Structure and thermal reactivity of Zn(II) salts of isocinchomeronic acid (2,5-pyridinedicarboxylic acid)

The synthesis, an improved reﬁned crystal and molecular structure re-determination, and the thermal decomposition behavior of two Zn(II) derivatives of isocinchomeronic acid (2,5-pyridinedicarboxylic acid or H 2 2,5-pydc) are presented. [Zn(2,5-pydc)(H 2 O) 3 Zn(2,5-pydc)(H 2 O) 2 ] 2 ( 1 ) crystallizes in the triclinic P-1 space group with a = 7.106(2), b = 11.450(2), c = 11.869(1) A˚, a = 107.29(1), b = 104.08(1), c = 90.32(2) (cid:2) , and Z = 2. [Zn(2,5-pydc)(H 2 O) 2 ] (cid:1) H 2 O ( 2 ) is orthorhombic (P2 1 2 1 2 1 space group), with a = 7.342(1), b = 9.430(1), c = 13.834(2) A˚, and Z = 4. The structures were reﬁned to agreement R 1 -factors of 0.0315 ( 1 ) and 0.0336 ( 2 ). Complex ( 1 ) is arranged as molecular Zn 4 (2,5-pydc) 4 (H 2 O) 10 tetramers, the cages of which deﬁne channels that remain unblocked by anions. Compound ( 2 ) is polymeric with Zn(2,5-pydc)(H 2 O) 2 and Zn(2,5-pydc)(H 2 O) 3 units linked through bridging ligands. Both compounds were synthesized under mild conditions in aqueous media, without need to resort to hydrothermal media. Changing the pH from 4.51 to 5.75 sufﬁces to direct the chemical processes toward the orthorhombic compound rather than to the triclinic one.


Introduction
During the last decade the series of metal complexes formed by the pyridine-dicarboxylic acids have drawn considerable interest because these ligands are able to form compounds of large structural diversity. All the anionic isomers (2,3-, 2,4-, 2,5-, 2,6-, 3,4-and 3,5-dicarboxylates) present several potential donor sites: the oxygen atoms of the carboxylic groups and the nitrogen atom in the aromatic ring. Depending on the relative position of the donor sites (angles and distances), the solid complexes formed by these ligands may be molecular solids, as in the case of 2,6and 2,4-pydc derivatives [1][2][3], or may contain chains [4] or complex three-dimensional structures [5] hence giving rise to a variety of materials with properties useful for potential applications. In particular, discrete macro-cyclic metal complexes may be tailored to form cavities, porous materials or channels that behave as hosts for small molecule guests. The synthesis of these open molecular frameworks has received much attention [6][7][8][9][10].
The structure of the isocinchomeronic acid (H 2 2,5-pydc) makes this ligand suitable for the synthesis of cage materials because the two carboxylate groups in the 2-and 5-ring positions may easily coordinate to different metal centers, and the relative position of the coordinative moieties is adequate to form supramolecular structures of varied structural features. The carboxylic groups also stabilize the 3-D structure acting as proton donors and H-acceptors as each carboxylate can accept up to four hydrogen bonds [11,12]. The formation of mono-dentate and/or multi-dentate M-O and M-N metal-ligand bonds [13] may also change the crystal properties. H 2 2,5-pydc complexes of first transition metal (II) ions having 1:1 ratio have been reported to be molecular, as in [Ni (2,5- 4 ] Á 2H 2 O [14], 1-D polymeric, as in Cu(2,5-pydc)(H 2 O) [15], 2D-polymeric, as in [Me (2,5- [16,17], Zn [18] and Ni [19]), and 3-D polymeric, as in [Fe (2,5- [20] A supramolecular structure has also been reported by Liang et al. [21], by Wang et al. [22] and by Mahata et al. [23]: the compound with stoichiometry Zn(2,5pydc)(H 2 O) 2 contains the discrete entity Zn 4 (2,5pydc) 4 (H 2 O) 8 where the four penta-coordinated Zn atoms are connected by four 2,5pydc ligands through bridges forming a rectangular structure. Stacking of the tetramers leads to the formation of channels.
We report here the synthesis, a more precise structure refinement and the thermal decomposition behavior of a complex containing Zn(2,5-pydc) tetramers (1) in which the channels remain unblocked by anions. After our own X-ray diffraction measurements were completed, the crystal structure of this compound was reported by Wei [24] with a rather limited crystallographic agreement factor (R = 0.0812). As the crystal structure of the coordination compounds depends also on the electronic configuration of the metal site, we chose Zn(II) as the metallic center because of its structural flexibility to adopt various coordination polyhedra. The supramolecular structure shown by the Zn(II) derivative is unusual and defined by hexa-and penta-coordinated zinc ions, conformational features that suggest the potential use of this material to host for small guests.
We also report the synthesis, structural and thermal study of another complex with stoichiometry Zn(C 7 H 3 O 4 N)(H 2 O) 3 (2). There are previous references to this compound in the literature [16][17][18], but only cell parameters are reported along with data for the isostructural Co and Ni compounds.
In this paper we show that adequate choice of pH during the preparation procedure provides a powerful tool to tailor the material's properties, not only because the protonation or deprotonation of the ligand may determine the possibility of obtaining 1:1 or 1:2 compounds, but also because the incorporation of water in the solid framework can be modulated by the pH.

Materials and methods
All chemicals were reagent grade and have been used as provided. The metal content of the solid was determined at INQUIMAE, Universidad de Buenos Aires, with a Varian Techtrom A-A5R atomic absorption spectrometer. Elemental analyses were performed in a Carlo Erba EA 1108 microanalyzer. The X-ray diffraction data were collected on an Enral-Nonius CAD-4 diffractometer working in the x-2h scanning mode. Thermal analyses were performed in a Shimadzu TGA-51 and DTA-50 thermal analyzers, under a nitrogen atmosphere, at a heating rate of 6°C min -1 . PXRD patterns were recorded in a Siemens D5000 diffractometer with a Bragg-Brentano geometry, using the Cu Ka radiation and a curved graphite-monochromator. [Zn (2,5-

Synthesis of the solids
The same procedure was used, but NaOH was added until pH 5.75 was reached. The crystals obtained were pale yellow and the chemical analyses indicated the empirical formula Zn (

Data collection and processing
Crystal data, data collection procedure, structure determination methods, and refinement results for the two complexes are summarized in Table 1 [25][26][27][28][29]. In 1, the hydrogen atoms of the 2,5-pydc group were positioned stereo-chemically while the ones of the water molecules located in a difference Fourier map. All H-atoms were refined with a common isotropic displacement parameter (which converged to 0.050(3) Å 2 ), the former riding on the corresponding carbon atom and the latter kept fixed at their found positions.
In 2, the H-atoms of the 2,5-pydc ligand were positioned on stereo-chemical basis and refined with the riding model. The water H-atoms were found in a difference Fourier map and refined isotropically with O-H and OÁÁÁO distances constrained to target values of 0.86(1) and 1.36(1) Å , respectively.

Results and discussion
Description of the structure of [Zn (2,5- The compound contains discrete [Zn 4 (pydc) 4 (H 2 O) 10 ] molecules, in which the four coplanar Zn atoms are connected by four 2,5-pydc ligands through bridges forming a centrosymmetric structure. The four Zn atoms define a parallelogram. Figure 1 shows the ORTEP [30] plot and the labeling scheme of the tetrameric complex. Selected bond distances and angles are shown in Table 2.
As stated above, the four Zn atoms define a parallelogram with Zn(1)-Zn(2) and Zn (2) (2) Zn (1) (5), see above) acts as a spacer between cages, and the robustness of the basic Zn tetramer opens the possibility to tailor other materials by introducing new spacers. As can be seen, different preparative techniques may lead to different solids. In hydrothermal media, one water molecule may be removed [21][22][23] or not [24], depending on the concentration of the reagents. High concentrations and slightly higher temperatures lead to the less hydrated material. At the concentrations used in our work, in hydrothermal media the weaker Zn-OW(4) or Zn-OW (5) bond is broken, and all the Zn atoms become penta-coordinated [21]; as stated, Zn-OW(3) bond is unaffected, reflecting a large stability of the water molecule protruding into the cavity.
Also in hydrothermal media, hexa-coordinated Zn(1) can only be prepared at much lower concentrations (tenfold decrease in Zn and ligand concentrations) [24]. Hence, it is concluded that the use of hydrothermal conditions favor the dehydration, although several other experimental conditions differ: the starting Zn salt and, very especially, the pH, which was very low in Liang 0 s synthesis. Indeed, even in our experimental conditions, raising the pH to 5.75 leads to the formation of orthorhombic Zn(2,5-pydc)(H 2 O) 2 Á H 2 O, to be described next.
The equatorial plane is defined by N and O(1) from the carboxylate in 2-position, and two water molecules (OW(1) and OW(2)) slightly protruding above and below the plane. The aromatic ring is slightly tilted from the equatorial plane  (13)8). This deviation is typical of 2,5-pydc compounds, as opposed, for instance to 2,6-pydc compounds featuring one or two ligands bound to one central metal ion [1][2][3].
The lack of coplanarity of the carboxylate in the 5-position determines in turn a three-dimensional structure that differs appreciably from that of 2,6-pydc compounds. The third water molecule is interstitial water, located between layers.
Alternate anion and cation polymeric layers are defined approximately along the a-(bidentate ring and O(4)) and b-axis (O(3)). Tian et al. [17] have shown that the isostructural Co(II) derivative corresponds to a chiral coordination polymer containing interconnected right-handed and lefthanded helical chains; thus, the compound exhibits strong signals in vibrational circular dichroism spectra.    (2.0174 and 2.038(3) Å ) and thus the compound may be viewed as formed from discrete monomers. There is an extensive hydrogen bonding network within each layer and between layers, linking coordinated water (OW(1) and OW(2)) with carboxylate groups (O(1) and O(2) for OW (1), and O(4) for OW (2)). Interstitial water (OW(3)) also links coordinated water (Ow(2)), and carboxylate group (O(3)), forming further weak hydrogen bonds with the polymeric layers.
Thermal properties Figure 4a shows the TG and DTA traces obtained for (1). Five well-defined weight losses are seen. The first endothermic step centered at 106. At 165°C, after the loss of four additional water molecules, the PXRD indicates the formation of a new crystallographic phase, probably containing tetra-coordinated Zn. This new phase does not correspond to any of the reported Zn 2,5-dipicolinate complexes. It is interesting to note that in hydrothermal media, (1) is stable at this temperature. The amorphous precursor of crystalline ZnO is seen in the PXRD of the solid formed at 300°C. The TG and DTA traces for [Zn(2,5-pydc)(H 2 O) 2 ] Á H 2 O (2) are shown in Fig. 4b. Only one endothermic dehydration step is found at 166.8°C, with a weight loss (18.7%) corresponding to the three water molecules. The organic ligand decomposes in one endothermic step characterized by two DTA peaks (376.4°C and 402.0°C, weight loss (31.2%), and by an exothermic step at 490.0°C (weight loss 20.2 %). At 543.0°C a new exothermic peak is detected. The final product at 650°C is also ZnO.

Conclusions
The tetramer [Zn(2,5-pydc)(H 2 O) n Zn(2,5-pydc)(H 2 O) 2 ] 2 is a robust entity that constitutes the basic structural unit of two different compounds with n = 2 or 3. The thermal treatment of the more hydrated solid leads to the less hydrated one, with retention of the cage structure. Further loss of water at higher temperatures is accompanied with the destruction of the cage. The decomposition of (1) when heated in dry atmosphere also indicates that the thermodynamic stability of the phase is not very large at a relativity high temperature.
The syntheses of the tetrameric material (n = 2) [21,24] is achieved if no base is added to the system. The formation of the Zn salt is therefore attended by strong acidification that compensates the increased rates of chemical reactions, and the tetramer forms.
Crystallographic data (excluding structure factors) for the structures in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication numbers CCDC-271469 for (1) and CCDC-271470 for (2). Copies of the data can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [fax: 1 44-(0)1223-336033 or e-mail: deposit@ccdc.cam.ac.uk].