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

(eta(5)-C5H5)Fe(CO)(2)H (FpH) is stable to weak acids such as acetic acid. However, reduction of FpH in acetonitrile in the presence of weak acids generates H-2 catalytically. Evidence for the catalytic generation of H-2 from just water also is observed. Since reduction of Fp(2) generates Fp(-), which can be protonated with weak acids, Fp(2) serves as a convenient procatalyst for the electrocatalytic production of H-2. Electrochemical simulations provide values for the key parameters of a catalytic mechanism for production of H-2 in this system. Protonation of Fp(-) is found to be the rate-determining step preceding H-2 production. The wealth of structural, spectroscopic, and thermodynamic information available on the key Fp(2)2, 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 Fp(2), the IR spectra of Fp(2), Fp(-), and FpH, and the photoelectron spectra of Fp(2) 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 H-2 and the Fp(center dot) 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.