H2 Binding, Splitting, and Net Hydrogen Atom Transfer at a Paramagnetic Iron Complex

The reactivity of H2 with abundant transition metals is crucial for developing catalysts for energy storage in chemical bonds. While diamagnetic transition metal complexes that bind and split H2 have been extensively studied, paramagnetic complexes that exhibit this behavior remain rare. We describe the reactivity of a square planar S = ½ FeI(P4N2)+ cation (FeI+) that reversibly binds H2/D2 in solution, exhibiting an inverse equilibrium isotope effect of KH2/KD2 = 0.58(4) at -5.0 °C. In the presence of excess H2, the dihydrogen complex FeI(H2)+ cleaves H2 at 25 °C in a net hydrogen atom transfer reaction to give the dihydrogen-hydride cation trans-FeII(H)(H2)+. The proposed mechanism of H2 splitting involves both intra- and intermolecular steps, resulting in a mixed firstand second-order rate law with respect to initial [FeI+]. The key intermediate is a paramagnetic dihydride complex, trans-FeIII(H)2+, whose weak FeIII-H bond dissociation free energy (calculated BDFE = 44 kcal/mol) leads to bimetallic H-H homolysis, generating trans-FeII(H)(H2)+. Reaction kinetics, thermodynamics, electrochemistry, EPR spectroscopy, and DFT calculations all support the proposed reaction mechanism.