Hydrophobic Vitamin B12. II. Coordination Geometry and Redox Behavior of Heptamethyl Cobyrinate in Nonaqueous Media

The nuclear cobalt(II) ion in heptamethyl cobyrinate iodide, Cob(II)ester·I, was found to assume a distorted square-planar geometry of Co–N4 type with iodide as a counterion in a polar solvent (methanol) and a square-pyramidal one of Co–N4I type with the iodide ion coordinated at one of the axial positions in apolar solvents such as dichloromethane, benzene, and toluene. The redox chemistry of Cob(II)ester·I and heptamethyl cobyrinate perchlorate, [Cob(II)ester]ClO4, was investigated in various organic solvents by means of cyclic voltammetry and controlled-potential electrolysis. The Co(II)/Co(I) and Co(III)/Co(II) redox potentials for [Cob(II)ester]ClO4 exhibited cathodic shifts as either solvent basicity or polarity was raised, and were found to be in the anodic side relative to the corresponding potentials for CoII(tpp) under identical conditions. The medium effect on the redox behavior of hydrophobic vitamin B12 was discussed in connection with the microenvironmental effect on vitamin B12 placed in the enzyme active sites.