Organometallic chemistry meets crystal engineering to give responsive crystalline materials.

Dynamically porous crystalline materials have been obtained by engineering organometallic molecules. This feature article deals with organometallic wheel-and-axle compounds, molecules with two relatively bulky groups (wheels) connected by a linear spacer. The wheels are represented by half-sandwich Ru(ii) moieties, while the spacer can be covalent or supramolecular in character. Covalent spacers are obtained using divergent bidentate ligands connecting two [(arene)RuX2] groups. Supramolecular spacers are instead obtained by exploiting the dimerization of COOH or C(O)NH2 groups appended to N-based ligands. A careful choice of ligand functional groups and X ligands leads to the isolation of crystalline materials with remarkable host-guest properties, evidenced by the possibility of reversibly capturing/releasing volatile guests through heterogenous solid-gas reactions. Structural correlations between the crystalline arrangement of the apohost and the host-guest compounds allow us to envisage the structural path followed by the system during the exchange processes.