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

Treatment of CpCeClK(THF) with alkali-metal alkoxides and siloxides in the presence of hexachloroethane generates the monomeric bis(pentamethylcyclopentadienyl) cerium(IV) complexes CpCe(OR) (Cp* = CMe; R = Et, Pr, CHBu, Bu, SiMe, or SiPh). Large substituents R trigger ligand scrambling to half-sandwich complexes CpCe(OR), which could be isolated for R = Bu and SiPh. Similar reactions with sodium aryloxide NaOAr (OAr = OCHPr-2,6) led to CpCe(OAr)Cl. Treatment of tris(cyclopentadienyl) complexes CpCeCl (Cp = Cp = CH; Cp = CHMe) with NaOAr afforded CpCe(OAr) and CpCe(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 CpCe(OR) than in the case of CpCe(OR). Enhanced stabilization of the cerium +IV oxidation state could be further demonstrated in the series alkoxy > siloxy > aryloxy as well as CMe > CHMe.