Investigations of e-h bond activation processes involving aluminium and 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.

Keywords: alane
oxidative addition
sigma complexes
gallane
bond activation
hydrides
aluminium
carbon dioxide reduction
gallium
Advisor: Aldridge, S