Local Structure and Xenon Adsorption Behavior of Metal−Organic

Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, ... Herbert C. Hoffmann , Bassem Assfour , Fanny Epperlein , Nicole K...
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J. Phys. Chem. C 2007, 111, 1524-1534

Local Structure and Xenon Adsorption Behavior of Metal-Organic Framework System [M2(O2CPh)4(pyz)]n (M ) Rh and Cu) As Studied with Use of Single-Crystal X-ray Diffraction, Adsorption Isotherm, and Xenon-129 NMR Takahiro Ueda,*,†,‡ Kenji Kurokawa,† Taro Eguchi,†,‡ Chihiro Kachi-Terajima,§ and Satoshi Takamizawa*,§ Department of Chemistry, Graduate School of Science, Osaka UniVersity, Toyonaka, Osaka 560-0043, Japan, The Museum of Osaka UniVersity, Toyonaka, Osaka 560-0043, Japan, and International Graduate School of Arts and Sciences, Yokohama City UniVersity, Kanagawa 236-0027, Japan ReceiVed: August 17, 2006; In Final Form: NoVember 7, 2006

The local structure and xenon adsorption behavior of a metal-organic framework system [M(II)2(bza)4(pyz)]n (bza and pyz ) benzoate and pyrazine, M ) Rh (1a) and Cu (1b)) were investigated by using single-crystal X-ray diffraction, xenon adsorption isotherm, and 129Xe NMR measurements. Single-crystal X-ray diffraction analysis revealed that rare gas atoms were accommodated in a one-dimensional (1D) nanochannel with the dimer structure. Xenon adsorption reached saturation at the xenon uptake of 1.93 Xe per molecular unit in 1a and of 1.85 Xe in 1b, suggesting accommodation of two Xe atoms per host formula unit. Analysis of the xenon adsorption isotherm based on the Fowler-Guggenheim equation gave the xenon-xenon interaction and the isosteric heat of adsorption. The 129Xe NMR spectrum suggested that the environment of the adsorption site for xenon in 1a is very tight and anisotropic. Furthermore, the temperature dependence of the 129Xe chemical shift was explained by using xenon loading, supporting the xenon dimer as the local structure of xenon in 1a. These aspects revealed that cooperative adsorption of xenon to 1a and 1b occurs according to the xenon-xenon interaction. Xenon is stabilized in the nanochannel through formation of a dimer structure.

Introduction In recent years, metal-organic frameworks have received considerable attention as new and attractive materials for gas storage, ion exchange, and use as potential catalysts. Many metal-organic frameworks, which reversibly occlude large amounts of gases such as N2, Ar, O2, CH4, and Xe, have been reported.1-7 Recently, some analogues have been used as single-crystal adsorbents. They provide a new observation field for light gas aggregation in homogeneous nanochannels. This approach is expected to accelerate investigation of the physicochemical properties of specific aggregate structures that are produced within the host solids. The empty crystal hosts of [M(II)2(bza)4(pyz)]n (bza and pyz ) benzoate and pyrazine, M ) Rh(1a) and Cu (1b)) have been reported by Takamizawa et al. to be ideal model compounds for observation of light gas arrangements.8,9 These compounds can accommodate some light gases such as H2, O2, CO2, N2O, CS2, CH4, and C2H5OH. The guest molecular arrangement in the one-dimensional (1D) nanochannel has been determined by using single-crystal X-ray diffraction.10-17 Observation of light aggregates with X-ray diffraction is only possible under conditions with sufficient periodicity of guest molecules. Such a situation is realized when adsorption of the guest molecule reaches saturation. However, in many cases, the guest molecules have no longrange order at the midway filling of gases under the adsorption * Address correspondence to this author. T.U.: Phone: +81-6-68505778. Fax: +81-6-6850-5785. E-mail: [email protected]. S.T.: Phone/fax: +81-45-787-2187. E-mail: [email protected]. † Osaka University. ‡ The Museum of Osaka University. § Yokohama City University.

process. The local structure of guest molecules in the nanochannel is important information to elucidate the guest-host and guest-guest interactions of adsorption processes. For such cases, NMR is a more suitable tool to examine the local structure of guest molecules than the X-ray diffraction method. Xenon-129 NMR spectroscopy is an excellent means to study the pore structure of porous materials.18-22 Recent theoretical investigations into the origin of the chemical shielding of xenon in the nanospace have yielded microscopic information such as the local structure surrounding a xenon atom trapped in a void space, through the temperature and the loading dependence of the 129Xe chemical shift.23-26 The local structure of xenon adsorbed in nanospace is crucial knowledge for elucidating the host-guest intermolecular interaction along with the adsorption behavior of the host materials. Especially, xenon undergoes socalled single-file diffusion (SFD) in a narrow 1D channel with a diameter smaller than the size of twice the van der Waals diameter of xenon.27,28 In such a system, the xenon-xenon interaction is restricted to the channel axis direction (1D), thereby simplifying the description of chemical shielding. In a previous study, we examined Xe-Xe interatomic interaction confined in a 1D nanochannel (()-[Co(en)3]Cl3 crystal through the 129Xe chemical shift tensor and its anisotropy.29 Chemical shift anisotropy has also been observed in other SFD systems such as AlPO and SAPO,24,30-33 TPP,34-36 and dipeptide37 crystals, in which symmetry of the channel cross section, the local structure, and diffusivity of the confined xenon atom in the channel were described based on Xe-Xe interatomic interaction. For the present study, we carried out single-crystal X-ray diffraction analyses on 1a and 1b with rare gases such as Ar, Kr, and Xe to characterize the adsorption structure of rare gases

10.1021/jp065321x CCC: $37.00 © 2007 American Chemical Society Published on Web 01/03/2007

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TABLE 1: Crystallographic Data for Single-Crystal Host 1a Under the Condition of Forcible Adsorption complex phase empirical formula crystal size/mm3 M/g mol-1 crystal system space group T/K a/Å b/Å c/Å R/deg β/deg γ/deg V/Å3 Z Dcalcd/Mg m-3 µ(Mo KR)/mm-1 no. of reflecns collected no. of independent reflecns (Rint) goodness of fit R1 (I > 2σ (all data)) wR2 (I > 2σ (all data)) least diff peak (hole)/e Å-3

1a‚1.15(Ar) gas (ca. 10.0 MPa) C32H24Ar1.15N2O8Rh2 0.25 × 0.20 × 0.04 816.29 triclinic P1h 298 9.588(2) 10.322(3) 10.861(3) 72.139(5) 64.783(5) 62.722(5) 856.0(4) 1 1.584 1.126 3229 1778 (0.0511) 1.015 0.0606 (0.1034) 0.1399 (0.1777) 0.806 (-1.018)

1a‚1.51(Kr) gas (ca. 4.8 MPa) C32H24Kr1.51N2O8Rh2 0.20 × 0.16 × 0.04 896.89 triclinic P1h 298 9.587(2) 10.364(2) 10.822(2) 71.867(5) 65.518(5) 63.237(5) 863.5(3) 1 1.725 2.927 5001 3029 (0.0913) 0.942 0.0840 (0.1787) 0.1705 (0.2334) 1.370 (-1.371)

1a‚2(Xe) gas (ca. 3.0 MPa) C32H24N2O8Rh2Xe2 0.30 × 0.21 × 0.05 1032.95 triclinic P1h 298 9.578(8) 10.408(10) 10.837(10) 70.593(19) 65.599(18) 63.48(2) 865.8(14) 1 1.981 2.927 5030 3063 (0.1478) 0.921 0.1244 (0.2406) 0.2874 (0.3415) 1.839 (-2.275)

in 1a and 1b. We also performed an Xe adsorption isotherm measurement for both compounds to characterize the adsorption behavior of xenon. Furthermore, 129Xe NMR spectra were measured to examine the local structure of xenon confined in 1a under static and magic-angle sample spinning conditions. The temperature dependence of the 129Xe NMR spectrum was also conducted to obtain contributions of the xenon-wall and xenon-xenon interactions to the chemical shielding. The local structure of the confined xenon atoms was evaluated based on the temperature dependence of the chemical shift tensor for xenon confined in the nanochannel constructed in 1a.

complex and 0.18175 g of Cu complex) was mounted on a sample stainless-steel basket. Before measurements, the system was evacuated for 2 h at 5 × 10-1 Pa. Adsorption isotherms were measured at 298 K at pressures of 0.05-4.8 MPa. The weight measurement resolution of the magnetic balance was (10-5 g. Equilibrium weights were achieved within 30 min. The observed change in the sample weight, ∆w, generally indicated the absolute amount of adsorbates, aG, according to the following relationship:38

Experimental Section

where m is the mass of an adsorbent sample, F and Fs are the density of the bulk gas and the picnometric density of an adsorbent, W0 is the pore volume, and V0 is the volume of the sample holder. The pore volume accessible by free gas is negligible in each metal complex because the 1D nanochannel is regarded to form a single-file diffusion system in which only the adsorbed xenon and argon atoms occupy the nanospace. Therefore, the second term, mW0F, on the right-hand side of eq 1 is less affected by the change in the sample weight. Consequently, only the correction of buoyancy attributable to the third term on the right-hand side of eq 1 indicates the absolute amount of adsorbate. The resultant isotherms for xenon gas were analyzed based on a two-dimensional (2D) lattice gas model,39 whereas the isotherms for argon gas were approximately fitted by the Langmuir equation. We carried out 129Xe NMR measurements using Bruker MSL200 and DSX-200 spectrometers. The temperature dependence of the static 129Xe NMR spectrum was measured by using the MSL200 spectrometer at a frequency of 55.4 MHz at 123295 K. The free induction decay (FID) signals were recorded by using the single pulse method with a delay time of 5 s and a pulse width of 5 µs for π/2 pulse. Temperatures were controlled to within (1 K by using a controller unit (Bruker BVT-1000). Magic-angle sample spinning (MAS) NMR spectra were measured with the DSX-200 spectrometer with a Larmor frequency of 55.3 MHz. That device is equipped with a pneumatic MAS control unit. The FID signal using the MAS probe under the static condition was recorded with a spinecho pulse sequence (90°-τ-180°-τ-acquisition) to avoid artifactual distortions of the powder pattern, in which the time

Empty single-crystal hosts [M(II)(bza)4(pyz)]n (M ) Rh (1a) and Cu (1b)) were synthesized with use of a method described in the literature.8,9 Single-crystal X-ray diffraction data were obtained with a Bruker SMART APEX CCD area (graphite-monochromated Mo KR radiation (λ ) 0.71073 Å)) with a nitrogen-flow temperature controller. Single crystals were sealed inside a thick-walled glass capillary with liquid rare gases of Ar, Kr, and Xe condensed in a liquid nitrogen bath. The inner pressure of each gas was 3.010.0 MPa at room temperature. Data collection was carried out at 298 K for all gas-inclusion crystals. Empirical absorption corrections were applied by using the SADABS program. The structures were solved with direct methods (SHELXS 97) and refined with full-matrix least-squares calculations on F2 (SHELXL-97), using the SHELX-TL program package. Nonhydrogen atoms were refined anisotropically. Hydrogen atoms were fixed at calculated positions and refined by using a riding model. Crystallographic data of the structural determination collected under different conditions are summarized in Table 1 for 1a and in Table 3 for 1b. High-pressure xenon and argon adsorption isotherms were measured with use of a high sensitivity magnetic suspended gravimetric system (Rubotherm Pra¨zisionsmesstechnik GmbH, Bochum, Germany). High-purity-grade xenon and argon gases were purchased from Japan Air Gases Ltd. A known amount of metal complex was heated to 100 °C under reduced pressure (P < 2 Pa) for 2 h before mounting on the sample basket. The pretreated sample (Xe adsorption: 0.09935 g of Rh complex and 0.12424 g of Cu complex; Ar adsorption: 0.11203 g of Rh

∆w ) maG - m(W0 + 1/FS)F - V0F

(1)

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delay between the first and second pulses is 100 µs. Under the sample spinning condition, the FID signals were recorded by using the single-pulse sequence (90°-acquisition). All measurements with the MAS probe were carried out at room temperature. Magic-angle sample spinning was achieved with a 1.1 kHz spinning rate. The spinning rate is controlled within the experimental error of (10 Hz. The powdered specimen was sealed in a glass ampule with Xe gas after evacuation at 373 K for 2 h. In the ampule, the respective equilibrium pressures of xenon are ca. 0.2 MPa for the variable-temperature experiment and 0.3 MPa for the MAS NMR experiment at room temperature. For MAS NMR measurement, a glass ampule was inserted into the ZrO2 rotor. The 129Xe chemical shift was inferred from a signal from bulk xenon gas at zero density.40 Results and Discussion Crystal Structure under Forcible Adsorption State. Structures of these empty crystal hosts were described in detail in refs 8 and 9. We determined the crystal structures of 1a and 1b including the rare gases (Ar, Kr, Xe) using single-crystal X-ray analyses, even at room temperature with pressurized gas. The ORTEP drawings and packing diagrams are shown in Figure 1 for 1a and in Figure 4 for 1b. All gas-inclusion crystals of 1a had undergone the same phase transition from an empty R to inclusion β phase at 298 K, as we observed previously in the gas-adsorbed crystals of 1a.12,13,16 In the P1h phase, the connected 1D channels were generated by host-lattice expansion because of its flexibility (Figures 2 and 3). Although the gas-inclusion crystal structure shows a similar arrangement of rare gases in these channels, the observed amounts of molecules differ from one another. The gas-inclusion crystal was determined as 1a‚1.15(Ar), 1a‚1.51(Kr), and 1a‚ 2.0(Xe) at 298 K. The number of guest molecules increases in the order of Ar, Kr, and Xe. The order of the adsorption amount is explainable by using the ease of condensation of guest gases. The result indicates that the gas adsorption equilibrium can be achieved through smooth diffusion of rare gases in single-crystal 1. The short contacts and relevant distances are listed in Table 2. Figure 3 shows that the adsorbed gas is stabilized in the cavity in the same manner throughout this series. The guest gas is surrounded by the three benzoate planes with the shortest interatomic distance (guest‚‚‚C(9)benzoate) of 3.61(2) Å for 1a(Ar), 3.630(13) Å for 1a(Kr), and 3.731(18) Å for 1a(Xe). Close contacts are also found in the guest‚‚‚H(5) (benzoate edge) with 3.082(17) Å for 1a(Ar), 3.256(4) Å for 1a(Kr), and 3.343(4) Å for 1a(Xe). Within the pockets, the adsorbed gases locate in separate positions, forming isolated pairs between the pockets. That process is observed as the guest size increases: the closest guest-guest contact is apparent for the crystal including the largest gas Xe, indicating that xenon dimer is a more favorable structure in the cavity under the forcible condition. The adsorbed xenon is packed in pockets at both sides of the neck, thereby surrounding the corner surface. The xenon atoms are captured by the π-orbital of the orthogonal conjugated planes of the benzoate moieties with atomic distances of ca. 3.72-4.50 Å through xenon-π interaction, as presented in Figure 3f. Consequently, the xenon dimer is stabilized in the cavity by xenon-π interaction and by interatomic interaction among xenon atoms. The R-β phase transition was not observed for 1b under Ar and Kr gas. The gas-inclusion crystals maintain the same space group of monoclinic C2/c. On the other hand, the crystal in a Xe atmosphere displays the phase transition from the C2/c phase to the P1h phase, as well as the gas-inclusion crystals of 1a.

Figure 1. Thermal ellipsoid drawing of the asymmetric units at the 50% probability level with the atom labeling scheme of 1a‚1.15(Ar) (a), 1a‚1.51(Kr) (b), and 1a‚2.0(Xe) (c) at 298 K.

The observed amounts of gases in the crystal under Ar, Kr, and Xe gas are respectively 1.04, 1.72, and 2.0 atoms per Cu2 unit at 298 K. The trend in the adsorbed amount resembles that observed in 1a with guests. A packing diagram is shown in Figures 5 and 6; the close host-guest and guest-guest contacts are summarized in Table 4. In spite of the difference of the phase transition, the guest arrangement in the channel is fundamentally the same over this series. The shortest hostguest distances (guest‚‚‚C(9)benzoate) are 3.493(12) Å for 1b(Ar), 3.60(1) Å for 1b(Kr), and 3.748(12) Å for 1b(Xe). The closest guest‚‚‚H(5) (the benzoate edge) distance is 2.973(8) Å for 1b(Ar), 3.181(2) Å for 1b(Kr), and 3.358(3) Å for 1b(Xe). Compared with the corresponding distances determined in 1a,

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Figure 2. Packing views of crystal 1a with included gases: for Ar (a, d), for Kr (b, e), and for Xe (c, f) at 298 K, showing the projection along the a-axis (top) and b-axis (bottom). Elements are color coded: Rh (magenta), C (gray), H (white), N (blue), O (red), Ar (light-blue), Kr (green), and Xe (pink).

Figure 3. Surface views of channels of 1a with included gases in a space-filling model at 298 K and the environment of adsorbed gas for Ar (a, d), for Kr (b, e), and for Xe (c, f). Elements are color coded: Ar (light-blue), Kr (green), and Xe (pink).

the short contacts between the guest and C(9) atom for 1b(Ar) and 1b(Kr) are shortened from 3.61(2) to 3.493(12) Å and from 3.630(13) to 3.60(1) Å, respectively. The intrapair distance in the neighboring cavity is also decreased from 5.16(3) to 4.596(12) Å for 1b(Ar) and from 4.645(7) to 4.569(2) Å for 1b(Kr). The crystal structure of 1b(Xe) displays opposite trends of hostguest and guest-guest distances, which are probably related to the structural phase transition on this crystal.

Xe and Ar Adsorption Isotherm. Figure 7 shows adsorption isotherms of 1a and 1b for Xe and Ar. Xenon adsorption is accelerated at more than about 0.2 MPa. The isotherms exhibit no hysteresis. Xenon adsorption reaches saturation at more than 2 MPa. Adsorption uptakes of xenon, for example, at 4 MPa correspond respectively to molar ratios of 1.93Xe and 1.85Xe in 1a and 1b. These are consistent with the composition of 1a‚ 2Xe and 1b‚2Xe, as shown in the crystal structure of the β-phase

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TABLE 2: The Host-guest and Guest-guest Short Contacts for 1a: Pertinent Interatomic Distances (Å) 1a‚1.15(Ar)

1a‚1.51(Kr)

1a‚2(Xe)

guest‚‚‚C(1) guest‚‚‚C(2) guest‚‚‚C(5)a guest‚‚‚C(7) guest‚‚‚C(8) guest‚‚‚C(9) guest‚‚‚C(10)b guest‚‚‚C(11)b guest‚‚‚C(13)c guest‚‚‚C(14)c guest‚‚‚H(5)a guest‚‚‚H(10)b guest‚‚‚H(11)b guest‚‚‚H(13)c guest‚‚‚H(14)c

guest‚‚‚host contact 3.744(19) 3.820(13) 3.756(19) 3.847(11) 3.72(2) 3.905(18) 3.96(2) 3.984(14) 3.61(2) 3.680(14) 3.61(2) 3.630(13) 3.84(3) 3.96(2) 3.87(3) 4.02(2) 3.87(2) 3.965(18) 3.71(2) 3.738(17) 3.082(17) 3.256(4) 3.18(2) 3.280(5) 3.23(2) 3.388(5) 3.398(18) 3.631(4) 3.106(17) 3.172(4)

3.928(18) 3.937(17) 4.03(3) 4.01(2) 3.85(2) 3.731(18) 4.00(3) 4.12(3) 4.06(2) 3.85(2) 3.343(4) 3.293(5) 3.511(4) 3.815(4) 3.344(4)

A‚‚‚Ba in cage A‚‚‚B′ c inter cage

guest‚‚‚guest contact 6.21(2) 6.592(6) 5.16(3) 4.645(7)

6.779(8) 4.484(6)

a Symmetric code (1 - x, 1 - y, 1 - z). bSymmetric code (2 - x, -y, 1 - z). cSymmetric code (1 - x, -y, 1 - z).

described above. Analogous isotherms were also observed in Xe and/or CH4 adsorption behavior in a Werner MX2A4 coordination complex such as β-Co(NCS)2(4-MePy)4 and β-Ni(NCS)2(4-MePy)4.41 The stepwise adsorption isotherm in the Werner MX2A4 coordination complex was explained by generation of new sites for adsorption: the first step of adsorption refers to a filling of the cage of fourfold symmetry up to 0.5 mol of the guest molecule per mole of β-sorbent; the second step corresponds to uptake from 0.5-2.0 mol of adsorbate arising from the rearrangement of the guest molecules in the nanochannel. However, it is difficult to consider the existence of multisites for adsorption in the metastable β-form of 1a and 1b because of a 1D nanochannel. In this case, it is likely that the adsorption behavior is analogous to the 2D condensation of the gaseous molecule on the flat surface, although the adsorption of xenon takes place in the 1D nanochannel formed on the bulk crystallites.42 To elucidate adsorption behavior qualitatively from a microscopic viewpoint, a 2D lattice gas model is applicable to describe the isotherm.39 This model is capable of qualitatively interpreting the order-disorder phase transition in the 2D condensed phase. The simplest and crudest treatment based on mean field (BraggWilliams) approximation is useful to verify the outline of the adsorption behavior. It uses the Fowler-Guggenheim equation, which describes the relationship between an equilibrium pressure, p2, and a filling, φ ()wXe/wXe(max)), as

p2 )

(

)

φ 2Zφ p (T) exp 1-φ 0 kBT

(2)

where  is the intermolecular interaction between the nearestneighboring adsorbed atoms, Z is the coordination number of the 2D lattice of adsorption sites, and p0(T) is the characteristic pressure. For 1a and 1b, a 1D nanochannel can accommodate small molecules one-dimensionally; the coordination number of adsorption sites, Z, is 2. Therefore, eq 2 can reproduce the xenon adsorption isotherms as described by the solid lines in Figure 7a,b. The two resultant parameters for reproduction of isotherms,  and p0 (298 K), are listed in Table 5. The positive intermolecular interaction energy indicates the attractive interaction between the nearest-neighbor adsorbed xenon atoms. The evaluated  values agree well with the

Figure 4. Thermal ellipsoid drawing of the asymmetric units at the 50% probability level with atom labeling schemes of 1b‚1.04(Ar) (a), 1b‚1.72(Kr) (b), and 1b‚2.0(Xe) (c) at 298 K.

Lennard-Jones potential parameter for gas-gas interaction: Xe-Xe/kB ) 221 K. When Z ) 2, the critical temperature, Tc, is equal to /kB. In addition, the  values satisfy the condition Tc/T < 1, suggesting that the xenon adsorption at 298 K takes place outside the coexisting region of two phases. In this region, the Gibbs phase rule provides two degrees of freedom. That is, at constant temperature, a given pressure indicates a unique uptake of xenon for 1a and 1b. In other words, 1a and 1b, whose nanochannels are partially filled with xenon, are regarded as single-component. The uptake of xenon changes continuously with equilibrium pressure of xenon over the entire region of filling. Regarding the adsorption process, the interatomic interaction among the adsorbed xenon atoms plays an important role by causing the condensation of xenon into the nanochannels.

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Figure 5. Packing views of crystal 1b with included gases: for Ar (a, d), for Kr (b, e), and for Xe (c, f) at 298 K showing the projection along the a-axis (c) and b-axis (a, b, f) and the [110] direction (d, e). Elements are color coded: Cu (orange), C (gray), H (white), N (blue), O (red), Ar (light-blue), Kr (green), and Xe (pink).

Figure 6. Surface views of channels of 1b with included gases in a space-filling model at 298 K and the environment of adsorbed gas for Ar (a, d), for Kr (b, e), and for Xe (c, f). Elements are color coded: Ar (light-blue), Kr (green), and Xe (pink).

In this condition, the kinetic energy of gaseous xenon (3kBT/2 for a xenon atom) is greater than the interatomic interaction among the adsorbed xenon atoms in the nanochannels. The adsorbed xenon in the nanochannel probably takes the dynamically disordered structure of xenon. The ascending uptake of xenon greater than 0.2 MPa might decrease the degree of disorder of xenon arrangement in the nanochannel because the

free space in which the adsorbed xenon atom can migrate freely is reduced. However, the thermodynamically identified phase, in which xenon is ordered in the nanochannel, is probably achieved below the critical temperature, Tc. Furthermore, p0(T) is related to both the kinetic energy of adsorbate atoms in gaseous phase and the binding energy of the adsorbate atoms, 2, through the following relationship:39

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TABLE 3: Crystallographic Data for Single-Crystal Host 1b Under the Condition of Forcible Adsorption complex phase empirical formula crystal size/mm3 M/g mol-1 crystal system space group T/K a/Å b/Å c/Å R/deg β/deg γ/deg V/Å3 Z Dcalcd/Mg m-3 µ(Mo KR)/mm-1 no. of reflecns collected no. of independent reflecns (Rint) goodness of fit R1 (I > 2σ (all data)) wR2 (I > 2σ (all data)) least diff peak (hole)/e Å-3

1b‚1.04(Ar) gas (ca. 8.6 MPa) C32H24Ar1.04Cu2N2O8 0.60 × 0.17 × 0.08 733.16 monoclinic C2/c 298 18.396(4) 9.702(2) 19.188(4) 90 98.001(5) 90 3391.4(13) 4 1.436 1.407 12261 4215 (0.0848) 1.029 0.0672 (0.1640) 0.1326 (0.1622) 0.501 (-0.475)

1b‚1.72(Kr) gas (ca. 7.1 MPa) C32H24Cu2Kr1.72N2O8 0.12 × 0.06 × 0.04 835.75 monoclinic C2/c 298 18.631(3) 9.7039(13) 19.276(3) 90 99.002(3) 90 3442.1(8) 4 1.613 3.482 9804 3053 (0.1234) 0.970 0.0659 (0.1696) 0.1419 (0.1803) 0.762 (-0.581)

1b‚2(Xe) gas (ca. 3.3 MPa) C32H24Cu2N2O8Xe2 0.60 × 0.18 × 0.06 954.21 triclinic P1h 298 9.740(6) 10.471(7) 10.867(7) 70.741(12) 65.519(12) 63.558(13) 888.7(10) 1 1.783 3.120 5237 3313 (0.1070) 0.963 0.1059 (0.2487) 0.2492 (0.2880) 2.003 (-0.896)

TABLE 4: The Host-Guest and Guest-Guest Short Contacts for 1b: Pertinent Interatomic Distances (Å) 1b‚1.04(Ar)

1b‚1.72(Kr)

guest‚‚‚C(1) guest‚‚‚C(2) guest‚‚‚C(5)a guest‚‚‚C(7) guest‚‚‚C(8) guest‚‚‚C(9) guest‚‚‚C(10)b guest‚‚‚C(11)b guest‚‚‚C(13)c guest‚‚‚C(14)c guest‚‚‚H(5)a guest‚‚‚H(10)b guest‚‚‚H(11)b guest‚‚‚H(13)c guest‚‚‚H(14)c

3.803(9) 3.84(1) 3.698(13) 3.76(1) 3.536(12) 3.493(12) 4.037(11) 4.148(12) 4.032(13) 3.763(12) 2.973(8) 3.316(8) 3.539(8) 3.687(8) 3.196(8)

guest‚‚‚host contact 3.874(9) 3.917(9) 3.874(12) 3.842(9) 3.67(1) 3.60(1) 4.06(1) 4.20(1) 4.06(1) 3.78(1) 3.181(2) 3.327(2) 3.619(2) 3.766(2) 3.205(2)

A‚‚‚Ba A‚‚‚B′ c

6.678(13) 4.596(12)

guest‚‚‚guest contact 6.801(3) 4.569(2)

1b‚2(Xe) guest‚‚‚C(1) guest‚‚‚C(2) guest‚‚‚C(5)c guest‚‚‚C(7) guest‚‚‚C(8) guest‚‚‚C(9) guest‚‚‚C(10)d guest‚‚‚C(11)d guest‚‚‚C(13)e guest‚‚‚C(14)e guest‚‚‚H(5)c guest‚‚‚H(10)d guest‚‚‚H(11)d guest‚‚‚H(13)e guest‚‚‚H(14)e

3.948(15) 3.964(13) 4.05(2) 4.032(16) 3.879(16) 3.748(12) 4.06(2) 4.16(2) 4.087(16) 3.855(14) 3.358(3) 3.358(3) 3.554(2) 3.782(2) 3.328(2)

A‚‚‚Bc A‚‚‚B′ e

6.780(5) 4.513(4)

a Symmetric code (1.5 - x, 1.5 - y, 1 - z). b Symmetric code (1 - x, 2 - y, 1 - z). c Symmetric code (1 - x, 1 - y, 1 - z). d Symmetric code (2 - x, -y, 1 - z). e Symmetric code (1 - x, -y, 1 - z).

p0(T) )

(

)

2πMkBT h2

3/2

( )

kBT exp -

2 kBT

(3)

where M is the molecular mass of adsorbate for the case where xenon M is 131.2 × 10-3 kg. This equation indicates the binding energy of xenon adsorbed in 1D nanochannels by using p0 (298 K), as listed in Table 5, which corresponds to about 38 kJmol-1. Considering , 2, and the kinetic energy of xenon gas, the isosteric heat of adsorption is given as

5 Qads ) -Na 2 + kBT + 2Zθ 2

(

)

(4)

outside the two-phase region.39 This relationship implies isosteric heat of adsorption of ca. 48 kJmol-1 at φ ) 0.5 in both compounds. The enthalpy of vaporization at the boiling point of xenon is 12.6 kJmol-1. Therefore, the estimated value in the isosteric heat of adsorption implies stabilization of about 35 kJmol-1 because of the xenon-wall interaction in the nanochannel of 1a and/or 1b. This stabilization energy might be somewhat

large for physisorption. It is considered that the xenon-π interaction engenders effective stabilization of xenon in addition to emphasizing the potential field between adsorbed xenon atoms and the pore wall because of the narrow channel. These effects cause the deep potential in the nanochannel of 1a and 1b. In contrast, Ar adsorption behavior is quite different from Xe adsorption. The adsorption amount of argon monotonically increases without hysteresis, and the resultant isotherm seems to be the type-I adsorption isotherm classified by IUPAC. In this case, the isotherm will generally obey the Langmuir equation, which is suitable for monolayer adsorption based on the interaction between adsorbent and adsorbate. The interatomic interaction of adsorbates (argon-argon) is not taken into account in the Langmuir equation. The argon adsorption isotherms in 1a and 1b can be reproduced by the Langmuir equation as shown in Figure 7. This aspect strongly suggests that the argon interatomic interaction rarely affects the adsorption behavior in 1a and 1b, but argon-wall interaction mainly dominates the adsorption behavior. The difference of the interatomic interactions in xenon and argon will reflect on the local structure of

[M2(O2CPh)4(pyz)]n

J. Phys. Chem. C, Vol. 111, No. 3, 2007 1531

Figure 7. Adsorption and desorption behavior of 1a (a) and 1b (b) at 298 K for Xe and Ar. Open circles and triangles represent the adsorption isotherm for xenon and argon gas, respectively, and the filled circles and triangles are the desorption process in each gas. The solid line in the xenon adsorption isotherm represents results of reproduction of the isotherm from eq 3, and in argon adsorption isotherm the solid line is approximated by the Langmuir equation.

TABLE 5: The Characteristic Parameters of Fowler-Guggenheim Isotherm for Adsorption and Desorption of Xenon in 1a and 1b M

nXe(max)/nadsorbent

/kB (K)

P0(298K) (MPa)

2/kB (K)

Rh Cu

1.94 1.90

209 218

1.05 1.45

4640 4540

adsorbents in 1a and 1b: The strong interatomic interaction between xenon atoms is advantageous to form the Xe dimmer as a local structure, leading to full accommodation of xenon (1a‚2(Xe) and 1b‚2(Xe)) even at 3 MPa. On the other hand, argon atoms cannot form dimmer structures in 1a and 1b because of weak interatomic interaction, giving a half of full accommodation of Ar (1a‚1.15(Xe) and 1b‚1.04(Xe)) even at 10 MPa. 129Xe Magic-Angle Sample Spinning (MAS) NMR Spectrum. Figure 8 shows the 129Xe NMR spectrum of xenon in 1a, which was observed under the static and the MAS conditions in the sample including 90 mg of 1a and 2 mg of Xe gas in an ampule. According to the Xe absorption isotherm, an equilibrium pressure in this ampule is estimated as about 0.3 MPa at room temperature, which corresponds to about 60% of the saturation amount of xenon in 1a. The powder pattern under the static condition seems to be broadened by the anisotropy of the chemical shift tensor, but it is somewhat distorted at the highest field side of the powder pattern. Under the MAS condition, the spectrum consists of an isotropic peak at 240 ppm and several sets of spinning side bands. No additional peaks are apparent in any MAS spectrum. The MAS spectra observed at the different spinning rate were reproduced by using the chemical shift anisotropy (∆CS) of 45 ppm and the asymmetric parameter (ηCS) of 0.95, although some line broadenings were taken into account. The spectral simulation was carried out by using the WIN-FIT program,42 which was supplied by Bruker Analytik GmbH. Line broadening results from the fluctuation of the spinning rate of the ZrO2 rotor. The resultant parameters indicate the principal axis components of the chemical shift tensor: δ11 ) 196 ppm, δ22 ) 239 ppm, and δ33 ) 285 ppm. By using these parameters, the calculated spectrum under the static condition is also shown in Figure 8b (bottom). The calculated spectrum seems to reproduce the experimental one approximately, but the shoulder at the highest field side of the powder pattern cannot be interpreted. That pattern discrepancy might arise from the existence of the minor component of xenon. At room temperature, the xenon atoms have sufficient kinetic energy to undergo fast diffusion

Figure 8. 129Xe MAS NMR spectrum of xenon confined in the nanochannel of the Rh complex: the observed spectrum (a) and the reproduced one (b). The MAS spectra observed at different spinning rates were well reproduced by using a set of parameters: δiso ) 240 ppm, ∆CS ) 45 ppm, and ηCS ) 0.95. For the static spectrum, the reproduced spectrum is calculated by using the parameters given above.

in the nanochannel. The fast exchange among the xenon atoms occupying nonequivalent sites is reflected in the powder pattern, even if some nonequivalent sites exist in the homogeneous nanochannel. The xenon adsorption isotherm also supports the dynamic disorder of the adsorbed xenon in the nanochannel. Therefore, it is considered that xenon in the homogeneous nanochannel should contribute to the powder pattern as the major component. Separation of peaks in these two components implies that the chemical exchange of xenon between these two components is slow in comparison with the NMR time scale, which corresponds to the inverse of the frequency difference between these peaks. To the extent that we can observe the static powder pattern, it seems that the population of such a minor component is extremely small. The discrepancy between the calculated and experimental powder patterns is only the shoulder at the highest field side. Furthermore, the minor component seems to have small anisotropy of the chemical shift tensor, implying a lesser contribution of the xenon-xenon interaction to the chemical shift, as explained later in this text. Therefore, the minor component might stem from the isolated xenon atom in the closed space in the crystal of 1a. Disorder in the nanochannel such as crystal structure defects and a blocking

1532 J. Phys. Chem. C, Vol. 111, No. 3, 2007

Figure 9. Temperature dependence of the 129Xe NMR spectrum of xenon confined in the nanochannel of the Rh complex: the observed spectrum (a) and the reproduced one (b). The powder spectrum was reproduced through simulation based on the chemical shift anisotropy by using the Bruker WIN-FIT spectral simulation program.

effect attributable to condensation of xenon at the channel entrance44 seem to create the closed spaces that accommodate the isolated xenon, but a detailed explanation for such spaces remains elusive. Temperature Dependence of the 129Xe NMR Spectrum. Figure 9a shows the temperature dependence of the 129Xe NMR spectrum. The temperature dependence of the chemical shift tensor reflects the xenon-wall and xenon-xenon interactions. It is expected to provide the details of the local structure of xenon in the nanochannel of 1a. A powder pattern characterized by anisotropy of the chemical shift tensor was observed in the chemical shift range greater than 200 ppm. The sample for the temperature dependence includes 1a of 192 mg and xenon gas of 31 mg in a glass ampule, which achieves an equilibrium pressure of about 0.2 MPa at room temperature in the glass ampule on the basis of the xenon adsorption isotherm. This value corresponds to about 30% of the saturation amount of xenon in 1a. In this case, the powder spectrum observed at 295 K shows less distortion than that in Figure 8a. Assuming that the Xe-129 nuclei are equivalent chemically and magnetically, the spectrum was well reproduced by the line shape simulation with 0.73 of the asymmetry parameter, 43 ppm of the anisotropy, and 238 ppm of the isotropic value of the chemical shift tensor (Figure 9b, bottom). These values differ somewhat from those determined from the MAS spectrum, although only the xenon uptakes differ in the samples, clearly indicating the xenon loading dependence of ∆CS and ηCS. The spectrum shifts toward the lower field side and the anisotropy of chemical shift tensor increases when the temperature decreases. In addition, it seems that the asymmetric parameter reaches 1. The remarkable temperature dependence of the spectrum strongly suggests that the chemical shift tensor is affected by dynamic processes as well as the local structure of xenon adsorbed in 1a through the xenon-xenon interaction. Figure 9b shows the powder spectrum reproduced by the simulation based on the chemical shift anisotropy at each temperature. At temperatures of 170-210 K, the spectrum cannot be reproduced completely with a single peak. Figure 8 shows that this discrepancy resembles that of the static spectrum. Figure 9b shows that, in the spectral analysis, we used an

Ueda et al.

Figure 10. Temperature dependence of the principal values of the 129Xe chemical shift tensor for the major spectral components: δ (0), 11 δ22 (4), and δ33 (O). Solid lines are shown as a guide to the eye.

additional minor component to reproduce the observed one. The population of the minor component is less than 5% of the total area of the powder pattern. The anisotropy and the isotropic value in the minor component (∆CS ) 6-13 ppm and δiso ) 206-212 ppm) are much smaller than those in the major component, which implies a smaller contribution of the xenonxenon interaction to the chemical shift in the minor component. Furthermore, the decreased temperature induces this minor component, suggesting that this component is related to the thermal effect of the local structure of the crystal framework in 1a as well as the increase of xenon loading. Figure 10 shows the temperature dependence of the principal value of the chemical shift tensor used for reproduction of the observed spectrum. For temperatures of 170-210 K, only the principal values in the major component are plotted in the figure. When the temperature decreases from 298 to 170 K, δ11 and δ22 increase, whereas δ33 decreases. At temperatures lower than 170 K, each principal value seems to be independent of temperature and has a constant value. Local Structure of Xenon in 1a. In the β-phase of 1a, a 1D nanochannel exists along the c-axis with a rectangular cross section of about 0.77 × 0.60 nm2, which can accommodate the xenon atom. The nanochannel with the anisotropic cross section is classified in the elliptical medium-bore pipe, as described by Jameson et al. in ref 23. Figure 11 depicts a schematic representation of the temperature and loading dependence of the xenon chemical shielding tensor in the elliptical mediumbore pipe. In this channel, the chemical shift tensor for an isolated xenon atom is characterized by the principal values: σ|| is a component along the channel axis; σt is a component along the longitudinal axis in the elliptical cross section; and σ⊥ is a component along the transverse axis in the elliptical cross section as shown in Figure 11. In xenon-129, δ is given by -σ. This situation is satisfied in the very low loading limit of xenon in the channel. In this case, the chemical shift is dominated by interaction between xenon and the channel walls. The order of magnitude in the tensor components was predicted to be |σ||| > |σt| > |σ⊥| by ab initio MO calculation.23,24 It is expected that σ|| and σt are more de-shielded with increasing interatomic interaction between the adsorbed xenon and the channel walls because the equilibrium position of the adsorbed xenon atom approaches the channel wall by cooling if the adsorption amount of xenon does not change by cooling (Figure 11a).

[M2(O2CPh)4(pyz)]n

Figure 11. Schematic representation of temperature (a) and loading (b) dependence of the components of the 129Xe chemical shielding tensor for xenon in the elliptical medium-bore pipe.

On the other hand, when the adsorption amount of xenon increases by cooling, by pressurization, or by loading of xenon, the interatomic interaction among the adsorbed xenon atoms gives rise to effective de-shielding of the chemical shift tensor. Figure 11b shows that the increase of the adsorption amount of xenon forms xenon dimer (Xe2) and trimer (Xe3) in the elliptical medium-bore pipe. In Xe2 species, interatomic interaction between two xenon atoms causes great de-shielding in σt and σ⊥. As a result, the chemical shift tensor component takes the order of |σt| > |σ⊥| > |σ|||. It is also expected that the formation of xenon trimer (Xe3) increases more de-shielding in σt and σ⊥ than in Xe2, thereby further increasing the anisotropy of the chemical shift tensor. The monomer (Xe), dimer (Xe2), and trimer (Xe3) contribute to the local structure of xenon under the dynamic disorder of the adsorbed xenon atom, as suggested by the Xe adsorption isotherm. Actually, a single peak will result from the fast exchange between the Xen species (n ) 1, 2, 3). However, the above-mentioned assignment of the principal value of the chemical shift tensor implies that the xenon dimer (Xe2) and trimer (Xe3) mainly contribute to the local structure of xenon, even in about 30% filling of xenon at room temperature. The temperature dependence of δii in 1a is also consistent with the latter case (formation of the Xe2 or Xe3 local structures by cooling). Furthermore, results show that the adsorbed xenon atoms form a dimer structure with an interatomic distance of 0.4485 nm at 3 MPa in the crystal structure of the β-phase resolved by single-crystal X-ray diffraction analysis, where the channel is fully occupied by xenon. The intermolecular distance between Xe2 dimers is 0.6779 nm. Therefore, it is concluded that the observed chemical shift tensor of xenon is provided mainly by interatomic interaction among xenon atoms in an isolated Xe2 dimer. That is, on average of the chemical shift tensor by the fast exchange of xenon, the population of Xe2 contributing to the chemical shift will be much larger than those of other species. The powder pattern therefore exhibits a large

J. Phys. Chem. C, Vol. 111, No. 3, 2007 1533 anisotropic and asymmetric chemical shift tensor, which reflects the average xenon-xenon interaction and the site symmetry of xenon in the 1a nanochannel. It is reasonable that δ11 and δ22 correspond respectively to tensor components -σt and -σ⊥ in this material. Cooling from 298 to 210 K increases the xenon loading and shortens the average interatomic distance between pairs of xenon atoms. These two effects increase the xenon-xenon interatomic interaction, engendering de-shielding in σt and σ⊥, i.e., in δ11 and δ22. Therefore, increased δ11 and δ22 at temperatures of 170-298 K is inferred to increase the contribution of the xenon-xenon interaction to the chemical shift (see Figure 11 and Figure 16 in ref 23). At temperatures less than 170 K, almost all the xenon coexisting with the powder specimen of 1a in the ampule is expected to be accommodated in the nanochannel. In this case, the xenon uptake in 1a is estimated to be about 50% of saturation. The complete adsorption of xenon from the gas phase to the nanochannel of 1a indicates the slight temperature dependence of the principal values of the chemical shift tensor. In contrast, δ33 principally supports σ||, mainly because of the interaction between the xenon atom and the pore wall. The channel cross section (0.77 × 0.60 nm2) of 1a is less than twice that of the van der Waals diameter of the xenon atom, where the adsorbed xenon atoms are restricted by a single-minimum potential well.32 In this case, the thermal vibration of the adsorbed xenon atom on the cross section causes fluctuation of the interaction between the xenon atom and the pore wall. A harmonic oscillator approximately describes vibration of xenon in the channel. The amplitude of the vibration then decreases with decreasing temperature.45 The chemical shift tensor component, which is dominated by the interaction between the xenon atom and the wall, is given by the statistical average of the interatomic distance between the xenon atom and the wall. In this case, the contribution of the short interatomic distance to the statistics increases when the amplitude of the vibration is larger. The amplitude of oscillation decreases with decreasing temperature. That small amplitude of oscillation increases the population of xenon in the long interatomic distance between the xenon atom and the walls. Therefore, the tensor component is more shielded by cooling. This aspect well explains the change in δ33 with temperature. Therefore, it is concluded that the oscillation of the xenon atom along the longitudinal and transverse axes of the channel cross section dominates the temperature dependence of δ33. The inhomogeneous distribution at low temperatures, e.g., 170-210 K, might be caused by slowing the exchange rate to the intermediate region. At such a time, the spectrum caused by the exchange between the local structures such as Xe, Xe2, and Xe3 shows a complex change depending on the exchange rate, as reported for aluminosilicates such as AlPO-11,24,30-32 which probably distorts the powder pattern. Conclusion Single-crystal X-ray diffraction analyses clarified the molecular arrangement of rare gases (Ar, Kr, Xe) and the crystal structure of the framework in 1a and 1b. In these compounds, rare gases form a dimer structure and align one-dimensionally, similarly to a zigzag chain. Results also showed that a phase transition from R-form to β-form takes place in 1a when the rare gas is accommodated. On the other hand, the R-form of 1b can accommodate Ar and Kr, without an accompanying phase transition. Xenon adsorption in 1a and 1b takes place stepwise without hysteresis, suggesting that xenon-xenon interaction plays an

1534 J. Phys. Chem. C, Vol. 111, No. 3, 2007 important role in adsorption. A 2D gas model (FowlerGuggenheim equation) was used to analyze the isotherms, evaluating the xenon-xenon interaction and the isosteric heat of adsorption. The isosteric heat of adsorption suggests that the large stabilization of xenon in 1a and 1b arises from the dispersion force among xenon atoms as well as the xenon-π interaction with benzoate moieties. Xenon in 1a gives rise to a very de-shielded 129Xe NMR resonance line that was broadened by the anisotropic chemical shift tensor. This phenomenon suggests that a xenon atom occupies the site of extremely small free volume in 1a. Furthermore, the temperature dependence of the 129Xe chemical shift suggests that the Xe2 dimer is a dominant local structure in 1a, even under a partial filling condition. This temperature dependence supports the results of single-crystal X-ray diffraction analyses as well as the xenon adsorption isotherm. This study therefore revealed that, in 1a and 1b, xenon adsorption takes place cooperatively because of xenon-xenon interaction. Furthermore, the adsorbed xenon atoms engender the formation of xenon dimer as a favorable local structure in the 1D nanochannel. These results are consistent with those for other rare gas atoms such as Ar and Kr. Acknowledgment. This work was partially supported by a Grant-in-Aid for Scientific Research (No. 16550013) from the Japanese Ministry of Education, Culture, Sports, Science, and Technology. Supporting Information Available: Crystallographic information files (CIF) for 1a and 1b. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Mori, W.; Takamizawa, S.; Kato, C. N.; Ohmura, T.; Sato, T. Microporous Mesoporous Mater. 2004, 73, 31. (2) Mori, W.; Takamizawa, S. Chapter 6. Suprazeolite Microporous Materials of Metal Carboxylates. In Organometallic Conjugations Structures, Reactions and Functions of d-d and d-π Conjugated Systems; Nakamura, A., Ueyama, N., Yamaguchi, K., Eds.; Springer-Verlag: Berlin, Germany, 2002. (3) Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; O’Keeffe, M.; Yaghi, O. M. Science 2002, 295, 18. (4) Noro, S.; Kitaura, R.; Kondo, M.; Kitagawa, S.; Ishii, T.; Matsuzaka, H.; Yamashita, M. J. Am. Chem. Soc. 2002, 124, 2568. (5) Kitaura, R.; Kitagawa, S.; Kubota, Y.; Kobayashi, T. C.; Kindo, K.; Mita, Y.; Matsuo, A.; Kobayashi, M.; Chang, H.-C.; Ozawa, T. C.; Suzuki, M.; Sakata, M.; Takata, M. Science 2002, 298, 2358. (6) Chui, S.-Y.; Lo, S. M.-F.; Chui, S. S.-Y.; Shek, L.-Y.; Lin, Z.; Zhang, X. X.; Wen, G.-H.; Williams, I. D. J. Am. Chem. Soc. 2002, 122, 6293. (7) Biradha, K.; Fujita, M. Chem. Commun. 2002 1866. (8) Takamizawa, S.; Hiroki, T.; Nakata, E.; Mochizuki, K.; Mori, W. Chem. Lett. 2002, 1208. (9) Takamizawa, S.; Nakata, E.; Yokoyama, H. Inorg. Chem. Commun. 2003, 6, 763. (10) Takamizawa, S.; Nakata, E. CrystEngComm 2005, 7, 476.

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