Self-Assembly of Polyoxometalate-Based Metal Organic Frameworks Based on Octamolybdates and Copper-Organic Units: from CuII, CuI,II to CuI via Changing Organic Amine

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Abstract

Six polyoxometalate (POM)-based hybrid materials have been designed and synthesized based on octamolybdate building blocks and copper-organic units at different pH values under hydrothermal conditions, namely, [H2bbi][Cu(II)(bbi)2(beta-Mo8O26)] (1), [Cu(II)(bbi)2(H2O)(beta-Mo8O26)0.5] (2), [Cu(II)(bbi)2(alpha-Mo8O26)][Cu(I)(bbi)]2 (3), [Cu(II)Cu(I)(bbi)3(alpha-Mo8O26)][Cu(I)(bbi)] (4), [Cu(I)(bbi)]2[Cu(I)2(bbi)2(delta-Mo8O26)0.5][alpha-Mo8O26]0.5 (5), and [Cu(I)(bbi)][Cu(I)(bbi)(theta-Mo8O26)0.5] (6), where bbi is 1,1'-(1,4-butanediyl)bis(imidazole). Their crystal structures have been determined by X-ray diffraction. In compound 1, the bbi ligands with bis-monodentate coordination modes link Cu(II) cations to generate a 2D copper-organic unit with (4, 4) net, which is pillared by the (beta-Mo8O26)(4-) anions to form a 3D framework with alpha-Po topology. The similar copper-organic units are connected alternately by (beta-Mo8O26)(4-) anions to generate a 3D 2-fold interpenetrating (4,6)-connected framework with (4(4) x 6(2))(4(4) x 6(10) x 8) topology in compound 2. Compounds 3 and 4 are supramolecular isomers with polythreaded topology. If Cu (I)...O interactions are considered, the structure of 3 is a novel self-penetrating (3,4,6)-connected framework with (5(2) x 8)2(5(4) x 6 x 8)(4(4) x 6(10) x 10) topology, and the structure of 4 is a (4,6)-connected framework with (4(2) x 6(3) x 7)(5.6(4) x 8)(4(2) x 5(6) x 6(6) x 8)(4(2) x 5(6) x 6(4) x 7 x 8(2)) topology. Different from compounds 3 and 4, compounds 5 and 6 are supramolecular isomers with polythreaded topology based on different octamolybdate isomers. By careful inspection of the structures of 1-6, it is believed that various copper-organic units, which are formed by bbi ligands combined with Cu(II)/Cu(I) cations, octamolybdates with different types and coordination modes, and the nonbonding interactions between polyanions and copper-organic units are important for the formation of the different structures. In addition, with step by step increasing of the amount of organic amine, we have achieved the transformation of Cu(II) ions into Cu(I) ones in different degrees in POMs-based metal-organic frameworks (MOFs) for the first time. The infrared spectra, X-ray powder diffraction, and thermogravimetric analyses have been investigated in detail for all compounds, and the luminescent properties have been also been investigated for compounds 3 and 4.

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