Bioactive heterobimetallic Re(I)/Au(I) complexes containing bidentate N‑heterocyclic carbenes

Abstract

The first cationic heterobimetallic complexes of the type fac-[Re(CO)3(NHC)(LAuPPh3)]+, where NHC is an imidazole pyridine-based carbene and L is 3-pyridylalkyne, 4-pyridylalkyne, or 5-ethynyl-1-methyl-1H-imidazole, have been synthesized together with their Re(I) precursors. All of them have showed similar emissive properties resulting from the presence of the NHC system within the Re(I) core. Thus, emission can be ascribed to a phosphorescent process with a mixture of a MLCT from the Re(dπ) → NHC(π*), LLCT from the imidazolyl/pyridyl to the NHC ligand, and LC (NHC derivative) transitions. In all cases, the emission maximum is blue-shifted in comparison with that observed in the typical diimine-Re(I) systems. Only the heterobimetallic species displayed antiproliferative activity against tumor lung A549 cells, which was increased after irradiation at 405 nm up to nearly 5 times for complexes 4 and 5. A necrotic process seems to be the preferred cell death mechanism. Fluorescence microscopy showed that only heterobimetallic complexes 4 and 5 were suitable for cell visualization. Their biodistribution pattern reveals accumulation within the cytoplasm close to the nucleus and some nucleus permeation. Overall it can be suggested that, whereas the emissive properties are dominated by the NHC-Re(I) fragment, the anticancer activity is mainly dependent on the Au(I) counterpart.