Synthesis, Characterization, and Catalytic Activities of Palladium Complexes with Phenylene-Bridged Bis(thione) Ligands

The neutral phenylene-bridged bis(thione) compounds, 1,3-bis(3'-ethylimidazolyl-2'-thione)benzene (Betb), 1,3-bis(3'-butylimidazolyl-2'-thione)benzene (Bbtb), and 1,3-bis(3'-allylimidazolyl-2'-thione)benzene (Batb), have been synthesized and characterized. Reactions of palladium precursor PdCl2(CH3CN)(2) with phenylene-bridged bis(thione) ligands in 1:2 ratio resulted in the formation of the complexes: PdCl2(L)(2) (L = Betb, 3a; L = Bbtb, 3b; L = Batb, 3c, respectively). In contrast, treatment of the ligands with PdCl2(CH3CN)(2) in 1:1 ratio gave cyclometalation palladium complexes Pd2+Cl(L-) (L = Betb-H, 4a; L = Bbtb-H, 4b; L = Batb-H, 4c) through the metal-induced C-H activation. Complexes 4a-c can also be obtained by the reaction of bis(thione) ligands and PdCl2 in 1:1 ratio. The reaction of 3a-c with additional PdCl2(CH3CN)(2) also afforded complexes 4a-c. All ligands and palladium complexes were fully characterized by one-/two-dimensional NMR spectra, mass spectrometry, and infrared spectrometry. And the molecular structures of 3a-c, 4a, and 4c have been determined by the single-crystal X-ray diffraction method. Furthermore, the detailed spectroscopic properties and catalytic activities of the complexes for the reduction of nitro compounds were discussed in terms of the modification of the coordination ligands to the center metal.