Heterometallic (3d-4f) Coordination Clusters with Unique Topology: Self-Assembly Synthesis, Structural Features, and Magnetic Properties

Three heterometallic Fe6Ln4 clusters were self-assembled by using N-methyldiethanolamine (H2mdea) and 3,5-dinitrobenzoic acid (Hdnbz) as principal building blocks and azide as an ancillary ligand. The obtained products 1-3 were isolated as air-stable microcrystalline solids and fully characterized. Single-crystal X-ray diffraction studies reveal similar type of molecular units [Fe6Ln4(μ4-O)2(μ3-OH)2(mdea)6(dnbz)8(N3)4]·2MeCN·xH2O·CH2Cl2 {Ln = Y (1), Gd (2), Dy (3); x = 4, 6, 10, respectively}. Topological analysis of 1-3 was performed following a method for high nuclearity coordination clusters. The [Fe6Ln4(μ-O)10(μ3-O)4(μ4-O)2] cores were identified in all structures and further simplified to graph topological skeletons, which were classified as pentanodal 2,3,3,5,6-connected motifs with the unique topology. The magnetic behavior of 1-3 was investigated in detail. The magnetic studies and magnetocaloric measurements reveal that the tetranuclear gadolinium centers of 2 exhibit a high cryogenic magnetocaloric effect (MCE) that reaches 13.66 J·kg-1·K-1 (25.18 mJ·cm-3·K-1) for ΔH = 90 kOe at 4.5 K. Among all the compounds, only 3 shows the ac-signals (ac, alternating current) and slow-relaxation behavior, which is a peculiar characteristic of a single-molecule magnet (SMM), giving a value of Ueff = 19.5(4) K. The present work thus contributes to the design of topologically unique discrete crystalline materials with notable functional properties. © 2020 American Chemical Society.