Received 16 March 2015; accepted 10 April 2015; published 13 April 2015

ABSTRACT

The chemical preparation, crystal structure and spectroscopic characterization of a novel organic- inorganic hybrid material, bis(4-dimethylaminopyridinium) tetrachlorocuprate, have been reported. This compound crystallizes in the monoclinic system in space group C2/c and cell parameters a = 12.4356 (18), b = 12.0901 (17), c = 14.094 (2) Å, β = 115.303 (2)˚, Z = 4 and V = 1915.8 (5) ų. In the title salt, (C₇H₁₁N₂)₂CuCl₄, both 4-dimethylaminopyridinium cations are protonated at their pyridine N atoms. The geometry of the ions is intermediate between tetrahedral and square planar. The atomic arrangement can be described by an alternation of inorganic layers built up by tetrachlorocuprate anions and organic layers formed by 4-dimethylaminopyridinium cations. The organic layers are located in sandwich between the inorganic layers. The anionic and cationic layers are held together by N-H・・・Cl and C-H・・・Cl hydrogen bonds into a three-dimensional network. The individual cations are π-π stacked with their neighbors at a distance of 3.7622 (5) Å. The vibrational absorption bands were identified by infrared spectroscopy and DFT calculations allowed their attribution.

Keywords:

Crystal Structure, 4-Dimethylaminopyridine, Hydrogen Bonds, π-π Interactions, DFT Calculations

1. Introduction

Non-covalent interactions, such as hydrogen bonding and π-π stacking, play a very prominent role in the organization of structural units in both biochemistry and material science [1] - [3] . These weak interactions exercise important effects on the architecture and properties of many materials in various fields such as biology [4] [5] and crystal engineering [6] [7] . The control of these interactions will be important for the design of materials with specific types of properties such as conductivity or magnetic behavior [8] . Several works were focused on copper (II) halides to study the influence of the organic cation features on the packing interactions that govern the structural organization and consequently on the properties of this kind of complex [9] [10] . Recently, some studies have examined how metal halide oligomers can be tied together via pyridinium derivative cations into chains, layers, etc. [11] . It is well-know that organic-inorganic materials with pyridine and its derivatives as template agents have led to the preparation of some materials with interesting physical properties [12] - [16] and biological activities [17] - [19] .

In connection with studies of the structural aspect of halo-metal anion salts containing pyridine derivatives [20] - [23] , we herein report the crystal structure of (C₇H₁₁N₂)₂CuCl₄ and its IR spectroscopic characterization.

2. Experimental Procedure

2.1. Chemical Preparation

CuCl₂∙2H₂O (1 mmol; 0.17 g) was added to a 4-dimethylaminopyridine (1 mmol; 0.12 mL) solution dissolved in 15 mL of absolute ethanol. The resulting solution was then acidified with 1 mL concentrated HCl and allowed to evaporate slowly at room temperature. Yellow block crystals were formed after 5 days (yield, 78.7%).

2.2. Investigation Techniques

2.2.1. X-Ray Single Crystal Structural Analysis

Single crystals were carefully selected under a microscope and mounted on a Mitegen micromesh mount with the help of a trace of mineral oil. X-ray diffraction data were collected at 100 K on a Bruker Smart APEX CCD area-detector diffractometer using the ω scan technique with MoKα radiation (λ = 0.7107 Å). Data were collected, the unit cells determined, and the data integrated and corrected for absorption and other systematic errors using the Apex2 suite of programs [24] . SHELXS-97 [25] was used to solve the structures using direct methods and SHELTXL6.14 [26] and SHELXLE [27] were used for refinement. The packing diagrams were generated using the Mercury 2.3 package [28] . Crystal data and experimental parameters used for the intensity data collection are summarized in Table 1.

2.2.2. Infrared Spectroscopy

The IR spectrum was recorded in the range 4000 - 400 cm⁻¹ with a “Perkin-Elmer FTIR-1000” spectrometer using samples dispersed in spectroscopically pure KBr pressed into a pellet.

2.2.3. DFT Calculations

The infrared spectrum was calculated with the Gaussian 09 software by assuming that in the 500 - 4000 cm⁻¹ range all bands were due to the cation. The positions of the protons were optimized at the B3LYP/6-311++G** level of theory and the infrared spectrum was then calculated with the same method. No imaginary frequency was found showing the accuracy of these calculations. For plotting the spectrum a line broadening of 10 cm⁻¹ was applied.

3. Results and Discussion

3.1. Crystal Structure

The structure of the title compound consists of discrete anions and 4-dimethylaminopyridinium cations (Figure 1). Only the pyridinium N atom is protonated, while the amine group remains unprotonated, resulting in a charge of +1 for the organic cation. Thus, to ensure charge equilibrium, the structure associates each tetrachlorocuprate anion with two 4-dimethylaminopyridinium cations. The copper ion lies on a special position (Table 2) and it has a coordination geometry intermediate between tetrahedral and square planar with the trans-

bond angles of 141.19 (1)˚ for both Cl₂-Cu₁- and -Cu₁-Cl₁ (i = −x + 1, y, −z + 1/2) (Table 3). The Cu-Cl bond distances are 2.2491( 3) and 2.2562 (4) Å. These bond distances and angles of anion are in agreement with those found in other compounds [29] .

Examination of the crystal structure of the title salt (Figure 2) reveals that the atomic arrangement can be described by inorganic layers parallel to (a, b) plane of anions alternated with layers of 4-dimethylami- nopyridinium cations. The organic layers are arranged in sandwich between the inorganic ones. Figure 3 shows that the inorganic layers are located at z = (2n + 1)/4, while the organic ones are situated at z = n/2. No significant inter-stack Cl・・・Cl interactions was observed (the shortest Cl・・・Cl contact is 4.459 (1) Å).

Anions and cations are linked by a network of N-H・・・Cl and C-H・・・Cl hydrogen bonds into a three dimensional network. Among these hydrogen bonds, one is bifurcated N₁-H₁A・・・(Cl₁, Cl₂) (Table 4).

The geometric features of the organic cations are given in Table 3. The values of the bond lengths and angles are similar to those observed in bis(4-dimethylaminopyridinium) tetrachlorozincate [30] . The sum of the angles around the N(2) nitrogen atom is close to 360˚. The C₃-N₂ bond distance of the NH₂ group is 1.341 (1) Å, which is short for a C-N single bond, but still not quite as contracted as one would expect for a fully established C=N double bond. This C-N bond length distance value is consistent with an imino resonance form as it is commonly found for a C-N single bond involving sp² hybridized C and N atoms [31] . The shortening of the C-N_amino bond distance is probably due to the delocalization of the ring π density with the p-orbital electrons of the

Symmetry code: (i) −x + 1, y, −z + 1/2.

Symmetry codes: (ii) x − 1/2, y + 1/2, z; (iii) x, −y + 1, z + 1/2; (iv) x − 1/2, −y + 3/2, z + 1/2; (v) ?x + 1/2, y + 1/2, −z + 1/2.

amino group. Consequently, the basicity of the N₂ nitrogen atom decreases, which is in favor of the protonation of the N₁ nitrogen atom of the pyridinic ring. Moreover, the C-N-C angles of the pyridine are very sensitive to protonation [32] [33] . A pyridinium cation always possesses an expanded C-N-C angle in comparison with the parent pyridine. The angle C₁-N₁-C₅ of 120.83 (10)˚ is typical for the protonated forms of dimethylpyridine reported in many structures [34] - [52] .

As it can be seen clearly from Figure 4 and Table 5, the two neighboring anti-parallel organic cations, which are not connected by hydrogen bonding, are stacked each other by turns in a face to face mode. The centroid- centroid distance is 3.7622 (5) Å, less than 3.8 Å, the maximum value accepted for π-π interactions [53] .

3.2. IR Spectroscopy

FTIR spectroscopy was used to identify the functional groups present in the crystal. The infrared absorption spectrum of (C₇H₁₁N₂)₂CuCl₄ is shown in Figure 5. A detailed assignment of all bands observed in the infrared spectrum of the 4-dimethylaminopyridinium cation in the title compound is based on the comparison with other compounds associated to the same cation [54] [55] . In the domain of high frequencies, the bands at 3447, 3195, 3112, 2964 cm⁻¹ are due to the stretching vibration frequencies of the N-H and C-H bonds. The band at 1644 cm⁻¹ is assigned to the N-H bending mode. The bands at 1571 and 1452 cm⁻¹ are attributed to the C=C and C=N stretching modes of the pyridine ring. The absorption band located at 1217 cm⁻¹ corresponds to the n (C-N) and

Symmetry codes: (i) 1/2 − x, 1/2 − y, −z. Cg1 is the centroid of N₁-C₅ ring. d(CgI…CgJ) is the distance between the centroids of rings. α is the dihedral angle between planes I and J. β is the angle between the vector CgI à CgJ and the normal to plane I. γ is the angle between the vector CgI àCgJ and the normal to plane J. Dist_perp is the interplanar distance between the parallel rings I and J.

n (C-C) modes. The band at 1070 cm⁻¹ can be attributed to the d (C-C) mode. The remaining bands in the range 1000 to 500 cm⁻¹ are assigned to γ (C-C), γ (C-H) and γ (C-N) out-of-plane bending modes.

DFT calculations showed that the inorganic entities lead to vibrations below 500 cm⁻¹ which are not observed experimentally in our conditions. So we focused only on the vibrations of the organic cation. X-ray diffraction data give C-H or N-H distances which are too small compared to what is usually observed (typically below 0.1 nm) due to the fact that this method is not sensitive to the nuclei but to the electrons and so gives values corresponding to distances between the barycenters of electronic charges. As a consequence, the positions of protons were first optimized, the C and N atoms being located at the positions given by the X-ray study. The resulting C-H and N-H distances corresponded to what is usually obtained (typically 0.109 nm for C-H and 0.104 nm for N-H) and the frequencies calculation was made on this semi-optimized geometry. The resulting IR spectrum, calculated by the B3LYP/6-311++G** method, is plotted on Figure 6 and is very similar to the experimental spectrum allowing a good correlation between the experimental and theoretical data as shown on Figure 7. Thus, the precision is well-sufficient to assign the experimental frequencies and to confirm the attributions proposed above.

4. Conclusion

Bis(4-dimethylaminopyridinium) tetrachlorocuprate was synthesized and its structure is reported for the first time in the literature. In the atomic arrangement, the organic layers, built up by the 4-dimethylaminopyridinium cations, are arranged in sandwich between the inorganic layers. Intermolecular N-H・Cl and C-H・Cl hydrogen bonds as well as π-π stacking interactions contribute to the structure cohesion. The vibrational absorption bands were identified by infrared spectroscopy and DFT calculations allowed their attributions.

Supplementary Data

Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC No 1047063. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK: fax: (+44) 01223-336-033; e-mail: deposit@ccdc.cam.ac.

Acknowledgements

We would like to acknowledge the support provided by the Secretary of State for Scientific Research and Technology of Tunisia. The X-ray diffractometer was funded by NSF Grant 0087210, Ohio Board of Regents Grant CAP-491, and by Youngstown State University of USA. A special thank you is needed for Dr. Matthias Zeller, who has helped in collecting and processing X-ray crystallography data.

References