| The focus of this study is to explore the structural features of polyoxometalates that are closely related to their catalytic properties and to better understand the mechanisms behind their reactivity. The synthetic work has centered on cluster systems based on trivacant Keggin and Wells-Dawson polyoxoanions. They have been proved to be very important synthons for the preparation of transition-metal-substituted complexes that can be extensively modified to impact their utility in catalysis, medicine and materials science.; The templating effects of a small dianion group, carbonate, in the coordination chemistry of lacunary tungstophophates have been investigated as part of our initial effort to incorporate functional groups that mimic the catalytic properties of metal oxide surfaces. Further studies utilizing chiral organic ligands, such as L or D-tartrate, allow the isolation of enantiomerically pure, nonracemizing polyanion complexes. Strong chiroptical effects are manifested by the very large metal oxide frameworks as a result of chirality transfer from the far smaller enantiopure organic molecule. NMR and chiroptical studies indicate that the complexes are configurationally stable in solution. In addition, complexes with multiple stereogenic centers have been prepared in similar ways. In these systems, specific enantiomers may be isolated by controlling the absolute configuration of given chiral inducer(s), and their conversion to meso stereoisomers has been observed by 31P NMR in cross-over experiments.; In our attempts to elucidate the mechanism of proton transfer on the surfaces of polyoxoanions, we found that two related polyoxotungstate complexes, {lcub}[P2W15O54(H2O)2] 2Zr{rcub}12- and {lcub}[P2W15O54(H 2O)2]Zr[P2W17O61]{rcub} 14-, exhibit extensive dynamic structural changes noted by 31P NMR and X-ray crystallography induced by unusual quantitatively reversible multiple protonation behavior. pH titration of {lcub}[P2W 15O54(H2O)2]2Zr{rcub} 12- indicates a two-step dissociation of all eight protons in agreement with the structural and spectroscopic studies. These systems offer structural insights into dynamic proton transfer for metal oxide clusters and provide a model to investigate proton-related heterogeneous catalysis at metal oxide interfaces. |
