Soft organic and metal-organic frameworks with porous architecture: From wheel-and-axle to ladder-and-platform design of host molecules

The family of wheel-and-axle host molecules is reviewed and the approaches to the creation of new host geometries for supramolecular materials based on weaker interactions are discussed. The combination of bulky groups (or platforms) and spacers yields various host geometries including humming-top molecules (Werner complexes, metal dibenzoylmethanates and other bis-chelates, dimeric metal carboxylates and macrocyclic complexes), wheel-and-axle and dumb-bell shaped molecules and their modifications, ladder-and-platform, shish-kebab, multi-decker, stair-case and double-strand ladder-and-platform oligomeric and polymeric structures. The host shapes are compared with cyclic and trifoil (trityl) host types. The use of metal centers, chelating and macrocyclic ligands (beta-diketones, carboxylic acids, Schiff bases, porphyrins, phthalocyanines, corroles, annulenes and their analogs), bridging ditopic ligands and other building elements in the engineering of host molecules is illustrated. The role of such factors as size and nuclearity of the platforms, rigidity/flexibility of the spacers and hydrogen bonding is discussed. The relation between porosity and packing efficiency, predisposition to self-inclusion, parallel alignment and interdigitation, dimensionality and secondary assembly of host molecules in the crystal is examined. Clathration and sorption abilities and other important qualities and functionalities of the new host materials are elucidated.