Synthesis and characterization of transition metal complexes bearing tetrafluoro-4-pyridyl substituent on the cyclopentadienyl ring

The reaction of sodium cyclopentadienide (NaCp) with pentafluoropyridine gives Na[4-(C5F4N)C5H4] (PyFCpNa, 1) contaminated with starting NaCp from which pure 1 could be extracted with Et2O. Hydrolysis of 1 and subsequent crystallization gives pure Diels–Alder dimer 1,4-bis(tetrafluoro-4-pyridyl)tricyclo[5.2.1.02,6]deca-3,8-diene (2). The reactions of 1 with FeCl2, [MnBr(CO)5], CoBr2, [Ni(NH3)6]Cl2, [TiCl4(THF)2] and [CpTiCl3] cleanly affords the corresponding metallocenes [Fe(PyFCp)2] (3), [(PyFCp)Mn(CO)3] (5), [Co(PyFCp)2] (6), [Ni(PyFCp)2] (8), [(PyFCp)2TiCl2] (9) and [(PyFCp)(Cp)TiCl2] (10), respectively. Tetrafluoro-4-pyridyl-substituted ferrocene 3 and [Fe(PyFCp)(Cp)] (4) can be alternatively prepared by the reaction of the respective lithioferrocenes with C5F5N in THF. Air-oxidation of complex 6 affords the corresponding cobaltocenium salt [Co(PyFCp)2]PF6 (7). All prepared compounds were characterized spectroscopically and by elemental analysis. The crystal structures of 3–7 were determined, revealing extensive arene π ⋯ π stacking and C–H ⋯ F–C contacts. Electrochemical studies supported with the spectroscopic data of the prepared metallocene complexes evidenced strong electron-withdrawing nature of the tetrafluoro-4-pyridyl substituent.