2 , (2), exhibit remarkably similar structures although they are not isomorphous. The whole molecule of compound (2) is generated by twofold symmetry, with the ether O atom located on the twofold axis. Both compounds have intramolecularinteractions between terminal phenyl rings with centroid-centroid distances of 3.6214 (16) and 3.8027 (14) Å in (1) and (2), respectively. In the crystal of (1), short C-HÁ Á ÁS hydrogen bonds link the molecules, forming chains along [001], while in (2) there are no analogous C-HÁ Á ÁSe interactions present. Chemical contextThe ligand bis[2-(diphenylphosphanyl)phenyl] ether (POP) and its congeners, including the more rigid Xantphos [(9,9-dimethyl-9H-xanthene-4,5-diyl)bis(diphenylphosphane)], comprise a series of chelating diphosphines with a range of flexibility to accommodate variable bonding geometries at transition metals. Experimental and theoretical studies of metal complexes with diphosphines have shown a strong correlation between diphosphine bite angle and selectivity in catalytic transformations (Dierkes & van Leeuwen, 1999;Gathy et al., 2011). Simple functionalization of these diphosphines to form diphosphine dioxides, disulfides, and diselenides has permitted further tuning of the bonding of these ligands to metals by changing the bite-angle range as well as the electronic properties of these ligands. The -accepting phosphorous donor atoms of the parent diphosphines are profoundly altered with the addition of -donor chalcogen donor atoms (Dairiki et al., 2009). Chalcogen-modified diphosphine ligands have been utilized in strategies to tune the catalytic behavior of systems including the Pd II -catalysed hydroamination of dienes (Jahromi et al., 2012) and Ru II transfer hydrogenation of aldehydes and ketones . Hemilability, implicated in the selectivity and reactivity of some catalytic reactions (Braunstein et al., 2001), can also result from the chalcogen functionalization of phosphines as well .Our interest in the application of chalcogen-substituted diphosphines to alter the electronic features of photoluminescent Cu I sensor materials (Smith et al., 2010) led us to study the solid-state structural features of the dichalcogen diphosphines, including the disulfide and diselenide of the ligand POP. We wanted to investigate the inter-and intramolecular features that dominate the solid-state structural behavior of these ligands. The molecular geometry and Structural commentaryThe molecular structures of (1) and (2) (2)], the intramolecular EÁ Á ÁE distances [E = S 6.636 (1) Å for (1); E = Se 6.8246 (7) Å for (2)], and the EPÁ Á ÁPE angles [158.29 (4) for (1); 158.44 (2) for (2)] all indicate a common geometry near the phosphorous-chalcogen bonds. This similarity extends to the phenyl ring orientations. A structural overlap calculation of the pairwise atomic coordinates of all related atoms of (1) and (2) (except the chalcogens) reveals an r.m.s. deviation of only 0.214 Å over 39 atom pairs (Fig. 3).The largest differences in the intramolecular features of (1) and (2) can...

The title salt, NH4 +·H+·2C14H9N4O2 −, is composed of an ammonium cation with a strong intermolecular negatively charge-assisted hydrogen-bonded acid/conjugate base-pair monoanion. The carboxylic acid H atom is located on an inversion center, while the N atom of the ammonium cation is located on a twofold rotation axis. In the crystal, the N—H bonds of each ammonium cation act as donors with carboxylate O-atom acceptors to form chains along the a-axis direction. The chains are linked by offset π–π interactions [intercentroid distances = 3.588 (2) and 3.686 (2) Å], forming layers parallel to the ab plane.