Dinuclear vs. mononuclear copper(ii) complexes with nitrophenylimino-benzylal- vs. -naphthylal-based Schiff base ligands

Abstract

The Schiff base ligands 2-((Z)-(2-methyl-4-nitrophenylimino)methyl)phenol (HL¹), 1-((Z)-(2-methyl-4-nitrophenylimino)methyl)naphthalen-2-ol (HL²) and 1-((Z)-(4-methyl-2-nitrophenylimino)methyl)naphthalen-2-ol (HL³) react with copper(II) acetate to provide the copper(II) complexes [Cu(L¹)₂]₂ (1), [Cu(L²)₂] (2) and [Cu(L³)₂] (3), respectively. Structural analysis reveals that the HL¹ exists as a usual (phenol)O–H⋯N(imine) (i.e., enolimine) form, while HL² and HL³ exist as a zwitterionic (phenolate)O⁻⋯H⁺–N(imine) (i.e., ketoimine) form. ¹³C-NMR studies suggest that HL¹ (enolimine) and HL² (ketoamine) preserve their structural integrity both in the solution and solid-state, while HL³ undergoes an interconversion from the ketoimine (solid-state) to enolimine (solution) form. Two Schiff base ligands chelate the copper ion with trans-N,N′ and -O,O′ configurations in structures of both 1 and 3. Remarkably, the asymmetric unit in 1 consists of two pairs of dimeric or dinuclear formula units [Cu(L¹)₂]₂ comprising altogether four symmetry-independent Cu(L¹)₂ groups. Each copper(II) in 1 adopts a geometry that is distorted from square-planar towards tetrahedral when considering only the four short Cu–N/O bonds of 1.9–2.0 Å in 1. There is, however, an additional long fifth Cu–O bond of ca. 2.5 Å as part of the dimer formation, which changes the geometry around Cu to a distorted square-pyramidal coordination. In contrast, in 3, the asymmetric unit contains one ligand and the Cu atom on a special position of an inversion center so that the second ligand is generated by symmetry, which gives an ideal square-planar coordination around Cu with an N₂O₂ chromophore. Thermal analyses by DSC revealed a reversible phase transformation from the crystalline solid to an isotropic liquid for the Schiff base ligands. Thermogravimetric analysis (TGA) revealed multi-step thermal decompositions with mass losses of ca. 18% (1), 36% (2), and 16% (3) within 200–340 °C, followed by continuous mass losses of ca. 59% (1), 44% (2), and 50% (3) up to 1000 °C. Cyclic voltammetry results indicate a redox reaction with a single reductive peak at Ec1 = −0.712 (1), −0.914 (2) and −1.176 V (3) and two well separated oxidative peaks at Ea1/Ea2 = +0.704/−0.399 (1), +0.677/−0.301 (2) and Ea1/Ea2 = +0.405/−0.469 V (3), suggesting two sequential one-electron charge transfer processes in DMF. The compounds showed significant antibacterial activity against E. coli, S. typhi, B. cereus and S. aureus bacteria in comparison to amoxicillin, ampicillin and chloramphenicol (10), with the highest activity observed for ampicillin. Computational modeling supports the experimental results of molecular and electronic structures.