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).