Supramolecular ribbons. Crystal structure and spectroscopic properties of 2,2 '-bipyrroyl

A crystal structure determination of 2,2'-bipyrroyl (1; 2,2'-dipynyl-diketo ne, bis (2-pyrrolyl)ethanedione) and its spectroscopic properties in soluti on are reported. In the crystal, 1 self-assembles via hydrogen bonding into supramolecular ribbons that extend indefinitely through the crystal lattic e. The observed molecular conformation is one where each pyrrole ring and a djacent carbonyl group are co-planar (torsion angle similar to 0.9 degrees) , with the N-H pointing in the same direction as the C=O. The two carbonyls have a transoid but not co-planar geometry with a torsion angle of similar to 128 degrees. Adjacent molecules in the crystal are linked by pairs of i ntermolecular hydrogen bonds, pyrrole NH to carbonyl oxygen, to form a matr ix of polymeric chains that lie like neatly stacked, parallel streams of ri bbons. Molecular mechanics calculations on the monomer indicate an intramol ecularly hydrogen bended planar conformation (sp, ap, sp) at the global ene rgy minimum. In CHCl3, 1 is monomeric according to Vapor pressure osmometry (MWobs = 179+/-10 vs MWcalc = 188). In THF, the measured molecular weight is 340+/-15, which corresponds best to one molecule of 1 solvated by two TH F molecules (MW = 322 for C10H8N2O4. 2 C4H8O) rather than to a dimer.