Synthesis and Structural Characterization of a Glycoconjugated Re(CO)₃⁺ Complex and Its Tendency for CO Release

Rhenium(I) tricarbonyl complexes have been extensively studied for their potential in biomedical applications, e.g., photoactivated chemotherapy, while glycoconjugated metal complexes gain increasing attention for development of therapeutics and imaging agents due to their water solubility and biocompatibility. To combine these favorable properties, we synthesized a water-soluble compound, fac-[Re(CO)3(GluP)Cl], where the GluP is a diimine (N,N') d-glucosamine Schiff base. Prior to exploring potential anticancer activities of this complex, we investigated its structure and stability in solutions using electro-spray ionization mass spectrometry (ESI-MS), NMR and other spectroscopic techniques. We found that the complex is relatively stable in water, keeping the GluP ligand intact and coordinated to the Re(I) ion, while the Re-Cl bond gradually hydrolyzes. The complex has two diastereomers due to the chirality of the Re(I) center. In each diastereomer the sugar C1-OH can exist as β-(major) or α-(minor) anomer. An ESI-MS study detected the formation of the dicarbonyl ion [Re(CO)2(GluP)]+ in protic solvents, with the associated peak gaining intensity over time. This phenomenon was attributed to the gradual loss of a CO ligand under ambient conditions as confirmed by gas chromatography. This is the first report on a solvent-assisted CO release from a Re(CO)3+ species without using ultraviolet (UV)-light.