Cadmium(II) and lanthanum(III) coordination architectures with anthracene-9,10-dicarboxylate: Crystal structures and photoluminescent properties

Inorganica Chimica Acta 385 (2012) 58–64

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Cadmium(II) and lanthanum(III) coordination architectures with anthracene-9,10-dicarboxylate: Crystal structures and photoluminescent properties Jun-Jie Wang a, Ze Chang b, Tong-Liang Hu b,⇑ a b

College of Chemistry and Chemical Engineering, Anyang Normol University, Anyang, Henan 455002, China Department of Chemistry, and Tianjin Key Laboratory of Metal & Molecule-Based Material Chemistry, Nankai University, Tianjin 300071, China

a r t i c l e

i n f o

Article history: Received 26 October 2011 Received in revised form 18 December 2011 Accepted 20 December 2011 Available online 30 December 2011 Keywords: Cd(II) and La(III) complexes Crystal structures Anthracene-9,10-dicarboxylate Luminescent property Topological structure

a b s t r a c t Two new coordination polymers, namely, [Cd(HL)2]1 (1) and [La2(L)3(DMAC)6]1 (2) (L = anthracene9,10-dicarboxylate and DMAC = N,N-dimethylacetamide), were synthesized. Their structures were characterized by elemental analyses, infrared spectra, and single crystal X-ray diffraction analysis. Complex 1 is found to assume a three-dimensional (3D) twofold interpenetrating diamond network, showing one-dimensional channels running along the [1 1 1] direction. On the other hand, 2 has a two-dimensional 63 honeycomb network, which is further linked into 3D supramolecular networks by C–Hp interactions. The results indicate that the different synthesis methods play important roles in controlling the formation of the final frameworks of 1 and 2. The luminescent properties of the corresponding complexes were also briefly investigated. Crown Copyright Ó 2011 Published by Elsevier B.V. All rights reserved.

1. Introduction The design and synthesis of metal–organic frameworks (MOFs) is currently gaining considerable interest owing not only to their intriguing structural diversities [1], but also to their potential applications in molecular adsorption and separation processes [2], gas storage [2,3], ion exchange [4], catalysis [5], sensor technology [6], among other. Although porous MOF materials have achieved a series of significant developments to date [7], the rational control of the construction of polymeric networks remains a great challenge in crystal engineering. The formation of these complexes mainly depends on the combination of two factors, namely, the coordination geometry of metal ions and the nature of ligands [8]. Now, the utilization of pillars as secondary building units has proven to be a versatile strategy in the construction of coordination frameworks, especially highly porous structures [9]. Structural characteristics such as pores and channels may also be controlled and adjusted using different metal ions [10]. However, methodologies using metal ions as connecting nodes for holding together organic ligands in predefined patterns within self -assembled oligomeric or polymeric aggregates still attract great interest [11]. Many other factors can affect the structures of the fi-

⇑ Corresponding author. Fax: +86 372 2900040. E-mail address: [email protected] (J.-J. Wang).

nal products, such as counteranions [12], pH [13], temperature [14], and solvent system [15]. As a typical bridging dicarboxylic acid ligand, anthracene-9, 10-dicarboxylic acid (H2L) can be partially or fully deprotonated to afford various coordination modes in mono-, di-, or multi-coordinated ways [16,17]. Hence, H2L can be regarded as a potential pillar candidate. In the previous works of our group, H2L has been successfully used to prepare a series of transition metal complexes exhibiting interesting magnetic and luminescent properties [17]. In the present work, to explore further the influence of different synthesis methods on the structures and properties of H2L complexes, we report the syntheses and crystal structures of two CdII and LaIII complexes based on H2L. The complexes are [Cd(HL)2]1 (1) and [La2(L)3(DMAC)6]1 (2) (L = anthracene-9,10-dicarboxylate and DMAC = N,N-dimethylacetamide). The solid-state luminescent properties of all the complexes and the corresponding ligands have been studied.

2. Experimental 2.1. Materials and general methods H2L [16–18] was synthesized according to a previously reported procedure. All other reagents and solvents for synthesis were commercially available and used either as received or purified by standard methods prior to use. Elemental analyses (C, H, and N) were

0020-1693/$ - see front matter Crown Copyright Ó 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2011.12.037

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performed using a Perkin–Elemer 240C analyzer. Infrared (IR) spectra were recorded within 4000 cm1 to 400 cm1 on a Tensor 27 OPUS (Bruker) Fourier-transform IR spectrometer with KBr pellets. The UV–Vis diffuse reflectance spectra of the samples were recorded using a solid-state UV–Vis spectrometer (UV–Vis, Lambda 20, Perkin–Elmer). The solid-state emission spectra at room temperature were obtained using a Cary Eclips fluorescence spectrophotometer.

Table 1 Crystallographic data and structure refinement summary for complexes 1 and 2.

Chemical formula Formula weight Crystal system Space group a (Å) b (Å) c (Å) b (°) V (Å3) Z Dcalc (g cm3) F(0 0 0) l (mm1) Collected reflections Unique reflections Rint Goodness-of-fit (GOF) T (K) R1 (I > 2r(I))a wR2 (all data)b

2.2. Syntheses of complexes 1 and 2 2.2.1. [Cd(HL)2]1 (1) A mixture of 0.05 mmol H2L (13.3 mg) and 0.1 mmol Cd(NO3)24H2O (30.8 mg) was dissolved in a mixed solution containing 2 mL of N,N-dimethylformamide (DMF) and 1 mL of CH3OH. The resultant solution was heated at 100 °C for 4 days. Light brown single crystals suitable for X-ray analysis were obtained after cooling to room temperature. The yield was 30% based on H2L.

0

2.2.2. [La2(L)3(DMAC)6]1 (2) A similar synthetic procedure as for 1 was used, except that Cd(NO3)24H2O and DMF were replaced by LaCl3 xH2O and DMAC. Brown single crystals suitable for X-ray analysis were obtained. The yield was 20% based on H2L. 2.3. X-ray powder diffraction (XPRD) The XRPD patterns of 1 and 2 were recorded on a Rigaku D/ Max-2500 diffractometer operated at 40 kV and 100 mA using a Cu-target tube and a graphite monochromator. The intensity data were recorded by continuous scanning in the 2h/h mode from 10° to 50°, with a step size of 0.02° and a scan speed of 8° min1. 2.4. X-ray crystallographic studies Single-crystal X-ray diffraction measurements for 1 and 2 were carried out on a Rigaku Saturn CCD diffractometer at 113(2) K and a Bruker Smart 1000 CCD diffractometer at 293(2) K, respectively. The determination of unit cell parameters and data collection were performed with Mo Ka radiation (k = 0.71073 Å). Unit cell dimensions were obtained using least-squares refinements. The program SAINT [19] was used for the integration of the diffraction profiles. All structures were solved by direct methods using the SHELXS program of the SHELXTL package, and refined with SHELXL [20]. The metal atoms in each complex were located from E maps. Other non-hydrogen atoms were located by successive difference Fourier syntheses and refined with anisotropic thermal parameters on F2. The hydrogen atoms on the carbon atoms of the ligands were placed in calculated, ideal positions and refined as riding on their respective carbon atoms. No hydrogen atom was located on the solvent molecules. The crystallographic data and experimental details of the structural analyses are summarized in Table 1. 3. Results and discussion 3.1. Synthesis considerations and general characterizations Solvothermal synthesis is widely used to produce new materials with diverse structural architectures, although its mechanism is not yet completely clear. This method can minimize the problems associated with ligand solubility, and enhance the reactivity of reactants. Our first attempts to react CdII or LaIII salts with H2L by solvothermal methods have yielded only some precipitates or microcrystalline products unsuitable for single crystal X-ray dif-

qmax/qmin (e Å A3) a b

1

2

C32H18O8Cd 642.90 tetragonal I4(1)/acd 21.122(3) 21.122(3) 16.7824(19) 90 7487.0(17) 8 1.137 2560 0.622 29 812 2239 0.0489 1.070 113(2) 0.0382 0.0975 0.257/0.785

C72H78N6O18La2 1593.25 trigonal  R3 16.211(5) 16.211(5) 23.312(6) 90 5306(3) 6 1.496 2430 1.265 18 272 2707 0.1717 1.243 293(2) 0.0905 0.2279 1.148/0.856

R = R(||F0|  |FC||)/R|F0|. wR = [Rw(|F0|2  |FC|2)2/Rw(F 20 )]1/2.

fraction analysis, which may be due to the inappropriate pH values of the reaction systems. Thus, the use of DMF or DMAC is the key to the formation of 1 and 2. DMF or DMAC not only adjusts the pH values of reaction systems, but also sometimes acts as a coordination group to change the structures. No complex suitable for X-ray analysis has been obtained using normal heating or basic compounds other than DMF or DMAC, such as NaOH or TMA. Complexes 1 and 2 are all stable in air. All general characterizations were carried out based on the crystal samples. Elemental analyses show that the components of these complexes are well consistent with the results of the structural analyses. In general, the IR spectra show features attributable to each component of the complexes [21]. The broad band at ca. 3436 cm1 to 3319 cm1 indicates the O–H stretching of the carboxylic group or water ligands [17,22]. For 1 and 2, the characteristic bands of the carboxylate groups appear in the usual region between 1606 and 1549 cm1 for the antisymmetric stretching vibrations, as well as between 1434 and 1431 cm1 for the symmetric stretching vibrations. The Dm values [Dm = mas(COO)  ms(COO)] were 161 cm1 for 1 as well as 57 and 172 cm1 for 2, attributable to the corresponding coordination modes of the dicarboxylate groups (Scheme 1). These findings agree with their solid structural features from the results of crystal structure analyses [17,22].

3.2. Description of crystal structures 3.2.1. [Cd(HL)2]1 (1) Complex 1 crystallizes in the tetragonal space group I4(1)/acd, which is assembled by L and CdII ions (Fig. 1), different from a previously reported metal–organic complex, {[Cd2(L)2(H2O)4] (C2H5OH)0.75(H2O)1.25}1. This complex exhibits an unusual (3,4)connected (6.82)2(6.85) topology by the undefiled test tube diffusion method [17b]. Each asymmetric unit of 1 contains one quarter of CdII ions lying at a site with 4 symmetry, half L ligand lying about an inversion center, and half free protonated aqua molecules (Fig. S1a) [23]. Each CdII is 0 eight-coordinated by eight carboxylate 0 O atoms [Cd–O = 2.391(3) Å A to 2.453(3) Å A] [17b,c] from four different L ligands in a bis-chelating coordination mode (Table 2). In other words, the Cd(1) ion is connected to four adjacent CdII centers via L bridges, resulting in a three-dimensional (3D) polymeric

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Fig. 1. View of (a) Coordination environment of CdII ions in 1 (H atoms and free protonated aqua molecules omitted for clarity); (b) the diamond structure motif and its schematic view (right); (c) schematic representations of twofold interpenetrating diamond network in 1; (d) space-filling representation of the 1D channels along [1 1 1] direction, free protonated aqua molecules which occupied the pores omitted for clarity.

network with a diamond structure. The first view of the structure displays a diamond cage with the adjacent Cd Cd distance of 11.364(1) Å. Such cages are formed into a twofold interpenetrated diamond network (Fig. 1c). If the L groups are omitted (Fig. 1b), the

CdCdCd angles in 1 range from 97.83° to 136.67° and significantly deviate from 109.45°, which is expected for an idealized diamond network [24]. To minimize the large void cavities in the diamond cages and stabilize the entire framework, a twofold

J.-J. Wang et al. / Inorganica Chimica Acta 385 (2012) 58–64 Table 2 Selected bond distances (Å) and angles (°) for complex 1.a Cd(1)–O(2) O(2)#1–Cd(1)–O(2)#2 O(2)–Cd(1)–O(1)#1 O(2)–Cd(1)–O(1) O(2)#1–Cd(1)–O(1)#2 a

2.391(2) 84.61(14) 87.91(9) 53.52(9) 137.97(10)

Cd(1)–O(1) O(2)#1–Cd(1)–O(2) O(2)#1–Cd(1)–O(1) O(1)#1–Cd(1)–O(1) O(1)#1–Cd(1)–O(1)#2

2.454(3) 123.16(8) 83.58(9) 90.575(14) 168.50(14)

Symmetry codes: #1 y + 3/4, x + 3/4, z + 1/4; #2 y  3/4, x + 3/4, z + 1/4.

Cd O

O

61

involvement of water molecules in the coordination, the complex exhibits an unusual (3,4)-connected (6.82)2(6.85) topology. In the present research, when the room temperature C2H5OH-H2O diffusion method was used instead of CH3OH-DMF solvothermal synthesis, a different 3D twofold interpenetrating diamond network [Cd(HL)2]1 was produced. When another ion with a higher valence, LaIII, took the place of CdII, a 2D 63 honeycomb network [La2(L)3(DMAC)6]1 was obtained. Although the ligands in both complexes are the same, their coordination chemistries are obviously different. The discrepancy is a result of the different synthesis methods using varied solvents and pH-regulating agents. The present work also provides a good comparison between two different synthesis methods and corresponding complexes. CdII or LaIII centers were pillared by H2L to construct different MOFs with varied topologies. The results reveal that different synthesis methods play important roles in the formation of different frameworks. The solvents used in the reaction significantly influenced the final structure. 4. XRPD results

O

O Cd

Scheme 1. View of bis-chelating coordination mode of L2.

interpenetrating diamond framework is generated [17a,23a–c]. The space-filling model clearly shows that even after the twofold interpenetration, the open space (void volume = 1195.4 Å3, 16.0% of the unit cell volume) still exists within the coordination network of 1. One-dimensional channels are shown along the [1 1 1] direction, which are occupied by free protonated aqua molecules [23d] (Fig. 1d).

3.2.2. [La2(L)3(DMAC)6]1 (2) Different from complex 1, complex 2 has a two-dimensional (2D) structure (Fig. 2a). X-ray single-crystal diffraction analysis reveals that each asymmetric unit contains one third LaIII center, half an L ligand, and one DMAC ligand (Fig. S1b). In other words, each LaIII center is nine-coordinated by six carboxylate O atoms from three chelating L ligands and three O atoms from three DMAC ligands. L also acts as a bis-chelating ligand coordinated (Scheme 1) to LaIII with La–O bond distances of 2.478(7) and 2.543(6) Å. L bridges adjacent LaIII centers, resulting in a 2D framework. All bond distances and angles around each LaIII center fall into the normal range (Table 3) [25]. Therefore, to describe the overall 2D coordination polymer, 2 must be reduced to its underlying topological network. LaIII can be regarded as a three-connecting node, whereas L acts as a topological two-connecting bridge and can be ignored. Given the three two-connecting L ligands and three terminal DMAC molecules on both sides of the LaIII ion, the L geometry around each LaIII can best be described as a distorted triangle-pyramid coordination environment. Hence, the 2D sheet is waved, and a 2D waved 63 honeycomb sheet [26] is formed (Fig. 2b and c). The 2D sheets are further assembled to form a 3D network by intermolecular C–Hp interactions via the edge-to-face orientation between DMF and the naphthalene rings of L [d = 2.853(2) Å and A = 1116.49(5)° in the CHp patterns] [27] (Fig. 2d). The previous CdII complex {[Cd2(L)2(H2O)4](C2H5OH)0.75 (H2O)1.25}1 [17b] has been constructed from H2L by the undefiled test tube C2H5OH–H2O diffusion method in the presence of excess 2,6-dimethylpyridine at room temperature. Considering the

To confirm whether the crystal structures are truly representative of the bulk materials, XRPD experiments on 1 and 2 were also performed. The XRPD and computer-simulated patterns of the corresponding complexes are shown in Fig. 3. Although the experimental patterns have a few unindexed diffraction lines and some are slightly broadened compared with those simulated from the single crystal modes, the bulk-synthesized materials and as-grown crystals can still be considered homogeneous. 5. Luminescent properties 5.1. UV–Vis spectra The solid-state UV–Vis absorption spectra of both 1 and 2 (Fig. 4) were recorded at room temperature. In the absorption spectra of H2L, there are two characteristic absorption peaks (252 and 372 nm) owing to the separate appearance of the K- and Bbands [17a]. Both bands correspond to p ? p⁄ transitions [17a]. Consequently, the absorption band profiles 1 and 2 are very similar. The characteristic K-band (227 nm for 1 and for 228 nm 2) and B-band (355 nm for 1 and for 353 nm 2) are also still typical, and are further blue-shifted compared with the absorption maxima of the corresponding free ligand [17a]. Therefore, the characteristic K- and B-band absorptions in the UV–Vis spectra of 1 and 2 are mainly assigned as the p ? p⁄ transitions of H2L, respectively [17a]. 5.2. Emission properties Luminescent compounds are currently of great interest because of their potential applications in chemical sensors, photochemistry, and electroluminescent displays [28]. To examine the luminescent properties of the IIB and IIIA metal complexes, the luminescent properties of 1 and 2 as well as those of the free ligand H2L were investigated at room temperature (Figs. S2 and S3). The free ligand H2L displays moderate luminescence in the solid state at kmax = 518 nm upon excitation at kEx = 444 nm [17]. Under the same experimental conditions, 1 and 2 exhibit intense luminescent emissions at kmax = 430/520 and 432/519 nm upon excitations at kEx = 380 and 380 nm in the purple and green fluorescent regions, respectively (Fig. 5). The emission peaks of 1 and 2 are similar to those of the H2L ligand, which are neither metal-to-ligand charge transfer nor ligand-to-metal charge transfer in nature because CdII or LaIII ions are difficult to oxidize or reduce to their d10 configuration

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Fig. 2. View of (a) Coordination environment of LaIII ions in 2 (H atoms omitted for clarity); (b) the 2D 63 honeycomb network motif schematic view in the c direction (H atoms omitted for clarity), and (c) the 2D waved schematic view in the a direction; (d) the 3D framework linked by inter-chain C–Hp interactions (partial H atoms omitted for clarity).

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Table 3 Selected bond distances (Å) and angles (°) for complex 2.a La(1)–O(3) La(1)–O(2) O(3)–La(1)–O(3)#1 O(3)–La(1)–O(1) O(1)#1–La(1)–O(1) O(1)–La(1)–O(2)#1 O(3)#1–La(1)–O(2) O(1)–La(1)–O(2) a

2.471(7) 2.652(6) 81.4(3) 129.5(2) 77.1(3) 70.46(19) 74.3(2) 50.30(19)

La(1)–O(1)

2.539(6)

O(3)–La(1)–O(1)#1 O(3)#1–La(1)–O(1) O(3)–La(1)–O(2)#1 O(3)–La(1)–O(2) O(1)#1–La(1)–O(2) O(2)#1–La(1)–O(2)

144.1(2) 85.7(3) 150.8(3) 79.3(2) 122.4(2) 115.71(10)

Symmetry codes: #1 y, x  y, z.

Fig. 4. Solid state UV–Vis spectra for of 1 and 2 at room temperature.

Fig. 5. Emission spectra of 1 and 2 in the solid state at room temperature (kex = 380 nm for 1, and 380 nm for 2, respectively).

diamond network and a 2D waved 63 honeycomb network, respectively, were successfully obtained. The structural discrepancies between the complexes can be attributed to the different synthesis methods using various solvents. The photoluminescence solidstate properties of complexes 1 and 2 were also studied at room temperature. Both complexes display purple or green emissions owing to their anthracene rings. Fig. 3. X-ray powder diffraction (XRPD) patterns of (a) for 1 and (b) for 2.

Acknowledgements [17d,23a]. This phenomenon can be tentatively assigned to intraligand transfer p⁄–p transitions, namely, ligand-to-ligand charge transfer, similar to previously reported results on coordination polymers with an anthracene backbone [17]. The different relative intensities of 1 and 2 may be assigned to structural diversities and different ratios of the H2L ligand [17d,23a], or to the strong conjugation and inter/intramolecular interaction between the molecule segments of ligands [17d].

This work was financially supported by the NNSF of China (No. 20801029 and 21071006), the NSF of Tianjin, China (10JCZDJC22100 and 11JCYBJC04100), the key project of Science and Technology Department of Henna Province (No. 112102210371) and the Industrial Research Project of Technology Bureau of Anyang (No. 39).

6. Conclusion

Appendix A. Supplementary Material

Two anthracene-9,10-dicarboxylate complexes with different metal ions, Cd1I and LaIII, possessing a 3D twofold interpenetrated

Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2011.12.037.

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