Thermal Stability of Bis(acetylacetonato)bis(substituted aniline)nickel(II) Complexes, [Ni(acac)₂B₂], in the Solid State

Abstract

The kinetics of aniline elimination reactions of [Ni(acac)₂B₂] complexes {Hacac=acetylacetone and B=aniline or its derivatives} have been studied in the solid state by the Coats Redfern method on the basis of the weight loss curves recorded at a heating rate of 1°C/min in nitrogen stream, 100 ml/min, :
[Ni(acac)₂B₂]_??_Ni(acac)₂B +B↑_??_Ni(acac)₂ +B↑
The values of the rate constant and the activation energy were determined from the slopes of the plots of log[-log(1-α)/(2.303. T²)] vs 1/T. Then, the activation free energies (ΔG) were calculated from the values of the activation energy and the frequency factor. The ΔG/values for the first and the second elimination reactions, ΔG(383K) and ΔG(435K), varied in the range of 109-127kJ.⋅mol^-1 and 130-439 kJ⋅mol^-1, respectively, depending on the nature of aniline derivatives. A linear correlation was observed between the ΔG values and pKa values of aniline derivative except for the complexes with p-chloroaniline and p-bromoaniline. These complexes with p-chloroaniline and p-bromoaniline were more stable (ca.26 kJ⋅mol^-1) than expected from their pKa, values. These results suggested that the kinetic stability of [Ni(acac)₂B₂] complexes mainly depends on the pKa and the heat of sublimation of aniline derivatives.