Mechanism of <i>n</i>‑Butane Hydrogenolysis Promoted by Ta-Hydrides Supported on Silica

The mechanism of hydrogenolysis of alkanes, promoted by Ta-hydrides supported on silica via 2 Si–O– bonds, has been studied with a density functional theory (DFT) approach. Our study suggests that the initial monohydride (Si–O−)₂Ta^(III)H is rapidly trapped by molecular hydrogen to form the more stable tris-hydride (Si–O−)₂Ta^(V)H₃. Loading of <i>n</i>-butane to the Ta-center occurs through C–H activation concerted with elimination of molecular hydrogen (σ-bond metathesis). Once the Ta-alkyl species is formed, the C–C activation step corresponds to a β-alkyl transfer to the metal with elimination of an olefin. According to these calculations, an α-alkyl transfer to the metal to form a Ta-carbene species is of higher energy. The olefins formed during the C–C activation step can be rapidly hydrogenated by both mono- and tris-Ta-hydride species, making the overall process of alkane cracking thermodynamically favored

Well-Defined Surface Species [(SiO)W(O)Me₃] Prepared by Direct Methylation of [(SiO)W(O)Cl₃], a Catalyst for Cycloalkane Metathesis and Transformation of Ethylene to Propylene

The silica-supported tungsten oxo-trimethyl complex [(SiO)­W­(O)­Me₃] was synthesized using a novel SOMC synthetic approach. By grafting the inexpensive stable compound WOCl₄ on the surface of silica, partially dehydroxylated at 700 °C (SiO_2‑700), a well-defined monopodal surface complex [(SiO)­W­(O)­Cl₃] was produced. The supported complex directly methylated with ZnMe₂ and transformed into [(SiO)­W­(O)­Me₃], which we fully characterized by microanalysis, IR, mass balance and SS NMR (¹H, ¹³C, ¹H–¹³C HETCOR, ¹H–¹H DQ and TQ). [(SiO)­W­(O)­Me₃] has two conformational isomers on the surface at room temperature. The conversion of one to the other was observed at 318 K by variable-temperature ¹³C CP/MAS and ¹H spin echo MAS solid-state NMR; this was also confirmed by NMR and DFT calculations. [(SiO)­W­(O)­Me₃] was found to be active in cyclooctane metathesis and to have a wide distribution range in ring-contracted and ring-expanded products. In addition, [(SiO)­W­(O)­Me₃] proved to be highly active for selective transformation of ethylene to propylene compared to other silica-supported organometallic complexes