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), [Co-2(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)-[Co-2(1a)(3)](4+) is roughly 10(5) 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)-[Co-2(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 [Co-2(1b)(3)](4+) (1b = bis[1-ethyl-2-(5'-methylpyrid-2'-yl)benzimidazol-5-yl]methane) and free la 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 [Co-2(1a)(3)](4+). The slow racemization of (-)(589)-[Co-2-(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.