Acetylacetonate ligand anionic structure

Acetylacetone is a weak acid K 8.8) (d), and the 3-proton can be lost easily to give the enolate anion. This enolate anion has a five-atom n network extending over the two oxygen and three non-terminal carbon atoms. Six electrons occupy the resulting jr-type molecular orbitals. The enolate anion thus has a delocalized symmetric (C2 ) structure (see later discussion) and is most often represented as illustrated in Fig. lA. Numerous derivatives of this anion can be formed giving rise to a variety of metal-to-ligand bonding structures. Also, acetylacetone as the neutral... 

Chromium, (ri6-benzene)tricarbonyl-stereochemistry nomenclature, 1,131 Chromium complexes, 3,699-948 acetylacetone complex formation, 2,386 exchange reactions, 2,380 amidines, 2,276 bridging ligands, 2,198 chelating ligands, 2,203 anionic oxo halides, 3,944 applications, 6,1014 azo dyes, 6,41 biological effects, 3,947 carbamic acid, 2,450 paddlewheel structure, 2, 451 carboxylic acids, 2,438 trinuclear, 2, 441 carcinogenicity, 3, 947 corroles, 2, 874 crystal structures, 3, 702 cyanides, 3, 703 1,4-diaza-1,3-butadiene, 2,209 1,3-diketones... 

Figure 1-11. The resonance forms of some ligands that cannot be represented by a single valence bond structure (acetate anion, acetylacetonate anion and pyridine).

In situ condensation of 2,6-diaminopyridine and acetylacetone has been proposed to yield compounds of type [MXL] (where LH2 is the N6-cyclic ligand formed by condensation of two molecules of each of the above precursors 718 M = Cr, Mn, Fe or Co and X is a range of mono anions). The structure of the Mn111 species (and, indeed of all of the compounds) was proposed to be square pyramidal the pyridine nitrogens of the organic ligand were assumed not to bind to the Mn"1. However non-coordination of these donors would appear to induce considerable strain into the macrocyclic system and hence, in the absence of full X-ray structural data, the assigned structure should be considered tentative. 

Other titanium complexes supported by acetylacetonate 17-19, " showing similar structural analogies with 12-16, were reported to have comparable moderate catalytic activities, stereoselectivity and polydispersity for the bulk ROP of rac-LA (Scheme 6.2). In totality, the activity of these complexes seems to be limited, probably by a substantial 7t-donation interaction from the anionic 0,0-bidentate-type ligands to the Ti " Lewis acid that consequently decreases the rate of polymerisation in addition to the sterically crowded complexes limiting the incoming monomer approach. 

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