Aqueous emissive cyclometalated iridium photoreductants: synthesis, computational analysis and the photocatalytic reduction of 4-nitrophenol

Abstract

Herein, we present luminescent mononuclear iridium complexes [1]³⁺–[4]³⁺ using NEt₃-appended C^N chelating benzimidazole (L¹–L⁴) and semi-flexible phenanthroline-pyrazine-based (phpy) ligands exhibiting photocatalytic reduction of 4-nitrophenol (4-NP) in the presence of NEt₃ in an aqueous medium. The formation of [1]³⁺–[4]³⁺ was confirmed by HRMS, ¹H–¹H COSY, and ¹³C and ¹⁹F NMR spectroscopy. The complex [4]³⁺ is water soluble, whereas the others ([1]³⁺–[3]³⁺) are partially soluble. The complexes are luminescent in both CH₃CN and H₂O media. The DFT study reveals that the HOMO of [1]³⁺ resides on the C^N chelating benzimidazole and iridium center. However, it moves to the pyrazine-pyridine of the phpy unit in the case of [2]³⁺–[4]³⁺. The LUMOs are localized on the phenanthroline unit of phpy for all the complexes. This suggests an important role of the fluorine atom on electron density distribution. Spin density analysis demonstrates that the emission bands of the complexes arise from ³MLLCT states. The complex [4]³⁺ displays promising photocatalytic activity towards 4-NP photoreduction, whereas complexes [1]³⁺–[3]³⁺ exhibit lower reactivity. The mechanistic study suggests that the reaction proceeds through an oxidative quenching pathway, where 4-NP is reduced by accepting an electron from excited [Ir(III)] and gets oxidized to Ir(IV), which comes back to its original Ir(III) state by accepting an electron from the sacrificial electron donor NEt₃.