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 C_4v symmetry. The IR spectra confirmed that the complexes are all monomeric except for [VO(Clsal)₂tn] which polymerizes via OV?VO 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.