Proton coupled electron transfer

Proton coupled electron transfer (PCET) is an electrochemical reaction mechanism in which an electron and proton is simultaneously moved in a concerted mechanism. [Huynh, M. H.; Meyer, T. "Chem. Rev." 2007, "107", 5004-5064.]

Mechanism

Proton coupled electron transfer (PCET) is defined as when a proton and electron starting in different orbitals are transferred to different end orbitals in a single concerted elementary step. This is a contrast to step wise mechanisms in which the electron and proton are transferred sequentially. It is also distinct from a hydrogen atom transfer (HAT) in which the proton and electron start in the same orbitals and move together to the final orbital commonly recognized as a radical pathway.

HAT

[HM] ⁺ + [M'] ²⁺ → [M] ⁺ + [HM'] ²⁺

initial ET

[HM] ⁺ + [M'] ²⁺ → [HM] ²⁺ + [M'] ⁺

[HM] ²⁺ + [M'] ⁺ → [M] ⁺ + [HM'] ²⁺

initial PT

[HM] ⁺ + [M'] ²⁺ → [M] ⁰ + [HM'] ³⁺

[M] ⁰ + [HM'] ³⁺ → [M] ⁺ + [HM'] ²⁺

PCET

"cis"- [(byp)₂(py)Ru^IV(O)] ²⁺ + "cis"- [(byp)₂(py)Ru^II(OH₂)] ²⁺ → 2"cis"- [(byp)₂(py)Ru^III(OH)] ²⁺

Evidence

It is relatively simple to demonstrate that the electron and proton begin and end in different orbitals, it is more difficult to prove that they do not move sequentially. General sequential pathways are lower in energy than concerted pathways. The main evidence that PCET exists is that a number of reactions occur faster than expected for the sequential pathways. In the initial electron transfer (ET) mechanism the initial redox event has a minimum thermodynamics barrier associate with the first step. Similarly the initial proton transfer (PT) mechanism has a minium barrier associated with the protons initial pK_a. Variations on these minimum barriers are also considered. The important finding is that there are number of reactions with rates greater than these minimum barriers would permit. This suggests a third mechanism lower in energy, the concerted PCET has been offered as this third mechanism. This assertion has also been supported by the observation of unusually large kinetic isotope effects (KIE).

Confusion

In some literature the definition of PCET has been extended to include the sequential mechanisms listed above. This confusion in the definition of PCET has led to the proposal of alternate names including electron transfer-proton transfer (ETPT), electron-proton transfer (EPT), and concerted electron-proton transfer (CEPT).

Context

PCET may be a significant mechanism in many important biological processes including photosynthesis specifically the oxygen evolving complex (OEC) of photosystem II. PCET was discovered and popularized by Thomas J. Mayer while studying ruthenium bipyridine (bpy) complexes and their redox chemistry.

References