Influence of Carboxylate Ligand Variation on Structure, Magnetic Exchange, and Magnetocaloric Performance in Hexanuclear Gd(III) Complexes

Hexanuclear cyclic Gd(III) coordination compounds are promising candidates for magnetic and magnetocaloric applications, yet systematic series remain rare due to synthetic challenges. We report the synthesis and characterization of five new complexes with the general formula [Gd(H₃bt)(carboxylate)]₆, employing four different carboxylate coligands. Single‑crystal X‑ray diffraction confirmed their isostructural nature, with polymorphism observed in the 4‑bromobenzoate derivatives. Magnetic properties were investigated using SQUID magnetometry and complemented by Broken Symmetry DFT calculations. The study reveals weak antiferromagnetic exchange interactions and notable magnetocaloric performance, with entropy changes approaching 28 J K^- ¹ kg^- ¹ at 2.7 K (other compounds measured at 2 K) and 9 T. Magneto‑structural correlations based on structural parameters and substituent effects were established, providing guidelines for tuning exchange interactions in cyclic Gd(III) systems. Incorporation of a thioether substituent further suggests potential for surface deposition, opening pathways toward device‑oriented applications.