Adducts of Nitrous Oxide and N-Heterocyclic Carbenes: Syntheses, Structures, and Reactivity

摘要

N-Heterocyclic carbenes (NHCs) react at ambient conditions with nitrous oxide to give covalent adducts. In the crystal, all compounds show a bent N_2O group connected via the N-atom to the former carbene carbon atom. Most adducts are stable at room temperature, but heating induces decomposition into the corresponding ureas. Kinetic experiments show that the thermal stability of the NHC-N_2O adducts depends on steric as well as electronic effects. The coordination of N_2O to NHCs weakens the N-N bond substantially, and facile N-N bond rupture was observed in reactions with acid or acetyl chloride. On the other hand, reaction with tritylium tetrafluoroborate resulted in a covalent modification of the terminal O-atom, and cleavage of the C-N_2O bond was observed in a reaction with thionyl chloride. The coordination chemistry of IMes-N_2O (IMes = 1,3-dimesitylimidazol-2-ylidene) was explored in reactions with the complexes CuOTf, Fe(OTf)_2, PhSnCl_3, CuCl_2, and Zn(C_6F_5)_2. Structural analyses show that IMes-N_2O is able to act as a N-donor, as an O-donor, or as a chelating N,O-donor. The different coordination modes go along with pronounced electronic changes as evidenced by a bond length analysis.