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

Oxo- and hydroxo-bridged hexanuclear Ce(IV) clusters surrounded by 12 carboxylate ligands, Ce 6 O 4 (OH) 4 (O 2 CR) 12 (L) n (R = 2,6-Me 2 -4-MeOC 6 H 2 ( 1a ), 2,6-Me 2 -4- t BuC 6 H 2 ( 1b ), 2,4,6-Me 3 C 6 H 2 ( 1c ), 2,6-Me 2 C 6 H 3 ( 1d ), 2,6-Me 2 -4-FC 6 H 2 ( 1e ), 2,6-Me 2 -4-ClC 6 H 2 ( 1f ), 9-anthracenyl ( 1g ), and CH 2 t Bu ( 1h ), L = H 2 O or RCO 2 H), were synthesized by treating Ce(O t Bu) 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, M 6 O 4 (OH) 4 (O 2 CR) 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(3 H )-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 [Cp 2 Fe] + /Cp 2 Fe for E 1/2 ). The one-electron-reduced Ce(III)Ce(IV) 5 clusters 3a-h were isolated by reducing 1a-h with Cp* 2 Co to give [Cp* 2 Co][Ce 6 O 4 (OH) 4 (O 2 CR) 12 (thf) n ] ( 3a-h ); cluster 3d was the first structurally determined hexanuclear cerium cluster containing a [Ce 6 O 4 (OH) 4 ] 11+ core.