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

Abstract

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.

Keywords:

• N₂O₂ and NNO Schiff base ligands
• Co^III complexes
• NMR
• cyclic voltammetry
• X-ray structure
• DFT and DNA binding

Acknowledgements

We thank Mr. V. Ramkumar (Department of Chemistry, IIT Madras) for assistance with the data collection and the XRD facility, Dr. J.D. Smith, University of Sussex, UK and Prof. Pavan Mathur, University of Delhi for discussion and referees for their constructive comments.