A new family of heterometallic [Cu₆M₄] (M = Gd, Tb, Dy and Y) clusters derived from the combined use of selected pyridyl poly-alcohol ligands

Abstract

The combined use of 2-(2-pyridyl)-1,3-propane-diol (pypdH₂) and 2-hydroxymethyl-2-(2-pyridyl)-1,3-propane-diol (pyptH₃) in Cu²⁺/4f chemistry has afforded a new family of isostructural [Cu₆M₄(pypt)₄(pypdH)₄(NO₃)₈] [M = Gd (1), Tb (2), Dy (3), and Y (4)] complexes. These compounds are based on an unprecedented three-layered symmetric [Cu₆M₄(μ-OR)₁₆]⁸⁺ structural core, formed from the connection of the metal ions by bridging alkoxide arms of the organic ligands. Direct current magnetic susceptibility studies for complexes 1–3 revealed the presence of dominant ferromagnetic exchange interactions, suggesting the existence of large spin ground state values. Alternating current magnetic studies indicate the presence of slow-magnetic relaxation in 1–3.

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Introduction

Heterometallic Cu²⁺/Ln³⁺ complexes represent a fascinating frontier in the field of coordination chemistry. The interest is driven by the potential of these molecules to exhibit unprecedented structural motifs and intriguing electronic and magnetic properties.^1–3 The growth of this area is largely attributed to two major discoveries that took place a long time ago: the observation of ferromagnetic Cu²⁺/Gd³⁺ coupling in a {Cu₂Gd} complex in 1986,⁴ and the detection of single-molecule magnetism behaviour (SMM) in a {Cu₂Tb₂} compound in 2004.⁵ Since then, tremendous effort has been put into the synthesis of Cu²⁺/Ln³⁺ complexes and research in this field has afforded many beautiful coordination complexes with large nuclearities^6–9 and aesthetically pleasing structural motifs not found in homometallic 3d- or 4f-metal clusters.^10–14 This structural variability of Cu²⁺/Ln³⁺ complexes arises from the versatile coordination geometries of Cu²⁺ and Ln³⁺ ions.

The interest of Cu²⁺/Ln³⁺ complexes in the field of molecular magnetism is mainly focused in the areas of single molecule magnets^15–17 and molecular magnetic refrigerants.^18,19 The intrinsic characteristics of lanthanide ions and particularly their unquenched orbital angular momenta and the presence of numerous unpaired electrons, endow these ions with significant magnetic moments and anisotropy.^20–23 However, polymetallic lanthanide complexes display very weak intramolecular exchange interactions, leading to fast magnetic relaxation through quantum tunnelling of magnetization (QTM).²⁴ One method that has been employed to afford 4f-containing clusters with stronger magnetic interactions as compared to these of purely 4f systems involves the incorporation of 3d metal ions into these systems. This method also allows the mitigation of QTM and, in the case of ferromagnetic interactions,^25–28 the formation of systems with large spin ground states.²⁹ Among the various families of 3d/4f metal clusters, Cu²⁺/4f compounds have attracted significant attention because of the strong and often ferromagnetic (vide supra) exchange interactions of Cu²⁺ and Ln³⁺ ions. Thus, complexes containing highly anisotropic lanthanide ions like Dy³⁺ along with Cu²⁺ ions, can meet the requirements of single-molecule magnets (SMMs) and exhibit slow magnetization relaxation,^30–34 whereas complexes consisting of isotropic Gd³⁺ ions (and Cu²⁺ ions) are good candidates to exhibit enhanced magnetocaloric effect (MCE) and act as molecular magnetic refrigerants.^17,35–39

The synthesis of Cu²⁺/4f clusters depends on numerous synthetic parameters, one of which is the choice of the organic ligand. In fact various organic ligands containing either O or O/N donor atoms have been employed in the synthesis of Cu²⁺/4f clusters including polydentate Schiff bases,^1,40,41 oximes,^38,42,43 alcohols,^35,44–47 and amino acids.^48–50 Given that alcohol containing ligands have afforded Cu²⁺/4f clusters with aesthetically pleasing structures^{11,33,35,44–47} and based on our experience in the mixed-ligand approach,⁵¹ we investigated reactions that included the combined use of 2-(2-pyridyl)-1,3-propane-diol (pypdH₂) and 2-hydroxymethyl-2-(2-pyridyl)-1,3-propane-diol (pyptH₃) in Cu²⁺/4f cluster chemistry (Scheme 1). These are flexible ligands, containing one aromatic N-donor atom and two (pypdH₂), or three (pyptH₃) aliphatic alcohol O atoms that can bind to various metal ions in many coordination modes, depending on the deprotonation level. Moreover, the versatility of the aliphatic alcohol oxygen atoms of these ligands enables them to accommodate metal ions across a wide range of ionic radii.⁵² To the best of our knowledge, these ligands are employed for the first time in coordination chemistry although there are very few manganese and copper metal complexes, bearing the methyl substituted analogue of pypdH₂ ligand (2-methyl-2-(2-pyridyl)-1,3-propan-diol) (Table S5†).^53,54

Scheme 1 Structural formulae and abbreviations of the ligands used in this work.

We report the syntheses, structures and magnetic properties of a new family of Cu²⁺/4f clusters with the formula [Cu₆M₄(pypt)₄(pypdH)₄(NO₃)₈] [M = Gd (1), Tb (2), Dy (3), and Y (4)]. They are the first examples of coordination complexes containing the ligands pypdH₂ and pyptH₃ and display an unprecedented three-layered symmetric [Cu₆M₄(μ-OR)₁₆]⁸⁺ structural core. Direct current (dc) and alternating current (ac) magnetic susceptibility studies suggest the presence of dominant ferromagnetic exchange interactions and of slow relaxation of the magnetization process for compounds 1–3.

Results and discussion

Synthetic comments

Our group as well as other ones have a continuing interest on the use of polyol-type ligands in 3d and 3d/4f metal cluster chemistry.^55–58 Indeed, the use of such ligands in both homometallic Mn and heterometallic Mn/4f cluster chemistry either as primary organic chelating ligands^52,59,60 or in combination with various phenolic oximes^61–63 or (py)₂CO derivatives,^64–67 yielded high nuclearity complexes with aesthetically pleasing structures and intriguing magnetic properties. An extension of these studies involves the use of pypdH₂ and pyptH₃ ligands, which bear resemblance to well-known polyol-type ligands including 1,3-propanediol (pdH₂) and 1,1,1-tris(hydroxymethyl)ethane (thmeH₃), respectively. These ligands were employed in 3d/4f cluster chemistry either as the main chelate or in combination with other chelates and we herein report the initial results of these efforts; a new family of Cu²⁺/4f compounds obtained from the combined use of pypdH₂ and pyptH₃.

The general reaction of Cu(NO₃)₂·5/2H₂O, M(NO₃)₃·xH₂O (M = Gd, Tb, Dy and Y), pypdH₂ and pyptH₃ in the presence of NEt₃ in a molar ratio of ∼1 : 1 : 1 : 1 : 2.5 in MeCN resulted in a blue solution that was left undisturbed at room temperature to afford, after 1 day, blue crystals of 1–4 in high yields (∼45–65%). The isolation of complexes 1–4 required the use of base. In fact, their isolation was independent of the organic/inorganic base that was used in the reaction mixture since they were isolated when Me₄NOH, Bu^t₄NOH, NaOH or Et₃N was used although the latter led to significantly higher reaction yield. In addition, the use of MeCN solvent, was essential for the isolation of 1–4 since the same reaction in other solvents (i.e., MeOH, EtOH, DMF, CH₂Cl₂) led to microcrystalline or amorphous solids that could not be further characterized. Furthermore, the presence of NO₃⁻ anions from both Cu²⁺ and M³⁺ starting materials is crucial for the synthesis of 1–4, since analogous reactions with different anions (i.e., Cl⁻, ClO₄⁻ and MeCO₂⁻) led to insoluble precipitates which we were unable to crystallize. Finally, the presence of both organic ligands in the reaction mixture was essential for the isolation of 1–4. The role of the two ligands, pypdH₂ and pyptH₃, was scrutinized through various modifications of the original synthetic procedure. The aim was to investigate whether analogous complexes containing only one of these ligands could be formed. However, these experiments did not result in the formation of crystalline materials. The formation of compounds 1–4 is summarized in eqn (1), where x = 5 or 6:

The reported compounds were characterized by single-crystal X-ray crystallography, powder diffraction, elemental analyses (C, H, N), thermogravimetric analysis and IR spectroscopy. Details on the IR, pXRD and TGA studies for complexes 1–4 are available in ESI (Fig. S1–S3 and Table S6†). The protonation levels of the O atoms of pypdH₂ and pyptH₃ ligands were determined by charge-balance considerations, inspection of the metric parameters and BVS calculations (Table S3†).^68,69

Description of structures

Compounds 1·3MeCN, 2·3MeCN, 3·4MeCN and 4·4MeCN are isostructural with their main difference being the lanthanide ions they contain and the number of lattice MeCN molecules. For this reason, only the structure of 1·3MeCN will be described in detail as a representative example. Partially labelled representations of the structure of 1·3MeCN are shown in Fig. 1. The coordination modes of the ligands pypdH⁻ and pypt³⁻ are illustrated in Scheme 2. Selected interatomic distances (Å) and angles (°) for 1·3MeCN, 2·3MeCN, 3·4MeCN and 4·4MeCN are shown in Table S4.† Complex 1·3MeCN (Fig. 1a) crystallizes in the tetragonal space group P4̄2₁c, with the asymmetric unit (Fig. 1b) featuring 1/4 of the neutral compound [Cu₆Gd₄(pypt)₄(pypdH)₄(NO₃)₈]. The compound consists of six Cu²⁺ and four Gd³⁺ ions bridged through the alkoxido arms of four singly deprotonated pypdH⁻ and four triply deprotonated pypt³⁻ ligands, possessing a symmetric [Cu₆Gd₄(μ-OR)₁₆]⁸⁺ metal core (Fig. 1c). The core of 1 displays a three-layered structure consisting of two equivalent [Cu₃Gd₂(μ-OR)₈]⁴⁺ units (Fig. 1d) that are held together by the four pypt³⁻ ligands. The upper and lower layers comprise linear {GdCuGd} sub-units displaying different orientations and are separated by a central {Cu₄} rectangle. Each pentametallic [Cu₃Gd₂(μ-OR)₈]⁴⁺ unit features an alternating Cu/Gd chain, where the three central metal ions, two Gd³⁺ ions and one Cu²⁺ ion, are arranged in an almost linear configuration. The remaining two Cu²⁺ ions are positioned 3.321 Å away from and on the same side of the plane formed by the metal ions of the linear {GdCuGd} sub-unit. The four Cu2 centers are located on the same plane, forming a rectangle, separating the two linear {GdCuGd} sub-units and giving rise to the three-layered structure of 1 (Fig. 1d). The four singly-deprotonated pypdH⁻ ligands bridge a Cu²⁺ and a Gd³⁺ ion in the same η¹:η¹:η²:μ fashion (Scheme 2). In addition, the four triply-deprotonated pypt³⁻ ligands bridge five metal ions, two Cu²⁺ and three Gd³⁺ ions, possessing the η¹:η²:η²:η²:μ₅ coordination mode. The coordination sphere of the Gd³⁺ ions is completed by 8 chelating NO₃⁻ ions (each Gd³⁺ ion is bound to two NO₃⁻ ions). Cu1 (and Cu1′) is four-coordinate, exhibiting a distorted square planar geometry. The cis- and trans-angles fall within the ranges of 86.0–96.3° and 161.6–165.5°, respectively, deviating slightly from the ideal 90° and 180° angles of a perfect square plane. Cu2 (and its symmetry equivalents) is five-coordinate, possessing a {CuO₃N₂} coordination sphere. Employing the methodology proposed by Reedijk and Addison,⁷⁰ a trigonality index (τ) of 0.02 was determined, suggesting a distorted square pyramidal geometry (where τ is 0 and 1 for perfect square pyramidal and trigonal bipyramidal geometries, respectively). The axial Cu2–O5 bond length of 2.391 Å is notably longer than the bond lengths found in the basal plane, which range from 1.923 to 2.018 Å. The crystallographically unique Gd³⁺ ion is 8-coordinate with its coordination sphere being {GdO₈}. Its coordination geometry was estimated as distorted triangular dodecahedral, using the SHAPE 2.1 ⁷¹ program (Table S2†). There are no significant intermolecular interactions between neighboring clusters in the crystal structure of 1·3MeCN, while the closest intermolecular metal⋯metal separation is 7.541 Å between Gd1 and Cu2 atoms of adjacent [Cu₆Gd₄] complexes. To the best of our knowledge complex 1 is the first example of a [Cu₆Gd₄] cluster and possesses an unprecedented metal/O core.

Fig. 1 Partially labelled representations of: (a) the molecular structure, (b) the asymmetric unit, (c) the metal/O core, (d) the pentanuclear [Cu₃Gd₂(μ-OR)₈]⁴⁺ sub-unit of the core and (e) the three-layered structure of 1·3MeCN. H atoms and MeCN solvent molecules are omitted for clarity. Colour code: Cu²⁺, cyan; Gd³⁺, yellow; N, green; O, red; C, black. Symmetry codes: (′) +x, 1 − y, 1 − z, (′′) 1 − x, +y, 1 − z, (′′′), 1 − x, 1 − y, +z.

Scheme 2 Crystallographically established coordination modes of pypdH⁻ and pypt³⁻ ligands in complex 1·3MeCN.

Magnetism studies

Static properties. Solid-state, variable-temperature direct current (dc) magnetic susceptibility measurements were performed on powdered polycrystalline samples of 1–4 in a 0.1 T field and in the 2.0–270 K range. The χ_MT versus T plots for all complexes are depicted in Fig. 2a (χ_M is the molar magnetic susceptibility). The experimental χ_MT values at 270 K (35.4 (1), 46.5 (2) and 57.1 (3) cm³ mol⁻¹ K) are comparable to the expected ones (33.75 (1), 49.53 (2) and 58.93 (3) cm³ mol⁻¹ K) for six Cu²⁺ (S = 1/2, g = 2.0) and four Gd³⁺ (⁸S_7/2, S = 7/2, L = 0, g = 2.0) or Tb³⁺ (⁷F₆, S = 3, L = 3, g = 3/2) or Dy³⁺ (⁶H_15/2, S = 5/2, L = 5, g = 4/3) non-interacting ions.⁷² The χ_MT product slightly increases upon cooling in all compounds to a value of 38.48 (1), 47.76 (2), and 57.64 (3) cm³ mol⁻¹ K at 100 K. Below this temperature, χ_MT increases more rapidly to a maximum value of 114.90 (1), 72.54 (2) cm³ mol⁻¹ K at 2 K, and 111.04 (3) cm³ mol⁻¹ K at 4.2 K. For 3, the χ_MT product, after reaching this peak, decreases to 102.74 cm³ mol⁻¹ K at 2.0 K. The large increase in χ_MT values at low temperatures suggests the presence of dominant ferromagnetic interactions, with large spin ground states for all complexes while the slight decrease below 4.0 K observed for 3 could be attributed to the depopulation of the m_j sublevels of the ground J state of the Dy³⁺ ions (Stark sublevels) and/or to weak antiferromagnetic interactions between the metal ions. For 4, the χ_MT product increases from 1.78 cm³ mol⁻¹ K at 270 K to 2.55 cm³ mol⁻¹ K at 6.5 K and then decreases to 1.74 cm³ mol⁻¹ K at 2.0 K. The value at 270 K is smaller than the spin-only (g = 2.0) value of 2.25 cm³ mol⁻¹ K for six Cu²⁺ (S = 1/2) non-interacting ions.

Fig. 2 (a) Temperature dependence of the χ_MT product for complexes 1–4 in a field of 0.1 T, (b) plot of the magnetization versus temperature data for 1, (c) plot of the magnetization versus temperature data for 4. Inset of 2a and 2c: representations of the isotropic exchange interactions included in spin-Hamiltonians used in the fit of the susceptibility and magnetization data for complexes 1 (a) and 4 (c). The red lines are a fit of the experimental data as described in the text.

Magnetization (M) studies were conducted for complexes 1–4, under various low temperatures and magnetic fields (Fig. 2b, c and S4, S5†). The data for compound 1 reveal a rapid increase of the magnetization values with increase of the magnetic field, leading to a magnetization saturation at a value of ∼34μ_B. This behaviour suggests the existence of a large spin ground state value, possibly S = 17 (assuming g ≈ 2) which is the maximum S value for a [Gd³⁺₄Cu²⁺₆] compound. This indicates the presence of entirely ferromagnetic exchange interactions between the metal ions in 1.

In contrast, the magnetization plots for 2 and 3 at 2.0 K exhibit a rapid increase up to 0.1 T, followed by a gradual, increase up to 22.76μ_B and 26.24μ_B at 5 T, respectively, without reaching saturation. The lack of saturation in magnetization of 2 and 3 is indicative of the presence of anisotropy. For 4, M increases rapidly with increasing H at 2 K to the value of 5.68μ_B at 5 T without reaching saturation.

The magnetic susceptibility data for compounds 4 and 1 were fit to evaluate the strength of the intramolecular Cu²⁺–Cu²⁺ and Cu²⁺–Gd³⁺ magnetic exchange interactions, respectively. For complex 4, the spin Hamiltonian used is shown below:

where the indices refer to the constituent Cu²⁺ ions, Ŝ is a spin operator and J₁ is the pairwise isotropic exchange interaction. For simplicity, we assume all magnetic exchange interactions between Cu²⁺–Cu²⁺ ions to be equivalent (Fig. 2c, inset) and neglect anisotropy terms in the spin-Hamiltonian. In addition, considering that the exchange interactions between Cu²⁺ ions are expected to be very weak due to the magnetic coupling occurring via polyatomic bridges or simply through space, only the low-temperature magnetization data were fitted. This resulted in the best-fit parameter J₁ = −0.20 cm⁻¹ (g = 2.0) indicating weak antiferromagnetic Cu²⁺–Cu²⁺ exchange.

The χ_MT and magnetization data for complex 1 were simultaneously fit to the following spin-Hamiltonian:

where the summation indices i, j run through the constitutive metal centres, D is the uniaxial single-ion anisotropy parameter for Gd³⁺ ions, Ŝ is a spin operator and J₂ is the isotropic Cu²⁺–Gd³⁺ exchange interaction parameter. To avoid overparameterization, we assumed all magnetic exchange interactions between Cu²⁺–Gd³⁺ ions to be equivalent (Fig. 2a, inset) and neglected Cu²⁺–Cu²⁺ interactions. The best-fit parameters were J₂ = +4 cm⁻¹ and |D_Gd| = 6.5 × 10⁻³ cm⁻¹ (g = 2.0). Including the Cu²⁺–Cu²⁺ interactions in the model resulted in values similar to those determined in the case of 4 but did not significantly modify the quality of the fit. Thus, 1 can be described as containing ferromagnetic Cu²⁺–Gd³⁺ interactions and consisting of two {Cu₃Gd₂} pentametallic units, each with an S = 17/2 spin value, with the m_S = 17 projection being the ground state in the presence of a magnetic field. The reported J-coupling constant of 1 is in agreement with J_ij values reported for other Cu/Gd compounds.²⁹

Dynamic properties. Alternating current (ac) magnetic susceptibility measurements were also performed to probe the magnetic dynamics of 1–3, using a 3.5 Oe ac field. Complexes 2 and 3 show ac signals either in the absence or presence of an applied dc field. However, no peak maxima were observed in both cases suggesting fast magnetic relaxation most likely through quantum tunnelling of magnetization (QTM) (Fig. S7–S9†). Notably, complex 1, containing isotropic Gd³⁺ ions, exhibits in-phase (χ′) and out-of-phase (χ′′) ac susceptibility signals under an applied dc field of 0.2 T (Fig. 3 and S6, S10†). Specifically, at frequencies below 60 Hz and temperatures below 5.3 K, complex 1 shows frequency dependent out-of-phase signals, indicative of magnetization relaxation at this time – scale. However, it was not possible to fit the experimental data and calculate the relaxation parameters since no peak maxima of the χ′′ signals were observed in the frequency range of 1–1488 Hz from 1.8 to 5.3 K. In addition, attempts to fit the data using Kramers–Kronig relations^73,74 failed to produce reasonable values.

Fig. 3 Plot of the out-of-phase (χ′′_M) signals vs. frequency (ν) for complex 1 under an applied dc field of 0.2 T.

Notably, compound 1 which is based on an isotropic lanthanide ion displays out-of-phase signals at higher temperatures than the corresponding analogues (2 and 3) based on anisotropic ions. This could be attributed to the existence of different relaxation mechanisms in these compounds; strong quantum tunnelling dominates in anisotropic Tb and Dy complexes, whereas in Gd complex relaxation does not occur via the Orbach process but involves other mechanisms related to spin-phonon interactions. This behaviour has been observed recently in other compounds containing Gd³⁺ ions.^75–79

Conclusions

The synthesis, crystal structures and magnetic studies of a new family of decanuclear [Cu²⁺₆M³⁺₄] complexes [M = Gd (1), Tb (2), Dy (3), and Y (4)] are reported. These complexes were successfully synthesized from the initial employment of 2-(2-pyridyl)-1,3-propane-diol (pypdH₂) and 2-hydroxymethyl-2-(2-pyridyl)-1,3-propane-diol (pyptH₃) organic ligands. Complexes 1–4 feature a new metal stoichiometry and an unprecedented three-layered symmetric [Cu₆M₄(μ-OR)₁₆]⁸⁺ structural core. Dc magnetic susceptibility studies revealed the presence of ferromagnetic interactions and large spin ground state values, possibly S = 17 for the case of 1. Ac studies revealed the presence of slow magnetic relaxation processes for all compounds. Work in progress includes the extension of the use of pypdH₂ and pyptH₃ ligands in 3d and/or 3d/4f cluster chemistry.

Experimental section

Materials, physical and spectroscopic measurements

Elemental analyses (C, H, and N) were performed by the in-house facilities of the University of Cyprus, Chemistry Department. IR spectra were recorded as ATR in the 4000–400 cm⁻¹ range using a Shimadzu Prestige – 21 spectrometer. Variable-temperature dc and ac magnetic susceptibility data were collected at the University of Copenhagen using a Quantum Design MPMS-XL SQUID magnetometer equipped with a 7 T magnet and capable of operating in the 1.8–400 K range. Prior to measurements crystalline samples were crushed to microcrystalline powders and fixed in n-hexadecane to avoid orientation of the sample in the magnetic field. The ac magnetic susceptibility measurements were performed in an ac field of 3.5 G and in the presence or absence of an applied dc field. The oscillation frequencies were in the 0.1–1488 Hz range. Pascal's constants⁸⁰ were used to estimate the diamagnetic corrections, which were subtracted from the experimental susceptibilities to give the molar paramagnetic susceptibility (χ_M). The program PHI⁸¹ was used to fit the magnetic data.

X-ray crystallography

Data were collected on a Rigaku XtaLAB Synergy-S single crystal X-ray diffractometer equipped with a CCD area detector and a graphite monochromator utilizing Cu Kα radiation (λ = 1.54184 Å). Selected crystals were attached to glass fiber with paratone-N oil and transferred to a goniostat for data collection. Empirical absorption corrections (multiscan based on symmetry-related measurements) were applied using CrysAlis RED software.⁸² The structures were solved by direct methods using SIR92 ⁸³ and refined on F² using SHELXL97,⁸⁴ SHELXL-2014/7,⁸⁵ and SHELXT.⁸⁶ Software packages used: CrysAlisCCD⁸² for data collection, CrysAlisRED⁸² for cell refinement and data reduction, WINGX for geometric calculations,⁸⁷ while MERCURY⁸⁸ and Diamond⁸⁹ were used for molecular graphics.

For all compounds, the non-H atoms were treated anisotropically, whereas the H atoms were placed in calculated, ideal positions and refined as riding on their respective C atoms. Unit cell parameters and structure solution and refinement data for complexes 1·3MeCN, 2·3MeCN, 3·4MeCN and 4·4MeCN are listed in Table S1 of the ESI.†

Syntheses

All manipulations were performed under aerobic conditions using materials (reagent grade) and solvents as received. The ligands 2-(2-pyridyl)-1,3-propane-diol (pypdH₂) and 2-hydroxymethyl-2-(2-pyridyl)-1,3-propane-diol (pyptH₃) were prepared, purified, and characterized as described elsewhere.⁹⁰Caution: Although no such behaviour was observed during the present work, nitrate salts are potentially explosive; they should be synthesized and used in small quantities and treated with care.

[Cu₆M₄(pypt)₄(pypdH)₄(NO₃)₈] [M = Gd (1), Tb (2), Dy (3), and Y (4)]. To a stirred, colourless solution of pypdH₂ (0.030 g, 0.2 mmol), pyptH₃ (0.040 g, 0.2 mmol) and NEt₃ (0.070 mL, 0.5 mmol) in MeCN/MeOH (2 : 1, 15 mL) were subsequently added solids M(NO₃)₃·xH₂O (x = 5 or 6) (0.2 mmol) and Cu(NO₃)₂·5/2H₂O (0.046 g, 0.2 mmol). The resulting blue solution was stirred for 30 min under mild heating (at ∼50 °C). Then, the solution was filtered off, and the filtrate was left undisturbed at room temperature. After 1 day, dark blue block – shaped crystals of 1·3MeCN, 2·3MeCN, 3·4MeCN and 4·4MeCN appeared, which were kept in mother liquor for X-ray analysis, or collected by filtration, washed with MeCN, and dried in air for other solid-state studies. The reaction yields were in the range of ∼45–65%. Anal. Calc. (found); C₆₈H₁₀₀N₁₆O₅₄Cu₆Gd₄ (1·10H₂O): C, 27.08 (27.32); H, 3.34 (3.52); N, 7.43 (7.67)%; C₆₈H₁₀₈N₁₆O₅₈Cu₆Tb₄ (2·14H₂O): C, 26.39 (26.67); H, 3.52 (3.73); N, 7.24 (7.58)%; C₆₈H₁₁₀N₁₆O₅₉Cu₆Dy₄ (3·15H₂O): C, 26.12 (26.31); H, 3.55 (3.51); N, 7.17 (7.38)%; C₆₈H₁₀₀N₁₆O₅₄Cu₆Y₄ (4·10H₂O): C, 29.78 (29.69); H, 3.68 (3.53); N, 8.17 (7.98)%.

Data availability

The data supporting this article (various structural tables, spectroscopic and magnetic figures and tables for complexes 1–4) have been included as part of the ESI.†

Crystallographic data for 1·3MeCN, 2·3MeCN, 3·4MeCN and 4·4MeCN has been deposited at the CCDC under 2370731–2370734† numbers and can be obtained from https://www.ccdc.cam.ac.uk/structures/.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Cyprus Research and Innovation Foundation Research Grant “EXCELLENCE/0421/0399” which is co-funded by the Republic of Cyprus and the European Regional Development Fund.

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Footnotes

† Electronic supplementary information (ESI) available: Various structural tables, spectroscopic and magnetic figures and tables for complexes 1–4. CCDC 2370731–2370734. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4dt02054e

‡ Current Address: Department of Chemistry, University of Patras, 26504 Patras, Greece.

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