Imidazolylidene Cu(II) Complexes: Synthesis Using Imidazolium Carboxylate Precursors and Structure Rearrangement Pathways

Copper(II) complexes of type (NHC)CuX2 (X = OAc, Cl, Br, BF4, and NO3) bearing monodentate Nheterocyclic
carbenes (NHCs) were prepared by in situ decarboxylation of imidazolium carboxylates as a new synthetic
methodology for Cu(II)-NHC complexes. In contrast to the classical deprotonation method, the decarboxylation protocol does
not require anaerobic conditions and provides access to complexes with NHCs that are unstable as free carbenes such as N,N′-
diisopropyl-imidazolylidene and N,N′-dimethyl-imidazolylidene. Spectroscopic evidence of the formation of the Cu−CNHC
bond is provided by UV−vis and EPR, in particular by the 44 MHz carbene hyperfine coupling constant using a 13C-labeled
imidazolylidene ligand. A variation of the nature of the carbene N-substituents and the anions bound to the Cu(II) center is
possible with this methodology. These variations strongly influence the stability of the complexes. Structural rearrangement and
ligand reorganization was observed during recrystallization, which are comprised of heterolytic Cu−CNHC bond dissociation for
unstable NHC ligands as well as homolytic Cu−X bond cleavage and disproportionation reactions depending on the nature of
the anion X in the copper complex.