Molecular Design of Al(II) Intermediates for Small Molecule Activation.

Promoting societally important small molecule activation processes with earth-abundant metals is foundational for a sustainable chemistry future. In this context, mapping new reaction pathways that would enable abundant main-group elements to mimic the behaviors of d- and f-block elements is facilitated by exploring unusual oxidation states. The most abundant metal on earth, aluminum, has been well studied in the Lewis acidic +III and Lewis basic +I oxidation states but rarely in the potentially biphilic +II oxidation state until recently, when a renaissance of Al-(II) chemistry emerged from a range of research groups. In this Perspective, we review the chemistry of mononuclear Al radicals, including both Al-centered radicals (i.e., Al-(II) compounds) and redox non-innocent systems (i.e., formally Al-(II) species that are physically Al-(III) with ligand-centered radicals), with an emphasis on small molecule reactivity. We also provide a meta-analysis of the Al-(II) literature to summarize how different design strategies (e.g., redox non-innocence, strained coordination geometries) have been shown to impart biphilic character to Al radicals and tune their behavior, thus allowing Al radicals to mimic the chemistry of certain d- and f-block metal ions such as Ti-(III) and Sm-(II). We expect these molecular design concepts to inform future Al-(II) studies as the chemistry of this unusual oxidation state of Al continues to grow.