Single-molecule Magnet Behavior Research Articles

A magnetic Ni-based metal-organic framework with interesting transformation dynamics

Herein, a flexible system composed of two Ni(II)-based coordination compounds: a 3D framework of {[Ni(μ-3isoani)2]·DMF}n (1) formula and a 0D monomeric [Ni(3isoani)2(H2O)4] (2) complex (where 3isoani = 3-aminoisonicotinato and DMF = dimethylformamide) which show mutual solvent-induced reversible transformations into each other is reported. The system shows reversible 1 ↔ 2 transformations upon the exposure of one compound to the solvent present in the other, that is, the MOF (1) in water and the monomer (2) in DMF. The process is easily followed by the naked eye because it involves a colour change from green (1)-to-brownish green (2), making the process traceable as previously observed for the Co(II)-based system. Moreover, these compounds present very different magnetic properties since 1 shows field-induced single-molecule magnet (SMM) behaviour, which corresponds to one of the few Ni(II)-based compounds showing single-ion behaviour (being Ni(II) the unique spin carrier), whereas 2 behaves as a regular paramagnet, all of which is explained according to a careful study of the magnetic properties by means of experimental direct-current (dc) and alternating-current (ac) measurements. Obtained experimental results are well supported by active space self-consistent field (CASSCF) calculations that allow understanding the slow relaxation occurring in the Ni(II) ion.

Structures and magnetic properties of a trinuclear angular [Ni3] and a heptanuclear wheel-like [Ni7] complexes with a Schiff base ligand

The presence of large magnetic anisotropy with nickel(II) ions facilitates the single-molecule magnet (SMM) behavior in multimetallic nickel(II) complexes. Herein syntheses under ambient conditions, characterizations, crystal structures, and experimental magnetic analyses of an angular [NiII3L4] and a wheel-like [NiII7L6] complexes of general formula [Ni3L2(μ-L)2(μ-OH2)2(CF3CO2)2] (1) and [Ni7(μ-L)6(μ-OMe)6]Cl2·6H2O (2) (HL = 2-{(3-ethoxypropylimino)methyl}-6-methoxyphenol) have been reported. The angular core of trimetallic neutral complex 1 is assembled using four L−, two bridging water molecules, and two bound trifluoroacetate anions. In contrast, the heptanuclear complex 2 features a planar wheel-like arrangement with a central NiII enclosed by a {NiII6} hexagon bound to six MeO− and six L−. The isolation of these two aggregates of varying nuclearity and metal ion: ligand anion ratio indicated the importance of reaction conditions and crystallization to follow the bottom-up assembly processes. Experimental magnetic studies showed antiferromagnetic interactions between the central and terminal NiII ions in complex 1 and competing ferro- and antiferromagnetic interactions in the wheel-like complex 2.

From a Dy2 single-molecule magnet (SMM) to a 1D triangle-based Dy(III) chain SMM utilizing formate ions as bridging units

The reaction of Dy(NO3)2·6H2O with a Schiff base ligand N1,N3-bis(5-chlorosalicyladehyde)diethylenetriamine (H2bcsdt) afforded a dinuclear DyIII complex [Dy2(bcsdt)2(NO3)2] (1). The addition of HCOOH to the reaction system from which 1 was prepared generated a coordination polymer with the formula {[Dy3(bcsdt)3(HCOOH)(HCOO)(CO3)]·EtOH}n (2). Single-crystal X-ray diffraction analysis revealed that complex 2 possesses a one-dimensional chain structure based on a {[Dy3(bcsdt)3(HCOOH)(HCOO)(CO3)]·EtOH} repeating unit. In this unit, the three DyIII ions are connected by three oxygen atoms of one CO32– ion and display a nearly perfect triangle topology. The magnetic properties of 1 and 2 were investigated. They both exhibited slow relaxation of magnetization, indicative of single molecule magnet (SMM) behavior.

Two Pairs of Homochiral Parallelogram-like Dy4 Cluster Complexes with Strong Magneto-Optical Properties.

Two pairs of homochiral Dy(III) tetranuclear cluster complexes derived from (+)/(-)-3-trifluoroacetyl camphor (D-Htfc/L-Htfc), [Dy4(OH)2(L1)4(D-tfc)2(DMF)2]·4DMF (D-1) [H2L1 = (E)-2-(2-hydroxy-3-methoxybenzylideneamino)phenol)]/[Dy4(OH)2(L1)4(L-tfc)2(DMF)2]·4DMF (L-1) and [Dy4(OH)2(L2)4(D-tfc)2(DMF)2]·2H2O·3MeCN (D-2) [H2L2 = (E)-3-(2-hydroxy-3-methoxybenzylideneamino)naphthalen-2-ol]/[Dy4(OH)2(L2)4(L-tfc)2(DMF)2]·2H2O·3MeCN (L-2), were synthesized at room temperature, which have a Dy4 parallelogram-like core. The magnetic studies revealed that D-1 exhibits single-molecule magnet (SMM) behavior under zero dc magnetic field, and its magnetic relaxation has a distinct Raman process in addition to the Orbach process, with the Ueff/k value of 57.5 K and the C value of 28.27 s-1K-2.14; while D-2 displays dual magnetic relaxation behavior at 0 Oe field, with the Ueff/k value 114.8 K for the slow relaxation process (SR) and the C value of 10.656 s-1K-5.80 for the fast relaxation process (FR), respectively. Theoretical calculations indicated that the conjugated groups (phenyl vs naphthyl) of the Schiff base bridging ligands (H2L1 and H2L2) significantly affect the intramolecular magnetic interactions between the Dy3+ ions and ultimately lead to different relaxations. Furthermore, magnetic circular dichroism (MCD) measurements showed that these two pairs of Dy4 enantiomers exhibit strong room temperature magneto-optical Faraday effects; notably, increasing the conjugated group on the Schiff base bridging ligand is beneficial to enhancing the magneto-optical Faraday effects.

A Pair of Multifunctional Cu(II)-Dy(III) Enantiomers with Zero-Field Single-Molecule Magnet Behaviors, Proton Conduction Properties and Magneto-Optical Faraday Effects.

Multifunctional materials with a coexistence of proton conduction properties, single-molecule magnet (SMM) behaviors and magneto-optical Faraday effects have rarely been reported. Herein, a new pair of Cu(II)-Dy(III) enantiomers, [DyCu2(RR/SS-H2L)2(H2O)4(NO3)2]·(NO3)·(H2O) (R-1 and S-1) (H4L = [RR/SS] -N,N'-bis [3-hydroxysalicylidene] -1,2-cyclohexanediamine), has been designed and prepared using homochiral Schiff-base ligands. R-1 and S-1 contain linear Cu(II)-Dy(III)-Cu(II) trinuclear units and possess 1D stacking channels within their supramolecular networks. R-1 and S-1 display chiral optical activity and strong magneto-optical Faraday effects. Moreover, R-1 shows a zero-field SMM behavior. In addition, R-1 demonstrates humidity- and temperature-dependent proton conductivity with optimal values of 1.34 × 10-4 S·cm-1 under 50 °C and 98% relative humidity (RH), which is related to a 1D extended H-bonded chain constructed by water molecules, nitrate and phenol groups of the RR-H2L ligand.

Magnetic refrigeration property and single-molecule magnetic behavior of three asymmetric Ln2 (LnIII = Gd, TB and Dy) compounds

Three new dinuclear Ln(III)-based compounds formulated as {Ln2(dbm)5L}·nMeOH (Ln = Gd (1), Tb (2), and Dy (3), Hdbm = dibenzoylmethane, HL = 2-hydroxyimino-propionic acid (6-methoxy-pyridin-2-ylmethylene)-hydrazide, MeOH = methanol) have been successfully synthesized under solvothermal conditions. The structures and magnetic properties of 1–3 have been systematically studied. The single-crystal X-ray diffraction analysis shows that compounds 1–3 are dinuclear structures with two asymmetric LnIII ion centres. The magnetic properties study suggests that compound 1 displays cryogenic magnetic refrigeration property, while compound 3 exhibits single-molecule magnetic behavior with an energy barrier (ΔE/kB) of 62.3 K and pre-exponential factor τ0 = 2.93 × 10-8 s under a zero direct-current field.

Two dysprosium complexes based on TEMPO radical derivative: synthesis, structures, magnetic and fluorescent properties

A new TEMPO derivative was synthesized and two Dy-radical complexes, [Dy(hfac)3(H2O)hmb-TEMPO] (1) and [Dy(hfac)3(hmb-TEMPO)2] (2, hmb-TEMPO = 4-((2-hydroxy-3-methoxybenzylidene)amino)-2,2,6,6-tetramethyl; hfac = hexafluoroacetylacetonate), were synthesized through reacting hmb-TEMPO radical and Dy(hfac)3 with 1:1 and 1:2 ratio, respectively. The structures of 1 and 2 were elucidated by single-crystal X-ray structural analysis. Both 1 and 2 feature mononuclear structures. Nonzero out-of-phase signals were observed for both Dy derivatives, indicating single-molecule magnet behavior. The room-temperature photoluminescence spectra of 1 and 2 exhibit strong emissions in the visible region.

Dehydration-actuated single-molecule magnet behavior in a cyanide-bridged [Fe2Co2] cluster featuring zigzag structure

A rare zigzag cyanide-bridged tetranuclear complex, {[(Tp*)FeIII(CN)3]2[CoII(tphz)1/2(SCN)(MeOH)]2}·4H2O (1·4H2O) (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate, tphz = tetrapyridophenazine) was successfully isolated and characterized structurally and magnetically. Single-crystal structural analysis indicates the Fe3+ and Co2+ ions are linked by cyanides while the inverse two Co2+ are bridged by the pyrazine of tphz. Magnetic studies for 1·4H2O revealed the ferromagnetic coupling (JFeCo = +3.2 cm−1) between the Fe3+ and Co2+ ions and antiferromagnetic interaction (JCoCo = −2.7 cm−1) between the Co2+ ions of the cluster. After the release of crystallized water molecules, however, the magnetic behaviors were drastically tuned indicated by the notable changes in the magnetic couplings for FeIIICoII (JFeCo = +3.9 cm−1) and CoIICoII (JCoCo = −3.5 cm−1). Reduced magnetization reveals the uniaxial magnetic anisotropy of the dehydrated molecule with the zero-field splitting parameter D = −10.3 cm−1. Additionally, field-supported single-molecule magnet behavior was observed with the effective energy barrier Ueff = 38.2 K, making complex 1 the first example of a zigzag Fe2Co2 single-molecule magnet. This work provides not only a rare dynamic switching molecular complex but also highlights the important influence of guest water molecules in magnetic behaviors.

A Japan-Poland Collaboration for Coupling Molecular Nanomagnetism and Luminescent Thermometry.

This invited Team Profile was created by the Laboratory of Advanced Functional Materials (Tokoro Lab), Division of Materials Science, Faculty of Pure and Applied Sciences, University of Tsukuba (Japan); the Multifunctional Luminescent Materials Group (Szymon Group, MLMG), Faculty of Chemistry, Jagiellonian University, Kraków (Poland); and the Solid State Physical Chemistry Group (Ohkoshi Lab), Department of Chemistry, School of Science, The University of Tokyo (Japan). They recently published an article on a bimetallic cyanido-bridged TbIII -CoIII framework exhibiting a desolvation-induced structural transformation providing high-symmetry trigonal-prismatic [TbIII (NC)6 ]3- complexes. This induces a strong magnetic anisotropy leading to single-molecule magnet behavior with an energy barrier of 854(26) K, which appears in the cryogenic temperatures range, together with the concomitant luminescent thermometry effect. "Desolvation-Induced Highly Symmetrical Terbium(III) Single-Molecule Magnet Exhibiting Luminescent Self-Monitoring of Temperature", J. Wang, J. J. Zakrzewski, M. Zychowicz, Y. Xin, H. Tokoro, S. Chorazy, S. Ohkoshi, Angew. Chem. Int. Ed. 2023, 62, e202306372.

Targeted Synthesis of End-On Dinitrogen-Bridged Lanthanide Metallocenes and Their Reactivity as Divalent Synthons.

High-yield syntheses of the lanthanide dinitrogen complexes [(Cp2tttM)2(μ-1,2-N2)] (1M, M = Gd, Tb, Dy; Cpttt = 1,2,4-C5tBu3H2), in which the [N2]2- ligands solely adopt the rare end-on or 1,2-bridging mode, are reported. The bulk of the tert-butyl substituents and the smaller radii of gadolinium, terbium, and dysprosium preclude formation of the side-on dinitrogen bonding mode on steric grounds. Elongation of the nitrogen-nitrogen bond relative to N2 is observed in 1M, and their Raman spectra show a major absorption consistent with N═N double bonds. Computational analysis of 1Gd identifies that the local symmetry of the metallocene units lifts the degeneracy of two 5dπ orbitals, leading to differing overlap with the π* orbitals of [N2]2-, a consequence of which is that the dinitrogen ligand occupies a singlet ground state. Magnetic measurements reveal antiferromagnetic exchange in 1M and single-molecule magnet (SMM) behavior in 1Dy. Ab initio calculations show that the magnetic easy axis in the ground doublets of 1Tb and 1Dy align with the {M-N═N-M} connectivity, in contrast to the usual scenario in dysprosium metallocene SMMs, where the axis passes through the cyclopentadienyl ligands. The [N2]2- ligands in 1M allow these compounds to be regarded as two-electron reducing agents, serving as synthons for divalent gadolinium, terbium, and dysprosium. Proof of principle for this concept is obtained in the reactions of 1M with 2,2'-bipyridyl (bipy) to give [Cp2tttM(κ2-bipy)] (2M, M = Gd, Tb, Dy), in which the lanthanide is ligated by a bipy radical anion, with strong metal-ligand direct exchange coupling.

Single-Molecule Magnet Properties in 3d4f Heterobimetallic Iron and Dysprosium Complexes Involving Hydrazone Ligand.

The reaction between the ((E)-N'-(2-hydroxy-3-methoxybenzylidene)pyrazine-2-carbohydrazide) (H2opch) ligand and the metallo-precursor [Dy(hfac)3]·2H2O led to the formation of an homometallic coordination complex with the formula [Dy2(hfac)3(H2O)(Hopch)2][Dy(hfac)4] (1). In presence of both [Dy(hfac)3] 2H2O and the Fe(II) salt, the heterobimetallic tetranuclear [FeDy3(hfac)8(H2O)2(opch)2] (2) was isolated, while the addition of the co-ligand 1,2-Bis(2-hydroxy-3-methoxybenzylidene) hydrazine (H2bmh) led to the formation of two heterobimetallic tetranuclear complexes with the formula [Fe3Dy(hfac)6(opch)2(H2bmh)] C6H14 (3) C6H14 and [Fe2Dy2(hfac)7(opch)2(H2bmh)] 0.5C7H16 (4) 0.5C7H16. Single crystal X-ray diffraction and dc magnetic investigation demonstrated that 3 and 4 involved the iron center in the +II and +III oxidation states. Dynamic magnetic measurements highlighted the single-molecule magnet behavior of 1 and 2 in a zero applied dc field primarily due to the ferromagnetic interactions taking place in these compounds.

Trityl-Based Lanthanide-Supramolecular Assemblies Exhibiting Slow Magnetic Relaxation.

The triphenylmethane (trityl) group has been recognized as a supramolecular synthon in crystal engineering, molecular machine rotors and stereochemical chirality inductors in materials science. Herein we demonstrate for the first time how it can be utilized in the domain of molecular magnetic materials through shaping of single molecule magnet (SMM) properties within the lanthanide complexes in tandem with other non-covalent interactions. Trityl-appended mono- (HL1 ) and bis-compartmental (HL2 ) hydrazone ligands were synthesized and complexated with Dy(III) and Er(III) triflate and nitrate salts to generate four monometallic (1-4) and two bimetallic (5, 6) complexes. The static and dynamic magnetic properties of 1-6 were investigated, revealing that only ligand HL1 induces assemblies (1-4) capable of showing SMM behaviour, with Dy(III) congeners (1, 2) able to exhibit the phenomenon also under zero field conditions. Theoretical ab initio studies helped in determination of Dy(III) energetic levels, magnetic anisotropic axes and corroborated magnetic relaxation mechanisms to be a combination of Raman and quantum tunnelling in zero dc field, the latter being cancelled in the optimum non-zero dc field. Our work represents the first study of magneto-structural correlations within the trityl Ln-SMMs, leading to generation of slowly relaxing zero-field dysprosium complexes within the hydrogen-bonded assemblies.

A family of mono-nuclear lanthanide-radical complexes: Synthesis, structures, magnetic and fluorescent properties

Two new rare earth complexes based on TEMPO derivative namely [Ln(hfac)3(H2O)hmb-TEMPO] (Ln = Gd 1, Tb 2; hmb-TEMPO = 4-((2-hydroxy-3-methoxybenzylidene) amino)-2,2,6,6-tetramethyl; hfac = hexafluoroacety lacetonate) were successfully synthesized through reacting hmb-TEMPO radical and Ln(hfac)3. Single-crystal X-ray diffraction reveals that 1 and 2 are isostructural complexes in which the LnIII ion is surrounded nine oxygen atoms and displayed distorted tricapped trigonal prism geometry. Magnetic studies show that antiferromagnetic interactions dominate in complex 1. Nonzero out-of-phase signals were observed for complex 2, indicating single-molecule magnet behavior. The low-temperature photoluminescence spectrum of complex 2 were investigated and discussed in detail.

Lanthanide dimers in coordination chains constructed by sole dicarboxylate ligand for single-molecule magnet behavior and magnetocaloric effect

The solvothermal reaction of Dy(III) and Gd(III) ions with 2,5-di‑hydroxy-benzene-1,4-di-carboxylic acid (H4dhbdc) produced two new isostructural lanthanide(III) coordination chains with the general formula {[Ln2(H2dhbdc)3(H2O)8]·6H2O}n [Ln = Dy (1) and Gd (2)]. The chains consist of carboxylate-bridged dilanthanide(III) units linked by H2dhbdc2− ligands in an end-to-end bridging mode. The Dy3+ and Gd3+ ions are located in slightly distorted biaugmented trigonal prism (C2v) and triangular dodecahedron (D2d) coordination environments, respectively. Magnetic studies reveal the extremely weak antiferromagnetic interaction (J = -0.041 cm−1) between the magnetically isotropic GdIII ions in 2. The dysprosium analogue was found to exhibit single-molecule magnet (SMM) behavior with a prominent QTM effect under zero dc field, which is largely due to large single-ion magnetic anisotropy of individual Dy3+ ions and weak antiferromagnetic interaction. For the gadolinium chain, the magnetocaloric effect (MCE) was observed with a magnetic entropy change of 20.2 J kg−1 K − 1. These results demonstrate a dicarboxylate synthetic route for the preparation of coordination polymers containing dilanthanide units for SMM and MCE applications.

Desolvation-Induced Highly Symmetrical Terbium(III) Single-Molecule Magnet Exhibiting Luminescent Self-Monitoring of Temperature.

A conjunction of Single-Molecule Magnet (SMM) behavior and luminescence thermometry is an emerging research line aiming at contactless read-out of temperature in future SMM-based devices. The shared working range between slow magnetic relaxation and the thermometric response is typically narrow or absent. We report TbIII -based emissive SMMs formed in a cyanido-bridged framework whose properties are governed by the reversible structural transformation from [TbIII (H2 O)2 ][CoIII (CN)6 ] ⋅ 2.7H2 O (1) to its dehydrated phase, TbIII [CoIII (CN)6 ] (2). The 8-coordinated complexes in 1 show the moderate SMM effect but it is enhanced for trigonal-prismatic TbIII complexes in 2, showing the SMM features up to 42 K. They are governed by the combination of QTM, Raman, and Orbach relaxation with the energy barrier of 594(18) cm-1 (854(26) K), one of the highest among the TbIII -based molecular nanomagnets. Both systems exhibit emission related to the f-f electronic transitions, with the temperature variations resulting in the optical thermometry below 100 K. The dehydration leads to a wide temperature overlap between the SMM behavior and thermometry, from 6 K to 42 K. These functionalities are further enriched after the magnetic dilution. The role of post-synthetic formation of high-symmetry TbIII complexes in achieving the SMM effect and hot-bands-based optical thermometry is discussed.

Desolvation‐Induced Highly Symmetrical Terbium(III) Single‐Molecule Magnet Exhibiting Luminescent Self‐Monitoring of Temperature

AbstractA conjunction of Single‐Molecule Magnet (SMM) behavior and luminescence thermometry is an emerging research line aiming at contactless read‐out of temperature in future SMM‐based devices. The shared working range between slow magnetic relaxation and the thermometric response is typically narrow or absent. We report TbIII‐based emissive SMMs formed in a cyanido‐bridged framework whose properties are governed by the reversible structural transformation from [TbIII(H2O)2][CoIII(CN)6] ⋅ 2.7H2O (1) to its dehydrated phase, TbIII[CoIII(CN)6] (2). The 8‐coordinated complexes in 1 show the moderate SMM effect but it is enhanced for trigonal‐prismatic TbIII complexes in 2, showing the SMM features up to 42 K. They are governed by the combination of QTM, Raman, and Orbach relaxation with the energy barrier of 594(18) cm−1 (854(26) K), one of the highest among the TbIII‐based molecular nanomagnets. Both systems exhibit emission related to the f–f electronic transitions, with the temperature variations resulting in the optical thermometry below 100 K. The dehydration leads to a wide temperature overlap between the SMM behavior and thermometry, from 6 K to 42 K. These functionalities are further enriched after the magnetic dilution. The role of post‐synthetic formation of high‐symmetry TbIII complexes in achieving the SMM effect and hot‐bands‐based optical thermometry is discussed.

Nine-Coordinate LnIII-Based One-Dimensional Complexes: Synthesis and Magnetic Properties

Three isostructural one-dimensional lanthanide complexes with formulae of [Dy(H2L)(H2O)2]·Cl·6H2O (1), [Dy0.067Y0.933(H2L)(H2O)2]·Cl·6H2O (1′,) and [Gd(H2L)(H2O)2]·Cl·6H2O (2), where H2L2– is the dianion of the bis-hydrazone (N′,N‴E,N′,N‴E)-N′,N‴-(pyridine-2,6-diylbis(methanylylidene))bis(2-hydroxybenzohydrazide), were synthesized and structurally characterized. Single-molecule magnet (SMM) behavior was observed for the DyIII compound under an applied dc field, which is rarely seen for a nine-coordinate DyIII compound with a hula-hoop coordination geometry. The SMM behavior is ascribed to the single-ion properties of DyIII, which is proved by comparison of the ac magnetic susceptibilities of 1 and the Y/Dy diluted complex 1′. Magnetic studies indicate that the GdIII analogue shows a magnetocaloric effect with a maximum entropy change −ΔSm of 21.32 J kg–1 K–1 at ΔH = 7 T and 3 K.

A Combined Synthetic, Magnetic, and Theoretical Study on Enhancing Ligand-Field Axiality for Dy(III) Single-Molecule Magnets Supported by Ferrocene Diamide Ligands.

Molecular design is crucial for improving the performance of single-molecule magnets (SMMs). For dysprosium(III) SMMs, enhancing ligand-field axiality is a well-suited strategy to achieve high-performance SMMs. We synthesized a series of dysprosium(III) complexes, (NNTIPS)DyBr(THF)2 (1, NNTIPS = fc(NSiiPr3)2; fc = 1,1'-ferrocenediyl, THF = tetrahydrofuran), [(NNTIPS)Dy(THF)3][BPh4] (2), (NNTIPS)DyI(THF)2 (3), and [(NNTBS)Dy(THF)3][BPh4] (4, NNTBS = fc(NSitBuMe2)2), supported by ferrocene diamide ligands. X-ray crystallography shows that the rigid ferrocene backbone enforces a nearly axial ligand field with weakly coordinating equatorial ligands. Dysprosium(III) complexes 1-4 all exhibit slow magnetic relaxation under zero fields and possess high effective barriers (Ueff) around 1000 K, comparable to previously reported (NNTBS)DyI(THF)2 (5). We probed the influences of structural variations on SMM behaviors by theoretical calculations and found that the distribution of negative charges defined by rq, i.e., the ratio of the charges on the axial ligands to the charges on the equatorial ligands, plays a decisive role. Moreover, theoretical calculations on a series of model complexes 1'-5' without equatorial ligands unveil that the axial crystal-field parameters B20 are directly proportional to the N-Dy-N angles and support the hypothesis that enhancing the ligand-field axiality could improve SMM performance.

A [Dy2] complex with single-molecule magnet behavior and its analogues based on transformed ligand induced by methanol in coordination process

Three [Ln2] complexes, [Ln2(dbm)4(L′)2] (Ln = Tb(1), Dy(2), Ho(3)), were synthesized, and structurally and magnetically analysed. The structural analysis revealed that a rare ligand-transformation in the field of organic chemistry and coordination chemistry occured during the coordination process, where the original Schiff-base ligand HL (2-[[(1,2,4-triazole)-4-imino]methyl]-8-hydroxy-quinoline) was transformed to coordinated ligand HL′ (methyl-8-hydroxyquinoline-2-carboxylate). Static and dynamic magnetic analysis of 1–3 demonstrated single-molecule magnet (SMM) behavior for complex 2 with effective energy barrier of 61.7 K at zero dc field.

Field-Induced Single-Ion Magnet Behavior in Nickel(II) Complexes with Functionalized 2,2':6'-2″-Terpyridine Derivatives: Preparation and Magneto-Structural Study.

Two mononuclear nickel(II) complexes of the formula [Ni(terpyCOOH)2](ClO4)2∙4H2O (1) and [Ni(terpyepy)2](ClO4)2 MeOH (2) [terpyCOOH = 4'-carboxyl-2,2':6',2″-terpyridine and terpyepy = 4'-[(2-pyridin-4-yl)ethynyl]-2,2':6',2″-terpyridine] have been prepared and their structures determined by single-crystal X-ray diffraction. Complexes 1 and 2 are mononuclear compounds, where the nickel(II) ions are six-coordinate by the six nitrogen atoms from two tridentate terpy moieties. The mean values of the equatorial Ni-N bond distances [2.11(1) and 2.12(1) Å for Ni(1) at 1 and 2, respectively, are somewhat longer than the axial ones [2.008(6) and 2.003(6) Å (1)/2.000(1) and 1.999(1) Å (2)]. The values of the shortest intermolecular nickel-nickel separation are 9.422(1) (1) and 8.901(1) Å (2). Variable-temperature (1.9-200 K) direct current (dc) magnetic susceptibility measurements on polycrystalline samples of 1 and 2 reveal a Curie law behavior in the high-temperature range, which corresponds to magnetically isolated spin triplets, the downturn of the χMT product at lower temperatures being due to zero-field splitting effects (D). Values of D equal to -6.0 (1) and -4.7 cm-1 (2) were obtained through the joint analysis of the magnetic susceptibility data and the field dependence of the magnetization. These results from magnetometry were supported by theoretical calculations. Alternating current (ac) magnetic susceptibility measurements of 1 and 2 in the temperature range 2.0-5.5 K show the occurrence of incipient out-phase signals under applied dc fields, a phenomenon that is characteristic of field-induced Single-Molecule Magnet (SMM) behavior, which herein concerns the 2 mononuclear nickel(II) complexes. This slow relaxation of the magnetization in 1 and 2 has its origin in the axial compression of the octahedral surrounding at their nickel(II) ions that leads to negative values of D. A combination of an Orbach and a direct mechanism accounts for the field-dependent relation phenomena in 1 and 2.