Proton-Coupled Electron-Transfer Reactions In [Mn-2(Iv)(Mu-O)(3)L'(2)](2+) (L'=1,4,7-Trimethyl-1,4,7-Triazacyclononane)

The pK(a) value of [Mn-2(IV)(mu-O)(3)L'(2)](2+) (L' = 1,4,7-trimethyl-1,4,7-triazacyclononane) has been determined spectrophotometrically by carrying out titration experiments with concentrated sulfuric acid. The extremely low pK(a) value of -2.0 suggests that the electron density on the bridging oxygen atoms is very small. The asymmetric Mn-O-Mn vibration is observed at 670 cm(-1), while the symmetric Mn-O-Mn vibration is present at 702 cm(-1). The unusually high frequencies of these vibrations are due to the small Mn-O-Mn angle of 78 degrees. Protonation of an oxygen bridge shifts both the asymmetric and symmetric vibrations to 683 cm(-1). Electrochemical experiments in acetonitrile have shown that one-electron reduction of the complex is chemically irreversible. IR, EPR, and UV-vis studies of the reduced species suggest the presence of a (MnMnIV)-Mn-III(mu-O)(2)(mu-OH) core. pH-dependent differential pulse voltammetry experiments in aqueous solutions have revealed an apparent pK(a) value of approximately 4.0 for the reduced mixed-valence species in various buffer systems. The reduction wave at pH > 4 is observed at around -0.10 V vs SCE. Cyclic voltammetry has revealed that the reduced species is prone to reaction with carboxylate groups. A bis(carboxylate)mono-oxo-bridged Mn(III)-Mn(III) species is formed in citric acid buffer which exhibits an anodic peak around +0.6 V vs SCE, and a UV-vis spectrum that is typical of such a species.