Solid State Separation and Isolation of Tautomers of Fused-Ring Triazolotriazoles

Fine control of the tautomeric forms of [1,2,4]­triazolo­[3,2-c]­[1,2,4]­triazole derivatives in acidic conditions has been achieved by acting on the electronic character of the substituent at position 7 of the heterobicycle and on the counterion. Strong electron releasing or electron withdrawing substituents lead almost exclusively to a single tautomeric form, the 1H-3H or the 2H-3H, respectively. In the case of the phenol substituent, both tautomeric forms are present in comparable amount in solution; the two tautomers can also be selectively precipitated in different crystalline salts using suitable counterions.