Tuning Organocerium Electrochemical Potentials by Extending Tris(cyclopentadienyl) Scaffolds with Terminal Halogenido, Siloxy, and Alkoxy Ligands

Treatment of cerous CpR3Ce(thf) (CpR = C5H4R; R = H, Me) with the halogenating reagents C2Cl6, TeBr4, and I2 afforded the ceric halides CpR3CeX (X = Cl, Br, I) in high yield. Subsequent salt metathesis with sodium alkoxides and siloxides led to a series of alkoxy and siloxy derivatives. Compounds CpR3CeOR′ with R′ = Me, Et, CH2tBu, iPr, tBu, SiMe3, SiEt3, Si(iPr)3 SiPh3 (and Si(OtBu)3) have been isolated and characterized by 1H, 13C, and 29Si NMR and DRIFT spectroscopy, magnetic measurements, X-ray structure analyses, cyclic voltammetry, and elemental analyses. The ceric complexes CpR3CeX and CpR3CeOR′ are isostructural, featuring terminal ligands X and OR′. The magnetic measurements revealed temperature-independent paramagnetism (TIP), with positive magnetic susceptibilities in the range χ0 (1.53–3.9) × 10–4 emu/mol. Cyclic voltammetry indicated two types of redox processes: (a) chemical and electrochemical reversibility for halide and siloxide complexes and (b) EC- or ECE-type mechanisms for the alkoxides (chemical reversibility at high scan rates). In all cases formal potentials could be determined ranging from −0.583 V vs Fc/Fc+ for Cp3CeI to −1.259 V vs Fc/Fc+ for CpMe3Ce(OEt). The electrochemical data revealed an increase in stabilization with respect to reduction of the cerium(IV) center in the series I < Br < Cl < siloxy < alkoxy ligand and a better stabilization with CpMe in comparison to Cp ligands by approximately 0.05–0.1 V. As a result, an improved stabilization of Ce(IV) was observed for more strongly electron donating ligands.