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

Fe-II complexes containing the fluorinated ligand 1,2-bis (perfluorophenyl)ethane-1,2-dionedioxime (dAr(F)gH(2); H = dissociable proton) exhibit relatively positive Fe-II/I reduction potentials. The air-stable difluoroborated species [(dAr(F)gBF(2))(2)Fe(py)(2)] (2) electrocatalyzes H-2 generation at -0.9 V vs SCE with i(cat)/i(p) approximate to 4, corresponding to a turnover frequency (TOP) of similar to 20 s(-1) [Faradaic yield (FY) = 82 +/- 13%]. The corresponding monofluoroborated, proton-bridged complex [(dAr(F)g(2)H-BF2)Fe(py)(2)] (3) exhibits an improved TOF of similar to 200 s(-1) (i(cat)/i(p) approximate to 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) approximate to 200 M-1 s(-1)) that undergoes hydride protonation to form H2. Complex 3 likely reacts via protonation of an Fe-1 intermediate that subsequently forms H, via a bimetallic mechanism (k(RLS) approximate to 2000 M-1 s(-1)). 3 catalyzes production at relatively positive potentials compared with other iron complexes.