Field-Induced Multiple Relaxation Mechanism of CoIII2DyIII Compound with the Dysprosium Ion in a Low-Symmetrical Environment

Structural, optical and magnetic properties of a new metal-organic CoII-based complex

A mononuclear cobalt(ii) complex [C5H8N3]2[CoCl4(C5H7N3)2] (I) was synthesized and structurally characterized. Single crystal X-ray diffraction analysis indicates that monometallic Co(ii) ions acted as coordination nodes in a distorted octahedral geometry, giving rise to a supramolecular architecture. The latter is made up of a ½ unit form composed of an anionic element [Co0.5Cl2(C5H7N3)]- and one 2-amino-4-methylpyrimidinium cation [C5H8N3]+. The crystalline arrangement of this compound adopts the sandwich form where inorganic parts are sandwiched between the organic sheets following the [100] direction. More information regarding the structure hierarchy has been supplied based on Hirshfeld surface analysis; the X⋯H (X = N, Cl) interactions play a crucial role in stabilizing the self-assembly process of I, complemented by the intervention of π⋯π electrostatic interaction created between organic entities. Thermal analyses were carried out to study the thermal behavior process. Static magnetic measurements and ab initio calculations of compound I revealed the easy-axis anisotropy character of the central Co(ii) ion. Two-channel field-induced slow-magnetic relaxation was observed; the high-frequency channel is characterized by underbarrier relaxation with U eff = 16.5 cm-1, and the low-frequency channel involves a direct relaxation process affected by the phonon-bottleneck effect.

Elevating the Performance of Heterometallic 3d/4f SMMs: The Role of Diamagnetic CoIII and ZnII Ions in Magnetization Dynamics

Recent advances in the synthesis of 3d/4f Single-Molecule Magnets (SMMs) have revealed the effective role of incorporating diamagnetic CoIII or ZnII ions to enhance the magnetic properties of LnIII ions. This concept highlights notable examples of CoIII/LnIII and ZnII/LnIII SMMs documented in the recent literature, illustrating how the selection of various peripheral and/or bridging ligands can modulate the magnetic anisotropy of 4f metal ions, thereby increasing their energy barriers.

Influence of the phonon-bottleneck effect and low-energy vibrational modes on the slow spin-phonon relaxation in Kramers-ions-based Cu(II) and Co(II) complexes with 4-amino-3,5-bis-(pyridin-2-yl)-1,2,4-triazole and dicyanamide

Two new complexes, bis-[4-amino-3,5-bis-(pyridin-2-yl)-1,2,4-triazole-κ2N2,N6]bis-(dicyanamide-κN8)copper(II), [Cu(abpt)2(dca)2] (1) and bis-[4-amino-3,5-bis-(pyridin-2-yl)-1,2,4-triazole-κ2N2,N6]bis-(dicyanamide-κN8)cobalt(II), [Co(abpt)2(dca)2] (2), have been prepared and magneto-structurally characterised. Single crystal studies of both complexes have shown that their crystal structures are molecular, in which the central atoms are six-coordinated in the form of a distorted octahedron by two bidentate abpt and two monodentate dca ligands. Even if both complexes have the same composition and crystallize in the same P1̄ space group, they are not isostructural. Both structures contain strong intermolecular N-H⋯N hydrogen bonds and π-π stacking interactions. IR spectra are consistent with the solved structures of both complexes and confirmed the terminal character of the dca ligands and the bidentate coordination of the abpt ligands. The analysis of the magnetic properties showed that both complexes exhibit field-induced slow spin-phonon relaxation. In both complexes, the slow spin-phonon relaxation is influenced by a severe phonon-bottleneck effect that affects the direct process, a dominant relaxation mechanism at low temperatures in both complexes. The phonon-bottleneck effect in 1 was suppressed by simply reducing the crystallite size, and further analysis of the field dependence of the relaxation time yielded the characteristic energy of vibrational modes of 11 cm-1 participating in the Raman process at low magnetic fields. The analysis of magnetic properties and ab initio calculations confirmed that 2 represents a system with a moderate uniaxial anisotropy yielding an average energy barrier of 82 cm-1 (from all four nonequivalent Co(II) sites in the structure of 2).

Slow Magnetization Relaxation in a Family of Triangular {CoIII 2 LnIII } Clusters: The Effect of Diamagnetic CoIII Ions on the LnIII Magnetic Dynamics

The first use of the Schiff base chelate N-naphthalidene-o-aminophenol (naphH2 ) in Co/Ln chemistry has afforded a family of isostructural [CoIII 2 LnIII (OMe)2 (naph)2 (O2 CMe)3 (MeOH)2 ] (Ln=Tb, Dy and Er) complexes, revealing a rare {CoIII 2 Ln(μ3 -OMe)}8+ triangular core composed of two diamagnetic CoIII ions and a 4f-ion with slightly distorted square antiprismatic geometry. Alternating current (ac) magnetic susceptibility studies revealed that {Co2 Dy}, and its magnetic diluted analogue {Co2 Dy0.05 Y0.95 }, behave as mononuclear single-molecule magnets (SMMs) with similar energy barriers for the magnetization reversal, Ueff , of ~85-90 K. SMM properties were also detected for {Co2 Er}, with the compound exhibiting a Ueff of 18.7 K under an applied magnetic field of 800 Oe. To interpret the experimental magnetic results, ab initio CASSCF/RASSI-SO and DFT calculations were performed as a means of exploring the single-ion characteristics of LnIII ions and comprehend the role of the diamagnetic CoIII ions in the magnetization relaxation of the three heterometallic compounds.

Dy3+ single ion magnet in the extended inorganic solid Ca(Y,Dy)AlO4

Confined in the extended solid with a widespread K2NiF4 structure type, cation Dy3+ exhibits magnetic bistability under a zero field.

Slow Magnetic Relaxation in a Co2 Dy Trimer and a Co2 Dy2 Tetramer

The combination of Co(III) and Dy(III) with a compartmental Schiff base ligand (H₃ L=3-[(2-Hydroxy-3-methoxy-benzylidene)-amino]-propane-1,2-diol), presenting three different coordinating pockets, has allowed the synthesis of two novel Co(III)-Dy(III) complexes: [Co₂ Dy(HL)₄ ]NO₃  ⋅ 2CH₃ CN (1), a rare example of trinuclear linear Co^III ₂ Dy^III complex (and the first with slow relaxation of magnetization in absence of a DC field) and [Co₂ Dy₂ (μ₃ -OH)₂ (HL)₂ (OAc)₆ ] ⋅ 4.6H₂ O (2), the first tetranuclear Co^III ₂ Dy^III ₂ cluster with a rhomb-like structure where the Co(III) ions are connected along the short diagonal of the rhomb. 1 presents two different relaxation processes: a fast relaxation dominated by Quantum tunnelling (QT) and a slow relaxation with an energy barrier of 40 K. 2 shows two close relaxation processes without applied DC fields that follow QT and Orbach mechanisms whereas for H_DC =500 Oe, the QT is cancelled and a direct term appears. Here we present the synthesis, X-ray structure and magnetic characterization of these two Co(III)-Dy(III) single-ion/molecule magnets.

Syntheses, structures and magnetic properties of two isostructural dicyanamide-bridged 2D polymers

Complexes [Co(biq)(μ_1,5-dca)₂]_n (1) and [Ni(biq)(μ_1,5-dca)₂]_n (2) (biq is 2,2′-biquinoline, dca is dicyanamide anion, N(CN)₂⁻) have been characterized by crystal structure analysis, and spectral and magnetic measurements.

Bulky ligands shape the separation between the large spin carriers to condition field-induced slow magnetic relaxation

Crystal engineering of magnetic relaxation in supramolecular networks based on almost isotropic cyanido-bridged {Mn9[W(CN)8]6L8(solv)8} clusters decorated by bulky 4,4'-di-tert-butyl-2,2'-bipyridine (But2bpy) and 4,7-diphenyl-1,10-phenanthroline (Ph2phen) ligands is presented. The three new compounds {MnII9[WV(CN)8]6(But2bpy)8(MeOH)8}·Pri2O·13MeOH (1), {MnII9[WV(CN)8]6(But2bpy)8(MeOH)6(H2O)2}·4Pri2O·2H2O (1a), and {MnII9[WV(CN)8]6(Ph2phen)8(MeOH)8}·29MeOH·6H2O (2) were characterized structurally and magnetically. Compound 1 exhibits unequivocal domination of repulsive intercluster contacts operating between the side But groups leading to intercluster distances exceeding 10 Å in all three dimensions. Compound 1a reveals a 1-dimensional (1D) supramolecular chain structure with very close intercluster distances of 6.7 Å realized through the direct W-CNHO-Mn hydrogen bonds along the chain, further isolated by the above repulsive ButBut synthons. Compound 2 shows significant separation in all three directions with the intercluster distances close to 10, 12 and 13.5 Å. However, in contrast to 1, these separations are accompanied by indirect hydrogen bond arrays and local π-π interactions of potential to assist in the transfer of weak magnetic interactions. The dc magnetic data show the signature of S = 39/2 in the ground state, which is typical in this group of compounds. The high-spin clusters are accompanied by different intercluster interactions, illustrated by the effective zJ' values of +0.010 cm-1 (1), +0.008 cm-1 (1a) and +0.001 cm-1 (2). The low temperature ac susceptibility measurements revealed a temperature- and field-dependent magnetic relaxation time for all 1, 1a and 2 compounds (τ1, τ1a-fast, and τ2-fast in the range 10-3-10-4 s). In contrast and only in the case of 1a and 2, an additional temperature independent slow process was detected (τ1a-slow and τ2-slow located between 0.1 s and 1 s). The magnetic relaxations were correlated with the obtained supramolecular networks, indicating the significant role of dipolar fields, weak non-covalent interactions, hydrogen bonds and π-π interactions.

Synthetic diversity and change in nuclearity in [Co-Dy] coordination aggregates: bridge removal, solvent induced structural reorganization and AC susceptibility measurements

Three new cobalt(ii/iii)-dysprosium(iii) complexes, [DyIII3CoII2CoIII2(L1)2(O2CCMe3)8(OH)4(OMe)2(H2O)4]·Dy(η1-O2CCMe3)2(η2-O2CCMe3)2(MeOH)2·4H2O (1), [DyIII3CoII2CoIII2(L2)2(O2CCMe3)8(OH)4(OMe)2(MeOH)2(H2O)2]·Dy(η1-O2CCMe3)2(η2-O2CCMe3)2(MeOH)2·4MeOH (2) and DyIII2CoII2CoIII2(L2)2(O2CCMe3)10(OH)2 (3) have been reported. In the heptanuclear 3d-4f monocationic aggregates in 1 and 2 the three dysprosium and four cobalt sites are arranged into a vertex shared dicubane structure, induced by two structure-directing ligands. Interestingly, a unique and previously unknown dysprosium(iii)-pivalate based counter anion, Dy(η1-O2CCMe3)2(η2-O2CCMe3)2(MeOH)2-, was trapped by the monocationic cores during crystallization. MeCN induced structural rearrangement of 2 through loss of OMe- bridges and dysprosium(iii) ions at the shared vertex resulted in the hexanuclear 3d-4f neutral aggregate 3, in which two dysprosium and four cobalt sites exhibit a near planar disposition. HRMS(+ve) of solutions of 1 and 2 revealed the pathway for aggregation processes and solvent induced structural transformation along with the importance of bridging OMe- in directing the formation of these compounds. Magnetic studies show a non-zero out-of-phase component in the AC susceptibility measurements of 1 but not in 2 and 3, although 1 and 2 have a very similar {CoIII2CoII2DyIII3} core and another DyIII center. Ab initio single-ion calculations point to the different single-ion anisotropic behavior for DyIII centers (energy in cm-1 and g-tensors) as well as negative and positive D values for CoII sites in 1 and 2 respectively reaffirming the experimental result. However, calculations envision that, zero-field out-of-phase signal and no out-of-phase signal in 1 and 2 respectively do not solely generate from the single-ion Dy/Co anisotropies and the overall relaxation mechanism can be understood by considering the exchange interactions between DyIII-DyIII and DyIII-CoII centres.

Counter anions influence the relaxation dynamics of phenoxy-bridged Dy2 single molecule magnets

Structural and magnetic investigations into four dinuclear dysprosium complexes reveal that the counter anion influences the dynamic relaxation process in these complexes.

New examples of triangular terbium(iii) and holmium(iii) and hexagonal dysprosium(iii) single molecule toroics

The structural, magnetic and theoretical aspects are described for three triangular lanthanide complexes, [Tb(OH)(teaH₂)₃(paa)₃]Cl₂ (1), [Dy(OH)(teaH₂)₃(paa)₃]Cl₂ (2) and [Ho(OH)(teaH₂)₃(paa)₃]Cl₂ (3), and a hexanuclear wheel of formula [Dy(pdeaH)₆(NO₃)₆] (4) [teaH₃ = triethanolamine, paaH = N-(2-pyridyl)-acetoacetamide and pdeaH₃ = 3-[bis(2-hydroxyethyl)amino]propan-1-ol]. Each complex displays single molecule toroidal behaviour as rationalised using high-level ab initio calculations. Complexes 2 and 3 are the first examples of mixed moment single molecule toroidal complexes featuring non-Kramers ions.

1D coordination polymer (OPD)2CoIISO4 showing SMM behaviour and multiple relaxation modes

A novel one-dimensional Co^II coordination polymer (OPD)₂Co^IISO₄ was formed from alternating tetrahedral sulphate anions, Co centers and molecules of 1,2-phenylenediamine (OPD). The thermal stability of the structure was confirmed up to ∼230 °C using thermogravimetry and non-ambient powder X-ray diffraction in air. The distorted pseudo-octahedral coordination sphere of Co^II ions promotes strong magnetic anisotropy. Therefore (OPD)₂Co^IISO₄ was subjected to thorough characterization with ab initio and DFT calculations along with dc magnetic measurements both of which point to strong easy-axis anisotropy (D/k_B≈-87 K, E/k_B≈ 23 K). The analysis of ac susceptibility revealed field assisted magnetic relaxation in two field regimes: a low field one with one relaxation time and a high field one where three relaxation times were distinguished. The main role in relaxations of the fastest process (τ₁) was attributed to the direct and Raman processes.

Two {ZnDyIII} complexes supported by monophenoxido/dicarboxylate bridges with multiple relaxation processes: carboxylato ancillary ligand-controlled magnetic anisotropy in square antiprismatic DyIII species

Two linear-shaped heterometallic trinuclear {ZnII2DyIII} clusters were prepared with a compartmental Schiff-base ligand and carboxylate as the co-ligand. The central DyIII ion is sandwiched by two [ZnHhms(RCOO)2] (H2hms = (2-hydroxy-3-methoxybenzylidene)-semicarbazide) units to form a distorted square antiprismatic (SAP) geometry with D4d symmetry. The DyIII and ZnII ions are triply bridged by one unique monophenoxido/dicarboxylate group, where one of the carboxylates shows a severe angular distortion. The magnetic analysis revealed a field-dependent change with two relaxation processes in 1 and 2, which originated from the molecular magnetic center and dipolar-dipolar coupling. The dipole-induced slow relaxation disappeared upon magnetic-site dilution, while another faster relaxation appeared in the high frequency region associated with the different distortions of the antiprismatic sites for 1 and disordered structures for 2. Ab initio calculations revealed that two {ZnII2DyIII}-carboxylate complexes are axial in nature, but the remarkable quantum tunnelling of the magnetization exists in the ground state, which lowers the anisotropic barrier and prohibits zero-field single-molecule magnet behavior. The crystal field of the carboxylato ancillary ligands produces a competitive effect with the phenoxido bridging ligands on the magnetic anisotropy of the lanthanide ions, which greatly affects the electrostatic distribution of this type of {ZnII2DyIII} single-ion magnets (SIMs). The results offer a model for further investigating structural factors in the relaxation mechanism of SIM systems.

Axial Ligand Field in D4d Coordination Symmetry: Magnetic Relaxation of Dy SMMs Perturbed by Counteranions

A series of mononuclear Dy^III complexes with the general formula [DyLz₂(salicylaldehyde)₂]·X·solvent (Lz = 6-pyridin-2-yl[1,3,5]triazine-2,4-diamine; X = OH^- (1·OH), Cl^- (2·Cl), Br^- (3·Br)) have been synthesized using mixed salicylaldehyde/pyridinyl-triazine ligands and discriminative counteranions. The Dy^III ion in these three complexes resides in a similar D_4d coordination geometry with counteranions perturbing the coordination environment and bond lengths and angles in the lattice. Magnetostructural studies reveal that the asymmetric distribution of salicylaldehyde/pyridinyl-triazine ligands and the presence of discriminative counteranions result in the coexistence of large anisotropy and quantum tunneling of magnetization. The magnetic anisotropy is dominated by the axial ligand field with short Dy-O_sali distances and large ∠O_sali-Dy-O_sali angles, while the quantum tunneling relaxation is probably dictated by the π-π stacking of the Lz ligands, which induces an axial constriction of the coordinating plane. Ab initio calculations substantiate the diversity of the magnetic behaviors in these complexes and highlight the importance of axial ligand field with short Dy-O_sali distances, large ∠O_sali-Dy-O_sali angles and less ligand stacking in these pseudo-D_4d-symmetrical single-molecule magnets.

Bis-phenoxido and bis-acetato bridged heteronuclear {Co(III)Dy(III)} single molecule magnets with two slow relaxation branches

Two bis-μ-phenoxido-bis-μ-acetato heterobimetallic {Co(III)Dy(III)} complexes and , formulated as [Co(III)Dy(III)L(μ-OAc)2(NO3)2] derived from the comparable hexadentate Schiff bases N,N'-ethylenebis(3-ethoxysalicylaldimine) and N,N'-ethylenebis(3-methoxysalicylaldimine) were synthesized and X-ray structure analysis confirms their nearly identical structures. These are the first examples of bis(μ-phenoxido)-bis(μ-carboxylato) {Co(III)Dy(III)} systems. The AC susceptibility measurements show that both complexes exhibit a field-induced slow magnetic relaxation with two relaxation branches. While the high-frequency process spans the usual range of the relaxation time for analogous single molecule magnets (τ0 ∼ 10(-7) s), the low-frequency branch is as slow as τ ∼ 0.1 s at T = 1.9 K and B = 0.2 T.

Fine Tuning of Multicolored Photoluminescence in Crystalline Magnetic Materials Constructed of Trimetallic EuxTb1-x[Co(CN)6] Cyanido-Bridged Chains

Coordination compounds built of trivalent lanthanide ions have been demonstrated as promising solid-state materials for diverse photoluminescent applications and as attractive magnetic objects with the prospective application in information storage and spintronics. We present a synthetic methodology in which both luminescent and magnetic functionalities are induced within lanthanide-based coordination polymers by the application of hexacyanocobaltate(III) anions and 3-hydroxypyridine (3-OHpy), both coordinated to 4f-metal ions modulating the lanthanide-centered properties. We report a series of trimetallic cyanido-bridged chains {[Eu^III_xTb^III_1-x(3-OHpy)₂(H₂O)₄][Co^III(CN)₆]}·H₂O (x = 1, 0.8, 0.5, 0.4, 0.3, 0.2, 0.1, 0; compounds 1, 2, ..., 7, 8). They reveal tunable visible photoluminescence ranging from green, through yellow and orange, to red color depending on the composition of material and the wavelength of UV excitation light. Such multicolored emission is realized by the adjusted ratio between red emissive Eu³⁺ and green emissive Tb³⁺ and by the selection of wavelengths of UV light controlling the intensities of Eu- and Tb-based components of visible luminescence. The photoluminescence is enhanced by the energy-transfer (ET) process from [Co^III(CN)₆]^3- and 3-OHpy to lanthanides, and the efficiencies of ET to Eu and Tb play an important role in the switchable emission. The whole family, 1-8, exhibits temperature-dependent paramagnetism due to the intrinsic property of lanthanide(3+) ions. Tb-containing 2-8 reveal the field-induced slow relaxation of magnetization due to the magnetic anisotropy of Tb³⁺. Moreover, the compounds built of large amounts of Tb reveal the double relaxation, the faster of a typical Tb^III single-ion origin, and the slower originating from the magnetic dipole-magnetic dipole interactions between neighboring Tb^III centers. Compound 2, which can be considered as a magnetically diluted sample, exhibits almost single relaxation process with the thermal energy barrier ΔE/k_B of 35.8(6) K and τ₀ = 1.1(2) × 10^-8 s at H_dc = 1500 Oe, indicating a single-molecule magnet behavior.

Structures and magnetic properties of dysprosium complexes: the effect of crystallization temperature

Two new dysprosium complexes, [Dy(H₅L)(NO₃)₂(CH₃OH)₂]·4CH₃OH (1) and [Dy₂(H₅L)₂(NO₃)₄(H₂O)₂]·10CH₃OH (2), were isolated from the reaction of a novel 1,8-naphthalenediol-based ligand N,N'-((1,8-dihydroxynaphthalene-2,7-diyl)bis(methanylylidene))bis(2-hydroxybenzohydrazide), H₆L with dysprosium(iii) nitrate upon crystallization at different temperatures. Because of a low-symmetrical coordination environment, both complexes display only field-induced single molecular magnetic (SMM) behavior. Interestingly, complex 2 containing two dysprosium ions shows field-induced multiple relaxation processes, whereas only one relaxation process is observed for complex 1. The remarkably different behavior observed in 2 is mainly ascribed to the weak intra- or intermolecular interactions between the two Dy^III centres in this complex.

Slow magnetisation relaxation in tetraoxolene-bridged rare earth complexes

Two tetraoxolene-bridged dinuclear Dy(iii) complexes exhibit slow relaxation in ac magnetic susceptibility studies with zero-field quantum tunnelling of the magnetisation that is suppressed by the application of a dc magnetic field.

Macroscopic Hexagonal Tubes of 3 d-4 f Metallocycles

The self-assembly of three giant hexagonal 3d-4f metallocycles with inner diameters of 16.4, 16.5, and 16.4 Å, is described. Hexagonal metallocycles were stacked along the crystallographic c axis, producing unique hexagonal macroscopic tubular single crystals. The assembly mechanism of the tubular crystals was investigated. Remarkably, all three hexagonal metallocycles show typical single-molecule magnet behavior, benefiting from the ferromagnetic couplings between the 3d and 4f ions.

Yellow to greenish-blue colour-tunable photoluminescence and 4f-centered slow magnetic relaxation in a cyanido-bridged Dy(III)(4-hydroxypyridine)-Co(III) layered material

A cyanido-bridged layered {[Dy(III)(4-OHpy)2(H2O)3][Co(III)(CN)6]}·0.5H2O (1) (4-OHpy = 4-hydroxypyridine) framework with dual photo-luminescence and magnetic properties was prepared. 1 exhibits visible emission whose color, yellow to greenish-blue, is switchable by selected wavelengths of UV excitation light. Magnetic data revealed that 1 shows not only the slow magnetic relaxation of a typical Dy(III) single-ion origin but also the relaxation process caused by the magnetic dipole-magnetic dipole interactions between the neighbouring Dy(III) centers.

Relaxation Dynamics and Magnetic Anisotropy in a Low-Symmetry Dy(III) Complex

The magnetic behaviour of a Dy(LH)3 complex (LH(-) is the anion of 2-hydroxy-N'-[(E)-(2-hydroxy-3-methoxyphenyl)methylidene]benzhydrazide) was analysed in depth from both theoretical and experimental points of view. Cantilever torque magnetometry indicated that the complex has Ising-type anisotropy, and provided two possible directions for the easy axis of anisotropy due to the presence of two magnetically non-equivalent molecules in the crystal. Ab initio calculations confirmed the strong Ising-type anisotropy and disentangled the two possible orientations. The computed results obtained by using ab initio calculations were then used to rationalise the composite dynamic behaviour observed for both pure Dy(III) phase and Y(III) diluted phase, which showed two different relaxation channels in zero and non-zero static magnetic fields. In particular, we showed that the relaxation behaviour at the higher temperature range can be correctly reproduced by using a master matrix approach, which suggests that Orbach relaxation is occurring through a second excited doublet.

Two dodecanuclear heterometallic [Zn6Ln6] clusters constructed by a multidentate salicylamide salen-like ligand: synthesis, structure, luminescence and magnetic properties

We present herein two dodecanuclear heterometallic [Zn₆Ln₆] clusters with interesting structures, luminescence and magnetic properties.

A nine-coordinated dysprosium(iii) compound with an oxalate-bridged dysprosium(iii) layer exhibiting two slow magnetic relaxation processes

A 2D oxalate-bridged dysprosium(iii) compound, formulated as [Dy(C₂O₄)_1.5(H₂O)₃]_n·2nH₂O (1), has been hydrothermally isolated.