Reductive Dynamic and Static Excited State Quenching of a Homoleptic Ruthenium Complex Bearing Aldehyde Groups.

A new homoleptic Ru polypyridyl complex bearing two aldehyde groups on each bipyridine ligand, [Ru(dab)](PF), where dab is 4,4'-dicarbaldehyde-2,2'-bipyridine, was synthesized, characterized, and utilized for iodide photo-oxidation studies. In acetonitrile (CHCN) solution, the complex displayed an intense metal-to-ligand charge transfer (MLCT) absorbance maximum at 475 nm (ε = 22,000 M cm) and an infrared (IR) band at 1712 cm assigned to the pendent aldehyde groups. Visible light excitation in air-saturated solution resulted in room temperature photoluminescence (PL) with a maximum at 675 nm, a quantum yield, ϕ = 0.

Reduction of CO to Methanol with Recyclable Organic Hydrides.

The reaction steps for the selective conversion of a transition metal carbonyl complex to a hydroxymethyl complex that releases methanol upon irradiation with visible light have been successfully quantified in acetonitrile solution with dihydrobenzimidazole organic hydride reductants. Dihydrobenzimidazole reductants have been shown to be inactive toward H generation in the presence of a wide range of proton sources and have been regenerated electrochemically or photochemically. Specifically, the reaction of -[Ru(bpy)(CO)] (bpy = 2,2'-bipyridine) with one equivalent of a dihydrobenzimidazole quantitatively yields a formyl complex, -[Ru(bpy)(CO)(CHO)], and the corresponding benzimidazolium on a seconds time scale.

Combined Effects of Hemicolligation and Ion Pairing on Reduction Potentials of Biphenyl Radical Cations.

Formal reduction potentials of highly oxidizing and short-lived radical cations of substituted biphenyls generated by pulse radiolysis in 1,2-dichloroethane (DCE) were measured using a redox equilibrium ladder method. The effect of halide ion-radical interactions on reduction potentials of biphenyls was examined by utilizing the ability of DCE to release Cl in the vicinity of the radical cation. The Hammett correlation of measured potentials across a range of over 700 mV shows saturation at high Hammett sigma values.

Lewis Acids and Electron-Withdrawing Ligands Accelerate CO Coordination to Dinuclear Cu Compounds.

A series of dinuclear molecular copper complexes were prepared and used to model the binding and Lewis acid stabilization of CO in heterogeneous copper CO reduction electrocatalysts. Experimental studies (including measurement of rate and equilibrium constants) and electronic structure calculations suggest that the key kinetic barrier for CO binding may be a σ-interaction between Cu and the incoming CO ligand. The rate of CO coordination can be increased upon the addition of Lewis acids or electron-withdrawing substituents on the ligand backbone.

Multi-Electron Transfer at H-Terminated p-Si Electrolyte Interfaces: Large Photovoltages under Inversion Conditions.

Photovoltages for hydrogen-terminated p-Si(111) in an acetonitrile electrolyte were quantified with methyl viologen [1,1'-(CH)-4,4'-bipyridinium](PF), abbreviated MV, and [Ru(bpy)](PF), where bpy is 2,2'-bipyridine, that respectively undergo two and three one-electron transfer reductions. The reduction potentials, °, of the two MV reductions occurred at energies within the forbidden bandgap, while the three [Ru(bpy)] reductions occurred within the continuum of conduction band states. Bandgap illumination resulted in reduction that was more positive than that measured with a degenerately doped n-Si demonstrative of a photovoltage, , that increased in the order MV (260 mV) < MV (400 mV) < Ru (530 mV) ∼ Ru (540 mV) ∼ Ru (550 mV).

A Dicationic -Re(bpy)(CO)Cl for CO Electroreduction at a Reduced Overpotential.

A dicationic Re bipyridine-type complex, -Re(6,6'-(2-((trimethylammonio)-methyl)phenyl)-2,2'-bipyridine )(CO)Cl hexafluorophosphate (), has been synthesized, and its electrochemical behavior under Ar and CO has been investigated. The presence of pendent tetra-alkylammonium cations induces an anodic shift in the electrocatalytic potential for CO reduction relative to structurally similar model complexes. The electrochemical mechanisms in anhydrous CHCN and in the presence of weak acids (water or trifluoroethanol) have been analyzed using cyclic voltammetry assisted by infrared spectroelectrochemistry and theoretical calculations.

Understanding the Role of Inter- and Intramolecular Promoters in Electro- and Photochemical CO Reduction Using Mn, Re, and Ru Catalysts.

Recycling of carbon dioxide to fuels and chemicals is a promising strategy for renewable energy storage. Carbon dioxide conversion can be achieved by (i) artificial photosynthesis using photoinduced electrons; (ii) electrolysis using electricity produced by photovoltaics; and (iii) thermal CO hydrogenation using renewable H. The focus of our group's research is on molecular catalysts, in particular coordination complexes of transition metals (e.

Structural and Electronic Influences on Rates of Tertpyridine-Amine Co -H Formation During Catalytic H Evolution in an Aqueous Environment.

In this work, the differences in catalytic performance for a series of Co hydrogen evolution catalysts with different pentadentate polypyridyl ligands (L), have been rationalized by examining elementary steps of the catalytic cycle using a combination of electrochemical and transient pulse radiolysis (PR) studies in aqueous solution. Solvolysis of the [Co -Cl] species results in the formation of [Co (κ -L)(OH )] . Further reduction produces [Co (κ -L)(OH )] , which undergoes a rate-limiting structural rearrangement to [Co (κ -L)] before being protonated to form [Co -H] .

Molecular Catalysts with Intramolecular Re-O Bond for Electrochemical Reduction of Carbon Dioxide.

A new Re bipyridine-type complex, namely, -Re(pmbpy)(CO)Cl (pmbpy = 4-phenyl-6-(2-hydroxy-phenyl)-2,2'-bipyridine), , carrying a single OH moiety as local proton source, has been synthesized, and its electrochemical behavior under Ar and under CO has been characterized. Two isomers of , namely, characterized by the proximity of Cl to OH and , are identified. The interconversion between and is clarified by DFT calculations, which reveal two transition states.

Unexpected Roles of Triethanolamine in the Photochemical Reduction of CO to Formate by Ruthenium Complexes.

A series of 4,4'-dimethyl-2,2'-bipyridyl ruthenium complexes with carbonyl ligands were prepared and studied using a combination of electrochemical and spectroscopic methods with infrared detection to provide structural information on reaction intermediates in the photochemical reduction of CO to formate in acetonitrile (CHCN). An unsaturated 5-coordinate intermediate was characterized, and the hydride-transfer step to CO from a singly reduced metal-hydride complex was observed with kinetic resolution. While triethanolamine (TEOA) was expected to act as a proton acceptor to ensure the sacrificial behavior of 1,3-dimethyl-2-phenyl-2,3-dihydro-1-benzo[]imidazole as an electron donor, time-resolved infrared measurements revealed that about 90% of the photogenerated one-electron reduced complexes undergo unproductive back electron transfer.

Self-Assembled Bilayers as an Anchoring Strategy: Catalysts, Chromophores, and Chromophore-Catalyst Assemblies.

Anchoring strategies for immobilization of molecular catalysts, chromophores, and chromophore-catalyst assemblies on electrode surfaces play an important role in solar energy conversion devices such as dye-sensitized solar cells and dye-sensitized photoelectrosynthesis cells. They are also important in interfacial studies with surface-bound molecules including electron-transfer dynamics and mechanistic studies related to small molecule activation catalysis. Significant progress has been made in this area, but many challenges remain in terms of stability, synthetic complexity, and versatility.

Hydrogen bonding between hydroxylic donors and MLCT-excited Ru(bpy)(bpz) complex: implications for photoinduced electron-proton transfer.

The equilibrium constants of the hydrogen bonding (HB) between hydroxylic donors, ROH, and an MLCT-excited Ru(bpy)(bpz) complex, 1(T), correlated with ROH empirical HB acidities, which could be used for evaluating the unimolecular rate constants of concerted electron-proton transfer within the H-bonded phenol-1(T) exciplexes.

Solvent-dependent transition from concerted electron-proton to proton transfer in photoinduced reactions between phenols and polypyridine Ru complexes with proton-accepting sites.

The bimolecular rate coefficients (kobsq) for quenching the metal-to-ligand charge transfer excited states of two Ru polypyridine complexes containing H-bond accepting sites by six p-substituted phenols exhibit abrupt deviations from the expected linear correlations of log kobsq with phenol's Hammett σp constant. This pattern is attributed to a transition of the quenching mechanism from a concerted electron-proton transfer (EPT) to a proton transfer (PT); the latter becomes predominant for the most acidic phenols in acetonitrile, but not in dichloromethane. This assertion is supported by a detailed thermochemical analysis, which also excludes the quenching pathways involving electron transfer from phenols with or without deprotonation of phenols to the solvent, either concerted or sequential.

Photocatalytic CO Reduction by Trigonal-Bipyramidal Cobalt(II) Polypyridyl Complexes: The Nature of Cobalt(I) and Cobalt(0) Complexes upon Their Reactions with CO, CO, or Proton.

The cobalt complexes CoL1(PF) (1; L1 = 2,6-bis[2-(2,2'-bipyridin-6'-yl)ethyl]pyridine) and CoL2(PF) (2; L2 = 2,6-bis[2-(4-methoxy-2,2'-bipyridin-6'-yl)ethyl]pyridine) were synthesized and used for photocatalytic CO reduction in acetonitrile. X-ray structures of complexes 1 and 2 reveal distorted trigonal-bipyramidal geometries with all nitrogen atoms of the ligand coordinated to the Co(II) center, in contrast to the common six-coordinate cobalt complexes with pentadentate polypyridine ligands, where a monodentate solvent completes the coordination sphere. Under electrochemical conditions, the catalytic current for CO reduction was observed near the Co(I/0) redox couple for both complexes 1 and 2 at E = -1.

Enhanced, robust light-driven H generation by gallium-doped titania nanoparticles.

The splitting of water into molecular hydrogen and oxygen with the use of renewable solar energy is considered one of the most promising routes to yield sustainable fuel. Herein, we report the H evolution performance of gallium doped TiO photocatalysts with varying degrees of Ga dopant. The gallium(iii) ions induced significant changes in the structural, textural and electronic properties of TiO nanoparticles, resulting in remarkably enhanced photocatalytic activity and good stability for H production.

Application of Pulse Radiolysis to Mechanistic Investigations of Catalysis Relevant to Artificial Photosynthesis.

Taking inspiration from natural photosystems, the goal of artificial photosynthesis is to harness solar energy to convert abundant materials, such as CO and H O, into solar fuels. Catalysts are required to ensure that the necessary redox half-reactions proceed in the most energy-efficient manner. It is therefore critical to gain a detailed mechanistic understanding of these catalytic reactions to develop new and improved catalysts.

Role of Hydrogen Bonding in Photoinduced Electron-Proton Transfer from Phenols to a Polypyridine Ru Complex with a Proton-Accepting Ligand.

Electron-proton transfer (EPT) from phenols to a triplet metal-to-ligand charge transfer (MLCT)-excited Ru polypyridine complex containing an uncoordinated nitrogen site, 1(T), can be described by a kinetic model that accounts for the H-bonding of 1(T) to phenol, 1(T) to solvent, and phenol to solvent. The latter plays a major role in the kinetic solvent effect and commonly precludes simultaneous determination of the EPT rate constant and 1(T)-phenol H-bonding constant. A number of these quantities previously reported for similar systems are shown to be in error due to inconsistent data analysis.

Three-dimensional ruthenium-doped TiO2 sea urchins for enhanced visible-light-responsive H2 production.

Three-dimensional (3D) monodispersed sea urchin-like Ru-doped rutile TiO2 hierarchical architectures composed of radially aligned, densely-packed TiO2 nanorods have been successfully synthesized via an acid-hydrothermal method at low temperature without the assistance of any structure-directing agent and post annealing treatment. The addition of a minuscule concentration of ruthenium dopants remarkably catalyzes the formation of the 3D urchin structure and drives the enhanced photocatalytic H2 production under visible light irradiation, not possible on undoped and bulk rutile TiO2. Increasing ruthenium doping dosage not only increases the surface area up to 166 m(2) g(-1) but also induces enhanced photoresponse in the regime of visible and near infrared light.

Mechanistic studies of hydrogen evolution in aqueous solution catalyzed by a tertpyridine-amine cobalt complex.

The ability of cobalt-based transition metal complexes to catalyze electrochemical proton reduction to produce molecular hydrogen has resulted in a large number of mechanistic studies involving various cobalt complexes. While the basic mechanism of proton reduction promoted by cobalt species is well-understood, the reactivity of certain reaction intermediates, such as Co(I) and Co(III)-H, is still relatively unknown owing to their transient nature, especially in aqueous media. In this work we investigate the properties of intermediates produced during catalytic proton reduction in aqueous solutions promoted by the [(DPA-Bpy)Co(OH2)](n+) (DPA-Bpy = N,N-bis(2-pyridinylmethyl)-2,20-bipyridine-6-methanamine) complex ([Co(L)(OH2)](n+) where L is the pentadentate DPA-Bpy ligand or [Co(OH2)](n+) as a shorthand).

Efficient water oxidation with organometallic iridium complexes as precatalysts.

Catalytic water oxidation has been investigated using five iridium complexes as precatalysts and NaIO4 as an oxidant at various pH conditions. An increase in the activity of all complexes was observed with increasing pH. A detailed analysis of spectroscopic data together with O2-evolution experiments using Cp*Ir(6,6′-dihydroxy-2,2′-bipyridine)(OH2)(2+) as a precatalyst indicate that the high catalytic activity is closely connected with transient species (A) that exhibits an absorption band at λmax 590 nm.

Reactivity of a fac-ReCl(α-diimine)(CO)3 complex with an NAD+ model ligand toward CO2 reduction.

The reactivity of a rhenium complex containing an NAD(+) model ligand was examined toward photochemical formation of the corresponding NADH-like dihydro form of the complex and electrochemical CO2 reduction. The hydricity of the NADH-like complex was estimated by a thermodynamic cycle and reaction with Ph3C(+).

Enabling light-driven water oxidation via a low-energy RuIV=O intermediate.

The discovery of catalysts capable of driving water oxidation at relatively low overpotential is a key challenge for efficient photoinduced water oxidation. The mononuclear Ru(II) polypyridyl complex (1) [Ru(NPM)(H2O)(pic)2](2+) (NPM = 4-tert-butyl-2,6-di-(1',8'-naphthyrid-2'-yl)-pyridine, pic = 4-picoline) has been examined as a catalyst for visible-light-driven water oxidation in a three-component homogeneous system containing [Ru(bpy)3](2+) as a photosensitizer, persulfate as a sacrificial electron acceptor and catalyst 1. In contrast to the well-established water oxidation mechanism via the nucleophilic attack of a water molecule on the high-energy [Ru(V)=O](3+) species, a lower-energy "direct pathway" for O-O bond formation via a [Ru(IV)=O](2+) intermediate was proposed for the first time for the catalyst 1 (Polyansky et al.