Diketimine

Diketimines 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.

Preparation
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 &alpha;,&beta;-diketones. These species can exist as a mixture of two tautomers.

Coordination complexes
The 1,2-diketimine ligands, also called &alpha;-diimines, include dimethylglyoxime as well as oxidized derivatives of 1,2-diaminobenzene. 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.

Uses
Substituted &alpha;-diimine and NacNac ligands are useful in the preparation of so-called Post-metallocene catalysts for the polymerization and copolymerization of ethylene and alkenes.