Magneto‐Structural Correlations, Substitution Effects and Slow Relaxation of the Magnetization on Trinuclear Linear Ni(II) Complexes: An Experimental and Theoretical Study

We report the preparation of three neutral linear trinuclear Ni(II) complexes [Ni₃(LX)₂] (1-3) by self-assembly of Ni(II) ions and N₃O₃-tripodal Schiff base ligands, LX, which were obtained by condensation between the triamine tris(methylhydrazine)phosphorylsulfide and salicylaldehyde derivatives with substituents X=H (L1) (1), Br (L2) (2) and NO₂ (L3) (3) in para position to the phenoxo group. Experimental magneto-structural studies carried out on these complexes indicate that the magnetic exchange interactions between the neighboring Ni(II) ions are ferromagnetic in nature, moderate in magnitude and clearly dependent of the electronic properties of the substituent in para position. Thus, the electron-withdrawing NO₂ group decreases the ferromagnetic coupling, whereas the Br substituent, with small electronic effects, does not significantly vary the exchange coupling observed for the unsubstituted complex. Theoretical calculations performed on these complexes, containing ligands with a variety of electron-donating and withdrawing groups in para position to the phenoxo group, support that the ferromagnetic coupling decreases on passing from strong electron-donating to electron-withdrawing groups. Interestingly, complexes 1-3 show weak slow relaxation of the magnetization, with relaxation times (τ) decreasing in the order: 1>2>3. These compounds represent some of the few examples of Ni(II) complexes exhibiting slow magnetic relaxation.