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

N-Heterocyclic carbenes
(NHCs) react at ambient conditions with
nitrous oxide to give covalent adducts. In the crystal, all compounds
show a bent N2O 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–N2O adducts depends on steric as well as electronic effects.
The coordination of N2O 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–N2O bond was observed in a reaction with thionyl chloride. The coordination
chemistry of IMes–N2O (IMes = 1,3-dimesitylimidazol-2-ylidene)
was explored in reactions with the complexes CuOTf, Fe­(OTf)2, PhSnCl3, CuCl2, and Zn­(C6F5)2. Structural analyses show that IMes–N2O 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.