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 CO₂ 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 = PhC{N(Dipp)C}₂; Dipp = 2,6-iPr₂C₆H₃). [(ADC)E]₂ readily undergo [4 + 2]-cycloadditions with CO₂ to result in barrelene-type bis-metallylenes [(ADC)E]₂(OC═O). The CO₂ addition is reversible for E = Ge; thus, CO₂ detaches under vacuum or at an elevated temperature and regenerates [(ADC)Ge]₂. [(ADC)Sn]₂(OC═O) is isolable but deoxygenates additional CO₂ to form [(ADC)Sn]₂(O₂CO) and CO. [(ADC)Si]₂(OC═O) is extremely reactive and could not be isolated or detected as it spontaneously reacts further with CO₂ to yield elusive monomeric Si(IV) oxides [(ADC)Si(O)]₂(CO_n) or carbonates [(ADC)Si(CO₃)]₂(CO_n) (n = 1 or 2) via the (de)oxygenation of CO₂. 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.