SYMBOLS xvi ACKNOWLEDGEMENTS xviii CHAPTER 1 INTRODUCTION 1-1. General introduction 1 1-2. Catalytic oxidation of hydrocarbons 3 1-3. Oxygen activation and selectivity of oxidation reactions 19 1-4. Dioxygen complexes of transition metals and oxygen atom transfer 26 1-5. Mechanistic studies on oxygen transfer from dioxygen complexes 30 1- 6. Purpose of present work and scope of this thesis 36 CHAPTER 2 EXPERIMENTAL PROCEDURES 39 2- 1. Materials 39 2-2. Iridium compounds 40 2-3. Platinum compounds 43 2-4. Instrumentation 45 2-5. Detection of H 2 0 2 47 2-5-1. A modified iodometric titration procedure 48 2-5-2. *H NMR detection of H 2 0 2 49 2-6. Determination of oxygen solubility in benzene and CH 2 C1 2 53 - vii -T A B L E O F C O N T E N T S C O N T I N U E D PAGE CHAPTER 3 A KINETIC AND MECHANISTIC STUDY OF TRANS-CHLOROCARBONYLBIS(TRIPHENYLPHOSPHINE)IRIDIUM(I), [IrCl(CO)(Ph3P)2], IN AN OXYGENATION-PROTONATION PROCESS 55 3-1. Introduction 55 3-1-1. Reversibility and stability of dioxygen carrier complexes 55 3-1-2. Iridium dioxygen carriers and 0-0 bond lengths 57 3-1-3. The nature of metal-dioxygen bonding 58 3-2. Kinetics of the reversible oxygenation of IrCl(CO)(PPh 3) 2, I 59 3-2-1. UV-vis spectroscopic characteristics of IrCl(CO)(PPh 3) 2, I 59 3-2-2. Pseudo first-order kinetics 61 3-2-3. Kinetic measurements and data treatment 64 3-2-4. Summary of the reversible oxygenation of IrCl(CO)(PPh 3) 2, I 70 3-3. Protonation of IrCl(0 2)(CO)(PPh 3) 2, II 71 3-3-1. Reaction between IrCl(0 2)(CO)(PPh 3) 2, II, and trifluoroacetic acid 71 3-3-1-1. Stoichiometry 71 3-3-1-2. Products 74 3-3-1-3. Primary and secondary reactions 83 3-3-1-4. Low temperature tests 83 3-3-2. Reaction between IrCl(0 2)(CO)(PPh 3) 2, II, and HCl 84 3-3-3. Reaction between IrCl(0 2)(CO)(PPh 3) 2, II, and other acids 95 3-3-4. Summary of the protonation reactions of IrCl(02)(CO)(PPh3)2, II 95 3- 4. Oxygen transfer from IrCl(0 2)(CO)(PPh 3) 2, II, to free PPh 3 96 CHAPTER 4 A KINETIC AND MECHANISTIC STUDY OF TRANS-CHLOROCARBONYLBIS(TRIPHENYLPHOSPHINE)IRIDIUM(I), [IrCl(CO)(Ph3P)2], IN A PROTONATION-OXYGENATION PROCESS 99 4- 1. Introduction 99 4-2. Characterization of IrHCl 2(CO)(PPh 3) 2, V 102 4-3. Reaction between IrHCl 2(CO)(PPh 3) 2, V, and 0 2 106 4-4. Kinetic study of the reaction between IrHCl 2(CO)(PPh 3) 2, V, and 0 2 109 - viii -T A B L E J D F CONTENTS CONTINUED PAGE 4-5. Mechanism of the reaction of IrHCl 2(C0)(PPh 3) 2 , V , with 0 2 115 4-6. Kinetics of the reversible oxidative-addition reaction of HC1 to IrCl(C0)(PPh 3) 2, I 116 4-7. Summary 124 4-8. Kinetic expressions 126 4-9. Study on the reaction of IrH(CO)(PPh 3) 3, VI , with 0 2 127 4-9-1. Characterization of IrH(CO)(PPh 3) 3, V I 127 4-9-2. Reaction between IrH(CO)(PPh 3) 3, VI , and 0 2 127 4- 10. Reaction between IrHCl 2(CO)(PPh 3) 2, V , and AgBF 4 134 CHAPTER 5 A STUDY OF THE EQUILIBRIUM BETWEEN TETRAKIS(TRIPHENYLPHOSPHINE)PLATINUM(0) AND TRIS(TRIPHENYLPHOSPHINE)PLATINUM(0) IN TOLUENE 136 5- 1. Introduction 136 5-2. Platinum(O) complexes of PPh 3 136 5-3. A 3 1 P{'H} study of the equilibrium established by Pt(PPh 3) 4 and Pt(PPh 3) 3 in toluene 137 5-.4. 1 9 5 Pt{ x H} NMR spectra of Pt(PPh 3) 3 and Pt(PPh 3) 4 in toluene 152 5-5. A study of the equilibrium of Pt(PPh 3) n species by electronic spectroscopy 153 5- 6. Summary 157 CHAPTER 6 A KINETIC AND MECHANISTIC STUDY OF TRIS- AND TETRAKIS-(TRIPHENYLPHOSPHINE)PLATINUM(0) SPECIES IN OXYGENATION-PROTONATION PROCESSES 158 6- 1. Introduction 158 6-2. Kinetics and mechanism of oxygenation reactions of Pt(PPh 3) n compounds 160 6-2-1. Reaction between Pt(PPh 3) 3 and 0 2 160 6-2-2. Kinetics of the reaction of Pt(0 2)(PPh 3) 2 with PPh 3 165 6-2-3. Kinetics of the reaction of Pt(PPh 3) 3 with 0 2 169 6-2-4. Mechanism of the reaction of Pt(PPh 3) 3 with 0 2 172 - ix -T A B L E O F C O N T E N T S C O N T I N U E D PAGE 6-2-5. Reaction between Pt(PPh 3) 4 and 0 2 172 6-3. Reaction between Pt(0 2)(PPh 3) 2 and HCl 173 6- 4. Summary 178 CHAPTER 7 A MECHANISTIC STUDY OF TRIS- AND TETRAKIS-(TRIPHENYLPHOSPHINE)PLATINUM(0) SPECIES IN PROTONATION-OXYGENATION PROCESSES 179 7- 1. Introduction 179 7-2. Synthesis and characterization of trans-PtHCl(PPh3)2 180 7-3. Reaction between trans-PtHCl(PPh3)2 and 0 2 182 7-4. Reaction between cationic platinum hydride and 0 2 187 7- 5. Summary 193 CHAPTER 8 GENERAL CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE WORK 194 8- 1. General conclusions 194 8-2. Recommendations for future work 196 REFERENCES 198 - X -LIST O F T A B L E S T A B L E P A G E 1-1 P r o p e r t i e s o f d i o x y g e n i n v a r i o u s o x i d a t i o n s ta tes 22 1- 2 A s u m m a r y o f h y d r o p e r o x i d e c o m p l e x e s o f g r o u p V I I I m e t a l s 36 2- 1 T h e p e a k w i d t h o f t h e p r o t o n resonance (6) o f H 2 0 2 a t d i f f e ren t [ H 2 0 2 ] i n C D 2 C 1 2 a t 25 °C 5 0 2- 2 S o l u b i l i t y coef f i c ien ts o f o x y g e n i n benzene a t d i f f e ren t t e m p e r a t u r e s 53 3- 1 M o l a r e x t i n c t i o n coef f ic ients o f I rCl(CO)(PPh3) 2 i n benzene a n d C H 2 C 1 2 a t 25 ° C u n d e r a r g o n a t m o s p h e r e 59 3-2 T h e p s e u d o first-order r a t e c o n s t a n t s f o r t h e o x y g e n a t i o n r e a c t i o n w i t h respec t t o I rCl(CO)(PPh3) 2 i n benzene a t 25 ° C 66 3-3 K i n e t i c m e a s u r e m e n t o f t h e reverse r a t e c o n s t a n t k _ i f o r d e o x y g e n a t i o n o f I r C l ( 0 2 ) ( C O ) ( P P h 2 ) 2 i n benzene a t 25 ° C 6 7 3-4 T h e p s e u d o first-order r a t e c o n s t a n t s o f t h e o x y g e n a t i o n r e a c t i o n w i t h respec t t o I r C l ( C O ) ( P P h 3 ) 2 i n C H 2 C 1 2 a t 25 ° C 68 3-5 T h e i n f r a r e d d a t a f o r t r i f l u o r o a c e t a t e s a n d f ree t r i f l u o r o a c e t i c a c i d 76 3- 6 F A B mass s p e c t r a l d a t a ( h i g h mass p o r t i o n ) f o r t h e res idue f r o m t h e p r o t o n a t i o n r e a c t i o n ( I r C l ( 0 2 ) ( C O ) ( P P h 3 ) 2 : T F A = 1 : 2 ) 8 1 4- 1 T h e s t r e t c h i n g f requenc ies f o r t h e c a r b o n y l g r o u p s o f I r H C l 2 ( C O ) ( P P h 3 ) 2 , I r C l ( 0 2 ) ( C O ) ( P P h 3 ) 2 a n d I r C l ( C O ) ( P P h 3 ) 2 a t 25 ° C 104 4-2 F A B mass s p e c t r a l d a t a ( h i g h mass p o r t i o n ) f o r t h e res idue f r o m t h e r e a c t i o n b e t w e e n I r H C l 2 ( C O ) ( P P h 3 ) 2 a n d 0 2 108 4-3 T h e p s e u d o first-order r a t e c o n s t a n t , k 0 j , , f o r t h e r e a c t i o n o f I r C l ( C O ) ( P P h 3 ) 2 w i t h H C 1 i n C H 2 C 1 2 a t 25 ° C 120 4-4 T h e f o r w a r d a n d reverse r a t e c o n s t a n t s f o r t h e r e a c t i o n b e t w e e n I r C l ( C O ) ( P P h 3 ) 2 a n d H C 1 i n C H 2 C 1 2 a t 25 ° C 121 4- 5 T h e J H a n d 3 1 P N M R a n d F T - I R d a t a f o r I r H ( 0 2 ) ( P P h 3 ) 2 131 5- 1 K va lues f o r t h e e q u i l i b r i u m P t ( P P h 3 ) 3 + P P h 3 ^ P t ( P P h 3 ) 4 a t 22 ° C i n t o l u e n e - d 8 146 K' 5- 2 T h e e q u i l i b r i u m c o n s t a n t K ' ( P t ( P P h 3 ) 4 ^ P t ( P P h 3 ) 3 + P P h 3 ) i n t o l u e n e a t v a r i o u s t e m p e r a t u r e s 149 6- 1 T h e p s e u d o first-order r a t e c o n s t a n t s f o r t h e r e a c t i o n b e t w e e n P t ( 0 2 ) ( P P h 3 ) 2 a n d PPh3 i n benzene a t 25 ° C 167 7- 1 T h e r e a c t i o n c o n d i t i o n s f o r t h e p r e p a r a t i o n o f t h e c a t i o n i c p l a t i n u m h y d r i d e species 188 7-2 T h e 3 1 P a n d * H N M R d a t a f o r t h e c a t i o n i c p l a t i n u m h y d r i d e species 191 - x i -LIST O F F I G U R E S F I G U R E P A G E 1-1 M e c h a n i s t i c f ea tu res o f t h e o x i d a t i o n o f e t h y l e n e b y p a l l a d i u m sa l ts i n w a t e r 5 1-2 M o l y b d a t e - c a t a l y z e d e p o x i d a t i o n o f a n o le f i n ( " b u t t e r f l y m e c h a n i s m " ) 13 1-3 M o l y b d a t e - c a t a l y z e d e p o x i d a t i o n o f a n o le f in ( " p e r o x o m e t a l l o c y c l e m e c h a n i s m " ) 14 1-4 T h e m e c h a n i s m o f o x y g e n a c t i v a t i o n a n d i t s t r a n s f e r t o s u b s t r a t e s b y c y t o c h r o m e P-450 16 1-5 T h e gene ra l s t r u c t u r e o f c a t i o n i c sa len c o m p l e x e s 17 1-6 P o r p h y r i n d i a n i o n l i g a n d s a n d F e ( T p i v P P ) ( N - M e I m ) ( 0 2 ) 18 1-7 T h e m e c h a n i s m f o r e p o x i d a t i o n o f o le f ins c a t a l y z e d b y F e ( T P P ) X ( X = h a l i d e ) 20 1-8 M o l e c u l a r o r b i t a l d i a g r a m f o r m o l e c u l a r o x y g e n i n t h e g r o u n d a n d e x c i t e d s ta tes 2 1 1-9 T h e g e o m e t r i e s o f T ; 1 - a n d 7 y 2 - d i o x y g e n c o m p l e x e s 27 1 - 10 T h e m e c h a n i s m o f c a t a l y t i c o x i d a t i o n o f t e r m i n a l o le f ins b y a p a l l a d i u m c a t a l y s t 33 2- 1 T h e J H N M R s p e c t r u m o f H 2 0 ( ~ 1 0 "4 M ) i n C D 2 C 1 2 a t 25 ° C 5 1 2-2 T h e a H N M R s p e c t r u m o f H 2 0 2 (4 .5 x 1 0 ~3 M ) i n C D 2 C 1 2 a t 25 ° C 5 1 2-3 T h e 1 H N M R s p e c t r u m o f H 2 G 2 ( 9 . 1 x 1 0 "3 M ) i n C D 2 C 1 2 a t 25 ° C 52 2- 4 T h e l i n e a r dependence o f t h e p e a k w i d t h f o r t h e p r o t o n resonance o f H 2 0 2 o n [ H 2 0 2 ] 52 3- 1 T h e n a t u r e o f o x y g e n - m e t a l b o n d i n g f o r a s ide-on d i o x y g e n m e t a l c o m p l e x 58 3-2 T h e U V - v i s s p e c t r u m o f I r C l ( C O ) ( P P h 3 ) 2 i n benzene a t 25 ° C 60 3-3 ( a ) T h e changes i n t h e a b s o r p t i o n c u r v e o f I rCl(CO)(PPh3) 2 over t i m e i n t h e o x y g e n a t i o n r e a c t i o n i n benzene a t 25 ° C 69 ( b ) T h e p s e u d o first-order k i n e t i c s f o r t h e o x y g e n a t i o n r e a c t i o n w i t h respec t t o I rCl(CO)(PPh3) 2 69 ( c ) T h e first-order d e p e n d e n c e o n 0 2 f o r t h e o x y g e n a t i o n r e a c t i o n o f I r C l ( C O ) ( P P h 3 ) 2 i n b e n z e n e a t 25 °C 70 3-4 T h e 3 1 P { 1 H } N M R s p e c t r u m f o r t h e r e a c t i o n o f I r C l ( 0 2 ) ( C O ) ( P P h 3 ) 2 w i t h T F A (1 : 1 ) i n C H 2 C 1 2 a t 25 °C; t h e s p e c t r u m was essen t ia l l y u n v a n a n t w i t h t i m e 73 3-5 T h e 3 1 P { 1 H } N M R s p e c t r u m f o r t h e r e a c t i o n o f I r C l ( 0 2 ) ( P P h 3 ) 2 w i t h T F A ( 1 : 2 ) i n C H 2 C 1 2 a t 25 ° C 73 3-6 ( a ) T h e F T - I R s p e c t r u m o f I r C l ( 0 2 ) ( C O ) ( P P h 3 ) 2 i n N u j o l m u l l 77 ( b ) T h e F T - I R s p e c t r u m o f t h e r e s i d u e i n N u j o l m u l l f r o m t h e r e a c t i o n b e t w e e n I r C l ( 0 2 ) ( C O ) ( P P h 3 ) 2 a n d T F A ( 1 : 1) a t 25 ° C 77 — xii — L I S T O F F I G U R E S C O N T I N U E D F I G U R E P A G E 3-6 ( c ) T h e F T - I R s p e c t r u m o f t h e res idue i n N u j o l m u l l f r o m t h e r e a c t i o n b e t w e e n I r C l ( 0 2 ) ( C O ) ( P P h 3 ) 2 a n d T E A (1 : 2 ) a t 25 ° C 78 ( d ) T h e F T - I R s p e c t r u m o f t h e res idue i n K B r p e l l e t f r o m t h e r e a c t i o n b e t w e e n I r C l ( 0 2 ) ( C O ) ( P P h 3 ) 2 a n d T E A (1 : 1) a t 25 °C 78 3-7 T h e " F - ^ H } N M R s p e c t r u m i n C D 2 C 1 2 o f t h e res idue f r o m t h e r e a c t i o n b e t w e e n I r C l ( 0 2 ) ( C O ) ( P P h 3 ) 2 a n d T E A ( 1 : 2 ) a t 25 ° C 80 3-8 T h e mass s p e c t r a o f ( a ) I r C l ( 0 2 ) ( C O ) ( P P h 3 ) 2 a n d t h e res idue f r o m t h e r e a c t i o n b e t w e e n I r C l ( 0 2 ) ( C O ) ( P P h 3 ) 2 a n d T F A ( 1 : 1 ( b ) ; 1 : 2 ( c ) ) i n t h e 250-300 (mass n u m b e r ) r e g i o n 80 3-9 T h e 3 1 P ^ H } N M R s p e c t r a f o r t h e r e a c t i o n s o f I r C l ( 0 2 ) ( C O ) ( P P h 3 ) 2 w i t h H C l a t H C l / I r r a t i o s ( a ) 0 : 1 , ( b ) 1 : 1 , ( c ) 2 : 1 , a n d ( d ) 2 : 1 ( a f t e r 24 h ) i n C D 2 C 1 2 a t 20 °C 86 3-10 T h e * H N M R s p e c t r a ( i n t h e 6 1.0 t o 6.0 p p m r e g i o n ) f o r t h e r e a c t i o n s o f I r C l ( 0 2 ) ( C O ) ( P P h 3 ) 2 w i t h H C l a t H C l / I r r a t i o s ( a ) 0 : 1 , ( b ) 1 : 1 , a n d ( c ) 2 : 1 i n C D 2 C 1 2 a t 20 ° C 87 3 -11 T h e * H N M R s p e c t r u m ( i n t h e 6 7.0 t o 8.0 p p m r e g i o n ) f o r m e r - I r C l 3 ( C O ) ( P P h 3 ) 2 i n C D 2 C 1 2 a t 20 ° C 88 3-12 T h e s t e r e o c h e m i s t r y i n v o l v e d i n t h e p r o p o s e d r e a c t i o n p a t h w a y f o r t h e r e a c t i o n b e t w e e n I r C l ( 0 2 ) ( C O ) ( P P h 3 ) 2 a n d H C l 9 1 3-13 T h e * H N M R s p e c t r u m ( i n t h e 6 7.0 t o 8.0 p p m r e g i o n ) f o r t h e r e a c t i o n o f I r C l ( 0 2 ) ( C O ) ( P P h 3 ) 2 w i t h H C l a t H C l / I r r a t i o 2 : 1 i n C D 2 C 1 2 a t -70 ° C 93 3-14 T h e * H N M R s p e c t r a ( i n t h e 6 7.0 t o 8.0 p p m r e g i o n ) f o r t h e r e a c t i o n s o f I r C l ( 0 2 ) ( C O ) ( P P h 3 ) 2 w i t h H C l a t H C l / I r r a t i o s ( a ) 0 : 1 , ( b ) 1 : 1 , ( c ) 2 : 1 , a n d ( d ) 2 : 1 ( a f t e r 24 h ) i n C D 2 C 1 2 a t 20 ° C 94 3- 15 ( a ) T h e 3 1 P { 1 H } N M R s p e c t r u m o f t h e f i l t r a t e f r o m t h e r e a c t i o n I r C l ( 0 2 ) ( C O ) ( P P h 3 ) 2 + P P h 3 , 1 : 10 , C H 2 C 1 2 98 ( b ) T h e 3 1 P { 1 H } N M R s p e c t r u m ( C D 2 C 1 2 ) o f t h e y e l l o w c r y s t a l s s e p a r a t e d f r o m t h e r e a c t i o n I r C l ( 0 2 ) ( C O ) ( P P h 3 ) 2 + P P h 3 , 1 : 10 , C H 2 C 1 2 98 4 - 1 T h e s t r u c t u r e o f I r H C l 2 ( C O ) ( P P h 3 ) 2 103 4 -2 T h e F T - I R s p e c t r u m o f I r H C l 2 ( C O ) ( P P h 3 ) 2 i n N u j o l m u l l 103 4 -3 T h e s o l u t i o n F T - I R a b s o r p t i o n 6pectra f o r t h e c a x b o n y l g r o u p s o f I r H C l 2 ( C O ) ( P P h 3 ) 2 , I r C l ( 0 2 ) ( C O ) ( P P h 3 ) 2 , a n d I r C l ( C O ) ( P P h 3 ) 2 i n C H 2 C 1 2 a t 25 °C 105 4-4 T y p i c a l 3 1 P { ' H } N M R s p e c t r u m o f t h e s o l u t i o n r e s u l t i n g f r o m t h e r e a c t i o n b e t w e e n I r H C l 2 ( C O ) ( P P h 3 ) 2 a n d 0 2 i n C H 2 C 1 2 a t 25 °C 107 - xiii -LIST O F F I G U R E S C O N T I N U E D FIGURE PAGE 4-5 The FT-IR spectrum of the residue in Nujol mull from the reaction between IrHCl 2(CO)(PPh 3) 2 and 0 2 107 4-6 The changes of the FT-IR absorption spectra of the solution for the reaction between IrHCl 2(CO)(PPh 3) 2 and 0 2 in CH 2 C1 2 at 25 °C 110 4-7 The time dependence of [IrHCl2(CO)(PPh3)2] for the reaction between IrHCl 2(CO)(PPh 3) 2 and 0 2 in CH 2 C1 2 at 25 °C 111 4-8 The time dependence of [IrCl(02)(CO)(PPh3)2] for the reaction between IrHCl 2(CO)(PPh 3) 2 and 0 2 in CH 2 C1 2 at 25 °C 111 4-9 The pseudo first-order treatment for the absorbance at 2047 c m - 1 versus time for the reaction between IrHCl 2(CO)(PPh 3) 2 and 0 2 in CH 2 C1 2 at 25 °C 113 4-10 The unchanged FT-IR solution absorption spectra for the reaction between IrHCl 2(CO)(PPh 3) 2 and 0 2 in acidic CH 2 C1 2 at 25 °C 114 4-11 The stopped-flow kinetic data monitored at 386 nm for the reaction of IrCl(CO)(PPh3) 2 at 1.62 x 10~4 M with HCl (1.22 x 10~2 M) in CH 2 C1 2 at 25 °C 118 4-12 The pseudo first-order kinetic behaviour of the reaction between IrCl(CO)(PPh3) 2 and HCl with respect to [IrCl(CO)(PPh3)2] 119 4-13 The linear dependence of the observed rate constants on [HCl] for the reaction between IrCl(CO)(PPh3)2 and HCl 119 4-14 The changes in the solution FT-IR absorption spectra for HCl elimination reaction from IrHCl 2(CO)(PPh 3) 2 in CH 2 C1 2 at 25 °C 122 4-15 The first-order kinetic behaviour for the HCl elimination reaction from IrHCl 2(CO)(PPh 3) 2 in CH 2 C1 2 at 25 °C 123 4-16 The J H NMR spectrum of the green solution obtained from the reaction of IrH(CO)(PPh3)3 with 0 2 in C 6 D 6 129 4-17 The 3 1 P{ J H} NMR spectrum of the green solution obtained from the reaction of IrH(CO)(PPh3)3 with 0 2 in C 6 D 6 129 4-18 The FT-IR spectrum of the green solution obtained from the reaction of IrH(CO)(PPh3)3 with 0 2 in C 6 D 6 130 4-19 The structure of IrH(0 2)(CO)(PPh 3) 2, VII 130 4- 20 The 3 1 P{ 1 H} NMR spectrum of the dark blue solution obtained from the reaction of IrH(CO)(PPh3)3 with 0 2 in C 6 D 6 after 20 h at room temperature 132 5- 1 (a) The 3 1P{ 1H} NMR spectrum of Pt(PPh 3) 3 in toluene-d8 at 20 °C 139 • (b) The 3 1P{ 1H} NMR spectrum of Pt(PPh 3) 3 in toluene-d8 at -90 °C 139 5-2 The 3 1 P{ : H} NMR spectra of a mixture of Pt(PPh 3) 3 and Pt(PPh 3) 4 (1 : 1.5 mole ratio) at 100, 20 and -90 °C in toluene-d8 140 - x i v -L I S T O F F I G U R E S C O N T I N U E D FIGURE PAGE 5-3 T h e 3 1 P { 1 H } N M R s p e c t r a o f a m i x t u r e o f P t ( P P h 3 ) 3 a n d P P h 3 ( 1 : 1 m o l e r a t i o ) a t d i f f e r e n t t e m p e r a t u r e s i n t o l u e n e - d 8 142 5-4 T h e 3 1 P { 1 H } N M R s p e c t r a o f t h e m i x t u r e o f P t ( P P h 3 ) 3 a n d P P h 3 a t d i f f e ren t m o l e r a t i o s , 22 ° C i n t o l u e n e - d 8 144 5-5 T h e 3 1 P { 1 H } N M R s p e c t r a o f P t ( P P h 3 ) 4 a t d i f f e ren t t e m p e r a t u r e s i n t o l u e n e - d g 148 5-6 T h e t e m p e r e t u r e d e p e n d e n c e o f t h e e q u i l i b r i u m c o n s t a n t , K ' , f o r P t ( P P h 3 ) 4 ^ P t ( P P h 3 ) 3 + P P h 3 151 5-7 T h e 1 9 5 P t { 1 H } N M R s p e c t r u m o f P t ( P P h 3 ) 3 i n t o l u e n e - d 8 a t 25 ° C 152 5-8 T h e 1 9 5 P t { 1 H } N M R s p e c t r u m o f P t ( P P h 3 ) 4 i n t o l u e n e - d 8 a t 25 °C 153 5-9 T h e U V - v i s s p e c t r u m o f P t ( P P h 3 ) 3 i n benzene a t 22 ° C 154 5-10 T h e U V - v i s s p e c t r a o f t h e m i x t u r e s o f P t ( P P h 3 ) 3 (4 .86 x IO - 4 M ) w i t h P P h 3 a t d i f f e ren t m o l e r a t i o s i n benzene a t 22 ° C 155 5- 11 T h e U V - v i s s p e c t r u m o f P t ( P P h 3 ) 4 i n benzene a t 22 ° C 156 6- 1 T h e 3 1 P { 1 H } N M R s p e c t r u m o f t h e res idue f r o m t h e r e a c t i o n o f P t ( P P h 3 ) 3 w i t h 0 2 , i n t o l u e n e - d 8 a t 20 ° C 161 6-2 T h e 3 1 P j 1 ! ! } N M R s p e c t r a f o r t h e r e a c t i o n o f P t ( P P h 3 ) 3 w i t h 0 2 ( a n e q u i m o l a r a m o u n t i n t h e gas p h a s e ) i n t o l u e n e - d 8 a t 20 ° C 162 6-3 T h e 3 1 P I 1 ! ! } N M R s p e c t r u m o f t h e s o l u t i o n r e s u l t i n g f r o m t h e r e a c t i o n b e t w e e n P t ( 0 2 ) ( P P h 3 ) 2 a n d P P h 3 ( 1 : 3.3 m o l e r a t i o ) i n t o l u e n e - d 8 a t 22 ° C 164 6-4 T h e 3 1 P { 1 H } N M R s p e c t r u m o f t h e s o l u t i o n r e s u l t i n g f r o m a m i x t u r e o f P t ( 0 2 ) ( P P h 3 ) 2 , P P h 3 a n d P t ( P P h 3 ) 3 i n t o l u e n e - d 8 a t 22 ° C , a t a 1 : 1.4 : 2.1 m o l e r a t i o 164 6-5 ( a ) T h e a b s o r b a n c e c u r v e o b t a i n e d o n a s p e c t r o p h o t o m e t e r f o r t h e r e a c t i o n b e t w e e n P t ( 0 2 ) ( P P h 3 ) 2 a n d P P h 3 ( 1 : 18.6 m o l e r a t i o ) i n benzene a t 25 ° C 166 ( b ) T h e p s e u d o f i r s t - o r d e r k i n e t i c b e h a v i o u r i n P t ( 0 2 ) ( P P h 3 ) 2 f o r t h e r e a c t i o n b e t w e e n P t ( 0 2 ) ( P P h 3 ) 2 a n d P P h 3 ( 1 : 18.6 m o l e r a t i o ) i n benzene a t 25 ° C 166 6-6 T h e f i r s t - o r d e r d e p e n d e n c e o n [ P P h 3 ] f o r t h e r e a c t i o n b e t w e e n P t ( 0 2 ) ( P P h 3 ) 2 a n d P P h 3 i n benzene a t 25 " C 168 6-7 ( a ) T h e a b s o r b a n c e c u r v e o b t a i n e d o n a s p e c t r o p h o t o m e t e r f o r t h e r e a c t i o n b e t w e e n P t ( P P h 3 ) 3 a n d 0 2 ( 1 a t m ) i n benzene a t 25 °C 170 ( b ) T h e p s e u d o first-order k i n e t i c b e h a v i o u r i n P t ( P P h 3 ) 3 f o r t h e r e a c t i o n b e t w e e n P t ( P P h 3 ) 3 a n d 0 2 (1 a t m ) i n benzene a t 25 ° C 171 6-8 T h e 3 1 P { J H } N M R s p e c t r u m o f t h e res idue f r o m t h e r e a c t i o n o f P t ( P P h 3 ) 4 w i t h 0 2 i n t o l u e n e - d 8 a t 20 ° C 172 - XV -LIST O F F I G U R E S C O N T I N U E D FIGURE PAGE 6-9 The 3 1P{ 1H} NMR spectra for the reaction between Pt(0 2)(PPh 3) 2 and HCl (1 : 1 and 1 : 2) in CD 2 C1 2 at 20 °C 174 6-10 The 3 1P{ 1H} NMR spectra for the reaction between Pt(0 2)(PPh 3) 2 and HCl (1 : 1 and 1 : 2) in CD 2 C1 2 at -72 °C 175 6- 11 The J H NMR spectrum at 20 °C for the solution resulting from the reaction between Pt(0 2)(PPh 3) 2 and HCl (1 : 2) in CD 2 C1 2 at -72 °C 177 7- 1 The FT-IR spectrum of PtHCl(PPh 3) 2 in Nujol mull 181 7-2 The 3 1P{ 1H} NMR spectrum of the residue in CD 2 C1 2 at 20 °C from reaction (1): PtHCl(PPh 3) 2 with 0 2 in CH 2 C1 2 at 20 °C 183 7-3 The 3 1 P{*H} NMR spectrum of the residue in CD 2 C1 2 at 20 °C from reaction (2): PtHCl(PPh 3) 2 with 0 2 in THF at 20 "C 183 7-4 The 3 1P{ 1H} NMR spectrum of the solution resulting from the decomposition of Pt(0 2)(PPh 3) 2 in CD 2 C1 2 at 20 °C 185 7-5 The 3 1P{ 1H} NMR spectrum of the solution resulting from reaction (3): PtHCl(PPh 3) 2 with 0 2 in CD 2 C1 2 at 20 °C in the presence of Pt(0 2)(PPh 3) 2 185 7-6 The 3 1P{ 1H} NMR spectrum of the solution resulting from reaction (4): PtHCl(PPh 3) 2 with 0 2 in CD 2 C1 2 at 20 °C in the presence of HCl 186 7-7 (a) The 3 1P{ 1H} NMR spectrum of the cationic hydrides in a solvent mixture of CD 2 C1 2 and acetone-d6 at -75 °C 189 (b) The 3 1P{ JH} NMR spectrum of the cationic hydrides in a solvent mixture of CD 2 C1 2 and acetone-d6 at 20 °C 189 (c) The *H NMR spectrum of the cationic hydrides in a solvent mixture of CD 2 C1 2 and acetone-d6 at -75 °C 190 (d) The X H NMR spectrum of the cationic hydrides in a solvent mixture of CD 2 C1 2 and acetone-d6 at 20 °C 190 - xvi-A B B R E V I A T I O N S A N D S Y M B O L S The following list of abbreviations and symbols, which are used conventionally in most scientific publications, will be employed without additional notations in this thesis. A definition will be given, however, following the appearance of a specified abbreviation or a symbol in the contents of this thesis. A angstrom(s) A absorbance °C Celsius cal calorie, 1 cal = 4.18 Joules d day(s): doublet FT Fourier Transform g gram(s) h hour(s) Hz Hertz, cycles per second IR infrared J coupling constant, in Hz k rate constant; kilo-K equibbrium constant; Kelvin log logarithm M molar, moles per liter mL milliliter mmol millimole - xvii-ms millisecond nm nanometers NMR nuclear magnetic resonance Ph phenyl PPh 3 triphenylphosphine ppm parts per million s second(s); singlet 5 solvent t time; triplet TMS tetramethylsilane 6 chemical shift in ppm with respect to TMS £ molar extinction coefficient, M - 1 c m - 1 A wavelength, nm A m u wavelength of maximum absorbance, nm v stretching modes in IR vibrations [ ] concentration •pH} broadband proton decoupled - xvii i-A C K N O W L E D G E M E N T S The author expresses his Bincerest appreciation and thanks to Professor B. R. James for his guidance and support throughout the course of this work. The author also thanks Chemistry Department and supporting staff for their assistance during his entire graduate program, and also the members of Professor James' research group with whose company the author has enjoyed for about 4 years. The skilful typing of this thesis was accomplished by Shao-feng Wu and must be mentioned with special gratitude by the author. Finally, this thesis is dedicated to Shao-feng, Ying-tao and Ying-jun as a testimony to their support and encouragement. -1 -C H A P T E R 1 I N T R O D U C T I O N 1-1. General introduction Current studies on selective oxidation of hydrocarbons remain intense both from industrial and biological viewpoints. 1 - 1 2 The key to selective oxidation is via a better understanding of the reaction mechanisms involved. The underlying theme of the work described in this thesis centres on the chemistry (syntheses and properties) of iridium and platinum metal hydroperox-ide species (M-OOH), which could be critical intermediates in homogeneous catalytic oxidation processes. Kinetic and mechanistic studies on dioxygen* activation by iridium and platinum complexes are also included as indispensable parts of the present work. A brief review of ho-mogeneous catalysis will first be presented (Section 1-1), followed by a closer look at oxidation processes (Section 1-2 to 1-6). Transition metals demonstrate chemical features that contrast with those of main group metals. In a simplified and rather general view, the former group exhibits variable valence and forms complexes with different ligands. 1 3 , 1 4 This characteristic makes the chemistry of transi-tion metals and their compounds rich and very attractive to chemists. In practice, transition metals and their complexes have been widely used in the chemical and petroleum industries as heterogeneous and homogeneous catalysts. Examples of these processes include oxidations * dioxygen: throughout this thesis, dioxygen ( 0 2 ) will be simply called oxygen. Free dioxygen is, therefore, called molecular oxgyen. - 2 -of olefins and aromatic compounds, polymerization and hydrogenation of olefins, asymmet-ric hydroformylation of alkenes, carbonylation of alcohols, and asymmetric hydrogenation of prochiral alkenes. 1 5 - 1 9 Most reactions in homogeneous catalysis, using transition metals and their complexes, can be described in terms of some fundamental reactions of coordination and organometallic chemistry. 1 6 , 2 0 These reactions (which appear in many combinations and sequences to complete a catalytic cycle or to provide the necessary transformations to access the catalytic cycle) are ligand replacement, oxidation and reduction, oxidative addition and reductive elimination, insertion or cis-migration, and /3-elimination. It is also believed that in both homogeneous and heterogeneous catalytic processes, these basic reaction patterns are involved even though the reaction conditions can be remarkably different. In terms of industrial operations, heterogeneous catalysts have great practical advantages over homogeneous ones. In a process of heterogeneous catalysis, the products and the substrates can be separated much more easily from the reaction system than in a process of homogeneous catalysis. Consequently, the purification of products can be accomplished in a more efficient and economic way. The major advantages of homogeneous catalysts, however, are the high rates and good selectivity and, especially, the mild conditions of the reactions. As a result, high yields of product become possible. Furthermore, mechanisms can be studied more conveniently for reactions of homogeneous catalysis and the reactivities of homogeneous catalysts can be modified by using different ligands. Therefore, it is not surprising that significant efforts have been made in the studies of homogeneous catalysts. Combination of the advantages of both homogeneous and heterogeneous catalysts creates - 3 -a possible approach to an ideal catalyst. The increasing research in development of supported catalysts, in which soluble catalysts are immobilized on insoluble but dispersive supporting bodies, has provided and will provide transition metals and their complexes more opportunities for industrial applications.20 Another apparent trend is the development of more active soluble catalysts through new concepts in catalyst design. For example, photoactivated catalysts open the way to reactions at even lower temperatures than those used with conventional homogeneous catalysts. The basic concept involved in photoactivated catalysts is the acquisition of the energy, by means of radiation, necessary to create a vacant coordination site for interaction of the substrate with the catalyst, particularly when the rate-determining step in the corresponding homogeneous catalytic reaction is dissociation of a ligand from the metal centre. Metal cluster catalysts, as another example, could provide multiple reaction sites for particularly difficult transformations such as alkane isomerization and hydrogenolysis.16 1-2. Catalytic oxidation of hydrocarbons Of the homogeneous catalytic processes employed in industry, oxidation of hydrocarbons is on the largest scale. Molecular oxygen is usually chosen as an oxidant because of its abundance and accessibility. The oxidations of cyclohexane to cyclohexanol and cyclohexanone, p-xylene to terephthalic acid, ethylene to acetaldehyde and butane to acetic a c i d 1 5 , 1 6 are homogeneously catalyzed by transition metal complexes using molecular oxygen. Often, instead of directly using molecular oxygen, other forms of oxygen sources, such as metal peroxides, hydrogen peroxide and alkylperoxides, are used as oxidants.3'4-9-15'16 - 4 -Oxidations of organic compounds catalyzed by transition metal complexes take place through various pathways and involve different reaction mechanisms. It would be impossi-ble to classify catalytic oxidations in terms of reaction mechanisms with definite boundaries. However, a general classification of the catalytic oxidation reactions can be made in order to provide an overall basic knowledge of the mechanisms involved. In this thesis, catalytic oxidations including enzymatic systems are divided into three classes of reactions. The first class is the Wacker-type oxidation of organic compounds. In this oxidative process, a metal such as Pd(II) (a lower oxidation state) plays a role as a particularly strong electrophile. The substrate, in most cases an olefin, is activated toward nucleophilic attack through 7r-complex coordination with the metal. The overall result is the formation of aldehydes or ketones with concomitant two-electron reduction of the metal catalyst. The well-known Wacker process for oxidation of ethylene to acetaldehyde, discovered in 1959, is one of the best studied examples of this c l a s s . 2 1 - 2 6 However, the stoichiometric oxida-tion of ethylene oxidized by palladium(II) chloride has been known since 1893.27 In a Wacker-type reaction, molecular oxygen is not directly involved in the substrate oxidation, but is used in regeneration of metal catalyst. A mechanistic representation is shown in Fig. l - l . 1 6 The major features of the mechanism of ethylene oxidation seem to be well established, but a lively controversy about the detailed mechanism has developed. There is general agreement that the initial step is the replacement of a chloride ion in [PdChj] 2 - (shown as P d 2 + , (1 ) in Fig. 1-1) by ethylene to form (2 ). Additionally, it is agreed that another chloride ion is replaced by water and hydroxide ligands. As a consequence, the rate of the reaction is sharply decreased by high chloride concentrations. -5-r 2 C u + p d 2 + /C2H4 ^ x . 2 tl P d 2 + C H 2 CH 2OH C H 2 — P d + CH 3 , V . CHOH I I - M + CH2 H Fig. 1-1. Mechanistic features of the oxidation of ethylene by palladium salts in water. - 6 -T h e m a j o r p o i n t o f d i s a g r e e m e n t is t h e m e c h a n i s m o f t h e a d d i t i o n o f 0 H ~ t o t h e c o o r d i -n a t e d e t h y l e n e t o g ive a h y d r o x y e t h y l p a l l a d i u m i n t e r m e d i a t e (3 ) . T h i s i n t e r m e d i a t e has neve r b e e n o b s e r v e d b u t is c o m m o n l y a s s u m e d t o b e p r e s e n t i n t h e r e a c t i o n m i x t u r e . I n i t i a l l y , t h e O H - a d d i t i o n w a s c o n s i d e r e d t o r e s u l t f r o m m i g r a t i o n o f a c o o r d i n a t e d 0 H ~ l i g a n d t o c a r b o n , as s h o w n i n e q . 1 -1 . T h i s m i g r a t i o n ( o r e t h y l e n e i n s e r t i o n i n t o a P d - O H b o n d ) is cons is ten t w i t h t h e k i n e t i c s r e p o r t e d . 1 6 H o w e v e r , s t e r e o c h e m i c a l s t u d i e s 2 8 , 2 9 o f a d d i t i o n t o c o o r d i n a t e d cis- o r t r a n s - C H D = C H D i n d i c a t e a t t a c k by an e x t e r n a l n u c l e o p h i l e as s h o w n i n eq . 1-2. I t is q u i t e p o s s i b l e t h a t b o t h m e c h a n i s m s (c is a n d t r a n s O H - a t t a c k ) o p e r a t e a n d t h a t r e a c t i o n c o n d i t i o n s d e t e r m i n e w h i c h p r e d o m i n a t e s . ( 1 - 1 ) ( 1 - 2 ) - 7 -One remarkable aspect of the ethylene oxidation is that no H/D exchange with solvent oc-curs. When the reaction with C 2 H 4 takes place in D 2 0, no deuterium appears in the product. Evidently the hydrogen migrations involved in transformation of the HOCB^CHjPd interme-diate to C H 3 C H O occur entirely within the coordination sphere of the palladium. ^ -Hydrogen elimination to give a vinyl alcohol complex (4) seems a likely pathway. Analogous platinum complexes of vinyl alcohol are well characterized.30 Readdition of the hydride ligand to the coordinated vinyl alcohol gives the a-hydroxyethylpalladium complex (5) shown in Fig. 1-1. A y3-hydrogen elimination from OH completes acetaldehyde formation and results in formation of zerovalent palladium (6 ) or a palladium(II) hydride. Either species is rapidly reoxidized by copper(II) in a chloride-containing medium.16 Molecular oxygen is used to oxidize copper(I) to copper(II) and, then, the catalytic cycle is established. The second class of reactions is the autoxidation of organic compounds, involving a Haber-Weiss31 mechanism to generate radicals when using molecular oxygen as oxidant. Autoxida-tions, in this text, refer to the liquid-phase oxidations proceeding under relatively mild tem-perature using either molecular oxygen, hydrogen peroxide or alkyl peroxides. Autoxidations take place through a free-radical chain process. When hydrogen peroxide is employed as an oxidant, the radicals are produced through a decomposition mechanism of hydrogen peroxide catalyzed by metal salts, known as the Haber-Weiss mechanism. This mechanism can be demonstrated using Fenton's reagent, which consists of ferrous salts and H2O2, as shown in eqs. 1-3 to 1-7.15 Fe11 + H2O2 — • FeinOH + SO' (1-3) - 8 -Fe111 + H 2 0 2 — • Fe11 + HO'2 + H + (1-4) Fen + HO'—>FeUIOH (1-5) Fe111 + E0\ — • Fe11 + 03 + H+ (1 - 6) HO' + H202 — • H20 + H0\ (1 - 7) In the presence of organic substrates, the hydroxy! radicals (HO*) produce organic free radicals as showjn in eq.1-8. RH + HO'—>R' + H20 (1-8) Further reactions will occur following the formation of the organic free radicals. For example, in the oxidation of tert-butyl alcohol using Fenton's reagent, 2,5-dimethyl-2,5-hexanediol is produced in 84 % yield by the dimerization of the intermediate primary alkyl radicals, as shown in eqs. 1-9 and 1-10.32 (CH3)3COH + HO' —• {CH3)2C{OH)CH'2 + H20 (1 - 9) 2{CH3)2C(OH)CH'2 atm<Zl±?™n (CH3)2C{OH)CH2CH2C(OH){CH3)2 (1 - 10) When alkyl peroxides are used as oxidants, two principal reactions are involved with metal complexes to generate organic free radicals, as shown in eqs. 1-11 and 1-12. ROiH + M ( n - 1 ) + —• RO' + M n + + HO- ( 1 - H ) - 9 -R02H + M n + — • R0'2 + M ( n _ 1 ) + +H+ (1-12) The metal-ion-promoted decomposition ol alkyl hydroperoxides can be utilized for the intro-duction of the alkylperoxy group into olefins, ethers, and alkylaromatic compounds. Copper salts are the catalysts of choice for this reaction, as shown in eq. 1-13.15 2R'02H + RHCuWCuVBR02R' + H20 + R'OH (1-13) When molecular oxygen is used as the oxidant, organic free-radicals are generated by the direct interaction of strong metal oxidants with organic substrates in two ways, electron transfer or electrophilic substitution. The industrially important oxidation of p-xylene to terephthalic acid with a cobalt catalyst is a relevant example. 1 5 , 1 6 , 3 3 The two routes producing organic free-radicals are given in eqs. 1-14 to 1-17. Electron transfer: M(OAc)3 + RH ^  RH+ + M(OAc)^ (1 - 14) RH+—>R'+H+ (1-15) Electrophilic substitution: RH + M{OAc)3 —• RM(OAc)2 + HO Ac (1 - 16) RM(OAc)2 —> R' + M(OAc)2 (1 - 17) The net result in both cases is an one-electron reduction of the metal oxidant with concomitant formation of the substrate radical (R*). This radical is readily trapped by molecular oxygen, so that successive reactions occur to generate products. A typical example is the oxidation of alkylbenzenes catalyzed by cobaltic acetate to form benzoic acid or analogues, as in Scheme 1-1." - 10 -Scheme 1-1: The oxidation of alky] aromatic compounds catalyzed by cobaltic acetate ArCH3 + Co111 ^ [ArCH3]+ + Co11 [ArCH3}+ — ArCH'2 + H+ ATCH\ + 02 -* ArCH20'2 ArCH20\ + Co11 -> ArCH202Coin ArCH202CoIU -> ArCHO + HOCo111 HO Co111 + HO Ac -» AcOCo"1 + H20 ArCHO ArCOOH Thus, in the second class of catalytic oxidations of organic compounds, reaction processes involve the fundamental steps of a radical chain-reaction mechanism including initiation, prop-agation and termination stages. The essential role of the metal catalysts is to produce radicals in the initiation stage. In general, the metal catalyst can be regenerated with peroxidic inter-mediates or molecular oxygen. The third class is that of oxygen atom transfer to organic substrates utilizing either a metal oxide or a metal peroxide (produced by the interaction between a metal complex and molecular oxygen or peroxo species). That is, the metal catalyst is used as a means of relaying (transferring) oxygen atoms to the organic substrates . Various mechanisms are operative in this class. A,free-radical chain reaction, in general, is not involved, although radical intermediates may be produced in some cases. The epoxidation of olefins catalyzed by molybdenum salts and complexes, or vanadium compounds, belongs to this c l a s s . 1 5 - 1 7 , 3 4 - 3 8 For instance, propylene oxide is manufactured industrially by the metal catalyzed epoxidation of propylene using an alkylhydroperoxide produced by hydrocarbon autoxidation. Processes employing molybdenum catalysts with either tert-butyl hydroperoxide or ethylbenzene hydroperoxide are used, as shown in eqs. 1-18 and 1-19.15 Indeed, oxygen can be transferred from its activated form in alkyl peroxides to a great variety of olefins to produce the corresponding epoxides under conditions similar to those used for propylene epoxidation. 16 CH3CH = CH2 + (CH3)3COOH CH3Clf—CH2 + (CH3)3COH (1 - 18) CH3CH = CH2 + PhCH{CH3)OOH ^ CH3ClT—CH2 -f PhCH{CH3)OH ( 1 - 1 9 ) Curiously, most molybdenum compounds have essentially the same activities as catalysts for epoxidation, even, for example, zerovalent Mo(CO)6 and hexavalent M 0 O 3 compounds. Therefore, the compounds added to the reaction mixture are almost certainly catalyst precur-sors rather than actual catalytic species per se. The mechanistic aspects of the third class of hydrocarbon oxidation involve either an intramolecular or intermolecular process in which an oxygen atom is transferred to a substrate. For catalytic epoxidations of olefins using molybdenum catalysts, there are two commonly accepted mechanisms as given in Fig. 1-2 and 1-3, that is, a "butterfly mechanism"39 and a "peroxometallocycle mechanism".35,36 Fig. 1-2 shows the mechanism proposed by Sharpless.16 An alkoxymolybdate ( 7 ) under-goes ester exchange with an alkylhydroperoxide to form an alkylperoxomolybdate ( 8 ) . Reaction of this complex with an olefin probably occurs by displacement of a neutral ligand such as ROH. In any event, an intermediate or transition state (9) develops in which the Mo-bound peroxy oxygen transfers to olefin. This transfer forms the epoxide, which is still complexed to the metal as in (10). Displacement by a neutral ligand completes the catalytic cycle. Fig. 1-3 illustrates a mechanism proposed by Kaloustian et al . 4 0 This mechanism suggests that the actual epoxidizing agent is a diperoxo complex (11) formed tn situ. The following steps are the replacement of a ligand by olefin to give (12) and the formation of a metal peroxometallocycle (13). Finally, the elimination of the epoxide leaves a monoperoxo complex (14) which then reacts with alkylperoxide to regenerate the actual active catalyst (11) and complete the catalytic cycle. Although a complete understanding of the mechanism of olefin epoxidation catalyzed by molybdate compounds requires further studies, the reaction is nevertheless highly stereoselec-tive. Further, in general, the reactivity of the metal catalyst increases with increasing nucle-ophilicity of the olefin and is inhibited by coordinating a-donor solvents and ligands. 3 5 , 3 6 - 13 -Fig. 1-2. Molybdate-catalyzed epoxidation of an olefin ("butterfly mechanism"). - 14 -Fig. 1-3. Molybdate-catalyzed epoxidation of an olefin ("peroxometallocycle mechanism"). - 15 -As an extension of the third class of catalytic oxidations, oxygen atom transfer using metal centres with macrocyclic nitrogen and oxygen ligands, including metalloporphyrins, is partic-ularly interesting and is a subject of intense current i n t e r e s t . 1 , 2 , 3 4 , 3 8 , 4 1 - 4 6 Nature has evolved such systems not only for binding and reversibly carrying O2 (e.g. myoglobin, hemoglobin) but also for activating O2 through enzymic oxygenases, which incorporate one or two atoms of O2 to a substrate, or through oxidases that convert both atoms of 0 2 to water or hydrogen peroxide. The heme unit (an iron-porphyrin moiety) is particularly prevalent and, for exam-ple, is found in myoglobin and hemoglobin, 4 7 - 4 9 the monooxygenase cytochrome P-450, 5 0 - 5 3 tryptophan dioxygenase, and in cytochrome c oxidase - the terminal enzyme in the respiratory redox chain that reduces O2 to water. 5 4 , 5 5 The basic mechanism of oxygen activation and its transfer to hydrocarbon substrates by cytochrome P-450 has been widely investigated. Although several aspects of the catalytic cycle for this enzyme are still controversial, the process outbned in Fig. 1-4 is consistent with existing data for most substrates. 4 1 , 4 3 The overall mechanism involves: (a) the addition of substrates to the enzyme (low spin Fe / 7 /) to give a high spin enzyme-substrate complex (15 ) ; (b) addition of one electron to give a high spin Fe J / system (16 ) ; (c) binding of molecular oxygen and addition of the second electron to give a high spin oxygenated complex ( 1 7 ) . This intermediate (17 ) breaks down to oxygenated product, water and the original iron(III) form of the enzyme, the incorporation of the oxygen atom into the hydrocarbon bkely proceeding through a high valent metal oxo species ( 1 8 ) ; this can be an Fe(V ) = 0 species as shown, or an 0=Fe ( rV)(porpt ) cation radical species. -16-Fig. 1-4. The mechanism of oxygen activation and its transfer to substrates by cytochrome P-450 (Substrate S throughout the cycle is held in close proximity to the iron centre via interaction with the protein). The development of new strategies for the selective oxidation of hydrocarbons has been motivated by, and closely related to, the naturally occurring process illustrated in Fig. 1-4. Metalloporphyrins have been studied extensively owing to their direct relationship to enzymatic oxidations with cytochrome P-450. Other macrocycles in which a more or less square-planar array of nitrogen and oxygen atoms is coordinated to the metal center are also employed in the studies of oxygen atom transfer. 9 , 3 4 , 4 1 ' 5 6 - 5 9 Fig. 1-5 gives an example of macrocyclic ligands, in terms of chromium(III) complexes.9 Fig. 1-6 shows the genera] structure of a porphyrin dianion ligand, including the positions of various substituents, and an iron "picket fence" porphyrin dioxygen complex, one of the best-characterized models for M b 0 2 , Fe(TpivPP)(N-MeIm)(02) (an iron porphyrin dioxygen complex with four pivalamido pickets and an appended axial imidazole). This complex re-versibly binds 0 2 at room temperature and is stable both in solution and in the solid state. It has been isolated and characterized both by *H NMR and by X-ray diffraction 6 0 5 3 4 Fig. 1-5. The general structure of cationic salen complexes (Different substituents can be in positions 1 to 8). -18-Fig. 1-6. Porphyrin dianion ligands and Fe(TpivPP)(N-MeIm)(02) ( Different substituents can be in positions 1 to 8 and a, p\ 7, and 6 in the porphyrin). - 19 -Studies of the epoxidation of olefins using metal porphyrin complexes as catalysts, with alkyl peroxides, periodate, and iodosylbenzene as the effective oxygen sources, have presented mechanisms similar to that in Fig. 1-4. Thus, for example, an iron(V)-oxo porphyrin complex 2 , 6 1 or a ruthenium(IV or VI)-oxo porphyrin complex 4 1 , 4 2 , 6 2 is likely the active intermediate respon-sible for oxygen atom transfer to the substrates. An example is given in Fig. 1-7 to illustrate the mechanism for epoxidation of olefins catalyzed by chloro(tetraphenylporphyrinato)iron(III) using iodosylbenzene as an oxygen source.44 The active intermediate is species 19, which reacts with olefin in a side-on approach arrangement (20) to accomplish the transfer of the oxygen atom. Overall, mechanistic studies of reactions of the third class have reached a point which in-dicates the capability of metal complexes to mediate the oxidative transformation of substrates through an intramolecular process (i.e, the oxidative transformation of substrate takes place within the coordination sphere). In particular, several mechanistic studies have been dedicated to metal dioxygen complexes with respect to their capabilities of transferring an oxygen atom to substrates, as described in the following sections (1-3, 1-4). 1-3. Oxygen activation and selectivity o f oxidation reactions Two fundamental problems, involved in homogeneous catalytic oxidation of organic com-pounds, have continued to challenge chemists. One is the activation of molecular oxygen, and the other is the selectivity of catalytic oxidation. 1 0 , 1 5 Both problems are intimately related. Molecular oxygen is normally in a triplet state ( 3 S j ) having unpaired electrons as illustrated in Fig. 1-8.63,64 The first excited state, a singlet C&g), is 22.5 kcal/mol higher in energy. -20-Fig. 1-7. The mechanism for epoxidation of olefins catalyzed by Fe(TPP)X (X = halide) 4 (TPP = tetraphenylporphyrin, with phenyl substituents at bridging a, (3, -y, 6 position and pyrrole 1-8 positions unsubstituted). - 21 -3<r* 17T, 2 P t n ^ : ' ; 2 p *9 H O M O 2 s ^<jt,;^4f 2s 1S 4 H : ; ^ ; > t t - 1 s 10g AO MO AO Fig. 1-8. Molecular orbital diagram for molecular oxygen in the ground and excited states. - 22 -Another singlet excited state ( ' S j ) lies 37.5 kcal/mol above the ground state. This second singlet state has a short lifetime (10~ 1 2 s) in solution, rapidly relaxing to the longer lived (10~ 3-10~ 6 s) first excited state through a spin-allowed transition. Since electrons must be added to antibonding orbitals, reduction of molecular oxygen results in longer, weaker O-O bonds. Similarly, oxidation of molecular oxygen removes an electron from an antibonding orbital, and the 0-0 bond is thus strengthened and shortened. These simple MO considerations are sufficient to explain the bond distances and IR frequencies of the various oxidation states of the oxygen molecule, as given in Table 1-1.65 Table 1-1. Properties of dioxygen in various oxidation states 0-0 i/(0-0), bond bond energy, species compd. distance, A cm"1 order kcal/mol o2 + 0 2AsF 6 1.123 1858 2.5 149.4 0 2 ( 3 SJ) o 2 1.207 1554.7 2 117.2 0 2( aA f l) o 2 1.216 1483.5 2 94.7 o2- K0 2 1.33 1145 1.5 Li0 2 1097 1.5 H 2 0 2 1.49 1 35 Na 2 0 2 1.49 842 1 48.8 L i 2 0 2 802 1 - 23 -Chemical reduction of molecular oxygen is of the highest importance for biological systems and for oxygenation reactions. The free energy change for the four-electron reduction of molec-ular oxygen to two water molecules is -316 kJ/mol (at pH=7, 25 °C). The potential is quite attractive for energy storage. Such a reduction process does not occur in one step but rather through a series of steps involving successive one- and two-electron transfers. The essential thermodynamic considerations are outlined in eqs. 1-20 to 1-23 (pH =7, 25 °C), 4 1 where it is seen that the one-electron reduction to superoxide is unfavorable, and the oxidizing free energy available resides in the conversion from O 2 - or peroxide to water. The reduction potentials apply, of course, to uncomplexed species, and the energetics will be modified considerably on coordination to metal centers; for example, bonding of O2 to an Fe(II) center gives a species commonly formulated as an Fe(III) superoxide; yet clearly a reaction such as given in eq. 1-24 is not feasible thermodynamically.41 02 + e =^  02" E° - -0AV (1 - 20) 0 2 _ + 2H+ + e =i H202 E° = +0.90F (1 - 21) H202 + 2H+ + 2e 2H20 E° = +1.35V* (1-22) 0 2 + 4J7+ + 4e 2J7 20 E° = +0.80F (1 - 23) Fe{II) + 03 *± Fe(III) + 02" (1-24) As mentioned above, the electronic configuration of molecular oxygen in its ground state has a multiplicity of three. 6 3 , 6 4 This triplet ground state provides a considerable kinetic barrier to the autoxidation of normally diamagnetic organic molecules, because reactions involving change of spin are generally very slow.65 The restriction of spin conservation prevents ground state molecular oxygen interacting directly with organic molecules66. This phenomenon is also associated with the fact that the first one-electron reduction of molecular oxygen is unfavourable thermodynamically. 4 1 , 6 3 There are, in principle, several possibilities to circumvent this restriction of spin conserva-tion. One is the photochemical activation of ground state molecular oxygen to the lowest excited singlet state, thus removing the spin conservation barrier. Secondly, ground state molecular oxygen could react with radical species or a free electron to produce a new radical-containing oxygen, which is then able to form a diamagnetic product through radical coupling. However, initiation of the formation of organic radical molecules can require a long period of time and high energy if there are no metal complexes to assist this process. The third possibility involves the formation of a metal dioxygen complex, where greater spin-orbital coupling between the ground state molecular oxygen and the transition metal considerably reduces the kinetic bar-rier to change of spin, and where the formation of a metal dioxygen complex may itself provide sufficient energy to pair the s p i n s . 6 5 , 6 7 - 7 0 The removing of the energy barrier from the ground state molecular oxygen is usually referred to as the activation of molecular oxygen. It is only after the molecular oxygen is activated that its direct participation in the oxidation of organic compounds becomes feasible. The activation of molecular oxygen is characterized by changes in the oxygen-oxygen bond length and in the bond energy (see Table 1-1). Thus, to activate ground state molecular oxygen requires weakening of the oxygen-oxygen bond and lengthening of the bond. These two important physical properties of the bond, correlated to each other, indicate the degree of activation of molecular oxygen. For example, the O-O bond length varies from 1.207 A in ground state molecular oxygen, through to 1.300 A in a superoxo species, and about 1.450 A in a peroxo species.67'71 In most autoxidations, radicals originally generated by either a metal catalyst or a hy-droperoxide will be trapped readily by molecular oxygen to form a superoxo radical. This occurs because molecular oxygen, in its ground state, is an efficient trap for radicals. As a result, the superoxo radical becomes a predominant species in the reaction, and a free-radical chain mechanism is often operative in autoxidation, as outlined in Scheme 1-2.72 Consequently, various transformations of the original organic compound can occur during the propagation and termination stages, and the reaction is not particularly selective. Scheme 1-2: A free-radical chain mechanism in autoxidation 7 2 In i t iat ion ( Initiator • { R ' + 0 2 " R'-R ' O — O Propagation I"'-Terminat ion 4 — O — O - + R - H — • R ' - O - O - H + R -R • + 0 2 — • R - O - O -_ > r R 0 0 R + 0 2 2 R - O - O — • R O O O O R — R O • 0 2 • O R ^ (R tert iary; 2R00 H CR primary) I H o-K ^2R0- +O2 V - C + O 2 + H O - C -II x o - 26 -An intramolecular process involving a metal complex as catalyst for oxidations of organic compounds, as discussed in section 1-2, presents an ideal approach towards selectivity. Strate-gically, it is essential to search for a mechanism in which molecular oxygen is activated in such a manner that an intramolecular process to transfer an oxygen atom to a substrate at a desired position takes place. With such an activation process, the various selectivities, such as chemos-electivity, regioselectivity, and stereoselectivity, are potentially realized. Further discussion of this topic is presented in the following sections. 1-4. Dioxygen complexes of transition metals and oxygen atom transfer When dioxygen acts as a ligand forming a complex with a transition metal, a term "dioxy-gen complex" of a transition metal will be used. Many transition metals form dioxygen complexes. 6 4 , 7 1 In principle, the formation of dioxygen complexes creates an avenue to acti-vate molecular oxygen, potentially leading to selective catalytic oxidations of organic substrates using molecular oxygen or peroxides. Nearly all currently known dioxygen metal complexes can be conveniently divided into two classes, according to the characteristics of the dioxygen l i g a n d . 6 3 - 6 5 , 7 1 They are superoxo com-plexes and peroxo complexes. Both types can exist within monomer or dinuclear species . Two types of mononuclear dioxygen complexes are known; unidentate, superoxide or ^-dioxygen complexes having an end-on geometry, and bidentate, peroxide or J72-dioxygen complexes hav-ing a side-on geometry, as shown in Fig.1-9. 6 3 , 6 4 , 7 3 - 8 0 A basic difference between the superoxo and peroxo metal complexes is the length of the oxygen-oxygen bond (Table 1-1). In an IR spectrum, the oxygen-oxygen bond stretch of a superoxo complex can be differentiated from - 2 7 -that of a peroxo complex, and vice versa. 63,73 o o o M M 2 1 2 2 Fig. 1-9. The geometries of T71- and T/2-dioxygen complexes The r^-dioxygen complexes (21) have the 0 2 ligand bound to the metal in an angular manner. The structural characteristics and physical properties of these derivatives are con-sistent with their formulation as ^-superoxide complexes; however, the strength and degree of covalency in the metal dioxygen bond are highly variable. Typically, such ^-dioxygen complexes are formed from metals having a single available coordination site and a favorable one-electron oxidation potential. The appropriate circumstances for forming T 7 1 -dioxygen com-plexes are often encountered with macrocyclic Schiff-base or porphyrin complexes of Cr(II), Fe(II), Ru(II), Co(II) and Ni(II) having a single axial ligand. This is exactly the situation in deoxyhemoglobin(Hb) and myoglobin(Mb); in each, a 5-coordinate, high-spin (S=2) iron(II) is bound to a porphyrinato group and a single axial imidazole provided by the "proximal histidine" group from the polypeptide globin. The r/2-dioxygen complexes (22) have isosceles triangular structures with O-O bond lengths - 28 -in the range 1.46 ± 0.04 A, comparable to 1.49 A in hydrogen peroxide. Complexes of the perox-ide 7/2-dioxygen class are typically prepared by the oxidative-addition of 0 2 to a coordinatively unsaturated basic metal having d 8 or d 1 0 configuration. Such compounds are known for Pt(0), Pd(0), Ni(0), Ir(I), and Rh(I). 6 5 , 7 1 , 8 1 Studies on electronic structures of dioxygen complexes in terms of molecular orbital theory, electronic spectra, and vibrational spectra, have given a fairly consistent picture of the nature of the bonding between dioxygen and a transition metal. The bonding arises essentially from the interaction of the molecular oxygen 7r* orbitals with occupied d-orbitals of the metal. The oxygen ligand acts principally as an electron acceptor. 7 1 , 8 2 - 9 2 Reactivity of dioxygen complexes with organic substrates is closely related to the nature of the dioxygen-metal bond. After molecular oxygen is bonded to the metal, the number of unpaired electrons on the oxygen molecule decreases by one or two. In other words, electron density of the metal d-orbitals is partially transferred to the oxygen molecule. In the first case (one-electron transfer), the kinetic barrier to spin changes is now removed.65 In the second case (two-electron transfer), the dioxygen ligand may acquire basic or nucleophilic character. Thus, dioxygen (peroxidic) complexes react readily with diamagnetic electrophiles that are otherwise inert to molecular oxygen. For example, a widely studied complex, Pt(0 2)(PPli3) 2 will react with a number of inorganic and organic compounds, and transfer both oxygen atoms to substrates stoichiometrically. The substrates will not react with molecular oxygen directly. 1 5 (eqs. 1-25 and 1-26) - 2 9 -V l V ^ V * o 2NC> L W O N O ONO l / ONOa | c 2 v L > / ° ) c = ° (1 - 25) COa L \ c=o PtTCHO L \ _ / ° - ° N • Pt/ / C H P h (1 - 26) \ CM, - 30 -The interaction between molecular oxygen and a metal centre is, nevertheless, much more complex than that suggested simply based on electron transfer. In practice, it remains useful to use nucleophilicity and electrophilicity to describe the reactivity of dioxygen in a metal complex towards organic or inorganic substrates. Generally, dioxygen in peroxometal complexes containing metals in high oxidation states is electrophilic, whereas in peroxometal complexes containing metals in lower oxidation states, the dioxygen is nucleophilic. This conclusion is consistent with the fact that dioxygen in low-valent peroxometal complexes fails to react readily with olefins, which are usually more susceptible to oxidation by electrophiles. 6 3 , 7 1 For example, mononuclear peroxo complexes of group VIII metals do not readily transfer oxygen to olefins, although stable five-membered peroxo metallocyclic adducts have been isolated with aldehydes, ketones, and activated electrophilic olefins (eq. 1-26), as well as iridium complexes bearing both dioxygen and ethylene.15 1-5. Mechanistic studies on oxygen transfer from dioxygen complexes Mechanistic studies of oxygen transfer from dioxygen complexes, especially dioxygen com-plexes of group VIII metals, when acting as catalysts through an intramolecular process, have suggested a peroxymetallocycle species as a possible intermediate to implement oxygen atom transfer. This idea can be illustrated using the first authentic example of a selective non-radical oxidation of an olefin catalyzed by a group VIII metal complex, RhCl(PPh3 ) 3, reported by Read and co-workers.93,94 In this reaction, terminal olefins were selectively oxidized to methyl ketones, and triphenylphosphine, as a coreductant, is oxidized to triphenylphosphine oxide. Oxygen la-beling studies 9 5 - 9 8 show that the oxygen atoms in both products are derived from dioxygen - 31 -when the reaction is carried out in the presence of a small amount of water. The mechanism proposed by Read and co-workers (Scheme 1-3) exhibits several interesting features. First, the triphenylphosphine appears to promote dissociation of the peroxometal group to the 1, 3-dipolar form (23). Second, a peroxymetallocycle ( 24) (a five-member ring peroxo metal species) is formed by an intramolecular attack of a peroxo ligand on a coordinated olefin. Such a nudeophilic addition is not expected for an intermolecular process, but the coordination of an olefin to rhodium could render the double bond electrophilic, as in the Pd(II)-catalyzed nudeophilic addition of OH group (as in eq. 1-1) to simple olefins in the well-known Wacker process. The decomposition of the peroxymetallocycle results in the formation of a dioxetane and a phosphine rhodium complex ( 25 ). Alternatively, a decomposition of ( 24 ) can be envisioned, in which /3-elimination from the Rh-alkyl bond affords the enol and an oxorhodium(III) species ( 26 ). Reduction of the latter by PPI13 then regenerates the Rh(I) catalyst. Scheme 1-3: A selective non-radical oxidation of an olefin catalyzed by RhCl(PPh3)3 R + RCH"~CHa I I O — O 25 R 24 26 - 32-Regardless of the unattractive use of stoichiometric amounts of triphenylphosphine, this reaction illustrates the important principle that oxygen atom transfer from 0 2 to simple olefins is feasible within a peroxymetallocycle, provided that the simultaneous transfer of the second oxygen atom to an electron acceptor can be accomplished. Recently, in their studies of selective oxidation of organic compounds, catalyzed by group VIII complexes, the groups of James,11 Mimoun 3 5 - 3 7 , 9 9 and S t r u k u l 1 0 0 - 1 0 2 have suggested pseudo five-membered peroxymetallocyclic intermediates for the oxygen atom transfer mech-anism. For example, catalytic oxidation of terminal olefins to methyl ketones by hydrogen peroxide using a palladium catalyst, operating in the absence of acid halide and co-metal (i.e. a non-Wacker process), was found to be a very efficient catalytic reaction. The mechanism proposed for this reaction by Mimoun 9 9 (Fig. 1-10) shows a pseudo peroxopalladium cyclic species (28) as the possible intermediate.. The decomposition of this intermediate generates a methyl ketone and a palladium hydroxide. In addition, a metal hydroperoxide intermediate (27) seems to be involved prior to the formation of the pseudo peroxymetallocycle. An instructive discussion on the pseudocyclic hydroperoxymetallation mechanism for oxy-gen transfer involving a metal hydroperoxide intermediate as precursor has been presented by James and co-workers in their puMication of a study on catalytic oxidation of cydooctene to cyclooctanone with an iridium hydride complex using O2/H2 mixtures.11 The formation of an iridium(III) hydroperoxide intermediate was suggested as a key step prior to formation of the pseudo peroxyiridium cyclic intermediate (eq. 1-27). - 33 -Net reaction : H202 + RCH = CH2 H20 + RCOCH3 A = CF3CO2 ,OAc~,acac~. Fig. 1-10. The mechanism of catalytic oxidation of terminal olefins by a palladium catalyst. - 34 -To produce the iridium(IIl) hydroperoxide, two possible routes were proposed. One involves insertion of molecular oxygen into a metal hydride bond, and the other involves protonation of a dioxygen complex, as shown in eqs. 1-28 and 1-29. IrHCl2(CsH12) Ir(OOH)Cl2(C8H12) (1 - 28) lr"H -^ -V Ir-1 - ^ i r 1 — O II O (1 - 29) ,ri«i I - l L _ M r i M — OOH The catalytic cycle was completed by using the co-reducing agent hydrogen (Scheme 1-4). Scheme 1-4: A catalytic cycle for cyclooctene oxidation using O2/H2 mixture catalyzed by an iridium hydride complex. Reports on group VIII metal hydroperoxides have appeared for cobalt, 1 0 3 , 1 0 4 rhodium, 1 0 5 , 1 0' platinum, 1 0 0 - 1 0 2 palladium, 3 5 , 3 6 , 9 9 and iridium; 7 , 1 1 they have sometimes been isolated and char-acterized, or proposed as reaction intermediates. Oxygenation of coordinated hydride and pro-tonation of coordinated peroxide are general routes to prepare metal hydroperoxides, and Table 1-2 gives a summary of the above group VIII metal hydroperoxides that have been isolated. The characterization of the -00H in a metal hydroperoxide depends mainly on IR measurements (i.e, i/(00-H) appears in the 3500 to 3600 cm - 1 region, while £(00-H) and i/(0-OH) appear at about 1260 and 820 cm - 1, respectively). 7 , 1 1 Obviously, one requires other means (such as NMR techniques), in combination with IR measurements, to identify the -OOH group of the metal hydroperoxide unambiguously. In addition, more study is needed in order to obtain a better knowledge of the chemistry of metal hydroperoxides, particularly, their roles in oxygen activation and transfer. - 36 -Table 1-2 A summary of hydroperoxide complexes of group VIII metals Metal hydroperoxide Preparation route Characterization of the -OOH Reference [Co(OOH)(CN) 5] 3- [CoH(CN) 5] 3- + 0 2 103,104 K 2[Rh(OOH)(CN) 4(H 20)I [RhH(CN) 4(H 20)] 2- +0 2 IR 105 trans-RhCl(OOH)(acac)(PPh 3) 2 0 2RhCl(PPh 3) 2 + acacH a IR 106a trans-Pt(OOH)(CF 3)(PPh 2Me) 2 trans-PtH(CF 3)(PPh 2Me) 2 IR 100 + H 2 0 2 a: acacH = acetylacetone Investigations of direct relevance to the chemistry involved in eqs. 1-28 and 1-29 are Kochi's observation of the reaction of Pt(0 2)(PPh 3) 2 with an acyl chloride (RCOC1) to give an acyl peroxy complex 1 0 6 6 and Groves's related observation of such a reaction with a Mn porphyrin peroxide; 1 0 6 c the acyl peroxy complexes then transfer oxygen atom to olefins to give corresponding epoxides with liberation of carboxylate (RCG* 2 -). Further, Balch's recent work 1 0 6 d on 0 2 insertion into iron porphyrin alkyl complexes to make alkylperoxy complexes might also be considered an analogy to 0 2 insertion into a metal-hydrogen bond. 1-6. Purpose of present work and scope of this thesis Most industrial oxidation reactions are energy intensive and, as discussed in previous sec-tions, those taking place by direct oxidation with molecular oxygen are not particularly selective. Hence, studies of oxygen activation and oxygen transfer particularly using group VIII metal complexes through homogeneous catalysis focus on development of more selective, milder oxi-dation reactions. The answer to selectivity, however, depends on a better understanding of the reaction mechanisms of oxidations using molecular oxygen or its alternatives. In fact, except for the Wacker type oxidation of olefins by "indirect" methods to aldehydes, ketones and related products, the oxidations by molecular oxygen are not well understood. This thesis covers research work undertaken to elucidate kinetic and mechanistic prob-lems within oxygen activation and oxygen transfer via posssible M-OOH species using iridium and platinum triphenylphosphine complexes in 'protonation-oxygenation' and 'oxygenation-protonation' processes. In this thesis, a protonation-oxygenation process refers to one in which a transition metal complex undergoes an oxidative addition reaction with HX (X = halide) to form a metal hydride, and, then the isolated metal hydride is treated in solution with molecular oxygen (eq. 1-30). An oxygenation-protonation process refers to one in which a transition metal complex undergoes an oxygenation reaction with molecular oxygen to form a dioxygen complex, and then the isolated dioxygen complex is treated with acid in solution (eq. 1-31). To our knowledge, no kinetic and mechanistic studies on such 'dual nature' processes at iridium and platinum centres (or other metals for that matter) have been reported. Furthermore, the protonation-oxygenation and oxygenation-protonation processes are thought to present ideal pathways for group VIII metals to activate and transfer oxygen through a pseudo-cyclic hy-droperoxymetallation mechanism. The present studies involved searching for possible synthetic routes (via eqs. 1-30, 1-31) to iridium and platinum hydroperoxide complexes, which can be the precursors of the pseudo-peroxymetallocycle intermediates in oxygen transfer. Therefore, - 38 -the present studies are considered as contributing to the ultimate goal of developing new ho-mogeneous catalysts for selective oxidation processes. IrJ + 02 — » l T n \ 0 \ - )^IrIU - OOH ? (1 - 30a) Pt° + 02—» Ptn(Ol-)^*Ptu -OOH ? (1 - 306) /r' + H + —» I r n i H ^ I r i n - OOH ? (1 -31a) Pt° + H+ —» Pt"H^Pt" - OOH ? (1 - 316) Work related to synthesis and characterization of the known, previously reported iridium and platinum complexes is included in the experimental chapter (Chapter 2). The kinetic and mechanistic work on iridium complexes is presented in Chapters 3 and 4, while studies involving platinum complexes are discussed in Chapters 5, 6 and 7. A general discussion, conclusion and future ideas section (Chapter 8) form the final part of this thesis. - 39 -C H A P T E R 2 E X P E R I M E N T A L P R O C E D U R E S 2-1. Materials Solvents were obtained as spectral or reagent grade from Aldrich, Eastman, Fisher, Mallinckrodt, British Drug Houses (BDH) or Matheson, Coleman and Bell Chemical Co. Ben-zene, hexane, toluene and tetrahydrofuran (THF) were distilled from sodium / benzophenone under one atmosphere of nitrogen. 1 0 9 Distillation under nitrogen of CH2CI2 was from P2O5, and of alcohols from the corresponding magnesium alkoxide. All dried solvents were stored under nitrogen or argon. Dimethylformamide (DMF) and chloroform as spectral grade were used without further treatment. Deuterated solvents used in the present work were acetone-d6, benzene-de, chloroform-di, dichloromethane-d2, and toluene-dg, and were obtained from Merck Frosst Canada Inc. When specified as dry, the deuterated solvent was treated with molecular sieves (4 A, Fisher) by the following procedure. First, the molecular sieves were heated to about 100 °C in a flask under vacuum using an electric mantle for several hours, and then the deuterated solvent was vacuum transferred into this flask. After several hours, the dried deuterated solvent was vacuum transferred into a Schlenk tube, where argon was finally introduced. Purified oxygen, nitrogen, argon and carbon dioxide were obtained from Union Carbide of Canada Ltd. Lecture bottles of anhydrous hydrogen chloride and chlorine were obtained from Matheson Gas Co. Argon was passed through a column of anhydrous calcium chloride to remove trace water.110 All other gases were used without further purification. - 4 0 -Triphenylphosphine obtained from BDH was reprecipitated from a saturated ethanol solu-tion by addition of hexane; the solid was then dried under vacuum and stored under argon. All synthetic reactions were carried out using degassed solvents under an atmosphere of argon by employing Schlenk techniques* unless specified otherwise. All products were dried under vacuum and stored under argon or nitrogen in the dark. Additional characterization data of obtained products are presented in Chapters 3-7, where details of studies on these compounds are described. 2-2. I r i d i u m c o m p o u n d s The iridium was obtained as I r C l 3 • 3H2O from Johnson Matthey Ltd. The compound trans-IrCl(CO)(PPh 3) 2 was either supplied by Johnson Matthey Ltd. or prepared from IrCl 3 • 3 H 20. Literature methods were followed to prepare IrCl(CO)(PPh 3) 2, 1 1 1 IrCl(0 2)(CO)(PPh 3) 2, 1 1 J IrHCl 2(CO)(PPh 3) 2, 1 1 3 and I r H ( C O ) ( P P h 3 ) 3 m . T r a n s - c h l o r o c a r b o n y l b i s ( t r i p h e n y l p h o s p h i n e ) i r i d i u m ( I ) , I r C l ( C O ) ( P P h 3 ) 2 A mixture of IrCl 3 • 3 H 20 (0.99 g, 2.8 mmol) and PPh 3 (3.8 g, 15 mmol) in 50 mL DMF * Schlenk techniques: techniques of vacuum / pressure operations using a combination of two pyrex glass manifolds, one with the connection to a vacuum pump and the other with the connection to an inlet of compressed inert gas supply through a gas regulator. - 41 -was vigorously refluxed for 12 hours under a nitrogen atmosphere at 154 °C. The resulting yellowish brown solution was filtered while hot. Warm methanol (50 mL) was then rapidly added into the resulting solution with stirring, and then the mixture was cooled in an ice bath. The resulting yellowish green crystals were collected on a filter and washed with cold methanol. Recrystallization was carried out in benzene or chloroform. Yield: 1.7 g (77%). C37H3o'OCTP2Ir requires:(%) C 56.97, H 3.85. Found: C 57.25, H 3.80. u(CO) 1954, i/(IrCl) 319 cm"1. 3 1 P^H}: 24.10 ppm (singlet, trans PPh 3, in CD 2C1 2) w.r.t. 85% H 3P0 4, downfield being positive. The IR data are in excellent agreement with those in the literature, 1 1 1 , 1 1 5 while 3 1 P NMR data, to our knowledge, have not been reported. Chloroperoxo(carbonyl)bis(triphenylphosphine)iridium(III), I r C l ( 0 2 ) ( C O ) ( P P h 3 ) 2 A solution of IrCl(CO)(PPh3)2 (0.50 g, 0.64 mmol) in 10 mL benzene was purged with oxygen at room temperature for about one hour during which time a pale orange precipitate formed. A condenser at 15 °C was used to prevent loss of benzene during the reaction. The condenser was then removed and the solution was purged again with oxygen for another 15 minutes to decrease the volume of the solution and to increase the amount of precipitate. The product was filtered off and washed with benzene. Yield: 0.3 g (60%). C 3 7H3o0 3ClP 2Ir requires: (%) C 54.73, H 3.69, O 5.91. Found: C 54.86, H 3.74, O 5.80. 1/(0-0) 858, u{CO) 1997, i/(IrCl) 314 cm - 1. "P^H}: 4.12 ppm (singlet, trans PPh 3, in CD 2C1 2). The IR and 3 1 P NMR data are in good agreement with those in the literature. 1 1 2- 1 1 6' 1 1 7 0 - 42 -Dichlorohydrido(carbonyl)bis(triphenylphosphine)iridium(III), I r H C l 2 ( C O ) ( P P h 3 ) 2 Some IrCl(CO)(PPh 3) 2 (0.20 g, 0.26 mmol) was dissolved in benzene (40 mL) and the solution was treated with anhydrous HCl at 1 atm for half an hour. A white precipitate appeared. Hexane (25 mL) was added to obtain more precipitate. After filtration, the white powder was washed with hexane. Yield: 0.2 g (96%). C 3 7H 3 1OCl 2P 2Ir requires: (%) C 54.41, H 3.83. Found: C 54.41, H 3.81. i/(IrH) 2238, i/(CO) 2022, i/(IrCl) 312 and 265 cm - 1, trans to CO and H respectively. i m 3 1P{ 1H}: -0.58 ppm (singlet, trans PPh 3, in CD 2C1 2); 1H: -15.30 ppm (triplet, Jp_// = 12 Hz). The complex has been shown previously to have trans PPh 3 and cis CI ligands; the IR spectral data agree well with those given in the literature. 1 1 7 b The *H and 3 1P{ 1H} NMR data are not available-in the literature. Hydrido(carbonyl)tris(triphenylphosphine)iridium(I), I r H ( C O ) ( P P h 3 ) 3 A yellow suspension of trans-IrCl(CO)(PPh 3) 2 (0.34 g, 0.44 mmol) in dry ethanol (40 mL) containing PPh 3 (0.45 g, 1.72 mmol) was heated with stirring under reflux and argon. A solution of sodium borohydride (0.16 g, 4.3 mmol) in dry ethanol (30 mL) was added slowly. The yellow suspension gradually changed to a paler yellow powder and the mixture was kept refluxing for about 5 hours. The powder was filtered off while hot, washed with hot ethanol (30 mL) and dried under vacuum. Yield: 0.40 g (92%). C 5 5H460P 3Ir requires: (%) C 65.54, H 4.57. Found: C 65.50, H 4.60. »/(IrH) 2119, £(IrH) 824, u(CO) 1921 cm - 1 (in Nujol mull); i/(IrH) 2075, t>(CO) 1931 cm" 1 (in C 6D 6); JH: -10.29 ppm (quartet, 3P_H = 22 Hz); 3 1P{ 1H>: 14.22 ppm (singlet). The IR and J H NMR data are in good agreement with those in the - 4 3 -literature, 1 1 7 c , d while, surprisingly, 3 1 P NMR data do not appear to have been reported. 2 - 3 . Platinum compounds The platinum was obtained as KjPtCLj from Johnson Matthey Ltd and cis-PtCl2(PPh3)2 was kindly donated by Strukul's group in Italy. Literature methods were followed to prepare Pt(PPh 3) 4, 1 1 8 Pt(PPh 3) 3, 1 1 9 PtHCl(PPh 3) 2, 1 2 0 and Pt ( 0 2)(PPh 3) 2. 1 2 1 Tetrakis(triphenylphosphine)platinum(0), Pt(PPh 3) 4 (a) By hydrazine hydrate reduction of cis - P t C l 2 ( P P h 3 ) 2 : 1 1 8 a • l ' A solution of N 2 H 4 H 2 O (0.5 mL) in ethanol (4.0 mL) was added dropwise into a suspension containing cis-PtCl2(PPh 3 )2 (0.70 g, 0.89 mmol) and PPh 3 (0.71 g, 2.7 mmol) in ethanol (10 mL) over 15 minutes at 80 °C. A condenser at 15 °C was used to prevent the loss of ethanol from vapourization. The white suspension of cis-PtCl 2(PPh 3 )2 turned to a bright yellow suspension; the solid was filtered off, dissolved in benzene (2 mL) and reprecipitated by addition of ethanol (2 mL). Yield: 0.8 g (72%); (b) From K 2 P t C l 4 using KOH and P P h 3 : 1 1 8 a , M An aqueous solution (1.5 mL) saturated with K 2 P t C l | (0.20 g, 0.48 mmol) was added dropwise into a solution consisting of PPh 3 (0.60 g, 2.3 mmol), KOH (0.05 g, 0.89 mmol) and ethanol (15 mL) at 60 °C. The pale yellow precipitate that formed was collected and reprecipitated from benzene (2 mL)/ethanol (2 mL). Yield : 0.2 g(34%); (c) By borohydride reduction of K 2PtCl4: 1 1 8 a' c An aqueous solution (5 mL) of K 2 P t C l 4 (0.71 g, 1.7 mmol) was mixed with a 25 mL ethanol solution containing PPh3 (2.2 g, 8.4 mmol) - 44 -at 50 °C, and a 10 mL aqueous solution containing NaBH* (0.20 g, 5.3 mmol) was then added dropwise. The resulting bright yellow powder was collected and reprecipitated from benzene (2 mL)/ethanol (2 mL). Yield: 1.6 g (77%). In the above procedures, the best quality product (as judged by elemental analysis) was obtained using method (b); however, the percentage yield was the lowest among the three methods. C 7 2H 6oP4Pt requires: (%) C 69.50, H 4.83. Found: C 69.28, H 4.70. Tris(triphenylphosphine)platinum(0), P t ( P P h 3 ) 3 Pt(PPh 3) 4 (0.50 g, 0.40 mmol) was added into dry ethanol (25 mL) to form a suspension, which was boiled for 2 hours when the colour changed from yellow to orange-yellow. After filtration, the orange-yellow powder was washed with warm, dry ethanol. Yield : 0.3 g (76%). C 54H 4 5 P 3 P t requires: (%) C 66.06, H 4.59. Found: C 65.81, H 4.59. Trans-chlorohydridobis(triphenylphosphine)platinum(II), PtHCl(PPh 3)2 Anhydrous HC1 gas was bubbled slowly through a solution of Pt(PPh 3 )4 (0.20 g, 0.16 mmol) in benzene (10 mL) until the solution became colourless. Hexane (5 mL) was then added to give a white precipitate which was reprecipitated from CH2CI2 (3 mL)/hexane (3 mL). Yield : 0.08 g (65%). C 3 6 H 3 a C l P 2 P t requires: (%) C 57.17, H 4.10. Found: C 56.92, H 3.96. j/(PtH) 2209, i/(PtCl) 290 cm - 1. ^P^H}: 28.07 ppm (one major singlet and two singlet satellites, Jpt-p = 3000 Hz, in CD2CI2), *H: -15.12 ppm (one major triplet and two triplet satellites, JP-H = 14 Hz, Jpt-H = 1170 Hz). The IR and *H NMR data are in good agreement with those in the literature, 1 2 2- 1 2 3 while 3 1 P NMR data are not available. Peroxobis(triphenylphosphine)platinum(II), P t ( 0 2 ) ( P P h 3 ) 2 Oxygen was bubbled through a solution of Pt(PPh 3)4 (0.20 g, 0.16 mmol) in benzene (6 mL) for 20 minutes. A pale orange powder that gradually formed was filtered off and washed with benzene and hexane. Yield: 0.07 g (57%). C 3 6 H 3 0 O 2 P 2 P t requires: (%) C 57.52, H 3.99. Found: C 57.64, H 3.78. v(0-0) 821 cm - 1. 3 1P{ 1H}: 14.48 ppm (one major singlet and two singlet satellites, 3pt-p = 4045 Hz, in CD 2C1 2). The IR and 3 1 P NMR data are consistent with those reported in the literature. 7 1 , 1 2 4 Other materials Liquid reagents, such as hydrazine monohydrate, hydrogen peroxide (30% by weight in wa-ter), trifluoromethanesulfonic acid (Aldrich), and trifluoroacetic acid (BDH), were used without further treatment. Reagent grade solids, such as potassium bromate, potassium hydrogen ph-thalate, phenolphthalein, silver trifluoroacetate, silver hexafluorophosphate, triphenylphosphine oxide (all Aldrich chemicals), potassium iodide (BDH), sodium thiosulfate and p-toluenesulfonic acid monohydrate (Fisher), were used as obtained. 2-4. Instrumentation Three types of samples were prepared generally for infrared measurements. A pellet sample was made from a mixture of dry Csl or KBr powder with the powdered compound using a Carver Laboratory Press; a Nujol mull sample, made from a mixture of dry Nujol with the powdered compound, was placed between Csl plates, while a solution sample employed a Csl or NaCl cell with path length of 0.5 mm. Infrared spectra were recorded on a Nicolet 5DX Infrared Fourier Transform Spectrometer with a nitrogen purged sample chamber. A Perkin-Elmer 1710 Infrared Fourier Transform Spectrometer was also used for some pellet samples. Visible and ultraviolet (UV-vis) spectra were recorded on a Perkin-Elmer 552 A Spectrophotometer. Anaerobic, quartz spectral cells with 0.1 cm path length were used and thermostated if necessary. A Durrum-Gibson Stopped-Flow Spectrophotometer was employed for rapid kinetic measurements in the milli-second to second time range, at a constant temperature. Gas detection was performed on a Carle Analytical Gas Chromatograph 311 using a ther-mal conductivity cell detector. The column {<f>2 mm x 3.7 m) was filled with a PPQ (Porapack Q) packing material maintained at 50 °C. Mass spectra were taken on a Kratos MS 50 Mass Spectrometer, while Fast Atom Bom-bardment (FAB) spectra were taken on an A E l MS9 Mass Spectrometer modified and converted by the electronics shop of this department. Proton NMR spectra were recorded on a Varian X L 300 with tetramethylsilane (TMS) as external standard. Fluorine NMR spectra were recorded on a Bruker WP 270 or a Varian X L 300 using trifluoroacetic acid as external standard and 3 1P{ 1H} NMR spectra were recorded on a Varian X L 300 spectrometer, unless specified otherwise. The standard used for 3 1P{ 1H} NMR spectra was triphenylphosphine which has a signal 5.88 ppm upfield from 85% H 3 P O 4 ; 1 2 5 all 3 1 P data are quoted relative to 85% H 3 P O 4 , downfield signals being positive. 1 9 5Pt{ 1H} NMR spectra were recorded on the Varian X L 300 spectrometer, the reference frequency (0.00 ppm) being set at 64.2 MHz. 1 2 6 All spectrometers were operated in the Fourier transform mode and were equipped with variable temperature attachments. Elemental analyses were carried out by Mr. Peter Borda of this department. - 47 -2-5. Detection of H 2C* 2 Hydrogen peroxide was one product involved in the present work, and reactions were car-ried out in organic solvents containing trace water even after being dried. Also, reactions were performed on a scale with starting compounds of 10-100 milligrams. Therefore, a method to de-tect hydrogen peroxide in the concentration range of ~ 10"2 to 10~ 3 M in organic solvents (with trace water) had to be sought. Literature methods for detection of H 2 0 2 in aqueous solution are iodometry, 1 2 7 spectrophotometric determination using titanium complexes,128 electrochem-ical detection using cyclic voltammetry,129 and a fluorometric method. 1 3 0 Each method has, however, its own limitation in terms of selectivity, sensitivity and feasibility. For example, iodometry is one of the most sensitive and widely used methods. It is not, however, specific for hydrogen peroxide. A blank sample must be tested at the time as a real sample in order to eliminate the possible interference of organic peroxides. Spectrophotometric detection us-ing titanium complexes and cyclic voltammetry have detection limits of about 10~4 to 10 - 5 M H 20 2. 1 2 7 But, the spectrophotometric method may be interfered with by other transition metal complexes in the solution. Cyclic voltammetry would be easy to use if the system contained no other reducible or oxidizable species other than H 20 2. Fluorometric methods, on the other hand, are able to detect H 2 0 2 to about 10~ 8 M, but the method is not simple. 1 2 8 , 1 2 9 Attempts to choose an appropriate method and to explore a new method for the detection of H 2 0 2 were undertaken in our studies . A modified iodometric procedure for the detection of H 2 0 2 in organic solvents containing coloured iridium complexes was used as described in Section 2-5-1. In addition, a JH NMR technique was employed for the estimation of H 2 0 2 as described in Section 2-5-2 and, to our knowledge, the application of NMR for H2O2 detection has not been described previously. 2-5-1. A modified iodometric titration procedure In iodometry, a solution to be analyzed should be colourless, in order to detect unambigu-ously an endpoint at which the blue colour of the iodine 6 tarch complex will disappear. This requirement should be satisfied particularly when a very small amount of H2O2 is to be detected. In CH2CI2 solutions containing dark green chloro-iridium(III) complexes, iodometry cannot be employed directly. A modified iodometric procedure was used as follows. Immediately after a reaction between IrCl(0 2)(CO)(PPh3) 2 and an acid was complete, the CH2CI2 solution of chloro-iridium(III) complexes was subjected to extraction by degassed water. Iodometry was then applied to the separated water layer to determine H2O2. Meantime, neat CH2CI2 solvent was also extracted by the same amount of degassed water and the separated water, as a blank sample, analyzed for H2O2. Theoretically, the difference in consumption of sodium thiosulfate between real and blank samples is taken as a measure of the H2O2. However, the blank samples were found to contain no H 20 2 in the present work. The main steps and chemical reactions involved in iodometry are as follows: Step 1: An excess amount of KI is added to the solution of interest, and an aqueous solution of HC1 is used as an acid medium. 27" + H202 + 2H+—>I2+2H20 (2-1) I2 + r ^ J3- (2 - 2) Step 2: Several drops of starch-ethanol solution is added into the solution, when a blue colour will be observed. I2 + starch — • complex with blue colour (2 — 3) Step 3: Aqueous sodium thiosulfate solution is used to titrate the test solution until the blue colour disappears. h + 2S20\- — 21- + S4Ol~ (2-4) The sodium thiosulfate aqueous solution is standardized using potassium bromate. The steps are the same as Steps (2-1) to (2-4) except that the unknown solution containing H2O2 is replaced by a solution of known concentration of potassium bromate. The different reaction involved is: BrOz + 6 7 " + 6 # + — » Br~ + 3/2 + 3H20 (2-5) 2-5-2. J H N M R detection of H 2 0 2 It is known and easily shown that the protons of H 2 0 in neat CD2CI2 appear at 1.55 ppm as a sharp peak (peak width at half height = 3 Hz) in a proton NMR spectrum (Fig. 2-1). When about 1 ~ 2 pl of 33% aqueous H 2 0 2 (by weight) is added to the CD 2C1 2 ([H 20 2] ~ I O - 3 M), a slightly broadened singlet at 1.60 ppm is found (about 10 Hz peak width at half height). One cannot depend on these observations for detection of H2O2 because the differences between the above two cases (H 20 in CD 2C1 2 and H 20-H 20 2 (33%) in CD 2C1 2) in both chemical shifts and the peak widths at half height are too small to be discriminated. However, the situation - 50 -is c h a n g e d i n CD2CI2 w h e n H2O2 is p resent i n excess over H 2 O . I t was observed t h a t w h e n H 2 0 2 was f o r m e d s t o i c h i o m e t r i c a l l y f r o m t h e r e a c t i o n o f Pt(0 2 )(PPh3)2 w i t h H C 1 (gas) a t m o l e r a t i o s 1 : 1 a n d 1 : 2 i n C D 2 C 1 2 , a b r o a d p e a k a p p e a r e d i n t h e r a n g e o f 1.50 t o 2.00 p p m , w h e r e a s o n l y a s h a r p p e a k ( c o r r e s p o n d i n g t o t h e t r a c e H 2 O i n CD2CI2) was observed be fo re t h e r e a c t i o n ( F i g s . 2 -1 t o 2 -3 ) . T h e w i d t h o f t h i s p e a k a t h a l f h e i g h t ( A ) was f o u n d t o d e p e n d l i n e a r l y o n t h e a m o u n t o f H 2 0 2 p r o d u c e d i n t h e r e a c t i o n ( F i g . 2 -4 ) . T a b l e 2-1 s u m m a r i z e s t h e e x p e r i m e n t a l d a t a a n d gives a n e m p i r i c a l express ion f o r t h e p e a k w i d t h ( o b t a i n e d o n t h e V a r i a n X L 300 s p e c t r o m e t e r ) i n t e r m s o f H2O2 c o n c e n t r a t i o n s . T h e exchange process o f p r o t o n s b e t w e e n H2O2 a n d t r a c e H 2 O i n CD2CI2 p r e s u m a b l y p r o d u c e s t h e b r o a d e n i n g effect a n d t h e degree o f b r o a d e n i n g b e c o m e s m o r e n o t i c e a b l e w h e n t h e H2O2 c o n c e n t r a t i o n increases. T a b l e 2 - 1 . T h e p e a k w i d t h o f t h e p r o t o n resonance (6) o f H2O2 a t d i f f e ren t [ H 2 0 2 ] i n CD2CI2 a t 25 °C [ H 2 0 2 ] [ H 2 0 ] 6(E20-E202) A ( p e a k w i d t h ) x 1 0 3 , M M p p m H z 0 t r a c e , 1 x 1 0 - 4 1.55 3 1.0 t r a c e , 1 x I O - 4 1.60 10 4 .5 t r a c e , 1 x I O - 4 1.63 36 9 .1 t r a c e , 1 x I O - 4 1.63 78 [H2O2] = ( 1 . 2 x 1 0 " 4 A - 1.9 x 1 0 ~ 4 ) ± 3 x I O - 4 M ( t h e e r r o r is e s t i m a t e d f r o m a l i n e a r regress ion a n a l y s i s o f t h e d a t a o f F i g . 2 - 4 ) . - 51 -5 32 lHaOl «~ lx10~ 4 M residue H20 1.55 I i i 1 1 11 i i i 111 i i 11 i i i | I I I | I I I I I I I I I I » I I I I I I I I | I I I I j ppjQ Fig. 2-1. The aH NMR spectrum of H 2 0 (~ IO" 4 M) in CD 2C1 2 at 25 °C (residue = CHDC1 2 and CH 2C1 2) 5.32 (H202)=4.5*10-3 M « (H 2 0 )»~1 x 10~4 M residue Jl 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 » i 1 1 1 1 1 1 1 1 1 1 1 1 1 5 1 3 1 P P m Fig. 2-2. The J H NMR spectrum of H 2 0 2 (4.5 x IO" 3 M) in CD 2C1 2 at 25 °C (X = unknown; residue = CHDCl 2 and CH 2Cl 2). - 52 -5 3 2 ( H 2 O 2 J » 9 . 1 x 1 0 - 3 M : ( H 2 0 ) - ~ 1 x 1<T4 M residue i i j M i i | i i M | i i i i | p p m i i i i | i i i i ( i i i i | i i i i | ' i i i | i i i i | 5 3 Fig . 2-3. The JH NMR spectrum of H 2 0 2 (9.1 x I O - 3 M) in CD 2C1 2 at 25 °C 4.0 8.0 [ H 2 0 2 ] X l O 3 , M Fig . 2-4. The linear dependence of the peak width for the proton resonance of H 2 0 2 on [H 20 2]. 2-6. Determination of oxygen solubility in benzene and C H 2 C I 2 Oxygen solubility can be measured using a constant pressure gas-uptake apparatus 1 3 1 or calculated using solubility coefficients provided in literature. 1 3 2 The solubilities of oxygen in benzene and CH2CI2 in this thesis were calculated using the solubility coefficients as follows. According to Landolt-Bornstein, 1 3 2 an equation and a host of data (solubility coefficient A, defined by eq. 2-8 at given temperature) were available for gas solubility calculations: where, A: solubility coefficient; Po: reference pressure, 735.56 mm Hg; P<32: partial pressure of oxygen in mm Hg; M o 2 : molecular weight of oxygen Ws: oxygen dissolved in per 100 g of benzene or CH2CI2 solution, in g The values of A at different temperatures in benzene are listed in Table 2-2. Table 2-2. Solubility coefficients of oxygen in benzene at different temperatures A = (22415tt^P0) (2-8) (MO2Po2)-(l00-Wg) Temp. (°C) 10 20 30 40 50 60 A 0.221 0.224 0.229 0.233 0.240 0.244 The calculations were carried out by foDowing Step 1 to Step 3: Step 1: A plot of A versus temperatures was constructed using the available data (for benzene solution, the data in Table 2-2) and the At value at a given temperature t was determined; Step 2: The Xt value was substituted into eq. 2-8 and W 0 calculated; Step 3: The W 0 value was converted into molarity of oxygen in benzene or CH2CI2 using eq. 2-9: ,^ . 10D* • WQ ,n n. [0 2] = ^ — M (2-9) where: [O2] is the oxygen solubility (M) in benzene or CH2CI2 at a given temperature t and an oxygen pressure P02', and D* is the density of benzene or CH2CI2 solution containing oxygen at a given temperature t, which is taken as nearly equal to benzene or CH2CI2 density at the same temperature, in g/cm3. A solubility of 9.11 x IO - 3 M for oxygen at 1 atm at 25 °C in benzene was obtained from the calculation. Noticeably, kinetic studies on the additions of hydrogen, oxygen, and methyl iodide to some square-planar iridium(I) complexes in benzene and dimethylformamide reported by Chock and Halpern 1 1 5 have employed similar calculations to find the solubility data for hydrogen or oxygen. The solubility of oxygen at 1 atm in CH2CI2 at 25 °C was 8.13 x 10" 3 M, obtained using the solubility coefficient for CHC1 3 at 16 °C, which was the only available data for a solvent of this type. Henry's Law was assumed to hold for the O2 solubility in both solvents. The solubility of 0 2 (1 atm) in benzene at 25 °C was also found to be (7.51 ± 0.02) x 10 - 3 M using a gas-uptake apparatus in the present work. Nevertheless, oxygen solubility data (in benzene and CH2CI2) for different oxygen pressures obtained using eqs. 2-8 and 2-9 were employed in order to have a common ground for comparison of kinetic results with the literature data. - 55 -C H A P T E R 3 A K I N E T I C A N D M E C H A N I S T I C S T U D Y O F T R A N S - C H L O R O C A R B O N Y L B I S ( T R I P H E N Y L P H O S P H I N E ) I R I D I U M ( I ) , [ I r C l ( C O ) ( P P h 3 ) 2 ] , I N A N O X Y G E N A T I O N - P R O T O N A T I O N P R O C E S S 3-1. Introduction Dioxygen metal complexes are potentially viable intermediates for direct activation of molecular oxygen. However, the chemical behaviour of 0 2 after coordination (that is, the activation of 0 2) is a complex subject not only because of the many pathways available for 0 2 reactivity 1 5 , 1 7 but also because of the different bonding strengths between dioxygen and metal centres.71 One reaction that has received considerable attention is the protonation of dioxygen metal complexes because a metal hydroperoxo complex (M-OOH) has been suggested as an intermediate of the reaction, which finally gives H 2O 2. 1 0 8 , 1 1 7 a However, the determination of the reaction stoichiometry, the analysis of the products, and the reaction mechanism, have not been systematically studied. Therefore, work on the oxygenation-protonation processes (as denned in Section 1-6) involving iridium(I) and platinum(O) complexes was undertaken. 3-1-1. Reversibil ity and stability of dioxygen carrier complexes Extensive studies have been carried out on the oxidative addition of 0 2 to metal complexes, particularly using complexes of platinum(O), 1 3 3- 1 3 5 i^i<uum(I) 7 1 , 1 1 5 , 1 1 7 a , 1 3 6 , 1 3 7 and rhodium(I). 1 3 7 , 1 3 8 Of these reactions, the oxygenation of Vaska's complex, trans-IrCl(C0)(Ph3P)2 is one of the most interesting, largely because the reaction system is simple and involves stable - 56 -species readily susceptible to comprehensive studies. In particular, the oxygenation reaction is reversible under ambient conditions.71 Reversibly formed dioxygen complexes, or oxygen carriers, are those in which bound dioxy-gen can be removed by a change in temperature, partial pressure of oxygen gas, pH, or other conditions. Formation of the dioxygen complex may be considered to be reversible only if the original metal complex (the oxygen carrier) is formed when dioxygen is removed.63 Completely reversible oxygenation requires that a compound is able to undergo repeated oxygenation/deoxygenation cycles without appreciable loss of activity. In general, this loss of activity is due to irreversible oxidation of the original metal complex to an inactive complex in which the metal is in a higher oxidation state. This reaction occurs with loss of the dioxygen ligand, either as superoxide or peroxide, or in oxidation products in which the oxygen is reduced to water and the chelating or coordinating ligand is oxidized. A reversible dioxygen complex can be described in terms of its thermodynamic stability (simplified as stability), i.e., the difference in free energies of the precursor complex and molecular oxygen relative to that of the dioxygen complex. Stability may be expressed by a stability constant, which is simply the equilibrium constant for the reaction between a metal complex and molecular oxygen (eq. 3-1). The K values reflect the strength of metal-dioxygen bonding. M L n + 0 2 £ MLn03 ( 3 - 1 ) A kinetic and thermodynamic study on the reversible reaction of molecular oxygen with a series of iridium complexes,90 trans-[IrCl(CO)(R3P)2] (R3P = tertiary phosphine) showed that the stability of resulting dioxygen complexes increases with increasing basicity of the substituent R, provided the substituents have comparable structures. In addition, the geometry of R could - 57 -sometimes exert a steric inhibition effect on the kinetics of the oxygenation reactions and affect the stability of the resulting product. For instance, when R3P = (CgFs^P (vCo, 1994 cm - 1) or (o-CH 3C 6H4) 3P (yco, 1946 cm - 1), no reaction with molecular oxygen takes place. The former illustrates a ligand with poor basicity and the latter represents a ligand with considerable bulk. 3-1-2. Iridium dioxygen carriers and O-O bond lengths The oxygenation shown in eq. 3-2 is a reversible oxidative addition reaction of the uni-valent (spin-paired cf8), four-coordinate iridium compound to the tervalent (spin-paired d 6), six-coordinate iridium peroxo complex, IrCl(02)(CO)(PPh3)2.7 1 IrCl(CO)(PPh3)2 + 0 2 JrCl(02){CO)(PPh3)2 (3 - 2) On the other hand, IrI(CO)(PPh3)2 binds dioxygen irreversibly.67 The degree of 'reversibil-ity' of the metal dioxygen complex is related to the different electron releasing ability of the halides in the complexes. That is, as the halide becomes more electron releasing, the greater is the back-donation from the metal to the O2 antibonding molecular orbitals and conse-quently the stronger the bonding between dioxygen and metal centre. The correlation of the reversibility of the metal dioxygen complex with the O-O bond length has been a controver-sial t o p i c . 6 7 , 1 3 9 , 1 4 0 It is now clear that no one-to-one correlation exists between reversibility of metal dioxygen complex and the O-O bond length. For example, a reversible dioxygen adduct IrCl ( 0 2)(CO)(PPh 2Et ) 2 has the 0 - 0 bond length of 1.46 A, 1 4 1 which is about the same as in the irreversible dioxygen adduct IrI(02)(CO)(PPh3)2 with the O-O bond length of 1.51 A. 1 4 2 Crystallographic investigations of the complexes [M ( 0 2)L 4]X (M = Rh, Ir, L = P M e 2 P h , X = BPlw; M = Ir, L2 = bis(diphenylphosphino)methane, X = PFe, CIO4) and [Rh(0 2)(AsMe 2Ph)4]C104, together with others of iridium, cobalt, and platinum, have found V the 0-0 bond lengths to be in a range approximately between 1.41 and 1.52 A , 1 4 0 implying that the 0-0 bond length is essentially independent of the metal and ligand involved. 3-1-3. T h e nature of metal-dioxygen bonding The nature of bonding between dioxygen and metal can be discussed at this point in terms of molecular orbital (MO) calculations, 8 1 , 1 4 3 which show that, for a side-on metal dioxygen complex, there are three interactions between the metal ion and the dioxygen ligand: the metal a\,-dioxygen 7r*, the metal 5pa-dioxygen 7ru, and the metal spCT-dioxygen ag interactions (Fig. 3-1). The 7r back-donation of electrons from metal occurs through the metal aVdioxygen T* interaction, which is the major component in the metal-oxygen bonding. The other two less significant interactions are regarded as a-donation of electron density from oxygen. All metal-0 2 associations are accompanied by an extensive redistribution of electrons from the metal to dioxygen. To obtain further understanding of the thermodynamic stability and reactivity of a metal dioxygen complex and the nature of metal-oxygen bonding, a study of an oxygenation-protonation process with IrCl(CO)(PPh3)2 was carried out. The observed kinetic and chemical behaviours are presented in the following sections. d 7 r - i r g s p a - i r u s p a - l o g F i g . 3-1. The nature of oxygen-metal bonding for a 6ide-on dioxygen metal complex.81 - 59 -3-2. Kinetics of the reversible oxygenation of IrCl(CO)(PPb.3 )2 , I Some kinetic measurements of k 2 and k_i (eq. 3-2) have been carried out by Chock and Halpern, 1 1 5 and Vaska and Chen. 1 4 3 Either a constant pressure gas-uptake apparatus or a spectrophotometer was employed. However, k_ i in benzene, and k 2 and k_j in CH2CI2 have not been reported, and these data were required for the present studies. 3-2-1. UV-vis spectroscopic characteristics of I r C l ( C O ) ( P P h 3 ) 2 , I Solutions of I are bright yellow in colour, and a UV-vis spectrum (Fig. 3-2) shows three strong absorption bands at 337, 386 and 438 nm (Table 3-1). The large molar extinction coefficients show that the bands are due to allowed charge-transfer transitions between metal orbitals to ligand orbitals (i.e, metal to ligand charge transfer, M L C T ) . 9 0 , 1 1 2 , 1 4 4 a Linearity of absorbance versus concentration of Ir for all three absorption bands was established over the concentration range of 5.00 x 10~ 4 to 7.00 x 10~ 3 M in benzene solution. Table 3-1. Molar extinction coefficients of IrCl(CO)(PPh 3) 2 in benzene and CH2CI2 at 25 °C under argon atmosphere*1 A, nm 337 386 438 Solvent c, M _ 1 c m _ 1 3.2 x 103 4.0x 103 7.3 x 102 benzene £, M - 1 c m _ 1 3.1 x 103 3.8x 103 7 x 102 benzene* £, M^cm" 1 3.4 x 103 3.8 x 103 7.4 x 102 CH2CI2  a: Errors in t values are ~ ±5 % . b: Literature data in benzene; 1 4 4 6 extinction coefficients in CH2CI2 are not reported in the literature. - 60 -f • i i ' ' 300 350 400 450 500 Wavelength» n m F i g . S-2. The UV-vis spectrum of IrCl(CO)(PPh 3)j in benzene at 25 °C. - 61 -3-2-2. Pseudo first-order kinetics In a reaction system like the oxygenation of I, only two reactants and one product are involved. If the reaction rate has a first order dependence on the concentration of a given reactant, regardless of the dependence on the other reactant if present in large excess and/or kept at a constant concentration, pseudo first-order kinetics apply. A pseudo first-order rate constant with respect to the given reactant can be obtained regardless of the reversibility or irreversibility of the reaction. For reaction 3-3, A+B^LC (3-3) the rate expression for pseudo first-order kinetics can be formulated as follows. Let [a]D= initial concentration of reactant A [a] = concentration of reactant A at a given time t [b] = concentration of reactant B ( to be maintained constant ) [c] 0= initial concentration of product C (in most case, it is zero) [c] = concentration of product Cat a given time t. The rate expression is • ^ = k2[a][b)-k-i[c] (3-4) In the reaction system, [c] = [o]o - [a] (3-5) - 62 -Therefore, - ^ = kob[a}-k-i[a]o (3-6) where *oo = * 2 [*>] + *-! (3-7) and k0b is called an observed rate constant or a pseudo first-order rate constant with respect to reactant A. An integral form for eq. 3-6 will be ln([a) - ^±[a]0) - ln([a}0 - ^-{a]0) = -kobt (3 - 8) In terms of the absorbance of the reaction solution recorded on a UV-vis spectrophotometer at a fixed wavelength during the reaction, eq. 3-9 can be established. 1 4 4 c ln{[a) - ^ [ a ] 0 ) - ln([a)0 - ^ [ a ] e ) = ln(A - Atq) - ln(A0 - Aeq) (3 - 9) where A = absorbance of reaction solution at a given time t A 0 = absorbance of reaction solution at t = 0 A e o = absorbance of reaction solution at equilibrium. Therefore, a final rate expression, containing terms for the absorbance of the reaction solution, the pseudo first-order rate constant k0(,, and reaction time can be established via combination of eq. 3-8 and eq. 3-9, as shown in eq. 3-10. (3-10) - 63 -According to eq. 3-10, k 0t can be obtained mathematically or diagrammatically by using the absorbance data of a reaction solution collected during a single experimental run at a constant temperature over a time length of about two to three half-lives, in order to observe a relatively completed reaction. The k 2 and k_i values at a given temperature are readily found from the slope and the intercept of the line constructed according to eq. 3-7, provided the reaction is first order in [b]. To obtain several k0b values, one carries out a series of individual runs with different [b] values. Furthermore, k 2 and k_j at various temperatures can be obtained by the same approach, and the kinetic and thermodynamic parameters can be acquired. Theoretically, k 2 and k _ i can be obtained according to eqs. 3-7 and 3-10. Practically, when k 2 is much greater than k _ i , the k_ i value obtained may be less accurate, because it appears as an intercept close to the origin in the plot of k0(, vs. [b]. In order to obtain a relatively reliable value for k _ i , one can either increase the number of individual runs using the pseudo first-order method, or measure k _ i directly starting from product C (eq. 3-11). C ^ A + B ( 3 - H ) If the reaction in eq. 3-11 is first order in [c] and starts when [c] = [c]0 ([a]0 and [b]0 are zero), an integral form of the rate expression is readily obtained (eq. 3-12). ln(l - = (3-12) In association with absorbance of the reaction solution at a given wavelength, eq. 3-12 can be converted to eq. 3-13 (this assumes that product B has negligible absorbance at the wavelength chosen). - 6 4 -l n { A ^ o ) s = j b _ i t ( 3 _ 1 3 ) where A = absorbance of reaction solution at a given time t A 0 = absorbance of reaction solution at t = 0 Aoo = absorbance of reaction solution at completion. 3-2-3. Kinetic measurements and data treatment In a special flask with an attachment of a UV-vis cell as a side arm, the oxygenation of I under several constant oxygen pressures (at pseudo first-order condition, because the number of moles of Ir present is relatively small) at 25 °C in either benzene or CH2CI2 was carried out and monitored by a UV-vis spectrophotometer. The reaction exhibited similar kinetic behaviour in both benzene and CH2CI2 and spectrophotometric plots are exemplified in Fig. 3-3 (a), showing a reaction in benzene. The appearance of an isosbestic point at 332 nm is consistent with the fact that, other than oxygen, there are two species equilibrating in the reaction solution 1 4 5 (eq. 3-2).* The absorbance data taken at a fixed wavenumber (386 nm in benzene and CH2CI2) were treated using eq. 3-10, and pseudo first-order kinetics were obeyed (Fig. 3-3 (b)). The k0(, values were readily obtained for the different individual runs and the k 2 and k _ i were found using eq. 3-7, the reaction being first order in molecular oxygen (Fig. 3-3 (c)). * It should be noted that a system in which three species were present in solution, with the ratio of lTCl(CO)(PPh 3) 2 : IrCl(02)(CO)(PPh 3) 2 being kept constant, could also give rise to isosbestic points. 1 4 5 - 65 -Table 3-2 summarizes the results for the oxygenation reaction in benzene at 25 °C. The k 2 (3.4 x 10~ 2 M - 1 s - 1 ) found in the present work is consistent with that reported in the literature (3.36 x I O - 2 M ~ 1 s - 1 ) , 1 1 5 while the k_i (~1.5 x IO - 5 s" 1) has not been previously reported. The reverse rate constant k _ i value was checked by a direct method, because only three individual runs were performed using the pseudo first-order method and the k 2 was found to be much greater than k _ i . The deoxygenation of IrCl(0 2)(CO)(PPh 3) 2, I I (eq. 3-14) was performed by removing the 0 2 using an Ar stream. IrCl(02)(CO)(PPh3)2 —> 0 2 + IrCl(CO)(PPh3)2 (3 - 14) The benzene solution of II was prepared in the same flask described above which was placed in a thermostated oil bath (25 °C). Argon saturated with benzene was continuously bubbled via a long stainless steel needle into the solution and vented through a bubbler. The reaction solution was monitored by a spectrophotometer at a fixed wavelength (386 nm) corresponding mainly to an absorption band of IrCl(CO)(PPh3)2 (the bubbling was stopped momentarily for measuring the absorption data). The results are given in Table 3-3 and the more accurate k_i ((1.39 ± 0.06) x 10 - 5 s _ 1 ) obtained was in a good agreement with the k_i value (about 1.5 x 10~ 5 s - 1 ) obtained using the pseudo first-order method; i.e. with the intercept of the line drawn in Fig. 3-3 (c). The agreement indicated that the pseudo first-order method gives a rebable k _ i value, when k 2 is greater than k _ i by three orders of magnitude for this particular system involving 0 2 concentration of the order of 10 - 3 to 10 - 2 M. However, one has to use the direct method to obtain k _ i when the difference between k 2 and k _ i is about five orders of magnitude, as described for a system in Chapter 4. In CH2CI2, the oxygenation reaction was also found to be first order in I and first order in - 66 -molecular oxygen, Table 3-4 summarizes the results of the kinetic measurements at 25 °C. The data (k 2 = 6.2 x 10~ 2 M - 1 s - 1 and k_ i = 1.6 x 10" 5 s _ 1 ) are comparable with those obtained in chlorobenzene at 40°C by Vaska and Chen 1 4 3 (k 2 = 9.93 x IO - 2 M _ 1 s _ 1 and k_ i = 1.39 x 10 - 5 s - 1 ) , although these workers did not quote the figure used for the solubility of 0 2 which could affect the absolute value of"k2 but not k _ i . Table 3-2. The pseudo first-order rate constants for the oxygenation reaction with respect to IrCl(CO)(PPh 3) 2 in benzene at 25 °C a k 0 b , s P 0 j,mmHg; [0 2], M x 104 x 103 3.15 760 9.11 2.50 665 7.97 1.73 405 4.86 k 2 = (3.4 ± 0.3) x 10~ 2 M - 1* - 1; k_ x = ~1.5 x 10" 5 s" 1 K = k 2 / k _ i = (2.3 ± 0.4) x 103 M - 1 - 67 -Table 3-3. Kinetic measurement of the reverse rate constant k_i for deoxygenation of IrCl(0 2)(CO)(PPh 3) 2 in benzene at 25 °C a Time, (s) A, at 386 nm in A~Ar x H P 4 x 102 x 10 0 2.4 0 0.366 3.7 -0.495 1.30 7.2 -1.97 1.80 8.4 -2.52 2.44 10.8 -3.74 3.87 13.4 -5.26 k_! = (1.39 ± 0.06) x 10~ 5 s" 1; correlation coefficient (r) for the linear ln ^^J? versus time plot = 0.9965. a: [IrCl(0 2)(CO)(PPh 3) 2] 0 = 6.72 x IO - 4 M; benzene: 10 mL; A T O = 0.293 (theoretical). - 68 -Table 3-4. The pseudo first-order rate constants of the oxygenation reaction with respect to IrCl(CO)(PPh 3) 2 in CH 2C1 2 at 25 " C Kb, s 1 Poj, mm Hg, [0 2], M x 104 x 103 2.87 410 4.39 2.49 360 3.85 2.32 330 3.53 2.16 294 3.15 1.55 215 2.30 k 2 = (6.6 ± 0.3) x IO" 7 s - 1 mm H g - 1 or (6.2 ± 0.3) x l O ^ M - V 1 ; k_j = (1.6 ± 0.1) x 10 5 s 1; correlation coefficient (r) of the linear regression analysis for the data in the table (k o i ) versus [0 2]) = 0.9967; K = k 2 / k_j = (3.9 ± 0.4) x 103 M" 1. - 69 -(IrCHCOJfPPhaJjJo - 2.09 x 10"» M P o 2 = 7 6 0 m m H g 3 0 0 3 5 0 4 0 0 Wavelength n m 4 5 0 Fig. 3 - 3 . (a) The changes in the absorption curve of IrCl(CO)(PPh 3) 2 over time in the oxygenation reaction in benzene at 25 °C. 1.6 -8 < 1.2 -1 1 o < < 0.8 " c 0.4 -2000 T i m e , s 4000 6000 Fig. 3 - 3 . (b) The pseudo first-order kinetics for the oxygenation reaction with respect to IrCl(CO)(PPh 3) 2. - 70 -0 2.0 0 4.0 _ 2 6.0 8.0 Po 2 X 10 Z , mm Hg Fig. 3-3. ( c ) The first-order dependence on 0 2 for the oxygenation reaction of IrCl(CO)(PPh3)2 in benzene at 25 °C. 3-2-4. Summary of the reversible oxygenation of IrCl(CO)(PPh . 3 ) 2 , I Through the re-examination of the kinetics for the reversible oxygenation of I in benzene and CH2CI2, k2 and k _ i data were obtained (Tables 3-2 and 3-4); these are not available in the literature with the exception of the k2 value in benzene at 25 °C. The k2 in CH2CI2 (6.2 x 10 - 2 M _ 1 s - 1 ) is somewhat greater than that in benzene (3.4 x 10 - 2 M _ 1 s - 1 ) , even allowing for uncertainty in the O2 solubility in CH2CI2, and this can be rationalized in terms of polarity effects. The cis addition of molecular oxygen to I has an unexpectedly large negative entropy of activation (-21 e.u. in benzene), reported by Chock and Halpern, 1 1 5 suggesting an increase in polarity in going from reactants to the transition state (towards a coordinated peroxide). Consequently, a polar solvent, even a slightly polar solvent such as CH2CI2 (dielectric constant ( € = 9.08, 20 ° C ) 1 4 6 , will favour the formation of the transition state and increase the rate of the oxygenation reaction. An increase in k 2 for the oxygenation reaction of I in going from benzene to the more polar solvent dimethylfonnamide (k 2 = 1.75 x 10"1 M _ 1 s _ 1 , 30 °C) is also reported in the literature. 1 1 5 3-3. Protonation of I r C l ( 0 2 ) ( C O ) ( P h 3 P ) 2 , II Initially, our interest in the protonation reaction of II focused on the synthetic aspects of hydroperoxo iridium complexes (Ir-OOH). This requires that only one of the side-on dioxygen-iridium bonds is broken to form the Ir-OOH species, via an electrophilic attack on the oxygen atom by a proton. In order to study this suggestion, the protonation reaction of the dioxygen complex using various acids was carried out. 3-3-1. Reaction between I r C l ( 0 2 ) ( C O ) ( P h 3 P ) 2 , II, and trifluoroacetic acid Trifluoroacetic acid (TFA) is a strong a c i d 1 1 0 and the anion is a ligand capable of forming complexes with iridium, 1 1 2 , 1 4 7 both as a mono- and bidentate ligand. The C F 3 group can be identified as that of a trifluoroacetato iridium complex or that of free TFA, by FT-IR and 1 9 F NMR spectroscopy. TFA was therefore used as one of the acids in the study of the protonation reaction. 3-3-1-1. Stoichiometry Titrations of TFA (benzene solution) against CH 2C1 2 solutions of II were carried out at 25 °C in Schlenk tubes under argon using mole ratios of TFA to II equal to 1 : 1 and 2 : 1. The reaction solutions changed from pale orange to dark green and were transferred to 10 mm NMR tubes for "P-^H} NMR analysis on a Bruker WP-80 NMR spectrometer. A 5 mm NMR tube, containing C^De as a locking solvent, was placed in the 10 mm NMR tube, and secured with Teflon ring spacer. At a TFA/Ir ratio of 1 : 1 (for example, II: 81 mg, 0.10 mmol; CH 2C1 2 = 5 mL; benzene solution of TFA, 0.27 M, 0.37 mL), about half the starting II complex remained unreacted, as shown in Fig. 3-4. Several unidentified products (detected in the regions from 6 -11.38 to -14.38 ppm and from 6 -29.88 to -33.88 ppm) and OPPh 3 at 6 27.12 ppm were found. The amounts of these products increased on addition of TFA. At an acid/Ir ratio of 2 : 1 (for example, II: 127 mg, 0.16 mmol, CH 2C1 2 = 8 mL; benzene solution of TFA, 0.27 M, 1.19 mL), the starting II complex had disappeared. A mixture of products similar to those of the 1 : 1 titration was observed (Fig. 3-5). Among the final products, species in the region from S -11.38 to -14.38 ppm were always dominant. The titrations show that complete reaction of 1 mole of IrCl(0 2)(CO)(PPh 3) 2 requires 2 moles of TFA. Furthermore, *K NMR spectra of the 2 : 1 reaction in CeD$ indicated the formation of H 20 (6 ~0.4 ppm). In these titration studies, very little IrCl(CO)(PPh 3) 2, I, 6 = 24.10 ppm, was found in CD 2C1 2 solutions of the oxygenated complex after about two hours in the absence of added TFA; the deoxygenation rate within the NMR tube is extremely small. Any I that was present remained in the solution even after addition of TFA approached a ratio of about 1:1. This fact indicated qualitatively that reaction between TFA and II was faster than that between TFA and I, although the latter is also a 'fast' reaction (see Chapter 4). - 73 -4-12 frafcHcoMPPb*)* 2 7 - 1 2 OPPh 3 produc ts i i i • • • i i i i i i 6 0 30 0 - 3 0 - 6 0 p p m Fig. 3-4. The ^ P-^H} NMR spectrum for the reaction of IrCl(0 2)(CO)(PPh 3) 2 with TFA (1 : 1) in CH 2C1 2 at 25 °C; the spectrum was essentially invariant with time. products 27-12 OPPh 3 1 6 0 30 - 3 0 - 6 0 ppm Fig. 3-5. The 3 1P{ ]H} NMR spectrum for the reaction of IrCl(0 2)(CO)(PPh 3) 2 with TFA (1 : 2) in CH 2C1 2 at 25 °C. - 74 -3-3-1-2. Products Because the titration studies resulted in a complex mixture of final products, further examination of products was undertaken. Protonation reactions of IrCl(02)(CO)(PPh3)2, II, were carried out in CH2CI2 or benzene using TFA : Ir ratios of 1 : 1 or 2 : 1; the colour of the reaction solution changed from pale orange to dark green in a few minutes. The solvent was then removed and a khaki powder obtained after drying under vacuum. The reaction residue was analyzed by FT-IR, mass and FAB spectroscopy, and 3 1P{ 1H} and 1 9 F NMR. FT-IR spectra Figs. 3-6 (a), (b) and (c) show the FT-IR spectra (in Nujol) of II and the residues from the reactions with TFA. Complex II has a very strong and sharp band at 1997 cm - 1 for the terminal v(CO). After the reaction with acid at a ratio of 1 : 1, two such bands at 1997 and 2060 cm - 1 with approximately equal intensity were observed. The residue of the reaction with the 2 : 1 acid/Ir ratio showed a relatively weak and broad band at 2076 cm - 1 with a shoulder at 2060 cm - 1. The band at 1997 cm" 1 had totally disappeared. Complex II also has a sharp, medium strength band at 858 cm - 1 for the coordinated dioxygen (Fig. 3-6 (a)), while residue from the 1 : 1 acid/Ir ratio reaction showed a band at 845 cm - 1 which partially overlapped with the band at 858 c m - 1 (Fig. 3-6 (b)), indicating the presence of OPPI13. For residue from the 2 : 1 reaction, only a weak, broad band at 845 cm - 1 was observed; the band at 858 cm - 1 had disappeared (Fig. 3-6 (c)). These observations support the 1 : 2 (II : TFA) stoichiometry of the reaction, and also indicate that different carbonyl complexes of iridium are formed. The higher v(CO) values (wavenumber) imply the presence of Ir(III) species. Furthermore, the presence of only a weak band in the terminal CO region in Fig. 3-6 (c) reflects the loss of a - 75-terminal CO in some products of the reaction (see the GC analysis). The carboxyl group refers to the CO2 of TFA or that of the trifluoroacetato ligand in a trifluoroacetato iridium complex. Both Fig. 3-6 (b) and (c) show a strong band at 1697 cm"1, assigned to the asymmetric stretch (va3ym(C02)) of a coordinated trifluoroacetato group. 1 4 8 However, the symmetric stretch (^ J v m(C02)) cannot be clearly assigned because of the strong Nujol bands present in the 1380 to 1480 cm - 1 region. Therefore, a residue from the 1 : 1 acid/Ir ratio reaction was analyzed by FT-IR in a KBr pellet (blank KBr pellets have zero absorption in the 2500 to 500 cm - 1 region) and the v 5 y m ( C 0 2 ) was identified at 1407 cm - 1 in addition to the i / a 3 y m ( C 0 2 ) at 1697 cm - 1 (Fig. 3-6. (d)). An empirical criterion states that a monodentate trifluoroacetato group has a higher uasym(C02) value and a lower vSym(C02) value, and hence a larger separation value (A) between the ^(C02) frequencies, relative to values for the free trifluoroacetato ion (~1680 and ~1445 cm - 1, respectively), usually taken as values for the sodium or potassium salts. 1 4 8 The separation of f ( C 0 2 ) values obtained from Fig. 3-6 (d) is 290 cm - 1, indicating the formation of monodentate trifluoroacetato iridium complexes (Table 3-5). In addition, very strong bands at 1190 and 1150 cm - 1, which are different from the 1200 and 1100 cm - 1 u(CFz) values for free TFA, also imply the presence of trifluoroacetato iridium complexes (Fig. 3-6 (b)-(d)). Literature v(Ir-Cl) values for chloro iridium complexes appear in the region of about 250 to 350 c m - 1 (in Nujol) 1 5 0. The changes from a single sharp i/(Ir-Cl) band at 314 cm - 1 (assigned to Ir-Cl for I I ) 1 5 0 to several bands between 300 to 340 cm - 1 in both Figs. 3-6 (b) and (c) indicate that different chloro iridium complexes were formed in the reaction with TFA. - 76 -Table 3 -5 . The infrared data for trifluoroacetates and free trifluoroacetic acid Compound ^ y m C C O j ) f » y m(C0 2) separation bonding mode cm - 1 cm - 1 A, cm - 1 reference K ( 0 2 C C F 3 ) ° 1678 Na(0 2CCF 3) f c 1680 H 0 2 C C F 3 c 1680 Residue of 1697 1 : 1 TFA/Ir reaction 0 1437 1457 1420 1407 241 223 260 290 ionic ionic 148 148 H-bonded 149 system monodentate this work a: K ( 0 2 C C F 3 ) on KBr window, b: Acetonitrile solution of Na(0 2CCF 3) in NaCl cell, c: Neat H 0 2 C C F 3 in NaCl ceU. d: In KBr pellet. - 7 7 -to E CO 3800 2 4 5 0 1550 Wavenumber c m - 1 8 7 5 2 0 0 F i g . 3-6. ( a ) The FT-IR spectrum of IrCl (0 2 ) (CO)(PPh 3 ) 2 in Nujol mull. « 0 C CO E CO e CO 3800 2 4 5 0 1550 875 Wavenumber. cm- 1 2 0 0 Fig. 3-6. (b) The F T - I R spectrum of the residue in Nujol mull from the reaction between IrCl (0 2 ) (CO)(PPh 3 ) 2 and T F A (1 : 1) at 25 °C. - 78 -s c CD E CO 3800 2450 Wavenumber 1550 c m * 1 875 200 Fig. 3-6. (c) The FT-IR spectrum of the residue in Nujol mull from the reaction between IrCl(0 2)(CO)(PPh 3) 2 and TFA (1 : 2) at 25 °C. E s 4OO0 2OO0 1 2 0 0 Wavenumber c m - 1 400 F i g . 3-6. (d) The FT-IR spectrum of the residue in KBr pellet from the reaction between IrCl(0 2)(CO)(PPh 3) 2 and TFA (1 : 1) at 25 °C. - 79 -3 1 Pi1!!} and 1 9 F NMR spectra The 3 1P{ 1iT} spectra in CD2CI2 of the residue of the reactions with acid : Ir ratios of 1 : 1 and 2 : 1 were similar to those in Figs. 3-4 and 3-5. The OPPh 3 (at 6 27.42 ppm) was identified, while species in the regions from S -11.38 to -14.43 ppm and from 6 -29.88 to -33.88 ppm were unidentified products. The 1 9 F NMR spectra in CD2CI2 of the residue of the reactions (e.g. Fig. 3-7) show that all the resonance peaks appear in pairs. For every such pair, the two equal intensity resonance peaks are separated by about 0.01 ppm. No free TFA (6 = 0.00 ppm) was observed. These 1 9F{ 1H} data suggest that the resulting trifluoroacetato iridium complexes contain two inequivalent trifluoroacetato ligands. Mass and FAB spectra Fig. 3-8 shows the mass spectral data under the same ionization conditions. Triphenylphos-phine oxide was found in the reaction residue. Tests of thin layer chromatography on AI2O3 gel or silica gel with CH2CI2 as a solvent, and solvent mixtures of CH2Cl2 and C 2H 5C0 2CH3 (1 : 1; 1 : 5; 1 : 10) as developers showed that the mixture of reaction products could not be easily separated . Therefore, FAB was employed in an attempt to find the possible components of the reaction residue. Table 3-6 bsts the high mass portion (mass number > 715) of the residue from the reaction with the acid : Ir ratio of 2 : 1. The FAB spectra support the suggestion that trifluoroacetato iridium(III) complexes are formed, and also imply that these might further lose either the PPI13 or CO ligands in the oxidation reaction between these coordinated ligands and H2O2 (see GC and Iodometry analyses). The presence of OPPI13 in the product mixture reinforced this suggestion. - 80 -I I | I I I I | I I I I | I I I I | I l i I 3 . 0 2 . 0 all the resonance peaks appear in pairs i • • 1 .0 I ' i " ' | 0. 0 P P m Fig. 3-7. The 1 9F{ 1H} NMR spectrum in CD 2C1 2 of the residue from the reaction between IrCl(0 2)(CO)(PPh 3) 2 and TFA (1 : 2) at 25 °C. PPhJ OPPhJ (a) 262 OPPh£ PPhJ I \ • 277 («>. 262 ( I I'" I I l " l I I I | I | I I 250 300 250 liu 277 PPh% OPPhJ V. / (<=) I I I I I I I I 300 278 j i 1 i i I I I I I | 260 300 Fig. 3-8. The mass spectra of (a) IrCl(0 2)(CO)(PPh 3) 2 and the residues from the reaction between IrCl(0 2)(CO)(PPh 3) 2 and TFA (1 : 1 (b); 1 : 2 (c)) in the 250-300 (mass number) region. - 81 -Table 3-6. FAB mass spectral data (high mass portion) for the residue from the protonation reaction (IrCl(0 2)(CO)(PPh 3) 2 : TFA = 1:2) Mass relative possible fragment (+) b munber intensity 0 1006 very weak (IrCl(OCOCF 3) 2(CO)(PPh 3) 2] + 893 medium [IrCl(OCOCF 3)(CO)(PPh 3) 2]+ 865 strong [IrCl(OCOCF 3)(PPh 3) 2]+ 829 medium [Ir(OCOCF 3)(PPh 3) 2]+ 751 very strong [IrCl(PPh 3) 2]+ 715 very strong [Ir(PPh 3) 2]+ and [IrCl(OCOCF 3) 2(PPh 3)]+ a: The intensity of [Ir(PPh 3)] + with a mass number of 453 was taken as the reference. b: The possible parent compounds are IrCl(OCOCF 3) 2(CO)(PPh 3) 2, IrCl(OCOCF 3) 2(PPh 3) 2, and IrCl(OCOCF 3) 2(CO)(PPh 3), or solvated species of the last two compounds. GC analysis In order to find evidence for the fate of the terminal CO, gas phase analysis of the reaction system with the acid : Ir ratio of 2 : 1 in CH 2C1 2 was performed using a PPQ column (described in Section 2-4) at 50 °C that is able to separate C 0 2 from argon and oxygen. Blank samples were also analyzed. With He carrier gas (25 mL/min), an injection gas volume of 0.2 mL, chart - 82 -speed 1 cm/min, and an attenuation of 16, retention times for Ar and CO2 were 4.10 and 15.20 min, respectively. Although no quantitative data for C 0 2 produced during the reaction were obtained because of the small scale of the reaction and lack of solubility data for C0 2, the GC results qualitatively show formation of C0 2. This explains why only a weak u(CO) band in the FT-IR spectra of the reaction residue was observed. The formation of CO2, as well as the formation of OPPI13, show that oxidation reactions must have occurred to convert the CO and PPh 3 ligands to their respective oxides. The IrCl(0 2)(CO)(PPh 3 )2 itself will not transfer the dioxygen to the PPh 3 or CO ligands (the dioxygen atoms do transfer to PPh 3 in the presence of free PPh 3 as described in Section 3-4). The test for the formation of H2O2 was performed by iodometry. Iodometric analysis Benzene solutions of TFA (0.27 M) were added to 5 mL of CH2CI2 solutions containing IrCl(0 2)(CO)(PPh 3) 2 to a 2 : 1 acid/Ir ratio (for example, II: 20 mg, 0.025 mmol; TFA, 0.19 mL) and the solutions stirred at room temperature for 10 minutes; the resulting solutions were analyzed using iodometry with the modification described in Section 2-5-1. The H2O2 detected was only 20-25% of the total expected based on the mass of the starting complex. This result is consistent with the concomitant formation of C 0 2 and OPPh 3 with consumption of H20 2. A study reported in the literature 1 0 8 shows that only trace H 2 0 2 is detectable by iodometry for the protonation of IrCl(0 2)(CO)(PPh 3) 2 using excess CH 2C1C00H (acid : Ir > 100). Although no product analysis is given in that study, based on the findings in this section, the H 2 0 2 is expected to be consumed in reactions with CO and PPh 3 ligands when an excess acid is present. - 83 -3-3-1-3. Primary and secondary reactions Studies on the stoichiometry and the products of the protonation reaction of IrCl(C>2)(CO)(PPh3)2 suggest that the reaction proceeds via a primary stage, in which H 2 0 2 and IrCl(OCOCF 3 )2(CO)(PPh 3 )2 are formed, and then secondary reactions in which CO and PPh 3 ligands are oxidized by the H 20 2. The CO2, OPPh 3 and several trifluoroacetato irid-ium(III) complexes are finally formed in the reaction system. The reaction of IrCl(CO)(PPh 3 )2, I, with Bu t02H has been reported to give CO2, OPPh 3 and other iridium species 1 5 1 but the mechanism for this reaction is not clear. The reaction of I with H2O2 also gives the same products (Section 4-5). 3-3-1-4. Low temperature tests Protonation of IrCl(0 2)(CO)(PPh 3 ) 2 with TFA was also performed at -30 °C and -57 °C (the low temperatures being obtained by mixing C 2 H 5 O H with dry-ice). Results at -30 and -57 °C, based on both 3 1P{ 1H} and FT-IR analyses of the resulting residue (obtained at as low a temperature as possible), showed that two equivalents of TFA were required to react completely with the starting complex. Analyses of the reaction residue by mass and FAB spectroscopy gave results similar to those obtained from the residue of the reactions at 25 °C. The tests for H2O2 at 7 °C were carried out using the 1 H NMR method (described in Section 2-5-2) for the 2 : 1 acid/Ir ratio reaction in CeDg. The amount of H2O2 detected was as twice high as that detected for reactions at 25 °C (40-50% of the total H2O2 expected based on the mass of the starting iridium complex) under similar reaction conditions. This fact indicates that at lower temperatures the secondary reactions are less significant. 3-3-2. Reaction between I r C K 0 2 ) ( C O ) ( P P h 3 ) 2 , II, and H C l Reactions of II with HCl were carried out at 1 : 1 and then 2 : 1 acid/Ir ratios in CD 2C1 2 at 20 and -70 °C, and were monitored by *H and 3 1 P N M R . The pale brown solutions of II were obtained by addition of CD 2C1 2 into a 5 mm N M R tube containing the iridium compound under argon and, then, the tube was stoppered with a rubber septum. A calculated amount of anhydrous HCl gas per iridium was next introduced into the N M R tube via a gas-tight syringe and the mixture shaken rapidly, when the solution changed to pale yellow. Fig. 3-9 shows the 3 1P{ 1H} N M R spectra for the 1 : 1 and 2 : 1 reactions at 20 °C (for example, II: 4.1 mg, 0.005 mmol; CD 2C1 2: 0.6 mL; HCl (1 atm): 112 pL, 0.005 mmol for the first addition, and after 30 minutes another 112 u-L addition). For the 1 : 1 reaction, Fig. 3-9 (b) ), shows that half of the starting compound II (6 4.12 ppm) has reacted to give nearly an exclusive formation of a phosphine-containing species at 6 -12.23 ppm; another 'negligible' species at 6 -17.82 ppm is seen also. The corresponding *H N M R spectrum, Fig. 3-10 (b), shows a broad band (~30 Hz at half-width) at 6 ~1.6 ppm, where ap impurity H 20 peak (6 1.56 ppm, ~15 Hz at half-width) was present before the reaction, Fig. 3-10 (a); the broadening indicates the formation of H 2 0 2 (see Section 2-5-2). The 3 1P{ 1H} N M R spectrum for the 2 : 1 reaction, Fig. 3-9 (c), obtained after the second addition of HCl, shows that no starting compound II is left, while the two species at 6 -12.23 and -17.82 ppm are again present, the intensity of the latter no longer being negligible. In addition, a broad band centred at 6 0.2 ppm (~160 Hz at half-width) is also observed. The corresponding 1H N M R spectrum, Fig. 3-10 (c), shows a more broadened band (~96 Hz at half-width) in the S 1.5 to 2.5 ppm region; the H 2 0 2 is formed in about 100% yield (based on II), according to the quantitative equation for H2O2 given in Table 2-1, Section 2-5-2. Further, the 3 1P{ 1H} NMR spectrum for the 2 : 1 reaction solution left in the dark for one day shows only one peak at S -17.82 ppm, while the peak at 6 -12.23 ppm as well as the broad band at 6 0.2 ppm have disappeared, Fig. 3-9 (d). The corresponding 1H NMR spectrum shows no significant loss of the H2O2. Results (*H and 3 1 P NMR data) obtained from the 1 : 1 and 2 : 1 (HC1 : II) reactions at -70 °C (for example, II: 3.8 mg, 0.0047 mmol; CD 2C1 2 : 0.6 mL; HC1 (1 atm): 105 /xL, 0.0047 mmol for the first addition, and after 30 minutes another 105 ul, addition) are very similar to those obtained at 20 °C; however, the 3 1 P NMR peak at 6 -17.82 ppm is much less noticeable. When the 2 : 1 reaction solution was warmed up to 20 °C, the intensity of the peak at 6 -17.82 ppm increased with respect to that at 6 -12.23 ppm, and the corresponding 1H NMR spectrum showed the formation of H2O2 in almost 100% yield. The known mer-IrCl3(CO)(PPh3)2, III, was obtained from the reaction between anhydrous C l 2 and IrCl(CO)(PPh 3 )2 in benzene 1 3 6 and analyzed by FT-IR, lK and 3 1 P NMR. The IR data obtained (i/(CO) = 2077 cm - 1, in KBr) are in good agreement with the literature data (i/(CO) = 2080 cm - 1), 1 3 6 while the 3 1 P NMR data (6 -17.82 ppm, in CD 2C1 2) and the *H NMR data (Fig. 3-11, two multiplets at 6 7.43 and 7.93 ppm in CD2CI2) are not reported in the literature. Reaction between III (~2 mg) and H2O2 (3 drops, 33% by weight in H 20) was attempted in CD2CI2 (0.6 mL) in a 5 mm NMR tube. After 2 days, 3 1 P NMR analysis showed that no phosphine-containing product was formed and the starting compound III remained. - 86 -4.12 <•>) i<i i-n i » ( m m i » • •Ui [ i i . . T ^ l l f< i.t J ) i ^ w i i f 4.12 IrClCOjKCOXPPhsJj i<w«l»ip«iffH"i"|4'tl"<'<'* fcc-iKn,(co)(PPh3H mer-lrCl3(CO)(PPh3)j -17.82 JL -12.23 V*c-lra»(CO)(PPh s)i 0.2 -17.82 mer-IrCl3(CO)(PPh3)2 -1782 »er-IrCl 3 (CO)(PPh 3 )j • I 1 1 1 1 —I — 30 20 10 o - 10 - 2 0 P P M Fig. 3-9. The 3 1P{ JH} NMR spectra for the reactions of IrCl(C-2)(CO)(PPh3)2 with HC1 at HCl/Ir ratios (a) 0 : 1, (b) 1 : 1, (c) 2 : 1, and (d) 2 : 1 (after 24 h) in CD 2C1 2 at 20 °C. - 87 -(•) 1.56 H 2 0 Y -A. l i | i i i i | i i i i | l i i i | l i i i ) i i i i | i i i i j i i i i | i i 6.0 5.0 4.0 3.0 " i | i i i l | i i i i | i l l i | i l l 2.0 1.0 PPM Y H 2 0 , H 2 0 2 i i i i i i i I i i i i i i i i i I i i i i i i i i 2.0 1.0 PPM l l | l l l I | I l l l | l l l l | I I l I | l l l 1 | l l I I | ' 6.0 5.0 4.0 3.0 X ( c ) Y Hfi.Hfiz i i | i i i i i i i i i | i i i i i i i i i | i i i i i i i i i | i i i i i i i i i I i i i i i i i i i [ i i i i i i i i 6.0 5.0 4.0 3.0 2.0 1.0 PPM x = u n k n o w r t ; y = residue in C D 2 C I 2 Fig. 3 - 1 0 . The *H NMR spectra (in the S 1 .0 to 6.0 ppm region) for the reactions of IrCl(0 2)(CO)(PPh 3) 2 with HCl at HCl/Ir ratios (a) 0 : 1, (b) 1 : 1, and (c) 2 : 1 in CD 2C1 2 at 20 °C. - 88 -A : B ( i n t e g r a t i o n ) = 3'-2 7.43(A: m e t a - , p a r a - H ) ( B . - o r t h o - H ) 7.93 Fig. 3-11. The *H NMR spectrum (in the 6 7.0 to 8.0 ppm region) for mer-IrC] 3(CO)(PPh 3) 2 in CD 2C1 2 at 20 °C. - 89 -The 1H and 3 1 P NMR studies described above show that complete reaction of one mole of II requires 2 moles of HCl with the formation of H2O2 in about 100% yield. Also, the fact that there was no significant loss of H2O2 in the resulting reaction solution even after several days excludes the involvement of secondary oxidation reactions of the Ir phosphine-containing species with H2O2. Therefore, the dominant phosphine-containing species at 6 -12.23 ppm is likely to be a trichloro iridium(III) complex. The 3 1 P NMR data for mer-IrCl 3(CO)(PPh 3 ) 2, III, is a singlet at 6 -17.82 ppm, and this species is a minor product during the early period of the HCl/iridium peroxide reaction; however the amount of III increases and becomes the only product upon the disappearance of the species at 6 -12.23 ppm during longer reaction times. Hence, the species at 6 -12.23 ppm is most likely an isomer of III, and is considered (see below) presumably to be fac-IrCl 3(CO)(PPh 3) 2, IV. The initial formation of a fac-trichloro species is given in the reaction pathway proposed in Scheme 3-1; this suggests that IrCl2(OOH)(CO)(PPh 3 )2 is formed, followed by a reaction between this hydroperoxo iridium species and HCl to give the fac- and mer-IrCl 3(CO)(PPh 3) 2 and H 20 2. The fac-IrCl 3(CO)(PPh 3) 3 seems to be favoured kinetically, and then isomerization gives the thermodynamically stable mer compound at 20 °C. The broad band at 6 0.2 ppm ( 3 1P NMR data) could refer to a fluxional intermediate such as a five-coordinate, cationic Ir(III) species, [IrCl 2(CO)(PPh 3) 2] +, involved in the isomerization reaction. An iridium(III) hydroperoxide intermediate has also been suggested by James and co-workers in a study on catalytic oxidation of cyclooctene to cyclooctanone with an iridium hydride complex using 0 2/H 2 mixtures.11 - 90 -Scheme 3-1 The proposed pathway for the reaction between IrCl(02)(CO)(PPh3)2 and HCl. IrCl{02){CO){PPh3)2 22* nIrCl2(OOH){CO)(PPh3)2n H202, "fac-IrChiCO^PPh^i" {major), mer - IrCl3(CO)(PPh3)2 (minor) "fac-lTCl3(CO){PPh3)2n » o m ! l l i a l i o n mer - IrCl3(CO)(PPh3)2 Correspondingly, the stereochemistry involved in the reaction pathway proposed in Scheme 3-1 is depicted in Fig. 3-12. The stronger trans effect of CO (compared to chloride) on the Ir-0 bonding in IrCl(02)(CO)(PPli3)2, II, leads on reaction with HCl to the formation of a hydroperoxo species, IrCl2(OOH)(CO)(PPli3)2, having the -OOH group trans to the chloride. Then, further HCl attack on the hydroperoxo species gives the fac and mer compounds presumed to be via an I a type of mechanism which is favoured for octahedral Ir(III) complexes1 5 2 1 1 (I a: an associative interchange mechanism for ligand replacement reactions). The rotation of the non-linear -OOH group along the Ir-OOH bond perhaps creates some steric hindrance for HCl attack adjacent to the -OOH group, and thus less steric hindrance for trans attack on the side opposite to the -OOH group might be expected. Such a speculative rationale could account for the formation of the fac compound being favoured kinetically. -91 -H 2 Q 2 (thermodynamically stable) (favoured kinetically) Fig. 3-12. The stereochemistry involved in the proposed pathway for the reaction between IrCl(0 2)(CO)(PPh 3 ) 2 and HC1. - 92 -Further support for the formation of the fac isomer was obtained from 1H NMR studies on the 2 : 1 HCl/iridium peroxide reaction at -70 °C. The trans phosphine ligands in mer-IrCl 3(CO)(PPh 3) 2 give rise to two multiplets at 6 7.43 and 7.93 ppm separated by 150 Hz (Fig. 3-11), while the J H NMR spectrum (Fig. 3-13) for the 2 : 1 reaction at -70 °C (which completely converts the IrCl(02)(CO)(PPh3)2 to one dominant phosphine-containing species at 6 -12.23 ppm ( 3 1P NMR)) shows two multiplets at 6 7.43 and 7.83 ppm with a smaller separation (120 Hz), suggesting the cis positions of phosphine ligands. 1 5 2 6 That is, the "fac" structure for the intermediate trichloro iridium complex. The ] H NMR data in the 6 7.0 to 8.0 ppm region for the reactions at 20 °C (Fig. 3-14) also support the proposed reaction pathway in Scheme 3-1. The phenyl groups appear as two multiplets at 8 7.43 and 7.53 ppm in CD2CI2 for IrCl(0 2)(CO)(PPh 3) 2, Fig. 3-14 (a). In the 1 : 1 reaction, Fig. 3-14 (b), a multiplet at 6 7.83 ppm, attributed to the fac isomer, appears, while the multiplet at 6 7.93 ppm corresponding to the mer isomer is very weak. In the 2 : 1 reaction, Fig. 3-14 (c), not only are the two multiplets at 6 7.83 and 7.93 ppm with comparable intensities observed, but also a multiplet is found at 6 7.73 ppm, attributed to the isomerization intermediate detected by the broad band at 6 0.2 ppm in 3 1 P NMR. The integration for the multiplets at 6 7.93, 7.83 and 7.73 ppm shows a molar ratio of 1 : 2.5 : 3.2, which is about the same as that (1 : 2.7 : 3.2) obtained from the integration for the corresponding 3 1 P peaks at S -17.82 and -12.23 ppm and the broad band at 6 0.2 ppm. This observation further supports that the 1H multiplet at 6 7.73 ppm and the 3 1 P band at 6 0.2 ppm are attributable to the presence of an isomerization intermediate. After 24 hours, the J H NMR spectrum for the same reaction solution shows two major multiplets at 6 7.93 and 7.43 ppm (the mer compound) and a weak multiplet at 6 7.73 ppm, while the - 93 -multiplet at 6 7.83 ppm for the fac compound is absent (Fig. 3-14 (d)). 7.4 3 7.83 J t i i i i i i i i i i i i 1 1 1 1 I 1 1 1 i i i i i i i I i i i i i i i i i 1 i i i i i i i i i | i i ' i i 9.0 8.0 7.0 1 6.0 s o p p m X n residue in CD 2CI 2 Fig. 3-13. The *H NMR spectrum (in the 6 7.0 to 8.0 ppm region) for the reaction of IrCl(02)(CO)(PPh3)2 with HC1 at HCl/Ir ratio 2 : 1 in CD2C12 at -70 °C. Fig. 3-14. The 1H NMR spectra (in the 6 7.0 to 8.0 ppm region) for the reactions of IrCl(0 2)(CO)(PPh 3) 2 with HCl at HCl/Ir ratios (a) 0 : 1, (b) 1 : 1, (c) 2 : 1, and (d) 2 : 1 (after 24 h) in CD 2C1 2 at 20 °C. - 95 -3-3-3. Reactions between I r C l ( 0 2 ) ( C O ) ( P P h 3 ) 2 , II, and other acids Two acids with relatively large anions, trifluoromethanesulfonic acid (triflic acid) and p-toluenesulfonic acid, were also used for the protonation reaction of II in order to study any possible steric effects of the anions on the reaction stoichiometry. Ethanol solutions of triflic acid and p-toluenesulfonic acid (6.0 x 10~ 2 M) were used. A solution (10 mL CH 2C1 2) of II (30 mg, 0.037 mmol) was mixed with triflic acid solution to a ratio of 1 : 1 at -57 °C. After 20 minutes, the solvent was removed at as low a temperature as possible and the dried residue analyzed by FT-IR, which showed that about half of II remained unreacted. Reactions between II and p-toluenesulfonic acid at the 1 : 1 ratio in CH 2C1 2 at both -57 and 25 °C were also carried out, and the FT-IR spectra of the resulting residues showed again that significant amounts (> 50%) of starting complex were left unreacted. The conclusion drawn is that an increase in the size of the anion of an acid does not alter the stoichiometry of the protonation reaction of II. 3-3-4. Summary of the protonation reactions of IrCl(0 2)(CO ) ( P P h 3 ) 2 , II Results obtained from the protonation reactions of II with HX reveal a stoichiometry of one mole of complex to two moles of HX between -57 to 25 °C for X = O C O C F 3 _ , p-CH 3C 6H 4-S0 3-, and C F 3 S 0 3 " , and between -70 to 20 °C for X = C l ~ . The imme-diate products of the protonation reaction of II with C F 3COOH are H 2 0 2 and probably IrCl(OCOCF 3) 2(CO)(PPh 3) 2. Oxidation reactions between IrCl(OCOCF 3) 2(CO)(PPh 3) 2 and H 2 0 2 then follow to produce OPPh 3, C0 2, H 20, and other iridium(III) compounds. Pathways are outlined in Scheme 3-2 for this particular reaction. The protonation reaction of II using HC1 gives H2O2 and IrCl3(CO)(PPh3)2 compounds. The fac-IrCb(C0)(PPh3)2 that is kinetically - 96 -favoured during the protonation reaction undergoes isomerization to form the thermodynami-cally stable mer-IrCl3(CO)(PPh3)2 isomer as outlined in Scheme 3-1, and oxidation reactions between H 2 0 2 and mer-IrCl3(CO)(PPh3)2 do not take place at room temperature (see Section 3-3-2). Although the hoped for possibility of detecting the intermediate iridium(III) hydroper-oxo complex was not realized, formation of fac-IrCl3(CO)(PPh3)2 in the reaction supports indirectly the involvement of the hydroperoxo iridium species (Scheme 3-1). Scheme 3-2. The protonation of IrCl(0 2)(CO)(PPh 3) 2 with CF 3COOH. IrCl(02){CO){PPh3)2 ^ r ^ u " nIrCl(OOH)(OCOCF3)(CO){PPh3)2n CFtCOOH « / rc/( O C O C F 3) 2( C O)(p F f t 3) 2« + H2Q2 —> OPPh3,C02,"Ir(III)n species, and H20 3-4. Oxygen transfer from I r C l ( 0 2 ) ( C O ) ( P P h 3 ) 2 , II, to free P P h 3 Oxidation of styrene to styrene oxide, benzaldehyde and acetophenone and the oxidation of PPh 3 to OPPh 3 using IrX(CO)(PPh 3 )2 (X = CI, Br, and I) and molecular oxygen have been reported in the literature, 1 5 3 but mechanisms involved in these oxidation reactions remain unclear. Although an attempt to obtain some mechanistic information on the oxygen transfer from II to PPh3 was made in the present work, no detailed 6tudy on this topic was carried out. However, 3 1P{ 1H} NMR evidence showed the ability of II to transfer both oxygen atoms to free PPh 3. Dichloromethane solutions of II (20-200 mg, 0.025-0.25 mmol) were mixed with PPh3 in Ir : phosphine ratios of 1 : 1; 1 : 2 and 1 : 10 under Ar at 25 °C. Yellow crystals were formed after two days and were identified as IrCl(CO)(PPh 3) 2 by FT-IR and 3 1P{ 1H} NMR. In the resulting solution, OPPh 3 was found to be the only phosphine-containing product by 3 1P{ ,H} NMR (Fig. 3-15 (a) and (b)). Clearly, oxygen is transferred to PPh 3 from the dioxygen complex, in the presence of excess free PPh 3, as described in eq. 3-15. Solutions of the dioxygen complex under Ar in the absence of the free PPh 3 are relatively stable and no OPPh 3 can be found under corresponding conditions. IrCl{02)(CO){PPh3)2 + 2PPh3 —+ IrCl(CO)(PPh3)2 + 20PPh3 (3 - 15) A mechanistic study 1 2 4 on the 0 2 — oxidation of PMePh 2 using Pt(0 2)(PPh 3) 2 in ethanol shows that direct oxygen transfer from the metal-dioxygen adduct to the PMePh 2 is not involved. Free peroxide, liberated by nucleophilic attack of the excess phosphine and generated as H0 2~, oxidizes the phosphine to the oxide. The concomitant Pt(II) phosphine product is then reduced back to the Pt(0) precursor via the phosphine (and OH -) generating a second mole of phosphine oxide. The process becomes catalytic in the presence of 0 2 (see eqs. 6-6 to 6-8 in Section 6-1). The iridium dioxygen complex probably transfers oxygen to the free PPh 3 via a similar reaction pathway involving the H 0 2 - species. However, mechanistic details on reaction 3-15 remain unsubstantiated. - 98 -PPh 3 -5-88 OPPh 3 27.12 Ira(02)(CO)(PPh,)2 4 1 2 4> i I i i i i | i i i ' | i i i i | i i i i I i i i i | i t i i I i i i i | I I I i I i i i i | I I i i | 40 30 20 10 0 - i I I I I I I I I I I 0 p p m Fig. 3-15. (a) The 3 1P{ 1H} NMR spectrum of the filtrate from the reaction IrCl(0 2)(CO)(PPh 3) 2 + PPh 3, 1 : 10, CH 2C1 2. I 24'02 IrCl(CO)(PPh«)2 I I | I I I 1 | t I 1 I | I I I I I I I I I I I I I I I I I I I | I I I 1 | I I I I | I Qrvrfl 40 30 20 10 H H Fig. 3-15. (b) The 3 1P{ ]H} NMR spectrum (CD 2C1 2) of the yellow crystals separated from the reaction IrCl(0 2)(CO)(PPh 3) 2 + PPh 3, 1 : 10, CH 2C1 2. - 99 -C H A P T E R 4 A K I N E T I C A N D M E C H A N I S T I C S T U D Y O F T R A N S - C H L O R O C A R B O N Y L B I S ( T R I P H E N Y L P H O S P H I N E ) I R I D I U M ( I ) , [I r C l ( C O ) ( P P h 3 ) 2 ] , IN A P R O T O N A T I O N - O X Y G E N A T I O N P R O C E S S 4-1. Introduction As discussed in the introduction to Chapter 3, the chemical behaviour of 0 2 after coordina-tion is a complex subject. The study of the IrCl(CO)(PPh3)2 (I) complex in an oxygenation-protonation process (Chapter 3) shows that the iridium dioxygen complex acts as a peroxo source to produce H 2 0 2 ) which is the actual oxidant that reacts with the oxidizable ligands. This study provided no direct evidence for formation of the possible oxygen transfer inter-mediate: the iridium hydroperoxo complex. Thus, an alternative approach, a protonation-oxygenation process has been studied. That is, I is used in reaction with an acid (HCl) to produce a hydrido iridium(III) complex, IrHCl 2(CO)(PPh3) 2 (V) and then molecular oxy-gen is admitted for reaction with the hydrido iridium complex. The purpose of studying this protonation-oxygenation process was to examine the possibility of producing an iridium hy-droperoxo complex via an insertion reaction of 0 2 into an iridium-hydrogen bond. Reactions between Vaska-type complexes and hydrogen halides are oxidative-addition re-actions which can produce cis and/or trans addition products. When solid, crystalline IrCl(CO)(PPh3)2 reacts with gaseous hydrogen halides, only cis-addition products (with respect to H and X) are found. 1 1 1 , 1 1 3 The stereochemical course of the oxidative-addition reaction in so-lution has been studied extensively. Collman and Sears 1 5 4 suggested that the oxidation-addition - 100 -of hydrogen halides, alkyl halides, and acetyl halides with trans-IrCl(CO)(PMePh 2) 2 is a ki-netically controlled trans addition with the stereochemistry of the addition being unaffected by the nature of the solvent. Subsequent isomerization was considered to occur to give product mixtures containing both the cis and trans adducts. However, Blake and Kubota 1 1 3" reported subsequently that the addition of hydrogen halide to trans-halo(carbonyl)bis(arylphosphine)iri-dium(I) is stereospecifically cis in benzene and chloroform, but in the presence of polar solvents such as methanol, acetonitrile, water, or dimethylformamide, which have the capacity for sol-vating ions, mixtures of cis and trans isomers are formed directly. It is now generally concluded that stereospecific trans addition occurs in polar solvents and cis addition in nonpolar or some weakly polar solvents (such as toluene, chloroform, dichloromethane and chlorobenzene). 1 5 5 , 1 5 6 Isomerization of trans products takes place in the polar solvents to form a mixture of cis and trans products. It is also known that product with H trans to CO is not formed using either crystalline or solutions of I, according to IR data on the products of the oxidative-addition reaction (i.e. the absence of a trans H-CO vibrational interaction), 1 1 3 6 as shown in eq. 4-1. When HX is HCl, the two "isomers" formed from the solution of I, of course, are identical; the characterization of this product, IrHCl2(CO)(PPh3)2, is presented in Section 4-2. The kinetic and mechanistic studies of the oxidative-addition reactions of I and analogues with HX (X = CI and Br) have indicated that an ionic mechanism (addition of an anion, or of solvent, followed by a protonation step) occurs in polar solvents leading to trans products, 1 5 5 while a concerted cis addition (with a fairly polar transition state) occurs in the nonpolar and weakly polar solvents given above.156" The reverse of the oxidative-addition reaction of acids (i.e. the reductive-elimination reaction), is also known, and the equilibrium between the - 101 -forward and reverse reactions depends on the acids and the reaction conditions. - | - H X (o r R X ) Ph3P PPh, (4-1) PPh a OC| " H ( R ) 0 C S | « H ( R ) HX = HYDROGEN HALIDES; RX = ALKYL HALIDES Another aspect involved in these protonation-oxygenation studies is the chemical behaviour of the hydride products in their reaction with O2. Pioneering work related to this topic has been carried out by James et al . , 7 , 1 1 in studies on the catalytic oxidation of cyclooctene to cyclooctanone using as catalyst I r HC^CsHnXDMA) (DMA = N,N'-dimethylacetamide) with H2/O2 gaseous mixtures, and in studies involving reactivity of IrHCl2(DMSO)3 (DMSO = S-bonded dimethylsulfoxide) with oxygen. - 102 -The purpose of the present work was to explore the chemistry of the interaction between oxygen and an iridium(III)-hydride complex, with the aim of producing an iridium(III) hy-droperoxo species. Knowledge of the fundamental reactivity between metal-hydrides and molec-ular oxygen is scant, and is important in terms of catalytic oxidations generally and production of H 2 0 2 (and H 20) from H 2 and 0 2 mixtures.42 4-2. Characterization of I r H C l 2 ( C O ) ( P P h 3 ) 2 , V The title compound V was prepared as described in Chapter 2, via the reaction between IrCl(CO)(PPh 3) 2 and HCl in CH 2C1 2 solution. The structure of the single product is shown in Fig. 4-1, based on the following evidence. The XH NMR spectrum of V (one triplet at 6 -15.27 ppm), the 3 1P{*H} NMR spectrum (one singlet at 6 -0.58 ppm) in CD 2C1 2, and the fact that the coupling constant between phosphorus and hydrogen is 11 Hz, which is typical for phosphorus cis to hydrogen,14 shows that the two phosphine ligands are mutually trans. The FT-IR spectrum (Fig. 4-2) shows an iridium-hydrogen stretch at 2238 cm - 1 and a carbonyl stretch at 2022 cm - 1. The values are consistent with values for complexes having a carbonyl group cis to hydrogen; trans disposed H and CO groups give a higher value for j/(CO). 1 5 0 The two assigned iridium-chloride stretching frequencies at 312 and 265 cm - 1 are also consistent with the literature values for complexes with a chloride trans to a carbonyl group (305 ~ 310 cm - 1) or trans to hydrogen (255 ~ 269 c m - 1 ) . i m The IR data have been reported previously 1 1 7 6 (see Section 2-2). - 103 -PPh3 Fig. 4-1. The structure of IrHCl 2(CO)(PPh 3) 2. 3800 2450 1550 875 200 Wavenumber, c m - 1 Fig. 4-2. The FT-IR spectrum o f IrHCl 2(CO)(PPh 3) 2 in Nujol mull. - 104 -A useful feature of V is the stretching frequency of the carbonyl group in solution. After subtraction of the solvent background from the solution IR spectrum, one can obtain the carbonyl band region in the absorption mode quantitatively, just as one can obtain UV-vis absorption bands in solution. Fig. 4-3 shows FT-IR absorption spectra for the carbonyl groups of IrHCl 2(CO)(PPh 3)2, IrCl(0 2)(CO)(PPh 3)2 and IrCl(CO)(PPh 3) 2 in C H 2C1 2. Table 4-1 summarizes the IR data for the carbonyl groups of the iridium complexes in C H 2C1 2 and in Nujol mulls. Table 4-1. The stretching frequencies for the carbonyl groups of IrHCl 2(CO)(PPh 3) 2, IrCl(0 2)(CO)(PPh 3) 2 and IrCl(CO)(PPh 3) 2 at 25 °C Compound ^(CO), cm - 1 i/(CO), cm - 1 e, M - 1cm- l a in Nujol mull in CH 2C1 2 in CH 2C1 2 IrHCl 2(CO)(PPh 3) 2 2022 2047 2430 IrCl(0 2)(CO)(PPh 3) 2 1997 2014 1540 IrCl(CO)(PPh 3) 2 1950 1965 2045 a: Values reported are an average of three measurements with a relative error of ± 10% and Beer's Law is applicable over the concentration range of ~1 x 10" 3 to ~5 x 10" 5 M. - 105 -A «= 0-104 IrCl(CO)(PPh 3) 2 1965 1 2014 1 IrCl(0 2)(CO)(PPh 3) 2 2047 i IrHCl 2 (CO)(PPh 3 ) 2 2088 1999 1950 C M " 1 Fig. 4-3. The solution FT-IR absorption spectra for the carbonyl groups of IrHCl 2(CO)(PPh 3) 2, IrCl(0 2)(CO)(PPh 3) 2, and IrCl(CO)(PPh 3) 2 in CH 2C1 2 at 25 °C. - 106 -4-3. Reaction between I r H C l 2 ( C O ) ( P P h 3 ) 2 , V, and 0 2 Molecular oxygen at 1 atm was introduced into a CH 2C1 2 solution (30 mL) containing V (for example, 30 mg, 0.037 mmol) at 25 °C, and the system left for 48 hours. The solution, which gradually changed from colourless to greenish brown, was studied by 3 1P{ 1H} NMR (on a Bruker WP-80 NMR spectrometer). No starting V (6 -0.58 ppm) was left, while OPPh 3 (6 27.12 ppm), IrCl(0 2)(CO)(PPh 3) 2, II, (6 4.12 ppm) and mer-IrCl 3(CO)(PPh 3) 2 {6 -17.82 ppm) were observed, in addition perhaps to other unknown trace species (presumably Ir(III) phosphine species) observed in the 6 -8 to -20 ppm region (Fig. 4-4). After CH 2C1 2 was removed, a greenish brown powder was obtained and analyzed by FT-IR, mass and FAB spectroscopy. In the FT-IR spectrum, three bands for terminal carbonyl groups were observed, 2075, 1997 and 1956 cm - 1 (Fig. 4-5). The band at 1997 cm - 1, correlating with the band at 858 cm" 1 (r>(lr02)), confirmed formation of II. The bands at 2075 and 1956 cm" 1 in the terminal CO region, as well as the multiple bands in the 330 to 280 cm" 1 region for Ir-Cl stretching, demonstrate that mer-IrCl 3(C0)(PPh 3) 2 (f(CO) = 2077 cm"1, see Section 3-3-2) and IrCl(C0)(PPh 3) 2, I, (f(CO) = 1954 cm - 1, see Section 2-2) are present in the residue. Compound I was not formed in the reaction but in the work-up process for the reaction residue because of the loss of the dioxygen from II during the removal of CH 2C1 2 (30 mL) under vacuum. No band at 2238 cm"1 corresponding to i/(IrH) of V was found. The mass and FAB spectra of the reaction residue also suggest the formation of OPPh 3, II and the chloro complexes of iridium(III). Table 4-2 shows the FAB spectral data of the high mass portion (> 590 mass number) of the residue. - 107-IK3.(Oa)(CO)(PPh«)2 4 . 1 J m e r - l r C I 3 ( C O K P P h 3 ) 2 -17.82 50 40 30 20 10 0 ppm -10 Fig. 4-4. Typical 3 1P{ 1H} NMR spectrum of the solution resulting from the reaction between IrHCl 2(CO)(PPh 3) 2 and 0 2 in CH 2C1 2 at 25 °C. CP o c CO E C O c CO 3800 1550 875 c m - 1 Fig. 4-5. The FT-IR spectrum of the residue in Nujol mull 2450 Wavenumber 200 from the reaction between IrHCl 2(CO)(PPh 3) 2 and 0 2. - 108 -Table 4-2. FAB mass spectral data (high mass portion) for the residue from the reaction between L*HCl 2(CO)(PPh 3) 2 and 0 2 Mass number relative intensity11 possible fragments (+) 6 851 weak [IrCl 3(CO)(PPh 3) 2] + 823 medium [IrCl 3(PPh 3) 2] + 815 medium [IrCl 2(CO)(PPh 3) 2] + 787 strong [IrCl 2(PPh 3) 2]+ 780 strong [IrCl(CO)(PPh 3) 2]+ 751 strong [IrCl(PPh 3) 2]+ 745 strong [Ir(CO)(PPh 3) 2]+ 590 strong [IrCl 3(CO)(PPh 3)]+ a: Relative to the intensity of [Ir(PPh 3) 2] + with a mass of 715. b: Possible parent compounds include: IrCl 3(CO)(PPh 3) 2 and IrCl(0 2)(CO)(PPh 3) 2. Other reactions (for example, V: 74 mg, 0.09 mmol; CH 2C1 2: 30 mL; 0 2: 1 atm; 20 hours at 25 °C) were carried out to test for production of any H 20 2, using iodometry. The amount detected was 14 % of that expected (with respect to the moles of starting V), if formed by the steps outlined below in eqs. 4-2 to 4-4 (See Section 4-4). Irin(H)Cl — * Ir1 + HCl (4-2) - 109 -Ir1 + 02 —» I r 1 1 1 {Of) (4-3) lr'"(Of) + 2 HCI—+ IrIUCl2 + B202 (4-4) Carbon dioxide in the gas phase was detected using gas chromatography with a PPQ column (carrier gas He, 25 mL/min; 50°C; injection volume 0.2 mL gas; chart speed 1 cm/min; attenuation 16. Retention times 15.20 min (C0 2), 4.10 min (0 2)). Further, when H 2 0 2 was reduced by the PPI13 and CO ligands, H 2 0 would be formed in the reaction system. Hence, the overall reaction between V and 0 2 in CH 2C1 2 can be qualitatively described by eq. 4-5. IrHCl2{CO){PPh3)2 ^OPPh3,C02,IrCl{02)(CO)(PPh3)2 (4-5) H202, H20, mer - IrCl3(CO)(PPh3)2 and other "Ir(III)" species 4-4. Kinetic study of the reaction between I r H C l 2 ( C O ) ( P P h 3 ) 2 , V, and 0 2 In order to learn more about reaction of the iridium hydride with 0 2, a kinetic study was carried out. Reaction solutions (for example, V: 16 mg, 0.019 mmol; CH 2C1 2: 20 mL; 0 2: 1 atm; 25 °C) were monitored (via 0.2 mL samples) by an FT-IR spectrometer operating in the absorption mode (Fig. 4-6). The y(CO) band of V at 2047 cm - 1 decreased gradually, while a band at 2014 cm - 1 (corresponding to the carbonyl group of IrCl(02)(PPh3)2, II), eventually appeared and increased continuously up to a final steady intensity. No band at 1965 cm - 1 (corresponding to the carbonyl group of IrCl(CO)(PPh 3) 2, I) was observed during the entire reaction process. The absorbance at 2047 cm - 1 was plotted versus reaction time, and the time dependence of [V] was obtained (Fig. 4-7). In the same way, using the absorbance A = 0-104 - n o -"2047 • I r H C l 2 ( C O ) ( P P h 3 ) 2 0 2 : 1 atm 2014 I r C l ( 0 2 ) ( C O ) ( P P h 3 ) 2 Time,h 0 1.00 6 .00 , 9 .00 . 18.50 . 22.50 . 26 50 . 32 .50 4 3 - 5 0 2119 2025 1966 CM - l Fig. 4-6. The changes in the FT-IR absorption spectra of the solution for the reaction between IrHCl 2(CO)(PPh 3) 2 and 0 2 in CH 2C1 2 at 25 °C. (The higher wavenumber bands in the 2060 to 2090 cm - 1 region are considered to be other Ir(III) carbonyl species.) - I l l -12.0 i CN O 8 0 0- 0 L n ^ C ^ C M H I M ^ M M M l c M I C ^ ^ ^ M C ^ ^ 0.0 10.0 20 0 30.0 40 0 50 0 Time, h Fig. 4-7. The time dependence of [IrHCl2(CO)(PPh3)2] for the reaction between IrHCl 2(CO)(PPh 3) 2 and 0 2 in CH 2C1 2 at 25 °C. Fig. 4-8. The time dependence of [IrCl(0 2)(CO)(PPh 3) 2] for the reaction between IrHCl 2(CO)(PPh 3) 2 and 0 2 in CH 2C1 2 at 25 °C. - 112 -at 2014 cm - 1, the time dependence of [II ] was also obtained (Fig. 4-8). Species II was not readily detected in the first few hours (about one third of the half-life of V). The absorbance data at 2047 cm - 1 were treated using pseudo first-order kinetics (under constant pressure of 0 2) using eq. 3-10. A linear relationship was only observed in about the second half of the reaction period, as shown in Fig. 4-9. From the linear portion of the curve, an observed pseudo first-order rate constant k0|, = (3.2 ± 0.3) x 10 - 5 s - 1 was obtained; the observed rate constant will be discussed in Section 4-6. It was concluded that the reaction of V with 0 2 results in the formation of II as one iridium-containing product, and the appearance of II leads to the occurrence of pseudo first-order kinetic behaviour with respect to V. The use of a UV-vis spectrophotometer for this kinetic study is not feasible because, although a CH2CI2 solution of V is "colourless", the UV-vis absorbance changes to the resulting brownish green solution containing several Ir(III) species have no simple correlation to the reaction kinetics. The above study shows that the dioxygen complex II is a major product from the reaction of V with 0 2, and this must be formed via the intermediate IrCl(CO)(PPli3)2,1. This suggested that elimination of HCl from V to produce II must be one of the reactions taking place. To test this suggestion, the reaction of V with O2 was performed under acidic conditions as follows. Gaseous HCl was introduced into the reaction solution (V: 8 mg, 0.01 mmol; CH2CI2: 20 mL; 0 2: 1 atm; HCl: 0.02 mmol; 25 °C). There was no change in the v(CO) absorption region over a period of 85 hours (Fig. 4-10), and the starting hydride was recovered after then removing the solvent under vacuum. This result shows that addition of HCl prevents any reaction of the hydride with 0 2, and strongly suggests that loss of HCl from IrHCl 2(CO)(PPh 3 )2 is necessary prior to reaction with O2. - 113 -Fig. 4-9. The pseudo first-order treatment of the absorbance at 2047 cm - 1 versus time for the reaction between IrHCl 2(CO)(PPh 3) 2 and 0 2 in CH 2C1 2 at 25 °C ( A 0 , A t , Aoo represent absorbance of the hydride at time = 0, t, and infinity, respectively). The linear slope was obtained using 4 points as indicated in the figure, and the standard deviation for the k0j, value is ± 3 x 10~ 6 s _ 1 . - 114 -2119 2025 1966 C M " 1 Fig. 4-10. The unchanged FT-IR solution absorption spectra for the reaction between IrHCl 2(CO)(PPh 3) 2 and 0 2 in acidic CH 2C1 2 at 25 °C. - 115 -4-5. Mechanism of the reaction of I r H C l 2 ( C O ) ( P P h 3 ) 2 , V, with 02 A mechanism for the reaction of V with 0 2 is proposed, as shown in Scheme 4-1, based on the following observations: (i) The dioxygen complex II is a major product and has to be formed from 0 2 and IrCl(CO)(PPh 3) 2, Step 2; (ii) The total inhibition by added free HCl suggests that the elimination of HCl from V is the first step, Step 1; (iii) Hydrogen peroxide is a product and has to be formed by protonation of II with the HCl formed from V, Step 3; removal of the HCl impedes the reverse of Step 1; (iv) In addition to the formation of II, the products C 0 2, OPPh 3, mer-IrCl 3(CO)(PPh 3) 2, III, and other chloro complexes of iridium(III) are also formed. However, the reaction between III and the H 2 0 2 does not take place readily as described in Section 3-3-2, and the 3 1 P NMR study shows no detectable change within several hours for V in CD 2C1 2 when H 2 0 2 (33% by weight in H 20) is added at 20 °C. Therefore, OPPh 3 and C 0 2 appeared to form from the reaction between I and H 20 2, and this was confirmed by a 3 1 P NMR study on the reaction of I in CD 2C1 2 with H 2 0 2 (33% by weight in H 20), in which the OPPh 3 (6 27.12 ppm), II (6 4.12 ppm), and phosphine-containing species detectable at 6 -7 to -10 ppm, were generated in about two hours, Step 4. The reaction of I with Bu'0 2H to give OPPh 3, C 0 2 and other iridium(III) species has been reported by Harvie and McQuillin. 1 5 1 Scheme 4-1. The mechanism of the reaction of IrHCl 2(CO)(PPh 3) 2 with 0 2 Step 1 lrHCl2(CO){PPh3)2 £=IrCl(CO)(PPh3)2 + HCl V I Step 2 2 I II - 116 -Step 3 IrCl(02)(CO)(PPh3)2 + 2HCI J—~ H202 + lrCl3{CO){PPh3)2 II Step A IrCl(CO)(PPh3)2 + H202 —• C02,OPPh3,H20, IrCl{02){CO)(PPh3)2, other "Ir{IIiy species II Once I is formed, the two oxidative-addition reactions (k_i and k2 steps) will compete with each other. The k 2 step is eventually favoured because the formation of II leads to an irreversible reaction (Step 3), that uses up the free HC1 to produce H2O2 which also irreversibly reacts with I leading to a further decrease in the concentration of I (Step 4). The sequence of steps 1 to 3 suggests that at the end of the reaction, the amount of compound II formed will approach a maximun value somewhat close to (but not equal to) half mole of II per initial mole of V and this is confirmed by the experimental findings. For example, at the end of the reactions described in Section 4-4, II was found to be present in about 0.35 equivalents based on the starting V; this figure is calculated from final optical density values for the 2014 cm - 1 band with e = 1540 M _ 1cm _ 1. 4-6. Kinetics of the reversible oxidative-addition reaction of HC1 to I r C l ( C O ) ( P P h 3 ) 2 , I In order to substantiate further the mechanism given in Scheme 4-1, the kinetics of Step 1 in both directions were studied in CH2CI2. Although the kinetics of the oxidative-addition reaction of I with HC1 in methanol-acetonitrile, toluene, chlorobenzene, and dichloromethane have been - 117-studied, 1 5 5 , 1 5 6 a the rate constant for HCl elimination from V in dichloromethane is not available in the literature. The HCI-CH2CI2 solutions containing various concentrations of HCl were freshly prepared by introducing the anhydrous gas into CH2CI2 under different HCl pressures using Schlenk techniques. The actual concentration of HCl was obtained from titration of the HCI-CH2CI2 solutions against an ethanol solution of known concentration in NaOH, this being standardized using potassium hydrogen phthalate in ethanol. The oxidative-addition reaction (eq. 4-6) was followed by a stopped-fiow spectrophotometer at 386 nm (corresponding to A m o i for I), at 25 °C under pseudo first-order conditions (i.e. [HCl] S> [I]). The absorbance vs. time .curve was recorded as a photograph (e.g. Fig. 4-11) and treated using the pseudo first-order method (i.e. eq. 3-10). The results show pseudo first-order kinetic behaviour with respect to [I] (Fig. 4-12). Table 4-3 lists the pseudo first-order rate constants, k0j,, for the forward reaction of eq. 4-6, at different [HCl], and first-order behaviour with respect to [HCl] is demonstrated by the plot of k o b versus [HCl] (Fig. 4-13); i.e. kob = k_j[HCl] + ki. From Fig.4-13, the rate constant for the oxidative-addition reaction, k_j, is (4.4 ± 0.2) x 1 0 2 M _ 1 s - 1 at 25 "C in CH 2C1 2. The literature data for k_ a in CH 2C1 2 at 25 °C is given as 7.1 x 10 - 2 M _ 1 s - 1 using a stopped-flow spectrophotometer;156" this is obviously incorrect for this fast reaction, and perhaps 7.1 x 102 M _ 1 s - 1 should have been written. JrCl(CO){PPh3h + HCl^IrHCh{CO)(PPh3)2 (4-6) - 118 -Fig. 4-11. The stopped-flow kinetic data monitored at 386 nm for the reaction of IrCl(CO)(PPh 3) 2 at 1.62 x IO" 4 M with HCl (1.22 x IO" 2 M) in CH 2C1 2 at 25 0 - 119 -(HCI) = 2 . 3 1 x l 0 - 2 M o X 1 0 0 [v]c = 1 . 6 2 x l O " 4 M 8 0 • 8 < 8 < 6 0 k c * - 1 2 . 7 s H 1 1 o < < 4 . 0 2 . 0 0 . 0 • • 0 2 0 4 0 6 0 8 0 Time, s (xlO 3) Fig. 4-12. The pseudo first-order kinetic behaviour for the reaction between IrCl(CO)(PPh 3 ) 2 and HC1, with respect to [IrCl(CO)(PPh 3) 2]. 2 0 — k 0b = k.,lHCIl + k , "o X 1.5 -k_,= 4 - 4 xlO 2 M"1s"1 i CO 1 0 * 0 .5 0 0 m i 0>0 10 2.0 3.0 (HCII. M ( X 1 0 2 ) Fig. 4-13. The linear dependence of the observed rate constants on [HC1] for the reaction between IrCl(CO)(PPh 3) 2 and HC1. - 120 -Table 4-3. The pseudo first-order rate constant, k 0b, for the reaction of IrCl(CO)(PPh 3) 2 with HCl in CH 2C1 2 at 25 °C Sample No. [HCl], M 1 3.08 x IO - 2 15.5 2 2.31 x 10~2 12.7 3 1.22 x 10~2 7.6 4 1.13 x 10~2 7.1 k_! = (4.4 ± 0.2) x 102 M - 1 s - 1 Because the intercept of Fig. 4-13 is close to the origin, and indeed within experimental error, the line could be drawn through the origin, it is impossible to estimate ki via the data. Of note, k_i (for I + HCl) is about 7 x 103 times greater than the rate constant for the addition reaction of 0 2 to I at 25 CC; and further, I is readily formed from an IrCl(0 2)(CO)(PPh 3) 2 solution in the absence of 0 2 at 25 °C (Section 3-3-1), while I is not detectable by 3 1P{ 1H} NMR in a solution of IrHCl 2(CO)(PPh 3) 2 at 25 °C, even after days. Thus, the on-rate for HCl is greater than for 0 2, and the off-rate is lower than for 0 2. That is, qualitatively, the equilibrium constant K for the reaction of I with 0 2 is less than that of the reaction with HCl. The ratio of k _ i and ki for the reaction of I with HCl is estimated to be > 10 6 M _ 1, because K for the 0 2 reaction is about 103 M"1; ki must be < 10 - 4 s _ 1 and consequently (see Fig. 4-13), to obtain a reliable value for kj for the HCl reaction requires use of a direct method (Section 3-2-2). The direct method used was as follows. In a CH2CI2 solution, the dioxygen - 121 -complex II ( V : II = 1) was added under oxygen at 1 atm to scavenge rapidly (Step 3, Scheme 4-1) the free HC1 released from the hydride. The oxygen atmosphere was necessary to prevent II from losing 0 2 and to convert I (released from V) to IrCl(0 2)(CO)(PPh 3) 2. Under these conditions, reaction 4-6 can occur only in the HC1 elimination direction, as shown in eq. 4-7; the kinetics were monitored at 2047 cm - 1 (corresponding to the carbonyl group of V), by an FT-IR spectrometer operating in the absorption mode under the reaction conditions mentioned above. Fig. 4-14 shows the FT-IR spectra obtained for one such kinetic measurement (V: 3 mg, 0.004 mmol;. II: 3 mg, 0.004 mmol; CH 2C1 2: 20 mL; 0 2: 1 atm; 25 °C). First-order kinetic behaviour in [V] is observed (Fig. 4-15). The rate constant ki was found to be (3.1 ± 0.1) x 10 - 5 s" 1, and the equilibrium constant K (= k_i/ki) for the reversible oxidative-addition reaction of I with HC1 is readily calculated (Table 4-4). IrHCl2{CO)(PPh3)2-^HCl + IrCl{CO){PPh3)2 (4-7) V I Table 4-4. The forward and reverse rate constants for the reaction between IrCl(CO)(PPh 3 )2 and HC1 in CH 2C1 2 at 25 °C k _ i , M - 1 s _ 1 K, M" 1 (4.4 ± 0.2) x 102 (3.1 ± 0.1) x IO" 5 (1.4 ± 0.1) x 107 a: Average of 2 experiments - 122-2047 A - 0 - 1 0 4 2014 I r H C l 2 ( C O ) ( P P h 3 ) 2 I I Time, h I r C l ( 0 2 ) ( C O ) ( P h 3 P ) 2 3.25 5-25 8.25 10-25 12-25 19-75 24-50 2119 2025 1966 C M " 1 Fig. 4-14. The changes in the solution FT-IR absorption spectra for the HCl elimination reaction from IrHCl 2(CO)(PPh 3) 2 in CH 2C1 2 at 25 °C. - 123 -Fig. 4-15. The first-order kinetic behaviour for the HC1 elimination reaction from lrHCl 2(CO)(PPh 3) 2 in CH 2C1 2 at 25 °C. - 124-4-7. Summary The overall scheme for the reaction of the hydride IrHCl2(CO)(PPh3)2 with 0 2 is sum-marized in Scheme 4-2. The rate constant data clearly show that at the concentrations used, the ki step is rate-determining and clearly I will not be detectable either during or at the end of the reaction. The dioxygen complex II builds up because the HCl regenerated in the rate-determining step is sufficient to react with only half of the amount of II formed. The K value (1.4 x 107 M - 1) shows that at 25 °C in a CH2CI2 solution containing initially about 5 x 10~ 3 M of V, the I at equilibrium (in the absence of 0 2) will be about 1.9 x 10 - 5 M, which cannot be detected using an IR spectrometer. Further, the rate constant for the oxygenation of I is about 2000 times greater than the rate constant for the HCl elimination from V so that when the k _ i step is eliminated (because the free HCl is scavenged by the base, II), the I becomes present in a state of almost zero concentration and the ki step is rate-determining. Comparison of the kinetic data for the reaction of V with 0 2 (Section 4-3), and the kinetics for the HCl elimination from V in the presence of II and O2 (1 atm), reveals the similarities between these two systems. The two reaction systems eventually display identical chemistry. The rate constant, found from the linear portion of Fig. 4-9 for the reaction between V and O2, has a value close to that estimated from Fig. 4-15, for the HCl elimination from V. This fact provides substantial evidence in support of the mechanism suggested in Scheme. 4-2. Further, the initial curved portion of the plot in Fig. 4-9 reflects the period in which the HCl addition to I becomes less competitive with the HCl reaction with II, which is rapid and irreversible. At this stage, the [II] is high enough to "drive" the reaction of the hydride with O2 through the irreversible HCl elimination, and the kinetic analysis gives rise to the linear protion l r H C I 2 ( C O ) ( P P h , ) 2 -- 125 -reduc t ive-el iminat ion ra te -de te rm in ing o x i d a t i v e -addi t ion l rCI(CO)(PPh 3 ) 2 + HCI I + 0 2 5 0-1 k, = 3 . 1 x 1 0 " 5 s revers ib le k_, = 4 . 4 x 1 0 2 M" 1 s ' 1 oxygenat ion k 2 s 6 . 2 x 1 0 "2 M - 1 s- 1 k . 2 = 1.6 x 1 05 s" 1 d r i v i n g force lrCICG£(CO)(PPh3)2 n 2HCI f a s t pro tonat ion ( ' i n s t a n t a n e o u s ' even a t -70°c) H 2 0 2 + lrCI,(CO)(PPti3) 2 l rCI(Cy (CO)(PPh a ) 2 , H l O , ^ irCI(CO)(PPh3)2 I ox ida t ion OOj, O P P h j . o t h e r - l r d l i r species Scheme 4 - 2 . The kinetics and mechanism of IrCl(CO)(PPh3)2 for the protonation-oxygenation process. - 126 -of Fig. 4-9. The observed reaction between IrHCl2(CO)(PPh3)2 and 0 2 is thus simply the elim-ination of HC1 followed by the formation of IrCl(0 2)(CO)(PPh 3) 2, if the subsequent reactions involving H 2 0 2 are not taken into account. 4-8. Kinetic expressions Use of the overall scheme in Scheme 4-2 allows the rate expression for the reaction to be formulated. _£V] dt and = k1{V]-k-1[I][HCt\- (4-8) = h[V] + fc_2[II] - *-i[I][2f"CZ] - fc2[02][I] (4 - 9) Because [I] is probably present in a steady-state in the initial reaction period for about one third of the half-life of V (see Sections 4-4 and 4-7), - f - 0 (4-..) Thus, fci[v] + fc-api] ( 4 _ n ) Substitution of eq. 4-11 into eq. 4-8 gives: <*[V] _ fcifc2[Vl[Oa]-fc-ifc-a[gC7][H] ( 4 _ n ) dt k-^HClj + kilOi} K ' This would be the general rate expression if HC1 were not scavenged. Under conditions when [HC1] is effectively zero (when scavenged by II), then clearly d[V] dt fci[V] (4-13) - 127 -4-9. Study on the reaction of I r H ( C O ) ( P P h 3 ) 3 , VI, with 0 2 Of key importance is the 'negative result' that there is no evidence for reaction of the hydride V with 0 2 to give an Ir-OOH species. The hydride is a coordinatively saturated 18-electron complex and, if such 0 2 insertion exists, prior coordination of the 0 2 molecule at the metal centre will be a requirement unless a H atom transfer process is available. The choice of the iridium(III) hydride was dictated more by the species chosen for the oxygenation-protonation process, Vaska's compound; and, in retrospect, V was probably ill-suited for the test studies. The well known, five-coordinate iridium(I) species IrH(CO)(PPh3)3 was sub-sequently selected for reactivity toward 0 2, as well as cationic species formed by removing chloride from IrHCl 2(CO)(PPh 3) 2, V, using AgBF 4. 4-9-1. Characterization of I r H ( C O ) ( P P h 3 ) 3 , V I The title compound was prepared as described in Section 2-2. The ]H NMR data in C^De show a quartet at S -10.29 ppm and a coupling constant of 22 Hz, indicating a cis geometric arrangement between the hydrogen and the phosphorus. The 3 1P{ 1H} NMR spectrum of V I in CeDe shows only a singlet at 6 14.22 ppm, implying that the three phosphine ligands are magnetically equivalent. A trigonal bipyramid structure with the three phosphine ligands arranged equatorially has been established previously for compound V I based on the *H NMR and IR data,117"" and the 3 1 P NMR data are consistent with such a structure. 4-9-2. Reaction between I r H ( C O ) ( P P h 3 ) 3 , VI, and 0 2 Yellow solutions of V I were prepared by vacuum transfer of dry C6D6 into Schlenk tubes - 128 -containing the iridium compound, and 0 2 (1 atm) was then introduced into the reaction sys-tems, that were left at room temperature with stirring. The colour of the solution changed gradually to green then to blue and finally to dark blue over a period of about 20 hours. For example, in one reaction (VI: 50 mg, 0.05 mmol, C^Dg: 3 mL; 0 2: 1 atm), the initial yellow solution changed to green after ~ 6 hours, then to blue after ~ 10 hours, while after 20 hours a dark blue solution was obtained. The 1H NMR spectrum of the green solution (Fig. 4-16) shows a new hydride resonance at 8 -14.45 ppm (triplet) with a coupling constant (Jp_n) of 14 Hz, indicating that a hydrogen is cis to two equivalent phosphine ligands for the new species. Also, the integration shows a molar ratio of about 1 : 20 for the new species versus the starting complex V I at this 6tage of the reaction. The 3 1P{ JH} NMR spectrum of the green solution (Fig. 4-17) shows a singlet at 8 6.33 ppm in addition to the singlet at 8 14.32 of the starting compound V I , and another singlet at 8 24.37 ppm attributed to OPPI13. The singlet at 8 6.33 ppm is assigned to two equivalent phosphine ligands in the new hydride species because the phosphorus integration shows a ratio of 1 : 30 for the 8 6.33 and 14.32 ppm singlets, which gives the same molar ratio of 1 : 20 as that obtained from the *H NMR data. The FT-IR spectrum of the green solution (Fig. 4-18) obtained from a reaction be-tween V I (130 mg, 0.13 mmol) and 0 2 (1 atm) in C^De (5 mL) at room temperature shows bands at 897 and 1990 cm - 1, assignable to i/(0-0) and v(CO) frequencies of the new species. The FT-IR spectrum also shows a significant amount of the starting IrH(CO)(PPh3)3 species (i/(IrH): 2075 cm"1; i/(CO): 1931 cm" 1). The *H and 3 1 P NMR, and FT-IR data (Table 4-5) suggest that the new species is (carbonyl)hydrido(peroxo)bis(triphenylphosphine)iridium(III), IrH(0 2)(CO)(PPh 3) 2, VII, with the structure shown in Fig. 4-19. - 129 -6-10-29 ppm J P - H = 2 2 H z IrH(CO)(PPh 3 )3 8—14.45 ppm J p _ H = 1 4 H z I r H ( 0 2 ) ( C O ) ( P P h 3 ) 2 A. F i g . 4 - 1 6 . The 1 H N M R spectrum of the green solution obtained from the reaction of IrH(CO)(PPh 3 ) 3 with 0 2 in C 6 D 6 . 14.32 IrH(CO)(PPh 3 ) 3 r O P P h 3 24.37 I iH(Oj) (CO)(PPh 3 ) 2 6.33 T W | l l f l | l l l l | l l l t | l l l l | l l l l | l l l l | l l l l | l l l t | l l l l | l l l l | l l l l | l l > l | l l l l | l l l l | l t l l | l l l l | l 84 14 F i g . 4 - 1 7 . The "P-^H} N M R spectrum of the green solution obtained from the reaction of IrH(CO)(PPh 3 ) 3 with 0 2 in C 6 D 6 . - 130 -* I I I i I I I 2 5 0 0 1 8 6 3 1 4 7 5 1 0 8 8 Wavenumber c m - 1 F i g . 4-18. The FT-IR spectrum of the green solution obtained from the reaction of IrH(CO)(PPh 3) 3 with G 2 in C 6D 6. H F i g . 4-19. The structure of IrH(0 2)(CO)(PPh 3) 2, V I I . - 131 -While studying this species, we became aware of a report by Drouin and Harrod, 1 1 6 who were using 0 2 to trap alkyl species formed from the hydrido(olefin) complexes IrH(CO)(olefin)-(PPh3)2; however, with the styrene derivative, the olefin was simply replaced by 0 2, and this led also to generation of VII. They also synthesized VII by treating IrH 3(CO)(PPh 3) 2 with 0 2, and noted that VII was the first known hydrido(dioxygen) complex.116 Table 4-5. The *H and 3 1 P NMR and FT-IR data for IrH(0 2)(CO)(PPh 3) 2 S^E) ^ ( 3 1 P { 1 H » u(02) u(CO) »/(IrH) reference ppm ppm c m - 1 cm - 1 cm - 1 -14.45(t) 6.34(B) 897 1990 2060 this work" Jp_H = 14 Hz -14.5(t)a 9.99(s)b 823c 1986c 2079c 116 Jp_H = 18.5 Hz a: in C^Dg; b: in CD 2C1 2; c: in Nujol mull. The band at 2060 cm which seems to overlap with a band at 2075 cm - 1 for the v(IrH) of IrH(CO)(PPh 3) 3, is assigned to the z/(IrH) of IrH(0 2)(CO)(PPh 3) 2 in C 6 D 6 (see Fig. 4-18). Analyses of the dark blue reaction solution (20 h) by *H and 3 1P{ 1H} NMR, and FT-IR showed the presence of OPPh 3 (6 24.37 ppm from 3 1 P NMR data; Fig. 4-21), H 20 (6 ~0.4 ppm from *H NMR data) and iridium(III) phosphine species (a very broad 3 1 P NMR band centred at 6 -26.4 ppm; Fig. 4-20) that contained no Ir-H (based on *H NMR) or Ir-carbonyl or -carboxylate moieties; (based on FT-IR data); presumably the CO is lost in the form of - 132 -OPPh,, 24.37 —r-40 —T-2 0 -r o - 2 0 - 4 0 ppm — r - 6 0 Fig. 4-20. The 3 1P{ 1H} NMR spectrum of the dark blue solution obtained from the reaction of lrrI(CO)(PPh 3) 3 with 0 2 in C 6 D 6 after 20 h at room temperature. - 133 -C0 2. Further NMR studies showed that IrH(CO)(PPh 3) 3 (20 m.3, 0.02 mmol) did not react with 0 2 (1 atm) in C 6 D 6 (2 mL) when excess free PPh 3 (50 mg, 0.2 mmol) was present, during a reaction period of more than 24 hours. Thus, dissociation of a phosphine ligand from V I is probably a prerequisite for reactivity with 0 2. The data overall indicate then that the reaction takes place via coordination of 0 2 to a bis(phosphine) intermediate to generate IrH(0 2)(CO)(PPh 3) 2, which subsequently decomposes to form the final products, as outlined in eqs. 4-14 to 4-16. IrH{CO){PPh3)3 *± IrH(CO)(PPh3)2 + PPh3 (4 - 14) IrH(CO)(PPh3)2 + 02 —* lrH{02)(CO){PPh3)2 (4 - 15) IrH{02){CO){PPh3)2 —• "Ir(OOH)(CO){PPh3)2n —• (4-16) OPPh3, C02, H20, and Ir(III) species There is no information on how IrH(0 2)(CO)(PPh 3) 2 decomposes to form the mixture of prod-ucts, but an iridium hydroperoxide species, Ir(OOH), formed by insertion of the 0 2 into the Ir-H bond is probably involved. For V I I an orthogonal structure with respect to the H and 0 2 was also suggested by Drouin and Harrod, 1 1 6 and they considered that migratory insertion to give the Ir(OOH) was unlikely just as insertion of an olefinic ligand will not occur if the double bond is orthogonal to the metal-hydrogen bond. 1 1 6 The attainment of the required cis-coplanar arrangement will depend on the rotational barrier of the coordinated dioxygen (peroxide). This could explain why an Ir(OOH) species is not detected, but does not rule out the possibility of it being an intermediate in reaction 4-16. - 134 -4-10. Reaction between I r H C l 2 ( C O ) ( P P h 3 ) 2 , V, and A g B F 4 The chloride ligand was removed from IrHCl 2(CO)(PPh3) 2 by addition of AgBF 4 stoichio-metrically (V : AgBF 4 = 1:1 and 1 : 2) in a solvent mixture (CD 2C1 2 and acetone-de), in attempts to generate cationic iridium hydrides with coordinated solvent, and possibly provide for 0 2 an easy access to coordinate at the metal centre. A C 3DeO solution (3 mL) of AgBF 4 (5 mg, 0.025 mmol) was added to a CD 2C1 2 solution (2 mL) containing V (20 mg, 0.025 mmol) under argon with stirring. A white precipitate was formed and the initial colourless solution changed to a yellow filtrate, which was then analyzed by *H and 3 1P{ XH} NMR. The JH NMR spectrum shows that the triplet (6 -15.30 ppm, 3p-H = 12 Hz) for the starting complex V has disappeared entirely and no other hydridoiridium species is present. Correspondingly, the 3 1P{ 1H} NMR spectrum shows only one singlet peak at 6 33.77 ppm and no starting complex (a singlet at 6 -0.58 ppm). Reaction of V with AgBF 4 using a 1 : 2 ratio ( V: 46 mg, 0.056 mmol, AgBF 4: 22 mg, 0.11 mmol; CD 2C1 2: 2 mL; C 3 D 6 O : 3 mL) also gives the same result of entire loss of the starting hydride V and no' formation of cationic hydridoiridium species. Further-more, the 3 1P{ XH} NMR spectrum again shows one peak at 6 33.77 ppm. These studies show that no cationic iridium hydride can be generated from the reaction of IrHCl2(CO)(PPh3)2 with AgBF 4. The fact that the 1 : 2 (V : AgBF 4) reaction gives the same data as the 1 : 1 suggests that the loss of hydride takes place via HCl elimination after removal of the first chlo-ride by the addition of one equivalent AgBF 4, as described in eqs. 4-17 and 4-18. The cationic iridium species with trans phosphine ligands (based on the 3 1 P NMR data) did react slowly (more than 20 hours) with oxygen (1 atm) at room temperature, based on UV-vis analysis: absorption maxima at A 430, 380 and 324 nm in the CD2CI2 and acetone-d6 mixture decreased - 135 -as the initial yellow solution changed to pale brown. Qualitatively, this corresponds to for-mation of a peroxy derivative from an iridium(I) complex (eq. 4-19); very similar electronic spectral changes were observed for formation of IrCl ( 0 2)(CO)(PPh 3) 2 from Vaska's compound although this was not confirmed, for example, by IR data. JrHCl2(CO){PPh3)2 + Ag+ S^nt[lTHCl(CO){PPh3)2{Solvent)}+ + AgCl (4 - 17) [JrHCl(CO){PPh3)2(Solvent))+—>[Ir{CO)(PPh3)2{Solvent)]+ + HCl (4 - 18) [Jr{CO)(PPh3)2{Solvent)}+ + 02 ^[Ir{02)(CO)(PPh3)2(Solvent)}+ (4-19) - 136 -C H A P T E R 5 A S T U D Y O F T H E E Q U I L I B R I U M B E T W E E N T E T R A K I S ( T R I P H E N Y L P H O S P H I N E ) P L A T I N U M ( 0 ) A N D T R I S ( T R I P H E N Y L P H O S P H I N E ) P L A T I N U M ( 0 ) I N T O L U E N E 5-1. Introduction For several reasons, platinum was the second metal chosen in the present studies considering routes to M-OOH species: (a) there has been a number of examples using platinum peroxide complexes to oxidize organic and inorganic compounds (as in eqs. 1-25 and 1-26, p.29); (b) studies on hydroperoxo complexes of platinum(II) have been reported; 1 0 1 , 1 0 2' 1 0 8 (c) a distinctive feature of the chemistry of platinum phosphine complexes is the accessibility of a range of stable coordination numbers via Pt(0), Pt(II) and Pt(IV) species. 1 5 7 Differences in the protonation-oxygenation and oxygenation-protonation processes for the platinum complexes compared to the iridium complexes (Chapters 3 and 4) might be expected. Throughout Chapters 5-7, the symbol P will be used for coordinated and free triphenylphosphine, PPI13. 5-2. Plat inum(O) complexes o f P P h 3 The zerovalent complexes PtP n, where n = 3 or 4, are widely used as starting materials for the preparation of divalent complexes of platinum, such as trans-PtHClP 2 and Pt(02)P2- 1 1 8 , 1 5 8 There is a general consensus that PtP 4 dissociates in benzene and toluene to give coordinatively unsaturated species as indicated in eq. 5-1, although dissociation of PtP 3 to PtP2 is considered to be neg l i g i b l e ; 1 1 7 a - 1 2 0 i , ' 1 5 9 _ 1 6 1 this conclusion is based on UV-vis, and NMR spectroscopy - 137 -studies, and measurement of the molecular weight in benzene solution of p t P 4. 1 2 0 a " i' 1 5 7 ' 1 6 0- 1 6 2 Also, the solution equilibria of PtL 4 (L = PMe 3, PMe 2Ph, PMePh 2, PEt 3 and PBu 3 n) have been studied quantitatively using 3 1P{ 1H} NMR. 1 2 6 6 Nevertheless, investigations of the species present in solutions of PtP 4 and of the equilibrium between PtP 4 and P t P 3 at various tem-peratures using NMR have been far from complete. 1 5 7 , 1 6 2 The thermodynamic data for the equilibrium between PtP 4 and PtP 3 are not available in the literature. PtP< Jp PtP3 =~ PtP2 (5-1) (Solvent molecules excluded) 5-3. A 3 1P{ 1H} N M R study of the equilibrium established by P t ( P P h 3 ) 4 and P t ( P P h 3 ) 3 in toluene The compounds PtP 3 and PtP 4 were prepared as described in Chapter 2; samples for 3 1P{ 1H} NMR studies were prepared in vacuum-sealed NMR tubes using dry, degassed toluene-d 8 as solvent. The 3 1P{ 1H> NMR spectrum of PtP 3 (25.3 mg, 0.0257 mmol) in toluene-d8 (0.4 mL) at 20 °C showed a singlet at 6 50.00 ppm and two singlet satellites at 6 68.36 and 31.44 ppm, of relative intensities 1:4:1 (Fig. 5-1 (a)). The spectrum is typical for platinum complexes with magnetically equivalent PPh 3 ligands. The natural abundance of Pt (isotopes 194,196 and 198) with nuclear spin of zero is about 66% and that of 1 9 5 P t with nuclear spin of one half is about 34%; 1 6 3 the Jpt-p coupling constant is 4467 Hz. Physical studies on solutions of zerovalent nickel, palladium, and platinum complexes containing ethylene and arylphosphine ligands (L = - 138 -PPh 3, P(p-C 6H 4CH 3 ) 3 or P(m-C 6H 4CH 3) 3) have been reported by Tolman et a l . , 1 6 2 but only 3 1 P NMR data for PtP 3 at temperatures below -50 °C are given (6 -55 ppm, 3Pt-P = 4370 Hz, in 1 : 1 toluene-CH 2 Cl2). Sen and Halpern 1 5 7 report that, at room temperature, a very broad band can be observed in the 3 1 P NMR spectrum for PtP 3, and that a temperature below -70 °C is required to obtain the characteristic line due to PtP3 (6 49.9 ppm) together with two 1 9 5 P t satallites (Jpt-P = 4438 Hz). Our results show that in order to obtain the 3 1 P NMR spectrum at room temperature for PtP3, the PtP 3 must contain no free PPI13; its presence gives rise to a broad band corresponding the occurrence of the associative exchange between the free phosphine and the PtP3 (see below). When our sample was cooled to -90°C, the spectrum (Fig. 5-1 (b)) was essentially identical with that at 20 °C, indicating that a non-fluxional species is present over this temperature range (the triangular planar molecular structure is reported 1 6 4 for PtPs). Also, dissociation of PtP3 to PtPj was not observed and this is consistent with the observations reported by Tolman et al.. 1 6 2 The 3 1P{ JH} NMR spectra given in Fig. 5-2 were obtained for a mixture of PtP3 and PtP 4 (PtP 3: 18.3 mg, 0.0187 mmol; PtP 4: 34.9 mg, 0.0281 mmol; toluene-d8: 0.4 mL). The spectrum at 20 °C shows a broad band centred at 6 40.1 ppm, which is broadened even further at 100 °C. At -90 °C, two distinct 6ets of peaks are observed. The first set (6 50.07 ppm with satellites at 6 68.40 and 31.75 ppm) corresponds to PtP3, while the second set (a singlet at 6 9.64 ppm with two singlet satellites at 6 25.34 and -6.03 ppm at an intensity ratio of 1 : 4 : 1) is assigned to PtP 4. These results show that PtP 3 and PtP 4 are involved in an exchange process, involving transfer of PPI13 ligand over the temperature range 100 to -85 °C; at -90°C, the 3 1P{ 1H} spectra of PtP 3 and PtP 4 are "frozen" out. - 139 -50.00 X = 0PPh 3 68.36 31.44 - A L -r— 60 T 40 ppm Fig. 5-1. (a) The 3 1P{ JH} NMR spectrum of Pt(PPh 3) 3 in toluene-d8 at 20 °C. 50.07 68.40 X 31.75 40 i 60 ppm Fig. 5-1. (b) The 3 1P{ 1H} NMR spectrum of Pt(PPh 3) 3 in toluene-dg at -90 °C. - 140 -ioocc x = 0 P P h 3 X m 40 0 ppm 20°C -90°C - T -40 1 0 ppm 9.64 60.07 68.40 26.34 31.75 | Awl •6.03 40 1 — 0 ppm F i g . 5 - 2 . The 3 1P{ 1H} NMR spectra of a mixture of Pt(PPh 3) 3 and Pt(PPh 3) 4, (1 : 1.5 mole ratio) at 100, 20 and -90 "C in toluene-d8. - 141 -When one equivalent of PtP 3 (6.7 mg, 0.0068 mmol) and one equivalent of PPh 3 (1.8 mg, 0.0068 mmol) are mixed in toluene-dg (0.4 mL) only a very broad and flat band is observed in the 3 1P{ 1H} NMR spectrum at 20 °C; the band covers a range of about 25 ppm, centred at 6 ~36 ppm (Fig. 5-3). When the sample is cooled down to -20 °C, the band becomes even flatter, with the centre moving up-field. At -80 "C, the peaks of PtP 4 at 6 25.45, 9.70 and -6.03 ppm emerge and, at -90 °C, are fully established. The broad bands must reflect the occurrence of an exchange process involving association of PPh 3 to PtP 3 in toluene, and the reverse, because PtP 4 is the only species observed in the solution at -90 °C and because PtP 3 itself is stable toward further dissociation at ambient or lower temperatures. However, no PtP 3, PtP 4 or PPh 3 could be identified as descrete moieties on the 3 1P{ 1H} NMR time scale 1 6 5 when the temperature is higher than -80 "C. The similarity between the 3 1 P NMR behaviour of a mixture of P t P 3 and PPh 3 with that of a mixture of PtP 3 and PtP 4 provides further evidence for the exchange process: (K) PtP3 + P ik PtPt (5-2) Kb where k/ = the rate constant of the association reaction, kj, = the rate constant of the dissociation reaction, and K = k//kj,. - 142 -20° C X = 0PPh3 -20'C i -I 0 ppm 1 -80°C 40 i 0 ppm 9.64 .1 — -" 1 90°C 40 9.64 0 . J . — _ — , 40 0 PPm Fig. 5-3. The 3 1P{ 1H} NMR spectra of a mixture of Pt(PPh 3) 3 and PPh 3 (1:1 mole ratio) at different temperatures in toluene-d8. - 143 -Equilibrium 5-2 was further studied using different mole ratios of PtP3 to PPI13 at a particular temperature (22 ° C ) and the equilibrium constant, K, was then obtained as described below. The first sample contained PtP 3 mixed with 0.09 equivalent of PPI13 in 0.4 mL toluene-d8 (PtP 3: 8.63 mg, 0.0088 mmol; PPh 3: 0.20 mg, 0.0008 mmol) and the 3 1P{ 1H} NMR spectrum (Fig. 5-4) showed a broad but still distinguishable (with respect to the satellites) set of peaks (a major 48.1 ppm peak of 363 Hz width at half height and two satellites at 6 68.1 and 28.1 ppm). The spectrum of the second sample, a mixture of PtP3 and 0.7 equivalent of PPI13 in 0.5 mL toluene-dg (PtP 3: 11.7 mg, 0.012 mmol; PPh 3: 2.1 mg, 0.008 mmol), showed a broad band centred at 37.9 ppm. A mixture of PtP 3 and one equivalent of PPI13 in 0.4 mL toluene-dg (PtP 3: 6.7 mg, 0.0068 mmol; PPh 3: 1.8 mg, 0.0068 mmol), corresponding to the sample used for the data of Fig. 5-3, gave an even broader band centred at 6 34.4 ppm. At a 5.5 : 1 ratio (PtP 3: 7.7 mg, 0.0079 mmol; PPh 3: 11.3 mg, 0.043 mmol; toluene-dg: 0.5 mL), the observed band appeared up-field (6 10.1 ppm) and was sharpened somewhat relative to the peak for the third sample. The final sample used was a mixture of PtP 3 with 16 equivalent of PPI13 in 0.5 mL toluene-d8 (PtP 3: 9.1 mg, 0.0093 mmol; PPh 3: 39.3 mg, 0.15 mmol). A stronger peak (363 Hz in width at half height) appeared at 6 -0.4 ppm. Figure 5-4 shows all the relevant spectra from which the equilibrium constant (K) is readily calculated. The chemical shifts of phosphorus in PtP4, PtP3 and PPI13 are as follows, assuming that the chemical shift of PtP 4 at 22 "C is the same as at -90 °C: 6ptpt = 9.64 ppm; Sptp3 = 50.00 ppm; Spph3 = -5.88 ppm 5 0 . 0 0 - 144 -P t P 3 '• PPh 3 1 : 0 A . J -A L X = OPPh 3 Ptp3 = Pt(ppb3)3 1 : 0 0 9 1 « 0 - 7 37.9 * 1 : L O i>init< T 34.4 1 : 5-5 iii^ i>iWi»>n<iii^ <i|»ii^ i»H(ii>i<MKiin^ w»iXiiwii mH'»» 1 0 . 1 i i i i i M d i m i^^^^Miii twa in 11 T T 1 : 16 —r~ -0.4 80 6 0 ~~T~ 4 0 20 0 P Pm Fig. 5-4. The 3 1P{ ]H} NMR spectra of mixtures of Pt(PPh 3) 3 and PPh 3 at different mole ratios, 22 °C in toluene-d8. - 145 -In an exchange process of PPI13 between PtP3 and P t P 4 , the observed chemical shift, 60i,s (the centre of the band), is the mole-fraction weighted average of the chemical shifts of the three individual species involved at a given temperature. 1 4 5 That is, , 4[PzP 4] W 4 + 3 [ f t P3]6Ptp3 + [PPh3]SpPhi  o h s ~ 4 [ P t P 4 ] + 3[Pf P 3] + [PPh3] { > where [ P t P 4 ] , [ P t P 3 ] and [PPh 3] are the equilibrium concentrations of P t P 4 , P t P 3 and P P h 3 . Because P t P 4 was not added as such, [PtP3] = [PtP3}0 - [PtP4] ( 5 - 4 ) and [PPh3] = [PPh3)0 - [PtPA] (5 - 5) where [ P t P 3 ] 0 and [PPh3]0 are the i n i t i a l concentrations of PtP3 and PPI13. Substitution of eqs. 5-4, 5-5 and the chemical shifts Sptpt,6ptp3 and 6pph3 into eq. 5-3, gives eq. 5-6. _ 150.00[P*P3]o ~ 105.00[PtP 4] - 5.88[PP/t 3] 0 f . b ° b s - 3 [ P t P 3 ] c + [PPh3)0 ( 5 6 j Once the equilibrium concentration of P t P 4 is found using eq. 5-6, the K of eq. 5-2 can be readily obtained. Table 5-1 lists the K values calculated from the data for the five individual - 146 -samples at 22 °C. The average K value for the equilibrium (Pt(PPh 3) 3 + PPh 3 ^ Pt(PPh 3) 4) is 4.9 ±0.5 M - 1. The data (Table 5-1) show, for example, that 30% of PtP 3, in the presence of a five-fold excess of PPh 3 (at ~ 1 0 - 1 M), is converted to PtP 4. Table 5-1. K values for the equilibrium Pt(PPh 3] l 3 + PPh 3 ^ Pt(PPh 3) 4 at 22 °C in toluene-dg [PtP 3] 0 [PPh 3] 0 [PtP4]° K [PPh 3] 0/[PtP 3] 0 Conversion % M M ppm M M" 1 [PtP 4]/[PtP 3] 0 2.20 x 10" -2 1.91 x IO" 3 48.1 1.99 x IO - 4 5.3 0.09/1 1 2.38 x 10" •2 1.60 x IO" 2 37.9 1.54 x IO - 3 4.8 0.67/1 6 1.72 x 10" •2 1.70 x IO" 2 34.4 1.13 x IO" 3 4.4 0.99/1 7 1.57 x 10" 2 8.66 x IO - 2 10.1 4.69 x IO" 3 5.2 5.52/1 30 1.85 x 10 - 2 3.02 x 10 - 1 -0.4 1.07 x 10 - 2 4.7 16.32/1 58 a: [PtP 4] = calculated concentration of Pt(PPh 3) 4 at equilibrium. - 147 -The compound PtP4 in toluene was found to have the same NMR behaviour as a 1 : 1 mixture of PtP 3 and PPh 3. The 3 1P{ 1H} NMR spectra of PtP 4 (11.9 mg, 0.0096 mmol) in 0.4 mL toluene-dg at different temperatures are given in Fig. 5-5 (cf. Fig. 5-3). The broad band becomes flatter, and the centre of the band moves up-field, as the temperature is decreased from 20 to -80 °C. The broad band disappears below -80°C, and at -90°C a distinct peak at 6 9.70 ppm together with two satellites at 6 25.45 and -6.03 ppm are observed. The equilibrium constant for the dissociation of phosphine (Pt(PPh 3) 4 ^  Pt(PPh 3) 3 + PPh 3) at different temperatures was estimated using the chemical shift data as described above for the phosphine association to PtP 4 (eq. 5-3). In a solution of PtP 4 with no initial P t P 3 and PPh 3 added, [PtP4]0 = [PtP4] + [PtP3] (5 - 7) and, [PtP3] = [PPh3] (5 - 8) where [PtP 4] 0 is the initial concentration of PtP 4 and [PtP 3] and [PPh 3] are the concentrations of P t P 3 and PPh 3 at equilibrium. Substitutions of eqs. 5-7 and 5-8 and the values of 6ptpt, 6ptP3 and 6pph3 into eq. 5-3, gives , 38.56[PtP4]0 + 105.78[PtP3] '*>= AJPIPZ (5" 9 ) - 148 -20° C X= OPPh, X T . i,- Urur ui. i , i r v i i f f - ' i i i i . HM'1*"* n, , .L.y. i, r . . . . . . n r r | r Y ^ . -1 80 - 4 0 ° C i 40 X ... T _ 0 ppm i 80 1 40 1 0 ppm - 7 0 ° C 5-5. The 3 1P{ 1H} NMR spectra of Pt(PPh 3) 4 at different temperatures in toluene-d; - 149 -Table 5-2 summarizes the relevant data and gives the values of the equilibrium constants at various temperatures. Table 5-2. The equilibrium constant K* ( Pt(PPh 3) 4 ^ Pt(PPh 3) 3 + PPh 3 ) in toluene at various temperatures'1 Temp. "C Sobs-, ppm K\ M [PtP4]>, M [PtP 3] c, M +22d 2.0 x 10"1 +20 33.9 2.4 x 10 _ 1 0.20 x 10 - 2 2.20 x 10~2 -20e l . l x l O " 1 -40 29.1 5.0 x IO" 2 0.63 x 10" 2 1.77 x 10~2 -70 27.1 3.1 x 10~ 2 0.81 x 10~ 2 1.59 x IO" 2 -90' 9.5 1.3 x IO" 2 2.19 x IO" 2 2.1 x IO" 3 a: [PtP 4] 0 = 2.40 x 10 - 2 M, initial concentration of Pt(PPh 3) 4. b: [PtP 4]: concentration of Pt(PPh 3) 4 at equilibrium, c: [PtP 3]: concentration of Pt(PPh 3) 3 at equilibrium, d: The K' value is obtained from the average K values (4.9 M - 1 ) in Table 5-1. e: Data from Fig. 5-3 for a 1 : 1 ratio of PtP 3 and PPh 3. f: Data obtained from Fig. 5-6 by extrapolation of the line to -90 °C. The K' value at 20 °C is in excellent agreement with the K value for the reverse equilibrium, 4.9 M" 1 at 20 °C. - 150 -The enthalpy (AH°) and entropy (A5°) of the reaction Pt(PPh 3 ) 4 Pt(PPh 3) 3 + PPh 3 was obtained from the K' values at different temperatures (Tables 5-1 and 5-2) by constructing a plot of ln K' versus 1/T (Fig. 5-6) according to the standard van't Hoff equation. 1 6 6 The AH0 value was found to be 2.7 i 0.3 kcal mol" 1 and the A S 0 value 5.9 ± 0.6 e.u. The small and positive AH0 value indicates that the dissociation of PPh 3 from PtP 4 is an endothermic process, with presumably only a small amount energy required to break the Pt-phosphorus bond. This is consistent with the fact that a temperature of -90 °C is necessary to prevent the dissociation of PtP 4 in toluene. The positive AS0 for the equilibrium reaction is consistent with a dissociation process. The quite small value of AS0 is perhaps a little surprising considering that the forward reaction generates two molecules from one reactant molecule. More effective solvation of the free (rather than coordinated) PPh 3 would lead to a lowering of the AS0 value. Ligand-exchange studies on zerovalent nickel complexes Ni(PR 3) 4 have shown that the phosphorus bond strength is primarily a consequence of steric effects. 1 6 2 The large ligand cone angle of 145° for PPh 3 makes coordination of four phosphin in a tetrahedral array extremely difficult. That dissociation is due to steric rather than electronic effects is nicely illustrated by the behaviour of Ni(PMe 3) 4. The PMe 3 ligand is a better donor than PPh 3 and builds more negative charge on the Ni centre so that the ligand dissociation of Ni(PMe 3) 4 could be easier than Ni(PPh 3) 4. However, Ni(PMe 3) 4 shows no evidence of ligand dissociation in solution because the ligand cone angle of PMe 3 is 118° having less steric effect on the ligand dissociation than that of PPh 3. 1 6 2 - 151 -2 . 0 0 0 0 0 A H 0 - 2 - 7 k c a l m o l AS" = 5 . 9 c a l m o f ' K " ' XL c 2-00 - 400 -6.00 1 300 4 0 0 5 0 0 i ( X 10 3 ) , K 600 Fig. 5-6. The temperature dependence of the equilibrium constant, K\ for Pt(PPh 3) 4 ^ Pt(PPh 3) 3 + PPh 3. - 152-5-4. ^ P t ^ H } N M R spectra of Pt(PPh 3 ) 3 and Pt(PPh 3) 4 in toluene In order to obtain additional evidence for the equilibrium taking place in a toluene solution of PtP 4, the 1 9 5Pt^H} NMR spectra of toluene-d8 solutions of PtP 3 and PtP4 were taken on a Varian XL 300 spectrometer at 25 °C. The scale of the chemical shift for the PtP n complexes was set by choosing a frequency of 64.2 MHz as the point of zero ppm for 1 9 5P{1H} NMR spectra;126" on this scale, 1 ppm equals 64.2 Hz. The 195Pt{1H} NMR spectrum of PtP3 shows a quartet centred at 6 -224 ppm with 3pt-p = 4492 Hz (Fig. 5-7); the data are consistent with the 31P{JH} NMR spectrum of the same sample, where Jpt-p was measured as 4467 Hz (Fig. 5-1 (a)). The 1 9 5Pt{1H} NMR spectrum of PtP4 shows a broad band centred at about S -233 ppm as presented in Fig. 5-8; the same sample displayed a broad band in the 31P{1H} NMR spectrum (Fig. 5-5). This observation is another indication of the presence of the equilibrium between PtP4 and PtP3, discussed in the previous sections. * - 2 2 4 ppm , J 4 4 9 2 Hz 4 0 0 2 0 0 0 - 2 0 0 - 4 0 0 p p m Fig. 5-7. The 1 9 5Pt{1H} NMR spectrum of Pt(PPh3)3 in toluene-d8 at 25 °C. - 153 -Fig. 5-8. The "Spt-pH} NMR spectrum of Pt(PPh 3) 4 in toluene-d8 at 25 °C. 5-5. A study of the equilibrium of P t ( P P h 3 ) n species by electronic spectroscopy Electronic spectroscopy studies (TJV-vis) on P t P n species were carried out in benzene because some subsequent solution (kinetics and oxygenation) were carried out in this solvent (Chapter 6); the NMR studies was performed in toluene-da in order to have a large range of temperature variation. A benzene solution of PtP 3 (PtP 3: 3.9 mg, 0.004 mmol; benzene: 4 mL) is brownish yellow and has two strong absorption bands at 298 and 332 nm as shown in Fig. 5-9; the extinction coefficients at the two wavelengths are 2.5 x 104 and 2.6 x 104 M - 1cm _ 1, respectively, and are indicative that the absorption bands result from charge transfer from metal to phosphine ligand. 1 6 2 The UV-vis data are comparable with the literature data (2.9 x 104 M _ 1 c m - 1 at - 154 -298 nm and 3.0 x 10* M - 1 c m _ 1 at 332 nm. 1 6 2 The trough at 282 nm perhaps qualitatively indicates that there is little free PPh 3 in the solution, because PPI13 has a high extinction cofficient (8.74 x 103 M _ 1 c m _ 1 ) at 282 nm. Addition of PPh 3 to a benzene solution of PtP 3 (9.53 mg, 0.0097 mmol in 20 mL benzene) gave the spectral changes shown in Fig. 5-10. The absorbance at 332 nm remained essentially unchanged during the addition of PPh 3 and indeed the spectra are, within experimental error, the sum of those of PtP 3 and PPI13. There is no evidence for formation of PtP 4 at the concentrations used. Using the K value of 4.9 M " 1 for the phosphine association measured in toluene at 22 °C leads to the same conclusion: even at the 5 : 1 phosphine excess, the amount of PtP 4 present would be about 1.2% of the total platinum. The UV-vis studies show that the addition of PPh 3 to a 5 : 1 ratio of [PPI13] : [PtP3] gives no significant formation of PtP 4 in benzene when the initial concentration of PtP3 is in the range of 1 x 10 - 3 to 1 x 10"2 M at ambient temperature. 2 - 0 0 2 9 8 3 3 2 8 1 1 % 100 c CO 3 0 0 4 0 0 5 0 0 Wavelength, nm Fig. 5-9. The UV-vis spectrum of Pt(PPh 3) 3 in benzene at 22 °C. - 155-248 312 408 5 0 4 Wavelength, nm Fig. 5-10. The UV-vis spectra of mixtures of Pt(PPh 3) 3 (4.86 x IO - 4 M) with PPh 3 at different mole ratios in benzene at 22 °C. - 156 -The UV-vis spectrum of a benzene solution of PtP 4 (PtP 4: 3.73 mg, 0.003 mmol; benzene: 3 mL) is similar to that of PtP 3, and is essentially simply the sum of the spectra of PtP 3 and one mole equivalent of PPh 3. At this concentration, the K value (toluene) indicates that about 99.5% of the original PtP 4 should dissociate into PtP 3 and PPh 3. Addition of PPh 3 to solutions of PtP 4 duplicated the data of Fig. 5-10. 300 400 500 Wavelength, nm Fig. 5-11. The UV-vis spectrum of Pt(PPh 3) 4 in benzene at 22 °C. - 157 -5-6. Summary Studies on the equilibrium of P t P n species in benzene and in toluene using UV-vis spec-trophotometry and 3 1P{ 1H} NMR, particularly the latter, provided quantitative knowledge about the nature of the well-known equilibrium Pt(PPh 3) 4 ^ Pt(PPh 3)3 + PPh 3. Thermo-dynamic data forthe equilibrium have been obtained; the data show that PtP 4 at concentra-tions upto ~ 1 0 - 2 dissociates 'completely' to PtP 3 and PPh 3 at ambient temperatures and the exchange rate of the PPh 3 between PtP 4 and PtP 3 depends on temperature. When the tem-perature is low enough (-90 °C), little dissociation takes place; at 22 °C, K for dissociation was determined as 0.2 M. The number may be compared with results of the studies on tertiary phosphine exchange for other P t L 4 species (L = PMe 2Ph, PEt 3 and PBu 3 n) in toluene; 1 2 6* here, the equilibrium constant for PtP 4 ^ P t L 3 + L varies from 8 x 10 - 6 M (L = PMePh 2) to 4 M ( L = PEt 3) to 24 M (L = PBu 3 n) at 22 °C. No correlation between the size of the tertiary phosphine and the equilibrium constant can be established. With a relatively high concentra-tion of PtP 3 (> IO - 2 M), PtP 3 will be transformed to PtP 4 to a significant degree when PPh 3 is added and PtP 4 will become a predominant species even at ambient temperatures, according to the equilibrium constant obtained in the present work (see Table 5-1). - 158 -C H A P T E R 6 A KINETIC A N D MECHANISTIC STUDY OF TRIS- A N D TETRAKIS-(TRIPHENYLPHOSPHINE)PLATINUM(0) SPECIES IN OXYGENATION-PROTONATION PROCESSES 6-1. Introduction The protonation of Pt(0 2)(PPh 3) 2 (abbreviated as 0 2 P t P 2 ) using carboxylic acids has been studied 1 0 8' 1 6 7 and a stepwise mechanism involving the formation of a hydroperoxo plat-inum complex was suggested (eq. 6-1) by Muto and Kamiya. 1 0 8 In support of this mechanism, they prepared the triphenylmethyl hydroperoxide in a yield of 83.5% by the successive equimo-lar addition of CH 2C1C00H and triphenylmethyl bromide (TPMB) to 0 2 P t P 2 in CH 2C1 2 at -70 °C (eq. 6-2), and then characterized the triphenylmethyl hydroperoxide using IR for the hydroperoxy moiety y(OO-H) at 3490 cm - 1; this band shifted to a higher frequency, 3540 cm - 1, when the hydroperoxide was treated with excess PPh 3 due to reduction to the alcohol. In the present studies, in attempts to obtain direct evidence for the formation of the Pt(OOH) species, the protonation of 0 2 P t P 2 using HCl was carried out at 20 and -70 °C, the systems being analyzed by 1E and 3 1 P NMR, FT-IR, and iodometry. Pt(02)(PPh3)2 CH2ClCOOH Pt(OOH)(OCOCH2Cl)(PPh3)2 CH2CICOOH ( 6 - 1 ) Pt(OCOCH2Cl)2{PPh3)2 + H202 Pt(02)(PPh3)2 CH7ClCOOH Pt(OOH)(OCOCH2Cl)(PPh3)2 ( 6 - 2 ) ™* Pt(OCOCH2Cl)Br(PPh3)2 + Ph3COOH - 159 -Another aspect related to the chemistry of platinum peroxides is the kinetics and mecha-nism for the oxygenation of P t P n (n=3 or 4). Halpern et al. have studied the kinetics (using UV-vis spectroscopy and gas-uptake techniques) and the mechanism of the oxygenation of PtP 4 in d e t a i l , 1 1 , 1 2 4 , 1 6 0 and suggest that PtP 3, formed from the dissociation of PtP 4, reacts with molecular oxygen to form 0 2 P t P 2 and PPh 3 in the first step (eq. 6-3), while the formation of 0 2 P t P 2 via a dissociative pathway from PtP 3 (eq. 6-4) is not involved. The reaction be-tween 0 2 P t P 2 and free PPh 3 then follows to give PtP 3 and OPPh 3 as the second step (eq. 6-5), which is considered to take place via free peroxide formation, similar to a mechanism estab-lished, based on 3 1 P NMR studies at low temperature, for the reaction between P t ( 0 2 ) ( P R 3 ) 2 and free PR 3 (PR 3 = PMePh 2 or PMe 2Ph) in ethanol (eqs. 6-6 to 6-8).12' Pt(PPh3)3 + 02 — > Pt{02)(PPh3)2 + PPh3 Pt(PPh3)3 ^ Pt(PPh3)2 + PPh3-^Pt(02)(PPh3)2 Pt(02)(PPh3)2 P-^3 Pt{PPh3)3 + OPPh3 Pt(02)(PR3)2 + PR3 —- Pt(02)(PR3)3 PRtjROH ptrPR3y+ + HO- + R>0-H02 + PR3 —• OPR3 + OH~ Pt{PR3)\+ PR><02L^>R0: pt(PR3)t + OPM3 + R'OH PR 3 = PMePh 2, PMe 2Ph, and R'OH = C 2H 5OH. (6-3) (6-4) (6-5) (6-6) (6-7) (6-8) - 160 -Direct oxygen transfer from the metal dioxygen adduct to the PPI13 substrate is not in-volved and the effective oxidants are platinum(II) and free peroxide generated by nudeophilic attack of the phosphine at the metal (eq. 6-6). Olefinic substrates are not sufficiently strong nucleophiles to replace peroxide and are not oxidized by C<2PtP 2 . 1 2 4 In the present work, results attained during the 3 1 PI1!!} NMR and UV-vis studies on the oxygenation-protonation process involving P t P n compounds supplement data available on the processes outlined in eqs. 6-1 to 6-5 and give more direct evidence for reactions 6-3 to 6-5. 6-2. Kinetics and mechanism of oxygenation reactions of Pt(PPh3)n compounds 6-2-1. Reaction between P t ( P P h 3 ) 3 and 0 2 A benzene solution (10 mL) of PtP 3 (PtPs: 11.3 mg, 0.012 mmol) was reacted with 1 atm oxygen at 25 "C for 20 minutes when the yellow colour changed to pale orange-brown, indicating the completion of the reaction and this was confirmed by UV-vis spectroscopy. The solvent was then removed under vacuum and a mixture of pale orange and white powders was obtained. The 3 1P{ JH} NMR spectrum of this mixture in toluene-dg indicated that the only phosphine-containing products were 0 2 P t P 2 and OPPh 3, as shown in Fig. 6-1; the 3 1 P NMR data for Pt(C>2)(PPh3)2 agree with those reported in the literature. 1 2 4 In order to learn more details about this reaction, a 1 : 1 Pt : 0 2 experiment was monitored by 3 1P{ 1H) NMR. A toluene-dg solution (0.4 mL) of PtP 3 (10.2 mg, 0.01 mmol) was prepared in an NMR tube under argon and the tube was stoppered by a rubber septum, and oxygen gas (1 atm, 0.23 mL, 0.01 mmol) introduced into the gas phase by means of a gas-tight syringe. The tube was shaken to obtain a good mixing of the O2 with the reaction solution. After 8 hours at 20 °C, a - 161 -s m a l l a m o u n t o f C*2PtP2 was p r e s e n t , a n d i n t e r e s t i n g l y , n o OPPI13 was observed ( F i g . 6-2 ( a ) ) . T h e peaks c o r r e s p o n d i n g t o PtP3 were b r o a d e n e d a n d , as d iscussed i n C h a p t e r 5, t h i s i n d i c a t e s a n e x c h a n g e o f P P h 3 b e t w e e n P t P 4 a n d P t P 3 ; t h a t i s , t h e P P h 3 f o r m e d c o n c o m i t a n t l y w i t h t h e 0 2 P t P 2 m u s t b e i n v o l v e d i n t h e r a p i d e x c h a n g e process w i t h t h e u n r e a c t e d P t P 3 . A f t e r 2 4 h o u r s , t h e C«2PtP2 h a d t o t a l l y d i s a p p e a r e d , PtP3 r e m a i n e d w i t h n o b r o a d e n i n g o f t h e 3 1 P p e a k s , a n d O P P h 3 w a s o b s e r v e d ( F i g . 6-2 ( b ) ) . T h e e x p e r i m e n t a l f i n d i n g s c o n f i r m t h a t t h e f i r s t s tep o f t h e r e a c t i o n be tween PtP3 a n d O2 is t h e f o r m a t i o n o f 0 2 P t P 2 a n d PPI13 ( e q . 6 - 3 ) , w h i l e t h e subsequen t f o r m a t i o n o f OPPI13 p r o b a b l y o c c u r r s v i a r e a c t i o n o f 0 2 P t P 2 a n d P P h 3 ( e q . 6 - 5 ) . R e a c t i o n b e t w e e n 0 2 P t P 2 a n d PtP3 does n o t t a k e p l a c e , as d iscussed b e l o w . A: Pt(Oa)(PPh3)2 17.13 ppm, J|»,_p m 4069 H z B : OPPhs, 24.31 ppm B A J l A A 60 30 0 ppm Fig. 6-1. T h e 31P{1H} N M R s p e c t r u m o f t h e res idue f r o m t h e r e a c t i o n o f P t ( P P h 3 ) 3 w i t h 0 2 , i n t o l u e n e - d g a t 20 ° C . - 162 -(a) After 8 h A: Pt(PPh3)3 + PPlia Pt(PPl»3)4 B: Pt(0 3)(PPh«) 2 17.13 ppm, 2pt-p m 4069 Hi (b) After 24k C: Pt(PPha)3 50.00 ppm, Jpt-p = 4467 Ha D: OPPhs, 24.31 ppm X J U , 6 0 3 0 0 ppm Fig. 6-2. The 3 1P{ 1H} NMR spectra for the reaction of Pt(PPh 3) 3 with 0 2 (an equimolar amount in gas phase) in toluene-ds at 20 °C. - 163 -No 3 1P{ aH} NMR study on the reaction between 0 2 P t P 2 and PPh 3 itself has been reported in the literature, and such evidence was thus sought to confirm the formation of PtP 3 and OPPh 3, as assumed in the earlier work. 1 6 0 , 1 6 1 The first test solution was prepared by dissolving one equivalent of 0 2 P t P 2 and 3.3 equivalents of PPh 3 in toluene-dg (0.4 mL) in a vacuum-sealed NMR tube ( 0 2 P t P 2 : 5.4 mg, 0.007 mmol; PPh 3: 6.3 mg, 0.024 mmol). At 22 °C, within the initial half an hour, the colour of the reaction solution changed from pale brown to yellow, while the 3 1P{ ]H} NMR showed a strong sharp peak corresponding to OPPh 3 and a very broad and flat band centred at 6 32.6 ppm, as shown in Fig. 6-3. No 0 2 P t P 2 remained and the flat band is attributed to phosphine exchange between PtP 3 and PtP 4. The data show that reaction of 0 2 P t P 2 with PPh 3 produces PtP 3 and OPPh 3. A second solution was prepared by dissolving one equivalent of 0 2PtP 2, 1.4 equivalents of PPh 3 and 2.1 equivalents of PtP 3 in toluene-ds (0.4 mL) in a vacuum-sealed NMR tube (0 2PtP 2: 3.2 mg, 0.0043 mmol; PPh 3: 1.6 mg, 0.0059 mmol; PtP 3: 8.9 mg, 0.0091 mmol). After one hour, the 3 1P{ JH} NMR data (Fig. 6-4) showed the presence of PtP 3, OPPh 3 and 0 2 P t P 2 and the absence of free PPh 3. Furthermore, the excess 0 2 P t P 2 did not disappear in the presence of the relatively large amount of PtP 3 even after a few days. This further confirms that PtP 3 does not readily dissociate to PtP 2 and PPh 3, even if there is a driving force (i.e., the reaction of 0 2 P t P 2 with PPh 3) that converts the PPh 3 to OPPh 3. Consequently, compatible with earlier conclusions based on kinetic data, 1 6 1 0 2 P t P 2 is not readily produced in the process described in eq. 6-4. - 164 -24.31 PtP8 = Pt(PPha)3 PtP4 = Pt(PPh«)4 OPPh3 PtP3 + PPh3 PtP4 unknown *Ui Mi*m*»Mim**mJ+ ****** ~ i — 60 1— 30 0 ppm Fig. 6-3. The 3 1P{ 1H} NMR spectrum of the solution resulting from the reaction between Pt(0 2)(PPh 3) 2 and PPh 3 (1 : 3.3 mole ratio) in toluene-d8 at 22 °C. A A Kill>»^'l^<'i»Mi»il)M1l# W ^iK'H|)t»'>|iiWMi^ A: Pt(PPh«)3 50.00 ppm, Jpt-P = 4467 Hz B: OPPhe, 14.31 ppm C: Pt(02)(PPha)2 17.13 ppm B C I 3 0 i 6 0 0 ppm Fig. 6-4. The 3 1P{ 1H} NMR spectrum of the solution resulting from a mixture of Pt(0 2)(PPh 3) 2, PPh 3 and Pt(PPh 3) 3 in toluene-dg at 22 °C, at a 1 : 1.4 : 2.1 mole ratio. - 165 -6-2-2 Kinetics of the reaction of P t ( 0 2 ) ( P P h 3 ) 2 with P P h 3 In order to obtain more information related to the mechanism of the reaction between PtP 3 and 0 2, some kinetics on the reaction between 0 2 P t P 2 and PPh 3 were re-examined. A general procedure for the kinetic study was as follows. A known amount of 0 2 P t P 2 was charged into a special round bottom flask having an optical cell (0.1 mm pathlength) as a side-arm. Benzene (5 mL) was added into the flask under argon, and a known amount of PPh 3 was placed in a small glass bucket (4 mm in diameter and 8 mm in height), which was suspended via a small hook, that was part of a ground glass stopper for the reaction flask. When the 0 2 P t P 2 had dissolved to form a pale brown solution, the bucket was dropped into the solution by turning the ground glass stopper and a vigorous shaking was applied to obtain a rapid dissolution of PPh 3. The flask was immediately placed into a spectrophotometer on which the kinetic data were recorded. Under the reaction conditions (Table 6-1), adequate kinetic data were obtained within a period of 20 minutes when the resulting solution was yellow in colour, indicating the presence of PtP 3. Five individual runs with different mole ratios of PPh 3 to 0 2 P t P 2 were performed at 25 °C. The curve recorded on the spectrophotometer at a fixed wavelength (400 or 360 nm at which the 0 2 P t P 2 solution has little absorbance) reflects the increasing concentration of PtP 3 formed as the reaction proceeds and the kinetics of the reaction were readily monitored. The concentration of PPh 3 was kept in great excess over that of 0 2 P t P 2 and the absorbance data obtained fitted nicely eq. 3-10, the expression for pseudo first-order kinetic behaviour in 0 2 P t P 2 (see Section 3-2-2). An example of the results obtained is given in Figs. 6-5 (a) and (b), the pseudo first-order rate constants being summarized in Table 6-1. - 166 -^ 0 3 0 E c o o g 0-15 CO -Q o G O .o < 0 .00 O 252 504 756 1008 1260 Time » s Fig. 6-5. (a) The absorption curve obtained on a spectrophotometer for the reaction between Pt(0 2)(PPh 3) 2 and PPh 3 (1 : 18.6 mole ratio) in benzene at 25 °C; the upper lines represent data at longer time periods. - 3 - 0 0 L 0 253 506 759 1012 1200 Time. s F i g . 6-5. (b) The pseudo first-order kinetic behaviour in Pt(0 2)(PPh 3) 2 for the reaction between Pt(0 2)(PPh 3) 2 and PPh 3 (1: 18.6 mole ratio) in benzene at 25 °C. - 167 -Table 6 -1 . The pseudo first-order rate constants for the reaction between Pt(0 2)(PPh 3) 2 and PPh 3 in benzene at 25 °C No. [ 0 2 P t P 2 ] 0 * x 10\ M [PPh 3] 0° x 10 2,M [PPh 3] 0/[0 2PtP 2] 0 Kb x 103, s _ 1 1 7.43 1.46 19.7/1 1.88 2 7.56 1.86 24.6/1 2.63 3 8.79 1.24 14.1/1 1.72 4 5.78 1.70 29.4/1 2.35 5 7.38 1.37 18.6/1 1.77 a: [ 0 2 P t P 2 ] 0 and [PPh 3] 0 are the initial concentrations of Pt(0 2)(PPh 3) 2 and PPh 3, respec-tively. The pseudo first-order rate constant is proportional to the concentration of PPh 3 (Table 6-1, Fig. 6-6). The second order rate constant for the reaction is 0.14 ± 0.01 M _ 1 s - 1 , which is in excellent agreement with the literature data (0.15 ± 0.01 M _ 1 s - 1 ) , also determined in benzene at 25 °C by UV-vis data; 1 6 0 however, no details (e.g. data on [PPh3]) were given in this report. 1 6 0 The kinetic information shows that the second step in the oxygenation of PtP 4 (i.e. the reaction of 0 2 P t P 2 with PPh 3) contains a rate-determining reaction which is first-order in both 0 2 P t P 2 and PPh 3, entirely consistent with the proposal that the step involves nudeophilic attack by PPh 3 to replace the dioxygen as peroxide. 1 2 4 The NMR data show that v the products are PtP 3 and OPPh 3. - 168 -Fig. 6-6. The first order-dependence on [PPI13] for the reaction between Pt(0 2)(PPh 3 )2 and PPh 3 in benzene at 25 °C. - 169 -6-2-3. Kinetics of the reaction of P t ( P P h 3 ) 3 with 0 2 As described in Section 6-2-1, the reaction of PtP 3 with 0 2 gives 0 2 P t P 2 and PPh 3 in the first step (eq. 6-3), and the 0 2 P t P 2 then reacts with free PPh 3 to form P t P 3 and OPPh 3 in the second step (eq. 6-5). Therefore, 'clear' kinetic information for the completed first step is not readily obtained experimentally. However, the rate of the second step is much slower than that of the first step; for example, complete conversion of 0 2PtP2 (~0.001 M) in the presence of excess PPh 3 (~0.02 M) takes ~50 minutes, while complete conversion of PtP 3 (~0.001 M) to the peroxide by 0 2 (1 atm) takes ~5 minutes under similar reaction conditions. Hence, the rate constant for the first step can be obtained using kinetic data for the initial period of the reaction (~3 minutes). The reactions were carried out using the same flask and a procedure similar to that described in Section 6-2-2; benzene in the flask was degassed three times, while PtP 3 was kept in the small bucket suspended via the glass hook of the flask. Oxygen (1 atm) was introduced into the flask, and then the bucket was dropped into the benzene with vigorous shaking to completely dissolve the PtP 3. The cell side-arm of the flask was immediately placed into a spectrophotometer on which the kinetic data were recorded as absorption curves versus time at a fixed wavelength (332 nm) (Fig. 6-7 (a)). Pseudo first-order kinetics (eq. 3-10, Section 3-2-2) with respect to PtP 3, under 0 2 at 1 atm, are obtained (Fig. 6-7 (b)) and the second-order rate constant (assuming a first-order dependence on [0 2]) is 0.9±0.2 M _ 1 s _ 1 using [0 2] = 0.009 M at 25 °C, the data being obtained from three individual runs (Run 1: PtP 3: 2.1 mg, 0.002 mmol; benzene: 5 mL; Run 2: PtP 3: 11.2 mg, 0.011 mmol; benzene: 10 mL; Run 3: PtP 3: 11.6 mg, 0.012 mmol; benzene: 10 mL). The [0 2] is that given in the literature (Section 2-6). The rate constant obtained in the present studies is lower than that (2.6 ± 0.1 M _ 1 s _ 1 using - 170 -an experimental [O2] = 0.0065 M) obtained by Halpern's group using a spectrophotometric method (UV-vis); 1 6 0 using Halpern's [O2] value 'adjusts' our rate constant to be 1.3 ± 0.3 M _ 1 s - 1 . The method used by Birk et a l 1 6 0 was less direct as far as the starting platinum complexes were concerned; an 1 : 1 mixture of Pt(PPh3)2(C 2H 4) and PPh 3 or Pt(PPh 3) 4 was used in Birk's work." CM CO CO 8 c CO o CO .0 < 2.57 1.71 0.86 0 Time, s 150 225 Fig. 6-7. (a) The absorption curve obtained on a spectrophotometer for the reaction between Pt(PPh 3) 3 and 0 2 (1 atm) in benzene at 25 °C; the lower lines represent data at longer time periods. - 171 -Fig. 6 - 7 . (b) The pseudo first-order kinetic behaviour in Pt(PPh 3) 3 for the reaction between Pt(PPh 3) 3 and 0 2 (1 atm) in benzene at 25 °C. - 172 -6-2-4. Mechanism of the reaction of P t ( P P h 3 ) 3 with 0 2 The findings reported in this thesis (Section 6-2-1 to 6-2-3) fully support the conclusions of Halpern et a l . 1 2 4 - 1 6 0 - 1 6 1 r e g a r ( } i n g the mechanism for the reaction of P t (PPh 3 ) n (n = 3 or 4) with 0 2. The 'extra' equilibrium Pt(PPh 3 ) 3 + P P h 3 ^ Pt(PPh 3 ) 4 should be included for a more correct overall picture but, as this equilibrium is rapid and lies very much to the left, its presence does not affect the 'practical' kinetics observed. 6-2-5 Reaction between P t ( P P h 3 ) 4 and 0 2 The study on the equilibrium between PtP 4 and P tP 3 described in Chapter 5 demonstrated that P tP 4 dissociates almost totally to P tP 3 and P P h 3 in benzene when the concentration of P tP 4 is < IO - 3 M. Hence, the chemical behaviour of P tP 4 in benzene towards 0 2 is identical to that of P t P 3 in benzene. A 3 1 P { J H } spectrum of the residue (in toluene-dg) obtained from the reaction of P tP 4 with 0 2 (1 atm) in benzene at 25 °C (PtP 4 : 14.9 mg, 0.012 mmol; benzene: 10 mL; reaction time: 20 minutes) is given in Fig. 6-8. Comparison between Fig. 6-1 and Fig. 6-8 confirms that the identical products ( 0 2 P t P 2 and OPPh 3 ) are formed. B A: Pt(02)(PPh3)2 17.13 ppm, ipt-p = 4069 Hz B: OPPhs, 24.31 ppm A A A X X 60 30 0 ppm Fig. 6-8. The 3 1 P { 1 H } NMR spectrum of the residue from the reaction of Pt(PPh 3 ) 4 with 0 2 , in toluene-dg at 20 °C. - 173 -6-3. Reaction between P t ( 0 2 ) ( P P h 3 ) 2 and HC1 The reaction of 0 2 P t P 2 with HC1 was carried out at 20 and -72 °C in order to find its stoichiometry and, possibly, mechanistic information on the reaction. A procedure for titration of HC1 against 0 2 P t P 2 was followed as described below. A CD 2C1 2 solution containing a known amount of 0 2 P t P 2 was prepared under argon in an NMR tube stoppered by a rubber septum. A known, calculated amount of HC1 gas at 1 atm was introduced into the NMR tube by means of a gas-tight syringe at 20 and -72 °C, and the 3 1 P{*H} spectra recorded for the resulting solutions of different molar ratios of HC1 to 0 2 P t P 2 at the corresponding temperature. Fig. 6-9 shows the results of the titration experiments (0 2PtP 2: 1.36 mg, 0.0018 mmol; CD 2C1 2: 0.4 mL) at one and two equimolar additions of HC1 at 20 "C. The only phosphine-containing product found was identified as cis-PtCl 2(PPli3) 2 by comparison with the 3 1P{ 1H} NMR data for an authentic sample. When the ratio of 0 2 P t P 2 to HC1 was 1 : 1, half of the starting 0 2 P t P 2 had reacted, and when the ratio was 1 : 2, all the starting 0 2 P t P 2 was converted to cis-PtCl 2(PPh 3) 2. At -72 °C, the 3 1P{ 1H} NMR spectrum (Fig. 6-10 (b)) for the 1 : 1 reaction ( 0 2 P t P 2 : 4.4 mg, 0.0058 mmol; CD 2C1 2: 0.6 mL) again shows that about half of the starting 0 2 P t P 2 has reacted to give cis-PtCl 2(PPh 3) 2, while the 3 1P{ 1H} NMR spectrum for the 2 : 1 acid/platinum reaction (Fig. 6-10 (c)) shows that nearly all the 0 2 P t P 2 has reacted and the only phosphine-containing product is cis-PtCl 2(PPh 3) 2. Therefore, at 20 and at -72 °C, the only stoichiometry observed for consumption of 0 2 P t P 2 by protonation using HC1 is two moles of acid per mole of oxygenated species. - 174 -Pt(02)(PPh8)2 : HCa (a) 1 • 0 A; Pt(02)(PPba)2 15.37 ppm, ht-P ~ 4031 Hz B: PtCl3(PPhs)2 14~32 ppm, Jpt-p = 8670 Hi —i— i —i— i f - ] —i i i | | — i —i i I—|—i—i—r—|—i—i—I I | |— i—i— i— I—I— •— i— I—I—I—• i I I—I—<—>—r _ r" - o ppm 30 (b) 1:1 1 5 A » B = 1 «1 t i i | i i l i | i i i i | i i h | i i i i | l i H n i i | i i i i | ) i > l | * i > | i | < i | 1 5 A B J ..iiu:<. . ii. iLlI , .-lit i. I . 1, I j i J l.l^y^lj^/fc^l B 30 (O 1 . 2 1 5 B Si ppm B B i " • . '!* T " i I I I i I • I I • • • I i • • j I I ' • • I I 1 ' ' ' ' 1 ' I ' ' ' ' p D m 3 0 1 5 0 P P M Fig. 6-9. The 3 1P{ 1H} NMR spectra for the reaction between Pt(0 2)(PPh 3) 2 and HC1 (1 : 1 and 1 : 2) in C D 2 C 1 2 at 20 ° C . Pt (0 2 ) (PPh 3 ) 2 « HC l ( a ) 1 0 175 -A A X J U L A « Pt(o2)(PPh3)2 15 37 p p m ^^=4031 Hz B : PtClalPPh»)a 14.32 PPm Jpt-P = 3670 Hz X V OPPhs, a n i m p u r i t y in the s ta r t ing mate r ia l A ,ui, „ 4 . J •»•> B rijx. B ( O 1:2 B / X • A i l - - . A NTVJ^ B B JL h ^ - r I ' M 30 i ' ' ' i ) ' < i ' i ' i i ' i i i ' ' i ' i i i i II i i i ' i ' i i ' • ' is o ppm Fig. 6-10. The 3 1P{ 1H} NMR spectra for the reaction between Pt(0 2)(PPh 3) 2 and HCl (1 : 1 and 1 : 2) in CD 2C1 2 at -72 °C. - 176 -The oxygen-containing products of the reaction of C*2PtP2 with HCl were also analyzed for by iodometry and *H NMR. A CH 2C1 2 solution (1.5 mL) of 0 2PtP2 was prepared under argon in a Shlenk tube stoppered with a rubber septum (0 2PtP 2: 3.4 mg, 0.0028 mmol) and two equivalents of HCl (1 atm) introduced using a gas-tight syringe. After 5 minutes, H2O2 was found in a yield of 88% based on the starting C*2PtP2 concentration. In a blank, control experiment, no H2O2 was found. Further evidence for the formation of H2O2 was obtained from a ! H NMR study of the reaction in CD2CI2 at 20 °C, showing the stoichiometrical formation of H2O2 in either 1 : 1 or 2 : 1 reaction, as described in Section 2-5-2. For the 2 : 1 reaction at -72 °C, the *H NMR analysis for H2O2 was carried out after the resulting solution was warmed to 20 °C. The ]H NMR spectrum (Fig. 6-11) shows a broad band in the 6 1.3 to 2.1 ppm region (90 Hz width at half height), where only a H2O peak (6 1.56 ppm) was observed before the reaction; this indicates the formation of H2O2 in a yield of 91% based on the starting C«2PtP2 (estimated using the equation in Table 2-1 in Section 2-5-2). Another reaction of 0 2 P t P 2 with HCl ( 0 2 P t P 2 : 7.0 mg, 0.0057 mmol; CD 2C1 2: 0.5 mL; 0 2 P t P 2 : HCl = 1 : 2; 20 °C) was carried out and analyzed by 3 1P{ 1H} to confirm the completion of the reaction. In this case, the solvent was removed to obtain the residue (a white powder) for FT-IR analysis and two bands at 315 and 294 cm - 1, corresponding to the symmetrical and asymmetrical stretching modes of the two cis Pt-Cl bonds, were observed (literature data are 319 and 295 c m - 1 1 6 8 ) . Therefore, reaction of 0 2 P t P 2 with HCl can be described as in eq. 6-6. Pt(02)(PPh3)2 + 2 HCl = H202 + cis - PtCl2(PPh3)2 (6 - 9) The results obtained in the present work agree with an earlier study of the reaction of - 177 -0 2 P t P 2 with C H 3 C O O H which gave cis-Pt(OCOCH 3) 2(PPh 3) 2 and H 20 2. 1 6 7 Also, our results agree with the work describing the reaction of other carboxylic acids with 0 2 P t P 2 at low temperature (-70 °C) to give selective formation of H 2 0 2 and the dicarboxylato platinum(II) complex, as reported by Muto and Kamiya. 1 0 8 However, direct evidence for the Pt-OOH species using 3 1 P NMR, to substantiate further the mechanism suggested by Muto and Kamiya (eq. 6-1), was not obtained. H 2 0 , H 2 0 2 T 1 I 1 1 1 1 I 1 1 1 1 1 1 1 1 1 1 1 1 r 1 1 1 1 1 1 1 1 1 1 1 I 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 5.0 4.0 3.0 2.0 1.0 PPM I I • I I 1 I I I 6.0 X = res idue in C D 2 C I 2 Fig. 6-11. The J H NMR spectrum at 20 °C for the solution resulting from the reaction between Pt(0 2)(PPh 3) 2 and HCl (1 : 2) in CD 2C1 2 at -72 °C. - 178-6-4. Summary The kinetic and mechanistic studies on the oxygenation of P t P n (n = 3, 4) compounds indicate that the overall reaction can be summarized by three consecutive steps. The first one is the formation of 0 2 P t P 2 and PPI13; the second step is the equilibrium between PtP 3 and PtP 4; and the third step is the formation of PtP 3 and OPPh 3 from 0 2 P t P 2 and PPh 3. However, 0 2 P t P 2 and OPPh 3 are the final products if excess of 0 2 is used. The protonation of the 0 2 P t P 2 consumes two equivalents of HCl per Pt to generate H 2 0 2 and cis-PtCl 2(PPh 3) 2. No evidence was found for the formation of an intermediate platinum hydroperoxo species. - 179 -CHAPTER 7 A MECHANISTIC STUDY OF TRIS- AND TETRAKIS-(TRIPHENYLPHOSPHINE)PLATINUM(0) SPECIES IN PROTONATION-OXYGENATION PROCESSES 7-1. Introduction Work described in this chapter is on the interaction of molecular oxygen with a platinum(II) hydride. The interest is focused on the possibility of the insertion reaction of molecular oxygen into a Pt-H bond to form a hydroperoxo platinum complex, and two reactions are principally in-volved in the present study. The first one is the reaction between molecular oxygen and the well known, 16e, square planar platinum hydride, trans-PtHCl(PPh3)2 (abbreviated as HPtP2Cl) , and the second one is the reaction between molecular oxygen and a cationic platinum hydride with coordinated solvent, trans-[PtH(Solv)(PPli3)2]+, which is formed in situ by removing the chloride ligand from HP tP2Cl . These two oxygenation reactions have been briefly mentioned by Strukul et a l . , 1 0 0 - 1 0 2 who reported that H P t P 2 C l is active toward molecular oxygen as outlined in eq. 7-1 but no details (such as spectroscopy or oxygen gas-uptake studies) were presented. They further reported that the reaction of [PtH(Solv)(PPli3)2]+ with O2 gave the fi-hydioxo complex [Pt(OH)(PPh 3 ) 2 ] 2 2 + , and suggested reactivity via insertion of 0 2 into the Pt-H bond to form a hydroperoxo platinum intermediate, that rapidly dimerized to form the platinum product with the elimination of O2 as described in eqs. 7-2 and 7-3. The present work was aimed at monitoring the postulated reaction processes (eqs. 7-1 to 7-3), particularly the last two, using both 3 1P{ 1H} and J H NMR to obtain more direct mechanistic information - 180 -of the possible oxygen insertion reaction. trans - PtHCl(PPh3)22^cis - PtCl2(PPh3)2,OPPh3,H2O,Pt(0)species trans - PtHCl(PPh3)2 + Ag+^trans - [PtH(Solv)(PPh3)2]+ + AgCl [PtH{Solv){PPh3)2}+ + 02~^Cn\Pt{OOH){Solv){PPh3)2)+" \[Pt(OHXPPhi)a$+ + \02 where Solv = solvent; 7-2. Synthesis and characterization of t rans-PtHCl(PPh3)2 The compound H P t P 2 C l was prepared as described in Chapter 2 and fully characterized using 3 1P{ aH} and 1E NMR, FT-IR and elemental analysis. The 3 1P{ 1H} NMR spectrum in CD2C12 (a singlet at 6 30.01 ppm with two singlet satellites at 6 42.40 and 17.61 ppm, Jp«_p = 3000 Hz), indicates that the two phosphine ligands were trans to each other, while the ]H NMR spectrum in CD2CI2 (a triplet at 6 -15.12 ppm with two triplet satellites, Jpt-w = 1170 and Jp_# = 14 Hz) confirms that the hydrogen is cis with respect to the two phosphine ligands. Finally, the FT-IR spectrum in Nujol (Fig. 7-1) shows one strong Pt-H band at 2209 cm - 1 and one strong Pt-Cl band at 290 cm - 1, the relatively low value for the latter being consistent with the strong trans effect of hydrogen weakening the Pt-Cl bond. The spectroscopic data are in excellent agreement with those reported earlier. 1 2 2' 1 2 3 (7-1) (7-2) (7 -3 ) - 181 -Fig. 7-1. The FT-IR spectrum of PtHCl(PPh 3) 2 in Nujol mull. -182-7-3. Reaction between tra n s - P t H C ] ( P P h 3 ) 2 and 0 2 The reaction between 0 2 and HPtP 2Cl was carried out under a range of conditions and the resulting solution, or the residue obtained from the solution was analyzed by 3 1P{ 1H} (the residue was dissolved in CD 2C1 2). Reaction (1) was carried out in 3 mL CH 2C1 2 with 1 atm oxygen at 20 °C for 4 days (HPtP 2Cl: 20.1 mg, 0.027 mmol); Reaction (2) Was carried out in 10 mL T H F with 4 atm oxygen at 20 "C for 4 days (HPtP 2Cl: 10.4 mg, 0.014 mmol); Reaction (3) was carried out in 0.5 mL CD 2C1 2 in the presence of 0 2 P t P 2 with 1 atm oxygen at 20 °C for 4 days (HPtP 2Cl: 7.8 mg, 0.01 mmol; 0 2PtP 2: "5.5 mg, 0.007 mmol); Reaction (4) was carried out in 0.5 mL CD 2C1 2 in the presence of HCl with 1 atm oxygen at 20 °C for 4 days (HPtP 2Cl: 4.47 mg, 0.006 mmol; HCl: 0.006 mmol). In addition, the decomposition of 0 2 P t P 2 under 0 2 at 1 atm was studied to find the decom-position products at 20 °C (0 2PtP 2: 4.3 mg, 0.006 mmol; CD 2C1 2: 0.3 mL; 4 days). In reaction (1), only cis-PtCl 2(PPh3) 2 (abbreviated as PtCl 2P 2) was observed as product and most of the starting HPtP 2Cl remained unreacted (Fig. 7-2). The P t C l 2 P 2 was identified by comparison with data for an authentic sample of P t C l 2 P 2 (Chapter 6). In reaction (2), more P t C l 2 P 2 was generated than in reaction (1) and, as well, OPPI13 and other unidentified species were formed; however, a significant amount of starting HPtP 2Cl still remained (Fig. 7-3). These observations show that the reaction between HPtP 2Cl and 0 2 is very slow. - 183 -A t P tHCl(PPh , ) , 19.01 ppm, lpt-p = 8000 H i B i P tCbfPPh , ) , \AM ppm, ipt-p = WBTO H i B I I I I I I I I I I I 4 5 • 1 1 i i i i • i i i i • i 111 j i i i i i i i II i | i i i i i 3 0 1 5 PP"1 Fig. 7-2. The 3 1P{ 1H} NMR spectrum of the residue in CD 2C1 2 at 20 °C from reaction (1): PtHCl(PPh 3) 2 with 0 2 in CH 2C1 2 at 20 °C. A t PtHCl(PPhj) , 39.01 ppm, ipx-p = 8000 H i B t PtC1s(PPh»)s 14L8S ppm, lpt-p = 8870 H i C t unknown, (L96 ppm D i OPPhs, 17J6Z ppm B, WL JU B C I B I I • I I I I • II I I "' I I ' • I | I • I I I I n • • I I I 11 • 11111 n u • 11111 n 111 • 11 n • 11111 | i 11' 4 5 3 0 1 5 0 ppm Fig. 7-3. The 3 1P{ 1H} NMR spectrum of the residue in C D 2 C l 2 at 20 °C from reaction (2): PtHCl(PPh 3) 2 with 0 2 in T H F at 20 °C. - 184 -The decomposition of 0 2 P t P 2 under 0 2 in CD 2C1 2 produced mainly PtCl 2P 2, 0PPh3 and a species at 6 6.96 ppm (unidentified), as shown in Fig. 7-4. The reaction between 0 2 and HPtP 2 C l gave the same products as those formed in the decomposition of 0 2 P t P 2 under 0 2, as well as other different products (compare Figs. 7-3 and 7-4). It was observed from the data for reaction (3) that the presence of 0 2 P t P 2 seemed to speed up the reaction of HPtP 2Cl with 0 2, because a greater amount of products relative to (i) the reaction without added 0 2PtP 2, and (ii) the 0 2 P t P 2 decomposition reaction, were observed (Fig. 7-5). In the presence of HCl (reaction (4)), the only product observed by 3 1P{*H} NMR was P t C l 2 P 2 , while most of the starting H P tP 2Cl remained (Fig. 7-6). The P t C l 2 P 2 observed in reaction (4) is presumably generated by the reaction of HPtP 2Cl with HCl (eq. 7-4), in agreement with the observation that the reaction of PtP 3 with excess HCl produces a mixture of HPtP 2Cl, PtCl 2P 2, and free PPh 3. Reaction between a chlorohydridobis(phosphine)platinum(II) complex with HCl to form a cis(dichloro)bis(phosphine) platinum complex has been reported (ref. 18, p.334) PtHCl{PPh3)2 + HCl = PtCl2(PPh3)2 + H2 (7 - 4) Reaction (4) shows that the presence of HCl decreases the reaction rate between HPtP 2Cl and 0 2, implying that the elimination of HCl from HPtP 2Cl to give "PtP 2" might be involved. This suggestion is further supported by the fact that the presence of a base (0 2PtP 2) in the system speeds up the reaction (reaction (3)). - 185 -B B B i PtOjfPPha), 14JJJ ppm, JM-P = 8670 H i C t unknown, 646 ppm D t OPPh,, 87.58 ppm «4» B J L 45 i i i i i i i i i i 1 1 1 I 1 15 T i | i I I I i I 0 3 0 ppm Fig. 7-4. The 3 1P{ ]H} NMR spectrum of the solution resulting from the decomposition of Pt(0 2)(PPh 3) 2 in C D 2 C l 2 at 20 °C. A A t PtHCl(PPh i), 39.01 ppm, lpt-p = 3000 H i B t PtCbfPPh,), 14J13 ppm, ipt-p — 8670 H i C t unknown, 6JM ppm D t OPPhj, 37.58 ppm B i i i i i I i i i i i i i i i i i i i i I i i i i i i i i i i i i i i I i i i 4 5 3 0 15 B LL i I i i i I 1 i i i i ppm Fig. 7-5. The 3 1P{ 1H} NMR spectrum of the solution resulting from reaction (3): PtHCl(PPh 3) 2 with 0 2 in C D 2 C l 2 at 20 °C in the presence of Pt(0 2)(PPh 3) 2. - 186 -A t PtHGlfPPhs), 19.01 ppm, lpt-p — 8000 H i B t PtCb(PPM» 14*81 ppm, lpt-p — 8870 H i 4 5 30 15 0 ppm Fig. 7-6. The 3 1P{ 1H} NMR spectrum of the solution resulting from reaction (4): PtHCl(PPh 3) 2 with 0 2 in CD 2C1 2 at 20 °C in the presence of HC1. -187-7-4. Reaction between cationic platinum hydride and 0 2 The cationic platinum hydride with coordinated solvent, trans-[PtH(solv)(PPh3)2]PF6, (abbreviated as [HPt(S)P2]PFe) was prepared in situ according to the literature method used for preparation of trans-[PtH(acetone)(PPh2Me) 2]PF6. 1 6 9 The solvents used for the preparation were CD2CI2, acetone-d6 and THF as indicated in Table 7-1. A general procedure, carried out under argon, was as follows. A known amount of H P t P 2 C l was charged into a Schlenk tube and dissolved in a degassed solvent. A corresponding solution of AgPF6 was prepared and a known volume of this, containing one equivalent of AgPF6 per HP tP2Cl , was taken and added dropwise into the Schlenk tube. Alternatively, both H P t P 2 C l and AgPF6 in solid form were mixed in a Schlenk tube at a 1 : 1 mole ratio and the appropriate solvent added. In every case a white precipitate was formed and filtered off, leaving a pale brown filtrate which was transferred into an NMR tube under argon which was then stoppered with a rubber septum. The solution sample was first analyzed by 3 1 P-^H} and 1E NMR at -75 and 20 °C before the argon was pumped off and oxygen at 1 atm admitted. Immediately, the NMR tube was placed in an ethanol/liquid N2 bath at -75 °C. The reaction solution was then monitored at -75 and then at 20 °C by 3 1P{ 1H} and 1H NMR. Four tests were performed according to this general procedure with the reaction conditions given in Table 7-1. The results obtained from the NMR studies of the reactions outlined in eqs. 7-2 and 7-3 are described below. The 3 1 P and *H NMR data at -75 and 20 °C confirm that in situ trans-solvent coordinated hydrides are generated. For example, in Test No. 2, cationic hydrides with coordinated acetone-d6 (the dominant species) or CD 2C1 2 (a minor species) are observed (Fig. 7-7 (a), (b), (c) and (d); Table 7-2). - 188 -Table 7-1. The reaction conditions for tht preparation of the cationic platinum hydride species Test A B HPtPjCl : AgPF 6 No. (mole ratio) HPtP 2Cl solvent AgPF 6 solvent x 102 g mL x 102 g mL 1 1.29 1.5 3.86 0.34 1:1 (1.71 x IO" 2 CD 2C1 2 (1.53 x 10"1 Acetone-d6 (1.5 mL of A + mmol) mmol) 0.04 mL of B) 2 1.81 0.5 6.10 0.5 1 : 1 (2.39 x IO" 2 CD 2C1 2; (2.41 x 10"1 Acetone-d6 (2 mL of A + mmol) 1.5 mmol) 0.05 mL of B) Acetone-d6 3 1.00 1.5 2.39 0.5 1 : 1 (1.32 x IO" 2 THF; (7.63 x 10"2 THF (2 mL of A + mmol) 0.5 mmol) 0.09 mL of B) Toluene-dg 4 1.93 0.733 1 : 1 (2.55 x 10" 2 (2.90 x IO" 2 (A + B) in mmol; solid) mmol; solid) 0.5 mL THF; 0.3 mL CD 2C1 2 - 189 -B B B Two tram cationic platinum hydrides with coordinated solvents: A and B A: 31.96 ppm Jpt-p = 2962 Hz; B: 30.46 ppm Jpt-p = 2962 Hz *PF 20 2~* species from AgPFe -13.88 ppm, Jp-r = 958 Hz 1 PPg: -142.32 ppm Jp-p m 706 Hz • ' i i . 4 0 T 0 T T A - 4 0 - 8 0 - 1 2 0 Fig. 7-7. (a) The 3 1P{ 1H} NMR spectrum of the cationic hydrides in a solvent mixture of CD2CI2 and acetone-d6 at -75 °C. B Two trans cationic platinum hydrides with coordinated solvents: A and B A: 31.72 ppm Jpt-P = 2962 Hz; B: 30.26 ppm Jpt-P = 2962 Hz •PFaOa"" species from AgPF« -13.64 ppm, Jp-p = 967 Hz ppm PFg: -142.08 ppm Jp.p « 706 Hz 1 4 0 T O T - 4 0 - 8 0 - 1 2 0 Fig. 7-7. (b) The 3 1P{ 1H} NMR spectrum of the cationic hydrides in a solvent mixture of CD2CI2 and acetone-d6 at 20 °C. - 190 -Two trans cationic platinum hydrides with coordinated solvents: A and B A: -22.36 p p m . Jp,-H=1397Hz B: -23.51 PPm, JPt.H=1226 Hz A. B I I | I I I I | I I I I | I I 1 I 1 I I I I | I I I I | I I I I | I I I I | H I 1 I I I I I I I I I I 1 I I I I I I I I I 1 I I I I I I I I I I I I I ' I ' ' ' ' I ' ' -20 - 2 2 - 2 4 - 2 6 p p m Fig. 7-7. (c) The *H NMR spectrum of the cationic hydrides in a solvent mixture of CD 2C1 2 and acetone-d6 at -75 °C. Two trans cationic platinum by dri des ^ with coordinated solvents: A and B A: -22.66 p p m , Jpt-H=1405 Hz B: -23.60 PPm, JK-H=1301 Hz l l l l l l l | l l l l | l l l l | l l l l | l l l l | l l l l | l l l l | l l l l | » l l l | l l l l | l l l l | l l l » | l l l l | l l » l | l l l l | I M I | ' l - 2 0 - 2 2 - 2 4 - 2 6 p p m Fig. 7-7. (d) The *H NMR spectrum of the cationic hydrides in a solvent mixture of CD 2C1 2 and acetone-d6 at 20 °C. - 191 -Table 7-2. The 3 1 P and JH NMR data for the cationic platinum hydride species0 Temp. 31 p *H °C 6, ppm Jpt-p» Hz 6, ppm Jpt-//j Hz J p - H , Hz -75 A \ 1: 4 : 1 triplet A, 1:4: 1 triplet (broadened) 31.96 2962 -22.36 1397 -75 BS 1: 4 : 1 triplet B, 1:4: 1 triplet (broadened) 30.46 2962 -23.51 1290 20 A\ 1: 4 : 1 triplet A, 1:4: 1 triplet of 1 : 2 : 1 triplet 31.72 2962 -22.66 1405 12 20 B c, 1 : 4 : 1 triplet B, 1:4: 1 triplet of 1 : 2 : 1 triplet 30.26 2962 -23.60 1301 a: in CD2CI2 (0.5 mL)/acetone-d6 (1.5 mL). b: A is the dominant species, c: B is the minor species. Both A and B were prepared under the reaction conditions given in Table 7-1, No. 2. When oxygen was admitted to the solutions of the cationic hydrides, no change in the 3 1P{*H} or a H spectra was observed at either -75 or 20 °C, even after several days. A triplet observed at about 6 -13.88 ppm (Figs. 7-8 (a) and (b)) is attributed to an unknown species present in the sample of AgPF6 (possibly, a " P F 2 O 2 " " species). Studies on all four test solu-tions show that the cationic hydrides are not active toward O2 at -75 or 20 °C. The literature procedure reported by Strukul and co-workers100 for the preparation of the cationic platinum hydride species from PtHCl(PPh3)2 and for the subsequent reaction of the cationic platinum - 192 -hydride with 0 2 in THF was also carefully repeated several times to verify the results obtained from the JH and 3 1 P NMR studies described above. The compounds HPtP 2Cl (11.3 mg, 0.015 mmol) and AgBF 4 (3.9 mg, 0.02 mmol) were charged in a Schlenk tube, which was evacuated and then filled with argon and placed in an ice-bath. Dry THF (2 mL) was added to the Schlenk tube under argon and the mixture stirred for about two hours. After the AgCl formed was filtered off under argon, a pale yellow filtrate containing the cationic platinum hydride was collected in a Schlenk tube and freeze-thaw degassed twice; the Schlenk tube was then placed in a dry-ice bath at -78 °C. Oxygen at 1 atm was introduced into the Schlenk tube with stirring and the dry-ice bath was replaced by an ice-bath after half an hour. The reaction solution was finally warmed to room temperature; however, no colour change and no precipitate were observed. Strukul's group 1 0 0 had reported precipitation of [Pt 2(OH) 2(PPh3) 4][BF 4] 2 at this stage. Another test (HPtP 2Cl: 16.7 mg, 0.022 mmol; AgBF 4: 4.4 mg, 0.023 mmol; THF: 1 mL) was carried out under argon to obtain the pale yellow filtrate, and one drop of H 20 (0.05 mL) was added to part of the filtrate (0.3 mL) with shaking at room temperature. Again, no precipitate was formed from the reaction solution. These observations further show that the cationic hydride is inactive not only toward oxygen but also toward H 2 0 in T H F at room tem-perature. The reported formation of the hydroxy-bridged complex [Pt 2(OH) 2 (PPli3) 4][BF 4] 2 was not substantiated in the present studies. It is worth noting that [Pt 2(OH) 2(PPh3) 4][BF 4] 2 is reported to be formed in the reaction of cis-PtCl2(PPh3)2 with AgBF 4 in moist acetone (~ 0.5% H 2O). 1 ? 0 Therefore, it seems possible that PtCl 2(PPh 3) 2, which, in the present work, was found to be common impurity in the PtHCl(PPh3)2 synthesized (Section 2-3); see eq. 7-4, Section 7-3, is the compound responsible for the formation of the hydroxy-bridged complex - 193 -([Pt 2(OH)2(PPri3)4][BF 4]2) reported in Strukul's work.100 The cationic hydrides certainly do not react with 0 2, and the suggestion of hydroxide formation from an intermediate hydroper-oxide seems to be unfounded. 7-5. S u m m a r y Reaction between O2 and trans-PtHCl(PPh3)2 does not take place readily: there are some very slow changes at 20 °C but, based on the results described in Section 7-3, more likely path-ways for this reactivity are those proposed in Scheme 7-1. The suggested pathways follow closely those established more definitively for the IrHCl(CO)(PPh3)2 system (Chapter 4). There is no reactivity whatsoever of O2 toward trans-PtH(solvent)(PPh3)2 species, i.e. the replacement of a chloride of trans-PtHCl(PPli3)2 by a solvent molecule does not enhance the coordination of O2 or any subsequent insertion reaction of the O2 into the platinum-hydrogen bond. Not even "very slow changes" are observed, implying that the tendency of [PtH(solvent)(PPh3)2]PF6 to undergo reductive elimination of a proton (as HPF6 or HBF 4, cf. Scheme 7-1) is less than that of the chlorohydrido complex. Scheme 7-1. The mechanism of the reaction between O2 and PtHCl(PPh3)2 PtHCl{PPh3)2 ^ nPt(PPh3)2n + HCl nPt{PPh3)2n + 0 2 Pt(02)(PPh3)2 Pt{02)(PPh3)2 + 1HCI — » PtCl2(PPh3)2 + H202 Pt(02)(PPh3)2 —> OPPh3, other unidentified decomposition products - 194 -C H A P T E R 8 G E N E R A L C O N C L U S I O N S A N D R E C O M M E N D A T I O N S F O R F U T U R E W O R K 8-1. Genera] conclusions In the oxygenation-protonation studies using iridium(I) and platinum(O) complexes, dioxy-gen is activated in the form of a dioxygen (peroxide) metal complex. The oxygenation of IrCl(CO)(PPh 3) 2 is reversible and that of Pt(PPh 3) n (n = 3 and 4) is irreversible, reflecting that the bonding of oxygen to platinum is stronger than that of oxygen to iridium. Both IrCl(0 2)(CO)(PPh 3) 2 and Pt(0 2)(PPh 3) 2 oxidize free PPh 3 to give OPPh 3, but the platinum system reacts more rapidly. Protonation of iridium or platinum dioxygen 'complexes produces H 2 0 2 stoichiometrically. In the TFA/IrCl(0 2)(CO)(PPh 3) 2 reaction system, the H 2 0 2 that forms will further oxidize the PPh 3 and CO ligands of IrCl(OCOCF 3) 2(CO)(PPh 3) 2, which is presumably formed as the immediate product of protonation. In the HCl/IrCl(0 2)(CO)(PPh 3) 2 reaction system, fac-IrCl 3(CO)(PPh 3) 2 is formed as a kinetic product that isomerizes to give mer-IrCl 3(CO)(PPh 3) 2, which is stable toward H 2 0 2 formed in the protonation reaction. The reaction of Pt(0 2)(PPh 3) 2 with HCl gives H 2 0 2 and cis-PtCl 2(PPh 3) 2 which shows no reac-tivity toward H 20 2. In the protonation-oxygenation studies, IrHCl 2(CO)(PPh 3) 2 and PtHCl(PPh 3) 2, of known geometries, do not react with 0 2 'directly'. Reductive elimination of HCl from these two complexes in rate-determining steps creates opportunities for the subsequent formation of IrCl(02)(CO)(PPh3)2 or Pt(02)(PPh3)2, which then reacts with the previously liberated HCl - 195 -in the same manner as in the protonation reaction of the corresponding dioxygen (peroxide) metal complex (eqs. 8-1 to 8-3, and 8-4 to 8-6, respectively). In addition, secondary reactions, such as reactions 8-7 and 8-8, take place in the iridium and platinum systems, respectively. IrHCl2(CO){PPh3)2 A= IrCl(CO)(PPh3)2 + HCl (8-1) A T - i IrCl(CO){PPh3)2 + 02 IrCl(02){CO){PPh3)2 (8-2) K-2 IrCl(02)(CO)(PPh3)2 + 2 HCl —> H202 + IrCl3(CO){PPh3)2 (8-3) PtHCl(PPh3)2 *± nPt(PPh3)2n + HCl (8-4) "Pt(PPh3)2n + 02 —» Pt(02){PPh3)2 (8-5) Pt(02)(PPh3)2 + 2 HCl —> H202 + PtCl2(PPh3)2 (8-6) lrCl{CO){PPh3)2 ^ 2 C02,OPPh3,IrCl{02){CO){PPh3)2 (8-7) H20, and other "7r(/77)" species Pt(02)(PPh3)2 —• OPPh3, and other decomposition products (8 — 8) Kinetic data for determination of the rate constants kj, k_i (in CH2CI2), k2 and k_2 (in benzene and in CH2CI2) have been obtained at 25 °C. Equilibria data on the reaction: Pt(PPh3)i ^ Pt{PPh3)3 + PPh3 in toluene as well as kinetic data in benzene for reaction of Pt(PPli3)3 with O2 and reaction of Pt(02)(P?h3)2 with PPh3 have been measured. - 196 -Removal of one chloride ligand from IrHCl2(CO)(PPh3)2 leads to subsequent rapid elim-ination of HCl, and formation of a cationic iridium(I) species with coordinated solvent; this does react with oxygen slowly at room temperature to generate a probable peroxide complex, but formation of an OOH species in this system is not possible. The removal of the chloride ligand from PtHCl(PPh3)2 gives a cationic platinum hydride with coordinated solvent, which is, however, inactive toward oxygen at room temperature; the findings are contrary to a literature report. The five-coordinate IrH(C0)(PPh 3) 3 reacts with oxygen to form IrH(0 2)(CO)(PPh 3)2 (eqs. 8-9, 8-10), this species decomposing to give a mixture of products via the possible pathway shown in eq. 8-11. IrH{CO)(PPh3)3 ^ JrH{CO)(PPh3)2 + PPh3 (8-9) JrH(CO)(PPh3)2 + 0 2 ^ IrH(02){CO)(PPh3)2 (8 - 10) IrH(02){CO)(PPh3)2 —>nIr(OOH){CO)(PPh3)2" (8-11) —• OPPh3, C02, H20, and nIr(JII) species'" Direct evidence for formation of a hydroperoxo species M(OOH) (M = Ir or Pt) via an oxygenation-protonation or a protonation-oxygenation process using IrCl(CO)(PPh3)2, Pt(PPh3) n (n = 3 and 4), and related complexes, has not been obtained. 8 -2. Recommendations for future work Although some basic chemistry behind protonation of coordinated peroxide and oxygena-tion of coordinated hydride, including useful kinetic and equilibria data, has been learned -197-through the present work, little information on the formation of hydroperoxo metal complexes via the processes described has been obtained. 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