Carboxylic-supported copper complexes as catalyst for the green oxidative coupling of 2,6-dimethylphenol: Synthesis, characterization and structure

Three new Cu(II) complexes with carboxylic ligand, namely {[Cu(qc)2(py)]·4H2O}∞ (1), [Cu(qc)2(4,4′-bpy)]∞ (2) and [Cu(pc)(2,2′-bpy)(H2O)]2·H2O (3) (Hqc = 3-hydroxy-2-quinoxalinecarboxylic acid, H2pc = 4-hydroxyphthalic acid, py = pyrazine) have been synthesized and characterized. In both 1 and 2, each Cu(II) ion is coordinated by two quinoxalinecarboxylate moieties in the equatorial plane and two 4,4′-bpy or pyrazine units provide coordination in the axial positions, thus, resulting in a 1-D polymeric chain structure. Complex 3 has a dimeric structure in which two Cu(II) cations are bridged by two deprotonated pc2− ligands and two 2,2′-bpy molecules. As heterogeneous catalysts, the title complexes showed high catalytic efficiency in the green oxidative polymerization of 2,6-dimethylphenol (DMP) to poly(1,4-phenylene ether) (PPE) in the presence of H2O2 as oxidant in water under mild conditions. Moreover, they allow reuse without significant loss of activity through four runs, which suggests that these catalysts are efficient, mild, and easily recyclable for the oxidative coupling of DMP. The preliminary study of the catalytic–structural correlations suggests that the coordination environment of the metal center plays an important role in the improvement of their catalytic efficiencies.