Reversible Insertion of CO$_2$ into a Bis(formazanate) Magnesium Complex

The capture, storage, and release of CO$_2$ is one of the most pressing issues in chemistry. Herein, we designed bis(formazanate) magnesium complexes as precursors, which were synthesized by straightforward transamination between the neutral formazan [PhNNC(4-$^t$BuPh)NNHPh] (LH) and [Mg{N(SiMe$_3$)$_2$}$_2$]. Depending on the solvent, either the homoleptic complex [L$_2$Mg] or the thf-coordinated complex [L$_2$Mg(thf)] was isolated. Reaction of the solvent-free complex [L$_2$Mg] with CO$_2$ led to stoichiometric insertion into one of the four Mg–N bonds with concomitant rearrangement of the ligand scaffold which adopts a trinuclear cagelike structure. The insertion reaction is reversible depending on thermolysis or treatment with tetrahydrofuran, which is corroborated by density functional theory calculations. This showcases the first example of reversible uptake of CO$_2$ by an s-block coordination compound.