Delineating the Role of Substituents on the Coordination Behavior of Aroylhydrazone Ligands in Pd-II Complexes and their Influence on Suzuki-Miyaura Coupling in Aqueous Media

摘要

Reactions of piperonal aroylhydrazone ligands (HL1-HL4) with [PdCl2(PPh3)(2)] in CH3OH and CHCl3 afforded four new Pd(II) complexes (1-4) featuring diverse coordination behaviors. These complexes 1-4 were fully characterized by elemental analyses, UV/Visible, FT-IR and H-1/C-13 NMR spectra. The variant coordination modes of piperonal aroylhydrazone ligands with Pd(II) ion in these complexes were unambiguously confirmed by single-crystal XRD study. Though the ligands HL1-HL4 are of similar nature, one of them (HL4) possessing an electron withdrawing nitro group as substitution in the peripheral part of the hydrazone displayed an uncommon ligation towards the palladium ion. An experimentally observed molecular architects were in turn rationalized by density functional theory (DFT) calculations as well as natural bonding analysis (NBO). All these Pd(II) complexes were evaluated for their catalytic potential towards Suzuki-Miyaura cross-coupling reaction of sterically congested arylboronic acids with (hetero)arylhalides under an open-flask conditions in aqueous media with low catalyst loading. Advantageously, the selected catalyst was active up to five cycles without significant loss in activity.