[Hf(CH2tBu)4], 1, has been found to react at room temperature with an aerosil silica dehydroxylated at different temperatures θ (θ = 800, 500, 200 °C) to give a modified surface referred to as 1/SiO2-(θ) 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(CH2tBu)3], 1-SiO2-(800), is obtained. Contrarily, with SiO2-(500) a monosiloxy 1-SiO 2-(800) and a bis-siloxy surface complex [(≡SiO) 2Hf(CH2tBu)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)2Hf(CH2tBu)2, 2-SiO2-(200), and (≡SiO)2(≡SiOH)Hf(CH 2tBu)2, 2′-SiO2-(200). Finally with SiO2-Al2O3.(500), two major neutral surface complexes are formed: the monosiloxy 1-SiO2-Al2O 3-(500) and the bisiloxy 2-SiO2-Al2O 3.(500), as well as a third complex, 3, which is not as well-defined and may be cationic. Under hydrogen at 150 °C, 17 h, both modified surface 1/SiO2-(θ) and 1/SiO2-Al2O 3-(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)2Si(H) 2] and [(≡SiO)3SiH]. With 1/SiO2-Al 2O3.(500), the formation of [(≡SiO)nAlH] is also observed. The major surface hydride in the hydrogenolysis of 1/SiO 2-(800), 1/SiO2-(500), and l/SiO2Al 2O3-(500) is (≡SiO)2Hf(H)2, whereas in the hydrogenolysis of 1/SiO2-(200), (≡SiO) 3Hf(H) forms preferentially. All these alkyl and hydride surface complexes have been fully characterized by elemental analysis, labeling experiments, infrared, 1H/13C solid-state NMR, and 1H DQ solid-state NMR.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry