The identification and separation of molecular isomers through modification of titanium-organic cages

Numerous studies have focused on discrete metal–organic cages (MOCs) with distinctive cavities. Because their accessible hydrophobic inner spaces are able to absorb and orient guest molecules through supermolecular interactions, they can be used for a wide range of things, including molecular encapsulation.

The host–guest interactions between the host MOC and adsorbed guest species are typically studied using nuclear magnetic resonance (NMR) spectroscopy and electrospray ionization mass spectrometry (ESI-MS) analyses. Notwithstanding, concentrating on how the host-visitor edifices act at the sub-atomic level remains part of the test since the top-notch single precious stones of the host MOC are expected to acknowledge visitor trade and construction investigation utilizing ordinary X-beam diffraction techniques.

The research team led by Prof. Zhang Jian from the Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences reported the post-assembly modification of homochiral titanium–organic cages for the recognition and separation of molecular isomers in a study that was published in the Angewandte Chemie International Edition.

The pre-synthesized chiral MOC was extended and fixed by the researchers into a porous framework, which made it possible to study host-guest behavior at the molecular level and improved its solid-state crystallinity.

A post-assembly reaction was used by the researchers to produce two size-tunable and homochiral Ti4L6 cage-based frameworks, polynuclear titanium cage (PTC)-236( and), using the chiral MOC as a precursor. They built the three-layered microporous system of PTC-236 by utilizing -or -[Ti4L6] confines with triple-abandoned left- or right-handed helical Zn-bimb chains (bimb = 4,4-di(1H-imidazol-1-yl)-1,1-biphenyl).

PTC-236 was found to have chiral pore channels in its host framework and numerous recognition sites provided by the cage moieties, making it highly stable in air, water, and other solvents. Through single-crystal-to-single-crystal transformation, the study of PTC-236 in host–guest chemistry was made easier thanks to these properties. PTC-236 was used for the X-ray diffraction analyses of more than 20 guests, including aromatic and aliphatic nitriles, aromatic alcohol molecules, and benzene (PhH) and its derivatives.

The material also showed selective recognition and separation of spice Cinnamonitrile cis-trans isomers with up to 95% separation, according to the researchers. The host-guest interactions between the titanium cage and captured guests are clearly visible to them.

This study develops a novel controlled multistage assembly system for isomer separation materials, expands the use of structurally well-defined MOCs in solid phases, and proposes a new strategy for the orderly combination of structurally well-defined MOCs into functional porous frameworks.

More information: Guang‐Hui Chen et al, Post‐Assembly Modification of Homochiral Titanium–Organic Cages for Recognition and Separation of Molecular Isomers, Angewandte Chemie International Edition (2023). DOI: 10.1002/anie.202300726

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