Water oxidation by a ruthenium complex with noninnocent quinone ligands: possible formation of an O-O bond at a low oxidation state of the metal

Bis(acetylacetonato)bis(pyrazolato)ruthenate(iii) as a redox-active scorpionate ligand

The potential of a new anionic octahedral metal complex [Ru(III)(acac)2(pz)2](-) ((-)) (pzH = pyrazole) as a ligand with a scorpionate coordination behaviour like tris(pyrazolyl)borate (tp) and reversible redox activity is presented. Trinuclear metal complexes, [Ru(III)2Zn(II)(acac)4(pz)4] () and [Ru(II)Ru(III)2(acac)4(pz)4] (), were each synthesized by the reaction of ZnCl2 or Ru3(CO)12 with [Ru(III)(acac)2(pz)(pzH)] (H) that is in situ deprotonated and acts as a precursor of (-). Single-crystal X-ray diffraction studies clarified that (-) acts as a scorpionate ligand; two (-) units in and one unit in function as bidentate ligands with two pyrazolates as pincers, while another (-) unit in functions as a tridentate ligand with one oxygen atom as a tail in addition to the two pyrazolate pincers. Moreover, and showed reversible multi-stage redox behaviours based on the Ru(II)/Ru(III) and Ru(III)/Ru(IV) couples of the (-) units in the cyclic voltammetry (CV) measurements. Based on the X-ray, IR, and CV measurements and the comparison with other Ru(ii) complexes with tp derivatives, the (-) unit was found to act as a redox-active scorpionate with electron withdrawing properties compared to the tp.

Water splitting with polyoxometalate-treated photoanodes: enhancing performance through sensitizer design

Visible light driven water oxidation has been demonstrated at near-neutral pH using photoanodes based on nanoporous films of TiO₂, polyoxometalate (POM) water oxidation catalyst [{Ru₄O₄(OH)₂(H₂O)₄}(γ-SiW₁₀O₃₆)₂]^10- (1), and both known photosensitizer [Ru(bpy)₂(H₄dpbpy)]²⁺ (P2) and the novel crown ether functionalized dye [Ru(5-crownphen)₂(H₂dpbpy)](H₂2). Both triads, containing catalyst 1, and catalyst-free dyads, produce O₂ with high faradaic efficiencies (80 to 94%), but presence of catalyst enhances quantum yield by up to 190% (maximum 0.39%). New sensitizer H₂2 absorbs light more strongly than P2, and increases O₂ quantum yields by up to 270%. TiO₂-2 based photoelectrodes are also more stable to desorption of active species than TiO₂-P2: losses of catalyst 1 are halved when pH > TiO₂ point-of-zero charge (pzc), and losses of sensitizer reduced below the pzc (no catalyst is lost when pH < pzc). For the triads, quantum yields of O₂ are higher at pH 5.8 than at pH 7.2, opposing the trend observed for 1 under homogeneous conditions. This is ascribed to lower stability of the dye oxidized states at higher pH, and less efficient electron transfer to TiO₂, and is also consistent with the 4^th1-to-dye electron transfer limiting performance rather than catalyst TOF_max. Transient absorption reveals that TiO₂-2-1 has similar 1^st electron transfer dynamics to TiO₂-P2-1, with rapid (ps timescale) formation of long-lived TiO₂(e^-)-2-1(h⁺) charge separated states, and demonstrates that metallation of the crown ether groups (Na⁺/Mg²⁺) has little or no effect on electron transfer from 1 to 2. The most widely relevant findings of this study are therefore: (i) increased dye extinction coefficients and binding stability significantly improve performance in dye-sensitized water splitting systems; (ii) binding of POMs to electrode surfaces can be stabilized through use of recognition groups; (iii) the optimal homogeneous and TiO₂-bound operating pHs of a catalyst may not be the same; and (iv) dye-sensitized TiO₂ can oxidize water without a catalyst.

Collecting meaningful early-time kinetic data in homogeneous catalytic water oxidation with a sacrificial oxidant

As the field of water oxidation catalysis grows, so does the sophistication of the associated experimental apparatuses. However, problems persist in studying some of the most basic aspects of catalytic water oxidation including acquisition of satisfactory early-reaction-time kinetics and rapid quantification of O2 concentration. We seek to remedy these problems and through better experimental design, elucidate mechanistic aspects of catalytic water oxidation with theory backed by experimental data. Two new methods for evaluating homogeneous water oxidation catalysts by reaction with a stoichiometric oxidant are presented which eliminate problems of incomplete fast mixing and O2 measurement response time. These methods generate early-reaction-time kinetics that have previously been unavailable.

Cooperative effects in homogenous water oxidation catalysis by mononuclear ruthenium complexes

The homogenous water oxidation catalysis by [Ru(terpy)(bipy)Cl](+) (1) and [Ru(terpy)(Me2bipy)Cl](+) (2) (terpy = 2,2':6',2''-terpyridine, bipy = 2,2'-bipyridine, Me2bipy = 4,4'-dimethyl-2,2'-bipyridine) under the influence of two redox mediators [Ru(bipy)3](2+) (3) and [Ru(phen)2(Me2bipy)](2+) (4) (phen = 1,10-phenanthroline) was investigated using Ce(4+) as sacrificial oxidant. Oxygen evolution experiments revealed that mixtures of both 2-4 and 2-3 produced more molecular oxygen than catalyst 2 alone. In contrast, the combination of mediator 4 and catalyst 1 resulted in a lower catalytic performance of 1. Measurements of the temporal change in the intensity of a UV transition at 261 nm caused by the addition of four equivalents of Ce(4+) to 2 revealed three distinctive regions-suggested to correspond to the stepwise processes: (i) [Ru(IV)=O](2+) → [Ru(V)=O](3+); (ii) [Ru(V)=O](3+) → [Ru(III)-(OOH)](2+); and (iii) [Ru(III)-(OOH)](2+) → [Ru(II)-OH2](2+). UV-Visible spectrophotometric experiments on the 1-4 and 2-4 mixtures, also carried out with four equivalents of Ce(4+), demonstrated a faster [Ru(phen)2(Me2bipy)](3+) → [Ru(phen)2(Me2bipy)](2+) reduction rate in 2-4 than that observed for the 1-4 combination. Cyclic voltammetry data measured for the catalysts and the mixtures revealed a coincidence in the potentials of the Ru(II)/Ru(III) redox process of mediators 3 and 4 and the predicted [Ru(IV)=O](2+)/[Ru(V)=O](3+) potential of catalyst 2. In contrast, the [Ru(IV)=O](2+)/[Ru(V)=O](3+) process for catalyst 1 was found to occur at a higher potential than the Ru(II)/Ru(III) redox process for 4. Both the spectroscopic and electrochemical experiments provide evidence that the interplay between the mediator and the catalyst is an important determinant of the catalytic activity.

New platinum and ruthenium Schiff base complexes for water splitting reactions

New Pt(ii) and Ru(ii) complexes with Schiff base ligands display effective visible-light catalytic water reduction and Ce⁴⁺-driven oxidation activities, respectively.

Design of mononuclear ruthenium catalysts for low-overpotential water oxidation

Water oxidation is a key reaction in natural photosynthesis and in many schemes for artificial photosynthesis. Inspired by energy challenges and the emerging understanding of photosystem II, the development of artificial molecular catalysts for water oxidation has become a highly active area of research in recent years. In this Focus Review, we describe recent achievements in the development of single-site ruthenium catalysts for water oxidation with a particular focus on the overpotential of water oxidation. First, we introduce the general scheme to access the high-valent ruthenium-oxo species, the key species of the water-oxidation reaction. Next, the mechanisms of the OO bond formation from the active ruthenium-oxo species are described. We then discuss strategies to decrease the onset potentials of the water-oxidation reaction. We hope this Focus Review will contribute to the further development of efficient catalysts toward sustainable energy-conversion systems.

What can density functional theory tell us about artificial catalytic water splitting?

Water splitting by artificial catalysts is a critical process in the production of hydrogen gas as an alternative fuel. In this paper, we examine the essential role of theoretical calculations, with particular focus on density functional theory (DFT), in understanding the water-splitting reaction on these catalysts. First, we present an overview of DFT thermochemical calculations on water-splitting catalysts, addressing how these calculations are adapted to condensed phases and room temperature. We show how DFT-derived chemical descriptors of reactivity can be surprisingly good estimators for reactive trends in water-splitting catalysts. Using this concept, we recover trends for bulk catalysts using simple model complexes for at least the first-row transition-metal oxides. Then, using the CoPi cobalt oxide catalyst as a case study, we examine the usefulness of simulation for predicting the kinetics of water splitting. We demonstrate that the appropriate treatment of solvent effects is critical for computing accurate redox potentials with DFT, which, in turn, determine the rate-limiting steps and electrochemical overpotentials. Finally, we examine the ability of DFT to predict mechanism, using ruthenium complexes as a focal point for discussion. Our discussion is intended to provide an overview of the current strengths and weaknesses of the state-of-the-art DFT methodologies for condensed-phase molecular simulation involving transition metals and also to guide future experiments and computations toward the understanding and development of novel water-splitting catalysts.

Water oxidation chemistry of a synthetic dinuclear ruthenium complex containing redox-active quinone ligands

We investigated theoretically the catalytic mechanism of electrochemical water oxidation in aqueous solution by a dinuclear ruthenium complex containing redox-active quinone ligands, [Ru2(X)(Y)(3,6-tBu2Q)2(btpyan)](m+) [X, Y = H2O, OH, O, O2; 3,6-tBu2Q = 3,6-di-tert-butyl-1,2-benzoquinone; btpyan =1,8-bis(2,2':6',2″-terpyrid-4'-yl)anthracene] (m = 2, 3, 4) (1). The reaction involves a series of electron and proton transfers to achieve redox leveling, with intervening chemical transformations in a mesh scheme, and the entire molecular structure and motion of the catalyst 1 work together to drive the catalytic cycle for water oxidation. Two substrate water molecules can bind to 1 with simultaneous loss of one or two proton(s), which allows pH-dependent variability in the proportion of substrate-bound structures and following pathways for oxidative activation of the aqua/hydroxo ligands at low thermodynamic and kinetic costs. The resulting bis-oxo intermediates then undergo endothermic O-O radical coupling between two Ru(III)-O(•) units in an anti-coplanar conformation leading to bridged μ-peroxo or μ-superoxo intermediates. The μ-superoxo species can liberate oxygen with the necessity for the preceding binding of a water molecule, which is possible only after four-electron oxidation is completed. The magnitude of catalytic current would be limited by the inherent sluggishness of the hinge-like bending motion of the bridged μ-superoxo complex that opens up the compact, hydrophobic active site of the catalyst and thereby allows water entry under dynamic conditions. On the basis of a newly proposed mechanism, we rationalize the experimentally observed behavior of electrode kinetics with respect to potential and discuss what causes a high overpotential for water oxidation by 1.

Insights into Ru-based molecular water oxidation catalysts: electronic and noncovalent-interaction effects on their catalytic activities

A series of Ru-bda water oxidation catalysts [Ru(bda)L2] (H2bda = 2,2'-bipyridine-6,6'-dicarboxylic acid; L = [HNEt3][3-SO3-pyridine], 1; 4-(EtOOC)-pyridine, 2; 4-bromopyridine, 3; pyridine, 4; 4-methoxypyridine, 5; 4-(Me2N)-pyridine, 6; 4-[Ph(CH2)3]-pyridine, 7) were synthesized with electron-donating/-withdrawing groups and hydrophilic/hydrophobic groups in the axial ligands. These complexes were characterized by (1)H NMR spectroscopy, high-resolution mass spectrometry, elemental analysis, and electrochemistry. In addition, complexes 1 and 6 were further identified by single crystal X-ray crystallography, revealing a highly distorted octahedral configuration of the Ru coordination sphere. All of these complexes are highly active toward Ce(IV)-driven (Ce(IV) = Ce(NH4)2(NO3)6) water oxidation with oxygen evolution rates up to 119 mols of O2 per mole of catalyst per second. Their structure-activity relationship was investigated. Electron-withdrawing and noncovalent interactions (attraction) exhibit positive effect on the catalytic activity of Ru-bda catalysts.

Excitonic states in a (Ti6O12)3 nanotube

The low-lying excited electronic states of a (Ti(6)O(12))(3) nanotube are investigated using ab initio self-consistent field configuration interaction theory. The transition energies and moments are calculated and the nature of the orbitals involved is discussed. Transitions correspond to an excitation from an O(2p) to a nearby Ti(3d) orbital and singlet-singlet transitions vary in excitation energy from 2.1 eV to 4.3 eV, depending on the oxygen site and environment of the titanium site. Two different structures for the three stacked Ti(6)O(12) rings are found. The occluded Ti structure is lower in energy than a staggered structure by 1.25 eV, only 0.02 eV/bond. Excited electronic states are found to correspond to highly localized holes on oxygen and a highly localized electron in a d orbital on a nearest neighbor titanium. The staggered structure has four absorptions that lie within the intense portion of the solar spectrum, at 585 nm, 472 nm, 471 nm, 423 nm; the occluded structure has one strong absorption, at 532 nm.

Polyoxometalate water oxidation catalysts and the production of green fuel

In the last five years and currently, research on solar fuels has been intense and no sub-area in this field has been more active than the development of water oxidation catalysts (WOCs). In this timeframe, a new class of molecular water oxidation catalysts based on polyoxometalates have been reported that combine the advantages of homogeneous and heterogeneous catalysts. This review addresses central issues in green energy generation, the challenges in water oxidation catalyst development, and the possible uses of polyoxometalates in green energy science.

Similarities of artificial photosystems by ruthenium oxo complexes and native water splitting systems

The nature of chemical bonds of ruthenium(Ru)-quinine(Q) complexes, mononuclear [Ru(trpy)(3,5-t-Bu(2)Q)(OH(2))](ClO(4))(2) (trpy = 2,2':6',2''-terpyridine, 3,5-di-tert-butyl-1,2-benzoquinone) (1), and binuclear [Ru(2)(btpyan)(3,6-di-Bu(2)Q)(2)(OH(2))](2+) (btpyan = 1,8-bis(2,2':6',2''-terpyrid-4'-yl)anthracene, 3,6-t-Bu(2)Q = 3,6-di-tert-butyl-1,2-benzoquinone) (2), has been investigated by broken-symmetry (BS) hybrid density functional (DFT) methods. BS DFT computations for the Ru complexes have elucidated that the closed-shell structure (2b) Ru(II)-Q complex is less stable than the open-shell structure (2bb) consisting of Ru(III) and semiquinone (SQ) radical fragments. These computations have also elucidated eight different electronic and spin structures of tetraradical intermediates that may be generated in the course of water splitting reaction. The Heisenberg spin Hamiltonian model for these species has been derived to elucidate six different effective exchange interactions (J) for four spin systems. Six J values have been determined using total energies of the eight (or seven) BS solutions for different spin configurations. The natural orbital analyses of these BS DFT solutions have also been performed in order to obtain natural orbitals and their occupation numbers, which are useful for the lucid understanding of the nature of chemical bonds of the Ru complexes. Implications of the computational results are discussed in relation to the proposed reaction mechanisms of water splitting reaction in artificial photosynthesis systems and the similarity between artificial and native water splitting systems.