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

The diphosphido-bridged diiron clusters syn-[{μ2-P­(CH2Fc)­H}2Fe2(CO)6] (2a) and anti-/syn-[{μ2-P­(CH2Fc)­Me}2Fe2(CO)6] (3), 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 2a 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 2a, unusually, undergoes one-electron reduction
at −2.18 V (vs Fc+/Fc) to give the anion [{μ2-P­(CH2Fc)}­{μ2-P­(CH2Fc)­H}­Fe2(CO)6]– ([2a – H]–), which was independently obtained by
deprotonation of 2a with excess 1,8-diazabicycloundec-7-ene
(DBU). The reduction proceeds through the radical anion 2a′•– intermediate, which
was detected by X-band EPR spectroscopy in situ during electrolysis.
The formation of [2a – H]– from the 2a′•– 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 2a, the mixture of dimers 3 exhibited a two-electron
reduction at −2.11 V (vs Fc+/Fc) that afforded 32–. Both dimers catalyze the reduction
of protons from p-toluenesulfonic acid, with 2a exhibiting higher catalytic currents at lower overpotential.