Three series of heterometallic Ni^II-Ln^III Schiff base complexes: Synthesis, crystal structures and magnetic characterization

Three series of Ni^II-Ln^III complexes were synthesized with the general formulae [(μ₃-CO₃)₂{Ni(HL)(CH₃-CH₂OH)Ln(CH₃COO)}₂]·2CH₃CH₂OH (1-6) (Ln = Tb (1), Dy (2), Ho (3), Er (4), Tm (5), Yb (6); H₃L = N,N′-bis(3-methoxysalicylidene)-1,3-diamino-2-prop-anol), [Ni(HL)Ln(dbm)₃]·CH₃OH₂·2CH₂Cl₂ (7-10) (Ln = Tb (7), Eu (8), Gd (9), Ho (10); Hdbm = 1,3-diphenyl-1,3-propanedione) and [Ni(HL)(H₂O)(tfa)Ln(hfac)₂] (11-15) (Ln = Tb (11), Dy (12), Eu (13), Gd (14), Ho (15); Hhfac = 1,1,1,5,5,5-hexafluoropentane-2,4-dione, tfa^- = trifluoroacetate) using compartmental Schiff base ligands in conjunction with auxiliary ligands. For the NiII2LnIII2 series, the tetranuclear structure could be considered as two Ni^II-Ln^III dinuclear subunits bridged by two carbonates derived from atmospheric carbon dioxide. The Ln^III ions of complexes 1-6 were octa-coordinated with distorted triangular dodecahedral geometry, while the Ln^III ions of the dinuclear complexes 7-15 were nona-coordinated with distorted muffin geometry. The magnetic properties of the three series complexes were studied using dc and ac magnetic measurements. For the Ni^II-Gd^III complexes, the dc magnetic susceptibility measurements suggested the existence of the anticipated ferromagnetic interaction between Ni^II and Gd^III ions. The fitting of the χ_MT vs. T data processed by PHI software provided the parameters g = 2.08 (J = +0.87 cm^-1) for 9 and g = 2.02 (J = +1.83 cm^-1) for 14. The interaction exchange was magneto-structurally correlated to the Ni-O-Gd angle (α) and Ni(μ-O)Gd dihedral angle (β). With an applied dc field, complexes 1 (Tb), 2 (Dy), 7 (Tb) and 12 (Dy) exhibited single magnetic relaxation with SMM parameters of U_eff/k = 13.60 K, 11.52 K, 7.69 K and 5.14 K, respectively. Analysis of the Cole-Cole plots for complexes 2 and 7 suggested that a single relaxation process was mainly involved in the relaxation process, with α values in the range of 0.37-0.17 and 0.14-0.11, respectively.