以三(4-咪唑基苯基)胺为配体的镉配合物的合成、晶体结构和荧光性质

李 健 喻 敏 刘光祥

（南京晓庄学院环境科学学院，新型功能材料南京市重点实验室，南京 211171）

As an emerging multifunctional solid crystalline materials over the last two decades,metal-organic frameworks （MOFs）have been a hotspot not only for the diversity of architectures and fascinating topol-ogies but also for potential applications in gas storage and separation,heterogeneous catalysis,fluorescence,chemical sensing,magnetism,electrical energy storage,and so forth[1-7].Although much progress has been achieved in this field over the past years,it is still a challenge for us to fabricate the desired MOFs with expected structures and properties,because there are varied factors that can affect the structure and property of MOFs[8-12].From the previously reported studies,it has been demonstrated that organic ligands are crucial in determining the formation of definite MOFs[13-15].Among various organic ligands,the N-donor ligands as good candidates for the construction of coordination polymers,have aroused a good deal of interests from chemists because of their diversities in coordination modes and conformations[16-17].It should be noted that,to date,the imidazole-containing N-donor ligands such as 1,4-di（1H-imidazol-1-yl）benzene,4,4′-di（1H-imidazol-1-yl）biphenyl,1,3,5-tri（1H-imidazol-1-yl）benzene,3,3′,5,5′-tetra （1H-imidazol-1-yl）biphenyl and 1,3-di（1H-imidazol-1-yl）benzene,have been widely used in the construction of various coordination polymers[18-22].However,the coordination polymers constructed by tri（imidazole）ligands are still in its infancy,although some intriguing examples have been reported[23-27].Tris（4-imidazolylphenyl）amine （TIPA）,as a tridentate bridging ligand,is rarely used in the construction of coordination networks.The TIPA ligand possesses three Ph-imidazole （Ph=phenyl）arms with conformational and geometrical flexibility.The Ph-imidazole arms can rotate freely and adjust itself sterically around the central N moiety when coordinating to the metals[28-32].

In recent years,a number of coordination polymers with various structural types and topological features have been documented[33-35].In this regard,entangled architectures,including interweaving,polyknotting,polythreading,interdigitation and polycatenation,have been deliberately designed and extensively discussed in several comprehensive reviews[36-37].Generally,the topological architectures of the coordination polymers can be controlled by the deliberate design and judicious choice ofthe organic ligands and coordination geometries of the metals[38-39].In this aspect,the structural features of the organic ligands,such as shape,functionality, flexibility, symmetry, length, and substituent group,can influence structure types of the coordination polymers directly[40-44].In this work,two coordination p olymers,[CdI（TIPA）（CDC）0.5]n（1）and{[Cd（TIPA）（MPDA）]·H2O}n（2）,have been synthesized by using TIPA ligand and different carboxylate anions,where H2CDC=1,4-cyclohexanedicarboxylic acid and H2MPDA=5-methylisophthalicacid.Theeffectsof anions on their complex structures are unraveled in detail.Further,the photoluminescent properties of two complexes have also been studied.

1 Experimental

1.1 Materials and general methods

All chemicals and solvents were of reagent grade and used as received without further purification.The TIPA ligand was synthesized according to the reported method[30].Elemental analyses （C,H and N）were performed on a Vario ELⅢelemental analyzer.Infrared spectra were recorded on KBr discs using a Nicolet Avatar 360 spectrophotometer in the range of 4000 ～400 cm-1.Thermogravimetric analyses （TGA）were performed on a Netzsch STA-409PC instrument in flowing N2with a heating rate of 10℃·min-1.The luminescent spectra for the powdered solid samples were measured at room temperature on a Horiba FluoroMax-4P-TCSPC fluorescence spectrophotometer with a xenon arc lamp as the light source.In the measurements of emission and excitation spectra the pass width is 5 nm.All the measurements were carried out under the same experimental conditions.

1.2 Synthesis of[CdI(TIPA)(CDC)0.5]n(1)

A mixture containing CdI2（36.6 mg,0.1 mmol）,H2CDC （17.2 mg,0.1 mmol）,and TIPA （44.3 mg,0.1 mmol）in DMF/H2O （1∶1,V/V）solvent （10 mL）was sealed in a Teflon-lined stainless steel container and heated at 120℃for 3 days.After being cooled down to room temperature,colorless block crystals of 1 were obtained in 57%yield based on TIPA.Anal.Calcd.for C31H25IN7O2Cd （%）：C,48.55;H,3.29;N,12.79.Found（%）：C,48.47;H,3.31;N,12.77.IR （KBr,cm-1）：3 601 （m）,3 089 （w）,1 581 （s）,1 519 （s）,1 364 （s）,1 307 （m）,1087 （m）,1 015 （w）,966 （w）,811 （m）,746（m）,697 （w）,657 （w）,543 （w）.

1.3 Synthesis of{[Cd(TIPA)(MPDA)]·H2O}n(2)

Complex 2 was prepared by a process similar to that yielding complex 1 at 120 ℃ by using CdI2（36.6 mg,0.1 mmol）,H2MPDA （17.2 mg,0.1 mmol）,and TIPA （44.3 mg,0.1 mmol）in DMF/H2O （1∶1,V/V）solvent （10 mL）.Colorless block crystals of 2 were collected by filtration and washed with water and ethanol several times with a yield of 53%based on TIPA ligand.Anal.Calcd.for C36H29N7O5Cd （%）：C,57.49;H,3.89;N,13.04.Found （%）：C,57.44;H,3.91;N,13.01.IR （KBr,cm-1）：3 416 （m）,3 091 （m）,1 623 （m）,1 553 （m）,1519 （s）,1 431 （m）,1 381 （s）,1 271 （m）,1 032 （m）,913 （m）,828 （w）,734 （s）,654（m）,542 （w）.

1.4 X-ray crystallography

Two block single crystals with dimensions of 0.27 mm×0.25 mm×0.22 mm for 1 and 0.25 mm×0.22 mm×0.19 mm for 2 were mounted on glass fibers for measurement,respectively.X-ray diffraction intensity data werecollectedonaBrukerAPEXⅡCCD diffractometer equipped with a graphite-monochromatic Mo Kα radiation （λ=0.071 073 nm）using the φω scan mode at 293（2）K.The diffraction data were integrated by using the SAINT program[45],which was also used for the intensity corrections for the Lorentz and polarization effects.Semi-empirical absorption corrections were applied using the SADABS program[46].The structures were solved by direct methods using SHELXS-2014[47]and all the non-hydrogen atoms were refined anisotropically on F2by the full-matrix leastsquares technique with the SHELXL-2014[48]crystallographic software package.The hydrogen atoms,except those of water molecules,were generated geometrically and refined isotropically using the riding model.The pertinent crystallographic data collection and structure refinement parameters are presented in Table 1 and selected bond lengths and angles are listed in Table 2.

CCDC：1557335,1;1557336,2.

Table 1 Crystal data and structure refinement for 1 and 2

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

2 Results and discussion

2.1 Crystal structure

Single crystal X-ray diffraction analysis revealed that complex 1 crystallizes in the monoclinic space group C2/c.The asymmetric unit of complex 1 contains one crystallography independent Cd（Ⅱ）ion,one unique TIPA ligand,one iodide ion and a half CDC2-anion which locates at an inversion center as illustrated in Fig.1.The Cd（Ⅱ）ion is five-coordinated by three nitrogen atoms from three TIPA ligands,one iodide ion and one oxygen atom in a distorted square pyramidal geometry with a τ value of 0.479[49].The Cd-N bond lengths vary from 0.229 6（3）to 0.236 9（4）nm,and N-Cd-N angles range from 88.79（13）°to 168.09（14）°.

Fig.1 Coordination environments of the Cd（Ⅱ）ion in 1 with the ellipsoids drawn at the 30%probability level

Fig.2 View of a single sheet formed the TIPA ligands and Cd（Ⅱ）ions in 1

Each TIPA ligand bridges three Cd（Ⅱ）ions to generate a highly undulant 2D sheet with a thickness of ca.1.13 nm （Fig.2）.From a topological viewpoint,the sheet can be considered as a 3-connected network with a Schlfli symbol of （4.82）.There are two kinds of large windows of Cd2（TIPA）2and Cd4（TIPA）4in the resulting layers.The Cd2（TIPA）2window is built up by two Cd（Ⅱ）ions and four Ph-imidazole arms of two different TIPA ligands with the dimension of 1.202 nm×1.529 nm,while the Cd4（TIPA）4unit is built up by four Cd（Ⅱ）ions and eight Ph-imidazole arms of four ligands with the dimension of 1.126 nm×3.335 nm.It is noteworthy that each Cd4（TIPA）4unit is divided into two Cd2（TIPA）2（CDC）subunit by CDC anion linking two Cd（Ⅱ）ions to give an undulate 2D layer.Topologically,the TIPA ligand can be considered as a 3-connected node,the CDC anions can be considered as linkers,and Cd（Ⅱ） ions can be regarded as 4-connected nodes.Therefore,the 2D layer of 1 is a（3,4）-connected topology with Schläfli symbol of （4.52）（4.53.72）（Fig.3）.

The most fascinating structural feature of 1 is that two such layers are interlaced each other in a parallel fashion to give a 2D→2D polyrotaxane network （Fig.4）.The Cd2TIPA2windows of each layer are passed by CDC anions of adjacent layer,and each Cd2（TIPA）2（CDC）unit of each layer is threaded through by one armed rod of the TIPA ligand from adjacent layers. More interestingly, the neighboring 2D polyrotaxane sheets are parallel with each other.

Fig.3 Undulate layer and schematic representation of the （3,4）-connected framework with （4.52）（4.53.72）topology of 1

Fig.4 2D→2D polyrotaxane network in 1

When the ligand H2MPDA was used instead of H2CDC to react with Cd（Ⅱ）salts under hydrothermal conditions,complex 2 was isolated.Complex 2 crystallizes in the monoclinic space group P21/c.The asymmetric unit consists of one Cd（Ⅱ）ion,one TIPA ligand,and one lattice water molecule.The local coordination geometry around the Cd（Ⅱ）ion is depicted in Fig.5.The Cd（Ⅱ）ion adopts a distorted octahedral coordination sphere that is defined by three oxygen atoms from two distinct MPDA2-anions and three nitrogen atoms from three different TIPA ligands;thus,the Cd（Ⅱ）ions can be considered as 5-connecting nodes.Each TIPA links three Cd（Ⅱ）ions,acting as a 3-connecting node to form a 1D ladder-like chain.Two carboxylate groups of the MPDA2-anions adopting monodentate and bidentate chelate coordination modes bridge two Cd（Ⅱ）ions,and MPDA2-joins adjacent parallel chains as a pillar connector to form a 2D grid like （4,4）bilayer（Fig.6）.The Schläfli symbol for this binodal net is （42·67·8）（42·6）.Viewed from the b axis,there exist rectangular channels with a cross section of approximately 2.04 nm×1.09 nm （excluding van der Waals radii）.The channel is so large that the two other equivalent bilayers can be accommodated in that channel.This may be the mainly reason to form the 2D→3D parallel entangled structure.Upon interpenetration,complex 2 just contains a small solvent accessible void space of 2.8%of the total crystal volume,according to a calculation performed using PLATON[50].

Fig.5 Coordination environments of the Cd（Ⅱ）ion in 2 with the ellipsoids drawn at the 30%probability level

Fig.6 View of the ladder-like chain （a）and perspective view of the 2D grid-like （4,4）bilayer（b）in 2

The topological feature of complex 2 is most unusual because it is a rarely observed bilayer motif which is parallel-parallel catenated with two other equivalent adjacent ones to form a 3D supramolecular structure （Fig.7）.Toourknowledge,this （3,5）-connected bilayer is yet to be reported.Compared to 2D→3D polycatenation systems in parallel-parallel inclined fashion or parallel-parallel highly undulating fashion,fewer examples of 2D→3D parallel entangled structures have been observed[51-52].

Fig.7 Perspective view of the 2D→3D parallel entangled structures in 2

2.2 FT-IR spectra

The IR spectra of 1 and 2 show the absence of the characteristic bands at around 1 700 cm-1attributed to the protonated carboxylate group,which indicates that the complete deprotonation of H2CDC and H2MPDA ligands upon reaction with Cd（Ⅱ）ion.The presence of vibrational bands of 1 650～1 550 cm-1are characteristic of the asymmetric stretching of the deprotonated carboxylic groups of CDC2-and MPDA2-anions.The difference between asymmetric and symmetric carbonyl stretching frequencies （Δν=νasymνsym）was used to fetch information on the metalcarboxylate binding modes.Complex 1 showed a pairs of νasymand νsymfrequencies at 1 584,1 354 cm-1corresponding to the carbonyl functionality of dicarboxylate ligand indicating a symmetric monodentate coordination mode （Δν=230 cm-1）.Complex 2 shows two pairs of νasymand νsymfrequencies at 1 623,1 431 cm-1（Δν=192 cm-1）and 1 553,1 381 cm-1（Δν=172 cm-1）for the carbonyl functionality indicating two coordination modes as observed in the crystal structure.OH stretching broad bands at 3 416 cm-1for 2 are attributable to the lattice water.The bands in the region of 640～1 250 cm-1are attributed to the-CH-in-plane or out-of-plane bend,ring breathing,and ring deformation absorptions of benzene ring,respectively.The IR spectra exhibit the characteristic peaks of imidazole groups at ca.1 520 cm-1[53].

2.3 Thermal stability

The thermal behaviors of complexes 1 and 2 were measured under a dry N2atmosphere at a heating rate of 10℃·min-1from 25 to 800℃ and the TG curves are presented in Fig.8.The TGA curve of 1 reveals that no obvious weight loss is observed until the temperature reaches 340℃.The anhydrous compound decomposes from 340 to 800℃,indicating the release of organic components.For 2,the first weight loss of 2.61% （Calcd.2.39%）occurs in the range of 60～150℃,indicating the loss of one free water molecules.Then,the framework of 2 decomposes gradually above 270℃.

Fig.8 TGA curves of complexes 1 and 2

2.4 Photoluminescent properties

Photoluminescence properties of Cd（Ⅱ）complexes have attracted intense interest due to their potential applications in photochemistry,chemical sensors,and electroluminescent display[54-55].The photoluminescent properties of 1,2 and TIPA ligand were investigated in solid state at room temperature （Fig.9）.The TIPA ligand exhibits emission band with a maximum at 422 nm upon excitation at 365 nm,which may be assigned to π*→n or π*→π transitions of the ligands[56-57].

Fig.9 Solid-state photoluminescent spectra of complexes 1～2 and TIPA ligand

The emission peaks of complexes occur at 429 nm （λex=370 nm）for 1,482 nm （λex=380 nm）for 2.Under the same experimental conditions,the emission intensities of free H2CDC and H2MPDA are weaker than that of TIPA ligand,so it is considered that it has no significant contribution to the fluorescent emission of the complexes with the presence of TIPA ligand.The emission of 1 can be essentially ascribed to the intraligand fluorescent emission.The red-shift of the emission can be attributed to the ligand coordination to the metal center,which effectively increases the rigidity of the ligand and reduces the loss of energy by radiationless decay.However,the intense blue emission peak at 481 nm （λex=380 nm）for 2 is highly red-shifted with respect to the free TIPA ligands.The intense fluorescence of 2 indicates that a strong interaction exists between the ligand and the Cd（Ⅱ）ion[58-60].

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