Adsorption and Catalytic Reduction of Nitrogen Oxides (NO, N₂O) on Disulfide Cluster Complexes of Cobalt and Iron-A Density Functional Study

The reactivity of nitrogen oxide, NO, as a ligand in complexes with [Fe2-S2] and [Co2-S2] non-planar rhombic cores is examined by density functional theory (DFT). The cobalt-containing nitrosyl complexes are less stable than the iron complexes because the Co-S bonds in the [Co2-S2] core are weakened upon NO coordination. Various positions of NO were examined, including its binding to sulfur centers. The release of NO molecules can be monitored photochemically. The ability of NO to form a (NO)₂ dimer provides a favorable route of electrochemical reduction, as protonation significantly stabilizes the dimeric species over the monomers. The quasilinear dimer ONNO, with trans-orientation of oxygen atoms, gains higher stability under protonation and reduction via proton-electron transfer. The first two reduction steps lead to an N₂O intermediate, whose reduction is more energy demanding: in the two latter reaction steps the highest energy barrier for Co₂S₂(CO)₆ is 109 kJ mol^-1, and for Fe₂S₂(CO)₆, it is 133 kJ mol^-1. Again, the presence of favorable light absorption bands allows for a photochemical route to overcome these energy barriers. All elementary steps are exothermic, and the final products are molecular nitrogen and water.