Unsymmetrical Pd(II) Pincer Complexes with Benzothiazole and Thiocarbamate Flanking Units: Expedient Solvent-Free Synthesis and Anticancer Potential.

Driven by the growing threat of cancer, many research efforts are directed at developing new chemotherapeutic agents, where the central role is played by transition metal complexes. The proper ligand design serves as a key factor to unlock the anticancer potential of a particular metal center. Following a recent trend, we have prepared unsymmetrical pincer ligands that combine benzothiazole and thiocarbamate donor groups. These compounds are shown to readily undergo direct cyclopalladation, affording the target S,C,N-type Pd(II) pincer complexes both in solution and in the absence of a solvent. The solid-phase strategy provided the complexes in an efficient and ecologically friendly manner. The resulting palladacycles are fully characterized using nuclear magnetic resonance (NMR) and infrared (IR) spectroscopy and, in one case, by single-crystal X-ray diffraction (XRD). The solvent-free reactions are additionally analyzed by powder XRD. The pincer complexes exhibit remarkable cytotoxicity against several solid and blood cancer cell lines, including human colorectal carcinoma (HCT116), breast cancer (MCF7), prostate adenocarcinoma (PC3), chronic myelogenous leukemia (K562), multiple plasmacytoma (AMO1), and acute lymphoblastic leukemia (H9), with the dimethylamino-substituted derivative being particularly effective. The latter also induced an appreciable level of apoptosis in both parental and doxorubicin-resistant cells K562 and K562/iS9, vindicating the high anticancer potential of this type of palladacycles.