Structural Snapshots of Reversible Carbon Dioxide Capture and (De)oxygenation at Group 14 Diradicaloids

Although diradicals should exhibit a rather small reaction barrier as compared to closed-shell species for activating kinetically inert molecules, the activation and functionalization of carbon dioxide with stable main-group diradicals remain virtually unexplored. In this work, we present a thorough study on CO 2 activation, reversible capture, and (de) oxygenation mediated by stable Group 14 singlet diradicals (i. e. , diradicaloids) (ADC) E 2 (E = Si, Ge, Sn) based on an anionic dicarbene (ADC) framework (ADC = PhCN (Dipp) C 2; Dipp = 2, 6- i Pr 2 C 6 H 3). (ADC) E 2 readily undergo 4 + 2-cycloadditions with CO 2 to result in barrelene-type bis-metallylenes (ADC) E 2 (OC═O). The CO 2 addition is reversible for E = Ge; thus, CO 2 detaches under vacuum or at an elevated temperature and regenerates (ADC) Ge 2. (ADC) Sn 2 (OC═O) is isolable but deoxygenates additional CO 2 to form (ADC) Sn 2 (O 2 CO) and CO. (ADC) Si 2 (OC═O) is extremely reactive and could not be isolated or detected as it spontaneously reacts further with CO 2 to yield elusive monomeric Si (IV) oxides (ADC) Si (O) 2 (CO n) or carbonates (ADC) Si (CO 3) 2 (CO n) (n = 1 or 2) via the (de) oxygenation of CO 2. The molecular structures of all isolated compounds have been established by X-ray diffraction, and a mechanistic insight of their formation has been suggested by DFT calculations.