Synthesis and characterization of gold(III) adducts and cyclometallated derivatives with 2-substituted pyridines. Crystal structure of [Au{NC5H4(CMe2C6H4)-2}Cl2]

Abstract

The 2-benzylpyridines HL [NC₅H₄-(CH₂Ph)-2, -(CHMePh)-2 and (CMe₂Ph)-2] reacted with AuCl₃·2H₂O or Na[AuCl₄] to give adducts [Au(HL)Cl₃] or cyclometallated derivatives [AuLCl₂]. The metallation reaction involves direct activation of a C–H bond of the phenyl substituent. With the 2-alkylpyridines NC₅H₄(CMe₃)-2 and NC₅H₄(CH₂CMe₃)-2 (HL) only the tetrachloroaurate(III) salts [H₂L][AuCl₄] were isolated. The structure of [Au{NC₅H₄(CMe₂C₆H₄)-2}Cl₂] has been determined by X-ray diffraction. It reveals a six-membered ring having a boat-like conformation: there is a rather short interaction between one hydrogen of the Me group in axial position and the gold atom, Au ⋯ H 2.56(5)Å. The cyclometallated species react with PPh₃ to give cationic complexes [AuL(PPh₃)Cl]⁺ and with 1,2-bis(diphenylphosphino)ethane (dppe) to yield [AuL(dppe)Cl]⁺. Under pressure (50 atm, 60 °C) of CO extrusion of the metal occurs. The organic products formed likely arise from insertion of CO into the Au–C bond. The reactivity of the cycloaurated species is compared with that of analogous palladium(II) derivatives described previously.