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·H ₂O (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 diastereomeric salt with [Sb₂{(R,R)-tart}₂]^2- [(R,R)-tart = (2R,3R)-tartrate(4-) ion]. From the less soluble part, Λ-[Co(H₂L)][Sb₂-{(R,R)-tart}₂] ·4H₂O (2) was isolated. Starting from 2, two optically active complexes, Λ-[Co(H₃L)](ClO₄)₃·1. 5H₂O (Λ-1) and Λ-[Co(L)] (Λ-3), were obtained. The crystal structures of these complexes are compared with those of the racemic structures. Λ-1 shows an unusually strong circular dichroism (λ = 488 nm, Δε = -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 Λ-[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 [Co^III/II(L)] ^0/- is not observed.