Synthesis, characterization and preliminary insulin-enhancing studies of symmetrical tetradentate Schiff base complexes of oxovanadium(IV)

A series of oxovanadium(IV) symmetrical tetradentate Schiff base complexes have been isolated from the reaction of VOSO4 with Schiff bases obtained from the condensation of 2-hydroxybenzophenone or 2-hydroxy-5-chlorosalicylaldehyde with various aliphatic diamines. The compounds were characterized by elemental analyses, 1H NMR, infrared, electron paramagnetic resonance, electronic spectral, cyclic voltammetry and room temperature magnetic susceptibility measurements. The solution EPR spectra are consistent with square pyramidal complexes with C4v symmetry. The IR spectra confirmed that the complexes are all monomeric except for [VO(Clsal)2tn] which polymerizes via Odouble bond; length as m-dashV⋯Vdouble bond; length as m-dashO linkages. The electronic spectra indicate a square pyramidal geometry in both non-coordinating and coordinating solvents except for [VO(bp2-pn)] which appears to be octahedral in DMSO. The room temperature magnetic moments of 1.7–1.8 B.M. are normal for V(IV) d1 configuration. Evidence for electrochemical pseudo-reversibility is presented for four of the complexes. In vitro studies revealed that two of the compounds, [VO(bp2-en] and [VO(bp2-tn)MeOH], significantly increased glucose uptake when compared to the basal glucose uptake in transformed and sensitized C1C12 cells, but not at the same level as insulin.