



Supporting information
![]() | Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270100012385/gg1018sup1.cif |
![]() | Structure factor file (CIF format) https://doi.org/10.1107/S0108270100012385/gg1018Isup2.hkl |
CCDC reference: 156170
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).
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.
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.
![]() | Fig. 1. Molecular configuration and atom-numbering scheme, showing 30% probability ellipsoids and hydrogen-bonding interactions as broken lines. |
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) Å3 | Needle, colourless |
Z = 4 | 0.77 × 0.14 × 0.14 mm |
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 on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.042 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.115 | H-atom parameters constrained |
S = 1.06 | w = 1/[σ2(Fo2) + (0.0652P)2 + 1.4716P] where P = (Fo2 + 2Fc2)/3 |
2827 reflections | (Δ/σ)max < 0.001 |
217 parameters | Δρmax = 0.62 e Å−3 |
0 restraints | Δρmin = −0.37 e Å−3 |
C7H6ClN2O+·C8H5Cl2O3− | V = 1550.6 (6) Å3 |
Mr = 389.61 | Z = 4 |
Monoclinic, P21/c | Cu Kα radiation |
a = 18.526 (4) Å | µ = 5.58 mm−1 |
b = 6.9600 (15) Å | T = 150 K |
c = 12.759 (3) Å | 0.77 × 0.14 × 0.14 mm |
β = 109.512 (16)° |
Stoe Stadi-4 diffractometer | 2430 reflections with I > 2σ(I) |
Absorption correction: integration (Stoe & Cie, 1995) | Rint = 0.05 |
Tmin = 0.311, Tmax = 0.615 | 3 standard reflections every 60 min |
3506 measured reflections | intensity decay: none |
2827 independent reflections |
R[F2 > 2σ(F2)] = 0.042 | 0 restraints |
wR(F2) = 0.115 | H-atom parameters constrained |
S = 1.06 | Δρmax = 0.62 e Å−3 |
2827 reflections | Δρmin = −0.37 e Å−3 |
217 parameters |
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 |
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) |
U11 | U22 | U33 | U12 | U13 | U23 | |
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) |
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) |
D—H···A | D—H | H···A | D···A | D—H···A |
N21A—H21A···O10Bi | 0.88 | 1.95 | 2.795 (3) | 161 |
N21A—H22A···O7Bii | 0.88 | 2.31 | 3.176 (3) | 170 |
N21A—H22A···O10Bii | 0.88 | 2.46 | 2.841 (3) | 106 |
N3A—H3A···O11Bi | 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. |
Experimental details
Crystal data | |
Chemical formula | C7H6ClN2O+·C8H5Cl2O3− |
Mr | 389.61 |
Crystal system, space group | Monoclinic, P21/c |
Temperature (K) | 150 |
a, b, c (Å) | 18.526 (4), 6.9600 (15), 12.759 (3) |
β (°) | 109.512 (16) |
V (Å3) | 1550.6 (6) |
Z | 4 |
Radiation type | Cu Kα |
µ (mm−1) | 5.58 |
Crystal size (mm) | 0.77 × 0.14 × 0.14 |
Data collection | |
Diffractometer | Stoe Stadi-4 diffractometer |
Absorption correction | Integration (Stoe & Cie, 1995) |
Tmin, Tmax | 0.311, 0.615 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 3506, 2827, 2430 |
Rint | 0.05 |
(sin θ/λ)max (Å−1) | 0.611 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.042, 0.115, 1.06 |
No. of reflections | 2827 |
No. of parameters | 217 |
H-atom treatment | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 0.62, −0.37 |
Computer programs: DIF4 (Stoe & Cie, 1990), DIF4, REDU4 (Stoe & Cie, 1990), SHELXS97 (Sheldrick, 1997), SHELXL97 (Sheldrick, 1997), PLATON97 (Spek, 1997), SHELXL97.
C2A—N21A | 1.301 (4) | C2A—N3A | 1.322 (3) |
C2B—C1B—O7B—C8B | 175.0 (2) | O7B—C8B—C9B—O11B | 170.3 (2) |
C1B—O7B—C8B—C9B | −179.9 (2) |
D—H···A | D—H | H···A | D···A | D—H···A |
N21A—H21A···O10Bi | 0.88 | 1.95 | 2.795 (3) | 161 |
N21A—H22A···O7Bii | 0.88 | 2.31 | 3.176 (3) | 170 |
N21A—H22A···O10Bii | 0.88 | 2.46 | 2.841 (3) | 106 |
N3A—H3A···O11Bi | 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. |
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.