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)](OTf) (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)](OTf) 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.