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

Abstract

<jats:p>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 LnIII 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 {LnIII2(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 GdIII∙∙∙GdIII antiferromagnetic exchange interactions in 1 [J/kB = −0.020(6) K based on the spin Hamiltonian Ĥ = −2J(ŜGd1∙ ŜGd2)] and probably weak antiferromagnetic LnIII∙∙∙LnIII 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 DyIII 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.</jats:p>

Novel Lanthanide(III) Porphyrin-Based Metal–Organic Frameworks: Structure, Gas Adsorption, and Magnetic Properties

“…The magnetization value of 0.82 N A μ B at 70 kOe corresponds to an average effective g -factor of the ground doublet of about 1.65. No significant reduction of χ T at the lowest temperatures was observed that rules out a significant influence of the exchange interaction, which is typically very low between lanthanide ions as shown for dimeric lanthanide complexes with a much shorter exchange path through a double oxygen bridge; ,− some exceptions are examples of Dy­(III)-based molecular magnets summarized in ref .…”

Dinuclear Lanthanide(III) Complexes from the Use of Methyl 2-Pyridyl Ketoxime: Synthetic, Structural, and Physical Studies

“…For a given bond, the Ln-O/N bond lengths follow the Nd>Dy order ( Table 1 ) due to lanthanide contraction; this trend is also observed for the Ln III… Ln III distances. The Ln-O/N bond lengths in 1 and 5 are typical for 9-coordinate Nd(III) [ 33 , 56 , 57 ] and Dy(III) [ 35 , 38 , 41 , 54 , 58 , 59 , 60 , 61 , 62 ]. The Ln1/Ln1′ coordination geometry was evaluated using the program SHAPE [ 61 , 63 ] and is best described as muffin-type ( Figure S1, Table S2 ); an alternative description of the polyhedron could be that of a spherical-capped square antiprism.…”

“…Ln III distances. The Ln-O/N bond lengths in 1 and 5 are typical for 9-coordinate Nd(III) [33,56,57] and Dy(III) [35,38,41,54,[58][59][60][61][62]. The Ln1/Ln1 coordination geometry was evaluated using the program SHAPE [61,63] and is best described as muffin-type (Figure S1, Table S2); an alternative description of the polyhedron could be that of a spherical-capped square antiprism.…”

“…Our efforts have led to dinuclear Ln(III) complexes with interesting structural features and properties; a point of interest is that the bridging acetato ligands in most of the complexes have resulted from the Ln III -assisted hydrolysis of the MeCN (solvent). This paper can also be considered as a continuation of the interest of our groups in the chemistry and properties (photophysical, magnetic, catalytic) of the Ln III 2 complexes [ 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 ]; such complexes can potentially combine both optical and magnetic properties within the same molecule leading to luminescent SMMs [ 45 , 46 ]. Ln III 2 compounds continue to attract the intense attention of many inorganic chemistry groups around the world because they are ideal model systems for providing answers to fundamental questions regarding the origin of a certain physical, spectroscopic, or catalytic property, i.e., if such properties arise from the whole molecule (two Ln III centers) or half of it (a single Ln III ion) [ 47 ].…”

Zero‐field Slow Magnetic Relaxation Behavior of Dy2 in a Series of Dinuclear {Ln2} (Ln=Dy, Tb, Gd and Er) Complexes: A Combined Experimental and Theoretical Study

“…Thus, designing and isolating dinuclear SMMs in a controlled manner, one can build larger motifs using a bottom‐up approach and construct SMMs with higher effective energy and blocking temperature. So, the synthesis of dinuclear complexes with small modifications in their coordination environments can be a promising approach to gain desired knowledge on complicated magnetic interactions and the effect of anisotropy on its magnetic dynamics [35–45] . For example, the modification of the terminal group on the ligand has been known to enhance the energy barrier dramatically by stabilizing the ground Kramers doublet (KDs) [46–48] .…”