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

The steric and electronic effects on the stability and reactivity of cerium(IV) aryloxide complexes supported by the Kläui tripodal ligand [Co(η5-C5H5){P(O)(OEt)2}3]− (LOEt–) have been investigated. A Ce(IV) oxo species (1) prepared in situ from [Ce(LOEt)2Cl2] and Ag2O 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: (A) [CeIV(LOEt)2(OAr)2] for Ar = Ph (2-H), 2,4-X2C6H3 [X = Br (2-Br2,4), Cl (2-Cl2,4)], 3,5-Br2C6H2 (2-Br3,5), or 2,6-F2C6H3 (2-F2,6); (B) [CeIII(LOEt)2(OAr)] for Ar = 2,4,6-X3C6H2 (X = tBu) or 2,6-X2C6H3 (X = tBu, Cl); or (C) [CeIV(LOEt)2(OAr)Cl] (3-Cl2,4,6) for Ar = 2,4,6-Cl3C6H2. The Ce(IV) aryloxide chloride complex [CeIV(LOEt)2(OC6H3Cl2-2,4)Cl] (3-Cl2,4) has been synthesized by ligand exchange between 2-Cl2,4 and [CeIV(LOEt)2Cl2]. 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-Br2,4, 2-Br3,5, 2-Cl2,4, and 2-F2,6. Among the Ce(IV) bis(aryloxide) complexes synthesized, 2-F2,6 was found to be most reactive with respect to Ce(IV/III) reduction. In hexane, 2-F2,6 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-F2,6 has a triplet excited state lying 7.1 kcal mol–1 above the singlet ground state. The oxidation of tbp by 2-F2,6 involves Ce–Oaryl 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.