Dimensionality Trends in Metal−Organic Frameworks Containing Perfluorinated or Nonfluorinated Benzenedicarboxylates

Abstract

A new family of hybrid inorganic-organic materials has been synthesized using a combination of flexible bis-pyridyl ligands in conjunction with a perfluorinated or nonfluorinated benzenedicarboxylate ligand. A significant difference between the carboxylate torsion angles of the fluorinated and nonfluorinated ligands leads to the formation of markedly different structures for the two groups of materials. No isostructural phases were found, and the use of perfluorinated ligands tends to increase the dimensionality of the resulting frameworks.Metal-organic frameworks containing fluorinated ligands are receiving increasing attention due to reports of interesting H 2 adsorption in these materials. 1-4 However, few have been synthesized to date and little is known of the structural chemistry of perfluorinated ligands in hybrid framework materials as compared to their nonfluorinated analogues. 5-13 Our current work involves developing a better understanding of the manner in which perfluorinated benzenedicarboxylates incorporate into hybrid structures and examining structural trends of the resulting materials. We have previously reported structures containing perfluorinated carboxylates in combination with nonfluorinated coligands such as imidazole, 14 triazole, 3 and both 2,2 0 -and 4,4 0 -bipyridine. 15,16 Here we extend this work to include the similar but longer and more flexible 1,2-bis(4-pyridyl)ethane (bpe) and 1,3-bis(4-pyridyl)propane (bpp) ligands in conjunction with tetrafluoroterephthalate (tftpa) and tetrafluoroisophthalate (tfipa). In contrast to our previous work, there are few reports of materials containing bpe or bpp and the nonfluorinated analogues of these dicarboxylates (tpa and ipa) for comparison. We have therefore undertaken a synthetic study of these materials as well.There are two important differences in the chemistry of perfluorinated benzenedicarboxylates as compared to their nonfluorinated analogues. First is their significantly enhanced acidity, which may contribute to the inability of compounds containing only transition metals and perfluorinated benzenedicarboxylates to crystallize, as hybrid frameworks are typically not obtained under strongly acidic conditions. This could explain the relative ease with which perfluorinated benzenedicarboxylates incorporate into hybrid materials when other basic ligands such as triazole and bipyridines are present in the reaction. The second difference involves the effect that the fluorine atoms have on the torsion angle by which the carboxylate groups are twisted out of the plane of the benzene ring. In structures containing tpa and ipa, the carboxylate group typically remains roughly in plane with the benzene ring to which it is attached (i.e., torsion angle near 0°). However, in structures containing tftpa and tfipa, the carboxylate groups are typically rotated between 45°and 60°with respect to the benzene ring. 12 This can be attributed both to an electrostatic repulsion between the highly electronegative fluorine atoms on the ring and the lone-pair ...

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