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 H<sub>2</sub> activation found to act as a catalyst for the reduction of CO<sub>2</sub> 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 Ï-donating and sterically shielding analogues of classical NHCs. These NHC adducts are found to be apposite for the formation of Ï-alane and Ï-gallane complexes at group 6 metal carbonyl fragments, which has allowed the characterisation of the first κ<sup>2</sup> Ï-gallane complexes. The attempted formation of a terminally coordinated κ<sup>3</sup> Ï-alane complex leads instead to the isolation of a novel dinuclear cluster featuring both μ:κ<sup>1</sup>,κ<sup>1</sup> and μ:κ<sup>2</sup>,κ<sup>2</sup> coordination to Mo(CO)<sub>3</sub> units. The work presented in Chapter 4 probes the interaction of the β-diketiminate stabilised gallane Dipp<sub>2</sub>NacNacGaH<sub>2</sub> with transition metal carbonyls. Far from simply mimicking the chemistry of the alane congener Dipp<sub>2</sub>NacNacAlH<sub>2</sub>, which forms simple κ<sup>1</sup> and κ<sup>2</sup> Ï-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 H<sub>2</sub>. The reaction of Dipp2NacNacAlH2 with Co<sub>2</sub>(CO)<sub>2</sub> is shown to yield an unusual alane complex which displays an unprecedented degree of Al-H activation in a Ï-alane complex. Chapter 5 represents an extension of the work described in Chapter 5, investigating the interaction of Dipp<sub>2</sub>NacNacMH<sub>2</sub> (M = Al, Ga) with cationic group 9 transition metal fragments supported by ancillary phosphine ligands. While attempts to isolate unsupported, cationic Ï-alane complexes prove unsuccessful, Dipp<sub>2</sub>NacNacGaH<sub>2</sub> readily binds to cationic rhodium and iridium centres, forming the first cationic Ï-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) Ï-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, Dipp<sub>2</sub>NacNacâ²Ga(<sup>t</sup>Bu), 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 H<sub>2</sub>, NH<sub>3</sub>, H<sub>2</sub>S and SiH4 may be cleaved under mild conditions. Moreover, the hydride product of H<sub>2</sub> activation is shown to be a competent catalyst in conjunction with HBpin for the reduction of CO<sub>2</sub> to the methanol derivative MeOBpin.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:711822 |
| Date | January 2015 |
| Creators | Abdalla, Joseph |
| Contributors | Aldridge, Simon |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://ora.ox.ac.uk/objects/uuid:ba3a0d95-4993-498b-8b15-e93da9a42aa3 |
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