Rivaling the ropework of a skilled sailor, chemists have tied the most complex molecular knot to date. The structure contains three overhand knots on a continuous 324-atom loop that weaves under and over itself at nine crossing points (Nat. Chem. 2018, DOI: 10.1038/s41557-018-0124-6).
David A. Leigh and colleagues at the University of Manchester created the knots from six long building blocks featuring alkene groups at each end and three bipyridyl groups in the middle. These ligands wrapped around six iron ions, binding through the bipyridyl nitrogen atoms to form a complex with a helical twist. Using a common ruthenium catalyst, the team connected the alkenes with a ring-closing metathesis reaction and then removed the iron to release the knotted loop. The process yielded a roughly 1:1 mixture of the knot and a catenane containing three twisted interlocking rings.
“The work is outstanding. It’s a very exciting development in the field,” says Edward E. Fenlon of Franklin & Marshall College, who works on molecular knots and was not involved in the study.
Connecting ligands in this way has become a well established method for making molecular knots. Just a few weeks ago, Leigh’s group unveiled a molecular granny knot containing six crossing points, which was made using the same tactic (Angew. Chem. Int. Ed. 2018, DOI: 10.1002/anie.201807135). Knots are chiral, Leigh adds, so it takes careful ligand design, based on a mixture of experience and molecular modeling, to ensure that they hook up without—ahem—any hitches.
Although creating these molecules is largely viewed as a knotty synthetic challenge, Leigh hopes that they could eventually lead to applications. “Any sailor or climber will tell you that you need a particular knot to do a particular job,” Leigh says. “We want to be able to produce different sorts of knots, learn about what they can do, and then exploit them in materials.”
Leigh has already shown that a pentafoil molecular knot catalyzes certain reactions thanks to its central cavity that binds halide ions. Meanwhile, Fenlon notes that knots often form in linear polymers, weakening the material. “When your plastic grocery bag splits open, it might have been caused by a knot impurity in the polyethylene,” he says. “Making molecular knots will allow us to test this hypothesis.”