Schiff base

General structure of an azomethine

General structure of a Schiff base (narrow definition)

A Schiff base, named after Hugo Schiff, is a compound with the general structure R₂C=NR' (R' ≠ H). They can be considered a sub-class of imines, being either secondary ketimines or secondary aldimines depending on their structure. The term is often synonymous with azomethine which refers specifically to secondary aldimines (i.e. R-CH=NR' where R' ≠ H).

A number of special naming systems exist for these compounds. For instance a Schiff base derived from an aniline, where R³ is a phenyl or a substituted phenyl, can be called an anil, while bis-compounds are often referred to as salen-type compounds.

The term Schiff base is normally applied to these compounds when they are being used as ligands to form coordination complexes with metal ions. Such complexes do occur naturally, for instance in Corrin, but the majority of Schiff bases are artificial and are used to form many important catalysts, such as Jacobsen's catalyst.

Synthesis

Schiff bases can be synthesized from an aliphatic or aromatic amine and a carbonyl compound by nucleophilic addition forming a hemiaminal, followed by a dehydration to generate an imine. In a typical reaction, 4,4'-diaminodiphenyl ether reacts with o-vanillin:

A mixture of 4,4'-oxydianiline 1 (1.00 g, 5.00 mmol) and o-vanillin 2 (1.52 g, 10.0 mmol) in methanol (40.0 mL) is stirred at room temperature for one hour to give an orange precipitate and after filtration and washing with methanol to give the pure Schiff base 3 (2.27 g, 97%)

Biochemistry

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 PLP forms a Schiff base with a lysine residue and is transaldiminated to the substrate(s). Similarly, the cofactor retinal forms a Schiff base in rhodopsins, including human rhodopsin (via Lysine 296), which is key in the photoreception mechanism.

An example where the substrate forms a Schiff base to the enzyme is in the fructose 1,6-bisphosphate aldolase catalyzed reaction during glycolysis and in the metabolism of amino acids.

Coordination chemistry

Schiff bases are common ligands in coordination chemistry. The imine nitrogen is basic and exhibits pi-acceptor properties. The ligands are typically derived from alkyl diamines and aromatic aldehydes.

Schiff base ligands
Copper(II) complex of the Schiff base ligand salicylaldoxime.  Salen is a common tetradentate ligand that becomes deprotonated upon complexation.  Jacobsen's catalyst is derived from a chiral salen ligand.

Chiral Schiff bases were one of the first ligands used for asymmetric catalysis. In 1968 Ryōji Noyori developed a copper-Schiff base complex for the metal-carbenoid cyclopropanation of styrene. For this work he was later awarded a share of the 2001 Nobel Prize in Chemistry.

Conjugated Schiff bases

Conjugated Schiff bases have interesting optoelectronic properties and have been used in organic electronics such as organic and perovskite solar cells, organic field-effect transistor (OFET) and electrochromic devices. Their simple and clean chemistry make them a promising candidate as a low cost alternative to currently used conjugated materials. Schiff base chemistry is also used to prepare covalent organic framework (COFs), which can be used for gas storage.