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Article

Self-Assembly Heterometallic Cu-Ln Complexes: Synthesis, Crystal Structures and Magnetic Characterization

by
Shaoliang Zhang
1,*,
Ruili Du
1,
Xiufang Fan
1,
Xinhua Zhao
2,
Yanlan Wang
1,* and
Shanshan Li
3,*
1
Institution of Functional Organic Molecules and Materials, School of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng 252059, China
2
Department of Chemistry, South University of Science and Technology of China, Shenzhen 518055, China
3
School of Geography and Environment, Liaocheng University, Liaocheng 252059, China
*
Authors to whom correspondence should be addressed.
Crystals 2023, 13(3), 535; https://doi.org/10.3390/cryst13030535
Submission received: 29 January 2023 / Revised: 22 February 2023 / Accepted: 16 March 2023 / Published: 21 March 2023
(This article belongs to the Special Issue Coordination Polymers: Design and Application)
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Abstract

:
Using N2O4 donor symmetric ligand H2L and dca co-ligand, two new isostructural dinuclear CuII–LnIII complexes [Cu(Cl)(L)Ln(NO3)(CH3OH)(H2O)(dca)] [Ln=Ho (1CuHo), Gd (2CuGd)] [H2L = 6,6′-((1E,1′E)-(ethane-1,2-diylbis(azaneylylidene))bis(methaneylylidene))bis(2-methoxyphenol); dca=dicyanamide] were designed, synthesized and studied. In the two isostructural compounds, the geometric environment around the nine-coordinated Ln(III) ions is muffin, whereas the geometry of the penta-coordinated Cu(II) ions is square pyramid. The magnetic properties of both complexes were also studied. Direct current magnetic susceptibility measurements indicate ferromagnetic interactions between the Cu(II) ion and Gd(III) ion in complex 2CuGd. Alternating current (ac) magnetic measurements indicate that complex 1CuHo displays slow magnetic relaxation behaviour.

1. Introduction

Molecule-based magnets [1,2,3,4,5], which are widely applied in molecular spintronics [6,7,8], quantum computations [9,10,11], and as magneto luminescent materials [12], have gained much attention in the field of functional molecular materials. Among these, heterometallic 3d-4f polynuclear complexes make up one of the most interesting research topics [13,14,15,16]. In these type of complexes, the 4f ions provide large ground state magnetic moments and magnetic anisotropy, whereas the 3d ions take part in magnetic coupling interactions. Single-ion anisotropy [17,18,19,20,21,22] and magnetic coupling interactions [23,24,25,26] are crucial to the scale of the magnetization relaxation barrier. When there is a stronger 3d-4f magnetic exchange interaction, the quantum tunneling of magnetization (QTM) is effectively restrained, and large energy barriers, hysteresis loops and relaxation times can be observed [23,24]. Therefore, the 3d-4f metal combinations are more suitable for constructing low-dimensional molecular magnets (single molecule magnets, SMMs or single chain magnets, SCMs) [25,26,27,28,29,30].
Cu-Ln heterometallic complexes, among 3d-4f complexes, have encouraged special curiosity due to the ferromagnetic coupling interaction between the Cu(II) ion and the Ln(III) ion [31]. The [Cu2Tb2] complex, the first 3d-4f complex observed to have SMM properties, demonstrated that the magnetic interaction between the Cu(II) ion and the Tb(III) ion is critical to its SMM behavior [32]. In this regard, the SMM behavior of complexes in this category could be improved because the employment of Cu-Ln magnetic coupling interactions increases the ground state spin multiplicity.
Salen-type Schiff-base ligands obtained from salicylaldehyde derivatives and various diamines are typical materials frequently used to synthesize different 3d-4f complexes [33,34]. It can be easily tailored by changing the diamines or the substituent groups on aromatic rings. N2O2 donor ligands and N2O4 donor ligands are the two main types of Schiff-base ligands. N2O2 donor ligands can produce trinuclear 3d-4f complexes [35], whereas N2O4 donor ligands can initiate the formation of di-, tri- and tetranuclear heterometallic complexes [13,28,33]. Furthermore, by varying 3d/4f/ligand molar ratios, tailoring Salen ligands and introducing different co-ligands, the structures of related 3d-4f complexes can be modulated [28]. Thus, it is compelling to research the structures and magnetic properties of 3d-4f-heterometallic-compound-based Salen ligands.
Dicyanamide (dca) has attracted extensive attention for the preparation of complexes with extended architectures because it is a utility ligand containing three nitrogen donor atoms, which make it possible to act as uni-, bi- and tridentate ligands in the area of coordination chemistry [36,37]. A dca ligand could be widely used to design SMMs [38], spin crossover complexes [39] and structural phase transition complexes [40].
Considering all the aspects stated above, we synthesized two Cu-Ln complexes [Ln=Ho(1CuHo), Gd(2CuGd)]. The crystal structures and magnetic properties of the two new compounds were studied in this paper.

2. Experimental Section

2.1. Materials and Methods

Chemicals used were the finished product of Energy Chemical Company. H2L was synthesized according to the method reported in reference [27]. Elements (e.g., C, H, N) in the two complexes were analysed with Elementar Vario MICRO analyzer (Langenselbold, Germany). IR data were collected on a Bruker Tensor 27 FT-IR spectrometer in the 4000–400 cm−1 range (Billerica, MA, USA). Their magnetic properties were investigated on crushed crystal samples by means of a Quantum Design SQUID VSM magnetometer (San Diego, CA, USA). Experimental data were corrected for diamagnetism of the sample holder and that of the complexes according to Pascal’s constants [41]. Powder X-ray diffraction (PXRD) of the two complexes were logged at 298 K on a Bruker D8 Advance diffractometer using Cu Kα X-ray source (operated at 40 kV and 40 mA).

2.2. Synthesis

2.2.1. H2L Synthesis

Hydroxy-3-methoxybenzaldehyde (20.00 mmol, 3.04 g) and ethylenediamine (10.00 mmol, 0.60 g) were mixed and dissolved in 50 mL ethanol. The above solution was refluxed for three hours and then cooled, the yellow crystal of H2L ligand was formed. Yield: 90%. IR (KBr, cm−1): 3425 (w), 2931 (w), 1632 (s), 1609 (m), 1467 (s), 1408 (m), 1249 (s), 1132 (w), 1080 (s), 1009 (m), 962 (s), 836 (m), 782 (m), 740 (s), 620 (m), 521 (w), 439 (w).

2.2.2. Complex 1CuHo Synthesis

A mixture of H2L (32.8 mg, 0.100 mmol) and Cu(NO3)2·3H2O (24.2 mg, 0.100 mmol) was dissolved in 3 mL of methanol, and mixed with a solution of HoCl3·6H2O (37.9 mg, 0.100 mmol in 3 mL of CH3OH-CH3CN with v/v = 1:1). After a 30 min stir of the above solution, a 3 mL methanol solution containing NaN(CN)2 (8.90 mg, 0.100 mol) was added. Then, it was filtered, and put into a test tube. Thus, diethyl ether vapor could be able to diffuse into the test tube. X-ray-suitable deep brown needle crystals were formed in the following seven days. Mass yield: 57%. Elemental analysis (%) calculated for C22H28N6O10ClCuHo: C, 33.0; H, 3.5; N, 10.5. Found: C, 33.1; H, 3.4; N, 10.6. IR (KBr, cm−1): 3405 (w), 2311 (m), 2270 (m), 2218 (m), 2154 (s), 1636 (s), 1607 (s), 1558 (w), 1473 (m), 1456 (w), 1385 (w), 1281 (m), 1242 (m), 1222 (m), 1170 (w), 1078 (m), 1042 (w), 979 (w), 956 (w), 854 (m), 781 (w), 737 (m), 646 (w), 617 (w), 510 (w).

2.2.3. Complex 2CuGd Synthesis

Complex 2CuGd deep brown crystals were prepared following a similar procedure as that of complex 1CuHo using GdCl3·6H2O (37.1 mg, 0.100 mmol) instead of HoCl3·6H2O. Mass yield: 60%. Elemental analysis (%) calculated for C22H28N6O10ClCuGd: C, 33.3; H, 3.6; N, 10.6. Found: C, 33.4; H, 3.5; N, 10.6. IR (KBr, cm−1): 3404 (w), 2269 (m), 2222 (m), 2162 (s), 1639 (s), 1609 (s), 1559 (w), 1473 (m), 1458 (w), 1386 (w), 1285 (m), 1241 (m), 1223 (s), 1171 (w), 1109 (m), 1077 (m), 1041 (w), 974 (m), 956 (w), 855 (m), 780 (w), 737 (s), 644 (m), 613 (w), 526 (m).

2.3. X-ray Crystallography

All reflections for single crystals of the complex 1CuHo and complex 2CuGd were collected at room temperature on a Bruker Apex II CCD detector with monochromated Mo Kα (λ = 0.71073 Å) radiation. The structures of the two complexes were solved by direct method and refined by full-matrix least-squares method on F2 through the SHELXTL software package [42,43,44,45]. The final crystallographic data and refinement parameters of the two complexes are listed in Table 1, and the data of selected bond distances and angles are listed in Tables S1 and S2.

3. Results and Discussion

3.1. IR Spectra

In the IR spectra, the strong absorption bands at 2154 cm−1 for complex 1CuHo and 2162 cm−1 for complex 2CuGd can be ascribed to the terminal ligands. One sole sharp peak can be observed at 1636 cm−1 (complex 1CuHo) and 1639 cm−1 (complex 2CuGd) for vC=N stretching, which indicates that the H2L ligands have a symmetrical nature. The absorption peaks at 510 cm−1 for complex 1CuHo and 526 cm−1 for complex 2CuGd can be attributed to the stretching vibrations of metal–oxygen or metal–nitrogen.

3.2. Crystal Structure Descriptions

The two complexes are isostructural and display similar dinuclear [CuIILLnIII] structures with a symmetric ligand (H2L), where Ln stands for Ho in complexes 1CuHo and Gd in complexes 2CuGd. The two complexes crystallized in the monoclinic space group with Z = 4. Here, we describe complex 1CuHo as a representative structure (Figure 1). On this molecule level, the central Ho(III) ion was coordinated to L2− via two μ2-phenoxido oxygen atoms (O1, O2) and two methoxy oxygen atoms (O3, O4). Along with this, one chelating bidentate nitrato ligand (O5, O6), one methanol molecule (O8), one water molecule (O9) and one terminal dca ligand (N4) make the Ho(III) centre nine-coordinated. The Cu(II) ion is five-coordinated surrounded by the four atoms (O1, O2, N1, N2) from the L2- ligands and one axial Cl1 atom.
We calculated the continuous shape measures (CShM) to estimate the coordination geometry of the metal ions (Ln and Cu in Figure 2) through the SHAPE 2.1 software [46]. The results (Tables S3 and S4, ESI†) show that the lowest CShM values for Ln1 are 2.243 for complex 1CuHo, and 2.298 for complex 2CuGd, which indicates the environment of Ln is a muffin geometry; the CShM values for Cu1 are 1.687 for complex 1CuHo, and 1.640 for complex 2CuGd, which indicates the environment of Cu is a square pyramid geometry. The bond lengths of Cu–O/N lie in the ranges of 1.888(14)–1.962(12) Å and the bond lengths of Ln-O/N range from 2.316(10) to 2.641(10) Å, which fall in the normal range compared to the other heterometallic dinuclear CuIILnIII (Ln = Tb and Dy) compounds originating from N2O3 donor unsymmetrical ligands [29,30] and N2O4 donor symmetrical ligands [27].
As is well noted, hydrogen bonding and π-π interactions are crucial to construct and stabilize supramolecular structures. In complex 1CuHo, the monomer is connected into a chain through O-H···Cl hydrogen bonds’ interaction along the a-axis direction. Furthermore, the two neighboring chains are interlinked into a supramolecular 2-D layer in the ab plane through the π-π stacking between the two phenyl rings defined by C1···C6 and C11···C16 from the H2L ligands. (Figure 3, Table 2)

3.3. Magnetic Properties

The temperature-dependent magnetic susceptibility data of the polycrystalline complexes 1CuHo-2CuGd were determined in the temperature interval of 2–300 K under an applied direct current (DC) 1000 Oe field. (Figure 4) For the complex 1CuHo, the value of χMT is 14.22 cm3 mol−1 K under room temperature, which is close to the theoretical value 14.445 cm3 mol−1 K. (Cu: g = 2.0, S = 1/2; Ho: g = 5/4, J = 8). Upon cooling from 300 K, the χMT decreases monotonically, which could be attributed to the depopulation of the Zeeman effect and possible weak antiferromagnetic interactions between the spin centers.
The value of χMT for complex 2CuGd at 300 K is 7.72 cm3 mol−1 K, in line with the expected value for one magnetically isolated CuIIGdIII unit 8.255 cm3 mol−1 K (Cu: g = 2.0, S = 1/2; Gd: g = 2, J = 7/2). In contrast to complex 1CuHo, the value of χMT for complex 2CuGd gradually increased up towards the maximum (9.16 cm3 mol−1 K at 5.0 K), which indicated the presence of a major ferromagnetic interaction through Gd-O-Cu, and then decreased to 9.10 cm3 mol−1 K at 2.0 K. This slight drop at the lower temperature indicated the existence of the intermolecular antiferromagnetic coupling [47]. The neighboring interdimer Gd⋯Gd distances were 8.819 Å along the b axis. Compared to the literature, the Gd⋯Gd distances are short enough to give an antiferromagnetic interaction. [47]. This can be described by a dinuclear model with a spin Hamiltonian as H = −2JSCuSGd. The dc magnetic data were fitted using PHI program [48], in which the best fit presented J = 2.97 cm−1, gCu = 1.92, and gGd = 1.92 for complex 2, revealing that there is ferromagnetic CuII-GdIII interaction in complex 2.
Several complexes with a magneto-structural relationship, where the Cu(II)- Gd(III) exchange interaction parameter is depicted as a function of Cu-O-Gd bond angles, have been reported [28]. Generally, the ferromagnetic interaction can be observed between heavy Ln(III) and Cu(II). Alternatively, with the increase in the number of unpaired electrons in the 4f orbitals, the probability of AF interaction between the dx2–y2 orbital of CuII and the magnetic orbitals of the 4f shell also goes up [30]. Thus, moving from TbIII to ErIII, the AF contribution is gradually increased. (Table 3, The complexes 3CuTb and 4CuDy are isostructural with the current complexes 1CuHo and 2CuGd.)
The field-dependent magnetization curves of these compounds at 2 K were measured at the range of 0–70 KOe. (Figure 5). The experimental magnetizations at 7 T are 6.22 and 7.19 μB for complexes 1CuHo-2CuGd, respectively (Table 4). The magnetization value of complex 1CuHo was apparently lower than the saturated magnetization value, which can be explained by the strong magnetic anisotropy of the Ho(III) ion.
The observation of an out–of–phase χM′′ ac susceptibility signal is one of the important characteristics of an SMM. Thus, alternating current (ac) magnetic susceptibilities were performed in the temperature range of 2.0–6.0 K without the applied DC field at different frequencies to analyse the dynamics of the magnetization of the two complexes. As can be seen from Figure 6, complex 1CuHo exhibited an out−of−phase χM′′ signal below 3.0 K. Although no maximum was observed down to 2.0 K, an obvious frequency dependence of the χM′′ signal was shown to be consistent with the SMM behaviour. It can be seen from Figure S1 that there were no observable out−of−phase signals down to 2.0 K, which rules out the SMM behaviour of complex 2CuGd.
This variance in SMM properties for these complexes under zero field can be interpretated through the spin parity effect of the 3d-4f ions in the ferro-/antiferromagnetically-coupled complexes (i.e., Kramer’s molecules, complex 1CuHo and complex 3CuTb, are likely to display SMM behaviours).

4. Conclusions

Two Cu(II)-Ln(III) (Ln=Ho and Gd) complexes were synthesized. The X-ray crystal structures revealed that they are both isostructural and display a similar dinuclear structure, where the Ln(III) ion was nine-coordinated and the Cu(II) ion was five-coordinated. Magnetic studies revealed that the Cu(II) and Gd(III) centers show a ferromagnetic interaction, and the Cu(II) and Ho(III) show an antiferromagnetic interaction. Complex 1CuHo exhibits a slow magnetic relaxation behaviour below 3K at zero field. They may provide an experimental basis for expanding the 3d-4f compound families and enlightening further studies on the magnetic property of 3d-4f compounds. In future, we will continue to study the effect of diverse 3d-4f structures on magnetic properties in depth. Hopefully, we will be able to effectually regulate magnetic properties by accommodating the N2O3 donor ligand with the absence of symmetry elements.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/cryst13030535/s1.

Author Contributions

S.Z. Methodology, Software, Supervision, Writing-original draft. R.D. Experimental and Data curation. X.F. Experimental and Data curation. X.Z. Software and Data curation. Y.W. Reviewing and Editing. S.L. Reviewing and Editing. All authors have read and agreed to the published version of the manuscript.

Funding

The research was funded by Natural Science Foundation of Shandong Province (grant number: ZR2016BB07) and National Natural Science Foundation of China (grant number: 21901097).

Data Availability Statement

The crystallographic data CCDC-2097679 (1), 2097680 (2) for this paper can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif (accessed on 28 January 2023).

Acknowledgments

We thank to Introduction and Cultivation Program for Young Innovative Talents in Shandong Provincial Colleges and Universities (Innovation Team of Functional Organometallic Materials presided by Yanlan Wang).

Conflicts of Interest

The authors declare no conflict of interest.

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Crystals 13 00535 g001 550
Figure 1. The crystal structure of complex 1CuHo (a), complex 2CuGd (b).
Figure 1. The crystal structure of complex 1CuHo (a), complex 2CuGd (b).
Crystals 13 00535 g001
Crystals 13 00535 g002 550
Figure 2. The coordination geometries of Cu(II) (a) and Ho(III) (b) in complex 1.
Figure 2. The coordination geometries of Cu(II) (a) and Ho(III) (b) in complex 1.
Crystals 13 00535 g002
Crystals 13 00535 g003 550
Figure 3. The 2-D layer built through hydrogen bond interactions and π-π stacking in complex 1.
Figure 3. The 2-D layer built through hydrogen bond interactions and π-π stacking in complex 1.
Crystals 13 00535 g003
Crystals 13 00535 g004 550
Figure 4. Temperature-dependent magnetic susceptibility data for complexes 1–2, solid lines show the best fits to the appropriate model.
Figure 4. Temperature-dependent magnetic susceptibility data for complexes 1–2, solid lines show the best fits to the appropriate model.
Crystals 13 00535 g004
Crystals 13 00535 g005 550
Figure 5. Isothermal magnetization of complexes 1–2 at 2 K with DC field of up to 70 KOe.
Figure 5. Isothermal magnetization of complexes 1–2 at 2 K with DC field of up to 70 KOe.
Crystals 13 00535 g005
Crystals 13 00535 g006 550
Figure 6. Frequency dependence of the out–of–phase χM′′ ac magnetic susceptibility versus T for complex 1CuHo measured under Hac = 2 Oe and Hdc = 0 Oe.
Figure 6. Frequency dependence of the out–of–phase χM′′ ac magnetic susceptibility versus T for complex 1CuHo measured under Hac = 2 Oe and Hdc = 0 Oe.
Crystals 13 00535 g006
Table
Table 1. Crystallographic data for the two complexes.
Table 1. Crystallographic data for the two complexes.
Complex12
FormulaC22H28N6O10ClCuHoC22H28N6O10ClCuGd
M800.42792.74
Crystal systemMonoclinicMonoclinic
Space groupP21/nP21/n
a [Å]8.8038(6)8.8191(7)
b [Å]16.3659(12)16.5598(14)
c [Å]19.8889(13)20.0787(18)
α [°]9090
β [°]91.5570(10)91.748(2)
γ [°]9090
V [Å3]2864.6(3)2931.0(4)
Z44
ρcalcd(g cm−3)1.8561.797
T/K
F(000)		298(2)
1580		298(2)
1568
Crystal size (mm)0.48×0.42×0.400.45×0.40×0.38
Reflections collected13,35813,751
Independent reflections49845137
Rint0.12620.1758
GOF
R1 a, wR2 b (I > 2σ(I))		0.865
0.0901, 0.1761		0.788
0.0822, 0.1644
a R1 = ∑||Fo| − |Fc||/∑|Fo| b wR2 = {∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]}1/2.
Table
Table 2. Hydrogen bonding and π-π interactions [Å] for the two complexes.
Table 2. Hydrogen bonding and π-π interactions [Å] for the two complexes.
ComplexD···AD(D···A)/Å
complex 1CuHoO9···Cl1 a3.023(2)
π···π b3.763(2)
complex 2CuGdO9···Cl1 a3.056(2)
π···π b3.872(2)
Symmetry codes: a: 1 + x, y, z; b: 1.5 − x, 0.5 + y, 0.5 − z.
Table
Table 3. Geometrical features and magnetic parameters of Cu-Ln bimetallic complexes.
Table 3. Geometrical features and magnetic parameters of Cu-Ln bimetallic complexes.
ComplexCu-Ln InteractionCu–Ophenol-Ln Bridging Angle (°)Cu-Ophenol Bond Length (Å)Ln-Ophenol Bond Length (Å)SMM Behavior under Zero DCc FieldRef
1CuHoantiferromagnetic105.15/104.061.929/1.9382.316/2.338yesThis work
2CuGdferromagnetic 104.96/105.411.948/1.9252.366/2.375noThis work
3CuTbferromagnetic105.43/104.481.907/1.9432.334/2.327yes27
4CuDyferromagnetic 104.52/105.421.942/1.9152.328/2.328no27
Table
Table 4. Experimental and theoretical values of χMT at room temperature and the saturation magnetization M (70 KOe, 2 K) for the two compounds.
Table 4. Experimental and theoretical values of χMT at room temperature and the saturation magnetization M (70 KOe, 2 K) for the two compounds.
ComplexGround State of LnχMT/cm3mol−1K (Theoretical)χMT/cm3mol−1K (Observed)M/μB (Theoretical)M/μB (Observed)
1CuHo5I814.44514.22116.22
2CuGd8S7/28.2557.7287.19
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Zhang, S.; Du, R.; Fan, X.; Zhao, X.; Wang, Y.; Li, S. Self-Assembly Heterometallic Cu-Ln Complexes: Synthesis, Crystal Structures and Magnetic Characterization. Crystals 2023, 13, 535. https://doi.org/10.3390/cryst13030535

AMA Style

Zhang S, Du R, Fan X, Zhao X, Wang Y, Li S. Self-Assembly Heterometallic Cu-Ln Complexes: Synthesis, Crystal Structures and Magnetic Characterization. Crystals. 2023; 13(3):535. https://doi.org/10.3390/cryst13030535

Chicago/Turabian Style

Zhang, Shaoliang, Ruili Du, Xiufang Fan, Xinhua Zhao, Yanlan Wang, and Shanshan Li. 2023. "Self-Assembly Heterometallic Cu-Ln Complexes: Synthesis, Crystal Structures and Magnetic Characterization" Crystals 13, no. 3: 535. https://doi.org/10.3390/cryst13030535

APA Style

Zhang, S., Du, R., Fan, X., Zhao, X., Wang, Y., & Li, S. (2023). Self-Assembly Heterometallic Cu-Ln Complexes: Synthesis, Crystal Structures and Magnetic Characterization. Crystals, 13(3), 535. https://doi.org/10.3390/cryst13030535

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