Proton-Coupled Electron Transfer Guidelines, Fair and Square.

Proton-coupled electron transfer (PCET) reactions are fundamental to energy transformation reactions in natural and artificial systems and are increasingly recognized in areas such as catalysis and synthetic chemistry. The interdependence of proton and electron transfer brings a mechanistic richness of reactivity, including various sequential and concerted mechanisms. Delineating between different PCET mechanisms and understanding why a particular mechanism dominates are crucial for the design and optimization of reactions that use PCET. This Perspective provides practical guidelines for how to discern between sequential and concerted mechanisms based on interpretations of thermodynamic data with temperature-, pressure-, and isotope-dependent kinetics. We present new PCET-zone diagrams that show how a mechanism can switch or even be eliminated by varying the thermodynamic (ΔGPT° and ΔGET°) and coupling strengths for a PCET system. We discuss the appropriateness of asynchronous concerted PCET to rationalize observations in organic reactions, and the distinction between hydrogen atom transfer and other concerted PCET reactions. Contemporary issues and future prospects in PCET research are discussed.

Strategies for switching the mechanism of proton-coupled electron transfer reactions illustrated by mechanistic zone diagrams.

The mechanism by which proton-coupled electron transfer (PCET) occurs is of fundamental importance and has great consequences for applications, e.g. in catalysis. However, determination and tuning of the PCET mechanism is often non-trivial. Here, we apply mechanistic zone diagrams to illustrate the competition between concerted and stepwise PCET-mechanisms in the oxidation of 4-methoxyphenol by Ru(bpy)3 3+-derivatives in the presence of substituted pyridine bases. These diagrams show the dominating mechanism as a function of driving force for electron and proton transfer (ΔG 0 ET and ΔG 0 PT) respectively [Tyburski et al., J. Am. Chem. Soc., 2021, 143, 560]. Within this framework, we demonstrate strategies for mechanistic tuning, namely balancing of ΔG 0 ET and ΔG 0 PT, steric hindrance of the proton-transfer coordinate, and isotope substitution. Sterically hindered pyridine bases gave larger reorganization energy for concerted PCET, resulting in a shift towards a step-wise electron first-mechanism in the zone diagrams. For cases when sufficiently strong oxidants are used, substitution of protons for deuterons leads to a switch from concerted electron-proton transfer (CEPT) to an electron transfer limited (ETPTlim) mechanism. We thereby, for the first time, provide direct experimental evidence, that the vibronic coupling strength affects the switching point between CEPT and ETPTlim, i.e. at what driving force one or the other mechanism starts dominating. Implications for solar fuel catalysis are discussed.

Understanding the Role of Surface States on Mesoporous NiO Films.

Surface states of mesoporous NiO semiconductor films have particular properties differing from the bulk and are able to dramatically influence the interfacial electron transfer and adsorption of chemical species. To achieve a better performance of NiO-based p-type dye-sensitized solar cells (p-DSCs), the function of the surface states has to be understood. In this paper, we applied a modified atomic layer deposition procedure that is able to passivate 72% of the surface states on NiO by depositing a monolayer of Al2O3. This provides us with representative control samples to study the functions of the surface states on NiO films. A main conclusion is that surface states, rather than the bulk, are mainly responsible for the conductivity in mesoporous NiO films. Furthermore, surface states significantly affect dye regeneration (with I-/I3- as redox couple) and hole transport in NiO-based p-DSCs. A new dye regeneration mechanism is proposed in which electrons are transferred from reduced dye molecules to intra-bandgap states, and then to I3- species. The intra-bandgap states here act as catalysts to assist I3- reduction. A more complete mechanism is suggested to understand the particular hole transport behavior in p-DSCs, in which the hole transport time is independent of light intensity. This is ascribed to the percolation hole hopping on the surface states. When the concentration of surface states was significantly reduced, the light-independent charge transport behavior in pristine NiO-based p-DSCs transformed into having an exponential dependence on light intensity, similar to that observed in TiO2-based n-type DSCs. These conclusions on the function of surface states provide new insight into the electronic properties of mesoporous NiO films.

Elucidating Proton-Coupled Electron Transfer Mechanisms of Metal Hydrides with Free Energy- and Pressure-Dependent Kinetics.

Proton-coupled electron transfer (PCET) was studied in a series of tungsten hydride complexes with pendant pyridyl arms ([(PyCH2Cp)WH(CO)3], PyCH2Cp = pyridylmethylcyclopentadienyl), triggered by laser flash-generated RuIII-tris-bipyridine oxidants, in acetonitrile solution. The free energy dependence of the rate constant and the kinetic isotope effects (KIEs) showed that the PCET mechanism could be switched between concerted and the two stepwise PCET mechanisms (electron-first or proton-first) in a predictable fashion. Straightforward and general guidelines for how the relative rates of the different mechanisms depend on oxidant and base are presented. The rate of the concerted reaction should depend symmetrically on changes in oxidant and base strength, that is on the overall ΔG0PCET, and we argue that an "asynchronous" behavior would not be consistent with a model where the electron and proton tunnel from a common transition state. The observed rate constants and KIEs were examined as a function of hydrostatic pressure (1-2000 bar) and were found to exhibit qualitatively different dependence on pressure for different PCET mechanisms. This is discussed in terms of different volume profiles of the PCET mechanisms as well as enhanced proton tunneling for the concerted mechanism. The results allowed for assignment of the main mechanism operating in the different cases, which is one of the critical questions in PCET research. They also show how the rate of a PCET reaction will be affected very differently by changes of oxidant and base strength, depending on which mechanism dominates. This is of fundamental interest as well as of practical importance for rational design of, for example, catalysts for fuel cells and solar fuel formation, which operate in steps of PCET reactions. The mechanistic richness shown by this system illustrates that the specific mechanism is not intrinsic to a specific synthetic catalyst or enzyme active site but depends on the reaction conditions.

Light Driven Electron Transfer in Methylbipyridine/Phenol Complexes Is Not Proton Coupled.

The pursuit of systems that undergo optical electron proton transfer (photo-EPT) is very attractive, due to the wealth of information contained in the absorption spectra of such complexes. However, separating photo-EPT transitions from conventional charge transfer states remains a major challenge. In this study, we show that optical charge transfer in a complex between 4-methoxyphenol and N-methyl-4,4'-bipyridyl, previously assigned to occur through photo-EPT involving a hydrogen bond between the reactants ( Gagliardi , C. J. ; ; Wang , L. ; ; Dongare , P. ; ; Brennaman , M. K. ; ; Papanikolas , J. M. ; ; Meyer , T. J. ; ; Thompson , D. W. Proc. Natl. Acad. Sci. U.S.A. 2016 , 113 , 11106 - 11109 ), does not lead to protonation of the acceptor molecule. Additionally, we propose that association of the complex is likely due to donor-acceptor interactions rather than hydrogen bonding.

Pourbaix Diagram, Proton-Coupled Electron Transfer, and Decay Kinetics of a Protein Tryptophan Radical: Comparing the Redox Properties of W32• and Y32• Generated Inside the Structurally Characterized α3W and α3Y Proteins.

Protein-based "hole" hopping typically involves spatially arranged redox-active tryptophan or tyrosine residues. Thermodynamic information is scarce for this type of process. The well-structured α3W model protein was studied by protein film square wave voltammetry and transient absorption spectroscopy to obtain a comprehensive thermodynamic and kinetic description of a buried tryptophan residue. A Pourbaix diagram, correlating thermodynamic potentials (E°') with pH, is reported for W32 in α3W and compared to equivalent data recently presented for Y32 in α3Y ( Ravichandran , K. R. ; Zong , A. B. ; Taguchi , A. T. ; Nocera , D. G. ; Stubbe , J. ; Tommos , C. J. Am. Chem. Soc. 2017 , 139 , 2994 - 3004 ). The α3W Pourbaix diagram displays a pKOX of 3.4, a E°'(W32(N•+/NH)) of 1293 mV, and a E°'(W32(N•/NH); pH 7.0) of 1095 ± 4 mV versus the normal hydrogen electrode. W32(N•/NH) is 109 ± 4 mV more oxidizing than Y32(O•/OH) at pH 5.4-10. In the voltammetry measurements, W32 oxidation-reduction occurs on a time scale of about 4 ms and is coupled to the release and subsequent uptake of one full proton to and from bulk. Kinetic analysis further shows that W32 oxidation likely involves pre-equilibrium electron transfer followed by proton transfer to a water or small water cluster as the primary acceptor. A well-resolved absorption spectrum of W32• is presented, and analysis of decay kinetics show that W32• persists ∼104 times longer than aqueous W• due to significant stabilization by the protein. The redox characteristics of W32 and Y32 are discussed relative to global and local protein properties.