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

The ligands yl-2-(X'-methyl-2'-pyridyl)benzimidazolyl)]methane (X' = 6 ', 6-bismbmp, 1; X' = 5', 5-bismbmp, 2) react with cobalt(II) perchlor ate in acetonitrile to give self-assembled dinuclear triple-helical co mplexes. 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) angs trom, 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) angs trom, 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, triclini c, 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 wra pped around a pseudo-C3 axis defined by the cobalt atoms. The coordina tion spheres around the high-spin cobalt (II) ions may best be describ ed as octahedra flattened along the C3 axis. Comparison of the structu res shows that steric repulsions between the methyl groups in the 6-po sitions of the pyridine rings in 6 result in a distortion of the coord ination spheres around Co(II) and increase in the Co-N(py) bond distan ces. 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 acetonitr ile and result from cooperative self-assembly processes. The greater d eformation of the coordination spheres around Co(II) in [Co2(6-bismbmp )3]4+ is responsible for a decrease of the stability of the triple-hel ical structure in solution and the failure to observe the oxidation of Co(II) in [Co2(6-bismbmp)3]4+.