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

Dioxygen activation at manganese centers is well known in nature, but synthetic manganese systems capable of utilizing O₂ 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 O₂ to form [Mn^III(OH)(dpaq)](OTf) according to a 4 : 1 Mn : O₂ 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 O₂ according to a 2 : 1 Mn : O₂ 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 d₃-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.