Coordination-triggered redox activity of early and late lanthanide calix[4]arene complexes

Abstract

The methylation of p-tert-butylcalix[4]arene in the distal 1,3-phenolic sites provides H₂L = {p-tert-butylcalix[4](OMe)₂(OH)₂arene}. This unit acts as a rigid coordinating ligand to early and late lanthanide metal ions, enabling the construction of two families of mononuclear compounds featuring (N(nBu)₄)[Ln^IIIL(acac)₂]·CH₃CN (Ln = Pr (1), Nd (2), Ho (3), and Er (4)) and (N(nBu)₄)₂[Ln^IIIL{Mo₅O₁₃(OMe)₄(NO)}]·CH₂Cl₂ (Ln = Nd (5) and Er (6)). The metal ions adopt distorted bicapped trigonal prismatic coordination environments, resulting in slow relaxation of the magnetization for 4. These compounds exhibit reversible redox waves at positive potentials, centered within the calix[4]arene ligand, representing a new type of calix[n]arene-based electrochemical activity induced by coordination to the metal centers.

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Introduction

Calix[4]arenes (Fig. 1) are typically cone-shaped molecules constructed by the repetition of four phenolic units linked by methylene bridges.¹ The organic skeleton is not electrochemically active, presenting an irreversible oxidation wave at high potentials.² However, their functionalization with external redox centers (e.g. ferrocene,³ quinone,⁴ bipyridine, TTF,⁵ and nitro⁶ groups) facilitates electrochemical investigation and potential exploitation in various fields, such as sensors, catalysis, ion selectivity, energy storage devices, and molecular electronics. Furthermore, the electrochemical activity is specifically localized within these newly introduced redox-active groups, and it is influenced by the surrounding structural environment and susceptible to alterations in response to external stimuli.⁷

Fig. 1 (a) Representation of calix[4]arene (C[4]) and p-tert-butylcalix[4]arene (TBC[4]), (b) p-tert-butylcalix[4](OMe)₂(OH)₂arene (H₂L) originating from bis-methylation at two distal positions of TBC[4].

Another important approach is the utilization of its hydroxy groups at the lower rim for complexation with lanthanide metal centers. While calix[4]arenes, and more specifically p-tert-butylcalix[4]arene (TBC[4]), are ideal to construct polynuclear complexes with lanthanide (Ln) metal ions⁸ (with 42 entries for polynuclear and 6 for mononuclear calix[4]arene-based structures with Ln ions in the Cambridge Structural Database), the bis-methylation at the distal positions provides a slightly different ligand, i.e. p-tert-butylcalix[4](OMe)₂(OH)₂ arene (referred to hereafter as H₂L; Fig. 1b), which favors the construction of monometallic Ln-based complexes.⁹ The design of compounds with single lanthanide metal ions has been specifically investigated in the design of single-ion magnets (SIMs) due to their large intrinsic magnetic anisotropy and the possibility to change the alignment of the anisotropy axis via judicious choice of the ligands.¹⁰ These molecules have been suggested for potential applications in molecular spintronics and quantum technologies, where specific symmetries such as D_4d, D_5h, and D_6d are proposed in the design of high-performance Ln^III-SIMs.¹¹ In 2013, Zuo et al. reported a seven-coordinate dysprosium ion encapsulated between an H₂L and a Kläui-type tripodal ligand displaying field-induced (dc = 900 Oe) SIM behavior with U_eff = 73.7 K and τ₀ = 9.1 × 10⁻⁹ s.¹² Recently we investigated the use of H₂L in combination with acetylacetonate (acac) and polyoxopentamolybdate to obtain (N(nBu)₄)[Tb^IIIL(acac)₂] and (N(nBu)₄)₂[Dy^IIIL{Mo₅O₁₃(OMe)₄(NO)}], where the Ln^III ions are eight-coordinate in distorted square-antiprismatic coordination geometries (D_4d symmetry), presenting slow relaxation of the magnetization.¹³

Early and late Ln ions also show potential for constructing SIMs, although they are the subject of significantly fewer studies. Herein, we have expanded our previous work to both early and late Ln metals featuring (N(nBu)₄)[Ln^IIIL(acac)₂]·CH₃CN (acac = acetylacetonate; Ln = Pr (1), Nd (2), Ho (3), and Er (4)) and (N(nBu)₄)₂[Ln^IIIL{Mo₅O₁₃(OMe)₄(NO)}]·CH₂Cl₂ (Ln = Nd (5) and Er (6)). While our previous reports concentrated on the magnetic properties resulting from coordination to Gd, Tb, and Dy ions, this study expands upon our prior investigations to offer a comprehensive understanding of the redox activity facilitated by metal coordination. The studied compounds display two reversible or quasi-reversible redox waves at positive potentials localized within the L²⁻ ligand. Given that the employed lanthanide centers are not redox-active, our investigations demonstrate that this unprecedented electrochemical activity on the calix[4]arene ligand is induced by the coordination of lanthanide metal centers.

Results and discussion

Structure descriptions

A two-step synthetic strategy (Fig. 2) was applied to obtain the hybrid complex compounds (N(nBu)₄)₂[Ln^IIIL{Mo₅O₁₃(OMe)₄(NO)}]. The first step involved the synthesis of a series of (N(nBu)₄)[Ln^IIIL(acac)₂] complexes by reacting (N(nBu)₄)(acac), Ln^IIICl₃ and, H₂L in dry toluene at reflux for 6 h under Schlenk conditions (Fig. 2, center). These complexes served as good starting materials for a further metathesis reaction with the monolacunary Lindqvist-type pentamolybdate¹⁴ [Mo₅O₁₃(OMe)₄(NO)(Na(MeOH))]²⁻ to obtain the targeted [Ln^IIIL{Mo₅O₁₃(OMe)₄(NO)}]²⁻ complexes (Fig. 2, right). The six complexes crystallize in the monoclinic system and structure solution was performed in the C2/c space group. The SCXRD data reveal that complexes within families 1–4 and 5, 6 are structurally analogous, we therefore only discuss the structural description of the representative erbium derivatives 4 and 6.

Fig. 2 The combination of acac and H₂L ligands under basic conditions yields [Er^IIIL(acac)₂]⁻. A metathesis reaction of [Er^IIIL(acac)₂]⁻ with [Mo₅O₁₃(OMe)₄(NO)Na(MeOH)]²⁻ in 1 : 1 stoichiometry provides [Er^IIIL{Mo₅O₁₃(OMe)₄(NO)}]²⁻. In both structures, Er^III resides in a distorted bicapped trigonal prismatic Er^IIIO₈ environment. Color code: Er: green, O: red, N: blue, Mo^IV: purple, Mo_NO: dark purple, C: gray. N(nBu)₄⁺, solvents and H atoms are omitted for clarity.

Colorless rod-like single crystals of 4 were obtained by slow diffusion of CH₃CN into a concentrated THF solution. The H₂L ligand is fully deprotonated (L²⁻) and coordinates to the erbium metal ion as a tetradentate ligand through the lower-rim O sites (Er^III–OPh: 2.147(4) Å; Er^III–OMePh: 2.600(4) Å). The L²⁻ does not adopt the typical bowl shape (C_4v) but a pinched conformation (C_2v) where the two methylated aromatic rings are nearly coplanar and the two phenoxide rings splay (Fig. 3). This conformation creates an O₄ pocket defined by two edge-sharing triangular units (O1/O1/O2, Fig. 3a) with a dihedral angle of 24.0°. The further coordination of two acac ligands (Er^III–O: 2.350(4) and 2.364(4) Å) completes the distorted bicapped trigonal prismatic O₈ environment of the Er^III center (Fig. S1†).

Fig. 3 Top view of the (a) [Er^IIIL(acac)₂]⁻ and (b) [Er^IIIL{Mo₅O₁₃(OMe)₄(NO)}]²⁻ displaying the typical pinched conformation of the L²⁻ ligand and its C_2v symmetry. Color code: Er = green, O = red, Mo^IV = purple, and C = gray. N(nBu)₄⁺ and H-atoms are omitted for clarity.

Brown rod-like single crystals of 6 were grown by slow evaporation of a concentrated solution of the complex in dichloromethane/n-hexane (1/3, v/v) for one week. The monolacunary Lindqvist pentamolybdate [Mo₅O₁₃(OMe)₄(NO)]³⁻ grows from a central oxygen atom (μ₅-O_c) bridging to four equatorial Mo^VI ions (Mo^VI–O_c: 2.296(5)–2.364(5) Å) and one axial [Mo(NO)]³⁺ unit (Mo–O_c: 2.109(5) Å). On the outside, four μ₂-O_b connect the central Mo^VI ions (Mo^VI–O_b: 1.900(5)–1.923(5) Å). The link between the central Mo^VI ions and the axial [Mo(NO)]³⁺ unit is completed by four μ₂-O_OMe methoxy groups (Mo^VI–O_OMe: 2.249(5)–2.282(5) Å; Mo_axial–O_OMe: 1.994(5)–2.012(5) Å). The remaining coordination sites are constituted of four terminal oxo groups (Mo^VI–O_t: 1.693(5)–1.701(5) Å) and one linear nitrosyl group (Mo_axial–N_NO: 1.755(7) Å). The lacunary Mo₅ provides four reactive oxygen atoms (Mo^VI–O: 1.741(5)–1.755(5) Å) in a nearly O₄ square (O⋯O: 2.754(7)–2.848(7) Å) to coordinate with the [Er^IIIL]⁺ unit. The calix[4]arene adopts the same pinched conformation as in complex 4, with a 24.3° dihedral angle between the two adjacent triangles O1/O3/O2 and O1/O3/O4 (Fig. 3b) formed by the lower-rim O₄ oxygen atoms (O⋯O: 2.835(7)–2.968(6) Å). The two tetradentate ligands generate a distorted bicapped trigonal prismatic Er^IIIO₈ environment (Fig. S1;† Er^III–O_POM: 2.408(5)–2.479(5) Å; Er^III–OPh: 2.107(5) and 2.120(5) Å; Er^III-OMePh: 2.518(4) and 2.552(5) Å).

In the crystal lattice, complexes 4 and 6 (Fig. S2 and S3†) pack in an antiparallel bilayer array arrangement in which the molecules interdigitate, with the presence of N(nBu₄)⁺ countercations in the interstitial space for charge balance. The shortest Er^III–Er^III distance is 11.204 Å (4) and 13.155 Å (6), and the closest intermolecular contacts between molecules are mediated by CH₃(^tBu)–CH₃(acac) and CH₃(^tBu)–CH(Ph) interactions at 3.643 and 3.744 Å, respectively (4) and NO–CH₃(POM) and CH₃(POM)–CH₃(POM) interactions at 3.855 and 4.194 Å, respectively (6).

ESI-HMRS, FT-IR, UV-Vis, and TGA experiments

The ESI-HRMS spectra in the negative-ion mode of 1–6 (Fig. S8–S11†) show fragmentations related to the loss of N(nBu₄)⁺ countercations. In 1–4, the presence of only a main ion is observed, assigned to [M − N(nBu₄)]⁻ (M = N(nBu)₄)[Ln^IIIL(acac)₂] in the m/z region of 1013–1041. Complexes 5 and 6 present two different fragmentations, the singly [M − N(nBu)₄]⁻ (m/z region of 1904–1926) and the doubly charged [M − 2N(nBu)₄]²⁻ (m/z region of 830–842) ions, where (M = N(nBu)₄)₂[Ln^IIIL{Mo₅O₁₃(OMe)₄(NO)}]. The isotopic distributions of the calculated species in 1–6 perfectly match the experimental values. In the FT-IR spectra of 1–6 (Fig. S12–S14†), the most relevant bands are related to vibrations associated with ν(C–H) ∼3000–2800 cm⁻¹ (s), overlapping vibrations of ν(arC–C)/δ(CH₂)/δ_as(CH₃) ∼1480–1410 cm⁻¹ (vs), δ_s(CH₃) ∼1332 cm⁻¹ (m), ν(C–O) ∼1210 cm⁻¹ (w). The strong stretching band appearing at ∼1597 cm⁻¹ in 1–4 is assigned to C–O_delocalized in the acac ligand. In 5 and 6, specific groups of the POM show vibrations related to ν(N–O) ∼1622 cm⁻¹ (m), ν(C–O) ∼1037 cm⁻¹ (m), ν(Mo–O_t) ∼927–838 cm⁻¹ (s), ν(Mo–O–Mo) ∼680 cm⁻¹ (vs). The TGA results for 1–4 (Fig. S15 and S16†) reveal thermal stability up to ∼160 °C for the complexes, characterized by a slight decline in mass given the loss of crystallization solvent within the calixarene cavities. Above this temperature, the complexes start to decompose. However, complexes 5 and 6 are thermally stable up to ∼280 °C (Fig. S17†). The electronic absorption spectra of 1–4 (Fig. S18 and S19†) in CH₃CN solutions display two intense absorption bands at ∼282 and 258 nm, corresponding to the typical π–π* electronic transitions centered on the phenolic rings of the L²⁻ ligand. Upon substitution of the acac by the {Mo₅} ligand in 5 and 6, the bands display a small red shift to ∼310 (5) and 258 nm (6), in agreement with the color change from colorless in complexes 1–4 to brown in 5, 6. In addition, a shoulder band at ∼258 nm can be attributed to the n–π* electron transition in the [Mo₅O₁₃(OMe)₄(NO)]³⁻ unit.

Magnetic studies

The magnetic properties of complexes 1–6 at static fields are shown in Fig. 4a as χ_mT vs. T at 0.1 T and M_mvs. B at 2.0 K and B = 0.1–5.0 T. At 290 K, the χ_mT values of 1.51 (1), 1.50 (2), 13.55 (3), 11.10 (4), 1.50 (5), and 11.21 cm³ K mol⁻¹ (6) are within the expected ranges¹⁵ for the respective isolated Ln^III centers: 1.45–1.62 (1, Pr^III), 1.45–1.53 (2 and 5, Nd^III), 13.26–13.78 (3, Ho^III), and 11.05–11.28 cm³ K mol⁻¹ (4 and 6, Er^III). χ_mT values remain nearly constant down to 100 K (1, 2 and 5) and 150 K (3, 4 and 6). Subsequently, complexes 1, 2 and 5 exhibit a faster decrease to minima values of 0.07 (1), 0.48 (2) and 0.67 cm³ K mol⁻¹ (5) at 2.0 K. While 3, 4 and 6 show a progressive decrease to 12.58 (3), 8.36 (4) and 9.36 (6) in the temperature range of 150–25 K, wherefrom they approach 8.13 (3), 6.06 (4) and 5.83 (6) at 2.0 K, respectively. Their behavior at lower temperatures (<25 K) is due to the thermal depopulation of the m_J energy sublevels of the ground terms (³H₄ for Pr^III, ⁴I_9/2 for Nd^III, ⁵I₈ for Ho^III, and ⁴I_15/2 for Er^III) arising from the spin–orbit coupling, interelectronic repulsion and ligand field. At 2 K, the molar magnetization M_mvs. applied field B (Fig. 4a, inset) of 1–6 show an approximately linear dependence in the range of 0–5 T. The M_m values are 0.32 (1), 1.2 (2), 6.5 (3), 4.6 (4), 1.4 (5), and 5.0N_Aμ_B (6) at 2 K and 5 T. They do not reach the saturation point (Pr^III 3.2, Nd^III 5.7, Ho^III 10.0, and Er^III 9.0N_Aμ_B) due to the measurement of ground/powdered samples, i.e. the determination of the mean value of randomly oriented crystallites consisting of magnetically anisotropic centers.

Fig. 4 (a) Susceptibility data for complexes 1–6: χ_mT vs. T at 0.1 (circles: 1 black, 2 green, 3 blue, 4 purple, 5 yellow, and 6 red) and M_mvs. B at 2.0 K (inset; same colour code as in 4a). (b) Out-of-phase molar magnetic susceptibility χ′′_mvs. frequency f for 4; (c) Cole–Cole plot of 4 in the range 1.89–7.5 K at a static bias field of 300 Oe (circles: data, lines: fits to a generalised Debye expression). (d) Plot of relaxation time τ vs. T⁻¹ (circles) for 4; the solid red line represents a fit considering quantum tunnelling of magnetisation and Raman relaxation processes.

In complex 4 out-of-phase (χ′′_m) signals were detected at a static bias field, with more pronounced curvature observed at 300 Oe in the Cole–Cole plot of the out-of-phase (χ′′_m) vs. the in-phase magnetic susceptibility (χ′_m) data (Fig. 4c). The simultaneous fitting of a generalized Debye expression¹⁶ to the χ′_mvs. f (Fig. S20†) and χ′′_mvs. f data, yields the solid lines shown in Fig. 4b, c and the relaxation times τ with the distribution α = 0.095 ± 0.082, indicating the presence of multiple relaxation pathways. The best reproduction of the Arrhenius plot τ vs. 1/T data, as shown in Fig. 4d, required considering a field-independent contribution of the quantum tunneling of magnetization (QTM) and a Raman slow relaxation process, as found for analog compounds before,^13a and as expressed in the formula τ⁻¹ = B + CTⁿ. The best-fit yields a QTM with B = (529 ± 10) s⁻¹ and, a constant C = (0.143 ± 0.008) s⁻¹ K⁻ⁿ and an exponent n = 7.2 ± 0.1 for the Raman process. An exponent of 7 is well-known from literature and first reported by Kronig.^17a It describes the typical Raman process by spin-two-phonon interaction for which the phonon energy is larger than the splitting of ground and excited state.^17b

Electrochemical studies

The electrochemical experiments of 1–6 (cyclic voltammetry – CV and rotating disk voltammetry – RDV) were recorded with GC-disk electrodes (stationary and rotating) in dry DMF solutions. The results of oxidation and reduction potentials are summarized in Table S5† and the voltammograms are presented in Fig. 5 and S24.† At positive potentials, the [Ln^IIIL(acac)₂]⁻ family displays two reversible oxidation steps (Fig. 5a and c), with potential differences of 200–270 mV between them. The first oxidation step in 1–4 appears at a potential range of 0.33–0.34 V, showing full reversibility with a peak separation of about 70 mV and a comparable cathodic/anodic peak current ratio. The second oxidation step in 1 and 2 occurs at approximately 60–70 mV less positive than for 3 and 4, and the cathodic counter peak is nearly absent, whereas in 3 and 4 the reversibility is still evident. The decrease of reversibility (quasireversibility) of the second oxidation step in 1–4 is caused by a combination of slower heterogeneous kinetics on the electrode (manifested by anodic/cathodic peak separation of ∼160 mV) and by a relatively fast follow-up reaction (displayed by nearly absent cathodic counterpeak at the standard scan rate of 200 mV s⁻¹). At negative potentials, no reduction behavior is observed. We postulate that the two (quasi)reversible redox couples represent the formation of the “phenoxy radical” in the calix[4]arene unit, which is further oxidized to “cyclohexadienone” in the second step (Fig. 5). Such electrochemical transformations related to these species have been previously reported in p-substituted phenolic molecules.¹⁸ Due to the absence of electronic communication between the two phenolic units within the C_2v symmetrical calix[4]arene, the oxidation reactions within each phenolic unit occur simultaneously, resulting in overlapping of the anodic peaks/waves.

Fig. 5 (a) Cyclic voltammograms (200 mV s⁻¹) of oxidation of 1–4 (1 light green; 2 dark green; 3 pink; 4 purple). (b) RD-voltammograms (1000 rpm) of oxidation of 1–4. (c) Cyclic voltammograms (200 mV s⁻¹) of oxidation of 5 (orange) and 6 (blue). (d) RD-voltammograms (1000 rpm) of oxidation of 5 and 6. (e) Proposed oxidation mechanism.

CV and RDV of the [Ln^IIIL{Mo₅O₁₃(OMe)₄(NO)}]²⁻ family (Fig. 5b and d) exhibit two fully reversible one-electron oxidation processes at 0.53 ± 10 mV and 0.78 ± 20 mV (CV-peak separation about 60–80 mV and comparable cathodic/anodic peak current ratio). The observed potential difference of approximately 260 mV between the two reversible oxidation steps in 5 and 6 is in good agreement with the range of values found in 1–4. This alignment suggests that 5 and 6 exhibit the same oxidation processes, with only a shift by about 200 mV towards more positive values, attributed to the slightly more intensive electron-withdrawing effect of the [Mo₅O₁₃(OMe)₄(NO)]³⁻ unit comparing to the acac ligands. At negative potentials, four reduction signals can be observed in 5 and 6 (Fig. S25†), probably related to the reduction and re-oxidation of four Mo^IV and one Mo^II ions in [Mo₅O₁₃(OMe)₄(NO)]³⁻, given the non-innocent nature of this ligand.

By comparing the oxidation processes in 1–6, we found that the first anodic potential in 1–4 (0.33 V), and 5 and 6 (0.53 V) are identical. This observation indicates that the type of lanthanides has a small influence on the electronic properties of the phenoxy radical. However, in the second oxidation step, early lanthanides Pr (1) and Nd (2, 5) complexes are oxidized at less positive potentials than late lanthanides Ho (3) and Er (4, 6) by approximately 60 mV. Here, the subtle yet consistent shift in the oxidation potentials is affected by the coordination to the lanthanide metals, which agrees with the different nature of the bond as depicted in Fig. 5e, right.

Conclusions

In summary, we expand our study on the two previously reported families of monometallic lanthanide complexes to early and late lanthanide metals featuring (N(nBu)₄)[Ln^IIIL(acac)₂]·CH₃CN (Ln = Pr (1), Nd (2), Ho (3), and Er (4)) and (N(nBu)₄)₂[Ln^IIIL{Mo₅O₁₃(OMe)₄(NO)}]·CH₂Cl₂ (Ln = Nd (5) and Er (6)). The erbium complex (4) shows typical SIM behavior in the presence of a bias field, where different relaxation processes are active. The analysis of the magnetization reversal indicates both QTM and Raman processes. Given that uncoordinated H₂L is not redox-active by itself, the coordination to the lanthanide metal ions triggers a new type of electroactivity in the calix[4]arene ligand without the necessity of its derivatization with redox-active groups (e.g. ferrocene, nitro, bipyridine, cobaltocenium). The redox waves observed by cyclic voltammetry are reminiscent of the ones obtained with the tetrathiafulvalene (TTF) ligand and derived compounds. This finding paves the way for future endeavors in the creation of new families of calix[n]arene-based monometallic lanthanide complexes, in which to explore the intrinsic electrochemical activity and to study shifts in the redox activity associated with their host/guest chemistry ability of the calix[4]arene unit.

Experimental section

All reagents (99.9%) used in this study were obtained from commercial sources. High-grade dry solvents were obtained using a MBRAUN MB-SPS 800 solvent purification system. (N(nBu)₄)[Ln^IIIL(acac)₂]·CH₃CN and (N(nBu)₄)₂[Ln^IIIL{Mo₅O₁₃(OMe)₄(NO)}]·CH₂Cl₂ were synthesized following our previous work.¹³

Synthesis of (N(nBu)₄)[Ln^IIIL(acac)₂]·CH₃CN

Acetylacetone (0.20 mL, 2.0 mmol) and N(nBu₄)OH (1.30 mL from a 40 wt% in MeOH, 2.0 mmol) were dissolved in 40 mL of MeOH and stirred at 60 °C for two hours. The solvent was then evaporated to dryness resulting in a yellow oil of N(nBu₄)(acac). The obtained product, Ln^IIICl₃ (74.72 mg for PrCl₃, 135.64 mg for HoCl₃, 125.29 mg for NdCl₃, 136.80 mg for ErCl₃, 0.50 mmol), and H₂L (338.49 mg, 0.50 mmol) were dissolved in 50 mL of dry toluene under Ar and stirred at 135 °C for six hours. The resulting solution was cooled, filtered, evaporated to dryness, and washed with CH₂Cl₂ to obtain a white powder. Colorless single crystals were obtained by slow diffusion of CH₃CN in a concentrated solution of THF containing the product. Yield (235 mg, 39% for 1); (277 mg, 42% for 2); (260 mg, 40% for 3), and (240 mg, 36% for 4).

Synthesis of (N(nBu)₄)₂[Ln^IIIL{Mo₅O₁₃(OMe)₄(NO)}]·CH₂Cl₂

(N(nBu)₄)[Ln^IIIL(acac)₂]·CH₃CN (0.1 mmol) and (N(nBu)₄)₂[Mo₅O₁₃(OMe)₄(NO)(Na(MeOH))] (113.52 mg, 0.1 mmol) were dissolved in 50 mL of dry CH₃CN and stirred under reflux for 12 hours. The resulting solution was then filtered and evaporated to yield a brown oil. Consecutive re-dissolution, filtration, and evaporation of this crude in CH₂Cl₂, CH₃CN, and THF yielded a brown solid free of unreacted starting materials and by-products. This solid was washed with 500 mL Et₂O, filtered, and evaporated to afford a brown solid. Brown single crystals were obtained by slow diffusion of hexane in a concentrated solution of CH₂Cl₂ containing the product. Yield (39 mg, 18% for 5) and (33 mg, 15% for 6).

Crystal data for (1) (CCDC 2299497†): C₇₄H₁₁₁N₂O₈Pr, M_r = 1297.55 g mol⁻¹, colorless plate, 0.07 × 0.10 × 0.27 mm³, monoclinic, space group C2/c, a = 23.746(5) Å, b = 16.124(3) Å, c = 21.064(4) Å, α = 90°, β = 120.16(3)°, γ = 90°, V = 6973(3) ų, Z = 4, STOE STADIVARI diffractometer, MoKα radiation (λ = 0.71073 Å), T = 100 K, 6124 reflections collected, 36 655 unique (R_int = 0.0828), 4056 observed (I > 2σ(I)). Final GooF = 1.018, R₁ = 0.0611 (I > 2σ(I)) and wR₂ = 0.1612 (all data).

Crystal data for (2) (CCDC 2299498†): C₇₄H₁₁₁N₂O₈Ho, M_r = 1321.57 g mol⁻¹, colorless plate, 0.05 × 0.05 × 0.20 mm³, monoclinic, space group C2/c, a = 23.831(5) Å, b = 16.057(3) Å, c = 21.071(4) Å, α = 90°, β = 120.42(3)°, γ = 90°, V = 6953(3) ų, Z = 4, STOE STADIVARI diffractometer, MoKα radiation (λ = 0.71073 Å), T = 100 K, 6136 reflections collected, 36 272 unique (R_int = 0.0697), 5110 observed (I > 2σ(I)). Final GooF = 1.043, R₁ = 0.0581 (I > 2σ(I)) and wR₂ = 0.1486 (all data).

Crystal data for (3) (CCDC 2299499†): C₇₄H₁₁₁N₂O₈Nd, M_r = 1300.88 g mol⁻¹, colorless plate, 0.05 × 0.11 × 0.32 mm³, monoclinic, space group C2/c, a = 23.771(5) Å, b = 16.146(3) Å, c = 21.069(4) Å, α = 90°, β = 120.26(3)°, γ = 90°, V = 6984(3) ų, Z = 4, STOE STADIVARI diffractometer, MoKα radiation (λ = 0.71073 Å), T = 100 K, 6166 reflections collected, 35 449 unique (R_int = 0.1083), 4033 observed (I > 2σ(I)). Final GooF = 1.035, R₁ = 0.0697 (I > 2σ(I)) and wR₂ = 0.1848 (all data).

Crystal data for (4) (CCDC 2299500†): C₇₄H₁₁₁N₂O₈Er, M_r = 1323.90 g mol⁻¹, colorless plate, 0.075 × 0.21 × 0.30 mm³, monoclinic, space group C2/c, a = 23.790(5) Å, b = 16.055(3) Å, c = 21.038(4) Å, α = 90°, β = 120.40(3)°, γ = 90°, V = 6931(3) ų, Z = 4, STOE STADIVARI diffractometer, MoKα radiation (λ = 0.71073 Å), T = 100 K, 6114 reflections collected, 32 953 unique (R_int = 0.0657), 5309 observed (I > 2σ(I)). Final GooF = 1.047, R₁ = 0.0579 (I > 2σ(I)) and wR₂ = 0.1487 (all data).

Crystal data for (5) (CCDC 2299501†): C₈₃H₁₄₄N₃O₂₂Cl₂Mo₅Nd, M_r = 2230.84 g mol⁻¹, brown needle, 0.10 × 0.14 × 0.41 mm³, monoclinic, space group C2/c, a = 67.598(14) Å, b = 12.541(3) Å, c = 23.425(5) Å, α = 90°, β = 103.49(3)°, γ = 90°, V = 19 311(7) ų, Z = 8, STOE STADIVARI diffractometer, MoKα radiation (λ = 0.71073 Å), T = 100 K, 16 875 reflections collected, 87 889 unique (R_int = 0.0652), 13 308 observed (I > 2σ(I)). Final GooF = 1.025, R₁ = 0.0490 (I > 2σ(I)) and wR₂ = 0.1430 (all data).

Crystal data for (6) (CCDC 2299502†): C₈₃H₁₄₄N₃O₂₂Cl₂Mo₅Er, M_r = 2253.86 g mol⁻¹, brown needle, 0.07 × 0.20 × 0.33 mm³, monoclinic, space group C2/c, a = 66.701(13) Å, b = 12.581(3) Å, c = 23.523(5) Å, α = 90°, β = 103.25(3)°, γ = 90°, V = 19 311(7) ų, Z = 8, STOE STADIVARI diffractometer, MoKα radiation (λ = 0.71073 Å), T = 100 K, 16 897 reflections collected, 85513unique (R_int = 0.0898), 11 966 observed (I > 2σ(I)). Final GooF = 1.005, R₁ = 0.0660 (I > 2σ(I)) and wR₂ = 0.1833 (all data).

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank the Chinese Scholarship Council (CSC) and the Punjab Educational Endowment Fund (PEEF) for Dr Yushu Jiao and Dr Sidra Sarwar's scholarships. J. Ludvík and L. Koláčná are grateful to the grant GAČR 230646 S and to the institutional support RVO 61388955.

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Footnote

† Electronic supplementary information (ESI) available: Additional figures and experimental data related to the discussion of results. CCDC 2299497–2299502. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3dt03828a

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