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

Abstract

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 1 H and 13 C{ 1 H} 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.