Ultrafast Electron Transfer from CuInS2 Quantum Dots to a Molecular Catalyst for Hydrogen Production: Challenging Diffusion Limitations.

Systems integrating quantum dots with molecular catalysts are attracting ever more attention, primarily owing to their tunability and notable photocatalytic activity in the context of the hydrogen evolution reaction (HER) and CO2 reduction reaction (CO2RR). CuInS2 (CIS) quantum dots (QDs) are effective photoreductants, having relatively high-energy conduction bands, but their electronic structure and defect states often lead to poor performance, prompting many researchers to employ them with a core-shell structure. Molecular cobalt HER catalysts, on the other hand, often suffer from poor stability. Here, we have combined CIS QDs, surface-passivated with l-cysteine and iodide from a water-based synthesis, with two tetraazamacrocyclic cobalt complexes to realize systems which demonstrate high turnover numbers for the HER (up to >8000 per catalyst), using ascorbate as the sacrificial electron donor at pH = 4.5. Photoluminescence intensity and lifetime quenching data indicated a large degree of binding of the catalysts to the QDs, even with only ca. 1 µM each of QDs and catalysts, linked to an entirely static quenching mechanism. The data was fitted with a Poissonian distribution of catalyst molecules over the QDs, from which the concentration of QDs could be evaluated. No important difference in either quenching or photocatalysis was observed between catalysts with and without the carboxylate as a potential anchoring group. Femtosecond transient absorption spectroscopy confirmed ultrafast interfacial electron transfer from the QDs and the formation of the singly reduced catalyst (CoII state) for both complexes, with an average electron transfer rate constant of ≈ (10 ps)-1. These favorable results confirm that the core tetraazamacrocyclic cobalt complex is remarkably stable under photocatalytic conditions and that CIS QDs without inorganic shell structures for passivation can act as effective photosensitizers, while their smaller size makes them suitable for application in the sensitization of, inter alia, mesoporous electrodes.

Electro- and photochemical H2 generation by Co(ii) polypyridyl-based catalysts bearing ortho-substituted pyridines.

Cobalt(ii) complexes featuring hexadentate amino-pyridyl ligands have been recently discovered as highly active catalysts for the Hydrogen Evolution Reaction (HER), whose high performance arises from the possibility of assisting proton transfer processes via intramolecular routes involving detached pyridine units. With the aim of gaining insights into such catalytic routes, three new proton reduction catalysts based on amino-polypyridyl ligands are reported, focusing on substitution of the pyridine ortho-position. Specifically, a carboxylate (C2) and two hydroxyl substituted pyridyl moieties (C3, C4) are introduced with the aim of promoting intramolecular proton transfer which possibly enhances the efficiency of the catalysts. Foot-of-the-wave and catalytic Tafel plot analyses have been utilized to benchmark the catalytic performances under electrochemical conditions in acetonitrile using trifluoroacetic acid as the proton source. In this respect, the cobalt complex C3 turns out to be the fastest catalyst in the series, with a maximum turnover frequency (TOF) of 1.6 (±0.5) × 105 s-1, but at the expense of large overpotentials. Mechanistic investigations by means of Density Functional Theory (DFT) suggest a typical ECEC mechanism (i.e. a sequence of reduction - E - and protonation - C - events) for all the catalysts, as previously envisioned for the parent unsubstituted complex C1. Interestingly, in the case of complex C2, the catalytic route is triggered by initial protonation of the carboxylate group resulting in a less common (C)ECEC mechanism. The pivotal role of the hexadentate chelating ligand in providing internal proton relays to assist hydrogen elimination is further confirmed within this novel class of molecular catalysts, thus highlighting the relevance of a flexible polypyridine ligand in the design of efficient cobalt complexes for the HER. Photochemical studies in aqueous solution using [Ru(bpy)3]2+ (where bpy = 2,2'-bipyridine) as the sensitizer and ascorbate as the sacrificial electron donor support the superior performance of C3.

Hydrogen evolution reaction mediated by an all-sulfur trinuclear nickel complex.

We report the synthesis and the characterization of a trinuclear nickel complex. Solid state and solution studies using X-ray diffraction, NMR and UV-vis spectroscopy highlight the square planar geometries around the metal centers and an all-sulfur coordination sphere. It exhibits significant electrocatalytic activity for hydrogen evolution in DMF using Et3NHCl as the proton source. DFT studies suggest that sulfur atoms act as proton relay, as proposed in [NiFe] hydrogenases.

Electrocatalytic Hydrogen Evolution with a Cobalt Complex Bearing Pendant Proton Relays: Acid Strength and Applied Potential Govern Mechanism and Stability.

[Co(bapbpy)Cl]+ (bapbpy: 6,6'-bis(2-aminopyridyl)-2,2'-bipyridine) is a polypyridyl cobalt(II) complex bearing both a redox-active bipyridine ligand and pendant proton relays. This compound catalyzes electro-assisted H2 evolution in DMF with distinct mechanisms depending on the strength of the acid used as the proton source (pKa values ranging from 3.4 to 13.5 in DMF) and the applied potential. Electrochemical studies combining cyclic voltammetry and bulk electrolysis measurements enabled one to bring out four distinct catalytic processes. Where applicable, relevant kinetic information were obtained using either foot-of-the-wave analysis (FOWA) or analytical treatment of bulk electrolysis experiments. Among the different catalytic pathways identified in this study, a clear relationship between the catalyst performances and stability was evidenced. These results draw attention to a number of interesting considerations and may help in the development of future adequately designed catalysts.

Proton reduction reaction catalyzed by homoleptic nickel bis-1,2-dithiolate complexes: Experimental and theoretical mechanistic investigations.

A series of homoleptic monoanionic nickel dithiolene complexes [Ni(bdt)2](NBu4), [Ni(tdt)2](NBu4), and [Ni(mnt)2](NBu4) containing the ligands benzene-1,2-dithiolate (bdt2-), toluene-3,4-dithiolate (tdt2-) and maleonitriledithiolate (mnt2-), respectively, have been employed as electrocatalysts in the hydrogen evolution reaction with trifluoroacetic acid as proton source in acetonitrile. All complexes were active catalysts with TONs reaching 113, 158 and 6 for [Ni(bdt)2](NBu4), [Ni(tdt)2](NBu4), and [Ni(mnt)2](NBu4), respectively. Faradaic yield for hydrogen evolution reaction reaches 88 % for 2- , which also displays the minimal overpotential requirement value (467 mV) within the series. Two pathways for H2 evolution can be hypothesized that differ on on the sequence of protonation and reduction steps. DFT calculations are in agreement with experimental data and indicate that protonation at sulfur follows reduction to the dianion. Hydrogen evolves from the direduced-diprotonated form via a highly distorted nickel hydride intermediate. The effects of acid strength and concentration in the hydrogen-evolving mechanism are also discussed.

Structural and functional characterization of the hydrogenase-maturation HydF protein.

[FeFe] hydrogenase (HydA) catalyzes interconversion between 2H+ and H2 at an active site composed of a [4Fe-4S] cluster linked to a 2Fe subcluster that harbors CO, CN- and azapropanedithiolate (adt2-) ligands. HydE, HydG and HydF are the maturases specifically involved in the biosynthesis of the 2Fe subcluster. Using ligands synthesized by HydE and HydG, HydF assembles a di-iron precursor of the 2Fe subcluster and transfers it to HydA for maturation. Here we report the first X-ray structure of HydF with its [4Fe-4S] cluster. The cluster is chelated by three cysteines and an exchangeable glutamate, which allows the binding of synthetic mimics of the 2Fe subcluster. [Fe2(adt)(CO)4(CN)2]2- is proposed to be the true di-iron precursor because, when bound to HydF, it matures HydA and displays features in Fourier transform infrared (FTIR) spectra that are similar to those of the native HydF active intermediate. A new route toward the generation of artificial hydrogenases, as combinations of HydF and such biomimetic complexes, is proposed on the basis of the observed hydrogenase activity of chemically modified HydF.

Carbon-Nanotube-Supported Bio-Inspired Nickel Catalyst and Its Integration in Hybrid Hydrogen/Air Fuel Cells.

A biomimetic nickel bis-diphosphine complex incorporating the amino acid arginine in the outer coordination sphere was immobilized on modified carbon nanotubes (CNTs) through electrostatic interactions. The functionalized redox nanomaterial exhibits reversible electrocatalytic activity for the H2 /2 H+ interconversion from pH 0 to 9, with catalytic preference for H2 oxidation at all pH values. The high activity of the complex over a wide pH range allows us to integrate this bio-inspired nanomaterial either in an enzymatic fuel cell together with a multicopper oxidase at the cathode, or in a proton exchange membrane fuel cell (PEMFC) using Pt/C at the cathode. The Ni-based PEMFC reaches 14 mW cm-2 , only six-times-less as compared to full-Pt conventional PEMFC. The Pt-free enzyme-based fuel cell delivers ≈2 mW cm-2 , a new efficiency record for a hydrogen biofuel cell with base metal catalysts.

Nickel-centred proton reduction catalysis in a model of [NiFe] hydrogenase.

Hydrogen production through water splitting is one of the most promising solutions for the storage of renewable energy. [NiFe] hydrogenases are organometallic enzymes containing nickel and iron centres that catalyse hydrogen evolution with performances that rival those of platinum. These enzymes provide inspiration for the design of new molecular catalysts that do not require precious metals. However, all heterodinuclear NiFe models reported so far do not reproduce the Ni-centred reactivity found at the active site of [NiFe] hydrogenases. Here, we report a structural and functional NiFe mimic that displays reactivity at the Ni site. This is shown by the detection of two catalytic intermediates that reproduce structural and electronic features of the Ni-L and Ni-R states of the enzyme during catalytic turnover. Under electrocatalytic conditions, this mimic displays high rates for H2 evolution (second-order rate constant of 2.5 × 104 M-1 s-1; turnover frequency of 250 s-1 at 10 mM H+ concentration) from mildly acidic solutions.

Experimental and Theoretical Insight into Electrocatalytic Hydrogen Evolution with Nickel Bis(aryldithiolene) Complexes as Catalysts.

A series of neutral and monoanionic nickel dithiolene complexes with p-methoxyphenyl-substituted 1,2-dithiolene ligands have been prepared and characterized with physicochemical methods. Two of the complexes, the monoanion of the symmetric [Ni{S2C2(Ph-p-OCH3)2}2] (3(-)) with NBu4(+) as a counterion and the neutral asymmetric [Ni{S2C2(Ph)(Ph-p-OCH3)}2] (2), have been structurally characterized by single-crystal X-ray crystallography. All complexes have been employed as proton-reducing catalysts in N,N-dimethylformamide with trifluoroacetic acid as the proton source. The complexes are active catalysts with good faradaic yields, reaching 83% for 2 but relatively high overpotential requirements (0.91 and 1.55 V measured at the middle of the catalytic wave for two processes observed depending on the different routes of the mechanism). The similarity of the experimental data regardless of whether the neutral or anionic form of the complexes is used indicates that the neutral form acts as a precatalyst. On the basis of detailed density functional theory calculations, the proposed mechanism reveals two different main routes after protonation of the dianion of the catalyst in accordance with the experimental data, indicating the role of the concentration of the acid and the influence of the methoxy groups. Protonation at sulfur seems be more favorable than that at the metal, which is in marked contrast with the catalytic mechanism proposed for analogous cobalt dithiolene complexes.

From enzyme maturation to synthetic chemistry: the case of hydrogenases.

Water splitting into oxygen and hydrogen is one of the most attractive strategies for storing solar energy and electricity. Because the processes at work are multielectronic, there is a crucial need for efficient and stable catalysts, which in addition have to be cheap for future industrial developments (electrolyzers, photoelectrochemicals, and fuel cells). Specifically for the water/hydrogen interconversion, Nature is an exquisite source of inspiration since this chemistry contributes to the bioenergetic metabolism of a number of living organisms via the activity of fascinating metalloenzymes, the hydrogenases. In this Account, we first briefly describe the structure of the unique dinuclear organometallic active sites of the two classes of hydrogenases as well as the complex protein machineries involved in their biosynthesis, their so-called maturation processes. This knowledge allows for the development of a fruitful bioinspired chemistry approach, which has already led to a number of interesting and original catalysts mimicking the natural active sites. More specifically, we describe our own attempts to prepare artificial hydrogenases. This can be achieved via the standard bioinspired approach using the combination of a synthetic bioinspired catalyst and a polypeptide scaffold. Such hybrid complexes provide the opportunity to optimize the system by manipulating both the catalyst through chemical synthesis and the protein component through mutagenesis. We also raise the possibility to reach such artificial systems via an original strategy based on mimicking the enzyme maturation pathways. This is illustrated in this Account by two examples developed in our laboratory. First, we show how the preparation of a lysozyme-{Mn(I)(CO)3} hybrid and its clean reaction with a nickel complex led us to generate a new class of binuclear Ni-Mn H2-evolving catalysts mimicking the active site of [NiFe]-hydrogenases. Then we describe how we were able to rationally design and prepare a hybrid system, displaying remarkable structural similarities to an [FeFe]-hydrogenase, and we show here for the first time that it is catalytically active for proton reduction. This system is based on the combination of HydF, a protein involved in the maturation of [FeFe]-hydrogenase (HydA), and a close mimic of the active site of this class of enzymes. Moreover, the synthetic [Fe2(adt)(CO)4(CN)2](2-) (adt(2-)= aza-propanedithiol) mimic, alone or within a HydF hybrid system, was shown to be able to maturate and activate a form of HydA itself lacking its diiron active site. We discuss the exciting perspectives this "synthetic maturation" opens regarding the "invention" of novel hydrogenases by the chemists.

Dye-sensitized PS-b-P2VP-templated nickel oxide films for photoelectrochemical applications.

Moving from homogeneous water-splitting photocatalytic systems to photoelectrochemical devices requires the preparation and evaluation of novel p-type transparent conductive photoelectrode substrates. We report here on the sensitization of polystyrene-block-poly-(2-vinylpyridine) (PS-b-P2VP) diblock copolymer-templated NiO films with an organic push-pull dye. The potential of these new templated NiO film preparations for photoelectrochemical applications is compared with NiO material templated by F108 triblock copolymers. We conclude that NiO films are promising materials for the construction of dye-sensitized photocathodes to be inserted into photoelectrochemical (PEC) cells. However, a combined effort at the interface between materials science and molecular chemistry, ideally funded within a Global Artificial Photosynthesis Project, is still needed to improve the overall performance of the photoelectrodes and progress towards economically viable PEC devices.

Artificially maturated [FeFe] hydrogenase from Chlamydomonas reinhardtii: a HYSCORE and ENDOR study of a non-natural H-cluster.

Hydrogenases are enzymes that catalyze the oxidation of H2 as well as the reduction of protons to form H2. The active site of [FeFe] hydrogenase is referred to as the "H-cluster" and consists of a "classical" [4Fe-4S] cluster connected via a bridging cysteine thiol group to a unique [2Fe]H sub-cluster, containing CN(-) and CO ligands as well as a bidentate azadithiolate ligand. It has been recently shown that the biomimetic [Fe2(adt)(CO)4(CN)2](2-) (adt(2-) = azadithiolate) complex resembling the diiron sub-cluster can be inserted in vitro into the apo-protein of [FeFe] hydrogenase, which contains only the [4Fe-4S] part of the H-cluster, resulting in a fully active enzyme. This synthetic tool allows convenient incorporation of a variety of diiron mimics, thus generating hydrogenases with artificial active sites. [FeFe] hydrogenase from Chlamydomonas reinhardtii maturated with the biomimetic complex [Fe2(pdt)(CO)4(CN)2](2-) (pdt(2-) = propanedithiolate), in which the bridging adt(2-) ligand is replaced by pdt(2-), can be stabilized in a state strongly resembling the active oxidized (Hox) state of the native protein. This state is EPR active and the signal originates from the mixed valence Fe(I)Fe(II) state of the diiron sub-cluster. Taking advantage of the variant with (15)N and (13)C isotope labeled CN(-) ligands we performed HYSCORE and ENDOR studies on this hybrid protein. The (13)C hyperfine couplings originating from both CN(-) ligands were determined and assigned. Only the (15)N coupling from the CN(-) ligand bound to the terminal iron was observed. Detailed orientation selective ENDOR and HYSCORE experiments at multiple field positions enabled the extraction of accurate data for the relative orientations of the nitrogen and carbon hyperfine tensors. These data are consistent with the crystal structure assuming a g-tensor orientation following the local symmetry of the binuclear sub-cluster.

Cobalt and nickel diimine-dioxime complexes as molecular electrocatalysts for hydrogen evolution with low overvoltages.

Hydrogen production through the reduction of water appears to be a convenient solution for the long-run storage of renewable energies. However, economically viable hydrogen production requests platinum-free catalysts, because this expensive and scarce (only 37 ppb in the Earth's crust) metal is not a sustainable resource [Gordon RB, Bertram M, Graedel TE (2006) Proc Natl Acad Sci USA 103:1209-1214]. Here, we report on a new family of cobalt and nickel diimine-dioxime complexes as efficient and stable electrocatalysts for hydrogen evolution from acidic nonaqueous solutions with slightly lower overvoltages and much larger stabilities towards hydrolysis as compared to previously reported cobaloxime catalysts. A mechanistic study allowed us to determine that hydrogen evolution likely proceeds through a bimetallic homolytic pathway. The presence of a proton-exchanging site in the ligand, furthermore, provides an exquisite mechanism for tuning the electrocatalytic potential for hydrogen evolution of these compounds in response to variations of the acidity of the solution, a feature only reported for native hydrogenase enzymes so far.

Cyclopentadienyl ruthenium-nickel catalysts for biomimetic hydrogen evolution: electrocatalytic properties and mechanistic DFT studies.

The new dinuclear nickel-ruthenium complexes [Ni(xbsms)RuCp(L)][PF(6)] (H(2)xbsms = 1,2-bis(4-mercapto-3,3-dimethyl-2-thiabutyl)benzene; Cp(-) = cyclopentadienyl; L = DMSO, CO, PPh(3), and PCy(3)) are reported and are bioinspired mimics of NiFe hydrogenases. These compounds were characterized by X-ray diffraction techniques and display novel structural motifs. Interestingly, [Ni(xbsms)RuCpCO][PF(6)] is stereochemically nonrigid in solution and an isomerization mechanism was derived with the help of density functional theory (DFT) calculations. Because of an increased electron density on the metal centers [Eur. J. Inorg. Chem. 2007, 18, 2613-2626] with respect to the previously described [Ni(xbsms)Ru(CO)(2)Cl(2)] and [Ni(xbsms)Ru(p-cymene)Cl](+) complexes, [Ni(xbsms)RuCp(dmso)][PF(6)] catalyzes hydrogen evolution from Et(3)NH(+) in DMF with an overpotential reduced by 180 mV and thus represents the most efficient NiFe hydrogenase functional mimic. DFT calculations were carried out with several methods to investigate the catalytic cycle and, coupled with electrochemical measurements, allowed a mechanism to be proposed. A terminal or bridging hydride derivative was identified as the active intermediate, with the structure of the bridging form similar to that of the Ni-C active state of NiFe hydrogenases.

Modelling NiFe hydrogenases: nickel-based electrocatalysts for hydrogen production.

NiFe hydrogenases are unique enzymes that catalyze the H+/H2 interconversion with remarkable efficiency. The determination of the tridimensional structure of their active site (a sulfur-rich dinuclear nickel-iron cluster with diatomic cyanide and carbonyl ligands) has stimulated the synthesis of a variety of nickel-based complexes as potential electrocatalysts for hydrogen production. These catalysts may provide an adequate alternative to platinum. This paper gives an historical perspective of this biomimetic structural approach and then focusses on recently reported bio-inspired functional mimics displaying electrocatalytic activity for hydrogen production.