The Effects of Molecular Conformation and Chain Length in Complexes of Tetra-n-alkylammonium Halide with Nonplanar Aromatic Molecule

Abstract

The crystal structures of tetrabutyl- and tetrapentylammionium bromide (C4H9)4N+Br-, and (C5H11)4N+Br- complexes with rac-1,1′-bi-2-naphthol (C20H14O2); TBAB/BNP and TPAB/BNP, respectively, have been solved by X-ray diffraction techniques. The molecules of TBAB/BNP and TPAB/BNP are held in their aggregates by strong hydrogen bond (O-H…Br) and weak hydrogen bonds; C-H…O, C-H…Br, and C-H…π. The packing structures are different from each other, as well as those observed in tetrabutylammonium bromide with planar aromatic molecules. The tetrabutylammonium cation exhibits conformational and orientational disorder in TBAB/BNP complex, while in TPAB/BNP the alkyl chains of the tetrapentylammonium cation adopt biradial conformation in which there are two sets of paired chains. The tetrapentylammonium cations in TPAB/BNP completely entrap the BNP molecules in a manner that is attributed to the effect of increase in chain length as well as influence of biradial conformation in the tetrapentylammonium cation. However, in TBAB/BNP the naphthol planes of BNP form a void and sandwiches the tetrabutylammonium cation. The patterns of hydrogen bonding network as well as conformation of the tetra-n-akylammonium cations contribute to the formation of the shape of the voids that entrap either BNP or tetrabutylammonium cation. The molecular conformation of the tetra-n-alkylammonium bromides does affect the packing fashion of the nonplanar-shaped BNP molecule and vice versa.