catenanes

Catenanes are molecules in which two or more macrocyclic rings are interlocked like links in a chain. The rings are held together by a mechanical bond rather than a covalent linkage, so the structure cannot be separated without breaking chemical bonds in the rings themselves. Catenanes are a prominent example of mechanically interlocked molecules (MIMs), a class that also includes rotaxanes and related topologies. A [2]catenane contains two interlocked rings; higher-order catenanes have more rings.

Synthesis and topology are central to catenane chemistry. The construction typically relies on template-directed strategies that use non-covalent interactions or metal coordination to preorganize components so that ring formation yields an interlocked structure. Approaches include metal-templated self-assembly, hydrogen-bond templating, and dynamic covalent or clipping methods, where a macrocycle is formed around a pre-threaded unit or around recognition sites positioned to yield interlocking. These methods have evolved from foundational demonstrations in the late 20th century to the production of increasingly complex interlocked architectures.

Properties and applications are largely driven by the mechanical bond. The rings in a catenane can move relative to each other, enabling potential uses as molecular switches, components of nanomachines, or responsive materials. Research often focuses on controlling motion, switching states with stimuli (such as chemical, photochemical, or redox inputs), and understanding how topology influences behavior. Characterization typically employs NMR spectroscopy, mass spectrometry, and X-ray crystallography; some catenanes are also studied by circular dichroism when chirality arises from their topology.

See also: rotaxanes. Unlike rotaxanes, where an axle and a ring are mechanically interlocked but the system may be disassembled by removing stoppers, catenanes consist of interlocked rings without an external thread.