Diimines are organic compounds containing two imine (RCH=NR') groups. Common 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.

1,2-Diimines

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

The 1,2-diketimine ligands also called α-diimines and 1,4-diazabutadienes. They are derived from the condensation of 1,2-diketones and glyoxal with amines, often 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.[3]

Synthesis of [tBuN-CH=CH-tBuN]Si.[4][5]
Synthesis of diiminopyridine complexes.
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 for the polymerization and copolymerization of ethylene and alkenes.[6][7]

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

References

  1. ↑ Wang, F.; Chen, C. (2019). "A Continuing Legend: The Brookhart-Type α-Diimine Nickel and Palladium Catalysts". Polymer Chemistry. 10 (19): 2354-2369. doi:10.1039/C9PY00226J.
  2. 1 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 (12): 1575–1577. Bibcode:2012JChEd..89.1575I. doi:10.1021/ed300243s.
  3. ↑ Mashima, Kazushi (2020). "Redox-Active α-Diimine Complexes of Early Transition Metals: From Bonding to Catalysis". Bulletin of the Chemical Society of Japan. 93 (6): 799–820. doi:10.1246/bcsj.20200056.
  4. ↑ Haaf, Michael; Schmedake, Thomas A.; West, Robert (2000-10-01). "Stable Silylenes". Accounts of Chemical Research. 33 (10): 704–714. doi:10.1021/ar950192g. ISSN 0001-4842. PMID 11041835.
  5. ↑ Asay, Matthew; Jones, Cameron; Driess, Matthias (2011-02-09). "N-Heterocyclic Carbene Analogues with Low-Valent Group 13 and Group 14 Elements: Syntheses, Structures, and Reactivities of a New Generation of Multitalented Ligands†". Chemical Reviews. 111 (2): 354–396. doi:10.1021/cr100216y. ISSN 0009-2665. PMID 21133370.
  6. ↑ Ittel, S. D.; Johnson, L. K.; Brookhart, M. (2000). "Late-Metal Catalysts for Ethylene Homo- and Copolymerization". Chemical Reviews. 100 (4): 1169–1203. doi:10.1021/cr9804644.
  7. ↑ Guo, Lihua; Dai, Shengyu; Sui, Xuelin; Chen, Changle (2016). "Palladium and Nickel Catalyzed Chain Walking Olefin Polymerization and Copolymerization". ACS Catalysis. 6: 428–441. doi:10.1021/acscatal.5b02426.
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