New zwitterionic mono-NHC ligands and related hydrophilic Au(I), Ag(I) and Cu(I) carbene complexes

N-heterocyclic carbenes (NHCs) are an interesting class of ligands with donor properties similar to phosphines. In particular, our research group has recently synthesized new Ag(I) carbene complexes with general formula {Ag3[HB(RIm)3]2}Br (Im = imidazole; R = Bn, Mes or tBu) which were successfully employed as carbene transfer reagents in the synthesis of related gold(I) complexes. We also reported the first discrete molecules with bis(triazolylidene)borate carbene scorpionate-type ligands, the metal(I) complexes M2[H2B(BnTz)2]2 (Tz = 1,2,4-triazole; M = Ag(I) or Au(I)). A limit of such complexes for biological studies is the poor solubility and stability in water. In order to evaluate the cytotoxic activity of metal adducts, the need to substitute organic solvents with water, solvent cheap and non-toxic, has received increasing attention; therefore, the typical strategy is to modify compounds by attaching phase tags such as esters, sulfonates and carboxylates. New N-heterocyclic carbene ligand precursors {H2C(HTzR)2} and {H2C(HImR)2} (R = PrSO3 or COOEt) were synthesized starting from the related bis(azolyl)methanes; they were employed to obtain water soluble bimetallic compounds.3 We are now developing the chemistry of new zwitterionic mono-NHC ligands, with water soluble side arms: we have synthesized the hydrophilic carbene ligand precursors {HIm4R,5R} (R = PrSO3; CH2COOEt or CH2CONEt2) and the related Ag(I) and Au(I) carbene complexes. These new compounds were evaluated for their cytotoxic activity towards a panel of several human tumour cell lines