Vanadium(V) oxyanions. The dependence of vanadate alkyl ester formation on the pKa of the parent alcohols

Abstract

The interaction of vanadate (VO4H21−) with a variety of alkyl alcohol's (ROH) covering about seven pKa units of acidity have been studied in aqueous solution by 51V nuclear magnetic resonance spectroscopy. A linear free energy relationship between the equilibrium constant for the formation of the vanadate monoester (HOVO3R1−) and the acidity of the alcohol was established at 23 °C. The correlation was found to be [Formula: see text] where K = [HOVO3R1−]/[VO4H21−][ROH]. This correlation was shown not to extend to ligands with π-bonding capability such as phenols or phosphates, where the products are highly favoured relative to the alkyl esters. An interesting correlation between pKa values of the product esters and the pKa of the parent alcohols was also observed. It was found that below a [Formula: see text] of about 15 the pKa values of the esters were essentially constant. However, above this value of 15 the pKa of the ester was found to increase rapidly with an increase in pKa of the alcohol. This result may indicate that the electron accepting ability of the metal is exhausted with the higher pKa alcohols, and the extra electron density is transferred to the oxygens, thus causing an increase in pKa of the ester.