The Effect of Pendent Groups upon Flexibility in Coordination Networks with Square Lattice Topology

Gas or vapor-induced phase transformations in flexible coordination networks (CNs) offer the potential to exceed the performance of their rigid counterparts for separation and storage applications. However, whereas ligand modification has been used to alter the properties of such stimulus-responsive materials, they remain understudied compared with their rigid counterparts. Here, we report that a family of Zn2+ CNs with square lattice (sql) topology, differing only through the substituents attached to a linker, exhibit variable flexibility. Structural and CO2 sorption studies on the sql networks, [Zn(5-Ria)(bphy)]n, ia = isophthalic acid, bphy = 1,2-bis(pyridin-4-yl)hydrazine, R = −CH3, −OCH3, −C(CH3)3, -N=N-Ph, and -N=N-Ph(CH3)2, 2–6, respectively, revealed that the substituent moieties influenced both structural and gas sorption properties. Whereas 2–3 exhibited rigidity, 4, 5, and 6 exhibited reversible transformation from small pore to large pore phases. Overall, the insight into the profound effect of pendent moieties of linkers upon phase transformations in this family of layered CNs should be transferable to other CN classes.


Single-crystal X-ray diffraction (SCXRD) measurements.
Single-crystal reflection data were collected on a Bruker Quest diffractometer equipped with a Photon 100 detector and IμS microfocus X-ray source (Cu Kα, λ = 1.54178Å; Mo Kα, λ = 0.71073 Å).Indexing was performed using APEX3 5 (Difference Vectors method).Absorption correction was performed by the multi-scan method implemented in SADABS. 6Space group was determined using XPREP implemented in APEX3. 5Structural solution and refinement against F 2 were carried out using the SHELXT 7 non-linear least squares implemented in Olex2 v1.2.10. 8,9All non-hydrogen framework atoms were refined with anisotropic parameters, while H atoms were placed in calculated positions and refined using a riding model.Some disordered atoms have been refined isotropically.Disordered water molecules of hydration were located for 2 and 3.In this case, some of this electron density could be modelled as a water molecule with partial occupancy, however, refinement was unsatisfactory and the atomic displacement parameters were unreasonable.The PLATON SQUEEZE 10 routine was performed to account for the electron density, resulting in satisfactory refinement.In 2, the electron count/unit cell agreed well with the presence of four water molecules (electron count/cell = 38, 38/10 = 3.8).In 3, the electron count/unit cell agreed well with the presence of three water molecules (electron count/cell = 29, 29/10 = 2.9).All solvated phases were collected under liquid N2 flow at temperatures between 100K to 136K to avoid phase transformation caused by guest molecule desorption.After the activation of all compounds, the samples were transferred quickly and coated with oil immediately for avoiding the exposure to atmosphere.Crystallographic data and structural refinement information are listed in Table S1.The structures.both as-synthesized and activated, of 2, 3, 6-α and 6-β were solved and refined in the P21/n space group; 4-α, and 4-α' were solved and refined in the P212121 space group; 4-β was solved and refined in the P21 space group, 5-α was solved and refined in the P21212 space group.Crystallographic data for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication No. CCDC 2243680-2243681, 2243684-2243687, 2243704, 2243706, 2248289-2248290.

Thermogravimetric analysis (TGA)
Thermogravimetric analyses (TGA) were performed under N2 using a TA Instruments Q50 system.Samples were loaded into aluminium sample pans and heated at 10 K min -1 from room temperature to 500 °C.

IR spectra Fourier Transform Infrared (FTIR) Spectroscopy
Spectra were obtained by using a FTIR spectrometer (Agilent technologies, Cary 630) in the range of wavelength 4000-650 cm -1 .

Powder X-ray diffraction (PXRD) measurements
Powder X-ray diffraction patterns were recorded on a PANalytical X'Pert MPD Pro (Cu Ka, λ = 1.5418Å) with a 1D X'Celerator strip detector.Experiments were conducted in continuous scanning mode with the goniometer in the theta-theta orientation.Incident beam optics included the Fixed Divergences slit with anti-scatter slit PreFIX module, with a 1/8°divergence slit and a 1/4° anti-scatter slit, as well as a 10 mm fixed incident beam mask and a Soller slit (0.04 rad).Divergent beam optics included a P7.5 anti-scatter slit, a Soller slit (0.04 rad), and a Ni β filter.The data were collected in the range of 2θ = 4 -40°.Raw data were then evaluated using the X'Pert HighScore Plus™ software V 4.1 (PANalytical, The Netherlands).

Variable Temperature Powder X-ray Diffraction (VT-PXRD)
Diffractograms at different temperatures were recorded using a PANalytical X'Pert Pro-MPD diffractometer equipped with a PIXcel3D detector operating in scanning line detector mode with an active length of 4 utilizing 255 channels.The condition of low temperature (-193°C -450°C) was controlled by Anton Paar LNC Nitrogen Suction Equipment with application of liquid N2 cooling to the sample chamber.Anton Paar TTK 450 stage coupled with the Anton Paar TCU 110 Temperature Control Unit was used to record the variable temperature diffractograms.The diffractometer is outfitted with an Empyrean Cu LFF (long fine focus) HR (9430 033 7300x) tube operated at 40 kV and 40 mA and CuKα radiation (λα = 1.54056Å) was used for diffraction experiments.Continuous scanning mode with the goniometer in the theta-theta orientation was used to collect the data.Incident beam optics included the Fixed Divergences slit, with a 1/4° divergence slit and a Soller slit (0.04 rad).Divergent beam optics included a P7.5 anti-scatter slit, a Soller slit (0.04 rad), and a Ni-β filter.In a typical experiment, 20 mg of sample was loaded on a zero-background sample holder made for Anton Paar TTK 450 chamber.The data were collected from 4 -40° (2θ) with a step size of 0.0167113° and a scan time of 50 seconds per step.Crude data were analyzed using the X'Pert HighScore Plus™ software V 4.1 (PANalytical, The Netherlands).The temperatures were controlled from 173K to 298K and 298K to 613 K-693K.

Scanning Electron Microscopy (SEM)
Scanning electron microscopy measurements were carried out for the activated samples to investigate particle size.The images were collected on a Hitachi SU-70 instrument, using a 3 kV acceleration voltage and a working distance of 15 mm.Before the measurement, the samples were dispersed on carbon tape attached to SEM stubs and were gold-coated for 50 seconds to enhance surface conductivity.

Modelling.
All calculations were carried out using the BIOVIA Materials Studio 2014 (MS) software suite. 11Sorption of CO2 in 4-6 was simulated using the Sorption module with the Condensed-phase Optimized Molecular Potentials for Atomistic Simulation Studies (COMPASS II) 12 forcefield and the charges automatically assigned.The simulations ran for 1 × 10 6 equilibration steps and 1 × 10 7 production steps at 195K.The Adsorption Isotherm task, which makes use of the grand canonical thermodynamic ensemble, was used to simulate the adsorption of the guest molecules at a fixed fugacity; the number of sorbate molecules was varied until equilibrium was reached in a series of fixed pressure runs.Guest molecule adsorption configurations were sampled by the Metropolis Monte Carlo method (MMC), 13 which filters allowable transformations.Trial configurations were generated without bias and transformations that resulted in a state with a higher probability were accepted, while others were rejected.Trial states were governed by the forcefield-derived potential energy.The sites of guest inclusion derived from these calculations are shown in Figure S15.

Figure S3 .
Figure S3.PXRD patterns of as-synthesized and activated samples (Activated samples were obtained by exchanging with DCM for 2 days and then heated under vacuum at 60 °C for 10 h; for 4-α', the sample was heated directly to 343K without solvent exchange) in 1 (yellow), 2 (black), 3 (blue), 4 (red), 5 (green), and 6 (purple) and comparison of calculated PXRD patterns from their SCXRD determined structures.

Figure S7 .
Figure S7.Variable-temperature PXRD (VT-PXRD) patterns of 1 from 298K to 673K and 3 from 298K to 673K under N2 atmosphere and comparison with calculated PXRD patterns from their SCXRD determined structures.VT-PXRD patterns of 4-6 at low temperatures from 173K to 298K under N2 atmosphere.

Figure S8 .
Figure S8.Comparison of the calculated and experimental PXRD patterns of 4-α, 5-α, and 6-α reveals matching of peak positions but variation in the intensities.

Figure S12 .
Figure S12.First and second cycles of CO2 sorption of 4 and 5 at 195K.

Figure S14 .
Figure S14.Isosteric enthalpy of adsorption, Qst, of 5 for CO2 from isotherms collected at 273 K and 298 K was determined to be 21.5 KJ/mol.Realistic values for 4 and 6 could not be obtained because they are non-porous at these temperatures.

*
Interactions appear in pairs with marked lengths.

Table S1 .
Crystallographic data and structure refinement summary for 1-6.