Med Library . org

Open Source Encyclopedia

Diketimine

Welcome to MedLibrary.org. For best results, we recommend beginning with the navigation links at the top of the page, which can guide you through our collection of over 14,000 medication labels and package inserts. For additional information on other topics which are not covered by our database of medications, just enter your topic in the search box below:

Diketimines or diimines are a family of ligands and ligand precursors derived from 1,2- and 1,3-diketones by replacement of the carbonyl oxygen atoms with NR groups, where R = aryl, alkyl. Two families of diketimines are important in coordination chemistry and catalysis: 1,2-diketimines and 1,3-diketimines.

Contents

Preparation

Main article: Imine

Diketimines are prepared by conventional condensation reactions that are used to convert aldehydes and ketones to imines, Schiff bases, and oximes. 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 derived from bulky amines, e.g. 2,6-disubstituted anilines, require prolonged reaction times.[1] 1,3-Diketimines are often referred to as "HNacNac", a modification of the abbreviation Hacac used for α,β-diketones. These species can exist as a mixture of tautomers.

Tautomers of a substituted HNacNac ligand precursor and an idealized complex (right) of the conjugate base (M = metal, L = other ligand)

Coordination complexes

The 1,2-diketimine ligands, also called α-diimines, include dimethylglyoxime as well as oxidized derivatives of o-phenylenediamine. The steric properties of the substituents on nitrogen provide a means to control the axial coordination sites on a square planar complex. Large planar substituents such as mesityl tend to be orthogonal to the MN2 plane. In this way, the axial coordination sites on a square planar complex are shielded. Such steric control is not possible for complexes of the related to 2,2'-bipyridine, glyoximate, and 9,10-phenanthroline ligands.

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

Deprotonation of HNacNac compounds affords anionic bidentate ligands that form a variety of coordination complexes.[2] Some derivatives with large R groups can be used to stabilize low valent main group and transition metal complexes.[3] Unlike the situation for the acetylacetonates, the steric properties of the coordinating atoms in NacNac- ligands is adjustable by changes in the R substituent. Attachment to a metal center is usually carried out by initial deprotonation of HNacNac with n-butyllithium; the lithium derivative is then treated with a metal chloride to eliminate LiCl. In some cases, HNacNacs also serve as charge-neutral 1,3-diimine ligands.

1,2-Diketimines, but not the 1,3-diketimines, are “non-innocent ligands”, akin to the dithiolenes.

Uses

Substituted α-diimine and NacNac ligands are useful in the preparation of so-called post-metallocene catalysts for the polymerization and copolymerization of ethylene and alkenes.[1][4]

References

  1. ^ a b Feldman, J.; McLain, S. J.; Parthasarathy, A.; Marshall, W. J.; Calabrese, J. C.; Arthur, S. D. (1997). "Electrophilic Metal Precursors and α-Diimine Ligand for Nickel(II)- and Palladium(II)-Catalyzed Ethylene Polymerization". Organometallics 16: 1514–1516. doi:10.1021/om960968x.
  2. ^ Bourget-Merle, L.; Lappert, M. F.; Severn, J. R. (2002). "The Chemistry of -Diketiminatometal Complexes". Chemical Reviews 102: 3031–3066. doi:10.1021/cr010424r.
  3. ^ Qian, B.; Ward, D. L.; Smith, M.R. (1998). "Synthesis, Structure, and Reactivity of β-Diketiminato Aluminum Complexes". Organometallics 17: 3070–3076. doi:10.1021/om970886o.
  4. ^ 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.