Homogeneous hydrogenation of nitrous oxide

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Nitrous oxide (N2O) is an abundant gas that accumulates in the Earth's atmosphere, leading to depletion of ozone in the stratosphere and contributing to global warming as a potent greenhouse gas. Although naturally occurring, increasing N2O emissions resulting from intensive agricultural fertilisation, industrial processes, and combustion of fossil fuels and biomass are a major cause for concern due to the detrimental impact this will have on our environment. The hydrogenation of nitrous oxide is an attractive chemical remediation method, driven by the release of environmentally benign dinitrogen and water (Scheme 1). Whilst a range of heterogenous systems have been shown to catalyse this reaction, the mild conditions and propensity for precise reaction control synonymous with homogenous catalysis make these variants the most desirable from a remediation perspective. Moreover, as the mechanisms of homogenous catalysts can be more readily studied, they can provide a platform for shedding light on important fundamental chemistry associated with how N2O can be activated and potentially exploited in (value added) synthetic applications. Whilst the chemistry of N2O has been of longstanding interest, its catalytic transformation using transition metal compounds has proven to be challenging due to the robust triatomic formulation of this gas. Building upon recent developments in the literature and underpinning fundamental work conducted in the Department of Chemistry at the Univeristy of Warwick (Scheme 1), this project will involve the development of new late-transition-metal-based catalysts for the homogenous hydrogenation of nitrous oxide using, in the first instance molecular dihydrogen, but with a long term view to the use of sustainable or biomass-derived chemicals, from which hydrogen can be extracted directly (transfer hydrogenation)Methodology: Recent work by Milstein has substantiated the propensity of metal hydrides for N-O bond activation of N2O and shown that in combination with bifunctional "pincer" ligands, catalytic hydrogenation can be achieved. On the basis of these design principles and facilitated by expertise in the Chaplin group, the first step of the project will be to systematic study the reaction of transition metal hydride complexes, supported by tridentate "pincer" ligands or other chelating variants bearing reactive amido (M=NR2) or phospido (M=PR2) linkages, with N2O. In combination with evaluation of catalytic activity, this work will be used to establish structure activity relationships for the hydrogenation of N2O to inform the design and synthesis of new more effective catalysts. Complementing this work, established transfer hydrogenation catalysts will be screened using N2O as the substrate, seeking to develop procedures for remediation that use sacrificial organic molecules, e.g. ethanol, as hydrogen reservoirs.