Polyelectrolyte acceleration of electron transfer reactions

Abstract

The kinetics of electron-transfer reactions of some Co(III) complexes with Ru(NH 3) 6 2+ (outer sphere), Cr(H 2O) 6 2+ (inner sphere), and V(H 2O) 6 2+ (inner sphere) have been studied in the presence of varying concentrations of the sodium salt of poly(methacryloxyethylsulfonic acid), (PMES), using the stopped-flow technique. A large acceleration of reaction rates was observed with Ru(NH 3) 6 2+ and V(H 2O) 6 2+, with acceleration factors up to 20,000. The accelerated rates of electron transfer show a nonlinear dependence on polyelectrolyte concentrations: first, increasing with increasing polyelectrolyte concentration then reaching a maximum and subsequently decreasing. The magnitude of acceleration depends strongly on the charge of oxidants, being much larger for dipositive than for unipositive ions. Acceleration specificity is shown by the reduction of cis-Co(en) 2Cl 2 + by Ru(NH 3) 6 2+, in which case the acceleration factor is much higher in the presence of PMES than in the case of the corresponding trans isomer. The acceleration of the reaction of cis-Co(en) 2PyCl 2+ with Ru(NH 3) 6 2+ by the polyelectrolyte is practically equal to the acceleration of the reduction of this complex by V(H 2O) 6 2+, whereas the reduction of cis-Co(en) 2NH 2Cl 2+ by Ru(NH 3) 6 2+ is more strongly accelerated than the reduction by V(H 2O) 6 2+. Activation energies have been determined for some of the reactions and have been interpreted in terms of a multiple equilibrium reaction scheme. At a given concentration of reactants and polyelectrolyte, the acceleration factor increases with decreasing temperature and, further, the concentration of polyelectrolyte at which the maximum acceleration occurs decreases with decreasing temperature. The reaction rates of inner sphere reduction using Cr(H 2O) 6 2+ exhibit an entirely different behavior in the presence of PMES; no acceleration effect is observed. Two distinct reaction paths can be identified, with evidence for the formation and decay of one or more binuclear intermediates. The electron-transfer rate corresponding to the decay of the intermediates is much slower in the presence of PMES.