SMM Behavior Research Articles

Anisotropic exchange interaction and field-induced SMM behaviour in a mixed valence {CoCo} complex.

We are reporting the synthesis and structural characterization of a new hexanuclear Co(ii)/Co(iii) complex starting from a versatile pivalate cobalt precursor and the racemic mixture of a chelating Schiff base type ligand. The main [CoII4CoIII2(μ3-OH)2(μ-OR)2(μ-OR')2(μ-OR'')2]6+ core is unprecedented and exhibits an inversion center that affords only two unique Co(ii) sites. We performed DC and AC magnetic measurements and analysed them in terms of the anisotropic exchange of ground Kramers doublets at each Co(ii) site due to their unquenched angular orbital contribution to the magnetic moment. Quantum computations support the experimental data treatment. The interplay of dominant antiferromagnetic exchange, inversion symmetry and a non-collinear main quantization axis affords an exchange energy spectrum with mostly non-magnetic states. Nevertheless, field induced SMM behaviour is observed at 1500 Oe and below 3 K which might be explained by the relaxation of the first excited magnetic state (which is populated enough) through the next closest excited state. The Orbach and/or Raman mechanism could be operative from the experimental and quantum computed results.

Coordination anion effects on the geometry and magnetic interaction of binuclear Dy2 single-molecule magnets.

Two new dimeric dysprosium(III) complexes, [Dy2(HL)2(SCN)2]·2CH3CN (1) and [Dy2(HL)2(NO3)2]·2CH3CN·2H2O (2), have been assembled using the H3L multidentate ligand (H3L = 2,2'-((((2-hydroxy-5-methyl-1,3-phenylene)bis(methylene))bis((pyridin-2-ylmethyl)azanediyl))bis(methylene))diphenol). The use of different coordination anions for the two complexes results in distinct coordination geometries of the metal sites. The Dy centers in complexes 1 and 2 display capped octahedron and triangular dodecahedron coordination geometries, respectively. Consequently, the two compounds exhibit distinct dc and ac magnetic properties. Complex 1 behaves as a single molecule magnet (SMM) while no SMM behavior is observed for complex 2. Although complexes 1 and 2 possess a similar core of Dy2O2, their different coordination anions lead to two distinct magnetic interactions, namely ferromagnetic and antiferromagnetic, respectively. Ab initio calculations reveal that these interactions may result from strong intramolecular dipolar couplings that are ferromagnetic for 1 but antiferromagnetic for 2, while exchange couplings are antiferromagnetic in both cases.

Observation of field-induced single-ion magnet behavior in an octahedral dysprosium complex with a strong ligand field

A mononuclear six-coordinate DyIII complex exhibiting slow magnetic relaxation provides a rare case for octahedral lanthanide complexes with SMM behavior.

Improving the single-molecule magnet properties of two pentagonal bipyramidal Dy3+compounds by the introduction of both electron-withdrawing and -donating groups

Modifying the electron withdrawing F atoms on the pyridine rings and the electron donating –CH3groups on the phenolic hydroxyl rings to improve the SMM behavior of two mononuclear Dy3+compounds obtained from the ligand H2bmbpen-F.

Assembly of tetra-nuclear YbIII-containing selenotungstate clusters: synthesis, structures, and magnetic properties.

Two tetra-nuclear YbIII-incorporated selenotungstate clusters, Keggin (C2H8N)6Na14[Yb4Se6W44O160(H2O)12]·40H2O (1) and Wells-Dawson (C2H8N)4Na14[Yb4Se6W45O159(OH)6(H2O)11]·38H2O (2), have been isolated through a pH-controlled assembly, which exhibit the first YbIII-containing polyoxotungstates with selenium heteroatoms. Their assemblies rely on the structure-directing effects of SeO32- anion templates to give rise to available Se-containing Keggin-/Wells-Dawson-type motifs. Both compounds were characterized by single-crystal X-ray diffraction, IR spectroscopy, power X-ray diffraction (XRD), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS) as well as electrospray ionization mass spectrometry (ESI-MS). Furthermore, systematic magnetic studies revealed that 1 exhibits field-induced single-molecule magnetic behavior with a pre-exponential factor of τ0 = 6.60(7) × 10-8 s and a relaxation energy barrier of ΔE/kB = 39.44(2) K, while 2 only displays antiferromagnetic interactions between the ytterbium centers.

Lanthanide-based bis-(3,5-dicarboxy-phenyl)terephthalamide metal–organic frameworks: slow relaxation of magnetization and detection of trace Fe2+ and Fe3+

{[Dy2(BDPT)1.5(DMF)4]·H2O}n (1) and [Sm2(BDPT)1.5(DMF)4]n (2) were prepared with highly sensitive fluorescent detection of trace Fe2+ and Fe3+. 1 exhibits weak antiferromagnetic interactions with signature of SMM behavior.

Regulating the solution structural integrity and slow magnetic relaxation behavior of two Dy6 clusters with a pyridine–triazole ligand

Two {Dy6} skeletons consolidated by nitrate (1) and hydroxide (2) have been isolated. HRESI-MS reveals that 1 retains a higher structural integrity and fragment stability. While 2 has a typical slow magnetic relaxation of SMM behavior.

Successive syntheses and magnetic properties of homodinuclear lanthanide macrocyclic complexes

Two series of homodinuclear lanthanide macrocyclic complexes with D4d symmetry have been successively synthesized. Magnetic studies reveal ferromagnetic interactions between Dy3+ or Tb3+ centres and typical SMM behaviors for Dy complexes 1 and 4.

Lanthanide-Based Single-Molecule Magnets Derived from Schiff Base Ligands of Salicylaldehyde Derivatives

The breakthrough in Ln(III)-based SMMs with Schiff base ligands have been occurred for the last decade on account of their magnetic behavior, anisotropy and relaxation pathways. Herein, we review the synthetic strategy, from a structural point of view and magnetic properties of mono, di, tri and polynuclear Ln(III)-based single-molecule magnets mainly with Schiff bases of Salicylaldehyde origin. Special attention has been given to some important breakthroughs that are changing the perspective of this field with a special emphasis on slow magnetic relaxation. An overview of 50 Ln(III)-Schiff base complexes with SMM behavior, covering the period 2008–2020, which have been critical in understanding the magnetic interactions between the Ln(III)-centers, are presented and discussed in detail.

A tetranuclear DyIII compound with in situ oxazolidine ligand derived from hydroxyquinoline carboxaldehyde: Synthesis, structure, and SMM behavior

Tetranuclear dysprosium(III) complex [Dy4(HL3)4(CH3COO)4(EtOH)2] (1) (H3L3 = 2-(4,4-bis(hydroxymethyl)oxazolidin-2-yl)quinolin-8-ol) was synthesized from 8-hydroxyquinoline-2-carbaldehyde (HL1), 2-amino-2-(hydroxymethyl)propane-1,3-diol (H3L2) ligands and Dy(CH3COO)3·6H2O in CH3OH and CH3CH2OH mixed solvent. The H3L3 ligand was obtained by the in situ Schiff base reaction of HL1 with H3L2 followed by the in situ cyclisation reaction catalyzed by Dy(III) ions. This is the first example of in situ cyclisation reactions of oxazolidine catalyzed by Dy(III) ion during the formation of complex. AC magnetic susceptibility measurements showed that complex 1 exhibited an obvious single-molecule magnetic behavior, which energy barrier was 37.3 (2) K under zero dc-field.

Hexanuclear NiII4LnIII2 Complexes with SMM Behavior at Zero Field for Ln = Tb, Dy, Ho

A mononuclear Ni(II) complex, [NiL2]·2H2O, was prepared by the reaction of a N2O2 donor monocondensed Schiff base ligand, 2-((3-aminopropylimino)methyl)-6-methoxyphenol (HL), with NiCl2·6H2O. The reaction of this complex with NiCl2·6H2O and LnCl3·6H2O (Ln = Gd, Tb, Dy, Ho) in a 1:1:1 molar ratio leads to four hexanuclear Ni4Ln2 complexes formulated as [{(NiL)2Gd}2(μ2-Cl)2(μ3-OH)4(OH2)4]Cl4·CH3CN·H2O (1), [{(NiL)2Tb}2(μ2-Cl)2Cl2(μ3-OH)4(OH2)2]Cl2·12H2O (2), [{(NiL)2Dy}2(μ2-Cl)2Cl2(μ3-OH)4(OH2)2]Cl2·16H2O (3), and [{(NiL)2Ho}2(μ2-Cl)2(μ3-OH)4(OH2)4]Cl4·CH3CN·1.8H2O (4). The Ln(III) centers are octacoordinated with a triangular-dodecahedral geometry, and the geometries around the Ni(II) centers are distorted octahedral in all four complexes. The temperature-dependent DC magnetic measurements reveal the existence of ferromagnetic interactions in all four complexes (1–4). The AC susceptibility measurements performed with and without an applied DC field show SMM behavior in complexes 2–4 with a single slow relaxation process in 2 (SR) but slow (SR) and fast (FR) relaxation processes in 3 and 4. The energy barriers of the relaxation times for the SR processes are ca. 13, 20, and 19 K in 2–4, respectively.

The Influence of Halide Substituents on the Structural and Magnetic Properties of Fe6Dy3 Rings.

We report the synthesis and magnetic properties of three new nine-membered Fe(III)-Dy(III) cyclic coordination clusters (CCCs), with a core motif of [Fe6Dy3(μ-OMe)9(vanox)6(X-benz)6] where the benzoate ligands are substituted in the para-position with X = F (1), Cl (2), Br (3). Single crystal X-ray diffraction structure analyses show that for the smaller fluorine or chlorine substituents the resulting structures exhibit an isostructural Fe6Dy3 core, whilst the 4-bromobenzoate ligand leads to structural distortions which affect the dynamic magnetic behavior. The magnetic susceptibility and magnetization of 1-3 were investigated and show similar behavior in the dc (direct current) magnetic data. Additional ac (alternating current) magnetic measurements show that all compounds exhibit frequency-dependent and temperature-dependent signals in the in-phase and out-of-phase component of the susceptibility and can therefore be described as field-induced SMMs. The fluoro-substituted benzoate cluster 1 shows a magnetic behavior closely similar to that of the corresponding unsubstituted Fe6Dy3 cluster, with Ueff = 21.3 K within the Orbach process. By increasing the size of the substituent toward 4-chlorobenzoate within 2, an increase of the energy barrier to Ueff = 36.1 K was observed. While the energy barrier becomes higher from 1 to 2, highlighting that the introduction of different substituents on the benzoate ligand in the para-position has an impact on the magnetic properties, cluster 3 shows a significantly different SMM behavior where Ueff is reduced in the Orbach regime to only 4.9 K.

Octanuclear Ni4 Ln4 Coordination Aggregates from Schiff Base Anion Supports and Connecting of Two Ni2 Ln2 Cubes: Syntheses, Structures, and Magnetic Properties.

A family of 3d-4f aggregates have been reported through guiding the dual coordination modes of ligand anion (HL- ) and in situ generated ancillary bridge driven self-assembly coordination responses toward two different types of metal ions. Reactions of lanthanide(III) nitrate (Ln=Gd, Tb, Dy, Ho and Yb), nickel(II) acetate and phenol-based ditopic ligand anion of 2-[{(2-hydroxypropyl)imino}methyl]-6-methoxyphenol (H2 L) in MeCN-MeOH (3 : 1) mixture and LiOH provided five new octanuclear Ni-4f coordination aggregates from two Ni2 Ln2 cubanes. Single-crystal X-ray diffraction analysis reveals that all the members of the family are isostructural, with the central core formed from the coupling of two distorted [Ni2 Ln2 O4 ] heterometallic cubanes [Ni2 Ln2 (HL)2 (μ3 -OH)2 (OH)(OAc)4 ]+ (Ln=Gd (1), Tb (2), Dy (3), Ho (4) and Yb (5)). Higher coordination demand of 4f ions induced the coupling of the two cubes by (OH)(OAc)2 bridges. Variable temperature magnetic study reveals weak coupling between the Ni2+ and Ln3+ ions. For the Tb (2) and Dy (3) analogs, the compounds are SMMs without an applied dc field, whereas the Gd (1) analogue is not an SMM. The observation revealed thus that the anisotropy of the Ln3+ ions is central to display the SMM behavior within this structurally intriguing family of compounds.

Synthesis, Structures, and Magnetic Properties of Zigzag Tetranuclear Lanthanide Complexes

Five isostructural tetranuclear lanthanide complexes with the general formula [Ln4(teaH2)2(teaH)2(NO3)6]·2CH3OH [Ln3+ = Dy3+ (1), Tb3+ (2), Ho3+ (3), Er3+ (4), and Gd3+ (5)] were successfully synthesized by the reaction of various lanthanide nitrate and triethanolamine (teaH3) ligand. Single crystal X‐ray analyses reveal the eight‐coordinate Ln3+ centers adopt a slightly distorted triangular dodecahedron geometry and nine‐coordinate Ln3+ ions own an approximately capped square antiprism environment in similar zigzag Ln4 core. Magnetic studies demonstrate the presence of anitferromagnetic interactions between Ln3+ centers without obvious SMM behavior.

Family of Chiral ZnII-LnIII (Ln = Dy and Tb) Heterometallic Complexes Derived from the Amine-Phenol Ligand Showing Multifunctional Properties.

A family of chiral 3d-4f heterometallic complexes, namely, [Zn2Ln(R,R-L)2(H2O)4](ClO4)3) [Ln = Dy (1), Tb (3)], [Zn2Ln(S,S-L)2(H2O)4](ClO4)3 [Ln = Dy (2), Tb (4)], [Zn2Ln2(R,R-L)2(CO3)2(NO3)2]·2CH3OH [Ln = Dy (5), Tb (7)], and [Zn2Ln2(S,S-L)2(CO3)2(NO3)2]·2CH3OH [Ln = Dy (6), Tb (8)] {H2L = cyclohexane-1,2-diylbis(azanediyl)bis(methylene)bis(2-methoxyphenol)}, has been synthesized and characterized. Crystal structure analysis reveals that complexes 1-4 are isostructural trinuclear clusters crystallized in chiral space group C2221, and 5-8 are isostructural tetranuclear clusters crystallized in chiral space group P1. Interestingly, the adjacent [ZnLn] units within the tetranuclear cluster in 5-8 are bridged by two carbonate anions via in situ incorporation of CO2 from air. Magnetic measurements indicate that complexes 1 and 3 exhibit field-induced single-molecule magnet behavior with energy barriers (Ueff) of 22.46 and 38.70 K (or 41.87 K), respectively. Complex 5 displays typical SMM behavior with Ueff = 19.61 K under zero dc field, while for complex 7, no obvious out-of-phase signals are observed even under 2 kOe dc field, the absence of SMM behavior. The solid-state luminescence studies reveal that all complexes display the characteristic fluorescence emission of lanthanide ions. Furthermore, the Kurtz-Perry measurements reveal these complexes are potential nonlinear optical materials.

Synthesis, crystal structure and magnetic properties of a pentanuclear Mn(III) cluster with 1,2,4-triazole based Schiff base ligand

The reaction of equimolar amounts of 2-hydroxy-5-iodobenzaldehyde and 5-(methylthio)-1H-1,2,4-triazol-3-amine in methanol yielded a new 1,2,4-triazole based Schiff base ligand, H2L, which was characterised by spectroscopic methods. The combination of this Schiff base ligand with Mn(OAc)2·4H2O and KSCN in methanol yielded dark brown crystals of [KMn5(L)4(O)2(OAc)4(CH3OH)2(H2O)]·5.25(CH3OH)·2.33(H2O) (1), which were characterised by elemental analysis, spectroscopic methods and single-crystal X-ray diffraction. The compound contains Mn(III) pentanuclear clusters in which two oxido, four acetate and four dinegative Schiff base ligands link the Mn(III) ions. The structure of the cluster consists of two perpendicular Mn3O units and one Mn(III) ion acting as a bridge between them. The study of the magnetic properties revealed the occurrence of antiferromagnetic intramolecular interactions among the Mn(III) atoms. Since this type of compounds are candidates to exhibit SMM-behavior, the presence of SMM behavior is also investigated.

Coumarin-lanthanide based compounds with SMM behavior and high quantum yield luminescence.

Coumarin-based lanthanide complexes of general formula [Ln(coum)3(phen)(H2O)x]·yH2O (Ln-phen, x = 0,1, 0 ≤ y ≤ 1.5; phen = 1,10-phenanthroline; coum = 3-acetyl-4-hydroxylato-coumarin; Ln = Eu, Tb, Dy, Tm) and [Ln(coum)3(batho)]·0.7EtOH (Ln-batho, batho = 4,7-diphenyl-1,10-phenanthroline; Ln = Eu, Tb, Dy, Tm) were synthesized. The magnetic relaxation and photoluminescence behavior of these complexes was compared with that of the related compounds [Ln(coum)3(EtOH)(H2O)]·EtOH (Ln-coum), so as to investigate the effects of incorporating a second chromophore, either the phen or batho ligand, to the original coordination scaffold, provided with three coumarin (coum) ligands. Slow relaxation of the magnetization under H = 0 with moderate activation energies was observed for the Dy-phen (U/kB = 99.1 K) and Dy-batho (U/kB = 67.1 K) compounds, whereas Tb analogues presented field-induced single molecule magnet (SMM) behavior, with U/kB = 11.7 K (16.6 K@3 kOe) for Tb-phen (Tb-batho), respectively. Luminescent emission in the visible range was observed for all the Ln-coumarin based compounds upon ligand sensitization, with high quantum yields of 45 (40%) for Eu-phen (Eu-batho) compounds and 65-76-58% for Tb-coum, phen, batho analogues. Sensitization is mainly provided by the coumarin ligand having the energy difference ΔE between its triplet state T1 and the lanthanide emitting level closest to the optimum, while the second ligand can play either a synergistic or competing sensitizing role. The Tb-phen/batho compounds presented simultaneously SMM and luminescent behavior, with excellent values of the bifunctional figure of merit (ηSMM-QY ∼ 1000% K). The reported compounds represent a new class of bifunctional materials with potential interesting application in various fields.

Slow magnetic relaxation in CoII-LnIII heterodinuclear complexes achieved through a functionalized nitronyl nitroxide biradical.

A new nitronyl nitroxide biradical ligand NITPh-PyPzbis (5-(3-(2-pyridinyl)-1H-pyrazol-1-yl)-1,3-bis(1'-oxyl-3'-oxido-4',4',5',5'-tetramethyl-4,5-hydro-1H-imidazol-2-yl)benzene) has been successfully applied for constructing a 3d-4f CoII-LnIII system, giving rise to a family of novel hetero-tri-spin complexes, namely, [LnCo(hfac)5(NITPh-PyPzbis)]·CH2Cl2 (LnCo = YCo 1, GdCo 2, TbCo 3, DyCo 4, and HoCo 5; hfac = hexafluoroacetylacetonate). In these hetero-tri-spin complexes, the NITPh-PyPzbis biradical chelates one LnIII and one CoII simultaneously by means of its adjacent aminoxyl moieties and two N donors from the 3-(2-pyridinyl)-1H-pyrazol-1-yl unit, respectively, realizing the unique biradical-Co-Ln heterodinuclear structure. Direct-current magnetic susceptibility investigations show that antiferromagnetic coupling is predominant in the YCo derivative, while the leading magnetic interaction in the GdCo analogue is ferromagnetic. Alternating-current data for the DyCo complex display visible temperature/frequency-dependent χ'' peaks, indicating the SMM behavior. In the fluorescence spectra, four characteristic emission bands of the TbIII ion have been detected for the TbCo analogue.

Luminescent Ir(iii)–Ln(iii) coordination polymers showing slow magnetization relaxation

Two structural types of iridium(iii)–lanthanide(iii) coordination polymers, single-chainIr2Lnand double-chainIr4Ln2(Ln = Gd, Dy, Er, and Yb), have been prepared. SMM behaviour and NIR luminescence were observed for the Ir–Er and Ir–Yb systems.

SMM behaviour of heterometallic dinuclear CuIILnIII (Ln = Tb and Dy) complexes derived from N2O3 donor unsymmetrical ligands

Four heterometallic dinuclear CuIILnIII (Ln = Tb and Dy) complexes derived from two different N2O3 donor unsymmetrical Schiff base ligands exhibit SMM behaviour.