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

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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.