A Comparison of the Lability of Mononuclear Octahedral and Dinuclear Triple-Helical Complexes of Cobalt(II)

The lability of the mononuclear octahedral complex tris(5-methyl-2-(1‘-methylbenzimidazol-2-yl)-pyridine)cobalt(II), [Co(2)3]2+, is compared with the dinuclear triple-helical complex tris[bis[1-methyl-2-(5‘-methylpyrid-2‘-yl]benzimidazol-5-yl]methane]dicobalt(II), [Co2(1a)3]4+. [Co(2)3]2+ undergoes rapid isomerization between mer and fac forms (k 298(mer − fac) = 1.6 ± 0.2 s-1) in acetonitrile while the racemization of (−)589-[Co2(1a)3]4+ is roughly 105 times slower (k 298 = 1.4 ± 2 10-5 s-1). The pressure dependence of the isomerization of [Co(2)3]2+ suggests a dissociatively activated process. The racemization of (−)589-[Co2(1a)3]4+ is found to be independent of pH above pH 4, and is not affected by added cobalt(II) or a change of solvent. Ligand exchange between [Co2(1b)3]4+ (1b = bis[1-ethyl-2-(5‘-methylpyrid-2‘-yl)benzimidazol-5-yl]methane) and free 1a may be followed by electrospray mass spectrocopy and establishes the mechanism of formation of the triple helix to be initial formation of mononuclear [Co(1a)3]2+ followed by capping by a second cobalt to give [Co2(1a)3]4+. The slow racemization of (−)589-[Co2(1a)3]4+ is attributed to the very slow dissociation of a cobalt ion from the triple helix. This inertness is attributed to the rigidity of the ligand and the tight pitch of the helix.