Structural Snapshots of Reversible Carbon Dioxide Capture and (De)oxygenation at Group 14 Diradicaloids
Falk Ebeler, Beate Neumann, Hans‐Georg Stammler, Israel Fernández, Rajendra S. Ghadwal
Abstract
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 = PhC{N(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.