Ruthenium Oxo Complexes Research Articles

C-H Bond Activation by a Seven-Coordinate Bipyridine-Bipyrazole Ruthenium(IV) Oxo Complex.

Ruthenium-oxo species with high coordination numbers have long been proposed as active intermediates in catalytic oxidation chemistry. By employing a tetradentate bipyridine-bipyrazole ligand, we herein reported the synthesis of a seven-coordinate (CN7) ruthenium(IV) oxo complex, [RuIV(tpz)(pic)2(O)]2+ (RuIVO) (tpz = 6,6'-di(1H-pyrazol-1-yl)-2,2'-bipyridine, pic = 4-picoline), which exhibits high activity toward the oxidation of alkylaromatic hydrocarbons. The large kinetic isotope effects (KIE) for the oxidation of DHA/DHA-d4 (KIE = 10.3 ± 0.1) and xanthene/xanthene-d2 (KIE = 17.2 ± 0.1), as well as the linear relationship between log (rate constants) and bond dissociation energies of alkylaromatics, confirmed a mechanism of hydrogen atom abstraction.

Water co-catalysis in aerobic olefin epoxidation mediated by ruthenium oxo complexes.

We report the development of a versatile Ru-porphyrin catalyst system which performs the aerobic epoxidation of aromatic and aliphatic (internal) alkenes under mild conditions, with product yields of up to 95% and turnover numbers (TON) up to 300. Water is shown to play a crucial role in the reaction, significantly increasing catalyst efficiency and substrate scope. Detailed mechanistic investigations employing both computational studies and a range of experimental techniques revealed that water activates the RuVI di-oxo complex for alkene epoxidation via hydrogen bonding, stabilises the RuIV mono-oxo intermediate, and is involved in the regeneration of the RuVI di-oxo complex leading to oxygen atom exchange. Distinct kinetics are obtained in the presence of water, and side reactions involved in catalyst deactivation have been identified.

High-valent nonheme Fe(IV)O/Ru(IV)O complexes catalyze C-H activation reactivity and hydrogen tunneling: a comparative DFT investigation.

A comprehensive density functional theory investigation has been presented towards the comparison of the C-H activation reactivity between high-valent iron-oxo and ruthenium-oxo complexes. A total of four compounds, e.g., [Ru(IV)O(tpy-dcbpy)] (1), [Fe(IV)O(tpy-dcbpy)] (1'), [Ru(IV)O(TMCS)] (2), and [Fe(IV)O(TMCS)] (2'), have been considered for this investigation. The macrocyclic ligand framework tpy(dcbpy) implies tpy = 2,2':6',2''-terpyridine, dcbpy = 5,5'-dicarboxy-2,2'-bipyridine, and TMCS is TMC with an axially tethered -SCH2CH2 group. Compounds 1 and 2' are experimentally synthesized standard complexes with Ru and Fe, whereas compounds 1' and 2 were considered to keep the macrocycle intact when switching the central metal atom. Three reactants including benzyl alcohol, ethyl benzene, and dihydroanthracene were selected as substrates for C-H activation. It is noteworthy to mention that Fe(IV)O complexes exhibit higher reactivity than those of their Ru(IV)O counterparts. Furthermore, regardless of the central metal, the complex featuring a tpy-dcbpy macrocycle demonstrates higher reactivity than that of TMCS. Here, a thorough analysis of the reactivity-controlling characteristics-such as spin state, steric factor, distortion energy, energy of the electron acceptor orbital, and quantum mechanical tunneling-was conducted. Fe(IV)O exhibits the exchanged enhanced two-state-reactivity with the quintet reactive state, whereas Ru(IV)O has only a triplet reactive state. Both the distortion energy and acceptor orbital energy are low in the case of Fe(IV)O supporting its higher reactivity. All the investigated C-H activation processes involve a significant contribution from hydrogen tunneling, which is more pronounced in the case of Ru, although it cannot alter the reactivity pattern. Furthermore, it has also been found that, independent of the central metal, aliphatic hydroxylation is always preferable to aromatic hydroxylation. Overall, this work is successful in establishing and investigating the cause of enzymes' natural preference for Fe over Ru as a cofactor for C-H activation enzymes.

Proton-Coupled Electron Transfer at the Surface of Polyoxovanadate-Alkoxide Clusters.

ConspectusProton-coupled electron transfer (PCET) is a fundamental process involved in all areas of chemistry, with relevance to biological transformations, catalysis, and emergent energy storage and conversion technologies. Early observations of PCET were reported by Meyer and co-workers in 1981 while investigating the proton dependence of reduction of a molecular ruthenium oxo complex. Since that time, this conceptual framework has grown to encompass an enormous scope of charge transfer and compensation reactions. In this Account, we will discuss ongoing efforts in the Matson Laboratory to understand the fundamental thermodynamics and kinetics of PCET processes at the surface of a series of Lindqvist-type polyoxovanadate clusters. This project aims to provide atomistic resolution of net H atom uptake and transfer at the surfaces of transition-metal oxide materials.First, we discuss our efforts aimed at understanding PCET at metal oxide surfaces using the Lindqvist-type polyoxovanadate-alkoxide (POV-alkoxide) cluster [nBu4N]2[V6O13(TRIOLNO2)2]. These clusters reversibly bind H atom equivalents at bridging oxide sites, mirroring the proposed uptake and release of e-/H+ pairs at transition-metal oxide surfaces. Summarized results include the measurement of bond dissociation free energies of surface hydroxide moieties (BDFE(O-H)) as well as mechanistic analyses that verify concerted proton electron transfer as the operative pathway for PCET at the surface of POV-alkoxide clusters.Next, we discuss net proton and H atom uptake at the surface of reduced variants of the Lindqvist-type POV-alkoxide cluster, [V6O7(OR)12]n (R = Me, Et; n = -2, -1, 0, + 1). In the case of these low-valent POV-alkoxide clusters, nucleophilic bridging sites are kinetically inhibited by functionalization of the cluster surface with organic ligands. This molecular modification enables site-selectivity in proton and H atom uptake to terminal oxide sites. The impact of reaction site and cluster electronics on reaction driving force of PCET is explored, with core electron density playing a critical role in dictating thermodynamics of H atom uptake and transfer. Additional work described here contrasts the kinetics of PCET at terminal oxide sites to the reactivity observed at bridging oxides in POV-alkoxide clusters.Overall, this Account summarizes our foundational knowledge regarding the assessment of PCET reactivity at the surfaces of molecular metal oxides. Drawing analogies between POV-alkoxide clusters and nanoscopic metal oxide materials provide design principles for the advancement of materials applications with atomic precision. These complexes are additionally highlighted as tunable redox mediators in their own right; our studies demonstrate how cluster surface reactivities can be optimized by modifying electronic structure and surface functionalities.

Unexpected Promotional Effects of Alkyl‐Tailed Ligands and Anions on the Electrochemical Generation of Ruthenium(IV)‐Oxo Complexes

AbstractElectro‐generation of RuIV=O species from the RuII‐aqua complex is a crucial step to excel the electrocatalytic water oxidation performance of ruthenium oxo complexes. We report herein the synthesis and X‐ray crystal structural characterizations of new RuII‐aqua complexes containing N‐substituted 2,2′‐dipyridylamine ligands (L) tagged with an alkyl chain of various lengths, [Ru(tpy)(L)(OH2)]2+. Cyclic voltammetric analyses show that the length of the alkyl chain exerts great influence on the electro‐generation of RuIV=O species. The L with a longer alkyl chain in an acidic aqueous medium promotes the conversion of RuIII−OH to RuIV=O efficiently. Surprisingly, this conversion can be further enhanced by the presence of perchlorate anions. Chronocoulometric data reveal that the alkyl chain on L promotes the adsorption of the ruthenium complex on the electrode surface. Bulk electrolysis results indicate that the [Ru(tpy)(dppa)(OH2)]2+ (dppa=(2,2′‐dipyridyl)‐n‐propylamine) gives the most active electrocatalytic water oxidation activity among the ruthenium‐aqua complexes investigated in this study.

Li4Ru2OCl10·10H2O: crystal structure, magnetic properties and bonding interactions in ruthenium-oxo complexes.

The results of the structural determination, magnetic characterization, and theoretical calculations of a new ruthenium-oxo complex, Li4[Ru2OCl10]·10H2O, are presented. Single crystals were grown using solvent methods and the crystal structure was characterized by single crystal X-ray diffraction. Li4[Ru2OCl10]·10H2O crystallizes into a low-symmetry triclinic structure (P1) due to the much smaller Li+ cation compared to K+ cation in the tetragonal complex K4[Ru2OCl10]·H2O. The X-ray photoelectron spectra confirm only the single valent Ru4+ in Li4[Ru2OCl10]·10H2O even though two distinct Ru sites exist in the crystal structure. Magnetic measurements reveal the diamagnetic property of Li4[Ru2OCl10]·10H2O with unpaired electrons existing on Ru4+. Furthermore, the molecular orbital analysis matches well with the observed UV and magnetic measurements.

High-valent ruthenium(IV)-oxo complex stabilized mesoporous carbon (graphitized)/nafion modified electrocatalyst for methanol oxidation reaction in neutral pH

Abstract Owing to its importance in the fuel cell, biofuel and bioinorganic chemistry, since 1981, there has been an immense interest in the development of surface-confined ruthenium-oxo complex and to utilize it as a model bio-mimicking system for oxidation of water and alcohol reactions. In this connection, ruthenium-oxo complex has been attached on solid-surfaces by various chemical approaches. Indeed, the preparation of stable and fouling-free surface-confined ruthenium-complex system is a challenging task. Herein, we report a new methodology based on a modification of [Ru(tpy)(DMSO)Cl2]-Nafion (Nf) precursor, wherein, tpy = 2,2′:6′,2″-terpyridine ligand, system on a graphitized mesoporous carbon (GMC) modified electrode followed by electrochemical treatment for in-situ conversion to a high-valent ruthenium oxo complex electrode, designated as GCE/GMC@[(tpy)RuIV/III=O]-Nf, in pH 7 phosphate buffer solution. It showed a stable and well-defined redox peak at a standard electrode potential, Eo’ = 0.780 V vs Ag/AgCl with a surface-excess value, 5.5 × 10−9 mol.cm−2. Some of the kinetic parameters such as transfer coefficient, α and heterogeneous electron-transfer rate constant, ks values were evaluated as 0.54 and 2.43 s−1 respectively for the redox couple. Collective physicochemical characterizations of the surface-confined system by TEM, Raman, UV–Vis and FTIR spectroscopic techniques and ESI-MS analysis, it has been revealed that [RuIII(tpy)(DMSO)(Cl)(OH)]+ complex is surface-confined on the electrode via π-π interaction, pore-encapsulation, and ionic interaction and further transformed to [(tpy)(DMSO)RuIVO]2+ like complex at an applied bias potential, 0.8 V vs Ag/AgCl. This new surface-confined electrode showed high efficient and enzyme-free electrocatalytic oxidation of methanol with a current sensitivity of 39 μA mM−1 in a neutral pH solution.

X-ray Photoelectron Fingerprints of High-Valence Ruthenium-Oxo Complexes along the Oxidation Reaction Pathway in an Aqueous Environment.

Recent advances in operando-synchrotron-based X-ray techniques are making it possible to address fundamental questions related to complex proton-coupled electron transfer reactions, for instance, the electrocatalytic water splitting process. However, it is still a grand challenge to assess the ability of the different techniques to characterize the relevant intermediates, with minimal interference on the reaction mechanism. To this end, we have developed a novel methodology employing X-ray photoelectron spectroscopy (XPS) in connection with the liquid-jet approach to probe the electrochemical properties of a model electrocatalyst, [RuII(bpy)2(py)(OH2)]2+, in an aqueous environment. There is a unique fingerprint of the extremely important higher-valence ruthenium-oxo species in the XPS spectra along the oxidation reaction pathway. Furthermore, a sequential method combining quantum mechanics and molecular mechanics is used to illuminate the underlying physical chemistry of such systems. This study provides the basis for the future development of in-operando XPS techniques for water oxidation reactions.

Cis-Oxoruthenium complexes supported by chiral tetradentate amine (N4) ligands for hydrocarbon oxidations.

We report the first examples of ruthenium complexes cis-[(N4)RuIIICl2]+ and cis-[(N4)RuII(OH2)2]2+ supported by chiral tetradentate amine ligands (N4), together with a high-valent cis-dioxo complex cis-[(N4)RuVI(O)2]2+ supported by the chiral N4 ligand mcp (mcp = N,N'-dimethyl-N,N'-bis(pyridin-2-ylmethyl)cyclohexane-1,2-diamine). The X-ray crystal structures of cis-[(mcp)RuIIICl2](ClO4) (1a), cis-[(Me2mcp)RuIIICl2]ClO4 (2a) and cis-[(pdp)RuIIICl2](ClO4) (3a) (Me2mcp = N,N'-dimethyl-N,N'-bis((6-methylpyridin-2-yl)methyl)cyclohexane-1,2-diamine, pdp = 1,1'-bis(pyridin-2-ylmethyl)-2,2'-bipyrrolidine)) show that the ligands coordinate to the ruthenium centre in a cis-α configuration. In aqueous solutions, proton-coupled electron-transfer redox couples were observed for cis-[(mcp)RuIII(O2CCF3)2]ClO4 (1b) and cis-[(pdp)RuIII(O3SCF3)2]CF3SO3 (3c'). Electrochemical analyses showed that the chemically/electrochemically generated cis-[(mcp)RuVI(O)2]2+ and cis-[(pdp)RuVI(O)2]2+ complexes are strong oxidants with E° = 1.11-1.13 V vs. SCE (at pH 1) and strong H-atom abstractors with DO-H = 90.1-90.8 kcal mol-1. The reaction of 1b or its (R,R)-mcp counterpart with excess (NH4)2[CeIV(NO3)6] (CAN) in aqueous medium afforded cis-[(mcp)RuVI(O)2](ClO4)2 (1e) or cis-[((R,R)-mcp)RuVI(O)2](ClO4)2 (1e*), respectively, a strong oxidant with E(RuVI/V) = 0.78 V (vs. Ag/AgNO3) in acetonitrile solution. Complex 1e oxidized various hydrocarbons, including cyclohexane, in acetonitrile at room temperature, affording alcohols and/or ketones in up to 66% yield. Stoichiometric oxidations of alkenes by 1e or 1e* in t BuOH/H2O (5 : 1 v/v) afforded diols and aldehydes in combined yields of up to 98%, with moderate enantioselectivity obtained for the reaction using 1e*. The cis-[(pdp)RuII(OH2)2]2+ (3c)-catalysed oxidation of saturated C-H bonds, including those of ethane and propane, with CAN as terminal oxidant was also demonstrated.

ChemInform Abstract: Characteristics and Reactivity of Ruthenium‐Oxo Complexes

AbstractReview: 240 refs.

Characteristics and reactivity of ruthenium-oxo complexes.

In this perspective, we have surveyed the synthetic procedure, characteristics, and reactivity of high-valent ruthenium-oxo complexes. The ruthenium-oxo complexes have served as ideal species to elucidate the characteristics of metal-oxo complexes in terms of not only geometrical and electronic structures but also oxidation reactivity and mechanisms of oxidation reactions. Due to the high stability and excellent reversibility of redox processes, ruthenium-oxo complexes have provided significant mechanistic insights into the oxidation of organic compounds including alcohols, alkenes, and alkanes and also water on the basis of detailed kinetic analysis.

Correction: Characteristics and reactivity of ruthenium-oxo complexes.

Correction for 'Characteristics and reactivity of ruthenium-oxo complexes' by Tomoya Ishizuka et al., Dalton Trans., 2016, 45, 16727-16750.

Where does the water go? A computational study on the reactivity of a ruthenium(V) oxo complex (bpc)(bpy)Ru(V)O.

Two possible reactive sites (the oxo site and the Ru site) for water on a high-valent ruthenium(V) oxo complex were examined. Our results suggest that the reaction on the ruthenium (via a seven coordinate intermediate) has both a lower barrier and a product with a lower free energy than the product of addition at the oxo.

Tri(pyridylmethyl)phosphine: The Elusive Congener of TPA Shows Surprisingly Different Coordination Behavior

Tri(pyridylmethyl)phosphine (TPPh), the remarkably elusive congener of tri(pyridylmethyl)amine (TPA), has been prepared, as well as the relative tri(N-methyl-pyridylamino)phosphine (TPAMP). The coordination properties of these new ligands have been evaluated for chromium(III), iron(II), and ruthenium(II) complexes and compared with the related TPA complexes. In all cases, a different coordination behavior has been observed whereby TPPh and TPAMP always act as tridentate ligands. A chromium(III) complex [Cr(TPPh)Cl3] has been prepared, which has shown low ethylene oligomerization activity. Octahedral low spin iron(II) complexes [Fe(TPPh)2](2+) and [Fe(TPAMP)2](2+) were obtained with two ligands bound to the metal center. Ruthenium(II) chloro complexes of TPA and TPPh undergo ligand exchange reactions in acetonitrile, and the ruthenium(II) complex [Ru(MeCN)2(TPA)](2+) can be oxidized by m-CPBA in acetonitrile to give a transient ruthenium(IV) oxo complex [Ru(O)(MeCN)(TPA)](2+). Attempts to generate high valent ruthenium(IV) oxo TPPh or TPAMP complexes could not be achieved, probably due to insufficient stabilization by these strong field ligands.

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.

Reply

Trust in experiments: As DFT calculations suggest that the hydrogen bonding of water molecules can stabilize a seven-coordinate ruthenium–oxo complex to exhibit a singlet spin state, experiments have shown that the novel seven-coordinate pentaganol-bipyramidal ruthenium–oxo complex has a singlet ground state. Nevertheless, it displays the same reactivity as an analogous triplet-state complex.

ChemInform Abstract: Ruthenium Oxo Complexes as Organic Oxidants

ChemInform Abstract: Ruthenium Oxo Complexes as Organic Oxidants

Characteristics of Mononuclear Ruthenium–Oxo Complexes Adjusted by Axial Ligand for the Catalysis of Oxygen-Transfer Reactions

Ruthenium(II) complexes, [Ru(babp)(dmso)L], consisting of tetradentate square-planar ligand (babp = 6,6′-bis(benzoylamino)-2,2′-bipyridinato) and two monodentate axial ones, DMSO and L (L = dmso or...

Stoichiometric oxidations of σ-bonds: Radical and possible non-radical pathways

Many transition metal complexes accomplish or catalyze the oxidation of C H, O H, and other σ-bonds. Under aerobic conditions, metal complexes typically modulate an autoxidation radical chain. In anaerobic reactions, a metal complex can be the reactive species that attacks the σ-bond, in many cases by abstracting a hydrogen atom from the substrate. Examples described here include the oxidation of alkylaromatic compounds by ruthenium oxo complexes and reactions of deprotonated iron(III) complexes. In general, these reactions occur with addition of H + to a ligand and e − to the metal center. Rate constants for such hydrogen-atom transfer reactions can, in many cases, be predicted by the Marcus cross relation. Autoxidation and metal-mediated radical mechanisms are so prevalent that proposals of non-radical oxidations of C H bonds carry a higher burden of proof. It is argued here that the oxidation of H 2 by OsO 4 occurs by a non-radical, [3 + 2] mechanism. OsO 4 oxidizes alkanes under similar aqueous conditions. For example, isobutane is oxidized to tert-butanol, and cyclohexane to adipate and succinate. The alkane oxidations do not have the hallmarks of a radical mechanism but sufficient questions remain that a radical pathway cannot be excluded at this time.

Electrocatalytic activity of a polypyridyl ruthenium-oxo complex covalently attached to a graphite felt electrode

The complex [RuIV(tpy)(bpy)(O)]2+ grafted to a graphite felt electrode shows electrocatalytic activity toward primary and secondary alcohols. Kinetic analyses highlights the contribution of the immobilized catalyst to the oxidation of benzenemethanol to the corresponding aldehyde, whereas the formation of benzoic acid from benzaldehyde is mostly due to a direct oxidation at the graphite electrode. In contrast to the catalyst in solution, the immobilized catalyst exhibits poor activity toward compounds containing C–H bonds adjacent to unsaturated bonds.