Supramolecular assembly of covalently modified Anderson-type polyoxometalates and crown ethers towards pseudo-rotaxane structures

The supramolecular assembly of a series of covalently modified Anderson-type polyoxometalates (POMs) and 18-crown-6 ethers was investigated for the first time. Seven new POM–crown ether complexes were prepared by varying the countercations and the modification mode of the Anderson clusters, and they were fully characterized by single-crystal X-ray diffraction, electrospray ionization time-of-flight mass spectrometry (ESI–TOF–MS), ¹H nuclear magnetic resonance (NMR), etc. Alkali metal cations within the cavities of the crown ethers served as supercations to bind to the POM hybrids, thereby forming supramolecular assemblies with different architectures and compositions. More interestingly, five of the seven POM–crown ether complexes possessed novel pseudo-rotaxane structures, which were obtained by changing the modification mode of the Anderson clusters from symmetric to asymmetric. These pseudo-rotaxanes have not been reported previously and are considered important intermediates in the construction of POM-based supramolecular architectures with complementary structural and chemical properties.