Hydrogenation reactions, addition of molecular hydrogen to other compounds in the presence of a catalyst, are one of the most fundamentally important chemical transformations, widely used by the agricultural, food, pharmaceutical and petroleum industries. The major role of the catalyst is to ‘activate’ hydrogen, i.e. weaken or break the H-H bond, allowing it to add to the compound of interest. Transition metals such as nickel, platinum and palladium are almost exclusively used as hydrogenation catalysts. These metals are costly to purchase and use due to their low abundance in the Earth’s crust and their toxicity which requires expensive and wasteful purification processes to remove residual metal. Hence, it is important to develop less toxic hydrogenation catalysts using more abundant elements, for safety, environmental and economic reasons.
The conduct of hydrogenation reactions using boron-containing compounds known as boranes, as catalysts has been demonstrated. This enables hydrogenation reactions to proceed without the use of metals. These Lewis acidic boranes are paired with bulky Lewis bases so that they cannot react directly with each other but, instead, their combined acidity/basicity allows them to react directly with small molecules such as hydrogen. Such hindered Lewis acid/Lewis base pairs are known as Frustrated Lewis Pairs (FLPs). These catalysts are currently limited by their sensitivity to many commonly used reaction solvents, including water; their slow reaction rates compared to transition metal catalysts; and the limited, but increasing, range of chemical functionalities which can be successfully hydrogenated. Many of these issues arise from the need to use highly Lewis acidic or Lewis basic catalysts for the hydrogen activation step which limits the reactivity of the resultant intermediate towards the hydrogenation of the desired substrate.
In this project, we explored the use of ionic liquids (ILs), low melting salts, as solvents for FLPs. As they are composed solely of ions, it was hypothesised that ILs will stabilise the key ionic intermediate of hydrogen activation by FLPs. This would enable boranes that are weaker Lewis acids to be used without a commensurate increase in the Lewis basicity of the other FLP component. Boranes that are weaker Lewis acids are typically more robust to contaminants such as water and lead to more reactive borohydride intermediates which in turn result in faster reaction rates and greater substrate scope.
We found that ILs are indeed viable solvents for hydrogen activation by FLPs. This stands them apart from common solvents such as acetonitrile, methanol and water which are incompatible with borane containing FLPs. Moreover, the key hydrogen activation intermediate is stabilised in ILs relative to solvents conventionally used for FLP chemistry, such as toluene. This indicates that for a given FLP, hydrogen activation will proceed to a greater extent in ILs than in toluene. This result indicates that ILs can allow boranes that are weaker Lewis acids to successfully engage in hydrogen activation reactions without the need to compensate through the use of a stronger Lewis base. However, reaction rates for FLP catalysed hydrogenations in ILs were found to be slower than conventional organic solvents. Ultimately it was found that ILs have the potential to increase the scope of metal-free hydrogenation chemistry but further optimisation of the reaction system and understanding of IL solvent effects on FLPs are required to bring this to fruition.
