Propylammonium 4,4,4-trifluoro-1-(naphthalen-2-yl)butane-1,3-dionate

The title salt, C3H10N+·C14H8F3O2 −, constitutes the first organic crystal containing a residue of 4,4,4-trifluoro-1-(naphthalen-2-yl)butane-1,3-dione. The terminal –CF3 group is disordered over two locations [occupancy ratio = 0.830 (7):0.170 (7)]. Bond delocalization involving the two carbonyl groups and the α-carbon was observed. The crystal packing is mediated by several supramolecular interactions, namely charged-assisted N—H⋯O hydrogen bonds, C—H⋯F and C—F⋯F short contacts and C—H⋯π interactions.

Our group has been interested in the photoluminescence behaviour of the latter family of compounds. In this context we have prepared a handful of photoluminescent materials comprising HNTA residues and lanthanides (Bruno, Coelho et al., 2008;Gago et al., 2005). As part of our on-going research we have isolated the title compound, which represents to the best of our knowledge the first organic crystal containing HNTA residues.
The asymmetric unit of the title salt comprises one propylammonium cation (PrNH 3 + ) and one naphthoyltrifluoroacetonate anion (NTA -) ( Fig. 1). The +synclinal torsion angle N1-C15-C16-C17 of PrNH 3 + has a value of 65.1 (3)°. The -CF 3 group in NTAexhibits rotational disorder, which was modelled by the superposition of two different parts whose rate of occupancy was refined and converged to 0.830 (7) and 0.170 (7), respectively. Most atoms of this anion are placed in two planes: the naphthyl carbon atoms (C5 to C14) and C4 define plane A [largest deviation of 0.042 (2) Å for C4]; C1 to C4, O1, F2' and F3 define plane B [largest deviation of 0.049 (3) Å for C1]. The angle subtended by planes A and B is 39.0 (2)°, while the bond C4-O2 subtends an angle of 16.2 (3)° with plane B (i.e., the bond is raised from the plane). Atoms O1, C2 to C4 and O2 are engaged in a system of delocalized bonds due to the proton transfer to propylamine. This feature is evident in the C-C and C-O distances which are intermediate between the expected values for single and double bonds (see Table 1 for geometric details; Allen et al., 1987).
The crystal structure is rich in several types of supramolecular contacts, particularly strong charge-assisted hydrogen bonds (N + -H···O -) and weaker short distance contacts, namely C-H···F, F···F and C-H···π interactions (see Table 2 for a full listing and division into families). Hydrogen bonds appear between the two charged species comprising the asymmetric unit, establishing direct connections between the positively charged ammonium cation and the ketonic oxygen atoms (with partial negative charge): N1 donates H1Z in a bifurcated hydrogen bonding interaction simultaneously to O1 and O2; this leads to the formation of two spiral chains with graph set motif C 2 1 (4) (Grell et al., 1999), with the N1-H1Z bond being common to both. We note that the importance of these spirals in the crystal structure can be distinguished due to the asymmetric nature of the aforementioned bifurcated hydrogen bond: the main spiral chain has a considerably shorter N···O distance, being thus formed by the sequence H1X-N1-H1Z···O1 (dashed pink lines in Figs 2 and 3); the other supplementary materials sup-2 supramolecular chain can be envisaged as formed by the sequence H1Y-N1-H1Z···O2 (dashed green lines in Figs 2 and 3). Noteworthy is that all these interactions are confined into a small hydrophilic space of the crystal structure which runs parallel to the b axis. The C-H···F contacts occur between H atoms from the aromatic rings and neighboring F atoms, being also close to the observed F···F contacts (dashed violet lines in Fig. 3). The C-H···π contacts arise from H atoms belonging to the terminal -CH 3 moiety of the PrNH 3 + cation, or from an aromatic ring, with the terminal ring of neighbouring naphthyl moiety. All the above mentioned supramolecular interactions contribute decisively for the crystal packing, in which the cations and anions are disposed into layers parallel to the (1 0 1) plane (Fig. 3).

Experimental
All chemicals were purchased from commercial sources and used as received.
Propylamine (0.34 g, 4.00 mmol) was added and a yellow precipitate formed. The resultant mixture was further stirred for 16 h. After evaporation of the solvent to dryness, the resulting yellow solid was dissolved in CH 2 Cl 2 at 40°C. Excess of dried MgSO 4 was added to the solution, which was then filtered off and evaporated to dryness. Suitable crystals of the title compound were isolated by slow cooling to ambient temperature of a hot concentrated solution in CH 2 Cl 2 .

Refinement
Hydrogen atoms bound to carbon were placed at their idealized positions and were included in the final structural model in riding-motion approximation with C-H = 0.95 Å (aromatic and methine C-H), 0.99 Å (-CH 2 -) and 0.98 Å (-CH 3 ).
Hydrogen atoms associated with the -NH 3 + group were directly located from difference Fourier maps and were included in the final structural model with the N-H and H···H distances restrained to 0.95 (1) and 1.55 (1) Å, respectively, as to ensure a chemically reasonable geometry for this moiety. The isotropic thermal displacement parameters for hydrogen atoms were fixed at 1.2×U eq (C-H and -CH 2 ) or 1.5×U eq (-CH 3 and -NH 3 + ) of the respective parent atoms.
The -CF 3 group was found to be disordered and was modelled over two distinct positions with complementary rates of occupancy calculated from unrestrained refinement. The site occupancies ultimately converged to 0.830 (7) and 0.170 (7). Fig. 1. Molecular units (anion plus cation) composing the asymmetric unit of the title compound. Displacement ellipsoids are drawn at the 50% probability level and the atomic labeling is provided for all non-hydrogen atoms. Hydrogen atoms are represented as small spheres with arbitrary radius.  Table 2. Symmetry operations used to generate equivalent atoms: (i) 1/2 -x, -1/2 + y, 3/2 -z; (ii) -1/2 -x, 1/2 + y, 3/2 -z.