Computational and experimental investigations of CO2 and N2O fixation by sterically demanding N-heterocyclic carbenes (NHC) and NHC/borane FLP systems

Abstract

The sterically demanding NHCs 1,3-di-tert-butylimidazolin-2-ylidene (1a), 1,3-di-tert-butyl-4,5-dimethylimidazolin-2-yildene (1b), and also the corresponding frustrated Lewis pair combinations 1a,b/B(C₆F₅)₃ react readily with CO₂ to form the NHC·CO₂ (5a,b) and the NHC·CO₂·B(C₆F₅)₃ (9a,b) adducts, respectively. However, N₂O activation and isolation of the NHC·N₂O adduct (6) was only possible for NHC 1a. On heating, the NHC·N₂O adduct 6 degrades to 1a, N₂O, N₂ and the urea derivative 7. Nevertheless, an NHC·N₂O adduct of 1b was obtained with the FLP system 1b/B(C₆F₅)₃. In contrast, for the FLP combination 1a/B(C₆F₅)₃, N₂O coordination appears to be slower than the self-deactivation. Hence, only the self-deactivation product 3 was observed under an N₂O atmosphere. DFT calculations give insights into the CO₂ and N₂O activation process with 1a,b and 1a,b/B(C₆F₅)₃.