Synthesis, characterization, and cyclometalation studies of benzo[1,2-h: 5,4-h′]diquinolines with palladium and platinum

Benzo[1,2-h: 5,4-h′]diquinoline(1a) represents a new family of tridentate NCN pincer ligand. We report the synthesis of the parent ligand (1a) and its derivatives (1b R = Me, 1c R = t-Butyl, 1d R = Phenyl). The ligands were characterized by 1H and 13C NMR, as well as mass spectral analysis, and X-ray structural determination. They readily undergo cyclometalation with LiPdCl4, Pd(OAc)2, and K2PtCl4 to form the cyclometalated Pd(NCN)Cl (2a–c, 3a), and Pt(NCN)Cl (4a) pincer complexes. These complexes have been characterized through NMR, and mass spectrometry. PdNCNCl (2a) structure was determined by single crystal X-ray diffraction. Complex 2a has shown to catalyze the Heck coupling reaction between bromobenzene and n-butylacyrlate in NMP at 140 °C, TON of 2506 were observed.

Graphical abstractA new series of tridentate NCN pincer ligands and cyclometalated Pd and Pt pincer complexes have been synthesized. The Pd(NCN)Cl complex shows catalytic activity for Heck reaction between bromobenzene and butylacrylate, and the Pt(NCN)Cl complex shows some activity for methane oxidation.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Benzo[1,2-h: 5,4-h′]diquinoline, a new family of NCN pincer ligands. ► Forms cyclometalated Pd and Pt(NCN) complexes. ► Pd(NCN)Cl complex shows catalytic activity for Heck reaction. ► Pt(NCN)Cl complex shows some activity for methane oxidation.