Crystal engineering of flexible metallorganic supramolecular networks

Flexibly porous crystalline materials have been obtained by engineering the structure of metal-containing molecules. In particular, the review deals with 'wheel-and-axle' compounds, molecules with two bulky and relatively rigid end groups (wheels) that are connected by a linear rigid link (axle). Owing to their irregular shape, 'wheel-and-axle' compounds do not pack efficiently, and for this reason they tend to form inclusion compounds. Here, we present the results obtained by using metal-containing 'wheel-and-axle' complexes, where metal ions are inserted in the axle or in the wheels. In particular, wheel-and-axle diols are obtained by inserting Pd(II) and Pt(II) ions into the axle, while metallorganic wheel-and-axle compounds are realized by using half-sandwich Ru(II) building units. A careful choice of metal coordination geometry and ligand functional groups leads to the isolation of crystalline materials with remarkable host-guest properties. Moreover, a close analysis of the crystal structures allows one to propose a model for guest uptake/release.