Polynuclear Gold(I) Complexes of Mercaptocarboxylic Acids

In a search for air-stable and water-soluble mercapto compounds of gold with a high gold content, benzylthiol, α-mercapto-acetic acid, β-mercapto-propionic acid, and thiosalicylic acid were converted into aurated sulfonium salts by treatment with reagents of the type [(R₃P)Au]₃O⁺ BF₄^- or [(R₃P)Au]⁺ BF₄^-. - BzSH served as a mono-functional model thiol, which was converted into BzS[Au(PMe₃)]₂⁺ BF₄^- (1). The small tertiary phosphine was chosen in order to minimize steric effects. In the crystal the cations are associated into centrosymmetrical climers through short Au-Au contacts (auriophilicity). - The two carboxylic acids with thiol functions, HSCH₂COOH and HSCH₂CH₂COOH, gave triply aurated species of the type {[(Ph₃P)Au]₂S(CH₂)_nCOOAu(PPh ₃)}⁺ BF₄^- (2: n = 2, 3: n = 3). The crystal structure analysis of the representative compound 2 showed again the presence of centrosymmetrical cation dimers with geometrical details of the sulfonium part of the molecules very similar to those of the reference compound 1. In addition, the carboxyl groups in 2 and 3 bear a (phosphine)gold unit mono-hapto bound at one of the oxygen atoms. - 2-HS-C₆H₄-COOH was converted into a sulfonium salt {[(Ph₃P)Au]₂S-C₆H₄-COOH}⁺ BF₄^- (4), which crystallizes free of solvent from dry dichloromethane, and as a monohydrate with one mole of dichloromethane (4a = 4·H₂O·CH₂Cl₂) from hydrous CH₂Cl₂. In the solvent-free crystalline phase (4) the cations are associated into dimers with the usual intermolecular double Au-Au contacts, but in 4a there are monomeric cations with the water molecule hydrogen-bonded to the carboxylic acid function.