If Bis-nitrotriazoles weren’t enough for you…

A couple weeks ago, I talked about a patent published by the Klapötke group in which a series of bis(aminotriazole) salts were prepared and characterized.  It’s pretty neat stuff, and the molecules showed pretty solid energetic performance across the board.

Well, as luck would have it, another publication from the same group came out last week.  This piece is a followup on some chemistry from back in 2015 [1], wherein they prepared some triazole tetrazoles bearing nitro, azido, and amino ring substituents (compounds 5–8).  You know, for the severely carbon-averse out there.  A quick snapshot of this chemistry is reproduced below.  Compound 5 is afforded in ~64% overall yield in 5 steps.  The process chemists out there will appreciate this detail: all compounds are prepared by chromatography-free means.

Reproduced from [1]

Get your azoles, now with 33% more nitrogen!  — Reproduced from [1]

As one might expect, the sensitivities of the four parent compounds to external stimuli (impact, friction, ESD) ranged from “quite” to “extremely.”

The scope of the recent offering is to use these materials as energetic anions with various metallic and non-metallic cations [2].  Since the tetrazole proton is basically holding on for dear life — as evidenced by a 1H chemical shift of 16.19 ppm — preparing these salts was a matter of treating the parent compounds with a basic solution of the desired cation.  The goal here being use metal cations (a–c) to produce sensitive primary explosives, and nitrogen-rich cations (d–h) to produce less sensitive secondary explosives.

Get your nitrogen heterocycles, now with 33% more nitrogen!

Nitrogenous energetic materials 5–8 and corresponding metal (a–c) and nitrogen rich (d–h) cations — Reproduced from [2]

Silver’s an interesting choice; I’m generally content to not find myself in the same room as large quantities of any compound with both “azido” and “silver” in its name, but nonetheless, compound 6c exists, albeit only briefly.  The compound has impact and friction sensitivities below the minimum test threshold using the BAM method.  To quote the text:

The yield [of 6c] was not determined owing to the extremely high friction sensitivity of this compound.

The cesium analog, 6b, fares slightly better.  It holds together long enough to get some spectroscopy data, and the impact, friction, and ESD sensitivities of this compound were high, but measurable.  The text notes the ESD value is slightly less sensitive than lead azide, which is commonly employed in blasting caps.  But high sensitivity alone does not necessarily make for a good primary explosive — the material must be capable of initiating detonation in a secondary explosive.  To that end, a mass of RDX was loaded into a copper tube sealed at one end, and 50 mg of compound 6b was layered on top.  Firing with an electrical detonator resulted in the image below:

rdx det

Left to right: intact copper tube; copper tube after deflagration of 100 mg 6b; copper tube fragments after detonation of 300 mg RDX initiated by 50 mg 6b — Reproduced from [2].

So it does in fact kick off secondary explosives quite nicely — fragments of metal casings are often a welcome sign in energetic research.  It is also noted that compounds 6a–c all undergo a deflagration to detonation transition (DDT), although it’s not clear how this was determined.  Small samples of explosives generally do not detonate unless they are confined or there is a critical diameter present.  But I digress.

Now onward to the so-called “nitrogen rich” salts.  A total of 14 nitrogenous salts were prepared, however four were only isolable as the corresponding dihydrates: 5d, and 8-2e–g (compound 8 formed the corresponding dianion).  The remaining ten demonstrated a very high degree of insensitivity.  With the exception of 7e, all were less sensitive than RDX in BAM impact and friction, and ESD tests.  Additionally, all were within a 10% margin of RDX in both detonation velocity and detonation pressure according to EXPLO5 calculations.  Likewise, almost all burned cooler than RDX.

One major advantage of using nitrogen-based HEDMs is that they are non-oxidative.  That is, no oxidizing salts (say, perchlorate, nitrate), and minimal organic nitro-groups, so metal components of gun barrels won’t break down as quickly.  Since deflagrations/detonations are far from ideal from a stoichiometry standpoint, you can end up with significant amounts of NO and NO2 (and even HCl if your propellant is ammonium perchlorate).  Those gases get pretty warm in armament combustion chambers and can seriously damage barrel bores, which can lead to critical failure.  But with nitrogen heterocylces, you reduce the formation of nitrogen oxides and instead form mostly N2, and eliminate chlorine entirely.

So to that end, the authors reformulated two propellants, HN-1 and HN-2 (HN = “High nitrogen”), which are used in very large bore guns.  The compositions are mostly RDX, with some TAGzT, a relatively sensitive triaminoguanidinium salt of 5,5′-azotetrazole.  Substituting compound 5h, for TAGzT resulted in about a 10% boost in the specific energy of the formulation, with a modest increase in combustion temperature.

This is a perfect example of one of the fundamental challenges of energetics research, which I mentioned in passing in my previous post: you can’t have it all.  You can always cram more energy density into a molecule — just stick more nitro groups on it, or replace a triazole with a tetrazole.  But increasing the energy density comes at a price, almost without exception: your molecule burns hotter, or is more sensitive, or both.  And the authors acknowledge this, stating that while the combustion temperature increases, it is still well below the critical temperature for gun propellants.

References

  1. Dalton Trans., 2015, 44, 17054, DOI: 10.1039/c5dt03044g
  2. Euro. J. Inorg. Chem., 2016, DOI: 10.1002/ejic.201600108
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Bigger booms, through chemistry

The Klapötke group at LMU is marching relentlessly onward with their quest to find new and interesting ways to stick as many nitrogen atoms onto one molecule in as close proximity as (barely) possible for long enough to get NMR data.

You may remember the Klapötke group from Derek’s post over at ItP in the “Azidoazide Azide” issue of Things I Won’t Work With.  This is the group that would look at pentazole and think “Gee, I wonder if we could replace that proton with an azide…”  I’ve always thought this kind of work was pretty cool; most of these crazy nitrogen heterocycles are practically useless but they serve the important purpose of giving us a better understanding of the nature of chemical bonds at the margin of what is possible.

Klapötke et al is back with a published patent application that showed up on my scanner.  This time, they’ve taken a step back from the realm of the ridiculous and have prepared a reasonable looking energetic active ingredient: 3,3′-dinitro-5,5′-bis-triazole-1,1′-diol (and a couple bis salts thereof).

untitled
And that structure looks not at all unreasonable.  Sure, electron deficient triazoles aren’t the most stable, but that hydroxyl contributes some electron density back to the ring system.  Oxygen balance looks good.  Slightly under-oxidized, actually, which as a rule gives you a bit of stability back.

But enough with speculation, let’s take a look at the thermal and sensitivity data provided in the text.  In energetics, RDX is commonly used as a benchmark: it has good (not great) explosive performance, and it reasonably insensitive to impact, friction, and electrostatic discharge.  Interestingly, the application does not present characterization data on the parent diol, but instead offers three salts: dihydroxylammonium (MAD-X1), diguanidinium (MAD-X2), and di-triaminoguanidinium (MAD-X3).

And the lead compound, MAD-X1, outperforms RDX across the board: better sensitivity in all three metrics, high detonation velocity (9.3 km/s to RDX’s 8.7), greater crystal density, higher thermal decomposition onset, larger heat of formation, and lower detonation temperature.  As anyone who works in the field knows, it’s really hard to have it all; you can always increase you explosive performance… at the expense of sensitivity.  And vice versa.  But, as far as performance metrics go, MAD-X1 seems to pretty handily have a leg up on the competition.

Even the synthesis is pretty straightforward and uses decidedly non-exotic reagents.  First, oxalic acid is condensed with aminoguanidinium bicarbonate in concentrated HCl, then worked up under basic conditions, affording 3,3′-diamino-5,5′-bis-(1H-1,2,4-triazole) (“DABT”).  DABT is then oxidized to the bis-nitro derivative as the corresponding dihydrate, which is fantastic from a energetics processing standpoint.  Treatment with potassium peroxymonosulfate affords the anhydrous diol, which reacts subsequently with an ethanolic solution of hydroxylamine, which yields MAD-X1 in 44% overall yield over four steps.

synthesis of MADx1

While not as concise as the two-step Bachmann process, which yields RDX from hexamethylenetetramine in 57% overall yield on an industrial scale, Klapötke’s preparation of MAD-X1 appears scalable.  Namely, it dispenses with the wildly exothermic nitrolysis process used to make nitramines — if you’ve ever had the pleasure of performing such a reaction you’ll know it’s incredibly easy to end up with a runaway reaction and a resultant yield rapidly approaching zero.  Do that on a large scale, and you’ll have a pilot plant rapidly approaching low earth orbit.

Overall, I’m pretty impressed with this compound’s prep and apparent utility.  My main criticism is: how’s that alkoxide salt going to hold up in an environment where metals are present?  Namely, in a casing or shell.  If the the use of picric acid has taught us anything, it’s that acidic energetics tend to not play well with metals.  I’d love to see some followup formulation work addressing this issue.

The Tools of the Trade

Ever wonder what an organic chemistry lab looks like?  Well, wonder no more!  I put together this virtual tour of sorts for the curious.  I’m going to take you through the facilities and equipment used by those who do what I do.

Tools of the trade: a virtual tour of an organic chemistry lab

The first stop every morning is the same: I boot up my computer and check Twitter.  Twitter has become an immensely useful tool for staying on top of new chemical research and gathering opinions from other scientists.  Other bloggers often discuss new methods from recent publications, and combined with scanning the feeds of major chemistry journals, I can get a rapid grasp on what’s important today.

That, and cracking lame chemistry jokes.

Next, it’s time to head down to my hood, where I spend the majority of my time.  For the uninitiated, a fume hood is a cabinet that pulls in air, creating negative pressure in the area where I’m doing experiments.  The air is then run through the ventilation system, and safely vented outside the building.  This prevents such unpleasant happenings as me getting a face-full of toxic hydrogen fluoride gas, or tripping out on diethyl ether fumes.

Where the magic (read: science) happens

Where the magic (read: science) happens

You’ll also notice a bunch of stuff sitting around my fume hood.  Starting from the top left, we have shelves of reagents and some glassware I use very frequently.  Then there’s the long, glass tube running horizontally across the top of the hood.  That’s called a Schlenk line; one end is hooked up to an inert gas (nitrogen or argon) and the other end to a vacuum pump.  This allows me to remove air and atmospheric water from a reaction flask, and replace it with inert gas.  Some compounds are highly sensitive to air.  Continuing to the right, you’ll see some waste containers, and a number of solvent bottles containing chemicals I use so frequently it makes sense to keep a stock of them in my hood.  Doubling back to the left (foreground) you’ll see a couple of hot plates, which double as magnetic stirrers.  These allow precise control of reaction temperature.

Everything so far is something you’d find in pretty much any organic chemist’s workspace.  However, you may have noticed the large aluminium plate making up the counter of the hood.  Since I work with explosive material, it is critical that the risk of static discharge be eliminated.  Rubbing your feet on carpet and then touching the compounds I work with would be inadvisable, to put it lightly.  So, my entire workspace is electrically grounded, including the floor in front of the hood.

We work with compounds that are SUPPOSED to go boom.  Ideally not mid-experiment though.

We work with compounds that are SUPPOSED to go boom. Ideally not mid-experiment though.

Another component of my workspace somewhat unique to explosive operations is steel armor plating.  The sides and rear of my fume hood are surrounded by ¼” thick steel plates.  In the unfortunate event of an accidental detonation, any fragmentation would be restricted from adjacent labs and hallways.

Rotary Evaporators

Rotary Evaporators

Next to my work area are two rotary evaporators or “RotaVaps.”  These allow chemists to rapidly remove solvent from reaction mixtures, isolating the products of the reaction.  By placing the mixture under a vacuum and providing gentle heating, the solvent boils much more rapidly (and at a lower temperature) than it otherwise would.  A cold trap then cools the gaseous solvent, where it condenses into a liquid in an adjacent flask.  Since these RotaVaps are also used for explosive material, they are also armored, just like my hood.

Now let’s check out the bread and butter of any organic chemist: the most powerful tool in my structural determination arsenal.  I am, of course, talking about nuclear magnetic resonance spectroscopy (NMR).  NMR is used to analyze the number and chemical environment of certain active nuclei.  Most commonly analyzed is hydrogen, followed by carbon; fluorine, nitrogen, and phosphorous are also active, but much less commonly analyzed.  NMR allows chemists to rapidly determine whether or not a reaction has worked as intended, and to verify the structure of compounds.

Our nuclear magnetic resonance spectrometer, AKA my one true love.

Our nuclear magnetic resonance spectrometer, AKA my one true love.

As always, thanks for reading!  Feel free to let me know how you felt about this piece, comments, questions, and suggestions are always welcome!

-Mitch

Safety First!

Safety First!