Theoretical exploration of single-molecule magnetic and single-molecule toroic behaviors in peroxide-bridged double-triangular {MII3LnIII3} (M = Ni, Cu and Zn; Ln = Gd, Tb and Dy) complexes

Detailed state-of-the-art ab initio and density functional theory (DFT) calculations have been undertaken to understand both Single-Molecule Magnetic (SMM) and Single-Molecule Toroic (SMT) behaviors of fascinating 3d-4f {M3Ln3} triangular complexes having the molecular formula [MII3LnIII3(O2)L3(PyCO2)3](OH)2(ClO4)2·8H2O (with M = Zn; Ln = Dy (1), Tb (2) & Gd (3) and M = Cu; Ln = Dy (4), Tb (5) & Gd (6)) and [Ni3Ln3(H2O)3(mpko)9(O2)(NO3)3](ClO4)·3CH3OH·3CH3CN (Ln = Dy (7), Tb (8), and Gd (9)) [mpkoH = 1-(pyrazin-2-yl)ethanone oxime]. All these complexes possess a peroxide ligand that bridges the {LnIII3} triangle in a μ3-η3:η3 fashion and the oxygen atoms/oxime of co-ligands that connect each MII ion to the {LnIII3} triangle. Through our computational studies, we tried to find the key role of the peroxide bridge and how it affects the SMM and SMT behavior of these complexes. Primarily, ab initio Complete Active Space Self-Consistent Field (CASSCF) SINGLE_ANISO + RASSI-SO + POLY_ANISO calculations were performed on 1, 2, 4, 5, 7, and 8 to study the anisotropic behavior of each Ln(III) ion, to derive the magnetic relaxation mechanism and to calculate the LnIII-LnIII and CuII/NiII-LnIII magnetic coupling constants. DFT calculations were also performed to validate these exchange interactions (J) by computing the GdIII-GdIII and CuII/NiII-GdIII interactions in 3, 6, and 9. Our calculations explained the experimental magnetic relaxation processes and the magnetic exchange interactions for all the complexes, which also strongly imply that the peroxide bridge plays a role in the SMM behavior observed in these systems. On the other hand, this peroxide bridge does not support the SMT behavior. To investigate the effect of bridging ions in {M3Ln3} systems, we modeled a {ZnII3DyIII3} complex (1a) with a hydroxide ion replacing the bridged peroxide ion in complex 1 and considered a hydroxide-bridged {CoIII3DyIII3} complex (10) having the formula [Co3Dy3(OH)4(OOCCMe3)6(teaH)3(H2O)3](NO3)2·H2O. We discovered that as compared to the LoProp charges of the peroxide ion, the greater negative charges on the bridging hydroxide ion reduce quantum tunneling of magnetization (QTM) effects, enabling more desirable SMM characteristics and also leading to good SMT behavior.

A Multiconfigurational Wave Function Theoretical Study on Electronic Structure and Magnetic Susceptibility of Dilanthanide Single Molecule Magnet

Single molecule magnets (SMMs) have been a promising material for next-generation high-density information storage and molecular spintronics. N23--bridged dilanthanide complexes, {[(Me3Si)2N]2Ln(THF)(μ-η2:η2-N2)(THF)Ln[(Me3Si)2N]2}1-, exhibit high blocking temperatures and have been one of the promising candidates for future application. Rational understanding should be established between the magnetic properties and electronic structure. However, the theoretical study is still challenging due to the complexities in their electronic structures. Here, we theoretically studied the magnetic susceptibility of dilanthanide SMMs based on the state-of-the-art multistate-complete active space self-consistent field and perturbation theory at the second order and restricted active space state interaction with spin-orbit coupling calculations. Temperature dependence of the magnetic susceptibility (χmT-T curve) was quantitatively reproduced by the theoretical calculations. The complexities in the electronic states of these dilanthanide complexes originate from significantly strong static electron correlations in the lanthanide 4f and N2 π* orbitals and the SOC effect. The temperature dependence of the magnetic susceptibility results from the energy levels and magnetic properties of the low-lying excited state. The χmT values below 50 K are dominated by the ground state, while thermal distribution in the low-lying excited state affects the χmT values over 50 K. Saturation magnetization at low temperatures was also evaluated, and the result agrees with the experimental observation.

Strategies to quench quantum tunneling of magnetization in lanthanide single molecule magnets

Enhancing blocking temperature (T_B) is one of the holy grails in Single Molecule Magnets(SMMs), as any future potential application in this class of molecules is directly correlated to this parameter. Among many factors contributing to a reduction of T_B value, Quantum Tunnelling of Magnetisation (QTM), a phenomenon that is a curse or a blessing based on the application sought after, tops the list. Theoretical tools based on density functional and ab initio CASSCF/RASSI-SO methods have played a prominent role in estimating various spin Hamiltonian parameters and establishing the mechanism of magnetization relaxation in this class of molecules. Particularly, various strategies to quench QTM effects go hand-in-hand with experiments, and different methods proposed to quell QTM effects are scattered in the literature. In this perspective, we have explored various approaches that are proposed in the literature to quench QTM effects, and these include the role of (i) local symmetry of lanthanides, (ii) super-exchange interaction in {3d-4f} complexes, (iii) direct-exchange interaction in {radical-4f} and metal-metal bonded complexes to suppress the QTM, (iv) utilizing external stimuli such as an electric field or pressure to modulate the QTM and (v) avoiding QTM effects by stabilising toroidal states in 4f and {3d-4f} clusters. We believe the strategies summarized here will help to design new-generation SMMs.

Are lanthanide-transition metal direct bonds a route to achieving new generation {3d-4f} SMMs?

Lanthanide based single-molecule magnets are gaining wide attention due to their potential applications in emerging technologies. One of the main challenges in this area is quenching quantum tunnelling of magnetisation (QTM), which often undercuts the magnetisation reversal barrier. Among the several strategies employed, enhancing exchange coupling has been studied in detail, with large exchanges resulting in stronger quenching of QTM effects. Lanthanides, however, suffer from weak exchanges offered by the deeply buried 4f orbitals and the numerous attempts to enhance the exchange coupling in the {3d-4f} pairs have not exceeded values larger than 30 cm^-1. In this work, using a combination of DFT and the ab initio CASSCF/RASSI-SO method, we have explored lanthanide-transition metal direct bonds as a tool to quench QTM effects. In this direction, we have modelled [PyCp₂LnMCp(CO)₂] (Ln = Gd(III), Dy(III), and Er(III) and M = V(0), Mn(0), Co(0) and Fe(I) and here PyCp₂ = [2,6-(CH₂C₅H₃)₂C₅H₃N]₂^- using [PyCp₂DyFeCp(CO)₂] as an example as reported by Nippe et al. (C. P. Burns, X. Yang, J. D. Wofford, N. S. Bhuvanesh, M. B. Hall and M. Nippe, Angew. Chem., Int. Ed. 2018, 57, 8144). Bonding analysis reveals a dative Ln-TM bond with a donation of π(V/Mnd_xy-π*CO) to 5d_z² (Gd) in the case of Gd-V and Gd-Mn and 4s(Co) to 5d_xy/5d_yz (Gd) for Gd-Co with the transition metal ion being found in the low-spin S = ½ configurations in all the cases. B3LYP/TZV (Gd;CSDZ) calculations on [PyCp₂GdMCp(CO)₂] yield J_Gd-V = -46.1 cm^-1, J_Gd-Mn = -57.1 cm^-1, J_Gd-Co = +55.3 cm^-1, J_Gd-Fe⁺ = +13.9 cm^-1, J_Gd-Vhs = -162.1 cm^-1 and J_Gd-Mnhs = -343.9 cm^-1 and unveiling record-high J values for {3d-4f} complexes. The mechanism of magnetic coupling is developed, which discloses the dominating presence of strong 3d-4f orbital overlaps in most of the cases studied, leading to antiferromagnetic exchange. When these overlaps are weaker and 3d to Gd(5d_z²), charge transfer dominates, yielding a ferromagnetic coupling for the Gd-Co/Gd-Fe⁺ complexes. Calculations performed on the anisotropic Dy(III) and Er(III) complexes reveal that the ground state g_zz axis lies along the Cp-Ln-Cp axis and the Ln-TM bonds, respectively. Thus the Ln-TM bond hinders the single-ion anisotropy of Dy(III) by offering equatorial ligation and lowering the m_J = ±½ state energy, and at the same time, helping in enhancing the axiality of Er(III). When strong {3d-4f} exchange couplings are introduced, record-high barrier heights as high as 229 cm^-1 were accomplished. Furthermore, the exchange coupling annihilates the QTM effects and suggests the lanthanide-transition metal direct bond as a viable alternative to enhance exchange coupling to bring {3d-4f} complexes back in the race for high-blocking SMMs.

Enhancing the barrier height for magnetization reversal in 4d/4f RuIII2LnIII2 "butterfly" single molecule magnets (Ln = Gd, Dy) via targeted structural alterations

A series of 4d-4f {RuIII2DyIII2} and {RuIII2GdIII2} 'butterfly' (rhombohedral) complexes have been synthesized and characterized and their magnetic properties investigated. Earlier, we have reported the first 4d/4f SMM - [RuIII2DyIII2(OMe)₂(O₂CPh)₄(mdea)₂(NO₃)₂] (1Dy) with a U_eff value of 10.7 cm^-1. As the structural distortion around the Dy^III centres and the Ru^III⋯Dy^III exchange interactions are key to enhancing the anisotropy, in this work we have synthesised three more {Ru₂Dy₂} butterfly complexes where structural alteration around the Dy^III centres and alterations to the bridging groups are performed with an aim to improve the magnetic properties. The new complexes reported here are [Ru₂Dy₂(OMe)₂(O₂C(4-Me-Ph)₄(mdea)₂(MeOH)₄], 2Dy, [Ru₂Dy₂(OMe)₂(O₂C(2-Cl,4,5-F-Ph)₄(mdea)₂(NO₃)₂], 3Dy, and an acac derivative [Ru₂Dy₂(OMe)₂(acac)₄(NO₃)₂(edea)₂], 4Dy, where acac^- = acetylacetonate, edea^2- = N-ethyldiethanolamine dianion. Complex 2Dy describes alteration in the Dy^III centers, while complexes 3Dy and 4Dy are aimed to alter the Ru^III⋯Dy^III exchange pathways. To ascertain the 4d-4f exchange, the Gd-analogues of 1Dy and 4Dy were synthesised [Ru₂Gd₂(OMe)₂(O₂CPh)₄(mdea)₂(NO₃)₂], 1Gd, [Ru₂Gd₂(OMe)₂(acac)₄(NO₃)₂(edea)₂], 4Gd. Both ac and dc susceptibility studies were performed on all these complexes, and out-of-phase signals were observed for 3Dy in zero-field while 2Dy and 4Dy show out-of-phase signals in the presence of an applied field. Complex 3Dy reveals a barrier height U_eff of 45 K. To understand the difference in the magnetic dynamic behavior compared to our earlier reported {RuIII2DyIII2} analogue, detailed theoretical calculations based on ab initio CASSCF/RASSI-SO calculations have been performed. Calculations reveal that the J_Ru⋯Dy value varies from -1.8 cm^-1 (4Dy) to -2.4 cm^-1 (3Dy). These values are also affirmed by DFT calculations performed on the corresponding Gd^III analogues. The origin of the largest barrier and observation of slow magnetic relaxation in 3Dy is routed back to the stronger single-ion anisotropy and stronger J_Ru⋯Dy exchange which quenches the QTM effects more efficiently. This study thus paves the way forward to tune local structure around the Ln^III center and the exchange pathway to enhance the SMM characteristics in other {3d-4f}/{4d-4f} SMMs.

Enhancing Magnetic Communication between Metal Centres: The Role of s-Tetrazine Based Radicals as Ligands

Although 1,2,4,5-tetrazines or s-tetrazines have been known in the literature for more than a century, their coordination chemistry has become increasingly popular in recent years due to their unique redox activity, multiple binding sites and their various applications. The electron-poor character of the ring and stabilization of the radical anion through all four nitrogen atoms in their metal complexes provide new aspects in molecular magnetism towards the synthesis of new high performing Single Molecule Magnets (SMMs). The scope of this review is to examine the role of s-tetrazine radical ligands in transition metal and lanthanide based SMMs and provide a critical overview of the progress thus far in this field. As well, general synthetic routes and new insights for the preparation of s-tetrazines are discussed, along with their redox activity and applications in various fields. Concluding remarks along with the limitations and perspectives of these ligands are discussed.

Heteroleptic Samarium(III) Chalcogenide Complexes: Opportunities for Giant Exchange Coupling in Bridging σ- and π-Radical Lanthanide Dichalcogenides

The introduction of (N2)3-• radicals into multinuclear lanthanide molecular magnets raised hysteresis temperatures by stimulating strong exchange coupling between spin centers. Radical ligands with larger donor atoms could promote more efficient magnetic coupling between lanthanides to provide superior magnetic properties. Here, we show that heavy chalcogens (S, Se, Te) are primed to fulfill these criteria. The moderately reducing Sm(II) complex, [Sm(N††)2], where N†† is the bulky bis(triisopropylsilyl)amide ligand, can be oxidized (i) by diphenyldichalcogenides E2Ph2 (E = S, Se, Te) to form the mononuclear series [Sm(N††)2(EPh)] (E = S, 1-S; Se, 1-Se, Te, 1-Te); (ii) S8 or Se8 to give dinuclear [{Sm(N††)2}2(μ-η2:η2-E2)] (E = S, 2-S2; Se, 2-Se2); or (iii) with Te═PEt3 to yield [{Sm(N††)2}(μ-Te)] (3). These complexes have been characterized by single crystal X-ray diffraction, multinuclear NMR, FTIR, and electronic spectroscopy; the steric bulk of N†† dictates the formation of mononuclear complexes with chalcogenate ligands and dinuclear species with the chalcogenides. The Lα1 fluorescence-detected X-ray absorption spectra at the Sm L3-edge yielded resolved pre-edge and white-line peaks for 1-S and 2-E2, which served to calibrate our computational protocol in the successful reproduction of the spectral features. This method was employed to elucidate the ground state electronic structures for proposed oxidized and reduced variants of 2-E2. Reactivity is ligand-based, forming species with bridging superchalcogenide (E2)-• and subchalcogenide (E2)3-• radical ligands. The extraordinarily large exchange couplings provided by these dichalcogenide radicals reveal their suitability as potential successors to the benchmark (N2)3-• complexes in molecular magnets.

Probing the strong magnetic exchange behaviour of transition metal–radical complexes: a DFT case study

Alteration of the structural parameters of metal–radical complexes may pave the way forward for fine tuning the magnetic exchange coupling value as high as >−500 cm⁻¹ – a much sought-after parameter in the area of SMMs.

Molecular and electronic structure of the dithiooxalato radical ligand stabilised by rare earth coordination

Heterometallic rare earth transition metal compounds of dithioxalate (dto)2-, [NiII{(dto)LnIIITp2}2] (Ln = Y (1), Gd (2); Tp = hydrotris(pyrazol-1-yl)borate) were synthesised. The Lewis acidic rare earth ions are bound to the dioxolene and chemical reduction of 1 and 2 with cobaltocene yielded [CoCp2]+[NiII{(dto)LnIIITp2}2]˙- Ln = Y (3), Gd (4). The reduction is ligand-based and 3 and 4 are the first examples of both molecular and electronic structural characterisation of the dithiooxalato radical (dto)3˙-.

Hard versus soft: zero-field dinuclear Dy(iii) oxygen bridged SMM and theoretical predictions of the sulfur and selenium analogues

Two dinuclear lanthanide complexes (Gd and Dy) were prepared and characterized by X-ray, magnetic and computational methods. The Dy analogue shows SMM behavior with an energy barrier of 98.4 K in the absence of an applied dc field. Theoretical calculations were performed on model complexes which support the hypothesis that the energy barrier will increase if the soft-donor atoms S and Se are used in lieu of an O donor.

Functionalization of Endohedral Metallofullerenes toward Improving Barrier Height for the Relaxation of Magnetization for Dy2@C80-X (X = CF3, C3N3Ph2)

We theoretically studied the electronic and magnetic properties of the exterior functionalized endohedral metallofullerenes (EMFs) of Gd₂@ I _h-C₈₀-X (where X is the exterior functional group). Molecular orbital analysis suggests that the presence of unpaired electron on the I _h-C₈₀ cage is not favoring the observation of stable species. One of the effective strategies to address this problem is by attaching an exterior functional group to the fullerene cage. Out of the studied exterior functionalized EMFs, we were successful in finding two stable species such as Gd₂@ I _h-C₈₀-CF₃ and Gd₂@ I _h-C₈₀-C₃N₃Ph₂ with no unpaired spin on the cage. Further, we utilized exterior functional groups such as -CF₃ (1) and -C₃N₃Ph₂ (2) to model and to stabilize dinuclear Dy₂@ I _h-C₈₀ species, and we thoroughly investigated their magnetic properties using ab initio calculations. Within the single-ion paradigm, Dy^III ions in 1 and 2 are magnetically anisotropic, and their magnetization-reversal energy barriers are estimated to be ∼698 and ∼705 cm^-1, respectively. Furthermore, beyond the single-ion paradigm, i.e., considering a ferromagnetic coupling (∼30 cm^-1) between the lanthanide ions and the radical spin, the energy barriers of 1 and 2 are estimated to be 79.8 and 73.0 cm^-1, respectively.

Solvent-tuned magnetic exchange interactions in Dy2 systems ligated by a μ-phenolato heptadentate Schiff base

A series of binuclear dysprosium compounds, namely, [Dy(api)]₂ (1), [Dy(api)]₂·2CH₂Cl₂ (2), [Dy(Clapi)]₂·2C₄H₈O (3), and [Dy(Clapi)]₂·2C₃H₆O (4) (H₃api = 2-(2-hydroxyphenyl)-1,3-bis[4-(2-hydroxyphenyl)-3-azabut-3-enyl]-1,3-imidazoline; H₃Clapi = 2-(2′-hydroxy-5′-chlorophenyl)-1,3-bis[3′-aza-4′-(2′′-hydroxy-5′′-chlorophenyl)prop-4′-en-1′-yl]-1,3-imidazolidine), have been isolated by the reactions of salen-type ligands H₃api/H₃Clapi with DyCl₃·6H₂O in different solvent systems. Structural analysis reveals that each salen-type ligand provides a heptadentate coordination pocket (N₄O₃) to encapsulate a Dy^III ion and all of the Dy^III centers in 1–4 adopt a distorted square antiprism geometry with D_4d symmetry. Magnetic studies showed that compound 1 did not exhibit single-molecule magnetic (SMMs) behavior. With the introduction of different lattice solvents, compounds 2–4 showed filed-induced slow magnetic relaxation with barriers U_eff of 18.2 K (2), 28.0 K (3) and 16.4 K (4), respectively. Ab initio calculations were employed to interpret the magnetization behavior of 1–4. The combination of experimental and theoretical data reveal the importance of the weak exchange interaction between the Dy^III ions in the observation of slow magnetic relaxation, and a relaxation mechanism has been developed to rationalize the observed difference in the U_eff values. The different lattice solvents influence Dy–O–Dy bond angles and thus alter the torsion of the square antiprism geometry, consequently resulting in distinct magnetic interactions and the magnetic behavior.

Solvent-tuning changes the magnetic exchange interaction and results in different magnetic relaxation dynamics in Dy₂ systems ligated by a μ-phenolato heptadentate Schiff base.

Enriching lanthanide single-ion magnetism through symmetry and axiality

Rapidly growing modern information technology demands energy and cost efficient tools that can efficiently store and process a large amount of data. However, the miniaturization technology that was being used to boost the performance of the electronic devices, keeping up with the pace as estimated by Moore's law, is reaching its limit. To overcome these challenges, several alternative routes that can eventually mimic the modern electronics fabrication using silicon have been proposed. Single molecule magnets (SMMs), being considered as one of the potential alternatives, have gone through significant progress and the focus has shifted from the use of polynuclear clusters to mononuclear complexes in the last few years. The recent frenzy in the field of SMMs is driven by a better understanding of the effects of crystal field (CF) and molecular symmetry on the magnetic properties, especially in the case of mononuclear paramagnetic complexes, apart from other controlling factors. This has led to the advent of highly anisotropic single-ion magnets (SIMs) with magnetic blocking temperatures as high as 60 K and anisotropic energy barriers over 1800 K. This article overviews our recent research in the light of the emergence of the importance of CF and symmetry in 4f ion based single-ion magnets (SIMs), especially in the context of SIMs with D5h symmetry, apart from commenting on the synthetic efforts adopted to place these metal ions in unusual coordination geometries.

Syntheses, crystal structures, magnetic properties and ESI-MS studies of a series of trinuclear CuIIMIICuII compounds (M = Cu, Ni, Co, Fe, Mn, Zn)

Six trinuclear CuIIMIICuII compounds (M = Cu, Ni, Co, Fe, Mn, Zn) derived from the Schiff base ligand, H2L (2 + 1 condensation product of salicylaldehyde and trans-1,2-diaminocyclohexane) are reported in this investigation. The composition of the metal complexes are [{CuIIL(ClO4)}2CuII(H2O)]·2H2O (1), [{CuIIL(ClO4)}{NiII(H2O)2}{CuIIL}]ClO4·CH3COCH3 (2), [{CuIIL(ClO4)}{CoII(CH3COCH3)(H2O)}{CuIIL(CH3COCH3)}]ClO4 (3) and isomorphic [{CuIIL(ClO4)}2MII(CH3OH)2] (4, M = Fe; 5, M = Mn; 6, M = Zn). Two copper(ii) ions in 1-6 occupy N2O2 compartments of two L2- ligands, while the second metal ion occupies the O(phenoxo)4 site provided by the two ligands, i.e., the two metal ions in both CuIIMII pairs are diphenoxo-bridged. Positive ESI-MS of 1-6 reveals some interesting features. Variable-temperature and variable-field magnetic studies reveal moderate or weak antiferromagnetic interactions in 1-6 with the following values of magnetic exchange integrals (H = -2JS 1 S 2 type): J 1 = -136.50 cm-1 and J = 0.00 for the CuIICuIICuII compound 1; J 1 = -22.16 cm-1 and J = -1.97 cm-1 for the CuIINiIICuII compound 2; J 1 = -14.78 cm-1 and J = -1.86 cm-1 for the CuIICoIICuII compound 3; J 1 = -6.35 cm-1 and J = -1.17 cm-1 for the CuIIFeIICuII compound 4; J 1 = -6.02 cm-1 and J = -1.70 cm-1 for the CuIIMnIICuII compound 5; J = -2.25 cm-1 for the CuIIZnIICuII compound 6 (J is between two CuII in the N2O2 compartments; J 1 is between CuII and MII through a diphenoxo bridge).

Modelling spin Hamiltonian parameters of molecular nanomagnets

Molecular nanomagnets encompass a wide range of coordination complexes possessing several potential applications. A formidable challenge in realizing these potential applications lies in controlling the magnetic properties of these clusters. Microscopic spin Hamiltonian (SH) parameters describe the magnetic properties of these clusters, and viable ways to control these SH parameters are highly desirable. Computational tools play a proactive role in this area, where SH parameters such as isotropic exchange interaction (J), anisotropic exchange interaction (Jx, Jy, Jz), double exchange interaction (B), zero-field splitting parameters (D, E) and g-tensors can be computed reliably using X-ray structures. In this feature article, we have attempted to provide a holistic view of the modelling of these SH parameters of molecular magnets. The determination of J includes various class of molecules, from di- and polynuclear Mn complexes to the {3d-Gd}, {Gd-Gd} and {Gd-2p} class of complexes. The estimation of anisotropic exchange coupling includes the exchange between an isotropic metal ion and an orbitally degenerate 3d/4d/5d metal ion. The double-exchange section contains some illustrative examples of mixed valance systems, and the section on the estimation of zfs parameters covers some mononuclear transition metal complexes possessing very large axial zfs parameters. The section on the computation of g-anisotropy exclusively covers studies on mononuclear Dy(III) and Er(III) single-ion magnets. The examples depicted in this article clearly illustrate that computational tools not only aid in interpreting and rationalizing the observed magnetic properties but possess the potential to predict new generation MNMs.

Understanding the Mechanism of Magnetic Relaxation in Pentanuclear {MnIVMnIII2LnIII2} Single-Molecule Magnets

A new family of heterometallic pentanuclear complexes of formulas [Mn^IVMn^III₂Ln^III₂O₂(benz)₄(mdea)₃(NO₃)₂(MeOH)] (Ln = Dy (1-Dy), Tb (2-Tb), Gd (3-Gd), Eu (4-Eu), Sm (5-Sm), Nd (6-Nd), Pr (7-Pr); benz(H) = benzoic acid; mdeaH₂= N-methyldiethanolamine) and [Mn^IVMn^III₂Ln^III₂O₂(o-tol)₄(mdea)₃(NO₃)₂(MeOH)] (Ln = Gd (8-Gd), Eu (9-Eu); o-tol(H) = o-toluic acid) have been isolated and structurally, magnetically, and theoretically characterized. dc magnetic susceptibility measurements reveal dominant antiferromagnetic magnetic interactions for each complex, except for 2-Tb and 3-Gd, which reveal an upturn in the χ_MT product at low temperatures. The magnetic interactions between the spin centers in the Gd derivatives, 3-Gd and 8-Gd, which display markedly different χ_MT vs T profiles, were found to be due to the interactions of the Gd^III-Gd^III ions which change from ferromagnetic (3-Gd) to antiferromagnetic (8-Gd) due to structural differences. ac magnetic susceptibility measurements reveal a nonzero out-of-phase component for 1-Dy and 7-Pr, but no maxima were observed above 2 K (H_dc = 0 Oe), which suggests single-molecule magnet (SMM) behavior. Out-of-phase signals were observed for complexes 2-Tb, 4-Eu, 8-Gd, and 9-Eu, in the presence of a static dc field (H_dc = 2000, 3000 Oe). The anisotropic nature of the lanthanide ions in the benzoate series (1-Dy, 2-Tb, 5-Sm, 6-Nd, and 7-Pr) were thoroughly investigated using ab initio methods. CASSCF calculations predict that the origin of SMM behavior in 1-Dy and 7-Pr and the applied field SMM behavior in 2-Tb does not solely originate from the single-ion anisotropy of the lanthanide ions. To fully understand the relaxation mechanism, we have employed the Lines model to fit the susceptibility data using the POLY_ANISO program, which suggests that the zero-field SMM behavior observed in complexes 1-Dy and 7-Pr is due to weak Mn^III/IV-Ln^III and Ln^III-Ln^III couplings and an unfavorable Ln^III/Mn^III/Mn^IV anisotropy. In complexes 4-Eu, 8-Gd, and 9-Eu ab initio calculations indicate that the anisotropy of the Mn^III ions solely gives rise to the possibility of SMM behavior. Complex 7-Pr is a Pr(III)-containing complex that displays zero-field SMM behavior, which is rare, and our study suggests the possibility of coupling weak SOC lanthanide metal ions to anisotropic transition-metal ions to derive SMM characteristics; however, enhancing the exchange coupling in {3d-4f} complexes is still a stubborn hurdle in harnessing new generation {3d-4f} SMMs.

Experimental and theoretical exploration of magnetic exchange interactions and single-molecule magnetic behaviour of bis(η1:η2:μ2-carboxylate)GdIII2/DyIII2 systems

This investigation demonstrates differences in SMM properties and nature of magnetic exchange in closely related DyIII2/GdIII2 compounds.

A nickel(ii)–manganese(ii)-azido layered coordination polymer showing a three-dimensional ferrimagnetic order at 35 K

The synthesis, crystal structure and magnetic properties of a two-dimensional nickel(ii)–manganese(ii)–azido system are described. It is an interesting molecular magnet because it exhibits long-range ferrimagnetic ordering at a T_c of 35 K and a hysteresis loop up to 20 K.

Elucidation of Dual Magnetic Relaxation Processes in Dinuclear Dysprosium(III) Phthalocyaninato Triple-Decker Single-Molecule Magnets Depending on the Octacoordination Geometry

When applying single-molecule magnets (SMMs) to spintronic devices, control of the quantum tunneling of the magnetization (QTM) as well as a spin-lattice interactions are important. Attempts have been made to use not only coordination geometry but also magnetic interactions between SMMs as an exchange bias. In this manuscript, dinuclear dysprosium(III) (Dy^III ) SMMs with the same octacoordination geometry undergo dual magnetic relaxation processes at low temperature. In the dinuclear Dy^III phthalocyaninato (Pc^2- ) triple-decker type complex [(Pc)Dy(ooPc)Dy(Pc)] (1) (ooPc^2- =2,3,9,10,16,17,23,24-octakis(octyloxy)phthalocyaninato) with a square-antiprismatic (SAP) geometry, the ground state is divided by the Zeeman effect, and level intersection occurs when a magnetic field is applied. Due to the ground state properties of 1, since the Zeeman diagram where the levels intersect in an H_dc of 2500 Oe, two kinds of QTM and direct processes occur. However, dinuclear Dy^III -Pc systems with C₄ geometry, which have a twist angle (ϕ) of less than 45° do not undergo dual magnetic relaxation processes. From magnetic field and temperature dependences, the dual magnetic relaxation processes were clarified.

Strong Ferromagnetic Exchange Coupling Mediated by a Bridging Tetrazine Radical in a Dinuclear Nickel Complex

The radical bridged compound [(Ni(TPMA))₂-μ-bmtz^•-](BF₄)₃·3CH₃CN (bmtz = 3,6-bis(2'-pyrimidyl)-1,2,4,5-tetrazine, TPMA = tris(2-pyridylmethyl)amine) exhibits strong ferromagnetic exchange between the S = 1 Ni^II centers and the bridging S = 1/2 bmtz radical with J = 96 ± 5 cm^-1 (-2J_Ni-radS_NiS_rad). DFT calculations support the existence of strong ferromagnetic exchange.

Quenching the Quantum Tunneling of Magnetization in Heterometallic Octanuclear {TMIII4 DyIII4 } (TM=Co and Cr) Single-Molecule Magnets by Modification of the Bridging Ligands and Enhancing the Magnetic Exchange Coupling

We report the synthesis, structural characterisation, magnetic properties and provide an ab initio analysis of the magnetic behaviour of two new heterometallic octanuclear coordination complexes containing Co^III and Dy^III ions. Single-crystal X-ray diffraction studies revealed molecular formulae of [Co^III₄ Dy^III₄ (μ-OH)₄ (μ₃ -OMe)₄ {O₂ CC(CH₃ )₃ }₄ (tea)₄ (H₂ O)₄ ]⋅4 H₂ O (1) and [Co^III₄ Dy^III₄ (μ-F)₄ (μ₃ -OH)₄ (o-tol)₈ (mdea)₄ ]⋅ 3 H₂ O⋅EtOH⋅MeOH (2; tea^3- =triply deprotonated triethanolamine; mdea^2- =doubly deprotonated N-methyldiethanolamine; o-tol=o-toluate), and both complexes display an identical metallic core topology. Furthermore, the theoretical, magnetic and SMM properties of the isostructural complex, [Cr^III₄ Dy^III₄ (μ-F₄ )(μ₃ -OMe)_1.25 (μ₃ -OH)_2.75 (O₂ CPh)₈ (mdea)₄ ] (3), are discussed and compared with a structurally similar complex, [Cr^III₄ Dy^III₄ (μ₃ -OH)₄ (μ-N₃ )₄ (mdea)₄ (O₂ CC(CH₃ )₃ )₄ ] (4). DC and AC magnetic susceptibility data revealed single-molecule magnet (SMM) behaviour for 1-4. Each complex displays dynamic behaviour, highlighting the effect of ligand and transition metal ion replacement on SMM properties. Complexes 2, 3 and 4 exhibited slow magnetic relaxation with barrier heights (U_eff ) of 39.0, 55.0 and 10.4 cm^-1 respectively. Complex 1, conversely, did not exhibit slow relaxation of magnetisation above 2 K. To probe the variance in the observed U_eff  values, calculations by using CASSCF, RASSI-SO and POLY_ANISO routine were performed on these complexes to estimate the nature of the magnetic coupling and elucidate the mechanism of magnetic relaxation. Calculations gave values of J_Dy-Dy as -1.6, 1.6 and 2.8 cm^-1 for complexes 1, 2 and 3, respectively, whereas the J_Dy-Cr interaction was estimated to be -1.8 cm^-1 for complex 3. The developed mechanism for magnetic relaxation revealed that replacement of the hydroxide ion by fluoride quenched the quantum tunnelling of magnetisation (QTM) significantly, and led to improved SMM properties for complex 2 compared with 1. However, the tunnelling of magnetisation at low-lying excited states was still operational for 2, which led to low-temperature QTM relaxation. Replacement of the diamagnetic Co^III ions with paramagnetic Cr^III led to Cr^III ⋅⋅⋅Dy^III coupling, which resulted in quenching of QTM at low temperatures for complexes 3 and 4. The best example was found if both Cr^III and fluoride were present, as seen for complex 3, for which both factors additively quenched QTM and led to the observation of highly coercive magnetic hysteresis loops above 2 K. Herein, we propose a synthetic strategy to quench the QTM effects in lanthanide-based SMMs. Our strategy differs from existing methods, in which parameters such as magnetic coupling are difficult to control, and it is likely to have implications beyond the Dy^III SMMs studied herein.

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.

Modulating spin dynamics of binuclear LnIII–radical complexes by using different indazole radicals

Four Ln^III–indazole–radical complexes have been synthesized. By changing the location of the radical in the indazole ring, they exhibit interesting magnetic properties.

Giant exchange interaction in mixed lanthanides

Combining strong magnetic anisotropy with strong exchange interaction is a long standing goal in the design of quantum magnets. The lanthanide complexes, while exhibiting a very strong ionic anisotropy, usually display a weak exchange coupling, amounting to only a few wavenumbers. Recently, an isostructural series of mixed (Ln = Gd, Tb, Dy, Ho, Er) have been reported, in which the exchange splitting is estimated to reach hundreds wavenumbers. The microscopic mechanism governing the unusual exchange interaction in these compounds is revealed here by combining detailed modeling with density-functional theory and ab initio calculations. We find it to be basically kinetic and highly complex, involving non-negligible contributions up to seventh power of total angular momentum of each lanthanide site. The performed analysis also elucidates the origin of magnetization blocking in these compounds. Contrary to general expectations the latter is not always favored by strong exchange interaction.

An air-stable Dy(iii) single-ion magnet with high anisotropy barrier and blocking temperature

A mononuclear Dy(iii) complex assembled just from five water molecules and two phosphonic diamide ligands combines the advantages of high anisotropy barrier, high blocking temperature and significant coercivity, apart from its remarkable air- and moisture-stability.

Herein we report air-stable Dy(iii) and Er(iii) single-ion magnets (SIMs) with pseudo-D_5h symmetry, synthesized from a sterically encumbered phosphonamide, ^tBuPO(NHⁱPr)₂, where the Dy(iii)-SIM exhibits a magnetization blocking (T_B) up to 12 K, defined from the maxima of the zero-field cooled magnetization curve, with an anisotropy barrier (U_eff) as high as 735.4 K. The Dy(iii)-SIM exhibits a magnetic hysteresis up to 12 K (30 K) with a large coercivity of ∼0.9 T (∼1.5 T) at a sweep rate of ∼0.0018 T s^–1 (0.02 T s^–1). These high values combined with persistent stability under ambient conditions, render this system as one of the best-characterized SIMs. Ab initio calculations have been used to establish the connection between the higher-order symmetry of the molecule and the quenching of quantum tunnelling of magnetization (QTM) effects. The relaxation of magnetization is observed via the second excited Kramers doublet owing to pseudo-high-order symmetry, which quenches the QTM. This study highlights fine-tuning of symmetry around the lanthanide ion to obtain new-generation SIMs and offers further scope for pushing the limits of U_eff and T_B using this approach.

A rare doubly nitrato and phenoxido bridged trimetallic CuII complex: EPR, antiferromagnetic coupling and theoretical rationalization

A new homometallic trinuclear Cu(ii) complex is afforded with a tetradentate Schiff base precursor. Complex 1 manifests a strong antiferromagnetic coupling which is in agreement with EPR and DFT computation to explain the magneto-structural correlation.