Catalysis gets all tied up in knots Over the past decade, chemists have used metal ion templating to prepare a wide variety of knotted molecular strands. Marcos et al. now show that one such pentafoil knot can be applied to catalysis. When held taut by zinc ions, the knot can capture a chloride or bromide ion from a halocarbon, thereby unleashing the reactivity of the residual cation for applications such as Lewis acid catalysis. Removing the zinc ions lowers the knot's affinity for the halides, offering a reversible modulation mechanism for the catalysis. Science , this issue p. 1555