Reactivity of Tetraneopentylhafnium, Hf(CH2tBu)4, with Silica Surfaces

The reaction of Hf(CH 2 t Bu) 4 with a silica surface treated at 800 °C affords a unique surface organometallic species in only one surface environment, (⋮SiO)Hf(CH 2 t Bu) 3 ( 1 -SiO 2 - (800) ). In contrast, with SiO 2 - (500) two surface species, (⋮SiO)Hf(CH 2 t Bu) 3 ( 1 -SiO 2 - (500) ) and (⋮SiO) 2 Hf(CH 2 t Bu) 2 ( 2 −SiO 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 C 1 to C 5 . 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(CH 2 t Bu) 3 indicated the hafnium complex exhibits in EXAFS shorter Hf−C and Hf−O bonds and a larger Hf−C α −C β angle and in 2D J -resolved NMR spectra a lower 1 J (C α −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(CH 2 t Bu) 3 and more active in alkane hydrogenolysis.