Palladium(II) Chloride Complexes of N,N'-Disubstituted Imidazole-2-thiones: Syntheses, Structures, and Catalytic Performances in Suzuki-Miyaura and Sonogashira Coupling Reactions.

Reactions of PdCl with 2 equiv of N,N'-disubstituted-imidazole-2-thiones RRCNS (R = R = Me (1a), Pr (1b), Cy (1c), CMeH (1d); R = Me, R = Ph (1e)) under the different conditions afford five mononuclear complexes trans-[(RRCNS)PdCl] (R = R = Me (2a), Pr (2b), Cy (2c), CMeH (2d); R = Me, R = Ph (2e)) and five binuclear Pd(II) complexes [(PdCl){μ-(RRCNS)}] (R = R = Me (3a), Pr (3b), Cy (3c), CMeH (3d); R = Me, R = Ph (3e)), respectively. Complexes 2a-2e are easily converted into the corresponding 3a-3e by adding equimolar PdCl in refluxing MeOH, while the reverse reaction is achieved at room temperature by addition of 2 equiv of 1a-1e. In 2b, 2d, and 2e, each Pd(II) holds a distorted square planar geometry completed by two trans Cl atoms and two trans S atoms. Complexes 3a-3e have a dimeric [PdS] structure in which two {PdCl} units are interlinked by two N,N'-disubstituted-imidazole-2-thiones. Each Pd(II) adopts a distorted square planar geometry accomplished by two cis Cl atoms and two cis bridging S atoms. Among them, complex 3d has the two largest CMeH groups on the 2 and 5 positions of imidazole-2-thione, the longest Pd-μ-S bond, the largest S-Pd-S angle, and displays the highest catalytic activity toward Suzuki-Miyaura and copper-free Sonogashira cross-coupling reactions, which are confirmed by density functional theory calculations. The results provide an interesting insight into the introduction of various substituent groups into the periphery ligands of coordination complex-based catalysts, which could tune their geometric structures to acquire the best catalytic activity toward organic reactions.