Hydrogen-bond assisted stabilization of the less favored conformation of a tridentate Schiff base ligand in dinuclear nickel(II) complex: an experimental and theoretical study

Abstract

The Schiff base ligand, HL (2-[1-(3-methylamino-propylimino)-ethyl]-phenol), the 1:1 condensation product of 2-hydroxy acetophenone and N-methyl-1,3-diaminopropane, has been synthesized and characterized by X-ray crystallography as the perchlorate salt [H₂L]ClO₄ (1). The structure consists of discrete [H₂L]⁺ cations and perchlorate anions. Two dinuclear Ni^II complexes, [Ni₂L₂(NO₂)₂] (2), [Ni₂L₂(NO₃)₂] (3) have been synthesized using this ligand and characterized by single crystal X-ray analyses. Complexes 2 and 3 are centrosymmetric dimers in which the Ni^II ions are in distorted fac- and mer-octahedral environments, respectively, bridged by two μ ₂-phenolate ions of deprotonated ligand, L. The plane of the phenyl rings and the Ni₂O₂ basal plane are nearly coplanar in 2 but almost perpendicular in 3. We have studied and explained this different behavior using high level DFT calculations (RI-BP86/def2-TZVP level of theory). The conformation observed in 3, which is energetically less favorable, is stabilized via intermolecular non-covalent interactions. Under the excitation of ultraviolet light, characteristic fluorescence of compound 1 was observed; by comparison fluorescence intensity decreases in case of compound 3 and completely quenched in compound 2.