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.