Monochiral Nano-Channels

ARTICLE pubs.acs.org/crystal

Metal
Organic Frameworks with Achiral/Monochiral Nano-Channels Xiao-Yi Yi,† Hua-Cai Fang,† Zhi-Gang Gu,† Zheng-Yuan Zhou,† Yue-Peng Cai,*,† Jian Tian, and Praveen K. Thallapally*,‡ †

School of Chemistry and Environment, Key Laboratory of Electrochemiscal Technology on Energy Storage and Power Generation of Guangdong Higher Education Institutes, South China Normal University, Guangzhou 510006, Guangzhou University City, Panyu district, Guangzhou, China ‡ Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, United States

bS Supporting Information ABSTRACT: Three pH/temperature-dependent 2D MOFs containing 1D nanotubular meso-helical chains were firstly synthesized from multidentate 2,4 0 -H2bpdc. Crystal structure analysis shows that 2 and 3 are monochiral and the resultant crystals were not racemic as evidenced by the observation of strong signals in vibrational circular dichroism (VCD) and circular dichroism (CD) spectra.

’ INTRODUCTION The search for new structural motifs with helical architectures has become an intensive research area because of their fascinating structural features and various functions in biological systems1,2 It is well-known that the artificial helical coordination polymers can be constructed by using chiral or achiral building blocks.3 In many cases, right-handed (here onward M) and left-handed (here onward P) helices are obtained in equal amounts as a racemate when achiral building blocks are used.4 In some cases, however, spontaneous resolution into enantiomeric chiral crystals occurs through an efficient transfer of stereochemical information between neighboring helices,3b,5 for example, the right/left handed helical chains packed in homochiral 2D sheet driven by temperature, though the phenomenon of spontaneous resolution.6 In recent years, monochiral 0D, 1D, 2D, and 3D structures have been well reported.7,8 The basic features to assemble discrete helices or low dimensional helical coordination polymers are now fairly well established, however the design and control of the helical unit into a multihelical-array still remains a challenge. On the other hand, metal
organic frameworks with nanochannels have attracted a great deal of attention,10 especially the helical tubular structure with chiral channels because of their potential applications in chiral separation and asymmetric heterogeneous catalysis.11 While most of the effort has so far focused on the assembly of the porous metal
organic frameworks with large and identical cavities for molecular adsorption and separation. MOFs with monochiral channels remain largely unexplored in spite that chiral building blocks are commonly used for the r 2011 American Chemical Society

generation of chiral solids. Apart from utilization of chiral building blocks, chiral solvents and or auxiliary ligands were commonly used but with limited success. Herein we report the construction of chiral helical chains/channels by self-assembling 2,40 -biphenyldicarboxylic acid (2,40 -H2bpdc) (Scheme 1) and cobalt clusters of chemical formula, C14H10O5Co (1
3). Compared to its isomers, such as 2,20 -biphenyldicarboxylic acid, 3,40 biphenyldicarboxylic acid and 4,40 -biphenyldicarboxylic acid, the coordination chemistry of 2,40 -H2bpdc was seldom investigated except one report on 2,40 -H2bpdc with other auxiliary ligands.12

’ EXPERIMENTAL SECTION Syntheses of Complexes 1
3. Three compounds 1
3 were synthesized by combining Co(NO3)2 (0.036 g) and 2,40 -pbdc (0.037 g) in a 40 mL Teflon-lined stainless-steel autoclave containing 33.3 mL of deionized water under different pH and temperatures (120 to 160 °C for 2 weeks). After two weeks the autoclave was cooled to room temperature at a rate of 0.5 °C min
1. An achiral MOF, was obtained by adjusting pH = 6.5 and temperature 120 °C, to give pink block crystals of 1 in a (0.0273 g) 63% yield (based on Co). Anal. Calcd for C14H10O5Co (%): C, 52.97; H, 3.15. Found: C, 52.82; H, 3.27. IR frequencies (KBr, cm
1): 3419(m), 3311(w), 1593(s), 1381(m), 1152(s), 1043(m), 801(w), 675(w). By adjusting the pH = 6.5 and temperature (160 °C) resulted in brown block crystals of 2 (0.0242 g) (yield 51% based on Co). Anal. Received: December 6, 2010 Revised: May 5, 2011 Published: May 10, 2011 2824

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Crystal Growth & Design Scheme 1. Construction Two Dimensional Metal Organic Frameworks by Adjusting Temperature and Reaction pH

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Scheme 2. Two Coordination Modes of Ligand 2,40 -bpdc in Complexes 1
3

Figure 2. 2D achiral MOF containing 1-D nanochannels with double helical chains in 1 (Nanochannels are shown in red).

Figure 1. Coordination environments of the CoII and 2,40 -bpdc2
ions in 1. Symmetry codes: (a) 0.5
x, 0.5 þ y, z; (b) 1
x, 0.5 þ y, 0.5
z; (c)
0.5 þ x, y, 0.5
z; (d)
x, 0.5 þ y, 0.5
z; (e) 0.5 þ x, y, 0.5
z; (f)
0.5 þ x, y, 0.5
z; (g)
x,
0.5 þ y, 0.5
z; (h) 1
x,
0.5 þ y, 0.5
z. Calcd for C14H10O5Co (%): C, 52.97; H, 3.15. Found: C, 52.85; H, 3.31. IR frequencies (KBr, cm
1): 3414(m), 3302(m), 1595(s), 1378(s), 1150(m), 1045(m), 798(w), 671(w). Similarly, by adjusting the resulting solution pH to 8 at temperature 160 °C, to give blue block crystals 3 (0.0198 g) (yield 41% based on Co). Anal. Calcd for C28H22O12Co3 (%): C, 46.20; H, 3.03. Found: C, 46.15; H, 3.22. IR frequencies (KBr, cm
1): 3419(m), 3308(m), 1590(s), 1372(s), 1153(w), 1049(m), 790(w), 677(w). Transition from 1 to 3. When the reaction solution containing single crystals of 1 (1 mmol) were heated at 160 °C in a Teflon-lined stainless steel autoclave for 72 h resulted in a brown block crystals of 2. Similar methods were applied to convert complexes 2 to 3 at pH = 8 and 160 °C, as well as 3 to 1 at pH = 5.5 and 140 °C. Composition and structures of 1, 2, and 3 were confirmed by elemental analysis, single crystal and powder X-ray diffraction.

’ RESULTS AND DISCUSSION We have successfully isolated three new MOFs with one being achiral and the other two being monochiral by hydrothermal reactions of Co(NO3)2 with ligand 2,40 -H2bpdc namely

[Co(2,40 -bpdc)H2O]n (achiral-1 and chiral-2) and [Co3(OH)2(2,40 bpdc)2(H2O)2]n (chiral-3). Single-crystal X-ray analysis suggest 1 and 2 have the same composition [Co(2,40 -bpdc)H2O]n, but crystallizes in achiral and chiral space group (1, Pbca and 2, P21212). Similarly, X-ray analysis of 3 indicates isostructural to complex 2. Interestingly, three compounds 1 f 2 f 3 f 1 exhibit unprecedented, irreversible transformations upon heating (120
160 °C) and adjusting pH (5.5
8.5) (Scheme 1). In depth analysis of 1 shows a 2D layer-like structure with 1D channels built from 2,40 -bpdc2- and CoII ions under acidic environment. Each Co2þ ion is hexa-coordinated by five oxygen atoms from four carboxylate groups of four different 2,40 -bpdc2ligands and one oxygen atom from one coordinated water molecule with the Co
O bond lengths in the range of 2.177(4) to 2.371(2) Å and the O
Co
O bond angles ranging from 54.8(2) to 162.6(3)° (Figure 1 and Supporting Information Table S1). Each 2,40 -bpdc2- ligand via μ4-η1:η1:η2:η1 coordination manner (mode I) links four Co(II) centers (Scheme 2) to generate a 22-membered ring. On the basis of the coordination mode I and conformation of ligand 2,40 -bpdc2-, these rings are connected in parrallel into square tubular structure to result in a 1D channels. (Supporting Information Figure S1 and Table S2). The two adjacent 1D tubes are alternately hinged into 2D layer through two parallel double-stranded helical chains (
Co
O
Co
O
Co
)n and (
Co
O
C
O
Co
O
C
O
Co
)n with sharing Co1 centers (Figures 2 and Supporting Information Figure S2). The twodimensional layers are further assembled into 3D supramolecular network with 1D parallel channels and meso-helical chains in bc plane (Supporting Information Figure S3). Upon increasing the temperature of the solution to 160 °C results in 2. The asymmetric unit of 2 consists of two crystallographically independent Co centers (Co1 and Co2) in a distorted octahedral coordination environment Co1 is sixcoordinated by four chelating carboxylate oxygen atoms from two 2,40 -bpdc and two monodentate carboxylate oxygen atoms from other two 2,40 -bpdc. Co2 is also six-coordinated by four 2825

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Crystal Growth & Design

Figure 3. 2D monochiral MOF containing 1D left-handed helical nanochannels in 2.

Figure 4. The coordination environments of the CoII and 2,40 -bpdc2ions in 3. Symmetry codes: (a) 1
x, y, z; (b) 0.5 þ x, 0.5 þ y, z; (c) 1.5 þ x, 0.5 þ y, z; (d) 2
x, y, 1.5
z; (e) 1 þ x, y, z; (f) 1.5
x, 0.5 þ y, 1.5
z; (g) 2
x, y, 1.5
z; (h) 1
x, y, 1.5
z; (i) 0.5
x, 0.5 þ y, 1.5
z; (g)
1 þ x, y, z.

monodentate carboxylate oxygen atoms from 2,40 -bpdc ligands and two coordination water molecules. The distances of Co
O bonds range from 2.014(2) to 2.153(2) Å, comparable to those in other Co(II) carboxylate complexes.6a The O
Co
O bond angles range from 60.94(8) to 174.76(10)°. Two adjacent Co centers are linked by two carboxylate groups from two different 2,40 -bpdc ligands with the Co1 3 3 3 Co2 separation of 3.588(3) Å. The 2,40 -bpdc2
ligands are full deprotonated and also adopt μ4η1:η1:η2:η1-coordination mode I (Scheme 2). In depth analysis of complex 2 shows 1D tube-like channel with approximate dimensions of 7.5  4.5 Å2. By taking Co1 ions as two alternating but parallel backbones along the b-axis, 1D tube-like channel can be viewed as 2,40 -bpdc wrapped around Co1 ions in a doublestranded left-handed helix(Supporting Information Figure S6). Through coordinated hinge of two double-strands double helical chainssharing the cobalt sites (Co1 and Co2), the adjacent helical channels are interconnected to each other to allow the lefthanded chirality to transfer uniformly along the bc plane, leading to the formation of an interesting homochiral 2D sheet shown in Figure 3, in which all the helical channels are left-handed chirality. The homochirality transfer within the homochiral 2D sheet has also been found in other coordination polymers.6a,12 The chiral sheets are further linked by weak hydrogen bonding O
H 3 3 3 O/C(O)-H 3 3 3 π to generate a 3D monochiral coordination framework containing 1D double-stranded left-handed helical tube-like channels(Supporting Information Figure S7).

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Figure 5. 2-D monochiral MOF containing 1-D left-handed helical nanochannels hinged by double-strands double helical chains in 3. Green indicates right-handed helix (P) and lavender left-handed helix (M). The μ3-bridging hydroxyl groups locking two adjacent doublestrand double helical chains were omitted for clarity.

Similar to complex 2, complex 3 is also 2D monochiral MOF containing 1D left-handed helical channels. Structural analysis reveals that there are three types of Co2þ centers (Co1, Co2, and Co3). Co1 and Co2 have an octahedral coordination environment and Co3 is in tetrahedral coordination geometry. As indicated in Figure 4, for six coordinated oxygen atoms around Co1 center, four oxygen atoms (O2, O2g, O4b, and O4f) from four bridging bidentate 40 -position carboxylic groups of four 2,40 bpdc2
ligands are located on the same plane, and two μ3bridging hydroxyl groups respectively occupy two the axial positions. As for Co2 center, six coordinated atoms come from two oxygen atoms of two 2,40 -bpdc2- ligands located in axial positions and four μ3-bridging hydroxyl groups distributed in the same plane. The distance between neighboring Co2 and Co3 ions in one square unit Co2(μ3-OH)2 are 2.973 Å, indicating existence of Co 3 3 3 Co weak interaction.14 Compared with 2, complex 3 was obtained under higher pH value condition, resulting in OH
anion coordinating to three different Co centers in μ3-bridging manner and 2,40 -bpdc2- adopting coordination mode II (Scheme 2). On the basis of the coordination fashions of Co2þ and 2,40 -bpdc2- ions, 2D MOF containing 22membered ring channels with pore size of ∼7.6  8.6  9.2 Å is built along [100] direction as depicted in Figure 5 (Supporting Information Figure S8), in which each 22-membered channel is surrounded by two the same single-strand left-handed helical chains presenting monochirality. These 1D tubular channels are arranged in parallel into 2-D MOF structure through the coordination hinges of two intertwined single-strand double helical chains As discussed above, complexes 1
3 are obtained from the same reactants and the reaction conditions but for a slight change in the pH values and the reaction temperatures, respectively. Obviously these facts prompted us to further investigate coordination chemistry based on 2,40 -H2bpdc ligand. To explore the influence of pH values and reaction temperature on three resulting complexes, the interconversions among these three complexes were carried out. The results show that on increasing reaction temperature to 160 °C, complex 1 convert to 2. When pH increased to 8, complex 2 converts to 3. Adjusting the pH back to 5.5, complex 3 transforms to 1 at 140 °C (Scheme 2). Although the conversions can be carried out in the solution, the single crystal transformations between the three complexes are not successful, possibly deriving from this process always being accompanied by single crystal cracking, so that the single crystallinity of the samples cannot be retained. The complexes 1
3 were confirmed by matching their X-ray powder patterns with those generated from the corresponding 2826

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Crystal Growth & Design

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Figure 7. Solid-state CD spectra of (a) complex 2 and (b) complex 3 for the bulk crystals (inset: the solid-state UV absorption spectra of 2 in (a) and 3 in (b), respectively).

Figure 6. Solid-state VCD and IR spectra of (a) complex 2 and (b) complex 3 for the bulk crystals.

single crystals (Figure S12 in the Supporting Information). Transformations by thermal treatment at temperatures range of 140
160 °C result in the significant changes in the powder patterns but acceptable matches were observed between the simulated and the experimental powder X-ray diffraction patterns for bulk crystalline samples as obtained from the synthesis of corresponding compounds 1, 2 and 3. These facts clearly indicate that the transition described for single crystals also occur in macroscopic powder samples and lead to monophasic products. In addition, many experiments show that reverse conversion of 1 f 3 f 2 f 1 is not allowed in this system, indicating that these transformations are irreversible cycles. Three compounds 1
3 potentially have one-dimensional channels, however, the coordination of water molecules to CoII ion in channels results in the small solvent-accessible volume versus the total volume of this crystal calculated from PLATON/ Solv.15 The thermal gravimetric analysis (TGA) (Supporting Information Figure S13) and X-ray powder diffraction experiments (Supporting Information Figure S14) revealed that the coordinated water molecule in 1
3 may be removed at temperature range from 180 to 250 °C and remain the integrity of the whole framework structure with the solvent accessible volume of 2.8% for 1, 8.3% for 2 and 4.7% for 3. To further support the nonporous nature of these materials, N2 adsorption/desorption experiments were conducted at 77K, which shows (Supporting Information Figure S15)the surface area of 2σ(I)]; R1 = 0.0641, R2 = 0.1513 (all data); residual electron density = 0.448 eA
3. C14H10O5Co for 2: Mr = 317.15; crystal size = 0.23 0.20  0.11 mm3; orthorhombic; space group P21212; a = 24.058(6) Å, b = 6.8774(16) Å, c = 7.7918(18) Å; R = β = γ = 90°; V = 1289.2(5) Å3; Z = 4; Fcalcd = 1.634 mg
3; μ = 1.346 mm
1; λ = 0.71073 Å; T = 298 (2) K; 3.38 < 2θmax < 50.5; 6717 reflections collected; 1399 unique reflections (Rint = 0.0415); R1 = 0.0249, R2 = 0.0544 [I > 2σ(I)]; R1 = 0.0300, R2 = 0.0569 (all data); residual electron density = 0.253 eA
3. C28H22O12Co3 for 3; Mr = 727.25; crystal size=0.27 0.20  0.16 mm3; orthorhombic; space group C2221; a = 6.3985(9) Å, b = 19.848(3) Å, c = 21.634(3) Å; R = β = γ = 90°; V = 2747.4(7) Å3; Z = 4; Fcalcd = 1.758 mg
3; μ = 1.857 mm
1; λ = 0.71073 Å; T = 298 (2) K; 4.10 < 2θmax < 50.5; 7140 reflections collected; 2501 unique reflections (Rint = 0.0200); R1 = 0.0209, R2 = 0.0506 [I > 2σ(I)]; R1 = 0.0225, R2 = 0.0513 (all data); residual electron density = 0.363 eA
3.

’ ASSOCIATED CONTENT

bS

Supporting Information. Further experimental details, including ligand synthesis, X-ray crystallography and mass

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Crystal Growth & Design spectroscopy. This material is available free of charge via the Internet at http://pubs.acs.org.CCDC-763805(1), 789247(2), and 763807(3) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via ww.ac.uk/ data_request/cif. The figure graphics were generated by using the w.ccdc.cam MERCURY 2.2 program supplied with the Cambridge Structural Database.

’ AUTHOR INFORMATION Corresponding Author

*Fax: (þ) 86-020-39310187. E-mail: [email protected]; [email protected].

’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (No.20772037), Science and Technology Planning Project of Guangdong Province (Grant No. 2006A10902002 and 2010B031100018), and the Natural Science Foundation of Guangdong Province (9251063101000006 and 06025033). PKT thank the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award KC020105-FWP12152. PNNL is a multiprogram national laboratory operated for DOE by Battelle under Contract DE-AC05-76RL01830 ’ REFERENCES (1) (a) Albrecht, M. Chem. Rev. 2001, 101, 3457–3498. (b) Moulton, B.; Zaworotko, M. J. Chem. Rev. 2001, 101, 1629–1658. (c) Schmuck, C. Angew. Chem., Int. Ed. 2003, 42, 2448–2452. (d) Berl, V.; Huc, I.; Khoury, R. G.; Krische, M. J.; Lehn, J.-M. Nature 2000, 407, 720–723. (e) Bu, X.-H.; Tong, M.-L.; Chang, H.-C.; Kitagawa, S.; Batten, S. R. Angew. Chem., Int. Ed. 2004, 43, 192–195. (f) Chen, J.-Q.; Cai, Y.-P.; Fang, H.-C.; Zhou, Z.-Y.; Zhan, X.-L.; Zhao, Gang; Zhang, Z. Cryst. Growth Des. 2009, 9, 1605–1613. (2) (a) Ye, B.-H.; Tong, M.-L.; Chen, X.-M. Coord. Chem. Rev. 2005, 249, 545–565. (b) Zheng, X.-D.; Lu, T.-B. CrystEngComm 2010, 12, 324–336. (c) Cui, Y.; Lee, S. J.; Lin, W. J. Am. Chem. Soc. 2003, 125, 6014–6015. (d) Azumaya, I.; Uchida, D.; Kato, T.; Yokoyama, A.; Tanatani, A.; Takayanagi, H.; Yokozawa, T. Angew. Chem., Int. Ed. 2004, 43, 1360–1363. (e) Rao, C. N. R.; Natarajan, S.; Vaidhyanathan, R. Angew. Chem., Int. Ed. 2004, 43, 1466–1496. (f) Cai, Y.-P.; Zhou, X.-X.; Zbou, Z.-Y.; Zhu, S.-Z.; Thallapally, P. K.; Liu, J. Inorg. Chem. 2009, 48, 6341–6343. (3) (a) Luan, X. J.; Wang, Y. Y.; Li, D. S.; Liu, P.; Hu, H. M.; Shi, Q. Z.; Peng, S. M. Angew. Chem., Int. Ed. 2005, 44, 3864–3867. (b) Perez-García, L.; Amabilino, D. B. Chem. Soc. Rev. 2002, 31, 342–356. (c) Anokhina, E. V.; Jacobson, A. J. J. Am. Chem. Soc. 2004, 126, 3044–3045. (d) Cai, Y.-P.; Yu, Q.-Y.; Zhou, Z.-Y.; Hu, Z.-J.; Fang, H.-C.; Wang, N.; Zhan, Q.-G.; Chen, L.; Su, C.-Y. CrystEngComm 2009, 11, 1006–1013. (4) (a) Gu, Z.-G.; Cai, Y.-P.; Fang, H.-C.; Zhou, Z.-Y.; Thallapally, P. K.; Tian, J.; Liu, J.; Exarhos, G. J. Chem. Commun. 2010, 46, 5373–5375. (b) Cai, Y.-P.; Su, C.-Y.; Chen, C.-L.; Li, Y.-M.; Kang, B.-S.; Chan, A. S. C.; Kaim, W. Inorg. Chem. 2003, 42, 163–168. (5) (a) Gao, E. Q.; Yue, Y. F.; Bai, S. Q.; He, Z.; Yan, C. H. J. Am. Chem. Soc. 2004, 126, 1419–1429. (b) Ezuhara, T.; Endo, K.; Aoyama, Y. J. Am. Chem. Soc. 1999, 121, 3279–3283. (6) (a) Tian, G.; Zhu, G.-S.; Yang, X.-Y.; Fang, Q.-R.; Xue, M.; Sun, J.-Y.; Wei, Y.; Qiu, S.-L. Chem. Commun 2005, 1396–1398. (b) Lu, W.-G.; Gu, J.-Z.; Jiang, L.; Tan, M.-Y.; Lu, T.-B. Cryst.Growth Des. 2009, 8, 192–199. (7) (a) Kofod, P.; Harris, P.; Larsen, S. Inorg. Chem. 1997, 36, 2258–2266. (b) Doxsee, K. M.; Hagadorn, J. R.; Weakley, T. J. R.

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