5-azido-4-dimethylamino-1-methyl-1,2,4-triazolium Hexafluoridophosphate and Derivatives

5-Azido-4-(dimethylamino)-1-methyl-1,2,4-triazolium hexafluoridophosphate was synthesized from the corresponding 5-bromo compound with NaN3. Reaction with bicyclo[2.2.1]hept-2-ene yielded a tricyclic aziridine, addition of an N-heterocyclic carbene resulted in a triazatrimethine cyanine, and reduction with triphenylphosphane gave the 5-amino derivative. The crystal structures of three nitrogen-rich salts were determined. Thermoanalysis of the cationic azide and triazene showed exothermal decomposition. The triazene exhibited negative solvatochromism in polar solvents involving the dipolarity π* and hydrogen-bond donor acidity α of the solvent.


Results and Discussion
The title compound 1 was synthesized from the corresponding bromo compound and sodium azide (Figure 1) by analogy with a similar imidazolium salt [11].The well-known versatility of azide chemistry [22] motivated us to attempt exemplary reactions of 1.Thus, dipolar cycloaddition of azides with alkenes is known to produce 1,2,3-triazolines or aziridines [23].In our hands, strained bicycloheptene (norbornene) [24,25] reacted with azidotriazolium salt 1 to give the tricyclic aziridine 2 in acceptable yield.Recently, "click chemistry" (Cu-catalyzed cycloaddition of azide and alkyne) was successful with 2-azido-1-methylimidazole [26], but not with quaternary 2-azidoimidazolium salts, which can be explained by electron-deficiency of the cationic azide.Attempts to add phenylethyne to 1 were also not met with success, possibly due to the same reason.
Reaction of azide 1 with the carbene derived from the correspondimg triazolium salt resulted in the formation of triazene 3 in modest yield.Staudinger reduction of the azide 1 with triphenylphosphane led to the 5-amino compound 4. The progress of this transformation could be visually followed as the mixture turned yellow (formation of triazene), molecular nitrogen gas was evolved, and the colour disappeared again (formation of iminophosphorane).The product was obtained after final hydrolysis.
The hexafluoridophophate was chosen as an advantageous anion because these salts crystallize readily and are non-hygroscopic.The role of fluorine in crystal structures of organic compounds has been reviewed [27].It has been concluded that short C-H ... F contacts between oppositely charged molecules are genuine interionic hydrogen bonds [28].On the other hand, these interactions are weak enough to escape the undesirable formation of distracting aggregates, thus allowing to concentrate on the geometry of the cation.Crystal data and refinement details are summarized in Table 1.Selected bond lengths and angles are collected in Table 2. Hydrogen bond geometries are outlined in Table 3.Recently, "click chemistry" (Cu-catalyzed cycloaddition of azide and alkyne) was successful with 2-azido-1-methylimidazole [26], but not with quaternary 2-azidoimidazolium salts, which can be explained by electron-deficiency of the cationic azide.Attempts to add phenylethyne to 1 were also not met with success, possibly due to the same reason.
Reaction of azide 1 with the carbene derived from the correspondimg triazolium salt resulted in the formation of triazene 3 in modest yield.Staudinger reduction of the azide 1 with triphenylphosphane led to the 5-amino compound 4. The progress of this transformation could be visually followed as the mixture turned yellow (formation of triazene), molecular nitrogen gas was evolved, and the colour disappeared again (formation of iminophosphorane).The product was obtained after final hydrolysis.
The hexafluoridophophate was chosen as an advantageous anion because these salts crystallize readily and are non-hygroscopic.The role of fluorine in crystal structures of organic compounds has been reviewed [27].It has been concluded that short C-H...F contacts between oppositely charged molecules are genuine interionic hydrogen bonds [28].On the other hand, these interactions are weak enough to escape the undesirable formation of distracting aggregates, thus allowing to concentrate on the geometry of the cation.Crystal data and refinement details are summarized in Table 1.Selected bond lengths and angles are collected in Table 2. Hydrogen bond geometries are outlined in Table 3.    2a), likely due to a repelling interaction between a lone electron pair on the N atom and the ring π system.The heteroaromatic C2-H donates bifurcated hydrogen bonds to F1 and F2, whereas H atoms of two methyl groups form contacts to F4 (Figure 2b).The C5H10N7 + cation contains 58.3 wt.% nitrogen and, thus, can be considered to be "nitrogen-rich".The azide group in 1 deviates considerably from linearity with an N5-N6-N7 angle of 168.8(2)°, it is rotated out of the ring plane by 26.4° (Figure 2a), likely due to a repelling interaction between a lone electron pair on the N atom and the ring π system.The heteroaromatic C2-H donates bifurcated hydrogen bonds to F1 and F2, whereas H atoms of two methyl groups form contacts to F4 (Figure 2b).In the crystal structure of aziridine 2, the asymmetric unit contained half a formula unit.The triazole rings were located in the crystallographic mirror plane, whereas the C4 methyl groups are situated out of this plane.The C3 methyl group was found to be disordered over two orientations related by mirror symmetry.Part of the cation showed a 1:1 positional disorder by a crystallographic mirror plane.In the tricyclic group including N5, only C7 lay exactly in the mirror plane.All other atoms of this group were refined in general positions with occupancies of 0.5 (Figure 3a, only one disorder component shown).In the crystal structure of aziridine 2, the asymmetric unit contained half a formula unit.The triazole rings were located in the crystallographic mirror plane, whereas the C4 methyl groups are situated out of this plane.The C3 methyl group was found to be disordered over two orientations related by mirror symmetry.Part of the cation showed a 1:1 positional disorder by a crystallographic mirror plane.In the tricyclic group including N5, only C7 lay exactly in the mirror plane.All other atoms of this group were refined in general positions with occupancies of 0.5 (Figure 3a, only one disorder component shown).A positional disorder of the PF6 anion with ratio 7:3 (ratio refined as free variable) was observed.Again, the heteroaromatic C2-H donates a hydrogen bond to F3, and aliphatic hydrogen atoms form contacts to F1 and F2 (Figure 3b).
The triazole-substituted triazene system in 3 is almost planar.The dihedral angle between the triazole rings is only 9.3°.The N5-N6 bond is twisted out of the plane of the adjacent triazole ring by 17.2°, and the N7-N6 bond out of the plane of the second triazole ring by 14.6° (Figure 4a).Several C-H ... F interactions involve the heteroaromatic C2-H and C7-H, as well as the C5 and C9 methyl groups.Additionally, a C5-H ... N5 interaction is observed (Figure 4b).

UV Spectroscopy
Compound 3 exhibited negative solvatochromism in polar solvents the extent of which could be described by a linear combination of specific and non-specific solute-solvent interactions.The electronic absorption spectrum showed a single band in the UV region (Figure 5a).This band was shifted bathochromically by 13 nm on changing the solvent from water to dichloromethane (see Experimental Section).The electronic transition leads to less charge separation in the excited state than in the more dipolar ground state, so that increased solvent polarity leads to higher transition energy.Transition energies ET were calculated from the wavelengths of the absorption maxima according to the equation ET/kJ•mol −1 = hcN/λ = 119625/(λ/nm).A linear solvation energy relationship A positional disorder of the PF 6 anion with ratio 7:3 (ratio refined as free variable) was observed.Again, the heteroaromatic C2-H donates a hydrogen bond to F3, and aliphatic hydrogen atoms form contacts to F1 and F2 (Figure 3b).
The triazole-substituted triazene system in 3 is almost planar.The dihedral angle between the triazole rings is only 9.3 ˝.The N5-N6 bond is twisted out of the plane of the adjacent triazole ring by 17.2 ˝, and the N7-N6 bond out of the plane of the second triazole ring by 14.6 ˝(Figure 4a).Several C-H...F interactions involve the heteroaromatic C2-H and C7-H, as well as the C5 and C9 methyl groups.Additionally, a C5-H...N5 interaction is observed (Figure 4b).A positional disorder of the PF6 anion with ratio 7:3 (ratio refined as free variable) was observed.Again, the heteroaromatic C2-H donates a hydrogen bond to F3, and aliphatic hydrogen atoms form contacts to F1 and F2 (Figure 3b).
The triazole-substituted triazene system in 3 is almost planar.The dihedral angle between the triazole rings is only 9.3°.The N5-N6 bond is twisted out of the plane of the adjacent triazole ring by 17.2°, and the N7-N6 bond out of the plane of the second triazole ring by 14.6° (Figure 4a).Several C-H ... F interactions involve the heteroaromatic C2-H and C7-H, as well as the C5 and C9 methyl groups.Additionally, a C5-H ... N5 interaction is observed (Figure 4b).

UV Spectroscopy
Compound 3 exhibited negative solvatochromism in polar solvents the extent of which could be described by a linear combination of specific and non-specific solute-solvent interactions.The electronic absorption spectrum showed a single band in the UV region (Figure 5a).This band was shifted bathochromically by 13 nm on changing the solvent from water to dichloromethane (see Experimental Section).The electronic transition leads to less charge separation in the excited state than in the more dipolar ground state, so that increased solvent polarity leads to higher transition energy.Transition energies ET were calculated from the wavelengths of the absorption maxima according to the equation ET/kJ•mol −1 = hcN/λ = 119625/(λ/nm).A linear solvation energy relationship

UV Spectroscopy
Compound 3 exhibited negative solvatochromism in polar solvents the extent of which could be described by a linear combination of specific and non-specific solute-solvent interactions.The electronic absorption spectrum showed a single band in the UV region (Figure 5a).This band was shifted bathochromically by 13 nm on changing the solvent from water to dichloromethane (see Experimental Section).The electronic transition leads to less charge separation in the excited state than in the more dipolar ground state, so that increased solvent polarity leads to higher transition energy.Transition energies E T were calculated from the wavelengths of the absorption maxima according to the equation E T /kJ¨mol ´1 = hcN/λ = 119625/(λ/nm).A linear solvation energy relationship was established by least-squares fitting of the data to the simplified solvatochromic equation E T = E T0 + sπ* + aα involving only the parameters π* [29] and α [30] which represent the dipolarity/polarizability and hydrogen-bond donor acidity of the solvent.The hydrogen-bond acceptor parameter β [31] did not significantly contribute to the relationship.The coefficients s and a reflect the sensitivity of the solute to these solvent properties.The π* and α values were taken from the compilation of Marcus [32].From the spectral data, the following equation was derived by multiple linear regression (Figure 5b) which describes the solvatochromic behavior of the triazene 3.

Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA)
Thermoanalysis of 1 showed a phase transition at 62 °C.Both compounds 1 and 3 exhibited exothermal decomposition at higher temperatures (decomposition maxima at 163 and 210 °C) with considerable mass loss (Figure 6).The melting point of compound 2 was unremarkable.

Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA)
Thermoanalysis of 1 showed a phase transition at 62 ˝C.Both compounds 1 and 3 exhibited exothermal decomposition at higher temperatures (decomposition maxima at 163 and 210 ˝C) with considerable mass loss (Figure 6).The melting point of compound 2 was unremarkable.

Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA)
Thermoanalysis of 1 showed a phase transition at 62 °C.Both compounds 1 and 3 exhibited exothermal decomposition at higher temperatures (decomposition maxima at 163 and 210 °C) with considerable mass loss (Figure 6).The melting point of compound 2 was unremarkable.
All other chemicals were purchased from Sigma-Aldrich (European affiliate, Steinheim, Germany).NMR spectra were recorded with a Bruker Avance DPX 300 spectrometer (Billerica, MA, USA).IR spectra were obtained with a Nicolet 5700 FT spectrometer (Thermo Fisher Scientific Inc., Waltham, MA, USA) in ATR mode.UV spectra were recorded with a Perkin-Elmer Lambda XLS+ spectrometer (Waltham, MA, USA); mean values of five replicates were taken.DSC and TGA were recorded with Perkin-Elmer DSC 7 and TGA 7 instruments (Waltham, MA, USA) at a heating rate of 10 ˝C¨min ´1.High-resolution mass spectra were measured with a Finnigan MAT 95 mass spectrometer.In general, spectra were recorded from the crude but pure compounds, not from single-crystalline material.Single crystal diffraction intensity data were recorded by ω scans with an Oxford Diffraction Gemini-R Ultra (Oxford Diffraction Ltd., Abingdon, Oxfordshire, UK) diffractometer at 173(2) K. Absorption corrections were applied in all cases (multi-scan).The structures were solved by direct methods and refined by full-matrix least-squares techniques on F 2 .CCDC 1444906-1444908 contains the supplementary crystallographic data for this paper.These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html.

Conclusions
Nitrogen-rich azidoazolium salts are not only of interest as structural motifs in the area of energetic materials but also represent valuable intermediates in synthetic chemistry.The versatility of azide chemistry definitely contributes to the increasing popularity of this functional group in the field of heterocyclic chemistry.

Figure 2 .
Figure 2. (a) ORTEP plot of the cation in 1; and (b) packing in the crystal structure of 1.

Figure 2 .
Figure 2. (a) ORTEP plot of the cation in 1; and (b) packing in the crystal structure of 1.

Figure 4 .
Figure 4. (a) ORTEP plot of the cation in 3. (b) Packing in the crystal structure of 3.

Figure 4 .
Figure 4. (a) ORTEP plot of the cation in 3. (b) Packing in the crystal structure of 3.

Figure 4 .
Figure 4. (a) ORTEP plot of the cation in 3. (b) Packing in the crystal structure of 3.

Figure 5 .
Figure 5. (a) Normalized UV spectra of triazene 3 in H 2 O and CH 2 Cl 2 ; (b) Correlation of observed vs. calculated UV transition energies of triazene 3 in seven polar solvents.

Figure 5 .
Figure 5. (a) Normalized UV spectra of triazene 3 in H2O and CH2Cl2; (b) Correlation of observed vs. calculated UV transition energies of triazene 3 in seven polar solvents.

Table 1 .
Crystal data and structure refinement details for compounds 1
2.1.Crystal StructuresThe C 5 H 10 N 7+ cation contains 58.3 wt.% nitrogen and, thus, can be considered to be "nitrogen-rich".The azide group in 1 deviates considerably from linearity with an N5-N6-N7 angle of 168.8(2) ˝, it is rotated out of the ring plane by 26.4 ˝(Figure