Structural variations of the adducts of insulin-enhancing VO(pic)2 compound with neutral O- and N-ligands: X-ray and DFT quantum-mechanical study

Oxidovanadium(IV) bispicolinato compounds with dimethyl sulfoxide (DMSO), pyridine, 4-aminopyridine, 4-(dimethylamino)pyridine, and imidazole were prepared and their crystal structures determined. The coordination of VO(IV) is octahedral with the neutral ligand in cis position to the V=O moiety. X-ray crystallographic studies revealed different arrangements of the ligands around the vanadium metal center. The DMSO adduct presents the first crystallographic evidence of VO(IV) picolinato complexes with a nitrogen atom occupying the axial position trans to the V=O, leading to a geometry with both oxygen atoms from the picolinato ligands in cis position to the V=O and trans to each other. On the other hand, the compounds with N-ligands have an arrangement similar to some known structures with one carboxylic group in trans position to the V=O moiety. Quantum-mechanical calculations were performed on six possible isomers of DMSO and pyridine adducts, and their energy differences were studied.