Cobalt(III) complexes of a tripodal ligand containing three imidazole groups: Properties and structures of racemic and optically active species

The complex [Co(H3L)](ClO4)(3)center dot H2O (1), where H3L (tris[2-(4-imidazoylmethylideneamino) ethyl] amine) is a tripodal ligand obtained by condensation of tris(2-aminoethyl)amine and 4-formylimidazole in a 1:3 molar ratio, was synthesized and optically resolved by fractional crystallization of the dia-stereomeric salt with [Sb-2{(R,R)-tart)(2)](2-) [(R,R)-tart = (2R,3R)-tartrate(4-) ion]. From the less soluble part, Lambda-[Co(H2L)][Sb-2- (R, R) -tart)(2)]center dot 4H(2)O (2) was isolated. Starting from 2, two optically active complexes, Lambda-[Co(H3L)]ClO4)(3)center dot 1.5H(2)O (Lambda-1) and Lambda-[Co(L)] (Lambda-3), were obtained. The crystal structures of these complexes are compared with those of the racemic structures. A-I shows an unusually strong circular dichroism (lambda = 488 nm, Delta epsilon = -7.74 M-1 cm(-1)) in the first d-d absorption band region. The effects of deprotonation-reprotonation of the uncoordinated imidazole NH groups of Lambda-[Co(H3L)](3+) on the UV/Vis and CD spectra and on the cyclic voltammograms were studied in methanol. Although the deprotonation-reprotonation reactions are reversible, the redox couple for the completely deprotonated species [CoIII/II(L)](0/-) is not observed. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008).