Non-symmetrically substituted N-heterocyclic carbene–Ag(I) complexes of benzimidazol-2-ylidenes: Synthesis, crystal structures, anticancer activity and transmetallation studies

Non-symmetrically substituted benzimidazolium salts (1 and 2) having bromide counterion undergo metallation with Ag2O at stiochiometric ratio, giving mononuclear bis-carbene Ag(I) complexes (3 and 4), active as anticancer agents against Human Colorectal (HCT 116) cell lines. Both the complexes provide good activity (IC50 = 13.9 μM for 3 and 14.6 μM for 4) in the anticancer studies and so distinctly better than the ligand precursors (IC50 = 119.3 μM for 1, and 70.0 μM for 2). The molecular structure of Ag complexes 3 and 4 is elucidated by X-ray diffraction studies. The Pd(II)– and Au(I)–NHC complexes (5 and 6) were synthesized from 4 via the technique of transmetallation. However, attempts to produce transmetallated complexes using 3 were unsuccessful.