Linear free energy relationships for Fe(III) complexes with aroylacetanilides

Acid dissociation constants (pK_a) of seven substituted benzoylacetanilides 1a-g and the stability constants (log B_n) of their 1:1 and 1:2 Fe(III) chelates were determined spectrophotometrically in 60% by volume ethanol-water mixture at ionic strength of 0.1 and 25°C. The correlations between log B_n and pK_a and Hammett substituent constant (σ) were examined. The relations follow the equations: log B₁ = 9.53−2.86 σ; log B₂ = 16.27−7.07 σ; log B₁ =1.26 pK_a-2.36; and log B₂=3.10 pK_a-13.09. The relations between the stability constants of Fe(III) chelates with the ligands 1a-g and benzoylacetarylamides 2a-g and those reported for Cu(II) chelates with 2a-g are also discussed. The results imply that Fe(III) chelates with the ligands 1a-g are subject to “normal” rather than “structural” substituent effects.