Unusual chemistry of Cu(ii) salan complexes: synthesis, characterization and superoxide dismutase activity

Abstract

The diaminobis(phenolato) “salan” ligand precursors, N,N′-dimethyl-N,N′-bis-(2-hydroxy-3-X-5-Y-6-Z-benzyl)-1,2-diaminoethane {X = ⁱPr, Y = Cl, Z = Me (H₂L¹); X = OCH₃, Y = allyl, Z = H (H₂L²)} and N,N′-bis-(2-hydroxy-3-X-5-Y-6-Z-benzyl)-4-methylimidazolidine {X = ⁱPr, Y = Cl, Z = Me (H₂L³); X = ^tBu, Y = CH₃, Z = H (H₂L⁴)} and their corresponding Cu(II) complexes are synthesized and characterized. [Cu(L¹)(DMSO)] and [Cu(L²)] show the expected geometry and coordination behavior. However, upon coordination to the Cu(II) centre in the presence of CH₃OH, ring cleavage in H₂L³ and H₂L⁴ occurs. Subsequent and unexpected coordination of the in situ formed pendant acetal fragment of the new ligand, formed via ring opening, leads to the formation of the phenolato bridged dimeric Cu(II) complex, [Cu(L^3A)]₂, and a monomeric Cu(II) complex [Cu(L^4A)], where an extra acetal fragment is present as a pendant group. An outline of the probable mechanism for this process is proposed. The superoxide dismutase activity of the Cu(II) complexes is also investigated.