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Formation of a One-Dimensional Array of Oxygen in a Microporous Metal-Organic Solid

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Science  20 Dec 2002:
Vol. 298, Issue 5602, pp. 2358-2361
DOI: 10.1126/science.1078481

Abstract

We report the direct observation of dioxygen molecules physisorbed in the nanochannels of a microporous copper coordination polymer by the MEM (maximum entropy method)/Rietveld method, using in situ high-resolution synchrotron x-ray powder diffraction measurements. The obtained MEM electron density revealed that van der Waals dimers of physisorbed O2 locate in the middle of nanochannels and form a one-dimensional ladder structure aligned to the host channel structure. The observed O–O stretching Raman band and magnetic susceptibilities are characteristic of the confined O2 molecules in one-dimensional nanochannels of CPL-1 (coordination polymer 1 with pillared layer structure).

The confinement of molecules into low-dimensional nanospace may alter their properties and reactivity, especially in the case of O2 molecules, which have rich redox and magnetic properties arising from their unpaired electrons (1, 2). In spite of many experimental and theoretical investigations, the adsorption mechanism and ordering state of adsorbed O2molecules that form a specific array in nanochannels are not yet clear (3, 4). In the past decade, various novel nanochannel structures constructed with metal-organic frameworks have been produced and studied with regard to their adsorption performance with various gas molecules (5–8). If the gas molecules could be trapped in mid-channel, low-dimensional nanostructures could be designed, depending on the host channel structure.

Our strategy to elucidate the specific structure of O2molecules adsorbed in nanochannels uses a crystalline microporous copper coordination polymer that forms a uniform nanosized one-dimensional (1D) channel. We obtained accurate powder x-ray diffraction (XRD) data by synchrotron powder diffraction with a large Debye-Scherrer camera and were able to image the diffraction data by the MEM (maximum entropy method)/Rietveld method, which enabled us to determine a precise electron density map (9–12).

The 3D structure of the porous Cu(II) coordination polymer, which we determined previously with single-crystal diffraction data (13), is shown in Fig. 1. A 2D sheet constructed by Cu(II) and pzdc (2,3-pyrazinedicarboxylate) were linked by pyz (pyrazine) to form a pillared layer structure, which we call CPL-1 (coordination polymer 1 with pillared layer structure). 1D channels with dimensions of 4.0 Å by 6.0 Å formed between the 2D sheets along the a axis, with one water molecule being included per Cu(II) ion.

Figure 1

Representation of the 3D porous pillared layer structure of CPL-1 down from the a axis (Cu, green; O, red; C, gray; N, blue). Water molecules are omitted for clarity. Crystal data were as follows: C16H12Cu2N6O10,M = 575.40, monoclinic, space groupP21/c (no. 14), a = 4.693(3) Å, b = 19.849(2) Å,c = 11.096(2) Å, β = 96.90(2)°,V = 1026.1(6) Å3, Z = 2,R 1 = 0.062, andR w = 0.062. The elemental analysis calculated for C16H12Cu2N6O10was as follows: C, 33.40%; H, 2.10%; and N, 14.61%. The amounts found were: C, 32.78%; H, 1.57%; and N, 14.34%.

In order to adsorb O2 molecules into the nanochannels, we heated CPL-1 under reduced pressure to remove water molecules from the channels, and then dosed CPL-1 with O2gas while cooling. The in situ powder XRD patterns of as-synthesized CPL-1 and of anhydrous CPL-1 with O2 at 80 kPa over the temperature range from 300 to 90 K are shown in Fig. 2. The changes can be categorized into three stages: (i) after heating under reduced pressure to remove water molecules; (ii) during the cooling process, between 130 and 150 K; and (iii) after the reheating process from 90 to 300 K. No change was observed in the absence of O2 over the full temperature range. LeBail fitting of the powder diffraction data of Fig. 2 revealed a cell volume contraction at (i), expansion at (ii), and recontraction at (iii). We attribute these changes to structure distortion arising from framework flexibility, which is triggered by (i) desorption of water molecules and then the (ii) adsorption and (iii) desorption of O2 molecules (14).

Figure 2

Synchrotron XRD patters of (A) as-synthesized CPL-1 at 300 K and anhydrous CPL-1 (after drying at 373 K under reduced pressure) with O2 of 600 torr with cooling from 300 to 90 K and reheating to 300 K. (B to H) represent XRPD patterns at 300, 200, 150, 130, 110, 90, and 300 K, respectively.

The crystal structure of anhydrous CPL-1 at 120 K without O2 molecules determined by Rietveld analysis of the powder data up to 53.3° (d > 0.89 Å) reveals that the porous structure was identical to that of as-synthesized CPL-1, with only slight structure distortions (15). The MEM electron density distribution map of the anhydrous CPL-1 (Fig. 3A), whose reliability factor based on structure factors, R F, is 1.6%, shows that no water molecules exist in the nanochannel (16,17).

Figure 3

MEM electron densities of (A) anhydrous CPL-1 without O2 molecules at 120 K and (B) CPL-1 with adsorbed O2 at 90 K as an equal-density contour surface. The equicontour level is 1.0 e Å−3.

The space group of the anhydrous CPL-1 with O2 at a pressure of 80 kPa at 90 K (Fig. 2G) is assigned as the same space group, P21/c, as CPL-1 without O2. The cell parameters were determined asa = 4.68759(4) Å, b = 20.4373(2) Å,c = 10.9484(1) Å, and β = 96.9480(6)° by Rietveld analysis. As the preliminary model, we used the same structure model corresponding to Fig. 3A for CPL-1 without O2, in the pre-Rietveld analysis for the MEM/Rietveld analysis. The reliability factors based on the powder pattern, R wp, and the integrated intensities, R I, were 18.5 and 54.2%, respectively. However, the MEM electron density visualized the O2 density feature in the middle of the nanochannel. After a revision based on the electron density, the Rietveld refinement dramatically improved, and the R wp andR I of the final Rietveld fitting became 2.1 and 3.9%, respectively. The final electron densities, obtained by MEM whose reliability factor, R F, was 1.5%, reveal the 3D pillared-layer structure, consistent with the single-crystal data (Fig. 3B).

The peanut-shaped electron densities, which are presumably due to O2 molecules, are clearly recognized in the middle of the channels. There is a total of 15.8(1) electrons, calculated on the basis of the MEM results, which virtually agrees with the number in the O2 molecule. Therefore, we deduced that the interlayer peanut-shaped electron densities are O2 molecules, and one O2 molecule was adsorbed per copper atom without any electron transfer between O2 molecules and/or O2 molecules and the pore wall.

The O2 adsorption isotherm measured at 77 K shows a type I isotherm with a saturated amount of adsorption of 1.0 O2 molecules per copper atom, which is in good agreement with the MEM/Rietveld analysis. The relatively small value of the isotropic displacement parameter [B = 4.1(2) Å2 ] and lack of disorder of O2 molecules indicate that O2 molecules adsorbed in the nanochannels resemble the solid state rather than the liquid state at 90 K, which is much higher than the freezing point of O2 under atmospheric pressure, 54.4 K (18). This freezing behavior should be attributed to the strong confinement effect of the nanochannel. The overall crystal structure and geometry of O2 molecules, based on the above analysis, are represented in Fig. 4. Two O2 molecules align parallel to each other along the a axis with an inclination of 11.8°, and the intermolecular distance is 3.28(4) Å, which is much smaller than the minimum of the Lennard-Jones potential ofR e = 3.9 Å (19). This intermolecular distance is close to the nearest distance in solid α-O2, which is stable below 24 K. This result indicates that O2 molecules adsorbed in nanochannels form van der Waals dimers, (O2)2, and their successful structural characterization has now been reported. Each dimer aligns along the a axis to form a 1D ladderlike structure. The interatomic distance of oxygen atoms in the adsorbed O2 molecule was found to be 1.245(50) Å.

Figure 4

Schematic representation of CPL-1 with adsorbed O2 at 90 K. (A) The perspective view down from the a axis. (B) and (C) Views down from the b axis and c axis, respectively.

In order to investigate the properties of adsorbed O2molecules, magnetic susceptibilities and Raman spectroscopic measurement were performed. The susceptibility for adsorbed O2 molecules, which is the difference in those between CPL-1 with O2 and CPL-1 without O2, approaches zero with decreasing temperature, indicating a nonmagnetic ground state (Fig. 5). Correspondingly, in the high-field magnetization process, the difference between CPL-1 with and without O2 can be observed above 50 T (Fig. 5, inset). The magnetization process of CPL-1 is a paramagnetic curve with a saturated moment of 1 bohr magneton for a Cu atom, which can be expected for the usual magnetic Cu ion. The CPL-1 with O2 shows a small difference in the low-field region but a steep increase above 50 T, indicating a magnetic transition from the nonmagnetic to the magnetic state. This magnetization process can be understood as a behavior of the antiferromagnetic dimer or the 1D antiferromagnetic chain with bond alternation. On the basis of structural information about the adsorbed O2 molecule in CPL-1, the nonmagnetic ground state is associated with the antiferromagnetic dimer. The antiferromagnetic interaction (ℋ = –2JS1S2 ) is estimated as J/kB ∼ –50 K, which is larger than that of the α phase of J/kB ∼ –30 K (20). Figure 6 shows Raman spectra of CPL-1 around 90 K with and without O2 molecules. A sharp peak due to the stretching mode of adsorbed O2molecules is observed at 1561 cm−1, which is slightly higher than that of liquid or solid oxygen at ambient pressure (21), comparable with that of α phase at a pressure of 2 GPa.

Figure 5

Temperature dependence of the susceptibility of (A) CPL-1 and (B) CPL-1 with O2 molecules, and (C) the differences between (A) and (B), correspond to the contribution from adsorbed O2 molecules. (Inset) High-field magnetization process of (A) CPL-1 and (B) with O2 molecules. χ, susceptibility;M, magnetization; μB, bohr magneton; f.u., formula unit; H, magnetic field T, temperature.

Figure 6

Raman spectra of (A) CPL-1 and (B) CPL-1 with O2 molecules. A peak due to the stretching of O2 molecules (marked by an arrow) is shown in (B). The abscissas were calibrated using the standard lines from a neon lamp, and the resolution of the data is 0.6 cm−1.

The confinement effect and the restricted geometry resulting from 1D nanochannels of CPL-1 lead to a specific molecular assembly: a 1D ladder structure constructed of O2 dimers, which is unlikely to be bulk fluid and/or solid.

  • * To whom correspondence should be addressed. E-mail: Kitagawa{at}sbchem.kyoto-u.ac.jp

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