Syntheses,Crystal Structures,Hirshfeld Surfaces Analyses and Fluorescence Properties of Two Tetranuclear Nickel（Ⅱ） and Zinc（Ⅱ） Complexes Based on an Unsymmetrical Salamo-like N2O4-Donor Ligand

WANG Yan-BinYU MengZHANG YuSU Qiong＊,DONG Wen-Kui＊,

(1Key Laboratory of Environmental Friendly Composite Materials and Biomass in Universities of Gansu Province,Chemical Engineering Institute,Northwest University for Nationalities,Lanzhou 730030,China)

(2School of Chemical and Biological Engineering,Lanzhou Jiaotong University,Lanzhou 730070,China)

Abstract:Two newly designed tetra-nuclear transition metal（Ⅱ） complexes,[Ni2(L)(μ-OAc)(CH3OH)]2·4CH3OH(1)and[Zn2(L)(μ-OAc)(CH3CH2OH)]2(2),derived from an unsymmetrical salamo-like donors-N2O4ligand(H3L=6-hydroxy-6′-methoxy-2,2′-(ethylenediyldioxybis(nitrilomethylidyne))diphenol)were synthesized and characterized by elemental analyses,infrared spectra,ultraviolet-visible spectra,fluorescence spectra,Hirshfeld surfaces analyses,and single crystal X-ray crystallography.X-ray crystallographic analyses show that complexes 1 and 2 are all symmetrical four-nuclear complexes.All hexa-coordinated nickel（Ⅱ）ions in complex 1 form distorted octahedral geometries,while all penta-coordinated zinc（Ⅱ）ions in complex 2 form twisted quadrangular pyramid and triangular biconical geometries.CCDC:1984199,1;1984198,2.

Keywords:salamo-like N2O4-donor ligand;complex;synthesis;crystal structure;Hirshfeld surface analysis

0 Introduction

It is common knowledge that Salen and its many different kinds of analogues(R-CH=N-(CH2)2-N=CHR)have been extensively investigated in modern coordination chemistry and organometallic chemistry for several decades[1-5].They are one of the most versatile multidentate chelating ligands and could form stable mono-or multi-nuclear metal（Ⅱ） complexes[6-9]with alkaline earth,rare earth and transition metal ions,and their potential applications have been explored in biological system[10-15].Moreover,they have certain advantages in luminescent[16-22]and magnetic[23]materials,ion recognitions[24-26],electrochemical fields[27-30],supramolecular constructions[31-37],and so on.

In recent years,our research mostly concentrated on the syntheses of salamo-like ligandsas derivatives of Salen[38-46].These compounds have been investigated in forming transition metal（Ⅱ）complexes with interesting properties[47-49].Some works have been devoted to synthesizeandcharacterizemono-,di-andmulti-nuclear metal（Ⅱ）complexes bearing salamo-like ligand and its derivatives[50-53].However,there are few researches on unsymmetric salamo-like ligands,and it is expected to obtain multi-nuclear transition metal（Ⅱ）complexes by introducing some groups such as alkoxy or hydroxyl substitute at 3-positions of salicylaldehyde derivatives in the salamo-like ligands.Herein,as part of our ongoinginterestinsalamo-liketransitionmetal（Ⅱ）complexes,X-ray crystal structures,spectroscopic properties and Hirshfeld surfaces analyses of two newly designed and synthesized transition metal（Ⅱ） complexes,{[Ni2(L)(μ-OAc)(CH3OH)]2·4CH3OH (1)and [Zn2(L)( μ-OAc)(CH3CH2OH)]2(2),derived from an unsymmetrical salamo-like N2O4-donor ligand(H3L=6-hydroxy-6′- methoxy-2, 2′-(ethylenediyldioxybis(nitrilomethylidyne))diphenol)have been investigated in detail.

1 Experimental

1.1 Materials and methods

3-Hydroxysalicylic aldehyde of 99% purity and 3-methoxysalicylaldehyde of 98% purity were purchased from Alfa Aesar and used without further purification.The other reagents and solvents were analytical grade reagents from Tianjin Chemical Reagent Factory.

C,H and N analyses were obtained using a GmbH VarioEL V3.00 automatic elemental analyzer.Elemental analysis for Ni（Ⅱ） and Zn（Ⅱ） were conducted using an IRIS ER/S·WP-1ICP atomic emission spectrometer.1H NMR spectrawererecorded usingaBruker AVANCE DRX-400 spectrometer.The melting points were determined by a microscopic melting point instrument made in Beijing Tektronix Instrument Limited Company and were uncorrected.IR spectra were recorded on a VERTEX70 FT-IR spectrophotometer,with samples prepared as KBr(500～4 000 cm-1)and CsI(100～500 cm-1)pellets.UV-Vis absorption spectra were recorded on a Shimadzu UV-2550 spectrometer.Fluorescent spectra were recorded on F-7000 spectrophotometer.X-ray single crystal structure determinations were carried out on a SuperNova,Dual Eos fourcircle diffractometer.Hirshfeld surfaces analyses and two-dimensional fingerprint plots of complexes 1 and 2 were calculated using Crystal Explorer program.

1.2 Synthesis of H3L

The unsymmetric salamo-like ligand H3L was synthesized by a modified method reported in the earlier literature(Scheme 1)[54].

Scheme 1 Synthetic route of H3L

1.3 Synthesis of complex 1

A methanol solution(3.0 mL)of nickel（Ⅱ）acetate tetrahydrate(4.92 mg,0.02 mmol)was added to a acetone solution(2.0 mL)of H3L(3.46 mg,0.01 mmol)at room temperature.After stirring for 2 h,the mixture was filtered off.The resulting filtrate was left undis-turbed for about a week to form block-like clear yellowish green crystals suitable for X-ray crystallographic analysis.Yield:45.8%.Anal.Calcd.for C44H60N4Ni4O22(%):C,42.91;H,4.91;N,4.55;Ni,19.06.Found(%):C,43.16;H,4.98;N,4.47;Ni,18.87.

1.4 Synthesis of complex 2

A solution of zinc（Ⅱ）acetate tetrahydrate(4.96 mg,0.02 mmol)in ethanol(2.5 mL)was added dropwise to a solution of H3L(3.5 mg,0.01 mmol)in dichloromethane(3 mL)at room temperature.The color of the mixing solution turned yellow immediately,and was stirred continually for 1 h at room temperature.The mixture was filtered and the filtrate was allowed to stand at room temperature for about two weeks.The solvent was partially evaporated,and several clear yellowish single crystals suitable for X-ray crystallographic analysis were obtained.Yield:47.3%.Anal.Calcd.for C42H48N4Zn4O18(%):C 43.55;H 4.18;N 4.84;Zn 22.58.Found(%):C 43.68;H 4.32;N 4.74;Zn 22.31.

1.5 Crystal structure determinations of complexes 1 and 2

Suitable crystals of complexes 1 and 2 with approximate dimensions of 0.18 mm×0.15 mm×0.12 mm and 0.11 mm×0.10 mm×0.08 mm,respectively,were mounted on glass rod for determining single crystal structures.X-ray diffraction data of complexes 1 and 2 were collected on a Bruker APEXⅡCCD diffractometer with a graphite monochromated Cu Kα radiation source(λ=0.154 184 nm)at 173.20(10)K,and SuperNova(Dual,Cu at zero,Eos)diffractometer using a graphite monochromated Mo Kα radiation source(λ=0.071 073 nm)at 100 K,respectively.The semi-empirical absorption corrections were applied using the SADABS program.The structures were solved by the direct methods(SHELXS-2014).All nonhydrogen atoms were refined anisotropically;the hydroxyl hydrogen atoms in the methanol molecules were located in difference Fourler maps,all other hydrogen atoms were generated geometrically and allowed to ride on their parent carbon atoms[55].The crystal data and experimental parameters relevant to the structure determinations are listed in Table 1.

CCDC:1984199,1;1984198,2.

Table 1 Crystal data and structure refinement parameters for complexes 1 and 2

Continued Table 1

2 Results and discussion

2.1 IR spectra

The FT-IR spectra of H3L with its corresponding complexes 1 and 2 exhibited various bands in the 400～4 000 cm-1region(Table 2).The hydroxyl group of H3L exhibited a characteristic absorption band at approximate 3 436 cm-1.An absorption band of the coordinated and crystallized methanol molecules was observed at approximate 3 387 cm-1in complex 1,and an absorption band of the coordinated ethanol molecules was observed at approximate 3 428 cm-1in complex 2,indicating that the Ni（Ⅱ） and Zn（Ⅱ） ions are coordinated to oxygen atoms in phenoxy groups of the fully deprotonated ligand L3-moieties.The results are in accordance with the elemental analysis results.A characteristic strong C=N stretching band of H3L emerged at approximate 1 613 cm-1,and those of complexes 1 and 2 emerged at approximate 1 602 and 1 597 cm-1,respectively[56].The changes in the spectra of complexes 1 and 2 shows that the oxime nitrogen atoms of the ligand H3L have coordinated with the Ni（Ⅱ） and Zn（Ⅱ） ions.The expected Ar-O vibration band of H3L was observed at approximate 1 253 cm-1;nevertheless,this band occurred at approximate 1 234 and 1 213 cm-1in complexes 1 and 2,respectively.The Ar-O stretching frequencies of phenoxy groups are shifted to low frequencies,which could be evidence of the Ni-O or Zn-O bond formation between the Ni（Ⅱ） or Zn（Ⅱ） ions and the oxygen atoms of phenoxy groups[57].

The far-IR spectra(500～100 cm-1)of complexes 1 and 2 were also obtained to identify the bonds of M-O and M-N frequencies.The νM-Obands at approximate 426 and 423 cm-1in complexes 1 and 2 can be assigned to νNi-Oand νZn-O,while the νM-Nbands at approximate 516 and 512 cm-1are attributed to νNi-Nand νZn-N,respectively[52].The results mentioned above are in accordance with the results of X-ray crystal diffractions.

Table 2 FT-IR spectra of H3L and complexes 1 and 2

2.2 UV-Vis spectra

The UV-Vis absorption spectra of the free ligand H3L with its corresponding complexes 1 and 2 in the ethanol solutions(10 μmol·L-1)at 298 K are shown in Fig.1.

Obviously,the absorption peaks of the free ligand H3L differ from those of complexes 1 and 2.The UVVis spectra of ligand H3L and its complexes 1 and 2 in the ethanol solutions are shown in Fig.1.From the Fig.1,we can see that the free ligand H3L showed two strong absorption peaks near 271 and 319 nm.The absorption peak at ca.271 nm can be assigned to the ππ*transitions of phenyl rings,and the absorption peak at ca.319 nm can be assigned to the intra-ligand π-π*transition of oxime group.Upon coordination of the free ligand,the π-π*transitions of phenyl rings in complexes 1 and 2 are bathochromically shifted to 274 and 278 nm,respectively,which indicates the coordination of ligand L3-moieties with Ni（Ⅱ） and Zn（Ⅱ） ions[49].Meanwhile,new absorption peaks appeared at 374 and 349 nm in complexes 1 and 2 may be accounted for the nπ*charge transfer transition from the filled p orbital of bridging phenoxo oxygen atoms to the vacant d-orbital of corresponding metal ions.

Fig.1 UV-Vis spectra of H3L and complexes 1 and 2

Fig.2 (a)Molecular structure of complex 1 with 30% probability displacement ellipsoids;(b)Coordination polyhedra for Ni（Ⅱ）ions of complex 1

2.3 Descriptions of crystal structures

X-ray crystallographic analyses reveal that complexes 1 and 2 form two different crystal structures,which are different from common tetra-nuclear structures of salamo-like complexes reported earlier[40].

2.3.1 Crystal structure of complex 1

As shown in Fig.2 and Table 3,in the crystal structure of complex 1,there are four Ni（Ⅱ）ions,two fully deprotonated L3-moieties,two coordinated MeOH molecules,two μ-acetate ligands and four crystallized MeOH molecules.Complex 1 possesses a highly symmetrical tetra-nuclear structure.The hexa-coordinated central Ni（Ⅱ）ion(Ni1)is surrounded by four oxygen atoms(O1,O1#1,O2 and O5)from the fully deprotonated L3-moiety,one oxygen atom(O7)of the μ-acetate ligand and one oxygen atoms(O6)from the coordinated methanol molecule adopting a twisted octahedral geometry.The hexa-coordinated terminal Ni（Ⅱ）ion(Ni2)lies in the N2O2-donor coordination sphere(N1,N2,O2 and O5)of the fully deprotonated L3-unit,and coordinates further to one oxygen atom(O8)from the μ-acetate ligand and one oxygen atoms(O9)from the coordinated methanolmolecule adopting a twisted octahedral geometry.

As illustrated in Fig.3 and Table 4,there are two intra-molecular hydrogen bonding inter-actions(C8-H8A…O9 and C20-H20B…O5)in the crystal structure of complex 1.

Fig.3 View of intra-molecular hydrogen bondings of complex 1

Table 4 Hydrogen bonding interaction parameters for complex 1

2.3.2 Crystal structure of complex 2

Fig.4 (a)Molecular structure of complex 2 with 30% probability displacement ellipsoids;(b)Coordination polyhedra for Zn（Ⅱ）ions of complex 2

X-ray crystallographic analysis reveals that complex 2 is a symmetric trinuclear structure.It crystallizes in the triclinic system,space group.As shown in Fig.4,the obtained complex 2 includs four Zn（Ⅱ）ions,two fully deprotonated L3-moieties,two coordinated ethanol molecules and two μ-acetate ligands.The penta-coordinated central Zn（Ⅱ）ion(Zn1)is surrounded by three oxygen atoms(O6,O6 and O5)from the fully deprotonated L3-moiety,one oxygen atom(O8)of the μ-acetate ligand and one oxygen atom(O9)from the coordinated ethanol molecule,forming a square pyramid geometry.Unlike the center Zn（Ⅱ）ion,the pentacoordinated terminal Zn（Ⅱ）ion(Zn2)lies in the N2O2-donor coordination sphere(N1,N2,O1 and O5)of the fully deprotonated L3-unit,and coordinates further to one oxygen atom(O7)from the μ-acetate ligand,adopting a twisted triangular bipyramidal geometry.Herein,we have synthesized a novel structural tetranuclear complex 2,where all the Zn（Ⅱ） ions are penta-coordinated with distorted tetragonal pyramidal and trigonal bipyramidal symmetries.In order to get the geometries adopted by Zn1 and Zn2,the τ values were estimated to be τ1=0.143 and τ2=0.725[58-59],respectively.

Table 5 Selected bond lengths(nm)and angles(°)for complex 2

As illustrated in Fig.5,there are three intramolecular(O9-H9…O1,O9-H9…O2,and C9-H9A…O7)hydrogen bonding interactions.As shown in Fig.6 and Table 6,complex 2 molecules are linked into an infinite 2D supramolecular structure by inter-molecular(C7-H7B…O3,C10-H10A…O4)hydrogen bonding interactions in the crystal structure of complex 2[60-64].

Fig.5 Intra-molecular C-H…O hydrogen bondings of complex 2

Fig.6 Two dimensional supramolecular structure linked by inter-molecular C-H…O hydrogen bondings in complex 2

Table 6 Hydrogen bonding interaction parameters for complex 2

2.4 Hirshfeld surfaces analyses

The Hirshfeld surfaces[65]of complexes 1 and 2 are illustrated in Fig.7.The surfaces have been mapped over dnorm,and the corresponding location in shape index exists the complementary region of red concave surface surrounded by receptors and the blue convex surface surrounding receptors,further proving that such hydrogen bonding exists.As for the large amount of white region in dnormsurfaces,it is suggested that there is a weaker and farther contact between molecules,rather than hydrogen bonding.

Fig.7 Hirshfeld surfaces analyses mapped with curvedness,dnormand shape index of complexes 1 and 2

Fig.8 Fingerprint plots of complexes 1(a)and 2(b)

Fig.8 shows the 2D plots generated[66]corresponding to the O…H,C…H and H…H interactions from the Hirshfeld surfaces of complexes 1 and 2.As shown in Fig.8a,for complex 1,the H…H interactions appear at(0.115 7 nm,0.114 7 nm)and account for 67% of the total area of Hirshfeld surfaces.The C…H/H…C interactions are in the range of(0.157 8 nm,0.106 7 nm)and appear as a pair of symmetrical wings,accounting for 11.4% of the total area of Hirshfeld surfaces.The proportions of O…H/H…O interactions are comprised of 17.7% of the total Hirshfed surfaces for each molecule of complex 1.As shown in Fig.8b,for complex 2,the interactions of H…H appear at(0.115 nm,0.115 nm)accounting for 45.4% of the total area of Hirshfeld surfaces.The C…H/H…C interactions in the range of(0.160 nm,0.105 nm)account for 24.2% of the total area of Hirshfeld surfaces.The proportions of O…H/H…O interactions are comprised of 24.3% of the total Hirshfed surfaces for each molecule of complex 2.It is because of the existence of these weaker hydrogen bonds that complexes 1 and 2 can be stable.

2.5 Fluorescence properties

The fluorescence spectra of H3L and its corresponding complexes 1 and 2 were investigated at room temperature and are shown in Fig.9.The free ligand H3L exhibited a relatively strong emission peak at ca.400 nm upon excitation at 370 nm,and it should be assigned to the intraligand π-π*transition.Complex 1 showed a lower photoluminescence with maximum emission at ca.391 nm.Compared with the ligand H3L,emission intensity of complex 1 reduced obviously,indicating that the Ni（Ⅱ）ions have a quality of fluorescent quenching.On the other hand,complex 2 showed an obvious fluorescence enhancement at ca.395 nm.The intense peak is likely due to the coordination of H3L with the Zn（Ⅱ）ions,which breaks the intramolecular hydrogen-bonding interactions of H3L and increases the coplanarity of the conjugated system.

Fig.9 Emission spectra of H3L and complexes 1 and 2

3 Conclusions

In summary,we have reported the successful syntheses and characterizations of two newly designed tetra-nuclear metal（Ⅱ） complexes,[Ni2(L)( μ-OAc)(CH3OH)]2·4CH3OH(1)and[Zn2(L)( μ-OAc)(CH3CH2OH)]2(2),derived from an unsymmetrical salamo-like ligand H3L.All the hexa-coordinated Ni（Ⅱ）ions in complex 1 adopt slightly twisted octahedral configurations.The penta-coordinated Zn（Ⅱ）ions(Zn1 and Zn1#1)in complex 2 form square pyramid geometries,while the other penta-coordinated Zn（Ⅱ）ions(Zn2 and Zn2#1)possess twisted triangular bipyramidal geometries.The inter-molecular hydrogen bonding interactions in crystal structure of 2 result in a self-assembled infinite 2D supramolecular network.

Acknowledgements:This work was supported by the National Natural Science Foundation of China (Grant No.21968032),the Fundamental Research Funds of Central Universities-Innovation Team Cultivation Project (Grant No.31920190012),the Northwest Minzu University′s Double First-class and Characteristic DevelopmentGuide Special Funds-Chemistry Key Disciplines in Gansu Province(Grant No.11080316),and the Teaching Quality and Reform Engineering Project of Gansu University(Grants No.2019GSSYJXSFZX-01,2019GSJXCGPY-16),which are gratefully acknowledged.