Endowing Metal-Organic Coordination Materials with Chiroptical Activity by a Chiral Anion Strategy

Recently, chiral metal-organic coordination materials have emerged as promising candidates for a wide range of applications in chiroptoelectronics, chiral catalysis, and information encryption, etc. Notably, the chiroptical effect of coordination chromophores makes them appealing for applications such as photodetectors, OLEDs, 3D displays, and bioimaging. The direct synthesis of chiral coordination materials using chiral organic ligands or complexes with metal-centered chirality is very often tedious and costly. In the case of ionic coordination materials, the combination of chiral anions with cationic, achiral coordination compounds through noncovalent interactions may endow molecular materials with desirable chiroptical properties. The use of such a simple chiral strategy has been proven effective in inducing promising circular dichroism and/or circularly polarized luminescence signals. This concept article mainly delves into the latest advances in exploring the efficacy of such a chiral anion strategy for transforming achiral coordination materials into chromophores with superb photo- or electro-chiroptical properties. In particular, ionic small-molecular metal complexes, metal clusters, coordination supramolecular assemblies, and metal-organic frameworks containing chiral anions are discussed. A perspective on the future opportunities on the preparation of chiroptical materials with the chiral anion strategy is also presented.

Introduction of Multicomponent Dyes into 2D MOFs: A Strategy to Fabricate White Light-Emitting MOF Composite Nanosheets

Metal-organic frameworks (MOFs) have been extensively studied in host-guest chemistry by means of ultrahigh porosities, tunable channels, and component diversities. As the host matrix, MOFs exhibit immense potential in the preparation of single-phase white light-emitting (SPWLE) materials. Nonetheless, it is a great challenge that the size of the introduced guest molecules is limited by MOF pores, which affects the WLE optimization. In this work, two-dimensional (2D) MOFs are first utilized as the host matrices to simultaneously encapsulate red-green-blue fluorescent dyes for SPWLE. Various dyes@2D MOF composites with high-quality WLE performances and ultrathin nanosheet morphologies are directly assembled from 2D MOF precursors and dyes in high yields. Owing to the flexible interlamellar space of 2D MOFs, different types and sizes of guests can be easily introduced, which greatly expands the range of available MOF hosts and guests, making the WLE much more tunable. The strategy of employing 2D MOFs as the host matrices to introduce multicomponent dyes for SPWLE nanosheets resolves the restriction of MOF pores on the guest molecule size and opens a new avenue to rationally design and prepare SPWLE nanosheets that are highly solution-processable.

A review on chiral metal-organic frameworks: synthesis and asymmetric applications

Chiral metal-organic frameworks (CMOFs) have the characteristics of framework structure diversity and functional tunability, and have important applications in the fields of chiral identification, separation of enantiomers and asymmetric catalysis. In recent years, the application of CMOFs has also been extended to other research fields, such as circularly polarized fluorescence and chiral ferroelectrics. Compared with achiral MOFs, the design of CMOFs only considers the modes of introduction of chirality, and also takes into account the crystallization and purification. Therefore, the synthesis and characterization of CMOFs face many difficult challenges. This review discusses three effective strategies for constructing CMOFs, including direct synthesis of chiral ligands, spontaneous resolution of achiral ligands or chiral template-induced synthesis, and post-synthetic chiralization of achiral MOFs. In addition, this review also discusses the recent application progress of CMOFs in chiral molecular recognition, enantiomer separation, asymmetric catalysis, circularly polarized fluorescence, and chiral ferroelectrics.

Homochiral porous coordination polymer of EuIII for metal ion sensing and enantioselective adsorption

A novel multifunctional homochiral porous metal–organic framework was obtained by combining the luminescent component Eu(iii) with an enantiopure triangular polycarboxylic ligand.

A new two-dimensional folding sheet-like coordination polymer assembled from cadmium(II) and (S)-2-(benzylamino)succinic acid: synthesis, structure and properties

A new two-dimensional (2D) coordination polymer, namely, poly[[diaqua[μ₃-(S)-2-(benzylamino)succinato-κ⁴N,O¹:O^1':O⁴]cadmium(II)] monohydrate], {[Cd(C₁₁H₁₁NO₄)(H₂O)₂]·H₂O}_n, has been synthesized by the solvothermal reaction of Cd(CH₃COO)₂·2H₂O with the synthesized ligand (S)-2-(benzylamino)succinic acid (H₂L). The title compound has been structurally characterized by elemental analysis, IR spectroscopy and single-crystal X-ray diffraction analysis. In the crystal structure, each Cd^II cation binds to three carboxylate groups from three symmetry-related L^2- dianions. The tetradentate L^2- ligand links three symmetry-related Cd^II cations into a 2D folding sheet, which can be simplified as a uninodal (3,3)-connected hcb net with the point symbol (6³). In the lattice, all the folding sheets are arranged in an interdigitated fashion and aggregate into zipper-like arrays through interlayer π-π interactions. The large and nonpolar side chain may play an important role in the formation and aggregation of the 2D sheet. The thermal stability and photoluminescence properties of the title compound were investigated, and it exhibits a blue emission with a quantum yield of 8%.

Synthesis of Chiral MOF-74 Frameworks by Post-Synthetic Modification by Using an Amino Acid

The synthesis of chiral metal-organic frameworks (MOFs) is highly relevant for asymmetric heterogenous catalysis, yet very challenging. Chiral MOFs with MOF-74 topology were synthesised by using post-synthetic modification with proline. Vibrational circular dichroism studies demonstrate that proline is the source of chirality. The solvents used in the synthesis play a key role in tuning the loading of proline and its interaction with the MOF-74 framework. In N,N'-dimethylformamide, proline coordinates monodentate to the Zn2+ ions within the MOF-74 framework, whereas it is only weakly bound to the framework when using methanol as solvent. Introducing chirality within the MOF-74 framework also leads to the formation of defects, with both the organic linker and metal ions missing from the framework. The formation of defects combined with the coordination of DMF and proline within the framework leads to a pore blocking effect. This is confirmed by adsorption studies and testing of the chiral MOFs in the asymmetric aldol reaction between acetone and para-nitrobenzaldehyde.

Three novel Co(ii)/Ni(ii)-based coordination polymers as efficient heterogeneous catalysts for dye degradation

Three novel Co(ii)/Ni(ii)-based coordination polymers have been synthesized and characterized. Compound 2 shows a rare 2D + 2D heterogeneous framework. Compound 3 is a chiral 0-D molecular complex driven by the solvent-assisted homochiral helix. The photocatalytic oxidation activities and mechanism have been studied.

Selective chiral symmetry breaking and luminescence sensing of a Zn(ii) metal–organic framework

The non-random handedness excess is observed in a chiral metal–organic framework [(CH₃)₂NH₂]₅[Zn_1.5(Zn₃O)(TATAT)₂]·9DMF·17H₂O (1, 1P ≫ 1M) without any chiral sources.

Spontaneous symmetry breaking of Co(ii) metal–organic frameworks from achiral precursors via asymmetrical crystallization

Enantioenriched 3D pseudo-enantiomorphs integrating porosity, chirality and magnetism together with different colour and morphology were obtained through spontaneous symmetry breaking.

Structural diversity and magnetic properties of three metal–organic frameworks assembled from a T-shaped linker

Three helical metal–organic frameworks were solvothermally synthesized by reacting a T-shaped linker, 2,2′-bipyridyl-5,5′-dicarboxylic acid ligand with transition metals, and the magnetic properties of the compounds were studied.

A two-dimensional flexible porous coordination polymer based on Co(ii) and terpyridyl phosphine oxide

A 2D framework with –P(O)Ph₂ phenyl groups on the layer surface has been obtained, which shows remarkable dynamic sorption behaviours.

Temperature-induced one-dimensional chiral Ag(i) linear chains and left-handed 21 helices: DFT studies, luminescence and SHG response

Two pairs of chiral coordination polymers have been synthesized with the same materials at 25 and 90 °C, respectively.