N-Heterocyclic Carbene Gold(I) Complexes: Mechanism of the Ligand Scrambling Reaction and Their Oxidation to Gold(III) in Aqueous Solutions

N-Heterocyclic carbene (NHC) gold(I) complexes offer great prospects in medicinal chemistry as antiproliferative, anticancer, and antibacterial agents. However, further development requires a thorough understanding of their reaction behavior in aqueous media. Herein, we report the conversion of the bromido[3-ethyl-4-(4-methoxyphenyl)-5-(2-methoxypyridin-5-yl)-1-propylimidazol-2-ylidene]gold(I) ((NHC)(AuBr)-Br-I, 1) complex in acetonitrile/water mixtures to the bis[3-ethyl-4-(4-methoxyphenyl)-5-(2-methoxypyridin-5-yl)-1-propylimidazol-2-ylidene]gold(I) ([(NHC)(2)Au-1](+), 7), which is subsequently oxidized to the dibromidobis[3-ethyl-4-(4-methoxyphenyl)-5-(2-methoxypyridin-5-yl)-1-propylimidazol-2-ylidene] gold-(III) ([(NHC)(2)(AuBr2)-Br-III](+), 9). By combining experimental data from HPLC, NMR, and (LC-)/HR-MS with computational results from DFT calculations, we outline a detailed ligand scrambling reaction mechanism. The key step is the formation of the stacked ((NHC)(AuBr)-Br-I)(2) dimer (2) that rearranges to the T-shaped intermediate Br(NHC)(2)Au-I-(AuBr)-Br-I (3). The dissociation of Br- from 3 and recombination lead to (NHC)(2)Au-I-(AuBr2)-Br-I (5) followed by the separation into [(NHC)(2)Au-I](+) (7) and [(AuBr2)-Br-I](-) (8). [(AuBr2)-Br-I](-) is not stable in an aqueous environment and degrades in an internal redox reaction to Au-0 and Br-2. The latter in turn oxidizes 7 to the gold(III) species 9. The reported ligand rearrangement of the (NHC)(AuBr)-Br-I complex differs from that found for related silver(I) analogous. A detailed understanding of this scrambling mechanism is of utmost importance for the interpretation of their biological activity and will help to further optimize them for biomedical and other applications.