New barium, strontium and strontium-doped barium squarates: synthesis, crystal structures and DNA/BSA binding, antioxidant and in vitro cytotoxicity studies

A new set of differently hydrated barium and strontium squarates, namely poly[[triaqua(mu-1,2-dioxocyclobut-3-ene-1,2-diolato)barium] monohydrate], {[Ba-(C4O4)(H2O)(3)]center dot H2O}(n) (1), poly[[diaqua(mu-1,2-dioxocyclobut-3-ene-1,2-diolato)strontium] monohydrate], {[Sr(C4O4)(H2O)(2)]center dot H2O}(n) (2), and poly[[triaqua(mu-1,2-dioxocyclobut-3-ene-1,2-diolato) barium/strontium(0.85/0.15)] monohydrate], {[Ba0.85Sr0.15(C4O4)(H2O)(3)]center dot H2O}(n) (3), is reported. The study of their crystal structures indicates that all the complexes crystallize in the triclinic space group P (1) over bar. Complexes 1 and 3 have a rare combination of squarate units coordinated through monodentate O atoms to two different metal atoms and through two bidentate O atoms to three different metal atoms. Furthermore, they have three coordinated water molecules to give a coordination number of nine. The squarate ligands in complex 2 exhibit two different coordination modes: (i) monodentate O atoms coordinated to four different Sr atoms and (ii) two monodentate O atoms coordinated to two different metal atoms with the other two O atoms bidentate to four different Sr atoms. All the compounds decompose to give the respective carbonates when heated to 800 degrees C, as evidenced by thermogravimetry/differential thermal analysis (TG-DTA), which are clusters of nanoparticles. Complexes 1 and 3 show additional endothermic peaks at 811 and 820 degrees C, respectively, indicating the phase transition of BaCO3 from an orthorhombic (alpha-Pmcn) to a trigonal phase (beta-R3m). All three complexes have significant DNA-binding constants, ranging from 2.45 x 10(4) to 9.41 x 10(4) M-1 against EB-CT (ethidium bromide-calf thymus) DNA and protein binding constants ranging from 1.1 x 10(5) to 8.6 x 10(5) with bovine serum albumin. The in vitro cytotoxicity of the complexes is indicated by the IC50 values, which range from 128.8 to 261.3 mu g ml(-1). Complex 3 shows better BSA binding, antioxidant activity against the DPPH radical and cytotoxicity than complexes 1 and 2.