Structural and selectivity of 18-crown-6 ligand in lanthanide-picrate complexes

The selectivity factor in the separation of lanthanide could be associated with the coordination behaviour. Thus, we observed the study in the solid phase to understand the coordination pattern of Ln(III) with the 18-crown-6 (18C6) ligand. Good selectivity of the rigid 18C6 ligand toward Ln(III) depends on gradually smaller their ionic radii of Ln(III) in the complexes formation in the presence of picrate anion (Pic⁻), i.e. lanthanide contraction and steric effects as clearly shown in the series of [Ln(Pic)₂(18C6)]⁺(Pic)⁻ {Ln = La, Ce, Pr, Nd, Sm, Gd} and [Ln(Pic)₃(OH₂)₃] · 2(18C6) · 4H₂O {Ln = Tb, Ho} complexes. The La-Gd complexes crystallized in an orthorhombic with space group Pbca, while the Ho complex crystallized in triclinic with space group . The lighter lanthanides complexes [La-Sm] had a 10-coordination number from the 18C6 ligand and the two picrates, forming a bicapped square-antiprismatic geometry. Meanwhile, the middle lanthanide complex [Gd] had a nine-coordination number from the 18C6 ligand and the two picrates, forming a tricapped trigonal prismatic geometry. The heavier lanthanide [Ho] is rather unique, since Ho(III) coordinated with nine oxygen atoms from three picrates and three water molecules in the opposite direction whereas three 18C6 molecules surrounded in the inner coordination sphere, forming a trigonal tricapped prismatic geometry. The 18C6 ligand is effective in controlling the molecular geometry and coordination bonding of Ln-O and can use a crystal engineering approach. No dissociation of Ln-O bonds in solution was observed in NMR studies conducted at different temperatures. The photoluminescence spectrum of the Pr complex has typical 4f-4f emission transitions, i.e. ³P₀ → ³F₂ (650 nm), ¹D₂ → ³F₂ (830 nm) and ¹D₂ → ³F₄ (950 nm).