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 Kl & auml;ui tripodal ligand [Co(eta(5)-C5H5){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 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) [Ce-IV(L-OEt)(2)(OAr)(2)] for Ar = Ph (2-H), 2,4-X2C6H3 [X = Br (2-Br-2,Br-4), Cl (2-Cl-2,Cl-4)], 3,5-Br2C6H2 (2-Br-3,Br-5), or 2,6-F2C6H3 (2-F-2,F-6); (B) [Ce-III(L-OEt)(2)(OAr)] for Ar = 2,4,6-X3C6H2 (X = Bu-t) or 2,6-X2C6H3 (X = Bu-t, Cl); or (C) [Ce-IV(L-OEt)(2)(OAr)Cl] (3-Cl-2,Cl-4,Cl-6) for Ar = 2,4,6-Cl3C6H2. The Ce(IV) aryloxide chloride complex [Ce-IV(L-OEt)(2)(OC6H3Cl2-2,4)Cl] (3-Cl-2,Cl-4) has been synthesized by ligand exchange between 2-Cl-2,Cl-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) & Aring;]/Ce-O-C angles [138.6(3)-166.8(3)degrees] and the substituents of the aryloxide ligands in 2-H, 2-Br-2,Br-4, 2-Br-3,Br-5, 2-Cl-2,Cl-4, and 2-F-2,F-6. Among the Ce(IV) bis(aryloxide) complexes synthesized, 2-F-2,F-6 was found to be most reactive with respect to Ce(IV/III) reduction. In hexane, 2-F-2,F-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,F-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,F-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.