Macrocyclic ligand

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In coordination chemistry, a macrocyclic ligand is a macrocyclic ring having at least nine atoms (including all hetero atoms) and three or more donor sites that serve as ligands. [1] Crown ethers and porphyrins are prominent examples. Macrocyclic ligands often exhibit high affinity for metal ions, the macrocyclic effect.

Contents

History

Porphyrins and phthalocyanines have long been recognized as potent ligands in coordination chemistry as illustrated by numerous transition metal porphyrin complexes and the commercialization of copper 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 a template for ring formation. [2]

14-membered N4 macrocyclic ligands, called Curtis macrocycles, arise from the condensation of acetone and a nickel complex of ethylenediamine. CurtisMac.png
14-membered N4 macrocyclic ligands, called Curtis macrocycles, arise from the condensation of acetone and a nickel complex of ethylenediamine.

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]

Macrocyclic effect

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 the chelate 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]

Synthesis

In general, macrocyclic complexes are synthesized by combining macrocyclic ligands and metal ions. [10]

In template 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 the coordination 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.

18-Crown-6 can be synthesized by the Williamson ether synthesis using potassium ion as the template cation. 18-crown-6 was synthesized using potassium ion as the template cation.png
18-Crown-6 can be synthesized by the Williamson ether synthesis using potassium ion as the template cation.

Phthalocyanines were the first macrocycles synthesized by template reaction. Featuring planar, dianionic, 18-membered rings with four nitrogenous ligands, phthalocyanines resemble porphyrins.

The size of the metal cation used as the template has proved to be of importance in directing the synthetic pathway for the Schiff 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]

Uses and occurrence

Phthalocyanines, as their metal complexes, are arguably the most commercially useful complex of a macrocyclic ligand. They are used as dyes and pigments such as phthalocyanine blue. [12]

Macrocyclic ligands occur in many cofactors in proteins and enzymes. Of particular interest are tetraazamacrocycles. [13]

Heme, the active site in the hemoglobin (the metalloprotein in blood that transports oxygen), is a porphyrin that contains iron. Chlorophyll, the green photosynthetic pigment found in plants, contains a chlorin ring. Vitamin B12 contains a corrin ring.

Related Research Articles

<span class="mw-page-title-main">Porphyrin</span> Heterocyclic organic compound with four modified pyrrole subunits

Porphyrins are a group of heterocyclic macrocycle organic compounds, composed of four modified pyrrole subunits interconnected at their α carbon atoms via methine bridges (=CH−). In vertebrates, an essential member of the porphyrin group is heme, which is a component of hemoproteins, whose functions include carrying oxygen in the bloodstream. In plants, an essential porphyrin derivative is chlorophyll, which is involved in light harvesting and electron transfer in photosynthesis.

<span class="mw-page-title-main">Charles J. Pedersen</span> American organic chemist (1904–1989)

Charles John Pedersen was an American organic chemist best known for discovering crown ethers and describing methods of synthesizing them during his entire 42-year career as a chemist for DuPont at DuPont Experimental Station in Wilmington, Delaware, and at DuPont's Jackson Laboratory in Deepwater, New Jersey. Often associated with Reed McNeil Izatt, Pedersen also shared the Nobel Prize in Chemistry in 1987 with Donald J. Cram and Jean-Marie Lehn. He is the only Nobel Prize laureate born in Korea other than Peace Prize laureate Kim Dae-jung.

Supramolecular chemistry refers to the branch of chemistry concerning chemical systems composed of a discrete number of molecules. The strength of the forces responsible for spatial organization of the system range from weak intermolecular forces, electrostatic charge, or hydrogen bonding to strong covalent bonding, provided that the electronic coupling strength remains small relative to the energy parameters of the component. While traditional chemistry concentrates on the covalent bond, supramolecular chemistry examines the weaker and reversible non-covalent interactions between molecules. These forces include hydrogen bonding, metal coordination, hydrophobic forces, van der Waals forces, pi–pi interactions and electrostatic effects.

<span class="mw-page-title-main">Crown ether</span> Ring molecules with several ether (–O–) groups

In organic chemistry, crown ethers are cyclic chemical compounds that consist of a ring containing several ether groups (R−O−R’). The most common crown ethers are cyclic oligomers of ethylene oxide, the repeating unit being ethyleneoxy, i.e., −CH2CH2O−. Important members of this series are the tetramer (n = 4), the pentamer (n = 5), and the hexamer (n = 6). The term "crown" refers to the resemblance between the structure of a crown ether bound to a cation, and a crown sitting on a person's head. The first number in a crown ether's name refers to the number of atoms in the cycle, and the second number refers to the number of those atoms that are oxygen. Crown ethers are much broader than the oligomers of ethylene oxide; an important group are derived from catechol.

<span class="mw-page-title-main">Phthalocyanine</span> Chemical compound

Phthalocyanine is a large, aromatic, macrocyclic, organic compound with the formula (C8H4N2)4H2 and is of theoretical or specialized interest in chemical dyes and photoelectricity.

<span class="mw-page-title-main">Catenane</span> Molecule composed of two or more intertwined rings

In macromolecular chemistry, a catenane is a mechanically interlocked molecular architecture consisting of two or more interlocked macrocycles, i.e. a molecule containing two or more intertwined rings. The interlocked rings cannot be separated without breaking the covalent bonds of the macrocycles. They are conceptually related to other mechanically interlocked molecular architectures, such as rotaxanes, molecular knots or molecular Borromean rings. Recently the terminology "mechanical bond" has been coined that describes the connection between the macrocycles of a catenane. Catenanes have been synthesised in two different ways: statistical synthesis and template-directed synthesis.

<span class="mw-page-title-main">Macrocycle</span> Molecule with a large ring structure

Macrocycles are often described as molecules and ions containing a ring of twelve or more atoms. Classical examples include the crown ethers, calixarenes, porphyrins, and cyclodextrins. Macrocycles describe a large, mature area of chemistry.

<span class="mw-page-title-main">Cyclen</span> Chemical compound

Cyclen (1,4,7,10-tetraazacyclododecane) is a aza-crown ether with the formula (CH2CH2NH)4. It is a white solid. Being structurally simple, symmetrical, and polyfunctional, cyclen has been widely investigated.

<span class="mw-page-title-main">Aza-crown ether</span> Ring molecule with several amine (–N– or >N–) groups

In organic chemistry, an aza-crown ether is an aza analogue of a crown ether. That is, it has a nitrogen atom in place of each oxygen atom around the ring. While the parent crown ethers have the formulae (CH2CH2O)n, the parent aza-crown ethers have the formulae (CH2CH2NH)n, where n = 3, 4, 5, 6. Well-studied aza crowns include triazacyclononane, cyclen, and hexaaza-18-crown-6.

<span class="mw-page-title-main">DOTA (chelator)</span> Chemical compound

DOTA (also known as tetraxetan) is an organic compound with the formula (CH2CH2NCH2CO2H)4. The molecule consists of a central 12-membered tetraaza (i.e., containing four nitrogen atoms) ring. DOTA is used as a complexing agent, especially for lanthanide ions. Its complexes have medical applications as contrast agents and cancer treatments.

<span class="mw-page-title-main">Template reaction</span>

In chemistry, a template reaction is any of a class of ligand-based reactions that occur between two or more adjacent coordination sites on a metal center. In the absence of the metal ion, the same organic reactants produce different products. The term is mainly used in coordination chemistry. The template effects emphasizes the pre-organization provided by the coordination sphere, although the coordination modifies the electronic properties of ligands.

<span class="mw-page-title-main">Metallacrown</span> Large ring molecules made of mainly inorganic and metal atoms

In chemistry, metallacrowns are a macrocyclic compounds that consist of metal ions and solely or predominantly heteroatoms in the ring. Classically, metallacrowns contain an [M–N–O] repeat unit in the macrocycle. First discovered by Vincent L. Pecoraro and Myoung Soo Lah in 1989, metallacrowns are best described as inorganic analogues of crown ethers. To date, over 600 reports of metallacrown research have been published. Metallacrowns with sizes ranging from 12-MC-4 to 60-MC-20 have been synthesized.

<span class="mw-page-title-main">Cyclam</span> Chemical compound

Cyclam (1,4,8,11-tetraazacyclotetradecane) is an organic compound with the formula (NHCH2CH2NHCH2CH2CH2)2. Classified as an aza-crown ether, it is a white solid that is soluble in water. As a macrocyclic ligand, it binds strongly to many transition metal cations. The compound was first prepared by the reaction of 1,3-dibromopropane and ethylenediamine.

<span class="mw-page-title-main">Weak-Link Approach</span>

The Weak-Link Approach (WLA) is a supramolecular coordination-based assembly methodology, first introduced in 1998 by the Mirkin Group at Northwestern University. This method takes advantage of hemilabile ligands -ligands that contain both strong and weak binding moieties- that can coordinate to metal centers and quantitatively assemble into a single condensed ‘closed’ structure. Unlike other supramolecular assembly methods, the WLA allows for the synthesis of supramolecular complexes that can be modulated from rigid ‘closed’ structures to flexible ‘open’ structures through reversible binding of allosteric effectors at the structural metal centers. The approach is general and has been applied to a variety of metal centers and ligand designs including those with utility in catalysis and allosteric regulation.

In chemistry, tetradentate ligands are ligands that bind four donor atoms to a central atom to form a coordination complex. This number of donor atoms that bind is called denticity and is a method of classifying ligands.

Cobalt(II)–porphyrin catalysis is a process in which a Co(II) porphyrin complex acts as a catalyst, inducing and accelerating a chemical reaction.

<span class="mw-page-title-main">Zirconocene</span> Chemical compound

Zirconocene is a hypothetical compound with 14 valence electrons, which has not been observed or isolated. It is an organometallic compound consisting of two cyclopentadienyl rings bound on a central zirconium atom. A crucial question in research is what kind of ligands can be used to stabilize the Cp2ZrII metallocene fragment to make it available for further reactions in organic synthesis.

<span class="mw-page-title-main">Thia-crown ether</span> Ring molecules with several sulfide (–S–) groups

In organic chemistry, thia-crown ethers are organosulfur compounds which are the thia analogues of crown ethers. That is, they have a sulfur atom in place of each oxygen atom around the ring. While the parent crown ethers have the formulae (CH2CH2O)n, the parent thia-crown ethers have the formulae (CH2CH2S)n, where n = 3, 4, 5, 6. They have trivial names "x-ane-Sy", where x and y are the number of atoms in the ring and the number of those atoms that are sulfur, respectively. Thia-crown ethers exhibit affinities for transition metals.

<span class="mw-page-title-main">Transition metal porphyrin complexes</span>

Transition metal porphyrin complexes are a family of coordination complexes of the conjugate base of porphyrins. Iron porphyrin complexes occur widely in Nature, which has stimulated extensive studies on related synthetic complexes. The metal-porphyrin interaction is a strong one such that metalloporphyrins are thermally robust. They are catalysts and exhibit rich optical properties, although these complexes remain mainly of academic interest.

<span class="mw-page-title-main">Phosphorus porphyrin</span> Organophosphorus compound

Phosphorus-centered porphyrins are conjugated polycyclic ring systems consisting of either four pyrroles with inward-facing nitrogens and a phosphorus atom at their core or porphyrins with one of the four pyrroles substituted for a phosphole. Unmodified porphyrins are composed of pyrroles and linked by unsaturated hydrocarbon bridges often acting as multidentate ligands centered around a transition metal like Cu II, Zn II, Co II, Fe III. Being highly conjugated molecules with many accessible energy levels, porphyrins are used in biological systems to perform light-energy conversion and modified synthetically to perform similar functions as a photoswitch or catalytic electron carriers. Phosphorus III and V ions are much smaller than the typical metal centers and bestow distinct photochemical properties unto the porphyrin. Similar compounds with other pnictogen cores or different polycyclic rings coordinated to phosphorus result in other changes to the porphyrin’s chemistry.

References

  1. ↑ Melson, GA (1979). Coordination Chemistry of Macrocyclic Compounds. New York: Plenum Press. p. 2. ISBN   0-306-40140-1.
  2. ↑ Curtis, N.F. (April 1968). "Macrocyclic Coordination Compounds Formed by Condensation of Metal-Amine Complexes with Aliphatic Carbonyl Compounds". Coordination Chemistry Reviews. 3 (1): 3–47. doi:10.1016/S0010-8545(00)80104-6.
  3. ↑ Pedersen, Charles J. (December 1967). "Cyclic polyethers and their complexes with metal salts". Journal of the American Chemical Society. 89 (26): 7017–7036. doi:10.1021/ja01002a035.
  4. ↑ Pedersen, C. J.; Frensdorff, H. K. (January 1972). "Macrocyclic Polyethers and Their Complexes". Angewandte Chemie International Edition in English. 11 (1): 16–25. doi:10.1002/anie.197200161. PMID   4622977.
  5. ↑ Antje Schrodt; Anton Neubrand; Rudi van Eldik (1997). "Fixation of CO2 by Zinc(II) Chelates in Alcoholic Medium. X-ray Structures of {[Zn(cyclen)]3(μ3-CO3)}(ClO4)4 and [Zn(cyclen)EtOH](ClO4)2". Inorg. Chem. 36 (20): 4579–4584. doi:10.1021/ic961368t. PMID   11670124.
  6. ↑ Collins, T. J. (2002), "TAML oxidant activators: a new approach to the activation of hydrogen peroxide for environmentally significant problems", Accounts of Chemical Research, 35 (9): 782–790, doi:10.1021/ar010079s, PMID   12234208
  7. ↑ Cabbines, D. K.; Margerum, D. W. (1969). "Macrocyclic effect on stability of copper(II) tetramine complexes". J. Am. Chem. Soc. 91 (23): 6540–6541. doi:10.1021/ja01051a091.
  8. ↑ Melson, GA (1979). Coordination Chemistry of Macrocyclic Compounds. New York: Plenum Press. p. 166. ISBN   0-306-40140-1.
  9. ↑ Weller M, Overton T, Rourke J, Armstrong F (2014). Inorganic Chemistry. OUP Oxford. p. 229. ISBN   978-0-19-964182-6.
  10. ↑ L.F Lindloy, The Chemistry of Macrocylic Ligand Complexes, Cambridge University Press, 1989, 20-50 ISBN   0-521-25261-X
  11. ↑ Alexander, V. (March 1995). "Design and Synthesis of Macrocyclic Ligands and Their Complexes of Lanthanides and Actinides". Chemical Reviews. 95 (2): 273–342. doi:10.1021/cr00034a002.
  12. ↑ Milgrom, L.R (1997). The Colours of Life: An Introduction to the Chemistry of Porphyrins and Related Compounds. New York: Oxford University Press. ISBN   0-19-855380-3. (hardbound) ISBN   0-19-855962-3 (pbk.)[ page needed ]
  13. ↑ S. J. Lippard, J. M. Berg "Principles of Bioinorganic Chemistry" University Science Books: Mill Valley, CA; 1994. ISBN   0-935702-73-3.[ page needed ]
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