Sequential Proton-coupled Electron Transfer Research Articles

Electronic inductive and resonance effects of substituents on concerted two-proton-coupled electron transfer between electrogenerated superoxide and hydroquinone derivatives in N,N-dimethylformamide

Improving energy conversion technologies is pivotal for reducing the effects of global warming. The use of electron transfer catalysts based on electron carriers found in organisms, such as redox biomimetic quinone systems, is a promising route for truly efficient energy conversion. Herein, we report the density functional theory (DFT)-based analysis of the reaction between electrogenerated superoxide radical anion (O2•−) and methyl- or chloro-substituted benzene-1,4-diol (hydroquinone) derivatives in N,N-dimethylformamide. We investigate the electrochemical scavenging of O2•− by hydroquinone derivatives involving two proton transfers (PTs) and one electron transfer (ET). We show that chloro and methyl substituents promote a proton-coupled electron transfer (PCET) reaction with the effect increasing with the number of substituent groups. Our DFT results demonstrate that the substituent effect promotes either of the PT or ET processes, along a sequential PCET pathway. The electrochemical and DFT analyses of the substituent effect indicate that the concerted two-proton-coupled electron transfer (2PCET) promoted by an increasing number of the substituents is a plausible pathway for the reaction between electrogenerated O2•− and hydroquinone derivatives. In addition, our findings show that the resonance effect is the main driving force for the promotion of 2PCET.

General Kinetic Model for pH Dependence of Proton-Coupled Electron Transfer: Application to an Electrochemical Water Oxidation System.

The pH dependence of proton-coupled electron transfer (PCET) reactions, which are critical to many chemical and biological processes, is a powerful probe for elucidating their fundamental mechanisms. Herein, a general, multichannel kinetic model is introduced to describe the pH dependence of both homogeneous and electrochemical PCET reactions. According to this model, a weak pH dependence can arise from the competition among multiple sequential and concerted PCET channels involving different forms of the redox species, such as protonated and deprotonated forms, as well as different proton donors and acceptors. The contribution of each channel is influenced by the relative populations of the reactant species, which often depend strongly on pH, leading to complex pH dependence of PCET apparent rate constants. This model is used to explain the origins of the experimentally observed weak pH dependence of the electrochemical PCET apparent rate constant for a ruthenium-based water oxidation catalyst attached to a tin-doped In2O3 (ITO) surface. The weak pH dependence is found to arise from the intrinsic differences in the rate constants of participating channels and the dependence of their relative contributions on pH. This model predicts that the apparent maximum rate constant will become pH-independent at higher pH, which is confirmed by experimental measurements. Our analysis also suggests that the dominant channels are electron transfer at lower pH and sequential PCET via electron transfer followed by fast proton transfer at higher pH. This work highlights the importance of considering multiple competing channels simultaneously for PCET processes.

Introducing Brønsted acid sites to accelerate the bridging-oxygen-assisted deprotonation in acidic water oxidation

Oxygen evolution reaction (OER) consists of four sequential proton-coupled electron transfer steps, which suffer from sluggish kinetics even on state-of-the-art ruthenium dioxide (RuO2) catalysts. Understanding and controlling the proton transfer process could be an effective strategy to improve OER performances. Herein, we present a strategy to accelerate the deprotonation of OER intermediates by introducing strong Brønsted acid sites (e.g. tungsten oxides, WOx) into the RuO2. The Ru-W binary oxide is reported as a stable and active iridium-free acidic OER catalyst that exhibits a low overpotential (235 mV at 10 mA cm−2) and low degradation rate (0.014 mV h−1) over a 550-hour stability test. Electrochemical studies, in-situ near-ambient pressure X-ray photoelectron spectroscopy and density functional theory show that the W-O-Ru Brønsted acid sites are instrumental to facilitate proton transfer from the oxo-intermediate to the neighboring bridging oxygen sites, thus accelerating bridging-oxygen-assisted deprotonation OER steps in acidic electrolytes. The universality of the strategy is demonstrated for other Ru-M binary metal oxides (M = Cr, Mo, Nb, Ta, and Ti).

Water Oxidation Catalyzed by a Bioinspired Tetranuclear Manganese Complex: Mechanistic Study and Prediction.

Density functional theory calculations were utilized to elucidate the water oxidation mechanism catalyzed by polyanionic tetramanganese complex a [MnIII 3 MnIV O3 (CH3 COO)3 (A-α-SiW9 O34 )]6- . Theoretical results indicated that catalytic active species 1 (Mn4 III,III,IV,IV ) was formed after O2 formation in the first turnover. From 1, three sequential proton-coupled electron transfer (PCET) oxidations led to the MnIV -oxyl radical 4 (Mn4 IV,IV,IV,IV -O⋅). Importantly, 4 had an unusual butterfly-shaped Mn2 O2 core for the two substrate-coordinated Mn sites, which facilitated O-O bond formation via direct coupling of the oxyl radical and the adjacent MnIV -coordinated hydroxide to produce the hydroperoxide intermediate Int1 (Mn4 III,IV,IV,IV -OOH). This step had an overall energy barrier of 24.9 kcal mol-1 . Subsequent PCET oxidation of Int1 to Int2 (Mn4 III,IV,IV,IV -O2 ⋅) enabled the O2 release in a facile process. Furthermore, apart from the Si-centered complex, computational study suggested that tetramanganese polyoxometalates with Ge, P, and S could also catalyze the water oxidation process, where those bearing P and S likely present higher activities.

Flexibility Enhances Reactivity: Redox Isomerism and Jahn-Teller Effects in a Bioinspired Mn4O4 Cubane Water Oxidation Catalyst.

Understanding how water oxidation to molecular oxygen proceeds in molecular metal-oxo catalysts is a challenging endeavor due to their structural complexity. In this report, we unravel the water oxidation mechanism of the highly active water oxidation catalyst [Mn4V4O17(OAc)3]3–, a polyoxometalate catalyst with a [Mn4O4]6+ cubane core reminiscent of the natural oxygen-evolving complex. Starting from the activated species [Mn44+V4O17(OAc)2(H2O)(OH)]1–, we scrutinized multiple pathways to find that water oxidation proceeds via a sequential proton-coupled electron transfer (PCET), O–O bond formation, another PCET, an intramolecular electron transfer, and another PCET resulting in O2 evolution, with a predicted thermodynamic overpotential of 0.71 V. An in-depth investigation of the O–O bond formation process revealed an essential interplay between redox isomerism and Jahn–Teller effects, responsible for enhancing reactivity in the catalytic cycle. This is achieved by redistributing electrons between metal centers and weakening relevant bonds through Jahn–Teller distortions, introducing flexibility to the otherwise rigid cubane core of the catalyst. These mechanistic insights are expected to advance the design of efficient bioinspired Mn cubane water-splitting catalysts.

Conformationally Dynamic Radical Transfer within Ribonucleotide Reductase.

Ribonucleotide reductases (RNR) catalyze the reduction of nucleotides to deoxynucleotides through a mechanism involving an essential cysteine based thiyl radical. In the E.coli class 1a RNR the thiyl radical (C439•) is a transient species generated by radical transfer (RT) from a stable diferric-tyrosyl radical cofactor located >35 Å away across the α2:β2 subunit interface. RT is facilitated by sequential proton-coupled electron transfer (PCET) steps along a pathway of redox active amino acids (Y122β ↔ [W48β?] ↔ Y356β ↔ Y731α ↔ Y730α ↔ C439α). The mutant R411A(α) disrupts the H-bonding environment and conformation of Y731, ostensibly breaking the RT pathway in α2. However, the R411A protein retains significant enzymatic activity, suggesting Y731 is conformationally dynamic on the time scale of turnover. Installation of the radical trap 3-amino tyrosine (NH2Y) by amber codon suppression at positions Y731 or Y730 and investigation of the NH2Y• trapped state in the active α2:β2 complex by HYSCORE spectroscopy validate that the perturbed conformation of Y731 in R411A-α2 is dynamic, reforming the H-bond between Y731 and Y730 to allow RT to propagate to Y730. Kinetic studies facilitated by photochemical radical generation reveal that Y731 changes conformation on the ns-μs time scale, significantly faster than the enzymatic kcat. Furthermore, the kinetics of RT across the subunit interface were directly assessed for the first time, demonstrating conformationally dependent RT rates that increase from 0.6 to 1.6 × 104 s-1 when comparing wild type to R411A-α2, respectively. These results illustrate the role of conformational flexibility in modulating RT kinetics by targeting the PCET pathway of radical transport.

Sequential Proton-Coupled Electron Transfer Mediates Excited-State Deactivation of a Eumelanin Building Block.

Skin photoprotection is commonly believed to rely on the photochemistry of 5,6-dihydroxyindole (DHI)- and 5,6-dihydroxyindole-2-carboxylic acid (DHICA)-based eumelanin building blocks. Attempts to elucidate the underlying excited-state relaxation mechanisms have been partly unsuccessful due to the marked instability to oxidation. We report a study of the excited-state deactivation of DHI using steady-state and time-resolved fluorescence accompanied by high-level quantum-chemistry calculations including solvent effects. Spectroscopic data show that deactivation of the lowest excited state of DHI in aqueous buffer proceeds on the 100 ps time scale and is 20 times faster than in methanol. Quantum-chemical calculations reveal that the excited-state decay mechanism is a sequential proton-coupled electron transfer, which involves the initial formation of a solvated electron from DHI, followed by the transfer of a proton to the solvent. This unexpected finding would prompt a revision of current notions about eumelanin photophysics and photobiology.

Insights into the Mechanism of Aromatic Ring Cleavage of Noncatecholic Compound 2-Aminophenol by Aminophenol Dioxygenase: A Quantum Mechanics/Molecular Mechanics Study

2-Aminophenol 1,6-dioxygenase (APD) is an extradiol dioxygenase responsible for the ring cleavage of 2-aminophenol (2AP) at the position ortho to the hydroxyl substituent. To elucidate the reaction mechanism, we conducted quantum mechanical/molecular mechanical (QM/MM) calculations. The mode of binding of the substrate (monodentate or bidentate) to the iron center was found to have a crucial role in dioxygen activation. The Fe–O2 adducts with 2AP bound bidentately has a quintet ground state having a FeIII–superoxo character, while the Fe–O2 adducts with a monodentately bound substrate has been characterized as a substrate radical–FeII–superoxide. Unlike other extradiol dioxygenases that cleave catechol analogues using the superoxo moiety of the Fe–O2 adducts to attack the substrate, we found here an FeII–O(H)O intermediate formed through two sequential proton-coupled electron transfer steps from the initial FeIII–superoxo species is responsible for the attack. Importantly, the second-sphere His195 residue...

Tipping the Balance between Concerted versus Sequential Proton-Coupled Electron Transfer.

We use quantized molecular dynamics simulations to investigate the competition between concerted and sequential proton-coupled electron-transfer (PCET) reaction mechanisms in inorganic catalysts. By analyzing reactive nonadiabatic PCET trajectories and computing both concerted and sequential rate constants, we characterize various molecular features that govern inorganic PCET reactions, including the solvent polarity, ligand-mediated electron-proton interactions, and intrinsic proton-transfer (PT) energy barrier. Using atomistic simulations with over 1200 atoms, we find that the symmetric iron biimidazoline system is extremely biased toward the concerted mechanism because of the strong ligand-mediated electron-proton interaction and the short PT distance. However, by investigating system-bath models in which electron-proton interactions are shielded, which are representative of ruthenium terpyridylbenzoates and iron (tetraphenylporphyrin)benzoates, we predict that a crossover between the concerted and sequential PCET mechanisms may be possible either by increasing the polarity of the solvent or by increasing the intrinsic PT energy barrier. In addition, we predict the possibility of a crossover in the PCET mechanism by directly varying the strength of the ligand-mediated electron-proton interactions. The results presented here reveal new strategies for altering the competition between the competing PCET mechanisms and design principles for controlling PCET in catalytic systems.

Photoinduced Water Oxidation at the Aqueous GaN (101̅0) Interface: Deprotonation Kinetics of the First Proton-Coupled Electron-Transfer Step

Photoelectrochemical water splitting plays a key role in a promising path to the carbon-neutral generation of solar fuels. Wurzite GaN and its alloys (e.g., GaN/ZnO and InGaN) are demonstrated photocatalysts for water oxidation, and they can drive the overall water splitting reaction when coupled with co-catalysts for proton reduction. The present work investigates the water oxidation mechanism on the prototypical GaN (101̅0) surface using a combined ab initio molecular dynamics and molecular cluster model approach taking into account the role of water dissociation and hydrogen bonding within the first solvation shell of the hydroxylated surface. The investigation of free-energy changes for the four proton-coupled electron-transfer (PCET) steps of the water oxidation mechanism shows that the first PCET step for the conversion of −Ga–OH to −Ga–O•– requires the highest energy input. The study further examines the sequential PCETs, with the proton transfer (PT) following the electron transfer (ET), and finds that photogenerated holes localize on surface −NH sites, and the calculated free-energy changes indicate that PCET through −NH sites is thermodynamically more favorable than −OH sites. However, proton transfer from −OH sites with subsequent localization of holes on oxygen atoms is kinetically favored owing to hydrogen bonding interactions at the GaN (101̅0)–water interface. The deprotonation of surface −OH sites is found to be the limiting factor for the generation of reactive oxyl radical ion intermediates and consequently for water oxidation.

An experimental and ab initio study on the abiotic reduction of uranyl by ferrous iron

It is important to understand the mechanisms controlling the removal of uranyl from solution from an environmental standpoint, particularly whether soluble Fe(II) is capable of reducing soluble U(VI) to insoluble U(IV). Experiments were performed to shed light into discrepancies of recent studies about precipitation of U-containing solids without changing oxidation states versus precipitation/reduction reactions, especially with respect to the kinetics of these reactions. To understand the atomistic mechanisms, thermodynamics, and kinetics of these redox processes, ab initio electron transfer (ET) calculations, using Marcus theory, were applied to study the reduction of U(VI)aq to U(V)aq by Fe(II)aq (the first rate-limiting ET-step). Outer-sphere (OS) and inner-sphere (IS) Fe–U complexes were modeled to represent simple species within a homogeneous environment through which ET could occur.Experiments on the chemical reduction were performed by reacting 1mM Fe(II)aq at pH 7.2 with high (i.e., 0.16mM) and lower (i.e., 0.02mM) concentrations of U(VI)aq. At higher U concentration, a rapid decrease in U(VI)aq was observed within the first hour of reaction. XRD and XPS analyses of the precipitates confirmed the presence of (meta)schoepite phases, where up to ∼25% of the original U was reduced to U4+ and/or U5+-containing phases. In contrast, at 0.02mM U, the U(VI)aq concentration remained fairly constant for the first 3h of reaction and only then began to decrease due to slower precipitation kinetics. XPS spectra confirm the partial chemical reduction U associated with the precipitate (up to ∼30%). Thermodynamic calculations support that the reduction of U(VI)aq to U(IV)aq by Fe(II)aq is energetically unfavorable. The batch experiments in this study show U(VI) is removed from solution by precipitation and that transitioning to a heterogeneous system in turn enables the solid U phase to be partially reduced.Ab initio ET calculations revealed that OS ET is strongly kinetically inhibited in all cases modeled. OS ET as a concerted proton-coupled ET reaction (ferrimagnetic spin configuration) is thermodynamically favorable (−35kJ/mol), but kinetically inhibited by concurrent proton-transfer (10−19s−1). OS ET as a sequential proton-coupled ET reaction is thermodynamically unfavorable (+102kJ/mol) as well as kinetically inhibited, where ET is the rate-limiting step (10−12s−1). In contrast, the reduction of U(VI)aq to U(V)aq by Fe(II)aq as an IS ET reaction is both thermodynamically favorable (−16kJ/mol) and kinetically rapid (108s−1); the IS ET rate is several orders of magnitude faster than the OS ET rate. Thus, reduction of U(VI)aq to U(V)aq by Fe(II)aq in a homogenous system could occur if an IS Fe–U complex can be achieved. However, the formation of IS Fe–U complexes in an homogeneous solution is predicted to be low; considerable thermodynamic and kinetic barriers exist to proceed from an OS ET reaction to an IS ET reaction, a process that needs to overcome dehydration of the first solvation shell (+96kJ/mol) and hydrolysis of Fe(II)aq. The computational results complement and further substantiate experimental results where the reduction of U(VI)aq by Fe(II)aq does not occur.

Importance of Proton-Coupled Electron Transfer from Natural Phenolic Compounds in Superoxide Scavenging.

The superoxide O2(·-) scavenging reaction of (+)-catechin (Cat), quercetin (Que), rutin, and α-tocopherol (α-TOH) as natural phenolic compounds is investigated on the basis of electrochemical and ESR spectral measurements with the aid of density functional theory (DFT) calculations. Reversibility of the O2/O2(·-) redox couple is significantly affected by the presence of the phenolic compounds. The catechol moiety of Cat, Que, and rutin plays an essential role in concerted proton-coupled electron transfer (PCET) to HO2(·) derived from O2(·-) to give H2O2 and the corresponding o-benzoquinone radical anions. On the other hand, the presence of α-TOH causes sequential electron and proton transfers to HO2(·) to give H2O2 and the α-tocopheroxyl radical. These electron transfers in the presence of the phenolic compounds are inferred from the ESR spectral measurements. The DFT calculation results suggest that the O2(·-) scavenging reaction of the natural phenolic compounds proceeds efficiently with the one-step concerted PCET or sequential PCET mechanism.

Which Oxidation State Leads to O–O Bond Formation in Cp*Ir(bpy)Cl-Catalyzed Water Oxidation, Ir(V), Ir(VI), or Ir(VII)?

Density functional calculations are used to revisit the reaction mechanism of water oxidation catalyzed by the Cp*Ir(bpy)Cl (Cp* = pentamethylcyclopentadienyl, bpy = 2,2′-bipyridine) complex. One of the experimentally suggested active species [(bpy)Ir(H2O)2(HCOO)Cl]+ can undergo very facile intramolecular formate oxidation at higher oxidation state even though it can also promote O–O bond formation. Therefore, [(bpy)Ir(H2O)2(CH3COO)Cl]+ is here proposed to be the most likely precatalyst as acetate was also experimentally observed after Cp* oxidation. O–O bond formation takes place at the high formal oxidation states of IrVI and IrVII, rather than that of IrV, as suggested before. Three sequential proton-coupled electron transfer oxidations result in the formation of a highly oxidized intermediate, [(bpy)IrVIO(OH)(CH3COO)Cl]+. From this formal IrVI intermediate, O–O bond formation takes place by a water attack on the IrVI=O moiety assisted by the acetate ligand, which abstracts a proton during the attack. The barrier was calculated to be very facile, being 14.7 kcal/mol, in good agreement with experimental kinetic results, which gave a barrier of around 18 kcal/mol. The attack leads to the formation of an IrIV-peroxide intermediate, which undergoes proton-coupled electron transfer to form an IrIII–O2 intermediate. Finally, O2 can be released, coupled with the binding of another water molecule, to regenerate the catalytic IrIII species. Water oxidation at IrVII has a slightly higher barrier, but it may also contribute to the activity. However, water oxidation at IrV has a significantly higher barrier. Acetate oxidation by C–H activation was found to have a much higher barrier, suggesting that [(bpy)Ir(H2O)2(CH3COO)Cl]+ is a remarkably stable catalyst. The possible catalytic species [(bpy-dc)IrIII(H2O)3Cl]2+ without acetate coordination has also been considered and also gave a reasonably feasible barrier for the water oxidation. O–O bond formation at IrVII is slightly preferred compared with at IrVI, which is different from the case with acetate.

An analysis of model proton-coupled electron transfer reactions via the mixed quantum-classical Liouville approach.

The nonadiabatic dynamics of model proton-coupled electron transfer (PCET) reactions is investigated for the first time using a surface-hopping algorithm based on the solution of the mixed quantum-classical Liouville equation (QCLE). This method provides a rigorous treatment of quantum coherence/decoherence effects in the dynamics of mixed quantum-classical systems, which is lacking in the molecular dynamics with quantum transitions surface-hopping approach commonly used for simulating PCET reactions. Within this approach, the protonic and electronic coordinates are treated quantum mechanically and the solvent coordinate evolves classically on both single adiabatic surfaces and on coherently coupled pairs of adiabatic surfaces. Both concerted and sequential PCET reactions are studied in detail under various subsystem-bath coupling conditions and insights into the dynamical principles underlying PCET reactions are gained. Notably, an examination of the trajectories reveals that the system spends the majority of its time on the average of two coherently coupled adiabatic surfaces, during which a phase enters into the calculation of an observable. In general, the results of this paper demonstrate the applicability of QCLE-based surface-hopping dynamics to the study of PCET and emphasize the importance of mean surface evolution and decoherence effects in the calculation of PCET rate constants.

Investigations of Model Proton-Coupled Electron Transfer Reactions from a Mixed Quantum-Classical Liouville Perspective

Many biological processes involve the transfer of electrons from a donor to an acceptor accompanied by the transfer of protons. This phenomenon, known as proton-coupled electron transfer (PCET), is at the heart of energy conversion reactions in photosynthesis and respiration. Theoretical methods, which treat the coupled dynamics of the transferring protons, electrons, and their environment in a quantum mechanical fashion, are required for accurately describing PCET reactions. However, the size and complexity of real systems render the application of a full quantum treatment computationally impossible. Hybrid methods which treat the transferring protons and electrons quantum mechanically, but treat the donor, acceptor, and solvent molecules classically, offer feasible yet accurate alternatives to full quantum treatments. In this study, for the first time, we adopt a surface-hopping approach based on the solution of the quantum-classical Liouville equation (QCLE) for the study of PCET reactions. The advantage of this approach over other nonadiabatic dynamics methods is that it inherently accounts for quantum coherence effects in the dynamics through phases accumulated during trajectory segments on the means of two adiabatic surfaces. As a starting point, we consider a simple model, which is comprised of three coupled degrees of freedom: an electronic coordinate, a protonic coordinate, and a solvent coordinate. Varying the parameters in the model allows us to study both concerted and sequential PCET mechanisms. For each mechanism considered, we investigate the role played by mean surface evolution in an ensemble of surface-hopping trajectories and discuss the implications of decoherence of the proton-electron subsystem on the rates of these reactions.

Theoretical Analysis of the Sequential Proton-Coupled Electron Transfer Mechanisms for H2 Oxidation and Production Pathways Catalyzed by Nickel Molecular Electrocatalysts

The design of electrocatalysts for the oxidation and production of H2 is important for the development of alternative energy sources. This Article focuses on the [Ni(P2RN2R′)2]2+ electrocatalysts, where P2RN2R′ denotes 1,5-diaza-3,7-diphosphacyclooctane ligands with substituent groups R and R′ covalently bound to the phosphorus and nitrogen atoms, respectively. Theoretical methods are used to investigate the mechanism of the step in the catalytic cycle corresponding to [HNiII(P2N2)2]+ – e– → [NiI(P2HN2)(P2N2)]2+ for H2 oxidation and the reverse reaction for H2 production. This step involves electron transfer (ET) between the Ni complex and the electrode as well as proton transfer (PT) between the Ni and the N. The sequential mechanisms, PT–ET and ET–PT, are investigated for the following (R,R′) substituents: (Me,Me), (Ph,Ph), and (Ph,Bz), where Me, Ph, and Bz denote methyl, phenyl, and benzyl substituents. Density functional theory is used to calculate reduction potentials, pKa values, and PT pathways, and the inner- and outer-sphere reorganization energies for electrochemical ET are calculated within the framework of Marcus theory. For the (Ph,Ph) and (Ph,Bz) systems, the sequential PT–ET mechanism for H2 production would require surmounting a large free energy barrier for the initial PT step, followed by thermodynamically favorable ET. The sequential ET–PT mechanism for these systems would require a moderate initial applied overpotential, followed by a PT reaction with a relatively low free energy barrier. Consistent with experimental data, the calculated overpotential required for the initial reduction in the ET–PT mechanism is lower for the (Ph,Bz) system than for the (Ph,Ph) system for H2 production. The concerted mechanism, in which the electron and proton transfer simultaneously without a stable intermediate, may be thermodynamically favorable and is a direction of future research.

Evidence for a One‐Electron Mechanistic Regime in Dirhodium‐Catalyzed Intermolecular CH Amination

Swift and energy efficient conversion of chemical feedstocks to pharmaceuticals and agrochemicals requires the development of new methods to add nitrogen functionality to unfunctionalized organic substrates. Dirhodium-catalyzed insertion of nitrene species into C-H bonds is a promising new method, the main drawback of which is the currently limited understanding of the catalytic mechanism. Herein, cyclic voltammetry and controlled potential electrolysis measurements have enabled us to solve many of the mechanistic mysteries of intermolecular C-H amination catalyzed by [Rh(2)(esp)(2)] (esp=α,α,α',α'-tetramethyl-1,3-benzenedipropanoate). The primary result is that, in addition to a simple nitrene-transfer mechanism that dominates the early stages of the reaction, another mechanism is available that relies on sequential proton-coupled electron transfer steps. Whereas the nitrene-transfer mechanism requires the use of expensive, atom-inefficient oxidants, we show that simple one-electron oxidants such as Ce(4+) may be used to achieve catalytic C-H amination via the one-electron mechanistic regime.

Theory of Proton-Coupled Electron Transfer in Energy Conversion Processes

Proton-coupled electron transfer (PCET) reactions play an essential role in a broad range of energy conversion processes, including photosynthesis and respiration. These reactions also form the basis of many types of solar fuel cells and electrochemical devices. Recent advances in the theory of PCET enable the prediction of the impact of system properties on the reaction rates. These predictions may guide the design of more efficient catalysts for energy production, including those based on artificial photosynthesis and solar energy conversion. This Account summarizes the theoretically predicted dependence of PCET rates on system properties and illustrates potential approaches for tuning the reaction rates in chemical systems. A general theoretical formulation for PCET reactions has been developed over the past decade. In this theory, PCET reactions are described in terms of nonadiabatic transitions between the reactant and product electron-proton vibronic states. A series of nonadiabatic rate constant expressions for both homogeneous and electrochemical PCET reactions have been derived in various well-defined limits. Recently this theory has been extended to include the effects of solvent dynamics and to describe ultrafast interfacial PCET. Analysis of the rate constant expressions provides insight into the underlying physical principles of PCET and enables the prediction of the dependence of the rates on the physical properties of the system. Moreover, the kinetic isotope effect, which is the ratio of the rates for hydrogen and deuterium, provides a useful mechanistic probe. Typically the PCET rate will increase as the electronic coupling and temperature increase and as the total reorganization energy and equilibrium proton donor-acceptor distance decrease. The rate constant is predicted to increase as the driving force becomes more negative, rather than exhibit turnover behavior in the inverted region, because excited vibronic product states associated with low free energy barriers and relatively large vibronic couplings become accessible. The physical basis for the experimentally observed pH dependence of PCET reactions has been debated in the literature. When the proton acceptor is a buffer species, the pH dependence may arise from the protonation equilibrium of the buffer. It could also arise from kinetic complexity of competing concerted and sequential PCET reaction pathways. In electrochemical PCET, the heterogeneous rate constants and current densities depend strongly on the overpotential. The change in equilibrium proton donor-acceptor distance upon electron transfer may lead to asymmetries in the Tafel plots and deviations of the transfer coefficient from the standard value of one-half at zero overpotential. Applications of this theory to experimentally studied systems illustrate approaches that can be utilized to tune the PCET rate. For example, the rate can be tuned by changing the pH or using different buffer species as proton acceptors. The rate can also be tuned with site-specific mutagenesis in biological systems or chemical modifications that vary the substituents on the redox species in chemical systems. Understanding the impact of these changes on the PCET rate may assist experimental efforts to enhance energy conversion processes.

Phenolate hydroxylation in a bis(mu-oxo)dicopper(III) complex: lessons from the guanidine/amine series.

A new hybrid permethylated-amine-guanidine ligand based on a 1,3-propanediamine backbone (2L) and its Cu-O2 chemistry is reported. [(2L)CuI(MeCN)]1+ complex readily oxygenates at low temperatures in polar aprotic solvents to form a bis(mu-oxo)dicopper(III) (O) species (2b), similar to the parent bis-guanidine ligand complex (1b) and permethylated-diamine ligand complex (3b). UV-vis and X-ray absorption spectroscopy experiments confirm this assignment of 2b as an O species, and full formation of the 2:1 Cu-O2 complex is demonstrated by an optical titration with ferrocene-monocarboxylic acid (FcCOOH). The UV-vis spectra of 1b and 2b with guanidine ligation show low-intensity visible features assigned as guanidine pi --> Cu2O2 core transitions by time-dependent density functional theory (TD-DFT) calculations. Comparison of the reactivity among the three related complexes (1b-3b) with phenolate at 195 K is particularly insightful as only 2b hydroxylates 2,4-di-tert-butylphenolate to yield 3,5-di-tert-butylcatecholate (>95% yield) with the oxygen atom derived from O2, reminiscent of tyrosinase reactivity. 1b is unreactive, while 3b yields the C-C radical-coupled bis-phenol product. Attenuated outer-sphere oxidative strength of the O complexes and increased phenolate accessibility to the Cu2O2 core are attributes that correlate with phenolate hydroxylation reactivity observed in 2b. The comparative low-temperature reactivity of 1b-3b with FcCOOH (O-H BDE 71 kcal mol(-1)) to form the two-electron, two-proton reduced bis(mu-hydroxo)dicopper(II,II) complex is quantitative and presumably precedes through two sequential proton-coupled electron transfer (PCET) steps. Optical titrations along with DFT calculations support that the reduced complexes formed in the first step are more powerful oxidants than the parent O complexes. These mechanistic insights aid in understanding the phenol to bis-phenol reactivity exhibited by 2b and 3b.