Pentagram-type Ln15 (Ln = Dy, Tb, Eu, Sm, Ho) clusters with different anion templates: magnetic and luminescence properties

Abstract

Five 15-nuclearity lanthanide clusters were obtained under solvothermal conditions based on N-tris(hydroxymethyl)methylglycine (H₅thmmg) with molecular formulas Ln₁₅(OH)₂₀(H₄thmmg)₁₀(H₂O)₁₀·xCl·(15 − x)NO₃ (Ln = Dy, x = 15 (1); Ln = Tb, x = 11 (2); Ln = Eu, x = 13 (3); Ln = Sm, x = 15 (4); and Ln = Ho, x = 15 (5)). They have the same pentagram-type metal cores made up of five cubane-like [Ln₄(μ₃-OH)₄] units by sharing vertical Ln³⁺ ions and their peripheries are all protected by ten H₄thmmg⁻ ligands and ten coordinated H₂O. Such structural Sm₁₅ and Ho₁₅ clusters are reported for the first time. For 1, 4 and 5, these metal-hydroxide cores are further stabilized only by free Cl⁻ ions, whereas NO₃⁻ and Cl⁻ ions both operate as anionic templates for 2 and 3 to balance the positive charge and to promote lanthanide metal ion aggregation. These are the first reported Ln₁₅ clusters self-assembled by NO₃⁻ and Cl⁻ anion templates. The luminescence behaviors of 2 and 3 show the characteristic emissions of the corresponding Ln³⁺ (Ln = Tb, Eu) ions. The temperature-dependent luminescence of 2 and 3 reveals that their emission intensities increase as the temperature drops, and good linear relationships exist in the 120–300 K range, so 2 and 3 are potential luminescent molecular thermometers. Moreover, 2 and 3 exhibit high quantum yields (QYs) (58.05% for 2 and 17.34% for 3) and long luminescence lifetimes (τ = 934 μs for 2, τ = 242 μs for 3). In addition, 1 displays slow magnetic relaxation behavior.