A macrocyclic ligand is a cyclic compound with nine or more members including all the heteroatoms and with three or more donor atoms. The increased thermodynamic stability of macrocyclic complexes over their linear analogs having the same number of chelate rings is known as the macrocyclic effect.
A macrocyclic ligand is a cyclic compound with nine or more members including all the heteroatoms and with three or more donor atoms. These donor atoms are usually positioned so that upon coordination preferably five or six membered chelate rings are formed with the metal ion. The macrocyclic cavity size is determined by the number of atoms in the macrocyclic ring. The cavity size is also affected by the backbone rigidity, and by the nature and the hybridization of the donor atom. Unsaturationin the macrocyclic ring can have significant effects. Extensive unsaturation results in the loss of flexibility, which in turn may lead to increased inertness of the complex. Based on donor atom type, macrocyclic ligands can be classified as crown ethers (oxygen donor atoms), polyaza macrocycles or azacrown ethers (nitrogen donor atoms), macrocycles containing sulfur, phosphorous, arsenic atoms and macrocyclic ligands with mixed donor atoms (polyaza polyoxa macrocycles, lariat ethers). Macrocycles with pendant functional groups include polyaza macrocylces that have coordinating sidearms attached to the nitrogen. These include NOTA, DOTA, DOTMA TETA (acetate sidearms), NOTP, DOTP, (phosphonate sidearms), HP-DO3A (acetate and hydroxypropyl sidearms). These ligands are capable of axial metal ion coordination. Chelation will be more efficient because the donor atoms are held near to the central metal ion. An important class of macrocyclic ligands consists of macrobicyclic polyaza polyoxa ligands called cryptands. Cryptands can completely encapsulate a coordinated metal ion. Cryptands have a sterically controlled coordination cavity that can exhibit enhanced specificity and stability for individual metal ions. The increased thermodynamic stability of macrocyclic complexes over their linear analogs having the same number of chelate rings is known as the macrocyclic effect. The kinetic macrocyclic effect refers to the slow kinetics of dissociation of macrocyclic compounds as compared to the open chain analogs.