Waterproof architectures through subcomponent self-assembly

Abstract

Metal–organic containers are readily prepared through self-assembly, but achieving solubility and stability in water remains challenging due to ligand insolubility and the reversible nature of the self-assembly process. Here we have developed conditions for preparing a broad range of architectures that are both soluble and kinetically stable in water through metal(II)-templated (M^II = Co^II, Ni^II, Zn^II, Cd^II) subcomponent self-assembly. Although these structures are composed of hydrophobic and poorly-soluble subcomponents, sulfate counterions render them water-soluble, and they remain intact indefinitely in aqueous solution. Two strategies are presented. Firstly, stability increased with metal–ligand bond strength, maximising when Ni^II was used as a template. Architectures that disassembled when Co^II, Zn^II and Cd^II templates were employed could be directly prepared from NiSO₄ in water. Secondly, a higher density of connections between metals and ligands within a structure, considering both ligand topicity and degree of metal chelation, led to increased stability. When tritopic amines were used to build highly chelating ligands around Zn^II and Cd^II templates, cryptate-like water-soluble structures were formed using these labile ions. Our synthetic platform provides a unified understanding of the elements of aqueous stability, allowing predictions of the stability of metal–organic cages that have not yet been prepared.