Abstract
The synthesis and reactivity of several aluminium complexes supported by distinct pyridine-based pincer ligand scaffolds, one with neutral phosphine arms and the other with anionic amide arms, have been reported in this thesis.
Chapter 3 reports the preparation and structural characterisation of dearomatised aluminium complexes supported by the tBuPNP* scaffold. The direct addition of AlMe3 and Me3N.AlH3 to the tBuPNP scaffold resulted in the coordination of the aluminium centre to phosphorus forming (tBuPNP).2AlMe3 (III) and the nucleophilic attack of the hydride at the pyridine ring forming {(C5H4N)(CH2P(tBu)2)2}AlH2 (VI). The preparation of complexes of the type (tBuPNP*)AlR2 (R = Me or H) was achieved by a two-step route: deprotonation of the tBuPNP ligand followed by the addition of 2.0 eq. of either AlMe3 or Me3N.AlH3 to yield (tBuPNP*)AlR2 (R = Me (V), H (VII)). Additionally, the in-situ preparation of the cation [(tBuPNP*)AlMe]+, by the addition of B(C6F5)3 to V was tentatively suggested by NMR spectroscopy.
Chapter 4 reports the preparation and structural characterisation of aluminium complexes supported by the dianionic DippNNN scaffold. The direct addition of Me3N.AlH3 or AlMe3 to the protonated DippNNN.H2 ligand yielded the complexes (DippNNN)Al(X)L (X = H, L = THF; X = Me, L = vacant). Additionally, the THF-stabilised iodide analogue (DippNNN)Al(I)THF was prepared by alkyl-halide exchange with the aluminium methyl complex. The unusual square planar four-coordinate aluminium methyl complex was shown to remain Lewis acidic, both experimentally binding THF and Et3P=O and computationally, due to the presence of a vacant orbital located principally on aluminium. Further experimental and theoretical studies revealed that all of the complexes (DippNNN)Al(X)L function as Lewis acids.
The research detailed in chapter 5 builds upon the preceding chapter and is focussed on investigating the reactivity of the Lewis acidic aluminium complexes supported by the DippNNN scaffold. The reactivity of the complexes (DippNNN)Al(X)L with carbonyl-containing substrates was explored. The aluminium methyl complex was shown to react via differing metal-ligand cooperative pathways depending on the substrate e.g. enolisable and non-enolisable ketones. Furthermore, both the methyl and THF-stabilised hydride complexes were able to catalyse the hydroboration of ketones and an ester. Remarkably, the methyl complex was also able to catalyse the reduction of CO2 to a methanol equivalent.