& Cerium(IV) Cluster Growth Cerium(IV) Hexanuclear Clusters from Cerium(III) Precursors: Molecular Models for Oxidative Growth of Ceria Nanoparticles Laurent Mathey, [a, b] Mitali Paul, [a] Christophe CopØret, [b] Hayato Tsurugi,* [a] and Kazushi Mashima* [a] Abstract: Reactions of cerium(III) nitrate, Ce(NO 3 ) 3 ·6 H 2 O, with different carboxylic acids, such as pivalic acid, benzoic acid, and 4-methoxybenzoic acid, in the presence of a triden- tate N,N,N-donor ligand, diethylenetriamine (L 1 ), under aero- bic conditions yielded the corresponding cerium hexamers Ce 6 O 8 (O 2 CtBu) 8 (L 1 ) 4 (1), Ce 6 O 8 (O 2 CC 6 H 5 ) 8 (L 1 ) 4 (2), and Ce 6 O 8 (O 2 CC 6 H 4 -4-OCH 3 ) 8 (L 1 ) 4 (3). Hexamers 1, 2, and 3 con- tain the same octahedral Ce IV 6 O 8 core, in which all interstitial oxygen atoms are connected by m 3 -oxo bridging ligands. In contrast, treatment of the Ce IV precursor (NH 4 ) 2 Ce(NO 3 ) 6 (CAN) with pivalic acid and the ligand L 1 under the same conditions afforded Ce 6 O 4 (OH) 4 (O 2 CtBu) 12 (L 1 ) 2 (4), exhibiting a deformed octahedral Ce IV 6 O 4 (OH) 4 core containing m 3 -oxo and m 3 -hydroxo moieties in defined positions. In contrast to the formation of 1–3, the use of N-methyldiethanolamine (L) in the reaction with Ce(NO 3 ) 3 ·6 H 2 O and pivalic acid afforded a previously reported Ce III dinuclear cluster, Ce 2 (O 2 CtBu) 6 L 2 , even in the presence of dioxygen. ESI-MS analysis of the re- action mixture clearly indicated the importance of the ligand L 1 in promoting oxidation of the Ce III aggregates, [Ce n (O 2 CtBu) 3n (L 1 ) 2 ], which is necessary for the formation of Ce IV hexamers. Introduction Among the lanthanides, cerium has attracted special interest because of its two stable oxidation states, Ce III and Ce IV , which are amenable to chemical redox processes. [1] In fact, Ce IV -based compounds, such as cerium ammonium nitrate [CAN; (NH 4 ) 2 Ce(NO 3 ) 6 ], are widely used as versatile oxidants in organ- ic synthesis, [2] catalysis, [3] bioinorganic reactions, [4] and electro- chemistry. [5] In materials chemistry, such properties make ceria (CeO 2 ) a unique support with oxygen storage capacity. [1, 6] CeO 2 also has high thermal stability, [6b] and has thus been used as an important support in catalysis and as a solid electrolyte materi- al. [6a, 7] Consequently, a great deal of research effort has been di- rected towards the synthesis of ceria nanoparticles. For exam- ple, such materials have been obtained by the calcination of cerium complexes, [8] an electrochemical method, [9] solvother- mal [10] and hydrothermal [7a,8f,g] syntheses, a sol–gel process, [11] pyrolysis, [12] thermal decomposition, [13] and laser ablation. [14] The mechanistic details of the growing nuclearity of cerium oxide from mono- and lower nuclear compounds is not well understood, though control of the growth process is a key factor in designing and constructing materials with a wide range of uses, from catalysis to electronic devices. [15] There is thus a need to understand the chemistry of [Ce x O y ] clusters. Recently, syntheses of multinuclear cerium complexes have been developed by carefully selecting Ce precursors, [16] Ce III[16, 17] or Ce IV , [18] and adequate ligands to ensure the forma- tion of a dinuclear Ce III cluster (Figure 1, A) [17] and oxo-/hy- droxo-bridged hexanuclear Ce IV clusters (Figure 1, B, [18] C, [19] and D [20] ), respectively. Similarly, heterometal clusters [19] can be prepared by using Ce III as a starting material and Mn(OAc) 2 as both a source of a different metal and as an oxidant (Figure 1, E). We thus focused our attention on the formation of larger nanometer-sized cerium clusters as analogues of small ceria nanoparticles, through a controlled assembly process of mono- nuclear cerium precursors. Herein, we report the synthesis of Figure 1. Examples of cerium clusters reported in the literature: dimer A, hexamers B, C, and D, and mixed-metal hexamer E. [a] Dr. L. Mathey, M. Paul, Prof. Dr. H. Tsurugi, Prof. Dr. K. Mashima Department of Chemistry, Graduate School of Engineering Science, Osaka University, and CREST, JST, Toyonaka, Osaka 560-8531 (Japan) E-mail : tsurugi@chem.es.osaka-u.ac.jp mashima@chem.es.osaka-u.ac.jp [b] Dr. L. Mathey, Prof. Dr. C. CopØret Department of Chemistry and Applied Biosciences ETH Zürich, 8037 Zürich (Switzerland) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201501731. Chem. Eur. J. 2015, 21, 13454 – 13461 # 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 13454 Full Paper DOI: 10.1002/chem.201501731