Nickel(II) and copper(II) complexes of unsymmetrical tetradentate reduced Schiff base ligands

Two new reduced Schiff base ligands, [HL1 = 4-{2-[(pyridin-2-ylmethyl)-amino]-ethylimino}-pentan-2-one and HL2 = 4-[2-(1-pyridin-2-yl-ethylamino)-ethylimino]-pentan-2-one] have been prepared by reduction of the corresponding tetradentate unsymmetrical Schiff bases derived from 1:1: 1 condensation of 1,2-ethanediamine, acetylacetone and pyridine-2-carboxaldehyde/2-acetyl pyridine. Four complexes, [Ni(L1)]ClO4 (1), [Cu(L1)]ClO4 (2), [Ni(L2)]ClO4 (3), and [Cu(L2)]ClO4 (4) with these two reduced Schiff base ligands have been synthesized and structurally characterized by X-ray crystallography. The mono-negative ligands L1 and L2 are chelated in all four complexes through the four donor atoms to form square planar nickel(II) and copper(II) complexes. Structures of 3 and 4 reveal that enantiomeric pairs are crystallized together with opposite chirality in the nitrogen and carbon atoms. The two CuII complexes (2 and 4) exhibit both irreversible reductive (CuII/CuI; Epc, −1.00 and −1.04 V) and oxidative (CuII/CuIII; Epa, +1.22 and +1.17 V, respectively) responses in cyclic voltammetry. The electrochemically generated CuI species for both the complexes are unstable and undergo disproportionation.

Graphical abstractThe monocondensation product of 1,2-diaminoethane and acetylacetone on reaction with pyridine-2-carboxaldehyde or 2-acetylpyridine, followed by the reduction with NaBH4 yielded two tetradentate reduced Schiff base ligands HL1 and HL2. The NiII and CuII complexes, [Ni(L1)]ClO4, [Cu(L1)]ClO4, [Ni(L2)]ClO4, and [Cu(L2)]ClO4 of these two ligands have been isolated and structurally characterized. Their spectral and electrochemical properties have also been reported.Figure optionsDownload full-size imageDownload as PowerPoint slide