Synthesis of two copper clusters and their catalysis towards the oxidation of benzene into phenol

Abstract

Reaction of a heptadentate ligand (H₂L^I) and a hexadentate ligand (H₂L^II) with three equivalents of Cu(ClO₄)₂·6H₂O in methanol under basic conditions afforded a hexanuclear cluster [Cu₆(L^I)₂(OH)₄](ClO₄)₄·2DMF·3Et₂O (1) and a nonanuclear cluster [(Cu₉(L^II)₃(OH)₇)](ClO₄)₅·0.25CH₃OH·1.15H₂O (2), respectively, where H₂L^I is 2,2′-(((pyridine-2,6-diylbis(methylene))bis((pyridin-2-ylmethyl)azanediyl))bis(methylene))diphenol and H₂L^II 2,2′-((((5-methyl-1,3-phenylene)bis(methylene))bis((pyridin-2-ylmethyl)azane-diyl))bis(methylene))bis(4-methylphenol). The structures of both clusters in their solid states have been determined by X-ray crystallography. Their magnetic susceptibilities reveal that both clusters are antiferromagnetic due to the coupling between the copper centers in the clusters. NMR spectroscopic analysis, conductivity measurements and ESI-MS analysis suggest that the clusters retain their structural integrities in solution. Both clusters show catalytic activity towards the hydroxylation of benzene into phenol with hydrogen peroxide (H₂O₂) as an oxidant at 80 °C in aqueous acetonitrile. The conversion rate is about 20% and their TON/TOF are 564/188 and 905/302 for clusters 1 and 2, respectively.