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

Abstract

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-2 H -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)Au I Br complexes ( 1a – 9a ) in terms of the conversion to the [(NHC) 2 Au I ] + ( 1b – 9b ) and [(NHC) 2 Au III Br 2 ] + ( 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)Au I Br) 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 Au III Br 2 ] + . 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)Au I Br 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)Au I I and finally to [(NHC) 2 Au I ] + .

Exploring the Coordination Properties of Phosphonium‐Functionalized N‐Heterocyclic Carbenes Towards Gold

“…The partial oxidation of gold(I) to gold(III) could be the result of a partial disproportionation of the gold(I) complex. This type of reactivity has already been reported in the literature [48–52] …”

“…This type of reactivity has already been reported in the literature. [48][49][50][51][52] Complex 3 presents a linear coordination geometry, as expected for a metal center in a d 10 electronic configuration, with one NHC and a bromide ligand coordinated to the gold(I) (CÀ AuÀ Br angle of 177.54°, see Figure 2). The AuÀ C and AuÀ Br bond distances are in agreement with literature data (1.984 Å and 2.3976 Å, respectively).…”

Reaction Behavior of [1,3-Diethyl-4,5-diphenyl-1H-imidazol-2-ylidene] Containing Gold(I/III) Complexes against Ingredients of the Cell Culture Medium and the Meaning on the Potential Use for Cancer Eradication Therapy

“…The latter finally decomposes to Au(0). 24,25 Especially, the mono-NHC-Au(I)-I complex displayed this reaction during the first minutes of incubation. Such transformations are of high importance for the interpretation of the biological results, because there are several clues that the antiproliferative activity of [(NHC) 2 Au-(I)] + species is generally more than 5-fold higher than that of the related (NHC)gold(I)-X (X = Cl, Br, I) complexes.…”

“…Such transformations are of high importance for the interpretation of the biological results, because there are several clues that the antiproliferative activity of [(NHC) 2 Au-(I)] + species is generally more than 5-fold higher than that of the related (NHC)gold(I)-X (X = Cl, Br, I) complexes. 26−31 In water-free organic solvent, e.g., dimethylformamide (DMF), the mono-NHC-Au(I)-X complexes (Chart 1) were stable for 72 h (degradation <2%), 24 so stock solutions for the in vitro testing can be prepared. The final concentrations for the cell culture experiments were achieved by dilution with the respective medium, to realize a maximum amount of organic solvent of 0.1%.…”

“…Instead, they show partially ligand scrambling resulting in the cationic [(NHC) 2 Au­(I)] + complex (bis-NHC-Au­(I), Chart ) and [Au­(I)­X 2 ] − ion. The latter finally decomposes to Au(0). , …”