Single-molecule Magnets Performance Research Articles

Thirty Years of Hide-and-Seek: Capturing Abundant but Elusive MIII@C3v(8)-C82 Isomer, and the Study of Magnetic Anisotropy Induced in Dy3+ Ion by the Fullerene π-Ligand.

Our knowledge about endohedral metallofullerenes (EMFs) is restricted to the structures with sufficient kinetic stability to be extracted from the arc-discharge soot and processed by chromatographic and structural techniques. For the most abundant rare-earth monometallofullerene MIII@C82, experimental studies repeatedly demonstrated C2v(9) and Cs(6) carbon cage isomers, while computations predicted equal stability of the "missing" C3v(8) isomer. Here we report that this isomer is indeed formed but has not been recovered from soot using standard protocols. Using a combination of redox extraction and subsequent benzylation and trifluoromethylation with single-crystal XRD analysis of CF3 adduct, we prove that Dy@C3v(8)-C82 is one of the most abundantly produced metallofullerenes, which was not identified in earlier studies because of the low kinetic stability. Further, using the Dy@C3v(8)-C82(CF3) and Dy@C3v(8)-C82(CH2Ph) monoadducts for the case study, we analyzed the role of metal-fullerene bonding on the single-ion magnetic anisotropy of Dy in EMFs. The multitechnique approach, combining ab initio calculations, EPR spectroscopy, and SQUID magnetometry, demonstrated that coordination of the Dy ion to the fullerene cage induces moderate, nonaxial, and very fluid magnetic anisotropy, which strongly varies with small alterations in the Dy-fullerene coordination geometry. As a result, Dy@C3v(8)-C82(CH2Ph) is a weak field-induced single-molecule magnet (SMM), whose signatures of magnetic relaxation are detectable only below 3 K. Our results demonstrate that metal-cage interactions should have a detrimental effect on the SMM performance of EMFs. At the same time, the strong variability of the magnetic anisotropy with metal position suggests tunability and offers strategies for future progress.

Auxiliary Rather Than Dominant. The Role of Direct Dy-S Coordination in Single-Molecule Magnet Unveiled via ab initio Study.

The role of Dy-S coordination in a single-molecule magnet (SMM) is investigated via an ab initio study in a group of mononuclear structures. The SMM performance of this group is well interpreted via a concise criterion consisting of long quantum tunneling of magnetization (QTM) time τQTM and high effective barrier for magnetic reversal Ueff. The best SMMs in the selected group, i.e., 1Dy (CCDC refcode: PUKFAF) and 2Dy (CCDC refcode: NIKSEJ), are just those holding the longest τQTM and the highest Ueff simultaneously. Further analysis based on the crystal field model and ab initio magneto-structural exploration indicates that the influence of Dy-S coordination on the SMM performance of 1Dy is weaker than that of axial Dy-O coordination. Thus, Dy-S coordination is more likely to play an auxiliary role rather than a dominant one. However, if placed at the suitable equatorial position, Dy-S coordination could provide important support for good SMM performance. Consequently, starting from 1Dy, we built two new structures where Dy-S coordination only exists at the equatorial position and two axial positions are occupied by strong Dy-O/Dy-F coordination. Compared to 1Dy and 2Dy, these new ones are predicted to have significantly longer τQTM and higher Ueff, as well as a nearly doubled blocking temperature TB. Thus, they are probable candidates of SMM having clearly improved performance.

Structure and assembly studies of two planar Dy(iii) single molecule magnets with double relaxations

We presented here the assembly mechanisms of two planar tetranuclear Dy4 and hexanuclear Dy6 single molecule magnets with double relaxations.

Synthesis, crystal structures and magnetic properties of two Ni-Dy heterometallic complexes with the structural topologies regulated by employing different Schiff base ligands

Regulation of the structural topologies of single molecule magnets (SMMs) is one of the most important approach for enhancing their SMM performance. In this work, we first obtained a Ni2Dy trinuclear complex, [Ni2Dy(Hhmmp)2{(py)2CO2}(CH3COO)2]OH (1, H2hmmp = 2-[(2-hydroxyethylimino)methyl]-6-methoxyphenol, (py)2C(OH)2 = the gem-diol form of di-2-pyridyl ketone (dpk)) by employing H2hmmp and dpk reacting with Dy(NO3)3·5H2O and Ni(CH3COO)2·4H2O in the MeCN solution containing triethylamine. However, when H2hmmp, triethylamine and MeCN in the reaction system of 1 were replaced by 2-(((2-hydroxy-3-methoxyphenyl)methylene)amino)-2-(hydroxymethyl)-1,3-propanediol (H4L), N(Me)4OH·5H2O and the mixture of MeCN and N,N-dimethylformamide (DMF), a Ni2Dy2 complex, [Ni2Dy2(HL)2{(py)2CO(OH)}2(CH3COO)2]·2H2O (2), was obtained. Structural analysis revealed that two NiII and one DyIII in 1 formed a “V-shaped” topology, while two NiII and two DyIII ions in 2 formed a butterfly-like topology with Ni and DyIII ions occupied on the body positions and the wing positions, respectively. Additionally, the coordination geometries of DyIII ion in 1 and 2 square antiprism geometry (D4d) and capped octahedron (C3v), respectively. Magnetic investigation indicated that 1 exhibits obviously slow magnetic relaxation behavior, with an energy barrier Ueff of ∼6 K. The orientations of the magnetic anisotropy of DyIII in 1 and 2 were estimated by Magellan software; the results indicate that few coordination atoms were located on the equatorial plane of DyIII in 1 and 2, which result in 1 and 2 exhibiting poor SMM performance.

A Combined Synthetic, Magnetic, and Theoretical Study on Enhancing Ligand-Field Axiality for Dy(III) Single-Molecule Magnets Supported by Ferrocene Diamide Ligands.

Molecular design is crucial for improving the performance of single-molecule magnets (SMMs). For dysprosium(III) SMMs, enhancing ligand-field axiality is a well-suited strategy to achieve high-performance SMMs. We synthesized a series of dysprosium(III) complexes, (NNTIPS)DyBr(THF)2 (1, NNTIPS = fc(NSiiPr3)2; fc = 1,1'-ferrocenediyl, THF = tetrahydrofuran), [(NNTIPS)Dy(THF)3][BPh4] (2), (NNTIPS)DyI(THF)2 (3), and [(NNTBS)Dy(THF)3][BPh4] (4, NNTBS = fc(NSitBuMe2)2), supported by ferrocene diamide ligands. X-ray crystallography shows that the rigid ferrocene backbone enforces a nearly axial ligand field with weakly coordinating equatorial ligands. Dysprosium(III) complexes 1-4 all exhibit slow magnetic relaxation under zero fields and possess high effective barriers (Ueff) around 1000 K, comparable to previously reported (NNTBS)DyI(THF)2 (5). We probed the influences of structural variations on SMM behaviors by theoretical calculations and found that the distribution of negative charges defined by rq, i.e., the ratio of the charges on the axial ligands to the charges on the equatorial ligands, plays a decisive role. Moreover, theoretical calculations on a series of model complexes 1'-5' without equatorial ligands unveil that the axial crystal-field parameters B20 are directly proportional to the N-Dy-N angles and support the hypothesis that enhancing the ligand-field axiality could improve SMM performance.

The importance of second sphere interactions on single molecule magnet performance

Secondary interactions occur beyond the primary coordination sphere and can influence the performance of single molecule magnets (SMMs). This article highlights the role that secondary interactions play in the synthesis and performance of SMMs.

Single-ion magnet behavior of Ln3+ encapsulated in carbon nanotubes: an ab initio insight.

Single-molecule magnets (SMMs) have attracted large interest owing to their capability to store information at the level of a single molecule, which has great potential for applications in information technology. The key characteristic required for SMM performance is the magnetization blocking barrier, and in the last decade, impressive efforts have been made to increase its height. Herein, we report an ab initio investigation of the SMM behavior of a series of lanthanide ions (Tb3+, Dy3+, Ho3+, Er3+, Tm3+ and Yb3+) encapsulated in zigzag carbon nanotubes (CNTs) of different diameters. The results show that despite the high symmetry of the Ln environment, none of the investigated systems, except for Er3+ encapsulated in the (7,0) CNT, exhibited any blocking behavior. This is mainly attributed to the strong competition between axial and equatorial contributions to the crystal field of these encapsulated ions, resulting in weak or lack of magnetic axiality. The presented results provide useful theoretical guidance for the design of high-performance SMMs via modulating the crystal field of the ligand environment.

Reaching the Maximal Unquenched Orbital Angular Momentum L = 3 in Mononuclear Transition-Metal Complexes: Where, When and How?

The conditions for achieving the maximal unquenched orbital angular momentum L = 3 and the highest magnetic anisotropy in mononuclear 3d complexes with axial coordination symmetry are examined in terms of the ligand field theory. It is shown that, apart from the known linear two-coordinate 3d7 complex CoII(C(SiMe2ONaph)3)2 characterized by record magnetic anisotropy and single-molecule magnet (SMM) performance (with the largest known spin-reversal barrier Ueff = 450 cm−1), the maximal orbital angular momentum L = 3 can also be obtained in linear two-coordinate 3d2 complexes (V3+, Cr4+) and in trigonal-prismatic 3d3 (Cr3+, Mn4+) and 3d8 (Co+, Ni2+) complexes. A comparative assessment of the SMM performance of the 3d2, 3d3 and 3d8 complexes indicates that they are unlikely to compete with the record linear complex CoII(C(SiMe2ONaph)3)2, whose magnetic anisotropy is close to the physical limit for a 3d metal.

Detailed magnetic properties and theoretical calculation in ferromagnetic coupling DyIII-MII 3d-4f complexes based on a 1,4,7,10‑tetraazacyclododecane derivative

Four Dy-based 3d-4f complexes [MIIDyIII(H2L)(NO3)3](CH3OH)2 (H4L = N,N',N'',N'''-tetra(2-hydroxy-3-methoxy-5-methylbenzyl)-1,4,7,10-tetraazacyclododecane, M = Mn (2), Fe (3), Co (4) and Ni (5), the solvent of compound 5 is uncertain) were synthesized through the reference method of complex [ZnIILnIII(H2L)(NO3)3](CH3OH)2 (1). The magnetic properties of all the five complexes were measured for comparison. The magnetic results show that all the complexes except 4 behave as single-molecule magnets (SMMs). The magnetostructural analysis and theoretical calculation indicate the ferromagnetic coupling in 2, 3 and 5 as well as the antiferromagnetic coupling in 4 between DyIII and transition metal ions, while the SMM properties of 1 are caused by the anisotropy of single DyIII ions only. Unexpectedly, neither of the magnetic anisotropic ions in one molecule, namely DyIII and CoII, provides greater advantages for SMM performance. This investigation experimentally proves that the coexistence of magnetic anisotropy of lanthanide ions and ferromagnetic coupling between lanthanide and transition metal ions is important for the design of 3d-4f SMMs.

Lanthanide SMMs Based on Belt Macrocycles: Recent Advances and General Trends.

Enhancement of axial magnetic anisotropy is the central objective to push forward the performance of Single-Molecule Magnet (SMM) complexes. In the case of mononuclear lanthanide complexes, the chemical environment around the paramagnetic ion must be tuned to place strongly interacting ligands along either the axial positions or the equatorial plane, depending on the oblate or prolate preference of the selected lanthanide. One classical strategy to achieve a precise chemical environment for a metal centre is using highly structured, chelating ligands. A natural approach for axial-equatorial control is the employment of macrocycles acting in a belt conformation, providing the equatorial coordination environment, and leaving room for axial ligands. In this review, we present a survey of SMMs based on the macrocycle belt motif. Literature systems are divided in three families (crown ether, Schiff-base and metallacrown) and their general properties in terms of structural stability and SMM performance are briefly discussed.

Dominance of Cyclobutadienyl Over Cyclopentadienyl in the Crystal Field Splitting in Dysprosium Single‐Molecule Magnets

AbstractReplacing a monoanionic cyclopentadienyl (Cp) ligand in dysprosium single‐molecule magnets (SMMs) with a dianionic cyclobutadienyl (Cb) ligand in the sandwich complexes [(η4‐Cb′′′′)Dy(η5‐C5Me4tBu)(BH4)]− (1), [(η4‐Cb′′′′)Dy(η8‐Pn†)K(THF)] (2) and [(η4‐Cb′′′′)Dy(η8‐Pn†)]− (3) leads to larger energy barriers to magnetization reversal (Cb′′′′=C4(SiMe3)4, Pn†=1,4‐di(tri‐isopropylsilyl)pentalenyl). Short distances to the Cb′′′′ ligands and longer distances to the Cp ligands in 1–3 are consistent with the crystal field splitting being dominated by the former. Theoretical analysis shows that the magnetic axes in the ground Kramers doublets of 1–3 are oriented towards the Cb′′′′ ligands. The theoretical axiality parameter and the relative axiality parameter Z and Zrel are introduced to facilitate comparisons of the SMM performance of 1–3 with a benchmark SMM. Increases in Z and Zrel when Cb′′′ replaces Cp signposts a route to SMMs with properties that could surpass leading systems.

Dominance of Cyclobutadienyl Over Cyclopentadienyl in the Crystal Field Splitting in Dysprosium Single-Molecule Magnets.

Replacing a monoanionic cyclopentadienyl (Cp) ligand in dysprosium single‐molecule magnets (SMMs) with a dianionic cyclobutadienyl (Cb) ligand in the sandwich complexes [(η4‐Cb′′′′)Dy(η5‐C5Me4 t Bu)(BH4)]− (1), [(η4‐Cb′′′′)Dy(η8‐Pn†)K(THF)] (2) and [(η4‐Cb′′′′)Dy(η8‐Pn†)]− (3) leads to larger energy barriers to magnetization reversal (Cb′′′′=C4(SiMe3)4, Pn†=1,4‐di(tri‐isopropylsilyl)pentalenyl). Short distances to the Cb′′′′ ligands and longer distances to the Cp ligands in 1–3 are consistent with the crystal field splitting being dominated by the former. Theoretical analysis shows that the magnetic axes in the ground Kramers doublets of 1–3 are oriented towards the Cb′′′′ ligands. The theoretical axiality parameter and the relative axiality parameter Z and Z rel are introduced to facilitate comparisons of the SMM performance of 1–3 with a benchmark SMM. Increases in Z and Z rel when Cb′′′ replaces Cp signposts a route to SMMs with properties that could surpass leading systems.

Tuning Magnetic Relaxation in Square-Pyramidal Dysprosium Single-Molecule Magnets Using Apical Alkoxide Ligands

Tuning Magnetic Relaxation in Square-Pyramidal Dysprosium Single-Molecule Magnets Using Apical Alkoxide Ligands

Magnetic Hysteresis at 10 K in Single Molecule Magnet Self-Assembled on Gold.

Tremendous progress in the development of single molecule magnets (SMMs) raises the question of their device integration. On this route, understanding the properties of low‐dimensional assemblies of SMMs, in particular in contact with electrodes, is a necessary but difficult step. Here, it is shown that fullerene SMM self‐assembled on metal substrate from solution retains magnetic hysteresis up to 10 K. Fullerene‐SMM DySc2N@C80 and Dy2ScN@C80 are derivatized to introduce a thioacetate group, which is used to graft SMMs on gold. Magnetic properties of grafted SMMs are studied by X‐ray magnetic circular dichroism and compared to the films of nonderivatized fullerenes prepared by sublimation. In self‐assembled films, the magnetic moments of the Dy ions are preferentially aligned parallel to the surface, which is different from the disordered orientation of endohedral clusters in nonfunctionalized fullerenes. Whereas chemical derivatization reduces the blocking temperature of magnetization and narrows the hysteresis of Dy2ScN@C80, for DySc2N@C80 equally broad hysteresis is observed as in the fullerene multilayer. Magnetic bistability in the DySc2N@C80 grafted on gold is sustained up to 10 K. This study demonstrates that self‐assembly of fullerene‐SMM derivatives offers a facile solution‐based procedure for the preparation of functional magnetic sub‐monolayers with excellent SMM performance.

Rationally Designing Metal-Organic Frameworks Based on [Ln2] Magnetic Building Blocks Utilizing 2-Hydroxyisophthalate and Fine-Tuning the Magnetic Properties of Dy Analogues by Terminal Coordinated Solvents.

By utilizing the 2-hydroxyisophthalic acid (H3ipO) ligand, 2D metal-organic frameworks (MOFs) featuring rare Ophenol-bridged [Ln2]-magnetic building blocks (MBBs), [Ln2(ipO)2(DMF)(H2O)] [Ln = Gd (1), Dy (2); DMF = N,N-dimethylformamide], were rationally designed and synthesized. When the reaction solvents that behave as terminal ligands were changed, the coordination geometries of LnIII ions and the arrangement fashion of [Ln2]-MBBs for these MOFs were modified accordingly. Another type of 2D MOF of [Ln2(ipO)2(H2O)4]·2H2O [Ln = Gd (3), Dy (4)] was thus obtained. MOFs 1 and 3 exhibited favorable magnetocaloric effect, whose maximum -ΔSm values reach 30.0 and 31.7 J kg-1 K-1, respectively. None of the single-molecule-magnet (SMM) behavior was observed in 2. However, from 2 to 4, the change of the terminal coordinated solvents brought obvious improvement of the magnetic properties. MOF 4 showed interesting relaxation behavior, in which dual relaxation was only visible under weak direct-current fields, and its highest effective energy barrier (Ueff) reached up to 243 K. Ab initio calculations revealed the tuning mechanism of the terminal coordinated solvents. Their change optimized the arrangements of the magnetic axis of the DyIII centers in both each MBB and the whole framework, thus improving the magnetic anisotropy and magnetic interactions of the system. Significantly, within the [Dy2]-MBBs of 4, the angle made by the individual magnetic axis and Dy···Dy' line is nearly 0°. This case favoring a high SMM performance not only was scarcely achieved in discrete {Ln2}-SMMs with numerous members but also has never been observed in any MBB-based MOFs as far as we know.

Magnetic Relaxation Dynamics of a Binuclear Diluted Er(III)/Y(III) Compound Influenced by Lattice Solvent.

It is crucial to investigate the slow relaxation mechanisms of binuclear ErIII -based single-molecule magnets (SMMs) and explore strategies for optimizing their magnetic properties. Herein, a doped compound, [Y1.75 Er0.25 (thd)4 Pc] ⋅ 2C6 H6 (YEr ⋅ 2C6 H6 , Hthd=2,2,6,6-tetramethylheptanedione, H2 Pc=phthalocyanine), was synthesized by doping the paramagnetic erbium(III) compound Er2 ⋅ 2C6 H6 in the diamagnetic yttrium(III) matrix Y2 ⋅ 2C6 H6 . The doping effect was studied using SQUID magnetization measurements. The results suggest that magnetic-site dilution improves the magnetic property from a fast relaxation of the pure ErIII compound to a typical SMM relaxation process of the doped sample. In this binuclear system, the dominant single-ion relaxation is entangled with the neighboring ErIII ion through the intramolecular ErIII ⋅⋅⋅ErIII interaction, which plays an important role in suppressing the quantum tunneling of the magnetization (QTM) process. Furthermore, the influence of lattice solvents on single-ion relaxation was studied. By releasing the benzene molecules, compound YEr ⋅ 2C6 H6 can be successfully transformed to a desolvated sample YEr accompanied by structural alteration and improved SMM performance.

An Inconspicuous Six-Coordinate Neutral DyIII Single-Ion Magnet with Remarkable Magnetic Anisotropy and Stability.

It is a crucial challenge to address both magnetic anisotropy and stability for single-molecule magnets (SMMs) used in next-generation nanodevices. Highly axial lanthanide SMMs with neutral charge and moderate coordination numbers represent promising magnetic materials. Here, using iodide ions with large volume and low surface charge density as weak donors, we report a six-coordinate neutral dysprosium SMM [Dy(Cy3PO)2I3(CH3CN)] with a certain degree of stability exhibiting a huge thermal barrier of 1062 K and hysteresis loops open up to 9 K. Through the elaborate reduction of ligand field strength, an apparent strongly axial crystal field is provided which elicits prominent crystal-field splitting and high axiality with the thermally activated relaxation via the third-excited Kramers' doublet. Moreover, the profound influence of strong equatorial ligand substitution on the electronic structure and relaxation pathway is clearly explored in DyIII analogues. The result suggests the great potential of the reducing the transverse ligand field in the improvement of SMMs performance.

Single-Molecule Magnets DyM2 N@C80 and Dy2 MN@C80 (M=Sc, Lu): The Impact of Diamagnetic Metals on Dy3+ Magnetic Anisotropy, Dy⋅⋅⋅Dy Coupling, and Mixing of Molecular and Lattice Vibrations.

The substitution of scandium in fullerene single‐molecule magnets (SMMs) DySc2N@C80 and Dy2ScN@C80 by lutetium has been studied to explore the influence of the diamagnetic metal on the SMM performance of dysprosium nitride clusterfullerenes. The use of lutetium led to an improved SMM performance of DyLu2N@C80, which shows a higher blocking temperature of magnetization (T B=9.5 K), longer relaxation times, and broader hysteresis than DySc2N@C80 (T B=6.9 K). At the same time, Dy2LuN@C80 was found to have a similar blocking temperature of magnetization to Dy2ScN@C80 (T B=8 K), but substantially different interactions between the magnetic moments of the dysprosium ions in the Dy2MN clusters. Surprisingly, although the intramolecular dipolar interactions in Dy2LuN@C80 and Dy2ScN@C80 are of similar strength, the exchange interactions in Dy2LuN@C80 are close to zero. Analysis of the low‐frequency molecular and lattice vibrations showed strong mixing of the lattice modes and endohedral cluster librations in k‐space. This mixing simplifies the spin–lattice relaxation by conserving the momentum during the spin flip and helping to distribute the moment and energy further into the lattice.

High local coordination symmetry around the spin center and the alignment between magnetic and symmetric axes together play a crucial role in single-molecule magnet performance.

A tetradentate 8-hydroxyquinoline-based acyl hydrazone ligand (HL1 = 8-hydroxyquinoline-2-carboxaldehyde-(aminourea)hydrochloride) was elaborately used to construct a mononuclear dysprosium complex DyCl3HL1·CH3OH (1) with a nearly ideal pentagonal bipyramid coordination geometry (D5h) surrounding the Dy(iii) ion to achieve the significant performance of single-molecule magnets (SMMs). Meanwhile, the isolated high local symmetry center was successfully kept intact and further bridged to a series of double bipyramid systems by two phenolic oxygen atoms of the acyl hydrazone ligands (HL1 and HL2 = 8-hydroxyquinoline-2-carboxaldehyde-(benzoyl)hydrazine), with the formulae [Dy2Cl4(L1)2(CH3OH)2]·4C5H5N (2) and [Dy2Cl4(L2)2]·2CH3CN (3). In addition, the monodentate co-ligand anion was replaced by a larger sterically hindered ligand and a bidentate monovalent β-diketonate anion to generate [Dy2(tfo)4(L2)2(EtOH)2] (4), [Dy2(tta)4(L2)2(EtOH)2]·2(EtOH) (5) and [Dy2(dbm)4(L2)2(EtOH)2] (6) (tfo = trifluoromethanesulfonic acid, dbm = dibenzoylmethane, tta = 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione) with eight-coordinate geometry. Strikingly, the dynamic magnetic measurements revealed that complexes 1-3 did not display the expected significant SMM performance albeit they had high local symmetry. In combination with ab initio calculation, the alignment of the coordination symmetric axis and the magnetic easy axis dominates the molecular magnetic anisotropy, and the magnetic easy axis could be modulated by the distribution of coordination atoms with different electrostatic properties.

Linear hexanuclear helical dysprosium single-molecule magnets: the effect of axial substitution on magnetic interactions and relaxation dynamics.

The reaction between the rigid Schiff-base ligand H4L ([3,6-bis(2-hydroxy-3-methoxybenzylidene)hydrazinecarbonyl]-pyridazine) and different dysprosium(iii) salts afforded two new linear hexanuclear helical clusters, [Dy6L3(SCN)6(DMF)8]·4DMF (1), and [Dy6L3(NO3)6(DMF)4(H2O)2]·8DMF (2), which possess a similar Dy6 core with [Dy6L3(PhCOO)6(CH3OH)6]·11CH3OH·H2O (3). Modulation of the axial ligands around DyIII sites causes different coordination geometries and magnetic interactions, resulting in distinct magnetic relaxation behaviors. Compounds 1 and 2 show typical single-molecule magnet (SMM) properties with effective barriers (Ueff) of 15 and 68 K, respectively, which could greatly profit from strong ferromagnetic interactions compared with compound 3. Presence of a hula-hoop-like geometry of the DyIII ions and stronger ferromagnetic interactions results in better SMM performance of compound 2 than that of 1.