Nickel(II) complexes of terdentate or symmetrical tetradentate Schiff bases: Evidence of the influence of the counter anions in the hydrolysis of the imine bond in Schiff base complexes

Two sets of ligands, set-1 and set-2, have been prepared by mixing 1,3-diaminopentane and carbonyl compounds (2-acetylpyridine or pyridine-2-carboxaldehyde) in 1:1 and 1:2 ratios, respectively, and employed for the synthesis of complexes with Ni(II) perchlorate, Ni(II) thiocyanate and Ni(II) chloride. Ni(II) perchlorate yields the complexes having general formula [NiL2](ClO4)2(L = L1 [N3-(1-pyridin-2-yl-ethylidene)-pentane-1,3-diamine] for complex 1 or L2[N3-pyridin-2-ylmethylene-pentane-1,3-diamine] for complex 2) in which the Schiff bases are monocondensed terdentate, whereas Ni(II) thiocyanate results in the formation of tetradentate Schiff base complexes, [NiL(SCN)2] (L = L3[N,N′-bis-(1-pyridin-2-yl-ethylidine)-pentane-1,3-diamine] for complex 3 or L4 [N,N′-bis(pyridin-2-ylmethyline)-pentane-1,3-diamine] for complex 4) irrespective of the sets of ligands used. Complexes 5 {[NiL3(N3)2]} and 6 {[NiL4(N3)2]} are prepared by adding sodium azide to the methanol solution of complexes 1 and 2. Addition of Ni(II) chloride to the set-1 or set-2 ligands produces [Ni(pn)2]Cl2, 7, as the major product, where pn = 1,3-diaminopentane. Formation of the complexes has been explained by the activation of the imine bond by the counter anion and thereby favouring the hydrolysis of the Schiff base. All the complexes have been characterized by elemental analyses and spectral data. Single crystal X-ray diffraction studies confirm the structures of three representative members, 1, 4 and 7; all of them have distorted octahedral geometry around Ni(II). The bis-complex of terdentate ligands, 1, is the mer isomer, and complexes 4 and 7 possess trans geometry.

Graphical abstractReaction of Schiff base ligands derived from pentane-1,3-diamine and carbonyl compounds (2-acetylpyridine or pyridine-2-carboxaldehyde) with Ni(II) perchlorate yields the bis-complexes of monocondensed terdentate ligands, [NiL2](ClO4)2, whereas with Ni(II) thiocyanate it results in the formation of tetradentate Schiff base complexes, [NiL′](SCN)2. The Schiff base complexes of Ni(II) chloride undergo hydrolysis during re-crystallization to form bis-diamine complex of Ni(II). [NiL2](ClO4)2 transforms to [NiL′](N3)2 on addition of NaN3.Figure optionsDownload full-size imageDownload as PowerPoint slide