A study of ligand exchange processes at early transition metal centres : synthesis, mechanism and applications to polymer synthesis
This thesis describes studies into the exchange reactivity of early transition metal species containing mono- and dianionic ligands, examining both the intermetal exchange of such ligands and the exchange to and from external substrates. Studies on the metathesis polymerisation of functionalised monomers using well-defined Schock initiators are described. Chapter 1 highlights the electronic and structural aspects of the ligands employed in this thesis (including oxo, imido, alkylidene, alkoxide and amide moieties) and briefly reviews intermetal ligand exchange reactions. In addition, applications of the olefin metathesis reaction, namely R.O.M.P and ADMET, are reviewed employing the well-defined Schrock type initiators. Chapter 2 describes the preparation of Mo(NBu(^t))(_2)(OBu(^t))(_2), and examines the reactivity of its imido ligands towards a series of organic substrates including aryl amines, benzaldehyde, dioxygen and isocyanates. Intermetal imido-imido exchange reactivity is inspected for Mo(NR)(_2)X(_2) and CpMo(NBu(^t))(_2)Cl systems. The intermetal oxo-imido exchange reaction between Mo(NBu(^1))(_2)(OR)(_2) and Mo(O)(_2)(OR)(_2) is investigated, in which the role of acid catalysis is examined. The effects of increasing the elecu-on withdrawing properties of the ancillary alkoxide ligands by progressive fluorination are probed. The favourable position of the oxo-imido exchange reaction is exploited in the synthesis of the oxo-imido complexes Mo(O)(NBu(^t))(OBu(^t))2, Mo(O)(NBu(^t))(OR(^6))(_2) and Mo(O)(NBu(^t))Cl(_2).dme. The intermetal exchange reactivity of mono-anionic X-type ligands is then explored in systems of the type MoQ(_2)X(_2) (Q = O, NBu(^t); Q(_2) = (NAr)(CHCMe(_2)Ph); X = CI, OR, NHR).Chapter 3 presents the attempted synthesis of a half-sandwich niobium bis (imido) complex of general formula CpNb(NR(^1))(NR(^2)), starting with the synthesis of imido-amide complexes CpNb(NR)(NR(^1)R(^2))Cl (R = Bu(^t), R(^1) = R(^2) = Et; R = Ar, R(^1) = Bu(^t) R(^2) = H) via the reaction of CpNb(NR)Cl(_2) with lithium amides. Further reaction of these imido-amide species with lithium amides result in the formation of CpNb(NBu(^t))(NHBu(^t))(_2) and CpNb(NBu(^t))(NHBu(^t)(NE(_t2)). The reactivity of the imido ligand in CpNb(NBu(^t))X(_2) is investigated towards aryl amines, butanol, dioxygen and H(_2)O. Complexes of the type CpNb(NBu(^t))X(_2) (X = Br, OPr(^3), OR(_f)(^3), OR(_f_(^6) and NC(_4)H(_4)) are isolated and employed in mono-anionic intermetal exchange reactions, the results from which are compared with those from analogous reactions in chapter 2.Chapter 4 outlines metathesis polymerisation studies on hetero-cyclopentenes using the well-defined molybdenum Schrock initiators. The R.O.M.P of 2,3- and 2,5- dihydrofurans are reported. A metallacycle adduct is isolated in the reaction of Mo(NAr)(CHCMe(_2)Ph)(OBu(^t))(_2) with vinylene carbonate. The reaction of l-(amino)-3- borolenes with the Schrock initiators is described and a product characterised as a 1- (amino)-2-borolene by X-ray crystallography. The factors that may effect the polymerisability of hetero-cyclopentenes are briefly considered. In addition, the reactivity of functionalised acyclic diallyl species of the type Q(CH(_2)CHCH(_2))(_2) (Q = O, NH, PPh, O(_2)CO) are explored with the Schrock initiators in acyclic diene metathesis reactions (ADMET). Diallyl ether is found to undergo catalytic ring closure to 2,5 dihydrofuran, while diallyl phenyl-phosphine gives l-phenyl-2,5 dihydrophosphole.