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

Abstract

A family of four Ln(III) complexes has been synthesized with the general formula [Ln₂(NO₃)₄(L)₂(S)] (Ln = Gd, Tb, Er, and S = H₂O; 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/H₂O 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^III₂(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 (H₂O 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.

Keywords: asymmetric dinuclear lanthanide(III) complexes; dysprosium(III) and erbium(III) single-molecule magnets; magnetic properties; magnetization relaxation pathways; metal complexes of N’-(1-(1-pyridin-2-yl)ethylidene)pyridine-2-carbohydrazide; single-crystal X-ray structures.