2,2′-Bipyridin-1′-ium 1-oxide bromide monohydrate

Structural disorder is observed due to the fact that protonation, as well as oxidation, of the N atoms of 2,2′-bipyridine occurs either at either of the N atoms. The disorder extends to the remainder of the cation, with a refined occupancy rate of 0.717 (4) for the major moiety.


Chemical context
Bipyridine ligands are an important class of ligands with respect to the synthesis of transition metal complexes. They are especially well-known for their use in the development of complexes with specific photophysical (Thompson et al., 2013;Sun et al., 2015, Dongare et al., 2017 and/or photocatalytic (Wenger, 2013;Fukuzumi et al., 2016;Knoll et al., 2015;Duan et al., 2015;Pal & Hanan, 2014) properties or for the construction of dye-sensitized solar cells (Happ et al., 2012;Bomben et al., 2012;Robson et al., 2012;Adeloye & Ajibade, 2014;Lu et al., 2016;Omae, 2016). During our attempts to introduce substituents to 2,2 0 -bipyrdines that would allow us to use them as monomers in copolymerization reactions (Heintz et al., 2017), we treated 2,2 0 -bipyridine with a mixture of hydrobromic acid and hydrogen peroxide with the aim of getting direct access to 4-bromo-2,2 0 -bipyridine-1-oxide. After recrystallization, the title compound turned out to be the only isolable product.

Structural commentary
The molecular structure of the cation of the title compound is depicted in Fig. 1, showing the disorder of the cations in which the oxygen atom and the proton are bonded to either N1 or N2. The two cation moieties are disordered over the same ISSN 2056-9890 position in an approximate 3:1 ratio, with a refined occupancy for the major moiety of 0.717 (4). The disorder has been refined in terms of a whole molecule disorder, thus leading virtually identical bond lengths which, in addition, are of expected values. See the Refinement section for details of the refinement of the disorder. The two pyridine subunits of the 2,2 0 -bipyridine exhibit an s-cis conformation, which is stabilized by an intramolecular N-HÁ Á ÁO hydrogen bond ( Table 1). The s-cis conformation also allows the cations to arrange themselves into dimeric aggregates via additional N-HÁ Á ÁO hydrogen bonds (cf. Supramolecular features).  (Table 1). In addition, the figure shows that hydrogen atoms in the 3, 3 0 , 4 and 4 0 positions of each bipyridine unit are engaged in C-HÁ Á ÁBr hydrogen bonds (Desiraju & Steiner, 2001) with the interactions of the 3 and 3 0 hydrogen atoms being part of a bifurcated hydrogen bond towards the bromide anion. Hydrogen atoms in the 6 and 6 0 positions are part of bifurcated hydrogen bonds towards the water molecule. Moreover, the hydrogen atoms of the water molecules are involved in hydrogen bonds of the O-HÁ Á ÁBr type. Bromide anions and water molecules form zigzag chains along the b-axis direction (Fig. 3). In summary, a complex network structure is realized by hydrogen bonds linking the constituents of this zigzag chain into dimers of cations.

Figure 2
Dimer of cations formed by N-HÁ Á ÁO hydrogen bonds (Table 1). Hydrogen-bonded bromide anions and water molecules are also shown. Disorder of the cation is omitted for clarity.

Figure 3
Zigzag chain of water molecules and bromide anions parallel to the b axis.

Figure 1
Molecular structure of the cation of the title compound. Non-hydrogen atoms showing displacement ellipsoids with octand shading represent the major component of the two disordered cations. Englert et al., 1993) and the triiodide (SINBIB; Lin et al., 2007). All of these compounds, as well as the title compound itself, show an s-cis conformation of the bipyridine. Moreover, in all compounds, both rings of the bipyridine show an almost perfect coplanar arrangement with dihedral angles well below 10 [title compound: molecule 1: 1.2 (6) , molecule 2: 2(2) ; ESUMEL 5.9 ; PEPDAP 3.9 ; SINBIB 2.7 ]. This arrangement is most probably caused by the short intramolecular N-HÁ Á ÁO hydrogen bond between the protonated nitrogen atom and the oxygen atom (title compound: molecule 1 1.76, molecule 2; 1.81 Å ; ESUMEL 1.73 Å ; PEPDAP 1.71 Å ; SINBIB 1.73 Å ). The supramolecular arrangement in ESUMEL and PEPDAP is identical, with the cations also forming hydrogenbonded dimers. Nevertheless, in contrast to the title compound, these dimers are formed by weak C-HÁ Á ÁO hydrogen bonds of aromatic C-H functions towards the oxygen atom. All other hydrogen bonds are realized by oxygen atoms of the counter-ions acting as the hydrogen-bond acceptor sites. In SINBIB, the cations form an infinite plane realized by bifurcated hydrogen bonds of the oxygen atoms with aromatic C-H functions. In addition, each cation shows a weak C-HÁ Á ÁI interaction. In ESUMEL and SINBIB, the protonated N-H groups are not involved in the hydrogenbond network, whereas in PEPDAP there is an N-HÁ Á ÁO hydrogen bond to one of the perrhenate counter-ions. In summary, the hydrogen-bond network observed for the title compound is unique compared to the situation for other closely related crystal structures.

Synthesis and crystallization
2,2 0 -Bipyridine (1 g, 6.5 mmol) was dissolved in 15 mL methanol. Then hydrobromic acid (0.58 g, 7.2 mmol) and a 30% solution of hydrogen peroxide (0.74 mL, 6.5 mmol) were added at 283 K. The solution was stirred at room temperature for 20 h. The clear solution turned yellow and a fine precipitate was formed, which dissolved again during the reaction time. After the solvent had evaporated, the colourless residue was dissolved in ethanol. Then water was added until a fine precipitate was formed. Storing the solution in the refrigerator (277 K) overnight led to the formation of crystals suitable for x-ray diffraction (yield: 126 mg, 0.3 mmol, 46%).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Data were corrected for Lorentz and polarization effects. Water H atoms were freely refined All other hydrogen atoms were placed in idealized positions (N-H = 0.88, C-H = 0.95 Å ) and refined using a riding model with U iso (H) = 1.2U eq (C or N). The disorder was refined in terms of a whole molecule disorder. The geometry of major and minor moieties were restrained to be similar (SAME restraint in SHELXL) and anisotropic displacement parameters of equivalent atoms in the two moieties were constrained to be identical. Site-occupation factors of the atoms of the two disordered cations were refined using the FVAR instruction and were calculated to be 0.717 (4) (O1 to H10) and 0.283 (4) (O1B to H10B).

Funding information
KH gratefully acknowledges a PhD grant from the 'Stiftung der deutschen Wirtschaft'. Data collection: COLLECT (Nonius, 1998); cell refinement: DENZO (Otwinowski & Minor, 1997); data reduction: DENZO (Otwinowski & Minor, 1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2018 (Sheldrick, 2015); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012); software used to prepare material for publication: Mercury (Macrae et al., 2006). 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. 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 > 2sigma(F 2 ) is used only for calculating R-factors(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.