Solid state selection between nearly isoenergetic tautomeric forms driven by right hydrogen-bonding pairing

We have prepared new hydroxyphenyl derivatives of [1,2,4]triazolo[3,2-c][1,2,4]triazole and investigated their tautomeric behavior, for the neutral and the monoprotonated forms, either computationally and experimentally in solution and the solid state. The results of our analysis indicate that the tautomeric behavior, in particular for the monoprotonated forms, is strongly dependent on the position of the OH group in the phenyl ring. The different position of the group (ortho, meta, para) has an effect on the relative energy of the various tautomeric forms of the cations but also on their ability to get an optimum packing, which is driven by directional hydrogen bonds. In particular, with the OH in para position, the two tautomers 1H-3H and 2H-3H of the singly protonated form have a similar energy and can be selectively isolated in the solid state with different counterions; with OH in the meta position, though the energy difference between the tautomers is even smaller, only the 2H-3H is present in the solid state, because of optimum packing interactions (hydrogen-bonding pairing); finally, with ortho OH, only the 1H-3H is isolated in the solid state because it has energy significantly lower than 2H-3H.