Distorted Fivefold-Diamond Structure of 4,4-Bis(2-carboxyethyl)pimelic Acid (`Methanetetrapropionic Acid')

The molecules of 4,4-bis(2-carboxyethyl)pimelic acid (=4,4-bis(2-carboxyethyl)heptanedioic acid; 1) assemble to build up hollow, five-fold interpenetrating diamond-like networks in tetragonal crystals via standard pairwise H-bonds between the carboxy groups. In keeping with the flat, extended molecular conformation of 1 of approximate D2d symmetry, the supramolecular diamondoid networks are tetragonally compressed. Their mode of interpenetration is unusual and corresponds to that of adamantane-1,3,5,7-tetracarboxylic acid, which has an undistorted five-fold diamond structure. Both the degree and the mode of interpenetration of 1 differ from those of the lower homologue 3,3-bis(carboxymethyl)glutaric acid (2), which adopts a more-common tetragonally elongated triple-diamond architecture in the solid state.