Synthesis , Characterization and DFT Studies of Two Zinc ( II ) Complexes Based on 2-Isopropylimidazole

Two novel coordination compounds, [Zn(L)2(OOCH)2] (1) and [Zn(L)3(OCHO)](OCHO)]·H2O (2) (where L = 2-isopropylimidazole, C6H10N2) have been prepared by reaction of 2-isopropylimidazole with zinc(II) formate at room temperature using toluene as solvent. These compounds were characterized by elemental and thermal analyses, IR, HNMR and CNMR spectroscopies, single crystal X-ray diffraction and DFT studies. The Zn centers in 1 and 2 adopt pseudo-tetrahedral coordination geometries. Compound 1 crystallizes in the monoclinic system P2/c space group whereas compound 2 crystallizes in the P-1 space group of the triclinic crystal system. Several types of hydrogen intra-/intermolecular interactions are observed in these materials and extend into a two-dimensional leaf like network in 1 and a two-dimensional lattice of rectilinear pillars in 2. Compounds 1 and 2 were also optimized and their frontier molecular orbitals, global reactivity descriptors, molecular electrostatic potential, natural bond orbitals were investigated using density functional theory (DFT). In fact the induced structural differences from complex 1 to complex 2 led to the reduction of the frontier molecular orbital energy gap by 1.338 eV and a decrease of the chemical hardness by 0.669 eV.

cation.The syntheses of the complexes were carried out in air.Their melting points were uncorrected and measured using an SMP3 Stuart Scientific instrument operating at a 1.5˚C/min ramp rate.Elemental analysis (C, H, N) was performed with a Fisson Instrument 1108 CHNS-O elemental analyzer, and the thermogravimetric analysis was performed using a Perkin-Elmer STA 6000 thermo-balance.The IR spectrum was recorded with a Perkin-Elmer System 100 FT-IR spectrophometer, meanwhile the NMR spectra (400 MHz, 1 H and 100 MHz, 13 C) were measured on a Mercury Plus Variant 400 spectrophotometer operating at room temperature.Proton chemical shift (δ) values are reported in parts per million (ppm) from SiMe 4 (calibrating by internal deuterium solvent lock).Peak multiplicities are abbreviated as: singlet, s; doublet, d; triplet, t; quartet, q and multiplet, m.Single crystals of the materials were coated with dry perfluoropolyether and placed at the tip of a glass fiber in a cold nitrogen stream [T = 173(2) K] and mounted on a goniometer.The intensity data were collected on a Bruker-Nonius X8ApexII CCD area detector diffractometer using Mo-K α -radiation source (λ = 0.71073 Å) fitted with a graphite monochromator.The data collection strategy used was ω and φ rotations with narrow frames (width of 0.50 degree).Instrument and crystal stability were evaluated from the measurement of equivalent reflections at different measuring times and no decay was observed.The data were reduced using SAINT [15] and corrected for Lorentz and polarization effects, and a semi-empirical absorption correction was applied (SADABS) [16].The structure was solved by direct methods using SIR-2002 [17] and refined against all F 2 data by full-matrix least-squares techniques using SHELXL-2016/6 [18] minimizing w[Fo 2 -Fc 2 ] 2 .All the non-hydrogen atoms were refined with anisotropic displacement parameters.The hydrogen atoms of the compound were included in the calculated positions and allowed to ride on the attached atoms with isotropic temperature factors (U iso values) fixed at 1.2 times those U eq values of the corresponding attached atoms.Theoretical studies were performed using the Gaussian 09 Revision-A.02-SMP program [19].The vibrational frequencies, natural bond orbitals, electronic structures and geometries of the compounds were computed using density functional theory (DFT) at the B3LYP level of theory of the Lanl2DZ basis set.Molecular orbitals (MO) were visualized using the Gauss View 5.0.8 program.Global reactivity descriptors (chemical potential (μ) chemical hardness (η) molecular electrophilicity (w) and chemical softness) were computed from the energies of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO).For a donor atom (i) and an acceptor atom (j), the stabilization energy E (2) associated with the electron delocalization between the donor and acceptor was obtained from the relationship; E (2) = ΔEij = q i F(ij)/(Ej-Ei) (where qi = orbital occupancy, Ej, Ei = diagonal elements and F(ij) = off diagonal NBO fock matrix element.

Spectroscopic Results
The spectrum of compound 1 displays a weak band at 3136 cm −1 attributable to the valence vibration of the N-H group of the 2-isopropylimidazole fraction.The displacement of this frequency to a lower value is an indication that this group is engaged in hydrogen bond formation.The variable broad bands occurring between 2967 -2823 cm −1 is due to C-H vibration of both the imidazole and the methyl groups of the ligand.The broad intense band observed between 1610 -1472 cm −1 is assignable to the vibrations of C=O of the formate group, the C=C and C=N fractions of the imidazole unit.The intense band observed in the interval 1376 -1298 cm −1 can be assigned to C-N and C-C vibrations of the imidazole ring.Even though the spectrum of 1 indicated the absence of O-H vibration of water, compound 2 shows a slight deflection at 3300 cm −1 due to O-H vibration, indicating the presence of uncoordinated water in the material.The medium band at 3074 cm −1 can be assigned to the valence vibration of N-H group of 2-isopropylimidazole.This band expected between 4000 -3200 cm −1 is shifted to lower frequency, indicating the involvement of this group in hydrogen bond formation.Furthermore, the broad band observed in the interval 2970 -2684 cm −1 is due to C-H vibration of the ligand.While the large intense bands appearing between 1595 -1473 cm −1 is attributable to the C=C and C=N vibrations of the imidazole ring.The bands occurring from 1398 cm −1 to 1295 cm −1 arises due to C-C and C-N vibrations of the imidazolic unit.
Complex 1 was also subject to 1 H and 13 C-NMR spectroscopies.The 1 HNMR spectrum shows five types of protons.The multiplet appearing at δ = 1.2 ppm is due to the resonance of the methyl protons of the isopropyl group (6H, m, −CH 3 ).Two singlets observed at δ = 13.3 ppm and δ = 8.5 ppm are attributable to the N-H imidazolyl proton (1H, s, −NH) and the proton of the carboxyllate group (1H, s, HCOO) respectively.The multiplet observed at δ = 6.8 ppm is due to the resonance vibration of the −CH=CH-protons of the imidazole cycle (2H, m, −CH=CH−) while that occurring at 3.1 ppm is assignable to the resonance vibration of the -CH-proton of the isopropyl substituent (1H, m, −CH−).On the other hand, the 13 CNMR spectrum indicated the presence of six types of carbon atoms in the complex.In fact, the peak at δ = 169.3ppm is due to the carbon atom of the carboxyllate group (1C), while the methyl carbons of the isopropyl group resonates at δ = 21.3 ppm (2C).This spectrum also shows two sp 2 carbon atoms resonating at δ = 117.5 ppm (1C) and 125.0 ppm (1C) due to the carbon atoms of the −CH=CH− groups of the imidazole ring and one sp 3   carbon atom at δ = 28.4ppm (1C) arising from the resonance of the isopropyle's −CH− carbon atom.Finally, the peak observed at δ = 155.9ppm is due to the −C=N carbon of the imidazolyl ring.

Results of Thermal Analyses
These complexes were subjected to thermal analyses during which complex 1 was heated from 25˚C -350˚C while 2 was heated from 25˚C to 450˚C and the results obtained are illustrated on Figure 1.The figures display two curves, one in red and the other in blue.The blue curves represent the change in mass of the samples as they are subjected to heat while the red curves indicate the variations in the heat content of the material during heating.Complex 1 (Figure 1(a)) is thermally stable up 100˚C, after which it underwent a 28.5% mass loss between 100˚C -230˚C corresponding to the departure of one 2-isopropylimidazole molecule (Calc.29.3%).Moreover, the curve indicating the variation of the heat content indicates two endothermic processes at 106˚C and 210˚C.The endothermic process observed at 106˚C is associated with the melting of the material and has an enthalpy, ∆Hf = 7.0 kJ•mol −1 .Meanwhile, Figure 1(b) shows that 2 is thermally stable up to 60˚C, followed by a significant 55% weight loss in the interval of 90˚C -230˚C attributable to the departure of two molecules of uncoordinated water, two 2-isopropylimidazole molecules and one formate anion (Calc.56%).The heat change curve indicates four endothermic processes, one at 70˚C and three between 185˚C and 230˚C.The endothermic process at 70˚C is associated with the melting of the material and has a transformation enthalpy, ΔH f = 3.75 kJ•mol −1 .The two endothermic processes around 185˚C indicate the beginning of the progressive departure of C 6 H 10 N 2 , HCOO − and H 2 O of the material.

Structural Determination
Single crystals of the complexes were subjected to X-ray analyses and their MERCURY and ORTEP views are shown in Figure 2 and Figure 3 respectively.
The unit cells in which these materials crystallize are shown in Figure 4 while the crystallographic data and the structural refinement details used in the full description of their molecular crystalline structures are summarized in Table 1.The crystallographic data indicate that complex 1 crystallizes in the P2/c space group of the monoclinic system while complex 2 is found in the triclinic system with the P-1 space group.Actually, these compounds consist of a zinc atom si-  + coordination entity, one anionic formate counter ion and two hydrated water molecules.Some selected bond lengths and angles in the compounds are summarized in Table 2.The arrangement of the 2-isopropylimidazole ligands in the molecular structure of 1 creates π-π interactions of the order 3.156 Å to 5.149 Å due to the proximity of the electronic cloud around the aromatic nuclei of 2-isopropylimidazoles.Moreover, some intermolecular hydrogen interactions observed in 1 and 2 are summarized on the Table 3 and Table 4.These interactions generate in the crystal lattice of 1 a two-dimensional leaf like structure shown in Figure 5 containing cavities of various sizes and shapes.Some pseudo-rectangular cavities of dimensions of 9.832 Å × 8.919 Å for example, found within this crystal structure and having vertices occupied by the zinc atoms, are likely to accommodate small molecules.
On the other hand, the interactions in 2 generate a two-dimensional crystalline lattice consisting of important rectilinear pillars (Figure 6), in which the anions and the cations alternate, leaving small cavities occupied by water molecules of crystallization and capable of inducing particular properties in the material.

DFT Studies
DFT studies were performed on both complexes, at the B3LYP level of theory, using the Lanl2DZ basis set in the gas phase.The optimized structures obtained are shown in Figure 7.The optimized structure of 1 shows a partial delocalization of the π-electron systems of the formate group and the imidazolyl rings.In the optimized structure of 2, the π-electron systems of the formate groups and those of two imidazole ligands are partially delocalized while that of one imidazole moiety is completely delocalized.The geometric parameters of these optimized structures are summarized on Table 5 and Table 6.All the bonds around the Zn(II) center in 1 and 2 are slightly elongated in the optimized structure with acceptable discrepancies ranging from 0.010 -0.083 Å in 1 and 0.024 -Open Journal of Inorganic Chemistry    within the acceptable range.In complex 2, all bond angles around the Zn(II) tetrahedron are increased in the optimized structure except the N 18 -Zn 1 -N 10 and N 18 -Zn 1 -N 2 angles which are compressed.The disparity between these bond Open Journal of Inorganic Chemistry   green colors represent the negative phases [22].The figure shows that major contributions to the HOMO of complex 1 are made by the formate ligands while the main contributions to the HOMO in compound 2 are made by one water molecule of crystallization.However, some small contributions to the HOMO of 2 were observed from the −C=N nitrogen atom of one imidazolyl fragment.Furthermore, major contributions to the LUMO of 1 came from the three 2-isopropylimidazolyl ligands meanwhile the major contributions to the LUMO of 2 were made by only one 2-isopropylimidazole fragment of the molecule.The global reactive descriptors of both complexes (Table 7) were obtained using the E HOMO and E LUMO .The global reactivity descriptors predict the reactivity and kinetic stability of a chemical special.In fact, molecules with large frontier molecular orbital gap or high values of chemical hardness are generally considered as hard molecules and are characterized by less polarizability, high kinetic stability and low chemical reactivity.Contrarily, molecules with small frontier molecular orbital gap or low values of chemical hardness are termed soft molecules.Such molecules are characterized by high degree of polarizability, low kinetic stability and high chemical reactivity.It is observed from Table 7 that compound 1 has a higher frontier molecular orbital gap and a higher value of chemical hardness than compound 2. Thus compound 2 is a softer molecule which is easily polarizable, with a low kinetic stability and a higher reactivity than complex 1.This implies that the replacement of one formate group in bisformatobis(2-isopropylimidazole)zinc(II) (1) by a 2-isopropylimidazole unit couple with the presence of one formate counter ion and two hydrated water molecules in its unit cell increases the reactivity of the complex.This is achieved by the reduction of the frontier molecular orbital energy gap by 1.338 eV and the decrease of its chemical hardness by 0.669 eV.Moreover, the analysis of the molecular electrostatic potential map shows that the formate ligands of 1 offer sites for electrophilic attack while in complex 2, the counter fomate anion is the only site susceptible to attack by electrophiles.Furthermore, the theoretical vibrational frequencies and corresponding assignments of the complexes were investigated using the LanL2DZ basis set and the results obtained are shown in Table 8.The O-H vibrations of complex 2 and the N-H vibrations of both complexes are shifted to higher values compared to the values obtained experimentally.Meanwhile, the theoretical C=C and C=N of both complexes are close to the experimental values and to those reported in literature.In order to comprehend the role of intermolecular orbital interaction in the material in terms of charge transfer, Natural Bond Orbital (NBO) analyses were carried out on both complexes.This was achieved by considering all possible interactions between filled donor and empty acceptor NBOs and by estimating their energetic importance by second-order perturbation theory [23].NBO analysis effectively studies intra and intermolecular binding through the examination of the degree of delocalization of electrons that usually occurs when ligand orbitals overlap with metal orbitals.When the interaction between the electron donor and electrons acceptor is strong, there is a greater extent of conjugation.This is often characterized by high values of the stabilization energy, E (2) .The stabilization energies, E (2) for the most important intramolecular charge transfer interactions are summarized in Table 9 for compound 1 and Table 10 for compound 2. The donation of lone pair of electrons from N7 to an anti-bonding orbital of Zn9 in compound  enzyme mimic, bisformatobis(2-isopropylimidazole)zinc(II) (1), by replacing one of its formate ligands by a 2-isopropylimidazole group.This, we thought could be achieved by varying the molar ratio of zinc(II) formate-water (1/2) and 2-isopropylimidazole from 1:2 in 1 to 1:4 in 2.Although the attempt was successful, the additional presence of one formate counter ion and two hydrated water molecules was observed in the crystal structure of 2. This structural differences induced in these complexes, affected the electronic and physicochemical properties of the complexes.For instance, while compound 1 appeared thermally stable up to 100˚C, compound 2 was only stable up to 60˚C.Furthermore, the intra-and intermolecular interactions observed in these materials generated a two-dimensional leaf like crystalline network in structure 1 and a two-dimensional crystalline lattice of rectilinear pillars in 2. The structural differences between 1 and 2 also reduced the frontier molecular orbital energy gap and the chemical hardness of compound 1.Thus the reactivity of the modified
tuated at the center of a tetrahedral.This geometry is similar to what is observed in [Zn(OOCCH 3 ) 2 (pzH) 2 ] (where pzH = pyrazole)[20].More so, the formate unit adopts a monodentate coordination mode, just as was noticed earlier in[Zn(N 2 H 8 C 5 ) 2 (OCHO) 2 ]•H 2 O[14].In the neutral compound 1, the geometry around the zinc center is constructed by two nitrogen atoms (Zn-N = 2.004 Å) from two 2-isopropylimidazole molecules and two oxygen atoms (Zn-O = 1.957Å) from two formate units.Meanwhile, in compound 2, the geometry around the zinc atom is established by three nitrogen atoms (Zn(2)-N(7) = 1.990Å, Open Journal of Inorganic Chemistry
Å in 2. However, the N 10 -Zn 9 -N 7 and N 10 -Zn 9 -O 23 angles in 1 are enlarged, while the O 23 -Zn 9 -O 18 angle is compressed in the optimized structure.In fact, the N 10 -Zn 9 -N 7 angle which is increased from 103.34˚ in the experimental structure to 115.86˚ in the optimized structure shows an exaggerated disparity of 12.19˚.Meanwhile, the discrepancy observed between the experimental angles and theoretical angles of N 10 -Zn 9 -O 23 and O 23 -Zn 9 -O 18 in the interval 4.06˚ -5.57˚ is

Table 1 .
Crystallographic data and structure refinement details of the complexes.

Table 2 .
Selected bond lengths and angles in the title complexes.

Table 3 .
Hydrogen bonding interaction in the complex 1.

Table 4 .
Hydrogen bonding interaction in the complex 2.

Table 7 .
Global reactivity descriptors of the complexes.

Table 8 .
Calculated IR vibrational frequencies of the complexes and their assignment.

Table 9 .
.064 and a stabilization energy of 72.970 kcal/mol.The donation of the third lone pair of electrons from O23 to π*C 21 -O 22 anti-bonding orbital stabilized the molecule by 71.700 kcal/mol while the donation of the first lone Stabilization energies E(2)(kcal/mol) of the most important charge transfer interactions (donor -acceptor) of compound 1.
matotris(2-isopropylimidazole)zinc(II) formate-water (1/2) (2) have been synthesized and characterized in an attempt to modify the active site present in the (*) indicates anti-bonding, LP(A) is a valence lone pair orbital on atom A, ED is electron delocalization, F(i,j) is the Fock matrix elements (a.u) between i and j NBO.