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 solution and solid-state, while HL3 undergoes an interconversion from ketoimine (solid-state) to enolimine (solution) form. Two Schiff base ligands chelate the copper ion with trans-N,N' and -O,O' configurations in both structures for 1 and 3. Remarkably, the asymmetric unit in 1 consists of two pairs of dimeric or dinuclear formula units [Cu(L1)2]2 of altogether four symmetry-independent Cu(L1)2 groups. Each copper(II) in 1 ion adopts a geometry which 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 then renders 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 the 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 a N2O2 chromophore. Thermal analyses by DSC revealed a reversible phase transformation from crystalline solid to isotropic liquid for the Schiff base ligands. Thermogravimetric analysis (TGA) exposes multi-step thermal decompositions with mass losses of ca. 18% (1), 36% (2), and 16 % (3) at 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), but 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 for Ampicillin. Computational modeling supports the experimental results of molecular and electronic structure.