Early-transition-metal silyl complexes free of anionic ligands : comparative studies of alkyl and silyl ligands

This dissertation describes the research results about chemistry of early- transition-metal silyl complexes, mechanisms of the formation of tantalum alkyl- alkylidene and silyl-alkylidene complexes and related organometallic chemistry. An overview and summary of my Ph.D. research project are provided in Chapter 1. Chapter 2 describes the synthesis and characterization of a new family of early- transition-metal silyl complexes free from anionic π-ligands. These silyl complexes are (Me₃CCH2₂)₃TiSi(SiMe₃)₃ (1), (Me₃ECH₂)₂Ta(=CHEMe₃)Si (SiMe₃)₃ (E = C, 2; Si, 3) and (Me₃CCH₂)2₂W(=CCMe₃)Si(SiMe₃)₃ (4). Complexes 2 and 4 represent, to our knowledge, the first stable alkylidene and alkylidyne silyl complexes of early- transition-metal elements. Chapter 3 illustrates the kinetic and mechanistic studies of the formation of Schrock-type alkylidene complexes (Me₃ECH₂)₃Ta-CHEMe₃ (E = C, 8; Si, 9) and a bridged-alkylidyne complex (Me₃SiCH₂)₂Ta(µ-CSiMe₃)₂Ta(CH₂SiMe₃)₂ (6). (Me₃ECH₂)₅Ta (E = C, 11; Si, 12) are the precursors to the alkylidene complexes 8 and 9. With the elimination of EMe4, the quantitative conversion of 12 to 9 follows first-order kinetics. The further conversion of 9 to 6 follows second-order kinetics. Chapter 4 presents the mechanistic investigation of the formation of silyl-alkylidene complex 3 from the reaction of (Me₃SiCH₂)₃ TaCl₂ with LiSi(SiMe₃)₃(THF)₃. An intermediate (Me₃SiCH₂)₃Ta(Cl)Si(SiMe₃)₃ (5) is formed in the first step. The decomposition of 5 with the elimination of HSi(SiMe₃)₃ gives "(Me₃SiCH₂)₂Ta(=CHSiMe₃)(Cl)" (10) which then reacts with LiSi(SiMe₃)₃(THF)₃, to form the final product 3. In the absence of LiSi(SiMe₃)₃(THF)₃, complex 10 will dimerize to produce an unusual bis(alkylidene) complex (Me₃SiCH₂)₄(Cl)₂Ta₂(=CHSiMe₃)₂ (7). The mechanisms presented in these two chapters demonstrate two different reaction pathways leading to two types of alkylidene complexes: alkyl alkylidene and silyl alkylidene complexes. The final chapter describes the further study of the bis(alkylidene) complex 7. Two nonsymmetric bridged-alkylidyne complexes (PMe₃)₂(CI)M(µ-CSiMe₃)₂M(CI) (CH₂SiMe₃)₂ (M = Ta, 13; Nb, 14) are synthesized. The characterization and reactivities of complexes 7, 13 and 14 are reported.