Cyano-bridged {Fe₂Co} assemblies showing metamagnetic transition and a magnetocaloric effect

The investigation of magnetic materials featuring unconventional magnetic topologies represents a forefront research area in the interdisciplinary fields of physics, chemistry, and materials science. Such systems hold considerable promise for applications in strongly correlated electron systems, spintronic devices, magnetic memory technologies, and magnetocaloric applications. Among them, cyano-bridged Prussian blue analogues (PBAs) have emerged as a prominent class of molecular magnetic materials, offering a versatile platform for the systematic modulation of magnetic interactions and topological architectures through the rational selection of paramagnetic metal centers and auxiliary ligands. Herein, we report two heterometallic molecular magnets based on tricyanoferrate bridges, namely, [(PzTp)Fe(CN)₃]₂[Co(dypu)]·H₂O (1) and (Tp*)[Fe(CN)₃]₂[Co(dypu)]·H₂O (2) (dypu = 1,3-di(pyridin-4-yl)urea), which exhibit field-induced magnetic phase transition. Structural characterization shows that compounds 1 and 2 exhibit one-dimensional double-zigzag chains, further connected into a two-dimensional network by the ditopic dypu ligand. Magnetic analysis reveals ferromagnetic coupling between the cyano-bridged Fe^III and Co^II centers in compound 1, whereas antiferromagnetic coupling occurs in 2. Interestingly, variable-temperature and variable-field magnetic susceptibility measurements reveal notable magnetic structure transitions in compound 1: (i) from spin-canted antiferromagnetism (AFM) to nearly collinear AFM at a critical field (H_C1) of 3.5 kOe, followed by (ii) a transition to a nearly ferromagnetic (FM) alignment at a second critical field (H_C2) of 18 kOe. Notably, a significant magnetocaloric effect is observed during the phase transition, with the change in entropy (ΔS) reaching 23.22 J K^-1 kg^-1. This study underscores that the rational modulation of auxiliary ligands enables the tuning of diverse magnetic interactions and structural topologies and advances the understanding of magneto-structural correlations in molecular magnetic systems.