Copper(II) Dicyanamide Complexes with N-Substituted Tetrazole Ligands – Energetic Coordination Polymers with Moderate Sensitivities

Following the useful concept of energetic coordination compounds (ECC), copper(II) dicyanamide was used as a building block for the synthesis of eight new complexes. As ligands, six different N-substituted tetrazoles were applied, leading to the formation of high-nitrogen containing complexes. The obtained compounds were characterized in detail by single crystal as well as powder XRD, IR, EA, DTA, and TGA. In addition, the sensitivities towards impact and friction were determined with BAM standard techniques as well as the sensitivity towards electrostatic discharges. All compounds show moderate sensitivities (IS>6, FS>80 N) and energetic properties but differ in their polymeric structures forming polymeric chains or layers up to 3D networks.


Introduction
The dicyanamide moiety belongs to the class of cyanamides, which have manifold applications and are used as precursors or intermediates in industry. Sodium dicyanamide (NaDCA) can be easily obtained by reacting cyanamide with cyanogen chloride and sodium hydroxide. [1] Dicyanamides can be used as starting materials for the synthesis of many different materials such as heterocycles, ionic liquids or transition metal complexes. [3] An interesting hetero-cycle with regards to its very high nitrogen content on the one hand side and its perfect stability and facile accessibility on the other hand side is 5,5'-bis(1H-tetrazol-5-yl)amine (H 2 bta). It can easily be prepared by a one-step reaction starting with NaDCA (Chart 1 A)). The neutral compound and its metal salts were discussed for different applications, especially for the use as energetic materials, e. g. as burn rate modifiers due to their moderate sensitivities. [4] Another important use of the DCA anion is its application in transition metal complexes. Due to its manifold coordination modes and bridging character it can connect between several metal centers forming polymeric compounds with unique features (Chart 1 B)). [2] An innumerable amount of complexes mainly based on 3d transition metals are discussed in literature for different applications such as magnetic switches. [5] Besides the commonly used central metals copper and cobalt there can also be found more exotic ones, like silver or uranium. [6] In the past, strong oxidizing anions such as nitrate or perchlorate were often used for the synthesis of energetic coordination compounds (ECC). The obtained complexes are powerful compounds with great performance but also come along with high sensitivities making their handling considerably more difficult. [7] In recent years, the focus of new energetic compounds shifted strongly towards substances that are safer to use and therefore possess decreased sensitivities. [8] For the synthesis of novel ECC, we focused on small anions with positive heat of formation (~H f ). Most of the anions such as (per)chlorate, nitrate, etc. are formed exothermically in the gas phase ( Figure 1). In contrast cyanide, fulminate, azide, and dicyanamide have different positive calculated~H f 's and allow therefore the tuning of the desired properties.
In the case of copper(II) complexes the choices are considerably narrowed. Both, fulminates and cyanides are leading to the reduction to Cu + , while copper azides are extremely sensitive and dangerous to handle. The optimal compromise for the synthesis of energetic copper complexes with moderate sensitivities is thus the dicyanamide anion showing a moderate positive heat of formation. For the DCA anion, both the pure copper(I) and copper(II) salts are stable at ambient conditions. [10] While many copper(II) dicyanamide complexes were investigated, nitrogen-rich representatives are very rare in literature. Solely the 1,5-diaminotetrazole compound is mentioned as an energetic combustion catalyst for propellant systems, indicating their potential as possible ECC. [11] Therefore, in this work, new copper(II) coordination polymers based on dicyanamide and tetrazole ligands were synthesized and their energetic properties investigated.

Synthesis
The copper(II) dicyanamide compounds were synthesized using six different tetrazole ligands and started with a metathesis reaction. Therefore, the corresponding copper(II) nitrate complexes were generated in situ and further reacted with aqueous sodium dicyanamide solutions (Scheme 1). The used ligands were available in the research group and prepared according to literature known syntheses. [12] All eight complexes possess polymeric structures due to the bridging nature of the DCA anion and therefore crystallize quickly after the synthesis.
During the synthesis of 6 a, sometimes the formation of a second species in very small traces was observed. The sidespecies 6 b represents a monohydrate of 6 a and is the only water-containing compound. All other complexes were obtained as anhydrous substances. Similar to 6 b only some single-crystals of complex 3 were able to be obtained. Therefore, both compounds could not be further characterized. The pure copper(II) dicyanamide (7) was obtained as the main product during the attempted synthesis of 3 and as the only product when 2-amino-5-methyltetrazole was applied as ligand.

Crystal Structures
All compounds were characterized by low-temperature singlecrystal X-ray diffraction and details on the measurement and refinement data are given in the Supporting Information (Table S1À 3). [13] In all eight complexes, the central Cu 2 + cations are octahedrally coordinated and show typical d 9 Jahn-Teller distortions. Due to the bridging dicyanamide anion, all structures are multidimensional. The spatial structures of compounds 1, 2 and 3 with the ligands 1-MTZ (1), AET (2) and 1-AMT (3) are quite similar. The copper atoms are all coordinated by two tetrazole ligands in equatorial positions and four bridging dicyanamido anions. The MTZ complex 1 is crystalizing in the monoclinic space group P2 1 /n and compounds 2 and 3 in the triclinic space group PÀ 1 ( Figure 2). They show comparable densities between 1.735 and 1.772 g cm À 3 .
In all three cases, the bridging leads to the formation of one-dimensional chains (Figure 3), which are caused by the binding of the two outer nitrogen atoms of the DCA to the central metals. Two anions each bridging between the same two cations. The heterocycles all bind to the copper via the N4 atom which is located next to the carbon atom. The bond lengths between the Cu1and the N4 atom are almost the same for all three complexes. The copper complex 4 with 1-AT as ligand has a nearly similar coordination environment as the previously described complexes 1-3. The central copper atom is coordinated by two 1-AT ligands via N4, and 4 DCA anions again via the outer two nitrogen atoms ( Figure 4). It crystallizes in the monoclinic space group P2 1 /c with two formula units per unit cell and a calculated density of 1.860 g cm À 3 at 143 K.
However, the compounds differ in their polymeric structure. While in complexes 1-3 the dicyanamide anions are bridging always between the same two central metals, the four DCAs in compound 4 link to four different copper cations. This creates a two-dimensional polymeric layer ( Figure 5).

ChemPlusChem
Full Papers doi.org /10.1002/cplu.202000156 difference between the two is the arrangement of the tetrazole ligands. While in 5 b the heterocycles of one side all point to the same side in 5 a they alternate, allowing a denser packing and explaining the higher density (Figure 7).
Compared to the other complexes, the structures of 6 a and 6 b are outstanding due to the insertion of an additional bridging ligand. The anhydrous compound 6 a crystallizes in the triclinic space group PÀ 1 with a density of 1.817 g cm À 3 (110 K) and the monohydrate in the monoclinic space group C2/c with a slightly lower density (1.752 g cm À 3 @ 127 K). In both cases, the central metals are coordinated by two tetrazoles and four DCA nitrogen atoms. Like in 5 a and 5 b, the anions are binding with one outer and the middle nitrogen atom. The molecular unit of 6 b is completed by an additional crystal water molecule ( Figure 8).
Nevertheless, 6 b is not just simply the monohydrate of 6 a. A closer examination of the crystal structures reveals the difference of 6 a forming a two-dimensional and 6 b a threedimensional structure. Similar to compounds 1-3, in 6 a two anions each are bridging between the same two central metals leading to the formation of polymeric chains. The additional linking of the ditetrazolyl ligand is causing an enhancement of the dimension to polymeric layers (Figure 9).
The dte ligand is also increasing the dimension in complex 6 b, but here the DCA anions are already forming 2D-layers, similar to complex 5 a and 5 b. This results in a three-dimensional polymeric structure of 6 b ( Figure 10). The structure favors the storage of water, which is why 6 b crystallizes as monohydrate.

Physiochemical Properties
All synthesized compounds, except 3 and 6 b, were investigated regarding their thermal properties and sensitivities towards external stimuli. In order to check the isomeric purity of compounds 5 a and 5 b, they were further investigated by powder X-ray diffraction.

Sensitivities
Except for 3 and 6 b, the energetic properties of all compounds were investigated. The sensitivities towards impact, friction, and electrostatic discharge are summarized in Table 1. The impact sensitivities are in the range of 6-10 J and therefore, the complexes have to be classified as sensitive according to the UN Recommendations on the Transport of Dangerous Goods. Only the salt 7 is classified as less sensitive with a value of 40 J.
Concerning the friction sensitivities, compounds 1, 6 a and 7 can be classified as insensitive (> 360 N). In contrast, complexes 2, 4, 5 a and 5 b have to be classified as sensitive, with values between 80 N (5 a, 5 b) and 192 N (2). It can be seen that the energetic character of pure copper(II) dicyanamide is getting increased through the coordination of nitrogen-rich ligands. Interestingly the two different polymorphs 5 a and 5 b do not show major aberrations in their energetic properties.

Powder diffraction
For a more detailed investigation, compounds 5 a and 5 b were applied to X-ray powder diffraction measurements ( Figure 13).
Small variations in the orthorhombic species 5 b can be attributed to the temperature differences of single-crystal and powder X-ray diffraction. Therefore, it is possible to synthesize 5 b isomerically pure. However, in the case of 5 a contamination with the second polymorph are observable and further research is necessary for selective synthesis of it.

Conclusion
In this work, eight new nitrogen-rich copper(II) dicyanamide complexes based on N-substituted tetrazole ligands were synthesized. Due to the good coordination behavior of dicyanamide, they can all be obtained without the inclusion of water. Only the formation of a 3D-polymeric side-species allows the inclusion of guest solvent molecules. All complexes were investigated by low-temperature single-crystal X-ray experiments and their polymeric structure examined in detail. The energetic properties and thermal behavior of the low watersoluble six ECC were determined and showed moderate sensitivities (IS > 6,FS > 80 N). This allows the safe handling of   the complexes, which could be used as potential energetic additives e. g. burn rate catalysts. Also, a non-energetic use as magnetic switches could be possible. Especially compounds 4 and 5 b show promising properties for the use as energetic metal-organic frameworks (EMOF).

Experimental Section
Caution! All investigated compounds are energetic materials, which partly show increased sensitivities towards various stimuli (e. g. elevated temperatures, impact, friction or electronic discharge). Although no incidents occurred, proper security precautions (safety glasses, face shield, earthed equipment and shoes, leather jacket, Kevlar sleeves, and earplugs) have to be worn while synthesizing and handling the described compounds.
Detailed information on chemicals, syntheses, as well as analytical data and methods of 1-6 can be found in the Supplementary Information.