Computational Study of Metal-Dinitrogen Keggin-Type Polyoxometalate Complexes [PW11O39M(II)N2)](5-) (M = Ru, Os, Re, Ir): Bonding Nature and Dinitrogen Splitting.

Molecular geometry, electronic structure, and metal-dinitrogen bonding nature of a series of metal-dinitrogen derivatives of Keggin-type polyoxometalates (POMs) PW11O39M(II)N2) (M = Ru, Os, Re, Ir) have been studied by using a density functional theory (DFT) method with the M06L functional. Among these Keggin-type POM complexes, Os- and Re-substituted POM complexes are the most active for N2 adsorption with considerable adsorption energy. The electronic structure analysis shows that Os(II) and Re(II) centers in their metal-dinitrogen POM complexes possess π(2)xzπ(2)yzπ(2)xy and π(2)xzπ(2)yzπ(1)xy configurations, respectively. DFT-M06L calculations show that the possible synthesis routes proposed in this work for the Ru-, Os-, and Re-dinitrogen POM complexes are thermodynamically feasible under various solvent environments. Meanwhile, the Re-dinitrogen POM complex was assessed for the direct cleavage of dinitrogen molecule. In the reaction mechanism, a dimeric Keggin-type POM derivative of rhenium could represent the intermediate which undergoes N-N bond scission. The calculated free energy barrier (ΔG(⧧)) for a transition state with a zigzag conformation is 16.05 kcal mol(-1) in tetrahydrofuran, which is a moderate barrier for the cleavage of the N-N bond when compared with the literature values. In conclusion, regarding the direct cleavage of the dinitrogen molecule, the findings would be very useful to guide the search for a potential N2 cleavage compound into totally inorganic POM fields.