Synthesis, structure and sulfonamide-sulfonimide tautomerism of sulfonamide-1,2,4-triazine derivatives

Two new 1,2,4-triazine-containing sulfonamide derivatives, namely, 4-bromo-N-(5,6-diphenyl-2H-1,2,4-triazin-3-ylidene)benzenesulfonamide, C₂₁H₁₅BrN₄O₄S, 3a, and methyl 2-{[(5,6-diphenyl-1,2,4-triazin-3-yl)sulfamoyl]methyl}benzoate, C₂₄H₂₀N₄O₄S, 3b, which crystallize in the different sulfonimide and sulfonamide tautomeric forms, respectively, were synthesized and characterized by spectroscopic, X-ray diffraction and theoretical calculation methods. Both molecules adopt a very similar conformation of the common part of the structure and the differences occur within the substituents on the sulfonamide group. The amino groups characteristic for the existing tautomeric forms are involved in strong intermolecular N-H...N and N-H...O hydrogen bonds in 3a and 3b, respectively. The Hirshfeld surface analysis showed that H...H contacts constitute a high percentage of the intermolecular interactions. Theoretical calculations at the ab initio DFT/B3LYP/6-311++G(d,p) level showed that the two tautomeric forms observed for 3a and 3b can co-exist in chloroform, ethanol and water solutions, with a distinct predominance of the sulfonamide form; the participation of the sulfonimide form increases with increasing solvent polarity.