Theoretical insights and predictions: Photocatalytic functionalization of C(sp3)-H bonds in methane using [W10O32]4-

Abstract

Methane activation catalyzed by [MW₉O₃₂]^n- (n = 3–5) and [MW₅O₁₉]^m- (m = 1–3) was investigated through Density Functional Theory (DFT) calculations. We identified the key active species, active site, and reaction mechanism by studying the model reaction of methane hydroalkylation catalyzed by [W₁₀O₃₂]^4-. Systematic comparisons across modified polyoxometalates (POMs) revealed multiple electronic and structural descriptors governing methane activation efficiency. A key discovery involves the role of photoinduced surface-bridging oxygen holes in [W₁₀O₃₂]^4-, which serve as active sites for cleaving the inert C(sp³)H σ-bond. While other POMs, including [W₆O₁₉]^2-, [PW₁₂O₄₀]^3-, and [P₂W₁₈O₆₂]⁶ ^--, also exhibit methane-activating oxygen holes, their requirement for higher-energy UV excitation (compared to the visible-light-responsive [W₁₀O₃₂]4^-) likely explains the latter’s broader utility in organic synthesis. Electronic structure analysis uncovered a near-linear correlation between triplet POM β-LUMO energies and C(sp³)H activation barriers. Crucially, the negative charge density of POMs inversely regulates β-LUMO energetics: lower anionic charges correspond to reduced β-LUMO energy levels and consequently lower activation barriers. This structure–activity relationship implies that strategic charge modulation in POM architectures can fine-tune frontier molecular orbital levels, thereby controlling catalytic performance in methane functionalization. Based on these principles, we propose the low-charge metal-substituted POM [ReW₅O₁₉]^- as a promising candidate for XH bond activation (X = S, C, N, O).