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

Abstract

There has been growing interest in the CO 2 capture and reduction by transition-metal-free catalysts. Here we performed a proof-ofconcept 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 CO 2 . 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 CO 2 activation mechanism. (BDI)Al(I) can effectively bind and activate CO 2 by transferring one electron to CO 2 , and the resulting complex can be best expressed as [(BDI)Al(I)] + [CO 2 ] − . The participation of (BDI)Ae + further enhances the capture and activation of CO 2 by (BDI)Al(I).

Rational Design of Main Group Metal-Embedded Nitrogen-Doped Carbon Materials as Frustrated Lewis Pair Catalysts for CO2 Hydrogenation to Formic Acid

“…The optimized structures of Al@N-Gr-1, Al@N-Gr-2, Al@C 2 N are shown in Figure , where the bond lengths between Al and adjacent pyridine N are 1.93, 1.99, and 1.96 Å, respectively. These parameters are consistent with the reported values in similar AlN 2 structures. , We note that the Al atom protrudes from the substrate surface by 1.24 Å in Al@N-Gr-1, while Al@N-Gr-2 and Al@C 2 N maintain the planar structure. Stability is one of the prerequisites for evaluating the catalyst performance.…”

“…These parameters are consistent with the reported values in similar AlN 2 structures. 63,65 We note that the Al atom protrudes from the substrate surface by 1.24 Å in Al@N-Gr-1, while Al@N-Gr-2 and Al@C 2 N maintain the planar structure. Stability is one of the prerequisites for evaluating the catalyst performance.…”

“…Notably, Cui et al synthesized a monovalent aluminum compound that contains an Al­(I) center coordinated with a bidentate ligand with two N atoms and is a stable analogue of a carbene. Our recent computational studies showed that this Al­(I) center can effectively catalyze the oxidation addition of σ bonds such as H 2 and even efficiently activate CO 2 . , However, there are few reports on the hydrogenation of CO 2 to HCOOH over heterogeneous catalysts that fall into the FLP category.…”

“…Our recent computational studies showed that this Al(I) center can effectively catalyze the oxidation addition of σ bonds such as H 2 and even efficiently activate CO 2 . 64,65 However, there are few reports on the hydrogenation of CO 2 to HCOOH over heterogeneous catalysts that fall into the FLP category.…”