Chemistry of Group IV Metal Ion‐Containing Polyoxometalates

Abstract

AbstractThe chemistry of Group IV metal ion (TiIV, ZrIV and HfIV)‐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 TiIV, ZrIV and HfIV atoms, are described in several sections, classified with Keggin and Dawson POM families. Because of the ionic radius of TiIV (0.75 Å), close to that of WVI (0.74 Å), the TiIV atom can fit nicely into the mono‐lacunary site of the POM, but ZrIV and HfIV atoms (0.85–0.86 Å), larger than TiIV 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 TiIV atom, whereas it acts as the tetradentate ligand to ZrIV and HfIV atoms. The TiIV atom in the POM takes on six‐coordinate geometry, whereas the ZrIV and HfIV 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.