Magnetocaloric effect and slow magnetic relaxation behavior in binuclear rare earth based RE2(L)2(DMF)4 (RE = Gd, Tb, and Dy) complexes☆
Graphical abstract
Three binuclear rare earth based complexes combining RE ions with semirigid tricarboxylic ligand (H3L), namely [RE2(L)2(DMF)4] [RE = Gd, Tb, and Dy; H3L = 5-((4-Carboxybenzyl)oxy)isophthalic acid; DMF = N,N-dimethylformamide] complexes, were fabricated successfully. Their structural, magnetic and magnetocaloric properties were investigated systematically. This work not only provides an effective strategy for obtaining molecular materials with high MCE, but also confirms that tricarboxylate ligands are the ideal choice for constructing stable high dimensional geometric structures.
Introduction
The molecule-based magnetic materials have attracted extensive attention in light of their various magnetic properties and charming topologies as emerging inorganic-organic hybrid materials in recent years.1, 2, 3 In particular, the rare earth (RE) ions are easy to be bonded with different organic ligands to form novel structures with unique properties due to their high coordination numbers and complex coordination environments, which facilitates their meaningful application prospects in various industrial fields including luminescent sensing, high-density information storage, cryogenic magnetic refrigeration, etc.1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16 In molecular magnetism, the introduction of RE ions is conducive to obtaining single molecule magnets (SMMs), which is related to the strong spin-orbital coupling and the unquenched orbital moment of RE ions. The slow relaxation of magnetization of SMMs is exhibited below the blocking temperature (TB), which provides the basis for applying in the information storage, quantum-computing, molecular spintronics and other fields.11, 12, 13 Recently, the RE-CPs based on multinuclear complexes, especially those for highly anisotropic Dy-based systems, have been found to be particularly significant as SMMs. They have attracted much attention since they can offer an ideal pattern for the synergism of RE ions anisotropy and configurations in enhancing the anisotropic barrier.14, 15, 16, 17 The slow magnetic relaxation phenomenon has been reported in the [Cr4Dy(CH3COO)(pyCOO)((py)2COO)4], [Cu3Dy3(O2)L3(PyCO2)3](OH)2(ClO4)2·8H2O, [Dy2(tda)3(H2O)2], (H6edte)0.5[Dy(ox)2(H2O)] and other high–dimensional complexes with Dy clusters as nodes, thus it is significant to explore and investigate such complexes.11, 12, 13, 14, 15, 16, 17, 18, 19, 20
In addition to the properties of SMMs, the MCE-based magnetic refrigeration technology is a potential substitute for the current vapor-cycle refrigeration technology. The MCE is an inherent attribute of the change of entropy (temperature) caused by varying the external magnetic fields.21, 22, 23, 24, 25, 26, 27, 28, 29 The MCE-based molecular magnetic refrigerant is a potential substitute for rare and expensive He-3 in the field of refrigeration at extremely low temperatures.27, 28, 29 It should be emphasized that the MCE is contrary to the requirements of SMMs, as it is more dependent on factors of instance low magnetic anisotropy, high spin ground state and small values in efficient magnetocaloric materials design and development.30,31 Thus, although transition metal-based molecular magnetic refrigeration materials have become a research hotspot because of the characteristic of high ground-state spin, they usually exhibit small MCEs due to their strong magnetic coupling.32 From this perspective, the target products can be obtained from the RE-CPs, especially the Gd-based complex, owing to the high spin ground state, weak super-exchange interactions and zero orbital momentum of the Gd ions. Nevertheless, considering RE ions with kinetic lability and weak stereochemical preferences, the development of RE-CPs has progressed less than that of transition metal systems.33, 34, 35
5-((4-Carboxybenzyl)oxy)isophthalic acid (H3L) acts as a semirigid tricarboxylic ligand and has the characteristics of multi-coordination and high negative charge, which is of great significance in meeting the requirements of high coordination and balancing the positive charge of RE ions to build a stable high-dimensional geometric structure. Furthermore, a multi-carboxylate system composed of RE ions can not only connect multiple RE ions to increase the ability of electron transfer between magnetic centers but also endow rigidity and stability of the complex36 As expected, we have reported the construction of a series of 3D RE-CPs, namely, [RE2(L)2(DMF)4] [RE = Gd, Tb, and Dy (1–3)]. The 3D network of these complexes can be represented by a distinctive topology point symbol of {42·6}2{44·62·87·102}. The complex 1 exhibits a competitive large magnetic entropy change in the reported binuclear coordination polymers, and the scarce slow magnetic relaxation is detected in complex 3 adopting {Dy2} as the nodes.
Section snippets
Materials and instruction
All purchased reactants and reagents have not been further purified. A Vario EL-III elemental analyzer was utilized to check the elemental analysis of C, H and N. The FTIR spectra of powder samples were conducted by a Nicolet iS10 apparatus equipped with a Smart ART appendix. The room temperature powder X-ray diffraction (PXRD) characterizations of all present RE2(L)2(DMF)4 complexes were investigated by a SmartLab-SE X-ray diffractometer adopting Cu/Kα radiation (Rigaku, Japan). The
Structural description
The present [RE2(L)2(DMF)4] [RE = Gd, Tb, and Dy (1–3)] complexes were constructed from H3L ligands and RE(III) ions under hydrothermal conditions. All present complexes exhibit identical monoclinic structure with the space group, so we only discuss the situation of complex 1 in detail as an example. The crystallographic minimal asymmetric unit of complex 1 is composed of two Gd3+ cations, two ligand anions and two DMF molecules (Fig. 1(a)). From Table S1, we can note that the O–Gd–O
Conclusions
Three binuclear 3D RE-based complexes derived from the semirigid tricarboxylic ligands H3L, RE2(L)2(DMF)4, were successfully prepared. The systematic investigations of structural, magnetic and magnetocaloric properties of the present RE2(L)2(DMF)4 complexes were implemented. RE1 and RE2 share the same coordination environment, exhibiting the irregular three-hat triangular prism coordination environment with approximately D3h axial symmetry. The AFM interaction between the central ions can be
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Foundation item: Project supported by the National Natural Science Foundation of China (91963123), the State Key Laboratory of Solidification Processing in NWPU (SKLSP202020), the Ten Thousand Talents Plan of Zhejiang Province of China (2018R52003), and the Fundamental Research Funds for the Provincial University of Zhejiang (GK199900299012-022)