The role of hydrogen bonding in pseudo-macrocyclic ring formation in 2,6-dimethyl-1,3,5,7-cyclooctatetraene-1,3,5,7-tetracarboxylic acid monohydrate

2,6-Dimethyl-1,3,5,7-cyclooctatetraene-1,3,5,7-tetracarboxylic acid 3, C14H 14O9, was prepared by thermolysis of the corresponding semibullvalene and c haracterized by spectroscopic and single-crystal X-ray diffraction studies. The monohydrate of 3 crystallizes in the tetrahedral space group P-4n2, a = 11.0924(3) Angstrom, c = 12.6799(5) Angstrom, V = 1560.15(9) Angstrom (3) , Z = 4. The cyclooctatetraene ring adopts a tub-shaped conformation with a crystallograpically imposed twofold rotational symmetry, and is composed o f localized C=C double bonds in the 1,3,5, and 7 positions with an average interatomic distance of 1.327 (5) Angstrom and C-C single bonds with an ave rage interatomic distance of 1.489(5) Angstrom. The average C=C-C angle in the ring is 122.6(3)degrees. Each symmetry generated eight-membered ring co ntains four carboxyl groups, two of which are coplanar with the methyl grou ps across the C=C ring atoms. However, across the C-C bonds the carboxyl gr oups and the methyl groups show a torsion angle of 64.3(4)degrees. The pres ence of a water molecule in the crystal lattice generates a three-dimension al network of close hydrogen bondings between water and the carboxyl groups of multiple rings. This creates a network of orthogonal 10-membered rings between the 8-membered rings. Two given cyclooctatetraene rings are intermo lecularly hydrogen bonded not only directly through their carboxyl groups b ut also via a bridging water molecule. This effect is rare in polycarboxyli c acids and their monohydrates which bond only with water or among themselv es.