Tetrazole derivatives have featured several times here in "Things I Won't Work With", which might give you the impression that they're invariably explosive. Not so - most of them are perfectly reasonable things. A tetrazole-for-carboxyl switch is one of the standard med-chem tricks, standard enough to have appeared in several marketed drugs. And that should be recommendation enough, since the FDA takes a dim view of exploding pharmaceuticals (nitroglycerine notwithstanding; that one was grandfathered in). No, tetrazoles are good citizens. Most of the time.
It's when they get put in with the wrong sort of company that they turn delinquent. What with four nitrogens in the ring and only one carbon, they do have a family history of possible trouble - several sections of this blog category could just as accurately be called Things That Suddenly Want To Turn Back Into Elemental Nitrogen. And thermodynamically, there aren't many gently sloping paths down to nitrogen gas, unfortunately. Both enthalpy and entropy tilt things pretty sharply. A molecule may be tamed because it just can't find a way down the big slide, but if it can, well, it's time to put on the armor, insert the earplugs, and get ready to watch the free energy equation do its thing right in front of your eyes. Your heavily shielded eyes, that is, if you have any sense at all.
Nitro groups are just the kind of bad company I mean, since they both bring their own oxygens to the party and pull electrons around in delightfully destabilizing ways. So nitrotetrazole is already not something I'd feel good about handling (its metal salts are primary explosives), but today's paper goes a step further and makes an N-oxide out of a nitrogen on a nitrotetrazole ring. This both adds more oxygen and tends to make the crystal packing tighter, which raises the all-important kapow/gram ratio. (There is, of course, little reason to do this unless you feel that life is empty without sudden loud noises). The paper mentions that "Introducing N-oxides onto the tetrazole ring may . . . push the limits of well-explored tetrazole chemistry into a new, unexplored, dimension.", but (of more immediate importance) it may also push pieces of your lab equipment into unexplored parts of the far wall.
Turns out that you can make the N-oxides through pretty mild chemistry (oxone at room temp), which is surprising. Until now, only a handful of the things had been made, most using an indirect route using hydrazoic acid (next!). The only direct oxidation of a tetrazole had been done with the relentlessly foul hypofluorous acid (next! keep moving!), which itself has to be made fresh from fluorine gas (next! thank you! next!). These recipes pretty much excluded most reasonable people from skipping through this green and sunny field of knowledge. Still, if you're a reasonable person, you're probably not yearning to make nitrotetrazole oxides in the first place. These things have a way of evening out.
So what are these fine new heterocycles like, anyway? Well, the authors prepared a whole series of salts of the parent compound, using the bangiest counterions they could think of. And it makes for quite a party tray: the resulting compounds range from the merely explosive (the guanidinium salts) to the very explosive indeed (the ammonium and hydroxyammonium ones). They checked out the thermal stabilities with a differential scanning calorimeter (DSC), and the that latter two blew up so violently that they ruptured the pans on the apparatus while testing 1.5 milligrams of sample. No, I'm going to have to reluctantly give this class of compounds a miss, as appealing as they do sound.
Several of the new compounds show similar detonation properties to RDX, albeit with less thermal stability. This is one reason we're reading about them in the open literature; the ideal explosive acts incredibly stable under a wide range of conditions, then loses its composure all at once at just the specified moment. We don't seem to be quite there yet. What I expect is that the authors are probably trying to work this same N-oxide magic on the azidotetrazolates instead of the nitro compounds. Now, that'll be a hoppin' bunch of compounds - for all I know, the research groups involved have already tried this and just haven't been able to get anything out past the lip of the flask yet. I'll be monitoring the literature for signs. On the other hand, if you live in Münich or College Park, you can probably monitor the progress of this work by listening for distant booming noises and the tinkle of glass.