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 PdCl2 with 2 equiv of N,N'disubstituted-imidazole-2-thiones (RRC3N2S)-R-1-C-2 (R-1 = R-2 = Me (1a), Pr-i (1b), Cy (1c), C6Me3H2 (1d); R-1 = Me, R-2 = Ph (1e)) under the different conditions afford five mononuclear complexes trans-[((RRC3N2S)-R-1-C-2)(2)PdCl2] (R-1 = R-2 = Me (2a), Pr-i (2b), Cy (2c), C6Me3H2 (2d); R-1= Me, R-2 = Ph (2e)) and five binuclear Pd(II) complexes [(PdCl2){mu-((RRC3N2S)-R-1-C-2)}](2) (R-1 = R-2 = Me (3a), Pr-i (3b), Cy (3c), C6Me3H2 (3d); R-1 = Me, R-2 = Ph (3e)), respectively. Complexes 2a-2e are easily converted into the corresponding 3a-3e by adding equimolar PdCl2 in refluxing MeOH, while the reverse reaction is achieved at room temperature by addition of 2 equiv of la le. 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 [Pd2S2] structure in which two {PdCl2} 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 C6Me3H2 groups on the 2 and 5 positions of imidazole-2-thione, the longest Pd-mu-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.