Hydroxide-Free Cubane-Shaped Tetranuclear [Ln₄] Complexes

Abstract

The reaction of the lanthanide(III) chloride salts [Gd(III), Tb(III), and Dy(III)] with a new chelating, flexible, and sterically unencumbered multisite coordinating compartmental Schiff-base ligand (E)-2-((6-(hydroxymethyl)pyridin-2-yl)methyleneamino)phenol (LH2) and pivalic acid (PivH) in the presence of triethylamine (Et3N) affords a series of tetranuclear Ln(III) coordination compounds, [Ln4(L)4(μ2-η(1)η(1)Piv)4]·xH2O·yCH3OH (1, Ln = Gd(III), x = 3, y = 6; 2, Ln = Tb(III), x = 6, y = 2; 3, Ln = Dy(III), x = 4, y = 6). X-ray diffraction studies reveal that the molecular structure contains a distorted cubane-like [Ln4(μ3-OR)4](+8) core, which is formed by the concerted coordination action of four dianionic L(2-) Schiff-base ligands. Each lanthanide ion is eight-coordinated (2N, 6O) to form a distorted-triangular dodecahedral geometry. Alternating current susceptibility measurements of complex 3 reveal frequency- and temperature-dependent two-step out-of-phase signals under zero direct current (dc) field, which is characteristic of single-molecule magnet behavior. Analysis of the dynamic magnetic data under an applied dc field of 1000 Oe to fully or partly suppress the quantum tunneling of magnetization relaxation process affords the anisotropic barriers and pre-exponential factors: Δ/kB = 73(2) K, τ0 = 4.4 × 10(-8) s; Δ/kB = 47.2(9) K, τ0 = 5.0 × 10(-7) s for the slow and fast relaxations, respectively.