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

Authors

Demyan E. Prokopchuk, Geoffrey M. Chambers, Eric D. Walter, Michael T. Mock, R. Morris Bullock

Publication

Journal of the American Chemical Society

Abstract

While diamagnetic transition metal complexes that bind and split H-2 have been extensively studied, paramagnetic complexes that exhibit this behavior remain rare. The square planar S = 1/2 Fe-I(P4N2)(+) cation (FeI+) reversibly binds H-2/D-2 in solution, exhibiting an inverse equilibrium isotope effect of K-H2/K-D2 = 0.58(4) at -5.0 degrees C. In the presence of excess H-2, the dihydrogen complex Fe-I(H-2)(+) cleaves H-2 at 25 degrees C in a net hydrogen atom transfer reaction, producing the dihydrogen-hydride trans-Fe-II(H)(H-2)(+). The proposed mechanism of H-2 splitting involves both intra- and intermolecular steps, resulting in a mixed first- and second-order rate law with respect to initial [FeI+]. 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)(H-2)(+). Reaction kinetics, thermodynamics, electrochemistry, EPR spectroscopy, and DFT calculations support the proposed mechanism.

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