Abstract
A new 2,6-bis aminomethyl piperidine decavanadate hydrate, (C7N3H20)2V10O28.4.21H2O, was synthesized by slow evaporation of a solution at room temperature. The molecular structure was investigated by single-crystal X-ray diffraction. In the crystal structure, the layers of decavanadate groups, organic cations, and water molecules are arranged parallel to the (010) plane. Also, the prepared compound has been analysed by FTIR spectroscopy and scanning electron microscopy (SEM). The crystal structure of the title compound is stabilized by hydrogen bonds and van der Waals interactions. The cohesion of the structure is ensured by O-H…O and N-H…O hydrogen bonds. The three-dimensional Hirshfeld surface (3D-HS) and the relative two-dimensional fingerprint plots (2D-FPs) of (C7N3H20)2V10O28.4.21H2O compound revealed that the structure is dominated by O…H/H…O (70.8%) and H…H (18.5%) contacts.
1. Introduction
Recently, the influence of noncovalent interactions has been widely studied due to their importance for the self-assembly and supramolecular stabilization [1]. However, hydrogen bonding has a significant function in crystal structure construction because of highly directive and relatively strong specific interactions [2]. These interactions exist in polyoxometalate (POM) groups, and the cohesion of these compounds is ensured by hydrogen bonds and van der Waals interactions, which encourage researchers to study this family especially in the field of biology [3]. Polyoxometalates (POMs) are a kind of inorganic clusters, which can be combined with organic ligands and metal ions to form supramolecular compounds. These groups have been extensively studied due to their potential applications in various fields such as catalysis [4], materials science [5], and antibacterial and antitumor activities [6]. Among the various types of POMs, polyoxovanadates (POVs) are an important part of the group and invariably contain highly symmetrical core assemblies of VOn units, which assemble into either discrete molecular clusters or link together to form one-dimensional chains, two-dimensional layers, or three-dimensional frameworks [7]. Several previous studies have shown that the decavanadate group has a significant role in some biological reactions [8, 9]. This role is derived mainly from interactions between anions and biological molecules such as proteins [10, 11]. These molecules have a nanomolar affinity for casein kinase 2 (CK2), making them the most potent inhibitors of CK2 to date [12]. Decavanadate compounds are known for their low toxicity and very potent antitumor activities. These results are verified by several authors such as Aissa et al. [13], Liu et al. [7], and Louati et al. [14].
In this paper, we report the synthesis, crystal structure, and Hirshfeld surface analysis of the novel decavanadate (C7N3H20)2V10O28·4.21H2O.
2. Experimental
2.1. Materials and Physical Measurements
The infrared spectrum was measured at room temperature, using a Perkin Elmer Spectrum™ 100 FTIR spectrometer (France, Perkin Elmer) in the 4000–500 cm−1 region, in an ATR cell.
The SEM-EDX (energy dispersive X-ray) was made of a single crystal chosen by a polarizing microscope. Scanning electron microscopy (SEM) coupled to energy dispersive X-ray analysis spectrometry is performed using an FEI Quanta 200 environmental apparatus (LEUVEN Belgium, PHILIPS/FEI).
2.2. Synthesis of 2,6-Bis Aminomethyl Piperidinium Decavanadate Hydrate (C7N3H20)2V10O28 4.21H2O
The crystals are obtained from a mixture of vanadium oxide (V2O5) (Fluka, 99.9%) and 2,6-bis (ammoniomethyl) piperidinium (Sigma–Aldrich, 96%) in water with the respective molar proportion of 1 : 4. The mixture obtained is stirred with magnetic stirring, along with heating for about 3 hours. The solution thus obtained is transferred to a Petri dish. After a few days, the solution allows to deposit orange-coloured crystals suitable for X-ray diffraction analysis.
2.3. X-Ray Diffraction Study
A single orange crystal of (C7N3H20)2V10O28·4.21H2O was selected and used for X-ray diffraction experiment. Intensity data were collected using an Enraf-Nonius CAD-4 automatic diffractometer equipped with graphite monochromatic MoKα (λ = 0.7107 Å). The data reduction was processed with XCAD4 [15] included in the WINGX software package [16]. The structure was solved by the direct method using the program SHELXL-97 [17] and refined against F2 data using the program SHELXL-2014 [18]. The reflections were corrected for Lorentz and polarization effects followed by psi-scan absorption correction [19]. All nonhydrogen atoms were refined with anisotropic atomic displacement parameters, whereas all hydrogen atoms were fixed using the HFIX instruction. The structure representation was prepared using DIAMOND [20]. The crystal data of (C7N3H20)2V10O28·4.21H2O are summarized in Table 1. The CIF file containing complete information about the structure of the obtained structure was deposited in the Cambridge Crystallographic Data Center (CCDC no. 1968413); the file is freely available upon request to the following website: http://www.ccdc.cam.ac.uk/data_request/cif.
2.4. Hirshfeld Surface Study
The molecular Hirshfeld surface is created based on the electron distribution of a molecule and has been calculated as the sum of electron densities of spherical atoms. The Hirshfeld surface was produced from the asymmetric unit of the (C7N3H20)2V10O28·4.21H2O molecule. The Hirshfeld surfaces are mapped with dnorm, and 2D fingerprint plots presented in this paper were generated using Crystal Explorer 3.1 [21]. The mapping of dnorm on the Hirshfeld surface highlights the directional intermolecular interactions. The value of the dnorm is negative or positive when intermolecular contacts are shorter or longer than van der Waals separations.
3. Results and Discussion
3.1. Crystal Structure
The single-crystal X-ray diffraction analysis shows that the formula unit of (C7N3H20)2V10O28·4.21H2O consists of one [V10O28]6− decavanadate group, two organic cations of 2,6-bis (ammoniomethyl) piperidinium, and 4.21 water molecules (Figure 1). The [V10O28]6− anions consist of ten sharing edges [VO6] which is similar to those reported in the literature [22]. The decavanadate group in this structure is noncentrosymmetric. The V-O distances in this group depend on the type of oxygen atoms involved: they are between 1.575(4) and 1.620(4) Å for terminal oxygen atoms, between 1.680(4) and 2.080(4) Å for doubly coordinated oxygen atoms, between 1.923(4) and 2.046(3) Å for triply coordinated oxygen atoms, and between 2.090(3) Å and 2.355(4) Å for hexacoordinated oxygen atoms. V-V distances range from 3.092(1) to 3.121(2) Å. These distances are consistent with those found in the compounds studied in the bibliography [23, 24].
The projection of the structure along the [100] direction shows that the decavanadate groups and organic cations form a three-dimensional framework that leaves free tunnels in which the water molecules are lodged (Figure 2).
The distortion indexes of VO6 octahedra ranged between 8.1% and 16.7% [11] (Table 2). The vanadium is in the +5 oxidation state. This result was confirmed by the bond valence sum calculations (Table 2) according to Brown [25] ().
The structure of (C7N3H20)2V10O28·4.21H2O compound can be described as a layered structure parallel to the (001) plane (Figure 3).
Organic cations are linked only by van der Waals interactions. The cohesion of the structure is ensured by hydrogen bonds, van der Waals interactions, and N-H…O and O-H…O hydrogen bonds.
In this structure, the hydrogen bonds are weak with the D-A bond lengths varying from 2.846 to 3.508 Å according to Brown [26] (Table 3). Mean values of distances C-C and C-N are, respectively, 1.512(2) and 1.509(2) Å. These bond lengths are in agreement with those reported in the literature [27, 28].
3.2. IR Spectroscopic Characterisation
The swept frequency range of the IR spectrum extends from 4000 to 500 cm−1 (Figure SI. 1). The bands at 825, 750, 676, 600, 546, and 398 cm−1 are assigned to the internal vibration modes of the VO6 octahedra [14]. The strong band at 966 cm−1 is assigned to the vibration modes (V = O). The broad band located in the 2800–3430 cm−1 domain is attributed to –OH groups involved in hydrogen bonds [29]. The vibration modes (C-H), (C-N), (C-C), and (N-H) are located around 1164, 1199, 1384, 1487, 1624, 1940, and 2098 cm−1 [30].
3.3. SEM-EDX Analysis
The scanning electron microscopy of single crystals of (C7N3H20)2V10O28.4.21H2O phase revealed the presence of vanadium (V), nitrogen (N), carbon (C), and oxygen (O) in the sample studied. The morphology of crystals and the spectrum obtained are presented in Figure 4.
3.4. Hirshfeld Surface Analysis
The Hirshfeld surfaces of (C7N3H20)2V10O28·4.21H2O, over a dnorm, are shown in Figure 5. In the crystal, molecules are linked into a three-dimensional network by N-H…O and O-H…O hydrogen bonds. In order to visualize the intermolecular interactions in the crystal of the title compound, the Hirshfeld surface (HS) analysis was carried out by using Crystal Explorer. Figure 5 shows the presence of O-H…O and N-H…O bonds and H…H interactions.
The total contacts are given in Figure 6(a). The structure is dominated by O…H/H…O (70.8%) (Figure 6(b)) and H…H (18.5%) (Figure 6(c)) contacts. The O…O-type interactions represent 10.7% (Figure 6(d)).
(a)
(b)
(c)
(d)
4. Conclusion
A novel organic-inorganic framework of (C7N3H20)2V10O28·4.21H2O has been successfully synthesized by evaporation of an aqueous solution at room temperature. Its single structure was determined using single-crystal X-ray diffraction, and it was characterized by FTIR and SEM analysis. The structure of the obtained compound is formed by the decavanadate groups and the organic cations. It can be described as a three-dimensional framework that contains tunnels in which the water molecules are located. The crystal structure was stabilized by O-H…O and N-H…O hydrogen bonds. The Hirshfeld surface analysis reveals that O…H/H…O (70.8%) and H…H (18.5%) intermolecular interactions are the most dominant in the crystal structure. In this perspective, we plan to perform further characterizations to explore the anticancer activity and therapeutic intervention of the title compound.
Data Availability
The CIF file containing complete information about the structure of the obtained structure was deposited with the Cambridge Crystallographic Data Center (CCDC no.1968413). The file is freely available upon request from the following web site: http://www.ccdc.cam.ac.uk/data_request/cif.
Conflicts of Interest
The authors declare no conflicts of interest.
Acknowledgments
This work was supported by a national federated project research under the code PRF2019-D3P2, granted by the Tunisian Ministry of Higher Education and Scientific Research. The authors sincerely thank Ksiksi Abd El-Hakim, for support in editing the article.
Supplementary Materials
Figure SI: infrared spectrum of the (C7N3H20)2V10O28·4.21H2O compound. (Supplementary Materials)