Hydrogen bonding, π–π stacking and van der Waals forces-dominated layered regions in the crystal structure of 4-aminopyridinium hydrogen (9-phosphonononyl)phosphonate

The structure of the title molecular salt, [C5H7N2][(HO)2OP(CH2)9PO2(OH)], shows a three-dimensional network with hydrogen bonding, π–π stacking, and van der Waals forces-dominated layered regions.


Chemical context
Salts of organophosphonic acids with organic cations, e.g. with protonated primary (Mahmoudkhani & Langer, 2002b), secondary (Wheatley et al., 2001) and tertiary amines (Kan & Ma, 2011) are of growing interest in supramolecular chemistry and crystal engineering. Besides their interesting topologies and structural diversity, they seem to be feasible model compounds for metal phosphonates as they exhibit similar structural characteristics but are less difficult to crystallize. Mostly, these organic solids establish extended hydrogenbonded networks which are characterized by a rich diversity of strong charge-supported hydrogen bonds (Aakerö y & Seddon, 1993) and can either be one-, two-or three-dimensional. This contribution forms part of our research on the principles of the arrangement of alkane-,!-diphosphonic acids (van Megen et al., 2015) and their organic aminium salts (van Megen et al., 2016). Moreover, aminopyridines and the related protonated cations are of crucial interest in the field of biochemistry (Muñ oz-Caro & Niñ o, 2002;Bolliger et al., 2011) and are also used as counter-cations to stabilize complex salts (Reiss & Leske, 2014a,b), in crystal engineering (Sertucha et al., 1998;Surbella III et al., 2016) as well as in polymer chemistry (Deng et al., 2015). ISSN 2056-9890

Structural commentary
The asymmetric unit of the title compound, [C 5 H 7 N 2 + ][(HO) 2 OP(CH 2 ) 9 PO 2 (OH) À ], consists of one 4-aminopyridinium cation and one hydrogen (9-phosphonononyl)phosphonate anion, both in general positions (Fig. 1). Generally, the first protonation of the 4-aminopyridine can take place at the exo-as well as at the endocyclic nitrogen atom. In the literature, all monoprotonated 4-aminopyridines characterized to date are protonated at the endocyclic nitrogen atom. Geometric parameters derived from the singlecrystal diffraction experiment for the title compound show a short exocyclic N-C bond length [1.324 (2) Å ] and slightly longer C-C and C-N bond lengths of the six-membered ring [1.350 (3)-1.425 (2) Å ]. The bonding properties of this cation are best described by a pair of mesomeric structures: the enamine and the imine form (Scheme 2), which have been discussed in detail before (Koleva et al., 2008).
For the designation of the title compound, the systematic name of the amino form is used throughout this article. The bond lengths and angles of the anion are unexceptional and lie within the expected ranges. The alkylene chain of the anion shows nearly antiperiplanar conformations. In detail, the P-OH distances of the phosphonate moieties have values between 1.5535 (13) and 1.5786 (14) Å , longer than the P O distances [1.5045 (13)-1.5149 (12) Å ].

Related structures
For related phosphonate and bis(phosphonate) salts, see:

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. 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 (AFIX23 option of the SHELX program for the methylene groups and AFIX43 option for the methine groups). Two-dimensional hydrogen-bonded networks composed of phosphonyl and hydrogen phosphonate groups. The graph set R 6 6 (24) is indicated by blue bonds.  The IR (blue) and Raman (red) spectra of the title compound.  SHELXL-2014/7 (Sheldrick, 2015b); molecular graphics: DIAMOND (Brandenburg, 2015).

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.