Lanthanide(III)-Based Single-Ion Magnets

Macrocycle based dinuclear dysprosium(III) single molecule magnets with local D5h coordination geometry

Targeted approaches for manipulating the coordination geometry of lanthanide ions are a promising way to synthesize high-performance single-molecule magnets (SMMs), but most of the successful examples reported to date focus on mononuclear complexes. Herein, we describe a strategy to assemble dinuclear SMMs with Dy^III ions in approximate D_5h coordination geometry based on pyrazolate-based macrocyclic ligands with two binding sites. A Dy4 complex with a rhomb-like arrangement of four Dy^III as well as two dinuclear complexes having axial chlorido ligands (Dy2·Cl and Dy2*·Cl) were obtained; in the latter case, substituting Cl^- by SCN^- gave Dy2·SCN. Magneto-structural studies revealed that the μ-OH bridges with short Dy-O bonds dominate the magnetic anisotropy of the Dy^III ions in centrosymmetric Dy4 to give a vortex type diamagnetic ground state. Dynamic magnetic studies of Dy4 identified two relaxation processes under zero field, one of which is suppressed after applying a dc field. For complexes Dy2·Cl and Dy2*·Cl, the Dy^III ions feature almost perfect D_5h environment, but both complexes only behave as field-induced SMMs (U_eff = 19 and 25 K) due to the weak axial Cl^- donors. In Dy2·SCN additional MeOH coordination leads to a distorted D_2d geometry of the Dy^III ions, yet SMMs properties at zero field are observed due to the relatively strong axial ligand field provided by SCN^- (U_eff = 43 K). Further elaboration of preorganizing macrocyclic ligands appears to be a promising strategy for imposing a desired coordination geometry with parallel orientation of the anisotropy axes of proximate Dy^III ions in a targeted approach.

Field-induced single-ion magnets exhibiting tri-axial anisotropy in a 1D Co(II) coordination polymer with a rigid ligand 4,4'-(buta-1,3-diyne-1,4-diyl)dibenzoate

Herein a 1D Co(II) coordination polymer of formula [Co(η¹-L1)(η²-L1)(py)₂(H₂O)]_n (CoCP) has been synthesised using the rigid H2L1 proligand, containing a long spacer bearing two triple bonds. Single-crystal X-ray diffraction showed that Co(II) adopts a distorted octahedral geometry. The state-averaged complete active self-consistent field (SA-CASSCF) calculation showed that the ground state of CoCP is a high spin quartet with a highly multiconfigurational character of its electronic structure. Due to the large intra- and intermolecular distances between the spin carriers, the magnetic interactions are negligible and the zero-field splitting (ZFS) effects of cobalt(II) ions are predominant. This behavior was confirmed by direct current (DC) magnetic measurements and theoretical calculations using the broken-symmetry approach. Quantum chemical calculations indicate that CoCP has a negative axial component possessing mixed tri-axial anisotropy. The DC magnetic susceptibility data were fitted with a Griffith-Figgis Hamiltonian and the obtained parameters are in good agreement with those simulated by the ab initio calculation. Alternating current (AC) magnetic measurements showed a field induced slow magnetic relaxation in CoCP, which is attributed to the hyperfine interaction effects.

Synthesis, characterization, magnetism and theoretical analysis of hetero-metallic [Ni2Ln2] partial di-cubane assemblies

A family of four isostructural [Ln₂Ni₂(L)₂(μ₃-OCH₃)₂(μ_1,3-PhCO₂)₂(PhCO₂)₂(CH₃OH)₄]·2CH₃OH [where Ln = Gd (1), Tb (2), Dy (3) and Ho (4)] complexes has been synthesized using Schiff base ligand 2-[{(2-hydroxybenzyl)imino}methyl]-6-methoxyphenol (H₂L). All the complexes possess a partial di-cubane core structure where the growth of the core is contingent upon the ligand anions and solvent generated μ₃-OCH₃ groups. DC magnetic analysis revealed dominating ferromagnetic interactions between the metal ions, however, we find no slow relaxation characteristics in the AC susceptibility. Further insight into the magnetic behavior of the reported complexes was achieved using DFT and CASSCF theoretical calculations, leading to the comprehension of the fast relaxation characteristics observed by magnetometry.

Probing optical and magnetic properties via subtle stereoelectronic effects in mononuclear DyIII-complexes

Tapping into the secondary coordination environment of mononuclear DyIII-complexes leads to drastic changes in luminescence and magnetism. Visualization of effects induced by stereoelectronics on the opto-magnetic properties was achieved through subtle modifications in the ligand framework.

Successive syntheses and magnetic properties of homodinuclear lanthanide macrocyclic complexes

A series of homodinuclear β-diketone lanthanide(III) complexes, formulated as [(acac)₄Ln₂(L1)] (Ln³⁺ = Dy³⁺ (1), Tb³⁺ (2), and Gd³⁺ (3), respectively) were first synthesized based on a closed-macrocyclic ligand (H₂L1) derived from the [2 + 2] cyclocondensation of 4-tert-butyl-2,6-diformylphenol and o-phenylenediamine in the presence of lanthanide acetylacetonates. Subsequently, by using the above compounds as building blocks to assemble directly with another Schiff base ligand, N,N'-bis(5-chlorosalicylidene)-o-phenylenediamine (H₂L2), three new homodinuclear sandwich-type lanthanide complexes with the general formula [Ln₂(L1)(L2)₂] (Ln³⁺ = Dy³⁺ (4), Tb³⁺ (5), and Gd³⁺ (6), respectively) were further designed and prepared. Single-crystal X-ray analyses show that the central Ln³⁺ ion adopts a distorted square antiprism conformation with D_4d local symmetry. Magnetic studies reveal ferromagnetic interaction between Dy³⁺ and Tb³⁺ centres and zero-field slow relaxation of magnetization for Dy complexes 1 and 4. The corresponding magneto-structural correlations of SMMs 1 and 4 were further discussed by theoretical calculations and with experimental outcomes.

Methods and Models of Theoretical Calculation for Single-Molecule Magnets

Theoretical calculation plays an important role in the emerging field of single-molecule magnets (SMMs). It can not only explain experimental phenomena but also provide synthetic guidance. This review focuses on discussing the computational methods that have been used in this field in recent years. The most common and effective method is the complete active space self-consistent field (CASSCF) approach, which predicts mononuclear SMM property very well. For bi- and multi-nuclear SMMs, magnetic exchange needs to be considered, and the exchange coupling constants can be obtained by Monte Carlo (MC) simulation, ab initio calculation via the POLY_ANISO program and density functional theory combined with a broken-symmetry (DFT-BS) approach. Further application for these calculation methods to design high performance SMMs is also discussed.

Cadmium-Inspired Self-Polymerization of {LnIIICd2} Units: Structure, Magnetic and Photoluminescent Properties of Novel Trimethylacetate 1D-Polymers (Ln = Sm, Eu, Tb, Dy, Ho, Er, Yb)

A series of heterometallic carboxylate 1D polymers of the general formula [Ln^IIICd₂(piv)₇(H₂O)₂]_n·nMeCN (Ln^III = Sm (1), Eu (2), Tb (3), Dy (4), Ho (5), Er (6), Yb (7); piv = anion of trimethylacetic acid) was synthesized and structurally characterized. The use of Cd^II instead of Zn^II under similar synthetic conditions resulted in the formation of 1D polymers, in contrast to molecular trinuclear complexes with Ln^IIIZn₂ cores. All complexes 1–7 are isostructural. The luminescent emission and excitation spectra for 2–4 have been studied, the luminescence decay kinetics for 2 and 3 was measured. Magnetic properties of the complexes 3–5 and 7 have been studied; 4 and 7 exhibited the properties of field-induced single-molecule magnets in an applied external magnetic field. Magnetic properties of 4 and 7 were modelled using results of SA-CASSCF/SO-RASSI calculations and SINGLE_ANISO procedure. Based on the analysis of the magnetization relaxation and the results of ab initio calculations, it was found that relaxation in 4 predominantly occurred by the sum of the Raman and QTM mechanisms, and by the sum of the direct and Raman mechanisms in the case of 7.

Azide-Coordination in Homometallic Dinuclear Lanthanide(III) Complexes Containing Nonequivalent Lanthanide Metal Ions: Zero-Field SMM Behavior in the Dysprosium Analogue

A series of homometallic dinuclear lanthanide complexes containing nonequivalent lanthanide metal centers [Ln₂(LH₂)(LH)(CH₃OH)(N₃)]·xMeOH·yH₂O [1, Ln = Dy^III, x = 0, y = 2; 2, Ln = Tb^III, x = 1, y = 1] have been synthesized [LH₄ = 6-((bis(2-hydroxyethyl)amino)-N'-(2-hydroxybenzylidene)picolinohydrazide] and characterized. The dinuclear assembly contains two different types of nine-coordinated lanthanide centers, because the nonequivalent binding of the azide co-ligand as well as the varying coordination of the deprotonated Schiff base ligand. Detailed magnetic studies have been performed on the complexes 1 and 2. Complex 1 and its diluted analogue (1_5%) are zero-field SMMs with effective energy barriers (U_eff) of magnetization reversal equal to 59(3) K and 66(3) K and relaxation times of τ₀ = 10(4) × 10^-6 s and 10(4) × 10^-8 s, respectively. On the other hand, complex 2 shows a field-induced SMM behavior. Combined ab initio and density functional theory calculations were performed to explain the experimental findings and to unravel the nature of the magnetic anisotropy, exchange-coupled spectra, and magnetic exchange interactions between the two lanthanide centers.

Mononuclear Dysprosium Alkoxide and Aryloxide Single-Molecule Magnets

Recent studies have shown that mononuclear lanthanide (Ln) complexes can be high‐performing single‐molecule magnets (SMMs). Recently, there has been an influx of mononuclear Ln alkoxide and aryloxide SMMs, which have provided the necessary geometrical control to improve SMM properties and to allow the intricate relaxation dynamics of Ln SMMs to be studied in detail. Here non‐aqueous Ln alkoxide and aryloxide chemistry applied to the synthesis of low‐coordinate mononuclear Ln SMMs are reviewed. The focus is on mononuclear Dy^III alkoxide and aryloxide SMMs with coordination numbers up to eight, covering synthesis, solid‐state structures and magnetic attributes. Brief overviews are also provided of mononuclear Tb^III, Ho^III, Er^III and Yb^III alkoxide and aryloxide SMMs.

Slow Relaxation of the Magnetization in Anilato-Based Dy(III) 2D Lattices

The search for two- and three-dimensional materials with slow relaxation of the magnetization (single-ion magnets, SIM and single-molecule magnets, SMM) has become a very active area in recent years. Here we show how it is possible to prepare two-dimensional SIMs by combining Dy(III) with two different anilato-type ligands (dianions of the 3,6-disubstituted-2,5-dihydroxy-1,4-benzoquinone: C₆O₄X₂²⁻, with X = H and Cl) in dimethyl sulfoxide (dmso). The two compounds prepared, formulated as: [Dy₂(C₆O₄H₂)₃(dmso)₂(H₂O)₂]·2dmso·18H₂O (1) and [Dy₂(C₆O₄Cl₂)₃(dmso)₄]·2dmso·2H₂O (2) show distorted hexagonal honeycomb layers with the solvent molecules (dmso and H₂O) located in the interlayer space and in the hexagonal channels that run perpendicular to the layers. The magnetic measurements of compounds 1, 2 and [Dy₂(C₆O₄(CN)Cl)₃(dmso)₆] (3), a recently reported related compound, show that the three compounds present slow relaxation of the magnetization. In compound 1 the SIM behaviour does not need the application of a DC field whereas 2 and 3 are field-induced SIM (FI-SIM) since they show slow relaxation of the magnetization when a DC field is applied. We discuss the differences observed in the crystal structures and magnetic properties based on the X group of the anilato ligands (H, Cl and Cl/CN) in 1–3 and in the recently reported derivative [Dy₂(C₆O₄Br₂)₃(dmso)₄]·2dmso·2H₂O (4) with X = Br, that is also a FI-SIM.

Structural Characterization, Magnetic and Luminescent Properties of Praseodymium(III)-4,4,4-Trifluoro-1-(2-Naphthyl)Butane-1,3-Dionato(1-) Complexes

Four new Pr(III) mononuclear complexes of formula [Pr(ntfa)3(MeOH)2] (1), [Pr(ntfa)3(bipy)2] (2), [Pr(ntfa)3(4,4′-Mt2bipy)] (3) and [Pr(ntfa)3(5,5′-Me2bipy)] (4), where ntfa = 4,4,4-trifuoro-1-(naphthalen-2-yl)butane-1,3-dionato(1-), 5,5′-Me2bipy = 5,5′-dimethyl-2,2′-dipyridine, 4,4′-Mt2bipy = 4,4′-dimethoxy-2,2′-dipyridine, have been synthesized and structurally characterized. The complexes display the coordination numbers 8 for 1, 3 and 4, and 10 for 2. Magnetic measurements of complexes 1–4 were consistent with a magnetically uncoupled Pr3+ ion in the 3H4 ground state. The solid state luminescence studies showed that the ancillary chelating bipyridyl ligands in the 2–4 complexes greatly enhance the luminescence emission in the visible and NIR regions through efficient energy transfer from the ligands to the central Pr3+ ion; behaving as “antenna” ligands.

Alignment of axial anisotropy of a mononuclear hexa-coordinated Co(ii) complex in a lattice shows improved single molecule magnetic behavior over a 2D coordination polymer having a similar ligand field

The parallel orientation of the anisotropic axes minimizes the transverse component and slow down the relaxation process and results in a higher energy barrier in 0D complex as compared to 2D framework where anisotropic axes are randomly oriented.

Magnetic investigation in di- and tetranuclear lanthanide complexes

Dinuclear and tetranuclear dysprosium-based complexes have been constructed by using a crab-like hydrazone ligand, with the former acting as a typical single-molecule magnet and the later showing diamagnetic ground state.

A new series of 3D lanthanide phenoxycarboxylates: synthesis, crystal structure, magnetism and photoluminescence studies

A new series of 3D Ln-carboxylates was synthesized, where the Tb one shows antiferromagnetic coupling and the Dy one shows ferromagnetic interaction, and with emission spectra combining the intra-4f emission of the Ln ions and that of the ligand.

Slow magnetic relaxation in dinuclear dysprosium and holmium phenoxide bridged complexes: a Dy2 single molecule magnet with a high energy barrier

High axiality in double unsupported phenoxide-bridged Dy^III and Ho^III complexes promotes slow relaxation of magnetisation and high U_eff of the dysprosium derivative.

Slow Magnetic Relaxation in a One-Dimensional Coordination Polymer Constructed from Hepta-Coordinate Cobalt(II) Nodes

A one-dimensional coordination polymer was synthesized employing hepta-coordinate CoII as nodes and dicyanamide as linkers. Detailed direct current (DC) and alternating current (AC) magnetic susceptibility measurements reveal the presence of field-induced slow magnetic relaxation behavior of the magnetically isolated seven-coordinate CoII center with an easy-plane magnetic anisotropy. Detailed ab initio calculations were performed to understand the magnetic relaxation processes. To our knowledge, the reported complex represents the first example of slow magnetic relaxation in a one-dimensional coordination polymer constructed from hepta-coordinate CoII nodes and dicyanamide linkers.

Influence of magnetic dilution on relaxation processes in a solid solution comprising tetrahedral Co/ZnII complexes

Single ion magnets have long been considered good prospective candidates to record a bit of information. One of the smallest known single ion magnets is CoBr₂(pyridine)₂. This molecular compound exhibits slow relaxation of magnetization mainly due to the thermally activated Orbach process, [A. M. Majcher et al., Chem. Sci., 2018, 9, 7277-7286]. However, the total relaxation time is dramatically shortened at low temperatures due to the direct, Raman, and quantum tunneling of magnetization processes. At low temperatures, the distribution of the probability of the possible relaxation pathways in this case favours QTM and the direct process over the Orbach process. To prolong the relaxation time, the compound was diluted with diamagnetic Zn^II, producing 5 analogues of the general formula: Co_xZn_1-xBr₂(pyridine)₂ (x = 0.91, 0.67, 0.43, 0.24, and 0.06), confirmed to be a solid solution by independent experimental techniques (powder X-ray diffraction, infrared spectroscopy). The presence of diamagnetic Zn^II ions changes the distribution of the dipolar interactions between the Co^II centres in the material, which results in a monotonous change in the relaxation times, which in turn become longer with increasing dilutions, which is explained by the diminishing QTM contribution. The appearance of multiple relaxation processes is also observed for higher x, which is explained as the creation of multiple, separate frequency domains, as a result of the competition between QTM and the direct process contributions. We present a thorough, systematic study of magnetic dilution, which will hopefully be useful to estimate optimal dilutions in similar solid solutions.

Pentagonal Bipyramidal Ln(III) Complexes Containing an Axial Phosphine Oxide Ligand: Field-induced Single-ion Magnetism Behavior of the Dy(III) Analogues

A series of neutral homologous complexes [(L)Ln(Cy₃PO)Cl] {where Ln = Gd (1), Tb (2), Dy (3), and Er (5)} and [(L)Dy(Ph₃PO)Cl] (4) [H₂L = 2,6-diacetylpyridine bis-benzoylhydrazone] were isolated. In these complexes, the central lanthanide ion possesses a pentagonal bipyramidal geometry with an overall pseudo D_5h symmetry. The coordination environment around the lanthanide ion comprises of three nitrogen and two oxygen donors in an equatorial plane. The axial positions are taken up by a phosphine oxide (O donor) and a chloride ion. Among these compounds, the Dy(III) (3 and 4) analogues were found to be field-induced single-ion magnets.

The effect of the electronic structure and flexibility of the counteranions on magnetization relaxation in [Dy(L)2(H2O)5]3+ (L = phosphine oxide derivative) pentagonal bipyramidal SIMs

The effects of the electronic structure and flexibility of triflate anions in a new high-U_eff TBPY-7 SMM, [Dy(OPCy₃)₂(H₂O)₅](CF₃SO₃)₃·2OPCy₃, have been analyzed.

Influence of anion induced geometry change in Zn(ii) on the magnetization relaxation dynamics of Dy(iii) in Zn–Dy–Zn complexes

A series of [Zn₂Dy(L₁)₂(OAc)₄](X) where X = (NO₃)_0.92(Br)_0.08 (1), ClO₄ (2), Cl (3) and PF₆ (4) complexes were synthesized and their dc and ac magnetic data investigated. Experimental results are validated through MOLCAS calculations.

Field-induced slow magnetic relaxation and luminescence thermometry in a mononuclear ytterbium complex

A mononuclear pentagonal bipyramidal Yb^III complex functions as a field-induced SIM luminescent thermometer.

Equatorial coordination optimization for enhanced axiality of mononuclear Dy(iii) single-molecule magnets

The optimization of equatorial sites leads to significant improvement of the magnetic properties of Dy(iii) SMMs.

End-to-End Azido-Bridged Lanthanide Chain Complexes (Dy, Er, Gd, and Y) with a Pentadentate Schiff-Base [N3O2] Ligand: Synthesis, Structure, and Magnetism

The syntheses, structure and magnetic properties are reported for five novel 1D polymeric azido-bridged lanthanide complexes with the general formula {[Ln(DAPMBH)(N₃)C₂H₅OH]C₂H₅OH}_n where H₂DAPMBH = 2,6-diacetylpyridine bis(4-methoxybenzoylhydrazone)-a new pentadentate pyridine-base [N₃O₂] ligand and Ln = Dy (1), Y_0.930Dy_0.070 (2), Er (3), Y_0.923Er_0.077 (4), and Gd (5). X-ray diffraction analysis of 1-5 show that the central lanthanide atoms are eight-coordinated with the N₅O₃ donor set originating from the ligand DAPMBH, one coordinated ethanol molecule and two end-to-end type N₃^- bridges connecting the metal centers into infinite chain. The [LnN₅O₃] coordination polyhedron can be regarded as a distorted dodecahedron (D_2d). AC magnetic measurements revealed that compounds 1-4 show field-induced single-molecule magnet behavior, with estimated energy barriers U_eff ≈ 47-17 K. The experimental study of magnetic properties was complemented by theoretical analysis based on crystal-field calculations. Direct current magnetic susceptibility studies revealed marginally weak intrachain exchange interaction between Ln³⁺ ions mediated by the end-to-end azide bridging groups (J ≈ -0.015 cm^-1 for 5). Comparative analysis of static and dynamic magnetic properties of magnetically concentrated (1, 3) and diluted (2, 4) Dy and Er compounds showed that, despite fascinating 1D azido-bridged chain structure, compounds 1 and 3 are not single-chain magnets; their magnetic behavior is largely due to single-ion magnetic anisotropy of individual Ln³⁺ ions.

Magnetocaloric behavior of REE porphyrin-based paramagnets at room temperature

REE complexes with cyclic aromatic ligands are ranked as the molecular materials with both electronic functionality and a single-molecule magnet behavior at temperatures lower 80 K. At the temperature close to room, they display a positive magnetocaloric effect (MCE) often comparable with one in ferromagnetics. We were determined MCE during the magnetization of both (5,10,15,20-tetra(4-tert-butylphenylporphinato))ytterbium(III)chloride, (Cl)YbT[Formula: see text]BuPP and (5,10,15,20-tetraphenylporphinato)ytterbium(III)chloride, (Cl)YbTPP in their aqueous suspensions over the temperature range of 278–320 K and in magnetic fields from zero to 1 T by the direct microcalorimetric method. Specific heat capacity in the solid of (Cl)YbT[Formula: see text]BuPP/(Cl)YbTPP has been directly determined in zero magnetic fields using differential scanning calorimetry. Thermodynamic parameters of ytterbium(III) complexes magnetization namely enthalpy/entropy change was determined. To improve understanding of the correlation between (Cl)YbT[Formula: see text]BuPP/(Cl)YbTPP magnetic properties and its electronic structure, we have compared magnetic behavior of paramagnets studied with those for (Cl)MTPP where M = Gd, Eu, Tm.

Phosphonate-assisted tetranuclear lanthanide assemblies: observation of the toroidic ground state in the TbIII analogue

The reaction of LnCl3·6H2O with a multidentate flexible Schiff base ligand (LH4), H2O3PtBu and trifluoroacetic acid (tfaH) afforded a series of homometallic tetranuclear complexes, [Ln4(LH2)2(O3PtBu)2(μ2-η1η1tfa)2][2Cl] (Ln = DyIII (1), TbIII (2) and GdIII (3)). The tetranuclear lanthanide core contains two structurally different lanthanide centres, one being in a distorted trigonal dodecahedron geometry and the other in a distorted trigonal prism. Complexes 1-3 were investigated via direct and alternating current (DC and AC) magnetic susceptibility measurements. Only 1 revealed a weak single-molecule magnet (SMM) behaviour. Alternating current (ac) magnetic susceptibility measurements on 1 reveal a frequency-dependent out-of-phase signal. However, the absence of distinct maxima in the χ'' peak (within the temperature/frequency range of our experiments) prevented deduction of the experimental energy barrier for magnetization reversal (Ueff) and the relaxation time. We have carried out extensive ab initio (CASSCF + RASSI-SO + SINGLE_ANISO + POLY_ANISO) calculations on complexes 1-2 to gain deeper insights into the nature of magnetic anisotropy. Our calculations yielded only one exchange coupling parameter between the two LnIII centres bridged by the ligand (neglecting the exchange between the LnIII centres that are not proximal wrt each other). All the extracted J values indicate a weakly antiferromagnetic coupling between the metal centres (J = -0.025 cm-1 for 1 and J = -0.015 cm-1 for 2). Calculated exchange coupled Ucal values of ∼5 and ∼1 cm-1 in 1 and 2 respectively nicely corroborated the experimental observations regarding weak and no SMM characteristics. Our calculations indicated the presence of a net single-molecule toroidal (SMT) behaviour in complex 2. On the other hand, fitting the magnetic data (susceptibility and magnetization) in the isotropic cluster 3 revealed weak AFM exchange couplings of J1 = 0.025 cm-1 and J2 = -0.020 cm-1 which are consistent with those for GdIII ions.

Series of Chloranilate-Bridged Dinuclear Lanthanide Complexes: Kramers Systems Showing Field-Induced Slow Magnetic Relaxation

A series of chloralilate-bridged dinuclear lanthanide complexes of formula [{LnIII(Tp)2}2(μ-Cl2An)]·2CH2Cl2, where Cl2An2− and Tp− represent the chloranilate and hydrotris (pyrazolyl)borate ligands, respectively, and Ln = Gd (1), Tb (2), Ho (3), Er (4), and Yb (5) was synthesized. All five complexes were characterized by an elemental analysis, infrared spectroscopy, single crystal X-ray diffraction, and SQUID measurements. The complexes 1–5 in the series were all isostructural. A comparison of the temperature dependence of the dc magnetic susceptibility data of these complexes revealed clear differences depending on the lanthanide center. Ac magnetic susceptibility measurements revealed that none of the five complexes exhibited a slow magnetic relaxation under a zero applied dc field. On the other hand, the Kramers systems (complexes 4 and 5) clearly displayed a slow magnetic relaxation under applied dc fields, suggesting field-induced single-molecule magnets that occur through Orbach and Raman relaxation processes.

Field Induced Single Ion Magnetic Behaviour in Square-Pyramidal Cobalt(II) Complexes with Easy-Plane Magnetic Anisotropy

Two penta-coordinate CoII complexes with formulae [Co(14-TMC)Cl](BF4) (1, 14-TMC = 1,4,8,11-Tetramethyl-1,4,8,11-tetraazacyclotetradecane) and [Co(12-TBC)Cl](ClO4)·(MeCN) (2, 12-TBC = 1,4,7,10-Tetrabenzyl-1,4,7,10-tetraazacyclododecane) were synthesized and characterized. Structural analysis revealed that ligand coordinates to the CoII centre in a tetradentate fashion and the fifth position is occupied by chloride ion and the geometries around CoII centres are best described as distorted square pyramidal. Detailed magnetic measurements disclose the presence of significant easy-plane magnetic anisotropy and field induced slow magnetic relaxation behaviours of the studied complexes. More insight into the magnetic anisotropy has been given using ab initio theory calculations, which agree well with the experimental values and further confirmed the easy-plane magnetic anisotropy.

Evidencing under-barrier phenomena in a Yb(iii) SMM: a joint luminescence/neutron diffraction/SQUID study

The synthesis, spectroscopic and magnetic characterisation and ab initio investigation of a new Yb based complex, [YbTp₂NO₃], are reported and exploited to rationalise its field-induced SMM behaviour.

Fine-tuning the type of equatorial donor atom in pentagonal bipyramidal Dy(iii) complexes to enhance single-molecule magnet properties

Upon fine-tuning an equatorial donor, the electronic effect is implemented to significantly enhance magnetic anisotropy in pentagonal bipyramidal Dy^III complexes.

The effect of the second coordination sphere on the magnetism of [Ln(NO3)3(H2O)3]·(18-crown-6) (Ln = Dy and Er)

The objective of this work was the exploration of the effect of the second coordination sphere on the magnetic properties of [Ln(NO₃)₃(H₂O)₃]·(18C6) (Ln = Dy (1) and Er (2)) compounds comprising co-crystallized 18-crown-6 ethers. Both compounds were identified as field-induced single molecule magnets (SMMs) with estimated magnetization reversal barriers U_eff = 66–71 K for 1 and U_eff = 21–24 K for 2. Theoretical calculations with the B3LYP functional revealed substantial change and redistribution of the electrostatic potential upon accounting for the second coordination sphere represented by two 18C6 molecules, which resulted in the change of the crystal-field around metal atoms. As a result, the multireference CASSCF calculations exposed significant impact of the second coordination sphere on the energy splitting of the respective ⁶H_15/2 (Dy^III) and ⁴I_15/2 (Er^III) ground states, the magnetization reversal barrier and the magnetic anisotropy parameters. Moreover, the calculated magnetization reversal barriers, U_calc. = 57 K for 1 and U_calc. = 16 K for 2, are in good agreement with the experimental values accentuating the importance of the second coordination sphere on the magnetic properties of SMMs.

The impact of the second coordination sphere on the magnetic properties of [Ln(NO₃)₃(H₂O)₃]·(18C6) compounds comprising co-crystallized 18-crown-6 ethers was investigated.

Boosting axiality in stable high-coordinate Dy(iii) single-molecule magnets

A new nine-coordinate, air-stable Dy(iii) single-ion magnet has been successfully isolated. Our in silico studies demonstrate that through carefully modulating the ligand electronics, the axiality can be boosted to generate Ucal barriers of over 600 K.

Two mononuclear dysprosium(iii) complexes with their slow magnetic relaxation behaviors tuned by coordination geometry

We report here two field-induced single-ion magnets of Dy(iii), which present octahedral and pentagonal–bipyramidal coordination geometries, respectively, with their magnetic performances tuned by the coordination geometries of Dy(iii).

Hybrid organic–inorganic mononuclear lanthanoid single ion magnets

The plasticity of the coordination chemistry of lanthanoid ions has allowed the design and synthesis for the first time of a family of mononuclear hybrid organic–inorganic lanthanoid complexes with slow relaxation of the magnetization.

Heterometallic MIILnIII (M = Co/Zn; Ln = Dy/Y) Complexes with Pentagonal Bipyramidal 3d Centers: Syntheses, Structures, and Magnetic Properties

We herein reported the syntheses, structures, and magnetic properties of three dinuclear heterometallic M^IILn^III complexes, namely, [M^IILn^III(H₂L)(CH₃OH)₂(NO₃)₂](NO₃)·S (M = Co, Ln = Dy, S = MeOH (1_CoDy); M = Zn, Ln = Dy, S = MeOH (2_ZnDy); M = Co, Ln = Y, S = MeNO₂ (3_CoY), H₄L = 2,6-diacetylpyridine bis[2-(semicarbazono) propionylhydrazone]. Synthesized from the predesigned multidentate ligand H₄L, which has two different coordination pockets (smaller N₃O₂ and larger N₂O₄ pockets) suitable for either a 3d or a 4f metal center, all these complexes have very similar structures, where the M^II centers possess a pentagonal bipyramidal (PBP) geometry and the Ln^III sites have a tetradecahedron geometry. Magnetic measurements on these compounds revealed the existence of weak ferromagnetic coupling between the Co²⁺ and Dy³⁺ centers and the field-induced slow magnetic relaxation of all three complexes. Furthermore, theoretical calculation on all these complexes indicates that although the change of the diamagnetic Zn²⁺ ion to the paramagnetic Co²⁺ ion only slightly modifies the local magnetic anisotropy of the Dy³⁺ ion, the weak Co-Dy magnetic interaction decreases the energy barrier. These compounds are the first systematic results of a heterometallic 3d-4f single-molecule magnet containing predesigned PBP 3d metal ions.

Synthesis, structure, photoluminescent, optical and magnetic properties of a novel thulium p-hydroxybenzenesulfonate complex

A novel thulium p-hydroxybenzenesulfonate complex [Tm(C6H5O4S)2(H2O)6](C6H5O4S)·3H2O (1) was solvothermally synthesized and characterized by elemental analyses, photoluminescence, solid-state UV/vis diffuse reflectance, magnetic measurements and single-crystal X-ray diffraction. Complex 1 features an ionic structure with the thulium ion possessing a square antiprismatic geometry. Complex 1 crystallized in the monoclinic system with space group P21. Photoluminescent measurements with solid-state sample demonstrate that the anti-Stokes emission bands in the red/NIR spectral region 710 and 812 nm are observed from the Tm3+ 4f intrashell transitions from the 3F2,3 excited states to the 3H6 and 1G4 excited state to the 3H5 state, respectively. Solid-state UV/vis diffuse reflectance spectra of complex 1 show the existence of a wide optical band gap of 3.56 eV. Variable-temperature magnetic susceptibility and field dependence magnetization measurements are also studied and the magnetic susceptibility obeys the Curie-Weiss law (χ m =c/(T−θ)) with the value C being of 8.6 K and a negative Weiss constant θ being of −0.2 K.