Structure and reactivity of silica-supported chromium(IV) complexes.

This thesis has been divided into four major sections. The first section deals with the reactions of CrR4 (R is neopentyl or trimethylsilylmethyl) with the surfaces of partially dehydroxylated silicas, leading to the formation of discrete mononuclear surface organometallic fragments. The reaction stoichiometry was found to depend on the density of surface hydroxyl groups. On silica subjected to prior dehydroxylation at 500°C, one Cr is grafted per hydroxyl group and one equiv. of alkane is evolved, leaving ≡SiOCrR3 on the surface. On silica dehydroxylated at 200°C, each Cr is grafted onto two hydroxyl groups and two equiv. of alkane are evolved, giving (≡SiO) 2CrR2. When CrR4 reacts with deuterated hydroxyl groups, monodeuteroalkane is liberated. The chemisorbed Cr species retain the oxidation state (IV) and nuclearity (mono-) of their parent molecular precursors. The second section examines the thermal transformations of silicasupported (≡SiO)2Cr(CH2CMe3)2, which undergoes a clean reaction at 69°C to generate a supported alkylidene complex, (≡SiO)2Cr=CHCMe3, with concurrent liberation of CMe4. The reaction is quantitative and kinetically first-order. Isotope-labeling and kinetics experiments support a mechanistic assignment of intramolecular alpha-H abstraction. The temperature dependence of the first-order rate constants is consistent with a two-step mechanism of reversible (alkyl)(alkyhdene)Cr(VI) hydride formation followed by reductive elimination of alkane. Reactions with acetone and Br2 are consistent with the alkylidene formulation. A related alkylidene complex was prepared by thermolysis of silicasupported (≡SiO)2Cr(CH2SiMe3) 2 at 150°C. The third section describes the reactions of supported alkylidenes with various olefins. In contrast to silica-supported bis(neopentyl)Cr(IV), the surface alkylidene complexes are efficient room temperature alpha-olefin polymerization catalysts. Based on kinetic and isotope effects, a mechanism of agostic-assisted migratory insertion is proposed for the propagation step. The overall oxidation state of the Cr remains (IV) during the reaction. The last part of this thesis deals with the study of silica-supported amido and alkoxo chromium complexes. The reactions of Cr(NEt2) 4 and Cr(OtBu)4 with the partially dehydroxylated silica surface at 200°C give exclusively (≡SiO)2Cr(NEt 2)2 and (≡SiO)2Cr(OtBu) 2, respectively. Both surface organometallic complexes undergo ligand exchange reactions with alcohols. Thermal treatment of (≡SiO)2 Cr(OtBu)2 followed by addition of benzyl alcohol shows evidence for oxidation to benzoic acid. However, the "Cr=O" intermediate has yet to be detected