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

Abstract

Tetravalent cerium alkoxide complexes supported by the Klaui tripodal ligand [Co(eta(8)-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)]center dot MeCONH2 (1) with (PrOH)-Pr-i or reaction of [Ce-IV(L-OEt)(2)Cl-2] (2) with Ag2O in (PrOH)-Pr-i afforded the Ce(IV) dialkoxide complex [Ce-IV(L-OEt)(2)((OPr)-Pr-i)(2)] (3-iPr). 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-iPr 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)-Pr-i)Cl] (4). Treatment of 3-iPr with HX and methyl triflate (MeOTf) afforded [Ce(L-OEt)(2)X-2] (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-iPr reacted with water in hexane to give a Ce(III) complex and a Ce(IV) species, presumably the reported tetranuclear oxo cluster ( [Ce-4(IV)(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-iPr 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)((Bu4Cl2H4O2H)-Bu-i)] (7) ((Bu4Cl2H4O2H2)-Bu-i = 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.