(R)-Doxylaminium (R,R)-tartrate

In the title compound (systematic name: (R)-dimethyl{2-[1-phenyl-1-(pyridin-2-yl)ethoxy]ethyl}azanium (R,R)-3-carboxy-2,3-dihydroxypropanoate), C17H23N2O+·C4H5O6 −, the doxylaminium cation is protonated at the N atom. The tartrate monoanions are linked by short, almost linear O—H⋯O hydrogen bonds into chains extended along [100]. These chains are interlinked by anion–pyridine O—H⋯N hydrogen bonds into a two-dimensional grid structure. WeakC—H⋯O interactions also play a role in the crystal packing. An intramolecular hydroxy–carboxylate O—H⋯O hydrogen bond influences the conformation of the anion: the hydrogen-bonded fragment is almost planar, the maximum deviation from the mean plane being 0.059 (14) Å. In the cation, the aromatic rings are almost perpendicular [dihedral angle = 84.94 (8)°] and the conformation of the O—C—C—N chain is gauche(−), the dihedral angle is −76.6 (2)°. The absolute configuration was assigned on the basis of known chirality of the parent compound.

There are not many crystal structures of related compounds in the Cambridge Crystallographic Database (Allen, 2002), and these are exclusively salts: monoprotonated doxylaminium hydrogen succinate (Parvez et al., 2001), and diprotonated doxylaminium tetrachlorocuprate(II) (Braitenbach & Parvez, 2001), and isostructural tetrachlorozincate(II) and tetrachlorocobaltate(II) (Parvez & Sabir, 1998). In view of the importance of doxylamine, we have determined the crystal and molecular structure of the title salt, (1, Scheme 1), (R)-doxylaminium (R,R)-tartarate. The absolute configuration was assigned on the basis of known chirality of the parent compound.
In 1 doxylamine is monoprotonated at N44, thus giving a quaternary ammonium cation (Fig. 1). This 'additional′ hydrogen atom was found in the difference Fourier map and successfully refined. The aromatic rings in the cation are almost perpendicular, the dihedral angle between the least-squares planes of phenyl (planar within 0.0064 (16) Å) and pyridine (0.0056 (16) Å) rings is 84.94 (8)°. The conformation along C-O-C-C-N chain is tg-, the appropriate torsion angles are -164.77 (18)° and -76.6 (2)°. Similar conformation has been observed in previously reported doxylamine salts, despite the presence of intra-cationic hydrogen bonds in the di-cations (Parvez & Sabir, 1998, Braitenbach & Parvez, 2001. In the anion the carbon chain is in an extended conformation (C-C-C-C torsion angle is -178.33 (16)°), and the overall conformation of the anion might be described by the dihedral angle betwen the two approximately planar 'halves′: C1A, O11A, O12A, C2A and C3A, C4A, O41A, O42A, which is 43.45 (8) °. It might be noted that due to the intramolecular O3A-H···O41A hydrogen bond (cf. Table 1), the O3A oxygen atom is almost coplanar with the CCOO plane (0.033 (4) Å), while O2A is significantly deviated from similar plane, by -0.420 (3) et al., 1990et al., , Bernstein et al., 1995. Repetition of these rings produces the chessboard-like pattern of anions and cations in the crystal structure (Fig.   3). Some secondary C-H···O interactions are also playing a role in the crystal packing.

Experimental
The title compound was obtained as a gift sample from R. L. Fine Chem., Bengaluru, India. The compound was recrystallized from methanol by slow evaporation (m.p: 388 K).

Refinement
Hydrogen atoms attached to O and N atoms were found in difference Fourier maps and isotropically refined, all other H atoms were put in the idealized positions, and refined as riding model. Their isotropic thermal parameters were set at 1.2 (1.5 for methyl groups) times U eq 's of appropriate carrier atoms. The Friedel pairs were not merged, however their coverage is relatively low, of ca 50%.   The hydrogen-bonded ring of cations and anions. Hydrogen bonds are drawn as dashed lines, symmetry codes: (i) x,y,z;

Computing details
(ii) 1 -x,1/2 + y,1 -z; (iii) -1 + x,y,z; (iv) -x,1/2 + y,1 -z.  The crystal packing as seen approximately along c-direction, hydrogen bonds, including weak C-H···O interactions listed in table 1) are drawn as dashed lines. 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 > σ(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.