Supramolecular "Shish Kebabs"

Noncovalent interactions and hierarchically assembled structures are often important in biological systems. Dimerization or further oligomerization into more sophisticated assemblies can be important for the function and stability of the resulting structures. Understanding and controlling such assembly processes would, thus, be useful. Employing abiotic chemical systems for understanding higher-order superstructures, however, can be challenging. In the field of supramolecular chemistry, such higher-order structures can, for example, be created using ring-in-ring systems based on host-guest recognition.

Xiaopeng Li, Shenzhen University, Guangdong, China, Cheng Yang, Lihua Yuan, Sichuan University, Chengdu, China, and colleagues have synthesized a ring-in-ring system that can form discrete higher-order structures. The team used cyclobis(paraquat-o-phenylene) tetracation (o-Box) or its isomer cyclobis(paraquat-p-phenylene) tetracation (p-Box) as a guest and a shape-persistent macrocycle as a host. The macrocycle was synthesized on a gram scale via the condensation of a trimeric diamine and a diacid chloride prepared from pyrimidine-4,6-dicarboxylic acid. The ca. 12 Å-sized cavity of the ring can accommodate a Box molecule in a pseudo-rotaxane-like structure.

The near-planar conformation of the rings allows the assembly of multiple rings (up to five) into a single ring to form a shish-kebab-like structure. This can be followed by further assembly into a higher-order dimeric structure (example pictured). The complexes are stabilized by the cooperative action of several non-covalent interactions, including π–π stacking and C–H···O and C–H···π interactions.

The team found that the conformation of each ring threaded on the Box molecule changes to a varying extent by adapting itself to maximize intermolecular interactions. Such higher-order dimeric structures with conformational adaptivity could be useful to understand sophisticated phenomena in Nature.