Reactivity of tetraneopentylhafnium, Hf(CH₂tBu)₄, with silica surfaces

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-SiO_2-(800)). In contrast, with SiO_2-(500) two surface species, (≡SiO)Hf(CH₂?Bu)₃ (1-SiO_2-(500)) and (≡SiO)₂Hf(CH₂tBu)₂ (2SiO _2-(500)), in a molar ratio of 70:30 are obtained. Thermal treatment of 1-SiO_2-(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-SiO_2-(800) and (≡SiO)Zr(CH₂tBu)₃ indicated the hafnium complex exhibits in EXAFS shorter and Hf - O bonds and a larger Hf - C_α - C_β 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-SiO_2-(800) was monitored by infrared spectroscopy, in batch and continuous flow reactors, and proved that 1-SiO_2-(800) was more stable than (≡SiO)Zr(CH₂tBu)₃ and more active in alkane hydrogenolysis.