鄒訓(xùn)重 吳 疆 顧金忠 趙 娜 馮安生 黎 彧＊，

(1廣東輕工職業(yè)技術(shù)學院，廣東省特種建筑材料及其綠色制備工程技術(shù)研究中心/佛山市特種功能性建筑材料及其綠色制備技術(shù)工程中心，廣州 510300)

(2青海民族大學藥學院，青藏高原植物資源化學省級重點實驗室，西寧 810007)

(3蘭州大學化學化工學院，蘭州 730000)

0 Introduction

In recent years，the design and hydrothermal syntheses of functional coordination polymers have attracted tremendous attention owing to their fascinating architectures and topologies，as well as potential applications in catalysis，magnetism，luminescence and gas absorption[1-10].Even after years of comprehensive study，it is difficult to predict the structures of coordination polymers，because a lot of factors influence the construction of complexes，such as the structural features of organic ligands，the coordination requirements of metal ions，solvent systems，temperatures，and pH values[11-17].

In this regard，various types of aromatic polycarboxylic acids have been proved to be versatile and efficient candidates for constructing diverse coordination polymers due to their rich coordination chemistry，tunable degree of deprotonation，and ability to act as H-bond acceptors and donors[3，13，17-20].

In order to extend our research in this field，we chose a rigid linear tricarboxylic acid ligand，2，5-di(4-carboxylphenyl)nicotinic acid(H3L)，to construct novel coordination compounds.The ligand possesses the following features:(1)it contains a pyridyl and two phenyl rings with structural flexibility and conformation，and rotation of the C-C single bond between pyridyl and phenyl rings could form numbers of coordination geometries of metal ions；(2)it has seven potential coordination sites，one N atom from pyridyl ring and six O atoms of three carboxylate groups，which is beneficial to construct coordination polymers with interesting structures by its rich coordination modes；(3)it can act as hydrogen-bond acceptor as well as donor，depending upon the degree of deprotonation.

Taking into account these factors，we herein report the syntheses，crystal structures，and magnetic properties of two Ni（Ⅱ） coordination compounds constructed from H3L.

1 Experimental

1.1 Reagents and physical measurements

All chemicals and solvents were of AR grade and used without further purification.Carbon，hydrogen and nitrogen were determined using an Elementar Vario EL elemental analyzer.IR spectra were recorded using KBr pellets and a Bruker EQUINOX 55 spectrometer.Thermogravimetric analysis(TGA)data were collected on a LINSEIS STA PT1600 thermal analyzer with a heating rate of 10℃·min-1.Magnetic susceptibility data were collected in the 2～300 K temperature range with a Quantum Design SQUID Magnetometer MPMS XL-7 with a field of 0.1 T.A correction was made for the diamagnetic contribution prior to data analysis.

1.2 Synthesis of[Ni2(μ-HL)2(2,2′-bipy)2(H2O)4]·6H2O(1)

A mixture of NiCl2·6H2O (0.024 g，0.10 mmol)，H3L (0.036 g，0.10 mmol)，2，2′-bipy (0.016 g，0.1 mmol)，NaOH(0.012 g，0.20 mmol)，and H2O(8 mL)was stirred at room temperature for 15 min，and then sealed in a 25 mL Teflon-lined stainless steel vessel，and heated at 120 ℃ for 3 days，followed by cooling to room temperature at a rate of 10 ℃·h-1.Blue block-shaped crystals of 1 were isolated manually，and washed with distilled water.Yield:60%(based on H3L).Anal.Calcd.for C60H58Ni2N6O22(%):C 54.08，H 4.39，N 6.31；Found(%):C 54.37，H 4.36，N 6.33.IR(KBr，cm-1):3 451m，3 277m，1 694m，1 599s，1 560s，1 476w，1 448m，1 392s，1 314w，1 280w，1 224w，1 174w，1 097w，1 052w，1 013w，918w，901w，873w，790w，762m，739w，707w，668w，657w.

1.3 Synthesis of{[Ni(μ-HL)(2,2′-bipy)(H2O)2]·H2O}n(2)

Synthesis of 2 was similar to 1 except using 160℃instead of 120℃as the temperature of hydrothermal reaction.Green block-shaped crystals of 2 were isolated manually，and washed with distilled water.Yield:57%(based on H3L).Anal.Calcd.for C30H25NiN3O9(%):C 57.17，H 4.00，N 6.67；Found(%):C 56.92，H 3.98，N 6.69.IR(KBr，cm-1):3 339w，3 033w，1 677m，1 604m，1 560s，1 521m，1 476w，1 431m，1 386s，1 319w，1 287m，1 192w，1 153w，1 125w，1 103w，1 058w，1 008w，968w，918w，857w，806w，778m，734w，711w，678w，650w.The complexes are insoluble in water and common organic solvents，such as methanol，ethanol，acetone，and DMF.

1.4 Structure determinations

The data of two single crystals with dimensions of 0.25 mm×0.24 mm×0.22 mm (1)and 0.26 mm×0.23 mm×0.22 mm(2)were collected at 293(2)K on a Bruker SMART APEXⅡCCD diffractometer with Mo Kα radiation(λ=0.071 073 nm).The structures were solved by direct methods and refined by full matrix least-square on F2using the SHELXTL-2014 program[21]. All non-hydrogen atoms were refined anisotropically.All the hydrogen atoms were positioned geometrically and refined using a riding model.A summary of the crystallography data and structure refinements for 1 and 2 is given in Table 1.The selected bond lengths and angles for complexes 1 and 2 are listed in Table 2.Hydrogen bond parameters of complexes 1 and 2 are given in Table 3 and 4.

CCDC:1909474，1；1909475，2.

Table 1 Crystal data for complexes 1 and 2

Table 2 Selected bond lengths(nm)and bond angles(°)for complexes 1 and 2

Table 3 Hydrogen bond parameters for complex 1

Table 4 Hydrogen bond parameters for complex 2

2 Results and discussion

2.1 Description of the structure

2.1.1 [Ni2(μ-HL)2(2，2′-bipy)2(H2O)4]·6H2O(1)

Single-crystal X-ray diffraction analysis reveals that complex 1 crystallizes in the triclinic space group P1.Its asymmetric unit contains one crystallographically unique Ni（Ⅱ） ion，one μ-HL2-block，one chelating 2，2′-bipy moiety，two H2O ligands，and three lattice water molecules.As depicted in Fig.1，the sixcoordinated Ni1 center is bound by two O atoms from two μ-HL2-blocks，two O atoms from two H2O ligands，and two N atoms from 2，2′-bipy moiety，thus resulting in an octahedral{NiN2O4}environment.The lengths of the Ni-O bonds range from 0.206 6(2)to 0.210 9(2)nm，whereas the Ni-N distances vary from 0.205 9(2)to 0.207 5(2)nm；these bonding parameters are comparable to those found in other reported Ni（Ⅱ）complexes[10，15].In 1，the HL2-block behaves as a μ-spacer(modeⅠ，Scheme 1).Its nicotinate N donor remains uncoordinated while two COO-groups are mono-dentate.The dihedral angles between pyridyl and phenyl rings in the HL2-are 49.84°and 54.79°.The μ-HL2-blocks connect two Ni1 ions to give a Ni2molecular unit having a Ni…Ni distance of 1.337 1(2)nm(Fig.2).These discrete Ni2units are assembled to a 3D supramolecular framework through O-H…O/N hydrogen bond(Fig.3 and Table 3).

Scheme 1 Coordination modes of HL2-ligands in complexes 1 and 2

Fig.1 Asymmetric unit of complex 1 with 30%probability thermal ellipsoids

Fig.3 Perspective of 3D supramolecular framework parallel to ac plane in 1

Fig.2 Dinuclear Ni（Ⅱ）unit of complex 1

2.1.2 {[Ni(μ-HL)(2，2′-bipy)(H2O)2]·H2O}n(2)

The asymmetric unit of 2 consists of one Ni（Ⅱ）ion，one μ-HL2-block，one 2，2′-bipy ligand，two coordinated and one lattice water molecules.As shown in Fig.4，six-coordinates Ni1 ion reveals a distorted octahedral{NiN2O4}environment，filled by two carboxylate O atoms from two individual μ-HL2-blocks，two O atoms from two H2O ligands，and a pair of N atoms from 2，2′-bipy ligand.The Ni-O distances range from 0.202 2(2)to 0.214 1(2)nm，whereas the Ni-N distances vary from 0.206 1(2)to 0.208 7(2)nm；these bonding parameters are comparable to those observed in other Ni（Ⅱ） complexes[15，17-18].In 2，the HL2-block acts as a μ-linker via monodentate COO-groups(modeⅡ，Scheme 1)，and the nicotinate N atom remains uncoordinated.In HL2-，two dihedral angles between pyridyl and benzene rings are 19.52°and 42.02°.The HL2-linkers connect the adjacent Nil centers to form a zigzag 1D chain with the Ni1…Ni1 separation of 0.923 5(2)nm(Fig.5).

Fig.4 Asymmetric unit of complex 2 with 30%probability thermal ellipsoids

Fig.5 One dimensional chain viewed along a axis in 2

The nickel（Ⅱ）compounds 1 and 2 were prepared hydrothermally under similar reaction conditions，except using different reaction temperatures (120℃for 1 and 160℃for 2).The HL2-ligands adopt different coordination modes at 120 and 160℃ (Scheme 1)，which results in distinct structures[22-25].

2.2 TGA analysis

Fig.6 TGA curves of complexes 1 and 2

To determine the thermal stability of complexes 1 and 2，their thermal behaviors were investigated under nitrogen atmosphere by thermogravimetric analysis(TGA).As shown in Fig.6，complex 1 lost its six lattice and four coordinated water molecules in the range of 36～178 ℃ (Obsd.13.2%；Calcd.13.5%)，followed by the decomposition at 278℃.The TGA curve of 2 revealed that one lattice and two coordinated water molecules were released between 142 and 218℃ (Obsd.8.9%；Calcd.8.6%)，and the dehydrated solid began to decompose at 271℃.

2.3 Magnetic properties

Variable-temperature magnetic susceptibility studies were carried out on powder sample of 2 in the 2～300 K temperature range.The χMT value at 300 K was 1.05 cm3·mol-1·K，which is close to the expected one (1.00 cm3·mol-1·K)for one magnetically isolated Ni（Ⅱ） ion (S=1，g=2.0).Upon cooling，the χMT value decreased very slowly from 1.05 cm3·mol-1·K at 300 K to 0.981 cm3·mol-1·K at 17 K，and then decreased steeply to 0.663 cm3·mol-1·K at 2 K.In the 2 ～300 K interval，the χM-1vs T plot for 2 obeys the Curie-Weiss law with a Weiss contant θ of-5.23 K and a Curie constant C of 1.05 cm3·mol-1·K.An empirical(Weng′s)formula can be applied to analyze the 1D systems with S=1，using numerical procedures[26-27]:

Fig.7 Temperature dependence of χM T(○)and 1/χM(□)vs T for complex 2

Using this method，the best-fit parameters for 2 were obtained:g=2.08，J=-0.94 cm-1，and R=7.7×10-5，where R=∑(Tobs-Tcalc)2/∑(Tobs)2.The J value of-0.94 cm-1indicates that the coupling between the Ni（Ⅱ）centers is antiferromagnetic.

3 Conclusions

In summary，we have synthesized two Ni（Ⅱ）coordination compounds whose structures depend on the hydrothermal reaction temperature.This work demonstrates that the hydrothermal reaction temperature has a significant effect on the structures of the coordination compounds.