Theoretical Investigations on the Mechanistic Aspects of O₂ Activation by a Biomimetic Dinitrosyl Iron Complex

Though dinitrosyl-iron complexes (DNICs) are largely believed to act as NO carriers, several experiments on model DNICs have suggested that they can also act as nitrating agents in presence of dioxygen. Oxygen activation by DNICs has been implicated as a possible route for protein tyrosine nitration (PTN), which leads to neurodegenerative disorders. Herein using static and dynamic theoretical techniques we unravel a previously unknown dual state mechanistic paradigm for dioxygen activation of a biomimetic nitrating DNIC complex leading to phenolic nitration. Our computations reveal that the model DNIC, the ground electronic state of which is singlet, has a low-lying triplet state and an inherent singlet–triplet spin-crossover of DNICs can be triggered by fluxional changes in the bite angle of the two NO ligands. The presence of a low-lying triplet state in the DNIC affords an avenue for O₂ activation other than a direct O₂ activation by O₂-induced spin-crossover of the singlet ground state. These two low-lying channels facilitate the formation of a peroxynitrite species. Nitration of phenolic substrates is facilitated by the release of NO₂. The corresponding minimum energy crossing points (MECP) have been located. Along the reaction path, the changes in the electronic structure scenarios have been studied and interpreted. Our report also sheds light on the plausible mechanistic pathway of PTN by reactive species formed once O₂ activation by DNICs have been achieved.