Synthesis, X-Ray Crystallography, Thermal Analysis, and DFT Studies of Ni(II) Complex with 1-Vinylimidazole Ligand

1 Department of Opticianry, Vocational High School of Health Services, Kilis 7 Aralık University, 79000 Kilis, Turkey 2Department of Chemistry, Arts and Sciences Faculty, Ondokuz Mayıs University, 55139 Samsun, Turkey 3 Department of Physics, Arts and Sciences Faculty, Ondokuz Mayıs University, 55139 Samsun, Turkey 4Department of Chemistry, Arts and Sciences Faculty, Giresun University, 28100 Giresun, Turkey


Introduction
Imidazole was first reported in 1858, although various imidazole derivatives had been discovered as early as the 1840s [1].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][3][4][5].For example, imidazole has been used extensively as a corrosion inhibitor on certain transition metals, such as copper in industry [1], and also the substituted imidazole derivatives are valuable in treatment of many systemic fungal infections in pharmacy [6].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 [7].Imidazole in polymers has traditionally been utilized in a variety of applications such as immobilized catalysts [8,9], redox reactions [10], water purification [11,12], hydrometallurgy/metal recovery [13,14], and ion and proton conductors [15].
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][17][18][19].Numerous complexes derived from -block metals and imidazole ligands are well known [7].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 [20].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 [23].(ClO 4 ) 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.
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 ( = 0.71073 Å) at 296 K.The structure was solved by direct methods using SHELXS-97 [24] and implemented in the WinGX [25] program suite.The refinement was carried out by full-matrix leastsquares method on the positional and anisotropic temperature parameters of the nonhydrogen atoms, or equivalently corresponding to 241 crystallographic parameters, using SHELXL-97 [26].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 CH 2 atoms, respectively.All other H atoms were positioned geometrically and refined with a riding model with U iso 1.2 times that of attached atoms.Data collection is by CrysAlis PRO [27], cell refinement by CrysAlis RED [28], and data reduction by CrysAlis RED [28].The general-purpose crystallographic tool PLATON [29] 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.

Computational Procedures.
All the calculations were performed by using Gaussian 03 package [30] and Gauss-View molecular visualization programs [31] 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.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 [35], 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⋅ ⋅ ⋅  interaction between the C2 atom and imidazole (N5, N6, C11-C13) ring (symmetry code: (b) −, −+1, and −+1).The distance of atom C2 between the centroids of these rings is 3.46 A.

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 [36].
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%).

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 ( 2 ) 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).[38][39][40][41][42].
The calculations of the mean linear polarizability ( tot ) and the mean first hyperpolarizability ( tot ) 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 [43].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 ( tot ), linear polarizability ( tot ), and first-order hyperpolarizability ( tot ) of the title compound were calculated with the UB3LYP/6-31G(d) and UB3LYP/LanL2DZ methods.The calculated values of  tot ,  tot , and  tot are 2.939 D, 59.95 A 3 , and 69.86 10 −30 cm 5 /esu for UB3LYP/6-31G(d) and 3.958 D, 65.52 A 3 , and 278.1 10 −30 cm 5 /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.

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][45][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 [47]: 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 [48].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.

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 [49].The HOMO and LUMO energies, the energy gap (Δ), the ionization potential (), the electron affinity (), the absolute electronegativity (), the absolute hardness (), and softness () for molecule have been calculated at

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][52][53][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.

Conclusions
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.

3. 1 .Figure 1 :
Figure 1: An ORTEP view of the title complex with the atomic numbering scheme.Displacement ellipsoids are shown at the 30% probability level.
Scheme 1: The chemical diagram of the title complex.

Figure 2 :
Figure 2: Part of the crystal packing of the title complex.For clarity, hydrogen atoms and hydrogen bonds are not shown.

Figure 7 :
Figure 7: The distributions and energy levels of the HOMO and LUMO orbitals computed at the UB3LYP/6-31G(d) and UB3LYP/LanL2DZ levels for the title complex.

Figure 8 :
Figure 8: The charge distribution calculated by the Mulliken method for complex.

Table 1 :
Crystal data and structure refinement parameters for the title complex.