Homochiral lanthanoid(iii) mesoxalate metal–organic frameworks: synthesis, crystal growth, chirality, magnetic and luminescent properties

Abstract

The achiral chelating and bridging mesoxalato ligand (H2mesox2−), the conjugate base of mesoxalic or dihydroxymalonic acid (H4mesox), is a new enantiopurity enforcer in extended structures by yielding the Λ/Δ-metal configured homochiral MOFs 2D-[Ln2(μ-H2mesox)3(H2O)6], [with Ln(III) = La (1), Ce (2), Pr (3), Nd (4), Sm (5), Eu (6), Gd (7), Tb (8), Dy (9), Er (10) and Yb (11)]; through self-resolution during crystallization. Single crystals of the compounds have been grown in agarose gel. All the compounds obtained are isostructural as deduced by means of single crystal and powder X-ray diffraction analysis and exhibit the Ln(III) ions covalently connected by the mesoxalato ligands into a corrugated grey arsenic-type (6,3)-net (or layer) with chair-shaped six-membered rings. Luminescence measurements reveal that the Eu(III) compound (6) exhibits several strong characteristic emission bands for isolated europium(III) ions in the visible region when excited between 350 and 420 nm; similarly the terbium(III) compound (8) displays the characteristic emission bands for isolated terbium(III) ions. Magnetic susceptibility measurements show deviations from the Curie law mainly owing to the split of the ground term due to the ligand field and spin–orbit coupling in the case of Sm(III) (4) and Eu(III) (6) compounds.