4,15-Dimethyl-7,12-diazoniatricyclo[10.4.0.02,7]hexadeca-1(12),2,4,6,13,15-hexaene dibromide monohydrate

The crystal structure of the viologen 4,4′-dimethyl-2,2′-dipyridyl-N,N′-tetramethylene dibromide monohydrate is presented, along with details of an improved synthesis and NMR spectroscopic analysis.

The title compound, C 16 H 20 N 2 2+ Á2Br À ÁH 2 O (1) is a member of the class of compounds called viologens. Viologens are quaternary salts of dipyridyls and are especially useful as redox indicators as a result of their large negative oneelectron reduction potentials. Compound 1 consists of a dication composed of a pair of 4-methylpyridine rings mutually joined at the 2-position, with a dihedral angle between the pyridine rings of 62.35 (4) . In addition, the rings are tethered via the pyridine nitrogen atoms by a tetramethylene bridge. Charge balance is provided by a pair of bromide anions, which are hydrogen bonded to a single water molecule [D OÁ Á ÁBr = 3.3670 (15) and 3.3856 (15) Å ]. The crystal structure of 1, details of an improved synthesis, and a full analysis of its NMR spectra are presented.

Chemical context
The title compound (1) is a member of the class of compounds called viologens. Viologens are quaternary salts of dipyridyls, which have proven useful as redox indicators as a result of their large negative one-electron reduction potentials (Anderson & Patel, 1984). The herbicides, paraquat, and diquat are viologens. We found that the literature synthesis of 4,4 0 -dimethyl-2,2 0 -dipyridyl-N,N 0 -tetramethylene dibromide, i.e., 1  could be improved by a change in the solvent. We report details of our improved synthesis of 1 along with the crystal structure and a full analysis of its NMR spectra.  give general directions for the syntheses of a series of bridged dimethyl 2,2 0 -dipyridyl salts. Our attempts to make the title compound by their directions failed; only a salt of the starting dipyridyl was recovered. Homer & Tomlinson (1960) noted that HBr is formed by dehydrohalogenation of the dibromide. We think that the conditions used by Anderson & Patel (1984), i.e., refluxing ISSN 2056-9890 o-dichlorobenzene, b.p. 453 K, produced a good deal of HBr, which protonated the dipyridyl, rendering it unreactive. Carrying out the reaction in refluxing xylene (mixed isomers, b.p. ca 413 K) does not produce HBr, but the reaction is slow; after five h, about 50% of the starting dipyridyl was recovered. The quaternization of tertiary amines is known as the Menschutkin reaction (Menschutkin, 1890). The velocity of this reaction shows a strong dependence on solvent (Abraham & Grellier, 1976), with about a 65,000-fold increase from hexane to DMSO. The addition of nitrobenzene to the solvent gave satisfactory yields of the product in a reasonable time (see Synthesis and crystallization section).

Structural commentary
The molecular structure of 1 is shown in Fig. 1. It consists of a dication composed of a pair of 4-methylpyridine rings mutually joined at their 2-positions, with a dihedral angle between the pyridine rings of 62.35 (4) . In addition, the rings are tethered via the pyridine nitrogen atoms by a tetramethylene bridge. There are no unusual bond lengths or angles. As a result of the two bridges between the pyridine rings, 1 occurs as two optical isomers, and therefore provides an example of atropisomerism (Eliel et al., 1994;Alkorta et al., 2012;Mancinelli et al., 2020). Crystals of 1, however, were centrosymmetric, with space group P2 1 /n, and are thus strictly racemic. Charge balance is provided by a pair of bromide anions, which are hydrogen bonded to a single water molecule of crystallization [D OÁ Á ÁBr = 3.3670 (15) and 3.3856 (15) Å ] (Table 1).

Supramolecular features
Aside from the hydrogen bonds between the water molecule and bromide anions, the only other notable intermolecular contacts are interactions of type C-HÁ Á ÁBr (Fig. 2 A view of 1 showing the atom-labeling scheme. Displacement ellipsoids are drawn at the 50% probability level. Hydrogen bonds between water and Br À are shown as dashed lines.

Figure 2
A packing plot of 1 viewed down the crystallographic a axis. Hydrogen bonds between water and Br À are shown as dashed lines, while weaker C-HÁ Á ÁBr interactions are shown as dotted lines. standard van der Waals radii of C, H, and Br (Bondi, 1964) are 1.2, 1.7, and 1.85 Å , respectively. The percentages of atomÁ Á Áatom contact types between asymmetric units were obtained from Hirshfeld-surface fingerprint plots (Figs. S1 and S2 in the supporting information; Spackman & McKinnon, 2002;McKinnon et al., 2004) using CrystalExplorer 17.5 (Turner et al., 2017), and are presented in Table 2.

Database survey
The most similar structures to 1 in the Cambridge Structural Database (CSD, V5.41, update of November 2019; Groom et al., 2016) are BIYTEL, BIYTUB, BIYTOV, BIYVAJ, and BIYTIP (Sanchez et al., 2019). BIYTEL has a trimethylene bridge, BIYTUB has a dimethylene bridge, BIYTOV has a trimethylene bridge but lacks the 4-Me substituents, BIYVAJ has a trimethylene bridge but 5-Me groups instead of 4-Me, and BIYTIP has a dimethylene bridge but is a methanol solvate. CSD entry TMEPYR (Derry & Hamor, 1970) contains a tetramethylene bridge, but lacks 4-Me subsituents. CSD entries DIQUAT (Derry & Hamor, 1969) and DQUATB (Sullivan & Williams, 1976), have dimethylene bridges but also lack the 4-Me substituents. Atomic coordinates for TMEPYR, DIQUAT and DQUATB are, however, not present in the CSD. CSD entry PICGAM (Talele et al., 2018) has a -CH 2 C 6 H 4 CH 2 -linker and is an acetonitrile solvate. These crystal structures have Br À anions for charge balance and (unless otherwise stated) include water of crystallization. The tetramethylene bridge is present in CSD entries HIJGAI (Hofbauer et al., 1996), YOBWAN , and YUFCOR , but these crystal structures feature complex organometallic anions rather than bromide and are not hydrates. The dihedral angle between the two pyridine rings in each structure is strongly dependent on the length of the bridging tether. These range between 15.78-19.01 for dimethylene, 49.40-53.96 for trimethylene, and 63.87-67.15 for tetramethylene [cf. 62.35 (4) in 1]. In PICGAM, the dihedral angle is 72.64 , presumably as a result of the increased rigidity of the tether.

Table 2
Percentage of atomÁ Á Áatom contacts between asymmetric units in 1.
Contact percentages were derived from Hirshfeld-surface fingerprint plots (Spackman & McKinnon, 2002;McKinnon et al., 2004) using CrystalExplorer 17.5 (Turner et al., 2017). Reciprocal contacts are included in the totals. The sum of all percentages in the table is 100.1% due to accumulation of rounding errors.

Figure 3
Analysis of 2-D NMR spectra: (a) HSQC and HMBC resonance assignments, (b) COSY resonance assignments. Peaks marked by an asterisk correspond to water or multiple quantum artifacts. 1-D traces are shown to the left and top of the figure.
methyl group exchange with deuterons in a base-catalyzed reaction (Zoltewicz & Jacobson, 1978). Our NMR sample, which also showed exchange, was neutral. Exchange was prevented by adjusting the 'pH' to $1 with DCl. This exchange with solvent deuterium led to some deuterium couplings with both protons and carbon and hence multiplicities in the NMR spectra, which were initially puzzling. Calder et al. (1967) discuss the effect of the length of the bridging group on the NMR spectra and the mobility of the structures.
There are eight resonance signals in the 1 H NMR spectrum recorded in D 2 O, including one on the downfield shoulder of the residual water resonance. All but one of the signals are of equal intensity and the one at 2.68 ppm is about three times larger. The 13 C NMR spectrum shows eight signals (C1-C8), two of which (C2 and C4) are barely separated. Quantitative measurement using inverse-gated decoupling with a long recycle delay (60 s) shows that the carbon signals are of equal intensity. The 1-D 13 C DEPT (Distortionless Enhancement by Polarization Transfer) and 2-D multiplicity-edited 1 H-13 C HSQC (Heteronuclear Single Quantum Coherence) establish a ratio of 3:2:1 for CH, CH 2 , and CH 3 , respectively. Further analysis of 2-D 1 H COSY (Correlation Spectroscopy) and 2-D 1 H-13 C HMBC (Heteronuclear Multiple-Bond Correlation spectroscopy) spectra led to the NMR assignments summarized in Table 3. A selective HMBC focusing on the C2/C4 region was recorded for unambiguous assignments of multiple-bond 1 H-13 C correlations related to these two carbons. These details together with the 2-D 1 H-15 N HMBC, which reveals stronger H2/N9 and H4/N9 cross-peaks than H1/ N9, clearly establish a symmetric three-ring molecular structure, as shown in Fig. 3, in full agreement with the crystal structure (Fig. 1).
The stereospecific assignment of the methylene protons was achieved by a systematic recording of 1-D selective NOESY (Nuclear Overhauser Effect Spectroscopy) and COSY spectra. A stronger NOE was observed between the proton at 4.73 ppm and H1, and thus this resonance was assigned to H6A while the geminal one at 4.03 ppm to H6B. The 1-D selective homonuclear decoupling 1 H NMR spectra led to the extraction of J-coupling constants between these methylene protons (Table 3). A large 3 J coupling exists between H6B and H7B (11.3 Hz), followed by a sizable 3 J coupling between H6A and H7A (6.1 Hz). As a result of the complexity of the spectra, the 3 J(H 6A H 7B ) and 3 J(H 6B H 7A ) could not be determined, but were estimated to be less than 2 Hz. Also, the 11.1 Hz coupling between H7A and H7B was tentatively assigned to the geminal coupling rather than the one across the C7-C7 0 bond.
All NMR spectra were recorded on a Bruker Ascend 700 MHz spectrometer equipped with a TXO cryoprobe at 298 K. Spectra were indirectly referenced to the deuterium lock frequency, set to 4.7 ppm.

Synthesis and crystallization
The starting materials were standard commercial samples of 95-98% purity. 4,4 0 -Dimethyl-2,2 0 -dipyridyl (0.92 g, 5 mmol) and 1,4-dibromobutane (0.6 mL, 1.08g, 5 mmol) were added to a mixture of 5 mL each of xylene (mixed isomers, b.p. ca 413 K) and nitrobenzene (b.p. 483 K). The mixture was refluxed for about 5 h, during which time a heavy precipitate formed. After cooling, the crude material was filtered and washed with acetone to yield 1.1 g of a tan-colored powder. Paper electrophoresis of this material at pH 7.5 showed (via UV) a small amount of starting material at R p ca zero and product at R p À2.2 (R p is movement relative to picric acid). Crystallization from methanol-acetone gave 0.5-0.6 g (ca 50%) of reddish crystals, m.p. 528-530 K [lit. 528-533 K;