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

Using ferrocenecarboxylic acid (FcCO2H) and triethanolamine (H3tea) as ligands, the isostructural heterotrimetallic complexes [LnIII2CrIII2(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 {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 (Δ/kB) 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 Cr0.75Fe1.25O3 and heterotrimetallic oxide LnCr1-xFexO3 (with x ≈ 0.75) phases.

Towards comparative investigation of Er- and Yb-based SMMs: the effect of the coordination environment configuration on the magnetic relaxation in the series of heteroleptic thiocyanate complexes.

We prepared and studied two similar series of Er and Yb thiocyanates, involving [Ln(H2O)5(NCS)3]·H2O (1Er, 1Yb) as well as the molecular and ionic complexes with 2,2'-bipyridine (bpy) and 1,10-phenantroline (phen), [Ln(H2O)(bpy)2(NCS)3]·0.5(bpy)·H2O (2Er, 2Yb), [Ln(H2O)(phen)2(NCS)3]·phen·0.5H2O (3Er, 3Yb), [Hbpy][Ln(bpy)2(NCS)4]·H2O (4Er, 4Yb) and [Hphen][Ln(phen)2(NCS)4] (5Er, 5Yb). All the complexes were found to exhibit the properties of field-induced single-molecule magnets. For 1Yb, the effective value of the energy barrier for magnetization reversal, Δeff/kB, equals to 50 K, which is among the highest ones currently known for molecular SMMs based on Yb3+. The obtained data are discussed involving essential structural features of the complexes, namely the configuration of the Ln environment, i.e. its composition and geometry as well as mutual distribution of different donating centers. To the best of our knowledge, this work also involves experimental investigation of the largest and thus sufficiently representative series of similar mononuclear SMMs based on Er and Yb within one study.