The Mechanism of Photocatalytic C(sp³)-H Activation by Decatungstate Anion: CPET or HAT?

The decatungstate anion ([W₁₀O₃₂]^4-) has demonstrated remarkable success in photocatalytic C(sp³)-H activation under mild conditions, however, its photocatalytic mechanism remains elusive. This work systematically studies the photocatalytic C(sp³)-H activation of [W₁₀O₃₂]^4- through combined theoretical calculations and known experimental evidence. Specifically, upon light irradiation, [W₁₀O₃₂]^4- undergoes excitation to its first singlet excited state (S₁, ¹A_2u), followed by intersystem crossing (ISC) to populate the second triplet excited state (T₂, ³A_1u). The T₂ state was first identified as the reactive electronic state, characterized by both a distinct long-range ligand to metal charge transfer (l-LMCT) and a rare nonradiative anti-Kasha behavior, which facilitates C(sp³)-H bond activation. Importantly, the block localized wave function (BLW) calculations prove that the photocatalytic C(sp³)-H activation proceeds via a concerted proton-electron transfer (CPET) mechanism: the σ(C-H) bonding electron of alkane mainly transfers to the p-orbital of inner-shell oxygen atoms in [W₁₀O₃₂]^4-, while the proton adsorbs the bridging oxygen site on the surface of [W₁₀O₃₂]^4-. This finding fundamentally challenges the conventional hydrogen atom transfer (HAT) mechanism paradigm for the activation C(sp³)-H bond by [W₁₀O₃₂]^4-.