Diimine

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.

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
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. 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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