3d single-ion magnets

A Nanosized {NiII 18} Cluster with a 'Flying Saucer' Topology Exhibiting Slow Relaxation of Magnetisation Phenomena at Both 15 K and 1.3 K

A high-nuclearity {Ni18} complex (1) with a unique 'flying saucer' motif has been prepared from the organic chelate, α-methyl-2-pyridine-methanol (mpmH), in conjunction with bridging azido (N3 -) and peroxido (O2 2-) ligands. Magnetic susceptibility measurements revealed the presence of both ferro- and antiferromagnetic exchange interactions between the metal centres in 1, and the stabilization of spin states with appreciable S values at two different temperature regimes. The end-on bridging azido and alkoxido groups are in all likelihood the ferromagnetic mediators, while the η3:η3:μ6-bridging peroxides most likely promote the antiparallel alignment of the metals' spin vectors, yielding an overall non-zero spin ground state for the centrosymmetric compound 1. Furthermore, the {Ni18} nanosized cluster behaves as a single-molecule magnet, exhibiting magnetic hysteresis at low temperatures and two relaxation processes at 15 K and 1.3 K, a very rare phenomenon in polynuclear magnetic 3d-metal clusters.

Quantifying Magnetic Anisotropy of Series of Five-Coordinate CoII Ions: Experimental and Theoretical Insights

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.

Polyoxometalates as advanced-performance anions for ∼D5h Dy(III) single-ion magnets

We enhance single-ion magnet (SIM) magnetisation reversal barriers by engineering the second coordination sphere, substituting conventional small monoanions with a bulky polyoxometalate (POM) trianion. Importantly, our approach serves as a model for creating new high-performance multifunctional hybrid materials.

Six-Coordinated CoII Single-Molecule Magnets: Synthetic Strategy, Structure and Magnetic Properties

The pursuit of molecule-based magnetic memory materials contributes significantly to high-density information storage research in the frame of the ongoing information technologies revolution. Remarkable progress has been achieved in both transition metal (TM) and lanthanide based single-molecule magnets (SMMs). Notably, six-coordinated CoII SMMs hold particular research significance owing to the economic and abundant nature of 3d TM ions compared to lanthanide ions, the substantial spin-orbit coupling of CoII ions, the potential for precise control over coordination geometry, and the air-stability of coordination-saturated structures. In this review, we will summarize the progress made in six-coordinated CoII SMMs, organized by their coordination geometry and molecular structure similarity. Valuable insights, principles, and new mechanism gleaned from this research and remaining issues that need to be addressed will also be discussed to guide future optimization.

Levamisole Based Co(II) Single-Ion Magnet

A new Co(II) complex, [Co(NCS)2(L)2] (1) has been synthesized based on levamisole (L) as a new ligand. Single-crystal X-ray diffraction analyses confirm that the Co(II) ion is having a distorted tetrahedral coordination geometry in the complex. Notably strong intramolecular S⋅⋅⋅S and S⋅⋅⋅N interactions has been confirmed by employing Quantum Theory of Atoms in Molecules (QTAIM). These intramolecular interactions occur among the sulfur and nitrogen atoms of the levamisole ligands and also the nitrogen atoms of the thiocyanate. Direct current (dc) magnetic analyses reveal presence of zero field splitting (ZFS) and large magnetic anisotropy on Co(II). Detailed ab initio ligand field theory calculations quantitatively predicted the magnitude of ZFS. Prominent field-induced single-ion magnet (SIM) behavior was observed for 1 from dynamic magnetization measurements. Slow magnetic relaxation follows an Orbach mechanism with the effective energy barrier Ueff=29.6 (7) K and relaxation time τo=1.4 (4)×10-9 s.

Control of the geometry and anisotropy driven by the combination of steric and anion coordination effects in CoII complexes with N6-tripodal ligands: the impact of the size of the ligand on the magnetization relaxation time

Four mononuclear CoII complexes of formula [Co(L)(SCN)2(CH3OH)0.5(H2O)0.5]·1.5H2O·0.75CH3OH (1), [Co(L1)Cl2]·H2O·2CH3CN (2), [Co(L1)(SCN)2]·1.5H2O·CH3OH (3) and [Co(L1)]ClO4·2CH3OH (4) were prepared from the N6-tripodal Schiff base ligands (S)P[N(Me)NC(H)2-Q]3 (L) and (S)P[N(Me)NC(H)1-ISOQ]3 (L1), where Q and ISOQ represent quinolyl and isoquinolyl moieties, respectively. In 1, the L ligand does not coordinate to the CoII ion in a tripodal manner but using a new N,N,S tridentate mode, which is due to the fact that the N6-tripodal coordination promotes a strong steric hindrance between the quinolyl moieties. However, L1 can coordinate to the CoII ions either in a tripodal manner using CoII salts with poorly coordinating anions to give 4 or in a bisbidentate fashion using CoII salt-containing medium to strongly coordinating anions to afford 2 and 3. In the case of L1, there is no steric hindrance between ISOQ moieties after coordination to the CoII ion. The CoII ion exhibits a distorted octahedral geometry for compounds 1-3, with the anions in cis positions for the former and in trans positions for the two latter compounds. Compound 4 shows an intermediate geometry between an octahedral and trigonal prism but closer to the latter one. DC magnetic properties, HFEPR and FIRMS measurements and ab initio calculations demonstrate that distorted octahedral complexes 1-3 exhibit easy-plane magnetic anisotropy (D > 0), whereas compound 4 shows large easy-axis magnetic anisotropy (D < 0). Comparative analysis of the magneto-structural data underlines the important role that is played not only by the coordination geometry but also the electronic effects in determining the anisotropy of the CoII ions. Compounds 2-3 show a field-induced slow relaxation of magnetization. Despite its large easy-axis magnetic anisotropy, compound 4 does not show significant slow relaxation (SMR) above 2 K under zero applied magnetic fields, but its magnetic dilution with ZnII triggers SMR at zero field. Finally, it is worth remarking that compounds 2-4 show smaller relaxation times than the analogous complexes with the tripodal ligand bearing in its arms pyridine instead of isoquinoline moieties, which is most likely due to the increase of the molecular size in the former one.

Structural, optical and magnetic properties of a new metal-organic CoII-based complex

A mononuclear cobalt(ii) complex [C5H8N3]2[CoCl4(C5H7N3)2] (I) was synthesized and structurally characterized. Single crystal X-ray diffraction analysis indicates that monometallic Co(ii) ions acted as coordination nodes in a distorted octahedral geometry, giving rise to a supramolecular architecture. The latter is made up of a ½ unit form composed of an anionic element [Co0.5Cl2(C5H7N3)]- and one 2-amino-4-methylpyrimidinium cation [C5H8N3]+. The crystalline arrangement of this compound adopts the sandwich form where inorganic parts are sandwiched between the organic sheets following the [100] direction. More information regarding the structure hierarchy has been supplied based on Hirshfeld surface analysis; the X⋯H (X = N, Cl) interactions play a crucial role in stabilizing the self-assembly process of I, complemented by the intervention of π⋯π electrostatic interaction created between organic entities. Thermal analyses were carried out to study the thermal behavior process. Static magnetic measurements and ab initio calculations of compound I revealed the easy-axis anisotropy character of the central Co(ii) ion. Two-channel field-induced slow-magnetic relaxation was observed; the high-frequency channel is characterized by underbarrier relaxation with U eff = 16.5 cm-1, and the low-frequency channel involves a direct relaxation process affected by the phonon-bottleneck effect.

Gaining Insights into the Interplay between Optical and Magnetic Properties in Photoexcited Coordination Compounds

We describe early and recent advances in the fascinating field of combined magnetic and optical properties of inorganic coordination compounds and in particular of 3d-4f single molecule magnets. We cover various applied techniques which allow for the correlation of results obtained in the frequency and time domain in order to highlight the specific properties of these compounds and the future challenges towards multidimensional spectroscopic tools. An important point is to understand the details of the interplay of magnetic and optical properties through performing time-resolved studies in the presence of external fields especially magnetic ones. This will enable further exploration of this fundamental interactions i. e. the two components of electromagnetic radiation influencing optical properties.

Spin-State Control in Dysprosium(III) Metallacrown Magnets via Thioacetal Modification

Integrating controllable spin states into single-molecule magnets (SMMs) enables precise manipulation of magnetic interactions at a molecular level, but remains a synthetic challenge. Herein, we developed a 3d-4f metallacrown (MC) magnet [DyNi5(quinha)5(Clsal)2(py)8](ClO4) ⋅ 4H2O (H2quinha=quinaldichydroxamic acid, HClsal=5-chlorosalicylaldehyde) wherein a square planar NiII is stabilized by chemical stacking. Thioacetal modification was employed via post-synthetic ligand substitutions and yielded [DyNi5(quinha)5(Clsaldt)2(py)8](ClO4) ⋅ 3H2O (HClsaldt=4-chloro-2-(1,3-dithiolan-2-yl)phenol). Thanks to the additional ligations of thioacetal onto the NiII site, coordination-induced spin state switching (CISSS) took place with spin state altering from low-spin S=0 to high-spin S=1. The synergy of CISSS effect and magnetic interactions results in distinct energy splitting and magnetic dynamics. Magnetic studies indicate prominent enhancement of reversal barrier from 57 cm-1 to 423 cm-1, along with hysteresis opening and an over 200-fold increment in coercive field at 2 K. Ab initio calculations provide deeper insights into the exchange models and rationalize the relaxation/tunnelling pathways. These results demonstrate here provide a fire-new perspective in modulating the magnetization relaxation via the incorporation of controllable spin states and magnetic interactions facilitated by the CISSS approach.

Halide mediated modulation of magnetic interaction and anisotropy in dimeric Co(II) complexes

The burgeoning interest in the field of molecular magnetism is to perceive the high magnetic anisotropy in different geometries of metal complexes and hence to draw a magneto-structural correlation. Despite a handful of examples to exemplify the magnetic anisotropy in various coordination geometries of mononuclear complexes, the magnetic anisotropies for two different coordination geometries are underexplored. Employing an appropriate synthetic strategy utilizing the ligand LH2 [2,2'-{(1E,1'E)-pyridine2,6-diyl-bis(methaneylylidine)}-bis(azaneylylidine)diphenol] and cobalt halide salts in a 1 : 2 stoichiometric ratio in the presence of triethylamine allowed us to report a new family of dinuclear cobalt complexes [CoII2X2(L)(P)(Q)]·S with varying terminal halides [X = Cl, P = CH3CN, Q = H2O, S = H2O (1), X = Br, P = CH3CN, Q = H2O, S = H2O (2), X = I, P = CH3CN, and Q = CH3CN (3)]. All these complexes are characterized through single crystal X-ray crystallography, which reveals their crystallization in the monoclinic system P21/n space group with nearly identical structural features. These complexes share vital components, including Co(II) centers, a fully deprotonated ligand [L]2-, halide ions, and solvent molecules. The [L]2- ligand contains two Co(II) centers, where phenolate oxygen atoms bridge the Co(II) centers, forming a Co2O2 four-membered ring. Co1 demonstrates a distorted pentagonal-bipyramidal geometry with axial positions for solvent molecules, while Co2 displays a distorted tetrahedral geometry involving phenolate oxygen atoms and halide ions. Temperature-dependent dc magnetic susceptibility measurements were conducted on 1-3 within a range of 2 to 300 K at 1 kOe. The χmT vs. T plots exhibit similar trends, with χmT values at 300 K higher than the spin-only value, signifying a significant orbital contribution. As the temperature decreases, χmT decreases smoothly in all the complexes; however, no clear saturation at low temperatures is observed. Field-dependent magnetization measurements indicate a rapid increase below 20 kOe, with no hysteresis and a low magnetic blocking temperature. DFT and CASSCF/NEVPT2 theoretical calculations were performed to perceive the magnetic interaction and single-ion anisotropies of Co(II) ions in various ligand-field environments.

Heterobimetallic 3d-4f complexes supported by a Schiff-base tripodal ligand

Complexes featuring multiple metal centres are of growing interest regarding metal-metal cooperation and its tuneability. Here the synthesis and characterisation of heterobimetallic complexes of a 3d metal (4: Mn, 5: Co) and lanthanum supported by a (1,1,1-tris[(3-methoxysalicylideneamino)methyl]ethane) ligand is reported, as well as discussion of their electronic structure via electron paramagnetic resonance (EPR) spectroscopy, electrochemical experiments and computational studies. Competitive binding experiments of the ligand and various metal salts unequivocally demonstrate that in these heterobimetallic complexes the 3d metal (Mn, Co) selectively occupies the κ6-N3O3 binding site of the ligand, whilst La occupies the κ6-O6 metal binding site in line with their relative oxophilicities. EPR spectroscopy supported by density functional theory analysis indicates that the 3d metal is high spin in both cases (S = 5/2 (Mn), 3/2 (Co)). Cyclic voltammetry studies on the Mn/La and Co/La bimetallic complexes revealed a quasi-reversible Mn2+/3+ redox process and poorly-defined irreversible oxidation events respectively.

Thermal processes in anisotropic metal complexes induced by non-adiabatic switching of magnetic field

In this article we analyze the thermal processes in magnetically anisotropic metal complexes under the action of non-adiabatic switching of magnetic field. Using the non-stationary perturbation theory for the case of sudden perturbation, we show that this field can cause not only heat release, but also heat absorption, interconnected with the axial zero field splitting (parameter D) in a paramagnetic metal complex. As an illustrative example we consider the simplest S = 1-complexes having "easy axis" and "easy plane" types of anisotropy influenced by the magnetic field that is suddenly turned off. We demonstrate that the character of the thermal processes (heat dissipation or absorption) depends on the sign of D and direction of applied field and so the analysis of these processes can be in principle used as a complementary tool (in addition to SQIUD magnetometry, EPR spectroscopy and INS) for studying magnetic anisotropy. The conditions under which the non-adiabatic switching of the magnetic field gives rise to the heat absorption are revealed. This unusual phenomenon, which can be called "nonadiabatic field switching cooling", may have practical applications.

Six-coordinated nickel(II) complexes with benzothiadiazole Schiff-base ligands: synthesis, crystal structure, magnetic and HFEPR study

Two new nickel(II) complexes, namely Ni(L1)2 (1) and Ni(L2)2·CH2Cl2(2) were obtained by reacting nickel(II) acetate tetrahydrate with the benzothiadiazole Schiff base ligands HL1 = 2-[4-(2,1,3-benzothiadiazole)imino]methyl-phenol or HL2 = 2-[(2,1,3-benzothiadiazol-4-ylimino)methyl]-6-methoxyphenol in the presence of Et3N. The tridentate NNO chelate ligands induce a distorted octahedral environment around the nickel(II) ions. Single crystal X-ray diffraction analysis reveals elongated Ni-N bonds with the nitrogen atom of the benzothiadiazole ring in both complexes. Intermolecular hydrogen bonds and π-π stacking interactions create two-dimensional and three-dimensional supramolecular arrays, respectively, for complexes 1 and 2. Magnetic susceptibility and high-field electron paramagnetic resonance measurements show the presence of significant magnetic anisotropy, with an axial distortion parameter D of -8--10 cm-1.

Magnetic Anisotropy and Relaxation in Four-Coordinate Cobalt(II) Single-Ion Magnets with a [CoIIO4] Core

A mononuclear four-coordinate Co(II) complex with a [CoIIO4] core, namely, PPN[Li(MeOH)4][Co(L)2] (1) (PPN = bis(phosphoranediyl)iminium; H2L = perfluoropinacol), has been studied by X-ray crystallography, magnetic characterization, and theoretical calculations. This complex presents a severely distorted coordination geometry. The O-Co-O bite angle is 83.42°/83.65°, and the dihedral twist angle between the O-Co-O chelate planes is 55.6°. The structural distortion results in a large easy-axis magnetic anisotropy with D = -104(1) cm-1 and a transverse component with |E| = +4(2) cm-1. Alternating current (ac) susceptibility measurements demonstrate that 1 exhibits slow relaxation of magnetization at zero static field. However, the frequency-dependent out-of-phase (χ"M) susceptibilities of 1 at 0 Oe do not show a characteristic maximum. Upon the application of a dc field or the dilution with a diamagnetic Zn matrix, the quantum tunneling of magnetization (QTM) process can be successfully suppressed. Notably, after dilution with the Zn matrix, the obtained sample exhibits a structure different from that of the pristine complex. In this altered sample, the asymmetric unit does not contain the Li(MeOH)4+ cation, resulting in an O-Co-O bite angle of 86.05° and a dihedral twist angle of 75.84°, thereby leading to an approximate D2d symmetry. Although such differences are not desirable for magnetic studies, this study still gives some insights. Theoretical calculations reveal that the D parameter is governed by the O-Co-O bite angle, in line with our previous report for other tetrahedral Co(II) complex with a [CoIIN4] core. On the other hand, the rhombic component is found to increase as the dihedral angle deviates from 90°. These findings provide valuable guidelines for fine-tuning the magnetic properties of Co(II) complexes.

Challenges for exploiting nanomagnet properties on surfaces

Molecular complexes with single-molecule magnet (SMM) or qubit properties, commonly called molecular nanomagnets, are great candidates for information storage or quantum information processing technologies. However, the implementation of molecular nanomagnets in devices for the above-mentioned applications requires controlled surface deposition and addressing the nanomagnets' properties on the surface. This Perspectives paper gives a brief overview of molecular properties on a surface relevant for magnetic molecules and how they are affected when the molecules interact with a surface; then, we focus on systems of increasing complexity, where the relevant SMMs and qubit properties have been observed for the molecules deposited on surfaces; finally, future perspectives, including possible ways of overcoming the problems encountered so far are discussed.

Modulated spin dynamics of [Co2] coordination helicates via differential strand composition

Coordination supramolecular chemistry provides a versatile entry into materials with functionalities of technological relevance at the nanoscale. Here, we describe how two different bis-pyrazolylpyridine ligands (L1 and L2) assemble with Co(II) ions into dinuclear triple-stranded helicates, in turn, encapsulating different anionic guests. These constructs are described as (Cl@[Co2(L1)3])3+, (SiF6@[Co2(L1)(L2)3])2+ and (ClO4@[Co2(L2)3])3+, as established by single-crystal X-ray diffraction. Extensive magnetic and calorimetric measurements, numerical treatments and theoretical calculations reveal that the individual Co(II) centers of these supramolecular entities exhibit field-induced slow relaxation of magnetization, dominated by direct and Raman mechanisms. While the small variations in the spin dynamics are not easily correlated with the evident structural differences among the three species, the specific heat measurements suggest two vibronic pathways of magnetic relaxation: one that would be associated with the host lattice and another linked with the guest.

Field induced slow magnetic relaxation in a linear homotrinuclear manganese heterospin coordination compound with S = 7/2 ground state and intriguing spin density distribution

We report a first example of field-induced (HDC = 2500 Oe) slow magnetization relaxation in the homotrinuclear linear heterospin manganese coordination compound with S = 7/2 ground state, based on the bidentate 3,5-di-tert-butyl-1,2-benzoquinone-1-monooxime (HL) ligand with composition {[MnL3]Mn[MnL3]}.

Solvation/desolvation induced reversible distortion change and switching between spin crossover and single molecular magnet behaviour in a cobalt(II) complex

Coexistence and switching between spin-crossover (SCO) and single molecular magnet (SMM) behaviours in one single complex may lead to materials that exhibit bi-stable and stimuli sensitive properties in a wide temperature range and under multiple conditions; unfortunately, the conflict and dilemma in the principle of approaching SCO and SMM molecules make it particularly difficult; at low temperature, low spin (LS) SCO molecules possess highly symmetrical geometry and isotropic spins, which are not suitable for SMM behaviour. Herein, we overcome this issue by using a rationally designed Co(II) mononuclear complex [Co(MeOphterpy)2] (ClO4)2 (1; MeOphterpy = 4'-(4-methoxyphenyl)-2,2':6',2''-terpyridine), the magnetic properties of which reversibly respond to desolvation and solvation. The solvated structure reinforced a low distortion of the coordination sphere via hydrogen bonding between ligands and methanol molecules, while in the desolvated structure a methoxy group flipping occurred, increasing the distortion of the coordination sphere and stabilising the HS state at low temperature, which exhibited a field-induced slow magnetic relaxation, resulting in a reversible switching between SCO and SMM properties within one molecule.

A tetravalent praseodymium complex with field-induced slow magnetic relaxation

Herein the synthesis, structural characterization, and magnetic properties of a Pr(IV) complex [Pr(OSiPh3)4(L)] (1, L = 4,4'-dimethoxy-2,2'-bipyridine) are reported. The stability of the Pr(IV) complex significantly enhanced with the use of the bidentate ligand L. Slow magnetic relaxation was observed at low temperatures, indicating that the complex may be the first single-ion magnet with a tetravalent lanthanide ion being the magnetic center.

How the spin state tunes the slow magnetic relaxation field dependence in spin crossover cobalt(II) complexes

A novel family of cobalt(II) compounds with tridentate pyridine-2,6-diiminephenyl type ligands featuring electron-withdrawing substituents of general formula [Co(n-XPhPDI)2](ClO4)2·S [n-XPhPDI = 2,6-bis(N-n-halophenylformimidoyl)pyridine with n = 4 (1-3) and 3 (4); X = I (1), Br (2 and 4) and Cl (3); S = MeCN (1 and 2) and EtOAc (3)] has been synthesised and characterised by single-crystal X-ray diffraction, electron paramagnetic resonance, and static (dc) and dynamic (ac) magnetic measurements combined with theoretical calculations. The structures of 1-4 consist of mononuclear bis(chelating) cobalt(II) complex cations, [CoII(n-XPhPDI)2]2+, perchlorate anions, and acetonitrile (1 and 2) or ethyl acetate (3) molecules of crystallisation. This unique series of mononuclear six-coordinate octahedral cobalt(II) complexes displays both thermally-induced low-spin (LS)/high-spin (HS) transition and field-induced slow magnetic relaxation in both LS and HS states. A complete LS ↔ HS transition occurs for 1 and 2, while it is incomplete for 4, one-third of the complexes being HS at low temperatures. In contrast, 3 remains HS in all the temperature range. 1 and 2 show dual spin relaxation dynamics under the presence of an applied dc magnetic field (Hdc), with the occurrence of faster- (FR) and slower-relaxing (SR) processes at lower (Hdc = 1.0 kOe) and higher fields (Hdc = 2.5 kOe), respectively. On the contrary, 3 and 4 exhibit only SR and FR relaxations, regardless of Hdc. Overall, the distinct field-dependence of the single-molecule magnet (SMM) behaviour along with this family of spin-crossover (SCO) cobalt(II)-n-XPhPDI complexes is dominated by Raman mechanisms and, occasionally, with additional temperature-independent Intra-Kramer [LS or HS (D > 0)] or Quantum Tunneling of Magnetisation mechanisms [HS (D < 0)] also contributing.

Tetranuclear CoII4O4 Cubane Complex: Effective Catalyst Toward Electrochemical Water Oxidation

The reaction of Co(OAc)2·6H2O with 2,2'-[{(1E,1'E)-pyridine-2,6-diyl-bis(methaneylylidene)bis(azaneylylidene)}diphenol](LH2) a multisite coordination ligand and Et3N in a 1:2:3 stoichiometric ratio forms a tetranuclear complex Co4(L)2(μ-η1:η1-OAc)2(η2-OAc)2]· 1.5 CH3OH· 1.5 CHCl3 (1). Based on X-ray diffraction investigations, complex 1 comprises a distorted Co4O4 cubane core consisting of two completely deprotonated ligands [L]2- and four acetate ligands. Two distinct types of CoII centers exist in the complex, where the Co(2) center has a distorted octahedral geometry; alternatively, Co(1) has a distorted pentagonal-bipyramidal geometry. Analysis of magnetic data in 1 shows predominant antiferromagnetic coupling (J = -2.1 cm-1), while the magnetic anisotropy is the easy-plane type (D1 = 8.8, D2 = 0.76 cm-1). Furthermore, complex 1 demonstrates an electrochemical oxygen evolution reaction (OER) with an overpotential of 325 mV and Tafel slope of 85 mV dec-1, required to attain a current density of 10 mA cm-2 and moderate stability under alkaline conditions (pH = 14). Electrochemical impedance spectroscopy studies reveal that compound 1 has a charge transfer resistance (Rct) of 2.927 Ω, which is comparatively lower than standard Co3O4 (5.242 Ω), indicating rapid charge transfer kinetics between electrode and electrolyte solution that enhances higher catalytic activity toward OER kinetics.

Spectroscopic techniques to probe magnetic anisotropy and spin-phonon coupling in metal complexes

Magnetism of molecular quantum materials such as single-molecule magnets (SMMs) has been actively studied for potential applications in the new generation of high-density data storage using SMMs and quantum information science. Magnetic anisotropy and spin-phonon coupling are two key properties of d- and f-metal complexes. Here, phonons refer to both intermolecular and intramolecular vibrations. Direct determination of magnetic anisotropy and experimental studies of spin-phonon coupling are critical to the understanding of molecular magnetism. This article discusses our recent approach in using three complementary techniques, far-IR and Raman magneto-spectroscopies (FIRMS and RaMS, respectively) and inelastic neutron scatterings (INS), to determine magnetic excited states. Spin-phonon couplings are observed in FIRMS and RaMS. DFT phonon calculations give energies and symmetries of phonons as well as calculated INS spectra which help identify magnetic peaks in experimental INS spectra.

Building Molecular Nanomagnets by Encapsulating Lanthanide Ions in Boron Nitride Nanotubes: Ab Initio Investigation

Lanthanide-based single-ion magnets have attracted much interest due to their great potential for information storage at the level of one molecule. Among various strategies to enhance magnetization blocking in such complexes, the synthesis of axially symmetric compounds is regarded as the most promising. Here, we investigate theoretically the magnetization blocking of several lanthanide ions (Tb3+, Dy3+, Ho3+, Er3+, and Tm3+) encapsulated in highly symmetric zigzag boron nitride nanotubes (BNNTs) of different diameters with ab initio methodology. We found that Tb3+@(7,0)BNNT, Dy3+@(7,0)BNNT, and Tm3+@(5,0)BNNT are suitable SIM candidates, while the other investigated complexes from this series show no signs of magnetization blocking owing to a hard competition between contributions to the crystal field of the lanthanide ion from neighboring and more distant atoms of the nanotube.

Spin canting and slow magnetic relaxation in mononuclear cobalt(II) sulfadiazine ternary complexes

Monomeric [Co(SDZ)2phen] (1) and [Co(SDZ)(bq)Cl] (2) complexes (SDZ = sulfadiazine, phen = 1,10-phenanthroline, and bq = 2,2'-biquinoline) have been synthesized and characterized. X-ray diffraction studies indicate that SDZ acts as a bidentate ligand coordinating through the sulfonamide and the pyrimidine N atoms in both compounds. In complex 1, the coordination sphere consists of two SDZ ligands and a bis-chelating phen ligand, giving rise to a CoN6 coordination sphere. On the other hand, 2 has a CoN4Cl core, with two N-atoms from SDZ and two from the bq ligand. Both compounds have been studied by dc and ac magnetometry and shown to display slow magnetic relaxation under an optimum external dc field (1 kOe) at low temperatures. Moreover, compound 2 displays long range magnetic ordering provided by spin-canted antiferromagnetism, which has been characterized by further field-dependent magnetic susceptibility measurements, FC/ZFC curves, hysteresis loops and frequency-independent ac curves. The signs of the calculated D parameters, positive in 1 and negative in 2, have been rationalized according to the two lowest-lying transitions in the orbital energy diagrams derived from ab initio ligand field theory (AILFT). In a subsequent attempt to reveal the possible hidden zero-field SMM behaviour, Ni(II)-based 3 and Co(II)-doped Ni(II)-based (with a Ni : Co ratio of 0.9 : 0.1) heterometallic compound 2Ni were synthesized.

Large Magnetic Anisotropy in Mono- and Binuclear cobalt(II) Complexes: The Role of the Distortion of the Coordination Sphere in Validity of the Spin-Hamiltonian Formalism

To get a better insight into understanding the factors affecting the enhancement of the magnetic anisotropy in single molecule (single ion) magnets, two cobalt(II) complexes based on a tridentate ligand 2,6-di(thiazol-2-yl)pyridine substituted at the 4-position with N-methyl-pyrrol-2-yl have been synthesized and studied by X-ray crystallography, AC and DC magnetic data, FIRMS and HFEPR spectra, and theoretical calculations. The change of the counteranion in starting Co(II) salts results in the formation of pentacoordinated mononuclear [Co(mpyr-dtpy)Cl2]·2MeCN (1) complex and binuclear [Co(mpyr-dtpy)2][Co(NCS)4] (2) compound. The observed marked distortion of trigonal bipyramid geometry in 1 and cationic octahedral and anionic tetrahedral units in 2 brings up a question about the validity of the spin-Hamiltonian formalism and the possibility of determining the value and sign of the zero-field splitting D parameter. Both complexes exhibit field-induced slow magnetic relaxation with two or three relaxation channels at BDC = 0.3 T. The high-frequency relaxation time in the reciprocal form τ(HF)-1 = CTn develops according to the Raman relaxation mechanism (for 2, n = 8.8) and the phonon-bottleneck-like mechanism (for 1, n = 2.3). The high-frequency relaxation time at T = 2.0 K and BDC = 0.30 T is τ(HF) = 96 and 47 μs for 1 and 2, respectively.

Zero-Field SMM Behavior Triggered by Magnetic Exchange Interactions and a Collinear Arrangement of Local Anisotropy Axes in a Linear Co3II Complex

A new linear trinuclear Co(II)3 complex with a formula of [{Co(μ-L)}2Co] has been prepared by self-assembly of Co(II) ions and the N3O3-tripodal Schiff base ligand H3L, which is obtained from the condensation of 1,1,1-tris(aminomethyl)ethane and salicylaldehyde. Single X-ray diffraction shows that this compound is centrosymmetric with triple-phenolate bridging groups connecting neighboring Co(II) ions, leading to a paddle-wheel-like structure with a pseudo-C3 axis lying in the Co-Co-Co direction. The Co(II) ions at both ends of the Co(II)3 molecule exhibit distorted trigonal prismatic CoN3O3 geometry, whereas the Co(II) at the middle presents an elongated trigonal antiprismatic CoO6 geometry. The combined analysis of the magnetic data and theoretical calculations reveal strong easy-axis magnetic anisotropy for both types of Co(II) ions (|D| values higher than 115 cm-1) with the local anisotropic axes lying on the pseudo-C3 axis of the molecule. The magnetic exchange interaction between the middle and ends Co(II) ions, extracted by using either a Hamiltonian accounting for the isotropic magnetic coupling and ZFS or the Lines' model, was found to be medium to strong and antiferromagnetic in nature, whereas the interaction between the external Co(II) ions is weak antiferromagnetic. Interestingly, the compound exhibits slow relaxation of magnetization and open hysteresis at zero field and therefore SMM behavior. The significant magnetic exchange coupling found for [{Co(μ-L)}2Co] is mainly responsible for the quenching of QTM, which combined with the easy-axis local anisotropy of the CoII ions and the collinearity of their local anisotropy axes with the pseudo-C3 axis favors the observation of SMM behavior at zero field.

Tuning Magnetic Anisotropy in Co(II) Tetrahedral Carbazole-Modified Phosphine Oxide Single-Ion Magnets: Importance of Structural Distortion versus Heavy-Ion Effect

Three mononuclear cobalt(II) tetrahedral complexes [Co(CzPh2PO)2X2] (CzPh2PO = (9H-carbazol-9-yl)diphenylphosphine oxide and X = Cl (1), Br (2), I (3)) have been synthesized using a simple synthetic approach to examine their single-ion magnetic (SIM) behavior. A detailed study of the variation in the dynamic magnetic properties of the Co(II) ion in a tetrahedral ligand field has been carried out by the change of the halide ligand. The axial zero-field splitting parameter D was found to vary from -16.4 cm-1 in 1 to -13.8 cm-1 in 2 and +14.6 cm-1 in 3. All the new complexes exhibit field-induced SIM behavior. The results obtained from ab initio CASSF calculations match well with the experimental data, revealing how halide ions induce a change in the D value as we move from Cl- to I-. The ab initio calculations further reveal that the change in the sign of D is due to the multideterminant characteristics of the ground state wave function of 1 and 2, while single-determinant characteristics are instead observed for 3. To gain a better understanding of the relationship between the structural distortion and the sign and magnitude of D values, magnetostructural D correlations were developed using angular relationships, revealing the importance of structural distortions over the heavy halide effect in controlling the sign of D values. This study broadens the scope of employing electronically and sterically modified phosphine oxide ligands in building new types of air-stable Co(II) SIMs.

Field induced single ion magnet behavior in CoII complexes in a distorted square pyramidal geometry

We report three CoII-based complexes with the general formula [CoII(L)(X)2] by changing the halide/pseudo-halide ions [X = NCSe (1SeCN); Cl (2Cl) and Br (3Br)]. The obtained τ5 and CShM values confirm a distorted square pyramidal geometry around the CoII ion in all these complexes. In these three complexes, the central CoII ion is situated above the basal plane of the square pyramidal geometry. The extent of distortion from the ideal SPY-5 geometry differs upon changing the coordinating halide/pseudo-halide ion in these complexes. This essentially results in the alteration of the anisotropic parameter D and hence impacts the magnetic properties in these complexes. This phenomenon has been corroborated with the aid of theoretical investigations. All these complexes display field-induced SIM behaviour with magnetic relaxation occurring through a combination of processes depending on the applied dc magnetic field values and dilution.

Slow magnetic relaxation in 8-coordinate Mn(II) compounds

The design and synthesis of high-spin Mn(II)-based single-molecule magnets (SMMs) have not been well developed to a great extent, as compared with a large number of SMMs based on the other first row transition metal complexes. In light of our success in designing Fe(II), Co(II) and Fe(III)-based SMMs with a high coordination number of 8, it is of great interest to design Mn(II) analogues with such a strategy. In this contribution, four Mn(II) compounds, [MnII(Ln)2](ClO4)2 (1-4) were obtained from reactions of neutral tetradentate ligands, L1-L4, with hydrated MnII(ClO4)2 (L1 = 2,9-bis(carbomethoxy)-1,10-phenanthroline, L2 = 2,9-bis(carbomethoxy)-2,2'-dipyridine, L3 = N2,N9-dibutyl-1,10-phenanthroline-2,9-dicarboxamide, L4 = 6,6'-bis(2-(tert-butyl)-2H-tetrazol-5-yl)-2,2'-bipyridine). Their crystal structures have been determined by X-ray crystallography and it clearly shows that the Mn(II) centers in these compounds have an oversaturated coordination number of 8. Their magnetic properties have been investigated in detail; to our surprise, all of these Mn(II) compounds show interesting slow magnetic relaxation behaviors under an applied direct current field, although they have very small negative D values.

Alkyl Chains Modulated Magnetization Dynamics of Mononuclear Trigonal Prismatic CoII Complexes

Four benzeneboron-capped mononuclear CoII complexes with different alkyl substitutions on the fourth position of phenylboronic acid were obtained. The CoII ions are all wrapped by the pocket-like ligands and located in trigonal prismatic coordination geometries. Alternating-current magnetic susceptibility measurements reveal that they show different magnetization dynamics, such as distinct relaxation rates at the same temperature, the faster QTM rates for the ethyl and propyl substituted complexes, as well as different relaxation processes. Magneto-structural correlation study reveals that the various deviations of coordination geometry of CoII ion, diverse crystal packings and possible different vibration modes of substituents caused by modifying alkyl chains are the key factors affecting the magnetization dynamics. This work demonstrates that the alkyl chains even locating far away from the metal center can have a large impact on the magnetic behavior of the CoII complex with a very rigid coordination geometry, offering a new perspective towards transition metal based single-molecule magnets.

Peripheral site modification in a family of dinuclear [Dy2(hynad)2-6(NO3)0-6(sol)0-2]0/2- single-molecule magnets bearing a {Dy2(μ-OR)2}4+ diamond-shaped core and exhibiting dissimilar magnetic dynamics

The first use of the organic chelate N-hydroxy-1,8-naphthalimide (hynadH) in DyIII chemistry has unveiled access to a synthetic 'playground' composed of four new dinuclear complexes, all of which possess the same planar {Dy2(μ-OR)2}4+ diamond-shaped core, resulting from the bridging and chelating capacity of the hynad- groups. The structural stability of the central {Dy2} core has allowed for the modulation of the peripheral coordination sites of the metal ions, and specifically the NO3-/hynad- ratio of capping groups, thus affording the compounds [Dy2(hynad)2(NO3)4(DMF)2] (1), (Me4N)2[Dy2(hynad)2(NO3)6] (2), [Dy2(hynad)4(NO3)2(H2O)2] (3), and [Dy2(hynad)6(H2O)2] (4). Because of the chemical and structural modifications in the series 1-4, the DyIII coordination polyhedra are also dissimilar, comprising the muffin (1 and 3), tetradecahedral (2), and spherical tricapped trigonal prismatic (4) geometries. Complexes 1, 2, and 4 exhibit a ferromagnetic response at low temperatures, while 3 is antiferromagnetically coupled. All compounds exhibit out-of-phase (χ''M) ac signals as a function of ac frequency and temperature, thus behaving as single-molecule magnets (SMMs), in the absence or presence of applied dc fields. Interestingly, the hynad--rich and nitrato-free complex 4, demonstrates the largest energy barrier (Ueff = 69.62(1) K) for the magnetization reversal which is attributed to the presence of the two axial triangular faces of the spherical tricapped trigonal prism by the negatively charged O-atoms of the hynad- ligands.

A tetracobalt(II) cluster with a two vertex truncated dicubane topology endogenously supported by carboxylate-based (2-pyridyl)methylamine ligands: magneto-structural and DFT studies

A reaction between CoCl2 and L3-(CO2-)2 (2 : 1 stoichiometry) in CH3OH affords a discrete complex [CoII4-{L3-(CO2-)2}2(μ3-OCH3)2(CH3OH)2(H2O)2Cl2] (1) [L3-(CO2-)2 = 3-[N-{2-(pyridin-2-yl)methyl}amino]-bis(propionate)]. The structure of 1 reveals two terminal mononuclear CoII{L3-(CO2-)2}Cl units connected by a dimeric CoII2(μ3-OCH3)2(CH3OH)2(H2O) unit present in the centre through two methoxo (μ3-OCH3)- and two carboxylate (μ-1,1-OCO-) bridges affording a tetranuclear coordination cluster of Co(II) with a defective dicubane topology. In 1, Co1 (terminal) has distorted octahedral CoIIN2O3Cl and the central Co2 has CoIIO6 coordination. Such coordination arrangements afford the observed topology. Variable-temperature magnetic studies reveal anti-ferromagnetic coupling in 1. Three magnetic exchange interactions (one anti-ferromagnetic and two ferromagnetic: J1 = +3.3 cm-1 (Co⋯Co 3.176 Å; μ-1,1-OCO- and μ3-OCH3 bridges), J2 = -2.5 cm-1 (Co⋯Co 3.228 Å; μ-1-OCO- and μ3-OCH3 bridges) and J3 = +10.6 cm-1 (Co⋯Co 3.084 Å; two μ3-OCH3 bridges)) have been identified, with the inclusion of the orbital reduction parameter (α = Aκ = 1.38), spin-orbit coupling (λ = -158 cm-1) and axial distortion (energy gap Δ = -975 cm-1 between singlet and doublet levels), rationalized by density functional theory (DFT) calculations.

Ultrafast Jahn-Teller Photoswitching in Cobalt Single-Ion Magnets

Single-ion magnets (SIMs) constitute the ultimate size limit in the quest for miniaturizing magnetic materials. Several bottlenecks currently hindering breakthroughs in quantum information and communication technologies could be alleviated by new generations of SIMs displaying multifunctionality. Here, ultrafast optical absorption spectroscopy and X-ray emission spectroscopy are employed to track the photoinduced spin-state switching of the prototypical complex [Co(terpy)2 ]2+ (terpy = 2,2':6',2″-terpyridine) in solution phase. The combined measurements and their analysis supported by density functional theory (DFT), time-dependent-DFT (TD-DFT) and multireference quantum chemistry calculations reveal that the complex undergoes a spin-state transition from a tetragonally elongated doublet state to a tetragonally compressed quartet state on the femtosecond timescale, i.e., it sustains ultrafast Jahn-Teller (JT) photoswitching between two different spin multiplicities. Adding new Co-based complexes as possible contenders in the search for JT photoswitching SIMs will greatly widen the possibilities for implementing magnetic multifunctionality and eventually controlling ultrafast magnetization with optical photons.

Tuning Crystal Packing and Magnetic Properties in a Series of [Dy12] Metallocubanes Based on Azobenzene Derivatives of Salicylic Acid

A series of four new Dy₁₂ dodecanuclear clusters based on azobenzene derivative ligands of salicylic acid (L1-L4) has been synthesized and characterized in the crystalline phase using X-ray diffraction on single crystal and powder, IR spectroscopy, elemental analysis, and DSC-TGA methods. It was revealed that all obtained clusters exhibit the formation of the similar metallic cluster nodes, as vertex-sharing heterocubanes, obtained from four Dy³⁺ cations, three bridging hydroxyl groups, and O atoms from the salicylic ligands. The coordination geometry around the Dy(III) centers has been carefully analyzed. Whereas Dy₁₂-L1 and Dy₁₂-L2 with L1 and L2 containing Me and OMe groups in para positions of the phenyl rings, respectively, form similar porous 3D diamond-like molecular networks due to CH-π interactions, for Dy₁₂-L3 with L3 bearing NO₂-electron-withdrawing group, the generation of 2D molecular grids assembled by π-π staking is observed, and for Dy₁₂-L4 with L4 bearing phenyl substituent, 3D hexagonal channels have been generated. The complexes Dy₁₂-L1, Dy₁₂-L2, and Dy₁₂-L3 exhibit a zero-field slow magnetic relaxation effect. After UV irradiation of Dy₁₂-L1, a decrease of the magnetic anisotropy energy barrier displaying the possibility of control over magnetic properties by the external stimulus has been observed.

Spin-state energetics and magnetic anisotropy in penta-coordinated Fe(III) complexes with different axial and equatorial ligand environments

The penta-coordinated trigonal-bi-pyramidal (TBP) Fe(III) complex (PMe₂Ph)₂FeCl₃ shows a reduced magnetic anisotropy in its intermediate-spin (IS) state as compared to its methyl-analog (PMe₃)₂Fe(III)Cl₃. In this work, the ligand environment in (PMe₂Ph)₂FeCl₃ is systematically altered by replacing the axial -P with -N and -As, the equatorial -Cl with other halides, and the axial methyl group with an acetyl group. This has resulted in a series of Fe(III) TBP complexes modelled in their IS and high-spin (HS) states. Lighter ligands -N and -F stabilize the complex in the HS state, while the magnetically anisotropic IS state is stabilized by -P and -As at the axial site, and -Cl, -Br, and -I at the equatorial site. Larger magnetic anisotropies appear for complexes with nearly degenerate ground electronic states that are well separated from the higher excited states. This requirement, largely controlled by the d-orbital splitting pattern due to the changing ligand field, is achieved with a certain combination of axial and equatorial ligands, such as -P and -Br, -As and -Br, and -As and -I. In most cases, the acetyl group at the axial site enhances the magnetic anisotropy compared to its methyl counterpart. In contrast, the presence of -I at the equatorial site compromises the uniaxial type of anisotropy of the Fe(III) complex leading to an enhanced rate of quantum tunneling of magnetization.

Site Selectivity for the Spin States and Spin Crossover in Undecanuclear Heterometallic Cyanido-Bridged Clusters

Polynuclear molecular clusters offer an opportunity to design new hierarchical switchable materials with collective properties, based on variation of the chemical composition, size, shapes, and overall building blocks organization. In this study, we rationally designed and constructed an unprecedented series of cyanido-bridged nanoclusters realizing new undecanuclear topology: Fe^II[Fe^II(bzbpen)]₆[W^V(CN)₈]₂[W^IV(CN)₈]₂·18MeOH (1), Na^I[Co^II(bzbpen)]₆[W^V(CN)₈]₃[W^IV(CN)₈]·28MeOH (2), Na^I[Ni^II(bzbpen)]₆[W^V(CN)₈]₃[W^IV(CN)₈]·27MeOH (3), and Co^II[Co^II(R/S-pabh)₂]₆[W^V(CN)₈]₂[W^IV(CN)₈]₂·26MeOH [4R and 4S; bzbpen = N¹,N²-dibenzyl-N¹,N²-bis(pyridin-2-ylmethyl)ethane-1,2-diamine; R/S-pabh = (R/S)-N-(1-naphthyl)-1-(pyridin-2-yl)methanimine], of size up to 11 nm³, ca. 2.0 × 2.2 × 2.5 nm (1-3) and ca. 1.4 × 2.5 × 2.5 nm (4). 1, 2, and 4 exhibit site selectivity for the spin states and spin transition related to the structural speciation based on subtle exogenous and endogenous effects imposed on similar but distinguishable 3d metal-ion-coordination moieties. 1 exhibits a mid-temperature-range spin-crossover (SCO) behavior that is more advanced than the previously reported SCO clusters based on octacyanidometallates and an onset of SCO behavior close to room temperature. The latter feature is also present in 2 and 4, which suggests the emergence of Co^II-centered SCO not observed in previous bimetallic cyanido-bridged Co^II-W^V/IV systems. In addition, reversible switching of the SCO behavior in 1 via a single-crystal-to-single-crystal transformation during desolvation was also documented.

Synergy of Magnetic Anisotropy and Ferromagnetic Interaction Triggering a Dimeric Cr(II) Zero-Field Single-Molecule Magnet

A novel Cr^II-dimeric complex, [Cr^IIN(SiⁱPr₃)₂(μ-Cl)(THF)]₂ (1), has been successfully constructed using a bulky silyl-amide ligand. Single-crystal structure analysis reveals that complex 1 exhibits a binuclear motif, with a Cr₂Cl₂ rhombus core, where two equivalent tetra-coordinate Cr^II centers in the centrosymmetric unit display quasi-square planar geometry. The crystal structure has been well simulated and explored by density functional theory calculations. The axial zero-field splitting parameter (D < 0) with a small rhombic (E) value is unambiguously determined by systematic investigations of magnetic measurements, high-frequency electron paramagnetic resonance spectroscopy, and ab initio calculations. Remarkably, ac magnetic susceptibility data unveil that 1 features slow dynamic magnetic relaxation typical of single-molecule magnet behavior with U_eff = 22 K in the absence of a dc field. This increases up to 35 K under a corresponding static field. Moreover, magnetic studies and theoretical calculations point out that a non-negligible ferromagnetic coupling (FMC) exists in the dimeric Cr-Cr units of 1. The coexistence of magnetic anisotropy and FMC contributes to the first case of Cr^II-based single-molecule magnets (SMMs) under zero dc field.

Magneto-structural studies on a number of doubly end-on cyanate and azide bridged dinuclear nickel(ii) complexes with {N3O} donor Schiff base ligands

Two new doubly μ 1,1-N3 bridged (1 and 3) and six new doubly μ 1,1-NCO bridged NiII complexes (2, 4-8) with six different N3O donor Schiff base ligands have been synthesized and magneto-structurally characterized. All these neutral complex molecules are isostructural and constitute edge sharing bioctahedral structures. Magnetic studies revealed that all these complexes exhibit ferromagnetic interaction through bridging pseudohalides with ferromagnetic coupling constant J being significantly higher for azide-bridged complexes than that of the cyanate analogues. This is consistent with the literature reported data and also the presence of polarizable π systems and two different N and O donor atoms in cyanate ion, rendering it a poor magnetic coupler in comparison to azide analogues. Although, the magneto-structurally characterized doubly μ 1,1-N3 bridged NiII complexes are abundant, only few such complexes with μ 1,1-bridging NCO- ions are reported in the literature. Remarkably, addition of these six new examples in this ever-growing series of doubly μ 1,1-NCO bridged systems gives us an opportunity to analyse the precise magneto-structural correlation in this system, showing a general trend in which the J value increases with an increase in bridging angles. Therefore, the high degree of structural and magnetic resemblances by inclusion of six new examples in this series is the major achievement of the present work. An elaborate DFT study was performed resulting in magneto-structural correlation showing that nature and value of the J-parameter is defined not only by Ni-Nb-Ni bond angles, but an important role is also played by the Ni1-Ni2-Nb-Xt dihedral angle (Nb and Xt are bridging N and terminal N or O atom of bridging ligands, respectively).

Spectroscopic and Magnetic Studies of Co(II) Scorpionate Complexes: Is There a Halide Effect on Magnetic Anisotropy?

The observation of single-molecule magnetism in transition-metal complexes relies on the phenomenon of zero-field splitting (ZFS), which arises from the interplay of spin-orbit coupling (SOC) with ligand-field-induced symmetry lowering. Previous studies have demonstrated that the magnitude of ZFS in complexes with 3d metal ions is sometimes enhanced through coordination with heavy halide ligands (Br and I) that possess large free-atom SOC constants. In this study, we systematically probe this "heavy-atom effect" in high-spin cobalt(II)-halide complexes supported by substituted hydrotris(pyrazol-1-yl)borate ligands (Tp^tBu,Me and Tp^Ph,Me). Two series of complexes were prepared: [Co^IIX(Tp^tBu,Me)] (1-X; X = F, Cl, Br, and I) and [Co^IIX(Tp^Ph,Me)(Hpz^Ph,Me)] (2-X; X = Cl, Br, and I), where Hpz^Ph,Me is a monodentate pyrazole ligand. Examination with dc magnetometry, high-frequency and -field electron paramagnetic resonance, and far-infrared magnetic spectroscopy yielded axial (D) and rhombic (E) ZFS parameters for each complex. With the exception of 1-F, complexes in the four-coordinate 1-X series exhibit positive D-values between 10 and 13 cm^-1, with no dependence on halide size. The five-coordinate 2-X series exhibit large and negative D-values between -60 and -90 cm^-1. Interpretation of the magnetic parameters with the aid of ligand-field theory and ab initio calculations elucidated the roles of molecular geometry, ligand-field effects, and metal-ligand covalency in controlling the magnitude of ZFS in cobalt-halide complexes.

Strategies to quench quantum tunneling of magnetization in lanthanide single molecule magnets

Enhancing blocking temperature (T_B) is one of the holy grails in Single Molecule Magnets(SMMs), as any future potential application in this class of molecules is directly correlated to this parameter. Among many factors contributing to a reduction of T_B value, Quantum Tunnelling of Magnetisation (QTM), a phenomenon that is a curse or a blessing based on the application sought after, tops the list. Theoretical tools based on density functional and ab initio CASSCF/RASSI-SO methods have played a prominent role in estimating various spin Hamiltonian parameters and establishing the mechanism of magnetization relaxation in this class of molecules. Particularly, various strategies to quench QTM effects go hand-in-hand with experiments, and different methods proposed to quell QTM effects are scattered in the literature. In this perspective, we have explored various approaches that are proposed in the literature to quench QTM effects, and these include the role of (i) local symmetry of lanthanides, (ii) super-exchange interaction in {3d-4f} complexes, (iii) direct-exchange interaction in {radical-4f} and metal-metal bonded complexes to suppress the QTM, (iv) utilizing external stimuli such as an electric field or pressure to modulate the QTM and (v) avoiding QTM effects by stabilising toroidal states in 4f and {3d-4f} clusters. We believe the strategies summarized here will help to design new-generation SMMs.

Magnetic Anisotropy and Relaxation of Pseudotetrahedral [N2 O2 ] Bis-Chelate Cobalt(II) Single-Ion Magnets Controlled by Dihedral Twist Through Solvomorphism

The methanol solvomorph 1 ⋅ 2MeOH of the cobalt(II) complex [Co(L^Sal,2-Ph )₂ ] (1) with the sterically demanding Schiff-base ligand 2-(([1,1'-biphenyl]-2-ylimino)methyl)phenol (HL^Sal,2-Ph ) shows the thus far largest dihedral twist distortion between the two chelate planes compared to an ideal pseudotetrahedral arrangement. The cobalt(II) ion in 1 ⋅ 2MeOH exhibits an easy-axis anisotropy leading to a spin-reversal barrier of 55.3 cm^-1 , which corresponds to an increase of about 17 % induced by the larger dihedral twist compared to the solvent-free complex 1. The magnetic relaxation for 1 ⋅ 2MeOH is significantly slower compared to 1. An in-depth frequency-domain Fourier-transform (FD-FT) THz-EPR study not only allowed the direct measurement of the magnetic transition between the two lowest Kramers doublets for the cobalt(II) complexes, but also revealed the presence of spin-phonon coupling. Interestingly, a similar dihedral twist correlation is also observed for a second pair of cobalt(II)-based solvomorphs, which could be benchmarked by FD-FT THz-EPR.

Slow magnetic relaxation in two mononuclear Mn(II) complexes not governed by the over-barrier Orbach process

Two hexacoordinate Mn(II) complexes containing a chelating residue of hexafluoroacetylacetone and (Cl-substituted) 4-benzylpyridine show DC magnetic functions typical for S = 5/2 spin systems: g ∼ 2, D - small. The AC susceptibility confirms a field supported slow magnetic relaxation in which the over-barrier Orbach relaxation process does not play a role. Both systems possess two or three slow relaxation channels.

Lanthanide Single-molecule Magnets: Synthetic Strategy, Structures, Properties and Recent Advances

Single-molecule magnets (SMMs) show wide potential applications in the field of ultrahigh-density storage materials, quantum computing, spintronics, and so on. Lanthanide (Ln) SMMs, as an important category of SMMs, open up a promising prospect due to their large magnetic moments and huge magnetic anisotropy. However, the construction of high performance for Ln SMMs remains an enormous challenge. Although remarkable advances are focused on the topic of Ln SMMs, the research on Ln SMMs with different nuclear numbers is still deficient. Therefore, this review summarizes the design strategies for the construction of Ln SMMs, as well as the metal skeleton types. Furthermore, we collect reported Ln SMMs with mononuclearity, dinuclearity, and multinuclearity (three or more Ln spin centers) and the SMM properties including energy barrier (U_eff ) and pre-exponential factor (τ₀ ) are described. Finally, Ln SMMs with low-nuclearity SMMs, especially for single-ion magnets (SIMs), are highlighted to understand the correlations between structures and magnetic behavior of the detail SMM properties are described. We expect the review can shed light on the future developments of high-performance Ln SMMs.

Slow Magnetic Relaxation and Modulated Photoluminescent Emission of Coordination Polymer Based on 3-Amino-4-hydroxybenzoate Zn and Co Metal Ions

As a starting point, a new 3D porous framework with the {[CoL]·0.5DMF·H₂O}_n chemical formula (where L = 3-amino-4-hydroxybenzoate) is described. Its performance as a single molecule magnet was explored. The study of magnetic properties reveals that Co-MOF shows no frequency-fdependant alternating current (ac) signals under zero direct current (dc) magnetic field, whereas single-molecule magnet behaviour is achieved when Co^II ions are diluted in a Zn^II based matrix. Interestingly, this strategy renders a bifunctional [Co_xZn_1-xL]_n material that is also characterized by a strong photoluminescent emitting capacity.

Magnetic anisotropy and structural flexibility in the field-induced single ion magnets [Co{(OPPh2)(EPPh2)N}2], E = S, Se, explored by experimental and computational methods

During the last few years, a large number of mononuclear Co(II) complexes of various coordination geometries have been explored as potential single ion magnets (SIMs). In the work presented herein, the Co(II) S = 3/2 tetrahedral [Co{(OPPh2)(EPPh2)N}2], E = S, Se, complexes (abbreviated as CoO2E2), bearing chalcogenated mixed donor-atom imidodiphosphinato ligands, were studied by both experimental and computational techniques. Specifically, direct current (DC) magnetometry provided estimations of their zero-field splitting (zfs) axial (D) and rhombic (E) parameter values, which were more accurately determined by a combination of far-infrared magnetic spectroscopy and high-frequency and -field EPR spectroscopy studies. The latter combination of techniques was also implemented for the S = 3/2 tetrahedral [Co{(EPiPr2)2N}2], E = S, Se, complexes, confirming the previously determined magnitude of their zfs parameters. For both pairs of complexes (E = S, Se), it is concluded that the identity of the E donor atom does not significantly affect their zfs parameters. High-resolution multifrequency EPR studies of CoO2E2 provided evidence of multiple conformations, which are more clearly observed for CoO2Se2, in agreement with the structural disorder previously established for this complex by X-ray crystallography. The CoO2E2 complexes were shown to be field-induced SIMs, i.e., they exhibit slow relaxation of magnetization in the presence of an external DC magnetic field. Advanced quantum-chemical calculations on CoO2E2 provided additional insight into their electronic and structural properties.

Towards Luminescent Vanadium(II) Complexes with Slow Magnetic Relaxation and Quantum Coherence

Molecular entities with doublet or triplet ground states find increasing interest as potential molecular quantum bits (qubits). Complexes with higher multiplicity might even function as qudits and serve to encode further quantum bits. Vanadium(II) ions in octahedral ligand fields with quartet ground states and small zero-field splittings qualify as qubits with optical read out thanks to potentially luminescent spin-flip states. We identified two V²⁺ complexes [V(ddpd)₂ ]²⁺ with the strong field ligand N,N'-dimethyl-N,N'-dipyridine-2-yl-pyridine-2,6-diamine (ddpd) in two isomeric forms (cis-fac and mer) as suitable candidates. The energy gaps between the two lowest Kramers doublets amount to 0.2 and 0.5 cm^-1 allowing pulsed EPR experiments at conventional Q-band frequencies (35 GHz). Both isomers possess spin-lattice relaxation times T₁ of around 300 μs and a phase memory time T_M of around 1 μs at 5 K. Furthermore, the mer isomer displays slow magnetic relaxation in an applied field of 400 mT. While the vanadium(III) complexes [V(ddpd)₂ ]³⁺ are emissive in the near-IR-II region, the [V(ddpd)₂ ]²⁺ complexes are non-luminescent due to metal-to-ligand charge transfer admixture to the spin-flip states.

Interplay of Anisotropic Exchange Interactions and Single-Ion Anisotropy in Single-Chain Magnets Built from Ru/Os Cyanidometallates(III) and Mn(III) Complex

Two novel 1D heterobimetallic compounds {[Mn^III(SB²⁺)M^III(CN)₆]·4H₂O}_n (SB²⁺ = N,N'-ethylenebis(5-trimethylammoniomethylsalicylideneiminate) based on orbitally degenerate cyanidometallates [Os^III(CN)₆]^3- (1) and [Ru^III(CN)₆]^3- (2) and Mn^III Schiff base complex were synthesized and characterized structurally and magnetically. Their crystal structures consist of electrically neutral, well-isolated chains composed of alternating [M^III(CN)₆]^3- anions and square planar [Mn^III(SB²⁺)]³⁺ cations bridged by cyanide groups. These -ion magnetic anisotropy of Mn^III centers. These results indicate that the presence of compounds exhibit single-chain magnet (SCM) behavior with the energy barriers of Δτ₁/k_B = 73 K, Δτ₂/k_B = 41.5 K (1) and Δτ₁/k_B = 51 K, Δτ₂ = 27 K (2). Blocking temperatures of T_B = 2.8, 2.1 K and magnetic hysteresis with coercive fields (at 1.8 K) of 8000, 1600 Oe were found for 1 and 2, respectively. Theoretical analysis of the magnetic data reveals that their single-chain magnet behavior is a product of a complicated interplay of extremely anisotropic triaxial exchange interactions in M^III(4d/5d)-CN-Mn^III fragments: -J_xS_M^xS_Mn^x-J_yS_M^yS_Mn^y-J_zS_M^zS_Mn^z, with opposite sign of exchange parameters J_x = -22, J_y = +28, J_z = -26 cm^-1 and J_x = -18, J_y = +20, J_z = -18 cm^-1 in 1 and 2, respectively) and single orbitally degenerate [Os^III(CN)₆]^3- and [Ru^III(CN)₆]^3- spin units with unquenched orbital angular momentum in the chain compounds 1 and 2 leads to a peculiar regime of slow magnetic relaxation, which is beyond the scope of the conventional Glaubers's 1D Ising model and anisotropic Heisenberg model.

The magnetic properties of MAl4(OH)12SO4·3H2O with M = Co2+, Ni2+, and Cu2+ determined by a combined experimental and computational approach

The magnetic properties of the nickelalumite-type layered double hydroxides (LDH), MAl₄(OH)₁₂(SO₄)·3H₂O (MAl₄-LDH) with M = Co²⁺ (S = 3/2), Ni²⁺ (S = 1), or Cu²⁺ (S = 1/2) were determined by a combined experimental and computational approach. They represent three new inorganic, low-dimensional magnetic systems with a defect-free, structurally ordered magnetic lattice. They exhibit no sign of magnetic ordering down to 2 K in contrast to conventional hydrotalcite LDH. Detailed insight into the complex interplay between the choice of magnetic ion (M²⁺) and magnetic properties was obtained by a combination of magnetic susceptibility, heat capacity, neutron scattering, solid-state NMR spectroscopy, and first-principles calculations. The NiAl₄- and especially CoAl₄-LDH have pronounced zero-field splitting (ZFS, easy-axis and easy-plane, respectively) and weak ferromagnetic nearest-neighbour interactions. Thus, they are rare examples of predominantly zero-dimensional spin systems in dense, inorganic matrices. In contrast, CuAl₄-LDH (S = 1/2) consists of weakly ferromagnetic S = 1/2 spin chains. For all three MAl₄-LDH, good agreement is found between the experimental magnetic parameters (J, D, g) and first-principles quantum chemical calculations, which also predict that the interchain couplings are extremely weak (< 0.1 cm^-1). Thus, our approach will be valuable for evaluation and prediction of magnetic properties in other inorganic materials.

Pushing up the easy-axis magnetic anisotropy and relaxation times in trigonal prismatic CoII mononuclear SMMs by molecular structure design

The replacement of pyridine by 1-methyl-imidazol in the arms of a N₆-tripodal ligand allows preparing two new Co^II complexes with quasi-ideal triangular prismatic geometry, which behave as SIMs (Single Ion Magnets) at zero dc field with enhanced axial magnetic anisotropy, magnetic relaxation times and magnetic hysteresis.

Pentagonal-bipyramidal 4d and 5d complexes with unquenched orbital angular momentum as a unique platform for advanced single-molecule magnets: current state and perspectives

This article overviews the current state and prospects of the concept of advanced single-molecule magnets (SMMs) based on low-spin (S = 1/2) pentagonal-bipyramidal (PBP) 4d³ and 5d³ complexes with unquenched orbital angular momentum. This approach is based on the unique property of PBP 4d³ and 5d³ complexes to cause highly anisotropic spin coupling of perfect uniaxial symmetry, -J_zSziSzj - J_xy(SxiSxj + SyiSyj), regardless of the local geometric symmetry. The M(4d/5d)-M(3d) exchange-coupled pairs in the apical positions of the PBP complexes produce Ising-type exchange interactions (|J_z| > |J_xy|), which serve as a powerful source of uniaxial magnetic anisotropy of a SMM cluster. In polynuclear heterometallic 4d/5d-3d complexes embodying PBP 4d/5d units and high-spin 3d ions, anisotropic Ising-type exchange interactions produce a double-well potential with high energy barriers U_eff, which is controlled by the anisotropic exchange parameters J_z, J_xy. Theoretical analysis shows that the barrier is proportional to the difference |J_z - J_xy| and to the number n of the apical 4d/5d-3d pairs in a SMM cluster, U_eff ∝ |J_z - J_xy|n, which provides an opportunity to scale up the barrier U_eff and blocking temperature T_B up to the record values. A novel family of 4d/5d complexes with forced PBP coordination provided by structurally rigid planar pentadentate Schiff-base ligands in the equatorial plane is discussed as a better alternative to the cyanometallates. The possibility of a significant increase in the anisotropic exchange parameters J_z, J_xy in PBP complexes with monoatomic apical μ-bridging ligands is examined. The basic principles of molecular engineering the highest barrier through anisotropic exchange interactions of PBP 4d/5d complexes are formulated. The theoretical and experimental results taken together indicate that the concept of high-performance SMMs based on 4d/5d PBP complexes with unquenched orbital angular momentum is an attractive alternative to the currently dominant lanthanide-based SMM strategy.