IONIC LIQUID ENERGETIC MATERIALS

Cross Reference to Related Application

This application claims the benefit of U.S. Application No. 60/564,370, filed April 22, 2004, the disclosure of which is incorporated by reference. Background of the Invention

Field of the Invention

The present application is directed to energetic materials, and more particularly, to ionic liquid energetic materials. Description of the Related Art Energetic materials are useful as propellants, gas generants, and the like. One class of propellant is known as a "monopropellant," which is a chemical propellant that does not require a separate oxidizer. Hydrazine, N ₂ H ₄ , is typically considered the current state of the art in monopropellants. Hydrazine has been studied for at least 30 years as a monopropellant, for example, as disclosed in G. P. Sutton Rocket Propulsion Elements An Introduction to the Engineering of Rockets, Sixth Edition John Wiley, New York: 1992, at 257. Briefly, hydrazine is decomposed by passing through a heated bed of an iridium- coated alumina catalyst, providing ammonia, nitrogen, and hydrogen gas. Notably, the decomposition of hydrazine may be controlled to provide the volatile products in varying stoichiometrics, which permits varying the specific impulse. These hydrazine systems are stable and dependable, and provide consistent and predictable results.

Hydrazine possesses significant drawbacks, however, which limit its use as a monopropellant. Hydrazine is a potential carcinogen and damages living tissue.

Furthermore, hydrazine has a high vapor pressure, which results in vapor toxicity in workers. Consequently, workers wear self-contained breathing suits, thereby raising the cost of working with hydrazine.

Summary of the Invention

Disclosed herein is a high-nitrogen-content energetic ionic liquid useful as an energetic material, for example, as a propellant and/or gas generant. In some embodiments, the cation of the energetic ionic liquid is a tetrazolium ion. In some embodiments, the anion of the energetic ionic liquid is a tetrazolide ion. In some embodiments, the energy impulse of the energetic ionic liquid is at least about 275 lb-s/lb.

An embodiment disclosed herein provides an energetic ionic liquid comprising an anion of formula I:

I and a cation, wherein R ¹ is selected from the group consisting of hydrogen; halogen; C ₁ to C ₂₀ alkyl, aralkyl, or aryl, optionally substituted with one or more substituents independently selected from fluorine and azide; and NR ³⁰ R ³¹ or OR ³² , wherein R ³⁰ , R ³¹ , and R ³² are independently selected from the group consisting of Ci to C ₂₀ alkyl, aralkyl, and aryl, each of which is independently optionally substituted with one or more substituents independently selected from fluorine and azide.

Another embodiment provides an energetic ionic liquid comprising a cation and an anion, wherein the cation is selected from the group consisting of formula H:

II wherein R ⁰ is selected from the group selected from hydrogen; halogen; Ci to C ₂₀ alkyl, aralkyl, or aryl, optionally substituted with one or more substituents independently selected from fluorine and azide; and NR ³³ R ³⁴ , OR ³⁵ , or SR ³⁶ wherein R ³³ , R ³⁴ , R ³⁵ , and R ³⁶ are independently selected from the group consisting of Ci to C ₂₀ alkyl, aralkyl, or aryl, each of which is independently optionally substituted with one or more substituents independently selected from fluorine and azide; R ⁷ and R ⁸ are independently selected from the group consisting of Ci to C ₂₀ alkyl, aralkyl, or aryl, each of which is optionally substituted with one or more substituents independently selected from fluorine and azide; and R ⁸ is attached at the 2-, 3-, or 4-position of the tetrazole ring, or a mixture thereof.

Also disclosed herein is a composition comprising a disclosed energetic ionic liquid. In some embodiments, the composition further comprises a non-energetic ionic liquid. In some embodiments, the composition further comprises an energetic salt.

Brief Description of the Drawings

FIG. 1 is a plot of the calculated energy impulses (Isp) for butylammonium 5- methyltetrazolide (BAMT), pentylammonium 5-methyltetrazolide (ABAMT), monomethyhydrazine (MMH), and 7V,7V-dimethylaminoethylazide (DMAZ).

FIG. 2 is a plot of the calculated density energy impulses (D*Isp) for butylammonium 5-methyltetrazolide (BAMT), pentylammonium 5 -methyl tetrazolide (ABAMT), monomethyhydrazine (MMH), and N,N-dimethylaminoethylazide (DMAZ).

FIG. 3 provides differential scanning calorimetry plots for butylammonium 5- methyltetrazolide and pentylammonium 5-methyltetrazolide.

Detailed Description of the Preferred Embodiment

Disclosed herein is a high-nitrogen-content energetic ionic liquid comprising a cation (M ⁺ ) and an anion (A ^" ), wherein at least one of the cation or anion is a high- nitrogen-content ion. Some embodiments of the disclosed energetic ionic liquid are useful in the formulation of monopropellants and/or bipropellants. In some embodiments, the energetic ionic liquid exhibits substantially no detectable vapor pressure. Some embodiments of the energetic ionic liquid have a high energy density.

In some embodiments, the melting point of the energetic ionic liquid is below ambient temperature. In other embodiments, the melting point of the energetic ionic liquid is about ambient temperature. In other embodiments, the melting point of the energetic ionic liquid is above ambient temperature. The fluid state of the energetic ionic liquid permits the filling of irregular volumes without leaving voids. Embodiments in which the energetic ionic liquid has a melting point above ambient temperature permits the molding or casting, at an elevated temperature, of a motor or other object that is solid at ambient temperature. For example, in some embodiments, the melting point of the energetic ionic liquid is below about 0 ⁰ C, about 10 °C, about 20 ⁰ C, about 30 °C, about 40 ⁰ C, about 50

⁰ C, about 60 °C, about 70 ⁰ C, about 80 °C, about 90 °C, or about 100 ⁰ C.

In some embodiments, the melting point of the energetic ionic liquid is modified using one or more additives that alters the melting-point to the energetic ionic liquid. Those skilled in the art will understand that the additive will typically depress the melting point of the energetic ionic liquid. Those skilled in the art will also realize that the energy density of the mixture of the energetic ionic liquid and the melting-point-altering additive will, in some cases, be different from the energy density of the energetic ionic liquid alone. In some embodiments, the additive is used primarily to modify the energy density rather than to alter the melting point of the resulting mixture.

In some embodiments, the additive is an ionic liquid. In embodiments in which the ionic liquid additive has a low vapor pressure, evaporation does not appreciably change the stoichiometry of the ionic liquid additive/energetic ionic liquid. In some embodiments, a

cation or an anion of the ionic liquid additive is the same as a cation or an anion of the energetic ionic liquid. In another embodiment, both the cation and the anion of the ionic liquid additive are different from the cation and anion of the energetic ionic liquid. In some embodiments, the ionic liquid additive is a non-energetic ionic liquid. In embodiments in which the non-energetic ionic liquid additive is non-flammable, the additive/energetic ionic liquid mixture is safer than embodiments in which the additive is flammable or inflammable.

In some embodiments, the energetic ionic liquid is dissolved in a solvent. In some embodiments, the solvent is a non-energetic ionic liquid. Some non-energetic ionic liquids are ideal solvents for energetic ionic liquids, because some non-energetic ionic liquids possess low vapor pressure and/or non-flammability. As discussed above, dissolving the energetic ionic liquid in a non-energetic ionic liquid provides a mixture with a lower energy density.

In some embodiments, the energetic ionic liquid has an energy impulse (Isp) of at least about 275 lb-s/lb, at least about 300 lb-s/lb, or at least about 325 lb-s/lb. The Isp is calculated or experimentally determined. Methods for determining Isp are known in the art, for example, using Chemical Equilibrium with Applications (CEA) software, developed by the U.S. National Aeronautics and Space Administration (NASA). Isp values are calculated using inhibited red fuming nitric acid (IRFNA) as the oxidant. As used herein, the term "alkyl" is used with its ordinary meaning, as well as referring to straight-chain (normal) alkyl groups, branched alkyl groups, cyclic alkyl groups, and combinations thereof. As used herein, the term "aryl" is used with its ordinary meaning, as well as to mean carbocyclic aryl groups as well as heterocyclic aryl groups, either of which is isolated or fused. An "optionally substituted" group is optionally substituted with any substituent known for that group.

Some embodiments of the disclosed energetic ionic liquid comprise a tetrazolide anion of formula I:

I and a cation, wherein R ¹ is selected from the group consisting of hydrogen; halogen;

nitro; azide;

Ci to C ₂₀ alkyl, aralkyl, or aryl, optionally substituted with one or more substituents independently selected from fluorine and azide; NH ³⁰ R ³¹ or OR ³² , wherein R ³⁰ , R ³¹ , and R ³² are independently selected from the group consisting of hydrogen and Ci to C ₂₀ alkyl, aralkyl, and/or aryl, each of which is independently optionally substituted with one or more substituents independently selected from fluorine and azide.

In some embodiments, R ³⁰ and R ³¹ together comprise a cyclic group. In some embodiments, R ¹ is 5'-tetrazolyl, which is optionally substituted. The properties of the energetic ionic liquid, for example, the melting point, density, viscosity, water miscibility, energy density, stability, and the like, depend on the identity of R ¹ . In some embodiments, R ¹ is selected to vary the energy density of the energetic ionic liquid.

In some embodiments, the cation is a monocation. In some embodiments, the cation is a dication, a trication, a tetracation, or a polycation. Some embodiments comprise a plurality of cations. The properties of the energetic ionic liquid depend on the identity of the cation or cations. By selecting an appropriate cation or cations, the properties of the energetic ionic liquid may be varied, including, for example, the melting point, density, viscosity, water miscibility, energy density, stability, and the like. In some embodiments, the cation is selected to provide an energetic ionic liquid with a high energy density.

Examples of suitable cations include ammonium cations, imidazolium cations, pyridinium cations, phosphonium cations, guanidinium cations, and uronium cations.

In some embodiments, the cation is an ammonium cation of formula IV:

IV wherein R ² , R ³ , R ⁴ and R ⁵ are each independently selected from the group consisting of hydrogen, Cj to C ₂₀ alkyl, aralkyl, and/or aryl, each of which is independently optionally substituted. Examples of suitable substituents include halogen, fluorine, azide, hydroxyl, alkoxy groups, amino groups, carbonyl groups, and the like. In some preferred embodiments, one or more of R ² , R ³ , R ⁴ and R ⁵ is and alkyl or aryl azide. In some embodiments, any of R ² , R ³ , R ⁴ , and/or R ⁵ together form one or more rings. For example, in some embodiments, R ² and R ³ together form a five-membered ring, which optionally

contains one or more additional heteroatoms, for example, oxygen and/or nitrogen. In some embodiments, R ² and R together form a six-membered ring, which optionally contains one or more additional heteroatoms, for example, oxygen and/or nitrogen. Examples of suitable ammonium cations include tetramethylammonium, tetraethylammonium, tetrabutylammonium, methyltrioctylammonium, 1,1-dimethylpyrrolidinium, 1 -ethyl- 1- methylpyrrolidinium, 1,1-dipropylpyrrolidinium, 1,1-dibutylpyrrolidiiiium, 1 -butyl- 1- methylpyrrolidinium, 1 -butyl- 1-ethylpynOlidiniuni, 1,1-dihexylpyrrolidinium, 1-hexyl-l- methylpyrrolidinium, and 1-octyl-l-methylpyrrolidinium. In some embodiments, the ammonium cation is derived from tris(2-aminoetliyi)amine [4097-89-6]. Suitable imidazolium cations include imidazolium cations of formula V:

V wherein R ¹⁰ , R ¹¹ , and R ¹² are each independently selected from the group consisting of hydrogen, and Ci to C ₂₀ alkyl, aralkyl, and aryl, each of which is independently optionally substituted with one or more substituents independently selected from fluorine and azide. In some embodiments, R ¹⁰ and R ¹² together form a ring. Examples of suitable imidazolium cations include 1-methylimidazolium, 1-butylimidazolium, 1,3-dimethylimidazolium, 1- ethyl-3-methylimidazolium, 1 -butyl-3-methylimidazolium, 1 -pentyl-3-methylimidazolium, 1 -hexyl-3-methylimidazolium, 1 -octyl-3-methylimidazolium, 1 -decyl-3- methylimidazolium, 1 -benzyl-3 -methylimidazolium, 1 -phenylpropyl-3 -methylimidazolium, l-ethyl-2,3-dimethylimidazolium, l-butyl-2,3-dimethylimidazolium, and l-hexyl-2,3- dimethylimidazolium.

Suitable pyridinium cations include pyridinium cations of formula VI:

wherein R ¹³ is selected from the group consisting of Ci to C ₂₀ alkyl _/ aralkyl, and aryl, optionally substituted with one or more substituents independently selected from fluorine and azide; and R ¹⁴ , R ¹⁵ , and R ¹⁶ are independently selected from the group consisting of hydrogen, and Ci to C ₂₀ alkyl, aralkyl, and aryl, each of which is independently optionally

substituted with one or more substituents independently selected from fluorine and azide. In some embodiments, R ¹⁴ and R ¹⁵ together form a ring. Examples of suitable pyridinium cations include N-ethylpyridinium, yV-butylpyridinium, N-hexylpyridinium, N- octylpyridinium, 3-methyl-N-butylpyridinium, 3-methyl-N-hexylpyridinium, 3-methyl-N- octylpyridinium, 3-ethyl-N-butylpyridinium, 4-methyl-N-butylpyridinium, 4-methyl-N- hexylpyridinium, 3,4-dimethyl-N-butylpyridinium, and 3,5-dimethyl-N-butylpyridinium. Suitable phosphonium cations include phosphonium cations of formula VII:

VII wherein R ¹⁷ , R ¹⁸ , R ¹⁹ , and R ²⁰ are independently selected from the group consisting of Ci to C ₂₀ alkyl, aralkyl, and aryl, each of which is independently optionally substituted with one or more substituents independently selected from fluorine and azide. In some embodiments, any of R ¹⁷ , R ¹⁸ , R ¹⁹ , and/or R ²⁰ together form one or more rings. Examples of suitable phosphonium cations include tetrabutylphosphonium, trihexyl(tetradecyl)phosphonium, benzyltriphenylphosphonium, and tri-z-propyl(methyl)phosphonium.

Suitable guanidinium, isouronium, and isothiouronium cations include cations of formula VIII:

VIII wherein A is selected from NR ²⁵ R ²⁶ , OR ²⁷ , and SR ²⁸ ; and R ²¹ , R ²² , R ²³ , R ²⁴ , R ²⁵ , R ²⁶ , R ²⁷ , and R ²⁸ are independently selected from hydrogen; and C ₁ to C ₂₀ alkyl, aralkyl, and/or aryl, each of which is independently optionally substituted with one or more substituents independently selected from fluorine and azide. In some embodiments, any of R ²¹ , R ²² , R ²³ , R ²⁴ , R ²⁵ , R ²⁶ , R ²⁷ , and/or R ²⁸ together form one or more rings. Examples of suitable cations of formula VIII include guanidinium, N,N,N',N'-tetramethyl-N"-ethylguanidinium, N,NN',N',N"-pentamethyl-N"-propylguanidinium, N,iV,N',N',N"-pentamethyl-N"-z- propylguanidinium, hexamethylguanidinium, O-methyl-N,N,N',N'-tetramethylisouronium, 0-ethyl-N,N,N',N'-tetramethylisouronium, and S-ethy\-N,N,N',N'- tetramethylisothiouronium . The melting points of selected ammonium tetrazolide salts are provided in TABLE

I. The ammonium tetrazolides are synthesized as described in greater detail below. The

tetrazolides are the dianion of 5,5'-bi-lH-tetrazole (BHT, R ¹ = 5-tetrazolyl) and the anion of 5-methyltetrazole (MT, R ¹ = CH ₃ ), which are illustrated below.

BHT MT TABLE I

Amine Tetrazole m.p.

Triniethylamine (2 eq.) BHT 38 °C

Dimethylamine MT 90 ⁰ C

Diethylamine MT 70 °C

Isopropylamine MT 58 ⁰ C

The identities of the R , R , R ₅ and R groups and the symmetry of the ammonium cation are believed to influence the melting point of the energetic ionic liquid. The identity of R ¹ on the tetrazolide is also believed to influence the melting point of the energetic ionic liquid.

Examples of some preferred embodiments of an ammonium tetrazolide energetic ionic liquid are provided in TABLE H

TABLE II

Ammonium

Tetrazolide R ² R ³ R ⁴ R ⁵

BHT Ethyl Ethyl Ethyl H

Isopropyl H H H

72-Butyl H H H

7Z-Butyl n-Butyl H H

MT Isobutyl H H H

Isopropyl Isopropyl H H

Isobutyl Isobutyl H H

Pentyl H H ' H

The tetrazolide anion is synthesized from a IH- or 2H-tetrazole, which is synthesized by any method known in the art. In some embodiments, the tetrazole is synthesized according to a method illustrated in SCHEME I.

SCHEME I

In some embodiments of the illustrated reaction, a nitrile (R ¹ CN) is treated with an azide, for example, sodium azide, a catalytic amount of a trialkylammonium salt, and a stoichiometric amount of a quaternary ammonium salt. The reaction mixture is acidified, for example using HCl gas, and the tetrazole product isolated. In some embodiments, sodium azide is the limiting reagent, thereby reducing the generation of hydrazoic acid. Yields of about 90% or greater of the tetrazole have been realized for this reaction.

The XH- or 2H-tetrazole is then deprotonated using a base, m some embodiments, the base is an amine corresponding to the ammonium cation of the ammonium tetrazolide energetic ionic liquid, as illustrated in SCHEME II. In these embodiments, the energetic ionic liquid is formed concomitantly with the formation of the tetrazolide anion. Note that the ammonium cation necessarily comprises a hydrogen ligand in these embodiments. In the illustrated embodiment, the reaction is performed in a solvent. In other embodiments, no solvent is used. In other embodiments, another base is used to form the tetrazolide anion and the cation is introduced by any means known in the art, for example, ion exchange or metathesis.

SCHEME II

Some embodiments of the disclosed high-nitrogen-content energetic ionic liquid comprise a 1,3,5-trisubstituted tetrazolium cation of formula II:

II wherein R ⁶ is selected from the group selected from hydrogen; halogen; nitro; azide;

Ci to C ₂ o alkyl, aralkyl, or aryl, optionally substituted with one or more substituents independently selected from fluorine and azide; and NR ³³ R ³⁴ , OR ³⁵ , or SR ³⁶ wherein R ³³ , R ³⁴ , R ³⁵ , and R ³⁶ are independently selected from the group consisting of hydrogen; and Ci to C ₂₀ alkyl, aralkyl, and/or aryl, each of which is independently optionally substituted with one or more substituents independently selected from fluorine and azide; and R ⁷ and R ⁸ are independently selected from the group consisting of Ci to C ₂₀ alkyl, aralkyl, and aryl, each of which is independently optionally substituted. Examples of suitable substituents include halogen, fluorine, azide, hydroxyl, alkoxy groups, amino groups, carbonyl groups, and the like.

The R ⁸ substituent on the tetrazole ring is at the 2-, 3-, or 4-position, i.e., 1,2,5-, 1,3,5- and 1,4,5-tetrazolium cations, respectively. Some embodiments comprise a mixture of any combination of these positional isomers. The substitution pattern of a cation of formula II influences the properties of the energetic ionic liquid, for example, the melting point, viscosity, water miscibility, water sensitivity, density, energy density, stability, and the like. In some embodiments, the substituents are selected to provide an energetic ionic liquid with a high energy density.

The anion in energetic ionic liquids comprising cations of formula II and/or III is any anion known in the ionic liquid art. In some embodiments, the anion is a monoanion. In other embodiments, the anion is a dianion, a trianion, a tetraanion, and/or a polyanion.

Some embodiments comprise a plurality of types of anions. The anion or anions influence the properties of the energetic ionic liquid, for example, the melting point, viscosity, water miscibility, water sensitivity, density, energy density, stability, and the like. In some embodiments, the anion is selected to provide a high energy density. Suitable anions include nitrate, nitrite, perchlorate, halides, sulfonates, sulfates, borates, phosphates, phosphinates, antimonates, amides, imides, carboxylates, alkyl anions, and coordination complexes. In another embodiment, the anion is a tetrazolide anion of formula I.

Examples of suitable halide anions include chloride and bromide. Examples of suitable sulfonates and sulfates include trifluoromethanesulfonate, A- methylbenzenesulfonate, and methyl sulfate. Examples of suitable borates include tetrafluoroborate, tetracyanoborate, bis(catecholato)borate, bis(salicylato)borate, bis(malonato)borate, bis(oxalato)borate, and bis(2,2'-biphenyldiolato)borate. Examples of suitable phosphate and phosphinate anions include hexafluorophosphate,

tris(pentafluoiOethyl)trifluorophosphate, tris(heptafluoiOpropyl)trifluorophosphate, tris(nonafluorobutyl)trifluorophosphate, bis(pentafluoroethyl)phosphinate, and bis(2,4,4- trimethylpentyl)phosphinate. Suitable antimonates include hexafluoroantimonate. Suitable amides and imides include dicyanamide, bis(trifluoromethyl)amide, and bis(trifluoiOmethanesulfonyl)imide. Suitable alkyl anions include bis(trifluoromethanesulfonyl)methide. Suitable carboxylate anions include lactate and decanoate. Suitable coordination complex anions include tetracarbonylcobaltide(I), dichlorocuprate(I), and tetrachloroaluminate.

Examples of embodiments of tetrazolium cations of formula II and/or formula III are provided in TABLE HI. In each of the embodiments illustrated in TABLE III, R ⁷ = R ⁸ . In other embodiments, R and R are different.

TABLE HI R^ PJ R^

Phenyl Butyl Butyl

Methyl Butyl Butyl

Phenyl Isopropyl Isopropyl

Methyl Isopropyl Isopropyl

The tetrazolium cation of an energetic ionic liquid of formula II is synthesized by any means known in the art. In some embodiments, a 1,5-disubstituted tetrazole is synthesized according to SCHEME III as disclosed in A. Onishi & H. Tanaka, "Method of Reducing the Physical Properties of Tetrazoles" EP 0 600 691 Al; R.R. Borch, "Nitrilium Salts. A New Method for the Synthesis of Secondary Amines" J. Org. Chem. 1969, 34, 627-629; L.A. Lee et al. "Reactions of Nitrilium Salts. I. With Sodium and Dimethylammoniuni Azide" J. Org. Chem. 1972, 37, 343-347; L.A. Lee & J.W. Wheeler, "Proton Magnetic Resonance Spectra of Some Tetrazoles, Triazoles, and Tetrazolium and Triazolium Salts" J. Org. Chem. 1972, 37, 348-351; R.N. Hanley et al. "Cyclic Meso-ionic Compounds. Part 18. The Synthesis and Spectroscopy Properties of 1,2,3, A- Thiatriazolium-5-arnmides and l,2,3,4-Tetrazolium-5-thiolates" J. Chem. Soc. Perlans Trans I 1979, 741-743; A. Araki & Y. Butsugan "Meso-ionic Fulvalene; Synthesis Properties of Anhydro-5-Cyclopentadienyl-l,3-diphenyl-l,2,3,4-Tetrazoliuni Hydroxide" J. Chem. Soc, Chem. Commun. 1983, 789-790; S. Araki & Y. Butsugan "Electrophile Substitution Reaction of Meso-ionic Sesquiulvalene" Tetrahedron Letters 1984, 25, 441- 444; S. Araki et al. "Nitrogen-Rich Mesoionic Compounds from l,3-Diaryl-5-

chlrotetrazolium Salts and Nitrogen Nucleophiles - Synthesis and Properties of 1,3-Diaryl- 5-azidotetrazolium Salts" Eur. J. Org. Chew. 1998, 121-127, the disclosures of which are incorporated by reference.

SCHEME HI

_R 6_ _C≡N

In the illustrated embodiment, a nitrile R ⁰ CN is alkylated using a trialkyloxonium tetrafluoroborate to provide a nitrilium ion. Those skilled in the art will understand that other alkylating agents are suitable for this transformation, for example, alkyl triflates.

Azide adds to the nitrilium ion and the resulting adduct undergoes an ring closure to provide a 1 ,5-disubstituted tetrazole.

Another synthesis for 1,5-tetrazoles is described in Tetrahedron Lett., 1995, 36(40), 7337-7340, and illustrated in SCHEME IV.

SCHEME IV

SiCI ₄ + 3 NaN ₃ In some embodiments, cation of formula II is synthesized according to SCHEME

V.

SCHEME V

wherein R ⁸ Z is a reagent capable of substituting the tetrazole. Such reagents are known in the art. In some embodiments, R ⁸ is a Ci to C ₂₀ alkyl or aralkyl, optionally substituted with one or more substituents independently selected from fluorine and azide, and Z is any suitable leaving group known in the art. Suitable leaving groups include halide, sulfonate, sulfamide, bissulfimide, sulfate, or carboxylate, for example, iodide, bromide, trifluoromethanesulfonate, 4-methylbenzenesulfonate, 3-nitrobenzenesulfonate, A- bromobenzenesulfonate, bis(trifluoromethanesulfonyl)imide, /?-nitrobenzoate, and trifluoroacetate. In some embodiments, R ⁸ Z is a cyclic alkylating agent, for example,

ethylene oxide or λf N-dimethylaziridinium ion. An example of an alkylation reaction of a tetrazole to form a tetrazolium is provided in J. pr. Ck, 1989, 331(6), 885.

In another embodiment, a disubstituted tetrazole is synthesized from a 5-substituted tetrazole, for example through a tetrazolide anion, the synthesis of which is discussed above. The disubstituted tetrazole is then further substituted to provide a tetrazolium cation of formula II. An example of this synthetic sequence is illustrated in SCHEME VI.

SCHEME VI

wherein R ⁷ Y and R ⁸ Z are reagents capable of transferring the R ⁷ and R ⁸ groups to the tetrazole, respectively. In some embodiments, R ⁷ Y and R ⁸ Z are added sequentially. In some embodiments, R ⁷ is a Ci to C ₂₀ alkyl or aralkyl, optionally substituted with one or more substituents independently selected from fluorine and azide, and Y is any suitable leaving group known in the art. Suitable leaving groups are described above. In some embodiments, R ⁷ Y is a cyclic alkylating agent, for example, ethylene oxide or iV,iV-dimethylaziridinium ion. R ⁸ Z is as described above.

As discussed above, also disclosed herein is a composition comprising an energetic ionic liquid as disclosed herein and An energetic salt. In some embodiments, the composition exhibits improved properties compared to the energetic ionic liquid alone, for example, energy density, energy impulse, processability, melting temperature, and the like. In some embodiments, the energetic salt is an oxidant, for example, salts comprising perchlorate, chlorate, nitrate, nitrite, percarbonate, dinitramide ((NOa) ₂ N ^" ), peroxide, persulfate, chromate, permanganate, and derivatives thereof. Other energetic salts comprising the azide anion. Suitable counterions are known in the art, and include ammonium, lithium, sodium, and potassium. EXAMPLE 1

General Procedure for Preparing Ammonium 5-Methyltetrazolide Salts

Reagents were purchased from Aldrich Chemical Co. (Milwaukee, WI) and used without purification.

A round bottom flask equipped with a temperature monitor was charged with 5- methyltetrazole (0.5 g, 6.0 mmol). Methanol (5 inL) was added and the solution stirred at 0 ⁰ C for 5 min. The amine (1.1 eq) was added dropwise at which the exothermic reaction

remained under control. At the end of the exotherm, the reaction mixture was heated for 2h at 65 ⁰ C, then allowed to cool to room temperature. Methanol and unreacted amine were removed under high vacuum. The resulting products were analyzed by NMR (DMSO-J _< j), melting point for solids, and density for liquids. N,N-Dimethylbutylammonium 5-methyltetrazolide: ¹ H, 5(DMSO- J _< j): 0.86 (t, 3H,

CH ₃ ), 1.27 (st, 2H, CH ₂ ), 1.56 (m, 2H, CH ₂ ), 2.46 (s, 6H, CH ₃ ), 2.68 (s, 3H, CH ₃ ), 2.92 (m,

2H, CH ₂ ). Mp = 101.3 ⁰ C (dec).

Isopropylammonium 5-methyltetrazolide: ¹ H, 5(DMSO-J ₁₅ ): 0.91 (d, 6H, CH ₃ ), 1.86

(sept, IH, CH) ₅ 2.25 (s, 3H, CH ₃ ), 2.66 (d, 2H, CH ₂ ), 6.36 (b, 3H, NH). Mp = 109.9 ⁰ C. Diisopropylammonium 5-methyltetrazolide: ¹ H, 5(DMSO-J ₆ ): 1.20 (d, 12H, CH ₃ ),

2.25 (s, 3H, CH ₃ ), 3.31 (m, 2H, CH). Mp = 121.6 ⁰ C. 5-methyltetrazolide: ¹ H, 5(DMSO-J ₀ ): 0.86 (t, 3H, CH ₃ ), 1.32

(m, 2H, CH ₃ ), 1.58 (m, 2H, CH ₂ ), 2.29 (s, 3H, CH ₃ ), 2.89 (t, 2H, CH ₂ ). Liquid at 20 ⁰ C, d =

1.0 ± 0.1 g/mL. w-Pentylammonium 5-methyltetrazolide: ¹ H, δ(DMSO-J^): 0.85 (t, 3H, CH ₃ ), 1.27

(m, 4H, CH ₂ ), 1.56 (m, 2H, CH ₂ ), 2.23 (s, 3H, CH ₃ ), 2.79 (t, 2H, CH ₂ ). Liquid at 20 °C, d =

1.0 ± 0.1 g/mL.

Theoretical Energy Impulses of Ammonium 5-Methyltetrazolide Salts

Theoretical energy impulses were determined for bipropellant systems in which the ammonium 5-methyltetrazolide salts were the fuel and inhibited red fuming nitric acid

(IRPNA) was the oxidant. The lowest energy conformations and relative energies of the ammonium 5-methyltetrazolide salts were calculated by Hartree-Fock (3.2 IG*) and density functional (B3LYP-3.21G*) methods. For the lowest energy conformers, enthalpies of formation were calculated using isodesmic equations. Energy impulses were then calculated using Chemical Equilibrium with Applications (CEA) software (NASA). The oxidant/fuel

(O/F) ratio was varied to determine the maximum value. The remaining parameters used in the calculation are set forth in TABLE IV. If the measured value for the density was used if known; if not determined, the density was assumed to be 1.

TABLE IV

Fuel Tetrazolide salt Oxidant IRFNA

Temperature 298.15 K

Chamber pressure 2000 psia

Pressure ratio 136.0919

Density 1 (if unknown)

For comparison, energy impulses were also calculated for monomethyhydrazine (MMH) and λζiV-dimethylaminoethylazide (DMAZ). The calculated energy impulses (Isp) for butylammonium 5-methyltetrazolide (BAMT) and pentylammonium 5- methyltetrazolide (ABAMT) are plotted against the O/F ratio in FIG. 1. FIG. 2 is a plot of the calculated density energy impulse (D*Isp) versus the O/F ratio for the for the same compounds.

Differential scanning calorimetry (DSC) data for butylammonium 5- methyltetrazolide and pentylammonium 5-methyltetrazolide are illustrated in FIG. 3. The exotherms were normalized to provide quantitative values. Exotherms were observed for both compounds at about 275 °C. The normalized exotheπn values are 218 J/g for butylammonium 5-methyltetrazolide and 147.5 J/g for pentylammonium 5- methyltetrazolide. For comparison, the exotherm for glycidyl azide polymer (GAP) is 2100 J/g.

EXAMPLE 2 1 ^-Dicyclopropyletrazole

A 3 -neck round-bottom flask equipped with a magnetic stir bar, a reflux condenser, and two rubber septa was filled with dry nitrogen. The flask was charged with NaN ₃ (12.7 g, 195 mmol) and dry acetonitrile (25 mL), then cooled to 0 ⁰ C. Silicon(IV) chloride (11.0 g, 65 mmol) was added dropwise, followed by dicyclopropyl ketone (7.2 g, 32.5 mmol).

The reaction mixture was then heated to 80 ⁰ C for 12 hours. The reaction mixture was poured into ice-cold saturated aqueous sodium carbonate and extracted with methylene chloride (3 x 60 mL). The methylene chloride extracts were combined and concentrated in vacuo. The 1,5-dicyclopropyltetrazole was purified by flash column chromatography (silica gel, 1:1 ethyl acetate : methylene chloride).

Using the same method, the following 1,5-tetrazoles were synthesized from the corresponding ketones:

1,5-Tetrazole(s) Ketone

1 ,5-Diphenyltetrazole Benzophenone

1 -Methyl-5-phenyltetrazole, Acetophenone 5-Methyl- 1 -phenyltetrazole

5 -methyl- 1 -tert-butyltetrazole Pinacolone l-(3-chloropropyl)-5-methyltetrazole, 5-Chloro-2-pentanone 5-(3-chloropropyl)-l-methyltetrazole

1 -Isopentyl-5-methyltetrazole, 5-Methyl-2-hexanone 5-Isopentyl- 1 -methyltetrazole

EXAMPLE 3

A 5-niL round-bottom flask equipped with a reflux condenser and magnetic stir bar was filled with dry nitrogen. The flask was charged with 5-methyl-l-te/Y-butyltetrazole

(0.506 g, 3.61 nimol). Dimethylsulfate (1.06 g, 8.4 mmol) was added by syringe and the reaction mixture heated at 80 ⁰ C for 4 hours. The reaction mixture was cooled and extracted several times with ether. The ether extracts were combined and concentrated under vacuum. The residue was purified by washing with NaClO ₄ solution. If no precipitate formed, methanol was added, whereupon a precipitate slowly formed. Alternatively, the compound was purified by dissolving in ethanol and adding 10% HClO ₄ .

The embodiments illustrated and described above are provided only as examples of certain preferred embodiments. Various changes and modifications can be made to the embodiments presented herein by those skilled in the art without departure from the spirit and scope of the disclosure, which is limited only by the appended claims.