GENERAL PRINCIPLES OF THE SYNTHESIS OF POLYOXOMETALATES IN AQUEOUS SOLUTION

摘要

The heteropolyoxotungstates constitute a large family of compounds which can be obtained by successive condensation and hydrolysis reactions starting from the mononuclear metal anion and a precursor of the heteroatom. Several fundamental features can be drawn: 1. The 9-metalli-oxoanions are the first species synthesized at pH 9-10 when the metal is in excess with respect to the heteroatom, and play a key role in the forthcoming condensation steps occuring at lower pH. 2. Each species has a pH range in which its stability is maximum but its kinetic or thermodynamic stability can strongly depends on the nature of the heteroatom and the alkaline cations present in the solution. This is due to the formation of complexes between the vacant heteropolyoxotungstates (considered as polydentate ligands) and the alkaline cations. The adaptation of the sizes of the coordination site and the cation is the main factor controlling the stability. 3. Several parallel condensation or hydrolysis reactions can occur when the metal atoms added or eliminated, respectively, are in non-equivalent sites in the structure. The selectivity of these reactions depends also on the nature of the alkaline cation. In some cases, the thermodynamic product is directly obtained but in other cases a kinetic intermediate is formed which transforms then into the stable product. Large variations of the rate of this transformation can be observed depending upon the nature of the alkaline cation in the solution. In conclusion, the nature of the alkaline cations in the solution is a fundamental parameter for the control the synthesis of the heteropolyoxotungstates in aqueous solution but the use of mixed aqueous/organic or pure organic solvents can strongly modify the kinetic and thermodynamic stability of these species.