Nitrogenase-mimic iron-containing chalcogels for photochemical reduction of dinitrogen to ammonia

A nitrogenase-inspired biomimetic chalcogel system comprising double-cubane [Mo₂Fe₆S₈(SPh)₃] and single-cubane (Fe₄S₄) biomimetic clusters demonstrates photocatalytic N₂ fixation and conversion to NH₃ in ambient temperature and pressure conditions. Replacing the Fe₄S₄ clusters in this system with other inert ions such as Sb³⁺, Sn⁴⁺, Zn²⁺ also gave chalcogels that were photocatalytically active. Finally, molybdenum-free chalcogels containing only Fe₄S₄ clusters are also capable of accomplishing the N₂ fixation reaction with even higher efficiency than their Mo₂Fe₆S₈(SPh)₃-containing counterparts. Our results suggest that redox-active iron-sulfide-containing materials can activate the N₂ molecule upon visible light excitation, which can be reduced all of the way to NH₃ using protons and sacrificial electrons in aqueous solution. Evidently, whereas the Mo₂Fe₆S₈(SPh)₃ is capable of N₂ fixation, Mo itself is not necessary to carry out this process. The initial binding of N₂ with chalcogels under illumination was observed with in situ diffuse-reflectance Fourier transform infrared spectroscopy (DRIFTS).¹⁵N₂ isotope experiments confirm that the generated NH₃ derives from N₂. Density functional theory (DFT) electronic structure calculations suggest that the N₂ binding is thermodynamically favorable only with the highly reduced active clusters. The results reported herein contribute to ongoing efforts of mimicking nitrogenase in fixing nitrogen and point to a promising path in developing catalysts for the reduction of N₂ under ambient conditions.