Hydrogen generation from weak acids: electrochemical and computational studies in the [(η⁵-C₅H₅)Fe(CO) ₂]₂ system

(η⁵-C₅H₅)Fe(CO)₂H (FpH) is stable to weak acids such as acetic acid. However, reduction of FpH in acetonitrile in the presence of weak acids generates H₂ catalytically. Evidence for the catalytic generation of H₂ from just water also is observed. Since reduction of Fp₂ generates Fp ^-, which can be protonated with weak acids, Fp₂ serves as a convenient procatalyst for the electrocatalytic production of H₂. Electrochemical simulations provide values for the key parameters of a catalytic mechanism for production of H₂ in this system. Protonation of Fp^- is found to be the rate-determining step preceding H₂ production. The wealth of structural, spectroscopic, and thermodynamic information available on the key Fp₂, Fp^-, and FpH species provide a variety of checkpoints for computational modeling of the catalytic mechanism. The computations give good agreement with the crystal structure of Fp₂, the IR spectra of Fp₂, Fp^-, and FpH, and the photoelectron spectra of Fp₂ and FpH. The computations also account well for the reduction potentials and equilibrium constants in the electrochemical simulations. The FpH^- anion is found to be susceptible to a direct and rapid attack by a proton to produce H₂ and the Fp radical, which is then reduced and protonated to continue the electrocatalytic cycle. This direct energetically downhill step of metal hydride protonation to produce molecular hydrogen may be common for sufficiently electron rich metal hydrides and/or sufficiently strong acids among many of the hydrogenase mimics reported thus far.