Aqueous chemistry of ammonium (dipicolinato)oxovanadate(V): The first organic vanadium(V) insulin-mimetic compound

The aqueous speciation, formation constants, and solution structure were determined for a new insulin-mimetic organic vanadium(V) compound (ammonium (dipicolinato)oxovanadate(V)). The solution properties of the system were characterized by using potentiometry,H-1, C-13, and V-51 NMR 1D and 2D spectroscopy, and UV/visible spectroscopy. These studies were conducted using the crystalline compound as well as combinations of the free ligand and the metal salt. The major complex is most stable in the acidic pH range, although it does protonate at low pH. It protonates at pH similar to 1 and decomposes below pH 0. The dipic ligand is coordinated in a tridentate manner throughout the pH range studied. Protonation at low pH takes place on one of the oxo groups. Dynamic processes were explored using H-1 and C-13 EXSY NMR spectroscopy. VO(2)dipic(-) was found to exchange between the complex and the ligand at high and at low pH values. In the intermediate-pH range, no evidence for exchange processes was obtained, documenting the inertness of the complex at pH 3-4. The high stability and inertness in the pH 3-4 region may be of biological significance since the combination of high stability and low lability suggests the complex will be more resistant to hydrolysis at the pH of the stomach.