Hydrogen Generation from Weak Acids: Electrochemical and Computational Studies in the [(η⁵-C₅H₅)Fe(CO)₂]₂ System

Abstract

C 5 H 5 )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 Fpis 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 , 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 FpHanion is found to be susceptible to a direct and rapid attack by a proton to produce H 2 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.

Diiron Dichalcogenolato (Se and Te) Complexes: Models for the Active Site of [FeFe] Hydrogenase

“…The computational methodology utilized by Weigand and co-workers to investigate these catalysts (12, 13, 23-27) has previously been shown to give good quantitative agreement with the structures, CO stretching frequencies (νCO), oxidation and reduction potentials, bond energies, and pK a values of a variety of catalysts inspired by [FeFe] hydrogenase active sites. [56,59,60,[101][102][103] The optimized geometries and computed νCO values agree very well with the experimental values, suggesting that the computations are able to yield sufficiently reasonable electron distributions and energies for catalysts 12, 14, and 23-27.…”

Synthesis and Characterization of [FeFe]‐Hydrogenase Models with Bridging Moieties Containing (S, Se) and (S, Te)

“…The DFT computational methodology utilized in this study has previously been shown to give good quantitative agreement with the structures, carbonyl stretching frequencies, oxidation potentials, reduction potentials, bond energies, and pKa values of a variety of [FeFe]-hydrogenase model compounds. [33,36,[54][55][56] It is important to re-examine the validity of a method for each new class of molecules.…”

“…[36, [54][55][56] DFT calculations were carried out with the Amsterdam density functional (ADF2009.01b) package. [82,83] Geometry optimizations and frequency calculations (with no imaginary frequencies in the final geometries) were carried out by using the VWN functional with the Stoll correction implemented.…”

Review of electrochemical studies of complexes containing the Fe2S2 core characteristic of [FeFe]-hydrogenases including catalysis by these complexes of the reduction of acids to form dihydrogen

“…This is particularly valuable for N,N-dimethylformamide, dimethylsulfoxide and acetonitrile for which pK a -values of many acids are known. Though we have promoted the use of the potential at half the catalytic peak height as the potential where catalysis occurs [12] such potentials are difficult to estimate from published voltammograms so in this review we will use the peak potential of the catalytic peak.…”