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

The 51V NMR chemical shifts of four peroxovanadium complexes, namely [VO(O2)pic] (1), [VO(O2)pyraz] (2), [VO(O2)aq1] (3) and [VO(O2)2aq] (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 51V NMR chemical shifts of the four complexes in the mixed solvent H2OCH3CN. In such a medium only one 51V 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 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 51V NMR 51V 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 molecular 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.