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 CuIII–OH complex and organic substrates, involving hydride
transfer coupled with a reversed electron transfer from CuIII–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 NiII–OH complex and TEMPOH,
where two β-electrons are engaged in the process: one transiently
and one completely transferred to a NiII-coordinating ligand.