Rational Construction of Wide Coordination Space and Control of

ARTICLE pubs.acs.org/crystal

Rational Construction of Wide Coordination Space and Control of Adsorption Properties in One-Dimensional Cu(II) Coordination Polymer Shin-ichiro Noro,*,†,‡ Katsuo Fukuhara,‡ Kazuya Kubo,†,‡ and Takayoshi Nakamura*,†,‡ † ‡

Research Institute for Electronic Science, Hokkaido University, Sapporo 001-0020, Japan Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan

bS Supporting Information ABSTRACT: A one-dimensional Cu(II) coordination polymer {[Cu(bpetha)(2,20 -bpy)(MeOH)] 3 2PF6}n (bpetha =1,2-bis(4-pyridyl)ethane; 2,20 -bpy =2,20 -bipyridine) with wide coordination space at the axial site was rationally synthesized and crystallographically characterized, in which [Cu(2,20 -bpy)] units are bridged by bpetha ligands, forming a one-dimensional zigzag chain that shows smooth adsorption for EtOH, MeOH, and H2O vapors.

’ INTRODUCTION Coordination compounds constructed from metal ions and organic ligands are of great interest because of their designable structures and functionalities such as magnetism and conductivity, dielectric and optical properties, and host
guest properties. In particular, Cu complexes have not only diverse valences (Cu(I), Cu(II), and Cu(III) states) and coordination environments but also unique axial Jahn
Teller bonds, which lead to unprecedented coordination structures, extraordinary electronic and optical properties, and host
guest functionalities.1 We have studied the host
guest properties of the one-dimensional Cu(II) coordination polymer formulated as [Cu(PF6)2(bpetha)2]n (1; bpetha =1,2-bis(4-pyridyl)ethane) that has a doubly linked chain structure,2 because they are now one of the main topics in the field of coordination polymers.3 Polymer 1 possesses weak and flexible coordination bonds, Cu
PF6, at the axial sites. The weakness of the axial bonds and the high electronegativity of the F atoms cause the Cu
PF6 units to have a polar structure. As a result, 1 shows highly selective adsorption for CO2 and C2H2 gases over other gases (e.g., N2, O2, CO, and H2) via electrostatic interactions.2b On the other hand, the flexible Cu
PF6 bonds of 1 enable a reversible cleavage and reformation of the bonds in response to specific Lewis base guests. When 1 is exposed to vapors such as Lewis base ketones and nitriles, their guests are coordinated to the Cu(II) axial sites instead of the PF6
anions. In contrast, Lewis base guests such as r 2011 American Chemical Society

alcohols and ethers cannot displace the PF6
anions from the Cu(II) axial sites. That is, 1 exhibits a high selectivity independent of the guests’ overall sizes. Such selectivity for specific Lewis base guests is attributed to steric crowding not only around the Cu(II) axial space but also around coordinated atoms of guests. Each pyridine ring of bpetha ligands is not coplanar to the basal plane of the Cu(II) ions. This is due to steric hindrance between neighboring pyridine rings, resulting in restricted coordination space at the axial sites. The surrounding of coordinated atoms with sp3 hybrid orbitals in alcohols and ethers are more crowded than those with sp2 and sp1 hybrid orbitals in ketones and nitriles, respectively. Hence, sterically crowded alcohols and ethers cannot approach the sterically restricted axial space.2c On the basis of these results, we formed the following hypothesis: if coordination space at the Cu(II) axial sites is broadened by a modification of the framework, guests with sp3 hybrid orbitals can easily approach and interact with the Cu(II) axial sites. On the basis of this background, we attempted to control the geometry of the coordination framework of 1 by adding another building block, 2,20 -bipyridine (2,20 -bpy). The rational construction of frameworks with many types of building units is an important method to control host
guest functionalities precisely. Received: January 27, 2011 Revised: April 15, 2011 Published: May 03, 2011 2379

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Figure 1. Reversible adsorption/desorption of Lewis base guests with structural changes in [Cu(PF6)2(bpetha)2]n (1, top) and {[Cu(PF6)(bpetha)(2,20 bpy)] 3 PF6}n (2, bottom).

For example, solid solutions of porous coordination polymers with more than two types of organic ligands and biporous coordination polymers with exchangeable cartridge molecules enable the finetuning of the channel environments and adsorption properties.4 It is known that the 2,20 -bpy molecule acts as a terminal ligand occupying the cis position, thereby forming zigzag chain structures with a combination of linear bridging ligands.5 The 2,20 -bpy ligand also provides a less crowded coordination space at the Cu(II) axial sites than 4,40 -bipyridine and its derivative ligands (bpetha, 1,2bis(4-pyridyl)ethene, etc.). In contrast to 4,40 -bipyridine and its derivative ligands, each pyridine ring of which is not coplanar to the basal plane of the Cu(II) ions for the above-mentioned reason, the 2,20 -bpy ligand is coplanar to the Cu(II) basal plane, hence affording wider coordination space at the axial sites. Here, we report on the synthesis, crystal structure, and adsorption properties of the one-dimensional Cu(II) coordination polymer {[Cu(bpetha)(2,20 -bpy)(MeOH)] 3 2PF6}n (2⊃MeOH; Figure 1), and demonstrate that its desolvated form 2 exhibits smooth adsorption for EtOH, MeOH, and H2O vapors with sterically crowded coordinated atoms. A detailed comparison of the properties of 2 with those of 1 is discussed.

’ EXPERIMENTAL SECTION Materials. All reagents and chemicals were obtained from commercial sources. Polymer 1 was synthesized according to the literature.2a

Synthesis of {[Cu(bpetha)(2,20 -bpy)(MeOH)] 3 2PF6}n (2⊃MeOH). A microcrystalline sample was synthesized as follows. KPF6 (735 mg, 4 mmol) was added to a MeOH solution (10 mL) containing Cu(ClO4)2 3 6H2O (371 mg, 1 mmol). The obtained suspension was stirred for 30 min and then filtered. The obtained sky-blue filtrate was slowly added to a MeOH solution (10 mL) containing 2,20 -bpy (156 mg, 1 mmol) and bpetha (184 mg, 1 mmol). The obtained sky-blue microcrystals were filtered, washed with MeOH, and dried at room temperature under vacuum. Coordinated MeOH guests were rapidly released from the microcrystals. The resulting desolvated form 2 adsorbed atmospheric H2O molecules. Yield: 548 mg, 77%. Calcd for C22H22Cu1F12N4O1P2 (2⊃H2O): C, 37.12; H, 3.11; N, 7.87. Found: C, 37.17; H, 3.27; N, 7.87. Single crystals of 2⊃MeOH suitable for X-ray diffraction analysis were prepared using the standard diffusion method, in an H-shaped cell (∼50 mL). X-ray Structural Analysis. X-ray diffraction measurements of 2⊃MeOH were performed using a Rigaku RAXIS-RAPID imaging plate diffractometer, using graphite-monochromated Mo KR radiation (λ = 0.71073 Å). The data were corrected for Lorentz and polarization effects. The structure was solved using direct methods (SIR2004)6 and expanded using Fourier techniques.7 All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were refined using the riding model. The refinements were carried out using full-matrix least-squares techniques on F2. All calculations were performed using the CrystalStructure crystallographic software package.8 The crystallographic data in CIF format are available from the Cambridge Crystallographic Data Centre, CCDC reference number 805055. 2380

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Crystal Growth & Design Crystal Data for 2⊃MeOH. C23H24CuF12N4OP2, FW = 725.94, monoclinic P21/n, T = 173 K, a = 12.8912(6) Å, b = 13.1722(5) Å, c = 17.1140(7) Å, β = 96.0184(14)°, V = 2890.0(2) Å3, Z = 4, Dcalc = 1.668 g 3 cm
3, F000 = 1460.00, 2θmax = 54.8°. Final R1 = 0.0442 (I > 2.00σ(I)), wR2 = 0.0587 (I > 2.00σ (I)), GOF = 1.209 for 447 parameters and a total of 26906 reflections, 6544 unique (Rint = 0.054). μ = 9.699 cm
1. Fmax = 0.65 e/Å3, and Fmin =
0.48 e/Å3. Physical Measurements. Elemental analysis (C, H, and N) was performed using a PerkinElmer model 240C elemental analyzer. Infrared (IR) spectroscopy measurements were carried out using a KBr disk on a Nicolet 6700 FT-IR spectrophotometer, with a resolution of 4 nm. Temperature-dependent IR spectra were measured using a high-temperature transmission cell, HT-32 (Thermo Spectra-Tech). The neat sample was set between two NaCl disks. X-ray diffraction (XRD) data were collected using a Rigaku RINT-Ultima III diffractometer employing Cu KR radiation. Temperature-dependent XRD data were collected using a Rigaku Ultima IV diffractometer employing Cu KR radiation and equipped with a temperature-controlled attachment. Thermogravimetric analysis and differential thermal analysis (TG
DTA) were performed using a Rigaku Thermo Plus 2/TG-DTA8120 over the temperature range 25
500 °C under an N2 atmosphere. The adsorption and desorption isotherms for N2 (195 K), CO2 (195 K), and MeOH (298 K) were recorded on a BELSORP-max volumetric adsorption instrument (BEL Japan, Inc.). The adsorption and desorption isotherms for EtOH and H2O at 298 K were measured with a BELSORP-aqua volumetric adsorption equipment (BEL Japan, Inc.). Prior to adsorption/desorption measurements, the sample was heated at 373 K under vacuum overnight. Although we have measured the adsorptions of H2O and MeOH vapors in 1,2c they were repeated under the same conditions as used for 2 because the gate-opening adsorption behavior is very sensitive to the conditions.

’ RESULTS AND DISCUSSION Figure 2 shows the crystal structure of 2⊃MeOH. Figure 2a indicates that the Cu(II) ion has a distorted square pyramidal environment with four nitrogen atoms from one 2,20 -bpy ligand, and two bpetha ligands in the basal plane and one MeOH molecule at the axial site (Cu
O distance = 2.454(2) Å). In this crystal, there are two types of PF6
anions: one (A) is near the Cu(II) axial site, and the other (B) is completely isolated and disordered. Although the remaining axial site is capped by the PF6
anion A, the Cu 3 3 3 F distance of 2.960(2) Å is clearly more than the sum of the van der Waals radii (2.87 Å). Therefore, the PF6
anion A interacts with the Cu(II) ion only through an electrostatic force. The coordination state of the PF6
anions can be also confirmed by a comparison of P
F bond distances. We found that the distances of P
F bonds whose F atoms weakly coordinate to the Cu(II) ions (1.622(2)
1.632(2) Å) are clearly longer than those of other P
F bonds (1.564(2)
1.598(2) Å) in the porous assembly of coordination complex [Cu(PF6)2(4methylpyridine)4].9 On the other hand, all P
F bond distances of the PF6
anion A in 2⊃MeOH range from 1.565(2) to 1.598(2) Å, and a similar trend to [Cu(PF6)2(4-methylpyridine)4] is not observed. Hence, it should be noted that the PF6
anion A of 2⊃MeOH undoubtedly exists in the noncoordinated state. On the other hand, the PF6
anion B interacts with the coordinated MeOH molecule via a moderate hydrogen bond (F 3 3 3 O distance = 2.89(1) Å). The bpetha ligands with an anti conformation bridge the [Cu(2,20 -bpy)] units from cis equatorial sites to form the one-dimensional zigzag chain structure as shown in Figure 2b. There are two types of orientations in one-dimensional zigzag chains (Figure 2c): one (green) extends

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along the a þ b axis and the other (pink) along the a
b axis. Each chain assembles as shown in Figure 2d, and coordinationfree PF6
anions are located in a space between the chains. No π
π and hydrogen-bonding interactions are observed between the chains. We previously determined the crystal structure of {[Cu(bpetha)2(acetone)2] 3 2PF6}n (1⊃2acetone) with the one-dimensional doubly linked chain motif.2a Figure 3 shows the structure around the Cu(II) center in 2⊃MeOH and 1⊃2acetone. In the case of 1⊃2acetone, each pyridine ring of coordinated bpetha ligands is not coplanar to the Cu(II) basal plane (dihedral angles between the N4 plane and the pyridine ring, 55°), and therefore, axial coordination space becomes sterically crowded. In fact, coordinated acetone molecules have a constrained coordination state with the Cu—OdC bond angle of 180°. In contrast, the 2,20 -bpy ligand of 2⊃MeOH is nearly coplanar to the Cu(II) basal plane (dihedral angles between the N4 plane and the pyridine ring, 11° and 13°), affording wider coordination space at the axial sites. Thereby, the MeOH molecules can coordinate to the Cu(II) axial site in a normal coordination state with the Cu
O
C bond angle of 126.3(2)°, which is similar to the values for other Cu(II) complexes having wide coordination space (125
133°).10 As mentioned in the Experimental Section, 2⊃MeOH easily loses its guest MeOH molecule and takes in atmospheric H2O guests, forming 2⊃xH2O. Results of the TG
DTA study of 2⊃xH2O suggest the release of guest H2O molecules in the temperature range 25
80 °C and that the resultant desolvated solid 2 is stable up to ∼170 °C, without further weight loss (see Supporting Information). The powder XRD pattern of the desolvated solid 2 shows sharp lines with shifting of some Bragg peak positions in comparison to the as-synthesized framework 2⊃xH2O and the simulated 2⊃MeOH from singlecrystal analysis (see Supporting Information), suggesting no destruction, but structural change of the framework upon desolvation. Temperature-dependent IR spectra of 2⊃xH2O were measured to obtain further insight into the structural transformation. We checked the region associated with PF6
anions (see Supporting Information), because it has been reported that vibration bands of a PF6
anion change depending on its coordination state.11 In 2⊃xH2O, a very strong ν3 vibration band of the PF6
anions is observed at around 840 cm
1. On the other hand, the corresponding band in the desolvated 2 is seen to be split into two peaks, separated by about 15 cm
1, suggesting that the PF6
anion B approaches the vacant Cu(II) axial site formed after removal of guest H2O molecules. As a result, the chemical formula of 2 can be described as {[Cu(PF6)(bpetha)(2,20 -bpy)] 3 PF6}n. Interestingly, the sensitivity for moisture in the desolvated 1 and 2 differs. The desolvated solid 1 is stable in the atmosphere, while the desolvated solid 2 quickly adsorbs H2O vapor, resulting in a color change from purple to bluish purple. Such a quick response for H2O vapor is caused by the presence of wide coordination space at the Cu(II) axial site (vide infra). The gas adsorption/desorption isotherms for 2 were measured and compared with 1. Neither coordination polymer shows adsorption for N2 at 195 K, indicating the absence of permanent pores. In contrast, CO2 gas is adsorbed to 1 and 2 at 195 K (see Supporting Information). Hence, the structural change from a closed to an open form accompanies the CO2 adsorption in 2. Moreover, because 2 has similar structural components to 1, it is likely that the polar PF6
anions interact with adsorbed CO2 gas. 2381

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Figure 2. Crystal structure of 2⊃MeOH: (a) the coordination environment around the Cu(II) ion, and (b) the zigzag chain structure in the projection along the c-axis (PF6
anions are omitted for clarity). In both panels, the hydrogen atoms are omitted for clarity. (c) The arrangement of two onedimensional zigzag chains, and (d) the packing structure viewed along the a axis. The chains running along the a þ b and a
b axes are colored green and pink, respectively. The vermilion, blue, gray, red, gold, and orange colors represent copper, nitrogen, carbon, oxygen, fluorine, and phosphorus, respectively.

Furthermore, to evaluate the difference in expanse of coordination space for 1 and 2, adsorption measurements for vapors were performed. Figure 4 shows the adsorption and desorption isotherms for EtOH, MeOH, and H2O at 298 K. In the case of 1,

which has a sterically crowded coordination space, the adsorption of EtOH guest is prevented due to the steric hindrance at the coordination space of Cu(II) axial sites.2c It is noteworthy that EtOH is undoubtedly adsorbed to 2, in which the gate-opening 2382

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Figure 3. Comparison of coordination space around the Cu(II) center in (a) 2⊃MeOH and (b) 1⊃2acetone. The coordinated guest molecules are omitted.

Figure 4. Adsorption (filled symbols) and desorption (open symbols) isotherms for EtOH (red), MeOH (blue), and H2O (green) vapors in 1 (circle) and 2 (square) at 298 K.

pressure is P/P0 ≈ 0.06. The point at which the increase in adsorption becomes gradual is about 1.0 mol 3 mol
1, whose value corresponds to one of the up and down Cu(II) axial sites. Furthermore, the desorption does not occur in the measurement range, because EtOH guests are strongly captured via coordination bonds. Therefore, these results suggest that the smooth adsorption is successfully achieved by the formation of wide coordination space at the Cu(II) axial site and one of the up and down Cu(II) axial sites, which may be capped by the PF6
anion B, is available for a guest capture. Similar observations were made for MeOH and H2O vapors: the gate-opening pressures change considerably, from P/P0 = ∼0.9 (1) to ∼0.007 (2) for MeOH and ∼0.8 (1) to ∼0.07 (2) for H2O. In the case of MeOH, the adsorbed amounts after gate-opening are ca. 2 and 1 mol 3 mol
1 for 1 and 2, respectively. These values are in good agreement with the expected number of axial Cu(II) sites available for a guest capture. Furthermore, the desorption behavior for MeOH is similar to that for EtOH in 2. Hence, we can conclude that EtOH and MeOH are adsorbed to one of the up and down Cu(II) axial sites in 2.

Adsorption of H2O vapor in 2 was further investigated by TG
DTA, XRD, and IR measurements. The H2O amount adsorbed of more than 1 mol 3 mol
1 is observed from the adsorption isotherms in 2 (ca. 1.8 mol 3 mol
1) and TG
DTA curves of 2 exposed to H2O vapor, and the XRD pattern of the exposed compound 2⊃xH2O is very similar to simulated 2⊃MeOH from single-crystal analysis. In the case of 1, both the up and down Cu(II) axial sites can catch Lewis base guests such as acetone, 2-butanone, and acetonitrile, resulting in an amount of ca. 2 mol 3 mol
1 being absorbed.2c However, 1 exceptionally adsorbs more than 2 mol 3 mol
1 H2O (ca. 3.6 mol 3 mol
1), indicating the presence of two types of adsorbed H2O molecules. Considering that one of the up and down Cu(II) axial sites is related to the EtOH and MeOH adsorption, the adsorbed H2O molecules are located at two types of adsorption sites in 2. In fact, the IR spectra of 2⊃xH2O show two types of δ(H2O) vibrations, at ca. 1660 and 1615 cm
1, both of which disappear after heating at 373 K (see Supporting Information). The band at 1660 cm
1 is unusually high for the bending mode of H2O (free H2O molecules in an inert solvent have δ(H2O) vibrations at 1598 cm
1).12a Such a blue shift is only observed when the H2O molecule is a strong hydrogen donor.12 The low value of the bending mode is related to the electron donor character of H2O, typical for H2O molecules interacting via the oxygen atom.13 Hence, these results suggest that one of the up and down Cu(II) axial sites is used for capture of H2O molecules and other H2O molecules interact with the coordinated H2O via the oxygen atom. The larger amount adsorbed for a H2O guest than for EtOH and MeOH guests may be related to the difference in their molecular sizes (cross-sectional areas of H2O, MeOH, and EtOH are 12.5, 21.9, and 28.3 Å2, respectively).14 Finally, we estimated the overall adsorption/desorption states in 2, as shown in Figure 5. Removal of guest MeOH molecules from the as-synthesized sample 2⊃MeOH affords the desolvated 2, in which the coordination-free PF6
anion B approaches the vacant Cu(II) site. When 2 is exposed to EtOH, MeOH, or H2O vapor, the one-sided Cu(II) axial site that is probably blocked by the PF6
anion B becomes an adsorption site for the guests. In all states depicted in Figure 5, the PF6
anion A remains in its original position. 2383

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Figure 5. Overall adsorption/desorption states in 2. The PF6
anions A and B are colored blue and red, respectively.

’ CONCLUSION We report the rational design and synthesis of the onedimensional Cu(II) coordination polymer {[Cu(bpetha)(2,20 bpy)(MeOH)] 3 2PF6}n (2⊃MeOH) using a 2,20 -bpy ligand as a space-controlling unit. This coordination polymer features wide coordination space at the Cu(II) axial sites derived from the planar and chelating 2,20 -bpy ligand. Thereby, its desolvated form [Cu(PF6)(bpetha)(2,20 -bpy)] 3 PF6}n (2) exhibits smooth adsorption for EtOH, MeOH, and H2O vapors compared with the prototype [Cu(PF6)2(bpetha)2]n (1) with restricted Cu(II) axial space. The temperature-dependent XRD and IR results suggest that the coordination mode of one PF6
anion in 2 changes in association with adsorption/desorption of guest molecules, reminiscent of the behavior of 1. The H2O adsorption behavior illustrates that one of the up and down Cu(II) axial sites catches a H2O molecule and extra H2O molecules form hydrogen bonds with the coordinated H2O via their own oxygen atoms. Further work to regulate the guest adsorption properties of Cu(II) coordination polymers is currently under way. ’ ASSOCIATED CONTENT

bS

Supporting Information. Selected bond distances for 2⊃MeOH, TG
DTA curves of 2⊃xH2O, XRD patterns of 2⊃xH2O under various conditions, temperature-dependent IR spectra of 2⊃xH2O, adsorption/desorption isotherms for N2 gas in 2 at 195 K, adsorption/desorption isotherms for CO2 gas in 1 and 2 at 195 K, and CIF file. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Mailing address: Research Institute for Electronic Science, Hokkaido University, N20W10, Kita-ku, Sapporo 001-0020,

Japan. Tel: þ81-11-706-9418. Fax: þ81-11-706-9420. E-mail addresses: [email protected] and [email protected].

’ ACKNOWLEDGMENT We would like to thank Ms. T. Hattori and Ms. M. Kiuchi (Instrumental Analysis Division, Equipment Management Center, Creative Research Institution, Hokkaido University) for carrying out elemental analyses. We thank Prof. H.-C. Chang (Faculty and Graduate School of Science, Hokkaido University) for the temperature-dependent XRD measurements. This work is supported by a Grant-in-Aid for Scientific Research, Young Scientists (B) (22750114) from MEXT, Japan. ’ REFERENCES (1) (a) Noro, S. Phys. Chem. Chem. Phys. 2010, 12, 2519. (b) Nomoto, K.; Kume, S.; Nishihara, H. J. Am. Chem. Soc. 2009, 131, 3830. (c) Kim, L. H.; Okubo, T.; Tanaka, N.; Mimura, N.; Maekawa, M.; Kuroda, T. Chem. Lett. 2010, 39, 792. (d) Kanoo, P.; Madhu, C.; Mostafa, G.; Maji, T. K.; Sundaresan, A.; Pati, S. K.; Rao, C. N. R. Dalton Trans. 2009, 5062. (e) Aum€uller, A.; Erk, P.; Klebe, G.; H€unig, S.; Sch€utz, J. U.; Werner, H.-P. Angew. Chem., Int. Ed. 1986, 25, 740. (f) Kato, R.; Kobayashi, H.; Kobayashi, A. J. Am. Chem. Soc. 1989, 111, 5224–5232. (g) Kitagawa, H.; Nagao, Y.; Fujishima, M.; Ikeda, R.; Kanda, M. Inorg. Chem. Commun. 2003, 6, 346. (h) Zhang, W.; Xiong, R.-G.; Huang, S. D. J. Am. Chem. Soc. 2008, 130, 10468. (i) Wu, H.; Simmons, J. M.; Liu, Y.; Brown, C. M.; Wang, X.-S.; Ma, S.; Peterson, V. K.; Southon, P. D.; Kepert, C. J.; Zhou, H.-C.; Yildirim, T.; Zhou, W. Chem.—Eur. J. 2010, 16, 5205. (h) Wang, X.-S.; Ma, S.; Forster, P. M.; Yuan, D.; Eckert, J.; Lopez, J. J.; Murphy, B. J.; Parise, J. B.; Zhou, H.-C. Angew. Chem., Int. Ed. 2008, 47, 7263. (i) Takaishi, S.; Hosoda, M.; Kajiwara, T.; Miyasaka, H.; Yamashita, M.; Nakanishi, Y.; Kitagawa, Y.; Yamaguchi, K.; Kobayashi, A.; Kitagawa, H. Inorg. Chem. 2009, 48, 9048. (j) Fielden, J.; Sprott, J.; Long, D.-L.; K€ogerler, P.; Cronin, L. Inorg. Chem. 2006, 45, 2886. (k) Abourahma, H.; Moulton, B.; Kravtsov, V.; Zaworotko, M. J. J. Am. Chem. Soc. 2002, 124, 9990. 2384

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