Asymmetric Dinuclear Lanthanide(III) Complexes from the Use of a Ligand Derived from 2-Acetylpyridine and Picolinoylhydrazide: Synthetic, Structural and Magnetic Studies †

A family of four Ln(III) complexes has been synthesized with the general formula [Ln(2)(NO(3))(4)(L)(2)(S)] (Ln = Gd, Tb, Er, and S = H(2)O; 1, 2 and 4, respectively/Ln = Dy, S = MeOH, complex 3), where HL is the flexible ditopic ligand N’-(1-(pyridin-2-yl)ethylidene)pyridine-2-carbohydrazide. The structures of isostructural MeOH/H(2)O solvates of these complexes were determined by single-crystal X-ray diffraction. The two Ln(III) ions are doubly bridged by the deprotonated oxygen atoms of two “head-to-head” 2.21011 (Harris notation) L¯ ligands, forming a central, nearly rhombic {Ln(III)(2)(μ-OR)(2)}(4+) core. Two bidentate chelating nitrato groups complete a sphenocoronal 10-coordination at one metal ion, while two bidentate chelating nitrato groups and one solvent molecule (H(2)O or MeOH) complete a spherical capped square antiprismatic 9-coordination at the other. The structures are critically compared with those of other, previously reported metal complexes of HL or L¯. The IR spectra of 1–4 are discussed in terms of the coordination modes of the organic and inorganic ligands involved. The f-f transitions in the solid-state (diffuse reflectance) spectra of the Tb(III), Dy(III), and Er(III) complexes have been fully assigned in the UV/Vis and near-IR regions. Magnetic susceptibility studies in the 1.85–300 K range reveal the presence of weak, intramolecular Gd(III)∙∙∙Gd(III) antiferromagnetic exchange interactions in 1 [J/k(B) = −0.020(6) K based on the spin Hamiltonian Ĥ = −2J(Ŝ(Gd1)∙ Ŝ(Gd2))] and probably weak antiferromagnetic Ln(III)∙∙∙Ln(III) exchange interactions in 2–4. Ac susceptibility measurements in zero dc field do not show frequency dependent out-of-phase signals, and this experimental fact is discussed for 3 in terms of the magnetic anisotropy axis for each Dy(III) center and the oblate electron density of this metal ion. Complexes 3 and 4 are Single-Molecule Magnets (SMMs) and this behavior is optimally observed under external dc fields of 600 and 1000 Oe, respectively. The magnetization relaxation pathways are discussed and a satisfactory fit of the temperature and field dependencies of the relaxation time τ was achieved considering a model that employs Raman, direct, and Orbach relaxation mechanisms.