Standard Electrode Potentials for the Reduction of CO₂ to CO in Acetonitrile–Water Mixtures Determined Using a Generalized Method for Proton-Coupled Electron-Transfer Reactions

Standard Electrode Potentials for the Reduction of CO₂ to CO in Acetonitrile–Water Mixtures Determined Using a Generalized Method for Proton-Coupled Electron-Transfer Reaction

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. However, our fundamental understanding of the dissolution of an electrolyte in an IL is still rather limited. For example, the activity of a charged species has frequently been assumed to be unity without a clear experimental basis. In this study, we have discussed a standard component-based scheme for the dissolution of an electrolyte in an IL, supported by our observation of ideal Nernstian responses for the reduction of silver and ferrocenium salts in a representative IL, 1-ethyl-3-methylimidazolium bis­(trifluoromethanesulfonyl)­imide ([emim⁺]­[NTf₂^–] or [emim⁺]­[TFSI^–]). Using this scheme, which was also supported by temperature-dependent measurements with ILs having longer alkyl chains in the imidazolium ring, and the solubility of the IL in water, we established the concept of Gibbs transfer energies of “pseudo-single ions” from the IL to conventional neutral molecular solvents (water, acetonitrile, and methanol). This concept, which bridges component- and constituent-based energetics, utilizes an extrathermodynamic assumption, which itself was justified by experimental observations. These energies enable us to eliminate inner potential differences between the IL and molecular solvents (solvent–solvent interactions), that is, on a practical level, conditional liquid junction potential differences, so that we can discuss ion–solvent interactions independently. Specifically, we have examined the standard electrode potential of the ferrocenium/ferrocene redox couple, Fc⁺/Fc, and the absolute intrinsic standard chemical potential of a proton in [emim⁺]­[NTf₂^–], finding that the proton is more acidic in the IL than in water by 6.5 ± 0.6 units on the unified pH scale. These results strengthen the progress on the physical chemistry of ions in IL solvent systems on the basis of their activities, providing a rigorous thermodynamic framework

Thermodynamic Aspects of Electrocatalytic CO₂ Reduction in Acetonitrile and with an Ionic Liquid as Solvent or Electrolyte

Thermodynamic Aspects of Electrocatalytic CO₂ Reduction in Acetonitrile and with an Ionic Liquid as Solvent or Electrolyt

Thermodynamic and Kinetic Hydricity of Ruthenium(II) Hydride Complexes

Despite the fundamental importance of the hydricity of a transition metal hydride (Δ<i>G</i>_H^–^°(MH) for the reaction M–H → M⁺ + H^–) in a range of reactions important in catalysis and solar energy storage, ours (<i>J. Am. Chem. Soc.</i> <b>2009</b>, <i>131</i>, 2794) are the only values reported for water solvent, and there has been no basis for comparison of these with the wider range already determined for acetonitrile solvent, in particular. Accordingly, we have used a variety of approaches to determine hydricity values in acetonitrile of Ru­(II) hydride complexes previously studied in water. For [Ru­(η⁶-C₆Me₆)­(bpy)­H]⁺ (bpy = 2,2′-bipyridine), we used a thermodynamic cycle based on evaluation of the acidity of [Ru­(η⁶-C₆Me₆)­(bpy)­H]⁺ p<i>K</i>_a = 22.5 ± 0.1 and the [Ru­(η⁶-C₆Me₆)­(bpy)­(NCCH₃)_1/0]^2+/0 electrochemical potential (−1.22 V vs Fc⁺/Fc). For [Ru­(tpy)­(bpy)­H]⁺ (tpy = 2,2′:6′,2″-terpyridine) we utilized organic hydride ion acceptors (A⁺) of characterized hydricity derived from imidazolium cations and pyridinium cations, and determined <i>K</i> for the hydride transfer reaction, S + MH⁺ + A⁺ → M­(S)²⁺ + AH (S = CD₃CN, MH⁺ = [Ru­(tpy)­(bpy)­H]⁺), by ¹H NMR measurements. Equilibration of initially 7 mM solutions was slowon the time scale of a day or more. When <i>E</i>°(H⁺/H^–) is taken as 79.6 kcal/mol vs Fc⁺/Fc as a reference, the hydricities of [Ru­(η⁶-C₆Me₆)­(bpy)­H]⁺ and [Ru­(tpy)­(bpy)­H]⁺ were estimated as 54 ± 2 and 39 ± 3 kcal/mol, respectively, in acetonitrile to be compared with the values 31 and 22 kcal/mol, respectively, found for aqueous media. The p<i>K</i>_a estimated for [Ru­(tpy)­(bpy)­H]⁺ in acetonitrile is 32 ± 3. UV–vis spectroscopic studies of [Ru­(η⁶-C₆Me₆)­(bpy)]⁰ and [Ru­(tpy)­(bpy)]⁰ indicate that they contain reduced bpy and tpy ligands, respectively. These conclusions are supported by DFT electronic structure results. Comparison of the hydricity values for acetonitrile and water reveals a flattening or compression of the hydricity range upon transferring the hydride complexes to water

Hydride Reduction of NAD(P)⁺ Model Compounds with a Ru(II)–Hydrido Complex

In order to better understand the regioselective hydride transfer of metal hydrido complexes to NAD­(P)⁺ model compounds, reactions of [Ru­(tpy)­(bpy)­H]⁺ (<b>Ru-H</b>: tpy = 2,2′:6″,2″-terpyridine, bpy = 2,2′-bipyridine) with various substituent NAD­(P)⁺ model compounds were investigated in detail. All of the NAD­(P)⁺ model compounds accepted hydride from <b>Ru-H</b>, yielding 1:1 adducts, where the dihydro form(s) of the model compounds coordinated with the carbamoyl group to the Ru­(II) center of [Ru­(tpy)­(bpy)]²⁺, with very different reaction rates. Some reactions produced the adduct with only the 1,4-dihydro structure, whereas others produced a mixture of two adducts, with a 1,4- or 1,2-dihydro structure. In particular, temperature-dependent adduct formation kinetics studies provided important information on the transition state(s) of the hydride transfer reactions and factors for determining the regioselectivity. Most adducts were cleaved to the corresponding free dihydro product(s) with the same distribution of the regioisomers to the adduct(s)

HCC risk mutations in genotype C sequences across different disease phases.

<p>We checked the distribution of HBV genotype C HCC risk mutations among sequences from different disease phases (ASC, 18; CHB, 38; LC, 27; HCC, 144). Those mutations showed characteristics among different phases: 1) except three mutations (1479T, 1631T, and 1800C) all the other mutations pre-existed in ASC, among which 1383C and 1719T were most pronounced. More than 50% ASC possessed either one of these two mutations and 42% ASC possessed both; 2) the frequency of four mutations (1485T, 1631T, 1762T, and 1764A) showed an increasing trend accompanied with the disease progression though the changes among groups were not significant (Jonckheere-Terpstra trend test, <i>P</i> > 0.05); and 3) ratios of 4 mutations (1383C, 1479C, 1479T, and 1719T) fluctuated among different disease phases. No mutants were observed in the cases denoted by asterisks.</p

Flowchart for screening HBx sequences downloaded from an online global database.

<p>We downloaded HBx sequences from Hepatitis Virus Database (HVDB, <a href="http://s2as02.genes.nig.ac.jp/" target="_blank">http://s2as02.genes.nig.ac.jp/</a>). The version we exploited was DDBJ Rel. 95, containing 5956 HBx sequences in total. Sequences were then screened successively by attached information such as publication, origin, and diagnosis. Sequences enrolled should be 1) Full length HBV X sequence; 2) human sera origin; and 3) with diagnosis information and thus could be classified to non-HCC or HCC group. Finally 1115 full length HBx sequences (HCC, 161; and Non-HCC, 954) from 57 publications were extracted for further analyses.</p

Seven nucleotide mutations of HBx sequences were independent risk factors for genotype C HBV-related <b>HCC</b>.

<p>^aMutated nucleotides are shown in bold.</p><p>B cell epitope: region (aa positions 29–48); BH3-like motif: region (aa positions 116–132); Box α, region (nt1646-1668); C/EBP, CCAAT/enhancing binding protein, region (nt1643-1658); CP, core promoter, region (nt1613-1849); Enh2: enhancer 2, region (nt1636-1744); HNF3, hepatocyte nuclear factor 3, region (nt1713-1723); NRE, negative regulatory element, region (nt1611-1634); T-cell epitope: region (aa positions 116–127).</p><p>Seven nucleotide mutations of HBx sequences were independent risk factors for genotype C HBV-related <b>HCC</b>.</p

Electrocatalytic CO₂ Reduction with a Homogeneous Catalyst in Ionic Liquid: High Catalytic Activity at Low Overpotential

We describe a new strategy for enhancing the efficiency of electrocatalytic CO₂ reduction with a homogeneous catalyst, using a room-temperature ionic liquid as both the solvent and electrolyte. The electrochemical behavior of <i>fac</i>-ReCl­(2,2′-bipyridine)­(CO)₃ in neat 1-ethyl-3-methylimidazolium tetracyanoborate ([emim]­[TCB]) was compared with that in acetonitrile containing 0.1 M [Bu₄N]­[PF₆]. Two separate one-electron reductions occur in acetonitrile (−1.74 and −2.11 V vs Fc^+/0), with a modest catalytic current appearing at the second reduction wave under CO₂. However, in [emim]­[TCB], a two-electron reduction wave appears at −1.66 V, resulting in a ∼0.45 V lower overpotential for catalytic reduction of CO₂ to CO. Furthermore, the apparent CO₂ reduction rate constant, <i>k</i>_app, in [emim]­[TCB] exceeds that in acetonitrile by over one order of magnitude (<i>k</i>_app = 4000 vs 100 M^–1 s^–1) at 25 ± 3 °C. Supported by time-resolved infrared measurements, a mechanism is proposed in which an interaction between [emim]⁺ and the two-electron reduced catalyst results in rapid dissociation of chloride and a decrease in the activation energy for CO₂ reduction

Formation of η²‑Coordinated Dihydropyridine–Ruthenium(II) Complexes by Hydride Transfer from Ruthenium(II) to Pyridinium Cations

Reactions between various pyridinium cations with and without a −CF₃ substituent at the 3-position and [Ru­(tpy)­(bpy)­H]⁺ (tpy = 2,2′:6′,2″-terpyridine and bpy = 2,2′-bipyridine) were investigated in detail. The corresponding 1,4-dihydropyridines coordinating to a Ru­(II) complex in η² mode through a CC bond were quantitatively formed at the initial stage. The only exception observed was in the case of the 1-benzylpyridinium cation, where a mixture of two adducts with 1,4-dihydropyridine and 1,2-dihydropyridine was formed in the ratio 96:4. Cleavage of the Ru–(CC) bond proceeded at a slower rate in all reactions, giving the corresponding dihydropyridine and [Ru­(tpy)­(bpy)­(NCCH₃)]²⁺ when acetonitrile was used as a solvent. Kinetic activation parameters for the adduct formation indicated that the 1,4-regioselectivities were induced by formation of sterically constrained structures