Helix-based supramolecular isomerism of metal–organic framework

Abstract

Reactions of Na₂Pra₂biim [Na₂Pra₂biim = 1, 1′-di(propionic acid)-2, 2′-biimidazole disodium salt] and Cd(NO₃)₂·4H₂O under different reaction conditions produce four helix-based (pseudo)supramolecular isomers of distinct structural topology: [Cd(Pra₂biim)(H₂O)]_n·2H₂O (1) with (4²·6²·12²) topology, [Cd(Pra₂biim)]_n·nH₂O (2) with (4¹·6⁵) topology of CrB₄, [Cd(Pra₂biim)]_n·2nH₂O (3) with (4³·12³) topology and [Cd(H_0.5Pra₂biim)(H₂O)₂]_2n·(ClO₄)_n (4) with (6⁵·8) topology. Three kinds of helical chains built of N,N′-biim–Cd, η²-Pra₂–Cd and η¹-Pra₂–Cd have been observed in 1–4, respectively. Compound 2 is the thermodynamically stable and compounds 1, 3, 4 are thermodynamically metastable. Conformation analysis of the ligand is presented to understand the flexibility of the Pra₂biim²⁻ that influences the assembly topological architectures. Chiral recognition between helical chains has been found in compound 3.