(R)-1-Phenyl­ethyl­ammonium trifluoro­acetate

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 R₄³(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.

Key indicators

• Single-crystal X-ray study
• T = 298 K, P = 0.0 kPa
• Mean [sigma](C-C) = 0.004 Å
• R factor = 0.041
• wR factor = 0.112
• Data-to-parameter ratio = 11.6

checkCIF/PLATON results

No syntax errors found



Alert level C
PLAT242_ALERT_2_C Check Low       Ueq as Compared to Neighbors for         C9
PLAT242_ALERT_2_C Check Low       Ueq as Compared to Neighbors for        C10

Alert level G
REFLT03_ALERT_4_G Please check that the estimate of the number of Friedel pairs is
            correct. If it is not, please give the correct count in the
            _publ_section_exptl_refinement section of the submitted CIF.
           From the CIF: _diffrn_reflns_theta_max           29.00
           From the CIF: _reflns_number_total               1808
           Count of symmetry unique reflns         1809
           Completeness (_total/calc)             99.94%
           TEST3: Check Friedels for noncentro structure
           Estimate of Friedel pairs measured         0
           Fraction of Friedel pairs measured     0.000
           Are heavy atom types Z>Si present         no
PLAT791_ALERT_4_G The Model has Chirality at C1      (Verify) ....          R

   0 ALERT level A = In general: serious problem
   0 ALERT level B = Potentially serious problem
   2 ALERT level C = Check and explain
   2 ALERT level G = General alerts; check

   0 ALERT type 1 CIF construction/syntax error, inconsistent or missing data
   2 ALERT type 2 Indicator that the structure model may be wrong or deficient
   0 ALERT type 3 Indicator that the structure quality may be low
   2 ALERT type 4 Improvement, methodology, query or suggestion
   0 ALERT type 5 Informative message, check

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S1. 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) → ··· → R_2n^2n-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.

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).

S2. 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.

S3. Refinement

As no heavy atoms are present in the crystal and data were measured at room-temperature using Mo Kα radiation, no absorption correction was applied to the raw data. Because of insufficient anomalous scattering effects, the Flack parameter could not be reliably determined, and measured Friedel pairs (796) were merged. Absolute configuration was assigned by reference to the chiral amine used as starting material, assuming that no inversion occurred during crystallization. Ammonium H atoms were refined with free coordinates, in order to get accurate dimensions for hydrogen bonds. Other H atoms were placed in idealized positions and refined as riding to their carrier atoms, with bond lengths fixed to 0.93 (aromatic CH), 0.96 (methyl CH₃) or 0.98 Å (methine CH). Isotropic displacement parameters for H atoms were calculated as U_iso(H) = 1.2U_eq(C) for aromatic CH groups and U_iso(H) = 1.5U_eq(C, N) for other groups. The methyl group was considered as a rigid group free to rotate about its C—C bond.

Computing details

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

Figures

	Fig. 1. The structure of the title compound, with displacement ellipsoids at the 50% probability level for non-H atoms.
	Fig. 2. The hydrogen-bonding network in the title compound (hydrogen bonds are dashed). The inset represent the packing structure viewed down the axis of the 21–column (b axis). H atoms have been omitted for clarity. In both figures, cations are green and anions blue.

(R)-1-Phenylethylammonium trifluoroacetate

Crystal data

C8H12N^+·C2F3O2^−	F(000) = 488
Mr = 235.21	Dx = 1.352 Mg m^−3
Orthorhombic, P212121	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: P 2ac 2ab	Cell parameters from 70 reflections
a = 6.7821 (5) Å	θ = 4.5–12.5°
b = 6.9887 (8) Å	µ = 0.13 mm^−1
c = 24.378 (2) Å	T = 298 K
V = 1155.49 (19) Å^3	Prism, colorless
Z = 4	0.60 × 0.44 × 0.40 mm

Data collection

Siemens P4 diffractometer	Rint = 0.020
Radiation source: fine-focus sealed tube	θmax = 29.0°, θmin = 1.7°
Graphite monochromator	h = −9→4
2θ/ω scans	k = −9→1
3079 measured reflections	l = −33→1
1808 independent reflections	3 standard reflections every 97 reflections
1288 reflections with I > 2σ(I)	intensity decay: 1%

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.041	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.112	w = 1/[σ^2(Fo^2) + (0.0464P)^2 + 0.1579P] where P = (Fo^2 + 2Fc^2)/3
S = 1.03	(Δ/σ)max < 0.001
1808 reflections	Δρmax = 0.16 e Å^−3
156 parameters	Δρmin = −0.13 e Å^−3
0 restraints	Extinction correction: SHELXTL-Plus (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 constraints	Extinction coefficient: 0.045 (4)
Primary atom site location: structure-invariant direct methods

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
N1	0.9254 (3)	0.1712 (3)	0.19995 (7)	0.0480 (4)
H1C	1.002 (5)	0.206 (4)	0.2286 (11)	0.072*
H1B	0.853 (5)	0.062 (5)	0.2039 (11)	0.072*
H1A	0.840 (5)	0.268 (5)	0.1966 (11)	0.072*
C1	1.0532 (3)	0.1429 (4)	0.15044 (8)	0.0513 (5)
H1	1.1377	0.0316	0.1571	0.062*
C2	1.1846 (5)	0.3153 (5)	0.14290 (11)	0.0818 (9)
H2C	1.2617	0.3000	0.1102	0.123*
H2B	1.2709	0.3273	0.1739	0.123*
H2A	1.1049	0.4283	0.1398	0.123*
C3	0.9260 (3)	0.1001 (4)	0.10055 (8)	0.0502 (5)
C4	0.9482 (5)	−0.0721 (4)	0.07311 (11)	0.0706 (7)
H4	1.0390	−0.1616	0.0857	0.085*
C5	0.8359 (6)	−0.1118 (5)	0.02695 (12)	0.0893 (10)
H5	0.8526	−0.2272	0.0086	0.107*
C6	0.7020 (5)	0.0165 (5)	0.00854 (11)	0.0819 (10)
H6	0.6265	−0.0114	−0.0223	0.098*
C7	0.6776 (4)	0.1863 (5)	0.03497 (10)	0.0743 (8)
H7	0.5859	0.2744	0.0221	0.089*
C8	0.7891 (4)	0.2282 (4)	0.08117 (9)	0.0633 (7)
H8	0.7710	0.3442	0.0992	0.076*
C9	0.6937 (4)	0.6770 (3)	0.19540 (9)	0.0482 (5)
O1	0.6911 (4)	0.5019 (3)	0.19191 (10)	0.0921 (7)
O2	0.8190 (3)	0.7849 (3)	0.21505 (7)	0.0620 (5)
C10	0.5086 (4)	0.7725 (4)	0.17224 (11)	0.0613 (6)
F1	0.4723 (3)	0.7214 (4)	0.12145 (7)	0.1175 (8)
F2	0.5178 (3)	0.9610 (2)	0.17221 (11)	0.1065 (8)
F3	0.3482 (2)	0.7254 (3)	0.20053 (8)	0.0914 (6)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
N1	0.0484 (10)	0.0459 (9)	0.0498 (9)	−0.0030 (9)	−0.0118 (9)	0.0002 (9)
C1	0.0420 (10)	0.0574 (13)	0.0545 (11)	0.0045 (11)	−0.0077 (10)	−0.0010 (10)
C2	0.0655 (16)	0.105 (2)	0.0744 (16)	−0.029 (2)	−0.0104 (14)	0.0154 (17)
C3	0.0429 (11)	0.0622 (13)	0.0455 (10)	0.0012 (12)	−0.0017 (9)	0.0003 (10)
C4	0.0760 (17)	0.0669 (16)	0.0689 (14)	0.0113 (16)	−0.0136 (15)	−0.0104 (13)
C5	0.116 (3)	0.082 (2)	0.0699 (15)	−0.003 (2)	−0.0199 (18)	−0.0186 (16)
C6	0.085 (2)	0.106 (3)	0.0544 (13)	−0.022 (2)	−0.0162 (15)	−0.0043 (16)
C7	0.0650 (16)	0.099 (2)	0.0585 (13)	0.0027 (18)	−0.0157 (13)	0.0114 (15)
C8	0.0618 (14)	0.0706 (16)	0.0576 (12)	0.0107 (14)	−0.0108 (12)	−0.0038 (12)
C9	0.0487 (12)	0.0465 (12)	0.0495 (10)	0.0049 (10)	−0.0017 (10)	−0.0005 (9)
O1	0.0946 (16)	0.0488 (11)	0.1330 (18)	0.0160 (11)	−0.0199 (16)	−0.0080 (11)
O2	0.0575 (10)	0.0609 (10)	0.0677 (9)	−0.0061 (9)	−0.0241 (8)	0.0114 (8)
C10	0.0477 (13)	0.0611 (14)	0.0753 (14)	−0.0048 (13)	−0.0089 (12)	0.0110 (13)
F1	0.1021 (14)	0.173 (2)	0.0768 (10)	−0.0089 (18)	−0.0389 (10)	0.0115 (13)
F2	0.0614 (10)	0.0583 (10)	0.200 (2)	0.0064 (9)	−0.0258 (13)	0.0349 (11)
F3	0.0526 (8)	0.0911 (13)	0.1303 (14)	−0.0024 (10)	0.0155 (9)	0.0106 (12)

Geometric parameters (Å, º)

N1—C1	1.499 (3)	C5—C6	1.352 (5)
N1—H1C	0.90 (3)	C5—H5	0.9300
N1—H1B	0.92 (3)	C6—C7	1.361 (4)
N1—H1A	0.90 (3)	C6—H6	0.9300
C1—C2	1.510 (4)	C7—C8	1.388 (3)
C1—C3	1.521 (3)	C7—H7	0.9300
C1—H1	0.9800	C8—H8	0.9300
C2—H2C	0.9600	C9—O1	1.227 (3)
C2—H2B	0.9600	C9—O2	1.233 (3)
C2—H2A	0.9600	C9—C10	1.530 (3)
C3—C8	1.374 (3)	C10—F1	1.312 (3)
C3—C4	1.385 (3)	C10—F2	1.319 (3)
C4—C5	1.387 (4)	C10—F3	1.329 (3)
C4—H4	0.9300
C1—N1—H1C	109.1 (18)	C5—C4—H4	119.8
C1—N1—H1B	106.6 (18)	C6—C5—C4	120.4 (3)
H1C—N1—H1B	117 (2)	C6—C5—H5	119.8
C1—N1—H1A	113.5 (18)	C4—C5—H5	119.8
H1C—N1—H1A	104 (2)	C5—C6—C7	120.2 (3)
H1B—N1—H1A	107 (2)	C5—C6—H6	119.9
N1—C1—C2	109.5 (2)	C7—C6—H6	119.9
N1—C1—C3	109.99 (17)	C6—C7—C8	120.1 (3)
C2—C1—C3	113.23 (19)	C6—C7—H7	119.9
N1—C1—H1	108.0	C8—C7—H7	119.9
C2—C1—H1	108.0	C3—C8—C7	120.7 (3)
C3—C1—H1	108.0	C3—C8—H8	119.7
C1—C2—H2C	109.5	C7—C8—H8	119.7
C1—C2—H2B	109.5	O1—C9—O2	130.3 (3)
H2C—C2—H2B	109.5	O1—C9—C10	113.4 (2)
C1—C2—H2A	109.5	O2—C9—C10	116.2 (2)
H2C—C2—H2A	109.5	F1—C10—F2	106.3 (2)
H2B—C2—H2A	109.5	F1—C10—F3	105.6 (2)
C8—C3—C4	118.3 (2)	F2—C10—F3	106.6 (3)
C8—C3—C1	122.0 (2)	F1—C10—C9	112.6 (2)
C4—C3—C1	119.7 (2)	F2—C10—C9	113.4 (2)
C3—C4—C5	120.4 (3)	F3—C10—C9	111.85 (19)
C3—C4—H4	119.8
N1—C1—C3—C8	61.4 (3)	C4—C3—C8—C7	−0.6 (4)
C2—C1—C3—C8	−61.5 (3)	C1—C3—C8—C7	178.9 (2)
N1—C1—C3—C4	−119.1 (2)	C6—C7—C8—C3	0.4 (4)
C2—C1—C3—C4	118.0 (3)	O1—C9—C10—F1	−54.5 (3)
C8—C3—C4—C5	0.7 (4)	O2—C9—C10—F1	126.5 (2)
C1—C3—C4—C5	−178.8 (3)	O1—C9—C10—F2	−175.2 (3)
C3—C4—C5—C6	−0.7 (5)	O2—C9—C10—F2	5.8 (3)
C4—C5—C6—C7	0.5 (5)	O1—C9—C10—F3	64.2 (3)
C5—C6—C7—C8	−0.4 (5)	O2—C9—C10—F3	−114.8 (3)

Hydrogen-bond geometry (Å, º)

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···O2^i	0.92 (3)	1.97 (3)	2.818 (3)	154 (3)
N1—H1C···O2^ii	0.90 (3)	1.92 (3)	2.816 (2)	175 (3)
N1—H1B···F2^i	0.92 (3)	2.50 (3)	3.202 (3)	134 (2)

Symmetry codes: (i) x, y−1, z; (ii) −x+2, y−1/2, −z+1/2.