A novel single-pot synthesis of dinuclear and mononuclear copper(II) complexes with sterically demanding Schiff bases: Structural, spectral, magnetic, electrochemical, DNA binding and theoretical investigation

Abstract

Two dinuclear CuII complexes, cyclo[Cu2(LtBu)Cl2]·4EtOH] (monoclinic P21/n) (2) and cyclo[Cu2(LMe)1.5Cl·0·5H2O]Cl·(H2O) (hexagonal P6) (3) (where cycloH2LtBu = cyclobis{2-[benz(N-ethan-1,2-diyl)imidoyl][6-benzimidoyl][4‑tert‑butyl]phenol}, and cycloH2LMe = cyclobis{2-[benz(N-propan-1,3-diyl)imidoyl][6-benzimidoyl][4-methylphenol}, and two mononuclear CuII complexes, [Cu(LtBu)(OH2)] (monoclinic P21/c) (4) and [Cu(mdbp)2] (monoclinic P21/c) (5) (where H2LtBu = bis[2-{6-benzoyl-4‑tert-butylphenol}benzimidoyl]-1,3-propane; mdbpH = 4-methyl-2,6-dibenzoylphenol) have been synthesized by a one–pot condensation involving CuII chloride, ethane-1,2-diamine, propane-1,3-diamine, 4-methyl-2,6-dibenzoylphenol (mdbpH) and 4‑tert‑butyl‑2,6-dibenzoylphenol (bdbpH). The macrocyclic Schiff bases, cyclo[H2LtBu] and cyclo[H2LMe], and the tetradentate Schiff base [H2LtBu], and 2 – 5, have been characterised by elemental analysis, conductivity measurements, mass spectrometry, IR, electronic, 1H and 13C NMR, and ESR spectroscopy, cyclic voltammetry, X-ray crystallography and variable temperature magnetic susceptibility. In 2 and 3, the macrocyclic N4O2 ligand wraps around square pyramidally coordinated copper in a binuclear phenoxo bridged complex. In 2 the apical chlorine atoms are trans with respect to the macrocyclic ligand whereas in 3 they are cis. In the crystalline solid Cl and H2O are disordered on one of the axial positions. Variable temperature magnetic properties indicate weak antiferromagnetic interactions in 1, 2 and 3. Mononuclear complex 4 possesses a tetradentate ONNO ligand with a square pyramidal configuration at Cu and H2O directed at the apical position. Complexes 2 and 3, and 5 are, respectively, examples of square pyramidal and square planar coordinated copper. Density functional theory (DFT) calculations for gas-phase complexes 1, 2, 3, 4 and 5 provide descriptions of electronic properties that are compared with spectroscopic, electrochemical and DNA binding measurements. The binding constants to calf thymus DNA decrease in the order, 2 > 4 > 3 >1 > 5.