Magnetocaloric effect and slow magnetic relaxation behavior in binuclear rare earth based RE₂(L)₂(DMF)₄ (RE = Gd, Tb, and Dy) complexes

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. The RE₂(L)₂(DMF)₄ complexes consist of two central RE ions with the same coordination environment which were connected by two tridentate bridging carboxylic groups and two syn–syn bidentate bridging carboxylic groups originating from the L³⁻ ligands to form the {RE₂} dimeric unit, and thus provides the basis for further constructing a dense three-dimensional (3D) network structure. Moreover, the present RE₂(L)₂(DMF)₄ complexes can be described by a topology diagram with the topology point symbol of {4²·6}₂{4⁴·6²·8⁷·10²}. Weak anti-ferromagnetic (AFM) coupling between the adjacent RE ions for all the present complexes was found according to the magnetic calculations. The observed significant cryogenic magnetocaloric effect (MCE) with the maximum magnetic entropy change $-\mathrm{\Delta }{S}_{\mathrm{M}}^{\max }$ to be 26.3 J/(kg·K) with $\mathrm{\Delta }H$ = 7 T in Gd₂(L)₂(DMF)₄ complex makes it competitive for the cryogenic magnetic refrigerant. Moreover, the slow magnetic relaxation behavior at 0.2 T dc field with an obvious large $U_{\mathrm{eff}}/k$ = 45(4) K and ${\tau }_0$ = 6.5(2) × 10⁻¹⁰ s was confirmed in Dy₂(L)₂(DMF)₄ complex. 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 (T_B), 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 $M_{\mathrm{w}}/N_{\mathrm{m}}$ 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.