Electron-Transfer Behavior of the Metal Complexes Attached to

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Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 11, 2016 | http://pubs.acs.org Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch005

Electron-Transfer Behavior of the Metal Complexes Attached to Polymer Matrices EISHUN TSUCHIDA and HIROYUKI NISHIDE Waseda University, Department of Polymer Chemistry, Tokyo 160, Japan Interfacial electron-transfer reactions between polymer-bonded metal complexes and the substrates in solution phase were studied to show colloid aspects of polymer catalysis. A polymer-bonded metal complex often shows a specifically catalytic behavior, because the electron-transfer reactivity is strongly affected by the polymer matrix that surrounds the complex. The electron-transfer reaction of the amphophilic block copolymer-bonded Cu(II) complex with Fe(II) (phenanthroline) proceeded due to a favorable entropic contribution, which indicated hydrophobic environmental effect of the copolymer. An electrochemical study of the electron-transfer reaction between a poly(xylylviologen) coated electrode and Fe(III) ion gave the diffusion constants of mass-transfer and electron-exchange and the rate constant of electron-transfer in the macromolecular domain. A polymer-Cu complex coated electrode was successfully applied as the interfacial catalyst for the oxidative polymerization of phenol. 3

The metal complex bound to a polymer o f t e n shows a s p e c i f i c c a t a l y t i c behavior compared with that of the corresponding monomeric complex, because the r e a c t i v i t i e s o f the complex are s t r o n g l y a f f e c t e d by the polymer chain that surrounds the complex ( 1 , 2 ) . The c a t a l y t i c c y c l e is illustrated in Scheme 1, the example used being the Cu complex c a t a l y z e d o x i d a t i o n o f 2,6-dimethylphenol (3). In the first step, the substrate phenol coordinates t o the Cu(II) complex and one e l e c t r o n t r a n s f e r s from the substrate to the Cu(II) i o n . Then the a c t i v a t e d substrate d i s s o c i a t e s from the c a t a l y s t and the reduced Cu(I) c a t a l y s t is r e o x i d i z e d to the o r i g i n a l Cu(II) complex. Among these elementary r e a c t i o n s , the e l e c t r o n - t r a n s f e r step is the most important process governing the c a t a l y t i c behavior o f a polymer-metal complex f o r the f o l l o w i n g reasons: ( i ) The e l e c t r o n - t r a n s f e r step is o f t e n the slowest 0097-6156/83/0212-0049$06.00/0 © 1983 American Chemical Society Bailey et al.; Initiation of Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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elementary r e a c t i o n i n the c a t a l y s i s . ( i i ) The e l e c t r o n - t r a n s f e r step i s an intracomplex process, and i t i s expected that the prope r t y of l i g a n d s i n the complex c a t a l y s t as w e l l as the property of the coordinated substrate d i r e c t l y a f f e c t s the r a t e of the e l e c t r o n - t r a n s f e r step, ( i i i ) The e l e c t r o n - t r a n s f e r r e a c t i o n i s s t r o n g l y i n f l u e n c e d by chemical environment around the complex, such as solvent e f f e c t and a l s o the environmental e f f e c t of a polymer, ( i v ) The e l e c t r o n - t r a n s f e r process r e q u i r e s rearrangement of the complex s t r u c t u r e , which accompanies conformational change of the polymer-ligand. (v) Metal complexes are concentrated w i t h i n the macromolecular domain, where an electron-exchange pathway i s i n f e r r e d . The present paper describes i n t e r f a c i a l e l e c t r o n - t r a n s f e r r e a c t i o n s between the metal complexes bound to polymer matrices and the substrates i n s o l u t i o n phase to show c o l l o i d aspects of polymer c a t a l y s i s . E l e c t r o n - t r a n s f e r Reaction of the Metal Complexes Attached to Polystyrene Beads The importance of the e l e c t r o n - t r a n s f e r step i s demonstrated, e.g. i n the o x i d a t i v e p o l y m e r i z a t i o n of 2,6-dimethylphenol c a t a l y z e d by the Cu complex attached to p o l y v i n y l p y r i d i n e beads. A metal complex c a t a l y s t i s o f t e n i n s o l u b i l i z e d by using a c r o s s l i n k e d polymer matrix i n order to handle i t e a s i e r , but the i n s o l u b i l i t y causes a decrease i n the c a t a l y t i c a c t i v i t y . The Cu c a t a l y s t bound to divinylbenzene c r o s s - l i n k e d p o l y ( 4 - v i n y l p y r i d i n e ) beads r e s u l t e d i n much lower c a t a l y t i c a c t i v i t y , which was caused by the slower e l e c t r o n - t r a n s f e r step and i t s higher a c t i v a t i o n energy ( 4 ) . The s t r u c t u r e of the Cu complex c a t a l y s t must be rearranged during the e l e c t r o n - t r a n s f e r step from the square planar s t r u c t u r e to the t e t r a g o n a l s t r u c t u r e (Scheme 2 ) . Thus, the energy r e q u i r e d to rearrange the complex s t r u c t u r e i s l a r g e r and e l e c t r o n - t r a n s f e r step occurs with more d i f f i c u l t y f o r the polymer complex i n which Cu ions are s i t u a t e d i n the c r o s s l i n k e d polymer matrix and i n which the polymer-ligand i s l a c k i n g in f l e x i b i l i t y . In order to improve t h i s p o i n t , the metal complex c a t a l y s t s have been attached to the polymer matrices with spacer groups (5, 6) . Cupric i o n was complexed with the p y r i d i n e d e r i v a t i v e to polystyrene beads with o l i g o (ethylene oxide) (1) ( 7 ) . The r a t e constant of the e l e c t r o n - t r a n s f e r step was increased s e v e r a l times f o r the c a t a l y s t ( 1 ) . T h i s agreed with the r e s u l t that 1 acted as an e f f e c t i v e c a t a l y s t f o r the o x i d a t i v e polymeri z a t i o n of dimethylphenol to y i e l d high molecular weight p o l y p h e n y lene oxide) ( 7 ) . The second example which emphasized the e l e c t r o n - t r a n s f e r step as a key process i n polymer c a t a l y s i s i s the R u i b i p y r i d i n e ) ^ complex s e n s i t i z e d photoreduction of methylviologen, which i s of i n t e r e s t as a means to reduce protons l e a d i n g to hydrogen e v o l u t i o n (Scheme 3). To suppress a spontaneous backward r e a c t i o n which consumes the acquired energy, a heterogeneous r e a c t i o n

Bailey et al.; Initiation of Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 11, 2016 | http://pubs.acs.org Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch005

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Electron-Transfer Behavior

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Scheme 1.

Ν

Scheme 2.

Ill-