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.

Molecular and electronic structural properties of the hydrogen-bonded complexes of p-quinone dianions (PQ(2)(-)) were investigated by electrochemistry and spectroelectrochemistry of PQ in MeCN combined with ab initio MO calculations. Hydrogen bonding between PQ(2)(-) and MeOH was measured as the continuous positive shift of the apparent second half-wave reduction potentials with increasing concentrations of MeOH. Detailed analyses of the behavior reveal that PQ(2)(-) forms the 1:2 hydrogen-bonded complexes at low concentrations of MeOH and the 1:4 complexes at high concentrations, yielding the formation constants. Temperature dependence of the formation constants allows us to yield the formation energy as 76.6 and 118.9 kJ mol(-)(1) for the 1:2 and 1:4 complex formation of the 1,4-benzoquinone dianion (BQ(2)(-)) with MeOH, respectively. These results show that the pi-dianions involving the quinone carbonyl groups exhibit very strong hydrogen-accepting ability. The longest wavelength band of the spectra of BQ(2)(-) and the chloranil dianion (CL(2)(-)) is assigned to the (1)B(3u) <-- (1)A(g) band mainly contributed from an intramolecular charge-transfer (CT) configuration. Hydrogen bonding allows the band of BQ(2)(-) and CL(2)(-) to be blue-shifted, depending on the strength of the hydrogen bonds. CNDO/S-CI calculations reveal that the blue shift is ascribed to stabilization of the ground state by the hydrogen bonding involving strong n-sigma-type CT interaction. The HF/6-31G(d) calculation results show that the structure of PQ(2)(-) is characterized by a lengthening of the C=O bonds and a benzenoid ring. The geometrical properties of the hydrogen-bonded complexes of PQ(2)(-) are a slight lengthening of the C=O bonds and a short distance of the hydrogen bonds. It is demonstrated that this situation is due to the strong n-sigma CT interaction in the hydrogen bonds. The results suggest that the differing functions and properties of biological quinones are conferred by the n-sigma CT interaction through hydrogen bonding of the dianions with their protein environment.

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.