Synthesis and Characterization of Gold(I) Complexes of Dibenzotropylidene-Functionalized NHC Ligands (Trop-NHCs)

Gold­(I)
complexes of dibenzotropylidene-functionalized N-heterocyclic
carbene ligands (Trop-NHCs) have been prepared in order to investigate
their structural features and to reveal possible interactions of the
olefin unit with the metal center. The precursor imidazolium chloride
salts (R-1) were generated in a single step using N-substituted imidazoles (R = H, Me, DiPP, Ad) and 1 or
2 equiv of Trop-Cl, generating unsymmetrical and symmetrical NHC-olefin
hybrids. The structural parameters of the ligands were determined
by synthesis and X-ray diffraction analysis of their corresponding
gold­(I) chloride complexes, revealing highly flexible steric demands
of the Trop unit. Conversion of these complexes with halide-abstracting
reagents such as AgNTf2 and NaBArF24 cleanly
gave neutral, NTf2-coordinated complexes of the type [(NHC)­Au­(NTf2)] (R-3) and the cationic bis-NHC-coordinated
complexes [(NHC)2Au]­BArF24 (R-4), respectively. The Gagosz-type complexes R-3 were
further tested in the hydration of diphenylacetylene, showing a clear
trend in activity depending on the ligand’s sensitivity to
hydrolysis.