Reactivity of Cobalt Clusters Con±/0 with Dinitrogen: Superatom Co6+ and Superatomic Complex Co5N6

J Phys Chem A. 2021 Mar 18;125(10):2130-2138. doi: 10.1021/acs.jpca.1c00483. Epub 2021 Mar 9.

Abstract

We report a joint experimental and theoretical study on the reactions of cobalt clusters (Con±/0) 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 Con (n = 2-30) and anionic Con- clusters (n = 7-53) which are relatively inert in reacting with nitrogen in the fast-flow tube, Con+ clusters readily react with nitrogen resulting in adducts of one or multiple N2 except Co6+ 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 Con+ clusters with nitrogen and unveiled the open-shell superatomic stability of Co6+ within a highly symmetric (D3d) structure. The D3d Co6+ bears an electron configuration of a half-filled superatomic 1P orbital (i.e., 1S21P3||1D0), 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 Con+ and N2 corresponding to covalent bonds, but the Co-N interactions in cationic Co2+N2 and Co6+N2 clusters are apparently weaker than those in the other systems. In addition, we identify a superatomic complex Co5N6+ which exhibits similar frontier orbitals as the naked Co5+ cluster, but the alpha HOMO-LUMO gap is nearly double-magnified, which is consistent with the high-abundance peak of Co5N6+ in the experimental observation. The enhanced stability of such a ligand-coordinated superatomic complex Co5N6+, along with the superatom Co6+ with aromaticity, sheds light on special and general superatoms.