Investigations of e-h bond activation processes involving aluminium and gallium
Keywords: alane
oxidative addition
sigma complexes
gallane
bond activation
hydrides
aluminium
carbon dioxide reduction
gallium

This thesis examines the interaction of hydrides of the group 13 metals aluminium and gallium with transition metal centres. Furthermore, a gallium-based system is developed which activates a wide range of E-H bonds, with the product of H2 activation found to act as a catalyst for the reduction of CO2 to a methanol derivative.
Chapter 3 details the synthesis of a number of alane and gallane adducts of expanded-ring N-heterocyclic carbene (NHC) ligands, which are more strongly σ-donating and sterically shielding analogues of classical NHCs. These NHC adducts are found to be apposite for the formation of σ-alane and σ-gallane complexes at group 6 metal carbonyl fragments, which has allowed the characterisation of the first κ2 σ-gallane complexes. The attempted formation of a terminally coordinated κ3 σ-alane complex leads instead to the isolation of a novel dinuclear cluster featuring both μ:κ1,κ1 and μ:κ2,κ2 coordination to Mo(CO)3 units.
The work presented in Chapter 4 probes the interaction of the β-diketiminate stabilised gallane Dipp2NacNacGaH2 with transition metal carbonyls. Far from simply mimicking the chemistry of the alane congener Dipp2NacNacAlH2, which forms simple κ1 and κ2 σ-alane complexes, the gallane shows a marked propensity towards dehydrogenation and formation of direct M-Ga(I) bonds. This represents a rare mode of reactivity among group 13 hydrides, being unprecedented beyond boron chemistry, and provides a new route to M-Ga bond formation. Experimental and computational investigations of the mechanism suggest that initial Ga-H oxidative addition is facile, and is generally followed by rate-limiting loss of H2. The reaction of Dipp2NacNacAlH2 with Co2(CO)2 is shown to yield an unusual alane complex which displays an unprecedented degree of Al-H activation in a σ-alane complex.
Chapter 5 represents an extension of the work described in Chapter 5, investigating the interaction of Dipp2NacNacMH2 (M = Al, Ga) with cationic group 9 transition metal fragments supported by ancillary phosphine ligands. While attempts to isolate unsupported, cationic σ-alane complexes prove unsuccessful, Dipp2NacNacGaH2 readily binds to cationic rhodium and iridium centres, forming the first cationic σ-gallane complexes as well as cationic gallylene complexes resulting from complete Ga-H oxidative addition. The extent of Ga-H bond activation is shown to be markedly dependent on the nature of the phosphine co-ligands. In particular, a series of rhodium complexes is reported which represents snapshots of the oxidative addition process, from a Rh(I) σ-gallane complex to a Rh(III) gallylene dihydride, with two further complexes which are on the cusp of these two oxidation states.
Described in Chapter 6 are the synthesis and reactivity studies of an ambiphilic system, Dipp2NacNac′Ga(tBu), featuring a three-coordinate gallium centre supported by a deprotonated NacNac ligand. The combination of this electrophilic gallium centre with the highly nucleophilic exocyclic alkene functionality facilitates the cooperative activation of protic, hydridic and apolar E-H bonds. Accordingly, molecules including H2, NH3, H2S and SiH4 may be cleaved under mild conditions. Moreover, the hydride product of H2 activation is shown to be a competent catalyst in conjunction with HBpin for the reduction of CO2 to the methanol derivative MeOBpin.