A theoretical study on the acid-catalyzed disproportionation reaction of a Mn(IV)-oxo porphyrin complex

Density functional theory (DFT) calculation were performed to study the disproportionation reaction of high-valent Mn(IV)(O) species, such as 2[Mn^IV(O)(Porp)] → [Mn^IV(O)(Porp^•+)] + [Mn^III(O)(Porp)] (Porp = porphine²⁻), which experimentally occurs (only) in the presence of acid. Energy calculations of all possible derivatives of the above species with regard to protonation and Cl⁻ and OTf⁻ axial ligation enabled us to calculate the approximate thermodynamics of all the possible reactions and pinpoint the most probable disproportionation reaction occurring for more detailed studies. Thus, we predict that the real disprotonation reaction occurs in four steps, such as (i) reactant protonation, (ii) axial ligand exchange, (iii) proton-coupled electron transfer mediated disproportionation, and finally (iv) product protonation. The conditions for the disproportionation reaction to occur in this system may be applicable for similar systems, such as other high-valent metal-oxo systems. Furthermore, the current study is an example where relatively rudimentary calculations can give surprisingly deep insights into a biochemically relevant reaction.