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

Abstract

Reactions of PdCl₂ with 2 equiv of N,N'-disubstituted-imidazole-2-thiones R¹R²C₃N₂S (R¹ = R² = Me (1a), ⁱPr (1b), Cy (1c), C₆Me₃H₂ (1d); R¹ = Me, R² = Ph (1e)) under the different conditions afford five mononuclear complexes trans-[(R¹R²C₃N₂S)₂PdCl₂] (R¹ = R² = Me (2a), ⁱPr (2b), Cy (2c), C₆Me₃H₂ (2d); R¹ = Me, R² = Ph (2e)) and five binuclear Pd(II) complexes [(PdCl₂){μ-(R¹R²C₃N₂S)}]₂ (R¹ = R² = Me (3a), ⁱPr (3b), Cy (3c), C₆Me₃H₂ (3d); R¹ = Me, R² = 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 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 [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-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.