Isolated Silver Intermediate of Gold Precatalyst Activation

The silver chloride and gold chloride complex of the extremely sterically shielding N-heterocyclic carbene ligand IPr** have been synthesized and characterized. Formally, eight methyl groups of IPr have been replaced by 4-tert-butylphenyl substituents, and two para-methyl groups have been added. The world record for the ligand's buried volume (55.4 % for IPr**AuCl, and 56.7 % for IPr**AgCl) has been determined by single-crystal X-ray analyses. Reaction of IPr**AuCl with AgSbF6 in toluene yields AgCl and a cationic gold complex with toluene coordination. The same reaction in 1,2-dichloroethane leads to precipitation of AgCl and an equilibrium of [IPr**Au]SbF6 and [IPr**AuClAg]SbF6. Single crystals of the latter were suitable for an X-ray structure analysis, which reveal an Au–Cl–Ag triangle with an almost undisturbed IPr**–Au–Cl fragment and the coordination of two IPr** arenes to a silver cation. The structural parameters emphasize the role of NHC-Au cations as highly electrophilic “soft protons”.