Biomimetic assembly and activation of [FeFe]-hydrogenases.

Hydrogenases are the most active molecular catalysts for hydrogen production and uptake, and could therefore facilitate the development of new types of fuel cell. In [FeFe]-hydrogenases, catalysis takes place at a unique di-iron centre (the [2Fe] subsite), which contains a bridging dithiolate ligand, three CO ligands and two CN(-) ligands. Through a complex multienzymatic biosynthetic process, this [2Fe] subsite is first assembled on a maturation enzyme, HydF, and then delivered to the apo-hydrogenase for activation. Synthetic chemistry has been used to prepare remarkably similar mimics of that subsite, but it has failed to reproduce the natural enzymatic activities thus far. Here we show that three synthetic mimics (containing different bridging dithiolate ligands) can be loaded onto bacterial Thermotoga maritima HydF and then transferred to apo-HydA1, one of the hydrogenases of Chlamydomonas reinhardtii algae. Full activation of HydA1 was achieved only when using the HydF hybrid protein containing the mimic with an azadithiolate bridge, confirming the presence of this ligand in the active site of native [FeFe]-hydrogenases. This is an example of controlled metalloenzyme activation using the combination of a specific protein scaffold and active-site synthetic analogues. This simple methodology provides both new mechanistic and structural insight into hydrogenase maturation and a unique tool for producing recombinant wild-type and variant [FeFe]-hydrogenases, with no requirement for the complete maturation machinery.

Spectroscopic investigations of a semi-synthetic [FeFe] hydrogenase with propane di-selenol as bridging ligand in the binuclear subsite: comparison to the wild type and propane di-thiol variants.

[FeFe] Hydrogenases catalyze the reversible conversion of H2 into electrons and protons. Their catalytic site, the H-cluster, contains a generic [4Fe-4S]H cluster coupled to a [2Fe]H subsite [Fe2(ADT)(CO)3(CN)2]2-, ADT = µ(SCH2)2NH. Heterologously expressed [FeFe] hydrogenases (apo-hydrogenase) lack the [2Fe]H unit, but this can be incorporated through artificial maturation with a synthetic precursor [Fe2(ADT)(CO)4(CN)2]2-. Maturation with a [2Fe] complex in which the essential ADT amine moiety has been replaced by CH2 (PDT = propane-dithiolate) results in a low activity enzyme with structural and spectroscopic properties similar to those of the native enzyme, but with simplified redox behavior. Here, we study the effect of sulfur-to-selenium (S-to-Se) substitution in the bridging PDT ligand incorporated in the [FeFe] hydrogenase HydA1 from Chlamydomonas reinhardtii using magnetic resonance (EPR, NMR), FTIR and spectroelectrochemistry. The resulting HydA1-PDSe enzyme shows the same redox behavior as the parent HydA1-PDT. In addition, a state is observed in which extraneous CO is bound to the open coordination site of the [2Fe]H unit. This state was previously observed only in the native enzyme HydA1-ADT and not in HydA1-PDT. The spectroscopic features and redox behavior of HydA1-PDSe, resulting from maturation with [Fe2(PDSe)(CO)4(CN)2]2-, are discussed in terms of spin and charge density shifts and provide interesting insight into the electronic structure of the H-cluster. We also studied the effect of S-to-Se substitution in the [4Fe-4S] subcluster. The reduced form of HydA1 containing only the [4Fe-4Se]H cluster shows a characteristic S = 7/2 spin state which converts back into the S = 1/2 spin state upon maturation with a [2Fe]-PDT/ADT complex.

Engineering n-p junction for photo-electrochemical hydrogen production.

The generation of hydrogen from water and sunlight offers a promising approach for producing scalable and sustainable carbon free fuels. One of the challenges of solar-to-fuel technology is the design of efficient, long-lasting and low-cost photocathodes, which are responsible for absorbing sunlight and driving catalytic hydrogen evolution. We report on the protection of a Cu/Cu2O/CuO photoelectrode against photocorrosion by a 200-300 nm-thick BaTiO3 perovskite layer, deposited using the sol-gel method. This photoelectrode mediates H2 production with a current density of ∼3.1 mA cm-2 at 0 V versus RHE under 3 Sun irradiation and in a pH = 6 aqueous electrolyte. While the unprotected Cu/Cu2O/CuO photoelectrodes show a rapid decay of activity, the BaTiO3-protected photoelectrodes exhibit ∼10% current decay over 20 min.

Noble metal-free hydrogen-evolving photocathodes based on small molecule organic semiconductors.

Organic semiconductors have great potential for producing hydrogen in a sustainable and economically-viable manner because they rely on readily available materials with highly tunable properties. We demonstrate here the relevance of heterojunctions to the construction of H2-evolving photocathodes, exclusively based on earth-abundant elements. Boron subnaphthalocyanine chloride proved a very promising acceptor in that perspective. It absorbs a part of the solar spectrum complementary to α-sexithiophene as a donor, thus generating large photocurrents and providing a record onset potential for light-driven H2 evolution under acidic aqueous conditions using a nanoparticulate amorphous molybdenum sulfide catalyst.

Recent Developments in Hydrogen Evolving Molecular Cobalt(II)-Polypyridyl Catalysts.

The search for efficient noble metal-free hydrogen-evolving catalysts is the subject of intense research activity. A new family of molecular cobalt(II)-polypyridyl catalysts has recently emerged. These catalysts prove more robust under reductive conditions than other cobalt-based systems and display high activities under fully aqueous conditions. This review discusses the design, characterization, and evaluation of these catalysts for electrocatalytic and light-driven hydrogen production. Mechanistic considerations are addressed and structure-catalytic activity relationships identified in order to guide the future design of more efficient catalytic systems.

Push-pull organic dyes and dye-catalyst assembly featuring a benzothiadiazole unit for photoelectrochemical hydrogen production.

In this work, we report the design and the preparation of two new dyes and a molecular dyad for the photoelectrochemical hydrogen production from water in a dye-sensitized photoelectrochemical cell (DSPEC). We designed dyes that include a benzothiadiazole (BTD) and an indacenodithiophene (IDT) units, and we obtained a new molecular dyad by covalent coupling with the cobalt diimine-dioxime catalyst. The introduction of the benzothiadiazole core in the structure improves the absorption properties and leads to an extension of the spectrum in the visible range up to 650 nm. The photoelectrochemical properties of the new dyad were evaluated on pristine and lithium-doped NiO electrodes. We demonstrate that increasing the light harvesting efficiency of the dyad by introducing a IDT-BTD chromophore is clearly beneficial for the photoelectrochemical activity. We also demonstrate that lithium doping of NiO, which improves the electronic conductivity of the mesoporous film, leads to a significant increase in performance, in terms of TON and F.E., more than doubled with our new dyad. This BTD-based molecular system outperforms the results of previously reported dyads using the same catalyst.

Short dental implants in patients with oral lichen planus: a long-term follow-up.

Oral lichen planus is associated with the Koebner phenomenon, and trauma may exacerbate oral lesions. Short dental implants, as alternatives to bony augmentation, would reduce the number of interventions and their morbidity. However, we know of no studies that have analysed the long-term outcomes of short implants in patients with oral lichen planus. We have therefore designed a retrospective study of such patients treated with short implants (≤8.5mm long), with survival of implants as the main outcome. The secondary outcomes were marginal bone loss and the development of complications. We calculated the implants' survival and compared the outcomes statistically between erosive and reticular oral lichen planus. Sixty-six short implants were placed in 23 patients with a mean (SD) age of 58 (7) years. The mean (SD) peri-implant bone loss was 0.96 (0.89) mm mesially and 0.99 (1.1) mm distally. Sixty-five of the 66 implants survived with a mean (SD) follow-up of 68 (32) months, and there were no significant differences between erosive and reticular disease. Stable long-term outcomes can be expected for short implants placed in patients with oral lichen planus, and graftless rehabilitation of missing teeth could be possible in these patients if short implants were used.

A noble metal-free proton-exchange membrane fuel cell based on bio-inspired molecular catalysts.

Hydrogen is a promising energy vector for storing renewable energies: obtained from water-splitting, in electrolysers or photoelectrochemical cells, it can be turned back to electricity on demand in fuel cells (FCs). Proton exchange membrane (PEM) devices with low internal resistance, high compactness and stability are an attractive technology optimized over decades, affording fast start-up times and low operating temperatures. However, they rely on the powerful catalytic properties of noble metals such as platinum, while lower cost, more abundant materials would be needed for economic viability. Replacing these noble metals at both electrodes has long proven to be a difficult task, so far incompatible with PEM technologies. Here we take advantage of newly developed bio-inspired molecular H2 oxidation catalysts and noble metal-free O2-reducing materials, to fabricate a noble metal-free PEMFC, with an 0.74 V open circuit voltage and a 23 µW cm-2 output power under technologically relevant conditions. X-ray absorption spectroscopy measurements confirm that the catalysts are stable and retain their structure during turnover.

Dye-Sensitized Nanostructured Crystalline Mesoporous Tin-doped Indium Oxide Films with Tunable Thickness for Photoelectrochemical Applications.

A simple route towards nanostructured mesoporous Indium-Tin Oxide (templated nano-ITO) electrodes exhibiting both high conductivities and optimized bicontinuous pore-solid network is reported. The ITO films are first produced as an X-ray-amorphous, high surface area material, by adapting recently established template-directed sol-gel methods using Sn(IV) and In(III) salts. Carefully controlled temperature/atmosphere treatments convert the as-synthesized ITO films into nano-crystalline coatings with the cubic bixbyite structure. Specially, a multi-layered synthesis was successfully undertaken for tuning the film thickness. In order to evaluate the performances of templated nano-ITO as an electrode substrate for photoelectrochemical applications, photoelectrodes were prepared by covalent grafting of a redox-active dye, the complex [Ru(bpy)2(4,4'-(CH2PO3H2)2-bpy)]Cl21 (bpy=bipyridine). Surface coverage was shown to increase with the film thickness, from 0.7 × 10-9 mol.cm-2 (one layer, 45 nm) to 3.5 × 10-9 mol.cm-2 (ten layers, 470 nm), the latter value being ~ 100 times larger than that for commercially available planar ITO. In the presence of an electron mediator, photocurrents up to 50 µA.cm-2 have been measured under visible light irradiation, demonstrating the potential of this new templated nano-ITO preparation for the construction of efficient photoelectrochemical devices.

[Ni(xbsms)Ru(CO)2Cl2]: a bioinspired nickel-ruthenium functional model of [NiFe] hydrogenase.

As a model of the active site of [NiFe] hydrogenases, a dinuclear nickel-ruthenium complex [Ni(xbsms)Ru(CO)2Cl2] was synthesized and fully characterized. The three-dimensional structure reveals a nickel center in a square-planar dithioether-dithiolate environment connected to a ruthenium moiety via a Ni(mu-SR)2Ru bridge. This complex catalyzes hydrogen evolution by electroreduction of the weakly acidic Et3NH+ ions in N,N-dimethylformamide and is therefore the first functional bioinspired model of [NiFe] hydrogenases.

Synthesis, characterization, and photochemical behavior of {Ru(arene)}2+ derivatives of alpha-[PW11O39]7-: an organometallic way to ruthenium-substituted heteropolytungstates.

Reaction of [Ru(arene)Cl(2)](2) (arene = benzene, toluene, p-cymene, hexamethylbenzene) with K(7)[PW(11)O(39)].14H(2)O provided two series of organometallic derivatives of heteropolytungstates: type-1 and type-2 complexes of general formulas [PW(11)O(39){Ru(arene)(H(2)O)}](5-) and [{PW(11)O(39){Ru(arene)}}(2){WO(2)}](8-), respectively. All compounds were characterized by infrared and multinuclear NMR ((1)H, (31)P, (183)W) spectroscopies. The crystal structures of Na(4)K(4)[{PW(11)O(39){Ru(benzene)}}(2){WO(2)}].6H(2)O (NaK-2a.6H(2)O), K(7)H[{PW(11)O(39){Ru(toluene)}}(2){WO(2)}].4H(2)O (K-2b.4H(2)O), and Cs(3)K(2)[PW(11)O(39){Ru(p-cymene)(H(2)O)}].4H(2)O (CsK-1c.4H(2)O) were obtained and revealed that the {Ru(arene)} fragment is supported on the oxometallic framework. Photochemical reactivity of [PW(11)O(39){Ru(arene)(H(2)O)}](5-) (arene = toluene, p-cymene) in the presence of various ligands L (L = H(2)O, dimethyl sulfoxide, tetramethylene sulfoxide, and diphenyl sulfoxide) was investigated, and led to the formation of [PW(11)O(39){Ru(L)}](5-), in which the ruthenium is incorporated into the lacunary [PW(11)O(39)](7-) anion.

Interplay of cubic building blocks in (et6-arene)ruthenium-containing tungsten and molybdenum oxides.

A series of molybdenum and tungsten organometallic oxides containing [Ru(arene)]2+ units (arene =p-cymene, C6Me6) was obtained by condensation of [[Ru(arene)Cl2]2] with oxomolybdates and oxotungstates in aqueous or nonaqueous solvents. The crystal structures of [[Ru(eta6-C6Me6]]4W4O16], [[Ru(eta6-p-MeC6H4iPr]]4W2O10], [[[Ru-(eta6-p-MeC6H4iPr)]2(mu-OH)3]2][[Ru(eta6-p-MeC6H4iPr)]2W8O28(OH)2[Ru(eta6-p-MeC6H4iPr)(H2O)]2], and [[Ru(eta6-C6Me6)]2M5O18[Ru(eta6-C6Me6)(H2O)]] (M = Mo, W) have been determined. While the windmill-type clusters [[Ru(eta6-arene)]4(MO3)4(mu3-O)4] (M = Mo, W; arene =p-MeC6H4iPr, C6Me6), the face-sharing double cubane-type cluster [[Ru(eta6-p-MeC6H4iPr)]4(WO2)2(mu3-O)4(mu4-O)2], and the dimeric cluster [[Ru(eta6-p-MeC6H4iPr)(WO3)3(mu3-O)3(mu3-OH)Ru(eta6-pMeC6H4iPr)(H2O)]2(mu-WO2)2]2- are based on cubane-like units, [(Ru(eta6-C6Me6)]2M5O18[Ru(eta6-C6Me6)(H2O)]] (M = Mo, W) are more properly described as lacunary Lindqvist-type polyoxoanions supporting three ruthenium centers. Precubane clusters [[Ru(eta6-arene)](MO3)2(mu-O)3(mu3-O)]6- are possible intermediates in the formation of these clusters. The cluster structures are retained in solution, except for [[Ru(eta6-p-MeC6H4iPr)]4Mo4O16], which isomerizes to the triple-cubane form.