Reduction of Nicotinamides, Flavins, and Manganese Porphyrins by Formate, Catalyzed by Membrane-Bound Rhodium Complexes

Article abstract of DOI:10.1021/jo00111a018

Two polymerizable cyclopentadienyl-2,2'-bipyridinerhodium complexes and one amphiphilic rhodium complex have been synthesized and incorporated into the bilayers of vesicles formed from a polymerizable ammonium surfactant.Incorporation was achieved by chemical linkage (via a copolymerization reaction) or by physical absorption.The complexes are located at different positions in the bilayer, viz. in the hydrophobic interior or at the aqueous interface.The anchored rhodium complexes efficiently catalyze the reduction of both two-electron substrates (e.g., nicotinamides and flavin) and one-electron substrates (e.g., manganese(III) porphyrins) by sodium formate.Kinetic studies indicate that the rate-limiting step of the reaction is the reduction of the rhodium center by formate, leading to a rhodium(III) hydride or a rhodium(I) species.The three complexes showed different catalytic activities.Cyclic voltammetry and UV-vis measurements revealed that their reduction potentials ((2+) + 2e(-) ->/<- Rh(I)) and the pK_a values ((+) ->/<- Rh(I) + H(+)) are also different.From the observed trends it is concluded that the catalytic activity of the rhodium complexes depends on its position in the bilayer.