Reversible Pressure-Magnetic Modulation in a Tetrathiafulvalene-Based Dyad Piezochromic Dysprosium Single-Molecule Magnet.

The extreme sensitivity of trivalent lanthanide ions to crystal field variations led to the emergence of single-molecule magnetic switching under various stimuli. The use of pressure as an external stimulus instead of classic light irradiation, oxidation or any chemical reactions allows a fine tuning of the magnetic modulation. Here the well-known pure isotopically enriched [162 Dy(tta)3 (L)]⋅C6 H14 (162 Dy) Single-Molecule Magnet (SMM) (tta- =2-2-thenoyltrifluoroacetonate and L=4,5-bis(propylthio)-tetrathiafulvalene-2-(2-pyridyl)benzimidazole-methyl-2-pyridine) was experimentally investigated by single-crystal diffraction and squid magnetometry under high applied pressures. Both reversible piezochromic properties and pressure modulation of the slow magnetic relaxation behavior were demonstrated and supported by ab initio calculations. The magnetic study of the diluted sample [162 Dy0.05 Y0.95 (tta)3 (L)]⋅C6 H14 (162 Dy@Y) indicated that variations in the electronic structure have mainly intermolecular origin with weak intramolecular contribution. Quantitative magnetic interpretation concludes to a deterioration of the Orbach process for the benefit of both Raman and QTM mechanisms under applied pressure.

Solvato Modulation of the Magnetic Memory in Isotopically Enriched Erbium Polyoxometalate.

Single-Molecule Magnet (SMM) property is by essence molecular, while commonly measured in solid crystalline state. Solvent crystallization molecules are usually neglected in the analysis and interpretation of solid-state properties. The solvation/desolvation process in the polyoxometalate(POM)-based Na9 [Er(W5 O18 )2 ] ⋅ 35 H2 O SMM demonstrates that the dehydrated form relaxes more than 1000 times faster than the initial state, while the rehydration process allows the quasi complete recovering of the initial magnetic behaviour. This dehydration process is monitored by thermogravimetric analysis (TGA) and temperature-dependent X-ray powder diffraction, and rationalized by periodic quantum chemical calculations evidencing the tremendous role of the labile water molecules in the stability of the edifice. Ab-initio calculations highlight that sodium ions localization in the structure drive the magnetic responses. Isotopic enrichment with nuclear spin free (166 Er, I=0) ErIII ions shows that the relaxation dynamics in the quantum regime depends on the nuclear spin.

Lanthanide(III) Hexanuclear Circular Helicates: Slow Magnetic Relaxation, Toroidal Arrangement of Magnetic Moments, and Magnetocaloric Effects.

Four hexanuclear circular helicates, {[Dy6L6(DMF)12]·6CF3SO3·12DMF}2 (1Dy), {[Gd6L6(DMF)12]·6CF3SO3·12DMF}2 (1Gd), [Dy6L6(DMF)10(H2O)2]·6ClO4·4H2O·10DMF (2Dy), and [Gd6L6(DMF)12]·6ClO4·2H2O·10DMF (2Gd), where DMF = N,N-dimethylformamide, were synthesized by employing a glutaratedihydrazide-bridged bis(3-methoxysalicylaldehyde) ligand (H2L) and characterized structurally and magnetically. Direct-current magnetic susceptibility studies indicated predominant weak antiferromagnetic exchange interactions among gadolinium analogues, which were quantified using the PHI software, giving J = -0.003 cm-1 with g = 2.00 for 1Gd and J = -0.001 cm-1 with g = 2.02 for 2Gd. Alternating-current magnetic susceptibility measurements indicated that complexes 1Dy and 2Dy show slow relaxation of magnetization behavior, further supported by theoretical calculations that also highlighted the toroidal arrangement of the magnetic moments.

Tetrathiafulvalene-Based Helicene Ligand in the Design of a Dysprosium Field-Induced Single-Molecule Magnet.

The design of a coordination complex that involves a ligand combining both a tetrathiafulvalene core and a helicene fragment was achieved thanks to the reaction between the new 2-{1-[2-methyl[6]helicene]-4,5-[4,5-bis(propylthio)tetrathiafulvalenyl]-1 H-benzimidazol-2-yl}pyridine ligand (L) and the Dy(hfac)3·2H2O metalloprecursor. Magnetic investigations showed field-induced single-molecule-magnet (SMM) behavior under an applied magnetic field of 1000 Oe for [Dy(hfac)3(L)]·0.5CH2Cl2, while experimentally oriented single-crystal magnetic measurements allowed for determination of the magnetic anisotropy orientation. The magnetic behavior was rationalized through ab initio CASSCF/SI-SO calculations. This redox-active chiral-field-induced SMM paves the way for the design of switchable-multiproperty SMMs.

Tuning the Magnetic Interactions in Dy(III)4 Single-Molecule Magnets.

The study of mononuclear lanthanide-based systems, where the observed single-molecule magnets (SMMs) properties originate from the local description of the magnetic properties of the lanthanide ion, has been widely investigated through the literature. The case of polynuclear SMMs becomes more challenging both experimentally and theoretically due to the complexity of such architectures involving interactions between the magnetic centers. Many efforts have been focused on the understanding of the nature of these interactions and their effects on the SMM properties. In this work, a series of three structurally related tetranuclear dysprosium(III) SMMs, namely, [Dy4(L)4(OH)2(DMF)4(NO3)2]·2(DMF)·(H2O) (1), [Dy4(L)4(OH)2(DMF)2(tfaa)2]·2(CH3CN) (2), and [Dy4(L)4(OH)2(DMF)2(acac)2]·2(DMF) (3) (H2L = 2-(2-hydroxy-3-methoxybenzylideneamino)phenol, Htfaa = trifluoroacetylactone, Hacac = acetylacetonate), has been synthesized and investigated. By a fine-tuning of the ligands on the changeable coordination sites in these Dy(III)4 SMMs, the intramolecular magnetic interactions can be modified, switching from antiferromagnetic (for 1 and 2) to ferromagnetic (for 3). Ab initio calculations support these statements. In addition, the formation of 1 has been analyzed by ESI-MS analysis of the reaction mixture, indicating rather quick and high-yield formation of the [Dy4] framework in solution. The combination of experimental work and ab initio calculations offers further insight into the relationship between structures and magnetic properties and sheds light on how to tune magnetic interactions in future polynuclear dysprosium complexes.

A Dy4 Cubane: A New Member in the Single-Molecule Toroics Family.

Molecular materials that possess a toroidal moment associated to a non-magnetic ground state are known as single-molecule toroics (SMTs) and are usually planar molecules. Herein, we report a Dy4 cubane, namely [Dy4 (Bppd)4 (µ3 -OH)4 (Pa)4 (H2 O)4 ]⋅0.333 H2 O (where BppdH=1,3-Bis(pyridin-4-yl)propane-1,3-dione and PaH=2-Picolinic acid) for which magnetometry measurements and state-of-art ab initio calculations highlight SMT behavior in a tridimensional structure (3D-SMT). The in-depth theoretical analysis on the resulting low-lying energy states, along with their variation in function of the magnetic exchange pathways, allows further light to be shed on the description of single-molecule toroics and identify the coupling scheme that better reproduces the observed data.

Circularly polarized luminescence in the one-dimensional assembly of binaphtyl-based Yb(iii) single-molecule magnets.

Lanthanide ions have attracted great interest owing to their optical and magnetic properties. Single-molecule magnet (SMM) behavior has been a fascinating science for thirty years. Moreover, chiral lanthanide complexes allow the observation of remarkable circularly polarized luminescence (CPL). However, the combination of both SMM and CPL behaviors in a single molecular system is very rare and deserves attention in the design of multifunctional materials. Four chiral one-dimensional coordination compounds involving 1,1'-Bi-2-naphtol (BINOL)-derived bisphosphate ligands and the Yb(iii) centre were synthesized and characterized by powder and single-crystal X-ray diffraction. All the Yb(iii)-based polymers displayed field-induced SMM behavior with magnetic relaxation occurring by applying Raman processes and near infrared CPL in the solid state.

Modulation of the magnetic and photophysical properties in 3d-4f and 4f-4f' heterobimetallic complexes involving a tetrathiafulvalene-based ligand.

The reaction between the 2-(1-(2,6-di(pyrazol-1-yl)-4-methylpyridyl)-4,5-(4,5-bis(propylthio)-tetrathiafulvalenyl)-1H-benzimidazol-2-yl)-pyridine ligand (L), 1 equivalent of Ln(hfac)3·2H2O/Dy(tta)3·2H2O (hfac- = 1,1,1,5,5,5-hexafluoroacetylacetonate, tta- = 2-thenoyltrifluoroacetonate) and M(hfac)2·2H2O leads to the formation of heteroleptic 3d-4f dinuclear complexes of formula [MLn(hfac)5(L)]n (M(II) = Cd, Zn, Co, Mn, Ni and Ln(III) = Dy, Yb, Nd) and [ZnDy(tta)2(hfac)3(L)]·(CH2Cl2). Their X-ray structures reveal that the two coordination sites are occupied by one Ln(III) ion and one M(II) transition metal respectively. The M(II) ions are coordinated to the benzoimidazolylpyridine (bzip) moiety in a N2O4 coordination sphere, while the Ln(III) ions are coordinated to the 2,6-di(pyrazol-1-yl)-4-pyridine (dpp) moiety in a N3O6 surrounding. When Dy(III) ion is used a field-induced Single-Molecule Magnet (SMM) behavior is detected with a magnetic relaxation time slightly dependent to the nature of the vicinal divalent transition metal. On the other hand, when the Yb(III) is used, intense, moderated or quenched 2F5/2 → 2F7/2 NIR luminescence is observed when the Yb(III) ion is respectively associated with the Zn(II), Mn(II) and Ni(II)/Co(II) ion. The emission intensity can be modulated in function of the metal-to-ligand charge transfer and d-d transition intensities. The replacement of the divalent transition metal by a trivalent lanthanide leads to the formation of heteroleptic 4f-4f' dinuclear complexes of formula [Ln2-xLn'x(hfac)6(L)]·a(CH2Cl2)·b(C6H14) and [Dy1.11Nd0.89(tta)3(hfac)3(L)]. The coordination selectivity is based on the radius. Among the 4f-4f' series, the Dy(III) derivatives displayed such ion in N2O6 eight-coordinated sphere allowing the observation of SMM behavior. The three compounds [Dy1.21Nd0.79(hfac)6(L)]·2(CH2Cl2)·(C6H14), [Yb1.04Nd0.96(hfac)6(L)] and [YbPr(hfac)6(L)] displayed respectively Nd(III), modarated Yb(III) and intense Yb(III) NIR emissions.

Dysprosium(III) compounds assembled via a versatile ligand incorporating salicylic hydrazide and 8-hydroxyquinolin units: syntheses, structures and magnetic properties.

Assembly of dysprosium(iii) salts with a multidentate ligand H3L ((2-hydroxy)-N'-((8-hydroxyquinolin-2-yl)methylene)-benzohydrazide) affords a variety of products with different topological structures, namely [Dy(H2L)(HL)]·CH3OH (1), [Dy2(HL)2(C6H5COO)2(CH3OH)2]·3CH3OH (2), [Dy2(HL)2(NO3)2(DMF)4]·4DMF (3), [Dy4L4(CH3OH)4]·2CH3OH·4H2O (4) and ([Dy4(HL)4(C6H5COO)4(CH3OH)(H2O)]·2CH3OH·CH3CN·H2O)n (5). The versatile and flexible coordination modes of phenoxo groups from salicylic hydrazide prove to be a key factor in the assembly of corresponding structures depending upon the reaction conditions. It is noteworthy that ligands HL2- act as a long-distance link and further connect the Dy2 fragments into an infinite 1D chain due to the conformational flexibility resulting from the rotatable C-C bond in 5. Furthermore, the magnetic measurements were performed on all complexes. The dc magnetic susceptibility data evidence distinct magnetic coupling interactions in the dinuclear complexes 2 (antiferromagnetic) and 3 (ferromagnetic) despite their similar structures, and only complex 3 shows slow relaxation behavior of magnetization. Ab initio calculations and electrostatic potential analysis on complexes 2, 3, and three other complexes (6, 7, 8) incorporating different kinds of ligands reveal the important interrelationship of magnetic anisotropy, magnetic coupling interactions and SMM properties.

High temperature quantum tunnelling of magnetization and thousand kelvin anisotropy barrier in a Dy2 single-molecule magnet.

We report here a dinuclear DyIII iodine-bridged single-molecule magnet self-assembled by cis/trans coordination chemistry that displays a large anisotropy barrier of ca. 1300 K and a hysteresis opening temperature of 16 K. High temperature quantum tunnelling of magnetization is observed up to 56 K in zero-field and explained by the combination of the large anisotropy barrier and the local transverse field at the trans site. The results provide a model for thorough understanding of the effect of electronic structure on the magnetic behavior of lanthanide complexes.

Tetranuclear dysprosium single-molecule magnets: tunable magnetic interactions and magnetization dynamics through modifying coordination number.

The study of mononuclear lanthanide-based systems, where the observed Single Molecule Magnets (SMMs) properties originate from the local magnetic anisotropy of the single lanthanide ion, has been extensively investigated in the literature. The case for polynuclear lanthanide SMMs becomes more challenging both experimentally and theoretically due to the complexity of such architectures involving interactions between the magnetic centers. Much interest was devoted to the study of the structural effect on the magnetic interactions and relaxation dynamics. However, the understanding of the structural influence on those two factors remains a difficult task. To address this issue, a system containing two structurally related tetranuclear Dy(iii) SMMs, namely [Dy4(L)4(OH)2(DMF)4(NO3)2]·2(DMF)·(H2O) (1) and [Dy4(L)4(OH)2(DMF)2(NO3)2] (2) (H2L = 2-(2-hydroxy-3-methoxybenzylideneamino)phenol), is introduced and investigated. Through modifying the ligands on the changeable coordination sites, the intramolecular magnetic interactions and relaxation dynamics in these two Dy(iii)4 SMMs can be tuned. Both complexes exhibit slow relaxation of their magnetization with a relaxation barrier of 114 K for complex 2 while a blocking temperature below 2 K is observed for complex 1. Ab initio calculations reveal that changes in coordination numbers and geometries on the Dy(iii) sites can significantly affect the magnetic interactions as well as single-ion anisotropy. The combination of experimental work and ab initio calculations offers insight into the relationship between structures and magnetic properties and sheds light on the rational design of future polynuclear lanthanide SMMs with enhanced magnetic properties.

Structural and magnetic investigations of a binuclear coordination compound of dysprosium(iii) dinitrobenzoate.

A centro-symmetric binuclear compound of formula [Dy(L)·(CH3COO)2·(H2O)2]2 (1) was isolated from the reaction between the 2,4-dinitrobenzoate anion (L) and the tris(acetate) of Dy(iii). Single crystal diffraction studies reveal a µ1-κ2,η1:η1 chelating binding mode of L while the binuclear compound is formed by the two bridging (µ2-κ3,η1:η2) acetate anions. The nona-coordinated sphere of each Dy(iii) ion is filled with a chelating (κ2,η1:η1) acetate anion and two terminal water molecules. Static magnetic measurements combined with ab initio SA-CASSCF/RASSI-SO calculations lead to two intramolecular competitive interactions i.e. ferromagnetic exchange interactions (0.04 cm-1) and antiferromagnetic dipolar interactions (-0.5 cm-1). Finally, dynamic magnetic measurements revealed a Single-Molecule Magnet behaviour in a zero-applied magnetic field with an effective energy barrier Δ = 21.5(2) cm-1 and τ0 = 7(3) × 10-6 s through Orbach and Quantum Tunnelling of the Magnetization relaxation mechanisms.

Bromine-bridged Dy2 single-molecule magnet: magnetic anisotropy driven by cis/trans stereoisomers.

We report the first bromine-bridged dinuclear [Dy(Cy3PO)2(µ-Br)(Br)2]2·2C7H8 single-molecule magnet with an effective energy barrier of 684 K and magnetic hysteresis below 3 K. The asymmetric DyIII centres present two unique stereoisomeric octahedral coordination environments depending on the cis/trans disposition of the Cy3PO ligands, leading to the orthogonality of the easy magnetic axes that annihilates the dipolar interactions.

Field-Induced Dysprosium Single-Molecule Magnet Based on a Redox-Active Fused 1,10-Phenanthroline-Tetrathiafulvalene-1,10-Phenanthroline Bridging Triad.

Tetrathiafulvalene and 1,10-phenanthroline moieties present, respectively remarkable redox-active and complexation activities. In this work, we investigated the coordination reaction between the bis(1,10-phenanthro[5,6-b])tetrathiafulvalene triad (L) and the Dy(hfac)3·2H2O metallo precursor. The resulting {[Dy2(hfac)6(L)]·CH2Cl2·C6H14}3 (1) dinuclear complex showed a crystal structure in which the triad L bridged two terminal Dy(hfac)3 units and the supramolecular co-planar arrangement of the triads is driven by donor-acceptor interactions. The frequency dependence of the out-of-phase component of the magnetic susceptibility highlights three distinct maxima under a 2000 Oe static applied magnetic field, a sign that 1 displays a Single-Molecule Magnet (SMM) behavior with multiple magnetic relaxations. Ab initio calculations rationalized the Ising character of the magnetic anisotropy of the DyIII ions and showed that the main anisotropy axes are perpendicular to the co-planar arrangement of the triads. Single-crystal rotating magnetometry confirms the orientation of the main magnetic axis. Finally combining structural analysis and probability of magnetic relaxation pathways through Quantum Tunneling of the Magnetization (QTM) vs. excited states (Orbach), each DyIII center has been attributed to one of the three observed magnetic relaxation times. Such coordination compound can be considered as an ideal candidate to perform redox-magnetic switching.