Kinetics and Mechanisms of Thermally Induced Alkane Eliminations from Silica-Supported Bis(alkyl)chromium(IV) and -vanadium(IV) Complexes

Silica-supported bis(alkyl) complexes of Cr(IV) and V(IV), (⋮SiO) 2 M(CH 2 R) 2, undergo surprisingly clean thermal transformations to generate supported alkylidene complexes, (⋮SiO) 2 M CHR, with concurrent liberation of RCH 3 . The reactions are quantitative and kinetically first order, suggesting that all surface organometallic complexes possess or can access very similar coordination environments. Isotope-labeling and kinetics experiments support a mechanistic assignment of intramolecular α-H elimination. In the case of the Cr complexes, the temperature dependences of the first-order rate constants give nonlinear Eyring plots that are concave downward. A two-step mechanism of reversible (alkyl)(alkylidene)Cr VI hydride formation followed by reductive elimination of alkane is proposed. This mechanism is not accessible for V(IV), therefore concerted α-H elimination is considered more likely. The activation parameters for the thermolysis of (⋮SiO) 2 V(CH 2 SiMe 3 ) 2, obtained from the linear Eyring plot, are Δ H ⧧ = (11.0 ± 1.0) kcal/mol and Δ S ‡ = (−43 ± 3) cal K - 1 mol - 1 . The large negative value of Δ S ‡ is suggestive of surface assistance for α-H elimination.