COBALT(III) COBALT(II) ELECTROCHEMICAL POTENTIAL CONTROLLED BY STERIC CONSTRAINTS IN SELF-ASSEMBLED DINUCLEAR TRIPLE-HELICAL COMPLEXES

The ligands bis[5-(1-methyl-2-(X'-methyl-2'-pyridyl)benzimidazolyl)]methane (X' = 6', 6-bismbmp, 1; X' = 5', 5-bismbmp, 2) react with cobalt(II) perchlorate in acetonitrile to give self-assembled dinuclear triple-helical complexes. The crystal structures of [Co2(6-bismbmp)3](ClO4)4(CH3CN)2.5 (6, Co2C92H85.5N20.5Cl4O16, a = 14.403(2) angstrom, b = 16.558(3) angstrom, c = 21.328(3) angstrom, alpha = 78.15(1)-degrees, beta = 81.19(1)-degrees, gamma = 89.02(1)-degrees, triclinic, P1BAR Z = 2) and [Co2(5-bismbmp)3](ClO4)4(CH3CN)6 (7, Co2C99H96N24Cl4O16, a = 13.648(5) angstrom, b = 16.917(4) angstrom, c = 22.801(8) angstrom, alpha = 92.45(1)-degrees, beta = 103.80(1)-degrees, gamma = 96.32(1)-degrees, triclinic, P1BAR, Z = 2) show dinuclear triple-helical cations [Co2(6-bismbmp)3]4+ and [Co2(5-bismbmp)3]4+ where the three bidentate ligands are wrapped around a pseudo-C3 axis defined by the cobalt atoms. The coordination spheres around the high-spin cobalt (II) ions may best be described as octahedra flattened along the C3 axis. Comparison of the structures shows that steric repulsions between the methyl groups in the 6-positions of the pyridine rings in 6 result in a distortion of the coordination spheres around Co(II) and increase in the Co-N(py) bond distances. H-1-NMR and electronic spectra and spectrophotometric titrations in solution indicate that the pseudo-D3 triple-helical structures of [Co2(6-bismbmp)3]4+ and [Co2(5-bismbmp)3]4+ are maintained in acetonitrile and result from cooperative self-assembly processes. The greater deformation of the coordination spheres around Co(II) in [Co2(6-bismbmp)3]4+ is responsible for a decrease of the stability of the triple-helical structure in solution and the failure to observe the oxidation of Co(II) in [Co2(6-bismbmp)3]4+.