Deeper Understanding of Mononuclear Manganese(IV)-Oxo Binding Brønsted and Lewis Acids and the Manganese(IV)-Hydroxide Complex.

Binding of Lewis acidic metal ions and Brønsted acid at the metal-oxo group of high-valent metal-oxo complexes enhances their reactivities significantly in oxidation reactions. However, such a binding of Lewis acids and proton at the metal-oxo group has been questioned in several cases and remains to be clarified. Herein, we report the synthesis, characterization, and reactivity studies of a mononuclear manganese(IV)-oxo complex binding triflic acid, {[(dpaq)Mn(O)]-HOTf} (-HOTf). First, -HOTf was synthesized and characterized using various spectroscopic techniques, including resonance Raman (rRaman) and X-ray absorption spectroscopy/extended X-ray absorption fine structure. In particular, in rRaman experiments, we observed a linear correlation between the Mn-O stretching frequencies of -HOTf (e.g., ν at ∼793 cm) and -M (M = Ca, Zn, Lu, Al, or Sc) and the Lewis acidities of H and M ions, suggesting that H and M bind at the metal-oxo moiety of [(dpaq)Mn(O)]. Interestingly, a single-crystal structure of -HOTf was obtained by X-ray diffraction analysis, but the structure was not an expected Mn(IV)-oxo complex but a Mn(IV)-hydroxide complex, (dpaq)Mn(OH) (), with a Mn-O bond distance of 1.8043(19) Å and a Mn-O stretch at 660 cm. More interestingly, reverted to -HOTf upon dissolution, demonstrating that -HOTf and are interconvertible depending on the physical states, such as -HOTf in solution and in isolated solid. The reactivity of -HOTf was investigated in hydrogen atom transfer (HAT) and oxygen atom transfer (OAT) reactions and then compared with those of -M complexes; an interesting correlation between the Mn-O stretching frequencies of -HOTf and -M and their reactivities in the OAT and HAT reactions is reported for the first time in this study.