Diimine

Diimines are organic compounds containing two imine (RCH=NR') groups. The most popular derivatives are 1,2-diketones and 1,3-diimines. These compounds are used as ligands and as precursors to heterocycles. Diimines are prepared by condensation reactions where a dialdehyde or diketone is treated with amine and water is eliminated. Similar methods are used to prepare Schiff bases and oximes.

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.

1,2-Diimines

The 1,2-diketimine ligands, also called α-diimines. They are derived from the condensation of 1,2-diketones with amines, using anilines.[1]

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

An example is glyoxal-bis(mesitylimine), a yellow solid that is synthesized by condensation of 2,4,6-trimethylaniline and glyoxal.[2] 2,2'-Bipyridine is a 1,2-diimine.

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

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 anionic bidentate anionic ligands.

Uses

Substituted α-diimine ligands are useful in the preparation of so-called post-metallocene catalysts for the polymerization and copolymerization of ethylene and alkenes.[3]

Diimines are precursors to NHC ligands by condensation with formaldehyde.[2]

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References
  1. Wang, F.; Chen, C. (2019). "A Continuing Legend: The Brookhart-Type α-Diimine Nickel and Palladium Catalysts". Polymer Chemistry. 10: 2354-2369. doi:10.1039/C9PY00226J.
  2. Ison, Elon A.; Ison, Ana (2012). "Synthesis of Well-Defined Copper N-Heterocyclic Carbene Complexes and Their Use as Catalysts for a "Click Reaction": A Multistep Experiment That Emphasizes the Role of Catalysis in Green Chemistry". J. Chem. Educ. 89: 1575–1577. doi:10.1021/ed300243s.
  3. Ittel, S. D.; Johnson, L. K.; Brookhart, M. (2000). "Late-Metal Catalysts for Ethylene Homo- and Copolymerization". Chemical Reviews. 100: 1169–1203. doi:10.1021/cr9804644.
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