1,3-Bis(4-bromophenyl)imidazolium chloride dihydrate

In the title hydrated salt, C15H11Br2N2 +·Cl−·2H2O, the complete imidazolium cation is generated by a crystallographic twofold axis, with one C atom lying on the axis. The chloride ion and both water molecules of crystallization also lie on a crystallographic twofold axis of symmetry. The cation is non-planar, the dihedral angle formed between the central imidazolium and benzene rings being 12.9 (3)°; the dihedral angle between the symmetry-related benzene rings is 25.60 (13)°. In the crystal, O—H⋯Cl hydrogen bonds result in supramolecular chains along c mediated by eight-membered {⋯HOH⋯Cl}2 synthons. These are consolidated by C—H⋯O and π–π [centroid–centroid distance = 3.687 (3) Å] interactions.

In the title hydrated salt, C 15 H 11 Br 2 N 2 + ÁCl À Á2H 2 O, the complete imidazolium cation is generated by a crystallographic twofold axis, with one C atom lying on the axis. The chloride ion and both water molecules of crystallization also lie on a crystallographic twofold axis of symmetry. The cation is non-planar, the dihedral angle formed between the central imidazolium and benzene rings being 12.9 (3) ; the dihedral angle between the symmetry-related benzene rings is 25.60 (13) . In the crystal, O-HÁ Á ÁCl hydrogen bonds result in supramolecular chains along c mediated by eightmembered {Á Á ÁHOHÁ Á ÁCl} 2 synthons. These are consolidated by C-HÁ Á ÁO and -[centroid-centroid distance = 3.687 (3) Å ] interactions.

Comment
The deprotonation of N,N-disubstituted imidazolium salts has been extensively used to generate imidazolylidene carbenes for use as ligands for metals or their salts in homogeneous catalysis (Nolan, 2006;Glorius, 2007). The structural motif can be readily varied so as to modify the electronic properties of the carbene and their complexes (Alcarazo et al., 2010;Leuthaeusser et al., 2007;Diez-Gonzalez & Nolan, 2007). Whereas structural studies are available for a number of derivatives (Luger & Ruban, 1975;Cole & Junk, 2004;Wan et al., 2008), little is known about simple 1,3-diphenyl derivatives that do not posses substituents in the 2,6-positions of the phenyl rings. As part of a study into structural effects of these carbenes, we have been able to prepare and crystallize for the first time the salt 1,3-di-(4-bromophenyl)imidazolium chloride, isolated as a dihydrate, (I).
The crystallographic asymmetric unit of (I) comprises half a 1,3-di-(4-bromophenyl)imidazolium cation, Fig. 1, half a chloride, and two half water molecules, as each of the aforementioned species lies on a two-fold axis of symmetry. The cation is non-planar with the dihedral angle formed between the central imidazolium ring [r.m.s. deviation = 0.005 Å] and the benzene ring (C3-C8) being 12.9 (3) °; the dihedral angle formed between the symmetry related benzene rings is 25.60 (13)°.
The twists between the rings allows for the close approach of a water molecule allowing the formation of a C1-H···O1 interaction, Table 1. This O1-water molecule also forms O-H···Cl interactions with the chloride which in turn is connected to the second water molecule leading to eight-membered {···HOH···Cl} 2 synthons aligned along the c axis, Fig. 2 and Table   1. The three-dimensional packing is consolidated by further C-H···O2 interactions, Fig. 3, as well as π-π contacts (along c) between the imidazolium and between rings [ring centroid···ring centroid distance = 3.687 (3) Å, angle of inclination = 12.9 (3) ° for i: x, y, 1+z].
Experimental p-Bromoaniline (50 mmol) was solubilised in AcOH/H 2 O (3:1 V/V, 40 ml). Aqueous formaldehyde (37%, 2 ml) was added to the solution resulting in the precipitation of a solid. Following this, aqueous glyoxal (40%, 3 ml) was added and the reaction mixture was subsequently warmed (333 K) for 30 minutes. Finally, aqueous HCl (3M, 10 ml) was added resulting in the formation of a homogeneous solution. Heating was continued for a further 30 min. The crude product was precipitated from the reaction by diluting with water. The solid was isolated by filtration and allowed to air dry. The product was recrystallized from 2-propanol to generate colourless needles of (I  3365, 3092, 3048, 1556, 1488, 1309, 1259, 1075, 1008, 824. supplementary materials sup-2

Refinement
The C-bound H atoms were geometrically placed (C-H = 0.95 Å) and refined as riding with U iso (H) = 1.2U eq (C). The water-bound H atoms were refined with O-H = 0.84±0.01 Å, and with U iso (H) = 1.5U eq (O). Fig. 1. The molecular structure of the cation in (I) showing displacement ellipsoids at the 50% probability level. The C1 atom lies on a two-fold axis. Symmetry operation i: y, x, 2-z.

Special details
Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'s involving l.s. planes.
Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.