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, H3A 0B8, Canada; bSchool of

Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK;c525

Solutions, Inc., P.O. Box 2206, Tuscaloosa, AL 35403, USA

Abstract

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 energetic 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.

Introduction

Energetic materials,1-5 i.e. controllable chemical energy storage systems, are central for a number

of civilian and military applications, including explosives, propellants and pyrotechnics. Current

research in this area is aimed towards materials with specific requirements, such as high energy

density, improved thermal stability, low cost, and environmental acceptability.6-9 An important

class of energetic materials are hypergolic materials, fuels10-13 which ignite in contact with an

oxidizer, such as white (WFNA) or red fuming nitric acid (RFNA). Hypergolic compounds are a

necessary component of spacecraft and launcher propellant systems,14 which require materials

with a low ignition delay (ID), defined as the time between ignition and the first contact of the fuel

with the oxidizer. The most popular hypergolic components of bipropellant systems today are

based on toxic and highly carcinogenic hydrazine and its derivatives,15-16 raising concerns of

environmental damage. As an example, the annual release of cancerogenic propellants in the

atmosphere is estimated at 12,000 tons for Europe alone, inspiring the search for safer hypergolic

fuels.17-19

Metal-organic frameworks (MOFs)20,21 are advanced materials of principal importance in

gas separation, storage,22-24 and catalysis,25 but have been also been explored as energetic

materials.26-28 In principle, MOFs should offer a uniquely modular platform29 for developing

hypergolic ignition and fuel systems, where the ignition and combustion properties of the solid

fuel can be fine-tuned by variation of both organic and inorganic components of the MOF.

However, exploration of MOFs as energetic materials has so far focused only on properties

relevant to explosives or pyrotechnical materials, such as thermal and shock sensitivity.26,30

Hypergolicity, which is a critical property of a potential fuel or propellant has not hey been

investigated in context of MOFs or coordination polymers in general.

We now present a strategy to induce hypergolic behavior in MOFs by incorporating an

acetylene or vinyl substituent as a trigger of hypergolicity in a zeolitic imidazolate framework

(ZIF, Figure 1a).31,32 Whereas ZIFs are generally not considered energetic materials and exhibit

2

high thermal stability,31,33 we noted a potential structural and electronic similarity between

imidazole linker in a ZIF, flanked on both nitrogen sites by cationic nodes, and energetic

imidazolium cations used in energetic ionic liquids, similarly flanked by alkyl substituents (Figure

1b). We hypothesized that the azolate linkers in a ZIF could exhibit latent energetic properties,

which might be unlocked by introducing triggers of hypergolic behavior, such as vinyl or acetylene

functionalities.34

Figure 1. a) Similarity between: (top) the organic linker in a ZIF and (bottom) a typical energetic

imidazolium cation. b) The ligands HAIm and HVIm used in the development of hypergolic ZIFs.

Results and discussion

Using imidazole ligands substituted in the 2-position by acetylene (HAIm) and vinyl (HVIm)

groups (Figure 1c), we targeted ZIFs based on Zn2+, Co2+ and Cd2+ cations which are popular in

ZIF synthesis, but also provide an opportunity to systematically vary size and electronic properties

of framework nodes. The framework Zn(VIm)2 was recently reported, but its ignition or energetic

properties were not explored. All six ZIFs were readily accessible by ion- and liquid-assisted

grinding (ILAG) mechanochemistry, i.e. by ball milling equimolar amounts of a metal source

(ZnO, CoCO3, Cd(OH)2 or CdO) and a substituted imidazole, in the presence of a liquid additive

and a salt catalyst (see SI). In all six cases, powder X-ray diffraction (PXRD) revealed quantitative

formation of products isostructural to ZIF-8 (CSD OFERUN0235), the popular sodalite (SOD)

topology ZIF based on 2-methylimidazole. The structure of guest-free SOD-Zn(VIm)2 was

recently reported (CSD GAZBOB),36 which facilitated the creation of a preliminary structural

model for each framework, and the final structures were obtained by Rietveld refinement of PXRD

data on MOF samples that were washed with methanol and evacuated (Figure 2). Compositions of

each MOF was verified by thermogravimetric analysis (TGA) in air, and the absence of starting

material was confirmed by infrared spectroscopy (see SI). Combined TGA and differential

scanning calorimetry (DSC) revealed high thermal stability in air for all MOFs, with exothermic

degradation above 200oC.

Hypergolic properties were evaluated by measuring the ID for each material using a standard drop

test,37 in which a single 10 μL drop of a liquid oxidizer was released from a fixed height of 5.0 cm,

using a 100 μL Hamilton micro-syringe, into a 4.5 cm high glass vial, containing 5.0 mg of the

solid fuel MOF or pure ligand) placed in the center. As oxidizers were used white (WFNA, 98%

HNO3) and less potent red fuming nitric acid (RFNA, 90+% HNO3), conventionally used in

3

propellant systems.11,18,38 Each drop test was done 3 times and recorded using a monochrome

Redlake MotionPro Y4 high-speed camera operating at 1000 frames/s. The tests were done on

Zn(AIm)2, Co(AIm)2, Cd(AIm)2, Zn(VIm)2, Co(VIm)2, Cd(VIm)2, as well as HAIm, HVIm

(Figure 2) and two ZIFs containing saturated hydrocarbon groups that were not expected to exhibit

hypergolicity: ZIF-8 due to isostructurality to herein prepared ZIFs, and open RHO-topology zinc

2-ethylimidazolate (RHO-Zn(EtIm)2) as a saturated analogue of Zn(AIm)2 and Zn(VIm)2.

Figure 2: (a) Final Rietveld fits for: (top row) Zn(AIm)2, Co(AIm)2, Cd(AIm)2, (bottom row) Zn(VIm)2,

Co(VIm)2 and Cd(VIm)2. (b) Examples of hypergolicity drop tests for:(top row) HAIm, Zn(AIm)2,

Co(AIm)2 and Cd(AIm)2, (bottom row) HVIm, Zn(VIm)2, Co(VIm)2 and Cd(VIm)2. The moment of

ignition for each experiment is indicated by a red star. Each drop test was conducted in triplicate (see Table

1).

As anticipated, drop tests with WFNA and RFNA (Figure 2, Table 1) revealed no ignition

for ZIF-8 and RHO-Zn(EtIm)2. In contrast, Zn(AIm)2, Co(AIm)2 and Cd(AIm)2 exhibited high

hypergolic behavior with WFNA, with ultra-short IDs (below 5 ms)19 and appearance of a flame

4-6 cm in height. Similar results were obtained for Zn(AIm)2 made from solution (Table 1, also

SI), indicating that method of preparation does not affect hypergolic behavior significantly. The

4

vinyl-substituted MOFs were also hypergolic with WFNA, with short IDs between 11 ms and 35

ms. For comparison, a target ID in the development of a functional hypergolic propellant is 50 ms

or below.18 Consistent results were obtained using RFNA, where drop tests showed highly

hypergolic behavior for Zn(AIm)2, Co(AIm)2 and Cd(AIm)2, and weak hypergolicity for

Co(VIm)2. The drop tests confirm that incorporating vinyl or acetylene groups onto the ZIF

scaffold promotes hypergolic behavior, previously not reported in ZIFs. Importantly, the pure

ligands HAim and HVim show poor (with WFNA) or no (with RFNA) hypergolic performance,

which is improved upon ZIF formation.

Table 1. Hypergolic properties of herein explored ZIFs in drop tests using WFNA and RFNA

as oxidizers.a

fuel drop test with WFNA drop test with RFNA

ID / ms

flame

duration /

ms

flame

heightb /

cm

flame

color ID / ms

flame

duration

/ ms

flame

heightb

/ cm

flame

color

Zn(AIm)2 2(1);

4(1)c

>600 4 red,

blue

12(2) < 150 4 red,

blue

Co(AIm)2 2(1) >200 4 orange 7(2) > 200 5 orange

Cd(AIm)2 5(1) >200 6 yellow 30(2) < 150 4 orange

Zn(VIm)2 29(1) sparks sparks red no

ignition

- - smoke

Co(VIm)2 11(5) >200 2 orange 164e sparks - orange

Cd(VIm)2 35(1) sparks sparks yellow no

ignition

- - smoke

ZIF-8 no

ignition

- - - no

ignition

- - -

Zn(EtIm)2d no

ignition

- - - no

ignition

- - -

HAIm 34(4) >100 7 red no

ignition

- - smoke

HVIm 34(2) sparks sparks red no

ignition

- - smoke

a each test was conducted in triplicate; b approximate values; csample made from solution; dRHO-

topology framework; eignited in only one out of three tests.

Particularly remarkable is the comparison of hypergolic activity of HAIm, which exhibits an ID

of 34(4) ms with WFNA and does not ignite at all with RFNA, to the corresponding ZIFs, which

exhibited ID times as low as 2(1) ms (with WFNA) and 7(2) ms (with RFNA). These results show

that the formation of coordination bonds in a ZIF can improve hypergolic behavior compared to

the free ligand. This makes the design of hypergolic MOFs different from strategies for making

5

energetic MOFs, where coordination bond formation typically leads to the stabilization of an

energetic ligand.26

The isostructurality of all six hypergolic MOFs enables a systematic analysis of hypergolic

behavior with respect to metal and linker choice. Overall, ZIFs derived from HAIm exhibited

shorter IDs and more vigorous combustion compared to the ones derived from HVIm (Figure 2,

Table 1), indicating the acetylene group is more efficient in inducing hypergolic behavior. The

zinc- and cobalt-based MOFs generally exhibited shorter IDs compared to cadmium ones, which

agrees with a larger radius and poorer electron-withdrawing properties of Cd2+. Importantly, Co2+

gave hypergolic properties comparable or superior to those of either zinc or cadmium frameworks:

ID for Co(AIm)2 is identical (with WFNA) or significantly shorter (with RFNA) compared to

Zn(AIm)2. Moreover, Co(VIm)2 was the only vinyl-substituted ZIF that developed a flame with

WFNA, and the only vinyl-substituted MOF that was hypergolic with RFNA. As Zn2+ and Co2+

are of similar size, we relate the high hypergolicity of Co(AIm)2 and Co(VIm)2 to the open d-shell

configuration of Co2+ and, potentially, to the ability of metal node to undergo redox reactions.

Drop tests with WFNA were also performed on Zn(AIm)2, Co(AIm)2 and Cd(AIm)2 made

from solution, again revealing ultra-short IDs below 5 ms. Hypergolic tests on Zn(AIm)2,

Co(AIm)2 and Cd(AIm)2 after one month of storage in a closed vial revealed a short ID of 4(1)

ms for Zn(AIm)2 but diminished activity for Co(AIm)2, (ID of 54(5) ms, see SI). Hypergolicity of

Cd(AIm)2 dropped significantly upon storage, producing only sparks in drop tests. The retention

of hypergolic activity upon storage, evident for Zn(AIm)2, is an important factor for applications

in solid fuels.

In order to verify whether the observed hypergolic behavior is indeed the result of using

trigger groups to unlock the suspected latent energetic behavior of ZIFs, we used CASTEP periodic

density functional theory (DFT)39 calculations to evaluate the combustion energies (ΔEc) for

reactions of hypergolic MOFs Zn(AIm)2, Co(AIm)2, Cd(AIm)2, Zn(VIm)2, Co(VIm)2, Cd(VIm)2

as well as the non-hypergolic ZIF-8 with O2 gas to produce solid ZnO, CdO or Co3O4 along with

CO2, N2, and H2O gas (equations given in SI). The calculations were performed using the GGA-

type PBE40 functional combined with Grimme D241 dispersion correction, and crystal structures

of ZIFs were geometry-optimized with respect to atom coordinates and unit cell parameters,

subject to space group symmetry constraints. The gravimetric (Eg) and volumetric (EV) energy

densities were computed from ΔEc, taking into account the calculated density of ZIFs. The

accuracy of calculations was verified by comparing the herein calculated ΔEc for ZIF-8 (-3916.0

kJ mol-1) to the combustion enthalpy (ΔHc) which was also measured herein by bomb calorimetry

(-4654.0 kJ mol-1). The comparison shows that calculated values are reasonably accurate and

probably underestimate true values by ca. 15%.

All oxidation reactions were calculated to be highly exothermic, with ΔEc ranging from -

4,700 kJ mol-1 to -4,800 kJ mol-1 (Table 2). Importantly, the ΔEc of vinyl- and acetylene-substituted

ZIFs were similar to that of ZIF-8, which confirms that acetylene and vinyl substituents act as

triggers for hypergolic behavior, without increasing significantly the energetic content of the ZIFs.

The energetic content is significant, as the calculated Eg and EV for all herein explored ZIFs,

including ZIF-8, are actually comparable to popular explosives (e.g. TNT, 14.9 kJ g-1)43 and

hydrazine fuels (19.5 kJ g-1, 19.3 kJ cm-3).44

Table 2. Calculated combustion energy ΔEc, gravimetric (Eg) and volumetric (EV) energy

density, and crystallographic unit cell parameter (a) for ZIFs.

6

ZIF ΔEc / kJ mol-1 Eg / kJ g-1 EV / kJ cm-3 a / Å

Zn(AIm)2 -4783.8 19.3 19.3 17.045(1)

Co(AIm)2 -4760.0 19.7 19.5 16.960(2)

Cd(AIm)2 -4799.9 16.3 16.5 17.9721(9)

Zn(VIm)2 -4789.9 19.0 18.9 17.147(2)a

Co(VIm)2 -4767.6 19.5 18.4 17.296(1)

Cd(VIm)2 -4808.4 16.1 15.8 18.234(2)

ZIF-8 -3916.0b 17.2c 15.9d 16.992(1)e

a) from CSD structure GAZBOB;36 b) measured enthalpy of combustion (ΔHc) is -4649.0 kJ mol-1; c) 20.6

kJ g-1 based on measured ΔHc; d) 18.9 kJ cm-3 based on measured ΔHc; e) CSD structure OFERUN02.35

In summary, we described a strategy to induce hypergolic behavior in MOFs, by using acetylene

and vinyl substituents to unlock the latent energetic properties of electron-deficient linkers in a

ZIF. This strategy is intrinsically different from approaches used to design other types of energetic

(e.g., explosive, pyrotechnic) MOFs, as it utilizes the formation of coordination bonds to modify

the electronic structure of the linker and enhance its hypergolic behavior, rather than to stabilize

an energetic ligand.26,45,46 The modularity of MOFs permits modification of the hypergolic

properties (ignition delay, flame color and duration) of the fuel to be modified, including achieving

ultra-fast ignition delays, without changing its overall structure. It is important to stress that the

acetylene or vinyl triggers induce hypergolicity without major changes to the overall energetic

content of the ZIF structure. The ability to achieve and fine-tune hypergolic performance,

combined with the absence of hydrazine-based carcinogens or explosive components, should make

hypergolic MOFs promising candidates for safer, environmentally-friendly propellants.

Supporting Information

Supporting information is available free of charge on the ACS Publications website (PDF of

additional experimental details and crystal structure descriptions, differential scanning calorimetry

and thermogravimetric analysis).

Corresponding Authors

Emails: tomislav.friscic@mcgill.ca, robin.rogers@525solutions.com

Notes

The authors declare no competing financial interests.

Acknowledgments

This research is supported by the Air Force Office of Scientific Research under AFOSR Award

No. FA9550-14-1-0306. This research was undertaken, in part, thanks to funding from the Canada

Excellence Research Chairs Program, the NSERC Discovery Grant (NSERC RGPIN-2017-

06467), the NSERC E. W. R. Steacie Memorial Fund (NSERC SMFSU 507347-17). and the

Canada CFI program.

7

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40. Perdew, J. P.; Burke K.; Ernzerhof, M. Generalized gradient approximation made simple. Phys.

Rev. Lett. 1996, 77, 3865-3868.

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dispersion correction J. Comput. Chem. 2006, 27, 1787-1799.

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Ma S. Imparting amphiphobicity on single-crystalline porous materials. Nat. Commun. 2016, 7,

13300.

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D. L., Lawrence Livermore National Laboratory, UCRL-52821, Livermore. CA, 1982.

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1

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

2

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

F(000) 1488 1452 1500 1704 1752 Rwp 0.118 0.048 0.065 0.068 0.076 Rp 0.092 0.036 0.050 0.055 0.058

RBragg 0.082 0.027 0.051 0.051 0.023 χ2 4.137 1.460 2.407 1.971 3.533

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.

5

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

6

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.

9

Figure S4: 13C NMR spectrum of HAIm recorded in DMSO-d6.

Figure S5: Solid-state CP-MAS 13C NMR spectrum of HAIm.

10

Figure S6: Solid-state CP-MAS 13C NMR of SOD-Zn(AIm)2.

11

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.

12

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%.

13

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%.

14

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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