DFT study on chemical N₂ fixation by using a cubane-type RuIr₃S₄ cluster: Energy profile for binding and reduction of N₂ to ammonia via Ru-N-NH_x (x = 1-3) intermediates with unique structures

The N-N bond activation of the dinitrogen ligand in the cubane-type mixed-metal sulfido cluster, [(Cp*Ir)₃{Ru(tmeda)(N ₂)}(μ₃-S)₄] (tmeda = Me₂NCH ₂CH₂NMe₂), is investigated by using DFT calculations at the B3LYP level of theory. The elongated N-N bond distance, red-shifted N-N stretching, and negatively charged N₂ ligand indicate that the dinitrogen is reductively activated by complexation. The degree of the N-N bond activation is classified into the "moderately activated" category, [Studt, F.; Tuczek, F. J. Comput. Chem. 2006, 27, 1278] as in the Mo-triamidoamine complex that can catalyze N₂ reduction [Yandulov, D. V.; Schrock, R. R. Science 2003, 301, 76]. Availability of the RuIr ₃S₄ cluster as a catalyst for N₂ reduction is discussed by optimizing possible intermediates in a catalytic cycle analogous to that proposed by Yandulov and Schrock. A calculated energy profile of the catalytic cycle demonstrates that the RuIr₃S₄ cluster can transform dinitrogen into ammonia in the presence of lutidinium cation and Cp*₂Co as proton and electron sources, respectively. The RuIr₃S₄ clusters with an NNH_x (x = 1-3) ligand, which are intermediates in the catalytic cycle, have a significantly bent Ru-N-N linkage, although precedent NNH_x complexes generally adopt a linear M-N-N array. The unique structures of the nitrogenous ligands in these intermediates are interpreted in terms of the bonding interaction between the hydrogen atom bonded to the N₂ ligand and the adjacent iridium atom in the cuboidal RuIr₃S₄ framework.