Is a strong axial crystal-field the only essential condition for a large magnetic anisotropy barrier? The case of non-Kramers Ho(iii) versus Tb(iii )

Pressure-Induced Structural, Optical and Magnetic Modifications in Lanthanide Single-Molecule Magnets

Lanthanide Single-Molecule Magnets are fascinating objects that break magnetic performance records with observable magnetic bistability at the boiling temperature of liquid nitrogen, paving the way for potential applications in high-density data storage. The switching of lanthanide SMM has been successfully achieved using several external stimuli such as redox reaction, pH titration, light irradiation or solvation/desolvation thanks to the high sensitivity of the magnetic anisotropy to any structural change in the lanthanide surrounding. Nevertheless, the use of applied high pressure as an external stimulus is largely underused, especially considering that it can be combined with high pressure X-ray diffraction to establish a complementary structure-property relationship. This Concept article summarizes the few relevant examples of investigations of lanthanide SMMs under applied high pressure, provides conclusions on the effect of such stimulus on molecular structures and magnetic anisotropy, and finally draws perspective on the future development of magnetic measurements under applied pressure.

Electron Paramagnetic Resonance Spectra of Pentagonal Bipyramidal Gadolinium Complexes

Gadolinium is a special case in spectroscopy because of the near isotropic nature of the 4f⁷ configuration of the +3 oxidation state. Gd³⁺ complexes have been studied in several symmetries to understand the underlying mechanisms of the ground state splitting. The abundance of information in Gd³⁺ spectra can be used as a probe for properties of the other rare earth ions in the same complexes. In this work, the zero-field splitting (ZFS) of a series of Gd³⁺ pentagonal bipyramidal complexes of the form [GdX₁X₂(L_eq)₅]ⁿ⁺ [n = 1, X = axial ligands: Cl^-, ^-O^tBu, ^-OArF₅ or n = 3, X = ^tBuPO(NHⁱPr)₂, L_eq = equatorial ligand: Py, THF or H₂O] with near fivefold symmetry axes along X¹-Gd-X² was investigated. The ZFS parameters were determined by fitting of room-temperature continuous wave electron paramagnetic resonance (EPR) spectra (at X-, K-, and Q-band) to a spin Hamiltonian incorporating extended Stevens operators compatible with C₅ symmetry. Examination of the acquired parameters led to the conclusion that the ZFS is dominated by the B₂⁰ term and that the magnitude of B₂⁰ is almost entirely dependent on, and inversely proportional to, the donor strength of the axial ligands. Surveying the continuous shape measure and the X¹-Gd-X² angle of the complexes showed that there is some correlation between the proximity of each complex to D_5h symmetry and the magnitude of the B₆⁵ parameter, but that the deformation of the X¹-Gd-X² angle is more significant than other distortions. Finally, the magnitude of B₂⁰ was found to be inversely proportional to the thermal barrier for the reversal of the magnetic moment (U_eff) of the corresponding isostructural Dy³⁺ complexes.

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.

Magnetic anisotropy of transition metal and lanthanide ions in pentagonal bipyramidal geometry

The magnetic anisotropy associated with a pentagonal bipyramidal (PBP) coordination sphere is examined on the basis of experimental and theoretical investigations. The origin and the characteristics of this anisotropy are discussed in relation to the electronic configuration of the metal ions. The effects of crystal field, structural distortion, and a second-coordination sphere on the observed anisotropies for transition meal and lanthanide ions are outlined. For the Ln derivatives, we focus on compounds showing SMM-like behavior (i.e. slow relaxation of their magnetization) in order to highlight the essential chemical and structural parameters for achieving strong axial anisotropy. The use of PBP complexes to impart controlled magnetic anisotropy in polynuclear species such as SMMs or SCMs is also addressed. This review of the magnetic anisotropies associated with a pentagonal bipyramidal coordination sphere for transition metal and lanthanide ions is intended to highlight some general trends that can guide chemists towards designing a compound with specific properties.

Effect of an axial coordination environment on quantum tunnelling of magnetization for dysprosium single-ion magnets with theoretical insight

Herein, we report two mononuclear dysprosium complexes [Dy(H₄L){B(OMe)₂(Ph)₂}₂](Cl)·MeOH (1) and [Dy(H₄L){MeOH)₂(NCS)₂}](Cl) (2) [where H₄L = 2,2'-(pyridine-2,6-diylbis(ethan-1-yl-1-ylidene))bis(N-phenylhydrazinecarboxamide)] with different axial coordination environments. The structural analysis revealed that the pentadentate H₄L ligand binds through the equatorial position in both complexes. In complex 1, the axial positions are occupied by bidentate dimethoxydiphenyleborate [B(OMe)₂(Ph)₂]^-. On the other hand, in complex 2, one axial position is occupied by two NCS^- and one MeOH molecule while another MeOH molecule is coordinated to the other axial position. Magnetic measurements disclose the presence of field-induced slow relaxation of magnetization with an energy barrier of U_eff = 30 K for 1 whereas no such effective barrier was observed in complex 2. Detailed analysis of field and temperature dependence of the relaxation time confirms the major role of Raman, QTM, and direct processes rather than the Orbach process in complex 1. It was observed that [B(OMe)₂(Ph)₂]^- provides higher axial anisotropy which slows down the QTM process (relaxation time for the QTM process is 2.70 × 10^-5 s) in 1 as compared to NCS anions and MeOH molecules in 2 (1.03 × 10^-8 s), and is responsible for the absence of an effective energy barrier in the latter complex as confirmed by ab initio calculations. The calculations also show that the presence of a large bidentate dimethoxydiphenyleborate ligand in axial positions may result in high-performance Dy-based single-ion magnets.

Air-stable chiral mono- and dinuclear dysprosium single-molecule magnets: steric hindrance of hexaazamacrocycles

Two pairs of air-stable chiral Dy-SMMs were constructed using different sterically hindered hexaazamacrocycles as equatorial ligands, leading to a nuclearity increase from 1 to 2.

Deciphering the Role of Anions and Secondary Coordination Sphere in Tuning Anisotropy in Dy(III) Air-Stable D5h SIMs*

Precise control of the crystal field and symmetry around the paramagnetic spin centre has recently facilitated the engineering of high-temperature single-ion magnets (SIMs), the smallest possible units for future spin-based devices. In the present work, we report a series of air-stable seven coordinate Dy(III) SIMs {[L₂ Dy(H₂ O)₅ ][X]₃ ⋅L₂ ⋅n(H₂ O), n = 0, X = Cl (1), n=1, X = Br (2), I (3)} possessing pseudo-D_5h symmetry or pentagonal bipyramidal coordination geometry with high anisotropy energy barrier (U_eff ) and blocking temperature (T_B ). While the strong axial coordination from the sterically encumbered phosphonamide, ^t BuPO(NHⁱ Pr)₂ (L), increases the overall anisotropy of the system, the presence of high symmetry significantly quenches quantum tunnelling of magnetization, which is the prominent deactivating factor encountered in SIMs. The energy barrier (U_eff ) and the blocking temperature (T_B ) decrease in the order 3>2>1 with the change of anions from larger iodide to smaller strongly hydrogen-bonded chloride in the secondary coordination sphere, albeit the local coordination geometry and the symmetry around the Dy(III) display only slight deviations. Ab initio CASSCF/RASSI-SO/SINGLE_ANISO calculations provide deeper insights into the dynamics of magnetic relaxation in addition to the role of the secondary coordination sphere in modulating the anisotropy of the D_5h systems, using diverse models. Thus, in addition to the importance of the crystal field and the symmetry to obtain high-temperature SIMs, this study also probes the significance of the secondary coordination sphere that can be tailored to accomplish novel SIMs.

Single-molecule magnets within polyoxometalate-based frameworks

As an extension of our interest in polyoxometalates (POMs) and lanthanoids, we report the design and synthesis of two polyoxometalate-based frameworks under hydrothermal conditions; [Ho₄(PDA)₄(H₂O)₁₁][(SiO₄)@W₁₂O₃₆]·8H₂O (1) and [Tb₄(PDA)₄(H₂O)₁₂][(SiO₄)@W₁₂O₃₆]·4H₂O (2) (H₂PDA = 1,10-phenanthroline-2,9-dicarboxylic acid). Both hybrids have been characterized by elemental analysis, Fourier transform infrared spectroscopy, thermogravimetric analysis, and powder/single-crystal X-ray diffraction. According to the structural analysis, 1 and 2 consist of 2D-cationic coordination polymers based on the respective lanthanoids and PDA^2- as well as Keggin anions that reside in the interspaces between two adjacent layers as discrete counterions connected by extensive hydrogen bonding. Although the overall structures of 1 and 2 are composed of cationic and anionic layers, there are many differences in the cationic layers such as various coordination modes of PDA^2-, different void shapes, and the existence of dinuclear Tb(III) units only in 2. Frameworks 1 and 2 were further characterized by dc and ac magnetic measurements and both exhibit slow relaxation of magnetization at low temperatures under an applied dc field. Their single-molecule magnet (SMM) properties are investigated, where weak field-induced SMM behaviour is observed at low temperatures in dynamic magnetic studies.

Magnetic anisotropies of Ho(III) and Dy(III) single-molecule magnets experimentally determined via polarized neutron diffraction

We present the magnetic anisotropy of two isostructural pentagonal-bipyramidal complexes, [Ln(H₂O)₅(HMPA)₂]I₃·2HMPA (HMPA = hexamethylphosphoramide, Ln = Dy, Ho). Using ac magnetic susceptibility measurements, we find magnetic relaxation barriers of 600 K and 270 K for the Dy- and Ho-compounds, respectively. This difference is supported by polarized neutron diffraction (PND) measured at 5 K and 1 T which provides the first experimental evidence that the transverse elements in the magnetic anisotropy of the Ho-analogue are significant, whereas the Dy-analogue has a near-axial magnetic anisotropy with vanishing transverse contributions. The coordination geometries of the two complexes are highly similar, and we attribute the loss of strong magnetic axiality as expressed in the atomic susceptibility tensors from PND, as well as the smaller relaxation barrier in the Ho-complex compared to the Dy-complex, to the less favorable interaction of the pentagonal bipyramidal crystal field with the characteristics of the Ho(III) 4f-charge distribution.

Enhancing the energy barrier and hysteresis temperature in two benchtop-stable Ho(iii) single-ion magnets

The energy barrier and hysteresis temperature in two benchtop-stable D_5h-symmetry Ho^III single-ion magnets were significantly enhanced via the variation of the halogen anion. The coexistence of a high energy barrier of 418 K and hysteresis temperature of 15 K was observed in the bromide-ion containing Ho^III single-ion magnet.

Strong Equatorial Crystal Field Enhances the Axial Anisotropy and Energy Barrier for Spin Reversal Process in Yb2 Single Molecule Magnets

The importance of equatorial crystal fields on magnetic anisotropy of ytterbium single molecule magnets (SMMs) is observed for the first time. Herein, we report three similar dinuclear ytterbium complexes with the formula [Yb₂ (3-OMe-L)₂ (DMF)₂ (NO₃ )₂ ]⋅DMF (1), [Yb₂ (3-H-L)₂ (DMF)₂ (NO₃ )₂ ]⋅DMF⋅H₂ O (2), and [Yb₂ (3-NO₃ -L)₂ (DMF)₂ (NO₃ )₂ ] (3), [where 3-X-H₂ L=N'-(2-hydroxy-3-X-benzylidene)picolinohydrazide, X=OMe (1), H (2) NO₂ (3)]. Detailed magnetic measurements reveal the presence of weak antiferromagnetic interactions between the Yb centers and a field-induced slow relaxation of magnetization in all complexes. A higher energy barrier for spin reversal was observed for complex 1 (U_eff =50 K) and it decreases in the order of 2 (47 K) to 3 (40 K). Notably, complex 1 shows a remarkable energy barrier within the frequency range of 1-850 Hz reported for Yb-based SMMs. Further, ab initio calculations show a higher axial anisotropy and lower quantum tunneling of magnetization (QTM) in the ground state for 1 compared to 2 and 3. It was also observed that the presence of a strong crystal field in the equatorial plane (when the ∡ O1-Yb-O3 bond angle is close to 90°) enhances the axial anisotropy and improves the SMM behavior in the studied complexes. Both the experimental and theoretical analysis of relaxation dynamics discloses that Raman and QTM play major role on slow relaxation process for all complexes. To provide more insight into the exchange interactions, broken-symmetry DFT calculations were performed.

Seven-coordinate LnIII complexes assembled from a bulky MesacacH ligand: their synthesis, structure, photoluminescence and SMM behaviour

The reaction of a bulky acetyl acetone ligand 1,3-dimesitylpropane-1,3-dione (MesacacH) with hydrated lanthanide chlorides in the presence of tetramethylammonium hydroxide afforded a new family of neutral mononuclear LnIII complexes [Ln(Mesacac)3(DMF)] (Ln = Dy (1); Tb (2); Y0.91Dy0.09 (3); and Er (4)). The molecular structures of these complexes were confirmed by single crystal X-ray diffraction studies. The coordination geometries of the LnIII centre were analysed by SHAPE analysis which revealed a capped octahedral geometry in 1-4. Photoluminescence studies showed ligand-sensitized green emissions for 2 with an appreciable quantum yield of 0.83%. Static (dc) and dynamic (ac) magnetic studies of complexes 1 and 3 were performed. The dynamic magnetic study revealed that complex 1 exhibits zero-field slow relaxation of the magnetization without showing a clear maximum in the out-of-phase ac susceptibility plots. However, magnetic dilution of 1 with the YIII metal ion (complex 3) and/or the application of a dc magnetic field induces a strong frequency dependence of the ac susceptibility signals with χ''M peaks in the 3-10 K temperature range, thus supporting field-induced SMM behaviour of 1. The relaxation process takes place through a combination of the Orbach and Raman mechanisms. The fitting of the temperature dependence of the relaxation time to the equation τ-1 = τ0-1 exp(-Ueff/kBT) + BTn, allows the extraction of the effective energy barrier Ueff/kB = 70 K (48.7 cm-1) and pre-exponential parameter of τ0 = 2.7 × 10-7 s for the Orbach mechanism (first term) and the parameters B = 0.04 s-1 K-n and n = 6.11, for the Raman mechanism (second term).

Enhancing the barrier height for Yb(III) single-ion magnets by modulating axial ligand fields

The effect of systematic modification of the axial ligand field X on Ueff values in Yb(iii)-based SIMs, [Yb(Ph3PO)4X2]X' (X, X' = NO3 (1), OTf (2) and X = I/Br/Cl; X' = I3 (3)), whose equatorial Ph3PO ligation remains unchanged, has been investigated. Combined magnetic studies coupled with ab initio calculations reveal weakening of the axial ligand fields leading to the increase in the energy barrier, apart from suggesting the operation of different relaxation pathways.

Asymmetric Dinuclear Lanthanide(III) Complexes from the Use of a Ligand Derived from 2-Acetylpyridine and Picolinoylhydrazide: Synthetic, Structural and Magnetic Studies

A family of four Ln(III) complexes has been synthesized with the general formula [Ln₂(NO₃)₄(L)₂(S)] (Ln = Gd, Tb, Er, and S = H₂O; 1, 2 and 4, respectively/Ln = Dy, S = MeOH, complex 3), where HL is the flexible ditopic ligand N’-(1-(pyridin-2-yl)ethylidene)pyridine-2-carbohydrazide. The structures of isostructural MeOH/H₂O solvates of these complexes were determined by single-crystal X-ray diffraction. The two Ln^III ions are doubly bridged by the deprotonated oxygen atoms of two “head-to-head” 2.21011 (Harris notation) L¯ ligands, forming a central, nearly rhombic {Ln^III₂(μ-OR)₂}⁴⁺ core. Two bidentate chelating nitrato groups complete a sphenocoronal 10-coordination at one metal ion, while two bidentate chelating nitrato groups and one solvent molecule (H₂O or MeOH) complete a spherical capped square antiprismatic 9-coordination at the other. The structures are critically compared with those of other, previously reported metal complexes of HL or L¯. The IR spectra of 1–4 are discussed in terms of the coordination modes of the organic and inorganic ligands involved. The f-f transitions in the solid-state (diffuse reflectance) spectra of the Tb(III), Dy(III), and Er(III) complexes have been fully assigned in the UV/Vis and near-IR regions. Magnetic susceptibility studies in the 1.85–300 K range reveal the presence of weak, intramolecular Gd^III∙∙∙Gd^III antiferromagnetic exchange interactions in 1 [J/k_B = −0.020(6) K based on the spin Hamiltonian Ĥ = −2J(Ŝ_Gd1∙ Ŝ_Gd2)] and probably weak antiferromagnetic Ln^III∙∙∙Ln^III exchange interactions in 2–4. Ac susceptibility measurements in zero dc field do not show frequency dependent out-of-phase signals, and this experimental fact is discussed for 3 in terms of the magnetic anisotropy axis for each Dy^III center and the oblate electron density of this metal ion. Complexes 3 and 4 are Single-Molecule Magnets (SMMs) and this behavior is optimally observed under external dc fields of 600 and 1000 Oe, respectively. The magnetization relaxation pathways are discussed and a satisfactory fit of the temperature and field dependencies of the relaxation time τ was achieved considering a model that employs Raman, direct, and Orbach relaxation mechanisms.

Enhancing the energy barrier by replacing the counterions in two holmium(iii)-pentagonal bipyramidal single-ion magnets

Based on a phosphine oxide ligand, HMPA (hexamethylphosphoric triamide), two mononuclear Ho^III-pentagonal bipyramidal complexes were synthesized with the formulas [Ho(HMPA)₂(H₂O)₅]₂Cl₆·2HMPA·2H₂O (1) and [Ho(HMPA)₂(H₂O)₅]Br₃·2HMPA (2). Single-crystal X-ray diffraction results show that all Ho^III ions in both the two complexes are hepta-coordinated and are located in pentagonal bipyramidal {HoO7} coordination polyhedrons constructed by two axial HMPA ligands and five equatorial water molecules. However, due to the employment of different halide ions as counterions, the second coordination sphere surrounding each [Ho(HMPA)₂(H₂O)₅]³⁺ moiety is different in the two complexes: in 1, three Cl^- ions, one water molecule and one HMPA ligand; in 2, three Br^- ions and two HMPA ligands. Ac magnetic susceptibilities under zero dc field show that both the two complexes are single-ion magnets with effective energy barriers of 290 K and 320 K for 1 and 2, respectively. Compared with 1, the enhancement in the energy barrier of 2 is believed to be induced mainly by the change in the second coordination sphere rather than the minor differences in the {HoO7} polyhedrons.

A single-ion single-electron cerrous magnet

Herein, we present monometallic Ln(iii) complexes [L₃Ln(NO₃)₃] [where Ln = Ce (1) and La (2)] assembled from a simple reaction of the respective lanthanide nitrate hydrate and a bulky phosphonic diamide ^tBuPO(NHⁱPr)₂ ligand (L), where complex 1 behaves as a single-ion single-electron magnet under a small applied magnetic field. The Ce(iii) ion occupies a nine-coordinate distorted muffin-like coordination environment. The combination of direct and Raman process dominates the relaxation dynamics in 1 under the applied dc field. The low-temperature measurements performed with oriented crystals on a micro-SQUID setup exhibits strong tunnelling at zero-field, consistent with the theoretical results where strong mixing of the ground state with higher excited m_J levels is detected and also throws additional insights on the relaxation dynamics of 1. Ab initio calculations have been performed to understand the origin of anisotropy and models have been proposed for future directions.

An imido ligand significantly enhances the effective energy barrier of dysprosium(iii) single-molecule magnets

We report herein an imido ligand 1,3-bis(2,6-diisopropylphenyl) imidazolin-2-imine (Im^DippNH) that can form a very short Dy-N bond (2.12 Å) with the dysprosium(iii) ion, which leads to a much larger effective energy barrier for magnetisation reversal (803 K) compared to the analogous alkoxide ligand (53 K). Moreover, we predict that a linear two-coordinate [Dy(Im^DippN)₂]⁺ complex may have an effective energy barrier larger than 4000 K.

Ln-Pt electron polarization effects on the magnetic relaxation of heterometallic Ho- and Er-Pt complexes

Heterometallic Ln-Pt complexes, with the formula [Ln₂Pt₃(H₂O)₂(SAc)₁₂] (Ln = Ho(1), Er(2); SAc = thioacetate), were synthesized. From natural bond orbital (NBO) and local orbital locator (LOL) analyses and X-ray absorption fine structure (XAFS) measurements, it was clear that the Ln-Pt interactions or electron polarization occurred. Butterfly-type hysteresis was observed for both 1 and 2. 1 and 2 underwent field-induced slow magnetic relaxation up to 4 K. These magnetic properties were induced by Ln-Pt electron polarization.

A new air- and moisture-stable pentagonal-bipyramidal DyIII single-ion magnet based on the HMPA ligand

A new pentagonal-bipyramidal Dy^III SIM was obtained with HMPA ligand, which is stable to air and moisture and shows slow relaxation of magnetization up to 36 K (1000 Hz) with U_eff of 556 K.

Field Induced Slow Magnetic Relaxation in a Non Kramers Tb(III) Based Single Chain Magnet

Herein, we report a novel Tb(III) single chain magnet with the chemical formulae [Tb(μ-OH2)(phen)(μ-OH)(nb)2]n by using 4-nitrobenzoic acid (Hnb) and 1,10-phenanthroline (phen) as ligand system. The single-crystal X-ray diffraction reveals that 4-nitrobenzoic acid acts as a monodentate ligand, water and hydroxyl ions are the bridging ligand and the phen serves as a bidentate chelating ligand. The static magnetic susceptibility measurement (from 2 K to 300 K) shows ferromagnetic interaction at very low temperature (below 6 K). The alternating current (AC) susceptibility data of the complex show temperature and frequency dependence under an applied 2000 Oe DC (direct current) field. The phen moiety behaves as an antenna and enables the complex to show the green light fluorescence emission by absorption-energy transfer-emission mechanism. To calculate the exchange interaction, broken symmetry density functional theory (BS-DFT) calculations have been performed on a model compound which also reveals weak ferromagnetic interaction. Ab initio calculations reveals the anisotropic nature (gz = 15.8, gy, gy = 0) of the metal centre and the quasi doublet nature of ground state with small energy gap and that is well separated from the next excited energy state.

Mononuclear Lanthanide(III)-Salicylideneaniline Complexes: Synthetic, Structural, Spectroscopic, and Magnetic Studies

The reactions of hydrated lanthanide(III) [Ln(III)] nitrates and salicylideneaniline (salanH) have provided access to two families of mononuclear complexes depending on the reaction solvent used. In MeCN, the products are [Ln(NO3)3(salanH)2(H2O)]·MeCN, and, in MeOH, the products are [Ln(NO3)3(salanH)2(MeOH)]·(salanH). The complexes within each family are proven to be isomorphous. The structures of complexes [Ln(NO3)3(salanH)2(H2O)]·MeCN (Ln = Eu, 4·MeCN_Eu, Ln = Dy, 7·MeCN_Dy; Ln = Yb, 10·MeCN_Yb) and [Ln(NO3)3(salanH)2(MeOH)]·(salanH) (Ln = Tb, 17_Tb; Ln = Dy, 18_Dy) have been solved by single-crystal X-ray crystallography. In the five complexes, the LnIII center is bound to six oxygen atoms from the three bidentate chelating nitrato groups, two oxygen atoms from the two monodentate zwitterionic salanH ligands, and one oxygen atom from the coordinated H2O or MeOH group. The salanH ligands are mutually “cis” in 4·MeCN_Eu, 7·MeCN_Dy and 10·MeCN_Yb while they are “trans” in 17_Tb and 18_Dy. The lattice salanH molecule in 17_Tb and 18_Dy is also in its zwitterionic form with the acidic H atom being clearly located on the imine nitrogen atom. The coordination polyhedra defined by the nine oxygen donor atoms can be described as spherical tricapped trigonal prisms in 4·MeCN_Eu, 7·MeCN_Dy, and 10·MeCN_Yb and as spherical capped square antiprisms in 17_Tb and 18_Dy. Various intermolecular interactions build the crystal structures, which are completely different in the members of the two families. Solid-state IR data of the complexes are discussed in terms of their structural features. 1H NMR data for the diamagnetic Y(III) complexes provide strong evidence that the compounds decompose in DMSO by releasing the coordinated salanH ligands. The solid complexes emit green light upon excitation at 360 nm (room temperature) or 405 nm (room temperature). The emission is ligand-based. The solid Pr(III), Nd(III), Sm(III), Er(III), and Yb(III) complexes of both families exhibit LnIII-centered emission in the near-IR region of the electromagnetic spectrum, but there is probably no efficient salanH→LnIII energy transfer responsible for this emission. Detailed magnetic studies reveal that complexes 7·MeCN_Dy, 17_Tb and 18_Dy show field-induced slow magnetic relaxation while complex [Tb(NO3)3(salanH)2(H2O)]·MeCN (6·MeCN_Tb) does not display such properties. The values of the effective energy barrier for magnetization reversal are 13.1 cm−1 for 7·MeCN_Dy, 14.8 cm−1 for 17_Tb, and 31.0 cm−1 for 18_Dy. The enhanced/improved properties of 17_Tb and 18_Dy, compared to those of 6_Tb and 7_Dy, have been correlated with the different supramolecular structural features of the two families. The molecules [Ln(NO3)3(salanH)2(MeOH)] of complexes 17_Tb and 18_Dy are by far better isolated (allowing for better slow magnetic relaxation properties) than the molecules [Ln(NO3)3(salanH)2(H2O)] in 6·MeCN_Tb and 7·MeCN_Dy. The perspectives of the present initial studies in the Ln(III)/salanH chemistry are discussed.

Ab Initio Prediction of Tunneling Relaxation Times and Effective Demagnetization Barriers in Kramers Lanthanide Single-Molecule Magnets

Single-molecule magnets (SMMs) are promising candidates for molecule-based quantum information devices. Their main limitation is their cryogenic operative temperature. To achieve devices performing at higher temperatures, demagnetization mechanisms must be suppressed by chemical tuning. Electronic structure calculations can provide useful information to rationalize SMM behavior, but they do not provide a direct prediction for the key experimental parameters describing magnetic relaxation (i.e., tunneling relaxation time (τ_QT) and effective demagnetization barrier ( U_eff)). In this Letter, a first-principles model is proposed to predict τ_QT and U_eff for mononuclear, half-integer spin SMMs, allowing direct comparison with experiment. Model accuracy was assessed against experimental data for 18 mononuclear Ln^III complexes (15 Dy^III and 3 Er^III) and applied to 3 of the current best-performing SMMs, correctly predicting nontrivial relaxation pathways. The model shows that the combination of single-ion anisotropy and spin-spin dipolar coupling can account for the major part of tunneling demagnetization for the studied systems.

Slow magnetic relaxation and luminescence properties in lanthanide(iii)/anil complexes

The initial use of anils, i.e. bidentate Schiff bases derived from the condensation of anilines with salicylaldehyde or its derivatives, in 4f-metal chemistry is described. The 1 : 1 reactions between Ln(NO3)3·xH2O (Ln = lanthanide) or Y(NO3)3·6H2O and N-(5-bromosalicylidene)aniline (5BrsalanH) in MeCN has provided access to complexes [Ln(NO3)3(5BrsalanH)2(H2O)]·MeCN (Ln = Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb) and [Y(NO3)3(5BrsalanH)2(H2O)]·MeCN, respectively, in good yields. The structures of the isomorphous complexes with Ln = Pr(1·MeCN), Sm(3·MeCN), Gd(5·MeCN), Dy(7·MeCN) and Er(9·MeCN) have been determined by single-crystal X-ray crystallography. The other complexes were proven to be isostructural with the fully structurally characterized compounds based on elemental analyses, IR spectra, unit cell determinations and powder X-ray patterns. The 9-coordinate LnIII centre in the [Ln(NO3)3(5BrsalanH)2(H2O)] molecules is bound to six oxygen atoms from the three bidentate chelating nitrato groups, two oxygen atoms that belong to the organic ligands and one oxygen atom from the aquo ligand. The 5BrsalanH molecules behave as monodentate O-donors; the acidic H atom is clearly located on the imino N atom and thus the formally neutral ligands adopt an extremely rare coordination mode participating in the zwitterionic form. The coordination polyhedra defined by the nine donor atoms around the LnIII centres are best described as spherical capped square antiprisms. Various intermolecular interactions build the crystal structures and Hirshfeld surface analysis was applied to evaluate the magnitude of interactions between the molecules. Solid-state IR and UV/VIS data are discussed in terms of structural features. 1H NMR data prove that the diamagnetic [Y(NO3)3(5BrsalanH)2(H2O)] complex decomposes in DMSO. Combined dc and ac magnetic susceptibility, as well as magnetization data for 7 suggest that this complex shows field-induced slow magnetic relaxation. Two magnetization relaxation processes are evident. The fit to the Arrhenius law has been performed using the 6.5-8.5 K ac data, affording an effective barrier for the magnetization reversal of 27 cm-1. Cole-Cole plot analysis in the temperature range in which the Orbach relaxation process is assumed, reveals a narrow distribution of relaxation times. The solid Dy(iii) complex 7 emits green light at 338 nm, the emission being ligand-centered. The perspectives of the present, first results in the lanthanide(iii)-anil chemistry are critically discussed.

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.