C-alkylpyrogallol[4]arenes (abbreviated as PgC_n, where n is the length of the associated alkyl tail), which are vase-shaped macrocyclic host molecules composed of 1,2,3-trihydroxybenzene units, have been determined over the past decade to be versatile building blocks for the construction of supramolecular complexes. Since the initial discovery by Atwood et al. in 2005 of PgC_n-based metal-organic nanocapsules (MONCs) constructed from six PgC_n units and 24 Cu²⁺ ions, a number of studies have demonstrated that PgC_n canassembled with Mg, Co, Ni, Cu, Zn and Ga ions to form MONCs in different shapes including hexameric octahedral, dimeric spherical and hexameric “rugby ball” shapes. Interestingly, the assemblies of the dimeric spherical and hexameric octahedral MONCs could be controlled by external stimuli including solvents and temperatures. However, PgC_n-based MONCs are limited to the aforementioned metal ions, and still have the possibility of synthesizing new PgC_n-based nanocapsules and exerting control over their self-assembly behavior.

A search of the CCDC database for metal complexes composed of the o-dihydroxybenzene and 1,2,3-trihydroxybenzene subunit reveals that they can also coordinate with trasition metals such as Ti, V, Mn, Fe and so on, and also lanthanide ions. In these results, we have pay much more attention to cyclotricatechylene (CTC), a macrocyclic molecule composed of o-dihydroxybenzene units, which is much simialr to PgC_n molecules. Notably, CTC can assemble with VO²⁺ centers to form tetrahedral MONCs. Bear this in mind, we have try to use PgC_n and VO²⁺ ions to construct MONCs. Fortunatly, we have obtained a ball-shaped capsule (V₂₄-ball) constructed from 24 vanadium centers and 6 PgC₃ units. In attempt to synthesize the dimeric spherical V₈ MONC is failed, but its quasi-isomer, hexameric octahedral V₂₄ capsule (V₂₄-oct) is accidentally obtained. Their structural differences are due to the coordination geometry differences in V centers. Speciafically, the V centrers adopt five-coordinated square-pyramidal and six-coordinated octahedral geometries for V₂₄-ball and V₂₄-oct, respectively.

It has been observed that the five-coordinate square pyramidal and six-coordinated octahedral oxidovanadium complexes can interconvert by associating and disassociating an axial molecule. With this in mind, we searched for conditions which promote the interconversion between the contracted V₂₄ octahedron and the expanded V₂₄ ball. Interestingly, we found that the axial water molecules of vanadium centers in V₂₄ octahedron are removed in DMF/CH₃OH (1:1, v/v) solution at 80 ^oC, the DMF working as a dehydrating agent; while those vanadium centers in V₂₄ ball-shaped capsule can capture the water molecules in DMF/CH₃CN/H₂O/NEt₃ (20:80:10:1, v/v/v/v) solution at 80 ^oC. Following such structural interconversions, the magnetic properties are significantly changed. Specifically, the V⁴⁺ centers in V₂₄ ball are antiferromagnetic coupled, while V₂₄ octahedron ferromagnetic at higher temperature.

Overall, our work not only provides a new strategy for efficient construction of metal-organic nanocapsule quasi-isomers by controlling the coordination environments of the metal centers, but also profoundly improves the understanding the transformation process and their structure-property relationships in stimuli-responsive assembly system.

For further information, please see our paper “Interconvertible vanadium-seamed hexameric pyrogallol[4]arene nanocapsules" published in Nature Communications from here.