Multiple single-crystal-to-single-crystal guest exchange in a dynamic 1D coordination polymer

A novel 1D coordination polymer that dynamically expands or shrinks upon the uptake of vapours of volatile small chlorinated molecules, such as 1,2-dichloroethane (DCE), dichloromethane (DCM) and trichloromethane (TCM), is reported. This system is robust enough to withstand multiple guest exchange via single-crystal-to-single-crystal transformation, as proved by 1 H-NMR and X-ray diﬀraction. The single crystal of guest-free, host framework, stable at 400 K, can also be obtained.

A novel 1D coordination polymer that dynamically expands or shrinks upon the uptake of vapours of volatile small chlorinated molecules, such as 1,2-dichloroethane (DCE), dichloromethane (DCM) and trichloromethane (TCM), is reported.This system is robust enough to withstand multiple guest exchange via singlecrystal-to-single-crystal transformation, as proved by 1 H-NMR and X-ray diffraction.The single crystal of guest-free, host framework, stable at 400 K, can also be obtained.
Coordination polymers having potential internal voids, often termed as metal organic frameworks (MOFs), are versatile hybrid materials, generated by the union of organic ligands and metal ions, that have shown important applications in many areas as diverse as catalysis, gas adsorption, conductivity, etc. 1 This versatility stems from the vast library of organic ligands that modern synthesis can develop and, at the same time, the different coordination modes that metals can adopt.Such combination provides highly tunable and diversified materials, a crucial difference with respect to conventional porous materials like zeolites.MOFs are particular, as their frameworks can be robust yet maintain a degree of flexibility, 2 allowing for dynamic structural transformations upon external stimuli. 3Such transformations have been essential to understand fundamental aspects of solid-state chemistry in general, and structure-function relationships in MOFs. 4 Not surprisingly, X-ray crystallography is a central technique in such investigations, as it allows for the visualization of the three dimensional molecular arrangement at the atomic scale, particularly when guest adsorption processes occur via singlecrystal-to-single-crystal (SC-SC) transformations. 5A less frequent type of reaction, which has been reported for 2D or 3D coordination polymers, 6 is represented by multiple SC-SC guest exchange processes, where different guest molecules are exchanged sequentially one after the other.These transformations require robust frameworks since structural modification could be quite demanding, especially if reiterated.This explains why multiple SC-SC processes are seldom reported in coordination networks formed by 1D polymer chains, which are generally less robust compared to the 2D or 3D counterparts. 7ere, we report on a new dynamic coordination polymer synthesized by the combination of ZnCl 2 with a linear nonchelating bidentate ligand (L) (Scheme 1).Its single crystal X-ray diffraction (SCXRD) characterization reveals 1D -Zn-L-Znchains that upon packing yield arrays of non-interpenetrated 1D channels.These channels are filled with TCM molecules (1ÁTCM).Notably, the same crystal can exchange TCM with DCM and DCE, via multiple SC-SC reactions whose products, 1ÁDCM and 1ÁDCE, respectively, were fully characterized by X-ray and NMR techniques.‡ This exchange is not performed by immersing the single crystals into a solution of the chosen solvent, but by directly exposing them to solvent vapours.Interestingly, solvent guest exchange is not always reversible, in that once 1ÁDCE is formed, exposure of its crystals to TCM or DCM vapours has no effect.This points to a marked selectivity in the solvent exchange process.Notably, the crystals are stable at room temperature, in contact with air for more than one month and are thermally stable up to 400 K.
The synthesis of L was performed by reacting 1,4-diaminocyclohexane and 4-(3-pyridinyl)benzaldehyde under refluxing conditions. 8Single crystals of 1ÁTCM were grown using a three layer TCM-nitrobenzene-methanol solution (ESI †). 8 The 1ÁTCM crystal structure was fully characterized at 100 K by SCXRD. 8As shown in Fig. 1a, the Zn-N coordination results in 1D linear polymer chains which in turn pack as a consequence of the bulkiness of the central cyclohexyl moieties and the 3-pyridinyl substitution.The tendency of p-p stacking aggregation of the flat aromatic region is hampered by this structural mismatch.Weak intermolecular C-HÁ Á Áp interactions between the cyclohexane ring in one polymer chain and the pyridine rings of the adjacent polymer chains can be also detected.As a result of the relative orientations among the stacked 1D chains, a channel structure including TCM guest molecules (disordered) is formed.The channels expand infinitely along the crystallographic a-axis (Fig. 1b) and account for ca.23.8% of the unit cell volume which corresponds to 791 Å 3 . 9he internal surface of the channels exposes L's cyclohexyl rings and iminic N atoms on one side (side A), while stacked aromatic rings and the Zn-centres point their C-Hs and Zncoordinated chloride units inwards at the other side (side B) (Fig. 1c).As a consequence of such a channel environment, NÁ Á ÁCl (d = 3.12(2) Å and C-ClÁ Á ÁN angle = 160.6(9)1)contacts between TCM and the nitrogen at the iminic bond are observed (see Fig. 1d). 10 In contact with air, at room temperature, the single crystallinity of 1ÁTCM is maintained.
The presence of 1D channels in 1ÁTCM allows for guest exchange studies.Although not very common, materials of this kind were indeed used to trap different guest molecules from the gas phase via gas/solid reaction. 11We started investigating the replacement of TCM with the structurally very similar DCM.Thus, a single crystal of 1ÁTCM glued in a glass capillary was placed in an enclosed chamber including a vial with 1 mL of DCM.The sealed chamber containing 1ÁTCM and DCM was then left overnight (ca.12 h).Once the crystal was removed from the chamber, it was observed under optical microscopy and showed no change in size, morphology and had no cracks.Then, the single crystal was mounted in the diffractometer and analysed by SCXRD at 100 K. 8 Crystallographic analysis revealed that DCM replaced the original TCM guest to give the new porous coordination polymer 1ÁDCM.As a result of the guest exchange, a small variation in the lattice parameters (Fig. 2b) occurs, resulting in a decrease of unit cell volume whilst maintaining the space group (Pnna).After the guest exchange, the variation in the channel shape is not very significant and a similar volume space occupied by DCM (23.6% of the unit cell (775 Å 3 )) is observed.While in 1ÁTCM the solvent is disordered over two positions, DCM is found over four positions in 1ÁDCM. 8Importantly, the guest exchange occurs without loss of crystallinity and, therefore, via a SC-SC transformation.Due to the similarity of the two guest solvents and their considerable disorder, additional confirmation by 1 H NMR is also provided (vide infra).
Since the crystallinity is maintained after the first SC-SC guest exchange, we attempted a second SC-SC transformation by exposing 1ÁDCM overnight to a third different chlorinated guest molecule: DCE.The single crystal maintained its integrity allowing complete single crystal structure elucidation after the second guest exchange process, again in a SC-SC fashion. 8CXRD clearly shows that the incoming DCE guest molecules are actually in the channel, therefore forming the coordination  polymer 1ÁDCE.We note that in this case, no specific host guest interactions can be detected. 12The channel shape shows a considerable change (24.2% of unit cell that corresponds to 823 Å 3 ) when compared to 1ÁDCM due to an anisotropic lattice variation (Fig. 2c).DCE molecules did not escape the channels even after keeping the crystals in contact with air at room temperature for one month.
The DCE molecules are tightly packed inside the MOF as their inclusion leads to the most anisotropic structural deformation (max a and c axis, min b axis) in the framework, which also presents the largest volume.Interestingly, we also notice that the above SC-SC transformations are not always reversible, viz., by exposing 1ÁDCE single crystals to vapours of DCM or TCM no reversed guest exchange was observed, 13 while 1ÁDCM can be converted back to 1ÁTCM by exposing it to TCM vapours.At the moment, we have no elements to exclude kinetic factors as being responsible for the observed lack of reversibility for the 1ÁDCE case.Also, the different vapour pressure of the three solvents, despite being remarkably different (P 25 C ¼ 87; 194; 432 mmHg, for DCE, DCM and TCM, respectively), does not have any impact on the data interpretation since each solvent is in large excess with respect to the MOF material, in all cases, under the experimental conditions.
Despite the fact that the X-ray characterization of 1ÁTCM, 1ÁDCM and 1ÁDCE is convincing about the identity of the solvent guest present, 14 we sought to gather additional confirmation by performing 1 H-NMR analysis on the three species.We collected ca. 5 mg of 1ÁTCM, as crystallized, and about the same quantity of 1ÁDCM and 1ÁDCE after a first and second solvent guest exchange, respectively.The samples were dissolved in DMSO-d 6 by heating and Fig. 3 shows the outcome of such analysis.In each case, the spectrum shows signals only of the solvent that was previously indicated by the X-ray analysis.TCM is observed in 1ÁTCM (Fig. 3a), but not in 1ÁDCM (Fig. 3b) where DCM appears, due to solvent exchange.Similarly, no DCM is present in 1ÁDCE, while the presence of DCE is confirmed (Fig. 3c).First of all, this rules out the possibility of an erroneous identification of the included solvent identities from the X-ray data.Secondly, this clearly confirms the occurrence of a complete solvent exchange, in accordance with the X-ray data.In all the spectra, contamination by nitrobenzene (NB) is observed, and that is due to the crystallization conditions employed.NB is a high boiling point solvent and it cannot be easily removed from the samples.Concerns about the possibility of actual inclusion of the NB evidenced by NMR into the channels can be dismissed by the fact that there is no evidence of NB from X-ray diffraction data.Moreover, attempts to include chlorobenzene, a molecule of similar size, also failed.We believe that substituted benzenes are too big to enter the MOF channels.As a reference, a spectrum of L is also shown (Fig. 3e).These above NMR analyses can also provide a quantitative evaluation of the guest solvent content of the three MOF samples, since an internal reference of known concentration, nitromethane, was added (signal at 4.41 ppm, final concentration 4.5 Â 10 À3 M).Data from two independent measurements indicate a solvent : ligand ratio, R, equal to 0.40, 0.15 and 0.55 for the TCM, DCM and DCE, respectively.The above figures are lower than those derived from X-ray diffraction analysis which indicated R = 1 in all cases.The need to heat the sample for a complete dissolution of the material could be the reason for such discrepancy in the solvent guest content in the MOF, especially for the DCM.
A very important aspect in functional materials is represented by their stability.Thermal stability, of course, but also the ability to refrain from structural collapse upon removal of eventually trapped guest molecules.Thermo-gravimetrical analysis carried out on a microcrystalline sample of 1ÁTCM (vide ante) shows that complete solvent guest release is obtained at ca. 137 1C. 8 Thus, the thermal stability of 1ÁTCM was tested by in situ heating a single crystal to 400 K overnight. 15he single crystal diffracted well at 400 K and therefore SCXRD was measured at that temperature (i.e., high temperature phase).Crystallographic analysis revealed that TCM was removed from the channels and the framework did not collapse allowing structure solution of the host framework 1 (Fig. 4a and b).We note that the changes in the lattice parameters are anisotropic and despite increasing the temperature to 400 K, the b-axis shows a length decrease of 0.28 Å.However, the channel volume after guest release is 858 Å 3 which corresponds to 25% of the unit cell volume. 9The anisotropic variation in the unit cell parameters is readily observed in the shift of the diffraction peaks in the simulated XRPD pattern (Fig. 4c and d).Finally, we also analyzed by 1 H-NMR a sample of 1 and, consistently, its spectrum shows no sign of TCM (Fig. 3d).
Another important feature of this coordination polymer is that it can be produced in larger scale using fast crystallization methods. 16The white microcrystalline powder obtained shows a good agreement with the simulated powder X-ray diffraction pattern of 1ÁTCM (Fig. 4e). 17Therefore, 1ÁTCM can be easily obtained in high phase purity, quickly and in high yields as a microcrystalline powder.This is an important aspect for the realization of any industrial applications.
In conclusion, we have synthesized a new 1D coordination polymer that contains 1D channels and displays significant thermal stability.According to its guest behaviour, 1ÁTCM fits in Kitagawa's third type classification18 displaying breathinglike dynamic behaviour.Indeed the channel structure can dynamically adapt to various guest molecules (i.e., TCM, DCM and DCE) that can be exchanged via a multi-SC-SC reaction.While this process is more favourable in 2D and 3D linked MOFs (i.e., due to the higher number of coordination bonds), the process reported herein is noteworthy as it occurs in a 1D coordination polymer.The resulting 1Ásolvent species are stable and can be stored for various weeks.Importantly, our data also show preferential absorption behaviour as 1ÁDCE is stable in the presence of vapours of DCM and TCM at room temperature.Finally, the host framework 1 is also remarkably stable up to 400 K and, in light of the properties described above, we believe that this material could be promising for the development of a selective chemical trap for volatile organic molecules and could thus constitute a novel non-destructive remediation method to remove pollutants from the atmospheric environment.
Notes and references ‡ Single crystal data collection of L, 1ÁTCM, 1ÁDCM, 1ÁDCE and 1 was recorded with a Bruker X8 Prospector APEX-II/CCD diffractometer equipped with a microfocusing mirror (Cu-K a radiation, l = 1.54178Å).XRPD experiments were carried out using a Bruker D2 diffractometer.

Fig. 1
Fig. 1 Single crystal structure of 1ÁTCM.(a) View of the 1D polymer chain in 1ÁTCM; (b) crystal packing viewed along the crystallographic a-axis showing the host guest interactions (dashed line).Hydrogen atoms have been omitted for clarity; (c) detailed view of the interior of a channel; (d) zoomed view of one of the disordered trichloromethane molecules showing the specific host-guest interactions in the channel.Colour code: carbon (gray); nitrogen (blue); chlorine (green); hydrogen (white).

Fig. 2
Fig. 2 Voids corresponding to the contact surface area depicting the framework adaptability to various guest molecules via multiple SC-SC gas-solid reactions and unit cell parameters: channel view along the a-axis of as synthesized 1ÁTCM (a); TCM replaced by DCM (SC-SC-1) (b); DCM replaced by DCE (SC-SC-2) (c).The data were recorded at 100 K. Hydrogen atoms are omitted for clarity.Colour code as in Fig. 1.