Computational Design of Neutral Heteronuclear Alkaline-Earth Metal Complexes Featuring a 2c─3e Bond and Their Application in CO2 Activation.

In contrast to electron-paired bonds, odd-electron bonding systems have garnered significant interest due to their distinctive bonding and radical properties. A series of neutral heteronuclear (AEM-AEM')AMD (AEM = Be, Mg, and Ca) complexes was theoretically designed by binding two distinct alkaline-earth-metal (AEM) atoms with amidinate radical ligands (AMD) via side-on coordination. Upon one-electron transfer from AEM-AEM' to AMD, the electron occupies an AMD nonbonding orbital, avoiding destabilization from rising ligand orbital energy, while a two-center three-electron (2c─3e) bond forms between the two AEMs. Subsequent CO2 activation studies show that (Mg─Ca)AMD exhibits the highest activity, a net electron transfer of 1.46 |e| derived from a two-electron redox process, accompanied by activation indicators: elongation of the C─O bond length (from 1.160 to 1.248 Å), reduction of the Wiberg bond index (from 1.816 to 1.454) and decrease of the flexible force constant (from 16.667 to 6.410 mDyn/Å). Investigation of CO2 conversion reveals CO formation with monotonically decreasing free energies. Although methane is the thermodynamically more favorable product, its formation encounters three energy-consuming steps, making the free-energy profile nonmonotonic. This work extends heteronuclear odd-electron bonding into the s-block and provides a route for CO2 activation using alkaline-earth systems beyond transition-metal or main-group catalysts.