Iridium(III) complexes based on cyanomethane and cyanamide ligands with luminescence quenching properties for Fe(III) sensing and biological activities

Abstract

Two organometallic iridium(III) complexes, Ir(ppy)2Cl(NCCH3) (1) and Ir(ppy)2Cl(NCNH2) (2), were accidentally synthesized from the reaction between the chloro-bridged dimer of (ppy)2Ir(μ-Cl)2Ir(ppy)2 and 2-aminobenzothiazole (abt, C7H6N2S) ligand refluxed in dichloromethane under Ar gas. In the obtained complexes, ppy is 2-phenylpyridine, NCCH3 is cyanomethane or acetonitrile, and NCNH2 is the cyanamide ligand. Complex 1 was received instead of the expected Ir(ppy)2Cl(abt) complex, after the reaction completed for 10 h. Acetonitrile entered as a ligand to the coordination sphere of Ir(III) during the precipitation stage. The reaction was repeated, refluxing for 20 h. This time, the abt ligand fragmented into the cyanamide molecule yielding complex 2 instead of the Ir(ppy)2Cl(abt) complex. Single-crystal X-ray analysis revealed the distorted octahedral geometries of both title complexes. The N and C donors of ppy were in their typical trans- and cis- orientations, respectively. The cyanomethane and cyanamide ligands coordinated via the N-atom. Both complexes exhibited similar photo-physical properties. Complexes 1 and 2 emitted green luminescence between 520 and 525 nm when excited with 325 and 350 nm, respectively, at room temperature in dichloromethane. The respective relative quantum efficiencies were 0.06 and 0.08, compared to coumarin 6 in ethanol. The luminescent emission of both complexes was selectively turn-offtoward increasing Fe3+ concentrations. The 1:1 binding stoichiometry between the complex and Fe3+ ion was determined by Job's plot. Benesi-Hildebrand plots were applied to determine the binding constants (Kb) of 5.86 × 104 and 9.50 × 104 M−1 for complexes 1 and 2, respectively. The complex 2 displayed in vitro cytotoxicity toward HCC1937 and MDA-MB-231 human breast cancer cells with IC50 values of 13.5 and 72.8 µM, respectively. These values indicated 2–3 folds higher sensitivity than cisplatin. In addition, complex 1 showed inhibitory activity against the Gram-positive bacteria Staphylococcus aureus (SA) and methicillin-resistant Staphylococcus aureus (MRSA), with MIC/MBC values of 32/32 and 32/64 μg. mL−1, respectively.