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

A nitrogenase-inspired biomimetic chalcogel system comprising double-cubane [Mo 2 Fe 6 S 8 (SPh) 3 ] and single-cubane (Fe 4 S 4 ) biomimetic clusters demonstrates photocatalytic N 2 fixation and conversion to NH 3 in ambient temperature and pressure conditions. Replacing the Fe 4 S 4 clusters in this system with other inert ions such as Sb 3+ , Sn 4+ , Zn 2+ also gave chalcogels that were photocatalytically active. Finally, molybdenum-free chalcogels containing only Fe 4 S 4 clusters are also capable of accomplishing the N 2 fixation reaction with even higher efficiency than their Mo 2 Fe 6 S 8 (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 NH 3 using protons and sacrificial electrons in aqueous solution. Evidently, whereas the Mo 2 Fe 6 S 8 (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). 15 N 2 isotope experiments confirm that the generated NH 3 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.