organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic matter, organic materials, i.e., matter in its various forms that contain ...

, a Schiff base (named after Hugo Schiff) is a compound with the general structure ( =

alkyl In organic chemistry, an alkyl group is an alkane missing one hydrogen. The term ''alkyl'' is intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of . A cycloalkyl group is derived from a cy ...

aryl In organic chemistry, an aryl is any functional group or substituent derived from an aromatic ring, usually an aromatic hydrocarbon, such as phenyl and naphthyl. "Aryl" is used for the sake of abbreviation or generalization, and "Ar" is used ...

, but not

hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

). They can be considered a sub-class of

imines In organic chemistry, an imine ( or ) is a functional group or organic compound containing a carbon–nitrogen double bond (). The nitrogen atom can be attached to a hydrogen or an organic group (R). The carbon atom has two additional single bo ...

, being either secondary ketimines or secondary aldimines depending on their structure. Anil refers to a common subset of Schiff bases: imines derived from

aniline Aniline (From , meaning ' indigo shrub', and ''-ine'' indicating a derived substance) is an organic compound with the formula . Consisting of a phenyl group () attached to an amino group (), aniline is the simplest aromatic amine. It is an in ...

s. The term can be synonymous with azomethine which refers specifically to secondary aldimines (i.e. where R' ≠ H).

Synthesis

Schiff bases can be synthesized from an

aliphatic In organic chemistry, hydrocarbons ( compounds composed solely of carbon and hydrogen) are divided into two classes: aromatic compounds and aliphatic compounds (; G. ''aleiphar'', fat, oil). Aliphatic compounds can be saturated (in which all ...

aromatic amine In organic chemistry, an aromatic amine is an organic compound Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be a ...

and a

carbonyl In organic chemistry, a carbonyl group is a functional group with the formula , composed of a carbon atom double bond, double-bonded to an oxygen atom, and it is divalent at the C atom. It is common to several classes of organic compounds (such a ...

compound by

nucleophilic addition In organic chemistry, a nucleophilic addition (AN) reaction is an addition reaction where a chemical compound with an electrophilic double or triple bond reacts with a nucleophile, such that the double or triple bond is broken. Nucleophilic addit ...

forming a

hemiaminal In organic chemistry, a hemiaminal (also carbinolamine) is a functional group or type of chemical compound that has a hydroxyl group and an amine attached to the same carbon atom: . R can be hydrogen or an alkyl group. Hemiaminals are intermediat ...

, followed by a

dehydration In physiology, dehydration is a lack of total body water that disrupts metabolic processes. It occurs when free water loss exceeds intake, often resulting from excessive sweating, health conditions, or inadequate consumption of water. Mild deh ...

to generate an

imine In organic chemistry, an imine ( or ) is a functional group or organic compound containing a carbon–nitrogen double bond (). The nitrogen atom can be attached to a hydrogen or an organic group (R). The carbon atom has two additional single bon ...

. In a typical reaction,

4,4'-oxydianiline 4,4′-Oxydianiline (ODA) is an organic compound with the chemical formula, formula Oxygen, O(Carbon, C6Hydrogen, H4Nitrogen, NH2)2. It is an ether derivative of aniline. This colourless solid is a useful monomer and Cross-link, cross-linking agent ...

reacts with ''o''-

vanillin Vanillin is an organic compound with the molecular formula . It is a phenolic aldehyde. Its functional groups include aldehyde, hydroxyl, and ether. It is the primary component of the ethanolic extract of the vanilla bean. Synthetic vanillin ...

Schiff bases can also be synthesized via the Aza-Wittig reaction.

Biochemistry

Schiff bases have been investigated in relation to a wide range of contexts, including antimicrobial, antiviral and anticancer activity. They have also been considered for the inhibition of

amyloid-β Amyloid beta (Aβ, Abeta or beta-amyloid) denotes peptides of 36–43 amino acids that are the main component of the amyloid plaques found in the brains of people with Alzheimer's disease. The peptides derive from the amyloid-beta precursor prot ...

aggregation. Schiff bases are common enzymatic intermediates where an amine, such as the terminal group of a lysine residue, reversibly reacts with an aldehyde or ketone of a cofactor or substrate. The common enzyme cofactor

pyridoxal phosphate Pyridoxal phosphate (PLP, pyridoxal 5'-phosphate, P5P), the active form of vitamin B6, is a coenzyme in a variety of enzymatic reactions. The International Union of Biochemistry and Molecular Biology has catalogued more than 140 PLP-dependen ...

(PLP) forms a Schiff base with a lysine residue and is transaldiminated to the substrate(s). Similarly, the cofactor

retinal Retinal (also known as retinaldehyde) is a polyene chromophore. Retinal, bound to proteins called opsins, is the chemical basis of visual phototransduction, the light-detection stage of visual perception (vision). Some microorganisms use ret ...

forms a Schiff base in

rhodopsin Rhodopsin, also known as visual purple, is a protein encoded by the ''RHO'' gene and a G-protein-coupled receptor (GPCR). It is a light-sensitive receptor protein that triggers visual phototransduction in rod cells. Rhodopsin mediates dim ...

s, including human rhodopsin (via Lysine 296), which is key in the photoreception mechanism.

Coordination chemistry

The term Schiff base is normally applied to these compounds when they are being used as

ligand In coordination chemistry, a ligand is an ion or molecule with a 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 el ...

s to form

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

with

metal ions A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. These properties are all associated with having electrons available at the Fermi level, as against n ...

. One example is

Jacobsen's catalyst Jacobsen's catalyst is the common name for N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminomanganese(III) chloride, a coordination compound of manganese and a salen-type ligand. It is used as an asymmetric catalyst in the Jacob ...

. The imine nitrogen is basic and exhibits pi-acceptor properties. Several, especially the diiminopyridines are

noninnocent ligand In chemistry, a (redox) non-innocent ligand is a ligand in a metal complex where the oxidation state is not clear. Typically, complexes containing non-innocent ligands are redox active at mild potentials. The concept assumes that redox reactions ...

s. Many Schiff base ligands are derived from alkyl

diamine A diamine is an amine with two amino groups. Diamines are used as monomers to prepare polyamides, polyimides, and polyureas. The term ''diamine'' refers mostly to Primary (chemistry), primary diamines, as those are the most reactive. In terms of ...

s and aromatic aldehydes. Chiral Schiff bases were one of the first ligands used for

asymmetric catalysis Enantioselective synthesis, also called asymmetric synthesis, is a form of chemical synthesis. It is defined by IUPAC as "a chemical reaction (or reaction sequence) in which one or more new elements of Chirality (chemistry), chirality are formed ...

. In 1968

Ryōji Noyori is a Japanese chemist. He won the Nobel Prize in Chemistry in 2001, Noyori shared a half of the prize with William S. Knowles for the study of chirally catalyzed hydrogenations; the second half of the prize went to K. Barry Sharpless for his ...

developed a copper-Schiff base complex for the metal-carbenoid cyclopropanation of

styrene Styrene is an organic compound with the chemical formula C6H5CH=CH2. Its structure consists of a vinyl group as substituent on benzene. Styrene is a colorless, oily liquid, although aged samples can appear yellowish. The compound evaporates easi ...

. Schiff bases have also been incorporated into

metal–organic framework Metal–organic frameworks (MOFs) are a class of porous polymers consisting of metal cluster compound, clusters (also known as Secondary Building Units - SBUs) coordinated to organic compound, organic ligands to form one-, two- or three-dimension ...

s (MOF).

Conjugated Schiff bases

Conjugated Schiff bases absorb strongly in the UV-visible region of the electromagnetic spectrum. This absorption is the basis of the anisidine value, which is a measure of oxidative spoilage for fats and oils.

Historic references

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References

{{reflist, refs= Functional groups Schiff bases Organometallic chemistry Ligands