ELECTROCHEMISTRY AND CATALYSIS WITH VITAMIN-B12 HEXACARBOXYLATE IN AN INSOLUBLE SURFACTANT FILM

Vitamin B12 hexacarboxylate, 8-aminocob(III)yrinic acid c-lactam, was used to incorporate catalytic activity into insoluble liquid crystal films of a cationic surfactant on carbon electrodes. The most incorporation into cast films of didodecyldimethylammonium bromide (DDAB) from aqueous buffers was found at pH 4.9 in solutions containing CN-. This pH is close to the pK(a) of carboxylic acid groups on the corrin ring. Co(III)B12 hexacarboxylate can exist in two interconvertable forms in DDAB films in solutions containing CN-. The dicyanoCo(III) complex, reduced irreversibly at -1.2 V vs SCE, is the main species at equilibrium. Repetitive cyclic voltammetry slowly converts dicyanoB12Co(III) hexacarboxylate to monocyanoB12Co(III) hexacarboxylate, which gives a chemically reversible two electron reduction to the four-coordinate Co(I) form of the complex (-0.66 V at pH 4.7). Upon standing in solutions containing CN-, the monocyano form of the complex in DDAB films converts back to dicyanoB12Co(III) hexacarboxylate. DDAB films loaded with B12Co(III) hexacarboxylate catalyzed reductions of trichloroacetic acid and 1,2-dibromoethane at potentials of Co(I) formation. Thus, vitamin B12 hexacarboxylate has similar catalytic activity to vitamin B12 (aquocobalamine) which cannot be incorporated into films at cationic sites.