Understanding the nature of charge carriers in nanoscale titanium dioxide is important for its use in solar energy conversion, photocatalysis, and other applications. UV-irradiation of aqueous, colloidal TiO2 nanoparticles in the presence of methanol gives highly reduced suspensions. Two distinct types of electron traps were observed and characterized by EPR and optical spectroscopies. The relative populations of the states depend on temperature, indicating a small energy difference, ΔH° = 3.0 ± 0.6 kcal/mol (130 ± 30 meV). Interconversion between the electron traps occurs slowly over the course of minutes to hours within the temperature range studied here, 0–50 °C. The slow time scale implies that interconversion involves changes in structure or stoichiometry, not just the movement of electrons. This occurrence of slow structural modification with changes in trap state occupancy is likely a general feature of reduced TiO2 systems at thermodynamic equilibria or photostationary states and should be considered in the design of TiO2-containing devices.

Samarium diiodide in the presence of water and THF (SmI2(H2O)n) has in recent years become a versatile and useful reagent, mainly for reducing carbonyl-type substrates. This work reports the reduction of several enamines by SmI2(H2O)n. Mechanistic experiments implicate a concerted proton-coupled electron transfer (PCET) pathway, based on various pieces of evidence against initial outer-sphere electron transfer, proton transfer, or substrate coordination. A thermochemical analysis indicates that the C–H bond formed in the rate-determining step has a bond dissociation free energy (BDFE) of ∼32 kcal mol–1. The O–H BDFE of the samarium aquo ion is estimated to be 26 kcal mol–1, which is among the weakest known X–H bonds of stable reagents. Thus, SmI2(H2O)n should be able to form very weak C–H bonds. The reduction of these highly electron rich substrates by SmI2(H2O)n shows that this reagent is a very strong hydrogen atom donor as well as an outer-sphere reductant.

Improving molecular catalysis for important electrochemical proton-coupled electron transfer (PCET) reactions, such as the interconversions of H+/H2, O2/H2O, CO2/CO, and N2/NH3, is an ongoing challenge. Synthetic modifications to the molecular catalysts are valuable but often show trade-offs between turnover frequency (TOF) and the effective overpotential required to initiate catalysis (ηeff). Herein, we derive a new approach for improving efficiencies—higher TOF at lower ηeff—by changing the concentrations and properties of the reactants and products, rather than by modifying the catalyst. The dependence of TOF on ηeff is shown to be quite different upon changing, for instance, the pKa of the acid HA versus the concentration or partial pressure of a reactant or product. Using the electrochemical reduction of dioxygen catalyzed by iron porphyrins in DMF as an example, decreasing [HA] 10-fold lowers ηeff by 59 mV and decreases the TOF by a factor of 10. Alternatively, a 10-fold decrease in Ka(HA) also lowers ηeff by 59 mV but only decreases the TOF by a factor of 2. This approach has been used to improve a catalytic TOF by 104 vs the previously reported scaling relationship developed via synthetic modifications to the catalyst. The analysis has the potential to predict improved efficiency and product selectivity of any molecular PCET catalyst, based on its mechanism and rate law.

The bulky 2,6-di-tert-butyl-4-nitrophenolate ligand forms complexes with [TptBuCuII]+ and [TptBuZnII]+ binding via the nitro group in an unusual nitronato-quinone resonance form (TptBu = hydro-tris(3-tert-butyl-pyrazol-1-yl)borate). The Cu complex in the solid state has a five-coordinate κ2-nitronate structure, while the Zn analogue has a four-coordinate κ1-nitronate ligand. 4-Nitrophenol, without the 2,6-di-tert-butyl substituents, instead binds to [TptBuCuII]+ through the phenolate oxygen. This difference in binding is very likely due to the steric difficulty in binding a 2,6-di-tert-butyl-phenolate ligand to the [TptBuMII]+ unit. TptBuCuII(κ2-O2NtBu2C6H2O) reacts with the hydroxylamine TEMPO–H (2,2,6,6-tetramethylpiperidin-1-ol) by abstracting a hydrogen atom. This system thus shows an unusual sterically enforced transition metal–ligand binding motif and a copper–phenolate interaction that differs from what is typically observed in biological and chemical catalysis.

Kinetic studies of the reactions of two previously characterized copper(III)-hydroxide complexes (LCuOH and NO2LCuOH, where L = N,N′-bis(2,6-diisopropylphenyl)-2,6-pyridine-dicarboxamide and NO2L = N,N′-bis(2,6-diisopropyl-4-nitrophenyl)pyridine-2,6-dicarboxamide) with a series of para substituted phenols (XArOH where X = NMe2, OMe, Me, H, Cl, NO2, or CF3) were performed using low temperature stopped-flow UV-vis spectroscopy. Second-order rate constants (k) were determined from pseudo first-order and stoichiometric experiments, and follow the trends CF3 < NO2 < Cl < H < Me < OMe < NMe2 and LCuOH < NO2LCuOH. The data support a concerted proton–electron transfer (CPET) mechanism for all but the most acidic phenols (X = NO2 and CF3), for which a more complicated mechanism is proposed. For the case of the reactions between NO2ArOH and LCuOH in particular, competition between a CPET pathway and one involving initial proton transfer followed by electron transfer (PT/ET) is supported by multiwavelength global analysis of the kinetic data, formation of the phenoxide NO2ArO− as a reaction product, observation of an intermediate [LCu(OH2)]+ species derived from proton transfer from NO2ArOH to LCuOH, and thermodynamic arguments indicating that initial PT should be competitive with CPET.

Protonation and reduction of pincer-ligated Rh– and Ir–N2 complexes have been studied by NMR spectroscopy and cyclic voltammetry to assess the capability of these complexes to activate or reduce N2. Protonation, which is a prerequisite to electrochemical reduction, results in a cationic metal-hydride that loses N2 under an atmosphere of Ar. Reduction of the metal-hydride results in fast disproportionation of an unobserved transient Ir2+ species. These studies suggest that the regioselectivity of initial protonation is a strong determinant for the ability of a system to facilitate the reduction of N2.

Charge carriers (electrons) were added to ZnO nanocrystals (NCs) using the molecular reductants CoCp*2 and CrCp*2 [Cp* = η5-pentamethylcyclopentadienyl]. The driving force for electron transfer from the reductant to the NCs was varied systematically by the addition of acid, which lowers the energy of the NC orbitals. In the presence of excess reductant, the number of electrons per NC (⟨ne–⟩) reaches a maximum, beyond which the addition of more acid has no effect. This ⟨ne–⟩max varies with the NC radius with an r3 dependence, so the density of electrons (⟨Ne–⟩max) is constant over a range of NC sizes. ⟨Ne–⟩max = 4.4(1.0) × 1020 cm–3 for CoCp*2 and 1.3(0.5) × 1020 cm–3 for the weaker reducing agent, CrCp*2. Up until the saturation point, the addition of electrons is linear with respect to protons added. This linearity contrasts with the typical description of hydrogen atom-like states (S, P, etc.) in the conduction band. The 1:1 relationship of ⟨ne–⟩ with protons per NC and the dramatic dependence of ⟨Ne–⟩max on the nature of the cation (H+ vs MCp*2+) suggest that the protons intercalate into the NCs under these conditions. The differences between the reductants, the volume dependence, calculations of the Fermi level using the redox couple, and a proposed model encompassing these effects are presented. This study illustrates the strong coupling between protons and electrons in ZnO NCs and shows that proton activity is a key parameter in nanomaterial energetics.

Early-transition-metal nitrides, including γ-Mo2N, are active and selective for a variety of reactions, including the hydrogenation of organics (e.g., hydrodeoxygenation), CO (e.g., Fischer–Tropsch synthesis), and CO2. In addition to adsorbing hydrogen onto the surface, some of these materials can incorporate hydrogen into subsurface, interstitial sites. Research described in this paper examined, experimentally and computationally, the nature of hydrogen on and in γ-Mo2N, with a particular focus on characterizing the interactions of these hydrogens with crotonaldehyde. Hydrogen was added to γ-Mo2N via exposure to gaseous hydrogen at elevated temperatures, forming γ-Mo2N-Hx, where 0.061< x < 0.082. Temperature-programmed desorption (TPD) experiments indicate that γ-Mo2N-Hx has at least two distinct hydrogen binding sites and that these sites can be selectively populated. Inelastic neutron scattering and density functional theory calculations indicate the presence of surface nitrogen-bound (κ1-NHsurf), surface Mo-bound (κ1-MoHsurf), and interstitial Mo-bound (μ6-Mo6Hsub) hydrogens. Selectivities for the hydrogenation of crotonaldehyde, a model of species in biomass-derived liquids, correlated with the populations at these sites. Importantly, materials with high densities of interstitial, hydridic hydrogen were selective for C═O hydrogenation (i.e., formation of crotyl alcohol). Collectively the results provide mechanistic insights regarding the desorption and reactivity of hydrogen on and in γ-Mo2N. Hydrogen adsorption/desorption to γ-Mo2N is heterolytic; in particular, H2 adds across a Mo–N bond. Because the surface Mo–H site is energetically unfavorable in comparison to the interstitial site, hydrogen migrates into interstitial sites once the surface NH sites are saturated. Crotonaldehyde adsorption facilitates migration of this interstitial hydrogen back to the surface, forming surface Mo–H that is selective for hydrogenation of the C═O bond. These insights will facilitate the design of γ-Mo2N and other early-transition-metal nitrides for catalytic applications.Keywords: interstitial hydrogen; molybdenum nitride; selective hydrogenation; surface hydrogen; α,β-unsaturated aldehydes

Anhydrous H2O2 reacts with organic colloidal solutions of ceria nanoparticles to form a stable surface peroxo/hydroperoxo species with the release of oleate capping ligands into solution. A new optical spectroscopic signature was identified for cerium-peroxo/hydroperoxo species in solution and correlated with solid-state IR spectroscopy and chemical reactivity.

Two new monomeric Cu(II) alkoxide complexes were prepared and fully characterized as models for intermediates in copper/radical mediated alcohol oxidation catalysis: TptBuRCuIIOCH2CF3 with TptBu = hydro-tris(3-tert-butyl-pyrazol-1-yl)borate 1 or TptBuMe = hydro-tris(3-tert-butyl-5-methyl-pyrazol-1-yl)borate 2. These complexes were made as models for potential intermediates in enzymatic and synthetic catalytic cycles for alcohol oxidation. However, the alkoxide ligands are not readily oxidized by loss of H; instead, these complexes were found to be hydrogen atom acceptors. They oxidize the hydroxylamine TEMPOH, 2,4,6-tri-t-butylphenol, and 1,4-cyclohexadiene to the nitroxyl radical, phenoxyl radical, and benzene, with formation of HOCH2CF3 (TFE) and the Cu(I) complexes TptBuRCuI-MeCN in dichloromethane/1% MeCN or 1/2 [TptBuRCuI]2 in toluene. On the basis of thermodynamics and kinetics arguments, these reactions likely proceed through concerted proton–electron transfer mechanisms. Thermochemical analyses give lower limits for the “effective bond dissociation free energies (BDFE)” of the O–H bonds in 1/2[TptBuRCuI]2 + TFE and upper limits for the free energies associated with alkoxide oxidations via hydrogen atom transfer (effective alkoxide α-C–H BDFEs). These values are summations of the free energies of multiple chemical steps, which include the energetically favorable formation of 1/2[TptBuRCuI]2. The effective alkoxide α-C–H bonds are very weak, BDFE ≤ 38 ± 4 kcal mol–1 for 1 and ≤44 ± 5 kcal mol–1 for 2 (gas-phase estimates), because C–H homolysis is thermodynamically coupled to one electron transfer to Cu(II) as well as the favorable formation of the 1/2[TptBuRCuI]2 dimer. Treating 1 with the H atom acceptor tBu3ArO• did not result in the expected alkoxide oxidation to an aldehyde, but rather net 2,2,2-trifluoroethoxyl radical transfer occurred to generate an unusual 2-substituted dienone–ether product. Treating 2 with tBu3ArO• gives no reaction, despite evidence that overall ligand oxidation and formation of 1/2[TptBuMeCuI]2 is significantly exoergic. The origin of this lack of reactivity may be due to insufficient weakening of the alcohol α-C–H bond upon complexation to copper.

Improved electrocatalysts for the oxygen reduction reaction (ORR) are critical for the advancement of fuel cell technologies. Herein, we report a series of 11 soluble iron porphyrin ORR electrocatalysts that possess turnover frequencies (TOFs) from 3 s–1 to an unprecedented value of 2.2 × 106 s–1. These TOFs correlate with the ORR overpotential, which can be modulated by changing the E1/2 of the catalyst using different ancillary ligands, by changing the solvent and solution acidity, and by changing the catalyst’s protonation state. The overpotential is well-defined for these homogeneous electrocatalysts by the E1/2 of the catalyst and the proton activity of the solution. This is the first such correlation for homogeneous ORR electrocatalysis, and it demonstrates that the remarkably fast TOFs are a consequence of high overpotential. The correlation with overpotential is surprising since the turnover limiting steps involve oxygen binding and protonation, as opposed to turnover limiting electron transfer commonly found in Tafel analysis of heterogeneous ORR materials. Computational studies show that the free energies for oxygen binding to the catalyst and for protonation of the superoxide complex are in general linearly related to the catalyst E1/2, and that this is the origin of the overpotential correlations. This analysis thus provides detailed understanding of the ORR barriers. The best catalysts involve partial decoupling of the influence of the second coordination sphere from the properties of the metal center, which is suggested as new molecular design strategy to avoid the limitations of the traditional scaling relationships for these catalysts.

Several substituted iron–porphyrin complexes were evaluated for oxygen reduction reaction (ORR) electrocatalysis in different homogeneous and heterogeneous media. The selectivity for four-electron reduction to H2O versus two-electron reduction to H2O2 varies substantially from one medium to another for a given catalyst. In many cases, the influence of the medium in which the catalyst is evaluated has a larger effect on the observed selectivity than the factors attributable to chemical modification of the catalyst. For instance, introduction of potential proton relays has variable effects depending on the catalyst medium. Thus, comparisons of selectivity results from supported and soluble molecular ORR electrocatalysts must be interpreted with caution, as selectivity is a property not only of the catalyst, but also of the larger mesoscale environment beyond the catalyst. Still, in all the direct pairwise comparisons in the same medium, the catalysts with potential proton relays have similar or better selectivity for the preferred 4e– path.

A variety of next-generation energy processes utilize the electrochemical interconversions of dioxygen and water as the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). Reported here are the first estimates of the standard reduction potential of the O2 + 4e– + 4H+ ⇋ 2H2O couple in organic solvents. The values are +1.21 V in acetonitrile (MeCN) and +0.60 V in N,N-dimethylformamide (DMF), each versus the ferrocenium/ferrocene couple (Fc+/0) in the respective solvent (as are all of the potentials reported here). The potentials have been determined using a thermochemical cycle that combines the free energy for transferring water from aqueous solution to organic solvent, −0.43 kcal mol–1 for MeCN and −1.47 kcal mol–1 for DMF, and the potential of the H+/H2 couple, – 0.028 V in MeCN and −0.662 V in DMF. The H+/H2 couple in DMF has been directly measured electrochemically using the previously reported procedure for the MeCN value. The thermochemical approach used for the O2/H2O couple has been extended to the CO2/CO and CO2/CH4 couples to give values of −0.12 and +0.15 V in MeCN and −0.73 and −0.48 V in DMF, respectively. Extensions to other reduction potentials are discussed. Additionally, the free energy for transfer of protons from water to organic solvent is estimated as +14 kcal mol–1 for acetonitrile and +0.6 kcal mol–1 for DMF.

An enormous variety of biological redox reactions are accompanied by changes in proton content at enzyme active sites, in their associated cofactors, in substrates and/or products, and between protein interfaces. Understanding this breadth of reactivity is an ongoing chemical challenge. A great many workers have developed and investigated biomimetic model complexes to build new ways of thinking about the mechanistic underpinnings of such complex biological proton-coupled electron transfer (PCET) reactions. Of particular importance are those model reactions that involve transfer of one proton (H+) and one electron (e–), which is equivalent to transfer of a hydrogen atom (H•). In this Current Topic, we review key concepts in PCET reactivity and describe important advances in biomimetic PCET chemistry, with a special emphasis on research that has enhanced efforts to understand biological PCET reactions.

The creation of systems that efficiently interconvert chemical and electrical energies will be aided by understanding proton-coupled electron transfers at solution–semiconductor interfaces. Steps in developing that understanding are described here through kinetic studies of reactions of photoreduced colloidal zinc oxide (ZnO) nanocrystals (NCs) with the nitroxyl radical TEMPO. These reactions proceed by proton-coupled electron transfer (PCET) to give the hydroxylamine TEMPOH. They occur on the submillisecond to seconds time scale, as monitored by stopped-flow optical spectroscopy. Under conditions of excess TEMPO, the reactions are multiexponential in character. One of the contributors to this multiexponential kinetics may be a distribution of reactive proton sites. A graphical overlay method shows the reaction to be first order in [TEMPO]. Different electron concentrations in otherwise identical NC samples were achieved by three different methods: differing photolysis times, premixing with an unphotolyzed sample, or prereaction with TEMPO. The reaction velocities were consistently higher for NCs with higher numbers of electrons. For instance, NCs with an average of 2.6 e–/NC reacted faster than otherwise identical samples containing ≤1 e–/NC. Surprisingly, NC samples with the same average number of electrons but prepared in different ways often had different reaction profiles. These results show that properties beyond electron content determine PCET reactivity of the particles.Keywords: multiexponential kinetics; proton-coupled electron transfer; protons; stopped-flow kinetics; TEMPO; zinc oxide;

We describe here a direct comparison of electrochemical and spectrochemical experiments to determine rates and selectivity of oxygen reduction catalyzed by iron 5,10,15,20-meso-tetraphenylporphyrin chloride. Good agreement was found between the two methods, suggesting the same mechanism is occurring under both conditions, with the same third-order rate law, similar selectivity, and the derived rate constants agreeing within a factor of at most 4, with kcat ≅ 2 × 106 M–2 s–1. This Communication provides a rare example of a redox catalytic process characterized by two common but very different methods.

The synthesis of a new tripodal ligand family that contains tertiary amine groups in the second-coordination sphere is reported. The ligands are tris(amido)amine derivatives, with the pendant amines attached via a peptide coupling strategy. They were designed to function as new molecular catalysts for the oxygen reduction reaction (ORR), in which the pendant acid/base group could improve the catalyst performance. Two members of the ligand family were each metalated with cobalt(II) and zinc(II) to afford trigonal-monopyramidal complexes. The reaction of the cobalt complexes [Co(L)]– with dioxygen reversibly generates a small amount of a cobalt(III) superoxo species, which was characterized by electron paramagnetic resonance (EPR) spectroscopy. Protonation of the zinc complex Zn[N{CH2CH2NC(O)CH2N(CH2Ph)2}3)]− ([Zn(TNBn)]–) with 1 equiv of acid occurs at a primary-coordination-sphere amide moiety rather than at a pendant basic site. The addition of excess acid to any of the complexes [M(L)]– results in complete proteolysis and formation of the ligands H3L. These undesired reactions limit the use of these complexes as catalysts for the ORR. An alternative ligand with two pyridyl arms was also prepared but could not be metalated. These studies highlight the importance of the stability of the primary-coordination sphere of ORR electrocatalysts to both oxidative and acidic conditions.

The ligand shell of colloidal nanocrystals can dramatically affect their stability and reaction chemistry. We present a methodology to quantify the dodecylamine (DDA) capping shell of colloidal zinc oxide nanocrystals in a nonpolar solvent. Using NMR spectroscopy, three different binding regimes are observed: strongly bound, weakly associated, and free in solution. The surface density of bound DDA is constant over a range of nanocrystal sizes, and is low compared to both predictions of the number of surface cations and maximum coverages of self-assembled monolayers. The density of strongly bound DDA ligands on the as-prepared ZnO NCs is 25% of the most conservative estimate of the maximum surface DDA density. Thus, these NCs do not resemble the common picture of a densely capped surface ligand layer. Annealing the ZnO NCs in molten DDA for 12 h at 160 °C, which is thought to remove surface hydroxide groups, resulted in a decrease of the weakly associated DDA and an increase in the density of strongly bound DDA, to ca. 80% of the estimated density of a self-assembled monolayer on a flat ZnO surface. These findings suggest that as-prepared nanocrystal surfaces contain hydroxide groups (protons on the ZnO surfaces) that inhibit strong binding of DDA.Keywords: capping ligands; colloidal semiconductors; ligand shell; NMR spectroscopy of nanocrystals; zinc oxide;

The preparation and full characterization of the 4-(nitrophenyl)phenoxyl radical, 2,6-di-tbutyl-4-(4′-nitrophenyl) phenoxyl radical (tBu2NPArO•) is described. This is a rare example of an isolable and crystallographically characterized phenoxyl radical and is the only example in which the parent phenol is also crystallographically well-defined. Analysis of EPR spectra indicates some spin delocalization onto the secondary aromatic ring and nitro group. Equilibrium studies show that the corresponding phenol has an O–H bond dissociation free energy (BDFE) of 77.8 ± 0.5 kcal mol–1 in MeCN (77.5 ± 0.5 kcal mol–1 in toluene). This value is higher than related isolated phenoxyl radicals, making this a useful reagent for hydrogen atom transfer (HAT) studies. Additional thermochemical and spectroscopic parameters are also discussed.

Anhydrous H2O2 reacts with organic colloidal solutions of ceria nanoparticles to form a stable surface peroxo/hydroperoxo species with the release of oleate capping ligands into solution. A new optical spectroscopic signature was identified for cerium-peroxo/hydroperoxo species in solution and correlated with solid-state IR spectroscopy and chemical reactivity.

The bulky 2,6-di-tert-butyl-4-nitrophenolate ligand forms complexes with [TptBuCuII]+ and [TptBuZnII]+ binding via the nitro group in an unusual nitronato-quinone resonance form (TptBu = hydro-tris(3-tert-butyl-pyrazol-1-yl)borate). The Cu complex in the solid state has a five-coordinate κ2-nitronate structure, while the Zn analogue has a four-coordinate κ1-nitronate ligand. 4-Nitrophenol, without the 2,6-di-tert-butyl substituents, instead binds to [TptBuCuII]+ through the phenolate oxygen. This difference in binding is very likely due to the steric difficulty in binding a 2,6-di-tert-butyl-phenolate ligand to the [TptBuMII]+ unit. TptBuCuII(κ2-O2NtBu2C6H2O) reacts with the hydroxylamine TEMPO–H (2,2,6,6-tetramethylpiperidin-1-ol) by abstracting a hydrogen atom. This system thus shows an unusual sterically enforced transition metal–ligand binding motif and a copper–phenolate interaction that differs from what is typically observed in biological and chemical catalysis.

Kinetic studies of the reactions of two previously characterized copper(III)-hydroxide complexes (LCuOH and NO2LCuOH, where L = N,N′-bis(2,6-diisopropylphenyl)-2,6-pyridine-dicarboxamide and NO2L = N,N′-bis(2,6-diisopropyl-4-nitrophenyl)pyridine-2,6-dicarboxamide) with a series of para substituted phenols (XArOH where X = NMe2, OMe, Me, H, Cl, NO2, or CF3) were performed using low temperature stopped-flow UV-vis spectroscopy. Second-order rate constants (k) were determined from pseudo first-order and stoichiometric experiments, and follow the trends CF3 < NO2 < Cl < H < Me < OMe < NMe2 and LCuOH < NO2LCuOH. The data support a concerted proton–electron transfer (CPET) mechanism for all but the most acidic phenols (X = NO2 and CF3), for which a more complicated mechanism is proposed. For the case of the reactions between NO2ArOH and LCuOH in particular, competition between a CPET pathway and one involving initial proton transfer followed by electron transfer (PT/ET) is supported by multiwavelength global analysis of the kinetic data, formation of the phenoxide NO2ArO− as a reaction product, observation of an intermediate [LCu(OH2)]+ species derived from proton transfer from NO2ArOH to LCuOH, and thermodynamic arguments indicating that initial PT should be competitive with CPET.