Reversible Insertion of CO2 into a Bis(formazanate) Magnesium Complex

The capture, storage, and release of CO2 is one of the most pressing issues in chemistry. Herein, we designed bis(formazanate) magnesium complexes as precursors, which were synthesized by straight-forward transamination between the neutral formazan [PhNNC(4-(BuPh)-Bu-t)NNHPh] (LH) and [Mg{N (SiMe3)(2)}(2)]. Depending on the solvent, either the homoleptic complex [L2Mg] or the thf-coordinated complex [L2Mg(thf)] was isolated. Reaction of the solvent-free complex [L2Mg] with CO2 led to stoichiometric insertion into one of the four Mg-Nbonds 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 CO2 by an s-block coordination compound.