Hydrogen bonds and van der Waals forces as tools for the construction of a herringbone pattern in the crystal structure of hexane-1,6-diaminium hexane-1,6-diyl bis(hydrogen phosphonate)

The solid-state structure of the title salt, [H3N(CH2)6NH3][(HO)O2P(CH2)6PO2(OH)], possesses a herringbone motif as a consequence of the interplay of strong hydrogen bonds and non-covalent interactions.


Chemical context
Salts which comprise organophosphonate anions and organic cations, e.g. protonated primary (Mahmoudkhani & Langer, 2002a,b,c), secondary (Wheatley et al., 2001) or tertiary amines (Kan & Ma, 2011) are of growing interest in supramolecular chemistry and crystal engineering. Compounds of this type possess interesting topologies and an extended structural diversity. Furthermore, they seem to be feasible model systems for metal phosphonates as they exhibit similar structural characteristics. Most of these salt-type solids show extended hydrogen-bonded networks which are characterized by a rich diversity of strong charge-supported hydrogen bonds (Aakerö y & Seddon, 1993;Białek et al., 2013) besides some weaker intermolecular interactions (van Megen et al., 2016a,b).
A search in the Cambridge Structure Database (Groom et al., 2016) yielded more than 180 entries for the hexane-1,6diaminium dication (H16AH). At this point it is not our aim to review all these structures, but we think it is worth highlighting some important classes of compounds and applications. The structures and properties of many simple salts of H16AH, like halides (van Blerk & Kruger, 2008), acetates (Paul & Kubicki, 2009) and salts with more complex inorganic anions such as hexafluoridosilcate (Ouasri et al., 2014), tetraiodide (Reiss & van Megen, 2012) or dihydrogen arsenate (Wilkinson & Harrison, 2007) have been extensively studied. Moreover, the H16AH dication is well known for its use in crystal engineering of hydrogen-bonded solids which contain unstable species (Frank & Reiss, 1997), in supramolecular chemistry (Assaf & Nau, 2015), as a tecton for the construction of layered materials (Bujoli-Doeuff et al., 2012), or as a cationic template for novel complex systems (Holtby et al., 2007). Finally, it should be stressed out that the H16AH cation is applied in the context of nylon-based hybride materials (Boncel et al., 2014).
This contribution is part of an ongoing study regarding the structural chemistry of alkane-,!-diphosphonic acids (van Megen et al., 2015) and their organic aminium salts (van Megen et al., 2016a,b).

Structural commentary
The asymmetric unit of [H 3 N(CH 2 ) 6 NH 3 ][(HO)O 2 P(CH 2 ) 6 -PO 2 (OH)] consists of one half of an H16AH dication and one half of a hexane-1,6-diyl bis(hydrogen phosphonate) dianion (16PHOS). Both ions are located around different inversion centres of space group type P2 1 /c (Wyckoff sites 2a and 2d, respectively). Bond lengths and angles in the dication as well as in the dianion are in the expected ranges (Table 1).
As shown in Fig. 1, the cation has a conformation best described as a double hook. In detail, atom C1 is turned out from the plane of the central four carbon atoms by about 6 (Table 1), whereas atom N1 is turned out significantly from the plane defined by the central four carbon atoms [N1-C1-C2-C3 = 69.9 (3) ]. The individual conformation of the cationic diaminium tecton seems to be a compromise between an effort to form the most stable conformation on the one hand, and intermolecular interactions, namely hydrogen bonding and van der Waals interactions, on the other hand (Frank & Reiss, 1996, 1997. The conformation of the anion is that of the energetically most stable all-transoid conformation of the hexane-1,6-diyl moiety (r.m.s. of the six carbon atoms and two phosphorus atoms: 0.2643 Å ), also expressed by the almost perfect antiperiplanar arrangement of each CH 2 group (cf. the torsion angles in Table 1). A detailed view of the hydrogen phosphonate groups shows the P-OH distance of 1.5817 (14) Å to be greater than the two other P-O distances [1.4977 (13) and 1.5112 (13) Å ].

Supramolecular features
Within the crystal of the title compound, the aminium groups of the cations as well as the hydrogen phosphonate groups of

Figure 1
The H16AH cation and the 16PHOS anion are shown together with their hydrogen bonds. Displacement ellipsoids are drawn at the 50% probability level; hydrogen atoms are drawn as spheres with arbitrary radii. [Symmetry codes: ( 0 ) Àx, Ày, Àz; the anions form hydrogen bonds with adjacent ions. In detail, each hydrogen atom of the NH 3 group and the OH group of the hydrogen phosphonate moiety donates a single hydrogen bond to a phosphoryl oxygen atom ( Fig. 1), whereby each phosphoryl oxygen atom accepts two hydrogen bonds. Anions and cations are connected by medium strong to strong, charge-supported N-HÁ Á ÁO and O-HÁ Á ÁO hydrogen bonds (Steiner, 2002; Table 2). The hydrogen-bonding interactions help to construct a two-dimensional network which propagates parallel to the ac plane ( Fig. 2). This network contains two characteristic types of meshes ( Fig. 2), which can be classified as ten-membered and twelve-membered hydrogen-bonded ring motifs with the first level graph-set descriptors R 3 3 (10) and R 4 5 (12), respectively (Etter et al., 1990). It is remarkable that the structure of NH 4 C 10 H 21 PO 2 OH (Boczula et al., 2012) possesses layers with a very similar topology [R 3 3 (10) and R 5 5 (12)]. Along the b axis of the unit cell, these hydrogen-bonded networks are linked by the alkylene chains of the anions as well as the cations, forming a three-dimensional network with a typical herringbone pattern.
We have already shown that ,!-diaminiumalkane tectons support the formation of salts with tailored, linear polyiodides (Reiss & Engel, 2002) showing a herringbone pattern with alternating cations and anions. Thus, the title structure is a further example for both the robustness of the herringbone motif and the structure-directing properties of ,!-functionalized alkylene tectons.
A comparison with the 'parent' structures, namely those of 1,6-diaminohexane (Thalladi et al., 2000) and hexane-1,6-di-phosphonic acid (van Megen et al., 2015) seems useful. A characteristic feature of each herringbone motif is the angle of the fishbones to each other. It is not surprising, then, that this angle in the title crystal structure is almost the average of those found for the parent structures (Fig. 3), which is another proof of the usefulness of ,!-diaminiumalkane tectons in crystal engineering. The two-dimensional hydrogen-bonded network composed of aminium and hydrogen phosphonate groups parallel to the ac plane. The R 3 3 (10) graph-set motif is indicated by green bonds and the R 4 5 (12) motif with blue bonds. [Symmetry codes: ( 0 ) x, Ày + 1 2 , z À 1 2 ; ( 00 ) x À 1, Ày + 1 2 , z À 1 2 .]

Synthesis and crystallization
For the preparation of the title compound, equimolar quantities (0.5 mmol) of hexane-1,6-diamine (58.1 mg) and hexane-1,6-bisphosphonic acid (123.1 mg) were dissolved in methanol, separately. The solutions were mixed and the resulting white precipitate was then dissolved in distilled water. Within several days, colourless crystals were obtained in an open petri dish by slow evaporation of the solvent. Hexane-1,6-diamine was purchased from commercial sources and hexane-1,6-bisphosphonic acid was synthesized according to the literature (Schwarzenbach & Zurc, 1950;Moedritzer & Irani, 1961;Griffith et al., 1998).

IR and Raman spectra
The IR and Raman spectra of the title compound are shown in Fig. 4. The vibration spectra of the title compound are in excellent accord with those of NH 4 C 10 H 21 PO 2 OH (Boczula et al., 2012). This is not particularly surprising as both structures are closely related, including the hydrogen-bonding schemes. Since Boczula et al. presented a detailed discussion of the spectra, we do not include a repeated discussion. An additional, often neglected feature of such IR spectra are the broad bands associated with the O-H stretching vibration indicating strong hydrogen bonds (Hadži, 1965;Baran et al., 1989). A detailed discussion has also been reported very recently (van Megen et al., 2016a) for this feature. In the IR spectrum of the title compound, the maxima of the so called A, B and C bands can be estimated to be at 2750, 2200 and 1600 cm À1 .

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All hydrogen atoms bound to either nitrogen or oxygen atoms were identified in difference syntheses and refined without any geometric constraints or restraints with individual U iso (H) values. Carbon-bound hydrogen atoms were included using a riding model (AFIX 23 option of the SHELX program for the methylene groups and AFIX 43 option for the methine groups).

Hexane-1,6-diaminium hexane-1,6-diyl bis(hydrogen phosphonate)
Crystal data 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 )