Solvation, preferential solvation and complexation by the solvent of peroxovanadium complexes studied by 51V NMR spectroscopy.: Correlations with the oxidative reactivity

The V-51 NMR chemical shifts of four peroxovanadium complexes, namely [VO(O-2)pic] (1), [VO(O-2)pyraz] (2), [VO(O-2)aq(+)] (3) and [VO(O-2)(2)aq(-)] (4), in twelve solvents have been measured. Excellent Kamlet-Taft-type correlations with both the polarity and the hydrogen bonding donor ability of the solvents have been found for complexes 1 and 2. Preferential solvation phenomena have been studied by measuring the V-51 NMR chemical shifts of the four complexes in the mixed solvent H2O-CH3CN. In such a medium only one V-51 NMR signal is observed thus suggesting the occurrence of fast exchange processes between solvent molecules and ligands of vanadium. Preferential solvation plots indicate that CH3CN may be considered a better solvent than H2O for complexes 1-3 while for complex 4 the ability of the two solvents is very similar. These results have been taken as an indication that CH3CN displays a well characterized coordinating ability toward vanadium. In two other mixed solvents, i.e. H2O-CH3OH and H2O-DMF, under identical experimental conditions, two V-51 NMR signals are observed. Also on the basis of the comparison with the behavior of a peroxovanadium complex containing a tridentate ligand, namely pyridine-2,6-dicarboxylic acid, it has been established that only the equatorial water molecule undergoes a slow exchange with the solvents. This appears to be related to the reactivity of peroxovanadium complexes as measured by their decomposition rate. In addition such a reactivity is also affected by the electron density on the metal caused by the nature of the ligands. (C) 1998 Elsevier Science S.A. All rights reserved.