Hydrogen-bonded molecular salts of reduced benzothiazole derivatives with carboxylates: a robust (8) supramolecular motif (even when disordered)

The syntheses and structures of five molecular salts of protonated 4,4,7,7-tetramethyl-3a,5,6,7a-tetrahydrobenzothiazol-2-ylamine (C11H19N2S+) with different deprotonated carboxylic acids (4-methylbenzoic acid, 4-bromobenzoic acid, 3,5-dinitrobenzoic acid, fumaric acid and succinic acid) are reported·In every case, the cation protonation occurs at the N atom of the thiazole ring and the six-membered ring adopts a half-chair conformation (in some cases, the deviating methylene groups are disordered over two sets of sites). The C—N bond lengths of the nominal –NH+=C—NH2 fragment of the cation are indistinguishable, indicating a significant contribution of the –NH—C=N+H2 resonance form to the structure.


Figure 2
The molecular structure of (II) showing 50% displacement ellipsoids with the hydrogen bonds indicated by double-dashed lines.

Figure 4
The molecular structure of (IV) showing the major disorder component only (50% displacement ellipsoids) with the hydrogen bonds indicated by double-dashed lines.

Figure 5
The molecular structure of (V) showing the major disorder component of the methylene groups only (50% displacement ellipsoids) and both disorder components for the N3-HÁ Á ÁO3 and N3Á Á ÁH-O3 hydrogen bonds; the hydrogen bonds are indicated by double-dashed lines.

Figure 3
The molecular structure of (III) showing 50% displacement ellipsoids with the hydrogen bonds indicated by double-dashed lines.
The 'anomalous' situation of incomplete proton transfer for (V) might be correlated with pK a values for the acids involved: 4-methylbenzoic acid (pK a = 4.25), 4-bromobenzoic acid (3.99), 3,5-dinitrobenzoic acid (2.77), fumaric acid (pK a1 = 3.03, pK a2 = 4.54) and succinic acid (4.21, 5.64). These data apply to the acids dissolved in water (Jover et al., 2008), but we might guess that a similar trend applies in the crystals and the pK a2 value for succinic acid is clearly larger than the others. Incomplete (or partial) proton transfer processes have been observed in other crystals (e.g. Biliškov et al., 2011) and can lead to interesting physical properties (e.g. Noohinejad et al., 2015).

Supramolecular features
The most notable supramolecular feature (which occur within the asymmetric units as defined here) of (I)-(V) is an R 2 2 (8) loop in which the protonated N1 + -H1 moiety of the thiazole ring and the syn H atom of the -N2H 2 amine group both form near-linear N-HÁ Á ÁO hydrogen bonds to the O atoms of the carboxylate group of an adjacent anion [Tables 1-5 for compounds (I)-(V), respectively]. In (V), the proton disorder associated with N3 and O3 leads to the same motif for both disorder components (two N-HÁ Á ÁO bonds or one N-HÁ Á ÁO and one NÁ Á ÁH-O bond). Despite the presumed electronic delocalization of the cation noted above, it may be seen that for (I)-(III), the HÁ Á ÁO distance for the charge-assisted hydrogen bond arising from N1 is notably shorter than the bond arising from N2. The situation for (IV) and (V) is less clear-cut: the HÁ Á ÁO separations for the N1 and N2 (and equivalent N3 and N4) hydrogen bonds tend to be closer in magnitude and indeed the N2 bond in (IV) is marginally shorter than the N1 bond. The intermolecular dihedral angles between the thiazole and benzoate rings are 17.13 (14), 16.42 (19) and 20.15 (8) for (I), (II) and (III), respectively, suggesting that the pairwise hydrogen bonds tend to align the aromatic rings of the cation and the anion in roughly the same plane.

Figure 6
Fragment of an [001] hydrogen-bonded chain in the crystal of (I); the chain in (II) is almost identical to this. C-bound H atoms omitted for clarity. Symmetry code: (i) x, 1 À y, 1 group and packing with slight differences in the unit-cell parameters to accommodate the different para-substituents of the benzoate anion), with the N2-H3N group linking the ion pairs into [001] chains ( Fig. 6), with adjacent molecules related by c-glide symmetry. It may be noted that O2 accepts both hydrogen bonds from the amide H atoms and O1 accepts the charge-assisted bond from the thiazole ring. The situation for (III) is quite different, with isolated centrosymmetric tetramers (two cations and two anions) arising ( Fig. 7) in which pairs of R 2 4 (8) loops linking one cation to two anions are apparent as well as the cation-to-anion R 2 2 (8) loops already mentioned. A weak C-HÁ Á ÁO interaction (Table 3) arising from a methyl group occurs between tetramers.
Crystals (IV) and (V) are isostructural and feature [100] chains in each case (Fig. 8). It may be seen that locally the cation has the same hydrogen-bonding pattern to the anion as in (I) and (II) but because the dianions accept hydrogen bonds at 'both ends', a different overall structure arises, which features the same R 2 4 (8) loop seen in (III), but is not generated by a crystallographic centre of symmetry.

Hirshfeld surface analyses
The Hirshfeld surfaces of the C 11 H 19 N 2 S + cations in (I)-(V) were calculated using CrystalExplorer (Turner et al., 2017) and fingerprint plots (McKinnon et al., 2007) were also generated. An example fingerprint plot for (I) is shown in Fig. 9; plots for (II)-(V) are available in the supporting information. The prominent 'spike' feature terminating at (d i , d e = $0.62, 0.98) corresponds to the N-HÁ Á ÁO hydrogen bonds. Less prominent spikes at (1.30, 2.05) and (2.05, 1.30) correspond to HÁ Á ÁS and SÁ Á ÁH contacts, respectively: if these are indicative of attractive directional interactions, they must be very weak at best, as the shortest HÁ Á ÁS/SÁ Á ÁH contact is 3.31 Å , compared to the van der Waals separation of 3.0 Å for these atoms.
The percentage surface contact data (Table 6) for the C 11 H 19 N 2 S + species in the five structures reveal a number of similarities but also some differences: HÁ Á ÁH contacts dominate the packing in each case, although the percentage for (III) is significantly less that for the others. The HÁ Á ÁO contacts associated with the hydrogen bonds are very consistent for (I), (II), (IV) and (V), but those for (II) are much higher and presumably reflect the presence of the 'extra' O atoms of the nitro substituents of the anion, although no significant directional interactions could be identified for these Fragment of a [100] hydrogen-bonded chain in the crystal of (IV); the chain in (V) is almost identical to this. C-bound H atoms omitted for clarity. Symmetry codes: (i) x + 1, y, z; (ii) x À 1, y, z.

Database survey
So far as we are aware, the only reported crystal structures to contain the 4,4,7,7-tetramethyl-3a,4,5,6,7,7a-hexahydrobenzothiazol-2-ylamine cation are those described recently by Sagar et al. (2017) (refcodes NEFTIE and NEFTOK), where it was crystallized with benzoate and picrate anions, respectively. The benzoate structure contains essentially the same hydrogen-bonded chains of cations and anions generated by c-glide symmetry as in (I) and (II) although it is not isostructural (space group Cc rather than Pc). The centrosymmetric, hydrogen-bonded tetramers in the picrate structure bear a resemblance to those in (III) but in the picrate anion, the acceptor oxygen atoms are the deprotonated phenol -OH group and adjacent nitro-group O atoms rather than carboxylate O atoms.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 7. The methylene groups in (I), (IV) and (V) were modelled as being disordered over two sets of sites and the C atoms were refined with isotropic displacement parameters. The N-bound H atoms were located in difference-Fourier maps and their positions were freely refined without difficulty in every case except for the proton associated with atoms N3 and O3 in compound (V). Careful scrutiny of difference maps indicated two electron density maxima in the vicinity of these two atoms, one in a reasonable location for an N3-HÁ Á ÁO3 hydrogen bond (i.e. proton transferred) and the other corresponding to an N3Á Á ÁH-O3 hydrogen bond (i.e. proton not transferred): for a detailed discussion of proton location in potentially disordered hydrogen bonds, see Fá bry (2018). Despite their feeble scattering power, when included in the atomic model these refined well as disordered H atoms: their occupancy sum was constrained to unity and revealed equal occupancies [0.50 (5):0.50 (5)] for the two sites. H atoms for all structures were placed geometrically (C-H = 0.95-0.99 Å ) and refined as riding atoms. The methyl groups were allowed to rotate, but not to tip, to best fit the electron density. In every case, the constraint U iso (H) = 1.2U eq (carrier) or 1.5U eq (methyl carrier) was applied. The absolute structures of (I), (II), (IV) and (V) were established by refinement of the Flack absolute structure parameter (Parsons et al., 2013). It may be noted that despite being isostrutural, the crystal of (IV) chosen for data collection was found to be an inversion twin, whereas the chosen crystal of (V) has a well-defined absolute structure (despite disorder). Acta Cryst. (2019). E75, 167-174 research communications Table 6 Hirshfeld contact interactions arising from the C 11 H 19 N 2 S + cation (%) in (I)-(V).    (7) 0.00 (3) Computer programs: CrystalClear-SM (Rigaku, 2017), CrysAlis PRO (Rigaku, 2017), SHELXS97 (Sheldrick, 2008), SHELXL2014 (Sheldrick, 2015), ORTEP-3 for Windows (Farrugia, 2012) and publCIF (Westrip, 2010 For all structures, data collection: CrystalClear-SM (Rigaku, 2017); cell refinement: CrysAlis PRO (Rigaku, 2017); data reduction: CrysAlis PRO (Rigaku, 2017); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012); software used to prepare material for publication: publCIF (Westrip, 2010).  Hydrogen site location: mixed H atoms treated by a mixture of independent and constrained refinement w = 1/[σ 2 (F o 2 ) + (0.0463P) 2 + 0.0807P]

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq C1 0.6357 (4) 0.6587 (3)  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.  Bis(2-amino-4,4,7,7-tetramethyl-4,5,6,7-tetrahydro-1,3-benzothiazol-3-ium)  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. Bis(2-amino-4,4,7,7-tetramethyl-4,5,6,7-tetrahydro-1,3-benzothiazol-3-ium) succinate-2-amino-4,4,7,7tetramethyl-4,5,6,7-tetrahydro-1,3-benzothiazol-3-ium hydrogen succinate 4,4,7,7-tetramethyl-3a,5,6,7a- 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.