XUE Li-PingLIU Yue-ChengHAN Yun-Hu TIAN Chong-Bin LI Qi-Peng LIN Ping DU Sho-Wu

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New Zn(II)/M(I) (M = Na, K and Rb) Metal-organic Frameworks Based on 5-Methylisophthalic Acid Ligand:Syntheses, Structures and Fluorescent Property①

XUE Li-Pinga, bLIU Yue-ChengaHAN Yun-Hua, bTIAN Chong-BinaLI Qi-Penga, bLIN Pinga②DU Shao-Wua

a(350002)b(100049)

Reactions of Zn(NO3)2·6H2O and MNO3(M = Na, K and Rb) with H2mip (H2mip = 5-methylisophthalic acid) in DMF (DMF = N,N′-dimethylformamide) resulted in the formation of novel heterometallic metal-organic frameworks (Me2NH2)[ZnM(mip)2] (1: M = Na; 2: M = K; 3: M = Rb, mip = 5-methylisophthalate dianion for 1～3). These complexesbelong to the monoclinic system, space group2/and have been fully characterized by satisfactory elemental analysis, FT-IR spectra, TGA and single-crystal X-ray diffraction. Single-crystal X-ray diffraction studies reveal that 1～3 are 3D isomorphic structures based on a trinuclear unit {ZnM2(2-COO)2(3- COO)2} and exhibittopological net and blue fluorescence.

heterometallic, solvothermal synthesis, photoluminescence

1 INTRODUCTION

The design and synthesis of metal-organic frame- works (MOFs) have become a significant research area today not only because of their appealing structural topologies but also due to their potential applications as functional materials[1-2]. In particular, much interest has been directed toward the research of heterometallic MOF tuned by secondary me- talions. Consequently, many transition-transition () and transition-rare earth metal () heterome- tallic frameworks have been reported, which possess not only interesting structural topologies but also significant properties, such as luminescence, ion exchange, nonlinear optics and magnetism. However, heterometallic frameworks tuned by alkali metal ions are relatively less explored[3-7]. Compared with transition metal and rare earth metal ions, the alkali metal ions have relatively larger radius and high affinity for oxygen donors, which will provide unique opportunities for the formation of extended transition metal carboxylates doped by alkali metal ions[8-11].

In our previous studies, we have focused our attention on mixed metal Zn/M (M = alkali metal ions) carboxylate systems. In this paper, we apply the same strategy to prepare novel MOFs structures with mixed metals of Zn(II) and alkali metal ions.Herein, we report the synthesis of three new 3Dcompounds {(Me2NH2)[ZnM(mip)2]}n(1: M = Na;2: M = K; 3: M = Rb).In these complexes, the alkali metal ions not only function as counterions but also participate in extending the 3D frameworks.Com- pounds 1～3 are isostructural and exhibit 3D frameworks with 6-connected {41263} topology[12].

2 EXPERIMENTAL

2.1 Materials and instruments

All the chemicals were purchased commercially and used as received. Thermogravimetric analysis- mass spectrometry analysis (TGA-MS) experiments were performed using a TGA/NETZSCH STA449C instrument heated from 30 to 800 ℃ (heating rate of 10 ℃/min, nitrogen stream). The powder X-ray diffraction (XRD) patterns were recorded on crushed single crystals in the 2range of 5～50o using Cu-radiation. The XRD were measured on a PANalytical X’pert PRO X-ray diffractometer. IR spectra using the KBr pellet technique were recorded on a Spectrum-One FT-IR spectrophotometer. Elemental analyses (C, H, and N) were measured with an Elemental Vairo EL III Analyzer. Fluore- scence spectra for the solid samples were performed on an Edinburgh Analytical instrument FLS920.

2.2 Synthesis

{(Me2NH2)[ZnM(mip)2]}n(1: M = Na; 2: M = K; 3: M = Rb). A mixture of Zn(NO3)2·6H2O (0.0892 g, 0.3 mmol),NaNO3(0.0425 g, 0.5 mmol) and H2mip (0.1661 g, 1.0 mmol) in DMF (10 mL) was stirred at room temperature for a few minutes. The resulting slurry was transferred into a 20 mL Teflon-lined stainless steel vessel, which was heated at 115 ℃and maintained at this temperature for 72 h. The system was cooled to room temperature at a rate of 1.25 ℃/h.Colorless crystals of 1 were obtained, washed with DMF and dried in air (yield 75% based on Zn). Elemental Anal. Calcd. for1C20H20NaNO8Zn (490.73): C, 48.95; H, 4.11; N, 2.85%. Found: C, 48.62; H, 3.99; N, 2.57%. IR (KBr, cm?1): 3438 m, 3070 s, 2796 m, 1629 s, 1595 m, 1568 s, 1427 s, 1348 m, 1244 w, 1103 s, 1018 w, 894 w, 777 s, 732 s, 594 w, 518 s. Colorless crystals of 2 (yield 65%) and 3 (yield 70%)were obtained from a way analogous to that for 1 except that KNO3and RbNO3are used respectively instead of NaNO3. Elemental Anal. Calcd. for 2C20H20KNO8Zn (506.84): C, 47.39; H, 3.98; N, 2.76%. Found: C, 46.89; H, 3.75; N, 2.61%. IR (KBr, cm?1): 3435 m, 3128 s, 2970 s, 1631 s, 1593 m, 1568 s, 1429 s, 1352 m, 1240 w, 1103 w, 1020 w, 894 w, 775 s, 729 s, 595 w, 514 s. Elemental Anal. Calcd. for3: C20H20NO8RbZn (553.21): C, 43.42; H, 3.64; N, 2.53%. Found: C, 42.79; H, 3.75; N, 2.33%.IR (KBr, cm?1): 3435 m, 3136 s, 3055 w, 1631 s, 1595 s, 1568 s, 1427 s, 1350 m, 1101 s, 1020 w, 891 m, 775 s, 729 m, 594 w, 518 s.

2.3 Structure determination

Single-crystal X-ray diffraction data were collec- ted on a Rigaku diffractometer with a Mercury CCD area detector (Mo,= 0.71073 ?) at room temperature. Empirical absorption corrections were applied to the data using the Crystal Clear program. The structures were solved by direct methods using SHELXS-97 and refined on2by full-matrix least- squares with the SHELXL-97 program package[13-14]. Metal atoms in each compound were located from the-maps and other non-hydrogen atoms were located in successive difference Fourier syntheses. All non-hydrogen atoms were refined anisotropically. Crystallographic data and other pertinent informa- tion are summarized in Table 1. The selected bond lengths and bond angles are listed in Tables 2～4.The topological analyses for these compounds were studied using TOPOS 4.0[15].

3 RESULTS AND DISCUSSION

3.1 Structural description

{(Me2NH2)[ZnM(mip)2]}n(1: M = Na; 2: M = K; 3: M = Rb) Single-crystal X-ray diffraction studies of 1～3 reveal that they are isostructural with different alkali metal ions. Therefore, only the crystal structure of 1 will be described in detail. Compound 1 crystallizes in the monoclinic space group2/The asymmetric unit of 1 consists of one Zn(II) ion, one K(I) ion, two mip ligands and one (Me2NH2)+cation. As depicted in Fig. 1a, Zn(1) is six-coordinated by six carboxylate oxygen atoms (O(1C), O(2C), O(1B), O(2B), O(8), O(8A)) from four distinctmip ligands and can be described as a badly distorted octahedron {Zn1O6}. Zn(2) is four-coordinated and can be regarded as a slightly distorted tetrahedralcoordination geometry. It is linked to four oxygen atoms (O(4), O(4D), O(6), O(6D)) from four different mip ligands. The Na(1) is hexa-coordinated, embraced by six carboxylic oxygen atoms (O(1), O(1E), O(7F), O(7G), O(8I), O(8H)) from six different mip ligands. Its coor- dination geometry can be considered as a standard octahedral sphere, and twocarboxylic oxygen atoms (O(1) and O(1E)) occupy the axial positions with the O(1)?Na(1)?O(1E) angle of 180.0°. The equatorial plane is defined by four oxygen atoms (O(7F), O(7G), O(8I), O(8H)). The Na(2) coordination environment is the same with Na(1), surroundedby six carboxylate oxygen atoms (O(3), O(3J), O(4D), O(4K), O(5D), O(5K)), which is defined by four carboxylate oxygen atoms (O(3), O(3J), O(5D) and O(5K)) of four separate mip ligands in the equatorial plane and two carboxylate oxygen atoms O(4D) and O(4K) from another two different mip ligands at the remaining apical positions with the O(4D)– Na(2)–O(4K) bond angle of 180.0°. The Zn–O distances are in the range of 1.9885(19)～2.333(2) ? and the Na–O distances are in the range of 2.418(2)～2.542(2) ?. They are all within the normal ranges[16].In 1,theZn(II) and Na(I) ions are connected by carboxylate groups adopting(k1-k1)- (k1-μ2)-5coordination mode (Fig. 1b).Compound 1 is 3D structure based on a trinuclear heterometallic unit {ZnNa2(2-COO)2(3-COO)2}(Fig. 1c).

Table 1. Crystallographic Data for Compounds 1～3

=?║│–│║/∑││,wR= [ ∑(2-2)2/∑(2)2]0.5

Table 2. Selected Bond Lengths (?) and Bond Angles (°) for 1

Symmetry codes: (A) ??2,, ?+1/2; (B) ??1,+1, ?+1/2; (C)?1,+1,; (D) ??1,, ?+1/2; (E) ?, ??2,?+1; (F)+1, ??1,+1/2; (G) ??1,?1, ?+1/2; (H) ??1, ??1, ?+1; (I)+1,?1,; (J) ??1, ??1, ?; (K), ??1,?1/2

Table 3. Selected Bond Lengths (?) and Bond Angles (°) for 2

Symmetry codes: (A) ?,, ?+3/2; (B),?1,; (C) ?+1,?1, ?+3/2; (D) ?+1,, ?+3/2; (E) ?+1, ?+1, ?+2; (F),+1,; (G) ?+1, ?+2, ?+2; (H), ?+1,+1/2; (I) ?+1,+1, ?+3/2; (J) ?, ?+1, ?+1; (K), ?+1,?1/2

Table 4. Selected Bond Lengths (?) and Bond Angles (°) for 3

Symmetry codes: (A) ??1,, ?+1/2; (B) ?,, ?+1/2; (C),?1,; (D) ?,?1, ?+1/2; (E) ??1, ?+1, ?; (F), ?+1,?1/2; (G) ?, ?+2, ?+1; (H) ?, ?+1, ?+1; (I),+1,; (J), ?+2,+1/2

Fig. 1. (a) View of the coordination environment of metal ions in 1. The labels for carbon atoms are not shown for clarity. Symmetry codes: (A) ??2,, ?+1/2; (B) ??1,+1, ?+1/2; (C)?1,+1,; (D) ??1,, ?+1/2; (E) ?, ??2, ?+1; (F)+1, ??1,+1/2; (G) ??1,?1, ?+1/2; (H) ??1, ??1, ?+1; (I)+1,?1,; (J) ??1, ??1, ?; (K), ??1,?1/2.(b) Coordination mode of the mipligand. (c) Trinuclear heterometallic [ZnNa2(COO)4] cluster (SBU)

The mip ligands, adopting a (k1-k1)-(k1-2)-5coordination mode, link the Zn(II) and Na(I) ions into a 3D anionic framework with large square channels when viewed along the c axis (Fig. 2a). From the topological point of view, the mipligands and Zn can be viewed as connectors and the Na atoms as nodes. Thus, the anionic framework can be abstracted into a 6-connected network with the Schl?fli symbol of {41263} (Fig. 2b). The network can be specified by the vertex symbol of 4?4?4?4?4?4?4?4?4?4?4?4?*?*?* analyzed by OLEX program[17], which is a typicaltopological network.

Fig. 2. (a)Polyhedral view of the 3D framework for 1. (b) Topological view showing the equivalent 3D framework for 1

3.2 Spectral and thermal analyses

The powder X-ray diffraction patterns of 1～3 are given in Fig. 3 with the pattern simulated on the basis of single crystal structures. The positions of diffraction peaks in both patterns correspond well, which indicate that the as-synthesized samples 1～3are pure. The IR spectra of 1～3 are very similar owing to their analogue structures (Fig. 4).In the IR spectra of 1～3, the absence of strong absorption associated with the carboxyl group at around 1701 cm–1indicates complete deprotonation of H2mip. The characteristic absorption bands of carboxylate groups are shown in the 1565～1635 cm–1range for the asymmetric stretching vibrationas(COO–) and 1345～430 cm–1for the symmetric stretching vibrations(COO–)[18]. The above results are all confirmed by single-crystal X-ray diffraction analy- sis. In order to examine the stability of the fram- ework, thermal gravimetric analyses (TGA) for 1～3were carried out in nitrogen gas from 30 to 800 ℃ (Fig. 5). The TGA curves for 1～3 are similar, which show almost no weight loss before the decompo- sition of the framework. The frameworks of 1～3begin to collapse at the temperature of 298, 284 and 273 ℃, respectively, which may be attributed to the different bond energies of M–O bonds (M = Na, K, Rb).

Fig. 3. Simulated and experimental XRD powder patterns

Fig. 4. IR spectra of 1～3

3.3 Luminescent properties

Solid-state luminescent properties of 1～3 were studied at room temperature. As shown in Fig. 6, for the compounds, these emission peaks are 515 nm (ex= 381 nm) for 1, 501 nm (ex= 369 nm) for 2, and 481 nm (ex= 371 nm) for 3. The free H2mip displays an intense ultraviolet emission at 357 nm upon excitation at 315 nm, which can be assigned to the intra-ligand fluorescence emission (→* transitions of the aromatic carboxylic acid)[19].It is evident that the fluorescent properties of Zn-based MOFs with mip ligands are effectively tuned by the Na+, K+and Rb+ions in hetero-MOFs and their peaks were blue-shifted ranging from Na+, K+to Rb+. According to the reported literatures, these emission peaks of 1, 2 and 3 should be interpreted as ligand-to-metal charge transition (LMCT),which is similar to those found for Zn-based MOFs[20].

Fig. 5. View of the TGA curve of the complexes

Fig. 6. Luminescent spectra at room temperature

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22 January 2014;

7 May 2014 (CCDC 981464～981466 for 1～3)

① This work was supported by the National Basic Research Program of China (973 Program, 2012CB821702), the National Natural Science Foundation of China (21233009 and 21173221) and the State Key Laboratory of Structural Chemistry

. E-mail: pinglin@fjirsm.ac.cn