Abstract
A new mercury (II) coordination polymer {2[Hg(L)I2](HgI2)}n (1) [L: (E)-N'-(1-(pyridine-3-yl)ethylidene)isonicotinohydrazide]was synthesized by the reaction of mercury(II) iodide with the ligand under thermal gradient conditions using the branch-tube method and fully characterized by infrared spectroscopy, 1H NMR, elemental analysis and single crystal X-ray diffraction. The determination of the structure by single crystal X-ray crystallography shows that the compound in the solid state consists of two distinct 1D polymeric chains, which will be denoted parts A and B. The coordination numbers of Hg (II) are four, (HgI4) of part A and (HgNI3) of part B with square planar and tetrahedral geometry around the mercury (II) ions. The supramolecular features in these complexes are guided and controlled by weak directional intermolecular interactions. The chains interact with each other through π–π and CH-π stacking interactions creating a 3D framework. The structure of 1 was optimized by Density Functional Theory Calculations (DFT). Computed bond lengths and angles and vibrational frequencies are in good agreement with the experimental data.
Keywords
Introduction
Coordination polymers and organic-inorganic hybrid materials are widely investigated in recent years, not only for their potential applications [1–4] but also for their fascinating versatility in building up extended structures [5–8]. Coordination polymers based on divalent mercury cations may exhibit strongly asymmetric Hg-Halide-Hg bridges. In addition, the complexes of the soft Hg (II) ion are much less predictable in respect of coordination number at the metal [9] and represent a synthetic challenge to crystal engineering. The organic part of a coordination polymer exhibits interestingly steric and electronic effects on the self-assembly of coordination polymers. Organic aromatic ligands are good candidates for spacer part of organic-inorganic materials, because not only they can act as hydrogen-bonding acceptors or donors, but also can provide recognition sites for π–π stacking interactions to form interesting supramolecular structures when coordinating to metal ions [10–15]. The spherical d10 configuration is associated with a flexible coordination environment so that the geometries of these complexes can vary from tetrahedral to octahedral and severe distortions in the ideal polyhedron occur easily. Furthermore, due to the general liability of the d10 metal ion complexes, the formation of coordination bonds is reversible, which enable metal ions and ligands to rearrange during the process of polymerization to give highly ordered network structures. Consequently, Zn, Cd and Hg can readily accommodate all kinds of architectures and a selection of the topological types of 1D, 2D and 3D polymers is given [16–19]. The electronic structure has been determined by the density functional theory (DFT) method. Currently DFT is commonly used to examine the electronic structure of metal complexes. It meets the requirements of being accurate, easy to use and fast enough to allow the study of relatively large molecules of metal complexes [20]. DFT has shown to be sufficient for the arrangement optimization and calculation of spectral properties. It gives good agreement with the experimental data and its use is justified in the case of large molecules. We herein report on the synthesis, characterization, X-ray crystal structure of a new coordination polymer {2[Hg(L)I2](HgI2)}n (1)and its DFT calculations.
Experimental
Materials and experimental techniques
All reagents for the synthesis and analysis were commercially available and used as received. Microanalyses were carried out using a Heraeus CHN-O-Rapid analyzer. Melting points were measured on an Electro thermal 9100 apparatus and are uncorrected.IR spectra were recorded using Perkin-Elmer 597 and Nicolet 510P spectrophotometers.
Synthesis
Synthesis of the schiff base ligand (L)
The Schiff base ligand (E)-N'-(1-(pyridine-3-yl)ethylidene) isonicotinohydrazide (L, Scheme 1) was prepared by the condensation of Isoniazid (1.37 g, 10 mmol) and methyl-3-pyridyl ketone (1.21 g, 10 mmol) in ethanol solution. The reaction occurred under reflux and after 7 h a white coloration was observed, indicating the formation of the schiff base ligand L [21–25]. The resulting white precipitate was filtered, washed with ethanol, dried in air and used without further purification. Yield: 2.24 g (87%). (m.p: 225–230°C). Anal. Calc. for C13H12N4O: C, 64.99; H, 5.03; N, 23.31. Found: C, 65.11; H, 4.98; N, 23.24%. Characteristic IR bands (cm–1): 3436 b, υ(NH); 2923 w, υ (CH); 1659.08 vs, υ(C = N); 1502 w, υ(C-N); 1449 w, υ(CN); 1298 w, υ(CH); 742 m, υ(CH) [29]. 1HNMR (CDCl3, ppm): 7.260–9.085 (Aromatic H, 8 H), 8.327 (Amide H, 1 H), 2.519 (s, 3 H).
Synthesis of {2[Hg(L)I2](HgI2)}n (1) single crystals
The crystals of title complex was obtained by the branch tube method [26–29]. Schiff base ligand (0.240 g, 1 mmol) and HgI2 (0.45 g, 1 mmol) was placed in the arm to be heated and methanol was carefully added to fill both arms. The tube was sealed and the ligand-containing arm was immersed in a bath at 60°C while keeping the other arm at ambient temperature. After 3 days, colorless crystals of 1 deposited in the cooler arm.They were filtered off, washed with acetone and ether, and dried in air. Yield, 0. 537 g (77.8%), (m.p. = 240°C). Anal. Calc. for C26 H24Hg3 I6 N8 O2: C, 16.94; H, 1.31; N, 6.07%. Found: C, 16.89; H, 1.22; N, 6.11%. Characteristic IR bands (cm–1): 3438.14 b,υ(NH), 3084.51b, υ(CH); 2923.36 w, υ(CH); 1660.01 vs, υ(C = N); 1502.22 m, υ(C-N); 1449.47w, υ(CN); 1377.73 vs, υ(CH); 1298.83w, υ(CH); 1042.73 m, υ (CH); 742.64w, υ(CH); 678.79w, υ(CH).
X-ray crystallography
Single crystal X-ray diffraction data were obtained with a Gemini four circle diffractometer manufactured by Oxford Diffraction equipped with an Atlas CCD detector, using graphite a monochromator and Mo Ka radiation (λ= 0.71073 A°). Standard data collection strategy of CrysAlis [30] was used for data collection; the same program was used for integration of CCD images and absorption correction. The crystal structure was solved by direct methods with program SIR2002 [31] and refined with Jana2006 [32] by full-matrix least-squares technique on F2. Crystallographic data and details of the data collection and structure refinement are listed in Table 1. The calculated structure factors and full lists of bond distances, bond angles and torsion angles are given in the supplementary data. X-ray crystal structure is shown in Fig. 1.
Computational details
The geometry of 1 has been optimized using the B3LYP density functional model. In these calculations the 3–21G* basis set was used for C and H atoms, while the 6–31G* basis set was used for N and O, atoms. For the Hg and I atoms, the LanL2DZ valence and effective core potential functions were used. All DFT calculations were performed using the Gaussian 98 R-A.9 package [33–37]. X-ray structures were used as input geometries when available.
Results and discussion
The reaction between the ligand with HgI2 led to the formation of the new Hg(II) coordination compound {2[Hg(L)I2](HgI2)}n (1) (Scheme 1).
Infrared spectra
The IR spectra display characteristic absorption bands for compound. For 1, the absorption band at 3084.51 (cm–1) are due to the C–H modes involving the aromatic ring hydrogen atoms [38]. The C–H modes involving the aliphatic hydrogen atoms of ligand L in compound 1 reveal around 2923.36 (cm–1) [39]. The absorption bands with variable intensity in the frequency range 1449.47–1502.22 (cm–1) correspond to ring vibrations of the pyridine moiety of the ligand [40–48].
Crystal structure
Single crystals of 1 were obtained by the reactions of mercury (II) iodide with the ligand L under thermal gradient conditions using the branch-tube method. After 3 days, X-ray quality crystals of {2[Hg(L)I2](HgI2)}n (1) were obtained. Structural analysis by single crystal X-ray diffraction revealed that 1 crystallizes in the monoclinic system with space group P 1 21/n1(Fig. 1) and showed that the compound in the solid state, consist of two distinct linear one dimensional polymer chains, which will be denoted as parts A and B. In part A with formula 2[Hg(I)2]n, the Hg (II) centers is chelated by two iodide atoms with distances of Hg1 ... I1(2.636 Å) and two iodide atoms with distances of Hg1 ... I1(3.368 Å). Coordination number for Hg1 centers is four with a HgI4 chromophore and the Hg (II) ions are in a distorted square planar geometry. Each ring of (HgI2)n polymeric chain is lozenge symmetry form (Fig. 2) and the two I1-Hg1-I’1 angles have opened up to 90.92°, respectively and the two Hg1-I1- Hg1 angles have reduced to 89.08° indicating a little distortion from a regular square planar but the sum of angels around the metal center is 360.0° that shows geometry is quite planar (Fig. 1).
In part B with formula 2[Hg(L)I2]n, single crystal analysis revealed a Hg (II) polynuclear complex forming linear one dimensional polymeric chain along a axis (Fig. 2) with Hg ... Hg separation 4.243 Å. In this structure only one of the pyridine, but not the imines nitrogen atoms of the ligand L with distance of Hg2 ... N4 (2.371 Å) is linked to each mercury atom, one iodide atom is terminally coordinated to the mercury atoms with distance of Hg2 ... I3 (2.624 Å) and are not bridging and two iodide atoms with distances of Hg2 ... I2 (2.641 Å) and (3.364 Å) are as a bridge to form a one-dimensional polymeric chain along to the a direction (Fig. 2). Coordination number for Hg2 centers is four (one pyridyl nitrogen atom and three iodide atoms) with the distorted tetrahedral geometry. The angles around the Hg2 center deviate significantly from 109.4° that indicating distortion from regular tetrahedral (for more information about the bonds distances and angles see Table.3). Thus we find both form of coordination number of four square planar and tetrahedral in this polymeric compound. The Hg ... Hg separation in part A and B is 4.243 Å. The individual 1D chains are parallel to each other and further linked by O1 ... H hydrogen bonds with bonds distances of O1 ... H8b of 2.686 Å, O1 ... H8c of 2.299Å and O1 ... H2N2 of 2.188 Å (Fig. 3). Compound 1 also exhibits weak CH ... I1 hydrogen bonds with bonds distances of C1H1 ... I1 of 3.129 Å and C5H5 ... I1 of 3.110 Å and intermolecular interaction between nitrogen from pyridine ring with Hg1 atoms with bond distance of N1 ... Hg1 of 2.870 Å Fig. 4. The details about the hydrogen bonds are summarized in Table 2. Whereas O1 ... H8b, O1 ... H8c and O1 ... H2N2 interconnect only crystallographic equivalent parts of type B, the remaining hydrogen bonds (C1H1 ... I1 and C5H5 ... I1) are interaction involving between parts A and B. A search was made for π... π stacking interactions in the complex, revealed that in the crystal packing of complex, the interplanar distance between L rings is 4.243 Å, The Parallel arrays of the planes of the aromatic moieties in the complex indicate that these interactions are of the slipped face-to-face stacking types. Also there is a strong C–H ... π stacking interaction named C8–H8a ... π, with a separation of 3.404 Å between the hydrogen atoms of the methyl with the pyridyl ring of the adjacent unit (Fig. 5). Thus compound 1 expands the 1D polymeric chains into 3D coordination compound via these intermolecular interactions. Consequently, the strong Hg–N, hydrogen bonding, π–π and C–H ... π stacking interactions may control the coordination sphere of mercury (II) ion in this complex.
DFT calculations
The calculated structural parameters are listed in Table 3. It should be noted that the experimental data belong to the solid phase, whereas the calculated data correspond to the isolated molecule in gas-phase. However, the experimental and computational data in Table 3 clearly show that both data only slightly differ from each other. For example, the largest difference between experimental and calculated Hg2-N4 length is about 0.027. As a result, the calculated geometrical parameters represent a good approximation. The computed IR frequencies are listed in Table 4 together with the experimentally determined frequencies. The assignment of the experimental ν(Hg–I) vibration is based on the theoretically calculated frequencies. Both calculated and experimental frequencies are found to be in agreement. The NBO charges of Hg(II) and the coordinated atoms were also calculated. The positive charge of the Hg(II) ions was 0.897 and 0637. The charges of the aliphatic N and O atoms of the ligands were –0.377, –0.305 and –0.607. The charges of aromatic N atoms of ligands were 0.585 and –0.510 while the iodide anions have charges between –0.320 ––0.509. The calculations indicate that complex 1 has 102 occupied molecular orbitals (MOs) for the per unit. The value of the energy separation between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) was calculated. Figure 6 shows the HOMO and LUMO for the Hg(II) complex. As will be seen in Fig. 1, the HOMO is principally localized among two aromatic carbon of ligand, whereas the LUMO is delocalized approximately on another aromatic carbon and nitrogen atoms of the ligand. The calculated HOMO–LUMO gap is 0.093 a.u.
Conclusion
A new Hg (II) coordination polymer, {2[Hg(L)I2](HgI2)}n (1) was synthesized and characterized with FT-IR, 1H NMR and single crystal X-ray diffraction analysis. The result of single crystal X-ray diffraction revealed that it takes the forms of linear one dimensional polymeric chains in the solid state and consists of both form of four coordination number, square planar and tetrahedral geometry around the mercury (II) ions. The strong Hg–N, hydrogen bonding, CH–π and π–π stacking interaction observed in the extended structure and seem these interactions play an important role to extend this 1D polymer into 3D structure. A theoretical study of this title complex was undertaken to examine its electronic structure. Camputed bond lengths, angles and vibrational frequencies are in good agreement with the experimental data.
Footnotes
Acknowledgments
The authors thank Tabriz University and Qom University for all the supports and also acknowledge the project Praemium Academiae of the Academy of Sciences of the Czech Republic.