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 metal oxo, 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, (Mn-IV(Me2EBC)(OH)(2)](PF6)(2), the Mn-IV-OH and Mn-IV=O moieties have similar hydrogen abstraction capabilities, but Mn-IV=O abstracts hydrogen at a more than 40-fold faster rate than the corresponding Mn-IV-OH. However, after the first hydrogen abstraction, the reduction product, Mn-III-OH2 from the Mn-IV-OH moiety, cannot transfer a subsequent OH group to the substrate radical. Instead the Mn-III-OH from the Mn-IV=O moiety reforms the OH group, generating the hydroxylated product. In the oxygenation of substrates such as triarylphosphines, the reaction with the Mn-IV=O moiety proceeds by concerted oxygen atom transfer, but the reaction with the Mn-IV-OH functional group proceeds by eledron transfer. In addition, the manganese(IV) species with a Mn-IV-OH group has a higher redox potential and demonstrates much more fadle electron transfer than the one that has the Mn-IV=O group. Furthermore, an increase in the net charge of the Me OH further accelerates its electron transfer rate. But its influence on hydrogen abstraction is minor because charge-promoted eledron transfer does not enhance hydrogen abstraction remarkably. The Mn-IV-OOH moiety with an identical coordination environment is a more powerful oxidant than the corresponding Mn-IV-OH and Mn-IV=O moieties in both hydrogen abstraction and oxygen atom transfer. With this full understanding of the oxidative reactivity of the Mn-IV-OH and Mn-IV=O moieties, we have darified the correlation between the physicochemical properties of these active intermediates, including net charge, redox potential, and metal oxygen bond order, and their reactivities. The reactivity differences between the metal oxo and hydroxo moieties on these manganese(IV) functional groups after the first hydrogen abstraction have provided dues for understanding their occurrence and functions in metalloenzymes. The P450 enzymes require an iron(IV) oxo form rather than an iron(IV) hydroxo form to perform substrate hydroxylation. However, the lipoxygenases use an iron(III) hydroxo group to dioxygenate unsaturated fatty adds rather than an iron(III) oxo species, a moiety that could fadlitate hydroxylation reactions. These distinctly different physicochemical properties and reactivities of the metal oxo and hydroxo moieties could provide clues to understand these elusive oxidation phenomena and provide the foundation for the rational design of novel oxidation catalysts.