Tandem templating strategies facilitate the assembly of calix[8]arene-supported Ln18 clusters

Abstract

Calix[n]arenes offer ideal chemical functionality through the polyphenolic lower rim to construct nano-sized coordination clusters with lanthanide (Ln) metal ions (e.g., Nd^III₁₀, Gd^III₈). However, the number of metal centers they can accommodate is still limited compared to that achievable with smaller ligands (e.g., Gd^III₁₄₀, Gd^III₁₀₄). Here, we exploit a combination of the “anion template strategy” and “templating ligands” to synthesise three highly symmetric (D_3h, trigonal planar) Ln^III₁₈ (Ln = La, Nd, and Gd) systems, representing the largest calix[n]arene-based coordination clusters yet. The Ln^III₁₈ fragment is templated by a chloride anion located at the center of the cluster, wherefrom twelve μ₃-OH⁻ ligands bind ‘internally’ to the eighteen Ln^III ions. ‘Externally’ the metallic skeleton is connected by p-tert-butylcalix[8]arene, oxo, chloro and carbonate ligands. The crystal packing in the lattice reveals large cylindrical channels of ∼26 Å in diameter, whose pore volume corresponds to ∼50% of the unit cell volume (using a 1.2 Å spherical probe radius). Magnetic measurements reveal the predominance of weak antiferromagnetic exchange in the Gd analog. Heat capacity data of Gd^III₁₈ reveal a high magnetic entropy with −ΔS_m = 23.7 J K⁻¹ kg⁻¹, indicating potential for engineering magnetic refrigerant materials with calix[8]arenes.