Bis(acetylamido)oxovanadium(IV) complexes: solid state and solution studies

The syntheses, solid state, spectroscopic and potentiometric solution characterizations of VO(acac-NH2)(2) (acac-NH2 = acetylacetamido) and VO(acac-NMe2)(2) (acac-NMe2 = N,N-dimethylacetylacetamido) have been investigated. In VO(acac-NMe2)(2) the vanadium ion exhibits a distorted square pyramidal coordination environment. Crystals of VO(acac-NMe2)(2) are monoclinic and structural studies were carried out. EPR and UV-visible spectroscopic and potentiometric methods were used to characterize the speciation, complex stability and structures in both aqueous and organic solutions. VO(acac-NH2)(2) and VO(acac-NMe2)(2) each form two species in solution: a 1 : 2 and a 1 : 1 metal : ligand complex. The complexes formed from VO(acac-NH2)(2) are slightly more stable than those of VO(acac-NMe2)(2); however, VO(acac)(2) forms more stable complexes. In noncoordinating solvents the 1 : 2 complexes maintain a five-coordinate vanadium atom, whereas in coordinating solvents such as water and pyridine the complexes become six-coordinate. Based on the similarity of the EPR parameters of VO(acac-NH2)(2) with those of the VO(acac)(2) complex, it is likely these species contain the coordinating solvent added trans to the V=O bond. These new VO( acac)(2) derivatives are improved with respect to water solubility. Although the solution stability is not as good as that of VO(acac)(2), it is higher than that of VO(acac-R)(2) (R = CH3 or C2H5). Thus, should the stability of the 1 : 1 complex be important for the compound's efficacy, both of these new complexes would be predicted to be less effective than VO(acac)(2) but more effective than VO(acac-Et)(2) (acac-Et = 3-ethylacetylacetonate) in lowering glucose levels in streptozotocin-induced diabetic rats.