Single-ion Magnets Research Articles

Insights into the Spin Dynamics of Mononuclear Cerium(III) Single-Molecule Magnets.

Four novel CeIII mononuclear complexes of formulas [Ce(ntfa)3(MeOH)2] (1), [Ce(ntfa)3(5,5′-Me2bipy)] (2), [Ce(ntfa)3(terpy)] (3), and [Ce(ntfa)3(bipy)2] (4), where ntfa = 4,4,4-trifluoro-1-(naphthalen-2-yl)butane-1,3-dionato, 5,5′-Me2bipy = 5,5′-dimethyl-2,2′-dipyridyl, terpy = 2,2′:6′,2″-terpyridine, and bipy = 2,2′-bipyridine, have been synthesized and structurally characterized with CeIII displaying coordination numbers of 8, 8, 9, and 10, respectively. Magnetic measurements indicate that all the complexes show a field-induced single-ion magnet behavior under a small applied dc field. The magnetic analysis shows the relevance of the different spin relaxation mechanisms in the magnetic relaxation of the CeIII compounds, with special emphasis on the local-mode process. Multiconfigurational calculations were also performed to get more information on the axiality of the compounds.

Modulating Two Pairs of Chiral DyIII Enantiomers by Distinct β-Diketone Ligands to Show Giant Differences in Single-Ion Magnet Performance and Nonlinear Optical Response.

Using Dy(dbm)3(H2O) and Dy(btfa)3(H2O)2 to react with enantiopure N-donors, (-)/(+)-4,5-pinenepyridyl-2-pyrazine (LR/LS), respectively, two pairs of chiral DyIII enantiomers, Dy(dbm)3LR/Dy(dbm)3LS (R-1-Dy/S-1-Dy) and Dy(btfa)3LR/Dy(btfa)3LS (R-2-Dy/S-2-Dy) were obtained, wherein one of the benzene rings of dbm- (dibenzoylmethanate) in R-1-Dy/S-1-Dy is displaced by the -CF3 group of btfa- (4,4,4-trifluoro-1-phenyl-1,3-butanedionate) in R-2-Dy/S-2-Dy. Interestingly, this substitution results not only in giant differences in their single-ion magnetic (SIM) performances but also in their completely different nonlinear optical (NLO) responses. R-1-Dy presents a large effective energy barrier (Ueff = 265.47 K) under zero applied field, being more than 4 × R-2-Dy (61.40 K). The discrepancy on their magnetic performances has been further elucidated by ab initio calculations. Meanwhile, R-1-Dy/S-1-Dy display the strongest third-harmonic generation responses (35/33 × α-SiO2) among the known lanthanide NLO-active coordination compounds (CCs). On the contrary, R-2-Dy/S-2-Dy exhibit moderate second-harmonic generation responses (0.65/0.70 × KDP). These results not only give the first example of the CCs with both SMM/SIM behavior and a THG response but also provide an efficient strategy for achieving the function regulation and switch in multifunctional CCs.

Competition between single ion and single chain magnetism in stoichiometric spin chain oxides [Sr4Mn2CoO9]n [Sr5Mn3CoO12

Stoichiometric spin chain “Mn4+-Co2+” oxides [Sr4Mn2CoO9]n[Sr5Mn3CoO12] have been synthesized for n ​= ​1, 2, 3, 4, 5, ∞. The X-ray powder diffraction study of these oxides also formulated Sr1+x(Mn1-xCox)O3, shows that their hexagonal aperiodic structure (a≈9.6 ​Å, c1 ​~ ​2.6 ​Å, c2 ​~ ​3.9 ​Å; modulation vector q ​= ​γc1∗, with γ ​= ​c1/c2) consists of blocks of multiple (n) Mn2CoO9 trimeric units that alternate with single Mn3CoO12 tetrameric units. Competition between single ion (SIM) and single chain magnetism (SCM) is established from ac-susceptibility magnetic measurements: introduction of tetrameric units in trimeric chains hinders SIM signal which decreases with n and disappears for n ​= ​1. A model is proposed for the origin of SIM based on the presence of [Mn4+2Co2+Mn4+2O18]18−groups in the structure. This scheme is supported by the SIM signal observed for the n ​= ​∞ member Sr2Ca2Mn2Co0.5Zn0.5O9 that contains a majority of isolated [Mn4+2Co2+Mn4+2O18] groups.

Co(II) single-ion magnets: synthesis, structure, and magnetic properties.

Magnetoactive coordination compounds exhibiting bi- or multistability between two or more magnetic stable states present an attractive example of molecular switches. Currently, the research is focused on molecular nanomagnets, especially single molecule magnets (SMMs), which are molecules, where the slow relaxation of the magnetization based on the purely molecular origin is observed. Contrary to ferromagnets, the magnetic bistability of SMMs does not require intermolecular interactions, which makes them particularly interesting in terms of application potential, especially in the high-density storage of data. This paper aims to introduce the readers into a basic understanding of SMM behaviour, and furthermore, it provides an overview of the attractive Co(II) SMMs with emphasis on the relation between structural features, magnetic anisotropy, and slow relaxation of magnetization in tetra-, penta-, and hexacoordinate complexes.Graphical abstract

Dielectric-Optical Switches: Photoluminescent, EPR, and Magnetic Studies on Organic-Inorganic Hybrid (azetidinium)2MnBr4.

A new organic–inorganic hybrid, AZEMnBr, has been synthesized and characterized. The thermal differential scanning calorimetry, differential thermal analysis, and thermogravimetric analyses indicate one structural phase transition (PT) at 346 and 349 K, on cooling and heating, respectively. AZEMnBr crystallizes at 365 K in the orthorhombic, Pnma, structure, which transforms to monoclinic P21/n at 200 K. Due to the X-ray diffraction studies, the anionic MnBr42– moiety is discrete. The azetidinium cations show dynamical disorder in the high-temperature phase. In the proposed structural PT, the mechanism is classified as an order–disorder type. The structural changes affect the dielectric response. In this paper, the multiple switches between low- and high- dielectric states are presented. In addition, it was also observed that the crystal possesses a mutation of fluorescent properties between phase ON and OFF in the PT’s point vicinity. We also demonstrate that EPR spectroscopy effectively detects PTs in structurally diverse Mn(II) complexes. AZEMnBr compounds show DC magnetic data consistent with the S = 5/2 spin system with small zero-field splitting, which was confirmed by EPR measurements and slow magnetic relaxation under the moderate DC magnetic field typical for a single-ion magnet behavior. Given the above, this organic–inorganic hybrid can be considered a rare example of multifunctional materials that exhibit dielectric, optical, and magnetic activity.

Slow-Relaxation Behavior of a Mononuclear Co(II) Complex Featuring Long Axial Co-O Bond.

Co(II) mononuclear complex with different coordination geometry would display various of field-induced single-ion magnet (SIM) behaviors. Here, we identify a field-induced single-ion magnet in a mononuclear complex Co(H2DPA)2·H2O (H2DPA = 2,6-pyridine-dicarboxylic acid) by the hydrothermal method. The long axial Co-O coordination bond (Co1‧‧‧O3) can be formed by Co1 and O3. Therefore, Co(II) ion is six-coordinated in a distorted elongated octahedron. AC magnetization susceptibilities show that the effective energy barrier is up to 43.28 K. This is much larger than most mononuclear Co(II). The distorted elongated octahedron caused by the axial Co-O coordination bond is responsible for the high effective energy barrier. The distribution of electron density in Co1 and O3 atoms in the long axial bond would influence the magnetic relaxation process in turn. Our work deepens the relationship between the effective energy barrier and the weak change of ligand field by long axial bonds, which would facilitate constructing SIM with high energy temperature.

Tuning the Equatorial Negative Charge in Hexagonal Bipyramidal Dysprosium(III) Single-Ion Magnets to Improve the Magnetic Behavior.

Taking advantage of the pentaethylene glycol (EO5) and deprotonation of EO5, a family of new structurally hexagonal bipyramidal Dy(III) complexes, [Dy(EO5)(2,6-dichloro-4-nitro-PhO)2](2,6-dichloro-4-nitro-PhO) (1), [Dy(EO5-BPh2)(2,6-dichloro-4-nitro-PhO)2] (2), and [Dy(EO5-BPh2)(2,6-dichloro-4-nitro-PhO)Cl] (3), were controbllably synthesized and structurally characterized. Magnetic measurements show that complex 1 is a zero-field SIM and has an observable hysteresis opening up to 4 K. Conversely, only under extra magnetic field is slow magnetic relaxation observed in 2 and 3. This considerable difference in the magnetic behavior is mainly caused by the change of the equatorial negative charge. Detailed ab initio calculations further elucidate that the quantum tunneling is induced by the presence of equatorial negative charge, and the magnetic anisotropy depends on the axial ligands. This work demonstrates that the absence of the equatorial negative charge should also be considered in the rational design of promising single molecular magnets based on the oblate ions.

Hydrogen-Bonded Framework of a Cobalt(II) Complex Showing Superior Stability and Field-Induced Slow Magnetic Relaxation.

A unique hydrogen-bonded organic-inorganic framework (HOIF) constructed from a mononuclear cobalt(II) complex, [Co(MCA)2·(H2O)2] (HMCA = 4-imidazolecarboxylic acid), via multiple hydrogen-bonding interactions was synthesized and structurally characterized. The Co(II) center in the HOIF features a highly distorted octahedral coordination environment. Remarkably, the CoII HOIF showed permanent porosity with superior stability as established by combined thermogravimetric analysis (TGA), variable-temperature infrared spectra (IR), variable-temperature powder X-ray diffraction data (PXRD), and a CO2 isotherm. Structural studies reveal that short multiple hydrogen bonds should be responsible for the superior thermal and chemical stability of a HIOF. Magnetic investigations reveal the large easy-plane magnetic anisotropy of the Co2+ ions with the fitted D values being 22.1 (magnetic susceptibility and magnetization data) and 29.1 cm-1 (reduced magnetization data). In addition, the HOIF exhibits field-induced slow magnetic relaxation at low temperature with an effective energy barrier of Ueff = 45.2 cm-1, indicative of a hydrogen-bonded framework single-ion magnet of the compound. The origin of the significant magnetic anisotropy of the complex was also understood from computational studies. In addition, BS-DFT calculations indicate that the superexchange interactions between the neighboring CoII ions are non-negligible antiferromagnetism with JCo-Co = -0.5 cm-1. The foregoing results provide not only a carboxylate-imidazole ligand approach toward a stable HOIF but also a promising way to build a robust single-ion magnet via hydrogen-bond interactions.

The Complete Series of Lanthanoid-Chloranilato Lattices with Dimethylsulfoxide: Role of the Lanthanoid Size on the Coordination Number and Crystal Structure

We report the synthesis, structural and magnetic characterization of the complete series of lanthanoid-based chloranilato 2D lattices with dimethylsulfoxide (dmso) formulated as: [Ln2(C6O4Cl2)3(dmso)6] with Ln = La(1), Ce(2), Pr(3), Nd(4), Sm(5), Eu(6), Gd(7) and Tb(8) or [Ln2(C6O4Cl2)3(dmso)4]·2dmso·2H2O with Ln = Dy(9), Ho(10), Er(11), Tm(12) and Yb(13); C6O4Cl22− = dianion of 3,6-dichloro-2,5-dihydroxy-1,4-benzoquinone = chloranilato. Single crystal X-ray analysis shows that the largest Ln(III) ions (La–Tb, 1–8) crystallise in the monoclinic P21/n space group (phase I), whereas the smallest ones (Dy–Yb, 9–13) crystallise in the triclinic P-1 space group (phase II). Both phases show a (6,3)-2D network with the typical hexagonal honeycomb lattice, although phase I presents important distortions, resulting in rectangular cavities with a brick-wall orientation. The largest ions (phase I) show a coordination number of nine with a capped square antiprismatic geometry in contrast to the smallest ions (phase II) that present a coordination number of eight with a triangular dodecahedral geometry. Magnetic measurements show that all the Ln(III) ions are magnetically well isolated, leading to the presence of a field induced single-ion magnet behaviour in the Er derivative, with an energy barrier of 23(2) K for DC fields of 20, 50 and 100 mT.

Trigonally Distorted Hexacoordinate Co(II) Single-Ion Magnets.

By simple reactions involving various cobalt(II) carboxylates (acetate and in situ prepared pivalate and 4-hydroxybenzoate salts) and neocuproine (neo), we were able to prepare three different carboxylate complexes with the general formula [Co(neo)(RCOO)2] (R = –CH3 for 1, (CH3)3C– for 2, and 4OH-C4H6– for 3). The [Co(neo)(RCOO)2] molecules in the crystal structures of 1–3 adopt a rather distorted coordination environment, with the largest trigonal distortion observed for 1, whereas 2 and 3 are similarly distorted from ideal octahedral geometry. The combined theoretical and experimental investigations of magnetic properties revealed that the spin Hamiltonian formalism was not a valid approach and the L-S Hamiltonian had to be used to reveal very large magnetic anisotropies for 1–3. The measurements of AC susceptibility showed that all three compounds exhibited slow-relaxation of magnetization in a weak external static magnetic field, and thus can be classified as field-induced single-ion magnets. It is noteworthy that 1 also exhibits a weak AC signal in a zero-external magnetic field.

Pseudo-mono-axial ligand fields that support high energy barriers in triangular dodecahedral Dy(iii) single-ion magnets.

The synthesis of air-stable, high-performance single-molecule magnets (SMMs) is of great significance for their practical applications. Indeed, Ln complexes with high coordination numbers are satisfactorily air stable. However, such geometries easily produce spherical ligand fields that minimize magnetic anisotropy. Herein, we report the preparation of three air-stable eight-coordinate mononuclear Dy(iii) complexes with triangular dodecahedral geometries, namely, [Dy(BPA-TPA)Cl](BPh4)2 (1) and [Dy(BPA-TPA)(X)](BPh4)2·nCH2Cl2 (X = CH3O- and n = 1 for 2; L = PhO- and n = 2 for 3), using a novel design concept in which the bulky heptadentate [2,6-bis[bis(2-pyridylmethyl)amino]methyl]-pyridine (BPA-TPA) ligand enwraps the Dy(iii) ion through weak coordinate bonds leaving only a small vacancy for a negatively charged (Cl-), methoxy (CH3O-) or phenoxy (PhO-) moiety to occupy. Magnetic measurements reveal that the single-molecule magnet (SMM) property of complex 1 is actually poor, as there is almost no energy barrier. However, complexes 2 and 3 exhibit fascinating SMM behavior with high energy barriers (U eff = 686 K for 2; 469 K for 3) and magnetic hysteresis temperatures up to 8 K, which is attributed to the pseudolinear ligand field generated by one strong, highly electrostatic Dy-O bond. Ab initio calculations were used to show the apparent difference in the magnetic dynamics of the three complexes, confirming that the pseudo-mono-axial ligand field has an important effect on high-performance SMMs compared with the local symmetry. This study not only presents the highest energy barrier for a triangular dodecahedral SMM but also highlights the enormous potential of the pseudolinear Dy-L ligand field for constructing promising SMMs.

Water-driven reversible switching of single-ion magnetism and proton conduction in a dysprosium sulfonate

A one-dimensional Dy(iii) sulfonate was reported to exhibit switched single-ion magnetism and proton conduction induced by water desorption/adsorption processes in a single-crystal-to-single-crystal transformation manner.

A single-ion magnet building block strategy toward Dy2 single-molecule magnets with enhanced magnetic performance.

A molecular dysprosium(III) complex [Dy(DClQ)3(H2O)2] (1) was used as a building unit for the construction of lanthanide SMMs, leading to the isolation of two dinuclear Dy(III) complexes, namely [Dy2(DClQ)6(MeOH)2] (2) and [Dy2(DClQ)6(bpmo)2]·6MeCN (3) (DClQ = 5,7-dichloro-8-hydroxyquinoline, bpmo = 4,4'-dipyridine-oxide). Structural analyses revealed the same N3O5 coordination environment of the Dy(III) centers with a distorted biaugmented trigonal prism (C2V symmetry) and triangular dodecahedron (D2d symmetry) for 2 and 3, respectively. Magnetic studies revealed the presence of ferromagnetic and weak antiferromagnetic exchange interactions between the Dy3+ centers in 2 and 3, respectively. Interestingly, slow relaxation of magnetization at zero fields was evidenced with an Ueff of 51.4 K and 159.0 K for complexes 2 and 3, respectively. The detailed analysis of relaxation dynamics discloses that the Orbach process is dominant for 2 whereas Raman and QTM play an important role in 3. Theoretical calculations were carried out to provide insight into the magnetic exchange interactions and relaxation dynamics for the complexes. Due to a single-ion magnet (SIM) of 1, the foregoing results demonstrate a SIM modular synthetic route for the preparation of dinuclear lanthanide SMMs.

Single-ion magnet behaviour in homoleptic Co(ii) complexes bearing 2-iminopyrrolyl ligands

Four-coordinate distorted tetrahedral bis(2-iminopyrrolyl)cobalt(ii) complexes behave as Single-Ion Magnets (SIMs) in the absence of an external magnetic field.

Lanthanide(iii)-oxamato complexes containing Nd3+ and Ho3+: crystal structures, magnetic properties, and ab initio calculations

Two series of chlorine or fluorine derivatives containing monooxamato ligands result in diamagnetic complexes of Y3+ and La3+ and paramagnetic complexes of Nd3+ and Ho3+ that behave as single-ion magnets at lower temperatures.

Phonon-induced relaxation mechanisms are changed by a chelating effect in a CoII single-ion magnet.

It is well known that phonon-induced relaxation processes play a significant role in accelerating magnetization relaxation in the low-temperature regime. Unfortunately, many SIMs (single-ion magnets) suffer from being quenched by these mechanisms such that neither out-of-phase signals nor magnetization hysteresis can be readily observed. Nevertheless, because it involves molecular motions at low-frequency (low-energy) levels, methods for synthetically controlling this factor have not yet been addressed by chemists. In this study, we prepared a series of three compounds in which one contains a rigid chelating ligand, and the other two contain analogous ligands that can coordinate more liberally. To our surprise, compound 1, with a rigid chelating ligand, displayed promising SIM behavior with out-of-phase signals up to 11 K in a zero d.c. magnetic field at an a.c. frequency of 1000 Hz. The other two (2 and 3) with dangling ligands failed to show significant out-of-phase signals until an extra d.c. field was applied. The results of magnetization relaxation studies suggest that the phonon-induced relaxation processes play an essential role in 2 and 3, even at very low temperatures. Nevertheless, the rigid chelating ligand in 1 prevents the molecule from being involved in phonon-induced relaxation processes that seriously interfere with the magnetization relaxation up to 5.6 K. Therefore, we concluded that the presence of a rigid chelating ligand can efficiently change the phonon-induced relaxation processes at low temperatures.

Slow relaxation of Dy(III) single-ion magnets dominated by the simultaneous binding of chelating ligands in low-symmetry ligand-fields.

Electronic effect and geometry distortion of low-symmetry ligand-field on the anisotropy barrier (Ueff) of spin reversal have been compared in three Dy(III) single-ion magnets through the simultaneous binding of chelating ligands. The substitution of N,O-salicylaldoxime by N,N'-1,10-phenanthroline in the distorted triangular-dodecahedronal field sharply decreases the Ueff by 286 K due to an increase in non-preferred transverse anisotropy, while the geometry distortion with CShM = 1.569 went down to 1.376 only lowering the Ueff by 12 K. The co-coordination strategy of heterodonor ligands highlights the importance of ligand-surroundings on the relaxation dynamics.

Slow magnetic relaxation of mononuclear complexes based on uncommon Kramers lanthanide ions CeIII, SmIII and YbIII.

Based on the uncommon Kramers ions CeIII, SmIII and YbIII, complexes [Ce(dppbO2)2Cl3] (1, dppbO2 = 1,2-bis(diphenylphosphino)benzene dioxide), [Sm(dppbO2)2Cl3] (2) and [Yb(dppbO2)2Cl2]Cl (3) toward single-ion magnets were obtained and fully characterized. Complexes 1 and 2 are isostructural seven-coordinate with distorted geometries between a capped trigonal prism and a capped octahedron, while 3 is six-coordinate with an octahedral geometry. Dynamic magnetic property measurements reveal their field induced slow magnetic relaxation behaviours. Fits to the temperature dependent relaxation time (τ) result in effective barriers of 38(2), 11.7(2) and 20.2(6) cm-1 for 1, 2 and 3, respectively, as well as relatively low Raman exponents (3.4(2) for 1 and 2.1(1) for 3). Ab initio calculations were then performed and they indicated that the first excited Kramers doublets (KDs) for 1, 2 and 3 lie at ca. 291, 63.5 and 137 cm-1, respectively, which are much higher than the fitting results. Combining the significant transverse magnetic moments between the ground state KDs and the first excited KDs, we thus attribute the dominant relaxation pathway of the three compounds to a Raman process, while quantum tunnelling of magnetization and direct relaxation processes may coexist. In this case, the obtained effective barriers in a high temperature region for 1 and 3 are actually the energies of the vibrational modes, which lead to second-order Raman processes with exponential temperature dependence.

Effects of strong coordination bonds at the axial or equatorial positions on magnetic relaxation for pentagonal bipyramidal dysprosium(III) single-ion magnets.

Three pentagonal bipyramidal mononuclear Dy(III) complexes based on amino-substituted nitrophenol and tetradentate amide ligands of formulas [Dy(Hbpen)(OPhNO2NH2Cl)Cl2] (1), [Dy(Hbpen)(OPhNO2NH2)Cl2] (2) and [Dy(Hbpen)(OPhNO2NH2Cl)3] (3) (Hbpen = N,N'-bis(2-pyridylmethyl)-ethylenediamine, OPhNO2NH2Cl = 2-amino-6-chloro-4-nitrophenol, and OPhNO2NH2 = 2-amino-4-nitrophenol) were isolated. X-ray diffraction studies illustrate that complexes 1 and 2 with one strongly coordinating phenol ligand at their equatorial positions have a similar structure except for a slight difference in the chloride substituent of the phenol ligand. Complex 3 possesses the same equatorial coordination as 1 but its apical positions are occupied by two other phenol ligands. Magnetic studies show that 1 and 2 are zero-field single-ion magnets (SIMs), and 3 exhibits field-induced SIM behavior. Upon removing the chloride substituent groups from the phenol ligand, the effective energy barrier enhances from 233.7 K (1) to 362.7 K (2) under external dc fields. The stronger quantum tunneling of magnetization observed for 3 in comparison with 1 shows the destructive influence of a strong phenoxyl oxygen ligand field contributing to the transverse component on the magnetic properties. A comparison of complex 2 and the reported Dy(III) analogue [Dy(Hbpen)Cl(OPhBr2NO2)2] with two phenol ligands (2,6-dibromo-4-nitrophenol) in the axial direction leads to the conclusion that the magnetic anisotropy is strongly dependent on the Dy-Ophenoxyl bond lengths. The results provide direct information vital to understanding how the strong coordination environment at the axial or equatorial positions influences the SIM behavior.

Supramolecular encapsulation of hexaaquacobalt(II) cations in a hydrogen-bonded framework for slow magnetic relaxation and high proton conduction.

The supramolecular assembly of hexaaquacobalt(II) nitrate and a tetradentate carboxylate ligand resulted in the isolation of a cobalt hydrogen-bonded organic framework (HOF). Variable-temperature X-ray diffraction experiments reveal high thermal stability of the framework sustained by charge-assisted, multiple hydrogen bonding interactions with the co-former. Interestingly, the material shows field-induced slow relaxation of magnetization originating from the magnetically anisotropic Co2+ ions within the supramolecular framework, revealing a rare single-ion magnet (SIM) HOF. Additionally, the HOF also exhibits high proton conductivity above 100 °C due to the extensive H-bond networks and high content of water and carboxylate within the material. More importantly, these results not only observe the magnetic and electrical properties of an old molecule but also demonstrate a significant turn-on effect of multifunctionalities from non-functional synthons achieved in a supramolecular approach.