Diimine

This article is about the class of compounds having two imine groups. For compound having the molecular formula N₂H₂ and often called "diimine", see diazene.

Diimines are organic compounds containing two imine (RCH=NR') groups. Common derivatives are 1,2-diimines and 1,3-diimines. These compounds are used as ligands, but they are also precursors to other organic compounds.^[1]

Preparation

Diimines are prepared by condensation reactions where a dialdehyde or diketone is treated with amine and water is eliminated.^[2] Many are derived from the condensation of 1,2-diketones and dialdehydes with amines, often anilines.^[3] The dialdehyde glyoxal is an especially common precursor. Similar methods are used to prepare Schiff bases and oximes.

1,2-Diimines

Sample of alpha-diimine derived from 2,6-diisopropylaniline and glyoxal.

A substituted 1,2-diimine ligand and an idealized metal complex.

The 1,2-diimines are also called α-diimines and 1,4-diazabutadienes. An example is glyoxal-bis(mesitylimine), a yellow solid that is synthesized by condensation of 2,4,6-trimethylaniline and glyoxal.^[4][2] 2,2'-Bipyridine is a 1,2-diimine.

1,2-Diketimines are “non-innocent ligands”, akin to the dithiolenes.^[5]

Synthesis of [tBuN-CH=CH-tBuN]Si.^[6][7]

Synthesis of diiminopyridine complexes.

1,3-Diimines

For example, acetylacetone (2,4-pentanedione) and a primary alkyl- or arylamine will react, typically in acidified ethanol, to form a diketimine. 1,3-Diketimines are often referred to as HNacNac, a modification of the abbreviation Hacac for the conjugate acid of acetylacetone. These species form bidentate anionic ligands.

Uses

Substituted α-diimine ligands are useful in the preparation of post-metallocene catalysts, which are used for the polymerization of alkenes.^[8][9]

1,2-Diimines are precursors to NHC ligands by condensation with formaldehyde.^[4]

Reduction of 1,2-diimines gives diamines.^[1]