Three Zn(II)–Ln(III) clusters based on Schiff base ligand: synthesis, structure, fluorescence, and antioxidant activity

Three heterometallic Zn(II)–Ln(III) clusters of formulas [ZnLnL(NO₃)₂(OAc)(MeOH)] × (MeOH) (<b>1</b> Ln = Eu, <i>x</i> = 4; <b>2</b> Ln = Gd, <i>x</i> = 3) and [Zn₂NdL₂(OAc)₂][Nd(MeOH)(NO₃)₄] · CH₂Cl₂ (<b>3</b>) (H₂L = bis(5-bromo-3-methoxysalicylaldehyde-3-oxopentane-1,5-diamine) have been synthesized and characterized via single-crystal X-ray diffraction. Clusters <b>1</b> and <b>2</b> are isostructural and heterodinuclear compounds, while <b>3</b> is composed of a heterotrinuclear [Zn₂NdL₂(OAc)₂]⁺ cation and a [Nd(MeOH)(NO₃)₄]⁻ anion. The difference in their structures can be attributed to the lanthanide contraction effect. Zn(II) and Ln(III) ions except for [Nd(MeOH)(NO₃)₄]⁻ anion were encapsulated in the inner and outer cavities of the deprotonated ligand L²⁻, respectively. In comparison with the free H₂L ligand, the fluorescence peaks of the three Zn(II)–Ln(III) clusters were significantly blue shifted and enhanced, which may be due to the coordination of ligand with metal ions. The antioxidant activity of clusters <b>1–3</b> and ligand H₂L were determined by measuring the scavenging of superoxide and hydroxyl radicals <i>in vitro</i>. The results showed that H₂L and clusters <b>1–3</b> possess favorable activities in scavenging hydroxyl radicals as well as superoxide radicals, and all clusters exhibited better scavenging activities than free H₂L.