Slow magnetic relaxation in mononuclear gadolinium(III) and dysprosium(III) oxamato complexes

A series of lanthanide(III) oxamate complexes of formula Na[Eu(4-HOpa)4(H2O)]∙2H2O (1), Na[Gd(4-HOpa)4(H2O)]∙2H2O (2), Na[Dy(4-HOpa)4(H2O)]∙2H2O (3), and Na[Ho(4-HOpa)4(H2O)]∙2H2O (4) (4-HOpa = N-4-hydroxyphenyloxamate) were synthesized and characterized. The crystal structures of complexes 1–4, as well as the oxamate ligand in the ethyl ester acid form (Et[4-HOpa]), were elucidated by single crystal X-ray diffraction. 1–4 are isostructural, and they crystallize in the P21/n space group of the monoclinic system. The crystal structures of 1–4 consist of mononuclear lanthanide(III) complex anions, [Ln(4-HOpa)4(H2O)]− (Ln = Eu3+, Gd3+, Dy3+, and Ho3+), coordinated sodium(I) countercations, and crystallization water molecules, resulting in a 2D NaILnIII supramolecular coordination network. The coordination sphere of Ln3+ ions consists of square-face capped square antiprism (SAPRS-9). Magnetic properties were investigated for 1–4 in the 2–300 K temperature range using polycrystalline samples. Complex 2 exhibits an unusual and rare slow-relaxation of the magnetization for Gd3+ ions under DC applied field of 1.0 kOe, behaving as a field-induced single-ion magnet as occurs for the Dy3+ complex 3. The Eu3+ complex 1 has a nonmagnetic ground state, while the Ho3+ compound 4 behave as a paramagnet, and do not exhibit relaxation of the magnetization with or without an applied field.