Solvent and Temperature Effects on Photoinduced Proton-Coupled Electron Transfer in the Marcus Inverted Region

Concerted proton-coupled electron transfer (PCET) in the Marcus inverted region was recently demonstrated (Science 2019, 364, 471-475). Understanding the requirements for such reactivity is fundamentally important and holds promise as a design principle for solar energy conversion systems. Herein, we investigate the solvent polarity and temperature dependence of photoinduced proton-coupled charge separation (CS) and charge recombination (CR) in anthracene-phenol-pyridine triads: 1 (10-(4-hydroxy-3-(4-methylpyridin-2-yl)benzyl)anthracene-9-carbonitrile) and 2 (10-(4-hydroxy-3-(4-methoxypyridin-2-yl)benzyl)anthracene-9-carbonitrile). Both the CS and CR rate constants increased with increasing polarity in acetonitrile:n-butyronitrile mixtures. The kinetics were semi-quantitatively analyzed where changes in dielectric and refractive index, and thus consequently changes in driving force (-ΔG°) and reorganization energy (λ), were accounted for. The results were further validated by fitting the temperature dependence, from 180 to 298 K, in n-butyronitrile. The analyses support previous computational work where transitions to proton vibrational excited states dominate the CR reaction with a distinct activation free energy (ΔG*CR ∼ 140 meV). However, the solvent continuum model fails to accurately describe the changes in ΔG° and λ with temperature via changes in dielectric constant and refractive index. Satisfactory modeling was obtained using the results of a molecular solvent model [J. Phys. Chem. B 1999, 103, 9130-9140], which predicts that λ decreases with temperature, opposite to that of the continuum model. To further assess the solvent polarity control in the inverted region, the reactions were studied in toluene. Nonpolar solvents decrease both ΔG°CR and λ, slowing CR into the nanosecond time regime for 2 in toluene at 298 K. This demonstrates how PCET in the inverted region may be controlled to potentially use proton-coupled CS states for efficient solar fuel production and photoredox catalysis.

Bimodal Evans-Polanyi Relationships in Hydrogen Atom Transfer from C(sp3)-H Bonds to the Cumyloxyl Radical. A Combined Time-Resolved Kinetic and Computational Study

The applicability of the Evans-Polanyi (EP) relationship to HAT reactions from C(sp3)-H bonds to the cumyloxyl radical (CumO•) has been investigated. A consistent set of rate constants, kH, for HAT from the C-H bonds of 56 substrates to CumO•, spanning a range of more than 4 orders of magnitude, has been measured under identical experimental conditions. A corresponding set of consistent gas-phase C-H bond dissociation enthalpies (BDEs) spanning 27 kcal mol-1 has been calculated using the (RO)CBS-QB3 method. The log kH' vs C-H BDE plot shows two distinct EP relationships, one for substrates bearing benzylic and allylic C-H bonds (unsaturated group) and the other one, with a steeper slope, for saturated hydrocarbons, alcohols, ethers, diols, amines, and carbamates (saturated group), in line with the bimodal behavior observed previously in theoretical studies of reactions promoted by other HAT reagents. The parallel use of BDFEs instead of BDEs allows the transformation of this correlation into a linear free energy relationship, analyzed within the framework of the Marcus theory. The ΔG⧧HAT vs ΔG°HAT plot shows again distinct behaviors for the two groups. A good fit to the Marcus equation is observed only for the saturated group, with λ = 58 kcal mol-1, indicating that with the unsaturated group λ must increase with increasing driving force. Taken together these results provide a qualitative connection between Bernasconi's principle of nonperfect synchronization and Marcus theory and suggest that the observed bimodal behavior is a general feature in the reactions of oxygen-based HAT reagents with C(sp3)-H donors.

Oriented Electrostatic Effects on O2 and CO2 Reduction by a Polycationic Iron Porphyrin

Next-generation energy technologies require improved methods for rapid and efficient chemical-to-electrical energy transformations. One new approach has been to include atomically positioned, electrostatic motifs in molecular catalysts to stabilize high-energy, charged intermediates. For example, an iron porphyrin bearing four cationic, o-N,N,N-trimethylanilinium groups (o-[N(CH₃)₃]⁺) has recently been used to catalyze the complex, multistep O₂ and CO₂ reduction reactions (ORR and CO₂RR) with fast rates and at low overpotentials. The success of this catalyst is attributed, at least in part, to specific charge-charge interactions between the atomically positioned o-[N(CH₃)₃]⁺ groups and the bound substrate. However, by nature of the mono-ortho substitution pattern, there are four possible atropisomers of this metalloporphyrin and thus four unique electrostatic environments. This work reports that each of the four individual atropisomers catalyzes both the ORR and CO₂RR with fast rates and low overpotentials. The maximum turnover frequencies vary among the atropisomers, by a factor of 60 for the ORR and a factor of 5 for CO₂RR. For the ORR, the αβαβ isomer is the fastest and has the highest overpotential, while for the CO₂RR the αααα isomer is the fastest and has the highest overpotential. The role of charge positioning is complex and can affect more than a single step such as CO₂ binding. These data offer a first-of-a-kind perspective on atomically positioned charge and highlight the significance of high charge density, rather than orientation, on the thermodynamics and kinetics of multistep molecular electrochemical transformations.

Interfacial Acid-Base Equilibria and Electric Fields Concurrently Probed by In Situ Surface-Enhanced Infrared Spectroscopy

Understanding how applied potentials and electrolyte solution conditions affect interfacial proton (charge) transfers at electrode surfaces is critical for electrochemical technologies. Herein, we examine mixed self-assembled monolayers (SAMs) of 4-mercaptobenzoic acid (4-MBA) and 4-mercaptobenzonitrile (4-MBN) on gold using in situ surface-enhanced infrared absorption spectroscopy (SEIRAS). Measurements as a function of the applied potential, the electrolyte pD, and the electrolyte concentration determined both the relative surface populations of acidic and basic forms of 4-MBA, as well as the local electric fields at the SAM-solution interface by following the Stark shifts of 4-MBN. The effective acidity of the SAM varied with the applied potential, requiring a 600 mV change to move the pK_a by one unit. Since this is ca. 10× the Nernstian value of 59 mV/pK_a, ∼90% of the applied potential dropped across the SAM layer. This emphasizes the importance of distinguishing applied potentials from the potential experienced at the interface. We use the measured interfacial electric fields to estimate the experienced potential at the SAM edge. The SAM pK_a showed a roughly Nernstian dependence on this estimated experienced potential. An analysis of the combined acid-base equilibria and Stark shifts reveals that the interfacial charge density has significant contributions from both SAM carboxylate headgroups and electrolyte components. Ion pairing and ion penetration into the SAM also influence the observed surface acidity. To our knowledge, this study is the first concurrent examination of both effective acidity and electric fields, and highlights the relevance of experienced potentials and specific ion effects at functionalized electrode surfaces.

Nanoparticle O-H Bond Dissociation Free Energies from Equilibrium Measurements of Cerium Oxide Colloids

A novel equilibrium strategy for measuring the hydrogen atom affinity of colloidal metal oxide nanoparticles is presented. Reactions between oleate-capped cerium oxide nanoparticle colloids (nanoceria) and organic proton-coupled electron transfer (PCET) reagents are used as a model system. Nanoceria redox changes, or hydrogen loadings, and overall reaction stoichiometries were followed by both ¹H NMR and X-ray absorption near-edge spectroscopies. These investigations revealed that, in many cases, reactions between nanoceria and PCET reagents reach equilibrium states with good mass balance. Each equilibrium state is a direct measure of the bond strength, or bond dissociation free energy (BDFE), between nanoceria and hydrogen. Further studies, including those with larger nanoceria, indicated that the relevant bond is a surface O-H. Thus, we have measured surface O-H BDFEs for nanoceria-the first experimental BDFEs for any nanoscale metal oxide. Remarkably, the measured CeO-H BDFEs span 13 kcal mol^-1 (0.56 eV) with changes in the average redox state of the nanoceria colloid. Possible chemical models for this strong dependence are discussed. We propose that the tunability of ceria BDFEs may be important in explaining its effectiveness in catalysis. More generally, metal oxide BDFEs have been used as predictors of catalyst efficacy that, traditionally, have only been accessible by computational methods. These results provide important experimental benchmarks for metal oxide BDFEs and demonstrate that the concepts of molecular bond strength thermochemistry can be applied to nanoscale materials.

Multiple selectivity-determining mechanisms of H2O2 formation in iron porphyrin-catalysed oxygen reduction

Multiple H2O2-forming mechanisms are accessible in Fe(porphyrin)-catalysed oxygen reduction, a key reaction in both fuel cell technologies and oxygen-utilizing enzymes. Our kinetic analysis reveals that the porphyrin secondary structure dictates the pathway for H2O2 formation. This approach is generalizable to other electrocatalytic processes and provides insight into the selectivity-determining steps.

Different Kinetic Reactivity of Electrons in Distinct TiO2 Nanoparticle Trap States

Electrons added to TiO₂ and other semiconductors often occupy trap states, whose reactivity can determine the catalytic and stoichiometric chemistry of the material. We previously showed that reduced aqueous colloidal TiO₂ nanoparticles have two distinct classes of thermally-equilibrated trapped electrons, termed Red/e ^- and Blue/e ^-. Presented here are parallel optical and electron paramagnetic resonance (EPR) kinetic studies of the reactivity of these electrons with solution-based oxidants. Optical stopped-flow measurements monitoring reactions of TiO₂/e ^- with sub-stoichiometric oxidants showed a surprising pattern: an initial fast (seconds) decrease in TiO₂/e ^- absorbance followed by a secondary, slow (minutes) increase in the broad TiO₂/e ^- optical feature. Analysis revealed that the fast decrease is due to the preferential oxidation of the Red/e ^- trap states, and the slow increase results from re-equilibration of electrons from Blue to Red states. This kinetic model was confirmed by freeze-quench EPR measurements. Quantitative analysis of the kinetic data demonstrated that Red/e ^- react ~5 times faster than Blue/e ^- with the nitroxyl radical oxidant, 4-MeO-TEMPO. Similar reactivity patterns were also observed in oxidations of TiO₂/e ^- by O₂, which like 4-MeO-TEMPO is a proton-coupled electron transfer (PCET) oxidant, and by the pure electron transfer (ET) oxidant KI₃. This suggests that the faster intrinsic reactivity of one trap state over another on the seconds-minutes timescale is likely a general feature of reduced TiO₂ reactivity. This differential trap state reactivity is likely to influence the performance of TiO₂ in photochemical/electrochemical devices, and it suggests an opportunity for tuning catalysis.

General Light-Mediated, Highly Diastereoselective Piperidine Epimerization: From Most Accessible to Most Stable Stereoisomer

We report a combined photocatalytic and hydrogen atom transfer (HAT) approach for the light-mediated epimerization of readily accessible piperidines to provide the more stable diastereomer with high selectivity. The generality of the transformation was explored for a large variety of di- to tetrasubstituted piperidines with aryl, alkyl, and carboxylic acid derivatives at multiple different sites. Piperidines without substitution on nitrogen as well as N-alkyl and aryl derivatives were effective epimerization substrates. The observed diastereoselectivities correlate with the calculated relative stabilities of the isomers. Demonstration of reaction reversibility, luminescence quenching, deuterium labeling studies, and quantum yield measurements provide information about the mechanism.

Two-Electron-Two-Proton Transfer from Colloidal ZnO and TiO2 Nanoparticles to Molecular Substrates

Transfers of multiple electrons and protons are challenging yet central to many energy-conversion processes and other chemical and biochemical reactions. Semiconducting oxides can hold multiple redox equivalents. This study describes the 2e^-/2H⁺ transfer reactivity of photoreduced ZnO and TiO₂ nanoparticle (NP) colloids with molecular 2e^-/2H⁺ acceptors, to form new O-H, N-H, and C-H bonds. The reaction stoichiometries were monitored by NMR and optical spectroscopies. Faster 2e^-/2H⁺ transfer rates were observed for substrates forming O-H or N-H bonds, presumably due to initial hydrogen bonding at the oxide surface. Chemically reduced ZnO NPs stabilized by Na⁺ or Ca²⁺ also engage in 2e^-/2H⁺ transfer reactivity, showing that protons transferred in these processes are inherent to the oxide nanoparticles and do not exclusively stem from photoreduction. These results highlight the potential of ZnO and TiO₂ for multiple proton-coupled electron transfer (PCET) reactions.

Theoretical Study of Shallow Distance Dependence of Proton-Coupled Electron Transfer in Oligoproline Peptides

Long-range electron transfer is coupled to proton transfer in a wide range of chemically and biologically important processes. Recently the proton-coupled electron transfer (PCET) rate constants for a series of biomimetic oligoproline peptides linking Ru(bpy)₃²⁺ to tyrosine were shown to exhibit a substantially shallower dependence on the number of proline spacers compared to the analogous electron transfer (ET) systems. The experiments implicated a concerted PCET mechanism involving intramolecular electron transfer from tyrosine to Ru(bpy)₃³⁺ and proton transfer from tyrosine to a hydrogen phosphate dianion. Herein these PCET systems, as well as the analogous ET systems, are studied with microsecond molecular dynamics, and the ET and PCET rate constants are calculated with the corresponding nonadiabatic theories. The molecular dynamics simulations illustrate that smaller ET donor-acceptor distances are sampled by the PCET systems than by the analogous ET systems. The shallower dependence of the PCET rate constant on the ET donor-acceptor distance is explained in terms of an additional positive, distance-dependent electrostatic term in the PCET driving force, which attenuates the rate constant at smaller distances. This electrostatic term depends on the change in the electrostatic interaction between the charges on each end of the bridge and can be modified by altering these charges. On the basis of these insights, this theory predicted a less shallow distance dependence of the PCET rate constant when imidazole rather than hydrogen phosphate serves as the proton acceptor, even though their pK_a values are similar. This theoretical prediction was subsequently validated experimentally, illustrating that long-range electron transfer processes can be tuned by modifying the nature of the proton acceptor in concerted PCET processes. This level of control has broad implications for the design of more effective charge-transfer systems.

Shallow Distance Dependence for Proton-Coupled Tyrosine Oxidation in Oligoproline Peptides

We have explored the kinetic effect of increasing electron transfer (ET) distance in a biomimetic, proton-coupled electron-transfer (PCET) system. Biological ET often occurs simultaneously with proton transfer (PT) in order to avoid the high-energy, charged intermediates resulting from the stepwise transfer of protons and electrons. These concerted proton-electron-transfer (CPET) reactions are implicated in numerous biological ET pathways. In many cases, PT is coupled to long-range ET. While many studies have shown that the rate of ET is sensitive to the distance between the electron donor and acceptor, extensions to biological CPET reactions are sparse. The possibility of a unique ET distance dependence for CPET reactions deserves further exploration, as this could have implications for how we understand biological ET. We therefore explored the ET distance dependence for the CPET oxidation of tyrosine in a model system. We prepared a series of metallopeptides with a tyrosine separated from a Ru(bpy)32+ complex by an oligoproline bridge of increasing length. Rate constants for intramolecular tyrosine oxidation were measured using the flash-quench transient absorption technique in aqueous solutions. The rate constants for tyrosine oxidation decreased by 125-fold with three added proline residues between tyrosine and the oxidant. By comparison, related intramolecular ET rate constants in very similar constructs were reported to decrease by 4-5 orders of magnitude over the same number of prolines. The observed shallow distance dependence for tyrosine oxidation is proposed to originate in part from the requirement for stronger oxidants, leading to a smaller hole-transfer effective tunneling barrier height. The shallow distance dependence observed here and extensions to distance-dependent CPET reactions have potential implications for long-range charge transfers.

Characterization of the non-covalent docking motif in the isolated reactant complex of a double proton-coupled electron transfer reaction with cryogenic ion spectroscopy

The solution kinetics of a proton-coupled electron transfer reaction involving two-electron oxidation of a Ru compound with concomitant transfer of two protons to a quinone derivative have been interpreted to indicate the formation of a long-lived intermediate between the reactants. We characterize the ionic reactants, products, and an entrance channel reaction complex in the gas phase using high-resolution mass spectrometry augmented by cryogenic ion IR photodissociation spectroscopy. Collisional activation of this trapped entrance channel complex does not drive the reaction to products but rather yields dissociation back to reactants. Electronic structure calculations indicate that there are four low-lying isomeric forms of the non-covalently bound complex. Comparison of their predicted vibrational spectra with the observed band pattern indicates that the C=O groups of the ortho-quinone attach to protons on two different -NH2 groups of the reactant scaffold, exhibiting strong O-H-N contact motifs. Since collisional activation does not lead to the products observed in the liquid phase, these results indicate that the reaction most likely proceeds through reorientation of the H-atom donor ligand about the metal center.

Developing Scaling Relationships for Molecular Electrocatalysis through Studies of Fe-Porphyrin-Catalyzed O2 Reduction

The oxygen reduction reaction (ORR) is a multiproton/multielectron transformation in which dioxygen (O₂) is reduced to water or hydrogen peroxide and serves as the cathode reaction in most fuel cells. The ORR (O₂ + 4e^- + 4H⁺ → 2H₂O) involves up to nine substrates and thus requires navigating a complicated reaction landscape, typically with several high-energy intermediates. Many catalysts can perform this reaction, though few operate with fast rates and at low overpotentials (close to the thermodynamic potential). Attempts to optimize these parameters, both in homogeneous and heterogeneous electrocatalytic systems, have focused on modifying catalyst design and understanding kinetic/thermodynamic relationships between catalytic intermediates. One such method for analyzing and predicting catalyst reactivity and efficiency has been the development of "molecular scaling relationships". Here, we share our experience deriving and utilizing molecular scaling relationships for soluble, iron-porphyrin-catalyzed O₂ reduction in organic solvents. These relationships correlate turnover frequencies (TOF_max) and effective overpotentials (η_eff), properties uniquely defined for homogeneous catalysts. Following a general introduction of scaling relationships for both homogeneous and heterogeneous electrocatalysis, we describe the components of such scaling relationships: (i) the overall thermochemistry of the reaction and (ii) the rate and rate law of the catalyzed reaction. We then show how connecting these thermodynamic and kinetic parameters reveals multiple molecular scaling relationships for iron-porphyrin-catalyzed O₂ reduction. For example, the log(TOF_max) responds steeply to changes in η_eff that result from different catalyst reduction potentials (18.5 decades in TOF_max/V in η_eff) but much less dramatically to changes in η_eff that arise from varying the pK_a of the acid buffer (5.1 decades in TOF_max/V in η_eff). Thus, a single scaling relationship is not always sufficient for describing molecular electrocatalysis. This is particularly evident when the catalyst identity and reaction conditions are coupled. Using these multiple scaling relationships, we demonstrate that the metrics of turnover frequency and effective overpotential can be predictably tuned to achieve faster rates at lowered overpotentials. This Account uses a collection of related stories describing our research on soluble iron-porphyrin-catalyzed ORR to show how molecular scaling relationships can be derived and used for any electrocatalytic reaction. Such scaling relationships are powerful tools that connect the thermochemistry, mechanism, and rate law for a catalytic system. We hope that this collection shows the utility and simplicity of the molecular scaling approach for understanding catalysis, for enabling direct comparisons between catalyst systems, and for optimizing catalytic processes.

Base-Directed Photoredox Activation of C-H Bonds by PCET

Photoredox catalysis using proton-coupled electron transfer (PCET) has emerged as a powerful method for bond transformations. We previously employed traditional chemical oxidants to achieve multiple-site concerted proton-electron transfer (MS-CPET) activation of a C-H bond in a proof-of-concept fluorenyl-benzoate substrate. As described here, photoredox oxidation of the fluorenyl-benzoate follows the same rate constant vs driving force trend determined for thermal MS-CPET. Analogous photoredox catalysis enables C-H activation and H/D exchange in a number of additional substrates with favorably positioned bases. Mechanistic studies support our hypothesis that MS-CPET is a viable pathway for bond activation for substrates in which the C-H bond is weak, while stepwise carboxylate oxidation and hydrogen atom transfer likely predominate for stronger C-H bonds.

Highly Diastereoselective Functionalization of Piperidines by Photoredox-Catalyzed α-Amino C-H Arylation and Epimerization

We report a photoredox-catalyzed α-amino C-H arylation reaction of highly substituted piperidine derivatives with electron-deficient cyano(hetero)arenes. The scope and limitations of the reaction were explored, with piperidines bearing multiple substitution patterns providing the arylated products in good yields and with high diastereoselectivity. To probe the mechanism of the overall transformation, optical and fluorescent spectroscopic methods were used to investigate the reaction. By employing flash-quench transient absorption spectroscopy, we were able to observe electron transfer processes associated with radical formation beyond the initial excited-state Ir(ppy)₃ oxidation. Following the rapid and unselective C-H arylation reaction, a slower epimerization occurs to provide the high diastereomer ratio observed for a majority of the products. Several stereoisomerically pure products were resubjected to the reaction conditions, each of which converged to the experimentally observed diastereomer ratios. The observed distribution of diastereomers corresponds to a thermodynamic ratio of isomers based upon their calculated relative energies using density functional theory (DFT).

Selectivity-Determining Steps in O2 Reduction Catalyzed by Iron(tetramesitylporphyrin)

The oxygen reduction reaction (ORR) is the cathode reaction in fuel cells and its selectivity for water over hydrogen peroxide production is important for these technologies. Iron porphyrin catalysts have long been studied for the ORR, but the origins of their selectivity are not well understood because the selectivity-determining step(s) usually occur after the rate-determining step. We report here the effects of acid concentration, as well as other solution conditions such as acid pKa, on the H2O2/H2O selectivity in electrocatalytic ORR by iron(tetramesitylporphyrin) (Fe(TMP)) in DMF. The results show that selectivity reflects a kinetic competition in which the dependence on [HX] is one order greater for the production of H2O than H2O2. Based on such experimental results and computational studies, we propose that the selectivity is governed by competition between protonation of the hydroperoxo intermediate, FeIII(TMP)(OOH), to produce water versus dissociation of the HOO- ligand to yield H2O2. The data rule out a bifurcation based on the regioselectivity of protonation of the hydroperoxide, as suggested in the enzymatic systems. Furthermore, the analysis developed in this report should be generally valuable to the study of selectivity in other multi-proton/multi-electron electrocatalytic reactions.

Combining scaling relationships overcomes rate versus overpotential trade-offs in O2 molecular electrocatalysis

The development of advanced chemical-to-electrical energy conversions requires fast and efficient electrocatalysis of multielectron/multiproton reactions, such as the oxygen reduction reaction (ORR). Using molecular catalysts, correlations between the reaction rate and energy efficiency have recently been identified. Improved catalysis requires circumventing the rate versus overpotential trade-offs implied by such "scaling relationships." Described here is an ORR system-using a soluble iron porphyrin and weak acids-with the best reported combination of rate and efficiency for a soluble ORR catalyst. This advance is achieved not by "breaking" scaling relationships but rather by combining two of them. Key to this behavior is a polycationic ligand, which enhances anionic ligand binding and changes the catalyst E _1/2. These results show how combining scaling relationships is a powerful way toward improved electrocatalysis.

Cooperation of cerium oxide nanoparticles and soluble molecular catalysts for alcohol oxidation

Nano-cerium oxide and organometallic catalysts cooperate in anaerobic and aerobic alcohol oxidations.

Low Reorganization Energy for Electron Self-Exchange by a Formally Copper(III,II) Redox Couple

The rate constant for electron self-exchange (k₁₁) between LCuOH and [LCuOH]^- (L = bis-2,6-(2,6-diisopropylphenyl)carboximidopyridine) was determined using the Marcus cross relation. This work involved measurement of the rate of the cross-reaction between [Bu₄N][LCuOH] and [Fc][BAr₄^F] (Fc⁺ = ferrocenium; BAr₄^F = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate)) by stopped-flow methods at -88 °C in CH₂Cl₂ and measurement of the equilibrium constant for the redox process by UV-vis titrations under the same conditions. A value of k₁₁ = 3 × 10⁴ M^-1 s^-1 (-88 °C) led to estimation of a value 9 × 10⁶ M^-1 s^-1 at 25 °C, which is among the highest values known for copper redox couples. Further Marcus analysis enabled determination of a low reorganization energy, λ = 0.95 ± 0.17 eV, attributed to minimal structural variation between the redox partners. In addition, the reaction entropy (ΔS°) associated with the LCuOH/[LCuOH]^- self-exchange was determined from the temperature dependence of the redox potentials, and found to be dependent upon ionic strength. Comparisons to other Cu redox systems and potential new applications for the formally Cu^III,II system are discussed.