Synthesis of contra-helical trefoil knots with mechanically tuneable spin-crossover properties

Molecular knots attract attention on account of their topological intricacies and potential application. Tying molecular knots with different topologies, on larger length scales, remains challenging, not to mention the difficulties with harnessing their topological characteristics in order to modulate their properties. Here, we report a general approach to construct torus knots from two coaxially nested multistranded contra-helices. As a proof of concept, a series of two iron(II)-templated contra-helical trefoil knots have been synthesized near-quantitatively in one step. Among these, one features a long trefoil knot—a 111-atom closed loop that is ~11 nm long. The thermally induced spin crossover of the two iron(II) centres in each knot can be modulated in opposing directions by changing the intramolecular mechanical strain. The synthesis of molecular knots with mechanically tuneable properties enables the unleashing of their stimuli-responsive multifunctionalities. One of these molecular knots exhibits, during crystallization, narcissistic self-sorting, which allows the manual separation of enantiomers. A purely organic trefoil knot, obtained by reductive demetallation of its precursor, is also characterized in the solid state by X-ray crystallography.