Anion-π Interaction Powered Perrhenate Recognition and Extraction in Aqueous Media

Selective anion recognition and extraction in aqueous media is a challenging research topic, and the anion-π interaction is an undetermined solution for the development of anion sorbent materials with better affinity and selectivity. Here, noncovalent anion-π interaction was introduced as the driving force for this purpose. A cage-based 2D cationic metal-organic framework, IPM-21, is featured with porous channels formed by complementary V-shape electron-deficient cavities. This 3D rhombic electron-deficient cavity can bind two anions with the clipped π-acidic surfaces, exhibiting much higher affinity toward ReO₄^- due to the strong complementary effect. This cavity was forced to expand its opening size to seamlessly adopt the ReO₄^- anion with a large volume. Experimental results found that the binding energy of IPM-21 with ReO₄^- is around 2.3 kJ/mol higher than that with ClO₄^-. Parts per million levels of the ReO₄^- anion in aqueous media can be effectively extracted by IPM-21 with a removal up to 99%, even with mixed competing anions. IPM-21 can be easily recycled and reused by treatment with high concentration aqueous NaClO₄. Due to the extremely low interlamellar interaction, the IPM-21 crystal exhibited enhanced ReO₄^- extraction performance with the recycling times due to self-exfoliation; as a result, ultrathin IPM-21 nanosheets with large lateral sizes were produced in this process.