Extended investigation of electrocatalytic generation of dihydrogen using [(mu-1,2-benzenedithiolato)][Fe(CO)3]2 has revealed that weak acids, such as acetic acid, can be used. The catalytic reduction producing dihydrogen occurs at approximately -2 V for several carboxylic acids and phenols resulting in overpotentials of only -0.44 to -0.71 V depending on the weak acid used. This unusual catalytic reduction occurs at a potential at which the starting material, in the absence of a proton source, does not show a reduction peak. The mechanism for this process and structures for the intermediates have been discerned by electrochemical and computational analysis. These studies reveal that the catalyst is the monoanion of the starting material and an ECEC mechanism occurs.

Diiron diselenolato complexes have been prepared as models of the active site of [FeFe]-hydrogenases. Treatment of Fe 3 (CO) 12 with 1 equiv of 1,3-diselenocyanatopropane (1) in THF at reflux afforded the model compound Fe 2 (µ-Se 2 C 3 H 6 )(CO) 6 (2) in 68% yield. The analogous methyl-substituted complex, Fe 2 (µ-Se 2 C 3 H 5 CH 3 )(CO) 6 (3), was obtained from the reaction of Fe 3 (CO) 12 with the in situ generated compound 3-methyl-1,2-diselenolane (4). In contrast, the reaction of Fe 3 (CO) 12 with 1,3,5-triselenacyclohexane (5) produced a mixture of Fe 2 (µ 2 ,κ-Se,C-SeCH 2 SeCH 2 )(CO) 6 (6), Fe 2 [(µ-SeCH 2 ) 2 Se](CO) 6 (7), and Fe 2 (µ-Se 2 CH 2 )(CO) 6 (8). Compounds 2, 3, 6, and 7 were characterized by IR, 1 H, 13 C, and 77 Se NMR spectroscopy, mass spectrometry, elemental analysis, and X-ray single-crystal structure analysis. The He I and He II photoelectron spectra for 3 are reported, and the electronic structure is further analyzed with the aid of DFT computations. The calculated reorganization energy of the cation of 3 to the "rotated" structure, which has a semibridging carbonyl ligand, is less than that of the analogous complexes with sulfur instead of selenium. Complexes 2 and 3 have been proved to be catalysts for electrochemical reduction of protons from the weak acids pivalic and acetic acid, respectively, to give hydrogen.

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.

A new synthetic method for annulating hydroquinones to Fe 2 S 2 (CO) 6 moieties is reported. Piperidine catalyzed a multistep reaction between Fe 2 (μ-SH) 2 (CO) 6 and quinones to afford bridged adducts in 26-76% yields. The hydroquinone adducts undergo reversible two-electron reductions. In the presence of acetic acid, hydrogen is produced catalytically with these adducts at potentials more negative than that of the initial reversible reduction. Spectroscopic studies suggest the presence of intramolecular hydrogen bonding between the phenolic OH groups and the adjacent sulfur atoms. Computations, which are in good agreement with the electrochemical studies and spectroscopic data, indicate that the hydrogen bonding is most important in the reduced forms of the catalysts. This hydrogen bonding lowers the reduction potential for catalysis but also lowers the basicity and thereby the reactivity of the catalysts.

Keywords: Iron / Hydrogenases / Substitution / Electrocatalysis / Ligand effects / Enzyme catalysis / SeleniumThe displacement of terminal CO ligands in Fe 2 (µ-Se 2 C 3 H 5 CH 3 )(CO) 6 (1) by triphenylphosphane, trimethyl phosphite, and bis(diphenylphosphanyl)ethane (dppe) ligands is investigated. Treatment of 1 with 1 equiv. of triphenylphosphane afforded Fe 2 (µ-Se 2 C 3 H 5 CH 3 )(CO) 5 (PPh 3 ) (2). The mono-and disubstituted phosphite complexes Fe 2 (µ-Se 2 C 3 H 5 CH 3 )(CO) 5 P(OMe) 3 (3)andFe 2 (µ-Se 2 C 3 H 5 CH 3 )(CO) 4 -[P(OMe) 3 ] 2 (4) were obtained from the reaction of 1 with excess P(OMe) 3 at reflux in toluene. In contrast, the reaction of 1 with 1 equiv. of dppe in the presence of Me 3 NO·2H 2 O gave

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The electronic interactions between metals and dithiolenes are important in the biological processes of many metalloenzymes as well as in diverse chemical and material applications. Of special note is the ability of the dithiolene ligand to support metal centers in multiple coordination environments and oxidation states. To better understand the nature of metal-dithiolene electronic interactions, new capabilities in gas-phase core photoelectron spectroscopy for molecules with high sublimation temperatures have been developed and applied to a series of molecules of the type Cp2M(bdt) (Cp = η5-cyclopentadienyl, M = Ti, V, Mo, and bdt = benzenedithiolato). Comparison of the gas-phase core and valence ionization energy shifts provides a unique quantitative energy measure of valence orbital overlap interactions between the metal and sulfur orbitals that is separated from the effects of charge redistribution. The results explain the large amount of sulfur character in the redox-active orbitals and the ‘leveling’ of oxidation state energies in metal-dithiolene systems. The experimentally-determined orbital interaction energies reveal a previously unidentified overlap interaction of the predominantly sulfur HOMO of the bdt ligand with filled π orbitals of the Cp ligands, suggesting that direct dithiolene interactions with other ligands bound to the metal could be significant for other metal-dithiolene systems in chemistry and biology.

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