Water-Soluble Heteronuclear [NaCu6II] Metallomacrocyclic Sandwich Complexes: Synthesis, Structure, Properties and In Vitro Biological Studies

The water-soluble heteronuclear metallomacrocyclic sandwich clusters [NaCu6(hpnbpda)3(OH)3(OH2)3](NO3)·4H2O (1), [NaCu6(hpnbpda)3(OH)3(OH2)3](ClO4) (2), and [NaCu6(hpnbpda)3(OH)3(OH2)3](PF6) (3) [H3hpnbpda = N,N′-bis(2-pyridylmethyl)-2-hydroxy-1,3-propanediamine-N,N′-diacetic acid] have been synthesized in methanol at room temperature and fully characterized using several analytical techniques including single-crystal X-ray diffraction. The molecular architecture of complex 1 is built from the template assembly of three dinuclear [Cu2(hpnbpda)]+ fragments through their weak oxophillic interactions with a central sodium(I) cation. A close analysis of the single-crystal X-ray structure reveals that the metallic core of cluster 1 consists of six highly distorted octahedral CuII ions arranged at the corners of a trigonal prism that encapsulates the sodium(I) ion. Complex 1 displays a rare μ3:η2:η1:η1 bridging mode of six carboxylate groups of three hpnbpda3– ligands with each bridging between two copper(II) atoms and the sodium(I) center. Variable-temperature magnetic-susceptibility measurements (2–300 K) on a powdered microcrystalline sample reveal that the paramagnetic copper centers in complex 1 are antiferromagnetically coupled to one another. Systematic biological investigation such as cytotoxicity assessment, DNA content analysis in terms of cell-cycle distribution by means of the fluorescence-activated cell sorting (FACS) method, DNA binding, DNA cleavage, and the underlying mechanism of possible apoptotic cell-death events in human cervical cancer cells (HeLa) were carried out in detail using complex 1. The in vitro assays using complex 1 in HeLa cells provided new findings that indicate its possible future therapeutic application. Theoretical calculations were carried out to find the Fukui functions at the metal sites in complex 1 to predict the possible metal centers involved in the DNA binding.