Research Article | Open Access
Synthesis, X-Ray Crystallography, Thermal Analysis, and DFT Studies of Ni(II) Complex with 1-Vinylimidazole Ligand
The paper presents a combined experimental and computational study of hexa(1-vinylimidazole)Ni(II) perchlorate complex. The complex was prepared in the laboratory and crystallized in the monoclinic space group P21/n with (5), (8), (9) Å, , (5), and . The complex has been characterized structurally (by single-crystal X-Ray diffraction) and its molecular structure in the ground state has been calculated using the density functional theory (DFT) methods with 6-31G(d) and LanL2DZ basis sets. Thermal behaviour and stability of the complex were studied by TGA/DTA analyses. Besides, the nonlinear optical effects (NLO), molecular electrostatic potential (MEP), frontier molecular orbitals (FMO), and the Mulliken charge distribution were investigated theoretically.
Imidazole was first reported in 1858, although various imidazole derivatives had been discovered as early as the 1840s . Derivatives of imidazole represent a class of heterocyclic compounds of great importance. Both imidazole and its derivatives have found widespread applications in industry and pharmacy [2–5]. For example, imidazole has been used extensively as a corrosion inhibitor on certain transition metals, such as copper in industry , and also the substituted imidazole derivatives are valuable in treatment of many systemic fungal infections in pharmacy . The imidazole ligand is of particular interest due to its important role in many biological systems, especially as the side group in histidine which plays an essential role in the active motif of many enzymes . Imidazole in polymers has traditionally been utilized in a variety of applications such as immobilized catalysts [8, 9], redox reactions , water purification [11, 12], hydrometallurgy/metal recovery [13, 14], and ion and proton conductors .
Imidazoles are useful ligands in coordination chemistry and the synthesis of the compounds containing the imidazole ring is an important area of scientific investigation [16–19]. Numerous complexes derived from -block metals and imidazole ligands are well known . A large number of investigations on the complexation of copper(II) with imidazole ligands and vinylimidazole ligands have been reported, with some of them reporting structures determined crystallographically . The title compound is a novel complex firstly synthesized by us.
Determination of the structural and spectroscopic properties of compounds using both experimental techniques and theoretical methods has attracted interest for many years. In recent years, among the computational methods calculating the electronic structure of molecular systems, DFT has been the favorite one due to its great accuracy in reproducing the experimental values of in molecule geometry, vibrational frequencies, atomic charges, dipole moment, and so forth [21, 22].
In this present study, hexa(1-vinylimidazole)Ni(II) perchlorate has been investigated both experimentally and theoretically. In experimental study, the complex was synthesized and characterized by single-crystal X-ray diffraction methods. In theoretical study, the geometric parameters of the title complex in the ground state have been calculated using the density functional method (DFT) (UB3LYP) with 6-31G(d) and LanL2DZ basis sets. The calculated optimized structures were compared with their X-ray structure. It was noted here that the experimental results belong to solid phase and theoretical calculations belong to gaseous phase. In the solid state, the existence of the crystal field along with the intermolecular interactions has connected the molecules together, which result in the differences of bond parameters between the calculated and experimental values .
2. Experimental and Theoretical Methods
Nickel(II) perchlorate hexahydrate [Ni(H2O)6] (ClO4)2, 1-vinylimidazole, and ethanol were purchased from commercial sources and used without further purification. Perkin Elmer Diamond TG/DTA thermal analyzer was used to record simultaneous TG, DTG, and DTA curves in the static air atmosphere at a heating rate of 10 K min 21 in the temperature range 30–750°C using platinum crucibles.
Nickel(II) perchlorate hexahydrate (0.36 g; 1.0 mmol) was dissolved in 20 mL ethanol and 1-vinylimidazole (0.56 g; 6.0 mmol) was added slowly. The mixture was stirred for 1 h, filtered, and left for crystallization. Single crystals of hexa(1-vinylimidazole)Ni(II) perchlorate suitable for X-ray analysis were obtained after one week (yield %56).
2.3. Crystal Structure Analysis
Diffraction data for complex was collected on Agilent Diffraction SuperNova (single source at offset) Eos diffractometer using graphite monochromated Mo Ka radiation ( Å) at 296 K. The structure was solved by direct methods using SHELXS-97  and implemented in the WinGX  program suite. The refinement was carried out by full-matrix least-squares method on the positional and anisotropic temperature parameters of the nonhydrogen atoms, or equivalently corresponding to 241 crystallographic parameters, using SHELXL-97 . All H atoms were positioned geometrically and treated using a riding model, fixing the bond lengths at 0.86, 0.93, 0.97, and 0.96 Å for NH, CH, and CH2 atoms, respectively. All other H atoms were positioned geometrically and refined with a riding model with Uiso 1.2 times that of attached atoms. Data collection is by CrysAlis PRO , cell refinement by CrysAlis RED , and data reduction by CrysAlis RED . The general-purpose crystallographic tool PLATON  was used for the structure analysis and presentation of the results. Details of the data collection conditions and the parameters of the refinement process are given in Table 1.
2.4. Computational Procedures
All the calculations were performed by using Gaussian 03 package  and Gauss-View molecular visualization programs  on the personal computer without restricting any symmetry for the title complex. For modeling, the initial guess of the complex was first obtained from the X-ray coordinates. The molecular structure of the title complex in the ground state (in vacuo) is optimized by UB3LYP methods with 6-31G(d) and LanL2DZ basis sets. Besides, the nonlinear optical effects, the molecular electrostatic potential (MEP), frontier molecular orbitals (FMO), and the Mulliken population analysis of the title complex were determined by theoretical calculation results.
3. Results and Discussion
3.1. Structural Description of the Complex
The title complex, a ORTEP-3  view of which is shown in Figure 1, is crystallizing in the monoclinic space group P21/n with four molecules in unit cell. The asymmetric unit in the crystal structure contains the Ni(II) that is a cation, perchlorate anions, and six 1-vinylimidazole molecules. The Ni(II) atom displays a Jahn-Teller distorted octahedral coordination geometry, with six N atoms from six 1-vinylimidazole ligands in the equatorial plane and in axial positions. Ni–N1, Ni–N3, and Ni–N5 bond lengths are 2.110(3) Å, 2.129(3) Å, and 2.125(3) Å, respectively. These Ni–N bond lengths are comparable with those reported by [33, 34]. When the bond lengths and angles of the imidazole rings in the title complex are compared with literature , it is seen that there are no significant differences.
The crystal packing is stabilized by a single intermolecular C–H⋯O hydrogen-bonding interaction (Table 2, Figure 2), giving a view of the crystal structure of complex approximately along the a axis. Also, in the structure, there is a C2–H2⋯π interaction between the C2 atom and imidazole (N5, N6, C11–C13) ring (symmetry code: (b) , , and ). The distance of atom C2 between the centroids of these rings is 3.46 A.
|Symmetry codes: ; ; .|
Cg1 is the centroid of the imidazole (N5, N6, C11–C13) ring.
3.2. Magnetic Properties and Thermal Analysis
Magnetic moment determined for the hexa(1-vinylimidazole)Ni(II) perchlorate complex at room temperature is 2.90 BM. These values are characteristic of high-spin octahedral complexes of metals .
The thermal decomposition behavior of the complex was followed up to 750°C in a static air atmosphere. The complex is thermally stable up to 178°C. The results of TGA/DTA curves of complex are illustrated in Figure 3. The TG curves exhibit a continuous mass loss. Therefore, it was almost impossible to calculate mass loss value for each step. The stages of the temperature range of 178–590°C are related to the repeatedly decomposition of the six 1-vinylimidazole ligands by giving both endo- and exothermic effects. The mass loss calculations suggest that the remainder is left as a final product. NiO is the end product (Teo: 11.1%, exp: 12.2%).
3.3. Quantum Chemical Computational Studies
3.3.1. Theoretical Structures
The molecular structure of the title complex was also investigated theoretically; see Scheme 1. The starting coordinates were those obtained from the X-ray structure determination and in the ground state (in vacuo) was optimized using DFT(UB3LYP) with the 6-31G(d) and LanL2DZ basis sets. However, it should not be forgotten in here that the experimental results belong to solid phase and theoretical calculations belong to gaseous phase. The optimized molecular structure of the title molecule was obtained from Gaussian 03 program as shown in Figure 4. The molecular structure of the complex belongs to C1 point group symmetry with 89 atoms composing the structure. Randomly selected geometric parameters (bond length, bond angle, and torsion angles) were experimentally obtained and theoretically calculated by UB3LYP methods with basis sets listed in Table 3. And these selected parameters are compared with their experimental data. Correlation values are 0.9949 and 0.9881 for bond lengths, 0.9522 and 0.9994 for bond angles, and 0.1988 and 0.1043 for torsion angles, respectively. Consequently, according to correlations values, for the bond length and torsion angle, the 6-31G(d) basis set method is more useful than the LanL2DZ basis set method. Conversely, for bond angles, the LanL2DZ basis set method is more useful than the 6-31G(d) basis set method.
A logical method for globally comparing the structures obtained with the theoretical calculations is by superimposing the molecular skeleton with that obtained from X-ray diffraction, giving RMSEs of 0.084 and 0.071 A for the same methods, respectively. As a result, 6-31G(d) correlates a little well with the geometrical parameters when compared with LANL2Z method (Figure 5).
3.3.2. Nonlinear Optical Effects
Nonlinear optical (NLO) effects arise from the interactions of electromagnetic fields in various media to produce new fields altered in phase, frequency, amplitude, or other propagation characteristics from the incident fields . In the recent years, because of potential applications in modern communication technology, data storage, telecommunication, and optical signal processing, a large number of research papers in new materials exhibiting efficient nonlinear optical (NLO) properties have been of great interest [38–42].
The calculations of the mean linear polarizability and the mean first hyperpolarizability from the Gaussian output have been explained in detail previously and DFT has been extensively used as an effective method to investigate the organic NLO materials . The values of the polarizability and the first hyperpolarizability of Gaussian 03 output are reported in atomic units (a.u.), so the calculated values have been converted into electrostatic units (esu) (: 1 a.u. = 0.1482 × 10−24 esu; : 1 a.u. = 8.6393 × 10−33 esu).
The total molecular dipole moment , linear polarizability (), and first-order hyperpolarizability of the title compound were calculated with the UB3LYP/6-31G(d) and UB3LYP/LanL2DZ methods. The calculated values of , , and are 2.939 D, 59.95 A3, and 69.86 10−30 cm5/esu for UB3LYP/6-31G(d) and 3.958 D, 65.52 A3, and 278.1 10−30 cm5/esu for UB3LYP/LanL2DZ, respectively. Urea is one of the prototypical molecules used in the study of the NLO properties of molecular systems. Therefore, it was used frequently as a threshold value for comparative purposes. Theoretically, the first-order hyperpolarizability of the title compound is of 12 and 14.4 times magnitude of urea at the same levels, respectively. According to these results, the title compound is a good candidate of NLO material.
3.3.3. Molecular Electrostatic Potential
The molecular electrostatic potential (MEP) is related to the electronic density and is a very useful descriptor in understanding sites for electrophilic attack and nucleophilic reactions as well as hydrogen-bonding interactions [44–46].
The molecular electrostatic potential, , at a given point in the vicinity of a molecule is defined in terms of the interaction energy between the electrical charge generated from the molecule electrons and nuclei and a positive test charge (a proton) located at . For the system studied, the values were calculated as described previously using the following : where is the charge of nucleus A located at , is the electronic density function of the molecule, and is the dummy integration variable. Being a real physical property, can be determined experimentally by diffraction or by computational methods . To predict reactive sites for electrophilic and nucleophilic attack for the title molecule, MEP was calculated at the 6-31G(d) and LanL2DZ optimized geometries. The negative (red) regions of MEP were related to electrophilic reactivity and the positive (blue) regions to nucleophilic reactivity shown in Figure 6. As can be seen from the figure, there is one possible site on the title complex for electrophilic attack. The negative region is localised on the carbon atom of the imidazole ring, C13, with a maximum value of −0.049 and −0.064 a.u. for UB3LYP/6-31G(d) and UB3LYP/LanL2DZ basis sets. These results provide information concerning the region where the complex can interact intermolecularly and bond metallically. Therefore, Figure 6 confirms the existence of an intermolecular C13–H13⋯O2 interaction between the O atoms perchlorate ion and C atoms of the imidazole ring.
3.3.4. HOMO-LUMO Analysis
The highest occupied molecular orbital (HOMO) and the lowest lying unoccupied molecular orbital (LUMO) are named as frontier molecular orbitals (FMO).
The distributions and energy levels of the HOMO and LUMO orbitals computed at the UB3LYP/6-31G(d) and UB3LYP/LanL2DZ level for the title complex are shown in Figure 7. The calculations indicate that the title complex has 89 and 84 occupied molecular orbitals and the value of the energy separation between the HOMO and LUMO are −1.5 and −1.14 eV for at the same levels, respectively.
A molecule with a small frontier orbital gap is more polarizable and is generally associated with a high chemical reactivity and low kinetic stability and is also termed as soft molecule . The HOMO and LUMO energies, the energy gap , the ionization potential , the electron affinity , the absolute electronegativity , the absolute hardness , and softness for molecule have been calculated at the same levels and the results are given in Table 4. By using HOMO and LUMO energy values for a molecule, electronegativity and chemical hardness can be calculated as follows: (electronegativity), (chemical hardness), and (chemical softness), where and are ionization potential and electron affinity, and , respectively .
3.3.5. The Mulliken Charge Population
The Mulliken atomic charge calculation has an important role in the application of quantum chemical calculation to molecular system because of atomic charges effect dipole moment, molecular polarizability, electronic structure, and a lot of properties of molecular systems.The charge distributions calculated by the Mulliken method [51–54] for the equilibrium geometry of the complex is given in Figure 8. The calculated Mulliken charges of C13 and H13 atoms are determined as −0.12 and 0.16 e and −0.21 and 0.26 e for the 6-31G(d) and LanL2DZ methods, respectively. These values confirm intermolecular hydrogen bond C13–H13⋯O2.
In this present investigation, molecular structure, nonlinear optical effects, molecular electrostatic potential, HOMO-LUMO analysis, and the Mulliken charge populations of hexa(1-vinylimidazole)Ni(II) perchlorate have been studied using DFT (UB3LYP/6-31G(d) and UB3LYP/LanL2DZ) calculations. They are compared with the calculated geometric parameters (bond lenght, bond angle, and torsion angle) with their experimental data. It is seen that there are no significant differences, when the experimental structure is compared with theoretical structures. It was noted here that the experimental results belong to solid phase and theoretical calculations belong to gaseous phase. The predicted nonlinear optical (NLO) properties of the complex are much greater than those of urea. The complex is a good candidate as second-order nonlinear optical material. Besides, The MEP map shows that the negative potential sites are on electronegative atoms and the positive potential sites are around the hydrogen atoms. These sites provide information concerning the region from where the compound can undergo intra- and intermolecular interactions. Similarly, the Mulliken charges confirm the intermolecular hydrogen bonds in the crystal.
Supplementary file (crystallographic data) for the structure reported in this paper has been deposited with the Cambridge Crystallographic Data Center as Supplementary Publication no. CCDC–940965. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: (+44)1223 336-033; e-mail: firstname.lastname@example.org).
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported by TÜBİTAK-The Scientific and Technologıcal Research Council of Turkey (Project no. 110T131 ).
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