Separation of isomers by enclathration

Abstract

The separation of isomers with similar chemical structures is one of the most difficult procedures in chemistry. This is because their physical and chemical properties are generally so similar that most of the common techniques are not successful. In such situations one employs host-guest chemistry, as a tool of separation technology.

In this thesis three aromatic hydrocarbon host compounds 9,9’-bianthryl (H1), 9,9’-spirobifluorene (H2) and trans-2,3-dibenzoylspiro(cyclopropane-1,9’-fluorene) (H3) were investigated in terms of their selectivity towards heterocyclic aromatic compounds (pyridine, PYR; piperidine, PIP; morpholine, MOR and 1,4-dioxane, DIO), cyclohexanone derivative compounds (cyclohexanone, CYHA; 2-methylcyclohexanone, 2-MCYHA; 3-methylcyclohexanone, 3-MCYHA and 4-methylcyclohexanone, 4-MCYHA) and the xylene isomers (ortho-xylene, ox; meta-xylene, mx and para-xylene, px). The H1, H2 and H3 host compounds were combined with a series of the heterocyclic compounds and six inclusion compounds were formed: H1•MOR, H2•2PYR, H2•PIP, H2•MOR, H2•DIO and H3•PYR. In the second part the derivatives of cyclohexanone formed inclusion compounds with the hosts H1 and H2. The H1•2CYHA, H1•ANT, H1•0.5(2-MCYHA) and H3•CYHA structures were obtained. In the third part the H1 formed clathrates with ox and px; H2 and H3 only formed clathrates with px and ox respectively. The following four structures were obtained: H1•0.5ox, H1•0.5px, H2•0.5px and H3•ox and were analysed by single crystal X-ray diffraction (SCXD), powder X-ray diffraction (PXRD), proton nuclear magnetic resonance spectroscopy (1H-NMR), thermal gravimetry (TG) and differential scanning calorimetry (DSC). It wasconcluded that H2 discriminates between the four heterocyclic compounds as follows: PIP > MOR ≈ DIO ≈ PYR and this can be explained by the 1H-NMR results and the packing features. In analysis of the xylene isomers, it was found that the three hydrocarbon host compounds H1, H2 and H3 efficiently discriminate between the isomers by forming inclusion compounds. H1 enclathrates both ox and px but prefers the former. This can be explained in terms of the packing features and lattice energies. H2 and H3 only enclathrate px and ox respectively. It was concluded that host compounds with small conformational movements are potentially good in selective inclusion.