Slow Magnetic Relaxation in a Co II –Y III Single‐Ion Magnet with Positive Axial Zero‐Field Splitting

Heterometallic Hexanuclear [Cu2 Ln4 ] Complexes Showing Zero-field SMM Behaviour and Magnetocaloric Effect

Three heterometallic hexanuclear 3d-4f complexes bearing the formula [Cu₂ (L)₂ Ln₄ (L)₄ (o-van)₂ ] [L=2-((E)-((2-hydroxyphenyl)imino]methyl)phenol; o-van=ortho-vanillin] (Ln^III =Gd^III (1), Dy^III (2), and Tb^III (3)) have been synthesized and characterized. DC magnetic susceptibility measurements reveal overall antiferromagnetic interactions in 1 and 3, whereas co-existence of ferro- as well as antiferromagnetic interactions were observed in 2. The magnetocaloric effect has been observed for 1 with an entropy change (-ΔS_m ) of 22.3 J kg^-1  K^-1 at 3 K and 7 T. Zero-field single molecule magnet (SMM) behaviour has been observed for 2, where Raman relaxation and quantum tunneling of magnetization (QTM) played a role in magnetization relaxation. The Cu-O-Ln angle well explains the magnetic exchange coupling occurring in the complexes. BS-DFT calculation for the complexes provides an estimate of the exchange interactions between the paramagnetic centres. Ab initio calculations performed for complex 2 established a good correlation to the experimental relaxation dynamics.

Applying Unconventional Spectroscopies to the Single-Molecule Magnets, Co(PPh3 )2 X2 (X=Cl, Br, I): Unveiling Magnetic Transitions and Spin-Phonon Coupling

Large separation of magnetic levels and slow relaxation in metal complexes are desirable properties of single-molecule magnets (SMMs). Spin-phonon coupling (interactions of magnetic levels with phonons) is ubiquitous, leading to magnetic relaxation and loss of memory in SMMs and quantum coherence in qubits. Direct observation of magnetic transitions and spin-phonon coupling in molecules is challenging. We have found that far-IR magnetic spectra (FIRMS) of Co(PPh₃ )₂ X₂ (Co-X; X=Cl, Br, I) reveal rarely observed spin-phonon coupling as avoided crossings between magnetic and u-symmetry phonon transitions. Inelastic neutron scattering (INS) gives phonon spectra. Calculations using VASP and phonopy programs gave phonon symmetries and movies. Magnetic transitions among zero-field split (ZFS) levels of the S=3/2 electronic ground state were probed by INS, high-frequency and -field EPR (HFEPR), FIRMS, and frequency-domain FT terahertz EPR (FD-FT THz-EPR), giving magnetic excitation spectra and determining ZFS parameters (D, E) and g values. Ligand-field theory (LFT) was used to analyze earlier electronic absorption spectra and give calculated ZFS parameters matching those from the experiments. DFT calculations also gave spin densities in Co-X, showing that the larger Co(II) spin density in a molecule, the larger its ZFS magnitude. The current work reveals dynamics of magnetic and phonon excitations in SMMs. Studies of such couplings in the future would help to understand how spin-phonon coupling may lead to magnetic relaxation and develop guidance to control such coupling.

Modulating Magnetic and Photoluminescence Properties in 2-Aminonicotinate-Based Bifunctional Coordination Polymers by Merging 3d Metal Ions

Herein, the synthesis and study of bifunctional coordination polymers (CPs) with both magnetic and photoluminescence properties, derived from a heterometallic environment, are reported. As a starting point, three isostructural monometallic CPs with the formula [M(μ-2ani)₂ ]_n (M^II =Mn (1_Mn ), Co (3_Co ) and Ni (4_Ni ); 2ani=2-aminonicotinate), crystallise as chiral 2D-layered structures stacked by means of supramolecular interactions. These compounds show high thermal stability in the solid state (above 350 °C), despite which, in aqueous solution, compound 1_Mn is shown to partially transform into a novel 1D chain CP with the formula [Mn(2ani)₂ (μ-H₂ O)₂ ]_n (2_Mn ). A study of the direct current (dc) magnetic properties of 1_Mn , 3_Co and 4_Ni reveals a spin-canted structure derived from antisymmetric antiferromagnetic weak exchanges along the chiral network (as confirmed by DFT calculations) and magnetic anisotropy of the ions, in such a way that long-range ordering is observed with variable magnitude for the spin carriers. Moreover, compounds 3_Co and 4_Ni show no frequency-dependent alternating current (ac) susceptibility curves under zero dc field; this is characteristic behaviour of a glassy state that may be partially supressed for 3_Co by applying an external dc field. To overcome long-range magnetic ordering, Co^II ions are diluted in a diamagnetic Zn^II -based matrix, which enables single-molecule magnet behaviour. Interestingly, this strategy allows a bifunctional Co_x Zn_1-x 2ani material, which is imbued with a strong photoluminescent emitting capacity, as characterised by an intense blue light followed by a green afterglow, to be obtained.

Alignment of Axial Anisotropy in a 1D Coordination Polymer shows Improved Field Induced Single Molecule Magnet Behavior over a Mononuclear Seven Coordinated FeII Complex

Herein, we report a CN-bridged alternating Fe^II -Ni^II 1D chain to ensure the alignment of axial anisotropy and improve the single molecule magnet (SMM) behavior in seven coordinated Fe^II compound. The chain was constructed from hepta coordinated Fe(II) complex as an anisotropic building unit and diamagnetic nickel tetra cyanate as a bridging ligand. The magnetic measurements show the easy-axis anisotropy of the seven coordinated Fe(II) complex and field induced SMM behavior with spin reversal energy barrier U_eff =61(2) K (42 cm^-1 ) and pre-exponential relaxation time τ₀ =1.9×10^-8  s. The detailed analysis of the relaxation dynamics discloses that the Orbach process plays an important role in slow relaxation of magnetization for this compound. Notably, this example represents a remarkable energy barrier observed in hepta coordinated Fe(II) SMMs. The ab initio calculations estimate the magnitude of axial anisotropy and show the parallel orientation of the anisotropic axis throughout the 1D polymeric chain. In addition, it is also reported that the presence of weak π accepter ligands in the distorted axial position enhance the easy-axis anisotropy.

Two Four-Coordinate and Seven-Coordinate CoII Complexes Based on the Bidentate Ligand 1, 8-Naphthyridine Showing Slow Magnetic Relaxation Behavior

For a long time, the cobalt(II) complex ([Co(napy)₄ ](ClO₄ )₂ ) (napy=1, 8-naphthyridine) has been considered as an eight-coordinate complex without any structural proof. After careful considerations, two complexes [Co(napy)₂ Cl₂ ] (1) and [Co(napy)₄ ](ClO₄ )₂ (2) based on the bidentate ligand napy were synthesized and structurally characterized. X-ray single-crystal structural determination showed that the cobalt(II) center in [Co(napy)₂ Cl₂ ] (1) is four-coordinate with a tetrahedral geometry (T_d ), while [Co(napy)₄ ](ClO₄ )₂ (2) is seven-coordinate rather than eight-coordinate with a capped trigonal prism geometry (C_2v ). Direct-current (dc) magnetic data revealed that complexes 1 and 2 possess positive zero-field splitting (ZFS) parameters of 11.08 and 25.30 cm^-1 , respectively, with easy-plane magnetic anisotropy. Alternating current(ac) susceptibility measurements revealed that both complexes showed slow magnetic relaxation behaviour. Theoretical calculations demonstrated that the presence of easy-plane magnetic anisotropy (D>0) for complexes 1 and 2 is in agreement with the experimental data. Furthermore, these results pave the way to obtain four-coordinate and seven-coordinate cobalt(II) single-ion magnets (SIMs) by using a bidentate ligand.

Phosphine Oxide Ligand Based Tetrahedral CoII Complexes with Field-induced Slow Magnetic Relaxation Behavior Modified by Terminal Ligands

Two isostructural mononuclear Co^II complexes, [Co(xantpo)(NCE)₂ ] (E=S (1) and O (2); xantpo=9,9-dimethyl-4,5-bis(diphenylphosphoryl) xanthene), supported by a bidentate phosphine oxide ligand are reported. The cobalt complexes exhibit characteristic tetrahedral structures coordinated with two oxygen and two nitrogen atoms. Magnetic property measurements show their similar static magnetic behaviours but very different dynamic magnetic behaviours. Both complexes show field-induced slow magnetic relaxation behaviours, but the relaxation of 2 is much slower than that of 1. Fittings to the magnetic data and ab initio CASSCF calculations reveal significant changes in the zero field splitting (ZFS) parameters (D and E), which can be attributed to the small geometrical changes of the Co ions and the different ligand field strength of the two terminal ligands.

A Trigonal Prismatic Cobalt(II) Complex as a Single Molecule Magnet with a Reduced Contribution from Quantum Tunneling

Herein, we report a new trigonal prismatic cobalt(II) complex that behaves as a single molecule magnet. The obtained zero-field splitting, which is also directly accessed by THz-EPR spectroscopy (-102.5 cm^-1 ), results in a large magnetization reversal barrier U of 205 cm^-1 . Its effective value, however, is much lower (101 cm^-1 ), even though there is practically no contribution from quantum tunneling to magnetization relaxation.

Field Induced Single Ion Magnetic Behaviour in Square-Pyramidal Cobalt(II) Complexes with Easy-Plane Magnetic Anisotropy

Two penta-coordinate CoII complexes with formulae [Co(14-TMC)Cl](BF4) (1, 14-TMC = 1,4,8,11-Tetramethyl-1,4,8,11-tetraazacyclotetradecane) and [Co(12-TBC)Cl](ClO4)·(MeCN) (2, 12-TBC = 1,4,7,10-Tetrabenzyl-1,4,7,10-tetraazacyclododecane) were synthesized and characterized. Structural analysis revealed that ligand coordinates to the CoII centre in a tetradentate fashion and the fifth position is occupied by chloride ion and the geometries around CoII centres are best described as distorted square pyramidal. Detailed magnetic measurements disclose the presence of significant easy-plane magnetic anisotropy and field induced slow magnetic relaxation behaviours of the studied complexes. More insight into the magnetic anisotropy has been given using ab initio theory calculations, which agree well with the experimental values and further confirmed the easy-plane magnetic anisotropy.

Dinuclear Dy2 Single-Molecule Magnets: Functional Modulation on the Bridging Ligand and Different Relaxation Performances within the Single-Crystal to Single-Crystal System

Crystal structures, single-molecule magnetic behavior, and ab initio calculations of four new phenoxo-bridged dinuclear dysprosium complexes and their gadolinium(III) analogues are explored. Complexes [Dy₂ (DMOMP)₂ (DBM)₄ ]₂ ⋅CHCl₃ (1; DMOMP=1-methyl-3,5-dimethoxy-4-hydroxybenzene, DBM=1,3-diphenylpropane-1,3-dione); [Dy₂ (DMOAP)₂ (DBM)₄ ]₂ ⋅CHCl₃ (2; DMOAP=syringaldehyde); Dy₂ (DMOEP)₂ (DBM)₄ (3; DMOEP=methyl syringate); and solvent-free Dy₂ (DMOMP)₂ (DBM)₄ (4), which is obtained by the transformation of single crystal into single crystal from 1, have nearly identical core structures and only differ in the substituents at the para position of the phenol moieties of the bridging ligand. In this system, the electronic effects are efficiently implemented to significantly modify the ligand field strength and exchange coupling by modulating the substituents on the phenol backbone. The effective energy barrier (U_eff ) of magnetization reversal is improved significantly to fivefold magnitude, at most, and the hysteresis temperature up to 3.5 K by deliberately using the electron-withdrawing substituent to replace the electron-donating one. The origin of the two relaxation processes in 1 is mostly attributed to the existence of two molecules in one unit, which is illuminated by means of the transformation of single crystal into single crystal.

A W-Shaped Ga-Dy-Dy-Dy-Ga Cluster: Synthesis, Characterization, and Magnetic Properties

A W-shaped {Ga-Dy-Dy-Dy-Ga} coordination cluster, [Ga₂ Dy₃ (L)₆ (MeO)₂ (MeOH)₃ ]⋅NO₃ ⋅7 MeOH (1) with H₂ L, (E)-2-(2-hydroxy-3-methoxy-benzylideneamino)phenol was synthesized, characterized, and the magnetic properties investigated. The magnetic measurements indicate that the compound shows single-molecule magnet (SMM) behavior under zero dc field. This suggests that the combination of Ga^III ions and Dy^III ions (4 f) is a successful strategy to assemble SMM clusters. The combination of these metal ions also offers prospects in dual functionality therapeutics for medical applications.

Single Ion Magnets from 3d to 5f: Developments and Strategies

Single-ion magnets (SIMs), exhibiting slow magnetization relaxation in the absence of the magnetic field, originate from their single spin-carrier centre. In pursuit of high-performance magnetic properties, such as high spin-reversal barrier and high blocking temperature, various metal centres were investigated to establish SIMs, including 3d and 5d transition metal ions, 4f lanthanide ions, and 5f actinide ions, which possess unique zero-field splitting and magnetic properties. Therefore, proper ligand field is of great importance to different types of metals. In the given great breakthroughs since the first SIM, [Pc₂ Tb]^- (Pc=dianion of phthalocyanine), was reported, strategies of ligand field design have emerged. In this review, the developments of SIMs with different metal centres are summarized, as well as the possible strategies.

Slow Magnetic Relaxation in a Mononuclear Ruthenium(III) Complex

The development of magnetic molecules with long spin reversal/decoherence times highly depends on the understanding of relaxation behavior under different external conditions. Herein, a magnetic study on a Ru^III complex (1) is presented. Detailed analysis of the relaxation time and the magneto-heat capacity data suggests that the resonant phonon trapping process dominates the magnetic relaxation in the crystalline sample of 1, slowing down the spin relaxation rate, as further confirmed by the measurements on a ground sample and frozen solution. Thus, it provides a rare example showing that 4d metal-centered mononuclear compounds without second-order anisotropy can display slow magnetic relaxation.

Strictly linear trinuclear Dy-Ca/Mg-Dy single-molecule magnets: the impact of long-range f-f ferromagnetic interactions on suppressing quantum tunnelling of magnetization leading to slow magnetic relaxation

The synthesis, structural characterization and magnetic properties of four heterometallic complexes with formulas Ln₂M(OQ)₈ [Ln(iii) = Dy, M(ii) = Ca (1), Mg (2); Ln(iii) = Er, M(ii) = Ca (3), Mg (4); HOQ = 8-hydroxyquinoline] are reported. Complexes display a perfectly linear arrangement of three metal ions involving two terminal Ln(iii) ions and one central alkaline earth M(ii) ion, and they are bridged by three phenolato oxygen atoms from three 8-hydroxyquinoline ligands. Direct-current (dc) magnetic susceptibility studies show that there exists a long-range ferromagnetic (FM) interaction between two f-electronic centers in Dy-based complexes 1 and 2 albeit they are separated by diamagnetic alkaline earth ions, and the FM interaction has been successfully reproduced by ab initio calculations. Alternating current (ac) magnetic susceptibility measurements indicate that complexes 1 and 2 exhibit significant single-molecule magnet (SMM) behaviors. This is mostly attributed to the parallel magnetic axes of individual Dy(iii) sites aligning closely to the Dy-M-Dy direction, and the linearly extending magnetic susceptibility tensors of each ion lead to the FM interaction. The dc and ac susceptibility studies of the intramolecularly diluted complexes reveal that the f-f coupling suppresses the QTMs significantly in the absence of the external magnetic field.

Three novel mononuclear Mn(iii)-based magnetic materials with square pyramidal versus octahedral geometries

Single crystal X-ray diffraction studies reveal that complexes 1 and 2 have square pyramidal geometry and 3 has octahedral geometry, which showed a dependence of negative anisotropy (D) values on the electronic, geometry and packing effects.

The ferromagnetic [Ln2Co6] heterometallic complexes

Four 3d–4f [Ln₂Co₆] complexes were fabricated. [Dy₂Co₆] (1) and [Ho₂Co₆] (2) displayed ferromagnetic coupling, and [Dy₂Co₆] (1) exhibited slow magnetic relaxation.

Large Easy-Plane Magnetic Anisotropy in a Three-Coordinate Cobalt(II) Complex [Li(THF)4 ][Co(NPh2 )3 ]

Magnetic anisotropy is the key element in the construction of single-ion magnets, a kind of nanomagnets for high-density information storage. This works describes an unusual large easy-plane magnetic anisotropy (with a zero-field splitting parameter D of +40.2 cm^-1 ), mainly arising from the second-order spin-orbit coupling effect in a trigonal-planar Co^II complex [Li(THF)₄ ][Co(NPh₂ )₃ ], revealed by combined studies of magnetism, high frequency/field electron paramagnetic resonance spectroscopy, and ab initio calculations. Meanwhile, the field-induced slow magnetic relaxation in this complex was mainly attributed to the Raman process.

A Low-Symmetry Dysprosium Metallocene Single-Molecule Magnet with a High Anisotropy Barrier

The single-molecule magnet (SMM) properties of the isocarbonyl-ligated dysprosium metallocene [Cp*2 Dy{μ-(OC)2 FeCp}]2 (1Dy ), which contains a rhombus-shaped Dy2 Fe2 core, are described. Combining a strong axial [Cp*](-) ligand field with a weak equatorial field consisting of the isocarbonyl ligands leads to an anisotropy barrier of 662 cm(-1) in zero applied field. The dominant thermal relaxation pathways in 1Dy involves at least the fourth-excited Kramers doublet, thus demonstrating that prominent SMM behavior can be observed for dysprosium in low-symmetry environments.

Field-Induced Slow Magnetic Relaxation in a Mononuclear Manganese(III)-Porphyrin Complex

We report on a novel manganese(III)-porphyrin complex with the formula [Mn(III) (TPP)(3,5-Me2 pyNO)2 ]ClO4 ⋅CH3 CN (2; 3,5-Me2 pyNO=3,5-dimethylpyridine N-oxide, H2 TPP=5,10,15,20-tetraphenylporphyrin), in which the Mn(III) ion is six-coordinate with two monodentate 3,5-Me2 pyNO molecules and a tetradentate TPP ligand to build a tetragonally elongated octahedral geometry. The environment in 2 is responsible for the large and negative axial zero-field splitting (D=-3.8 cm(-1) ), low rhombicity (E/|D|=0.04) of the high-spin Mn(III) ion, and, ultimately, for the observation of slow magnetic-relaxation effects (Ea =15.5 cm(-1) at H=1000 G) in this rare example of a manganese-based single-ion magnet (SIM). Structural, magnetic, and electronic characterizations were carried out by means of single-crystal diffraction studies, variable-temperature direct- and alternating-current measurements and high-frequency and -field EPR spectroscopic analysis followed by quantum-chemical calculations. Slow magnetic-relaxation effects were also observed in the already known analogous compound [Mn(III) (TPP)Cl] (1; Ea =10.5 cm(-1) at H=1000 G). The results obtained for 1 and 2 are compared and discussed herein.