Oxidizing Cerium(IV) Alkoxide Complexes Supported by the Kläui Ligand [Co(η⁵-C₅H₅){P(O)(OEt)₂}₃]^-: Synthesis, Structure, and Redox Reactivity

Tetravalent cerium alkoxide complexes supported by the Kläui tripodal ligand [Co(η⁵-C₅H₅){P(O)(OEt)₂}₃]^- (L_OEt^-) have been synthesized, and their nucleophilic and redox reactivity have been studied. Treatment of the Ce(IV) oxo complex [Ce^IV(L_OEt)₂(O)(H₂O)]·MeCONH₂ (1) with PrOH or reaction of [Ce^IV(L_OEt)₂Cl₂] (2) with Ag₂O in PrOH afforded the Ce(IV) dialkoxide complex [Ce^IV(L_OEt)₂(OPr)₂] (3-Pr). The methoxide and ethoxide analogues [Ce^IV(L_OEt)₂(OR)₂] (R = Me (3-Me), Et (3-Et)) have been prepared similarly from 2 and Ag₂O in ROH. Reaction of 3-Pr with an equimolar amount of 2 yielded a new Ce(IV) complex that was formulated as the chloro-alkoxide complex [Ce^IV(L_OEt)₂(OPr)Cl] (4). Treatment of 3-Pr with HX and methyl triflate (MeOTf) afforded [Ce(L_OEt)₂X₂] (X^- = Cl^-, NO₃^-, PhO^-) and [Ce^IV(L_OEt)₂(OTf)₂], respectively, whereas treatment with excess CO₂ in hexane led to isolation of the Ce(IV) carbonate [Ce^IV(L_OEt)₂(CO₃)]. 3-Pr reacted with water in hexane to give a Ce(III) complex and a Ce(IV) species, presumably the reported tetranuclear oxo cluster [Ce^IV₄(L_OEt)₄(O)₅(OH)₂]. The Ce(IV) alkoxide complexes are capable of oxidizing substituted phenols, possibly via a proton-coupled electron transfer pathway. Treatment of 3-Pr with ArOH afforded the Ce(III) aryloxide complexes [Ce^III(L_OEt)₂(OAr)] (Ar = 2,4,6-tri-tert-butylphenyl (5), 2,6-diphenylphenyl (6)). On the other hand, a Ce(III) complex containing a monodeprotonated 2,2'-biphenol ligand, [Ce^III(L_OEt)₂(Bu₄C₁₂H₄O₂H)] (7) (Bu₄C₁₂H₄O₂H₂ = 4,4',6,6'-tetra-tert-butyl-2,2'-biphenol), was isolated from the reaction of 3-Pr with 2,4-di-tert-butylphenol. The crystal structures of complexes 3-Pr, 3-Me, 3-Et, and 5-7 have been determined.