Guest-Induced Helical Metallacyclic Chains in a Porous Coordination Solid Constructed from a Flexible Ester-Containing Ligand Kexuan Huang,† Zhengjiang Xu,† Yizhi Li,† and Hegen Zheng*,†,‡ State Key Laboratory of Coordination Chemistry, Coordination Chemistry Institute, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, P. R. China, and State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, P. R. China
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 2 202-204
ReceiVed October 2, 2006; ReVised Manuscript ReceiVed December 3, 2006
ABSTRACT: A porous metal-organic framework [Co(SCN)2(EDPC)2(H2O)3]n (1; EDPC ) 1,2-ethanediyl di-3-pyridinecarboxylate) has been synthesized and structurally characterized. Immersing the crystals of 1 in THF solution led to the dynamic framework of 1 undergoing a crystal-to-crystal guest-exchange process to afford the helical metallacyclic polymer [Co(SCN)2(EDPC)2(THF)]n [1⊃THF]. The guest THF molecules were accommodated in the cavities of the metallacyclic chains and showed the handness of the helical chains by their oxygen atoms as probes. In the past decade, crystalline nanoporous coordination polymers have been extensively studied for their potential applications in magnetism,1 catalysis,2 and gas adsorption or separation.3 It was believed that robustness and stability of the porous frameworks were essential for their large pores and high surface area, and therefore a combination of rigid organic ligands and metal cluster have been widely used in self-assembly with edge-directed or face-directed strategies.4 On the other hand, the use of flexible organic linkers or metal ions with adaptive coordination geometries is less common, because flexible organic components are generally less predictable, or could even lead to a collapse of the framework.5 However, recently, inspired by the molecular recognition capability of metalloproteins,6 porous coordination polymers with dynamic frameworks, or so-called “third generation compounds”,7 have attracted the attention of chemists. Their dynamic frameworks exhibit crystal-to-crystal or amorphous-to-crystal transformation accompanying the adsorption and desorption of guest molecules, with concomitant pore shrinkage or expansion.8 Such “shaperesponsive fitting” porous solids, in certain cases, are beyond the scope of zeolites, for their potential applications in specific sensing and gas separation.9 Ethanediyl pyridinecarboxylate ligands have been used as flexible linkers to generate metallacyclic ensembles, which showed hysteretic adsorption properties.10 The ester function of the ligand, especially the carbonyl group, was chosen because of its role as a good hydrogen-bond acceptor.11 In addition, several two-dimensional porous frameworks based on the self-assembly of flexible bidentate ligands with cobalt(II) thiocyanate have recently been reported to show selective guest adsorption.12 Herein, we report a rare example of a dynamic porous coordination polymer [Co(SCN)2(EDPC)2(H2O)3]n (1; EDPC ) 1,2ethanediyl di-3-pyridinecarboxylate), which forms double-stranded helical metallacyclic chains induced by the guest THF molecules (THF ) tetrahydrofuran). Compound 1 was synthesized by the reaction of 2 equiv of EDPC with 1 equiv of Co(SCN)2 in a H2O/CH3OH (v/v 3:1) solution at ambient temperature (Scheme 1). Red well-shaped crystals, suitable for single-crystal X-ray structure analysis, were obtained after several days.13 X-ray analysis revealed that14 compound 1 crystallizes in the monoclinic C2/m space group and exhibits a 1D metallacyclic chain architecture composed of cobalt(II) centers linked via EDPC ligands. (Figure 1a) The cobalt(II) center is octahedrally coordinated to four equatorial pyridyl nitrogen atoms from the EDPC ligands; the Co-N distance * To whom correspondence should be addressed. E-mail: zhenghg@ nju.edu.cn. Fax: 86-25-83314502. † Nanjing University. ‡ Chinese Academy of Sciences.
Figure 1. (a) Comparison of metallacyclic chains between compound 1 (left) and 1⊃THF (right) along the c axis. (H atoms are omitted for clarity; Co, turquoise; N, blue; C, gray; O, red; S, yellow) (b) Representation of the pore structures of 1 (left) and 1⊃THF (right) displayed by stick and van der Waals surface models.
Scheme 1. One-Dimensional Metallacyclic Chain and Infinite Channels of 1 (H atoms are omitted for clarity; Co, turquoise; N, blue; C, gray; O, red; S, yellow)
is 2.207(2) Å. The NCS groups are coordinated axially in a slightly bent mode with angles of Co-N(3)-C(8) ) 149.0(3)° and N-C-S ) 178.6(3)°. The cobalt ions are linked by EDPC ligands to form a 1D metallacyclic chain architecture. The closest intrachain Co‚‚‚Co distance is 14.01 Å. The individual links in the chains consist of macrocyclic Co2(EDPC)2 units, which can be viewed as being 22-membered macrocycles with dimensions of 7.8 × 7.8 Å2. Metallacyclic chains in 1 stack together in an ABAB fashion to generate infinite channels along the c axis, with a cavity volume of 429.2 Å,3 which is ca. 22.8% of the total crystal volume (Platon15; see the Supporting Information).
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Figure 2. TGA data for 1 and 1⊃THF.
Figure 4. Guest-induced helical metallocyclic chains with different handedness. The guest THF molecules are accommodated in the cavities of the metallocyclic chains serving as probes.
Figure 3. Nitrogen sorption isotherm (77 K) for compound 1.
Figure 5. Four kinds of THF molecular columns in 1⊃THF with THF oxygen atoms pointing at different directions in the chiral channels along the c axis. (H atoms are omitted for clarity; Co, turquoise; N, blue; C, gray; O, red; S, yellow).
Thermogravimetric analysis (TGA) of compound 1 (Figure 2) revealed a weight loss of 7.25% in the region of 105-150 °C, consistent with the loss of three lattice water molecules from the channels of compound 1 (calcd 6.98%). A plateau region in the temperature range of 150-240 °C indicates that the molecular architecture is stable up to 240 °C in the absence of guests. To evaluate the porosity of the framework of 1, we performed nitrogen gas sorption isotherm measurements (Figure 3) on the desolvated crystalline sample at 77 K. It showed a typical Type I gas sorption behavior and an N2 uptake of approximately 21.53 cm3 (STP)/g, with a Langmuir surface area of 71.45 m2/g. To study the guest-exchange property of compound 1, we immersed crystals of 1 in a THF solution at room temperature for several days to generate dark red crystals of [Co(SCN)2(EDPC)2(THF)]n [1⊃THF].16 The X-ray analysis indicated that 1⊃THF also exhibits a 1D metallacyclic chain architecture with a coordination environment around the cobalt ion similar to that of 1 (Figure 1a).17 The guest THF molecules are located in the cavity of the metallamacrocycles, stacked parallel to one of the pyridine groups (at a distance of ca. 3.8 Å). The weak interactions between the THF and pore-wall molecules give rise to the deformation of the metallamacrocycle framework with the rotation glycol part of the ligand. The O-C-C-O dihedral angle changed from 0.0(4) to 65.5(7)° (Figure 1b). The distortion of the ligands ultimately transforms the metallacyclic chains into a helix and, as a result, 1⊃THF crystallizes in the P21/c space group. The most fascinating feature of 1⊃THF is that the oxygen atom of each guest THF molecule serves as a “compass” or a “probe” to show the different handedness of the helical metallacyclic chains (Figure 4). In a single helical chain, the oxygen atoms of the THF molecules are inclined in the same direction. We propose that when the first guest THF molecule is introduced into the cavity, it causes a distortion of a single metallamacrocycle unit; a cooperative effect may exist, and the distortion may spread over the metallacyclic chain to afford metallamacrocycles with the same shape, which results in the arrangement of the THF molecules being unidirec-
tioned. Moreover, helical metallacyclic chains stack along the c axis to form four kinds of chiral channels with different geometries that accommodate the guest THF columns as detectors pointing in four different directions (Figure 5). The TGA data for 1⊃THF indicate guest weight loss over the temperature range 120-160 °C, immediately followed by decomposition of the framework. Thus, the guest THF molecules play an important role in the stabilization of the compound. This may explain why the whole transformation process seems to be irreversible. In conclusion, we have shown that a porous coordination polymer has been synthesized using a flexible ester-containing ligand. The dynamic property of the pores has been confirmed by a crystalto-crystal guest-exchange process. The guest THF molecules are located in the cavities of the metallamacrocycles to change the dynamic framework into helical chains. Therefore, even a weak interaction between the guest and the pore-wall molecules could cause catastrophic change throughout the crystal, which results in the molecular recognition property of the dynamic frameworks. Acknowledgment. This work was supported by the National Natural Science Foundation (20571039, 20171020), the Ministry of Education of China (20050284031), and the Nature Science Foundation of Jiangsu province (BK2006124). Supporting Information Available: XRD patterns (PDF) and X-ray crystallographic files (CIF format) for the structure determination of compounds 1 and 1⊃THF. This material is available free of charge via the Internet at http://pubs.acs.org.
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