Abstract
Copper(II) complexes of type (NHC)CuX2 (X = OAc, Cl, Br, BF4, and NO3) bearing monodentate N-heterocyclic 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.
Highlights
N-Heterocyclic carbenes (NHCs)[1] have emerged as ligands for transition metals to perform a multitude of catalytic transformations.[2]
While soft-acid soft-base Cu(I)‒C interactions lead to highly stable Cu(I)-NHC complexes, less favourable intermediate/hard-soft Cu(II)‒C interactions make the isolation of Cu(II)-NHC complexes difficult
Synthesis of complex (1)Cu(OAc)[2] following either the deprotonation or the decarboxylation routes starting from ligands 1H.Cl and 1-CO2, respectively
Summary
N-Heterocyclic carbenes (NHCs)[1] have emerged as ligands for transition metals to perform a multitude of catalytic transformations.[2]. Most of the few examples reported so far employ tetheredNHC ligands to favour coordination to the Cu(II) center (for example complexes A–D,6–9 Fig. 1). These Cu(II)-NHC complexes are typically prepared by transmetalation using an NHC transfer reagent or by direct complexation to the isolated free carbene.[6,7,8,9,10,11,12,13,14,15,16] Complexation of Cu(II) by unsupported monodentate NHC ligands has been explored little with only (1)Cu(OAc)[2] being authenticated by X-ray crystallography (E, Fig. 1).[12]
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