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 MIr(3)S(4) ligating N(2) (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 N(2) beyond the RuIr(3)S(4) cluster prepared by Mizobe and co-workers [Angew. Chem. Int. Ed. 2007, 46, 5431]. The degree of N(2) activation in the metal-N(2) complexes was evaluated based on the N-N bond distance and vibrational frequency and the gross atomic charge on N(2). The degree of N(2) activation strongly depends on the metal atoms at the N(2)-binding site, and the MoIr(3)S(4) and WIr(3)S(4) clusters exhibit significant N(2)-activation ability. The reactivity of the MIr(3)S(4)-N(2) 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-N(2) 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-N(2) complex has not been protonated at room temperature. On the other hand, the protonation of the Mo-N(2) and W-N(2) 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 N(2), which is the first step toward nitrogen fixation.