Pentamethylcyclopentadienyl Complexes of Cerium(IV): Synthesis, Reactivity, and Electrochemistry

Treatment of Cp*₂CeCl₂K(THF) with alkali-metal alkoxides and siloxides in the presence of hexachloroethane generates the monomeric bis(pentamethylcyclopentadienyl) cerium(IV) complexes Cp*₂Ce(OR)₂ (Cp* = C₅Me₅; R = Et, iPr, CH₂tBu, tBu, SiMe₃, or SiPh₃). Large substituents R trigger ligand scrambling to half-sandwich complexes Cp*Ce(OR)₃, which could be isolated for R = tBu and SiPh₃. Similar reactions with sodium aryloxide NaOAr (OAr = OC₆H₃iPr₂-2,6) led to Cp*₂Ce(OAr)Cl. Treatment of tris(cyclopentadienyl) complexes Cp^R₃CeCl (Cp^H = Cp = C₅H₅; Cp^Me = C₅H₄Me) with NaOAr afforded Cp^Me₂Ce(OAr)₂ and Cp₃Ce(OAr). The cerium(IV) complexes display a pseudotetrahedral geometry in the solid state. Cyclic voltammetry revealed mostly chemically reversible as well as electrochemically quasi-reversible redox processes with potentials ranging from -0.84 to -1.61 V versus Fc/Fc⁺. Switching from sandwich to half-sandwich complexes decreased the electrochemical potentials drastically, showing better stabilization of the cerium(IV) center in the case of Cp*Ce(OR)₃ than in the case of Cp*₂Ce(OR)₂. Enhanced stabilization of the cerium +IV oxidation state could be further demonstrated in the series alkoxy > siloxy > aryloxy as well as C₅Me₅ > C₅HMe₄.