Anchored homogeneous catalysts: the role of the heteropoly acid anchoring agent

Heteropoly acids (HPAs) are used to anchor catalytically active homogeneous complexes to supports by an interaction between the HPA and the metal atom of the complex. In an attempt to determine the nature of the HPA/metal bonding several substrates were hydrogenated using chiral and achiral complexes anchored to alumina by the commercially available Keggin HPAs, phosphotungstic acid (PTA), phosphomolybdic acid (PMA), silicotungstic acid (STA) and silicomolybdic acid (SMA). It was found that the nature of the HPA can have a significant influence on both the activity and selectivity of reactions run using these HPA anchored homogeneous catalysts. The results obtained indicate that there is a direct interaction between the HPA and the metal atom of the active organometallic complex. While these data do not unequivocally establish that there is a bond between the metal atom and an oxygen on the surface of the HPA, the extent to which the activity and selectivity change when different HPAs are used as anchors is more like what would be expected when one uses a series of ligands attached to the metal atom rather than changing the counterions associated with a charged organometallic species. (C) 2003 Elsevier B.V. All rights reserved.