Hydrogen Generation Catalyzed by Fluorinated Diglyoxime–Iron Complexes at Low Overpotentials

FeII complexes containing the fluorinated ligand 1,2-bis(perfluorophenyl)ethane-1,2-dionedioxime (dArFgH2; H = dissociable proton) exhibit relatively positive FeII/I reduction potentials. The air-stable difluoroborated species [(dArFgBF2)2Fe(py)2] (2) electrocatalyzes H2 generation at −0.9 V vs SCE with i cat/i p ≈ 4, corresponding to a turnover frequency (TOF) of ∼20 s–1 [Faradaic yield (FY) = 82 ± 13%]. The corresponding monofluoroborated, proton-bridged complex [(dArFg2H-BF2)Fe(py)2] (3) exhibits an improved TOF of ∼200 s–1 (i cat/i p ≈ 8; FY = 68 ± 14%) at −0.8 V with an overpotential of 300 mV. Simulations of the electrocatalytic cyclic voltammograms of 2 suggest rate-limiting protonation of an Fe“0” intermediate (k RLS ≈ 200 M–1 s–1) that undergoes hydride protonation to form H2. Complex 3 likely reacts via protonation of an FeI intermediate that subsequently forms H2 via a bimetallic mechanism (k RLS ≈ 2000 M–1 s–1). 3 catalyzes production at relatively positive potentials compared with other iron complexes.