Cooperative Bond Scission in a Soft Porous Crystal Enables

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Cooperative Bond Scission in a Soft Porous Crystal Enables Discriminatory Gate-Opening for Ethylene over Ethane Susan Sen, Nobuhiko Hosono, Jia-Jia Zheng, Shinpei Kusaka, Ryotaro Matsuda, Shigeyoshi Sakaki, and Susumu Kitagawa J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b10110 • Publication Date (Web): 13 Nov 2017 Downloaded from http://pubs.acs.org on November 13, 2017

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Journal of the American Chemical Society

Cooperative Bond Scission in a Soft Porous Crystal Enables Discriminatory Gate-Opening for Ethylene over Ethane Susan Sen,† Nobuhiko Hosono,*† Jia-Jia Zheng,†,§ Shinpei Kusaka,† Ryotaro Matsuda,†,‡ Shigeyoshi Sakaki,§ Susumu Kitagawa*† †

Institute for Integrated Cell-Material Sciences, Kyoto University Institute for Advanced Study, Kyoto University, Yoshida Ushinomiya-cho, Sakyo-ku, Kyoto 606-8501, Japan.

§

Fukui Institute for Fundamental Chemistry, Kyoto University, Takano Nishihiraki-cho 34-4, Sakyo-ku, Kyoto 606-8103, Japan.

ABSTRACT: Here we report a soft porous crystal possessing hemilabile crosslinks in its framework that exhibits exclusive gate opening for ethylene, enabling the discriminatory adsorption of ethylene over ethane. A Co-based porous coordination polymer (PCP) bearing vinylogous tetrathiafulvalene (VTTF) ligands, [Co(VTTF)], forms Co–S bonds as intermolecular crosslinks in its framework in the evacuated closed state. The PCP recognizes ethylene via d–π complexation on the accessible metal site that displaces and cleaves the Co–S bond to “unlock” the closed structure. This ethylene-triggered unlocking event facilitates remarkable nonporous-to-porous transformations that open up accessible void space. This structural transformation follows a two-step gate-opening process. Each phase, including the intermediate structure, was successfully characterized by single-crystal X-ray diffraction analysis, which revealed an intriguing “half-open” structure suggestive of a disproportionate gate-opening phenomenon. The gate-opening mechanism was also investigated theoretically; density functional and Monte Carlo calculations reveal that the unique “half-open” phase corresponds to a substantially stable intermediate over the possible transformation trajectories. While ethylene opens the gate, ethane does not because it is unable to coordinate to the Co center. This feature is maintained even at pressures above 1 MPa and at a temperature of 303 K, demonstrating the potential of the “gate-locking/unlocking” mechanism that exploits the hemilabile crosslinking in soft porous crystals.

INTRODUCTION Flexible porous coordination polymers (PCPs) or metal-organic frameworks (MOFs) are soft nanoporous crystals that exhibit guest-dependent structural transformations.1 The reversible nature of the coordination bond and access of the transition metal center to multiple coordination geometries allow a PCP to show different interconvertible structures depending on external parameters including temperature, pressure, and the guest molecule.2 Often termed the “gate-opening behavior” of a PCP, these small perturbations trigger changes in the framework structure, opening up the pore so that it can abruptly adsorb gas molecules under certain conditions (i.e., pressure).3 The gate-opening event of a PCP is considered to be a cooperative effect induced by gas adsorption. Such cooperative gate-opening behavior is of great importance in the development of molecular recognition and separation technologies; however, a rational design that realizes materials that exhibit this cooperative gate effect is nonetheless challenging. In general, the gate-opening mechanism can be explained by thermodynamic interactions involving guest molecules, and the framework-deformation energy. The open structure is stabilized by the energy gained by gas adsorption; hence the gate-opening event is dependent on temperature and dosing pressure, as well as interactions between the gas species and the framework.4 This structural transformation occurs in a concerted manner at a specific threshold of the balanced global energy.

On the other hand, gate-opening behavior in another mechanism has been reported, which provided us with the inspiration to realize the guest-specific cooperative response in this study. This phenomenon has been observed in some PCPs that exhibit gateopening adsorption exclusively for a specific guest that acts as a “key” for the structural transformation;5 the selective adsorption of water in a Ce-based PCP,6 {[Ce(tci)]}n (tci: tris(2carboxyethyl)isocyanurate), is such an example. The adsorption event is triggered by the coordination of water to Ce-metal ions that facilitates changes in the coordination environment resulting in an acceleration of subsequent water-molecule binding. The framework only opens the gate for the key molecule and remains closed for any other potential guest, including methanol, nitrogen, and carbon dioxide. Such allosteric-like behavior in a crystalline solid can be considered a cooperative effect that is induced by a local structural change at a specific adsorption site upon binding by the key guest. The exclusivity of the latter cooperative gate-opening behavior appears to be very effective for the realization of highly challenging absorptive separation of competitive small molecules, as demonstrated recently for the separation of nitrogen and carbon monoxide.7 However, the design of a PCP that exhibits such a specific adsorbate-dependent structural transformation remains a formidable task. Herein, we report the design and synthesis of a PCP that exerts a guest-specific cooperative gate opening; this opening features a

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concerted unlocking/locking mechanism that involves the reversible disruption/formation of a hemilabile crosslink. In this PCP, an ethylene molecule acts as the “key” that opens the gate, hence realizing the discriminatory adsorption of ethylene over ethane. We synthesized a new PCP embodied with Co2+ and vinylogous tetrathiafulvalene (VTTF, 2,2'–[1,2–bis(4–benzoic acid)–1,2– ethanediylidene]bis–1,3–benzodithiole) linkers (Figure 1). In this PCP, hereafter referred to as “Co(VTTF)”, VTTF and Co2+ ions form a 1D chain that stack together via intermolecular C–H···π interactions. Interestingly, we observed the formation of a bond between the Co and the sulfur atom of the benzodithiole moiety of VTTF in the evacuated state of the PCP, where the Co–S bond tethers the skeletal 1D chains to form an overall nonporous 3D framework. In gas sorption experiments involving Co(VTTF), we found that ethylene molecules “unlock” and open the gates of the entire PCP structure.

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transformation; ethane is ineffective despite its physical properties being similar to those of ethylene. Hence, the PCP recognizes ethylene and allows it to open the gate via the disruption of the hemilabile link at the commencement of the sorption process. Remarkably, the material response towards ethylene and ethane is well maintained even at high pressure and ambient temperature, which is desirable for practical gas-separation applications.

RESULTS AND DISCUSSION Synthesis and Structure of Co(VTTF) PCP. We synthesized the vinylogous tetrathiafulvalene dicarboxylate ligand (VTTF) using a conventional oxidative coupling reaction. The VTTF linker was treated with Co(NO3)2·6H2O in a 1:1 (v/v) mixture of N,Ndimethylacetamide (DMAc) and methanol as solvent, under solvothermal conditions to give the solvated framework 1 {[Co(VTTF)(H2O)]·(Solv)n, (where n is the number of solvent molecules in the crystal)} (Figures 1 and S1) as a brown prismatic crystal. SXRD analysis of the as-synthesized 1 reveals the angular geometry of the ligand that allows it to form a 1D chain structure, [Co(VTTF)]n, with Co2(COO)4 paddle-wheel sub-building units (SBUs), in which the VTTF moiety adopts a cisoid conformation typical of neutral TTF vinylogues (Figure 1b).8 The axial positions of the Co paddle-wheel units are capped with solvent molecules. The stacking together of these 1D chains through C–H···π interactions extends the framework into three dimensions to form infinite 1D rectangular channels along the b-axis with diagonal cross sections of 14.6 × 9.8 Å. The axially coordinated solvent molecules are directed toward the channels that are decorated with electron-rich VTTF moieties. The total solvent-accessible void is calculated to be 1212.8 Å3, which is 32.1% of the unit cell volume in the assynthesized framework.

Figure 1. (a) Chemical structure and (b) single crystal structure of the building unit of Co(VTTF) PCP (1). Atoms are represented by colors: S, yellow; O, red; C, gray; Co, blue. (c) View down the b-axis of the single [Co(VTTF)]n chain in 1. (d) Side view of the [Co(VTTF)]n chain in 1. (e) Stacking structure of the columnar array of the [Co(VTTF)]n chain in 1. Lattice and coordinated solvent molecules are omitted for clarity.

Thermogravimetric (TG) analysis and variable temperature PXRD study confirm the high thermal stability (> 350 °C) of 1 (Figure S2). The coordinated and lattice solvent molecules were exchanged with methanol prior to vacuum activation, which is necessary to fully activate the PCP by removing the coordinating guest molecules under mild conditions. TG analysis of the methanolexchanged 1 confirms the removal of all solvent molecules below 100 °C with no further weight loss up to 380 °C (Figure S2).

More interestingly, the gate-opening event was found to follow a two-step process, in which a “half-open” intermediate state is formed, followed by full gate opening. Each state in this two-step gate-opening process was individually characterized by single crystal X-ray diffraction (SXRD) analysis, which revealed that the nonporous-to-porous single-crystal-to-single-crystal (SCSC) transformation is triggered by ethylene molecules that disrupt Co–S bonds by displacing S atoms with themselves. The “half-open” structure gives rise to a disproportionate splitting of the crosslinked framework, which is substantially stabilized in the adsorption process, as demonstrated by theoretical calculations. Hence, this unlocking event, triggered by ethylene, facilitates a remarkable nonporous-toporous transformation in a single-crystalline phase that opens up additional void space to accommodate further ethylene.

SXRD structural analysis of the fully evacuated phase of Co(VTTF) (2) reveals a nonporous structure with only 41.1 Å3 (1.5%) of solvent-accessible void volume in the unit cell, indicating that the porous-to-nonporous SCSC transformation takes place during the guest-evacuation process (Figures 2a, d, and S3). This structural transformation is accompanied by the deconstruction of the paddle-wheel SBU on removal of the coordinating solvent molecules (Figure 2a). Whereas two square-pyramidal Co(II) metal ions are bridged by four different carboxylate groups to form the paddle-wheel SBU in 1, each Co(II) metal ion in 2 adopts a distorted trigonal bipyramidal (tbp) geometry, with pairs bridged by two carboxylate groups; the Co–Co separation increased from 2.898(1) Å (1) to 3.453(4) Å (2) during the SCSC transformation (Figure S4).

The metal-coordinating ability of the benzodithiole moiety allows the Co–S link to form; this link plays a crucial role in stabilizing the evacuated nonporous phase. Only ethylene, with its πcoordinating ability, triggers the nonporous-to-porous structural


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Figure 2. Structural transformations of Co(VTTF) PCP. (a) Crystal structure of 2 (evacuated phase), (b) 3 (“half-open” phase), and (c) 4 (fully open phase). The coordination environment of the Co paddle-wheel SBU in each phase is shown in each inset. Atoms are represented by colors: S, yellow; O, red; C, gray; Co, blue. Void spaces in structures 2, 3, and 4 are depicted in (d), (e), and (f), respectively. The void spaces were visualized using the Mercury CSD 3.5.1 software with a probe radius of 1.2 Å.

Figure 3. (a) Adsorption (filled squares) and desorption (open squares) isotherms of 2 for ethylene (blue square) and ethane (red circle) measured at 170 K and 185 K, respectively. The inset shows a magnified view of the isotherms in the low-pressure region. (b) Coincident in-situ adsorption/PXRD patterns during ethylene adsorption measured at 170 K at given equilibrium pressures.

Interestingly, one of the axial positions of each Co(II) metal ion in 2 is coordinated to a S atom of a VTTF moiety in a neighboring [Co(VTTF)]n chain. The coordination of S to Co metal ion is well investigated for organometallic complexes.9 The coordination properties of the dithiole ring of the VTTF moiety, however, has been realized for the first time; a Co–S equilibrium bond length of 2.506(5) Å was observed in the single-crystal structural (Figure S4), which is comparable to the reported bond length of axially coordinated Co–S bonds in pentacoordinated Co(II) complexes.9 The neighboring [Co(VTTF)]n chains in 2 are tethered to each other via Co–S bonds that stabilize the nonporous form and, as a consequence, remove the accessible free space. PCP 2 maintains the high

thermal stability of 1 and does not show any appreciable weight loss before framework decomposition at 380 °C (Figure S2). The bulk phase purity of 2 was confirmed by its powder X-ray diffraction (PXRD) pattern, which is consistent with the simulated PXRD pattern calculated from the single crystal X-ray data (Figure S5). It is interesting to note that the reverse SCSC transformation takes place immediately to restore the Co paddle-wheel SBU upon exposure of 2 to air. The recovered structure (1’) is quite close to that of the as–synthesized 1, with the exception that a slightly shorter c-axis is observed (Figure S6 and Table S1). The guest water molecules are coordinated to the open metal site of the paddle wheel SBU of 1’. This difference in guest-molecule orientation



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possibly contributes to the observed differences in the overall packing structure and cell size between 1 and 1’. On repeated activation by heating under dynamic vacuum, 1’ is again transformed to the nonporous phase 2, confirming that the SCSC transformations are reversible.

mL(STP)/g) in the low-pressure region (< 9.0 kPa) (Figure 3a and inset). This implies that surface adsorption possibly takes place first, then triggers the sudden opening of the entire crystal at a certain threshold pressure (i.e., 9.0 kPa) with a corresponding degree of bond cleavage.

Gas Sorption Properties and SCSC Transformations. Gas sorption studies with ethylene at 170 K reveal that the nonporousto-porous phase transformation occurs upon ethylene adsorption. Coincident in-situ gas sorption/PXRD studies for ethylene at 170 K reveal that 2 hardly adsorbs ethylene at low pressure, but abruptly begins to adsorb at 9.0 kPa, with sharp changes in the PXRD pattern highlighting the SCSC transformation and the gate opening (Figure 3). The structural changes continue until a pressure of 21.1 kPa is reached and a major change in structure is observed.

With increasing ethylene-dosing pressure, the “half-open” structure is fully transformed into the saturated adsorbed-phase structure (Figures 2 and 3), which has an adsorption capacity of 110 mL (STP)/g at an ethylene pressure of 100 kPa, which is equivalent to 12.4 ethylene molecules per unit cell (3.1 ethylene molecules per Co2+). The transformation of 3 to the fully open-phase structure was also investigated by in-situ gas-sorption/PXRD analysis. As expected, the PXRD pattern of 2 at an ethylene-dosing pressure of 100 kPa (point “j” in Figure 3b) was close to the simulated pattern of the as-synthesized open structure 1, except slight deviations around 10°-12°. Those small differences in peak positions can be due to the highly flexible nature of this PCP as the open-phase structure is a packing of 1D [Co(VTTF)]n chains stabilized by weak interactions between the chains and guest molecules. We successfully determined the adsorbed-phase single-crystal structure, hereafter referred to as 4, by SXRD analysis of a crystal of 2 sealed in an ethylene-filled glass capillary, which revealed that 4 has an analogous open-phase structure to that of 1 (Figure 2c) (See Supporting Information).

Interestingly, the in-situ gas sorption/PXRD study on the PCP reveals that the structural transformation occurs in two steps. The first step occurs from 9.0 to 21.1 kPa, whereas the second step proceeds gradually until the saturated point is reached (Figure 3a). We envisage that ethylene molecules interact with the framework to open the gate during the first step, and we assume the involvement of the “half-open” structure 3 as the intermediate. Gratifyingly, 3 was successfully captured by SXRD analysis of a single crystal of 2 sealed in a glass capillary partially filled with ethylene (Figure 2b and 2e) (see supporting information for experimental detail). In 3, the Co paddle-wheel moiety is recovered and is slightly distorted, with a Co–Co distance of 2.785(4) Å, which is shorter than that observed in as-synthesized 1 (Figure S7). The structure of 3 is intriguing as one of the axial positions of the paddle wheel unit is still connected to the S atom of a VTTF ligand of an adjacent chain, while the other is occupied by an ethylene molecule to form the “half-open” configuration (Figure 3b). Although we could not reasonably refine the positions of the coordinated ethylene molecule due to severe disorder, the electrons corresponding to one ethylene molecule were observed at the axial position of the open-metal site. Herein we define the fully closed structure of 2 to be the “aaaa” configuration, where “a” refers to a unit of closed pairs of [Co(VTTF)]n chains (Figure 2). The half-open structure of 3 is referred to as “abab”, where “b” denotes a unit of separated (open) pairs of chains. Thus, the fully open structure is referred to as the “bbbb” configuration. The “abab” structure is the result of the disproportionate splitting of the “aaaa” structure, which is unique and raises questions about the splitting mechanism (vide infra). The Co–C bond distance in an organometallic Co–ethylene complex is reported to be 2.02 Å.10 The half-open structure 3 is stabilized by the two bonds formed between Co and ethylene through the σ–donating and π–accepting abilities of the ethylene molecule. The PXRD pattern of 2 at point “g” (at an equilibrium ethylene pressure of 10.7 kPa at 170 K, Figure 3b) is close to that calculated from the single-crystal structure of 3, indicating that the structural transformation surely occurs through intermediate 3 (Figure S8). It should be noted that the peak observed at 2θ = 4.7° in the in-situ gas sorption/PXRD pattern is characteristic of structure 3 (Figure 3b). We deduce that the SCSC transformation of 2 to 3 occurs at the surface of each crystallite at the very beginning of the adsorption process. In fact, we found that the PCP structure begins to change from 2 to 3 with a very small uptake of ethylene (~1.1

In sharp contrast to ethylene, ethane was incapable of unlocking the nonporous phase and was negligibly adsorbed at low temperature (185 K) (Figure 3a) as well as at ambient temperature and at (high) pressures above 1 MPa (vide infra). This observation suggests that the nonporous-to-porous SCSC transformation is triggered exclusively by ethylene to further enhance its uptake capacity. Therefore, the hemilabile Co–S crosslink in the PCP acts as the “lock” and only ethylene is the key that disrupts this bond by displacing it with itself. Theoretical Calculations. The results of the coincident in-situ gas sorption/PXRD studies and the SXRD analyses indicate that PCP 2 follows a two-step gate-opening process during ethylene adsorption. The “aaaa” phase is first transformed into the intermediate “abab” half-open phase, and then into the fully open phase with the “bbbb” configuration. Structural disproportionation during the SCSC transformation of the PCP is barely observed, which is unique in the Co(VTTF) PCP. The disproportionate configuration, namely “abab”, is somehow substantially stabilized by several ethylene molecules, which results in the “aaaa” to “abab” trajectory being preferred to the direct “aaaa” to “bbbb” transformation (Figures 2). To understand this intriguing SCSC transformation process, we carried out theoretical calculations to predict the energy required for each deformation step. The canonical Monte–Carlo (MC) method11 was first used for searching and locating the most probable ethylene adsorption site. Single-crystal structures 2–4 were used as the initial structures of the “aaaa”, “abab”, and “bbbb” configured starting points, respectively. Subsequent density functional theory (DFT) calculations with various ethylene loadings enabled us to determine the deformation energy (ΔEdef), interaction (stabilization) energy (EINT), and adsorption (binding) energy (Eads) of each structure; Eads = EINT + ΔEdef. Detailed descriptions of the MC and DFT methodologies and definitions of Eads, EINT, and


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ΔEdef are provided in the Experimental Section and Supporting Information. The MC simulations followed by DFT geometry optimizations reveal three different adsorption sites in the void space, namely Sites I, II, and III in the fully open “bbbb” configuration, when loaded with a total of 12 ethylene molecules, as shown in Figure 4. For the initial conditions, the single-crystal structure of 4, into which 12 ethylene molecules were loaded (i.e., three ethylene molecules per Co2+ in the unit cell), was employed. Site I corresponds to the open metal site of the Co paddle-wheel SBU. Sites II and III are located in the void spaces around Site I. This fully open structure (“bbbb”) with 12 ethylene molecules is referred to as “case G”, whereas the evacuated structure with the “aaaa” configuration is “case A”. The calculation results are summarized in Table 1.

possible intermediate structures with site-occupancy combinations of II×4 or III×4 were excluded when loaded with four or six ethylenes. Hence, seven possible and representative states are envisaged, namely A to G, where A and G correspond to the evacuated (2) and fully adsorbed (4) phases, respectively (Figure 5). Cases C/D and E/F have equal numbers of ethylenes, four and six respectively, but differ in their site occupancies and framework structures. The adsorption energetics for these envisaged combinations of adsorption site occupancies and ethylene loadings are given in Table 1. Considering these combinations and structures, two pathways for reaching G are proposed: Pathway 1: A →B→C→E→G, and Pathway 2: A→B→D→F→G (Figure 5). Table 1. Adsorption energiesa (E ads, kcal mol–1) of ethylene in Co(VTTF). n

A

0

–

B

2

I×2

C

4

I×2, II×1, III×1

D

4

I×4

E

6

F

6

G

12

Figure 4. Case G structure containing total 12 ethylene molecules. The rectangle depicts the one unit cell, in which four ethylene molecules are present at each Site I (magenta), II (green), and III (orange).

To elucidate the details of the A to G SCSC transformation process, we investigated the geometries and stabilities of possible intermediates with different numbers of loaded ethylene molecules (0, 2, 4, and 6 per unit cell) (Table 1). We also considered several combinations with the same loading number but different adsorption-site occupancies. Because the adsorption energies of Sites II and III were almost identical, we treated these two sites as energetically equal. In addition, since Sites II and III in the “half-open” phase can accommodate a maximum of two ethylenes per site,

EINT Adsorption Site Occupancies ()

Case

I×2, II×2, III×2 I×4, II×1, III×1 I×4, II×4, III×4

ΔEdef Eads () ()

–

0

–

–43.0 (–21.5) –65.5 (–16.4) –83.2 (–20.8) –88.6 (–14.8) –107.2 (–17.9) –172.7 (–14.4)

21.8 (10.9) 22.4 (5.6) 60.4 (15.1) 22.8 (3.8) 62.3 (10.4) 63.0 (5.3)

–21.1 (–10.6) –43.1 (–10.8) –22.8 (–5.71) –65.8 (–11.0) –44.9 (–7.5) –109.7 (–9.1)

a

= + , where and are the average interaction energy (EINT) per ethylene and the deformation energy (ΔEdef) with respect to the number of adsorbed ethylene molecules, respectively. Note: Another possible state of I×4, II×2, III×2 was not examined because of the similarity with case G where only full-open structure is allowed.

Figure 5. Energy diagram of ΔEdef as a function of the number of ethylenes loaded into the unit cell. The color coding is same as that in Figure 2. Ethylene molecules in Sites I, II, and III are depicted as space-filling models and are colored magenta, green, and orange, respectively. Possible transformation trajectories: Pathways 1 and 2, are indicated by red and blue dashed lines, respectively. The structures of two unit cells are depicted.



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Case B, in which two ethylene molecules are loaded into one unit cell, was considered the possible meta-stable structure involved in the transformation process. Each ethylene molecule occupies only Site I but Site II and III are unoccupied because ethylene adsorption energy at Site I is much larger than that at Site II and III. This means that case B adopts "abab" configuration. MC followed by DFT calculations reveal that B is 21.1 kcal/mol more stable than A (“aaaa” configuration without ethylene) (Table 1). Subsequent structures containing four ethylene molecules were considered. In this case, two configurations, “abab” and “bbbb”, are possible; in the former two ethylenes occupy Site I and two more ethylenes occupy Sites II and III (case C), while in the latter four ethylene molecules remain at Site I (case D), as shown in Figure 5. Case C was calculated to be 43.1 kcal/mol more stable than A (Table 1). On the other hand, D was calculated to be 22.8 kcal/mol more stable than A (Table 1). We next considered further adsorbed state loaded with six ethylene molecules. In this state, two configurations (cases E and F) are possible, as shown in Table 1 and Figure 5. Case E, with the “abab” configuration, was calculated to be 65.9 kcal/mol more stable than A, while F, with the “bbbb” configuration, was calculated to be 45.0 kcal/mol more stable than A and less stable than E by 25.9 kcal/mol. On the basis of these results, we conclude that ethylene adsorption occurs via Pathway 1 (A→B→C→E→G) because the system is unlikely to form the energetically less stable D and F when four and six ethylene molecules are loaded. These results suggest that gate-opening occurs in a stepwise manner; i.e., during the A-to-B (E) and E-to-G transitions. According to the calculated energies, the energy gained by ethylene adsorption, , is insensitive to the number of ethylenes over the entire process, as they range between –14.4 to –21.5 kcal/mol (Table 1), which is ascribed to the difference in stabilization energy between adsorptions at Site I and Site II/III. However, the deformation energy, , varies significantly between 3.8 and 15.10 kcal/mol (per ethylene molecule; see Table 1). Hence, the differences in the adsorption energy, Eads, among the various cases arise mostly from framework deformation. The deformation energy of the framework was evaluated for each case, as shown in Figure 5. Interestingly, the deformation energy is 21.8 kcal/mol for the transformation of A to B, which corresponds to the first gate opening. The deformation energy changes little on progressing from B to C and from C to E because the half-open “abab” configuration is conserved during these transformations. On the other hand, the deformation energy was calculated to increase significantly when going from B to D because the second gate-opening alters the configuration from “abab” to “bbbb”; subsequently, the deformation energy changes little during the transformations from D to F and from F to G, because the “bbbb” configuration is maintained. These results lead to the conclusion that gate-opening is destabilizing from an energy perspective; in particular, the second gate-opening is more destabilizing than the first. When four ethylene molecules are loaded, two ethylenes occupy Site I, as in case B, and another two occupy Sites II and/or III without gate-opening, leading to case C. If two more ethylenes occupy Site I then the second gate opening must occur, leading to the “bbbb” configuration. However, this scenario induces a larger destabilization energy; hence the transformation from B to D is energetically disfavored.

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The possible pathways discussed above are supported by experimental evidence, namely the SXRD analysis of 3, which corresponds to case E. This, in turn, discounts the involvement of F and supports Pathway 1 as the true trajectory for the adsorption process. Both theoretical studies and experimental observations indicate that ethylene adsorption involves a multi-step structural transformation (Pathway 1), which is facilitated by increasing ethylene pressure. In contrast, ethane molecules are unable to unlock the nonporous phase, as evidenced by the significantly low interaction energy of ethane towards the Co open metal site compared to ethylene (See Experimental Section and Supporting Information). The ethylene molecules that break down the crosslinks in the flexible framework trigger the global gate-opening event, facilitating accelerated gas sorption in a cooperative fashion. The “abab” disproportionate splitting of the framework occurs in a cooperative manner over the entire PCP crystallite, as observed in the SXRD and PXRD studies. This suggests that the PCP molecular structure (namely the [Co(VTTF)]n chains) transforms synergistically to adopt the structure of the regular “abab” intermediate, rather than undergo random splitting. This is a reminder of the pivotal role that framework flexibility (degree of freedom) plays in mediating the global SCSC transformation that results in the nonlinear response in the solid phase. Temperature-Swing Adsorption Experiment. In order to confirm the existence of the three different proposed adsorption sites in the framework, temperature-swing adsorption (TSA) experiments were performed starting with a 10:90 ethylene/ethane gas mixture. This ratio was chosen to maintain the minimum partial pressure of ethylene over 10 kPa that is above the gate-opening pressure at 170 K for ethylene. Details of the experimental TSA protocols are provided in the Supporting Information. Briefly, evacuated PCP 2 was placed in the chamber into which the feed gas was introduced at a constant flow rate, at 190 K and 1 bar, for 12 h. We selected this temperature for the TSA experiment in order to maintain the adsorption temperature well above the boiling point of ethane as well as to attain the adsorption capacity close to maximum. Analysis of the adsorbed phase by gas chromatography (GC) following the experimental TSA cycle reveals that the adsorbed phase was ethylene enriched, with an apparent ethylene content of 36% (Figures 6a and S13a). It should be noted that the gate-opening pressure of the PCP 2 at 190 K may be shifted to rather higher pressure than that of at 170 K although we could observe consistent adsorption of ethylene by the TSA with the composition of 10:90 ethylene/ethane. This fact is possibly due to the difference in the experimental condition between the single-component gas sorption experiment and the TSA experiment. It is to be noted that the exact value of gate opening pressure is dependent on various parameters of experimental condition as well as crystallite sizes of PCP.12 Further, we carried out the similar TSA experiment with 50:50 ethylene/ethane gas mixture that also confirmed ethylene enrichment in the adsorbed phase with ethylene content of 65 % (Figures 6b and S13b) as expected.


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Figure 6. Schematic presentations of ethylene and ethane fractions in the feed and adsorbed gases with a feed gas of C2H4:C2H6 mixture in (a) 10:90 and (b) 50:50 ratio. Once the ethylene had triggered the structural transformation that opens the framework, the co-adsorption of both gases is possible. However, the different adsorption sites available within the framework provide control over the ethylene-to-ethane gas ratio in the adsorbed phase. Site I, being the open metal site, preferably adsorbs ethylene, whereas Sites II and III are assumed to be similarly accessible to both ethylene and ethane. In this hypothesis, Site I accommodates four ethylenes while Sites II and III can host a combined total of eight ethylene and ethane molecules, preserving the 10:90 feed gas ratio. This provides an expected 40:60 ratio of ethylene/ethane in the adsorbed phase (See Table S2 for detail estimations). Similarly, the expected ethylene/ethane ratio in the adsorbed phase is 67:33 for the 50:50 ethylene/ethane gas mixture (Table S3), which are close to the observed value, in good agreement with the proposed mechanism. High Pressure and High Temperature Adsorption Behavior. According to the ethylene adsorption isotherms acquired at different temperatures, the gate-opening pressure increases with increasing temperature. The gate-opening phenomenon was observed at above 9.0, 57, 165, 255, and 272 kPa at 170, 273, 283, 298, and 303 K, respectively (Figures 7 and S9). Interactions between the gas molecules and the framework decrease with increasing temperature. Hence, the energy gain required to balance framework deformation is lowered, which results in higher gate-opening pressures at elevated temperatures. The number of adsorbed ethylene molecules at temperatures above 273 K was determined to be ~0.8 ethylenes per Co2+ at the saturation pressure. This trend was maintained even at higher pressure (1 MPa). It should be noted that ethane never opened the gate, even at pressures above 1 MPa at 283 K (Figure 7). The in-situ PXRD pattern of 2 with ethylene at 1 bar at 273 K was found to be well matched to the simulated pattern of the “halfopen” structure 3 (Figure S10), indicating that the PCP adopts the structure corresponding to case C (Figure 5). This observation also indicates that Pathway 1 is a reasonable choice at higher temperatures. In the adsorption isotherm at 273 K (Figure 7a), the first gate opening event, with abrupt uptake, was observed at 57 kPa, in which the number of ethylenes adsorbed was ~0.43 per Co2+. This first gate-opening event presumably corresponds to the A-to-B transition since structure B is able to accommodate 0.5 ethylenes per Co2+, which is in good agreement with the observed value. This abrupt uptake suggests that the A-to-B gate opening occurs in a highly cooperative manner that is driven by the recognition of ethylene followed by synergistic structural changes. Subsequently, the second step occurs with the gradual uptake of up to 0.85 ethylenes per Co2+, which corresponds to the B-to-C transition with occupation of the voids in the “half-open” phase (Figure 5).

Figure 7. Adsorption/desorption isotherms for 2 with ethylene (blue) and ethane (red) at (a) 273 K to 100 kPa and (b) 283 K to 1 MPa.

Cooperative Effect on the SCSC Transformation Behavior. The most intriguing feature of the Co(VTTF) ethylene-adsorption isotherm is the steep rise in the adsorption isotherm observed once the ethylene pressure reaches a threshold value (above 9.0 kPa at 170 K and above 57 kPa at 273 K) (Figures 3 and 7), which is a strong indicator of structural changes associated with a high degree of cooperativity. We analyzed the ethylene adsorption isotherm using the Hill model, at both 170 and 273 K. The Hill coefficient (n), which is the measure of the degree of cooperativity, for the first step in the ethylene-adsorption isotherm at 170 K was determined to be 27.6 (>1) (Figure 8a), confirming a highly positive cooperative-adsorption phenomenon. This first step corresponds to the rapid overall transformation of A to E, as evidenced by the in-situ gas sorption/PXRD analysis (Figure 3). This step is followed by a less-cooperative second step that corresponds to the E-to-G transformation. At the higher temperature (273 K), the positive cooperativity in the first step is even stronger (n = 129.5) (Figure 8b). At this temperature, the first step corresponds to the transformation of A to B, which is followed by the second B-to-C step (vide supra). It should be noted that the B-to-C step involves no structural transformation. The difference in cooperativity in each step is due to the temperature, as well as the transition structures and stabilizing abilities of the guest molecules. The desorption isotherm at 273 K exhibits a large hysteresis associated with steep desorption (n = 2.9) at low pressure, which is also an indication of cooperativity during the reverse structural transformation (Figure S11). The highly cooperative effects achieved by Co(VTTF) PCP arise from the scission of the hemilabile Co–S crosslink induced by the ethylene guest itself. This ensures that the proposed “unlocking” gate-opening mechanism is able to regulate molecular adsorption behavior using weak interactions as effective triggers.



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Figure 8. Hill plot of the ethylene adsorption isotherm at (a) 170 K and (b) 273 K, where the vertical axes display fractional ethylene loadings. Dashed lines indicate the fitting results for each gate-opening step. and n is the Hill coefficient calculated for a given regime.

CONCLUSION The softness of porous materials can be effectively used for the selective adsorption of small molecules by exploiting their abilities to “unlock” the framework when comparable sizes and similar physicochemical properties limit their discrimination. We designed and demonstrated a guest-specific gate opening by cooperative Co–S bond scission in a crosslinked framework that recognizes ethylene over ethane and discriminately adsorbs ethylene through nonporous–porous SCSC transformations. Moreover, the ethylene-exclusive adsorption properties remain unaffected at high pressure and ambient temperature, which illustrates the importance of the unlocking mechanism in the design of materials. The use of hemilabile links in flexible frameworks is a promising approach for the design of cooperative system through controlled structural transformations. The mechanism revealed in this study provides new directions for the design of soft porous crystals for the development of absorptive-based separation technologies.

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finement using SHELXL 2017/1. Due to severe disorder, the guest and solvent molecules were removed using the SQUEEZE routine of the PLATON software when necessary, after which the structures were refined again using the data generated. All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were added via a riding model. The refinement results for 1, 1’, 2, 3, and 4 are provided in Table S1. Coincident in-situ Gas Sorption/PXRD. Coincident in-situ gas sorption/PXRD experiments were performed using a Rigaku SmartLab diffractometer (with Cu Kα radiation) connected to BELSORP–18PLUS volumetric-adsorption equipment (MicrotracBEL Corp. Japan,). The pieces of equipment were automated and synchronized with each other. A powder X-ray diffraction pattern was obtained at each equilibrium point in the gas sorption isotherm. The BELSORP–18PLUS volumetric adsorption apparatus was equipped with a cryostat system for the control of the experiment temperature. Theoretical Calculations. To determine the possible ethylene occupations, classical Monte-Carlo (MC)11 simulations were carried out using the standard universal force field (UFF),13 where the atomic charges were calculated using the charge-equilibration method,14 and the electrostatic interactions were evaluated using the Ewald summation method. The adsorption structures and adsorption energies were calculated using a spinpolarized density functional theory (DFT) method with periodic boundary conditions, as implemented in the Vienna ab initio simulation package (VASP 5.4.1).15,16 The Perdew–Burke–Ernzerhof functional17 with Grimme’s semiempirical “D3” dispersion term18 (PBE–D3) was employed in these calculations. A plane-wave basis set with an energy cutoff of 500 eV was used to describe the valence electrons, while core electrons were described by projector–augmented–wave pseudopotentials.19,20 Γ–point sampling for the Brillouin zone was used. During the geometry optimization, both cell parameters and atomic positions were fully optimized until all atomic forces were below 0.01 eV/Å. A Hubbard U correction,21 with a U value of 5.3 eV,22 was applied to the localized d-electrons of the Co2+ center. Adsorption energies were evaluated using the same method. Details of the computational methods are provided in the Supporting Information.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Synthetic procedures and characterizations of the ligand and PCPs, computational methods, TSA procedure, SXRD refinement results, PXRD patterns, TG, and GC data (PDF). X-ray crystallographic data (CIF, 1) X-ray crystallographic data (CIF, 1’) X-ray crystallographic data (CIF, 2) X-ray crystallographic data (CIF, 3) X-ray crystallographic data (CIF, 4)

AUTHOR INFORMATION

EXPERIMENTAL SECTION Synthesis of the ligand. The protocol for the synthesis of the VTTF ligand is provided in the Supporting Information. Synthesis of [Co(VTTF)(H2O)(Solvent)]n (1). An equimolar amount of the VTTF ligand (50 mg) and Co(NO3)·6H2O (25.57 mg) were added to a mixture of DMAc (20 mL) and methanol (20 mL), followed by heating at 80 °C for 48 h in a glass vial. The dark-brown crystals were collected and thoroughly washed with methanol. The washed crystals were stored in methanol (50 mL) at 25 °C. The solvent was decanted and replaced three times a day for 5 d. SXRD Analysis. SXRD experiments were performed at 93 K with a Rigaku XtaLab P200 diffractometer and a Dectris Pilatus 200K CCD system equipped with a MicroMax–007 HF/VariMax rotating anode X-ray generator with confocal monochromated Mo Kα radiation. The crystal structure was solved directly and refined by full matrix least–squares re-

Corresponding Authors *[email protected]; [email protected]

Present Address ‡

Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS We thank M. Gochomori, N. Shimanaka, and A. Terashima for their expert technical synthetic and SXRD analysis support. Dr. A. Hori and


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Y. Sato are acknowledged for useful discussions surrounding the experiments. This work was supported by a KAKENHI Grant-in-Aid for Specially Promoted Research (No. 25000007) from the Japan Society of the Promotion of Science (JSPS). N.H. acknowledges JSPS for a KAKENHI Grant-in-Aid for Young Scientists (B) (No. 16K17959) and the Regional Innovation Strategy Support Program (Nextgeneration Energy System Creation Strategy for Kyoto) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. S. Kitagawa acknowledges the ACCEL program (JPMJAC1302) of the Japan Science and Technology Agency for financial support. The synchrotron radiation experiments were performed at the BL02B1 beamline of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2016B1075).

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Cooperative Bond Scission in a Soft Porous Crystal Enables

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