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 activation, reversible capture, and (de)oxygenation mediated by stable Group 14 singlet diradicals (i.e., diradicaloids) [(ADC)E] (E = Si, Ge, Sn) based on an anionic dicarbene (ADC) framework (ADC = PhC{N(Dipp)C}; Dipp = 2,6-PrCH). [(ADC)E] readily undergo [4 + 2]-cycloadditions with CO to result in barrelene-type bis-metallylenes (ADC)E. The CO addition is reversible for E = Ge; thus, CO detaches under vacuum or at an elevated temperature and regenerates [(ADC)Ge]. (ADC)Sn is isolable but deoxygenates additional CO to form (ADC)Sn and CO. (ADC)Si is extremely reactive and could not be isolated or detected as it spontaneously reacts further with CO to yield elusive monomeric Si(IV) oxides (ADC)Si(O) or carbonates (ADC)Si(CO) ( = 1 or 2) via the (de)oxygenation of CO. 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.