NMR Studies of a Mn-hydroxo Adduct Reveal an Equilibrium between Mn-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) (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) also shows a large number of proton resonances in the diamagnetic region, suggesting the presence of multiple species in CDCN solution. The majority of the signals in the diamagnetic region disappear when a small amount of water is added to a CHCN solution of Mn(OH)(dpaq). 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) in CDCN 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.