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

The sterically demanding NHCs 1,3-di-tert-butylimidazolin-2-ylidene (1a), 1,3-di-tert-butyl-4,5-dimethyl-imidazolin-2-yildene (1b), and also the corresponding frustrated Lewis pair combinations 1a, b/B(C6F5)(3) react readily with CO2 to form the NHC center dot CO2 (5a, b) and the NHC center dot CO2 center dot B(C6F5)(3) (9a, b) adducts, respectively. However, N2O activation and isolation of the NHC center dot N2O adduct (6) was only possible for NHC 1a. On heating, the NHC center dot N2O adduct 6 degrades to 1a, N2O, N-2 and the urea derivative 7. Nevertheless, an NHC center dot 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).