Solventless synthesis of poly(pyrazolyl) phenyl-methane ligands and thermal transformation of tris(3,5-dimethylpyrazol-1-yl)phenylmethane

The solventless synthesis of tris(pyrazolyl)phenylmethane ligands of formula C₆H₅C(Pz^R2)₃ (R = H, Me), starting from PhCCl₃ and 3,5-dimethylpyrazole (Pz^Me2) or pyrazole (Pz) was performed. The sterically crowded C₆H₅C(Pz^Me2)₃ is thermally transformed into the bis(pyrazolyl)(p-pyrazolyl)phenylmethane ligand Pz^Me2-C₆H₄CH(Pz^Me2)₂. In this compound both Pz^Me2 rings are linked through the N-atom to the methine C-atom. At higher temperatures, the binding mode of Pz^Me2 changes from N1 to C4. All transformations occurred via quinonoid carbocation intermediates that undergo an aromatic electrophilic substitution on the 4-position of Pz^Me2. Reaction conditions were established to obtain five tris(pyrazolyl)phenylmethane ligands in moderate to good yields. ¹H- and ¹³C-NMR spectroscopy and X-ray diffraction of single crystals support the proposed structures.