Formation of a Mn^III-O-Ce^IV species from a Mn^III-hydroxo complex and ceric ammonium nitrate

The addition of 1.0 equiv. ceric ammonium nitrate (CAN) to the Mn^III-hydroxo complex [Mn^III(OH)(PaPy₂Q)]⁺ (1) yields a new species (2) that contains a Mn^III-O-Ce^IV core. The Mn^III oxidation state in 2 is supported by ¹H NMR data, which reveal hyperfine-shifted protons from the PaPy₂Q ligand, and a solution magnetic moment of 4.87μ_B, consistent with an S = 2 Mn^III ion. Analysis of Mn K-edge EXAFS data for 2 shows a Ce ion 3.8 Å from the Mn center, supportive of the Mn^III-O-Ce^IV core. Complex 2 reacts with excess ferrocene to generate only 1.0 equiv. of the ferrocenium ion. Because 1 is unreactive with ferrocene, we attribute this stoichiometry to the presence of one Ce^IV ion in 2. Complex 2 can oxidize PPh₃ and tri-tert-butylphenol, with enhanced reactivity relative to 1. We find no evidence that CAN oxidizes the Mn^III center in 1, even when using excess CAN at low temperatures. This result stands in contrast to the reactivity of similar systems, where excess CAN converted a Mn^III-hydroxo complex to a complex with a Mn^IV-O-Ce^IV core (Karmalkar et al. Angew. Chem., Int. Ed., 2019, 58, 16124-16129). We discuss the basis for these different reactivities.