Group 4 alkyl complexes as precursors of silica anchored molecular catalysts for the reduction of ketones by hydrogen transfer

The silica anchored mononuclear isopropoxides of the elements of group 4, = SiOM(OiPr)(3), M=Ti, Zr, Hf, were synthesized via a two-step procedure, com prising: (i) the reaction of the tetraneopentyl complexes MNp4 with a silic a partially dehydroxylated at 500 degrees C; (ii) the reaction of the surfa ce complexes thus obtained with isopropanol. The supported Ti complex is to tally inactive for the reduction of ketones by isopropanol; furthermore a s ignificant fraction of titanium leaches into the solution. On the contrary, the zirconium and hafnium complexes are efficient catalysts for the same r eaction. Their properties are dependent upon the nature of the ketone, the nature of the solvent and are sensitive to the presence of water. Under all conditions so far tested, the supported hafnium catalyst exhibits a higher activity than the zirconium one. The very similar catalytic behaviors of = SiOHf(OiPr)(3) and =SiOHf(OH)(3), a complex obtained by mild hydrolysis of =SiOHfNp3, are interpreted by the easy substitution of the hydroxy ligands by isopropanol, evidenced by in situ IR spectroscopy; unexpectedly, the rev erse reaction is much more difficult. Finally, the better performances of = SiOZr(OiPr)(3) when compared to those of a formally similar solid synthesiz ed from Zr(OiPr)4 and silica highlights the importance of the choice of the precursor and of the surface state of silica to obtain a stable, mononucle ar active species. (C) 1999 Elsevier Science B.V. All rights reserved.