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 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 [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.