Three
Cu(II) coordination compounds with 4-methyl imidazole were
obtained, such as [Cu(C4H6N2)4(NO3)2], [Cu(C4H6N2)4Br2], and [Cu(C4H6N2)4Cl2]. Crystallographic
studies confirmed their structural similarity with Cu(II) in the active
site of endogenous copper–zinc superoxide dismutase (CuZn-SOD).
The superoxide anion radical (O2
•–) scavenging activity was evaluated by the non-enzymatic experimental
assay and followed the trend [Cu(C4H6N2)4(NO3)2] > [Cu(C4H6N2)4Br2] > [Cu(C4H6N2)4Cl2]. The
density
functional theory and the hard and soft acids and bases principle
showed the importance of the electron-deficient character of Cu(II)
in the chemical reactivity of the coordination compounds; Cu(II) is
the softest site in the molecule and it is preferred for the nucleophilic
and radical attacks of the soft O2
•–. A simple rule was obtained: “the electron-deficient character
of Cu(II) is the key index for the O2
•– scavenging activity and is modulated by the electron-releasing counteranion
effect on the coordination compound”.