Acetylacetone, metal derivatives structure

The structure of bis(salicylaldoxime)beryllium has been proposed as being trans octahedral by comparison of the space group and unit cell volume with those of related transition metal complexes it is presumably a dihydrate if it is indeed octahedral in geometry.297 Stability constants have been reported for a range of beryllium /3-ketoamines derived from both salicylaldehyde and acetylacetone precursors. They show strong complexes which are stable to hydrolysis under the conditions used.298,299... 

Free ligands have been studied in order to obtain an insight into their structure, both in solution and in the solid state, and for comparison with their metal complexes. H NMR spectroscopy has been used to investigate the keto-enol equilibrium and the nature of the hydrogen bonds. In the case of optically active Schiff bases UV and CD spectra provided information about structure in solution. The Schiff bases that have been most widely examined are derivatives of acetylacetone, salicyl-aldehyde and hydroxymethylenecamphor, whose prototypes with en are shown in Figure 13. 

Acetylacetonate and substituted acac derivatives are attractive because of their versatility and stability under normal conditions, as well as their ability to deposit metals cleanly under relatively mild conditions . The dipivaloylmethanato (dpm) derivative from stable and volatile lanthanide compounds, e.g. Lu(dpm)3, have in the gas phase D3 symmetry of the coordination polyhedron. According to Kepert s model, bidentate ligands can be approximated by diatomic molecules and it is completely predictable for the structures of these complexes in the gas phase, but the solid-state structures might be different. 

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... 

By far the most frequently occurring acetylacetonate derivatives are those in which the enolate anion is coordinated to a central metal atom through both oxygen atoms. A classic example of this is the well-known tris(acetylacetonato)iron(III) complex (Fig. IB) whose structure has been determined by single crystal X-ray structure analysis (7). Crystallographic data show that the two chelate ring C—C bond distances are... 

A final and unusual type of carbon-bonded acetylacetonate is that in which the metal atom forms a derivative at a terminal or 1-carbon. The only compounds in this case are three tellurium derivatives shown in Fig. 1G, H, I. These compounds were first prepared by Morgan and Drew (33) in the 1920 s, and on the basis of chemical evidence they correctly postulated the structure of 1G, but not of 1H or 11. Recent spectroscopic studies by Dewar et al. (34) have confirmed the structure of 1G and determined the structures illustrated in Fig. IH and II. It is interesting to note here that sulfur and selenium form bonds only to the 3-carbon atom. At present it is difficult to rationalize the loss of hydrogen for a terminal methyl group which is many orders of magnitude less reactive than the 3-protons. 

The structures of typical ligands were shown earlier in Figure 1.9 and will also be shown later in Figures 3.1 and 3.2. Complexes include some metal halides, hydrates, amines, amides and imides, such as Ti(NR2)4 (R is an alkyl group), oxides, H3B NR3 (a borane-amine adduct), Co(MNT)2 (MNT = maleonitrile dithiolate), Cupc (pc = phthalocyanine), Mo(CO)6, cluster carbonyls, and metal acetylacetonate derivatives. 

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