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Triazolyl–Based Copper–Molybdate Hybrids: From Composition Space Diagram to Magnetism and Catalytic Performance

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journal contribution
posted on 06.10.2014 by Ganna A. Senchyk, Andrey B. Lysenko, Artem A. Babaryk, Eduard B. Rusanov, Harald Krautscheid, Patrícia Neves, Anabela A. Valente, Isabel S. Gonçalves, Karl W. Krämer, Shi-Xia Liu, Silvio Decurtins, Konstantin V. Domasevitch
The multicomponent mixed-metal CuII/MoVI oxides/1,3-bis­(1,2,4-triazol-4-yl)­adamantane (tr2ad) system was thoroughly studied employing a compositional diagram approach. The concept allowed us to prepare three layered copper–molybdate hybrid solids [CuII2(tr2ad)4]­(Mo8O26) (1), [Cu4II4-O)­(tr2ad)2(MoO4)3]·7.5H2O (2), and [CuI2(tr2ad)2]­(Mo2O7)·H2O (3), and to elucidate the relationship between initial reagent concentration/stoichiometry and the stability of the resultant structural motifs. Compounds 1 and 2 were found to dominate throughout a wide crystallization range of the concentration triangle, whereas compound 3 was formed by redox processes in the narrow crystallization area having a high excess of Cu­(OAc)2·H2O. Independent experiments carried out with Cu­(OAc)2 and (NH4)6Mo7O24 in the absence of tr2ad, under the same conditions, revealed the formation of low-valent and bimetallic oxides, including Cu2O, MoO2, Cu­(Mo3O10)·H2O, and Cu3(MoO4)2(OH)2. Compounds 1 and 2 show high thermal and chemical stability as examined as catalysts in the epoxidation of cis-cyclooctene and the oxidation of benzyl alcohol (BzOH) with different types of oxidants. The oxidation reaction of BzOH using tert-butyl hydroperoxide (TBHP) as the oxidant, in the presence of 1 or 2, led to benzaldehyde and benzoic acid (PhCO2H), with the latter being formed in up to 90% yield at 24 h. The results suggest that 1 and 2 may be favorable heterogeneous catalysts for the synthesis of PhCO2H. Whereas compound 1 only reveals a weak ferromagnetic coupling between neighboring CuII centers (J = 0.41 cm–1), compound 2 shows distinct intracluster antiferromagnetic exchange interactions (J = −29.9 cm–1, J′ = −25.7 cm–1), which consequently results in a diamagnetic ground state.