Synthesis, reactivity and mechanistic studies on oxo, imido and alkylidene complexes of the early transition metals
This thesis describes studies directed towards the synthesis and reactivity of transition metal species containing multiply bonded oxo, imido and alkylidene ligands, with particular emphasis on the use of alkylidene complexes of the type Mo(NAr)(CHCMe(_2)R)(O-t-Bu)(_2) (R = Me, Ph) as initiators in living ring opening metathesis polymerization (ROMP).Chapter 1 highlights areas of transition metal chemistry of relevance to the general theme of this thesis, including recent advances in the ROMP of functionalized norbornene and norbornadiene monomers. Chapter 2 shows how a "Wittig like" capping reaction may be used to introduce a wide range of potentially useful functional groups onto the end of polymer chains generated via living ROMP. Amongst the functional groups introduced in this manner are CI, Me, OMe, CHO, CN, NMe(_2), NO(_2), NH(_2), CF(_3) and CO(_2)Me. Unusual 2-oxametallacycle intermediates have been observed in solution for benzaldehyde derivatives possessing electron withdrawing substituents. Chapter 3 develops the use of styrene and several of its derivatives as chain transfer agents in living ROMP. The use of substituted styrenes allows for functionalizadon of the start of the polymer chain. Chapter 4 investigates the "heteroatom exchange" reactivity of a variety of four coordinate molybdenum complexes. The exchange of multiply bonded oxo, imido and alkqrlidene units between such species, and to and from external substrates, has been investigated, and such studies have helped to identify bimolecular decomposition pathways responsible for the high molecular weight impurity sometimes observed in polymer samples prepared by living ROMP. Chapter 5 describes the synthesis and characterization of several niobium and tantalum half-sandwich imido complexes of the type Cp*M(NR)Cl(_2) (M = Nb, R = Me, 2,6-iPr(_2)C(_6)H(_3); M = Ta, R = 2,6-iPr(_2)C(_6)H(_3)). The "heteroatom exchange" reactivity of the imido unit in this environment has been investigated. The related half-sandwich alkylidene complex CpTa(CHCMe(_2)Ph)Cl(_2) has also been prepared and the exchange reactivity of the alkylidene ligand studied. Chapter 6 gives experimental details for chapters 2-5.