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.

Figure 1

Structural formula of the free ligand N’-(1-(pyridin-2-yl)ethylidene)pyridine-2-carbohydrazide, drawn in its enol-imino tautomer, and its abbreviation.

Figure 2

To date the crystallographically confirmed ligation modes of HL or L¯, and the Harris notation that describes these modes. The neutral ligand exists in the keto-amino form in the complexes. In the anionic ligand, the central OCNNC backbone has been drawn in a manner that emphasizes its delocalized nature which appears in most complexes. The coordination bonds are drawn with bold lines. M = metal ion. The structurally characterized complexes are listed in Table 3.

Figure 3

Solid-state (diffuse reflectance) electronic spectra of complexes 2 (top left), 3 (top right) and 4 (bottom) in the 250–2000 nm range.

Figure 4

Molecular structure of [Dy₂(NO₃)₄(L)₂(MeOH)] as found in 3∙2.5MeOH at 120 K. Thermal ellipsoids are depicted at 50% probability level. Hydrogen atoms are omitted for clarity. Note that only the major complex is depicted here, i.e., with coordinated MeOH instead of H₂O, C1M having an occupancy of ca. 0.7.

Figure 5

Molecular structure of [Er₂(NO₃)₄(L)₂(H₂O)] as found in 4∙3MeOH∙0.5H₂O. Thermal ellipsoids are depicted at 50% probability level. Hydrogen atoms are omitted for clarity.

Figure 6

Sphenocoronal and spherical capped square antiprismatic coordination geometries of Dy1 and Dy2, respectively, in the structure of 3∙2.5MeOH. The plotted polyhedra are the ideal, best-fit polyhedra using the program SHAPE [74].

Figure 7

Temperature dependence of the χT product for the four complexes (1∙2MeOH∙2H₂O, in black; 2∙2MeOH∙1.5H₂O in orange; 3∙2.5MeOH in red and 4∙3MeOH∙0.5H₂O in blue) discussed in this paper at 0.1 T (χ is defined as M/H per mole of the respective complex). The solid black line is the fit of the data to the theoretical Heisenberg model for a dinuclear Gd^III₂ complex; see the text for details.

Figure 8

Frequency dependence of the real, in-phase (χ′, top) and imaginary, out-of-phase (χ″, bottom) components of the ac susceptibility under an external dc field of 600 Oe (0.06 T) at the indicated temperatures for complex 3∙2.5MeOH. Solid lines are visual guides on the left plots, while they show the generalized Debye fit of the ac data on the right.

Figure 9

Frequency dependence of the real, in-phase (χ′, top) and imaginary, out-of-phase (χ″, bottom) components of the ac susceptibility under an external dc field of 1000 Oe (0.1 T) at the indicated temperatures for complex 4∙3MeOH∙0.5H₂O. Solid lines are visual guides on the left plots, while they show the generalized Debye fit of the ac data on the right.

Figure 10

Field (left) and temperature (right) dependencies of the relaxation time (τ) at 2.0 K and in the presence of an applied static field of 0.06 and 1 T for complexes 3∙2.5MeOH (top) and 4∙3MeOH∙0.5H₂O (bottom), respectively. The relaxation time was estimated from the generalized Debye fits of the ac susceptibility data shown in Figure 8 and Figure 9, Figures S10 and S13. The estimated standard deviations of the relaxation time (vertical solid bars) have been calculated from the α parameters of the generalized Debye fit (Figures S11–S15) and the log-normal distribution as described in ref. [81]. The solid red lines are the best fit discussed in the text.