Strong Ferromagnetic Exchange Interactions in Hinge-like Dy(O₂Cu)₂ Complexes Involving Double Oxygen Bridges

Two trinuclear isomeric compounds, [{(Cu^II(salpn))­(Me­(CO)­Me)}₂Dy^III(NO₃)₃] (<b>1</b>) and [{Cu^II(salpn)}₂Dy^III(H₂O)­(NO₃)₃]·MeOH (<b>2</b>), along with one polymeric compound, {[{Cu^II(salpn)}₂Dy^III(NO₃)₃bpy]·MeOH·H₂O}_<i>n</i> (<b>3</b>), were synthesized using a metalloligand, [Cu^II(salpn)], where H₂salpn and bpy stand for <i>N</i>,<i>N</i>′-bis­(salicylidene)-1,3-propanediamine and 4,4′-bipyridine, respectively. Compounds <b>1</b> and <b>2</b> were selectively prepared with two solvents: the less polar acetone led to the exclusive crystallization of <b>1</b> with a <i>transoid</i> trinuclear architecture, while more polar solvent methanol provided sole construction of <b>2</b> with a <i>cisoid</i> trinuclear architecture. Compound <b>3</b> was prepared from <b>1</b> or <b>2</b> after bpy was introduced as a bridge. The Dy and Cu ions are doubly bridged with oxygen atoms, and the core DyO₂Cu skeletons are characterized by different “butterfly angles” of 140.9(1)°, 147.1(19)°, and 142.4(2)° for <b>1</b>, <b>2</b>, and <b>3</b>, respectively. We have examined the molecular structures and magnetic properties of <b>1</b>–<b>3</b> using high-frequency electron paramagnetic resonance (HF-EPR), magnetization, and magnetic susceptibility techniques. These compounds showed slow magnetization reversal in the measurements of alternating current magnetic susceptibility. We analyzed EPR frequency-field diagrams using an effective spin-Hamiltonian including only one doublet of Dy sublevels and found that the exchange couplings are ferromagnetic in all compounds. The exchange coupling parameters <i>J</i>_Dy–Cu of <b>1</b>, <b>2</b>, and <b>3</b> were determined as 2.25 ± 0.05, 1.82 ± 0.04, and 1.79 ± 0.04 K, respectively. These values are larger than those found in previous research using EPR analysis on [Cu^II(L^A)­(C₃H₆O)­Dy^III(NO₃)₃] (H₂L^A = <i>N</i>,<i>N</i>′-bis­(3-methoxysalicylidene)-1,3-diamino-2,2-dimethylpropane) and [Dy^IIIL^B₂(NO₃)₂{Cu^II(CH₃OH)}₂]­(NO₃)­(CH₃OH) (H₂L^B = 2,6-bis­(acetylaceto)­pyridine). The present result shows an advantage of doubly oxygen-bridged motifs to built strong ferromagnetic interactions between lanthanide and transition metal ions. We found that the exchange coupling strength is sensitive to the structural parameters such as bond angles, bond lengths, and butterfly angles. Precise determination of the exchange parameters would contribute to development of exchange-coupled 4f–3d heterometallic complexes

Strong Ferromagnetic Exchange Interactions in Hinge-like Dy(O₂Cu)₂ Complexes Involving Double Oxygen Bridges

Two trinuclear isomeric compounds, [{(Cu^II(salpn))­(Me­(CO)­Me)}₂Dy^III(NO₃)₃] (<b>1</b>) and [{Cu^II(salpn)}₂Dy^III(H₂O)­(NO₃)₃]·MeOH (<b>2</b>), along with one polymeric compound, {[{Cu^II(salpn)}₂Dy^III(NO₃)₃bpy]·MeOH·H₂O}_<i>n</i> (<b>3</b>), were synthesized using a metalloligand, [Cu^II(salpn)], where H₂salpn and bpy stand for <i>N</i>,<i>N</i>′-bis­(salicylidene)-1,3-propanediamine and 4,4′-bipyridine, respectively. Compounds <b>1</b> and <b>2</b> were selectively prepared with two solvents: the less polar acetone led to the exclusive crystallization of <b>1</b> with a <i>transoid</i> trinuclear architecture, while more polar solvent methanol provided sole construction of <b>2</b> with a <i>cisoid</i> trinuclear architecture. Compound <b>3</b> was prepared from <b>1</b> or <b>2</b> after bpy was introduced as a bridge. The Dy and Cu ions are doubly bridged with oxygen atoms, and the core DyO₂Cu skeletons are characterized by different “butterfly angles” of 140.9(1)°, 147.1(19)°, and 142.4(2)° for <b>1</b>, <b>2</b>, and <b>3</b>, respectively. We have examined the molecular structures and magnetic properties of <b>1</b>–<b>3</b> using high-frequency electron paramagnetic resonance (HF-EPR), magnetization, and magnetic susceptibility techniques. These compounds showed slow magnetization reversal in the measurements of alternating current magnetic susceptibility. We analyzed EPR frequency-field diagrams using an effective spin-Hamiltonian including only one doublet of Dy sublevels and found that the exchange couplings are ferromagnetic in all compounds. The exchange coupling parameters <i>J</i>_Dy–Cu of <b>1</b>, <b>2</b>, and <b>3</b> were determined as 2.25 ± 0.05, 1.82 ± 0.04, and 1.79 ± 0.04 K, respectively. These values are larger than those found in previous research using EPR analysis on [Cu^II(L^A)­(C₃H₆O)­Dy^III(NO₃)₃] (H₂L^A = <i>N</i>,<i>N</i>′-bis­(3-methoxysalicylidene)-1,3-diamino-2,2-dimethylpropane) and [Dy^IIIL^B₂(NO₃)₂{Cu^II(CH₃OH)}₂]­(NO₃)­(CH₃OH) (H₂L^B = 2,6-bis­(acetylaceto)­pyridine). The present result shows an advantage of doubly oxygen-bridged motifs to built strong ferromagnetic interactions between lanthanide and transition metal ions. We found that the exchange coupling strength is sensitive to the structural parameters such as bond angles, bond lengths, and butterfly angles. Precise determination of the exchange parameters would contribute to development of exchange-coupled 4f–3d heterometallic complexes

Structure and Magnetism of [<i>n</i>-BuNH₃]₁₂[Cu₄(GeW₉O₃₄)₂]·14H₂O Sandwiching a Rhomblike Cu₄⁸⁺ Tetragon through α-Keggin Linkage

A sandwich-type polyoxometalate, [Cu₄(GeW₉O₃₄)₂]¹²⁻ (<b>1a</b>), in which two B-α-[GeW₉O₃₄]¹²⁻ ligands sandwich a rhomblike Cu₄⁸⁺ tetragon through α-Κeggin linkage, is first isolated as a [<i>n</i>-BuNH₃]⁺ salt, [<i>n</i>-BuNH₃]₁₂[Cu₄(GeW₉O₃₄)₂]·14H₂O (<b>1</b>). A Cu₄O₁₄ cluster for the rhomblike Cu₄⁸⁺ tetragon in <b>1a</b> with <i>C</i>_2<i>h</i> local symmetry consists of two Jahn−Teller (JT) distorted CuO₆ octahedra (at internal sites) with a short diagonal Cu_int···Cu_int distance of 3.10−3.11 Å and two CuO₅ square pyramids (at external site) with a long diagonal Cu_ext···Cu_ext distance of 5.34−5.35 Å, the feature of which is different from [Cu₄(H₂O)₂(GeW₉O₃₄)₂]¹²⁻ (<b>2a</b>), comprising the four JT-distorted CuO₆ octahedral Cu₄⁸⁺ tetragons through β-Keggin linkage: the axial Cu_ext−O bond distance (2.27−2.29 Å) for <b>1a</b> is shorter than the corresponding JT-axial distance (2.36 Å) for <b>2a</b>. Measurements of magnetic susceptibility, magnetization, and electron spin resonance spectroscopy for <b>1</b> are carried out for better understanding of the molecular magnetism of the Cu₄⁸⁺ tetragon in comparison with <b>2a</b>. The analysis of the magnetic behavior, based on the isotropic Heisenberg spin Hamiltonian comprising three exchange parameters (<i>J</i>, <i>J′</i>, and <i>J′′</i>), gives <i>J</i> = −24.1 cm⁻¹ for the Cu_ext··Cu_int sides, <i>J′</i> = −99.1 cm⁻¹ for the Cu_int···Cu_int diagonal, and <i>J′′</i> = +0.04 cm⁻¹ for the Cu_ext···Cu_ext diagonal of the Cu₄⁸⁺ rhombus. The <i>S</i> = 1 ground state of <b>1</b> displays <u><i>g</i></u>_|| = 2.42, <i>g</i>_⊥= 2.07, <i>D</i> = −1.44 × 10⁻² cm⁻¹, and |<i>A</i>_Cu||| = 46.5 × 10⁻⁴ cm⁻¹. An observation of the asymmetric magnetization between a positive and a negative pulsed field (up to 10³ T/s) at 0.5 K on the hysteresis loop indicates the quantum tunneling at zero field. The magnetic exchange interactions of four unpaired d_{<i>x</i>²−<i>y</i>²}-electron spins are discussed in terms of the point-dipole approximation, and the primary contribution to <i>D</i> is implied to come from the magnetic dipole−dipole interaction between two spins at the Cu_ext centers

Structure and Magnetism of [<i>n</i>-BuNH₃]₁₂[Cu₄(GeW₉O₃₄)₂]·14H₂O Sandwiching a Rhomblike Cu₄⁸⁺ Tetragon through α-Keggin Linkage

A sandwich-type polyoxometalate, [Cu₄(GeW₉O₃₄)₂]¹²⁻ (<b>1a</b>), in which two B-α-[GeW₉O₃₄]¹²⁻ ligands sandwich a rhomblike Cu₄⁸⁺ tetragon through α-Κeggin linkage, is first isolated as a [<i>n</i>-BuNH₃]⁺ salt, [<i>n</i>-BuNH₃]₁₂[Cu₄(GeW₉O₃₄)₂]·14H₂O (<b>1</b>). A Cu₄O₁₄ cluster for the rhomblike Cu₄⁸⁺ tetragon in <b>1a</b> with <i>C</i>_2<i>h</i> local symmetry consists of two Jahn−Teller (JT) distorted CuO₆ octahedra (at internal sites) with a short diagonal Cu_int···Cu_int distance of 3.10−3.11 Å and two CuO₅ square pyramids (at external site) with a long diagonal Cu_ext···Cu_ext distance of 5.34−5.35 Å, the feature of which is different from [Cu₄(H₂O)₂(GeW₉O₃₄)₂]¹²⁻ (<b>2a</b>), comprising the four JT-distorted CuO₆ octahedral Cu₄⁸⁺ tetragons through β-Keggin linkage: the axial Cu_ext−O bond distance (2.27−2.29 Å) for <b>1a</b> is shorter than the corresponding JT-axial distance (2.36 Å) for <b>2a</b>. Measurements of magnetic susceptibility, magnetization, and electron spin resonance spectroscopy for <b>1</b> are carried out for better understanding of the molecular magnetism of the Cu₄⁸⁺ tetragon in comparison with <b>2a</b>. The analysis of the magnetic behavior, based on the isotropic Heisenberg spin Hamiltonian comprising three exchange parameters (<i>J</i>, <i>J′</i>, and <i>J′′</i>), gives <i>J</i> = −24.1 cm⁻¹ for the Cu_ext··Cu_int sides, <i>J′</i> = −99.1 cm⁻¹ for the Cu_int···Cu_int diagonal, and <i>J′′</i> = +0.04 cm⁻¹ for the Cu_ext···Cu_ext diagonal of the Cu₄⁸⁺ rhombus. The <i>S</i> = 1 ground state of <b>1</b> displays <u><i>g</i></u>_|| = 2.42, <i>g</i>_⊥= 2.07, <i>D</i> = −1.44 × 10⁻² cm⁻¹, and |<i>A</i>_Cu||| = 46.5 × 10⁻⁴ cm⁻¹. An observation of the asymmetric magnetization between a positive and a negative pulsed field (up to 10³ T/s) at 0.5 K on the hysteresis loop indicates the quantum tunneling at zero field. The magnetic exchange interactions of four unpaired d_{<i>x</i>²−<i>y</i>²}-electron spins are discussed in terms of the point-dipole approximation, and the primary contribution to <i>D</i> is implied to come from the magnetic dipole−dipole interaction between two spins at the Cu_ext centers

Slow Relaxation of the Magnetization of an Mn^III Single Ion

A Mn^III-salen-type complex with a diamagnetic [Co^III(CN)₆]^3– moiety, [Mn^III(5-TMAM­(<i>R</i>)-salmen)­(H₂O)­Co^III(CN)₆]·7H₂O·MeCN [<b>1</b>; 5-TMAM­(<i>R</i>)-salmen = (<i>R</i>)-<i>N</i>,<i>N</i>′-(1-methylethylene)­bis­(5-trimethylammoniomethylsalicylideneiminate], was prepared. From direct-current magnetic susceptibilities, magnetization, and high-field and multifrequency electronic spin resonance measurements on powdered samples, <b>1</b> has a significant uniaxial anisotropy. Frequency-dependent alternating-current susceptibility signals were clearly observed, indicating slow magnetic relaxation. Thus, complex <b>1</b> behaves as a single-ion magnet

Exchange Coupling and Its Chemical Trend Studied by High-Frequency EPR on Heterometallic [Ln₂Ni] Complexes

We applied high-frequency electron paramagnetic resonance to trinuclear 4f–3d heterometallic complexes, [{Ln­(hfac)₃}₂{Ni­(dpk)₂(py)₂}] (Ln = Y, Gd, Tb, and Ho; hfac = hexafluoroacetylacetonate, dpk = di-2-pyridyl ketoximate, and py = pyridine), and determined the exchange parameter <i>J</i>_Ln–Ni as well as nickel­(II) zero-field splitting parameters. In contrast to the antiferromagnetic Dy analogue, ferromagnetic couplings were precisely characterized as <i>J</i>_Gd–Ni/<i>k</i>_B = +0.301(4) K, <i>J</i>_Tb–Ni/<i>k</i>_B = +0.216(12) K, and <i>J</i>_Ho–Ni/<i>k</i>_B = +0.110(3) K (defined as −<i>J</i>_Ln–Ni∑<i>J</i>_Ln^<i>z</i><i>S</i>_Ni)

Ferromagnetic Exchange Couplings Showing a Chemical Trend in Cu–Ln–Cu Complexes (Ln = Gd, Tb, Dy, Ho, Er)

Exchange couplings in isomorphous [LnCu₂] were evaluated by high-frequency electron paramagnetic resonance and magnetization studies. The exchange parameter <i>J</i>_Ln–Cu was decreased with an increase in the atomic number; <i>J</i>_Ln–Cu/<i>k</i>_B = 4.45(11), 2.27(6), 0.902(10), 0.334(3), and 0.136(8) K for Ln = Gd, Tb, Dy, Ho, and Er, respectively

Ferromagnetic Exchange Couplings Showing a Chemical Trend in Cu–Ln–Cu Complexes (Ln = Gd, Tb, Dy, Ho, Er)

Exchange couplings in isomorphous [LnCu₂] were evaluated by high-frequency electron paramagnetic resonance and magnetization studies. The exchange parameter <i>J</i>_Ln–Cu was decreased with an increase in the atomic number; <i>J</i>_Ln–Cu/<i>k</i>_B = 4.45(11), 2.27(6), 0.902(10), 0.334(3), and 0.136(8) K for Ln = Gd, Tb, Dy, Ho, and Er, respectively

Exchange Coupling and Its Chemical Trend Studied by High-Frequency EPR on Heterometallic [Ln₂Ni] Complexes

We applied high-frequency electron paramagnetic resonance to trinuclear 4f–3d heterometallic complexes, [{Ln­(hfac)₃}₂{Ni­(dpk)₂(py)₂}] (Ln = Y, Gd, Tb, and Ho; hfac = hexafluoroacetylacetonate, dpk = di-2-pyridyl ketoximate, and py = pyridine), and determined the exchange parameter <i>J</i>_Ln–Ni as well as nickel­(II) zero-field splitting parameters. In contrast to the antiferromagnetic Dy analogue, ferromagnetic couplings were precisely characterized as <i>J</i>_Gd–Ni/<i>k</i>_B = +0.301(4) K, <i>J</i>_Tb–Ni/<i>k</i>_B = +0.216(12) K, and <i>J</i>_Ho–Ni/<i>k</i>_B = +0.110(3) K (defined as −<i>J</i>_Ln–Ni∑<i>J</i>_Ln^<i>z</i><i>S</i>_Ni)

Exchange Coupling and Its Chemical Trend Studied by High-Frequency EPR on Heterometallic [Ln₂Ni] Complexes

We applied high-frequency electron paramagnetic resonance to trinuclear 4f–3d heterometallic complexes, [{Ln­(hfac)₃}₂{Ni­(dpk)₂(py)₂}] (Ln = Y, Gd, Tb, and Ho; hfac = hexafluoroacetylacetonate, dpk = di-2-pyridyl ketoximate, and py = pyridine), and determined the exchange parameter <i>J</i>_Ln–Ni as well as nickel­(II) zero-field splitting parameters. In contrast to the antiferromagnetic Dy analogue, ferromagnetic couplings were precisely characterized as <i>J</i>_Gd–Ni/<i>k</i>_B = +0.301(4) K, <i>J</i>_Tb–Ni/<i>k</i>_B = +0.216(12) K, and <i>J</i>_Ho–Ni/<i>k</i>_B = +0.110(3) K (defined as −<i>J</i>_Ln–Ni∑<i>J</i>_Ln^<i>z</i><i>S</i>_Ni)