Synthesis, characterization and anti-diabetic effect of bis[(1-R-imidazolinyl)phenolato]oxovanadium(IV) complexes

The five-coordinate oxovanadium(IV) complexes; [VO(pimin)2] (1a), [VO(Etpimin)2] (2) and [VO(EtOHpimin)2] (3), were prepared by reacting the ligands; 2-(2′-hydroxyphenyl)-1H-imidazoline (piminH), 2-(2′-hydroxyphenyl)-1-ethylimidazoline (EtpiminH) and 2-(2′-hydroxyphenyl)-1-ethanolimidazoline (EtOHpiminH), with VOSO4. The complexes were characterized by elemental analysis, IR, UV–Vis and cyclic voltammetry. All complexes show VO stretching vibrations between 932 and 987 cm−1. The presence of three d–d transition occurring between 400 and 625 nm and the irreversible oxidation (VIV → VV) between 400 and 490 mV confirm the d1 electronic configuration of the complexes. The solid state structures of [VO(pimin)2] (1a) and its autoxidation hydrolysis product [VO2(pimin)(piminH′)] (1b) were determined by single crystal X-ray diffraction. The geometry of [VO(pimin)2] was found to be intermediate between trigonal bipyramidal and square pyramidal and sits on a crystallographic twofold axis, while the geometry of [VO2(pimin)(piminH′)] was distorted trigonal bipyramidal. Potentiometric titrations were used to determine the protonation and stability constants for the ligands and oxovanadium(IV) complexes, respectively. The species existing over a biological pH range were also investigated. The in vitro studies indicated that the oxovanadium(IV) complexes were effective in enhancing glucose uptake in the 3T3-L1 adipocytes, C2C12 muscle cells and Chang liver cell lines. In these cell lines, the anti-hyperglycemic effect was equivalent to or surpassed the effect of metformin.