Tetrameric gold(III) complexes with L-histidine-containing dipeptides: crystal structure of the highly hydrated chloride salt of [Au(L-Val-L-His)]₄²⁺ tetramer

A chloride salt of the tetrameric gold(III) complex with the L-valyl-L-histidine dipeptide (L-Val-L-His), [Au(L-Val-L-His)]₄Cl₂·14.5H₂O, was obtained from the mononuclear [Au(L-Val-L-His)Cl]Cl·2H₂O species at pH 7.00 in aqueous solution at room temperature. The tetrameric gold(III) complex was characterized by ¹H NMR and IR spectroscopic techniques, while its structure was determined by single-crystal X-ray diffraction analysis. In the title compound, each gold(III) ion is coordinated by three nitrogen atoms, as in the corresponding mononuclear gold(III) complex (amine N_A, deprotonated amide N_P, and N3 imidazole nitrogen atoms), and in addition by the N1 nitrogen of the deprotonated imidazole ring from the second mononuclear gold(III) unit, which acts as a bridging ligand between two gold(III) ions. The chloride anions, by being involved in Au∙∙∙Cl∙∙∙Au interactions, help to bridge the neighboring gold(III) centers both inside and outside the tetramer.