Structural and magnetic properties of lanthanide–tetraphenol–cyclen complexes: slow magnetic relaxation of the Nd(III) complex

Abstract

Reactions of the macrocycle 1,4,7,10-tetrakis(2,4-dimethyl-1-hydroxy-6-methyl-benzyl)-1,4,7,10-tetraazacyclododecane (H₄L^Me,Me) with LnCl₃·6H₂O afforded the complexes [Ln(HL^Me,Me)]·MeOH (Nd: 1, Gd: 2, Tb: 3, Ho: 4) and the corresponding analogous ligand 1,4,7,10-tetrakis(2-chloro-4-dimethyl-1-hydroxy-6-methyl-benzyl)-1,4,7,10-tetraazacyclodo decane (H₄L^Me,Cl) with HoCl₃·6H₂O yielded [Ho(HL^Me,Cl)]·2MeOH (5). All complexes were spectroscopically and structurally characterized, and their magnetic properties were investigated over a temperature range of 2–300 K. They display similar geometrical structural features with coordination number seven around the central Ln(III) ion. The Nd(III) complex, [Nd(HL^Me,Me)]·MeOH (1), revealed a single-magnet behavior with magnetic relaxation occurring through a combination of Orbach and Raman processes. Both static (dc) and dynamic (ac) magnetic properties were rationalized using ab initio calculations. Complex 1 represents one of the rare Nd(III) single-ion magnets exhibiting slow relaxation dynamics.

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Introduction

The coordination compounds of lanthanide ions, Ln^III, show potential for a wide range of applications in the industrial and medical fields. For example, single-molecule magnets (SMMs) of lanthanides due to their large magnetic moments and magnetic anisotropy have emerged as promising ultra-high-density storage materials in quantum computers and spintronics.^1–8 Among lanthanide ions, the most used are heavy elements such as Tb^III,^9,10 Dy^III,^11–13 and Er^III14–16 because they have produced SMMs with magnetic bistability from liquid helium up to liquid nitrogen temperature. Nevertheless, SMMs involving light lanthanide ions are starting to emerge and are of fundamental interest to increase the scientific level of understanding of their magnetic properties.¹⁷ Moreover, due to their luminescence emission properties, they were found to be interesting in a variety of fields, which include bioimaging and sensing in biology,^18–23 fluorescent lamps and lasers²⁴ in light devices, biomarkers, and on/off switches,²⁵ in monitoring the binding sites of proteins, and as novel contrast agents in cancer diagnosis,^18,20,26 magnetic resonance imaging (MRI),^21,22,27 and sensing,^{18,19,21,24,28} as well as in monitoring the progress of photopolymerization processes (e.g., controlling the thickness of polymer coatings using the fluorescence probe technique (FPT)).²⁹

The most common organic chromophores employed to coordinate Ln(III) ions are those containing O-donors or mixed N-, O-donor atoms, where many of these compounds resulted in the construction of numerous Ln(III) complexes with interesting physicochemical properties. These complexes resulted not only in different geometrical structures, but also in luminescent emission molecules in the Vis-NIR regions and they exhibited a slow magnetization of relaxation phenomenon. These properties attracted attention towards designing Ln(III) complexes that are based on novel organic ligands. One of these ligands is 1,4,7,10-tetraazacyclododecane (cyclen), which was extensively functionalized with carboxylates,^27,30 amides,³¹ phosphonates and phosphinate³² arms, as well as mixed groups.^22,28,33 The coordination compounds of Ln(III) complexes of cyclen bearing phenolic arms have received little attention.^27,33–38

Therefore, the focus of this work is to synthesize symmetrical cyclen-tetraphenol derivatives (H₄L^X,Y, Scheme 1) and their corresponding Ln(III) complexes with the hope that the O-, N-donors of the ligand, H₄L^X,Y, may tune the binding, geometrical and magnetic properties of the resulting products. In particular, the following section focuses on the single X-ray diffraction structures of [Nd(HL^Me,Me)]·MeOH (1), [Gd(HL^Me,Me)]·MeOH (2), [Ho(HL^Me,Me)]·MeOH (4) and [Ho(HL^Me,Cl)]·2MeOH (5) complexes, and their magnetic properties were investigated by measuring the magnetic susceptibility. Finally, ab initio calculations were carried out to rationalize both the static (dc) and dynamic (ac) magnetic properties of the field-induced SMM [Nd(HL^Me,Me)]·MeOH (1).

Scheme 1 Synthesis of the cyclen-tetraphenol, H₄L^X,Y.

Results and discussion

Synthesis and basic characterization

1,4,7,10-Tetrakis(4-chloro-1-hydroxy-2-methyl-6-benzyl)-1,4,7,10-tetraazacyclo-dodecane (H₄L^Me,Cl) was synthesized using a procedure similar to that recently described for 1,4,7,10-tetrakis(2,4-dimethyl-1-hydroxy-6-benzyl)-1,4,7,10-tetraazacyclododecane (H₄L^Me,Me), outlined in Scheme 1. The interaction of equimolar amounts of Ln(III) nitrates or chlorides (Ln = Sm or Yb) and the tetraphenol ligands H₄L^X,Y in methanol and in the presence of four equivalents of Et₃N yielded the neutral species [Ln(HL^X,Y)]·solvent, where (HL^X,Y)³⁻ is the monoprotonated tribasic ligand. The isolated complexes were structurally characterized by elemental microanalyses, spectroscopic techniques (IR and ESI-MS) and single crystal X-ray diffraction. The preparation of the complexes under investigation was straightforward and was performed through heating a methanolic solution containing LnCl₃·6H₂O, H₄L^X,Y, and Et₃N in a stochiometric ratio of 1 : 1 : 4. Single crystals of X-ray quality were obtained from diluted solutions or recrystallization from CH₃CN. In fact, the preparation was also conducted in the presence of a slight excess of tetramethyl ammonium chloride, (CH₃)₄NCl, in an attempt to isolate the anionic complexes, (CH₃)⁴⁺[Ln(L^X,Y)]⁻, but the reactions resulted in the formation of the [Ln(HL^X,Y)]·solvent, in which their non-electrolytic nature was confirmed by measuring their molar conductivity in acetonitrile (Λ_M < 7 Ω⁻¹ m² mol⁻¹). This behaviour was also observed in other related Ln(III)-based cyclen-tetra-phenolates [Ln^III(HL^X,Y)] with Ln^III = La, Pr, Tb, Er, Dy; and HL^X,Y = HL^Me,Me.^16–19 The IR spectra of the complexes 1–5 together with the ligand H₄L^Me,Cl are illustrated in Fig. S1–S6, respectively; they display a similar characteristic pattern with a very weak broad vibrational band over the 3190–3235 cm⁻¹ region, attributable to the ν_as(O–H) stretching vibration of the remaining uncoordinated phenolic group and CH₃OH. A series of weak bands were found in the 2980–2850 cm⁻¹ region, which are assigned to ν_as(C–H) of the aromatic and aliphatic C–H bonds. The ESI-MS of the complexes in CH₃CN exhibit m/z values, which were in complete agreement with those of the protonated complex ion [Ln(HL^x,y) + H]⁺ (Fig. S7–S10). The UV spectra of the compounds, and H₄L^Me,Cl, measured in CH₃CN (Fig. S11–14), display a similar spectral pattern where three relatively strong broad bands were observed around 200, 250 and 300 nm, most likely attributed to σ → π*, π →π* and n → π* transitions, respectively, within the ligand.

Description of the structures

The four complexes, [Ln(HL^Me,Me)]·MeOH (Ln = Nd, 1; Gd, 2; Ho, 4) and [Ho(HL^Me,Cl)]·2MeOH (5), together with the earlier published [Tb(HL^Me,Me)]·MeOH (3) by Nakai et al.^22a show quite similar molecular structures. Only the structural features of the Nd complex 1 are described here. Compound 1 crystallizes in the orthorhombic space group Pca2₁ with Z = 4 (Table S1). A perspective view is presented in Fig. 1a. The structure consists of a monomeric and neutral [Nd(HL^Me,Me)] unit and one non-coordinated methanol solvent molecule. The Nd(III) centre is coordinated by the four N donor atoms (N1–N4) of the 1,4,7,10-tetraazacyclododecane ring with Nd–N bond lengths of 2.756(2), 2.714(2), 2.696(2), and 2733(2) Å, respectively, and by three oxygen donor atoms (O2–O4) of the 2,4-dimethylphenolic arms with Nd–O bond lengths of 2.239(13), 2.292(2), and 2206(2) Å, respectively, while the protonated oxygen atom O1 of the last neutral 2,4-dimethylphenol arm is not coordinated to the metal centre (Table S2). The NdN₄O₃ coordination figure (Table S3) can be described as a distorted capped octahedron of C_3v symmetry (CShM(C_3v) 1.438). The distortion is visualized by continuous shape measurement performed using SHAPE 2.1 (Table S4).³⁹ The bond angles N–Nd–N, N–Nd–O and O–Nd–O are 66.06(6)–103.70(7), 73.74(7)–174.41(7) and 93.10(7)–97.67(7)°, respectively (Table S2). O1 and O5 of the methanol molecule act as donors for hydrogen bonds of type O–H⋯O to O3 of the phenolic arms and O5 of the adjacent methanol molecule to generate a supramolecular system oriented along the b-axis of the unit cell (Fig. 1b and Table S5).

Fig. 1 (a) A perspective view of the molecular structure of 1. (b) Crystal packing of 1 highlighting the hydrogen bonds.

The molecular structures of compounds 2, 4 and 5, and their corresponding packing plots are shown in Fig. S15–S17, respectively, and their SHAPE measurements (Table S4) and possible hydrogen bonding are given in Tables S6–S8, respectively.

Moreover, one could remark that the change of one methyl group by one chloride leads to a change of space group of crystallization from orthorhombic Pca2₁ to monoclinic P2₁/n. From a molecular point of view, such chemical modification leads to a slight decrease in the coordination polyhedron distortion (from 1.243 to 0.904) around the Ho(III) centre.

Magnetic properties of the compounds

Static (dc) magnetic investigation. The temperature dependence of χ_MT (where χ_M is the magnetic susceptibility and T is the temperature) for the samples [Nd(HL^Me,Me)]·MeOH (1), [Gd(HL^Me,Me)]·MeOH (2), [Tb(HL^Me,Me)]·MeOH (3), [Ho(HL^Me,Me)]·MeOH (4) and [Ho(HL^Me,Cl)]·2MeOH (5) is represented in Fig. 2, showing room temperature values of 1.52 cm³ K mol⁻¹, 7.83 cm³ K mol⁻¹, 11.67 cm³ K mol⁻¹, 13.76 cm³ K mol⁻¹ and 13.75 cm³ K mol⁻¹, respectively. These values are in agreement with the expected values of 1.64 cm³ K mol⁻¹ (for one Nd(III) with a ⁴I_9/2 ground state (GS) and g_J = 8/11), 7.88 cm³ K mol⁻¹ (for one Gd(III), ⁸S_7/2 GS, g_J = 2), 11.82 cm³ K mol⁻¹ (for one Tb(III) with a ⁷F₆ ground state (GS) and g_J = 3/2) and 14.07 cm³ K mol⁻¹ (for one Ho(III) with a ⁵I₈ ground state (GS) and g_J = 5/4).⁴⁰ Upon cooling, χ_MT decreases monotonically down to 0.46 cm³ K mol⁻¹, 6.12 cm³ K mol⁻¹, 8.70 cm³ K mol⁻¹ and 7.80 cm³ K mol⁻¹ at 2 K for [Nd(HL^Me,Me)]·MeOH, and 3–5, respectively. It is worth noticing that the χ_MT product remains almost constant for the entire temperature range of 2–300 K because of the isotropic character of the Gd(III) ion, and such magnetic behaviour indicates an absence of significant magnetic interaction. The decreases observed for the other compounds could be mainly attributed to thermal depopulation of the m_J states of the lanthanide ions.

Fig. 2 Thermal dependence of the χ_MT product between 2 and 300 K for [Nd(HL^Me,Me)]·MeOH 1 (purple), [Gd(HL^Me,Me)]·MeOH 2 (black), [Tb(HL^Me,Me)]·MeOH 3 (green), [Ho(HL^Me,Me)]·MeOH 4 (red) and [Ho(HL^Me,Cl)]·2MeOH 5 (blue). The full orange line corresponds to the calculated data.

Magnetization for the five complexes at 2 K is depicted in Fig. S18 with experimental values of 4.86 Nβ, 1.47 Nβ, 5.02 Nβ, 1.46 Nβ and 9.89 Nβ at 50 kOe, respectively.

Dynamic (ac) magnetic investigation. The in-phase and out-of-phase components of the ac susceptibility for all the compounds were measured using immobilized crystalline powders. No out-of-phase signal was detected in the zero magnetic field in the 1–1000 Hz frequency range. Such absence of out-of-phase component of the ac susceptibility in 0 Oe is commonly attributed to efficient quantum tunnelling of the magnetization (QTM) leading to fast magnetic relaxation. It is well-known that such QTM can be cancelled by applying an external magnetic field.⁴¹ Indeed the field dependence of the magnetic susceptibility for [Nd(HL^Me,Me)]·MeOH (Fig. 3a and Fig. S19) revealed the appearance of an out-of-phase component of the magnetic susceptibility. Both in-phase and out-of-phase components of magnetic susceptibility can be simultaneously fitted using an extended Debye model²⁹ to extract the relaxation time (τ) (eqn (S1) and Table S7). The best fit of the resulting field dependence of the relaxation times τ vs. H was obtained using eqn (1) (Fig. 3b):

Fig. 3 (a) Field dependence of the out-of-phase component of the magnetic susceptibility at 2 K in the 0–4000 Oe field range for [Nd(HL^Me,Me)]·MeOH. (b) Field dependence of the magnetic relaxation time (full black circles) at 2 K in the field range of 0–4000 Oe with the corresponding best fit depicted in the full red line. The separated thermally activated Orbach + Raman, QTM and direct processes are drawn in dashed blue, black and green lines, respectively. (c) Frequency dependence of the out-of-phase component of the magnetic susceptibility at 3200 Oe in the 2–7 K temperature range. (d) Thermal dependence of the magnetic relaxation time for [Nd(HL^Me,Me)]·MeOH under a 3200 Oe applied magnetic field in the 2–4.8 K temperature range (full black circles). Full red line is the best-fitted curve (see text) while the dashed lines are the Orbach (black) and Raman (blue) contributions.

From left to right, the terms are the expressions of QTM, direct and thermally activated (Orbach + Raman) contributions. The best-fitted parameters are: B₁ = 260.5(12) s⁻¹, B₂ = 2.02(19) × 10⁻⁶ Oe⁻², B₃ = 4.84(41) × 10⁻¹⁴ s⁻¹ K⁻¹ Oe⁻⁴ and B₄ = 77.1(17) s⁻¹ for [Nd(HL^Me,Me)]·MeOH. Under an applied field of 3200 Oe, the complex shows frequency dependences of the out-of-phase signal of magnetization (Fig. 3c and Fig. S20), which can be analysed in the framework of the extended Debye model.⁴² The temperature dependence of the relaxation time is plotted and depicted in Fig. 3d (Table S8) and the normalized Argand (Fig. S21) concluded that more than 90% of the sample was slowly relaxing under the selected applied dc field. It is worth noticing that the remaining non-relaxing fraction is given by the interception of the semi-circle shape curve with the x axis (Fig. S21).

The thermal variation of the relaxation time could be fitted using eqn (2) considering a combination of Raman and Orbach processes.

The best fit was obtained with C = 13.8(14) K⁻ⁿ s⁻¹ and n = 2.69(12), and τ₀ = 2.20(32) × 10⁻⁷ s and U_eff = 33.5(9) K, where C and n are the constant and exponent factor of the Raman process, τ₀ and U_eff are the relaxation time and the effective energy barrier of the Orbach relaxation process. The expected n value for Kramers ions should be 9,⁴³ but it is well-known that for molecular systems, the presence of both acoustic and optical phonons can lead to lower values, between 2 and 7.^16,44,45 It is worth noticing that 1 is one of the few examples of a Nd(III) complex displaying slow magnetic relaxation reported in the literature (Table 1).^17a–h,46 In such systems, an efficient magnetic relaxation through a QTM process was identified due to the transversal components of the magnetic anisotropy, leading to a lack of out-of-phase contribution of the magnetic susceptibility in zero applied field.

Table 1 Literature review of Nd(III)-based SMMs

Compounds	Orbach process	Under-energy processes	Magnetic behaviour	Ref.
Li(DME)3[Nd(L^1)2]	U eff = 21.0K	—	Field-induced	48
L^1 = 1,4-bis(trimethylsilyl)-cyclooctatetraenyl dianion	τ 0 = 5.5 × 10^−5 s		SMM (H = 1000 Oe)
[Nd(L^2)3] (L^2 = trispyrazolylborate)	U eff = 4.0K	—	Field-induced	49
	τ 0 = 4.2 × 10^−5 s		SMM (H = 100 Oe)
{[Nd2(L^3)6(H2O)4]·2H2O}n (HL^3 = cyanoacetic acid)	U eff = 26.6K	—	Field-induced	50
	τ 0 = 1.75 × 10^−7 s		SMM (H = 1500 Oe)
{[Nd(L^4)3(H2O)2]·2CH3CN}n	U eff = 27.4K	—	Field-induced	51
L^4 = 3,5-dinitrobenzoic acids	τ 0 = 4.1 × 10^−7 s		SMM (H = 2500 Oe)
{[Nd(L^5)2(CH3COO)(H2O)2]}n	U eff = 28.8K	—	Field-induced	51
L^5 = 2,4-dinitrobenzoic acids	τ 0 = 3.1 × 10^−7 s		SMM (H = 3500 Oe)
(NH2Me2)3{[Nd(Mo4O13)(DMF)4]3(L^6)2}·8DMF	U eff = 26.7K	Raman	Field-induced	17a
L^6 = 1,3,5-benzenetricarboxylate	τ 0 = 1.41 × 10^−7 s		SMM (H = 500 Oe)
[Nd(L^7)3(H2O)2]	U eff = 84.1K	QTM, Raman, direct	Field-induced	17b
L^7 = C4H3OCOO	τ 0 = 1.04 × 10^−13 s		SMM (H = 1200 Oe)
[Nd6]	U eff = 3.4K	—	Field-induced	17d
(L = pyridyl-pyrazolyl-based ligand, (9E)-N′-(1-(6-((E)-1-(5-(pyridine-2-yl)-1H-pyrazole-3-carboylimino)ethyl)pyridin-2-yl)-ethenamine)-3-(pyridine-2-yl)-1H-pyrazole-5-carbohydrazide)	τ 0 = 3.1 × 10^−4 s		SMM (H = 3000 Oe)
β-Ln(L^8)(SO4)(H2O)2	U eff = 33.9K	QTM, Raman, direct	Field-induced	17e
HL^8 = 2-quinolinephosphonic acid	τ 0 = 2.44 × 10^−8 s		SMM (H = 2000 Oe)
γ-Ln(L^8)(SO4)(H2O)2	U eff = 18.2K	QTM, Raman, direct	Field-induced	17e
HL^8 = 2-quinolinephosphonic acid	τ 0 = 8.57 × 10^−7 s		SMM (H = 2000 Oe)
[Nd2(L^9)2(acac)2]	—	QTM, Raman	Field-induced	17f
H2L^9 = (N1,N3-bis(3-ethoxysalicylidene)diethylenetriamine)			SMM (H = 2400 Oe)
[Nd2(L^9)2(NO3)2]	—	QTM, Raman	Field-induced	17f
H2L^9 = (N1,N3-bis(3-ethoxysalicylidene)diethylenetriamine)			SMM (H = 2200 Oe)
K14Na6H4[Nd{(As2W19O67(H2O))(H2O)2}2(C2O4)]·64H2O	U eff = 6.8K	—	Field-induced	17g
	τ 0 = 5.48 × 10^−12 s		SMM (H = 500 Oe)
[Nd2(L^10)2(H2O2)2]Cl2	U eff = 32.3K	QTM	Field-induced	17h
H2L^10 = N,N′-bis(2-hydroxybenzyl)-N,N′-bis(2-methyl-pyridyl)ethylenediamine	τ 0 = 1.48 × 10^−6 s		SMM (H = 1500 Oe)
{[Ln(µ2-L^11)3·(H2O)2]·H2O}n	U eff = 28K	QTM	Field-induced	17i
HL^11 = 3,5-dinitrobenzoic acid	τ 0 = 7.29 × 10^−7 s		SMM (H = 1500 Oe)

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Moreover, the slow magnetic relaxation under an applied field was attributed to an Orbach process with significant contribution of an under-energy barrier such as Direct or Raman processes. A literature search reveals that Nd(III)-based SMMs display field-induced slow magnetic relaxation with a lack of Orbach process (or very weak effective energy barrier). The U_eff limit value reported in the literature for such complexes is about 35 K making 1 one of the Nd(III) field-induced SMMs with the best performance. When determined, the under-energy relaxation processes are efficient as found for 1.

Computational results

To understand the origin of the slow relaxation dynamics of 1, we performed ab initio calculations using OpenMOLCAS 24.02.⁴⁷ The computed five Kramers doublets (KDs) arising from the ⁴I_9/2 ground state of Nd(III) extend up to 503.5 cm⁻¹ (Table S9), suggesting significant electrostatic interactions exerted by the ligand field. The g tensors of the ground state KDs are found to be relatively axial (g_xx = 1.09, g_yy = 1.61 and g_zz = 3.50) with sizable transverse components. The g_zz axis of the ground state is found to lie close to one of the phenolic oxygen atoms and approximately aligned with the cyclen ligand backbone (Fig. 4a). The first excited state KD lies at 91 cm⁻¹ and is characterized by large transverse terms (g_xx = 2.74, g_yy = 2.63 and g_zz = 0.09), revealing easy plane character, and it can facilitate under barrier relaxation via excited state admixture when phonons are available. The angle between the g_zz axis of the first excited KDs and the ground state KDs is 52.8°. The calculated relaxation dynamics of higher excited KDs at 174.4, 268.6 and 503.5 cm⁻¹ establishes the energetic landscape for Orbach-type processes.

Fig. 4 Electronic structure and energy level for complex 1. (a) CASSCF computed g_zz orientation of the ground state KD of complex 1 (H atoms are omitted for clarity). (b) Computed ab initio relaxation dynamics for complex 1. The thick black lines are the Kramers doublets as a function of the magnetic moment. The dashed black lines show possible Orbach pathways. The blue lines are the most probable relaxation pathways for magnetization reversal, and the dotted red lines represent the presence of QTM/TA-QTM between the connecting pairs. The numbers at each arrow are the mean absolute values for the corresponding matrix element of the transition magnetic moment.

Transverse magnetic moments calculated between the ground and excited states confirm efficient tunnelling pathways with QTM (0.448 µ_B) and TA-QTM (0.056 µ_B) contributions, consistent with the experimental data. The computed χT increases from ∼0.38 cm³ K mol⁻¹ at ∼0 K to ∼1.576 cm³ K mol⁻¹ at 300 K, close to the expected free-ion value, while the strong low temperature drop reflects an isolated ground state KD with limited thermal population of excited states. The computed magnetic data agrees with the experimental observations (Fig. 2). Importantly, the transverse components of the ground state KD account for quantum tunnelling of the magnetization (QTM) at zero field, explaining the absence of signals in zero DC field and the appearance of slow relaxation only under an applied bias. These results are in excellent agreement with the experimental fit (Δ = 33.5 K, n = 2.7), which points to a relaxation mechanism dominated by Raman and QTM pathways rather than a high-barrier Orbach process. To further rationalize these findings, we examined the ab initio crystal-field (CF) parameters (Table S10). The CF parameters show that anisotropy is dominated by k = 2 contributions with |B^±1₂| and |B^±2₂| significantly larger than the axial B⁰₂, indicating a strongly rhombic rather than purely axial environment. The higher-order terms (k = 4) are much smaller in magnitude, acting only as fine corrections to the overall CF splitting. This CF landscape leads to a ground state KD with moderate axial character (g_z > g_x,y) but substantial transverse components, consistent with the computed g-tensor and the observed magnetic behaviour. The sizable transverse CF terms rationalize the efficiency of QTM and the dominance of under-barrier relaxation processes such as Raman and field-assisted tunnelling. The wave functional analysis shows that the ground state is predominantly in the m_J = ±9/2 state (Table S11). Overall, the experimental and theoretical results establish complex 1 as one of the rare Nd(III) single-ion magnets exhibiting slow relaxation dynamics driven by a rhombic CF environment and transverse magnetic anisotropy.

Conclusions

The macrocycle 1,4,7,10-tetrakis(2,4-dimethyl-1-hydroxy-6-methyl-benzyl)-1,4,7,10-tetraaza-cyclododecane (H₄L^Me,Me) reacts with LnCl₃·6H₂O and Et₃N in methanol in a 1 : 1 : 4 molar ratio and affords the 7-coordinate complexes [Ln(HL^Me,Me)]·MeOH (Nd: 1, Gd: 2, Tb: 3, Ho: 4), which are isostructural with the previously synthesized complexes with Ln = Dy,²⁴ Er²⁴ and Pr.²⁵ The second ligand H₄L^Me,Cl gave the complex [Ho(HL^Me,Cl)]·2MeOH (5). The application of an external 3200 Oe dc magnetic field for 1 induced the slow relaxation of the magnetization through a combination of Orbach and Raman processes. It is worth noticing that 1 is one of the few examples of Nd(III) complexes displaying slow magnetic relaxation reported in the literature.⁴⁵ A quantitative interpretation of the ac magnetic investigation highlighted one of the highest effective energy barriers among the Nd(III)-based SMMs. Modification of the macrocyclic ligands is in progress to induce other stable seven-coordinated lanthanide complexes with a high symmetry coordination sphere (for instance D_5h).

Experimental section

Materials and physical measurements

4-Chloro-2-methylphenol and 2,4-dimethylphenol were purchased from TCI, whereas 1,4,7,10-tetraazacyclododecane (cyclen) and LnCl₃·6H₂O were purchased Strem Chemicals. All other chemicals were of reagent grade quality. The infrared spectra were recorded on a Cary 630 (ATR-IR) spectrometer. The UV spectra were recorded using an Agilent 8453 HP diode array UV-Vis spectrophotometer. The ¹H NMR spectra were collected on a Bruker AvNeo 700 MHz spectrometer at room temperature. ¹H NMR chemical shifts (δ) are reported in ppm and were referenced internally to residual solvent resonances (CDCl₃: δ_H = 7.26 ppm). The conductivity measurements were performed using a Mettler Toledo Seven Easy conductivity meter, calibrated using the 1413 µS cm⁻¹ conductivity standard. The molar conductivity of the complexes was determined from Λ_M = (1.0 × 10³κ)/[Complex)], where κ is the specific conductance and [Complex] is the molar concentration of the complex. ESI-MS of the compounds was measured in MeOH or CH₃CN on an LC-MS Varian Saturn 2200 Spectrometer. Elemental microanalyses were performed by the Atlantic Microlaboratory, Norcross, Georgia, U.S.A.

Preparation of the compounds

1,4,7,10-Tetrakis(4-chloro-1-hydroxy-2-methyl-6-benzyl)-1,4,7,10-tetraazacyclodo-decane (H₄L^Me,Cl). To a mixture of 1,4,7,10-tetraazacyclododecane (cyclen) (1.723 g, 0.01 mol), 4-chloro-2-methylphenol (5.68 g, 0.04 mol) and Et₃N (4.08 g, 0.04 mol) dissolved in MeOH (60 mL), an aqueous solution of 37% formaldehyde (4.08 g, 0.04 mol HCHO) was added and the resulting solution was refluxed with stirring for four days, during which the colour turns reddish orange. The reaction mixture was allowed to stand at room temperature, and the resulting precipitate was collected by filtration and recrystallized using CHCl₃ and activated charcoal to yield a white precipitate, which was washed with Et₂O and dried in air (yield: 3.53 g, 45.0%). Characterization: m.p. 210–212 °C. Anal. calcd for C₄₀H₄₈Cl₄N₄O₄ (790.646 g mol⁻¹): C, 60.76; H, 6.12; N, 7.09%. Found: C, 60.60; H, 6.12; N, 6.92%. Selected IR bands (ATR, cm⁻¹): 2978 (vw), 2901 (vw), 2848 (w) ν(C–H); 1469 (s), 1362 (m), 1283 (vs), 1229 (vs) ν(C=, C–N and C–O); 1092 (s), 1019 (s), 964 (s), 876 (s), 766 (m). 736 (vs), 570 (m), 542 (m), 470 (m) (Fig. S6). ESI-MS (MeOH): m/z = 791.2500 (100%) (Fig. S22), calcd for [H₄L^Me,Cl + H]⁺ = 791.248 ¹H NMR (CDCl₃), 700 MHz, δ in ppm): δ = 7.03 (s, 1H, Ar-OH protons), 6.98 (s, 1H, Ar-OH); 4.08 centred (HO-phenol); 3.61, 3.19 (s, 2H, CH₂-phenolate); 2.77, 2.51 (s, 4H, CH₂-cyclen); 2.11 (s, 3H, CH₃ protons) (Fig S23). UV (CH₃CN) (λ_nm, ε M⁻¹ cm⁻¹): 205 (2.75 × 104), 247 (1.82 × 104), 286 (7.32 × 103) (Fig. S14).

A general method was used to synthesize the Ln(III) complexes 1–4 as follows: to a mixture containing H₄L^Me,Me (0.138 g, 0.2 mmol) and Et₃N (0.081 g, 0.8 mmol) dissolved in MeOH (25–30 mL), LnCl₃·6H₂O (0.20 mmol) was added and the resulting solution was heated with stirring for 30 min in a steam-bath, then filtered through Celite and allowed to stand at room temperature. The white crystalline compound that separated was collected by filtration, washed with propan-2-ol and Et₂O, and air dried. Single X-ray quality crystals were obtained from dilute solutions. Complex 2 was recrystallized from CH₃CN.

[Nd(HL^Me,Me)]·MeOH (1). Light bluish single crystals (yield: 80 mg, 45.3%). Characterization: anal. calcd for C₄₅H₆₁N₄NdO₅ (882.23 g mol⁻¹): C, 61.26; H, 6,97; N, 6.35%. Found: C, 60.76.42; H, 6.91; N, 6.42%. Selected IR bands (ATR, cm⁻¹): ∼3179 (sh, w) ν(O–H); 2907 (vw) 2849 (w) ν(C–H); 1607 (m), 1469 (vs), 1312 (v), 1271 (s), 1250 (s) ν(C=C, C–N and C–O); 857 (s), 795 (s), 779 (s), 472 (vs) (Fig. S1). ESI-MS (CH₃CN): m/z = 850.3580 (100%), 709.4698 (32.02%) (Fig. S7). Calcd m/z 850.3692 for [Nd(HL^Me,Me) + H]⁺ and 709.4693 for H₄L^Me,Me + H⁺. UV (CH₃CN) (λ_nm, ε M⁻¹ cm⁻¹): 203 (3.22 × 10⁴), 247 (2.64 × 104), 299 (1.03 × 104) (Fig. S11). Λ_M (CH₃CN) = 7.2 Ω⁻¹ cm² mol⁻¹.

[Gd(HL^Me,Me)]·MeOH (2). Shiny white single crystals (yield: 100 mg, 55.9%). Characterization: anal. calcd for C₄₅H₆₁N₄GdO₅ (895.24 g mol⁻¹): C, 60.37; H, 6,87; N, 6.26%. Found: C, 59.90; H, 6.64; N, 6.20%. Selected IR bands (ATR, cm⁻¹): ∼3226 (b, m) ν(O–H); 2907 (vw) 2852 (w) ν(C–H); 1608 (w), 1471 (vs), 1275 (s), 1252 (s), 1078 (s) ν(C=C, C–N and C–O); 526 (m), 488 (vs) (Fig. S2). ESI-MS (CH₃CN): m/z = 864.3718 (100%) (Fig. S8), calcd m/z 864.3699 for [Nd(HL^Me,Me) + H]⁺. UV (CH₃CN) (λ_nm, ε M⁻¹ cm⁻¹): 203 (4.07 × 104), 250 (1.23 × 104), 298 (5.53 × 103) (Fig. S12). Λ_M (CH₃CN) = 4.7 Ω⁻¹ cm² mol⁻¹.

[Tb(HL^Me,Me)]·MeOH (3). This complex was synthesized and characterized before.⁴⁴ It was isolated as colourless single crystals. Characterization: anal. calcd for C₄₅H₆₁N₄O₅Tb (896.92g mol⁻¹): C, 60.26; H, 6,86; N, 6.25%. Found: C, 59.97; H, 6.92; N, 6.34%. Selected IR bands (ATR, cm⁻¹): 3192 (b,w) ν(O–H); 2907 (vw) 2851 (w) ν(C–H); 1608 (m), 1470 (vs), 1315 (s), 1252 (s), 1159 (s), 1078 (s) ν(C=C, C–N and C–O); 856 (m), 812 (m), 745 (s), 489 (vs) (Fig. S3). Λ_M (CH₃CN) = 5.6 Ω⁻¹ cm² mol⁻¹.

[Ho(HL^Me,Me)]·MeOH (4). Shiny white single crystals (yield: 100 mg, 55.9%). Characterization: anal. calcd for C₄₅H₆₁HoN₄O₅ (902.92 g mol⁻¹): C, 59.86; H, 6.81; N, 6.10%. Found: C, 59.81; H, 6.67; N, 6.07%. Selected IR bands (ATR, cm⁻¹): ∼3211 (b, m) ν(O–H); 2908 (vw) 2853 (w) ν(C–H); 1608 (w), 1470 (vs), 1317 (s), 1278 (m), 1253 (s), 1159 (m), 1078 (s) ν(C=C, C–N and C–O); 856 (m), 813 (m), 781 (m), 527 (m), 489 (vs), 471 (m) (Fig. S4). ESI-MS (CH₃CN): m/z = 871.3789 (100%) (Fig. S9), calcd m/z 871.3762 for [Ho(HL^Me,Me) + H]⁺. Λ_M (CH₃CN) = 3.8 Ω⁻¹ cm² mol⁻¹.

[Ho(HL^Me,Cl)]·2MeOH (5). Pale pinkish single crystals (yield: 26 mg, 12.8%). Characterization: anal. calcd for C₄₂H₅₃·Cl₄HoN₄O₆ (1016.636 g mol⁻¹): C, 49.62; H, 5.25; N, 5.51%. Found: C, 49.24; H, 4.94; N, 5.76%. Selected IR bands (ATR, cm⁻¹): ∼3274 (b, w) ν(O–H); 2976 (vw) 2853 (vw), 2897 (vw), 2852 (w) ν(C–H); 1697 (w), 1586 (w), 1461 (vs), 1318 (m), 1289 (m), 1247 (s), 1152 (m), 1075 (m) ν(C=C, C–N and C–O); 937 (m), 904 (m), 876 (m), 853 (m), 791 (m), 771 (s), 738 (s), 651 (m), 575 (s), 415 (vs) (Fig. S5). ESI-MS (CH₃CN): m/z = 953.1569 (100%) (Fig. S10), calcd m/z 953.1547 for [Ho(HL^Me,Cl) + H]⁺. UV (CH₃CN) (λ_nm, ε M⁻¹ cm⁻¹): 204 (1.60 × 105), 256 (5.48 × 104), 300 (1.74 × 104) (Fig. S13). Λ_M (CH₃CN) = 6.7 Ω⁻¹ cm² mol⁻¹.

Single crystal X-ray crystallography

All crystals suitable for single crystal X-ray diffractometry were removed from a vial or a Schlenk and immediately covered with a layer of silicone oil. A single crystal was selected, mounted on a glass rod on a copper pin, and placed in the cold N₂ stream provided by an Oxford Cryosystems cryostream. XRD data collection was performed for compounds 1, 2, 4 and 5 on a Bruker APEX II diffractometer⁵² with the use of an IµS microsource (Incoatec microfocus) sealed tube of Mo Kα radiation (λ = 0.71073 Å) and a CCD area detector. The unit-cell constants and the orientation matrices were determined by the program CELL_NOW.⁵³ Data integration was carried out using SAINT.⁴⁸ Empirical absorption corrections were applied using SADABS.^54,55 TWINABS was used for scaling, empirical absorption corrections and the generation of two different data files, one with detwinned data for structure solution and refinement and a second one for (usually more accurate) structure refinement against total integrated intensities. CELL_NOW⁵³ was used for the determination of the unit cell and twin law, while TWINABS⁵⁶ was used for integration and generation of hkl4- and hkl5-files, which were used in structure solution and refinement. The structures were solved using the intrinsic phasing option in SHELXT⁵⁷ and refined by the full-matrix least-squares procedures in SHELXL^58–61 as implemented in the program SHELXLE.⁶² The space group assignments and structural solutions were evaluated using PLATON.^63,64 Non-hydrogen atoms were refined anisotropically. Several hydrogen atoms of OH moieties were located on a difference map. All other hydrogen atoms were placed in calculated positions corresponding to standard bond lengths and angles and refined using a riding model. Disorder was handled by modelling the occupancies of the individual orientations using free variables to refine the respective occupancy of the affected fragments (PART).⁶⁵ Disordered positions for the solvent of crystallization (2·MeOH) in compound 4 were refined using 60/40 split positions, respectively. Compound 1 was refined as a 2-component inversion twin (BASF 0.030(9)). Centroids and planes were determined and evaluated by the features of the program Olex2.⁶⁶ All crystal structure representations were made with the program Diamond⁶⁷ with all non-carbon atoms displayed as 30% ellipsoids. CIF files were edited, validated and formatted either with the programs encifer,⁶⁸ publCIF,⁶⁹ or Olex2.⁶⁶ CCDC 2489782–2489785 contain the supplementary crystallographic data for compounds 1, 2, 4 and 5, respectively. The Table S1 contains crystallographic data and details of measurements and refinement for compounds 1, 2, 4 and 5.

Magnetic analysis

The dc magnetic susceptibility measurements were performed on a solid polycrystalline sample using a Quantum Design MPMS-XL SQUID magnetometer between 2 and 300 K in an applied magnetic field of 0.02 T for temperatures between 2 and 20 K, 0.2 T for temperatures between 20 and 80 K and 1 T for temperatures between 80 and 300 K. AC magnetic susceptibility measurements were performed using a Quantum Design MPMS-XL SQUID for frequencies between 1 and 1000 Hz and an oscillating field of 3 Oe. These measurements were all corrected for diamagnetic contribution, as calculated with Pascal's constants.

Computational details

All ab initio calculations were performed using OpenMOLCAS 24.02.⁷⁰ OpenMOLCAS is a quantum chemistry package based on a multiconfigurational approach. In this approach, relativistic effects are treated based on a Douglas–Kroll Hamiltonian. The ANO-RCC⁷¹ basis set was employed for all calculations. The following contraction schemes were used: [8s7p5d3f2g1h] for Nd, [4s3p2d1f] for N, [4s3p2d1f] for O, [3s2p] for C and [2s] for H. For Nd, complete active space self-consistent field (CASSCF) calculations were carried out. The ground state atomic term is ⁴I_9/2, which results in five low-lying Kramers doublets. The CASSCF calculation employed an active space of three active electrons in the seven active orbitals, CAS (3,7). With this active space, 35 quartet and 112 doublet states were computed in the CI procedure. The RASSI-SO (restricted active space state interaction - spin orbit coupling)⁷¹ module was then used to mix the spin-free states and account for spin–orbit coupling effects. Finally, the SINGLE_ANISO code⁷² as implemented in MOLCAS was used to compute the g-tensors of Nd(III).

Author contributions

S. S. M., P. C. and F. P.: writing – review and editing, writing – original draft, validation, supervision, project administration, methodology, funding acquisition, formal analysis, data curation, and conceptualization; F. R. L.: synthesis and characterizations of the compounds, writing – original draft, validation, supervision, methodology, and formal analysis; T. G.: writing – original draft, data curation and formal analysis of magnetic measurements; R. C. F., A. T. and F. A. M.: writing – original draft, validation, software, methodology, investigation, formal analysis, and data curation (X-ray data); A. M. S.: investigation and data curation; N. M. H. S.: writing – original draft and formal analysis; G. V.: writing – review and editing, writing – original draft, validation, software, and methodology (computational study). All authors have read and agreed on the published version of the manuscript.

Conflicts of interest

The authors declare no conflicts of interest.

Data availability

The authors will provide the relevant data upon reasonable request.

The data that support the findings of this study are available in the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5nj03826j.

CCDC 2489782–2489785 contain the supplementary crystallographic data for this paper.^73a–d

Acknowledgements

F. P. and T. G. would like to thank the CNRS, Univ. Rennes and the European Commission through the ERC-CoG 725184 MULTIPROSMM (project n. 725184) for the financial support. G. V. and P. C. thank the Heidelberg University and the German Science Foundation (DFG) for the financial support. This study was conducted within the Max Planck School Matter to Life, supported by the German Federal Ministry of Education and Research (BMBF) in collaboration with the Max Planck Society. The authors acknowledge support by the state of Baden-Württemberg through bwHPC and the German Research Foundation (DFG) through grant no INST 40/575-1 FUGG (JUSTUS 2 cluster).

Notes and references

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