Pd(II)-Schiff base complexes containing hydroxyl group for Suzuki-Miyaura and Mizoroki-Heck cross-coupling reactions in aqueous system

Abstract

L1-Pd(II) and L2-Pd(II) complexes, where L1: (E)-methyl 6-benzyl-2-(2-hydroxybenzylideneamino)-4,5,6,7-tetrahydrothieno[2,3-c]pyridine-3-carboxylate, L2: (E)-methyl 6-benzyl-2-(2-hydroxy-5-methylbenzylideneamino)4,5,6,7-tetrahydrothieno[2,3-c]pyridine-3-carboxylate, have demonstrated high catalytic activity in SuzukiMiyaura (SM) and Mizoroki-Heck C-C cross-coupling reactions. Notably, aryl bromides bearing both electron-donating and electron-withdrawing substituents were effectively employed under the optimized conditions. The reaction parameters, including substrate-to-base ratio, temperature, solvent, catalyst loading, and the steric and electronic effects of phenylboronic acid and aryl bromides, were thoroughly investigated. Optimal conditions for enhanced catalytic performance were established. The geometries of both Pd(II) complexes were elucidated using a combination of 1H and 13C NMR, FT-IR, UV-Vis, mass spectrometry, thermogravimetric analysis (TGA), and microanalysis. Among the two, the L1-Pd(II) complex exhibited superior performance, particularly in SM cross-coupling reactions. Under optimized conditions, this catalyst enabled the coupling of aryl bromides using K2CO3 as the base, with a low catalyst loading of 0.01 mol%, in an EtOH/H2O (1:3) solvent mixture at 80 degrees C over 2 h, achieving up to 100 % yield.