Turnover Number in Photoinduced Molecular Catalysis of Hydrogen Evolution: a Benchmarking for Catalysts?

Development of devices for production of H2 using light and a sustainable source of electrons may require the design of molecular systems combining a molecular catalyst and a photosensitizer. Evaluation of the efficiency of hydrogen production is commonly performed in homogeneous solution with a sacrificial electron donor and the report of the maximal turnover number vs catalyst ( T O N c a t lim ${TON_{cat}^{\lim } }$ ). This figure of merit is strongly dependent on deactivation pathways and does not by itself provide a benchmarking for catalysts. In particular, when the photosensitizer degradation is the primary source of limitation, a kinetic model, rationalizing literature data, shows that a decrease of the catalyst concentration leads to an increase of T O N c a t lim ${TON_{cat}^{\lim } }$ . It indicates that exceptionally high T O N c a t lim ${TON_{cat}^{\lim } }$ obtained at very low catalyst concentration shall not be considered as an indication of an exceptional catalytic system. We advocate for a systematic kinetic analysis in order to get a quantitative measure of the competitive pathways leading to T O N c a t lim ${TON_{cat}^{\lim } }$ values and to provide keys for performance improvement.

Electrochemical Properties of a Rhodium(III) Mono-Terpyridyl Complex and Use as a Catalyst for Light-Driven Hydrogen Evolution in Water.

Molecular hydrogen (H2) is considered one of the most promising fuels to decarbonize the industrial and transportation sectors, and its photocatalytic production from molecular catalysts is a research field that is still abounding. The search for new molecular catalysts for H2 production with simple and easily synthesized ligands is still ongoing, and the terpyridine ligand with its particular electronic and coordination properties, is a good candidate to design new catalysts meeting these requirements. Herein, we have isolated the new mono-terpyridyl rhodium complex, [RhIII(tpy)(CH3CN)Cl2](CF3SO3) (Rh-tpy), and shown that it can act as a catalyst for the light-induced proton reduction into H2 in water in the presence of the [Ru(bpy)3]Cl2 (Ru) photosensitizer and ascorbate as sacrificial electron donor. Under photocatalytic conditions, in acetate buffer at pH 4.5 with 0.1 M of ascorbate and 530 µM of Ru, the Rh-tpy catalyst produces H2 with turnover number versus catalyst (TONCat*) of 300 at a Rh concentration of 10 µM, and up to 1000 at a concentration of 1 µM. The photocatalytic performance of Ru/Rh-tpy/HA-/H2A has been also compared with that obtained with the bis-dimethyl-bipyridyl complex [RhIII(dmbpy)2Cl2]+ (Rh2) as a catalyst in the same experimental conditions. The investigation of the electrochemical properties of Rh-tpy in DMF solvent reveals that the two-electrons reduced state of the complex, the square-planar [RhI(tpy)Cl] (RhI-tpy), is quantitatively electrogenerated by bulk electrolysis. This complex is stable for hours under an inert atmosphere owing to the π-acceptor property of the terpyridine ligand that stabilizes the low oxidation states of the rhodium, making this catalyst less prone to degrade during photocatalysis. The π-acceptor property of terpyridine also confers to the Rh-tpy catalyst a moderately negative reduction potential (Epc(RhIII/RhI) = -0.83 V vs. SCE in DMF), making possible its reduction by the reduced state of Ru, [RuII(bpy)(bpy•-)]+ (Ru-) (E1/2(RuII/Ru-) = -1.50 V vs. SCE) generated by a reductive quenching of the Ru excited state (*Ru) by ascorbate during photocatalysis. A Stern-Volmer plot and transient absorption spectroscopy confirmed that the first step of the photocatalytic process is the reductive quenching of *Ru by ascorbate. The resulting reduced Ru species (Ru-) were then able to activate the RhIII-tpy H2-evolving catalyst by reduction generating RhI-tpy, which can react with a proton on a sub-nanosecond time scale to form a RhIII(H)-tpy hydride, the key intermediate for H2 evolution.

Self-Assembled Heterometallic Complexes by Incorporation of Calcium or Strontium Ion into a Manganese(II) 12-Metallacrown-3 Framework Supported by a Tripodal Ligand with Pyridine-Carboxylate Motifs: Stability in Their Manganese(III) Oxidized Form.

We report on the isolation of a new family of µ-carboxylato-bridged metallocrown (MC) compounds by self-assembly of the recently isolated hexadentate tris(2-pyridylmethyl)amine ligand tpada2- incorporating two carboxylate units with metal cations. Twelve-membered MCs of manganese of the type 12-MC-3, namely, [{MnII(tpada)}3(M)(H2O)n]2+ (Mn3M) (M = Mn2+ (n = 0), Ca2+ (n = 1), or Sr2+ (n = 2)), were structurally characterized. The metallamacrocycles connectivity consisting in three -[Mn-O-C-O]- repeating units is provided by one carboxylate unit of the three tpada2- ligands, while the second carboxylate coordinated a fourth cation in the central cavity of the MC, Mn2+ or an alkaline earth metal, Ca2+ or Sr2+. Mn3Ca and {Mn3Sr}2 join the small family of heterometallic manganese-calcium complexes and even rarer manganese-strontium complexes as models of the OEC of photosystem II (PSII). A 8-MC-4 of strontium of the molecular wheel type with four -[Sr-O]- repeating unit was also isolated by self-assembly of the tpada2- ligand with Sr2+. This complex, namely, [Sr(tpada)(OH2)]4 (Sr4), does not incorporate any cation in the central cavity but instead four water molecules coordinated to each Sr2+. Electrochemical investigations coupled to UV-visible absorption and EPR spectroscopies as well as electrospray mass spectrometry reveal the stability of the 12-MC-3 tetranuclear structures in solution, both in the initial oxidation state, MnII3M, as well as in the three-electrons oxidized state, MnIII3M. Indeed, the cyclic voltammogram of all these complexes exhibits three-successive reversible oxidation waves between +0.5 and +0.9 V corresponding to the successive one-electron oxidation of the Mn(II) ion into Mn(III) of the three {Mn(tpada)} units constituting the ring, which are fully maintained after bulk electrolysis.

Electrophotocatalysis: Cyclic Voltammetry as an Analytical Tool.

Electrophotocatalysis (e-PC) is currently experiencing a renewed interest. By taking advantage of the highly oxidizing or reducing power of excited state of electrogenerated ion radicals, it allows thermodynamically difficult redox reactions to be performed. However, e-PC is facing various specific issues, such as its fundamentally heterogeneous nature, implying that mass transport is coupled to chemical reactions and light absorption; back electron transfer of the ion radical excited state with the electrode; and local heating near the electrode surface modifying mass transport conditions. Herein, we address these issues in the context of cyclic voltammetry as an analytical tool and we provide a rational framework for kinetic studies of electrophotocatalytic reactions under realistic conditions and hypothesis based on literature data. This approach may be beneficial to rationalize the design and the efficiency of present and future e-PC systems.

Seven Reversible Redox Processes in a Self-Assembled Cobalt Pentanuclear Bis(triple-stranded helicate): Structural, Spectroscopic, and Magnetic Characterizations in the CoICoII4, CoII5, and CoII3CoIII2 Redox States.

We report on the synthesis and structural characterization of the cobalt pentanuclear helicate complex from the rigid tetradentate bis(2-pyridyl)-3,5-pyrazolate ligand bpp-, namely, [{CoII(µ-bpp)3}2CoII3(µ3-OH)]3+ (13+), in which a trinuclear {CoII3(µ3-OH)} core is wrapped by two {CoII(µ-bpp)3} units. The cyclic voltammogram of 13+ in CH3CN revealed seven successive reversible one-electron waves, in the 0 and -3.0 V potential range, highlighting the remarkable stability of such architecture in several redox states. Two mixed-valent states of this complex, the two-electron-oxidized CoII3CoIII2 (15+) and the one-electron-reduced species CoICoII4 (12+), were generated by bulk electrolyses and successfully characterized by single-crystal X-ray diffraction among the eight redox levels between CoI5 and CoII3CoIII2 that can be accessed under electrochemical conditions. Because of the crystallographic characterization of 15+ and 12+, the five reduction processes located at E1/2 values of -1.63 (13+/2+), -1.88 (12+/+), -2.14 (1+/0), -2.40 (10/-), and -2.60 V (1-/2-) versus Ag/AgNO3 were unambiguously assigned to the successive reduction of each of the five Co(II) ions to Co(I), starting with the three ions located in the central core followed by the two apical ions. The two other redox events at E1/2 values of -0.21 (14+/3+) and -0.11 V (15+/4+) are assigned to the successive oxidation of the apical Co(II) ions to Co(III). The Co(I) complexes are rare, and the stabilization of a Co(I) within a trinuclear µ-hydroxo core in the reduced species, 12+, 1+, 10, 1-, and 12-, is probably the result of the particular structure of this complex in the presence of the two apical sites that maintain the trinuclear core through the six bridging bpp- ligands. The spectroscopic characteristics of 12+, 13+, and 15+ (ultraviolet-visible and X-band electron paramagnetic resonance) are also described as well as their magnetic properties in the solid state.

Catalytic Light-Triggered Reduction Promoted by a Dithienylethene Derivative.

A pyridinium substituted dithienylethene derivative was used for the first time as an efficient photoreducing agent of two different substrates. This reaction exhibits high catalytic yields due to the continuous regeneration of the initial state of the photochromic molecule.

Cobalt(II) Pentaaza-Macrocyclic Schiff Base Complex as Catalyst for Light-Driven Hydrogen Evolution in Water: Electrochemical Generation and Theoretical Investigation of the One-Electron Reduced Species.

We previously reported that the tetraazamacrocyclic Schiff base complex [CoIII(CR14)(X)2]n+ (CR14 = 2,12-dimethyl-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),2,11,13,15-pentaene, X = Cl (n = 1) (1-Cl2) or H2O (n = 3) (1-(H2O)2)) is a very efficient H2-evolving catalyst (HEC) in fully aqueous solutions at pH 4.0-4.5 when used in a photocatalytic system including a photosensitizer and ascorbate as sacrificial electron donor. The excellent H2-evolving activity of this complex, compared to other cobalt and rhodium catalysts studied in the same photocatalytic conditions, can be related to the high stability of its two-electron reduced form, the putative "Co(I)" state. These very interesting results led us to investigate the H2-evolving performances of a series of compounds from a close-related family, the pentaaza-macrocyclic cobalt [CoII(CR15)(H2O)2]Cl2 complex (2, CR15 = 2,13-dimethyl-3,6,9,12,18-pentaazabicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentaene), which comprises a larger macrocycle with five nitrogen atoms instead of four. Electrochemical as well as spectroscopic investigations in CH3CN coupled to density functional theory (DFT) calculations point to decoordination of one of the amine upon reduction of Co(II) to the low-valent "Co(I)" form. The resulting unchelated amine could potentially act as a proton relay promoting the H2 formation via proton-coupled-electron transfer (PCET) reactions. Besides, the iron, manganese, and zinc analogues, [FeII(CR15)(X)2]n+ (X = Cl (n = 0) or H2O (n = 2)) (3), [MnII(CR15)(CH3CN)2](PF6)2 (4), and {[ZnII(CR15)Cl](PF6)}n (5) were also synthesized and investigated. The photocatalytic activity of 2-5 toward proton reduction was then evaluated in a tricomponent system containing the [RuII(bpy)3]Cl2 photosensitizer and ascorbate, in fully aqueous solution. The photocatalytic activity of 2 was also compared with that of 1 in the same experimental conditions. It was found that the number of catalytic cycles versus catalyst for 2 are slightly lower than that for 1, suggesting that if the amine released upon reduction of 2 plays a role in promoting the H2-evolving catalytic activity, other factors balance this effect. Finally, photophysical and nanosecond transient absorption spectroscopies were used to investigate the photocatalytic system.

Electrochemical Generation and Spectroscopic Characterization of the Key Rhodium(III) Hydride Intermediates of Rhodium Poly(bipyridyl) H2-Evolving Catalysts.

We previously reported that the [RhIII(dmbpy)2Cl2]+ (dmbpy = 4,4'-dimethyl-2,2'-bipyridine) complex is an efficient H2-evolving catalyst in water when used in a molecular homogeneous photocatalytic system for hydrogen production with [RuII(bpy)3]2+ (bpy = 2,2'-bipyridine) as photosensitizer and ascorbic acid as sacrificial electron donor. The catalysis is believed to proceed via a two-electron reduction of the Rh(III) catalyst into the square-planar [RhI(dmbpy)2]+, which reacts with protons to form a Rh(III) hydride intermediate that can, in turn, release H2 following different pathways. To improve the current knowledge of these key intermediate species for H2 production, we performed herein a detailed electrochemical investigation of the [RhIII(dmbpy)2Cl2]+ and [RhIII(dtBubpy)2Cl2]+ (dtBubpy = 4,4'-di- tert-butyl-2,2'-bipyridine) complexes in CH3CN, which is a more appropriate medium than water to obtain reliable electrochemical data. The low-valent [RhI(Rbpy)2]+ and, more importantly, the hydride [RhIII(Rbpy)2(H)Cl]+ species (R = dm or dtBu) were successfully electrogenerated by bulk electrolysis and unambiguously spectroscopically characterized. The quantitative formation of the hydrides was achieved in the presence of weak proton sources (HCOOH or CF3CO3H), owing to the fast reaction of the electrogenerated [RhI(Rbpy)2]+ species with protons. Interestingly, the hydrides are more difficult to reduce than the initial Rh(III) bis-chloro complexes by ∼310-340 mV. Besides, 0.5 equiv of H2 is generated through their electrochemical reduction, showing that Rh(III) hydrides are the initial catalytic molecular species for hydrogen evolution. Density functional theory calculations were also performed for the dmbpy derivative. The optimized structures and the theoretical absorption spectra were calculated for the initial bis-chloro complex and for the various rhodium intermediates involved in the H2 evolution process.

Intramolecular Electron Transfers Thwart Bistability in a Pentanuclear Iron Complex.

With the intention to investigate the redox properties of polynuclear complexes as previously reported for the pentamanganese complex [{Mn(II)(µ-bpp)3}2Mn(III)Mn(II)2(µ3-O)](3+) (2(3+)), we focused on the analogous pentairon complex that was previously isolated as all-ferrous. As Masaoka and co-workers recently published, aerobic synthesis leads to the [{Fe(II)(µ-bpp)3}2Fe(III)Fe(II)2(µ3-O)](3+) complex (1(3+)). This species exhibits in acetonitrile solution four reversible one-electron oxidation waves. Accordingly, the three oxidized species 1(4+), 1(5+), and 1(6+) with a 3Fe(II)2Fe(III), 2Fe(II)3Fe(III), and 1Fe(II)4Fe(III) composition, respectively, were generated by bulk electrolysis and isolated. Mössbauer spectroscopy allowed us to determine the spin states of all the iron ions and to unambiguously locate the sites of the successive oxidations. They all occur in the µ3-oxo core except for the 1(4+) to 1(5+) process that presents a striking electronic rearrangement, with both metals in axial position being oxidized while the core is reduced to the [Fe(III)Fe(II)2(µ3-O)](5+) oxidation level. This strongly differs from the redox behavior of the Mn5 system. The origin of this electronic switch is discussed.

Synthesis, Characterization, and Photocatalytic H2-Evolving Activity of a Family of [Co(N4Py)(X)](n+) Complexes in Aqueous Solution.

A series of [Co(III)(N4Py)(X)](ClO4)n (X = Cl(-), Br(-), OH(-), N3(-), NCS(-)-κN, n = 2: X = OH2, NCMe, DMSO-κO, n = 3) complexes containing the tetrapyridyl N5 ligand N4Py (N4Py = 1,1-di(pyridin-2-yl)-N,N-bis(pyridin-2-ylmethyl)methanamine) has been prepared and fully characterized by infrared (IR), UV-visible, and NMR spectroscopies, high-resolution electrospray ionization mass spectrometry (HRESI-MS), elemental analysis, X-ray crystallography, and electrochemistry. The reduced Co(II) and Co(I) species of these complexes have been also generated by bulk electrolyses in MeCN and characterized by UV-visible and EPR spectroscopies. All tested complexes are catalysts for the photocatalytic production of H2 from water at pH 4.0 in the presence of ascorbic acid/ascorbate, using [Ru(bpy)3](2+) as a photosensitizer, and all display similar H2-evolving activities. Detailed mechanistic studies show that while the complexes retain the monodentate X ligand upon electrochemical reduction to Co(II) species in MeCN solution, in aqueous solution, upon reduction by ascorbate (photocatalytic conditions), [Co(II)(N4Py)(HA)](+) is formed in all cases and is the precursor to the Co(I) species which presumably reacts with a proton. These results are in accordance with the fact that the H2-evolving activity does not depend on the chemical nature of the monodentate ligand and differ from those previously reported for similar complexes. The catalytic activity of this series of complexes in terms of turnover number versus catalyst (TONCat) was also found to be dependent on the catalyst concentration, with the highest value of 230 TONCat at 5 × 10(-6) M. As revealed by nanosecond transient absorption spectroscopy measurements, the first electron-transfer steps of the photocatalytic mechanism involve a reductive quenching of the excited state of [Ru(bpy)3](2+) by ascorbate followed by an electron transfer from [Ru(II)(bpy)2(bpy(•-))](+) to the [Co(II)(N4Py)(HA)](+) catalyst. The reduced catalyst then enters into the H2-evolution cycle.

Synthesis, structure, spectroscopy and reactivity of new heterotrinuclear water oxidation catalysts.

Four heterotrinuclear complexes containing the ligands 3,5-bis(2-pyridyl)pyrazolate (bpp-) and 2,2':6',2''-terpyridine (trpy) of the general formula {[RuII(trpy)]2(µ-[M(X)2(bpp)2])}(PF6)2, where M = CoII, MnII and X = Cl-, AcO- (M = CoII, X = Cl-: Ru2Co-Cl2 ; M = MnII, X = Cl-: Ru2Mn-Cl2 ; M = CoII, X = AcO-: Ru2Co-OAc2 ; M = MnII, X = AcO-: Ru2Mn-OAc2 ), have been prepared for the first time. The complexes have been characterized using different spectroscopic techniques such as UV-vis, IR, and mass spectrometry. X-Ray diffraction analyses have been used to characterize the Ru2Mn-Cl2 and Ru2Mn-OAc2 complexes. The cyclic voltammograms (CV) for all four complexes in organic solvent (CH3CN or CH2Cl2) display three successive reversible oxidative waves corresponding to one-electron oxidations of each of the three metal centers. The oxidized forms of the complexes Ru2Co-OAc2 and Ru2Mn-OAc2 are further characterized by EPR and UV-vis spectroscopy. The magnetic susceptibility measurements of all complexes in the temperature range of 2-300 K reveal paramagnetic properties due to the presence of high spin Co(ii) and Mn(ii) centers. The complexes Ru2Co-OAc2 and Ru2Mn-OAc2 act as precatalysts for the water oxidation reaction, since the acetato groups are easily replaced by water at pH = 7 generating the active catalysts, {[Ru(H2O)(trpy)]2(µ-[M(H2O)2(bpp)2])}4+ (M = CoII: Ru2Co-(H2O)4 ; M = MnII: Ru2Mn-(H2O)4 ). The photochemical water oxidation reaction is studied using [Ru(bpy)3]2+ as the photosensitizer and Na2S2O8 as a sacrificial electron acceptor at pH = 7. The Co containing complex generates a TON of 50 in about 10 minutes (TOFi = 0.21 s-1), whereas the Mn containing complex only generates a TON of 8. The water oxidation reaction of Ru2Co-(H2O)4 is further investigated using oxone as a sacrificial chemical oxidant at pH = 7. Labelled water oxidation experiments suggest that a nucleophilic attack mechanism is occurring at the Co site of the trinuclear complex with cooperative involvement of the two Ru sites, via electronic coupling through the bpp- bridging ligand and via neighboring hydrogen bonding.

How Accurately Can Extended X-ray Absorption Spectra Be Predicted from First Principles? Implications for Modeling the Oxygen-Evolving Complex in Photosystem II.

First principle calculations of extended X-ray absorption fine structure (EXAFS) data have seen widespread use in bioinorganic chemistry, perhaps most notably for modeling the Mn4Ca site in the oxygen evolving complex (OEC) of photosystem II (PSII). The logic implied by the calculations rests on the assumption that it is possible to a priori predict an accurate EXAFS spectrum provided that the underlying geometric structure is correct. The present study investigates the extent to which this is possible using state of the art EXAFS theory. The FEFF program is used to evaluate the ability of a multiple scattering-based approach to directly calculate the EXAFS spectrum of crystallographically defined model complexes. The results of these parameter free predictions are compared with the more traditional approach of fitting FEFF calculated spectra to experimental data. A series of seven crystallographically characterized Mn monomers and dimers is used as a test set. The largest deviations between the FEFF calculated EXAFS spectra and the experimental EXAFS spectra arise from the amplitudes. The amplitude errors result from a combination of errors in calculated S0(2) and Debye-Waller values as well as uncertainties in background subtraction. Additional errors may be attributed to structural parameters, particularly in cases where reliable high-resolution crystal structures are not available. Based on these investigations, the strengths and weaknesses of using first-principle EXAFS calculations as a predictive tool are discussed. We demonstrate that a range of DFT optimized structures of the OEC may all be considered consistent with experimental EXAFS data and that caution must be exercised when using EXAFS data to obtain topological arrangements of complex clusters.

Calcium and heterometallic manganese-calcium complexes supported by tripodal pyridine-carboxylate ligands: structural, EPR and theoretical investigations.

Carboxylate-bridged Mn(II)-Ca(II) complexes are potentially relevant for mimicking the first stages of the Oxygen-Evolving Complex (OEC) assembly process. Here, we report on new homonuclear Ca(II) and heteronuclear Mn(II)­Ca(II) complexes with carboxylate-functionalized tripodal tris(2-pyridylmethyl)amine ligands, the heptadentate H3tpaa, previously reported, and the new hexadentate H2tpada, containing respectively three and two carboxylate units. The mononuclear [Ca(Htpaa)(OH2)] (Ca1) and dinuclear [Ca(tpada)(OH2)2]2 (Ca2) calcium complexes, as well as the tetranuclear [{Mn(tpaa)}2{Ca(OH2)5(µ-OH2)}2][Mn(tpaa)]2 (Mn2Ca2·2Mn) and dinuclear [Mn(tpada)ClCa(OH2)2.67(MeOH)2.33]Cl (MnCa) heterometallic species have been structurally characterized; the syntheses of Ca1 and Mn2Ca2·2Mn being previously reported by us (Inorg. Chem., 2015, 54, 1283). The Mn(II) and Ca(II) are linked by two µ1,1-bridging carboxylates in MnCa, while only one µ1,3-carboxylate bridge connects each Ca2+ ion to each Mn(II) in Mn2Ca2. A variable number of water molecules (n = 1 to 7) are coordinated to Ca in all complexes, most of them being involved in hydrogen-bond networks, in analogy to what occurs in the photosystem II. All donor atoms of the tpaa3- and tpada2- ligands are coordinated to the Mn2+ ions, despite the unusually long distance between the Mn2+ ion and the tertiary amine imposed by the constraining nature of the ligands, as supported by theoretical calculations. Solid state EPR spectroscopy, in combination with DFT calculations, has also shown that the Ca2+ ion has an effect on the electronic parameters (zero field splitting) of the linked Mn(II) in the case of MnCa (µ1,1-carboxylate bridges). In Mn2Ca2 (µ1,3-carboxylate bridge) the Ca2+ ion induces only slight structural changes in the Mn coordination sphere.

Cobalt(III) tetraaza-macrocyclic complexes as efficient catalyst for photoinduced hydrogen production in water: Theoretical investigation of the electronic structure of the reduced species and mechanistic insight.

We recently reported a very efficient homogeneous system for visible-light driven hydrogen production in water based on the cobalt(III) tetraaza-macrocyclic complex [Co(CR)Cl2](+) (1) (CR=2,12-dimethyl-3,7,11,17-tetra-azabicyclo(11.3.1)-heptadeca-1(17),2,11,13,15-pentaene) as a noble metal-free catalyst, with [Ru(II)(bpy)3](2+) (Ru) as photosensitizer and ascorbate/ascorbic acid (HA(-)/H2A) as a sacrificial electron donor and buffer (PhysChemChemPhys 2013, 15, 17544). This catalyst presents the particularity to achieve very high turnover numbers (TONs) (up to 1000) at pH 4.0 at a relative high concentration (0.1mM) generating a large amount of hydrogen and having a long term stability. A similar activity was observed for the aquo derivative [Co(III)(CR)(H2O)2](3+) (2) due to substitution of chloro ligands by water molecule in water. In this work, the geometry and electronic structures of 2 and its analog [Zn(II)(CR)Cl](+) (3) derivative containing the redox innocent Zn(II) metal ion have been investigated by DFT calculations under various oxidation states. We also further studied the photocatalytic activity of this system and evaluated the influence of varying the relative concentration of the different components on the H2-evolving activity. Turnover numbers versus catalyst (TONCat) were found to be dependent on the catalyst concentration with the highest value of 1130 obtained at 0.05 mM. Interestingly, the analogous nickel derivative, [Ni(II)(CR)Cl2] (4), when tested under the same experimental conditions was found to be fully inactive for H2 production. Nanosecond transient absorption spectroscopy measurements have revealed that the first electron-transfer steps of the photocatalytic H2-evolution mechanism with the Ru/cobalt tetraaza/HA(-)/H2A system involve a reductive quenching of the excited state of the photosensitizer by ascorbate (kq=2.5×10(7) M(-1) s(-1)) followed by an electron transfer from the reduced photosensitizer to the catalyst (ket=1.4×10(9) M(-1) s(-1)). The reduced catalyst can then enter into the cycle of hydrogen evolution.

A computational mechanistic investigation of hydrogen production in water using the [Rh(III)(dmbpy)2Cl2](+)/[Ru(II)(bpy)3](2+)/ascorbic acid photocatalytic system.

We recently reported an efficient molecular homogeneous photocatalytic system for hydrogen (H2) production in water combining [Rh(III)(dmbpy)2Cl2](+) (dmbpy = 4,4'-dimethyl-2,2'-bipyridine) as a H2 evolving catalyst, [Ru(II)(bpy)3](2+) (bpy = 2,2'-bipyridine) as a photosensitizer and ascorbic acid as a sacrificial electron donor (Chem. - Eur. J., 2013, 19, 781). Herein, the possible rhodium intermediates and mechanistic pathways for H2 production with this system were investigated at DFT/B3LYP level of theory and the most probable reaction pathways were proposed. The calculations confirmed that the initial step of the mechanism is a reductive quenching of the excited state of the Ru photosensitizer by ascorbate, affording the reduced [Ru(II)(bpy)2(bpy˙(-))](+) form, which is capable, in turn, of reducing the Rh(III) catalyst to the distorted square planar [Rh(I)(dmbpy)2](+) species. This two-electron reduction by [Ru(II)(bpy)2(bpy˙(-))](+) is sequential and occurs according to an ECEC mechanism which involves the release of one chloride after each one-electron reduction step of the Rh catalyst. The mechanism of disproportionation of the intermediate Rh(II) species, much less thermodynamically favoured, cannot be barely ruled out since it could also be favoured from a kinetic point of view. The Rh(I) catalyst reacts with H3O(+) to generate the hexa-coordinated hydride [Rh(III)(H)(dmbpy)2(X)](n+) (X = Cl(-) or H2O), as the key intermediate for H2 release. The DFT study also revealed that the real source of protons for the hydride formation as well as the subsequent step of H2 evolution is H3O(+) rather than ascorbic acid, even if the latter does govern the pH of the aqueous solution. Besides, the calculations have shown that H2 is preferentially released through an heterolytic mechanism by reaction of the Rh(III)(H) hydride and H3O(+); the homolytic pathway, involving the reaction of two Rh(III)(H) hydrides, being clearly less favoured. In parallel to this mechanism, the reduction of the Rh(III)(H) hydride into the penta-coordinated species [Rh(II)(H)(dmbpy)2](+) by [Ru(II)(bpy)2(bpy˙(-))](+) is also possible, according to the potentials of the respective species determined experimentally and this is confirmed by the calculations. From this Rh(II)(H) species, the heterolytic and homolytic pathways are both thermodynamically favourable to produce H2 confirming that Rh(II)(H) is as reactive as Rh(III)(H) towards the production of H2.

Ca K-edge XAS as a probe of calcium centers in complex systems.

Herein, Ca K-edge X-ray absorption spectroscopy (XAS) is developed as a means to characterize the local environment of calcium centers. The spectra for six, seven, and eight coordinate inorganic and molecular calcium complexes were analyzed and determined to be primarily influenced by the coordination environment and site symmetry at the calcium center. The experimental results are closely correlated to time-dependent density functional theory (TD-DFT) calculations of the XAS spectra. The applicability of this methodology to complex systems was investigated using structural mimics of the oxygen-evolving complex (OEC) of PSII. It was found that Ca K-edge XAS is a sensitive probe for structural changes occurring in the cubane heterometallic cluster due to Mn oxidation. Future applications to the OEC are discussed.

Efficient and limiting reactions in aqueous light-induced hydrogen evolution systems using molecular catalysts and quantum dots.

Hydrogen produced from water and solar energy holds much promise for decreasing the fossil fuel dependence. It has recently been proven that the use of quantum dots as light harvesters in combination with catalysts is a valuable strategy to obtain photogenerated hydrogen. However, the light to hydrogen conversion efficiency of these systems is reported to be lower than 40%. The low conversion efficiency is mainly due to losses occurring at the different interfacial charge-transfer reactions taking place in the multicomponent system during illumination. In this work we have analyzed all the involved reactions in the hydrogen evolution catalysis of a model system composed of CdTe quantum dots, a molecular cobalt catalyst and vitamin C as sacrificial electron donor. The results demonstrate that the electron transfer from the quantum dots to the catalyst occurs fast enough and efficiently (nanosecond time scale), while the back electron transfer and catalysis are much slower (millisecond and microsecond time scales). Further improvements of the photodriven proton reduction should focus on the catalytic rate enhancement, which should be at least in the hundreds of nanoseconds time scale.

A bio-inspired switch based on cobalt(II) disulfide/cobalt(III) thiolate interconversion.

Disulfide/thiolate interconversion supported by transition-metal ions is proposed to be implicated in fundamental biological processes, such as the transport of metal ions or the regulation of the production of reactive oxygen species. We report herein a mononuclear dithiolate Co(III) complex, [Co(III)LS(Cl)] (1; LS=sulfur containing ligand), that undergoes a clean, fast, quantitative and reversible Co(II) disulfide/Co(III) thiolate interconversion mediated by a chloride anion. The removal of Cl(-) from the Co(III) complex leads to the formation of a bis(µ-thiolato) µ-disulfido dicobalt(II) complex, [Co2(II,II)LSSL](2+) (2(2+)). The structures of both complexes have been resolved by single-crystal X-ray diffraction; their magnetic, spectroscopic, and redox properties investigated together with DFT calculations. This system is a unique example of metal-based switchable M(n)2-RSSR/2 M((n+1))-SR (M=metal ion, n=oxidation state) system that does not contain copper, acts under aerobic conditions, and involves systems with different nuclearities.

An efficient Ru(II) -Rh(III) -Ru(II) polypyridyl photocatalyst for visible-light-driven hydrogen production in aqueous solution.

The development of multicomponent molecular systems for the photocatalytic reduction of water to hydrogen has experienced considerable growth since the end of the 1970s. Recently, with the aim of improving the efficiency of the catalysis, single-component photocatalysts have been developed in which the photosensitizer is chemically coupled to the hydrogen-evolving catalyst in the same molecule through a bridging ligand. Until now, none of these photocatalysts has operated efficiently in pure aqueous solution: a highly desirable medium for energy-conversion applications. Herein, we introduce a new ruthenium-rhodium polypyridyl complex as the first efficient homogeneous photocatalyst for H2 production in water with turnover numbers of several hundred. This study also demonstrates unambiguously that the catalytic performance of such systems linked through a nonconjugated bridge is significantly improved as compared to that of a mixture of the separate components.

Long-Lived Charge Separated State in NiO-Based p-Type Dye-Sensitized Solar Cells with Simple Cyclometalated Iridium Complexes.

Three new cyclometalated iridium complexes were prepared and investigated on nanocrystalline NiO cathodes. Nanosecond transient absorption spectroscopy experiments show they present a surprisingly slow geminate charge recombination upon excitation on NiO, representing thus the first examples of simple sensitizers with such feature. These complexes were used in dye-sensitized solar cells using nanocrystalline NiO film as semiconductor. The long-lived charge separated state of these Ir complexes make them compatible with other redox mediators than I3(-)/I(-), such as a cobalt electrolyte and enable to reach significantly high open circuit voltage.