Synthesis and absolute structure of (R)-2-(benzylselanyl)-1-phenylethanaminium hydrogen sulfate monohydrate: crystal structure and Hirshfeld surface analyses

A hydrogen sulfate salt, [BnSeCH2CH(Ph)NH3 +](HSO4 −), of a chiral selenated amine (R)-2-(benzylselanyl)-1-phenylethanamine (BnSeCH2CH(Ph)NH2) has been synthesized and characterized by elemental analysis,1H and 13C{1H} NMR, FT–IR analysis, and single-crystal X-ray diffraction studies. This selenated salt crystallizes as a monohydrate. In the crystal, several O—H⋯O and N—H⋯O hydrogen bonds and C–H⋯π and Se⋯O weak interactions result in a complex two-dimensional sheet-like supramolecular architecture.


Structural commentary
The title salt ( Fig. 1) is formed by the transfer of a proton from sulfuric acid to the chiral selenated amine C 15 H 17 SeN. The asymmetric unit of the structure consists of one (C 15 H 18 SeN) + cation, one HSO 4 À anion and a solvent water molecule with no direct hydrogen-bonding interactions between them. In the HSO 4 À ion, three of the S-O bond lengths are almost the same, falling in the range of 1.447 (4)-1.452 (5) Å , while the fourth is slightly elongated at 1.527 (5) Å . This suggests that the three nearly identical S-O bonds have partial doublebond character owing to resonance, while the fourth S-O bond has single-bond character. This validates the formation of the salt via single proton transfer from sulfuric acid to the amine. The title salt crystallizes in the monohydrate form in the non-centrosymmetric monoclinic P2 1 space group. The cation is somewhat W shaped ( Fig. 1) with the dihedral angle between the two aromatic rings being 60.9 (4) . The carbon atom attached to the amine nitrogen atom is a chiral atom with an R configuration and the -C-C-bond of the -CH 2 -CHfragment has a staggered conformation.

Supramolecular features
The crystal structure features, by virtue of its salt form, several strong-to-moderate hydrogen bonds, which are not seen to the same extent in the reported freebase structure of the closely related compound (S)-1-(benzylselanyl)-3-phenylpropan-2amine (Prabhu Kumar et al., 2019). The general rule that all strong hydrogen-bond donors participate in hydrogen bonding with strong hydrogen-bond acceptors is totally satisfied in this salt, with all the strong donors and acceptors in the cation, anion and the solvent being involved in at least one hydrogen bond. In the crystal structure, two HSO 4 À anions and two water molecules are interconnected to form a tetrameric type of assembly comprising of alternating HSO 4 À anions and water molecules via discrete D(2) O1-H1DÁ Á ÁO2, O1-H1EÁ Á ÁO5 and O3-H3AÁ Á ÁO1 hydrogen bonds (Fig. 2, Table 1), with the O1-H1EÁ Á ÁO5 hydrogen bond appearing twice. This tetrameric type of assembly having a R 4 4 (12) graphset notation aggregates along the b-axis direction as an infinite one dimensional tape, with adjacent tetrameric units in the tape glued to each other through the common O1-H1EÁ Á ÁO5 hydrogen bonds (Fig. 2) A partial view along the c axis of the crystal packing of the title salt, showing the propagation of the one-dimensional tape along the b-axis direction. The various intermolecular interactions (Table 1) are shown as dashed lines. Colour key: green, anions; red, water; blue spheres, cations. Table 1 Hydrogen-bond geometry (Å , ).

Figure 1
A view of the molecular structure of the title salt, with atom labelling. Displacement ellipsoids are drawn at the 50% probability level. apart (i.e. half of the unit-cell length a) along the a axis, are interconnected via discrete D(2) N1-H1AÁ Á ÁO4, N1-H1BÁ Á ÁO4 and N1-H1CÁ Á ÁO2 hydrogen bonds (Fig. 3, Table 1) between the three amino hydrogen atoms of the cation sandwiched between the two tapes and the three HSO 4 À anions of the nearest asymmetric units (two HSO 4 À anions belong to one tape and two to the other), resulting in a complex two-dimensional sheet along the ab plane (Fig. 3). The cations serve as pendants to the complex sheet. The N1-H1AÁ Á ÁO4, N1-H1BÁ Á ÁO4 and N1-H1CÁ Á ÁO2 interactions are not structure-directing hydrogen bonds of themselves, but structure-directional characteristics are induced to them via the O1-H1DÁ Á ÁO2 and O1-H1EÁ Á ÁO5 hydrogen bonds. The pendant-type arrangement of cations is stabilized by C15-H15Á Á Á ( electrons of the C1-C6 ring) interactions between adjacent cations running as chains down the [010] axis. Secondary Se1Á Á ÁO4(x À 1, y, z) [3.1474 (4) Å ] interactions are also observed in the crystal structure.

Hirshfeld surface analyses
The Hirshfeld surfaces including d norm and shape-index and fingerprint (FP) analyses of the cation, anion and the solvent are shown in Figs Jerry P. Jasinski tribute    of C-HÁ Á Á interactions between the cations. Thus, the Hirshfeld surface analysis provides adequate and reliable evidence, both qualitatively (in terms of pictorial depiction) and quantitatively, for the various interactions in which the cations participate. Analysis of the Hirshfeld surfaces of the anion and the solvent molecule gives similar results (Fig. 5). In the case of the anion, the highest contribution to the Hirshfeld surface is from OÁ Á ÁH/HÁ Á ÁO contacts, contributing 88.6%, while for the Hirshfeld surface of water, 61.6% is from OÁ Á ÁH/ HÁ Á ÁO contacts and the remaining 38.4% is from HÁ Á ÁH dispersions.

Database survey
The cation of the reported structure is somewhat similar to that observed in a closely related structure, (S)-1-(benzylselanyl)-3-phenylpropan-2-amine (Prabhu Kumar et al., 2019), which is homologous to the cation of the title salt with one additional -CH 2 -group between the chiral carbon atom and its nearest aromatic ring. The configurations of the chiral carbon atom are different in the two structures. The dihedral angle between the aromatic rings in the related molecule is 66.49 (12) , which is very similar to that observed in the title structure. No intramolecular N-HÁ Á ÁSe interaction is observed in the molecular cation of the present structure, unlike in the related molecule where one is observed. In the crystal of the related amine, the molecules are linked by weak N-HÁ Á ÁN interactions, generating chains along the [100] direction.

Materials and methods
Chemical reagents were purchased from Sigma-Aldrich (India) and used without further purification unless stated otherwise. For chemical synthesis, reactions were carried out in distilled water or in laboratory-grade solvents at room temperature. Melting points were determined in capillary tubes closed at one end and were reported uncorrected. IR spectra were recorded on a Jasco FT-IR-4100 spectrometer. Specific optical rotations (SOR) were measured on a Rudolph Autopol-I automatic polarimeter using a cell of 100 mm path length. 1 H and 13 C{ 1 H} NMR spectra were recorded on an AVANCE-II Bruker 400 MHz spectrometer. (R)-1-(Benzylselanyl)-2-phenylethan-2-amine was synthesized according to our reported literature procedure (Revanna et al., 2015).
To an ice-cold methanolic (5 mL) solution of (2R)-1-(benzylselanyl)-2-phenylethan-2-amine (0.291 g, 1 mmol) was added 5 M of H 2 SO 4 (2 mL) under stirring. The resulting precipitate was stirred for a further hour at the same temperature. Then the precipitate was filtered and washed twice with cold methanol (10 mL Â 2). The white solid obtained was recrystallized from hot methanol (10 mL), which afforded colourless crystals of the title salt. The salt is soluble in water, dimethyl formamide (DMF) and dimethyl sulfoxide (DMSO), but insoluble in methanol, chloroform, dichloromethane, ether, tetrahydrofuran (THF) and hydrocarbon solvents such as n-hexane, benzene and toluene.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The C-bound H atoms were positioned with idealized geometry and refined using a riding model: C-H = 0.93 Å and U iso (H) = 1.2U eq (C) for aromatic H atoms, C-H = 0.97 Å and U iso (H) = 1.2U eq (C) for methylene H atoms and C-H = 0.98 Å and U iso (H) = 1.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )