Reactivity of Cobalt Clusters Co with Dinitrogen: Superatom Co and Superatomic Complex CoN.

We report a joint experimental and theoretical study on the reactions of cobalt clusters (Co) with nitrogen using the customized reflection time-of-flight mass spectrometer combined with a 177.3 nm deep-ultraviolet laser. Comparing to the behaviors of neutral Co ( = 2-30) and anionic Co clusters ( = 7-53) which are relatively inert in reacting with nitrogen in the fast-flow tube, Co clusters readily react with nitrogen resulting in adducts of one or multiple N except Co which stands firm in the reaction with nitrogen. Detailed quantum chemistry calculations, including the energetics, electron occupancy, and orbital analysis, well-explained the reasonable reactivity of Co clusters with nitrogen and unveiled the open-shell superatomic stability of Co within a highly symmetric (D) structure. The D Co bears an electron configuration of a half-filled superatomic 1P orbital (i.e., 1S1P||1D), a large α-highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap, symmetric multicenter bonds, and reasonable electron delocalization pertaining to metallic aromaticity. Topology analysis by atom-in-molecule illustrates the interactions between Co and N corresponding to covalent bonds, but the Co-N interactions in cationic CoN and CoN clusters are apparently weaker than those in the other systems. In addition, we identify a superatomic complex CoN which exhibits similar frontier orbitals as the naked Co cluster, but the alpha HOMO-LUMO gap is nearly double-magnified, which is consistent with the high-abundance peak of CoN in the experimental observation. The enhanced stability of such a ligand-coordinated superatomic complex CoN, along with the superatom Co with aromaticity, sheds light on special and general superatoms.