Hosting of [Cs(crown ether)2]+ type host–guest complexes by a nano-sized molecular cuboid

Abstract

Encapsulation of a guest by host-forming (G-in-H)-type host–guest complexes is the central theme of supramolecular chemistry. Binding of typical hosts (H_in) in the cavity of a larger host (H_out) to form (H_in-in-H_out) type assemblies is also known. However, the incorporation of host–guest complexes in a larger host to form [(G-in-H_in)-in-H_out] type assemblies, where (G-in-H_in) is composed of one guest and one host is relatively unexplored. A recent report describes the binding of host–guest complexes consisting of one guest and two hosts in a larger host molecule. In the present work, we have prepared a nano-sized polycationic molecular cuboid of the type Pd(L′)₈(L)₄ as the H_out, which is capable of encapsulating two units of H_in such as dibenzo[3n]crown-n (where n ranges from 6 to 8). The molecular cuboid is found to selectively recognize the larger-sized dibenzo[24]crown-8 and dibenzo[21]crown-7 over the smaller-sized dibenzo[18]crown-6. The polycationic molecular cuboid can bind a family of [Cs(crown ether)₂]⁺ type cationic host–guest complexes composed of one unit of a guest (Cs⁺) and two units of the H_in (crown ether). All these complexes are prepared via self-assembly in a water medium; the surrogate nuclear waste Cs⁺ is deeply internalized, being encapsulated by two units of crown ether, which are in turn sheltered by the molecular cuboid. All the [G-in-(H_in)₂-in-H_out] type assemblies are characterized using spectroscopic techniques and single-crystal structures. The relative orientation of the pair of crown ethers around the Cs⁺ ion is found to be different in the native form compared to the encapsulated form in the molecular cuboid.