Synthesis, characterization, crystal structure and antimicrobial activity of copper(II) complexes with the Schiff base derived from 2-hydroxy-4-methoxybenzaldehyde

Abstract

A novel Schiff base, ethyl 4-[(E)-(2-hydroxy-4-methoxyphenyl)methylene-amino]benzoate (HL), was prepared and structurally characterized on the basis of elemental analyses, (1)H NMR, (13)C NMR, UV-Vis and IR spectral data. Six new copper(II) complexes, [Cu(L)(NO3)(H2O)2] (1), [Cu(L)2] (2), [Cu(L)(OAc)] (3), [Cu2 (L)2Cl2(H2O)4] (4), [Cu(L)(ClO4)(H2O)] (5) and [Cu2(L2S)(ClO4)(H2O)]ClO4·H2O (6) have been synthesized. The characterization of the newly formed compounds was done by IR, UV-Vis, EPR, FAB mass spectroscopy, elemental and thermal analysis, magnetic susceptibility measurements and molar electric conductivity. The crystal structures of Schiff base and the complex [Cu2(L2S)(ClO4)(H2O)]ClO4·H2O (6) have been determined by single crystal X-ray diffraction studies. Both copper atoms display a distorted octahedral coordination type [O4NS]. This coordination is ensured by three phenol oxygen, two of which being related to the µ-oxo-bridge, the nitrogen atoms of the azomethine group and the sulfur atoms that come from the polydentate ligand. The in vitro antimicrobial activity against Escherichia coli ATCC 25922, Salmonella enteritidis, Staphylococcus aureus ATCC 25923, Enterococcus and Candida albicans strains was studied and compared with that of free ligand. The complexes 1, 2, 5 showed a better antimicrobial activity than the Schiff base against the tested microorganisms.

Figure 1

Schiff base ligand (HL).

Figure 2

Proposed structures for the copper(II) complexes 1–5.

Figure 2

Proposed structures for the copper(II) complexes 1–5.

Figure 3

X-ray molecular structure of (HL). Thermal ellipsoids are drawn at 50% probability level. H-bond parameters: O3−H∙∙∙N1 [O3−H 0.86 Å, H∙∙∙N1 1.87 Å, O3∙∙∙N1 2.602(2) Å, O3−H∙∙∙N1 140.7°].

Figure 4

π-π staking interactions in the crystal structure (HL).

Figure 5

View of the supramolecular ribbon in the crystal structure (HL). C17−H∙∙O2 [C17−H 0.96 Å, H∙∙∙O2 2.44 Å, C17∙∙∙O2(−x, −1 + y, 1.5 – z) 3.378(4) Å, C17−H∙∙∙O2 164.4°].

Figure 6

The X-ray structure of the complex cation [Cu₂(L₂S)₂(ClO₄)(H₂O)]⁺ (6). H-bonds parameters: N1−H∙∙∙O1 [N1−H 0.86 Å, H∙∙∙O1 1.79 Å, N1∙∙∙O1 2.526(7) Å, N1−H∙∙∙O1 141.7°]. N4−H∙∙∙O13 [N1−H 0.85 Å, H∙∙∙O13 1.80 Å, N4∙∙∙O13 2.522(7) Å, N4−H∙∙∙O13 140.0°]. O1w−H∙∙∙O19 [N1−H 0.85 Å, H∙∙∙O19 1.93 Å, O1w ∙∙∙O19 2.74(1) Å, O1w−H∙∙∙O19 161.3°].

Figure 7

Structure of the two ligands (L₂S) in the complex cation [Cu₂(L₂S)₂(ClO₄)(H₂O)]⁺.

Figure 8

The coordination mode for the copper atoms of in the binuclear complex [Cu₂(L₂S)₂(ClO₄) (H₂O)]⁺.

Figure 9

View of 2D supramolecular layer in the crystal structure 6.

Figure 10

EPR spectra for the complexes 2, 3 and 5 in powder, at room temperature.

Figure 11

EPR spectrum of the complex 1, in DMSO solution, registered at 77 K and (d2) second derivative spectra.

Figure 12

EPR spectrum (the derived signal) for complex 5 in DMSO solution at 77.

Figure 13

EPR spectrum of the complex 4, in solution, at room temperature and 77 K.