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

Given the immense challenge of excessive accumulation of carbon dioxide (CO₂) in the earth's atmosphere, an extensive search is under way to convert atmospheric CO₂ to compounds of more utility. With CO₂ being thermodynamically extremely stable, activation of CO₂ is the first and most important step toward its chemical conversion. Building upon our earlier model for the anionic activation of CO₂ with azabenzene and inspired by the work of others on metal atom-CO₂ complexes, we investigated the possibility of anionic activation of CO₂ on small anionic metal clusters, which would have implications for catalytic conversion of CO₂ 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 CO₂, with the sign of CO₂ being chemically activated. We found that a class of anionic metal clusters M^- with 1, 2, and 6 atoms consistently produced the activated complex (M-CO₂)^- for all three metals. There exists a strong interaction between the CO₂ moiety and M^- 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^- to the LUMO of CO₂ as in metal-CO₂ 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.