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Solar conversion of water into the storable energy carrier H

Cobalt polypyridyl complexes efficiently catalyze hydrogen evolution in aqueous media and exhibit high stability under reducing conditions. Their stability and activity can be tuned through electronic and steric considerations, but the rationalization of these effects requires detailed mechanistic understanding. As an example, tetradentate ligands with two non-permanently occupied coordination sites show higher activity with these sites in cis compared to trans configuration. Here reaction mechanisms of the Co-polypyridyl complex [CoII(bpma)Cl2] (bpma = bipyridinylmethyl-pyridinylmethyl-methyl-amine) have been studied using hybrid density-functional theory. This complex has two exchangeable cis sites, and provides a flexible ligand environment with both pyridyl and amine coordination. Two main pathways with low barriers are found. One pathway, which includes both open sites, is hydrogen evolution from a CoII-H intermediate with a water ligand as the proton donor. In the second pathway H–H bond formation occurs between the hydride and the protonated bpma ligand, with one open site acting as a spectator. The two pathways have similar barriers at higher pH, while the latter becomes more dominant at lower pH. The calculations consider a large number of interconnected variables; protonation sites, isomers, spin multiplicities, and the identities of the open binding sites, as well as their combinations, thus exploring many simultaneous dimensions within each pathway. The results highlight the effects of having two open cis-coordination sites and how their relative binding affinities change during the reaction pathway. They also illustrate why CoII-H intermediates are more active than CoIII-H ones, and why pyridyl protonation gives lower reaction barriers than amine protonation.

The design of molecular water oxidation catalysts (WOCs) requires a rational approach that considers the intermediate steps of the catalytic cycle, including water binding, deprotonation, storage of oxidizing equivalents, O-O bond formation, and O2 release. We investigated several of these properties for a series of base metal complexes (M = Mn, Fe, Co, Ni) bearing two variants of a pentapyridyl ligand framework, of which some were reported previously to be active WOCs. We found that only [Fe(Py5OMe)Cl]+ (Py5OMe = pyridine-2,6-diylbis[di-(pyridin-2-yl)methoxymethane]) showed an appreciable catalytic activity with a turnover number (TON) = 130 in light-driven experiments using the [Ru(bpy)3]2+/S2O82- system at pH 8.0, but that activity is demonstrated to arise from the rapid degradation in the buffered solution leading to the formation of catalytically active amorphous iron oxide/hydroxide (FeOOH), which subsequently lost the catalytic activity by forming more extensive and structured FeOOH species. The detailed analysis of the redox and water-binding properties employing electrochemistry, X-ray absorption spectroscopy (XAS), UV-vis spectroscopy, and density-functional theory (DFT) showed that all complexes were able to undergo the MIII/MII oxidation, but none was able to yield a detectable amount of a MIV state in our potential window (up to +2 V vs SHE). This inability was traced to (i) the preference for binding Cl- or acetonitrile instead of water-derived species in the apical position, which excludes redox leveling via proton coupled electron transfer, and (ii) the lack of sigma donor ligands that would stabilize oxidation states beyond MIII. On that basis, design features for next-generation molecular WOCs are suggested.

Solar conversion of water into the storable energy carrier H-2 can be achieved through photoelectrochemical water splitting using light adsorbing anodes and cathodes bearing O-2 and H-2 evolving catalysts, respectively. Herein a novel photocathode nanohybrid system is reported. This photocathode consists of a dye-sensitized p-type nickel oxide (NiO) with a perylene-based chromophore (PCA) and a tetra-adamantane modified cobaloxime reduction catalyst (Co) that photo-reduces aqueous protons to H-2. An original supramolecular approach was employed, using beta-cyclodextrin functionalized gold nanoparticles (beta-CD-AuNPs) to link the alkane chain of the PCA dye to the adamantane moieties of the cobaloxime catalyst (Co). This new architecture was investigated by photoelectrochemical measurements and via femtosecond-transient absorption spectroscopy. The results show that irradiation of the complete NiO|PCA|beta-CD-AuNPs|Co electrode leads to ultrafast hole injection into NiO (pi = 3 ps) from the excited dye, followed by rapid reduction of the catalyst, and finally H-2 evolution.

Heterobimetallic cofactors containing one manganese and one iron ion have recently been found within the di-metal carboxylate protein family. Herein we report the synthesis and characterization of ...

Developing new transition metal catalysts requires understanding of how both metal and ligand properties determine reactivity. Since metal complexes bearing ligands of the Py5 family (2,6-bis-[(2-pyridyl)methyl] pyridine) have been employed in many fields in the past 20 years, we set out here to understand their redox properties by studying a series of base metal ions (M = Mn, Fe, Co, and Ni) within the Py5OH (pyridine-2,6-diylbis[di-(pyridin-2-yl)methanol]) variant. Both reduced (M-II) and the one-electron oxidized (M-III) species were carefully characterized using a combination of X-ray crystallography, X-ray absorption spectroscopy, cyclic voltammetry, and density-functional theory calculations. The observed metal-ligand interactions and electrochemical properties do not always follow consistent trends along the periodic table. We demonstrate that this observation cannot be explained by only considering orbital and geometric relaxation, and that spin multiplicity changes needed to be included into the DFT calculations to reproduce and understand these trends. In addition, exchange reactions of the sixth ligand coordinated to the metal, were analysed. Finally, by including published data of the extensively characterised Py5OMe (pyridine-2,6-diylbis[di-(pyridin-2-yl)methoxymethane])complexes, the special characteristics of the less common Py5OH ligand were extracted. This comparison highlights the non-innocent effect of the distal OH functionalization on the geometry, and consequently on the electronic structure of the metal complexes. Together, this gives a complete analysis of metal and ligand degrees of freedom for these base metal complexes, while also providing general insights into how to control electrochemical processes of transition metal complexes.

Understanding the structural features of molecular electrocatalysts for carbon dioxide reduction and water oxidation is essential for manufacturing next generation catalysts for renewable energy. We will discuss the crucial structural motifs of those catalysts that have shown novel characteristics in recent years in terms of electrocatalytic efficacy (high TON, TOF and low overpotential), product selectivity and mechanisms. Both inorganic and organic homogeneous catalysts are scrutinized in this review. We will also highlight electrocatalysts with dual activity (i.e. they are able to catalyze both water oxidation and CO2 reduction) as an interesting prospect from the point of view of a single catalyst electrolyzer: a possible design for future easy-to-manufacture effective electrolyzers. This discussion will enrich the overall knowledge on the electrocatalyst design, an important step towards the development of efficient catalysts with cutting edge designs for a renewable energy future and practical applications.

N,N′-Diphenyl-3,7-diazacyclooctane and structurally related N,N′-diphenylbispidine derivatives react with silver(I) ions in a high-yielding C–C coupling reaction to produce dication–diradical species, with the silver ions serving a double function both as template and as an oxidant. The resulting bis(benzidino)phane derivatives are persistent organic radicals, stable for several months in solution as well as in the solid state, at room temperature and above, as well as being exposed to the atmosphere. The molecular structure features a double-decker cyclophane motif, stabilized by intramolecular π-dimerization of two delocalized benzidinium radical segments. Intermolecular π-dimers are formed in the solid state. Graphical abstract Electronic supplementary material The online version of this article (10.1007/s00706-018-2298-4) contains supplementary material, which is available to authorized users.

Ligand modifications with electron-withdrawing and electron-donating groups were applied to afford three novel mononuclear cobalt-based catalysts [Co(TPA-R)]2+ (TPA = tris(2-pyridylmethyl) amine; R ...

Mesoporous NiO photocathodes containing the push-pull dye PB6 and alkyl-derivatized cobaloxime catalysts were prepared using surface amide couplings and analyzed for photocatalytic proton reduction catalysis. The length of the alkyl linker used to derivatize the cobalt catalysts was found to correlate to the photocurrent with the highest photocurrent observed using shorter alkyl linkers but the lowest one for samples without linker. The alkyl linkers were also helpful in slowing dye-NiO charge recombination. Photoelectrochemical measurements and femtosecond transient absorption spectroscopic measurements suggested electron transfer to the surface-immobilized catalysts occurred; however, H2 evolution was not observed. Based on UV-vis, X-ray fluorescence spectroscopy (XRF), and X-ray photoelectron spectroscopy (XPS) measurements, the cobalt catalyst appeared to be limiting the photocathode performance mainly via cobalt demetallation from the oxime ligand. This study highlights the need for a deeper understanding of the effect of catalyst molecular design on photocathode performance.

International audience; A facile surface amide-coupling method was examined to attach dye and catalyst molecules to silatrane-decorated NiO electrodes. Using this method, electrodes with a push–pull dye were assembled and characterized by photoelectrochemistry and transient absorption spectroscopy. The dye-sensitized electrodes exhibited hole injection into NiO and good photoelectrochemical stability in water, highlighting the stability of the silatrane anchoring group and the amide linkage. The amide-coupling protocol was further applied to electrodes that contain a molecular proton reduction catalyst for use in photocathode architectures. Evidence for catalyst reduction was observed during photoelectrochemical measurements and via femtosecond-transient absorption spectroscopy demonstrating the possibility for application in photocathodes.

This work demonstrates that simple copper-bipyridine compounds and atmospheric CO2 react to produce useful/complex materials under appropriate conditions. Starting from a distorted square planar copper(ii) complex, [(tbubpy)CuCl2](tbubpy = 4-tert-butyl-2-(4-tert-butylpyridin-2-yl)pyridine), an air-sensitive, copper(i) complex, [(tbubpy)2CuI][BF4], which exhibits a distorted tetrahedral geometry, was synthesized and characterized. Reactions of [(tbubpy)2CuI][BF4] with CO2 inside a sealed tube and with air were carried out over a week and three weeks, respectively. A new tricopper(ii)-carbonato cluster, [{(tbubpy)2Cu}3(μ-CO3)][PF6]4, was isolated with three distorted octahedral copper(ii) centres bound by a carbonate-bridge formed from atmospheric CO2. NMR and UV-Vis spectroscopic analyses coupled with previous reports point to a multi-step process in the formation of a trinuclear CuII-carbonato cluster that includes the probable involvement of μ-hydroxo-bridged dicopper(ii) type intermediates.

The heterometallic complexes (NH4)2[Co(H2O)6]2[V10O28]⋅4 H2O (1) and (NH4)2[Co(H2O)5(β-HAla)]2[V10O28]⋅4 H2O (2) have been synthesized and used for the preparation of mixed oxides as catalysts for water oxidation. Thermal decomposition of 1 and 2 at relatively low temperatures (

Two iron(II) complexes with tetraazadentate ligands have been synthesised, characterised and evaluated as water oxidation catalysts. The two ligands, N,N′-diisopropyl-N,N′-bis(2-pyridylmethyl)-1,2-diaminoethane (1), and N-methyl-N-(2-pyridinylmethyl)-2,2′-bipyridine-6-methanamine (2), both give iron(II) complexes (1Fe and 2Fe, respectively) with an octahedral coordination geometry with two labile triflate ligands in a cis configuration in the crystal structures. When treated with cerium(IV) as a chemical oxidant in aqueous solution at room temperature the complexes form semi-stable FeIV(O) species that can be detected in the UV/Visible region. Both 1Fe and 2Fe can act as catalysts for water oxidation when treated with a large excess of oxidant, but 1Fe is a better catalyst than 2Fe. Possible geometrical factors behind this difference in reactivity are discussed and compared with literature data.

A Ni-PY5 [PY5 = 2,6-bis(1,1-bis(2-pyridyl)ethyl)pyridine)] complex has been found to act as an electrocatalyst for oxidizing water to dioxygen in aqueous phosphate buffer solutions. The rate of water oxidation catalyzed by the Ni-PY5 is remarkably enhanced by the proton acceptor base HPO42−, with rate constant of 1820 M−1 s−1. Controlled potential bulk electrolysis with Ni-PY5 at pH 10.8 under an applied potential of 1.5 V vs. normal hydrogen electrode (NHE) resulted in dioxygen formation with a high faradaic efficiency over 90%. A detailed mechanistic study identifies the water nucleophilic attack pathway for water oxidation catalysis.

In this paper a novel synthetic route, being a paradigm of the “direct synthesis” approach, is proposed for the preparation of heterometallic Mn/V compounds by a one-pot reaction. Two synthesized complexes, (NH4)2[Mn2(HGly)(H2O)10][V10O28]·(HGly)·2H2O (1) and (NH4)2[Mn(β-HAla)(H2O)5]2[V10O28]·2H2O (2) (HGly = glycine, β-HAla = β-alanine) have been fully characterized by elemental analysis, single-crystal X-ray diffraction, cyclic voltammetry, magnetic susceptibility, FTIR and EPR spectroscopy. Thermal degradation of these compounds lead to the formation of porous, solid mixed oxides V2O5/MnV2O6 in a ratio of 3:2, which were analyzed by X-ray phase analysis and scanning electron microscopy with energy dispersive X-ray microanalysis (SEM/EDX). Additionally the ability of 1 and 2 to act as oxygen evolving water oxidation catalysts under visible light-driven conditions have been studied in a Clark type cell and by ex situ EPR spectroscopy.

Co-oxides are promising water oxidation catalysts for artificial photosynthesis devices. Presently, several different proposals exist for how they catalyze O2 formation from water. Knowledge about this process at molecular detail will be required for their further improvement. Here we present time-resolved 18O-labelling isotope-ratio membrane-inlet mass spectrometry (MIMS) experiments to study the mechanism of water oxidation in Co/methylenediphosphonate (Co/M2P) oxide nanoparticles using [Ru(bpy)3]3+ (bpy = 2,2'-bipyridine) as chemical oxidant. We show that 16O–Co/M2P-oxide nanoparticles produce 16O2 during their first turnover after simultaneous addition of H218O and [Ru(bpy)3]3+, while sequential addition with a delay of 3 s yields oxygen reflecting bulk water 18O-enrichment. This result is interpreted to show that the O–O bond formation in Co/M2P-oxide nanoparticles occurs via intramolecular oxygen coupling between two terminal Co–OHn ligands that are readily exchangeable with bulk water in the resting state of the catalyst. Importantly, our data allow the determination of the number of catalytic sites within this amorphous nanoparticular material, to calculate the TOF per catalytic site and to derive the number of holes needed for the production of the first O2 molecule per catalytic site. We propose that the mechanism of O–O bond formation during bulk catalysis in amorphous Co-oxides may differ from that taking place at the surface of crystalline materials.

The dinuclear Co complex [(TPA)Co(μ-OH)(μ-O2 )Co(TPA)](ClO4 )3 (1, TPA=tris(2-pyridylmethyl)amine) catalyzes the oxidation of water. In the presence of [Ru(bpy)3 ](2+) and S2 O8 (2-) , photoinduced oxygen evolution can be observed with a turnover frequency (TOF) of 1.4±0.1 mol(O2 ) mol(1)(-1) s(-1) and a maximal turnover number (TON) of 58±5 mol(O2 ) mol(1)(-1) . The complex is shown to act as a molecular and homogeneous catalyst and a mechanism is proposed based on the combination of EPR data and light-driven O2 evolution kinetics.

Photo-induced water oxidation based on first row transition metal complexes has drawn much attention recently as a part of the efforts to design systems for solar fuel production. Here, the classic tetradentate ligand TPA (tris(2-pyridylmethyl)amine) is used together with cobalt(II) in CH3CN to form a mononuclear cobalt complex [Co(TPA)Cl]Cl. Single crystal X-ray diffraction shows that [Co(TPA) Cl] Cl is composed of discrete cationic units with a penta-coordinate cobalt center, along with chloride counter ions. In borate buffer, the Co complex acts as a water oxidation catalyst, as shown by the presence of a catalytic wave in electrochemistry. Under visible light irradiation, in the presence of photosensitizer and electron acceptor, the Co complex catalyzes O-2 evolution with a turnover frequency (TOF) of 1.0 mol(O-2)center dot mol(Co)(-1)center dot s(-1) and a turnover number (TON) of 55 mol(O-2)center dot mol(Co)(-1) in pH 8 borate buffer.

The coordination compound of Ru(II) with three 2,2′-bipyridine ligands possesses a potent photosensitization capacity for electron- and energy-transfer processes. In combination with salts of peroxydisulfate acid as sacrificial electron acceptors, Ru(bpy)32+ is widely used for photocatalytic oxidative transformations in organic synthesis and water splitting. The drawback of this system is that bipyridine degrades under the resulting strongly oxidative conditions, the concentration of Ru(bpy)32+ diminishes, and the photocatalytic reaction eventually stops. A commonly employed assay for the determination of Ru(bpy)32+, UV-Vis spectroscopy, has low selectivity and does not distinguish between the intact complex and its decayed forms. Here, we report a matrix assisted laser desorption/ionisation mass spectrometric method for the quantitative analysis of Ru(bpy)32+ in photochemical reaction mixtures. The developed method was successfully used for the determination of intact Ru(bpy)32+ during the course of the water photooxidation reaction. The significant difference between the results of MALDI MS and UV-Vis analyses was observed.

Herein, we report three (IrCp)-Cp-III* complexes with hydroxy-or amino-functionalized N-heterocyclic carbene (NHC) ligands that catalyze efficient water oxidation induced by addition of ceric ammonium nitrate (CAN). The pendant hydroxy or amino groups are very important for activity, and the complexes with heteroatom-functionalized NHC ligands show up to 15 times higher rates of oxygen evolution in CAN-induced water oxidation than a reference (IrCp)-Cp-III* complex without heteroatom functionalization. The formation of molecular high-valent Ir intermediates that are presumably involved in the rate-determining step for water oxidation is established by UV/Vis spectroscopy and ESI-MS under turnover conditions. The hydroxy groups on the NHC ligands, as well as chloride ligands on the iridium center are proposed to structurally stabilize the highvalent species, and thereby improve the catalytic activity. The Ir-III complex with a hydroxy-functionalized NHC shows the highest catalytic activity with a TON of 2500 obtained in 3 h and with >90% yield relative to the amount of oxidant used.

(NH4)2[Co(H2O)6]2 [V10O28]·4H2O (I) and (NH4)2 [Co(H2O)5(β-HAla)]2 [V10O28]·4H2O (II) are used for the preparation of mixed oxides as catalysts for water oxidation.

The influence of molybdenum content in the solid solutions of Bi1-x/3V1-xMoxO4 (x = 0.05-0.20) on the morphology, band gap, structure and light-driven water oxidation properties has been studied by scanning electron microscopy, X-ray powder diffraction and vibrational spectroscopy (Raman and infrared). To find out the peculiarities of structural changes for bismuth scheelite-related oxides containing both vanadium and molybdenum crystals of Bi0.98V0.93Mo0.07O4 have been grown from a K-Bi-V-Mo-O high-temperature melt and characterized by single crystal X-ray diffraction. For the scheelite-related framework both V and Mo were found to occupy the same positions lowering the point group symmetry of tetrahedra from 4/m to 2/m giving monoclinic distortion for solid solutions with x = 0.05-0.10. The most promising photocatalytic performance was obtained for Bi0.96Mo0.10V0.90O4, in which the oxygen evolution could reach 21 μM in 50 s under visible light of LEDs, λ = 470 ± 10 nm, and 820 μE cm(-2) s(-1). The changes in catalytic properties are shown to be governed by a crystal structure strain with a maximum obtained for the boundary sample between the monoclinic and tetragonal phase.

The atomic structure of water-oxidizing nanoparticles (10–60 nm) formed from cobalt(II) salts and methylenediphosphonate (M2P) is investigated. These amorphous nanoparticles are of high interest for production of solar fuels. They facilitate water oxidation in a directly light-driven process using [Ru(bpy)3]2+ (bpy = 2,2’-bipyridine) as a photosensitizer and persulfate (S2O82−) as an electron acceptor. By X-ray absorption spectroscopy (XAS) at the cobalt K-edge, cobalt L-edge and oxygen K-edge, we investigate the light- driven transition from the CoII/M2P precursor to the active catalyst, which is a layered cobalt(III) oxide with structural similarities to water-oxidizing electrocatalysts. The M2P ligand likely binds at the periphery of the nanoparticles, preventing their further agglomeration during the catalytic reaction. This system opens a possibility to link the catalytically active nanoparticles via a covalent bridge to a photosensitizer and build an artificial photosynthetic system for direct utilization of solar energy for fuel production without production of electricity as an intermediate step. This article is part of a Special Issue entitled: Photosynthetic and Biomimetic Hydrogen Production.

The carboxylate stretching frequencies of two high-valent, di-mu-oxido bridged, manganese dimers has been studied with IR spectroscopy in three different oxidation states. Both complexes contain one monodentate carboxylate donor to each Mn ion, in one complex, the carboxylate is coordinated perpendicular to the Mn-(mu-O)(2)-Mn plane, and in the other complex, the carboxylate is coordinated in the Mn-(mu-O)(2)-Mn plane. For both complexes, the difference between the asymmetric and the symmetric carboxylate stretching frequen-cies decrease for both the Mn-2(IV,IV) to Mn-2(III,IV) transition and the Mn-2(III,IV) to Mn-2(III,III) transition, with only minor differences observed between the two arrangements of the carboxylate ligand versus the Mn-(mu-O)(2)-Mn plane. The IR spectra also show that both carboxylate ligands are affected for each one electron reduction, i.e., the stretching frequency of the carboxylate coordinated to the Mn ion that is not reduced also shifts. These results are discussed in relation to FTIR studies of changes in carboxylate stretching frequencies in a one electron oxidation step of the water oxidation complex in Photosystem II.

The synthesis, isolation, and characterization of two high-valent manganese dimers with isomeric ligands are reported. The complexes are synthesized and crystallized from solutions of low-valent precursors exposed to tert-butyl hydroperoxide. The crystal structures display centrosymmetric complexes consisting of Mn(2)(IV,IV)(μ-O)(2) cores, with one ligand coordinating to each manganese. The ligands coordinate with the diaminoethane backbone, the carboxylate, and one of the two pyridines, while the second pyridine is noncoordinating. The activity of these complexes, under water oxidation conditions, is discussed in light of a proposed mechanism for water oxidation, in which this type of complexes have been suggested as a key intermediate.

The development of molecular water oxidation catalysts based on earth-abundant, non-noble metals is essential for artificial photosynthesis research. Iron, which is the most abundant transition metal in the earth's crust, is a prospective candidate for this purpose. Herein, we report two iron complexes based on the polypyridyl ligand Py5OH (Py5OH=pyridine-2,6-diylbis [di(pyridin-2-yl)methanol]) that can catalyse water oxidation to produce O2 in Ru(III) -induced (at pH 8, highest turnover number (TON)=26.5; turnover frequency (TOF)=2.2 s(-1) ), Ce(IV) -induced (at pH≈1.5 highest TON=16; TOF=0.75 s(-1) ) and photo-induced (at pH 8, highest TON=43.5; TOF=0.6 s(-1) ) reactions. A chloride ligand in one of the iron complexes is shown to affect the activity strongly, improve stability and, thereby, the performance at pH 8 but it inhibits oxygen evolution at pH≈1.5. The observations are consistent with a change in mechanism for catalytic water oxidation with the Fe(Py5OH) complexes between acidic (Ce(IV) ) and near-neutral pH (Ru(III) ).

A mononuclear cobalt(II) complex as a homogeneous molecular catalyst for photochemically, electrochemically and chemically induced oxygen evolution reactions is presented. Experimental evidence points towards the presence of a chloride ligand at the cobalt centre throughout the catalytic cycle, and the temporary detachment of a pyridine ligand to open a coordination site for substrate binding.

Two arsenite-inhibited forms of each of the aldehyde oxidoreductases from Desulfovibrio gigas and Desulfovibrio desulfuricans have been studied by X-ray crystallography and electron paramagnetic resonance (EPR) spectroscopy. The molybdenum site of these enzymes shows a distorted square-pyramidal geometry in which two ligands, a hydroxyl/water molecule (the catalytic labile site) and a sulfido ligand, have been shown to be essential for catalysis. Arsenite addition to active as-prepared enzyme or to a reduced desulfo form yields two different species called A and B, respectively, which show different Mo(V) EPR signals. Both EPR signals show strong hyperfine and quadrupolar couplings with an arsenic nucleus, which suggests that arsenic interacts with molybdenum through an equatorial ligand. X-ray data of single crystals prepared from EPR-active samples show in both inhibited forms that the arsenic atom interacts with the molybdenum ion through an oxygen atom at the catalytic labile site and that the sulfido ligand is no longer present. EPR and X-ray data indicate that the main difference between both species is an equatorial ligand to molybdenum which was determined to be an oxo ligand in species A and a hydroxyl/water ligand in species B. The conclusion that the sulfido ligand is not essential to determine the EPR properties in both Mo-As complexes is achieved through EPR measurements on a substantial number of randomly oriented chemically reduced crystals immediately followed by X-ray studies on one of those crystals. EPR saturation studies show that the electron transfer pathway, which is essential for catalysis, is not modified upon inhibition.

Aldehyde oxidoreductase (AOR) activity has been found in a number of sulfate-reducing bacteria. The enzyme that is responsible for the conversion of aldehydes to carboxylic acids is a mononuclear molybdenum enzyme belonging to the xanthine oxidase family. We report here the purification and characterization of AOR isolated from the sulfate-reducing bacterium Desulfovibrio (D.) aminophilus DSM 12254, an aminolytic strain performing thiosulfate dismutation. The enzyme is a homodimer (ca. 200 kDa), containing a molybdenum centre and two [2Fe-2S] clusters per monomer. UV/Visible and electron paramagnetic resonance (EPR) spectra of D. aminophilus AOR recorded in as-prepared and reduced states are similar to those obtained in AORs from Desulfovibrio gigas, Desulfovibrio desulfuricans and Desulfovibrio alaskensis. Despite AOR from D. aminophilus is closely related to other AORs, it presents lower activity towards aldehydes and no activity towards N-heterocyclic compounds, which suggests another possible role for this enzyme in vivo. A comparison of the molecular and EPR properties of AORs from different Desulfovibrio species is also included.

The triallylphosphine-substituted clusters [Ru3(CO)11{P(CH2CH=CH2)3}], [Ru3(CO)10{P(CH2CH=CH2)3}2] and [Ru3(CO)10{;-CH2=CHCH2P(CH2CH=CH2)2}] have been prepared; reaction of [Ru3(CO)10{;-CH2=CHCH2P(CH2CH=CH2)2}] with phosphines/phosphites leads to cleavage of the Ru–allyl -bond and formation of [Ru3(CO)11{P(CH2CH=CH2)3}(PR3)].

Density functional calculations have been used to investigate oxygen atom transfer reactions from the biological oxygen atom donors trimethylamine N-oxide (Me(3)NO) and dimethyl sulfoxide (DMSO) to the molybdenum(IV) complexes [MoO(mnt)(2)](2-) and [Mo(OCH(3))(mnt)(2)](-) (mnt = maleonitrile-1,2-dithiolate), which may serve as models for mononuclear molybdenum enzymes of the DMSO reductase family. The reaction between [MoO(mnt)(2)](2-) and trimethylamine N-oxide was found to have an activation energy of 72 kJ/mol and proceed via a transition state (TS) with distorted octahedral geometry, where the Me(3)NO is bound through the oxygen to the molybdenum atom and the N-O bond is considerably weakened. The computational modeling of the reactions between dimethyl sulfoxide (DMSO) and [MoO(mnt)(2)](2-) or [Mo(OCH(3))(mnt)(2)](-) indicated that the former is energetically unfavorable while the latter was found to be favorable. The addition of a methyl group to [MoO(mnt)(2)](2-) to form the corresponding des-oxo complex not only lowers the relative energy of the products but also lowers the activation energy. In addition, the reaction with [Mo(OCH(3))(mnt)(2)](-) proceeds via a TS with trigonal prismatic geometry instead of the distorted octahedral TS geometry modeled for the reaction between [MoO(mnt)(2)](2-) and Me(3)NO.

Two types of manganese oxides have been prepared by hydrolysis of tetranuclear Mn(iii) complexes in the presence or absence of phosphate ions. The oxides have been characterized structurally using X-ray absorption spectroscopy and functionally by O2 evolution measurements. The structures of the oxides prepared in the absence of phosphate are dominated by di-μ-oxo bridged manganese ions that form layers with limited long-range order, consisting of edge-sharing MnO6 octahedra. The average manganese oxidation state is +3.5. The structure of these oxides is closely related to other manganese oxides reported as water oxidation catalysts. They show high oxygen evolution activity in a light-driven system containing [Ru(bpy)3](2+) and S2O8(2-) at pH 7. In contrast, the oxides formed by hydrolysis in the presence of phosphate ions contain almost no di-μ-oxo bridged manganese ions. Instead the phosphate groups are acting as bridges between the manganese ions. The average oxidation state of manganese ions is +3. This type of oxide has much lower water oxidation activity in the light-driven system. Correlations between different structural motifs and the function as a water oxidation catalyst are discussed and the lower activity in the phosphate containing oxide is linked to the absence of protonable di-μ-oxo bridges.

The oxo-sulfido functional group cis-MoVIOS is essential to the activity of the xanthine oxidase family of enzymes but has proven elusive to synthesis in molecules containing no other four-electron ligands. A direct route to molecules containing this group has been achieved. The reaction system [MoO2(OSiPh3)2]/L in dichloromethane yields the complexes [MoVIO2(OSiPh3)2L] (L = phen (1), Me4phen (2), 4,4'-Me2bpy (3), 5,5'-Me2bpy (4), 2 py (5)) (74-96%), which are shown to have a distorted octahedral structure of crystallographically imposed C2 symmetry (1, 5) with cis oxo and trans silyloxy ligands. The related reaction system [MoO3S]2-/2Ph3SiCl/L in acetonitrile affords the complexes [MoVIOS(OSiPh3)2L] (L = phen (6), Me4phen (7), 4,4'-Me2bpy (8), 5,5'-Me2bpy (9)) (36-69%). From the collective results of elemental analysis, mass spectrometry, 1H NMR, and X-ray structure determinations (6, 7), complexes 6-9 are shown to contain the cis-MoVIOS group in molecules with the same overall stereochemistry as dioxo complexes 1-5. The crystal structures of 6 and 7 exhibit O/S disorder, which was modeled in refinements with 50% site occupancies. The Mo=O (1.607(5) (6), 1.645(5) (7) A) and Mo=S (2.257(3) (6), 2.203(2) (7) A) bond distances obtained in this way are somewhat shorter and longer, respectively, than expected. Distances obtained by molybdenum EXAFS analysis using the GNXAS protocol for 6-9 (Mo=O 1.71-1.72 A; Mo=S 2.18-2.19 A) are considered more satisfactory and are in good agreement with EXAFS values for xanthine oxidase. Molybdenum K-edge data for 1 and 6-9 are reported. Reaction of 7 with Ph3P in dichloromethane results in sulfur abstraction and formation of [MoVOCl(OSiPh3)2(Me4phen)] (10), which has a distorted octahedral structure with cis O/Cl and cis silyloxy ligands. Sulfur rather than oxygen abstraction is favored by relative Mo=O/Mo=S bond strengths. Complexes 6-9 should allow exploration of the biologically significant cis-MoVIOS group. (Less)

The oxidation of HSO3- by [MoVIO2(mnt)2]2- (mnt2- = 1,2-dicyanoethylenedithiolate) has been investigated using density functional and extended Huckel calculations. The results indicate that the favored pathway involves attack of the sulfur atom of HSO3- on one of the oxo groups of the molybdenum dioxo unit with concomitant hydrogen atom transfer to the other oxo group.

On the path to an energy transition away from fossil fuels to sustainable sources, the European Union is for the moment keeping pace with the objectives of the Strategic Energy Technology-Plan. For this trend to continue after 2020, scientific breakthroughs must be achieved. One main objective is to produce solar fuels from solar energy and water in direct processes to accomplish the efficient storage of solar energy in a chemical form. This is a grand scientific challenge. One important approach to achieve this goal is Artificial Photosynthesis. The European Energy Research Alliance has launched the Joint Programme “Advanced Materials & Processes for Energy Applications” (AMPEA) to foster the role of basic science in Future Emerging Technologies. European researchers in artificial photosynthesis recently met at an AMPEA organized workshop to define common research strategies and milestones for the future. Through this work artificial photosynthesis became the first energy research sub-field to be organised into what is designated “an Application” within AMPEA. The ambition is to drive and accelerate solar fuels research into a powerful European field – in a shorter time and with a broader scope than possible for individual or national initiatives. Within AMPEA the Application Artificial Photosynthesis is inclusive and intended to bring together all European scientists in relevant fields. The goal is to set up a thorough and systematic programme of directed research, which by 2020 will have advanced to a point where commercially viable artificial photosynthetic devices will be under development in partnership with industry.

In order to gain insights into the interplay between Cu(I) and Cu(II) in sulfur-rich protein environments, the first preparation and characterization of copper-substituted forms of the wild-type rubredoxin (Rd) from Desulfovibrio vulgaris Hildenborough are reported, as well as those of its variant C42A-Rd. The initial products appear to be tetrahedral CuI(S–Cys)n species for the wild type (n = 4) and the variant C42A (n = 3, with an additional unidentified ligand). These species are unstable to aerial oxidation to products, whose properties are consistent with square planar CuII(S–Cys)n species. These Cu(II) intermediates are susceptible to auto-reduction by ligand S–Cys to produce stable Cu(I) final products. The original Cu(I) center in the wild-type system can be regenerated by reduction, suggesting that the active site can accommodate CuI(S–Cys)2 and Cys–S–S–Cys fragments in the final product. The absence of one S–Cys ligand prevents similar regeneration in the C42A–Rd system. These results emphasize the redox instability of CuII–(S–Cys)n centers. Fil: Thapper, Anders. Uppsala University. Department of Photochemistry and Molecular Science; Suecia; Fil: Rizzi, Alberto Claudio. Universidad Nacional del Litoral. Facultad de Bioquímica y Ciencias Biológicas. Departamento de Física; Argentina; Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico - CONICET - Santa Fe; Argentina; Fil: Brondino, Carlos Dante. Universidad Nacional del Litoral. Facultad de Bioquímica y Ciencias Biológicas. Departamento de Física; Argentina; Fil: Wedd, Anthony G.. University of Melbourne. School of Chemistry and Bio Molecular Science and Biotechnology Institute; Australia; Fil: Pais, Ricardo J.. Universidade Nova de Lisboa. Faculdade de Ciências e Tecnologia. Departamento de Química; Portugal; Fil: Maiti, Biplab K.. Universidade Nova de Lisboa. Faculdade de Ciências e Tecnologia. Departamento de Química; Portugal; Fil: Moura, Isabel. Universidade Nova de Lisboa. Faculdade de Ciências e Tecnologia. Departamento de Química; Portugal; Fil: Pauletta,Sofia R.. Universidade Nova de Lisboa. Faculdade de Ciências e Tecnologia. Departamento de Química; Portugal; Fil: Moura, José J. G.. Universidade Nova de Lisboa. Faculdade de Ciências e Tecnologia. Departamento de Química; Portugal

Background The validity and reproducibility of gene expression studies depend on the quality of extracted RNA and the degree of genomic DNA contamination. Cyanobacteria are gram-negative prokaryotes that synthesize chlorophyll a and carry out photosynthetic water oxidation. These organisms possess an extended array of secondary metabolites that impair cell lysis, presenting particular challenges when it comes to nucleic acid isolation. Therefore, we used the NHM5 strain of Nostoc punctiforme ATCC 29133 to compare and improve existing phenol based chemistry and procedures for RNA extraction. Results With this work we identify and explore strategies for improved and lower cost high quality RNA isolation from cyanobacteria. All the methods studied are suitable for RNA isolation and its use for downstream applications. We analyse different Trizol based protocols, introduce procedural changes and describe an alternative RNA extraction solution. Conclusion It was possible to improve purity of isolated RNA by modifying protocol procedures. Further improvements, both in RNA purity and experimental cost, were achieved by using a new extraction solution, PGTX.

The far-red limit of photosystem II (PSII) photochemistry was studied in PSII-enriched membranes and PSII core preparations from spinach (Spinacia oleracea) after application of laser flashes between 730 and 820 nm. Light up to 800 nm was found to drive PSII activity in both acceptor side reduction and oxidation of the water-oxidizing CaMn4 cluster. Far-red illumination induced enhancement of, and slowed down decay kinetics of, variable fluorescence. Both effects reflect reduction of the acceptor side of PSII. The effects on the donor side of PSII were monitored using electron paramagnetic resonance spectroscopy. Signals from the S2-, S3-, and S0-states could be detected after one, two, and three far-red flashes, respectively, indicating that PSII underwent conventional S-state transitions. Full PSII turnover was demonstrated by far-red flash-induced oxygen release, with oxygen appearing on the third flash. In addition, both the pheophytin anion and the Tyr Z radical were formed by far-red flashes. The efficiency of this far-red photochemistry in PSII decreases with increasing wavelength. The upper limit for detectable photochemistry in PSII on a single flash was determined to be 780 nm. In photoaccumulation experiments, photochemistry was detectable up to 800 nm. Implications for the energetics and energy levels of the charge separated states in PSII are discussed in light of the presented results.

Two proteins were purified and preliminarily characterized from the soluble extract of cells (310 g, wet weight) of the aminolytic and peptidolytic sulfate-reducing bacterium Desulfovibrio (D.) aminophilus DSM 12254. The iron-sulfur flavoenzyme adenylylsulfate (adenosine 5'-phosphosulfate, APS) reductase, a key enzyme in the microbial dissimilatory sulfate reduction, has been purified in three chromatographic steps (DEAE-Biogel A, Source 15, and Superdex 200 columns). It contains two different subunits with molecular masses of 75 and 18 kDa. The fraction after the last purification step had a purity index (A(278nm) / A(388nm)) of 5.34, which was used for further EPR spectroscopic studies. The D. aminophilus APS reductase is very similar to the homologous enzymes isolated from D. gigas and D. desulfuricans ATCC 27774. A tetraheme cytochrome c(3) (His-heme iron-His) has been purified in three chromatographic steps (DEAE- Biogel A, Source 15, and Biogel-HTP columns) and preliminarily characterized. It has a purity index ([A(553nm) - A(570nm)](red) / A(280nm)) of 2.9 and a molecular mass of around 15 kDa, and its spectroscopic characterization (NMR and EPR) has been carried out. This hemoprotein presents similarities with the tetraheme cytochrome c(3) from Desulfomicrobium (Des.) norvegicum (NMR spectra, and N-terminal amino acid sequence).

The effect of exposing Photosystem II (PSII) enriched membranes to far-red laser flashes has been investigated by flash-induced variable Chla fluorescence and EPR spectroscopy. An enhancement of variable fluorescence of PSII together with a slowing down of the variable fluorescence decay kinetics was observed after application of laser flashes at a defined wavelength between 650–800 nm. In addition, the EPR signals from the S0−, S1− and S2-states of the water oxidizing complex were detected after application of far-red laser flashes.

Four manganese complexes were screened for O2 formation under coherent experimental conditions. We identified several new oxygen evolving reactions and in one case 18O labelling with the expected isotope ratio indicates observation of water oxidation.

Two monomeric fractions of photosystem II (PS II) core pacticles from the thermophilic cyanobacterium Thermosynechococcus elongatus have been investigated using flash-induced variable fluorescence kinetics and EPR spectroscopy. One fraction was highly active in oxygen evolution and contained the extrinsic protein subunits PsbO, PsbU, and PsbV. The other monomeric fraction lacked oxygen evolving activity as well as the three extrinsic subunits, but the luminally located, extrinsic Psb27 lipoprotein was present. In the monomeric fraction with bound Psb27, flash-induced variable fluorescence showed an absence of oxidizable Mn on the donor side of PS II and impaired forward electron transfer from the primary quinone acceptor, QA. These results were confirmed with EPR spectroscopy by the absence of the "split S1" interaction signal from YZ* and the CaMn4 cluster and by the absence of the S2-state multiline signal. A different protein composition on the donor side of PS II monomers with Psb27 was also supported by the lack of an EPR signal from cytochrome c550 (in the PsbV subunit). In addition, we did not observe any oxidation of cytochrome b559 at low temperature in this fraction. The presence of Psb27 and the absence of the CaMn4 cluster did not affect the protein matrix around YD or the acceptor side quinones as can be judged from the appearance of the corresponding EPR signals. The diminished electron transport capabilities on both the donor and the acceptor side of PS II when Psb27 is present give further indications that this PS II complex is involved in the earlier steps of the PS II repair cycle.

The molybdenum cofactor (Moco)-containing enzymes are divided into three classes that are named after prototypical members of each family, viz. sulfite oxidase, DMSO reductase and xanthine oxidase. Functional or structural models have been prepared for these three prototypical enzymes: (i) The complex [MoO2(mnt)2]2- (mnt2- = 1,2-dicyanoethylenedithiolate) has been found to be able to oxidize hydrogen sulfite to HSO4- and is thus a functional model of sulfite oxidase. Kinetic and computational studies indicate that the reaction proceeds via attack of the substrate at one of the oxo ligands of the complex, rather than at the metal. (ii) The coordination geometries of the mono-oxo [Mo(VI)(O-Ser)(S2)2] entity (S2 = dithiolene moiety of molybdopterin) found in the crystal structure of R. sphaeroides DMSO reductase and the corresponding des-oxo Mo(IV) unit have been reproduced in the complexes [M(VI)O(OSiR3)(bdt)2] and [M(VI)O(OSiR3)(bdt)2] (M = Mo,W; bdt = benzene dithiolate). (iii) A facile route has been developed for the preparation of complexes containing a cis-Mo(VI)OS molybdenum oxo, sulfido moiety similar to that detected in the oxidized form of xanthine oxidase.

Early intermediates of catalytic water reduction by a Co(II)-polypyridyl species have been characterized. Electrochemical detection of the Co(III)-hydride and time-resolved spectroscopic detection of the Co(I)-ligand intermediates provide an understanding of their reactivity in electrolytic or light-driven reduction of protons to hydrogen.

In this work we report the synthesis of two novel manganese complexes, [L1(3)Mn(II)(6)](ClO(4))(6) (1·(ClO(4))(6)) and [L2Mn(II)(2)(μ-OAc)(μ-Cl)](ClO(4))(2) (2·(ClO(4))(2)), where L1(2-) is the 2,2'-(1,3-phenylenebis(methylene))bis((2-(bis(pyridin-2-ylmethyl)amino)ethyl)azanediyl)diacetic acid anion and L2 is N1,N1'-(1,3-phenylenebis(methylene))bis(N2,N2'-bis(pyridin-2-ylmethyl)ethane-1,2-diamine). The ligands Na(2)L1 and L2 are built on the same backbone, L2 only contains nitrogen donors, while two carboxylate arms have been introduced in Na(2)L1. The two complexes have been characterized by single-crystal X-ray diffraction, magnetic susceptibility, EPR spectroscopy, and electrochemistry. X-Ray crystallography revealed that 1 is a manganese(II) hexamer and 2 is a manganese(II) dimer featuring an unprecedented mono-μ-acetato, mono-μ-chlorido bridging motif. The ability of the complexes to catalyse H(2)O(2) disproportionation, thereby acting as models for manganese catalases, has been investigated and compared to the activity of two other related manganese complexes. The introduction of carboxylate donors in the ligands, leading to increased denticity, resulted in a drop in H(2)O(2) disproportionation activity.