Hydrogen Bonds in Bis(1 H -benzimidazole-κN 3 )cadmium(II) Dibenzoate: Hirshfeld Surface Analysis and AIM Perspective

The coordination complex bis(1 H -benzimidazole-κN 3 )cadmium(II) dibenzoate has been synthesized and characterized by single crystal diffraction analysis. Cadmium center is six coordinated and formed a distorted octahedron coordinated geometry. The Hirshfeld analysis shows that in the d norm -surface of the compound, there are dark red spots near the hydrogen-bonds acceptor and donor atoms, while intermolecular interactions result in faint-red spots. The AIM analysis was performed, there exist a BCP in each N(C)–H∙∙∙O hydrogen bond, the bond paths also can be seen, the |V(b)|/G(b) < 1 and the H(b) > 0, the interaction is indicative of being a closed shell. The TG results are consistent with the X-ray diffraction structure.


Introduction
Crystal engineering based on coordination complexes 1 have been fast developing in the last few years owing to their potential applications such as absorption, 2 catalysis, 3 luminescence, 4 etc. Coordination complexes have already been applied in all aspects of life, and also used for removing the highly toxic heavy metal ions. 5 The benzimidazoles (Bzim) possess a phenyl ring fused with imidazole ring. 6 Many substances with benzimidazole scaffold have been deeply participated in variety of life activities. For instance, mebendazole is used for treatment of parasitic diseases, 7 and omeprazole is used for gastric ulcers, 8 etc. Many other important properties of metal complexes containing benzimidazoles have also been investigated by researchers. The antibacterial activity of Bzim Zn(II) and Co(II) complexes have been investigated. 9,10 For Bzim's Ir(III) N-heterocyclic carbene complexes' anticancer and antitumor properties were evaluated. 11,12 Bzim derivate's Pt(II) and Cu(II) complexes have DNA binding and/or antioxidant activity. 13,14 Even the Bzim-functional-ized ruthenium complexes can be used for dye-sensitized solar cells (DSSC) or energy-storage. [15][16][17] Thus, exploring the benzimidazole metal coordination complexes is extremely important.
The hydrogen bonds are critical in life science 18 and chemistry. 19 The Hirshfeld surface analysis 20 and the Atom in Molecule (AIM) theory 21 are two powerful tools to research hydrogen bonds from different perspectives. The Hirshfeld sufaces analysis is able to be utilized to identify a type and region of intermolecular interactions (including hydrogen bonds). Hirshfeld surface analyses comprises d norm -and shape index surfaces, and 2D fingerprint plots (FP). While the AIM theory enables us to analyze the properties of hydrogen bonds. In AIM view, the main concept is based on the bond critical point (BCP), they are the evidence of hydrogen bond is the existence of a bond path and a BCP between the donor hydrogen and the acceptor. People also developed the criteria for the assessment of the existence of hydrogen bonds. 22 In this article, we synthesized a new benzimidazole coordinated cadmium(II) dibenzoate (Cd(Bzim) 2 (BA) 2 Wang et al.: Hydrogen Bonds in Bis(1H-benzimidazole-κN 3 (where Bzim is benzimidazole and BA is benzoate) through the low temperature hydrothermal method, obtained the crystal structure, calculated the molecular orbitals, carried out the Hirshfeld analysis and the AIM analysis, and we mainly focused on the analysis of the hydrogen bonds.

Experiments
Cd(Ac) 2 · 2H 2 O (0.235 g, 1.0 mmol), benzimidazole (Bzim, 0.236 g, 2.0 mmol), benzoic acid (BA, 0.244g, 2.0 mmol) and 15 mL water were mixed with stirring followed by adjusting the pH value to 6.5 with an aqueous solution of NaOH. Then the mixture was sealed in a 25 mL Teflon-lined stainless-steel reactor and heated at 100 °C for 96 h to give dark yellow crystals of the title compound after cooling. Yield: 32% (based on Cd). IR

Structure Determination and Physical Measurements
A yellow block crystal for 1 was chosen for X-ray diffraction analysis. Crystal structure measurement was performed on a Bruker SMART APEX II CCD diffractometer equipped with a graphite-monochromatic MoKα (λ = 0.71073 Å) radiation. Data integration and reduction were performed using SaintPlus 6.01. 23 Absorption corrections were applied with a multi-scan mode. 24 The structure was solved by direct methods with SHELXS 25 and refined by full-matrix least-squares techniques using SHELXL-2018 26 within WINGX. 27 All non-hydrogen atoms were refined anisotropically. All H atoms on C atoms were positioned geometrically and refined as riding, with C-H = 0.93 and U iso (H) = 1.2U eq (C). The molecular graphics was prepared using program Diamond 3.2. 28 A perspective view of the compound, with the atomic numbering scheme, is shown in Fig. 1. The compound crystallizes in the space group Pbca (Table 1). Asymmetric unit consists of a cadmium, two benzimidazole molecules and two benzoic acid ions. The cadmium center is six coordinated and connected four oxygen from two benzoic acids and two nitrogen atoms from two Bzims. Bzim coordinates Cd center in κN coordinate mode and each benzoate ion is bonded to Cd center in a chelating mode. So, the compound can be described as bis(1H-benzimidazole-κN 3 )cadmium(II) dibenzoate and possess an distorted octahedron geometry.
In d norm -surface, hydrogen-bonds result in dark red spots near the hydrogen-bonds acceptor and donor atoms while intermolecular interactions result in faint-red spots. Further, the presence of patterns of red-and blue triangles on the same region of the shape-index surfaces is characteristic of π•••π stacking. The d norm -and shape index-surfaces of the compound are shown in Fig. 3 and Fig. 4.   The N-H•••O hydrogen-bonds in title compound can be seen in the d norm -surfaces as bright-red spots marked as a, b, c and g (the spots located near the atom each correspondence are: a → N1, b → N4, c → O2, g → O3) in Fig. 3, so that the a, b, c and g spots are due to N1-H1•••O2 and N4-H4•••O3 hydrogen bonds.
The weak C-H•••O hydrogen bond interactions (including intramolecular and intermolecular hydrogen bonds) in the compound are also demonstrated in the d norm -surface as faint-red color spots (Fig. 3) and signed as e, f, h, i. (that is, each correspondence are: e → O4, f → C22, h → O1, i → C3). The faint-red spots above the center of the benzimidazole ring in the d norm -surface of the compound (Fig. 3), all marked as d, arise from the C20-H20•••π(Cg5) and C20-H20•••π(Cg6) interaction, observed in crystal structure. The offset π•••π interactions observed in the crystal structure are identified by distinctive patterns of red-and blue-triangles across the respective phenyl rings on the shape-index surface plots of the molecule (see the circle in Fig. 4).
The quantitative analysis of the intermolecular interactions apparent in crystal structure was attempted by observing the 2D FP's (Fig. 5) Fig.5 (c)). The C-H•••π interactions (Table SI)

3. Atom in Molecules Analysis
The topology analysis proposed by Bader was initially used for researching electron density in "atoms in Fig. 4. The shape-index-surfaces.

Fig. 5a
The full image of Fingerprint Plot. Fig. 5b The H···H contacts show the contribution of 47.2%. Fig. 5c The H···C contacts have a contribution of 24.7%. Fig. 5d The H···O contacts have a contribution of 14.0% which is manifested by the interactions due to hydrogen bonds of N-H···O and C-H···O type. Fig. 5e The C···C contacts display the contribution of 6.1%. Fig. 5f The H···N contacts exhibit the contribution of 4.2%. Fig. 5g The bar graph shown the proportion of each contacts. molecules" (AIM) theory. 21,42,43 In this theory, based on the bond critical point (BCP), Popelier 22 proposed the hydrogen bond's electron density at the BCP (ρ BCP ) should be in the scope of 0.002-0.035 a.u. and the electron density Laplacian value (∇ 2 ρ BCP ) should be confined in the region from 0.024 to 0.139 a.u.
For investigating the hydrogen bonds, we select three coordination units to form the model (it contain three molecules), in which it can exhibit the hydrogen bonds and the π•••π interactions from the cif file (see supporting information) and calculate the single point to get the fchk file (see supporting information) from Gaussian 09 program 44 at ωB97XD 45 /GenECP (6-31+G** basis set for C, H, O, N and Lanl2dz 46 basis set for Cd), finally, we use the Multiwfn program 47 to study the topological properties of the hydrogen bonds in title compound (The Multiwfn produced CPs.pdb, Path.pdb and CPprop.txt are also in supporting information).  48 for the hydrogen bond. The two negative Hessian matrix eigenvalues of electron density, λ 1 and λ 2 , can measure the scope of contraction of ρ BCP which is perpendicular to the bond toward the critical point, while the positive λ 3 eigenvalue weighs the extent of contraction parallel to the bond and from the BCP toward each of the adjacent nuclei. The sum of eigenvalues λ 1 , λ 2 , and λ 3 is ∇ 2 ρ BCP . In addition, the numeric value of   the ionic bonds, hydrogen bonds, and van der Waals interactions) the ∇ 2 ρ BCP value is positive. 21,43,[49][50][51][52][53][54] On the basis of  Table 5 The local properties at BCP: the potential energy density, V(b); the Lagrangian kinetic energy density, G(b); and the total energy density H(b) a . The calculations at ωB97XD/GenECP level (6-31+G(d) for C, H, O, N and LanL2DZ for Cd). Morrison, 55 and Espinosa 56 et al. 57 suggested that bond interactions are sorted according to the |V(b)|/G(b) ratio, the ratio |V(b)|/G(b) < 1, the bonded interaction is taken for the closed shell, when |V(b)|/G(b) > 2, it is typically covalent interaction; and when 1 < |V(b)|/G(b) < 2, it is the intermediate character. As it is depicted in Table 5, the Lagrangian kinetic energy density is only slightly larger than the potential energy density for all the (N)C-H

Thermogravimetric Analysis
The TG of the title compound at the heating rate of 10 °C/min in nitrogen atmosphere is shown in Fig. 7.
There are two weight loss steps. The first weight loss stage mainly takes place from 301 to 393 °C, it may be related to the removal of the coordinated benzimidazole molecules (found: 41.14%; calcd: 39.99%). The second weight loss step mainly takes place from 405 to 636 °C, it corresponds to the release of the coordinated benzoate anion (found: 36.3%, calcd.: 40.99%). After 650 °C, nearly no weight loss is observed and the residue weight is 20.59%, which suggests it may be the cadmium oxide (CdO, calcd.: 21.73%).

Conclusion
The coordination complex bis(1H-benzimidazole-κN 3 )cadmium(II) dibenzoate has been synthesized, the crystal structures were characterized by the single crystal diffraction analysis, Hirshfeld analysis and the AIM analysis. The structure analysis reveals that the cadmium center is six coordinated and forms an distorted octahedron coordinated geometry, and the compound's formula is Cd(Bzim) 2 (C 6 H 5 COO) 2 . The Hirshfeld analysis shows that in the d norm -surface, there are dark red spots near the hydrogen-bond acceptor and donor atoms, while intermolecular interactions result in faint-red spots, the offset π•••π interactions observed are identified by distinctive patterns of red-and blue-triangles across the respective phenyl rings on the shape-index surface plots, and the 2D fingerprint plots were also investigated. The AIM analysis shows there exists a BCP in each N-H•••O and each C-H•••O hydrogen bond, the bond paths associated with the hydrogen bonds can be seen, and the |V(b)|/G(b) < 1.0 and the H(b) > 0, these results manifested that the interaction is a closed shell system. The TG plot shows two weight loss steps (corresponding to BA and Bzim), the residue may be the cadmium oxide (CdO).