The reactions of CrR4 (1, R is neopentyl or trimethylsilylmethyl) with the surfaces of partially dehydroxylated silicas lead to the formation of discrete mononuclear surface organometallic fragments. The reaction stoichiometry depends on the density of surface hydroxyl groups. On silica subjected to prior dehydroxylation at 500 °C, one Cr is grafted per hydroxyl group and 1 equiv of alkane is evolved, giving ⋮SiOCrR3, 2. On silica dehydroxylated at 200 °C, each Cr is grafted onto two hydroxyl groups and 2 equiv of alkane are evolved, giving (⋮SiO)2CrR2, 3. When CrR4 reacts with deuterated hydroxyl groups, unlabeled and monodeuteroalkanes are liberated. A chemisorption mechanism is proposed with competing protonation by surface hydroxyl groups and intramolecular α-H elimination. The chemisorbed Cr species retain the oxidation state and nuclearity of their parent molecular precursors, as shown by UV−vis and EPR spectra and magnetic susceptibility measurement. The surface organometallic complexes are unreactive toward water, CO, and olefins but highly sensitive to O2. While 2 is thermally stable up to 100 °C, 3 smoothly eliminates one alkyl ligand as alkane at 60 °C.

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Silica-supported bis(alkyl) complexes of Cr(IV) and V(IV), (tSiO) 2 M(CH 2 R) 2 , undergo surprisingly clean thermal transformations to generate supported alkylidene complexes, (tSiO) 2 MdCHR, 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 R-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 R-H elimination is considered more likely. The activation parameters for the thermolysis of (tSiO) 2 V(CH 2 -SiMe 3 ) 2 , obtained from the linear Eyring plot, are ∆H q ) (11.0 ( 1.0) kcal/mol and ∆S ‡ ) (-43 ( 3) cal K -1 mol -1 . The large negative value of ∆S ‡ is suggestive of surface assistance for R-H elimination.

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