In search for less-explored and non-classical N-heterocyclic carbenes (NHCs),1 we
have studied the use of pyrazoles and their benzannulated analogues (indazoles) as
ligand precursors. In analogy to complexes of imidazolinylidenes, those of their
isomeric pyrazolinylidenes2 can be conveniently accessed by in situ deprotonation or
oxidative addition of their N-heterocyclic salts in the presence of suitable metal
precursors. The structural and electronic diversity of this rare class of NHC is
exemplified by the isolation of non-classical NHC complexes with both normal and
mesoionic1 coordination modes. An overview of our previous and more recent efforts
in exploring the chemistry of these rare NHCs will be given including attempts at their
stereoelectronic profiling.3
References
1. Huynh, H. V. The Organometallic Chemistry of N-heterocyclic Carbenes. John
Wiley & Sons, Ltd. 2017 (and references therein).
2. Dordevic, N.; Huynh, H. V. Organometallics 2022, 41, 335–344 (and
references therein).
3. Huynh, H. V. Chem. Rev. 2018, 118, 9457–9492 (and references therein).