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Attenuated conjugation in a Ni(ii) catalyst simultaneously facilitates the molecule–support interactions and the intrinsic activity, unexpectedly displaying a superior performance in CO2 electroreduction.

An oxoiron(iv) cation radical is generated upon two-electron oxidation of an iron(iii) complex bearing an electron-rich methoxy substituted bTAML framework and characterized via multiple spectroscopic techniques and density functional theory (DFT).

Mononuclear nickel(II) and nickel(III) complexes of a bisamidate-bisalkoxide ligand, (NMe

Picosecond optical and X-ray absorption spectroscopies with time- dependent density functional theory revealed the reaction pathways, electronic and structural conformations of Ir–Co hydrogen evolution photocatalysts. The dyad bearing 2-phenylpyridine ancillary ligands produced more photoreduced Co(II) than its 2-phenylisoquinoline analogue. These findings are important for designs of earth- abundant photosensitizers for photocatalytic applications.

The utilization of a fully noble-metal-free system for photocatalytic CO2 reduction remains a fundamental challenge, demanding the precise design of photosensitizers and catalysts, as well as the exploitation of their intermolecular interactions to facilitate electron delivery. Herein, we have implemented triple modulations on catalyst, photosensitizer and coordinative interaction between them for high-performance light-driven CO2 reduction. In this study, heteroleptic copper and cobalt phthalocyanine complexes were selected as photosensitizers and catalysts, respectively. An over ten-fold improvement in light-driven reduction of CO2 to CO is achieved for the catalysts with appending electron-withdrawing substituents for optimal CO-desorption ability. In addition, pyridine substituents were implanted at the backbone of the phenanthroline moiety of the Cu(I) photosensitizers and the effect of their axial coordinative interaction with the catalyst was tested. The combined results of 1H NMR titration experiment, steady-state/transient photoluminescence, and transient absorption spectroscopy confirm the coordinative interaction and reductive quenching pathway in photocatalysis corroboratively. It has been found that the catalytic performances of the coordinatively interacted systems are unexpectedly reverse to those with the pyridine-free Cu(I) photosensitizers. Moreover, the latter system enables a very high quantum efficiency up to 63.5% at 425 nm with a high selectivity exceeding 99% for CO2-to-CO conversion. As determined by time-resolved X-ray absorption spectroscopy and DFT calculation, the replacement of phenyl by pyridyl groups in the Cu(I) photosensitizer favors a stronger flattening and larger torsional angle change of the overall excited state geometry upon photoexcitation, which explains the decreased lifetime of the triplet excited state. Our work promotes the systematic multi-pathway optimizations on the catalyst, photosensitizer and their interactions for advanced CO2 photoreduction.

Photoelectrochemical cells that utilize water as a source of electrons are one of the most attractive solutions for the replacement of fossil fuels by clean and sustainable solar fuels. To achieve this, heterogeneous water oxidation catalysis needs to be mastered and properly understood. The search continues for a catalyst that is stable at the surface of electro(photo)anodes and can efficiently perform this reaction at the desired neutral pH. Here, we show how oligomeric Ru complexes can be anchored on the surfaces of graphitic materials through CH–π interactions between the auxiliary ligands bonded to Ru and the hexagonal rings of the graphitic surfaces, providing control of their molecular coverage. These hybrid molecular materials behave as molecular electroanodes that catalyse water oxidation to dioxygen at pH 7 with high current densities. This strategy for the anchoring of molecular catalysts on graphitic surfaces can potentially be extended to other transition metals and other catalytic reactions. Efficient and stable water oxidation catalysts are important if photoelectrochemical cells are to be used to provide clean and sustainable solar fuels. A water oxidation catalyst that operates at neutral pH has now been developed that features ruthenium coordination oligomers anchored onto the surfaces of graphitic materials through CH–π interactions.

Time-resolved X-ray (tr-XAS) and optical transient absorption (OTA) spectroscopy in the picosecond time scale coupled with Density Functional theory (DFT) and X-ray absorption near-edge structure (XANES) calculations are applied to study three homoleptic Cu(I) dimeric chromophores with ethyl and longer propyl spacers, denoted as [Cu2(mphenet)2]Cl2 (C1), [Cu2(mphenet)2](ClO4)2 (C2) and [Cu2(mphenpr)2](ClO4)2 (C3) (where mphenet = 1,2-bis(9-methyl-1,10-phenanthrolin-2-yl)ethane and mphenpr = 1,3-bis(9-methyl-1,10-phenanthrolin-2-yl)propane). Tr-XAS analysis after light illumination at ∼ 100 ps illustrate the formation of a flattened triplet excited state in all 3 complexes. Optical transient absorption (OTA) analysis for C1 monitored in water and C2 and C3 measured in acetonitrile reveals distinct excited-state lifetimes of 169 ps, 670 ps and 1600 ps respectively. These differences are associated to changes in the solvent (comparing C1 and C2) and the flexibility of the ligand to adapt after Cu flattening upon excitation (C2 and C3). Our results are important for the improved structural dynamics of these types of Cu-based dimeric compounds, and can guide the integration of these chromophores into more complex solar energy conversion schemes.

A new Ru oligomer of formula {[Ru II (bda-κ-N 2 O 2)(4,4'-bpy)](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'(H O) @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/cm 2 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.

A terminal FeIIIOH complex, [FeIII(L)(OH)]2− (1), has been synthesized and structurally characterized (H4L = 1,2-bis(2-hydroxy-2-methylpropanamido)benzene). The oxidation reaction of 1 with one equiv. of tris(4-bromophenyl)ammoniumyl hexachloroantimonate (TBAH) or ceric ammonium nitrate (CAN) in acetonitrile at −45 °C results in the formation of a FeIIIOH ligand radical complex, [FeIII(L˙)(OH)]− (2), which is hereby characterized by UV-visible, 1H nuclear magnetic resonance, electron paramagnetic resonance, and X-ray absorption spectroscopy techniques. The reaction of 2 with a triphenylcarbon radical further gives triphenylmethanol and mimics the so-called oxygen rebound step of Cpd II of cytochrome P450. Furthermore, the reaction of 2 was explored with different 4-substituted-2,6-di-tert-butylphenols. Based on kinetic analysis, a hydrogen atom transfer (HAT) mechanism has been established. A pKa value of 19.3 and a BDFE value of 78.2 kcal/mol have been estimated for complex 2., One-electron oxidation of an FeIII–OH complex (1) results in the formation of a FeIII–OH ligand radical complex (2). Its reaction with (C6H5)3C˙ results in the formation of (C6H5)3COH, which is a functional mimic of compound II of cytochrome P450.

Catalytic water splitting is a viable process for the generation of renewable fuels. Here it is reported for the first time that a trinuclear supramolecular Ru(bda) (bda: 2,2′‐bipyridine‐6,6′‐dicarboxylate) catalyst, anchored on multi‐walled carbon nanotubes and subsequently immobilized on glassy carbon electrodes, shows outstanding performance in heterogeneous water oxidation. Activation of the catalyst on anodes by repetitive cyclic voltammetry (CV) scans results in a catalytic current density of 186 mA cm\(^{−2}\) at a potential of 1.45 V versus NHE. The activated catalyst performs water oxidation at an onset overpotential of 330 mV. The remarkably high stability of the hybrid anode is demonstrated by X‐ray absorption spectroscopy and electrochemically, revealing the absence of any degradation after 1.8 million turnovers. Foot of the wave analysis of CV data of activated electrodes with different concentrations of catalyst indicates a monomolecular water nucleophilic attack mechanism with an apparent rate constant of TOFmax (turnover frequency) of 3200 s\(^{−1}\).

Excited with copper: A multi‐method approach was used to determine the ground and excited state structures of a series of CuI photosensitizers containing additional donor moieties. For the first time a pentacoordinated copper center, corresponding to a 4+1‐fold coordination mode, was found in the excited state of the homoleptic complexes. A systematic series of four novel homo‐ and heteroleptic CuI photosensitizers based on tetradentate 1,10‐phenanthroline ligands of the type X^N^N^X containing two additional donor moieties in the 2,9‐position (X=SMe or OMe) were designed. Their solid‐state structures were assessed by X‐ray diffraction. Cyclic voltammetry, UV‐vis absorption, emission and X‐ray absorption spectroscopy were then used to determine their electrochemical, photophysical and structural features in solution. Following, time‐resolved X‐ray absorption spectroscopy in the picosecond time scale, coupled with time‐dependent density functional theory calculations, provided in‐depth information on the excited state electron configurations. For the first time, a significant shortening of the Cu−X distance and a change in the coordination mode to a pentacoordinated geometry is shown in the excited states of the two homoleptic complexes. These findings are important with respect to a precise understanding of the excited state structures and a further stabilization of this type of photosensitizers., publishedVersion

Chemistry - a European journal 26(47), 10801 - 10810 (2020). doi:10.1002/chem.202001393, Time���resolved X���ray (Tr���XAS) and optical transient absorption (OTA) spectroscopy on the pico���microsecond timescale coupled with density functional theory calculations are applied to study the light���induced spin crossover processes of a Fe���based macrocyclic complex in solution. Tr���XAS analysis after light illumination shows the formation of a seven���coordinated high���spin quintet metastable state, which relaxes to a six���coordinated high���spin configuration before decaying to the ground state. Kinetic analysis of the macrocyclic complex reveals an unprecedented long���lived decay lifetime of approximately 42.6 ��s. Comparative studies with a non���macrocyclic counterpart illustrate a significantly shortened approximately 568���fold decay lifetime of about 75 ns, and highlight the importance of the ligand arrangement in stabilizing the reactivity of the excited state. Lastly, OTA analysis shows the seven���coordinated high���spin state to be formed within approximately 6.2 ps. These findings provide a complete understanding of the spin crossover reaction and relaxation pathways of the macrocyclic complex, and reveal the importance of a flexible coordination environment for their rational design., Published by Wiley-VCH, Weinheim

International audience; Inspired by the sulphur-rich environment found in active hydrogenase enzymes, a novel Ni-based proton reduction catalyst with pentadentate N2S3 ligand was synthesized. When coupled with a [Ru(bpy)3] 2+ photosensitiser and ascorbate as electron donor in a 1:1 mixture of dimethylacetamide and aqueous ascorbic acid/ascorbate buffer, the catalyst showed improved photocatalytic activity compared to a homologous counterpart with a tetradentate N2S2 ligand. The mechanistic pathway of photo-induced hydrogen evolution was comprehensively analysed through optical transient absorption (OTA) and time-resolved X-ray absorption spectroscopy (tr-XAS) revealing important electronic and structural changes in the catalytic system during photo-irradiation. It was found that the Ni (II) catalyst undergoes a photo-induced metal-centred reduction to form a Ni (I) intermediate bearing a distorted square bipyramidal geometry. Further kinetic analyses pointed out differences in charge separation dynamics between the pentadentate and tetradentate homologues.

Water oxidation catalysis stands out as one of the most important reactions to design practical devices for artificial photosynthesis. The use of late 1st row transition metal (TM) complexes provides an excellent platform for the development of inexpensive catalysts with an 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, tetraamidate macrocyclic ligand (TAML) as water oxidation catalysts, 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 in 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 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, what we associate to impossibility of efficiently stabilize several oxidative equivalents via charge delocalization, resulting in highly reactive oxidized ligand. Finally, the acyclic structure of 32- prevent its use at neutral pH due to acidic demetallation, highlighting the importance of the macrocyclic stabilization

Inspired by the sulfur-rich environment found in active hydrogenase enzymes, a Ni-based proton reduction catalyst with pentadentate N

We report the use of time-resolved X-ray absorption spectroscopy in the ns-μs time scale to track the light induced two electron transfer processes in a multi-component photocatalytic system, consisting of [Ru(bpy)3](2+)/ a diiron(iii,iii) model/triethylamine. EXAFS analysis with DFT calculations confirms the structural configurations of the diiron(iii,iii) and reduced diiron(ii,ii) states.

The realization of artificial photosynthesis carries the promise of cheap and abundant energy, however, significant advances in the rational design of water oxidation catalysts are required. Detailed information on the structure of the catalyst under reaction conditions and mechanisms of O–O bond formation should be obtained. Here, we used a combination of electron paramagnetic resonance (EPR), stopped flow freeze quench on a millisecond–second time scale, X-ray absorption (XAS), resonance Raman (RR) spectroscopy, and density functional theory (DFT) to follow the dynamics of the Ru-based single site catalyst, [RuII(NPM)(4-pic)2(H2O)]2+ (NPM = 4-t-butyl-2,6-di(1′,8′-naphthyrid-2′-yl)pyridine, pic = 4-picoline), under the water oxidation conditions. We report a unique EPR signal with g-tensor, gx = 2.30, gy = 2.18, and gz = 1.83 which allowed us to observe fast dynamics of oxygen atom transfer from the RuIV═O oxo species to the uncoordinated nitrogen of the NPM ligand. In few seconds, the NPM ligand modific...

We report the light-induced electronic and geometric changes taking place in “real time” of a multimolecular [Ru(bpy)3]2+/[Ni(PPh2NPh2)2(CH3CN)]2+/ascorbic acid photocatalytic system by time-resolved X-ray absorption spectroscopy (tr-XAS) in the nano- to microsecond time regime. Using tr-XAS allows us to observe the diffusion-governed electron transfer between the excited photosensitizer and the nickel(II) proton reduction catalyst on the nanosecond time scale followed by formation of a transient distorted tetrahedral Ni(I) intermediate. A 50-fold increase in the decay lifetime of the Ni(I) species, in the presence of the electron donor, shows that the favored catalytic pathway occurs through reductive quenching of the excited photosensitizer followed by electron transfer to the catalyst. Lack of protonation of the Ni(I) amine groups within our experimental tr-XAS time window suggests that proton binding is the rate limiting step for H2 photocatalysis by this system. This study is supported by molecular o...

International audience; Inspired by the sulphur-rich environment found in active hydrogenase enzymes, a novel Ni-based proton reduction catalyst with pentadentate N 2 S 3 ligand was synthesized. When coupled with a [Ru(bpy) 3 ] 2+ photosensitiser and ascorbate as electron donor in a 1:1 mixture of dimethylacetamide and aqueous ascorbic acid/ascorbate buffer, the catalyst showed improved photocatalytic activity compared to a homologous counterpart with a tetradentate N 2 S 2 ligand. The mechanistic pathway of photo-induced hydrogen evolution was comprehensively analysed through optical transient absorption (OTA) and time-resolved X-ray absorption spectroscopy (tr-XAS) revealing important electronic and structural changes in the catalytic system during photo-irradiation. It was found that the Ni (II) catalyst undergoes a photo-induced metal-centred reduction to form a Ni (I) intermediate bearing a distorted square bipyramidal geometry. Further kinetic analyses pointed out differences in charge separation dynamics between the pentadentate and tetradentate homologues.

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.

Modern chemistry's grand challenge is to significantly improve catalysts for water splitting. Further progress requires detailed spectroscopic and computational characterization of catalytic mechanisms. We analyzed one of the most studied homogeneous single-site Ru catalysts, [Ru(II)(bpy)(tpy)H2O](2+) (where bpy = 2,2'-bipyridine, tpy = 2,2';6',2″-terpyridine). Our results reveal that the [Ru(V)(bpy)(tpy)═O](3+) intermediate, reportedly detected in catalytic mixtures as a rate-limiting intermediate in water activation, is not present as such. Using a combination of electron paramagnetic resonance (EPR) and X-ray absorption spectroscopy, we demonstrate that 95% of the Ru complex in the catalytic steady state is of the form [Ru(IV)(bpy)(tpy)═O](2+). [Ru(V)(bpy)(tpy)═O](3+) was not observed, and according to density functional theory (DFT) analysis, it might be thermodynamically inaccessible at our experimental conditions. A reaction product with unique EPR spectrum was detected in reaction mixtures at about 5% and assigned to Ru(III)-peroxo species with (-OOH or -OO- ligands). We also analyzed the [Ru(II)(bpy)(tpy)Cl](+) catalyst precursor and confirmed that this molecule is not a catalyst and its oxidation past Ru(III) state is impeded by a lack of proton-coupled electron transfer. Ru-Cl exchange with water is required to form active catalysts with the Ru-H2O fragment. [Ru(II)(bpy)(tpy)H2O](2+) is the simplest representative of a larger class of water oxidation catalysts with neutral, nitrogen containing heterocycles. We expect this class of catalysts to work mechanistically in a similar fashion via [Ru(IV)(bpy)(tpy)═O](2+) intermediate unless more electronegative (oxygen containing) ligands are introduced in the Ru coordination sphere, allowing the formation of more oxidized Ru(V) intermediate.

Density functional theory (DFT) is now widely used for analysis of the electronic structure and reactivities of transition metal complexes. However, large variability in how well different combinations of exchange-correlation potentials/basis sets reproduce real molecular geometries and electronic configurations remains a problem. Experimental X-ray absorption near-edge structure (XANES) spectra directly reflect the electronic structure of transition metal complexes. Combined analysis of theoretical calculations and experimental data is highly beneficial for DFT validation as well as for understanding limitations of the DFT. Ruthenium-based molecular water oxidation catalyst cis,cis-[(bpy)2(H2O)RuIIIORuIVO(OH)(bpy)2]4+ is a complex coordination compound with two Ru centers in different oxidation states bound by μ-oxo bridge. Multiple DFT calculations of this catalyst in different oxidation states have been reported previously but it was never clear whether DFT is truly capable of describing its geometry a...

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 oxi-dation 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 sys-tem, the electronic perturbation provided by the pyrene functionality translates into a 150 mV lower overpotential for 22- respect to 12- and an impressive increase in the kcat from 6 s-1 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

Density functional theory (DFT)-based relativistic calculations were performed to model the Ru L-edge X-ray absorption near edge structure (XANES) spectra of the hexaammineruthenium complex [Ru(NH{sub 3}){sub 6}]{sup 3+} and 'blue dimer' water oxidation catalyst, cis,cis- [(bpy){sub 2}(H{sub 2}O)Ru{sup III}ORu{sup III}(OH{sub 2})(bpy){sub 2}]{sup 4+} (bpy is 2,2-bipyridine). Two computational approaches were compared: simulations without the core-hole and by modeling of the core-hole within the Z+1 approximation. Good agreement between calculated and experimental XANES spectra is achieved without including the core-hole. Simulations with algorithms beyond the Z+1 approximation were only possible in a framework of the scalar relativistic treatment. Time-dependent DFT (TD-DFT) was used to compute the Ru L-edge spectrum for [Ru(NH{sub 3}){sub 6}]{sup 3+} model compound. Three different core-hole treatments were compared in a real-space full multiple scattering XANES modeling within the Green function formalism (implemented in the FEFF9.5 package) for the [Ru(Mebimpy)(bpm)(H{sub 2}O)]{sup 2+} complex. The latter approaches worked well in cases where spin-orbit treatment of relativistic effects is not required.

A method for the theoretical simulation of X-ray absorption near edge structure (XANES) spectra at the Ru L2,3 edges has been developed using relativistic density functional theory (DFT) calculations. The effect of the parameters of DFT calculations on the shape of theoretical curves has been comparatively analyzed for XANES spectra of a water oxidation catalyst and hexaammineruthenium complexes. Recommendations for the choice of the best parameters ensuring good agreement with the experimental data, including the most correct exchange-correlation potential, have been made.

Catalytic O(2) evolution with cis,cis-[(bpy)(2)(H(2)O)Ru(III)ORu(III)(OH(2))(bpy)(2)](4+) (bpy is 2,2-bipyridine), the so-called blue dimer, the first designed water oxidation catalyst, was monitored by UV-vis, EPR, and X-ray absorption spectroscopy (XAS) with ms time resolution. Two processes were identified, one of which occurs on a time scale of 100 ms to a few seconds and results in oxidation of the catalyst with the formation of an intermediate, here termed [3,4]'. A slower process occurring on the time scale of minutes results in the decay of this intermediate and O(2) evolution. Spectroscopic data suggest that within the fast process there is a short-lived transient intermediate, which is a precursor of [3,4]'. When excess oxidant was used, a highly oxidized form of the blue dimer [4,5] was spectroscopically resolved within the time frame of the fast process. Its structure and electronic state were confirmed by EPR and XAS. As reported earlier, the [3,4]' intermediate likely results from reaction of [4,5] with water. While it is generated under strongly oxidizing conditions, it does not display oxidation of the Ru centers past [3,4] according to EPR and XAS. EXAFS analysis demonstrates a considerably modified ligand environment in [3,4]'. Raman measurements confirmed the presence of the O-O fragment by detecting a new vibration band in [3,4]' that undergoes a 46 cm(-1) shift to lower energy upon (16)O/(18)O exchange. Under the conditions of the experiment at pH 1, the [3,4]' intermediate is the catalytic steady state form of the blue dimer catalyst, suggesting that its oxidation is the rate-limiting step.

Nature Communications 7, 11652 (2016). doi:10.1038/ncomms11652, New capabilities at X-ray free-electron laser facilities allow the generation of two-colour femtosecond X-ray pulses, opening the possibility of performing ultrafast studies of X-ray-induced phenomena. Particularly, the experimental realization of hetero-site-specific X-ray-pump/X-ray-probe spectroscopy is of special interest, in which an X-ray pump pulse is absorbed at one site within a molecule and an X-ray probe pulse follows the X-ray-induced dynamics at another site within the same molecule. Here we show experimental evidence of a hetero-site pump-probe signal. By using two-colour 10-fs X-ray pulses, we are able to observe the femtosecond time dependence for the formation of F ions during the fragmentation of XeF$_2$ molecules following X-ray absorption at the Xe site., Published by Nature Publishing Group, London

DOI: 10.1002/anie.201609167 URL: http://onlinelibrary.wiley.com/doi/10.1002/anie.201609167/abstract;jsessionid=D9AA38A4118537C982E63F8A1157F99C.f01t03?systemMessage=Wiley+Online+Library+will+be+unavailable+on+Saturday+17th+December+2016+at+09%3A00+GMT%2F+04%3A00+EST%2F+17%3A00+SGT+for+4hrs+due+to+essential+maintenance.Apologies+for+the+inconvenience Filiació URV: SI Inclòs a la memòria: SI Molecular ruthenium-based water oxidation catalyst precursors of general formula [Ru(tda)(Li)2] (tda2¿ is [2,2¿:6¿,2¿¿-terpyridine]-6,6¿¿-dicarboxylato; L1=4-(pyren-1-yl)-N-(pyridin-4-ylmethyl)butanamide, 1 b; L2=4-(pyren-1-yl)pyridine), 1 c), have been prepared and thoroughly characterized. Both complexes contain a pyrene group allowing ready and efficiently anchoring via ¿ interactions on multi-walled carbon nanotubes (MWCNT). These hybrid solid state materials are exceptionally stable molecular water-oxidation anodes capable of carrying out more than a million turnover numbers (TNs) at pH 7 with an Eapp=1.45 V vs. NHE without any sign of degradation. XAS spectroscopy analysis before, during, and after catalysis together with electrochemical techniques allow their unprecedented oxidative ruggedness to be monitored and verified.

An end-on superoxido complex with the formula {[CoIII(OH2)(trpy)][CoIII(OO•)(trpy)](μ-bpp)}4+ (34+) (bpp– = bis(2-pyridyl)-3,5-pyrazolate; trpy = 2,2′;6′:2″-terpyridine) has been characterized by resonance Raman, electron paramagnetic resonance, and X-ray absorption spectroscopies. These results together with online mass spectrometry experiments using 17O and 18O isotopically labeled compounds prove that this compound is a key intermediate of the water oxidation reaction catalyzed by the peroxido-bridged complex {[CoIII(trpy)]2(μ-bpp)(μ-OO)}3+ (13+). DFT calculations agree with and complement the experimental data, offering a complete description of the transition states and intermediates involved in the catalytic cycle.

Physical review / A covering atomic, molecular, and optical physics and quantum information 94(1), 013426(2016). doi:10.1103/PhysRevA.94.013426, The capability of generating two intense, femtosecond x-ray pulses with a controlled time delay opens the possibility of performing time-resolved experiments for x-ray-induced phenomena. We have applied this capability to study the photoinduced dynamics in diatomic molecules. In molecules composed of low-Z elements, K-shell ionization creates a core-hole state in which the main decay mode is an Auger process involving two electrons in the valence shell. After Auger decay, the nuclear wave packets of the transient two-valence-hole states continue evolving on the femtosecond time scale, leading either to separated atomic ions or long-lived quasibound states. By using an x-ray pump and an x-ray probe pulse tuned above the K-shell ionization threshold of the nitrogen molecule, we are able to observe ion dissociation in progress by measuring the time-dependent kinetic energy releases of different breakup channels. We simulated the measurements on N$_2$ with a molecular dynamics model that accounts for K-shell ionization, Auger decay, and the time evolution of the nuclear wave packets. In addition to explaining the time-dependent feature in the measured kinetic energy release distributions from the dissociative states, the simulation also reveals the contributions of quasibound states., Published by Inst., Woodbury, NY

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 [LCoIIICl2]+ (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.

Water oxidation is critically important for the development of energy solutions based on the concept of artificial photosynthesis. In order to gain deeper insight into the mechanism of water oxidation, the catalytic cycle for the first designed water oxidation catalyst, cis,cis-[(bpy)2(H2O)RuIIIORuIII(OH2)(bpy)2]4+ (bpy is 2,2-bipyridine) known as the blue dimer (BD), is monitored in D2O by combined application of stopped flow UV-Vis, electron paramagnetic resonance (EPR) and resonance Raman spectroscopy on freeze quenched samples. The results of these studies show that the rate of formation of BD[4,5] by Ce(IV) oxidation of BD[3,4] (numbers in square bracket denote oxidation states of the ruthenium (Ru) centers) in 0.1 M HNO3, as well as further oxidation of BD[4,5] are slower in D2O by 2.1–2.5. Ce(IV) oxidation of BD[4,5] and reaction with H2O result in formation of an intermediate, BD[3,4]′, which builds up in reaction mixtures on the minute time scale. Combined results under the conditions of these experiments at pH 1 indicate that oxidation of BD[3,4]′ is a rate limiting step in water oxidation with the BD catalyst.

Water oxidation is critically important for the development of energy solutions based on the concept of artificial photosynthesis. In order to gain deeper insight into the mechanism of water oxidation, the catalytic cycle for the first designed water oxidation catalyst, cis,cis-[(bpy)2(H2O)RuIIIORuIII(OH2)(bpy)2]4+ (bpy is 2,2-bipyridine) known as the blue dimer (BD), is monitored in D2O by combined application of stopped flow UV-Vis, electron paramagnetic resonance (EPR) and resonance Raman spectroscopy on freeze quenched samples. The results of these studies show that the rate of formation of BD[4,5] by Ce(IV) oxidation of BD[3,4] (numbers in square bracket denote oxidation states of the ruthenium (Ru) centers) in 0.1 M HNO3, as well as further oxidation of BD[4,5] are slower in D2O by 2.1–2.5. Ce(IV) oxidation of BD[4,5] and reaction with H2O result in formation of an intermediate, BD[3,4]′, which builds up in reaction mixtures on the minute time scale. Combined results under the conditions of these experiments at pH 1 indicate that oxidation of BD[3,4]′ is a rate limiting step in water oxidation with the BD catalyst.

L-2,L-3-edge X-ray absorption spectroscopy (XAS) has demonstrated unique capabilities for the analysis of the electronic structure of di-Ru complexes such as the blue dimer cis,cis-[(Ru2O)-O-III(H2O)(2)(bpy)(4)](4+) water oxidation catalyst. Spectra of the blue dimer and the monomeric [Ru(NH3)(6)](3+) model complex show considerably different splitting of the Ru L-2,L-3 absorption edge, which reflects changes in the relative energies of the Ru 4d orbitals caused by hybridization with a bridging ligand and spin-orbit coupling effects. To aid the interpretation of spectroscopic data, we developed a new approach, which computes L-2,L-3-edges XAS spectra as dipole transitions between molecular spinors of 4d transition metal complexes. This allows for careful inclusion of the spin-orbit coupling effects and the hybridization of the Ru 4d and ligand orbitals. The obtained theoretical Ru L-2,L-3-edge spectra are in close agreement with experiment. Critically, existing single-electron methods (FEFF, FDMNES) broadly used to simulate XAS could not reproduce the experimental Ru L-edge spectra for the [Ru(NH3)(6)](3+) model complex nor for the blue dimer, while charge transfer multiplet (CTM) calculations were not applicable due to the complexity and low symmetry of the blue dimer water oxidation catalyst. We demonstrated that L-edge spectroscopy is informative for analysis of bridging metal complexes. The developed computational approach enhances L-edge spectroscopy as a tool for analysis of the electronic structures of complexes, materials, catalysts, and reactive intermediates with 4d transition metals. (Less)

Several mono and carboxylate-bridged diiron nuclear enzymes are critical in activating dioxygen in biological processes such as DNA synthesis, hydrocarbon metabolism and cell proliferation1. Iron-containing enzymes such as cytochrome P450, peroxidases, catalases and methane monooxygenase (MMO) have been shown to activate dioxygen by using two electrons and protons to produce iron (IV) oxo intermediates2. The remarkable efficiency of these enzymes is attributed to formation of iron(IV) cations which serve as active oxidants in enzymatic reactions and can attack C-H bonds of a wide range of hydrocarbon substrates. It is widely postulated that ferryl-oxo species are key intermediates in the mechanism of cytochrome P450 and MMO. However due to the complexity of protein environments in biological iron enzymatic systems, monitoring the structural changes occurring during dioxygen activation is a complex undertaking.This project serves to study the light driven activation of a well characterized artificial analogue of an diiron MMO enzyme, (µ-peroxo) (µ-carboxylato) diiron(III) complex, [Fe2(µ-O2)(N-EtHPTB)(µ-PhCO2)]2+ in a chromophore/diiron complex assembled unit. Such types of assemblies provide new avenues for study of catalytic reaction mechanisms. They are promising examples of artificial molecular systems leading to dioxygen activation as visible light is efficiently absorbed at the chromophore, and light energy is in turn converted into a chemical potential via an electron relay through charge accumulation processes at the iron metal center. Formation of high valent iron peroxo species have been shown by UV-Vis, EPR and Resonance Raman spectroscopy as well as X-ray spectroscopic analysis. XANES and EXAFS revealed formation of an iron peroxo and oxo species with shorter bond distances and different coordination numbers.(1) Do, L. H.; Hayashi, T.; Moenne-Loccoz, P.; Lippard, S. J. J.Am.Chem.Soc. 2010, 132, 1273.(2) Lawrence Que, J.; Y.N.Ho, R. Chem.Rev 1996, 96, 2607.

Technology to convert Solar energy into fuel is being developed to search for alternate energy sources. This includes electrochemical cells based on artificial water splitting catalysts. By studying their water splitting mechanism we can hope to optimize the energy conversion process and also learn more natural water splitting complexes like Photosystem II. In this study, we analyzed two reported single site Ruthenium based catalysts, [RuII(bpy)(tpy)H2O]2+ and [RuII(bpy)(tpy)Cl]+. [RuII(bpy)(tpy)H2O]2+ is a widely studied water splitting catalyst for which [RuV(bpy)(tpy)= O]3+ has been detected as a rate limiting intermediate. However, using EPR, we see that this intermediate is not present when the sample is measured with CeIV in D2O or HNO3. Using, Ru-K edge EXAFs, we also see that the Ru=O distance measured after adding CeIV is more consistent with its assignment as a [RuIV(bpy)(tpy)=O]2+ intermediate and not with RuV=O.We also found that [RuII(bpy)(tpy)Cl]+ is not a catalyst and that its oxidation past RuIII is impeded by a lack of proton coupled electron transfer. Using L-edge and K-edge XANES spectra, we show that the edge position of the [RuII(bpy)(tpy)Cl]+ spectra is more consistent with the presence of [RuIII(bpy)(tpy)Cl]+. EXAFS analysis of [RuII(bpy)(tpy)Cl]+ also show shortening of the Ru-Cl bond (2.41 A to 2.34 A), implying a transition from RuII to RuIII. This is further tested in an Oxygen evolution assay where the sample is incubated in H2O for different time periods. We show that samples that are not incubated for long times show no activity, hence implying that a Cl- to H2O ligand exchange is necessary for [RuII(bpy)(tpy)Cl]+ to show catalytic activity. These studies give us crucial insight into the water splitting mechanism of these catalysts.

Phase transitions of solids and structural transformations of molecules are canonical examples of important photo-induced processes whose underlying mechanisms largely elude our comprehension due to our inability to correlate electronic excitation with atomic position in real time. Here, we present a decisive step towards such new methodology based on water-window covering (284–543 eV) attosecond soft x-ray pulses that can simultaneously access electronic and lattice parameters via dispersive x-ray absorption fine-structure (XAFS) spectroscopy. We validate this approach with an identification of the σ* and π* orbital contributions to the density of states in graphite simultaneously with its lattice’s four characteristic bonding distances. This work demonstrates the potential of dispersive XAFS, in combination with attosecond pulses, as a powerful investigative tool that is equally applicable to gas, liquid, and condensed phase.

Photosynthesis is one of the most important chemical processes on our planet. Splitting water into O2 and H2 using sunlight is a clean and sustainable proposition for addressing energy and environmental problems encountered in our society. Mimicking this reaction in a manmade device will allow for sunlight-to-energy conversion with water providing electrons and protons for production of chemical fuels. About 30 years ago Meyer and coworkers reported the first ruthenium-based catalyst for water oxidation, known as the “blue dimer”. This catalyst may be considered as an artificial analog of the oxygen-evolving complex (OEC) in Photosystem II (PS II). Recently it has been demonstrated that single-site Ru and Ir catalysts are also active in water oxidation. These single-site catalysts are attractive model compounds for both experimental and theoretical studies of mechanism of water oxidation and show improved catalytic activity compared to “blue dimer”. A better understanding of this mechanism and identification of the rate-limiting steps could pave the way to light-driven generation of molecular hydrogen by water splitting.A mononuclear ruthenium complex [Ru(bda)(pic)2] (pic=4-picoline) was found to show high catalytic activity as well as high chemical stability. EPR, Raman and XAS characterization of the electronic structure and molecular geometry of Ru(III)-OH2,Ru(IV)=O and Ru(IV)-OO-Ru(IV) are reported in the single site water oxidizing complex. Formation of metal bound peroxides as the result of O-O coupling has been implicated in the mechanism of catalytic water oxidation by Photosystem II oxygen evolving complex (OEC) and in Ru-based catalysts. However, such intermediates were never isolated and their structural and electronic characterization has not been reported. We believe that the intermediates described here are direct products of the O-O bond formation step in the studied catalyst.