Isopropyltriphenylphosphonium bromide monohydrate

In the title water-solvated salt, C21H22P+·Br−·H2O, the ionic components are linked by short C—H⋯Br contacts along the a-axis direction. The two half occupied water molecules are connected to each other by strong O—H⋯O hydrogen bonds and they are also linked to the bromide anion by short O—H⋯Br contacts.

In the title water-solvated salt, C 21 H 22 P + ÁBr À ÁH 2 O, the ionic components are linked by short C-HÁ Á ÁBr contacts along the a-axis direction. The two half occupied water molecules are connected to each other by strong O-HÁ Á ÁO hydrogen bonds and they are also linked to the bromide anion by short O-HÁ Á ÁBr contacts.

Related literature
For information on phase-transfer catalysts, see: Asai et al. (1994). For the crystal structure of tetraphenylphosphoniuum bromide, see: Alcock et al. (1985). For standard bond lengths, see: Allen et al. (1987).

Comment
The title compound belongs to a family of phase transfer catalysts, which usually contain large alkyl or aryl ions, such as R 4 N + ,R 4 P + ,R 4 Betc. Because of their ease of dissolution in water, such salts have a conveninient to satisfactory hydrophobic hydration (Asai, et al., 1994).
The asymmetry unit of the title structure consists of one isopropyltriphenylphosphonium cation, one bromide anion and two half occupied water molecules (Fig. 1). In contrast with tetraphenylphosphonium salts, the major character of the cation of the title compound is that one phenyl group has been substituted by an isopropyl group. The fluctuation of the C phenyl -P1 bond lengths in the title compound [from 1.789 (4)-1.806 (4) Å] is similar to that in the crystal structure of tetraphenylphosphonium bromide [1.801 (3)Å; Alcock, et al., 1985]. The bonds between P1 and the C atoms of phenyl rings are Csp 2 -Psp 3 bonds, but the connection between P1 and isopropyl group is typically an Csp 3 -Psp 3 bond (Allen, et al., 1987). The bond length of C19-P1 [1.818 (3) Å] associated with the isopropyl group is longer than that involving the phenyl groups, which vary from 1.789 (4) -1.806 (4) Å.
In the crystal the water molecules are linked to the Br anion by short O-H···Br contacts, and the two half occupied water molecules are connected to one another by strong O-H···O hydrogen bonds (Table 1 and Fig. 2). The large cations and bromide anions are linked by short C19-H19···Br1 contacts (Table 1 and Fig. 2). These weak interactions also that play an important role in the stabilization of the crystal structure.

Experimental
Triphenyl phosphine (10.5 g) and 2-bromopropane (4.2 mL) were placed in a teflon lined tube. The sealed tube was placed in an autoclave and heated to 433 K for 48 h, then cooled at a rate of 10 K/min. Colourless block-like crystals of the title compound were obtained.