Perbromate. A riddle, wrapped in a mystery, inside an enigma; but perhaps there is a key.

Chemists love a mystery as much as anyone. And gaps in patterns can be mysterious. Mendeleev’s period table had famous gaps which led to new discovery. And so from the 1890s onwards, chemists searched for the perbromate anion, BrO₄^–. It represented a gap between perchlorate and periodate, both of which had long been known. As the failure to turn up perbromate persisted, the riddle deepened. Finally, in 1968, the key was found, but talk about sledgehammer to crack a nut! It was done by alchemical-like radioactive transmutation of selenium into bromine:

Se⁸³O₄^2- → Br⁸³O₄^– + β^–

Once the psychological barrier had been surmounted, a chemical synthesis provided enough perbromic acid to show it was a stable, high boiling liquid. So, the failure to make it was not because it was unstable!

XeF₂ + NaBrO₃ → NaBrO₄

Once quantities were available, the thermodynamic and redox properties could be measured. This did little to solve the riddle. Although it was found to be a better oxidant than periodate, this was not considered enough to explain why it had proved so elusive. The theoreticians got in on the act, but their article too did little to resolve matters; the calculations merely verified the experimental measurements.

To this day, little perbromate has been made, and so much of its chemistry remains a mystery. Only in 2011 has a synthesis appeared which could potentially result in large and hence cheap quantities, by formation through the carefully controlled reaction of hypobromite and bromate ions in an alkaline sodium hypobromite solution.

Periodate has found much utility in organic synthesis as an oxidant, and perchlorate is a very interesting non-coordinating counter ion in metal catalysis. Who knows what use might transpire for perbromate!

So, unlike the gaps in the periodic table, plugging the perbromate gap has not yet resulted in unexpected discoveries. But it is worth speculating why any given compound may be non-existent. It may be thermodynamically unstable, and hence have too short a lifetime to be isolated (not the case for perbromate). Or all of the possible kinetic pathways to its formation may have unfeasibly large barriers. The key here is the word all; if one searches long enough, a route that works will probably be found.

The other side of the coin is novel types of compounds that may well exist, but no-one has anticipated trying to make them precisely because they are so novel. I am thinking here of the wonderfully entitled article “Mindless Chemistry” where systematic exploration of ALL possible minima for a given molecular formula revealed a whole zoo of species which the speculative chemist would never have dreamt of trying to make (in other words, they did not manifest as obvious gaps in the patterns that constitute our present chemical knowledge). I do often think about all of these undiscovered molecules, and if they could indeed be synthesised and their properties studied. One such occurred in silicon chemistry; truly the existence of an isomer of hexasilabenzene was not predicted before it was made, and its properties (aromaticity) did indeed prove fascinating and new.

Too long the focus of synthetic chemistry has been to try to make molecules that nature has already synthesized. Perhaps we should focus as well on molecules that nature has never deigned to make, but which are nevertheless entirely viable (as was the case for perbromate).