Iron(III) complexes of tridentate (NNO) ligands: Efficient catalysts for the synthesis of aminophenoxazinones in aqueous medium.

Ligand systems with precisely tailored electronic and steric properties are crucial for developing complexes that serve as effective biomimetic models. Keeping this in mind, a series of iron(III) complexes [Fe(L)Cl2] 1-3, featuring ligands L1(H) = 2-(1-(pyridin-2-yl)imidazo[1,5-a]pyridin-3-yl)phenol, L2(H) = 4-methoxy-2-(1-(pyridin-2-yl)imidazo[1,5-a] pyridin-3-yl)phenol and L3(H) = 4-bromo-2-(1-(pyridin-2-yl)imidazo[1,5-a]pyridin-3-yl)phenol were synthesized, characterized, and utilized as biomimetic models for the phenoxazinone synthase. Complexes 1-3, facilitated the oxidation of different aminophenol derivatives in aqueous medium under ambient conditions. Among these, 3 with bromo-substituted ligand achieved the highest product yield (92 %), while the methoxy-substituted complex exhibited a lower product yield (50 %). The parent o-aminophenol showed superior conversion with complex 3, compared to its derivatives. Remarkably, the complexes demonstrated high catalytic efficiencies with kcat values ranging from 9.4 × 105 to 5.8 × 104 h-1 in aqueous media. Mechanistic investigations, including both kinetic and mass spectrometric analysis, provided the evidence for the formation of two key adducts, such as [FeIII(L1-L3)(IPO)]•+ (4) and [FeIII(L1-L3)(2IPO)]••- (5), which were further supported by DFT analysis. Furthermore, radical trapping experiments with 5,5-dimethyl-1-pyrroline N-oxide reveal the involvement of radical species in the reaction. Overall, this study highlights the potential of these iron(III) complexes as highly efficient biomimetic catalysts.