The reaction of Hf(CH2tBu)4 with a silica surface treated at 800 °C affords a unique surface organometallic species in only one surface environment, (≡SiO)Hf(CH2tBu)3 (1-SiO2-(800)). In contrast, with SiO2-(500) two surface species, (≡SiO)Hf(CH2?Bu)3 (1-SiO2-(500)) and (≡SiO)2Hf(CH2tBu)2 (2SiO 2-(500)), in a molar ratio of 70:30 are obtained. Thermal treatment of 1-SiO2-(800) at increasing temperatures leads to the successive evolution of neopentane, isobutene, and isobutane as well as several alkanes varying from C1 to C5. Polyisobutenes are also formed on the surface. The mechanism by which such decomposition occurs suggests a succession of γ-H eliminations with formation of neopentane followed by β-methyl transfer and formation of isobutene and [Hf] - Me. This isobutene is reinserted into [Hf] - Me with formation of isopentene and [Hf] - H. A comparison of the analytical data of 1-SiO2-(800) and (≡SiO)Zr(CH2tBu)3 indicated the hafnium complex exhibits in EXAFS shorter and Hf - O bonds and a larger Hf - Cα - Cβ angle and in 2D J-resolved NMR spectra a lower 1J(Cα - H) value. These differences underlined a larger steric hindrance in the coordination sphere of the Hf metal. The thermal stability of 1-SiO2-(800) was monitored by infrared spectroscopy, in batch and continuous flow reactors, and proved that 1-SiO2-(800) was more stable than (≡SiO)Zr(CH2tBu)3 and more active in alkane hydrogenolysis.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry