Transition metal imido complexes : synthesis and applications to polymerisation catalysis
This thesis describes studies into Group 5 and Group 6 transition metal imido complexes, with particular emphasis on the development of complexes which can be applied to catalytic processes. Chapter 1 highlights the electronic and structural aspects of the imido and alkylidene ligands. The isolobal analogy between Group 4 bent metallocene. Group 5 half-sandwich imido and Group 6 bis(imido) metal fragments is outlined. In addition, Ziegler-Natta type a-olefin polymerisation and Ring Opening Metathesis Polymerisation (ROMP) are briefly reviewed. Chapter 2 describes initial screening of half-sandwich vanadium imido and chromium bis((^t)butylimido) dichloride complexes as catalyst precursors. Synthesis of the chromium bis(imido) dialkyl complex Cr(N(^t)Bu)(_2)(CH(_2)Ph)(_2) (1) is described, its conversion to a cationic alkyl species is probed and the polymerisation activity associated with the resultant compound is addressed. Finally this chapter details the synthesis and characterisation of a range of bis(adamantylimido) chromium complexes. Chapter 3 presents a synthetic entry point into the bis(arylimido) chemistry of chromium. The complex Cr(NAr)(_2)(NH(^t)Bu)Cl (12) is described (Ar = 2,6-(^i)Pr(_2)C(_6)H(_3)) and its conversion to the dichloride complex Cr(NAr)(_2)Cl(_2) (14) is examined. 14 forms the stable monoadduct with pyridine, the X-ray crystallographic study of which reveals a distorted square based pyramidal geometry about the chromium atom. The inclusion of the arylimido ligand at the metal centre allows stabilisation of the chromium bis- phosphine complexes Cr(NAr)(_2)(PMe(_3))(_2) (18) and Cr(NAr)(_2)(PMe(_2)Ph)(_2) (19). The reactivity of 18 towards unsaturated hydrocarbon substrates is briefly investigated. Chapter 4 focuses on the organometallic chemistry of the [Cr(NAr)(_2)] moiety. A range of dialkyl derivatives are isolated and the molecular structures of a selection are solved. The generation of the nascent species [Cr(NAr)(_2)(=CHCMe(_3))] is investigated and the conversion of Cr(NAr)(_2)(CH(_2)CMe(_3))(_2) (24) to Cr(NAr)(-2)(CHDCMe(_3))(C(_6)D(_5)) (25) is the subject of a kinetic study. In chapter 5, the ROMP of a series of amino acid derived norbomene monomers is studied. The resultant polymers are fully characterised and a brief molecular modelling study is carried out on representative polymers chain lengths. Chapter 6 contains experimental details to chapters 2-5.