Structural Characterization and Computational Studies of Bis ( 2-Ethylimidazole ) Bis ( Formato ) Zinc ( II )-Water ( 1 / 1 )

The reaction of 2-ethylimidazole and zinc formate monohydrate in 1:2 ratio in toluene leads to the formation of bis(2-ethylimidazole)bis(formato)zinc(II)-water (1/1), [Zn(N2H8C5)2(OCHO)2]·H2O, 1 which has been characterized by several techniques, including elemental and thermal analyses, IR, HNMR and CNMR spectroscopies, single crystal X-ray diffraction and DFT studies. The results obtained show that this complex crystallizes in the orthorhombic crystal system of the Pbca space group, with cell parameters a = 14.7230(2) Å, b = 7.3880(10) Å, c = 29.0843(4) Å, α = 90 ̊, β = 90 ̊, γ = 90 ̊, V = 3163.73 Å and Z = 8. The zinc center is bound to two molecules of 2-ethylimidazole, two formate molecules in a tetrahedral coordination geometry. One water of crystallization is present in the coordination sphere of the compound. Its molecular crystalline structure is strengthened by O/N-H...O, O-H...π, O-H...H, C-H...O, H...π, π...O and π...π interactions. The optimized structure, frontier molecular orbitals, global reactivity descriptors, molecular electrostatic potential, natural bond orbitals and the Mulliken atomic charges were investigated through theoretical studies.


Introduction
Metals have been known to play major roles in many biology and living systems.Some of these metals being transition elements are used by cells in structurally-constrained binding sites in metalloproteins, where they carry out structural, regulatory or catalytic functions [1].The biological activities of these metals can be influenced both by speciation and the ligands coordinated to them [2].For example, zinc which regulates the structure and catalytic action of over 300 enzymes can be found coordinated to different ligand sets such as histidine, glutamate, aspartate, cysteine, etc.The major goal of some bioinorganic chemists is to describe the coordination environment zinc adopts in proteins.Many comprehensive reviews have described the active sites of the metallozinc-enzymes, carboxypeptidase A and carbonic anhydrase [3].In carboxypeptidase A, zinc is coordinated to two nitrogen atoms from two histidine (His) residues and a glutamate's (Glu) oxygen atom, leaving an open site in the tetrahedral array for the catalytically important water molecule.Meanwhile the active site of carbonic anhydrase comprises of a Zn 2+ situated at the apex of three histidine residues with an available water molecule for catalysis [3].Attempts to understand the detailed reaction mechanisms at these zinc-binding centers, alongside the exact nature of the metal coordination environments, are only partially successful in spite of the several spectroscopic and crystallographic investigations executed on these enzymes [4].The synthesis, study of structures and properties of metal complexes with biologically relevant ligands are currently attracting much attention on account of their promising contribution to understanding the active mechanism of metalloenzymes by means of modeling their metal binding site [5] [6].Most recently we found profound interest in the reproduction of the active site of carboxypeptidase A through a judicious choice of transition metal that is present in the natural enzyme and a ligand containing nitrogen and/or oxygen donor atoms.In order to expand this family of compounds, we again report the reproduction of the active site of this zinc enzyme by replacing the isopropyl substituent on the imidazole ring in [3] by an ethyl residue.This is realized through the synthesis, characterization and computational studies of bis(2-ethylimidazole)bis(formato)zinc(II)-water (1/1),

Materials and Method
All chemicals used in the isolation and description of this complex were purchased from Aldrich and used without further purification.2) K] and mounted on top of a goniometer head.The intensity data were collected on a Bruker-Nonius X8 ApexII 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 [7] and corrected for Lorentz and polarization effects, and a semi-empirical absorption correction was applied (SADABS) [8].The structure was solved by direct methods using SIR-2002 [9] and refined against all F 2 data by full-matrix least-squares techniques using SHELXL-2016/6 [10] 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 [11].The vibrational frequencies, natural bond orbitals, Mulliken atomic charges, electronic structure and geometries of the isolated compound were computed within the density functional theory (DFT) at the hybrid Becke 3-Lee-Yang-Parr (B3LYP) exchange-correlation function with the Lanl2DZ basis set for all the atoms.Molecular orbitals (MO) were visualized using the GaussView 5.0.8 program.Global reactivity descriptors (Ionization energy (I), electron affinity (A), chemical potential (μ), chemical hardness (η), molecular electrophilicity (w), and chemical softness) were computed directly from the energies of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) using Equations ( 2)-( 6) [12].
( ) where, E HOMO and E LUMO are respectively energy values of HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital).
Chemical hardness (η) is associated with the stability of the compound, since it measures the ability of a compound to resist changes in the electron distribution or charge transfer.This implies that high values of chemical hardness indicate more stability and low reactivity of the compound, while molecules with small values of η are said to be chemically soft (S) and are highly polarizable and more chemically reactive.Chemical potential (μ) measures the escaping tendency of electrons from a compound in its ground state.High values of chemical potential signify that the molecule is less stable and more reactive [13].The global electrophilicity index (ω) measures the stabilization in energy of a system, when it acquires an additional electronic charge from the environment [14].
NBO analysis, which figures out the delocalization of electrons during coordination of metal to a ligand was performed using the Gaussian 09 Revision -A.02-SMP program using the Lanl2DZ basis set.For each donor (i) and acceptor (j) atoms, the stabilization energy E (2) associated with the electron delocalization between the donor and acceptor was calculated 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).

Physical Properties and Elemental Analysis
The synthesized complex appeared as colorless crystals which melted between

IR Spectrum of [Zn(N2H8C5)2(OCHO)2]•H2O, 1
The FT-IR spectrum of the compound (Figure 1) displays a slight brought band at 3300 cm −1 due O-H vibration of uncoordinated water molecules, while the weak absorption band between 3150 -3000 cm −1 can be attributed to N-H stretching vibration.This observed band, expected around 3300 cm −1 is shifted to lower wave number, due to the involvement of these protons in hydrogen bond formation with the oxygen atoms of the formate anion and that of water of crystallization.The weak multiple bands in the interval 2900 -2800 cm −1 are assignable to the valence vibrations of C-H of imidazole or that of the ethyl group.This range is similar to the C-H vibrational frequency observed between 2953 -2832 cm −1 by Fomuta and collaborators when they synthesized the complex salt, [Ag(N 2 H 10 C 11 ) 2 ]PF 6 [15].Moreover, the sharp absorption obtained at 1600 cm −1 is due to C=O vibrations while the sharp multiple bands appearing in the interval 1497 and 1473 cm −1 correspond to the vibrations of C = C and C = N of the imidazole unit.Furthermore, the broad band between 1396 -1297 cm −1 is due to the vibrations of C-N and N-N of the imidazole ring.

1 H and 13 C Nuclear Magnetic Resonance Spectrum (NMR)
The 1 H NMR spectrum shows five families occurring from the weak field to the strong field.In fact, the singlet at δ = 8.5 ppm (1 H, s) is attributable to the N-H imidazolyl proton whereas that observed at δ = 6.

Thermogravimetric Analysis
The thermogravimetric curve (Figure 2

Crystal Structure Analysis [Zn(N2H8C5)2(OCHO)2]•H2O, 1
Results from X-ray crystal analysis reveals that the synthesized material is a mo- whose MERCURY and ORTEP views are shown in Figure 3.The crystal data and other structural refinement details of the material are summarized in Table 1 while the selected bond lengths and bond angles are shown in Table 2.The unit cell in which the material crystallizes is shown in    with those reported in literature [16].

Theoretical Studies
The theoretical studies of [Zn(N 2 H 8 C 5 ) 2 (OOCH) 2 ]•H 2 O, 1 was performed using the density functional theory (DFT) using the Lanl2DZ basis set at the B3LYP level of theory in the gas phase.The optimized structure of the title compound is  shown in Figure 8. From the figure, it can be deduced that the π-electron system in both the imidazole ring and the formate group are only partially delocalized.However, the experimental geometry is well reproduced in the optimized structure with a comparison presented on Table 4 6.This difference in the theoretical and experimental geometries could be attributed on one hand to the difference between the experimental model which is in the solid phase and the theoretical model which is in the gas phase.On the other hand, this difference arises from the optimization of the experimental model in a bit to obtain the most stable structure of the material.The frontier molecular orbitals (HOMO and LUMO) (Figure 9) analyses of [Zn(N 2 H 8 C 5 ) 2 (OOCH) 2 ]•H 2 O, 1 and their energy gap reflect the chemical reactivity of the molecule.The HOMO and LUMO energies were obtained from an empirical formula based on the onset of the oxidation and reduction peaks measured by cyclic voltammetry.Recently this energy gap has been used to prove the bioactivity of molecules from intramolecular charge transfer [18].The HOMO represents the molecular orbital with the ability to donate electrons while the LUMO acts as the electron acceptors.The highest occupied molecular orbital, the 86 th , has energy of −6.093 eV while the lowest unoccupied molecular orbital, the 87 th has energy of 0.143 eV.The red regions of the molecular orbitals (MO) represent the positive phases while the green regions indicate the negative phases.The HOMO-LUMO transition indirectly explains the interactive ability of the target molecule [19].Moreover, the formate anion makes major contributions to the HOMO with some minor contributions from the carbon atoms of one imidazolyl ring while the major contributions to the LUMO are made by the imidazolyl ring and the water of crystallization, with minor contributions from the formate ion.The molecular electrostatic potential map analysis shows that the oxygen atoms of the formate group constitute the region with the most negative surface potential while the hydrogen atoms of water and that of the N-H imidazole ring possess the most positive electrostatic surface potential.Hence, the oxygen atoms of the formate groups are susceptible to electrophilic attacks while the aforementioned hydrogen atoms are susceptible to attacks by nucleophiles.The global reactive descriptors of this material were calculated using the E HOMO and E LUMO .The results obtained are presented on Table 7.In addition, the theoretical vibrational frequencies and corresponding assignments of the complex were investigated using LanL2DZ basis set and the results obtained are shown on interaction in the material, particularly the charge transfer was carried out by considering all possible interactions between filled donor and empty acceptor NBOs and by estimating their energetic importance by second-order perturbation theory [20].This analysis which also studies the delocalization of electrons when the hybrid orbitals of the ligands are overlapped with the hybrid orbitals of the metal ions is effective for the study of intra and intermolecular binding.When the stabilization energy, E (2) is high, the interaction between the electron donor  and acceptor is said to be strong.This also indicates a greater extent of conjugation in the whole system.The stabilization energies, E (2) deduced from the NBO calculations for the most significant intramolecular charge transfer interactions are reported in Table 9 for the coordinating atoms and the Zn(II) metal atom.Meanwhile, Table 10 indicates the stabilization energies, E (2) for the coordinating atoms and other atoms or group of atoms in the molecules.The strongest interaction in [Zn(N 2 H 8 C 5 ) 2 (OCHO) 2 ]•H 2 O, 1 is that involving the donation of a lone pair of electrons from N4 to the Zn8 anti-bonding orbital with an electron delocalization of 0.287, thereby stabilizing the material by 35.17 kcal/mol.The second strongest interaction is due to the delocalization of the electrons from the lone pair of O19 with LP*(6)Zn8 with occupancy of 0.287, stabilizing the complex by 34.48 kcal/mol, while the third strongest interaction involves the lone pair of N9 with LP*(6)Zn8 with an ED value of 0.287, stabilizing the complex by 30.96 kcal/mol.In addition, the donation of the lone pair of O16 to LP*(7)Zn8 and an ED value of 0.126 stabilized the complex by 30.97 kcal/mol.Again, the NBO results further indicated the absence of metal-ligand charge transfer with all E (2) ≤ 2.5 kcal/mol, except that of LP*(6)Zn8 to Ry*(1)N4 whose E (2)   from both O16 and O19 to C17-O18 and C20-O21 respectively.The Mulliken atomic charges of [Zn(N 2 H 8 C 5 ) 2 (OCHO) 2 ]•H 2 O, 1 were also calculated by NBO analysis using B3LYP method and the results obtained are presented in Table 11.The atoms, C1, C2, N4, N7, N9, C10, C11, N12, C14, C15, O16, O18, O2 and O22 possess negative charges while C3, Zn8, C13, C17, C20 are positively charged.The maximum negative charge is found on water's O22 atom with a value of 0.439.Meanwhile, the maximum positive charge resides on C3 of the imidazolyl ring with a value of 0.318.
(a)) which measures the effect of heat on the mass of the sample reveals that [Zn(N 2 H 8 C 5 ) 2 (OCHO) 2 ]•H 2 O, 1 is thermally (a) (b)
Analysis of the crystal structure of the title compound reveals that the water molecule is uncoordinated but remains isolated in the external coordination sphere of the material.More so, it is found to establish strong intermolecular O/N-H…O (1.917 Å, 1.983 Å, 2.009 Å) interactions, involving its H or O atoms and either the O atom of the formate or an N-H hydrogen atom of the 2-ethylimidazolyl fraction of another [Zn(N 2 H 8 C 5 ) 2 (OCHO) 2 ]•H 2 O, 1 molecule (Figure 5(a)).Furthermore, the oxygen atoms of the uncoordinated water are also involved in weak intermolecular O-H…π (2.832 Å), O-H…H (2.394 Å) and C-H…O (2.695 Å) hydrogen interactions (Figure 5(b)).Other observed interactions, not involving the water of crystallization, in the material include weak intermolecular N-H…O (2.037Å), C-H…O (2.378 Å, 2.643 Å) (Figure 6(a)) established between the hydrogen atoms of 2-ethylimidazole and formate's oxygen atoms, N-H…π (2.886 Å) involving N-H hydrogen and π-electron system of the formate, π…O (3.201 Å) between an oxygen atom and the π-electrons of two formate ions of different molecules and π…π (3.312 Å) occurring between two π-electron systems of different formate ligands (Figure 6(b)

Figure 5 .Figure 6 .
Figure 5. (a) Strong O/N-H…O and (b) Weak O-H…π, O-H…H, C-H…O intermolecular hydrogen interactions involving the oxygen/hydrogen atoms of the uncoordinated water.

Figure 9 .
Figure 9.The contour plots of (a) HOMO (−6.093 eV) and (b) LUMO (0.143 eV) orbitals of the studied compound and their energies.
intermolecular O/N-H…O, O-H…π, O-H…H and C-H…O hydrogen interactions and other unexpected N-H…π, π…O and π…π interactions insure the crystal packing.Moreover, the coordination environment around the Zn(II) centre reproduces well the active sites of carboxypeptidase A. DFT results revealed the most stable structural arrangement of the title compound with an acceptable disparity between theoretical and experimental bond lengths and angles, even though the disparity between the experimental and the theoretical O 16 -Zn 8 -N 4 and O 16 -Zn 8 -O 19 angles was exaggerated.The theoretical IR vibrational frequencies and the global reactivity descriptors were computed.The descriptors indicated that [Zn(N 2 H 8 C 5 ) 2 (OCHO) 2 ]•H 2 O, 1 is a soft molecule with a low kinetic stability and a high chemical reactivity.The strongest interaction in this material involved the donation of lone pair of electrons from N4 to Zn8 and there was the absence of metal-ligand back bonding charge transfer.
and Table 5.While all the bond N. O. Ngnabeuye et al.DOI: 10.4236/csta.2018.7100111 Crystal Structure Theory and Applications

Table 5 .
Comparison between experimental and theoretical bond angles of [Zn(N 2 H 8 C 5 ) 2 (OCHO) 2 ]•H 2 O, 1. around the Zn(II) tetrahedron experienced slight elongation in the optimized structure with a recorded acceptable disparity between 0.014 -0.084, the O 19 -Zn 8 -N 9 , O 16 -Zn 8 -N 4 and N 4 -Zn 8 -O 19 angles were enlarged while N 9 -Zn 8 -O 16 , N 9 -Zn 8 -N 4 and O 16 -Zn 8 -O 19 angles were compressed in the optimized structure.The discrepancy in the O 16 -Zn 8 -O 19 and O 16 -Zn 8 -N 4 bond angles was exaggerated in the optimized structure.In fact, the experimental O 16 -Zn 8 -O 19 angle which is 100.18˚widens up to 111.10˚ in the optimized structure, while the experimental O 16 -Zn 8 -N 4 angle of 125.32˚ is compressed to 113.54˚ in the optimized structure, giving discrepancies of 10.92˚ and 11.78˚ respectively.Some theoretically observed dihedral angles between different atomic planes are shown in Table Figure 8. Optimized structure of [Zn(N 2 H 8 C 5 ) 2 (OOCH) 2 ]•H 2 O, 1 showing its most stable structure and atomic labeling.lengths

Table 8 .
The table shows a shift

Table 7 .
Global reactivity descriptors of the complexes.

Table 9 .
Furthermore, electron delocalization also occurred from the second lone pair of electrons from O18 to O16-C17 and the third lone pair of electrons Crystal Structure Theory and Applications The second order perturbation energies E(2)(kcal/mol) of the most important charge transfer interactions (donor-acceptor) of 1.

Table 10 .
The second order perturbation energies E(2)(kcal/mol) of the most important charge transfer interactions for other atoms/groups within 1.
(*) 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.