C2N14 An Energetic and Highly Sensitive Binary Azidotetrazole

Binary CN Compounds

DOI:10.1002/anie.201100300

C 2N 14:An Energetic and Highly Sensitive Binary Azidotetrazole**

Thomas M.Klap?tke,*Franz A.Martin,and J?rg Stierstorfer

Although binary CN compounds are of great interest,only a few examples are known,which is mostly due to the fact that their chemistry is very challenging.Binary CN compounds exhibit a large variety of characteristics;they can be very harmful owing to their toxicity,such as dicyanogen,[1]and they are thought to be very hard,as calculated for b -C 3N 4,[2]or show graphite-like nanostructures with good electric and catalytic properties,such as mpg-C 3N 4.[3]Furthermore,binary CN compounds composed of azides are highly sensitive towards shock,friction,and electrostatic discharge.

Investigations on these compounds started at the begin-ning of the 20th century when Ott and Ohse presented C 3N 12in 1921(Scheme 1)as the first binary azido heterocyclic system.[4]Research into heterocyclic azides was recently intensified,[5]as they present very good systems to study highly energetic materials enabled by high positive heats of formation.[5c,6]The high heats of formation derive from the energy input of the azide substituents (70kJ mol à1)[7]and from the large number of energetic N àN and C àN bonds combined in the heterocyclic ring systems.Non-heterocyclic binary CN systems have also attracted much interest,such as tetraazidomethane,[8,5e]which has an extreme sensitivity towards shock and friction,or the open form of the title compound C 2N 14,isocyanogentetraazide,[9]which is somewhat less sensitive than the title compound.[10]

To date,only the open form of C 2N 14was known,which can be synthesized by a metathesis reaction of isocyanogen-tetrabromide with sodium azide.[9]Herein,the synthesis of the closed form of C 2N 14,1-diazidocarbamoyl-5-azidotetrazole (1),is presented for the first time,being synthesized by diazotation of triaminoguanidinium chloride in water with two equivalents of sodium nitrite.A suggested mechanism of this reaction is presented in Scheme 2.

Various attempts using different reaction conditions always yielded 1as the kinetically stable product,but in different yields.To initiate the dimerization reaction and the following ring-closure reaction,[11]respectively,the acidic reaction solution is brought to pH 8slowly with 0.1m sodium hydroxide solution.Basic reaction conditions are very important in this reaction step,otherwise residual sodium nitrite can decompose the azide groups partially,forming amines as https://www.sodocs.net/doc/aa6649741.html,pound 1can be easily isolated by extraction of the reaction solution with diethyl ether followed by a purification step using short-column chromatography with CHCl 3as solvent to remove the decomposition products mentioned above.[12]Compound 1is obtained as a colorless crystalline solid after recrystallization from diethyl ether,and has a melting point at 788C and decomposition starting at 1108C.

Single crystals of 1suitable for X-ray diffraction measure-ments were obtained by recrystallization from diethyl https://www.sodocs.net/doc/aa6649741.html,pound 1crystallizes in the orthorhombic space group Pbcn with a cell volume of 1697.6(2) 3and eight molecules in the unit cell.[13]The bond lengths and angles in the tetrazole rings are in the normal range expected for an azidotetra-zole.[14]The N1àN8bond (1.403(4) )only slightly shorter than a formal N àN single bond (1.48 ),[15]while the N8àC2bond (1.288(5) )is in the range of a C àN double bond (1.22 ).[15]As shown for 5-azido-1H -tetrazole,the

azide

Scheme 1.Selected binary CN compounds:a)dicyanogen,b)tetraazi-domethane,c)triazidotriazine,d)diazidotetrazine,e)tetraazidoazotri-azine (TAAT),and f)C 2N 14(open

form).

Scheme 2.Possible reaction pathway leading to the formation of 1.

[*]Prof.Dr.T.M.Klap?tke,F.A.Martin,Dr.J.Stierstorfer Ludwig Maximilian University Munich (LMU)Department of Chemistry

Butenandtstrasse 5-13,Haus D,81377Munich (Germany)Fax:(+49)89-2180-77492

E-mail:tmk@cup.uni-muenchen.de

Homepage:http://www.chemie.uni-muenchen.de/ac/klapoetke/[**]Financial support of this work by the Ludwig-Maximilian University

of Munich (LMU)and the U.S.Army Research Laboratory (ARL)is gratefully

acknowledged.

Supporting information for this article is available on the WWW under

https://www.sodocs.net/doc/aa6649741.html,/10.1002/anie.201100300.

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group located on the 5position lies perfectly within the plane of the tetrazole ring.[5d]The asymmetric unit of 1is presented in Figure 1.

The carbamoyl diazide group in 1itself is twisted out of the plane of the tetrazole ring (N1,N2,N3,N4,C1)by 66.128

relative to the plane formed by N12,C2,N9,and N8.This twist within the molecule results in the buildup of 2D chains along the c axis which show a zigzag conformation with an angle of 113.228(Supporting Information,Figure S4).

Calculations of the electrostatic potential at the B3L YP/cc-pVDZ level of theory [16]in the gas phase show a clear charge distribution within 1,which is reflected in the structure.The positive charge is located on the azide moieties,with N b exhibiting the highest positive charge compared to N a and N g .The negative charge is mainly located on the N4,N3,and N2nitrogen atoms of the tetrazole ring,hence exhibiting a large inhomogeneity in the charge distribution (Supporting Information,Figure S5).

Short contacts are found between terminal nitrogen atoms N11and N13(3.125(6) )and between N7and N3(3.047(5) ),which are much shorter than the sum of van der Waals radii for nitrogen atoms (2r w (N)=3.2 ).[15]The bonding situation is shown in Figure 2.A very rare bonding situation can be observed in which the structure is formed exclusively by interactions between partially charged nitrogen atoms.

The 2D chains are stacked along the b axis with a distance of 5.993 between coplanar chains (every second chain,chains in between are rotated by 1808;Figure 3).The very dense packing is represented by a high density of 1=1.723g cm à3.The chains are connected through short N àN contacts,namely N9···N3at 3.051 and N9···N2at 3.001 ,also showing very strong electrostatic interactions between negatively and positively charged nitrogen atoms.[17]

IR and Raman spectra of 1were recorded in the solid state.For safety reasons,only a small number of crystals were measured (the compound decomposes explosively upon irradiation by a Nd:YAG laser with an intensity of only 150mW!).The IR frequencies were also calculated using the B3L YP/cc-pVDZ level of theory and fitted according to Witek and Keiji with a scaling factor of 0.9704.[18]The

theoretical values are in good agreement with the exper-imental data,in which the stretching modes of the azide groups were observed in the region between 2100and 2200cm à1.In both Raman and IR spectra,a splitting was observed.Stretching modes of the azide groups are observed at 2179cm à1,2165cm à1,and 2133cm à1(Raman)and 2175cm à1,2155cm à1,and 2133cm à1(IR;Figure 4).Even though we performed computational calculations regarding the stretching modes,we cannot clearly distinguish between the stretching modes for each individual azide group because the difference in the wavenumbers is too small.From the calculations of the IR spectra,we were able to see stretching motions of all three azide groups in 1for each of the frequencies mentioned above.For each IR band however,one azide group shows a much larger stretching motion than the other two.The calculated frequencies and intensities are compiled in the Supporting Information,Table

S3.

Figure 2.Short N àN contacts,which correspond to electrostatic interactions.Thermal ellipsoids are set at 50%

probability.

Figure 1.ORTEP representation of 1.Thermal ellipsoids are set at 50%probability.Selected crystallographic data:orthorhombic,Pbcn ;Z =8,a =18.1289(1),b =8.2128(7),c =11.4021(9) ,a =b =g =908,V =1697.6(2) 3

.

Figure 3.Stacking of 2D chains along the b axis.ORTEP representation shown along the a axis with ellipsoids set at 50%probability.

Communications

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C and 14N NMR spectroscopy studies reveal clearly assignable peaks for the corresponding carbon or nitrogen atoms.As the carbamoyl diazide group can rotate freely around the N1àN8bond in solution,only two signals are observed in the 14N spectra regarding the N b nitrogen atoms of the three azide groups (N6,N10,N13).The N a signals can be observed,but lead to a very broad signal.The N g signals were also observed as a very broad signal,but are overlapped by the two N b peaks.If the solvent is changed from CDCl 3to [

D 6]DMSO,only one broader peak can be observed for the three N b atoms.

The sensitivity of C 2N 14is beyond our capabilities of measurement.The smallest possible loadings in shock and friction tests led to explosive decomposition.It must be stated that the shock and friction sensitivity of 1no doubt lies well under the limits of 0.25J in impact and 1N in friction sensitivity that can be experimentally determined (Table 1).

This sensitivity is thought to be due to the enormous inequality in the charge distribution,which is known to be responsible for such an increase in sensitivity.[19]Additionally,owing to the extremely high heat of formation (1495kJ mol à1),which is higher than most known heats of formation for CN systems,[5c]and the very high nitrogen content of 89.08%,compound 1is very powerful and has to be handled with extreme care!

Received:January 13,2011Published online:April 6,2011

.

Keywords:azides ·binary CN compounds ·energetic materials ·heterocycles ·nitrogen

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along with the complete compilation of analytical data are included in the Supporting Information.

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Supporting Information.

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Figure https://www.sodocs.net/doc/aa6649741.html,parison of the IR and Raman spectra of 1.The three individual stretching modes for each of the azide groups are identified (expansion shown in the ellipse).

Table 1:Compiled sensitivities,calculated heats of formation,and detonation parameters for 1.[a]IS [J]FS [N]1[gcm à3]D H f 0(s)[kJ mol à1]Q v [kJ kg à1]P C–J [kbar]V det [m s à1]<0.25

<1

1.723

1495

à6855

339

8960

[a]IS =impact sensitivity,FS =friction sensitivity,D H f 0=heat of forma-tion,Q v =heat of explosion,P C–J =detonation pressure at the Chapman–Jouguet point,V det =detonation

velocity.

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