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 C(6)H(5)C(Pz(R2))(3) (R = H, Me), starting from PhCCl(3) and 3,5-dimethylpyrazole (Pz(Me2)) or pyrazole (Pz) was performed. The sterically crowded C(6)H(5)C(Pz(Me2))(3) is thermally transformed into the bis(pyrazolyl)(p-pyrazolyl)phenylmethane ligand Pz(Me2)-C(6)H(4)CH(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. (1)H- and (13)C-NMR spectroscopy and X-ray diffraction of single crystals support the proposed structures.