Anti-Electrostatic Main Group Metal-Metal Bonds That Activate CO₂

There has been growing interest in the CO₂ capture and reduction by transition-metal-free catalysts. Here we performed a proof-of-concept study using an ab initio valence bond method called the block-localized wave function (BLW) method. The integrated BLW and density function theory (DFT) computations demonstrated that heterobimetallic Ae⁺/Al(I) (Ae represents alkaline earth metals Mg and Ca) Lewis acid/base combinations without transition metals can facilely capture and activate CO₂. There are two remarkable findings in this study. The first concerns the ionic nature of the metal-metal bonds. The experimentally synthesized low valent aluminum compound with a bidentate β-diketiminate (BDI) ligand, or (BDI)Al(I) in brief, is a Lewis base due to the lone pair on the aluminum cation though overall Al(I) is positively charged. Al(I) can form ionic metal-metal bonds with the alkaline earth metals of the positively charged Lewis acids (BDI)Ae⁺. This type of ionic metal-metal bonds is counterintuitive and antielectrostatic as both metals carry positive charges. The second finding is the CO₂ activation mechanism. (BDI)Al(I) can effectively bind and activate CO₂ by transferring one electron to CO₂, and the resulting complex can be best expressed as [(BDI)Al(I)]⁺[CO₂]^-. The participation of (BDI)Ae⁺ further enhances the capture and activation of CO₂ by (BDI)Al(I).