Role of Magnetic Exchange Interactions in the Magnetization Relaxation of {3d-4f} Single-Molecule Magnets: A Theoretical Perspective

Chiral Star-Shaped [CoIII3LnIII] Clusters with Enantiopure Schiff Bases: Synthesis, Structure, and Magnetism

Two enantiomeric pairs of new 3d-4f heterometallic clusters have been synthesized from two enantiomer Schiff base derivatives: (R/S)-2-[(2-hydroxy-1-phenylethylimino)methyl] phenol (R-/S-H2L). The formulae of the series clusters are Co3Ln(R-L)6 (Ln = Dy (1R), Gd (2R)), Co3Ln (S-L)6 (Ln = Dy (1S), Gd (2S)), whose crystal structures and magnetic properties have been characterized. Structural analysis indicated that the above clusters crystallize in the chiral P213 group space. The central lanthanide ion has a coordination geometry of D3 surrounded by three [CoIII(L)2]- anions using six aliphatic oxygen atoms of L2- featuring a star-shaped [CoIII3LnIII] configuration. Magnetic measurements showed the presence of slow magnetic relaxation with an effective energy barrier of 22.33 K in the DyIII derivatives under a zero-dc field. Furthermore, the circular dichroism (CD) spectra of 1R and 1S confirmed their enantiomeric nature.

Halide mediated modulation of magnetic interaction and anisotropy in dimeric Co(II) complexes

The burgeoning interest in the field of molecular magnetism is to perceive the high magnetic anisotropy in different geometries of metal complexes and hence to draw a magneto-structural correlation. Despite a handful of examples to exemplify the magnetic anisotropy in various coordination geometries of mononuclear complexes, the magnetic anisotropies for two different coordination geometries are underexplored. Employing an appropriate synthetic strategy utilizing the ligand LH2 [2,2'-{(1E,1'E)-pyridine2,6-diyl-bis(methaneylylidine)}-bis(azaneylylidine)diphenol] and cobalt halide salts in a 1 : 2 stoichiometric ratio in the presence of triethylamine allowed us to report a new family of dinuclear cobalt complexes [CoII2X2(L)(P)(Q)]·S with varying terminal halides [X = Cl, P = CH3CN, Q = H2O, S = H2O (1), X = Br, P = CH3CN, Q = H2O, S = H2O (2), X = I, P = CH3CN, and Q = CH3CN (3)]. All these complexes are characterized through single crystal X-ray crystallography, which reveals their crystallization in the monoclinic system P21/n space group with nearly identical structural features. These complexes share vital components, including Co(II) centers, a fully deprotonated ligand [L]2-, halide ions, and solvent molecules. The [L]2- ligand contains two Co(II) centers, where phenolate oxygen atoms bridge the Co(II) centers, forming a Co2O2 four-membered ring. Co1 demonstrates a distorted pentagonal-bipyramidal geometry with axial positions for solvent molecules, while Co2 displays a distorted tetrahedral geometry involving phenolate oxygen atoms and halide ions. Temperature-dependent dc magnetic susceptibility measurements were conducted on 1-3 within a range of 2 to 300 K at 1 kOe. The χmT vs. T plots exhibit similar trends, with χmT values at 300 K higher than the spin-only value, signifying a significant orbital contribution. As the temperature decreases, χmT decreases smoothly in all the complexes; however, no clear saturation at low temperatures is observed. Field-dependent magnetization measurements indicate a rapid increase below 20 kOe, with no hysteresis and a low magnetic blocking temperature. DFT and CASSCF/NEVPT2 theoretical calculations were performed to perceive the magnetic interaction and single-ion anisotropies of Co(II) ions in various ligand-field environments.

From field-induced to zero-field SMMs associated with open/closed structures of bis(ZnDy) tetranuclear complexes: a combined magnetic, theoretical and optical study

We have prepared a bis(compartmental) Mannich base ligand H4L (1,4,8,11-tetraaza-1,4,8,11-tetrakis(2-hydroxy-3-methoxy-5-methylbenzyl)cyclotetradecane) specifically designed to obtain bis(TMIILnIII) tetranuclear complexes (TM = transition metal). In this regard, we have succeeded in obtaining three new complexes of the formula [Zn2(μ-L)(μ-OAc)Dy2(NO3)2]·[Zn2(μ-L)(μ-OAc)Dy2(NO3)(OAc)]·4CHCl3·2MeOH (1) and [TM2(μ-H2L)2(μ-succinate)Ln2(NO3)2] (NO3)2·2H2O·6MeOH (TMII = Zn, LnIII = Dy (2); TMII = Co, LnIII = Dy (3)). Compound 1 contains two different bis(ZnDy) tetranuclear molecules that cocrystallize in the structure, in which acetato bridging ligands connect the ZnII and DyIII ions within each ZnDy subunit. This compound does not exhibit slow magnetic relaxation at zero field, but it is activated in the presence of an applied dc magnetic field and/or by Dy/Y magnetic dilution, showing two relaxation processes corresponding to each of the two different bis(ZnDy) units found in the structure. As revealed by the theoretical calculations, magnetic relaxation in 1 is single-ion in origin and takes place through the first excited state of each DyIII ion. When using the succinato dicarboxylate bridging ligand instead of acetate, compounds 2 and 3 were serendipitously formed, which have a closed structure with the succinate anion bridging two ZnDy subunits belonging to two different ligands. It should be noted that only compound 2 exhibits slow relaxation of magnetization in the absence of an external magnetic field. According to experimental and theoretical data, 2 relaxes through the second excited Kramers doublet (Ueff = 342 K). In contrast, 3 displays field-induced SMM behaviour (Ueff = 203 K). However, the Co/Zn diluted version of this compound 3Zn shows slow relaxation at zero field (Ueff = 347 K). Ab initio theoretical calculations clearly show that the weak ferromagnetic coupling between CoII and DyIII ions is at the origin of the lack of slow relaxation of this compound at zero field. Compound 2 and its diluted analogues 2Y and 3Zn show hysteresis loops at very low temperature, thus confirming their SMM behaviour. Finally, compounds 1 and 2 show DyIII based emission even at room temperature that, in the case of 2, allows us to extract the splitting of the ground 6H15/2 term, which matches reasonably well with theoretical calculations.

Magneto-structural maps and bridged-ligand effect for dichloro-bridged dinuclear copper(ii) complexes: a theoretical perspective

Theoretical understanding of magneto-structural correlations in dichloro-bridged dicopper(ii) complexes can guide the design of magnetic materials having broad-scale applications. However, previous reports suggest these correlations are complicated and unclear. To clarify possible correlations, magnetic coupling constants (J _calc) of variants of a representative {Cu-(μ-Cl)₂-Cu} complex A were calculated through BS-DFT. The variation of the Cu-(μ-Cl)-Cu angle (α), Cu⋯Cu distance (R ₀), and Cu-Cl-Cu-Cl dihedral angle (τ) followed by structural optimization and calculation of the magnetic coupling constant (J _calc) revealed several trends. J _calc increased linearly with R ₀ and τ, and initially increased and then decreased with α. Further, bridging ligand effects on J _calc for dicopper(ii) complexes were evaluated through BS-DFT; the results revealed that J _calc increased with increasing ligand field strength (I^- < Br^- < Cl^- < N₃ ^- < F^-). Furthermore, a linear relationship was found between the spin density of the bridging ligand and J _calc.

DFT computations combined with semiempirical modeling of variations with temperature of spectroscopic and magnetic properties of Gd3+-doped PbTiO3

The rare-earth or 3d transition metal dopants in perovskites have potential to induce interesting features, thus opening opportunities for investigations and applications. Hence, understanding some features, i.e., defect structure, site of incorporation, valence state, and mechanism of charge compensation, in a wide range of temperature is crucial for their technological applications. A comprehensive understanding of the mechanism of structural changes in PbTiO₃ doped with trivalent rare-earths is significant for their potential applications in photonics. To unravel the structural changes, we utilize the density functional theory (DFT) to optimize structural data, which then serve as input for the semiempirical superposition model (SPM) analysis of spectroscopic and magnetic properties of Gd³⁺-doped PbTiO₃. We compute the formation energies of the doped compounds with and without O-vacancy to determine the stable composition. Analysis of the Bader electron charges computed using DFT plus quantum theory of atoms in molecules enables elucidating the effects of the Gd dopant and O-vacancy on the ionic and covalent bonds and, thereby, chemical stability of the compositions. To explain and corroborate the zero-field splitting parameters (ZFSPs) measured by EMR and the lattice parameter changes obtained from XRD, we employ SPM. The optimized structures obtained from ab initio computations for various structural models of Gd³⁺ doped PbTiO₃ are utilized as input data for SPM calculations of ZFPs. This enables theoretical analysis of variations of ZFSPs from 5 to 780 K. The results were fine-tuned by matching with available experimental EMR data for Gd³⁺ probes in PbTiO₃ nanoparticles. Modeling has been carried out considering several possible structural models and the role of an O-vacancy around Gd³⁺ centers. The results show that the two-fold modeling approach, combining DFT and SPM, provides a reliable description of experimental data. Comparative analysis indicates that the Ti-site is less favorable for being replaced by Gd³⁺ with/without O-vacancy. This analysis confirms the plausibility of the Pb²⁺ site for Gd³⁺ dopants and sheds light on the changes of crystal structure during the phase transitions occurring in PbTiO₃ with decreasing temperature.

New Heterotrinuclear CuIILnIIICuII (Ln = Ho, Er) Compounds with the Schiff Base: Syntheses, Structural Characterization, Thermal and Magnetic Properties

New heterotrinuclear complexes with the general formula [Cu₂Ln(H₂L)(HL)(NO₃)₂]·MeOH (Ln = Ho (1), Er (2), H₄L = N,N′-bis(2,3-dihydroxybenzylidene)-1,3-diaminopropane) were synthesized using compartmental Schiff base ligand in conjugation with auxiliary ligands. The compounds were characterized by elemental analysis, ATR-FTIR spectroscopy, X-ray diffraction, TG, DSC, TG-FTIR and XRD analysis. The N₂O₄ salen-type ligand coordinates 3d and 4f metal centers via azomethine nitrogen and phenoxo oxygen atoms, respectively, to form heteropolynuclear complexes having CuO₂Ln cores. In the crystals 1 and 2, two terminal Cu(II) ions are penta-coordinated with a distorted square-pyramidal geometry and a Ln^III ion with trigonal dodecahedral geometry is coordinated by eight oxygen atoms from [Cu^II(H₂L)(NO₃)]⁻ and [Cu^II(HL)(NO₃)]²⁻ units. Compounds 1 and 2 are stable at room temperature. During heating, they decompose in a similar way. In the first decomposition step, they lose solvent molecules. The exothermic decomposition of ligands is connected with emission large amounts of gaseous products e.g., water, nitric oxides, carbon dioxide, carbon monoxide. The final solid products of decomposition 1 and 2 in air are mixtures of CuO and Ho₂O₃/Er₂O₃. The measurements of magnetic susceptibilities and field dependent magnetization indicate the ferromagnetic interaction between Cu^II and Ho^III ions 1.

Experimental and theoretical insights into Co-Ln magnetic exchange and the rare slow-magnetic relaxation behavior of [CoII2Pr]2+ in a series of linear [CoII2Ln]2+ complexes

We herein report a series of near-linear trinuclear complexes [Co₂Ln(HL)₄(NO₃)](NO₃)₂ (where HL = (2-methoxy-6-[(E)-2'-hydroxymethyl-phenyliminomethyl]-phenolate) with Ln(III) = La (1), Ce (2), Pr (3)). For the comparative study, we have also included the recently reported analogous complexes of Gd(III), Tb(III), and Dy(III) (complexes 4-6) with the same H₂L ligand. The experimental nature of the dc magnetic susceptibilities profile and an empirical approach revealed that the magnetic exchange interaction between Co(II) and Ln(III) having <4f⁷ (complexes 2 and 3) is antiferromagnetic while the dominant interaction between Co(II) and Ln(III) having ≥4f⁷ (complexes 4-6) is ferromagnetic. Dynamic magnetic relaxation studies on complexes 1-3 revealed the field induced single-molecule magnetic (SMM) behavior of 1 and 3 with effective energy barriers of 10.65 K and 15.03 K respectively, for magnetic relaxation. To the best of our knowledge, 3d-Pr(III) based zero or field induced SMMs have not been reported to date. CASSCF/SO-RASSI/SINGLE_ANISO based ab initio calculations on the X-ray structures of complexes 1-6, followed by POLY_ANISO simulations, estimated the magnetic exchange coupling constants J_Co-Ln and J_Co-Co and also rationalized our experimental findings for the dynamic magnetic properties.

Synergistic Experimental and Theoretical Studies of Luminescent-Magnetic Ln2Zn6 Clusters

The present work is part of our ongoing quest for developing functional inorganic complexes using unorthodox pyridyl-pyrazolyl-based ligands. Accordingly, we report herein the synthesis, characterization, and luminescence and magnetic properties of four 3d-4f mixed-metal complexes with a general core of Ln₂Zn₆ (Ln = Dy, Gd, Tb, and Eu). In stark contrast to the popular wisdom of using a compartmental ligand with separate islands of hard and soft coordinating sites for selective coordination, we have vindicated our approach of using a ligand with overcrowded N-coordinating sites that show equal efficiency with both 4f and 3d metals toward multinuclear cage-cluster formation. The encouraging red and green photolumiscent features of noncytotoxic Eu₂Zn₆ and Tb₂Zn₆ complexes along with their existence in nanoscale dimension have been exploited with live-cell confocal microscopy imaging of human breast adenocarcinoma (MCF7) cells. The magnetic features of the Dy₂Zn₆ complex confirm the single-molecule-magnet behavior with befitting frequency- and temperature-dependent out-of-phase signals along with an U_eff value of ∼5 K and a relaxation time of 8.52 × 10^-6 s. The Gd₂Zn₆ complex, on the other hand, shows cryogenic magnetic refrigeration with an entropy change of 11.25 J kg^-1 K^-1 at a magnetic field of 7 T and at 2 K. Another important aspect of this work reflects the excellent agreement between the experimental results and theoretical calculations. The theoretical studies carried out using the broken-symmetry density functional theory, ORCA suite of programs, and MOLCAS calculations using the complete-active-space self-consistent-field method show an excellent synergism with the experimentally measured magnetic and spectroscopic data.

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.

Modulation of Magnetic Anisotropy and Exchange Interaction in Phenoxide-Bridged Dinuclear Co(II) Complexes

We report a new class of four dimeric Co(II) complexes [Co₂(bbpen)(X)₂] (H₂bbpen = N,N'-bis(2-hydroxybenzyl)-N,N'-bis(2-methylpyridyl)ethylenediamine) [X^- = SCN (1), Cl (2), Br (3), and I (4)] with different coordination geometry of two Co(II) centers (trigonal-prismatic and pseudo-tetrahedral) and their magnetic study. Interestingly, the two Co(II) centers show two different types of magnetic anisotropy. State of the art ab initio CASSCF analysis reveals that the six-coordinate or the trigonal-prismatic Co(II) center possesses a consistently large negative axial zero-field splitting (negative D) parameter (∼-60 cm^-1), while the four-coordinate or the pseudo-tetrahedral Co(II) center exhibits a range of D values from +13 to -23 cm^-1. Ab initio calculations employing the lines model were used to estimate the magnetic exchange as both the Co(II) centers possess significant magnetic anisotropy. All the complexes display rare ferromagnetic interaction, and the strength of this interaction decreases as the ligand field on the pseudo-tetrahedral Co(II) center decreases from SCN^- > Cl^- > Br^- > I^-.

Remarkable Energy Barrier for Magnetization Reversal in 3D and 2D Dysprosium-Chloranilate-Based Coordination Polymers

Herein, two coordination polymers (CPs) [{Dy(Cl₂ An)_1.5 (CH₃ OH)}⋅4.5 H₂ O]_n (1) and [Dy(Cl₂ An)_1.5 (DMF)₂ ]_n (2), in which Cl₂ An is chloranilate (2,5-dihydroxy-1,4-benzoquinone dianion), exhibiting field-induced single-molecule magnet behavior with moderate barrier of magnetization reversal are reported. Detailed structural and topological analysis disclosed that 1 has a 3D network, whereas 2 has a 2D layered-type structure. In both CPs, magnetic measurements showed weak antiferromagnetic exchange interaction between the dysprosium centers and field-induced slow magnetic relaxation with barriers of 175(9)K and 145(7)K for 1 and 2, respectively. Notably, the energy barriers of magnetization reversal of 1 and 2 are remarkable for metal-chloranilate-based 3D (1) and 2D (2) CPs. The temperature and field dependence of relaxation time indicate the presence of multiple relaxation pathways, such as direct, quantum tunneling of magnetization, Raman, and Orbach processes, in both CPs. Ab initio theoretical calculations reinforced the experimentally observed higher energy barrier in 1 as compared with 2 due to the presence of large transverse anisotropy in the ground state in the latter. The average transition magnetic moment between the computed low-lying spin-orbit states also rationalized the relaxation as Orbach and Raman processes through the first excited state. BS-DFT calculations were carried out for both CPs to provide more insight into the exchange interaction.

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.

The differential magnetic relaxation behaviours of slightly distorted triangular dodecahedral dysprosium analogues in a type of cyano-bridged 3d–4f zig-zag chain compounds

Differential magnetic properties were observed for triangular dodecahedral dysprosium analogues in a type of cyano-bridged 3d–4f zig-zag chain compound.

Slow magnetic relaxation in CoII–LnIII heterodinuclear complexes achieved through a functionalized nitronyl nitroxide biradical

Five nitronyl nitroxide biradical-Co^II–Ln^III heterodinuclear complexes were achieved and the DyCo derivative displayed visible temperature/frequency-dependent χ′′ peaks, indicating the SMM behavior.

How important is the coordinating atom in controlling magnetic anisotropy in uranium(iii) single-ion magnets? A theoretical perspective

Theoretical investigation of actinide based nanomagnets is of paramount interest in the field of molecular magnetism as they offer remarkable properties compared to their lanthanide counterparts. Unlike lanthanides, the magnetic properties of actinides can be fine-tuned by modulating the ligand field as they possess a large metal-ligand covalency. In this regard, two complexes reported earlier have gained attention: [U(BcMe)₃] (1) has been found to show Single-ion Magnet (SIM) characteristics whereas isomeric [U(BpMe)₃] (2) does not exhibit any SIM behaviour. To unravel the origin of the differences observed in magnetic anisotropy, a detailed ab initio CASSCF study has been undertaken on the X-ray structure of complexes 1 and 2. Since actinide compounds exhibit strong covalency, the desired active space needs to be benchmarked to address this issue. Here, we have enlarged the active space systematically from CAS(3,7) to CAS(3,12) where all 5f electrons in 5f orbitals are sequentially expanded to include five formally empty 6d orbitals. Our calculations reveal that the incorporation of the 6d_z² orbital is vital in reproducing many experimental observables such as temperature dependent susceptibility, g-factors, ground state m_J level, and ground-state-excited-state gap. Inclusion of this orbital in the reference space is found to describe better the UH-BH agostic interactions leading to significant variations in the computed parameters. Gaining from this understanding, we have carried out extensive bonding analysis within the DFT framework using tools such as Natural Bond Orbital (NBO) and Atoms In Molecule (AIM) to further probe these weak agostic interactions. Also, predictions to enhance the U-ligand covalency using U-sulphur bonds and the role of the U-C distance and C-U-C bite angles in the nature of anisotropy have been studied, and relevant magneto-structural correlations have been developed. Thus our results for the first time provide a comprehensive understanding of uranium based SMMs and offer ways to fine tune the anisotropy for experimental chemists.

Influence of ancillary ligands and solvents on the nuclearity of Ni-Ln complexes

A Schiff base ligand resulting from the reaction of ovanillin and 2,2-dimethyl-1,3-diaminopropane allows the preparation of hetero-dinuclear [Ni-Ln]³⁺ or -trinuclear [Ni-Ln-Ni]³⁺ complexes. Although empirical parameters for rationalizing the strength of the ferromagnetic Ni-Gd interaction have already been discussed in several papers, no systematic study has been devoted to the control of the nuclearity of such complexes. With the help of structural determinations, we demonstrate the role of solvent and of the nature of ancillary ligands, linked to the Ln ions, in nuclearity. For instance, the presence of one chelating nitrato ligand is already sufficient to impede an increase in the nuclearity, while the replacement of nitrato ligands by chloride anions still yields dinuclear Ni-Ln complexes. This experimental result evidences the role of protic solvents. In contrast, the use of lanthanide salts, soluble in non-protic solvents, allows the isolation of dinuclear [Ni-Ln]³⁺ or trinuclear cationic [LNi-Ln-NiL]³⁺ complexes, depending on the Ni/Ln ratio. A further synthetic step can be overtaken by the reaction of a Ni-Ln complex, soluble in a non-protic solvent, with a LM complex (M = Cu, Zn). By doing so, a heterotrinuclear complex made of three different metal ions, two distinct 3d ions and a 4f one, has been isolated and structurally characterized. Note that the Ni coordination number decreases from 6 to 5 on going from the dinuclear complex to the trinuclear one. Also, the replacement of water molecules by chloride ligands in the hexacoordinate Ni complexes induces a net increase of the positive zero-field splitting parameter D to 20 cm^-1, which is supported by ab initio calculations. Although the Ni-Ln (Ln = Gd, Tb, Dy) magnetic interactions are ferromagnetic, the corresponding trinuclear complexes are devoid of SMM properties in the absence of an applied magnetic field.

Incorporating polyoxometalates and organic ligands to pursue 3d–4f heterometallic clusters: a series of {Cr4Ln4} clusters stabilized by phthalic acid and [SiW12O40]4−

By introduction of trilacunary Keggin-type polyoxometalate to the hydrothermal reaction system of Cr³⁺, Ln³⁺ and phthalic acid, a series of novel {Cr₄Ln₄} heterometallic clusters with the formula Cs₂[Cr₄Ln₄(μ₄-O)₄(μ₃-O)₄(C₈H₄O₄)₄(H₂O)₁₂](H₃SiW₁₂O₄₀)Cl·23H₂O (1-Ln, Ln = Ce, Pr, Nd) and [Cr₄Ln₄(μ₄-O)₄(μ₃-O)₄(C₈H₄O₄)₄(H₂O)₁₀](H₆SiW₁₂O₄₀)Cl₂·18H₂O (2-Ln, Ln = Sm, Eu, Gd, Tb, Dy, Ho, Er) have been obtained. Single-crystal structural analyses show that 1-Ln and 2-Ln constitute the first cases of Cr–Ln heterometallic clusters stabilized by inorganic polyoxometalate anions and organic ligands. Optical spectra studies demonstrate that 1-Ln and 2-Ln are narrow-gap semiconductors with band gaps of about 1.5 eV. Magnetic investigation shows that compound 2-Dy is a potential single molecule magnet.

By introduction of trilacunary Keggin-type polyoxometalate to the hydrothermal reaction system of Cr³⁺, Ln³⁺ and phthalic acid, a series of novel {Cr₄Ln₄} heterometallic clusters have been obtained.

Facile and environmentally friendly synthesis of six heterometallic dumbbell-shaped MLn (M = Co, Ni; Ln = Eu, Gd, Dy) clusters as cryogenic magnetic coolants and molecular magnets

Six analogous dumbbell-shaped 3d-4f complexes MLn were obtained by the reaction of Ln(NO₃)₃ (Ln = Eu, Gd, Dy) and M(OAc)₂ (M = Co, Ni) without any other organic ligands, and M₅Gd₄ exhibited a large magnetocaloric effect (-ΔS_m = 31.0, 37.3 J kg^-1 K^-1 for Co₅Gd₄ and Ni₅Gd₄ respectively), while M₅Dy₄ and Co₅Eu₄ showed single molecule magnet behaviour. To the best of our knowledge, Co₅Eu₄ is the first Eu-based cluster to display single molecule magnet behaviour (U_eff = 16.4 K).

Butterfly M2 IIIEr2 (MIII = Fe and Al) SMMs: Synthesis, Characterization, and Magnetic Properties

The reaction of N-(2-pyridylmethyl)iminodiethanol (H₂L, pmide), FeCl₂·4H₂O or AlCl₃·6H₂O with ErCl₃·6H₂O and p-Me-PhCO₂H in the ratio of 2:1:1:4 in the presence of Et₃N in MeOH and MeCN yielded compounds [Fe₂Er₂(μ₃-OH)₂(pmide)₂(p-Me-PhCO₂)₆]·2MeCN (1) and [Al₂Er₂(μ₃-OH)₂(pmide)₂(p-Me-PhCO₂)₆]·2MeCN (2). These two complexes are isostructural, possessing a planar butterfly motif with the Er^III ions in the wingtip positions. Both compounds show single molecule magnet (SMM) behavior. For the [Al₂Er₂] compound, the slow relaxation of the magnetization under zero applied direct current (dc) field does not show maxima, but the relaxation processes could be analyzed using an applied dc field of 1000 Oe. In-depth alternating current measurements under different dc fields on the [Fe₂Er₂] compound reveals that the Fe-Fe and Fe-Er interactions speed up the relaxation and decrease the energy barrier height of the SMM in comparison with the [Al₂Er₂] case.

Acquiring a record barrier height for magnetization reversal in lanthanide encapsulated fullerene molecules using DFT and ab initio calculations

Ab initio calculations performed on a series of lanthanide encapsulated fullerenes reveal {DyOSc}@C₈₂ to be one of the best host-guest pairs to offer a barrier height exceeding 1400 cm^-1. The high-symmetry environment preserved inside the cage quenches the QTM effects up to third-excited states leading to very larger barrier heights and this opens up a new possibility of obtaining attractive SMMs based on lanthanide based endohedral metallo-fullerenes (EMFs).

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.

Influence of alcoholic solvent and acetate anion coordination mode variations on structures and magnetic properties of heterometallic Zn2Dy2 tetranuclear clusters

We clarify that slight modifications of the synthetic conditions generate two Zn₂Dy₂ clusters with acetate anion coordination mode variation and different magnetic behaviors.

Heterometallic lanthanide-centred [NiII6LnIII] rings

The syntheses, structures and magnetic properties of three new hetero-heptametallic [Ni₆Ln] complexes are reported.

Heterometallic Heptanuclear [Cu5Ln2] (Ln = Tb, Dy, and Ho) Single-Molecule Magnets Organized in One-Dimensional Coordination Polymeric Network

The reaction of a multisite coordination ligand, LH₃, with Cu(II) salts and Ln(NO₃)₃·nH₂O in a 1:2:1 stoichiometric ratio in the presence of triethylamine was found to afford a series of one-dimensional heterometallic [{Cu₅Ln₂(L)₂(μ₃-OH)₄(ClO₄)(NO₃)₃(OH₂)₅}(ClO₄)₂(H₂O)_x]_∞ [Ln = Tb(1), Dy(2) and Ho(3), x = 4.25, 5.5, and 5 for 1-3, respectively] coordination polymers. Complexes 1-3 have been characterized by single crystal X-ray crystallography. The detailed study of the magnetic properties has also been performed and compared with the parent [Cu₅Ln₂] molecular analogues. The ac susceptibility measurements for complexes 1-3 confirm their SMM behavior with the following characteristics: Δ_eff/k_B = 23.4 K, τ₀ = 1.1 × 10^-6 s and Δ_eff/k_B = 27.9 K, τ₀ = 6.6 × 10^-7 s under 0 and 1200 Oe dc fields, respectively for 1; Δ_eff/k_B = 8.3 K, τ₀ = 3.1 × 10^-6 s for 2 under 0 dc field. For 3, the fast QTM below 4 K prevents the estimation of the SMM energy barrier. Remarkably, the magnetic and SMM properties of the previously reported molecular [Cu₅Ln₂] analogues are preserved after their assembly in these coordination networks.

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.

Tetranuclear Ni(ii) and Co(ii) Schiff-base complexes with an M4O6 defective dicubane-like core: zero-field SMM behavior in the cobalt analogue

Two tetranuclear Ni₄ and Co₄ complexes were prepared and characterized. Their magnetic properties were thoroughly studied and it was revealed that the Co₄ compound behaves as a zero-field single-molecule magnet.

Solution and solid state structures and magnetism of a series of linear trinuclear compounds with a hexacoordinate LnIII and two terminal NiII centers

Reported are the syntheses, structures and magnetic properties, also by NMR spectroscopy in solution, of a series of 13 linear trinuclear 3d-4f compounds with a lanthanide(iii) surrounded by two Ni^II ions, NiLn^III, where the central Ln^III is hexacoordinate. For three of the crystal structures, an additional H₂O molecule is coordinated to the central Ln^III ion, leading to a monocapped trigonal prismatic structure. However, NMR spectroscopy indicates that in solution, these complexes also have a hexacoordinate Ln^III center. The solution magnetic anisotropies, determined by NMR spectroscopy, indicate that the axial components of the anisotropies are relatively small and that the Dy^III derivative might therefore not exhibit single molecule magnetism. The axial anisotropies determined by NMR spectroscopy are in good agreement with the expectations based on the distorted trigonal prismatic ligand field.