Apr 18, 2014

Solid-State Magnetic Switching Triggered by Proton-Coupled Electron-Transfer Assisted by Long-Distance Proton-Alkali Cation Transport

Publication Date (Web): April 14, 2014 (Communication)
DOI: 10.1021/ja502294x
 
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Acidity of water molecules coordinated to Co ions in CoFe Prussian blue analogues (PBA) has been used to reversibly activate the CoIIIFeII ↔ CoIIFeIII electron transfer. The study of the structure and the electronic structure shows that the process implies an original PCET reaction between a solid-state porous coordination polymer and hydroxide ions in solution. The PCET reaction spreads throughout the solid network thanks to a long-range H+ and Rb+ transport within the pore channels of PBA taking advantage of the hydrogen-bonding network of zeolitic water molecules acting as proton wires.
 

Apr 15, 2014

Kinetics and Thermodynamics of H–/H•/H+ Transfer from a Rhodium(III) Hydride

Publication Date (Web): March 25, 2014 (Article)
DOI: 10.1021/ja412309j
 
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The thermodynamics and kinetics of all three cleavage modes for Rh–H, the transfer of H, H+, or H•, have been studied for the Rh(III) hydride complex Cp*Rh(2-(2-pyridyl)phenyl)H (1a). The thermodynamic hydricity, ΔG°H, for 1a has been measured (49.5(1) kcal/mol) by heterolytic cleavage of H2 with Et3N in CH3CN. The transfer of H from 1a to 1-(1-phenylethylidene)pyrrolidinium is remarkably fast (kH = 3.5(1) × 105 M–1 s–1), making 1a a very efficient catalyst for the ionic hydrogenation of iminium cations. The pKa of 1a in CH3CN has been measured as 30.3(2) with (tert-butylimino)tris(pyrrolidino)phosphorane (12), and the rate constant for H+ transfer from 1a to 12 has been estimated (kH+ = 5(1) × 10–4 M–1 s–1) from the half-life of the equilibration. Thus, 1a is a poor H+ donor both thermodynamically and kinetically. However, 1a transfers H to TEMPO smoothly, forming a stable Rh(II) radical Cp*Rh(2-(2-pyridyl)phenyl) (14a) that can activate H2 at room temperature and 1 atm. The metalloradical 14a has a g value of 2.0704 and undergoes reversible one-electron reduction at −1.85 V vs Fc+/Fc in benzonitrile, implying a bond-dissociation enthalpy for the Rh–H bond of 1a of 58.2(3) kcal/mol—among the weakest Rh(III)–H bonds reported. The transfer of H from 1a to Ar3C• (Ar = p-tBuC6H4) is fast, with kH• = 1.17(3) × 103 M–1 s–1. Thus, 1a is a good H and H• donor but a poor H+ donor, a combination that reflects the high energy of the Rh(I) anion [Cp*Rh(2-(2-pyridyl)phenyl)].
 

Mechanistic Aspects of Hydration of Guanine Radical Cations in DNA

Mechanistic Aspects of Hydration of Guanine Radical Cations in DNA

Publication Date (Web): April 1, 2014 (Article)
DOI: 10.1021/ja412471u


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Abstract Image
The mechanistic aspects of hydration of guanine radical cations, G•+ in double- and single-stranded oligonucleotides were investigated by direct time-resolved spectroscopic monitoring methods. The G•+ radical one-electron oxidation products were generated by SO4•– radical anions derived from the photolysis of S2O82– anions by 308 nm laser pulses. In neutral aqueous solutions (pH 7.0), after the complete decay of SO4•– radicals (5 μs after the actinic laser flash) the transient absorbance of neutral guanine radicals, G(-H) with maximum at 312 nm, is dominant. The kinetics of decay of G(-H) radicals depend strongly on the DNA secondary structure. In double-stranded DNA, the G(-H) decay is biphasic with one component decaying with a lifetime of 2.2 ms and the other with a lifetime of 0.18 s. By contrast, in single-stranded DNA the G(-H) radicals decay monophasically with a 0.28 s lifetime. The ms decay component in double-stranded DNA is correlated with the enhancement of 8-oxo-7,8-dihydroguanine (8-oxoG) yields which are 7 greater than in single-stranded DNA. In double-stranded DNA, it is proposed that the G(-H) radicals retain radical cation character by sharing the N1-proton with the N3-site of C in the [G•+:C] base pair. This [G(-H):H+C G•+:C] equilibrium allows for the hydration of G•+ followed by formation of 8-oxoG. By contrast, in single-stranded DNA, deprotonation of G•+ and the irreversible escape of the proton into the aqueous phase competes more effectively with the hydration mechanism, thus diminishing the yield of 8-oxoG, as observed experimentally.

Apr 12, 2014

Self-Assembly of Tetrahedral CdSe Nanocrystals: Effective “Patchiness” via Anisotropic Steric Interaction

Publication Date (Web): March 22, 2014 (Communication)
DOI: 10.1021/ja501596z


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Controlling the spontaneous organization of nanoscale objects remains a fundamental challenge of materials design. Here we present the first characterization of self-assembled superlattices (SLs) comprised of tetrahedral nanocrystal (NCs). We observe self-assembly of CdSe nanotetrahedra into an open structure (estimated space-filling fraction φ ≈ 0.59) which has not been anticipated by many recent theoretical studies and simulations of tetrahedron packings. This finding highlights a gap in the understanding of the hierarchy of energy scales acting on colloidal NCs during self-assembly. We propose a strong dependence of ligand–ligand interaction potential on NC surface curvature. This effect favors spatial proximity of vertices in the dense colloidal crystal and may be considered an emergent “patchiness” acting through chemically identical ligand molecules.

Apr 10, 2014

A Student-Made Silver–Silver Chloride Reference Electrode for the General Chemistry Laboratory: 10 min Preparation

http://pubs.acs.org/doi/abs/10.1021/ed400722e

A Student-Made Silver–Silver Chloride Reference Electrode for the General Chemistry Laboratory: 10 min Preparation

Department of Chemistry and Biochemistry, Ohio University, Athens, Ohio 45701, United States
J. Chem. Educ., Article ASAP
DOI: 10.1021/ed400722e
Publication Date (Web): April 10, 2014
Copyright © 2014 The American Chemical Society and Division of Chemical Education, Inc.
*E-mail: barlag@ohio.edu.

Abstract

A student-prepared silver–silver chloride reference electrode is described. The chemical deposition of AgCl(s) onto Ag(s) is accomplished in 30–50 s by placement of a Ag(s) wire in laundry bleach. An autopipettor tip with an agarose gel plug serves as the electrode housing; the agarose gel contains predissolved KNO3. Reference electrode preparation is completed in about 10 min, allowing enough time for laboratory exercises that utilize the electrode. Preparation and operation of the electrode and recovery of the Ag(s) are designed to teach a number of important chemistry principles.

Direct Detection of Key Reaction Intermediates in Photochemical CO2 Reduction Sensitized by a Rhenium Bipyridine Complex

Direct Detection of Key Reaction Intermediates in Photochemical CO2 Reduction Sensitized by a Rhenium Bipyridine Complex

Publication Date (Web): April 1, 2014 (Article)
DOI: 10.1021/ja500403e
 
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Photochemical CO2 reduction sensitized by rhenium–bipyridyl complexes has been studied through multiple approaches during the past several decades. However, a key reaction intermediate, the CO2-coordinated Re–bipyridyl complex, which should govern the activity of CO2 reduction in the photocatalytic cycle, has never been detected in a direct way. In this study on photoreduction of CO2 catalyzed by the 4,4′-dimethyl-2,2′-bipyridine (dmbpy) complex, [Re(dmbpy)(CO)3Cl] (1), we successfully detect the solvent-coordinated Re complex [Re(dmbpy)(CO)3DMF] (2) as the light-absorbing species to drive photoreduction of CO2. The key intermediate, the CO2-coordinated Re–bipyridyl complex, [Re(dmbpy)(CO)3(COOH)], is also successfully detected for the first time by means of cold-spray ionization spectrometry (CSI-MS). Mass spectra for a reaction mixture with isotopically labeled 13CO2 provide clear evidence for the incorporation of CO2 into the Re–bipyridyl complex. It is revealed that the starting chloride complex 1 was rapidly transformed into the DMF-coordinated Re complex 2 through the initial cycle of photoreduction of CO2. The observed induction period in the time profile of the CSI-MS signals can well explain the subsequent formation of the CO2-coordinated intermediate from the solvent-coordinated Re–bipyridyl complex. An FTIR study of the reaction mixture in dimethyl sulfoxide clearly shows the appearance of a signal at 1682 cm–1, which shifts to 1647 cm–1 for the 13CO2-labeled counterpart; this is assigned as the CO2-coordinated intermediate, ReII–COOH. Thus, a detailed understanding has now been obtained for the mechanism of the archetypical photochemical CO2 reduction sensitized by a Re–bipyridyl complex.
 

Apr 9, 2014

Electrochemical coating of [trans-L14CoIIICNFeII(CN)5]Na on ITO/Au electrode and its electrocatalytic properties towards nitrite reduction

Link 
Journal of Electroanalytical Chemistry
Volumes 722–723, 1 May 2014, Pages 1–6

César Cáceres, Manuel Martínez, Carlos Rodríguez, Paulina Dreyse, Victoria Ortega, Mauricio Isaacs

The preparation of an ITO (Indium Tin Oxide) modified electrode with a mixed valence CoIII/FeII complex is described. The ITO electrode was initially modified with Au nanoparticles by dip coating in order to obtain and ITO/Au surface over which a film of the [trans-L14CoIIINCFeII(CN)5]Na (L14CoIII–FeII; L14 = 6-methyl-1,4,8,11-tetraazacyclotetradecan-6-amine) complex was deposited electrochemically, thus obtaining an ITO/Au/L14CoIII–FeII modified electrode. The modified electrode has been characterized by cyclic voltammetry, Raman spectroscopy, AFM and SEM, thus confirming the presence of the mixed valence complex as a modifier species. The electrode prepared shows good stability in aqueous solution and its activity as electrocatalyst in the reduction of nitrites has been evaluated. Cyclic voltammetry experiments run in presence of nitrites show a current enhancement on the ITO/Au/L14CoIII–FeII modified electrodes with respect to the ITO/Au initial electrode. After controlled potential electrolysis experiments, analysis of the products indicate the presence of hydroxylamine as the main nitrite reduction species, although small quantities of hydrazine and ammonia where also obtained.

 

Apr 8, 2014

New Reactions of Terminal Hydrides on a Diiron Dithiolate

New Reactions of Terminal Hydrides on a Diiron Dithiolate

Publication Date (Web): March 22, 2014 (Article)
DOI: 10.1021/ja501366j
 
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Mechanisms for biological and bioinspired dihydrogen activation and production often invoke the intermediacy of diiron dithiolato dihydrides. The first example of such a Fe2(SR)2H2 species is provided by the complex [(term-H)(μ-H)Fe2(pdt)(CO)(dppv)2] ([H1H]0). Spectroscopic and computational studies indicate that [H1H]0 contains both a bridging hydride and a terminal hydride, which, notably, occupies a basal site. The synthesis begins with [(μ-H)Fe2(pdt)(CO)2(dppv)2]+ ([H1(CO)]+), which undergoes substitution to afford [(μ-H)Fe2(pdt)(CO)(NCMe)(dppv)2]+ ([H1(NCMe)]+). Upon treatment of [H1(NCMe)]+ with borohydride salts, the MeCN ligand is displaced to afford [H1H]0. DNMR (EXSY, SST) experiments on this complex show that the terminal and bridging hydride ligands interchange intramolecularly at a rate of 1 s–1 at −40 °C. The compound reacts with D2 to afford [D1D]0, but not mixed isotopomers such as [H1D]0. The dihydride undergoes oxidation with Fc+ under CO to give [1(CO)]+ and H2. Protonation in MeCN solution gives [H1(NCMe)]+ and H2. Carbonylation converts [H1H]0 into [1(CO)]0.

Molecular Hydrogen Formation from Proximal Glycol Pairs on TiO2(110)

Molecular Hydrogen Formation from Proximal Glycol Pairs on TiO2(110)

Publication Date (Web): April 4, 2014 (Communication)
DOI: 10.1021/ja500992b
 
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Understanding hydrogen formation on TiO2 surfaces is of great importance, as it could provide fundamental insight into water splitting for hydrogen production using solar energy. In this work, hydrogen formation from glycols having different numbers of methyl end-groups has been studied using temperature-programmed desorption on reduced, hydroxylated, and oxidized rutile TiO2(110) surfaces. The results from OD-labeled glycols demonstrate that gas-phase molecular hydrogen originates exclusively from glycol hydroxyl groups. The yield is controlled by a combination of glycol coverage, steric hindrance, TiO2(110) order, and the amount of subsurface charge. Combined, these results show that proximal pairs of hydroxyl-aligned glycol molecules and subsurface charge are required to maximize the yield of this redox reaction. These findings highlight the importance of geometric and electronic effects in hydrogen formation from adsorbates on TiO2(110).

A Simple Complex on the Verge of Breakdown: Isolation of the Elusive Cyanoformate Ion

A CO2 Cloak for the Cyanide Dagger

  • Igor Alabugin and
  • Rana K. Mohamed
Science 4 April 2014: 45-46.
The fleeting stability of the cyanoformate ion formed from CO2 and cyanide has implications for plant enzymology and CO2 sequestration. [Also see Report by Murphy et al.]
  • Summary
  • Full Text
  • Full Text (PDF) 
  • Characterization of a cyanide–carbon dioxide adduct bolsters its possible role in protecting a plant enzyme from cyanide inhibition. [Also see Perspective by Alabugin and Mohamed]
     Why does cyanide not react destructively with the proximal iron center at the active site of 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase, an enzyme central to the biosynthesis of ethylene in plants? It has long been postulated that the cyanoformate anion, [NCCO2], forms and then decomposes to carbon dioxide and cyanide during this process. We have now isolated and crystallographically characterized this elusive anion as its tetraphenylphosphonium salt. Theoretical calculations show that cyanoformate has a very weak C–C bond and that it is thermodynamically stable only in low dielectric media. Solution stability studies have substantiated the latter result. We propose that cyanoformate shuttles the potentially toxic cyanide away from the low dielectric active site of ACC oxidase before breaking down in the higher dielectric medium of the cell.

Manganese Catalysts with Bulky Bipyridine Ligands for the Electrocatalytic Reduction of Carbon Dioxide: Eliminating Dimerization and Altering Catalysis

Manganese Catalysts with Bulky Bipyridine Ligands for the Electrocatalytic Reduction of Carbon Dioxide: Eliminating Dimerization and Altering Catalysis

Publication Date (Web): March 18, 2014 (Article)
DOI: 10.1021/ja501252f
 
 
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With the goal of improving previously reported Mn bipyridine electrocatalysts in terms of increased activity and reduced overpotential, a bulky bipyridine ligand, 6,6′-dimesityl-2,2′-bipyridine (mesbpy), was utilized to eliminate dimerization in the catalytic cycle. Synthesis, electrocatalytic properties, X-ray diffraction (XRD) studies, and infrared spectroelectrochemistry (IR-SEC) of Mn(mesbpy)(CO)3Br and [Mn(mesbpy)(CO)3(MeCN)](OTf) are reported. Unlike previously reported Mn bipyridine catalysts, these Mn complexes exhibit a single, two-electron reduction wave under nitrogen, with no evidence of dimerization. The anionic complex, [Mn(mesbpy)(CO)3], is formed at 300 mV more positive potential than the corresponding state is formed in typical Mn bipyridine catalysts. IR-SEC experiments and chemical reductions with KC8 provide insights into the species leading up to the anionic state, specifically that both the singly reduced and doubly reduced Mn complexes form at the same potential. When formed, the anionic complex binds CO2 with H+, but catalytic activity does not occur until a 400 mV more negative potential is present. The Mn complexes show high activity and Faradaic efficiency for CO2 reduction to CO with the addition of weak Brønsted acids. IR-SEC experiments under CO2/H+ indicate that reduction of a Mn(I)–CO2H catalytic intermediate may be the cause of this unusual “over-reduction” required to initiate catalysis.

Ion-Gated Synthetic Photosystems

Ion-Gated Synthetic Photosystems

Publication Date (Web): April 1, 2014 (Communication)
DOI: 10.1021/ja501389g


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Abstract Image
Herein, molecular strings of ions built along charge-transporting channels are shown to dramatically increase photocurrents and enable charge transport over long distances, thus confirming the existence and significance of ion-gated photosystems. For their synthesis, ordered and oriented stacks of naphthalenediimides were grown on indium tin oxide by ring-opening disulfide-exchange polymerization. To these charge-transporting channels, coaxial strings of anions or cations—fixed, mobile, complete, partial, pure, or mixed—were added by orthogonal hydrazone exchange. The presence of partially protonated carboxylates was found to most significantly increase activity, implying that they both attract holes and repel electrons, that is, facilitate photoinduced charge separation and hinder charge recombination at the same time. As a result of this quite remarkable situation, photocurrents increased rather than decreased with increasing charge stabilization on their “stepping stones.” The presence of mobile anions facilitated long-distance charge transport through thick films. Turned off by inhibited anion mobility, that is, proton hopping, hole/proton antiport is identified to account for long-distance charge transport in ion-gated photosystems.

Core/Shell Au/CuPt Nanoparticles and Their Dual Electrocatalysis for Both Reduction and Oxidation Reactions

Core/Shell Au/CuPt Nanoparticles and Their Dual Electrocatalysis for Both Reduction and Oxidation Reactions

Publication Date (Web): March 20, 2014 (Article)
DOI: 10.1021/ja500590n
 
Abstract Image
We report a facile synthesis of monodisperse core/shell 5/1.5 nm Au/CuPt nanoparticles by coreduction of platinum acetylacetonate and copper acetylacetonate in the presence of 5 nm Au nanoparticles. The CuPt alloy effect and core/shell interactions make these Au/CuPt nanoparticles a promising catalyst for both oxygen reduction reaction and methanol oxidation reaction in 0.1 M HClO4 solution. Their specific (mass) reduction and oxidation activities reach 2.72 mA/cm2 (1500 mA/mg Pt) at 0.9 V and 0.755 mA/cm2 (441 mA/mg Pt) at 0.8 V (vs reversible hydrogen electrode), respectively. Our studies show that the existence of the Au nanoparticle core not only minimizes the Pt usage but also improves the stability of the Au/CuPt catalyst for fuel cell reactions. The results suggest that the core/shell design is indeed effective for optimizing nanoparticle catalysis. The same concept may be extended to other multimetallic nanoparticle systems, making it possible to tune nanoparticle catalysis for many different chemical reactions.
 

Effect of Protonation State and Interposed Connector Groups on Bond Dissociation Enthalpies of Alcohols and Related Systems


J. Phys. Chem. A, Article ASAP
DOI: 10.1021/jp501256f
Publication Date (Web): April 7, 2014

Abstract Image
High-level quantum chemical procedures have been used to study how the C–H bond dissociation enthalpies (BDEs) of alcohols and related systems are affected by changes to their protonation state. The high-level procedures used have been determined from a benchmark of 25 neutral, protonated, and deprotonated substituted methanes. The benchmark calculations suggest that the experimental C–H BDEs for CH3NH2 and CH3SH should be reassessed. We confirm previous findings that protonation increases the BDEs of alcohols, while deprotonation decreases the BDEs. For the prototypical alcohol, methanol, reducing the strength of the proton donor or acceptor leads to a smaller change in the BDE, and a smooth variation of C–H bond strength with the extent of protonation or deprotonation is observed. Changes in the BDE with protonation state are reduced for alcohols with a connector group separating the oxygen center and the site of C–H bond scission. These changes are rationalized through introduction of three new quantities, termed the effect of protonation state on dissociation energies, the alcohol radical connector energy, and the alcohol molecule connector energy. Gas-phase acidities and proton affinities for all relevant alcohols have been computed and compared with experiment. The agreement between theory and experiment is generally reasonable, with just one notable outlier (the proton affinity of CH3CH2CH2OH). In this case, we suggest that the experimental value should be reevaluated.

Apr 7, 2014

Electrocatalytic Water Oxidation by a Monomeric Amidate-Ligated Fe(III)–Aqua Complex

Electrocatalytic Water Oxidation by a Monomeric Amidate-Ligated Fe(III)–Aqua Complex

Publication Date (Web): March 26, 2014 (Communication)
DOI: 10.1021/ja412822u
 
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The six-coordinate FeIII-aqua complex [FeIII(dpaq)(H2O)]2+ (1, dpaq is 2-[bis(pyridine-2-ylmethyl)]amino-N-quinolin-8-yl-acetamido) is an electrocatalyst for water oxidation in propylene carbonate–water mixtures. An electrochemical kinetics study has revealed that water oxidation occurs by oxidation to FeV(O)2+ followed by a reaction first order in catalyst and added water, respectively, with ko = 0.035(4) M–1 s–1 by the single-site mechanism found previously for Ru and Ir water oxidation catalysts. Sustained water oxidation catalysis occurs at a high surface area electrode to give O2 through at least 29 turnovers over an 15 h electrolysis period with a 45% Faradaic yield and no observable decomposition of the catalyst.
 

N-Acyloxyphthalimides as Nitrogen Radical Precursors in the Visible Light Photocatalyzed Room Temperature C–H Amination of Arenes and Heteroarenes

Publication Date (Web): April 4, 2014 (Communication)
DOI: 10.1021/ja501906x
 
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This paper reports a room temperature visible light photocatalyzed method for the C–H amination of arenes and heteroarenes. A key enabling advance in this work is the design of N-acyloxyphthalimides as precursors to nitrogen-based radical intermediates for these transformations. A broad substrate scope is presented, including the selective meta-amination of pyridine derivatives. A radical aromatic substitution mechanism is proposed.
 

Electrocatalytic Oxygen Evolution with an Immobilized TAML Activator

Electrocatalytic Oxygen Evolution with an Immobilized TAML Activator

Publication Date (Web): April 7, 2014 (Communication)
DOI: 10.1021/ja5015986



Abstract Image


Iron complexes of tetra-amido macrocyclic ligands are important members of the suite of oxidation catalysts known as TAML activators. TAML activators are known to be fast homogeneous water oxidation (WO) catalysts, producing oxygen in the presence of chemical oxidants, e.g., ceric ammonium nitrate. These homogeneous systems exhibited low turnover numbers (TONs). Here we demonstrate immobilization on glassy carbon and carbon paper in an ink composed of the prototype TAML activator, carbon black, and Nafion and the subsequent use of this composition in heterogeneous electrocatalytic WO. The immobilized TAML system is shown to readily produce O2 with much higher TONs than the homogeneous predecessors.

Mechanism of the Formation of a Mn-Based CO2 Reduction Catalyst Revealed by Pulse Radiolysis with Time-Resolved Infrared Detection

Mechanism of the Formation of a Mn-Based CO2 Reduction Catalyst Revealed by Pulse Radiolysis with Time-Resolved Infrared Detection

Publication Date (Web): March 28, 2014 (Communication)
DOI: 10.1021/ja501051s
 
Abstract Image
Using a new technique, which combines pulse radiolysis with nanosecond time-resolved infrared (TRIR) spectroscopy in the condensed phase, we have conducted a detailed kinetic and mechanistic investigation of the formation of a Mn-based CO2 reduction electrocatalyst, [Mn(tBu2-bpy)(CO)3]2 (tBu2-bpy = 4,4′-tBu2-2,2′-bipyridine), in acetonitrile. The use of TRIR allowed, for the first time, direct observation of all the intermediates involved in this process. Addition of excess [nBu4N][HCO2] to an acetonitrile solution of fac-MnBr(tBu2-bpy)(CO)3 results in its quantitative conversion to the Mn–formate complex, fac-Mn(OCHO)(tBu2-bpy)(CO)3, which is a precatalyst for the electrocatalytic reduction of CO2. Formation of the catalyst is initiated by one-electron reduction of the Mn–formate precatalyst, which produces the bpy ligand-based radical. This radical undergoes extremely rapid (τ = 77 ns) formate dissociation accompanied by a free valence shift to yield the five-coordinate Mn-based radical, Mn(tBu2-bpy)(CO)3. TRIR data also provide evidence that the Mn-centered radical does not bind acetonitrile prior to its dimerization. This reaction occurs with a characteristically high radical–radical recombination rate (2kdim = (1.3 ± 0.1) × 109 M–1 s–1), generating the catalytically active Mn–Mn bound dimer.
 

Non-innocent Additives in a Palladium(II)-Catalyzed C–H Bond Activation Reaction: Insights into Multimetallic Active Catalysts

Publication Date (Web): April 3, 2014 (Communication)
DOI: 10.1021/ja412770h
 
Abstract Image
The role of a widely employed additive (AgOAc) in a palladium acetate-catalyzed ortho-C–H bond activation reaction has been examined using the M06 density functional theory. A new hetero-bimetallic Pd-(μ-OAc)3-Ag is identified as the most likely active species. This finding could have far-reaching implications with respect to the notion of the active species in palladium catalysis in the presence of other metal salt additives.
 

RH and H2 Production in Reactions between ROH and Small Molybdenum Oxide Cluster Anions

Department of Chemistry, Indiana University, 800 East Kirkwood Avenue, Bloomington, Indiana 47405, United States
J. Phys. Chem. A, Article ASAP
DOI: 10.1021/jp502021k
Publication Date (Web): March 24, 2014
Copyright © 2014 American Chemical Society
*E-mail: cjarrold@indiana.edu. Fax: 812-855-8300.

Abstract

Abstract Image
To test recent computational studies on the mechanism of metal oxide cluster anion reactions with water [RamabhadranR. O.; et al. J. Phys. Chem. Lett. 201013066;RamabhadranR. O.; et al. J. Am. Chem. Soc. 201313517039], the reactivity of molybdenum oxo–cluster anions, MoxOy (x = 1 – 4; y ≤ 3x) toward both methanol (MeOH) and ethanol (EtOH) has been studied using mass spectrometric analysis of products formed in a high-pressure, fast-flow reactor. The size-dependent product distributions are compared to previous MoxOy + H2O/D2O reactivity studies, with particular emphasis on the Mo2Oy and Mo3Oy series. In general, sequential oxidation, MoxOy + ROH → MoxOy+1 + RH, and addition reactions, MoxOy + ROH → MoxOy+1RH, largely corresponded with previously studied MoxOy + H2O/D2O reactions [RothgebD. W.MannJ. E., and JarroldC. C. J. Chem. Phys. 2010133054305], though with much lower rate constants than those determined for MoxOy + H2O/D2O reactions. This finding is consistent with the computational studies that suggested that −H mobility on the cluster–water complex was an important feature in the overall reactivity. There were several notable differences between cluster–ROH and cluster–water reactions associated with lower R–OH bond dissociation energies relative to the HO–H dissociation energy.

Covalent O–H Bonds as Electron Traps in Proton-Rich Rutile TiO2Nanoparticles

Department of Chemistry, Columbia University, New York, New York 10027, United States
Nano Lett., Article ASAP
DOI: 10.1021/nl404307n
Publication Date (Web): March 10, 2014
Copyright © 2014 American Chemical Society
*Corresponding Author E-mail: rich@chem.columbia.edu.

Abstract

Abstract Image
The cation in the electrolyte of the dye-sensitized solar cell (DSSC) has a profound effect on electron trapping and transport behavior in TiO2 nanocrystalline film; this is one of the important factors that determines the overall efficiency of DSSCs. Here, we present a quantum mechanical investigation on the structures and energetics of proton-induced electron trap states and the thermodynamical barrier heights for the ambipolar diffusion of proton/electron pair using a large cluster model for the computations. Our calculations indicate that protons react with TiO2 to form covalent O–H bonds. This is in contrast to the reaction of Li+ with TiO2, in which case the alkali metal is more accurately described as a simple coordinating cation. The covalent O–H bonding leads both to deeper electron trap states and to significantly higher barriers for the diffusion of carriers. These results are qualitatively consistent with experimental observations, and they extend our understanding of the cation effect in DSSCs at an atomic level of detail.