Two New Coordination Polymers Containing Metal-carboxylate Helix①

LI Gui-Lian LIU Guang-Zhen XIN Ling-Yun LI Xiao-Ling



Two New Coordination Polymers Containing Metal-carboxylate Helix①

LI Gui-Lian LIU Guang-Zhen②XIN Ling-Yun LI Xiao-Ling

(471022)

Two novel compounds, {[Cd(nbdc)(bpp)(H2O)]·H2O}n1 and {[Cd(nbdc)(dpds)-(H2O)]·H2O}n2 (H2nbdc = 4-nitro-1,2-benzenedicarboxylic acid, bpp = 1,3-bis(4-pyridyl)propane and dpds = 4,4΄-dipyridyldisulfide), were solvothermally synthesized and characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis (TGA), fluorescent analysis and single-crystal X-ray diffraction. Complex 1 is of monoclinic system, space group21/with= 14.4370(17),= 8.4090(10),= 19.168(2) Å,= 104.5050(10)°,= 2252.8(5) Å3,D= 1.639 g/cm3,M= 555.81,= 4,(000) = 1120,= 1.022 mm-1, the final= 0.0269 and= 0.0599 for 16656 observed reflections with2(). 2isisostructuralto 1 with= 14.4175(11),= 8.4737(7),= 18.0120(14) Å,= 106.7220(10)°,= 2107.5(3) Å3,D= 1.821 g/cm3,M= 577.85,= 4,(000) = 1152,= 1.287 mm-1, the final= 0.0280 and= 0.0705 for 15136 observed reflections with2(). Both complexespresent intimately related structures featuring infinite Cd-carboxylate helix of [CdⅡ(nbdc)(H2O)]nconnected by bpp (or dpds) molecule to produce the 2D layer frameworks.

1,2-benzenedicarboxylate, solvothermal, helix

1 INTRODUCTION

Recent interest in metal-organic frameworks (MOFs) is rapidly increasing not only for their appealing structures, but also for their potential applications in magnetism, ion exchange, catalysis, photoluminescence, gas storage, electric conduc- tivity and so on[1-5]. It is well-known that organic ligands play crucial roles in the design and con- struction of desirable frameworks. With regard to the aromatic benzenedicarboxylic acid and its deri- vatives (such as 1,n-benzenedicarboxylic acid, n = 2, 3, 4) are widely used as building blocks to link metalions to produce metal-organic frameworks with interesting structures and properties[6-10]. However, the coordination chemistry based on 4-ni- tro-1,2-benzenedicarboxylic acid (H2nbdc), a deri- vative of 1,2-benzenedicarboxylic acid, has rarely been studied[11]. Though the nitro group is not tangled in coordination, H2nbdc may provide the potential to construct unpredictable and interesting network structures due to the existence of a non- coordinating electron-withdrawing nitro-group on the aromatic backbone, which will have a profound impact on the electron density of such a ligand and therefore different physical and chemical properties[12].

On the other hand, among diverse elegant efforts to find key factors in their development, the choice of bridging ligand backbone is worthy of close attention as a rational design strategy. As a result, many types of bridging ligands, such as bpe (1,2- bi(4-pyridyl)ethene), bpa (1,2-bi(4-pyridyl)ethane) and bpp (1,3-di(4-pyridyl)propane) with different carbon backbones between the two 4-pyridyl rings, have been extensively used in the construction of diverse multi-dimensional architectures[2-4]. How- ever, dpds has seldom been reported not only because of its twisted conformation, with a C–S– S–C torsion angle of ca. 90° and axial chirality that potentially generate M and P enantiomers in chiral crystal engineering, but also of the easy cleavage of its S–S bond[13, 14]. As reported, the dpds ligand in most compounds tends to adopt the pyridyl form that coordinates with metal atoms through the nitrogen donor without S–S bond cleavage under room temperature[15].

We chose 1,2-benzenedicarboxylate acid with the electron-withdrawing group (-NO2) and diverse N- donor coligands (bpp, dpds) to construct new coor- dination polymers. We herein obtain the closely related Cd (II)complexes {[Cd(nbdc)(bpp)(H2O)]·H2O}nand {[Cd(nbdc)(dpds)(H2O)]·H2O}n. The dpds did not transfer into 4,4΄-dipyridylsulfide (dps) in com- plex 2. The two complexes are 2D layer structures with metal-carboxylate helix. Furthermore, PXRD, IR, thermal stability and fluorescence properties of the two complexes are given.

2 EXPERIMENTAL

2. 1 Reagents and instruments

All chemicals for synthesis were of reagent grade and used as received without further purification.Elemental analyses for C, H and N were carried out using a Flash 2000 organic elemental analyzer. Infrared spectra (4000～600 cm-1) were recorded on powdered samples using a NICOLET 6700 FT-IR spectrometer. The thermogravimetric analyses (TGA) were performed on a SII EXStar 6000 TG/DTA6300 analyzer with a heating rate of 5 ℃/min up to 900 ℃under N2atmosphere. Powder X-ray diffraction (PXRD) patterns were taken on a Bruker D8-ADVANCEX ray diffractometer equip- ped with Curadiation (= 1.5418 Å). The luminescence spectra were performed on an Aminco Bowman Series 2 Luminescence spectrometer at room temperature.

2. 2 Synthesis of 1

A mixture of Cd(OAc)2·2H2O (0.027 g, 0.10 mmol), H2nbdc (0.021 g, 0.10 mmol), bpp (0.040 g, 0.20 mmol), KOH (0.006g, 0.10 mmol) and H2O (7 mL) was placed in a 23 mL Teflon-lined autoclave. The vessel was heated to 120 ℃ for 4 days, and then cooled to room temperature. Colorless block crystals were obtained. Elemental analysis calcd. (%) forC21H21N3O8Cd: C, 45.39; H, 3.78; N, 7.54%. Found: C, 45.38; H, 3.81, N 7.56%. Selected IR (KBr, cm–1): 3000～3600(m), 1596(s), 1574(s), 1519(s), 1481(w), 1404(s), 1342(s), 1229(m), 1166(w), 1070(m), 1036(w), 924(w), 868(w), 849(m), 832(m), 794(m), 738(s), 669(m).

2. 3 Synthesis of 2

A mixture of Cd(OAc)2·2H2O (0.027 g, 0.10 mmol), H2nbdc (0.021 g, 0.10 mmol), dpds (0.022 g, 0.10 mmol), EtOH (3 mL) and H2O (3 mL) was placed in a 23 mL Teflon-lined autoclave. The vessel was heated to 120 ℃ for 4 days, and then cooled to room temperature, obtaining colorless block crystals. Elemental analysis calcd. (%) for C18H15N3O8S2Cd: C, 37.53; H, 2.58; N, 7.21. Found: C, 37.41; H, 2.62; N, 7.27. Main bands of IR (cm-1): 3305(m), 1598(s), 1578(s), 1558(s), 1514(s), 1480(m), 1407(m), 1371(m), 1345(s), 1321(w), 1226(w), 1063(m), 1018(m), 919(w), 852(w), 835(w), 813(s), 785(w), 750(w), 737(m), 709(s), 668(m).

2. 4 Structure determination

Crystals of the two title complexes with suitable sizes were mounted on a Bruker Smart APEX II CCD diffractometer equipped with graphite-mono- chromated Mo-(= 0.71073 Å) radiation by using a-scan technique at room temperature. Absorption corrections were based on symmetry equivalent reflections using the SADABS pro- gram[16]. A total of 16656 reflections for 1 and 15136 for 2 were collected, of which 4200 were unique (int= 0.0279) and 3466 were observed with> 2() for the former while 3923 were unique (int= 0.0164) and 3701 were observed for the latter. The structures were solved by direct methods with SHELXS-97 and refined on2by full-matrix least- squares using the SHELXL-97 program package[17]. All non-hydrogen atoms in complexes 1 and 2 were refined with anisotropic thermal parameters. The hydrogenatoms were placed in the calculated positions and refined isotropically with a riding model except for those bound to water molecules and hydroxyl groups, which were initially located in a difference Fourier map and included in the final refinement by use of geometrical restraints with the O–H distances fixed at 0.85 Å for water molecules, andiso(H) = 1.5eq(O). The crystal data as well as details of data collection and refinements for 1 and 2 are listed in Table 1, respectively. Selected bond lengths and bond angles are given in Table 2.

Table 1. Crystal and Structure Refinement Data for Complexes 1 and 2

= 1/[2(F2) + (0.0275)2+ 1.2829], where= (F2+ 2F2)/3) for 1;= 1/[2(F2) + (0.0312)2+ 2.9602], where= (F2+ 2F2)/3) for 2

Table 2. Selected Bond Lengths (Å) and Bond Angles (°)

Symmetry transformation for compound1: a: –+1,+1/2, –+1/2; b: –, –+2, –; 2: a: –+1,+1/2, –+1/2; b: –, –+1, –

3 RESULTS AND DISCUSSION

3. 1 Description of the crystal structures

The structure of complex 1 is a 2D thick-layer structure with mononuclear cadmium(II) as nodes and nbdc2-and bpp as linkers. The asymmetric unit consists of one crystallographically equivalent Cd(II) atom, one nbdc2-dianion, one bpp molecule and one coordination water, in addition to one guest water, as shown in Fig. 1a. The Cd(II) centers are distorted octahedral [CdO4N2] coordinated by four oxygen atoms from two nbdc2-anions and one coordination water and two nitrogen atoms from two bpp molecules. All the Cd–O bond lengths are between 2.244(2) and 2.535(2) Å, and the Cd–N bond lengths are 2.292(2) and 2.343(2) Å, respectively.

Two carboxylate groups of the nbdc2-anions adopt monodentate bridging and chelating-bidentate coordination modes, respectively. The Cd(II) ions are bridged by the nbdc2-anions to generate 1D helix[18]along thedirection with the Cd···Cd separation of 5.8779(5) Å, as shown in Fig. 1b. A careful examination shows that the helix Ais exclusively left-handed (L), whereas those in the neighboring chains Bare right-handed (R) (Fig. 1b).These helix chains array in a parallel fashion and are further linked by flexible bpp ligands to produce a 2D layer.

Fig. 1. (a) View of the asymmetric unit showing the coordination environment of Cd cation in 1. Symmetry codes: A = 1–, 0.5+, 0.5–; B = –, 2–, –. The partial hydrogen atoms are omitted. (b) View of a layer structure with metal-carboxylate helices in 2. All the -NO2groups are omitted

The abundant hydrogen bonds which play an important role in forming by self-assembly are observed in the intra-layer: (a) H-bonding between the guest water and carboxylate O atom of the nbdc2-anions (O(8)–H(3W)×××O(2):= 3.001(6) Å,= 179.5°); (b) H-bonding between the guest and coordination water (O(8)–H(4W)×××O(7):= 2.839(5) Å,= 176.8°); (c) H-bonding between the coordination water and the carboxylate O atoms (O(7)–H(1W)×××O(1):= 2.744(3) Å,= 159.2°; O(7)–H(2W)×××O(3):= 2.893(3) Å,= 146.4°), as shown in Table 3. Individual layers mutually hang together with an -ABCD- sequence and are entirely cohered together by van der Waals interactions to form a 3D supramolecular network.

Compound 2 isisostructuralto 1 andalso crys- tallizes in space group21/and produces a 2D layer containing the left-, right-handed metal-car- boxylate helices. The major difference for both two structures, resulting from the replacement of bpp ligand by the dpds ligand, is the distance of adjacent chains connected by auxiliary ligands (11.3271 Å for 1 and 10.5954 Å for 2).

Interestingly, the dpds auxiliary ligand in 2 still maintained the original structure in CH3CH2O- H-H2O solution at 120 ℃, different from the decomposition of the dpds ligand in some reported literatures. For examples, Tongreported, where the reactions ofthe dpds ligand with CuI were conducted in CH3CN solution, the dpds rea- gent was unprecedentedly converted intotwo iso- meric ligands, 4,4΄-dipyridylsulfide (dps) and 1-(4-pyridyl)-4-thiopyridine (ptp) at 120 and 160 ℃,respectively[19].Carla Aragonialso reported the reactionsbetween dpds and dithiophosphato NiIIcomplexes, yielding threedifferent bridging ligands from in situ chemical rearrangement ofthe starting dpds reagent (dpds, dps, and dipyridyltrisulfide) inCH3OH-CH2Cl2or CH3CH2OH-CH2Cl2mixed solution[20]. Ma. carried out the reactions of dpds with CoIIand H2tbip in CH3OH-H2O solution wheredpds can unpredictably partly convertinto dps at 120 ℃and predictably wholly transferinto dps at 160 ℃[13].Although detailed studies are still required to better understand thereason for the generation of various results from a dpds precursor,the results show that the solvent is an important factor for theformation of the final structures.

Table 3. Hydrogen Bonds for 1 and 2 (Bond Å and Angle o)

Symmetry transformations used to generate the equivalent atoms: a: –+1,+1/2, –+1/2; b:,+1,

3. 2 IR, TGA and PXRD

The IR spectrum of two compounds corres- ponded with their single crystal structures. The IR spectra show several characteristic bands: broad bands observed in the region of 3000～3600 cm-1represent O–H stretching modes within the coor- dinated or free water molecules. The strong absorp- tion at about 1596 and 1404 cm-1is attributed to the asymmetric stretching vibrationas(COO-) and the symmetric stretching vibrations(COO-), respec- tively. Medium intensity bands at about 1574 and 1229 cm-1can be ascribed to stretching modes of the pyridyl rings of bpp or dpds ligands. Puckering modes of the pyridyl rings are observed at about 832 and 669 cm-1. The characteristic bands of -NO2group were observed at about 1519 and 1342 cm-1.

Thermogravimetric analyses (TGA) were carried out to examine the thermal stability of 1 and 2. The experiments were performed on the samples con- sisting of numerous single crystals from 25 to 900 ℃ at a heating rate of 5 ℃·min-1under nitrogen atmosphere, as shown in Fig. 2. For 1, the first weight loss of 6.7% from 25 to 140 ℃ agrees approximately with the release of one guest and one coordinated water molecules per formula unit (calcd: 6.5%). The second weight loss is observed from 250 ℃ and organic components are further decomposed until 900℃. The final residue of 22.9% is CdO (calcd. 23.1%). The TGA curve for 2suggests that the first weight loss of 6.2% in the region of 30～190℃ corresponds to the release of one coordinated water and one guest water per formula unit (calcd. 6.2%), which are also in agreement with the results of elemental analysis. The residual framework starts to decompose with a series of complicated weight loss and does not stop until the heating ends at 900℃.

In order to check the phase purity of these two compounds, their PXRD patterns were checked at room temperature (Fig. 3). The peak positions of the simulated and experimental PXRD patterns are in agreement with each other, simultaneously the PXRD pattern of complex 1 is very similar with that of 2, demonstrating the good phase purity and the isomerism of two compounds. The differences in intensity may be due to the preferred orientation of the crystalline powder samples.

Fig. 2. TGA curves for complexes 1 and 2

Fig. 3. PXRD patterns for complexes 1 and 2

Fig. 4. Luminescence curves for complexes 1, 2 and free H2nbdc

3. 3 Luminescent properties

The luminescent properties of complexes with10metal centers are of great interest for their potential applications as photoactive materials. The photolu- minescence properties of 1, 2 and free H2nbdc were investigated in the solid state at room temperature, as illustrated in Fig. 4. Interestingly, the emission spectrum for compound 1 shows the main broad peaks at 375 and 395 nm (ex= 300 nm) produce a slight blue-shift compared with the emission shown by the H2nbdc ligand. Thereinto, due to very weak fluorescent emission of the bpp coligands, this emission can still be assigned to free H2nbdc ligand photoluminescence. And the blue-shift may be due to the bad conjugation effects compared with those in the free ligand. Complex 2 exhibits relatively weak fluorescence emission bands at ca. 430 and 468 nm (ex= 320 nm), and agrees with that of free H2nbdc ligand (ex= 340 nm). This can probably be assigned to the intraligand charge transfer ofH2nbdc ligand.

4 CONCLUSION

In summary, two new layer materials are de- signed by adopting 4-nitro-1,2-benzenedicarboxylic acid with a noncoordinating electron-withdrawing and N-donor coligand under solvothermal con- ditions. The two complexes have similar structures containing the same metal-carboxylate helix by different N-donor coligands, providing an example to study the effect of ancillary ligands on the structures and properties of the resulted coor- dination polymers. The structural disparity for two complexes is mainly ascribed to the difference of N-donor coligands. The dpds auxiliary ligand still maintains a high thermostability in the reaction process at 120 ℃ in the CH3CH2OH-H2O mixed solution. Further work in this area is in progress.

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24 October 2013;

17 March 2014 (CCDC 967506 for 1 and 966261 for 2)

the Foundation for University Young Key Teacher by Henan Province (No. 2011GGJS-153) and the Foundation of Science and Technology of Henan province (No. 112102310638 and 142300410301)

. Liu Guang-Zhen, born in 1972, associate professor, majoring in microporous materials. E-mail: gzliuly@126.com