Base-Directed Self-Assembly of Octamolybdate-Based Frameworks Decorated by Flexible N-Containing Ligands

Abstract

Eight polyoxometalate (POM)-based frameworks were synthesized under hydrothermal conditions, using different octamolybdate isomers (Mo8) and copper-organic fragments or organic units in the presence of different bases. These compounds are named as [CuI(bix)][(CuIbix)(δ-Mo8O26)0.5] (1), H(CuIbix)[(CuIbix)2(β-Mo8O26)]·2H2O (2), H3[CuI(bix)2(β-Mo8O26)0.5](β-Mo8O26)0.5·2H2O (3), (H2bix)2(β-Mo8O26)0.5(β-Mo8O26)0.5·H2O (4), (H2bix)[(Hbix)2(γ-Mo8O26)]·2H2O (5), H2bix(γ-Mo8O26)0.5·H2O (6), [CuI(bpp)][CuI(bpp)(α-Mo8O26)0.5]·H2O (7), and [(H2bpp)2(β-Mo8O26)] (8) [bix = 1,4-bis(imidazole-1-ylmethyl)benzene, bpp = 1,3-bis(4-pyridyl)propane]. Their structures were determined by single-crystal X-ray diffraction. In compounds 1 and 2, four copper cations are linked by octamolybdate isomers to generate a 2D layer with (4,4) sheet, which is pillared by bix ligands to form a 3D (4,4)-connected framework with (64·82)(84·122) and (62·81)2(62·82·102) topologies, respectively. With the peneation of the copper-organic polymeric chains, a 3D polythreaded framework was formed. Compound 3 has a 3D supramolecular network based upon (β-Mo8O26)4− anions and copper-organic units, with guest (β-Mo8O26)4− anions included. In compound 4, a 3D supramolecular arrangement is constructed by the hydrogen-bond interactions between (β-Mo8O26)4− polyanions from different 2D structures and bix ligands. Compound 5 exhibits a unique bisupported structure built from Mo−N covalent bonds between (γ-Mo8O26)4- anions and bix ligands. This compound represents a rare example of organic-molecule-functionalized POM clusters that can be synthesized under hydrothermal conditions. In compound 6, a 3D supramolecular net is generated by hydrogen bonding interactions among water molecules, oxygen atoms of octamolybdate and nitrogen atoms of bix ligands. In compound 7, the bpp ligands link Cu cations and (α-Mo8O26)4− anions to generate a 1D ladder-like chain. [Cu(bpp)]22+ fragments further connect with (α-Mo8O26)4- anions from different chains to form a 2D (3,4)-connected layer with (4281)(6184101) topology. Compound 8 also has 3D supramolecular framework based on C−H···O hydrogen bonds between bpp and (β-Mo8O26)4− polyoxoanions. It is proposed that the different basicity, steric hindrance, and reducing strength of the bases used are the key factors in generating different Mo8 isomers in the reaction process, and thus the different frameworks as seen in 1−8. A ligand synergetic effect is also taken into account to explain the presence of the same Mo8 isomers in 7 and 8, which were synthesized with different bases. IR, TG, and electrochemistry study were performed to further characterize these compounds.