Controlled Reversible Crystal Transformation of Cu(I

Reversible crystal-to-crystal transformations can be observed under .... (d)], 2 [(b), (e)], and 3 [(c), (f)] showing intermolecular interactions (sky...
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Controlled Reversible Crystal Transformation of Cu(I) Supramolecular Isomers Youngeun Jeon, Sanghun Cheon, Seonghwa Cho, Kang Yeol Lee, Tae Ho Kim,* and Jineun Kim* Department of Chemistry and Research Institute of Natural Science, Gyeongsang National University, 501 Jinju Daero, Jinju 660-701, S. Korea S Supporting Information *

ABSTRACT: Four copper(I) coordination polymers (CPs), {[CuIL]·CH3CN]}n (1), {[CuIL]·CHCl3}n (2), {[CuIL]· CH2Cl2}n (3), and [CuIL]n (4), were prepared by self-assembly reactions between CuI and (2-pyrazinylcarbonyl)thiomorpholine (L). CPs 1−4 are interconnected by rhomboid Cu−I2−Cu units. CPs 1 and 4 have one-dimensional loop-chain structures, and 2 and 3 adopt two-dimensional network structures. CPs 1−4 are pseudopolymorphic supramolecular isomers. CPs 2′ and 3′ are prepared by removal of solvate molecules from CPs 2 and 3, which are polymorphic supramolecular isomers with CP 4. Reversible crystal-to-crystal transformations were observed under appropriate conditions such as solvent or heat.

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example in which supramolecular isomerization has been achieved by the removal of solvate molecules at room temperature. Our results indicate that solvent exchange occurs even in nonporous CPs when interactions between solvent molecules and the CP frameworks are present. Such intermolecular interactions also affect the photoluminescence spectra of the CPs. An N/S donor ligand, 2-pyrazinyl-4-thiomorpholinylmethanone (L), was prepared from 2-pyrazinecarboxylic acid and thiomorpholine, according to the literature method (Scheme S1 of the Supporting Information).14 Four photoluminescent Cu(I) CPs {[Cu2I2L2]·MeCN}n (1), {[Cu2I2L2]·CHCl3}n (2), {[Cu2I2L2]·CH2Cl2}n (3), and [CuIL]n (4) were prepared by the reaction of CuI and L in a 1:1 molar ratio under controlled solvent conditions. Green luminescent crystalline 1 was obtained by slow evaporation from an acetonitrile solution. Orange luminescent crystals of 2 were formed by slow evaporation from an acetonitrile/chloroform (1:5) solution, while red-orange luminescent crystalline CP 3 was gained by slow evaporation from an acetonitrile/dichloromethane (1:3) solution. Yellow-green luminescent CP 4, as a quickly precipitated powder, was obtained by addition of diethyl ether or by ultrasonication (Scheme 1). Remarkably, single crystals of CP 4 were obtained by crystal transformation from single crystals of 3 after heating at 120 °C for 12 h; however, single crystals of 4 could not be grown from solutions of L and CuI. CPs 1−4 were characterized by elemental analysis (EA), thermogravimetric (TGA) and differential thermal (DTA) analyses, photoluminescence (PL) spectroscopy, single crystal (SCXRD) and powder X-ray diffraction (PXRD) analyses,

ver the past two decades, a number of diverse coordination polymers (CPs) or metal−organic frameworks (MOFs) have received considerable attention not only because of their structural properties but also because of their numerous potential applications in gas/solvent storage and separation, organic light-emitting diodes, catalysis, molecular switches, and sensors.1−10 Among CP research groups, supramolecular isomerism has been of particular interest, as the isomers show differential responses to physical stimuli owing to their crystal packing.11 In general, the structures of such compounds are affected by many factors such as the coordination environment of the metal ion, ligand structure, solvent, temperature, and anion identity. The prediction and control over a particular structure becomes more difficult since the formation energy differences between the supramolecular isomers are very small. In comparison with investigations of supramolecular isomerism in organic fields, significantly less research has been devoted to the study of the phenomenon in CPs and MOFs or the resulting changes in properties. In our group, we have been interested in the development of photoluminescent copper(I) CPs based on CuI and dithioether ligands.12−14 Our studies have focused on reversible/irreversible structural transformations and luminescence changes in these compounds as functions of temperature and solvent. Some of the structural transformations are related to conformational changes of the sulfur-containing organic ligands and the lability of the Cu−S bond. However, studies of reversible crystal transformations and solvent exchange processes for copper(I) CPs are limited. Therefore, we were motivated to study the reversible structural transformations between supramolecular isomers and solvent exchange in CPs based on Cu(I) and N/Sdonor ligands. Herein, we report the structures and photoluminescence and thermal properties of four Cu(I) CPs, and their reversible crystal transformations between supramolecular isomers by the control of conditions such as solvent, vacuum, and temperature. To the best of our knowledge, this is the first © 2014 American Chemical Society

Received: January 24, 2014 Revised: March 3, 2014 Published: March 11, 2014 2105

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Scheme 1. Controlled Reversible Crystal Transformation between Cu(I) Coordination Polymers

scanning electron microscopy (SEM), and energy dispersive Xray spectroscopy (EDS) (in the Supporting Information). CPs 1−4 show both reversible supramolecular isomerism by solvation.11 Pseudopolymorphic CPs 1 and 4 adopt onedimensional (1D) loop chain structures, and pseudopolymorphic CPs 2 and 3 have two-dimensional (2D) network structures. Different conformations of L engender supramolecular isomers. CPs 1 and 4 are nonporous; although it seems that there are empty spaces in the CPs in Scheme 1, the spaces in the loops are filled with atoms. In contrast to 1 and 4, dichloromethane and chloroform molecules occupy the empty spaces in the 2D networks of CPs 2 and 3. Solvate molecules serve as templates in the formation of the 2D networks. SCXRD and PXRD data for L and CPs 1−4 are listed in Figures S1−S11 and Tables S1−S3 of the Supporting Information). The copper ions in 1−4 exist in distorted tetrahedral environments with two iodide ions, one sulfur atom, and one nitrogen atom in the coordination shell. The Cu···Cu distances (2.6070−2.6571 Å) are shorter than the sum of the van der Waals radii (2.80 Å).15 The Cu−S (2.2862−2.3163 Å) and Cu−N bond distances (2.055−2.087 Å) are within the range of known values.16 It is worth noting that there are relatively weak C−H···X (X = Cl and I), C−H···N, and C−H··· O hydrogen bonds and C−Cl···π interactions in crystals 1−3 (Figure 1), since the interactions cause shifts in the luminescence spectra. For 1, intermolecular C1−H1a···N4 [3.431(5) Å] and C11−H11c···O1 [3.087(7) Å] hydrogen bonds with an acetonitrile molecule are present. The crystal structure of 2 is stabilized by chloroform molecules trapped by a C10−H10···O1 [3.132(8) Å] hydrogen bond and intermolecular C10−Cl2···Cg1 (C6, C7, N1, C8, C9, N2, and Cl2··· ring centroid = 3.523 Å) and C1−H1a···Cl1 [3.734(5) Å] interactions. In the crystal structure of 3, dichloromethane molecules are trapped in peanut-shaped cavities by the C10− H10b···O1 [3.240(9) Å] hydrogen bond and intermolecular C10−H10a···I1 [4.09(1) Å] and C2−H2a···Cl1 [3.824(6) Å] interactions. The main difference in the intermolecular interactions between 2 and 3 is that chloroform molecules

Figure 1. Crystal and packing structures of 1 [(a), (d)], 2 [(b), (e)], and 3 [(c), (f)] showing intermolecular interactions (sky blue: Cu; pink: I; orange: S; red: O; blue: N; gray: C; light gray: H; and green: Cl). Symmetry codes: (i) 1 − x, −y, 1 − z; (ii) 2 − x, 1 − y, 2 − z; (iii) 1.5 − x, 1.5 − y, 2.5 − z; (iv) 0.5 + x, 1.5 − y, 0.5 + z; (v) 0.5 + x, y, 0.5 − z.

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Figure 2. Photographs of CPs 1−4 before heating (a) without and (b) with UV irradiation and (c) after heating with UV irradiation. PL spectra of polycrystalline CPs 1−4 (d). (e) PL spectra of polycrystalline CPs 1−4 after heating (λex = 350 nm).

excited states with some mixing of the halide-to-ligand chargetransfer (XLCT) character.17 The emission maximum of 1 was blue-shifted in comparison with 4, probably due to electron donation to ligand molecules in the CP framework by acetonitrile. The emission maximum of 3 was red-shifted in comparison to that of 2 and likely resulted from electron withdrawal from the ligand molecules in the CP frameworks by dichloromethane. After removal of the solvates from 2 and 3, the luminescence spectra of 2′ and 3′ closely resembled that of 2. In 2, chloroform molecules withdraw and donate electrons by C−H···O hydrogen bonds and by C−Cl···π and C−H···Cl interactions, respectively. The electron density in the CP frameworks of 2 does not change before or after the removal of chloroform. However, dichloromethane withdraws more electrons by C−H···I and C−H···O intermolecular interactions than it donates electrons to the CP frameworks by C−H···Cl hydrogen bonds in 3. Thus, the emission maximum of CP 3 was red-shifted more than that of CP 2. In addition, the luminescence spectrum of 3 is shifted to that of 2′ upon removal of dichloromethane (Figure S13 of the Supporting Information). Although the coordination environments for 2′ (3′) and 4 are very similar, 2′ and 3′ emit longer wavelengths of light than 4 does. It might be suggested that the difference in the luminescence wavelengths of 2′ (3′) and 4 comes from the difference in the dimensionality of 2′ (3′) and 4; electrons are delocalized in 2D CP frameworks for 2′ and 3′ and in a 1D CP framework for 4. Figure 3 shows photographs of the samples under UV irradiation as well as their PL spectra, demonstrating the

donate electrons to aromatic rings via the C−Cl···π interactions in 2, whereas dichloromethane molecules withdraw electrons from I− ions via the C−H···I interactions in 3. Figure 1a shows the packing structure of 1, in which the 1D loop chain CPs are arranged along the [101] direction, with acetonitrile molecules trapped in-between. Figure 1e shows the 2D network structure of 2 parallel to the (−101) plane and channels parallel to the [−101] direction, which are occupied by chloroform molecules. Figure 1f shows the 2D network structure of 3 parallel to the ac plane and channels parallel to the b axis, which are occupied by dichloromethane molecules. Figure S5 (in the Supporting Information) shows the packing structure of 4, in which 1D loop chain CPs are packed in the (022) plane. The 1D loop chain structure of 4 parallel to the a axis is similar to that of 1. The donor N and S atoms of the Ls in 1 and 4 are on the opposite side (anti conformation) with respect to the amide bond plane, while those of 2 and 3 are on the same side (syn conformation) (Figure S6 of the Supporting Information). This implies that the syn conformation can induce the 2D peanut-shaped network structure, different from the 1D loop chain CPs 1 and 4. Photographs and PL spectra for 1−4 before and after heating at about 120 °C in air are shown in Figure 2. Figure 2b shows the different luminescent colors for 1−4 before heating. After thermal treatment, samples 1−3 showed the same luminescent color as 4 (Figure 2c). PL spectra before and after heating are shown in Figure 2 (panels d and e). Although the PL spectra for 1−4 were all different before heating, the PL spectra for 1− 3 were converted into that of 4 after thermal treatment, and 4 remained unchanged. The PXRD pattern changes for 1−3 agree with the changes observed in the PL spectra (Figure S7 of the Supporting Information). Therefore, it is concluded that 4 has the highest thermal stability. The PL spectra of CPs 1−4 in the solid state at room temperature were investigated. The emission spectra are shown in Figure 2d. CPs 1−4 exhibited strong photoluminescence with emission maxima at ca. 530, 558, 575, and 542 nm upon excitation at 350 nm, respectively. The emissions from 1−4 could be assigned to metal-to-ligand charge-transfer (MLCT)

Figure 3. Photographs of CPs 1−4 under UV irradiation showing reversible crystal transformations under appropriate solvent conditions (λex = 350 nm). 2107

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semiempirical absorption correction were carried out using APEX2.18 All of the calculations for the structure determination were carried out using SHELXTL.19 In all cases, all nonhydrogen atoms were refined anisotropically and all hydrogen atoms were placed in calculated positions and refined isotropically in a riding manner along with their respective parent atoms. In summary, we synthesized four supramolecular isomeric Cu(I) CPs which showed reversible crystal transformations upon the exchange of solvent molecules or application of heat. Supramolecular isomerization was shown upon the removal of solvate molecules at room temperature. In addition, we demonstrated that luminescence colors were controlled by intermolecular interactions between the CP frameworks and solvate molecules. Differences in the luminescence wavelengths for 2′ (3′) and 4 were suggested by the differences in charge distributions in the 2D and 1D CP frameworks, respectively. Further study of the dynamic natures of CPs 2 and 3 is under way.

solvent exchange and crystal transformation from 1 to 4 and, conversely, from 4 to 1 via 2 and 3. As can be seen, the crystalto-crystal transformations among CPs 1−4 are totally reversible. Polycrystalline samples of 1 and 3 transform into 2 in the MeCN/CHCl3 solvent system, since CP 2 contains chloroform solvates as discussed in the previous paragraph. Similarly, 2 and 4 were converted into 3 in a MeCN/CH2Cl2 solvent system. These solvent exchanges and crystal transformations did not occur unless acetonitrile was added in the CHCl3 or CH2Cl2. This implies that acetonitrile plays an important role in these processes. Presumably, acetonitrile disrupts the crystal structure, opening spaces within the CP framework that allow facile solvent exchange. Single crystals of 4 suitable for SCXRD were obtained by crystal-to-crystal transformation from CP 3 at 120 °C for 12 h. In order to test guest exchange in other solvents, CP 4 was soaked for 24 h in various acetonitrile/guest solvent combinations (1:5) such as diethyl ether, n-hexane, EtOH, MeOH, and acetone. However, these molecules were not incorporated into CP 4. These experiments were monitored by PXRD (Figure S12 of the Supporting Information). CP 4 was obtained from CP 1 under rough vacuum at room temperature or on heating at 120 °C. As shown in Scheme 1 and Figure 3, any CP can be transformed into any other CP under specific conditions. CP 1 changes into CP 4 when the solvent molecules are removed by vacuum or heat, as CP 1 is composed of 1D loop chains that contain solvate guest molecules in contrast to 4. Fast solvate removal from 2 and 3 under vacuum at room temperature yields 2′ and 3′, which are metastable (Figures S13 and S14 of the Supporting Information). CPs 4, 2′, and 3′ are polymorphic supramolecular isomers. Unfortunately, 2′ and 3′ could not be obtained from solutions of L and CuI. Amazingly, polycrystalline CPs 2 and 3 were slowly and partially converted to 4 via 2′ and 3′ after standing at room temperature for about three weeks, accompanied by the loss of solvate molecules (Figure S15 of the Supporting Information). This requires S−Cu bond breaking, conformational change, and new S−Cu bond formation. To investigate the thermal stabilities of 1−4, TGA and DTA were carried out at a rate of 10 °C/min in a N2 atmosphere (Figure S16 of the Supporting Information). All the TGA curves exhibited very similar weight loss patterns: solvate molecules were removed in the range of 80−120 °C, and loss of L by decomposition occurred at temperatures ranging from 200 to 350 °C. DTA curves of 1−3 show two peaks at 100 and 210 °C. The peaks at 100 °C are important indications for the crystal transformations of 1, 2, and 3 to 4, which are accompanied by significant weight losses from the solvent molecules in the TGA curves. The endothermic DTA signals at 200−210 °C correspond to the energy required for the decomposition and removal of L. Crystallographic details: The powder X-ray diffraction experiment was performed with a Bruker SMART APEX II ULTRA diffractometer equipped with graphite monochromated Mo Kα radiation (λ = 0.71073 Å) radiation generated by a rotating anode and a CCD detector. The powder diffraction frame was collected at a 25° interval in the 2θ range of 0−30° for 30 s at the detector distance of 5 cm. The one frame was integrated from 3° to 30°. Single crystal diffraction data for L and 1−4 were also collected with a Bruker SMART APEX II ULTRA diffractometer. The cell parameters for the compounds were obtained from a least-squares refinement of the spots (from 36 collected frames). Data collection, data reduction, and

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details for the preparation of ligand L and compounds 1−4. Relevant crystal data collection, refinement data for the crystal structures, and the selected bond lengths and angles of L and 1−4 are summarized in Table S1−S3. This material is available free of charge via the Internet at http:// pubs.acs.org. Copies of the data CCDC 660759−660768 can be obtained free of charge from the Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif.

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AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (Grants 2012R1A1B3003337 and 2012M2B2A4029305).

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