Decoupling Thermodynamic and Kinetic Controls in Methane Activation on High-Spin Rhenium Centers: The Interplay of Electronic Accumulation and Relativistic Spin-Orbit Coupling.

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.