Mono- and Digold(I) Complexes with Mesoionic Carbenes: Structural Characterization and Use in Catalytic Silver-Free Oxazoline Formation

Triazolylidenes are a prominent class of mesoionic carbenes that have found use as supporting ligands in homogeneous catalysis in recent years. We present here the syntheses of three new mononuclear gold­(I) chlorido and two new dinuclear gold­(I) chlorido complexes. The ligands in the aforementioned complexes are derived from either the corresponding monotriazolium or the bitriazolium salts. All complexes have been characterized by ¹H and ¹³C­{¹H} NMR spectroscopy, mass spectrometry, and single-crystal X-ray diffraction studies. Structural characterization delivers a delocalized bonding situation within the triazolylidene ligands and a linear coordination at the gold­(I) centers. The gold­(I) centers in all cases are bound to one triazolylidene-C donor and a chlorido ligand. Additionally, for the digold­(I) complexes large Au–Au distances were observed, ruling out the existence of aurophilic interactions in these digold complexes in the solid state. All of the gold­(I) complexes were tested as (pre)­catalysts for the cyclization reaction of propargylic amides to form oxazolines. We show here that the steric bulk of the substituents on the triazolylidene ligands plays a decisive role in the catalytic efficiency of the gold­(I) complexes. Copper­(II) triflate is shown as a viable alternative to silver­(I) salts as an additive for the oxazoline formation. Mechanistic studies show the detection of a gold­(I) triazolylidene vinyl complex as an intermediate in the catalytic synthesis of oxazoline with these complexes. These results thus establish copper­(II) triflate as an alternative to silver­(I) salts as an additive in gold­(I) triazolylidene catalysis. Furthermore, it also shows that steric tuning of triazolylidene ligands can indeed be utilized for increasing the catalytic efficiency of the corresponding complexes.