NMR Studies of a Mn III -hydroxo Adduct Reveal an Equilibrium between Mn III -hydroxo and μ-Oxodimanganese(III,III) Species

The solution properties of Mn -hydroxo and Mn -methoxy complexes featuring N amide-containing ligands were investigated using H NMR spectroscopy. The H NMR spectrum for one of these complexes, the previously reported [Mn (OH)(dpaq)](OTf) (dpaq = 2-[bis(pyridin-2-ylmethyl)]amino- N-quinolin-8-yl-acetamidate) shows hyperfine-shifted signals, as expected for this S = 2 Mn -hydroxo adduct. However, the H NMR spectrum of [Mn (OH)(dpaq)](OTf) also shows a large number of proton resonances in the diamagnetic region, suggesting the presence of multiple species in CD CN solution. The majority of the signals in the diamagnetic region disappear when a small amount of water is added to a CH CN solution of [Mn (OH)(dpaq)](OTf). Electronic absorption and Mn K-edge X-ray absorption experiments support the formulation of this diamagnetic species as the μ-oxodimanganese(III,III) complex [Mn (μ-O)(dpaq) )] . On the basis of these observations, we propose that the dissolution of [Mn (OH)(dpaq)](OTf) in CD CN results in the formation of mononuclear Mn -hydroxo and dinuclear μ-oxodimanganese(III,III) species that are in equilibrium. The addition of a small amount of water is sufficient to shift this equilibrium in favor of the Mn -hydroxo adduct. Surprisingly, electronic absorption experiments show that the conversion of [Mn (μ-O)(dpaq) )] to [Mn (OH)(dpaq)] by added water is relatively slow. Because this dimer to monomer conversion is slower than TEMPOH oxidation by [Mn (OH)(dpaq)] , the previously observed TEMPOH oxidation rates for [Mn (OH)(dpaq)] reflected both processes. Here, we report the bona fide TEMPOH oxidation rate for [Mn (OH)(dpaq)] , which is significantly faster than previously reported. H NMR spectra are also reported for the related [Mn (OMe)(dpaq)] and [Mn (OH)(dpaq )] complexes. These spectra only show hyperfine-shifted signals, suggesting the presence of only mononuclear Mn -methoxy and Mn -hydroxo species in solution. Measurements of T relaxation times and proton peak integrations for [Mn (OMe)(dpaq)] provide preliminary assignments for H NMR resonances.