A series of Mn(i) photo-activated carbon monoxide-releasing molecules with benzimidazole coligands: synthesis, structural characterization, CO releasing properties and biological activity evaluation

Abstract

Five Mn(I) photo-activated carbon monoxide-releasing molecules (photo-CORMs) with benzimidazole coligands, namely [MnBr(CO)₃L1] (1, L1 = 2-(2-pyridyl)benzimidazole), [Mn(CO)₂L1(PPh₃)₂](ClO₄) (2), [MnBr(CO)₃L2] (3, L2 = 2,2′-bisbenzimidazole), [MnBr(CO)₃L3]·CH₃OH (4, L3 = 2,6-bis(benzimidazole-2′-yl)pyridine) and fac-[MnBr(CO)₃L4] (5, L4 = 2,4-bis(benzimidazole-2′-yl) pyridine) were synthesized by reactions of MnBr(CO)₅ with complexes L1–L4, respectively, and characterized via single crystal X-ray diffraction, elemental analysis, ¹H-NMR, ¹³C-NMR, IR, UV-vis and fluorescence spectroscopy. The CO-release properties of 1–5 were investigated using the myoglobin assay and CO detection, and the results show that all of the complexes could release CO rapidly upon exposure to 365 nm UV light. Comparing their half-lives of CO release, we found that increasing the degree of unsaturation and conjugation of the ligand frame could be advantageous for prolonging the time of CO-release, and that the luminescence intensity of 1–5 could gradually be enhanced. The cellular fluorescence imaging tests demonstrate that these Mn(I) photo-CORMs can be taken up by human liver cells (HL-7702) and liver cancer cells (SK-Hep1), and exhibit good capabilities for bioimaging. A cell viability assay for SK-Hep1 shows that the anticancer activity of 3 is better than that of other complexes.