Single-ion Magnet Behavior Research Articles

Field-induced slow magnetic relaxation in an octahedral high-spin Co(II) complex

A Co(II)-containing complex of formula [Co2(3-fba)4(4,4′-bpy)3(CH3OH)2] (1), has been prepared and structurally characterized by single-crystal X-ray crystallography (3-Hfba = 3-fluorobenzoic acid, 4,4′-bpy = 4,4′-bipyridine). The title complex is composed of well-isolated bimetallic moieties where two octahedral high-spin Co(II) ions are spatially separated from each other by bipyridyl bridges. As a result, the complex features field-induced single-ion magnet behavior with the effective energy barrier of 22.8 K.

Multifunctional Properties of a 1D Helical Co(II) Coordination Polymer: Toward Single-Ion Magnetic Behavior and Efficient Dye Degradation

This contribution deals with the synthesis and utilization of a new pyrazole-based unsymmetrical ligand, 3-(3-carboxyphenyl)-1H-pyrazole-5-carboxylic acid (H2CPCA), for generating multifunctional materials. The reaction with the Co(II) salt in the presence of a co-ligand 2,9-dimethyl phenanthroline (dmphen) results in the formation of the helical compound {[Co2(dmphen)2(CPCA)2]DMF}n (1). However, two isostructural monomeric complexes are formed {[M(HCPCA)2(H2O)2], M = Co(II), (2) and Mn(II) (3)} when reactions were carried out in the absence of dmphen. Compound 1 shows some highly encouraging single-ion magnetic (SIM) properties. Detailed magnetic studies unveil slow relaxation of magnetization of compound 1, driven by the higher magnetic anisotropy of the cobalt ion, with the energy barrier of ∼9.2 K and relaxation time of 9.1 × 10–5 s, suggesting a SIM behavior. Moreover, UV–vis and fluorescence studies confirm the selective dye degradation of compound 1 with methylene blue both in the presence and absence of H2O2, with the remarkable degradation efficiency of ∼98 and ∼82%, respectively.

Lanthanide(III) (Eu, Gd, Tb, Dy) Complexes Derived from 4-(Pyridin-2-yl)methyleneamino-1,2,4-triazole: Crystal Structure, Magnetic Properties, and Photoluminescence.

The reaction of lanthanide(III) nitrates with 4-(pyridin-2-yl)methyleneamino-1,2,4-triazole (L) was studied. The compounds [Ln(NO3 )3 (H2 O)3 ]⋅2 L, in which Ln=Eu (1), Gd (2), Tb (3), or Dy (4), obtained in a mixture of MeCN/EtOH have the same structure, as shown by XRD. In the crystals of these compounds, the mononuclear complex units [Ln(NO3 )3 (H2 O)3 ] are linked to L molecules through intermolecular hydrogen-bonding interactions to form a 2D polymeric supramolecular architecture. An investigation into the optical characteristics of the Eu3+ -, Tb3+ -, and Dy3+ -containing compounds (1, 3, and 4) showed that these complexes displayed metal-centered luminescence. According to magnetic measurements, compound 4 exhibits single-ion magnet behavior, with ΔEeff /kB =86 K in a field of 1500 Oe.

Magnetic Metal-Organic Framework Exhibiting Quick and Selective Solvatochromic Behavior along with Reversible Crystal-to-Amorphous-to-Crystal Transformation.

By utilizing a flexible tetrapyridinate ligand, tetrakis(4-pyridyloxymethylene) methane(L), a novel multifunctional soft porous framework, [Co(NCS)2(L)]·2H2O·CH3OH (1), was constructed. This framework exhibits quick and selective solvatochromic and vapochromic behavior during a reversible crystal-to-amorphous-to-crystal (CAC) transformation. Importantly, the rapid CAC transition can selectively be triggered by methanol molecules, even at low concentrations of liquid or gaseous methanol. This reproducible transition can be monitored by single-crystal and power X-ray analysis, IR, and UV-vis, which all powerfully illustrate the selectivity and sensitivity of this system. In addition, the typical magnetic behavior of single ion magnets (SIMs) has been successfully introduced into this 3D framework, and the modified dynamic relaxations have been investigated via experimental and theoretical analysis. The consistent observations of both experimental and theoretical results support that the distortions of the metal coordination environments should be responsible for the finely tuned SIM behavior.

The Exploration and Analysis of the Magnetic Relaxation Behavior in Three Isostructural Cyano-Bridged 3d–4f Linear Heterotrinuclear Compounds

Three isostructural cyano-bridged 3d–4f linear heterotrinuclear compounds, (H2.5O)4{Ln[TM(CN)5(CNH0.5)]2(HMPA)4} (Ln = YIII, TM = [FeIII]LS (1); Ln = DyIII, TM = [FeIII]LS (2); Ln = DyIII, TM = CoIII (3)), have been synthesized and characterized by single-crystal X-ray diffraction. Due to the steric effect of the HMPA ligands, the central lanthanide ions in these compounds possess a low coordination number, six-coordinate, exhibiting a coordination geometry of an axially elongated octahedron with a perfect D4h symmetry. Four HMPA ligands situate in the equatorial plane around the central lanthanide ions, and two [TM(CN)5(CNH0.5)]2.5− entities occupy the apical positions to form a cyano-bridged 3d–4f linear heterotrinuclear structure. The static magnetic analysis of the three compounds indicated a paramagnetic behavior of compounds 1 and 3, and possible small magnetic interactions between the intramolecular DyIII and [FeIII]LS ions in compound 2. Under zero dc field, the ac magnetic measurements on 2 and 3 revealed the in-phase component (χ′) of the ac susceptibility without frequency dependence and silent out-of-phase component (χ″), which was attributed to the QTM effect induced by the coordination geometry of an axially elongated octahedron for the DyIII ion. Even under a 1 kOe applied dc field, the χ″ components of 2 were revealed frequency dependence without peaks above 2 K. And under a 2 kOe and 3 kOe dc field, the χ″ components of 3 exhibited weak frequency dependence below 4 K with the absence of well-shaped peaks, which confirmed the poor single-ion magnetic relaxation behavior of the six-coordinate DyIII ion excluding any influence from the neighboring [FeIII]LS ions as that in the analogue 2.

Low-Coordinate Single-Ion Magnets by Intercalation of Lanthanides into a Phenol Matrix.

It is very challenging to synthesize stable trivalent rare-earth complexes in which the coordination number is lower than 3 for the high oxidation state, there is a large ion radius and nearly non-bonding character of trivalent lanthanide ions. The bulky phenol ligand ArOH (Ar=2,6-Dipp2 C6 H3 , Dipp=2,6-diisopropylphenyl) was utilized to construct low-coordinate lanthanide compound [(ArO)Ln(OAr')] (Ar'=6-Dipp-2-(2'-i Pr-6'-CHMe(CH2- )C6 H3 )C6 H3 O- ; Ln=Tb, Dy, Ho, Er, Tm). These complexes and the free ligand ArOH were isostructural. Magnetic measurements and theoretical studies demonstrated that both the oblate-type dysprosium and prolate-type erbium analogues exhibited single-ion magnet (SIM) behavior. The bulky phenol ligands provided strong uniaxial ligand field, making the dysprosium SIM possessing blocking barrier up to 961 K.

Slow Magnetic Relaxation in a Series of Mononuclear 8-Coordinate Fe(II) and Co(II) Complexes.

A series of homoleptic mononuclear 8-coordinate FeII and CoII compounds, [FeII(L2)2](ClO4)2 (2), [FeII(L3)2](ClO4)2 (3), [FeII(L4)2](ClO4)2 (4), [CoII(L1)2](ClO4)2 (5), [CoII(L2)2](ClO4)2 (6), [CoII(L3)2](ClO4)2 (7), and [CoII(L4)2](ClO4)2 (8) (L1 and L2 are 2,9-dialkylcarboxylate-1,10-phenanthroline ligands; L3 and L4 are 6,6'-dialkylcarboxylate-2,2'-bipyridine ligands), have been obtained, and their crystal structures have been determined by X-ray crystallography. The metal center in all of these compounds has an oversaturated coordination number of 8, which is completed by two neutral homoleptic tetradentate ligands and is unconventional in 3d-metal compounds. These compounds are further characterized by electronic spectroscopy, cyclic voltammetry (CV), and magnetic measurements. CV measurements of these complexes in MeCN solution exhibit rich redox properties. Magnetic measurements on these compounds demonstrate that the observed single-ion magnetic (SIM) behavior in the previously reported [FeII(L1)2](ClO4)2 (1) is not a contingent case, since all of the 8-coordinate compounds 2-8 exhibit interesting slow magnetic relaxation under applied direct current fields.

Chiral mononuclear Dy(III) complex based on pyrrolidine-dithiocarboxylate S-donors with field-induced single-ion magnet behavior

Abstract A new chiral mononuclear Dy(III) complex, [Dy((-)-pbipy)(pdtc)3] (1) (pbipy = 4,5-pinene-2,2′-bipyridine, pdtc = pyrrolidine-dithiocarbamate) has been synthesized and structurally characterized. Magnetic studies show that complex 1 exhibits field-induced single-ion magnet behavior with τ0 = 4.2 × 10−7 s and Ueff/k = 56.7 K, which is the first example of homochiral complex derived from S-donor pyrrolidine-dithiocarboxylate with slow magnetic relaxation.

An intense luminescent Dy(iii) single-ion magnet with the acylpyrazolonate ligand showing two slow magnetic relaxation processes

A new Dy(iii)-acylpyrazolonate complex displays both intense yellow-light emission and single-ion magnet behavior with two slow magnetic relaxation processes, acting as a potential bifunctional material.

Mixed chelating ligands used to regulate the luminescence of Ln(iii) complexes and single-ion magnet behavior in Dy-based analogues.

The organic ligands 5,7-dibromo-2-methyl-8-quinolinol (L1), 1,10-phenanthroline (L2), and 5,7-dichloro-2-methyl-8-quinolinol (L3) were used to react with Dy(NO3)3·6H2O under solvothermal conditions at 80 °C to obtain the complexes [Dy(L1)3(H2O)] (1), [Dy(L2)2(NO3)3] (2), and [Dy(L3)3(H2O)] (3), respectively. The reaction of L1 and L2 with lanthanide(iii) nitrate salts in the presence of triethylamine as a base afforded four mononuclear complexes, namely, [Ln(L1)2(L2)(NO3)] [Ln = Dy (4), Ho (5), Er (6), and Tb (7)]. Complexes 1 and 2 emitted yellow-green and red light under excitation with light of a certain wavelength. Interestingly, 4-7 exhibited a superimposition of the luminescence of 1 and 2. To our knowledge, this is the first example of the use of different organic light-emitting ligands to adjust the fluorescence of Ln(iii) complexes. Moreover, the series of complexes [Ln(L3)2(L2)(NO3)] [Ln = Dy (8), Ho (9), Er (10), and Tb (11)] were also obtained under the same conditions by replacing L1 with L3. In the way that was expected, 8-11 exhibited a superimposition of the luminescence of 2 and 3. Density functional theory (DFT) calculations of electron cloud density showed that the electron cloud densities of complexes 4 and 8 are mainly concentrated in the quinoline rings. Furthermore, analysis of the molecular ion peaks of complexes 4-11 obtained by electrospray mass spectrometry (ESI-MS) showed that only the 1,10-phenanthroline ligand was discovered to dissociate in the solution state. Magnetic measurements of the Dy-containing complexes revealed features of field-induced single-ion magnet behavior.

Investigation of magneto-structural correlation based on a series of seven-coordinated β-diketone Dy(iii) single-ion magnets with C2v and C3v local symmetry.

A series of four β-diketone mononuclear Dy(iii) complexes, namely Dy(Hthd)3(MeOH)·2,5-Py (1), Dy(Hthd)3(Tppo) (2), Dy(Hthd)3(PyNO) (3), and Dy(Hthd)3(4-PyNO) (4) (Hthd = 2,2,6,6-tetramethyl-3,5-heptanedione, 2,5-Py = 2,5-pyridine, Tppo = triphenylphosphine oxide, PyNO = pyridine N-oxide, 4-PyNO = 4-methylpyridine N-oxide), have been isolated by reactions of Hthd, DyCl3·6H2O and an auxiliary ligand. X-ray crystallographic analysis revealed that complexes 1-4 are all seven-coordinated mononuclear structures with C2v or C3v local symmetry. The magnetic measurements of complexes 1-4 revealed their field-induced single-ion magnet (SIM) behavior, due to mixed low-lying states. The crystal field parameters and orientations of the magnetic easy axes were obtained from simulations of the DC magnetic data and an electrostatic model calculation. The magneto-structural correlation of seven-coordinated Dy(iii) SIMs with low-symmetry coordination environments was further discussed.

Slow magnetic relaxation in mononuclear complexes of Tb, Dy, Ho and Er with the pentadentate (N3O2) Schiff-base dapsc ligand

AC measurements revealed field-induced single-ion magnet behavior of Dy and Er Kramers ion complexes. The properties of complexes were rationalized by superposition CF model.

Chiral bis(phthalocyaninato) terbium double-decker compounds with enhanced single-ion magnetic behavior

Introducing chirality into bis(phthalocyaninato) terbium single-ion magnets was revealed to be an alternative method towards enhancing magnetic behavior.

Field-Induced Single-Ion Magnet Behaviour in Two New Cobalt(II) Coordination Polymers with 2,4,6-Tris(4-pyridyl)-1,3,5-triazine

We herein reported the syntheses, crystal structures, and magnetic properties of a two-dimensional coordination polymer {[CoII(TPT)2/3(H2O)4][CH3COO]2·(H2O)4}n (1) and a chain compound {[CoII(TPT)2(CHOO)2(H2O)2]}n (2) based on the 2,4,6-Tris(4-pyridyl)-1,3,5-triazine (TPT) ligand. Structure analyses showed that complex 1 had a cationic hexagonal framework structure, while 2 was a neutral zig-zag chain structure with different distorted octahedral coordination environments. Magnetic measurements revealed that both complexes exhibit large easy-plane magnetic anisotropy with the zero-field splitting parameter D = 47.7 and 62.1 cm−1 for 1 and 2, respectively. This magnetic anisotropy leads to the field-induced slow magnetic relaxation behaviour. However, their magnetic dynamics are quite different; while complex 1 experienced a dominating thermally activated Orbach relaxation at the whole measured temperature region, 2 exhibited multiple relaxation pathways involving direct, Raman, and quantum tunneling (QTM) processes at low temperatures and Orbach relaxation at high temperatures. The present complexes enlarge the family of framework-based single-ion magnets (SIMs) and highlight the significance of the structural dimensionality to the final magnetic properties.

Designing Single-Ion Magnets and Phosphorescent Materials with 1-Methylimidazole-5-carboxylate and Transition-Metal Ions.

Detailed structural, magnetic, and photoluminescence (PL) characterization of four new compounds based on 1-methylimidazole-5-carboxylate (mimc) ligand and transition metal ions, namely [Ni(mimc)2(H2O)4] (1), [Co(μ-mimc)2]n (2), {[Cu2(μ-mimc)4(H2O)]·2H2O}n (3), and [Cd(μ-mimc)2(H2O)]n (4) is reported. The structural diversity found in the family of compounds derives from the coordination versatility of the ligand, which coordinates as a terminal ligand to give a supramolecular network of monomeric entities in 1 or acts as a bridging linker to build isoreticular 2D coordination polymers (CPs) in 2-4. Magnetic direct-current (dc) susceptibility data have been measured for compounds 1-3 to analyze the exchange interactions among paramagnetic centers, which have been indeed supported by calculations based on broken symmetry (BS) and density functional theory (DFT) methodology. The temperature dependence of susceptibility and magnetization data of 2 are indicative of easy-plane anisotropy (D = +12.9 cm-1, E = +0.5 cm-1) that involves a bistable Ms = ±1/2 ground state. Alternating-current (ac) susceptibility curves exhibit field-induced single-ion magnet (SIM) behavior that occurs below 14 K, which is characterized by two spin relaxation processes of distinct nature: fast relaxation of single ions proceeding through multiple mechanisms (Ueff = 26 K) and a slow relaxation attributed to interactions along the polymeric crystal building. Exhaustive PL analysis of compound 4 in the solid state confirms low-temperature phosphorescent green emission consisting of radiative lifetimes in the range of 0.25-0.43 s, which explains the afterglow observed during about 1 s after the removal of the UV source. Time-dependent DFT and computational calculations to estimate phosphorescent vertical transitions have been also employed to provide an accurate description of the PL performance of this long-lasting phosphor.

Influence of the Ligand Field on the Slow Relaxation of Magnetization of Unsymmetrical Monomeric Lanthanide Complexes: Synthesis and Theoretical Studies.

A series of monomeric lanthanide Schiff base complexes with the molecular formulas [Ce(HL)3(NO3)3] (1) and [Ln(HL)2(NO3)3], where LnIII = Tb (2), Ho (3), Er (4), and Lu (5), were isolated and characterized by single-crystal X-ray diffraction (XRD). Single-crystal XRD reveals that, except for 1, all complexes possess two crystallographically distinct molecules within the unit cell. Both of these crystallographically distinct molecules possess the same molecular formula, but the orientation of the coordinating ligand distinctly differs from those in complexes 2-5. Alternating-current magnetic susceptibility measurement reveals that complexes 1-3 exhibit slow relaxation of magnetization in the presence of an optimum external magnetic field. In contrast to 1-3, complex 4 shows a blockade of magnetization in the absence of an external magnetic field, a signature characteristic of a single-ion magnet (SIM). The distinct magnetic behavior observed in 4 compared to other complexes is correlated to the suitable ligand field around a prolate ErIII ion. Although the ligand field stabilizes an easy axis of anisotropy, quantum tunnelling of magnetization (QTM) is still predominant in 4 because of the low symmetry of the complex. The combination of low symmetry and an unsuitable ligand-field environment in complexes 1-3 triggers faster magnetization relaxation; hence, these complexes exhibit field-induced SIM behavior. In order to understand the electronic structures of complexes 1-4 and the distinct magnetic behavior observed, ab initio calculations were performed. Using the crystal structure of the complexes, magnetic susceptibility data were computed for all of the complexes. The computed susceptibility and magnetization are in good agreement with the experimental magnetic data [χMT(T) and M(H)] and this offers confidence on the reliability of the extracted parameters. A tentative mechanism of magnetization relaxation observed in these complexes is also discussed in detail.

Spin Crossover in Double Salts Containing Six- and Four-Coordinate Cobalt(II) Ions.

The preparation and spectroscopic and structural characterization of three cobalt(II) complexes of formulas [Co(tppz)2](dca)2 (1), [Co(tppz)2][Co(NCS)4]·MeOH (2), and [Co(tppz)2][Co(NCO)4]·2H2O (3) [tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine and dca = dicyanamide] are reported here. Compounds 1-3 have in common the presence of the cationic [Co(tppz)2]2+ entity where each mer-tridentate tppz ligand coordinates to the cobalt(II) ion equatorially through two pyridyl donors and axially via the pyrazine, completing the six-coordination. The electroneutrality is achieved by the organic dca group (1) and the anionic tetrakis(thiocyanato-κN)cobaltate(II) (2) and tetrakis(cyanato-κN)cobaltate(II) (3) complexes. Direct current (dc) magnetic susceptibility measurements of 1 in the temperature range 1.9-400 K show the occurrence of a thermally induced spin crossover behavior of the [Co(tppz)2]2+ unit from a high spin (S = 3/2) at higher temperatures to a low-spin (S = 1/2) at lower temperatures, with the low spin phase being reached at T ≤ 200 K. X-band electron paramagnetic resonance (EPR) measurements in solution at low temperatures were used to characterize the low spin state. An analytical expression based on the combination of the spin-orbit coupling and both first- and second-order Zeeman effects for a d7 electronic configuration was used to fit the magnetic data of 1, the values of the best-fit parameters being Cvib = 0.1367(9), λ = -168(2) cm-1, α = 1.12(1), Δ = 1626(15) cm-1, and gLS = 2.12(1). The magnetic behavior of the four-coordinate cobalt(II) ions [Co(NCS)4]2- (2) and [Co(NCO)4]2- (3) with a 4A2 ground state overlaps with the spin crossover of the [Co(tppz)2]2+ entity, the abrupt decrease of the χMT product below 15.0 K being due to zero-field splitting effects between the spin components |±1/2> and |±3/2>. The combined analysis of the dc magnetic data and the Q-band EPR spectra in the solid state of 2 and 3 led to the following sets of best-fit parameters: Cvib = 0.105(5), λ = -170(4) cm-1, α = 1.10(2), Δ = 1700(25) cm-1, gLS = 2.10(1), gHS = 2.27(1), and |D| = 3.80(2) cm-1 (2) and Cvib = 0.100(1), λ = -169(5) cm-1, α = 1.10(3), Δ = 1500(30) cm-1, gLS = 2.10(1), gHS = 2.28(1), and |D| = 4.30(2) cm-1 (3). Some evidence of slowing of the relaxation of the magnetization has been found in the out-of-phase ac signal at very low temperatures under applied dc fields of 0.1-0.4 T for 3, suggesting the occurrence of single-ion magnet behavior of its [Co(NCO)4]2- anionic entity.

Designing Ligands to Isolate ZnLn and Zn2Ln Complexes: Field-Induced Single-Ion Magnet Behavior of the ZnDy, Zn2Dy, and Zn2Er Analogues.

A new H3L Schiff base ligand with three defined compartments, namely, two internal NNO and one external O6, was designed to allocate metal ions of different size. This ligand allows isolating heterodinuclear [ZnLn(HL)(NO3)(OAc)(D)](NO3) (Ln = Tb, D = H2O, ZnTb; Ln = Dy, D = CH3OH, ZnDy; and Ln = Er, D = CH3OH, ZnEr) complexes, where one of the NNO pockets allocates a zinc(II) ion, while the other one is empty, or heterotrinuclear [Zn2Ln(L)(NO3)2(OAc)2(H2O)] (Ln = Dy, Zn2Dy and Ln = Er, Zn2Er) compounds, where each NNO compartment accommodates ZnII. All these compounds crystallize with different solvates, and their structures were unequivocally determined by single-crystal X-ray diffraction studies. Complexes ZnDy, Zn2Dy, and Zn2Er behave as single-molecule magnets in the presence of an external dc field of 1000 Oe, with Ueff values of 41.05, 47.69, and 20.81 K, respectively, while ZnTb and ZnEr do not.

Mononuclear Dy(III) complex based on bipyridyl-tetrazolate ligand with field-induced single-ion magnet behavior and luminescent properties

A new mononuclear Dy(III) complex, namely {[Dy(tbpy)2(NO3)(DMF)]⋅DMF} (1) (tbpy=6-(tetrazol-5-yl)-2,2′-bipyridine, DMF=N,N-dimethylformamide), has been synthesized and characterized. Structural measurements reveal that the Dy(III) ion displays a distorted tricapped trigonal prism with two mono-anionic tridentate chelating ligands, nitrate anion and N,N-dimethylformamide molecule. Magnetic investigation indicates that the title complex 1 exhibits field-induced single-ion magnet behavior. Moreover, the studies on photoluminescence properties show that complex 1 displays relatively strong fluorescent emission, which are typical narrow emission bands of lanthanide ion.

Photoluminescent and Slow Magnetic Relaxation Studies on Lanthanide(III)-2,5-pyrazinedicarboxylate Frameworks.

In the series described in this work, the hydrothermal synthesis led to oxidation of the 5-methyl-pyrazinecarboxylate anion to the 2,5-pyrazinedicarboxylate dianion (2,5-pzdc) allowing the preparation of three-dimensional (3D) lanthanide(III) organic frameworks of formula {[Ln2(2,5-pzdc)3(H2O)4]·6H2O}n [Ln = Ce (1), Pr (2), Nd (3), and Eu (4)] and {[Er2(2,5-pzdc)3(H2O)4]·5H2O}n (5). Single-crystal X-ray diffraction on 1-5 reveals that they crystallize in the triclinic system, P1̅ space group with the series 1-4 being isostructural. The crystal structure of the five compounds are 3D with the lanthanide(III) ions linked through 2,5-pzdc2- dianions acting as two- and fourfold connectors, building a binodal 4,4-connected (4·648)(426282)-mog network. The photophysical properties of the Nd(III) (3) and Eu(III) (4) complexes exhibit sensitized photoluminescence in the near-infrared and visible regions, respectively. The photoluminescence intensity and lifetime of 4 were very sensitive due to the luminescence quenching of the 5D0 level by O-H oscillators of four water molecules in the first coordination sphere leading to a quantum efficiency of 11%. Variable-temperature magnetic susceptibility measurements for 1-5 reveal behaviors as expected for the ground terms of the magnetically isolated rare-earth ions [2F5/2, 2H4, 4I9/2, 7F0, and 4I15/2 for Ce(III), Pr(III), Nd(III), Eu(III), and Er(III), respectively] with MJ = 0 (2 and 4) and ±1/2 (1, 3, and 5). Q-band electron paramagnetic resonance measurements at low temperature corroborate these facts. Frequency-dependent alternating-current magnetic susceptibility signals under external direct-current fields in the range of 100-2500 G were observed for the Kramers ions of 1, 3, and 5, indicating slow magnetic relaxation (single-ion magnet) behavior. In these compounds, τ-1 decreases with decreasing temperature at any magnetic field, but no Arrhenius law can simulate such a dependence in all the temperature range. This dependence can be reproduced by the contributions of direct and Raman processes, the Raman exponent (n) reaching the expected value (n = 9) for a Kramers system.