Synthesis of Imidazolium Compounds as Model Substrates for Biomimetic Mono-Iron Hydrogenase Complexes

As the demand for carbon-free fuel sources has increased, hydrogen gas has become a promising alternative since it can be produced from water. However, efficient production of H2 requires the use of expensive metal catalysts such as platinum. Because of the cost and low availability of these metals, scientists have turned to nature as inspiration for the development of metal catalysts that use cheaper, more abundant transition metals. One such example of a natural H2 catalyst is the mono-iron hydrogenase enzyme, which heterolytically cleaves H2 into a proton and hydride. The active site of this enzyme contains a central iron atom surrounded by a nitrogen-bound pyridone, an acyl carbon, and a cysteine thiolate donor. This unique arrangement of ligands allows the enzyme to activate H2 with the help of the substrate, methenyl-H4MPT+. Previous research by the Rose Group has led to the development of a functional biomimetic model of the mono-Fe hydrogenase active site that is capable of activating H2. This biomimetic model can accept a hydride from the substrate, but the reverse reaction of hydride transfer to the substrate had yet to be observed. To achieve this, obtaining more information about the role of the substrate in H2 activation was necessary. This research therefore focused on the synthesis of a model imidazolium substrate based on methenyl- H4MPT+. In reactivity studies with an enzyme model, it was determined that altering the Lewis acidity of the model substrate promoted hydride transfer from the model complex. With this insight, it is hoped that the interaction between the endogenous substrate and the mono-iron hydrogenase active site can be better understood.