Hydrogen generation catalyzed by fluorinated diglyoxime-iron complexes at low overpotentials

Abstract

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) ≈ 4, corresponding to a turnover frequency (TOF) of ∼20 s(-1) [Faradaic yield (FY) = 82 ± 13%]. The corresponding monofluoroborated, proton-bridged complex [(dAr(F)g(2)H-BF(2))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 H(2). Complex 3 likely reacts via protonation of an Fe(I) intermediate that subsequently forms H(2) via a bimetallic mechanism (k(RLS) ≈ 2000 M(-1) s(-1)). 3 catalyzes production at relatively positive potentials compared with other iron complexes.