Design and Syntheses of Palladium Complexes of NNN/CNN Pincer Ligands: Catalyst for Cross Dehydrogenative Coupling Reaction of Heteroarenes

Abstract

This report describes simple syntheses of a new class of palladium(II) pincer complexes having NNN and CNN coordination modes. The new complexes were fully characterized with the help of 1H and 13C{1H} NMR, HRMS, and IR spectroscopy. The structure and bonding modes of complexes were further authenticated with the help of single-crystal X-ray diffraction. Thermally robust and moisture-/air-insensitive palladium pincer complexes C1–C4 have been used as catalysts for cross dehydrogenative coupling (CDC) reactions of two heteroarenes and found to be very efficient. The catalyst was successful in activating the C–H bond of a variety of heteroarenes, including benzimidazole, imidazole, benzothiazole, imidazopyridine, thiophene, and furan, with only 1.0 mol % of catalyst, achieving excellent yields of the coupled products. The catalyst showed excellent tolerance toward functional groups such as CHO, COMe, COOMe, COOEt, CONHPh, Me, CN, Br, and Cl. Further, the catalyst is reusable up to four reaction cycles with only a minor loss in its efficiency. The mechanism of the CDC reaction was investigated through control experiments, and it has been suggested that a palladium acetate analogue of the NNN pincer ligand (C5) is the active catalyst.