Synthesis, reactivity and bonding of compounds containing transition metal-nitrogen multiple bonds and related studies
This thesis describes studies directed towards the preparation of new transition metal species containing multiple bonds to nitrogen, with a view to learning more about the structure and reactivity of metal bound nitrogen: Chapter 1 highlights areas of transition metal nitrido and imido chemistry of relevance to the general theme of the thesis. Chapter 2 describes the use of silyl amines and amides for convenient solution syntheses of half-sandwich imido complexes of niobium and tantalum of the type CpM(NR)Cl(_2) (R = Me, Bu, 2-6-(^i)Pr(_2)C(_6)H(_3)). In addition, the preparation and characterisation of alkyl, phenoxide and alkoxide derivatives are presented. The bis-neopentyl derivative, CpNb(N(^t)Bu)(CH(_2)CMe(_3))(_2), is found to possess two a-agostic C- H interactions. Finally, attempts to prepare a bis imido species of the type [CpNb(NR)(_2)] are discussed. Chapter 3 employs Fenske-Hall quantum chemical MO calculations (FHMO), to learn more about the nature of the imido bonding in CpNb(NMe)Cl(_2), and compares this class of compound with Group 4 metallocenes (CP2MX2) and complexes of the type Cp*Ta(η(^2)-CHPMe(_2))X(_2). Chapter 4 describes a search for metal-nitrogen complexes exhibiting the unusual phenomenon of bond-stretch isomerism. Modified preparations of molybdenum and tungsten nitrido starting materials have been developed and their reactions with tertiary phosphines examined. Attempts to prepare M(N)Cl(_3)(PMe(_3))(_3) led to the isolation of the phosphiniminato species [Mo(NPMe(_3))Cl(_2)(PMe(_3))(_3)]Cl, which converts under mild conditions to the binuclear nitrido complex [(PMe(_3))(_2)Cl(_3)MoNMoCl(_3)(PMe(_3))(_2)][Me(_3)PNPMe(_3)]. The X-ray structures, reactivity and bonding of these unusual species are discussed. Other attempts to isolate bond-stretch isomers, from tungsten and rhenium nitride starting materials, resulted in nitride complexes such as W(N)Cl(_3)(PR(_3))(_2) (PR(_3) = PMe(_3), PMe(_2)Ph, PPh(_3)) and Re(N)Cl(_2)(PR(_3))(_n) (n = 3, PR(_3) = PMe(_2)Ph; n = 2, PR(_3) = PMePh(_2)). Chapter 5 presents a detailed analysis of the 0x0 bonding in the isomers of Nb(O)Cl(_3)(PMe(_3))(_3) arising by lengthening of the metal-oxygen bond (bond-stretch isomers). The bonding has been analysed over a range of metal-oxygen distances and destabilising anti-bonding interactions found at shorter Nb-O bond lengths. These observations are compared with the results of calculations on the hypothetical nitride species Mo(N)Cl(_3)(PMe(_3))(_3). Chapter 6 gives experimental details for chapter 2-5.