Protonation of N-heterocyclic carbene ligand coordinated to copper(I): Coordination mode of imidazolium cation as a function of counterion as determined by solid-state structures

Reactions of (IPr)Cu(X) (X = Cl or trifluoromethanesulfonate, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) complexes with the strong acids HOTf or HCl result in protonation of the C2 carbon of the IPr ligand to form imidazolium cations. Coordination of the imidazolium to the resulting CuI system depends upon the identity of the two counterions (chloride or triflate). The copper complexes [(IPrH)Cu(OTf)(μ-OTf)]2 and [IPrH][CuCl2] as well as the imidazolium salt [IPrH][OTf] have been characterized by NMR spectroscopy and single crystal X-ray diffraction studies.