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) (1 Ln = Eu, x = 4; 2 Ln = Gd, x = 3) and [Zn2NdL2(OAc)2][Nd(MeOH)(NO3)4] · CH2Cl2 (3) (H2L = bis(5-bromo-3-methoxysalicylaldehyde-3-oxopentane-1,5-diamine) have been synthesized and characterized via single-crystal X-ray diffraction. Clusters 1 and 2 are isostructural and heterodinuclear compounds, while 3 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 1–3 and ligand H2L were determined by measuring the scavenging of superoxide and hydroxyl radicals in vitro. The results showed that H2L and clusters 1–3 possess favorable activities in scavenging hydroxyl radicals as well as superoxide radicals, and all clusters exhibited better scavenging activities than free H2L.本研究合成并通过单晶X射线衍射表征了三种异金属Zn(II)-Ln(III)团簇，其通式分别为[ZnLnL(NO3)2(OAc)(MeOH)]·(MeOH)（1：Ln=Eu，x=4；2：Ln=Gd，x=3）与[Zn2NdL2(OAc)2][Nd(MeOH)(NO3)4]·CH2Cl2（3），其中配体H2L为双(5-溴-3-甲氧基水杨醛缩-3-氧代戊烷-1,5-二胺)。团簇1与2为同构异双核化合物，而团簇3由异三核阳离子[Zn2NdL2(OAc)2]+与阴离子[Nd(MeOH)(NO3)4]−构成。其结构差异可归因于镧系收缩效应。除阴离子[Nd(MeOH)(NO3)4]−中的Nd(III)外，其余Zn(II)与Ln(III)离子分别被包封于去质子化配体L²⁻的内、外空腔中。相较于游离的H2L配体，三种Zn(II)-Ln(III)团簇的荧光峰发生显著蓝移且强度增强，该现象可归因于配体与金属离子的配位作用。本研究通过体外检测超氧阴离子自由基与羟基自由基的清除能力，测定了团簇1~3及配体H2L的抗氧化活性。结果表明，H2L与团簇1~3均具备良好的羟基自由基与超氧阴离子自由基清除活性，且所有团簇的清除活性均优于游离的H2L配体。