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Synlett 2013; 24(17): 2183-2187
DOI: 10.1055/s-0033-1339854
synpacts
© Georg Thieme Verlag Stuttgart · New York

Recent Advanced Strategies for Extending the Nitrogen Chain in the Synthesis of High Nitrogen Compounds

Yongxing Tang
School of Chemical Engineering, Nanjing University of Science and Technology, Xiaolingwei 200, Nanjing, Jiangsu, P. R. of China   Fax: +86(25)84303286   Email: hyang@mail.njust.edu.cn   Email: gcheng@mail.njust.edu.cn
,
Hongwei Yang*
School of Chemical Engineering, Nanjing University of Science and Technology, Xiaolingwei 200, Nanjing, Jiangsu, P. R. of China   Fax: +86(25)84303286   Email: hyang@mail.njust.edu.cn   Email: gcheng@mail.njust.edu.cn
,
Guangbin Cheng*
School of Chemical Engineering, Nanjing University of Science and Technology, Xiaolingwei 200, Nanjing, Jiangsu, P. R. of China   Fax: +86(25)84303286   Email: hyang@mail.njust.edu.cn   Email: gcheng@mail.njust.edu.cn
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Further Information

Publication History

Received: 08 June 2013

Accepted after revision: 26 August 2013

Publication Date:
20 September 2013 (online)

 
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Abstract

The synthesis and handling of compounds containing long catenated nitrogen chains are challenging for researchers in this field. These challenges arise from their high endothermic, thermodynamic instability, and a serious lack of nitrogen–nitrogen bond-forming reactions. This paper addresses techniques of azo-coupling reaction between the diazonium salt of N-NH2 and amine derivative to form the longest nitrogen chain (N11). This type of nitrogen–nitrogen bond formation opens a new strategy for the construction of novel compounds containing odd or even numbers of catenated nitrogen atoms (≥9) especially for polynitrogen compounds.


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Key words

azo coupling - energetic materials - catenated nitrogen-atom chain - formation of nitrogen–nitrogen bond - tetrazole

Compounds containing long catenated nitrogen-atom chains and polynitrogen compounds are attracting and challenging in research areas such as propellants, explosives, and pyrotechnics. This attraction is due to their high heat of formation and environmentally bening decomposition products of nitrogen gas.

However, there are also two challenges in synthesizing and handling high nitrogen compounds containing directly linked N–N single or double bonds for chemists in this fascinating field. One arises from their high endothermic and thermodynamic unstability. The triple bond in molecular nitrogen (N2) with 942 kJ/mol bond energy is characterized as the strongest bond in nature. The sum of the bond energies of the N–N single and double bonds possesses much less than that of triple bonds. As a result, energetic compounds containing catenated N–N single and double bonds could be easily triggered to produce diatomic nitrogen gas accompanied by a very large energy release.[2] Therefore, the larger the string of the catenated nitrogen–nitrogen bond, the more unstable the compound is. The other challenge results from a serious lack of nitrogen–nitrogen bond-forming reactions in comparison with carbon analogues. With the development of synthetic organic chemistry, chemists have a wealth of methods to construct C–C and C–N bonds. In comparison, methods for the synthesis of nitrogen–nitrogen bonds are extremely limited.

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Yongxing Tang (left)was born in Anhui, China in March, 1987. He received his BSc from Xihua University. Now he is perusing his doctoral research under the supervision of both Prof. Guangbin Cheng and Associate Prof. Hongwei Yang.
Guangbin Cheng (middle) was born in Anhui, China in October, 1967. He received his MSc from East China Normal University in 1997 and PhD from Nanjing University of Science and Technology in applied chemistry in 2002 under the supervision of Prof. Chunxu Lu. Then he joined the faculty in School of Chemical Engineering at Nanjing University of Science and Technology. He has been at the Georg-August-University of Goettingen in Germany to do research as a visiting scholar from August, 2004 to August, 2005. His current research interest is the green nitration and the synthesis of novel energetic compounds by exploring new methods of N–N formation.
Hongwei Yang (right) was born in Xinjiang, China in March, 1979. She received her MSc from Xinjiang University in 2004 and PhD from Nanjing University in organic chemistry in 2007 under the supervision of Prof. Chengjian Zhu. Then she has been at the Lab for Translational Research at Harvard University to do postdoctoral research with Prof. Michael Chroev from March 2008 to October 2010. In November 2010, she joined the faculty in School of Chemical Engineering at Nanjing University of Science and Technology. Her current research interest is the synthesis of novel energetic compounds and asymmetric catalysis of organic chemistry especially molybdenum complexes as catalyst.

Recently, the oxidative azo coupling of the N-NH2 functionality of heteroaromatic rings has been developed to be an efficient method to form a rather long chain of catenated tetrazene structure (N–N=N–N linkage, Scheme [1]).[3] In 2010, an important breakthrough was achieved by the group of Pang et al. They reported this effective synthesis method for 1,1′-azobis-1,2,3-triazole, which contains eight directly linked nitrogen atoms (N8 structure), by the treatment of 1-amino-1,2,3-triazole with sodium dichloroisocyanurate (SDCI) at low temperature (1a → 1, Scheme [1]).[3a] [b] Subsequently, 1,1′-azobistetrazole (2) with N10 structure[3c] and 2,2′-azobis(5-nitrotetrazole) (3) with catenated N8 structure[3d] were synthesized sequentially with the same azo-coupling reagent SDCI. Unfortunately, the nitrogen chains of both 2 and 3 are thermally unstable with low decomposition temperature and physical unstability in solution. Recently, our group synthesized 1,1′-azobis(5-methyltetrazole) (4) containing the relative stable N10 structure.[3e] We also expanded the azo-coupling reagents of the N-NH2 functionality, including trichloroisocyanuric acid (TCICA), SDCI, and tert-butyl hypochlorite (t-BuOCl). Although compound 4 exhibits better thermal stability and physical stability in various solvents than 2 and 3, compound 4 is still very sensitive toward impact, friction, and electrostatic discharge.

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Scheme 1 Tetrazene structure by oxidative azo-coupling reaction[3]

Another efficient way to increase the length of a nitrogen chain is the electrophilic N-amination of heterocycles including pyrazoles,[4] triazoles,[5] and tetrazoles.[6] The addition of an amino group to the nitrogen atom of energetic azoles not only improves the stability but also increases the heat of formation. Aromatic heterocycles with two or more linked cyclic nitrogen atoms are aminated with O-hydroxylamine derivatives[7] to form N-amino derivatives. In 2010, Graindorge et al. reported a successful example of N-amination for 3,4,5-trinitropyrazolate with low reactivity of electron-poor systems by O-picrylhydroxylamine (Pic-O-NH2) as amine reactant in 26% yield (Scheme [2]).[8] Subsequently, groups of Klapötke[3a] [9] and Shreeve[10] reported that a series of nitrogen-rich anionic heterocycles was aminated with O-tosylhydroxyamine (THA) and ­Pic-O-NH2, respectively (Scheme [3]). In particular, triazoles and tetrazoles bearing strong electron-withdrawing groups were also well aminated, which expand the amination substrate scope.

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Scheme 2 Extending the nitrogen chain by N-amination
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Scheme 3 Amination reations with different amine reagents

In addition, the prepared N-amino energetic materials could be reacted with a nitrating reagent to form the N-bound heterocyclic nitramine compounds with longer nitrogen chains. The introduction of the nitro group tends to decrease in thermal stability and increase in sensitivity, but contributes significantly to the overall energetic performance.[11] Klapötke reported that the 1,3-bis(nitro­imido)-1,2,3-triazolate anion, containing a chain of seven nitrogen atoms with alternating positive/negative charges, was synthesized by the nitration of 1,3-diamino-1,2,3-triazolium nitrate with nitronium tetrafluoroborate (Scheme [4]).[9a] This methodology offers unique insight into extending the nitrogen chain for high-nitrogen systems from both academic and practical perspectives. Klapötke and co-authors also synthesized the N-nitrimine compounds 5-amino-1-nitrimino-4H-tetrazole and 5-amino-4-methyl-1-nitriminotetrazole by the nitration of 1,5-diaminotetrazole (DAT) and 1,5-diamino-4-methyltetrazole (MeDAT) using one equivalent of nitronium tetrafluoroborate in dry acetonitrile at 0 °C (Scheme [4]).[12] Additionally, Li et al. found that a mixture of fuming nitric acid and concentrated sulfuric acid (98%) could be a suitable nitration agent to obtain 5-amino-1-nitrimino-4H-tetrazole.[13]

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Scheme 4 Extending nitrogen chain by nitration reaction

In recent years, Christe successfully synthesized and characterized several novel polynitrogen cations, such as the V-shaped N5 + cation,[14] the N3NOF+ cation both as a Z- and E-isomers,[15] and the N7O+ cation.[16] All the catenated nitrogen cations were obtained by the substitution reaction of [N–F]+ with HN3 to replace the F atom by an azido group (Scheme [5]). In synthesizing these cations, it is useful to find energetic starting materials that already possess the energy-enhancing weakened bonds, the required formal charges, and suitable ligands that allow for an exothermic and facile coupling reaction. The N2F+ cation, the [F2NO]+ cation, and the [N3NFO]+ cation are very suitable as starting materials because they already possess the desired types of bonds and provide the formal positive charge. In view of the weak N–F and strong H–F bond, the HF elimination reaction is expected to be exothermic. It is worth noting that all attempts to isolate N7O+ had been unsuccessful since the N7O+ cation is thermally very unstable and decomposes instantaneously to N5 + and N2O.

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Scheme 5 Extending the nitrogen chain by substitution reaction

In this Synpacts article we wish to describe our efforts on developing a new nitrogen–nitrogen bond-forming method to synthesize a nitrogen-rich energetic salt containing the longest reported nitrogen chain (N11).[17]

Initially, 3H-tetrazolo[1,5-d]tetrazole (5, Scheme [6]) with the N7 structure aroused our interest.[18] Our attempts to prepare 5 following the reported literature procedure failed by treatment of DAT with sodium nitrite in concentrated HCl solution.[21] However, TLC demonstrated that the reaction occurred. We tried to change the reaction conditions by regulating the molar ratio of DAT to NaNO2 and the concentration of the HCl solution during the reaction. To our surprise, high yield of 7 (i.e. chloride salt of 6 together with 5-aminotetrazole) was separated from solution after six optimal concentration of HCl solution trials. The single crystals of 7, suitable for X-ray crystallographic analysis, were obtained by slow recrystallization from ethanol at room temperature (Figure [1]). By substituting concentrated HCl with 10% aqueous H2SO4 or HNO3 in the reaction, the salt of 6 was not obtained and decomposed into DAT and ATA.

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Scheme 6 Possible reaction pathway to the formation of 7
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Figure 1 Molecular structure of 7 at 50% probability ellipsoids

A possible reaction pathway to the formation of 7 is proposed in Scheme [6]. The formation of 6 is probably due to the azo-coupling reaction of the diazonium salt of DAT and undiazotized DAT in acidic solution. The amino group in the 1-position of DAT, the amino group attached to the nitrogen atom, is easier to transform into the diazonium ion compared with C-NH2 under acidic conditions. The amino group at the 1-position of DAT is ‘hydrazinic’ in character so that it has a higher reactivity than that of at the 5-position.[19] Electrophilic attack of the amino group of DAT with a diazonium salt gives rise to the corresponding product. Formation of a new nitrogen–nitrogen bond is the result of the reaction between relatively electron-poor and electron-rich nitrogen atoms. The adduct of ATA was formed following the diazotization–deamination sequence in which the diazonium group of the unreacted diazonium salt DAT was replaced by a hydrogen atom through treatment with ethanol.

Azo-coupling reaction between the diazonium salt of C-NH2 and amine derivative to form the triazene structure (N=N–NH linkage) has been well investigated,[20] while that of N-NH2, instead of C-NH2 was scarcely reported. We think this is the first successful example of an azo-coupling reaction between the diazonium salt of N-NH2 and an amine derivative.

The principal synthesis of triazene is the coupling of a diazonium salt with primary and secondary amines in weak acidic aqueous solution. Compared with the diazonium salt of C-NH2 that of N-NH2 is quite unstable toward acids and decomposes into nitrogen gas and amine derivatives. Therefore, in the course of the azo-coupling reaction between the diazonium salt of N-NH2 and the amine derivative, acidity and low temperature should be strictly controlled. In addition, substrates which are easily protonated under weak acid conditions are not expedient for azo coupling. When the aqueous solution of 7 was adjusted into a neutral pH value by treatment of NaHCO3, the organic compound 6 was decomposed immediately into DAT and ATA, along with the relase of N2 gas. This fact illustrates that the organic molecule 6 with an acyclic–cyclic eleven nitrogen chain is unstable, which is in good agreement with the experiment.

The discovery of 7 was an unexpected success, with the development of the first example for an azo-coupling reaction between the diazonium salt of N-NH2 and amine derivative to form the longest nitrogen chain (N11). We think that at least three aspects are worth future endeavor: (1) Further studies on stabilization of the organic N11 compound like 6. Experiments and theory showed that compound 6 with the N11 structure is unstable. This problem may be also solved by coordination with a metal atom. (2) Further expanding application scope of this new nitrogen–nitrogen bond-formation reaction. The compounds with an odd number of catenated nitrogen atoms (≥9) are difficult to be synthesized by existing nitrogen–nitrogen bond-forming reactions. This type of nitrogen–nitrogen bond-formation opens a new method for the syntheses of the compound containing the catenated odd or even number nitrogen (≥9) especially for polynitrogen compounds. (3) Continuous exploration of new nitrogen–nitrogen bond-forming reactions to extend nitrogen chains. The goal of this exploration of the high nitrogen compounds is to provide the diversity of strategies to synthesize compounds with catenated linked nitrogen atoms.


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Acknowledgment

This work was supported by the Natural Science Foundation of Jiangsu Province (BK2011696) and the National Natural Science Foundation of China (No. 21376121).



   Permissions and Reprints All articles of this category
Zoom Image
Yongxing Tang (left)was born in Anhui, China in March, 1987. He received his BSc from Xihua University. Now he is perusing his doctoral research under the supervision of both Prof. Guangbin Cheng and Associate Prof. Hongwei Yang.
Guangbin Cheng (middle) was born in Anhui, China in October, 1967. He received his MSc from East China Normal University in 1997 and PhD from Nanjing University of Science and Technology in applied chemistry in 2002 under the supervision of Prof. Chunxu Lu. Then he joined the faculty in School of Chemical Engineering at Nanjing University of Science and Technology. He has been at the Georg-August-University of Goettingen in Germany to do research as a visiting scholar from August, 2004 to August, 2005. His current research interest is the green nitration and the synthesis of novel energetic compounds by exploring new methods of N–N formation.
Hongwei Yang (right) was born in Xinjiang, China in March, 1979. She received her MSc from Xinjiang University in 2004 and PhD from Nanjing University in organic chemistry in 2007 under the supervision of Prof. Chengjian Zhu. Then she has been at the Lab for Translational Research at Harvard University to do postdoctoral research with Prof. Michael Chroev from March 2008 to October 2010. In November 2010, she joined the faculty in School of Chemical Engineering at Nanjing University of Science and Technology. Her current research interest is the synthesis of novel energetic compounds and asymmetric catalysis of organic chemistry especially molybdenum complexes as catalyst.
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Scheme 1 Tetrazene structure by oxidative azo-coupling reaction[3]
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Scheme 2 Extending the nitrogen chain by N-amination
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Scheme 3 Amination reations with different amine reagents
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Scheme 4 Extending nitrogen chain by nitration reaction
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Scheme 5 Extending the nitrogen chain by substitution reaction
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Scheme 6 Possible reaction pathway to the formation of 7
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Figure 1 Molecular structure of 7 at 50% probability ellipsoids