Crystal Structure and Supramolecular Architecture of Inorganic Ligand-Coordinated Salen-Type Schiff Base Complex: Insights into Halogen Bond from Theoretical Analysis and 3D Energy Framework Calculations

: To identify the e ﬀ ects of halogen bonding in the architecture of Schi ﬀ base complex supramolecular networks, we introduced halogenated Schi ﬀ -base 3-Br-5-Cl-salen as ligand and isolated a new salen-type manganese(III) complex [Mn III (Cl)(H 2 O)( 3-Br-5-Cl-salen )] ( 1 ) where 3-Br-5-Cl-salen = N,N’ - bis (3-bromo-5-chlorosalicylidene)-1,2-diamine. The complex was investigated in the solid-state for halogen bonds (XBs) by single crystal X-ray structure analysis. Meanwhile, theoretical calculations were carried out to rationalize the formation mechanism of di ﬀ erent types of XBs in the complex. The analysis result of electronic structure of the halogen bonds indicated that the chlorine atom coordinated to the Mn(III) center possesses the most negative potential and acts as anionic XB acceptor (electron donor) to the adjacent substituted halogens on the ligand, meanwhile the intermolecular Mn-Cl ··· X-C halogen bonding plays a signiﬁcant role in directing the packing arrangement of adjacent molecules and linking the 2D layers into a 3D network. In order to verify the above results and acquire detailed information, the title complex was further analyzed by using the Hirshfeld surface analyses.


Introduction
Engineering well-defined molecules into specific crystal network has been one of the frontiers in material science [1][2][3]. Although the strength and energy of non-covalent interactions are relatively small in comparison with those of covalent bond, large numbers of non-covalent interactions not only play important roles in the construction of crystal frameworks but also relate to various physicochemical properties, among which the most classic and widely studied intermolecular interaction is hydrogen bond [4][5][6]. Because of its high energy and directional character, the hydrogen bond is considered to be a structure-directing agent and, to some extent, can be used to construct specific molecular assemblies [7,8]. With the further development of research, however, because the constitution of hydrogen bond is relatively simple, in some cases it is difficult to achieve the goal of fine tuning the

Synthesis of [Mn III (3-Br-5Cl-salen)(Cl)(H 2 O)] (1)
3-bromo-5-chlorosalicylaldehyd (0.160 g, 1.0 mmol) (Aladdin, Shanghai, China) and diamine (Aladdin, Shanghai, China) (0.03 g, 0.5 mmol) were dissolved in methanol (Aladdin, Shanghai, China) (30mL) under stirring and the resulting solution refluxed at 80 • C for 1 h. Then manganese chloride (0.197g) (Aladdin, Shanghai, China) was added in one portion to the solution under stirring at 60 • C for another 2 h. After the reaction was cooled down at room temperature, the brownish red filtrate was left to crystallize for one week in a beaker at room temperature. Brown crystals were filtered and dried in air.

X-Ray Crystal Structure Analysis
Single-crystal datasets were collected at 293(2) K on an automated diffractometer equipped with MoK α radiation (λ = 0.71073 Å) and Cryostream cooler (Oxford Cryosystem). The title compound was investigated via polarizability using the CrysAlis Pro software according to the classic Lorentz equation [32]. Crystal structures of 1 were solved by direct methods and refined with full-matrix on F 2 computations using the Olex2 software program [33]. In the final refinement, all the non-H atoms were anisotropically refined. All H atoms were refined using a riding model with C-H = 0.97 Å and U iso (H) = 1.5 U eq (C) or 1.2 U eq (C), with aromatic C-H = 0.93 Å. Crystal data and details of structural determination and refinement for 1 are summarized in Table 1. Selected bond lengths and angles for 1 are collected in Table S1.  [34]. Hirshfeld surfaces analyses were carried out using CrystalExplorer 17.5 software [35] and the TONTO system [36].
Crystallographic data (excluding structure factors) for the structure in this paper have been deposited with the Cambridge Crystallographic Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies of the data can be obtained free of charge on quoting the depository number CCDC-1958103 (Fax: +44-1223-336-033; E-Mail: deposit@ccdc.cam.ac.uk, http://www.ccdc.cam.ac.uk) [37].
The complex crystallizes in monoclinic space group P21/c. There is one manganese(III) center, one 3-Br-5Cl-salen salen-type ligand, one chloride anion, and one water molecule in the asymmetric unit (see Figure 1a). The basal plane of Mn III center of 1 is chelated by two oxygen and two nitrogen atoms from 3-Br-5Cl-salen unit, with Mn-O(N) bond distances ranging from 1.864(5) to 2.027(5) Å, and O(N)-Mn-O bond angles of 80.8(2) to 173.0(2). The length of Mn-N is, on average, 2.1395(5) Å longer than that of Mn-O [1.898(6) Å]. The dihedral angle between two benzene rings is 8.38(58) • . Thus, the molecule exhibits an overall coplanar architecture, which is, as can be clearly observed, in accordance with those reported in non-halogenated Schiff base complexes [38,39]. The apical positions of 1 are occupied by one oxygen atom O(3) from a water molecule and one chlorine ligand Cl (3). From which we can clearly see that owing to the Jahn-Teller effect of high-spin d4 metal center, the hexa-coordinate Mn III center possesses an elongated axis, furnishing a distorted octahedral geometry. The bond lengths of Mn(1)-O(3) and Mn(1)-Cl(3) are 2.301(6) and 2.500(3) Å , respectively, much longer than those of the basal plane.
It is revealed by PLATON software that a self-assembled supramolecular dimer is built up of two adjacent monomers which are linked by π-π, O-H···O and O-H···Br intermolecular hydrogen interactions (see Figure 1b and Figure S1 Table 2.  The apical positions of 1 are occupied by one oxygen atom O(3) from a water molecule and one chlorine ligand Cl (3). From which we can clearly see that owing to the Jahn-Teller effect of high-spin d4 metal center, the hexa-coordinate Mn III center possesses an elongated axis, furnishing a distorted octahedral geometry. The bond lengths of Mn(1)-O(3) and Mn(1)-Cl(3) are 2.301(6) and 2.500(3) Å, respectively, much longer than those of the basal plane.
It is revealed by PLATON software that a self-assembled supramolecular dimer is built up of two adjacent monomers which are linked by π-π, O-H···O and O-H···Br intermolecular hydrogen interactions (see Figure 1b and Figure S1 (Supplementary Materials)) [40]. Then, the coordination oxygen atoms O(3) form intermolecular hydrogen bonds with the adjacent coordinated chlorine atom Cl(3) i [i: 1-x, 0.5+y, 1.5-z] which links the neighboring self-assembled supramolecular dimers to form a one-dimensional chain structure (Figures 1b and 2a). In the bc plane, the 1D chains were further linked together through the O-H···Cl H-bonds (O(3)-H(3A)···Cl(3) iii [iii: 1-x, −0.5+y, 1.5-z]) to form supramolecular dimer 2D layers. Details of intermolecular hydrogen bonding interactions are given in Table 2.  It is well-known that PLATON software is effective in qualitative analyses of classic intermolecular contacts. However, the information it can give about non-classical weak contacts and quantitative results is very limited. Platon only shows the 1D to 2D intermolecular contacts, this seems to suggest that Van der Waals forces play a dominating role in the organization of the 2D layer into 3D network. However, further theoretical calculation shows that there is a close  It is well-known that PLATON software is effective in qualitative analyses of classic intermolecular contacts. However, the information it can give about non-classical weak contacts and quantitative results is very limited. Platon only shows the 1D to 2D intermolecular contacts, this seems to suggest that Van der Waals forces play a dominating role in the organization of the 2D layer into 3D network. However, further theoretical calculation shows that there is a close relationship between XB and 3D crystal structure. Detailed structural discussion will be presented in the theoretical analysis section.

Optimized Structure and HOMO-LUMO
It is known that the electronic properties and reactivity of complex can be elucidated by the analysis of the frontier molecular orbitals [41]. The fit view of experimental and optimized structures of the title compound is given in Figure 3 and Figure S2 (Supplementary Materials). The bond distances and angles of the optimized molecular structure are basically consistent with the experimental structure; generally slight differences are observed because the optimized geometry was obtained by gas-phase DFT optimizations [42]. Selected frontier molecular orbitals three-dimensional plots of 1 are given in Figure 4. The HOMO surface is mainly localized around benzene rings and the coordination chlorine atom, and the LUMO around the metal centers and -C=N-double bond showing an effective communication from π-orbitals throughout the ligand.
Crystals 2020, 10, x FOR PEER REVIEW 6 of 14 relationship between XB and 3D crystal structure. Detailed structural discussion will be presented in the theoretical analysis section.

Optimized Structure and HOMO-LUMO
It is known that the electronic properties and reactivity of complex can be elucidated by the analysis of the frontier molecular orbitals [41]. The fit view of experimental and optimized structures of the title compound is given in Figure 3 and Figure S2 (Supplementary Materials). The bond distances and angles of the optimized molecular structure are basically consistent with the experimental structure; generally slight differences are observed because the optimized geometry was obtained by gas-phase DFT optimizations [42]. Selected frontier molecular orbitals three-dimensional plots of 1 are given in Figure 4. The HOMO surface is mainly localized around benzene rings and the coordination chlorine atom, and the LUMO around the metal centers and -C=N-double bond showing an effective communication from π-orbitals throughout the ligand.  relationship between XB and 3D crystal structure. Detailed structural discussion will be presented in the theoretical analysis section.

Optimized Structure and HOMO-LUMO
It is known that the electronic properties and reactivity of complex can be elucidated by the analysis of the frontier molecular orbitals [41]. The fit view of experimental and optimized structures of the title compound is given in Figure 3 and Figure S2 (Supplementary Materials). The bond distances and angles of the optimized molecular structure are basically consistent with the experimental structure; generally slight differences are observed because the optimized geometry was obtained by gas-phase DFT optimizations [42]. Selected frontier molecular orbitals three-dimensional plots of 1 are given in Figure 4. The HOMO surface is mainly localized around benzene rings and the coordination chlorine atom, and the LUMO around the metal centers and -C=N-double bond showing an effective communication from π-orbitals throughout the ligand.  In order to evaluate the weak contacts and electrochemical reactivity of the title complex, ESP and MEP maps were created by using the B3LYP/6-311G level in ground state [43]. The region shown as convex surface that represents negative electrostatic potential, which wholly covers around the chlorine ligand (Cl (3)) (see Figure 5a), mostly covers phenoxo oxygen atoms (O(1) and O(2)), and partially covers bromine atoms (Br(1) and Br (2)) in a small semicircle region near the benzene ring, indicating the most highly negative sites are localized around the atoms. Noteworthy, compared with bromine atoms, the substitute chlorine atoms (Cl(1) and Cl (2)) on the benzene rings do not display a higher negative potential, similar results can be found in the molecular electrostatic potential (MEP) map.  In order to evaluate the weak contacts and electrochemical reactivity of the title complex, ESP and MEP maps were created by using the B3LYP/6-311G level in ground state [43]. The region shown as convex surface that represents negative electrostatic potential, which wholly covers around the chlorine ligand (Cl (3)) (see Figure 5a), mostly covers phenoxo oxygen atoms (O(1) and O(2)), and partially covers bromine atoms (Br(1) and Br (2)) in a small semicircle region near the benzene ring, indicating the most highly negative sites are localized around the atoms. Noteworthy, compared with bromine atoms, the substitute chlorine atoms (Cl(1) and Cl (2)) on the benzene rings do not display a higher negative potential, similar results can be found in the molecular electrostatic potential (MEP) map.  The MEP for the title compound is scaled from −200 kJ mol −1 to +220 kJ mol −1 , and the quantitative molecular electrostatic potential surface is displayed with color: red indicates the negative regions and blue indicates the positive regions. As is shown in Figure 5b and Figure S3 (Supplementary Materials), the region around the halogen atom (Cl (3)) that coordinated to the Mn(III) center (−200 kJ·mol −1 ) exhibits the most highly negative electrostatic potential. Meanwhile, this region is also influenced by the phenoxo oxygen atom (−158 kJ·mol −1 ). Consequently, we can predict that the atom in the region acts as an electron donor in intermolecular interactions. The region of hydrogen atoms on the coordination water molecule (220 kJ·mol −1 ) and the amine group at the molecular plane (84 kJ·mol −1 ) exhibits the most highly positive electrostatic potential. Further study indicated that the bromine and chlorine atoms on the benzene rings present quite different values in different directions. More specifically, along the C-X bond axis, the MEP potential is positive, the MEP values for Br(1) and Cl(2) are calculated to be 70 kJ·mol −1 and 51 kJ·mol −1 , respectively; whereas the potentials in the direction perpendicular to the axis are negative, that for Br(1) and Cl(2) to be −51 kJ·mol −1 and −48 kJ·mol −1 , respectively. That is to say along C-X bond direction halogens is electrophilic (δ + ) which can act as σ-hole to interact with negatively charged electrostatic nucleophile Figure 6. In addition, although Br atom is close to the most negative region(chlorine ligand and phenoxo oxygen atoms) of the whole molecule, the positive potential of Br along C-Br bond direction is still higher than that in the C-Cl direction of Cl atom. These observation show that the electrostatic potential distribution of halogen atoms on the salen type ligand is basically consistent with other types of halogenated organic compounds. The MEP for the title compound is scaled from −200 kJ mol −1 to +220 kJ mol −1 , and the quantitative molecular electrostatic potential surface is displayed with color: red indicates the negative regions and blue indicates the positive regions. As is shown in Figure 5b and Figure S3 (Supplementary Materials), the region around the halogen atom (Cl (3)) that coordinated to the Mn(III) center (−200 kJ·mol −1 ) exhibits the most highly negative electrostatic potential. Meanwhile, this region is also influenced by the phenoxo oxygen atom (−158 kJ·mol −1 ). Consequently, we can predict that the atom in the region acts as an electron donor in intermolecular interactions. The region of hydrogen atoms on the coordination water molecule (220 kJ·mol −1 ) and the amine group at the molecular plane (84 kJ·mol −1 ) exhibits the most highly positive electrostatic potential. Further study indicated that the bromine and chlorine atoms on the benzene rings present quite different values in different directions. More specifically, along the C-X bond axis, the MEP potential is positive, the MEP values for Br (1) and Cl(2) are calculated to be 70 kJ·mol −1 and 51 kJ·mol −1 , respectively; whereas the potentials in the direction perpendicular to the axis are negative, that for Br(1) and Cl(2) to be −51 kJ·mol −1 and −48 kJ·mol −1 , respectively. That is to say along C-X bond direction halogens is electrophilic (δ + ) which can act as σ-hole to interact with negatively charged electrostatic nucleophile Figure 6. In addition, although Br atom is close to the most negative region(chlorine ligand and phenoxo oxygen atoms) of the whole molecule, the positive potential of Br along C-Br bond direction is still higher than that in the C-Cl direction of Cl atom. These observation show that the electrostatic potential distribution of halogen atoms on the salen type ligand is basically consistent with other types of halogenated organic compounds.

Hirshfeld Surface Analysis
Hirshfeld surface analysis provides insights into the detailed information about the strength of intermolecular interactions. A better understanding of the issue can contribute to addressing the challenge of quantitatively understanding the intermolecular contacts using visual information of color and shades on the surfaces [35]. Therefore, in this work, Hirshfeld surface analysis was used to analyze the intermolecular interactions in the crystal packing. Moreover, visualized images of the molecular structure and the positioning of Hirshfeld surfaces were also employed to elucidate the interaction of specific atoms and orientations.
The interactions with normalized contact distance in crystal structure shorter than the sum of the corresponding van der Waals radii of the atoms, are highlighted by red spots and mapped with negative dnorm value on the Hirshfeld surface, whereas the contacts around that of van der Waals radii

Hirshfeld Surface Analysis
Hirshfeld surface analysis provides insights into the detailed information about the strength of intermolecular interactions. A better understanding of the issue can contribute to addressing the challenge of quantitatively understanding the intermolecular contacts using visual information of color and shades on the surfaces [35]. Therefore, in this work, Hirshfeld surface analysis was used to analyze the intermolecular interactions in the crystal packing. Moreover, visualized images of the molecular structure and the positioning of Hirshfeld surfaces were also employed to elucidate the interaction of specific atoms and orientations.
The interactions with normalized contact distance in crystal structure shorter than the sum of the corresponding van der Waals radii of the atoms, are highlighted by red spots and mapped with negative d norm value on the Hirshfeld surface, whereas the contacts around that of van der Waals radii with zero d norm value are plotted in white color, and the longer contacts with the positive d norm value are represented in blue color ( Figure S4 (Supplementary Materials)) [44]. The largest red region in  Herein we carried out the mapping of the electrostatic potential on Hirshfeld surfaces at the B3LYP/3-21G level of theory (see Figure 8) [45]. It is clearly shown that the chlorine ligands act as halogen bonding acceptors and link three adjacent molecules participating in the formation of halogen bonding. Although these molecules are arranged in different directions, all the C-X..Cl-Mn angles are close to 180° (Figure 8a, Figure S5 and Table S2 (Supplementary Materials)). Therefore, it is concluded that this is due to the key role of XBs that leads to the organization of the 2D layered sheets of 1 (Figure 8b). Furthermore, structural analyses show that 2D layered structures are bound to form a 3D supramolecular framework via multiply halogen bonding (Figure 9). Herein we carried out the mapping of the electrostatic potential on Hirshfeld surfaces at the B3LYP/3-21G level of theory (see Figure 8) [45]. It is clearly shown that the chlorine ligands act as halogen bonding acceptors and link three adjacent molecules participating in the formation of halogen bonding. Although these molecules are arranged in different directions, all the C-X . . . Cl-Mn angles are close to 180 • (Figure 8a, Figure S5 and Table S2 (Supplementary Materials)). Therefore, it is concluded that this is due to the key role of XBs that leads to the organization of the 2D layered sheets of 1 (Figure 8b). Furthermore, structural analyses show that 2D layered structures are bound to form a 3D supramolecular framework via multiply halogen bonding (Figure 9).

Intermolecular Interaction Energy
In this work, we used the CrystalExplorer software to calculate and display the interaction energy in a molecular crystal by using the B3LYP method with 3-21G basis set level [46]. Analyzing and quantifying intermolecular interactions in a crystal lattice, including repulsion, electrostatic, dispersion, and polarization etc., could help researchers gain an insight into the underlying interaction energy that happens and leads to the organization of crystal packing into supramolecular architectures in crystalline materials [47].
The molecular environment of the title structure was built at its center and around with a maximum distance of 3.8 Å. The energies' benchmark was calculated according to Mackenzie's method to scale different energy frameworks, which shows that the scale factors for electrostatic, dispersion, polarization, and repulsion are 1.057, 0.740, 0.871, and 0.618, respectively.
The relative strengths in interaction energy of molecular packing in all directions obtained here exhibit cylinder-shaped energy frameworks (see Figure 10). Some insignificant contacts weaker than the threshold energy value of 10kcal/mol have been omitted from the original calculation for clarity. From the perspective of the computational results of the interaction energies, it is also revealed that XB interaction should be of fundamental importance because of the fact that XBs conducting electrostatic interactions (−224.4 kJ/mol) play a dominant role in the crystal structure of the investigated system. Additionally, the polarization, repulsion, and total interaction energies are found to be −89.6 kJ/mol, 275.5 kJ/mol, −326.9 kJ/mol, respectively. Details of the interaction energy calculation for 1 are presented in Table S1.
From the perspective of the computational results of the interaction energies, it is also revealed that XB interaction should be of fundamental importance because of the fact that XBs conducting electrostatic interactions (−224.4 kJ/mol) play a dominant role in the crystal structure of the investigated system. Additionally, the polarization, repulsion, and total interaction energies are found to be −89.6 kJ/mol, 275.5 kJ/mol, −326.9 kJ/mol, respectively. Details of the interaction energy calculation for 1 are presented in Table S1. Figure 10. Visualization of the intermolecular interaction energy: red for Coulomb, green for dispersion, and blue for total interaction energies of the title structure along a axis.

Conclusions
In order to get insights of the electronic structure and topologies of XBs in Schiff base complexes, a new hetero-halogenated Schiff-base complex was synthesized and determined by single crystal X-ray diffraction. Crystal structure analyses revealed that classic H-bond is the main driving force in Figure 10. Visualization of the intermolecular interaction energy: red for Coulomb, green for dispersion, and blue for total interaction energies of the title structure along a axis.

Conclusions
In order to get insights of the electronic structure and topologies of XBs in Schiff base complexes, a new hetero-halogenated Schiff-base complex was synthesized and determined by single crystal X-ray diffraction. Crystal structure analyses revealed that classic H-bond is the main driving force in linking the molecules into 1D and 2D structures, and no significant interactions are found from 2D layers to 3D network. Whereas, theoretical calculation gives us a further information about the energy architecture of the 3D structure of this halogenated compound.
Theoretical calculations indicated that the chlorine ligands have the most negative potential, which acts as anion XB acceptors to the adjacent halogens to form multiple XBs. This kind of interaction not only directs the adjacent molecules to form a 2D layered structure but also links the 2D layers into a 3D framework. The intermolecular interaction energy was also evaluated by Hirshfeld surface analysis, and the results indicate that the XB conducting electrostatic interactions are predominant over the crystal packing. Because the strength of XBs could be mediated by different halogen atoms, we could regulate the expansion of 2D layers into 3D frameworks by changing the halogen atoms and/or coordinated ones to form different XBs with different strength.