This paper presents the study of mixed-ligand complexes of Ln(III), which are used as volatile precursors in CVD processes for the preparation of lanthanide-containing films and coatings. New mixed-ligand acetylacetonate complexes of lanthanides (III) with acetic (propionic) acid and acetonitrile or dimethylformamide were synthesized and investigated by physic-chemical methods of analysis (elemental analysis, differential thermal analysis, IR spectroscopy, powder X-ray diffraction). Using mixed-ligand complexation, the properties of the initial lanthanide β-diketonates (in particular, chemical and thermal stability) can be changed significantly.
To assess the stability of the synthesized mixed-ligand complexes, their quantum-chemical modeling was performed using the semi-empirical method Sparkle/PM7. Standard changes of the Gibbs energy ∆G0298 were calculated for the solution reaction of (1) synthesis of mixed-ligand complexes and (2) substitution of one of the β-diketonate ligands in the Ln(III) tris-acetylacetonates dihydrates by an acetate ion or propionate ion. The ∆G0298 values for the syntesis reaction mainly increases with increasing donor basicity and decreasing ionic radii Ln(III) in the La>Gd>Lu series. For all mixed ligand complexes of Ln(III), the heats of formation are negative, which indicates their thermodynamic stability in solution.
It was established that the obtained complexes have the same composition of the general formula [Ln(AA)2·L·2D], where Ln (III) = La, Gd; НАА- acetylacetonе; L - anion of acetic (HAc) or propionic (HРrop) acids, D- acetonitrile (AN), dimethylformamide (DMFA).
The results of the thermal analysis confirm the computational data: in the case of the lanthanum mixed-ligand complexes, the carboxylic acid is coordinated to the central ion through bridging carboxylate-ions, which contributes to the formation of oligomers. The lanthanum mixed-ligand complexes are not volatile due to their oligomeric structure. On the other hand, similar gadolinium complexes are monomeric and sublimate at 180 - 350 °C.
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