What Singles out Aluminyl Anions? A Comparative Computational Study of the Carbon Dioxide Insertion Reaction in Gold-Aluminyl, -Gallyl, and -Indyl Complexes.

Anionic aluminum(I) anions ("aluminyls") are the most recent discovery along Group 13 anions, and the understanding of the unconventional reactivity they are able to induce at a coordinated metal site is at an early stage. A striking example is the efficient insertion of carbon dioxide into the Au-Al bond of a gold-aluminyl complex. The reaction occurs via a cooperative mechanism, with the gold-aluminum bond being the actual nucleophile and the Al site also behaving as an electrophile. In the complex, the Au-Al bond has been shown to be mainly of an electron-sharing nature, with the two metal fragments displaying a diradical-like reactivity with CO. In this work, the analogous reactivity with isostructural Au-X complexes (X = Al, Ga, and In) is computationally explored. We demonstrate that a kinetically and thermodynamically favorable reactivity with CO may only be expected for the gold-aluminyl complex. The Au-Al bond nature, which features the most (nonpolar) electron-sharing character among the Group 13 anions analyzed here, is responsible for its highest efficiency. The radical-like reactivity appears to be a key ingredient to stabilize the CO insertion product. This investigation elucidates the special role of Al in these hetero-binuclear compounds, providing new insights into the peculiar electronic structure of aluminyls, which may help for the rational control of their unprecedented reactivity toward carbon dioxide.