The unusual intermediate species in the formation of Ni(II) complexes of unsymmetrical Schiff bases by Elder’s method: structural, electrochemical and magnetic characterizations

Abstract

During the synthesis of unsymmetrical salen type Schiff base ligand using Ni(II) template by Elder’s method, we isolated three isostructural dinuclear Ni(II)-complexes as possible intermediates of the reaction upon decreasing the reaction time. These three complexes [Ni₂(L₁)₂(o-Hap) ₂] (2), [Ni₂(L₁)₂(o-Hnap) ₂] (3) and [Ni₂(L₁) ₂(Sal)₂] (4) [where HL₁ = 2-((E)-(3-aminopentylimino)methyl)phenol is a monocondensed tridentate NNO-donor Schiff base ligand, o-Hap = o-hydroxyacetophenone, o-Hnap = o-hydroxynapthaldehyde and Sal = salicylaldehyde] have been synthesized by putting the precursor complex, [Ni(L₁)₂] (1) and the respective carbonyl compound under reflux in methanol for two hours. The single crystal structural analyses reveal that intriguingly in all three complexes, the primary amine group of the mono-condensed Schiff base (L₁¹⁻) and the carbonyl group remain uncondensed although they are present as coordinated ligands at the same Ni(II) center. In these complexes, Ni(II) centers possess a distorted octahedral geometry with centrosymmetric phenoxido bridged dinuclear Ni₂O₂ core. Electrochemical and mass-spectrometric studies revealed that these dinuclear complexes are stable in solution. However, removal of Ni(II) ions by dimethylglyoxime leads to exclusive formation of the desired unsymmetrical N₂O₂-donor Schiff base. The variable-temperature magnetic susceptibility measurements on solid samples reveal that all the compounds are antiferromagnetically coupled with exchange coupling constants (J) of −16.87, −10.14 and −13.78 cm⁻¹ for 2, 3 and 4, respectively. These values are in accordance with the magnetostructural correlation of previously reported phenoxido bridged polynuclear Ni(II) complexes. The nature and magnitude of exchange interactions are further corroborated by density functional calculations.