Treatment of cerous Cp R 3 Ce(thf) (Cp R = C 5 H 4 R; R = H, Me) with the halogenating reagents C 2 Cl 6 , TeBr 4 , and I 2 afforded the ceric halides Cp R 3 CeX (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 Cp R 3 CeOR′ with R′ = Me, Et, CH 2 tBu, iPr, tBu, SiMe 3 , SiEt 3 , Si(iPr) 3 SiPh 3 (and Si(OtBu) 3 ) have been isolated and characterized by 1 H, 13 C, and 29 Si NMR and DRIFT spectroscopy, magnetic measurements, X-ray structure analyses, cyclic voltammetry, and elemental analyses. The ceric complexes Cp R 3 CeX and Cp R 3 CeOR′ are isostructural, featuring terminal ligands X and OR′. The magnetic measurements revealed temperature-independent paramagnetism (TIP), with positive magnetic susceptibilities in the range χ 0 (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 Cp 3 CeI to −1.259 V vs Fc/Fc + for Cp Me 3 Ce(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 Cp Me 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.

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, CH2 tBu, 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 = OC6H3 iPr2-2,6) led to Cp*2Ce­(OAr)­Cl. Treatment of tris­(cyclopentadienyl) complexes CpR 3CeCl (CpH = Cp = C5H5; CpMe = C5H4Me) with NaOAr afforded CpMe 2Ce­(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.

The first structurally characterized fluorenyl (Flu) complexes of cerium are reported, bearing one, two, and three fluorenyl ligands. The reaction of CeX 3 (THF) x (X = Cl and I) with KFlu led to the half-sandwich complexes FluCeX 2 (THF) 3 . The chloride derivative was utilized in salt metathesis reactions, affording complexes FluCeR 2 (THF) x with R = OtBu, OSiMe 3 , OC 6 H 3 iPr 2 -2,6, Me 2 Pz, and Cp (x = 1 or 2; Me 2 Pz = 3,5dimethylpyrazolato, Cp = C 5 H 5 ). The mono(fluorenyl) complexes FluCeR 2 (THF) x are prone to ligand redistribution at ambient temperature, leading to the respective sandwich complexes Flu 2 CeR(THF). Utilization of K(2,10-tBu 2 Flu) (KFlu tBu ) afforded tris(fluorenyl) complex Flu tBu 3 Ce(THF) instead, showcasing two η 5 -and one η 1 -bound fluorenyl ligands. Treatment of FluCeCl 2 (THF) 3 with halogenating oxidants such as C 2 Cl 6 , I 2 , or TeBr 4 did not afford stable cerium(IV) species but mixtures of 9halogenidofluorene and 1,1′-bifluorene. Selective fluorenyl coupling reactions could be achieved for C 2 Cl 6 . Respective investigations with lanthanum and neodymium complexes support σ-bond metathesis as the main reaction path for C−C-bond formation.