Method for the preparation of tetranitrodibenzotetrazapentalene

Tetranitrodibenzo-1,3a,4,6a-tetrazapentalene is prepared from diaminobenzene by reacting diaminobenzene and lead dioxide in ethylacetate to produce diaminoazobenzene, isolating the product by silica-gel elution, reacting diaminoazobenzene with an inorganic azide in a dilute aqueous nitrous acid solution at a temperature not in excess of 10.degree. C. to produce diazidoazobenzene, isolating the product, heating diazidoazobenzene slowly to about 190.degree. C. until nitrogen stops evolving to form dibenzotetrazapentalene, isolating the product, nitrating dibenzotetrazapentalene with concentrated sulfuric acid and forming nitric at a temperature not in excess of about 65.degree. C. to product tetranitrodibenzo-1,3a,4,6a-tetrazapentalene.

BACKGROUND OF THE INVENTION 
The present invention pertains generally to organic energetic material 
synthesis and in particular to the synthesis of nitrated aromatic 
aza-compounds from diaminobenzene. 
One nitrated aromatic aza-compound that has exceptional thermal stability 
and high energy content is tetranitrodibenzo-1,3a,4,6a-tetrazapentalene. 
This compound is used as a primer for energetic gas-producing devices, 
especially for cartridges used in cartridge-activated devices, e.g. 
canopy-release for fighter aircraft. 
Presently the use of this compound has been restricted on account of the 
high cost. The compound is usually prepared by nitrating 
dibenzo-1,3a,4,6a-tetrazapentalene which is prepared by the method 
disclosed in U.S. Pat. No. 2,984,544, now U.S. Pat. No. Re. 25,238 and in 
U.S. Pat. No. 3,197,475 by Rudolf A. Carboni. Dibenzotetrazapentalene is 
prepared by the steps comprising oxidizing diaminoazobenzene to 
diazidoazobeneze by sodium azide and nitrous acid, and thermally cracking 
the azide substituents of diazidoazobenzene to form 
dibenzotetrazapentalene. 
The diaminoazobenzene synthesis is disclosed in Carboni et al. JACS 89 p. 
2618 (1967). By this method oxidation is carried out in benzene and 
product purification or isolation is achieved by extraction with methylene 
chloride. These two techniques contribute much to the high cost of the 
final product in that the reaction proceeds slowly, the yields are low, 
the purification is extremely slow, and the use of benzene requires many 
safety precautions. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to synthesize 
tetranitrodibenzo-tetrazapentalene from diaminobenzene without the use of 
benzene. 
Another object of this invention is to prepare tetranitrodibenzo 
tetrazapentalene with simplified purification procedures. 
A further object of the invention is to increase the yield and decrease the 
synthesis time for the preparation of tetranitrodibenzotetrazapentalene. 
These and other objects are achieved by oxidizing diaminobenzene with lead 
oxide in ethyl acetate to obtain diaminoazobenzene, purifying 
diaminoazobenze by extraction in methylene chloride through silica gel, 
diazidoing diaminoazobenzene by oxiding with nitrous acid followed by 
slowly reacting with sodium azide under high-shear mixing, slowly heating 
the diazide product to a temperature from about 170.degree. to about 
185.degree. C. to form dibenzotetrazapentalene, and nitrating 
dibenzotetrazapentalene by mixing therewith concentrated sulfuric acid 
under high-shear mixing following by a slow addition fuming nitric acid at 
a temperature from about -7.degree. to about 0.degree. C., followed by a 
rapid heating to a temperature from about 50.degree. to 65.degree. C. to 
quickly produce the tetra nitro product. 
DETAILED DESCRIPTION OF THE INVENTION 
The synthesis of the present invention proceeds in several stages, i.e. 
through several intermediate products. The principal synthesis steps are 
represented by the formulae: 
##STR1## 
The first intermediate is diaminoazobenzene DAAB which is prepared by 
forming a suspension of o-phenylenediamine in ethylacetate, admixing 
therewith an approximately stoichiometric amount of lead dioxide, and 
refluxing until the reaction is complete. The suspension is a dark color. 
The insoluble lead salts are easily separated by filtration, leaving a 
deep red orange filtrate. Purification of diaminoazobenzene proceeds by 
evaporating the filtrate to dryness, dissolving the filtrate in preferably 
methylene chloride, eluting the solution on silica gel (at least nine 
grams of silica gel per gram of solution and preferably from 10 to 15 
grams of silica gel per gram of solution, collecting the red fraction, and 
drying the fraction. Additional details of the purification technique is 
found in Chang et al. Functionality Distribution of Carboxyl- and 
Hydroxyl-Terminated Polybutadiene J. Macromol. Sci.-Chem A7(8), p. 1727-49 
(1973). 
The reaction forming diaminoazobenzene also forms the double azo compound 
and polymers. The use of ethyl acetate is most preferred because it allows 
the reaction to proceed at a relatively fast rate and high yield at a 
temperature that minimizes double azoing and polymerizing, and has few 
safety problems. Fluids, such as methylene chloride, cyclohexane, and 
tetrahydrofuran can produce a yield, but it is too low to significantly 
reduce the high cost of the end product. The reaction must be run at a 
temperature from about 20.degree. C. to about 100.degree. C. Thus solvents 
with a boiling point below 20.degree. C. can not be used. 
Diaminoazobenzene is reacted to O,O'-diazidoazobenzene by the prior art 
method with the one exception of high-shear mixing the reaction. By 
high-shear mixing, reference is made to mixers operating at least 4000 RPM 
to produce turbulent conditions in the fluid being mixed. An example of a 
high-shear mixer is the mixer by the Premier Mill Corporation that is 
marked as the Disperoator and uses a duplex head. The synthesis comprises 
the steps of forming a suspension of diaminoazobenzene, concentrated 
hydrochloric acid or sulfuric acid, and water at a temperature from about 
-5.degree. to 5.degree. C. under high-shear agitation, slowly admixing 
therewith sodium or potassium nitrate to form nitrous acid, slowly 
admixing therewith an aqueous solution of sodium azide at a temperature 
from about 05.degree. to about 10.degree. C., and continuing to high-shear 
mix until the reaction mixture reaches room temperature. The rate of 
addition of sodium azide, the temperature, and the high-shear mixing 
controls the evolution of nitrogen to an acceptable level. Nitrous acid 
can be used directly, but would present safety and corrosion problems. 
Instead of sodium azide, lithium or potassium azide can be used, but these 
azides are dangerous and much more expensive. The amount of water to be 
used is not critical but enough to keep the reaction mixture fluid. 
The diazide precipitates as yellow crystals, which are isolated in high 
purity by a simple filtration. Of course other separation techniques used 
to separate precipitates can be used. 
Dibenzo-1,3a,4,6a-tetrazapentalene is prepared by the steps comprising 
forming a suspension of diazide in decalin, heating the suspension to 
about 58.degree. C., whereupon nitrogen begins to evolve, slowly 
increasing the temperature at a rate of about 0.5 to about 2 degrees per 
minute to a temperature from about 160.degree. C. to about 190.degree. C., 
preferably from 175.degree. to 185.degree. C., and continue heating until 
nitrogen ceases evolving. 
Again the product can be easily isolated by filtration. Purity can be 
increased by recrystalizing the tetrazapentalene from cyclohexane. Other 
solvents, e.g. benzene, can be used but are not preferred on account of 
safety or cost. 
Nitration of the tetrazapentalene is difficult. The nitration method of 
this invention comprises slowly adding a suspension of 
dibenzotetrazapentalene and an concentrated solution of sulfuric acid to 
fuming nitric acid at a temperature from about -5.degree. C. to about 
5.degree. C., and heating the reaction mixture to a temperature from about 
55.degree. to about 65.degree. C. until the reaction is complete. The 
fuming nitric acid comprises concentrated nitric acid with dissolved 
nitrogen oxides. The amount of fuming acid is at least 25 weight percent 
of stoichiometry and preferably from 35 to 75 weight percent above 
stoichiometry. The amount of the sulfuric acid is the amount needed to 
dissolve pentalene compound. The concentration of the sulfuric acid is 
from about 80 to about 100 percent and preferably from 90 to 100 percent. 
The preferred separation method comprises pouring the reaction mixture in 
ice water to precipitate the tetra nitro compound, filtering, and 
recrystalizing from dimethyl formamide. The final purity is in excess of 
99.9 percent. 
To further illustrate the practice of the present invention the following 
example is given. It is understood that the example is given by way of 
illustration and is not meant to limit this disclosure or the claims to 
follow in any manner.

EXAMPLE 
A. Preparation of O,O'-Diaminoazobenzene (Solvent-Ethyl Acetate) (I) 
To a stirred mixture of 4.5 g (0.05 mole) of O-phenylenediamine and 200 ml 
of ethyl acetate was added 23.9 g (0.1 mole) of lead dioxide. The mixture 
was immediately heated to reflux for 2 hours. The insoluble lead salts 
were removed by filtration and the deep red orange filtrate was evaporated 
to dryness. The solid, was dissolved in methylene chloride and eluted on a 
silica gel column (10 g silica gel/g sample) with methylene chloride. 
Collecting the red fraction, and evaporating the methylene chloride to 
dryness gave 4 g (98% yield) of DAAB with mp 133.degree.-134.degree.. 
______________________________________ 
Anal. calcd. for C.sub.12 H.sub.12 N.sub.4 : 
C 67.92 Found C 67.86 
H 5.66 H 5.68 
N 26.42 N 26.45 
______________________________________ 
A'. Preparation of O,O'-Diaminoazobenzene (Solvent-1. tetrahydrofuran, 2. 
cyclohexane, 3. methylene chloride) 
The above procedure was repeated for each of the above solvents. 
Cyclohexane gave a 11 percent yield of low purity after about two hours of 
refluxing tetrahydrofuran gave a 24 percent yield of low purity after 
about three to four hours of refluxing. Methylene chloride gave an 80 
percent yield with a 90 percent purity after four hours of refluxing. 
B. Preparation of O'O'-Diazidoazobenzene (II) 
A solution of 3.5 g (0.05 mole) of sodium nitrite in 20 ml of water was 
added dropwise to a stirred mixture of 4.3 g (0.02 mole) of DAAB, 30 ml of 
concentrated hydrochloric acid, and 40 ml of water at 0.degree.-2.degree. 
C. The temperature was maintained below 5.degree. during the addition, 
stirring was continued for an additional 1/2 hour after the nitrite 
addition. Sodium azide (3.25 g, 0.05 mole) in 20 ml of water was slowly 
added to the diazonium solution at 0.degree.-5.degree. C. with continued 
vigorous stirring. Nitrogen evolved, and the yellow diazide precipitated 
during the addition. The mixture was stirred for an additional 2 hours (or 
until reaches to room temperature), then filtered to obtain the 
diazidoazobenzene (4.9 g) mp 100.degree.-111.degree. C. 
C. Preparation of Dibenzo-1,3a,4,6a-Tetrazapentalene (III) 
A suspension of 100 g of O,O'-diazidoazobenzene in 100 ml decalin in a 3000 
ml three-neck flask was slowly heated to 175.degree.-185.degree. C. One 
mole of nitrogen evolved at 58.degree. C.; the second mole of nitrogen 
evolved at 160.degree. C. The suspension was continued to be heated at 
175.degree.-185.degree. C. for three hours, cooled, and then filtered to 
obtain (III). It was recrystallized from cyclohexane or benzene. The 
yellow solid melted at 236.degree.-237.degree. C., the yield was 60 g. 
______________________________________ 
Anal. Calcd. for C.sub.12 H.sub.8 N.sub.4 
C 69.22 Found C 69.28 
H 3.87 H 3.95 
N 26.91 N 26.82 
______________________________________ 
D. Preparation of Tetranitrodibenzo-1,3a,4,6a-Tetrazapentalene (IV) 
A 1.0 g sample of dibenzotetrazapentalene in 10 ml of concentrated sulfuric 
acid was added dropwise under stirring to 15 ml of fuming nitric acid 
which had been cooled to 5.degree. C. After the addition was completed, 
the mixture was heated to 60.degree. C. for 5 minutes, and then poured 
into ice water. A orange red solid was separated and collected by 
filtration and dried to give 1.4 g of the (IV) which can be recrystallized 
from dimethyl formamide. Melting point above 360.degree. C. 
______________________________________ 
Found 
Found (recrystallized 
Anal. Calcd. for C.sub.12 H.sub.4 N.sub.8 O.sub.8 
(crude sample) 
sample) 
______________________________________ 
C 37.11 C 37.01; 37.00 
C 36.95; 37.19 
H 1.03 H 0.92; 0.96 
H 0.97; 0.95 
N 28.87 N 28.61; 28.75 
N 28.59; 28.61 
______________________________________ 
Preliminary estimates of the cost of 
tetranitrodibenzo-1,3a,4,6a-tetrazapentalene show the cost of this 
material by the subject method to be one twentieth to one thirtieth of the 
commercial cost of this material. The synthesis time has been decreased by 
a factor of 3 to 5 by this method. The nitration technique of the subject 
synthesis yields the tetranitro product in a high yield. By this technique 
a large portion of dibenzotetrazapentalene is nitrated to the tetranitro 
product rather than the more readily produced trinitro product without 
decomposing the starting material. 
Obviously, many modifications and variations of the present invention are 
possible in light of the above teachings. It is therefore to be understood 
that, within the scope of the appended claims, the invention may be 
practiced otherwise than as specifically described.