Inorganic Amino-nitro-guanidinium Derivatives

1-Amino-3-nitroguanidine (ANQ, 1) was synthesized by hydrazinolysis of nitroguanidine (NQ) with hydrazine hydrate. Four different amino-nitroguanidinium salts (chloride (2), bromide (3), iodide (4) and sulfate (5)) were synthesized and structurally characterized by low-temperature X-ray diffraction. The halides 2–4 could only be obtained crystalline as monohydrates. In addition, they were characterized by NMR and vibrational spectroscopy, elemental analysis and the sensitivities towards impact, friction and electrostatic discharge were determined. The compounds can be used in silver (AgX, X = Cl, Br, I) and barium (BaSO 4) based metathesis reactions in order to form more complex salts of 1-amino-nitroguanidine.


Introduction
1-Amino-3-nitroguanidine (1, ANQ) can be described as the aminated sister compound of the famous 1-nitroguanidine (NQ) [1].The next step in the preparation of mixed aminonitroguanidines would be the synthesis of 1,3-diamino-5-nitroguanidine (DANQ), which is completely unknown yet (Figure 1).Nitroguanidine is used in airbags and many other double-and triple-based propellant applications [2].Interestingly, only very little correspondence on aminonitroguanidine is found in the literature [3][4][5].We recently investigated the use of ANQ in energetic materials in its neutral as well as OPEN ACCESS protonated form [6].For example, highly energetic 1-amino-3-nitroguanidinium dinitramide has been described [6].Because dinitraminic acid (HN(NO 2 ) 2 ) and concentrated aqueous solutions can explode spontaneously, the reaction pathway using a metathesis reaction of amino-nitroguanidinium chloride with silver dinitramide was chosen.This example shows the utility of the herein described compounds as precursor materials for the synthesis of ionic energetic materials containing the 1-amino-3-nitroguanidinium cation when using metathesis reaction protocols to precipitate low soluble silver halides in case of reacting the ANQ halides with the corresponding silver salts of the used acids or to precipitate BaSO 4 in case the corresponding Ba-salts are reacted with the herein described sulfate salt of ANQ.Here we present the synthesis and characterization of three amino-nitroguanidinium halides and bis(amino-nitroguanidinium) sulfate.

Synthesis
The synthesis of 1-amino-3-nitroguanidine (1: ANQ) was achieved starting from commercially available nitroguanidine (NQ) by treatment with hydrazine hydrate.In a hydrazinolysis reaction, the aminated nitroguanidine is formed after the elimination of ammonia [5].Unprotonated ANQ shows rather poor solubility in water, so that it can be isolated from the reaction mixture after neutralization by suction filtration.Unlike in water or buffered neutral solutions, in acidic media ANQ is dissolved comparatively easily upon the formation of the protonated ANQ + species.Therefore, the halides as well as the sulfate salt can be prepared by dissolving ANQ in dilute aqueous solutions of the respective mineral acids HCl, HBr, HI and H 2 SO 4 .To fully synthesize and characterize all halides (F − , Cl − , Br − , I − ) of ANQ, it was also dissolved in 40% aqueous HF.Unfortunately it was only possible to isolate unprotonated ANQ from this reaction mixture (Scheme 1).The reason for this behavior is easily found in the pK a values of the used mineral acids.ANQ, due to the presence of the electron withdrawing character of the nitro group, is a comparatively weak base and requires strong mineral acids for protonation in aqueous media.Since the acidic strength of the hydrohalides in aqueous solution decreases in the order HI > HBr > HCl > HF, the latter one is not able to protonate ANQ in aqueous solution any more.However, 40% aqueous HF proved to be an excellent solvent for the recrystallization of the poorly water soluble ANQ, especially if single crystals, e.g., for single crystal X-ray diffraction, are needed.Scheme 1. Synthesis of 1 (ANQ) and investigated amino-nitroguanidinium salts 2-5.
The storage stability of the halides decreases with increasing molecular weight of the anion.Whereas the hydrochloride remains a colorless crystalline material even after several months, the hydrobromide discolors slightly and the hydroiodide turns completely dark after the release of I 2 indicating a decomposition of the material.

Single Crystal X-ray Structure Analysis
The low temperature determination of the crystal structures of 2-5 was performed on an Oxford Xcalibur3 diffractometer with a Spellman generator (voltage 50 kV, current 40 mA) and a KappaCCD detector.The data collection and reduction was carried out using the CRYSALISPRO software [7].The structures were solved either with SHELXS-97 [8] or SIR-92 [9], refined with SHELXL-97 [10] and finally checked using the PLATON [11] software integrated in the WINGX [12] software suite.The non-hydrogen atoms were refined anisotropically and the hydrogen atoms were located and freely refined.The absorptions were corrected with a Scale3 Abspack multi-scan method [13].Selected data and parameters of the X-ray determinations are given in Table 1.Crystallographic data for the structures have been deposited with the Cambridge Crystallographic Data Centre [14].The structure of the 1-amino-3-nitroguanidinium cation is similar in all four structures investigated in this work However, in all structures the bond length between C1 and N5 is observed to be the shortest one.Also the bonds N2-O1, N2-O2 and N1-N2 are significantly shorter than single bonds.These structural details are in agreement with our previously described investigations of this cation in the literature [6].Selected bond lengths and angles of the cation of all structurally investigated compounds 2-5 are gathered in Table 2.The halides, which were obtained crystalline under their monohydrated form, crystallize in common space groups (2: P2 1 /n, 3: P2 1 /c, 4: P-1) and follow the trend of rising densities (2: 1.73 < 3: 2.02 < 4: 2.34 g cm -3 ).All three structures are dominated by many hydrogen bonds [16][17][18] involving the crystal water molecules.Figure 2 shows the molecular structure of 2.

D-H•••A d(D-H) d(H••
Figure 4 shows the molecular moiety of 3 while Figure 5 shows the same pseudo-octahedral coordination sphere of the bromide anion as we could observe in the chloride structure.The bond lengths and angles of selected hydrogen bonds are listed in Tables 5,6          However the iodine structure 4 crystallizes in the triclinic space group P-1 with two formula units in the unit cell.The crystal packing is different from the described chloride (2) and bromide (3) structure.Two molecular moieties of 4 connected to dimers are depicted in Figure 6.These dimers are packed in layers which are linked by the water molecules.

D-H•••A d(D-H) d(H•••A) d(D•••A) <(D-H•••A) N4 iv -H4a
The iodide anions participate in four hydrogen bonds listed in Table 7.The N•••I and O•••I distances observed reveal ordinary values comparable to those found in 3-Cyano-anilinium iodide monohydrate [19].Bis(amino-nitroguanidinium) sulfate crystallizes in the non-centrosymmetric orthorhombic space group Fdd2 with eight formula units in the unit cell.The density of 1.97 g cm −3 is slightly lower than that of the bromide structure 3 but higher than that of 2. The molecular structure of 5 is depicted in Figure 7.The sulfur atoms lie on the special position ½,0,z.The S-O bonds are uniform with a length of 1.48 Å, which is in agreement to many sulfate structures in the literature, e.g., that of potassium sulfate [20].[21,22].

Analytical Characterization
All investigated compounds were characterized using vibrational (IR and Raman) and NMR ( 1 H, 13 C) spectroscopy.Also mass spectrometry and elemental analysis were employed to identify the corresponding materials.For analytical details of the respective compounds see the Experimental Section.The decomposition temperatures were determined by differential scanning calorimetry on a Linseis PT 10 DSC [23].

Infrared Spectroscopy
The strongest observed absorptions in the IR spectra of the ANQ + -species are the symmetric N-H-valence vibrations of the -NH 3 + functionality at 3379-3414 cm −1 .In the IR spectrum of neutral ANQ, an additional strong absorption at 3551 cm −1 is observed.Here, the symmetric N-H-valence vibrations are found at higher energy since the amine group is not protonated.Further strong absorptions indicate the presence of the nitramine moiety found in all investigated compounds, which reveals the antisymmetric N-O-valence vibration of the nitro group at 1632-1639 cm −1 and the symmetric N-O-valence vibration at 1278-1280 cm −1 .The IR spectrum of the sulfate salt additionally shows the strong antisymmetric S-O-valence vibration of the sulfate anion at 1065 cm −1 .

Differential Scanning Calorimetry
Unlike unprotonated ANQ, which decomposes at 180 °C, the protonated ANQ + salts show thermal stabilities, which are far below that value.The halides decompose at 80 °C (2, 3) and 86 °C (4) respectively, whereas the sulfate salt 5 is stable up to 144 °C.

Sensitivity Testing
Since 1-amino-3-nitroguanidinium salts show enhanced sensitivity towards outer stimuli such as impact and friction, the impact and friction sensitivities were determined and carried out according to STANAG 4489 [24] and 4487 [25] modified instructions [26,27] using a BAM (Bundesanstalt für Materialforschung) drophammer [28][29][30] and a BAM friction tester [28][29][30].Regarding the sensitivities of the salts, a large difference between the halides 2-4 and the sulfate salt 5 is also observed.Whereas the halides reveal sensitivities of 25 J (impact sensitivity) and 288 N (friction sensitivity), the sulfate salt 5 is much more sensitive, having 6 J (impact sensitivity) and 120 N (friction sensitivity).This can primarily be explained by the formation of monohydrates, which is true for all halides 2-4, whereas the sulfate salt 5 crystallizes water-free.The determined values imply a classification [25] of the tested materials as "sensitive" towards both impact and friction.The higher sensitivities of the protonated species discussed herein compared to the neutral compound ANQ can be correlated to a lowered C-N bond order of the hydrazine moiety of the molecule, which appears upon protonation of the molecule as it was found during the structural investigation of neutral ANQ and its protonated species in reference 4. The same argumentation can also be applied to the thermal stabilities of the compounds, whereas a weaker C-N bond of the hydrazine moiety facilitates the loss of the hydrazine moiety and thus, decomposition of the material.

Experimental Section
All reagents and solvents were used as received (Sigma-Aldrich, Fluka, Acros Organics) unless stated otherwise.Melting and decomposition points were measured with a Linseis PT10 DSC using a heating rate of 5 °C min −1 , which were checked with a Büchi Melting Point B-450 apparatus. 1H and 13 C NMR spectra were measured with a JEOL instrument.All chemical shifts are quoted in ppm relative to TMS ( 1 H, 13 C).Infrared spectra were measured with a Perkin-Elmer Spektrum One FT-IR instrument.Raman spectra were measured with a Perkin-Elmer Spektrum 2000R NIR FT-Raman instrument equipped with a Nd:YAG laser (1064 nm).Elemental analyses were performed with a Netsch STA 429 simultaneous thermal analyzer.Friction and Impact sensitivity data were determined using a BAM drophammer and a BAM friction tester [25].The electrostatic sensitivity tests were carried out using an Electric Spark Tester ESD 2010 EN (OZM Research) operating with the "Winspark 1.15" software package [31].
Figure4shows the molecular moiety of 3 while Figure5shows the same pseudo-octahedral coordination sphere of the bromide anion as we could observe in the chloride structure.The bond lengths and angles of selected hydrogen bonds are listed in Tables5,6.Also in this structure the ions are held together by a three dimensional network of moderate N-H•••O (d(N-O) ~2.82-3.08Å), N-H•••Cl (d(N-Br) ~3.36-3.49Å) and O-H•••Cl (d(O-Cl ~3.20-3.28Å) hydrogen bonds.The crystal packing is identical to the chloride structure 2. The N-Br and O-Br hydrogen bonds are approximately 0.15 Å longer than those in the chloride structure 2.

Table 6 .
Selected hydrogen bonds in the structure of 3. iv