Syntheses and Redox Properties of Carboxylate-Ligated Hexanuclear Ce(IV) Clusters and Their Photoinduced Homolysis of the Ce(IV)–Ligand Covalent Bond

Abstract

Oxo- and hydroxo-bridged hexanuclear Ce(IV) clusters surrounded by 12 carboxylate ligands, Ce6O4(OH)4(O2CR)12(L)n (R = 2,6-Me2-4-MeOC6H2 (1a), 2,6-Me2-4-tBuC6H2 (1b), 2,4,6-Me3C6H2 (1c), 2,6-Me2C6H3 (1d), 2,6-Me2-4-FC6H2 (1e), 2,6-Me2-4-ClC6H2 (1f), 9-anthracenyl (1g), and CH2tBu (1h), L = H2O or RCO2H), were synthesized by treating Ce(OtBu)4 with the corresponding carboxylic acids (2–3 equiv.) in acetone or toluene, and the molecular structures of 1d and 1g were clarified by X-ray diffraction studies. UV–vis analyses of the clusters showed broad absorption corresponding to the ligand-to-metal charge transfer (LMCT) in the ultraviolet A (315–400 nm) to blue light region; density functional theory (DFT) studies of the simplified Ce(IV) and related Zr(IV) clusters, M6O4(OH)4(O2CR)12 (M = Ce, Zr, R = Ph, Me), revealed that the low-lying vacant 4f-orbitals of the Ce(IV) were responsible for absorption in the ultraviolet A to blue light region. Irradiation of blue LED light to 1a–f under an argon atmosphere resulted in the formation of 7-methylisobenzofuran-1(3H)-one (2a–f), which involved the following four steps: photoinduced homolysis of the Ce(IV)–OCOR bond, intramolecular hydrogen atom transfer to generate the corresponding benzyl radical, oxidation to the benzyl cation, and intramolecular cyclization. Cyclic voltammetry of cerium clusters 1a–f having 2,6-dimethyl-4-substituted arylcarboxylate ligands showed electrochemically irreversible redox waves in the range of −0.79 to −0.38 V (vs [Cp2Fe]+/Cp2Fe for E1/2). The one-electron-reduced Ce(III)Ce(IV)5 clusters 3a–h were isolated by reducing 1a–h with Cp*2Co to give [Cp*2Co][Ce6O4(OH)4(O2CR)12(thf)n] (3a–h); cluster 3d was the first structurally determined hexanuclear cerium cluster containing a [Ce6O4(OH)4]11+ core.

Topics & Concepts