macrocyclic

Macrocyclic compounds are organic molecules that contain a large, ring-shaped core formed by covalently linked atoms. In most definitions, the ring includes 12 or more atoms, although the precise lower bound varies by field. The ring structure creates a preorganized cavity that can host ions or other molecular guests. Macrocycles often possess heteroatoms such as oxygen, nitrogen, or sulfur within the ring, contributing to binding properties and coordination chemistry.

Formation is typically challenging due to entropic penalties of cyclization, and many macrocycles arise from templated or high-dilution syntheses. The so-called macrocyclic effect describes the observation that macrocycles often bind guests more strongly than their acyclic counterparts because the ring preorganizes donor atoms around the bound species. Rigid or semi-rigid backbones further enhance selectivity and affinity by reducing conformational freedom.

Prominent examples include crown ethers (for example 18-crown-6), porphyrins and related expanded macrocycles, cyclodextrins, calixarenes, and cucurbiturils. These structures have found broad utility as host molecules, ligands in coordination chemistry, catalysts, and carriers in drug delivery and imaging. Many macrocycles are synthesized to tune cavity size, shape, and functional groups, enabling selective binding of particular ions or neutral guests.

Macrocyclic chemistry intersects with supramolecular chemistry and materials science, contributing to sensors, separation technologies, and functional materials. The term also encompasses macrocyclic ligands for metal centers, where the ring constrains geometry and oxidation state, often enabling unusual reactivity or stabilizing reactive intermediates.