Brønsted and Lewis Acid Activation of a Reactive Mn(III)-O-Ce(IV) Intermediate

High-valent Mn-oxo species play a pivotal role as reactive intermediates in oxidative transformations, particularly in biological systems such as the oxygen-evolving complex (OEC) of Photosystem II. The redox-inactive Lewis acid Ca²⁺, a conserved component of the OEC, is essential for catalytic function; however, its precise mechanistic role remains elusive. Herein, we report the generation and characterization of a reactive Mn(III)-O-Ce(IV) species, [(2N3Py)Mn(III)-O-Ce(IV)(NO₃)₄]⁺ (1-Ce^IV), formed by treating the (2N3Py)Mn(II), (2N3Py = 1-(pyridin-2-yl)-N-(pyridin-2-ylmethyl)-N-((1-(pyridin-2-ylmethyl)piperidin-2-yl)methyl)methanamine) with ceric ammonium nitrate in acetonitrile with trace water. Characterization of 1-Ce^IV carried out by UV-vis absorption, resonance Raman (ν_Mn-O = 696 cm^-1; Δ¹⁸O = -29 cm^-1), EPR (g = 5.83), ¹H NMR, X-ray absorption spectroscopy, and ESI-MS confirm the presence of a Mn(III)-O-Ce(IV) unit. Reactivity studies reveal that 1-Ce^IV exhibits enhanced reactivity compared to the reported (BnTPEN)Mn(III)-O-Ce(IV) species. Addition of HClO₄ and Sc(III)(OTf)₃ to 1-Ce^IV yields [(2N3Py)Mn(IV)═O]²⁺-H⁺ (2) and [(2N3Py)Mn(IV)═O]²⁺-Sc(III) (3), respectively, with distinct spectroscopic features. Species 2 and 3 exhibit enhanced reactivity, with 3 displaying a 10-fold higher rate (k₂ = 1 M^-1 s^-1) in the sulfoxidation of thioanisole compared to 1-Ce^IV. These results underscore the critical role of redox-inactive Lewis acids and Brønsted acids in tuning the electronic structure and reactivity of high-valent Mn-oxo species, providing insight into bioinspired catalyst design.