MÉTODO DE ESTUDIO

Since the tetrazole derivatives represent fuels, appropriate oxidizers are important in order to realize corresponding propellant formulations. The parent aminotetrazoles can be protonated, and therefore salts with energetic oxygen rich cations, such as nitrate (NO3-), dinitramide (N(NO2)2-) or

the perchlorate (ClO4-) anion are sought.7

1.2 Energetic salts

Nitrate, dinitramide, perchlorate and azide salts of nitrogen rich cations have received major attention for a number of reasons: a high oxygen balance (nitrate, dinitramide and perchlorate salts), a high heat of formation ∆fH°, the release of large amounts of gases (e.g. N2) as favored

explosion products and high values of the density ρ. At present, the search for high energy compounds is mainly directed towards molecular crystals made from neutral molecules. The reason for this is that ionic crystals normally have poor values of ∆fH°solid (solid state formation enthalpy

may be estimated as ∆fH°solid = ∆fH°(gas)-Ecoh-RT) coming from high contribution of crystal

cohesive energy (Ecoh). This contribution is comparably small for molecular crystals of covalent

compounds, typically lower than 0.12 kcal/g,8 but for ionic crystals, Ecoh is typically one order of

magnitude larger owing to the long-range electrostatic interactions between ions (e.g. low

∆fH°solid(AN, ammonium nitrate) = -1.09 kcal/g due to significant cohesion of the crystal Ecoh(AN)

increase the distance between charged groups. Calculations on ethane and H3C-NH3+ suggest a

very high contribution of the ammonium group –NH3+ to ∆fH° (~145 kcal/mol).9 Thus, some ionic

crystals might provide valuable energetic constituents of propellants provided their cohesive energy is not too large. For example, the best known of the highly energetic nitrate and dinitramide salts are the oxidizers ammonium nitrate (AN) and ammonium dinitramide (ADN).10 In the case of nitrogen rich anions, hydrazinium azide11 and its organic derivatives12 were extensively investigated, which are in most cases unfortunately volatile and hygroscopic and also have relatively low densities.

1.3 Crystal building units

To understand the interplay of cations and anions within a network and predict important values like densities, Ecoh and ∆fH°, an algorithm is necessary for a predictable and controllable

long-range molecular organization.13 _{Basically, the crystal structure of the material is a result of}

iterative self-assembling of the constituent molecular, co-molecular (bimolecular)14 or ion pair subunits, considered as fundamental crystal building units. However, the prediction of solid-state structure of crystals is commonly frustrated by the complexity and lack of directionality of intermolecular forces. The packing control in three dimensions is elusive, owing to the numerous possible intermolecular interactions and multiplicity of structural possibilities. Therefore it is important to have a closer look into the structural aspects which account for the interplay between such factors, as directional demands of the interactions and geometrical dictates the close-packing. In this context, it is also of interest to investigate known and new salts of aminotetrazoles especially with nitrate, dinitramide15 and azide as counter ions especially focused in the formation of robust hydrogen-bonded multidimensional networks within these salts. The dimensionality and general structural feature of a multidimensional network depends on the modules which serve as ‘topological directors’ and strongly depends on the symmetry of the ions. A search in the Cambridge Structural Database (CSD) revealed that the crystal structure of guanidinium nitrate [HGN+NO3-] consists of hydrogen-bonded polar layers, stacked in the third dimension by van der

Waals interactions (Figure 4.2, I).16 The hydrogen bonded ring motif, in the formalism of graph-set analysis of hydrogen-bond patterns,17 which are found for [HGN+NO3-] are the R63(12)and the very common )2(8

2

R . The latter was also observed in other structures e.g. discussed in Chapter II (2.2.2) and Chapter III (3.1.3,4 and 3.2.3).

Figure 4.2. Scheme of the 2D organization pattern of [HGN+NO3-] I and [HAGN+NO3-] II through

intermolecular hydrogen bonds.

As the bond pattern strongly depends on the symmetry of the cations, lowering symmetry by a formal introduction of an amino group should modify the hydrogen-bond connectivity pattern. In the case of aminoguanidinium nitrate [HAGN+NO3-],18 the terminal NH2 group is positioned in

such a fashion that the hydrogen atoms appear above and below the plane of the rest of the molecule, and the lone pair is directed towards the hydrogen atom of one of the C=NH2 moieties

forming an intramolecular bond with the motifs S(5). The nitrate and aminoguanidinium moieties are approximately coplanar. Within the same plane the nitrate groups are linked through hydrogen bonds to the N atoms by hydrogen bonded ring motifs 2(4)

1 R , )2(4 2 R , )2(6 2 R , )2(8 2 R and ) 12 ( 2 4

R (Figure 1, II).Above and below this plane, the groups are bonded intermolecularly through the H atoms of the terminal amino group. In comparison to the above mentioned salts, and in order to gain further details of topological similarities of diaminotetrazolium salts, a closer inspection of the two- and three-dimensional hydrogen-bonded networks are going to be discussed for the new synthesized energetic salts.

2. HDAT+ / MeDAT+ salts

Aminotetrazoles are heterocycles, rich in electron pairs. The reaction with weak or strong acids leads with 14 (R = H, discussed elsewhere)19 and 55 (R = NH2) only with strong acids (X- =

Cl, Br, I, NO3, ClO4, SO4, picrate)4a,20 to the formation of the corresponding salts (Scheme 4.3).

Scheme 4.3. Protonation of 5-AT and DAT

Protonation of 14 and 55 can proceed both on the nitrogen atoms of the tetrazole ring and on the amino group(s). It was determined to proceed unambiguously at the N4 atom of the ring (Scheme 4.3).21 _{Nothing is known in the literature about the corresponding azide or dinitramide}

salts concerning the isolation of such salts.

The protonation of 55 with HNO3 (59a) and HClO4 (59b) as well as the quaternization with

MeI and subsequently metathesis of the iodide with corresponding silver salts and a new derivative of 55, the 1-amino-4-methyl-5-imino-4,5-dihydro-1H-tetrazole 60 as its iodide, azide, nitrate and dinitramide salt (61a-d) were investigated during this thesis (Figure 4.3). Also a new and improved synthesis of 55 was developed in order to make 55 accessible on a larger scale.

2.1 Synthesis

The synthesis strategy for 55 is based on the studies of Lieber et al. who treated diaminoguanidine nitrate (AGN, 12) with one and two molar portions of nitrous acid in a buffered acetic acid media.22 According to Lieber, the treatment of 12 with two moles of nitrous acid (MNO2 (M = Na, K) in acetic solution), yielded the corresponding alkali metal salt of tetrazolyl

azide (62) as the only isolable product.23