Internal and External Influences on Stability and Ligand Exchange Reactions in Bromido[3-ethyl-4-aryl-5-(2-methoxypyridin-5-yl)-1-propyl-1,3-dihydro-2H-imidazol-2-ylidene]gold(I) Complexes

The ligand scrambling reaction of gold(I) complexes is a phenomenon occurring primarily in L-Au-I-X (L = phosphine, N-heterocyclic carbene (NHC), and thiol; X = halide and thiol) complexes and has been observed among others for e.g., the bromido[3-ethyl-4-(4-methoxyphenyl)-5-(2-methoxypyridin-5-yl)-1-propyl-1,3-dihydro-2H-imidazol-2-ylidene]gold(I) complex (7a), which underwent ligand rearrangement in aqueous solutions. In this study, we investigated the influence of substituents on the 4-aryl ring of the related (NHC)(AuBr)-Br-I complexes (1a-9a) in terms of the conversion to the [(NHC)(2)Au-I](+) (1b-9b) and [(NHC)(2)(AuBr2)-Br-III](+) (1c-9c) species. Furthermore, the influence of external factors such as solvent, temperature, concentration, and presence of halides (Cl-, Br-, and I-) or hydroxyl ions was studied to gain a deeper understanding of the ligand rearrangement reaction. The substituent on the 4-aryl ring has a marginal impact on the scrambling reaction. Out of the investigated organic solvents (dimethylformamide (DMF), dimethyl sulfoxide (DMSO), ethanol (EtOH), methanol (MeOH), and acetonitrile (ACN)), only ACN separates single complex molecules. In all other solvents, relatively stable ((NHC)(AuBr)-Br-I)(2) dimers are present. The addition of water to ACN solutions forces the formation of such dimeric units, starting the transformation to [(NHC)(2)Au-I](+) and [(NHC)(2)(AuBr2)-Br-III](+). The rate-determining step is the release of Br- from a T-shape intermediate because an excess of KBr terminates this reaction. Furthermore, it is obvious that only single molecules react with halides. The aurophilic interactions between two (NHC)(AuBr)-Br-I molecules are too strong in the presence of water and largely impeded reaction with halides. As a single molecule, the reaction with Cl- (e.g., in a 0.9% NaCl solution) is notable, while I- even leads to a fast and quantitative conversion to (NHC)(AuI)-I-I and finally to [(NHC)(2)Au-I](+).

Automated summary

This study investigates the ligand scrambling reaction of gold(I) complexes and its influence by substituents and external factors like solvents and halides. The results show that substituents have a marginal impact on the reaction, adding water to ACN can force the conversion from dimers to single molecules, and the reactions with chloride and iodide ions exhibit different rates and product selectivity.