Early-transition-metal alkylidene chemistry. Synthesis of tantalum silyl alklidene complexes and reactions of tantalum alkylidene complexes with silanes

This dissertation describes research conducted on the chemistry of tantalum silyl complexes, reactions of alkylidene complexes with phenylsilanes, kinetic and mechanistic studies of the reaction of a tantalum alkylidene complex with phenylsilanes, and related organometallic chemistry. An overview and summary of the Ph.D. research project is given in Chapter 1. Chapter 2 describes the synthesis and characterization of the thermally unstable tantalum silyl alkylidene complexes (Me₃ECH₂)₂Ta[=CHEMe₃]SiPh₂Bu^t (E = C, 1; Si, 2). Complexes 1 and 2 were found to decompose by elimination of HSiPh₂Bu^t to unidentified products and (Me₃SiCH₂)₄Ta₂(µ-CSiMe₃)₂, respectively. Reaction of 1 or 2 with PMe₃ leads to elimination of HSiPh₂Bu^t and the formation of bis(phosphine)bis(alkylidene) complexes (Me₃ECH₂)Ta(PMe₃)₂[=CHEMe₃]₂ (E = C, 3; Si, 4). Chapter 3 presents the reactions of the tantalum alkylidene complexes (Me₃SiCH₂)₃ Ta(PMe₃)=CHSiMe₃ (7), 4, and (Me₃SiCH₂)Ta(PMe₃)₂[=CHCMe₃]₂ (8) with phenylsilanes PhRSiH₂ (R = Ph, Me) and (PhSiH₂)₂CH₂ to yield novel bis(silyl)-substituted alkylidene complexes as well as the first reported 1, 1'metalla-3-silacyclobutadiene and 1, 1 '-metalla-3,5-disilacyclohexadiene complexes through the preferential reaction of the silanes with the alkylidene ligands of 4, 7, and 8. Chapter 4 describes the mechanistic investigation of the formation of the bis(silyl)-substituted alkylidene complex (Me₃SiCH₂)₃Ta=C(SiMe₃)SiPhMeH (9a) from the reaction of 7 with PhMeSiH₂. Kinetic studies were found to be consistent with a dissociative mechanism in which a PMe₃ molecule dissociated from the tantalum center in 7, forming an open coordination site on the metal for the reaction with PhMeSiH₂. Deuterium labeling studies were consistent with a mechanism involving hydride and silyl intermediates. The final chapter discusses attempted syntheses of dianionic bis(silyl)lithium and -potassium compounds as potential ligands for the synthesis of poly- or persilyl early-transition-metal complexes. Two previously unreported bis(silyl)methane and -ethane complexes [(Me₃Si)₃Si]₂CH₂ (18) and (Me₃Si)₃SiCH₂CH₂Si(SiMe₃)₃ (19) were synthesized as possible ligand precursors.