Metal–organic frameworks with solvent-free lanthanide coordination environments: synthesis from aqueous ethanol solutions

Abstract

A series of 12 new metal–organic frameworks based on lanthanide cations (Ln = Nd, Sm–Dy), octahedral cluster anions [Re₆S₈(CN)₆]⁴⁻ or [Re₆Se₈(CN)₆]⁴⁻ and adipamide (adp) were synthesized under mild conditions in aqueous ethanol solutions. Compounds of the general composition Cs[Ln(adp)₂{Re₆Q₈(CN)₆}]·3H₂O (Ln = Nd, Sm–Dy; Q = S or Se) crystallize in the hexagonal space group P6₄22. Despite the presence of solvate H₂O molecules in structure cavities, the crystal structures revealed that the coordination environment of lanthanide cations in these 3D polymers did not contain H₂O ligands. This rare feature is attributed to the combination of a bulk cluster anion and flexible adp molecules, which occupy the lanthanide coordination sphere to yield a stable lanthanide-based building block. This assumption was confirmed by synthesis of a second crystalline modification of the compound Cs[Nd(adp)₂{Re₆S₈(CN)₆}]·3H₂O (orthorhombic space group Cccm). Luminescence studies evidence the presence of the typical red luminescence of hexarhenium cluster complexes and the absence of observable luminescence of lanthanide cations for all the compounds except the Nd derivatives. Magnetic data indicate very weak antiferromagnetic exchange interactions between paramagnetic Ln³⁺ centers.