RESEARCH ARTICLE
Dinuclear Copper(II) 3,4,5-Tri--benzylgallate

Ellagitannins have attracted much attention because of the biological and pharmacological activities. In the total synthesis of ellagitannins, 3,4,5-tri-O-benzylgallic acid has been a key compound to introduce the protected galloyl group. From the perspective of coordination chemistry, 3,4,5-tri-O-benzylgallic acid is an interesting carboxylate ligand which might be capable of dinuclear carboxylate complex. Such a dinuclear carboxylate complex might be interesting as a new example of copper acetate type complexes.

The objective of the work is to synthesize a copper acetate type complex by using 3,4,5-tri-O-benzylgallic acid and to see a new feature of copper acetate compounds, by elucidating the crystal structure, magnetic property, and adsorption property for N₂ gas.

Copper(II) 3,4,5-tri-O-benzylgallate has been synthesized by a reaction of 3,4,5-tri-O-benzylgallic acid and copper(II) nitrate at pH=9 condition. The isolated complex was characterized using single-crystal X-ray structure analysis, XRD analysis, UV-visible spectroscopy, IR spectroscopy, and temperature dependence of magnetic susceptibility.

The crystal structure shows a crystallographically centrosymmetric dinuclear molecule with four carboxylato-bridges and axial dimethylformamide molecules and crystal dmf molecules [Cu···Cu 2.6345(12) Å]. In the crystal, 1D supramolecular assembly by π-π interaction between the benzyl aromatic rings of dinuclear molecules was observed. Temperature dependence of magnetic susceptibilities showed a considerable antiferromagnetic interaction between the two copper(II) ions (2J = –214 cm^–1). Type-II gas-adsorption property was observed for N₂.

A key-compound for the total synthesis of ellagitannins, 3,4,5-tri-O-benzylgallic acid (Htbng), was shown to be a new ligand for the synthesis of dinuclear copper acetate analogue with a lantern-type core, extending the realm of copper acetate clusters.