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 N-2 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 N-2 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 N-2 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 N-2 fixation, Mo itself is not necessary to carry out this process. The initial binding of N-2 with chalcogels under illumination was observed with in situ diffuse-reflectance Fourier transform infrared spectroscopy (DRIFTS). N-15(2) isotope experiments confirm that the generated NH3 derives from N-2. Density functional theory (DFT) electronic structure calculations suggest that the N-2 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 N-2 under ambient conditions.