Reversible Switching between Nonporous and Porous Phases of a New SIFSIX Coordination Network Induced by a Flexible Linker Ligand

Abstract

Closed-to-open structural transformations in flexible coordination networks are of potential utility in gas storage and separation. Herein, we report the first example of a flexible SiF₆²⁻-pillared square grid material, [Cu(SiF₆)(L)₂]_n (L = 1,4-bis(1-imidazolyl)benzene), SIFSIX-23-Cu. SIFSIX-23-Cu exhibits reversible switching between nonporous (β1) and several porous (α, γ1, γ2, and γ3) phases triggered by exposure to N₂, CO₂, or H₂O. In addition, heating β1 to 433 K resulted in irreversible transformation to a closed polymorph, β2. Single-crystal X-ray diffraction studies revealed that the phase transformations are enabled by rotation and geometrical contortion of L. Density functional theory calculations indicated that L exhibits a low barrier to rotation (as low as 8 kJmol⁻¹) and a rather flat energy surface. In situ neutron powder diffraction studies provided further insight into these sorbate-induced phase changes. SIFSIX-23-Cu combines stability in water for over a year, high CO₂ uptake (ca. 216 cm³/g at 195 K), and good thermal stability.

Metal–organic materials (MOMs)¹ such as metal–organic frameworks (MOFs)² and porous coordination polymers (PCPs)³ offer potential solutions to the high-energy footprint, costs, and risks associated with storage or purification of gases and vapors.⁴ Whereas rigid microporous MOMs typically display type I isotherms, flexible MOMs (FMOMs)⁵ can exhibit stepped isotherms from structural contraction (expansion) under reduced (increased) pressure, i.e., breathing or swelling.⁶ FMOMs that exhibit stepped type F–IV isotherms, sudden switching from a nonporous (closed) activated phase to a porous (open) phase, are of particular interest for pressure swing adsorption (PSA) gas storage,⁷ as negligible adsorbate is present at low pressure.⁸ Closed-to-open transformations might also facilitate gas separation if adsorbates selectively induce transformations.⁹ Thus far, only ca. 150 FMOMs of the >20 000 porous MOMs reported are known to be flexible,^5b,10 and just a handful exhibit type F-IV isotherms with saturated uptake of >200 cm³/g.¹¹ None of these high-uptake FMOMs are sustained by inorganic linker ligands, a matter we address herein.

The “pillared sheet” platform of materials comprised of cationic square grid lattice (sql) sheets and hexafluorosilicate (SiF₆²⁻, “SIFSIX”) pillars¹² exhibit benchmark performance with respect to separation of industrial gases such as CO₂, SO₂, and small-molecule hydrocarbons.¹³ Whereas modeling studies indicate that rotation of pyrazine rings in SIFSIX-3-M (M = Ni, Fe) can cause inflections in Xe adsorption isotherms, the framework remains rigid.¹⁴ SIFSIX nets can also transform into nonporous sql or sql-c* nets under humid conditions.¹⁵ Herein, we introduce the first flexible SIFSIX net, [Cu(SiF₆)-(L)₂]_n (L = 1,4-bis(1-imidazolyl)benzene), SIFSIX-23-Cu (Figure 1).

Figure 1.

(a) Self-assembly strategy used to design SIFSIX-23-Cu. (b) 1D channels along the a-axis in the as-synthesized form, SIFSIX-23-Cu-α.

Layering 1:1 MeOH/H₂O onto CuSiF₆ in H₂O followed by further layering of a solution of L in MeOH afforded purple, needle-shaped crystals of SIFSIX-23-Cu·xMeOH·yH₂O (see Supporting Information (SI)), SIFSIX-23-Cu-α. Cu(II) cations are octahedrally coordinated to four equatorial L ligands (two L(syn) and two L(anti)), while SIFSIX anions occupy the axial sites to afford a noninterpenetrated pcu topology framework (Figure 1a). The CuL₂ sql net undulates due to out-of-plane protrusion of “V”-shaped L(syn) linker ligands. The pillars adopt a cis-bridging mode, unlike other coordination networks involving SIFSIX anions (Figures S1 and S2).^12,13,16 SIFSIX-23-Cu-α possesses a 1D channel of diameter 5.4 × 4.0 Ų along the a-axis (Figure 1b) with a calculated accessible void volume of 41.8% (Figure S3).

We found that desolvation induces SIFSIX-23-Cu-α to undergo multiple phase transformations to SIFSIX-23-Cu-γ1, -γ2, -γ3, and a solvent-free closed form, SIFSIX-23-Cu-β1 (Figures 2a and S4, Table S1). Whereas the space group and unit cell parameters vary, the connectivity of the SIFSIX-23-Cu network is preserved even with extreme deformation (Figure 2b, Table S2). The α to γ3 transformation caused the parallelogram of the sql net to undergo hinge-like motion with negligible change in edge length (Δd_max = 0.2 Å) but a decrease in ∠Cu–Cu–Cu from 90° (α) to 53.4° (γ3) (Figure 2b and c). Concomitantly, one diagonal of the parallelogram reduced from 18.46 Å (α) to 11.67 Å (γ3), whereas the other expanded from 18.46 Å (α) to 23.19 Å (γ3). An overall reduction in guest-accessible volume from 41.8% (α) to 20.4% (γ3) was calculated by PLATON.¹⁷ Removal of solvent from γ3 afforded SIFSIX-23-Cu-β1, a nonporous phase (Figures 2b and S5 and S6) with 34.5% volume reduction vs the α phase. The transformation from γ3 to β1 differs from the other phase changes (Figure 2b and d). FMOMs rarely exhibit such extreme structural changes.¹¹ β1 reverted to α after immersion in MeOH, EtOH, or CH₃CN (Figure S7).

Figure 2.

(a) Guest-induced reversible structural transformations occur between the α, γ1, γ2, γ3, and β1 phases. (b) A parallelogram defines the structural parameters of each phase. Deformation of the parallelogram during the α to γ3 (c) and the γ3 to β1 (d), transformations. (e) Overlapping parallelograms of the α and β1 phases highlight structural distortions.

Structural analysis of the SIFSIX-23-Cu phases (Figures 2e and S8–S11, Table S2 and S3)¹⁸ reveals hinge-like motion in the α to γ1 transformation that originates from rotation of imidazolyl rings and reorientation of phenyl rings. Indeed, rotation of L is a feature of each transformation (Figure S8). In the γ1 to β1 transformation, the coordinated N atom acts as a hinge or “kneecap” at which the Cu-imidazolyl junction bends (Figure S9). For the γ3 to β1 transformation, one imidazolyl ring of L(syn) bends at the N(1)_imidazolyl atom to subtend an angle of 16.9° between the C_phenyl–N_imidazolyl bond and the imidazolyl plane (Figure S10),^8d,11d,19 enabling shrinkage of the Cu–L(syn)–Cu edge in β1. These structural changes are mainly associated with L(syn) (Figure S11). C–H···F hydrogen bonds between SIFSIX anions and CH moieties of aromatic groups (Table S4) are often present in SIFSIX coordination networks (Figure S12).¹³ Density functional theory (DFT) calculations indicated that L exhibits a low rotational barrier (<8 kJ/mol) and a rather flat energy surface with minima corresponding to L(syn) and L(anti). A Cambridge Structural Database (CSD)²⁰ survey revealed that dihedral angles between phenyl and imidazolyl rings are consistent with those in L(syn) and L(anti) (Figures S13–S15). The energy difference of L(syn) calculated before and after bending from γ3 to β1 is also small (<16 kJ/mol) (Figure S16). The nature of L helps to explain the “softness” of SIFSIX-23-Cu.²¹

Variable-temperature PXRD studies of the α to β1 transformation (Figures 3a and S17) revealed a new phase at 425 K as also suggested by DSC (Figure 3b). That no weight loss was seen in the corresponding TGA curve indicates that this was a temperature-controlled rather than guest-controlled transformation. Heating β1 or any of the α phases at 433 K afforded single crystals of SIFSIX-23-Cu-β2. Single-crystal X-ray diffraction (SCXRD) characterization revealed that β2 exhibits a slightly smaller cell volume than that of β1. The connectivity is unchanged if anti-to-syn switching of L(anti) and swing motion of SIFSIX anions (Figure 3c) had occurred. That β2 is more stable than β1 is supported by its higher density, structural parameters which suggest less strain (Figure S11) and lack of reversibility after immersion in water, MeOH, EtOH, or CH₃CN (Figure S18) and periodic DFT calculations (PBE and PBE-dDsC). The DFT calculations revealed that small global energy differences between the α to γ3 phases (Figure 3d) are consistent with the reversibility of the structural transformations (Table S5, Figure S19). The energy difference between β1 and γ3 is 86.7 kJ/mol, a value that can be offset by adsorption. The β2 phase was determined to be relatively more stable (−222 kJ/mol vs α).

Figure 3.

(a) VT-PXRD of SIFSIX-23-Cu from 295 to 515 K. (b) TGA and DSC of SIFSIX-23-Cu. (c) Structural changes in the β1 to β2 transformation. (d) Relative energies of the structurally characterized phases (kJ/mol, PBE and PBE-dDsC).

Since the closed-to-open transformation from β1 to α was induced by solvent, we anticipated that gases might also trigger switching. The CO₂ isotherm of β1 at 195 K indeed displayed switching with two steps (Figure 4a). The first step at ca. 2.4 mmHg (adsorbed CO₂ = 84 cm³/g) gave a micropore volume of 0.16 cm³/g, close to that estimated from the crystal structure of γ3 (0.15 cm³/g). The second step occurred after saturation (ca. 216 cm³/g) with a micropore volume of 0.40 cc/g, in good agreement with the structure of α (0.40 cm³/g). The Langmuir surface areas for the phases in the first and second steps calculated from the CO₂ isotherm at 195 K are 451 and 941 m² g⁻¹, respectively (Figures S20 and S21). β1 also displayed gate opening for N₂ at 77 K with an onset pressure of 142 mmHg and saturated uptake of 160 cm³/g at 738 mmHg (Figure 4a). In contrast, the N₂ (77 K) and CO₂ (195 K) isotherms for β2 revealed no uptake (Figure S22).

Figure 4.

(a) Gas sorption isotherms of β1 at 195 K (CO₂) and 77 K (N₂). (b) CO₂ sorption isotherms of β1 at 273 and 298 K. (c) Adsorption/desorption cycles of β1 at 273 K. (d) High-pressure CO₂ sorption isotherms at 25 to 55 °C. (e) In situ NPD patterns at various conditions. At 77 K, 1 bar N₂ trace β1 is present because of large sample size (>1 g) and slow kinetics; equilibrium took >24 h. (f) Water sorption isotherm of β1 at 298 K.

In situ neutron powder diffraction (NPD) data were collected under various conditions to further study these switching events (Figure 4e). The diffraction pattern obtained at 0 bar matched that calculated for β1. For P = 0.02 bar at 195 K (the first step), the NPD pattern resembled that calculated for γ3. For P = 0.2 bar (the second step), the NPD pattern matched that calculated for α. The α phase reverted to β1 at P = 0 bar and 298 K. NPD data of samples collected at 77 K under 1 bar N₂ validated the formation of α. The saturated uptake of N₂ was lower than that calculated from the pore volume based on the CO₂ sorption data (258 cm³/g), perhaps because at 77 K sorbate–sorbent interactions are strong enough to hinder diffusion.²² This guest-dependent switching behavior suggests that structural transformations are driven by host–guest interactions.

The low-pressure CO₂ sorption isotherms at near ambient temperature revealed no switching behavior induced by CO₂ at 298 K below 1 bar (Figures 4b and S23). However, β1 exhibited a single step type F-IV isotherm at 273 K with a gate-opening pressure of 505 mmHg and saturated uptake of 77 cm³/g at 759 mmHg. The corresponding desorption isotherm displayed hysteresis. The calculated micropore volume of 0.15 cm³/g agrees with that calculated from the crystal structure of γ3 (0.15 cm³/g). In situ NPD data collected at 273 K and 1 bar CO₂ showed a similar NPD pattern to that obtained at 195 K for 0.02 bar CO₂, i.e., the γ3 phase (Figure 4e). That this phase switching is reversible was demonstrated by 41 cycles of CO₂ adsorption/desorption (Figures 4c and S24–S26). PXRD confirmed structural integrity (Figure S27). CO₂ triggered closed to open switching with an onset gate-opening pressure of <1 bar at 273 K observed in the ELM family of materials and zeolitic imidazolate frameworks (ZIFs).^8b,23

High-pressure CO₂ sorption on β1 (Figure 4d) resembled the isotherm obtained at 195 K. The onset gate-opening pressures for the first and second step at 298 K are around 2 and 5.8 bar, respectively. Interestingly, even at 55 °C CO₂ triggers gate-opening with a threshold pressure of ca. 5.3 bar, a pressure suitable for high-temperature gas sorption and separation. CO₂-induced closed-to-open switching from β1 to α at high pressure was found to be reversible (Figures S28–S30).

SIFSIX-23-Cu-β1 also underwent closed-to-open switching induced by water vapor at 298 K (Figure 4f). The steep water uptake (2.8%) at low humidity matches the SCXRD results, which indicate that β1 can capture one water molecule without a structural change. β1 sequentially formed γ3, γ2, and γ1 under atmospheric humidity (Figures S31 and S32) whereas water immersion caused β1 to revert to α (Figures S7). SIFSIX-23-Cu-α retained single-crystallinity after immersion in water for one year (Figures 5 and S33 and S34). This strong hydrolytic stability contrasts with that of other SIFSIX materials.¹⁵

Figure 5.

(a) As-synthesized crystals compared to those soaked in water for one year. (b) PXRD pattern of SIFSIX-3-Cu soaked in water for one year compared with that calculated for α. (b) The 195 K CO₂ and 77 K N₂ adsorption isotherms of samples before and after being soaked in water for one year.

In summary, the new pillared square grid coordination network SIFSIX-23-Cu exhibits an uncommon cis-bridging coordination mode of SIFSIX anions and is to our knowledge the first hybrid coordination network that undergoes reversible guest-induced closed-to-open transformations. It is also one of the highest uptake materials with a type F-IV isotherm. We attribute these facile and reversible transformations to the low rotational barrier of L and the correspondingly flat energy surface.