Enhancing the Magnetic Behaviors of Dy2 Complexes by Modulating the Crystal Field Environment with Different μ-O Bridging Ligands

Four similar dinuclear lanthanide complexes have been synthesized by linking two [Ln(hfac)2-3] units (hfac stands for hexafluoroacetylacetone) with different μ-O bridging ligands. The 2,2'-bipyridine-N-oxide ligand (bmpo) constructed two centrosymmetric complexes [Ln2(hfac)6(bmpo)2] (Ln = Dy(1), Tb(2)), with nine-coordinated LnIII ions showing Cs low symmetry, while the ligand di(2-pyridyl)methanediol (py2C(OH)2) formed another two compounds [Ln2(hfac)4(py2C(OH)O)2] (Ln = Dy(3), Tb(4)), with two kinds of eight-coordinated LnIII ions exhibiting improved symmetries of D4d and D2d. Magnetic analysis reveals that Dy2 complex 1 shows intramolecular antiferromagnetic coupling (J = -1.07 cm-1) and no relaxation process above 2.0 K even in a 1000 Oe dc field, owing to the low symmetry of DyIII ions, while the similar Dy2 complex 3 with improved DyIII symmetry shows ferromagnetic coupling (J = 1.17 cm-1), which induces a 1000 Oe dc field-induced two-step magnetization relaxation processes with effective energy barrier Ueff = 47.4 K and 25.2 K for the slow relaxation and fast relaxation processes, respectively. This study proves again that the improved symmetry combined with intramolecular ferromagnetic interactions, both mediated by bridging ligands, can enhance the DyIII anisotropy, further quench the quantum tunneling of the magnetization, and finally, enhance the magnetic behavior of LnIII-based systems.

Heterometallic clusters based on an uncommon asymmetric "V-shaped" [Fe3+(μ-OR)Ln3+(μ-OR)2Fe3+]6+ (Ln = Gd, Tb, Dy, Ho) structural core and the investigation of the slow relaxation of the magnetization behaviour of the [Fe2Dy] analogue

The synthesis, crystal structures, Mössbauer spectra and variable temperature dc and ac magnetic susceptibility studies of a new family of trinuclear heterometallic Fe³⁺/Ln³⁺ complexes, [Fe₂Ln(PhCO₂)₃((py)₂CO₂)((py)₂C(OMe)O)₂(NO₃)Cl] (Ln = Gd (1/Gd), Tb (1/Tb), Dy (1/Dy), and Ho (1/Ho)), where (py)₂CO₂^2- and (py)₂C(OMe)O^- are the anions of the gem-diol and hemiketal derivatives of di-2-pyridyl ketone, are reported. Compounds 1/Ln are based on an asymmetric "V-shaped" [Fe³⁺(μ-OR)Ln(μ-OR)₂Fe³⁺]⁶⁺ structural core formed from the connection of the two terminal Fe³⁺ centers to the central Ln³⁺ ion either through one or two alkoxide groups originating from the alkoxide-type bridging ligands. Direct current magnetic susceptibility studies reveal the presence of weak antiferromagnetic interactions between the Fe³⁺ ions. Alternating current magnetic studies indicate the presence of a slow-magnetic relaxation process in 1/Dy with an energy barrier U_eff = 6.7 (±0.3) K and a pre-exponential factor, τ₀ = 2.2 (±0.4) × 10^-7 s. The electronic, magnetic and relaxation properties of the complexes were further monitored by variable temperature ⁵⁷Fe Mössbauer spectroscopy. At T > 80 K the spectra from the complexes comprise two quadrupole doublets the hyperfine parameters of which reflect the distinct coordination environment of the two Fe³⁺ terminal sites. At T < 20 K, the Mössbauer spectra for 1/Dy are affected by magnetic relaxation effects. At 1.5 K, the spectrum of 1/Dy comprises well defined magnetic sextets indicating relaxation times slower than the characteristic time of the Mössbauer technique (10^-7 s) in agreement with the dynamic magnetic measurements. 1/Gd exhibits broad unresolved magnetic sextets at 1.5 K indicating that the spin relaxation time is of the order of the Mössbauer characteristic time at this temperature. For 1/Tb, 1/Ho the Mössbauer spectra exhibit slight broadening even at the lowest available temperature consistent with magnetic relaxation times less than 10^-7 s.

Slow magnetic relaxation in a Dy3 triangle and a bistriangular Dy6 cluster

Two lanthanide single-molecule magnets (SMMs) [Dy₃(μ₃-OH)(HL-1)₃(H₂O)₃](NO₃)₂·3H₃O (1, H₃L-1 = (E)-3-(((8-hydroxyquinolin-2-yl)methylene)amino)propane-1,2-diol) and [Dy₆(μ₃-OH)₄(H₂L-2)₄(HL-2)₂(L-2)₂] (2, H₃L-2 = (E)-2-hydroxy-N'-(2-hydroxy-3-methoxybenzylidene)benzohydrazide) were synthesized and characterized structurally and magnetically. Complex 1 contains a triangular Dy₃ core in which the three Dy³⁺ ions share a μ₃-OH^- anion and the deprotonated ligands of (HL-1)^2- serve both capping and bridging functions, while 2 displays a centrosymmetric hexanuclear Dy^III structure with two similar Dy₃ triangular cores ligated by two fully deprotonated (L-2)^3- ligands, each of which shares two μ₃-OH^- anions. All the Dy^III ions are eight-coordinated with quasi D_2d or C_2v symmetry. Magnetic studies reveal that 1 exhibited two-step magnetic relaxation under an applied dc field of 800 Oe, with effective energy barriers of 40.1 and 31.0 K for the slow relaxation (SR) and fast relaxation regimes (FR), respectively. Meanwhile, 2 only showed a tail of slow magnetic relaxation at above 2 K. Ab initio calculations have been carried out to show the nature of their different magnetic properties.

Oxazolidine Nitroxide Transformation in a Coordination Sphere of the Ln3+ Ions

Upon the interaction of the hydrated lanthanide(III) salts found in acetonitrile solution with a tripodal paramagnetic compound, 4,4-dimethyl-2,2-bis(pyridin-2-yl)-1,3-oxazolidine-3-oxyl (Rad), functionalized by two pyridyl groups, three neutral, structurally characterized complexes with diamagnetic polydentate ligands—[Dy(RadH)(hbpm)Cl₂], [Yb₂(ipapm)₂(NO₃)₄], and [Ce₂(ipapm)₂(NO₃)₄(EtOAc)₂]—were obtained. These coordination compounds are minor uncolored crystalline products, which were formed in a reaction mixture due to the Rad transformation in a lanthanide coordination sphere, wherein the processes of its simultaneous disproportionation, hydrolysis, and condensation proceed differently than in the absence of Ln ions. The latter fact was confirmed by the formation of the structurally characterized product of the oxazolidine nitroxide transformation during its crystallization in toluene solution. Such a conversion in the presence of 4f elements ions is unique since no similar phenomenon was observed during the synthesis of the 3d-metal complexes with Rad.

Significantly Enhancing the Single-Molecule-Magnet Performance of a Dinuclear Dy(III) Complex by Utilizing an Asymmetric Auxiliary Organic Ligand

In this work, we employed an asymmetric auxiliary organic ligand (1,1,1-trifluoroacetylacetone, Htfac) to further regulate the magnetic relaxation behavior of series of Dy₂ single-molecule magnets (SMMs) with a N1,N3-bis(3-methoxysalicylidene)diethylenetriamine (H₂L) ligand. Fortunately, an air-stable Dy₂ complex, [Dy₂(L)₂(tfac)₂] (1; Htfac = 1,1,1-trifluoroacetylacetone) was obtained at room temperature. A structural analysis indicated that some Dy-O or Dy-N bond lengths for 1 are not in the range of those for the complexes [Dy^III₂(L)₂(acac)₂]·2CH₂Cl₂ (Dy₂-acac; Hacac = acetylacetone) and [Dy^III₂(L)₂(hfac)₂] (Dy₂-hfac; Hhfac = hexafluoroacetylacetone), although the electron-withdrawing ability of tfac^- is stronger than that of acac^- but weaker than that of hfac^-. Additionally, the Dy-O3/O3a (the two O atoms bridged to Dy^III ions) bond lengths are also affected by the asymmetrical Htfc ligand. This indicated that the charge distribution of the coordination atoms around Dy^III has been modified in 1, which leads to the fine-tuning of the magnetic relaxation behavior of 1. Magnetic studies indicated that the values of effective energy barrier (U_eff) for 1 and its diluted sample (2) are 234.8(3) and 188.0(6) K, respectively, which are both higher than the reported value of 110 K for the complex Dy₂-hfac. More interestingly, 1 exhibits a magnetic hysteresis opening when T < 2.5 K at zero field, while the hysteresis loops of 2 are closed at a zero dc field. This discrepancy is due to the weak intramolecular exchange coupling in 2, which cannot overcome the QTM of the single Dy^III ion. Ab initio calculations for 1 revealed that the charge distributions of the coordination atoms around Dy^III ions were regulated and the intramolecular exchange coupling was indeed improved when the asymmetrical Htfc was employed as a ligand for the synthesis of this kind of Dy₂ SMM.

Influence of the Different Types of Auxiliary Noncarboxylate Organic Ligands on the Topologies and Magnetic Relaxation Behavior of Zn-Dy Heterometallic Single Molecule Magnets

In this work, we first synthesized a Zn-Dy complex, [Zn₆Dy₂(L)₆(tea)₂(CH₃OH)₂]·6CH₃OH·8H₂O (H₂L = N-3-methoxysalicylidene-2-amino-3-hydroxypyridine, teaH₃ = triethanolamine, 1), by employing H₂L, anhydrous ZnCl₂, and Dy(NO₃)₃·5H₂O reacting with auxiliary ligand teaH₃ in the mixture of CH₃OH and DMF. When teaH₃ and the solvent CH₃OH in the reaction system of 1 were replaced by the auxiliary ligand 2,6-pyridinedimethanol (pdmH₂) and the solvent MeCN, another Zn-Dy complex, [Zn₄Dy₄(L)₆(pdm)₂(pdmH)₄]·10CH₃CN·5H₂O (2), was obtained. For 1, its crystal structure can be viewed as a dimer of two Zn₃Dy^III units. However, for 2, four Dy^III form a zigzag arrangement, and each of its terminals linked two Zn^II ions. Interestingly, although the structural topologies of 1 and 2 are different, the coordination geometries of Dy^III in 1 and 2 are all triangular dodecahedron (TDD-8). The difference is that the continuous shape measure (CShM) values of Dy^III in 1 are larger than the corresponding values in 2. Magnetic investigation revealed that the diluted sample 1@Y exhibits two magnetic relaxation processes, while 2 only exhibits a single relaxation process. Ab initio calculations indicated that, in the crystal lattice of 1, two complexes exhibiting slightly different CShM values of Dy^III result in the double relaxation behavior of 1@Y. However, for 2, one of two Dy^III fragments possesses a fast quantum tunneling of magnetization (QTM), resulting in its magnetic process presented at T < 1.8 K, so 2 exhibits single relaxation behavior. More importantly, the theoretical calculations also clearly indicated that the weak ligation at equatorial sites of Dy^III in 1 and 2 ensure 1@Y and 2 possess SMM behavior, although the coordination geometry of Dy^III (TDD-8) in 1 and 2 severely deviates from the ideal polyhedron and its axial symmetry is low.

Regulating the distortion degree of the square antiprism coordination geometry in Dy–Na single ion magnets

A DyNa complex (1) and a DyNa₂ 1D chain (2) have been obtained; their square antiprism coordination geometries were changed from the bicapped triangular prism of the reported DyNa₂ complex. Compounds 1 and 2 exhibit single ion magnet behavior.

Modulating the structural topologies and magnetic relaxation behaviour of the Mn–Dy compounds by using different auxiliary organic ligands

A MnIII4Dy^III complex and a one-dimensional chain containing MnIII2Dy^III units have been obtained by using different combinations of organic ligands, and a slow magnetic relaxation behavior was observed for both complexes.

Synthesis, crystal structures and magnetic properties of a series of chair-like heterometallic [Fe4Ln2] (Ln = GdIII, DyIII, HoIII, and ErIII) complexes with mixed organic ligands

Four chair-like hexanuclear Fe-Ln complexes containing mixed organic ligands, namely, [Fe₄Ln₂{(py)₂CO₂}₄(pdm)₂(NO₃)₂(H₂O)₂Cl₄]·xCH₃CN·yH₂O (Ln = Gd^III (1, x = 1, y = 0), Dy^III (2, x = 1, y = 1), Ho^III (3, x = 0, y = 2), and Er^III (4, x = 1, y = 3); (py)₂CO₂H₂ = the gem-diol form of di-2-pyridyl ketone and pdmH₂ = 2,6-pyridinedimethanol) have been obtained by employing di-2-pyridyl ketone and 2,6-pyridinedimethanol reacting with FeCl₃ and Ln(NO₃)₃ in MeCN. The structures of 1-4 are similar to each other except for the number of lattice solvent molecules. Four Fe^III and two Ln^III in these complexes comprise a chair-like core with the "body" constructed by four Fe^III ions and the "end" constructed by two Ln^III ions. Among the four compounds, 2 shows field-induced single molecule magnet behavior as revealed by ac magnetic susceptibility studies, with the effective energy barrier and the pre-exponential factor of 22.07 K and 8.44 × 10^-7 s, respectively. Ab initio calculations indicated that, among 2_Dy, 3_Ho and 4_Er fragments, the energy gap between the lowest two spin-orbit states for 2_Dy is the largest, while the tunneling gap for 2 is the smallest. These might be the reasons for complex 2 exhibiting SMM behavior. Additionally, the orientations of the magnetic anisotropy of Dy^III in 2 were obtained by electrostatic calculations and ab initio calculations, both indicating that the directions of the main magnetic axis of Dy1 ions are almost aligned along Dy1-O5 (O5 from the pdm^2- ligand).