One ferromagnetic and two antiferromagnetic dinuclear nickel(ii) complexes derived from a tridentate N,N,O-donor Schiff base ligand: a density functional study of magnetic coupling

Abstract

Three new dinuclear Ni_II complexes, [Ni₂L₂(NO₃)₂] (1), [Ni₂L₂(NO₂)₂] (2), and [Ni₂L₂(CH₃COO) ₂(H₂O)] (3), have been synthesized by using a tridentate Schiff base ligand, 2-({[3-(dimethylamino)propyl]imino}methyl)phenol (HL), along with a nitrate, nitrite or acetate ion, respectively, as co-ligand. These three complexes were characterized by spectral analysis, X-ray crystallography and variable-temperature magnetic susceptibility measurements. The structural analyses revealed that the Ni^II ions are coordinated by the deprotonated chelating tridentate Schiff base and possess a distorted octahedral geometry in all three complexes. Complexes 1 and 2 are two di-μ ₂-phenoxido-bridged species in which the nitrate and nitrite act as chelating co-ligands. However, in complex 3, in which the acetate anion is monodentate, an additional water bridge is present along with two μ ₂-phenoxido bridges making the complex a face-sharing bi-octahedron. Magnetic susceptibility measurements indicate an antiferromagnetic intradimer interaction in complexes 1 and 2 with J values of –20.34(5) and –25.25(4) cm^–1, respectively, whereas complex 3 shows a dominant ferromagnetic exchange coupling with J = 19.11(9) cm^–1. DFT calculations were performed and the theoretically obtained J values of –19.99 (for 1), –24.19 (for 2) and 18.81 cm^–1 (for 3) corroborate very well the experimental results. An attempt has also been made to correlate the effect of Ni•••Ni distances and bridging Ni–O–Ni angles on the coupling constants of the Ni^II complexes through DFT calculations. The relative energy calculations show that the diphenoxido-bridged complexes are stable at larger bridging angles and consequently the coupling is antiferromagnetic, whereas with an additional water bridge, the formation of complexes with the Ni–O–Ni bridging angle in the ferromagnetic region is energetically profitable.