Synthesis and reactivity of transition metal-group 13 complexes
Keywords: main group
sigma complex
aluminium hydride
transition metal

The synthesis and reactivity of a number of mixed transition metal-aluminium and σ-alane complexes are detailed in this thesis.
Chapter III reports on the formation and structural characterisation of N,N'-chelated aluminium dihalide precursors featuring amidinate and guanidinate substituents. These precursors of the type RC(R'N)2AlX2 (R = iPr2N or Ph; R' = Cy or iPr or Dipp; X = hal), readily react with Na[CpFe(CO)2] via salt elimination to form the corresponding mixed iron-aluminium complexes CpFe(CO)2[(X)Al(NR')2CR] which have been characterised both spectroscopically and by X-ray diffraction. The reactivity of the novel mixed aluminium-iron complexes towards halide abstraction agents has been investigated and a propensity for augmented coordination at the aluminium centre observed. Furthermore, complementary syntheses of the methyl substituted complex CpFe(CO)2[(Me)Al(NCy)2CNiPr2] have been developed. This can be achieved either via the reaction between the related chloride complex and MeLi, or from the reaction between iPr2C(CyN)2Al(Me)Cl and Na[CpFe(CO)2].
The research detailed in Chapter IV builds on the previous chapter and is focussed on the use of more sterically demanding substituents at both aluminium and transition metal, as well as more electron rich transition metal fragments. The transition metal anions Na[Cp*Fe(CO)2] and Na[CpSiFe(CO)(PPh3)] react with the aluminium precursors forming related mixed iron-aluminium complexes which have been structurally characterised. The Dipp2NacNacAlCl2 precursor has been shown to undergo reaction with both Na[CpFe(CO)2] and Na[Cp*Fe(CO)2]. The halide abstraction chemistry of the latter utilising both Lewis acid and salt metathesis based abstraction approaches has been investigated. The dehydrohalogenation chemistry of the Dipp2NacNacAlCl2 precursor was investigated and the ligand activated products of reactions with both alkyl lithium and alkyl potassium reagents characterised.
Chapter V reports the extension of salt metathesis for the formation of an Al-H-Mn interaction, and the product has been fully characterised. In addition, the coordination of Al-H bonds from a number of alane precursors to in situ generated 16-electron fragments has allowed the structural characterisation of a number of novel σ-alane complexes. The incorporation of the transition metal fragments [Cp'Mn(CO)2] and [W(CO)5] permit comparison to archetypal borane and silane σ-complexes. Quantum chemical calculations suggest that the alane ligand has a binding energy close to that of dihydrogen but significantly less than that of CO, consistent with a predominant σ-donor role of the Al-H bond. The formation and structural characterisation of the κ2-complexes (OC)4M[κ2-H2AlDipp2NacNac] (M = Cr, Mo or W) define an unprecedented binding motif for the alane ligand. In the cases of chromium and molybdenum the κ2-complexes can be prepared either photolytically or via alkene displacement from the corresponding (OC)4M(cod) reagent. In the case of tungsten the alkene displacement route yields the desired product, but only under more forcing conditions. Spectroscopic characterisation of the related κ1-complex (OC)5Cr[κ1-H2AlDipp2NacNac], which readily forms the κ2-complex in solution without photolysis, has enabled the kinetics of chelate ring closure to be investigated. This analysis further characterises the formation of the unprecedented κ2-binding motif for the alane ligand.