A COMPARISON OF THE LABILITY OF MONONUCLEAR OCTAHEDRAL AND DINUCLEAR TRIPLE-HELICAL COMPLEXES OF COBALT(II)

The lability of the mononuclear octahedral complex (1'-methylbenzimida zol-2-yl)-pyridine)-cobalt(II), [Co(2)(3)](2+), is compared with the d inuclear triple-helical complex yrid-2'-yl]benzimidazol-5-yl]methane]d icobalt(II), [Co-2(1a)(3)](4+).[Co(2)(3)](2+) undergoes rapid isomeriz ation 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+) i s roughly 10(5) times slower (k(298) = 1.4 +/- 2 10(-5) s(-1)). The pr essure dependence of the isomerization of [Co(2)(3)](2+) suggests a di ssociatively activated process. The racemization of (-)(589)-[Co-2(1a) (3)](4+) is found to be independent of pH above pH 4, and is not affec ted by added cobalt(II) or a change of solvent. Ligand exchange betwee n [Co-2(1b)(3)](4+) (1b = 2-(5'-methylpyrid-2'-yl)benzimidazol-5-yl]me thane) and free la may be followed by electrospray mass spectrocopy an d establishes the mechanism of formation of the triple helix to be ini tial 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 o f a cobalt ion from the triple helix: This inertness is attributed to the rigidity of the ligand and the tight pitch of the helix.