General structure of a imine. Schiff bases are imines in which R3 is an alkyl or aryl group (not a hydrogen). R1 and R2 may be hydrogens
General structure of an azomethine compound
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 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.
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 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 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.
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.
Formation of metal clusters in halloysite nanotubes via a Schiff base reaction on example of ruthenium.
Schiff bases can be used to mass-produce nanoclusters of transition metals inside halloysite. This abundant mineral naturally has a structure of rolled nanosheets (nanotubes), which can support both the synthesis and the metal nanocluster products. These nanoclusters can be made of Ag, Ru, Rh, Pt or Co metals and can catalyze various chemical reactions.
^Bajema, Elizabeth A.; Roberts, Kaleigh F.; Meade, Thomas J. (2019). "Chapter 11. Cobalt-Schiff Base Complexes:Preclinical Research and Potential Therapeutic Uses". In Sigel, Astrid; Freisinger, Eva; Sigel, Roland K. O.; Carver, Peggy L. (Guest editor) (eds.). Essential Metals in Medicine:Therapeutic Use and Toxicity of Metal Ions in the Clinic. Metal Ions in Life Sciences. 19. Berlin: de Gruyter GmbH. pp. 267–301. doi:10.1515/9783110527872-017. ISBN978-3-11-052691-2. PMID30855112.
J.C. Hindson; B. Ulgut; R.H. Friend; N.C. Greenham; B. Norder; A. Kotlewskic; T.J. Dingemans (2010). "All-aromatic liquid crystal triphenylamine-based poly(azomethine)s as hole transport materials for opto-electronic applications". J. Mater. Chem. 20 (5): 937–944. doi:10.1039/B919159C.
L. Sicard; D. Navarathne; T. Skalski; W. G. Skene (2013). "On-Substrate Preparation of an Electroactive Conjugated Polyazomethine from Solution-Processable Monomers and its Application in Electrochromic Devices". Adv. Funct. Mater. 23 (8): 3549–3559. doi:10.1002/adfm.201203657.