Stoichiometric and catalytic reactivity of half-sandwich imido complexes of the group 5 metals

Abstract

This thesis describes investigations into the chemistry of half-sandwich imido complexes of the Group 5 metals, with particular emphasis on the development of complexes which can be applied to catalytic processes. Chapter 1 provides an introduction to the imido ligand and highlights aspects of the reactivity of half-sandwich transition metal imido and related complexes. The isolobal analogy between Group 4 bent me'tallocene. Group 5 half-sandwich imido and Group 6 bis(imido) metal fragments is outlined. In Chapter 2, reactions of CpV(NR)Cl(_2) (R=2,6-(^i)Pr(_2)C(_6)H(_3), 2,6-Me(_2)C(_6)H(_3)) with alkylating agents are described, yielding a number of highly unusual products. The novel structures of [CpV(N-2,6-(^i)Pr(_2)C(_6)H(_3))(|i-Me)2]2(l^-Mg) and [CpV(fx-N-2,6-iPr2C6H3)]2(|i-Me), featuring bridging methyl and imido substituents, have been determined. The former also possesses the first crystallograghically characterised V-Mg bond and multiple agostic interactions. The aim of Chapter 3 was to evaluate the steric and electronic influence of the imido substituent in half-sandwich imido complexes of niobium. For the [CpNb(N-2-(^t)BuC(_6)H(_4))] system, the successful isolation of a number of previously unstable species has been reported. The orientations of the diphenylacetylene and the ethylene ligands in CpNb(N-2-(^t)BuC(_6)H(_4))(PhC=CPh)(PMe(_3)) and CpNb(N-2,6-Cl(_2)C(_6)H(_3))(C(_2)H(_4))(PMe(_3)) respectively have been established through crystal structures and lend support to the isolobal relationship between the [Cp(_2)Zr] and [CpNb(NR)] fragments. In Chapter 4, the tantalum dialkyl complexes Cp*Ta(N(^t)Bu)(CH(_2)R)(_2) R=Ph, CMe(_2)Ph, CMe(_3)] are described and the molecular structure of Cp*Ta(NtBu)(CH(_2)CMe(_3))') reveals multiple agostic interactions. Reactions of Cp*Ta(N(^t)Bu)(CH(_2)R)(_2) with excess C(_6)F(_5)OH yield a number of crystallographically characterised pentafluorophenoxide and oxo-bridged products, namely [Cp*Ta(CH(_2)Ph)(0C(_6)F(_5))(µ-O)](_2), Cp*Ta(OC(_6)F(_5))(_4) and [Cp*Ta(OC(_6)F(_5))(_2)(µ-0)](_2). Chapter 5 discusses attempts to develop and explore the oxidative coupling chemistry at the half-sandwich tantalum imido fragment. Hence, a number of new tantalacyclopentane complexes and related species have been prepared, including Cp*Ta(N-2,6-(^i)Pr(_2)C(_6)H(_3))(σ-l,4-(2-Et)C(_4)H(_7)) and Cp*Ta(N- 2,6-(^i)Pr(_2)C(_6)H(_3))[a-l,6-C(0)(3-Et)C(_4)H(_7)C(0)].Chapter 6 contains the experimental details for Chapter 2-5.