Mechanical interlocking of metal organic cages

[2]-Catenanes formed from three-dimensional (3D) metal organic cages (MOCs) self-assembled from transition metals and organic ligands, hereafter called interlocked MOCs, constitute a relatively new class of mechanically interlocked materials (MIMs) that is receiving considerable research attention. This interest is mainly due to their esthetic synthetic aspects and mechanical properties, but also due to their potential applications in nanotechnological areas, including magnetic materials, guest selectivity, allosteric binding, or thermoresponsive behavior in elastomeric materials. In this article, a perspective on the research on interlocked MOCs (i.e., [2]-catenanes), covering from the first examples of interlocked MOCs to the latest research in this area is presented. The emphasis is in the synthetic methods used for their preparation and the structure‒function correlation aspects. The combination of experimental techniques in the solution state (i.e., NMR, ESI-MAS, etc.) and solid-state X-ray structural data, in combination with theoretical calculations have been very important to getting a rationalization of the experimental results. The described [2]-catenanes are formed from mechanically bonded hollow cages, therefore they can be exploited in host-guest chemistry applications typical from MOCs, with enhanced physical properties ranging from the mechanical bonds like enhanced cage's strength and dynamic behavior, which are needed for developing new smart functional materials.