Bis[(3-chlorobenzyl)ammonium] 2-phenylpropanedioate dihydrate

In the asymmetric unit of the title compound, 2C7H9ClN+·C9H6O4 2−·2H2O, there are two crystallographically independent cations, one dianion and two water molecules. The dihedral angle between the two carboxylate groups of the dianion is 78.1 (2)°. In the crystal, the components are held together by N—H⋯O, O—H⋯O and C—H⋯O hydrogen bonds, forming a layer parallel to the bc plane, with the hydrophilic and hydrophobic groups located in the inner and outer regions of the layers, respectively.


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The molecular structure of the compound and the crystal packing of the molecules viewed along the crystallographic b axis are shown in Figures 1 and 2, respectively. In its asymmetric unit the title compound has two crystallographically independent cations, one dianion and two water molecules. The two 3-chlorobenzylammonium cations have inverted but otherwise virtually indentical conformations as shown by the rotational angle of the ammonium group against the remainder of the molecule, N1-C16-C10-C11 = -119.04 (12)° and N2-C23-C17-C18 = 113.49 (12)° for the two groups (the two crystallographically independent ions are opposite enantiomers, but as as the structure is centrosymmetric both ions are present as racemic pairs throughout the structure).
The C-O bond length in one of the carboxylates is slightly longer than in the other with the respective values of O1-C1 and O2-C1 being 1.2714 (14) and 1.2501 (14) Å. The O3-C3 and O4-C3 bond lengths in the other carboxylates are 1.2559 (14) and 1.2538 (14) Å, respectively, making the carbonyl distance in the latter indistinguishable from the single bond due to resonance. The slight difference in the two sets of carbonyl distances may be attributed to the number of H bonds their respective O atoms are involved in, as H bonds tend to stabilize negative charge at the O atoms. The three O atoms with only two strong H bonds have C-O distances below 1.6 Å, the one with three strong H bonds has a C-O distance larger than 1.7 Å.
The molecules associate by placement of all the phenyl rings in one direction, while the hydrophilic ammonium and the carboxylate ends are oriented towards the other end and are hydrogen bonded to water molecules, resulting in alternating hydrophobic and hydrophylic regions respectively in the crystal packing. In the hydrophilic regions the water molecules act as donors and acceptors in an extended hydrogen bonding network. Each water molecule serves as a bridge that links a 3-chlorobenzylammonium moiety and a phenylmalonate group. This arrangement affords an interconnectivity of water and donor protons such that two of the ammonium H atoms are donated to the water O atoms in a short N-H···O contact of comparable length [N2-H2B···O5 iii = 1.98Å and N2-H2C···O5 i = 2.01Å;  (17) and O6-H6B···O3 iv = 1.945 (17) Å; Table 1].
In the hydrophobic regions the chloro-substituted aromatic rings show some stacking of parallel-shifted aromatic rings with each other, but the shortest centroid to centroid distance is larger than 4.3 Å and the interplanar separation is with greater than 3.6 Å also rather long for an attractive π-π stacking interaction. In the absence of any other directional interactions in the hydrophobic section of the structure it thus can be assumed that the crystal packing in these layers is likely to be dominated by shape recognition via dispersion forces.
supplementary materials sup-2 Experimental Single crystals of the title compound were grown by slow evaporation of a water-ethanol solution (1:1 v/v; 20 ml) of 3chlorobenzylamine (0.71 g, 5.0 mmol) and phenylmalonic acid (0.45 g, 0.25 mmol) at room temperature.

Refinement
All H atoms were found in a difference Fourier map. Positional parameters of the water H atoms were refined, with U iso (H) = 1.2U eq (O). The NH 3 + groups were refined as rigid groups (N-H = 0.91 Å), with U iso (H) = 1.5U eq (N), allowing for rotation around the C-N bonds. Other C-bound H atoms were refined as riding, with C-H = 0.95-1.00 Å, and with U iso (H) = 1.2U eq (C).