Synthesis, Crystal Structures and Magnetic Properties of Trinuclear {Ni₂Ln} (Ln^III = Dy, Ho) and {Ni₂Y} Complexes with Schiff Base Ligands

Abstract

The reaction of the Schiff base ligand o-OH-C₆H₄-CH=N-C(CH₂OH)₃, H₄L, with Ni(O₂CMe)₂∙4H₂O and lanthanide nitrate salts in a 4:2:1 ratio lead to the formation of the trinuclear complexes [Ni₂Ln(H₃L)₄(O₂CMe)₂](NO₃) (Ln = Dy (1), Ho (2), and Y (3)) which crystallize in the non-centrosymmetric space group Pna2₁. The complex cation consists of the three metal ions in an almost linear arrangement. The {Ni₂Ln} moieties are bridged through two deprotonated O_phenolato groups from two different ligands. Each terminal Ni^II ion is bound to two ligands through their O_phenolato, the N_imino atoms and one of the protonated O_alkoxo groups in a distorted octahedral. The central lanthanide ion is coordinated to four O_phenolato oxygen from the four ligands, and four O_carboxylato atoms from two acetates which are bound in the bidentate chelate mode, and the coordination polyhedron is biaugmented trigonal prism, which probably results in a non-centrosymmetric arrangement of the complexes in the lattice. The magnetic properties of 1–3 were studied and showed that 1 exhibits field induced slow magnetic relaxation.

1. Introduction

The study of multifunctional materials, with new or combined properties, is a very active area in material science, as new materials are expected to have a big impact on new technologies that possibly could be the driving force for economic growth in the near future [1]. In multifunctional materials, two or more physical properties are combined on the same entity or different entities or subunits of the material, and one could be used to monitor the other by external or internal stimuli [2]. In this respect, molecular materials could play a vital role, as their diverse properties/functions can be tailored by chemical design and synthesis, following a bottom-up approach and by using principles of coordination chemistry, i.e., the appropriate metal or metals coordinated with the proper ligands. The field of multifunctional Single Molecule Magnets (SMMs) is growing rapidly as it is proved by the pioneering works that have resulted on the synthesis of complexes that combine the SMM behavior with conductivity [3], ferroelectricity [4], dielectricity [5], luminescence [6] and chirality [5,7]. Many examples of pure transition metal or lanthanide complexes exist, which exhibit, in addition to the SMM properties, an extra property that classify them as multifunctional materials [3,6,8].

After the observation of the ferromagnetic coupling of Cu^II with Gd^III in two mixed metal complexes, a route was opened and the 3d/4f mixed metal complexes prepared afterwards have provided excellent examples with SMM properties [9,10,11]. Mixed 3d/4f metal complexes present a strong magnetocaloric effect [12] or luminescence properties [13]. In addition, enantiopure mixed metal complexes combine the SMM properties with ferroelectricity [14] or, in the case of the Zn/Dy [15] complex, it presents additionally luminescence properties. The last examples indicate that the introduction of chirality in a magnetic system is a probable route to multifunctionality as in the case of transition metal complexes [2]. As the trinuclear mixed 3d/4f metal systems have provided interesting examples with SMM properties [16,17], recently we have focused our research on the Ni^II-Ln^III-Ni^II system [18,19]. In these series of compounds, there are examples which present also SMM properties [20,21,22,23] or the magnetocaloric effect [24]. The crystal structure study of the series of [Ni₂Ln(H₃L)₄(O₂CMe)₂](NO₃) (Ln = Sm, Eu, Gd, Tb)] complexes [19] indicate that all the compounds are isomorphous and they crystallize in the centrosymmetric orthorhombic space group Pnnn and the study of their magnetic properties study indicate that the Ln^III = Gd complex presents magnetocaloric behavior. Based on the literature survey for the linear Ni^II-Ln^III-Ni^II system presented in [18], there are cases where the complexes are chiral and, as they crystallize in centrosymmetric space groups, both enantiomers exist in the structure [25,26].

In this study we present the synthesis, crystallographic characterization and magnetic properties of three complexes [Ni₂Ln(H₃L)₄(O₂CMe)₂](NO₃) (Ln = Dy (1), Ho (2), and Y (3)) which were synthesized following the same synthetic paths as previously described [19], but all three complexes crystallize in the non-centrosymmetric Pna2₁ space group which belongs to the mm² point group (following the standard notation). This point group, along with the axial and the m point groups, belong to the seven point groups which lie in the section where interesting electrical or optical properties could coexist. Thus, either the size of lanthanide(III) ion or intermolecular interactions probably result in the crystallization of these complexes to a space group which belongs to the appropriate point group, for the studied material to have multifunctional properties. The crystal structure characteristics of the three complexes are also discussed within this context.

2. Materials and Methods

All manipulations were performed under aerobic conditions using materials as received (Aldrich Co., London, UK). All chemicals and solvents were of reagent grade. The ligand OH-C₆H₄-CH=NC(CH₂OH)₃, H₄L was synthesized as described previously [27]. Elemental analysis for carbon, hydrogen, and nitrogen was performed on a PerkinElmer (Waltham, MA, USA) 2400/II automatic analyzer. Infrared spectra were recorded as KBr pellets in the range 4000–400 cm⁻¹ on a Bruker Equinox 55/S FT-IR spectrophotometer. Variable-temperature magnetic susceptibility measurements were carried out on polycrystalline samples of 1, 2 and 3 by using a Quantum Design PPMS 9T and a SQUID magnetometer (Quantum Design MPMS 5.5). Diamagnetic corrections were estimated from Pascal’s constants.

2.1. Compound Preparations

2.1.1 [Ni₂Dy(H₃L)₄(O₂CMe)₂](NO₃)·3MeOH·H₂O (1·3MeOH·H₂O). Solid Dy(NO₃)₃∙5H₂O (0.25 mmol, 0.1096 g) was added under stirring to a yellow solution of H₄L (1.00 mmol, 0.2252 g) in MeOH (20 mL). The solution was refluxed for three hours until solid Ni(O₂CMe)₂∙4H₂O (0.50 mmol, 0.1244 g) was added and continue refluxing for two more hours. The final green solution was layered with a mixture of Et₂O/n-hexane (1:1 v/v) to afford light green X-ray quality single crystals after approximately two weeks. The material was filtered off and dried in vacuo. Yield: 0.058 g, ~17% based on Dy^III. C₄₈H₆₂N₅O₂₃Ni₂Dy (1) (fw = 1356.96) requires C, 42.49; H, 4.61; N, 5.16%. Found: C, 42.40; H, 4.59; N, 5.14%. FT-IR (KBr pellets, cm⁻¹): 3416 (br), 2980 (w), 2935 (w), 2886 (w), 1600 (vs), 1556 (vs), 1524 (m), 1475 (vs), 1446 (vs), 1408 (m), 1384 (vs), 1349 (m), 1320 (s), 1283 (vs), 1253 (m), 1200 (s), 1166 (m), 1149 (s), 1140 (s), 1053 (vs), 1040 (vs), 977 (m), 946 (s), 922 (m), 891 (s), 864 (m), 816 (s), 775 (m), 761 (vs), 744 (s), 680 (s), 636 (s), 580 (m), 524 (m), 457 (s).

2.1.2 [Ni₂Ho(H₃L)₄(O₂CMe)₂](NO₃)∙2EtOH∙3H₂O (2∙2EtOH∙3H₂O). Solid Ho(NO₃)₃∙5H₂O (0.25 mmol, 0.1103 g) was added under stirring to a yellow solution of H₄L (1.00 mmol, 0.2252 g) in EtOH (20 mL). The solution was refluxed for three hours until solid Ni(O₂CMe)₂∙4H₂O (0.50 mmol, 0.1244 g) was added and refluxing was then continued for two more hours. The final green solution was layered with a mixture of Et₂O/n-hexane (1:1 v/v) to afford light green X-ray quality single crystals after approximately two weeks. The material was filtered off and dried in vacuo. Yield: 0.071 g, ~21% based on Ho^III. C₄₈H₆₂N₅O₂₃Ni₂Ho (2) (fw = 1359.39) requires C, 42.41; H, 4.60; N, 5.15%. Found: C, 42.32; H, 4.58; N, 5.13%. FT-IR (KBr pellets, cm⁻¹): 3443 (br), 2919 (m), 2846 (m), 1602 (s), 1556 (s), 1473 (s), 1444 (s), 1384 (vs), 1353 (sh), 1320 (w), 1284 (s), 1251 (m), 1199 (m), 1150 (m), 1124 (m), 1048 (s), 942 (m), 890 (m), 855 (w), 810 (m), 768 (m), 741 (w), 730 (w), 717 (w), 684 (w), 670 (w), 632 (w), 612 (w), 586 (w), 544 (w), 481 (w), 453 (w).

2.1.3 [Ni₂Y(H₃L)₄(O₂CMe)₂](NO₃)·3MeOH·H₂O (3·3MeOH·H₂O). Solid Y(NO₃)₃∙5H₂O (0.25 mmol, 0.0912 g) was added under stirring to a yellow solution of H₄L (0.50 mmol, 0.1126 g) and NaO₂CMe·3H₂O (0.50 mmol, 0.0680 g) in MeOH (20 mL). The solution was refluxed for three hours until solid Ni(O₂CMe)₂∙4H₂O (0.25 mmol, 0.0622 g) was added and refluxing was then continued for two more hours. The final green solution was layered with a mixture of Et₂O/n-hexane (1:1 v/v) to afford light green X-ray quality single crystals after approximately two weeks. The material was filtered off and dried in vacuo. Yield: 0.048 g, ~18% based on Y^III. C₄₈H₆₂N₅O₂₃Ni₂Y (3) (fw = 1283.37) requires C, 44.92; H, 4.87; N, 5.46%. Found: C, 44.83; H, 4.85; N, 5.44%. FT-IR (KBr pellets, cm⁻¹): 3443 (br), 2940 (w), 2855 (w), 1599 (vs), 1556 (vs), 1474 (vs), 1446 (s), 1384 (vs), 1341 (m), 1317 (m), 1285 (vs), 1253 (m), 1201 (s), 1150 (s), 1125 (m), 1045 (vs), 943 (m), 889 (s), 859 (m), 809 (s), 774 (vs), 740 (m), 687 (m), 634 (m), 611 (m), 583 (m), 546 (w), 523 (w), 478 (w), 453 (m).

2.2. Single Crystal X-ray Crystallography

Crystals of 1∙3MeOH∙H₂O (0.15 × 0.37 × 0.52 mm), 2∙2EtOH∙3H₂O (0.06 × 0.09 × 0.26 mm) and 3∙3MeOH∙H₂O (0.09 × 0.14 × 0.29 mm) were taken from the mother liquor and immediately cooled to −113 °C. Diffraction measurements were made on a Rigaku R-AXIS SPIDER Image Plate diffractometer using graphite monochromated Cu Kα radiation. Data collection (ω-scans) and processing (cell refinement, data reduction and empirical/numerical absorption correction) were performed using the CrystalClear program package [28]. The structures were solved by direct methods using SHELXS v.2013/1 and refined by full-matrix least-squares techniques on F² with SHELXL ver2014/6 [29,30]. The absolute structure of 1–3, which crystallizes in the non-centrosymmetric space group, was determined by refined as 2-component inversion twin using TWIN and BASF instructions, with Flack parameter x, equals to 0.016(4), 0.554(8) and 0.12(2), respectively. Important crystallographic and refinement data are listed in

Table 1. Crystallographic data (see Appendix A) for 1∙3MeOH∙H₂O, 2∙2EtOH∙3H₂O and 3∙3MeOH∙H₂O.

	1∙3MeOH∙H2O	2∙2EtOH∙3H2O	3∙3MeOH∙H2O
Formula	C48H62DyN5Ni2O23·3(CH4O)·(H2O)	C48H62HoN5Ni2O23·2(C2H6O)·3(H2O)	C48H62N5Ni2O23Y·3(CH4O)·(H2O)
F.w.	1471.08	1505.56	1397.49
Space group	Pna21	Pna21	Pna21
a (Å)	26.4216(5)	26.4211(5)	26.4052(5)
b (Å)	10.1714(2)	10.1651(2)	10.1074(2)
c (Å)	22.9439(4)	22.8249(4)	22.8177(4)
V (Å3)	6166.1(2)	6130.2(2)	6089.8(2)
Z	4	4	4
T (°C)	−113	−113	−113
Radiation	Cu Kα 1.54178	Cu Kα 1.54178	Cu Kα 1.54178
ρcalcd, g cm−3	1.585	1.631	1.524
μ, mm−1	7.845	3.806	2.728
Reflections with I > 2σ(I)	9688	9548	9392
R1 a	0.0457	0.0319	0.0611
wR2 a	0.1251	0.0793	0.1651

^a w = 1⁄[σ²(F_o²) + (αP)² + bP] and P = (max F_o²,0) + 2F_c²)/3, a = 0.0786, b = 4.6125 (1); a = 0.0428, b = 6.5995 (2); a = 0.1023, b = 14.2398 (3), R₁ = Σ(|F_o| − |F_c|)/Σ(|F_o|) and wR₂ = {Σ[w(F_o² − F_c²)²]/Σ[w(F_o²)²]}^1/2.

. Further experimental crystallographic details for 1∙3MeOH∙H₂O: 2θ_max = 130°; reflections collected/unique/used, 75,433/10,191 [R_int = 0.0672]/10,191; 793 parameters refined; (Δ/σ)_max = 0.001; (Δρ)_max/(Δρ)_min = 1.298/−1.191 e/ų; R1/wR2 (for all data), 0.0474/0.1269. Further experimental crystallographic details for 2∙2EtOH∙3H₂O: 2θ_max = 130°; reflections collected/unique/used, 65,395/9926 [R_int = 0.0267]/9926; 812 parameters refined; (Δ/σ)_max = 0.003; (Δρ)_max/(Δρ)_min = 0.753/−0.426 e/ų; R1/wR2 (for all data), 0.0337/0.0808. Further experimental crystallographic details for 3∙3MeOH∙H₂O: 2θ_max = 130°; reflections collected/unique/used, 56,144/10,094 [R_int = 0.0430]/10,094; 736 parameters refined; (Δ/σ)_max = 0.003; (Δρ)_max/(Δρ)_min = 1.626/−1.195 e/ų; R1/wR2 (for all data), 0.0659/0.1731. Hydrogen atoms were introduced at calculated positions as riding on bonded atoms. All non-hydrogen atoms were refined anisotropically. Plots of the structure were drawn using the Diamond 3 program package [31].

3. Results

3.1. Synthesis and IR Spectroscopic Characterization

Previously we have reported the trinuclear complexes [Ni₂Ln(H₃L)₄(O₂CMe)₂](NO₃) (Ln^III = Sm, Eu, Gd, Tb), which were isolated by the 4:2:1 H₄L/Ni(O₂CMe)₂∙4H₂O/Ln(NO₃)₃∙6H₂O molar ratio reaction in EtOH [19]. The crystal structure of the {Ni₂Tb} complex revealed that the Tb^III ion is linked to each of the Ni^II ions via two O_phenolato atoms of the (H₃L)⁻ ligands and contains a strictly linear Ni^II-Tb^III-Ni^II moiety. An extensive network of hydrogen bonds, due to the presence of protonated alkoxide groups of the ligands, gave rise to the lattice structure of the complex which consists of two 3D interpenetrating supramolecular diamond-like lattices. The magnetic study of the {Ni₂Gd} complex revealed ferromagnetic coupling between the metal ions leading to ground state of total spin S = 11/2, whereas antiferromagnetic coupling was found for the remaining three complexes. To study the influence of the lanthanide to the nature of the magnetic exchange between the metal ions we have performed analogous reactions with Ln^III = Dy, Ho and the diamagnetic Y^III ion.

The trinuclear complexes 1 and 2 were prepared by the stoichiometric reaction of two equivalents of Ni(O₂CMe)₂∙4H₂O with one equivalent of Ln(NO₃)₃∙5H₂O (1: Dy^III; 2: Ho^III) and four equivalents of H₄L in MeOH (1) or EtOH (2), respectively. The 1:1:2 Ni(O₂CMe)₂∙4H₂O/Y(NO₃)₃∙5H₂O/H₄L molar ratio reaction in MeOH in the presence of two equivalents of NaO₂CMe·3H₂O afforded the trinuclear complex 3. Precipitation of single-crystals of 1–3 was achieved by layering of the reaction solution with mixture of Et₂O/n-hexane. If compounds 1–3 are the only products of the reaction, their formation can be described by the following equation:

$$\begin{array}{l}{2\mathrm{Ni}(\mathrm{O}}_2{\mathrm{CMe})}_2\cdot {4\mathrm{H}}_2\mathrm{O}+{{\mathrm{Ln}(\mathrm{NO}}_3)}_3\cdot {5\mathrm{H}}_2{\mathrm{O}+4\mathrm{H}}_4\mathrm{L}\stackrel{\mathrm{ROH}}{\to }\\ \\ {[\mathrm{Ni}}_2{\mathrm{Ln}(\mathrm{H}}_3{\mathrm{L})}_4{{(\mathrm{O}}_2\mathrm{CMe})}_2{\left]\right(\mathrm{NO}}_3{)+2\mathrm{HNO}}_3{+2\mathrm{HO}}_2{\mathrm{CMe}+13\mathrm{H}}_2\mathrm{O}\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em} \mathbf{1}-\mathbf{3}\end{array}$$

(1)

The excess of acetates in the reaction solution which afforded single crystals of 3 did not increased the deprotonation state of the ligand or the identity of the product. Moreover, complex 3 was the only product of the reaction which was isolated under the specific crystallization conditions.

The IR spectra of the three complexes exhibit broad bands at 3416 and 3443 cm⁻¹ attributed to the ν(OH) vibrations due to the presence of protonated alkoxide groups of the ligands. The band at ~1600 cm⁻¹ is attributed to the ν(C=N) vibration. The shifting of this band to a lower frequency with respect to that of the free ligand (~1635 cm⁻¹) in the spectra of 1–3 suggests coordination of the metal ions through the imino nitrogen. The ν(C-O) stretching frequency of the phenolic oxygen of the ligand is seen at 1395 cm⁻¹ and shifts to a lower frequency in the spectra of all complexes, in the range of 1313–1320 cm⁻¹, indicating coordination to the metal ions [32]. The strong bands at ~1555 and ~1444 cm⁻¹ are attributed to the ν_as(CO₂) and ν_s(CO₂) stretching vibrations of the bidentate chelate acetato ligands. The difference Δ = ν_as(CO₂) − ν_s(CO₂) is 110–112 cm⁻¹ and is in agreement with the low difference values found in rare earth acetates with chelating coordination mode [33,34,35]. The strong band at 1384 cm⁻¹ in the spectra 1–3 is attributed to the presence of the ν₃(E′) [ν_d(NO)] mode of the uncoordinated D_3h ionic nitrates [36].

3.2. Description of the Structures

Compounds [Ni₂Ln(H₃L)₄(O₂CMe)₂](NO₃) (Ln = Dy^III (1), Ho^III (2), Y^III (3)) are isomorphous and crystallize in the non-centrosymmetric orthorhombic space group Pna2₁. The asymmetric unit cell contains one trinuclear cation, one nitrate counteranion and solvate molecules; the latter will not be discussed further. The Ni₂Ln moiety is almost linear with Ni∙∙∙Ln∙∙∙Ni angles 174.6 (1: Dy^III), 173.8 (2: Ho^III) and 174.5° (3: Y^III). The interatomic distances Ni∙∙∙Ln are Dy(1)···Ni(1) = 3.421(1) Å, Dy(1)···Ni(2) = 3.441(1) Å, Ho(1)···Ni(1) = 3.403(1) Å, Ho(1)···Ni(2) = 3.436(1) Å, Y(1)···Ni(1) = 3.405(2) Å, and Y(1)···Ni(2) = 3.433(2) Å. The central Ln^III is bridged to each of the terminal Ni^II ions through two deprotonated O_phenolato atoms from two different ligands (Figure 1a,

Table 2. Selected bond distances (Å) in 1∙3MeOH∙H₂O, 2∙2EtOH∙3H₂O and 3∙3MeOH∙H₂O.

	1∙3MeOH∙H2O	2∙2EtOH∙3H2O	3∙3MeOH∙H2O
Ln(1)-O(11)	2.312(4)	2.293(3)	2.298(6)
Ln(1)-O(31)	2.306(5)	2.300(4)	2.297(6)
Ln(1)-O(21)	2.354(5)	2.347(3)	2.341(6)
Ln(1)-O(1)	2.355(5)	2.330(4)	2.341(6)
Ln(1)-O(41)	2.429(5)	2.421(4)	2.419(6)
Ln(1)-O(44)	2.435(5)	2.416(3)	2.425(6)
Ln(1)-O(43)	2.454(5)	2.446(4)	2.443(6)
Ln(1)-O(42)	2.468(5)	2.443(4)	2.446(6)
Ni(1)-N(1)	2.033(6)	2.023(4)	2.038(8)
Ni(1)-N(11)	2.048(7)	2.044(4)	2.041(8)
Ni(1)-O(11)	2.056(5)	2.049(4)	2.050(7)
Ni(1)-O(1)	2.053(5)	2.053(4)	2.045(6)
Ni(1)-O(2)	2.079(6)	2.075(4)	2.076(7)
Ni(1)-O(12)	2.085(6)	2.090(4)	2.116(7)
Ni(2)-N(31)	2.037(7)	2.050(4)	2.038(8)
Ni(2)-N(21)	2.035(6)	2.037(4)	2.024(8)
Ni(2)-O(21)	2.057(5)	2.053(4)	2.055(6)
Ni(2)-O(31)	2.062(5)	2.063(4)	2.062(7)
Ni(2)-O(32)	2.082(5)	2.076(4)	2.069(7)
Ni(2)-O(22)	2.091(6)	2.091(4)	2.086(8)

). The Ni^II ions are coordinated to two ligands through their O_phenolato, the N_imino nitrogen and one of the protonated O_alkoxo groups in distorted octahedral geometry. The Ln^III ion is coordinated to four O_phenolato atoms from the four ligands, and four O_carboxylato atoms from two acetates which are bound in the bidentate chelate mode. Continuous Shape Measures by using the program SHAPE [37] show that the best-fit polyhedron around the Ln^III ion is the biaugmented trigonal prism, BTPR-8, (Figure 1b) with CShM = 2.89088 (1), 2.82693 (2) and 2.83263 (3).

The arrangement of ligands around the metal center’s creates a Δ-Δ-Δ configuration for the complex at (x, y, z) position (left image at Figure 2) and a Λ-Λ-Λ one for its mirror image (0.5 − x, −0.5 + y, 0.5 + z position, right image, Figure 2). The complexes studied in the present study are isostructural with the complex of {Ni₂Tb} studied previously [19], as is revealed in Figure S1. Although, the CShM parameter differs slightly for the two polyhedrons around Tb^III in {Ni₂Tb} and Dy^III in 1 (Table S1, Figure S2), the movement of O(1) and O(21) (Figure 1a) atoms toward the center of the trigonal prism faces, transforms a square antiprism polyhedron ({Ni₂Tb} case) to a biaugmented trigonal prism one (Dy, Ho and Y in the present study).

In all three structures, different types of hydrogen bonds are observed, i.e., intramolecular ones (

Table 3. Hydrogen bonds in 1∙3MeOH∙H₂O, 2∙2EtOH∙3H₂O and 3∙3MeOH∙H₂O.

Interaction	D···A (Å)	H···A (Å)	D-H···A (°)	Symmetry Operation
1∙3MeOH∙H2O
Intramolecular
O(4)-H(4O)···O(42)	2.695	1.879	163.5	x, y, z
O(13)-H(13O)···O(44)	2.672	1.837	172.4	x, y, z
O(22)-H(22O)···O(52)	2.947	2.154	157.2	x, y, z
O(23)-H(23O)···O(43)	2.666	1.921	147.2	x, y, z
O(34)-H(34O)···O(41)	2.670	1891	153.1	x, y, z
Intermolecular among complexes
O(3)-H(3O)···O(33)	2.703	1.911	156.7	0.5 − x, −0.5 + y, −0.5 + z
O(14)-H(14O)···O(34)	2.672	1.836	173.6	0.5 + x, 2.5 − y, z
O(24)···O(14)	2.919			1 − x, 2 − y, 0.5 + z
O(33)-H(33O)···O(13)	2.678	1.715	161.0	−0.5 + x, 2.5 − y, z
Intermolecular among complexes and lattice solvents
O(12)-H(12O)···O(1 m)	2.731	2.242	117.2	0.5 − x, 0.5 + y, −0.5 + z
O(2)-H(2O)···O(1 w)	2.618	1.858	149.7	0.5 − x, 1.5 − y, z
O(32)-H(32O)···O(2 m)	2.627	1.797	169.5	x, 1 + y, z
2∙2EtOH∙3H2O
Intramolecular
O(4)-H(4O)···O(42)	2.712	1.882	169.6	x, y, z
O(13)-H(13O)···O(44)	2.682	1.849	170.9	x, y, z
O(22)-H(22O)···O(53)	2.855	2.061	157.5	x, y, z
O(23)-H(23O)···O(43)	2.685	1.850	172.0	x, y, z
O(34)-H(34O)···O(41)	2.669	1.837	170.4	x, y, z
Intermolecular among complexes
O(3)-H(3O)···O(33)	2.726	1.895	169.6	0.5 − x, −0.5 + y, −0.5 + z
O(14)-H(14O)···O(34)	2.661	1.823	174.7	0.5 + x, 2.5 − y, z
O(24)-H(24O)···O(14)	2.805	1.979	167.6	1 − x, 2 − -y, 0.5 + z
O(33)-H(33O)···O(13)	2.679	1.841	176.0	−0.5 + x, 2.5 − y, z
Intermolecular among complexes and lattice solvents
O(12)-H(12O)···O(2 e)	2.649	1.835	162.6	0.5 − x, 0.5 + y, −0.5 + z
O(2)-H(2O)···O(1 e)	2.623	1.812	161.7	0.5 + x, 1.5 − y, z
O(32)-H(32O)···O(1 w)	2.657	1.833	166.3	x, 1 + y, z
O(1e)-H(1Oe)···O(2 w)	2.651	1.843	160.9	0.5 − x, 0.5 + y, −0.5 + z
O(2e)-H(2Oe)···O(23)	2.774	1.951	166.2	−0.5 + x, 1.5 − y, z
3∙3MeOH∙H2O
Intramolecular
O(4)-H(4O)···O(42)	2.685	1.854	170.1	x, y, z
O(13)-H(13O)···O(44)	2.672	1.865	160.8	x, y, z
O(22)-H(22O)···O(52)	2.994	2.199	157.8	x, y, z
O(23)-H(23O)···O(43)	2.686	1.894	156.5	x, y, z
O(34)-H(34O)···O(41)	2.657	1.872	154.8	x, y, z
Intermolecular among complexes
O(3)-H(3O)···O(33)	2.710	1.871	177.1	0.5 − x, −0.5 + y, −0.5 + z
O(14)···O(34)	2.645			0.5 + x, 2.5 − y, z
O(24)···O(14)	2.927			1 − x, 2 − y, 0.5 + z
O(33)-H(33O)···O(13)	2.671	1.926	147.2	−0.5 + x, 2.5 − y, z
Intermolecular among complexes and lattice solvents
O(12)-H(12O)···O(2)	3.060	2.593	116.4	x, y, z
O(2)-H(2O)···O(1 w)	2.568	1.771	157.6	0.5 + x, 1.5 − y, z
O(32)-H(32O)···O(2 m)	2.690	1.865	167.3	x, 1 + y, z

) which stabilize the observed configuration of the complexes, as well as intermolecular hydrogen bonds which built the supramolecular structure. Each trinuclear cation acts as 6-connected node and is linked to six neighboring clusters through ten hydrogen bonds developed between the protonated pendant alkoxide groups of (H₃L)⁻ and form an overall 3D supramolecular network (Figure 3). Figure S3 compares the packing observed in the structure of {Ni₂Tb} [19] and 1 where we observe important differences. Although in both structures the hydrogen bonds are among protonated pendant alkoxide groups of (H₃L)⁻ ligand, in the case of the {Ni₂Tb} complex, the axes of the trinuclear complexes are parallel to each other along all crystallographic directions. In the case of 1, they remain parallel to each other only along the c axis, whereas along the other directions the axes are inclined as compared to the central one (Figure S3). Probably the slight differences in the type of polyhedron around the Ln^III atom have caused small differences in the arrangement of protonated pendant alkoxide groups which play an anchor role, and thus promote a different arrangement of neighboring complexes, which results in different packing and, finally, in a different space group. In all studied structures from this family of compounds, solvents are trapped in the lattice, which interact through intermolecular bonds with the complexes (Table 3) and thus, in addition to the other physical properties, these compounds could be useful as host lattices of different solvents.

3.3. Magnetic Measurements

The temperature dependence of the magnetic susceptibility in the form of χ_MT vs. T under a magnetic field of 1 kOe is shown in Figure 4. The χ_MT product of complex 1 at 300 K is 16.87 cm³Kmol⁻¹ which compares with the theoretical value of 16.15 cm³Kmol⁻¹ for two non-interacting Ni^II (S = 1, g = 2.0) and one Dy^III (S = 5/2, J = 15/2, g = 4/3) ions. Upon cooling, the χ_MT product decreases slowly to the value of 14.54 cm³Kmol⁻¹ at 10 K and then drops more rapidly to the value of 12.36 cm³Kmol⁻¹ at 2 K (Figure 4). The slight decrease in the value of the χ_MT product down to 10 K is probably due to the thermal depopulation of the dysprosium excited states. The field dependence of the magnetization at 2 K reaches a value of 9.48 μ_B at 80 kOe without reaching saturation (Figure 4, inset).

The magnetic relaxation properties of 1 were monitored by ac magnetic susceptibility measurements. In the absence of an external magnetic field, no out-of-phase signals are observed down to 2 K when applying frequencies of up to 10 kHz. Slow relaxation is induced in the presence of small magnetic fields. Good conditions for monitoring the relaxation properties were obtained for H_DC = 1.0 kOe. The plots of the temperature dependence of χ′ and χ″ at an external field of 1.0 kOe for various frequency values, and of the frequency dependence of χ′ and χ″ at an external field of 1.0 kOe for various temperature ranges, are given in Figure 5 and Figure 6, respectively. The relevant Cole-Cole plots are shown in Figure 7. The frequency and temperature dependence indicate thermally assisted relaxation. From the analysis of these plots based on the generalized Debye model [38], the temperature dependence of the relaxation time in the 2.0–4.5 K temperature range was obtained. This dependence is shown in Figure 8 by plotting the quantity ln(τ) as a function of 1/T. For a thermal activation process obeying the Orbach law, the plots would follow a linear relationship. Clearly this is not the case, as seen in Figure 8. Deviation from the Orbach process is often observed in 3d/4f heteronuclear complexes and has been attributed to the involvement of multiple relaxation mechanisms [39,40,41,42]. The temperature dependence of the relaxation time, τ, for 1 can be fitted with the Equation (2):

$$\frac{1}{\tau }=AT+B{T}^n+\left(\frac{1}{{\tau }_0}\right)\exp \left(-\frac{U}{k_{B}T}\right)$$

(2)

In this equation the first term in the right part of Equation (2) represents the direct process, the second term the Raman mechanism and the third the Orbach mechanism [43]. Consideration of the three mechanisms simultaneously constitutes an over-parametrization. If we consider the direct and Raman mechanisms, a good fit is obtained with A = 11.8(2) × 10³ K⁻¹s⁻¹, n = 6.4(3) and B = 4(2) K^−6.4s⁻¹. For the direct and Orbach mechanisms, a good fit is obtained with A = 13.0(2) × 10³ K⁻¹s⁻¹, a preexponential factor τ₀ = 4(2) × 10⁻⁸ s, and a thermal barrier U = 26(2) K.

The temperature dependence of the magnetic susceptibility in the form of χ_MT vs. T under a magnetic field of 1 kOe is shown in Figure 9. The χ_MT product of complex 2 at 300 K is 16.29 cm³Kmol⁻¹, which compares to the theoretical value of 16.06 cm³Kmol⁻¹, for two non-interacting Ni^II (S = 1, g = 2.0) ions and one Ho^III (S = 2, J = 8, g = 5/4) ion (Figure 9). The χ_MT product of 2 decreases slightly upon decreasing of the temperature, reaching the value of 11.93 cm³Kmol⁻¹ at 20 K, and then drops rapidly to the value of 5.46 cm³Kmol⁻¹ at 2 K. The decrease of the χ_MT product at low temperatures is mainly governed by the thermal depopulation of the ground state sublevels that result from spin-orbit coupling of the Ho^III ion and a low symmetry crystal field. The field dependence of the magnetization for 2 is shown as inset in Figure 9. The magnetization of 2 at 2 K, reaches the value of 9.43 μ_B at 80 kOe without reaching saturation, which suggests that the ground spin state is not fully populated because other excited states remain populated to some extent even at 2 K.

The magnetic relaxation properties of 2 was explored by AC susceptibility measurements. No slow relaxation is observed either in the absence or in the presence of an external dc magnetic field.

The temperature dependence of the magnetic susceptibility in the form of χ_MT vs. T and 1/χ_M vs. T under a magnetic field of 1 kOe is shown in Figure 10. For T > 80 K the 1/χ_M vs. T data exhibits a linear behavior that could be fitted assuming the Curie Weiss law (1/χ_M = (T − θ/C). From the fit, a value of C = 2.18 cm³Kmol⁻¹ is determined. This value compares to two Ni^II ions (S = 1) with g > 2.0 and a diamagnetic Y^III ion. The χ_MT product remains practically constant down to ~80 K and below this temperature slowly increases reaching a broad maximum value of ~2.21 cm³Kmol⁻¹ at 10–15 K. This increase suggests weak intramolecular ferromagnetic interactions between the Ni^II ions. At lower temperatures, χ_MT decreases rapidly reaching a value of 1.58 cm³Kmol⁻¹ at 2 K (Figure 10). The drop of the χ_MT product at low temperatures is consistent with the single ion magnetic anisotropy (zero field splitting, zfs) of the Ni^II ions and/or intermolecular antiferromagnetic interactions between the molecules. Field dependent magnetization measurements at 2.2 K were performed up to 50 kOe for 3 and are shown as inset in Figure 10. The magnetization increases in low magnetic fields and reaches a final value of 3.24 μ_B for 3 at 50 kOe, without reaching saturation. The simultaneous fit of the χ_MT vs. T and M vs. H data was obtained using the spin Hamiltonian:

$$H=-2J{S}_{Ni1}{S}_{Ni2}+2D{S}_{Ni}^2$$

(3)

where J is the exchange coupling constant and D is the axial term of the zfs interaction. The obtained parameters, using the program PHI [44], are J = +0.36 cm⁻¹, D = +5.13 cm⁻¹ with g = 2.08 (solid lines in Figure 10). Weak magnetic interactions between distanced Ni(II) ions have been observed also in Ni₂Ln triads [20,23] for Ln = La. In particular in the case of ref. [23] the interactions were found to be weak ferromagnetically and the parameters are very close to those observed in the present work.

As in the case of 2, no slow relaxation is observed for 3, either in the absence or in the presence of an external dc magnetic field.

4. Concluding Comments

We have extended the series of trinuclear complexes [Ni₂Ln(H₃L)₄(O₂CMe)₂](NO₃) (Ln^III = Sm, Eu, Gd, Tb) previously reported [19], by using Ln^III = Dy, Ho and the diamagnetic ion Y^III. The trinuclear complexes [Ni₂Ln(H₃L)₄(O₂CMe)₂](NO₃) (Ln^III = Dy (1), Ho (2), Y (3)), which contain the monoanion of the tetradentate Schiff base ligand H₄L = o-C₆H₄-CH=NC(CH₂OH)₃, consist of almost linear trinuclear cations {Ni₂Ln} and nitrate anions. The molecular structures of 1–3 are isostructural to those of the complexes in [19] considering the coordination environment of the metal ions, but differ in the coordination geometry around the lanthanide ion and the pattern of intermolecular interactions. These differences may affect the crystallization of both types of complexes in different space groups, namely complexes in [19] crystallize in the centrosymmetric orthorhombic space group Pnnn, whilst 1–3 crystallize in the non-centrosymmetric orthorhombic space group Pna2₁. The latter belongs to one of the seven point groups which is synonymous to multifunctionality, as it could combine the magnetic properties with different other physical properties, which are compatible with non-centrosymmetry. It is interesting to note the fact that the studied complexes present chiral configurations without using chiral ligands, however both enantiomers coexist due to the presence of mirror plane symmetry. The magnetic study of 1 by ac susceptibility measurements revealed frequency and temperature dependent out-of-phase signals under H_DC = 1.0 kOe. The relaxation properties for 1 are governed by the direct and Orbach processes, or the direct and Raman processes. The magnetic behavior of 2 at low temperatures is governed by the thermal depopulation of the ground state sublevels due to the spin-orbit coupling of the Ho^III ion and a low symmetry crystal field. The magnetic properties of 3, which contains the diamagnetic Y^III ion, revealed ferromagnetic coupling between the Ni^II ions and zfs effects. The absence of slow magnetic relaxation effects in 2 and 3, as well as in the previously reported isostructural trinuclear clusters [19], suggests that the unique relaxation behavior of 1 is inherent to the Dy^III ion in the specific crystal field environment.

Further work is under way in an attempt to successfully isolate enantiopure crystals by following different synthetic paths or by choosing the appropriate ligands.