Crystal structure of (R)-3-fluoro­pyrrolidine hydro­chloride

This article is part of the collection Early Career Scientists in Structural Science.

The title compound [also called (R)-3-fluoro­pyrrolidin-1-ium chloride], C₄H₉FN⁺·Cl⁻, crystallizes in the monoclinic Sohncke space group P2₁ (#4). The asymmetric unit contains one 3-fluoro­pyrrolidinium cation and one chloride anion. The pyrrolidinium ring adopts a puckered conformation, and the fluorine substituent is bonded to C3 at a distance of 1.396 (3) Å. In the crystal, the ammonium N—H groups form hydrogen bonds with the chloride anions, creating an extended hydrogen-bonded arrangement. Although 3-fluoro­pyrrolidinium derivatives have been investigated as chiral organic components in hybrid materials, including magnetic and ferroelectric systems, the crystallographic characterization of the hydro­chloride salt itself has not been reported.

1. Chemical context

In recent years, chiral 3-fluoro­pyrrolidinium cations have attracted inter­est as the organic components in organic–inorganic hybrid materials with magnetic and ferroelectric properties. The addition of (R)- and (S)-3-fluoro­pyrrolidinium in cyanido-bridged dimetallic materials has led to the creation of homochiral multiferroic substances that exhibit ferroelectric and ferroelastic behavior (Mao et al., 2022).

(R)- and (S)-3-fluoro­pyrrolidinium cations have also been integrated into quasi-two-dimensional copper(II) chloride hybrid perovskites, materials, which, with the mol­ecule's addition, exhibit magnetic chirality (Zhang et al., 2026). In addition, the cation has been used in the context of alkaline-earth-metal hybrid perovskites, one of which being [(R)-3-fluoro­pyrrolidinium]Ba(ClO4)₃ where mol­ecular fluorination demonstrated an enhancement of several ferroelectric properties relative to the corresponding unsubstituted pyrrolidinium material (Zhou et al., 2025). (R)-3-Fluoro­pyrrolidinium has also been used to create hexa­gonal perovskite-type manganese(II) chloride ferroelectrics (Ai et al., 2019).

Despite the incorporation of 3-fluoro­pyrrolidinium in many chiral organic–inorganic hybrid materials, the hydro­chloride salt of (R)-3-fluoro­pyrrolidine [also known as (R)-3-fluoro­pyrrolidinium chloride], does not have a publicly reported crystal structure. The structure presented here therefore provides a crystallographic characterization of the organic salt, which enables the conformation, stereochemistry, and inter­molecular inter­actions of the 3-fluoro­pyrrolidinium cation to be independently examined, not in the context of an inorganic hybrid framework.

2. Structural commentary

The title compound crystallizes in the monoclinic Sohncke space group P2₁ (#4). The asymmetric unit contains one (R)-3-fluoro­pyrrolidinium cation and one chloride anion (Fig. 1). Two formula units compose the unit cell. The pyrrolidinium ring has a non-planarenvelope conformation, with C3 out of plane. The C1—C2—C3—C4 and C2—C3—C4—N1 torsion angles are 36.2 (2)° and −33.8 (2)°, respectively, indicating that the five-membered ring is puckered rather than planar. The C3 atom is bonded to fluorine, with a bond length of 1.396 (3) Å, and the geometry around this carbon is approximately tetra­hedral with F1—C3—C2, F1—C3—C4, and C2—C3—C4 angles of 109.2 (2), 109.1 (2), and 103.98 (16)°, respectively. The protonated nitro­gen also has approximately tetra­hedral geometry.

Figure 1 Mol­ecular structure of the title compound with displacement ellipsoids drawn at the 50% probability level.

3. Supra­molecular features

The cations and chloride anions are connected through N—H⋯Cl hydrogen bonds (Fig. 2, Table 1). Each pyrrolidinium cation contains two N—H groups, which both participate in hydrogen bonding with the chloride anions. Nearly linear, these inter­actions connect neighboring ions and allow for an extended hydrogen-bonded structure throughout the crystal.

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
N1—H1A⋯Cl1	0.87 (3)	2.20 (3)	3.0770 (18)	178 (2)
N1—H1B⋯Cl1^i	0.85 (3)	2.22 (3)	3.0686 (18)	174 (2)
Symmetry code: (i) .

Figure 2 Hydrogen-bonding network in the title compound viewed down the b axis, with Cl^− ions shown in yellow.

4. Database survey

Unfluorinated pyrrolidine hydro­chloride crystallizes in the ortho­rhom­bic space group Pnma (#62) with V/Z = 155.877 ų (Giglmeier et al., 2009). Fluorination causes V/Z to increase to 159.532 ų in (R)-3-fluoro­pyrrolidine hydro­chloride, though thermal expansion may account for some of the difference in volume as the crystal structure of pyrrolidine hydro­chloride was collected at 200 K whereas the structure of (R)-3-fluoro­pyrrolidine hydro­chloride was collected at 295 K.

5. Synthesis and crystallization

1 mmol NiCl₂·6H₂O (Thermo Scientific) and 2 mmol (R)-3-fluoro­pyrrolidine hydro­chloride (Ambeed) were dissolved in 0.25 mL of methanol. Crystals were obtained by slow vapor diffusion of diethyl ether into the solution. After five days, the crystals were rinsed with the anti­solvent several times and dried under vacuum. The obtained crystals were found to be recrystallized (R)-3-fluoro­pyrrolidine hydro­chloride; an organic-inorganic hybrid material containing both nickel chloride and (R)-3-fluoro­pyrrolidinium did not form.

6. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2. Carbon-bonded hydrogen atoms were refined using a riding model with the C—H bond length allowed to vary using the HFIX 24 (two sp³ hydrogens for C1, C2, and C4) or HFIX 14 (one sp³ hydrogen for C3) commands in SHELXL. The positions of the nitro­gen-bonded hydrogen atoms (H1A and H1B) were allowed to freely refine, and their isotropic thermal parameters were set to 1.2U_eq(N). The structure was refined using the OLEX2 GUI (Dolomanov et al., 2009).

Table 2
Experimental details

Crystal data
Chemical formula	C4H9FN^+·Cl^−
Mr	125.57
Crystal system, space group	Monoclinic, P21
Temperature (K)	295
a, b, c (Å)	5.1390 (1), 10.6704 (2), 6.3479 (1)
β (°)	113.564 (1)
V (Å^3)	319.06 (1)
Z	2
Radiation type	Mo Kα
μ (mm^−1)	0.50
Crystal size (mm)	0.38 × 0.21 × 0.16
Data collection
Diffractometer	Bruker D8 Quest
Absorption correction	Multi-scan (SADABS; Krause et al., 2015)
Tmin, Tmax	0.932, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections	18053, 1857, 1781
Rint	0.024
(sin θ/λ)max (Å^−1)	0.705
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.025, 0.068, 1.07
No. of reflections	1857
No. of parameters	74
No. of restraints	1
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
Δρmax, Δρmin (e Å^−3)	0.23, −0.13
Absolute structure	Flack x determined using 806 quotients [(I^+)−(I^−)]/[(I^+)+(I^−)] (Parsons et al. (2013)
Absolute structure parameter	−0.017 (16)
Computer programs: BIS (Bruker, 2026), SAINT (Bruker, 2016), SHELXT2018/2 (Sheldrick, 2015a), SHELXL2025/1 (Sheldrick, 2015bb), PLATON (Spek, 2020) and publCIF (Westrip, 2010).

Supporting information

Computing details

(R)-3-Fluoropyrrolidin-1-ium chloride

Crystal data

C4H9FN^+·Cl^−	F(000) = 132
Mr = 125.57	Dx = 1.307 Mg m^−3
Monoclinic, P21	Mo Kα radiation, λ = 0.71073 Å
a = 5.1390 (1) Å	Cell parameters from 9897 reflections
b = 10.6704 (2) Å	θ = 3.5–28.2°
c = 6.3479 (1) Å	µ = 0.50 mm^−1
β = 113.564 (1)°	T = 295 K
V = 319.06 (1) Å^3	Prism, brown
Z = 2	0.38 × 0.21 × 0.16 mm

Data collection

Bruker D8 Quest diffractometer	1857 independent reflections
Radiation source: sealed tube	1781 reflections with I > 2σ(I)
Flat graphite monochromator	Rint = 0.024
Detector resolution: 7.40 pixels mm^-1	θmax = 30.1°, θmin = 3.5°
ω and phi scans	h = −7→7
Absorption correction: multi-scan (SADABS; Krause et al., 2015)	k = −15→15
Tmin = 0.932, Tmax = 1.000	l = −8→8
18053 measured reflections

Refinement

Refinement on F^2	Hydrogen site location: mixed
Least-squares matrix: full	H atoms treated by a mixture of independent and constrained refinement
R[F^2 > 2σ(F^2)] = 0.025	w = 1/[σ^2(Fo^2) + (0.040P)^2 + 0.016P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.068	(Δ/σ)max < 0.001
S = 1.07	Δρmax = 0.23 e Å^−3
1857 reflections	Δρmin = −0.13 e Å^−3
74 parameters	Absolute structure: Flack x determined using 806 quotients [(I^+)-(I^-)]/[(I^+)+(I^-)] (Parsons et al. (2013)
1 restraint	Absolute structure parameter: −0.017 (16)
Primary atom site location: dual

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 (Ų)

	x	y	z	Uiso*/Ueq
Cl1	0.50723 (8)	0.66790 (4)	0.02568 (7)	0.05125 (14)
F1	0.6093 (3)	0.4266 (3)	0.5424 (3)	0.1009 (6)
N1	0.0917 (3)	0.54125 (16)	0.1987 (3)	0.0477 (3)
H1A	0.213 (6)	0.577 (2)	0.153 (4)	0.057*
H1B	−0.073 (6)	0.574 (2)	0.140 (4)	0.057*
C1	0.0736 (5)	0.4059 (3)	0.1332 (4)	0.0642 (5)
H1C	−0.120 (5)	0.3842 (6)	0.027 (3)	0.077*
H1D	0.204 (3)	0.3871 (5)	0.0589 (18)	0.077*
C2	0.1553 (6)	0.3349 (2)	0.3517 (5)	0.0704 (6)
H2A	0.259 (3)	0.264 (2)	0.3500 (5)	0.084*
H2B	−0.005 (5)	0.3108 (8)	0.3758 (9)	0.084*
C3	0.3344 (4)	0.4251 (3)	0.5356 (4)	0.0625 (5)
H3	0.3345 (4)	0.4034 (7)	0.686 (5)	0.075*
C4	0.1987 (5)	0.5491 (2)	0.4540 (4)	0.0609 (5)
H4A	0.335 (3)	0.6162 (15)	0.5129 (14)	0.073*
H4B	0.044 (3)	0.5631 (4)	0.5017 (11)	0.073*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cl1	0.04294 (19)	0.0534 (2)	0.0592 (2)	0.00460 (17)	0.02232 (15)	0.0093 (3)
F1	0.0422 (6)	0.1661 (18)	0.0836 (11)	0.0214 (10)	0.0139 (7)	0.0195 (13)
N1	0.0365 (6)	0.0502 (7)	0.0559 (8)	0.0036 (6)	0.0179 (6)	0.0085 (6)
C1	0.0690 (12)	0.0578 (10)	0.0551 (11)	−0.0026 (10)	0.0135 (9)	−0.0105 (10)
C2	0.0733 (15)	0.0504 (11)	0.0848 (17)	0.0087 (10)	0.0289 (13)	0.0116 (10)
C3	0.0512 (10)	0.0869 (15)	0.0486 (10)	0.0086 (11)	0.0189 (9)	0.0120 (10)
C4	0.0596 (11)	0.0633 (12)	0.0561 (11)	−0.0061 (9)	0.0193 (9)	−0.0136 (9)

Geometric parameters (Å, º)

F1—C3	1.396 (3)	C2—H2A	0.93 (3)
N1—H1A	0.88 (3)	C2—H2B	0.93 (3)
N1—H1B	0.85 (3)	C2—C3	1.509 (4)
N1—C1	1.496 (3)	C3—H3	0.98 (3)
N1—C4	1.491 (3)	C3—C4	1.489 (4)
C1—H1C	0.98 (2)	C4—H4A	0.97 (2)
C1—H1D	0.98 (2)	C4—H4B	0.97 (2)
C1—C2	1.486 (4)
H1A—N1—H1B	113 (2)	H2A—C2—H2B	108.9
C1—N1—H1A	107.8 (16)	C3—C2—H2A	110.8
C1—N1—H1B	109.6 (17)	C3—C2—H2B	110.8
C4—N1—H1A	109.4 (15)	F1—C3—C2	109.2 (2)
C4—N1—H1B	108.6 (18)	F1—C3—H3	111.4
C4—N1—C1	108.14 (16)	F1—C3—C4	109.1 (2)
N1—C1—H1C	110.6	C2—C3—H3	111.4
N1—C1—H1D	110.6	C4—C3—C2	103.98 (16)
H1C—C1—H1D	108.7	C4—C3—H3	111.4
C2—C1—N1	105.65 (18)	N1—C4—H4A	110.9
C2—C1—H1C	110.6	N1—C4—H4B	110.9
C2—C1—H1D	110.6	C3—C4—N1	104.23 (18)
C1—C2—H2A	110.8	C3—C4—H4A	110.9
C1—C2—H2B	110.8	C3—C4—H4B	110.9
C1—C2—C3	104.7 (2)	H4A—C4—H4B	108.9
F1—C3—C4—N1	82.6 (2)	C1—C2—C3—C4	36.2 (2)
N1—C1—C2—C3	−24.0 (2)	C2—C3—C4—N1	−33.8 (2)
C1—N1—C4—C3	19.2 (2)	C4—N1—C1—C2	3.1 (2)
C1—C2—C3—F1	−80.2 (2)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1A···Cl1	0.87 (3)	2.20 (3)	3.0770 (18)	178 (2)
N1—H1B···Cl1^i	0.85 (3)	2.22 (3)	3.0686 (18)	174 (2)

Symmetry code: (i) x−1, y, z.