Bridgehead Hydrogen Atoms Are Important: Unusual Electrochemistry and Proton Reduction at Iron Dimers with Ferrocenyl-Substituted Phosphido Bridges

The diphosphido-bridged diiron clusters <i>syn</i>-[{μ_<i>2</i>-P­(CH₂Fc)­H}₂Fe₂(CO)₆] (<b>2a</b>) and <i>anti</i>-/<i>syn</i>-[{μ_<i>2</i>-P­(CH₂Fc)­Me}₂Fe₂(CO)₆] (<b>3</b>), containing covalently linked ferrocenyl (Fc) groups, were synthesized in order to explore the effect of having a redox-active ligand bound to a Fe₂P₂ core as in the covalently linked Fe₄S₄-{μ₂-S­(Cys)}-Fe₂S₂ cofactor of [FeFe]-hydrogenases. The X-ray crystal structure of <b>2a</b> shows an Fe–Fe bond length of 2.630(1) Å and confirms that the two P–H bonds of the bridging 1°-phosphido groups are parallel and are separated by 2.683(2) Å. Cyclic voltammetry and spectroelectrochemistry studies revealed that <b>2a</b>, unusually, undergoes one-electron reduction at −2.18 V (vs Fc⁺/Fc) to give the anion [{μ_<i>2</i>-P­(CH₂Fc)}­{μ_<i>2</i>-P­(CH₂Fc)­H}­Fe₂(CO)₆]^<b>–</b> ([<b>2a</b> – H]^<b>–</b>), which was independently obtained by deprotonation of <b>2a</b> with excess 1,8-diazabicycloundec-7-ene (DBU). The reduction proceeds through the radical anion <b>2a</b>′^{<b>•–</b>} intermediate, which was detected by X-band EPR spectroscopy in situ during electrolysis. The formation of [<b>2a</b> – H]^<b>–</b> from the <b>2a</b>′^{<b>•–</b>} radical formally equates to loss of a hydrogen atom from the bridging P–H group. The result suggests that a new low-energy route for evolution of molecular hydrogen is available in Fe₂E₂ dimers with bridgehead hydrogen atomsi.e. dimers with hydrogen directly bonded to the bridging nonmetal atoms (E = P, S). In contrast to the one-electron reduction behavior of <b>2a</b>, the mixture of dimers <b>3</b> exhibited a two-electron reduction at −2.11 V (vs Fc⁺/Fc) that afforded <b>3</b>^2–. Both dimers catalyze the reduction of protons from <i>p</i>-toluenesulfonic acid, with <b>2a</b> exhibiting higher catalytic currents at lower overpotential