Sulfonated Water-Soluble N‑Heterocyclic Carbene Silver(I) Complexes: Behavior in Aqueous Medium and as NHC-Transfer Agents to Platinum(II)

This report describes the synthesis of water-soluble silver(I) and platinum(II) complexes bearing sulfonated mono- or dianionic N-heterocyclic carbene ligands. Thus, treatment of the corresponding zwitterionic imidazolium derivative with silver(I) oxide in water afforded the light-sensitive bis(carbene) complexes Ag[Ag(NHC)2] (2 Ag+ ), which were transformed into the stable salts Na[Ag(NHC)2] (2) by addition of sodium chloride. In contrast, the same reaction in dmso afforded mono(carbenes) of general formula Na[AgCl(NHC)] (3). The solvent-dependence of the reaction product can be rationalized on the basis of the equilibrium [AgCl2]− ↔ AgCl + Cl–. The precipitation of silver chloride is more favored in protic solvents than in aprotic solvents such as dmso, thus explaining the formation of bis(carbenes) in water. The formation of silver chloride may also promote the hydrolysis of silver NHC complexes under some conditions. The water-soluble platinum(II) complexes Na[PtCl2(dmso)(NHC)] were synthesized by using either mono(carbene) silver complexes 3 as carbene-transfer agents or by direct metalation of the imidazolium salt with cis-[PtCl2(dmso)2] in the presence of NaHCO3 as base. The (NHC)Pt(II) complexes were tested as catalysts for the hydration of alkynes in the aqueous phase and found to be active in neat water without the need for acidic cocatalysts.