Theoretical and experimental study of the protonated 2,4,6-tri(2-pyridyl)- 1,3,5-triazine [TPTZH₂]²⁺

The reaction between 2,4,6-tri(2-pyridyl)-1,3,5-triazine (TPTZ) and sulfuric acid in the presence of NH₄PF₆ yielded crystals of [TPTZH₂](PF₆)₂·H₂O, characterized by spectroscopic methods and single-crystal X-ray diffraction. The structural data indicate that the diprotonated form, [TPTZH₂] ²⁺, is more planar than the neutral TPTZ molecule. Due to the presence of PF₆ ^- anions located between neighboring dications, the triazine and/or pyridyl rings in each dication cannot interact in a π-π fashion. The proton affinity of TPTZ was computed using density function theory (B3LYP hybrid functional). Calculated thermochemical results and proton affinities indicate that for the first protonation, the favorable N atoms are different from those obtained from calculations based on Merz-Kollman atomic charges. For the second protonation, the proton affinities show a clear preference of the protonation on two N atoms, this time not contradicted by Merz-Kollman atomic charges, and in agreement with the experimental data. In the UV-Vis spectrum, two visible absorption bands of [TPTZH₂](PF ₆)₂·H₂O are most likely arising from an outer-sphere charge-transfer. The photoluminescence properties of TPTZ and [TPTZH₂](PF₆)₂ were studied at room temperature in CH₃CN solution. Graphical abstract: [Figure not available: see fulltext.]