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

Abstract

The multicomponent mixed-metal Cu-II/Mo-VI oxides/1,3-bis(1,2,4-triazol-4-yl)adamantane (tr(2)ad) system was thoroughly studied employing a compositional diagram approach. The concept allowed us to prepare three layered copper-molybdate hybrid solids [Cu-2(II)(tr(2)ad)(4)](Mo8O26) (1), [Cu-4(II)(mu(4)-O)(tr(2)ad)(2)(MoO4)(3)]center dot 7.5H(2)O (2), and [Cu-2(I)(tr(2)ad)(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)center dot H2O. Independent experiments carried out with Cu(OAc)(2) and (NH4)(6)Mo7O24 in the absence of tr(2)ad, under the same conditions, revealed the formation of low-valent and bimetallic oxides, including Cu2O, MoO2, Cu(Mo3O10)center dot H2O, and Cu-3(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 hydroperoidde (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 Cu-II centers (J = 0.41 cm(-1)), compound 2 shows distinct intraduster antiferromagnetic exchange interactions (J = 29.9 cm(-1), J' = 25.7 cm(-1)), which consequently results in a diamagnetic ground state.