Fast Catalysis at Low Overpotential: Designing Efficient Dicationic Re(bpy)(CO)I Electrocatalysts for CO Reduction.

We report a series of isomeric, dicationic Re(bpy)(CO)I complexes with bpy (2,2'-bipyridine) modified by two phenyl-CH-(NMe) pendants with cations located at variable distances from the active site for electrocatalytic CO reduction in CHCN/2.8 M HO. The position of the cationic groups dramatically increases the rate of catalysis by ∼800-fold, from 1.

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.

Reactivity of radiolytically and photochemically generated tertiary amine radicals towards a CO2 reduction catalyst.

Homogeneous solar fuels photocatalytic systems often require several additives in solution with the catalyst to operate, such as a photosensitizer (PS), Brønsted acid/base, and a sacrificial electron donor (SED). Tertiary amines, in particular triethylamine (TEA) and triethanolamine (TEOA), are ubiquitously deployed in photocatalysis applications as SEDs and are capable of reductively quenching the PS's excited state. Upon oxidation, TEA and TEOA form TEA•+ and TEOA•+ radical cations, respectively, which decay by proton transfer to generate redox non-innocent transient radicals, TEA• and TEOA•, respectively, with redox potentials that allow them to participate in an additional electron transfer step, thus resulting in net one-photon/two-electron donation.

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.

Mechanistic roles of metal- and ligand-protonated species in hydrogen evolution with [Cp*Rh] complexes.

Protonation reactions involving organometallic complexes are ubiquitous in redox chemistry and often result in the generation of reactive metal hydrides. However, some organometallic species supported by η-pentamethylcyclopentadienyl (Cp*) ligands have recently been shown to undergo ligand-centered protonation by direct proton transfer from acids or tautomerization of metal hydrides, resulting in the generation of complexes bearing the uncommon η-pentamethylcyclopentadiene (Cp*H) ligand. Here, time-resolved pulse radiolysis (PR) and stopped-flow spectroscopic studies have been applied to examine the kinetics and atomistic details involved in the elementary electron- and proton-transfer steps leading to complexes ligated by Cp*H, using Cp*Rh(bpy) as a molecular model (where bpy is 2,2'-bipyridyl).

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).

Steric and Lewis Basicity Influence of the Second Coordination Sphere on Electrocatalytic CO Reduction by Manganese Bipyridyl Complexes.

This study aims to provide a greater insight into the balance between steric (bpy vs (Ph)bpy vs mesbpy ligands) and Lewis basic ((Ph)bpy vs (MeOPh)bpy vs (MeSPh)bpy ligands) influence on the efficiencies of the protonation-first vs reduction-first CO reduction mechanisms with [Mn(Rbpy)(CO)(CHCN)] precatalysts, and on their respective transition-state geometries/energies for rate-determining C-OH bond cleavage toward CO evolution. The presence of only modest steric bulk at the 6,6'-diphenyl-2,2'-bipyridyl ((Ph)bpy) ligand has here allowed unique insight into the mechanism of catalyst activation and CO binding by navigating a perfect medium between the nonsterically encumbered bpy-based and the highly sterically encumbered mesbpy-based precatalysts. Cyclic voltammetry conducted in CO-saturated electrolyte for the (Ph)bpy-based precatalyst confirms that CO binding occurs at the two-electron-reduced activated catalyst in the absence of an excess proton source, in contrast to prior assumptions that all manganese catalysts require a strong acid for CO binding.

Coupling Pulse Radiolysis with Nanosecond Time-Resolved Step-Scan Fourier Transform Infrared Spectroscopy: Broadband Mid-Infrared Detection of Radiolytically Generated Transients.

We describe the first implementation of broadband, nanosecond time-resolved step-scan Fourier transform infrared (S-FT-IR) spectroscopy at a pulse radiolysis facility. This new technique allows the rapid acquisition of nano- to microsecond time-resolved infrared (TRIR) spectra of transient species generated by pulse radiolysis of liquid samples at a pulsed electron accelerator. Wide regions of the mid-infrared can be probed in a single experiment, which often takes < 20-30 min to complete.

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.

Solvated Electron in Acetonitrile: Radiation Yield, Absorption Spectrum, and Equilibrium between Cavity- and Solvent-Localized States.

The equilibrium between a solvent cavity-localized electron, e, and a dimeric solvent anion, (CHCN), which are the two lowest energy states of the solvated electron in acetonitrile, has been investigated by pulse radiolysis at 233-353 K. The enthalpy and entropy for the e to (CHCN) conversion amount to -11.2 ± 0.

IR linewidth and intensity amplifications of nitrile vibrations report nuclear-electronic couplings and associated structural heterogeneity in radical anions.

Conjugated molecular chains have the potential to act as "molecular wires" that can be employed in a variety of technologies, including catalysis, molecular electronics, and quantum information technologies. Their successful application relies on a detailed understanding of the factors governing the electronic energy landscape and the dynamics of electrons in such molecules. We can gain insights into the energetics and dynamics of charges in conjugated molecules using time-resolved infrared (TRIR) detection combined with pulse radiolysis.

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.

Vibrational Spectroscopy Reveals Effects of Electron Push-Pull and Solvent Polarity on Electron Delocalization in Radical Anions of Donor-Acceptor Compounds.

The nature of excess electrons has been studied in donor-acceptor (D-A) compounds based on substituted triarylamines and a nitrile-functionalized fluorene by changing the substituents on the triarylamines and also the solvent polarity. We observed that both electron push-pull capability at the distant location in the amine donor unit and solvation in solvents of varying polarity significantly affect the nitrile ν(C≡N) vibrations of the fluorene acceptor unit in radical anions of these D-A compounds. Quantum calculations show that the push-pull capability translates the position of an excess electron while keeping its width relatively constant.

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.

An Investigation of Electrocatalytic CO Reduction Using a Manganese Tricarbonyl Biquinoline Complex.

The subject of this study [-Mn(bqn)(CO)(CHCN)] (bqn = 2,2'-biquinoline), is of particular interest because the bqn ligand exhibits both steric and electronic influence over the fundamental redox properties of the complex and, consequently, its related catalytic properties with respect to the activation of CO. While not a particularly efficient catalyst for CO to CO conversion, generation and activity measurements of the [-Mn(bqn)(CO)] active catalyst allows for a better understanding of ligand design at the Mn center. By making direct comparisons to the related 2,2'-bipyridyl (bpy), 1,10-phenanthroline (phen), and 2,9-dimethyl-1,10-phenanthroline (dmphen) ligands via a combination of voltammetry, infrared spectroelectrochemistry, controlled potential electrolysis and computational analysis, the role of steric vs.

Nitrile Vibration Reports Induced Electric Field and Delocalization of Electron in the Charge-Transfer State of Aryl Nitriles.

An electric field is created upon photoinduced charge separation in electron donor-acceptor (D-A) molecules. The photophysics of a prototypical D-A molecule, 4-(dimethylamino)-benzonitrile (DMABN), has been under extensive investigation for decades. Here, by using the framework of the vibrational Stark effect (VSE), we show that the nitrile vibration quantifies a significant induced electric field in the intramolecular charge-transfer state of DMABN.

Investigation of excited state, reductive quenching, and intramolecular electron transfer of Ru(ii)-Re(i) supramolecular photocatalysts for CO reduction using time-resolved IR measurements.

Supramolecular photocatalysts in which Ru(ii) photosensitizer and Re(i) catalyst units are connected to each other by an ethylene linker are among the best known, most effective and durable photocatalytic systems for CO reduction. In this paper we report, for the first time, time-resolved infrared (TRIR) spectra of three of these binuclear complexes to uncover why the catalysts function so efficiently. Selective excitation of the Ru unit with a 532 nm laser pulse induces slow intramolecular electron transfer from the MLCT excited state of the Ru unit to the Re unit, with rate constants of (1.

Radiolytic formation of the carbon dioxide radical anion in acetonitrile revealed by transient IR spectroscopy.

The solvated electron in CH3CN is scavenged by CO2 with a rate constant of 3.2 × 1010 M-1 s-1 to produce the carbon dioxide radical anion (CO2˙-), a strong and versatile reductant. Using pulse radiolysis with time-resolved IR detection, this radical is unambiguously identified by its absorption band at 1650 cm-1 corresponding to the antisymmetric CO2˙- stretch.

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.

Probing Intermolecular Electron Delocalization in Dimer Radical Anions by Vibrational Spectroscopy.

Delocalization of charges is one of the factors controlling charge transport in conjugated molecules. It is considered to play an important role in the performance of a wide range of molecular technologies, including organic solar cells and organic electronics. Dimerization reactions are well-suited as a model to investigate intermolecular spatial delocalization of charges.

Turning on the Protonation-First Pathway for Electrocatalytic CO Reduction by Manganese Bipyridyl Tricarbonyl Complexes.

Electrocatalytic reduction of CO to CO is reported for the complex, {fac-Mn([(MeO)Ph]bpy)(CO)(CHCN)}(OTf), containing four pendant methoxy groups, where [(MeO)Ph]bpy = 6,6'-bis(2,6-dimethoxyphenyl)-2,2'-bipyridine. In addition to a steric influence similar to that previously established [Sampson, M. D.

Experimental Insight into the Thermodynamics of the Dissolution of Electrolytes in Room-Temperature Ionic Liquids: From the Mass Action Law to the Absolute Standard Chemical Potential of a Proton.

Room-temperature ionic liquids (ILs) are a class of nonaqueous solvents that have expanded the realm of modern chemistry, drawing increasing interest over the last few decades, not only in terms of their own unique physical chemistry but also in many applications including organic synthesis, electrochemistry, and biological systems, wherein charged solutes (i.e., electrolytes) often play vital roles.