Two Isostructural 3D Lanthanide Coordination Networks (Ln = Gd 3+ , Dy 3+ ) with Squashed Cuboid-Type Nanoscopic Cages Showing Significant Cryogenic Magnetic Refrigeration and Slow Magnetic Relaxation Soumava Biswas, Himanshu Sekhar Jena, Amit Adhikary, and Sanjit Konar* Department of Chemistry, IISER Bhopal, Bhopal 462066, India * S Supporting Information ABSTRACT: Two isostructural lanthanide-based 3D coordination networks [Ln = Gd 3+ (1), Dy 3+ (2)] with densely packed distorted cuboid nanoscopic cages are reported for the first time. Magnetic characterization reveals that complex 1 shows a significant cryogenic magnetocaloric effect (-ΔS m = 44 J kg -1 K -1 ), whereas 2 shows slow relaxation of magnetization. L anthanide-based molecular magnetic materials are a forefront area of research owing to their proposed applications in future devices like molecular magnets, 1 magnetic refrigerants, 2 etc. Magnetic refrigeration happens because of a phenomenon known as the magnetocaloric effect (MCE), and the magnitude of the MCE of a magnetic material is characterized by ΔS m , the isothermal magnetic entropy change, and ΔT ad , the adiabatic temperature change following a change in the applied magnetic field. Gadolinium is often present in magnetic refrigerant materials because of its large spin ground state, quenched orbital momentum, and weak superexchange inter- actions. 3 To date, some impressive gadolinium-based com- pounds associated with molecular magnetic cooling have been reported. 4 However, the controlled and long-range dense spatial assembly of the Gd 3+ spin centers is vital for the design of an ideal magnetic refrigerant, and therefore higher-dimensional coordi- nation polymers should also play an important role. The magnetic density can also be maximized by, for example, limiting the amount of nonmagnetic elements, which act passively in the physical process. Therefore, the small size of the ligands should be preferable. On the other hand, the intrinsic magnetic anisotropy and the increased number of unpaired f electrons of dysprosium may be responsible for the large energy barrier for reversal of magnetization, especially when combined with suitable ligands. The present work stems from our earlier investigation of the MCE on a gadolinium squarate 2D coordination polymer, 5a and a high-symmetry 3D iron(II)-based squashed cuboctahedral nanoscopic cage behaves like a spin-canting antiferromagnet. 5b Herein we report the synthesis, characterization, and magnetic property investigation of two heteroleptic isostructural lantha- nide-based 3D coordination networks [Ln(C 4 O 4 )- (C 2 O 2 ) 0.5 (H 2 O) 2 ] n [Ln = Gd 3+ (1) and Dy 3+ (2)]. Structural investigations reveal that the 3D framework consists of extended cuboid nanoscopic cages. Magnetic characterizations show that complex 1 acts as an efficient cryogenic magnetic refrigerant (-ΔS m = 44 J kg -1 K -1 ), whereas 2 shows slow relaxation of magnetization. Because complexes 1 and 2 are isostructural in nature, the structural description of complex 1 has been illustrated with suitable comparison with complex 2. Both of the complexes were crystallized in space group P2 1 /c, and the relevant structural refinement parameters are listed in Table S1 in the Supporting Information (SI). The asymmetric unit of complex 1, [Ln(C 4 O 4 )(C 2 O 4 ) 0.5 (H 2 O) 2 ], contains one Gd 3+ ion, one squarate, half of the oxalate ligand, and two coordinated water molecules (Figure S1a in the SI). Each lanthanide center is eight-coordinated with four oxygen atoms from four squarate ligands, two oxygen atoms from one oxalate ligand, and two other oxygen atoms from coordinated water molecules. Thus, the coordination geometry around the gadolinium centers can be described as a distorted square- antiprismatic (Figure S1b in the SI). The relevant bond parameters around the lanthanide centers in complexes 1 and 2 are listed in Table S2 in the SI and are found to be in the range of reported analogues of a Tb 3+ -based 3D framework containing similar set of ligands. 6 It can be seen that each squarate ion acts as a tetradentate ligand (μ 4 -connected) and binds four lanthanide ions to form lanthanide rectangles (Figure 1a). Four lanthanide rectangles are face-shared by a common squarate ion and result in Received: December 10, 2013 Published: April 1, 2014 Figure 1. Schematic representations of (a) the bridging mode of a squarate ion, (b) a distorted lanthanide cuboid nanocage showing four close faces by squarate ligands and two open faces, (c) the bridging mode of an oxalate ion, and (d) two adjacent cuboid nanocages connected by an oxalate ion, which closes the open face of each cuboid. Color code: Ln, blue; O, red; C, gray; squarate bridge, yellow rod; oxalate bridge, pink. Communication pubs.acs.org/IC © 2014 American Chemical Society 3926 dx.doi.org/10.1021/ic4030316 | Inorg. Chem. 2014, 53, 3926-3928