Chemistry Meets Topology: Metal Template Synthesis of a Molecular Trefoil Knot

Abstract

The construction of the simplest molecular knot, a trefoil (3₁) knot, through a metal template approach has been incorporated into an organic chemistry experiment for advanced undergraduate students. The experimental procedure begins with the formation of a trimeric circular helicate directed by zinc(II) template multicomponent self-assembly, and the subsequent covalent capture via ring-closing metathesis produces the trefoil knot. The highly efficient reaction conditions yield 90% in a two-step synthesis, with both the helicate and knotted products confirmed by Nuclear Magnetic Resonance (NMR) and Electrospray Ionization Mass Spectrometry (ESI-MS) spectroscopies. To clearly elucidate the structural characteristics of the trefoil knot, including the inherent topological chirality and its topological isomer macrocycle, a rope model and a stick-ball model are built during the seminar session. Further analysis of the knotted architecture through single-crystal diffraction is established as a long-term goal for the entire term. The sessions are structured with a proactive methodology, empowering undergraduate students to take on an active and engaged role. This laboratory course introduces students to the intricate chemistry of molecular knots, while reinforcing their foundational understanding of core organic laboratory procedures. Additionally, it strengthens their proficiency in structural characterization techniques, including NMR, MS and X-ray Diffraction (XRD) analysis. The experiment is designed to be readily implementable, facilitating the adaptation of both the experimental procedures and laboratory materials to a wide array of undergraduate course curricula.