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 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 (mu-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.