Mechanisms and Kinetics of 'Frustrated Lewis Pair' Hydrogenation

Catalytic hydrogenation reactions, which are of immense importance throughout the chemical sciences (from bulk chemical to pharmaceutical synthesis), are largely dependent on the use of rare, expensive, and often toxic precious metal catalysts (e.g. Ru, Rh, Pd, Pt). There is a huge drive to develop cheaper and more benign alternatives. In recent years substantial progress has been made with'frustrated Lewis pair' (FLP) catalysts, which consist of a sterically bulky Lewis acid and base, which are precluded from forming stable adducts. These systems, typically based on innocuous, inexpensive main group elements, are capable of heterolytically cleaving H2 into formal H+/H-fragments, which can then be transferred to a variety of reducible substrates. Rapid advances in the past six years (by our groups and others) have seen the scope of this methodology expand from simple imines and aziridines to N-heteroaromatics, alkenes and carbonyls, among many other substrates.
Because of the speed with which this area has developed, many important aspects of FLP-catalysed hydrogenation chemistry have received limited attention; for example, there has been very little direct experimental investigation into reaction mechanisms and kinetics, leading to uncertainty in, for example, the nature of the rate-determining step. For more complex FLP reactions such as carbonyl hydrogenations, even less experimental information is available, leading to much greater uncertainty. Importantly, knowledge of reaction and catalyst deactivation/inhibition mechanisms is fundamental to reactor and process design and scale-up, as well as for designing more effective catalysts. Measurements of intrinsic kinetics can be difficult to achieve; often, it is necessary to develop apparatus and techniques tailored to a specific reaction scheme, to ensure that mass transfer does not control the measured rates.
This research project will investigate FLP catalysed hydrogenations, measuring intrinsic reaction rates and the evolution of reaction intermediates and products in-situ. From these studies, the qualitative reaction mechanism, together with transient measurements of reactant and product concentrations, may be combined with a mathematical framework to extract kinetics of individual reaction pathways. This information will be used to optimise the catalyst structure.