Crystal structure, quantum mechanical investigation

Journal of Molecular Structure 1161 (2018) 486e496

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Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc

Crystal structure, quantum mechanical investigation, IR and NMR spectroscopy of two new organic perchlorates: (C6H18N3)$(ClO4)3H2O (I) and (C9H11N2)$ClO4(II) I. Bayar a, L. Khedhiri a, S. Soudani a, F. Lefebvre b, V. Ferretti c, C. Ben Nasr a, * Laboratoire de Chimie des Mat eriaux, Facult e des Sciences de Bizerte, Universit e de Carthage, 7021 Zarzouna, Tunisie Laboratoire de Chimie Organom etallique de Surface (LCOMS), Ecole Sup erieure de Chimie Physique Electronique, 69626 Cedex, Villeurbanne, France c Department of Chemical and Pharmaceutical Sciences and Center for Structural Diffractometry, via Fossato di Mortara 17, I-44121 Ferrara, Italy a

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 December 2017 Received in revised form 6 February 2018 Accepted 8 February 2018 Available online 10 February 2018

The reaction of perchloric acid with 1-(2-aminoethyl)piperazine or 5,6-dimethyl-benzimidazole results in the formation of 1-(2-amonioethyl)piperazine-1,4-dium triperchlorate hydrate (C6H18N3)$(ClO4)3$H2O (I) or 5,6-dimethyl-benzylimidazolium perchlorate (C9H11N2)$ClO4 (II). Both compounds were fully structurally characterized including single crystal X-ray diffraction analysis. Compound (I) crystallizes in the centrosymmetric triclinic space group P 1 with the lattice parameters a ¼ 7.455 (2), b ¼ 10.462 (2), c ¼ 10.824 (2) Å, a ¼ 80.832 (2), b ¼ 88.243 (2), g ¼ 88.160 (2) , Z ¼ 2 and V ¼ 832.77 (3) Å3. Compound (II) has been found to belong to the P21/c space group of the monoclinic system, with a ¼ 7.590 (3), b ¼ 9.266 (3), c ¼ 16.503 (6) Å, b ¼ 107.38 (2) , V ¼ 1107.69 (7) Å3 and Z ¼ 4. The structures of (I) and (II) consist of slightly distorted [ClO4]- tetrahedra anions and 1-(2-amonioethyl)piperazine-1,4-dium trication (I) or 5,6-dimethyl-benzylimidazolium cations (II) and additionally a lattice water in (I). The crystal structures of (I) and (II) exhibit complex three-dimensional networks of H-bonds connecting all their components. In the atomic arrangement of (I), the ClO 4 anions form corrugated chains, while in (II) the atomic arrangement exhibits wide pseudo-hexagonal channels of ClO4 tetrahedra including the organic entities. The lattice water serves as a link between pairs of cations and pairs of anions via several OeH/O and N-H/O interactions in compound (I). The vibrational absorption bands were identified by infrared spectroscopy. These compounds were also investigated by solid-state 13C, 35Cl and 15N NMR spectroscopy. DFT calculations allowed the attribution of the IR and NMR bands. Intermolecular interactions were investigated by Hirshfeld surfaces. Electronic properties such as HOMO and LUMO energies were derived. © 2018 Elsevier B.V. All rights reserved.

Keywords: Single crystal X-ray diffraction analysis Solid-state NMR Hirshfeld surface DFT calculations

1. Introduction Recently, much attention has been devoted to simple molecularionic crystals containing organic cations and acid radicals due to the tenability of their special structural features and their interesting physical properties [1e3]. In these compounds, there are several types of intermolecular interactions, including electrostatic forces, stacking and hydrogen-bonding interactions. The H-bonds play an important role in the construction of three-dimensional architectures and the stabilization of supramolecular crystal structures. Organic perchlorates resulting from the interaction between

* Corresponding author. E-mail address: [email protected] (C. Ben Nasr). https://doi.org/10.1016/j.molstruc.2018.02.038 0022-2860/© 2018 Elsevier B.V. All rights reserved.

perchloric acid and organic molecules, such as amines, amino alcohols and amino acids represent a class of high-performance materials due to their interesting physicochemical properties [4,5]. Piperazine and its derivatives attracted much attention during recent years due to their crucial role in many pharmaceutical industries [6]. Piperazine derivatives are found in biologically active compounds across a number of diverse therapeutic areas such as antifungal, antibacterial, antimalarial, antipsychotic, antidepressant and antitumor activity against colon, prostate, breast, lung and leukemia tumors [7,8]. On the other hand, imidazole and its derivatives display favorable physical properties such as high conductivities, solvation ability and wide range of Lewis acidity which, in addition to electrochemical applications [9], allow for their use in a variety of contexts such as cellulose dissolution [10], metal-free

I. Bayar et al. / Journal of Molecular Structure 1161 (2018) 486e496

catalysis [11], lubricants [12] and for CO2 separations [13], to name just a few. As a contribution in this field and in order to enhance the varieties of materials and to examine the influence of hydrogen bonds on the chemical and structural features, we report herein the chemical preparation and crystal structure of two new organic perchlorates, 1-(2-amonioethyl)piperazine-1,4-dium triperchlorate hydrate (C6H18N3)$(ClO4)3$H2O (I) and 5,6-dimethyl-benzylimidazolium perchlorate (C9H11N2)$ClO4 (II).

Table 1 Crystal data and structure refinement of (C6H18N3)$(ClO4)3$H2O (I) and (C9H11N2)$ ClO4 (II).

2. Experimental 2.1. Materials

Empirical formula

C6H18N3$3(ClO4)$H2O (I)

C9H11N2$ClO4 (II)

Formula weight [g mol1] Crystal color, habit Crystal temperature [K] Crystal size [mm] Radiation, wavelength [Å] Crystal system Space group

448.60

246.65

Plate, colorless 295

Prism, colorless 293

0.50 0.24 0.15 MoKa, 0.71073

0.55 0.35 0.31 MoKa, 0.71073

Triclinic

Monoclinic P21/c

Unit-cell dimensions:

1-(2-aminoethyl)piperazine (Sigma-Aldrich), 5,6-dimethylbenzimidazole (Sigma-Aldrich), HClO4 (70%, Aldrich) were obtained from commercial sources and used as received. Volume [Å3] Z Density calc. [g cm3] Reflections for cell determination q -range for cell determination [ ] Absorption coefficient m [mm1] F (000) q -Range for data collection [ ] Limiting indices

2.2. Chemical preparation The compounds (C6H18N3)$(ClO4)3$H2O (I) and (C9H11N2)$ClO4 (II) were obtained by slow evaporation, at room temperature of an aqueous solution of perchloric acid HClO4 and the corresponding amine, 1-(2-aminoethyl)piperazine (I) or 5,6-dimethyl-benzimidazole (II) in the stoichiometric ratio 3:1 (I) and 1:1 (II). The solutions were stirred for 15 min and allowed to stand at room temperature. Single crystals having the form of sticks appeared after a few days which could be subjected to X-ray diffraction analysis. The products were then filtered off and washed with a small amount of distilled water. Schematically the reactions can be written:

C6H15N3 + 3HClO4

C9H10N2 + HClO4

H2O

(C6H18N3)(ClO4)3. H2O

H2O

(C9H11N2)ClO4

Reflections collected/unique Refinement method Data, restrains, parameters (I > 2 s) Goodness-of-fit on F2 R indices (all data, on F2)

Dr(min, max) [e Å3]

C6H15N3 + 3HClO4

H2O

C9H10N2 + HClO4

(C6H18N3)(ClO4)3. H2O

H2O

(C9H11N2)ClO4

The entire molecular structures of the obtained salts (I) and (II) are: H

H

NH+ 33.ClO ClO- -..H 2O 4 H2O

(I)

NH+

N

+

ClO4

(II)

N NH3+

H

2.3. Investigation techniques 2.3.1. X-ray single crystal structural analysis Suitable crystals of (I) and (II) were selected and mounted on a Nonius Kappa CCD diffractometer and a Bruker APEX2 CCD areadetector, respectively, using Mo radiation (l ¼ 0.71073 Å). The intensities of I were collected at 295 K, integrated using the DenzoSMN package [14] and corrected for Lorentz-polarization and absorption effects [15]. The intensities of (II) were collected at 292 K,

487

P1 a ¼ 7.4553 (2) Å b ¼ 10.4624 (2) Å c ¼ 10.8238 (2) Å a ¼ 80.8320 (15) b ¼ 88.2430 (15) g ¼ 88.1600 (16) 832.77 (3) 2 1.789

a ¼ 7.5902 (3) Å b ¼ 9.2660 (3) Å c ¼ 16.5030 (6) Å

b ¼ 107.378 (2) 1107.69 (7) 4 1.479

25

25

8e10

8e10

0.62

0.35

464 2e28

512 2.8e27.7

9 h 9 13 k 13 14 l 14 (Rint ¼ 0.037)

9 h 9 12 k 12 21 k 21 (Rint ¼ 0.024)

Full matrix least-squares on F2 3470, 11, 259

Full matrix least squares on F2 2308, 24, 147

1.09 R ¼ 0.048, wR ¼ 0.144

1.09 R ¼ 0.060, wR ¼ 0.189 0.58 and 0.81

0.81 and 0.74

data collection, reflection indexing, unit-cell parameters refinement, Lorentz-polarization correction, peak integration and background determination were carried out with the CrysalisPro software [16]. An analytical absorption correction was applied using the modeled faces of the crystal [17]. Both structures were solved by direct methods with SIR97 [18] and the least-square refinement on F2 was achieved using SHELXL-97 [19] implemented in the WINGX system of programs [20] for I and the CRYSTALS software [21] for II with all non-hydrogen atoms anisotropic. All the hydrogen atoms were situated in geometrically optimized positions and treated as riding atoms, apart from those bound to N/O atoms in I that were found in the difference Fourier map and refined isotropically with restrained distances. The drawings were made with Diamond [22]. Experimental details, crystallographic and processing data are reported in Table 1. Crystallographic data for the structural analysis have been deposited at the Cambridge Crystallographic Data Center, CCDC No 1551511 for (I) and CCDC No 1551513 for (II). 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: (þ44) 01223-336-033; e-mail: [email protected]. 2.3.2. NMR and IR measurements The 13C NMR spectra were recorded on a solid-state high-resolution Bruker Avance-300 spectrometer operating at 75.47 MHz.

488


The 15N and 35Cl NMR spectra were recorded on a Bruker Avance500 spectrometer operating at 50.67 MHz for 15N and 49.00 MHz for 35Cl. In all cases a classical 4 mm probehead allowing spinning rates up to 10 KHz was used. 13C and 15N NMR chemical shifts are given relative to tetramethylsilane and neat nitromethane, respectively (precision 0.5 ppm). The spectra were recorded by use of cross-polarization (CP) from protons (contact time 2 ms) and magic angle spinning (MAS). Before recording each spectrum it was checked that there was a sufficient delay between the scans allowing a full relaxation of the protons (typically 10 s). The 35Cl NMR spectra were recorded by use of a single pulse of 0.5 ms (corresponding to p/12). Typically 50000 to 100000 scans were accumulated with a recycle time of 1 s. The chemical shifts are given relative to aqueous NaCl. FT-IR analysis was carried out at room temperature in the range 400e4000 cm1 using a NICOLET IR 200 FT-IR infrared spectrometer. 3. Results and discussion 3.1. X-ray diffraction study Details of data collection, refinement and crystallographic parameters of (I) and (II) are summarized in Table 1 whereas selected hydrogen-bond geometries are given in Table 2. A perspective view of an asymmetric unit of (C6H18N3)$(ClO4)3$H2O (I), built of one 1(2-ammoniumethyl)piperazin-1-ium cation, three perchlorate anions (ClO 4 ) and one water molecule, is depicted in Fig. 1, while the complete unit-cell content is shown in Fig. 2. In this atomic arrangement, there is one independent organic group in which all nitrogen atoms are protonated then corresponding to a trivalent [C6H18N3]3þ cation. The conformation of the piperazinedium sixmembered ring can be described in terms of Cremer and Pople puckering coordinates [23], through the parameters Q (total puckering amplitude), q2, q3, q and f. The calculated values for the N1eC2eC1eN2eC4eC3 ring, Q ¼ 0.578 Å, q2 ¼ 0.029 Å, q3 ¼ 0.577 Å, q ¼ 2.94 , correspond to the most stable chair conformation. In the organic moiety, NeC/CeC distances and

Fig. 1. View of the asymmetric unit of (C6H18N3)$(ClO4)3$H2O (I). Displacement ellipsoids are drawn at the 40% probability level and H atoms are shown as small spheres of arbitrary radii.

Table 2 Hydrogen-bond geometry (Å, ) for (C6H18N3)$(ClO4)3$H2O (I). and (C9H11N2)$ClO4 (II). DdH$$$A

DdH

(C6H18N3)·(ClO4)3·H2O (I) N1eH1N/O12 0.88 C4eH4B/O11 0.97 O1WeH2W/O9 0.88 O1WeH2W/O12 0.88 i N2eH3N/O1 0.87 C3eH3B/O11ii 0.97 N3eH4N/O5ii 0.90 N3eH4N/O1Wii 0.90 C2eH2A … O6ii 0.97 ii C2eH2B/O2 0.97 iii N2eH2N/O3 0.88 iii C1eH1B/O6 0.97 N3eH6N/O9iv 0.89 C4eH4A … O11v 0.97 N3eH5N/O5vi 0.89 N3eH5N/O8vi 0.89 (C9H11N2)·ClO4(II) i N1eH1A$$$O1 0.86 N2eH2/O4ii 0.86 C1eH1A$$$O2i 0.93 C1eH1A$$$O1iii 0.93 C8eH8B/O4ii 0.96

(3) (5) (5) (3) (2) (2)

(2) (2) (4) (4)

H$$$A

D$$$A

2.06 2.52 2.29 2.41 2.15 2.55 2.29 2.12 2.59 2.53 2.14 2.53 2.12 2.61 2.19 2.38

2.837 3.456 3.050 3.188 2.893 3.422 2.896 2.811 3.417 3.369 2.894 3.395 2.952 3.535 2.994 3.100

2.00 2.00 2.54 2.50 2.62

(3) (6) (6) (3) (3) (3)

(3) (2) (4) (5)

2.848 2.781 3.151 3.379 3.498

DdH$$$A (3) (4) (4) (4) (3) (4) (3) (4) (6) (3) (3) (5) (3) (4) (4) (4)

148 161 144 148 142 149 125 132 143 144 144 147 156 159 149 136

(3) (4) (4) (3) (2) (2)

(2) (2) (3) (3)

169 150 124 157 157

Equivalent positions: (i) -x,-y,-z; (ii) xþ1,y,z; (iii) 1-x,-y,-z; (iv) 1-x,1-y,-z-1; (v) 1x,1-y,-z; (vi) 1-x,-y,-z-1.

Fig. 2. Projection of the (C6H18N3)$(ClO4)3$H2O (I) structure on the (b, c) plane.

NeCeC/CeNeC angles, spreading within the respective ranges 1.482 (3) - 1.516 (3) Å and 108.96 (2) 113.50 (2) , are similar to those observed in other compounds with the same amine such as in [1-(2-ammoniumethyl) piperazinium] sulfate [24]; moreover, the torsion angles are comparable to the mean values calculated for other perchlorates with piperazinium cations [25,26]. Projection of the crystal structure of (C6H18N3)$(ClO4)3$H2O (I) along the an axis (Fig. 2) shows the organization of the different molecular moieties; the inorganic entities (i.e. ClO 4 anions and water molecules) form corrugated chains around the x ¼ 1/2 planes (Fig. 3), with the organic groups located in between. The inorganic/ organic layers interact mainly through N/OeH/O hydrogen bonds, in which all the OeH/NeH hydrogens are involved; out of them, three are bifurcated: O1WeH2W … (O9, O12), N3eH4N … (O5, O1W), N3eH5N … (O5, O8) Moreover, some weaker CeH/O


489

Fig. 3. Projection along the c axis of the inorganic chains in (C6H18N3)$(ClO4)3$H2O (I). A polyhedral representation is used for ClO4.

Fig. 5. Projection along the an axis of the atomic arrangement of (C9H11N2)$ClO4 (II). Dotted lines represent the various hydrogen bond interactions.

Fig. 4. View of the asymmetric unit of (C9H11N2)$ClO4 (II). Displacement ellipsoids are drawn at the 40% probability level and H atoms are shown as small spheres of arbitrary radii.

interactions contribute to the crystal robustness. (Fig. 2, Table 2). Compound (II) crystallizes in the centrosymmetric monoclinic space group P21/c with four formula units in the unit cell (Z ¼ 4). The crystal structure of (II) has an asymmetric unit that includes one crystallographically ClO 4 anion and one 5,6-dimethyl-benzylimidazolium cation (Fig. 4) to perform the electric neutrality of the total structure. The atomic arrangement exhibits wide pseudo-hexagonal channels of ClO4 tetrahedra including the organic entities (Fig. 5). These organic cations are themselves connected to the inorganic network through NeH/O and CeH/O hydrogen bonds. Fig. 6 shows clearly the disposition of the organic and inorganic parts in the structure. The geometrical features of the various hydrogen bonds are given in Table 2. Examination of the organic entity geometry shows that the atoms N1, C1, N2, C2, C3 of the imidazole group and the atoms C2, C3, C4, C5, C6, C7 of the phenyl ring have a good coplanarity. The bond lengths and angles do not show any unusual values, the mean values of CeC and CeN bond lengths are 1.513 and 1.491 Å respectively, which are between those of a single bond and a double bond and agree with literature. In the atomic arrangement of (I) and (II), the ClO 4 groups exhibit a compact assembly of oxygen atoms in which the chlorine atom shows a slight displacement from the center of gravity of the tetrahedron; this displacement doesn't exceed 0.08 Å (Table 3). The CleO distances range from 1.402 (2) to 1.428 (2) Å. The calculated average values of the distortion indices corresponding to the different angles and distances in the ClO4 tetrahedra, DI(OClO)ee, DI(ClO), DI(OO) exhibit a pronounced distortion of the OClO angles if compared to OO and ClO distances (Table 3). It could be of interest to compare the packing features of the

Fig. 6. Pseudo-hexagonal channels of ClO4 tetrahedra in (C9H11N2)$ClO4 (II). Projection along the c-axis of corrugated organic entities in (C9H11N2)$ClO4 (II).

present structure with that of a related structure recently published, i.e. the perchlorate salt of 3-chloroaniline [27]. In all cases the most important hydrogen bonding interactions involve the oxygens of the perchlorate anion and the protonated nitrogens of the cations, the longest N/O distances being associated to NHþ 3 … ClO 4 interactions due to the formation of bifurcated/trifurcated hydrogen bonds. The 3D-arrangement of perchlorate anions in the chloroanilinium-perchlorate salt leads to the formation of organic/ inorganic parallel layers, not observed in the present structures. The differences among the three crystalline architectures can be mainly ascribed to two reasons: the presence of a different number of hydrogen bonding donors (in compound I, for instance, there is a cocrystallized water molecule not present in the other structures) and the different size and shape of the organic anions.

490


Table 3 Calculated average values of the distortion indices corresponding to the different angles and distances in the ClO4 tetrahedra for compounds I and II. Compound

Cl-Om (Å)

O-Om (Å)

OeCl-Om ( )

ID (ClO)

ID (OO)

ID (OClO)

d(Å)

(I)

1.428 1.402 1.428 1.404

2.332 2.286 2.333 2.29

109.45 109.39 109.46 109.36

0.0036 0.02 0.003 0.017

0.0037 0.0118 0.003 0.018

0.0076 0.033 0.005 0.0289

0.019 0.078 0.011 0.07

(II)

3.2. Hirshfeld surfaces and fingerprint plot analysis of the intermolecular hydrogen bond interactions for both compounds (I) and (II) Recently, a method for studying crystal structures has been developed and involves calculation of molecular surfaces based on Hirshfeld's stockholder partitioning [27]. The Hirshfeld surface plots are generated using the CrystalExplorer software [28] and represent the partition into molecular fragments of the total crystalline electron density. One of the structural properties that can be mapped onto the Hirshfeld surface is a normalized measure of the distance between atoms which uses the van der Waals radii (rvdW) of the nearest atom inside the surface and the nearest atom outside. The dnorm property is a recently developed addition to the functions that can be displayed on the Hirshfeld surface by mapping dnorm, a normalized measure of the distance between atoms which uses the van der Waals radii (rvdW) of the nearest atom inside the surface and the nearest atom outside, with a color scale from blue (contacts longer than vdW separation), through white (around the vdW separation) to red (shorter than vdW separation). Another tool has been developed to allow easy comparison of structures by condensing the distance information calculated into a twodimensional plot of de, the distance to the nearest atom outside the molecule, against di, represents the distance from the surface to the nearest atom in the molecule itself. This plot is a histogram of intermolecular contact distances and provides a ‘fingerprint’ of the crystal packing pattern. Besides, the enrichment ratios (E) are the ratios between the actual contact surfaces and those computed as if all contacts were equiprobable [29e31]. Then, an enrichment ratio larger than unity reveals that a pair of elements has a high propensity to form contacts in crystals, while pairs which tend to avoid contacts with each other should yield an E value lower than unity [31]. The three-dimensional Hirshfeld surfaces and two-dimensional fingerprint plots of (I) are shown in Figs. 7 and 8, respectively. Intermolecular interactions were analyzed around the asymmetric unit for compound (I). The red spots on the surface represent shorter contacts to neighboring molecules and they refer to the hydrogen bond interactions O/H/H/O between the molecules inside the surface and the molecules which surround it. Hirshfeld surface two-dimensional fingerprint plots for the compound (I), showing the (a) O/H/H/O, (b) O/O and (c) H/H contacts. The contributions of the interactions to the crystal packing amount to 82, 10.4 and 7.6%, respectively. The de (y axis) and di (x axis) values are the closest external and internal distances (Å) from given points on the Hirshfeld surface contacts (Fig. 8). The contribution of the O/H/H/O intermolecular interactions for the crystal structure cohesion amounts to 82% (Fig. 8a) and these contacts are attributed to NeH/O, CeH/O and OeH/O hydrogen-bonding interactions and appear as two sharp symmetric spikes (1) in the twodimensional fingerprint maps with a prominent long spike at de þ di ~ 2 Å (Fig. 8a). They have the most significant contribution to the total Hirshfeld surface (82%) and this type appear with high enrichment EO … H ¼ 1.64. So, the oxygen and hydrogen atoms are often mutual partners in the crystal contacts and they are electrostatically favorable due to the partial positive charge, dþ of H

Fig. 7. (a)Hirshfeld surface displayed using the (dnorm) property in the transparent mode of compound (I). (b) Hirshfeld surface shown on the right with neighboring molecules.

atoms. Also, this type of contacts are most frequent interactions due to the abundance of oxygen and hydrogen on the molecular surface (% SO ¼ 51.4% and % SH ¼ 48.6%) (Table 4). The O/O interaction covers 10.4% of the total surface and appear in the middle of the scattered points in the two-dimensional fingerprint maps with a single broad peak at de ¼ di ~1.5 Å (Fig. 8b). These contacts are the second most frequent interactions but they are impoverished in the crystal with a value of enrichment equal to 0.39 (Table 4). The O/O contacts are electrostatically repulsive. The fingerprint plots corresponding to the H/H contacts shows also the presence of the scattered points in the middle of the two-dimensional fingerprint maps with a single broad peak respectively at de ¼ di ¼ 1.4 Å (Fig. 8c) and which represent only 7.6%, which are impoverished and are under-represented (E ¼ 0.32). In conclusion, the three types of contacts contribute significantly to the stability of the crystal structure and the enriched O/H contacts are the driving forces in

Fig. 8. Two-dimensional fingerprint plots for compound (I) with a histogram shows the percentages of contributions of contacts.

I. Bayar et al. / Journal of Molecular Structure 1161 (2018) 486e496 Table 4 Chemical proportions on the Hirshfeld surface and enrichment ratios of compound (I). Surface (%) Major contacts Proportion (%) Enrichment (Exy/Exx)

H 48.6

O 51.4

O/H 82 1.64

O/O 10.4 0.39

Table 5 Chemical proportions on the Hirshfeld surface and enrichment ratios of compound (II). Atoms

C

H

O

Enrichment

C H O N Cl 10.45 H/O 53 H/N 2.7

4.76 0.58 0.03 5.11 0 58.75 H …. H 27.3 O/O 1.3

0.79 1.61 0.79 0 27.9 H/C 7.2 C/O 0.2

H/H 7.6 0.32

the molecular arrangement and crystal packing formation. For compound (II), the H/O contacts which appear as a pair of very sharp spikes at de þ di ¼ 2.2 Å (Figs. 9 and 10a) are the most frequent interactions due to the abundance of hydrogen and oxygen on the molecular surface (58.75% and 27.9% respectively) and they are over-represented with an enrichment ratio of 1.61 in agreement with the presence of NeH/O and CeH/O hydrogen bonds in the crystal structure. The H/H contacts amounts to 27.3% appear with a single broad peak at de ¼ di ~1.1 Å (Fig. 10b) show an enrichment ratio around 0.79 and become the second most frequent interactions. The H/C hydrogen bonds account for 7.2% of the total Hirshfeld surface and were exhibited as a pair of too wide and blunt spikes of light sky-blue color with de þ di ¼ 3.4 Å (Fig. 10c) and these contacts are under-represented (EHC ¼ 0.58) which correspond to CeH/C and CeH … p hydrogen bonds in the crystal structure (Table 5). The C/C contacts between the cationic molecules, represent only 5.2% of the Hirshfeld surface, but they are extremely enriched (enrichment Ecc ¼ 4.76) (Table 5), while the O/C contacts are

491

Surface (%) Contacts (%)

N

Cl

0.16 0 0 2.9 C/C 5.2

0 0 0 N/C 3.1

impoverished with enrichment equal to 0.03. The N/C interactions show a very high enrichment equal to 5.11 and refer to interactions between the organic cations. The contact analysis for this compound suggests that the enriched H/O, H/H, H/C and N/C hydrogen bonds are the driving forces in the molecular arrangement and crystal packing formation.

3.3. HOMO-LUMO analysis for both compounds (I) and (II) Calculation of the HOMO/LUMO energy levels are very helpful to predict the chemical behavior of the desired compound. HOMOLUMO orbitals were calculated from the crystal data by DFT with the B3LYP/6e311þþG** method for both compounds (I) and (II) using the Gaussian 09 program [32] and are displayed in Figs. 11 and 12. In the both compounds the highest occupied molecular orbital (HOMO) is located mainly on one perchlorate anion which behaves as an electron donor with calculated energy of 6.9 and 7.15 eV respectively, and the lowest unoccupied molecular

Fig. 9. (a)Hirshfeld surface displayed using the dnorm property in the transparent mode of compound (II). (b) Hirshfeld surface shown on the right with neighboring molecules.

Fig. 10. Two-dimensional fingerprint plots for compound (II) with a histogram shows the percentages of contributions of contacts.

Fig. 11. Frontier molecular orbitals (HOMO and LUMO) of compound (I).

492


Fig. 14.

Fig. 12. Frontier molecular orbitals (HOMO and LUMO) of compound (II).

orbitals (LUMO) is concentrated on the organic cation which behaves as an electron acceptor with a calculated energy of 1.67 and 2.29 eV. The large energy gaps between the HOMO and LUMO orbitals in the two compounds are 5.23 and 4.86 eV, and characterize a high kinetic stability and high chemical hardness [33,34]. Indeed, it is energetically unfavorable to add electron to a highlying LUMO or to extract electrons from a low-lying HOMO [35]. 3.4. NMR results The 13C CPeMAS NMR spectrum of (I) is shown in Fig. 13. It exhibits six resonances corresponding to the six crystallographically independent carbon atoms. This is in agreement with one organic molecule being present in the asymmetric unit cell as revealed by X-ray structure determination. The 13C CP-MAS NMR spectrum of (II) is shown in Fig. 14. In the resonance zone of the aliphatic carbons, the spectrum shows two lines relating to the two methyl groups indicating the presence of a single organic cation in the asymmetric unit of the compound. In

Fig. 13.

13

13

C CP-MAS NMR spectrum of (II).

the aromatic carbon resonance region, between 115 and 140 ppm, the spectrum contains five lines, one of which being approximately twice the intensity of the others. The signal at 4 ppm is a spinning side band. This number of NMR components, which is less than seven, also proves the present of a single organic entity in the unit cell of the compound, a result in full agreement with the crystallographic data. The 15N CP-MAS NMR spectrum of compound (I), shown on (Fig. 15), is in good agreement with the structure determined by Xray diffraction. Indeed it contains three peaks at 352.5, 366.3 and at 372.0 ppm corresponding to the three crystallographically independent nitrogen atoms, also in full agreement with the presence of a single organic entity in the asymmetric unit of the compound. The 15N CP-MAS NMR spectrum of compound (II), presented in (Fig. 16), is in good agreement with the structure determined by Xray diffraction. It exhibits two well resolved peaks at 250 and 253 ppm, corresponding to the two crystallographically independent nitrogen sites, which proves the presence of a single organic moiety in the asymmetric unit of the compound. The 35Cl MAS NMR spectrum obtained for compound (I) is displayed on (Fig. 17). It shows a signal at ca. 1000 ppm, typical of perchlorate [36]. However, the signal cannot be simulated as originating from only one quadrupolar nucleus (isotropic chemical shift 1313 ppm, CQ ¼ 1.35 MHz and etaQ ¼ 0.25) and it is necessary to add a small isotropic signal at 994 ppm. This signal arises probably

C CP-MAS NMR spectrum of (I).


Fig. 18. Experimental (a) and simulated (b)

Fig. 15.

15

N CP-MAS NMR spectrum of (I).

15

N CP-MAS NMR spectrum of (II).

35

Cl MAS NMR spectra of (II).

atoms were first optimized with the B3LYP/6e311 þþ G ** method, the other atoms being frozen. Then the absolute chemical shifts were calculated using the GIAO method. Finally, the calculated values were calibrated relative to tetramethylsilane with dexp ¼ 0 ppm. The atoms are labeled as depicted below:

(I)

Fig. 16.

493

(II)

The results are listed on Tables 6 and 7. Clearly, there is a very good agreement between the experimental and theoretical values calculated after optimization of the position of the protons, allowing unambiguously the attribution of the different NMR signals. The discrepancy between the calculated and experimental values is mainly due to the fact that an isolated molecule was taken into account. 3.5. IR spectroscopy

Fig. 17. Experimental (a) and simulated (b)

35

Cl MAS NMR spectra of (I).

from some isolated perchlorate which is not in the crystalline phase. Its amount cannot be determined with precision but it corresponds probably only to a few percentage of the total chlorine and is enhanced due to different relaxation times. The 35Cl MAS NMR spectrum of (II) is displayed on (Fig. 18). As above the signal is located at ca. 1000 ppm and corresponds to a perchlorate anion. Its shape is typical for a quadrupolar nucleus and a simulation leads to the following parameters: isotropic chemical shift 1009 ppm; CQ ¼ 1.277 MHz and etaQ ¼ 0.74. Theoretical calculations were undertaken in order to assign the NMR resonances to the different crystallographic non-equivalent carbon atoms of the unit cell. The chemical shifts calculations were made on the organic molecules only. X-ray diffraction data give CeH or NeH 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 the hydrogen

FT-IR spectroscopy is a widely used technique for the characterization of new solid phases. The IR spectra of compounds (I) and (II) are given in Figs 19-a and 20-a, respectively. The characteristic vibrational modes of the two compounds can be compared to those of similar perchlorate compounds [24,37]. For compound (I), the very large band in the high-frequency region, spreading between 3665 cm1 and 3010 cm1, corresponds to the stretching vibrations of the NeH, CeH and OeH groups interconnected by a system of hydrogen bonds in the crystal. However, the strong peaks situated at 3657 and 3582 cm1 are attributed to stretching modes of OH of crystallization water molecule. The bands in the 1635-1300 cm1 region correspond to þ the NeH bending modes of NHþ 2 and NH , the aromatic CeC and Table 6 Comparison of calculated and experimental chemical shift values of the carbon atoms in (I). Atoms

d Experimental (ppm)

d calculated (ppm)

C1 C10 C2 C20 C3 C4 N1 N2 N3

41.0 48.4 50.7 51.1 52.5 34.2 366.3 352.3 372.0

47.3 47.8 53.6 57.4 57.5 39.3 310.7 289.4 325.8

494


Table 7 Comparison of calculated and experimental chemical shift values of the carbon atoms in (II). Atoms

d calculated (ppm)

d Experimental (ppm)

C1 C1′ C2 C2′ C3 C3′ C4 C4′ C5 N1 N′1

23.9 24.1 153.7 152.8 117.5 117.0 130.9 131.2 134.0 92.2 92.2

18.8 19.5 e e 112.5 110.7 125.5 126.5 135.9 250/-253 253/250

the aliphatic CeC and CeN groups. The bands between 970 and 700 cm1 are assigned to the out-of-plane bending modes g(CaryeH) and g(CaryeC). thylbenzylmidazolium cation (II), the broad For the 5,6-dime band seen between 3555 and 2800 cm1 in the high-frequency region is attributed to the valence vibrations of CeH, NeH and CH3 groups [38]. The bands in the 1650e1320 cm1 region correspond to the bending vibrations of NH groups and the aromatic CeC and CeN groups. IR bands observed in the region 935e780 cm1

are assigned to rocking deformations: r(NH2) and to the d(CeC), d(CeN) and d(CeCeN) vibrations. We examine the modes and frequencies observed for the ClO4 anion. A free ClO 4 ion with Td symmetry has four fundamental vibrations, the non-degenerate symmetric stretching mode n1 (A1), the doubly degenerate bending mode n2(E), the triply asymmetric stretching mode n3(F2), and the triply degenerate asymmetric bending mode n4(F2). All the modes are Raman active, whereas only n3 and n4 are active in the IR. The average frequencies observed for these modes are 981, 451, 1104, and 614 cm1, respectively [39]. For compound (I), the v3 mode appears as one intense band at 1094 cm1 and the v4 mode is observed at 625 cm1. The asymmetric stretching vibration n3 of the ClO4 anion in compound (II) is observed at 1053 cm1. The peak at 624 cm1 is assigned to the asymmetric bending mode n4 of the anion. DFT calculations of the frequencies were made on the geometry obtained after optimization of the protons. An additional calculation was also made on the anion and the contributions of the two components were summed to lead the full spectrum. The resulting IR spectra between 500 and 4000 cm1 are shown on Fig. 19-b for (I) and Fig. 20-b for (II), respectively, and are very similar to the experimental ones. A close agreement between the experimental and theoretical wave numbers is mostly achieved in the finger print region as shown in Fig. 21-a for (I) and Fig. 21-b for (II). Thus, the

Fig. 19. Experimental (a) and calculated (b) IR spectra of (I).


(a)

495

precision is well-sufficient to assign the experimental frequencies and to confirm the attributions proposed above. 4. Conclusion

(b)

Both compounds, (C6H18N3)$(ClO4)3$H2O (I) and (C9H11N2)$ClO4 (II), are characterized by single crystal X-ray diffraction, Hirshfeld surface analysis, DFT, Solid State NMR and FT-IR spectroscopy. According to our X-ray structural results, in the structure of comþ pound (I) the inorganic entities, ClO groups and 4 anions, NH water molecules, form corrugated chains around the x ¼ 1/2 planes. While, the atomic arrangement of compound (II) exhibits wide pseudo-hexagonal channels of ClO4 tetrahedra. The HOMO-LUMO energy gaps suggest a good stability of both compounds. NMR signals are in full agreement with the crystallographic data. DFT calculations allow the attribution of the experimental NMR lines and of IR bands at low frequencies. References

Fig. 20. Experimental (a) and calculated (b) IR spectra of (II).

Fig. 21. Comparison between experimental and calculated IR frequencies. (a) (I) and (b) (II).

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