Unusual coordination of mononuclear cobalt(III) complexes with sterically demanding Schiff bases: synthesis, spectroscopy, electrochemistry, crystallography, DNA binding, and theoretical investigation

Three mononuclear Co^III complexes, [Co(L^{1 tBu})(bdbp)]·2EtOH (1), [Co(L^{2 tBu})(bdbp)] (2), and [Co(L^{3 tBu})₂]ClO₄ (3) (where H₂L^{1 tBu} = bis[{6-benzoyl-4-tert-butyl-2-hydroxyphenyl}benzi-midoyl]-1,2-ethane, H₂L^{2 tBu} = bis[{6-benzoyl-4-tert-butyl-2-hydroxyphenyl}benzimidoyl]-1,3-propane, bdbpH = 4-tert-butyl-2,6-dibenzoylphenol and HL^{3 tBu} = 2-{N-(4-amino-butyl)benzimidoyl}-6-benzoyl-4-tert-butylphenol), have been synthesized by one-pot condensation involving Co^II perchlorate, ethane-1,2-diamine, propane-1,3-diamine, and butane-1,4-diamine. The Schiff bases, H₂L² and HL³, and 1–3 have been characterized by elemental analyses, conductivity measurements, mass spectrometry, IR, electronic, ¹H and ¹³C NMR spectroscopy, cyclic voltammetry, and X-ray crystallography. In 1 and 2, the tetradentate ONNO ligand wraps around cobalt in a rare β cis-conformation. Complex 3 possesses a tridentate NNO ligand leading to two seven-membered chelate rings in a fac-CoN₄O₂ octahedral configuration. Density functional theory (DFT) calculations provide a satisfactory description of the electronic properties of the new compounds. All the complexes are redox active and display the Co(III)–Co(II) couple in the potential range −0.6– + 0.3 V (vs. Ag/AgNO₃). Interaction of the complexes with calf thymus DNA (CT-DNA) has been investigated by a spectrophotometric method and cyclic voltammetry. Based on these observations, an intercalative binding mode of DNA has been proposed. The order of their intrinsic DNA binding constants K_b is K_b (1) < K_b (2) < K_b (3). The observed trend in the binding constant values is consistent with the energy gap between the HOMO and LUMO molecular orbitals.