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 group P2(1)/c with cell dimensions a = 6.503(1), b = 19.173(1), c = 10.527(1) Angstrom, beta = 101.60(1). The details of this structure have reaffirmed the fact th at the cubane skeleton is a flexible entity which reflects its steric and electronic environment. Of the four carboxylic acid groups three a dopt a syn conformation while the fourth adopts an anti 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 perpendi cular to the carboxylic acid groups are the longest and those which ar e 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 ecl ipsed with a C-C bond. The tetrahedral orientation of the substitutent s does not express itself in a three dimensional supramolecular assemb ly; however, all four carboxylic acid groups are involved in very stro ng donor hydrogen bonds which result in a two dimensional array parall el to (100). An additional surprising result is the bet that none of t he four substituents participate in traditional hydrogen bonded carbox ylic acid dimeric moieties.