Topological isomers in lanthanide–chloranilate coordination polymers: synthesis, structure and CO2 adsorption properties

Abstract

The effect of lanthanide contraction on the self-assembly of two-dimensional (2D) topological isomers in lanthanide coordination polymers (LnCPs) is presented in this work. Hydrothermal reactions involving LnCl₃ and chloranilic acid (H₂ca) afforded good yields of ten LnCPs with the general formula [Ln(ca)_1.5(H₂O)₃]·xH₂O (1Ln; Ln = Pr, Nd; 2Ln; Ln = Sm, Eu, Gd, Tb, Dy, Er, Tm, Yb; x = 3.5–5). The isostructural compounds crystallize in the centrosymmetric triclinic P space group and display 2D layered networks featuring topologically distinct isomers, specifically honeycomb for 1Ln and brick-wall for 2Ln. The lattice water molecules are located in interlayered spaces with the number of water molecules per formula unit increasing as the ionic radii of Ln³⁺ decrease. The incorporation of the semi-rigid rod 4,4′-bipyridine (4,4′-bipy) into brick-wall 2D networks aims to establish a porous 3D pillared framework, leading to the creation of an intercalation network comprising the guest 4,4′-bipy and lattice water molecules, [Ln(ca)_1.5(H₂O)₂]·2H₂O·4,4′-bipy (3Ln; Ln = Tm, Yb). The physicochemical properties of the isomers 1Ln and 2Ln were clearly illustrated, highlighting their unique structural features, thermal stability, and adsorption characteristics. At both low pressure (1 bar) and elevated pressure (up to 20 bar), the thermally activated brick-wall network 2Yb demonstrated better absorption of CO₂ relative to the honeycomb network 1Pr. This study illustrates that minor variations in the size of lanthanide metal ions significantly influence the architecture and application of innovative materials through structural topological isomers.