ABSTRACT The design of mononuclear complexes that simultaneously exhibit multiple functionalities represents a rapidly advancing frontier in molecular materials research. Achieving strong easy‐axis magnetic anisotropy within inherently polar crystal lattices remains a significant challenge, yet it offers a promising pathway toward next‐generation spin–electric materials. Comparative studies of isostructural systems with subtle structural variations provide key insight into molecular tuning strategies for practical implementation. In this context, we report a family of five‐coordinate, square‐pyramidal Fe(II) complexes, [Fe(L)(X) 2 ]·CHCl 3 (L = tridentate Schiff‐base ligand; X = Cl ( 1 ), Br ( 2 )), crystallizing in the polar, non‐centrosymmetric triclinic space group P1 and exhibiting pronounced easy‐axis magnetic anisotropy. Cantilever torque magnetometry on single crystals reveals a negative axial Zero Field Splitting parameter D = −25.6 cm − 1 ( 1 ) and −19.8 cm − 1 ( 2 ), among the largest reported for square‐pyramidal Fe(II). Ab initio CASSCF/NEVPT2 calculations reproduce the experimental Spin Hamiltonian parameters and show that subtle steric and electronic effects, particularly the out‐of‐plane displacement of Fe(II), critically govern the magnitude and sign of D . Complementary piezoresponse force microscopy and Polarization vs Electric field measurements confirm intrinsic polarization and a pronounced nanoscale piezoelectric response consistent with computed dipole moments, establishing a compelling platform for multifunctional spin–electric materials and multifunctional molecular architectures.

Understanding how electric fields interact with molecular spins is critical for advancing quantum technologies. This review focuses on spin-electric coupling (SEC) in paramagnetic molecules. We examine how electric fields affect spin-Hamiltonian parameters, highlighting experimental techniques to investigate this phenomenon, such as electric field-modulated electron paramagnetic resonance and spin echo under pulsed electric fields. Key mechanisms, including spin-orbit coupling and through-bond interactions, are discussed across various systems, from lanthanides to frustrated spin triangles and helical chains. We present here recent studies demonstrating that SEC can induce measurable shifts in magnetic resonance spectra, revealing new strategies for electric field-based spin control. These insights pave the way for enhanced spin-based data storage and quantum computation, emphasizing the importance of symmetry, polarizability, and molecular design in optimizing SEC effects.

Lanthanide complexes exhibiting marked easy-axis magnetic anisotropy are the main targets to achieve highly performant single molecule magnets, pseudo contact shift agents and rotating magnetic refrigerants. To experimentally map the magnetic anisotropy orientation and magnitude, a growing portfolio of experimental techniques is now available, including powder and single crystal magnetometry, cantilever torque magnetometry, and X-ray magnetic circular dichroism. While these methods provide complementary information, reference values for ideal systems are not always applied consistently, possibly introducing ambiguity in data interpretation. Here we present a unified and quantitative set of benchmark magnetic observables for trivalent lanthanide ions exhibiting ideal easy-axis anisotropy. For each ion we report: the orientation-dependent saturation magnetization and low-temperature magnetic susceptibility; the field-dependent maximum magnetic torque signal; and the angular- and field-dependent normalized XMCD response at the M4,5 edges. The resulting tables and figures provide a simple, ready-to-use reference framework for direct comparison with experimental measurements of lanthanide-based magnetic systems.

In this work, we report the preparation of a chiral Er(III) complex specifically designed to assess the influence of the magnetic anisotropy axes orientation on its magneto-chiral dichroism (MChD). High resolution MChD measurements performed along the three principal crystallographic axes revealed pronounced anisotropic effects on both MChD signal line shapes and intensities. By combining, for the first time, angle-resolved torque magnetometry, comprehensive magneto-optical spectroscopy on oriented single crystals, absorption measurements under magnetic fields up to 30 T and theoretical calculations, we provide clear experimental insight into the anisotropic nature of the MChD in lanthanide complexes. Specifically, we evaluate the extent to which Er moderate anisotropy enables signal persistence across a wide range of crystallographic orientations. These results allow the identification of the key role played by both the orientation and the magnitude of magnetic anisotropy in the MChD response of chiral lanthanide complexes, representing a significant step toward the directional control of magneto-chiroptical responses in lanthanide-based molecular materials.

We report the archetypal representatives of single-molecule magnets (SMMs) based on the “most prolate” lanthanide ion: Yb(BHT)3 (1) and Yb(DBP)3 (2) (BHT = 2,6-di-tert-butyl-4-methylphenolate, DBP = 2,6-di-tert-butylphenolate). The coordination sphere of the metal center was designed to accommodate only three aryloxide ligands yielding highly symmetric nearly trigonal planar geometry which maximizes the magnetic anisotropy associated with the prolate character of the mJ = |7/2| of YbIII. Detailed magnetic and optical studies combined with cantilever torque magnetometry measurements and ab initio calculations, unambiguously unravel the giant magnetic easy axis anisotropy, with the easy magnetization axis perpendicular to the trigonal coordination plane. Our results highlight an exceptional magnetic splitting of the 2F7/2 state (> 1500 cm-1), close to the maximum theoretical value, as well as extremely pure composition of the wavefunction describing each Kramers doublets (KD). This best possible electrostatic environment for the stabilization of the prolate f-electron density does not favour Orbach relaxation, but rather a Raman relaxation process. Nevertheless, the low temperature emission spectra demonstrate that the very strong YbIII anisotropy indeed induces an extremely rare and strong splitting of the 2F7/2 KD in both compounds, as the near infra-red (IR) emissions arising from radiative deexcitation toward the 2F7/2 ground state span over 1695 (1) and 1573 cm−1 (2) at 4 K. Based on this observation, a direct link between the YbIII position with respect to the trigonal coordination environment created by the oxygen atoms and the IR emission can be drawn in solid-state, but also in solution. This allows for going well beyond the crystal structure determination and provides hints about the geometry of such systems in solution.

Hydroxypyridinones represent a versatile class of bidentate ligands for the construction of coordination compounds with tuneable structural and magnetic properties. In this work, we systematically investigate the coordination chemistry and magnetic behaviour of two group VIII trivalent transition metals (Fe3+ and Ru3+) with 1,2-dimethyl-3-hydroxy-4-pyridinone and its thione analogue, 1,2-dimethyl-3-hydroxy-4-pyridinethione. Tris-chelated octahedral complexes are readily obtained with the oxygen-donor ligand, yielding isostructural compounds stabilized by extended hydrogen-bond networks in the solid state. Substitution of the ketonic oxygen with sulphur markedly alters the reactivity, leading to the formation of a tris-chelated Fe3+ complex and an unprecedented sodium-bridged binuclear Ru3+ species. Magnetic measurements reveal high-spin (S = 5/2) Fe3+ behaviour with significant intermolecular antiferromagnetic interactions, while the Ru3+ derivatives exhibit a low-spin S = 1/2 character. In the sulphur-containing Ru system, the data suggest partial spin delocalization onto the ligand framework. These results elucidate how subtle changes in the donor atom identity and metal electronic structure govern coordination modes, solid-state organization, and magnetic properties, providing valuable insights for the rational design of hydroxypyridinone-based molecular magnetic materials.

Abstract The determination of the orientations of the individual DyIII anisotropy axes in polynuclear complexes is challenging but crucial for the understanding of systems showing Single Molecule Magnet or Single Molecule Toroic behavior. In particular, the experimental proof of a toroidal ground state from magnetization data often remains ambiguous. Here, we report the coordination cluster [CoIII 3DyIII 3(µ3-OH)4(O2C-C6H4-p-Me)6(pmide)3(H2O)3]Cl2 · 10MeCN (1) (H2pmide = N-2-pyridylmethyldiethanolamine) which crystallizes with threefold symmetry and contains an equilateral DyIII 3 triangle surrounded by a triangle of diamagnetic CoIII ions. We also report a multi-technique investigation of its toroidal magnetic spin structure, including 161Dy Synchrotron Mössbauer Spectroscopy which shows an abrupt transition from a non-magnetic to a magnetic state. The experimental orientations of the individual DyIII magnetic axes were assessed using torque magnetometry and micro-SQUID measurements and both experiments converged on a spin structure that is in very good agreement with ab initio calculations. Such a multi-technique approach, including 161Dy Synchrotron Mössbauer Spectroscopy, provides a roadmap for the unambiguous identification of such toroidal states.

We report an unprecedented one-pot route synthesis and in-depth characterizations of a new binuclear copper(II) complex with the tridentate PAN ligand (1-(2-pyridylazo)-2-naphthol). The compound of formula [Cu2(μ-SO4)(PAN)2(H2O)2] was isolated as a highly crystalline material, where the two copper(II) centers are connected by a sulfate bridge. Single-crystal X-ray diffraction (SC-XRD) combined with UV-Vis spectroscopy confirms a square-pyramidal coordination around each Cu(II) center in both the solid state and solution. Remarkably, magnetic studies revealed an unconventional slow magnetic relaxation under applied dc fields, characterized by three field-dependent processes spanning more than four orders of magnitude in frequency. These findings broaden the design space of transition-metal molecular magnets by showing that simple sulfate bridging and π-conjugated ligands can engender complex relaxation dynamics in Cu(II) dimers.

Precise and experimentally accessible determination of the electronic structure of transition metal complexes remains a challenge in the development of molecular qubits, particularly for leading candidates with integer spin. Existing techniques often require large-scale facilities and substantial sample quantities or offer limited spectral access and sensitivity to subtle anisotropies. Here, we demonstrate that cantilever torque magnetometry (CTM) overcomes these limitations by combining high sensitivity to magnetic anisotropy with wide sample compatibility, minimal sample demands, and true laboratory-scale accessibility. By exploiting the distinct temperature dependences of g-tensor anisotropy and zero-field splitting (ZFS), CTM enables their experimental decoupling, yielding exceptionally precise bulk-mean value determination of spin Hamiltonian parameters from microgram-scale single crystals. The parameters extracted by CTM were found to be qualitatively consistent but quantitatively different from those determined using high-frequency electron paramagnetic resonance spectroscopy (∼1% for g and ∼5-15% for ZFS), highlighting that perfect agreement between magnetometric and resonance techniques is not guaranteed. Our study establishes CTM as a powerful and broadly accessible complement to magnetic resonance methods, opening new routes for high-precision characterization of low-anisotropy spin systems in molecular quantum information science.

Metal-mono(imido) linkages have been known for seven decades, and they are found in transition metal, main group, lanthanide, thorium, and uranium complexes. However, transuranium-mono(imido) complexes remain unknown in any scenario. Here, we present evidence for transient neptunium(V)-mono(imido) complexes. Treatment of [NpIII(TrenTIPS)] (1, TrenTIPS = {N(CH2CH2NSiPri3)3}3-) with N3R (R = SiMe3; 1-adamantyl, Ad) results in N2 evolution and dark purple solutions consistent with the formation of [NpV(TrenTIPS)(NR)] (3NpNR). However, solutions of 3NpNR rapidly turn orange, where for R = SiMe3 the isolated 1:1 products are [NpIV(TrenTIPS){N(H)SiMe3}] (4a) and [NpIV(TrenTIPS-2H){N(H)SiMe3}] (4b, TrenTIPS-2H = {N(CH2CH2NSiPri3)2(NCH2CH2NSiPri2C[Me]=CH2)}3-). The latter contains a dehydrogenated-Pri vinyl functionality accounting for the source of the two amido H atoms. The reaction for R = Ad proceeds similarly, but only [NpIV(TrenTIPS){N(H)Ad}] (5a) could be unequivocally confirmed, though its isolation suggests generality of the imido-to-amido functional group transformation. Complexes 4a/4b exhibit slow relaxation of their magnetization, adding to the small number of transuranium single ion magnets. Experimental and computational analysis suggests that the amido products are formed by C-H activation and two sequential hydrogen atom transfer reactions involving a three-step proton-coupled electron-transfer sequence of H• radical abstraction, electron transfer, then another H• radical abstraction step. In contrast to transient 3NpNR, the 5f2 uranium(IV)-imido complex [K(2.2.2-cryptand)][UIV(TrenTIPS)(NSiMe3)] (8UNSiMe3) is robust, even in boiling THF, suggesting the transience of 5f2 3NpNR is not due to the 5fn-count but the increased effective nuclear charge of neptunium vs uranium. This work highlights divergence of uranium- and neptunium-imido stabilities, emphasizing that the latter is an inherently challenging synthetic target.

The high sensitivity of EPR spectroscopy under modulated electric field applied to a non-centrosymmetric Cr 3 spin-frustrated triangle reveals weak but quantifiable spin–electric effects, highlighting how ligand choice governs spin–electric coupling.

The synergistic use of synthesis, single crystal X-ray Diffraction, torque and SQUID magnetometry, luminescence, and ab initio calculations is presented here to unambiguously unfold the magnetic anisotropy of an apparently ordinary low-symmetry eight-coordinated DyIII complex. Surprisingly, we found two low-lying states extremely close in energy, both displaying a strong easy axis anisotropy, tilted by ∼90°. Such a result has been rationalized, and its validity was extended upon targeted molecular modifications. These findings open new possibilities for the exploitation of such a kind of magnetic anisotropy in molecular materials.

Spin-electric effects are crucial for quantum technologies, offering several advantages over standard magnetic field-based spin control. Seeking a mechanism independent of spin-orbit interaction, here we report the detection of a spin-electric effect in the [Cu3(saltag)(py)6]ClO4 spin triangle. The effect is investigated by electron paramagnetic resonance under electric field modulation on single crystals. The anisotropy of the magnetic response to the electric field is addressed, and comprehensive ab initio calculations are performed to elucidate its origin. We demonstrate that when the electric field is applied in the plane of the triangle, the dominant contribution to the observed spin-electric signal arises from a variation of the isotropic exchange interaction. Our combined theoretical and experimental approach demonstrates that, in our system, there is no evidence of antisymmetric exchange (Dzyaloshinskii-Moriya) interaction, confirming that electric-field control of magnetic exchange is achievable in the absence of significant spin-orbit coupling. Moreover, we underscore the crucial role of the bridging ligand, which opens new avenues for chemically optimizing spin-electric coupling.

In this study, we employed cantilever torque magnetometry to probe the magnetic anisotropy of a single crystal of U(DOTA)(H2O) (DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid), representing, to the best of our knowledge, the first application of this technique to an actinide-based molecular system. Combining cantilever torque magnetometry data with ab initio calculations allowed us to determine the magnetic anisotropy tensor of the uranium center. The resulting parameters enabled direct comparison with the isoelectronic lanthanide complex previously reported in the literature as well as with ab initio predictions for the theoretical Es4+ isostructural analogue. We find that the magnetic anisotropy axes of U(DOTA)(H2O) (5f2) closely align with those of the Pr3+ (4f2) and Dy3+ (4f9) analogues and with those predicted for Es(DOTA)(H2O) (5f9). The combination of CTM and electronic structure calculations was essential to describe the weakly paramagnetic, EPR-silent, non-Kramers ground state of U(DOTA)(H2O), despite the added complexity of four noncollinear molecules in the unit cell. Overall, these results demonstrate that cantilever torque magnetometry provides a powerful route to characterize the magnetic anisotropy of actinide complexes and offers a valuable experimental benchmark for validating computational approaches to 5f-element magnetism.

Single-molecule magnets (SMMs) offer promise for high-density data storage, but suppressing fast magnetization relaxation remains a key challenge in synthetic design. Changing coordinating atoms across a group offers an avenue to control the phononic spectrum and crystal field. We report dysprosium(III) and erbium(III) tris(tetraphenyl-dichalcogenoimidodiphosphinate) complexes with O (1-Ln), S (2-Ln), or Se (3-Ln) donors, examining their influence on the anisotropy and relaxation behavior. The products exhibit distinct geometries, 6-coordinate 1-Ln, 9-coordinate 2-Ln, and 3-Ln complexes that vary from 9-coordinate 3-Tb to 7-coordinate 3-Er and 3-Dy. The geometry dominates the vibrational effects in defining the magnetic relaxation. The 7-coordinate 3-Dy, with one short Ln-N bond, produces an axial crystal field and the best field-induced SMM behavior (Ueff = 72(6) cm-1), while the high D3 symmetry in 2-Dy suppresses slow relaxation. Low-lying excited states in 1-3-Er promote rapid Orbach relaxation, masking Raman processes in 2-Er and 3-Er. In 1-Ln, low-energy vibrations drive Raman relaxation, and contrary to expectations, 3-Dy exhibits higher vibrational energy contributing to this process. This indicates that heavy chalcogen atoms in the first coordination sphere are a valuable resource, as they can drive crystal field changes that effectively counterbalance the onset of low-energy phonons.

The Spin-Electric Effect (SEE) on molecules─the influence of external electric fields on molecular spin states─offers a compelling route toward low-power molecule-based spintronic applications. However, SEE remains elusive in molecular systems due to typically weak spin-electric field coupling. In this study, we observe a relevant SEE in a mononuclear lanthanide complex using Electric Field Modulated Electron Paramagnetic Resonance spectroscopy. We reveal a marked anisotropy of the SEE, evidencing that the most perturbed g tensor component is the one perpendicular to the electric field, providing hints for the most convenient experimental configuration to tune ad hoc spin transitions. Ab initio calculations in synergy with the experimental results revealed that molecular symmetry breaking plays a fundamental role. We also point out the crystal field parameters that are most strongly modulated by the presence of an electric field. These parameters are all off-diagonal, indicating effective electric-field-mediated state mixing.

The mixture of manganese acetates and the asymmetrical benzoylacetone ligand (bzacH) crystallized into a monomeric Mn(III)-tris(benzoylacetonato) complex, [Mn(bzac)3]. The crystal structure of [Mn(bzac)3] revealed a static rotational disorder, where half [Mn(bzac)3] molecule adopts the Λ configuration and the other is in the Δ configuration. Both molecules are mer-isomers, exhibiting Jahn-Teller elongation along two opposing Mn–O–CMe bonds (mer-Me-Me). [Mn(bzac)3] was further characterized by IR spectroscopy, thermogravimetric analysis (TGA/DTA), magnetic susceptibility measurements, UV-Vis spectroscopy, Hirshfeld surface analysis and Density functional theory (DFT) calculations. The Hirshfeld surface analysis, complemented by 2D fingerprint plots, confirms the presence of significant intermolecular interactions, including H···H, H···O, H···C hydrogen bonding, and C—H···π stacking interactions. Magnetic measurements reveal a high-spin Mn(III) center with an S = 2 state with relevant Zero-Field Splitting (D = -4.50 cm⁻¹, g = 2.00) and slow magnetization relaxation at low temperatures. DFT calculations support the experimentally observed high-spin state of [Mn(bzac)₃] and provide insights into its electronic structure. HOMO analysis suggests that the Jahn-Teller elongation along the axial Mn–O bonds arises from electrostatic interactions between the Mn 3dz² orbital and the pz orbitals of the axial oxygen atoms, leading to the energetic stabilization of the HOMO relative to the dx²−y² LUMO. DFT also identifies four possible Jahn-Teller elongation isomers: one facial (fac) and three meridional (mer) forms. Among these, the experimentally observed mer-Me-Me isomer becomes increasingly favorable at elevated temperatures.

In this paper, we investigate the magnetic exchange interaction and magnetization dynamics of two new members of the [LnRad(NO3)3] family, where Rad is a tripodal nitroxide, and Ln is Er(III) or Yb(III), having the prolate type electron density. Single OK crystal and powder X-ray diffraction studies showed that these complexes are isostructural with their previously investigated Y, Gd, Dy, Tm, Tb, Eu, and Lu congeners. A magnetometric investigation, supported by ab initio calculations, showed the presence of antiferromagnetic coupling between the lanthanide ion and the radical in both compounds with estimated J values of ≈7 and ≈20 cm−1 for Er and Yb, respectively (+J Seff∙ S formalism).

Stabilizing large easy-axis type magnetic anisotropy in molecular complexes is a challenging task, yet it is crucial for the development of information storage devices and applications in molecular spintronics. Achieving this requires a deep understanding of electronic structure and the relationships between structure and properties to develop magneto-structural correlations that are currently unexplored in the literature. Herein, a series of five-coordinate distorted square pyramidal CoII complexes [Co(L)(X2)].CHCl3 (where X = Cl (1), Br (2), or I (3)) is reported, all exhibiting easy-axis magnetic anicotropy. The size of the zero field splitting axial parameter (D) is quantitatively determined (1 = -72; 2 = -67 and 3 = -25 cm-1) using a cantilever torque magnetometry which is further firmly supported by magnetic susceptibility, and EPR measurements. The study of the magnetization relaxation dynamics reveals field-induced slow relaxation of magnetization due to the predominant Raman relaxation process. Theoretical calculations on 1-3 and optimized model complexes of 1 reveal insights into the electronic structure and highlight the impact of steric and electronic effects on modulating the D values. Overall, the studies reported pave the way for designing a new generation of CoII complexes with enhanced axiality and a lower rhombicity.