Catalytic Hydrogenolysis and Isomerization of Light Alkanes over the Silica-Supported Titanium Hydride Complex (⋮SiO)₃TiH

Abstract

We have recently reported the hydrogenolysis of light alkanes under very mild conditions of temperature and pressure catalyzed by (tSiO) 3 ZrH 1 and (tSiO) 3 HfH. 2 This remarkable reactivity can be attributed to the fact that the active species are formally 8 eand extremely coordinatively unsaturated. We proposed, on the basis of the primary products observed, that the hydrogenolysis proceeds by stepwise cleavage of carboncarbon bonds by -alkyl elimination from surface metal-alkyl intermediates. We now report that under identical conditions the reactions of neopentane and isobutane with hydrogen over (tSiO) 3 TiH cannot be simply described as hydrogenolysis but that alkane skeletal isomerization also occurs.The title complex, (tSiO) 3 TiH, is synthesized in two steps from TiNp 4 [Np ) CH 2 C(CH 3 ) 3 ] and silica (Degussa Aerosil, 200 m 2 /g). All reactions are performed in sealed tube reactors pre-equipped with break-seal tubes for the introduction of nongaseous reactants and CaF 2 windows to allow aquisition of infrared spectra, as described elsewhere. 3 A silica wafer was dehydroxylated under dynamic vacuum for 15 h at 500°C (silica 500 ). The infrared spectrum indicates the presence of surface silanols free of any hydrogen bonding. Tetraneopentyl titanium 4 is sublimed (60°C, 10 -4 mm Hg) onto the disk at room temperature. The disk turns from white to yellow, neopentane is given off, the ν(SiO-H) band (3747 cm -1 ) is consumed, and new bands corresponding to the neopentyl ligand ν(CH) (2956, 2905, 2867, and 2800 cm -1 ) and δ(CH) (1466, 1393, 1365 cm -1 ) vibrations appear. When the reaction is performed on a fully deuterated silica disk, 1 equiv of neopentane-d 1 was evolved. It should be noted that the surface complex is never isolated, per se, but rather the formulations proposed would represent the vast majority of titanium centers on the surface. Spectroscopy, 5 other chemical tests and elemental analyses 6 are consistent with a surface species in which a trineopentyl metal complex is linked to the surface by one siloxy bridge, tSiOTiNp 3 (1), as was the case for the analogous zirconium and hafnium reactions.This alkyl complex (20 mg, 1 wt % Ti, 9.5 µmol) was treated under hydrogen (1 atm, 10 mmol) at 150°C for 1 h. The solid turned from pale yellow to brownish yellow. In the infrared spectrum, the bands associated with the neopentyl ligands disappear with the formation of four bands associated with ν-(TiH) at 1706, 1692, 1679, and 1647 cm -1 and two bands associated with ν(SiH) at 2263 and 2196 cm -1 . Methane and ethane were formed in a 1:1 ratio (total carbon given off corresponds to the release and hydrogenolysis of 3 equiv of neopentane). Reaction of this complex with deuterium gas (1 atm, 10 mmol) showed rapid disappearance of the ν(TiH) bands (within 15 min at room temperature) with no modification of ν(SiH) bands: the expected ν(TiD) bands were not observed as the silica is opaque in the region corresponding to these bands (around 1200 cm -1 ). Subsequent reaction with hydrogen gas completely ...

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