Synthesis and characterization of novel rhenium(I) complexes towards potential biological imaging applications

Re(I) tricarbonyl complexes exhibit immense potential as fluorescence imaging agents. However, only a handful of rhenium complexes have been utilized in biological imaging. The present study describes the synthesis of four novel rhenium complexes, their characterization and preliminary biological studies to assess their potential as biological imaging agents.

Four facial rhenium tricarbonyl complexes containing a pyridyl triazine core, (L1 = 5,5'(3-(2-pyridyl)-1,2,4-triazine-5,6-diyl)-bis-2-furansulfonic acid disodium salt and L2 = (3-(2- pyridyl)-5,6-diphenyl-1,2,4-triazine-4',4''-disulfonic acid sodium salt) have been synthesized by utililzing two different Re metal precursors, Re(CO)₅Br and [Re(CO)₃(H₂O)₃]OTf in an organic solvent mixture and water, respectively. The rhenium complexes [Re(CO)₃(H₂O)L1]⁺ (1), Re(CO)₃L1Br (2), [Re(CO)₃(H₂O)L2]⁺ (3), and Re(CO)₃L2Br (4), were obtained in 70-85% yield and characterized by ¹H NMR, IR, UV, and luminescence spectroscopy. In both H₂O and acetonitrile, complexes display a weak absorption band in the visible region which can be assigned to a metal to ligand charge transfer excitation and fluorescent emission lying in the 650-710 nm range. Cytotoxicity assays of complexes 1, 3, and 4 were carried out for rat peritoneal cells. Both plant cells (Allium cepa bulb cells) and rat peritoneal cells were stained using the maximum non-toxic concentration levels of the compounds, 20.00 mg ml^-1 for 1 and 3 and 5.00 mg ml^-1 for 4 to observe under the epifluorescence microscope. In both cell lines, compound concentrated specifically in the nuclei region. Hence, nuclei showed red fluorescence upon excitation at 550 nm.

Four novel rhenium complexes have been synthesized and characterized. Remarkable enhancement of fluorescence upon binding with cells and visible range excitability demonstrates the possibility of using the new complexes in biological applications.Graphical abstractMicrograph of rat peritoneal cells incubated with novel rhenium complex under epifluorescence microscope.