Structural studies and conductivity of [ Fe ( O 3 C 4 ) ( COO ) ] · H 2 O based H 4 btec ( H 4 btec = 1 , 2 , 4 , 5-benzenetetracarboxylic acid )

A new metal-organic hybrid compound [Fe(O3C4)(COO)]·H2O I has been hydrothermally synthesized and characterized by single-crystal X-ray diffraction. Rust crystals crystallize in the monoclinic system, space group I2/a, a = 6.9651(2) Å, b = 8.12630(10) Å, c = 19.4245(2) Å, β = 92.6600(10) ̊; V = 1098.25(4) Å; Z = 2 and Dx = 3.63 g/cm. The refinement converged into R = 0.042; Rw = 0.058. The structure, determined by single crystal X-ray diffraction, consists of a network of FeO6 centers, octahedral coordinated by btec (btec = 1,2,4,5-benzenetetracarboxylic acid) anions giving rise to a two-dimensional sheet structure. In the compound I, [Fe(O3C4)(COO)]·H2O, the FeO6 group bridged by the 1,2,4,5-benzenetetracarboxyl anion exist in a unit cell, with each anion lying about an inversion centre. One of the FeO2 a distance [1.965(2)] significantly corresponds to the shortest distance as the other and the distances found in the axial direction of compound I.

In spite of the rich coordination chemistry exhibited by H 2 bdc and H 3 btc (Chart 1) in the presence of auxiliary ligands and coordinated solvents, barring a few sporadic reports, which mainly concentrate on the direct interaction between the metal ion and the ligand, there have been no serious attempts to prepare metal-organic polymeric or supramolecular structures based on 1,2,4,5benzene tetracarboxylic acid (H 4 btec) [47][48][49][50][51][52][53][54][55][56][57][58][59][60][61][62][63][64][65].It may be argued that, due to steric reasons, all the four carboxyl groups of H 4 btec are unlikely to take part in coordination to the metal.However, even the presence of free -COOH groups (especially in the vicinity of coordinated water molecules, donor solvents and added amines) would lead to formation of new extended structures aided by hydrogen bonding inter-actions.Moreover, the presence of a large number of uncoordinated water molecules within the lattice, may in turn lead to interesting possibilities for the preparation of porous solids.1,2,4, 5-Benzenetetracarboxylic acid (H 4 btec) has been well known to be an ideal ligand to encapsulated metal nodes, forming coordination compounds with unique structures and interesting properties [66][67][68][69][70], which may have the following advantages: 1) Higher symmetry of the ligand may cause the generation of regular structures; 2) The rigidity of the ligand may reduce the possibility of lattice interpenetration in the products [71,72]; 3) The multidentate carboxylate is known to be essential in chelating metal ions to form chain-like units with M-O-M connectivity; 4) The four carboxylic groups of this ligand may chelate to metal ions by using various coordination modes to form fascinating multidimensional compounds.In consideration of the varieties of topologies and properties, incorporating functional moieties into MOFs is often a popular method used in crystal engineering.

Synthesis and Initial Characterization
The title compound was synthesized under hydrothermal conditions in the presence of tetramethylammonium nitrate.In a typical synthesis, 0.1158 g of pyromellitic acid (Acros Organics) was dispersed in 9 ml of H 2 O.To this, 0.1975 g of iron nitrate monohydrate (Prolabo) was added under constant stirring.Finally, we add 0.062 g of tetramethylammonium nitrate (Alfa Aesar) and the mixture was homogenized for 15 min at room temperature, was sealed in a 23-ml PTFE-lined stainless steel autoclave and heated at 120˚C for 60 h.Then the product obtained is filtered and washed with a small amount of distilled water.The chemical purity of the product was tested by EDAX measurements.[Figure 1

Single Crystal Structure Determination
The unit-cell dimensions were refined using X-ray diffraction data collected with a Kappa CCD Enraf Noninus diffractometor using Mo Kα radiation .The structure, [Fe(O 3 C 4 )(COO)]•H 2 O, was analyzed with the crystallographic CRYSTALS program [73].The structure was solved by conventional Patterson and difference-Fourier techniques.The chemical crystal data, the parameters used for X-ray diffraction data collection and strategy used for the crystal structure determination and their results, are listed in Table 1.Table 2 shows the atomic coordinates and equivalent isotropic displacement.The anisotropic displacement parameters are listed in Table 3. Selected bond distances and angles are given in Table 4. Structural graphics were created by the DIAMOND program [74].The asymmetric unit is shown in (Figure 2).

RESULTS AND DISCUSSION
The structure of compound I is representatively described in detail here.In an asymmetrical unit of 2, The   ters coordinated by the btec carboxylate units (Figure 2).The connectivity between these units gives rise to a two dimensional hybrid layered structure in the ac and bc planes as shown in (Figures 3 and 4).The structure re- sembles a 4-connected network in which each Fe atom is connected to four btec anions and each btec anion is linked to two Fe 2+ ions.The water molecules also occupy the interlamellar region.

CONCLUSION
In this work, we report a novel metal-organic complex [Fe(O 3 C 4 )(COO)]•H 2 O (I), which is prepared by the hydrothermal synthesis route.It crystallizes in the mono-clinic symmetry, space group I2/a.Compound I exhibits a novel bi-dimensional network constructed from bridging btec ligand.The successful isolation of compound I not only confirms that such metal-organic compounds may be designed and synthesized according to the inherent stereo and interactive information stored in the organic ligands and metal ions [75], but also further proves the strong capability of hydrothermal reactions in preparing novel metal-organic materials with mixed or-  ganic ligands.

ACKNOWLEDGEMENT
This work is supported by the minister of superior education and research.

4 .
structure of [Fe(O 3 C 4 )(COO)]•H 2 O , Compound I, is formed of a network of octahedral coordination by Fe carboxylate units btec.It also contains 11 non-hydrogen atoms.The Fe atom is octahedrally coordinated by six oxygen atoms from four different carboxylic groups.The Fe-O distances are in the range 1.965(11) -2.038(4) (av.2.013 Å) and the O-Fe-O angles are in the range 86.34(16) -179.994° (av.102.856°).Bond valence sum calculations 23 indicated that the valence states of the Fe, C and O in II were +2, +4 and −2 respectively.Their selected bond lengths and bond angles are listed in Table The bonded oxygen atoms of the carboxylate groups have C-O distances in the range 1.276(5) -1.290(5)Å.The O-C-O bond angles have an average value of 125.16 • .The present compound adds another example to this family of compounds.The various structural parameters observed in the present compound are in agreement with those observed before.The present compoun d adds another example to this family of compounds.The various structural parameters observed in the present compound are in agreement with those ob-

Figure 3 .
Figure 3.The projection structure of [Fe(O 3 C 4 )(COO)]•H 2 O, in the ac plane showing a single layer.

Figure 4 .
Figure 4.The projection structure of [Fe(O 3 C 4 )(COO)]•H 2 O, in the bc plane showing a single layer.