A zirconium squarate metal-organic framework with modulator-dependent molecular sieving properties†

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Synthesis
All chemicals used to synthesize ZrSQU and HfSQU were commercially obtained from ABCR and Sigma-Aldrich and used without further purification. A typical synthesis of ZrSQU is performed in an 11 mL borosilicate glass reactor wherein 0.24 mmol (56 mg) ZrCl4 is dissolved in a mixture of 64 mmol (5 mL) of N,Ndimethylformamide (DMF) and 70 mmol (4 mL) of acetic acid (AA). To this solution 0.72 mmol (82 mg) of SQA is added together with 12 mmol of HCl (1 ml HCl 37%). The molar ratio ZrCl4/SQA/HCl/H2O/DMF/AA equals 1/3/50/174/267/292. This mixture is reacted at 383 K for 2-3 h, after which a white precipitate has formed (92 % yield based on Zr). The as-synthesized material is separated by centrifugation and washed several times with DMF and acetone followed by a drying step in air at 333 K for 24 h. Replacing ZrCl4 with HfCl4 enables the formation of the isostructural hafnium squarate. The use of a monocarboxylic modulator as well as HCl is required to obtain powder of sufficient crystallinity. ZrSQU can also be synthesized using formic acid (FA). In this case the optimal molar ratio ZrCl4/SQA/HCl/H2O/DMF/FA is found to be 1/3/25/87/373/276. The synthesis and washing procedures are in each case identical as in the previously described method. In absence of modulator no product was formed. Similarly, in absence of DMF no crystallization occurred due to the low solubility of squaric acid in the resulting synthesis solution.
For reference, UiO-66 was prepared according to modified literature procedures 1,2 . Zirconium fumarate was prepared according to the procedure published by Wißmann et al. 3 employing 50 equivalents of acetic acid as modulator. Both materials were activated for gas sorption experiments by solvent exchange with DMF and methanol, followed by in air at 473 K for 24 h.

Materials characterization
Scanning electron microscopy (SEM) images and energy-dispersive X-ray spectroscopy (EDX) data were recorded on a Philips XL30 FEG microscope after sputtering with carbon. Thermogravimetric analyses were performed on a TA instruments TGA Q500. Samples were heated at a rate of 1 K min -1 to 673 K followed by 10 K min -1 to 1073 K under O2. Fourier transform infrared (FTIR) spectra were recorded on a NICOLET 6700 spectrometer (Thermo-Fischer) within the 500 cm -1 -4000 cm -1 range (256 scans; 2 cm -1 resolution). Samples were analyzed as a self-supporting wafer of pure MOF or as a pellet of 5 wt% MOF in KBr. 1 H liquid phase NMR spectra were obtained from activated ZrSQU samples (see section 'sorption experiments') dissolved in dimethylsulfoxide-d6 (DMSO-d6) according to the procedure proposed by Roy et al. 4 . Data was recorded on a Bruker AMX-300 spectrometer at 300 MHz for 1 H (16 scans).
Samples were calibrated using a known amount of toluene to quantify the amount of modulator.

Crystal structure determination
High-resolution powder x-ray diffraction (PXRD) data were recorded on a STOE Stadi P in Bragg-Brentano mode (2θ-θ geometry; monochromated CuKαradiation, λ = 1.54060 Å) equipped with a linear PSD detector system in transmission geometry. High-throughput PXRD data were obtained on a STOE COMBI P diffractometer (monochromated CuKα-radiation, λ = 1.54060 Å) equipped with an IP-PSD detector in transmission geometry. All cell indexing and Rietveld refinements were carried out using TOPAS, version 4.1 6 . As a starting point for the Rietveld-refinement, a structural model was set up using Materials Studio, version 4.3 7 . Pore accessible volume and pore sizes were analyzed with the PLATON 8 and Zeo++ 9 software using a 1.2 Å probe.
High resolution PXRD diffractograms for both ZrSQU and HfSQU synthesized with acetic acid as modulator were successfully indexed with a cubic cell (ZrSQU: a = 15.784(3) Å; HfSQU: a = 15.690(3) Å) and extinction conditions consistent with the space group Fm-3m (n° 225). The broad reflection centered around 5.5° 2θ was omitted for structure solution. As the Fm-3m space group is identical to that of the zirconium terephthalate UiO-66 1 , its crystal structure was taken as a basis for structure solution. An initial structural model was set up using the Materials Studio software by substituting the C-C6H4-C linker fragments in UiO-66(Hf) by the cyclic C4 fragment in SQU, effectively conserving the positions of the carboxylate oxygen atoms. This model was subsequently relaxed by force field geometry optimization (Forcite module) while imposing the cell parameters obtained by powder pattern indexing. The resulting model was Rietveld refined using the TOPAS software. Any residual electron density in the pores of the framework, as found by Fourier synthesis, was modelled by oxygen atoms partially occupying these positions. The refined structure of HfSQU was used as a starting model in the Rietveld refinement of ZrSQU after substitution of Hf for Zr.
The broad reflection centered around 5.5 Å, which is forbidden for Fm-3m, is believed to be caused by the presence of a primitive superstructure of the ZrSQU lattice. This has been described previously for UiO-66 where additional forbidden reflections were fitted by a model with the same cell parameters in the primitive Pm-3m space group 10,11 Final Rietveld plot of the refinement of ZrSQUA. Observed intensities, calculated intensities and the difference curve are represented in black, red and blue respectively.
Final Rietveld plot of the refinement of HfSQUA. Observed intensities, calculated intensities and the difference curve are represented in black, red and blue respectively.

Comparison of sorption properties to other Zr-MOFs
The tunable microporosity of ZrSQU can be exploited in gas purifications. In order to appreciate ZrSQU's potential, the amount of adsorbed gas molecules per unit cell (equivalent to two octahedral and four tetrahedral cages) was calculated based on the experimental results. For comparison, the same values were determined for the isoreticular UiO-66 (linker length 7.01 Å) and Zr-fumarate (linker length 5.11 Å) MOFs. As can be seen from     Figure S13: Hydrogen adsorption isotherms for UiO-66 (triangles) and Zrfumarate (squares) at 77 K.