Ceric Ammonium Nitrate Oxidation of a Mn Complex Generates a Bis(μ-oxo)dimanganese(IV,IV) Product via a Mn-Oxo Intermediate.

The oxidation of manganese complexes using ceric ammonium nitrate (CAN) is often complicated by the fact that cerium(IV) can serve as both an oxidant and a Lewis acid. In this work, we explore the reaction of CAN with the Mn complex Mn(OTf)(N4py) (N4py = ,-bis(4-methoxy-3,5-dimethyl-2-pyridylmethyl)--bis(2-pyridyl)methylamine). We chose this complex as multiple oxidation products, including oxomanganese(IV) and bis(μ-oxo)dimanganese(III,IV) complexes, have previously been reported. We envisioned that knowledge of the spectral properties of these intermediates would aid in understanding the potential complexities of CAN oxidation reactions. The oxidation of Mn(OTf)(N4py) with 2.0 equiv CAN in 9:1 (v/v) MeCN:HO at 25 °C transiently forms the Mn-oxo complex, but this species is formed in low yields and is unstable. The Mn-oxo complex evolves to a new intermediate that had not been previously observed. At lower temperatures, the formation of this new intermediate is preceded by formation of the previously reported bis(μ-oxo)dimanganese(III,IV) complex. EPR and X-ray absorption experiments for the new intermediate provide strong evidence for its formulation as [MnMn(μ-O)(N4py)]. This work establishes dinuclear MnMn species as products that must be considered in CAN oxidation reactions of Mn complexes and shows that both mononuclear Mn-oxo and dinuclear MnMn complexes can be intermediates in such reactions.