HIGH-VALENT OXO, METHOXORHENIUM COMPLEXES - MODELS FOR INTERMEDIATES AND TRANSITION-STATES IN PROTON-COUPLED MULTIELECTRON TRANSFER-REACTIONS

trans-Dioxorhenium(V) tetrapyridyl species are currently under active investigation as model systems for interfacial two-electron, two-proto n transfer reaction sequences (Jones-Skeens et al. Inorg. Chem. 1992, 31, 3879). We now find that the corresponding ore, methoxo complexes c an be prepared from the dioxo species and methyl trifluoromethanesulfo nate. The new complexes behave nearly identically with the analogous o re, hydroxo complexes-with one important exception: CH3+, unlike H+, d oes not dissociate from the pro ligand. As a direct consequence, the u sually elusive rhenium oxidation state IV is stabilized with respect t o 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(-) redo x couples comprising the: overall two-electron transfer (key informati on for understanding multi-electron transfer kinetics), (2) elucidatio n of the profound structural and energetic consequences of the initial protonation (methylation) step in the dioxorhenium(V) reduction kinet ics, (3) estimates for pK(a) of (O)(HO)Re(VI)L(4)(3+) (exceptionally n egative), and (4) estimates for the first pK(a) of(HO)(2)Re(III)L(4)() (extremely large and positive). The combination of (1) and (2), in p rinciple, provides sufficient information to characterize completely t he energetic accessibility of key intermediate species lying just befo re or just after the transition state for the two-electron, two-proton reduction of dioxorhenium(V) at electrochemical interfaces.