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A family of 3d-4f Cu-Ln ladder-like complexes: Synthesis, structures and magnetic properties
Journal Pre-proofs A family of 3d-4f Cu-Ln ladder-like complexes: synthesis, structures and magnetic properties Meng Yang, Xiaohong Liang, Yandie Zhang, Zhijian Ouyang, Wen Dong PII: DOI: Reference:
S0277-5387(20)30092-9 https://doi.org/10.1016/j.poly.2020.114435 POLY 114435
To appear in:
Polyhedron
Received Date: Revised Date: Accepted Date:
16 January 2020 8 February 2020 10 February 2020
Please cite this article as: M. Yang, X. Liang, Y. Zhang, Z. Ouyang, W. Dong, A family of 3d-4f Cu-Ln ladder-like complexes: synthesis, structures and magnetic properties, Polyhedron (2020), doi: https://doi.org/10.1016/j.poly. 2020.114435
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A family of 3d-4f Cu-Ln ladder-like complexes: synthesis, structures and magnetic properties Meng Yang, Xiaohong Liang, Yandie Zhang, Zhijian Ouyang, Wen Dong Guangzhou Key Laboratory for Environmentally Functional Materials and Technology, School of Chemistry and Chemical Engineering, Guangzhou University, 230 Wai Huan Xi Road, Guangzhou Higher Education Mega Center, Guangzhou 510006, P. R.China.
Abstract
A
family
of
rare
2p-3d-4f
ladder-like
one-chain
complexes
namely
[LnCu(hfac)5NIT-Ph-p-OCH2trz0.5C6H14]n (Ln = Er (1), Ho (2), Yb (3) ; NIT-Ph-p-OCH2trz
=
2-(4-((1H-1,2,4-triazol-1-yl)methoxy)phenyl)-4,4,5,5-tetra-
methylimidazoline-1-oxyl-3-oxide; hfac = hexafluoroacetylacetonate) have been successfully synthesized simultaneously through reacting nitronyl nirtroxide radical NIT-Ph-p-OCH2trz with Cu(hfac)2 and Ln(hfac)3. The structures of complexes of 13 were elucidated by single-crystal X-ray structural analysis and the result show that all complexes feature ladder-like chain structures. Nonzero out-of-phase signals are observed for Ho derivatives (2) indicating single-chain magnet behavior.
Keywords: Functionalized nitronyl nitroxides; 2p-3d-4f; Ladder-like structure; Magnetic properties
1. Introduction
The heterospin complex, which including different spin carrier in one molecule has attracted enormous interest due to it is valuable synthetic strategy to obtain molecular magnetic materials. So far, a great variety of reported single chain magnets (SCMs), and single molecule magnets (SMMs) are 2p-3d [14] and 2p-4f [59] heterobispin complexes.
Since
the
first
2p-3d-4f
heterotrispin
complex
named
[{CuL}2Gd(TCNQ)2]·TCNQ-·CH3OH·2CH3CN was report [10], many similar complexes based on nitronyl nitroxide radical have been obtained following a one-pot procedure through reacting the radical with 3d and 4f hexafluoroacetylacetonato complexes in the last years.[1116] In the 2p-3d-4f complexes, the R groups in the radical can be strongly influence the structure of the complexes. To design 2p-3d-4f heterotrispin complexes, special attention has been paid to functionalized radicals. The nitronyl nitroxide radical that contain a triazole ring is a good derivative because these free radicals can bond to the metal ions with not only the triazole nitrogen atoms but also the radical oxygen atoms, which may cause the formation of polynuclear clusters.[17,18] Using a
functional NITR containing triazole ring named
NIT-Ph-p-OCH2trz (scheme 1), a family of ladder-like one-chain 2p-3d-4f complexes namely [LnCu(hfac)5NIT-Ph-p-OCH2trz0.5C6H14]n ( Ln = Er (1), Ho (2), Yb (3), NIT-Ph-p-OCH2trz = (1H-1,2,4-triazol-1- yl)methoxy]phenyl]-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) were obtained. All complexes feature ladder-like chain structures. Nonzero out-of-phase signals are observed for Ho derivatives (2) exhibit frequency-dependent out-of-phase signals indicating single-chain magnet behavior.
Scheme 1 NIT-Ph-p-OCH2trz radical ligand.
2. Results and discussions
2.1. Description of the Crystal Structure. Complexes of 13 possess similar structures and are solved in triclinic Pī group space，therefore, only the structure of 1 is described as the representative. As shown in Figure 1, each Er(III) ion occupies one O atom from the NO group and one N atom form the triazole ring of the nitronyl nitroxide radical ligand, and it displays an eight-coordinate distorted square-antiprismatic geometry (D4d; CShM = 0.890, the CshM values are calculated by Shape software[19,20], Table S4) with the other six O atoms from the hfac ligands making up the coordination sphere. The Er−O(radical) and Er−N distances are 2.351(3) and 2.569(4) Å, respectively, while the Tb−O(hfac) distances ranging from 2.314(3) to 2.351(3) Å. The Cu(II) ion occupies the inner O4 site of the hfac ligand, with a six coordinate, elongated octahedral geometry (CShM = 1.265, Table S5), with the O4 coordination atoms of hfac ligand forming the equatorial plane and the O atom from NO group and N atom from triazole ring of the radical ligand occupying the apical position of the coordination geometry. The distance of Cu−O and Cu−N are 2.407(4) and 2.463(4) Å in the axis positions while the Cu−O bond lengths are ranging from 1.939(3) to 1.949(4) Å. The paramagnetic
NIT ligand is coordinated two Er(III) and two Cu(II) ions with the μ4-η1:η1:η1:η1 coordinate mode,
resulting in a ladder-like arrangement in which the anisole
moieties form the rungs of the ladder. In complex 1, the Er···Cu separations through the NIT motif and the triazole ring are of 8.52 and 6.93 Å, respectively while the Er···Er and Cu···Cu distances across the rungs are 8.98 and 10.58 Å, respectively.
Figure 1 Molecular structure of complex 1 (Hydrogen and Fluorine atoms are not shown for the sake of clarity).
Figure 2 Crystal packing of complex 1 (all hydrogen and fluorine atoms are omitted for clarity).
2.2. Magnetic properties The variable temperature magnetic susceptibility data for compounds 13 were measured in the 2300 K range under a 1000 Oe dc field. As shown in Figure 3, the
MT values are 12.98, 15.47 and 3.64 cm3Kmol1 at room temperature, which are close to the expected values of 12.23, 14.81, and 3.32 cm3Kmol1 for one uncorrelated Ln(III) ion (Er : 4I15/2, S = 3/2, L = 6, g = 6/5, C = 11.475 cm3 K/mol; Ho : 5I8, S = 2, L = 6, g = 5/4, C = 14.0625 cm3 K/mol; Yb : 2F7/2, S = 1/2, L = 3, g = 8/7, C = 2.5714 cm3K/mol) plus one isolated Cu(II) ion (S = 1/2, g = 2.0) and one radical (S = 1/2, g = 2.0). For complex 1, the MT value is a constant from 300 to 100 K, and diminished slowly up 25 K and then accelerated appropriately until it reached the minimum value of 8.92 cm3·K·mol−1 at 2 K. Complex 2 displayed a similar phenomenon. But for complex 3, the value of χMT diminished slowly at first and then accelerated appropriately until it reached the minimum value of 1.81 cm3·K·mol−1 at 2 K. According to our previous report [21], there exist ferromagnetic coupling between the Cu(II) ion and the axial NO unit results from the orthogonality of the magnetic orbital (dx2-y2) of the Cu(II) ion and the radical * magnetic orbital. The interactions between the Ln(III) ions and the coordinated NO group of the organic radical are ferromagnetic. For complexes 13, their magnetic behaviors do not display increase of the MT values when the temperature is decreased. This implies that the ferromagnetic Ln-radical and Cu-radical contributions are weaker for these three rare earth ions and the stronger crystal field effect of three lanthanides just overwhelms the ferromagnetic Ln-radical contributions on MT curve [22].
Such behavior is
consistent with the M versus H/T plots of 1−3 at 2 K, which displayed a rapid increase in the magnetization at low fields, and the magnetization reached the maximum
values of magnetization with 7.40, 8.20 and 3.46 Nβ at 7 T (Figure 4, Figures S8 and S9). The saturation values were much lower than the theoretical values (11, 12 and 6 Nβ for 13), which are likely due to crystal-field effect and the contribution of low lying excited states.
Figure 3 Plot of χMT versus T for complexes 13.
Figure 4 M versus H plot at 2 K for complex 1.
In order to explore spin dynamic behaviors of complex 2, the ac susceptibilities were measured utilizing an oscillating 3.0 Oe field from 2 to 10 K at different frequencies. It exhibits clear frequency-dependent χ″ signals in the zero dc fields, which could be due to slow magnetic relaxation behavior (Figure 5). But no peak maxima value of χ″ was observed in the available window above 2 K. And then the ac data were collected in 2 kOe dc field to reduce or suppress the possible quantum tunneling process (QTM). Unfortunately, no visible χ″ peaks were found yet. Thus, the activation barrier (Ueff) and relaxation time (τ0) cannot be obtained from the fit of Arrhenius law. If there is only one relaxation process in 2, the equation ln(χ″/χ′) = ln(ωτ0)+ Ea/kBT could be used to roughly estimate the Ueff and τ0 values.[23,24] The best fitting ac data of complex 2 yield the energy barrier (Ea/kB) 4.93 K with the relaxation time (τ0) 1.64 × 10−6 s at 2 kOe dc field (Figure 6). The frequency dependence of the χ′ and χ″ signals were also examined in a frequency range of 100−10000 Hz with 2 kOe dc fields. As shown in Figure 7, the χ″ component of the alternating-current (ac) susceptibility feature strong frequency-dependent phenomena, displaying the maxima for all frequencies. The Cole−Cole plot was also shown in Figure 7, and it was analyzed with the generalized Debye model [25] and the obtained α value vary from 0.12 to 0.54, indicating a rather broad distribution of relaxation times.
Figure 5 Temperature dependence of the out-phase in zero (left) and 2 kOe field (right) for complex 2.
Figure 6 Natural logarithm of the ratio of χ"/χ' vs. 1/T for 2. The solid line represents the fitting results.
Figure 7. (left) Frequency dependence of the out-of-phase components for the ac magnetic susceptibility in a 2 kOe dc field for 2. (right) Cole−Cole plots for 2. The solid lines represent the best fits with modified Debye functions.
3. Conclusions A family of Cu-Ln complexes based on the functional nitronyl nitroxide radical named [LnCu(hfac)5NIT-Ph-p-OCH2trz0.5C6H14]n ( Ln = Er (1), Ho (2), Yb (3) ) have been obtained. These complexes all possess ladder-like chain structures in which the anisole moieties form the rungs of the ladder. Magnetic measures show that the Ho derivatives (2) exhibits frequency-dependent out-of-phase signals indicating single-chain magnet behavior. This work demonstrates that using functionalized nitronyl nitroxide radicals is an appealing strategy for design 2p-3d-4f hetero-tri-spin compounds. It is not only to enrich the structural diversity of 2p-3d-4f compounds but also for modulating the magnetic behaviors of 2p-3d-4f systems.
4. Experimental
4.1 Materials and Physical Measurements All reagents were purchased from commercial sources and used without further purification. Elemental analysis for C, H, and N were carried out using a Perkin-Elmer elemental analyzer model 240. Magnetic measurements were measured on PPMS-9 magnetometer. The magnetic susceptibilities were corrected for the diamagnetic contribution of the constituent atoms by using of Pascal’s constants. 4.2 Preparation of complexes of 13. 20 mL dry boiling hexane with Ln(hfac)32H2O (0.02 mmol) was kept to reflux for 3
hours. Then a solution of NIT-Ph-p-OCH2trz (0.0064 g, 0.02mmol) in 3 mL dry CHCl3 was added and refluxed for 30 minutes. After that, 0.0098 g solid Cu(hfac)2 (0.02 mmol) was added. The resulting solution was stirred for 20 minutes, and then cooled it to room temperature and filtered off. The filtrate was kept at room temperature, after two days, blue needle-like crystals suitable for X-ray diffraction were obtained through evaporation method. Anal. calcd for C44H32CuF30N5O13Er (%): C, 32.23; H, 1.97; N, 4.27. Found: C, 32.32; H, 1.95; N, 4.26. IR(KBr): 1651(s), 1503(m), 1475(m), 1256(s), 1226(s), 1142(s), 801(m), 661(m), 589(m) cm−1. Anal. calcd for C44H32CuF30N5O13Ho (%): C, 32.28; H, 1.97; N, 4.28. Found: C, 32.30; H, 1.95; N, 4.29. IR(KBr): 1655(s), 1503(m), 1477(m), 1256(s), 1226(s), 1142(s), 799(m), 663(m), 589(m) cm−1. Anal. calcd for C44H32CuF30N5O13Yb (%): C, 32.12; H, 1.96; N, 4.27. Found: C, 32.31; H, 1.95; N, 4.26. IR(KBr): 1651(s), 1503(m), 1476(m), 1257(s), 1228(s), 1142(s), 803(m), 662(m), 590(m) cm−1. 4.3 X-ray structure determination All the crystal data were collected at 113 K using the Rigaku Saturn CCD diffractometer employing graphite-monochromated Mo-Kα radiation. The structures of all complexes were solved by direct methods with SHELXS-2014 program and refined by full-matrix least-squares on F2 with SHELXL-2014.[26,27] All the non-hydrogen atoms are refined anisotropically and the hydrogen atoms are added geometrically and refined as riding atoms with a uniform value of Uiso. A summary of the detailed crystallographic data and structure refinement of 1–3 is given in Table S6.
Appendix A.Supplementary data
CCDC 19744771974479 contain the supplementary crystallographic data for complexes 1, 3 and 2 respectively. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html,
or
from
the
Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected].
Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21271052) and Science and Technology Program Foundation of Guangdong Province (No.2015A030313502).
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[7] K. Bernot, L. Bogani, A. Caneschi, D. Gatteschi, R. Sessoli, J. Am. Chem. Soc. 128 (2016) 79477956. [8] H.D. Li, J. Sun, M. Yang, Z. Sun, J. Xie, Y. Ma, L.C. Li, New J. Chem. 41 (2017) 1018110188. [9] J. Sun, M. Yang, L. Xi, Y. Ma, L.C. Li, Dalton Trans. 47 (2018) 81428148. [10] A.M. Madalan, H.W. Roesky, M. Andruh, M. Noltemeyer, N. Stanica, Chem. Commun. 15 (2002) 1638−1639 [11] X.F. Wang, P. Hu, Y.G. Li, L.C. Li, Chem. Asian J. 2 (2015) 325328. [12] X.F. Wang, P. Hu, L.C. Li, J.P. Sutter, Inorg. Chem. 54 (2015) 96649669. [13] M. Zhu, Y.G. Li, Y. Ma, L.C. Li, D.Z. Liao, Inorg. Chem. 52 (2013) 1232612328. [14] M. Yang, J. Sun, J.N. Guo, G.F. Sun, L.C. Li, CrystEngComm 18 (2016) 93459356. [15] M. Zhu, J.J. Wang, M. Yang, Y. Ma, L.C. Li, Dalton Trans. 44 (2015) 98159822. [16] G.F. Sun, J.N. Guo, M. Yang, L. Xi, L.C. Li, J.P. Sutter, Dalton Trans. 47 (2018) 46724677. [17] T. Li, X.J. Shi, P.Y. Chen, S.J. Yu, L. Tian, Inorg. Chim. Acta 461 (2017) 206212. [18] J.Y. Shi, M.Z. Wu, P.Y. Chen, T. Li, L. Tian, Y.Q. Zhang, Inorg. Chem. 58 (2019) 1428514288
[19] D. Casanova, M. Llunell, P. Alemany, S. Alvarez, Chem. Eur. J. 11 (2005) 14791494. [20] M. Llunell, D. Casanova, J. Cirera, P. Alemany, S. Alvarez, 2.1 ed., University of Barcelona, Barcelona, 2013. [21] M. Yang, J. Xie, Z. Sun, L.C. Li, J.P. Sutter, Inorg. Chem. 56 (2017) 1348213490. [22] J.P. Sutter, M.L. Kahn, O. Kahn, Adv. Mater. 11(1999) 863865. [23] F. Luis, J. Bartolomé, J.F. Fernández, J. Tejada, J.M. Hernández, X.X. Zhang, R. Ziolo, Phys. Rev. B 55 (1997) 1144811456. [24] J. Bartolomé, G. Filoti, V. Kuncser, G. Schinteie, V. Mereacre, C. E. Anson, A. K. Powell, D. Prodius, C. Turta, Phys. Rev. B 80 (2009) 014430/1014430/16. [25] K.S. Cole, R.H. J. Cole, Chem. Phys. 9 (1941) 341351. [26] G. M. Sheldrick, SHELXS-2014, Program for structure solution, Universität of Göttingen, Germany, (2014). [27] G. M. Sheldrick, SHELXL-2014, Program for structure refinement, Universität of Göttingen, Göttingen, Germany, (2014).
A family of ladder-like 1-D 2p-3d-4f complexes based on functional nitronyl nitroxide radical are synthesized.
A family of 2p-3d-4f complexes which possess rare Ladder-like 1-D structures are synthesized. The Ho complex displays single-chain magnet behavior.
We declared that we have no conflicts of interest to this work.
Meng Yang: Conceptualization, Methodology, Software; Xiaohong Liang: Prepare the ligand and complexes; Yangdie Zhang: Writing Original draft preparation. Zhijian Ouyang: Stuctures and properties of the complexes; Wen Dong: Writing- Reviewing and Editing,
S0277-5387(20)30092-9 https://doi.org/10.1016/j.poly.2020.114435 POLY 114435
To appear in:
Polyhedron
Received Date: Revised Date: Accepted Date:
16 January 2020 8 February 2020 10 February 2020
Please cite this article as: M. Yang, X. Liang, Y. Zhang, Z. Ouyang, W. Dong, A family of 3d-4f Cu-Ln ladder-like complexes: synthesis, structures and magnetic properties, Polyhedron (2020), doi: https://doi.org/10.1016/j.poly. 2020.114435
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2020 Published by Elsevier Ltd.
A family of 3d-4f Cu-Ln ladder-like complexes: synthesis, structures and magnetic properties Meng Yang, Xiaohong Liang, Yandie Zhang, Zhijian Ouyang, Wen Dong Guangzhou Key Laboratory for Environmentally Functional Materials and Technology, School of Chemistry and Chemical Engineering, Guangzhou University, 230 Wai Huan Xi Road, Guangzhou Higher Education Mega Center, Guangzhou 510006, P. R.China.
Abstract
A
family
of
rare
2p-3d-4f
ladder-like
one-chain
complexes
namely
[LnCu(hfac)5NIT-Ph-p-OCH2trz0.5C6H14]n (Ln = Er (1), Ho (2), Yb (3) ; NIT-Ph-p-OCH2trz
=
2-(4-((1H-1,2,4-triazol-1-yl)methoxy)phenyl)-4,4,5,5-tetra-
methylimidazoline-1-oxyl-3-oxide; hfac = hexafluoroacetylacetonate) have been successfully synthesized simultaneously through reacting nitronyl nirtroxide radical NIT-Ph-p-OCH2trz with Cu(hfac)2 and Ln(hfac)3. The structures of complexes of 13 were elucidated by single-crystal X-ray structural analysis and the result show that all complexes feature ladder-like chain structures. Nonzero out-of-phase signals are observed for Ho derivatives (2) indicating single-chain magnet behavior.
Keywords: Functionalized nitronyl nitroxides; 2p-3d-4f; Ladder-like structure; Magnetic properties
1. Introduction
The heterospin complex, which including different spin carrier in one molecule has attracted enormous interest due to it is valuable synthetic strategy to obtain molecular magnetic materials. So far, a great variety of reported single chain magnets (SCMs), and single molecule magnets (SMMs) are 2p-3d [14] and 2p-4f [59] heterobispin complexes.
Since
the
first
2p-3d-4f
heterotrispin
complex
named
[{CuL}2Gd(TCNQ)2]·TCNQ-·CH3OH·2CH3CN was report [10], many similar complexes based on nitronyl nitroxide radical have been obtained following a one-pot procedure through reacting the radical with 3d and 4f hexafluoroacetylacetonato complexes in the last years.[1116] In the 2p-3d-4f complexes, the R groups in the radical can be strongly influence the structure of the complexes. To design 2p-3d-4f heterotrispin complexes, special attention has been paid to functionalized radicals. The nitronyl nitroxide radical that contain a triazole ring is a good derivative because these free radicals can bond to the metal ions with not only the triazole nitrogen atoms but also the radical oxygen atoms, which may cause the formation of polynuclear clusters.[17,18] Using a
functional NITR containing triazole ring named
NIT-Ph-p-OCH2trz (scheme 1), a family of ladder-like one-chain 2p-3d-4f complexes namely [LnCu(hfac)5NIT-Ph-p-OCH2trz0.5C6H14]n ( Ln = Er (1), Ho (2), Yb (3), NIT-Ph-p-OCH2trz = (1H-1,2,4-triazol-1- yl)methoxy]phenyl]-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) were obtained. All complexes feature ladder-like chain structures. Nonzero out-of-phase signals are observed for Ho derivatives (2) exhibit frequency-dependent out-of-phase signals indicating single-chain magnet behavior.
Scheme 1 NIT-Ph-p-OCH2trz radical ligand.
2. Results and discussions
2.1. Description of the Crystal Structure. Complexes of 13 possess similar structures and are solved in triclinic Pī group space，therefore, only the structure of 1 is described as the representative. As shown in Figure 1, each Er(III) ion occupies one O atom from the NO group and one N atom form the triazole ring of the nitronyl nitroxide radical ligand, and it displays an eight-coordinate distorted square-antiprismatic geometry (D4d; CShM = 0.890, the CshM values are calculated by Shape software[19,20], Table S4) with the other six O atoms from the hfac ligands making up the coordination sphere. The Er−O(radical) and Er−N distances are 2.351(3) and 2.569(4) Å, respectively, while the Tb−O(hfac) distances ranging from 2.314(3) to 2.351(3) Å. The Cu(II) ion occupies the inner O4 site of the hfac ligand, with a six coordinate, elongated octahedral geometry (CShM = 1.265, Table S5), with the O4 coordination atoms of hfac ligand forming the equatorial plane and the O atom from NO group and N atom from triazole ring of the radical ligand occupying the apical position of the coordination geometry. The distance of Cu−O and Cu−N are 2.407(4) and 2.463(4) Å in the axis positions while the Cu−O bond lengths are ranging from 1.939(3) to 1.949(4) Å. The paramagnetic
NIT ligand is coordinated two Er(III) and two Cu(II) ions with the μ4-η1:η1:η1:η1 coordinate mode,
resulting in a ladder-like arrangement in which the anisole
moieties form the rungs of the ladder. In complex 1, the Er···Cu separations through the NIT motif and the triazole ring are of 8.52 and 6.93 Å, respectively while the Er···Er and Cu···Cu distances across the rungs are 8.98 and 10.58 Å, respectively.
Figure 1 Molecular structure of complex 1 (Hydrogen and Fluorine atoms are not shown for the sake of clarity).
Figure 2 Crystal packing of complex 1 (all hydrogen and fluorine atoms are omitted for clarity).
2.2. Magnetic properties The variable temperature magnetic susceptibility data for compounds 13 were measured in the 2300 K range under a 1000 Oe dc field. As shown in Figure 3, the
MT values are 12.98, 15.47 and 3.64 cm3Kmol1 at room temperature, which are close to the expected values of 12.23, 14.81, and 3.32 cm3Kmol1 for one uncorrelated Ln(III) ion (Er : 4I15/2, S = 3/2, L = 6, g = 6/5, C = 11.475 cm3 K/mol; Ho : 5I8, S = 2, L = 6, g = 5/4, C = 14.0625 cm3 K/mol; Yb : 2F7/2, S = 1/2, L = 3, g = 8/7, C = 2.5714 cm3K/mol) plus one isolated Cu(II) ion (S = 1/2, g = 2.0) and one radical (S = 1/2, g = 2.0). For complex 1, the MT value is a constant from 300 to 100 K, and diminished slowly up 25 K and then accelerated appropriately until it reached the minimum value of 8.92 cm3·K·mol−1 at 2 K. Complex 2 displayed a similar phenomenon. But for complex 3, the value of χMT diminished slowly at first and then accelerated appropriately until it reached the minimum value of 1.81 cm3·K·mol−1 at 2 K. According to our previous report [21], there exist ferromagnetic coupling between the Cu(II) ion and the axial NO unit results from the orthogonality of the magnetic orbital (dx2-y2) of the Cu(II) ion and the radical * magnetic orbital. The interactions between the Ln(III) ions and the coordinated NO group of the organic radical are ferromagnetic. For complexes 13, their magnetic behaviors do not display increase of the MT values when the temperature is decreased. This implies that the ferromagnetic Ln-radical and Cu-radical contributions are weaker for these three rare earth ions and the stronger crystal field effect of three lanthanides just overwhelms the ferromagnetic Ln-radical contributions on MT curve [22].
Such behavior is
consistent with the M versus H/T plots of 1−3 at 2 K, which displayed a rapid increase in the magnetization at low fields, and the magnetization reached the maximum
values of magnetization with 7.40, 8.20 and 3.46 Nβ at 7 T (Figure 4, Figures S8 and S9). The saturation values were much lower than the theoretical values (11, 12 and 6 Nβ for 13), which are likely due to crystal-field effect and the contribution of low lying excited states.
Figure 3 Plot of χMT versus T for complexes 13.
Figure 4 M versus H plot at 2 K for complex 1.
In order to explore spin dynamic behaviors of complex 2, the ac susceptibilities were measured utilizing an oscillating 3.0 Oe field from 2 to 10 K at different frequencies. It exhibits clear frequency-dependent χ″ signals in the zero dc fields, which could be due to slow magnetic relaxation behavior (Figure 5). But no peak maxima value of χ″ was observed in the available window above 2 K. And then the ac data were collected in 2 kOe dc field to reduce or suppress the possible quantum tunneling process (QTM). Unfortunately, no visible χ″ peaks were found yet. Thus, the activation barrier (Ueff) and relaxation time (τ0) cannot be obtained from the fit of Arrhenius law. If there is only one relaxation process in 2, the equation ln(χ″/χ′) = ln(ωτ0)+ Ea/kBT could be used to roughly estimate the Ueff and τ0 values.[23,24] The best fitting ac data of complex 2 yield the energy barrier (Ea/kB) 4.93 K with the relaxation time (τ0) 1.64 × 10−6 s at 2 kOe dc field (Figure 6). The frequency dependence of the χ′ and χ″ signals were also examined in a frequency range of 100−10000 Hz with 2 kOe dc fields. As shown in Figure 7, the χ″ component of the alternating-current (ac) susceptibility feature strong frequency-dependent phenomena, displaying the maxima for all frequencies. The Cole−Cole plot was also shown in Figure 7, and it was analyzed with the generalized Debye model [25] and the obtained α value vary from 0.12 to 0.54, indicating a rather broad distribution of relaxation times.
Figure 5 Temperature dependence of the out-phase in zero (left) and 2 kOe field (right) for complex 2.
Figure 6 Natural logarithm of the ratio of χ"/χ' vs. 1/T for 2. The solid line represents the fitting results.
Figure 7. (left) Frequency dependence of the out-of-phase components for the ac magnetic susceptibility in a 2 kOe dc field for 2. (right) Cole−Cole plots for 2. The solid lines represent the best fits with modified Debye functions.
3. Conclusions A family of Cu-Ln complexes based on the functional nitronyl nitroxide radical named [LnCu(hfac)5NIT-Ph-p-OCH2trz0.5C6H14]n ( Ln = Er (1), Ho (2), Yb (3) ) have been obtained. These complexes all possess ladder-like chain structures in which the anisole moieties form the rungs of the ladder. Magnetic measures show that the Ho derivatives (2) exhibits frequency-dependent out-of-phase signals indicating single-chain magnet behavior. This work demonstrates that using functionalized nitronyl nitroxide radicals is an appealing strategy for design 2p-3d-4f hetero-tri-spin compounds. It is not only to enrich the structural diversity of 2p-3d-4f compounds but also for modulating the magnetic behaviors of 2p-3d-4f systems.
4. Experimental
4.1 Materials and Physical Measurements All reagents were purchased from commercial sources and used without further purification. Elemental analysis for C, H, and N were carried out using a Perkin-Elmer elemental analyzer model 240. Magnetic measurements were measured on PPMS-9 magnetometer. The magnetic susceptibilities were corrected for the diamagnetic contribution of the constituent atoms by using of Pascal’s constants. 4.2 Preparation of complexes of 13. 20 mL dry boiling hexane with Ln(hfac)32H2O (0.02 mmol) was kept to reflux for 3
hours. Then a solution of NIT-Ph-p-OCH2trz (0.0064 g, 0.02mmol) in 3 mL dry CHCl3 was added and refluxed for 30 minutes. After that, 0.0098 g solid Cu(hfac)2 (0.02 mmol) was added. The resulting solution was stirred for 20 minutes, and then cooled it to room temperature and filtered off. The filtrate was kept at room temperature, after two days, blue needle-like crystals suitable for X-ray diffraction were obtained through evaporation method. Anal. calcd for C44H32CuF30N5O13Er (%): C, 32.23; H, 1.97; N, 4.27. Found: C, 32.32; H, 1.95; N, 4.26. IR(KBr): 1651(s), 1503(m), 1475(m), 1256(s), 1226(s), 1142(s), 801(m), 661(m), 589(m) cm−1. Anal. calcd for C44H32CuF30N5O13Ho (%): C, 32.28; H, 1.97; N, 4.28. Found: C, 32.30; H, 1.95; N, 4.29. IR(KBr): 1655(s), 1503(m), 1477(m), 1256(s), 1226(s), 1142(s), 799(m), 663(m), 589(m) cm−1. Anal. calcd for C44H32CuF30N5O13Yb (%): C, 32.12; H, 1.96; N, 4.27. Found: C, 32.31; H, 1.95; N, 4.26. IR(KBr): 1651(s), 1503(m), 1476(m), 1257(s), 1228(s), 1142(s), 803(m), 662(m), 590(m) cm−1. 4.3 X-ray structure determination All the crystal data were collected at 113 K using the Rigaku Saturn CCD diffractometer employing graphite-monochromated Mo-Kα radiation. The structures of all complexes were solved by direct methods with SHELXS-2014 program and refined by full-matrix least-squares on F2 with SHELXL-2014.[26,27] All the non-hydrogen atoms are refined anisotropically and the hydrogen atoms are added geometrically and refined as riding atoms with a uniform value of Uiso. A summary of the detailed crystallographic data and structure refinement of 1–3 is given in Table S6.
Appendix A.Supplementary data
CCDC 19744771974479 contain the supplementary crystallographic data for complexes 1, 3 and 2 respectively. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html,
or
from
the
Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected].
Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21271052) and Science and Technology Program Foundation of Guangdong Province (No.2015A030313502).
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A family of ladder-like 1-D 2p-3d-4f complexes based on functional nitronyl nitroxide radical are synthesized.
A family of 2p-3d-4f complexes which possess rare Ladder-like 1-D structures are synthesized. The Ho complex displays single-chain magnet behavior.
We declared that we have no conflicts of interest to this work.
Meng Yang: Conceptualization, Methodology, Software; Xiaohong Liang: Prepare the ligand and complexes; Yangdie Zhang: Writing Original draft preparation. Zhijian Ouyang: Stuctures and properties of the complexes; Wen Dong: Writing- Reviewing and Editing,