Relationship between Hydrogen-Atom Transfer Driving Force and Reaction Rates for an Oxomanganese(IV) Adduct.

Hydrogen atom transfer (HAT) reactions by high-valent metal-oxo intermediates are important in both biological and synthetic systems. While the HAT reactivity of Fe-oxo adducts has been extensively investigated, studies of analogous Mn-oxo systems are less common. There are several recent reports of Mn-oxo complexes, supported by neutral pentadentate ligands, capable of cleaving strong C-H bonds at rates approaching those of analogous Fe-oxo species. In this study, we provide a thorough analysis of the HAT reactivity of one of these Mn-oxo complexes, [Mn(O)(2pyN2Q)], which is supported by an N5 ligand with equatorial pyridine and quinoline donors. This complex is able to oxidize the strong C-H bonds of cyclohexane with rates exceeding those of Fe-oxo complexes with similar ligands. In the presence of excess oxidant (iodosobenzene), cyclohexane oxidation by [Mn(O)(2pyN2Q)] is catalytic, albeit with modest turnover numbers. Because the rate of cyclohexane oxidation by [Mn(O)(2pyN2Q)] was faster than that predicted by a previously published Bells-Evans-Polanyi correlation, we expanded the scope of this relationship by determining HAT reaction rates for substrates with bond dissociation energies spanning 20 kcal/mol. This extensive analysis showed the expected correlation between reaction rate and the strength of the substrate C-H bond, albeit with a shallow slope. The implications of this result with regard to Mn-oxo and Fe-oxo reactivity are discussed.