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 Klaüi tripodal ligand [Co(η 5 -C 5 H 5 ){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)(H 2 O)]•MeCONH 2 (1) with i PrOH or reaction of [Ce IV (L OEt ) 2 Cl 2 ] (2) with Ag 2 O in i PrOH afforded the Ce(IV) dialkoxide complex [Ce IV (L OEt ) 2 (O i Pr) 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 Ag 2 O 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 (O i Pr)Cl] (4). Treatment of 3-iPr with HX and methyl triflate (MeOTf) afforded [Ce(L OEt ) 2 X 2 ] (X − = Cl − , NO 3 − , PhO − ) and [Ce IV (L OEt ) 2 (OTf) 2 ], respectively, whereas treatment with excess CO 2 in hexane led to isolation of the Ce(IV) carbonate [Ce IV (L OEt ) 2 (CO 3 )]. 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 IV 4 (L OEt ) 4 (O) 5 (OH) 2 ]. The Ce(IV) alkoxide complexes are capable of oxidizing substituted phenols, possibly via a protoncoupled 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 ( t Bu 4 C 12 H 4 O 2 H)] (7) ( t Bu 4 C 12 H 4 O 2 H 2 = 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.

Synthesis and Reactivity of Redox-Active Cerium(IV) Aryloxide Complexes

“…2-F 2,6 was reduced by decamethylferrocene (E 1/2 = −0.59 V vs Fc +/0 , MeCN) 29 in hexane, but it did not react with 1,1′-dimethylferrocene (E 1/2 = −0.11 V vs Fc +/0 , MeCN), 30 suggesting that its reduction potential should lie between −0.59 and −0.10 V vs Fc +/0 . Under the same conditions, no reaction was found between decamethylferrocene and [Ce IV (L OEt ) 2 (O i Pr) 2 ], 15 indicating the redox potential of the Ce(IV) alkoxide complex should be <−0.59 V. This result is consistent with the order of donor strength of the ligand: alkoxide > aryloxide.…”

“…Reaction of 1 with 2,4,6-trisubstituted phenols, except 2,4,6-trichlorophenol (vide infra), also yielded Ce­(III) aryloxides. For example, treatment of 1 with 2,4,6-tri- tert -butylphenol (tbp) afforded [Ce III (L OEt ) 2 (OC 6 H 2 t Bu 3 -2,4,6)], which has been previously synthesized from [Ce IV (L OEt ) 2 (O i Pr) 2 ] and tbp …”

“…The possibility of generating a reactive aryloxyl radical via Ce–O aryl homolysis of 2-F 2,6 led us to explore its reactivity toward organic substrates bearing reactive X–H bonds such as substituted phenols (Scheme ). Similar to [Ce IV (L OEt ) 2 (O i Pr) 2 ], 2-F 2,6 can oxidize di-ortho-substituted phenols such as 2,6-di- tert -butylphenol (dbp) and 2,4,6-tri- tert -butylphenol (tbp) to give the quinone products.…”

“…Ce­(IV) aryloxide complexes have been synthesized by (a) protonolysis of Ce­(IV) silylamides with aryl alcohols, (b) salt metathesis of Ce­(IV) chloride with group 1 metal aryloxides, and (c) oxidation of Ce­(III) complexes with 1,4-benzoquinones. ,, Recently, we found that while [Ce IV (L OEt ) 2 (O i Pr) 2 ] reacted with phenol to give the bis­(phenoxide) complex [Ce IV (L OEt ) 2 (OPh) 2 ], it was reduced rapidly to Ce­(III) by substituted phenols, such as 2,4,6-tri- tert -butylphenol (tbp), 2,4-di- tert -butylphenol (dbp), and 2,6-diphenylphenol, presumably via Ce–O aryl homolysis of reactive Ce­(IV) aryloxide intermediates (Scheme ). The latter result is unexpected given the reported ability of aryloxide ligands to stabilize Ce­(IV). In fact, the oxidation of Ce­(III) complexes by 1,4-benzoquinones to give Ce­(IV) aryloxides is well precedented. , The steric and electronic factors that affect the outcome of reactions of Ce­(III) complexes with substituted 1,4-benzoquinones have been elucidated by Anwander and co-workers recently …”

“…4c,12,14 Recently, we found that while [Ce IV (L OEt ) 2 (O i Pr) 2 ] reacted with phenol to give the bis(phenoxide) complex [Ce IV (L OEt ) 2 (OPh) 2 ], it was reduced rapidly to Ce(III) by substituted phenols, such as 2,4,6-tri-tert-butylphenol (tbp), 2,4-di-tert-butylphenol (dbp), and 2,6-diphenylphenol, presumably via Ce−O aryl homolysis of reactive Ce(IV) aryloxide intermediates (Scheme 1). 15 The latter result is unexpected given the reported ability of aryloxide ligands to stabilize Ce(IV). In fact, the oxidation of Ce(III) complexes by 1,4benzoquinones to give Ce(IV) aryloxides is well precedented.…”