Steric control of dioxygen activation pathways for Mn complexes supported by pentadentate, amide-containing ligands.

Dioxygen activation at manganese centers is well known in nature, but synthetic manganese systems capable of utilizing O as an oxidant are relatively uncommon. These present investigations probe the dioxygen activation pathways of two mononuclear Mn complexes supported by pentacoordinate amide-containing ligands, Mn(dpaq) and the sterically modified Mn(dpaq). Dioxygen titration experiments demonstrate that Mn(dpaq) reacts with O to form Mn(OH)(dpaq) according to a 4 : 1 Mn : O stoichiometry. This stoichiometry is consistent with a pathway involving comproportionation between a Mn-oxo species and residual Mn complex to form a (μ-oxo)dimanganese(iii,iii) species that is hydrolyzed by water to give the Mn-hydroxo product. In contrast, the sterically modified Mn(dpaq) complex was found to react with O according to a 2 : 1 Mn : O stoichiometry. This stoichiometry is indicative of a pathway in which a Mn-oxo intermediate abstracts a hydrogen atom from solvent instead of undergoing comproportionation with the Mn starting complex. Isotopic labeling experiments, in which the oxygenation of the Mn complexes was carried out in deuterated solvent, supported this change in pathway. The oxygenation of Mn(dpaq) did not result in any deuterium incorporation in the Mn-hydroxo product, while the oxygenation of Mn(dpaq) in d-MeCN showed [Mn(OD)(dpaq)] formation. Taken together, these observations highlight the use of steric effects as a means to select which intermediates form along dioxygen activation pathways.