Decoupling Thermodynamic and Kinetic Controls in Methane Activation on High-Spin Rhenium Centers: The Interplay of...

Uncovering how ligand environments modulate the reactivity of transition metal centers is pivotal for rational design of methane conversion catalysts. Herein, we report a striking reactivity dichotomy in the thermal gas-phase reactions of Re cluster cations with methane: while [Re(H₂O)]⁺ efficiently dehydrogenates methane, whereas [Re(CO₂)]⁺ remains inert, as revealed by FT-ICR mass spectrometry. Guided by extensive theoretical evaluation across a broadened ligand space (H₂O, CO₂, H₂S, SiO₂, HCl, HBr, etc.), we reveal a synergistic thermodynamic-kinetic controls mechanism governing methane activation. Thermodynamically, moderate σ-donation induces electron-rich Re center, raising d-orbital energies to match with methane C-H breaking. Kinetically, the rate-determining spin-forbidden transition is regulated by the competition between the "spin-orbit dilution" and "heavy-atom auxiliary" effects. High reactivity requires either strong orbital localization to preserve the metal's intrinsic spin orbit coupling (SOC) (e.g., H₂O), or incorporating heavy atoms (e.g., S, Cl) to provide auxiliary relativistic potential. Notably, this auxiliary strategy is limited by the nephelauxetic effect, as excessive covalency (e.g., in [Re(HBr)]⁺) quenches effective SOC. These insights challenge the conventional view that neutral ligands diminish reactivity without spin-state changes, offering quantitative electronic descriptors for the design of condensed-phase catalysts via surface hydroxyl engineering or heavy-atom doping.