A New Decavanadate with Organic Cation: Synthesis, Crystal Structure, and Hirshfeld Surface Analysis

University of Tunis El Manar, Faculty of Sciences of Tunis, Laboratory of Materials, Crystal Chemistry and Applied ermodynamics, 2092 El Manar II, Tunis, Tunisia University of Carthage, High Institute of Preparatory Studies in Biology and Geology (ISEP-BG) of Soukra, Soukra, Tunisia Faculty of Sciences of Sfax, Solid State Laboratory, Faculty of Sciences, University of Sfax, 1171 Sfax 3000, Tunisia


Introduction
Recently, the influence of noncovalent interactions has been widely studied due to their importance for the selfassembly 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]. ese 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. ese 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 VO n units, which assemble into either discrete molecular clusters or link together to form onedimensional chains, two-dimensional layers, or threedimensional frameworks [7]. Several previous studies have shown that the decavanadate group has a significant role in some biological reactions [8,9]. is role is derived mainly from interactions between anions and biological molecules such as proteins [10,11]. ese 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. ese 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 (C 7 N 3 H 20 ) 2 V 10 O 28 ·4.21H 2 O.

Materials and Physical Measurements.
e 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. e 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). e crystals are obtained from a mixture of vanadium oxide (V 2 O 5 ) (Fluka, 99.9%) and 2,6-bis (ammoniomethyl) piperidinium (Sigma-Aldrich, 96%) in water with the respective molar proportion of 1 : 4. e mixture obtained is stirred with magnetic stirring, along with heating for about 3 hours. e 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.

X-Ray Diffraction Study.
A single orange crystal of (C 7 N 3 H 20 ) 2 V 10 O 28 ·4.21H 2 O 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 � 0.7107Å). e data reduction was processed with XCAD4 [15] included in the WINGX software package [16]. e structure was solved by the direct method using the program SHELXL-97 [17] and refined against F 2 data using the program SHELXL-2014 [18]. e 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. e structure representation was prepared using DIAMOND [20]. e crystal data of (C 7 N 3 H 20 ) 2 V 10 O 28 ·4.21H 2 O are summarized in Table 1. e 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.

Hirshfeld Surface Study.
e 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. e Hirshfeld surface was produced from the asymmetric unit of the (C 7 N 3 H 20 ) 2 V 10 O 28 ·4.21H 2 O molecule. e Hirshfeld surfaces are mapped with d norm , and 2D fingerprint plots presented in this paper were generated using Crystal Explorer 3.1 [21]. e mapping of d norm on the Hirshfeld surface highlights the directional intermolecular interactions. e value of the d norm is negative or positive when intermolecular contacts are shorter or longer than van der Waals separations.

Crystal Structure.
e single-crystal X-ray diffraction analysis shows that the formula unit of (C 7 N 3 H 20 ) 2 V 10 O 28 ·4.21H 2 O consists of one [V 10 O 28 ] 6− decavanadate group, two organic cations of 2,6-bis (ammoniomethyl) piperidinium, and 4.21 water molecules (Figure 1). e [V 10 O 28 ] 6− anions consist of ten sharing edges [VO 6 ] which is similar to those reported in the literature [22]. e decavanadate group in this structure is noncentrosymmetric. e 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     Advances in Materials Science and Engineering 5 from 3.092(1) to 3.121(2)Å. ese distances are consistent with those found in the compounds studied in the bibliography [23,24]. e 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). e distortion indexes of VO 6 octahedra ranged between 8.1% and 16.7% [11] (Table 2). e vanadium is in the +5 oxidation state.
is result was confirmed by the bond valence sum calculations (Table 2) according to Brown [25] e structure of (C 7 N 3 H 20 ) 2 V 10 O 28 ·4.21H 2 O 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. e 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 3.508Å 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. ese bond lengths are in agreement with those reported in the literature [27,28].
e strong band at 966 cm −1 is assigned to the vibration modes υ(V � O). e broad band located in the 2800-3430 cm −1 domain is attributed to -OH groups involved in hydrogen bonds [29].

Conclusion
A novel organic-inorganic framework of (C 7 N 3 H 20 ) 2 V 10 O 28 ·4.21H 2 O has been successfully synthesized by evaporation of an aqueous solution at room temperature. Its single structure was determined using singlecrystal X-ray diffraction, and it was characterized by FTIR and SEM analysis. e 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. e crystal structure was stabilized by O-H.  6 Advances in Materials Science and Engineering further characterizations to explore the anticancer activity and therapeutic intervention of the title compound.

Data Availability
e CIF file containing complete information about the structure of the obtained structure was deposited with the Cambridge Crystallographic Data Center (CCDC no.1968413). e file is freely available upon request from the following web site: http://www.ccdc.cam.ac.uk/data_request/cif.