Electrochemical reduction and protonation of a biomimetic diiron azadithiolate hexacarbonyl complex: Mechanistic insights.

The electrochemical reduction and protonation of [Fe2(adtH)(CO)6] (1, adtH = SCH2N(H)CH2S) and [Fe2(pdt)(CO)6] (2, pdt = SCH2CH2CH2S) in the presence of moderately strong acid in acetonitrile was investigated by cyclic voltammetry (CV), focusing on the catalysis of hydrogen evolution reaction (HER) by a {2e-,2H+} pathway. The turnover frequencies at zero overpotential (TOF0) of the N-protonated product 1(H)+ and 2 for the HER were estimated from simulations of the catalytic CV responses at low acid concentration using a simple ECEC mechanism (two electrochemical and chemical steps). This approach confirmed that 1(H)+ is clearly a better catalyst than 2, pointing to a possible role of the protonable and biologically relevant adtH ligand in the enhancement of the catalytic performances. Density functional theory (DFT) calculations further suggested that, owing to a strong structural rearrangement in the course of the catalytic cycle, the HER catalysis by 1(H)+ only involves the iron center adjacent to the amine group in adtH and not the two iron centers as in 2. Since terminal hydride species (FeFe-H) are known to more easily undergo protonolyse to H2 than their bridging hydride isomers (Fe-H-Fe), this may explain here the enhanced activity of 1(H)+ over 2 for the HER.

Mechanistic insights into the catalysis of electrochemical proton reduction by a diiron azadithiolate complex.

Cyclic voltammetry experiments and DFT calculations allowed us to establish a complete mechanism of the catalysis of electrochemical proton reduction by [Fe2(µ-SCH2N(H)CH2S)(CO)6] (Fe-adt) in acetonitrile. The proposed mechanism is fully consistent with the observed dependence of the voltammetric responses on the strength of the acid used as a proton source. Addition of moderately strong acids, such as CCl3CO2H (pK(a) = 10.7) or HOTs·H2O (pK(a) = 8.6), triggers the occurrence of new reduction events at potentials less negative than the reduction of Fe-adt, therefore ascribed to reduction of the protonated forms of the complex. Reduction of the N-protonated form seems to favor a tautomerization reaction leading to a Fe-H intermediate. On the other hand, addition of weak acids, such as ClCH2CO2H (pK(a) = 15.3), leads to direct protonation on the diiron site subsequently to reduction of the catalyst. A better understanding of the mechanism of proton reduction by the biologically relevant Fe-adt derivative could impact the design of improved catalysts inspired by FeFe-hydrogenase.

Manganese carbonyl terpyridyl complexes: their synthesis, characterization and potential application as CO-release molecules.

Mn(I) carbonyl terpyridyl complexes have been synthesized and characterized. The tricarbonyl derivative exhibits interesting behaviors for controlled CO-release by both thermal and photosynthetic pathways.

Pulsed-EPR evidence of a manganese(II) hydroxycarbonyl intermediate in the electrocatalytic reduction of carbon dioxide by a manganese bipyridyl derivative.

A key intermediate in the electroconversion of carbon dioxide to carbon monoxide, catalyzed by a manganese tris(carbonyl) complex, is characterized. Different catalytic pathways and their potential reaction mechanisms are investigated using a large range of experimental and computational techniques. Sophisticated spectroscopic methods including UV/Vis absorption and pulsed-EPR techniques (2P-ESEEM and HYSCORE) were combined together with DFT calculations to successfully identify a key intermediate in the catalytic cycle of CO2 reduction. The results directly show the formation of a metal-carboxylic acid-CO2 adduct after oxidative addition of CO2 and H(+) to a Mn(0) carbonyl dimer, an unexpected intermediate.

Concerted proton-coupled electron transfer from a metal-hydride complex.

Metal hydrides are key intermediates in the catalytic reduction of protons and CO2 as well as in the oxidation of H2. In these reactions, electrons and protons are transferred to or from separate acceptors or donors in bidirectional protoncoupled electron transfer (PCET) steps. The mechanistic interpretation of PCET reactions of metal hydrides has focused on the stepwise transfer of electrons and protons. A concerted transfer may, however, occur with a lower reaction barrier and therefore proceed at higher catalytic rates. Here we investigate the feasibility of such a reaction by studying the oxidation­deprotonation reactions of a tungsten hydride complex. The rate dependence on the driving force for both electron transfer and proton transfer­employing different combinations of oxidants and bases­was used to establish experimentally the concerted, bidirectional PCET of a metal-hydride species. Consideration of the findings presented here in future catalyst designs may lead to more-efficient catalysts.

[Mn(bipyridyl)(CO)3Br]: an abundant metal carbonyl complex as efficient electrocatalyst for CO2 reduction.

Manganese at work: carbonyl bipyridyl complexes based on manganese, a non-noble abundant and inexpensive metal, have been proved to be excellent molecular catalysts for the selective electrochemical reduction of CO(2) to CO under mild conditions. Another advantage of manganese complexes over rhenium complexes is that these catalysts operate at markedly less overpotential (0.40 V gain).