Dinitramide anions

The dinitramide ion is stable in both acidic and basic solutions between pH 1-15 at room temperature but is slowly decomposed in the presence of strong concentrated acid. In contrast to alkyl V,V-dinitramines (Section 6.11.2) where the central nitrogen atom is highly electron deficient, the dinitramide anion has its negative charge delocalized over both nitrogen and oxygen atoms with the consequence that the N-N bonds are less susceptible to rupture. However, the dinitramide anion is not as stable as the nitrate anion ammonium dinitramide melts at 92 °C and decomposition starts at 130 °C. 

Numerous synthetic routes to the dinitramide anion have been reported. Cesium dinitramide (149) has been synthesized via the fluoride-catalyzed /1-elimination of 1-(V,A1-dinitramino)-2-trimethylsilylethane (148) with cesium fluoride the latter prepared by treating 2-(trimethylsilyl)ethyl isocyanate with a solution of nitronium tetrafluoroborate and pure nitric acid in acetonitrile.  

Ammonium dinitramide (152) is synthesized by treating a solution of ammonium ni-trourethane (150) with nitronium tetrafluoroborate or dinitrogen pentoxide in methylene chloride at -30 °C, followed by ammonolysis of the resulting ethyl A,lV-dinitrourethane (151)7 Ammonium dinitramide can be prepared from the nitration of ethyl carbamate and ammonium carbamate with the same reagents. This is currently the most efficient route to ammonium dinitramide and is used for its manufacture (Section 9.11). 

The nitration of nitramine (153) with nitronium tetrafluoroborate, followed by neutralization of the resulting dinitraminic acid with ammonia, also generates ammonium dinitramide (152). Neutralization of this reaction with alkylamines, instead of ammonia, yields the corresponding alkylammonium salts of dinitramide. The nitration of ammonia with dinitrogen pentoxide (15 %) or nitronium salts like the tetrafluoroborate (25 %) yield ammonium dinitramide (152) through the initial formation of nitramine. 

Ammonium dinitramide has been synthesized from the nitration of ammonium sulfamate with strong mixed acid at -35 to -45 °C followedby neutralization of the resulting dinitraminic acid with ammonia. The yield is 45 % when the mole ratios of sulfuric acid to nitric acid is 2 1 and ammonium sulfamate to total acid is 1 6. The nitration of other sulfonamide derivatives, followed by hydrolysis with metal hydroxides, also yields dinitramide salts.  

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The general-base-catalysed formation of dinitramide anion, (N02N ), on reaction of 2-(Af,Af-dinitroamino)propionitrile (16) in aqueous buffer solutions (pH 9.5-11.5), has been ascribed to the E cB mechanism ( 2 > -i[BH+]), for which fcap = oh-[HO ] -I- b[B] -I- h20- The Brpnsted p value is close to unity and the entropy of activation, = 10 1 calmoH K for reaction with hydroxide ion is consistent with the combined effects of bimolecular collision (ca -11 calmoH K ) and near-complete desolvation of HO (ca -1-20 cal mol ... 

Ammonium dinitramide (ADN) is a crystalline oxidizer with the formula NH4N(N02)2, that is, it is composed ofionicaUy bonded ammonium cations, NH4, and dinitramide anions, N(N02)2- Though ADN is crystalline and has a high oxygen content, similar to AP and KP, it has no halogen or metal atoms within its structure. ADN is used as an oxidizer in smokeless composite propellants, similar to AN and HNF. It melts at about 364 K, accompanied by the latent heat of fusion. 

In many energetic materials, significant hydrogen bonding is observed. This is particularly true of a number of salts of the dinitramide anion. In... 

The existence and preparation of stable dinitramides was disclosed publicly in two patent applications filed by the US military contractor SRI [1,2]. They cover a series of dinitramide salts, including ADN. The anion, N(N02)2", was named here for the first time as dinitramide, in contrast to the covalent alkyl compounds called dinitramines. The protonated form of the dinitramide anion was named dinitraminic acid, HN(N02)2. 

Thus it is notable that the dinitramide anion has likely been first synthesized twice, and was patented long after it was already being... 

Nitrourea has also been shown to be a starting material [5]. Nitration with N02BF4 in acetonitrile followed by neutralization with dry ammonia gave the dinitramide anion with a yield of 20.2%. Nitrourea is made by nitration of urea in mixed acid. 

Currently the most economical route to making dinitramides involves a mixture of nitric and sulfuric acids as the nitrating agent. The mixed acid is drastically less expensive than the nitration agents mentioned above. The substrate is potassium sulfaminate, which, due to its basicity, is suitable for nitration in a strongly acidic solution. Since the dinitramide anion is thermally unstable in acidic environment at room temperature, the nitration is carried out at -30°C to -40°C. 

This repulsion is diminished in the dinitramide ion since they are farther apart, due to sp2 hybridization of the central nitrogen. An additional factor is the energetically favorable distribution of the net electronic charge throughout the dinitramide anion. This is formally pictured by the resonance structures ... 

The stabilized valence structure in the dinitramide anion results in low reactivity toward electrophiles. Only under special conditions does the anion react as a nucleophile. For example, silver dinitramide is alkylated by alkyl halides in high yields to form the covalent dinitramine [7]. 

Dinitramic acid (88) and the dinitramide anion were discovered in 1971 (Figure 13). However, it was then only known in the former Soviet Union and was independently discovered in the United States in 1989. 

In the gas phase, the dinitramide ion N(N02)2 has symmetry. In the solid state, the symmetry is often distorted by interactions with the counterion, for example, hydrogen bonding with the ammonium ion in ammonium dinitramide (see Table 33). The N-NO2 rotation barrier is very low. In fact, though the bond lengths of the nitramide ion remain fairly constant, no two-dinitramide crystal structures show the same geometry of the dinitramide anion. The crystal stmcture of lithium dinitramide shows the symmetric anion. 

Table 33 Bonding parameters of the dinitramide anion in ammonium and lithiiun dinitramide...

The dinitramide anion is a relatively new entry in the field of energetic materials (Bottaro etal. 1991 Russell etal. 1997 and references therein) and a number of salts have been prepared and structurally characterized (Gilardi and Butcher l99Sb and references 7-12 therein Sitzmann et al. 2000, and references 2b-i therein), including some polymorphic ones, described briefly here. 

The largest group in this category are the 14 salts (and 16 structures) of the dinitramide anion (one, Rb[N(N02)2], melting at 102-106°C [690]), most of which have already been discussed... 

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