Anionic Activation of CO2 via (Mn–CO2)− Complex on Magic-Numbered Anionic Coinage Metal Clusters Mn– (M = Cu, Ag, Au)

Given the immense challenge of excessive accumulation of carbon dioxide (CO 2 ) in the earth’s atmosphere, an extensive search is under way to convert atmospheric CO 2 to compounds of more utility. With CO 2 being thermodynamically extremely stable, activation of CO 2 is the first and most important step toward its chemical conversion. Building upon our earlier model for the anionic activation of CO 2 with azabenzene and inspired by the work of others on metal atom–CO 2 complexes, we investigated the possibility of anionic activation of CO 2 on small anionic metal clusters, which would have implications for catalytic conversion of CO 2 on metal surfaces with atomic-scale structural irregularities. We carried out theoretical calculations using density functional theory to examine small anionic metal clusters of Cu, Ag, and Au to check whether they form a complex with CO 2, with the sign of CO 2 being chemically activated. We found that a class of anionic metal clusters M n – with 1, 2, and 6 atoms consistently produced the activated complex (M n –CO 2 ) − for all three metals. There exists a strong interaction between the CO 2 moiety and M n – via a partially covalent M–C bond with a full delocalization of the electronic charge, as a result of electron transfer from the HOMO of M n – to the LUMO of CO 2 as in metal–CO 2 π-backbonding. We examined the interaction of frontier orbitals from the viewpoints of the orbital geometry and orbital energetics and found that the above magic numbers are consistent with both aspects.