Syntheses, Structures, and Structural Transformations of Mixed Na(I

ARTICLE pubs.acs.org/crystal

Syntheses, Structures, and Structural Transformations of Mixed Na(I) and Zn(II) Metal
Organic Frameworks with 1,3,5-Benzenetricarboxylate Ligands Ying Fu,† Jie Su,† Sihai Yang,‡ Zhibo Zou,§ Guobao Li,*,† Fuhui Liao,† Ming Xiong,|| and Jianhua Lin*,† †

)

Beijing National Laboratory for Molecular Sciences (BNLMS), State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China ‡ School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, U.K. § Beijing Huiwen Middle School, Beijing 100061, P. R. China X-ray Laboratory, China University of Geoscience, Beijing 100083, P. R. China

bS Supporting Information ABSTRACT: Three new compounds, [ZnNa(BTC)(H2O)2] 3 H2O (1), [Zn3Na2(BTC)2(HCOO)2(H2O)8] (2), and [Zn3Na(BTC)2 (HCOO)(H2O)3] 3 2H2O (3) (H3BTC = 1,3,5-benzenetricarboxylate acid), were synthesized under hydrothermal conditions and were characterized by the single crystal and powder X-ray diffraction, IR spectra, elemental analyses, ICP measurements, and coupled TG-MS analyses. 1, 2, and 3 crystallize in the space groups P1, C2/c, and P1, respectively. They are three-dimensional (3D) frameworks, consisting of zigzag chains ((Na2Zn2O14)n for 1, (Na2Zn3O18)n for 2, or (NaZn3O14)n for 3) and bridging BTC ligands. Powder X-ray diffraction studies reveal that 1 and 2 exhibit reversible dehydration
rehydration behaviors with a crystal-tocrystal transformation observed for 2.

’ INTRODUCTION The studies on metal
organic frameworks (MOFs) have undergone rapid development due to their attractive topologies1 and beneficial properties in gas adsorption, separation, catalysis, magnetism, nonlinear optics and so on.2 For having potential applications as sensors,3 highly selective guest inclusion,4 magnetic bistability,5 and delivery of specific chemicals,6 MOFs with flexible frameworks (flexible metal
organic frameworks, FMOFs) that refer to the ability of reversibly structural transformations to be induced by external stimuli,7 are of great interest. Generally, hydrogen bonds and weak interactions with flexible lengths and angles are responsible for most of the reported dynamic nature of FMOFs.8 Most drastically, the reconstruction of the coordinated bonds, involving the loss of terminal and/or bridging aqua ligands, and/or coordination bonds to metal ions, is found to have a contribution to the flexibility of FMOFs.9 In recent years, crystalto-crystal transformations of FMOFs have received increasing attention, since these processes can directly and accurately indicate essentially dynamic structural changes. Here, we report the syntheses, structures, and thermal properties of three new compounds, [ZnNa(BTC)(H2O)2] 3 H2O (1), [Zn3Na2(BTC)2(HCOO)2(H2O)8] (2), and [Zn3Na(BTC)2(HCOO)(H2O)3] 3 2H2O (3). Two of them, 1 and 2, can undergo reversible dehydration
rehydration of both guest and coordinated water molecules. Particularly, the crystal-to-crystal transformation of 2 r 2011 American Chemical Society

indicates that the breakage and formation of coordinated bonds contributes a lot to the flexibility of 2.

’ EXPERIMENTAL SECTION Materials and Characterizations. All solvents and reagents for the syntheses were of analytical grade and used as received from commercial sources without further purification. Powder X-ray diffraction data of the studied samples were collected on a Rigaku D/Max-2000 diffractometer with Cu KR radiation (λ = 1.5418 Å) at 40 kV, 100 mA, and a graphite monochromator at the secondary beam. IR spectra were recorded on a Magna-IR 750 FTIR spectrophotometer in the region 4000
650 cm
1. Elemental analyses for C and H were carried out on an Elementar Vario EL III microanalyzer. Thermogravimetric
mass spectrometric (TG-MS) analyses were performed in an air atmosphere with a heating rate of 10 °C min
1 from 50 to 800 °C, using a NETZSCH STA449C instrument. Inductively coupled plasma (ICP) optical emission spectroscopy for the ratio of Na/Zn were measured with an ESCALAB2000 analyzer. Synthesis of [ZnNa(BTC)(H2O)2] 3 H2O (1) and [ZnNa(BTC)] (1a). A mixture of Zn(CH3COO)2 3 2H2O (1.0 mmol, 0.2195 g),

H3BTC (1.0 mmol, 0.2101 g), NaOH (2.0 mmol, 0.0800 g), and distilled water (10.0 mL) was placed in a 23 mL Teflon-lined stainless Received: December 4, 2010 Revised: April 24, 2011 Published: April 27, 2011 2243

dx.doi.org/10.1021/cg101610k | Cryst. Growth Des. 2011, 11, 2243–2249

Crystal Growth & Design

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Table 1. Crystallographic and Structural Refinement Parameters for 1, 2, 3, and 2a 1

2

3

2a

formula

ZnNaC9H9O9

Zn3Na2C20H24O24

Zn3NaC19H17O19

Zn3Na2C20O18H12

fw

349.52

890.48

768.43

782.39

space group

P1

C2/c

P1

C2/c

a (Å)

9.672(2)

7.511(2)

7.902(2)

7.353(15)

b (Å)

9.682(2)

19.151(4)

9.378(2)

19.332(4)

c (Å)

7.742(2)

21.213(4)

9.709(2)

16.542(3)

R (deg)

99.39(3)

90.00

61.65(3)

90.00

β (deg) γ (deg)

104.54(3) 60.72(3)

93.42(3) 90.00

71.60(3) 80.38(3)

99.00(3) 90.00

V (Å3)

611.4(2)

3046.0(10)

600.8(2)

2322.4(8)

Z

2

4

1

4

Dcalc (g 3 cm
3)

2.238

1.899

1.942

2.124

λ (Mo KR) (Å)

0.71073

0.71073

0.71073

0.71073

μ (mm
1)

2.086

2.474

3.086

3.210

GOF on F2

1.007

1.008

1.002

1.033

Rint R1, wR2 [I > 2σ(I)]

0.1591 0.0680, 0.2192

0.0179 0.0688, 0.2245

0.0156 0.0308, 0.0798

0.2314 0.1041, 0.2638

R1, wR2 (all data)

0.0734, 0.2300

0.0842, 0.2509

0.0350, 0.0823

0.1307, 0.2817

autoclave. The autoclave was sealed, heated at 170 °C for 5 days, and then cooled to room temperature. Colorless crystals were filtered, washed with distilled water, and then dried in air to give about 0.28 g of 1 (yield 80% based on H3BTC). Anal. Calcd for ZnNaC9H9O9 (fw 349.52): C 30.92, H 2.59. Found: C 30.19, H 2.87 (see Supporting Information for details). ICP analysis shows the ratio of Na/Zn in 1 is about 1.02 (calcd 1.00). The crystals of 1 were heated to 80 °C in air for 2 h to produce 1a. Anal. Calcd for ZnNaC9H3O6 (fw 295.47): C 36.58, H 1.02. Found: C 36.24, H 1.27.

Synthesis of [Zn3Na2(BTC)2(HCOO)2(H2O)8] (2) and [Zn3Na2(BTC)2(HCOO)2(H2O)2] (2a). 2 was synthesized from ZnCl2 (3.0 mmol, 0.4088 g), H3BTC (2.0 mmol, 0.4202 g), HCOONa (2.0 mmol, 0.1360 g), DMF (3.0 mL), and distilled water (7.0 mL). The mixture was sealed in a 23 mL Teflon-lined stainless autoclave and heated at 120 °C for 5 days. After cooling to room temperature, colorless crystals were filtered, washed with distilled water, and left to air-dry to give about 0.67 g of 2 (yield about 75% based on H3BTC). Anal. Calcd for Zn3Na2C20H24O24 (fw 890.48): C 26.97, H 2.72. Found: C 26.93, H 2.91. ICP analyses indicate the ratio of Na/Zn is 0.69 (calcd 0.67) for 2. The crystals of 2 were heated to 80 °C in air for 2 h to produce 2a. Anal. Calcd for Zn3Na2C20O18H12 (fw 782.39): C 30.70, H 1.54. Found: C 30.38, H 1.78. Synthesis of [Zn3Na(BTC)2(HCOO)(H2O)3] 3 2H2O (3). 3 was hydrothermally prepared from Zn(CH3COO)2 3 2H2O (6.0 mmol, 1.3169 g), H3BTC (2.0 mmol, 0.4202 g), NaOH (3.0 mmol, 0.1600 g), HCOOH (1.0 mL), and distilled water (9.0 mL) (see Supporting Information for details). The mixture was sealed in the 23 mL Teflonlined stainless autoclave and heated at 120 °C for 5 days. After cooling, colorless crystals were received, about 0.52 g (yield 67% based on H3BTC), by washing with distilled water and drying in air. Anal. Calcd for Zn3NaC19H17O19 (fw 768.43): C 29.70, H 2.23. Found: C 29.62, H 2.02. ICP analyses indicate the ratio of Na/Zn is 0.35 (calcd 0.33) for 3. Crystallographic Studies. Suitable single crystals of 1, 2, and 3 were carefully selected under an optical microscope and glued to thin glass fibers with epoxy resin. All data were collected using graphitemonochromatic Mo KR (λ = 0.71073 Å) radiation. X-ray single-crystal diffraction data of 1 and 2 were collected on a Rigaku AFC6S diffractometer by using the ω
2θ scan method at room temperature. Their PSI absorption corrections were applied using the TEXSAN program.10 The intensity data of 3 were collected on a Bruker SMART

X-ray diffractometer, equipped with an APEX-CCD area detector at room temperature. The data absorption correction of 3 was applied on the basis of symmetry-equivalent reflections using the ABSCOR program.11 The single crystal of 2 was heated at 353 K for 10 h and then 383 K for 1 h to produce 2a, and it was kept at the temperature during collection of diffraction data on a Bruker P4 X-ray diffractometer equipped with an APEX-CCD area detector. The data absorption correction of 2a was applied using the SADABS program.12 All structures were solved by the direct method and refined on F2 with full-matrix least-squares methods using the SHELXS-97 and SHELXL97 programs, respectively.13 All of the non-hydrogen atoms were refined anisotropically, except that the O atoms of uncoordinated water were refined isotropically for 1. Hydrogen atoms were added in the riding model and refined isotropically with O
H = 0.82 Å and C
H = 0.93 Å. The crystallographic data and structural refinement parameters are presented in Table 1, and the selected bond lengths are listed in Table S1 in the Supporting Information. CCDC 759101, 759102, 759103, and 783280 contain the supplementary crystallographic data for compounds 1, 2, 3, and 2a. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif.

’ RESULTS AND DISCUSSION Infrared (IR) Spectra. IR spectra of 1, 2, and 3 (shown in Figure S1 of the Supporting Information) confirm the presence of the organic ligands used in the synthesis (through the characteristic bands of aromatic and carboxylate groups). The broad absorption bands of the asymmetric and symmetric stretching vibrations of water appear at 3700
2700 cm
1.14a
d The bands at 1619
1560 cm
1 and 1442
1348 cm
1 correspond to the asymmetric and symmetric stretching vibrations of the bound carboxylate groups (CO2M), respectively.14 And the bands at 951
831 cm
1 and 791
722 cm
1 are related to the stretching vibrations of the C
C groups and the out-of-plane deformation vibrations of the C
H groups in the benzene ring.14b
d The absence of the absorption bands from 1680 to 1800 cm
1 indicates the complete deprotonation of BTC ligands.14e 2244

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Figure 1. Structural details of 1. (a) Thermal ellipsoid plot (50%) drawing of the coordination environment of Zn(II) and Na(I) centers. Symmetry codes: (i)
x,
y,
z. (b) 3D framework viewed along the [010] direction. (c) Zigzag chain of (Na2Zn2O14)n along the [111] direction. (d) 3D framework viewed along the [001] direction. (Color codes: Zn(1), ciel; Na(1), purple; Na(2), yellow; C, gray; O, red; Zn(1)O5 trigonal bipyramids, ciel; Na(1)O6 octahedra, purple; Na(2)O6 octahedra, yellow. The free water molecules and hydrogen atoms are omitted for clarity.)

Crystal Structure of [ZnNa(BTC)(H2O)2] 3 H2O (1). 1 crystal-

lizes in the triclinic space group P1. In the asymmetric unit of 1, there are one crystallographic independent Zn(II) cation, two Na(I) ions, one BTC ligand, two μ2-H2O molecules, and one free water molecule scattered at several sites (see Figure 1a for reference). Each Zn(II) is coordinated with three carboxyl oxygen atoms (O(2), O(3), and O(5)) from three BTC ligands and two oxygen atoms (O(1), O(7)) from two μ2-H2O molecules, to form a trigonal bipyramid, where the lengths of the Zn
O bonds are in the range 1.988
2.161 Å (Table S1 of the Supporting Information). Each Na(I) cation lies at an inversion center and exhibits a six-coordinated configuration to construct a distorted octahedron with the length of Na
O bonds ranging from 2.328 to 2.586 Å (Table S1). Na(1) and Na(2) have the same arrangement of ligands: each of them is bound with four carboxyl oxygen atoms (Na(1): O(3), O(4), O(3)i, and O(4)i; Na(2): O(2), O(6), O(2)i, and O(6)i) from four BTC linkers and two oxygen atoms (Na(1): O(1) and O(1)i; Na(2): O(7) and O(7)i) from two μ2-H2O molecules. Each bridging BTC in 1 is completely deprotonated, binding to three separate Zn(1) and two Na(1) and two Na(2) atoms (see Figure S3a in the Supporting Information), where one carboxyl group is coordinated with one Zn(1) and one Na(1) and one Na(2) atoms, adopting the bridging bidentate fashion, the second links one Zn(1) and one Na(1) atoms with the bidentate mode, while the third uses the bridging unidentate configuration, connecting one Zn(1) and one Na(2) atoms (according to the nomenclature summarized in Figure S2 in the Supporting Information). In 1, NaO6 octahedra and ZnO5 trigonal bipyramid are linked to each other by edge-sharing to form a zigzag chain of

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Figure 2. Structural details of 2. (a) Thermal ellipsoid plot (50%) drawing of the coordination environment of Zn(II) and Na(I) ions. Symmetry codes: (i)
x, y,
z þ 1/2; (ii) x þ 1/2, y þ 1/2, z; (iii)
x,
y,
z; (iv) x,
y, z
1/2. (b) 3D framework viewed along the [010] direction. (c) Zigzag chain of (Na2Zn3O18)n along the [100] direction. (d) 3D framework viewed along the [100] direction. (Color codes: Zn(1), ciel; Zn(2), green; Na(1), purple; C, gray; O, red; Zn(1)O6 octahedra, ciel; Zn(2)O4 tetrahedra, green; Na(1)O6 octahedra, purple. The hydrogen atoms are omitted for clarity.)

(Na2Zn2O14)n along the [111] direction (Figure 1c). Such zigzag chains are connected by the bridging BTC ligands parallel the ab plane to form a 3D network (Figure 1b and d). The guest water molecules occupy the void of the structure and interact with the framework through hydrogen bonds. Crystal Structure of [Na2Zn3(BTC)2(HCOO)2(H2O)8] (2). Crystallographic analysis reveals that compound 2 crystallizes in the monoclinic space group C2/c. It builds up from two independent Zn(II), one Na(I), one unique BTC ligand, one 2.20 formate ion,15 and five aqua ligands in the asymmetric unit, as shown in Figure 2a. Of the five kinds of aqua ligands, three are μ2-H2O bridging ligands and the remaining two coordinate to metal centers as terminal water. The length of the Zn
O bonds ranges from 1.955 to 2.143 Å, and the length of the Na
O bonds is in the range 2.334
2.473 Å (Table S1 of the Supporting Information). Zn(1) centers, lying at the 2-fold axis, connect two BTC ligands (O(1) and O(1)i), two μ2-H2O bridging ligands (O(11) and O(11)i), and two terminal waters (O(10) and O(10)i) to form six-coordinated octahedra. Each Zn(2) center shows a four coordinated tetrahedron configuration composed by two carboxyl oxygen atoms (O(6), O(7)) from two BTC ligands, one carboxyl oxygen atom (O(5)) from one 2.20 formate ion, and one oxygen atom (O(2)) from one terminal water. Na(1) ions also exhibit six-coordinated fashions to form distorted octahedra bound with two BTC (O(1), O(9)), one 2.20 formate ligand (O(5)), and three μ2-H2O molecules (O(11), O(12), and O(13)). The structure presents one type of completely deprotonated BTC bridging one Zn(1), two Zn(2), and two Na(1) atoms (see Figure S3b): the first carboxyl group is of the bidentate mode linking one Zn(2) and one Na(1) atoms, the second uses the bridging unidentate fashion to connect with 2245

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Figure 3. Structural details of 3: (a) thermal ellipsoid plot (50%) drawing of the coordination environment of Zn(II) and Na(I) centers; (b) 3D framework viewed along the [001] direction; (c) zigzag chain of (NaZn3O14)n along the [1 11] direction; (d) 3D framework viewed along the [100] direction. (Color codes: Zn(1), ciel; Zn(2), green; Zn(3), blue; Na(1), purple; C, gray; O, red; Zn(1)O6 octahedra, ciel; Zn(2)O5 trigonal bipyramids, green; Zn(3)O5 trigonal bipyramids, blue; Na(1)O6 octahedra, purple. The free water molecules and hydrogen atoms are omitted for clarity.)

one Zn(1) and one Na(1) atoms, while the third adopts the unidentate way, connecting one Zn(2) atom. And there is one kind of formate ion, being the 2.20 bridging configuration, to connect one Zn(2) and one Na(1) cations. As shown in Figure 2b, the Na(1)O6 and Zn(1)O6 octahedra in 2 link to each other to form an edge -shared primary chain of (Na2ZnO12)n along the a-axis (Figure 2c). Meanwhile, Zn(2)O4 tetrahedra join such chains through carboxyl oxygen atoms from 2.20 formate linkers to share vertexes with the Na(1)O6 octahedra acting as side chains. Therefore, there are zigzag chains of (Na2Zn3O18)n formed by such primary chains and side chains. The overall 3D net of 2 is constructed by the zigzag chains and the bridging BTC ligands as presented in Figure 2b and d. Crystal Structure of [Zn3Na(BTC)2(HCOO)(H2O)3] 3 2H2O (3). The single crystal X-ray diffraction analysis suggests compound 3 is in the triclinic space group P116 with the Flack parameter about 0.00(8), indicating that the absolute structure given by the structure refinement is likely correct. As illustrated in Figure 3a, the asymmetric unit of 3 contains three crystallographic unique Zn(II) cations, one Na(I) ion, two BTC ligands, one 2.20 formate ion, three μ2-H2O molecules, and two free water molecules. In the structure of 3, Zn(1) centers are six-coordinated by six oxygen atoms (O(1), O(2), O(6), O(7), O(11), and O(14)) to construct distorted octahedral geometries with the length of the Zn
O bonds ranging from 1.988 to 2.409 Å (Table S1 of the Supporting Information). The O(1), O(2), O(6), and O(7) atoms come from four separate BTC ligands; the O(11) atom comes from a μ2-H2O molecule, whereas the O(14) atom comes from a 2.20 formate ion. Both Zn(2) and Zn(3) atoms are five coordinated to form trigonal bipyramidal

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geometries with Zn
O bond distances in the range 1.946
2.332 Å. Zn(2) atoms connect three BTC ligands (O(5), O(7), O(10)), one 2.20 formate ion (O(14)), as well as one μ2-H2O molecule (O(3)). Besides connecting three BTC ligands (O(6), O(8), O(9)), Zn(3) is coordinated with two μ2-H2O molecules (O(11), O(12)). Na(1) cation is six-coordinated by six oxygen atoms from four BTC (O(4), O(9), O(10), O(16)) and two μ2H2O molecules (O(3), O(12)) and constructs a distorted octahedron with the length of Na
O bonds ranging from 2.311 to 2.565 Å. In addition, two independent BTC ligands, which are both completely deprotonated, have the similar arrangement of metal cations (Figure S3c). For the first one type of BTC ligand, the first carboxyl group uses the bridging bidentate configuration to connect one Zn(1), one Zn(2), and one Na(1) atoms, the second is coordinated with one Zn(1) and one Zn(2) atoms adopting the bridging unidentate fashion, while the third links one Zn(2) and one Na(1) atoms with the bidentate mode. The other type of BTC linker owns the same coordination fashion, except connecting Zn(3) instead of Zn(2). And each formate ion has the 2.20 bridging mode to bridge one Zn(1) and one Zn(2) cations. An edge-shared zigzag chain of (NaZn3O14)n is found in the structure of 3 along the [111] direction (Figure 3c), which includes Zn(2)O5, Zn(1)O6, Zn(3)O5, and Na(1)O6 polyhedra. The bridging BTC ligands, which parallel the bc plane, link those chains to build up a 3D network (Figure 3b and d). The uncoordinated water resides in the void of the structure. Thermal Properties and the Reversible Dehydration
Rehydration Phase Transition. The TG-MS data of 1, 2, and 3 under an air atmosphere at a heating rate of 10 °C min
1 (shown in Figure S4 of the Supporting Information) and the powder X-ray diffraction patterns of 1, 2, and 3 treated at different temperatures (shown in Figure S4) indicated that 1 was stable up to 380 °C and that both 2 and 3 were stable up to 240 °C. 1 lost the free and coordinated water molecules at 80
160 °C (found 15.11 wt %, calcd 15.46 wt %), and no further weight loss was found up to 380 °C (Figure S4a of the Supporting Information). The ion currents showed peaks at these temperatures only for ions with m/z = 17 and 18, which may be assigned to HOþ and H2Oþ, respectively. The second step between 380 and 520 °C corresponded to the release of the organic part, with the measured weight loss of about 47.01 wt % (calcd 46.09 wt %). The MS peaks of ions with m/z = 17, 18, 28, and 44 were observed during the above decomposition, which may correspond to HOþ, H2Oþ, COþ, and CO2þ, respectively. The residual weight about 37.87 wt % (calcd 38.44 wt %) corresponded to ZnO and Na2CO3, which agreed well with X-ray diffraction patterns as illustrated in Figure S5 of the Supporting Information. Interestingly, the PXRD analysis for 1 revealed that it undergoes a distinct structural transformation completed at 80 °C (Figure S4b). And the obtained phase (1a) was stable up to 380 °C. After exposing 1a in saturated moisture overnight, the original composition of 1 was recovered. It appeared clearly that compound 1 demonstrated a fully reversible dehydration
rehydration process, as reported in a few references.7g,17 The weight loss of 2 (depicted in Figure S4c of the Supporting Information), corresponding to the release of water molecules, was observed from 80 to 240 °C (found 16.50 wt %, calcd 16.18 wt %). During this step, only HOþ and H2Oþ (m/z = 17 and 18, respectively) were detected by MS. In 2, the absorption and deabsorption of the water molecule were also reversible, as for 1 (illustrated in Figure S4d). The crystal phase transformation of 2 2246

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Figure 4. Structural details of 2a. (a) Coordination environment of Zn(II) and Na(I) ions. Symmetry codes: (i)
x, y,
z þ 1/2; (ii)
x þ 1 /2,
y þ 1/2,
z; (iii) x þ 1/2,
y þ 1/2, z
1/2; (iv) x þ 1/2, y þ 1/2, z. (b) 3D framework viewed along the [010] direction. (c) Zigzag chain in the arrangement of (Na2Zn2O14)n along the [100] direction. (d) 3D framework viewed along the [100] direction. (Color codes: Zn(1), ciel; Zn(2), green; Na(1A), purple; Na(1B), yellow; C, gray; O, red; Zn(1)O4 tetrahedra, ciel; Zn(2)O4 tetrahedra, green; Na(1A)O6 octahedra, purple; Na(1B)O6 octahedra, yellow. The hydrogen atoms are omitted for clarity.)

appeared at 80 °C to give the dried form 2a, which could be stable up to 240 °C. The dehydrated compound collapsed in the region 240
500 °C (found 43.94 wt %, calcd 44.50 wt %), founding HOþ, H2Oþ, COþ, CO2þ, and HCO2þ (m/z = 17, 18, 28, 44, and 45, respectively) by MS. The final residue is ZnO and Na2CO3, corresponding to 39.54 wt % of the weight loss (calcd 39.32 wt %), which was confirmed by X-ray diffraction patterns (Figure S5). The TG-MS analysis of 3 was demonstrated in Figure S4e of the Supporting Information, with the weight loss attributed to the departure of water molecules (found 11.42 wt %, calcd 11.72 wt %) observed in the range 80
240 °C without any structure change, which was confirmed by the X-ray diffraction patterns (shown in Figure S4f). The destruction of the material occurred from 240 to 500 °C (found 49.63 wt %, calcd 49.61 wt %), leading to the formation of ZnO and Na2CO3 as the residue (found 38.94 wt %, calcd 38.66 wt %) (shown in Figure S5). The ion currents of m/z = 17 (HOþ) and 18 (H2Oþ) appeared in both above steps, whereas the MS peaks of ions with m/z = 28 (COþ), 44 (CO2þ), and 45 (HCO2þ) were observed in the second process. Crystal-to-Crystal Transformation. As discussed above, both compounds 1 and 2 show reversible dehydration
rehydration behaviors. Many attempts to obtain single-crystals of 1a were all unsuccessful. In 1a, we speculate that the 3D host framework of 1 remains after removal of water molecules, except for shrinkage of the framework indicated by an apparent shift toward a higher angle 2θ of the peaks in the PXRD patterns (Figure S4b of the Supporting Information). Thus, the molecular formula of 1a may be [ZnNa(BTC)], in accordance with the result of elemental analysis. Fortunately, compound 2 retains single crystallinity when dehydrated at high temperature (80 °C) under air atmosphere,

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Figure 5. Coordination environments of the metal atoms in (a) 2 and (b) 2a. Symmetry codes: (i)
x, y,
z þ 1/2; (ii) x þ 1/2, y þ 1/2, z; (iii)
x þ 1/2, y þ 1/2,
z þ 1/2. (Color codes: Zn(1), ciel; Zn(2), green; Na(1) and Na(1A), purple; Na(1B), yellow; C, gray; O, red; Zn(1)O4 tetrahedra, ciel; Zn(2)O4 tetrahedra, green; Na(1)O6 and Na(1A)O6 octahedra, purple; Na(1B)O6 octahedra, yellow. The hydrogen atoms are omitted for clarity.)

and gives the dried form 2a, [Zn3Na2(BTC)2(HCOO)2(H2O)2]. The single-crystal X-ray diffraction of 2a reveals that the crystal retains the monoclinic crystal system and the C2/c space group, with the unit cell parameters being changed (Table 1). The a-axis and c-axis are shortened from 7.511(2) to 7.353(15) Å and from 21.213(4) to 16.542(3) Å, respectively, while the b-axis is lengthened from 19.151(4) to 19.332(4) Å, and the β is enlarged from 93.42(3)° to 99.00(3)°. There are two independent Zn(II), two Na(I), one unique BTC ligand, one 3.21(syn, syn, anti-) formate ion, and one terminal aqua ligand in the asymmetric unit, as shown in Figure 4a. The Zn(2) center is also coordinated by two BTC ligands (O(6), O(7)), one formate ion (O(5)), and one terminal water (O(2)) to show a tetrahedron configuration, while the Zn(1) center now has a distorted tetrahedral geometry, connecting four BTC ligands (O(3), O(3)i, O(8), and O(8)i). It is interesting that there are two Na(I) centers (Na(1A) and Na(1B)) in 2a, both lying at the 2-fold axes and exhibiting distorted octahedra bound with four BTC (O(1), O(1)i, O(9), and O(9)i) and two formate ligands (Na(1A): O(5), O(5)i; Na(1B): O(4), O(4)i). Each BTC ligand bridges two separate Zn(1), two Zn(2), two Na(1A), and two Na(1B) atoms (see Figure S3d of the Supporting Information): the first carboxyl group is of the bidentate mode, linking one Zn(1) and one Zn(2) atoms, the second uses the bridging bidentate fashion with one Zn(1), one Na(1A), and one Na(1B) atoms, while the third adopts the same method as the second, connecting one Zn(2), one Na(1A), and one Na(1B) atoms. Each formate ion uses the 3.21(syn, syn, anti-) bridging configuration to link one Zn(2), one Na(1A), and one Na(1B) cations. As a result, the zigzag chain contains an edgeshared primary chain (Na2O6)n from Na(1A)O6 and Na(1B)O6 octahedra and side chains from Zn(2)O4 tetrahedra attached by sharing vertexes with Na(1A)O6 (Figure 4c). Therefore, there are zigzag chains of (Na2Zn2O14)n formed by such primary chains and side chains. Further, the bridging BTC ligands link both the zigzag chains and Zn(1)O4 tetrahedra to construct a whole 3D framework (Figure 4b and d). It is found that the atoms in 2a selected by us should be rotated on the ab plane for 180° to fit the atoms in 2 very well. In order to have a 2247

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Crystal Growth & Design clear understanding of the transition of 2 to 2a, the atoms in 2 and 2a have been set in the same orthogonal coordinate system (using eqs S1 and S2 shown in the Supporting Information) with the coordinates of Zn(2) atoms in both 2 and 2a defined as the original points, which are shown in Figure 5 (the details are listed in Table S2 in the Supporting Information). The same numbers are given to similar atoms during the transition. The atoms of BTC and HCOO
have the same numbers for both 2 and 2a. There are two Zn(II) in both 2 and 2a. One Zn(II) has the unchanged coordination environment during the transition and is numbered as Zn(2). The other Zn(II) is numbered as Zn(1), and its coordination environment changes during the transition. In addition, the one crystallographically independent Na(1) in 2 changes to two crystallographically independent Na(1A) and Na(1B) in 2a. Compared with structures 2 and 2a, it is not difficult to rationalize the structural transformation process. When the coordinated water molecules (O(10), O(11), O(12), and O(13)) are removed, the coordination environment of the Zn(1) center is changed due to the loss of the coordinated water molecules, while that of Zn(2) is preserved. One crystallographically unique Na(1) atom in 2 splits into Na(1A) and Na(1B) atoms in 2a. The connectivity modes of BTC and formate ligands are also changed. And it is quite different that Zn(1)O4 tetrahedra separate from the zigzag chains in 2a rather than joining them, as in 2. It should be noticed that the breakage and formation of coordinated bonds has a contribution to the reversible change of the framework 2.

’ CONCLUSIONS The combination of transition metals and alkali metals to react with the aromatic BTC ligands and formats affords three novel three-dimensional metal
organic frameworks through hydrothermal syntheses. Each compound presents a 3D framework formed by zigzag chains containing MOn polyhedra and bridging BTC anions. Moreover, both compounds 1 and 2 show reversible dehydration
rehydration processes. Notably, the framework of 2 clearly shows the crystal-to-crystal transformation to give the dried form 2a in response to removal of coordinated water molecules, rebinding of Zn(1) atoms, and spitting of Na(1) atoms, involving mainly breakage and formation of coordinate bonds. The illustration of these compounds demonstrates that combining transition metals and alkali metals may result in priming constructions and find potential applications in moisture sensing devices. This material is available free of charge via the Internet at http://pubs.acs.org. ’ ASSOCIATED CONTENT

bS Supporting Information. Crystallographic data for 1, 2, 3, and 2a in CIF format; IR, TG-MS, PXRD, and detail about selected bond lengths, connectivity modes of 1,3,5-benzenetricarboxylate ligands, and crystal-to-crystal transformation. This material is available free of charge via the Internet at http://pubs.acs.org. ’ AUTHOR INFORMATION Corresponding Author

*G.B.L.: telephone, (8610) 62750342; fax, (8610) 62753541; e-mail, [email protected]. J.H.L.: e-mail, [email protected].

’ ACKNOWLEDGMENT This work is supported by the National Natural Science Foundation of China (Grants 20771008 and 20821091) and

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the National Key Basic Research Project of China (Grant 2010CB833103).

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ion on the inversion center. Therefore, if P1 is used to describe the structure of 3, a disorder model should be used. However, these uncertainties can be omitted when P1 is used to describe the structure of 3. (17) (a) Lee, E. Y.; Suh, M. P. Angew. Chem., Int. Ed. 2004, 43, 2798. (b) F
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