Chemists have a familiarity with many elements and many compounds, from having worked with them or studied them in the literature. You get a feel for what’s “normal” and for what’s unusual, and there are quite a few degrees of the latter. Take compounds of bromine, for example. Most any working chemist will immediately recognize bromine (there are exceptions) because we don’t commonly encounter too many opaque red liquids with a fog of corrosive orange fumes above them in the container. Which is good. That’s bromine in oxidation state zero, elemental, and then you have bromide (oxidation state -1), one of the most common anions around. “Chlorides are rabble”, said Primo Levi in one of my favorite lines from The Periodic Table, and he was right about that, but bromides are not of much higher social standing. Every cation has a bromide salt, and it’s usually one of the cheaper ones in the catalog.
So far, so good. But bromine can also go up to +3 and +5 oxidation states, and there things start to get interesting. You can have various mixed-halogen things, all of which are reactive and toxic and are distinguished by their various degrees of vileness. And you can get all sorts of bromine-oxygen species, ranging from the pretty well-known ones like bromate ion (BrO3–) all the way up to. . .well, to the stuff described in this new paper., from Konrad Seppelt at the Freie Universität Berlin. It contains a whole list of new compounds that send my chemical intuition completely off the rails.
I have no “feel” for them whatsoever except a strong desire never to prepare any of them. Prepare any of them? I don’t even want to make the starting material. You know you’re in for a bumpy ride when your work needs something like bromine fluorine dioxide (bromyl fluoride, BrO2F); no one can claim that they weren’t warned. There hasn’t even been a reliable synthesis of that stuff until now – Seppelt describes a new one, from the aforementioned sodium bromate, which is fine, and bromine pentafluoride, which is not fine, because it’s a hideous oxidizing and fluorinating agent fit to fluorinate you right into the afterlife and whose attempted use in liquid rocket propellent mixtures was abandoned because it was too foul to work with, and, oh yeah, redistilled pure hydrogen fluoride, which is also about as far from “fine” as you can get. The SI of the paper casually mentions that you can use double vacuum distillation in a metal line to get your HF sufficiently anhydrous for the reaction, and you can go ahead and get cranking on that without waiting for me to show up.
You condense the latter two reagents onto a solid charge of the bromate at liquid nitrogen temperatures, and then let it warm all the way up to -78C, at which point a “vigorous” reaction sets in. Imagine running these things for the first time, waiting for said reaction and wondering if it’s going to stay inside your apparatus or invigorate itself all over the ceiling. Once you have made your bromyl fluoride, you raise the temperature a bit more to -40 and pump off the excess HF and pentafluoride, and you will want an extremely capable trap on the other end of that process, which according to the paper can take several hours, and probably had better. Finally, you sublime off the product from the solid residue in the tropical warmth of -10 or so and seal off that part of the tube.
You have now prepared the colorless solid bromine fluorine dioxide. What to do with it? Well, what you don’t do is let it warm up too far past +10C, because it’s almost certainly going to explode. Keep that phrase in mind, it’s going to come in handy in this sort of work. Prof. Seppelt, as the first person with a reliable supply of the pure stuff, set forth to react it with a whole list of things and has produced a whole string of weird compounds with brow-furrowing crystal structures. I don’t even know what to call these beasts: how, for example, do you refer to the cation in Br3O6+ triflate? What’s the name for the compound shown at right? Very few of us will have the need to name it, though – you make that one by condensing trifluoroacetic anhydride onto the bromyl fluoride at -196C, then warming and recrystallizing the solid from liquified freon to give yellow crystals. Those melt at -12C, and according to the paper and its SI, “The molten red liquid starts to gas slowly” and “inevitably explodes upon further warming“. Further experimentation runs a risk of exposure to further inevitabilities, and I’m glad that Prof. Seppelt’s expertise in the lab got him through all this.
The SI strongly warns readers that the preparations therein must not under any circumstances be scaled up, and that is clearly the advice of someone who has has your best interests at heart. Even at the amounts described, you will want an excellent and well-maintained vacuum line, access to noncommon nonhousehold reagents like the aforementioned bromine pentafluoride, a willingness to do things like redistill anhydrous HF, and you will at all times want to be suited up like you’re going to going to spay a velociraptor. Ah, the halogen chemist’s life for me, me hearties, yo-ho-ho and a barrel of. . .well, we still don’t know what to name it. Dang.