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 Klaui tripodal ligand [Co(η5-C5H5){P(O)(OEt)2}3]- (LOEt-) have been synthesized, and their nucleophilic and redox reactivity have been studied. Treatment of the Ce(IV) oxo complex [CeIV(LOEt)2(O)(H2O)]·MeCONH2 (1) with iPrOH or reaction of [CeIV(LOEt)2Cl2] (2) with Ag2O in iPrOH afforded the Ce(IV) dialkoxide complex [CeIV(LOEt)2(OiPr)2] (3-iPr). The methoxide and ethoxide analogues [CeIV(LOEt)2(OR)2] (R = Me (3-Me), Et (3-Et)) have been prepared similarly from 2 and Ag2O in ROH. Reaction of 3-iPr with an equimolar amount of 2 yielded a new Ce(IV) complex that was formulated as the chloro-alkoxide complex [CeIV(LOEt)2(OiPr)Cl] (4). Treatment of 3-iPr with HX and methyl triflate (MeOTf) afforded [Ce(LOEt)2X2] (X- = Cl-, NO3-, PhO-) and [CeIV(LOEt)2(OTf)2], respectively, whereas treatment with excess CO2 in hexane led to isolation of the Ce(IV) carbonate [CeIV(LOEt)2(CO3)]. 3-iPr reacted with water in hexane to give a Ce(III) complex and a Ce(IV) species, presumably the reported tetranuclear oxo cluster [CeIV4(LOEt)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-iPr with ArOH afforded the Ce(III) aryloxide complexes [CeIII(LOEt)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, [CeIII(LOEt)2(tBu4C12H4O2H)] (7) (tBu4C12H4O2H2 = 4,4',6,6'-tetra-tert-butyl-2,2'-biphenol), was isolated from the reaction of 3-iPr with 2,4-di-tert-butylphenol. The crystal structures of complexes 3-iPr, 3-Me, 3-Et, and 5-7 have been determined.