Crystal structure of fluroxypyr

In the title pyridine herbicide {systematic name: 2-[(4-amino-3,5-di­chloro-6-fluoro­pyridin-2-yl)­oxy]acetic acid}, C₇H₅Cl₂FN₂O₃, the mean plane of the carb­oxy­lic acid substituent and the pyridyl ring plane subtend a dihedral angle of 77.5 (1)°. In the crystal, pairs of O—H⋯O hydrogen bonds form inversion dimers with R₂²(8) ring motifs. These are extended into chains along [011] by N—H⋯F hydrogen bonds. In addition, inter­molecular N—H⋯O hydrogen bonds and weak π–π inter­actions [ring centroid separation = 3.4602 (9) Å] connect these chains into a three-dimensional network.

1. Chemical context

Fluroxypyr belongs to the pyridine family of herbicides. It is widely used on cereal crops, olive trees and fallow croplands to control broad-leaf weeds (Moreno-Castilla et al., 2012; Wang et al., 2011). Pyridine herbicides such as fluroxypyr are effective and popular chemicals for post-emergence broad-leaf weed control, particularly in turf during cool seasons. The efficacy of this herbicide may be affected by environmental conditions including the relative humidity, temperature and soil moisture. Because of this, its application often provides inconsistent broad-leaf weed control in winter or early spring (Reed & McCullough, 2012). Until now, its crystal structure had not been reported and we describe it herein.

2. Structural commentary

The structure of fluroxypyr is shown in Fig. 1. The dihedral angle between the mean plane of the carb­oxy­lic acid group (C6/C7/O2/O3) and the pyridyl ring (N1/C1–C5) is 77.5 (1)°. All bond lengths and bond angles are normal and comparable to those observed in the crystal structure of a related pyridine-containing herbicide (Cho et al., 2015).

Figure 1 The structure of the title compound, with displacement ellipsoids drawn at the 50% probability level. H atoms are shown as small spheres of arbitrary radius.

3. Supra­molecular features

In the crystal, the solid-state structure is stabilized by pairs of N2—H2B⋯O2 hydrogen bonds, forming inversion dimers with R₂²(18) ring motifs (Table 1 and Fig. 2). These dimers are linked by pairs of O3—H3⋯O2ⁱ/O2 hydrogen bonds that form classical carb­oxy­lic-acid-based inversion dimers with R₂²(8) ring motifs. These contacts form chains propagating along [011] (yellow dashed lines in Fig. 2). In addition, inter­molecular N2—H2A⋯F1 hydrogen bonds connect these chains, yielding sheets extending parallel to the bc plane (red dashed line in Fig. 3). These sheets are further linked by weak inter­molecular π–π inter­actions between the pyridyl rings (N1/C1–C5) [Cg1⋯Cg1^iv = 3.4602 (9) Å; symmetry code: (iv) −x, −y + 2, −z], resulting in a three-dimensional network structure (black dashed lines in Fig. 4).

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O3—H3⋯O2^i	0.84	1.84	2.6801 (15)	174
N2—H2A⋯F1^ii	0.88	2.39	2.9950 (15)	126
N2—H2B⋯O2^iii	0.88	2.25	3.0201 (16)	146
Symmetry codes: (i) -x+1, -y+1, -z+1; (ii) x, y+1, z; (iii) -x+1, -y+2, -z.

Figure 2 A view along the b axis of the crystal packing of the title compound. The chains are formed through inter­molecular O—H⋯O and N—H⋯O hydrogen bonds (yellow dashed lines). H atoms not involved in these inter­actions have been omitted for clarity.

Figure 3 The two-dimensional network formed through inter­molecular N—H⋯F hydrogen bonds (red dashed lines). Inter­molecular O/N–H⋯O hydrogen bonds within a chain are shown as yellow dashed lines. H atoms not involved in these inter­actions have been omitted for clarity.

Figure 4 A packing diagram showing the three-dimensional architecture formed by weak π–π inter­actions (black dashed lines). Inter­molecular O—H⋯O, N—H⋯O and N—H⋯F hydrogen bonds within a sheet are shown as yellow and red dashed lines. H atoms not involved in inter­molecular inter­actions have been omitted for clarity.

4. Database survey

We have reported the crystal structure of several pesticides including compounds with pyridine rings (Cho et al., 2015; Kang et al., 2015; Kwon et al., 2016; Park et al., 2016). In addition, a database search (CSD; Groom et al., 2006) yielded two other comparable structures, 2-[(3,5,6-tri­chloro­pyridin-2-yl)­oxy]acetic acid (Cho et al., 2014) and 2,4,5-tri­chloro­phen­oxy­acetic acid (Smith et al., 1976).

5. Synthesis and crystallization

The title compound was purchased from Dr. Ehrenstorfer GmbH. Colorless single crystals suitable for X-ray diffraction were obtained from a CH₃CN solution by slow evaporation at room temperature.

6. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms were positioned geometrically and refined using a riding model with d(O—H) = 0.84 Å, U_iso = 1.5U_eq(C) for the O—H group, d(N—H) = 0.88 Å, U_iso = 1.2U_eq(C) for the amine group, and d(C—H) = 0.99 Å, U_iso = 1.2U_eq(C) for the CH₂ group.

Table 2
Experimental details

Crystal data
Chemical formula	C7H5Cl2FN2O3
Mr	255.03
Crystal system, space group	Triclinic, P
Temperature (K)	173
a, b, c (Å)	7.1116 (9), 7.6131 (9), 8.9414 (11)
α, β, γ (°)	86.927 (6), 80.354 (6), 72.587 (5)
V (Å^3)	455.38 (10)
Z	2
Radiation type	Mo Kα
μ (mm^−1)	0.72
Crystal size (mm)	0.23 × 0.22 × 0.04
Data collection
Diffractometer	Bruker APEXII CCD
Absorption correction	Multi-scan (SADABS; Bruker, 2014)
Tmin, Tmax	0.690, 0.746
No. of measured, independent and observed [I > 2σ(I)] reflections	8052, 2092, 1972
Rint	0.023
(sin θ/λ)max (Å^−1)	0.650
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.028, 0.076, 1.12
No. of reflections	2092
No. of parameters	137
H-atom treatment	H-atom parameters constrained
Δρmax, Δρmin (e Å^−3)	0.29, −0.38
Computer programs: APEX2 and SAINT (Bruker, 2014), SHELXS97 and SHELXTL (Sheldrick, 2008), SHELXL2014 (Sheldrick, 2015) and DIAMOND (Brandenburg, 2010).

Supporting information

Computing details

Data collection: APEX2 (Bruker, 2014); cell refinement: SAINT (Bruker, 2014); data reduction: SAINT (Bruker, 2014); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg, 2010); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

2-[(4-Amino-3,5-dichloro-6-fluoropyridin-2-yl)oxy]acetic acid

Crystal data

C7H5Cl2FN2O3	Z = 2
Mr = 255.03	F(000) = 256
Triclinic, P1	Dx = 1.860 Mg m^−3
a = 7.1116 (9) Å	Mo Kα radiation, λ = 0.71073 Å
b = 7.6131 (9) Å	Cell parameters from 6070 reflections
c = 8.9414 (11) Å	θ = 2.8–27.5°
α = 86.927 (6)°	µ = 0.72 mm^−1
β = 80.354 (6)°	T = 173 K
γ = 72.587 (5)°	Plate, colourless
V = 455.38 (10) Å^3	0.23 × 0.22 × 0.04 mm

Data collection

Bruker APEXII CCD diffractometer	1972 reflections with I > 2σ(I)
φ and ω scans	Rint = 0.023
Absorption correction: multi-scan (SADABS; Bruker, 2014)	θmax = 27.5°, θmin = 2.3°
Tmin = 0.690, Tmax = 0.746	h = −9→8
8052 measured reflections	k = −9→9
2092 independent reflections	l = −10→11

Refinement

Refinement on F^2	0 restraints
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.028	H-atom parameters constrained
wR(F^2) = 0.076	w = 1/[σ^2(Fo^2) + (0.0371P)^2 + 0.2062P] where P = (Fo^2 + 2Fc^2)/3
S = 1.12	(Δ/σ)max < 0.001
2092 reflections	Δρmax = 0.29 e Å^−3
137 parameters	Δρmin = −0.38 e Å^−3

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	0.43701 (6)	0.90760 (5)	−0.32138 (4)	0.02990 (12)
Cl2	0.09018 (6)	1.29195 (5)	0.19394 (4)	0.02692 (11)
F1	0.37852 (14)	0.60390 (12)	−0.12163 (11)	0.0298 (2)
O1	0.08333 (16)	0.93076 (14)	0.31061 (11)	0.0234 (2)
O2	0.41952 (15)	0.68493 (14)	0.39455 (11)	0.0225 (2)
O3	0.24193 (16)	0.50096 (15)	0.50373 (13)	0.0278 (2)
H3	0.3515	0.4408	0.5295	0.042*
N1	0.23296 (18)	0.76320 (16)	0.09169 (14)	0.0202 (2)
N2	0.2865 (2)	1.24580 (17)	−0.12976 (14)	0.0244 (3)
H2A	0.2414	1.3515	−0.0813	0.029*
H2B	0.3429	1.2417	−0.2255	0.029*
C1	0.3133 (2)	0.76709 (19)	−0.05118 (17)	0.0202 (3)
C2	0.3356 (2)	0.91926 (19)	−0.13244 (15)	0.0192 (3)
C3	0.26931 (19)	1.08983 (18)	−0.05718 (15)	0.0177 (3)
C4	0.1811 (2)	1.08826 (18)	0.09480 (15)	0.0176 (3)
C5	0.16793 (19)	0.92352 (19)	0.16331 (15)	0.0178 (3)
C6	0.0683 (2)	0.7624 (2)	0.38160 (17)	0.0242 (3)
H6A	0.0248	0.6917	0.3114	0.029*
H6B	−0.0345	0.7900	0.4736	0.029*
C7	0.2634 (2)	0.64680 (19)	0.42518 (15)	0.0197 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cl1	0.0345 (2)	0.0351 (2)	0.01671 (18)	−0.00767 (16)	0.00124 (14)	−0.00224 (14)
Cl2	0.0359 (2)	0.01784 (17)	0.02395 (19)	−0.00397 (14)	−0.00232 (14)	−0.00355 (13)
F1	0.0372 (5)	0.0182 (4)	0.0318 (5)	−0.0048 (4)	−0.0035 (4)	−0.0076 (4)
O1	0.0270 (5)	0.0214 (5)	0.0177 (5)	−0.0044 (4)	0.0008 (4)	0.0058 (4)
O2	0.0245 (5)	0.0241 (5)	0.0208 (5)	−0.0101 (4)	−0.0048 (4)	0.0060 (4)
O3	0.0243 (5)	0.0232 (5)	0.0355 (6)	−0.0081 (4)	−0.0051 (5)	0.0130 (4)
N1	0.0209 (6)	0.0169 (5)	0.0232 (6)	−0.0058 (4)	−0.0055 (5)	0.0035 (4)
N2	0.0318 (7)	0.0200 (6)	0.0216 (6)	−0.0103 (5)	−0.0013 (5)	0.0055 (5)
C1	0.0195 (6)	0.0167 (6)	0.0242 (7)	−0.0035 (5)	−0.0058 (5)	−0.0025 (5)
C2	0.0190 (6)	0.0226 (7)	0.0158 (6)	−0.0060 (5)	−0.0025 (5)	−0.0002 (5)
C3	0.0170 (6)	0.0189 (6)	0.0183 (6)	−0.0065 (5)	−0.0053 (5)	0.0037 (5)
C4	0.0186 (6)	0.0155 (6)	0.0181 (6)	−0.0038 (5)	−0.0035 (5)	0.0002 (5)
C5	0.0150 (6)	0.0200 (6)	0.0173 (6)	−0.0036 (5)	−0.0037 (5)	0.0033 (5)
C6	0.0236 (7)	0.0258 (7)	0.0218 (7)	−0.0081 (6)	−0.0014 (5)	0.0090 (6)
C7	0.0252 (7)	0.0202 (6)	0.0132 (6)	−0.0073 (5)	−0.0005 (5)	0.0007 (5)

Geometric parameters (Å, º)

Cl1—C2	1.7181 (14)	N2—C3	1.3506 (17)
Cl2—C4	1.7216 (14)	N2—H2A	0.8800
F1—C1	1.3403 (16)	N2—H2B	0.8800
O1—C5	1.3499 (17)	C1—C2	1.370 (2)
O1—C6	1.4243 (17)	C2—C3	1.4080 (19)
O2—C7	1.2143 (17)	C3—C4	1.3990 (19)
O3—C7	1.3158 (17)	C4—C5	1.3877 (19)
O3—H3	0.8400	C6—C7	1.505 (2)
N1—C1	1.3117 (19)	C6—H6A	0.9900
N1—C5	1.3268 (18)	C6—H6B	0.9900
C5—O1—C6	117.32 (11)	C5—C4—C3	119.78 (12)
C7—O3—H3	109.5	C5—C4—Cl2	121.01 (11)
C1—N1—C5	116.03 (12)	C3—C4—Cl2	119.20 (10)
C3—N2—H2A	120.0	N1—C5—O1	119.54 (12)
C3—N2—H2B	120.0	N1—C5—C4	123.51 (13)
H2A—N2—H2B	120.0	O1—C5—C4	116.96 (12)
N1—C1—F1	115.22 (12)	O1—C6—C7	112.12 (12)
N1—C1—C2	126.49 (13)	O1—C6—H6A	109.2
F1—C1—C2	118.29 (13)	C7—C6—H6A	109.2
C1—C2—C3	118.00 (13)	O1—C6—H6B	109.2
C1—C2—Cl1	122.11 (11)	C7—C6—H6B	109.2
C3—C2—Cl1	119.88 (10)	H6A—C6—H6B	107.9
N2—C3—C4	122.43 (12)	O2—C7—O3	124.35 (13)
N2—C3—C2	121.39 (12)	O2—C7—C6	124.46 (13)
C4—C3—C2	116.17 (12)	O3—C7—C6	111.18 (12)
C5—N1—C1—F1	179.85 (11)	C2—C3—C4—Cl2	−178.35 (10)
C5—N1—C1—C2	−0.2 (2)	C1—N1—C5—O1	179.97 (12)
N1—C1—C2—C3	0.9 (2)	C1—N1—C5—C4	0.1 (2)
F1—C1—C2—C3	−179.10 (12)	C6—O1—C5—N1	−0.13 (18)
N1—C1—C2—Cl1	−178.05 (11)	C6—O1—C5—C4	179.71 (12)
F1—C1—C2—Cl1	1.94 (19)	C3—C4—C5—N1	−0.9 (2)
C1—C2—C3—N2	179.92 (13)	Cl2—C4—C5—N1	179.01 (10)
Cl1—C2—C3—N2	−1.09 (18)	C3—C4—C5—O1	179.27 (11)
C1—C2—C3—C4	−1.55 (19)	Cl2—C4—C5—O1	−0.82 (17)
Cl1—C2—C3—C4	177.43 (10)	C5—O1—C6—C7	78.48 (15)
N2—C3—C4—C5	−179.93 (12)	O1—C6—C7—O2	−4.9 (2)
C2—C3—C4—C5	1.56 (19)	O1—C6—C7—O3	173.75 (12)
N2—C3—C4—Cl2	0.16 (18)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O3—H3···O2^i	0.84	1.84	2.6801 (15)	174
N2—H2A···F1^ii	0.88	2.39	2.9950 (15)	126
N2—H2B···O2^iii	0.88	2.25	3.0201 (16)	146

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