A 2D Copper(II) Complex with Half Paddlewheel-like Building Block Controlled by 1,2,4,5-Benzenetetracarboxylic Acid: Synthesis, Crystal Structure and Luminescence Property①

ZHAO Hong-Kun YANG Han-Wen YANG En-Cui ZHAO Xiao-Jun

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A 2D Copper(II) Complex with Half Paddlewheel-like Building Block Controlled by 1,2,4,5-Benzenetetracarboxylic Acid: Synthesis, Crystal Structure and Luminescence Property①

ZHAO Hong-Kun②YANG Han-Wen YANG En-Cui ZHAO Xiao-Jun

(300387)

Self-assembly reaction of 1,2,4,5-benzenetetracarboxylic acid (H4btc) with CuCl2in mixedN,N-dimethylformamide (DMF), methanol and aqueous solution at room temperature generated a 2D complex, [Cu(btc)0.5(DMF)(H2O)]n(1) which was further characterized by X-ray diffraction, IR spectrum, elemental analysis and TG-DTA experiments.The structure determination reveals that the fundamental unit of complex 1 contains one Cu(II) ion, half a btc4–, one DMF molecule and one water molecule, forming a half paddlewheel-like building block [Cu2(btc)(DMF)2(H2O)2] which is different from those previous Cu-btc(II) polymers obtained either in water or other media without water.It crystallizes in monoclinic system, space group2/with22.8209(14),9.2244(6),10.8876(7) ?,117.5210(10)o,2032.6(2) ?3,2, C8H10CuNO6,M= 279.71,D= 1.828 g/cm3,= 2.162 mm-1,(000) = 1136, the final= 0.0225 and= 0.0652 with> 2().Furthermore, the thermal stability of complex 1 is observed to be dependent on the polymeric structure nature.And the luminescent measurements show that complex 1 produces strong emission at room temperature.

paddlewheel-like, synthesis, crystal structure, thermal stability;

1 INTRODUCTION

During the past decades, the coordination poly- mers derived from-block metal ions and 1,2,4,5- benzenetetracarboxylic acid (H4btc) via solution or hydrothermal synthetic methods have been reported with quite important applications in many areas such as photocatalysts[1], magnetism[2, 3], luminescent properties[4], photoluminescence[5, 6]gas sorption[7]and so on[8-10].H4btc is especially of interest because of its high symmetry and various coordination fea- tures toward the transition metal ions[11-13]and they can also act as hydrogen bond donors and/or acep- tors through inter- and/or intramolecular fashion.It should be noted that among those reported metal coordination polymers based on btc4-, lattice or coor- dinated water molecules[14-18]play a quite important role in the metal-organic frameworks through coor-dination bonds or secondary hydrogen bonding interactions.Very recently, we have reported a new two-dimensional (2-D) copper(II)complex [Cu(btc)0.5(DMF)]nbased on the paddlewheel-like [Cu2(-CO2)4(DMF)2] building blocks[19]which has no lattice or coordinated water molecules.Herein we describe the preparation, structural characterization and heat stability properties of the title complex by adding the third solvent molecules based on [Cu(btc)0.5(DMF)]n.In this paper, copper salts and H4btc were reacted in mixed DMF, methanol and aqueous solution to generate a new 2Dcopper(II)complex[Cu(btc)0.5(DMF)(H2O)]n(1), which exhibits a two-dimensional layer structure.

2 EXPERIMENTAL

2.1 Materials and methods

1,2,4,5-Benzenetetracarboxylic acid (H4btc) was purchased from Acros.Other analytical-grade rea- gents were used as received without further purifi- cation.Doubly deionized water was used for the conventional synthesis.Elemental analyses of carbon, hydrogen and nitrogen were carried out with a CE- 440 (Leeman-Labs) analyzer.Fourier transform (FT) IR spectrum (KBr pellets) was taken on an AVATAR-370 (Nicolet) spectrometer in the range of 4000～400 cm–1.Thermogravimetric analysis (TGA) experiments were carried out on Shimadzu simul- taneous DTG-60A thermal analysis instrument in the N2atmosphere ata heating rate of 5°C·min–1.

2.2 Synthesis of [Cu(btc)0.5(DMF)(H2O)]n (1)

CuCl2·H2O(17.05 mg, 0.1 mmol) was dissolved in a mixture solution of CH3OH/H2O (5 mL, v:v = 3:2).The DMF solution (5 mL) of H4btc (12.70 mg, 0.05 mmol) was then added dropwise with stirring.After 1 h, the resulting mixture was filtered and left under the ambient environment for evaporation.Blue block-shaped crystals suitable for X-ray analysis were obtained within two weeks in 55% yield (based on H4btc).Anal.Calcd.for C8H10CuNO6: C, 34.35; H, 3.60; N, 5.01%.Found: C, 34.07; H, 3.71; N, 5.13%.FT-IR (KBr pellets, cm-1): 3468(b), 1633(s), 1558(w), 1493(w), 1437(w), 1370(s), 1139(w), 1105(w), 868(w), 818(w), 754(w), 694(w), 669(w), 596(w), 535(w).

2.3 Crystal structure determination

Diffraction intensities for the title compound were collected on a Bruker APEX-II CCD diffractometer with a graphite-monochromated Moradiation (= 0.71073 ?) by using a-scan mode at ambient temperature.There is no evidence of crystal decay during data collection.Semiempirical absorption corrections were applied (SADABS), and the pro- gram SAINT was used for integration of the diffraction profiles[20].The structures were solved by direct methods using the SHELXS program of SHELXTL package and refined with SHELXL[21].The non-H atoms were modeled with anisotropic displacement parameters and refined by full-matrix least-squares methods on2.Generally, C-bound H atoms of the ligand were placed geometrically and refined as riding atoms.The starting positions of H attached to oxygen atom were located in difference Fourier syntheses and then fixedgeometrically as riding atoms.Isotropic displacement parameters of the H atoms were derived from the parent atoms.Crystal parameters and details of the data collections and refinement are listed in Table 1.Selected bond distances and bond angles are listed in Table 2.

Table 1. Selective Bond Lengths (?) and Bond Angles (°) for Complex 1

Symmetry operations: #1:,+ 1,; #2:-+ 1,+ 1,-+ 5/2

Table 2. Hydrogen Bond Geometries for Complex 1

Symmetry operations: #1:, –,+ 1/2; #2: 1/2 –, 1/2 –, 2 –

3 RESULTS AND DISCUSSION

Blue block-shaped crystals 1 were obtained by the slow evaporation of a mixture of H4btc and metal salt in a molar ratio of 1:2.The reaction of H4btc and metal salt in DMF and methanol solution gave rise to a 2D structure of [Cu(btc)0.5(DMF)]n[19].The sole difference for the preparation is the solvent mole- cules.Two carboxyls of btc4-in [Cu(btc)0.5(DMF)]nare replaced by two water molecules in 1by adding water into the DMF and methanol mixed solvent.

3.1 Crystal structure of [Cu(btc)0.5(DMF)(H2O)]n (1)

X-ray single-crystal diffraction studies confirm that complex 1 crystallizes in monoclinic2/space group with= 22.8209(14),= 9.2244(6),= 10.8876(7) ?,117.5210(10)o and= 2.As shown in Fig.1, the fundamental unit of 1contains one Cu(II) ion, half a btc4–, one DMF molecule and one water molecule.Each Cu(II)ion is five-coor- dinated in a distort square pyramidal geometry.The equatorial basal plane of each Cu(II) ion is surroun- ded by three oxygen atoms from three different btc4–and one oxygen atom from coordination water molecule with the Cu–O distances varying from 1.9543(16) to 1.9826(15) ?.The axial position is occupied by a DMF oxygen atom with the Cu–O distances of 2.1897(18) ?.Two neighboring five- coordinated Cu(II)ions are linkedtwo btc4–carboxylate groups with the Cu···Cu separation being 3.0657(2) ?, suggesting the weak Cu···Cu interactions forming a binuclear [Cu2(btc)(DMF)2(H2O)2] building block.By adding water into the solvent, two carboxyls of btc4–in [Cu(btc)0.5(DMF)]nwere substituted by two water molecules for1, forming a half paddlewheel-like [Cu2(btc)(DMF)2(H2O)2] building block instead of paddlewheel-like [Cu2(-CO2)4(DMF)2] building blocks.Each btc4–ligand molecule links four neighboring building blocks and alternatively each building block links four neighboring btc4–ligand molecules, forming a 2D layer structure as shown in Fig.2.All carboxylic groups of btc4–in complex 1 are deprotonated.Two-carboxylate groups of btc4–adopt a bidentate bridging coordination mode, while the remaining two adopt a monodentate coordination mode, and the whole ligand acts as6-bridge linking six Cu(II) ions.Between the coordination water molecule and carboxylate oxygen atom of non-coor- dination btc4–has strong O–H···O hydrogen bonds which make the 2D layer structure muchsteadier.

Fig.1. Fundamental unit of complex 1 (Hydrogen atoms are omitted for clarity except among the water molecule)

Fig.2. View of the 2-D layer structure of complex 1 along the-axis (DMF is predigested for one oxygen atom)

As depicted in Fig.3, classical C-H···O hydrogen bonding interactions from hydrogen atoms of DMF within one 2-D layer and oxygen atoms of btc4-from neighboring 2-D layer were observed, indicating the absence of DMF molecules dramatically affects the hydrogen bonded surrounding of the overall frame- work.The distances of H atom(H···O) and(C···O) are 2.494 and 2.407 ?, respectively, and the angle(C–H×××O) is 158.79, which obviously play an important role in stabilizing the 3-D packing structure[22, 23].

Fig.3. 3-D packing structure formed by C–H···O weak interactions (black dashed lines)

3.2 IR spectrum

As shown in Fig.4, the stretching vibration of carboxyl groups is easily assigned as they are very intense and appear in a characteristic region.For bect4–, all carboxylic groups of the organic ligand in complex 1 are deprotonated in agreement with the IR spectrum, where no absorption peak around 1730 cm-1for a protonated carboxylic group is observed.The asymmetrical stretching frequencyν(COO-) is 1558 and 1493 cm-1, the symmetrical stretching frequencyν(COO-) is 1370 and 1437 cm-1.The values of Δshow that the carboxylate groups coordinate to the Cu(II) atoms both in monodentate (188 cm-1> 95 cm-1)and bidentate (56 cm-1< 95 cm-1)fashions[24].

Fig.4. Infrared spectroscopy of complex 1

3.3 Powder X-ray diffraction (PXRD) and thermal analysis

To confirm the phase purity of complex 1, powder X-ray diffraction (PXRD) patterns were recorded for 1, and it was comparable to the corresponding simulated patterns calculated from single-crystal diffraction data (Fig.5).

The thermal analysis was carried out to explore the heat stability of complex 1.The first weight-loss process was observed ranging from 89 to 98 ℃, which corresponds to the loss of one coordinated water molecule (calcd.6.43%; obs.6.73%).The water-free material is then thermally stable up to 326 ℃, followed bythe breaking of Cu–O bonds and the decomposition of DMF and btc4-to give the final residual CuO (calcd.28.60%, obs.30.05%), as can be seen in Fig.6.

Fig.5. PXRD analysis of the title complex: bottom-simulated, top-experimental

Fig.6. TGA diagram for complex 1

3.4. Luminescent property

At room temperature, the pure H4btc in an aqueous solution exhibits strong emission at 442 nm under 325 nm excitation.Complex 1 excited at 425 nm produces an intense emission band with a maximum at 325 nm.Compared with the emission spectra of H4btc ligand, a certain degree of red shift in complex 1 was observed.

Fig.7. Fluorescence spectra of 1 and H4btc in an aqueous solution at room temperature

4 CONCLUSION

A new complex [Cu(btc)0.5(DMF)(H2O)]n(1) has been synthesized and the structure was determined and characterized by X-ray diffraction, IR spectrum, elemental analysis and TG-DTA experiments.The crystal structure of complex 1 is a two-dimensional layer structure, which reveals that the fundamental unit of complex 1 is different from those Cu-btc(II) polymers previously obtained either in water or other media without water.Each btc4–ligand molecule links four neighboring half paddlewheel-like [Cu2(btc)(DMF)2(H2O)2] building blocks and alternatively each building block links four neighboring btc4–ligand molecule, forming a 2D layer structure.Further analysis indicates C–H···Oweak interactions are the primary driving forces for the 3-D packing structure.Furthermore, the thermal stability of complex 1 is observed to be dependent on the polymeric structure nature.And the luminescent measurements show that complex 1 produces strong emission at room temperature.

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17 January 2018;

18 April 2018 (CCDC 1816020)

①This work was supported by the National NaturalScience Foundation of China (No.21531005)

.Zhao Hong-Kun (1982-), functional materials.E-mail: hongkun82@163.com

10.14102/j.cnki.0254-5861.2011-1954