Metamorphic oxygen-evolving molecular Ru and Ir catalysts.

Today sustainable and clean energy conversion strategies are based on sunlight and the use of water as a source of protons and electrons, in a similar manner as it happens in Photosystem II. To achieve this, the charge separation state induced by light has to be capable of oxidising water by 4 protons and 4 electrons and generating molecular oxygen. This oxidation occurs by the intermediacy of a catalyst capable of finding low-energy pathways via proton-coupled electron transfer steps. The high energy involved in the thermodynamics of water oxidation reaction, coupled with its mechanistic complexity, is responsible for the difficulty of discovering efficient and oxidatively robust molecules capable of achieving such a challenging task. A significant number of Ru coordination complexes have been identified as water oxidation catalysts (WOCs) and are among the best understood from a mechanistic perspective. In this review, we describe the catalytic performance of these complexes and focus our attention on the factors that influence their performance during catalysis, especially in cases where a detailed mechanistic investigation has been carried out. The collective information extracted from all the catalysts studied allows one to identify the key features that govern the complex chemistry associated with the catalytic water oxidation reaction. This includes the stability of trans-O-Ru-O groups, the change in coordination number from CN6 to CN7 at Ru high oxidation states, the ligand flexibility, the capacity to undergo intramolecular proton transfer, the bond strain, the axial ligand substitution, and supramolecular effects. Overall, combining all this information generates a coherent view of this complex chemistry.

Interplay between ß-Diimino and ß-Diketiminato Ligands in Nickel Complexes Active in the Proton Reduction Reaction.

Two Ni complexes are reported with κ4-P2N2 ß-diimino (BDI) ligands with the general formula [Ni(XBDI)](BF4)2, where BDI is N-(2-(diphenylphosphaneyl)ethyl)-4-((2-(diphenylphosphaneyl)ethyl)imino)pent-2-en-2-amine and X indicates the substituent in the α-carbon intradiimine position, X = H for 1(BF4)2 and X = Ph for 2(BF4)2. Electrochemical analysis together with UV-vis and NMR spectroscopy in acetonitrile and dimethylformamide (DMF) indicates the conversion of the ß-diimino complexes 12+ and 22+ to the negatively charged ß-diketiminato (BDK) analogues (1-H)+ and (2-H)+ via deprotonation in DMF. Moreover, further electrochemical and spectroscopy evidence indicates that the one-electron-reduced derivatives 1+ and 2+ can also rapidly evolve to the BDK (1-H)+ and (2-H)+, respectively, via hydrogen gas evolution through a bimolecular homolytic pathway. Finally, both complexes are demonstrated to be active for the proton reduction reaction in DMF at Eapp = -1.8 V vs Fc+/0, being the active species the one-electron-reduced derivative 1-H and 2-H.

Unravelling the Mechanistic Pathway of the Hydrogen Evolution Reaction Driven by a Cobalt Catalyst.

A cobalt complex bearing a κ-N3 P2 ligand is presented (1+ or CoI (L), where L is (1E,1'E)-1,1'-(pyridine-2,6-diyl)bis(N-(3-(diphenylphosphanyl)propyl)ethan-1-imine). Complex 1+ is stable under air at oxidation state CoI thanks to the π-acceptor character of the phosphine groups. Electrochemical behavior of 1+ reveals a two-electron CoI /CoIII oxidation process and an additional one-electron reduction, which leads to an enhancement in the current due to hydrogen evolution reaction (HER) at Eonset =-1.6 V vs Fc/Fc+ . In the presence of 1 equiv of bis(trifluoromethane)sulfonimide, 1+ forms the cobalt hydride derivative CoIII (L)-H (22+ ), which has been fully characterized. Further addition of 1 equiv of CoCp*2 (Cp* is pentamethylcyclopentadienyl) affords the reduced CoII (L)-H (2+ ) species, which rapidly forms hydrogen and regenerates the initial CoI (L) (1+ ). The spectroscopic characterization of catalytic intermediates together with DFT calculations support an unusual bimolecular homolytic mechanism in the catalytic HER with 1+ .

Surface-Promoted Evolution of Ru-bda Coordination Oligomers Boosts the Efficiency of Water Oxidation Molecular Anodes.

A new Ru oligomer of formula {[RuII(bda-κ-N2O2)(4,4'-bpy)]10(4,4'-bpy)}, 10 (bda is [2,2'-bipyridine]-6,6'-dicarboxylate and 4,4'-bpy is 4,4'-bipyridine), was synthesized and thoroughly characterized with spectroscopic, X-ray, and electrochemical techniques. This oligomer exhibits strong affinity for graphitic materials through CH-π interactions and thus easily anchors on multiwalled carbon nanotubes (CNT), generating the molecular hybrid material 10@CNT. The latter acts as a water oxidation catalyst and converts to a new species, 10'(H2O)2@CNT, during the electrochemical oxygen evolution process involving solvation and ligand reorganization facilitated by the interactions of molecular Ru catalyst and the surface. This heterogeneous system has been shown to be a powerful and robust molecular hybrid anode for electrocatalytic water oxidation into molecular oxygen, achieving current densities in the range of 200 mA/cm2 at pH 7 under an applied potential of 1.45 V vs NHE. The remarkable long-term stability of this hybrid material during turnover is rationalized based on the supramolecular interaction of the catalyst with the graphitic surface.

Synthesis, Structure, and Ammonia Oxidation Catalytic Activity of Ru-NH3 Complexes Containing Multidentate Polypyridyl Ligands.

Ammonia (electro)oxidation with molecular catalysts is a rapidly developing topic with wide practical applications ahead. We report here the catalytic ammonia oxidation reaction (AOR) activity using [Ru(tda-κ-N3O)(py)2], 2, (tda2- is 2,2':6',2''-terpyridine-6,6''-dicarboxylate; py is pyridine) as a catalyst precursor. Furthermore, we also describe the rich chemistry associated with the reaction of Ru-tda and Ru-tPa (tPa-4 is 2,2':6',2''-terpyridine-6,6''-diphosphonate) complexes with NH3 and N2H4 using [RuII(tda-κ-N3O)(dmso)Cl] (dmso is dimethyl sulfoxide) and [RuII(tPa-κ-N3O)(py)2], 8, as synthetic intermediates, respectively. All the new complexes obtained here were characterized spectroscopically by means of UV-vis and NMR. In addition, a crystal X-ray diffraction analysis was performed for complexes trans-[RuII(tda-κ-N3)(py)2(NH3)], 4, trans-[RuII(tda-κ-N3)(N-NH2)(py)2], 5, cis-[RuII(tda-κ-N3)(py)(NH3)2], 6 (30%), and cis-[RuII(tda-k-N3)(dmso)(NH3)2], 7 (70%). The AOR activity associated with 2 and 8 as catalyst precursors was studied in organic and aqueous media. For 2, turnover numbers of 7.5 were achieved under bulk electrolysis conditions at an Eapp = 1.4 V versus normal hydrogen electrode in acetonitrile. A catalytic cycle is proposed based on electrochemical and kinetic evidence.

Synthesis, Characterization, and Water Oxidation Activity of Isomeric Ru Complexes.

The synthesis and characterization of the isomeric ruthenium complexes with the general formula cis- and trans-[Ru(trpy)(qc)X]n+ (trpy is 2,2':6',2″-terpyridine, qc is 8-quinolinecarboxylate, cis-1 and trans-1, X = Cl, n = 0; cis-2 and trans-2, X=OH2, n = 1) with respect to the relative disposition of the carboxylate and X ligands are reported. For comparison purposes, another set of ruthenium complexes with general formula cis- and trans-[Ru(trpy)(pic)(OH2)]+ (pic is 2-picolinate (cis-3, trans-3)) have been prepared. The complexes with a qc ligand show a more distorted geometry compared to the complexes with a pic ligand. In all of the cases, the trans isomers show lower potential values for all of the redox couples relative to the cis isomers. Complexes cis-2 and trans-2 with six-member chelate rings show higher catalytic activity than cis-3 and trans-3. Overall, it was shown that the electronic perturbation to the metal center exerted by different orientation and geometry of the ligands significantly influences both redox properties and catalytic performance.

Seven Coordinated Molecular Ruthenium-Water Oxidation Catalysts: A Coordination Chemistry Journey.

Molecular water oxidation catalysis is a field that has experienced an impressive development over the past decade mainly fueled by the promise of generation of a sustainable carbon neutral fuel society, based on water splitting. Most of these advancements have been possible thanks to the detailed understanding of the reactions and intermediates involved in the catalytic cycles. Today's best molecular water oxidation catalysts reach turnover frequencies that are orders of magnitude higher than that of the natural oxygen evolving center in photosystem II. These catalysts are based on Ru complexes where at some stage, the first coordination sphere of the metal center becomes seven coordinated. The key for this achievement is largely based on the use of adaptative ligands that adjust their coordination mode depending on the structural and electronic demands of the metal center at different oxidation states accessed within the catalytic cycle. This Review covers the latest and most significant developments on Ru complexes that behave as powerful water oxidation catalysts and where at some stage the Ru metal attains coordination number 7. Further it provides a comprehensive and rational understanding of the different structural and electronic factors that govern the behavior of these catalysts.

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.

Redox Metal-Ligand Cooperativity Enables Robust and Efficient Water Oxidation Catalysis at Neutral pH with Macrocyclic Copper Complexes.

Water oxidation catalysis stands out as one of the most important reactions to design practical devices for artificial photosynthesis. Use of late first-row transition metal (TM) complexes provides an excellent platform for the development of inexpensive catalysts with exquisite control on their electronic and structural features via ligand design. However, the difficult access to their high oxidation states and the general labile character of their metal-ligand bonds pose important challenges. Herein, we explore a copper complex (12-) featuring an extended, π-delocalized, tetra-amidate macrocyclic ligand (TAML) as water oxidation catalyst and compare its activity to analogous systems with lower π-delocalization (22- and 32-). Their characterization evidences a special metal-ligand cooperativity in accommodating the required oxidative equivalents using 12- that is absent in 22- and 32-. This consists of charge delocalization promoted by easy access to different electronic states at a narrow energy range, corresponding to either metal-centered or ligand-centered oxidations, which we identify as an essential factor to stabilize the accumulated oxidative charges. This translates into a significant improvement in the catalytic performance of 12- compared to 22- and 32- and leads to one of the most active and robust molecular complexes for water oxidation at neutral pH with a kobs of 140 s-1 at an overpotential of only 200 mV. In contrast, 22- degrades under oxidative conditions, which we associate to the impossibility of efficiently stabilizing several oxidative equivalents via charge delocalization, resulting in a highly reactive oxidized ligand. Finally, the acyclic structure of 32- prevents its use at neutral pH due to acidic demetalation, highlighting the importance of the macrocyclic stabilization.

Second Coordination Sphere Effects in an Evolved Ru Complex Based on Highly Adaptable Ligand Results in Rapid Water Oxidation Catalysis.

A new Ru complex containing the deprotonated 2,2':6',2''-terpyridine-6,6''-diphosphonic acid (H4tPa) and pyridine (py) of general formula [RuII(H3tPa-κ-N3O)(py)2]+, 2+, has been prepared and thoroughly characterized by means of spectroscopic and electrochemical techniques, X-ray diffraction analysis, and density functional theory (DFT) calculations. Complex 2+ presents a dynamic behavior in the solution that involves the synchronous coordination and the decoordination of the dangling phosphonic groups of the tPa4- ligand. However, at oxidation state IV, complex 2+ becomes seven coordinated with the two phosphonic groups now bonded to the metal center. Further, at this oxidation state at neutral and basic pH, the Ru complex undergoes the coordination of an exogenous OH- group from the solvent that leads to an intramolecular aromatic O atom insertion into the CH bond of one of the pyridyl groups, forming the corresponding phenoxo-phosphonate Ru complex [RuIII(tPaO-κ-N2OPOC)(py)2]2-, 42-, where tPaO5- is the 3-(hydroxo-[2,2':6',2''-terpyridine]-6,6''-diyl)bis(phosphonate) ligand. This new in situ generated Ru complex, 42-, has been isolated and spectroscopically and electrochemically characterized. In addition, a crystal structure has been also obtained using single-crystal X-ray diffraction techniques. Complex 42- turns out to be an exceptional water oxidation catalyst achieving record maximum turnover frequencies (TOFmax) on the order of 16 000 s-1. A mechanistic analysis complemented with DFT calculations has also been carried out, showing the critical role of intramolecular second coordination sphere effects exerted by the phosphonate groups in lowering the activation energy at the rate-determining step.

A Ru-bda Complex with a Dangling Carboxylate Group: Synthesis and Electrochemical Properties.

Ruthenium complexes containing the tetradentate 2,2'-bipyridine-6,6'-dicarboxylato (bda2-) equatorial ligand and ortho-subsituted pyridines in the axial position have been prepared and characterized using spectroscopic, crystallographic and electrochemical techniques. Complexes [Ru(Hbda)(DMSO)(pyC)] (1) and [Ru(bda)(DMSO)(pyA)] (2) (where pyC is 2-pyridinecarboxylate, pyA is pyridine-2-ylmethanol and DMSO is dimethyl sulfoxide) have been isolated in moderate to high yields. The solid state structures of (1-H)- and 2 reveal the strong chelate effect of the axial pyridine ligand that coordinates in a bidentate fashion leaving the bda2- equatorial ligand coordinating in a tridentate mode. In solution, compound 2 shows a dynamic equilibrium between different coordination modes of the bda2- and pyA ligands. This phenomenon does not occur for 1 because the carboxylate binds stronger than the labile alcohol in 2. Cyclic voltammetry analysis of 1 reveals a complex behavior with a pH-independent wave at E1/2 = 1.12 V that is tentatively associated with the two-electron RuIV/II couple. In sharp contrast, complex 2 shows a pH-dependent one-electron wave at E1/2 = 0.83 V (pH 1), assigned to the proton-coupled electron transfer process of the RuIII/II couple and a pH-independent wave at E1/2 = 1.06 V assigned to the RuIV/III couple. Compound 2 is used to prepare complex [Ru(bda)(pic)(pyA)] (4). This complex is air sensitive and converts to complex [Ru(bda)(pic)(pyE)] (5) (where pyE is methyl 2-pyridine carboxylate) in the presence of methanol. This oxidation also occurs by applying a positive potential to an aqueous solution of 4, producing the derivative [Ru(bda)(pic)(pyC)] (3). Cyclic voltammetry of 3 shows two pH-independent one-electron oxidation waves at E1/2 = 0.64 V and E1/2 = 1.0 V, corresponding to the RuIII/II and RuIV/III couples, respectively. In addition, a water oxidation catalytic wave appears at Eonset ≈ 1.4 V. Foot-of-the-wave analysis of this catalytic wave based on a water nucleophilic attack accounts for a TOFmax = 0.63-0.74 s-1.

Synthesis, Electrochemical Characterization, and Water Oxidation Chemistry of Ru Complexes Containing the 2,6-Pyridinedicarboxylato Ligand.

The tridentate meridional ligand pyridyl-2,6-dicarboxylato (pdc2-) has been used to prepare complexes [RuII(pdc-κ3-N1O2)(DMSO)2Cl]- (1II), [RuII(pdc-κ3-N1O2)(bpy)(DMSO)] (2II), and {[RuIII(pdc-κ3-N1O2)(bpy)]2(µ-O)} (5III,III), where bpy = 2,2'-bipyridine. All complexes have been fully characterized through spectroscopic, electrochemical, and single-crystal X-ray diffraction techniques. Compounds 1II and 2II show S → O linkage isomerization of the DMSO ligand upon oxidation from RuII to RuIII, and thermodynamic and kinetic data have been obtained from cyclic voltammetry experiments. Dimeric complex 5III,III is a precursor of the monomeric complex [RuII(pdc-κ3-N1O2)(bpy)(H2O)] (4II) which is a water oxidation catalyst. The electrochemistry and catalytic activity of 4II has been ascertained for the first time and compared with related Ru-aquo complexes that are also active for the water oxidation reaction. It shows a TOFmax = 0.2 s-1 and overpotential of 240 mV in pH = 1. The overpotential shown by 4II is one of the lowest reported in the literature and is associated with the role of the two carboxylato groups of the pdc ligand, providing high electron density to the ruthenium complex.

Effect of Ligand Chelation and Sacrificial Oxidant on the Integrity of Triazole-Based Carbene Iridium Water Oxidation Catalysts.

We report the effect of replacing the pyridine group in the chelating trz Ir-water oxidation catalysts by a benzoxazole and a thiazole moiety. We have also evaluated if the presence of bidentate ligands is crucial for high activities and to avoid the decomposition into undesired heterogeneous layers. The catalytic performance of these benzoxazole/thiazole-triazolidene Ir-complexes in water oxidation was studied at variable pH using either CAN (pH = 1) or NaIO4 (pH = 5.6 and 7). Electrocatalytic experiments indicated that while CAN-mediated water oxidation led to catalyst heterogeneization irrespective of the triazolylidene substituent, periodate as sacrificial oxidant preserved a homogeneously active species. Repetitive additions of sacrificial oxidant indicates higher integrity of the Ir-complex with a thiazole-substituted triazolylidene compared to ligands featuring a benzoxazole as chelating donor or no chelating group at all. Rigid chelation of the thiazole group was also established from stability measurements under highly acidic, oxidizing, and high ionic strength conditions.

Elucidating the Nature of the Excited State of a Heteroleptic Copper Photosensitizer by using Time-Resolved X-ray Absorption Spectroscopy.

We report the light-induced electronic and geometric changes taking place within a heteroleptic CuI photosensitizer, namely [(xant)Cu(Me2 phenPh2 )]PF6 (xant=xantphos, Me2 phenPh2 =bathocuproine), by time-resolved X-ray absorption spectroscopy in the ps-µs time regime. Time-resolved X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analysis enabled the elucidation of the electronic and structural configuration of the copper center in the excited state as well as its decay dynamics in different solvent conditions with and without triethylamine acting as a sacrificial electron donor. A three-fold decrease in the decay lifetime of the excited state is observed in the presence of triethylamine, showing the feasibility of the reductive quenching pathway in the latter case. A prominent pre-edge feature is observed in the XANES spectrum of the excited state upon metal to charge ligand transfer transition, showing an increased hybridization of the 3d states with the ligand p orbitals in the tetrahedron around the Cu center. EXAFS and density functional theory illustrate a significant shortening of the Cu-N and an elongation of the Cu-P bonds together with a decrease in the torsional angle between the xantphos and bathocuproine ligand. This study provides mechanistic time-resolved understanding for the development of improved heteroleptic CuI photosensitizers, which can be used for the light-driven production of hydrogen from water.

Behavior of Ru-bda Water-Oxidation Catalysts in Low Oxidation States.

The Ru complex [RuII (bda-κ-N2 O2 )(N-NH2 )2 ] (1; bda2- =2,2'-bipyridine-6,6'-dicarboxylate, N-NH2 =4-(pyridin-4-yl)aniline) was used as a synthetic intermediate to prepare new RuII and RuIII bda complexes that contain NO+ , MeCN, or H2 O ligands. In acidic solution complex 1 reacts with an excess of NO+ (generated in situ from sodium nitrite) to form a new Ru complex in which the aryl amine ligand N-NH2 is transformed into a diazonium salt [N-N2+ =4-(pyridin-4-yl)benzenediazonium)] together with the formation of a new Ru(NO) moiety in the equatorial zone, to generate [RuII (bda-κ-N2 O)(NO)(N-N2 )2 ]3+ (23+ ). Here the bda2- ligand binds in a κ-N2 O tridentate manner with a dangling carboxylate group. Similarly, complex 1 can also react with a coordinating solvent, such as MeCN, at room temperature to give [RuII (bda-κ-N2 O)(MeCN)(N-NH2 )2 ] (3). In acidic aqueous solutions, a related reaction occurs in which solvent water coordinates to the Ru center to form {[RuII {bda-κ-(NO)3 }(H2 O)(N-NH3 )2 ](H2 O)n }2+ (42+ ) and is strongly hydrogen-bonded with additional water molecules in the second coordination sphere. Furthermore, under acidic conditions the aniline ligands are also protonated to form the corresponding anilinium cationic ligands N-NH3+ . Additionally, the one-electron oxidized complex {[RuIII {bda-κ-(NO)3.5 }(H2 O)(N-NH3 )2 ](H2 O)n }3+ (53+ ) was characterized, in which the fractional value in the κ notation indicates the presence of an additional contact to the pseudo-octahedral geometry of the Ru center. The coordination modes of the complexes were studied in the solid state and in solution through single-crystal XRD, X-ray absorption spectroscopy, variable-temperature NMR spectroscopy, and DFT calculations. While κ-N2 O is the main coordination mode for 23+ and 3, an equilibrium that involves isomers with κ-N2 O and κ-NO2 coordination modes and neighboring hydrogen-bonded water molecules is observed for 42+ and 53+ .

Synthesis, Structure, and Redox Properties of a trans-Diaqua Ru Complex That Reaches Seven-Coordination at High Oxidation States.

In this work we have prepared and characterized two Ru complexes that contain the pentadenatate tda2- ligand (tda2- = [2,2':6',2″-terpyridine]-6,6″-dicarboxylate) that occupies the equatorial positions and two monodentate ligands aqua and/or dmso that occupy the axial positons: [trans-RuIII(tda-κ-N3O)(OH2ax)2]+, 3III(OH2)2+, and [RuII(tda-κ-N3O)(dmso)(OH2ax)], 4II. The latter is a useful synthetic intermediate for the preparation of Ru-tda complexes with different axial ligands. The two complexes have been characterized in the solid state by single-crystal XRD and by elemental analysis. In solution, complex 4II has been characterized by NMR spectroscopy as well as the one-electron reduction of complex 3III(OH2)2+. The electrochemical properties of 3III(OH2)2+ and 4II have been assessed by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). Complex 3III(OH2)2+ shows the presence of four redox waves that are assigned to the VI/V, V/IV, IV/III, and III/II redox couples. The variation of the redox potentials is analyzed as a function of pH and is graphically presented as a Pourbaix diagram. Finally, the redox potentials displayed by both 3III(OH2)2+ and 4II are compared to related complexes previously reported in the literature and rationalized on the basis of the electron donating or withdrawing capacity of the auxiliary ligands as well as with regard to their ability to undergo seven-coordination at high oxidation states.

How to make an efficient and robust molecular catalyst for water oxidation.

Energy has been a central subject for human development from Homo erectus to date. The massive use of fossil fuels during the last 50 years has generated a large CO2 concentration in the atmosphere that has led to the so-called global warming. It is very urgent to come up with C-neutral energy schemes to be able to preserve Planet Earth for future generations to come and still preserve our modern societies' life style. One of the potential solutions is water splitting with sunlight (hν-WS) that is also associated with "artificial photosynthesis", since its working mode consists of light capture followed by water oxidation and proton reduction processes. The hydrogen fuel generated in this way is named as "solar fuel". For this set of reactions, the catalytic oxidation of water to dioxygen is one of the crucial processes that need to be understood and mastered in order to build up potential devices based on hν-WS. This tutorial describes the different important aspects that need to be considered to come up with efficient and oxidatively robust molecular water oxidation catalysts (Mol-WOCs). It is based on our own previous work and completed with essential contributions from other active groups in the field. We mainly aim at describing how the ligands can influence the properties of the Mol-WOCs and showing a few key examples that overall provide a complete view of today's understanding in this field.

Photoelectrochemical Behavior of a Molecular Ru-Based Water-Oxidation Catalyst Bound to TiO2-Protected Si Photoanodes.

A hybrid photoanode based on a molecular water oxidation precatalyst was prepared from TiO2-protected n- or p+-Si coated with multiwalled carbon nanotubes (CNT) and the ruthenium-based water oxidation precatalyst [RuIV(tda)(py-pyr)2(O)], 1(O) (tda2- is [2,2':6',2″-terpyridine]-6,6″-dicarboxylato and py-pir is 4-(pyren-1-yl)-N-(pyridin-4-ylmethyl)butanamide). The Ru complex was immobilized by π-π stacking onto CNTs that had been deposited by drop casting onto Si electrodes coated with 60 nm of amorphous TiO2 and 20 nm of a layer of sputtered C. At pH = 7 with 3 Sun illumination, the n-Si/TiO2/C/CNT/[1+1(O)] electrodes exhibited current densities of 1 mA cm-2 at 1.07 V vs NHE. The current density was maintained for >200 min at a constant potential while intermittently collecting voltammograms that indicated that over half of the Ru was still in molecular form after O2 evolution.

Electronic π-Delocalization Boosts Catalytic Water Oxidation by Cu(II) Molecular Catalysts Heterogenized on Graphene Sheets.

A molecular water oxidation catalyst based on the copper complex of general formula [(Lpy)CuII]2-, 22-, (Lpy is 4-pyrenyl-1,2-phenylenebis(oxamidate) ligand) has been rationally designed and prepared to support a more extended π-conjugation through its structure in contrast with its homologue, the [(L)CuII]2- water oxidation catalyst, 12- (L is o-phenylenebis(oxamidate)). The catalytic performance of both catalysts has been comparatively studied in homogeneous phase and in heterogeneous phase by π-stacking anchorage to graphene-based electrodes. In the homogeneous system, the electronic perturbation provided by the pyrene functionality translates into a 150 mV lower overpotential for 22- with respect to 12- and an impressive increase in the kcat from 6 to 128 s-1. Upon anchorage, π-stacking interactions with the graphene sheets provide further π-delocalization that improves the catalytic performance of both catalysts. In this sense, 22- turned out to be the most active catalyst due to the double influence of both the pyrene and the graphene, displaying an overpotential of 538 mV, a kcat of 540 s-1 and producing more than 5300 TONs.

Tracking the Structural and Electronic Configurations of a Cobalt Proton Reduction Catalyst in Water.

X-ray transient absorption spectroscopy (X-TAS) has been used to study the light-induced hydrogen evolution reaction catalyzed by a tetradentate macrocyclic cobalt complex with the formula [LCo(III)Cl2](+) (L = macrocyclic ligand), [Ru(bpy)3](2+) photosensitizer, and an equimolar mixture of sodium ascorbate/ascorbic acid electron donor in pure water. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analysis of a binary mixture of the octahedral Co(III) precatalyst and [Ru(bpy)3](2+) after illumination revealed in situ formation of a Co(II) intermediate with significantly distorted geometry and electron-transfer kinetics of 51 ns. On the other hand, X-TAS experiments of the complete photocatalytic system in the presence of the electron donor showed the formation of a square planar Co(I) intermediate species within a few nanoseconds, followed by its decay in the microsecond time scale. The Co(I) structural assignment is supported by calculations based on density functional theory (DFT). At longer reaction times, we observe the formation of the initial Co(III) species concomitant to the decay of Co(I), thus closing the catalytic cycle. The experimental X-ray absorption spectra of the molecular species formed along the catalytic cycle are modeled using a combination of molecular orbital DFT calculations (DFT-MO) and finite difference method (FDM). These findings allowed us to assign the full mechanistic pathway, followed by the catalyst as well as to determine the rate-limiting step of the process, which consists in the protonation of the Co(I) species. This study provides a complete kinetics scheme for the hydrogen evolution reaction by a cobalt catalyst, revealing unique information for the development of better catalysts for the reductive side of hydrogen fuel cells.