2-Amino-5-chloro-1,3-benzoxazol-3-ium 2-(3,4-di­chloro­phenoxy)­acetate

The 1:1 organic salt of the title compound, C₇H₆ClN₂O⁺·C₈H₅Cl₂O₃⁻ or [(2-ABOX)(3,4-D)], comprises the two constituent mol­ecules associated by an R₂²(8) graph-set interaction through the carboxyl­ate group of 3,4-D across the protonated N/N sites of 2-ABOX [N...O 2.546 (3) and 2.795 (3) Å]. Cation/anion pairs associate across an inversion centre forming discrete tetramers via an additional three-centre hydrogen-bonding association from the latter N amino proton to a phenoxy O atom [N...O 3.176 (3) Å] and a carboxyl­ate O atom [N...O 2.841 (3) Å]. This formation differs from the polymeric hydrogen-bonded chains previously observed for adduct structures of 2-ABOX with carboxyl­ic acids.

S1. Comment

2-Amino-5-chloro-1,3-benzooxazole (2-ABOX), first prepared in 1953 (Nagana et al., 1953), was formerly used as a skeletal muscle relaxant and is a uricosuric agent in gout. It is commonly known as Zoxazolamine, other names include Deflexol, Flexilon, Flexin, Zoxamin, Zoxine, and McN-485 (Stecher, 1968). 2-ABOX is the only commercially available 2-amino-1,3-oxazole, which we have investigated in tandem with the more readily available 2-amino-1,3-thiazole derivatives. Studies on the carboxylic acid adducts of 2-amino-1,3-thiazole derivatives (Lynch et al., 1998; Lynch, Nicholls et al., 1999; Lynch, Cooper et al., 1999) have shown that there is a significant difference in the two distances between the non-H atoms involved in the dominant R²₂(8) graph-set association (Etter, 1990). For the 2-amino-1,3-thiazole series, the average distance difference [i.e. N21A—O(acid) minus N3A—O(acid)] from thirteen examples is 0.109 Å, whereas for three 2-ABOX complexes the equivalent average difference is 0.213 Å (Lynch et al., 2000). Thus, to increase the data set of 2-amino-1,3-oxazole complexes we report here the 2-ABOX complex with 2-[(3,4-dichlorophenyl)oxy]acetic acid (3,4-D), as [(2-ABOX)(3,4-D)], (I), which itself is an active member of the herbicidal phenoxyacetic acid series (Crafts, 1957). \sch

Phenoxyacetic acids primarily exist in the solid state in either one of two conformations, synplanar (i.e. hooked) or antiperiplanar (i.e. flat). Depending on the adducting molecule, examples of 2-[(2,4-dichlorophenyl)oxy]acetic acid and 2-[(2,4,5-trichlorophenyl)oxy]acetic acid in both conformations are known (Lynch, Nicholls et al., 1999). The two previously reported adducts of 3,4-D, those with triphenylphosphine oxide (Lynch et al., 1993) and 2-aminopyrimidine (Lynch et al., 1994), as well as the parent structure of 3,4-D (Smith et al., 1981), only show 3,4-D in the antiperiplanar conformation. This is also the case in the 3,4-D molecule reported here; the three important torsion angles are listed in Table 1 and the dihedral angle between the plane of the carboxylate group and the phenyl ring is 15.67 (6)°. The carboxylate group of 3,4-D associates with the protonated N3A/N21A site of 2-ABOX, details listed in Table 2.

The solid-state packing of the two associated molecules in (I) deviates from that observed for both previously reported 2-ABOX and 2-amino-1,3-thiazole complexes because in these structures the second amino proton hydrogen bonds to an adjacent carboxylate oxygen thus propagating a one-dimensional hydrogen-bonded chain. However, the two coplanar molecules in (I) lie near an inversion centre which in the unit cell produces a hydrogen-bonded cyclic tetramer, as shown in figure 1. The associations proceed via a three-centre interaction from the second N21 proton to both the phenoxy oxygen (O7B) and a carboxylate oxygen (O10B) [R²₁(5) graph set], details listed in Table 2.

An interesting observation that has been made with the 2-amino substituted heterocyclic bases used in these series of studies is that the C2A—N21A bond, which should be a sp³ C—N, is consistently shorter than the adjacent C2A—N3A bond, which should be a sp² C═N; relevant bond distances for (I) are listed in Table 1. This has been observed in all carboxylic acid complexes of 2-aminopyridine, 2-aminopyrimidine, 3-amino-1,2,4-triazole, 2-amino-1,3-thiazole derivatives and 2-amino-5-chloro-1,3-benzooxazole but not in their parent structures where the bonding distances are as expected with C—NH₂ > C═N. The occurrence of the shorter C—NH₂ bond cannot be explained in terms of whether the bases exist as cations. If this were so then the double bond could shift to form C═NH₂⁺ with the heterocyclic nitrogen still retaining three bonds. However, the inconsistency in bond distances is observed irrespective of the acid proton location. We currently have no explanation for this interesting phenomenon but before any meaningful molecular modelling results can be obtained we need an extensive and diverse database of good quality co-crystal complexes from which to extract information.

S2. Experimental

Synthesis was carried out by refluxing equimolar amounts (2 mmol) of 2-amino-5-chloro-1,3-benzoxazole (Aldrich) and 2-(3,4-dichlorophenyloxy)acetic acid (Lancaster) for 15 min at ca 350 K in 40 ml of 95% ethanol. Crystals were obtained by the total evaporation of the solvent at room temperature (m.p. 438–439 K).

S3. Refinement

After establishing that complex (I) was a salt by initially locating the acid proton by difference syntheses, all H atoms were then included in the refinement, at calculated positions, as riding models with C—H in the range 0.95 to 0.99 and N—H at 0.88 Å.

Computing details

Data collection: DIF4 (Stoe & Cie, 1990); cell refinement: DIF4; data reduction: REDU4 (Stoe & Cie, 1990); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: PLATON97 (Spek, 1997); software used to prepare material for publication: SHELXL97.

Figures

Fig. 1. Molecular configuration and atom-numbering scheme, showing 30% probability ellipsoids and hydrogen-bonding interactions as broken lines.

'2-Amino-5-chloro-1,3-benzoxazol-3-ium 2-[(3,4-dichlorophenyl)oxy]acetate'

Crystal data

C7H6ClN2O^+·C8H5Cl2O3^−	F(000) = 792
Mr = 389.61	Dx = 1.669 Mg m^−3
Monoclinic, P21/c	Cu Kα radiation, λ = 1.54180 Å
a = 18.526 (4) Å	Cell parameters from 74 reflections
b = 6.9600 (15) Å	θ = 20–22°
c = 12.759 (3) Å	µ = 5.58 mm^−1
β = 109.512 (16)°	T = 150 K
V = 1550.6 (6) Å^3	Needle, colourless
Z = 4	0.77 × 0.14 × 0.14 mm

Data collection

Stoe Stadi-4 diffractometer	2430 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.05
Graphite monochromator	θmax = 70.4°, θmin = 2.5°
ω–θ scans	h = −4→22
Absorption correction: integration (Stoe & Cie, 1995)	k = −8→7
Tmin = 0.311, Tmax = 0.615	l = −15→14
3506 measured reflections	3 standard reflections every 60 min
2827 independent reflections	intensity decay: none

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.042	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.115	H-atom parameters constrained
S = 1.06	w = 1/[σ^2(Fo^2) + (0.0652P)^2 + 1.4716P] where P = (Fo^2 + 2Fc^2)/3
2827 reflections	(Δ/σ)max < 0.001
217 parameters	Δρmax = 0.62 e Å^−3
0 restraints	Δρmin = −0.37 e Å^−3

Special details

Geometry. Mean plane data ex SHELXL97 ###########################

Least-squares planes (x,y,z in crystal coordinates) and deviations from them (* indicates atom used to define plane)

− 0.5034 (0.0260) x + 6.7475 (0.0029) y + 3.0465 (0.0155) z = 1.7494 (0.0082)

* −0.0044 (0.0015) O1A * 0.0019 (0.0016) C2A * 0.0015 (0.0016) N3A * −0.0042 (0.0016) C9A * 0.0053 (0.0016) C8A −0.0066 (0.0043) N21A −0.0324 (0.0046) C4A −0.0391 (0.0057) C5A −0.0002 (0.0058) C6A 0.0179 (0.0049) C7A −0.1338 (0.0072) Cl5A

Rms deviation of fitted atoms = 0.0038

- 0.5556 (0.0216) x + 6.7126 (0.0029) y + 3.2852 (0.0144) z = 1.6696 (0.0049)

Angle to previous plane (with approximate e.s.d.) = 1.13 (0.09)

* −0.0081 (0.0020) C8A * 0.0065 (0.0019) C9A * 0.0015 (0.0020) C4A * −0.0080 (0.0021) C5A * 0.0065 (0.0021) C6A * 0.0015 (0.0021) C7A −0.0744 (0.0039) Cl5A −0.0375 (0.0039) O1A −0.0183 (0.0047) C2A −0.0400 (0.0059) N21A 0.0072 (0.0040) N3A

Rms deviation of fitted atoms = 0.0060

0.8086 (0.0216) x + 6.8271 (0.0025) y + 2.0936 (0.0148) z = 1.6264 (0.0067)

Angle to previous plane (with approximate e.s.d.) = 6.02 (0.08)

* −0.0004 (0.0020) C1B * −0.0002 (0.0020) C2B * 0.0025 (0.0020) C3B * −0.0041 (0.0020) C4B * 0.0035 (0.0021) C5B * −0.0012 (0.0021) C6B −0.0053 (0.0038) Cl3B −0.0017 (0.0039) Cl4B −0.0168 (0.0041) O7B 0.0940 (0.0054) C8B 0.0615 (0.0067) C9B −0.2540 (0.0068) O10B 0.3492 (0.0074) O11B

Rms deviation of fitted atoms = 0.0025

1.5451 (0.0297) x − 6.9265 (0.0022) y + 0.2617 (0.0198) z = 0.1601 (0.0153)

Angle to previous plane (with approximate e.s.d.) = 15.67 (0.06)

* 0.0023 (0.0007) C8B * −0.0084 (0.0024) C9B * 0.0032 (0.0009) O10B * 0.0029 (0.0008) O11B −0.2575 (0.0069) C1B −0.5997 (0.0083) C2B −0.6362 (0.0103) C3B −0.3230 (0.0110) C4B 0.0065 (0.0101) C5B 0.0470 (0.0080) C6B −0.2361 (0.0048) O7B −1.0562 (0.0120) Cl3B −0.3649 (0.0135) Cl4B

Rms deviation of fitted atoms = 0.0049

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

	x	y	z	Uiso*/Ueq
O1A	−0.25135 (10)	0.4484 (3)	−0.46197 (15)	0.0246 (4)
C2A	−0.31073 (15)	0.4278 (4)	−0.4240 (2)	0.0208 (5)
N21A	−0.38058 (13)	0.4516 (4)	−0.49098 (18)	0.0266 (5)
H21A	−0.4193	0.4365	−0.4663	0.033*
H22A	−0.3891	0.4827	−0.5609	0.033*
N3A	−0.28884 (12)	0.3813 (3)	−0.31752 (17)	0.0194 (5)
H3A	−0.3188	0.3615	−0.2776	0.024*
C4A	−0.15629 (14)	0.3236 (4)	−0.1789 (2)	0.0209 (6)
H4A	−0.1710	0.2922	−0.1163	0.026*
C5A	−0.08004 (15)	0.3259 (4)	−0.1736 (2)	0.0235 (6)
Cl5A	−0.01031 (4)	0.26101 (12)	−0.04948 (6)	0.0326 (2)
C6A	−0.05706 (15)	0.3740 (4)	−0.2637 (2)	0.0265 (6)
H6A	−0.0041	0.3763	−0.2553	0.033*
C7A	−0.11124 (16)	0.4187 (4)	−0.3657 (2)	0.0259 (6)
H7A	−0.0972	0.4511	−0.4286	0.032*
C8A	−0.18611 (15)	0.4134 (4)	−0.3704 (2)	0.0211 (5)
C9A	−0.20910 (14)	0.3697 (4)	−0.2806 (2)	0.0183 (5)
C1B	0.36559 (15)	0.1065 (4)	0.2882 (2)	0.0221 (6)
C2B	0.29056 (15)	0.1366 (4)	0.2192 (2)	0.0215 (6)
H2B	0.2797	0.1612	0.1422	0.027*
C3B	0.23186 (15)	0.1304 (4)	0.2632 (2)	0.0211 (5)
Cl3B	0.13917 (3)	0.16706 (10)	0.17578 (5)	0.02584 (18)
C4B	0.24750 (15)	0.0929 (4)	0.3762 (2)	0.0221 (6)
Cl4B	0.17422 (4)	0.08477 (11)	0.43234 (6)	0.0293 (2)
C5B	0.32169 (16)	0.0645 (4)	0.4440 (2)	0.0263 (6)
H5B	0.3326	0.0408	0.5211	0.033*
C6B	0.38109 (16)	0.0702 (4)	0.4001 (2)	0.0261 (6)
H6B	0.4324	0.0492	0.4471	0.033*
O7B	0.41975 (10)	0.1135 (3)	0.23644 (15)	0.0266 (5)
C8B	0.49773 (14)	0.0992 (4)	0.3062 (2)	0.0221 (6)
H81B	0.5060	−0.0240	0.3473	0.028*
H82B	0.5101	0.2051	0.3611	0.028*
C9B	0.54962 (15)	0.1096 (4)	0.2364 (2)	0.0214 (5)
O10B	0.52241 (11)	0.0980 (3)	0.13409 (16)	0.0319 (5)
O11B	0.61991 (10)	0.1259 (3)	0.29384 (15)	0.0294 (5)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1A	0.0208 (9)	0.0337 (12)	0.0204 (9)	−0.0023 (8)	0.0081 (7)	0.0006 (8)
C2A	0.0233 (13)	0.0170 (14)	0.0242 (12)	−0.0020 (10)	0.0108 (10)	−0.0035 (10)
N21A	0.0226 (11)	0.0347 (15)	0.0218 (11)	0.0006 (10)	0.0067 (9)	0.0025 (10)
N3A	0.0171 (10)	0.0228 (12)	0.0209 (10)	−0.0005 (9)	0.0095 (8)	0.0005 (9)
C4A	0.0208 (12)	0.0202 (15)	0.0240 (13)	0.0002 (10)	0.0107 (10)	0.0003 (10)
C5A	0.0194 (12)	0.0231 (15)	0.0266 (13)	0.0022 (11)	0.0059 (10)	0.0005 (11)
Cl5A	0.0200 (3)	0.0410 (5)	0.0333 (4)	0.0043 (3)	0.0043 (3)	0.0080 (3)
C6A	0.0181 (12)	0.0294 (16)	0.0365 (15)	−0.0003 (11)	0.0152 (11)	0.0003 (12)
C7A	0.0258 (14)	0.0272 (16)	0.0294 (14)	−0.0038 (12)	0.0155 (11)	−0.0009 (11)
C8A	0.0214 (13)	0.0212 (15)	0.0226 (12)	−0.0003 (10)	0.0098 (10)	−0.0014 (10)
C9A	0.0188 (12)	0.0155 (14)	0.0229 (12)	−0.0001 (10)	0.0101 (10)	−0.0026 (10)
C1B	0.0184 (12)	0.0261 (15)	0.0247 (13)	−0.0013 (11)	0.0111 (10)	−0.0047 (11)
C2B	0.0249 (13)	0.0212 (15)	0.0201 (12)	−0.0005 (11)	0.0096 (10)	−0.0026 (10)
C3B	0.0198 (12)	0.0186 (14)	0.0243 (13)	−0.0012 (10)	0.0067 (10)	−0.0038 (10)
Cl3B	0.0177 (3)	0.0302 (4)	0.0287 (3)	0.0019 (3)	0.0064 (2)	0.0008 (3)
C4B	0.0262 (13)	0.0184 (14)	0.0273 (13)	−0.0011 (11)	0.0166 (11)	−0.0024 (11)
Cl4B	0.0285 (3)	0.0335 (4)	0.0339 (4)	0.0013 (3)	0.0209 (3)	0.0019 (3)
C5B	0.0285 (14)	0.0321 (17)	0.0217 (13)	0.0000 (12)	0.0127 (11)	−0.0007 (11)
C6B	0.0192 (12)	0.0336 (17)	0.0256 (13)	0.0029 (11)	0.0076 (10)	−0.0003 (12)
O7B	0.0167 (9)	0.0440 (13)	0.0208 (9)	0.0002 (8)	0.0086 (7)	0.0001 (8)
C8B	0.0161 (12)	0.0290 (16)	0.0205 (12)	−0.0003 (11)	0.0050 (10)	−0.0032 (11)
C9B	0.0186 (12)	0.0238 (15)	0.0227 (12)	−0.0008 (11)	0.0081 (10)	−0.0016 (10)
O10B	0.0207 (9)	0.0532 (14)	0.0229 (10)	−0.0038 (9)	0.0087 (8)	−0.0043 (9)
O11B	0.0186 (9)	0.0480 (14)	0.0228 (9)	−0.0046 (9)	0.0086 (7)	−0.0028 (9)

Geometric parameters (Å, º)

O1A—C2A	1.349 (3)	C1B—O7B	1.374 (3)
O1A—C8A	1.394 (3)	C1B—C6B	1.382 (4)
C2A—N21A	1.301 (4)	C1B—C2B	1.390 (4)
C2A—N3A	1.322 (3)	C2B—C3B	1.381 (4)
N21A—H21A	0.8800	C2B—H2B	0.9500
N21A—H22A	0.8800	C3B—C4B	1.397 (4)
N3A—C9A	1.395 (3)	C3B—Cl3B	1.726 (3)
N3A—H3A	0.8800	C4B—C5B	1.372 (4)
C4A—C9A	1.377 (4)	C4B—Cl4B	1.734 (3)
C4A—C5A	1.392 (4)	C5B—C6B	1.392 (4)
C4A—H4A	0.9500	C5B—H5B	0.9500
C5A—C6A	1.394 (4)	C6B—H6B	0.9500
C5A—Cl5A	1.736 (3)	O7B—C8B	1.425 (3)
C6A—C7A	1.387 (4)	C8B—C9B	1.514 (3)
C6A—H6A	0.9500	C8B—H81B	0.9900
C7A—C8A	1.369 (4)	C8B—H82B	0.9900
C7A—H7A	0.9500	C9B—O10B	1.235 (3)
C8A—C9A	1.382 (3)	C9B—O11B	1.268 (3)
C2A—O1A—C8A	105.24 (19)	O7B—C1B—C6B	124.7 (2)
N21A—C2A—N3A	127.0 (2)	O7B—C1B—C2B	115.3 (2)
N21A—C2A—O1A	120.1 (2)	C6B—C1B—C2B	120.0 (2)
N3A—C2A—O1A	112.8 (2)	C3B—C2B—C1B	119.6 (2)
C2A—N21A—H21A	120.0	C3B—C2B—H2B	120.2
C2A—N21A—H22A	120.0	C1B—C2B—H2B	120.2
H21A—N21A—H22A	120.0	C2B—C3B—C4B	120.4 (2)
C2A—N3A—C9A	106.7 (2)	C2B—C3B—Cl3B	118.8 (2)
C2A—N3A—H3A	126.6	C4B—C3B—Cl3B	120.9 (2)
C9A—N3A—H3A	126.6	C5B—C4B—C3B	119.7 (2)
C9A—C4A—C5A	115.6 (2)	C5B—C4B—Cl4B	119.5 (2)
C9A—C4A—H4A	122.2	C3B—C4B—Cl4B	120.7 (2)
C5A—C4A—H4A	122.2	C4B—C5B—C6B	120.2 (2)
C4A—C5A—C6A	123.2 (3)	C4B—C5B—H5B	119.9
C4A—C5A—Cl5A	118.4 (2)	C6B—C5B—H5B	119.9
C6A—C5A—Cl5A	118.4 (2)	C1B—C6B—C5B	120.1 (2)
C7A—C6A—C5A	120.2 (2)	C1B—C6B—H6B	120.0
C7A—C6A—H6A	119.9	C5B—C6B—H6B	120.0
C5A—C6A—H6A	119.9	C1B—O7B—C8B	116.6 (2)
C8A—C7A—C6A	116.1 (2)	O7B—C8B—C9B	109.9 (2)
C8A—C7A—H7A	121.9	O7B—C8B—H81B	109.7
C6A—C7A—H7A	121.9	C9B—C8B—H81B	109.7
C7A—C8A—C9A	123.9 (3)	O7B—C8B—H82B	109.7
C7A—C8A—O1A	127.9 (2)	C9B—C8B—H82B	109.7
C9A—C8A—O1A	108.2 (2)	H81B—C8B—H82B	108.2
C4A—C9A—C8A	120.9 (2)	O10B—C9B—O11B	126.5 (2)
C4A—C9A—N3A	132.1 (2)	O10B—C9B—C8B	120.2 (2)
C8A—C9A—N3A	107.0 (2)	O11B—C9B—C8B	113.3 (2)
C8A—O1A—C2A—N21A	−179.9 (3)	C2A—N3A—C9A—C8A	0.5 (3)
C8A—O1A—C2A—N3A	−0.6 (3)	O7B—C1B—C2B—C3B	179.4 (2)
N21A—C2A—N3A—C9A	179.3 (3)	C6B—C1B—C2B—C3B	0.2 (4)
O1A—C2A—N3A—C9A	0.1 (3)	C1B—C2B—C3B—C4B	−0.4 (4)
C9A—C4A—C5A—C6A	0.9 (4)	C1B—C2B—C3B—Cl3B	−179.7 (2)
C9A—C4A—C5A—Cl5A	−177.8 (2)	C2B—C3B—C4B—C5B	0.8 (4)
C4A—C5A—C6A—C7A	−1.4 (5)	Cl3B—C3B—C4B—C5B	−179.9 (2)
Cl5A—C5A—C6A—C7A	177.2 (2)	C2B—C3B—C4B—Cl4B	−180.0 (2)
C5A—C6A—C7A—C8A	0.5 (4)	Cl3B—C3B—C4B—Cl4B	−0.7 (3)
C6A—C7A—C8A—C9A	1.0 (4)	C3B—C4B—C5B—C6B	−0.9 (4)
C6A—C7A—C8A—O1A	−178.8 (3)	Cl4B—C4B—C5B—C6B	179.9 (2)
C2A—O1A—C8A—C7A	−179.3 (3)	O7B—C1B—C6B—C5B	−179.4 (3)
C2A—O1A—C8A—C9A	0.9 (3)	C2B—C1B—C6B—C5B	−0.3 (4)
C5A—C4A—C9A—C8A	0.5 (4)	C4B—C5B—C6B—C1B	0.6 (5)
C5A—C4A—C9A—N3A	179.5 (3)	C6B—C1B—O7B—C8B	−5.9 (4)
C7A—C8A—C9A—C4A	−1.5 (4)	C2B—C1B—O7B—C8B	175.0 (2)
O1A—C8A—C9A—C4A	178.3 (2)	C1B—O7B—C8B—C9B	−179.9 (2)
C7A—C8A—C9A—N3A	179.3 (3)	O7B—C8B—C9B—O10B	−11.3 (4)
O1A—C8A—C9A—N3A	−0.9 (3)	O7B—C8B—C9B—O11B	170.3 (2)
C2A—N3A—C9A—C4A	−178.6 (3)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N21A—H21A···O10B^i	0.88	1.95	2.795 (3)	161
N21A—H22A···O7B^ii	0.88	2.31	3.176 (3)	170
N21A—H22A···O10B^ii	0.88	2.46	2.841 (3)	106
N3A—H3A···O11B^i	0.88	1.68	2.546 (3)	168

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