Synthesis and Structural Characterization of a New Two-Dimensional Polymeric Hybrid Material {[Cu 3 ( μ 3 -1,2,3-BTC)( μ 2 -1,2,3-BTC) (en) 3 (OH 2 ) 2 (OH)( μ 2 -OH 2 )] 2 ·5H 2 O·O} n

The reaction of benzene-1,2,3-tricarboxylate copper(II) trihydrate (Cu 3 (1,2,3-BTC) 2 ·3H 2 O) and ethylenediamine (en) gave after recrystallization from methanol at room temperature violet crystalline solid of the polymeric two dimensional material {[Cu 3 (μ 3 -1,2,3-BTC)(μ 2 -1,2,3-BTC)(en) 3 (OH 2 ) 2 (OH)(μ 2 -OH 2 )] 2 2 O·O} n (1) as identified by single crystal X-ray diffraction analysis. The compound was also characterized by elemental analysis, FT-IR spectroscopy, UV-vis spectroscopy, thermogravimetric analysis (TGA) and magnetic susceptibility. The molecular structure indicates crystallization in the triclinic space group P-1 with the unit cell parameters a = 12.1205(3) Å, b = 12.7026(3) Å, c = 13.9890(3) Å, α = 75.034(1)˚, β = 74.957(1)˚, γ = 68.415(1)˚. Structural analysis also revealed that the dimeric compound is connected by Cu-O-Cu bridges and the dimer is further converted into a polymeric chain structure through bridging of one benzene-1,2,3-tricarboxylate ligand. Different coordination modes are observed at the copper atom. The Cu atom at the center is octahedral while the two other ones are pentacoordinated. Thermal analysis of the compound showed decomposition processes, while magnetic studies indicated a magnetic susceptibility μ of 3.21 BM.


Introduction
Polyfunctional hybrid organo-inorganic materials such as metal-organic frameworks (MOFs), microporous organic materials (MOMs) and coordination polymers (CPs) continue to be of great interest for the scientific community owing to their structural and unusual properties as compared with those of activated carbon, silica gel and zeolite [1]. Attractions to this class of materials result not only from the several advantages associated with their intrinsic structural characteristics, but also from their rich physico-chemical properties with potential applications in gas storage, molecular recognition and separation of substrates, and in heterogeneous catalysis [2] [3] [4]. Gas storage is of critical importance in many industrial processes both from an economic and safety point of view. For instance, metal carboxylates such as [Cu 2 (pzdc) 2 (pyz)] (where pzdc 2− = pyrazine-2,3-dicarboxylate, pyz = Pyrazine) and [Cu 3 (BTC) 2 ] (BTC 3− = benzene-1,3,5tricarboxylate), characterized by pores radius of 4 × 6 Å and 12 × 12 Å, respectively, are well suitable for acetylene and CO 2 adsorption [5] [6]. These abilities of MOFs, MOMs, and CPs to store gases such as methane, ethylene, dihydrogen, carbon dioxide or dinitrogen have led to the replacement of compressed natural gas (CNG) by adsorbed natural gas (ANG), which therefore represents an alternative source of energy in the production of fuels [7]. Continuous research works on materials able to favor gas storage by adsorption are then expected to provide a great contribution in the mitigation of environmental pollution and global warming. In that respect, metal di-or tricarboxylates in association with bidentate amines represent excellent precursors for the generation of coordination polymers that hold potential as inorganic material in the adsorption of small molecules. Some recent examples include the synthesis of 3D-{[Cu(NCCH 3 ) 4 ][Cu 3 (μ 6 -btc) 2 ] 2 (O 2 CCH 3 )(H 2 O,CH 3 CN) x (btrip) 1.5 } n from 1,2-bis(1,2,4-triazol-4-yl)propane, in combination with benzene-1,3,5-tricarboxylate [8].
We recently investigated the reaction of several diamine precursors such as 1,3-diaminepropane (tn) with Cu(NDC)·3H 2 O (NDC 2− = naphthalenedicarboxylate) and isolated the complex {[Cu(NDC)(OH 2 )(μ-OH 2 )(tn)] n ·2H 2 O} that has been characterized in the solid state by single crystal X-ray diffraction analysis. Its molecular structure revealed a 2D coordination polymer in which the monomeric units are associated by intermolecular hydrogen bonding interactions [9]. In the continuation of this work, we report here the synthesis and structural characterization of a novel CPs based on benzene-1,2,3-tricarboxylatecopper(II) trihydrate (Cu 3 (1,2,3-BTC) 2 ·3H 2 O) and ethylenediamine (en) with the former as primary building unit.
was of analytical grade. Elemental analysis was performed using a Fisons Instruments 1108 CHNS-O analyzer. FT-IR spectra were recorded using a Perkin-Elmer FT-IR 100 spectrometer. UV-visible measurements were carried out by the aid of a Jenway 6715 UV-vis spectrophotometer. The melting point of the compound was obtained using an SMP3 Stuart Scientific Melting Point Apparatus while thermogravimetric analysis was carried out using a Perkin-Elmer STA 6000 thermobalance. Magnetic susceptibility was carried out using a Gouy magnetic balance. Reflections were collected on a Bruker SMART APEX II CCD Diffractometer using CuKα (1.54178 Å) radiation at 100 K. Dataprocessing, Lorentz-polarization, and face-indexed numerical absorption corrections were performed using SAINT, APEX, and SADABS computer programs [10] [11] [12]. The structures were solved by direct methods and refined by full-matrix least squares based on F2 with all reflections using the SHELXTL V6.14 program package [13] [14]. Non-hydrogen atoms were refined with anisotropic displacement coefficients. All H atoms were found in electron-density difference maps and carbon-bound H atoms were treated as idealized contribution unless noted differently. Structural data have been deposited with the Cambridge Structure Database as supplementary publications CCDC 1990458.

Synthetic Aspects
Complex 1 was obtained by mixing benzene-1,2,3-tricarboxylatecopper(II) trihydrate, prepared from benwene-1,2,3-tricarboxyates and copper(II) chloride, with excess equivalent of ethylene diamine in methanol at room temperature. The crude product was recrystallized from methanol by slow evaporation and violet crystalline solid of 1 was isolated in good yield (Scheme 1). Compound 1 displayed a violet color and showed some solubility in organic solvent such as methanol, dimethylformamide and acetonitrile. The material can be handled and stored at air for months without any sign of decomposition. Open Journal of Inorganic Chemistry Scheme 1. Synthesis of the two dimensional polymeric complex 1.

Thermal Properties
TGA (light red curve)/heat flow (dark red curve) profiles of 1 ( Figure 1) showed processes with weight loss. The compound showed a 16% weight loss between 40˚C -120˚C, which can be assigned to the departure of six water molecules (calc. 11.2%) and one molecule of ethylene diamine (6.2%). Above 200˚C, further decomposition of the compound occurs with a final weight loss of 32% at 243˚C and corresponding to the departure of a benzene-1,2,3-tricarboxylic acid molecule (calc. 21.43%) and two molecules of ethylenediamine (calc.12.4%).
These decompositions are supported by two endothermic effects at around 97˚C and 210˚C.

Magnetic Properties
The magnetic properties of the polymeric complex 1 was evaluated by determining the effective magnetic moment μ eff of the compound according to the formula μ eff = 2.828 × (χ Mcor T) 1/2 , where χ Mcor is the corrected molar susceptibility. Table 1 gives the diamagnetic contribution χ a of each atom or group of atoms in 1. The measured value χ g was found to be 0.437 × 10 −5 and the magnetic susceptibility χ M of the compound was calculated to be 0.437 × 10 −5 × 1015.38 = 4377 × 10 −6 .

IR Spectroscopy
The FT-IR spectrum of 1 ( Figure 2  and of the methyl and ethylenediamine groups [19]. The large and intense absorption band observed between 1595 -1559 cm −1 is the result of valence vibrations for both C=C and C=O bonds of benzenetricarboxylate. The valence vibration of the C=O group is shifted to lower frequency due to hydrogen bonding interactions between oxygen atoms of the anion and the hydrogen atoms of ethylenediamine or of water. Such a behaviour was also observed in some copper complexes derived from pyridine-2-carboxylate and quinoline-2-carboxylate.
The absorption band between 1280 -1363 cm −1 is due to the stretching and bending vibration of the C=C conjugated bond of the aromatic ring.

UV-Vis Spectroscopy
UV-vis spectroscopy analysis of 1 ( Figure 3) showed no absorption band in the visible region expected in the range 350 -800 nm. However, the spectrum shows a single absorption band at 296 nm which is attributed to intraligand π→π* electronic transitions due to C=O, C=C bonds. The broadness of the band and the absorption peak observed is due to the presence of copper that is covalently bound to oxygen.

X-Ray Single Crystal Structure
Single crystals X-ray diffraction analysis of 1 ( Figure 4 and Figure 5, Table 2 and Table 3