Understanding the Oxidative Relationships of the Metal Oxo, Hydroxo, and Hydroperoxide Intermediates with Manganese(IV) Complexes Having Bridged Cyclams: Correlation of the Physicochemical Properties with Reactivity

Multiple transition metal functional groups including metaloxo, hydroxo, and hydroperoxide groups play significant roles in various biological and chemical oxidations such as electron transfer, oxygen transfer, and hydrogen abstraction. Further studies that clarify their oxidative relationships and the relationship between their reactivity and their physicochemical properties will expand our ability to predict the reactivity of the intermediate in different oxidative events. As a result researchers will be able to provide rational explanations of poorly understood oxidative phenomena and design selective oxidation catalysts. This Account summarizes results from recent studies of oxidative relationships among manganese(IV) molecules that include pairs of hydroxo/oxo ligands. Changes in the protonation state may simultaneously affect the net charge, the redox potential, the metal–oxygen bond order (M–O vs MO), and the reactivity of the metal ion. In the manganese(IV) model system, [MnIV(Me2EBC)(OH)2](PF6)2, the MnIV–OH and MnIVO moieties have similar hydrogen abstraction capabilities, but MnIVO abstracts hydrogen at a more than 40-fold faster rate than the corresponding MnIV–OH. However, after the first hydrogen abstraction, the reduction product, MnIII–OH2 from the MnIV–OH moiety, cannot transfer a subsequent OH group to the substrate radical. Instead the MnIII–OH from the MnIVO moiety reforms the OH group, generating the hydroxylated product. In the oxygenation of substrates such as triarylphosphines, the reaction with the MnIVO moiety proceeds by concerted oxygen atom transfer, but the reaction with the MnIV–OH functional group proceeds by electron transfer. In addition, the manganese(IV) species with a MnIV–OH group has a higher redox potential and demonstrates much more facile electron transfer than the one that has the MnIVO group. Furthermore, an increase in the net charge of the MnIV–OH further accelerates its electron transfer rate. But its influence on hydrogen abstraction is minor because charge-promoted electron transfer does not enhance hydrogen abstraction remarkably. The MnIV–OOH moiety with an identical coordination environment is a more powerful oxidant than the corresponding MnIV–OH and MnIVO moieties in both hydrogen abstraction and oxygen atom transfer. With this full understanding of the oxidative reactivity of the MnIV–OH and MnIVO moieties, we have clarified the correlation between the physicochemical properties of these active