Understanding the Catalase-Like Activity of a Bioinspired Manganese(II) Complex with a Pentadentate NSNSN Ligand Framework. A Computational Insight into the Mechanism

The mechanism of H2O2 dismutation catalyzed by the recently reported 2,6-bis[((2-pyridylmethyl)thio)methyl]pyridine-Mn(II) complex ([MnS2Py3(OTf)(2)]) has been investigated by density functional theory using the S12g functional. The complex has been analyzed in terms of its coordination properties and the reaction of [MnS2Py3](2+) in a distorted square pyramidal coordination geometry with two hydrogen peroxide molecules has been investigated in our calculations. The sextet, quartet, and doublet potential energy profiles of the catalytic reaction have been explored. In the first dismutation process, the rate-determining step (RDS) is found to be the asymmetric O-O bond cleavage, which occurs on the sextet potential energy profile. A subsequent spin crossover from sextet to quartet, associated with a coordination rearrangement around the metal, can take place to generate a stable Mn(IV) dihydroxo intermediate. This could disfavor the ping-pong mechanism commonly considered to describe the H2O2 dismutation reaction, where the binding of the first H2O2 substrate leads to the release of one H2O product and the conversion of the catalyst into a Mn(W) oxo complex. The formation of this stable intermediate, featuring a peculiar trigonal prismatic coordination geometry, paves the way for an alternative reaction pathway for the second dismutation process, termed the dihydroxo mechanism, where two water molecules and dioxygen are easily and simultaneously formed. The competing channels have different spin states: the sextet reaction pathway corresponds to the ping-pong mechanism, whereas the quartet reaction follows preferably the dihydroxo mechanism. The doublet reaction path is energetically disfavored for both channels. For the ping-pong mechanism, the RDS in the second dismutation process is represented by the second hydrogen-abstraction from H2O2, with a calculated energy barrier very close to that of the RDS in the first dismutation reaction. Explicit solvent molecules, counterions, and trace amounts of water are found to further support the preference for the asymmetric O-O bond breaking by favoring the end-on coordination mode of the first H2O2 to the catalyst.