A variety of new tri- and tetranuclear Mn–Ln and Fe–Ln (Ln=lanthanide) complexes

Heterometallic clusters based on an uncommon asymmetric "V-shaped" [Fe3+(μ-OR)Ln3+(μ-OR)2Fe3+]6+ (Ln = Gd, Tb, Dy, Ho) structural core and the investigation of the slow relaxation of the magnetization behaviour of the [Fe2Dy] analogue

The synthesis, crystal structures, Mössbauer spectra and variable temperature dc and ac magnetic susceptibility studies of a new family of trinuclear heterometallic Fe³⁺/Ln³⁺ complexes, [Fe₂Ln(PhCO₂)₃((py)₂CO₂)((py)₂C(OMe)O)₂(NO₃)Cl] (Ln = Gd (1/Gd), Tb (1/Tb), Dy (1/Dy), and Ho (1/Ho)), where (py)₂CO₂^2- and (py)₂C(OMe)O^- are the anions of the gem-diol and hemiketal derivatives of di-2-pyridyl ketone, are reported. Compounds 1/Ln are based on an asymmetric "V-shaped" [Fe³⁺(μ-OR)Ln(μ-OR)₂Fe³⁺]⁶⁺ structural core formed from the connection of the two terminal Fe³⁺ centers to the central Ln³⁺ ion either through one or two alkoxide groups originating from the alkoxide-type bridging ligands. Direct current magnetic susceptibility studies reveal the presence of weak antiferromagnetic interactions between the Fe³⁺ ions. Alternating current magnetic studies indicate the presence of a slow-magnetic relaxation process in 1/Dy with an energy barrier U_eff = 6.7 (±0.3) K and a pre-exponential factor, τ₀ = 2.2 (±0.4) × 10^-7 s. The electronic, magnetic and relaxation properties of the complexes were further monitored by variable temperature ⁵⁷Fe Mössbauer spectroscopy. At T > 80 K the spectra from the complexes comprise two quadrupole doublets the hyperfine parameters of which reflect the distinct coordination environment of the two Fe³⁺ terminal sites. At T < 20 K, the Mössbauer spectra for 1/Dy are affected by magnetic relaxation effects. At 1.5 K, the spectrum of 1/Dy comprises well defined magnetic sextets indicating relaxation times slower than the characteristic time of the Mössbauer technique (10^-7 s) in agreement with the dynamic magnetic measurements. 1/Gd exhibits broad unresolved magnetic sextets at 1.5 K indicating that the spin relaxation time is of the order of the Mössbauer characteristic time at this temperature. For 1/Tb, 1/Ho the Mössbauer spectra exhibit slight broadening even at the lowest available temperature consistent with magnetic relaxation times less than 10^-7 s.

Tetranuclear Cr-Ln ferrocenecarboxylate complexes with a defect-dicubane structure: synthesis, magnetism, and thermolysis

Using ferrocenecarboxylic acid (FcCO₂H) and triethanolamine (H₃tea) as ligands, the isostructural heterotrimetallic complexes [LnIII2CrIII2(OH)₂(FcCO₂)₄(NO₃)₂(Htea)₂]·2MePh·2THF (Ln = Tb (1), Dy (2), Ho (3), Er (4), and Y (5); Fc = (η⁵-C₅H₄)(η⁵-C₅H₅)Fe; H₃tea = N(CH₂CH₂OH)₃) were obtained. In all of the complexes which possess a defective dicubane structure, two doubly deprotonated triethanolamine ligands chelate the chromium ions. However, during the synthesis of 1, an isomeric complex 1a in which Tb³⁺ is chelated by triethanolamine as a tetradentate ligand, was also isolated as a few single crystals. Magnetic susceptibility measurements revealed dominant antiferromagnetic interactions in the {LnIII2CrIII2} cores of 1-4 leading to the formation of complexes with an uncompensated magnetic moment, while weak Cr-Cr ferromagnetic interactions were detected in the Y analogue. Complexes 1, 2, and 3 exhibit single-molecule magnet properties dominated by an Orbach-type relaxation mechanism with magnetization reversal barriers (Δ/k_B) estimated around 54, 75, and 47 K, respectively. The Dy complex exhibits a magnetization hysteresis in an applied magnetic field at temperatures below 4 K. Thermolysis of the complexes was studied by TGA and DSC techniques; the final products obtained under an air atmosphere contain mixed oxide Cr_0.75Fe_1.25O₃ and heterotrimetallic oxide LnCr_1-xFe_xO₃ (with x ≈ 0.75) phases.

From the {FeIII 2 Ln2 } Butterfly's Perspective: the Magnetic Benefits and Challenges of Cooperativity within 3 d-4 f Based Coordination Clusters

In this Review we discuss the tuning handles which can be used to steer the magnetic properties of Fe^III‐4 f “butterfly” compounds. The majority of presented compounds were produced in the context of project A3 “Di‐ to tetranuclear compounds incorporating highly anisotropic paramagnetic metal ions” within the SFB/TRR88 “3MET”. These contain {Fe^III₂Ln₂} cores encapsulated in ligand shells which are easy to tune in a “test‐bed” system. We identify the following advantages and variables in such systems: (i) the complexes are structurally simple usually with one crystallographically independent Fe^III and Ln^III, respectively. This simplifies theory and anaylsis; (ii) choosing Fe allows ⁵⁷Fe Mössbauer spectroscopy to be used as an additional technique which can give information about oxidation levels and spin states, local moments at the iron nuclei and spin‐relaxation and, more importantly, about the anisotropy not only of the studied isotope, but also of elements interacting with this isotope; (iii) isostructural analogues with all the available (i. e. not Pm) 4 f ions can be synthesised, enabling a systematic survey of the influence of the 4 f ion on the electronic structure; (iv) this cluster type is obtained by reacting [Fe^III₃O(O₂CR)₆(L)₃](X) (X=anion, L=solvent such as H₂O, py) with an ethanolamine‐based ligand L′ and lanthanide salts. This allows to study analogues of [Fe^III₂Ln₂(μ₃‐OH)₂(L′)₂(O₂CR)₆] using the appropriate iron trinuclear starting materials. (v) the organic main ligand can be readily functionalised, facilitating a systematic investigation of the effect of organic substituents on the ligands on the magnetic properties of the complexes. We describe and discuss 34 {M^III₂Ln₂} (M=Fe or in one case Al) butterfly compounds which have been reported up to 2020. The analysis of these gives perspectives for designing new SMM systems with specific electronic and magnetic signatures

Magneto-structural studies of an unusual [MnIIIMnIIGdIII(OR)4]4- partial cubane from 2,2'-bis-p-tBu-calix[4]arene

Reaction of 2,2'-bis-p-tBu-calix[4]arene (H8L) with MnCl2·4H2O, GdCl3·6H2O and 2,6-pyridinedimethanol (H2pdm) affords [MnIIIMnIIGdIII(H3L)(pdmH)(pdm)(MeOH)2(dmf)]·3MeCN·dmf (3·3MeCN·dmf) upon vapour diffusion of MeCN into the basic dmf/MeOH mother liquor. 3 crystallises in the tetragonal space group P41212 with the asymmetric unit comprising the entire cluster. The highly unusual core contains a triangular arrangement of MnIIIMnIIGdIII ions housed within a [MnIIIMnIIGdIII(OR)4]4- partial cubane. Magnetic susceptibility and magnetisation data reveal best fit parameters JMn(II)-Mn(III) = +0.415 cm-1, JMn(III)-Gd(III) = +0.221 cm-1, JMn(II)-Gd(III) = -0.258 cm-1 and DMn(III) = -4.139 cm-1. Theoretically derived magnetic exchange interactions, anisotropy parameters, and magneto-structural correlations for 3 are in excellent agreement with the experimental data.

Heterometallic 3d-4f Complexes as Air-Stable Molecular Precursors in Low Temperature Syntheses of Stoichiometric Rare-Earth Orthoferrite Powders

Four 3d-4f hetero-polymetallic complexes [Fe₂Ln₂((OCH₂)₃CR)₂(O₂C^tBu)₆(H₂O)₄] (where Ln = La (1 and 2) and Gd (3 and 4); and R = Me (1 and 3) and Et (2 and 4)) are synthesized and analyzed using elemental analysis, Fourier transform infrared spectroscopy, thermogravimetric analysis, and SQUID magnetometry. Crystal structures are obtained for both methyl derivatives and show that the complexes are isostructural and adopt a defective dicubane topology. The four heavy metals are connected with two alkoxide bridges. These four precursors are used as single-source precursors to prepare rare-earth orthoferrite pervoskites of the form LnFeO₃. Thermal decomposition in a ceramic boat in a tube furnace gives orthorhombic LnFeO₃ powders using optimized temperatures and decomposition times: LaFeO₃ formed at 650 °C over 30 min, whereas GdFeO₃ formed at 750 °C over 18 h. These materials are structurally characterized using powder X-ray diffraction, Raman spectroscopy, scanning electron microscopy, energy-dispersive X-ray map spectroscopy, and SQUID magnetometry. EDX spectroscopy mapping reveals a homogeneous spatial distribution of elements for all four materials consistent with LnFeO₃. Magnetic measurements on complexes 1-4 confirm the presence of weak antiferromagnetic coupling between the central Fe(III) ions of the clusters and negligible ferromagnetic interaction with peripheral Gd(III) ions in 3 and 4. Zero-field-cooled and field-cooled measurements of magnetization of LaFeO₃ and GdFeO₃ in the solid-state suggest that both materials are ferromagnetic, and both materials show open magnetic hysteresis loops at 5 and 300 K, with M_sat higher than previously reported for these nanomaterials. We conclude that this is a new and facile low temperature route to these important magnetic materials that is potentially universal, limited only by what metals can be programmed into the precursor complexes.

Three new heterometallic ZnII–LnIII complexes with a windmill-like framework and field-induced SMM behavior

We have prepared three 3d–4f heterometallic complexes by the reaction of o-vanillin oxime (H₂L) with Zn(NO₃)₂·6H₂O and Ln(NO₃)₃·6H₂O salts in the presence of triethylamine.

Structural diversity and photo-physical and magnetic properties of dimeric to 1D polymeric coordination polymers of lighter lanthanide(iii) dinitrobenzoates

Single crystal diffraction studies reveal the formation of the following 10 new complexes of lighter Ln(iii) ions with general formulas {[Ln(μ2-L1)3·(H2O)2]·H2O}n (Ln = Nd (1) and Eu (2)), [Nd(μ2-L2)2·(CH3COO)·(H2O)2]n (3), [Ln2(μ2-L2)5·(L2)·(H2O)4]n (Ln = Sm (4), Ce (5), and Pr (6)), [La2(μ2-L2)6·(H2O)3·(DMF)]n (7) (DMF = dimethylformamide), [Ln(μ2-L2)2·(L2)·(H2O)3]2 (Ln = Eu (8) and Gd (9)) and [Gd(L2)·(CH3COO)2·(H2O)2]2 (10), where L1 and L2 are anions of 3,5- and 2,4-dinitrobenzoic acid, respectively. Complexes 1-7 are 1D coordination polymers, while 8-10 are dinuclear complexes. The luminescence properties of Nd(iii) and Eu(iii) analogues displayed metal-centred emission with L1 exhibiting weak but more efficient sensitization than L2. A study of the magnetic properties of the compounds clearly demonstrated the field-induced single ion magnet behaviour of the Nd(iii) compounds 1 and 3. Their behaviour has been compared to previously reported analogous Nd(iii) complexes and the role of the lattice solvent and polymorphism on the magnetic behaviour has been evaluated.

Synthesis and magnetochemistry of heterometallic triangular FeLnIII (Ln = La, Gd, Tb, Dy, and Ho) and FeYIII complexes

A series of Fe₂Ln (Ln = La, Gd, Tb, Dy and Ho) and Fe₂Y complexes have been synthesized via metal substitution and characterized by single crystal X-ray diffraction. All the molecules are isostructural and have a Fe₂LnO triangular core with the oxygen atom existing as an μ₃-oxo^2- anion. DC and AC magnetic susceptibility studies were performed on all the molecules. For Fe₂Ln (Ln = Gd, La) and Fe₂Y, the data were fitted to Van Vleck equations and the magnetic coupling constants were obtained. In all cases, the two Fe(iii) spins were found to be antiparallel to each other in the ground state leaving the heterometal to remain essentially uncoupled.

Understanding the magnetism of {Fe2Ln} dimers, step-by-step

A hierarchical, comparative approach to the complex magnetochemistry of {Fe₄M₂}-type coordination clusters (M = Y, Eu, Gd, Dy) allows determination of intramolecular exchange energies.

Multiple superhyperfine fields in a {DyFe2Dy} coordination cluster revealed using bulk susceptibility and 57Fe Mössbauer studies

A study which demonstrates the decisive role that the timescale of measurement techniques play in helping to unravel the details of relaxation processes in magnetic materials is presented.

The coordination chemistry and magnetism of some 3d-4f and 4f amino-polyalcohol compounds

Triethanolamine, teaH3, and diethanolamine, RdeaH2, 3d-4f and 4f compounds demonstrate an enormous variety in their structure and bonding. This review examines the synthetic strategies to these molecules and their magnetic properties, whilst trying to assess these ligands' suitability towards new SMMs and magnetic refrigerants.

Tuning the magnetic behaviors in [FeIII12LnIII4] clusters with aromatic carboxylate ligands

Six new Fe–Ln clusters composed of a Fe₁₂Ln₄ core derived from two monocarboxylate ligands have been successfully obtained with a step-by-step strategy. Magnetic analyses suggest that 1–6 show different magnetic properties, and 2 and 6 display SMM-like behaviors.

A family of novel Mn3Ln4 clusters displaying single-molecule magnet behavior

A family of novel Mn₃Ln₄ clusters: single-molecule magnets and the first structurally characterized 3d/4f clusters containing the 3-methyloxysalicylaldoximate ligand.

Syntheses, structures and magnetic properties of a family of heterometallic [Mn(II)2Mn(III)2Ln(III)2] clusters

A new family of isostructural Mn-Ln heterometallic clusters, [Mn(II)(2)Mn(III)(2)Ln(III)(2)(Piv)(8)(thme)(2)(H(2)tea)(2)] (H(3)thme = 1,1,1-tris(hydroxymethyl)ethane, Hpiv = pivalic acid, H(3)tea = triethanolamine, and Ln = Pr, Nd, Sm, Eu, Gd, Tb, and Dy) have been prepared by reaction of the hexanuclear manganese precursor, [Mn(6)O(2)(Piv)(10)(4-Me-Py)(2.5)(Hpiv)(1.5)], with H(3)thme, H(3)tea, and Ln(NO(3))(3)·6H(2)O under the solvothermal conditions. Complexes 1Pr-7Dy were characterized by single-crystal X-ray diffraction and their core [Mn(2)(III)Mn(2)(II)Ln(2)(III)(μ(3)-OR)(4)(μ(2)-OR)(6)](6+) composed of four face-sharing defected cubane units. Antiferromagnetic (AF) exchange interactions were suggested for all of the complexes by solid-state dc magnetic susceptibility measurements. Complexes 6Tb and 7Dy, show frequency-dependent ac susceptibility signals suggestive of slow magnetic relaxation.

Coordination modes, spectral, thermal and biological evaluation of hetero-metal copper containing 2-thiouracil complexes

Mononuclear copper complex [CuL(NH(3))(4)]Cl(2)·0.5H(2)O and three new hetero-metallic complexes: [Cu(2)Ni(L)(2)(NH(3))(2)Cl(2)·6H(2)O] 2H(2)O(,) [Cu(3)Co(L)(4)·8H2O]Cl·4(·)5H(2)O, and [Cu(4)Co(2)Ni(L)(3)(OH)(4)(NH(3))Cl(4)·3H(2)O]4H(2)O where L is 2-thiouracil, were prepared and characterized by elemental analyses, molar conductance, room-temperature magnetic susceptibility, spectral (IR, UV-Vis and ESR) studies and thermal analyses techniques (TG, DTG and DTA). The molar conductance data revealed that [CuL(NH(3))(4)]Cl(2)·0.5H(2)O and [Cu(3)Co(L)(4)·8H2O]Cl·4.5H(2)O are electrolytes, while, [Cu(2)Ni(L)(2)(NH(3))(2)Cl(2·)6H(2)O]·2H(2)O and [Cu(4)Co(2)Ni(L)(3)(OH)(4)(NH(3))Cl(4)·3H(2)O]4H(2)O are non-electrolytes. IR spectra showed, that 2-thiouracil ligand behaves as a bidentate or tetradentate ligand. The geometry around the metal atoms is octahedral in all the prepared complexes except in [Cu(4)Co(2)Ni(L)(3)(OH)(4)(NH(3))Cl(4)·3H(2)O]4H(2)O complex where square planar environment around Co(II), Ni(II) and Cu(II) were suggested. Thermal decomposition study of the prepared complexes was monitored by TG, DTG and DTA analyses under N(2) atmosphere. The decomposition course and steps were analyzed. The order of chemical reactions (n) was calculated via the peak symmetry method and the activation parameters of the non- isothermal decomposition were computed from the thermal decomposition data. The negative values of ΔS(∗) deduced the ordered structures of the prepared complexes compared to their starting reactants. The antimicrobial activity of the prepared complexes were screened in vitro against a Gram positive, a Gram negative bacteria, a filamentous fungi and a yeast. The antimicrobial screening data showed that the studied compounds exhibited a good level of activity against Escherichia coli, Staphylococcus aureus and Candida albicans but have no efficacy against Aspergillus flavus. It was observed that [Cu(4)Co(2)Ni(L)(3)(OH)(4)(NH(3))Cl(4)·3H(2)O]4H(2)O complex showed the most intensive activity against the tested microorganisms. Trials to prepare single crystals from complexes were failed.

Carboxylate-free Mn(III)2Ln(III)2 (Ln = lanthanide) and Mn(III)2Y(III)2 complexes from the use of (2-hydroxymethyl)pyridine: analysis of spin frustration effects

The initial employment of 2-(hydroxymethyl)pyridine for the synthesis of Mn/Ln (Ln = lanthanide) and Mn/Y clusters, in the absence of an ancillary organic ligand, has afforded a family of tetranuclear [Mn(III)(2)M(III)(2)(OH)(2)(NO(3))(4)(hmp)(4)(H(2)O)(4)](NO(3))(2) (M = Dy, 1; Tb, 2; Gd, 3; Y; 4) anionic compounds. 1-4 possess a planar butterfly (or rhombus) core and are rare examples of carboxylate-free Mn/Ln and Mn/Y clusters. Variable-temperature dc and ac studies established that 1 and 2, which contain highly anisotropic Ln(III) atoms, exhibit slow relaxation of their magnetization vector. Fitting of the obtained magnetization (M) versus field (H) and temperature (T) data for 3 by matrix diagonalization and including only axial anisotropy (zero-field splitting, ZFS) showed the ground state to be S = 3. Complex 4 has an S = 0 ground state. Fitting of the magnetic susceptibility data collected in the 5-300 K range for 3 and 4 to the appropriate van Vleck equations revealed, as expected, extremely weak antiferromagnetic interactions between the paramagnetic ions; for 3, J(1) = -0.16(2) cm(-1) and J(2) = -0.12(1) cm(-1) for the Mn(III)···Mn(III) and Mn(III)···Gd(III) interactions, respectively. The S = 3 ground state of 3 has been rationalized on the basis of the spin frustration pattern in the molecule. For 4, J = -0.75(3) cm(-1) for the Mn(III)···Mn(III) interaction. Spin frustration effects in 3 have been quantitatively analyzed for all possible combinations of sign of J(1) and J(2).