The present application is a continuation-in-part of copending application Ser. No. 08/101,396 now allowed, filed Aug. 2, and entitled "BITETRAZOLEAMINE GAS GENERANT COMPOSITIONS AND METHODS OF USE," which application is incorporated herein by this reference.
FIELD OF THE INVENTIONThe present invention relates to novel gas generating compositions for inflating automobile air bags and similar devices. More particularly, the present invention relates to the use of substantially anhydrous aminotetrazole (5-aminotetrazole) as a primary fuel in gas generating pyrotechnic compositions, and to methods of preparation of such compositions.
BACKGROUND OF INVENTIONGas generating chemical compositions are useful in a number of different contexts. One important use for such compositions is in the operation of "air bags." Air bags are gaining in acceptance to the point that many, if not most, new automobiles are equipped with such devices. Indeed, many new automobiles are equipped with multiple air bags to protect the driver and passengers.
In the context of automobile air bags, sufficient gas must be generated to inflate the device within a fraction of a second. Between the time the car is impacted in an accident, and the time the driver would otherwise be thrust against the steering wheel, the air bag must fully inflate. As a consequence, nearly instantaneous gas generation is required.
There are a number of additional important design criteria that must be satisfied. Automobile manufacturers and others set forth the required criteria which must be met in detailed specifications. Preparing gas generating compositions that meet these important design criteria is an extremely difficult task. These specifications require that the gas generating composition produce gas at a required rate. The specifications also place strict limits on the generation of toxic or harmful gases or solids. Examples of restricted gases include carbon monoxide, carbon dioxide, NOx, SOx, and hydrogen sulfide.
The automobile manufacturers have also specified that the gas be generated at a sufficiently and reasonably low temperature so that the occupants of the car are not burned upon impacting an inflated air bag. If the gas produced is overly hot, there is a possibility that the occupant of the motor vehicle may be burned upon impacting a just deployed air bag. Accordingly, it is necessary that the combination of the gas generant and the construction of the air bag isolates automobile occupants from excessive heat. All of this is required while the gas generant maintains an adequate burn rate. In the industry, burn rates in excess of 0.5 inch per second (ips) at 1,000 psi, and preferably in the range of from about 1.0 ips to about 1.2 ips at 1,000 psi, are generally desired,
Another related but important design criteria is that the gas generant composition produces a limited quantity of particulate materials. Particulate materials can interfere with the operation of the supplemental restraint system, present an inhalation hazard, irritate the skin and eyes, or constitute a hazardous solid waste that must be dealt with after the operation of the safety device. These features are undesirable aspects of the present sodium azide materials, but are presently tolerated in the absence of an acceptable alternative.
In addition to producing limited, if any, quantities of particulates, it is desired that at least the bulk of any such particulates be easily filterable. For instance, it is desirable that the composition produce a filterable, solid slag. If the solid reaction products form a stable material, the solids can be filtered and prevented from escaping into the surrounding environment. This also limits interference with the gas generating apparatus and the spreading of potentially harmful dust in the vicinity of the spent air bag which can cause lung, mucous membrane and eye irritation to vehicle occupants and rescuers.
Both organic and inorganic materials have also been proposed as possible gas generants. Such gas generant compositions include oxidizers and fuels which react at sufficiently high rates to produce large quantities of gas in a fraction of a second.
At present, sodium azide is the most widely used and accepted gas generating material. Sodium azide nominally meets industry specifications and guidelines. Nevertheless, sodium azide presents a number of persistent problems. Sodium azide is relatively toxic as a starting material, since its toxicity level as measured by oral rat LD50 is in the range of 45 mg/kg. Workers who regularly handle sodium azide have experienced various health problems such as severe headaches, shortness of breath, convulsions, and other symptoms.
In addition, sodium azide combustion products can also be toxic since molybdenum disulfide and sulfur are presently the preferred oxidizers for use with sodium azide. The reaction of these materials produces toxic hydrogen sulfide gas, corrosive sodium oxide, sodium sulfide, and sodium hydroxide powder. Rescue workers and automobile occupants have complained about both the hydrogen sulfide gas and the corrosive powder produced by the operation of sodium azide-based gas generants.
Increasing problems are also anticipated in relation to disposal of unused gas-inflated supplemental restraint systems, e.g. automobile air bags in demolished cars, The sodium azide remaining in such supplemental restraint systems can leach out of the demolished car to become a water pollutant or toxic waste. Indeed, some have expressed concern that sodium azide, when contacted with battery acids following disposal, forms explosive heavy metal azides or hydrazoic acid.
Sodium azide-based gas generants are most commonly used for air bag inflation, but with the significant disadvantages of such compositions many alternative gas generant compositions have been proposed to replace sodium azide. Most of the proposed sodium azide replacements, however, fail to deal adequately with each of the selection criteria set forth above.
One group of chemicals that has received attention as a possible replacement for sodium azide includes tetrazoles and triazoles. These materials are generally coupled with conventional oxidizers such as KNO3 and Sr(NO3)2. Some of the tetrazoles and triazoles that have been specifically mentioned include 5-aminotetrazole, 3-amino-1,2,4-triazole, 1,2,4-triazole, 1H-tetrazole, bitetrazole and several others. However, because of poor ballistic properties and/or high gas temperatures, none of these materials has yet gained general acceptance as a sodium azide replacement.
It will be appreciated, therefore, that there are a number of important criteria for selecting gas generating compositions for use in automobile supplemental restraint systems. For example, it is important to select starting materials that are not toxic. At the same time, the combustion products must not be toxic or harmful. In this regard, industry standards limit the allowable amounts of various gases produced by the operation of supplemental restraint systems.
It would, therefore, be a significant advancement in the art to provide compositions capable of generating large quantities of gas that would overcome the problems identified in the existing art. It would be a further advancement to provide gas generating compositions which are based on substantially nontoxic starting materials and which produce substantially nontoxic reaction products. It would be another advancement in the art to provide gas generating compositions which produce limited particulate debris and limited undesirable gaseous products. It would also be an advancement in the art to provide gas generating compositions which form a readily filterable solid slag upon reaction.
Such compositions and methods for their use are disclosed and claimed herein.
SUMMARY AND OBJECTS OF THE INVENTIONThe novel solid compositions of the present invention include a non-azide fuel and an appropriate oxidizer. Specifically, the present invention is based upon the discovery that improved gas generant compositions are obtained using substantially anhydrous 5-aminotetrazole, or a salt or a complex thereof, as a non-azide fuel. The compositions of the present invention are useful in supplemental restraint systems, such as automobile air bags.
It will be appreciated that 5-aminotetrazole generally takes the monohydrate form. However, gas generating compositions based upon hydrated tetrazoles have been observed to have unacceptably low burning rates. Accordingly, the present invention is related to the use of 5-aminotetrazole in its anhydrous or substantially anhydrous form.
The methods of the present invention teach manufacturing techniques whereby the processing problems encountered in the past can be minimized. In particular, the present invention relates to methods for preparing acceptable gas generating compositions using anhydrous 5-aminotetrazole. In one embodiment, the method entails the following steps:
(a) obtaining a desired quantity of gas generating material, said gas generating material comprising an oxidizer and hydrated 5-aminotetrazole;
(b) preparing a slurry of said gas generating material in water;
(c) drying said slurried material to a constant weight;
(d) pressing said material into pellets in hydrated form; and
(e) drying said pellets such that the gas generating material is in anhydrous or substantially anhydrous form.
Importantly, the methods of the present invention provide for pressing of the material while still in the hydrated form. Thus, it is possible to prepare acceptable gas generant pellets. If the material is pressed while in the anhydrous form, the pellets are generally observed to powder and crumble, particularly when exposed to a humid environment.
Following pressing of the pellets, the gas generating material is dried until the tetrazole is substantially anhydrous. Generally, the hydrated 5-aminotetrazole composition loses about 3% to 5% of its weight during the drying process. The 5-aminotetrazole itself loses about 17% of its weight (theoretical weight loss is 17.5%). This is found to occur, for example, after drying at 110° C. for 12 hours. A material in this state can be said to be anhydrous for purposes herein. Of course the precise temperature and length of time of drying is not critical to the practice of the invention, but it is presently preferred that the temperature not exceed 150° C. FIG. 1 illustrates a typical 5-aminotetrazole drying curve at 35° C.
Pellets prepared by this method are observed to be robust and maintain their structural integrity when exposed to humid environments. In general, pellets prepared by the preferred method exhibit crush strengths in excess of 10 lb load in a typical configuration (3/8 inch diameter by 0.07 inches thick). This compares favorably to those obtained with commercial sodium azide generant pellets of the same dimensions, which typically yield crush strengths of 5 lb to 15 lb load.
The present compositions are capable of generating large quantities of gas while overcoming various problems associated with conventional gas generating compositions. The compositions of the present invention produce substantially nontoxic reaction products. The present compositions are particularly useful for generating large quantities of a nontoxic gas, such as nitrogen gas. Significantly, the present compositions avoid the use of azides, produce no sodium hydroxide by-products, generate no sulfur compounds such as hydrogen sulfide and sulfur oxides, and still produce a nitrogen containing gas.
The compositions of the present invention also produce only limited particulate debris, provide good slag formation and substantially avoid, if not avoid, the formation of nonfilterable particulate debris. At the same time, the compositions of the present invention achieve a relatively high burn rate, while producing a reasonably low temperature gas. Thus, the gas produced by the present invention is readily adaptable for use in deploying supplemental restraint systems, such as automobile air bags.
BRIEF DESCRIPTION OF THE DRAWINGFIG. 1 is a graph of a drying curve for 5-aminotetrazole at 35° C.
DETAILED DESCRIPTION OF THE INVENTIONThe present invention relates to the use of substantially anhydrous 5-aminotetrazole (sometimes referred to herein as "5-AT"), or a salt or a complex thereof, as the primary fuel in a novel gas generating composition. As used herein, substantially anhydrous 5-aminotetrazole is defined as hydrated 5-AT which has lost not less than about 14% of its weight during drying and more preferably about 17% of its weight during drying. The salts or complexes of 5-aminotetrazole may including, for example, those of transition metals such as copper, cobalt, iron, titanium, and zinc; alkali metals such as potassium and sodium; alkaline earth metals such as strontium, magnesium, and calcium; boron; aluminum; and nonmetallic cations such as ammonium, hydroxylammonium, hydrazinium, guanidinium, aminoguanidinium, diaminoguanidinium, triaminoguanidinium, or biguanidinium.
In the compositions of the present invention, the fuel is paired with an appropriate oxidizer. Inorganic oxidizing agents are preferred because they produce a lower flame temperature and an improved filterable slag. Such oxidizers include metal oxides and metal hydroxides. Other oxidizers include metal nitrates, metal nitrites, metal chlorates, metal perchlorates, metal peroxides, ammonium nitrate, ammonium perchlorate and the like. The use of metal oxides or hydroxides as oxidizers is particularly useful and such materials include for instance, the oxides and hydroxides of copper, cobalt, manganese, tungsten, bismuth, molybdenum, and iron, such as CuO, Co2 O3, Fe20 O3, MoO3, Bi2 MoO6, Bi2 O3, and Cu(OH)2. The oxidizer may also be a mixture of the above-referenced oxidizing agents, or the above-referenced oxidizing agents and other oxidizing agents. For example, the oxide and hydroxide oxidizing agents mentioned above can, if desired, be combined with other conventional oxidizers such as Sr(NO3)2, NH4 ClO4, and KNO3, for a particular application, such as, for instance, to provide increased flame temperature or to modify the gas product yields.
A presently preferred oxidizer is cupric oxide. It has been found that gas generant compositions prepared from pyrometallurgical grade cupric oxide produce faster burn rates compared to hydrometallurgical grade cupric oxide. In addition, faster burn rates have been observed with ground cupric oxide compared to unground cupric oxide. An average oxidizer particle size of less than about 4 microns is presently preferred.
The 5-AT fuel is combined, in a fuel-effective amount, with an appropriate oxidizing agent to obtain a gas generating composition. In a typical formulation, the tetrazole fuel comprises from about 10 to about 50 weight percent of the composition and the oxidizer comprises from about 50 to about 90 weight percent thereof. More particularly, a composition can comprise from about 15 to about 35 weight percent fuel and from about 60 to about 85 weight percent oxidizer.
An example of the reaction between the anhydrous tetrazole and the oxidizer is as follows: ##STR1##
The present compositions can also include additives conventionally used in gas generating compositions, propellants, and explosives, such as binders, burn rate modifiers, slag formers, release agents, and additives which effectively remove NOx. Typical binders include lactose, boric acid, silicates including magnesium silicate, polypropylene carbonate, polyethylene glycol, and other conventional polymeric binders. Typical burn rate modifiers include Fe2 O3, K2 B12 H12, Bi2 MoO6, and graphite carbon fibers.
A number of slag forming agents are known and include, for example, clays, talcs, silicon oxides, alkaline earth oxides, hydroxides, oxalates, of which magnesium carbonate, and magnesium hydroxide are exemplary. A number of additives and/or agents are also known to reduce or eliminate the oxides of nitrogen from the combustion products of a gas generant composition, including alkali metal salts and complexes of tetrazoles, aminotetrazoles, triazoles and related nitrogen heterocycles of which potassium aminotetrazole, sodium carbonate and potassium carbonate are exemplary. The composition can also include materials which facilitate the release of the composition from a mold such as graphite, molybdenum sulfide, calcium stearate, or boron nitride.
The present compositions produce stable pellets. This is important because gas generants in pellet form are generally used for placement in gas generating devices, such as automobile supplemental restraint systems. Gas generant pellets should have sufficient crush strength to maintain their shape and configuration during normal use and withstand loads produced upon ignition since pellet failure results in uncontrollable internal ballistics.
The present invention relates specifically to the preparation of anhydrous 5-AT gas generant compositions. Anhydrous 5-AT compositions produce advantages over the hydrated form. For example, a higher (more acceptable) burn rate is generally observed. At the same time, the methods of the present invention allow for pressing the composition in the hydrated form such that pellets with good integrity are produced.
As discussed above, a gas generating composition comprises anhydrous 5-AT coupled with an acceptable oxidizer. At the stage of formulating the composition, the 5-aminotetrazole may be in the hydrated form which is generally available as a monohydrate. The components of the gas generant are mixed, for example by dry blending.
A water slurry of the gas generant composition is then preferably prepared. Generally the slurry comprises from about 3% to about 40% water by weight, with the remainder of the slurry comprising the gas generating composition. Although other materials may be used to prepare the slurry, such as ethanol and methanol, water is presently preferred. The slurry will generally have a paste-like consistency, although under some circumstances a damp powder consistency is desirable.
The mixture is then dried to a constant weight. This preferably takes place at a temperature less than about 110° C., and preferably less than about 45° C. For instance, a 5-AT/CuO composition mixture will generally establish an equilibrium moisture content in the range of from about 3% to about 5%, with the 5-AT being in the hydrated form (typically monohydrated). 5-AT monohydrate has a moisture content of approximately 17%.
Next, the material is pressed into pellet form in order to meet the requirements of the specific intended end use. As mentioned above, pressing the pellets while the 5-AT is hydrated results in a better pellet. In particular, crumbling of the material after pressing and upon exposure to ambient humidities is substantially avoided. It will be appreciated that if the pellet crumbles it generally will not burn in the manner required by automobile air bag systems.
After pressing the pellet, the material is dried such that the composition becomes anhydrous or substantially anhydrous. For instance, the above mentioned 5-AT/CuO material typically loses between 3% and 5% by weight water during this transition to the anhydrous state. It is found to be acceptable if the material is dried for a period of about 12 hours at about 110° C., or until the weight of the material stabilizes as indicated by no further weight loss at the drying temperature. For the purposes of this application, the material in this condition will be defined as "anhydrous."
Following drying it may be preferable to protect the material from exposure to moisture, even though the material in this form has not been found to be unduly hygroscopic at humidities below 20% Rh at room temperature. Thus, the pellet may be placed within a sealed container, or coated with a water impermeable material.
One of the important advantages of the anhydrous 5-AT gas generating compositions of the present invention, is that they are stable and combust to produce sufficient volumes of substantially nontoxic gas products. 5-AT has also been found to be safe when subjected to conventional impact, friction, electrostatic discharge, and thermal tests.
These anhydrous 5-AT compositions also are prone to form slag, rather than particulate debris. This is a further significant advantage in the context of gas generants for automobile air bags.
An additional advantage of an anhydrous 5-AT fueled gas generant composition is that the burn rate performance is good. As mentioned above, burn rates above 0.5 inch per second (ips) are preferred. Burn rates in these ranges are achievable using the compositions and methods of the present invention.
Anhydrous 5-AT compositions compare favorably with sodium azide compositions in terms of burn rate as illustrated in Table 1.
TABLE 1
______________________________________
Relative Vol. Gas
Gas Generant
Burn Rate at psi
Per Vol. Generant
______________________________________
Sodium azide
1.2 ± 0.1 ips
0.97
(baseline)
Sodium azide
1.3 ± 0.2 ips
1.0
low sulfur
Anhydrous 0.75 ± 0.05 ips
1.2
5-AT/CuO
______________________________________
An inflatable restraining device, such as an automobile air bag system comprises a collapsed, inflatable air bag, a means for generating gas connected to that air bag for inflating the air bag wherein the gas generating means contains a nontoxic gas generating composition which comprises a fuel and an oxidizer therefor wherein the fuel comprises anhydrous S-AT or a salt or complex thereof.
Suitable means for generating gas include gas generating devices which are used in supplemental safety restraint systems used in the automotive industry. The supplemental safety restraint system may, if desired, include conventional screen packs to remove particulates, if any, formed while the gas generant is combusted.
The present invention is further described in the following non-limiting examples.
EXAMPLES EXAMPLE 1Gas generating compositions were prepared utilizing 5-aminotetrazole as the fuel. Commercially obtained 5-aminotetrazole monohydrate was recrystallized from ethanol, dried in vacuo (1 mm Hg) at 170° F. for 48 hours and mechanically ground to a fine powder. Cupric oxide (15.32 g, 76.6%) and 4.68 g (23.4%) of the dried 5-aminotetrazole were slurried in 14 grams of water and then dried in vacuo (1 mm Hg) at 150° F. to 170° F. until the moisture content was approximately 25% of the total generant weight. The resulting paste was forced through a 24 mesh screen to granulate the mixture, which was further dried to remove the remaining moisture. A portion of the resulting dried mixture was then exposed to 100% relative humidity at 170° F. for 24 hours during which time 3.73% by weight of the moisture was absorbed. The above preparation was repeated on a second batch of material and resulted in 3.81% moisture being retained.
Pellets of each of the compositions were pressed and tested for burning rate and density. Burning rates of 0.799 ips at 1,000 psi were obtained for the anhydrous composition, and burning rates of 0.395 ips at 1,000 psi were obtained for the hydrated compositions. Densities of 3.03 g/cc and 2.82 g/cc were obtained for the anhydrous and hydrated compositions respectively. Exposure of pellets prepared from the anhydrous condition to 45% and 60% Rh at 70° F. resulted in complete degradation of pellet integrity within 24 hours.
EXAMPLE 2In this example compositions within the scope of the invention were prepared. The compositions comprised 76.6% CuO and 23.4% 5-aminotetrazole. In one set of compositions, the 5-aminotetrazole was received as a coarse material. In the other set of compositions, the 5-aminotetrazole was recrystallized from ethanol and then ground.
A water slurry was prepared using both sets of compositions. The slurry comprised 40% by weight water and 60% by weight gas generating composition. The slurry was mixed until a homogenous mixture was achieved.
The slurry was dried in air to a stable weight and then pressed into pellets. Four pellets of each formulation were prepared and tested. Two pellets of each composition were dried at 110° C. for 18 hours and lost an average of 1.5% of their weight.
Burn rate was determined at 1,000 psi and the following results were achieved:
______________________________________
Burn Rate (ips)
Sample (ips @ psi)
Density (gm/cc)
______________________________________
Coarse 5-AT/no post drying
0.620 2.95
Coarse 5-AT/post drying
0.736 2.94
Fine 5-AT/no post drying
0.639 2.94
Fine 5-AT/post drying
0.690 2.93
______________________________________
Overall, improved results were observed using the post drying method of the present invention.
EXAMPLE 3Commercially obtained 5-aminotetrazole monohydrate was prepared to be utilized as a fuel for use in gas generant compositions. Approximately five pounds of aminotetrazole monohydrate (Aldrich) was ground in a fluid energy mill. Using a Microtrac Standard Range Particle Analyzer it was determined that 10% of the resulting fuel particles had a diameter less than 2.2 microns and that 50% of the fuel particles had a diameter less than 5.6 microns. The ground aminotetrazole hydrate was dried at 220 F for at least four hours. A weight loss of approximately 14% was observed. The resulting anhydrous aminotetrazole powder was forced through a 60 mesh sieve before use.
EXAMPLE 4Three gas generating compositions were prepared utilizing the anhydrous 5-aminotetrazole powder from Example 3 as the fuel and three different types of cupric oxide as the oxidizer. The three types of cupric oxide were obtained from the American Chemet Corporation. They consisted of a ground cupric oxide of pyrometallurgical origin (grd pyro) with a mean particle size of 3.6 microns, a cupric oxide of hydrometallurgical origin (ungrd hydro) with a mean particle size of 9.5 microns, and a ground cupric oxide of hydrometallurgical origin (grd hydro) with a mean particle size of 3.6 microns. The respective cupric oxide (22.98 g, 76.60%) was stirred into 7.02 g (23.40%) of the aminotetrazole, the composition was shaken in an enclosed container for approximately two minutes and then slurried with 12 g of water. The three compositions were dried overnight at 73° F., and granulated through an 18 mesh sieve. Samples therefrom were pressed into 1/2" diameter cylindrical pellets with a weight of three grams each. The resulting burn rate data are summarized in Table 2. The burn rates were a function of the type of cupric oxide used as the oxidizer and increased in the burn rate as follows: ungrd hydro<<grd hydro<grd pyro.
Boraychem supply professional and honest service.
EXAMPLE 5Samples of granules prepared according to the procedure of Example 4 were dried further at 220° F. The accompanying weight losses are summarized in Table 2. Samples of the 5-AT/CuO composition were pressed into 1/2" diameter cylindrical pellets with a weight of three grams each. The resulting burn rate data are summarized in Table 2. The burn rates were a function of the type of cupric oxide used as the oxidizer and increased in burn rate as follows: ungrd hydro<<grd hydro<grd pyro. The burn rates were about twice as high as those obtained for pellets derived from granules dried at 73° F. as described in Example Samples of the granules prepared in Example 4 that were dried at 220° F. were pressed into 1/2" diameter cylindrical pellets with a weight of one gram each. These pellets were placed in a humidity chamber held at 60% humidity. Over a period of 67 hours, the pellets had gained between 3.7 and 4.3% of their original weight and were seriously delaminated with several large circumferential cracks.
EXAMPLE 6Samples of granules prepared according to the procedure of Example 4 were pressed into 1/2" diameter cylindrical pellets with a weight of three grams each. The pellets were dried overnight at 220° F. The accompanying weight losses are reported in Table 2 as well as the resulting burn rate data. The burn rates were a function of the type of cupric oxide used as the oxidizer and increased in burn rate as follows: ungrd hydro<<grd hydro<grd pyro. Furthermore, the burn rates are consistently higher than those of the corresponding pellets prepared as in Example 5. A sample of granules prepared in Example 4 were pressed into 1/2" diameter cylindrical pellets with a weight of one gram each. These pellets were dried at 220° F. and then placed in a humidity chamber held at 60% humidity. Over a period of 67 hours, the pellets gained between 4.2 and 4.5% of their original weight. These pellets appeared to be unchanged and showed no signs of cracking or delamination. Pellets processed by this method appear to be much more robust under conditions of high humidity than those prepared by the method of Example 5.
EXAMPLE 7A gas generating composition was prepared utilizing anhydrous 5-aminotetrazole powder from Example 3 as the fuel. The grd pyro cupric oxide described in Example 4 (22.98 g, 76.60%) was stirred into 7.02 g (23.40%) of the aminotetrazole. The composition was shaken in an enclosed container for approximately two minutes. However, this particular sample was not slurried in water or any other solvent. The resulting powder was pressed into 1/2" diameter cylindrical pellets with a weight of three grams each. The resulting burn rate data are summarized in Table 2. The burn rate of the composition was significantly lower than that of the corresponding grd pyro composition which was slurried in water and dried at 220° F. as pellets. One gram pellets of this material gained 4.7% of their original weight over a period of 67 hours in an atmosphere containing 60% humidity. In addition, pellets of this material delaminated during this humidity aging.
EXAMPLE 8A composition containing aminotetrazole from Example 3 (23.40 g, 23.40%) and the grd pyro cupric oxide described in Example 4 (76.60 g, 76.60%) was mixed and dried as in Example 4. Three gram pellets were produced according to procedures in Examples 4, 5, and 6, respectively. The burn rate data obtained from the 100 g mix are summarized in Table 2. Again, pellets produced from the completely dried granules delaminated, while pellets pressed from slightly moist granules and then dried as pellets remained intact during humidity aging.
EXAMPLE 9A crystalline sample of aminotetrazole hydrate (Dynamit Nobel) was dehydrated at 220° F. losing 17.1% of it original weight (17.5% being theoretical weight loss). A portion of this anhydrous aminotetrazole was recrystallized from methanol and an additional portion was recrystallized from ethanol. The resulting solids were heated at 220° F. to a constant weight. Each type of aminotetrazole was forced through a 60 mesh sieve. Three compositions containing grd pyro cupric oxide (38.30 g, 76.60%) and aminotetrazole (11.70 g, 23.40%) were mixed and processed in the solvent from which the aminotetrazole was last crystallized: water, methanol and ethanol, respectively. The cupric oxide and aminotetrazole were dry blended and mixed by shaking, followed by slurrying in 19 g, 11 g and 13 g of water, methanol an ethanol, respectively. The mixes were dried partially, granulated, dried completely, and then allowed to take up solvent in solvent-saturated air over a three day period. The formulations gained 3.6%, 2.1%, and 1.1% water, methanol and ethanol, respectively. Pellets were pressed from -18 mesh solvated granules. The pellets lost 4.2%, 0.6%, and 0.2% of their weight upon drying at 220° F. Burn rate data are summarized in Table 2. Burn rate for pellets derived from water-processing are significantly higher than those derived from alcohol processing.
TABLE 2
__________________________________________________________________________
Cupric Oxide/Aminotetrazole Formulations*
Burn Rate Variations with Processing and Cupric Oxide Grade
Mix Final
Final
Final
Pellets
Example
CuO Size
Slurry
Dry Dry Dry in Rb (in/s) at
Number
Grade (gm)
Media
Form Temp.
Wt. Loss
Humidity
P.sub.ave (psi)
__________________________________________________________________________
Ex. 4
grd pyro
30 water
granules
73° F.
NA NA 0.329 at
Ex. 4
grd hydro
10 water
granules
73° F.
NA NA 0.309 at
Ex. 4
ungrd hydro
30 water
granules
73° F.
NA NA 0.229 at
Ex. 5
grd pyro
30 water
granules
220° F.
5.3% crumbled
0.711 at
Ex. 5
grd hydro
30 water
granules
220° F.
6.0% crumbled
0.634 at
Ex. 5
ungrd hydro
30 water
granules
220° F.
7.4% crumbled
0.497 at
Ex. 6
grd pyro
30 water
pellets
220° F.
5.3% intact
0.787 at
Ex. 6
grd hydro
30 water
pellets
220° F.
4.8% intact
0.731 at
Ex. 6
ungrd hydro
30 water
pellets
220° F.
6.0% intact
0.537 at
Ex. 7
grd pyro
30 dry powder
NA NA crumbled
0.565 at
Ex. 8
grd pyro
100
water
granules
73° F.
NA NA 0.325 at
Ex. 8
grd pyro
100
water
granules
220° F.
4.6% crumbled
0.735 at
Ex. 8
grd pyro
100
water
granules
220° F.
4.7% intact
0.815 at
Ex. 9
grd pyro
50 water
pellets
220° F.
4.25%
NA 0.757 at
Ex. 9
grd pyro
50 methanol
pellets
220° F.
0.64%
NA 0.537 at
Ex. 9
grd pyro
50 ethanol
pellets
220° F.
0.16%
NA 0.540 at
__________________________________________________________________________
*76.60% cupric oxide, 23.40% anhydrous aminotetrazole.
EXAMPLE 10
A gas generating composition consisting of 55.78% (11.16 g) grd pyro cupric oxide as described in Example 4, 26.25% (5.25 g) of the 5.6 micron, partially dehydrated aminotetrazole (AT 0.8H2 O) described in Example 3, and 17.96% (3.59 g) of a ground sample of strontium nitrate was slurried with five grams of water and dried at 135° F. to a constant weight. Pellets were pressed from a portion of this gas generant material exhibiting a pellet density of 2.8 g/cc and a burn rate of 0.886 ips at Pave of psi. Additional generant was dried further at 220° F. with a corresponding weight loss of 2%. The density of pellets therefrom remained at 2.8 g/cc while the burn rate increased to 0.935 ips at a Pave of psi. The theoretical flame temperature of the anhydrous formulation is ° K.
EXAMPLE 11Three gas generating compositions were prepared utilizing the anhydrous 5-aminotetrazole powder prepared in Example 3 as the fuel (21.24%, 10.62 g), the three different types of cupric oxide described in Example 4, as the oxidizer (54.72%, 27.36 g), and ground strontium nitrate as the co-oxidizer (24.04%, 12.02 g). The formulation was mixed, slurried, dried, and granulated according to the procedure in Example 4, with a drying temperature of 122° F. Pellets were formed and processed similarly to those described in Example 4, 5, and 6. The results are summarized in Table 3. As with the cupric oxide/aminotetrazole formulations, burn rate values are dependent on the type of cupric oxide and follow the same trend: ungrd hydro<<grd hydro<grd pyro. Pellets from hydrated granules exhibit a lower burn rate than pellets derived from granules dried at 220° F. or from pellets dried at 220° F. The latter two types of pellets have comparable burn rates. This may be due in part to the fact that the weight loss from the hydrated compositions is much smaller than for the cupric oxide/aminotetrazole series of compositions in Example 4-6. One gram pellets that were formed and processed similarly to those of Examples 4-6, were placed in closed chamber with 60% humidity. After aging for 90 hours, weight gains of 3-4.5% were observed. Furthermore, all of the pellets showed signs of delamination except for the pellets containing the grd pyro cupric oxide that had been dried in the pellet form (See, Table 3). The granules of this particular mix had been pressed with the highest moisture content.
TABLE 3
__________________________________________________________________________
Cupric Oxide/Strontium Nitrate/Aminotetrazole Formulations*
Burn Rate Variations with Processing and Cupric Oxide Grade
Mix Final
Final
Final
Pellets
Example
CuO Size
Slurry
Dry Dry Dry in Rb (in/s) at
Number
Grade (gm)
Media
Form Temp.
Wt. Loss
Humidity
P.sub.ave (psi)
__________________________________________________________________________
Ex. 11
grd pyro
50 water
granules
122° F.
NA NA 0.793 at
Ex. 11
grd hydro
50 water
granules
122° F.
NA NA 0.753 at
Ex. 11
ungrd hydro
50 water
granules
122° F.
NA NA 0.655 at
Ex. 11
grd pyro
50 water
granules
220° F.
1.6% crumbled
0.986 at
Ex. 11
grd hydro
50 water
granules
220° F.
0.7% crumbled
0.758 at
Ex. 11
ungrd hydro
50 water
granules
220° F.
0.6% crumbled
0.736 at
Ex. 11
grd pyro
50 water
granules
220° F.
1.9% intact
0.950 at
Ex. 11
grd hydro
50 water
granules
220° F.
0.9% crumbled
0.772 at
Ex. 11
ungrd hydro
50 water
granules
220° F.
0.7% crumbled
0.678 at
__________________________________________________________________________
*54.72% cupric oxide, 24.04 strontium nitrate, 21.24% anhydrous
aminotetrazole.
EXAMPLE 12
A gas generating composition was prepared utilizing anhydrous 5-aminotetrazole powder (9.86%, 0.54 g, Fairmont), 8.7 micron ungrd hydro cupric oxide (55.30%, 3.04 g, Aldrich) as the oxidizer, ground strontium nitrate as the co-oxidizer (24.52%, 1.35 g), and sodium dicyanamide (NaDCA) as a ballistic modifier (10.32%, 0.57 g Aldrich Lot). The formulation was mixed as a water slurry, dried completely and pressed into pellets. The burn rate was 0.567 ips at Pave of psi with a calculated flame temperature of ° K.
EXAMPLE 13A gas generating composition was prepared utilizing anhydrous 5-aminotetrazole powder (12.64%, 1.27 g, Fairmont), 8.7 micron ungrd hydro cupric oxide (31.52%, 3.15 g, Aldrich) as the cooxidizer, ground strontium nitrate as the oxidizer (42.59%, 4.26 g), and sodium dicyanamide as a ballistic modifier (13.23%, 1.32 g, Aldrich Lot). The formulation was mixed as a water slurry, dried completely, and pressed into pellets. The burn rate was 0.817 ips with a Pave of psi. The theoretical flame temperature is ° K. Mixes producing the fastest burn rate are summarized in Table 4 for each of the formulation types described in the above examples.
TABLE 4
__________________________________________________________________________
Cupric Oxide, Aminotetrazole Formulations
Effect of Additives on Burn Rate
Mix
Flame
Example Size
Temp.
Density
Weight %
Rb (in/s) at
Number
Formulation
(gm)
(°K.)
(g/cc)
Gas P.sub.ave (psi)
__________________________________________________________________________
Ex. 8
23.40%
AT 100
2.95 39 0.815 at
76.60%
CuO
Ex. 11
21.24%
AT 50
2.86 39 0.950 at
54.72%
CuO
24.04%
Sr(NO.sub.3).sub.2
Ex. 10
23.34%
AT 20
2.83 41 0.935 at
57.99%
CuO
18.67%
Sr(NO.sub.3).sub.2
Ex. 12
9.86%
AT 5.5
3.11 34 0.567 at
55.31%
CuO
24.52%
Sr(NO.sub.3).sub.2
10.31%
NaDCA
Ex. 13
12.64%
AT 10
2.58 45 0.817 at
31.53%
CuO
42.60%
Sr(NO.sub.3).sub.2
13.23%
NaDCA
__________________________________________________________________________
The present invention may be embodied in other specific forms without departing from its essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description.
As a whole, the results described herein, based on molecular orbital calculations and the matrix-isolation technique coupled to FTIR and EPR spectroscopies, pave the way towards a better elucidation of the mechanistic pathways followed by 5-aminotetrazoles upon UV-irradiation and of the effect of the ring substitution pattern on the photoreactivity of these compounds.
Here we report the UV-induced photochemistry of monomeric 3-(2-methyl-(2H)-tetrazole-5-amino)-1,2-benzisothiazole 1,1-dioxide 4 (herein termed as 2-methyl-(2H)-tetrazole-5-amino-saccharinate, 2MTS ) isolated in solid argon. Compound 4 is one of our most promising ligands, since it proved to be non-toxic and selective towards Cu (II) and could be developed in the context of a new anti-tumor therapeutic approach based on selective copper chelation [ 11 ]. The matrix photochemistry of 4 is compared with that of other 5-aminotetrazole derivatives ( Scheme 2 ), including 2-methyl-5-aminotetrazole 2 , used as building block for the preparation of the ligand 4 , and its isomer, 1-methyl-5-aminotetrazole 5 . It is worth noting that 5-aminotetrazoles are of interest as gas-generating compositions [ 22 ], [ 23 ], as high energy materials (e.g. as air-bag insufflators and as propellants components for missiles) [ 24 ], [ 25 ], [ 26 ], [ 27 ] and, accordingly to recent findings, are being developed as powerful tools for proteome profiling in living cells [ 28 ], [ 29 ], [ 30 ], [ 31 ], [ 32 ]. Thus, a deeper investigation of the photochemistry of 5-aminotetrazoles and their derivatives is also especially relevant and timely.
Despite the promising properties observed for these new tetrazole-saccharinate ligands, the tetrazole ring is known to undergo easy cleavage, induced thermally or photochemically [ 19 ], [ 20 ], [ 21 ]. Thus, when considering the applications of tetrazole-saccharinates, the low stability of this heterocycle steams as a concern. In this context we considered relevant to investigate the photochemistry of tetrazole-saccharinates, as the electron withdrawing saccharyl system, known to be relatively photostable, will probably affect the photostability of the tetrazole moiety [ 10 ].
An investigation on the chelating ability of the new compounds revealed that selected N-linked tetrazole-saccharinates ( Scheme 1 ; X=&#;NH&#; or &#;N=) exhibit strong binding selectivity to copper ions [ 11 ]. This property was considered of interest for therapeutic applications [ 12 ], mainly because of the recent findings that link elevated levels of copper to cancer progression [ 13 ], [ 14 ], boosting the interest in selective copper chelators [ 12 ], [ 15 ], [ 16 ]. The copper complexes prepared from selected N-linked tetrazole-saccharinates were tested in vitro against cancer cell lines and have shown a considerable increase in cytotoxicity against tumoral cells, compared to the corresponding nontoxic ligands [ 11 ]. Also, in a different line of investigation, two mononuclear copper (II) and cobalt (II) complexes based on the tetrazole-saccharinate ligand 4 ( Scheme 2 ) were used as effective catalysts for selective oxidation of diverse secondary alcohols [ 17 ], while the ligand 4 alone proved effective as organo-catalyst for the oxidation of benzyl alcohols [ 18 ].
Tetrazoles exhibit excellent coordination abilities through their four nitrogen atoms, acting either as multidentate ligands or as bridging building blocks in supramolecular assemblies [ 1 ], [ 2 ], [ 3 ], [ 4 ]. Additionally, the structure of the tetrazolyl-based coordination complexes can be tailored by employing functionalysed tetrazoles in the assembly process [ 5 ], increasing the versatility and widening the applications of the tetrazole chemotype in coordination and supramolecular chemistry. Likewise, benzisothiazoles (known as saccharins) bear interesting coordination abilities and are thermally and photochemically much more stable than tetrazoles, although the saccharyl system is comparatively more difficult to functionalise [ 6 ], [ 7 ]. Taking advantage of the properties of both classes of heterocycles, we designed new scaffolds, termed as tetrazole-saccharinates ( Scheme 1 ), with the aim of exploring their applications as multidentate nitrogen ligands. In recent years we prepared a representative library of tetrazole-saccharinates and investigated their structure in detail, prior to exploring their reactivity and applications [ 8 ], [ 9 ], [ 10 ].
As described in the Experimental Section, a sample of crystalline 2-methyl-(2H)-tetrazole-5-amino-saccharinate 4 was sublimated under reduced pressure (at &#;150°C) and the vapors of the compound were co-deposited with argon (ca. 1: molar ratio) onto a CsI substrate kept at 15 K. In previous studies, 4 was found to undergo a complete amino&#;imino tautomerization under the conditions described, where the amino-bridged tautomeric form existing in the crystalline phase of the compound was completely converted into the theoretically predicted most stable imino-bridged tautomer [9]. In this most stable tautomeric form, observed in the matrix isolation experiments, the labile hydrogen atom is connected to the saccharyl nitrogen and the two heterocyclic fragments are linked by an imino moiety in which the double-bond is established with the carbon atom belonging to the saccharyl fragment (see structure 4 in Scheme 3).
Scheme 3:
To investigate the photochemistry of 4, the deposited matrix was irradiated with a tunable UV-laser source, starting at λ=330 nm and gradually decreasing until λ=222 nm (the shortest wavelength available in our experimental setup), with the sample being followed after each irradiation by recording its infrared spectrum. It was observed that irradiations at around λ~290 nm and λ~250 nm were the most effective in inducing changes in the spectrum of 4.
Irradiations at λ=290 nm, during up to 100 min., resulted in a decrease in the intensity of the bands due to 4 (indicating that the compound was being consumed) and, simultaneously, in a continuous increase of a distinctive absorption band at around cm&#;1. Further irradiations of this same matrix at λ=250 nm resulted in several new bands, with complete consumption of the reagent 4 after 10 min. of irradiation. Figure 1 presents the spectral changes in the range &#; cm&#;1: (i) after 100 min. of irradiation at λ=290 nm, when around 60% of 4 was consumed and a chemical species, with a characteristic absorption at cm&#;1, was produced, which could be identified as the 1H-diazirene 7 (see Scheme 3); (ii) after 5 and 10 min. of irradiation at λ=250 nm, subsequent to the irradiation at λ=290 nm, showing bands due to the different photoproducts generated at this wavelength. As reported for the photochemistry of an S-linked 1-methyltetrazole-saccharinate [10], the presence of the saccharyl ring, which seems to be unaffected by irradiation under the experimental conditions used, results in extensive overlap of the bands of 4 with those of the photoproducts, especially in the low-frequency spectral range (below cm&#;1), hampering the interpretation of the data based on the low frequency spectral region. However, the most characteristic bands of these photoproducts are expected to appear in the clean &#; cm&#;1 spectroscopic window.
Fig. 1:
As it can be observed in Fig. 1, it is clear that in the &#; cm&#;1 spectral range no other bands increased besides the distinctive cm&#;1 absorption due to the diazirene 7, even after 100 min. of irradiation at λ=290 nm. Subsequent irradiation of the same matrix at λ=250 nm resulted in a fast consumption of both the reactant 4 (see absorption at cm&#;1) and the diazirene 7 while, simultaneously, several new bands (, , , cm&#;1), due to other photoproduced species, appeared in the spectrum. The identification of the photoproducts corresponding to these characteristic absorption bands could be easily achieved by comparison with the reported results obtained for the photolysis of an S-linked tetrazole-saccharinate and the parent 2-methyl-5-aminotetrazole [10], [33]. The band at cm&#;1 was ascribed to the νNCN antisymmetric stretching of carbodiimide 8 (see Scheme 3), the band at cm&#;1 was ascribed to the νC&#;N stretching of nitrile 9 and the band at cm&#;1 was ascribed to the νCN stretching of CNH 12. The distinctive absorption at cm&#;1, which increased during the first 5 min. of irradiation and then started decreasing with further irradiations at λ=250 nm, was assigned to the νC=N stretching mode of the nitrile imine 6, calculated at cm&#;1. The spectral changes observed in the range &#; cm&#;1, after 5 and 10 min of irradiation at λ=250 nm, are shown in detail in Fig. 2. The calculated IR spectra for the proposed photoproducts are also shown in this figure, for comparison.
Fig. 2:
The mechanisms of fragmentation of tetrazoles remain under debate and the formation of nitrile imines from thermal and photochemical decomposition of tetrazoles has been subject of intense investigation over the last decades [19]. From the very first studies [32], it was postulated that the parent tetrazole in its gas phase most stable isomeric 2H-form, as well as 2-, 5- and 2,5-substituted tetrazoles, undergo fragmentation through formation of a nitrile imine intermediate that cyclizes to a 1H-diazirene, then leading to a final carbodiimide through rearrangement. However, this pattern of reactivity was demonstrated experimentally only very recently, during studies on the photochemistry of tetrazoles under low-temperature matrix isolation conditions [5], [13], [15], [21], [33], [35]. The observed outline of photo-fragmentation of 4 upon irradiation at λ=250 nm follows this general pattern, which, as also found in other cases [10], [33], is accompanied by an additional reaction path involving as reactant the 1H-diazirene (see Scheme 3).
The initial step of the photochemistry of matrix-isolated 4 corresponds to selective photoinduced cleavage of the C5&#;N4 and N2&#;N3 bonds of the tetrazole ring, leading to extrusion of molecular nitrogen and production of nitrile imine 6, which reached its maximum amount after the first 5 min. of irradiation at λ=250 nm, and then was gradually consumed with increased irradiation times (see band cm&#;1 in Fig. 2; blue and red lines represent 5 min and 10 min of irradiation at λ=250 nm, respectively), generating the 1H-diazirene 7 through a ring closing process.
It shall be noticed that, as mentioned above, upon irradiation of 4 at λ=290 nm, the diazirene 7 was observed as the sole photoproduct. Although around 40% of 4 remained after 100 min. of irradiation, suggesting that the efficiency of the reaction is rather low at this wavelength, it should be noticed that the capture of antiaromatic (i.e. 4π systems) structures such as 7 proved to be quite challenging, and has only been observed in rare cases, mostly upon isolation in cryogenic matrices [10], [33], [35], [36], [37]. Nevertheless, under the experimental conditions used, the antiaromatic three membered ring 7 could indeed be generated as the sole photoproduct and was found to be photostable. This result is even more remarkable because, contrarily to what is observed in this case, the diazirene derivative of 2-methyl-5-aminotetrazole was found to react upon irradiation at 325 nm [33]. On the other hand, the observed photostability of the antiaromatic three membered diazirene 7 follows the trend observed for a previously studied S-linked tetrazole-saccharinate [10], and appears to be a common phenomenon on this type of conjugates, probably induced by the stabilizing effect of the electron-withdrawing saccharyl moiety.
It is also interesting to note that during the irradiation experiments performed at 290 nm no spectroscopic evidence of any intermediate from 4 to the diazirene 7 was found, in particular no bands ascribable to nitrile imine 6, which is observed upon irradiation at λ=250 nm. This may be explained considering that the longer wavelength of excitation (λ=290 nm), being closer to the absorbance maximum of the expected preceding nitrile imine 6 [38], facilitates its fast photoconversion into the diazirene 7.
Upon irradiation at λ=250 nm, the diazirene 7 undergoes subsequent photoconversion into carbodiimide 8 (pathway a in Scheme 3), in a process that will be discussed in more detail below. A second pathway was also perceived, pathway b, involving concomitant decomposition of the diazirene 7 and formation of nitrile 9 and CNH 12.
Concerning pathway b, it should be noticed that the formation of nitrile 9 could in principle result either from (i) the cleavage of the C5&#;N4 and N1&#;N2 bonds of the tetrazole ring of 4, generating the nitrile 9 and methyl azide, or from (ii) cleavage of the diazirene ring 7, generating nitrile 9 and methyl nitrene 10, the latter undergoing subsequent isomerization to methylenimine 11, from which isocyanic acid (CNH) can be produced [39], [40]. However, formation of nitrile 9 via cleavage of the tetrazole ring can be ruled out, since no evidence of the intense νNNN antisymmetric mode of methyl azide could be found [40]. On the other hand, isocyanic acid was identified beyond doubt [39] and its formation from methylenimine 11 is expected based on the available knowledge regarding the photochemistry of this last compound [39], [40]. This mechanistic proposal is also supported by the results gathered from the photochemical investigation of the matrix-isolated parent 2-methyl-5-aminotetrazole 2, which included EPR measurements, enabling the identification of methyl nitrene 10 (as described in detail below).
The present study revealed that photolysis of 4 results in a selective fragmentation of the tetrazole ring, while the saccharyl system seems to be completely photostable under the conditions used. As such, comparison of the photochemistry of 4 with that reported for the parent 2-methyl-5-aminotetrazole 2 [33] may be viewed as an efficient approach to support the interpretation of the present spectroscopic data, and to evaluate the photochemical stability induced by the electron withdrawing saccharyl system into the photolabile tetrazole.
The photochemistry of 4, isolated in solid argon at 15 K, was investigated under similar conditions to those used for the parent tetrazole 2. Irradiation was performed with a tunable laser at λ=250 nm, with an output power of ~40 mW. It should be noted that both 5-aminotetrazole derivatives 2 and 4 show an absorption maximum at ~250 nm.
Comparison of the photochemistry of the conjugate 4 with that of the parent 2-methyl-5-aminotetrazole 2, in the same experimental conditions, revealed that the reaction pathways and obtained photoproducts are equivalent for both compounds (Scheme 4). However, kinetic studies on the photodegradation of both compounds unfolded interesting differences in the kinetic profiles. Photolysis of matrix-isolated 2 at λ=250 nm (~40 mW) generated the corresponding nitrile imine 13 in a maximal amount after 2 s of irradiation, which was totally consumed after 4 min. of irradiation. Also, ca. 50% of the initial compound 2 was consumed after 4 s of irradiation [33]. On the other hand, photolysis of 4 under the same conditions (λ=250 nm, ~40 mW) generated the corresponding nitrile imine 6 in a maximal amount after 5 min. of irradiation, and only after 60 min. of irradiation the nitrile imine was completely consumed. Moreover, ca. 50% of the initial 4 was consumed only after 2 min. of irradiation. These results suggest that the saccharyl system increases the photostability of the tetrazole ring and also of the nitrile imine intermediate by more than 20×, compared to parent tetrazole 2. These results are in keeping with the above noticed stabilization of the diazirene formed from 4 in result of the presence of the saccharyl substituent.
Scheme 4:
The general pattern of photoreactivity of 2-, 5- and 2,5-substituted tetrazoles has been described above. For 1,5-disubstituted tetrazoles, earlier studies [34], [38] postulated direct formation of the carbodiimide through an imidoylnitrene intermediate (see Fig. 3). In addition, more recent investigations demonstrated that 1H-diazirenes are common intermediates on the photochemistry of both 1,5- and 2,5-disubstituted tetrazoles [33].
Fig. 3:
During a study on the photochemistry of 5-methyl-tetrazole, Nunes et al. [35] have shown that 1H-diazirenes exhibit a close structural relation to imidoylnitrenes. Indeed, geometry optimization at the B3LYP/cc-pVTZ level on the triplet state of 5-methyl-1H-diazirene revealed convergence to the respective imidoylnitrene structure [35]. However, these imidoylnitrenes were only observed using internal or external traps [36], [37], since in the absence of these traps the reactive singlet state imidoylnitrene shall promptly cyclize to the 1H-diazirene or undergo a Wolff-type rearrangement into carbodiimide [41]. Recent studies on the photolysis of matrix-isolated tetrazoles allowed to identify several isomers formed from elimination of molecular nitrogen, but attempts to trap the putative imidoylnitrene intermediates were unsuccessful [35], [42].
Following the reactivity patterns described above, two important observations result from recent studies regarding the effect of the ring substitution outline on the photofragmentation pathways of 5-aminotetrazoles [33]: (i) the 1H-diazirene was observed as a common intermediate from photolysis of both 1-methyl- and 2-methyl-5-aminotetrazole, subsequently isomerizing to carbodiimide as final photoproduct; (ii) upon irradiation at short wavelengths (222 nm), an amino cyanamide was obtained, together with the 1H-diazirene and carbodiimide, from photolysis of 1-methyl-5-aminotetrazole, unraveling a new reaction pathway; this cyanamide isomerizes to the carbodiimide upon irradiation at longer wavelengths (325 nm).
In order to deepen our understanding of the effect of the ring substitution pattern on the mechanistic pathways followed by 5-aminotetrazole derivatives, further studies were undertaken. The C2H5N3 isomers were calculated at the B3LYP/6-311++G(d,p) level and the structures are represented in Fig. 3. The calculated energies indicate that 1H-diazirene (14 in Fig. 3) is the most energetic species in the singlet state; ~22 and ~11 kJ mol&#;1 below the imidoylnitrene triplet states TN1 and TN2, respectively. The high energetic character of 14, 94.1 kJ mol&#;1 above the amino cyanamide 17 (calculated as the most stable isomer in the singlet state potential energy surface), is conceivably due to both ring strain and antiaromatic destabilization. The observed trends for these isomeric species are similar to the trends found in calculations for H4C2N2 and H2CN2 isomers, which can be formed from the photolysis of 5-methyl-tetrazole and of the parent unsubstituted tetrazole, respectively [35], [42]. The optimized isomeric form E of triplet imidoylnitrene (TN2) is more stable than form Z (TN1) by ~10 kJ mol&#;1.
In Scheme 5, a mechanistic proposal for the photochemical transformations of 5 and 2 isomers is presented, which is also in agreement with the observed photochemistry of 4, described above. This mechanism considers as pivotal intermediate the highly reactive open-shell singlet state imidoylnitrene species, sN. Structurally, sN can be visualized as delocalized resonance structures with some biradical character at the two nitrogen atoms, and a central CN bond with appreciable double bond character [43], [44], [45].
Scheme 5:
As reported before [33], the λ=250 nm photolysis of 2 results in the cleavage of the tetrazole ring, with initial formation of nitrile imine 13 that develops to diazirene 14. This last one is expected to generate the imidoylnitrene in the geometrically most accessible Z isomeric form SN1, through ring opening. The reactive Z-sN1 species can then rearrange to the observed carbodiimide 15, via a direct R-group 1,2-shift (R=amino group) from the carbon of the imidoylnitrene to the sterically accessible nitrene moiety (in this isomeric form of the imidoylnitrene, the methyl substituent precludes the 1,2-shift to the methyl-substituted nitrogen). Several thermal and photochemical reactions of tetrazoles involving imidoylnitrenes&#; rearrangements were postulated based on this 1,2-shift [19]. Indeed, imidoylnitrenes are aza analogs of vinylnitrenes and acylnitrenes, thus these reactions can be expected to follow the same Wolff-type rearrangement to carbodiimides [41].
Compared to 2, the first step of the photolysis of 5 leads to a different product, though in both cases cleavage of the tetrazole ring, with extrusion of molecular nitrogen, takes place. In the case of 5, this process directly generates the postulated imidoylnitrene intermediate Z-sN1. Once formed, Z-sN1 can undergo the above mentioned Wolff-type rearrangement to carbodiimide 15, or collapse to give the 1H-diazirene 14, which are then common photoproducts from photolysis of both 5-aminotetrazole isomers 2 and 5. The observation of the cyanamide 17 photoproduct during the short wavelength irradiation (222 nm) [33] of 5 can be explained by considering that at this irradiation wavelength the channel for photoisomerization between Z and E forms of the imidoylnitrene is accessible. In the E imidoylnitrene isomeric form sN2, the methyl substituent is no longer blocking the 1,2-shift from the amino group to the methyl-substituted nitrogen, thus allowing generation of the cyanamide 17 by a mechanism similar to that discussed above leading to rearrangement of the imidoylnitrene into the carbodiimide 15.
As mentioned above, it was also shown [33] that upon subsequent irradiation at 325 nm of a photolysed argon matrix of 5, the formed cyanamide 17 (and also the diazirene 14) converts into the carbodiimide 15, a result that is also explained by the mechanism shown in Scheme 4 and involves participation of the imidoylnitrene intermediate. Recently, Abe et al. [46] reported the formation of an imidoyl nitrene from photolysis of 1-methyl-5-phenyl-tetrazole, which was observed by EPR (electron paramagnetic resonance) spectroscopy.
We performed EPR (electron paramagnetic resonance) spectroscopy for the photolysis of 5 and 2 in cryogenic conditions, aimed at trapping and identifying putative nitrene species. According to the reported data [47], we can expect to observe different EPR signatures for nitrenes and diradicals and they can be distinguished by EPR, where nitrenes give rise to transitions at high field (typically ~ G) and zero-field splitting parameters D of around 1 cm&#;1, whereas diradicals resonate at much lower field (typically ~ G) and show much smaller D values.
In these EPR studies, ca. 50 mM 2-methyltetrahydrofuran (MTHF) solution of 2 and the suspension of 5 were degassed under high vacuum at 1.0×10&#;2 Pa and sealed under the vacuum conditions, respectively. The solubility of tetrazole 5 was quite low. The EPR sample of 2 was placed in the EPR cavity, cooled to 5 K, and then irradiated at 266 nm (~10 mJ). After irradiation of compound 2 under these conditions for 60 min., the EPR spectrum of photolyzed 2 evidenced a signal at around G, at the resonance frequency of 9.39 GHz (Fig. 4a), which was identified as methyl nitrene 10, with |D/hc|=1.61 cm&#;1 and |E/hc|=0. cm&#;1, which are consistent with reported values in organic matrix [48]. This observation brings further support to our proposal that nitriles 9 and 16 result from photolysis of diazirenes 7 and 14, respectively (see Schemes 2 and 3), with concomitant formation of methyl nitrene 10 that rearranges to methylene imine 11, then affording isocyanic acid 12.
Fig. 4:
For compound 5, the irradiation with a broadband Xe-light source had to proceed for 11 h until a very weak signal could be observed, at ~ G, also ascribed to a triplet nitrene (Fig. 4b). It should be noted that the irradiation wavelength (λ=266 nm) is far from the maximum absorption (~222 nm) observed for 5, and even the Xe-lamp has a very weak light intensity below ~250 nm. In addition, the solubility of tetrazole 5 proved to be quite low, as mentioned above.
In consonance with the reported information, these EPR results seem to be the first direct observation of the postulated methylnitrene intermediate from the photolysis of diazirenes. Unfortunately, we could not observe evidence of the postulated diradical imidoyl nitrene SN1 which, according to the reported data, can be expected to be mixed with the strong signals at ~ G, which were derived from doublet impurities.
For more information, please visit 5-Aminotetrazole Anhydrous.