Self-Assembly of Four Coordination Polymers in Three-Dimensional

Jul 10, 2014 - National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan .... out on a Siemens SMART diffractomer with a CCD detector with...
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Self-Assembly of Four Coordination Polymers in Three-Dimensional Entangled Architecture Showing Reversible Dynamic Solid-State Structural Transformation and Color-Changing Behavior upon Thermal Dehydration and Rehydration Szu-Yu Ke,† Ya-Fan Chang,† Hsin-Yu Wang,† Ching-Chun Yang,† Cheng-Wei Ni,† Gu-Ying Lin,† Tzu-Ting Chen,† Mei-Lin Ho,*,† Gene-Hsiang Lee,§ Yu-Chun Chuang,*,‡ and Chih-Chieh Wang*,† †

Department of Chemistry, Soochow University, Taipei 215006, Taiwan Instrumentation Center, National Taiwan University, Taipei 10617, Taiwan ‡ National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan §

S Supporting Information *

ABSTRACT: A unique three-dimensional (3D) supramolecular compound, [Co(dpe)(BTC)(H2O)][Co(dpe)(BTC)(H2O)3][Co(dpe)(HBTC)(H2O)][Co(dpe)2(H2O)3.5(EtOH)0.5]·1.5H2O (1; dpe = 1,2-bis(4-pyridyl)ethane and H3BTC = benzenetricarboxylic acid), has been synthesized and structurally characterized by the single-crystal X-ray diffraction method. Compound 1 consists of four coordination polymers (CPs), two are two-dimensional (2D) layered metal−organic frameworks (MOFs) with (4,4) topology of [Co(dpe)(BTC)(H2O)]− and [Co(dpe)(HBTC)(H2O)], whereas the other two are onedimensional (1D) polymeric chains of [Co(dpe)(BTC)(H2O)3]− and [Co(dpe)(H2O)3.5(EtOH)0.5]2+. The 3D supramolecular architecture of 1 is constructed via the penetration of interdigitated double-layered 2D rectangular-grid frameworks by two 1D coordination polymeric chains and entangled tightly by the subtle combination of intermolecular hydrogen bonding and π−π interactions among the four CPs. Controlled heating of the as-synthesized crystal 1 at ∼160 °C produces a desolvated 1 and accompanying color-changing behavior from pink to deep-blue, and the deep-blue desolvated 1 regenerates the pink rehydrated crystal with the chemical formula of [Co(dpe)(BTC)(H2O)][Co(dpe)(BTC)(H2O)3][Co(dpe)(HBTC)(H2O)][Co(dpe)2(H2O)4]·3H2O (2) upon exposure to water vapor. The structural determination of 2 shows almost the same structural characteristics as that of 1 with the only difference being the replacement of disordered coordinated solvent (half H2O and half EtOH molecules) by H2O and the numbers of solvated water molecules. The cyclic thermogravimetric analysis and powder X-ray diffraction measurements of desolvated 1 demonstrate a reversible rehydration/dehydration property, which is associated with solid-state structural transformation and thermally induced UV−vis absorption properties.

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INTRODUCTION Crystal engineering of three-dimensional (3D) networks assembled by coordination polymers (CPs)1 or metal−organic frameworks (MOFs)1 with different structural topologies is an important topic not only for their potential applications as functional materials but also for their intriguing variety of 3D architectures in the solid-state studies of “self-assembly” or “supramolecular chemistry”.2 Many current investigations are focused on the design of new molecule-based functional materials. A variety of supramolecular architectures with © 2014 American Chemical Society

promising properties have been obtained on the basis of crystal engineering concepts supported by a large range of bonding forces; depending on the system, the interactions can range from classic M−L covalent bonds, to strong halogen3−5 or hydrogen bonds,6 to much weaker forces such as weak Received: April 30, 2014 Revised: June 30, 2014 Published: July 10, 2014 4011

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pellet): ν = 3468 (w), 3461 (w), 3037 (w), 1698 (s), 1635 (m), 1618 (m), 1559 (vs), 1427 (m), 1236 (s), 766 (m) cm−1. Crystallographic Data Collection and Refinement of 1. Single-crystal structure analysis for compound 1 was performed out on a Siemens SMART diffractomer with a CCD detector with Mo radiation (λ = 0.71073 Å) at 100 K. A preliminary orientation matrix and unit cell parameters were determined from 3 runs of 15 frames each; each frame corresponds to a 0.3° scan in 10 s, followed by spot integration and least-squares refinement. For each structure, data were measured using ω scans of 0.3° per frame for 20 s until a complete hemisphere had been collected. Cell parameters were retrieved using SMART21 software and refined with SAINT22 on all observed reflections. Data reduction was performed with the SAINT23 software and corrected for Lorentz and polarization effects. Absorption corrections were applied with the program SADABS.23 Direct phase determination and subsequent difference Fourier map synthesis yielded the positions of all atoms, which were subjected to anisotropic refinements for non-hydrogen atoms and isotropic for hydrogen atoms. The final full-matrix, least-squares refinement on F2 was applied for all observed reflections [I > 2σ(I)]. All calculations were performed by using the SHELXTL-PC V 5.03 software package.24 Crystal data and details of the data collection and structure refinements for 1 are summarized in Table 1.

hydrogen bonds7 and π−π stacking of small aromatics.8 Entangled systems, serving as an important subject in the area of supramolecular chemistry, are common in crystal engineering of coordination polymers (CPs) as seen in interpenetration, polycatenane, interdigitation, polythreading, and other species.2,9,10 Interpenetration has been recognized as one of the major types of entangled system and comprehensively investigated by Batten and Robson.9b,c,10e Recently, a variety of appealing interpenetrated or catenated 3D supramolecular networks have been reported, in which two or more coordination polymers with different dimensionality within the crystal were found in the formation of their 3D architectures, such as 1D chain + 1D chain ⇒ 3D,11a 1D chains + 1D ladders ⇒ 3D,11b 1D double-chain + 2D net ⇒ 2D,11c 1D chains + 2D layers ⇒ 3D,12,13 1D rings + 2D layers ⇒ 3D,14,15 1D chains + 1D rings + 2D layers ⇒ 3D,16 2D layers + 2D layers ⇒ 2D,17,18 or 3D19 and 2D layers + 3D CdSO4 topology ⇒ 3D20 supramolecular networks. In this contribution, we report here on the exploration of an unprecedented {[ 2D + 2D] doublelayers + 1D chains + 1D chains} ⇒ 3D supramolecular architecture, [Co(dpe)(BTC)(H 2 O)][Co(dpe)(BTC)(H2O)3][Co(dpe)(HBTC)(H2O)][Co(dpe)(H2O)3.5(EtOH)0.5]·1.5H2O (1) (dpe = 1,2-bis(4-pyridyl)ethane, and H3BTC = benzenetricarboxylic acid), obtained by the reaction of Co(II) chloride with dpe and H3BTC ligands. This example represents a remarkable three-dimensional (3D) supramolecular architecture sustained by four crystallographically independent CPs, that is, two two-dimensional (2D) (4,4) layered CPs, and two one-dimensional (1D) linear chain-like CPs, which are entangled together via the penetration of the rectangular channels of interdigitated 2D double-layered frameworks by two independent 1D polymeric chains. Controlled heating of 1 at 160 °C leads to desolvated species 1, which shows remarkable color-changing behavior and reversibility to give a rehydrated structure of 2 with molecular formula of [Co(dpe)(BTC)(H2O)][Co(dpe)(BTC)(H2O)3][Co(dpe)(HBTC)(H2O)][Co(dpe)(H2O)4]·3H2O, when exposed to water vapor, indicating a reversible solid-state dynamic structural transformation.

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Table 1. Crystal Data and Refinement Details of 1 empirical formula cryst syst a, Å b, Å c, Å V, Å3 ρc, g cm−3 μ, mm−1 total no. of data collected R1, wR2 (I > 2σ(I))a GOFb a b

C76H80.50Co4N8O28.50 orthorhombic 20.4932(8) 13.7177(6) 27.248(1) 7660.0(5) 1.559 0.942 57516 0.0586, 0.1233 1.026

formula wt space group α (deg) β. (deg) γ (deg) Z 2θ range T, K no. of obsd data R1, wR2 (all data)a no. of variables

1797.70 Pna21 90 90 90 4 1.49−27.50 150(2) 17454 0.0863, 0.1360 1094

R1 = ∑||Fo − Fc||/∑|Fo|; wR2(F2) = [∑w|Fo2 − Fc2|2/∑w(Fo4)]1/2. GOF = {Σ[w|Fo2 − Fc2|2]/(n − p)}1/2.

CCDC-965281 for 1 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge at www. ccdc.cam.ac.uk/conts/retrieving.html [or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax (internat.) +44-1223/336-033; e-mail [email protected]. In Situ X-ray Powder Diffraction of 1. The synchrotron powder X-ray diffraction data of 1 were collected at the BL01C2 beamline at the National Synchrotron Radiation Research Center (NSRRC) in Taiwan. The wavelength of the incident X-rays is 1.03321 Å, and the diffraction patterns were recorded with a Mar345 imaging plate detector approximately 331 mm from sample positions. The onedimensional powder diffraction profile was converted with program FIT2D25 and cake-type integration, where the diffraction angles were calibrated according to Bragg positions of Ag-benhenate and Si powder (SRM640c) standards. An in situ temperature dependent experiment for 1 was performed from 28 to 300 °C with a heating rate 10 °C/min. The powder sample was packed in a glass capillary (0.3 mm diameter) and heated in a stream of hot air; each pattern was exposed for about 1.2 min. The capillary of dehydrated sample was immersed in water for 1 h and then the powder pattern was measured again. Detailed structural analysis of rehydrated sample 2 was carried out using the Rietveld method within the profile refinement program GSAS.26 The Rietveld refinement is based on the calculated diffraction patterns according to the known structure model from single-crystal diffraction measurement. The powder pattern quality of the rehydrated sample is not as good as the sample before dehydration process. The

EXPERIMENTAL SECTION

Materials and Physical Techniques: General Considerations. All chemicals were of reagent grade and were used as commercially obtained without further purification. The elemental analyses of carbon, hydrogen, and nitrogen were determined with a PerkinElmer model 2400 series II analyzer. Infrared spectra were measured on a Nicolet Fourier transform IR, MAGNA-IR 500 spectrometer in the range of 500−4000 cm−1 using the KBr disc technique. UV−vis−NIR diffusive reflectance spectra of 1 were obtained with a HITACHI U4100 spectrophotometer equipped with an integrating sphere accessory (Al2O3 was used as a reference). Thermogravimetric analysis (TGA) of 1 was performed on a computer-controlled PerkinElmer 7 Series/UNIX TGA7 analyzer. Single-phased powder sample of 1 was loaded into alumina pans and heated with a ramp rate of 5 °C/min from 30 to 600 °C under nitrogen atmosphere. Synthesis of 1. A solution of trimesic acid (H3BTC, 4.2 mg, 0.02 mmol) and 1,2-bis(4-pyridyl)ethane (dpe, 5.2 mg, 0.03 mmol) in mixed solvents of distilled water and EtOH (1:1, v/v) (18 mL) was added to a solution of CoCl2 (3.8 mg, 0.03 mmol) in mixed solvents of distilled water and EtOH (1:1, v/v) (9 mL) at room temperature to give a pale-pink solution. Pink plate-shaped crystals were obtained after several days in 48.8% yield. The resulting crystals of 1 were collected by filtration, washed several times with distilled water, and dried in air. Anal. Calcd for C76H80.50N8O28.50Co4 (Mw = 1797.70): C 50.78, N 6.23, H 4.51. Found: C 49.90, N 6.15, H 4.23. IR (KBr 4012

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Figure 1. Molecular structures of (a) CP A, [Co(dpe)(BTC)(H2O)]−; (b) CP B, [Co(dpe)(BTC)(H2O)3]−; (c) CP C, [Co(dpe)(HBTC)(H2O)]; and (d) CP D, [Co(dpe)(H2O)3.5(EtOH)0.5]2+. ORTEP drawing with 30% thermal ellipsoids. The H atoms are omitted for clarity. smaller S/N ratio is attributed to the worse long-range ordering. In the refinement, the refined parameters are included the scale factor, zero point shift, background, lattice constants, profile parameters (pseudoVoigt function), atomic positions, isotropic displacement parameters, and occupancy.

both contain a six-coordinate Co(II) ion bonded with two antidpe, one water molecule, and two BTC3− ligands in CP A and two HBTC2− ligands in CP C to form a distorted octahedral geometry. The related bond lengths around the Co(II) ions are listed in Table 2. The anti-dpe acts as the bridge ligand with bismonodentate coordination mode connecting the Co(II) ion to form a 1D linear chain. Adjacent chains are then mutually connected via the bridges between the Co(II) ions and BTC3− (CP A) or HBTC2− (CP C) ligands with chelating/ monodentate coordination mode to generate 2D layered MOFs with 44 structural topology and rectangular grid as the basic building block. The grid dimensions are 10.25 Å × 13.72 Å (Figure 2a, yellow) via the bridges of BTC3− and anti-dpe in A and 10.29 Å × 13.72 Å (Figure 2a, green) via the bridges of HBTC2− and anti-dpe in C. Interestingly, CPs A and C are then mutually interdigitated into each other along the direction of the uncoordinated carboxylate groups of the lateral BTC3− and HBTC2− ligands to fabricate a 2D double-layered supramolecular network (Figure 2b), resulting in the formation of inner rectanglular cavities with dimensions of ca. 6.86 Å × 10.27 Å. The lateral BTC3− in CP A and HBTC2− in CP C are oriented vertically up and down, respectively, into the doublelayered network as the walls of the cavities as shown in Figure 2b. The molecular structures of CPs B and D are shown in Figure 1b,d, respectively, in which the Co(II) ion in CP B has a

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RESULTS AND DISCUSSION Synthesis and Structural Description of 1. The mixing of a colorless solution of dpe and H3BTC in ethanol/water solution and a pale-pink solution of cobalt(II) chloride in ethanol/water solution with CoCl2/dpe/H3BTC molar ratios of 3:3:2 resulted in the formation of pink crystals of compound 1 formulated with [Co(dpe)(BTC)(H2O)][Co(dpe)(BTC)(H2O)3][Co(dpe)(HBTC)(H2O)][Co(dpe)(H2O)3.5(EtOH)0.5]·1.5H2O, which are suitable for X-ray diffraction analysis. Structural determination reveals that the crystal structure of 1 shows the presence of four crystallographically independent polymeric structures (Figure 1) packed together: two 2D layered CPs of anionic [Co(dpe)(BTC)(H2O)]− (A) and neutral [Co(dpe)(HBTC)(H2O)] (C) and two 1D polymeric chain-like CPs of anionic [Co(dpe)(BTC)(H2O)3]− (B) and cationic [Co(dpe)(H2O)3.5(EtOH)0.5]2+ (D). In total, 1 can be formulated as [(A)(B)(C)(D)]· 1.5H2O, and the oxidation states of all independent cobalt centers in CPs A, B, C, and D are 2+. The molecular structures of CPs A and C are similar (shown in Figure 1a,c, respectively); 4013

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Table 2. Bond Lengths (Å) around Co(II) Ions in 1a

ligands and four oxygen atoms of three waters and one disordered coordinated solvent of ethanol and water molecules with half occupancy. The related bond lengths around the Co(II) ions are listed in Table 2. CPs B and D are both 1D linear chain-like CPs via the connectivity between the Co(II) ions and anti-dpe ligands with bis-monodentate coordination mode, which are arranged in parallel in an alternate BDBD sequence extending along the c axis as shown in Figure 2b. The most remarkable and interesting structural feature of 1 is that two 1D polymeric chains of [Co(dpe)(BTC)(H2O)3]− and [Co(dpe)(H2O)3.5(EtOH)0.5]2+ (CPs B and D) and 2D double-layers of {[Co(dpe)(BTC)(H2O)][Co(dpe)(HBTC)(H2O)]−} (CPs A and C) are tightly entangled together in a unique 3D array as shown in Figure 2c. First of all, adjacent double-layered frameworks of CPs A and C are arranged parallel in an ABAB manner along the c axis (Figure 2b), generating 1D rectangular channels. These 1D channels are then fully occupied by the 1D chains of CPs B and D in an alternate BDBD sequence to complete the unique 3D array. To the best of our knowledge, this is the first example with a supramolecular entangled architecture constructed by four crystallographically independent CPs as [neutral 2D + anionic 2D] double-layers + anionic 1D chains + cationic 1D chains ⇒ 3D via both interdigitation and penetration modes. It is also noteworthy that the subtle combination of hydrogen-bonding interactions and π···π stacking interactions among the four CPs play important roles in the tight entangling in the unprecedented 3D supramolecular architecture. First, inter-

CP A Co(1)−O(1) Co(1)−O(5)i Co(1)−N(1)

2.154(3) 2.045(3) 2.188(4)

Co(2)−O(7) Co(2)−O(22) Co(2)−N(3)

2.046(3) 2.117(3) 2.153(4)

Co(3)−O(16)iv Co(3)−O(14) Co(3)−O(13)

2.058(3) 2.150(4) 2.183(3)

Co(4)−O(24) Co(4)−O(26) Co(4)−N(8)v

2.064(4) 2.102(4) 2.134(5)

Co(1)−O(2) Co(1)−O(19) Co(1)−N(2)ii

2.151(4) 2.079(4) 2.174(4)

Co(2)−O(20) Co(2)−O(21) Co(2)−N(4)iii

2.110(3) 2.130(4) 2.162(4)

Co(3)−O(23) Co(3)−N(6)ii Co(3)−N(5)

2.084(4) 2.154(4) 2.193(4)

Co(4)−O(25) Co(4)−O(27) Co(4)−N(7)

2.072(4) 2.108(4) 2.136(5)

CP B

CP C

CP D

Symmetry transformations used to generate equivalent atoms: i = x − 1/2, −y + 5/2, z; ii = x, y + 1, z; iii = −x + 1, −y + 2, z − 1/2; iv = x − 1/2, −y + 3/2, z; v = −x + 2, −y + 2, z − 1/2. a

distorted {CoN2O4} octahedral coordination environment (Figure 1b) bonded to two nitrogen atoms of two anti-dpe ligands and four oxygen atoms of one BTC3− ligand and three water molecules, while the Co(II) ion in CP D also has a distorted {CoN2O4} octahedral coordination environment (Figure 1d) bonded to two nitrogen atoms of two anti-dpe

Figure 2. (a) Two-dimensional rectangle-grid layers of CP A (yellow), with grid-dimensions of 10.25 Å × 13.72 Å, and CP C (green), with griddimensions of 10.29 Å × 13.72 Å; (b) 2D bilayer of CPs A and C viewed along the c axis (top, left) and a axis (top, right) and two 1D linear chains of CP B (purple) and CP D (red) (bottom); (c) side view showing the interpenetration of two 1D chains (CPs B and D, spacing filling mode) into the rectangular channels formed by 2D bilayered network (CPs A and C) to form 3D supramolecular assembly in 1. CP A = yellow, CP C = green, CP B = purple, and CP D = red. 4014

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Table 3. Related Parameters of O−H···O Hydrogen Bonds for 1a D−H···A

D−H (Å)

H···A (Å)

D···A (Å)

D−H···A (deg)

O(19)−H(19A)···O(9) O(19)−H(19B)···O(6)iii O(21)−H(21B)···O(11)ii O(21)−H(21C)···O(17)ii O(22)−H(22C)···O(6)iii O(23)−H(23B)···O(9)iv O(23)−H(23C)···O(15)ii O(24)−H(24A)···O(4) O(24)−H(24B)···O(11)i O(25)−H(25B)···O(3) O(25)−H(25C)···O(29)v O(26)−H(26B)···O(15) O(26)−H(26C)···O(10)i O(29)−H(29)···O(12) O(27)−H(27D)···O(13) O(28)−H(28)···O(10)vi

0.85(4) 0.82(4) 0.87(4) 0.84(4) 0.86(4) 0.83(4) 0.86(4) 0.84(4) 0.86(4) 0.84(6) 0.90(6) 0.82(4) 0.87(4) 0.94(4) 0.85(6) 1.13(5)

2.04(4) 1.99(4) 1.93(4) 1.76(4) 2.14(4) 2.15(4) 2.04(4) 1.89(4) 1.93(4) 2.00(6) 2.34(6) 2.09(4) 1.97(4) 2.11(4) 2.00(6) 2.39(5)

2.711(6) 2.641(6) 2.574(6) 2.597(6) 2.810(6) 2.738(6) 2.701(6) 2.612(6) 2.771(6) 2.612(7) 2.787(7) 2.721(6) 2.691(6) 2.713(6) 2.783(7) 2.808(7)

135(4) 137(4) 130(4) 169(4) 134(4) 128(4) 133(4) 144(4) 166(4) 129(5) 111(5) 133(4) 140(4) 121(4) 153(5) 100(5)

Symmetry code: i = 2 − x, 2 − y, −1/2 + z; ii = −1/2 + x, 3/2 − y, z; iii = −1/2 + x, 5/2 − y, z; iv = 3/2 − x, −1/2 + y, −1/2 + z; v = 3/2 − x, 1/2 + y, −1/2 + z; vi = x, −1 + y, z. a

thermogravimetric analysis (TGA) and in situ temperature dependent XRD measurements of compound 1 were performed on single-phase polycrystalline samples. During the heating process, the TG analysis (Figure 3a) revealed that 1

molecular O−H···O type hydrogen bonding interactions between the coordinated water molecules and uncoordinated oxygen atoms of BTC3− or HBTC2− ligands among the four CPs with O···O distances in the range of 2.574−2.810 Å provide the extra energy for the stabilization of the entangled architecture. The common hydrogen-bonding distances and angles existing in 1 are listed in Table 3. The second key factor in the stabilization of the 3D entangled assembly of 1 is four sets of π−π stacking interactions existing between the pyridine rings of anti-dpe ligands and the benzene rings of the HBTC2− or BTC3− ligands with the ring centroid distances in the range of 3.525−4.025 Å. The related interplanar parameters are listed in Table 4. Furthermore, the 3D supramolecular architecture is Table 4. π−π Interactions (Face-to-Face) in 1a

ring(i) → ring(j)

slip angleb (i,j), deg

interplanar (i,j) distance,c Å

horizontal shift between the (i,j) ring centroids,d Å

distance between the (i,j) ring centroids, Å

→ → → →

20.8(4) 23.4(4) 35.1(4) 19.0(4)

3.529(7) 3.306(7) 3.292(7) 3.332(7)

1.340(7) 1.430(7) 1.970(7) 1.150(7)

3.775(7) 3.602(7) 4.025(7) 3.525(7)

R(1) R(2) R(2) R(5)

R(4) R(3)i R(6) R(6)

a Symmetry code: i = 3/2 + x, 1/2 − y, z − 1; R(1) = N(1)−C(1)− C(2)−C(3)−C(4)−C(5); R(2) = C(14)−C(15)−C(16)−C(18)− C(19)−C(21); R(3) = N(4)−C(29)−C(30)−C(31)−C(32)−C(33); R(4) = C(35)−C(36)−C(37)−C(39)−C(40)−C(42); R(5) = C(56)−C(57)−C(58)−C(60)−C(61)−C(63); R(6) = N(7)− C(64)−C(65)−C(66)−C(67)−C(68). bSlip angle: the angle formed between the ring-centroid vector (CC) and the ring normal to one of the croconate planes. cInterplanar distance: the perpendicular distance between two parallel rings. dHorizontal shift between the ring centroids: a shift from the face-to-face alignment.

reinforced by the O−H···O type hydrogen bonding interactions among the solvated, coordinated water molecules and BTC3− ligands with O···O distances in the range of 2.713−2.808 Å. The common hydrogen-bonding distances and angles existing in 1 are listed in Table 3. Thermogravimetric and in Situ Powder X-ray Diffraction Analyses of 1. To assess the thermal stability and structural variation as a function of the temperature,

Figure 3. (a) Thermogravimetric (TG) analysis of 1 and (b) TG measurements of cyclic dehydration/rehydration processes repeated 4 times. Red line, the variation of weight loss with time; black line, the variation of temperature with time. 4015

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was thermally stable up to 70 °C and underwent a one-step weight loss of 12.0%, which corresponded to the loss of 1.5 solvated and 8.5 coordinated water molecules and 0.5 ethanol molecules (calcd 11.3%), in the range of approximate 70−140 °C. In the temperature range of approximate 140−250 °C, the desolvated 1 was stable without any weight loss. On further heating, these samples decomposed at approximate 250 °C. In order to verify the reversibility of rehydration/dehydration property of desolvated 1, cyclic TG measurements have been performed under water vapor by thermal treatment shown in Figure 3b. Desolvated 1 after the desolvation process by the thermal treatment (up to 150 °C) shows a reversible reydration/dehydration behavior. The water molecules can be readsorbed by exposing the desolvated 1 to water vapor forming a rehydrated crystal 2 with weight increase of 11.9% corresponding to 12 water molecules (calcd 11.9% including 9 coordinated and 3 solvated water molecules) when the sample was cooled to room temperature. Such heating (up to 150 °C) and cooling (down to RT) procedures have been repeated for four cycles (Figure 3b) with almost the same weight-increase and weight-lose percentages (11.9−11.4%) to demonstrate the stable reversibility of the thermal rehydration/dehydration processes. This result reveals that compound 1 undergoes a reversible solid-state structural transformation between desolvated 1 and rehydrated 2 driven by thermal water absorption/ desorption processes. The structural determination of rehydrated 2 by single-crystal X-ray diffraction method reveals that the cell parameters (see Table S1 in the Supporting Information) are almost recovered, and the preliminary structural determination reveals that the molecular structure of rehydrated 2 can be formulated as [Co(dpe)(BTC)(H2O)][Co(dpe)(BTC)(H2O)3][Co(dpe)(HBTC)(H2O)][Co(dpe)(H2O)4]·3H2O, which is in consistent with the result with the absorption of 12 water molecules from cyclic rehydration/ dehydration TG measurements. It is unfortunate that the crystal quality of rehydrated 2 is not sufficient for further anisotropic refinements of the heavy atoms and determination of the hydrogen atoms. Further structural characterization of rehydrated 2 will be studied by PXRD measurement described in the next section. To determine the structural changes in depth upon desorption of water and ethanol molecules, in situ synchrotron X-ray powder diffraction patterns of 1 were collected continuously from 25 to 300 °C, and the results at some specific temperatures are shown in Figure 4. Comparisons of crystal data between the single-crystal and powder diffraction measurement are listed in Table S1, Supporting Information. The powder pattern of the fresh sample at room temperature matches well with the simulated pattern based on the single crystal structure. Comparison of the patterns as the temperature increases (Figure 4), the peak width becomes broader, and the intensity becomes much weaker, which indicates that compound 1 starts to lose its crystallinity and only can be retained up to 270 °C. As the temperature increased, a phase transition occurred from 140 °C, and a metastable phase arose at 160 °C, which is in consistent with the TG result. It is also unfortunate that the unit cell of desolvated 1 could not be determined due to the poor long-range ordering of PXRD data. Solid-State Structural Transformation by Thermal Dehydration/Rehydration Processes. The most promising feature of crystals of 1 is that they undergo a reversible solidstate structural transformation driven by thermal desolvation and rehydration/dehydration processes, which has been

Figure 4. In situ PXRD patterns of 1 at different temperatures and simulated PXRD pattern of 1 from single-crystal X-ray diffraction data.

identified by cyclic TG measurements. Such a property can also be demonstrated by PXRD analysis shown in Figure 5.

Figure 5. Simulated PXRD patterns of (a) 1 from single-crystal X-ray diffraction data, (b) fresh samples of 1 at RT, (c) desolvated samples of 1 at 160 °C, and (d) rehydrated samples of 2 obtained by exposure of the desolvate sample of 1 to water vapor.

When a desolvated sample of 1 at 160 °C (Figure 5c) is cooled to RT and exposed to water (i.e., the desolvated sample is placed in a glass capillary, beside a beaker filled with water), it reabsorbs the lost coordinated and solvated water molecules. The PXRD pattern of the rehydrated species 2 (Figure 5d) shows that the sample regains the original structure of 1 and is re-established as the rehydrated species 2 showing almost the same PXRD diffractogram as that of the freshly synthesized material (Figure 5a and Table S1, Supporting Information). The structure of 2 was obtained by Rietveld refinement (see Figure S1 in the Supporting Information). Immersion of dehydrated powder 1 into water revealed that the cell parameters and structure are almost recovered, but there is a slight difference between 1 and 2. The coordination environment of the Co(4) atom in 2 is coordinated by two anti-dpe linkers and four H2O molecules. The ethanol molecule coordinating to Co(4) in 1 is completely replaced by the water molecule. Moreover, not only the main structure but also the packing water molecules are recovered. More packing H2O 4016

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desolvated crystal was either immersed in water immediately (Figure 6h) or exposed to the air at RT standing for 1 day (Figure S2 in the Supporting Information). Furthermore, during the heating process, the color change of 1 was further characterized with UV−vis−NIR diffuse reflectance spectroscopy (see Figure 7). The UV−vis−NIR

molecules are reabsorbed into the crystal lattice leading to a slight cell expansion. Because the structure of high temperature phase cannot be characterized directly by PXRD data, the solidstate structural transformation mechanism of desolvated 1 for the thermal desolvation processes is not clear. However, based on our previous study on the identification of the reversible solid-state structural transformation mechanism between the 1D ladder-like chains of [Zn(dpe)(HBTC)(H2O)]·(H2O) and 2D dehydrated framework of [Zn(dpe)(HBTC)],27 the possible and sensible route for the reversible solid-state structural transformation between desolvated 1 and rehydrated 2 could be deduced: the removal of the coordinated water molecules creates open sites of Co(II) centers for the approach of neighboring uncoordinated oxygen atoms of the BTC3− and HBTC 2− ligands, which are also oriented toward the coordination water molecules of neighboring units via O− H···O hydrogen bonds. These contacts shorten in the course of the solid-state reaction and turn into bonds with Co(II) atom by removal of the water molecules to create the desolvated 1 and then water molecules are reabsorbed to form rehydrated 2 after the dehydrated sample is exposed to water vapor. Color-Changing Behavior and UV−Vis Absorption Properties of Desolvation/Rehydration for Crystal 1. It is important to note that the desolvation and rehydration processes of crystal 1 were accompanied by morphological changes, which can be monitored visually with photographs and by UV−vis absorption measurements of the crystal during the in situ heating/cooling process. A well-formed pink crystal of 1 was obtained as a plate-shaped sheet as shown in Figure 6a at

Figure 7. UV−vis−NIR diffuse reflectance spectra of 1 upon heating from 20 to 300 °C and spectrum of rehydration from 150 °C cooling back to 20 °C (dashed line).

spectrum of 1 exhibits two lower-lying absorption bands with 400 < λ < 700 nm and 800 < λ < 1700 nm, the spectral features of which are both assigned to d−d transitions. These transitions are consistent with the characteristic octahedral geometry of 1 (vide supra).28 Upon heating from 20 to 300 °C, the absorbance at 514 and 1200 nm tended to increase, accompanied by the gradual red-shift of the band at 514 nm. Upon decreasing temperature from 150 to 20 °C, the absorbance spectra shifted back to the shorter wavelength (see dashed line). Note that the absorption spectrum of rehydration is similar to the absorption spectrum at 20 °C. This reversible absorption change behavior, that is, color-changing behavior, is attributed to the change of coordination environments around Co(II) centers and hence change in the d-orbital energy level during the desolvation and rehydration processes.29 In addition, we also performed a control experiment to acquire the spectra of 1 under irradiation at 325 nm for 10 min, 30 min, and 6 h (data are not shown here). There was no significant change in absorption spectra of 1 under irradiation at 325 nm. Accordingly, this chromatic behavior is a thermocontrolled process but not a photocontrolled process.

Figure 6. Photographs of plate-shaped crystal of 1 at different temperatures: (a) fresh pink crystal at RT; (b) air-dried at 70 °C; (c) air-dried at 85 °C; (d) air-dried at 100 °C; (e) air-dried at 140 °C; (f) air-dried at 150 °C; (g) after maintaining at 150 °C for 5 min, cooled back to RT; (h) after immersion of the dehydrated crystal in water at RT.

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CONCLUSIONS In conclusion, the 3D penetrated structure of compound 1 can be considered as a prototype of a new nonporous supramolecular architecture, which is constructed via the interdigitation and penetration of four crystallographically independent CPs with porous [2D + 2D] double-layers penetrated by two 1D independent chains to generate its 3D supramolecular entangled architecture. The subtle combination of intermolecular hydrogen bonding and π−π interactions provide extra energy for the stabilization of the 3D entangled architecture showing high thermal stability. Notably, this compound shows an interesting color-changing behavior associated with solid-state structural transformation between the desolvated 1 and rehydrated 2 during the reversible thermal

RT. Compound 1 shows a reversible color-changing behavior when the temperature rises with the color of 1 gradually shifting from pink to deep blue (Figure 6). As the temperature was raised from RT to 70, 85, and 100 °C, the color of dried crystal became deeper than that at RT and many chaps with random cracks on the crystal surface are observed (Figure 6b,c,d). The color-changing behavior of 1 from pink to dark blue took place at 140 and 150 °C (Figure 6e,f) upon the loss of coordinated or solvated water and ethanol molecules. It is worth noting that the dark-blue desolvated crystal cannot turn back to pink (Figure 6g) as the temperature decreased to RT immediately. However, the color of the desolvated sample was changed from dark blue to its original pink (Figure 6a) after the 4017

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rehydration/dehydration processes, which is demonstrated by cyclic TG, PXRD, and UV−vis absorption measurements.

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ASSOCIATED CONTENT

* Supporting Information S

X-ray crystallographic file for compounds 1 in CIF format and figures showing powder X-ray diffraction patterns. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Ministry of Science and Technology, Taiwan, for financial support.

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