Hexakis(propylammonium) benzene-1,2,4,5-tetracarboxylate 2,5-dicarboxybenzene-1,4-carboxylate tetrahydrate

The title organic salt, 6C3H10N+·C10H2O8 4−·C10H4O8 2−·4H2O, contains seven independent entities in the asymmetric unit which comprises three propylammonium cations, two water molecules, half a 2,5-dicarboxybenzene-1,4-carboxylate dianion (H2btc2−) and half a benzene-1,2,4,5-tetracarboxylate tetraanion (btc4−), the latter two anions being located about centres of inversion. One of the water molecules is disordered over two positions in a 0.55 (2):0.45 (2) ratio. The combination of molecular ions and water molecules results in an extensive and complex three-dimensional network of hydrogen bonds, the network being made up of nine unique N—H⋯O interactions between the ammonium cations and the anions, as well as four unique O—H⋯O interactions between the water molecules and the anions.

The title organic salt, 6C 3 H 10 N + ÁC 10 H 2 O 8 4À ÁC 10 H 4 O 8 2À Á4H 2 O, contains seven independent entities in the asymmetric unit which comprises three propylammonium cations, two water molecules, half a 2,5-dicarboxybenzene-1,4-carboxylate dianion (H 2 btc 2À ) and half a benzene-1,2,4,5-tetracarboxylate tetraanion (btc 4À ), the latter two anions being located about centres of inversion. One of the water molecules is disordered over two positions in a 0.55 (2):0.45 (2) ratio. The combination of molecular ions and water molecules results in an extensive and complex three-dimensional network of hydrogen bonds, the network being made up of nine unique N-HÁ Á ÁO interactions between the ammonium cations and the anions, as well as four unique O-HÁ Á ÁO interactions between the water molecules and the anions.
The crystal structure contains a very extensive hydrogen bonded network based on O-H···O and N-H···O interactions. Several of these involve water molecules. Water molecule O1W accepts a hydrogen from N1C located on propylammonium cation C (N1C-H1C···O1W), and donates H atoms to both aromatic anions (molecules A and B). It is therefore involved in hydrogen bonding to an ammonium cation and two aromatic anions (Fig. 2). Figure 3 shows the hydrogen bonding between the O2WA water molecule and adjacent aromatic anions. In this case the disordered water molecule only forms intermolecular hydrogen bonds with the aromatic anions as both donor and acceptor. Hydrogen bonds involving O2WA as hydrogen donor consist of O2WA-H2WA···O4B and O2WA-H2WB···O3A, and as acceptor consists of O1A-H1A···O2WA (Table 1). The combination of two O2WA water molecules and the two aromatic anions (molecules A and B) forms a hydrogen bonded ring described by the graph set R 4 4 (18) (Etter et al., 1990;Bernstein et al., 1995). This extends as a chain of rings along the a axis. There are no intramolecular hydrogen bonds in this structure due to the syn orientation of the carboxyl hydrogen atoms. Each of the three independent propylammonium cations (molecules C, D, and E) donate three hydrogen atoms to various molecules and hence do not participate in hydrogen bond interactions with each other. Cations D and E hydrogen bond exclusively to the two aromatic anions: cation D hydrogen bonds to one B tetraanion and two A dianions, while cation E hydrogen bonds to one A dianion and two B tetraanions. The environment around propylammonium cation C is different from D and E in that it is involved in hydrogen bonding to a water molecule in addition to two B tetraanions.

Experimental
The title organic salt was synthesized by reacting propylamine (0.27 g) with pyromellitic dianhydride (0.50 g) in the presence of THF (5 ml; not anhydrous) as a solvent, at room temperature -the presence of water resulting in ring opening of the pyromellitic dianhydride and subsequent salt formation. The solid was filtered and recrystallized in methanol, yielding colourless crystals suitable for analysis by X-ray diffraction.

Refinement
All H atoms attached to C atoms were positioned geometrically, and allowed to ride on their parent atoms, with C-H bond lengths of 0.95 (aromatic CH), 0.99 (methylene CH 2 ), or 0.98 Å (methyl CH 3 ), and isotropic displacement parameters set to 1.2 (CH and CH 2 ) or 1.5 times (CH 3 ) the U eq of the parent atom. Amine H atoms were placed from the difference map and refined freely. SADI (SAme DIstance restraint; Sheldrick, 2008) was used in the final refinements to restrain all the N-H bond lengths to reasonable values. Water H atoms were placed from the difference map and refined freely. One of the water molecules is disordered over two positions, O2WA and O2WB, in a 0.55 (2):0.45 (2) ratio.

Figure 3
Hydrogen bond environment around the water molecule O2W. Here the water molecule hydrogen bonds to two aromatic dianions (molecule A) as both H-bond acceptor and donor. It also H-bonds to two aromatic tetra-anion molecules as Hbond donor. The combination of these interactions results in a R 4 4 (18) ring which extends along the a axis upon translation of the unit cell. -1,2,4,5-