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

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