- Email: [email protected]
An unusual 3D metal-organic framework with multiform helical chains
Inorganic Chemistry Communications 14 (2011) 519–521
Contents lists available at ScienceDirect
Inorganic Chemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i n o c h e
An unusual 3D metal-organic framework with multiform helical chains Jian-Qiang Liu a,⁎, Yao-Yu Wang b,⁎, Zhen-Bin Jia a a
Guangdong Medical College, School of Pharmacy, Dongguan, 523808, PR China Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education, Shaanxi Key Laboratory of Physico-Inorganic Chemistry, Department of Chemistry, Northwest University, Xi'an 710069, PR China b
a r t i c l e
i n f o
Article history: Received 1 November 2010 Accepted 7 January 2011 Available online 20 January 2011 Keywords: Metal-organic framework Helix Interpenetration Structure
a b s t r a c t A new metal-organic framework, namely, {[Co2(oba)2(bib)]·H2O}n (1) (H2oba = 4,4′-oxybis(benzoic acid) and bib = 1,4-bis(2-methyl-imidazol-1-yl)butane), was prepared via self-assembly of the bib with cobalt salt in the presence of V-shaped oba ligand under mild condition. The structure of polymer 1 presents a sixconnected self-interpenetrating 44.610.8 topological network with interweaving helical chains. In addition, the most stable conformation of bib has been analyzed by B3LYP calculation. © 2011 Elsevier B.V. All rights reserved.
The field in the synthesis and characterization of metal-organic framework (MOF) materials is growing exponentially because of their potential utility in catalysis, hydrogen storage and magnetism [1–3]. A helix is a geometric motif, which is ubiquitous in nature as well as in human art [4]. Some discrete helices or low-dimensional helical polymers have been presented [5], while the occurrence of pillaredlayer MOFs with helical character is still a challenge [6]. Only few examples have been reported concerning multiple helices in 3D pillared MOFs (usually, layers with helical chains were found in biology), which were assembled from V-shaped tetracarboxylate ligands and transitional metal salts in the presence of N-donor ligand [7]. To the best of our knowledge, such studies in 3D pillared network with multiple helices remain unexplored. Yaghi et al. have proposed the secondary building units crystal engineering strategy for the MOFs synthesis [8]. This, in effect, can pave the way for strategic modulation of vertex geometry in MOFs. A classic metal paddle-wheel structure with a pseudo metal–metal bond bridged by four μ2-carboxylate ends can be easily resolved by using V-shaped flexible carboxylates [9]. Moreover, several reports reveal that in most cases the two axial sites of paddle-wheel unit would be occupied by N-containing donors and formulated as [M2(O2CR)4(L)2] (Scheme 1c–d). Such a strategy may induce new products combining respective merits of 3D pillared and helical motifs. Inspired by the above-mentioned considerations, a more flexible 1,4-bis(2-methyl-imidazol-1-yl)butane(bib) was chosen as the axial linker. Firstly, the two imidazole moieties of bib are linked with four methylene groups. This, in fact, should invoke more flexibility, which
⁎ Corresponding authors. E-mail address: [email protected] (J.-Q. Liu). 1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2011.01.011
may improve the helicity of polymeric chains and favor the formation of helical structures. Secondly and most importantly, there are two methyl groups juxtaposed to the N atoms of the bib linker, and these methyl groups are expected to produce steric hindrance to restrict another ligands (such as aqua ligand) bonding to metal centers and restrain the occurrence of interpenetration among the motifs. This consideration led to our successful construction of a new 3D interpenetrating MOF of {[Co2(oba)2(bib)]·H2O}n (1), which presents a six-connected self-interpenetrating 44.610.8 topological network with interweaving helical chains. Solvothermal reaction of H2oba, Co(NO3)2 and bib in CH3OH/H2O at 150 °C gave the pink crystals of 1 [10]. Single crystal X-ray diffraction study reveals that the compound 1 contains one crystallographically independent Co(II) atom, one oba ligand, a half bib ligand and a half lattice water molecule. The 3D framework of compound 1 is built up from two subunits: one is a 2D grid sheet with Co(II) paddle-wheel units, and the other is a 1D chain connected by bridging bib ligands. Bib ligand has carried out at DFT levels using single-point energy calculations and charge distributions, which prove the trans-conformation stability (Fig. S1 and Table S1). Fig. 1 displays the coordination environment of the Co(II) atom and the sixfold connectivity of the bimetallic unit. Each Co(II) atom in the binuclear unit is coordinated by four carboxylic oxygen atoms of oba ligands (Co–O 2.026(3) to 2.054(3) Å) and one nitrogen atom of a bib ligand (Co–N 2.048(3) Å) to complete a square-pyramidal geometry. Two crystallographycially equivalent Co(II) atoms are linked by four carboxylate ends bonded in bis(bridging bidentate) mode to give a paddle-wheel formed [Co2(CO2)4] fragment in which the Co···Co distance is 2.9107(12) Å (Fig. 2a). Interestingly, the open nature of the (4,4) sheets, and their corrugation, allow the sheets to interpenetrate in an usual 2D → 2D parallel fashion as shown in Fig. 2b–c. In a
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Scheme 1. The conformation mode of bib and the second building unit of the paddlewheel core.
[Co(oba)(bpe)]n (bpe = 1,2-bis(4-pyridyl)ethane) polymer, the oba also exhibits similar mode and links the [Co2(CO2)4] fragment secondary building unit into 2D layers with rhombus-shaped cavities of about 13 × 15 Å2. These layers are pillared by bpe ligands, resulting in the formation of a 3D net that can be regarded as possessing a three interpenetrating α-polonium-related topology [11]. Although the bpe and bib ligands are both N-containing spacers, they are quite different. The bib has two different conformations; moreover, the distance between the two nitrogen atoms is larger than that of bpe. So for a special MII/oba system, the ancillary ligand has a significant effect on the formation of coordination polymers. The axis sites of each paddle-wheel unit are connected by two additional bib ligands. Thus, the bib ligands bridge two dimers, with each pair of dimers connected by a pair of parallel bib bridges, generating a novel 3D framework (Fig. 3a). In other words, each dimer is connected to six identical units (Fig. S2), four by individual oba ligands, and the other two by bib ligands, giving a 6-connected net with 48.66.8 topology (Fig. 3b) [12]. This motif is different to the usual Pcu topology with 6-connected network. In Pcu topology, the linkers are all parallel, however, in the presented structure of 6-connected 48.66.8 topology, the sheets bridge in two inclined directions. Moreover, the net is self-penetrating, each six-membered ring includes a link within a layer and a link between the layers is penetrated by two interlayer rods
Fig. 1. The coordination environments of Co(II) ions in 1 [symmetric codes: (i) −x, −y+ 1, −z; (ii) −x, 1 + y, 1/2 − z; (iii) x, −y, z − 1/2].
Fig. 2. (a) Perspective view of the 2D sheet directed by Co(II) and oba ligands; (b) view of the (4,4) grid sheet; (c) schematic view of 2D → 2D parallel interpenetration with catenane character; (d) view of the left- and right-handed double chains.
[2d]. Such net is also observed in the structure of Mn(dca)2L (L= 1,2-bis(4-pyridyl)ethane-N,N′-dioxide) [13]. Another noteworthy feature of 1 is the presence of helices within the (4,4) sheets directed by metal centers and oba ligands. The dihedral angle between the two phenyl rings within the oba ligand is 101.9°, which may be significant in forming the helical chain. Based on this connection, the Co(II) atoms are linked by the V-shaped oba ligands to shape the left- and right-handed helical chains running along the a axis with a pitch of 17.2 Å as described in Fig. 2d. The most fascinating structural feature of 1 is that the four diverse helical chains running along the crystallographyic a axis coexist in the 3D framework. Besides the single-helical chains in the 2D helical sheet, there are one type of quadruple-stranded helix with a pitch of 25.9 Å (Fig. 4a), two types of triple-stranded helices with pitches of 27.2 Å and one single meso-helix (tri-flexural helix) in 1 [4b,14]. As illustrated in Fig. 4a, the quadruple-stranded helix is built from two pairs of helices of 2D helical sheets and one from the [Co–bib–Co–bib–Co]n chain. Compared with the few occurrences of double- and triple-stranded helical architectures, the unusual intertwined quadruple-stranded helix is seldom in MOFs. The other two types of triple-stranded helices are directed by oba and bib ligands bridged by metal centers (Fig. 4b–c). Surprisingly, a further investigation reveals that the configuration of the forth type of helix displays a unique feature and the term “tri-flexural helix” is suggested for it, meaning consisting of three flexures in one single strand (Fig. 4d). To the best of our knowledge, only one instance of such helix was observed in MOF, thus this new helical motif may broaden insight into the isomers related to helices. To study the stability of the polymer, thermogravimetric analysis (TGA) of complex 1 was performed (Fig. S3). The compound 1 shows two main steps of weight loss. The first weight loss began at 30 °C and
Fig. 3. (a) The 3D framework with different tubular channels; (b) schematic view of 3D six-connected 48.66.8 topology net with self-interpenetration.
J.-Q. Liu et al. / Inorganic Chemistry Communications 14 (2011) 519–521
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Fig. 4. (a) View of the quadruple-stranded helix; (b) and (c) two types of triplestranded helices; and (d) view of one single meso-helix.
completed at 98 °C. The observed weight loss of 2.8% is corresponding to the loss of the water molecule (calcd 2.2%). Additionally, to confirm the phase purity and stability of compound 1, the original sample was both characterized by X-ray powder diffraction (XRPD) at room temperature. The pattern that was simulated from the single-crystal X-ray data of compound 1 was in agreement with those that was observed (Fig. S4). In summary, a rational synthetic strategy was employed to generate a new 3D pillared MOF with unprecedented channels and interweaving helices, which presents a six-connected self-interpenetrating 44.610.8 topological network. The compound represents the first example of interweaving helices based on cooperative flexible ligands. Further endeavors will focus on the nature of cooperative flexible ligands within such a system. Acknowledgments We gratefully acknowledge the financial support of this work by Guangdong Medical College and the National Natural Science Foundation of China (Grant No. 20771090). Appendix A. Supplementary material CCDC number 780704 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif. This includes DFT calculations for bib ligand, additional structural figures and the results of TGA and XRD. This material is available free of charge via the internet. Supplementary materials related to this article can be found online at doi:10.1016/j.inoche.2011.01.011.
[1] (a) J.M. Lehn, Supramolecular Chemistry: Concepts and Perspectives, VCH, Weinheim, 1995; (b) L.J. Murray, M. Dincaă, J.R. Long, Chem. Soc. Rev. 38 (2009) 1294; (b) X.H. Bu, M.L. Tong, H.H. Chang, S. Kitagawa, S.R. Batten, Angew. Chem. Int. Ed. 43 (2004) 192; (c) S.R. Batten, R. Robson, Angew. Chem. Int. Ed. 37 (1998) 1460; (d) L. Carlucci, G. Ciani, D.M. Proserpio, Coord. Chem. Rev. 246 (2003) 247; (e) D.J. Tranchemontagne, Z. Ni, M. O'Keeffe, O.M. Yaghi, Angew. Chem. Int. Ed. 47 (2008) 5136; (f) J.J. Perry IV, J.A. Perman, M.J. Zaworotko, Chem. Soc. Rev. 38 (2009) 1400. [2] (a) S.B. Ren, L. Zhou, J. Zhang, Y.L. Zhu, Y.Z. Li, H.B. Du, X.Z. You, Cryst. Eng. Commun. 12 (2010) 1635; (b) K.L. Mulfort, O.K. Farha, C.D. Malliakas, M.G. Kanatzidis, J.T. Hupp, Chem. Eur. J. 16 (2009) 276; (c) H.D. Guo, D.F. Qiu, X.M. Guo, S.R. Batten, H.J. Zhang, Cryst. Eng. Commun. 11 (2009) 2611; (d) S.R. Batten, S.M. Neville, D.R. Turner, Coordination Polymers: Design, Analysis and Application, Royal Society of Chemistry, Cambridge, 20088 ch. 3. [3] (a) S.R. Batten, R. Robson, in: J.-P. Sauvage, C. Dietrich-Buchecker (Eds.), Molecular Catenanes, Rotaxanes and Knots: A Journey Through the World of Molecular Topology, Wiley-VCH, Weinheim, 1999, pp. 77–105; (b) Q.R. Fang, G.S. Zhu, M. Xue, J.Y. Sun, Y. Wei, S.L. Qiu, R.R. Xu, Angew. Chem. Int. Ed. 44 (2005) 3845; (c) X.L. Wang, C. Qin, E.B. Wang, Z.M. Su, Chem. Eur. J. 12 (2006) 2680; (d) L.F. Ma, C.P. Li, L.Y. Wang, M. Du, Cryst. Growth Des. 10 (2010) 2641; (e) L.F. Ma, Q.L. Meng, C.P. Li, B. Li, L.Y. Wang, M. Du, Cryst. Growth Des. 10 (2010) 3036. [4] (a) K.P. Meurer, F.P. Vögtle, TOP. Curr. Chem. 127 (1985) 1; (b) L. Han, M.C. Hong, Inorg. Chem. Commun. 8 (2005) 406. [5] (a) V. Berl, I. Huc, R.G. Khoeuy, M.J. Krische, J.M. Lehn, Nature 407 (2000) 720; (b) X.M. Chen, G.F. Liu, Chem. Eur. J. 8 (2002) 4811; (c) L. Pan, K.M. Adams, H.E. Hernandez, X. Wang, C. Zheng, Y. Hattori, K. Kaneko, J. Am. Chem. Soc. 125 (2003) 3062; (d) X.J. Luan, Y.Y. Wang, D.S. Li, P. Liu, H.M. Hu, Q.Z. Shi, S.M. Peng, Angew. Chem. Int. Ed. 44 (2005) 3864; (e) L. Plasseraud, H. Maid, F. Hample, R.W. Saalfrank, Chem. Eur. J. 7 (2001) 4007. [6] S.N. Wang, H. Xing, Y.Z. Li, J.F. Bai, M. Scheer, Y. Pan, X.Z. You, Chem. Commun. (2007) 2293. [7] (a) D.R. Xiao, E.B. Wang, H.Y. An, Y.G. Li, Z.M. Su, C.Y. Sun, Chem. Eur. J. 12 (2006) 6528; (b) L.F. Ma, B. Liu, L.Y. Wang, C.P. Li, M. Du, Dalton Trans. (2010) 2301; (c) L.F. Ma, Q.L. Meng, L.Y. Wang, B. Liu, F.P. Liang, Dalton Trans. (2010) 8210. [8] (a) N.W. Ockwig, O. D- Friedrichs, M. O'Keeffe, O.M. Yaghi, Acc. Chem. Res. 38 (2005) 176; (b) M. Eddaoudi, J. Kim, M. O'Keeffe, O.M. Yaghi, J. Am. Chem. Soc. 124 (2001) 376; (c) O.M. Yaghi, H. Li, C. Davis, D. Richardson, T.L. Groy, Acc. Chem. Res. 31 (1998) 474. [9] (a) R. Mondal, M.K. Bhuia, K. Dhara, Cryst. Eng. Commun. 10 (2008) 1167; (b) W.B. Yuan, T. FriŠčić, D. Apperley, S.L. James, Angew. Chem. Int. Ed. 49 (2010) 18. [10] Synthesis of 1: A mixture of Co(NO3)2∙6H2O (0.030 g, 0.1 mmol), H2oba (0.019 g, 0.1 mmol), bib (0.022 g, 0.1 mmol), CH3OH (2 mL) and NaOH (0.5 M, 0.4 mL) and deionised water (10 mL) was stirred for 30 min in air, then transferred and sealed in a 25-mL Teflon reactor, which was heated at 150 °C for 72 h. The solution was then cooled to room temperature at the rate of 5 °C h−1, to yield fine pink crystalline product 1 in 50% yield. C40H36Co2N4O11 (866.59) (1): Anal. Calc. C, 55.44; H, 4.19; N, 6.46. Found: C, 55.26; H, 4.21; N, 6.39%. IR(cm−1): 3488 s, 3011 m, 1538 m, 1422 m, 1355 w, 1023 w, 812 m. Crystal data for 1: C40H36Co2N4O11, Mr= 866.59, monoclinic space group C2/c, a = 21.211(7), b = 8.718(3), and c = 22.820(9)Å, α = γ = 90° β = 101.394(8)° V = 4137(3) Å3, Z = 4, T = 298(2)K, Dc = 1.388 g/cm3, 12864 reflections collected, 3724 independent reflections, wR2= 0.1287, R1[IN 2σ(I)]= 0.0407, and GOF = 0.852. The hydrogen atoms of lattice water molecule O1W in compound 1 were not located using the different Fourier method. [11] C.Y. Sun, X.J. Zheng, S. Gao, L.C. Li, L.P. Jin, Eur. J. Inorg. Chem. (2005) 4150. [12] O.V. Dolomanov, A.J. Blake, N.R. Champness, M. Schröder, J. Appl. Crystallogr. 36 (2003) 1283. [13] H.L. Sun, S. Gao, B.Q. Ma, S.R. Batten, Cyrst. Eng. Commun. 6 (2004) 579. [14] C. Qin, X.L. Wang, L. Yuan, E.B. Wang, Cryst. Growth Des. 8 (2008) 2093.
Contents lists available at ScienceDirect
Inorganic Chemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i n o c h e
An unusual 3D metal-organic framework with multiform helical chains Jian-Qiang Liu a,⁎, Yao-Yu Wang b,⁎, Zhen-Bin Jia a a
Guangdong Medical College, School of Pharmacy, Dongguan, 523808, PR China Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education, Shaanxi Key Laboratory of Physico-Inorganic Chemistry, Department of Chemistry, Northwest University, Xi'an 710069, PR China b
a r t i c l e
i n f o
Article history: Received 1 November 2010 Accepted 7 January 2011 Available online 20 January 2011 Keywords: Metal-organic framework Helix Interpenetration Structure
a b s t r a c t A new metal-organic framework, namely, {[Co2(oba)2(bib)]·H2O}n (1) (H2oba = 4,4′-oxybis(benzoic acid) and bib = 1,4-bis(2-methyl-imidazol-1-yl)butane), was prepared via self-assembly of the bib with cobalt salt in the presence of V-shaped oba ligand under mild condition. The structure of polymer 1 presents a sixconnected self-interpenetrating 44.610.8 topological network with interweaving helical chains. In addition, the most stable conformation of bib has been analyzed by B3LYP calculation. © 2011 Elsevier B.V. All rights reserved.
The field in the synthesis and characterization of metal-organic framework (MOF) materials is growing exponentially because of their potential utility in catalysis, hydrogen storage and magnetism [1–3]. A helix is a geometric motif, which is ubiquitous in nature as well as in human art [4]. Some discrete helices or low-dimensional helical polymers have been presented [5], while the occurrence of pillaredlayer MOFs with helical character is still a challenge [6]. Only few examples have been reported concerning multiple helices in 3D pillared MOFs (usually, layers with helical chains were found in biology), which were assembled from V-shaped tetracarboxylate ligands and transitional metal salts in the presence of N-donor ligand [7]. To the best of our knowledge, such studies in 3D pillared network with multiple helices remain unexplored. Yaghi et al. have proposed the secondary building units crystal engineering strategy for the MOFs synthesis [8]. This, in effect, can pave the way for strategic modulation of vertex geometry in MOFs. A classic metal paddle-wheel structure with a pseudo metal–metal bond bridged by four μ2-carboxylate ends can be easily resolved by using V-shaped flexible carboxylates [9]. Moreover, several reports reveal that in most cases the two axial sites of paddle-wheel unit would be occupied by N-containing donors and formulated as [M2(O2CR)4(L)2] (Scheme 1c–d). Such a strategy may induce new products combining respective merits of 3D pillared and helical motifs. Inspired by the above-mentioned considerations, a more flexible 1,4-bis(2-methyl-imidazol-1-yl)butane(bib) was chosen as the axial linker. Firstly, the two imidazole moieties of bib are linked with four methylene groups. This, in fact, should invoke more flexibility, which
⁎ Corresponding authors. E-mail address: [email protected] (J.-Q. Liu). 1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2011.01.011
may improve the helicity of polymeric chains and favor the formation of helical structures. Secondly and most importantly, there are two methyl groups juxtaposed to the N atoms of the bib linker, and these methyl groups are expected to produce steric hindrance to restrict another ligands (such as aqua ligand) bonding to metal centers and restrain the occurrence of interpenetration among the motifs. This consideration led to our successful construction of a new 3D interpenetrating MOF of {[Co2(oba)2(bib)]·H2O}n (1), which presents a six-connected self-interpenetrating 44.610.8 topological network with interweaving helical chains. Solvothermal reaction of H2oba, Co(NO3)2 and bib in CH3OH/H2O at 150 °C gave the pink crystals of 1 [10]. Single crystal X-ray diffraction study reveals that the compound 1 contains one crystallographically independent Co(II) atom, one oba ligand, a half bib ligand and a half lattice water molecule. The 3D framework of compound 1 is built up from two subunits: one is a 2D grid sheet with Co(II) paddle-wheel units, and the other is a 1D chain connected by bridging bib ligands. Bib ligand has carried out at DFT levels using single-point energy calculations and charge distributions, which prove the trans-conformation stability (Fig. S1 and Table S1). Fig. 1 displays the coordination environment of the Co(II) atom and the sixfold connectivity of the bimetallic unit. Each Co(II) atom in the binuclear unit is coordinated by four carboxylic oxygen atoms of oba ligands (Co–O 2.026(3) to 2.054(3) Å) and one nitrogen atom of a bib ligand (Co–N 2.048(3) Å) to complete a square-pyramidal geometry. Two crystallographycially equivalent Co(II) atoms are linked by four carboxylate ends bonded in bis(bridging bidentate) mode to give a paddle-wheel formed [Co2(CO2)4] fragment in which the Co···Co distance is 2.9107(12) Å (Fig. 2a). Interestingly, the open nature of the (4,4) sheets, and their corrugation, allow the sheets to interpenetrate in an usual 2D → 2D parallel fashion as shown in Fig. 2b–c. In a
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Scheme 1. The conformation mode of bib and the second building unit of the paddlewheel core.
[Co(oba)(bpe)]n (bpe = 1,2-bis(4-pyridyl)ethane) polymer, the oba also exhibits similar mode and links the [Co2(CO2)4] fragment secondary building unit into 2D layers with rhombus-shaped cavities of about 13 × 15 Å2. These layers are pillared by bpe ligands, resulting in the formation of a 3D net that can be regarded as possessing a three interpenetrating α-polonium-related topology [11]. Although the bpe and bib ligands are both N-containing spacers, they are quite different. The bib has two different conformations; moreover, the distance between the two nitrogen atoms is larger than that of bpe. So for a special MII/oba system, the ancillary ligand has a significant effect on the formation of coordination polymers. The axis sites of each paddle-wheel unit are connected by two additional bib ligands. Thus, the bib ligands bridge two dimers, with each pair of dimers connected by a pair of parallel bib bridges, generating a novel 3D framework (Fig. 3a). In other words, each dimer is connected to six identical units (Fig. S2), four by individual oba ligands, and the other two by bib ligands, giving a 6-connected net with 48.66.8 topology (Fig. 3b) [12]. This motif is different to the usual Pcu topology with 6-connected network. In Pcu topology, the linkers are all parallel, however, in the presented structure of 6-connected 48.66.8 topology, the sheets bridge in two inclined directions. Moreover, the net is self-penetrating, each six-membered ring includes a link within a layer and a link between the layers is penetrated by two interlayer rods
Fig. 1. The coordination environments of Co(II) ions in 1 [symmetric codes: (i) −x, −y+ 1, −z; (ii) −x, 1 + y, 1/2 − z; (iii) x, −y, z − 1/2].
Fig. 2. (a) Perspective view of the 2D sheet directed by Co(II) and oba ligands; (b) view of the (4,4) grid sheet; (c) schematic view of 2D → 2D parallel interpenetration with catenane character; (d) view of the left- and right-handed double chains.
[2d]. Such net is also observed in the structure of Mn(dca)2L (L= 1,2-bis(4-pyridyl)ethane-N,N′-dioxide) [13]. Another noteworthy feature of 1 is the presence of helices within the (4,4) sheets directed by metal centers and oba ligands. The dihedral angle between the two phenyl rings within the oba ligand is 101.9°, which may be significant in forming the helical chain. Based on this connection, the Co(II) atoms are linked by the V-shaped oba ligands to shape the left- and right-handed helical chains running along the a axis with a pitch of 17.2 Å as described in Fig. 2d. The most fascinating structural feature of 1 is that the four diverse helical chains running along the crystallographyic a axis coexist in the 3D framework. Besides the single-helical chains in the 2D helical sheet, there are one type of quadruple-stranded helix with a pitch of 25.9 Å (Fig. 4a), two types of triple-stranded helices with pitches of 27.2 Å and one single meso-helix (tri-flexural helix) in 1 [4b,14]. As illustrated in Fig. 4a, the quadruple-stranded helix is built from two pairs of helices of 2D helical sheets and one from the [Co–bib–Co–bib–Co]n chain. Compared with the few occurrences of double- and triple-stranded helical architectures, the unusual intertwined quadruple-stranded helix is seldom in MOFs. The other two types of triple-stranded helices are directed by oba and bib ligands bridged by metal centers (Fig. 4b–c). Surprisingly, a further investigation reveals that the configuration of the forth type of helix displays a unique feature and the term “tri-flexural helix” is suggested for it, meaning consisting of three flexures in one single strand (Fig. 4d). To the best of our knowledge, only one instance of such helix was observed in MOF, thus this new helical motif may broaden insight into the isomers related to helices. To study the stability of the polymer, thermogravimetric analysis (TGA) of complex 1 was performed (Fig. S3). The compound 1 shows two main steps of weight loss. The first weight loss began at 30 °C and
Fig. 3. (a) The 3D framework with different tubular channels; (b) schematic view of 3D six-connected 48.66.8 topology net with self-interpenetration.
J.-Q. Liu et al. / Inorganic Chemistry Communications 14 (2011) 519–521
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Fig. 4. (a) View of the quadruple-stranded helix; (b) and (c) two types of triplestranded helices; and (d) view of one single meso-helix.
completed at 98 °C. The observed weight loss of 2.8% is corresponding to the loss of the water molecule (calcd 2.2%). Additionally, to confirm the phase purity and stability of compound 1, the original sample was both characterized by X-ray powder diffraction (XRPD) at room temperature. The pattern that was simulated from the single-crystal X-ray data of compound 1 was in agreement with those that was observed (Fig. S4). In summary, a rational synthetic strategy was employed to generate a new 3D pillared MOF with unprecedented channels and interweaving helices, which presents a six-connected self-interpenetrating 44.610.8 topological network. The compound represents the first example of interweaving helices based on cooperative flexible ligands. Further endeavors will focus on the nature of cooperative flexible ligands within such a system. Acknowledgments We gratefully acknowledge the financial support of this work by Guangdong Medical College and the National Natural Science Foundation of China (Grant No. 20771090). Appendix A. Supplementary material CCDC number 780704 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif. This includes DFT calculations for bib ligand, additional structural figures and the results of TGA and XRD. This material is available free of charge via the internet. Supplementary materials related to this article can be found online at doi:10.1016/j.inoche.2011.01.011.
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