High-Valent Oxo, Methoxorhenium Complexes: Models for Intermediates and Transition States in Proton-Coupled Multi-Electron Transfer Reactions

trans-Dioxorhenium(V) tetrapyridyl species are currently under active investigation as model systems for interfacial two-electron, two-proton transfer reaction sequences (Jones-Skeens et al. Inorg. Chem. 1992, 31, 3879). We now find that the corresponding oxo, methoxo complexes can be prepared from the dioxo species and methyl trifluoromethanesulfonate. The new complexes behave nearly identically with the analogous oxo, hydroxo complexes—with one important exception: CH3⁺, unlike H⁺, does not dissociate from the oxo ligand. As a direct consequence, the usually elusive rhenium oxidation state IV is stabilized with respect to redox disproportionation and is observable for several complexes at high pH. The ability to detect this state, in turn leads to (1) direct access to the formal reduction potentials for the isolated 1e^- redox couples comprising the overall two-electron transfer (key information for understanding multi-electron transfer kinetics), (2) elucidation of the profound structural and energetic consequences of the initial protonation (methylation) step in the dioxorhenium(V) reduction kinetics, (3) estimates for pK_a of (O)(HO)Re^VIL4³⁺ (exceptionally negative), and (4) estimates for the first pK_a of (HO)₂Re^IIIL4⁺(extremely large and positive). The combination of (1) and (2), in principle, provides sufficient information to characterize completely the energetic accessibility of key intermediate species lying just before or just after the transition state for the two-electron, two-proton reduction of dioxorhenium(V) at electrochemical interfaces.