Anionic N-Heterocyclic Carbene Gold(I) Complexes from Sydnones as a Precarbenic Unit: A Comprehensive Study of the Au-C sp² Bond Stability Compared to Its "Relative" Imidazole

A series of sydnones was used as N-heterocyclic carbene (NHC) precursors to synthesize gold-(I) complexes by a two-step reaction. The first reaction step involves the deprotonation of the sydnone to obtain the corresponding anionic NHC. Second, the metal precursor was added to afford the corresponding gold-(I) complex. At this point, the complex obtained depends on the nature of the metal precursor. The use of AuCl-(tht) allows one to obtain the monocarbene complex (sydnone-Au-tht), and by contrast, the use of a gold-(I) imidazole complex allows one to obtain the bis-carbene complex (syndone-Au-imidazole). Furthermore, the use of sydnone as a metal precursor (sydnone-Au-tht) yields the formation of asymmetric bis-sydnones complexes (sydnone-Au-sydnone'). All gold-(I) complexes have been characterized by NMR, Fourier transform infrared spectroscopy (FT-IR), ultraviolet-visible (UV-vis), and elemental analyses. Density functional theory (DFT) methods were used in order to theoretically establish a structure/stability relationship and describe the catalytic effect of these complexes and their imidazole analogues.