Network hydrogen bonding: the crystal and molecular structure of cubane-1,3,5,7-tetracarboxylic acid dihydrate

The structure of cubane-1,3,5,7-tetracarboxylic acid dihydrate (1) has been determined. It crystallizes in the space groupP21/c with cell dimensionsa=6.503(1),b=19.173(1),c=10.527(1) A, β=101.60(1). The details of this structure have reaffirmed the fact that the cubane skeleton is a flexible entity which reflects its steric and electronic environment. Of the four carboxylic acid groups three adopt asyn conformation while the fourth adopts ananti conformation. The orientation of these groups with respect to the cubane skeleton is reflected in the C−C bond lengths. Those C−C bonds which are perpendicular to the carboxylic acid groups are the longest and those which are most nearly eclipsed are the shortest in the cubane skeleton. In all cases it is the C=O rather than the C−O bond which is most nearly eclipsed with a C−C bond. The tetrahedral orientation of the substitutents does not express itself in a three dimensional supramolecular assembly; however, all four carboxylic acid groups are involved in very strong donor hydrogen bonds which result in a two dimensional array parallel to (100). An additional surprising results is the fact that none of the four substituents participate in traditional hydrogen bonded carboxylic acid dimeric moieties.