Molecular and Electronic Structures, Spectra, Electrochemistry and Anti‐bacterial Efficacy of Novel Heterocyclic Hydrazones of Phenanthrenequinone and Their Nickel(II) Complexes

A pair of tridentate ligands incorporating heterocyclic hydrazones of 9,10-phenanthrenequinone viz,. pyridyl-hydrazino-phenathrenequinone HLPy 1a and benzothiazolyl-hydrazino-phenathrenequinone HLBenz 1b have been synthesized and both behave as monoanionic towards nickel(II), forming meridional octahedral complexes of type [Ni(L-Py)(2)] 2a and [Ni(L-Benz)(2)] 2b as evident from single crystal X-ray diffraction studies. The complexes are electro-active in solution and nature of redox orbitals has been analysed by theoretical means. They display two oxidative and two reductive responses that have been ascribed to the redox processes of coordinated ligands. The electronic absorption spectral patterns of two complexes are analogous barring the fact that lower energy transition for 2b is marginally bathochromically shifted relative to that of 2a and it has been clarified by TD-DFT studies. Anti-bacterial efficacy of the ligands HLPy 1a, HLBenz 1b and complexes [Ni(L-Py)(2)] 2a, [Ni(L-Benz)(2)] 2b against four Gram-positive and four Gram-negative strains has been explored. [Ni(L-Py)(2)] 2a exhibits more pronounced efficacy than [Ni(L-Benz)(2)] 2b and these are greater than those of the corresponding ligands. The mode of action of 2a is essential via DNA damage while protein leakage and membrane lipid damage were observed upon treatment with 2b.