Bidentate [C,N] and Tridentate [C,N,S] Palladium Cyclometallated Complexes as Pre-Catalysts in Cross-Coupling Reactions

Treatment of halide substituted bidentate [C,N] and tridentate [C,N,S] Schiff base ligands with palladium(II) acetate or with lithium tetrachloropalladate, as appropriate, afforded through C−H activation of the organic ligand, the dinuclear (1 a, 1 b) and mononuclear (1 c, 1 d, 1 e) palladacycles, respectively. Reaction of the μ-acetate dinuclear complexes with aqueous sodium chloride in a metathesis process, gave the μ-chloride dinuclear complexes (2 a, 2 b). Treatment of the latter with triphenylphosphine or with bis(diphenylphosphino)methane (dppm)/ammonium hexafluorophosphate in complex/ phosphine 1 : 2 ratio, gave the phosphine derivatives (3 a, 3 b, 4 a, 4 b). Reaction of 1 c and of 1 e with triphenylphosphine/silver perchlorate and dppm/silver perchlorate, respectively, gave compounds 2 c and 2 e with removal of the chloride ligand by precipitation of the silver halide. The compounds were characterized by microanalysis (CHN), IR and ¹H and ³¹P{¹H} NMR spectroscopy. The crystal structures of three complexes, 4 a, 1 c and 1 e were studied by single-crystal X-ray diffraction. Furthermore, the performance of the palladacycles were tested as pre-catalysts, namely Suzuki-Miyaura cross-coupling reaction and those furnishing the better results are also reported.