吳 瓊 唐亞芳 資巧麗

（1昆明學(xué)院化學(xué)化工學(xué)院，昆明 650214）

（2云南省塑料薄膜制品工程技術(shù)研究中心，昆明 650214）

0 Introduction

Supramolecular chemistry isan important research field including multiple-subject ranging from pharmaceutics to nanotechnology industries[1-4].During the last decade,great efforts have been devoted to the rational design and synthesis of supramolecular aggregates with desired structure and properties[5-7].

Different supramolecular interaction resulting in different solid geometry and even physicochemical properties, such as activity of pharmaceutical ingredients （APIs）,magnetic behavior of molecular magnets.As is known to all,chemical reactivity and properties of organic compound are differed wildly by substitution of halogen atom[8-10].Furthermore,halogen atom has recently been recognized as an important factor in mediation the supramolecular network of crystalline materials[11-12].However,compared with the classic hydrogen-bond interactions,the contact formed by halogen atom is much weaker,so it is difficult to carry out qualitative and quantitative research,especially for the N-and O-rich ligands constructing supramolecular complex[13-15].Hence,related work is still at a very early stage.

Therefore,establishing a series of halogenated ligands constructing complexes with similar structure and composition is a fundamental way for detailed recognition of the intermolecular interactions formed by halogen atom.In this aspect,tetradentate Schiff base （salen）complex is an ideal candidate,not only because the complex has diverse structures and tunable constitution[16-18],but also because it is a well researched subject.Thus,subtle difference between analogs can be easily detected and studied[19-20].

Based on the aforementioned considerations,recently,we have initiated exploitation of the relationship between the composition of salen-type halogenated Schiff-base ligands and the structure of their complexes.Herein,we specifically introduced isomorphic halogenated salen as starting material,and isolated two nickelギ complexes[Ni（3,5-Cl-salcy）] （1）and[Ni（3-Cl-salcy）] （2）（3,5-Cl-salcyH2=N,N′-（± ）-bis（3,5-dichlorosalicylidene）cyclohexane-1,2-diamine and 3-Cl-salcyH2=N,N′-（± ）-bis（3-chlorosalicylidene）cyclohexane-1,2-diamine）.Theoretical analysis results reveal that halogen atoms play a key role in stabling the solid structures of title complexes.

1 Experimental

1.1 M aterial and methods

All chemicals were commercially available and used without further purification.Elemental analyses（C,H and N）were carried out on a Perkin-Elmer 240C instrument.Ni was analyzed on a PLASMA-SPECガICP atomic emission spectrometer.IR spectra were recorded in a range of 400～4 000 cm-1on an Alpha Centaurt FT/IR Spectrophotometer using KBr pellets.1H NMR spectra have been recorded by using Bruker 600 MHz Digital NMR Spectrometer resonance instrument（AVANCE Ⅲ 600 MHz）.

1.2 Synthesis

1.2.1 Synthesis of halogenated Schiff base ligand

The halogenated tetradentate Schiff base ligand 3,5-Cl-salcyH2and 3-Cl-salcyH2were prepared by mixing salicylaldehyde（3,5-dichlorosalicylaldehyd and 3-chlorosalicylaldehyd for 3,5-Cl-salcyH2and 3-ClsalcyH2,respectively） （0.2 mmol）and 1,2-diaminocyclohexane （0.012 mL,0.1 mmol,mixture of cis-and trans-form）in 30mL of methanol.The obtained solution was stirred and refluxed at 80℃for three hours,then the solvent was removed by rotary evaporator to give yellow powder product （Scheme 1）.

Scheme 1 Schematic drawing of halogenated tetradentate Schiff base ligands 3,5-Cl-salcyH2 and 3-Cl-salcyH2

3,5-Cl-salcyH2:Yield:0.035 2 g （75%）.Elemental analysis for C20H18Cl4N2O2（%）:C:52.2;H:3.94;N:6.08.Found（%）:C:51.2;H:3.93;N:6.13.1H NMR（600 MHz,methanol-d4）:δ8.38 （d,J=62.4 Hz,1H,-N=CH-）,7.48～7.10 （m,2H,ArH）,4.50 （s,1H,Ar-OH）,3.85 （dt,J=6.7,3.1 Hz,1H,-C*H-）,2.06～1.77 （m,2H,-CH2-）,1.76～1.18 （m,2H,-CH2-）.FT-IR （cm-1）:3 419（O-H）,1 598 （C=N）,1 389 （-CH2-）.

3-Cl-salcyH2:Yield:0.034 3 g （86%）.Elemental analysis for C20H20Cl2N2O2（%）:C:61.39;H:5.15;N:7.15.Found（%）:C:61.57;H:4.60;N:7.27.1H NMR（600 MHz,methanol-d4）:δ8.38 （d,J=68.5 Hz,1H,-N=CH-）,7.57～6.92 （m,2H,ArH）,6.65 （dt,J=34.9,7.8 Hz,1H,Ar-OH）,4.09～3.68 （m,1H,-C*H-）,2.19～1.79 （m,2H,-CH2-）,1.78～1.43 （m,2H,-CH2-）.FT-IR （cm-1）:3 432 （O-H）,1 598 （C=N）,1 389 （-CH2-）.

1.2.2 Synthesis of[Ni（3,5-Cl-salcy）]（1）

Ni（NO3）2·6H2O （0.060 0 g,0.2 mmol）wasdissolved in 10 mL methanol,and the solution was added to 20 mLof a methanolic solution of 3,5-Cl-salcyH2（0.031 6 g,0.1 mmol）,the resulting dark red suspension was stirred for 2 h.After cooling,the reddish filtrate was sealed in a beaker and kept undisturbed at room temperature.The red block crystals of complex 1 were afforded after one week.Yield:35%.Anal.Calcd.for C20H16Cl4N2NiO2（%）:C,46.47;H,3.12;N,5.42.Found（%）:C,47.13;H,3.31;N,5.14.

1.2.3 Synthesis of[Ni（3-Cl-salcy）] （2）

Complex 2 was prepared in a similar method as that of complex 1,except the ligand 3-Cl-salcyH2（0.024 5 g,0.1 mmol）was used to replace 3,5-ClsalcyH2.The red block crystals of 2 were afforded after oneweek.Yield:42%.Anal.Calcd.For C20H18Cl2N2NiO2（%）:C,53.62;H,4.05;N,6.25;Found:C,53.93;H,4.12;N,6.14.

1.3 X-ray crystal structure analysis

The data set of reflections were collected at 293（2）K on an Xcalibur Eos automated diffractometer with Mo Kα radiation （λ=0.071 073 nm）and a low-temperature device Cryostream cooler （Oxford Cryosystem）.Integration of the intensities and correction for Lorentz and polarization effects were performed using the CrysAlisPro software[21-22].The central nickel atom was located by direct method,and successive Fourier syntheses revealed the remaining atoms.Refinements were achieved by the full-matrix method on F2using the Olex2 software package[23].In the final refinement,all the non-H atoms were anisotropically refined.All H atoms were placed in calculated positions and refined using the riding model approximation.For the cyclohexane,C-H bonds were fixed at 0.097 nm,and the Uisovalues of the hydrogen atoms of methyl groups were set to 1.5Ueq,Cand the Uisovalues of all other hydrogen atoms were set to 1.2Ueq,C,with aromatic CH distances of 0.093 nm.The detailed crystal data and structure refinement for 1 and 2 are given in Table 1.Selective bond lengths and angles of 1 and 2 are listed in Table 2.

CCDC:1892682,1;1855958,2.

Table 1 Crystallographic data and structure refinement for com plexes 1 and 2

Continued Table 1

Table 2 Selected bond lengths（nm）and angles for com plexes 1 and 2

2 Results and discussion

2.1 Structure description

The identity of 1 and 2 were elucidated by single-crystal X-ray diffraction,elemental analysis results of were fully consistent with the crystallographic formulation.Single-crystal X-ray diffraction reveals that complex 1 crystallizes in the monoclinic P21/c（No.14）space group,which unit cell contains one crystallography independent Niギforming two racemic neutral monomers[Ni（（+）-3,5-Cl-salcy）]and[Ni（（-）-3,5-Cl-salcy）].As shown in Fig.1,the central Niギexhibits tetra-coordinated environment which is defined by[N2O2]in the equatorial plane from Schiff-base ligand.The bond lengths of Ni-O（N）are in a range of 0.183 7（3）～0.185 5（4）nm,and bond angles of O（N）-Ni-O vary from 84.23（15）°to 178.38（16）°,which are favorably comparable with the corresponding values observed in salen-type Niギa(chǎn)nalogous[24-25].

Fig.1 （a）Asymmetric unit of 1 with displacement ellipsoids drawn at 30%probability level;（b）π…π non-bonding contact of self-assembled supramolecular dimer built up by symmetry operation:1-x,-y,-z

The dihedral angle calculated between the planes of two benzene rings for 1 is 8.23（10）°,thus,the whole molecule exhibit coplanar configuration.Owing to the π-π interactions （the distances of Cg1i…Cg1 and Cg2i…Cg2 are equal to each other with centroid-to-centroid distance of 0.387 4（32）nm,Cg1:centroid of C1～C6 ring,Cg3:C15～C20 ring,Symmetry code:i1-x,-y,-z）,two neighboring molecules are further linked to each other to form a self-assembled supramolecular dimer（Fig.1（b））, in which the Ni…Ni distance of ca.0.331（26）nm （Fig.2）is smaller than the sum of two nickel atoms′van der Waals radius[26].

Fig.2 Ni…Ni no bond interactions in 1,built up by symmetry operation:1-x,-y,-z

It is noteworthy that according to the analysis result of PLATON software[27],exceptπ-π interactions,there is not any classical or unclassical hydrogen bond in the crystal structure.By comparison,Hirshfeld surface gives us qualitative and quantitative contributions of the interactions in the crystal packing.Detailed discussion is addressed in Hirshfeld surface analysis section.

Complex 2 crystallizes in the triclinic P1 （No.2）space group,and its asymmetric unit contains two identical,but crystallogaphically independent mononuclear Niギ unit.The Niギ centers present same coordination environment with 1 by chelating with chloride salen-type ligand via [N2O2]in the equatorial plane.The bond lengths of Ni-O and Ni-N in the two Niギmoieties are in a range of 0.183 7（2）～0.184 4（2）nm and 0.184 6（3）～0.185 5（3）nm,respectively.The bond angles around Niギcenters have been found to be 84.02（10）°to 94.90（11）°,respectively （Fig.3）.

Fig.3 Molecular structure of 2 showing the atom-labeling scheme

Intermolecular interaction analysis shows that the distance between Cg1ii… Cg1 is equal to Cg2ii… Cg2（Symmetry code:ii-x,1-y,-z）and Cg3iii… Cg3 is equal to Cg4iii…Cg4 （Symmetry code:iiix,-y,1-z）,with centroid-to-centroid distance being 0.419 6 （2）and 0.382 2（2）nm,respectively,where Cg1,Cg2,Cg3 and Cg4 are the centroids of the C1～C6,C5～C20,C21～C26 and C35～C40,respectively.Beside these offsetπ-π stacking created by neighboring monomers,hydrogen bonds formed by phenolic oxygen atoms also play an important part in linking the neighboring racemic monomers to form self-assembled supramolecular dimers（C8-H8…O1iii:D…A 0.339 3（5）nm,D-H… A 145°）.According to the analysis result of PLATON software[27],although there is no classic H-bond interactions in 2,C-H…Cl hydrogen bonds play a key role in stabilizing the crystal structure.As illustrated in Fig.4,the adjacent monomers are connected by multiple C-H…Cl hydrogen bonds interactions to generate a two-dimensional layered structure.Detailed hydrogen-bond geometry information is summarized in Table 3.

Fig.4 Two-dimensional layer structure of complex 2 formed by intramolecular hydrogen bonding

Table 3 PLATON analysis result of hydrogen bond parameters for comp lex 2

2.2 IR absorbance spectrum

The IR spectra of the complexes are very similar in the given region,and therefore,the spectrum of complex 1 is described here representatively.The IR spectrum （Fig.5）showed a broad band centered in a range of 2 950～2 850 cm-1and a strong peak at 1 447 cm-1are assigned to ν（-CH2-）vibrations indicating the presence of-CH2-group of the cyclohexane unit.The strong peak at 1 632 cm-1corresponds to the vibration of the C=N bonds.The characteristic peaks at 1 132 and 575 cm-1are attributed to the vibrations of ν（Ni-O）and ν（Ni-O）,respectively,and the peak at 729 cm-1is attributed to the ν（C-Cl）.These results are consistent with the structural analysis.

Fig.5 IR spectrum of 1

2.3 Hirshfeld surface analysis

The bundle of inter-related weak molecular interactions is difficult to be unraveled by traditional methods[28].Hirshfeld surface analysis provides us a new perspective on the issue.This analysis method can provide detailed explanation about the nearby environment of the molecules in a crystal,and the comparison between similar structures can give a more specific description of the difference[29].Although hydrogen bond interaction formed by halogen is weak and cannot be easily find out by traditional methods,Hirshfeld surface in the 2D fingerprint plots show that,for complex 1,the Cl…H-C interaction （Fig.6a）covers the highest proportion （42.5%）of the total plots,whereas the C…H-C （Fig.6b）contacts only cover less than a third proportion of Cl…H-C interaction （14.0% ）.These results indicate that the halogen atoms in 1 play the most important part in stabilizing the solid stature.

Fig.6 Two-dimensional fingerprint plots of complex 1 resolved into Cl…H/H…Cl（a）and C…H/H…C （b）

Fig.7 Two dimensional fingerprint plots of complex 2 resolved into Cl…H/H…Cl（a）and C…H/H…C （b）contacts

As shown in Fig.7,Cl…H/H…Cl bonding still appears to be a major contributor in the crystal packing of 2,and the proportion is significantly lower than that in 1,merely comprising 26.1%of the total Hirshfeld surfaces,whereas C…H/H…C close contacts contribution appear higher in the 2D fingerprint plot（19.8%）.These results indicate that although the structure and composition of 2 is similar with 1,different substitution number of chlorine atoms may result in an entirely different intermolecular interactions in solid structures.

3 Conclusions

In summary,two new salen-type halogenated Schiff-base Niギcomplexes have been synthesized by conventional method.The structures of the complexes were determined by single-crystal X-ray diffractions.A detailed comparison of Hirshfeld surface analysis result indicate that although the interaction formed by halogen atom is weak,halogen atoms can play an important role in supramolecular interaction.As different supramolecular interaction can lead a totally different physicochemical properties,further work will continue focus on the synthesis of halogenate ligands constructed complexes to systematically explore the regulating effect of halogen atoms.

Acknow ledgement:This work was supported by Fund for Less Developed Regions of the National Natural Science Foundation of China （Grant No.31760257）;Joint Research Projects of Yunnan province of Local of institutions （partial）of higher education （Grant No.2017FH001-002）;Agricultural plastic film and products research and development of Plateau area（Grant No.2016DH006）;The Kunming University Chemistry&Chemical engineering students′science and technology innovation project（Grant No.HXHG1808）.