Brønsted Acids as Direct C–H Bond Activators in Conjunction with High-Valent Metal–Oxo Catalysts: Revisiting Metal-Oxo Centered Mechanisms

High-valent metal-oxo species are ubiquitous in chemistry due to their versatile catalytic reactions and their widespread presence in metalloenzymes. Among the metal-oxo species, Fe^IV = O and Mn^IV = O species are the most widely studied, and several biomimic models of such species have been created over the years. While various factors such as spin state, ligand design, and redox potential influence the reactivity of these species, a dramatic enhancement of reactivity by <i>a</i> factor of 10² to 10⁸ upon the addition of Lewis (LA) and Brønsted acids (BA) stands out as a striking phenomenon, whose underlying mechanism remains largely unexplored. In this work, we explored the mechanism of BA-promoted C-H activation using [(N4Py)Mn^IV(O)]²⁺ (<b>1</b>) and [(N4Py)Fe^IV(O)]²⁺ (<b>2</b>) species to arrive at a generic mechanism for these catalytic transformations. We have explored three possible mechanistic routes: (i) a mechanism of C-H activation followed by -OH rebound without the BA (triflic acid) for the toluene hydroxylation reaction, (ii) a mechanism where triflic acid is a spectator, and (iii) a mechanism where triflic acid directly participates in both electron transfer/proton transfer and C-H bond activation steps. Our calculations reveal that when BAs are added, it is no longer the metal-oxo species that activates the C-H bond (as known conventionally), rather it is the BA that directly performs the C-H activation through an unprecedented mechanistic route. The direct involvement of triflic acid was found to lower the C-H bond activation barrier by approximately 20-30 kJ/mol compared to when it is absent. This reduction is attributed to the triflate anion performing direct C-H bond activation from the toluene radical cation, rather than the conventionally assumed metal-oxo moiety. Among many factors, the formation of ion-pair and the consequent electronic changes incurred, and large localized electric field effect around the S-O bond of the triflic acid was found to be the driving force for the calculated lower barrier height. The theoretical findings corroborate experimental observations, providing the first comprehensive explanation for the enhanced reactivity in the presence of LA/BA acids. These findings have direct implications for enzymatic systems such as the oxygen-evolving complex and open an uncharted path in the catalytic design.