Redox Halido-Based Reactivity on the Addition of Bromine to Linear NHC Gold Halides

Halogen addition to a linear gold complex to form a square-planar species is one of the most representative examples of what is generally classified as an “oxidative addition” process. This work examines the addition of bromine to the linear complexes labeled as NHC^{Me, Bn}AuX (with NHC^{Me, Bn}AuX indicating complexes obtained both with 1,3-dimethylimidazol-2-yl and 1-benzyl-3-dimethylimidazol-2-yl and X = Cl or I), which generally yield heterohalide square-planar gold derivatives. The products have been confirmed by X-ray structural characterization, highlighting the possibility of isomerism and the key role of both the halogen atom and the substituents on the carbene ligand in favoring the formation of one product over another. From this perspective, a distinctive reactivity emerges, revealing the possibility of obtaining homohalide derivatives containing three bromide ligands under different experimental conditions, depending on the nature of the halide in the initial linear complex. This reactivity implies a redox process with an electron transfer between the halide ligand and a Br₂ molecule. Detailed DFT calculations reveal the occurrence of the inverted ligand field for all square-planar products and intermediates. The analysis, also supported by Raman experiments, shows that the homohalide derivatives can form via halogen-bonded intermediates, accompanied by the elimination of IBr or BrCl moieties.