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(PzR2)₃ (R = H, Me), starting from PhCCl₃ and 3,5-dimethylpyrazole (PzMe2) or pyrazole (Pz) was performed. The sterically crowded C₆H₅C(PzMe2)₃ is thermally transformed into the bis(pyrazolyl)(p-pyrazolyl)phenylmethane ligand PzMe2-C₆H₄CH(PzMe2)₂. In this compound both PzMe2 rings are linked through the N-atom to the methine C-atom. At higher temperatures, the binding mode of PzMe2 changes from N1 to C4. All transformations occurred via quinonoid carbocation intermediates that undergo an aromatic electrophilic substitution on the 4-position of PzMe2. 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.