Abstract
The complexation of the V^IV^O ion with four amino acid derivatives of bis(imidazol‐2‐yl)methylamine [N‐glycyl‐bis(imidazol‐2‐yl)methylamine = Gly‐BIMA, N‐α‐aspartyl‐bis(imidazol‐2‐yl)methylamine = α‐Asp‐BIMA, N‐α‐glutamyl‐bis(imidazol‐2‐yl)methylamine = α‐Glu‐BIMA and N‐histidyl‐bis(imidazol‐2‐yl)methylamine = His‐BIMA] was studied in aqueous solution through the combined application of potentiometric and spectroscopic (UV/Vis and EPR) techniques. For comparison, the complexing capability of three simple bis(imidazol‐2‐yl) derivatives [bis(imidazol‐2‐yl)methane = BIM, bis(imidazol‐2‐yl)methylamine = BIMA and bis(imidazol‐2‐yl)nitromethane = BINM] and two benzyloxycarbonyl (Z) derivatives (Z‐Gly‐BIMA and Z‐Ala‐BIMA) was reported. Mono‐ and bis‐chelated species with the (N~im~, N~im~) donor set were formed in both acid and neutral pH conditions, with the bis‐chelated complexes being characterised by a cis‐trans isomerism. In the basic pH range the complexation process continues with the formation of a mono‐hydroxo cis‐VOL~2~H~–1~ complex in systems with BIM, BIMA and BINM, and with the deprotonation and coordination of the amide nitrogen to give VOLH~–1~ and VOLH~–2~ in those with Gly‐BIMA, α‐Asp‐BIMA, α‐Glu‐BIMA and His‐BIMA. The results demonstrate that the bis(imidazol‐2‐yl)methyl residue is an anchoring group of intermediate strength, capable of avoiding extensive hydrolysis of the V^IV^O ion in the presence of a slight excess of ligand (L/M from 3:1 to 5:1). DFT calculations with progressively more complex basis sets were performed in order to obtain information on the structure of the VOLH~–1~ and VOLH~–2~ complexes. Finally, a discussion on the ^51^V anisotropic parallel hyperfine coupling constant (A~∥~) of VOLH~–1~ and VOLH~–2~ and on the EPR properties connected to the V–N^–^(amide) bond in V^IV^O complexes is presented. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)