Crystal structure of tris(piperidinium) hydrogen sulfate sulfate

A novel mixed hydrogen sulfate–sulfate piperidinium salt comprises three protonated piperidinium cations, one hydrogen sulfate anion and one sulfate anion in the asymmetric unit. Strong hydrogen bonds exist between the cations and the anions giving rise to a three-dimensional structure.


Chemical context
Hydrogen bonding is a powerful and versatile tool commonly used in crystal engineering to design, combine and organize individual organic molecules in solids, thus creating new materials with tunable physical properties. Simple organicinorganic salts seem to be good candidates for this purpose because of the flexibility of their special structural features such as polarity and their promising potential applications in chemistry. Not of less importance would be the use of inorganic oxyanions, which are very attractive as inorganic building blocks due to their shapes and diverse reactivity in aqueous solutions. In recent years, sulfates and hydrogen sulfates of organic bases have found applications as ionic liquids (George et al., 2015). Therefore, the results of a structural study on a new molecular salt obtained from piperidine and sulfuric acid are reported here.

Supramolecular features
The crystal structure of (I) features N-HÁ Á ÁO and O-HÁ Á ÁO hydrogen bonds (Table 1, Fig. 1). The N atoms of the piperidinium cations are involved in hydrogen-bond formation, as donors with oxygen atoms of the sulfate and hydrogen sulfate anions. The sulfate-bound O atoms, which act as acceptors, link the organic molecules through rather strong hydrogen bonds, forming a two-dimensional network of hydrogen bonds giving rise to layers parallel to (100). The hydrogen sulfate ion accepts four hydrogen bonds from three cations, whereas the sulfate ion, as an acceptor, binds to five piperidinium ions, forming seven hydrogen bonds in the overall three-dimensional structure (Fig. 3).

Figure 1
The asymmetric unit of (I), showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level. Hydrogen bonds are denoted by cyan dashed lines.

Figure 3
Projection of the crystal structure of (I) on the (100) plane. Hydrogen bonds are denoted by cyan dashed lines.
3.1 ml (0.012 mol) of 30% aqueous sulfuric acid solution. The reaction mixture was continuously stirred for 15 minutes at 323 K and then allowed to cool down to room temperature. The final pH value was 2. The mixture was kept at room temperature over a period of several months, after which it was cooled in a refrigerator (T ' 278 K), giving colourless crystals of the title compound after a few weeks.