Unsymmetrical NNE (E = S, Se) Pincer Palladium(II) Complexes: Syntheses, Structure, and Catalytic Activity in Decarboxylative Heteroarylation of Coumarin-3-carboxylic Acids

This report describes the syntheses of two novel benzoxazolyl-derived unsymmetrical NNE (E = S, Se) pincer ligands and their palladium(II) complexes. Treatment of benzoxazolyl-derived pincer ligands L1 and L2 with PdCl2(CH3CN)2 provided complexes C1 and C2, respectively, in which the NNE ligands coordinated to Pd in a tridentate pincer manner. The ligands and complexes were characterized by 1H and 13C Nuclear Magnetic Resonance (NMR), Fourier-transform infrared spectroscopy (FTIR), high-resolution mass spectrometry (HRMS), UV–visible, X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry techniques. The structure of the ligand and its coordination mode with the palladium center was established using a single-crystal X-ray diffraction technique. The complex C2 showed a pincer coordination mode with a distorted square planar geometry. The catalytic performance of palladium complexes C1 and C2 was investigated for the decarboxylative heteroarylation of coumarin-3-carboxylic acid. Notably, heteroarenes could give up to 80% of heteroarylation products in the presence of 5 mol % of the catalyst. A broad substrate of heteroarenes showed tolerance toward the developed protocol. Among complexes, the selenium ligand coordinated complex (C2) outperformed the sulfur ligand coordinated complex C1, which may be due to stronger σ-donation by the Se atom.