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

Abstract

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.

Cerium Fluorenyl Complexes Including CC Coupling Reactions

“…11−13 The electron-donating capability of the ligands was ascribed an important role in stabilizing the cerium(IV) oxidation state. 10,14,15 In the case of unsymmetrical ruthenocene complexes, the relative electron-donating power of Cp-type ligands was shown to increase in the order pentachlorocyclopentadienyl, acetylcyclopentadienyl, cyclopentadienyl, indenyl, pentamethylcyclopentadienyl, and fluorenyl by X-ray photoelectron spectroscopy and electrochemical measurements. 16 This is consistent with our findings regarding the redox potentials of cyclopentadienyl and methylcyclopentadienyl complexes of cerium(IV).…”

“…Cyclopentadienyl (Cp R ) ligands (Cp R = C 5 H 5 , C 5 H 4 Me, C 5 H 4 SiMe 3 , C 5 H 3 (SiMe 3 ) 2 , and C 5 Me 5 ) were shown to be capable of stabilizing the +IV oxidation state of cerium. Since these Cp R ligands also support chemically and often electrochemically reversible cerium-centered redox properties, it stirred up the question of the feasibility of a similar ceric fluorenyl (Flu) chemistry. , As aforementioned, in contrast to the strictly η 5 -bound cyclopentadienyl ligands, fluorenyl is prone to η 5 → η 3 → η 1 coordination switches, thus tailoring metal centers to enhanced reactivity and alternative reaction pathways. ,, …”

“…Since these Cp R ligands also support chemically and often electrochemically reversible cerium-centered redox properties, it stirred up the question of the feasibility of a similar ceric fluorenyl (Flu) chemistry. 14,15 As aforementioned, in contrast to the strictly η 5 -bound cyclopentadienyl ligands, fluorenyl is prone to η 5 → η 3 → η 1 coordination switches, thus tailoring metal centers to enhanced reactivity and alternative reaction pathways. 20,28,29 When treating CeCl 3 (THF) 1.04 with 1−5 equiv.…”

“…Targeted Cl/R exchange should further improve the thermodynamic and electrochemical stabilization of any envisioned/putative Ce(IV) fluorenyl complexes, particularly in the case of R = alkoxy or siloxy. 14,15,39 Accordingly, the following ligand types could be implemented in complexes 3 R : cyclopentadienyl (using NaCp), aryloxy (NaOC 5 H 3 iPr 2 -2,6 = NaOAr), alkoxy (NaOtBu), siloxy (NaOSiMe 3 ), and pyrazo- The η 5 -fluorenyl ligand is sterically more demanding than the aryloxy ligand, giving space to the additional coordination of only two THF molecules. 40 For further comparison, the half-sandwich complex Cp*Ce(OC 5 H 3 tBu 2 -2,6) 2 synthesized by Heeres et al, bearing a slightly bulkier aryloxy ligand, shows significantly longer Ce−O distances (avg.…”

“…Since cerium provides ready access to the oxidation state +IV, we were tempted to investigate into the redox chemistry of cerium-fluorenyl complexes. The redox potential of cerium strongly depends on its ligand environment, but organocerium­(IV) compounds such as Cp 3 CeCl are isolable and have been fully characterized. − The electron-donating capability of the ligands was ascribed an important role in stabilizing the cerium­(IV) oxidation state. ,, In the case of unsymmetrical ruthenocene complexes, the relative electron-donating power of Cp-type ligands was shown to increase in the order pentachlorocyclopentadienyl, acetylcyclopentadienyl, cyclopentadienyl, indenyl, pentamethylcyclopentadienyl, and fluorenyl by X-ray photoelectron spectroscopy and electrochemical measurements . This is consistent with our findings regarding the redox potentials of cyclopentadienyl and methylcyclopentadienyl complexes of cerium­(IV) .…”