New Heterotrinuclear CuIILnIIICuII (Ln = Ho, Er) Compounds with the Schiff Base: Syntheses, Structural Characterization, Thermal and Magnetic Properties

New heterotrinuclear complexes with the general formula [Cu2Ln(H2L)(HL)(NO3)2]·MeOH (Ln = Ho (1), Er (2), H4L = N,N′-bis(2,3-dihydroxybenzylidene)-1,3-diaminopropane) were synthesized using compartmental Schiff base ligand in conjugation with auxiliary ligands. The compounds were characterized by elemental analysis, ATR-FTIR spectroscopy, X-ray diffraction, TG, DSC, TG-FTIR and XRD analysis. The N2O4 salen-type ligand coordinates 3d and 4f metal centers via azomethine nitrogen and phenoxo oxygen atoms, respectively, to form heteropolynuclear complexes having CuO2Ln cores. In the crystals 1 and 2, two terminal Cu(II) ions are penta-coordinated with a distorted square-pyramidal geometry and a LnIII ion with trigonal dodecahedral geometry is coordinated by eight oxygen atoms from [CuII(H2L)(NO3)]− and [CuII(HL)(NO3)]2− units. Compounds 1 and 2 are stable at room temperature. During heating, they decompose in a similar way. In the first decomposition step, they lose solvent molecules. The exothermic decomposition of ligands is connected with emission large amounts of gaseous products e.g., water, nitric oxides, carbon dioxide, carbon monoxide. The final solid products of decomposition 1 and 2 in air are mixtures of CuO and Ho2O3/Er2O3. The measurements of magnetic susceptibilities and field dependent magnetization indicate the ferromagnetic interaction between CuII and HoIII ions 1.

As a continuation of the investigation on salen-type Schiff base complexes, the aim of this work was to obtain heteronuclear species with N,N -bis(2,3-dihydroxybenzylidene)-1,3diaminopropane (the ligand is characterized by the presence in a meta position of a benzene ring -OH substituent instead of -OCH 3 ) and study their properties, as well as investigate the influence of the kind of the additional functional groups in the ligand and kind of lanthanide(III) ions on the structure and feature of the 3d-4f compounds. So far, starting from the N,N -bis(2,3-dihydroxybenzylidene)-1,3-diaminopropane and respective Cu 2+ and Ln 3+ salts, we have synthesized heteropolynuclear complexes with different structures and physicochemical properties [28][29][30]. In the case of the first half of the lanthanide row (La III , Pr III , Nd III ), the inert heterotrinuclear compounds Cu II Ln III Cu II which differ only in the amount and type of solvent molecules in the outside coordination sphere were obtained. In the crystals of copper(II) and praseodymium(III)/neodymium(III) complexes, the antiferromagnetic coupling of magnetic centers occurred [28]. The hexanuclear cation complex resulted from the simultaneous coordination of two dianionic Schiff bases to Cu II and Gd III ions and the forming of trinuclear units [Cu II 2 Gd III ] that were connected through bridging nitrate ions. The interaction between neighboring Cu II and Gd III ions was ferromagnetic [29]. We also obtained and characterized the heterodinuclear Cu II Dy III compound. Its magnetic measurements showed the weak ferromagnetic interaction between Cu II and Dy III ions [30]. It was noticed that in the heterodi-, heterotri-and heterohexanuclear complex crystals reported by us so far, the N,N -bis(2,3-dihydroxybenzylidene)-1,3-diaminopropane was double deprotonated.
Herein, we report the synthesis and crystal characterization, along with the spectral, thermal and magnetic properties of new heterotrinuclear compounds [Cu 2 Ln(H 2 L)(HL) (NO 3 ) 2 ]·MeOH (where Ln = Ho, Er) contained in the crystals dianionic H 2 L 2− and trianionic HL 3− Schiff base ligands. The complexes were synthesized in a step-wise manner without the isolation of the mononuclear complex.

Complexes [Cu 2 Ln(H 2 L)(HL)(NO 3 ) 2 ]·MeOH (1, 2)
The complexes were prepared following the same general procedure: a methanol solution (10 mL) of copper acetate monohydrate (0.4 mmol, 0.0799 g) was added to a hot methanol solution (50 mL) of the Schiff base H 4 L (0.4 mmol, 0.1248 g) to produce a green colored mixture which was magnetically stirred. After 30 min, a methanol solution (5 mL) containing dissolving Ho(NO 3 ) 3 ·5H 2 O (0.2 mmol, 0.0887 g) or Er(NO 3 ) 3 ·5H 2 O (0.2 mmol, 0.0882 g) was added and the resulting mixture was stirred for about 30 min. The resulting clear, deep green solutions were left undisturbed in a refrigerator at~4 • C. The X-ray quality green crystals of the desired compounds were obtained over a period of several days.

Methods
A CHN 2400 Perkin Elmer analyzer was used for determination of C, H and N contents. The metal amounts were determined on an ED XRF spectrophotometer (Canberra-Packard, Schwadorf, Austria). The ATR-FTIR spectra were recorded on a Nicolet 6700 spectrophotometer equipped with the Smart iTR attachment (diamond crystal) over 4000-525 cm −1 . Thermal analysis was carried out by the thermogravimetric (TG) and differential scanning calorimetry (DSC) methods using the SETSYS 16/18 analyzer (Setaram, Lyon, France). The samples 7.61 mg (1) and 6.66 mg (2) were heated in open Al 2 O 3 crucibles in air at the range of 20-1000 • C at a heating rate of 10 • C·min −1 . TGA Q5000 analyzer (TA Instruments, New Castle, DE, USA) interfaced to the Nicolet 6700 FTIR spectrophotometer (Thermo Scientific, Waltham, MA, USA) were applied for the TG-FTIR analysis. The samples in an open platinum crucible were heated from room temperature to 700 • C (heating rate was 20 • C·min −1 ). The temperature in the gas cell and transfer line was set to 250 and 240 • C, respectively. XRD analysis of the solid residue was carried out by using PAN Analytical/Empyrean spectrophotometer. The dc magnetic susceptibilities of the compounds were measured on Quantum Design SQUID-VSM magnetometer in a range of 1.8-300 K. The magnetization curves were recorded at 2K in an applied field up to 5 T. Diamagnetic corrections were estimated from Pascal's constants [32].

X-ray Crystal Structure Determination
Single-crystal data for the complexes were collected on an Oxford Diffraction Xcalibur CCD diffractometer (MoKα radiation, λ = 0.71073Å). The program CrysAlis [33] was used for collecting frames of data, cell refinement and data reduction. A multi-scan absorption correction was applied. Crystal data, data collection and structure refinement details are summarized in Table 1. The structures were solved by direct methods using SHELXS-2018 and refined by the full-matrix least-squares on F 2 using the SHELXL-2018 [34] (both programs implemented in WinGX software [35]). All the non-hydrogen atoms were refined with anisotropic displacement parameters. The H-atoms attached to carbon were positioned geometrically and refined applying the riding model [C-H = 0.93-0.99 Å and with U iso (H) = 1.2 or 1.5 Ueq(C)]. The O-bound H atoms were located on a difference Fourier map and refined freely or with O-H distances restrained to 0.82 Å using DFIX command. The following programs were used to prepare the molecular graphics: ORTEP3 [35] and Mercury [36]. The geometrical calculations were performed using PLATON program [37].

Results and Discussion
N,N -bis(2,3-dihydroxybenzylidene)-1,3-diaminopropane H 4 L is a multidentate ligand which possess six donor atoms, i.e., two imino nitrogen atoms and four oxygen atoms coming from hydroxyl groups. The ligand can act as a bridge between metal ions through phenoxy groups so as to link the 3d and 4f ions together, therefore, it can be used to synthesize 3d-4f complexes. The inner, smaller N 2 O 2 compartment of the Schiff base may accommodate a borderline acid, e.g., copper(II) ion, whereas the other, bigger O 2 O 2 site selectively binds to hard acids, such as lanthanide(III) ion. Using H 4 L, the copper(II) acetate and the holmium(III)/erbium(III) nitrate, we obtained the discrete heterotrinuclear complexes of the general formula [Cu 2 Ln(H 2 L)(HL)(NO 3 ) 2 ]·MeOH (Ln = Ho 1, Er 2) (Figure 3). The neutral complexes are isostructural, crystalize with one CH 3 OH solvent molecule and are characterized by the molar ratio between the Schiff base ligand and the 3d and 4f metal ions 2:2:1. The similar values of ionic radious of Ho III and Er III cations and the same molar ratio of the starting compounds may be the origin of the same crystal structure of 1 and 2.

Infrared Spectra
The FTIR spectra of 1 and 2 (Table 2, Figure S1) are similar. A broad absorption bands in the 2500-3300 cm −1 region can be attributed to the O-H stretching vibrations of methanol molecule (it interferes with the protonated hydroxyl groups of the N 2 O 4 ligand) that are involved in the strong hydrogen bonds.
A broad absorption bands in the 2500-3300 cm −1 region can be attributed to the O-H stretching vibrations of methanol molecule (it interferes with the protonated hydroxyl groups of the N 2 O 4 ligand) that are involved in the strong hydrogen bonds. This feature is in accordance with the X-ray structures, i.e., methanol molecule acts as a proton acceptor as well as a proton donor. The FTIR spectra of complexes have in common the occurrence of a strong absorption band at 1618 cm −1 1 and 1616 cm −1 2 which is characteristic of the presence of the azomethine group C=N. These bands are shifted towards lower frequencies relative to the free Schiff base 1632 cm −1 . This phenomenon is due to the coordination of azomethine nitrogen to the 3d metal ion. The strong bands situated at 1467 cm −1 , 1285 cm −1 and 1024 cm −1 , respectively, in the spectrum of 1 and 1465 cm −1 , 1287 cm −1 and 1024 cm −1 in the spectrum of 2 may be assigned to the monodentate nitrate ligand. The involvement of the phenolic oxygen atoms in the metal-ligand bonding is confirmed by the strong doublet bands observed at 1251 cm −1 , 1218 cm −1 1 and 1248 cm −1 , 1219 cm −1 2, respectively. The typical absorption band of the ν aryl -O vibration is identified in the free ligand spectrum at 1233 cm −1 [4,13,[38][39][40][41][42][43][44]. All these spectroscopic features are confirmed by the X-ray structures.

Crystal and Molecular Structure
The reaction of the Schiff base ligand H 4 L with copper(II) acetate and lanthanide(III) nitrate result in formation of the trinuclear complexes 1 and 2 which are isomorphous and crystallize in the centrosymmetric monoclinic space group P2 1 /c ( Table 1). The asymmetric unit cell of both complexes contains one neutral complex, which consists of two Cu(II) ions, one Ln(III) ion, a dianionic ligand H 2 L 2− , a trianionic ligand HL 3− , two nitrite ions and methanol molecule (Figures 4 and S2).    Cu (2) The Ho III and Er III ion assume a trigonal dodecahedron [O 8 ] configuration ( Figures 5 and S3), while both the partially deprotonated Schiff base ions (H 2 L 2− and HL 3− ) act in similar chelating coordination modes, i.e., lanthanide(III) ion is coordinated by four oxygen atoms of phenoxide or phenol groups of each ligand. A similar binding type of lanthanide (with partially deprotonated Schiff bases, i.e., one dianionic and one trianionic ligand) was reported for a trinuclear complex of Zn II -Tb III -Zn II ions [45]. The Cu(1) and Cu (2)    The crystal structures of 1 and 2 reveal the presence of intramolecular and intermolecular hydrogen bonds (Table S1). In the crystals 1 and 2, the molecules are linked by O(1)-H(1o)···O(1m) and O(1m)-H(1m)···O(5n) a (symmetry code (a): x−1,y,z) hydrogen bonds forming columns propagating along [100] with C 2 2 (10) motifs (Figures 6 and S4). Additional classical hydrogen bonds are supported by weaker non-classical C-H···O contacts, which linked formed columns in 3D supramolecular structure. The partial view of crystal packing for compound 1 and 2 are illustrated in Figures S5 and S6.

Thermal Analysis
In order to examine the thermal behavior of the heteronuclear complexes 1 and 2, the thermogravimetric analysis was carried out (Figures 7 and S7). The results of the thermal analysis allow to confirm/evaluate the presence of solvents (e.g., methanol, water) in the structure of compounds and to establish the endothermic and/or exothermic effects connected with different processes such as dehydration, desolvation, melting or decomposition. The TG and DSC curves recorded for both complexes are similar. The mass of samples decreases slowly with the increasing temperature. The first mass loss occurs up to 90 • C and it is assigned to the elimination of one methanol molecule (mass loss: observed 2.60% 1, 2.70% 2, calculated 2.99% 1; 2.98% 2). The small endothermic effect seen on the DSC curves confirms this process. In the case of compound 1, the decomposition process begins immediately after desolvation. The next mass losses recorded at above 200 • C and accompanied with exothermic effects seen on the DSC curves is connected with gradual decomposition of the samples. Additionally, this process is also confirmed by the TG-FTIR analysis ( Figure S8). The recorded TG-FTIR spectra show that carbon dioxide, carbon monoxide and nitric oxide are mainly emitted during this process. The characteristic doublet bands seen at 2240-2400 cm −1 and 670 cm −1 , respectively, are assigned to stretching and deformation vibrations of carbon dioxide molecules. The specific bands at 2060-2240 cm −1 are characteristic of carbon monoxide [59]. The solid intermediate products for thermal decomposition could not be identified. The residual mass is about 29.5% 1 and 30.6% 2 (the theoretical values are 31.5% 1 and 32.6% 2). At high temperature, the sublimation of the copper(II) oxide takes place and the differences between values calculated and found can be caused by this process. Mixtures of metal oxides CuO and Ho 2 O 3 /Er 2 O 3 , experimentally verified by X-ray diffraction powder patterns ( Figures S9 and S10) are the final solid products of thermal decomposition of 1 and 2 in air [60].

Magnetic Properties
Temperature-dependent molar susceptibility measurements of Cu II 2 Ho III 1 and Cu II 2 Er III 2 were carried out in a magnetic field of 0.1 T at 1.8-300 K. The χ M T vs. T curves for 1 and 2 are shown in Figure 8. The magnetic properties of heteronuclear Cu II Ln III compounds are governed by three factors: the thermal population of the Stark components of Ln III , the Cu II ···Cu II interactions (including intermolecular interaction) and the Cu II Ln III interactions. For Cu II 2 Ho III 1, the χ M T value experimentally determined at 300 K is equal to 14.17 cm 3 Kmol −1 , which is slightly smaller than the value 14.82 cm 3 Kmol −1 calculated for one Ho III ( 5 I 8 , J = 8, L = 6, S = 2, g = 5/4) and the two Cu II (S = 1 / 2 , g = 2) free ions. As the temperature is lowered, χ M T keeps a constant value until 150 K, then begins to decrease to 13.91 cm 3 Kmol −1 at 19 K, next increases to reach a value of 14.70 cm 3 Kmol −1 at 5.9 K and finally, shows a small decrease to 12.95 cm 3 Kmol −1 at 1.8 K. The shape of the χ M T vs. T curve is strongly suggestive of the occurrence of two competitive phenomena. The decrease of χ M T on lowering of the temperature is most probably governed by the depopulation of the Ho Stark sublevels, or the presence of magnetic anisotropy, or the antiferromagnetic interaction between metal centers, while the increase of the χ M T at lower temperatures may be attributed to a ferromagnetic Cu II Ho III coupling. For a Cu 2 II Er III 2, the experimental value of χ M T product at room temperature is equal to 12.18 cm 3 Kmol −1 and approximately corresponds to the value 12.23 cm 3 Kmol −1 calculated for one independent Er III ( 4 I 15/2 , S = 3/2, L = 6, J = 15/2, g = 6/5) and two independent Cu II ions (S = 1/2, g = 2). As shown in Figure 9, this value decreases by lowering the temperature to 8.05 cm 3 Kmol −1 at 1.8 K. The reduction of χ M T at low temperature should mainly arise from the crystal field splitting of Ln III ions and/or combine the contribution of the overall antiferromagnetic interactions among the metal ions. These results are compatible with the empirical investigations of heterometallic Cu II -4f compounds, in which the 4f ions show a spin-orbit coupling [32,[61][62][63].
The M vs. H plots (at 2 K) for 1 and 2 are presented in Figure 9. The values of magnetization rise quickly at the low magnetic field whereas at the high magnetic field, the increase of magnetization is slow and linear. The magnetization reaches the values 6.5 µ B for 1 and 7.0 µ B for 2, respectively, at 5T; these are far from the theoretical saturated values anticipated for one uncoupled lanthanide(III) ion and two copper(II) ions [32,[61][62][63].

Conclusions
In summary, neutral, heteronuclear Cu II Ln III Cu II complexes were obtained in a stepwise manner. In the crystal structures of 1 and 2, the smaller Cu II ion is exclusively coordinated to the N 2 O 2 compartment of the hexadentate Schiff base ligand, while the O 2 O 2 compartment accommodates a bigger Ho III /Er III ion. The Ho III /Er III and Cu II ions are double-bridged by two phenoxo oxygen atoms of the N 2 O 4 ligand. The complexes 1 and 2 crystallize as stable at room temperature solvates and their desolvation process is consistent with the loss of methanol molecules. The similar values of ionic radious of Ho III and Er III cations led to the same coordination number and the same coordination geometry of Ln III ions as well as the similar spectral and thermal properties. The Cu II and Ho III centers are ferromagnetically coupled which is in agreement with earlier observations in similar Cu II Ho III compounds. The structural investigations indicate that different (heterodi-, heterotri, -heterohexanuclear) coordination architectures can be received using the same Schiff base as a ligand, but changing the lanthanide(III) ions.

Supplementary Materials:
The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/ma15124299/s1, Figure S1: FTIR spectra of the free Schiff base H 4 L and complexes 1 and 2, Figure S2: The molecular structure of 2. Displacement ellipsoids are drawn at the 30% probability level, Figure S3: Coordination polyhedra of Cu(II) and Er(III) cations in the trinuclear complex 2, Figure S4: (a) A partial viewed along the b-axis direction of the crystal packing of 2 with hydrogen bonds shown as dashed lines; (b) A partial viewed along the a-axis direction of the crystal packing of 2 with hydrogen bonds shown as dashed lines, Figure S5: The overall crystal packing of compound 1 showing formation of 3D supramolecular structure, viewed along the a-axis. Hanging contacts were omitted for clarity, Figure S6: The overall crystal packing of compound 2 showing formation of 3D supramolecular structure, viewed along the c-axis. Hanging contacts were omitted for clarity, Figure S7: TG and DCS curves of thermal decomposition of the complex 1 in air, Figure S8: FTIR spectra of gaseous products involved during the complex 2 decomposition, Figure S9: The X-ray powder diffraction patterns of the final products of complex 1 decomposition in air, Figure S10: The X-ray powder diffraction patterns of the final products of complex 2 decomposition in air, Table S1: Hydrogen-bond geometry [Å, • ] for compounds 1 and 2.