We present results from two studies of two very similar catalysts, that catalyze the same reaction, but with qualitatively different mechanisms. The first system we have studied one by Beller based on an iron catalyst with a tripodal tetradentate phosphine. In tetrahydrofuran (THF), the catalyst was found to be very active in dehydrogenation of formic acid. Our theoretical modeling found that all the reactivity was on a neutral iron dihydride Fe(H)<sub>2</sub> species, and as for most catalysts we have studied the reaction proceeds at one single coordination site. This single site reactivity point towards the possibility of using less noble reactants than the most efficient catalysts that are based on Ir and Ru. The high catalytic activity of transition metals is often attributed to the presence of d-electrons in the valence shell. Due to the symmetry of the d-orbitals, their involvement in the reaction can make reactions that are normally symmetry forbidden allowed. Another feature of transition metals is that they can have several oxidation states in a range that is suitable for catalyzing certain reactions. However, in the hydrogenation of CO<sub>2</sub> or dehydrogenation/electrocatalytic oxidation of formic acid, it appears that all that is needed is one site where a hydride can form that is reactive enough. In principle there is no need for d-electrons, and hence, the site could potentially also be located on a main group element. The key is to find the right properties and sufficient robustness of the catalyst, which is the case for all the catalysts aforementioned. The second catalytic system we studied was close to identical to the Beller one, but with a linear tetradentate phosphine. The system was developed by Gonsalvi and co-workers and was found to catalyze hydrogenation of bicarbonate to formate and the reverse reaction. We tested several different reaction mechanisms, and interestingly, this catalyst reacts at two coordination sites, with an iron monohydride Fe-H intermediate as the active catalyst.

Bicarbonate hydrogenation by iron: Effects of solvent and ligand on the mechanism

255th National Meeting (2018)

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