Metallacrown of Ce^IIICu^II₅: Synthesis, Structural Characterization and Insights for Nanoparticles ^†

Abstract

The heterobimetallic 15-MC-5 metallacrown of formula [CeCu₅(5mpzHA)₅(NO₃)(H₂O)₇]·2NO₃·7H₂O, designated MC-Ce, was synthesized using 5-methyl-2-pyrazinehydroxamic acid (5mpzHA) as a linker, reacting with Ce^III and Cu^II salts under mild conditions. Single-crystal X-ray diffraction analysis reveals a crown-like [Cu₅Ce(5mpzHA)₅] core, characteristic of a 15-MC-5 system, with five Cu^II atoms at the rim of the crown and the Ce^III ion occupying the dome of the crown, with water molecules, oxygen atoms and one nitrate anion filling the nine-coordination sphere around the Ce^III ion, which exhibits a distorted spherical tricapped trigonal prism geometry. The thermogravimetric analysis evidences successive mass losses due to the removal of water molecules and decomposition of the structure after 217 °C, whereas the PXRD analysis of the thermal decomposition residue reveals the presence of copper and copper/cerium oxide particles. These nanocomposite materials were also synthesized using the metallacrown MC-Ce under a hydrothermal method in the presence of multi-walled carbon nanotubes (MWCNTs), affording insights that this metallacrown can act as a source precursor for the synthesis of these mixed cerium/copper oxide nanomaterials. The experimental χMT value in MC-Ce at room temperature is 3.175 cm³ mol⁻¹ K, which is higher than the calculated one for one magnetically isolated Ce^III plus five Cu^II ions, probably due to the antiferromagnetic interactions among Cu^II ions within the metallacrown hoop plus the thermal depopulation of JZ sublevels of Ce^III ground state (5/2), which exhibit a small splitting under the anisotropic ligand field effects. The χMT decreases continuously until it reaches the value of 0.80 cm³ mol⁻¹ K at 10 K, reinforcing the presence of intramolecular antiferromagnetic interactions.

1. Introduction

Metallacrowns (MCs) are structures containing a cyclic core formed by repeating –[M–N–O]_n– units with an X-MC-Y topological representation [1] that have been developed very rapidly within the metallamacrocycle field [2]. The scope of the size of MCs in the solid state built by the Cu^II center, for example, has been found to be 9-MC-3, 12-MC-4, Ln^III [15-MC-5], and Ln^III [18-MC-6], as summarized by M. V. Marinho et al. [3]. We have focused on MC chemistry with one particular 15-metallacrown-5 [3], assembled by the reaction between 5-methyl-2-pyrazinehydroxamic acid (5mpzHA) with copper(II) and europium(III) salts.

Examining the formation of MCs based on the Ce^III ion into the MC ring, there are only two examples [4,5], and even though it is well known that the Ce^III ion has only one unpaired electron, 4f¹, the expected ground state (L = 3, and S = ½) is ²F_5/2, [6] which contributes to its striking feature of magnetic anisotropy [7]. Interestingly Ce^III-based SMMs are found [7,8,9,10], as well as templates for the synthesis of copper and cerium oxide nanoparticles [4,5], which opened new avenues for applications in sensors, fuel cells, coatings, and catalysts. [5]

Thus, in this contribution, we present the synthesis, results of thermal analysis, and spectroscopic, X-ray diffraction, and magnetic properties of a 15-MC-5 metallacrown of formula [CeCu₅(5mpzHA)₅(NO₃)(H₂O)₇]·2NO₃·7H₂O (MC-Ce). In addition, we prepared nanocomposites of cerium and copper oxides supported on multi-walled carbon nanotubes (MWCNTs) under hydrothermal conditions, using water as the sole solvent in a stainless-steel autoclave at 190 °C. This synthetic approach was motivated by the nature of the metallacrown precursor MC-Ce, which contains metal–oxygen bonds with required metal atoms and undergoes thermal decomposition at relatively low temperatures. These characteristics enabled it to function as a single-source precursor, offering strong support for forming Cu₂O/CeO₂/MWCNTs nanocomposites.

2. Experimental

2.1. Materials

Cerium(III) nitrate hexahydrate (99.9%), 5-methyl-2-pyrazinecarboxylic acid (5mpca) (98%), and multi-walled carbon nanotube (MWCNT, 50–90 mm diameter, >95% carbon) were purchased from Sigma-Aldrich. Copper(II) acetate hydrate (98%) was purchased from Synth (Brazil). All chemicals were used as purchased, without further purification. 5-methyl-2-pyrazinehydroxamic acid (5mpzHA) was synthesized as described in the literature [3]. MC-Ce was synthesized following a reported method for the Eu^III analog [3], reacting Ce(NO₃)₃·6H₂O salt with Cu(OAC)₂·H₂O salt and 5mpzHA acid, as described above:

[CeCu₅(5mpzHA)₅(NO₃)(H₂O)₇]·2NO₃·7H₂O (MC-Ce): 5mpzHA (0.017 g, 0.110 mmol), and Cu(AcO)₂·H₂O (0.023 g, 0.110 mmol) were dissolved in methanol (10 mL) and the solution was kept under stirring at 60 °C for 1 h. A methanolic solution (10 mL) of Ce(NO₃)₃·6H₂O (0.009 g, 0.021 mmol) was added and the reaction was stirred at 60 °C for an additional 1 h. The solution color changed from brown to emerald green 15 min after adding cerium(III) salt. Thus, the solution was filtered and undisturbed at 18 °C, yielding dark green plate-like crystals within 2–4 weeks. Yield: 45%. ATR-FTIR (ν/cm⁻¹): 3355_(νO-H); 3239_(νO-H); 3033_(νC-H); 1604_(νC=O); 1580_(νCC/CN); 1544_(νC-Car); 1481_{(νN-O(nitrate))}; 1453_(δC-H); 1382_{(νN-O(nitrate))}; 1299, 1250_{(νN-O(nitrate))}; 1199_(δC-N-O); 1070_(δC-H); 1047_(νN-O); 991_{(νN-O(nitrate))}.

Copper and cerium oxides on MWCNTs were synthesized following a procedure analogous to that described in the literature [4,5] with only minor adjustments to the quantities of precursors. A quantity of MWCNTs (0.015 g) was added to aqueous metallacrown MC-Ce (0.015 g) in 5 mL of water, and the mixture was stirred and sonicated for 1.5 h at room temperature to obtain a suspension. The mixture was transferred to a 25 mL Teflon-lined stainless steel autoclave and heated at 190 °C for one day. The resulting precipitate was separated by centrifugation, washed with distilled water and acetone, and dried in an oven at 110 °C.

2.2. Characterization

The FT-IR spectrum was collected using a Shimadzu model FT-IR-Prestige 21 ATR and spectral scan between 4000 and 400 cm⁻¹. Thermal analyses (TGA) (Netzsch, Germany, STA 449 F3 Jupiter) were performed with the simultaneous thermal analyzer in the temperature range 30–1100 °C by using alumina crucibles and ca. 10 mg of the sample. A dinitrogen flow of 100 cm³ min⁻¹ with a heating rate of 10 °C min⁻¹ was used. The Powder X-ray diffraction data were collected at room temperature using a Rigaku Ultima IV (Rigaku, Japan) with Cu-Kα radiation (1.5418 Å, 40 kV, 30 mA) from 3° to 50° and 10 to 100° (2θ) using a step size of 0.02° (Figure S1, in Supporting Information). The magnetic measurements were performed in a SQUID magnetometer (model MPMS3), Quantum Design Inc., San Diego, CA, USA) for dc magnetic measurements and a PPMS for ac magnetic susceptibility measurements, both from the Quantum Design company. Data were collected for the polycrystalline sample crushed in a VSM powder sample holder.

2.3. X-Ray Crystallography

X-ray diffraction data collection for MC-Ce was performed with a GEMINI-A Ultra diffractometer (Rigaku-Oxford Diffraction, USA) diffractometer equipped with Mo-Kα radiation (λ = 0.71073 Å). Data integration and scaling of the reflections were performed with CrysAlisPro 1.171.40.42a [11]. Final unit cell parameters were based on the fitting of all reflection positions. Analytical absorption corrections were performed using the CrysAlisPro 1.171.40.42a. [12] Compound MC-Ce displayed large accessible voids, which are filled with disordered crystallization solvent molecules or counterions. Efforts to unambiguously identify all of them were unsuccessful and the solvent mask routine implemented on the Olex2 program was used [13]. One void presenting a volume of 552 ų was found per unit cell, accounting for 182 electrons per unit cell. This number of electrons is consistent with the presence of one nitrate and six crystallization water molecules per formula unit. This assumption is consistent with elemental analysis and thermal results. The MERCURY [14] program was used to prepare artwork representations. Crystal data and details of the data collection and refinement for MC-Ce are listed in

Table 1. Summary of the crystal data and refinement details for MC-Ce.

Formula	C30H53Cu5CeN18O33
Formula weight (g·mol−1)	1651.69
Crystal system	Triclinic
Space group	P1
T/K	293
Wavelength (Å)	0.71073
a/Å	11.6894(2)
b/Å	15.4536(4)
c/Å	16.1745(5)
α (°)	79.469(2)
β (°)	87.062(2)
γ (°)	81.301(2)
V (Å3)	2838.72(13)
Z	2
ρ/mg cm−3	1.733
μ (mm−1)	2.71
F(000)	1470
Crystal size (mm)	0.35 × 0.25 × 0.03
θ range/(°)	2.2–29.6
Reflns. with I > 2σ(I)	9434
Independent reflns.	13,295
Rint	0.047
N°. of parameters/GOF on F2	736/1.11
R, wR [I > 2σ(I)]	0.045, 0.105
Largest diff. peak/hole (e Å−3)	0.97/−0.79

w = 1/[σ²(Fo²) + (0.0405P)² + 0.0994P] where P = (Fo² + 2Fc²)/3.

. All crystallographic data are available in the supporting information section as CIF files. CCDC number is 2388656.

3. Results and Discussion

The main characteristic observed in the IR spectrum is the vibrational stretching frequency at 1604 cm⁻¹ of the ν_C=O band, which indicates the double deprotonation of the 5mpzHA ligand and the coordination of the amide-type nitrogen and oxygen atoms to metal ions. The coordination of the ligand can also be noted by the presence of the νCC/CN at 1580 cm⁻¹ as well as bands at 1480, 1398, 1337, and 1297 cm⁻¹ due to the presence of nitrate anions [3].

The thermogravimetric analysis (Figure 1) shows an initial weight loss between 39 and 217 °C, attributed to removing coordinated and uncoordinated water molecules, totaling 14 water molecules (obsd. 14.6; calcd. 15.3%). Subsequent weight loss steps are observed up to 900 °C, likely corresponding to the decomposition of MC-Ce, involving the loss of nitrate anions (3 molecules) and the 5-methyl-2-pyrazinehydroximate (5mpzA²⁻, 3 molecules), resulting in 41.4% of the compound (obsd. 38.7%). The significant residue can be associated with the presence of copper, copper oxide, and cerium oxide phases in the sample [Cu + 2 Cu₂O + CeO₂] [4,5], as evidenced by the PXRD analysis (Figure 2).

In addition, our powder X-ray investigation indicates that the solid obtained from the reaction of the precursor MC-Ce with MWNTs contains copper and cerium oxides, as previously reported for other lanthanide-copper 15-MC-5 compounds [4,5] (see Figure 3).

Thus, our findings based on an X-ray investigation of the residue from the MC-Ce thermal decomposition in an inert atmosphere show the obtaining of cerium and copper oxide mixture, whose nanocomposite materials were also evidenced by reacting with the MC-Ce with MWCNT under hydrothermal conditions. To our knowledge, this is the third report involving metallacrown systems for preparing cerium–copper mixed oxides. Earlier, 15-MC-5 complexes and MWCNTs were used as unique precursors via a simple hydrothermal method [4,5] by S. Y. Ketkov et al. using a polynuclear metallamacrocyclic complex and Ce(H₂O)₄[15MCCuGlyha-5]Cl₃ [4] with glycinehydroxamic acid (Glyha), as well as by I. L. Eremenko et al. [5] using a macrocycle [Ce(H₂O)₄[15-MCCuRha-5] with R = Glyha and α-phenylalaninehydroxamic acid (Phalaha). In this case, cerium and copper oxide nanocrystals of various sizes and morphologies were obtained [5]. In our study, we employ the metallacrown [CeCu₅(5mpzHA)₅(NO₃)(H₂O)₇]·2NO₃·7H₂O (MC-Ce) leveraging its high solubility in mixtures of water and polar solvents, such as ethanol and methanol, as well as its decomposition properties at a relatively low temperature (around 217 °C) and under hydrothermal method also utilizing MWCNTs as templates, to achieve similar objectives.

In addition, PXRD experiments were also carried out with the crushed crystals of MC-Ce, showing a good coincidence between the experimental and calculated patterns (Figure S1, Supporting Information), confirming that the single-crystal X-ray structure is the same as the as-synthesized bulk material.

3.1. X-Ray Diffraction

The crystal structure of MC-Ce was solved in the centrosymmetric triclinic P1 space group, probing the formation of a typical 15-MC-5 metallacrown with the general formula [CeCu₅(5mpzHA)₅(NO₃)(H₂O)₇]·2NO₃·7H₂O. The asymmetric unit is composed of a crown-like [Cu₅Ce(5mpzHA)₅] core, containing five deprotonated 5mpzHA²⁻ ligands, five Cu^II ions, one Ce^III ion through a μ₃-κ²N,N′:κ²O,O′:κO′ chelating–bridging mode (Figure 4). Thermal ellipsoids are shown in Figure S2 (ESI).

The Ce^III ion is nine-coordinated to five 5mpzHA oxygen atoms, two water molecules oxygen atoms, and two nitrate group oxygen atoms, exhibiting a coordination environment of a spherical tricapped trigonal prism geometry. The SHAPE software (Version 2.1) [15] was used to determine the degree of distortion of the coordination polyhedral of the central Ce^III ion to MC-Ce concerning the ideal nine-vertex coordination polyhedral CeO₉. The lowest continuous shape measurement (CShM) values correspond to the spherical tricapped trigonal prism (D_3h) (CShM = 5.874), spherical capped square antiprism (C_4v) (CShM = 6.649), and Muffin (C_s) (CShM = 6.549) (values in Table S1). The Ce–O bond distances in MC-Ce are in the range from 2.489(3) to 2.653(3) Å, where the distortion observed around the Ce^III geometry can be associated with the chelating nitrate ion that fills the coordination sphere, showing mostly longer bond distances [2.617(3) to 2.653(3) Å] [16].

Four copper (II) ions are pentacoordinate, exhibiting a distorted square pyramidal geometry, where each one is coordinated to the 5mpzHA ligands (basal plane) and one water molecule (apical position). The values for the trigonality parameter τ [17] are 0.11, 0.05, 0.011, and 0.042 for Cu1, Cu2, Cu4, and Cu5, respectively. The copper(II) ions are shifted from the mean basal plane by 0.043 (Cu1), 0.125 (Cu2), 0.052 (Cu3), 0.108 (Cu4), and 0.168 (Cu5) Å toward the apical position.

One copper(II) ion is coordinated to the 5mpzHA ligands in the basal plane, one water molecule, and one nitrate anion in the apical position, resulting in a distorted octahedral environment. The aqua ligands completing the copper(II) coordination sphere exhibit relatively longer bond distances [Cu1-O(24) = 2.784(4); Cu2-O(11) = 2.388(3); Cu3-O(22) = 2.531(3); Cu4-O(23) = 2.443(4); and Cu5-O(14) = 2.370(3) Å] compared to the bond distances in 5mpzHA (see

Table 2. Selected distances (in Å) for MC-Ce.

Atoms Label	Distance	Atoms Label	Distance
Cu1—N13	2.007(4)	Cu5—O9	1.944(3)
Cu1—N15	1.908(3)	Cu5—O10	1.928(4)
Cu1—O1	1.941(3)	Cu5—O14	2.370(3)
Cu1—O2	1.922(3)	Ce1—O2	2.522(3)
Cu1—O24	2.784(4)	Ce1—O4	2.489(3)
Cu2—N1	2.015(3)	Ce1—O6	2.496(3)
Cu2—N3	1.918(4)	Ce1—O8	2.502(3)
Cu2—O3	1.931(3)	Ce1—O10	2.505(3)
Cu2—O4	1.927(3)	Ce1—O12	2.530(3)
Cu2—O11	2.388(3)	Ce1—O13	2.593(3)
Cu3—N4	2.029(4)	Ce1—O15	2.617(3)
Cu3—N6	1.919(3)	Ce1—O16	2.653(3)
Cu3—O5	1.963(3)	Ce1···Cu1	3.9426(7)
Cu3—O6	1.930(3)	Ce1···Cu2	3.9567(6)
Cu3—O18b	2.69(1)	Ce1···Cu3	3.9704(8)
Cu3—O22	2.531(3)	Ce1···Cu4	3.8821(6)
Cu4—N7	1.992(3)	Ce1···Cu5	3.8944(7)
Cu4—N9	1.927(3)	Cu1···Cu2	4.5727(9)
Cu4—O7	1.946(3)	Cu2···Cu3	4.5987(8)
Cu4—O8	1.928(3)	Cu3···Cu4	4.6205(8)
Cu4—O23	2.443(3)	Cu4···Cu5	4.5929(8)
Cu5—N10	1.996(4)	Cu5···Cu1	4.5880(7)
Cu5—N12	1.919(3)

).

The average shortest intramolecular Ce···Cu and Cu···Cu distances are 3.9292(6) Å and 4.5945(8) Å, respectively (Table 2). The shortest intermolecular distances between metal ions in the crystal packing are 5.8957(7) Å (Cu4···Cu5i), 6.5831(6) Å (Ce1···Cu5ⁱ), and 7.9041(5) Å (Ce1···Ce1ⁱ), where I = 1 − x, 1 − y, 1 − z. In addition, the O-Ce-O angles of the 5mpzHA²⁻ ligands range from 68.71(9)° (O2-Ce1-O10) to 71.68(9)° (O6-Ce-O8).

In MC-Ce, supramolecular dimers with a Ce···Ce separation of 7.9041(5) Å are formed via a series of O−H···O hydrogen bonds [O13−H13a···O14ⁱ = 170(6)°, O13···O14ⁱ = 2.857(5) Å; O23−H23b···O24ⁱ = 172(4)°, O23···O24ⁱ = 2.855(5) Å; and O14-H14a···O23ⁱ = 156(7), O14···O23i = 2.835(5); O14-H14b···O18bi = 143(5), O14···O18bi = 2.711(13) (i) = 1 − x, 1 − y, 1 − z)], as shown in Figure 5 (top). Moreover, a network of hydrogen bonds involving the crystallization water molecule (O25), aqua ligands (O11, O22, and O24), and the pyrazine nitrogen atoms (N5 and N14) contributed to the stabilization of the crystal packing (Figure 5, bottom). Geometric parameters for these intermolecular interactions are provided in Table S3-ESI.

3.2. Magnetic Properties

dc Magnetic susceptibility was recorded in the 2–300 K temperature range for MC-Ce under a dc magnetic field of 1000 Oe. The thermal dependence of the χ_MT product is shown in Figure 6, and it has a temperature behavior similar to that observed for metallacrown with other lanthanide metallacrown complexes [3,18]. The experimental χ_MT value at room temperature is 3.175 cm³ mol⁻¹ K, which is higher than the calculated one for one magnetically isolated Ce^III (J = 5/2, g = 6/7, χ_MT = 0.80 cm³ mol⁻¹) plus five Cu^II ions (S = 1/2, g = 2, χ_MT = 1.875 cm³ mol⁻¹ K). Upon cooling, χ_MT decreases continuously until it reaches the value 0.80 cm³ mol⁻¹ K at 10 K; this magnetic behavior is attributed to the antiferromagnetic interactions among Cu^II ions within the metallacrown hoop plus the thermal depopulation of J_Z sublevels of Ce^III ground state (5/2), which exhibit a small splitting under the anisotropic ligand field effects. At around 10 K, it exhibits a further slight decrease, which also suggests the presence of intramolecular antiferromagnetic interactions. The experimentally obtained χ_MT data for the MC-Ce were fitted using the previously reported additive model by means of the PHI program with a Hamiltonian (Equation (1)) that considers the exchange interactions between adjacent Cu^II ions (Ĥ_Ex(Cu-Cu)) to be dominant. The magnetic contribution of Ce^III was considered by adding to the fitting model a free ion approximation to account for the magnetic anisotropy by using the anisotropic Hamiltonian (Ĥ_CF(Ce)). The exchange interaction between Cu^II and Ce^III is expected to be very weak, and it was neglected to avoid overparameterization. To account for weak magnetic interactions in the system, including intermolecular magnetic interactions and exchange interactions between Cu^II and Ce^III, a molecular field model was applied (Equation (4)):

Ĥ = Ĥ_Ex(Cu-Cu) + Ĥ_CF(Ce) + Zeeman terms

(1)

Ĥ_Ex(Cu-Cu) = 2J (Ŝ₁·Ŝ₂ + Ŝ₂·Ŝ₃ + Ŝ₃·Ŝ₄ + Ŝ₄·Ŝ₅ + Ŝ₅·Ŝ₁)

(2)

Ĥ_CF(Ce) = B₂⁰Ô₂⁰ + B₄⁰Ô₄⁰

(3)

χ_MT = χ_MT/[1 − (2zJ′χ_MT/N_Ag² μ_B²)]

(4)

in which J is the isotropic exchange interaction parameter between adjacent Cu^II ions in the metallacrown unit, and Ŝ_i is the spin operator for the corresponding Cu^II ions. The anisotropic Hamiltonian considers the second and fourth-order axial anisotropy terms (B₂⁰ and B₄⁰) with the corresponding Stevens’ operator equivalents (Ô₂⁰ and Ô₄⁰). As shown in Figure 6, the calculated curve (solid line) matches the experimental data in the investigated temperature range quite well. The best fit was achieved with J = −51.8(1) cm⁻¹, zJ′ = −0.90(5) cm⁻¹, TIP = 0.003 cm³ mol⁻¹ (TIP is the temperature-independent paramagnetism), g_Cu= 2.148(2), and g_Ce= 0.857(3). These values are close to previously reported data for similar metallacrown core Ln^III-Cu^II5 [9] obtained for the anisotropy coefficients (B₂⁰ = −13.0(2), B₄⁰ = −0.468(10) cm⁻¹), and are in the same order of values observed for other complexes based on Ce^III ions [18].

The experimental magnetic field dependence of the magnetization at three different temperatures (2, 4, and 8 K) is shown in Figure 7. When the external magnetic field reaches 7 T, the magnetization attains a maximum value of 1.45 NμB. Despite the gradual increases at a high field, no saturation of the magnetization is observed, suggesting significant anisotropy. At low temperatures, the contribution of the adjacent uncompensated Cu^II ion (S = 1/2) is expected alongside the contribution of the Ce^III ion (S = 1/2), which would result in a saturation magnetization of 2 NμB. In addition to the anisotropy effect, as highlighted in the inset of Figure 7, the large discrepancy between the experimental value and the expected saturation magnetization value may be attributed to a frustration effect on the uncompensated Cu^II ions [18]. This discrepancy is evident when the simulated magnetization curves, calculated using PHI with parameters derived from the susceptibility fit, are compared to the experimental magnetization data, shown in Figure S3.

To explore the possible dynamics of the magnetization of the Ce–Cu5 complex, alternating current (ac) magnetic susceptibility measurements were conducted in the temperature range of 2.0–10.0 K under an applied field up to 10 kOe. This study showed the lack of any significant out-of-phase signal for MC-Ce under zero and non-zero applied dc fields, evidencing that the MC-Ce does not exhibit slow magnetic relaxation.

4. Conclusions

This work describes a new 15-MC-5 metallacrown containing Ce^III and Cu^II centers of the formula [CeCu₅(5mpzHA)₅(NO₃)(H₂O)₇]·2NO₃·7H₂O (MC-Ce). The Ce^III ion features a nine-vertex CeO₉ spherical tricapped trigonal prism coordination environment. The thermal analysis revealed the collapse of the structure after 217 °C, and the phase composition of the residue was determined using PXRD, showing the presence of copper and copper/cerium oxide particles. These nanocomposite materials were synthesized using the metallacrown MC-Ce via a hydrothermal method at 190 °C in the presence of MWCNTs, providing evidence that this metallacrown can serve as a template for the synthesis of nanoparticles. Inspired by this result, further investigations into the functionality of these nanocomposites could reveal their potential applications. The experimental χ_MT value at room temperature was 3.175 cm³ mol⁻¹ K, higher than the calculated one for one magnetically isolated Ce^III plus five Cu^II ions (χ_MT = 1.875 cm³ mol⁻¹ K). The χ_MT decreases continuously until it reaches the value of 0.80 cm³ mol⁻¹ K at 10 K, where this magnetic behavior was attributed to the antiferromagnetic interactions among Cu^II ions within the metallacrown hoop plus the thermal depopulation of J_Z sublevels of Ce^III ground state (5/2), which exhibit a small splitting under the anisotropic ligand field effects.