H ₂ Binding, Splitting, and Net Hydrogen Atom Transfer at a Paramagnetic Iron Complex

Abstract

While diamagnetic transition metal complexes that bind and split H₂ have been extensively studied, paramagnetic complexes that exhibit this behavior remain rare. The square planar S = 1/2 Fe^I(P₄N₂)⁺ cation (Fe^I+) reversibly binds H₂/D₂ in solution, exhibiting an inverse equilibrium isotope effect of K_H2/ K_D2 = 0.58(4) at -5.0 °C. In the presence of excess H₂, the dihydrogen complex Fe^I(H₂)⁺ cleaves H₂ at 25 °C in a net hydrogen atom transfer reaction, producing the dihydrogen-hydride trans-Fe^II(H)(H₂)⁺. The proposed mechanism of H₂ splitting involves both intra- and intermolecular steps, resulting in a mixed first- and second-order rate law with respect to initial [Fe^I+]. The key intermediate is a paramagnetic dihydride complex, trans-Fe^III(H)₂⁺, whose weak Fe^III-H bond dissociation free energy (calculated BDFE = 44 kcal/mol) leads to bimetallic H-H homolysis, generating trans-Fe^II(H)(H₂)⁺. Reaction kinetics, thermodynamics, electrochemistry, EPR spectroscopy, and DFT calculations support the proposed mechanism.