Syntheses and structural characteristics of copper(II)-organic polymers based on N-heterocyclic ligands: A study on the importance of steric factors in the design of potent catalysts

Two new Cu(II)-organic polymers supported by different N-heterocyclic ligands, namely {[Cu(btx)2]·Cl2}n1 (btx = 1,4-bis(triazol-1-ylmethyl)-benzene) and {[Cu(bbbm)(Ac)2]·(CH3OH)2}n2 (bbbm = 1,1′-(1,4-butanediyl)bis-1H-benzimidazole) are designed and synthesized in order to ascertain the influence of coordination polymeric structure on catalytic activity. Single-crystal X-ray diffraction shows that 1 assumes a 2D cationic rhombohedral grid network arrangement, whereas 2 features a 1D zigzag chain structural motif in which Cu atoms are bridged by μ2-bbbm ligands. Both of them reveal impressive catalytic capability for the selectively oxidative coupling of 2,6-di-t-butylphenol to the corresponding diphenoquinone in excellent yield using H2O2 as oxidant and KOH as co-catalyst in water and allow reuse without a significant loss of activity through three runs with the hindered phenol. Under the similar reaction conditions, the two polymers also accelerate the selectively coupling of 2,4-di-t-butylphenol to generate the relevant biphenol with moderate to high efficiencies. The transformation of 2,6-dimethylphenol mediated by polymers 1 and 2 proceeds smoothly with 100% and 94.5% conversion, respectively. It is found that polymer 1 offers the superior results in terms of reactivity and selectivity to 2 for such processes, indicating the crucial role of structural parameters in catalytic performance of polymers. Furthermore, the ability of 1 and 2 to undergo homogeneous oxidation couplings of these substituted phenols in 1.5:1 (v/v) methanol–toluene mixture is explored as well to demonstrate the advantages of heterogeneous catalysis.

Two new Cu(II)-organic polymers containing coordinatively unsaturated metal sites are designed and synthesized as competent heterogeneous catalysts for the oxidative coupling of various substituted phenols in aqueous medium. It is suggested that the combination of the physical and chemical properties of inorganic and organic components can allow the achievement of the targeted polymeric materials furnishing unusual catalytic characters. Figure optionsDownload as PowerPoint slide