STRUCTURAL, MAGNETIC, AND ELECTROCHEMICAL PROPERTIES OF DINUCLEAR TRIPLE HELICES - COMPARISON WITH THEIR MONONUCLEAR ANALOGS

A series of dinuclear triple-helical complexes of iron(II) and cobalt( III) with bis[2-(pyrid-2'-yl)benzimidazol-5-yl]methane ligands 2 were compared with the analogous mononuclear complexes of Fe(II) and Co(III ) with 2-(pyrid-2'-yl)benzimidazole ligands 1, With the dinucleating l igands 2 only one complex is formed, in contrast to the mononuclear co mplexes, for which stepwise formation is observed. The crystal structu res of fac-[Co(1a)(3)](ClO4)3 . EtCN and [Fe-2(2c)(3)](ClO4)(4) . 4CH( 3)CN are reported and compared with previously determined triple-helix structures to show that the formation of the helicate does not involv e significant distortion of the metal coordination sphere, and that a decrease in metal-nitrogen bond length results in a longer metal -meta l distance, Magnetic susceptibilities were measured between 243 and 32 3 K in CD3CN solution for [Co(1a)(3)](2+), [Co-2(2a)(3)](4+), [Fe(1b)( 3)](2+) [Fe-2(2b)(3)](4+), [Fe(1a)(3)](2+) and [Fe-2(2a)(3)](4+). Coba lt(II) complexes and iron(II) complexes with methyl substituents at th e 6-position of the pyridine rings are high spin and show Curie parama gnetism with no significant met al-metal interaction. Complexes [Fe-2( 2a)(3)](4+) and [Fe(1a)(3)](2+) are spin-crossover systems; the dinucl ear complex shows greater stability in the lowspin form, Electrochemis try does not allow the separation of the two oxidation waves of [M-2(2 a)(3)](4+), but the H-1 NMR spectrum of [(CoCoIII)-Co-II(2a)(3)](5+) s hows the system to be a class II mixed-valent system. The oxidation of the metal ions is not a cooperative phenomenon.