Cobalt(III) Complexes of a Tripodal Ligand Containing Three Imidazole Groups: Properties and Structures of Racemic and Optically Active Species

Abstract

AbstractThe complex [Co(H3L)](ClO4)3·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 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 [CoIII/II(L)]0/– is not observed. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)