coordination chemistry A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of chemical bond, bound molecules or ions, that are in turn known as ''ligands'' or complexing ...

, a template reaction is any of a class of

ligand In coordination chemistry, a ligand is an ion or molecule ( functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's ele ...

-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

reactant In chemistry, a reagent ( ) or analytical reagent is a substance or compound added to a system to cause a chemical reaction, or test if one occurs. The terms ''reactant'' and ''reagent'' are often used interchangeably, but reactant specifies a ...

s produce different products. The template effects emphasizes the pre-organization provided by the coordination sphere, although the coordination modifies the electronic properties (

acidity In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a sequ ...

, electrophilicity, etc.) of ligands. An early example is the dialkylation of a nickel dithiolate: : Busch-LindoyTempRxn

The corresponding alkylation in the absence of a metal ion would yield

polymer A polymer (; Greek ''poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic and ...

Crown ether In organic chemistry, crown ethers are cyclic chemical compounds that consist of a ring containing several ether groups (). The most common crown ethers are cyclic oligomers of ethylene oxide, the repeating unit being ethyleneoxy, i.e., . I ...

s arise from dialkylations that are templated by alkali metals. Other template reactions include the Mannich and

Schiff base In organic chemistry, a Schiff base (named after Hugo Schiff) is a compound with the general structure ( = alkyl or aryl, but not hydrogen). They can be considered a sub-class of imines, being either secondary ketimines or secondary aldimine ...

condensations. The condensation of

formaldehyde Formaldehyde ( , ) ( systematic name methanal) is a naturally occurring organic compound with the formula and structure . The pure compound is a pungent, colourless gas that polymerises spontaneously into paraformaldehyde (refer to section ...

ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogeno ...

, and

tris(ethylenediamine)cobalt(III) Tris(ethylenediamine)cobalt(III) chloride is an inorganic compound with the formula o(en)3l3 (where "en" is the abbreviation for ethylenediamine). It is the chloride salt of the coordination complex o(en)3sup>3+. This trication was important in ...

to give a

clathrochelate In coordination chemistry, clathrochelates are ligands that encapsulate metal ions. Chelating ligands bind to metals more strongly than related monodentate ligands, and macrocyclic ligands bind more strongly than typical chelating ligands. It ...

complex is one example. 18-crown-6 was synthesized using potassium ion as the template cation

The phosphorus analogue of an aza crown can be prepared by a template reaction. Where it is not possible to isolate the phosphine itself. Phosphacrown

Limitations

Many template reactions are only stoichiometric, and the decomplexation of the "templating ion" can be difficult. The alkali metal-templated syntheses of crown ether syntheses are notable exceptions. Metal

Phthalocyanine Phthalocyanine () is a large, aromatic, macrocyclic, organic compound with the formula and is of theoretical or specialized interest in chemical dyes and photoelectricity. It is composed of four isoindole units linked by a ring of nitrogen atom ...

s are generated by metal-templated condensations of phthalonitriles, but the liberation of metal-free phthalocyanine is difficult. Some so-called template reactions proceed similarly in the absence of the templating ion. One example being the condensation of acetone and ethylenediamine, which yields isomeric 14-membered tetraaza rings. Similarly,

porphyrin 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−). The parent of porphyrin is porphine, a rare chemical ...

s, which feature 16-membered central rings, form in the absence of metal templates. CurtisMacrocycle

Concept in catalysis

In a general sense, transition metal-based

catalysis Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycl ...

can be viewed as template reactions: Reactants coordinate to adjacent sites on the metal ion and, owing to their adjacency, the two reactants interconnect (insert or couple) either directly or via the action of another reagent. In the area of

homogeneous catalysis In chemistry, homogeneous catalysis is catalysis by a soluble catalyst in a solution. Homogeneous catalysis refers to reactions where the catalyst is in the same phase as the reactants, principally in solution. In contrast, heterogeneous catalysi ...

, the cyclo-oligomerization of acetylene to

cyclooctatetraene 1,3,5,7-Cyclooctatetraene (COT) is an unsaturated derivative of cyclooctane, with the formula C8H8. It is also known as nnulene. This polyunsaturated hydrocarbon is a colorless to light yellow flammable liquid at room temperature. Because ...

at a

nickel Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow ...

(II) centre reflects the templating effect of the nickel, where it is supposed that four acetylene molecules occupy four sites around the metal and react simultaneously to give the product. This simplistic mechanistic hypotheses was influential in the development of these catalytic reactions. For example, if a competing ligand such as

triphenylphosphine Triphenylphosphine (IUPAC name: triphenylphosphane) is a common organophosphorus compound with the formula P(C6H5)3 and often abbreviated to P Ph3 or Ph3P. It is widely used in the synthesis of organic and organometallic compounds. PPh3 exists ...

were added to occupy one coordination site, then only three molecules of acetylene could bind, and these come together to form

benzene Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen ato ...

(see Reppe chemistry).{{cite journal , doi = 10.1002/jlac.19485600102 , title = Cyclisierende Polymerisation von Acetylen I Über Cyclooctatetraen , year = 1948 , last1 = Reppe , first1 = Walter , last2 = Schlichting , first2 = Otto , last3 = Klager , first3 = Karl , last4 = Toepel , first4 = Tim , journal = Justus Liebigs Annalen der Chemie , volume = 560 , pages = 1–92

References

Polymer chemistry Chemical reactions of ethers