VIVO Complexes of Bis(imidazol-2-yl) Derivatives: a Potentiometric, Spectroscopic and DFT Study

The complexation of the (VO)-O-IV ion with four amino acid derivatives of bis(irnidazol-2-yl)methylamine [N-glycyl-bis(imidazol-2-yl)methylamine = Gly-BIMA, N-alpha-aspartyl-bis(imidazol-2-yl)methylamine = alpha-Asp-BIMA, N-alpha-glutamyl-bis(imidazol-2-yl)methylamine = alpha-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, Nim) 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-VOL2H-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, alpha-Asp-BIMA, alpha-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 (VO)-O-IV 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 V-51 anisotropic parallel hyperfine coupling constant (All) of VOLH-1 and VOLH-2 and on the EPR properties connected to the V-N-(amide) bond in (VO)-O-IV complexes is presented.