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

Using ferrocenecarboxylic acid (FcCOH) and triethanolamine (Htea) as ligands, the isostructural heterotrimetallic complexes [LnIII2CrIII2(OH)(FcCO)(NO)(Htea)]·2MePh·2THF (Ln = Tb (1), Dy (2), Ho (3), Er (4), and Y (5); Fc = (η-CH)(η-CH)Fe; Htea = N(CHCHOH)) 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 Tb 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 {LnIII2CrIII2} 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 (/) 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 CrFeO and heterotrimetallic oxide LnCrFeO (with ≈ 0.75) phases.