(R)-1-Phenyl­ethyl­ammonium trifluoro­acetate

María-Guadalupe Hernández Linares; Gabriel Guerrero Luna; Sylvain Bernès

doi:10.1107/S1600536810013565

. 2010 Apr 21;66(Pt 5):o1118. doi: 10.1107/S1600536810013565

(R)-1-Phenylethylammonium trifluoroacetate

María-Guadalupe Hernández Linares ^a, Gabriel Guerrero Luna ^a, Sylvain Bernès ^b,^*

PMCID: PMC2979250 PMID: 21579169

Abstract

In the crystal structure of the title salt, C₈H₁₂N⁺·C₂F₃O₂ ⁻, all of the ammonium H atoms serve as donors for hydrogen bonds to carboxylate O atoms, forming an R ₄ ³(10) ring motif based on two cations and two anions. Since both cations and anions act as inter-ion bridging groups, R(10) rings aggregate in a one-dimensional supramolecular network by sharing the strongest N—H⋯O bond. Edge-sharing motifs lie on the twofold screw axis parallel to [010], and antiparallel packing of these 2₁-column structural units results in the crystal structure. This arrangement is one of the most commonly occurring in conglomerates of chiral 1-phenylethylamine with achiral monocarboxylic acids, confirming that these ionic salts are particularly robust supramolecular heterosynthons useful in crystal engineering.

Related literature

For graph-set analysis, see: Etter (1990 ▶); Bernstein et al. (1995 ▶). For characteristic structural patterns found in crystal salts of 1-phenylethylamine and monocarboxylic acids, see: Kinbara, Hashimoto et al. (1996 ▶); Kinbara, Kai et al. (1996 ▶); Lemmerer et al. (2008 ▶). For related chiral salt structures, see: Johansen et al. (1998 ▶); Boussac et al. (2002 ▶); Lemmerer et al. (2008 ▶).

Experimental

Crystal data

C₈H₁₂N⁺·C₂F₃O₂ ⁻
M _r = 235.21
Orthorhombic,
a = 6.7821 (5) Å
b = 6.9887 (8) Å
c = 24.378 (2) Å
V = 1155.49 (19) Å³
Z = 4
Mo Kα radiation
μ = 0.13 mm⁻¹
T = 298 K
0.60 × 0.44 × 0.40 mm

Data collection

Siemens P4 diffractometer
3079 measured reflections
1808 independent reflections
1288 reflections with I > 2σ(I)
R _int = 0.020
3 standard reflections every 97 reflections intensity decay: 1%

Refinement

R[F ² > 2σ(F ²)] = 0.041
wR(F ²) = 0.112
S = 1.03
1808 reflections
156 parameters
H atoms treated by a mixture of independent and constrained refinement
Δρ_max = 0.16 e Å⁻³
Δρ_min = −0.13 e Å⁻³

Data collection: XSCANS (Siemens, 1996 ▶); cell refinement: XSCANS; data reduction: XSCANS (Siemens, 1996 ▶); program(s) used to solve structure: SHELXTL-Plus (Sheldrick, 2008 ▶); program(s) used to refine structure: SHELXTL-Plus; molecular graphics: Mercury (Macrae et al., 2008 ▶); software used to prepare material for publication: SHELXTL-Plus.

Supplementary Material

Crystal structure: contains datablocks I, global. DOI: 10.1107/S1600536810013565/pb2025sup1.cif

e-66-o1118-sup1.cif^{(15.4KB, cif)}

Structure factors: contains datablocks I. DOI: 10.1107/S1600536810013565/pb2025Isup2.hkl

e-66-o1118-Isup2.hkl^{(89.1KB, hkl)}

Additional supplementary materials: crystallographic information; 3D view; checkCIF report

Table 1. Hydrogen-bond geometry (Å, °).

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
N1—H1A⋯O1	0.90 (3)	1.92 (3)	2.812 (3)	171 (3)
N1—H1B⋯O2ⁱ	0.92 (3)	1.97 (3)	2.818 (3)	154 (3)
N1—H1C⋯O2ⁱⁱ	0.90 (3)	1.92 (3)	2.816 (2)	175 (3)

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Symmetry codes: (i) Inline graphic ; (ii) .

Acknowledgments

This work was supported by CONACyT (grant 83049).

supplementary crystallographic information

Comment

In their works about optical resolution of conglomerates, Kinbara et al. noted that characteristic hydrogen-bond networks were formed in the salt crystals of 1-phenylethylamine and 1-(4-isopropylphenyl)ethylamine with cinnamic acid (Kinbara, Kai et al., 1996). They suggested that "the pattern of hydrogen bonds plays a significant role in the formation of conglomerates" (Kinbara, Hashimoto et al., 1996). In the specific case of salts of chiral 1-phenylethylamine with achiral monocarboxylic acids, a number of structural determinations indeed showed that two predominant supramolecular arrangements are favored by the charge assisted N—H···O hydrogen bonds, which result in crystals belonging to P2₁ or P2₁2₁2₁ space groups (Lemmerer et al., 2008): cations and anions associate through quite strong hydrogen bonds to form C₂¹(4)C₂²(6)[R₄³(10)] motifs (Etter, 1990; Bernstein et al., 1995). This basic unit has hydrogen bonds with translational units, forming an infinite columnar structure, which generates a screw axis in the crystal structure (invariably a 2₁ axis). This supramolecular structure, referred as '2₁–column' in the Kinbara's reports, may be arranged in a parallel packing in the crystal, which then belongs to P2₁ space group, or in an antiparallel fashion, generating P2₁2₁2₁ crystals.

The chiral title salt (Fig. 1) clearly falls in the latter category. Both the cation and anion are placed in general positions in an orthorhombic unit cell. All ammonium H atoms form hydrogen bonds with carboxylate O atoms, giving a ring motif R₄³(10), as shown in Fig. 2. The strongest hydrogen bond, N1—H1C···O2ⁱⁱ is common to two rings motifs. The repetition of the motif in the [010] direction generates homochiral (R)–2₁–columns. This 1D supramolecular network includes larger ring motifs, which appear if shared contacts are omitted. The sequence of sub-rings nest is R₄³(10) →R₆⁵(16) →R₈⁷(22) →R₁₀⁹(28) → ··· →R₂_n²ⁿ^-1(6n-2) [with n > 1]. The shortest contact between neighboring 2₁–columns is N1—H1B···F2ⁱ, which should be regarded as a van der Waals contact rather than as an actual hydrogen bond. As a consequence, an antiparallel arrangement of 2₁–columns is favored (Fig. 2, inset), which is, in turn, reflected in the P2₁2₁2₁ space group. Such crystal structures were obtained for numerous 1-phenylethylamine salts including different anions, e.g. bromofluoroacetate (Boussac et al., 2002), m-iodobenzoate (Lemmerer et al., 2008) or more complex, bulky carboxylate derivatives (Johansen et al., 1998).

The above description is thus in line with expectations from previous reported structures, and confirms that salts based on chiral 1-phenylethylamine and achiral monocarboxylic acids are robust heterosynthons, useful for crystal engineering and crystal structure prediction. The feature should however not been transferred to other salts (or worse, to cocrystals) of 1-phenylethylamine, which stabilize different supramolecular motifs, if any.

Experimental

The title salt crystallized when attempting to synthesize a diimine organic ligand. A mixture of (S)-6-acetyloxy-5-methyl-2,3-hexanedione (1 g, 5.37 mmol) and Na₂SO₄ (4 g) in chloroform (10 ml) was stirred for 5 min. A catalytic amount of trifluoroacetic acid and 2 equiv. of (R)-(+)-α-phenylethylamine (10.6 mmol) were added and the mixture was refluxed (ca. 353 K) under inert atmosphere, until starting materials were not detected by TLC (ca. 2 h). After evaporation under reduced pressure, the crude was recrystallized from CH₂Cl₂ at 298 K, affording, among other products, the title salt.