Dicyanamide-interlaced assembly of Zn(II)-schiff-base complexes derived from salicylaldimino type compartmental ligands: Syntheses, crystal structures, FMO, ESP, TD-DFT, fluorescence lifetime, in vitro antibacterial and anti-biofilm properties

Two new dicyanamide-interlaced tetranuclear Zn(II)-Schiff-base complexes [Zn-2(L-OMe)(mu_dca-kappa N-1 kappa N-5)(mu-dca-kappa N-1)](2) (1) and [Zn-2(L-OEt)(mu_dca-kappa N-1 kappa N-5)(mu-dca-kappa N-1)](2) (2) were synthesized by using salicylaldimino type Schiff bases (H2LOMe) and (H2LOEt) respectively. Schiff base ligands and the complexes were characterized by elemental analyses, powder X-ray diffraction, FT-IR, FT-Raman, H-1 NMR, C-13 NMR, UV-Vis, TGA and fluorescence spectroscopy. Dicyanamide modulated complexes were structurally characterized by single crystal X-ray diffraction studies. X-ray crystal structure divulges that the two complexes are isostructural. In both the complexes, the Zn1 metal centers fulfill 5-coordinated distorted square pyramidal geometry having ZnN(3)O(2 )chromospheres where Schiff bases are mainly trapped in their complete deprotonated dianionic forms [L](2-), whereas Zn2 metal center attained distorted octahedral geometry. In both complexes two asymmetric units are connected by double mu(1,5)-dicyanamide ion thus forming Zn-4-nuclear metal complex. B3LYP/def2-TZVP level of theory (DFT) successfully applied in both complexes. The complexes (1-2) exhibit intraligand (pi -> pi *) fluorescence in DMSO solvent with lifetimes in the range 0.66-0.82 ns. In vitro antibacterial, membrane damage assay and anti-biofilm properties of both complexes are evaluated against some important Gram-positive and Gram-negative bacterial strains. Finally, the UV-Vis experimental spectral findings are well rationalized with the electronic distribution of HOMO-LUMO through TD-DFT level of calculations.