Phenazine-Based Homogeneous Photocatalysts for Visible-Light-Driven Hydrogenation of Nitroarenes under Mild Conditions

Phenazine derivatives are attractive organic chromophores due to their redox activity and photophysical properties, yet their application in photocatalytic hydrogenation reactions remains underexplored. In this work, a homogeneous phenazine-based photocatalytic system was developed and applied to the visible-light-driven hydrogenation of nitro compounds under mild conditions. The photocatalysts’ activity was evaluated using nitrobenzene as a model substrate in the presence of triethanolamine as a sacrificial hydrogen and electron donor. Reaction parameters including photocatalyst structure, solvent, hydrogen source, irradiation wavelength, and catalyst loading were systematically investigated. Under optimized conditions, nitrobenzene was converted to aniline with yields of up to 81% after 12 h of irradiation at ambient temperature. Kinetic studies revealed that prolonged irradiation does not enhance conversion and can lead to decreased yields due to the instability and reconversion of azo-type intermediates. Substrate scope investigations demonstrated higher reductive efficiency for nitroarenes bearing electron-withdrawing substituents, whereas aliphatic nitro compounds were only partially reduced, often yielding oxime or N–OH intermediates. UV–Vis, fluorescence, and EPR spectroscopy provided mechanistic insight and confirmed the involvement of radical species generated upon light irradiation. Overall, this study establishes phenazine-based photocatalysts as effective metal-free systems for the hydrogenation of nitroarenes under visible light and mild reaction conditions.