Proton-Coupled Electron Transfer Guidelines, Fair and Square

Abstract

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 (ΔG_PT^° and ΔG_ET^°) 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.

Figure 1

Examples of PCET reactions range from (A) the long-range coupling of ET and PT in hydrogenases and (B) biomimetic PT wires to (C) water oxidation on metal oxides and (D) (photo)redox catalysis by PCET activation. Images were adapted from the original referenced papers: (A) copyright American Association for the Advancement of Science, 2020; (B–D) copyright American Chemical Society, 2018, 2011, and 2013, respectively.

Figure 2

(Left) Square scheme that summarizes the mechanisms by which proton-coupled electron transfer can proceed. The edges of the square show the sequential mechanisms with ETPT and PTET on the top and bottom, respectively. The pathway bisecting the square is concerted, where e^– and H⁺ are transferred without the formation of an intermediate species. Note that the donor and acceptor units for ET and PT can be the same or different species. (Right) Illustration of the three main mechanisms for PCET, each with a distinct transition state.

Figure 3

(Left) Square scheme for phenol oxidation. Approximate E° and pK_a values in water, vs NHE, are given. (Right) Pourbaix diagram for a tyrosine derivative in water, adapted from ref (8), copyright American Chemical Society, 2005.

Figure 4

Lewis structure of phenol and the resonance structures of phenoxyl radical.

Figure 5

(Left) Free energy parabolas for the reactant and product states along the nuclear coordinate for ET. (Right) Electron potentials for R and P states. At the intersection region the energies of donor and acceptor states are equal (dashed line) and the electron can tunnel through the barrier. From ref (23), copyright Uppsala University, 2010.

Figure 6

(Left) Free energy parabolas for the reactants and products as a function of reaction coordinate of a CEPT reaction. (Right) Schematic proton potentials (tunneling coordinate) for the reactants (A), transition state (B), and products (C) (potentials illustrate an electronically adiabatic case, with a single potential surface). From ref (23), copyright Uppsala University, 2010.

Figure 7

(A) Compounds for studies of PT distance dependence for CEPT, where Δr_O··N is the range of O–N distances within each series. (B) Data and linear fits for series C with two different oxidants.

Figure 8

(Top) Structure and (bottom) free-energy dependence for photoinduced CEPT in anthracene–phenol–pyridine molecules. The blue (charge separation) and red (charge recombination) regions indicate qualitative free-energy dependences according to eqs 6 and 7, for three different solvents with different λ. Bottom panel reprinted from ref (25), copyright American Association for the Advancement of Science, 2019.

Figure 9

Qualitative illustration of the different driving force dependencies for the sequential and concerted mechanisms when either ΔG_ET^° or ΔG_PT^° is varied. In each case, one of the rate-limiting mechanisms has a similar dependence as CEPT but lower driving force and larger vibronic coupling.

Figure 10

Structures of the Ru-Tyr compounds.

Figure 11

pH dependence of the rate constant for PCET oxidation of tyrosine in Ru_bpy-Tyr. The dominating mechanism was assigned to ETPT (A), CEPT (B), PTET (C), and ET from tyrosinate (D). Reprinted from ref (46), copyright American Chemical Society, 2012.

Figure 12

pH dependence of tryptophan oxidation in Ru-Trp complexes analogous to the complexes in Figure 10: Ru_bpy-Trp (red), Ru_tmb-Trp (green), and the corresponding bromotryptophan complexes (dark and light blue). Reprinted from ref (75), copyright American Chemical Society, 2011.

Figure 13

Structures of tungsten hydrides, oxidants, and bases.^,,,

Figure 14

Second-order PCET rate constant for oxidation of the W-H compounds 2 by oxidants 4 versus pyridinium pK_a (top) and versus oxidant E° (bottom). (Top) The lines are linear fits to the data with the same oxidant: [Fe((OMe)₂bpy)₃]³⁺, α = 1.03 (gray line, PTET_pre-eq); [Ru(Me₂bpy)₃]³⁺, α = 0.51 (blue line, CEPT); and [Ru(bpy)₃]³⁺, α < 0.1 (orange dashed lines, ETPT). KIE values are given where measured. (Bottom) Linear fits for 2a–d ordered from low to high pK_a value. With the weaker bases (a, b), the mechanism changes from CEPT for oxidant E° = 0.50–0.73 (α = 0.41) to ETPT_pre-eq for E° = 0.73–0.9 (α = 1.03). With the stronger bases (c, d), the reaction was assigned to PTET_pre-eq with a weak dependence on E° (α ≈ 0.08). At E° > 0.9 V, the initial ET is downhill, and the rate levels off. Reprinted from ref (61), copyright American Chemical Society, 2019.

Figure 15

Zone diagrams for oxidative PCET. Thermochemical data for these examples is taken from ref (76), and kinetic factors for the different scenarios are summarized in Table 1.

Figure 16

Electronic ground- and excited-state square schemes (cf. Figure 1) illustrating the different pathways of ground-state CEPT (orange arrow), excited-state CEPT (red arrow), ESIPT (blue arrow), and “photo-EPT” (purple arrow).

Figure 17

Areas of chemistry involving electron transfer as described by Rudolph A. Marcus in his Nobel lecture (redrawn from the original).