Synthesis, Crystal Structures, and DNA Cleavage Activities of Manganese(II) Complexes: A Good Example of the Synergy between Metal Ions Prompting DNA Cleavage

The potential synergy between metal centers makes multinuclear metal complexes promising candidates for artificial nucleases. However, few multinuclear mimics have shown higher DNA cleavage efficiencies than their mononuclear analogues. By using 1,8‐naphthalene dicarboxylic acid (1,8‐H2ndc) and 1,10‐phenanthroline (phen) ligands, one mononuclear complex [Mn(1,8‐ndc)(phen)2(H2O)]·CH3OH·2H2O (1), two dinuclear complexes [Mn(1,8‐Hndc)(phen)2]2·2ClO4·4H2O (2) and [Mn(1,8‐ndc)(phen)(H2O)2]2·CH3OH·H2O (3), and one hexanuclear complex [Mn(1,8‐ndc)(phen)(H2O)2]6·11H2O (4) were synthesized and structurally characterized by single‐crystal X‐ray analysis. Complexes 2–4 have closed‐ring structures bridged by 1,8‐ndc ligands that adopt a peculiar μ2‐η1:η1 bridging mode, which has never been reported before. Circular dichroism and fluorescence studies show that the four complexes interacted with calf thymus DNA through the intercalative binding mode, and the binding ability order is 2 > 1 > 3 > 4. All of the complexes can efficiently promote the oxidative cleavage of plasmid pBR 322 DNA in the presence of H2O2 with an efficiency order 3 > 4 > 2 > 1. The higher DNA cleavage abilities of the multinuclear complexes than the mononuclear one, combined with the fact that 3 with the shortest intramolecular Mn···Mn distance and the strongest magnetic coupling between MnII ions shows the highest DNA cleavage efficiency, confirm the great effect of the synergy between metal centers on the DNA cleavage efficiency of complexes. The four complexes also show strong concentration‐dependent cytotoxicity to human breast cancer cells.