Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - IILSCFLP (The Influence of Ionic Liquid Solvation on the Chemistry of Frustrated Lewis Pairs)

Teaser

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...

Summary

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.

Work performed

Work performed for this project involved studying the ability of several different FLPs to activate hydrogen within IL and organic solvent systems. As described above, the major finding of these studies was that ILs improved the ability of the FLP to activate hydrogen relative to organic solvents such as toluene. Studies on hydrogenation reaction rates of a common substrate were also performed to compare the effect of these solvent systems. These results indicated that the reactions in ILs are substantially slower than within organic solvents. These results are likely to arise in part from the physical properties of the ILs used (high viscosity, relatively poor hydrogen solubility) and in part from the experimental design which restricted the rate of hydrogen diffusion. Hence it is likely that further optimisation of these experimental conditions and solvent systems will be able to significantly improve upon this outcome.

The results of these experiments have been disseminated through conference poster presentations at the Molten Salt Discussion Group in London, December 2016 and at the 8th International Conference on Green and Sustainable Chemistry in Melbourne, July 2017 as well as during invited presentations at the University of Auckland, July 2017 and at Scion (a New Zealand forestry research institute), November 2017. The concept of using ILs as solvents was formulated into an outreach activity which was presented at the Imperial Fringe, March 2017, the Imperial Festival, May 2017 and Sydenham High School, March 2017.

Final results

The work performed within this project has illustrated, for the first time, that ILs are feasible solvents for hydrogen activation by FLPs and their subsequent hydrogenation reactions. Given there are only a fairly limited range of conventional organic solvents that are compatible with many of the boranes commonly used as Lewis acids in FLPs, this substantially increases the number of solvents that can be considered for the conduct of FLP chemistry. The ability of ILs to substantially increase the stability of the hydrogen activation intermediate creates the possibility of these solvents facilitating the use of weaker Lewis acidic boranes for successful hydrogen activation. As discussed above, weaker Lewis acids tend to be more robust towards contaminants such as water and residual functionalities of substrates such as alcoholic or other donor groups. Such Lewis acids also tend to generate more reactive borohydride intermediates which are capable of reacting with a wider array of functional groups and substrates. While the kinetics observed in ILs are more sluggish than in common organic solvents, it is likely that these effects could be substantially mitigated by improving the experimental design and through more highly developed solvent selection including the use of mixed solvents.

While these results are still at a preliminary stage, should future optimisation be successful there is the potential for this approach to replace transition metal hydrogenation catalysts for fine chemical production including pharmaceuticals. The replacement of these metals is advantageous from an environmental and economic perspective as most metals used for hydrogenation catalysis are scarce and toxic and hence their continued use leads to a depletion of natural resources and the generation of hazardous waste. Furthermore, ILs are electrically conductive and hence they could be easily integrated into devices such as electrochemical hydrogen fuel cells where the FLP could act as the hydrogen activation catalyst instead of platinum. Given the substantial cost saving relative to the use of platinum, this approach could assist with the greater uptake of such technology although considerable research effort is required before this can become a reality.