Topological Isomerism between 2D and 3D Hydrogen-Bonded Networks Using Conformationally Flexible Tetracarboxylic Acids

Topological isomerism between 2D and 3D networks in hydrogen-bonded organic frameworks (HOFs) has rarely been shown. Here, we show that a conformationally flexible tetracarboxylic acid PBQX assembles into either a 3D cds or two 2D sql networks, depending on the guest solvent used in crystallization. PBQX possesses a quinoxaline core with heterotype peripheral arms, 4-carboxyphenyl and 4-carboxybiphenyl groups. Single-crystal X-ray diffraction analysis revealed that bending distortions of the arm units and hydrogen bonding motifs allow the molecules to construct both 2D and 3D networks. Guest exchange of these crystals induced structural transformations without a loss of crystallinity, generating two additional isomers. Only the sql network preserved its topology during the transformation, indicating that the cds network is stabilized by interactions with specific guest molecules. Interaction energy calculations also confirmed that these guest–framework interactions compensate for the weaker framework stacking in the cds network. To probe the structure-topology correlation, we further synthesized an analogue, PBBrPQ, which possesses the same heterotype arms on a dibromopyrazinoquinoxaline core. PBBrPQ also gives two sql forms, but, in contrast to PBQX, does not access the cds network. These results indicate that “heterotype arm modulation” is an effective molecular design strategy to increase framework conformational flexibility and expand the structural diversity of HOFs.