Spontaneous Resolution and Structure Transformation of Ni^II Metal–Organic Frameworks from an Achiral Precursor

Abstract

Metal-center-driven spontaneous resolution of a pair of chiral doubly interpenetrated metal−organic frameworks (MOFs), [Ni 3 (ptd) 6 ]• 15.5H 2 O (1), which was synthesized from achiral precursors [ptd = 3-(pyridin-3-yl)-1,2,4-triazine-5,6-diol], shows vertical interpenetrated 2D → 3D frameworks with 1D channels. Interestingly, proceeding with solvothermal recrystallization in methanol, 1, with a high-symmetry tetragonal I4 space group, gives another tilted interpenetrated MOF of [Ni 3 (ptd) 2), with low-symmetry monoclinic C2 space group. Accompanied with the structure transformation, a red shift in circular dichroism (CD) spectrum and a magnetic response (weak ferromagnetic interactions in 1 to weak antiferromagnetic interactions in 2) can be observed. Strikingly, 1 and 2 feature heterogeneous catalytic activities for the Knoevenagel reaction.

Spontaneous Deracemizations

“…They are constructed by metal ions/clusters and organic linkers . Because of their considerable variety of architectures and topologies, MOFs have drawn considerable attention as possible functional materials. , Special efforts have been directed to the design and synthesis of chiral MOFs due to their attracting potential applications in enantioselective catalysis, − chiral separation, , chiral recognition, , nonlinear optics, − and magnetic materials. , …”

“…Four are Λ (red lines) and the others are Δ (blue lines), and a mixture of few randomly picked up crystals exhibit no CD signals (green line). Reprinted from ref . Copyright 2019 American Chemical Society.…”

“…When adequately substituted by polar and/or lipophilic lateral chains, amphiphilic planar aromatics give rise to bidirectional growth. Among them are the porphyrinoids, , cyanine dyes, tetrathiofulvalene, perylene, ,, and phenyl and polyphenyl based amphiphiles. ,, Molecules showing C 3 symmetry, e.g., triphenylenes, and similar star-shaped structures are also extensively used for the self-assembly of achiral compounds to chiral mesophases …”

A Highly Symmetric Bimetallic-Tetracarboxylate Framework: Two-Step Crystallization and Gas Separation Properties

“…Various factors including temperature, light, solvent exchange, ,, ligands exchange/insertion, sorption of metal ions/metal clusters, ,, and pressure effect have been widely studied. Due to the existence of the cleavage and formation of metal–ligand bonds simultaneously in the solid state structure, the stepwise synthesis process is generally accompanied by the appearance of recrystallization, single-crystal-to-single-crystal (SCSC), − and even crystal-to-amorphous-to-crystal (CAC) transitions. At the same time, the crystal structure before and after the initial crystallization shows not only 0D-to-2D, 1D-to-2D, and 2D-to-3D , but also 3D-to-3D conversions.…”

“…At the same time, the crystal structure before and after the initial crystallization shows not only 0D-to-2D, 1D-to-2D, and 2D-to-3D , but also 3D-to-3D conversions. It is obvious that a stepwise strategy can effectively overcome the hydrolysis of metal ions , and the interaction between ligands and obtain novel structures that usually cannot be obtained by a one-step reaction. − ,,,− In addition, it should be noted that the stepwise synthesis process can give rise to more stable structures, which provides greater possibilities for the application of materials.…”