1‐(Azidomethyl)‐5H‐Tetrazole: A Powerful New Ligand for Highly Energetic Coordination Compounds

Abstract Highly energetic 1‐(azidomethyl)‐5H‐tetrazole (AzMT, 3) has been synthesized and characterized. This completes the series of 1‐(azidoalkyl)‐5H‐tetrazoles represented by 1‐(azidoethyl)‐5H‐tetrazole (AET) and 1‐(azidopropyl)‐5H‐tetrazole (APT). AzMT was thoroughly analyzed by single‐crystal X‐ray diffraction experiments, elemental analysis, IR spectroscopy and multinuclear (1H, 13C, 14N, 15N) NMR measurements. Several energetic coordination compounds (ECCs) of 3d metals (Mn, Fe, Cu, Zn) and silver in combination with anions such as (per)chlorate, mono‐ and dihydroxy‐trinitrophenolate were prepared, giving insight into the coordination behavior of AzMT as a ligand. The synthesized ECCs were also analyzed by X‐ray diffraction experiments, elemental analysis, and IR spectroscopy. Differential thermal analysis for all compounds was conducted, and the sensitivity towards external stimuli (impact, friction, and ESD) was measured. Due to the high enthalpy of formation of AzMT (+654.5 kJ mol−1), some of the resulting coordination compounds are extremely sensitive, yet are able to undergo deflagration‐to‐detonation transition (DDT) and initiate pentaerythritol tetranitrate (PETN). Therefore, they are to be ranked as primary explosives.


Introduction
The design of novel energetic materials is mostly subjected to a purely synthetic approach, generally delimited by the same synthetic strategies. While some of those strategies are known and have been applied since the early 19th century, such as the introduction of several nitro groups to a molecular framework, [1][2][3][4][5] the concept of using energetic materials as ligands for energetic coordination compounds (ECCs) is relatively new. [6,7] By combining a variety of metal cations (Cu 2 + , Fe 2 + , Zn 2 + , Mn 2 + , Ag + ) with energetic anions (e. g., NO 3 À , ClO 3 À , ClO 4 À , N 3 À , and nitrophenolates) as well as a practically unlimited number of energetic ligands, ECCs offer a great adjustability and tuning of the energetic properties. It therefore expands the traditional approaches of synthesizing energetic materials, now used as ligands, by the possibility of further tuning the properties by the formation of coordination compounds.
Here, next to the synthesis and characterization of highly energetic 1-(azidomethyl)-5H-tetrazole, several energetic coordination compounds of the new ligand were synthesized. Single-crystal X-ray diffraction experiments were conducted, and all compounds were analyzed by elemental analysis and IR spectroscopy. The ligand was characterized by multinuclear NMR ( 1 H, 13 C, 14 N, and 15 N) spectroscopy, and the thermal and physicochemical properties of all compounds were determined.

Results and Discussion
Synthesis CAUTION! The synthetic work described in this section involves the handling of very sensitive intermediates such as 1-(azidomethyl)-5H-tetrazole (3) and ECCs 5-14. Proper protective measurements and equipment must be used! The synthesis of 1-(azidomethyl)-5H-tetrazol (3) is shown in Scheme 1. The known literature [30] hydroxy methylation of 1,5Htetrazole with formaldehyde (37 %) in aqueous solution was optimized towards a 66 % shorter reaction time with an improved yield of 80 % (over 65 %). An isomeric mixture of 1and 2-(hydroxymethyl)-5H-tetrazole (1 a, 1 b) was obtained as a colorless oil. Due to the polarity of the alcohol functionality, a separation of the isomers by column chromatography was not possible at this stage. The isomeric mixture 1 was mesylated by methanesulfonyl chloride in CH 2 Cl 2 at room temperature with a good overall yield of 64 %. After the reaction solution had been quenched in water, pure 1-(methanesulfonyl-methyl)-5H-tetrazole (2 a) precipitated and was filtered off. Compound 2 a was obtained as a colorless solid in moderate yield (48 %). The organic layer was separated, washed with hydrochloric acid (2 m) and water, and the solvent was removed in vacuo to obtain crude 2-(methanesulfonyl-methyl)-5H-tetrazole (2 b) in poor yield (18 %). After refluxing 2 a with an excess of sodium azide in a water/acetone mixture (1 : 1) overnight, the reaction solution was extracted with CH 2 Cl 2 and the organic-phase was washed with water and dried over magnesium sulfate. By removing the solvent in vacuo, 1-(azidomethyl)-5H-tetrazole (3) was obtained as colorless solid in quantitative yield. Crystals suitable for single-crystal X-ray diffraction experiments were obtained by recrystallization from acetone.

NMR spectroscopy
The 1 H NMR of the crude isomeric mixture 1 in [D 6 ]DMSO shows two signals at δ = 9.31 and 8.77 ppm attributed to the tetrazole protons of 1 a and 1 b isomers, respectively. Integration of the signals reveals a distribution of 4/1 favoring 1 a over 1 b. The two signals at δ = 6.11 and 6.03 ppm are attributed to the respective CH 2 group of 1 a and 1 b. The 1 H NMR of 2 a in [D 6 ]acetone shows three signals at δ = 9.43, 6.67, and 3.27 ppm attributed to the CH, the CH 2 and the CH 3 group, respectively. The 1 H NMR of compound 3 in [D 6 ]DMSO shows two signals at δ = 9.60 and 6.02 ppm attributed to the CH and CH 2 group. Compared to 1 a, the tetrazole CH is downfield shifted by 0.29 ppm whereas the upfield shift of the CH 2 unit (0.01 ppm) is negligible. The 13 C NMR spectrum also shows two signals at δ = 144.2 and 60.5 ppm, marking the tetrazole and the methylene carbon atoms, respectively. 14 N NMR measurement of 3 in [D 6 ]DMSO shows two broadened signals at δ = À 135.6 and À 160.4 ppm corresponding to the N β atom of the azide moiety and to the N γ atom of the tetrazole.

Detonation properties
The detonation performance parameters of 3 were calculated with the Explo5 program code, [31] for which the enthalpy of formation was determined by applying the atomization method using room temperature CBS-4M enthalpy. As the performance properties for AET and APT are not reported in literature, their enthalpies of formation were also calculated to further obtain their detonation performances. Table 1 compares all calculated values for 3, AET and APT. A small increase in density of 0.06 g cm À 3 (Table 1) is observed by decreasing the alkyl-chain by one CH 2 group from APT to AET, whereas further shortening the alkyl chain by a second CH 2 group to AzMT causes a more drastic increase in density by 0.24 g cm À 3 . The higher increase in density can be explained due to the liquid state of APT and AET at room temperature, whereas AzMT is already a solid at room temperature. Additionally, the enthalpy of formation from 3 (654.5 kJ mol À 1 ) to APT (568.4 kJ mol À 1 ) also decreases drastically. As the performance of energetic compounds depends on their enthalpy of formation and their densities, combining both   (3), compared to AET [23] and APT. [22] AzMT (3) AET [23] APT [22] Formula into an enthalpy density offers an appropriate value to quickly elucidate the differences. Here, AzMT shows a very high enthalpy density of 8110 J cm À 3 , followed by AET (5630 J cm À 3 ) and APT (4639 J cm À 3 ). Therefore, the calculated detonation performances show a drastic increase in performance from APT (6757 m s À 1 ) to AzMT (8124 m s À 1 ), hence a significantly higher performance of the corresponding ECCs can also be estimated.

Energetic coordination compounds
Due to the absence of acidic protons in compound 3, it can be used as a well-suited neutral ligand for ECCs. The application of several oxidizing anions such as nitrate, chlorate, perchlorate, fulminate, picrate, styphnate, and trinitro phloroglucinol, and reducing anions such as azide, enables the tuning of the energetic properties of the resulting coordination compound. The energetic coordination compounds 4-9 were obtained by dissolving the corresponding metal(II) salts in water at elevated temperatures (60-80°C) and adding 3 in stoichiometric amounts (Scheme 2). Due to the commercial unavailability of copper(II) chlorate, it was synthesized according to a previously published procedure. [23] The copper(II) nitroaromatic salts for compounds 10-12 were synthesized by dissolving basic copper(II) carbonate in water and adding stoichiometric amounts of the free acids of the corresponding nitroaromatic anion (Scheme 3). Compound 13 was synthesized by dissolving copper(II) chloride dihydrate in water, adding one equivalent of 3, followed by adding two equivalents of sodium azide dissolved in water. Immediately after the addition of the azide, a brown precipitate of 13 formed, was filtered off and washed with little hot water.
Compound 14 a/b was obtained by dissolving silver(I) fulminate in acetonitrile at 30°C and adding one equivalent of ligand. Crystals of compound 14 a start to form in the mother liquor after one day. By filtering off the platelets, crystals suitable for single-crystal X-ray diffraction are obtained. When leaving the mother liquor for several days, white needles, suitable for X-ray diffraction, of compound 14 b start to crystallize on top of the crystals of 14 a, ultimately completely turning 14 a into 14 b. Crystallization of the coordination compounds 4-12 was achieved by slowly evaporating the mother liquor for 1-2 days, and collecting the formed solids by filtration with subsequent drying in air. Single crystals suitable for X-ray diffraction experiments were also obtained from the mother liquor for compounds 4, 9, and 10. Single crystals of compound 13 were obtained by a three-layered crystallization approach. Sodium azide together with 3 was dissolved in water and added to a test tube. A second layer consisting of a 1 : 1 mixture of water and ethanol was layered on top, and the third layer of copper(II) chloride dissolved in ethanol was slowly added. The test tube was carefully stoppered as not to mix the three layers and was set aside. After several days, crystals of compound 13 had formed and were collected by filtration and washed with little hot water. It was not possible to obtain crystals suitable for single-crystal X-ray diffraction of compounds 5-8, 11, and 12; their composition was analyzed and proven by elemental analysis. Generally, all energetic coordination compounds were fully characterized by elemental analysis, infrared spectroscopy, differential thermal analysis, as well as their sensitivity towards impact (IS), friction (FS), and electrostatic discharge (ESD) were tested. Hot needle (HN) and hot plate (HP) tests were performed to obtain insight into their initiation capabilities, which were then tested for several chosen compounds.

Crystal structures
Solid-state crystal structures of compounds 3, 4, 9, 10, and 14 a/ b were determined by using low-temperature single-crystal Xray diffraction. All of the data and parameters of the measurements as well as of the refinements are given in Table S1 in the Scheme 2. Synthesis of energetic coordination compounds of copper(II) nitrate (4), 3d-metal perchlorates (5-8) and copper(II) chlorate (9) with 1-(azidomethyl)-5H-tetrazole. Scheme 3. Synthesis of nitroaromatic coordination compounds 10-12, as well as the coordination compounds of copper(II) azide (13) and silver(I) fulminate (14 a/b) with AzMT. Supporting Information. All crystal densities were recalculated to their respective room temperature crystal density.
Compound 3 crystallizes in the orthorhombic space group Pbca with eight formula units in the unit cell and a density of 1.55 g cm À 3 ( Figure 3). All bond lengths are in the typical range for CÀ N, NÀ N single and double bonds. The tetrazole moiety forms a nearly perfectly level plane (C1-N1-N2-N3 = 0.3°) from which the methylene group protrudes slightly by an angle of C1-N1-N2-N3 = 1.4°. Typical for covalently bound azides, the À N 3 unit protrudes from the tetrazole plane with an angle of N1-C2-N5 = 111°as shown in Figure 3.
Compound 4 crystallizes in the monoclinic space group Cc with four formula units in the unit cell and a density of 1.66 g cm À 3 (Figure 4). The copper(II) cation is sixfold coordinated by 1-(azidomethyl)-5H-tetrazole ligands whereas the two nitrate anions are non-coordinating. While the bond lengths within the six ligands are equivalent as for pure compound 3, the bond lengths towards the copper(II) center range between 2.02-2.33 Å with angles of nearly perfect 180°between opposing ligands. Two of them (N4 and N25) show a longer bond distance towards the copper(II) cation than the four other ligands. Hence, the copper(II) cation is coordinated in a square bipyramidal conformation, caused by a typical Jahn-Teller-like distortion, due to the electron configuration of [Ar]3d 9 of the copper(II) cation.
The chlorate complex 9 crystallizes in the triclinic space group P1̄with two formula units in the unit cell and a density of 1.71 g cm À 3 ( Figure 5). The copper(II) cation is sixfold coordinated by AzMT ligands, all binding via their respective N4-nitrogen atom. Like in 4, the coordination sphere is a square bipyramid due to Jahn-Teller-like distortion along the N1-Cu1-N1 i axis. The change of anion from nitrate (4) to chlorate (9) does not induce a change of bond lengths within the ligands. The bond lengths between the ligands and the copper(II) center are in the same range as in 4 (2.02-2.36 Å). The two chlorate anions are also non-coordinating.
Copper(II) picrate 10 ( Figure 6) crystallizes in the monoclinic space group P2 1 /c with two formula units in the unit cell and a density of 1.83 g cm À 3 . The copper(II) cation is coordinated by two equatorial AzMT ligands and two coordinating picrate anions, acting as bidentate ligands. The two picrates coordinate via their deprotonated hydroxy group as well as a second oxygen atom of one of the ortho-nitro groups. The bond lengths between the copper(II) and the AzMT ligands (2.01 Å) are in the same range as for the copper(II) nitrate (4) and   chlorate (9). The deprotonated hydroxy groups also coordinate in the equatorial plane, whereas the nitro groups oxygen atoms are axial standing with a bond length of 2.35 Å, resulting in a typical Jahn-Teller-like distortion along the O2-Cu1-O2 i axis.
The copper(II) azide complex 13 crystallizes in the monoclinic space group P2 1 /c with four formula units in the unit cell and a density of 1.93 g cm À 3 . The copper(II) cation is coordinated by five azide anions as well as one AzMT (3) ligand ( Figure 7) forming a strongly distorted octahedron due to high deviations from the 90°coordination angles, observed in compounds 4, 9, and 10. The deviation of up to 13.5°(N1-Cu1-N11 ii ) from a linear arrangement of two opposing coordinating compounds is caused by two different bridging modes of the azide anions. Two of the coordinating azide anions bridge between two copper cations, thereby forming strands that are interconnected by the other three anions, which bridge between three copper cations.
Interestingly, the coordination of the copper(II) cation, as well as the two different bridging modes of the azide anions, are nearly identical to those of the 1-MTZ analogous, proving the structural similarity of the 1-MTZ and AzMT ligand. [32] For the silver(I) fulminate complexes, crystallization in two different space groups was observed. The initially formed platelets (14 a) crystallized in the monoclinic space group I2/a with eight formula units in the unit cell and a density of 2.71 g cm À 3 , the highest of all herein investigated compounds (Figure 8). The molecular unit consists of a tetrameric cluster of silver fulminate (SF), with the silver cations forming a nearly planar rhombus with angles of 75.72°(Ag2 i -Ag1-Ag2) and 104.28°(Ag1-Ag2-Ag1 i ), and a torsion angle of 3.32°. The fulminate anions bridge between the silver cations with their terminal carbon and oxygen atoms, having shorter Ag-C (2.17-2.27 Å) interactions than AgÀ O interactions (2.50-2.73 Å). Like in previously reported silver fulminate complexes, [33] the carbon atoms of the fulminate anions bridge between two silver cations of a tetramer. While the same bridging between two silver cations of the tetramer is observed for the oxygen atoms, a third interaction with a silver cation of a second tetrameric unit causes the formation of an extended three-dimensional structure. Due to the bridging character of the fulminate anions, AgÀ Ag interactions (Figure 8) are observed within the tetrameric unit. Comparable to previously reported coordination compounds of SF with monodentate ligands, [33] the bond lengths of these argentophilic interactions (2.91-2.77 Å) are in the range to be classified as sub-van der Waals contacts (< 3.44 Å). [34] Additionally, the shortest argentophilic interaction observed has a distance of 2.7675(7) Å, being close to the shortest reported distance of 2.746(1) Å. [34] After the mother liquor had stood for several days, white crystalline needles started to form on top of the platelets. These crystalline needles were also suitable for single-crystal X-ray diffraction experiments, showing compound 14 b crystallizing in the triclinic space group P1̄with two formula units in the unit cell and a density of 2.30 g cm À 3 . In contrast to 14 a, the

Chemistry-A European Journal
Research Article doi.org/10.1002/chem.202200492 molecular unit consists of a trimeric cluster of silver fulminate coordinated by three AzMT ligands (Figure 9). Here, the fulminate anions also bridge between silver cations, but in contrast to 14 a, only by their terminal carbon atom, forming monomers with argentophilic interactions between the silver cations, slightly longer (2.81-2.87 Å) than those in the tetrameric structure.
As in the tetrameric structure, the silver fulminate cluster is nearly planar with a torsion angle of 7.8°(C6-Ag2-Ag1-C9). Interestingly, a different protrusion for every AzMT ligand from the plane, formed by the SF trimer, is observed. While the first AzMT (via N4) is nearly perpendicular with the plane, the AzMT ligands (coordinated via N12 and N20) protrude from the plane by 8.0°and 46.5°, respectively.

Thermal behavior
Differential thermal analyses of compounds 3-14 are shown in Figures 10 and 11. DTA plots for compounds 4, 9 and 12 are shown in the Supporting Information. Compound 3 shows an endothermic event at 54°C, due to a melting of the compound, and an exothermic decomposition temperature of 167°C, lower than the comparable 1-(azidoethyl)-5H-tetrazole (AET) derivative (193°C). [23] All energetic coordination compounds show

Chemistry-A European Journal
Research Article doi.org/10.1002/chem.202200492 lower thermal stability, which is also observed for ECCs of AET. Compound 4 shows a small endothermic event at 60°C, attributing a melting of the compound, with a first exothermic event happening already at 123°C contributing to explosive decomposition. The same behavior is observed for compound 9, showing an endothermic event at 112°C, immediately evolving into an exothermic event at 123°C. The metal perchlorate compounds 5-8 exhibit exothermic decompositions between 127 (6) and 137°C (5) where all four compounds violently detonate. The nitroaromatic compounds 10 (165°C), 11 (154°C), and 12 (151°C) exhibit slightly higher thermal stabilities, yet they are less stable than pure 3. The coordination compounds of copper(II) azide (13) and silver(I) fulminate (14 b) are thermally stable up to 147 and 109°C, respectively. Here the formation of complexes with 1-(azidomethyl)-5H-tetrazole (3) leads to a more drastic drop in thermal stability compared to their parent salts copper(II) azide (205°C) and silver(I) fulminate (196°C). (3) is a very sensitive compound with an impact and friction sensitivity of 2 J and 1 N ( Table 2).

1-(Azidomethyl)-5H-tetrazole
(Both values are considered as very sensitive.) Although it is not sensitive (540 mJ) towards electrostatic discharge (ESD), due to its mechanical sensitivities, it has to be ranked as a primary explosive. By applying 3 as ligand in ECCs of copper(II) nitrate (4), picrate, (10), styphnate (11), and trinitrophloroglycinolate (12), it is possible to stabilize the friction sensitivity of 3. While 4 is only slightly less sensitive (15 N) than pure 3, compound 10 is completely insensitive (360 N). With an increasing amount of hydroxy groups, 11 shows a higher sensitivity (192 N) and 12 the highest friction sensitivity of 45 N. Yet all nitroaromatic compounds are far less sensitive than the copper(II) nitrate complex 4. When changing from copper(II) nitrate to perchlorate (7) or chlorate (9) the resulting coordination compounds exhibits drastically increased sensitivities (< 1 J, < 0.1 N) compared to pure 3. Changing to manganese(II) (5), iron(II) (6), or zinc(II) perchlorate (8) the same increase in sensitivities is observed. While all compounds show very high sensitivities towards impact (< 1 J), compound 5 towards friction (0.5 N) than 6, 7, and 8 (< 0.1 N). Compound 13 also shows a very high mechanical sensitivity of < 1 J and < 0.1 N. Yet it seems to be more stable than pure copper(II) azide, derived from the fact that we had no problems handling compound 13, while several minor explosions happened when handling pure copper(II) azide. The coordination compound of silver(I) fulminate with 3 leads to an increase in impact sensitivity (< 1 J) compared to pure silver fulminate (5 J), whereas the friction sensitivity is approximately equal to pure SF (< 0.1 N). The same trends of stabilization or destabilization as for the mechanical sensitivities of the ligand (3) is observed comparing the ESD sensitivities. Compounds 4, 10, and 11 are completely insensitive towards ESD (> 1500 mJ), while compound 12 (1220 mJ) is nearly insensitive. Changing to metal perchlorates or chlorates the ESD sensitivity of the resulting complex (6: 380 mJ; 7: 480 mJ; 8: 380 mJ; 9: 380 mJ) is increased compared to the pure ligand (540 mJ). Here, only compound 5, the manganese perchlorate complex shows a slightly lower sensitivity (740 mJ) than 3. The highest sensitivity towards ESD is observed for the silver(I) fulminate complex 14 b (10 mJ), followed by the copper(II) azide complex 13 (38 mJ). Here, the addition of 3 to the corresponding salt results in a stabilization of the otherwise sensitive parent compound. Due to the high sensitivities of the metal (per-)chlorates, as well as 14 b towards impact (< 1 J), ball drop impact measurements were performed, allowing a more detailed differentiation of the compounds. For compounds 5 and 7 (Table 1) a ball drop impact sensitivity (BDIS) of 25 mJ was observed, while compounds 8, and 14 b are slightly more sensitive (20 mJ) and iron(II) perchlorate (6), as well as the copper(II) chlorate (9) showed the highest sensitivity (16 mJ).  6)).
[g] Ball drop impact sensitivity (OZM BIT-132) determined with the 1-of-6 method in accordance with the MIL-STD 1751 A (method 1016).

Initiation capabilities
To get a further insight on the behavior upon fast heating with and without confinement, hot plate (HP) and hot needle (HN) tests were carried out. This also allows for a first evaluation of the applicability of the tested compounds as primary explosives. Detailed experimental procedures are given in the Supporting Information, the results are summarized in Table 1. Pure compound 3 showed a detonation in the HP test and a deflagration in the HN test. While the copper(II) nitrate (4) and the nitroaromatic compounds 10, 11, and 12 show deflagration or decomposition (10,11), the (per)chlorate metal complexes show a violent detonation (6, 7, and 9) or sharp deflagration (5 and 8) in the HP test ( Figure 12). The complexes of copper(II) azide (13) and silver(I) fulminate (14) detonate both in the HP and HN test. For compounds 3, 5, 7, 8, and 12 initiation tests were performed. The detailed testing setup and procedure are given in the Supporting Information. Compounds 3 and 12 showed no positive initiation of PETN, which is indicated by the still intact witness plate ( Figure 13). Therefore, neither compound was able to undergo deflagration to detonation transition (DDT) in this setup, which is a necessary characteristic for a compound to be used as a primary explosive. Nonetheless, the perchlorates 5, 7, and 8 were able to undergo DDT and initiate the PETN sample, indicated by a perforation of the copper witness plate ( Figure 14).

Conclusion
Successful hydroxymethylation of 1,5H-tetrazole with aqueous formalin solution can only be achieved by the addition of The time between each frame is 10 ms and shows the moment before (t 0 ), at the onset of (t 1 ) and during (t 2 ) decomposition.  catalytic amounts (3-5 wt %) of sodium hydroxide to the reaction solution. Subsequent protection of the alcohol by methanesulfonyl chloride provided the precursor compound needed to successfully synthesize the highly energetic 1-(azidomethyl)-5H-tetrazole (3) for the first time. While its structural isomer 5-(azidomethyl)-1H-tetrazole has already been reported, 3 represents the smallest tetrazole derivate with the shortest N-alkyl-azide functionality reported, showing a very high positive heat of formation. Single-crystal X-ray analysis of 3 was supplemented by elemental analysis, infrared spectroscopy as well as multinuclear NMR ( 1 H, 13 C, 14 N, and 15 N) spectroscopy. The thermal behavior was determined by DTA measurement, and the sensitivities towards external stimuli were determined. Like other azido alkyl-tetrazoles, it performs well as a ligand in the formation of energetic coordination compounds. Here, the ECCs of copper(II) nitrate (4), chlorate (9), perchlorate (7), azide (13), manganese(II) (5), iron(II) (6), and zinc(II) perchlorate (8) were synthesized. The ECCs of copper(II) nitroaromatic salts (10)(11)(12), as well as that of silver(I) fulminate (14 a/b) were prepared. Suitable crystals were analyzed by single-crystal X-ray diffraction experiments, giving insight into the molecular structure of the ligand itself, as well as its coordination behavior towards 3d and 4d metals. The successful syntheses of the ECCs are supported by elemental analysis as well as infrared spectroscopy. Analysis of the sensitivities towards external stimuli showed the high sensitivity (impact: 2 J, friction: 1 N) of the ligand, which is generally increased by the formation of ECCs. While the impact sensitivity of the ligand was increased by the formation of ECCs (< 1 J (5-14)), the friction sensitivity can be decreased by forming the copper(II) picrate (360 N), styphnate (192 N), and trinitrophloroglycinolate (45 N) complexes. The same trend was observed for the thermal stability of 3 (177°C). Analyzing the thermal stabilities of the ECCs by DTA, showed decreasing thermal stabilities of the ECCs compared to the ligand itself. Initiation testing of the ligand itself and the ECC of copper(II) trinitrophloroglycinolate (12) showed no DDT, thus no initiation of the PETN sample. Testing the perchlorate salts of manganese, copper, and zinc showed a DDT with a positive initiation of PETN.

Experimental Section
CAUTION! This paper contains a large amount of extremely sensitive compounds. Full-body protection containing Kevlar® gloves, Kevlar® sleeves, face shield, leather coat and ear protection must be worn when manipulating those compounds! The equipment used must be earthed! [30] (1): 1,5H-tetrazole (20 g, 285 mmol), synthesized according to literature, [35][36] and sodium hydroxide (0.5 g, 12.5 mmol) were dissolved in water (200 mL), and formalin solution (37 %, 66 mL, 917 mmol) was added. The reaction mixture was stirred over night at room temperature and was extracted into ethyl acetate (3x400 mL). The organic phase was dried over MgSO 4 , and the solvent was removed in vacuo, resulting in crude isomeric mixture 1 as a colorless oil in good yield (22.1 g, 221 mmol, 80 %). 1  (Tetrazol-1-yl)methyl methanesulfonate (2 a): Crude isomeric mixture 1 (5.0 g, 50 mmol) was suspended in CH 2 Cl 2 (100 mL), cooled to 0°C. Methanesulfonyl chloride (6.87 g, 60 mmol, 1.2 equiv.) was added, the reaction mixture was stirred for 1 h while cooling, and pyridine (5.93 g, 75 mmol, 1.5 equiv.) was added, keeping the temperature below 5°C. The reaction mixture was allowed to slowly warm and was stirred for 18 h at room temperature. The reaction mixture was then transferred to a separatory funnel and washed with water (100 mL), hydrochloric acid (2 m, 50 mL), and water (50 mL). Colorless crystals of compound 2 a slowly start to crystallize from the aqueous phase in moderate yield (3.12 g, 17.5 mmol, 35 %). The organic phase was dried over MgSO 4 , and the solvent was removed in vacuo, leaving a colorless oil consisting of compounds 2 a and 2 b, which were separated by flash column chromatography over silica with n-hexane/ethyl acetate (50 : 50) as the eluting solvent. After removing the solvent in vacuo, additional compound 2 a was obtained in a yield of 13 %, whereas crude compound 2 b was still obtained. 1

Synthesis of energetic coordination compounds
General procedure: Compound 3 (125 mg, 1 mmol) was dissolved in water at 60-80°C. In a second container, the corresponding equivalents of metal salt (4-9: 0.16 mmol, 10-12: 0.5 mmol, 13 and 14: 1 mmol) was dissolved in water (4)(5)(6)(7)(8)(9)(10)(11)(12)(13) or acetonitrile (14) at 60-80°C. After all solids dissolved, the solution containing compound 3 was added to the metal salt solution with stirring. After stirring for 5 min at 60°C, any undissolved solids were filtered off, and the solution was left to crystallize for compounds 4-12, and 14 a/b. After the solvent was evaporated, the remaining solids were dried under high vacuum for 1 h. Compound 13 was obtained by combining the two solutions, stirring for 5 min at 60°C, filtering off the precipitate and washing with cold water.