Steric control of dioxygen activation pathways for MnII 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 2 as an oxidant are relatively uncommon. These present investigations probe the dioxygen activation pathways of two mononuclear Mn II complexes supported by pentacoordinate amide-containing ligands, [Mn II (dpaq)](OTf) and the sterically modified [Mn II (dpaq 2Me )](OTf). Dioxygen titration experiments demonstrate that [Mn II (dpaq)](OTf) reacts with O 2 to form [Mn III (OH)(dpaq)](OTf) according to a 4 : 1 Mn : O 2 stoichiometry. This stoichiometry is consistent with a pathway involving comproportionation between a Mn IV -oxo species and residual Mn II complex to form a (μ-oxo)dimanganese(iii,iii) species that is hydrolyzed by water to give the Mn III -hydroxo product. In contrast, the sterically modified [Mn II (dpaq 2Me )](OTf) complex was found to react with O 2 according to a 2 : 1 Mn : O 2 stoichiometry. This stoichiometry is indicative of a pathway in which a Mn IV -oxo intermediate abstracts a hydrogen atom from solvent instead of undergoing comproportionation with the Mn II starting complex. Isotopic labeling experiments, in which the oxygenation of the Mn II complexes was carried out in deuterated solvent, supported this change in pathway. The oxygenation of [Mn II (dpaq)](OTf) did not result in any deuterium incorporation in the Mn III -hydroxo product, while the oxygenation of [Mn II (dpaq 2Me )](OTf) in d 3 -MeCN showed [Mn III (OD)(dpaq 2Me )] + formation. Taken together, these observations highlight the use of steric effects as a means to select which intermediates form along dioxygen activation pathways.