Uncovering Nitrosyl Reactivity at N-Heterocyclic Carbene Center

N-heterocyclic carbenes (NHCs) have garnered much attention due to their unique properties, such as strong sigma-donating and pi-accepting abilities, as well as their transition-metal-like reactivity toward small molecules. In 2015, we discovered that NHCs can react with nitric oxide (NO) gas to form radical adducts that resemble transition metal nitrosyl complexes. To elucidate the analogy between NHC and transition metal NO adducts, here we have undertaken a systematic investigation of the electron- and proton-transfer chemistry of [NHC-NO]& sdot; (N-heterocyclic carbene nitric oxide radical) compounds. We have accessed a suite of compounds, comprised of [NHC-NO]+, [NHC-NO]-, [NHC-NOH]0, and [NHC-NHOH]+ species. In particular, [NHC-NO]- was isolated as potassium and lithium ion adducts. Most interestingly, a monomeric potassium [NHC-NO]- compound was isolated with the assistance of 18-crown-6, which is the first instance of a monomeric alkali N-oxyl compound to the best of our knowledge. Our results demonstrate that [NHC-NO]& sdot; exhibits redox behavior broadly similar to metal nitrosyl complexes, which opens up more possibilities for utilizing NHCs to build on the known reactivity of metal complexes. N-heterocyclic carbene (NHC) nitrosyl derivatives were prepared from redox chemistries of NHC nitric oxide radical ([NHC-NO]& sdot;). Under electrochemical conditions, [NHC-NO]& sdot; underwent reversible one-electron oxidation to lead [NHC-NO]+ while the reduction led to [NHC-NO]- and was found irreversible. These reactivities were further studied and found to resemble those of transition metal nitrosyl complexes while differing from oxime radicals.image