DFT studies of the effectiveness of p-phenylenediamine antioxidants through their Cu(II) coordination ability

In B3LYP optimized structures of Cu2+ complexes with a series of p-phenylenediamine (PPD) antioxidants the copper atom prefers bonding to the nitrogen atom between aromatic rings (N1 site). The PPD antioxidant effectiveness rises with increasing PPD affinity to Cu2+ ions, decreasing charges and spin densities at Cu atoms and the increasing electron density transfer in Cu-N bonds in the 2[PPD⋯Cu]2+ complexes with copper atoms bonded at N1 site unlike remaining N sites. Cu bonding to tertiary carbon atoms is excluded due to the absence of frontier molecular orbitals density at these atoms in PPD molecules.