鞠豐陽(yáng) 李云平 劉廣臻＊,

(1洛陽(yáng)師范學(xué)院食品與藥品學(xué)院，洛陽(yáng) 471934)

(2洛陽(yáng)師范學(xué)院化學(xué)化工學(xué)院，河南省功能導(dǎo)向多孔材料重點(diǎn)實(shí)驗(yàn)室，洛陽(yáng) 471934)

Coordination polymers (CPs)are crystalline materials constructed by conformationally rigid or flexible organic ligands with different inorganic metal centers［1-3］.Typically,the self-assembly process of CPs is controllable to achieve targeting CPs with intriguing topologies,high thermal stabilities and desirable attributes through rational design of rigid organic components［4-5］.So,rigid ligands with aromatic multicarboxyl,pyridyl or imidazolyl moieties are employed widely,yielding prolific production of CPs［6-10］.And the self-assembly process becomes challengeable when the flexible organic components are used［11-12］.As we all known,molecular flexibility is derived from the sp3-hybridized carbon atom,allowing rotation about C-C single bond and different conformational preference［13］.In terms of CPs,the ultimate conformations of flexible ligands are sensitive to many factors,such as the versatile coordination geometries of metal ions,coexisting substituent groups,auxiliary rigid ligands,counter ions,pH value and temperature［14-17］.Indeed,it has been demonstrated that conformational freedom may provide rare opportunities to generate CPs with structural diversity which is inaccessible from rigid ligands,chiral centers and dynamic behavior［18-21］.

Particularly,others and our team have endeavored to the investigations of CPs assembled from homophthalic acid (H2hmph)［22-30］.As an asymmetric flexible ligand possessing both rigid-COOH group and flexible-CH2COOH group,homophthalic acid has been used to construct CPs with structures from discrete,1D chain,2D lamella,2D thick-layer,3D microporous framework to 3D→3D self-penetrated net.And these limited studies have proven that homophthalic acid is an efficient flexible ligand,which can provide strong coordination capability and versatile coordination modes.To broaden our knowledge of the coordination chemistry of homophthalic acid ligand,1,4-bis(1-imidazolyl)benzene(bib)and 3,5-bis(1-imidazolyl)pyridine (bip)were introduced into the assembled systems of CPs.As novel diimidazolyl-type ligands,two imidazolyl rings in their structures are spaced by phenyl ring and pyridiyl ring,respectively.Additionally,the places of imidazolyl rings attached to phenyl ring and pyridiyl ring are different from each other.Distinct electronic cloud densities and spatial effects can influence the structures and properties of the resulted CPs.

In this work,we obtained three coordination polymers,namely{［Ni2(hmph)2(bib)2(H2O)2］·3H2O}n(1),［Cd(hmph)(bib)］n(2)and{［Zn(hmph)(bip)］·H2O}n(3).And the preparations,crystal structures,thermal stabilities,magnetic property forcomplex 1 and fluorescent properties for 2 and 3 are described as below.

1 Experimental

1.1 Materials and methods

All chemical reagents were purchased from reagent companies and used directly without any purification.Elemental analyses (C,H and N)were performed on a Flash EA 2000 elemental analyzer.Infrared spectra(IR)were measured by a Nicolet 6700 FT-IR spectrophotometer.The powder X-ray diffraction determinations (PXRD)were recorded by using a Bruker AXS D8 Advance diffractometer with monochromated Cu Kα radiation(λ=0.154 18 nm;generator current:40 mA;generator voltage:40 kV;scanning range:5°～50°).The thermogravimetric analysis experiments (TGA)were carried out on a SII EXStar6000 TG/DTA6300 analyzer at a heating rate of 10℃·min-1under N2atmosphere.Variable-temperature magnetic susceptibility measurement was performed on a Quantum Design SQUID MPMS XL-7 instrument from 2.5 to 300 K under an applied magnetic field of 2 000 Oe.Luminescence spectra were performed on a Hitachi F-7000 spectrophotometer at room temperature.

1.2 Preparation of the complexes

All three complexes were realized through facile solvothermal method.Starting materials were placed in a 7 mL penicillin bottle and sealed in 23 mL PTFE-lined stainless steel autoclave.The reactor was heated to given temperature (120℃for 1 and 2,160℃for 3)under autogenous pressure for 3 days and then cooled naturally to room temperature.The crystals were dried and collected after they were washed with distilled water and ethanol.The phase purities of all three complexes were confirmed by PXRD measurements at room temperature.Each PXRD pattern of the as-synthesized sample agrees well with the simulated one based on the structure solutions,indicative of the purity of the bulk sample(Supporting information,Fig.S1,S2 and S3).

Synthesis of{［Ni2(hmph)2(bib)2(H2O)2］·3H2O}n(1):In the presence of aqueous NaOH solution(0.5 mol·L-1,0.1 mL),the mixture of homophthalic acid(0.1 mmol,18.0 mg),1,4-bis(1-imidazolyl)benzene(0.1 mmol,21.0 mg),Ni(OAc)2·4H2O(0.2 mmol,49.8 mg)was suspended in distilled water(6.0 mL),giving green sheet crystals after solvothermal reaction (56%based on Ni).Anal.Calcd.for C42H42Ni2N8O13(%):C,51.25;H,4.30;N,11.39.Found(%):C,51.22;H,4.36;N,11.34.IR(cm-1):3 427(w),3 130(w),1 551(s),1 521(s),1 379(s),1 303(m),1 228(w),1 062(s),960(m),934(m),863(w),829(m),814(m),728(s),651(s).

Synthesis of ［Cd(hmph)(bib)］n(2).In the presence of aqueous HAc solution (0.5 mol·L-1,0.2 mL),the mixture of homophthalic acid(0.1 mmol,18.0 mg),1,4-bis(1-imidazolyl)benzene(0.2 mmol,42.0 mg),Cd(OAc)2·2H2O(0.2 mmol,53.3 mg)and H2O(6.0 mL)gave colourless blocked crystals after solvothermal reaction(72%based on Cd).Anal.Calcd.for C21H16CdN4O4(%):C,50.37;H,3.22;N,11.19.Found(%):C,50.32;H,3.25;N,11.27.IR(cm-1):3 119(w),1 576(s),1 524(s),1 490(m),1 384(s),1 302(s),1 271(m),1 133(s),1 103(s),1 065(s),1 051(s),955(s),928(m),839(s),821(s),732(s),701(m),667(s).

Synthesisof{［Zn(hmph)(bip)］·H2O}n(3).The mixture of homophthalic acid(0.1 mmol,18.0 mg),3,5-bis(1-imidazolyl)pyridine(0.2 mmol,42.0 mg),Zn(OAc)2·2H2O (0.2 mmol,43.9 mg)was suspended in H2O-methanol solution (6 mL,2∶1,V/V),giving colourless blocked crystals after solvothermal reaction (54%based on Zn).Anal.Calcd.for C20H17ZnN5O5(%):C,50.81;H,3.62;N,14.81.Found(%):C,50.77;H,3.65;N,14.76.IR(cm-1):3 441(w),3 128(w),1 634(s),1 585(s),1 559(s),1 503(s),1 383(s),1 361(s),1 312(m),1 256(s),1 110(s),1 061(s),1 005(m),956(m),941(m),878(s),840(s),728(s),691(s),646(s).

1.3 X-ray crystallography

Single-crystal X-ray diffraction data for complexes 1～3 were collected on a Bruker SMART APEXⅡCCD diffractometer equipped with graphitemonochromated Mo Kα radiation(λ=0.071 073 nm).Using Olex2 software,all three structures were solved by direct method and refined with SHElXL-2015 refinement package by least squares minimisation［31-33］.Hydrogen atoms were fixed attheir calculated positions with the riding model.The Uisovalues for H atoms of organic ligands were 1.2 times Ueqof their attached carbon atoms,and they were 1.5 times Ueqof their attached oxygen atoms for the hydrogen atoms at water.The details of the structure solution and final refinements are listed in Table 1.

CCDC:1935595,1;1935596,2;1935599,3.

Table 1 Crystal and structure refinement data for complexes 1～3

Continued Table 1

2 Results and discussion

2.1 Crystal structure

2.1.1 Structural description of{［Ni2(hmph)2(bib)2(H2O)2］·3H2O}n(1)

Fig.1 (a)View of coordination environment of Ni（Ⅱ）in complex 1;(b)View of Ni-carboxylate chain spaced by carboxylate ligands;(c)View of monolayer structure lying in bc plane;(d)3D supramolecular structure fabricated by H-bonding interactions labeled as dark dotted lines

Complex 1 belongs to the monoclinic crystal system of P21/n space group.The asymmetric unit consists of one Ni（Ⅱ）cation,one completely deprotonated hmph2-dianion,one bib molecule,one coordinating water molecule and one and a half guest water molecules,as shown in Fig.1a.The unique Ni atom displays a ［NiO4N2］octahedral geometry with the coordination sphere defined by three oxygen atoms from two symmetry related hmph2-moieties,the last one oxygen atom from one coordinating water and two N atoms from two bib.The Ni-O bond lengths range from 0.206 17(19)to 0.209 57(17)nm,while the Ni-N bond lengths are 0.206 3(2)and 0.207 5(2)nm.The［NiO4N2］units are connected by the flexible-CH2COO-groups in bridging bidentate mode to form a metalcarboxylate chain with the intrachain Ni…Ni separation of 0.533 83(5)nm,wherein the O-C-C-C-C-C-ONi eight-membered rings are fabricated by the cooperation of the rigid -COO-groups in monodentate mode and the-CH2COO-groups(Fig.1b).And the 1D motifs are further bridged by the bib molecules along c direction to generate an irregular metal-organic monolayer paralleling to bc plane,in which the Ni…Ni separations are 1.361 95(9)nm and 1.356 28(10)nm (Fig.1c).This monolayer employs the mentioned eight-membered rings acting as pendent arms above and below.Moreover,the adjacent bib bridges are of certain angle and every other bib bridges are parallel,probably because of the spatial effect resulting from the eight-membered rings.Particularly,the presence of coordinated and free water molecules leads to the formation of abundant H-bonding interactions.Through the cooperations of O(5W)-H(5WB)…O(6W)(O…O 0.280 3(3)nm, ∠O-H…O=173.1°)with O(6W)-H(6WA)…O(4)iii(O…O 0.326 0(4)nm,∠O-H…O=150.8°),O(6W)-H(6WB…O(3)iii(O…O 0.282 3(4)nm,∠O-H…O=128.2°),O(7W)-H(7WA)…O(6W)(O…O 0.298 3(8)nm, ∠O-H…O=152.6°)and O(7W)-H(7WB)…O(3)iii(O…O 0.278 2(7)nm,∠O-H…O=159.3°),individual layers are stacked to create the whole 3D supramolecular structure(Fig.1d).

2.1.2 Structural description of ［Cd(hmph)(bib)］n(2)

Complex 2 crystallizes in the triclinic crystal system of P1 space group.The asymmetric unit is comprised of one Cd（Ⅱ） cation,one completely deprotonated hmph2-dianion and one bib molecule,as shown in Fig.2a.The center Cd ion displays a［CdO4N2］octahedral geometry with the coordination sphere defined by four oxygen atoms from three symmetry related hmph2-dianions,and two terminal N atoms from two bib molecules.All Cd-O bond lengths fall in a range from 0.224 9(2)to 0.277 39(14)nm,while the Cd-N bond lengths are 0.228 0(2)and 0.230 2(2)nm.Two Cd octahedra are double bridged by two-CH2COO-groups to form one dinuclear kernel with the Cd…Cd separation of 0.398 31(21)nm (Fig.2b).These adjacent dinuclear units are further connected to generate a double-stranded chain along a direction through μ2-hmph2-dianion with -COO-groups in chelating coordination mode and-CH2COO-groups in bridging bidentate mode.And in such manner,an alternating arrangement of 8-membered and 16-membered metal-organic ring is fabricated.Furthermore,the 1D chains are bridged by μ2-bib ligands to produce a 2D bilayer (Fig.2c).Individual layers are stacked to create the ultimate 3D supramolecular structure by weak van der Waals forces since there are only intralayer π-π stacking interactions between the phenyl rings of bib molecules with the centroid-centroid distances of 0.378 33(19)nm.

Fig.2 (a)View of coordination environment of Cd（Ⅱ）in complex 2;(b)View of Cd-carboxylate chain containing an alternating arrangement of 8-membered and 16-membered rings;(c)View of 2D bilayer with intralayer π-π interactions

2.1.3 Structural description of{［Zn(hmph)(bip)］·H2O}n(3)

Complex 3 also crystallizes in the triclinic crystal system ofspace group.The asymmetric unit comprises one Zn（Ⅱ）cation,one completely deprotonated hmph2-dianion,one bip molecule and one uncoordinated water molecule,as shown in Fig.3a.The centre Zn ion exhibits a ［ZnN2O2］tetrahedral geometry with the coordination sphere defined by two oxygen atoms from two symmetry related hmph2-dianions,and two N atoms from two bip molecules.The Zn-O bond lengths are 0.194 06(13)and 0.194 72(12)nm.The Zn-N bond lengths are 0.200 76(14)and 0.204 89(14)nm.Two neighbor tetrahedra are double bridged by two μ2-hmph2-dianions adopting monodentate mode to form one dinuclear unit with the Zn…Zn separation of 0.537 52(2)nm(Fig.3b).Meanwhile,one 16-membered metal-organic ring is observed.The ancillary μ2-bip ligand molecules link adjacent dinuclear units to form a double-stranded chain structure.Individual chains are propagated to 2D layer structure through only two kinds of H-bonds originating from the uncoordinated water molecules and organic ligands(Fig.3c).Moreover,there are obvious intralayer π-π stacking interactions between the pyridine rings of bip molecules with the centroid-centroid distances of 0.390 97(2)nm(Fig.3d).So,the ultimate 3D supramolecular structure of complex 3 is maintained by weak van der Waals forces only.

Fig.3 (a)View of coordination environment of Zn（Ⅱ）in complex 3;(b)View of Zn-organic chain based on dinuclear units containing 16-membered rings;(c)View of 2D layer structure propagated through only two kinds of H-bonds labeled as dark dotted lines;(d)Intralayer π-π stacking interaction between the pyridine rings of bip molecules

2.2 Thermogravimetric analyses

TGA experiments for complexes 1～3 were carried out to monitoring their thermal stability,as shown in Fig.4.For complex 1,the TGA curve illustrated one distinct weight loss from 160 to 197℃,equivalent to two and a half water molecules(Calcd.9.14%，Obsd.8.70%).And the succeeding weight-loss between 267 and 402℃can be assigned to the decomposition of organic components.The final residue is attributed to NiO phase(Calcd.15.18%,Obsd.16.29%).Complex 2 can survive before 304℃and then decomposed with a two-step weight loss until 515℃.The residue weighing 25.49%of the total sample corresponds to the CdO component(Calcd.25.64%).The TGA curve of complex 3 showed the removal of one guest water molecule from 155 to 227℃with a slight weight loss(Calcd.3.81%,Obsd.3.59%).And two sharp weightloss processes were obseved within a range of 292 to 545℃,which are attributed to the collapse of the framework.The remnant holding a weight of 25.37%may be the mixture of ZnO (Calcd.17.21%)and unburned carbon［34］.

Fig.4 TGA curves for complexes 1～3

2.3 Magnetic property

In the metal-carboxylate chain structure of complex1,the separation distance between two adjacent Ni(Ⅱ)ions is 0.533 83(5)nm,indicating that there are magnetic interactions exchanged by carboxylate bridges of the-CH2COO-groups along the chain.So the temperature-dependent magnetic susceptibility was measured at an applied magnetic field of 2 000 Oe from 2.5 to 300 K.And the curves of χMT vs T and χMvs T are shown in Fig.5.The observed χMT at 300 K was 1.05 cm3·K·mol-1,which was near to the expected spin-only product of 1.00 cm3·mol-1·K for a uncoupled Ni（Ⅱ） ion.Upon cooling,it kept almost the same level in a wide temperature range of 76 to 300 K.Below 76 K,the χMT increased dramatically and reached to a maximum of 1.86 cm3·mol-1·K at 2.5 K.And similar change occurred in the curve of χMvs T.Because complex 1 can magnetically be handled as infinite uniform chains,the magnetic interaction(J)between two adjacent Ni（Ⅱ）ions can be estimated by using the classical spin expression derived from isotropic Heisenberg spin Hamiltonian chains［35-36］:

Fig.5 Temperature dependences of magnetic susceptibility χM(○)and χMT(□)for complex 1

In Eq.(1),u is the Langevin function defined as u=coth［JS(S+1)/(kT)］-kT/［JS(S+1)］with S=1,while other parameters represent their usual meanings.The magnetic data of complex 1 fitted well with Eq.(1)in the whole temperature range.And the best fit led to fitting parameters as following:J=1.01 cm-1,g=2.04 and R=2.8×10-4.Furthermore,these magnetic data obey the Curie-Weiss law χM=C/(T-θ)from 2.5 to 300 K with Weiss constant θ=0.992 2 K,Curie constant C=1.044 3 cm3·K·mol-1and R=0.999 96.The positive J value and θ value,together with the increase of χMT value indicate that there are typical ferromagnetic exchange coupling between two adjacent Ni（Ⅱ）ions in uniform chain［37］.

2.4 Fluorescent properties

The fluorescent properties of complexes 2 and 3 in the solid state were investigated at room temperature,as illustrated in Fig.6.Under excitation at 305 nm,the maximum emission peak for complex 2 appearred at～341 nm,and complex 3 exhibited the strongest peak at～332 nm with excitation wavelength at 275 nm.For comparison,the fluorescent behaviors of H2hmph,bib and bip ligands were detected.Featureless emissions with the maximum at～452 nm(λex=390 nm)for H2hmph,～385 nm(λex=314 nm)for bib and ～418 nm (λex=290 nm)for bip were observed,respectively.Taking account of the inherent properties of Cd/Zn metal centers and similar emission spectra shapes of the related complexes,the fluorescent properties of complexes 2 and 3 can probably be attributed to the ligand localized emission［38］.In comparison with the free organic precursors,obvious blue shifts of emission bands for 2 and 3 have been observed,which may be attributed to the deprotonated effect of H2hmph ligand,and the coordination interactions of the hmph2-and diimidazolyl-type ligands around the central metal ions［39］.

Fig.6 Solid-state emission spectra of complexes 2～3 and the related ligands at ambient temperature

3 Conclusions

Overall,treating Ni（Ⅱ）/Cd（Ⅱ）/Zn（Ⅱ） acetate with flexible homophthalic acid and diimidazolyl-type ligands yielded three novel coordination polymers under mild solvothermal routes.Complexes 1 and 2 possess 2D monolayer and bilayer structure featuring metal-carboxylate chains cross-linked by bib coligands,respectively.Complex 3 is double-stranded chain featuring Zn-carboxylate binuclear units extended by bip co-ligands.All three complexes have high thermal stabilities since their frameworks begin to collapse at 267℃for 1,304℃for 2 and 292℃for 3.In complex 1,typical ferromagnetic exchange coupling exists between two adjacent Ni（Ⅱ）ions in uniform chain.Moreover,the solid state luminescence of complexes 2 and 3 is attributed to the ligand localized emission with significant blue-shift.

Supporting information is available at http://www.wjhxxb.cn