Dinuclear Copper ( II ) 3 , 4 , 5-TriO-benzylgallate

Department of Applied Chemistry for Environment and Research Center for Coordination Molecule-based Devices, School of Science and Technology, Kwansei Gakuin University, 2-1 Gakuen, Sanda 669-1337, Japan Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, 2-1 Gakuen, Sanda 669-1337, Japan Department of Chemistry, Graduate School of Science, Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima 739-8526, Japan Natural Science Center for Basic Research and Development, Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima 739-8526, Japan Department of Chemistry, Interdisciplinary Graduate School of Science and Engineering, Shimane University, 1060 Nishikawatsu, Matsue 690-8504, Japan


INTRODUCTION
Ellagitannins are a class of tannins containing galloyl and hexahydroxydiphenoyl groups, in which more than 1000 analogues have been found in natural systems [1].These tannins have attracted much attention for a long period, because they have interesting biological and pharmacological activities.So far, total synthesis has been attempted for more than 20 ellagitannins.The benzyl group was often used as an effective protecting group of the phenol groups in the total syntheses [2,3], where 3,4,5-tri-O-benzylgallic acid (Fig. 1) is a key compound to introduce the protected galloyl group.From the perspective of coordination chemistry, 3,4,5-tri-Obenzylgallic 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.Copper acetate is one of the oldest metal complexes with a unique Cu 2 cluster and has attracted much attention since the discovery of the lantern-like acetatobridged cluster and antiferromagnetic spin-coupling [4 -13].
Recently, a unique di-µ-acetato-bridged dinuclear cluster, [Cu 2 (CH 3 CO 2 ) 4 (EtimH) 2 ] (EtimH = 2-ethylimidazole), was reported by Hernandez et al. [14].They aimed to attempt to form porous copper acetate-2-ethylimdazole.We have been engaged in the synthesis of copper acetate analogues and their metal-assembled complexes [13, 15 -31].During the course of our studies, we found that copper(II) benzoate forms a chain compound with pyrazine and the assembled chain complex has a porous structure and an adsorption property for N 2 , where the aromatic benzoate groups form a hydrophobic micropore [19,30].This is another interesting feature of copper acetate analogues.In this study, we synthesized a new copper acetate analogue by the reaction of 3,4,5-tri-O-benzylgallic acid (abbreviated as Htbng) with a copper(II) salt.The bulky aromatic groups of tbng -ligand may be expected to form enough space for gas adsorption property in the copper carboxylate.

MATERIALS AND METHODS
All the chemicals except for the Htbng ligand were commercial products and were used as supplied.Elemental analyses for C, H, and N were performed using a Thermo-Finnigan FLASH EA1112 series CHNO-S analyzer.Infrared spectra were measured with a JASCO MFT-2000 FT-IR Spectrophotometer in the 4000-600 cm -1 region.Diffused reflectance spectrum was measured with a Shimadzu UV-vis-NIR Recording Spectrophotometer Model UV-3100 in the 200-1500 nm region.Magnetic susceptibilities were measured with a Quantum Design MPMS-XL7 SQUID susceptometer over a temperature range of 4.5-300 K. Powder X-ray Diffraction (PXRD) data were collected in the θ range of 5-40°on a RIGAKU RINT2000/PC diffractometer with Cu Kα radiation (λ = 1.5418Å).Adsorption measurement for N 2 was performed by a MicrotracBEL BELSORP-mini II.Prior to the adsorption, the sample was evacuated at 298 K for 2h.

Synthetic Procedures
The Htbng ligand was synthesized according to the previously reported method [32].Copper(II) carboxylate was synthesized by the following method.[Cu 2 (tbng) 4 (dmf) 2 ]•H 2 O: A 0.198 g (0.45 mmol) portion of Htbng and 10 cm 3 of methanol were added to a 6 cm 3 of 0.10 M sodium hydroxide solution.The mixed solution was adjusted to pH = 9 by adding a small amount of nitric acid.To this solution, a solution of copper(II) nitrate trihydrate (0.0618 g, 0.25 mmol) in water (3 cm 3 ) was added with stirring to give a yellowish-green precipitate.The precipitate was collected and dried under

Crystallography
Single-crystals suitable for X-ray analysis were obtained due to very few amount of small crystals by slow diffusion of a dmf solution of [Cu 2 (tbng) 4 (dmf) 2 ]•H 2 O. Single-crystal diffraction data were measured on a Bruker Smart APEX-II ULTRA diffractometer equipped with a multilayered confocal mirror monochrometer and a Mo-Kα radiation source (λ = 0.71073 Å).Crystal data and details concerning data collection are given in Table 1.The structure was solved by direct methods, and refined by full-matrix least-squares methods.The hydrogen atoms were inserted at their calculated positions and fixed there.All of the calculations were carried out utilizing the SHELXTL software package [33].Crystallographic data have been deposited with Cambridge Crystallographic Data Centre: Deposit number CCDC-1815964.Copies of the data can be obtained free of charge viahttp://www.ccdc.cam.ac.uk/conts/ retrieving.html(or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge, CB2 1EZ, UK; Fax: +44 1223 336033; e-mail: deposit@ccdc.cam.ac.uk).

RESULTS AND DISCUSSIONS
Elemental analysis data of the isolated compound are in accordance with the formulation [Cu 2 (tbng) 3).The asymmetric unit contains two crystal dmf molecules and one-half of dinuclear [Cu 2 (tbng) 4 (dmf) 2 ] unit with a crystallographic inversion center at the midpoint of the Cu1 and Cu1 i atoms.The dinuclear unit has a lantern-like dinuclear core bridged by four tbng -ligands in a syn-syn fashion [13].The Cu1•••Cu1 i distance is 2.6345(12) Å, which is in the range found in dinuclear copper(II) carboxylates [6 -13].The coordination geometry around each copper atom is an elongated square-pyramid.The bond distances of the Cu1 and basal O atoms are 1.957(3)-1.977(3)Å, which are within the normal range found in copper(II) carboxylates [6 -13].The fifth position of the Cu1 atom is occupied by a DMF molecule with the Cu1-O11 distance of 2.166(3) Å, which is also in the normal range as axial bonding for the copper(II) carboxylates [6 -13].In the crystal, DMF molecules are trapped into the small space between the dinuclear units (Fig. 4).Each dinuclear molecule is associated with π•••π interaction of one of the three benzyl aromatic rings to form one-dimensional chain supramolecular network as shown in Fig. (5).
The magnetic data of [Cu 2 (tbng) 4 (dmf) 2 ]•H 2 O are shown in the form of χ A T vs. T plot in Fig. (6).The χ A T value is 0.341 cm 3 mol -1 K (per Cu II unit) at 300 K.This corresponds to the magnetic moment of 1.65 µ B , which is lower than the spin-only value (1.73 µ B ) as Cu II (d 9 , S = 1/2) ion.The χ A T decreases with lowering of the temperature and reaches value of 0.0046 cm 3 mol -1 K at 4.5 K, showing an antiferromagnetic interaction between the two copper(II) ions.The magnetic data were analyzed by the Bleaney-Bowers equation based on the Heisenberg model (H = -2JS 1 •S 2 (S 1 = S 2 = 1/2)) [5]:  Where J is an exchange coupling constant for the two copper(II) ions, p is the fraction of mononuclear impurity, and Nα is the temperature-independent paramagnetism, which was set to 60 x 10 -6 cm 3 mol -1 for each copper(II) ion [8,25,30].
The best-fitting parameters were 2J = -214 cm -1 , g = 2.13, p = 0.07), showing an antiferromagnetic coupling between the two copper(II) ions.The -2J value is a little smaller than those of copper(II) acetate (~ 284 cm -1 ) [6] and most of copper(II) 200 400 600 800 1000 1200 1400 Absorbance / arbitrary unit Wavelength / nm benzoates (-2J = 316-350 cm -1 ) [10], showing a relatively weak antiferromagnetic coupling.It is known that the larger bending angle of the OCO moiety of the benzoate group relative to the Cu-O•••O-Cu plane of the carboxylate-bridge, ϕ bend , plays an important role to weaken the antiferromagnetic coupling via the benzoate bridges [10].In the case of the most related copper(II) benzoate with 3,4,5-trimethoxybenzote,  [Cu 2 3,4,5-[Cu 2 {(CH 3 O) 3 C 6 H 2 CO 2 } 4 (CH 3 OH) 2 ]•2dmf, the ϕ bend values are 2.8 and 2.9°, and shows a stronger antiferromagnetic interaction between the two copper(II) ions (2J = -292 cm -1 ) [31].The crystal structure of the present complex revealed that the benzoate group has a considerable bending with ϕ bend = 9.4 and 5.9° possibly because of the packing effect of the dinuclear clusters with bulky galloyl groups, resulting in a weaker antiferromagnetic coupling between the two copper(II) ions.The crystal structure of [Cu 2 (tbng) 4 (dmf) 2 ]•4dmf did not show large voids, although it has many small cavities.The PXRD data of [Cu 2 (tbng) 4 (dmf) 2 ]•H 2 O did not coincide with the simulated data from the crystal structure of [Cu 2 (tbng) 4 (dmf) 2 ]•4dmf, suggesting that the packing of the dinuclear molecules was influenced by the incorporation of the solvent.Therefore, we expected an adsorbing property for the present complex, if the bulky galloyl groups work well for porous structure.However, the adsorption isotherm on [Cu 2 (tbng)

CONCLUSION
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.