Boosting the Oxygen Evolution Activity of FeNi Oxides/Hydroxides by Molecular and Atomic Engineering.

FeNi oxides/hydroxides are the best performing catalysts for oxidizing water at basic pH. Consequently, their improvement is the cornerstone to develop more efficient artificial photosynthetic systems. During the last 5 years different reports have demonstrated an enhancement of their activity by engineering their structures via: (1) modulation of the number of oxygen, iron and nickel vacancies; (2) single atoms (SAs) doping with metals such as Au, Ir, Ru and Pt; and (3) modification of their surface using organic ligands. All these strategies have led to more active and stable electrocatalysts for oxygen evolution rection (OER). In this Concept, we critically analyze these strategies using the most relevant examples.

Consecutive Ligand-Based Electron Transfer in New Molecular Copper-Based Water Oxidation Catalysts.

Water oxidation to dioxygen is one of the key reactions that need to be mastered for the design of practical devices based on water splitting with sunlight. In this context, water oxidation catalysts based on first-row transition metal complexes are highly desirable due to their low cost and their synthetic versatility and tunability through rational ligand design. A new family of dianionic bpy-amidate ligands of general formula H2 LNn- (LN is [2,2'-bipyridine]-6,6'-dicarboxamide) substituted with phenyl or naphthyl redox non-innocent moieties is described. A detailed electrochemical analysis of [(L4)Cu]2- (L4=4,4'-(([2,2'-bipyridine]-6,6'-dicarbonyl)bis(azanediyl))dibenzenesulfonate) at pH 11.6 shows the presence of a large electrocatalytic wave for water oxidation catalysis at an η=830 mV. Combined experimental and computational evidence, support an all ligand-based process with redox events taking place at the aryl-amide groups and at the hydroxido ligands.

Synthesis and Isomeric Analysis of RuII Complexes Bearing Pentadentate Scaffolds.

A RuII-pentadentate polypyridyl complex [RuII(κ-N5-bpy2PYMe)Cl]+ (1+, bpy2PYMe = 1-(2-pyridyl)-1,1-bis(6-2,2'-bipyridyl)ethane) and its aqua derivative [RuII(κ-N5-bpy2PYMe)(H2O)]2+ (22+) were synthesized and characterized by experimental and computational methods. In MeOH, 1+ exists as two isomers in different proportions, cis (70%) and trans (30%), which are interconverted under thermal and photochemical conditions by a sequence of processes: chlorido decoordination, decoordination/recoordination of a pyridyl group, and chlorido recoordination. Under oxidative conditions in dichloromethane, trans-12+ generates a [RuIII(κ-N4-bpy2PYMe)Cl2]+ intermediate after the exchange of a pyridyl ligand by a Cl- counterion, which explains the trans/cis isomerization observed when the system is taken back to Ru(II). On the contrary, cis-12+ is in direct equilibrium with trans-12+, with absence of the κ-N4-bis-chlorido RuIII-intermediate. All these equilibria were modeled by density functional theory calculations. Interestingly, the aqua derivative is obtained as a pure trans-[RuII(κ-N5-bpy2PYMe)(H2O)]2+ isomer (trans-22+), while the addition of a methyl substituent to a single bpy of the pentadentate ligand leads to the formation of a single cis isomer for both chlorido and aqua derivatives [RuII(κ-N5-bpy(bpyMe)PYMe)Cl]+ (3+) and [RuII(κ-N5-bpy(bpyMe)PYMe)(H2O)]2+ (42+) due to the steric constraints imposed by the modified ligand. This system was also structurally and electrochemically compared to the previously reported [RuII(PY5Me2)X]n+ system (X = Cl, n = 1 (5+); X = H2O, n = 2 (62+)), which also contains a κ-N5-RuII coordination environment, and to the newly synthesized [RuII(PY4Im)X]n+ complexes (X = Cl, n = 1 (7+); X = H2O, n = 2 (82+)), which possess an electron-rich κ-N4C-RuII site due to the replacement of a pyridyl group by an imidazolic carbene.

Molecular water oxidation mechanisms followed by transition metals: state of the art.

One clean alternative to fossil fuels would be to split water using sunlight. However, to achieve this goal, researchers still need to fully understand and control several key chemical reactions. One of them is the catalytic oxidation of water to molecular oxygen, which also occurs at the oxygen evolving center of photosystem II in green plants and algae. Despite its importance for biology and renewable energy, the mechanism of this reaction is not fully understood. Transition metal water oxidation catalysts in homogeneous media offer a superb platform for researchers to investigate and extract the crucial information to describe the different steps involved in this complex reaction accurately. The mechanistic information extracted at a molecular level allows researchers to understand both the factors that govern this reaction and the ones that derail the system to cause decomposition. As a result, rugged and efficient water oxidation catalysts with potential technological applications can be developed. In this Account, we discuss the current mechanistic understanding of the water oxidation reaction catalyzed by transition metals in the homogeneous phase, based on work developed in our laboratories and complemented by research from other groups. Rather than reviewing all of the catalysts described to date, we focus systematically on the several key elements and their rationale from molecules studied in homogeneous media. We organize these catalysts based on how the crucial oxygen-oxygen bond step takes place, whether via a water nucleophilic attack or via the interaction of two M-O units, rather than based on the nuclearity of the water oxidation catalysts. Furthermore we have used DFT methodology to characterize key intermediates and transition states. The combination of both theory and experiments has allowed us to get a complete view of the water oxidation cycle for the different catalysts studied. Finally, we also describe the various deactivation pathways for these catalysts.

Powerful Bis-facially Pyrazolate-Bridged Dinuclear Ruthenium Epoxidation Catalyst.

A new bis-facial dinuclear ruthenium complex, {[Ru(II)(bpy)]2(µ-bimp)(µ-Cl)}(2+), 2(2+), containing a hexadentate pyrazolate-bridging ligand (Hbimp) and bpy as auxiliary ligands has been synthesized and fully characterized in solution by spectrometric, spectroscopic, and electrochemical techniques. The new compound has been tested with regard to its capacity to oxidize water and alkenes. The in situ generated bis-aqua complex, {[Ru(II)(bpy)(H2O)]2(µ-bimp)}(3+), 3(3+), is an excellent catalyst for the epoxidation of a wide range of alkenes. High turnover numbers (TN), up to 1900, and turnover frequencies (TOF), up to 73 min(-1), are achieved using PhIO as oxidant. Moreover, 3(3+) presents an outstanding stereospecificity for both cis and trans olefins toward the formation of their corresponding epoxides due to specific interactions transmitted by its ligand scaffold. A mechanistic analysis of the epoxidation process has been performed based on density functional theory (DFT) calculations in order to better understand the putative cooperative effects within this dinuclear catalyst.

His-containing plant metallothioneins: comparative study of divalent metal-ion binding by plant MT3 and MT4 isoforms.

Metallothioneins (MTs) are a superfamily of Cys-rich, low-molecular weight metalloproteins that bind heavy metal ions. These cytosolic metallopeptides, which exist in most living organisms, are thought to be involved in metal homeostasis, metal detoxification, and oxidative stress protection. In this work, we characterise the Zn(II)- and Cd(II)-binding abilities of plant type 3 and type 4 MTs identified in soybean and sunflower, both of them being His-containing peptides. The recombinant metal-MT complexes synthesised in Zn(II) or Cd(II)-enriched Escherichia coli cultures have been analysed by ESI-MS, and CD, ICP-AES, and UV spectroscopies. His-to-Ala type 3 MT mutants have also been constructed and synthesised for the study of the role of His in divalent metal ion coordination. The results show comparable divalent metal-binding capacities for the MTs of type 3, and suggest, for the first time, the participation of their conserved C-term His residues in metal binding. Interesting features for the Zn(II)-binding abilities of type 4 MTs are also reported, as their variable His content may be considered crucial for their biological performance.

Comparative insight into the Zn(II)-, Cd(II)- and Cu(I)-binding features of the protozoan Tetrahymena pyriformis MT1 metallothionein.

Tetrahymena pyriformis MT1 (TpyMT1) is a model among ciliate metallothioneins (MTs). Here, we report on the analytic (ICP-AES, GC-FPD), spectroscopic (CD, UV-Vis, Raman) and spectrometric (ESI-MS) characterization of its recombinant Cd(II)-, Zn(II)- and Cu(I)-complexes, and of those formed during in vitro Zn/Cd and Zn/Cu replacement. In the presence of Cd(II), TpyMT1 renders a major Cd 11-TpyMT1 species, which is also the final step reached in the in vitro Zn/Cd exchange process in Zn 11-TpyMT1. Spectroscopic data supports a different folding of the isostoichiometric Cd 11- and Zn 11-TpyMT1 complexes. Unexpectedly, TpyMT1 biosynthesis in Zn(II)-rich cultures was sensitive to the aeration degree, so that high oxygenation rendered undermetalated, partially-oxidized, complexes (Zn9-TpyMT1). Biosynthesis in Cu(I)-rich media rendered extremely heterogeneous mixtures of CuxZny-species (x+y=8-20), where the higher the aeration, the higher the Zn(II) content. The complexity of these samples was reproduced during the Zn/Cu replacement, as the number of generated species increased gradually with the addition of copper to Zn(11)-TpyMT1. According to our results, a clear preference of TpyMT1 for Cd(II) binding, rather than for Zn(II), and especially Cu(I) can be postulated. This character is totally consistent with the induction pattern of the TpyMT1 gene and the postulated role of TpyMT1 in Cd-detoxification.

Ruthenium Nanoparticles for Catalytic Water Splitting.

Both global warming and limited fossil resources make the transition from fossil to solar fuels an urgent matter. In this regard, the splitting of water activated by sunlight is a sustainable and carbon-free new energy conversion scheme able to produce efficient technological devices. The availability of appropriate catalysts is essential for the proper kinetics of the two key processes involved, namely, the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). During the last decade, ruthenium nanoparticle derivatives have emerged as true potential substitutes for the state-of-the-art platinum and iridium oxide species for the HER and OER, respectively. Thus, after a summary of the most common methods for catalyst benchmarking, this review covers the most significant developments of ruthenium-based nanoparticles used as catalysts for the water-splitting process. Furthermore, the key factors that govern the catalytic performance of these nanocatalysts are discussed in view of future research directions.

Rational design of a neutral pH functional and stable organic photocathode.

In this work we lay out design guidelines for catalytically more efficient organic photocathodes achieving stable hydrogen production in neutral pH. We propose an organic photocathode architecture employing a NiO hole selective layer, a PCDTBT:PCBM bulk heterojunction, a compact TiO2 electron selective contact and a RuO2 nanoparticle catalyst. The role of each layer is discussed in terms of durability and function. With this strategically designed organic photocathode we obtain stable photocurrent densities for over 5 h and discuss routes for further performance improvement.

Dissimilar catalytic behavior of molecular or colloidal palladium systems with a new NHC ligand.

In this work, we describe the synthesis of a new N-heterocyclic carbene (NHC) ligand, derived from a hybrid pyrazole-imidazolium scaffold, namely 1-[2-(3,5-dimethylpyrazol-1-yl)ethyl]-3-((S)-1-phenylethyl)-3H-imidazol-2-ylidene (L). This ligand has been used as a stabilizer for the organometallic synthesis of palladium(0) nanoparticles (Pd NPs). L presents a better stabilizing effect than its pre-carbenic HLCl counterpart, allowing the formation of isolated Pd NPs while HLCl yields aggregated ones. Additionally, molecular Pd(ii) coordination compounds of L and HLCl were synthesized and characterized to better understand the coordination modes of these ligands. Both molecular and colloidal Pd systems have been further tested in catalytic C-C coupling processes. Three different types of reactions have been observed depending on the catalytic system: (i) the Suzuki-Miyaura reaction takes place with Pd molecular complexes; (ii) a secondary reaction, the dehalogenation of the substrate, is always detected and (iii) the C-C homocoupling between two molecules of bromoarenes is observed with colloidal catalysts.

Ruthenium Water Oxidation Catalysts based on Pentapyridyl Ligands.

Ruthenium complexes containing the pentapyridyl ligand 6,6''-(methoxy(pyridin-2-yl)methylene)di-2,2'-bipyridine (L-OMe) of general formula trans-[RuII (X)(L-OMe-κ-N5 )]n+ (X=Cl, n=1, trans-1+ ; X=H2 O, n=2, trans-22+ ) have been isolated and characterized in solution (by NMR and UV/Vis spectroscopy) and in the solid state by XRD. Both complexes undergo a series of substitution reactions at oxidation state RuII and RuIII when dissolved in aqueous triflic acid-trifluoroethanol solutions as monitored by UV/Vis spectroscopy, and the corresponding rate constants were determined. In particular, aqueous solutions of the RuIII -Cl complex trans-[RuIII (Cl)(L-OMe-κ-N5 )]2+ (trans-12+ ) generates a family of Ru aquo complexes, namely trans-[RuIII (H2 O)(L-OMe-κ-N5 )]3+ (trans-23+ ), [RuIII (H2 O)2 (L-OMe-κ-N4 )]3+ (trans-33+ ), and [RuIII (Cl)(H2 O)(L-OMe-κ-N4 )]2+ (trans-42+ ). Although complex trans-42+ is a powerful water oxidation catalyst, complex trans-23+ has only a moderate activity and trans-33+ shows no activity. A parallel study with related complexes containing the methyl-substituted ligand 6,6''-(1-pyridin-2-yl)ethane-1,1-diyl)di-2,2'-bipyridine (L-Me) was carried out. The behavior of all of these catalysts has been rationalized based on substitution kinetics, oxygen evolution kinetics, electrochemical properties, and density functional theory calculations. The best catalyst, trans-42+ , reaches turnover frequencies of 0.71 s-1 using CeIV as a sacrificial oxidant, with oxidative efficiencies above 95 %.

Mononuclear ruthenium compounds bearing N-donor and N-heterocyclic carbene ligands: structure and oxidative catalysis.

A new CNNC carbene-phthalazine tetradentate ligand has been synthesised, which in the reaction with [Ru(T)Cl3] (T = trpy, tpm, bpea; trpy = 2,2';6',2''-terpyridine; tpm = tris(pyrazol-1-yl)methane; bpea = N,N-bis(pyridin-2-ylmethyl)ethanamine) in MeOH or iPrOH undergoes a C-N bond scission due to the nucleophilic attack of a solvent molecule, with the subsequent formation of the mononuclear complexes cis-[Ru(PhthaPz-OR)(trpy)X]n+, [Ru(PhthaPz-OMe)(tpm)X]n+ and trans,fac-[Ru(PhthaPz-OMe)(bpea)X]n+ (X = Cl, n = 1; X = H2O, n = 2; PhthaPz-OR = 1-(4-alkoxyphthalazin-1-yl)-3-methyl-1H-imidazol-3-ium), named 1a+/2a2+ (R = Me), 1b+/2b2+ (R = iPr), 3+/42+ and 5+/62+, respectively. Interestingly, regulation of the stability regions of different Ru oxidation states is obtained by different ligand combinations, going from 62+, where Ru(iii) is clearly stable and mono-electronic transfers are favoured, to 2a2+/2b2+, where Ru(iii) is almost unstable with regard to its disproportionation. The catalytic performance of the Ru-OH2 complexes in chemical water oxidation at pH 1.0 points to poor stability (ligand oxidation), with subsequent evolution of CO2 together with O2, especially for 42+ and 62+. In electrochemically driven water oxidation, the highest TOF values are obtained for 2a2+ at pH 1.0. In alkene epoxidation, complexes favouring bi-electronic transfer processes show better performances and selectivities than those favouring mono-electronic transfers, while alkenes containing electron-donor groups show better performances than those bearing electron-withdrawing groups. Finally, when cis-ß-methylstyrene is employed as the substrate, no cis/trans isomerization takes place, thus indicating the existence of a stereospecific process.

Engineering stabilising beta-sheet interactions into a conformationally flexible region of the folding transition state of ubiquitin.

Protein engineering studies suggest that the transition state for the folding of ubiquitin is highly polarised towards the N-terminal part of the sequence and involves a nucleus of residues within the beta-hairpin (residues 1-17) and main alpha-helix (residues 23-34). In contrast, the observation of small phi-values for residues in the C-terminal portion of the sequence (residues 35-76), coupled with a folding topology that results in a much higher contact order, suggests that fast folding of ubiquitin is dependent upon configurational flexibility in the C-terminal part of the polypeptide chain to ensure passage down a relatively smooth folding funnel to the native state. We show that the introduction of a small mini-hairpin motif as an extension of the native 43-50 hairpin stabilises local interactions in the C-terminal part of the sequence, resulting largely in a deceleration of the unfolding kinetics without perturbing the apparent two-state folding mechanism. However, a single-point Leu-->Phe substitution within the engineered hairpin sequence leads to the premature collapse of the denatured ensemble through the stabilisation of non-native interactions and the population of a compact intermediate. Non-linear effects in the kinetic data at low concentrations of denaturant suggest that the collapsed state, which is further stabilised in the presence of cosmotropic salts, may subsequently fold directly to the native state through a "triangular" reaction scheme involving internal rearrangement rather than unfolding and refolding.

Extending the folding nucleus of ubiquitin with an independently folding beta-hairpin finger: hurdles to rapid folding arising from the stabilisation of local interactions.

The N-terminal beta-hairpin sequence of ubiquitin has been implicated as a folding nucleation site. To extend and stabilise the ubiquitin folding nucleus, we have inserted an autonomously folding 14-residue peptide sequence beta4 which in isolation forms a highly populated beta-hairpin (>70%) stabilised by local interactions. NMR structural analysis of the ubiquitin mutant (Ubeta4) shows that the hairpin finger is fully structured and stabilises ubiquitin by approximately 8kJmol(-1). Protein engineering and kinetic (phi(F)-value) analysis of a series of Ubeta4 mutants shows that the hairpin extension of Ubeta4 is also significantly populated in the transition state (phi(F)-values >0.7) and has the effect of templating the formation of native contacts in the folding nucleus of ubiquitin. However, at low denaturant concentrations the chevron plot of Ubeta4 shows a small deviation from linearity (roll-over effect), indicative of the population of a compact collapsed state, which appears to arise from over-stabilisation of local interactions. Destabilising mutations within the native hairpin sequence and within the engineered hairpin extension, but not elsewhere, eliminate this non-linearity and restore apparent two-state behaviour. The pitfall to stabilising local interactions is to present hurdles to the rapid and efficient folding of small proteins down a smooth folding funnel by trapping partially folded or misfolded states that must unfold or rearrange before refolding.

Comparative Raman study of four plant metallothionein isoforms: Insights into their Zn(II) clusters and protein conformations.

Four Metallothioneins (MTs) from soybean (Glycine max) were heterologously synthesized and comparatively analysed by Raman spectroscopy. The participation of protein donor groups (S-thiol and N-imidazol) in Zn(II) chelation, as well as the presence of secondary structure elements was comparatively analysed. Metal clusters with different geometry can be hypothesised for the four GmMTs: a cubane-like or an adamantane-like metal cluster in Zn-GmMT1, and dinuclear Zn-S clusters in Zn-GmMT2, Zn-GmMT3 and Zn-GmMT4. The latter have also a similar average Cys/Zn content, whereas a lower ratio is present in Zn-GmMT1. This is possible thanks to the involvement in metal coordination of a greater number of bridging Cys, as well as of some carboxylate groups. As regards secondary structure elements, a large content of ß-turn segments is present in all four Zn-GmMTs, especially for isoforms 1 and 4. ß-strands give a contribution to the folding of three GmMTs isoforms, and the highest percentage was found in Zn-GmMT2 (~45%). Conversely, the α-helix content is negligible in all the GmMTs except in Zn-GmMT3, where this peculiar feature coincides with the possible involvement of the two His residues in metal coordination. Conversely, His is predominantly free and present as tautomer I in Zn-GmMT4. In conclusion, this work illustrates the attractive potential of Raman spectroscopy, combined with other techniques, to be a very informative tool for evidencing structural differences among in vivo synthesized metal-MT complexes.

Chemical, electrochemical and photochemical molecular water oxidation catalysts.

Hydrogen release from the splitting of water by simply using sunlight as the only energy source is an old human dream that could finally become a reality. This process involves both the reduction and oxidation of water into hydrogen and oxygen, respectively. While the first process has been fairly overcome, the conversion of water into oxygen has been traditionally the bottleneck process hampering the development of a sustainable hydrogen production based on water splitting. Fortunately, a revolution in this field has occurred during the past decade, since many research groups have been conducting an intense research in this area. Thus, while molecular, well-characterized catalysts able to oxidize water were scarce just five years ago, now a wide range of transition metal based compounds has been reported as active catalysts for this transformation. This review reports the most prominent key advances in the field, covering either examples where the catalysis is triggered chemically, electrochemically or photochemically.

The sea urchin metallothionein system: Comparative evaluation of the SpMTA and SpMTB metal-binding preferences.

Metallothioneins (MTs) constitute a superfamily of ubiquitous metal-binding proteins of low molecular weight and high Cys content. They are involved in metal homeostasis and detoxification, amongst other proposed biological functions. Two MT isoforms (SpMTA and SpMTB) have been reported in the echinoderm Strongylocentrotus purpuratus (sea urchin), both containing 20 Cys residues and presenting extremely similar sequences, although showing distinct tissular and ontogenic expression patterns. Although exhaustive information is available for the Cd(II)-SpMTA complex, this including the full resolution of its 3D structure, no data has been reported concerning either SpMTA Zn(II) and Cu(I) binding properties, or the characterization of SpMTB at protein level. In this work, both the SpMTA and SpMTB isoforms, as well as their separate α and ß domains, have been recombinantly synthesized in the presence of Zn(II), Cd(II) or Cu(II), and the corresponding metal complexes have been analyzed using electrospray mass spectrometry, and CD, ICP-AES and UV-vis spectroscopies. The results clearly show a better performance of isoform A when binding Zn(II) and Cd(II), and of isoform B when coordinating Cu(I). Thus, our results confirm the differential metal binding preference of SpMTA and SpMTB, which, together with the reported induction pattern of the respective genes, highlights how also in Echinodermata the MT polymorphism may be linked to the evolution of different physiological roles.

Independent metal-binding features of recombinant metallothioneins convergently draw a step gradation between Zn- and Cu-thioneins.

Data on the metal-binding behaviour of circa 20 recombinant metallothioneins (MTs) from evolutionary divergent organisms, gathered after years of systematic research, are here comprehensively analyzed. The consideration of four independent in vivo and in vitro metal-binding features reveals a gradation of the metal-binding character of the MTs considered that significantly coincides in a robust new classification: a stepwise gradation between Zn- and Cu-thioneins. The intermediate positions in this list are occupied by a group of polyvalent MTs, exhibiting a merging Zn-/Cu-thionein character that would suit general metal handling purposes. In contrast, the extreme positions are respectively occupied by those MTs that would have evolved to fulfil specialized Zn- or Cu-related physiological roles. Overall, the analyzed trends allow the proposal of a chemically- and biologically-sound new reflection on MT classification criteria.