Homochiral metal-organic frameworks for enantioseparation

Obtaining homochiral compounds is of high importance to human health and environmental sustainability. Currently, enantioseparation is one of the most effective approaches to obtain homochiral compounds. Thanks to their controlled synthesis and high efficiency, homochiral metal-organic frameworks (HMOFs) are one of the most widely studied porous materials to enable enantioseparation. In this review, we discuss the chiral pocket model in depth as the key to unlock enantioselective separation mechanisms in HMOFs. In particular, we classify our discussion of these chiral pockets (also regarded as "molecular traps") into: (a) achiral/chiral linker based helical channels as a result of packing modality; and (b) chiral pores inherited from chiral ligands. Driven by a number of mechanisms of enantioseparation, conceptual advances have been recently made in the design of HMOFs for achieving high enantioseparation performances. Herein, these are systematically categorised and discussed. Further we elucidate various applications of HMOFs as regards enantioseparation, systematically classifying them into their use for purification and related analytical utility according to the reported examples. Last but not the least, we discuss the challenges and perspectives concerning the rational design of HMOFs and their corresponding enantioseparations.

Ancillary ligand modulated stereoselective self-assembly of triple-stranded Eu(iii) helicate featuring circularly polarized luminescence

Creating optically pure metal assemblies is a hot research topic in the realms of chiral supramolecules. Here, three new triple-stranded europium(iii) helicates Eu₂L₃(L′)₂ [L = 4,4′-bis(4,4,4-trifluoro-1,3-dioxobutyl)diphenyl sulphide; L′ = 1,10-phenanthroline (Phen) or R/S-2,2′-bis(diphenylphosphinyl)-1,1′-binaphthyl (R/S-BINAPO)] were synthesized in order to investigate the effects of ancillary ligands on controlling the stereoselective self-assembly of lanthanide helicates. X-ray single crystal structure analysis showed that Eu₂L₃(Phen)₂ crystalized in an achiral space group P1̄ with the equivalent amount of P and M helicates in one single cell. The isolated Eu₂L₃(S-BINAPO)₂ and Eu₂L₃(R-BINAPO)₂ were verified to be enantiopure by ¹H, ¹⁹F, ³¹P NMR and DOSY NMR analyses. Additionally, the mirror-image CD spectra also demonstrated the successful syntheses of the enantiomers and the presence of an effective chirality transformation from BINAPO to achiral L. Furthermore, the perfect mirror-image circularly polarized luminescence (CPL) spectra of Eu₂L₃(S-BINAPO)₂ and Eu₂L₃(R-BINAPO)₂ indicated the existence of the excited state chirality of the Eu³⁺ center associated with |g_lum| values reaching 0.112. In addition, the photophysical properties of three helicates were also discussed.

Chiral ancillary ligands (R/S-BINAPO) modulated the stereoselective self-assembly of lanthanide helicates, which presented strong CPL with |g_lum| values up to 0.112 and high luminescence quantum yield up to 34%.

An Li+-enriched Co2+-induced metallogel: a study on thixotropic rheological behaviour and conductance

An alkali base and counterion-selective red metallogel (1% w/v) has been synthesized by mixing the adipic acid-derived ligand H₂AL with LiOH, followed by the addition of 1 equivalent of Co(OAc)₂ in DMF. The addition of Co(OAc)₂ not only resulted in the formation of a 2 : 2 (M : L) complex, but also led to the consecutive steps of aggregation, fiber creation, entrapment of the solvent and eventually gelation. The metallogel formation and the mechanism behind gelation have been well characterized and established using various instrumental techniques such as FTIR spectroscopy, UV-vis spectroscopy, FE-SEM, TEM, PXRD, ESI-mass spectrometry, Job's plot and rheology analysis. Nyquist plots suggested a large decrease in the resistance value from 11.3 kΩ to 4.2 kΩ for the solution obtained from the ligand deprotonated by LiOH (AL^2-) and Co(OAc)₂ containing the metallogel. The Nyquist plot and resistance of the metallogel have also been studied under the influence of temperature and ultrasound stimuli. The extensive rheological measurements provide information about the strength of the gel network and the highly reversible nature and thixotropic behaviour of the metallogel.

Semi-conductive helical homochiral metal–organic frameworks based on enantiomeric proline derivatives

Chiral ligands (S)-H₂PCA and (R)-H₂PCA were synthesized by attaching (S)- and (R)-proline to 1,4-dicarboxybenzene. They further reacted with Cu(ii) and Co(ii) ions to obtain two pairs of helical homochiral MOFs with photocatalytic behaviours.

Nanofabrication of Au nanoparticles over conductive metallohydrogel nanofibers for nanocatalysis application

A conductive and fluorescent metallohydrogel (1% w/v, CPH) has been synthesized and utilized for nanofabrication of AuNps and nanocatalysis applications.

One, two, and three-dimensional metal–organic coordination polymers derived from enantiopure organic phosphorate: homochirality, water stability and proton conduction

A multifunctional ligand reacts with metal ions to generate three new coordination polymers, where 3 has a high water stability, a moderate proton conductivity and a lower activation energy.