The development of chiral metal-organic frameworks for enantioseparation of racemates

MIL-101(Cr), an achiral metal-organic framework, made up of a terephthalic acid ligand and a metal chromium ion was selected as a template. Its structural features are unsaturated Lewis acid sites that can be easily activated and it has an extremely high specific surface area, big pore size, and good thermal/chemical/water stability. This achiral framework was modified to introduce chirality within the structure to develop chiral metal-organic frameworks (CMOFs). Here, natural chiral ligands, amino acids (l-proline, l-thioproline and l-tyrosine), were selected for post synthetic modification (PSM) of MIL-101(Cr). This is a very simple, clean and facile methodology with respect to the reactants and reaction conditions. CMOFs 1-3 abbreviated as MIL-101-l-proline (CMOF-1), MIL-101-l-thioproline (CMOF-2) and MIL-101-l-tyrosine (CMOF-3) were prepared by introducing l-proline, l-thioproline and l-tyrosine as chiral moieties within the framework of (Cr). These CMOFs were characterized by FTIR, PXRD, SEM, and thermo gravimetric analysis. Chirality within these CMOFs 1-3 was established by circular dichroism (CD) and polarimetric methods. These three CMOFs 1-3 showed enantioselectivity towards RS-ibuprofen, RS-mandelic acid and RS-1-phenylethanol to varying extents. Their enantioselectivity towards racemates was studied by chiral HPLC and polarimetry.

Chiral Metal-Organic Frameworks

In the past two decades, metal-organic frameworks (MOFs) or porous coordination polymers (PCPs) assembled from metal ions or clusters and organic linkers via metal-ligand coordination bonds have captivated significant scientific interest on account of their high crystallinity, exceptional porosity, and tunable pore size, high modularity, and diverse functionality. The opportunity to achieve functional porous materials by design with promising properties, unattainable for solid-state materials in general, distinguishes MOFs from other classes of materials, in particular, traditional porous materials such as activated carbon, silica, and zeolites, thereby leading to complementary properties. Scientists have conducted intense research in the production of chiral MOF (CMOF) materials for specific applications including but not limited to chiral recognition, separation, and catalysis since the discovery of the first functional CMOF (i.e., d- or l-POST-1). At present, CMOFs have become interdisciplinary between chirality chemistry, coordination chemistry, and material chemistry, which involve in many subjects including chemistry, physics, optics, medicine, pharmacology, biology, crystal engineering, environmental science, etc. In this review, we will systematically summarize the recent progress of CMOFs regarding design strategies, synthetic approaches, and cutting-edge applications. In particular, we will highlight the successful implementation of CMOFs in asymmetric catalysis, enantioselective separation, enantioselective recognition, and sensing. We envision that this review will provide readers a good understanding of CMOF chemistry and, more importantly, facilitate research endeavors for the rational design of multifunctional CMOFs and their industrial implementation.

Luminescent metal–organic frameworks with a 2-(4-pyridyl)-terephthalic acid ligand for detection of acetone

Employing an unsymmetrical pyridyl-dicarboxylate ligand, three MOFs with diverse structures were obtained. The Cd-MOF shows excellent selective sensing towards acetone molecules.

Molecular tectonics: enantiomerically pure chiral crystals based on trans-1,2-cyclohexanediol

Enantiomerically pure trans-1,2-cyclohexanediol ((R,R) or (S,S)) based organic ligands act self-complementary units. In addition, their coordination behavior towards Cd²⁺ cation in the presence of N donor ancillary ligand was investigated.

A chiral metal–organic framework with polar channels: unique interweaving six-fold helices and high CO2/CH4 separation

A chiral Cu(ii) metal–organic framework, possesses interesting polar channels based on interweaving heterochiral [4 + 2] helices, exhibiting multiple CO₂ binding sites and highly selective capture for CO₂ over CH₄.

A three-dimensional cadmium(II) coordination polymer with unequal homochiral double-stranded concentric helical chains

A homochiral helical three-dimensional coordination polymer, poly[[(μ2-acetato-κ(3)O,O':O)(hydroxido-κO)(μ4-5-nicotinamido-1H-1,2,3,4-tetrazol-1-ido-κ(5)N(1),O:N(2):N(4):N(5))(μ3-5-nicotinamido-1H-1,2,3,4-tetrazol-1-ido-κ(4)N(1),O:N(2):N(4):N(5))dicadmium(II)] 0.75-hydrate], {[Cd2(C7H5N6O)2(CH3COO)(OH)]·0.75H2O}n, was synthesized by the reaction of cadmium acetate, N-(1H-tetrazol-5-yl)isonicotinamide (H-NTIA), ethanol and H2O under hydrothermal conditions. The asymmetric unit contains two crystallographically independent Cd(II) cations, two deprotonated 5-nicotinamido-1H-1,2,3,4-tetrazol-1-ide (NTIA(-)) ligands, one acetate anion, one hydroxide anion and three independent partially occupied water sites. The two Cd(II) cations, with six-coordinated octahedral and seven-coordinated pentagonal bipyramidal geometries are located on general sites. The tetrazole group of one symmetry-independent NTIA(-) ligand links one of the independent Cd(II) cations into 61 helical chains, while the other NTIA(-) ligand links the other independent Cd(II) cations into similar but unequal 61 helical chains. These chains, with a pitch of 24.937 (5) Å, intertwine into a double-stranded helix. Each of the double-stranded 61 helices is further connected to six adjacent helical chains through an acetate μ2-O atom and the tetrazole group of the NTIA(-) ligand into a three-dimensional framework. The helical channel is occupied by the isonicotinamide groups of NTIA(-) ligands and two helices are connected to each other through the pyridine N and carbonyl O atoms of isonicotinamide groups. In addition, N-H···O and O-H···N hydrogen bonds exist in the complex.

Chiral or achiral: four isomeric Cd(ii) coordination polymers based on phenylenediacrylate ligands

The isomeric coordination polymers derived fromp- andm-phenylenediacrylates contain similar homochiral motifs (hydrogen-bonded layers or μ-O_carboxylatebridged helical chains), but only themligand, with assistance of hydrogen bonds, induces spontaneous resolution to generate 3D homochiral networks.

A pair of dinuclear Re(i) enantiomers: synthesis, crystal structures, chiroptical and ferroelectric properties

A pair of dinuclear Re(i) enantiomers have been successfully synthesized, representing the first example of polynuclear Re(i) complexes with ferroelectric properties and possessing a multifunctional feature combining chirality, NLO-activity and ferroelectricity within one molecule.

An unusual chiral 3D inorganic connectivity featuring a {Pb₁₈} wheel: rapid and highly selective and sensitive sensing of Co(II)

A pair of enantiomorphic frameworks featuring rare chiral 3D inorganic connectivity and a {Pb18} wheel, has been obtained via spontaneous resolution. They exhibit rapid and highly selective and sensitive sensing of Co(2+) ions.

Crystal structure of 1H-imidazo(4,5-f)-1,10-phenanthroline-terphenyl- 2,5,2',5'-tetracarboxylic acid monohydrate (2:1:1), (C13H8N4)2·(C22H14O8)·(H2O), C48H32N8O9

C48H32N8O9, monoclinic, C2/c (no. 15), a = 26.901(8) Å, b = 7.068(2) Å, c = 20.247(6) Å, β = 93.796(3)°, V = 3841.2 Å3, Z = 4, Rgt(F) = 0.0689, wRref(F2) = 0.1822, T = 296 K.