Metal–organic framework-based separation columns: fundamental study for molecular recognition and potential for separation of linear polymers with close terminal structures

We report stationary phases using metal-organic frameworks (MOFs) composed of metal ions and organic ligands in HPLC. We mainly applied [Zn₂(1,4-ndc)₂ted]_<i>n</i> (ZnJAST4) (ndc: naphthalenedicarboxylate, ted: triethylenediamine) as the stationary phase. To inspect how metal ions and organic ligands affect retention behavior, three different types of MOF, [Cu₂(1,4-ndc)₂ted]_<i>n</i> (CuJAST4) with zinc ions of ZnJAST4 substituted with copper ions, [Zn₂(1,4-bdc)₂ted]_<i>n</i> (ZnJAST1) (bdc: benzenedicarboxylate) with the naphthalene of ZnJAST4 substituted with benzene, and ZnJAST4 were packed into columns, and then HPLC using benzene derivatives was conducted to evaluate the recognition ability of these MOFs toward functional groups and shapes of molecules using hexane as the mobile phase. The result revealed that the retention behavior depended on various factors such as halogen bonding, π stacking, molecular shapes, and molecular sizes. Furthermore, to evaluate retention selectivity toward functional groups, HPLC analyses were conducted with a ZnJAST4-packed column with a mobile phase, <i>N</i>,<i>N</i>-dimethylformamide (DMF), using poly(ethylene glycol) (PEG) derivatives, which have different polar terminal groups, as analytes. The result showed that when the polarity of terminal groups was low, the retention to ZnJAST4 increased.