Symmetry of octa-coordination environment has a substantial influence on dinuclear TbIII triple-decker single-molecule magnets

Spin dynamics phenomena of a cerium(III) double-decker complex induced by intramolecular electron transfer

Switchable spin dynamic properties in single-molecule magnets (SMMs) via an applied stimulus have applications in single-molecule devices. Many SMMs containing heavy lanthanoid ions with strong uniaxial magnetic anisotropy have been reported to exhibit SMM characteristics in the absence of an external magnetic field. On the other hand, SMMs containing light lanthanoid cerium(III) (Ce3+) ions exhibit field-induced slow magnetic relaxation. We investigated the chemical conversion of a diamagnetic Ce4+ ion (4f0) to a paramagnetic Ce3+ ion (4f1) in Ce-phthalocyaninato double-decker complexes (TBA+[Ce(obPc)2]- (1) and TBA+[Ce(Pc)2]- (2)) which exhibit field-induced SMM behaviour due to a 4f1 system. The phthalocyaninato ligands with electron-donating substituents (obPc2- = 2,3,9,10,16,17,23,24-octabutoxyphthalocyaninato) in 1 have a significant effect on the valence state of the Ce ion, which is reflected in its magnetic properties due to the mixed valence state of the Ce ion. Given that Ce double-decker complexes with π-conjugated ligands undergo intramolecular electron transfer (IET) to the Ce ion mixed valence state, characterised by a mixture of 4f0 and 4f1 configurations, we examined the dynamic disorder inherent in IET influencing magnetic relaxation.

Magnetic Anisotropy of Homo- and Heteronuclear Terbium(III) and Dysprosium(III) Trisphthalocyaninates Derived from Paramagnetic 1H-NMR Investigation

1H-NMR spectroscopy of lanthanide complexes is a powerful tool for deriving spectral-structural correlations, which provide a clear link between the symmetry of the coordination environment of paramagnetic metal centers and their magnetic properties. In this work, we have first synthesized a series of homo- (M = M* = Dy) and heteronuclear (M ≠ M* = Dy/Y and Dy/Tb) triple-decker complexes [(BuO)8Pc]M[(BuO)8Pc]M*[(15C5)4Pc], where BuO- and 15C5- are, respectively, butoxy and 15-crown-5 substituents on phthalocyanine (Pc) ligands. We provide an algorithmic approach to assigning the 1H-NMR spectra of these complexes and extracting the axial component of the magnetic susceptibility tensor, χax. We show how this term is related to the nature of the lanthanide ion and the shape of its coordination polyhedron, providing an experimental basis for further theoretical interpretation of the revealed correlations.

LnIII/MnII-LnIII complexes derived from a salicylic azo dye ligand: synthesis, structures, magnetic and fluorescence properties

Two LnIII complexes Ln(HTMSA)3(H2O)2·5.5H2O (Ln = Dy (1) and Tb (2), H2TMSA = 5-azotriazolyl-3-methoxysalicylaldehyde) and two MnII-LnIII clusters [Mn(H2O)6][MnLn2(TTMSA)4(HTTMSA)2(H2O)6]·4H2O (Ln = Dy (3) and Tb (4), H2TTMSA = 5-azotetrazolyl-3-methoxysalicylaldehyde) have been synthesized and structurally characterized. Single-crystal X-ray diffraction reveals that 1 and 2 are isostructural complexes in which the LnIII ions are surrounded by six oxygen atoms from three chelate HTMSA ligands and two oxygen atoms from two coordinated water molecules forming a distorted square-anti-prismatic geometry. In complexes 3 and 4, the MnII ions adjust two LnIII mononuclear anion clusters into tri-nuclear LnIII-MnII-LnIII anion clusters, with an additional [Mn(H2O)6]2+ as a counter ion to maintain the electroneutrality of the compound. Magnetic studies reveal that all the complexes 1-4 show nonzero out-of-phase signals, indicating single-molecule magnet behavior. The photoluminescence spectra of all the complexes were investigated and are discussed in detail.

Synthesis of Isomeric Tb3+ -Phthalocyanine Double-Decker Complexes Depending on the Difference in the Direction of Coordination Plane and Their Magnetic Properties

A C_4h symmetrically substituted phthalocyanine, 1,8,15,22-tertrakis(2,4-dimethylpent-3-oxy)phthalocyanine (H₂ TdMPPc), was used to synthesize Tb³⁺ -phthalocyanine double-decker complexes ([Tb(TdMPPc)₂ ]s). Because H₂ TdMPPc has C_4h symmetry, S,S, R,R, and meso isomers of [Tb(TdMPPc)₂ ] were obtained depending on the difference in the direction of the coordination plane of two C_4h -type phthalocyanines with respect to a central Tb³⁺ ion. We investigated the physical properties of these [Tb(TdMPPc)₂ ] isomers, including their single-ion magnetic properties, and found that the spin-reversal energy barrier (U_eff ) of the meso isomer was apparently higher than that of the enantiomers. Detailed crystal structural analyses indicated that the meso isomer has a more symmetrical structure than do the enantiomers, thereby suggesting that the higher U_eff of the meso isomer originated from the more highly symmetrical structure.

Redox-Triggered Switching of Conformational State in Triple-Decker Lanthanide Phthalocyaninates

Double- and triple-decker lanthanide phthalocyaninates exhibit unique physical-chemical properties, particularly single-molecule magnetism. Among other factors, the magnetic properties of these sandwiches depend on their conformational state, which is determined via the skew angle of the phthalocyanine ligands. Thus, in the present work we report the comprehensive conformational study of substituted terbium(III) and yttrium(III) trisphthalocyaninates in solution depending on the substituents at the periphery of molecules, redox-states and nature of solvents. Conjunction of UV-vis-NIR spectroscopy and quantum-chemical calculations within simplified time-dependent DFT in Tamm-Dancoff approximation provided the spectroscopic signatures of staggered and gauche conformations of trisphthalocyaninates. Altogether, it allowed us to demonstrate that the butoxy-substituted complex behaves as a molecular switcher with controllable conformational state, while the crown-substituted triple-decker complex maintains a staggered conformation regardless of external factors. The analysis of noncovalent interactions within the reduced density gradient approach allowed to shed light on the nature of factors stabilizing certain conformers.

Nonplanar porphyrins: synthesis, properties, and unique functionalities

Porphyrins are variously substituted tetrapyrrolic macrocycles, with wide-ranging biological and chemical applications derived from metal chelation in the core and the 18π aromatic surface. Under suitable conditions, the porphyrin framework can deform significantly from regular planar shape, owing to steric overload on the porphyrin periphery or steric repulsion in the core, among other structure modulation strategies. Adopting this nonplanar porphyrin architecture allows guest molecules to interact directly with an exposed core, with guest-responsive and photoactive electronic states of the porphyrin allowing energy, information, atom and electron transfer within and between these species. This functionality can be incorporated and tuned by decoration of functional groups and electronic modifications, with individual deformation profiles adapted to specific key sensing and catalysis applications. Nonplanar porphyrins are assisting breakthroughs in molecular recognition, organo- and photoredox catalysis; simultaneously bio-inspired and distinctly synthetic, these molecules offer a new dimension in shape-responsive host-guest chemistry. In this review, we have summarized the synthetic methods and design aspects of nonplanar porphyrin formation, key properties, structure and functionality of the nonplanar aromatic framework, and the scope and utility of this emerging class towards outstanding scientific, industrial and environmental issues.

A Nd6 molecular butterfly: a unique all-in-one material for SMM, MCE and maiden photosensitized opto-electronic device fabrication

Besides iron, ironically neodymium (Nd) is the most ubiquitously used metal for magnetic purposes, even among the lanthanides, when it comes to the field of molecular magnetism, yet it ranks among the least studied metals. However, strong apathy towards this magnetic lanthanide means that vital information will be missed, which is required for the advancement of the subject. Herein, we have successfully demonstrated the usefulness of a hexanuclear neodymium complex as a magnetic material, and also in electronic device fabrication. A {NdIII6} cage with an aesthetically pleasing butterfly topology was synthesized using a rather non-conventional N-rich pyridyl-pyrazolyl based ligand. The cage shows single molecule magnet (SMM) properties, with an effective energy barrier, U_eff, value of 3.4 K and relaxation time, τ₀, of 3.1 × 10^-4 s, originating from an unusual occurrence of metal centres with different coordination environments. Furthermore, magnetic studies reveal significant cyrogenic magnetic cooling, with a magnetic entropy change of 8.28 J kg^-1 K^-1 at 5 T and 3 K. To the best of our knowledge, the titular compound is the only example of a Nd-complex that exhibits concomitant magnetocaloric effect (MCE) and SMM properties. Complete active space self-consistent field (CASSCF) calculations were carried out to shed light on the origin of the magnetic anisotropy and magnetic relaxation of the compound. The same uniqueness is also true for the first electronic investigation carried out on the Nd complex. The maiden electronic device fabricated using the Nd complex shows an interesting intertwining of electronic and optical features, which contribute towards its improved photosensitized optoelectronic data.

Rare-earth based tetrapyrrolic sandwiches: chemistry, materials and applications

This review summarises advances in chemistry of tetrapyrrole sandwiches with rare earth elements and highlights the current state of their use in single-molecule magnetism, organic field-effect transistors, conducting materials and nonlinear optics.

Influence of the Different Types of Auxiliary Noncarboxylate Organic Ligands on the Topologies and Magnetic Relaxation Behavior of Zn-Dy Heterometallic Single Molecule Magnets

In this work, we first synthesized a Zn-Dy complex, [Zn₆Dy₂(L)₆(tea)₂(CH₃OH)₂]·6CH₃OH·8H₂O (H₂L = N-3-methoxysalicylidene-2-amino-3-hydroxypyridine, teaH₃ = triethanolamine, 1), by employing H₂L, anhydrous ZnCl₂, and Dy(NO₃)₃·5H₂O reacting with auxiliary ligand teaH₃ in the mixture of CH₃OH and DMF. When teaH₃ and the solvent CH₃OH in the reaction system of 1 were replaced by the auxiliary ligand 2,6-pyridinedimethanol (pdmH₂) and the solvent MeCN, another Zn-Dy complex, [Zn₄Dy₄(L)₆(pdm)₂(pdmH)₄]·10CH₃CN·5H₂O (2), was obtained. For 1, its crystal structure can be viewed as a dimer of two Zn₃Dy^III units. However, for 2, four Dy^III form a zigzag arrangement, and each of its terminals linked two Zn^II ions. Interestingly, although the structural topologies of 1 and 2 are different, the coordination geometries of Dy^III in 1 and 2 are all triangular dodecahedron (TDD-8). The difference is that the continuous shape measure (CShM) values of Dy^III in 1 are larger than the corresponding values in 2. Magnetic investigation revealed that the diluted sample 1@Y exhibits two magnetic relaxation processes, while 2 only exhibits a single relaxation process. Ab initio calculations indicated that, in the crystal lattice of 1, two complexes exhibiting slightly different CShM values of Dy^III result in the double relaxation behavior of 1@Y. However, for 2, one of two Dy^III fragments possesses a fast quantum tunneling of magnetization (QTM), resulting in its magnetic process presented at T < 1.8 K, so 2 exhibits single relaxation behavior. More importantly, the theoretical calculations also clearly indicated that the weak ligation at equatorial sites of Dy^III in 1 and 2 ensure 1@Y and 2 possess SMM behavior, although the coordination geometry of Dy^III (TDD-8) in 1 and 2 severely deviates from the ideal polyhedron and its axial symmetry is low.

Heteroleptic Crown-Substituted Tris(phthalocyaninates) as Dynamic Supramolecular Scaffolds with Switchable Rotational States and Tunable Magnetic Properties

Herein we report single-crystal X-ray diffraction characterization and complementary solution studies of supramolecular interaction between potassium salts and heteroleptic homo- and heteronuclear triple-decker crown phthalocyaninates [(15C5)₄Pc]M*[(15C5)₄Pc]M(Pc) or [M*,M], where M* and M = Y and/or Tb. Our results evidence that, in contrast to the previously studied crown-substituted phthalocyanines, the interaction of K⁺ cations with [M*,M] does not induce their intermolecular aggregation. Instead, the cations reversibly intercalate between the crown-substituted phthalocyanine ligands, resulting in switching of the coordination polyhedron of the metal center M* from square-antiprismatic to square-prismatic. In the case of terbium(III) complexes, such a switching alters their magnetic properties, which can be read-out by ¹H NMR spectroscopy. For [Tb*,Y], such a switching causes an almost 25% increase in the axial component of the magnetic susceptibility tensor. Even though the polyhedron of the paramagnetic center in [Y*,Tb] is not switched, minor structural perturbations associated with the overall reorganization of the receptor also cause smaller, but nevertheless appreciable, growth of the axial anisotropy. The observed effects render the studied complexes as molecular switches with tunable magnetic properties.

The construction of dynamic dysprosium-carboxylate ribbons by utilizing the hybrid-ligand conception

By utilizing the hybrid-ligand conception, three novel dysprosium complexes Dy(2-py-4-pmc)(L)(H2O) (H2-py-4-pmc = 2-(2-pyridyl)pyrimidine-4-carboxylic acid; L = fumarate (fum, 1), succinate (suc, 2), or pimelate (pim, 3)) have been successfully synthesized. Structural analysis reveals that the dicarboxylate ligands connect 2-py-4-pmc--protected Dy3+ to form one-dimensional molecular ribbons. Magnetic measurements indicate that the three complexes exhibit typical slow magnetic relaxation under a zero dc field with effective reversal barriers Ueff of 180 K, 145 K and 137 K for 1-3, respectively, which is mainly attributed to the strong Ising anisotropy of dysprosium ions induced by the appropriate arrangement of carboxylate groups. Ab initio calculations demonstrate that the charge distribution around dysprosium ions and the magnetic interactions between them are key contributions to their different dynamic behaviour.

Opening Magnetic Hysteresis by Axial Ferromagnetic Coupling: From Mono-Decker to Double-Decker Metallacrown

Combining Ising-type magnetic anisotropy with collinear magnetic interactions in single-molecule magnets (SMMs) is a significant synthetic challenge. Herein we report a Dy[15-MC_Cu -5] (1-Dy) SMM, where a Dy^III ion is held in a central pseudo-D_5h pocket of a rigid and planar Cu₅ metallacrown (MC). Linking two Dy[15-MC_Cu -5] units with a single hydroxide bridge yields the double-decker {Dy[15-MC_Cu -5]}₂ (2-Dy) SMM where the anisotropy axes of the two Dy^III ions are nearly collinear, resulting in magnetic relaxation times for 2-Dy that are approximately 200 000 times slower at 2 K than for 1-Dy in zero external field. Whereas 1-Dy and the Y^III -diluted Dy@2-Y analogue do not show remanence in magnetic hysteresis experiments, the hysteresis data for 2-Dy remain open up to 6 K without a sudden drop at zero field. In conjunction with theoretical calculations, these results demonstrate that the axial ferromagnetic Dy-Dy coupling suppresses fast quantum tunneling of magnetization (QTM). The relaxation profiles of both complexes curiously exhibit three distinct exponential regimes, and hold the largest effective energy barriers for any reported d-f SMMs up to 625 cm^-1 .

Quintuple-Decker Heteroleptic Phthalocyanine Heterometallic Samarium-Cadmium Complexes. Synthesis, Crystal Structure, Electrochemical Behavior, and Spectroscopic Investigation

A one-pot synthesis methodology was employed for obtaining diverse quintuple-decker phthalocyanine heterometallic lanthanide-cadmium complexes. By using the reaction of a double-decker homoleptic/heteroleptic phthalocyanine samarium compound with metal-free phthalocyanine and cadmium acetate in 1,2,4-trichlorobenzene at 200 °C, two novel quintuple-decker heteroleptic phthalocyanine heterometallic samarium-cadmium compounds, {(Pc)Sm(Pc)Cd(Pc*)Cd(Pc)Sm(Pc)} (1) and {(Pc)Sm(Pc*)Cd(Pc*)Cd(Pc*)Sm(Pc)} (2), together with one homoleptic phthalocyanine species, {(Pc*)Sm(Pc*)Cd(Pc*)Cd(Pc*)Sm(Pc*)} (3), were successfully fabricated, where H₂Pc and H₂Pc* represent unsubstituted phthalocyanine and 2,3,9,10,16,17,23,24-octakis(n-pentyloxy)phthalocyanine, respectively. Their quintuple-decker structures have been disclosed by various spectroscopic techniques and single-crystal X-ray diffraction. In addition, valence tautomerization of these three quintuple-decker complexes has been achieved by the addition of phenoxathiin hexachloroantimonate, giving three oxidized forms including one-, two-, and three-electron oxidation products. From 1 to 3 with the same oxidation state, the increased number of n-pentyloxy substituents of phthalocyanine ligands induces the blue shift of electronic absorption in the IR region due to the increased gap associated with the introduction of electron-donating substituents. In particular, the electronic absorption spectra of one- and two-electron oxidation products for 1 exhibit a rare band in the middle-IR region around 3000 nm, being one of the farthest electronic transitions captured by UV-vis spectroscopy. The three-electron oxidation product of 1 displays two bands at 2231 and 2740 nm, respectively. These data are well confirmed by IR spectroscopic data and theoretical calculation results.

Dinuclear Tb and Dy complexes supported by hybrid Schiff-base/calixarene ligands: synthesis, structures and magnetic properties

The synthesis of the new lanthanide complexes [HNEt3][Dy2(HL1)(L1)] (5), and [Ln2(L2)2] (Ln = TbIII (7), DyIII (8)) supported by the hybrid Schiff-base/calix[4]arene ligands H4L1 (25-[2-((2-methylphenol)imino)ethoxy]-26,27,28-trihydroxy-calix[4]arene) and H3L2 (25-[2-((2-methylpyridine)imino)ethoxy]-26,27,28-trihydroxy-calix[4]arene) are reported. Spectroscopic data (for 5) and X-ray crystallographic analysis (for 7·4MeCN, 8·4MeCN) reveal the presence of dimeric structures, featuring doubly-bridged NO4Ln(μ-O)2LnO4N (5) or N2O3Ln(μ-O)2LnO3N2 cores (7, 8) with seven-coordinated Ln3+ ions. The magnetic properties of polycrystalline samples of 5, 7 and 8 were studied by variable temperature dc and ac magnetic susceptibility measurements. The χ''(T) vs. T plots show no maxima in zero field, but the maxima can be detected under a 3 kOe dc field. The relaxation times τ obey the Arrhenius law above 5 K. Anisotropy barriers of ∼18 cm-1 (26 K) for 5 and ∼23 cm-1 (33 K) for 8 were determined.

Fulvalene as a platform for the synthesis of a dimetallic dysprosocenium single-molecule magnet

The dinucleating fulvalenyl ligand [1,1',3,3'-(C5 t Bu2H2)2]2- (Fvtttt) was used to synthesize the dimetallic dysprosocenium cation [{Dy(η5-Cp*)}2(μ-BH4)(η5:η5-Fvtttt)]+ (3) as the salt of [B(C6F5)4]- (Cp* = C5Me5). Compound [3][B(C6F5)4] was obtained using a method in which the double half-sandwich complex [{Dy(BH4)2(THF)}2(Fvtttt)] (1) was reacted with KCp* to give the double metallocene [{Dy(Cp*)(μ-BH4)}2(Fvtttt)] (2), followed by removal of a bridging borohydride ligand upon addition of [(Et3Si)2(μ-H)][B(C6F5)4]. The dimetallic fulvalenyl complexes 1-3 give rise to single-molecule magnet (SMM) behaviour in zero applied field, with the effective energy barriers of 154(15) cm-1, 252(4) cm-1 and 384(18) cm-1, respectively, revealing a significant improvement in performance across the series. The magnetic properties are interpreted with the aid of ab initio calculations, which show substantial increases in the axiality of the crystal field from 1 to 2 to 3 as a consequence of the increasingly dominant role of the Fvtttt and Cp* ligands, with the barrier height and hysteresis properties being attenuated by the equatorial borohydride ligands. The experimental and theoretical results described in this study furnish a blueprint for the design and synthesis of poly-cationic dysprosocenium SMMs with properties that may surpass those of benchmark systems.

Manipulation of the Coordination Geometry along the C4 Rotation Axis in a Dinuclear Tb3+ Triple-Decker Complex via a Supramolecular Approach

A supramolecular complex (1⋅C₆₀ ) was prepared by assembling (C60-Ih)[5,6]fullerene (C₆₀ ) with the dinuclear Tb³⁺ triple-decker complex [(TPP)Tb(Pc)Tb(TPP)] (1: Tb³⁺ =trivalent terbium ion, Pc^2- =phthalocyaninato, TPP^2- =tetraphenylporphyrinato) with quasi-D_4h symmetry to investigate the relationship between the coordination symmetry and single-molecule magnet (SMM) properties. Tb³⁺ -Pc triple-decker complexes (Tb₂ Pc₃ ) have an important advantage over Tb³⁺ -Pc double-decker complexes (TbPc₂ ) since the magnetic relaxation processes correspond to the Zeeman splitting when there are two 4f spin systems. The two Tb³⁺ sites of 1 are equivalent, and the twist angle (φ) was determined to be 3.62°. On the other hand, the two Tb³⁺ sites of 1⋅C₆₀ are not equivalent. The φ values for sites Tb1 and Tb2 were determined to be 3.67° and 33.8°, respectively, due to a change in the coordination symmetry of 1 upon association with C₆₀ . At 1.8 K, 1 and 1⋅C₆₀ undergo different magnetic relaxations, and the changes in the ground state affect the spin dynamics. Although 1 and 1⋅C₆₀ relax via QTM in a zero applied magnetic field (H), H dependencies of the magnetic relaxation times (τ) for H>1500 Oe are similar. On the other hand, for H<1500 Oe, the τ values have different behaviors since the off-diagonal terms ( B k q ; q ≠ 0 ) affect the magnetic relaxation mechanism. From temperature (T) and H dependences of τ, spin-phonon interactions along with direct and Raman mechanisms explain the spin dynamics. We believe that a supramolecular method can be used to control the magnetic anisotropy along the C₄ rotation axis and the spin dynamic properties in dinuclear Ln³⁺ -Pc multiple-decker complexes.

Crystallization and single molecule magnetic behavior of quadruple-stranded helicates: tuning the anisotropic axes

It has been successfully proven that the bis-β-diketone ligands with proper lengths and flexibility are essential for the construction of multiple-stranded helicates, where two Dy3+ centers in subtly different environments allow the tuning of the anisotropic axes. Based on our previous work, we have designed a flexible bis-β-diketone ligand BTT (BTT = 3,3''-bis(4,4,4-trifluoro-1,3-dioxobutyl)-m-terphenyl), which is successfully utilized to construct quadruple-stranded helicates with the formula of [C6H16N]2[Ln2(BTT)4]·2CH2Cl2·4CH3OH [Ln = Ce (1), Dy (2)]. Structural analysis indicates that they crystallize in the tetragonal space group P4/n, and each Ln3+ center is chelated by four diketonate moieties from four ligands, giving rise to a dinuclear quadruple-stranded helicate. Magnetic measurements show that 2 displays single molecular magnet behavior under an applied DC field of 2000 Oe. Further investigations indicate that the anisotropic axes of the Dy3+ centers can be tuned depending on the bis-β-diketonate ligands used to assemble the Dy helicates.

Relationship between the Coordination Geometry and Spin Dynamics of Dysprosium(III) Heteroleptic Triple-Decker Complexes

When using single molecule magnets (SMMs) in spintronics devices, controlling the quantum tunneling of the magnetization (QTM) and spin-lattice interactions is important. To improve the functionality of SMMs, researchers have explored the effects of changing the coordination geometry of SMMs and the magnetic interactions between them. Here, we report on the effects of the octa-coordination geometry on the magnetic relaxation processes of dinuclear dysprosium(III) complexes in the low-temperature region. Mixed ligand dinuclear Dy3+ triple-decker complexes [(TPP)Dy(Pc)Dy(TPP)] (1), which have crystallographically equivalent Dy3+ ions, and [(Pc)Dy(Pc)Dy(TPP)] (2), which have non-equivalent Dy3+ ions, (Pc2− = phthalocyaninato; TPP2− = tetraphenylporphyrinato), undergo dual magnetic relaxation processes. This is due to the differences in the ground states due to the twist angle (φ) between the ligands. The relationship between the off-diagonal terms and the dual magnetic relaxation processes that appears due to a deviation from D4h symmetry is discussed.

Solvent-tuned magnetic exchange interactions in Dy2 systems ligated by a μ-phenolato heptadentate Schiff base

A series of binuclear dysprosium compounds, namely, [Dy(api)]₂ (1), [Dy(api)]₂·2CH₂Cl₂ (2), [Dy(Clapi)]₂·2C₄H₈O (3), and [Dy(Clapi)]₂·2C₃H₆O (4) (H₃api = 2-(2-hydroxyphenyl)-1,3-bis[4-(2-hydroxyphenyl)-3-azabut-3-enyl]-1,3-imidazoline; H₃Clapi = 2-(2′-hydroxy-5′-chlorophenyl)-1,3-bis[3′-aza-4′-(2′′-hydroxy-5′′-chlorophenyl)prop-4′-en-1′-yl]-1,3-imidazolidine), have been isolated by the reactions of salen-type ligands H₃api/H₃Clapi with DyCl₃·6H₂O in different solvent systems. Structural analysis reveals that each salen-type ligand provides a heptadentate coordination pocket (N₄O₃) to encapsulate a Dy^III ion and all of the Dy^III centers in 1–4 adopt a distorted square antiprism geometry with D_4d symmetry. Magnetic studies showed that compound 1 did not exhibit single-molecule magnetic (SMMs) behavior. With the introduction of different lattice solvents, compounds 2–4 showed filed-induced slow magnetic relaxation with barriers U_eff of 18.2 K (2), 28.0 K (3) and 16.4 K (4), respectively. Ab initio calculations were employed to interpret the magnetization behavior of 1–4. The combination of experimental and theoretical data reveal the importance of the weak exchange interaction between the Dy^III ions in the observation of slow magnetic relaxation, and a relaxation mechanism has been developed to rationalize the observed difference in the U_eff values. The different lattice solvents influence Dy–O–Dy bond angles and thus alter the torsion of the square antiprism geometry, consequently resulting in distinct magnetic interactions and the magnetic behavior.

Solvent-tuning changes the magnetic exchange interaction and results in different magnetic relaxation dynamics in Dy₂ systems ligated by a μ-phenolato heptadentate Schiff base.

Enhancing the energy barrier of dysprosium(iii) single-molecule magnets by tuning the magnetic interactions through different N-oxide bridging ligands

The synergistic effect of strengthened Dy-Dy magnetic interactions and slightly enhanced symmetry of Dy^III ions results in a great U_eff enhancement from 11.79 K to 226.73 K.

From zero-dimensional to one-dimensional chain N-oxide bridged compounds with enhanced single-molecule magnetic performance

A series of zero-dimensional dinuclear dysprosium complexes bridged by pyridine-NO ligands were extended by double N-oxide bridged ligand to series of one-dimensional chain complexes with repeated Dy2 unit, they were structurally and magnetically characterized in this work.

Observation of Cooperative Electronic Quantum Tunneling: Increasing Accessible Nuclear States in a Molecular Qudit

As an extension of two-level quantum bits (qubits), multilevel systems, so-called qu dits, where d represents the Hilbert space dimension, have been predicted to reduce the number of iterations in quantum-computation algorithms. This has been tested in the well-known [TbPc₂]⁰ single-molecule magnet (SMM), which allowed implementation of the Grover algorithm in a single molecular unit. In the quest for molecular systems possessing an increased number of accessible nuclear spin states, we explore herein a dimeric Tb₂-SMM via single-crystal μ-SQUID measurements at sub-Kelvin temperatures. We observe ferromagnetic interactions between the Tb^III ions and cooperative quantum tunneling of the electronic spins with spin ground state | J _z = ±6⟩. Strong hyperfine coupling with the Tb^III nuclear spins leads to a multitude of spin-reversal paths, leading to seven strong hyperfine-driven tunneling steps in the hysteresis loops. Our results show the possibility of reading out the Tb^III nuclear spin states via cooperative tunneling of the electronic spins, making the dimeric Tb₂-SMM an excellent nuclear spin qu dit candidate with d = 16.

Redox-Driven Symmetry Change for Terbium(III) Bis(porphyrinato) Double-Decker Complexes by the Azimuthal Rotation of the Porphyrin Macrocycles

Molecular structures for three oxidation forms (anion, radical, and cation) of terbium(III) bis(porphyrinato) double-decker complexes have been systematically studied. We found that the redox state controls the azimuthal rotation angle (φ) between the two porphyrin macrocycles. For [Tb^III (tpp)₂ ]ⁿ (tpp: tetraphenylporphyrinato, n=-1, 0, and +1), φ decreases at each stage of the oxidation process. The decrease in φ is due to the higher steric repulsion between the phenyl rings on the porphyrin macrocycle and the β hydrogen atoms on the other porphyrin macrocycle, which results from the shorter interfacial distance between the two porphyrin macrocycles. Conversely, φ=45° for both [Tb^III (oep)₂ ]^-1 and [Tb^III (oep)₂ ]⁰ (oep: octaethylporphyrinato), but φ=36° for [Tb^III (oep)₂ ]⁺¹ . Theoretical calculations suggest that the smaller azimuthal rotation angle of the cation form is due to the electronic interaction in the doubly oxidized ligand system.

Comparison of the Magnetic Anisotropy and Spin Relaxation Phenomenon of Dinuclear Terbium(III) Phthalocyaninato Single-Molecule Magnets Using the Geometric Spin Arrangement

Herein we report the synthesis and characterization of a dinuclear Tb^III single-molecule magnet (SMM) with two [TbPc₂]⁰ units connected via a fused-phthalocyaninato ligand. The stable and robust complex [(obPc)Tb(Fused-Pc)Tb(obPc)] (1) was characterized by using synchrotron radiation measurements and other spectroscopic techniques (ESI-MS, FT-IR, UV). The magnetic couplings between the Tb^III ions and the two π radicals present in 1 were explored by means of density functional theory (DFT). Direct and alternating current magnetic susceptibility measurements were conducted on magnetically diluted and nondiluted samples of 1, indicating this compound to be an SMM with improved properties compared to those of the well-known [TbPc₂]^-/0/+ and the axially symmetric dinuclear Tb^III phthalocyaninato triple-decker complex (Tb₂(obPc)₃). Assuming that the probability of quantum tunneling of the magnetization (QTM) occurring in one TbPc₂ unit is P_QTM, the probability of QTM simultaneously occurring in 1 is P_QTM², meaning that QTM is effectively suppressed. Furthermore, nondiluted samples of 1 underwent slow magnetic relaxation times (τ ≈ 1000 s at 0.1 K), and the blocking temperature (T_B) was determined to be ca. 16 K with an energy barrier for spin reversal (U_eff) of 588 cm^-1 (847 K) due to D_4d geometry and weak inter- and intramolecular magnetic interactions as an exchange bias (H_bias), reducing QTM. Four hyperfine steps were observed by micro-SQUID measurement. Furthermore, solution NMR measurements (one-dimensional, two-dimensional, and dynamic) were done on 1, which led to the determination of the high rotation barrier (83 ± 10 kJ/mol) of the obPc ligand. A comparison with previously reported Tb^III triple-decker compounds shows that ambient temperature NMR measurements can indicate improvements in the design of coordination environments for SMMs. A large U_eff causes strong uniaxial magnetic anisotropy in 1, leading to a χ_ax value (1.39 × 10^-30 m³) that is larger than that for Tb₂(obPc)₃ (0.86 × 10^-30 m³). Controlling the coordination environment and spin arrangement is an effective technique for suppressing QTM in TbPc₂-based SMMs.

Modulating the Magnetic Interaction in New Triple-Decker Dysprosium(III) Single-Molecule Magnets

A new type of dinuclear dysprosium(III) complex based on phthalocyanine and salicylaldehyde derivatives (HL-R), [Dy₂(Pc)₂(L-R)₂(H₂O)]·2THF (R = OCH₃ (1), OC₂H₅ (2); H₂Pc = phthalocyanine; HL-OCH₃ = 2-hydroxy-3-methoxybenzaldehyde; HL-OC₂H₅ = 3-ethoxy-2-hydroxybenzaldehyde), was successfully synthesized and structurally characterized. Complex 1 features a sandwich-type triple-decker structure, where two coplanar L-OCH₃ ligands lie in the middle layer shared by two eight-coordinated Dy^III ions and two Pc ligands are located in the outer layer. In 2, the introduction of an ethoxy group generates a noncoordination mode for the O_alkoxy atom. Magnetic studies indicate that complex 1 behaves as a zero-field single-molecule magnet with a higher energy barrier, while 2 exhibits a fast tunneling relaxation process. Theoretical calculations revealed that changes in the ligand field environment around Dy^III ions can significantly affect the arrangement of the main magnetic axes and further result in distinct magnetic interactions as well as different relaxation behaviors.

Supramolecular Approach for Enhancing Single-Molecule Magnet Properties of Terbium(III)-Phthalocyaninato Double-Decker Complexes with Crown Moieties

A Tb^III -phthalocyaninato double-decker ([1]⁰ ) single-molecule magnet (SMM) having four 15-crown-5 moieties in one of the ligands was synthesized, and its dimerization and magnetic properties were studied in an attempt to utilize the supramolecular aggregation for enhancing the SMM properties. Aggregation of [1]⁰ to form [1₂ K₄ ]⁴⁺ in the presence of K⁺ ions was studied by using UV/Vis-NIR absorption and NMR spectroscopies. For the magnetic measurements, [1]⁰ and [1₂ K₄ ]⁴⁺ were dispersed in poly(methyl methacrylate) (PMMA). UV/Vis-NIR absorption measurements on the PMMA dispersed samples were used to track the formation of [1₂ K₄ ]⁴⁺ . Direct current (DC) magnetic susceptibility measurements revealed that there were ferromagnetic Tb-Tb interactions in [1₂ K₄ ]⁴⁺ , whereas there was no indication of ferromagnetic interactions in [1]⁰ . Upon the formation of [1₂ K₄ ]⁴⁺ from [1]⁰ and K⁺ ions, the temperature at which the magnetic hysteresis occurred increased from 7 to 15 K. In addition, the area of magnetic hysteresis became larger for [1₂ K₄ ]⁴⁺ , meaning that SMM properties of [1₂ K₄ ]⁴⁺ are superior to those of [1]⁰ . Alternating current (AC) magnetic measurements were used to confirm this observation. Magnetic relaxation times at 2 K increased 1000-fold upon dimerization of [1]⁰ to [1₂ K₄ ]⁴⁺ , demonstrating the effectiveness of using K⁺ ions to induce dimer formation for the improvement of the SMM properties.

Consecutive one-/two-step relaxation transformations of single-molecule magnets via coupling dinuclear dysprosium compounds with chloride bridges

A dimer of a dinuclear dysprosium(iii) SMM has been successfully isolated by assembling Dy₂ SMMs. Such structural transformation involves the generation and cleavage of chloride bridges and leads to consecutive transformations of one- and two-step slow relaxation.

Elucidation of the two-step relaxation processes of a tetranuclear dysprosium molecular nanomagnet through magnetic dilution

A new centrosymmetric tetranuclear aggregate [Dy₄(L)₂(OAc)₈(CH₃OH)₂] (1) was assembled using a unique symmetrical Schiff base ligand 1,5-bis(salicylidene)-carbohydrazide (H₂L).

Anion-driven structures and SMM behavior of dinuclear terbium and ytterbium complexes

SMM behavior is observed for [Ln₂(HL)₄(NO₃)₆] (Ln = Tb and Yb, HL = 8-hydroxyquinaldine) complexes having nine-coordinated lanthanide centres, whereas, [Ln₂(HL)₄Cl₆]·2EtOH complexes, having hexa-coordinated lanthanide centres do not show any SMM property.

Two Series of Homodinuclear Lanthanide Complexes: Greatly Enhancing Energy Barriers through Tuning Terminal Solvent Ligands in Dy2 Single-Molecule Magnets

The utilization of 2-ethoxy-6-{[(2-hydroxy-3-methoxybenzyl)imino]methyl}phenol (H₂ L) as a chelating ligand, in combination with the employment of alcohols (EtOH and MeOH) as auxiliary ligands, in 4 f-metal chemistry afforded two series of dinuclear lanthanide complexes of compositions [Ln₂ L₂ (NO₃ )₂ (EtOH)₂ ] (Ln=Sm (1), Eu (2), Gd (3), Tb (4), Dy (5), Ho (6), Er (7)) and [Ln₂ L₂ (NO₃ )₂ (MeOH)₂ ] (Ln=Sm (8), Eu (9), Gd (10), Tb (11), Dy (12), Ho (13), Er (14)). The structures of 1-14 were determined by single-crystal X-ray crystallography. Complexes 1-7 are isomorphous. The two lanthanide(III) ions in 1-7 are doubly bridged by two deprotonated aminophenoxide oxygen atoms of two μ₂ :η⁰ :η¹ :η² :η¹ :η¹ :η⁰ -L^2- ligands. One nitrogen atom, two oxygen atoms of the NO₃^- anion, two methoxide oxygen atoms of two ligand sets, and one oxygen atom of the terminally coordinated EtOH molecule complete the distorted dodecahedron geometry of each lanthanide(III) ion. Compounds 8-14 are isomorphous and their structures are similar to those of 1-7. The slight difference between 1-7 and 8-14 stems from purposefully replacing the EtOH ligands in 1-7 with MeOH in 8-14. Direct-current magnetic susceptibility studies in the 2-300 K range reveal weak antiferromagnetic interactions for 3, 4, 7, 10, 11, and 14, and ferromagnetic interactions at low temperature for 5, 6, 12, and 13. Complexes 5 and 12 exhibit single-molecule magnet (SMM) behavior with energy barriers of 131.3 K for 5 and 198.8 K for 12. The energy barrier is significantly enhanced by dexterously regulating the terminal ligands. To rationalize the observed difference in the magnetic behavior, complete-active-space self-consistent field (CASSCF) calculations were performed on two Dy₂ complexes. Subtle variation in the angle between the magnetic axes and the vector connecting two dysprosium(III) ions results in a weaker influence on the tunneling gap of individual dysprosium(III) ions by the dipolar field in 12. This work proposes an efficient strategy for synthesizing Dy₂ SMMs with high energy barriers.

Elucidation of Dual Magnetic Relaxation Processes in Dinuclear Dysprosium(III) Phthalocyaninato Triple-Decker Single-Molecule Magnets Depending on the Octacoordination Geometry

When applying single-molecule magnets (SMMs) to spintronic devices, control of the quantum tunneling of the magnetization (QTM) as well as a spin-lattice interactions are important. Attempts have been made to use not only coordination geometry but also magnetic interactions between SMMs as an exchange bias. In this manuscript, dinuclear dysprosium(III) (Dy^III ) SMMs with the same octacoordination geometry undergo dual magnetic relaxation processes at low temperature. In the dinuclear Dy^III phthalocyaninato (Pc^2- ) triple-decker type complex [(Pc)Dy(ooPc)Dy(Pc)] (1) (ooPc^2- =2,3,9,10,16,17,23,24-octakis(octyloxy)phthalocyaninato) with a square-antiprismatic (SAP) geometry, the ground state is divided by the Zeeman effect, and level intersection occurs when a magnetic field is applied. Due to the ground state properties of 1, since the Zeeman diagram where the levels intersect in an H_dc of 2500 Oe, two kinds of QTM and direct processes occur. However, dinuclear Dy^III -Pc systems with C₄ geometry, which have a twist angle (ϕ) of less than 45° do not undergo dual magnetic relaxation processes. From magnetic field and temperature dependences, the dual magnetic relaxation processes were clarified.

Changing Single-Molecule Magnet Properties of a Windmill-Like Distorted Terbium(III) α-Butoxy-Substituted Phthalocyaninato Double-Decker Complex by Protonation/Deprotonation

Synthesis, structures, and magnetic properties of α-butoxy-substituted phthalocyaninato double-decker complexes Tb(α-obPc)₂ (1^-) (α-obPc: dianion of 1,4,8,11,15,18,22,25-octa(n-butoxy)phthalocyaninato) with protonated (1H), deprotonated (1[HDBU]), and diprotonated forms (1H₂⁺) are discussed. X-ray analysis was used to confirm the position of the proton in 1H, and it was revealed that the protonation induced asymmetric distortion in 1H. In contrast, 1[HDBU] was distorted in a highly symmetric windmill-like fashion. 1H is arranged in a slipped column array in the crystal packing, whereas 1[HDBU] is arranged in a one-dimensional fashion, in which the magnetic easy axes of 1[HDBU] lie along the same line. From direct-current (dc) magnetic measurements, ferromagnetic Tb-Tb interactions occur in both 1H and 1[HDBU], and magnetic hysteresis was observed. However, the area of the magnetic hysteresis in 1[HDBU] is larger than that in 1H, meaning that magnetic relaxation time (τ) is longer in 1[HDBU]. In addition, the results of alternating-current magnetic measurements in a zero dc magnetic field indicate that τ of 1[HDBU] is longer as compared to 1H. In other words, protonation/deprotonation affects not only the molecular structures and crystal packing but also the single-molecule magnet properties.

Molecular Orientation of a Terbium(III)-Phthalocyaninato Double-Decker Complex for Effective Suppression of Quantum Tunneling of the Magnetization

Single-molecule magnet (SMM) properties of crystals of a terbium(III)-phthalocyaninato double-decker complex with different molecular packings (1: TbPc₂, 2: TbPc₂·CH₂Cl₂) were studied to elucidate the relationship between the molecular packing and SMM properties. From single crystal X-ray analyses, the high symmetry of the coordination environment of 2 suggested that the SMM properties were improved. Furthermore, the shorter intermolecular Tb–Tb distance and relative collinear alignment of the magnetic dipole in 2 indicated that the magnetic dipole–dipole interactions were stronger than those in 1. This was confirmed by using direct current magnetic measurements. From alternating current magnetic measurements, the activation energy for spin reversal for 1 and 2 were similar. However, the relaxation time for 2 is three orders of magnitude slower than that for 1 in the low-T region due to effective suppression of the quantum tunneling of the magnetization. These results suggest that the SMM properties of TbPc₂ highly depend on the molecular packing.

Hyperfine-Interaction-Driven Suppression of Quantum Tunneling at Zero Field in a Holmium(III) Single-Ion Magnet

An extremely rare non-Kramers holmium(III) single-ion magnet (SIM) is reported to be stabilized in the pentagonal-bipyramidal geometry by a phosphine oxide with a high energy barrier of 237(4) cm^-1 . The suppression of the quantum tunneling of magnetization (QTM) at zero field and the hyperfine structures originating from field-induced QTMs can be observed even from the field-dependent alternating-current magnetic susceptibility in addition to single-crystal hysteresis loops. These dramatic dynamics were attributed to the combination of the favorable crystal-field environment and the hyperfine interactions arising from ¹⁶⁵ Ho (I=7/2) with a natural abundance of 100 %.

Metal-Organic Framework of Lanthanoid Dinuclear Clusters Undergoes Slow Magnetic Relaxation

Lanthanoid metal-organic frameworks (Ln-MOFs) can adopt a variety of new structures due to the large coordination numbers of Ln metal ions, and Ln-MOFs are expected to show new luminescence and magnetic properties due to the localized f electrons. In particular, some Ln metal ions, such as Dy(III) and Tb(III) ions, work as isolated quantum magnets when they have magnetic anisotropy. In this work, using 4,4′,4″-s-triazine-2,4,6-triyl-tribenzoic acid (H₃TATB) as a ligand, two new Ln-MOFs, [Dy(TATB)(DMF)₂] (1) and [Tb(TATB)(DMF)₂] (2), were obtained. The Ln-MOFs contain Ln dinuclear clusters as secondary building units, and 1 underwent slow magnetic relaxation similar to single-molecule magnets.

End-to-end azido-pinned interlocking lanthanide squares

A self-assembled end-to-end azido-pinned interlocking lanthanide square displays a record energy barrier of 152(4) K among lanthanide azido-bridged SMMs in a zero dc field.

A series of six-membered lanthanide rings based on 2,2-bis(hydroxymethyl)-2,2′,2″-nitrilotriethanol: synthesis, crystal structures and magnetic properties

A series of six-membered ring cyclic lanthanide(iii) clusters have been constructed by utilizing the pentol ligand.

Phthalocyanine supported dinuclear LnIIIcomplexes: the solvent-induced change of magnetic properties in dysprosium(iii) analogues

Three phthalocyanine-supported dinuclear Ln^IIIcomplexes were obtained and characterized. The Dy^IIIanalogue exhibits reversible solvent dependent single-molecule magnetic properties between a single relaxation and a dual relaxation process.

Dysprosium Acetylacetonato Single-Molecule Magnet Encapsulated in Carbon Nanotubes

Dy single-molecule magnets (SMMs), which have several potential uses in a variety of applications, such as quantum computing, were encapsulated in multi-walled carbon nanotubes (MWCNTs) by using a capillary method. Encapsulation was confirmed by using transmission electron microscopy (TEM). In alternating current magnetic measurements, the magnetic susceptibilities of the Dy acetylacetonato complexes showed clear frequency dependence even inside the MWCNTs, meaning that this hybrid can be used as magnetic materials in devices.