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

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, comprising: (i) the reaction of the tetraneopentyl complexes MNp4 with a silica partially dehydroxylated at 500°C; (ii) the reaction of the surface complexes thus obtained with isopropanol. The supported Ti complex is totally inactive for the reduction of ketones by isopropanol; furthermore a significant fraction of titanium leaches into the solution. On the contrary, the zirconium and hafnium complexes are efficient catalysts for the same reaction. 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 reverse reaction is much more difficult. Finally, the better performances of SiOZr(OiPr)3 when compared to those of a formally similar solid synthesized 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, mononuclear active species.