Reactions of Tetraalkylchromium(IV) with Silica:  Mechanism of Grafting and Characterization of Surface Organometallic Complexes

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.