Hydrogen Atom Abstraction via Hydride-Coupled Electron Transfer and Its Origin

This study explores hydride-coupled electron transfer (HCET) as a fundamentally distinct mechanism alternative to proton-coupled electron transfer (PCET). HCET was identified in the reaction between a Cu^III-OH complex and organic substrates, involving hydride transfer coupled with a reversed electron transfer from Cu^III-OH to the substrate in a single-barrier step. First, we identified the connection between the thermodynamic cycles and reactivity and showed that the mechanism is dictated by the cycle with more favorable off-diagonal thermodynamics. As evidenced by electronic-structure-based descriptors, the transferred hydrogen atom in HCET gains electron density and volume at the transition state, indicating hydride character, while in PCET, it loses electron density and volume, signaling proton character. Second, intrinsic bond orbital analysis confirmed that HCET is a two-electron process: it involves the complete transfer of the proton and the C-H α-electron from the substrate to the Cu ion, while the β-electron undergoes a transient exchange, initially migrating alongside the α-electron to the Cu center before returning to substrate. An analogous HCET mechanism was identified in the reaction between a Ni^II-OH complex and TEMPOH, where two β-electrons are engaged in the process: one transiently and one completely transferred to a Ni^II-coordinating ligand.