Synthesis, characterization, and biological properties of rhenium(I) tricarbonyl complexes bearing nitrogen-donor ligands

Abstract

Rhenium(I) tricarbonyl complexes have properties that make them valuable for various biomedical applications, such as imaging, cancer treatment, and bactericidal uses. The ability to modify the ligand coordination sphere of these complexes enables researchers to fine-tune and optimize their properties for biological use. In this study, we explored the role of axial nitrogen-donor ligands. Specifically, the compounds fac-[Re(CO)3(phen)(L)]+, where phen = 1,10-phenanthroline and L = pyridine (Re-py), piperidine (Re-pip), morpholine (Re-morph), and thiomorpholine (Re-thio), were synthesized and characterized. X-ray crystal structures of these complexes show that they obtain an expected pseudo-octahedral geometry with the three CO ligands arranged in a facial manner. Additionally, the X-ray crystal structure of a byproduct from these reactions, the hydroxo-bridged dinuclear Re compound [(CO)3(phen)Re(μ-OH)Re(phen)(CO)3]+, is described. The photophysical properties of these complexes were investigated in detail, revealing that they are photoluminescent in air-equilibrated pH 7.4 phosphate-buffered saline with quantum yields ranging from 1.7 to 3.1%. Both the quantum yields and emission energies were found to correlate with the basicity of the axial nitrogen donor, whereby more basic ligands give rise to smaller quantum yields and lower-energy emissions. These four compounds were further evaluated for their potential as fluorescence microscopy imaging agents. Of the four compounds, only Re-py showed detectable intracellular luminescence in HeLa cells. Lastly, the cytotoxicities of these compounds in HeLa cells were determined. None of the four compounds is significantly cytotoxic as reflected by their 50% growth inhibitory concentrations that exceed 30 μM.