Ethyltriphenylphosphonium bromide dihydrate

In the crystal structure of the title hydrated bromide salt, C20H20P+·Br−·2H2O, O—H⋯Br and O—H⋯O hydrogen bonds as well as C—H⋯Br contacts connect the different components into a three-dimensional network. In the cation, the aromatic rings make dihedral angles of 55.24 (5), 76.16 (4) and 85.68 (4)°.

In the crystal structure of the title hydrated bromide salt, C 20 H 20 P + ÁBr À Á2H 2 O, O-HÁ Á ÁBr and O-HÁ Á ÁO hydrogen bonds as well as C-HÁ Á ÁBr contacts connect the different components into a three-dimensional network. In the cation, the aromatic rings make dihedral angles of 55.24 (5), 76.16 (4) and 85.68 (4) .

Related literature
For the crystal structures of the monohydrate as well as the dihydrate of tetraphenylphosphonium bromide, see: Vincent et al. (1988); Krug & Mü ller (1990). For graph-set analysis of hydrogen bonds, see: Etter et al. (1990); Bernstein et al. (1995).

Comment
The crystallization of ionic compounds is strongly influenced by the relative spatial size ratio of anion to cation, and the presence of different anions may influence the conformation and as well as metric parameters of the cation in the case of bigger, organic cations. At the beginning of a comprehensive study of the influence of various anions on bond lengths and angles among a series of tetra-organo phosphonium compounds, we determined the molecular and crystal structure of the title compound. The molecular structure of the monohydrate as well as the dihydrate of tetraphenylphosphonium bromide are apparent in the literature (Vincent et al., 1988;Krug & Müller, 1990).
The molecular geometry around the P atom is tetrahedral with the respective C-P-C angles covering a range of 105.57 (5)-112.48 (6) °, where the biggest as well as the smallest angle are enclosed between two phenyl groups. The least-squares planes defined by the carbon atoms of the aromatic moieties intersect at angles of 55.24 (5) °, 76.16 (4) °a nd 85.68 (4) °. The methyl group of the ethyl substituent adopts a staggered conformation with respect to two of the three phenyl groups if the cation is projected along the phosphorus-ethyl bond.
In the molecule, hydrogen bonds are apparent between the water molecules as well as the bromide anion. The latter also serves as acceptor for C-H···Br contacts that stem from one of the H atoms on a phenyl group's meta-position as well as one of the H atoms of the ethyl substituent's CH 2 group. While the first of these C-H···Br contacts falls only by about 0.1 Å below the sum of van der Waals radii, the latter one shows a shortening by more than 0.3 Å. In terms of graph-set analysis (Etter et al., 1990;Bernstein et al., 1995), the descriptor for the hydrogen bonds is DDDD on the unitary level, while the C-H···Br contacts necessitate a DD descriptor on the same level. In total, the components of the crystal structure are connected to form a three-dimensional network with the water molecules and bromide anions forming strands along the crystallographic a axis (Fig. 2). The closest intercentroid distance between two π-systems was measured at 4.5109 (7) Å.
The packing of the title compound is shown in Figure 3.

Experimental
The compound was obtained commercially (KEK). Crystals suitable for the X-ray diffraction study were obtained upon recrystallization from boiling water with subsequent evaporation of the solvent at ambient temperature.

Refinement
Carbon-bound H atoms were placed in calculated positions (C-H 0.95 Å for aromatic C atoms and C-H 0.99 Å for methylene groups) and were included in the refinement in the riding model approximation, with U(H) set to 1.2U eq (C).
The hydrogen atoms of the methyl group were allowed to rotate with a fixed angle around the C-C bond to best fit the experimental electron density [HFIX 137 in the SHELX program suite (Sheldrick, 2008)]. The H atoms of the water mo-supplementary materials sup-2 lecules were located on a difference Fourier map and refined using a DFIX instruction (d O-H set to 0.83 Å), with individual thermal parameters. Fig. 1. The molecular structure of the title compound, with atom labels and anisotropic displacement ellipsoids (drawn at 50% probability level).