Understanding the Selectivity of a Moderate Oxidation Catalyst: Hydrogen Abstraction by a Fully Characterized, Activated Catalyst, the Robust Dihydroxo Manganese(IV) Complex of a Bridged Cyclam

摘要

This manganese complex is a highly selective oxidation catalyst because of mechanistic constraints, both in oxygen atom transfer and hydrogen abstraction. Polanyi correlations are observed for both groups (Mn~Ⅳ-OH and Mn~Ⅳ=O), but hydrogen abstractions for the Mn~Ⅳ=O group average ～ 14 times faster than those for Mn~Ⅳ-OH. This illustrates intrinsically different oxidizing abilities for distinctly different functional groups when all else is about equal. Remarkably, the hydrogen abstracting abilities of both manganese complexes disappear when the thermal driving force becomes very small (i.e., BDE_(CH) for the substrate ≥ BDE_(OH) for the manganese complex). Although the slopes of the Polanyi correlations are substantially different, the thermodynami-cally controlled termination of oxidation is approximately the same since the BDE_(OH) values are similar for the two oxidizing groups. While intuitively appealing, the therrnodynamic control of hydrogen abstraction described here appears to be rare. Finally, its selective moderate oxidizing power makes this catalyst valuable for demanding applications.