Trapping of a Methanoato Bridge in µ-1,1,3,3 Mode for [Cu4] Aggregate Formation: Synthesis, Steric Control on Nuclearity, Antimicrobial Activity, and DNA-Interaction Properties

The tetranuclear coordination aggregate [Cu4(µ-L)2(µ1,1,3,3-O2CH)](OH)·6H2O (1) with the 1,3-bis[3-aza-3-(1-methyl-3-oxobut-1-enyl)prop-3-en-1-yl]-2-(2-hydroxyphenyl)-1,3-imidazolidine (H3L) ligand was isolated in good yield, and the HCO2– anion is the central bridging unit bound to the four copper centers in a unique µ-1,1,3,3 coordination mode. On the other hand, the use of bulky RCO2– anions (R = CMe3, CF3, and Ph) resulted in the dinuclear complexes [Cu2(µ-L)(µ1,3-O2CCMe3)]·3.5H2O (2a), [Cu2(µ-L)(µ1,3-O2CCF3)]·2H2O (2b), and [Cu2(µ-L)(µ1,3-O2CPh)]·4H2O (2c), in which the carboxylate anions bridge in µ-1,3 mode. All of the complexes were characterized by several physicochemical techniques and single-crystal X-ray structure analysis. The in vitro antimicrobial activities of the complexes were compared with those of free ligand H3L and Cu(HCO2)2·4H2O. They showed antibacterial and antifungal activities against various strains of Gram-negative (Escherichia coli and Pseudomonas aeruginosa) and Gram-positive (Bacillus subtilis and Streptomyces aureus) bacteria and a yeast (Candida albicans). Among them, complex 1 showed the highest antimicrobial activity against the above-mentioned strains. The interaction of 1 with calf thymus DNA in tris(hydroxymethyl)aminomethane (Tris) buffer was examined by electronic absorption spectroscopy, fluorescence emission spectroscopy and circular dichroism.