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Temperature dependent CO2 behavior in microporous 1-D channels of a metal-organic framework with multiple interaction sites

The MOF with the encapsulated CO2 molecule shows that the CO2 molecule is ligated to the unsaturated Cu(II) sites in the cage using its Lewis basic oxygen atom via an angular η1-(OA) coordination mode and also interacts with Lewis basic nitrogen atoms of the tetrazole ligands using its Lewis acidic carbon atom. Temperature dependent structure analyses indicate the simultaneous weakening of both interactions as temperature increases. Infrared spectroscopy of the MOF confirmed that the CO2 interaction with the framework is temperature dependent. The strength of the interaction is correlated to the separation of the two bending peaks of the bound CO2 rather than the frequency shift of the asymmetric stretching peak from that of free CO2. The encapsulated CO2 in the cage is weakly interacting with the framework at around ambient temperatures and can have proper orientation for wiggling out of the cage through the narrow portals so that the reversible uptake can take place. On the other hand, the CO2 in the cage is restrained at a specific orientation at 195 K since it interacts with the framework strong enough using the multiple interaction sites so that adsorption process is slightly restricted and desorption process is almost clogged.

The properly activated sample, 1a, was prepared by vacuuming (~10 −2 torr) (MeOH) 2 @1a at 160 °C for 7 d (Figs S3-S7). Single crystals of the CO 2 -bound 1a ((CO 2 ) 0.8 @1a-195K) were obtained by keeping single crystals of 1a in CO 2 at ~1.5 bar and at ambient temperature for an hour, and then cooling down to 195 K. The structure analysis revealed that CO 2 was ligated to unsaturated metal site using its Lewis basic oxygen atom in an angular η 1 -(O A ) coordination mode with a 0.40(1) site occupancy factor (0.8 CO 2 molecule per cage) (Figs 2f and S8). The Cu− O CO2 distance, 2.94(2) Å, is slightly longer than the Cu− O MeOH distance, 2.617(4) Å, of the bound MeOH in the as-synthesized crystal but still significantly shorter than the sum of their van der Waals radii sum (3.55 Å) 14 . The Cu− O CO2 distance is, much longer than the corresponding Ni-O CO2 bond distances in Ni-MOF-74 structure obtained from the PXRD experiment, 2.29(2) Å 22 , the corresponding Mg− O CO2 bond distances in Mg-MOF-74 structures obtained from the NPD experiments, from 2.24(3) Å to 2.39(6) Å 31 , but is comparable to that of Cu-MOF-74, 2.86(3) Å 32 . It is well-known that the Lewis acidic carbon atom of the CO 2 molecule can interact with the Lewis basic nitrogen atom in the pore environment [16][17][18][19]29 . Similar Lewis acid-base interaction is also observed in the CO 2 -bound 1a. The Lewis acidic carbon atom (C1C) of the bound CO 2 molecule is interacting with the two Lewis basic nitrogen atoms (N3) of two symmetry-related tz ligands. The distance between C1C and N3 (3.21(2) Å) is slightly shorter than their van der Waals radii sum (3.30 Å). The Cu2-O1C-C1C bond angle (124(2)°) is in the range those in Ni-MOF-74, Mg-MOF-74, and Cu-MOF-74 structures (from 117(2) to 144(2)°). The observed site occupancy factor of the ligated CO 2 molecule, 0.40(1), is due to steric repulsion between the CO 2 molecules bound to the symmetry-related Cu(II) sites in a cage. The interatomic distance between the symmetry-related ligated oxygen atoms of the bound CO 2 molecules (2.647(2) Å) is much closer than their van der Waals radii sum (3.1 Å) (Fig. 2f). The steric repulsion between the CO 2 molecules hinders simultaneous ligation of two CO 2 molecules in the same cage. The geometry of the bound CO 2 at 195 K is similar to that of a reported free CO 2 structure. The C− O bond distances, 1.12(3) Å and 1.13(3) Å, in the bound CO 2 structure are not distinguishable to that in the free CO 2 structure, 1.155(1) Å 33 . The little variation in the C− O bond distance in the bound CO 2 is due to the combined effect of two different interactions between the CO 2 molecule and the framework. The CO 2 molecule donates its σ electron to the metal center through the ligating oxygen atom but simultaneously accepts σ electron from the two Lewis basic nitrogen atoms of the tz ligands through the Lewis acidic carbon atom of the bound CO 2 . The O− C− O bond angle in the bound CO 2 structure, 176(3)°, is also nearly linear as that in the free CO 2 structure.
The single crystal of (CO 2 ) 0.26 (H 2 O) 0.15 @1a-296K-5h (the single crystal of CO 2 @1a-195K stood at ambient condition for five hours) showed the bound CO 2 molecule in the cage with reduced site occupancy, 0.13(2) (Fig. 2g). Though the CO 2 molecule is still ligated to the metal center, the Cu− O bond distance of (CO 2 ) 0.26 (H 2 O) 0.15 @1a-296K-5h at 296 K, 3.09(5) Å, is elongated by ~0.15 Å than that of (CO 2 ) 0.8 @1a-195K at 195 K but is still slightly shorter than the van der Waals sum of Cu and O atoms. However, interestingly, the Lewis acidic carbon atom (C1C) of the bound CO 2 molecule is no longer interacting with the Lewis basic nitrogen atom (N3) of the ligand. The distance between C1C and N3 (3.34(4) Å) is slightly longer than their van der Waals radii sum, 3.30 Å. The complete replacement of the bound CO 2 molecules in the pore by the water molecules in air took 10 days at ambient condition (Figs S8 and S9).
The vibrational mode analysis of the CO 2 molecule encapsulated in a cage. Figure 3 shows temperature-dependent infra-red (IR) spectra of the MOF, 1a, filled with 12 CO 2 molecules in the bending and asymmetric stretch regions (see Fig. S10 for whole-range spectra). In the IR spectra of the bending region given in Fig. 3a, two separate peaks are observed, which is distinctively different from the case of a free CO 2 molecule where only a single bending peak appears because its two bending modes are degenerate. This observation suggests that the interaction between a CO 2 molecule and 1a disturbs its bending motion to generate two separate peaks. The center frequency of the two peaks is also red-shifted by 11.7 cm −1 compared to the degenerate bending frequency of a free CO 2 of 667.3 cm −1 34 . Similar splitting of the bending modes and red-shift of the center frequency has been reported in the IR spectra of the CO 2 bound in an η 1 -(O A ) coordination mode in Ni-MOF-74 10 . The splitting of the two bending modes must be related to the binding energy of a CO 2 molecule to the framework and a larger separation indicates stronger interaction between them. The separation between the two peaks decreases as the temperature increases from 5.49 cm −1 at 103 K to 4.92 and 4.67 cm −1 , respectively, at 293 and 423 K. Provided that the binding energy is proportional to the separation, the binding energy at 293 and 423 K, respectively, decreases by 11 and 15% compared with that at 103 K. The CO 2 molecule encapsulated in the cage is less strongly bound to the framework at elevated temperatures. These observations are in agreement with the temperature dependent crystal structures of CO 2 @1a. Though the bound CO 2 structures themselves from the temperature dependent single crystal structure study are not as sensitive as the bending modes of the bound CO 2 molecule, the temperature dependent interactions between the CO 2 and the framework are in good agreement with the temperature dependent bending modes of the bound CO 2 molecule. While the peak separation of the two symmetric bending modes of the bound CO 2 molecule is temperature dependent, the center frequency is insensitive to temperature. Temperature insensitivity of the center frequency is due to the combined result of simultaneous weakening of both the sigma electron donation to the Cu(II) ion through the Lewis basic oxygen atom of the bound CO 2 and the sigma electron acceptance from the nitrogen atoms of the tz ligands via the Lewis acidic carbon atom of the bound CO 2 molecule. Figure 3b shows that the main asymmetric stretching band at ~2335 cm −1 with a FWHM of ~7 cm −1 is red-shifted by 14 cm −1 compared to the corresponding band of free 12 CO 2 at 2349 cm −1 33 . The red-shift indicates that the σ electron donation through the Lewis basic oxygen atom of the bound CO 2 to the Cu(II) center is slightly stronger than the σ electron acceptance through the carbon atom of the bound CO 2 from the nitrogen atoms of the tz ligands. An asymmetric stretching band shift is not a good indicator of the binding strength of an encapsulated CO 2 to a framework. It is a combined effect of electron donation and acceptance of the encapsulated CO 2 to and from the framework. The encapsulated CO 2 in the series of isostructural MOFs, M-MOF-74 (where, M = Ni(II), Mg(II), Zn(II), and Co(II)), showed different shifts of the asymmetric CO 2 stretching band depending on metal ions even though the frameworks are isostructural 35 .
The side bands at the high-frequency side are likely to originate from slightly different local structures of CO 2 inside the cage. At the lower-frequency side of the main band, two peaks, each of which is enclosed with a rounded rectangle, are observed. The temperature dependent peak at ~2323 cm −1 was assigned as a vibrational hot band coming from the transition, (ν 1 , ν 2 , ν 3 ) = (0, 1, 0) → (0, 2, 0) 36 . The assignment is further clarified by the hot-band spectra in the bending region shown in Fig. 3c, where the magnitude of the peaks follows the Boltzmann distribution. However, the peak at ~2327 cm −1 , which shows similar temperature dependence as the peak at ~2323 cm −1 , was assigned to be a peak arising from CO 2 double occupancy in a single cage (two CO 2 molecules in a single cage interacting to each other). Figure 3d shows the asymmetric stretch region of another , and Δ ν  423 K (4.67 cm −1 ) represent the separation between the two peak maxima at 103, 293, and 423 K, respectively. Similar spectra are reported for 1a filled with 13 CO 2 in Fig. S11. (b) Spectra in the asymmetric stretch region of 12 CO 2 . The peaks enclosed with rounded rectangles in the spectral region of ~2323 and ~2327 cm −1 represent a vibrational hot band (starting from the first excited state of the bending mode) and a peak originating from the existence of 2 CO 2 molecules in a single cavity, respectively. (c) Spectra in the spectral region of v 2 = 1 → v 2 = 2 transition (hot band). (d) Spectra of 1a with low CO 2 content in the spectral region of the asymmetric stretch. Note that the spectra in (a,b and c) were obtained from the same sample (high content of CO 2 ), whereas the spectra in (d) were obtained from a different sample (low content of CO 2 ).
Scientific RepoRts | 7:41447 | DOI: 10.1038/srep41447 sample that contains less amount of CO 2 by a factor of 3. The integrated areas of the peaks in the spectra of the low CO 2 content sample are about 1/3 of those of their corresponding spectra (Fig. S12). The double-occupancy peak is hardly observable at 2327 cm −1 indicating that the integrated area of the peak is not linearly proportional to CO 2 concentration and the peak must be from the double occupancy. In the spectra of the low CO 2 content sample, the peaks are narrower than their corresponding ones in the high-content spectra. This observation suggests that the skeleton of 1a is rather flexible and the structural distribution of the encapsulated CO 2 becomes more inhomogeneous with increase in CO 2 content 37 .
The observation that the magnitude of the double-occupancy peak increases as the temperature increases (Fig. 3b) should also be paid attention to. A single CO 2 molecule in a single cage is preferred at lower temperature under lower concentration of CO 2 . The steric repulsion between the CO 2 molecules ligated to the two symmetry-related metal sites in the same cage at low temperature is responsible for this single occupancy. Two CO 2 molecules in a single cage are allowed at high temperature under higher CO 2 content. The reduced restraint on the position of the weakly interacting CO 2 molecule in the cage at high temperature allows two CO 2 molecules in a single cage. The existence of the double-occupancy peak helps us to understand the process of CO 2 filling into the 1-D microporous channels, made of small cages interlinked with small neck-like portal windows. Not only a hopping process of a CO 2 molecule into an empty cage but also a filling process of a second CO 2 molecule into the cage with an encapsulated CO 2 are operating. CO 2 sorption behaviors. Even though 1a had microporous 1-D channel, it does not show any N 2 (at 77 K and 308 K), H 2 (at 77 K) and CH 4 (at 195 K and 308 K) adsorptions (Fig. S13). However, 1a shows reversible CO 2 uptake at 273 K even though the kinetic diameter of CO 2 (3.3 Å) is much larger than the dimension of the portal window and the isotherms are of a typical type I (Fig. 4a). The large quadruple moment of CO 2 might be responsible for such uptake behavior. The CO 2 molecule can interact with the framework strong enough to induce the portal dimension of the 1-D microporous channels large enough for the reversible CO 2 adsorption and desorption. The total CO 2 uptake amount of 1a at 273 K and 1 bar, 33.9 cm 3 /g, corresponds to 0.82 CO 2 molecules per cage (or 0.41 CO 2 molecules per unsaturated Cu(II) center). The CO 2 sorption isotherms of 1a at 298 K are also reversible type I isotherms and the total CO 2 uptake amount at ~1 bar is 37.4 cm 3 /g, which is slightly larger than the CO 2 uptake amount at 273 K and ~1 bar. The increased CO 2 uptake amount at the higher temperature is due to increased double occupancy of CO 2 molecules in a single cage and subsequent pore filling up to the cages located at the inner parts of the microporous 1-D channels. The total CO 2 uptake amount at 313 K and ~1 bar, 31.3 cm 3 /g, is slightly smaller than that at 298 K. The reduced CO 2 uptake amount is due to decreased CO 2 -framework interaction at the higher temperature. The adsorption enthalpy (− Δ H ads ) of CO 2 on 1a calculated using the adsorption isotherms at 298 K and 313 K is − 32-− 25 kJ/mol (Fig. S14), which will be the lower bound because the isotherms obtained are not in the true equilibrium condition due to slow adsorption kinetics of CO 2 . The adsorption enthalpy (− Δ H ads ) of CO 2 estimated using desorption isotherms at 298 K and 313 K is − 41-− 35 kJ/mol, which is comparable to the CO 2 adsorption enthalpy on Mg-MOF-74, ~40 kJ/mol 22,32 , but is much larger than that on Cu-MOF-74, ~22.1 kJ/mol 32 .
Interestingly, the CO 2 sorption on 1a at 195 K is irreversible (Fig. 4b). The CO 2 adsorption isotherm at 195 K shows steep rise at very low pressure. The adsorption reaches to its maximum amount, 24.3 cm 3 /g, at 0.022 bar. The amount of adsorbed CO 2 corresponds to ~0.60 CO 2 molecule per cage (or ~0.30 CO 2 molecule per unsaturated metal site), which is much smaller than the expected maximum amount, one CO 2 molecule per cage (or 0.5 CO 2 molecule per unsaturated metal site). Significant portions of the inner cages in the 1-D porous channel may not be accessible for CO 2 molecule because the channels are already blocked by the CO 2 molecules bound strongly to the framework as shown in Fig. 2d. There is no indication of CO 2 desorption even at 0.0015 bar.
At high temperature, not only hopping process of a CO 2 molecule into an empty cage but also filling process of a second CO 2 molecule into the cage with an encapsulated CO 2 molecule are allowed so that even most of the empty inner cages of the 1-D channels could be occupied with CO 2 molecules and the CO 2 molecules in the inner cages the 1-D channels can also be emptied via a reverse process. However, at low temperature, the filling of a second CO 2 molecule into the cage with an encapsulated CO 2 molecule is severely hindered because the CO 2 molecule bound strongly at the open metal center does not allow the second CO 2 molecule into the same cage. At low temperature, the main pore filling is via hopping process so that the maximum amount of CO 2 captured in the 1-D channels is significantly smaller than the expected maximum amount of CO 2 . When multiple consecutive cages in a 1-D channel are filled with CO 2 molecules, desorption of CO 2 is also severely hindered, which is the cause of the irreversible CO 2 sorption behavior of 1a at 195 K.

Conclusions
Although each cage contains two unsaturated Cu(II) sites and the cage size is large enough for two CO 2 molecules, the CO 2 uptake amount is only ~0.9 CO 2 molecule per cage at 298 K and 1 bar, which is due to the steric repulsion between the two bound CO 2 molecules in the cage. Temperature dependent single crystal structure analyses revealed that the interaction between the bound CO 2 molecule and the framework of the MOF is temperature-dependent. As temperature increases, both the CO 2 coordination to the unsaturated Cu(II) center and the Lewis acid− base interaction between the carbon atom of the CO 2 molecule and the nitrogen atom of the ligand are simultaneously weakened.
The red-shifted vibrational peaks of the bound CO 2 molecule indicate that the electron donation to the metal center through the oxygen atom of the CO 2 molecule is slightly larger than the electron acceptance from the nitrogen atom of the ligand through the carbon atom of the CO 2 molecule. The temperature dependent separation of the two bending peaks also supports the temperature dependent interaction between the CO 2 molecule and the framework of the MOF. As temperature increases, the separation of the bending peaks decreases. The observation tells that the CO 2 -framework interaction decreases as the temperature increases as observed in the temperature dependent structure analyses of a single crystal of CO 2 @1a.
The reversible CO 2 uptakes on 1a were observed at 298 and 313 K, respectively, even though the portal dimension of the cage aligned along the microporous 1-D channel is smaller than the kinetic diameter of CO 2 molecule. The CO 2 uptake amount per unsaturated Cu(II) center at 298 K, 1 bar is 0.45 molecule, which is only slightly smaller than the probable maximum value, 0.5. On the other hand, the irreversible CO 2 uptake was observed at 195 K and the CO 2 uptake amount per unsaturated Cu(II) center is only 0.30 molecule, which is much smaller than the expected value, 0.5. The decreased uptake amount at the lower temperature is due to the strongly interacting CO 2 molecules in the cages aligned along the microporous 1-D channel hindering the other CO 2 molecules to access the available inner cages in the 1-D channel. The different uptake behaviour at different temperature is related to both the small portal dimension of the cages aligned along the 1-D channel and the different strength of CO 2 − framework interaction in the cage. The reversible CO 2 uptakes were observed at the higher temperature since the CO 2 molecule interacting weakly with the framework could have proper orientation for the escape from the cavity through the small portal of the cages. However, the irreversible CO 2 uptake was observed at the lower temperature since the CO 2 molecule interacting strongly enough with the metal center in the cage could not have proper orientation for the escape from the cavity through the small portal.

Methods
Synthesis of the MOF, Cu 3 Cl 2 (tz) 4 (MeOH) 2 , 1. A 27.3 mg amount of CuCl 2 (0.203 mmol) was dissolved in 4 mL MeOH in a 10 mL vial, and then 0.90 mL 0.45 M tetrazole (tz) acetonitrile solution (0.41 mmol) was added into the solution. The Teflon-sealed vial was heated to 70 °C for 7 days and then slowly cooled down to ambient temperature. The block-shaped blue crystals obtained were washed using 10 mL fresh methanol and filtered. IR spectroscopy. The samples were in the form of a KBr pellet of a ~200-μ m thickness and a 13-mm diameter. The samples were located in a cell composed of two circular KBr windows of a 25-mm diameter. The windows were separated by a 200-μ m thick spacer with 25 and 20 mm outer and inner diameters, respectively. FTIR spectra were recorded by a Varian 7000e FTIR spectrometer. The samples with KBr windows were placed in a home-made oxygen-free high thermal conductivity (OFHC) copper cell, and the temperature of the samples was controlled and maintained in a variable temperature cell holder (GS21525, Specac, UK). Details of the temperature-controllable cell assembly have been reported elsewhere 38 .
Gas sorption measurements. All gas sorption isotherms were measured using a BELSORP-max (BEL Japan, Inc.) with a standard volumetric technique.