Chemistry of group IV metal ion-containing polyoxometalates

摘要

The chemistry of Group IV metal ion (Ti~(IV), Zr~(IV) and Hf~(IV))-containing polyoxometalates (POMs) is presented as a sharply focused microreview, introducing an aspect of our own research. The synthesis, structure, and solid/solution state behavior of the POMs, which are prepared by the reactions of various lacunary species of POMs with Ti~(IV), Zr ~(IV) and Hf~(IV) atoms, are described in several sections, classified with Keggin and Dawson POM families. Because of the ionic radius of Ti~(IV) (0.75 ?), close to that of WVI (0.74 ?), the Ti~(IV) atom can fit nicely into the mono-lacunary site of the POM, but Zr~(IV) and Hf~(IV) atoms (0.85-0.86 ?), larger than Ti~(IV) atom, do not fit into the mono-lacunary site of the POM. Thus, the mono-lacunary site of the POM acts as the oxygen-donor pentadentate ligand to the Ti~(IV) atom, whereas it acts as the tetradentate ligand to Zr~(IV) and Hf~(IV) atoms. The Ti~(IV) atom in the POM takes on six-coordinate geometry, whereas the Zr~(IV) and Hf ~(IV) atoms have higher coordination numbers (6, 7 and 8) due to their larger ionic radii. Consequently, most Ti-substituted Keggin POMs are isolated as oligomers formed by corner-sharing Ti-O-Ti bonds, whereas Zr/Hf-containing Keggin/Dawson POMs are usually isolated as di-, tri-, tetra-Zr/Hf cluster cations sandwiched between two lacunary POMs, the cluster cations of which are formed by edge-sharing M(OH)_2M (M = Zr, Hf) bonds. These compounds show quite different behavior under pH-dependent conditions. The pH-dependent interconversion between the dimeric and monomeric species of Ti-substituted Dawson POMs is quite an opposite tendency from those of Zr/Hf-containing Dawson POMs. The Ti-substituted Dawson POM oligomers have a tendency to undergo base hydrolysis to give monomers, whereas the Zr/Hf-containing Dawson POM oligomers have a tendency to undergo acid hydrolysis to provide monomers. In the Group ~(IV) metal ion-containing POMs, the Zr/Hf atoms function very similarly to each other, but show quite different behavior from the Ti atom. Site-selective lacunary species of WVI atoms in polyoxometalates (POMs) can be utilized as a support for various metal ions and cationic species. This microreview is focused on the syntheses, molecular structures and properties of Group IV metal ion-containing POMs, which were prepared by the reactions of various lacunary species of POMs with Ti~(IV), Zr~(IV) and Hf~(IV) atoms.