doi.org/10.26434/chemrxiv.7235555.v1 Hypergolic Zeolitic Imidazolate Frameworks (ZIFs) as Next-Generation Solid Fuels: Unlocking the Latent Energetic Behavior of ZIFs Tomislav Friscic, Hatem M. Titi, Mihails Arhangelskis, Dayaker Gandrath, Cristina Mottillo, Andrew Morris, Robin Rogers, Joseph Marrett, Giovanni Rachiero Submitted date: 29/10/2018 • Posted date: 30/10/2018 Licence: CC BY-NC-ND 4.0 Citation information: Friscic, Tomislav; Titi, Hatem M.; Arhangelskis, Mihails; Gandrath, Dayaker; Mottillo, Cristina; Morris, Andrew; et al. (2018): Hypergolic Zeolitic Imidazolate Frameworks (ZIFs) as Next-Generation Solid Fuels: Unlocking the Latent Energetic Behavior of ZIFs. ChemRxiv. Preprint. We present the first strategy to induce hypergolic behavior, i.e. spontaneous ignition and combustion in contact with an external oxidizer, into metal-organic frameworks (MOFs). The strategy uses trigger acetylene or vinyl substituents to unlock the latent hypergolic properties of linkers in zeolitic imidazolate frameworks, illustrated here by six hypergolic MOFs of zinc, cobalt and cadmium. Varying the metal and linker enabled the modulation of ignition and combustion properties, leading to ultrashort ignition delays (down to 2 ms), on par with popular propellants, but without requiring highly energetic or carcinogenic hydrazine components found in conventional hypergols. File list (2) download file view on ChemRxiv Manuscript_ChemRxiv.pdf (528.42 KiB) download file view on ChemRxiv ESI_ChemRxiv.pdf (1.28 MiB)
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doi.org/10.26434/chemrxiv.7235555.v1
Hypergolic Zeolitic Imidazolate Frameworks (ZIFs) as Next-GenerationSolid Fuels: Unlocking the Latent Energetic Behavior of ZIFsTomislav Friscic, Hatem M. Titi, Mihails Arhangelskis, Dayaker Gandrath, Cristina Mottillo, Andrew Morris,Robin Rogers, Joseph Marrett, Giovanni Rachiero
Submitted date: 29/10/2018 • Posted date: 30/10/2018Licence: CC BY-NC-ND 4.0Citation information: Friscic, Tomislav; Titi, Hatem M.; Arhangelskis, Mihails; Gandrath, Dayaker; Mottillo,Cristina; Morris, Andrew; et al. (2018): Hypergolic Zeolitic Imidazolate Frameworks (ZIFs) as Next-GenerationSolid Fuels: Unlocking the Latent Energetic Behavior of ZIFs. ChemRxiv. Preprint.
We present the first strategy to induce hypergolic behavior, i.e. spontaneous ignition and combustion incontact with an external oxidizer, into metal-organic frameworks (MOFs). The strategy uses trigger acetyleneor vinyl substituents to unlock the latent hypergolic properties of linkers in zeolitic imidazolate frameworks,illustrated here by six hypergolic MOFs of zinc, cobalt and cadmium. Varying the metal and linker enabled themodulation of ignition and combustion properties, leading to ultrashort ignition delays (down to 2 ms), on parwith popular propellants, but without requiring highly energetic or carcinogenic hydrazine components found inconventional hypergols.
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Hypergolic Zeolitic Imidazolate Frameworks (ZIFs) as Next-Generation Solid Fuels: Unlocking
the Latent Energetic Behavior of ZIFs
Hatem M. Titi,a Joseph M. Marrett,a Gandrath Dayaker,a Mihails Arhangelskis,a Cristina Mottillo,a Andrew J. Morris,b Giovanni P. Rachiero,a Tomislav Friščić,a,* and Robin D.
Rogersa,c,*
aDepartment of Chemistry, McGill University, Montreal, QC, H3A 0B8, Canada; bSchool of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK; c525 Solutions, Inc., 720 2nd
Street, Tuscaloosa, AL 35401, USA
Table of Contents
1. Materials and methods 2
2. Computational methods 4
3. Synthesis 5
4. Nuclear magnetic resonance (NMR) spectroscopy 8
5. Infrared spectroscopy 11
6. Thermal analysis 12
7. Example drop tests with RFNA oxidant 14
8. References 15
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1. Materials and methods All reactions were performed under an atmosphere of argon unless otherwise specified. Reactions run under argon were conducted in oven-dried glassware (120 °C, minimum 12 hours). Tetrahydrofuran (THF), Methanol, dimethylformamide (DMF), acetone and chloroform were obtained from a PureSolv™ PS-400 solvent purification system. Thin-layer chromatography (TLC) was performed on aluminum pre-coated silica gel plates from Merck, and developed plates were visualized by UV light (254 nm), Column chromatography was performed using flash chromatography with the indicated eluent on SiliaFlash P60 40-63 µm silica gel. Zinc oxide (Sigma-Aldrich, St. Louis, MO, USA), ammonium acetate (Sigma-Aldrich), ammonium sulfate (Sigma-Aldrich), cadmium oxide (Sigma-Aldrich), cadmium nitrate tetrahydrate (Macco), cobalt(II) carbonate (Alfa Aesar), and cobalt(II) nitrate hexahydrate (Sigma-Aldrich) were used as received. 1.1. Nuclear magnetic resonance (NMR) spectroscopy Solution 1H NMR spectra (300 MHz) and 13C NMR spectra (75 MHz) were recorded on Bruker AMX300 spectrometer. Chemical shifts are reported relative to DMSO-d6 (δ 2.50 ppm) for 1H NMR spectra and DMSO-d6 (δ 39.5 ppm) for 13C spectra. The 1H NMR spectra data are presented as follows: chemical shift, multiplicity (s = singlet, br. = broad), and integration.
Solid-state NMR spectra were acquired on a Varian VNMRS 400 MHz NMR spectrometer operating at 100.53 MHz for 13C and 399.77 MHz for 1H using a wide bore 4mm T3 double-resonance probe spinning at 14 kHz. Cross-polarization using RAMP CP for 5 ms at an rf field of approximately 62 kHz on 1H was used. SPINAL-64 1H decoupling was performed during acquisition using a 90 kHz rf field.
1.2. Fourier-transform attenuated total reflectance infrared (FTIR-ATR) spectroscopy The FTIR-ATR spectra were obtained using a Bruker Vertex 70 FTIR spectrometer equipped with the Platinum ATR accessory.
1.3. Mass spectrometry (MS) Mass spectrometry on HAIm and HVIm was performed using an Exactive Plus Orbitrap mass spectrometer from ThermoFischer Scientific.
1.4. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) Simultaneous thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were conducted on a TGA/DSC 1 (Mettler-Toledo, Columbus, Ohio, USA), with
samples (2 mg to 10 mg) placed in open 70 L alumina crucibles. All measurements were done in a dynamic atmosphere of air, with a gas flow of 60-65 mL min-1, and the samples were heated up to 700 °C at a constant rate of 10 °C min−1. 1.5. Powder X-ray diffraction (PXRD) Powder X-ray diffraction (PXRD) data were collected on a Bruker D2 Phaser diffractometer equipped with a LYNXEYE linear position sensitive detector (Bruker AXS, Madison, WI), using Ni-filtered CuKα radiation. Powder X-ray diffraction pattern of
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Co(Aim)2 was collected on a Bruker D8 Advance diffractometer equipped with a LYNXEYE-XE linear position sensitive detector (Bruker AXS, Madison, WI), using Ni-filtered CuKα radiation.
Rietveld refinement (Table S1) was performed using the software TOPAS Academic v. 6 (Coehlo Software). All ZIF structures were refined with a cubic I-43m space group. Diffraction peak shapes were described by a pseudo-Voigt function, while the background was modelled with a Chebyshev polynomial function. The linker geometry was defined by a rigid body, which was given rotational and translational degrees of freedom, subject to the space group symmetry constraints. For the vinyl-substituted ligand, a flexible torsion angle allowing rotation of the vinyl group relative to the imidazole plane, was defined. Finally, a soft restraint was applied to the Metal-N bond, (2.0 Å for Zn-N and Co-N bonds, 2.2 Å for Cd-N bond). Table S1: Summary of crystallographic and Rietveld refinement parameters.
Material Zn(AIm)2 Co(AIm)2 Co(VIm)2 Cd(AIm)2 Cd(VIm)2 chemical formula Zn(C5H3N2)2 Co(C5H3N2)2 Co(C5H5N2)2 Cd(C5H3N2)2 Cd(C5H5N2)2
Mr / g mol-1 247.56 241.12 245.15 294.60 298.63 Crystal system cubic cubic cubic cubic cubic
a / Å 17.0454(11) 16.9604(17) 17.2960(14) 17.9712(9) 18.234(2) b / Å 17.0454(11) 16.9604(17) 17.2960(14) 17.9712(9) 18.234(2) c / Å 17.0454(11) 16.9604(17) 17.2960(14) 17.9712(9) 18.234(2) α / ° 90 90 90 90 90 β / ° 90 90 90 90 90 γ / ° 90 90 90 90 90
V / Å3 4952.5(9) 4878.8(14) 5174.1(13) 5804.0(9) 6062(2) Space group I -4 3 m I -4 3 m I -4 3 m I -4 3 m I -4 3 m
Density / g cm-3 0.996 0.985 0.944 1.011 0.982 Radiation type CuKα CuKα CuKα CuKα CuKα
1.6. Hypergolic testing The compounds were exposed to one drop (approx. 10 μL) of white fuming nitric acid (WFNA) or red fuming nitric acid (RFNA) via a 100 μL Hamilton syringe (Hamilton, Reno, NV, USA). The liquid was dropped from a fixed height (5 cm) into a 4.5 cm vial containing approx. 5 mg of the particular material being tested. A Redlake MotionPro Y4 (Tallahassee, FL, USA) high speed CCD camera was used to capture the event at 1000 frames/s. Ignition delay was considered to be the time between the contact of the oxidizing liquid with the solid material and the subsequent ignition, and was measured by counting the frames between the two events. The test was repeated three times, the values for ignition delay were averaged, and a standard deviation was calculated.
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1.7. Bomb Calorimetry The bomb calorimetry measurement on ZIF-8 was carried out on a 6200 Isoperibol Calorimeter (Parr Instrument Company, Moline, IL), which is a microprocessor controlled, isoperibol oxygen bomb calorimeter.
2. Computational methods Periodic DFT calculations were performed using a plane-wave DFT code CASTEP 16.11. The input files were prepared from experimental crystal structures using a program cif2cell.1 Structures were geometry-optimized with respect to atom coordinates and unit cell parameters, subject to the symmetry constraints of I-43m space group. In case of vinyl-substituted ZIFs, the symmetry was lowered to P1 in order to resolve the disorder of the vinyl group (Figure S1).
Figure S1: Illustration of the orientation of vinyl substituents (green) in the SOD-Co(VIm)2 structure: a) experimental structure with vinyl group disordered by mirror plane symmetry; b) ordered model missing mirror symmetry, used in periodic DFT calculations. Hydrogen atoms were omitted for clarity.
Calculations were performed with PBE functional, van der Waals interactions were modelled using Grimme D2 dispersion correction. The plane wave basis set was truncated at 750 eV, and norm-conserving pseudopotentials were used to modify the Coulomb potentials in the atom core regions. The Brillouin zone was sampled with a 0.03 Å-1 Monkhorst-Pack k-point grid. The following convergence criteria were used: maximum energy change 10-5 eV/atom, maximum force on atom 0.01 eV/Å, maximum atom displacement 0.001 Å and residual stress 0.05 GPa. We have optimized the structures of all herein prepared ZIFs. In addition, calculations were performed for the compounds involved in the combustion reaction (ZnO, Co3O4, CdO, O2, CO2 and N2 and H2O). The gas-phase reaction components (O2, CO2, N2 and H2O) were modelled by placing a molecule in a cubic box of 30 Å length. Such a box size was found to be sufficient to make interactions between the periodic images negligibly small. The cell size was kept fixed, and the calculation was performed at the gamma point only, to reflect the lack of periodicity in the system. The reaction equations used to calculate combustion energies are given in Table S2.
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Table S2. Reaction equations used in the calculation of combustion energies.
Fuel material Reaction equation
ZIF-8 Zn(C4H5N2)2 (s) + 11 O2 (g) → ZnO (s) + 8 CO2 (g) + 2N2 (g) + 5H2O (g) Zn(VIm)2 Zn(C5H5N2)2 (s) + 13 O2 (g) → ZnO (s) + 10 CO2 (g) + 2N2 (g) + 5H2O (g) Zn(AIm)2 Zn(C5H3N2)2 (s) + 12 O2 (g) → ZnO (s) + 10 CO2 (g) + 2N2 (g) + 3H2O (g)
Co(VIm)2 Co(C5H5N2)2 (s) + 13⅙ O2 (g) → ⅓Co3O4 (s) + 10 CO2 (g) + 2N2 (g) + 5H2O (g)
Co(AIm)2 Co(C5H3N2)2 (s) + 12⅙ O2 (g) → ⅓Co3O4 (s) + 10 CO2 (g) + 2N2 (g) + 3H2O (g)
Cd(VIm)2 Cd(C5H5N2)2 (s) + 13 O2 (g) → CdO (s) + 10 CO2 (g) + 2N2 (g) + 5H2O (g) Cd(AIm)2 Cd(C5H3N2)2 (s) + 12 O2 (g) → CdO (s) + 10 CO2 (g) + 2N2 (g) + 3H2O (g)
The calculated combustion energies were used to derive the energy densities of ZIF materials.
3. Synthetic methods
3.1 Synthesis of 2-substituted imidazoles Compound HVIm was synthesized following the previously reported procedure.2 Compound HAIm was synthesized based on a modified procedure reported by Shaw and co-workers (Figure S2).3 To a solution of aldehyde (0.336 g, 3.5 mmol) and dimethyl-1-diazo-2-oxopropylphosphonate (1.0 g, 5.25 mmol) in MeOH (15 mL) was added K2CO3 (0.967 g, 7.0 mmol) in one portion at 0 °C under argon. The resulting yellow mixture was allowed to stir at room temperature for 12 h. The reaction mixture was filtered through a Celite pad and the solvent was concentrated under reduced pressure. The resulting crude mixture was diluted with 20 mL of water and extracted with EtOAc (2 x 30 mL) and the combined organic layers were washed with sat. NaHSO3 (1 x 20 mL) and sat. NaCl (1 x 20 mL), dried over MgSO4 and concentrated. Purification by flash chromatography on silica gel (methanol/dichloromethane: 1.5/98.5 to 2.5/97.5) afforded pure alkyne (0.138 g, 43%) as a white solid. Mp: 146.7-148.3. Rf: 0.5 (eluent: methanol/dichloromethane - 05:95). FTIR-ATR (cm-1, Figure S7): 3273, 3141, 3112, 3015, 2122, 1568, 1420, 1299, 1106, 986, 899, 757, 700, 610, 597. 1H NMR: (300 MHz, DMSO-d6, Figure S3): δ 12.79 (br. s, 1H), 7.05 (br.s, 2H), 4.30 (s, 1H) ppm. 13C NMR: (75 MHz, DMSO-d6, Figure S4): d 128.4, 123.5, 80.4, 75.3. ppm. HRMS: (Figure S7) calculated for C5H5N2 (M+H)+ 93.04472, found 93.04509. Analysis by solid-state CP-MAS 13C NMR (100 MHz): 74.8 ppm, 80.6 ppm, 118.7 ppm, 128.5 ppm (Figure S5).
Figure S2. Reaction scheme for the synthesis of HAIm.
3.2. Synthesis of hypergolic ZIFs
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3.2.1. Synthesis of SOD-Zn(AIm)2 and SOD-Zn(MeIm)2 (ZIF-8) Mechanochemical synthesis. These zinc-based ZIFs were synthesized using the ILAG methodology4 by milling of ZnO (1 mmol) with an equivalent amount (2 mmol) of the imidazole ligand HAIm or HMeIm, in the presence of ammonium acetate as the catalytic salt (0.12 mmol) and 100 µL ethanol as the liquid additive (absolute, kept over molecular sieves), in a 15 mL stainless-steel milling jar using two stainless-steel milling balls of 7 mm diameter (1.34 grams each). The mixture was milled for 30 minutes at frequency of 30 Hz in a (Retsch MM400 mixer mill). After preliminary analysis by PXRD, the product was stirred overnight with 15 mL methanol, filtered and evacuated under vacuum at 80 °C. Solid-state CP-MAS 13C NMR (100 MHz) for Zn(AIm)2: 75.4 ppm, 125.5 ppm, 135.7 ppm (Figure S6). 3.2.2 Solution synthesis of SOD-Zn(AIm)2:
A mixture of 2 mmol HAIm and 2 mmol triethylamine was added to 15 mL DMF and stirred at room temperature. A solution of 1 mmol zinc nitrate hexahydrate in 5 mL DMF was added dropwise, over 30 seconds, to the stirred solution of HAIm and triethylamine causing the precipitation of the target framework. This mixture was capped and placed at 60 °C for 3 hours, the product was isolated by filtration, and suspended in 20 mL methanol. This suspension was kept at 60 °C overnight, the product was again filtered, and stirred in chloroform for 3 hours to remove any residual DMF. Finally, the framework was isolated by filtration and evacuated at 80 °C under vacuum overnight. 3.2.3. Synthesis of SOD-Co(AIm)2 Mechanochemical synthesis. The framework SOD-Co(AIm)2 was obtained mechanochemically using the LAG methodology,5 by ball milling 0.25 mmol Cobalt(II) carbonate, 0.6 mmol HAIm, and 25 µL of ethanol in a 15 mL stainless-steel milling jar, using two stainless steel milling balls of 7 mm diameter (1.34 grams each). The reaction mixture was milled for 60 minutes at a milling frequency of 30 Hz. After preliminary PXRD analysis, the product was washed overnight with 15 mL of methanol, filtered and evacuated overnight under vacuum at 80 ⁰C.
Solution synthesis. 1 mmol of HAIm and 1.1 mmol of triethylamine were added to 15 mL of DMF and stirred at room temperature. A solution of 0.25 mmol of cobalt(II) nitrate hexahydrate in 20 mL of DMF was added dropwise to the ligand solution over a 10 minutes period, causing the precipitation of the deep-purple target framework. This mixture was allowed to stir for an additional 10 minutes, then the solid was isolated by filtration and rinsed with acetone. After stirring overnight in 20 mL of chloroform, the product was again isolated by filtration, rinsed with acetone, and placed under vacuum at 80 ⁰C overnight, affording the activated final product. The isolated yield was 82%.
3.2.4. Synthesis of SOD-Cd(AIm)2
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Mechanochemical synthesis. The framework SOD-Cd(AIm)2 was obtained mechanochemically using the ILAG methodology4 by ball milling 0.5 mmol cadmium(II) oxide, 1.5 mmol HAIm, 5 mg (NH4)2SO4 as the catalytic additive, and 100 µL of methanol as the milling liquid in a 15 mL stainless-steel milling jar, using two stainless steel milling balls of 7 mm diameter (mass 1.34 grams). After 70 minutes milling at a frequency of 30 Hz, PXRD analysis of the reaction mixture revealed complete disappearance of Bragg reflections of CdO, as well as the appearance of the ZIF product. The product was washed overnight with methanol and evacuated overnight under vacuum at 80 ⁰C.
Solution synthesis. 1 mmol of HAIm and 1.1 mmol of triethylamine were added to 15 mL of DMF and stirred at room temperature. A solution of 0.25 mmol of cadmium(II) nitrate tetrahydrate in 20 mL of DMF was added dropwise to the ligand solution over a 10 minute period, causing the precipitation of the target framework. The mixture was allowed to stir for an additional 10 minutes, then the solid was isolated by filtration and rinsed with acetone. After stirring overnight in 20 mL of chloroform, the product was again isolated by filtration, rinsed with acetone, and placed under vacuum at 80 °C overnight, affording the activated final product. The isolated yield was 65%. 3.2.5. Synthesis of SOD-Zn(VIm)2 The framework SOD-Zn(VIm)2 was obtained using the ILAG methodology4 by milling of zinc(II) oxide (1 mmol) with a slight excess of HVIm (2.1 mmol) in the presence of ammonium acetate as the catalytic salt additive (0.12 mmol) and 100 µL ethanol as the milling liquid (absolute, kept over molecular sieves), in a 15 mL stainless-steel milling jar using two stainless-steel milling balls of 7 mm diameter (1.34 grams each). The mixture was milled for 30 minutes at frequency of 30 Hz. After preliminary analysis by PXRD, the product was stirred overnight with 15 mL methanol, filtered and evacuated overnight under vacuum at 80 °C. Alternative syntheses of this material have been reported.1,6 3.2.6. Synthesis of SOD-Co(VIm)2 Mechanochemical synthesis. The framework SOD-Co(VIm)2 was obtained mechanochemically using the LAG methodology5 by ball milling 0.5 mmol Cobalt(II) carbonate, 1.2 mmol HVIm, and 50 µL of ethanol in a 15 mL stainless-steel milling jar, using two stainless steel milling balls of 7 mm diameter (1.34 grams each). After 60 minutes milling at a frequency of 30 Hz. After PXRD analysis, the product was washed overnight with 15 mL of methanol, filtered and evacuated overnight under vacuum at 80 ⁰C.
Solution synthesis. Solution synthesis for SOD-Co(VIm)2 was identical to that of SOD-Co(AIm)2, replacing 1 mmol of HAIm with 1 mmol of HVIm. The isolated yield was 32%. 3.2.7. Synthesis SOD-Cd(VIm)2 Mechanochemical synthesis. The framework SOD-Cd(VIm)2 was obtained mechanochemically using the ILAG methodology4 by ball milling 0.5 mmol CdO, 1.2 mmol
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HVIm, 5 mg ammonium methanesulfonate as the catalytic salt additive and 75 μL of methanol in a 15 mL stainless-steel milling jar, using two stainless steel milling balls of 7 mm diameter (1.34 grams each). The mixture was milled for 25 minutes at a frequency of 30 Hz. After preliminary PXRD analysis of the reaction mixture, the product was washed overnight with 15 mL methanol and evacuated overnight under vacuum at 80 ⁰C.
Solution synthesis. Solution synthesis for SOD-Cd(VIm)2 was identical to that of SOD-Cd(AIm)2, replacing 1.1 mmol of HAIm with 1.1 mmol of HVIm. The total isolated yield was found to be 30%. 4. Nuclear magnetic resonance (NMR) spectroscopy
Figure S3: 1H NMR spectrum of HAIm recorded in DMSO-d6.
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Figure S4: 13C NMR spectrum of HAIm recorded in DMSO-d6.
Figure S5: Solid-state CP-MAS 13C NMR spectrum of HAIm.
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Figure S6: Solid-state CP-MAS 13C NMR of SOD-Zn(AIm)2.
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5. Fourier-transform attenuated total reflectance (FTIR-ATR) infrared spectroscopy
Figure S7. Overlay of FTIR-ATR spectra of (from top to bottom): HAIm, Zn(AIm)2,
Co(AIm)2, and Cd(AIm)2.
Figure S8. Overlay of FTIR-ATR spectra of (from top to bottom): HVIm, Zn(VIm)2,
Co(VIm)2, and Cd(VIm)2.
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6. Thermal analysis
Figure S9. The TGA (left) and DSC (right) thermograms of SOD-Zn(AIm)2 in air. Experimental residue:
32.9%; calculated for ZnO: 33.6%.
Figure S10. The TGA (left) and DSC (right) thermograms of SOD-Co(AIm)2 in air. Experimental residue:
33.3%; calculated for Co3O4: 33.7%.
Figure S11: The TGA (left) and DSC (right) thermograms of SOD-Cd(AIm)2 in air. Experimental residue:
43.5%; calculated for CdO: 44.6%.
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Figure S12: The TGA (left) and DSC (right) thermograms of SOD-Zn(VIm)2 in air. Experimental residue:
32.3%; calculated for ZnO: 33.6%.
Figure S13: The TGA (left) and DSC (right) thermograms of SOD-Co(VIm)2 in air. Experimental residue:
32.7%; calculated for Co3O4: 33.0%.
Figure S14: The TGA (left) and DSC (right) thermograms of SOD-Cd(VIm)2 in air. Experimental residue:
42.9%; calculated for CdO: 43.9%.
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7. Example drop tests with RFNA
Figure S15. Example drop tests with RFNA. In each case, the first image corresponds to the first observable frame in which ignition is visible, and the number below indicates the ID. The second image corresponds to an arbitrary point after ignition, illustrating the induced flame.
Figure S16. The hypergolicity of SOD-Zn(AIm)2 (left) and SOD-Co(AIm)2 (right) after one month using WFNA as an oxidizer. For each material, the first image corresponds to the first observable frame in which ignition is visible, and thus the number below that image is the ignition delay. The second image corresponds to an arbitrary point after ignition, and illustrates the intensity and character of the flame.
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8. References: 1. Björkman, T. Comput. Phys. Commun. 2011, 182, 1183. 2. Sun, Q.; He, H.; Gao, W.-Y.; Aguila, B.; Wojtas, L.; Dai, Z.; Li, J.; Chen, Y.-S.; Xiao, F.-S.; Ma. S. Nat. Commun. 2016, 7, 13300. 3. Dirat, O.; Clipson, A.; Elliott, J. M.; Garrett, S.; Jones, A. B.; Reader, M.; Shaw, D. Tetrahedron Lett. 2006, 47, 1729. 4. Friščić, T.; Reid, D. G.; Halasz, I.; Stein, R. S.; Dinnebier, R. E.; Duer, M. J. Angew. Chem. Int. Ed. 2010, 49, 712. 5. Friščić, T.; Fábián, L. CrystEngComm 2009, 11, 743. 6. Marrett, J.; Mottillo, C.; Girard, S.; Do, J.-L.; Nickels, C. W.; Gandrath, D.; Germann, L. S.; Dinnebier, R. E.; Howarth, A. J.; Farha, O. K.; Friščić, T.; Li, C.-J. Cryst. Growth Des. 2018, 18, 3222.
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