Half-Sandwich Imido into related complexes of niobium and tantulum - relative of the zirconocene family

Abstract

This thesis describes studies directed towards the preparation of half- sandwich niobium and tantalirai compounds containing imido and phosphino-carbene ligands, with particular emphasis on the relationship of such species with bent metallocene complexes of the Group 4 triad. Chapter 1 highlights areas of transition metal chemistry of relevance to the general theme of this thesis, including reviews of metal imido and zirconocene chemistry. Chapter 2 describes the use of silylated anilines for convenient solution syntheses of half-sandwich imido complexes of niobium and tantalum of the type Cp'M(N-2,6-(^i)Pr(_2)-C(_6)H(_3))Cl(_2)(Cp' = Cp, Cp*). In addition, the syntheses and reactivities of mono- and bis-alkyl derivatives (methyl, neopentyl, and benzyl) are presented. The bis-neopentyl complexes CpNb(NR)(CH(_2)CMe(_3))(_2) (R = CMe(_3); 2,6-(^i)Pr(_2)-C(_6)H(_3)), reveal multiple a-agostic interactions which have been primarily studied via an X-ray crystal structure determination and NMR spectroscopy. Thermolysis of Cp*Nb(N-2,6-(^i)Pr(_2)-C(_6)H(_3))(CH(_2)Ph)(_2) in die presence of PMe(_3) affords die benzylidene complex Cp*Nb(N-2,6-(^i)Pr(_2)-C(_6)H(_3))(η(^1)-CHPh)(PMe3) whose X-ray crystal structure has been determined. Chapter 3 describes the preparation of the niobium and tantalum imido complexes Cp'M(N-2,6-(^i)Pr(_2)-C(_6)H(_3))(L)(PMe(_3)) (M = Nb, L = C(_2)H(_4), C(_3)H(_6), CO, Me(_2)C(_2). Ph(_2)C(_2), C(_6)H(_4), PMe(_3); M = Ta, L = C(_2)H(_4), C(_3)H(_6), CO). Single crystal structure determinations on CpNb(N-2,6-(^i)Pr(_2)-C(_6)H(_3))(η(^2)-C(_3)H(_6))(PMe(_3)) and CpNb(N-2,6-(^i)Pr(_2)-C(_6)H(_3))(η(_2)-C(_6)H(_4))(PMe(_3)) have been undertaken and their relationship to Group 4 metallocenes noted. Treatment of Cp*Ta(N-2,6-(^i)Pr(_2)-C(_6)H(_3))(L)(PMe(_3)) (L = C(_2)H(_4),C(_3)H(_6)) with a-olefins was found to lead to displacement of PMe(_3) and the generation of tantallacycle containing species. Chapter 4 compares the reactivity of tantalum imido and phosphino-carbenederivatives of the form Cp*Ta(E)(H)(X)(PMe(_3)) (E = N-2,6-(^i)Pr2-C(_6)H(_3), η(^2)-CHPMe(_2); X = H, I) with a number of a-olefins. Investigations into die mechanism of catalytic oligomerisation of a-olefins by Cp*Ta(η(^2)-CHPMe(_2))(H)(_2)(PMe(_3)) reveal that pathways involving metallacycle intermediates are most probable, whereas Cp*Ta(N-2,6-(^i);Pr(_2)-C(_6)H(_3))(H)(_2)(PMe(_3)) reacts with a-olefins to afford stable tantallacycle complexes. The reactivity of die dihydrido species has been moderated by the preparation of mono- iodide derivatives and their reactivity towards a-olefins studied. Cp*Ta(η(^2)-CHPMe(_2))(H)(I)(PMe(_3)) dimerises ediylene selectively to but-1-ene, while Cp*Ta(N-2,6-(^i)Pr(_2)-C(_6)H(_3))(H)(I)(PMe(_3)) reacts with ethylene to form die stable ethyl species Cp*Ta(N-2,6-(^i)Pr(_2)-C(_6)H(_3))(Et)(I). Furtherrmore, studies investigating a variety of niobium and tantalum imido species as possible catalysts for die oligomerisation and polymerisation of a-olefins under industrially relevant conditions have been undertaken in collaboration with B.P. Chemicals Ltd.. Chapter 5 gives experimental details for chapter 2-4.