Intramolecular Magnetic Interactions Research Articles

Endohedral metallofullerene molecular nanomagnets.

Magnetic lanthanide (Ln) metal complexes exhibiting magnetic bistability can behave as molecular nanomagnets, also known as single-molecule magnets (SMMs), suitable for storing magnetic information at the molecular level, thus attracting extensive interest in the quest for high-density information storage and quantum information technologies. Upon encapsulating Ln ion(s) into fullerene cages, endohedral metallofullerenes (EMFs) have been proven as a promising and versatile platform to realize chemically robust SMMs, in which the magnetic properties are able to be readily tailored by altering the configurations of the encapsulated species and the host cages. In this review, we present critical discussions on the molecular structures and magnetic characterizations of EMF-SMMs, with the focus on their peculiar molecular and electronic structures and on the intriguing molecular magnetism arising from such structural uniqueness. In this context, different families of magnetic EMFs are summarized, including mononuclear EMF-SMMs wherein single-ion anisotropy is decisive, dinuclear clusterfullerenes whose magnetism is governed by intramolecular magnetic interaction, and radical-bridged dimetallic EMFs with high-spin ground states that arise from the strong ferromagnetic coupling. We then discuss how molecular assemblies of SMMs can be constructed, in a way that the original SMM behavior is either retained or altered in a controlled manner, thanks to the chemical robustness of EMFs. Finally, on the basis of understanding the structure-magnetic property correlation, we propose design strategies for high-performance EMF-SMMs by engineering ligand fields, electronic structures, magnetic interactions, and molecular vibrations that can couple to the spin states.

Two Pairs of Homochiral Parallelogram-like Dy4 Cluster Complexes with Strong Magneto-Optical Properties.

Two pairs of homochiral Dy(III) tetranuclear cluster complexes derived from (+)/(-)-3-trifluoroacetyl camphor (D-Htfc/L-Htfc), [Dy4(OH)2(L1)4(D-tfc)2(DMF)2]·4DMF (D-1) [H2L1 = (E)-2-(2-hydroxy-3-methoxybenzylideneamino)phenol)]/[Dy4(OH)2(L1)4(L-tfc)2(DMF)2]·4DMF (L-1) and [Dy4(OH)2(L2)4(D-tfc)2(DMF)2]·2H2O·3MeCN (D-2) [H2L2 = (E)-3-(2-hydroxy-3-methoxybenzylideneamino)naphthalen-2-ol]/[Dy4(OH)2(L2)4(L-tfc)2(DMF)2]·2H2O·3MeCN (L-2), were synthesized at room temperature, which have a Dy4 parallelogram-like core. The magnetic studies revealed that D-1 exhibits single-molecule magnet (SMM) behavior under zero dc magnetic field, and its magnetic relaxation has a distinct Raman process in addition to the Orbach process, with the Ueff/k value of 57.5 K and the C value of 28.27 s-1K-2.14; while D-2 displays dual magnetic relaxation behavior at 0 Oe field, with the Ueff/k value 114.8 K for the slow relaxation process (SR) and the C value of 10.656 s-1K-5.80 for the fast relaxation process (FR), respectively. Theoretical calculations indicated that the conjugated groups (phenyl vs naphthyl) of the Schiff base bridging ligands (H2L1 and H2L2) significantly affect the intramolecular magnetic interactions between the Dy3+ ions and ultimately lead to different relaxations. Furthermore, magnetic circular dichroism (MCD) measurements showed that these two pairs of Dy4 enantiomers exhibit strong room temperature magneto-optical Faraday effects; notably, increasing the conjugated group on the Schiff base bridging ligand is beneficial to enhancing the magneto-optical Faraday effects.

In One Fell Sweep: Modeling Exchange, Hyperfine and Dipolar Interactions from EPR Spectra of Copper(II) Spin Triangles

The weak intramolecular magnetic interactions within a series of CuII3 complexes based on the trinucleating 2,4,6-tris(di-2-pyridylamino)-1,3,5-triazine (dipyatriz) ligand were investigated via Electron Paramagnetic Resonance (EPR) spectroscopy. X- and Q-band EPR spectroscopy in powders and frozen solutions were recorded and the Q-band spectra were interpreted by a multispin Hamiltonian model comprising exchange, dipolar and hyperfine interactions. The described methodology is suitable for the elucidation of weak intramolecular interactions which are not amenable to analysis via magnetic susceptibility studies.

Slow magnetic relaxation behaviors and diamagnetic dilution studies of a carboxyl bridged dinuclear dysprosium(III) complex

By using a rigid bulky carboxylic ligand α-cyanocinnamic acid (CCA), a dinuclear dysprosium (III) complex [Dy2(CCA)6 (MeOH)4] (1) was synthesized. Single crystal X-ray crystallography reveals that the two eight-coordinate dysprosium ions are bridged by four deprotonated carboxyl groups, forming a centrosymmetric paddle-wheel-like structure. Dynamic magnetic property measurements indicate that comple×1 displays field-induced slow magnetic relaxation. The temperature-dependent relaxation times can be fitted using Orbach and Raman processes with parameters of n = 2.8 (2), C = 27 (8) s−1/Kn, τ0 = 5 (2) × 10−10 s and Ueff = 40 (3) cm−1. Magnetic studies on the diamagnetic YIII-diluted analogue [Dy0.206Y1.794(CCA)6 (MeOH)4] (2) reveal its slow magnetic relaxation behavior without external dc field and the antiferromagnetic coupling between the DyIII ions in 1. Fits on the obtained relaxation times of 2 lead to the parameters of n = 4.5 (3), C = 0.7 (2) s−1/Kn, τ0 = 2.8 (2) × 10−9 s and Ueff = 38 (2) cm−1. The results suggest that slow magnetic relaxation originates from the single-ion relaxation of DyIII ions. Moreover, the diamagnetic dilution can suppress other fast relaxation pathways at low temperature, on account of the elimination of magnetic coupling and dipolar interaction. Ab initio calculations were then performed on the single DyIII ion species {YDy} and indicate that the first excited Kramers doublets (KDs) lie at ca. 76 cm−1, which is slightly higher than the experimental Ueff value. The intramolecular magnetic interactions were also calculated and indicate a weak ferromagnetic dipole-diploe magnetic interaction and an antiferromagnetic exchange coupling.

Dysprosium Clusters from 3-Ethoxysalicylidene Terminal-Decorated Acylhydrazone Ligands: In Situ Ligand Transformation-Assisted Assembly and Zero-Field Single-Molecule Magnet Behaviors

High-performance polynuclear lanthanide single-molecule magnets (Ln-SMMs) are still scarce now. Herein, we report the assembly of three novel DyIII clusters, where in situ ligand transformation-assisted assembly offered a system with high SMM performance successfully. These clusters, [Dy6(bepho)3(NO3)6(CH3OH)3(H2O)3]·5CH3OH·CH3CN·2H2O (1), [Dy4(bepho)2(H3epho)2 (CH3OH)2]·2CH3OH (2), and [Dy5(eehz)3(eqzl)(μ-CH3O)(μ3-CH3O)2(μ4-O)(CH3OH)]·2CH3OH (3), were all assembled from 3-ethoxysalicylidene terminal-decorated acylhydrazones. H4bepho, H5epho, H3eehz, and Heqzl represent N,N′-bis(3-ethoxysalicylicdene)pyridine-2,6-dicarbohydrazide N-oxide, N-(3-ethoxysalicylidene)pyridine-2,6-dicarbohydrazide N-oxide, N-(3-ethoxysalicylicdene)-2-(3-ethoxysalicylideneamino)benzohydrazide, and 3-(3-ethoxysalicylicdeneamino)quinazolinone, respectively. Cluster 1 is built by H4bepho directly, showing a hexanuclear zig-zag chain structure, while for 2 and 3, their original ligands all undergo unprecedented in situ transformations during the assembly. H5epho featuring asymmetric chelating arms and Heqzl bearing the quinazolinone parent are formed in 2 and 3, respectively, and participate in the assembly of clusters along with original ligands. In 2, H4bepho and H5epho construct a quasi-linear {Dy4} topology. A unique pentanuclear core of 3 is assembled by H3eehz and Heqzl. It consists of a [Dy4] tetrahedron and a [Dy3] triangle unit sharing an edge. Clusters 1–3 all show zero-field SMM behaviors, and 2 displays two distinct relaxation processes. Strikingly, the quantum tunneling of magnetization (QTM) effect in 3 might be suppressed by intramolecular magnetic interactions associated with its unique cluster topology. The zero-field energy barrier (Ueff) of 3 reaches up to 211.3 K, which is among the high values for all polynuclear Ln-SMM species and also the second largest one in the {Ln5}-SMM family.

Through-Space Magnetic Interaction of cis-Azobenzene Biradical

Two chiral PROXYL groups bridged with azobenzene show the change of the magnetic properties with the trans-cis photoisomerization of the azobenzene moiety. The isomerization brought the radicals closer together and increased the exchange interactions. The spin state of the cis-isomer was determined from two-dimensional nutation spectra as a mixture of S = 1/2 and S = 1. This is an example of the optical control of through-space intramolecular magnetic interactions.

Pseudohalide coligands guided structural motifs, magnetism and BSA-interactions of copper(II) complexes derived from 1,3-dimethy-6-aminouracil

Two novel copper(II) complexes with mononuclear (1) and 1D polynuclear (2) structural motifs were synthesized using the novel planar tridentate ligand 1,3-dimethyl-5-(thiazolyl-azo)-6-aminouracil, H2L (triclinic, PĪ space group and Z = 2) and pseudohalide coligands, namely azido and thiocyanato ligands. 1 is a monomeric-azo-azido complex with the monoclinic crystal system and P21/n space group, whereas 2 is a 1D polynuclear coordination polymer of Cu(II)-azo-µ-SCN (µ-SCN is bonded in the end-to-end (anti mode) manner) with the orthorhombic crystal system and Pbca space group. The crystallographic study, magneto-structural study and EPR spectra revealed distinctly that both complexes possess a slightly distorted square-pyramidal geometry about the Cu(II) ion. The observed and/or theoretical TDDFT bands show the nature of multiple intraligand, ligand-to-metal, metal-to-ligand charge transfer and d-d transitions. Direct current (dc) variable-temperature magnetic susceptibility measurements on polycrystalline samples of 1 and 2 were carried out in the temperature range 1.8–300 K. The copper(II) ions remain non-interacting at 300 K (χmT: 0.417 cm3 mol−1 K for 1 and 0.361 cm3 mol−1 K for 2). The Weiss constant for 1 is −2.3 K and for 2 it is 0.96 K, indicating that weak antiferromagnetic and weak ferromagnetic interactions exist in 1 and 2, respectively. As expected, complex 1 exhibits an intermolecular type of interaction (zJ′ = −1.37(1) cm−1), whereas 2 exhibits an intramolecular magnetic interaction (J = 1.17(2) cm−1) between the Cu(II) ions. The cause of the weak ferromagnetic coupling in 2 may be due to the orthogonality of the magnetic orbitals (dx2-y2) involved in the equatorial-axial singly bridging SCN moiety. All the compounds show excellent binding interactions (KA) with BSA, with the order complex 1 (1.993 × 104 M−1) > complex 2 (6.072 × 103 M−1) > free ligand (2.619 × 103 M−1).

Dramatic Impact of Auxiliary Ligands on the Dynamic Magnetic Relaxation in Tetranuclear DyIII2ZnII2 Single Molecule Magnets

Comprehensive SummaryBetter understanding the determining factors of dynamic magnetic relaxation in polynuclear lanthanide based single‐molecule magnets (SMMs) remains a challenge due to the complexity of such architectures involving interactions between the magnetic centers. To address this issue, two structurally related heterometal DyIII2ZnII2 SMMs, [Zn2Dy2(L)4(Ac)2(DMF)(CH3OH)]·CH3OH·2H2O (1) and [Zn2Dy2(L)4(Ac)2(DMF)2]·4CH3CN (2) (H2L = (E)‐2‐((2‐hydroxy‐3‐methoxybenzylidene)amino)‐4‐methyphenol, DMF = N,N‐dimethylformamide), are introduced and investigated. Through modifying the auxiliary ligands on one DyIII site while retaining that on the other DyIII, the intramolecular magnetic interactions and relaxation dynamics in these two heterometallic‐DyIII2ZnII2 SMMs can be tuned, demonstrating a dramatic change in the magnet relaxation behavior with energy barrier changing from a negligible value for 1 to 305 K for 2. Ab initio calculations reveal that changing the coordination geometries on the DyIII sites can significantly affect the magnetic interactions as well as single‐ion anisotropy.

Enhancing magnetic coupling through protonation of benzylideneaniline-bridged diradicals and comparison with stilbene- and azobenzene-based diradicals.

Taking nitroxide radicals as spin sources, we explore the intramolecular magnetic coupling interactions of the trans- and cis-forms of benzylideneaniline (BA)-bridged diradicals, in which the central -CH[double bond, length as m-dash]N- unit can undergo single protonation to convert to its protonated counterpart or vice versa. The calculated results for these two pairs of diradicals (protonated versus unprotonated trans and cis forms) verify that the signs of their magnetic coupling constants J do not change, but the magnitudes significantly increase after protonation. In the structure, the better conjugation of the protonated trans diradical and two reduced CCNC and CCCN torsion angles of the protonated cis one make for a more efficient spin transport, promoting the spin polarization, thus leading to larger spin couplings. In terms of mechanism, the proton-induced magnetic enhancement should be attributed to strong participation of the coupler BA through its lowest unoccupied molecular orbital (LUMO) with a lower energy level after protonation, and the small HOMO-LUMO (HOMO: highest occupied molecular orbital) gap of the coupler BA through protonation is crucial in explaining such remarkable spin-coupling enhancement. Furthermore, different linking modes of the radical groups to the couplers are also considered to confirm our conclusions. In addition, we also make a comparison of the magnetic coupling strengths among their isoelectronic analogues of BA-, AB- and stilbene-bridged nitroxide diradicals before or after protonation, and find a linear correlation among them. It should be noted that the magnetic behaviors of all these diradicals obey the spin alternation rule and singly occupied molecular orbital (SOMO) effect. This work provides helpful information for the rational design of promising magnetic molecular switches.

Low-Coordinate Dinuclear Dysprosium(III) Single Molecule Magnets Utilizing LiCl as Bridging Moieties and Tris(amido)amine as Blocking Ligands

A low-coordinate dinuclear dysprosium complex {[Dy(N3N)(THF)][LiCl(THF)]}2 (Dy2) with a double bridging ‘LiCl’ moiety and tris(amido)amine (N3N)3− anions as a blocking ligand is synthesized and characterized structurally and magnetically. Thanks to the use of the chelating blocking ligand (N3N)3− equipped with large steric –SiMe3 groups, the coordination sphere of both DyIII ions is restricted to only six donor atoms. The three amido nitrogen atoms determine the orientation of the easy magnetization axes of both DyIII centers. Consequently, Dy2 shows slow magnetic relaxation typical for single molecule magnets (SMMs). However, the effective energy barrier for magnetization reversal determined from the AC magnetic susceptibility measurements is much lower than the separation between the ground and the first excited Kramers doublet based on the CASSCF ab initio calculations. In order to better understand the possible influence of the anticipated intramolecular magnetic interactions in this dinuclear molecule, its GdIII-analog {[Gd(N3N)(THF)][LiCl(THF)]}2 (Gd2) is also synthesized and studied magnetically. Detailed magnetic measurements reveal very weak antiferromagnetic interactions in Gd2. This in turn suggests similar antiferromagnetic interactions in Dy2, which might be responsible for its peculiar SMM behavior and the absence of the magnetic hysteresis loop.

Single-Ion Anisotropy and Intramolecular Interactions in CeIII and NdIII Dimers.

This article reports the syntheses, characterization, structural description, together with magnetic and spectroscopic properties of two isostructural molecular magnets based on the chiral ligand N,N′-bis((1,2-diphenyl-(pyridine-2-yl)methylene)-(R,R/S,S)-ethane-1,2-diamine), L1, of general formula [Ln2(RR-L1)2(Cl6)]·MeOH·1.5H2O, (Ln = Ce (1) or Nd (2)). Multifrequency electron paramagnetic resonance (EPR), cantilever torque magnetometry (CTM) measurements, and ab initio calculations allowed us to determine single-ion magnetic anisotropy and intramolecular magnetic interactions in both compounds, evidencing a more important role of the anisotropic exchange for the NdIII derivative. The comparison of experimental and theoretical data indicates that, in the case of largely rhombic lanthanide ions, ab initio calculations can fail in determining the orientation of the weakest components, while being reliable in determining their principal values. However, they remain of paramount importance to set the analysis of EPR and CTM on sound basis, thus obtaining a very precise picture of the magnetic interactions in these systems. Finally, the electronic structure of the two complexes, as obtained by this approach, is consistent with the absence of zero-field slow relaxation observed in ac susceptibility.

Intramolecular Magnetic Interaction in a Photogenerated Dual Angular Momentum System in a Terbium-Phthalocyaninato 1:1 Complex.

Intramolecular magnetic interaction between a localized open-shell 4f-electronic system and a photoexcited macrocyclic π-conjugate system in terbium-phthalocyaninnato (Tb-Pc) 1:1 complex was investigated using variable-temperature variable-field magnetic circular dichroism (VTVH MCD) spectroscopy. The 1:1 complex [Tb(Pc)(cyclen)]Cl (Pc2- = phthalocyaninato dianion, cyclen = 1,4,7,10-tetraazacyclododecane) with the capping ligand providing an exact fourfold symmetry showed a significant temperature dependence and a nonlinear field dependence in the MCD intensity of the Pc-centered highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) π-π* transition, while a diamagnetic congener [Y(Pc)(cyclen)]Cl showed a temperature-independent MCD with a linear-field dependence. This indicates that the (4f)8 system of the Tb ion with a total angular momentum J and the photoexcited π-system of the Pc macrocycle with an orbital angular momentum L are magnetically coupled. By numerical simulation using a model where ground doublet state |Jz⟩ = |±6⟩ and excited quartet state |Jz, Lz⟩ = |±6, ±|Lz|⟩ are included, the J-L interaction magnitude ΔJL and the Pc-centered orbital angular momentum |Lz|ℏ were determined to be 1.1 cm-1 and 2.0 ℏ, respectively. From ab initio restricted active space self-consistent field (RASSCF)-restricted active space state interaction (RASSI) calculations on the π-π* excited states of the Tb complex, the magnitude of the J-L interaction was estimated. The comparison between the calculations on the Y and Tb complexes revealed that the ferromagnetic-type coupling occurs between the orbital component in the J of Tb and the L on Pc, supporting the model that we employed for the analysis of the experimental data.

N^N Pt(II) Bisacetylide Complexes with Oxoverdazyl Radical Ligands: Preparation, Photophysical Properties, and Magnetic Exchange Interaction between the Two Radical Ligands.

To study the effect of a stable radical on the photophysical properties of a phosphorescent Pt(II) coordination framework and the intramolecular magnetic interaction between radical ligands in the N^N Pt(II) bisacetylide complexes, we prepared a series of N^N Pt(II) bis(acetylide) complexes with oxoverdazyl radical acetylide ligands. The linker between the Pt(II) center and the spin carrier was systematically varied, to probe the effect on the sign and magnitude of the spin exchange interactions between the radical ligands and photophysical properties. The complexes were studied with steady-state and femtosecond/nanosecond transient absorption spectroscopy, continuous-wave electron paramagnetic resonance (EPR) spectroscopy, and density functional theory (DFT) computations. The transient absorption spectral studies show that the doublet excited state of the radicals are short-lived (τD ≈ 2 ps) and nonfluorescent. Moreover, the intrinsic long-lived triplet excited state (τT = 1.2 μs) of the Pt(II) coordination center was efficiently quenched by the radical (τT = 6.9 ps for one representative radical Pt(II) complex). The intramolecular magnetic interaction between the radical ligands through the diamagnetic Pt(II) atom was studied with temperature-dependent EPR spectroscopy; antiferromagnetic exchange interaction (-J S1S2, J = -5.4 ± 0.1 cm-1) for the complex with the shortest radical-radical distance through bridge fragments was observed. DFT computations give similar results for the sign and magnitude of the J values. For complexes with larger inter-radical distance, however, very weak coupling between the radical ligands was observed (|J| < 0.7 cm-1). Our results are useful for the study of the effect of a radical on the photophysical properties of the phosphorescent transition-metal complexes.

A perfect triangular dysprosium single-molecule magnet with virtually antiparallel Ising-like anisotropy

A perfect triangular Dy3 single-molecule magnet was reported. Each Dy(iii) magnetic axis is oriented almost normal to the plane of Dy3, and the intramolecular magnetic dipole interaction gives rise to a virtually antiparallel Ising-like ground state.

Binuclear iminopyridine-bridged 3d late transition metal complexes with o-semiquinones

Four novel binuclear high spin manganese(II), iron(II), cobalt(II) and nickel(II) complexes containing 1,2-bis(pyridin-2-ylmethylene)-para-phenylenediamine as a bridging ligand have been synthesized and investigated. Molecular structures of manganese and cobalt complexes were established by single crystal X-ray structural analysis. For all the complexes the magnetic susceptibility measurements have been performed. According to the obtained magnetic data the energies of intramolecular magnetic exchange interactions strongly depend on the electron structure of metal ion. In addition, all the obtained complexes demonstrate very weak antiferromagnetic metal-metal interaction.

A methylenediphosphonate bridged copper(II) tetramer: Synthesis, structural, thermal, and magnetic characterization of [Cu4(H2O)2(phen)4(μ-P2O6CH2)2]·21H2O

A novel tetracopper(II) complex namely, [Cu4(H2O)2(phen)4(μ-P2O6CH2)2]·21H2O (1) was synthesized by using copper(II) chloride, 1,10-phenanthroline (phen), and methylene diphosphonic acid (MDP). The structure of 1 was characterized by single crystal X-ray diffraction and spectroscopy. Compound 1 crystallizes in the triclinic system, P1¯ space group and its crystal structure consists of neutral centrosymmetric tetranuclear copper(II)-methylene diphosphonate units where each peripheral {Cu(phen)(P2O6CH2)}2− fragment adopts the bidentate(Oeq-P-Oeq)/monodentate (Oax) bridging mode towards each of the two inner {Cu(phen)}2+ entities resulting in a “clamshell”-like orientation. The two crystallographically independent copper(II) ions in 1 (Cu1/Cu2) are five-coordinate with two nitrogen atoms from a bidentate phen molecule and two phophonate-oxygens building the basal plane (Cu1/Cu2) and either a water molecule (Cu2) or a phosphonate-oxygen (Cu1) filling the apical position of somewhat distorted square pyramidal surroundings. The values of the intramolecular copper–copper separation are 3.3219(8) and 3.2062(8) Å for Cu1⋯Cu2 and Cu1⋯Cu1i, respectively. Cryomagnetic measurements on a polycrystalline sample of 1 reveal the occurrence of an overall antiferromagnetic behaviour leading to a diamagnetic low-lying spin state. The values of the intramolecular magnetic interactions are J1 = −22.60(4) cm−1 (across the bis-bidentate methylene diphosphonate) and J2 = +4.9(3) cm−1 (through the double-oxo(phosphonate) bridge), the spin Hamiltonian being defined as H = −J1(SCu1·SCu2 + SCu1i·SCu2i) − J2(SCu1·SCu1i). These values are analyzed through simple orbital symmetry considerations and compared with those for related systems in the literature.

Luminescent single-molecule magnet of metallofullerene DyErScN@Ih-C80

Magneto-luminescent molecules have significant applications in data storage and quantum computing. However, design of these bi-functional molecules coupled with magnetic behavior and photoluminescence is still challenging. In this work, we report a metallofullerene DyErScN@Ih-C80 exhibiting single-molecule magnet (SMM) behavior and near-infrared emission. For DyErScN@Ih-C80, two functional lanthanide metal ions of Dy3+ (SMM function) and Er3+ (luminescent function) are integrated inside a fullerene cage using a trimetallic nitride template, and its structure has been unambiguously characterized by single-crystal X-ray diffraction. Magnetic measurements revealed that DyErScN@Ih-C80 behaves as a SMM with a blocking temperature up to 9 K resulting from the intramolecular magnetic interaction between Dy3+ and Er3+ ions. Moreover, DyErScN@Ih-C80 exhibits temperature-dependent near-infrared emission around 1.5 μm with multiple splitting peaks from Er3+, which arises from the influence of Dy3+ ion. This study provides a new strategy to synthesize new magneto-luminescent molecule materials.

Using Redox-Active π Bridging Ligand as a Control Switch of Intramolecular Magnetic Interactions.

Intramolecular magnetic interactions in the dinuclear complexes [(tpy)Ni(tphz)Ni(tpy)] n+ ( n = 4, 3, and 2; tpy, terpyridine; tphz, tetrapyridophenazine) were tailored by changing the oxidation state of the pyrazine-based bridging ligand. While its neutral form mediates a weak antiferromagnetic (AF) coupling between the two S = 1 Ni(II), its reduced form, tphz•-, promotes a remarkably large ferromagnetic exchange of +214(5) K with Ni(II) spins. Reducing twice the bridging ligand affords weak Ni-Ni interactions, in marked contrast to the Co(II) analogue. Those experimental results, supported by a careful examination of the involved orbitals, provide a clear understanding of the factors which govern strength and sign of the magnetic exchange through an aromatic bridging ligand, a prerequisite for the rational design of strongly coupled molecular systems and high TC molecule-based magnets.

EPR and DEER Characterization of New Mixed Weakly Coupled Nitroxide Triradicals for Molecular Three-Spin Qubits

Three new mixed triradicals with small exchange coupling parameters (J ≪ AN) were obtained on the base of a coupling reaction between the derivative of spirofused 2,5-dihydroimidazol-type monoradical and the two mol. equivalents of carboxylic acid derivatives of PROXYL-, TEMPO- or 2,5-dihydro-1H-pyrrol-type nitroxides. Their intramolecular magnetic interactions were characterized in terms of comparison of the CW X, Q- and W-band EPR spectra with those of the monoradical precursors. The dipole–dipole coupling parameters of the triradicals were estimated on the base of the quantum chemical calculations at UB3LYP/6-31G(d) level of theory. Two types of the spin distances were found in the triradicals: short—with the distance of 13–17 A (D ≈ 11–24 MHz) and long—with the distance of 21–23 A (D ≈ 4–6 MHz). The longest spin–spin as well as spin–lattice relaxation times at 50 K were detected for the triradical carrying the two TEMPO fragments, indicating the potential usage of three-spin qubit models for quantum gate operations.

Effect of nuclearity and symmetry on the single-molecule magnets behavior of seven-coordinated β-diketonate Dy(III) complexes

A couple of mononuclear and dinuclear lanthanide complexes, Dy(thd)3(Py) (1), Ln2(thd)6(4,4′-Bpy) (Ln = Dy (2), Gd (3), Py = Pyridine, 4,4′-Bpy = 4,4′-Bipyridine) have been designed and isolated from the reaction of β-diketonate-based ligand 2,2,6,6-tetramethyl-3,5-heptanedione (Hthd), LnCl3∙6H2O and neutral ligands. The single-crystal X-ray diffraction analysis confirms that the Dy(III) ions of complexes 1 and 2 are seven-coordinated with N1O6 coordination environment, forming C3v and C2v local symmetry. Magnetic studies reveal that mononuclear complex 1 displays butterfly-shaped hysteresis loops at 1.8 K and slow relaxation of magnetization with energy barrier 34.7 K in optimized external filed while dinuclear complex 2 not exhibits single-molecule magnet (SMM) behavior. The variable temperature magnetic data of dinuclear Gd(III) analogues (3) indicate the negligible intramolecular magnetic interactions between the two Gd(III) centers in structure due to long Ln⋅⋅⋅ Ln distance. The diluted sample by Y(III) ions of complexes 1′ and 2′ and electrostatic model calculation were employed to elucidate distinction of magnetic properties. Investigation of magneto-structural correlation emphasizes that the orientation of the magnetic axis affects the magnetic properties, which is further associated with local symmetry of the Dy(III) ions.