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 Formally, eight methyl groups of IPr have been replaced by 4‐tert‐butylphenyl substituents, and two para‐methyl groups have been added.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”.

A sterically shielded cationic NHC gold complex IPr**Au-BArF(24) without solvent coordination has been prepared in situ in CH(2)Cl(2). The monovalent transition metal electrophile, a "soft proton", heterolytically activates the C-B bond of the weakly coordinating counterion tetrakis[3,5-bis(trifluoromethyl)phenyl]borate at room temperature.

An N‐heterocyclic carbene gold complex IPr**AuNTf2 has been synthesized, spectroscopically investigated, structurally characterized, and used as a highly active and stable catalyst in the Hashmi phenol synthesis (IPr**=1,3‐di‐p‐tolylimidazol‐2‐ylidene with four di‐tert‐butylbenzhydryl ortho substituents, Tf=trifluoromethansulfonyl). A side reaction comprises an irreversible arene oxide ring opening with subsequent 1,2 methyl shift. The advantage of the steric demand of the ancillary ligand was explained by higher equilibrium concentrations of the cationic gold species, the circumvention of inactive dinuclear intermediates, and the inhibition of catalyst decomposition pathways. The methanol addition–hydration of alkynes featured a turnover‐limiting proton transfer step of an alkenylgold catalyst resting state, indicated by a large primary kinetic isotope effect and an alkyne competition experiment.

The N-heterocyclic carbene (NHC) ligand IPr** features substituents of unprecedented steric demand (IPr** = 1,3-bis[2,6-bis[(4-tert-butylphenyl)methyl]-4-methylphenyl]imidazol-2-ylidene). The NHC structure is an advanced derivative of the IPr system, and of the IPr* by Berthon-Gelloz and Markó. In the IPr** ancillary ligand, two para-methyl are introduced and eight methyl groups of IPr are formally replaced by a 4-tert-butylphenyl substituent, respectively, thereby sterically shielding both a coordinated metal and its second ligand. Favorable features of ligand and late transition metal complexes include high solubility, aesthetic NMR spectra, and a tendency towards crystallization.

No abstract