N4Mg6M (M = Li, Na, K) superalkalis for CO2 activation.

Abstract

Superatoms have exciting properties, including diverse functionalization, redox activity, and magnetic ordering, so the resulting cluster-assembled solids hold the promise of high tunability, atomic precision, and robust architectures. By utilizing adamantane-like clusters as building blocks, a new class of superatoms N4Mg6M (M = Li, Na, K) is proposed here. The studied superalkalis feature low adiabatic ionization energies, an antibonding character in the interactions between magnesium and nitrogen atoms, and highly delocalized highest occupied molecular orbital (HOMO). Consequently, the N4Mg6M superalkalis might easily lose their HOMO electrons when interacting with superhalogen electrophiles to form stable superatom [superalkali]+[superhalogen]- compounds. Moreover, the studied superalkalis interact strongly with carbon dioxide, and the resulting N4Mg6M/CO2 systems represent two strongly interacting ionic fragments (i.e., N4Mg6M+ and CO2 -). In turn, the electron affinity of the N2 molecule (of -1.8 eV) is substantially lower than that observed for carbon dioxide (EA = -0.6 eV) and consequently, the N2 was found to form the weakly bound [N4Mg6M][N2] complex rather than the desired ionic [N4Mg6M]+[N2]- product. Thus, the N4Mg6M superalkalis have high selectivity over N2 when it comes to CO2 reduction and also are themselves stable. We believe that the results described within this paper will be useful for understanding CO2 activation, which is the first step for producing fuels from CO2. Moreover, we demonstrate that designing novel superatomic systems and exploring their physicochemical features might be used to create desirable functional materials.