Incoordination chemistry, amacrocyclic ligand[citation needed]is amacrocyclic ring having at least nine atoms (including allhetero atoms) and three or moredonor sites that serve asligands.[1]Crown ethers andporphyrins are prominent examples. Macrocyclic ligands often exhibithigh affinity for metal ions, themacrocyclic effect.
Porphyrins andphthalocyanines have long been recognized as potent ligands in coordination chemistry as illustrated by numeroustransition metal porphyrin complexes and the commercialization ofcopper phthalocyanine pigments. In the 1960s the synthesis of macrocylic ligands received much attention. One early contribution involved the synthesis of the "Curtis macrocycles", in which a metal ion serves as atemplate for ring formation.[2]
Polyether macrocycles - or "crown" ligands - were also developed at that time.[3] A few years later, three-dimensional analogs of crown ethers called "cryptands" were reported by Lehn and co-workers.[4]
![The silver complex of a thia-crown ether [Ag(18-ane-S6)]2+](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aAg(18-ane-S6)_dication.png)
![Crystal structure of a Zn(II)-cyclen-ethanol dication.[5]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aCyclen_Zinc_InorgChem_1997_4579_commons.jpg)
![Fe-TAML complex.[6]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aFeTAML.png)
The macrocyclic effect is the high affinity of metal cations for macrocyclic ligands, compared to their acyclic analogues.[7][8] The high affinity of macrocyclic ligands is thought to be a combination of the entropic effect seen in thechelate effect, together with an additional energetic contribution that comes from the preorganized nature of the ligating groups (that is, no additional strains are introduced to the ligand on coordination).[9]
In general, macrocyclic complexes are synthesized by combining macrocyclic ligands and metal ions.[10]
Intemplate reactions, macrocyclic ligands are synthesized in the presence of metal ions. In the absence of the metal ion, the same organic reactants may produce different, often polymeric, products. The metal ion may direct the condensation preferentially to cyclic rather than polymeric products (the kinetic template effect) or stabilize the macrocycle once formed (the thermodynamic template effect). The template effect makes use of the pre-organization provided by thecoordination sphere of the metal. The coordination modifies the electronic properties such as the acidity and electrophilicity of the ligands. When the metal atom is not desired in the final product, a disadvantage of templated synthesis is the difficulty in removing the templating metal from the macrocyclic ligand.
Phthalocyanines were the first macrocycles synthesized by template reaction. Featuring planar, dianionic, 18-membered rings with four nitrogenous ligands, phthalocyanines resembleporphyrins.
The size of the metalcation used as the template has proved to be of importance in directing the synthetic pathway for theSchiff base systems. The compatibility between the radius of the template cation and the "hole" of the macrocycle contributes to the effectiveness of the synthetic pathway and to the geometry of the resulting complex.[11]
Phthalocyanines, as their metal complexes, are arguably the most commercially useful complex of a macrocyclic ligand. They are used as dyes and pigments such asphthalocyanine blue.[12]
Macrocyclic ligands occur in many cofactors in proteins and enzymes. Of particular interest are tetraazamacrocycles.[13]
Heme, the active site in thehemoglobin (themetalloprotein in blood that transports oxygen), is aporphyrin that contains iron.Chlorophyll, the greenphotosynthetic pigment found inplants, contains achlorin ring.Vitamin B12 contains acorrin ring.