Equatorial Ligand Perturbations Influence the Reactivity of Manganese(IV)‐Oxo Complexes

Manganese(IV)‐oxo complexes are often invoked as intermediates in Mn‐catalyzed C−H bond activation reactions. While many synthetic MnIV‐oxo species are mild oxidants, other members of this class can attack strong C−H bonds. The basis for these reactivity differences is not well understood. Here we describe a series of MnIV‐oxo complexes with N5 pentadentate ligands that modulate the equatorial ligand field of the MnIV center, as assessed by electronic absorption, electron paramagnetic resonance, and Mn K‐edge X‐ray absorption methods. Kinetic experiments show dramatic rate variations in hydrogen‐atom and oxygen‐atom transfer reactions, with faster rates corresponding to weaker equatorial ligand fields. For these MnIV‐oxo complexes, the rate enhancements are correlated with both 1) the energy of a low‐lying 4E excited state, which has been postulated to be involved in a two‐state reactivity model, and 2) the MnIII/IV reduction potentials. Schwächeres Feld, schnellere Reaktion: Perturbationen des äquatorialen Ligandenfeldes bei einer Serie von MnIV‐Oxo‐Komplexen verschieben die Energie eines angeregten 4E‐Zustands, für den postuliert wurde, dass er Wasserstoff‐ und Sauerstoffatomtransferreaktionen vermittelt. Die experimentelle 4E‐Energie wird mit Steigerungen der Reaktionsgeschwindigkeit korreliert.