A few electrons are all it takes to significantly boost the rate of supramolecular assembly of one molecule threading through another. The finding extends electron catalysis, which is used in synthetic covalent chemistry, to noncovalent chemistry, and may lead to new complex forms of materials.
Non-covalent bonding processes, such as molecular recognition and supramolecular assembly, occur widely in chemistry and biology. The rates of these phenomena, which can be sluggish, can be jumpstarted, but doing so generally requires complex catalytic systems.
On Wednesday at the American Chemical Society Spring 2022 meeting, Yang Jiao of Northwestern University reported that the assembly rate of a host-guest complex can be increased by a factor of 640 in the presence of simple catalysts—electrons.
Speaking in a session organized by the Division of Organic Chemistry, Jiao described a study involving the assembly of a complex consisting of a ring-shaped molecular host and a dumbbell-shaped guest. The host contains two bipyridinium (BIPY) radical cations. The guest consists of three units; a BIPY radical cation binding site in the center, which drives assembly with the host via radical-pairing interactions; a bulky diisopropylphenyl group on one end that cannot be threaded through the ring; and a dimethylpyridinium (PY) cation on the other end. Under normal conditions, repulsion between the PY cation and the BIPY radical cations prevents the host and guest from assembling.
The researchers found that catalytic quantities of various types of chemical electron sources, including metals, metal complexes, and common reducing agents, rapidly increase the reaction rate with little dependence on the type of source. Applying electric current to a solution of the molecules also catalyzed the assembly process. Adding electrons to the system lowers Coulombic repulsion, enabling the PY end of the dumbbell to thread the ring, as shown by quantum calculations.
Jiao and others, including Northwestern’s Sir J. Fraser Stoddart and William A. Goddard III of the California Institute of Technology, recently published this work in Nature (2022, DOI: 10.1038/s41586-021-04377-3).
Redox-driven formation of mechanically interlocked molecules has been known for a long time, but “the involvement of electron catalysis during the process is remarkable and eye-opening,” said Rafal Klajn of the Weizmann Institute of Science. He hypothesizes that the catalytic activity could be further increased if either the ring or the thread component were surface-immobilized.
This story was updated on March 23, 2022, to clarify Rafal Klajn's comment. He thinks the catalytic activity would increase if either the ring or the thread component of the complex were surface-immobilized, not just the thread.