Theoretical study on activation and protonation of dinitrogen on Cubane-Type MIr₃S₄ Clusters (M = V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Ru, and W)

Density functional theory (DFT) calculations on cubane-type metal-sulfido clusters MIr₃S₄ ligating N₂ (M = V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Ru, and W) have been performed for the proposal of new clusters that can highly activate N₂ beyond the RuIr₃S₄ cluster prepared by Mizobe and co-workers [Angew. Chem. Int. Ed. 2007, 46, 5431]. The degree of N₂ activation in the metal-N₂ complexes was evaluated based on the N-N bond distance and vibrational frequency and the gross atomic charge on N₂. The degree of N₂ activation strongly depends on the metal atoms at the N₂-binding site, and the MoIr₃S₄ and Wlr₃S₄ clusters exhibit significant N₂-activation ability. The reactivity of the MIr₃S₄-N₂ complexes (M = Ru, Mo, and W) with a proton donor (lutidinium) has been discussed from a kinetic aspect by exploring a possible reaction pathway of proton transfer. The protonation of the Ru-N₂ complex would not occur due to a very high-activation barrier and to an instability of the Ru-NNH⁺ complex, which is consistent with our present experimental result that the Ru-N₂ complex has not been protonated at room temperature. On the other hand, the protonation of the Mo-N₂ and W-N₂ complexes would proceed smoothly from DFT criteria. The result of calculations indicates that the Mo and W clusters are best suited for the protonation of N₂, which is the first step toward nitrogen fixation.