Iron(III) and aluminium(III) complexes with substituted salicyl-aldehydes and salicylic acids

The chelating properties toward iron(III) and aluminium(III) of variously substituted salicyl-aldehydes and salicylic
acids have been evaluated, together with the effect of methoxy and nitro substituents in ortho and para position
with respect to the phenolic group. The protonation and iron and aluminium complex formation equilibria have
been studied by potentiometry, UV–visible spectrophotometry and 1H NMR spectroscopy. The overall results
highlight that salicyl-aldehydes present good chelating properties toward ironIII, with pFe ranging from 14.2
with nitro to 15.7 with methoxy substituent, being ineffective toward aluminium; the pFe values for salicylic
acids are generally lower than those for salicyl-aldehydes, and about 4 units higher than the corresponding pAl
values. The effect of the two substituents on the chelating properties of the ligands can be rationalized in
terms of the Swain–Lupton treatment which accounts for the field and resonance effects. The structural characterization
of the 1:2 iron complex with p-nitro salicylic acid shows that ironIII ion exhibits an octahedral surrounding
where two salicylate chelating ligands supply two O-phenolate and two O-carboxylate donor atoms
in a roughly equatorial plane. The trans-apical sites are occupied by two aqua ligands.