Closely-Related ZnII2LnIII2 Complexes (LnIII = Gd, Yb) with Either Magnetic Refrigerant or Luminescent Single-Molecule Magnet Properties

Cu(II)-Ln(III) (Ln = Gd, Tb and Dy) complexes of an unsymmetrical N2O3 donor ligand: field induced SMM behaviour of Cu(II)-Tb(III) complexes

Three new hetero-metallic CuII-LnIII complexes [(CuL)Gd(NO3)3(CH3OH)]n (1), [(CuL)Tb(NO3)3(H2O)]·[CuL] (2) and [(CuL)Dy(NO3)3(H2O)]·[CuL] (3) have been synthesized using a mono-nuclear Cu(II) complex, [CuL], of an unsymmetrically di-condensed N2O3 donor Schiff base ligand, N-(3-methoxysalicylidene)-N-(salicylidene)-1,2-ethylenediamine (H2L). Single crystal X-ray crystallography revealed that complex 1 is a nitrate bridged 1D chain of dinuclear Cu(II)-Gd(III) units whereas in 2 and 3, the dinuclear Cu(II)-Ln(III) units are co-crystallized with a [CuL] unit. The Ln(III) centers are nine coordinated with the geometry of a spherical capped square antiprism for Gd and spherical tricapped trigonal prism for Tb and Dy. The geometry of the Cu(II) center is distorted octahedral for complex 1 and distorted square planar for complexes 2 and 3. Temperature-dependent molar magnetic susceptibility measurements in 1-3 revealed the presence of overall ferromagnetic coupling between the Cu(II) and Ln(III) centers. Notably, field induced single-molecule magnet behavior was witnessed in the Tb(III) derivative (2). The ab initio calculations indicated that upon application of an external magnetic field, the tunneling in the ground state of complex 2 gets reduced and thereby field-induced SMM behaviour is observed. Besides, in the case of complex 1, BS-DFT calculations were carried out to gain further insights into the magnetic exchange coupling interactions between the Cu(II) and Gd(III) centers.

Effect of diamagnetic Zn(II) ions on the SMM properties of a series of trinuclear ZnDy2 and tetranuclear Zn2Dy2 (LnIII = Dy, Tb, Gd) complexes: combined experimental and theoretical studies

To study the effect of diamagnetic ions on magnetic interactions, utilizing a compartmental ligand (Z)-2-(hydroxymethyl)-4-methyl-6-((quinolin-8-ylimino)methyl)phenol (LH2), two different series of ZnII-LnIII complexes, namely the trinuclear series of [DyZn2(L)2(μ2-OAc)2(CH3OH)2]·NO3·MeOH (1), [TbZn2(L)2(μ2-OAc)2(CH3OH)2]·NO3·5MeOH·H2O (2), and [GdZn2(L)2(μ2-OAc)2(CH3OH)2]·NO3·MeOH·CHCl3 (3) and the tetranuclear series of [Dy2Zn2(LH)4(NO3)4(μ2OAc)]·NO3·MeOH·H2O (4), [Tb2Zn2(LH)4(NO3)4(μ2-OAc)]·NO3·MeOH·2H2O (5), and [Gd2Zn2(LH)4(NO3)4(μ2-OAc)]·NO3·MeOH·2H2O (6), were synthesized. Trinuclear ZnII-LnIII complexes 1-3 consist of one LnIII ion sandwiched between two peripheral ZnII ions forming a bent type ZnII-DyIII-ZnII array with an angle of 110.64°. Tetranuclear ZnII-LnIII complexes 4-6 are basically a combination of two dinuclear moieties of [LnZn(LH)2(NO3)2]+ connected by one bidentate bridging acetate ion in μ2-OAc coordination mode. The detailed magnetic analysis reveals that complexes 1 and 4 are single molecule magnets having energy barriers of 34.98 K and 46.71 K with relaxation times (τ0) of 5.05 × 10-4 s and 5.24 × 10-4 s, respectively. Ab initio calculations were employed to analyze the magnetic anisotropy and magnetic exchange interaction between the ZnII and LnIII centers with the aim of gaining better insights into the magnetic dynamics of complexes 1-6.

Slow magnetic relaxation in dinuclear Co(III)-Co(II) complexes containing a five-coordinated Co(II) centre with easy-axis anisotropy

Two air-stable Co(III)-Co(II) mixed-valence complexes of molecular formulas [Co^IICo^III(L)(DMAP)₃(CH₃COO)]·H₂O·CH₃OH (1) and [Co^IICo^III(L)(4-Pyrrol)₃ (CH₃COO)]·0.5CH₂Cl₂ (2) (H₄L = 1,3-bis-(5-methyl pyrazole-3-carboxamide) propane; DMAP = 4-dimethylaminopyridine; and 4-Pyrrol = 4-pyrrolidinopyridine) were synthesized and characterized by single-crystal X-ray crystallography, high-field electron paramagnetic resonance (HFEPR) spectroscopy, and magnetic measurements. Both complexes possess one five-coordinated paramagnetic Co(II) ion and one six-coordinated Co(III) ion with octahedral geometry. Direct-current magnetic susceptibility and magnetization measurements show the easy-axis magnetic anisotropy that is also confirmed by low-temperature HFEPR measurements and theoretical calculations. Frequency- and temperature-dependent alternating-current magnetic susceptibility measurements reveal their field-assisted slow magnetic relaxation, which is a characteristic behavior of single-molecule magnets (SMMs), caused by the individual Co(II) ion. The effective energy barrier of complex 1 (49.2 cm^-1) is significantly higher than those of the other dinuclear Co(III)-Co(II) SMMs. This work hence presents the first instance of the dinuclear Co(III)-Co(II) single-molecule magnets with a five-coordinated environment around the Co(II) ion.

Slow Magnetic Relaxation and Luminescent Properties of Mononuclear Lanthanide-Substituted Keggin-Type Polyoxotungstates with Compartmental Organic Ligands

The reaction of mid to late lanthanide ions with the N,N′-dimethyl-N,N′-bis(2-hydroxy-3-formyl-5-bromobenzyl)ethylene-diamine organic ligand and monolacunary Keggin type [α-SiW₁₁O₃₉]^8– anion affords a series of isostructural compounds, namely, K₅[Ln^III(α-SiW₁₁O₃₉)(C₂₀H₂₂Br₂N₂O₄)]·14H₂O (1-Ln, Ln = Sm to Lu). The molecular structure of these sandwich-type complexes is formed by the Ln^III ion in a biaugmented trigonal prismatic geometry, which occupies the external O₄ site of the organic ligand and the vacant site of the lacunary polyoxometalate (POM) unit. The empty N₂O₂ coordination site of the organic ligand allows its unprecedented folding, which displays a relative perpendicular arrangement of aromatic groups. Weak Br···Br and π–π interactions established between adjacent molecular units govern the crystal packing, which results in the formation of assemblies containing six hybrid species assembled in a chairlike conformation. 1-Gd and 1-Yb display slow relaxation of the magnetization after the application of an external magnetic field with maxima in the out-of-phase magnetic susceptibility plots below ∼5–6 K, which is ascribed to the presence of various relaxation mechanisms. Moreover, photoluminescent emission is sensitized for 1-Sm and 1-Eu in the visible region and 1-Er and 1-Yb in the NIR. In contrast, the quenching of metal-centered luminescence in the 1-Tb derivative has been attributed to the out-of-pocket coordination mode of the lanthanide center within the POM fragment. It is demonstrated that the 1-Yb dual magneto-luminescent material represents the first lanthanide-containing POM reported to date with simultaneous slow magnetic relaxation and NIR emission. Solution stability of the hybrid molecular species in water is also confirmed by ESI-mass spectrometry experiments carried out for 1-Tb and 1-Tm.

A series of solution stable, molecular polyoxometalate hybrids, namely, [α-SiW₁₁O₃₉Ln^III(C₂₀H₂₂N₂Br₂O₄)]⁵⁻ (Ln^III = Sm to Lu) have been synthesized from the combination of mid-to-late lanthanides, lacunary Keggin-type anions, and a compartmental organic ligand. Metal-centered luminescence in both the visible and NIR region as well as slow relaxation of magnetization was found for different members of this family, including the Yb derivative, which represents the first POM-based system with slow magnetic relaxation and sensitized emission in the NIR region.

Field-Induced SMM and Vis/NIR Luminescence on Mononuclear Lanthanide Complexes with 9-Anthracenecarboxylate and 2,2′:6,2″-Terpyridine

Five new mononuclear lanthanide complexes are synthesized by adding the several lanthanide nitrate hexahydrate salts, which for lanthanide (Ln) are Eu, Tb, Dy, Er, and Yb, with 9-anthracenecarboxylic acid (9-Hanthc) and 2,2′:6,2″-terpyridine (TPY) in mixed solution of methanol and dimethylformamide (DMF). The general formula is [Eu(9-anthc)3(TPY)(DMF)]·H2O (1Eu) where Eu(III) is ennea-coordinated or [Ln(9-anthc)3(TPY)(H2O)]·H2O·DMF (Ln = Tb (2Tb), Dy (3Dy), Er (4Er), and Yb (5Yb)) where Ln(III) is octa-coordinated. For compounds 3Dy, 4Er, and 5Yb, the dynamic ac magnetic study indicated field-induced single molecule magnet (SMM) behavior. The photoluminescence studies in the solid state of these complexes show the sensitization of 4f-4f transitions for 4Er and 5Yb in the NIR region.

Strong Equatorial Crystal Field Enhances the Axial Anisotropy and Energy Barrier for Spin Reversal Process in Yb2 Single Molecule Magnets

The importance of equatorial crystal fields on magnetic anisotropy of ytterbium single molecule magnets (SMMs) is observed for the first time. Herein, we report three similar dinuclear ytterbium complexes with the formula [Yb₂ (3-OMe-L)₂ (DMF)₂ (NO₃ )₂ ]⋅DMF (1), [Yb₂ (3-H-L)₂ (DMF)₂ (NO₃ )₂ ]⋅DMF⋅H₂ O (2), and [Yb₂ (3-NO₃ -L)₂ (DMF)₂ (NO₃ )₂ ] (3), [where 3-X-H₂ L=N'-(2-hydroxy-3-X-benzylidene)picolinohydrazide, X=OMe (1), H (2) NO₂ (3)]. Detailed magnetic measurements reveal the presence of weak antiferromagnetic interactions between the Yb centers and a field-induced slow relaxation of magnetization in all complexes. A higher energy barrier for spin reversal was observed for complex 1 (U_eff =50 K) and it decreases in the order of 2 (47 K) to 3 (40 K). Notably, complex 1 shows a remarkable energy barrier within the frequency range of 1-850 Hz reported for Yb-based SMMs. Further, ab initio calculations show a higher axial anisotropy and lower quantum tunneling of magnetization (QTM) in the ground state for 1 compared to 2 and 3. It was also observed that the presence of a strong crystal field in the equatorial plane (when the ∡ O1-Yb-O3 bond angle is close to 90°) enhances the axial anisotropy and improves the SMM behavior in the studied complexes. Both the experimental and theoretical analysis of relaxation dynamics discloses that Raman and QTM play major role on slow relaxation process for all complexes. To provide more insight into the exchange interactions, broken-symmetry DFT calculations were performed.

5-Aminopyridine-2-carboxylic acid as appropriate ligand for constructing coordination polymers with luminescence, slow magnetic relaxation and anti-cancer properties

Five new coordination polymers (CPs) constructed of aminopyridine-2-carboxylate (ampy) ligand have been synthesized and fully characterized. Three of them correspond to metal-organic chains built from the coordination of ampy to sodium and lanthanides with formulae [MNa(ampy)₄]_n (M = terbium (2), erbium (1) and ytterbium (3)) resembling a previously reported dysprosium material which shows anticancer activity. On another level, the reaction of Hampy with cobalt and copper ions ({[CoK(ampy)₃(H₂O)₃](H₂O)₃}_n (4) and [Cu(ampy)₂]_n (5)) lead to CPs with variable dimensionalities, which gives the opportunity of analyzing the structural properties of this new family. Lanthanide materials display solid state intense photoluminescent emissions in both the visible and near-infrared region and exhibit slow relaxation of magnetization with frequency dependence of the out-of-phase susceptibility. More interestingly, in our search for multifunctional materials, we have carried out antitumor measurements of these compounds. These multidisciplinary studies of metal complexes open up the possibility for further exploring the applications in the fields of metal-based drugs.

Zn2Ln2 complexes with carbonate bridges formed by the fixation of carbon dioxide in the atmosphere: single-molecule magnet behaviour and magnetocaloric effect

In the self-assembly process, CO₂ in the air is automatically fixed to Zn₂Ln₂ cluster molecular materials.

Synthesis, structure and magnetic properties of a series of dinuclear heteroleptic Zn2+/Ln3+ Schiff base complexes: effect of lanthanide ions on the slow relaxation of magnetization

We report an investigation on the synthesis, structure and magnetic properties of a series of heteroleptic Zn²⁺/Ln³⁺ complexes of the general formula [ZnLClLn(o-van)(H₂O)(NO₃)]·H₂O (H₂L = N,N'-bis(3-methoxysalicylidene)-1,2-diaminoethane; o-van = o-vanillinate; Ln = Tb (1), Dy (2), Er (3)). All the compounds exhibit a slow relaxation of their magnetization and the dysprosium analogue shows one of the highest anisotropic barriers for ZnDy complexes.

Slow Magnetic Relaxation in Dinuclear CoIIYIII Complexes

Four new dinuclear complexes, [Co(μ-L)(μ-CCl₃COO)Y(NO₃)₂]·2CHCl₃·CH₃CN·2H₂O (1), [Co(μ-L)(μ-CH₃COO)Y(NO₃)₂]·CH₃CN (2), [Co(μ-L)(μ-PhCOO)Y(NO₃)₂]·3CH₃CN·2H₂O (3), and [Co(μ-L)(μ-^tBuCOO)Y(NO₃)₂]·CHCl₃·2H₂O (4), having a Co^IIY^III core, have been synthesized by employing a ferrocene based compartmental ligand which was synthesized by the reaction of diacetyl ferrocene with hydrazine hydrate followed by a condensation reaction with o-vanillin. A general synthetic protocol was employed to synthesize complexes 1-4, where the metallic core was kept the same with changing the bridging carboxylate groups. In all the complexes, the main structural motif is kept similar by only slightly varying the substitution on the bridging acetate groups. This variation has resulted in a small but subtle influence on the magnetic relaxation of all these four compounds. Ab initio CASSCF/NEVPT2 calculations were carried out to assess the effect of the different substitutions of the bridging ligands on the magnetic anisotropy parameters and on orbital arrangements. Ab initio calculations yield a very large positive D value, which is consistent with the geometry around the Co^II ion and easy plane anisotropy (g_xx, g_yy > g_zz), with the order of the calculated D in the range of 72.4 to 91.7 cm^-1 being estimated in this set of complexes. To ascertain the sign of zero-field splitting in these complexes, EPR spectra were recorded, which support the sign of D values estimated from ab initio calculations.

Europium and ytterbium complexes with o-iminoquinonato ligands: synthesis, structure, and magnetic behavior

Complexes of divalent ytterbium (1) and europium (2) with a dianionic o-amidophenolate ligand were prepared by both the direct reduction of 4,6-di-tert-butyl-N-(2,6-diisopropylphenyl)-o-iminobenzoquinone (dpp-IQ) and the salt metathesis reaction of potassium o-amidophenolate with LnI2 (Ln = Yb, Eu). Oxidation of o-amidophenolates 1, 2 with one equivalent of dpp-IQ as well as the salt metathesis reaction of potassium o-iminosemiquinolate with LnI2 afforded ligand mixed-valent o-iminosemiquinonato-amidophenolato complexes of trivalent ytterbium (3) and europium (4). All novel complexes 1-4 were fully characterized, including the solid state structures of 1 and 2 determined by single crystal X-ray diffraction. The magnetic properties of paramagnetic 2-4 were examined.

Lanthanide anthracene complexes: slow magnetic relaxation and luminescence in DyIII, ErIII and YbIII based materials

Four mononuclear lanthanide complexes, [Ln(depma)2(H2O)6]Cl3·xH2O·yCH3OH [Ln = Dy (1Dy), x = 6, y = 0; Gd (2Gd), x = 3, y = 1; Er (3Er), x = 6, y = 0; Yb (4Yb), x = 3, y = 1; depma = 9-diethylphosphono-methylanthracene], are reported and structurally characterized. The octa-coordinated environment of LnIII is composed of two oxygen atoms from two different depma ligands and six oxygen atoms from each bound water molecule. All four complexes show photoluminescence in the solid state at room temperature, originating from the ligand-centered emissions of depma. Furthermore, slow magnetization relaxation is found in 1Dy, 3Er and 4Yb with the energy barriers (Ueff) of 40.5 K (1 kOe), 9.8 K (1 kOe) and 28.9 K (0.75 kOe), respectively.

Field-induced slow magnetic relaxation in the first Dy(iii)-centered 12-metallacrown-4 double-decker

The first double-decker Ga(iii)/Dy(iii) 12-MC-4 complex has been isolated and magnetic studies revealed its SMM properties.

The role of 3d–4f exchange interaction in SMM behaviour and magnetic refrigeration of carbonato bridged CuII2LnIII2 (Ln = Dy, Tb and Gd) complexes of an unsymmetrical N2O4 donor ligand

The role of exchange interaction between Cu(ii) and Ln(iii) ions in SMM behaviour and magnetocaloric effects has been extensively investigated by both experimental and theoretical CASSCF/RASSI-SO/SINGLE_ANISO methods.

A series of CuII–LnIII complexes of an N2O3 donor asymmetric ligand and a possible CuII–TbIII SMM candidate in no bias field

Among four isostructural heterometallic Cu^II–Ln^III complexes (Ln = Tb, Dy, Ho or Er) of an N₂O₃ donor asymmetric ligand, only the Cu^II–Tb^III complex shows SMM behavior at zero bias field but at applied bias field both the Cu^II–Tb^III and Cu^II–Dy^III complexes show SMM behavior.

Closing the Circle of the Lanthanide-Murexide Series: Single-Molecule Magnet Behavior and Near-Infrared Emission of the NdIII Derivative

Up to now, even if murexide-based complexometric studies are performed with all 3d or 4f ions, the crystal structures of the light-lanthanide derivatives of the lanthanide-murexide series are unknown. In this work, we report the crystal structure of the NdIII derivative named NdMurex. Contrary to all known complexes of the 3d or 4f series, a dimeric compound was obtained. As for its already reported DyIII and YbIII parents, the NdIII complex responsible for the color-change behaves as a single-molecule magnet (SMM). This behavior was observed on both the crystalline (NdMurex: Ueff = 6.20(0.80) K, 4.31 cm−1; τ0 = 2.20(0.92) × 10−5 s, Hdc = 1200 Oe) and anhydrous form (NdMurexAnhy: Ueff = 6.25(0.90) K, 4.34 cm−1; τ0 = 4.85(0.40) × 10−5 s, Hdc = 1200 Oe). The SMM behavior is reported also for the anhydrous CeIII derivative (CeMurexAnhy: Ueff = 5.40(0.75) K, 3.75 cm−1; τ0 = 3.02(1.10) × 10−5 s, Hdc = 400 Oe). The Near-Infrared Emission NIR emission was observed for NdMurexAnhy and highlights its bifunctionality.

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.

New field-induced single ion magnets based on prolate Er(iii) and Yb(iii) ions: tuning the energy barrierUeffby the choice of counterions within an N3-tridentate Schiff-base scaffold

Counterions modulate the structure and magnetic properties of rarely observed high-coordinate SIM species.

Syntheses, crystal structures and magnetic properties of a series of ZnII2LnIII2 compounds (Ln = Gd, Tb, Dy, Ho and Er): contrasting structural and magnetic features

In the five ZnII2LnIII2 compounds – (i) the Tb^III and Er^III analogues show slow relaxation of the magnetization, while the Ho^III system and, surprisingly, the Dy^III analogue don’t; (ii) the Gd^III system shows the MCE; and (iii) interestingly, the Ln–O bond length increases with the increase of atomic number.

A luminescent Schiff-base heterotrinuclear Zn2Dy single-molecule magnet with an axial crystal field

A luminescent Zn₂Dy single-molecule magnet.

Heterometallic 3d–4f single molecule magnets containing diamagnetic metal ions

This perspective deals with the synthesis and study of SMM properties of heterometallic 3d–4f complexes containing diamagnetic 3d metal ions.

A Chinese Pane-Like 2D Metal-Organic Framework Showing Magnetic Relaxation and Luminescence Dual-Functions

The discovery of graphene kicked off the curtain of atom-type two-dimensional (2D) materials. Layered metal-organic frameworks (MOFs) as parallel molecule-based 2D materials are more designable and more diverse, and magnetism may be induced by their metal ion nodes. However, the multifunctional 2D plane-like MOFs are very difficult to obtain. Here we describe a Chinese pane-like 2D MOF constructed from the Ln3+ cation and the nanoscale luminescent tritopic ligand tris(4'-carboxybiphenyl)-amine, responding to the slow magnetic relaxation and luminescence properties, respectively. Notably, the Dy-Dy distances separated by the tritopic ligand are up to 2 nm. Such a 2D molecular material is expected to have potential applications in optoelectronics and multimodal sensing.

Tm(iii) complexes undergoing slow relaxation of magnetization: exchange coupling and aging effects

The present study focuses on the dynamic magnetic behaviour of exchange coupled 3d-4f complexes containing the scarcely investigated non-Kramers Tm³⁺ center, the 3d metal ions being either the low-spin Fe³⁺ (1) or the diamagnetic Co³⁺ (2) ion. Both complexes display field-induced slow relaxation of magnetization. The field and temperature dependences of the relaxation rate provided indication of relevant contributions from quantum tunnelling, direct, Orbach and Raman processes, with only minor effects from exchange coupling interactions. Furthermore, the aged sample of 2 exhibited an additional relaxation process, possibly due to structural modifications accompanied by solvent loss, highlighting the importance of a careful consideration of this factor when analysing the magnetization dynamics in solvated systems.

A topologically unique alternating {CoIII3GdIII3} magnetocaloric ring

A building block approach to molecule-based refrigerants: using diamagnetic Co(iii) to decrease the interaction between Gd(iii) ions and hence, enhance the adiabatic temperature change.

Three sandwich-type zinc(ii)–lanthanide(iii) clusters: structures, luminescence and magnetic properties

We present herein three sandwich-type luminescent Zn^II–Ln^III clusters with Zn^II–Dy^III being SMMs with a slow zero-field relaxation.

Double-decker luminescent ytterbium and erbium SMMs with symmetric and asymmetric Schiff base ligands

Multifunctional molecules that respond both to magnetic fields and light are subject of study due to possible applications in fields as diverse as imaging or information processing and storage.