Self-Process-Programmed Structural Diversity in Supramolecular Assembly Based on Polyoxometalate Anion and Halogensubstituted Bipyridine Cation

Abstract

To investigate the relationship between network connectivity and the types of anions and cations, we synthesized a series of supramolecular assemblies based on different polyoxometalate (POM) anions and halogen-substituted bipyridine cations, namely, (Hcpy)6H2[PMo12O40] (1), (Hcpy)3[PMo12O40]·2.2H2O (2), (Hcpy)2(Hbppy)2[PMo12O40]·2H2O (3), (Hcpy)2(Hbppy)H2[PMo12O40]·H2O (4), (Hbppy)4[GeW12O40]·3H2O (5), (Hbppy)4[β-Mo8O26] (6), and (H6/7bppy)7[P2W18O60]·5H2O (7) (cpy = 4-(5-chloropyridin-2-yl)pyridine and bppy = 4-(5-(4-bromophenyl)pyridin-2-yl)pyridine). These compounds have been structurally characterized by elemental analyses, IR spectra, thermogravimatric analysis, and X-ray crystallography. All supramolecular frameworks of 1−7 are guided by nonconvalent interactions, especially directional hydrogen bonds and halogen bonds. Research results reveal that decorating nitrogen heterocycles with inductive electron withdrawal halogen groups increases the tendency to form face-to-face π-stacking interactions between them. Anionic clusters also present different and interesting structural features: in 1 and 5 Keggin-type anions form a one-dimensional pillar-like structure, while forming a two-dimensional waved sheet in 4, and only isolated clusters in 2 and 3. Compounds 1−5 consist of Keggin-type clusters with different organic cpy and bppy cations. Complex 6 is a supramolecular assembly based on [β-Mo8O26]4− and bppy, whereas complex 7 is an assembly containing Dawson-type clusters in which anions allow the assembly of bppy cations into distorted-wave organic layers. These results show that the types and ratios of cations and anions play significant roles in fabricating inorganic−organic arrangements. The thermal and electrochemistry properties of these compounds are discussed.