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).
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.
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.