When Al 3+ is present in solution, different coordination complexes with solvent molecules and the redox species (i.e. DPPH• and QH 2 ) can be formed. Apart from the formation of organometallic complexes with a positive trivalent charge, aluminum, as a strong Lewis acid, can reduce the pKa of the coordinated ligands, and prompt their deprotonation and the formation of coordination complexes of lower charge.