Synthesis and characterization of dihydroxo-bridged dicopper(II) complex and its catalytic properties for catechol oxidation

Abstract

Three dihydroxo-bridged dicopper(II) complex catalysts have been synthesized, namely [Cu 2(μ-OH) 2L 2](ClO 4) 2 [L=phen ( 1a), NO 2-phen ( 1b) and bipy ( 1c)]. Diphenic acid dianion (DPHA) was found to have the ability to form mononuclear complex of [CuDPHA] ( 2a), dinuclear complex of [Cu 2DPHA(phen) 2](ClO 4) 2.2H 2O ( 2b) and dihydroxo-bridged dicopper(II) complex of [Cu(μ-OH) 2DPHA(H 2O) 2] ( 2c). Based on IR, elemental analysis and electronic spectra, complexes 1a, 1b and 1c were supposed to have dihydroxo-bridged dinuclear planar structure and consist of two copper(II) ions in planar square environment, 2b was supposed to have extended DPHA-bridged structure with two copper(II) ions in saturated five-fold coordination environment, whereas 2c was found to have a heterocyclic structure with both DPHA and dihydroxo as bridging ligands. The temperature dependence of magnetic susceptibilities of 2b and 2c has been studied, giving the exchange integral of J=−1.5 cm −1 and J=−120 cm −1, respectively. This result is in agreement with proposed dihydroxo-bridged structure of 2c, since complex 2b was weakly antiferromagnetically coupled by DPHA ligand and Cu 2O 2 unit normally undergoes strong antiferromagnetic interaction as a rule. In methanol solution, catalytic properties of these complexes were investigated for the oxidation of catechol, revealing that 2c is an effective catalyst for the oxidation of catechol to benzoquinone. Complexes 1a, 1b and 1c are also capable of catalyzing this reaction, but at a low reaction rate.