Cerium(IV) Hexanuclear Clusters from Cerium(III) Precursors: Molecular Models for Oxidative Growth of Ceria Nanoparticles.

Reactions of cerium(III) nitrate, Ce(NO3 )3 ⋅6 H2 O, with different carboxylic acids, such as pivalic acid, benzoic acid, and 4-methoxybenzoic acid, in the presence of a tridentate N,N,N-donor ligand, diethylenetriamine (L(1) ), under aerobic conditions yielded the corresponding cerium hexamers Ce6 O8 (O2 CtBu)8 (L(1) )4 (1), Ce6 O8 (O2 CC6 H5 )8 (L(1) )4 (2), and Ce6 O8 (O2 CC6 H4 -4-OCH3 )8 (L(1) )4 (3). Hexamers 1, 2, and 3 contain the same octahedral Ce(IV) 6 O8 core, in which all interstitial oxygen atoms are connected by μ3 -oxo bridging ligands. In contrast, treatment of the Ce(IV) precursor (NH4 )2 Ce(NO3 )6 (CAN) with pivalic acid and the ligand L(1) under the same conditions afforded Ce6 O4 (OH)4 (O2 CtBu)12 (L(1) )2 (4), exhibiting a deformed octahedral Ce(IV) 6 O4 (OH)4 core containing μ3 -oxo and μ3 -hydroxo moieties in defined positions. In contrast to the formation of 1-3, the use of N-methyldiethanolamine (L) in the reaction with Ce(NO3 )3 ⋅6 H2 O and pivalic acid afforded a previously reported Ce(III) dinuclear cluster, Ce2 (O2 CtBu)6 L2 , even in the presence of dioxygen. ESI-MS analysis of the reaction mixture clearly indicated the importance of the ligand L(1) in promoting oxidation of the Ce(III) aggregates, [Cen (O2 CtBu)3n (L(1) )2 ], which is necessary for the formation of Ce(IV) hexamers.