Benzoylacetanilide complexes of iron(III): Structure-stability relationships

Abstract

Acid dissociation constants, pK ∗ , of a series of benzoylacetarylamides (Ia-h) and acetoacetanilide (IId) have been determined spectrophotometrically in 60 vol.% ethanol-water medium. The pK ∗ values of both Id and IId were similarly determined in 50 vol.% dioxane-water medium. The stability constants, log β n , of the ferric chelates with such ligands have also been determined. The effects of substituents in the keto anilides on the equilibria are discussed. Both values of pK ∗ and log β n were linearly correlated with Hammett substituent constant, σ. The equations of the straight lines obtained are: pK ∗ = 9.48 − 0.933 σ ( r = 0.979); log β 1 = 9.60 − 1.693 σ ( r = 0.989); log β 2 = 16.05 − 2.265 σ, ( r = 0.957); log β 3 = 19.02 − 1.657 σ, ( r = 0.939). The values of the stability constants of ferric chelates of Id and IId indicate that the former ligand (Id) forms weaker complexes than the latter (IId) with iron(III) irrespective of the nature of the solvent, contrary to what has previously been reported.