Solvent- and Vapor-Mediated Solid-State Transformations in 1,3,5

Article pubs.acs.org/crystal

Solvent- and Vapor-Mediated Solid-State Transformations in 1,3,5Benzenetricarboxylate Metal−Organic Frameworks Kate Davies, Susan A. Bourne,* and Clive L. Oliver Centre for Supramolecular Chemistry Research, Department of Chemistry, University of Cape Town, Rondebosch 7701, South Africa S Supporting Information *

ABSTRACT: The reaction of 1,3,5-benzenetricarboxylic acid (H3BTRI) with ZnSO4·7H2O in DMF and H2O afforded [Zn2(μ2-OH2)(HBTRI)(BTRI)(H2O)2]·DMA·3H2O (1). Compound 1 displays self-healing properties upon dehydration and rehydration as revealed by scanning electron microscopy. Further studies showed that 1 is flexible and able to absorb additional water molecules in high vapor pressure environments. The reaction of H3BTRI with Gd(NO3)3·6H2O in DMF and H2O resulted in [Gd(BTRI)(H2O)6] (2). It is also possible to form compound 2 through a solid-state process when crystals of 1 are placed into a solution of Gd(NO3)3·6H2O in DMF and H2O. The reverse process does not occur.

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INTRODUCTION Coordination polymers and metal−organic frameworks (MOFs) are a supramolecular form of coordination complexes, with a change in focus from convergent ligands to divergent ones.1 The processes that coordination polymers and MOFs undergo during gas/solvent storage and sensing are supramolecular ones,1 wherein noncovalent interactions between the host and guest molecules are formed and broken. It is possible to obtain a wide range of structures of both coordination polymers and MOFs from very simple starting materials.2 This allows for uses in many diverse areas including chemical separation,3−8 catalysis,5,9,10 nonlinear optics,11,12 and gas storage.13−23 1,3,5-Benzenetricarboxylic acid (H3BTRI) is a commonly used starting material and has been used to prepare many novel coordination polymers and interesting MOFs.24−30 H3BTRI has the ability to increase the thermal stability of frameworks31 and carboxylate moieties form particularly strong coordinate bonds with metal ions making them well suited to create robust structures. Aromatic carboxylates are able to aggregate metal ions into M−O−C clusters, thereby forming secondary-building units (SBUs) in situ.32 Transition metals are known to form SBU clusters with carboxylate moieties and have well-known coordination states, which is particularly useful for MOF synthesis.33,34 Lanthanide ions have large radii and high, variable coordination numbers, as well as variable geometries.32,35,36 They have a high affinity for the hard oxygen donor atoms of carboxylate moieties. 32,37−40 Although inorganic materials are generally considered rigid in the solid state, it is possible for them to display flexibility. This flexibility occurs with the exposure of the sample to external stimuli including temperature and pressure.41 This process has been termed breathing,41 the dynamic framework effect,42 and springlike behavior,43 among others. In general, these effects are caused by the removal of guest molecules with strong host− guest interactions.41 We recently reported the structures of two anionic Zn-BTRI MOFs, which demonstrate intriguing dehydration/rehydration behavior.44 The nature of the © 2012 American Chemical Society

rehydration behavior of one of these, [Zn2(μ2-OH2)(HBTRI)(BTRI)(H2O)2]·DMA·3H2O (1), was studied further and these results are presented here. A new one-dimensional neutral coordination polymer [Gd(BTRI)(H2O)6] (2) is also reported in this article. The solid-state transformation of compound 1 into 2 is described.

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EXPERIMENTAL SECTION

Materials. Gadolinium(III) nitrate hexahydrate (99.9% purity) and 1,3,5-benzenetricarboxylic acid (H3BTRI) (>95% purity) were purchased from Sigma Aldrich (Germany). Zinc(II) sulfate heptahydrate (>99.5% purity) was obtained from Hopkin and Williams Ltd. (United Kingdom). All starting materials were used without further purification. Thermal Techniques. Hot-stage microscopy (HSM) was used to visually follow the transformation process. The temperature was kept at a constant 30 °C throughout these experiments. Changes were studied using a Nikon SMZ-10 stereoscopic microscope fitted with a Sony Digital Hyper HAD color video camera connected to a Linkam THMS600 hot stage and a Linkam TP92 controlling unit. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used to determine guest loss and decomposition temperatures. Samples were dried on filter paper. For TGA, samples were placed in an open aluminum crucible. A TA-Q500 Thermogravimetric Analyzer from TA Instruments was used with a 50 mL·min−1 purging gas flow of dry N2. For DSC experiments, samples were placed in crimped, vented aluminum pans. The experiments were performed on a TA Instruments DSC-Q200 machine under a dry N2 atmosphere at 50 mL·min−1. TGA and DSC traces are provided in the Supporting Information. Scanning Electron Microscopy (SEM). Scanning electron microscopy (SEM) images were collected at the University of Cape Town Electron Microscope Unit by Dr Miranda Waldron. Single crystals were placed into a FEI Nova NanoSEM 230 under vacuum, and the crystal surface was studied at 2000× magnification. Received: December 27, 2011 Revised: February 11, 2012 Published: February 24, 2012 1999

dx.doi.org/10.1021/cg201707e | Cryst. Growth Des. 2012, 12, 1999−2003

Crystal Growth & Design

Article

X-ray Diffraction (XRD). Compounds 1 and 2 were analyzed with powder X-ray diffraction (PXRD). PXRD was used to confirm that the single crystal structure was representative of the bulk material, to determine that the original structure was regained with rehydration for compound 1, and to follow the transformation process of compound 1 into compound 2. Samples were mounted on a Huber D-83253 Imaging Plate appliance fitted with a Guinier Camera 670, a Huber MC9300 power supply unit and a Philips PW1120/00 X-ray generator. The generator was fitted with a Huber long fine-focus PW2273/20 tube and a Huber Guinier Monochromator Series 611/15. The samples were exposed to nickel-filtered CuKα1 radiation (λ = 1.5406 Å) produced at 20 mA and 40 kV. Those relevant PXRD traces not shown in the article are provided in the Supporting Information. A Nonius Kappa CCD single crystal X-ray diffractometer was used for single crystal data collection. This utilized MoKα radiation (λ = 0.71069 Å) was produced at 54 kV and 23 mA with a Nonius FR590 generator. Full crystallographic data for compound 1 can be found in a recent article.44 Full crystallographic data for compound 2 are provided in Table 1, and the single crystal data is accessible in the Supporting Information.

obtained. The single crystal structure revealed six water molecules coordinated to each metal center. TGA, DSC, and elemental analysis confirmed this. Found %C, 22.38; %H, 3.75. GdC9O12H15 requires % C, 22.88; %H, 3.20. Decomp. > 500 °C. PXRD was used to confirm that the crystal selected for analysis matched the bulk material.

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RESULTS AND DISCUSSION SEM Studies of 1. [Zn 2 (μ2 -OH 2 )(HBTRI)(BTRI)(H2O)2]·DMA·3H2O (1) is an anionic MOF previously reported by these authors44 that reversibly dehydrates and rehydrates. Variable temperature powder X-ray diffraction (PXRD) revealed that 1 is almost amorphous when dehydrated at 160 °C. The original structure is regained after exposure to water vapor for thirty minutes. Scanning electron microscopy (SEM) was performed on crystals of 1 before heating (Figure 1a), after heating (Figure 1b), and upon rehydration (Figure 1c). Three guest and two terminally coordinated water molecules are modeled in the single crystal structure, while a further water bridges the two zinc ions (see Figure 2a). The cations and guest water molecules are located in channels along [100] and are shown in Figure 2b. TGA demonstrates the possibility of additional absorbed water molecules located in the channels. In fact, the structure can absorb up to 1.5 extra water molecules per asymmetric unit (at 30 °C in a high vapor pressure environment). This was confirmed by rehydration studies. The compound is not stable in this state, however, and the guest molecules additional to those modeled in the single crystal structure are lost when the vapor pressure is reduced. There is no accessible void space in the single crystal structure, which implies that 1 expands (breathes) to absorb the additional water molecules. The water molecules observed in TG analysis, but not modeled in the single crystal XRD, are lost if left under nitrogen at 20 °C. Therefore, they could easily be removed under vacuum. All SEM experiments were performed under vacuum, and this vacuum facilitates the removal of the extra water in a short period of time. This may account for the several large cracks seen in Figure 1a. The extra water is held loosely enough that when removed under ambient conditions the crystal remains undeformed and retains its single crystallinity. Upon dehydration through heating to 160 °C, the cracks in the surface of the crystal become more pronounced and uniformly cover the entire area. These cracks are shown in Figure 1b. They are regular and all extend along the same direction. Dehydrated crystals of 1 were then exposed to water vapor at 25 °C, and the result is shown in Figure 1c. There are some cracks remaining in the surface, probably due to the high vacuum. However, the majority of the cracks are no longer present. Light scars are visible on the surface where water has been reabsorbed into the channels. The structure has healed itself upon rehydration and regains its original crystal structure with the restoration of the host−guest hydrogen bonds between the water molecules and the framework (see Figure 3). There are small channels shown in Figure 4a that extend along the [100] axis. The guest water molecules as well as the terminally coordinated water molecules are located in these channels. When compound 1 is dehydrated, it is likely that the structure contracts along the channels. It is the shrinking of these channels that causes the cracks in the surface of the crystal seen in the SEM images. When the dehydrated sample is exposed to water vapor again, the channels expand to accommodate the water, and the sample regains crystallinity. Figure 4b shows the crystal morphology of 1 obtained by face-

Table 1. Crystal Data and Refinement Parameters for [Gd(BTRI)(H2O)6] (2) empirical formula formula weight (g·mol−1) temperature (k) wavelength (Å) crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Z Dcalcd (g·cm−3) crystal size (mm) θ-range scanned (deg) index range

reflections collected independent reflections restraints parameters Rint goodness-of-fit R1 [I > 2σ(I)] wR2 R indices (all data) largest diff. peak and hole (e·A−3)

C9H15GdO12 472.46 173(2) 0.71073 monoclinic Cc 11.3160(3) 17.8157(5) 7.1635(2) 90 118.972(1) 90 1263.45(6) 4 2.484 0.60 × 0.15 × 0.10 3.10−26.02 −13 ≤ h ≤ 13 −21 ≤ k ≤ 21 −8 ≤ l ≤8 18 562 2482 21 242 0.0822 1.076 0.0243 0.0544 R1 = 0.0263 wR2 = 0.0558 1.219 and −1.079

Sample Preparation. Compound 1 was prepared from a 1:1 metal to ligand ratio of zinc(II) sulfate heptahydrate and 1,3,5-benzenetricarboxylic acid (H3BTRI) in water and N,N′-dimethylformamide (DMF). For full details, see Davies et al.44 For compound 2, 43 mg (0.0953 mmol) of gadolinium(III) nitrate heptahydrate was dissolved in 1 mL of distilled water with heating and stirring. Twenty milligrams (0.0952 mmol) of H3BTRI was dissolved in 1 mL of DMF with heating and was stirred. The vial was sealed and placed in a Dewar containing water at a temperature of 70 °C. This was left to cool slowly to room temperature. Thin, colorless needle-like crystals were 2000



Article

Figure 1. Scanning electron microscopy (SEM) pictures of 1. (a) Straight from the mother liquor, (b) after heating to 160°, and (c) after exposure to water vapor, when rehydration is complete.

Figure 2. (a) Asymmetric unit of 1; (b) packing of 1 along the [100] direction with the framework shown as a stick diagram and the guest water molecules and DMA cation shown with van der Waals radii. Reprinted with permission from ref 44. Copyright 2010 Royal Society of Chemistry.

isostructural with two previously reported structures.37,45 The six water molecules are removed from compound 2 in a twostep process as can be observed in the TGA trace. DSC analysis reveals that the amount of energy required to remove the sixth water molecule is comparable to that required to remove all of the previous five water molecules. Variable temperature PXRD shows a structural rearrangement occurring with the removal of five of the water molecules. This rearrangement persists after the removal of the sixth water molecule implying that this new structure is stable with only one water molecule. It is possible to regain the original structure when the dehydrated sample is exposed to water vapor at 30 °C for approximately two hours. Transformation of 1 to 2. Gadolinium nitrate was dissolved in a 1:1, v/v DMF/H2O solvent system to obtain a concentration of 0.1 mol·dm−3. Single crystals of 1 were submerged in this solution. Over time, the sample transforms from compound 1 to compound 2. The crystals of 1 retain their shape but lose single crystallinity. The PXRD experiments used to track the transformation progress are shown in Figure 6a. For each PXRD pattern, a different sample was removed from the solution. After 7 weeks in solution, there are some peaks of compound 1 observed in the PXRD pattern, but these are dramatically reduced. The majority of the peaks match those of compound 2. Crystal size has an effect on the time required for the transformation to take place. The larger the crystal, the longer the transformation process. This implies that the process begins at the surface and moves inward. A hot-stage microscopy study of a single crystal placed into the 0.1 mol·dm−3 solution and held at 30 °C is shown in Figure 6b. Cracks, similar to those caused by dehydration, form on the surface of the crystal. The process can be followed visually: a dark mark forms on the surface at ca. 30 min and moves across it until the entire crystal becomes opaque. Even after 2.5 days of submersion, the crystal

Figure 3. PXRD patterns of 1 (a) before heating, (b) after heating to 160 °C, and (c) after exposure to water vapor for 30 min.44

indexing of the crystal, and Figure 4c is an SEM image in the same orientation showing that the cracks run along the largest, (001)̅ , face of the crystal. Preparation of 2. Compound 2 was prepared from gadolinium(III) nitrate hexahydrate and 1,3,5-benzenetricarboxylic acid (H3BTRI). This compound can be prepared from a range of metal to ligand ratios and solvent volumes. Compound 2 crystallizes in the monoclinic crystal system with the space group Cc. The asymmetric unit of 2 is given in Figure 5 with the hydrogen atoms omitted. The asymmetric unit consists of one nine-coordinated Gd metal center, one fully deprotonated BTRI unit, and six terminally coordinated water molecules. The BTRI units bridge metal centers to form parallel, onedimensional chains. The chains extend along [202̅] and are staggered with the uncoordinated O1−C7−O2 carboxylate moiety located between the gadolinium metal centers on the adjoining chains (±1/2 along the b and c axes). Compound 2 is 2001



Article

Figure 4. (a) Packing diagram of 1 viewed onto the (001̅) face, (b) a face-indexed crystal of 1, and (c) SEM image of crystal in the same orientation showing the orientation of cracks across the (001̅) face.

shell of the crystal breaks away, exposing the rest of the sample to the solution. No cracks are observed on the surface of the crystal in those instances. This suggests that with higher concentrations of the gadolinium solution, the process occurs more rapidly with the effect of shattering the crystals. The reverse experiment was attempted several times, immersing crystals of 2 in solutions of zinc sulfate. No structural change was effected.

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CONCLUSIONS

Compound 1 is able to expand as necessary to accommodate extra water molecules and can contain a total of 4.5 guest water molecules at 30 °C. Cracks form on the surface of the crystal with dehydration but self-heal with rehydration. This, combined with the loss/restoration of crystallinity upon dehydration/ rehydration implies a breathing mechanism in this structure. When compound 1 is submerged in a solution of gadolinium nitrate, it converts to compound 2. This is at least partially a

Figure 5. Compound 2 with the asymmetric unit labeled and hydrogen atoms omitted for clarity.

retains its shape. It is probable that the exchange pathway is similar to that observed for dehydration/rehydration experiments. When the concentration of the gadolinium solution is increased, however, the crystals disintegrate and lose their shape. Instead, a white powder forms on the surface. The outer

Figure 6. (a) Variation in the PXRD pattern of 1 when placed into a 0.1 mol·dm−3 DMF/H2O solution of Gd(NO3)3: (i) calculated pattern of 1, (ii) 4 h in solution, (iii) 24 h in solution, (iv) combination of the calculated patterns of 1 and 2, (v) 3 days in solution, (vi) 7 weeks in solution, and (vii) the calculated pattern of 3. (b) Hot-stage microscopy pictures showing the transition over 2.5 days of a ca. 0.80 × 0.65 × 0.40 mm crystal of 1 (top left) placed into the 0.1 mol·dm−3 solution. 2002



Article

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solid-state transformation. Although single crystallinity is lost, the crystal morphology is retained when the concentration of the gadolinium solution is low enough. Compound 2 is generally microcrystalline. As compound 2 retains the morphology of compound 1 when formed in low concentration solutions, it is probable that the compound does not dissolve and recrystallize in the DMF/H2O solution. Compound 2 does not transform into compound 1 when placed in a solution of zinc sulfate but instead retains the structure of 2.

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ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic information files (CIF) for compound 2. TG analysis of 1 showing the additional water molecules as well as a graph of the rehydration study. TG/DSC analyses of 2. Relevant PXRD traces including variable temperature PXRD of 2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +27 21 650 2563. Fax: +27 21 650 5195. E-mail: susan. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support was received from the South African National Research Foundation (Grant CPR20100409000010269). We thank the University of Cape Town (UCT) Electron Microscope Unit for the scanning electron microscopy (SEM) images. K.D. thanks UCT, the UCT Department of Chemistry Equity and Development Program (EDP), and the National Research Foundation for funding.

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REFERENCES

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Solvent- and Vapor-Mediated Solid-State Transformations in 1,3,5

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