Direct Observation of Enhanced Iodine Binding within a Series of Functionalized Metal–Organic Frameworks with Exceptional Irradiation Stability

Optimization of active sites and stability under irradiation are important targets for sorbent materials that might be used for iodine (I2) storage. Herein, we report the direct observation of I2 binding in a series of Cu(II)-based isostructural metal–organic frameworks, MFM-170, MFM-172, MFM-174, NJU-Bai20, and NJU-Bai21, incorporating various functional groups (–H, −CH3, – NH2, –C≡C–, and −CONH–, respectively). MFM-170 shows a reversible uptake of 3.37 g g–1 and a high packing density of 4.41 g cm–3 for physiosorbed I2. The incorporation of −NH2 and –C≡C– moieties in MFM-174 and NJU-Bai20, respectively, enhances the binding of I2, affording uptakes of up to 3.91 g g–1. In addition, an exceptional I2 packing density of 4.83 g cm–3 is achieved in MFM-174, comparable to that of solid iodine (4.93 g cm–3). In situ crystallographic studies show the formation of a range of supramolecular and chemical interactions [I···N, I···H2N] and [I···C≡C, I–C=C–I] between −NH2, –C≡C– sites, respectively, and adsorbed I2 molecules. These observations have been confirmed via a combination of solid-state nuclear magnetic resonance, X-ray photoelectron, and Raman spectroscopies. Importantly, γ-irradiation confirmed the ultraresistance of MFM-170, MFM-174, and NJU-Bai20 suggesting their potential as efficient sorbents for cleanup of radioactive waste.


Experimental section 1.1 Synthesis of MOF materials
All materials were purchased from commercially available sources and used without further purification.Iodine (99.8% ACS) was purchased from Sigma-Aldrich.The synthesis of MFM-170, NJU-Bai20 and NJU-Bai21 were synthesised by using the reported methods [1][2][3] .
The solution was heated in 8 mL Wheaton vials at 353K for 1 day.The blue crystals were separated by filtration, washed with hot DMF, acetone exchanged and dried in air (9.5 mg, 80%

BET measurements
The acetone-exchanged MOF samples were dried in air and treated at 150 °C and 10 −10 bar for 12 h to yield the fully desolvated samples, which were then loaded in 3-flex instruments (Micrometrics company) for porosity characterization.The BET surface areas and total pore volume were calculated using the N2 isotherms measured at 77 K.

Powder X-ray diffraction
Powder X-Ray diffraction (PXRD) data were collected in flat plate mode over the 2θ range 5-30° on an X'pert multipurpose Diffractometer using Cu-Kα radiation (λ = 1.54056Å) at 45 kV and 40 mA.

SEM-EDX experiments
SEM measurements were undertaken on a Quanta 650 at a working voltage of 20 kV with a scale bar up to 10 microns.Cu and I element were detected by energy-dispersive X-ray spectroscopy.

Iodine adsorption
Prior to adsorption, the acetone-exchanged samples were heated under vacuum (1 mbar) for 8 h at 150 °C.The activated MOF sample (100 mg) was transferred into a vessel containing solid I2 (3.0 g) in an open vial.The vessel was charged with dry N2 at atmospheric pressure to allow adsorption of I2 vapour by the MOF under the same I2 pressure for all samples.To determine the background, an empty glass vial was loaded into the vessel.The time-resolved I2 adsorption profiles of MFM-170, MFM-174, NJU-Bai20 and for the blank vial are shown in Figure S15.The vessel was sealed and heated at 80 °C for 0.5-58 h under N2 to allow adsorption of I2 into the desolvated MOFs.The I2-loaded samples were cooled to room temperature and collected for further analysis.For the cycling experiment, the sample was reactivated under vacuum and 150 °C between each cycle.Physisorbed I2 can be fully removed by reactivation by heating at 80-180 °C; chemisorbed I2 bound to the framework cannot be removed in this way and requires higher temperatures.

Thermogravimetric analysis
Thermogravimetric analysis (SDT650 TA Instruments company) was used to determine the uptake of adsorbed I2 molecules within these MOF materials.Samples were heated from room temperature to 600 °C at a rate of 5 °C min −1 under a flow of air.The TGA plots of activated MOF samples were obtained by in situ activation on TA instrument.The bare sample (~10 mg) was loaded onto the pan and the sample was heated to 150 °C under N2 for 2 h, and the temperature was decreased to room temperature.After in situ activation, the temperature was increased to 600 °C at a rate of 5 °C min −1 under a flow of air.

X-ray photoelectron spectroscopy (XPS)
XPS spectra were measured using a Kratos Axis Ultra instrument equipped with a monochromatic Al Kα Xray source (E = 1486.6eV).A charge neutraliser was used to minimise charging and spectra were aligned to the binding energy scale relative to the hydrocarbon C-C/C-H peak at 284.8 eV.Spectra were fitted using the CASA XPS software using Voigt-like peak shapes.Spin-orbit splitting ratios and splitting energies were constrained to obtain physically meaningful fits.

MAS NMR spectroscopy
Magic angle spinning (MAS) NMR spectra were recorded using a Bruker 9.4 T (400 MHz 1 H Larmor frequency) AVANCE III spectrometer equipped with a 4 mm HFX MAS probe.Samples were treated and packed into 4 mm o.d.zirconia rotors under inert conditions and sealed with a Kel-F rotor cap.Experiments were acquired at ambient temperature using a MAS frequency of 12 kHz. 1 H-pulses of 100 kHz were used for the 1 H MAS NMR spectra that employed a Hahn-echo sequence with an inter-pulse delay of one rotor period, giving a total echo time of 0.167 ms.64 transients were co-added for each spectrum, with a recycle delay of 0.1 s used between each scan.{ 1 H-} 13 C cross-polarisation (CP)MAS NMR spectra were acquired with 1 Hpulses of 100 kHz and 13 C spin-locking at ~50 kHz that was applied for 1 ms, with corresponding ramped (70-100 %) 1 H spin-locking at ~75 kHz with 100 kHz of SPINAL-64 4 heteronuclear 1 H decoupling used throughout.A Hahn-echo sequence with an inter-pulse delay of one rotor period was used after CP in the CPMAS NMR experiments where between 1024 and 4800 transients were co-added for each spectrum, with a recycle delay of 1 s used between each scan.

Synchrotron single crystal X-ray diffraction
Data collection.X-ray data for I2@MFM-170, I2@MFM-172, I2@MFM-174 and I2@NJU-Bai20 were collected at 100-150 K using synchrotron radiation at the single crystal X-ray diffraction beamline I19 at Diamond Light Source 5 , equipped with a Pilatus 2M detector and an Oxford Cryosystems nitrogen flow gas system.Data were measured using GDA suite of programs.
Crystal structure determinations and refinements.X-ray data were processed and reduced using CrysAlisPro and dials 6,7 .Absorption corrections were performed using empirical methods (SCALE3 ABSPACK and SADABS) based upon symmetry-equivalent reflections combined with measurements at different azimuthal angles.The crystal structure was solved and refined against all F 2 values using the SHELX and Olex2 suite of programmes [8][9][10] .Hydrogen atoms were placed in calculated positions and refined using idealised geometries and assigned fixed isotropic displacement parameters.
Crystal structure of I2@MFM-170: All atoms were refined anisotropically with the exception of the highly disordered I2 molecules.The structure contained disordered phenyl moieties which were constrained to have an ideal geometry, and were modelled over two sites with 50% occupancies related through an inversion center.I-I bond distances were restrained using SHELX distance fix (DFIX) command.The occupancies of the coordinated water molecule and the I2 molecules were refined, and atomic displacement parameters were restrained using similar and rigid body approach using SHELX SIMU and RIGU commands.The swap parameter was refined to stabilise the molecules modelled in the pores.Alternatively, solvent mask protocol was applied to the crystals structure without any I2 in the pores to obtain an electron count consistent with 3 I2 molecules per formula unit.The difference electron density map obtained after applying the solvent mask protocol showed a large concentration of electron density in the pore close to the phenyl groups.The mismatch between the amounts of iodine obtained in the model and from the solvent mask protocol arise from the proximity of the disordered phenyl moieties with the electron density corresponding to a disordered I2.As consequence, the solvent mask protocol could not take into account all the remaining electron density corresponding to I2.
Crystal structure of I2@MFM-172: Data resolution was found to be 1.1 Å.All atoms were refined anisotropically with the exception of the disordered I2 molecules and phenyl moieties.I-I bond distances were restrained using SHELX distance fix (DFIX) command, and the Uiso parameters were constrained to be 0.3.
The occupancies of the coordinated water molecule and the I2 molecules were refined.Alternatively, solvent mask protocol was applied to the crystal structure without any I2 in the model to obtain 2 I2 molecules per formula unit.
Crystal structure of I2@MFM-174: Data resolution was found to be 1.15 Å.All atoms were refined anisotropically with the exception of the disordered I2 molecules and 4-aminopyridyl moieties.I-I bond distances were restrained using SHELX distance fix (DFIX) command, and Uiso parameters were constrained to be 0.3.The occupancies of the coordinated water molecule and the I2 molecules were refined.The ill-shaped remaining electron density of the pores was accounted using solvent mask protocol implemented in Olex2.
The number of electrons found was 612 per unit cell corresponding to 0.25 I2 molecules per formula unit.
Alternatively, solvent mask protocol was applied to the crystals structure without any I2 in the model to obtain 2.24 I2 molecules per formula unit.
Crystal structure of I2@NJU-Bai20: All atoms were refined anisotropically with the exception of the disordered I2 molecules and water molecules.I-I bond distances were restrained using SHELX distance fix (DFIX) command.Uiso parameters were constrained to be 0.3, and the occupancies of the coordinated water and I2 molecules were refined.Alternatively, solvent mask protocol was applied to the crystals structure without any I2 in the model to obtain 3.36 I2 molecules per formula unit.

γ-Irradiation
The FTS Model 812 γ-irradiator is designed specifically to support a wide range of research applications in order to understand the mechanistic effects of γ radiation on exposed materials.Here, the effects of γ radiation on waste storage media including MFM-170, MFM-174 and NJU-Bai20 were investigated.The 60 Co sources are arranged in a circle allowing for a uniform dose to the materials being irradiated.The samples were loaded in the sample irradiation chamber (200 x 250 x 270 mm), and a chamber dose rate of 340 Gy/hr was applied for 215 hours.

Raman spectroscopy
Raman spectra were obtained using a Renishaw inVia microscope with a 532 nm with an acquisition time of 90 s and accumulated for 3 cycles.Initially, the reflections file (.mtz) obtained from CrysAlisPro was edited to include the map coefficients.The maps were calculated from the edited reflections file and the model (pdb file) obtained from structure solution software (OLEX 2) using phenix.mapssoftware.

Figure S1 .
Figure S1.Views of structures of cage C in I2-loaded MFM-172.Left: view of I2 binding sites in cage C of MFM-172 (Site C-I: Teal; Site C-II: pink; Site C-III: orange); middle: view of intermolecular interactions between I2 C-I and MFM-172; right: view of intermolecular interactions between I2 C-II , I2 C-III and MFM-172 (C, grey; O, red; Cu, cyan; N, blue; H, white) all unit is Å.

Figure S2 .
Figure S2.Views of structures of cage C in I2-loaded MFM-174.Left: view of I2 binding sites in cage C of MFM-174 (Site C-I: Teal; Site C-II: pink); middle: view of intermolecular interactions between I2 C-I and MFM-174; right: view of intermolecular interactions between I2 C-II and MFM-174 (C, grey; O, red; Cu, cyan; N, blue; H, white) all unit is Å.

Figure S3 .
Figure S3.Views of structure of cage C in I2-loaded NJU-Bai20.Left: view of I2 binding sites in cage C of NJU-Bai20 (Site C-I: Teal; Site C-II: pink); middle: view of intermolecular interactions between I2 C-I and NJU-Bai20; right: view of intermolecular interactions between I2 C-II and NJU-Bai20 (C, grey; O, red; Cu, cyan; N, blue; H, white; all unit is in Å).

Figure S5 .
Figure S5.Views of structure of cage B in MFM-170.Left: view of intermolecular interactions between I2 B-III and MFM-170; middle: view of intermolecular interactions between I2 B-V and MFM-170; right: view of intermolecular interactions between I2 B-VI and MFM-170 (C, grey; O, red; Cu, cyan; N, blue; H, white; all unit is in Å).

Figure S6 .
Figure S6.Views of structures of cage B in MFM-172.Left: view of intermolecular interactions between I2 B-II and MFM-172; middle: view of intermolecular interactions between I2 B-III and MFM-172; right: view of intermolecular interactions between I2 B-IV and MFM-172 (C, grey; O, red; Cu, cyan; N, blue; H, white; all unit is in Å).

Figure S7 .
Figure S7.Views of structure of cage B in MFM-174.Top left: view of intermolecular interactions between I2 B-II and MFM-174; top right: view of intermolecular interactions between I2 B-III and MFM-174; bottom left: view of intermolecular interactions between I2 B-IV and MFM-174; bottom right: view of intermolecular interactions between I2 B-VI and MFM-174 (C, grey; O, red; Cu, cyan; N, blue; H, white; all unit is in Å.

Figure S9 .
Figure S9.Views of structure of cage A in MFM-170.Left: View of I2 binding sites in cage A of MFM-170; right: view of intermolecular interactions between I2 A-I and MFM-170 (C, grey; O, red; Cu, cyan; N, blue; H, white; all unit is in Å).

Figure S12 .
Figure S12.2Fo-Fc electron density map representation of I2-loaded MFM-172.Electron density map pictures (4 msd) are represented in blue, carbon atoms are represented in green, oxygen in light red, iodine in grey and copper as white crosses.2Fo-Fc electron density maps were obtained using Phenix software.Initially, the reflections file (.mtz) obtained from CrysAlisPro was edited to include the map coefficients.The maps were calculated from the edited reflections file and the model (pdb file) obtained from structure solution software (OLEX 2) using phenix.mapssoftware.

Figure S13 .
Figure S13.2Fo-Fc electron density map representation of I2-loaded MFM-174.Electron density map pictures (4 msd) is represented in blue, carbon atoms and nitrogen are represented in green, oxygen in light red, iodine in grey and copper as white crosses.2Fo-Fc electron density maps were obtained using Phenix software.Initially, the reflections file (.mtz) obtained from CrysAlisPro was edited to include the map coefficients.The maps were calculated from the edited reflections file and the model (pdb file) obtained from structure solution software (OLEX 2) using phenix.mapssoftware.

Figure S14 .
Figure S14.2Fo-Fc electron density map representation of I2-loaded NJU-Bai20.Electron density map pictures is represented in blue, carbon atoms and nitrogen are represented in green, oxygen in light red, iodine in grey and copper as white crosses.2Fo-Fc electron density maps were obtained using Phenix software.

Figure S22 .
Figure S22.SEM images of fresh MFM-174 and NJU-Bai20 and post γ-irradiation confirming the retention of the morphology.

Table S1 .
Data for crystal structures of MFM-172 and MFM-174

Table S2 .
Comparison of unit cells and BET surface area

Table S3 .
Data for crystal structures of I2-loaded MOFs

Table S4 .
Summary of binding sites in MFM-17x and NJU-Bai20 (occupancy: I2/{Cu2}) a.The two iodine atoms at B V , B VI and A I show different occupancies.The atom with the lower occupancy was included at binding site and the residue was included in disorder.

Table S6 .
Summary of iodine adsorption in MOFs via vapor diffusion