Crystal structures of (S)- and (R,S)-2,2'-bipyridine-3,3'-dicarboxylic acid 1,1'-dioxides and of their barium salts: Absolute configuration and molecular distortion enforced by supramolecular self-assembly

Crystal structures of the title compounds were determined by single crystal X-ray diffraction. Absolute configuration of the barium salt of (+)-(S)-acid was revealed by means of the Bijvoet's method. Compared to theoretical calculations, the conformations in the solid state are severely influenced by hydrogen bonding and/or coordination to barium ion. In all the crystal structures investigated except the racemic acid, there are two distinct bipyridine rotamers differing in the torsion angle around the biaryl axis and corresponding to syn-clinal and anticlinal arrangements. The self-assembly patterns of the free (optically active as well as racemic) acids are closely similar: in crystal, their molecules are organized in layers by short intermolecular hydrogen bonds between the hydroxyl and the N-oxide. In contrast, the supramolecular patterns of the corresponding barium salts differ totally from each other. The anions are coordinated to barium through their oxygen atoms by four different modes, from monodentate to eta(6), mu(6). This, together with the coordination of terminal and bridging water molecules, results in octa-, nona- and decacoordinated barium ions arranged in polymeric chains (in the S-salt) or layers (in the R,S-salt), linked by hydrogen bonds.