PAPER www.rsc.org/dalton | Dalton Transactions

Safe 5-nitrotetrazolate anion transfer reagents†

Thomas M. Klapotke* and Carles Miro Sabate

Received 24th October 2008, Accepted 19th December 2008First published as an Advance Article on the web 27th January 2009DOI: 10.1039/b818900p

Silver 5-nitrotetrazolate (1) and copper(II) 5-nitrotetrazolate 5-nitrotetrazole dihydrate (2) are usefulreagents for the synthesis of 5-nitrotetrazole (NT) salts. Both compounds were synthesized andcharacterized by vibrational spectroscopy (IR and Raman) and differential scanning calorimetry(DSC). In addition, their sensitivity towards friction, shock and electrostatic discharge was tested bystandard BAM methods. The extremely high sensitivity of both materials makes the transfer of the NT-

anion using 1 and 2 hazardous and not suitable for up-scaling. In order to diminish the hazardsinvolved with the transfer of the energetic anion and to render the synthesis of NT salts suitable for anindustrial scale the two compounds were stabilized by coordination with a chelating ligand andsilver(ethylendiamine) 5-nitrotetrazolate (3) and bis(ethylendiamine)copper(II) 5-nitrotetrazolate (4)were synthesized in high yields. Both the stabilized NT- anion transfer reagents were characterized byanalytical and spectroscopic methods. In addition, the crystal structure of the ethylendiamine coppercomplex (4) was determined: orthorombic, Pbca; a = 7.5200(1), b = 14.0124(2), c = 14.7740(2) A;V = 1556.78(4) A3. Furthermore, we synthesized triaminocopper(II) 5-nitrotetrazolate (5), which haspotential as a more environmentally-friendly primary explosive. Lastly, the synthetic potential of theethylediamine adducts 3 and 4 to form energetic salts of NT was investigated.

Introduction

The synthesis of highly energetic materials has been a long-termgoal in our research group.1–4 Azide, nitrate and perchlorate andmore recently dinitramide (-N(NO2)2) or azotetrazolate ([C2N10]2-)have classically been used as counter-anions for the synthesisof explosives.5 Recently, the use of 5-nitrotetrazolate (NT-) asan organic anion for the synthesis of energetic salts has gainedinterest.6–8 The NT- anion has a low carbon content, which isbalanced out by a good oxygen balance at the same time it hasa high nitrogen content (~61% in 5-nitrotetrazole, NT).9 In 1963Harris et. al. reported the synthesis of iron(II) 5-nitrotetrazolate(Fe(NT)2)10 by the reaction of sodium 5-nitrotetrazolate (NaNT)with a solution of FeCl2 in diluted HCl and characterized thecompound only by IR and elemental analysis. Around a decadelater, Raymond et al.11 patented the synthesis of the Hg+, Hg2+

and Ag+ salts from the reaction of a copper(II) 5-nitrotetrazolatecomplex with ethylenediamine (4) with a suitable metal nitrateand the thermal decomposition of these salts was studied lateron.12 The first structural report of a salt containing the NT-

anion was that of a nickel(II) complex.13 It was not until thebeginning of this century that Zhilin et al.14 synthesized theinteresting tetrammine-cis-bis(nitro-2H-tetrazolato-N2)cobalt(III)perchlorate (BNCP) and Hiskey et al. patented salts of theformula [Cat]z+M2+(NT)x (where Cat = NH4, Na, K, Rb, Cs andM = Fe, Co, Ni, Cu, Zn, Cr, Mn)15 and published their iron(II)nitrotetrazolate salts hierarchy ([FeIINT3]-, [FeIINT4]2-, [FeIINT5]3-

and [FeIINT6]4-).16 Some of these materials have found practicalapplications. For example, NaNT has been used as a stand-alone

Butenandtstr. 5–13 (D), 81377 Munich, Germany. E-mail: tmk@cup.uni-muenchen.de; Fax: +49 89 2180 77492; Tel: +49 89 2180 77491† CCDC reference number 704807. For crystallographic data in CIF orother electronic format see DOI: 10.1039/b818900p

energetic material and as an additive in explosive and propellantsmixtures,17 whereas BNCP has found application in detonatorsthat are integral to the fire suppression systems for military andcommercial aircrafts.14

We recently reported the synthesis of nitrogen-rich7,8 andalkali metal18 salts with the NT- anion, which have interestingproperties as high-explosives and/or propellants and as initiators,respectively. However, the preparation of such materials ofteninvolves handling highly sensitive species with associated haz-ards. Silver 5-nitrotetrazolate (1) and copper(II) 5-nitrotetrazolate5-nitrotetrazole dihydrate (2) are two of these highly sensitivecompounds. We envisioned 1 as a useful NT- anion transferreagent due to the high insolubility of silver halogenides in mostcommon solvents and also the readily precipitation of copper(II)oxide should render 2 a useful starting material for the synthesisof NT- salts. Unfortunately, the above mentioned high sensitivityof the compounds would render up-scaling reactions pointing atthe synthesis of salts of NT unsafe, when using 1 or 2. In thiscontext we would like to present our results on the synthesis andcharacterization of two ethylendiamine adducts, which are usefuland non-hazardous transfer reagents of the energetic NT- anionas exemplified by the hereby reported alternative synthesis of the5-aminotetrazolium and 1,3-dimethyl-5-aminotetrazolium salts.

Results and discussion

Synthesis

Silver 5-nitrotetrazolate (1) was prepared by precipitationfrom the sodium salt (NaNT)18 and silver nitrate in wa-ter as a white, highly sensitive powder. Copper(II) 5-nitrotetrazolate 5-nitrotetrazole dihydrate (2) was prepared asa turquoise highly insoluble and highly sensitive powder from

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5-amino-1H-tetrazole (5-At) according to a known literatureprocedure.19 Silver(ethylendiamine) 5-nitrotetrazolate (3) andbis(ethylendiamine)copper(II) 5-nitrotetrazolate (4) were synthe-sized from NaNT by addition of an excess of ethylendiamine (en)in hot ethanol and subsequent reflux with an equivalent amountof silver nitrate or copper(II) chloride, respectively. Slightly yellowcrystals of 3 precipitated upon cooling whereas 4, insoluble in hotethanol, separated from the hot reaction mixture as a lilac powder(eqn (1)).

In addition, the reaction between NaNT and copper(II) sulfatein ammonia led to the formation of a blue compound, whichseparated as a powder upon cooling the reaction mixture. El-emental analysis of this solid pointed, somewhat unexpectedly,at the formation of a copper complex containing three aminoligands instead of the expected four and obeying to the formulatriaminocopper(II) 5-nitrotetrazolate (5, eqn (2)). Compound 5might have either a square planar configuration completed by co-ordination to three ammonia ligands and one m1-5-nitrotetrazolateligand or a distorted octahedron geometry with the ammonialigands and one chelate and one m1-5-nitrotetrazolate ligands.

(2)

The ethylendiamine complexes 3 and 4 are readily soluble inpolar solvents such as DMSO, DMF and hot water (3 also in hotethanol) and in coordinating solvents (e.g., ammonia, pyridine,ethylendiamine etc.) and insoluble in most other common solvents.In contrast, compounds 1, 2 and 5 do not appreciably dissolveneither in polar nor in less polar or apolar solvents (only 1 dissolvesslightly in acetonitrile).

As mentioned above both the silver and the copper ethylendi-amine complexes (3 and 4, respectively) show promise because theyoffer an alternative safer synthesis of previously reported com-

pounds such as tetrazolium 5-nitrotetrazolate salts 6–10 (eqn (3)).Compounds 3 and 4 can be normally used like 1 and 2, respectively,and the ethylendiamine ligand can be conveniently removed afterthe reaction by applying a vacuum.

(1)

Spectroscopic discussion

Due to the broad NMR signals expected for copper(II) complexesand the low solubility of the silver salt 1 in every solvent tried, weonly measured the NMR spectra for compound 3, which showsproton resonances for the ligand (i.e., ethylendiamine) at d = 2.7(CH2) and 3.4 (NH2) ppm. In the 13C NMR, the resonance forthe ring carbon atom is found at d = 168.4 ppm, to low fieldin respect to that of metal salts of 5-amino-1H-tetrazole20 andto high field compared to that of metal salts containing the 5,5¢-azotetrazolate anion,21 in keeping with nitrogen-rich salts with thesame anion.8 Additionally, the methylene carbon atom resonancein 3 is observed at d = 43.3 ppm. The high symmetry in the anionallows to distinguish the three expected nitrogen atom resonancesin the 14N NMR spectrum. They are observed at d = +19 (N2/N3),-21 (NO2) and -62 (N1/N4) ppm and are broad (n1/2 ~420, 60 and400 Hz, respectively).

(3)

The most significant IR and Raman frequencies can be assignedby comparison with the calculated values.8 The Raman spectraof the complexes 3, 4 and 5 are dominated by three bands at~1410, 1050 and 1030 cm-1 corresponding to the coupled (inphase) NO2 and N1–C–N4 stretching, N1–C–N4 deformation andNO2 stretching and to the asymmetric ring deformation modes,respectively, observed as one strong and two weak/medium bandsat ~1420, 1050 and 1030 cm-1 in the IR spectra. On the otherhand, the spectra of the “pure” NT salts 1 and 2 are characterizedby an strong signal at ~1430 cm-1 (Raman), which has mediumintensity in the IR spectra (~1425 cm-1) and also correspond to acoupled NO2 and N1–C–N4 stretching vibration. In addition, theIR spectra are dominated by the nitro-group asymmetric stretch-ing (~1540 cm-1, 1565 cm-1 for compound 2), nitro-group andN1–C–N4 (out of phase) symmetric stretching (~1320 cm-1) and

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Table 1 Selected bond distances (A) and angles (◦) in 4

Bond distances/A

C1–N1 1.327(2) O2–N5 1.226(2)C1–N4 1.326(2) O1–N5 1.236(2)C1–N5 1.441(2) N6–C2 1.480(2)N1–N2 1.340(2) C3–C2 1.517(2)N3–N2 1.335(2) N7–C3 1.482(2)N4–N3 1.341(2)

Bond angles/◦

C1–N1–N2 103.0(1) N1–C1–N5 122.6(1)N3–N2–N1 109.4(1) O2–N5–C1 118.2(1)N2–N3–N4 109.9(1) O1–N5–C1 117.7(1)C1–N4–N3 102.7(1) O1–N5–O2 124.1(1)N4–C1–N1 115.0(1) N6–C2–C3 107.5(1)N4–C1–N5 122.3(1) N7–C3–C2 107.4(1)

the nitro-group and N1–C–N4 (in phase) deformation (~840 cm-1)modes.5c,22

Crystal structure of 4†

The structure of compound 4 was determined by X-ray analysisusing an Oxford Xcalibur3 diffractometer with a Spellman gener-ator (voltage 50 kV, current 40 mA) and a KappaCCD detector.‡Suitable crystals were grown as described in the Experimental. Thedata collection was performed using the CrysAlis CCD software,23

and the data reduction was performed with the CrysAlis REDsoftware.24 The structure was solved by direct methods using thestandard software implemented in the WinGX package25–28 andfinally checked with PLATON.29 All non-hydrogen atoms wererefined anisotropically. All hydrogen atoms were located fromdifference Fourier electron density maps and refined isotropically.The absorptions were corrected with the SCALE3 ABSPACKmulti-scan method.30 Selected bond lengths and angles are re-ported in Table 1 and coordination around the copper atoms inTable 2. In addition, hydrogen bonding geometries and graph-setsare tabulated in Tables 3 and 4, respectively.

‡ Structure refinement of solution for compound 4: chemical formula:C6H16N14O4Cu, Mr = 411.87 g mol-1, crystal system: orthorhombic, spacegroup: Pbca, a = 7.5200(1) A, b = 14.0124(2) A, c = 14.7740(2) A, a = b =g = 90◦, V = 1556.78(4) A3, Z = 4, rcalcd = 1.757 g cm-3, m = 1.455 mm-1,F(000) = 844, q range: 3.87–30.11◦, T = 100(2) K, index range: -10 ≤h ≤ 10, -19 ≤ k ≤ 19, -20 ≤ l ≤ 20, reflections collected: 18 690, uniquereflections: 2290, Rint = 0.0292, data/restraints/parameters: 2290/0/147,GOF = 1.184, R1 [I > 4s(I)] = 0.0302, wR2 [I > 4s(I)] = 0.0606, R1(alldata): 0.0448 and wR2 (all data): 0.0710.

Table 2 Geometry for the coordination around the copper cations in 4

Bond distances/A

Cu–N1 2.578(1) Cu–N1i 2.578(1)Cu–N6 2.018(1) Cu–N6i 2.018(1)Cu–N7 2.022(1) Cu–N7i 2.022(1)

Bond angles/◦

N1–Cu–N7 91.2(1) N1–Cu–N6i 90.7(1)N1–Cu–N6 89.3(1) N7–Cu–N1i 88.8(1)N6–Cu–N7 84.6(1) N6–Cu–N7i 95.4(1)C2–N6–Cu 109.0(1) N1–Cu–N1i 180.0(1)C3–N7–Cu 108.2(1) N7–Cu–N7i 180.0(1)N1–Cu–N7i 88.8(1) N6–Cu–N6i 180.0(1)N7–Cu–N6i 95.4(1)

Symmetry code: (i) 1 - x, -y, 1 - z.

Table 3 Selected hydrogen bonds for 4 (see Fig. 1 for labelling scheme)

D–H ◊ ◊ ◊ A D–H/A H ◊ ◊ ◊ A/A D ◊ ◊ ◊ A/A D–H ◊ ◊ ◊ A/◦

N6–H6B ◊ ◊ ◊ O1 0.88(2) 2.29(1) 3.083(2) 149(2)N7–H7A ◊ ◊ ◊ N3ii 0.85(1) 2.40(1) 3.128(1) 144(1)N7–H7B ◊ ◊ ◊ N3iii 0.87(1) 2.42(1) 3.282(1) 168(1)N7–H7B ◊ ◊ ◊ N4iii 0.87(1) 2.57(1) 3.371(1) 153(1)N6–H6A ◊ ◊ ◊ N4iv 0.84(2) 2.41(1) 3.199(2) 158(2)N6–H6A ◊ ◊ ◊ O2iv 0.84(1) 2.59(1) 3.226(1) 134(2)

Symmetry codes: (ii) -x + 3/2, y - 1/2, z; (iii) x + 1/2, -y + 1/2, -z + 1;(iv) -x + 1/2, y - 1/2, z.

Compound 4 forms violet crystals, which arrange in anorthorhombic cell in the space group Pbca and contain fourmolecules in the unit cell. Half of the molecule can be generatedby symmetry (Fig. 1a, symmetry code: (i) 1 - x, -y, 1 - z) and thecopper atoms are surrounded by two ethylendiamine (en) moietiesand two anions with a total coordination number of six. Theen ligands are disposed in an envelope-type conformation withthe central copper atom with C3 and C2 pointing upwards anddownwards, respectively. The two ligands sit on the equatorialplane with distances ~2.02 A to the metal, whereas the two anionswith a distance of 2.578(1) A to the central atom occupy the axialpositions (Fig. 1b). The geometry around the copper atom is thatof a slightly distorted octahedron with little Jahn–Teller effectdue to the high symmetry of the molecule (N1–Cu–N1i =N6–Cu–N6i = N7–Cu–N7i ~180◦). The geometry of the coordi-nation around the metal centre is summarized in Table 2. Themaximum variations in the angles are found in the equatorialplane where the angles are between 84.6(1) and 95.4(1)◦, whereas

Table 4 Graph-set table for 4 showing the unitary (diagonal) and secondary (off-diagonal) hydrogen bonding networks (see Fig. 1 for labelling scheme)

A B C D E F

N6–H6B ◊ ◊ ◊ O1 (A) D1,1(2)N7–H7A ◊ ◊ ◊ N3ii (B) C2,2(11) D1,1(2)N7–H7B ◊ ◊ ◊ N3iii (C) R4,4(22) R2,4(8) D1,1(2)N7–H7B ◊ ◊ ◊ N4iii (D) R4,4(20) R4,4(10) R2,1(3) D1,1(2)N6–H6A ◊ ◊ ◊ N4iv (E) R2,1(5) C2,2(8) R4,4(16) R2,4(14) D1,1(2)N6–H6A ◊ ◊ ◊ O2iv (F) C2,2(6) C2,2(11) C2,2(11) C2,2(10) C2,2(7) D1,1(2)

Symmetry codes: (ii) -x + 3/2, y - 1/2, z; (iii) x + 1/2, -y + 1/2, -z + 1; (iv) -x + 1/2, y - 1/2, z.

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Fig. 1 (a) Asymmetric unit showing the coordination around the copper atoms and (b) simplified coordination around the copper atoms in the crystalstructure of 4 (symmetry code: (i) 1 - x, -y, 1 - z).

the axial nitrogen atom (N1) forms angles between 88.8 and 91.2◦

with this plane. The distances for the coordination of the enligands to the copper atoms are comparable to those foundin copper salts of N,N,-bis-(1H-tetrazol-5-yl)-amine with amineligands,31 whereas those for the 5-nitrotetrazolate ring are betweencommon single and double bonds32 indicating delocalization of thenegative charge, in keeping with other crystallographic studies oncomparable compounds.7

Fig. 2 shows a view of the unit cell in 4. One host copper atomis found at each corner and in each face (Z = 1/8·8 + 1/2·6 = 4)forming a face centered cubic (fcc) unit cell and the axial axis ofthe octahedrons are oriented (alternating along c) in two differentdirections cutting the b axis at angles of 27.13(3)◦ and -47.31(2)◦.The two amino groups in the ligands participate in the formationof medium to weak hydrogen bonds with the nitrotetrazolate ringnitrogen atoms and the nitro-group oxygen atoms, which linkthe octahedrons formed by the coordination around the copperatoms together (Table 2). Fig. 3a shows the hydrogen bonding in

Fig. 2 View of the unit cell of 4 along the a axis (the hydrogen atoms havebeen left out for simplicity).

Fig. 3 (a) Hydrogen bonding in the structure of 4 (the copper atoms andsome of the hydrogen bonds have been omitted for simplicity). (b) View ofan R4,4(10) hydrogen bonding network. Symmetry codes: (i) 1 - x, -y, 1- z; (ii) -x + 3/2, y - 1/2, z; (iii) x + 1/2, -y + 1/2, -z + 1; (iv) -x + 1/2,y - 1/2, z; (v) 2 - x, -y, 1 - z; (vi) 1/2 + x, 1/2 - y, 1 - z; (vii) 1 - x, 1 - y,1 - z; (viii) -1/2 + x, 1/2 - y, 1 - z; (ix) -x + 3/2, y + 1/2, z.

the structure of the compound. Both amino groups in the ligand(N6H2 and N7H2) participate in the formation of three hydrogenbonds so that in total six different (weak) hydrogen bonds arefound in the structure. The NT- anions interact via hydrogenbonding by using all nitrogen atoms but N5,7,18 N1 (involved incoordination to the copper cations) and N2. The next atom toN2 is the hydrogen atom labelled as H6A (attached to N6) placedat 2.86(2) A, whereas the rest of the interactions summarized inTable 3 have distances between the hydrogen and the acceptoratoms below 2.6 A (sum of the van der Waals radii: rH + rN =2.75 A and rH + rO = 2.72 A).33

Using graph-set analysis as introduced by Bernstein34 the com-puter program RPLUTO35 allows to identify hydrogen bondingpatterns in molecules.8,36 The program finds the six hydrogen bondstabulated in Table 3 to form common dimmeric interactions of thetype D1,1(2) (Table 4). The hydrogen bond between N6 and O2iv

(symmetry code: (iv) -x + 1/2, y - 1/2, z) combines with the restat the secondary level to form exclusively chain motifs of the typeC2,2(X) where X = 6, 7, 10 and 11. The remainder of hydrogenbonds interact forming (mainly) ring graph-sets of variable sizes,which take the following labels: R2,1(X) (X = 3, 5), R2,4(X) (X =8, 14) and R4,4(X) (X = 10, 16, 20, 22). Fig. 3b show some ofthese hydrogen bonding networks. The R2,1(5) motif is formed

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Table 5 Physical properties and initial safety testing of the NT transition metal salts and comparison with lead azide

1 2 3 4 5 Pb(N3)2

Formula CN5O2Ag C3H5N15O8Cu C3H8N7O2Ag C6H16N14O4Cu C2H9N13O4Cu N6PbMW 221.91 441.47 282.01 411.83 342.02 291.24Tm/◦Ca — — — — — —Tdec/

◦Cb 273 230 212 225 245 315–360g

N (%)c 31.56 51.66 34.77 47.62 53.22 28.86X (%)d -7.2 -5.9 -51.1 -66.0 -25.7 -5.5Impact/Je <1 <3 >40 <7 <2 2.5–4.0h

Friction/Ne <5 <5 <240 <252 <18 0.1–1.0h

ESD (±)f + + - - - +Thermal shock Explodes Explodes Explodes Explodes Explodes Explodes

a Melting point (DSC onset) from measurement with b = 5 ◦C min-1 (NT salts) and from ref. 42 (lead azide). b Decomposition point (DSC onset) frommeasurement with b = 5 ◦C min-1 (NT salts) and from ref. 42 (lead azide). c Nitrogen content. d Oxygen balance, according to ref. 43. e Impact and frictionsensitivities determined by standard BAM methods (see ref. 37–39). f Rough sensitivity to 20 kV electrostatic discharge (ESD), + sensitive, - insensitivefrom an HF-Vacuum-Tester type VP 24. g Deflagration point. h Values for technically pure compound.

by chelation of one of the amino group protons in the en ligand(N6 ◊ ◊ ◊ N4iv = 3.199(2), N6 ◊ ◊ ◊ O2iv = 3.226(1) A; symmetry code:(iv) -x + 1/2, y - 1/2, z) whereas the R2,2(10) network is formedby the side-on interaction of two NT- anions and two en ligands(N7 ◊ ◊ ◊ N3ii = 3.128(1), N7 ◊ ◊ ◊ N4iii = 3.371(1) A; symmetry codes:(ii) -x + 3/2, y - 1/2, z; (iii) x + 1/2, -y + 1/2, -z + 1) and theyboth have been observed before in nitrogen-rich NT salts.8

Energetic properties

In order to assess the energetic properties of the NT salts, the ther-mal stability (decomposition points from DSC measurements),sensitivity to friction, impact, electrostatic discharge and thermalshock of each material was experimentally determined (Table 5).DSC measurements made on samples of ~1 mg of each energeticcompound in this study, show no melting points and high thermalstabilities above 210 ◦C, which are above those values for nitrogen-rich salts with the same anion.8 In addition to DSC analysis, allcompounds were tested by placing a small sample (~0.5–1.0 mg)of compound in the flame. In all five cases this resulted in a loudexplosion. Commonly used primary explosives such as AgN3 andPb(N3)2 also explode upon rapid heating. The data collected forfriction, impact and electrostatic discharge sensitivity testing arealso summarized in Table 5.37–39 The compounds in this studyare (in general) significantly more sensitive to friction and impactthan known nitrogen-rich salts of the NT- anion,7,8 similar toother metal salts with the same anion.18,40,41 Also, each compoundwas roughly tested for sensitivity to electrostatic discharge (ESDtesting) by spraying sparks across a small (3–5 crystals) sampleof material using a Tesla coil (~20 kV electrostatic dischargefrom an HF-Vacuum-Tester type VP 24). The sensitivity towardselectrostatic discharge seems to correlate well with the sensitivitytowards friction and the most friction sensitive compounds 1 and2 gave a positive test whereas the en adducts (3 and 4) and thecopper complex 5 failed to explode. From these results it seemslike the en adducts (3 and 4) are safer to handle than the “pure”5-nitrotetrazolate salts (1 and 2), which puts into perspectivetheir potential as NT- anion transfer reagents (see Discussionabove and Experimental). Note that, interestingly, compound 5can be easily initiated by impact, similar to lead azide (both~2 J)42 but is much less sensitive to friction, which has led to manyaccidental explosions associated with the use of lead azide in the

past. In addition, a radiation of ~300 mW can be used to detonatecompound 5 (copper salt 2 also explodes under these conditions).Lastly, from the sensitivity values reported here it can be derivedthat 1 and 2 fall under the category of primary explosives, i.e., theycan easily be initiated by impact, friction or heat. On the otherhand, 3 is safe for transport under the UN recommendations onthe transport of dangerous goods,39 whereas the diethylenamineadduct 4 is much less sensitive to impact and friction than coppersalt 2. Also compound 5 can easily be initiated by impact and anX-ray beam and therefore, this material has potential as a saferand more environmentally-friendly alternative to lead-based saltsin initiating systems.

Experimental

Caution!

All compounds described here are energetic materials, which mightexplode under certain conditions. Although we had no difficultiesduring the preparation and handling of the compounds describedbelow, they are nevertheless powerful explosives and their synthesisshould be carried out by experienced personnel, specially that ofcompounds 1, 2 and 5. In any case, proper protective measuressuch as Kevlar gloves, ear protection, safety shoes and plasticspatulas, should be taken at all times especially when working ona large scale (>1 g).

General

All chemical reagents and solvents of analytical grade were ob-trained from Sigma-Aldrich Inc. and used as supplied. 5-Amino-1H-tetrazolium bromide44 and 1,3-dimethyl-5-aminotetrazoliumiodide7a were prepared according to literature procedures. 1H, 13Cand 14N NMR spectra were recorded on a JEOL Eclipse 400instrument. The spectra were measured in DMSO-d6 at 25 ◦C.The chemical shifts are given relative to tetramethylsilane (1H,13C) or nitromethane (14N) as external standards. Infrared (IR)spectra were recorded on a Perkin-Elmer Spectrum One FT-IRinstrument as KBr pellets at room temperature.45 Raman spectrawere recorded on a Perkin-Elmer Spectrum 2000R NIR FT-Raman instrument equipped with a Nd:YAG laser (1064 nm).The intensities are reported in percentages relative to the most

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intense peak and are given in parentheses. Elemental analyses wereperformed with a Netsch Simultanous Thermal Analyzer STA429. Melting points were either determined using a melting pointapparatus (uncorrected) or by differential scanning calorimetry(Linseis DSC PT-10 instrument, calibrated with standard pureindium and zinc).46 Measurements were performed at a heatingrate of b = 2 ◦C min-1 in closed aluminium containers with a hole(1 mm) on the top for gas release with a nitrogen flow of 20 mLmin-1. The reference sample was a closed aluminium container.

Silver 5-nitrotetrazolate (1). Sodium 5-nitrotetrazolate dihy-drate (0.173 g, 1.00 mmol)9 was dissolved in 10 mL water and asmall excess of silver nitrate was added portion-wise (0.187 g,1.10 mmol) causing the precipitation of a white powder. Thereaction mixture was carefully stirred for 30 min under theexclusion of light and the insoluble solid was filtered off, washedwith water and left to air dry (0.212 g, 95%). CN5O2Ag (221.91 gmol-1, calcd/found): C 5.41/5.22, H–/0.04, N 31.56/31.42, Ag48.61/48.25%; DSC (5 ◦C min-1/◦C): 273 (dec.); m/z (FAB-,xenon, 6 keV, m-NBA matrix): 114.0 [CN5O2]-. Raman (rela-tive intensities) n/cm-1: 1567(5), 1432(100), 1414(50), 1330(21),1203(32), 1089(34), 1070(58), 1017(12), 837(18), 738(2), 545(9),256(2), 168(7). IR (KBr, relative intensities) n/cm-1: 1648 (w),1537 (vs), 1508 (w), 1446 (m), 1422 (s), 1384 (m), 1319 (m), 1187(w), 1170 (w), 1060 (w), 1050 (w), 1035 (w), 841 (m), 669 (w), 476(w).

Copper(II) 5-nitrotetrazolate 5-nitrotetrazole dihydrate (2). Asmall amount of 2 was isolated during the synthesis of sodium 5-nitrotetrazolate dihydrate18 and carefully analyzed. C3H5N15O8Cu(441.97 g mol-1, calcd/found): C 8.14/7.91, H 1.14/1.29, N47.52/47.41, Cu 14.24/13.88%; DSC (5 ◦C min-1/◦C): ~230 (dec.).Raman (relative intensities) n/cm-1: 3063(2), 2822(26), 1885(30),1871(33), 1435(100), 1130(56), 776(44), 420(46). IR (KBr, relativeintensities) n/cm-1: 3565 (s), 3478 (m), 1614 (w), 1565 (vs), 1445(m), 1435 (m), 1384 (m), 1328 (m), 1236 (w), 1065 (w), 842 (s), 664(w), 565 (w), 475 (w).

Silver(ethylendiamine) 5-nitrotetrazolate (3). Sodium 5-nitrotetrazolate dihydrate (0.173 g, 1.00 mmol) was dissolvedin 5 mL ethanol and a solution of ethylendiamine (0.085 g,1.41 mmol) in 2 mL ethanol was added at 60 ◦C. The solutionwas heated to boiling and a saturated solution of silver nitrate(0.170 g, 1.00 mmol) in 10 mL hot ethanol was added slowly. Thesolution turned brown immediately and it was stirred at reflux for2.5 h. After this time, the brown precipitate formed was filteredand discarded yielding a clear colourless solution from whichcrystals separated out upon cooling (0.221 g, 78%). C3H8N7O2Ag(282.01 g mol-1, calcd/found): C 12.78/12.68, H 2.86/2.77, N34.77/34.59, Ag 38.25/37.91%; DSC (5 ◦C min-1/◦C): 108.1(-en), ~212 (dec.); m/z (FAB+, xenon, 6 keV, m-NBA matrix):60.1 [Ag(en)]+; m/z (FAB-, xenon, 6 keV, m-NBA matrix):114.0 [CN5O2]-; 1H NMR (DMSO-d6, 400.18 MHz, 25 ◦C,TMS) d/ppm: 3.4 (s, 4H, NH2), 2.7 (s, 4H, CH2); 13C{1H} NMR(DMSO-d6, 100.63 MHz, 25 ◦C, TMS) d/ppm: 168.4 (1C, C-NT),43.3 (2C, CH2); 14N{1H} NMR (DMSO-d6, 40.55 MHz, 25 ◦C,MeNO2) d/ppm: +19 (2 N, n1/2 ~420 Hz, N2/3), -21 (1 N, n1/2

~60 Hz, NO2), -62 (2 N, n1/2 ~400 Hz, N1/4). Raman (relativeintensities) n/cm-1: 3284(3), 2986(36), 2946(5), 2877(1), 1600(3),1534(7), 1453(6), 1444(6), 1407(100), 1371(7), 1309(5), 1239(4),

1161(5), 1121(3), 1069(5), 1050(36), 1029(43), 1003(5), 845(5),832(9), 769(2), 537(3), 449(3), 262(3), 239(5). IR (KBr, relativeintensities) n/cm-1: 3427 (s), 3308 (s), 1587 (m), 1543 (s), 1484(m), 1444 (m), 1419 (m), 1384 (m), 1318 (m), 1172 (w), 1078 (w),1001 (w), 840 (m), 670 (w), 522 (w).

Bis(ethylendiamine)copper(II) 5-nitrotetrazolate (4). Sodium5-nitrotetrazolate dihydrate (0.177 g, 1.02 mmol) was dissolvedin 10 mL ethanol and a solution of ethylendiamine (0.100 g,1.67 mmol) in 5 mL ethanol was added dropwise under stirringat room temperature. The solution was heated to 70 ◦C anda solution of copper(II)chloride (0.068 g, 0.51 mmol) in 5 mLethanol was added causing immediate precipitation of a blue-lilac solid. The reaction mixture was boiled for 1 h and the lilacprecipitate was filtered and boiled again in fresh ethanol. Lastly, itwas filtered, washed with boiling ethanol and ether and left to airdry yielding the title compound (0.163 g, 77%). Crystals suitablefor X-ray analysis were obtained when an aqueous solutionof the compound was left to slowly evaporate. C6H16N14O4Cu(411.83 g mol-1, calcd/found): C 17.51/17.37, H 3.92/3.84, N47.69/47.85, Cu 15.31/15.02%; DSC (5 ◦C min-1, ◦C): ~225 (dec.);m/z (FAB+, xenon, 6 keV, m-NBA matrix): 183.2 [Cu(en)2]2+;m/z (FAB-, xenon, 6 keV, m-NBA matrix): 114.1 [CN5O2]-;Raman (relative intensities) n/cm-1: 3282(3), 2956(4), 1605(4),1535(7), 1460(8), 1442(6), 1407(100), 1319(7), 1284(5), 1174(3),1155(4), 1113(7), 1054(42), 1032(44), 886(10), 862(3), 839(12),774(2), 537(6), 476(8), 448(3), 299(7), 245(15). IR (KBr, relativeintensities) n/cm-1: 3450 (s), 3331 (vs), 3278 (s), 3154 (m), 2979(m), 2966 (m), 2893 (w), 2439 (w), 1593 (m), 1537 (vs), 1504 (w),1459 (w), 1441 (s), 1410 (s), 1384 (m), 1316 (m), 1281 (w), 1268(w), 1172 (w), 1155 (w), 1084 (w), 1053 (w), 1033 (s), 969 (w), 839(m), 719 (w), 669 (w), 536 (m), 471 (w).

Triaminocopper(II) 5-nitrotetrazolate (5). Sodium 5-nitrotetra-zolate dihydrate (0.346 g, 2.00 mmol) was dissolved in 3 mLof ammonia 35% and slowly added to a solution of copper(II)sulfate (0.160 g, 1.00 mmol) in 3 mL ammonia 35%. Afterrefluxing the reaction mixture for 15 min and stirring for further1 h at room temperature, a blue solid precipitated that waswashed with water and ethanol and left to air dry (0.269 g,79%). C2H9N13O4Cu (342.02 g mol-1, calcd/found): C 7.02/7.07,H 2.65/2.75, N 53.22/53.38, Cu 18.40/18.12%; DSC (5 ◦Cmin-1/◦C): ~245 (dec.); m/z (FAB+, xenon, 6 keV, m-NBA matrix):177.6 [Cu(CN5O2]+; m/z (FAB-, xenon, 6 keV, m-NBA matrix):114.1 [CN5O2]-, 405.7 [Cu(CN5O2)3]-. Raman (relative intensities)n/cm-1: 3351(3), 3282(12), 1540(10), 1445(15), 1416(100), 1317(6),1236(2), 1098(10), 1056(52), 1031(37), 839(14), 772(3), 536(5),449(4), 415(5), 264(4). IR (KBr, relative intensities) n/cm-1: 3355(s), 3278 (m), 2846 (w), 1612 (m), 1538 (vs), 1504 (w), 1442 (s),1427 (m), 1416 (s), 1384 (m), 1318 (s), 1264 (s), 1247 (s), 1213 (w),1186 (w), 1151 (w), 1054 (w), 1034 (w), 1021 (w), 840 (s), 772 (w),701 (m), 684 (m), 667 (m), 534 (w).

5-Amino-1H-tetrazolium 5-nitrotetrazolate (6). An improvedmethod to that reported in ref. 6 is presented here. 5-Amino-1H-tetrazolium bromide (1–10 mmol) was dissolved in waterand added to a suspension of 3 (1.0 equiv.) in the same solvent.Immediate precipitation of yellow silver bromide was observed andthe reaction mixture was stirred for 0.5–2.0 h under the exclusionof light. After this time the solvent was carefully evaporated

1840 | Dalton Trans., 2009, 1835–1841 This journal is © The Royal Society of Chemistry 2009

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under vacuum and at 50 ◦C leaving an off-white solid behind(quantitative yield). C2H4N10O2 (200.12 g mol-1, calcd/found): C12.00/11.73, H 2.01/1.72, N 69.99/69.45%; m.p. (uncorrected):157.0–158.7 ◦C (literature: 159 ◦C).

1,3-Dimethyl-5-aminotetrazolium 5-nitrotetrazolate (8). Animproved method to that reported in ref. 7a is presented here.Compound 8 was obtained as described for 6 from 1,3-dimethyl-5-aminotetrazolium iodide and 3 in an improved 94% yield.C4H8N10O2 (228.17 g mol-1, calcd/found): C 21.05/20.84, H3.53/3.32, N 61.39/61.07%; m.p. (uncorrected): 160.0–161.4 ◦C(literature: 160.5 ◦C).

Conclusions

The synthesis and characterization of two very sensitive transitionmetal 5-nitrotetrazole (NT) salts (1 and 2) and that of thecorresponding adducts with ethylendiamine (3 and 4) is presented.Compounds 1 and 2 have proofed to be very useful startingmaterials for the synthesis of NT salts, however, their increasedsensitivity makes their use on a larger scale problematic. Com-pounds 3 and 4 are introduced here as safer NT- anion transferreagents and the potential of 3 for the synthesis of tetrazolium saltsis exemplified. In addition, we have prepared a copper complexwith amino ligands containing the NT- anion (5), which can beeasily initiated by impact and by laser induction and offers a saferand more environmentally friendly alternative to commonly usedcompounds (e.g., lead azide).

Notes and references

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This journal is © The Royal Society of Chemistry 2009 Dalton Trans., 2009, 1835–1841 | 1841

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