Reactivity of silica-supported hafnium tris-neopentyl with dihydrogen: Formation and characterization of silica surface hafnium hydrides and alkyl hydride

Abstract

Surface organometallic chemistry represents an approach to the preparation of well-defined single sites for catalysis, the possibility of observing some elementary reaction steps, and the development of a fundamental basis for the synthesis of tailor-made catalysts. Silica-supported metal hydrides are an important class of new catalysts for alkane metathesis, methanolysis of alkanes, Ziegler-Natta depolymerization, alkane hydrogenolysis, etc. Understanding their mechanism of formation and aging is crucial. In the work presented here, the reaction of the well-defined silica surface organometallic complex [(≡SiO)Hf(CH2tBu)3], 1, ((≡SiO) = silica surface ligand) with dihydrogen has been performed at different temperatures (θ). At θ < 100°C, there is formation of a stable hafnium neopentyl dihydride, [(=SiO)-Hf(CH2tBu)(H)2], 2. For 100 < 0 < 200°C, 2 affords, via a succession of β-methyl transfer and subsequent hydrogenolysis of the resulting Hf-alkyl bonds, the formation of [(≡SiO)2Hf(H)2], 3, and [(=SiO)3SiH] with evolution of methane (C1) and ethane (C2). For 150 < θ < 300°C, 3 is totally converted into [(≡SiO) 3Hf(H)], 4, and [(≡SiO)2Si(H)2]. For θ > 300°C, [(≡SiO)3Hf(H)], 4, is transformed into [(≡ SiO)4HfJ, 5, and [(≡SiO)2Si(H) 2] into [(≡SiO)3SiH]. At this temperature, [(≡SiO)3SiH] is the only hydride remaining on the surface. All these species have been characterized with a multitude of techniques such as elemental analysis and infrared, 1H solid-state NMR, 1H DQ solid-state NMR, and EXAFS spectroscopies. The results elucidate a complete mechanism of surface organometallic chemistry by which one observes the stepwise transformation of a hafnium tris-neopentyl to hafnium neopentyl hydrides, hafnium mono- and bis-hydrides, silicon bis-hydride with the ultimate formation of tetrasiloxy surface species [(≡SiO)4Hf)], and silicon mono-hydride, the only hydride stable at very high temperature. It is suggested that the formation of these surface silicon hydrides is responsible for the aging of such catalysts in any reaction involving dihydrogen. © 2007 American Chemical Society.