Tetranuclear Cr–Ln ferrocenecarboxylate complexes with a defect-dicubane structure: synthesis, magnetism, and thermolysis

Abstract

Using ferrocenecarboxylic acid (FcCO₂H) and triethanolamine (H₃tea) as ligands, the isostructural heterotrimetallic complexes [Ln^III₂Cr^III₂(OH)₂(FcCO₂)₄(NO₃)₂(Htea)₂]·2MePh·2THF (Ln = Tb (1), Dy (2), Ho (3), Er (4), and Y (5); Fc = (η⁵-C₅H₄)(η⁵-C₅H₅)Fe; H₃tea = N(CH₂CH₂OH)₃) were obtained. In all of the complexes which possess a defective dicubane structure, two doubly deprotonated triethanolamine ligands chelate the chromium ions. However, during the synthesis of 1, an isomeric complex 1a in which Tb³⁺ is chelated by triethanolamine as a tetradentate ligand, was also isolated as a few single crystals. Magnetic susceptibility measurements revealed dominant antiferromagnetic interactions in the {Ln^III₂Cr^III₂} cores of 1–4 leading to the formation of complexes with an uncompensated magnetic moment, while weak Cr–Cr ferromagnetic interactions were detected in the Y analogue. Complexes 1, 2, and 3 exhibit single-molecule magnet properties dominated by an Orbach-type relaxation mechanism with magnetization reversal barriers (Δ/k_B) estimated around 54, 75, and 47 K, respectively. The Dy complex exhibits a magnetization hysteresis in an applied magnetic field at temperatures below 4 K. Thermolysis of the complexes was studied by TGA and DSC techniques; the final products obtained under an air atmosphere contain mixed oxide Cr_0.75Fe_1.25O₃ and heterotrimetallic oxide LnCr_1−xFe_xO₃ (with x ≈ 0.75) phases.