A cobalt complex with a bioinspired molybdopterin-like ligand: a catalyst for hydrogen evolution.

Cobalt dithiolene complexes are a new class of H2-evolving catalysts. Here we describe the preparation, the structure and the catalytic activity of an original cobalt complex using a bioinspired ligand, a quinoxaline-pyran-fused dithiolene derivative (qpdt(2-)) that mimics the molybdopterin cofactor present in the active sites of formate dehydrogenases. This complex displays very good activity for electrochemical proton reduction under weak acid conditions in terms of turnover frequency, faradic yields and stability. Density functional theory calculations show that protonation of a nitrogen atom of the ligand decreases overpotentials by 520 mV and H2 formation proceeds via protonation of an intermediate Co-H hydride, with an adjacent S atom of the dithiolene ligand serving as a proton relay.

Synthesis and Reactivity of a Bio-inspired Dithiolene Ligand and its Mo Oxo Complex.

An original synthesis of the fused pyranoquinoxaline dithiolene ligand qpdt(2-) is discussed in detail. The most intriguing step is the introduction of the dithiolene moiety by Pd-catalyzed carbon-sulfur coupling. The corresponding Mo(IV)O complex (Bu4N)2 [MoO(qpdt)2] (2) underwent reversible protonation in a strongly acidic medium and remained stable under anaerobic conditions. Besides, 2 was found to be very sensitive towards oxygen, as upon oxidation it formed a planar dithiin derivative. Moreover, the qpdt(2-) ligand in the presence of [MoCl4 (tBuNC)2] formed a tetracyclic structure. The products resulting from the unique reactivity of qpdt(2-) were characterized by X-ray diffraction, mass spectrometry, NMR spectroscopy, UV/Vis spectroscopy, and electrochemistry. Plausible mechanisms for the formation of these products are also proposed.

A Bioinspired Molybdenum Complex as a Catalyst for the Photo- and Electroreduction of Protons.

A molybdenum-dithiolene-oxo complex was prepared as a model of some active sites of Mo/W-dependent enzymes. The ligand, a quinoxaline-pyran-fused dithiolene, mimics molybdopterin present in these active sites. For the first time, this type of complex was shown to be active as a catalyst for the photoreduction of protons with excellent turnover numbers (500) and good stability in aqueous/organic media and for the electroreduction of protons in acetonitrile with remarkable rate constants (1030 s(-1) at -1.3 V versus Ag/AgCl). DFT calculations provided insight into the catalytic cycle of the reaction, suggesting that the oxo ligand plays a key role in proton exchange. These results provide a basis to optimize this new class of H2 -evolving catalysts.

Bioinspired Tungsten Dithiolene Catalysts for Hydrogen Evolution: A Combined Electrochemical, Photochemical, and Computational Study.

Bis(dithiolene)tungsten complexes, W(VI)O2 (L = dithiolene)2 and W(IV)O (L = dithiolene)2, which mimic the active site of formate dehydrogenases, have been characterized by cyclic voltammetry and controlled potential electrolysis in acetonitrile. They are shown to be able to catalyze the electroreduction of protons into hydrogen in acidic organic media, with good Faradaic yields (75-95%) and good activity (rate constants of 100 s(-1)), with relatively high overpotentials (700 mV). They also catalyze proton reduction into hydrogen upon visible light irradiation, in combination with [Ru(bipyridine)3](2+) as a photosensitizer and ascorbic acid as a sacrificial electron donor. On the basis of detailed DFT calculations, a reaction mechanism is proposed in which the starting W(VI)O2 (L = dithiolene)2 complex acts as a precatalyst and hydrogen is further formed from a key reduced W-hydroxo-hydride intermediate.