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

Abstract

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 πbackbonding. 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.

Spontaneous Reduction by One Electron on Water Microdroplets Facilitates Direct Carboxylation with CO2

“…So the CO 2 •– anion is metastable with a short lifetime of ∼90 ms. , As a result, despite being highly reactive, the direct use of CO 2 •– for further CO 2 fixation has been difficult and scarce . Instead of directly using electrons, another strategy is to use anions or partial negative charges to attack the carbon atom on CO 2 , yielding stable products with carboxylic groups. − …”

Impacts of the Catalyst Structures on CO2 Activation on Catalyst Surfaces

“…In an aqueous solution of transition metal salts, a CO 2 molecule can be activated by forming a transition metal-CO 2 complex via direct coordination [4,22,[76][77][78]. This can be considered a combination of π electrons transfer to the metal and the oxidative addition of the C-O bond to the metal (Scheme 2) [22].…”

“…Catalytic reduction of CO 2 to methanol and other products has been observed with transition metal complexes [22]; it is possible to develop highly active catalysts based on early transition metal complexes. It was found that anionic metal clusters (M n − ) with 1, 2, and 6 atoms formed the activated complex (M n -CO 2 ) − for Cu, Ag, and Au [4]. The activation was a result of a strong interaction between the CO 2 moiety and M n − via the formation of a partial covalent M-C bond with a full delocalization of the electronic charge due to electron transfer from the HOMO of M n − to the LUMO of CO 2 as in metal-CO 2 π-backbonding.…”

“…The activation was a result of a strong interaction between the CO 2 moiety and M n − via the formation of a partial covalent M-C bond with a full delocalization of the electronic charge due to electron transfer from the HOMO of M n − to the LUMO of CO 2 as in metal-CO 2 π-backbonding. This can easily be understood from the perspective of the orbital geometry and energetics following orbital mixing (orbital interaction between HOMO of M n − and LUMO of CO 2 ) [4,29].…”

“…CO 2 is a chemically inert molecule due to its kinetically stable nature; thus, its conversion by reduction to economically viable products relies on its activation to kinetically vibrant species [4]. The stable nature of CO 2 and high activation barrier (1.9 eV) implies that its transformation over catalyst surfaces should create a unique environment for facilitating activation pathways [5][6][7][8][9].…”