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. 2024 Nov 4;63(46):22315–22322. doi: 10.1021/acs.inorgchem.4c04286

Revealing Disorder, Sorption Locations and a Sorption-Induced Single Crystal–Single Crystal Transformation in a Rare-Earth fcu-Type Metal–Organic Framework

A R Bonity J Lutton-Gething †, Fajar I Pambudi †, Ben F Spencer ‡,§, Daniel Lee ∥, George F S Whitehead †, Inigo J Vitorica-Yrezabal †, Martin P Attfield †,*
PMCID: PMC11577313  PMID: 39494500

Abstract

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Rare-earth metal–organic frameworks (RE-MOFs) formed in the presence of fluoride donors are a group of complex and applicable MOFs. Determining structural complexity is crucial in applying such MOFs and has been achieved to uncover framework disorders in the important fcu framework topology MOF, Y-ndc-fcu-MOF (1). 1 is found to contain F– groups disordered over the μ3-face-capping sites in its secondary building unit (SBU) and framework distortions upon sorption of different guest molecules. The favored location of the guests is within the octahedral cage of 1 where they interact with the Y3+ centers. The size, shape, and interactions of the different guests lead to subtle distortions within the SBU and adoption of specific orientations of the naphthalene group of the 1,4-naphthalenedicarboxylate framework linkers. The sorption of DMF(l)/H2O(l) lowers the symmetry from cubic Fm3̅m (for MeOH(l), N2(g), CO2(g or l)) to cubic Pa3̅ (for DMF(l)/H2O(l)) symmetry with retention of the fcu topology, and conversion between the Pa3̅ and Fm3̅m structures is induced by solvent exchange. Such disorder and sorption locations and transformation are important considerations during the optimization and application of MOFs for sorption-based technologies.

Short abstract

Structural reinvestigation of the metal−organic framework Y-ndc-fcu-MOF finds μ3-F− groups disordered over face-capping sites in its secondary building block (SBU) and framework distortions upon sorption of guest molecules. Guests are located within the octahedral framework cages, where they interact with the Y3+ centers. The different guests lead to subtle distortions within the SBU and specific orientations of the framework linker with an example of an accompanying Fm3̅m to Pa3̅ symmetry change of the fcu topology framework.

Introduction

An exciting new group of functional metal–organic framework (MOF) that is emerging are rare-earth (RE) MOFs formed from synthesis mixtures containing modulators or linkers possessing terminal fluorine atoms.1,2 These RE-MOFs demonstrate great promise for numerous applications3−10 and possess complex framework structures that contain various degrees of disorder involving the constituent organic linker or the secondary building unit (SBU). Such disorder is particularly apparent in RE-MOFs containing the cuboctahedrally connected RE6XaR12 SBU that consists of six RE3+ ions organized in an octahedral or trigonal antiprismatic formation face-capped by μ3-X groups (X is potentially F–, (OH)− or O2–, a ≤ 8) and edge-bridged by the bidentate groups of ditopic organic linkers, R.11 The three-dimensional framework is formed by the connection of the SBUs by the organic linkers. SBUs containing solely μ3-face-capping (OH)− or F– groups have been reported,1,8 in addition to those containing a mixture of μ3-face-capping (OH)− and F– groups,12 and those containing a combination of μ3-face-capping F– groups and assumed (OH)− or O2– species.13,14 Disorder of the organic linker is observed through the adoption of different connection modes of the linker bidentate groups to the SBU12,15 and orientations of the noncoordinating spacer portion of the linker.16 The disorder involving the SBU can occur during initial synthesis of the MOF, while that involving the noncoordinating spacer of the linker will potentially depend upon any host–guest relationships within the void space of the framework. However, the latter has not been experimentally determined within this family of RE-MOF.

The cuboctahedrally connected RE6XaR12 SBU is structurally equivalent to that found in the significant fcu topological MOF group that includes UiO-66 and its numerous derivatives among its members.17,18 An archetypal member of this RE-fcu-MOF group is Y-ndc-fcu-MOF (1).16 The reported crystallography derived composition of as-synthesized 1 is (DMA)2[Y6(μ3-(OH))8(ndc)6]·(H2O)6(solvent)x (DMA = dimethylammonium, ndc = 1,4-naphthalenedicarboxylate), with each Y3+ ion coordinated by four O atoms from different carboxylate groups, four μ3-(OH)− groups, and a water molecule.16 The location of the DMA was not determined in the reported crystal structure. The reported framework of 1 consists of a network of face-sharing tetrahedral and octahedral cages which are reachable through three-sided apertures between the cages as shown in Figure 1. 1 and its homologues have been shown to be of particular interest for the selective separation of gas molecules, including to separate carbon dioxide (CO2) from dinitrogen (N2),16 to separate alkane mixtures,16,19 to separate hydrogen disulfide and CO2 from methane20 and to selectively detect ammonia in the presence of other gases.21 Surprisingly given the degree of interest in 1, and other RE-fcu-MOFs formed from synthesis mixtures containing modulators or linkers possessing terminal fluorine atoms, there is little direct experimental structural insight concerning the sorption of such guest species within the void space of the framework12 and the influence that such species may have on the framework structure.

Figure 1.

Figure 1

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Simplified framework of 1 illustrating the face-centered cubic arrangement of SBUs and the distribution of octahedral and tetrahedral cages. Color key: framework atoms, bonds and simplified organic linker groups = black lines; tetrahedral cages = blue spheres; octahedral cage = orange sphere.

To these ends, the chemical composition and structure of 1 has been redetermined in the presence of various proportions of a variety of guest species to try to identify and characterize any inherent framework disorder, host–guest interactions and resultant guest-induced framework disorder. We find that the SBU of 1 contains μ3-F– groups and that the framework structure undergoes a symmetry changing distortion or different degrees of naphthalene group rotation as a function of the nature of the guest species indicating the subtle framework flexibility of this fcu-based system.

Experimental Methods

Reagents

Y(NO3)3·6H2O (99.8%, Aldrich) 2-fluorobenzoic acid (97%, Aldrich), H2ndc (98+%, Alfa Aesar), anhydrous N,N-dimethylformamide (DMF) (99.8%, Alfa Aesar), nitric acid (HNO3) (70%, Fischer Chemical) and methanol (MeOH) (99.8%, Aldrich) were used as received with no further purification. Distilled water was obtained using a Milli-Q system (18 MΩ cm resistivity at 25 °C).

Synthesis and Activation of 1

9.40 mg (0.0435 mmol) H2ndc, 48.7 mg (0.348 mmol) 2-fluorbenzoic acid, 2.2 mL anhydrous DMF, 16.7 mg (0.0435 mmol) Y(NO3)3·6H2O, 0.3 mL of 3.5 M HNO3 in DMF, and 0.5 mL H2O were combined in a 20 mL scintillation vial.16 The solution was then sonicated for several minutes to ensure dissolution of the precursors and the sealed vial placed in a preheated oven at 115 °C for 72 h. Once cooled to room temperature, the colorless octahedral crystals of 1 DMF/H2O were collected by suction filtration, washed with DMF and dried in air.

1·DMF/H2O was then solvent exchanged to remove DMF and form MeOH-exchanged 1·MeOH by covering the sample with 3 mL MeOH twice daily for 5 days. 1·MeOH was heated in air to 373 K and cooled to room temperature where it adsorbed H2O from air to form 1·H2O.

Single Crystal X-ray Diffraction

Single crystal X-ray diffraction data collection, data reduction and the experimental equipment used were as previously reported.12 All crystal structures were initially solved using the SHELXT22 program using an intrinsic phasing method on an hklf4 file implemented through OLEX2 (v1.5),23 and refined using the SHELXL24 program using least-squares refinement methods against all F2 values. All non-hydrogen framework atoms were refined with anisotropic displacement parameters or isotropically when the latter was not possible.

Naphth group linker disorder was determined from residual electron density peaks observed after refinement of the ordered component of the structure. The occupancies of the disordered naphth group linker atoms within a particular linker arrangement were fixed or refined against a single free variable assuming that the sum of the occupancies of the linker positions was 1. Geometric restraints were applied to the naphth linker to maintain a suitable linker geometry during refinement. Hydrogen atoms were placed in calculated positions and refined with idealized geometries, assigned fixed occupancies and isotropic displacement parameters. Nonframework species were located, modeled and refined after completion of the framework structure. The nonframework atoms were modeled anisotropically with their occupancies refined against a single free variable when possible. The ordered DMF molecule in 1 DMF/H2O was modeled by fitting a rigid body model to electron density resembling DMF. The SQUEEZE25 procedure was applied to estimate the electrons within the void space for structures where guest species were particularly disordered (1·DMF/H2O, 1·MeOH and 1·N2vide infra) but was not applied to the CO2 containing structures to ensure the most accurate identification and estimation of the electron density associated with the coordinated CO2 molecule.

In Situ Single Crystal X-ray Diffraction Gas Adsorption Studies

(i) N2(g) A suitable single crystal of 1·H2O was chosen and activated by heating at 500 K for 2 h before being rapidly cooled (360 K hr–1) to 100 K prior to diffraction data collection. The crystal, 1·N2, was kept under a stream of dry N2(g) from the cryostream throughout the whole experiment. (ii) CO2(g) The description of the homemade gas rig and adapted Huber goniometer head “’gas cell’” have been previously reported.12,26

A suitable single crystal of 1·MeOH was selected and activated in situ by heating to 500 K under dry N2(g) followed by fitting the gas cell capillary over the crystal. A vacuum was then applied at 500 K to ensure that no N2(g) was trapped in the capillary and the gas cell was cooled to 298 K. Once the desired temperature of 298 K was reached, 5 bar CO2(g) was then allowed to enter the cell. Data were then collected at 298 and 200 K allowing 30 min equilibrium time at each temperature. The crystal was held at 200 K under 5 bar CO2 overnight, and data collected the next morning. During this time, a solid CO2(s) crystal formed at the top of the gas cell capillary. The gas cell and crystal were then warmed to 216 K (the liquid/vapor equilibrium point for 5 bar CO2)27 and as expected the solid CO2(s) slowly started melting, washing the crystal with liquid CO2(l). After data collection at 216 K, the gas cell was warmed further to 298 K and vacuum applied, and a final data set collected. The structures were reduced, solved and refined against one or two components where nonmerohedral twining was observed. The isotropic atomic displacement parameter for coordinating oxygen atoms of CO2 molecules was fixed to 0.1 Å2 to avoid atomic displacement parameter-occupancy correlation effects during refinement and to maintain chemically reasonable values of occupancy parameters.

Crystallographic information is provided in Tables S1–S2 and crystallographic information files CCDC 2372655–2372663 contain full details for all crystal structures reported.

Solid-State NMR Spectroscopy

Magic angle spinning (MAS) NMR spectra were recorded in two regimes. High-field 19F MAS NMR spectra were recorded on a Bruker 20.0 T (850 MHz 1H Larmor frequency) AVANCE NEO spectrometer equipped with a 1.3 mm HXY MAS probe that was used in 19F/13C double resonance mode. Experiments were acquired at ambient temperature using a MAS frequency of 60 kHz. 19F-pulses of 91 kHz were used, and an echo sequence was employed to reduce interference from the probe background and used a free-evolution delay of 10 rotor periods on either side of the π-pulse, giving a total echo duration of 0.333 ms. Moderate-field 19F and 13C MAS NMR spectra were recorded on a Bruker 9.4 T (400 MHz 1H Larmor frequency) AVANCE III spectrometer equipped with a 4 mm HFX MAS probe that was used in 1H/19F/13C triple resonance mode. Experiments were acquired at ambient temperature using MAS frequencies of 14 and 12 kHz for 19F and 13C MAS NMR spectra, respectively. 1H-, 19F-, and 13C-pulses of 100, 52, and 50 kHz were used, respectively, and 13C NMR spectra were recorded after {1H-}13C cross-polarization (CP) and echo sequences that used a free-evolution delay of 1 rotor period either side of the π-pulse were employed. For CP, a 70–100% ramp was used for 1H (∼73 kHz νrf at 100%) to match 50 kHz 13C spin-locking. 13C chemical shifts were referenced to TMS and 19F chemical shifts were referenced to CFCl3, both at 0 ppm.

XPS, Elemental Analysis, Powder X-ray Diffraction (PXRD), and Thermogravimetric Analysis (TGA)

Experimental details for XPS, elemental analysis and PXRD are similar to those previously reported.12 Le Bail fitting of the PXRD data were performed using GSASII software.28 Thermogravimetric analyses were measured on a Mettler-Toledo TGA/DSC instrument under flowing N2 from room temperature to 600 °C at a heating rate of 5 °C min–1.

Results and Discussion

SBU μ3-X Group Disorder

The presence of μ3-F– groups in the framework of 1 was determined from a variety of techniques. Elemental analyses of phase pure 1·H2O (see Figure S1) gave a Y:F ratio of 0.77 (Y6F4.6) indicating the incorporation of fluorine into this previously reported nonfluorine containing MOF. The XPS survey spectrum of evacuated 1·H2O indicated the occurrence of fluorine and the high-resolution XPS spectra of Y and F identified peaks at 158.7 (Y 3d5/2), 160.7 (Y 3d3/2) and 685.2 (F) eV respectively (see Figure S2). The values of the Y and F binding energies found for 1 and those reported for Y-fum-fcu-MOF,12 Y–BCA-3D13 and RE-frt-MOF-129 suggest very similar chemical states for these atoms in these MOFs that all contain a (Y6Fa) (a ≤ 8) core within the SBU and μ3-F– groups. 19F magic angle spinning (MAS) NMR spectra of 1·DMF/H2O and 1·MeOH show a single peak with chemical shift δ{19F} = −65 ppm (see Figure S3a) indicating the presence of only one F species in the Y6F4.6 core of the SBU. This shift matches those reported for the μ3-F– group of this SBU type present in other RE-F-fcu-MOFs, namely Y-fum-fcu-MOF12 and Y-DOBDC MOF.14 The presence of μ3-F– groups in the Y6(μ3-X)a core of the SBU is also supported by the X-ray diffraction determined crystal structures of 1 in the presence of different sorbates (vide infra). The atomic displacement parameters of the μ3-groups within the crystal structures at 100 K for nonthermally activated samples are most consistent with the presence of F atoms with 100% site occupancy (see Table S3 for 1·DMF/H2O), implying a majority of face-capping μ3-F– sites in the Y6(μ3-X)a core (X = F–, (OH)− or O2–, a ≤ 8) supporting the above results. Hence, the μ3-X sites in the subsequent sections are named for simplicity in terms of the major μ3-X species, F. These results demonstrate that the SBU of 1 contains a majority of μ3-F– groups disordered over the μ3-X positions and provides another example of a RE-MOF for which the fluoro-containing modulator molecule provides fluoride ions to the SBU.1 The amount of F– incorporated into the SBU (Y6F4.6) is less than that reported in Y-fum-fcu-MOF (Y6F5.6),12 and Y-DOBDC MOF (Y6F∼6)14 that both possess the same type of SBU. This demonstrates that a variable degree of fluoride inclusion occurs that is presumably dependent on the synthetic conditions, for example reagents, pH, temperature, and pressure, employed to prepare the RE-MOF as examined by Balkus et al.30 The degree of fluoride inclusion is a critical parameter to control as it has potentially important ramifications on the properties of the resultant MOFs including those of hydrophobicity, photoluminescence and guest sorption.1,12

Guest-Induced Framework Phase Transitions

1·DMF/H2O

The crystal structure of 1·DMF/H2O was solved and refined in the cubic Pa3̅ space group instead of the previously reported cubic Fm3̅m space group16 after initial inspection of the diffraction data showed the presence of weak diffraction spots that break the F-centering of the pattern and consideration of observed reflection condition data (as shown in Figure S4a–b and given in Tables S4–S7). The Le Bail fit using the PXRD data collected from a bulk powder sample of 1·DMF/H2O shown in Figure S5 supports this space group assignment. The crystallographically determined framework formula of 1·DMF/H2O is [Y6(μ3-F)8(ndc)6]2–·(DMF)1.38(6)(H2O)1.7(1) where framework interacting guest molecules have not been included as part of the framework formula and the charge balancing DMF-derived DMA ions were not located. The presence of DMA and DMF in 1·DMF/H2O are seen in the 13C MAS NMR spectrum as shown in Figure S3b.

The Y6 core of the SBU possesses trigonal antiprismatic geometry with two equilateral and six isosceles triangular faces. The isosceles and equilateral triangular faces are capped by the two crystallographically distinct μ3-F– sites, F1 and F2, respectively as seen in Figure 2a. The framework of 1·DMF/H2O contains one type of octahedral and tetrahedral cage with the latter being lined internally with three F1 and one F2 ions at their vertices. Despite the crystallographic observation of two distinct μ3-F– sites, a single Lorentzian peak is found in the 19F MAS NMR spectrum of 1·DMF/H2O (and at high field for 1·MeOH, vide supra). The presence of a single peak only was attributed to insufficient 19F NMR spectral resolution to resolve the signals from the two distinct μ3-F– sites with nearly identical chemical environments. This is in contrast to the distinguishable μ3-F– sites of Y-fum-fcu-MOF,12 which results from the different local environments of the two μ3-F– sites with regards to the distinct orientations of the fumerate linker in the framework.

Figure 2.

Figure 2

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Representations of the Y6F8 core of the SBU (a), a triangular aperture found within the framework showing the two crystallographically distinct naphth linker groups (b) and the two possible crystallographically distinct naphth linker groups shown at one linker position viewed along the axis formed through the C atoms of the two carboxylate groups of the linker (c) in 1·DMF/H2O. H atoms are omitted for clarity.

The framework of 1·DMF/H2O contains disordered naphth groups of the ndc linker. The naphth linker groups are disordered over two crystallographically distinct orientations. The major naphth orientation has an occupancy of 77.7(4) % and the minor component has an occupancy of 22.3(4) %, as shown in Figure 2b,c. Framework coordinated DMF and H2O molecules were located within the octahedral cages where the electron rich oxygen atoms of both molecules interact with the electron deficient Y3+ ions as shown in Figure 3a–c. Both molecules are relatively weakly coordinated with Y–O bond lengths of 2.61(1) and 2.60(2) Å for DMF and H2O respectively. The occupancy of the DMF and H2O molecules are 23(1) and 28(1) % respectively, occupying 51(2) % of the available open Y sites.

Figure 3.

Figure 3

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Representations of the coordination of a DMF molecule to a Y center within a simplified octahedral framework cage (a) and in greater detail at a Y center (b) and the coordination of a H2O molecule to a Y center (c) in 1·DMF/H2O. H atoms are omitted for clarity.

The ordered nature of the major naphth orientation component, in comparison to the symmetry disordered naphth linker in the previously reported Fm3̅m structure and described vide infra,16 most likely results from interactions between the naphth group and disordered guest molecules present in the void volume such as DMF, H2O or DMA ions, with the DMF molecules most likely to be instrumental in the ordering of the naphth groups due partially to their larger size. The amount of the minor naphth orientation component (22.3(4)%) appears to precisely correlate with the occupancy of the framework-bound DMF molecules (23(1) %) implying that the localized DMF molecules lead to the adoption of the minor naphth component orientation and the reduction of symmetry to the cubic Pa3̅ space group. There are no obvious favorable or unfavorable intermolecular interactions between the framework coordinated DMF molecules and the naphth moieties suggesting that the steric effect induced by the presence of the coordinated DMF molecules influence the orientation of the naphth groups in the ndc linker.

1·MeOH

The TGA and PXRD results for 1·MeOH shown in Figure S7 and S6 show marked differences to those for 1·DMF/H2O indicating successful solvent exchange of DMF for MeOH. The crystal structure of 1·MeOH was solved and refined in the cubic Fm3̅m space group as no weak F-symmetry breaking diffraction peaks were observed in the diffraction data (see Table S7). The Le Bail fit using the PXRD data collected from a bulk powder sample of 1·MeOH shown in Figure S6 again supports this space group assignment. The crystallographically determined framework formula of 1·MeOH is [Y6(μ3-F)8(ndc)6]2–·(MeOH)1.8(1) where the charge balancing DMA ions were not located. The Y6 core of the SBU possesses octahedral geometry with the eight equilateral faces capped by the μ3-F– crystallographically distinct F1 site as seen in Figure 4a. The framework of 1·MeOH contains one type of octahedral and tetrahedral cage with the latter being lined internally with four F1 ions at their vertices.

Figure 4.

Figure 4

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Representations of the Y6F8 core of the SBU (a), triangular apertures found within the framework showing the four possible symmetry related orientations of the naphth linker groups (b) and the four possible symmetry related orientations of the naphth groups of the crystallographically unique ndc linker shown at one framework linker position viewed along the axis formed through the C atoms of the two carboxylate groups of the linker (c) in 1·MeOH. Split occupancy atom sites are represented with two colors in (c) and H atoms are omitted for clarity.

The framework of 1·MeOH contains disordered naphth groups of the linker over four symmetry related orientations as shown in Figure 4b–c, and as previously reported.16 Framework coordinated MeOH molecules were located within the octahedral cages where the electron rich oxygen atoms of the MeOH molecules interact with the electron deficient Y3+ ions as shown in Figure 5a. Eight equally probable orientations of the MeOH molecules were located for which the MeOH molecules are coordinated with Y–OMeOH-CMeOH angles of 129(3) (for C6) or 137(3) ° (for C7) relative to the Y site. The MeOH is relatively weakly coordinated with a Y–O bond length of 2.64(2) Å and occupies 30(2) % of the available open Y sites. The high degree of disorder of the naphth group and MeOH suggests that interaction between the two is negligible and the presence of MeOH as the guest does not reduce the symmetry of the structure from the Fm3̅m space group due in part to its small molecular size compared to DMF.

Figure 5.

Figure 5

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Representations of the coordination of the two crystallographically distinct MeOH molecules to a Y center in 1·MeOH (a) the closest interaction of a N2 molecule to a Y center in 1·N2 (b) and the closest interaction of the end O atom of a CO2 molecule to a Y center in 1·CO2-200 K (c). Selected closest interaction distances are shown as red dotted lines and H atoms are omitted for clarity.

1·N2

The crystal structure of 1·N2 was solved and refined in the cubic Fm3̅m space group. The crystallographically determined framework formula of 1·N2 is [Y6(μ3-F)8(ndc)6]2–·(N2)3.2(2) where the charge balancing DMA ions were not located. The framework of 1·N2 is like that of 1·MeOH with an octahedral Y6 core of the SBU and the disorder of the naphth groups of the linker over four symmetry related orientations in a similar manner to that shown in Figure 4a–c, respectively. Framework coordinated N2 molecules were located within the octahedral cages where the electron rich nitrogen atoms of the N2 molecules interact with the Lewis acidic Y3+ centers and the N2 molecule is arranged linearly with respect to the Y3+ center as shown in Figure 5b. The refined atomic nitrogen positions gave a N–N distance of 1.05(4) Å matching that expected within the triply bonded N2.31,32 The N2 is weakly coordinated as indicated by the long Y···N distance of 3.03(3) Å and occupies 53(3) % of the available open Y sites. The reported N2 BET isotherm for 1(16) shows significant adsorption at a relative pressure (P/Po) of ∼0.15 corresponding well with the physisorption observed here at 100 K (P/Po = 0.128, assuming 1 bar N2). Similar low pressure physisorption behavior of N2(g) has also been observed for Y-fum-fcu-MOF.12 Again, the high degree of disorder of the naphth group suggests that the interaction between the N2 and linker is negligible and the linear nature of the small N2 molecule does not induce a reduction of the symmetry from the Fm3̅m space group for 1·N2.

In Situ CO2 Adsorption

A summary of the conditions and crystallographic results for 1 obtained from the in situ CO2 adsorption experiment are presented in Table 1. The crystal structures of 1 with sequence numbers 1–6 in Table 1 were solved and refined in the cubic Fm3̅m space group with similar frameworks to that described for 1·MeOH and 1·N2 (vide supra) and shown in Figure 4a–c. The nonframework DMA ions and CO2 molecules were not located, however electron density at ∼2.9 Å from the Y3+ centers was observed after CO2 was admitted to the crystal. This electron density was modeled as the coordinating oxygen atom of a CO2 molecule as shown in Figure 5c and was located at a distance in the range 2.80(4) – 2.96(2) Å within 1 at the different points in the in situ experiment, indicating that CO2 molecules interact with the Y3+ centers with strength intermediate between H2O/DMF/MeOH and N2.

Table 1. Crystallographic Occupancy of the Coordinating O Atom of a CO2 Molecule within 1 and the Unit Cell Volume of 1 under Different Applied Conditions during an In Situ CO2 Gas Adsorption Experiment.
sequence number and sample name conditions CO2 P/Po coordinating O atom occupancy unit cell volume (Å3)
1 vacuum, 500 K, <5 mbar 0 0 9607(1)
1·vac-500 K
2 5 bar CO2, 298 K 0.077 0.40(2) 9783(1)
1·CO2-298 K
3 5 bar CO2, 200 K >1 0.82(3) 9928.8(3)
1·CO2-200 K
4 5 bar CO2, 200 K, overnight >1 0.86(3) 9866(1)
1·CO2-200 K-overnight
5 5 bar CO2, 216 K 0.99 0.76(2) 9798.8(4)
1·CO2-216 K
6 vacuum, <5 mbar, 298 K 0 0.39(3) 9720.1(3)
1·vac-298 K
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The occupancy of the coordinating O atom of the CO2 increases as the temperature at which the crystal is held decreases with an accompanying increase in unit cell volume for the higher occupancies as seen in Table 1. On increasing the temperature from 200 to 216 K at 5 bar CO2 pressure the accompanying change from solid to liquid CO2 within the gas cell capillary causes nonmerohedral twinning of the crystal presumably due to reorientation of part of the crystal during desorption with possible accompanying strain relief. This nonmerohedral twinning persists during subsequent reheating of the crystal to 298 K and exposure to vacuum for 30 min. Significant electron density was observed at 2.7(1) Å from the Y3+ atoms in 1·vac-298 K suggesting that not all CO2 or potentially other residual guest molecules were removed during this treatment.

The location of the coordinating O atom of CO2 is similar to that found in Y-fum-fcu-MOF12 and that suggested in the closely related MOF (DMA)2[Tb6[μ3-(OH)]8(ftzb)6(H2O)6] (ftzb = 2-fluoro-4-(1H-tetrazol-5yl)benzoate) where the extension of the CO2 molecules into the octahedral cage is stabilized by additional interactions with the linkers.11 The favored position of CO2 within 1, Y-fum-fcu-MOF and (DMA)2[Tb6[μ3-(OH)]8(ftzb)6(H2O)6] contrast with the location of CO2 molecules within UiO-66 where the adsorbed CO2 molecules are found in the tetrahedral cages of the framework interacting with the μ3–OH groups.33 Again, the high degree of disorder of the naphth group suggests that the interaction between the CO2 and linker is negligible and the linear nature of the small CO2 molecule does not induce a reduction of the symmetry from the Fm3̅m space group.

Consideration of this set of host–guest compounds and the structure of the framework of 1 in the presence of different types of guest molecule reveal certain aspects of structural flexibility within the SBU and linker components of the fcu framework. The SBU of 1 is found to subtly flex when different guest molecules interact with the open Y3+ centers with the strongest interaction causing larger distortion of the Y-centered square antiprism. This is exemplified in the geometric parameters presented in Table 2 and Scheme 1, where the greater the interaction of the guest molecule with the Y3+ center, the greater the Y–Ondc bond length and narrower the Ondc–Y-Ondc angles. Simultaneously, little happens to the Y–F bond length but there is a noticeable expansion in all the F–Y–F-angles. Small changes occur for the Ondc–Y-F angles as the interaction with the guest molecule increases. The presence of guest species in the void space of 1 also lead to a guest-induced framework distortion resulting primarily from different orientations, and for the Pa3̅ structure, positions of the naphth groups of the linker. These orientations are shown in Figures 2b–c and 4b–c and quantified by the torsion angle (φ) between the naphth and the carboxylate groups of each ndc linker given in Table 3. The variation in φ with the size and orientation of the guest species relative to the Y3+ center suggest that nonlinear guest molecules that coordinate to the Y3+ center in a nonlinear manner appear to cause the greater rotation of the naphth group from the plane of the associated carboxylate groups, most noticeably for the 1·DMF/H2O minor naphth group orientation. However, further correlations are difficult to determine due to the large number of positionally undetermined molecules in the void space of 1 within these host–guest compounds.

Table 2. Geometric Parameters of a YO4F4 Square Antiprism as It Interacts with Various Guest Molecules at 100 K.
sample Y–O/Nguest distance (Å) Y–Ondc distance (Å) Y–F distance (Å) cis -Ondc–Y-Ondc angle (deg) cis-Ondc–Y-F angle (deg) cis–F–Y-F angle (deg) trans-Ondc–Y-Ondc angle (deg) trans-Ondc–Y-F angle (deg) trans-F–Y-F angle (deg)
1·N2 3.03(3) 2.257(3) 2.302(2) 80.88(7) 78.2(2) 62.4(2) 133.1(2) 138.27(7) 94.3(3)
1·MeOH 2.64(2) 2.313(3) 2.296(1) 79.61(7) 76.9(1) 66.3(1) 129.7(2) 139.85(5) 101.4(2)
1·DMF/H2Oa 2.61(2) 2.314(3) 2.293(1) 79.74(6) 76.87(8) 66.22(7) 130.1(1) 139.75(5) 101.2(1)
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a

Averaged geometric parameters.

Scheme 1. Schematic of the Motion of Various Atoms in the YO4F4 Polyhedron as an Atom of a Guest Molecule Approaches the Y3+ Center.

Scheme 1

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Color key: Y = cyan, F = green, O = red, atom from guest = purple.

Table 3. Torsion Angle (φ) between the Naphth Group and the Carboxylate Group of an Ndc Linker as Defined in Figure S8 and Shown in Figure 4c.
sample φ (deg)
1·DMF/H2O major naphth orientation 46.2(6), 47.3(9)
1·DMF/H2O minor naphth orientation 72(2), 72(4)
1·MeOH 51.7(7)
1·N2 43.3(6)
1·CO2-200 K 45.3(7)
1·CO2-200 K-overnight 42(1)
1·CO2-216 K 41(1)
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Conclusions

This work provides another example of a RE-MOF, prepared in the presence of a modulator molecule possessing a terminal fluorine atom, that contains μ3-face-capping F– groups within its SBU suggesting that many of the already reported RE-MOFs synthesized with this protocol might contain μ3-F– groups.1 It is also apparent that the degree of incorporation of μ3-F– groups within the same Y6Fa (a ≤ 8) core in the SBU unit is dependent on the reagents and conditions utilized during synthesis of the MOF. This indicates that it should be possible to tailor the composition of the Y6Fa (a ≤ 8) core in this SBU in a chemically controlled manner for a desired functional application as has been shown possible for other cores in RE-MOFs.34 The most favored location of guest molecules containing electron rich atoms is found to be within the octahedral cages of the 1 where they interact most strongly with the Lewis acidic Y3+ centers. The size, shape, and interactions of the guest species leads to subtle distortions within the SBU and specific orientations of the linker naphth group within the framework of 1. This demonstrates that the framework of a MOF is a relatively flexible entity that responsively adapts to the type of guest molecule and that such guest-induced framework transitions may be an important consideration in both understanding and optimizing the performance of these compounds during commercially relevant applications.

Acknowledgments

A.R.B.J.L.-G. is grateful to EPSRC and the University of Manchester for the award of a DTG PhD studentship (EPSRC EP/R513131/1) and funding the dual source Rigaku FR-X diffractometer (EPSRC EP/P001386/1). This work was supported by the Henry Royce Institute, funded through EPSRC grants EP/R00661X/1, EP/P025021/1 and EP/P025498/1. The UK High-Field Solid-State NMR Facility used in this research was funded by EPSRC and BBSRC (EP/T015063/1), as well as the University of Warwick including via part funding through Birmingham Science City Advanced Materials Projects 1 and 2 supported by Advantage WestMidlands (AWM) and the European Regional Development Fund (ERDF). Collaborative assistance from the Facility Manager Team (Dinu Iuga, University of Warwick) and Thomas J. Duddles (University of Manchester) is acknowledged. F.I.P. is grateful for funding from the Indonesia Endowment Fund for Education (LPDP) under the Ministry of Finance, Indonesia.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.4c04286.

  • Additional structural and diffraction information, crystallographic information files CCDC 2372655–2372663, XPS spectra, NMR spectra and TGA traces (PDF)

Author Present Address

⊥ School of Chemistry, The University of Birmingham, Edgbaston, Birmingham, B15 2TT, U.K

Author Present Address

# Department of Chemistry, Universitas Gadjah Mada, Sekip Utara, Yogyakarta, 55281, Indonesia

Author Present Address

∇ Fuente Nueva SN, Facultad de Ciencias, Departamento de Quimica Inorganica Universidad de Granada, Granada, 18071, Spain

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.

Supplementary Material

ic4c04286_si_001.pdf (1.1MB, pdf)

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Supplementary Materials

ic4c04286_si_001.pdf (1.1MB, pdf)

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