Mononuclear Single-molecule Magnets Research Articles

Organolanthanide Single-Molecule Magnets with Heterocyclic Ligands.

Lanthanide (Ln) based mononuclear single-molecule magnets (SMMs) provide probably the finest ligand regulation model for magnetic property. Recently, the development of such SMMs has witnessed a fast transition from coordination to organometallic complexes because the latter provides a fertile, yet not fully excavated soil for the development of SMMs. Especially those SMMs with heterocyclic ligands have shown the potential to reach higher blocking temperature. In this minireview, we give an overview of the design principle of SMMs and highlight those "shining stars" of heterocyclic organolanthanide SMMs based on the ring sizes of ligands, analysing how the electronic structures of those ligands and the stiffness of subsequently formed molecules affect the dynamic magnetism of SMMs. Finally, we envisaged the future development of heterocyclic Ln-SMMs.

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

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

Aggregation‐induced suppression of quantum tunneling by manipulating intermolecular arrangements of magnetic dipoles

AbstractThe relaxation time under zero field reflects the memory retention capabilities of single‐molecule magnets (SMMs) when used as storage devices. Intermolecular magnetic dipole interaction is ubiquitous in aggregates of magnetic molecules and can greatly influence relaxation times. However, such interaction is often considered harmful and challenging to manipulate in molecular solids, especially for high‐performance lanthanide single‐ion magnets (SIMs). By an elaborately designed combination of ion pairing and hydrogen bonding, we have synthesized two pseudo‐D5h SIMs with supramolecular arrangements of magnetic dipoles in staggered and side‐by‐side patterns, the latter of which exhibits a 104‐fold slower zero‐field relaxation time at 2 K. Intriguingly, the side‐by‐side complex exhibits a significantly accelerated magnetic relaxation upon diamagnetic dilution, contrary to the general trend observed in the staggered complex. This strongly reveals the presence of aggregation‐induced suppression of quantum tunneling in a side‐by‐side arrangement, which has not been observed in mononuclear SMMs. By leveraging ion‐pairing aggregation and converting to a side‐by‐side pattern, this study successfully demonstrates an approach to transform a harmful intermolecular dipole interaction into a beneficial one, achieving a τQTM of 980 s ranking among the best‐performance SMMs.

A Dicobalt(II) Single-Molecule Magnet via a Well-Designed Dual-Capping Tetrazine Radical Ligand

The recent years have witnessed the glory development for the construction of high-performance mononuclear single molecule magnets (SMMs) within a specific coordination geometry, which, however, is not well applied in cluster-based SMMs due to the synthetic challenges. Given that the monocobalt(II) complexes within a trigonal-prismatic (TPR) coordination geometry have been classified as excellent SMMs with huge axial anisotropy (D ≈ -100 cm-1), here we designed and synthesized a new dual-capping tetrazine ligand, 3,6-bis(6-(di(1H-pyrazol-1-yl)methyl)pyridin-2-yl)-1,2,4,5-tetrazine (bpptz), and prepared a novel dicobalt(II) complex, [Cp2CoIII][{(hfac)CoII}2(bpptz•-)][hfac]2·2Et2O (1, hfac = hexafluoroacetylacetonate). In the structure of 1, the bpptz•- radical ligand enwraps two Co(II) centers within quasi-TPR geometries, which are further bridged by the tetrazine radical in the trans mode. The magnetic study revealed that the interaction between the Co centers and the tetrazine radical is strongly antiferromagnetic with a coupling constant (J) of -65.8 cm-1 (in the -2J formalism). Remarkably, 1 exhibited the typical SMM behavior with an effective energy barrier of 69 cm-1 under a 1.5 kOe dc field, among the largest for polynuclear transition metal SMMs. In addition, DFT and ab initio calculations suggested that the presence of a strong Co(II)-radical magnetic interaction effectively quenches the QTM effect and enhances the barrier height for the magnetization reversal.

Redox-Active Dysprosium Single-Molecule Magnet: Spectro-Electrochemistry and Theoretical Investigations

The mononuclear single-molecule magnet (SMM) [Dy(tta)3(L)]⋅C6H14 (1) (where tta− = 2-thenoyltrifluoroacetonate and L = 4,5-bis(propylthio)-tetrathiafulvalene-2-(2-pyridyl)benzimidazole-methyl-2-pyridine) was studied by spectro-electrochemistry. The resulting electronic spectra of the three oxidation states 1, 1+∙, and 12+ were rationalized by time-dependent density functional theory (TD-DFT) calculations starting from the DFT optimized structures. The modulation of the magnetic anisotropy of the DyIII center upon oxidation was also inspected at the Complete Active Space Self-Consistent Field (CASSCF) level of calculation.

Effect of Low Spin Excited States for Magnetic Anisotropy of Transition Metal Mononuclear Single Molecule Magnets

Rational, fine tuning of magnetic anisotropy is critical to obtain new coordination compounds with enhanced single molecule magnet properties. For mononuclear transition metal complexes, the largest contribution to zero-field splitting is usually related to the excited states of the same spin as the ground level. Thus, the contribution of lower multiplicity roots tends to be overlooked due to its lower magnitude. In this article, we explore the role of lower multiplicity excited states in zero-field splitting parameters in model structures of Fe(II) and Co(II). Model aquo complexes with coordination numbers ranging from 2 to 6 were constructed. The magnetic anisotropy was calculated by state of the art ab initio methodologies, including spin-orbit coupling effects. For non-degenerate ground states, contributions to the zero-field splitting parameter (D) from highest and lower multiplicity roots were of the same sign. In addition, their relative magnitude was in a relatively narrow range, irrespective of the coordination geometry. For degenerate ground states, the contribution from lower multiplicity roots was significantly smaller. Results are rationalized in terms of general expressions for D and are expected to be reasonably transferable to real molecular systems.

Structural diversity and photo-physical and magnetic properties of dimeric to 1D polymeric coordination polymers of lighter lanthanide(iii) dinitrobenzoates

Single crystal diffraction studies reveal the formation of the following 10 new complexes of lighter Ln(iii) ions with general formulas {[Ln(μ2-L1)3·(H2O)2]·H2O}n (Ln = Nd (1) and Eu (2)), [Nd(μ2-L2)2·(CH3COO)·(H2O)2]n (3), [Ln2(μ2-L2)5·(L2)·(H2O)4]n (Ln = Sm (4), Ce (5), and Pr (6)), [La2(μ2-L2)6·(H2O)3·(DMF)]n (7) (DMF = dimethylformamide), [Ln(μ2-L2)2·(L2)·(H2O)3]2 (Ln = Eu (8) and Gd (9)) and [Gd(L2)·(CH3COO)2·(H2O)2]2 (10), where L1 and L2 are anions of 3,5- and 2,4-dinitrobenzoic acid, respectively. Complexes 1-7 are 1D coordination polymers, while 8-10 are dinuclear complexes. The luminescence properties of Nd(iii) and Eu(iii) analogues displayed metal-centred emission with L1 exhibiting weak but more efficient sensitization than L2. A study of the magnetic properties of the compounds clearly demonstrated the field-induced single ion magnet behaviour of the Nd(iii) compounds 1 and 3. Their behaviour has been compared to previously reported analogous Nd(iii) complexes and the role of the lattice solvent and polymorphism on the magnetic behaviour has been evaluated.

One mononuclear single-molecule magnet derived from Dy(III) and dmbpy (dmbpy = 4,4′-dimethyl-2,2′-dipyridyl)

Three β–diketonate mononuclear Ln-based compounds, formulated as [TbIII(hfac)3(dmbpy)](1), [DyIII(hfac)3(dmbpy)](2) and [ErIII(hfac)3(dmbpy)](3) (hfac=hexafluoroacetylacetone, dmbpy=4,4′-dimethyl-2,2′-dipyridyl) have been synthesized and structurally characterized by single crystal X-ray diffraction. Structure characterization reveals that complexes 1, 2 and 3 are isomorphous. Shape measurement shows the coordination geometry of complex 2 is trigonal dodecahedron (D2d). Magnetic property study indicates complex 2 shows SIM behavior with observed frequency-dependent signals at low temperature.

Field-Induced Slow Magnetic Relaxation in an Octacoordinated Fe(II) Complex with Pseudo-D2d Symmetry: Magnetic, HF-EPR, and Theoretical Investigations

An octacoordinated Fe(II) complex, [FeII(dpphen)2](BF4)2·1.3H2O (1; dpphen = 2,9-bis(pyrazol-1-yl)-1,10-phenanthroline), with a pseudo-D2d-symmetric metal center has been synthesized. Magnetic, high-frequency/-field electron paramagnetic resonance (HF-EPR), and theoretical investigations reveal that 1 is characterized by uniaxial magnetic anisotropy with a negative axial zero-field splitting (ZFS) (D ≈ -6.0 cm-1) and a very small rhombic ZFS (E ≈ 0.04 cm-1). Under applied dc magnetic fields, complex 1 exhibits slow magnetic relaxation at low temperature. Fitting the relaxation time with the Arrhenius mode combining Orbach and tunneling terms affords a good fit to all the data and yields an effective energy barrier (17.0 cm-1) close to the energy gap between the ground state and the first excited state. The origin of the strong uniaxial magnetic anisotropy for 1 has been clearly understood from theoretical calculations. Our study suggests that high-coordinated compounds featuring a D2d-symmetric metal center are promising candidates for mononuclear single-molecule magnets.

Systematic Study of Open-Shell Trigonal Pyramidal Transition-Metal Complexes with a Rigid-Ligand Scaffold.

A family of distorted trigonal pyramidal transition-metal complexes [MII (N3 N)Li(THF)] (M=Mn, Fe, Co, Ni) have been studied as candidates for mononuclear single-molecule magnets. Magnetic anisotropy of the family depends on the electronic configuration of the central ion, with the Co analogue exhibiting pronounced SMM behavior.

Rational Design of a Lanthanide-Based Complex Featuring Different Single-Molecule Magnets.

The rational synthesis of the 2-{1-methylpyridine-N-oxide-4,5-[4,5-bis(propylthio)tetrathiafulvalenyl]-1H-benzimidazol-2-yl}pyridine ligand (L) is described. It led to the tetranuclear complex [Dy4(tta)12(L)2] (Dy-Dy2-Dy) after coordination reaction with the precursor Dy(tta)3⋅2 H2O (tta(-) = 2-thenoyltrifluoroacetonate). The X-ray structure of Dy-Dy2-Dy can be described as two terminal mononuclear units bridged by a central antiferromagnetically coupled dinuclear complex. The terminal N2O6 and central O8 environments are described as distorted square antiprisms. The ac magnetism measurements revealed a strong out-of-phase signal of the magnetic susceptibility with two distinct sets of data. The high- and low-frequency components were attributed to the two terminal mononuclear single-molecule magnets (SMMs) and the central dinuclear SMM, respectively. A magnetic hysteresis loop was detected at very low temperature. From both structural and magnetic points of view, the tetranuclear SMM Dy-Dy2-Dy is a self-assembly of two known mononuclear SMMs bridged by a known dinuclear SMM.

Multiple single-molecule magnet behaviors in dysprosium dinuclear complexes involving a multiple functionalized tetrathiafulvalene-based ligand.

The reaction between the 2-(1-(2,6-di(pyrazol-1-yl)-4-methylpyridyl)-4,5-(4,5-bis(propylthio)-tetrathiafulvalenyl)-1H-benzimidazol-2-yl)-pyridine ligand (L) and 2 equiv of Dy(hfac)3·2H2O (hfac(-) = 1,1,1,5,5,5-hexafluoroacetylacetonate) and 1 equiv each of Dy(hfac)3·2H2O and Dy(tta)3·2H2O (tta(-) = 2-thenoyltrifluoroacetonate) metallic precursors leads to two dinuclear complexes, [Dy2(hfac)6(L)]·(CH2Cl2)2·C6H14 (1) and [Dy2(hfac)3(tta)3(L)] (2), respectively. Their X-ray structures reveal that the two coordination sites are occupied by one Dy(III) ion. The Dy(III) ion coordinated to the benzoimidazolylpyridine (bzip) moiety adopts a D4d coordination sphere, while the Dy(III) ion coordinated to the 2,6-di(pyrazol-1-yl)-4-pyridine (dpp) moiety is in a D3h surrounding. In a zero dc field, the dynamic magnetic measurements show a slow relaxation for the D4d eight-coordination Dy(III) magnetization for 1 and 2. Application of an external dc field induces multirelaxation signals of the magnetic susceptibility for both compounds. The low frequency and high frequency of the out-of-phase magnetic signals are attributed to the Dy(III) ion in D4d and D3h surroundings, respectively. The two complexes can be described as double induced-field mononuclear single-molecule magnets.

A Trigonal-Pyramidal Erbium(III) Single-Molecule Magnet.

Given the recent advent of mononuclear single-molecule magnets (SMMs), a rational approach based on lanthanides with axially elongated f-electron charge cloud (prolate) has only recently received attention. We report herein a new SMM, [Li(THF)4[Er{N(SiMe3)2}3Cl]⋅2 THF, which exhibits slow relaxation of the magnetization under zero dc field with an effective barrier to the reversal of magnetization (ΔEeff/kB =63.3 K) and magnetic hysteresis up to 3 K at a magnetic field sweep rate of 34.6 Oe s(-1). This work questions the theory that oblate or prolate lanthanides must be stabilized with the appropriate ligand framework in order for SMM behavior to be favored.

Two mononuclear single molecule magnets derived from dysprosium(III) and tmphen (tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline).

Two mononuclear Dy(III) complexes, [Dy(III)(hfac)3(tmphen)] (1) and [Dy(III)(acac)3(tmphen)]·2H2O (2) (tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline, hfac = hexafluoroacetylacetone, acac = acetylacetone) have been synthesized and structurally characterized by single crystal X-ray diffraction. Magnetic properties indicate that both of the complexes exhibit SMM behavior, and complex 1 is the first typical derivative of phenanthroline containing D2d-Dy(III) based mononuclear single molecule magnets. The energy barrier (Ueff/kB) of complex 2 (130.42 K) is much higher than that of complex 1 (35.09 K), indicating that the local symmetry of Dy(III) ions (D2d for 1, D4d for 2) plays an important role in magnetic behaviors.

Transition Ion Strikes Back: Large Magnetic Susceptibility Anisotropy in Cobalt(II) Clathrochelates.

Transition-metal complexes are rarely considered as paramagnetic tags for NMR spectroscopy due to them generally having relatively low magnetic anisotropy. Here we report cobalt(II) cage complexes with the largest (among the transition-metal complexes) axial anisotropy of magnetic susceptibility, reaching as high as 12.6 × 10(-32) m(3) at room temperature. This remarkable anisotropy, which results from an unusual trigonal prismatic geometry of the complexes and translates into large negative value of the zero-field splitting energy, is high enough to promote reliable paramagnetic pseudocontact shifts at the distance beyond 2 nm. Our finding paves the way toward the applications of cobalt(II) clathrochelates as future paramagnetic tags. Given the incredible stability and functionalization versatility of clathrochelates, the fine-tuning of the caging ligand may lead to new chemically stable mononuclear single-molecule magnets, for which magnetic anisotropy is of importance.

Molecular anisotropy analysis of single-ion magnets using an effective electrostatic model.

Simple electrostatic models have been shown to successfully rationalize the magnetic properties of mononuclear single molecule magnets based on f-elements and even to predict the direction of the magnetic anisotropy axis in these nanomagnets. In this Article, we go a step forward by showing that these models, conveniently modified to account for the covalency effects, are able to predict not only the easy axis direction but also the three components of the magnetic anisotropy. Thus, by using a lone pair effective charge (LPEC) model we can fully reproduce the angular dependence of the magnetic susceptibility in single crystals of pentamethylcyclopentadienyl-Er-cyclooctatetraene single-ion magnet. Furthermore, the parametrization of the ligands obtained in this study has been extrapolated to successfully reproduce spectroscopic data of a set of mononuclear lanthanoid complexes based on the same kind of ligands, thus emphasizing the predictive character of this model.

A new D2d-symmetry DyIII mononuclear single-molecule magnet containing a monodentate N-heterocyclic donor ligand

By adopting pyrazole to the system of Ln(hfac)3·2H2O with the strong electron-withdrawing effect, a D2d-symmetry DyIII mononuclear single-molecule magnet containing a monodentate N-heterocyclic ligand was designed and synthesized.

Highly Anisotropic Rhenium(IV) Complexes: New Examples of Mononuclear Single-Molecule Magnets

The rhenium(IV) complex (NBu4)2[ReBr4(ox)] (1) (ox = oxalate and NBu4(+) = tetra-n-butylammonium cation) has been prepared and its crystal structure determined by X-ray diffraction. The structure is made up of discrete [ReBr4(ox)](2-) anions and bulky NBu4(+) cations. Each [ReBr4(ox)](2-) anion is surrounded by six NBu4(+) cations, which preclude any significant intermolecular contact between the anionic entities, the shortest rhenium···rhenium distance being 9.373(1) Å. Variable temperature dc and ac magnetic susceptibility measurements and field-dependent magnetization experiments on polycrystalline samples of 1 reveal the occurrence of highly anisotropic magnetically isolated Re(IV) centers (S(Re) = 3/2), which exhibit slow relaxation of the magnetization at very low temperatures in a dc field. Ac measurements conducted on a polycrystalline sample of the complex (NBu4)2[ReCl4(ox)] (2) [compound isostructural to 1 whose structure and dc magnetic susceptibility study were previously reported in Tomkiewicz, A.; Bartczak, T. J.; Kruszyński, R.; Mroziński, J. J. Mol. Struct. 2001, 595, 225] show a similar behavior, both complexes thus constituting new examples of mononuclear single-molecule magnets. High-frequency and -field electron paramagnetic resonance on polycrystalline samples of 1 and 2 and on single crystals of 2 allowed for the determination for the first time of the negative sign and confirmed a significant magnitude and rhombicity (E/D) of the zero-field splitting tensor of the [ReCl4(ox)](2-) and [ReBr4(ox)](2-) centers, originating from a combination of spin-orbit coupling and low molecular symmetry. D and E values of 1 and 2 were estimated through magnetization measurements and theoretically calculated through complete active space and density functional theory methodologies.

Dilution-Induced Slow Magnetic Relaxation and Anomalous Hysteresis in Trigonal Prismatic Dysprosium(III) and Uranium(III) Complexes

Magnetically dilute samples of complexes Dy(H(2)BPz(Me2)(2))(3) (1) and U(H(2)BPz(2))(3) (3) were prepared through cocrystallization with diamagnetic Y(H(2)BPz(Me2)(2))(3) (2) and Y(H(2)BPz(2))(3). Alternating current (ac) susceptibility measurements performed on these samples reveal magnetic relaxation behavior drastically different from their concentrated counterparts. For concentrated 1, slow magnetic relaxation is not observed under zero or applied dc fields of several hundred Oersteds. However, a 1:65 (Dy:Y) molar dilution results in a nonzero out-of-phase component to the magnetic susceptibility under zero applied dc field, characteristic of a single-molecule magnet. The highest dilution of 3 (1:90, U:Y) yields a relaxation barrier U(eff) = 16 cm(-1), double that of the concentrated sample. These combined results highlight the impact of intermolecular interactions in mononuclear single-molecule magnets possessing a highly anisotropic metal center. Finally, dilution elucidates the previously observed secondary relaxation process for concentrated 3. This process is slowed down drastically upon a 1:1 molar dilution, leading to butterfly magnetic hysteresis at temperatures as high as 3 K. The disappearance of this process for higher dilutions reveals it to be relaxation dictated by short-range intermolecular interactions, and it stands as the first direct example of an intermolecular relaxation process competing with single-molecule-based slow magnetic relaxation.

Detailed Ab Initio First-Principles Study of the Magnetic Anisotropy in a Family of Trigonal Pyramidal Iron(II) Pyrrolide Complexes

A theoretical, computational, and conceptual framework for the interpretation and prediction of the magnetic anisotropy of transition metal complexes with orbitally degenerate or orbitally nearly degenerate ground states is explored. The treatment is based on complete active space self-consistent field (CASSCF) wave functions in conjunction with N-electron valence perturbation theory (NEVPT2) and quasidegenerate perturbation theory (QDPT) for treatment of magnetic field- and spin-dependent relativistic effects. The methodology is applied to a series of Fe(II) complexes in ligand fields of almost trigonal pyramidal symmetry as provided by several variants of the tris-pyrrolylmethyl amine ligand (tpa). These systems have recently attracted much attention as mononuclear single-molecule magnet (SMM) complexes. This study aims to establish how the ligand field can be fine tuned in order to maximize the magnetic anisotropy barrier. In trigonal ligand fields high-spin Fe(II) complexes adopt an orbitally degenerate (5)E ground state with strong in-state spin-orbit coupling (SOC). We study the competing effects of SOC and the (5)E⊗ε multimode Jahn-Teller effect as a function of the peripheral substituents on the tpa ligand. These subtle distortions were found to have a significant effect on the magnetic anisotropy. Using a rigorous treatment of all spin multiplets arising from the triplet and quintet states in the d(6) configuration the parameters of the effective spin-Hamiltonian (SH) approach were predicted from first principles. Being based on a nonperturbative approach we investigate under which conditions the SH approach is valid and what terms need to be retained. It is demonstrated that already tiny geometric distortions observed in the crystal structures of four structurally and magnetically well-documented systems, reported recently, i.e., [Fe(tpa(R))](-) (R = tert-butyl, Tbu (1), mesityl, Mes (2), phenyl, Ph (3), and 2,6-difluorophenyl, Dfp (4), are enough to lead to five lowest and thermally accessible spin sublevels described sufficiently well by S = 2 SH provided that it is extended with one fourth order anisotropy term. Using this most elementary parametrization that is consistent with the actual physics, the reported magnetization data for the target systems were reinterpreted and found to be in good agreement with the ab initio results. The multiplet energies from the ab initio calculations have been fitted with remarkable consistency using a ligand field (angular overlap) model (ab initio ligand field, AILFT). This allows for determination of bonding parameters and quantitatively demonstrates the correlation between increasingly negative D values and changes in the σ-bond strength induced by the peripheral ligands. In fact, the sigma-bonding capacity (and hence the Lewis basicity) of the ligand decreases along the series 1 > 2 > 3 > 4.