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

Abstract

The steric and electronic effects on the stability and reactivity of cerium(IV) aryloxide complexes supported by the Klaüi tripodal ligand [Co(η 5 -C 5 H 5 ){P(O)(OEt) 2 } 3 ] − (L OEt − ) have been investigated. A Ce(IV) oxo species (1) prepared in situ from [Ce(L OEt ) 2 Cl 2 ] and Ag 2 O has been used as a starting material for the synthesis of Ce(IV) aryloxide complexes. The reactions of 1 with aryl alcohols (ArOH) in hexane resulted in formation of three possible products:) has been synthesized by ligand exchange between 2-Cl 2,4 and [Ce IV (L OEt ) 2 Cl 2 ]. No obvious correlation was found between the Ce−O bond lengths [2.138(6)−2.215(3) Å]/Ce−O−C angles [138.6(3)−166.8( 3)°] and the substituents of the aryloxide ligands in 2-H, 2-Br 2,4 , 2-Br 3,5 , 2-Cl 2,4 , and 2-F 2,6 . Among the Ce(IV) bis(aryloxide) complexes synthesized, 2-F 2,6 was found to be most reactive with respect to Ce(IV/III) reduction. In hexane, 2-F 2,6 is capable of oxidizing 2,4,6-tri-tert-butylphenol (tbp) and 9,10-dihydroanthrancene, presumably via an H-atom abstraction pathway. Density functional theory calculations indicated that 2-F 2,6 has a triplet excited state lying 7.1 kcal mol −1 above the singlet ground state. The oxidation of tbp by 2-F 2,6 involves Ce−O aryl bond homolysis of the triplet excited state and H-atom transfer between the resulting 2,6-difluorophenoxyl radical and tbp. This work highlights the influence of the steric effect on the stability and reactivity of redox-active Ce(IV) aryloxide complexes.

Reactivity of [TismPriBenz]MgH and [TismPriBenz]MgMe towards Carbonyl Compounds: Access to Terminal Alkoxide and Enolate Complexes

“…While it is possible to infer that an increase in M–O–R bond angle is associated with an increase in oxygen to metal π-donation, previous studies have noted that there is often relatively little correlation between the M–O–R bond angle and M–OR bond length in structurally characterized compounds . In this regard, rather than invoking π-bonding, the shorter M–OR bond lengths in linear alkoxides have been attributed to an increased ionic component. − Supporting this suggestion, a molecular orbital analysis indicates that the lone pair p orbitals on oxygen in [Tism Pr i Benz ]­MgOCH 2 Ph exhibit little interaction with the magnesium center (Figure ).…”