Making room for reactivity in topochemical transformations under pressure.

The orderly arrangement of molecules in crystals provides a unique platform for the control of reactivity, where spatial vicinity and orientation of the molecules determine the course of the reaction. Common wisdom assumes that the densest crystal packings of the reactant molecules are most favorable for pressure-induced topochemical reactions. Based on thermodynamic and spatial arguments, here we show that the densest crystal packing may not be the most optimal platform for chemical synthesis. Instead, introducing void space within the crystal lattice significantly improves and even enables chemical reactions. In the case of sorbic acid confined between brucite-type layers, this reactivity is used to modify the optical, spectroscopic and magnetic properties of 2D layers and to synthesize a retrievable polymeric product.