Slow Magnetic Relaxation Behavior Research Articles

Cyanido-Bridged {LnIIIWV} Heterobinuclear Complexes: Synthesis and Magneto-Structural Study

A new series of cyanido-bridged {LnIIIWV} heterobinuclear complexes of formula [LnIII(pyim)2(i-PrOH)(H2O)2(μ-CN)WV(CN)7]·2H2O [Ln = Gd (1), Tb (2), Dy (3), Ho (4), and Er (5); pyim = 2-(1H-imidazol-2-yl)-pyridine) and i-PrOH = isopropyl alcohol] were synthesized by one-pot reaction between (NH3Bu)3[W(CN)8] and [Ln(pyim)2]2+ complexes (generated in situ by mixing the corresponding LnIII ions and the pyim ligand). Compounds 1-5 are isomorphous and crystallize in the monoclinic system P21/n space group. Their crystal structure consists of binuclear units in which the octacyanotungstate(V) anion coordinates to the corresponding LnIII ion through a single cyanide ligand. The tungsten(V) and lanthanide(III) ions are eight-coordinated, in distorted square antiprism (WV) and distorted trigonal dodecahedron (LnIII) geometries, respectively. The direct-current (dc) magnetic properties for 1-5 reveal the occurrence of weak antiferromagnetic interactions between WV and LnIII cation, with 8S7/2, 7F6, 6H15/2, 5I8, and 4I15/2 as ground terms for GdIII, TbIII, DyIII HoIII, and ErIII, respectively [JWLn = -1.19(1) (1), -1.02(2) (2), -1.10(2) (3), -1.30(2) (4), and -1.50(3) cm-1 (5), the spin Hamiltonian being defined as H = -JWLn SW·SLn]. The fit of the χMT data of 2-4 points out a positive value for the energy gap between the ML components (Δ). This feature is corroborated by their Q-band electron paramagnetic resonance spectra at low temperature, which clearly show MJ = 0 (2 and 4) and ±1/2 (3 and 5). Incipient frequency-dependent alternating-current magnetic susceptibility signals are observed for 3 and 5 under applied dc fields supporting the presence of slow magnetic relaxation behavior, the blocking temperatures being below 2.0 K. This new series of {LnIIIWV} heterobinuclear compounds provides more insights into the exchange magnetic interaction between 5d and 4f centers via the cyanide-bridge, for which scarce information is available to date.

Syntheses, structures, photoluminescence, and magnetism of a series of lanthanide 1,3-adamantanedicarboxylate coordination polymers

Five one-dimensional (1D) lanthanide dicarboxylate coordination polymers (CPs) have been successfully synthesized under hydrothermal conditions, namely, [Ln2(1,3-adc)3(H2O)4]·xH2O [Ln=Eu (1, x=1), Tb (2, x=1), Dy (3, x=2), and Er (4, x=1), 1,3-H2adc=1,3-adamantanedicarboxylic acid] and [Yb(1,3-adc)(1,3-Hadc)(H2O)3]·H2O (5). Structural characterization reveals that CPs 1–4 exhibit isostructural 1D chain formed by 1,3-adc linking alternately two mononuclear Ln(COO)4 units and one dinuclear Ln2(COO)4 cluster. CP 5 shows a 1D zigzag chain constructed from 1,3-adc connecting Yb(III), and the 1,3-Hadc anions are regularly arranged on two sides of the 1D chain. In 1–5, the neighboring 1D chains are extended into a two-dimensional (2D) supramolecular layer by hydrogen-bonding interactions. Property investigation indicates that CPs 1–3 and 5 display characteristic luminescence of the Eu(III), Tb(III), Dy(III), and Yb(III) ions, respectively. CP 3 shows a field-induced slow magnetic relaxation behavior.

One new planar Dy4 compound: Synthesis, structure and its magnetic dynamics behaviors

The self-assembly of tetranuclear dysprosium based on triangular Dy3 system by using 5-(4-methoxybenzylidene)-8-hydroxylquinoline (HL) and 1,3-diphenyl-1,3-propanedione (dppd) afford a new compound [Dy4(μ3-OH)2L6(dppd)4]·2CH3CN (1). It was structurally and magnetically characterized. The results indicate that the four Dy ions is of typical rhombus topology and weak ferromagnetic interactions exists between adjacent Dy3+ ions. Additionally, it also displays slow magnetic relaxation behavior with energy barrier Ueff of 62.36K.

Quantitative Estimation of Ising-Type Magnetic Anisotropy in a Family of C3 -Symmetric CoII Complexes.

In this paper, the influence of the structural and chemical effects on the Ising-type magnetic anisotropy of pentacoordinate CoII complexes has been investigated by using a combined experimental and theoretical approach. For this, a deliberate design and synthesis of four pentacoordinate CoII complexes [Co(tpa)Cl]⋅ClO4 (1), [Co(tpa)Br]⋅ClO4 (2), [Co(tbta)Cl]⋅(ClO4 )⋅(MeCN)2 ⋅(H2 O) (3) and [Co(tbta)Br]⋅ClO4 (4) by using the tripodal ligands tris(2-methylpyridyl)amine (tpa) and tris[(1-benzyl-1 H-1,2,3-triazole-4-yl)methyl]amine) (tbta) have been carried out. Detailed dc and ac measurements show the existence of field-induced slow magnetic relaxation behavior of CoII centers with Ising-type magnetic anisotropy. A quantitative estimation of the zero-field splitting (ZFS) parameters has been effectively achieved by using detailed ab initio theory calculations. Computational studies reveal that the wavefunction of all the studied complexes has a very strong multiconfigurational character that stabilizes the largest ms =±3/2 components of the quartet state and hence produce a large negative contribution to the ZFS parameters. The difference in the magnitudes of the Ising-type anisotropy can be explained through ligand field theory considerations, that is, D is larger and negative in the case of weak equatorial σ-donating and strong apical π-donating ligands. To elucidate the role of intermolecular interactions in the magnetic relaxation behavior between adjacent CoII centers, a diamagnetic isostructural ZnII analog (5) was synthesized and the magnetic dilution experiment was performed.

Synthesis, structure and single-molecule magnet characterization of Dy2 dinuclear complexes with an 8–hydroxyquinoline carbohydrazone ligand

The employment of an 8-hydroxyquinoline carbohydrazone tetradentate ligand, 8-hydroxyquinoline-2-carboxaldehyde-(benzoyl)hydrazone (H2L), DyCl3·6H2O and Et3N has led to the formation of dinuclear Dy(III) complex [Dy2(HL)2Cl4(CH3OH)2], (1) and [Dy2(HL)2((PhO)2PO2)4]·2(CH3OH) (2) in the presence of excess amount of bridging diphenyl phosphate anion, (PhO)2PO2−. X-ray crystallography data revealed that two DyIII atoms in 1 are doubly bridged by the two phenolato oxygen atoms of HL− ligands, however, the DyIII atoms are bridged by additional two deprotonated μ1,3-phosphate anions in 2. Each of the two lanthanide ions is eight-coordinated and is located in the different symmetry with triangular dodecahedron (D2d) for DyIII in 1 and elongated triangular bipyramid (D3h) for DyIII in 2. Dc magnetic susceptibility studies in the 2–300 K range reveal the weak antiferromagnetic interaction for 1 and 2. Both of them showed field-induced slow magnetic relaxation behavior with the Ueff for 1 of 46.0 K higher than 36.8 K for 2.

Probing the Effects of Ligand Field and Coordination Geometry on Magnetic Anisotropy of Pentacoordinate Cobalt(II) Single-Ion Magnets.

In this work, the effects of ligand field strength as well as the metal coordination geometry on magnetic anisotropy of pentacoordinated CoII complexes have been investigated using a combined experimental and theoretical approach. For that, a strategic design and synthesis of three pentacoordinate CoII complexes [Co(bbp)Cl2]·(MeOH) (1), [Co(bbp)Br2]·(MeOH) (2), and [Co(bbp)(NCS)2] (3) has been achieved by using the tridentate coordination environment of the ligand in conjunction with the accommodating terminal ligands (i.e., chloride, bromide, and thiocyanate). Detailed magnetic studies disclose the occurrence of slow magnetic relaxation behavior of CoII centers with an easy-plane magnetic anisotropy. A quantitative estimation of ZFS parameters has been successfully performed by density functional theory (DFT) calculations. Both the sign and magnitude of ZFS parameters are prophesied well by this DFT method. The theoretical results also reveal that the α → β (SOMO-SOMO) excitation contributes almost entirely to the total ZFS values for all complexes. It is worth noting that the excitation pertaining to the most positive contribution to the ZFS parameter is the dxy → dx2-y2 excitation for complexes 1 and 2, whereas for complex 3 it is the dz2 → dx2-y2 excitation.

2p-4f MOFs based on naphthalene-1,4,5,8-tetracarboxylate with magnetocaloric effect and slow magnetic relaxation properties

A family of new three-dimensional (3D) 2p-4f metal-organic frameworks (MOFs), namely [Na2Ln(L)(OH)(H2O)3]n (H4L=naphthalene-1,4,5,8-tetracarboxylic acid, Ln=Gd (1), Tb (2) and Dy (3)) have been synthesized by using hydrothermal method and structurally characterized by single-crystal X-ray diffraction, powder X-ray diffraction and IR spectra. Complexes 1–3 are isostructural and crystallize in the space group Cmca of orthorhombic system. Their crystal structures can be described as a kind of homeomorphism configurations with 3D framework. Magnetic measurements indicated that complex 1 exhibits antiferromagnetic behavior and magnetocaloric effect with −ΔSmmax=28.8Jkg−1K−1 (ΔH=7T and T=2K), and complex 2 exhibits field-induced slow magnetic relaxation behaviors.

Photoluminescent and Slow Magnetic Relaxation Studies on Lanthanide(III)-2,5-pyrazinedicarboxylate Frameworks.

In the series described in this work, the hydrothermal synthesis led to oxidation of the 5-methyl-pyrazinecarboxylate anion to the 2,5-pyrazinedicarboxylate dianion (2,5-pzdc) allowing the preparation of three-dimensional (3D) lanthanide(III) organic frameworks of formula {[Ln2(2,5-pzdc)3(H2O)4]·6H2O}n [Ln = Ce (1), Pr (2), Nd (3), and Eu (4)] and {[Er2(2,5-pzdc)3(H2O)4]·5H2O}n (5). Single-crystal X-ray diffraction on 1-5 reveals that they crystallize in the triclinic system, P1̅ space group with the series 1-4 being isostructural. The crystal structure of the five compounds are 3D with the lanthanide(III) ions linked through 2,5-pzdc2- dianions acting as two- and fourfold connectors, building a binodal 4,4-connected (4·648)(426282)-mog network. The photophysical properties of the Nd(III) (3) and Eu(III) (4) complexes exhibit sensitized photoluminescence in the near-infrared and visible regions, respectively. The photoluminescence intensity and lifetime of 4 were very sensitive due to the luminescence quenching of the 5D0 level by O-H oscillators of four water molecules in the first coordination sphere leading to a quantum efficiency of 11%. Variable-temperature magnetic susceptibility measurements for 1-5 reveal behaviors as expected for the ground terms of the magnetically isolated rare-earth ions [2F5/2, 2H4, 4I9/2, 7F0, and 4I15/2 for Ce(III), Pr(III), Nd(III), Eu(III), and Er(III), respectively] with MJ = 0 (2 and 4) and ±1/2 (1, 3, and 5). Q-band electron paramagnetic resonance measurements at low temperature corroborate these facts. Frequency-dependent alternating-current magnetic susceptibility signals under external direct-current fields in the range of 100-2500 G were observed for the Kramers ions of 1, 3, and 5, indicating slow magnetic relaxation (single-ion magnet) behavior. In these compounds, τ-1 decreases with decreasing temperature at any magnetic field, but no Arrhenius law can simulate such a dependence in all the temperature range. This dependence can be reproduced by the contributions of direct and Raman processes, the Raman exponent (n) reaching the expected value (n = 9) for a Kramers system.

Butterfly shaped tetranuclear dysprosium compound displaying slow magnetic relaxation features

One new tetranuclear dysprosium compound, namely [Dy4(μ3-OH)2(L)6(tmhd)4]·CH3CN (1) (HL=5-(4-methylbenzylidene)-8-hydroxylquinoline; tmhd=2,2,6,6-tetramethyl-3,5-heptanedione) was successfully synthesized and characterized. The structure of 1 was established by the single crystal X-ray diffraction technique, and the center Dy(III) ions was bridged by μ3-OH molecules and phenoxide oxygen atoms, displaying a ‘butterfly’ arrangement. Magnetic studies reveal that weak ferromagnetic interaction between adjacent Dy(III) exists in 1. Additionally, it also exhibits slow magnetic relaxation behaviors with the energy barrier of 46.4K.

Structures and Magnetostructural Correlation Analyses for Two Novel Hexanuclear Complexes Based on a Pentadentate Schiff Base Ligand

Abstract]have been synthesized, structurally characterized and magnetically studied. X‐ray diffraction analyses indicated that the potentially pentadentate symmetric ligand affords variable coordination conformations. The copper ions are bound into hexameric moieties with helix‐like arrangement whereas cobalts with four face‐sharing monovacant distorted cubanes. Magnetic investigations revealed that within the hexanuclear unit, compound 1 exhibits antiferromagnetic interactions with the best fit parameters of J1=‐17.96 cm−1, J2=‐22.26 cm−1, zJ=‐0.56 cm−1 and R=1.1 × 10−5, while compound 2 affords ferromagnetic intramolecular couplings with the best simulation parameters of J1=+0.10 cm−1; J2=+0.15 cm−1; Jx=Jy =0.1 cm−1; Jz=0.02 cm−1; D1=‐0.50; D2=‐0.45; g1=2.67; g2=2.57. Furthermore, the compound 2 displays slow magnetic relaxation behaviour detected by ac magnetic measurement under a 600 Oe field.

A unique dysprosium selenoarsenate(iii) exhibiting a photocurrent response and slow magnetic relaxation behavior.

A redox, substitution and self-assembly reaction offers a novel dysprosium selenoarsenate(iii) with As3+ [Dy2(tepa)2(μ2-OH)2Cl2]3[As3Se6]2 (1, tepa = tetraethylenepentamine), which provides the first example of chair conformation ring [As3Se6]3- combined with lanthanide complexes as counterions. 1 exhibits a photocurrent response and slow magnetic relaxation behavior.

Embedding 1D or 2D cobalt-carboxylate substrates in 3D coordination polymers exhibiting slow magnetic relaxation behaviors: crystal structures, high-field EPR, and magnetic studies.

By utilizing well-designed bifunctional ligands derived from 1H-imidazole-4,5-dicarboxylic acid, magnetic coordination polymers (CPs) that exhibit slow magnetic relaxation at the low temperature regions were constructed and further structurally characterized. In 1, 1D cobalt-carboxyl chains were stabilized in the final structure. In contrast, by adjusting the length of the substituted arms on imidazole-4,5-dicarboxylic acid, a novel 3D CP, 2 containing 2D 63 cobalt-carboxyl layer was obtained. A combination of Quantum Monte Carlo (QMC) simulations and the first-principles Density functional theory (DFT) calculations showed that compound 2 features weak ferro- and antiferro-magnetic coupling mechanisms with two different super-exchange paths of -/+/- for syn-anti carboxylate bridges and -/-/- for syn-syn carboxylate bridges. Through HF-EPR measurements performed on polycrystalline samples over the frequency range of 60-260 GHz and field range of 0-12 T, the effective g-values of 1 and 2 were all larger than 2.00, and the signs of their D values were probably positive.

From isolated to 2D coordination polymers based on 6-aminonicotinate and 3d-metal ions: towards field-induced single-ion-magnets

We report herein the synthesis and structural and chemical characterization of six new compounds consisting of 6-aminonicotinate (6ani) ligands and first-row transition metal ions, namely, [Mn2(μ-6ani)2(H2O)8](6ani)2 (1), [Co(6ani)(bipy)2(H2O)3](6ani)·5H2O (2), [M(μ-6ani)(6ani)(H2O)3]·2H2O [MII = Co (3), Ni (4)], and [M(μ-6ani)2]·H2O [MII = Co (5), Cu (6)] (where bipy = 4,4′-bipyridine). Compounds 1 and 2 consist of isolated dimeric and monomeric entities held together by supramolecular interactions governed by 6ani free anions. The rigid and low symmetry of 6ani ligands, in addition to their coordination as terminal and bridging ligands to octahedral metal geometries, gives rise to chiral 1D chains of compounds 3 and 4. 2D sql layers are formed (5 and 6) when 6ani ligands act as ligands bridging the metal ions, which precludes the presence of water molecules in the framework. Despite the wide structural diversity observed, all architectures share the occurrence of magnetically isolated 3d metal ions, given the poor exchange achieved through 6ani bridges, as confirmed by dc susceptibility measurements and DFT calculations. Dynamic ac susceptibility measurements reveal best-in-class field-induced slow magnetic relaxation behaviour in Co-based compounds with easy-plane magnetic anisotropy. The effective energy barriers (Ueff) of 39.6 K and 18.7 K for 3 and 5, respectively, estimated using the Arrhenius law are remarkably high among CoII-SIM coordination polymers reported so far.

Syntheses, structures, and magnetic properties of two series of copper-lanthanide heterometallic coordination polymers based on pyridine-2,3-dicarboxylic acid and succinic acid

Two series of copper-lanthanide heterometallic coordination polymers (CPs), [LnCu(2,3-pydc)2(suc)0.5(H2O)3]·H2O [Ln = La (I), Pr (II), Nd (III), Sm (IV), Gd (V), and Eu (VI), 2,3-H2pydc = pyridine-2,3-dicarboxylic acid, H2suc = succinic acid], [Ln2Cu3(2,3-pydc)6(H2O)10]·8H2O [Ln = Eu (VII), Tb (VIII), Dy (IX), and Er (X)], [Ln2Cu(2,3-pydc)2(suc)(μ2-OH)2(H2O)2]·2H2O [Ln = Er (XI), Yb (XII), and Lu (XIII)], [Ln2Cu(2,3-pydc)2(suc)2(H2O)6]·2H2O [Ln = Sm (1), Eu (2), Gd (3), Tb (4), Dy (5), and Er (6)], and Ln2Cu(2,3-pydc)2(suc)2(H2O)2 [Ln = Er (7), Yb (8), and Lu (9)], were successfully synthesized by the hydrothermal reactions of Cu(NO3)2·3H2O, LnCl3·nH2O, 2,3-H2pydc, and H2suc. CPs 1–9 show two types of similar two-dimensional (2D) structures. In 1–9, the 2,3-pydc anions bridge the CuII ions into a one-dimensional (1D) loop chain, and the suc anions link the LnIII ions into a 1D double chain. The 1D loop chains and the 1D double chains connect each other to generate a 2D layer. The adjacent 2D layers are further packed into a three-dimensional (3D) supramolecular architecture by hydrogen-bonding interactions. CPs 4 and 6 display ferromagnetic interactions between the paramagnetic centers, and CP 5 shows a field-induced slow magnetic relaxation behavior. The structures and magnetic properties of I–XIII have been reported in previous work (Eur. J. Inorg. Chem., 2016, 1060).

2D carboxylate-bridged LnIII coordination polymers: displaying slow magnetic relaxation and luminescence properties in the detection of Fe3+, Cr2O72- and nitrobenzene.

Eight new 2D isostructural lanthanide coordination polymers (Ln-CPs), [Ln(HL)(H2O)2(NO3)]·NO3 (1-Ln), Ln = NdIII, SmIII, EuIII, GdIII, TbIII, DyIII, HoIII, and YbIII ions, H2L = 4-(3,5-dicarboxylphenyl)-2-methylpyridine, were synthesized by using solvothermal methods and studied by structural analyses, magnetic analyses and luminescent probes. Crystallographic studies revealed that these compounds are 2D frameworks in which dinuclear units with double μ-syn,syn-carboxylate bridges are interlinked by single μ-trans,trans-carboxylate bridges from organic spacers. The layers are further stabilized and combined into 3D architectures through intra- and interlayer ππ stacking interactions and hydrogen bonding, respectively. Magnetic investigations indicated that the carboxylate bridges transmit intralayer antiferromagnetic coupling in 1-Nd, 1-Gd and 1-Ho but transmit intralayer ferromagnetic coupling in 1-Dy. Furthermore, 1-Dy also displays slow magnetic relaxation behavior with a high relaxation energy barrier (ΔUeff) of 100.7 K and a pre-exponential factor (τ0) of 1.4 × 10-8 s under zero dc field. The luminescence investigations showed that CPs 1-Eu and 1-Tb can serve as highly selective and recyclable sensing materials for Fe3+, Cr2O72- and nitrobenzene. Thus, both 1-Eu and 1-Tb should be excellent candidates for multifunctional sensors.

Functionalized nitronyl nitroxide biradical bridged one-dimensional lanthanide chains: slow magnetic relaxation in the Tb and Dy analogues

The first examples of Ln-nitronyl nitroxide 1D chains based on the functionalized nitronyl nitroxide biradical have been obtained, and complexes Dy and Tb exhibit slow magnetic relaxation behavior.

Slow magnetic relaxation based on the anisotropic Ising-type magnetic coupling between the MoIII and MnII centers.

Three new complexes containing MnMoIII zigzag chains have been synthesized and structurally characterized. The crystal structure of complex 3 consists of two linkage isomers of the MoIIIMn chains, which correspond to 1 and 2, respectively. Both complex 1 and 3 show slow magnetic relaxation behavior with an energy barrier of Δ/kB = 69.5 K and 68.8 K, respectively, whereas complex 2 is a simple paramagnet. The linear apical MoIII-CN-MnII pairs provide the required magnetic anisotropy for the slow magnetic relaxation.

Light Lanthanide Complexes with Crown Ether and Its Aza Derivative Which Show Slow Magnetic Relaxation Behaviors.

Two sets of isostructural Ln(III) mononuclear complexes, [Ln(NO3)3(18-crown-6)] (Ln = Ce (1), Pr (2), and Nd (3)) and [Ln(NO3)3(1,10-diaza-18-crown-6)] (Ln = Ce (4), Pr (5), and Nd (6)), were synthesized, and their slow magnetic relaxation behavior was investigated. Since Ln(III) ions are located in an axially stressed ligand field in both sets of complexes, they can exhibit single-molecule magnet (SMM) behavior owing to the oblate-type electronic distributions of the ground sublevels found in Ce(III), Pr(III), and Nd(III). Field-induced slow magnetic relaxation was observed for Ce(III) and Nd(III) complexes 1, 3, 4, and 6 under an applied bias dc field of 1000 Oe, whereas no slow relaxation was observed for Pr(III) complexes 2 and 5. The slow magnetic relaxation behavior of 1, 3, 4, and 6 was correlated with the even-numbered Jz sublevels of Ce(III) and Nd(III) ions, known as the Kramers system.

Slow Magnetic Relaxation of Lanthanide(III) Complexes with a Helical Ligand

Isostructural Ln(III) mononuclear complexes [Ln(NO3)2L]PF6·MeCN (Ln = Nd, Tb, or Dy; L denotes a helical hexa-dentate ligand) were synthesized, and their slow magnetic relaxation behavior was investigated. In these complexes, oblate-type Ln(III) ions are located in an axially stressed ligand field with two nitrate anions, and can exhibit single-molecule magnet (SMM) behavior. Field-induced SMM behavior was observed for Nd(III) and Dy(III) complexes under an applied bias DC field of 1000 Oe.

Syntheses, structures, and magnetic properties of a cubane-like {Co4O4} cluster and a dinuclear CuII compound constructed from a tridentate Schiff base ligand

The employment of a tridentate Schiff base ligand, 1-(((2-hydroxy-2,3-dihydro-1H-inden-1-yl)imino)methyl)naphthalen-2-ol (H2L) in the coordination chemistry of cobalt(II) acetate and copper(II) acetate is reported. One cubane-like cluster of formula [Co4L4]·CH3OH (1·CH3OH) and one alkoxo-bridged dinuclear copper compound of composition [Cu2L2] (2) were achieved by the reactions of metal acetates with H2L under solvothermal conditions. The structures of 1·CH3OH and 2 have been established by single-crystal X-ray diffraction studies. Complex 1·CH3OH has a cubane-like {Co4O4} core with divalent metal atoms and alkoxo atoms (originated from the L2− ligands) occupying alternating vertices. The magnetic properties of 1·CH3OH and 2 have been investigated. The variable-temperature dc magnetic susceptibility studies indicate ferromagnetic CoII⋯CoII exchange interactions for 1·CH3OH and antiferromagnetic CuII⋯CuII couplings for 2. The ac magnetic susceptibility investigation reveals that complex 1·CH3OH shows the slow magnetic relaxation (SMM) behavior.