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] (2Ag+), 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.