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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 primary alkyl- or arylamine react, typically in acidified ethanol. 1,3-Diketimines derived from bulky amines, e.g. 2,6-disubstituted anilines, require prolonged reaction times. 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 two tautomers.
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-phenanthrolene ligands.
Deprotonation of HNacNac compounds affords anionic bidentate ligands that form a variety of coordination complexes. Some derivatives with large R group can been used to stabilize low-valent main group and transition metal complexes. 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 butyllithium; the lithium derivative is then treated with a metal chloride to eliminate LiCl. In some cases, HNacNac’s 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.
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.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Diketimine". A list of authors is available in Wikipedia.|