3.1. Structures description
3.1.1. (CBMH2)ZnCl4.2H2O , (CP1).
The single-crystal X-ray diffraction investigation has exhibited that the new organic-inorganic hybrid compound CP1 crystallizes in the monoclinic system with space group P21/c. The asymmetric unit is made up ofone(H2CHBMA)2+organiccation, an isolate[ZnCl4]2-anion and two crystallographically independent uncoordinated molecules water as shown in Fig. 1a.The cohesion between these entities is ensured by a complex hydrogen bonding system and by Columbic interactions. The distribution of the different species is shown in Fig. 2c.
The central divalent Zn is surrounded by four chlorine atoms in a slightly distorted tetrahedral geometry.The bond length of the Zn-Cl lie in the range 2.2590 (5) - 2.2989 (5) Å (Zn-Clav = 2.274Å) and the bond angles Cl-Zn-Cl vary from 104.23 (2) to 114.49 (2)°(Cl-Zn-Clav = 109.44°), are in agreement with those observed in analogous compounds [22-25]. These geometric parameters in ZnCl42- are related to the number of hydrogen bonds accepted by the Cl atoms. To quantify the distortion of the [ZnCl4]2- anion from the ideal tetrahedral conformation,4 is a structural parameter introduced by Yang et al. (2007) [26] which can be used to gauge mainly the geometries of four-coordinate metal complexes. This parameter is defined as4=, where and are the two largest valence angles around the central atom and = 109.5° is the ideal tetrahedral angle. 4 can vary from 0 to 1, passing from an absolute square planar to an ideal tetrahedral conformation. The 4 value of the present structure is 0.952 which proves a little distortion of the Zn atom from a regular tetrahedron. BVS calculations using the Brown and Altermatt method [27]revealed that the zinc atom have valence sums equal with a value of 2.045, close to the ideal value of 2 for ZnII.
In the crystal structure, the four chlorine atoms of the [ZnCl4]2- anion are acting as acceptors of hydrogen bonds. Each ZnCl42- anion bridge the water molecules (O1W, O2W) by means of O-HCl hydrogen bonds, forming an infinite two dimensional supramolecular (ZnCl4-OW)nsheets extending parallel to (001) plane (Table 2, Fig. 2b). Within the anionic supramolecular layers, rings with graph-set motifs [28]of and are observed, lying at x = ½.
The (CHBMAH2)2+cations are arranged on both side of these anionic layers through non covalent interactions, to form 3-dimensional supramolecular structure. Hence, each (CBMH2)2+cation engages its hydrogen atoms bonded to N1 and N2 atoms in hydrogen bonds and participates in the structure stability via inter molecular H-bonds established between the organic and inorganic layers.The hydrogen-bonds details are given in Table 2 and are shown in Fig. 1c.The hydrogen bond interactions link the three components into a supramolecular 3D network.
In CP1,the C6-ring of the template cation adopts a typical chair conformation, four carbon atoms are coplanar, and the other two are puckered out of this plane, with normal distances C–C, C-N and angles C–C–C, C-C-N (Table 3). Interestingly, although CBM was employed as a mixture of cis and trans isomers, the organic cation (CBMH2)2+ present is conformation in the title compound (Fig.1a).Similar phenomenon was observed in the preparation of TJPU3, in which solely cis-CBM is used as template [29].
3.1.2. (H2CHBMA)CdI4.2H2O , (CP2).
CP2 crystallizes in the monoclinic centrosymmetric space group P21/m, the formula unit contains one (CBMH2)2+cation, one CdI42- anion and two molecules water of crystallization (Fig.2a). As shown in Fig. 2b, the central CdII atom, together with I2 and I3 atoms of the CdI42-anion and carbon atoms C4 and C5 of the cyclohexane ring lie in a specific crystallographic location, namely on mirror plane m. The remaining atoms of the cation, anion and water molecules, lying outside the plane, are necessarily mirror images of each other. Crystal structure study proves that in CP2, the CdII ion is in a slightly distorted tetrahedral geometry, with a four coordinate index,τ4 [26], of 0.97. Selected bond lengths and angles are listed in Table 5and are in agreement with the values reported for analogous compounds [30-32]. The bond angles implying the CdII atom range between 100.13(3) and 111.585(18) Å. The short valor, significantly smaller than all the other bond angles, is observed for the I2-Cd-I3 angle. This distortion can be explained by the fact that the iodine atoms are involved in a complex system of hydrogen bonds O(N)-H···I responsible for the phenomenon.
In the material, the discrete CdI42- anions alternately bridge the water molecules (OW) via O-H···I hydrogen bonds, building an infinite two dimensional supramolecular{CdI4-OW}nundulatedlayers extending parallel to (001) plane (Fig.2c).Within the anionic supramolecular layer, rings with a graph-set ring motif [28] ofandare observed, lying at x =½ (Fig. 2c). The (H2CHBMA)2+ cations are confined between these anionic layers to maximize the electrostatic interactions and are connected to CdI42- anion and H2O molecule via N-H1C···OW,N-H1A···I1, N-H1B···I3 and C5-H5A···I3 hydrogen bonds, thus forming a three-dimensional supramolecular structure.
It is worth to note that the existence of the uncoordinated water molecule consolidates the H-bonding network. It can participate as H-bond donor with the iodide ligands of the isolated tetrahedral CdI42- units (OW-H2···I1,OW-H2···I2, OW-H1···I3) and as H-bond acceptor with the positively charged ammonium groups of (CBMH2)2+cations ((N-H1C···OW). The hydrogen-bond details are given in Table 4 and are shown in Fig. 2d. The hydrogen bond interactions link the CdI42- anion, (CBMH2)2+cations and H2O molecule, to create a two-dimensional (2-D) hydrogen bond network.
Other interesting noticing is the template CHBMA molecule.As in the case of compound CP1, the cyclohexane ring of the organic cation in CP2 adopts also a chair conformation which is the most stable. Furthermore, although CBM molecule was used as a mixture of cis and trans isomers, the two methylamine groups exist in the cis conformation in both CP1 and CP2.For the same cis conformation, the two terminal ammonium groups in organic moiety are pointing upward for CP1 and downward for CP2. This difference has an impact on the topological nature of the anionic layers and is reflecting in a different anchoring of the organic cations on the anionic layers, built a three-dimensional hydrogen bonded network in the case of CP1 and two-dimensional hydrogen bonded network in the case of CP2.
Interestingly, the synthesis of CP1 and CP2 show selectivity for cis CBM isomers. The trans conformation has been also observed with the same organic cation (CBMH2)2+ in other compounds [29, 33].This may be a possible technique for separation and recognition of cis and trans isomers of CBM.
3.2. Hirshfeld Surface Analysis
Hirshfeld surface analysis have been carried out in order to decrypt and quantify the intermolecular interactions involved in the crystal packing of compounds CP1-CP2. Fig. 3 displays Hirshfeld surfaces mapped over the dnorm (normalized contact distance) property in two orientations and the associated full 2-dimensional fingerprint plots. The surfaces are represented as transparent to permit visualization all atoms of the asymmetric unit for the title compounds. To visualize the intermolecular interactions in the dnorm map, a color scale is used. The redareas imply contacts with distances shorter than the sum of van der Waals (vdW) radii, while the blue and white regions indicate contacts with distances longer than and equal to the sum of Van der waals radii, respectively [34]. For both compounds, the different intermolecular contacts are quantified with the breaking down of the full fingerprint region andtheir relative contributions, in percentage terms, to the overall crystal packingare exhibited as histograms in Fig. 3.
In compound CP1, the large red spots labeled 1’ in Fig. 3 represent H···Cl/Cl···H contacts, which are relevant in the dnorm maps. These contacts are attributed to N-H···Cl and O-H···Cl hydrogen bonds, which can be seen in the finger print as a pair of symmetrical spikes at (de + di2.30 Å) in agreement with the expected higher strength for N1-H1E···Cl3hydrogen bond (Table 2). The H···Cl/Cl···H contacts are dominant for the complex CP1 with higher contributions of 55.6% of the overall packing (Fig. 3). The large deep-red spots labeled 2’ in the dnorm show also strong H···O/O···H contacts attributed to N-H···O hydrogen bonds, which are viewed as a pair of symmetrical spikes with minimum (de + di1.94 Å) in agreement with the anticipated higher strength for N2-H2B···O2W. The H···O/O···H represent 5.5% of the total Hirshfeld surface area. The H···H interactions labeled 3’ in Figure 3, contributing 36.90% to the overall crystal packing. The two symmetrical broad peaks in the middle region of fingerprint with minimum contact (de + di 2.4 Å)is due to a short interatomic H···H contact.
In compound CP2, the H···I/I···H contacts labeled 1 in Figure 3 have the highest contribution (61.90%) to the overall Hirshfeld surface area. These contacts which appear as tiny red spots in the dnorm surface are attributed to N-H···I and O-H···I hydrogen bonds. The next most contribution to the overall crystal packing arise from the H···H and H···O/O···H interactions, respectively. In fact, the H···H contacts labeled3in the middle region of FP, are showed as two symmetrical broad peaks with minimum de + di 2.1 Å (shorter than the sum of van der Waals radii),representing a 29.2% contribution to the entire HS area in this Cd complex. Finally, like in the structure CP1, the large deep-red spots labeled 2 in the dnorm map (Figure 3) indicate also strong H···O/O···H interactions corresponding to the strongestN-H1C···OW hydrogen bond (Table 4), which also appear as two sharp symmetrical spikes at de + di 1.90Å in the FP with 5.5% contribution to the overall crystal packing.
The dominant XH/HX (X : Cl for Zn complex and I for Cd complex), OH/HO and HH interactions, suggest that hydrogen bonding and van der Waals interactions play the main roles in the crystal packing of the two compounds.
3.3. Thermal analysis (DTA/TGA)
To investigate the thermal stability of compounds CP1 (Figure 4) and CP2 (Figure 5), thermal analysis (DTA/TGA) were carried out under an N2 atmosphere with a heating rate of 10°C/min in the temperature range from 25° to 600 °C. Overall, the TGA curves for these compounds indicate that the two H2O molecules per formula unit are released between 25 and 130°C.The experimental mass losses of 9.27% and 4.49% are in good agreement with the calculated values of 9.29% and 4.50% for CP1 and CP2, respectively. After dehydration, the anhydrous compounds derivatives of CP1 and CP2 remain stable up to their melting points at 225 °C and 153°C, respectively. These melting points are confirmed by an additional thermal treatment in a separate furnace with run heating of 5°C/min, from room temperature to 250 °C for CP1 and from room temperature to200°C for CP2. After that, the compounds undergo a considerable decomposition, suggesting the loss, amongst others, organic cations and the halogen atoms.
3.4. Spectroscopic study
3.4.1. 13C NMR spectralanalysis
High-resolution NMR spectroscopy is a powerful technique for the characterization of organic-inorganic hybrid materials.
The13C NMR solution spectrum of (CBMH2)ZnCl4.2H2O (CP1) (Figure 6) shows five signals at 24.094, 28.905, 33.076, 34.873 and 45.077 ppm due tothe C4, (C3, C5), C1, (C2, C6) and (C7, C8), respectively. In fact, among the eight crystallographically independent carbons, we find the pairs (C3, C5), (C2, C6) and (C7, C8) that each one presents the same electronic environment. Therefore, they resonate at the same frequency and are caracterized by the same chemical shift.
The13C NMR solution spectrum of (CBMH2)CdI4.2H2O (CP2) (Figure 7)exhibits five signals corresponding to five carbon atoms crystallographically independent. From the spectrum, the signalscentered at24.102, 28.915, 33.076, 34.876 and 44.787 ppm ppm are attributed to C1, C2, C3, C4 and C5 respectively.
It is clear that the high values of chemical shifts are attributed to carbons (C7, C8) for CP1 C1 for CP2,which are the close stto coordination place (N). This is explained by effect of the electronegative nitogen atoms.
The low intensity signal observed experimentally at chemical shift 33.076 ppm, assigned to (C2, C6) carbon atoms for CP1 and to C2 carbon atoms for CP2, is due to the lack of attached protons.
3.4.2. Infrared spectra
To obtain further information upon the crystal structures of CP1 and CP2, we have undertaken vibrational analyses using infrared spectroscopy and Raman scattering. The IR absorption spectra of the two compounds CP1 (Figure S1) and CP2 (Figure S2) are similar, each shows clearly the bands corresponding only to vibrations of the 1,3-cyclohexanebis(methylammonium) cation, because the vibrational modes of ZnCl42- or CdI42- anions appear below 400 cm-1 in IR spectrum. Therefore, the following IR analysis for the CP1 compound equally applies to the CP2 other compound.
Tentative attribution of the observed absorption bands are based on comparisons with the free organic molecule 1,3-cyclohexanebis(methylamine) and previously reported homologous compounds[35].
The high-frequency region between 4000-2000 cm-1in the spectrum is characterized by OH, NH and CH stretching modes, harmonics and combination bands.
Water molecules vibration modes: Three bands are given for free water vibrational modes in the Tables of Molecular Vibrational Frequencies [36]. These frequencies are at 3756 cm-1, 3657 cm-1 and 1595 cm-1 for the asymmetric stretching (as), symmetric stretching (s) and the bending (scissoring) (ip) modes, respectively. The observed FTIR bands at 3487 cm-1 and 3422 cm-1for CP1 and at 3514 cm-1and 3438cm-1for CP2 are attributed respectively to asymmetric and symmetric stretching vibrations (O-H) of the crystallization water.As O-H of the crystallization H2O is involved in hydrogen bonds OH···N and OH···Cl(I), the wave numbers of these modes are shifted towards lower values.The (H2O) deformation mode is determined at 1608 cm-1 for CP1and at 1595cm-1for CP2 in th eFT-IR spectra.
Vibration of NH3+ groups: The amino groups of 1,3-cyclohexanebis(methylamine) are protonated. The NH3+ group may vibrate mainly in three different modes namely: asymmetric stretching (as), symmetric stretching (s) and the in plane bending mode (ip). The observed FTIR bands at 3133 cm-1 and 3077 cm-1 for CP1 (Fig. S1) and those observed at 3153 cm-1 and 3028 cm-1 for CP2 (Fig. S2) are assigned to asymmetric and symmetric NH3+ stretching, respectively. These frequencies of the stretching modes of NH3+ groups in the title compounds are lower compared to -NH2 frequencies (as = 3371cm-1, s = 3292cm-1) in 1,3-cyclohexanebis(methylamine). The most reasonable reason for this difference is the modification of NH2 chemical environment as a result of N-H···Cl(I) and N-H···O hydrogen bonds formation. The NH3+ in plane bending vibration is found to be at 1502 cm-1for CP1 and 1489 cm-1 for CP2.
- Vibration of CH2 group: The CH2 group may vibrate in different fashions namely: asymmetric stretching, symmetric stretching and the in plane bending mode: In this instance, the CH2asymmetric and symmetric stretch vibrations appear respectively at 2922 cm-1 and 2879 cm-1 for CP1 and at 2921 cm-1 and 2854cm-1 for CP2. The band located at 1450cm-1(Fig. S1) and that located at 1441 cm-1 (Fig. S2)are due to the(-CH2-) scissor mode, while the bands observed at 1439 cm-1and1390cm-1(Fig. S1) and those observed at 1434 cm-1 and 1392 cm-1 (Fig. S2) are ascribed to asymmetric and symmetric deformation modes of the (-CH2-), respectively.
- Harmonics and combination bands: The bands situated in the 25502000 cm-1 region are ascribed to the harmonics and combination bands.
- Vibrations of C-N and C-C groups: The medium intensity absorption band observed at 1137 cm-1for CP1 and that observed at 1125cm-1for CP2 correspond to the stretching vibration mode of C–N, while the stretching vibration mode of C–C bond is not detected on the IR spectra because of the low intensity of the corresponding absorption band.
3.4.3. Raman spectra
The presence of a heavy metal atom in a tetrahedral coordination such as ZnCl42- or CdI42-, gives rise to low frequency vibration modes which appear below 350 cm-1 in Raman spectrum.
Typically, the ideally tetrahedral MX4 (ZnCl42-, CdI42-) generate nine basal vibrations, as follows; 1(A1) and 3(F2) correspond to M-X (Zn-Cl, Cd-I) stretching vibrations, 2(E) and 4(F2) correspond to X-M-X (Cl-Zn-Cl, I-Cd-I) deformation vibrations. In the crystal, the ideally tetrahedral MX4 (ZnCl42-, CdI42-) is deformed and as a result there is a splitting on the vibrational modes of E and F2 symmetries.
Figure 8 shows the Raman spectrum of (CBMH2)ZnCl4.2H2O. According to previous studies and by comparison with the Raman spectrum of analogous compounds containing ZnCl42-, we may propose a tentative attribution of the observed bands [14, 37]. The bands observed at 244 and 276 cm-1 can be allocated to the symmetric and asymmetric Zn-Cl stretching modes of ZnCl42-. The bands at 137 cm-1 and 176.5 cm-1 can be assigned to the symmetric and asymmetric bending mode of Cl-Zn-Cl bonds.
Figure 9 shows The Raman spectrum of (CBMH2)CdI4.2H2O. The bands at 115.4 and 125.3 cm-1 can be attributed to the symmetric and asymmetric Cd-I stretching modes that are in good agreement with other tetrahedral compound with CdI42-[30, 38]. The band at 54 cm-1 can be assigned to the I-Cd-I deformation mode.
As is clear from the above discussion, these FT–IR and Raman spectroscopic results were coherent with the single-crystal X-ray results.