Surface Organometallic Chemistry of Hf(CH(2)tBu)(4) on Silica and Silica-Alumina: Reaction of the Resulting Grafted Hafnium Neopentyl with Dihydrogen

摘要

[Hf(CH(2)tBu)(4)], t, has been found to react at room temperature with an aerosil silica dehydroxylated at different temperatures theta (theta = 800, 500, 200 degrees C) to give a modified surface referred to as 1/SiO2-(theta) and with a silica-alumina to give a modified surface referred to as 1/SiO2-Al2O3-(500). With SiO2-(800) a single-site monosiloxy surface complex [( SiO)Hf(CH(2)tBu)(3)], 1/SiO2-(800), is obtained. Contrarily, with 1/SiO2-(500) a monosiloxy 1/SiO2-(500) and a bis-siloxy surface complex [( SiO)(2)Hf(CH(2)tBu)(2)], 2-SiO2-(500), are formed in a ratio of 70%:30%. With SiO2-(200), there is mainly the formation of the bis-siloxy surface complex (up to 90%) but in different local environments: ( SiO)(2)Hf(CH(2)tBu)(2), 2-SiO2-(200), and ( SiO)(2)( SiOH)Hf(CH(2)tBu)(2)], 2'-SiO2-(200). Finally with SiO2-Al2O3-(500), two major neutral surface complexes are formed: the monosiloxy 1-SiO2-Al2O3-(500) and the bisiloxy 2-SiO2-Al2O3-(500), as well as a third complex, 3, which is not as well-defined and may be cationic. Under hydrogen at 150 degrees C, 17 h, both modified surface 1/SiO2-(theta) and 1/SiO2-Al2O3-(500) afford the same surface hydrides but in different proportion and diverse surface "local environments". The formation of these hydrides is concomitant with the formation of [( SiO)(2)Si(H)(2)] and [( SiO)(3)SiH]. With 1/SiO2-Al2O3-(500), the formation of [( SiO)(n)AlH] is also observed. The major surface hydride in the hydrogenolysis of 1/SiO2-(800), 1/SiO2-(500), and 1/SiO2-Al2O3-(500) is ( SiO)(2)Hf(H)(2), whereas in the hydrogenolysis of 1/SiO2-(200) ( SiO)(3)Hf(H) forms preferentially. All these alkyl and hydride surface complexes have been fully characterized by elemental analysis, labeling experiments, infrared, H-1/C-13 solid-state NMR, and H-1 DQ solid-state NMR.