Chelate ring stacking interactions in the supramolecular assemblies of ZnIJII) and CdIJII) coordination compounds: a combined experimental and theoretical study†

The self-assembly of ZnIJII) and CdIJII) ions with two isomeric tetradentate ligands, 2-pyridylisonicotinoylhydrazone (HL) and 2-benzoylpyridyl-picolinoylhydrazone (HL), was studied by elemental analysis, FT-IR spectroscopy and single-crystal X-ray diffraction. The reaction of zincIJII) and cadmiumIJII) salts with HL and HL in methanol under solvothermal conditions produced six monomer and one tetranuclear zincIJII) complexes, namely, ZnIJHL)Br2 (1), ZnIJHL )Cl2 (2), [CdIJHL )2]IJNO3)2·H2O (3), CdIJHL)Br2IJ4), ZnIJHL )Cl2 (5), ZnIJHL )Br2 (6) and [Zn4IJL )4I2]ijZnI4]·2H2O (7). The structure of 7 includes a cationic tetranuclear cluster of four zinc ions, four ligands, and two anions, counterbalanced by ZnI4 2− ions. However, the reaction of zincIJII) and cadmiumIJII) salts with HL under the same conditions produced monomer compounds. Herein, the ligand effects on the complex structures were studied. Hirshfeld surface analysis and fingerprint plots facilitate the comparison of intermolecular interactions in compounds 1–7, which are crucial in building supramolecular architectures.


Introduction
2][13][14][15][16] The most important factor among these that controls molecular structures is ligands with suitably disposed bridging groups.Generally, ligands appended with potentially endogenous bridging groups linking metal ions in a closed-cluster system (e.g.cubane, rectangular, chair, boat cluster) have been widely used for the design and synthesis of coordination clusters. 17On the other hand, pyridine base ligands containing amide groups generally are coordinated to the metal centers through their pyridyl nitrogen atoms and interact with each other via hydrogen bonds involving the amide groups, which are important for molecular recognition and constructing supramolecular arrays. 18However, there is still a very long way to go to develop new architectures of coordination polymers using specific spacer ligands in order to rationalize the design of compounds with well-defined This journal is © The Royal Society of Chemistry 2017 structures and useful functions.The ability to predict and control the structure and topology of coordination clusters and polymers remains an elusive goal, and much more work is required to understand the inter-and intra-molecular forces that determine the patterns of molecular structure and crystal packing.
In this work, we chose two pyridine base ligands, HL 1 and HL 2 (Scheme 1), as chelating-bridging ligands whose main difference is the position of pyridinic nitrogen, and this slight difference leads to interestingly big differences on the structures of the products.Ligand flexibility, combined with the donor-rich nature of this type of ligand, leads to a situation where, in addition to rotational variations, different structural motifs occur through various combinations of the diazine and other donor groups.Ligands such as HL 2 (Scheme 1) have O (CO) and/or N (py) groups adjacent to the diazine.This rich coordination ability gives the possibility to generate, through self-assembly reactions with MĲII) salts, polynuclear clusters with four and five metals. 19On the other hand, ligands such as HL 1 have great bridging-chelating ability that is adequate for the design and construction of metalorganic coordination polymers because of the para position of the pyridinic nitrogen. 19n this manuscript, we report on the systematic syntheses and structural characterization of ZnĲII) and CdĲII) complexes of a series of unsymmetrical Schiff base ligands.The aim of this study is to analyse the competition between anions and ligands HL n (Scheme 1) for the coordination sites at the metalĲII) centre and to probe how the nature of the anion affects the crystal packing.The structural descriptions have been corroborated with calculations of Hirshfeld surfaces, which reveal a strong effect of noncovalent interactions on the properties of the surfaces.Finally, the interesting conventional and unconventional π-π stacking interactions observed in the solid state of some compounds have been analysed both energetically and using Bader's theory of atoms in molecules by means of DFT calculations.

Experimental and theoretical methods
Experimental Materials and measurements.All the reagents were commercially available and employed without further purifica-tion.Microanalyses were carried out using a Heraeus CHN-O-Rapid Analyser.FT-IR spectra were recorded on a Bruker Tensor 27 FT-IR spectrometer with KBr disks in the range 4000-400 cm −1 .Single crystals suitable for X-ray analyses were used for intensity data collection on a Bruker AXS SMART APEX CCD diffractometer using graphite monochromated MoKα radiation (λ = 0.71073 Å) at different temperatures (Table S1 †).
CdĲHL 2 )Br 2 Ĳ4).Yield: 64% (0.184g  (7).A solution of the ligand HL 2 (0.151 g, 0.5 mmol) in methanol was treated with a methanolic solution of ZnĲOAc) 2 2H 2 O (0.109 g, 0.5 mmol).The solution was heated under reflux, and sodium iodide (0.149 g, 1 mmol) was added in portions to the solution and then further refluxed for 3 h.The resulting solution was allowed to stand at room temperature, and upon slow evaporation, gave crystals.The crystals that separated out were collected, washed with ether and dried over P 4 O 10 in vacuo.Yield: 59% (0.156g

X-ray crystallography
Suitable crystals of 1-7 (see Scheme 2) were selected for data collection which was performed using Bruker APEX-II, STOE IPDS and Supernova diffractometers equipped with graphite monochromatic Mo-K α radiation.The structures were solved by direct methods using SHELXS-97 (ref.22) and refined by the full-matrix least-squares methods on F 2 using SHELXL-97 (ref.22) from within the WINGX (ref.23) suite of software.All non-hydrogen atoms were refined with anisotropic parameters.The H atoms were located from different maps and then treated as riding atoms with C-H distances of 0.93 Å and N-H distances of 0.86 Å.For compounds 6 and 7 of triclinic cells, the STOE IPDS-1 equipment did not allow us to collect over 91-92% fraction of the reflections.However, the structural characterization of these compounds (with a final R factor of ca.3%) is conclusive as to the general shape of the complexes.A suitable absorption correction was applied to all data sets.Molecular diagrams were created using MER-CURY. 24Supramolecular analyses were conducted and the diagrams were prepared with the aid of PLATON. 25The details of the crystal parameters, data collection and refinements are summarized in Table S1.† The selected lengths and angles are listed in Table S2.† Hirshfeld surface analysis.The Hirshfeld (HF) surfaces 26 and the related 2D-fingerprint plots 27 were calculated using Crystal Explorer. 28The CIF file of each structure was imported into Crystal Explorer and high resolution Hirshfeld surfaces were mapped with the function d norm .Before starting the calculations, the bond lengths to hydrogen atoms were set to standardized neutron values (O-H = 0.983Å, N-H = 1.009Å and C-H = 1.083Å).Then, the HF surfaces were resolved into 2D-fingerprint plots, in order to quantitatively determine the nature and type of all intermolecular contacts experienced by the molecules in the crystal.

Computational methods
The geometries of the complexes included in this study were computed at the wB97XD/6-31+G* level of theory using the crystallographic coordinates within the Gaussian-09 program. 29This level of theory which includes the dispersion correction (D) is adequate for studying noncovalent interactions dominated by dispersion effects like π-stacking. 30We have used the crystallographic coordinates instead of optimized complexes because we are interested in estimating the binding energies of several assemblies as they stand in the crystal structure, instead of investigating the most favourable geometry for a given complex.The "atoms-inmolecules" (AIM) analysis of the electron density has been performed at the same level of theory using the AIMAll program. 31

Results and discussion
Crystal structures [ZnĲHL 1 )X 2 ] (1-2) Compounds 1 and 2 crystallize in the P2 1 /c space group, in which the zinc cation is neutralized by two Br − and Cl − anions, respectively.The compounds are composed of the mononuclear unit, [ZnĲHL 1 )X 2 ] X = Br − (1) and Cl − (2), in which the ligand adopts an extended conformation with the aryl ring in the trans position with respect to the imine moiety (see Fig. 1).The linker coordinates the ZnĲII) through three coplanar ligating sites involving the carbonyl O, the hydrazine N and the pyridyl nitrogen, with bond distances shown in Table 1, generating two five-membered chelate rings.Moreover, two X − anions are located in essentially apical positions, above and below, the mean plane defined by the donating centers of the ligand resulting in a distorted tetragonal pyramidal geometry.
In both compounds, the N-H groups are involved in intermolecular hydrogen bonds to the bromide/chloride atoms with distances of 3.385Ĳ4)/3.472Ĳ4)for 1 and 3.225Ĳ3)/3.318Ĳ3)Å for 2 (see Fig. 2).These H-bonds facilitate the formation of infinite 1D columns with an antiparallel arrangement of the mononuclear complexes.These supramolecular 1D columns are further stabilized by stacking π-π interactions between the coordinated and uncoordinated pyridyl rings with centroid-to-centroid distances ranging from 3.68 to 3.96 Å Scheme 2 Complexes 1-7 reported herein.
(see Fig. 2).The atoms belonging to the chelate ring also participate in this stacking interaction, which is further discussed below in the theoretical study.Finally, the crystal structures are also stabilized by weak C-H⋯X (X = Cl, Br) and C-H⋯O noncovalent interactions.
Finally, the crystal packing of both compounds presents self-assembled dimers governed by chelate ring⋯chelate ring interactions (see Fig. 5).These dimers are only formed between the units of the crystal with a larger dihedral angle (see Fig. 4, right), which is likely due to the almost orthogonal arrangement of the phenyl ring with respect to the chelate ring.This facilitates the antiparallel approximation of the chelate rings (centroid-to-centroid distances are 3.45 Å in 4, 3.50 Å in 5 and 3.49 Å in 6).This chelate ring⋯chelate ring interaction is further studied below.This type of selfassembled dimers has been recently studied in HgĲII) complexes with the same ligand.21b The crystal structures are further stabilized by weak C-H⋯X and C-H⋯O noncovalent interactions.S2 †) ranged between 2.044Ĳ5)-2.174Ĳ6)Å and the other Zn-O bond lengths ranged between 2.121Ĳ5)-2.235Ĳ4)Å, respectively.

Crystal structures of [Zn
It should be mentioned that the isolation of a tetrahedral tetraiodozincate anion in the solid state of 7 is quite surprising since its stability is the lowest of the tetrahalozincate ZnX 4 2− series (X = Cl, Br, I).Therefore, formation of such a complex in the case of iodide, but not chloride or bromide (complexes 5 and 6) is rather unexpected.We do not have a convincing explanation for this experimental finding, although the different behaviour of 7 with respect to 5 and 6 could be related to the different protonation state of the ligand.The different behaviour of a series of tetrahalometallate anions MX 4 2− (M = Zn, Cd and Hg; X = Cl, Br and I) determining the solid state architecture of   different 2-phenylethylammonium salts has been analysed. 32oreover, the different behaviour of tetraiodozincate with respect to Cl and Br analogues in the synthesis of hybrid metal-organic salts has also been reported. 33Although the isolation of tetraiodozincate is not very common, several works have appeared in the literature, including tetrahedral ZnI 4 2− solid state X-ray structures, in the past decade. 34

Hirshfeld surface analysis
The intermolecular interactions in crystal structures 1-7 were quantified using Hirshfeld surface analysis and fingerprint plots (FP).The dominant intermolecular interactions are viewed as a bright red area on the d norm surface.Fig. 8 illustrates samples of Hirshfeld surfaces for structures 3 and 7.In general, the Hirshfeld surface analysis suggests that the crystal packing in structures 1-7 is largely dominated by the common planar components of ligands, leading to close H⋯H intercontacts as well as interesting C-H⋯π and π⋯π stacking interactions.In 3, we observe a high level of O⋯H interactions due to the hydrogen bonds between solvents molecules and the complex.The Cl⋯H, Br⋯H and I⋯H H-bonding interactions are also very important.
The two-dimensional fingerprint plots of the HS for structures 1-7 are shown in Fig. S1 (ESI †).The quantitative comparison of the intercontacts for all structures and the relevant intermolecular interactions are presented in Table S3 (ESI †).From this analysis, the division of contributions is possible for different interactions, including H⋯H, O⋯H, C⋯H, C⋯C, N⋯C, N⋯H, O⋯N, N⋯N (for all compounds), Cl⋯H, Cl⋯C [in 2 and 5], Br⋯H, Br⋯C [in 1, 4 and 6] and I⋯H and I⋯C [in 7], which commonly overlap in the full fingerprint plots.The fingerprint plots of compounds 1-7 show that the dominant interactions are H⋯H (22.5-41.1%)and C⋯H (7.2-20.4%).The C⋯H contacts represent the C-H⋯π interactions in the crystals, and the highest values were measured in 4, 5 and 6.For π-π interactions which correspond to the C⋯C contacts, the highest values were measured in 1, 2 and 3.The O⋯H and X⋯H hydrogen bonding (where X = Cl, Br, I) also play very important roles in stabilizing the structures.The O⋯H interactions vary from 5.1-5.3% for 4, 5 and 6 to 31.7% for 3, while the X⋯H contacts vary from 17.9 % for 7 to 35.9% for 1.

Theoretical Study
We have focused the theoretical study on the comparison of the energetic features of the different types of π-stacking interactions (chelate ring-π and π-π) observed in the crystal packing of compounds 1-2 and 4-6 described above (see Fig. 2 and 5).In particular, we have analysed the π-π and chelate ring⋯chelate ring stacking interactions which are crucial to understanding the crystal packing of complexes 1-7, as discussed above.Predictable π-stacking 35 interactions involve organic aromatic molecules; however, other planar molecular fragments can also participate in more "unpredictable" stacking interactions. 36Among them, chelate rings with delocalized π bonds establish stacking 36 interactions similar to those of aromatic organic molecules 35i-l in transition metal complexes.The existence of chelate-ring-π interactions is associated with the aromaticity of planar chelate rings with delocalized π bonds. 37irst of all, in order to study the donor-acceptor ability of the ZnHL 1 X 2 and MHL 2 X 2 (M = Cd, Zn) complexes, we have computed the molecular electrostatic potential (MEP) surface of a model system (compound 2), which is shown in Fig. 9.As expected, the most negative electrostatic potential corresponds to the region of the Cl ligands while the most positive part is located in the region of the N,C-H groups at the molecular plane.Therefore, H-bonding interaction between these groups (N-H⋯X) should be electrostatically favoured.
Furthermore, perpendicularly to the molecular plane, we found that each 5-membered chelate ring has almost negligible MEP values (−5 kcal mol −1 ).The MEP values are positive over the pyridine rings due to the effect of the coordination to the Zn.Therefore, pyridine-pyridine interactions (conventional π-stacking) should be electrostatically less favoured (electrostatic repulsion) than chelate ring⋯chelate ring interactions.
In isostructural compounds 1-2, we have computed the interaction energy of the self-assembled π-stacked dimers shown in Fig. 10a   and ΔE 4 = −46.7 kcal −1 , respectively) are very large due to the contribution of both H-bonding and π-π interactions, the latter involving a very extended π-system and the former encompassing the most positive part of the complex (N-H group) and the most negative (belts of the halido ligands).In an effort to calculate the contribution of the different forces that govern the formation of the self-assembled dimers, we have computed a theoretical model where the uncoordinated pyridine rings have been replaced by H atoms (see the small arrows in Fig. 10b) and consequently, the π-π stacking interactions between the coordinated and uncoordinated pyridine rings are not formed.As a result, the interaction energies are reduced to ΔE 2 = −45.3kcal mol −1 and ΔE 5 = −36.0kcal mol −1 in 1 and 2, respectively.Therefore, the contribution of both symmetrically equivalent π-π stacking interactions can be roughly estimated by difference (they are −13.4 and 10.7 kcal mol −1 for 1 and 2, respectively).Furthermore, we have used an additional dimer, where the halido ligand that participates in the H-bonding interactions has been replaced by hydride, and consequently, the H-bonding interactions are not formed.The resulting interaction energies are further reduced to ΔE 3 = −25.2kcal mol −1 and ΔE 6 = −24.7 kcal mol −1 for 1 and 2, respectively, which corresponds to the contribu-tion of the π-π stacking interactions between the CN-N-C(O) part and other long range van der Waals interactions.The contribution of both H-bonding interactions can be estimated by difference (they are −20.1 and −11.3 kcal mol −1 for 1 and 2, respectively).Therefore, the H-bonding interactions are stronger in compound 1, which is likely due to the larger polarizability of Br − with respect to Cl − .
In the isostructural compounds 4-6, the π-stacking binding mode is different to the one observed for 1 and 2. As previously mentioned, a chelate ring⋯chelate ring π-π interaction is formed, in addition to the H-bonding and conventional π-π interactions (see Fig. 11a).We have studied theoretically the energetic features of the dimers of compounds 4 (Cd) and 6 (Zn) to analyse the effect of the metal center.Interestingly, the chelate ring⋯chelate ring distance is significantly shorter (3.44 Å for 4 and 3.50 Å for 5 and 6) than the distance of the π-π stacking interaction between the CN-N-C(O) moieties in 1 and 2 (see Fig. 10a).Moreover, the H-bonding distances are longer in compounds 4 and 6 with respect to compounds 1 and 2. As a consequence, the computed interaction energies of the self-assembled dimers 4 and 6 (ΔE 7 = −52.7 kcal mol −1 and ΔE 10 = −53.9kcal mol −1 , respectively) are similar to those computed for 1 and 2 due to a  compensating effect between the longer H bond and the shorter chelate ring⋯chelate ring interaction.an effort to calculate the contribution of the different interactions, we have computed a theoretical model where the uncoordinated pyridine rings have been replaced by H atoms (see the small arrows in Fig. 11b) and consequently the conventional π-π stacking interactions are not formed.As a result, the interaction energies are reduced to ΔE 8 = −46.5 kcal mol −1 and ΔE 11 = −41.6 kcal mol −1 in 4 and 6, respectively.Therefore, this contribution (both π-π interactions) can be roughly estimated by difference (−6.2 and −12.3 kcal mol −1 for 4 and 6, respectively).These values are likely underestimated because the substitution of the uncoordinated pyridine by a H-atom reinforces the N-H⋯X H-bonding due to the elimination of the intramolecular N-H⋯NĲPy) H-bond.Furthermore, we have used an additional dimer, where the halido ligands that participate in the H-bonding interactions have been replaced by hydride, and consequently, the H-bonding interactions are not formed.The interaction energies are further reduced to ΔE 9 = −22.8kcal mol −1 and ΔE 6 = −27.7 kcal mol −1 for 4 and 6, respectively, which corresponds to the contribution of the chelate ring⋯chelate ring π-π stacking interactions and other long range van der Waals interactions.
In order to provide additional evidence of the existence of unconventional π-π stacking interactions between the CN-N-C(O) moieties and the chelate-ring interactions, we have analysed the self-assembled π-stacked dimers of compounds 2 and 5 (as exemplifying models) using Bader's theory of "atoms in molecules" (AIM), 38 which provides an unambiguous definition of chemical bonding.The AIM theory has been successfully used to characterize and understand a great variety of interactions including those described herein. 39In Fig. 12, we show the AIM analysis of compounds 2 and 5.In 2, it can be observed that each conventional π-π interaction (pyridine rings) is characterized by the presence of one bond critical point that interconnects two carbon atoms of the coordinated and uncoordinated pyridine rings, thus confirming the interaction.Furthermore, the distribution of critical points reveals the existence of two symmetrically related N-H⋯Cl H-bonding interactions.Each one is characterized by a bond critical point and a bond path connecting one H atom of the NH group with the Cl ligand.Finally, the unconventional π-π interactions between the CN-N-C(O) moieties is confirmed by the presence of four bond critical points interconnecting the CN-N-C(O) groups.In 5, the π-π interaction (pyridine rings) is characterized by the presence of two bond critical points and bond paths that connect two carbon atoms of the coordinated pyridine to two carbon atoms of the uncoordinated one.Furthermore, the distribution of critical points reveals the existence of two types of H-bonding interactions: N-H⋯Cl and C-H⋯O (chelate ring).This ancillary C-H⋯O interaction explains the large interaction energy obtained for the chelate ring⋯chelate ring π-π interaction (see Fig. 11c).Finally, the chelate ring⋯chelate ring interaction is characterized by two bond critical points and bond paths that connect the O atom of one ring to the nitrogen atom of the other chelate ring and vice versa, thus validating the existence of the interaction.The value of the Laplacian of the charge density at the bond critical points is positive, as is common in closed-shell interactions.

Concluding remarks
We reported the syntheses and X-ray structural characterization of seven new metal complexes of CdĲII) and ZnĲII) metal centers with two hydrazine-based ligands.Most compounds exhibit remarkable chelate ring-chelate ring and π-π stacking interactions in the solid state that have been studied using DFT calculations and Hirshfeld analysis.These interactions are crucial for the formation of supramolecular selfassembled dimers in the solid state.The energies associated with the interactions, including the contribution of the different forces, have been evaluated.In general, the chelate-chelate interactions are stronger than those those reported for conventional π-π complexes, 40 which is attributed to the coexistence of other long range van der Waals interactions.The results reported herein might be useful to understand the solid state architecture of MOF materials that contain MĲII)rings and organic aromatic molecules.
†) range between 2.358Ĳ3)-2.412Ĳ3)Å, both Cd-O carboxyl bond distances are 2.398(2) and 2.547(2) Å, and the Cd-O nitrate bond distances are 2.454(3) and 2.570(3) Å.The non-coordinated nitrate anion establishes a hydrogen bonding interaction with the acidic N6-H group.Crystal structures of CdĲHL2 )Br 2 (4), ZnĲHL 2 )Cl 2 (5) and ZnĲHL 2 )Br 2 (6) Isostructural compounds 4 (M = Cd, X = Br), 5 (M = Zn, X = Cl) and 6 (M = Zn, X = Br) crystallize in the P1 ¯space group, in which MĲII) (M = Cd, Zn) is neutralized by two Cl − or two Br − anions.The asymmetric unit of 4-6 is composed of two mononuclear Zn complexes (see Fig. 4 for an illustration of complex 4).The ligand coordinates the CdĲII) atom through three coplanar ligating sites involving the carbonyl O, the hydrazine N and the pyridyl nitrogen forming two fivemembered chelate rings.The main difference between the mononuclear complexes A and B present in the asymmetric unit is the dihedral angle between the imino group and the

Fig. 1
Fig. 1 X-ray structures of isostructural complexes 1 and 2 and the atomic numbering scheme.

Fig. 3 Fig. 4
Fig. 3 Perspective view of Cd-complex 3 and the atomic numbering scheme.

4
ĲL 2 ) 4 I 2 ]ĳZnI 4 ].2H 2 O Compound 7 (Fig. 6) crystallizes with Z′ = 1/2 in the space group C2/c.The asymmetric unit of 7 contains three ZnĲII) ions, two L 2 ligands, three I anions and two non-coordinated water molecules.The structure of 7 includes a cationic tetranuclear cluster of four ZnĲII) ions, four L 2 ligands, and two I anions, counterbalanced by a ZnI 4 2− ion.The Zn⋯Zn separations are 4.132 Å and 5.019 Å.The L 2 ligand self-assembles in the presence of ZnĲII) to give a rectangular [2 + 2] grid complex (Fig. 7), with two ligands bridging adjacent ZnĲII) ions on the short sides of the rectangle with an alkoxide oxygen, and two bridging the adjacent ZnĲII) ions on the long sides of the rectangle with N-N diazine groups.In 7, the ZnĲII) ions are of three coordination types.Firstly, the Zn1 atom is coordinated by three nitrogen atoms and two oxygen atoms from two different L 2 ligands and one coordinated I atom, thus showing an octahedral coordination geometry.Secondly, the Zn2 atom is coordinated by four nitrogen atoms and one oxygen atom from two different L 2 ligands, thus showing a square pyramidal coordination geometry.Finally, the Zn3 atom is located on a symmetry center and is coordinated by four I atoms, thus showing a tetrahedral coordination geometry.The bond distances of Zn-N (see Table

Fig. 6
Fig. 6 Perspective view of complex 7 and the atom numbering scheme.H-atoms are omitted for clarity.

Fig. 8
Fig. 8 Views of the Hirshfeld surfaces for 3 (left) and 7 (right) mapped with d norm .

Fig. 9
Fig. 9 MEP surface of compound 2. The MEP values at selected points are given in kcal mol −1 .

Fig. 10
Fig. 10 (a) Interaction energies of the self-assembled π-stacked dimers observed in the solid state of compounds 1-2.(b and c) Interaction energies in several theoretical models of 1 and 2.

Fig. 11
Fig. 11 (a) Interaction energies of the self-assembled π-stacked dimers observed in the solid state of compounds 4 and 6. (b and c) Interaction energies in several theoretical models of 4 and 6.