Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270108023421/dn3091sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108270108023421/dn30911asup2.hkl | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108270108023421/dn30911bsup3.hkl |
CCDC references: 703720; 703721
For related literature, see: Alarcon et al. (2004); Antonov & Stoyanov (1995); Antonov, Stoyanov & Stoyanova (1995); Barrow et al. (2002, 2003); Becker et al. (2001); Birkett et al. (2000); Chippendale et al. (1981, 1999); Diamantis et al. (1992); Gilli et al. (2002, 2005); Gridunova et al. (1991); Guggenberger & Teufer (1975); Harada et al. (1997); Herbst & Hunger (2004); Hihara et al. (2003); Kelemen et al. (1982); Kobelt et al. (1972, 1974); Kuder (1972); Lycka et al. (2000); Machacek et al. (2000); McGeorge et al. (1996, 1998); Mustroph (1987); Olivieri et al. (1989); Paulus (1982); Paulus et al. (1983); Salmen et al. (1988); Traven et al. (1980, 1985); Whitaker (1978, 1995).
For the synthesis of (1a), 4-(dimethylamino)aniline was diazotized with NaNO2 in aqueous HCl and coupled with β-naphthol under basic conditions (Becker et al., 2001). A red powder of (1a) was obtained. IR(KBr): 3049(w), 2966(w), 2925(w), 2894(w), 2866(w), 2358(w), 2341(w), 1990(w), 1890(w), 1683(w), 1652(w), 1596(s), 1558(m), 1515(m), 1506(w), 1467(w), 1402(s), 1375(m), 1352(w), 1344(w), 1313(w), 1272(m), 1245(m), 1193(w), 1164(m), 1153(m), 1135(w), 1080(s), 1010(m), 977(m), 958(w), 918(w), 864(w), 813(s), 783(w), 750(s), 729(w), 676(w), 669(w), 632(w), 565(w), 534(w), 511(w), 459(w), 447(w), 414(w), 1H-NMR (250 MHz, DMSO, 298 K, TMS): δ = 3.09 (s, 6H, H17A—C, H18A—C), 6.90 (dm, 2H, H13, H15, 3J = 9.3 Hz), 7.21 (d, 1H, H7, 3J = 9.25 Hz), 7.47 (ddd, 1H, H3, 3J1 = 8.0 Hz, 3J2 = 6.9 Hz, 3J3 = 1.2 Hz), 7.64 (ddd, 1H, H2, 3J1 = 8.4 Hz, 3J2 = 6.9 Hz, 3J3 = 1.3 Hz), 7.91 (m, 4H, H4, H6, H12, H16), 8.77 (d, 1H, H1A, 3J = 8.61 Hz), 14.9 (s, 1H, H1); the correlations were proven by COSY experiments.
(1a) was dissolved in boiling benzene and cooled to room temperature. The solvent was slowly evaporated at room temperature. Crystals in the form of red needles up to 2 x 0.32 x 0.25 mm3 were obtained.
The synthesis of (1b) was as described for (1a). A red powder of (1b) was obtained. IR(KBr): 3074(w), 3049(w), 2966(m), 2925(w), 2894(w), 2866(w), 2671(w), 2611(w), 2360(w), 1596(s), 1554(w), 1515(m), 1508(w), 1483(w), 1467(m), 1456(w), 1446(w), 1402(s), 1375(s), 1352(m), 1346(w), 1313(m), 1274(s), 1247(s), 1193(m), 1166(s), 1153(m), 1135(w), 1080(s), 1010(s), 977(m), 958(w), 918(w), 864(w), 813(s), 761(w), 750(s), 729(w), 676(w), 632(s), 565(m), 536(w), 520(m), 459(m), 447(w), 414(m); 1H-NMR (250 MHz, DMSO, 298 K, TMS): δ = 1.18 (t, 6H, H18A—C, H20A—C, 3J = 7 Hz), 3.49 (q, 4H, H17A—B, H19A—B, 3J = 6.81 Hz), 6.87 (d, 1H, H13, H15, 3J = 8.92), 7.19 (d, 1H, H7, 3J = 8.7 Hz), 7.47 (ddd, 1H, H3, 3J1 = 8.1 Hz, 3J2 = 6.9 Hz, 3J3 = 1.2 Hz), 7.64 (ddd, 1H, H2, 3J1 = 8.3 Hz, 3J2 = 6.8 Hz, 3J3 = 1.3 Hz), 7.91 (m, 4H, H4, H6, H12, H16), 8.77 (d, 1H, H1,3J = 8.34 Hz), 14.88 (s, 1H, H1O); the correlations were proven by COSY experiments.
(1b) was dissolved in boiling acetone and cooled to room temperature. The solvent was slowly evaporated at room temperature. Crystals in the form of red needles up to 0.2 x 0.4 x 0.2 mm3 were obtained.
For both compounds, all H atoms were located in a difference map, but those bonded to C were refined with fixed individual displacement parameters [U(H) = 1.2 Ueq(C)] or [U(H) = 1.5 Ueq(Cmethyl)] using a riding model with Caromatic—H = 0.95 Å, Cmethyl—H = 0.98 Å and Cmethylene—H = 0.99 Å. The methyl groups in (1a) were allowed to rotate but not to tip. The hydroxyl H atoms were refined isotropically.
For both compounds, data collection: X-AREA (Stoe & Cie, 2001); cell refinement: X-AREA (Stoe & Cie, 2001); data reduction: X-RED (Stoe & Cie, 2001); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: Mercury (Macrae et al., 2006); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008) and publCIF (Westrip, 2008).
C18H17N3O | F(000) = 616 |
Mr = 291.35 | Dx = 1.304 Mg m−3 |
Monoclinic, P21/n | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: -P 2yn | Cell parameters from 12577 reflections |
a = 7.6443 (6) Å | θ = 3.2–28.5° |
b = 8.0127 (6) Å | µ = 0.08 mm−1 |
c = 24.512 (2) Å | T = 173 K |
β = 98.640 (6)° | Rod, black |
V = 1484.4 (2) Å3 | 0.32 × 0.14 × 0.13 mm |
Z = 4 |
Stoe IPDSII two-circle diffractometer | 2762 reflections with I > 2σ(I) |
Radiation source: fine-focus sealed tube | Rint = 0.082 |
Graphite monochromator | θmax = 29.6°, θmin = 3.6° |
ω scans | h = −10→9 |
25765 measured reflections | k = −11→11 |
4153 independent reflections | l = −33→34 |
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)] = 0.058 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.138 | H atoms treated by a mixture of independent and constrained refinement |
S = 1.02 | w = 1/[σ2(Fo2) + (0.0622P)2 + 0.2854P] where P = (Fo2 + 2Fc2)/3 |
4153 reflections | (Δ/σ)max < 0.001 |
205 parameters | Δρmax = 0.28 e Å−3 |
0 restraints | Δρmin = −0.21 e Å−3 |
C18H17N3O | V = 1484.4 (2) Å3 |
Mr = 291.35 | Z = 4 |
Monoclinic, P21/n | Mo Kα radiation |
a = 7.6443 (6) Å | µ = 0.08 mm−1 |
b = 8.0127 (6) Å | T = 173 K |
c = 24.512 (2) Å | 0.32 × 0.14 × 0.13 mm |
β = 98.640 (6)° |
Stoe IPDSII two-circle diffractometer | 2762 reflections with I > 2σ(I) |
25765 measured reflections | Rint = 0.082 |
4153 independent reflections |
R[F2 > 2σ(F2)] = 0.058 | 0 restraints |
wR(F2) = 0.138 | H atoms treated by a mixture of independent and constrained refinement |
S = 1.02 | Δρmax = 0.28 e Å−3 |
4153 reflections | Δρmin = −0.21 e Å−3 |
205 parameters |
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. |
Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger. |
x | y | z | Uiso*/Ueq | ||
O1 | 0.03506 (16) | 0.50773 (19) | 0.56917 (5) | 0.0414 (3) | |
H1 | 0.124 (4) | 0.448 (4) | 0.5507 (12) | 0.088 (9)* | |
N1 | 0.41528 (16) | 0.44165 (16) | 0.59796 (5) | 0.0257 (3) | |
N2 | 0.33170 (16) | 0.39380 (16) | 0.55129 (5) | 0.0269 (3) | |
N3 | 0.68152 (18) | 0.03381 (19) | 0.40367 (5) | 0.0331 (3) | |
C1 | 0.5859 (2) | 0.5583 (2) | 0.70036 (7) | 0.0328 (3) | |
H1A | 0.6513 | 0.4977 | 0.6769 | 0.039* | |
C2 | 0.6697 (2) | 0.6165 (3) | 0.75018 (7) | 0.0432 (4) | |
H2 | 0.7919 | 0.5937 | 0.7611 | 0.052* | |
C3 | 0.5763 (3) | 0.7099 (3) | 0.78547 (8) | 0.0448 (5) | |
H3 | 0.6356 | 0.7502 | 0.8198 | 0.054* | |
C4 | 0.4001 (3) | 0.7418 (2) | 0.76994 (7) | 0.0387 (4) | |
H4 | 0.3383 | 0.8059 | 0.7935 | 0.046* | |
C5 | 0.3078 (2) | 0.6808 (2) | 0.71930 (6) | 0.0303 (3) | |
C6 | 0.1234 (2) | 0.7097 (2) | 0.70287 (7) | 0.0358 (4) | |
H6 | 0.0595 | 0.7718 | 0.7264 | 0.043* | |
C7 | 0.0371 (2) | 0.6501 (2) | 0.65415 (7) | 0.0370 (4) | |
H7 | −0.0864 | 0.6692 | 0.6447 | 0.044* | |
C8 | 0.1285 (2) | 0.5597 (2) | 0.61717 (6) | 0.0305 (3) | |
C9 | 0.31105 (19) | 0.52865 (18) | 0.63135 (6) | 0.0255 (3) | |
C10 | 0.4024 (2) | 0.58772 (19) | 0.68341 (6) | 0.0269 (3) | |
C11 | 0.43021 (19) | 0.30679 (18) | 0.51625 (6) | 0.0252 (3) | |
C12 | 0.6134 (2) | 0.27770 (19) | 0.52718 (6) | 0.0273 (3) | |
H12 | 0.6806 | 0.3190 | 0.5602 | 0.033* | |
C13 | 0.6956 (2) | 0.1888 (2) | 0.48986 (6) | 0.0296 (3) | |
H13 | 0.8196 | 0.1703 | 0.4976 | 0.036* | |
C14 | 0.5992 (2) | 0.12433 (19) | 0.44025 (6) | 0.0274 (3) | |
C15 | 0.4157 (2) | 0.1594 (2) | 0.42935 (6) | 0.0290 (3) | |
H15 | 0.3478 | 0.1209 | 0.3961 | 0.035* | |
C16 | 0.33468 (19) | 0.2492 (2) | 0.46669 (6) | 0.0289 (3) | |
H16 | 0.2116 | 0.2723 | 0.4585 | 0.035* | |
C17 | 0.8651 (2) | −0.0188 (2) | 0.41808 (7) | 0.0376 (4) | |
H17A | 0.9414 | 0.0797 | 0.4243 | 0.056* | |
H17B | 0.8996 | −0.0853 | 0.3879 | 0.056* | |
H17C | 0.8776 | −0.0863 | 0.4518 | 0.056* | |
C18 | 0.5782 (2) | −0.0343 (2) | 0.35379 (7) | 0.0372 (4) | |
H18A | 0.4968 | −0.1195 | 0.3639 | 0.056* | |
H18B | 0.6580 | −0.0847 | 0.3307 | 0.056* | |
H18C | 0.5103 | 0.0554 | 0.3333 | 0.056* |
U11 | U22 | U33 | U12 | U13 | U23 | |
O1 | 0.0268 (6) | 0.0592 (9) | 0.0372 (6) | 0.0114 (6) | 0.0019 (5) | −0.0009 (6) |
N1 | 0.0266 (6) | 0.0241 (6) | 0.0277 (6) | 0.0029 (5) | 0.0078 (5) | 0.0028 (5) |
N2 | 0.0270 (6) | 0.0255 (6) | 0.0289 (6) | 0.0028 (5) | 0.0059 (5) | 0.0028 (5) |
N3 | 0.0321 (7) | 0.0366 (8) | 0.0309 (7) | 0.0068 (6) | 0.0054 (5) | −0.0049 (6) |
C1 | 0.0280 (7) | 0.0393 (9) | 0.0323 (8) | 0.0001 (7) | 0.0088 (6) | −0.0014 (7) |
C2 | 0.0323 (8) | 0.0588 (12) | 0.0381 (9) | −0.0076 (8) | 0.0042 (7) | −0.0051 (8) |
C3 | 0.0471 (10) | 0.0547 (12) | 0.0340 (9) | −0.0162 (9) | 0.0108 (7) | −0.0096 (8) |
C4 | 0.0501 (10) | 0.0347 (9) | 0.0360 (8) | −0.0079 (8) | 0.0216 (7) | −0.0059 (7) |
C5 | 0.0366 (8) | 0.0262 (8) | 0.0314 (7) | −0.0008 (6) | 0.0158 (6) | 0.0018 (6) |
C6 | 0.0384 (9) | 0.0340 (9) | 0.0397 (9) | 0.0090 (7) | 0.0206 (7) | 0.0047 (7) |
C7 | 0.0294 (8) | 0.0444 (10) | 0.0398 (9) | 0.0115 (7) | 0.0140 (7) | 0.0091 (7) |
C8 | 0.0268 (7) | 0.0332 (8) | 0.0326 (8) | 0.0067 (6) | 0.0083 (6) | 0.0064 (6) |
C9 | 0.0255 (7) | 0.0232 (7) | 0.0293 (7) | 0.0022 (6) | 0.0092 (5) | 0.0043 (6) |
C10 | 0.0292 (7) | 0.0242 (7) | 0.0293 (7) | −0.0005 (6) | 0.0112 (6) | 0.0033 (6) |
C11 | 0.0260 (7) | 0.0226 (7) | 0.0276 (7) | 0.0035 (5) | 0.0061 (5) | 0.0028 (5) |
C12 | 0.0263 (7) | 0.0282 (8) | 0.0267 (7) | 0.0028 (6) | 0.0012 (5) | 0.0008 (6) |
C13 | 0.0251 (7) | 0.0315 (8) | 0.0320 (7) | 0.0052 (6) | 0.0036 (6) | 0.0010 (6) |
C14 | 0.0309 (7) | 0.0242 (7) | 0.0278 (7) | 0.0028 (6) | 0.0071 (6) | 0.0030 (5) |
C15 | 0.0280 (7) | 0.0310 (8) | 0.0271 (7) | 0.0016 (6) | 0.0008 (6) | 0.0001 (6) |
C16 | 0.0232 (7) | 0.0309 (8) | 0.0322 (8) | 0.0030 (6) | 0.0031 (6) | 0.0022 (6) |
C17 | 0.0337 (8) | 0.0377 (9) | 0.0434 (9) | 0.0076 (7) | 0.0122 (7) | −0.0029 (7) |
C18 | 0.0461 (9) | 0.0357 (9) | 0.0301 (8) | 0.0033 (8) | 0.0062 (7) | −0.0054 (7) |
O1—C8 | 1.347 (2) | C7—C8 | 1.423 (2) |
O1—H1 | 1.00 (3) | C7—H7 | 0.9500 |
N1—N2 | 1.2823 (18) | C8—C9 | 1.409 (2) |
N1—C9 | 1.4095 (18) | C9—C10 | 1.439 (2) |
N2—C11 | 1.4091 (18) | C11—C16 | 1.398 (2) |
N3—C14 | 1.3766 (19) | C11—C12 | 1.405 (2) |
N3—C17 | 1.457 (2) | C12—C13 | 1.383 (2) |
N3—C18 | 1.458 (2) | C12—H12 | 0.9500 |
C1—C2 | 1.373 (2) | C13—C14 | 1.421 (2) |
C1—C10 | 1.421 (2) | C13—H13 | 0.9500 |
C1—H1A | 0.9500 | C14—C15 | 1.416 (2) |
C2—C3 | 1.416 (3) | C15—C16 | 1.382 (2) |
C2—H2 | 0.9500 | C15—H15 | 0.9500 |
C3—C4 | 1.367 (3) | C16—H16 | 0.9500 |
C3—H3 | 0.9500 | C17—H17A | 0.9800 |
C4—C5 | 1.419 (2) | C17—H17B | 0.9800 |
C4—H4 | 0.9500 | C17—H17C | 0.9800 |
C5—C6 | 1.426 (2) | C18—H18A | 0.9800 |
C5—C10 | 1.430 (2) | C18—H18B | 0.9800 |
C6—C7 | 1.361 (3) | C18—H18C | 0.9800 |
C6—H6 | 0.9500 | ||
C8—O1—H1 | 103.8 (17) | C1—C10—C5 | 118.33 (14) |
N2—N1—C9 | 114.62 (12) | C1—C10—C9 | 122.06 (13) |
N1—N2—C11 | 116.84 (12) | C5—C10—C9 | 119.61 (14) |
C14—N3—C17 | 120.82 (13) | C16—C11—C12 | 119.11 (13) |
C14—N3—C18 | 120.02 (13) | C16—C11—N2 | 115.83 (13) |
C17—N3—C18 | 118.39 (13) | C12—C11—N2 | 125.05 (13) |
C2—C1—C10 | 120.82 (15) | C13—C12—C11 | 119.89 (14) |
C2—C1—H1A | 119.6 | C13—C12—H12 | 120.1 |
C10—C1—H1A | 119.6 | C11—C12—H12 | 120.1 |
C1—C2—C3 | 120.79 (17) | C12—C13—C14 | 121.63 (14) |
C1—C2—H2 | 119.6 | C12—C13—H13 | 119.2 |
C3—C2—H2 | 119.6 | C14—C13—H13 | 119.2 |
C4—C3—C2 | 119.66 (17) | N3—C14—C15 | 121.22 (14) |
C4—C3—H3 | 120.2 | N3—C14—C13 | 121.32 (14) |
C2—C3—H3 | 120.2 | C15—C14—C13 | 117.45 (14) |
C3—C4—C5 | 121.23 (15) | C16—C15—C14 | 120.55 (14) |
C3—C4—H4 | 119.4 | C16—C15—H15 | 119.7 |
C5—C4—H4 | 119.4 | C14—C15—H15 | 119.7 |
C4—C5—C6 | 122.11 (15) | C15—C16—C11 | 121.30 (14) |
C4—C5—C10 | 119.14 (15) | C15—C16—H16 | 119.3 |
C6—C5—C10 | 118.75 (15) | C11—C16—H16 | 119.3 |
C7—C6—C5 | 121.29 (14) | N3—C17—H17A | 109.5 |
C7—C6—H6 | 119.4 | N3—C17—H17B | 109.5 |
C5—C6—H6 | 119.4 | H17A—C17—H17B | 109.5 |
C6—C7—C8 | 121.17 (15) | N3—C17—H17C | 109.5 |
C6—C7—H7 | 119.4 | H17A—C17—H17C | 109.5 |
C8—C7—H7 | 119.4 | H17B—C17—H17C | 109.5 |
O1—C8—C9 | 122.46 (14) | N3—C18—H18A | 109.5 |
O1—C8—C7 | 117.82 (14) | N3—C18—H18B | 109.5 |
C9—C8—C7 | 119.72 (15) | H18A—C18—H18B | 109.5 |
C8—C9—N1 | 124.71 (14) | N3—C18—H18C | 109.5 |
C8—C9—C10 | 119.44 (13) | H18A—C18—H18C | 109.5 |
N1—C9—C10 | 115.85 (13) | H18B—C18—H18C | 109.5 |
C9—N1—N2—C11 | 179.75 (12) | C6—C5—C10—C9 | 1.3 (2) |
C10—C1—C2—C3 | −1.2 (3) | C8—C9—C10—C1 | 179.12 (14) |
C1—C2—C3—C4 | 0.4 (3) | N1—C9—C10—C1 | −0.7 (2) |
C2—C3—C4—C5 | 1.0 (3) | C8—C9—C10—C5 | −1.7 (2) |
C3—C4—C5—C6 | 178.69 (17) | N1—C9—C10—C5 | 178.48 (13) |
C3—C4—C5—C10 | −1.5 (3) | N1—N2—C11—C16 | 177.19 (13) |
C4—C5—C6—C7 | −179.95 (16) | N1—N2—C11—C12 | −4.0 (2) |
C10—C5—C6—C7 | 0.3 (2) | C16—C11—C12—C13 | −1.9 (2) |
C5—C6—C7—C8 | −1.4 (3) | N2—C11—C12—C13 | 179.27 (14) |
C6—C7—C8—O1 | −178.57 (16) | C11—C12—C13—C14 | −0.3 (2) |
C6—C7—C8—C9 | 1.0 (3) | C17—N3—C14—C15 | −172.56 (15) |
O1—C8—C9—N1 | −0.1 (2) | C18—N3—C14—C15 | −2.8 (2) |
C7—C8—C9—N1 | −179.59 (15) | C17—N3—C14—C13 | 8.6 (2) |
O1—C8—C9—C10 | −179.92 (15) | C18—N3—C14—C13 | 178.43 (15) |
C7—C8—C9—C10 | 0.6 (2) | C12—C13—C14—N3 | −179.03 (15) |
N2—N1—C9—C8 | 1.7 (2) | C12—C13—C14—C15 | 2.1 (2) |
N2—N1—C9—C10 | −178.44 (13) | N3—C14—C15—C16 | 179.44 (15) |
C2—C1—C10—C5 | 0.6 (2) | C13—C14—C15—C16 | −1.7 (2) |
C2—C1—C10—C9 | 179.84 (16) | C14—C15—C16—C11 | −0.5 (2) |
C4—C5—C10—C1 | 0.7 (2) | C12—C11—C16—C15 | 2.3 (2) |
C6—C5—C10—C1 | −179.48 (15) | N2—C11—C16—C15 | −178.74 (14) |
C4—C5—C10—C9 | −178.54 (15) |
D—H···A | D—H | H···A | D···A | D—H···A |
O1—H1···N2 | 1.00 (3) | 1.64 (3) | 2.5429 (17) | 148 (3) |
C20H21N3O | F(000) = 1360 |
Mr = 319.40 | Dx = 1.260 Mg m−3 |
Monoclinic, C2/c | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: -C 2yc | Cell parameters from 6089 reflections |
a = 24.813 (3) Å | θ = 3.6–25.7° |
b = 10.7144 (10) Å | µ = 0.08 mm−1 |
c = 13.8806 (19) Å | T = 173 K |
β = 114.150 (9)° | Rod, black |
V = 3367.3 (7) Å3 | 0.24 × 0.13 × 0.12 mm |
Z = 8 |
Stoe IPDSII two-circle diffractometer | 2205 reflections with I > 2σ(I) |
Radiation source: fine-focus sealed tube | Rint = 0.048 |
Graphite monochromator | θmax = 25.6°, θmin = 3.5° |
ω scans | h = −30→30 |
8777 measured reflections | k = −11→13 |
3140 independent reflections | l = −16→14 |
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)] = 0.045 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.113 | H atoms treated by a mixture of independent and constrained refinement |
S = 0.97 | w = 1/[σ2(Fo2) + (0.0649P)2] where P = (Fo2 + 2Fc2)/3 |
3140 reflections | (Δ/σ)max < 0.001 |
221 parameters | Δρmax = 0.12 e Å−3 |
0 restraints | Δρmin = −0.19 e Å−3 |
C20H21N3O | V = 3367.3 (7) Å3 |
Mr = 319.40 | Z = 8 |
Monoclinic, C2/c | Mo Kα radiation |
a = 24.813 (3) Å | µ = 0.08 mm−1 |
b = 10.7144 (10) Å | T = 173 K |
c = 13.8806 (19) Å | 0.24 × 0.13 × 0.12 mm |
β = 114.150 (9)° |
Stoe IPDSII two-circle diffractometer | 2205 reflections with I > 2σ(I) |
8777 measured reflections | Rint = 0.048 |
3140 independent reflections |
R[F2 > 2σ(F2)] = 0.045 | 0 restraints |
wR(F2) = 0.113 | H atoms treated by a mixture of independent and constrained refinement |
S = 0.97 | Δρmax = 0.12 e Å−3 |
3140 reflections | Δρmin = −0.19 e Å−3 |
221 parameters |
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. |
Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger. |
x | y | z | Uiso*/Ueq | ||
O1 | 0.59046 (6) | 0.42443 (13) | 0.81624 (11) | 0.0422 (3) | |
H1O | 0.5664 (11) | 0.479 (3) | 0.755 (2) | 0.081 (8)* | |
N1 | 0.60552 (5) | 0.52327 (12) | 0.63180 (10) | 0.0256 (3) | |
N2 | 0.55964 (5) | 0.55367 (12) | 0.64777 (11) | 0.0262 (3) | |
N3 | 0.39342 (5) | 0.88487 (12) | 0.36783 (11) | 0.0270 (3) | |
C1 | 0.70163 (7) | 0.43599 (17) | 0.59303 (15) | 0.0352 (4) | |
H1 | 0.6742 | 0.4918 | 0.5441 | 0.042* | |
C2 | 0.74883 (8) | 0.39222 (19) | 0.57493 (18) | 0.0476 (5) | |
H2 | 0.7538 | 0.4185 | 0.5137 | 0.057* | |
C3 | 0.78990 (8) | 0.3090 (2) | 0.6460 (2) | 0.0534 (6) | |
H3 | 0.8221 | 0.2793 | 0.6322 | 0.064* | |
C4 | 0.78341 (7) | 0.27104 (18) | 0.73458 (19) | 0.0483 (6) | |
H4 | 0.8115 | 0.2154 | 0.7823 | 0.058* | |
C5 | 0.73529 (7) | 0.31355 (15) | 0.75651 (15) | 0.0350 (4) | |
C6 | 0.72643 (8) | 0.27174 (17) | 0.84611 (15) | 0.0421 (5) | |
H6 | 0.7540 | 0.2155 | 0.8941 | 0.051* | |
C7 | 0.67930 (8) | 0.31047 (16) | 0.86473 (14) | 0.0390 (4) | |
H7 | 0.6746 | 0.2815 | 0.9255 | 0.047* | |
C8 | 0.63725 (7) | 0.39342 (16) | 0.79446 (13) | 0.0311 (4) | |
C9 | 0.64416 (6) | 0.43945 (14) | 0.70563 (13) | 0.0261 (3) | |
C10 | 0.69364 (6) | 0.39808 (15) | 0.68467 (14) | 0.0290 (4) | |
C11 | 0.51996 (6) | 0.63610 (14) | 0.57426 (13) | 0.0244 (3) | |
C12 | 0.52580 (6) | 0.68268 (14) | 0.48452 (13) | 0.0262 (4) | |
H12 | 0.5583 | 0.6571 | 0.4697 | 0.031* | |
C13 | 0.48492 (7) | 0.76516 (15) | 0.41759 (13) | 0.0277 (4) | |
H13 | 0.4902 | 0.7965 | 0.3580 | 0.033* | |
C14 | 0.43488 (6) | 0.80471 (14) | 0.43549 (12) | 0.0235 (3) | |
C15 | 0.42941 (6) | 0.75475 (15) | 0.52571 (13) | 0.0260 (4) | |
H15 | 0.3968 | 0.7785 | 0.5407 | 0.031* | |
C16 | 0.47055 (6) | 0.67245 (14) | 0.59190 (12) | 0.0259 (4) | |
H16 | 0.4653 | 0.6395 | 0.6510 | 0.031* | |
C17 | 0.40040 (7) | 0.94022 (16) | 0.27703 (13) | 0.0305 (4) | |
H17A | 0.3610 | 0.9646 | 0.2237 | 0.037* | |
H17B | 0.4166 | 0.8762 | 0.2447 | 0.037* | |
C18 | 0.44072 (8) | 1.05429 (18) | 0.30459 (16) | 0.0423 (5) | |
H18A | 0.4437 | 1.0858 | 0.2406 | 0.063* | |
H18B | 0.4801 | 1.0309 | 0.3565 | 0.063* | |
H18C | 0.4243 | 1.1194 | 0.3343 | 0.063* | |
C19 | 0.34556 (6) | 0.93391 (15) | 0.39363 (13) | 0.0292 (4) | |
H19A | 0.3281 | 0.8639 | 0.4176 | 0.035* | |
H19B | 0.3143 | 0.9684 | 0.3286 | 0.035* | |
C20 | 0.36477 (7) | 1.03509 (16) | 0.47845 (15) | 0.0362 (4) | |
H20A | 0.3307 | 1.0623 | 0.4916 | 0.054* | |
H20B | 0.3808 | 1.1063 | 0.4545 | 0.054* | |
H20C | 0.3952 | 1.0016 | 0.5438 | 0.054* |
U11 | U22 | U33 | U12 | U13 | U23 | |
O1 | 0.0520 (7) | 0.0472 (8) | 0.0341 (7) | 0.0103 (6) | 0.0245 (6) | 0.0078 (7) |
N1 | 0.0262 (6) | 0.0219 (7) | 0.0262 (7) | −0.0013 (5) | 0.0080 (5) | −0.0023 (6) |
N2 | 0.0274 (6) | 0.0231 (7) | 0.0274 (7) | 0.0004 (5) | 0.0106 (6) | −0.0016 (6) |
N3 | 0.0256 (6) | 0.0265 (7) | 0.0270 (7) | 0.0019 (5) | 0.0089 (5) | 0.0028 (6) |
C1 | 0.0318 (8) | 0.0325 (9) | 0.0432 (11) | −0.0068 (7) | 0.0172 (8) | −0.0072 (8) |
C2 | 0.0377 (9) | 0.0476 (12) | 0.0649 (14) | −0.0142 (9) | 0.0284 (10) | −0.0184 (10) |
C3 | 0.0283 (9) | 0.0490 (12) | 0.0846 (18) | −0.0049 (8) | 0.0246 (10) | −0.0276 (12) |
C4 | 0.0259 (8) | 0.0351 (11) | 0.0696 (15) | 0.0011 (7) | 0.0048 (9) | −0.0175 (11) |
C5 | 0.0257 (8) | 0.0248 (9) | 0.0421 (11) | −0.0008 (7) | 0.0012 (7) | −0.0097 (8) |
C6 | 0.0396 (9) | 0.0276 (9) | 0.0375 (11) | 0.0024 (7) | −0.0062 (8) | −0.0009 (8) |
C7 | 0.0487 (10) | 0.0325 (10) | 0.0250 (9) | −0.0001 (8) | 0.0042 (8) | 0.0022 (8) |
C8 | 0.0370 (8) | 0.0270 (9) | 0.0248 (9) | −0.0013 (7) | 0.0082 (7) | −0.0036 (7) |
C9 | 0.0267 (7) | 0.0204 (8) | 0.0254 (8) | −0.0008 (6) | 0.0048 (6) | −0.0026 (7) |
C10 | 0.0249 (7) | 0.0220 (8) | 0.0345 (10) | −0.0046 (6) | 0.0065 (7) | −0.0073 (7) |
C11 | 0.0251 (7) | 0.0196 (8) | 0.0256 (8) | −0.0015 (6) | 0.0074 (6) | −0.0010 (7) |
C12 | 0.0264 (7) | 0.0254 (8) | 0.0290 (9) | 0.0010 (6) | 0.0135 (7) | −0.0018 (7) |
C13 | 0.0317 (8) | 0.0298 (9) | 0.0243 (9) | 0.0001 (6) | 0.0142 (7) | 0.0014 (7) |
C14 | 0.0230 (7) | 0.0202 (8) | 0.0240 (8) | −0.0030 (6) | 0.0062 (6) | −0.0029 (7) |
C15 | 0.0231 (7) | 0.0264 (8) | 0.0302 (9) | −0.0010 (6) | 0.0127 (7) | −0.0009 (7) |
C16 | 0.0294 (7) | 0.0259 (8) | 0.0244 (8) | −0.0039 (6) | 0.0129 (7) | 0.0002 (7) |
C17 | 0.0312 (8) | 0.0308 (9) | 0.0246 (9) | 0.0018 (7) | 0.0065 (7) | 0.0041 (7) |
C18 | 0.0450 (9) | 0.0422 (11) | 0.0410 (11) | −0.0108 (8) | 0.0190 (8) | 0.0015 (9) |
C19 | 0.0224 (7) | 0.0286 (9) | 0.0333 (9) | 0.0021 (6) | 0.0080 (7) | 0.0009 (7) |
C20 | 0.0363 (8) | 0.0306 (9) | 0.0423 (11) | −0.0004 (7) | 0.0167 (8) | −0.0055 (8) |
O1—C8 | 1.355 (2) | C9—C10 | 1.442 (2) |
O1—H1O | 1.00 (3) | C11—C16 | 1.400 (2) |
N1—N2 | 1.2876 (16) | C11—C12 | 1.403 (2) |
N1—C9 | 1.404 (2) | C12—C13 | 1.379 (2) |
N2—C11 | 1.403 (2) | C12—H12 | 0.9500 |
N3—C14 | 1.374 (2) | C13—C14 | 1.427 (2) |
N3—C17 | 1.466 (2) | C13—H13 | 0.9500 |
N3—C19 | 1.4706 (19) | C14—C15 | 1.419 (2) |
C1—C2 | 1.377 (2) | C15—C16 | 1.377 (2) |
C1—C10 | 1.424 (2) | C15—H15 | 0.9500 |
C1—H1 | 0.9500 | C16—H16 | 0.9500 |
C2—C3 | 1.408 (3) | C17—C18 | 1.526 (2) |
C2—H2 | 0.9500 | C17—H17A | 0.9900 |
C3—C4 | 1.366 (3) | C17—H17B | 0.9900 |
C3—H3 | 0.9500 | C18—H18A | 0.9800 |
C4—C5 | 1.422 (3) | C18—H18B | 0.9800 |
C4—H4 | 0.9500 | C18—H18C | 0.9800 |
C5—C6 | 1.421 (3) | C19—C20 | 1.526 (2) |
C5—C10 | 1.428 (2) | C19—H19A | 0.9900 |
C6—C7 | 1.362 (3) | C19—H19B | 0.9900 |
C6—H6 | 0.9500 | C20—H20A | 0.9800 |
C7—C8 | 1.413 (2) | C20—H20B | 0.9800 |
C7—H7 | 0.9500 | C20—H20C | 0.9800 |
C8—C9 | 1.402 (2) | ||
C8—O1—H1O | 103.0 (14) | C13—C12—C11 | 120.74 (14) |
N2—N1—C9 | 115.08 (13) | C13—C12—H12 | 119.6 |
N1—N2—C11 | 116.16 (13) | C11—C12—H12 | 119.6 |
C14—N3—C17 | 121.45 (12) | C12—C13—C14 | 121.71 (15) |
C14—N3—C19 | 120.69 (13) | C12—C13—H13 | 119.1 |
C17—N3—C19 | 117.13 (13) | C14—C13—H13 | 119.1 |
C2—C1—C10 | 120.36 (18) | N3—C14—C15 | 121.60 (13) |
C2—C1—H1 | 119.8 | N3—C14—C13 | 121.89 (14) |
C10—C1—H1 | 119.8 | C15—C14—C13 | 116.50 (14) |
C1—C2—C3 | 120.9 (2) | C16—C15—C14 | 121.20 (14) |
C1—C2—H2 | 119.5 | C16—C15—H15 | 119.4 |
C3—C2—H2 | 119.5 | C14—C15—H15 | 119.4 |
C4—C3—C2 | 120.04 (17) | C15—C16—C11 | 121.64 (14) |
C4—C3—H3 | 120.0 | C15—C16—H16 | 119.2 |
C2—C3—H3 | 120.0 | C11—C16—H16 | 119.2 |
C3—C4—C5 | 121.04 (19) | N3—C17—C18 | 113.86 (14) |
C3—C4—H4 | 119.5 | N3—C17—H17A | 108.8 |
C5—C4—H4 | 119.5 | C18—C17—H17A | 108.8 |
C6—C5—C4 | 122.02 (17) | N3—C17—H17B | 108.8 |
C6—C5—C10 | 118.95 (15) | C18—C17—H17B | 108.8 |
C4—C5—C10 | 119.00 (19) | H17A—C17—H17B | 107.7 |
C7—C6—C5 | 121.50 (16) | C17—C18—H18A | 109.5 |
C7—C6—H6 | 119.2 | C17—C18—H18B | 109.5 |
C5—C6—H6 | 119.2 | H18A—C18—H18B | 109.5 |
C6—C7—C8 | 120.63 (17) | C17—C18—H18C | 109.5 |
C6—C7—H7 | 119.7 | H18A—C18—H18C | 109.5 |
C8—C7—H7 | 119.7 | H18B—C18—H18C | 109.5 |
O1—C8—C9 | 122.28 (15) | N3—C19—C20 | 114.40 (12) |
O1—C8—C7 | 117.26 (16) | N3—C19—H19A | 108.7 |
C9—C8—C7 | 120.46 (15) | C20—C19—H19A | 108.7 |
C8—C9—N1 | 125.04 (14) | N3—C19—H19B | 108.7 |
C8—C9—C10 | 119.31 (14) | C20—C19—H19B | 108.7 |
N1—C9—C10 | 115.64 (14) | H19A—C19—H19B | 107.6 |
C1—C10—C5 | 118.62 (15) | C19—C20—H20A | 109.5 |
C1—C10—C9 | 122.24 (15) | C19—C20—H20B | 109.5 |
C5—C10—C9 | 119.12 (16) | H20A—C20—H20B | 109.5 |
C16—C11—N2 | 116.88 (14) | C19—C20—H20C | 109.5 |
C16—C11—C12 | 118.17 (14) | H20A—C20—H20C | 109.5 |
N2—C11—C12 | 124.94 (13) | H20B—C20—H20C | 109.5 |
C9—N1—N2—C11 | −179.21 (12) | N1—C9—C10—C1 | −2.3 (2) |
C10—C1—C2—C3 | 0.3 (3) | C8—C9—C10—C5 | −1.6 (2) |
C1—C2—C3—C4 | −0.3 (3) | N1—C9—C10—C5 | 179.47 (13) |
C2—C3—C4—C5 | 0.4 (3) | N1—N2—C11—C16 | −178.51 (13) |
C3—C4—C5—C6 | 177.63 (18) | N1—N2—C11—C12 | 2.5 (2) |
C3—C4—C5—C10 | −0.5 (3) | C16—C11—C12—C13 | 2.1 (2) |
C4—C5—C6—C7 | −178.18 (17) | N2—C11—C12—C13 | −178.92 (14) |
C10—C5—C6—C7 | 0.0 (3) | C11—C12—C13—C14 | −1.1 (2) |
C5—C6—C7—C8 | 0.4 (3) | C17—N3—C14—C15 | 177.54 (14) |
C6—C7—C8—O1 | 177.78 (16) | C19—N3—C14—C15 | 7.6 (2) |
C6—C7—C8—C9 | −1.5 (3) | C17—N3—C14—C13 | −3.9 (2) |
O1—C8—C9—N1 | 1.7 (3) | C19—N3—C14—C13 | −173.86 (14) |
C7—C8—C9—N1 | −179.13 (14) | C12—C13—C14—N3 | −178.64 (14) |
O1—C8—C9—C10 | −177.19 (14) | C12—C13—C14—C15 | 0.0 (2) |
C7—C8—C9—C10 | 2.0 (2) | N3—C14—C15—C16 | 178.64 (14) |
N2—N1—C9—C8 | −2.9 (2) | C13—C14—C15—C16 | 0.0 (2) |
N2—N1—C9—C10 | 175.97 (13) | C14—C15—C16—C11 | 1.1 (2) |
C2—C1—C10—C5 | −0.4 (2) | N2—C11—C16—C15 | 178.82 (14) |
C2—C1—C10—C9 | −178.64 (16) | C12—C11—C16—C15 | −2.1 (2) |
C6—C5—C10—C1 | −177.73 (15) | C14—N3—C17—C18 | −81.70 (19) |
C4—C5—C10—C1 | 0.5 (2) | C19—N3—C17—C18 | 88.61 (17) |
C6—C5—C10—C9 | 0.6 (2) | C14—N3—C19—C20 | 74.87 (19) |
C4—C5—C10—C9 | 178.80 (15) | C17—N3—C19—C20 | −95.52 (17) |
C8—C9—C10—C1 | 176.66 (15) |
D—H···A | D—H | H···A | D···A | D—H···A |
O1—H1O···N2 | 1.00 (3) | 1.64 (3) | 2.5510 (19) | 149 (2) |
Experimental details
(1a) | (1b) | |
Crystal data | ||
Chemical formula | C18H17N3O | C20H21N3O |
Mr | 291.35 | 319.40 |
Crystal system, space group | Monoclinic, P21/n | Monoclinic, C2/c |
Temperature (K) | 173 | 173 |
a, b, c (Å) | 7.6443 (6), 8.0127 (6), 24.512 (2) | 24.813 (3), 10.7144 (10), 13.8806 (19) |
β (°) | 98.640 (6) | 114.150 (9) |
V (Å3) | 1484.4 (2) | 3367.3 (7) |
Z | 4 | 8 |
Radiation type | Mo Kα | Mo Kα |
µ (mm−1) | 0.08 | 0.08 |
Crystal size (mm) | 0.32 × 0.14 × 0.13 | 0.24 × 0.13 × 0.12 |
Data collection | ||
Diffractometer | Stoe IPDSII two-circle diffractometer | Stoe IPDSII two-circle diffractometer |
Absorption correction | – | – |
No. of measured, independent and observed [I > 2σ(I)] reflections | 25765, 4153, 2762 | 8777, 3140, 2205 |
Rint | 0.082 | 0.048 |
(sin θ/λ)max (Å−1) | 0.695 | 0.608 |
Refinement | ||
R[F2 > 2σ(F2)], wR(F2), S | 0.058, 0.138, 1.02 | 0.045, 0.113, 0.97 |
No. of reflections | 4153 | 3140 |
No. of parameters | 205 | 221 |
H-atom treatment | H atoms treated by a mixture of independent and constrained refinement | H atoms treated by a mixture of independent and constrained refinement |
Δρmax, Δρmin (e Å−3) | 0.28, −0.21 | 0.12, −0.19 |
Computer programs: X-AREA (Stoe & Cie, 2001), X-RED (Stoe & Cie, 2001), SHELXS97 (Sheldrick, 2008), Mercury (Macrae et al., 2006), SHELXL97 (Sheldrick, 2008) and publCIF (Westrip, 2008).
D—H···A | D—H | H···A | D···A | D—H···A |
O1—H1···N2 | 1.00 (3) | 1.64 (3) | 2.5429 (17) | 148 (3) |
D—H···A | D—H | H···A | D···A | D—H···A |
O1—H1O···N2 | 1.00 (3) | 1.64 (3) | 2.5510 (19) | 149 (2) |
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Azo pigments are widely used for the colouration of coatings, plastics and printing inks, with an annual sales volume of more than one billion Euros (Herbst & Hunger, 2004). In the literature, most azo pigments are drawn with an N═N double bond (1-azo to 4-azo, see Fig. 1). However, all commercial pigments based on β-naphthol adopt the hydrazone tautomeric form in the solid state (1-hydrazone). The same holds true for pigments with naphthol-AS (2), pyrazolone (3) and acetoacetylanilide groups (4), as proven by many X-ray structure determinations of β-naphthol pigments (Guggenberger & Teufer, 1975; Whitaker et al., 1978; Salmen et al., 1988; Olivieri et al., 1989; Diamantis et al., 1992, Gilli et al., 2002), naphthol-AS pigments (Kobelt et al., 1972, 1974; Paulus et al., 1982), pyrazolone pigments (Whitaker, 1995) and acetoacetanilide pigments (Paulus et al., 1983; Gridunova et al., 1991; Barrow et al., 2002, 2003). Consequently, all these pigments should not be called 'azo pigments' but 'hydrazone pigments'.
The preference for the hydrazone tautomeric form can be explained by the fact that generally a C═N double bond is energetically more stable than an N═ N double bond. In the case of β-naphthol and naphthol-AS pigments, the formation of the hydrazone tautomer leads to a partial loss of aromaticity in the naphthalene moiety; but this does not have a major impact on the energy. For acetoacetylanilide (4) the hydrazone form is additionally stabilized by the enhanced conjugation of the π-systems; furthermore, planar molecules can generally be packed more densely in the crystalline state, leading to higher lattice energies.
In solution, azo and hydrazone tautomers exist in an equilibrium depending on temperature, solvent and substitution pattern. The equilibrium in solution has been studied by UV–visible (Traven et al., 1980; Antonov et al., 1995; Antonov & Stoyanov, 1995) and NMR spectroscopy (Chippendale et al., 1999; Lycka et al., 2000; Birkett et al., 2000; Machacek et al., 2000; Alarcon et al., 2004). Solid-state NMR investigations have also been carried out (Chippendale et al.,1981; Olivieri et al.,1989; McGeorge et al., 1996, 1998). Quantum mechanical calculations have been performed (Kuder,1972; Gilli et al., 2005; Hihara et al., 2003; Alarcon et al., 2004). A review with 100 references is given by Mustroph (1987).
The crystal structure of 1-(4-(diethylamino)phenylazo)-2-naphthol (1b) has been determined by single-crystal X-ray analysis (Traven et al., 1985). From the N—N, C—N and C—O bond lengths, they concluded that the compound exists 'in a form intermediate between the boundary structures' of the azo and hydrazone tautomeric forms. The X-ray structure determination of the dimethylamino derivative (1a) in the range 100–295 K has been performed (Gilli et al., 2005), showing that the proton of the hydroxyl group is disordered over two positions with a prevalence of the azo tautomer. The occupancies of the proton in the O—H versus N—H forms were determined to be 79 (1):21 (1).
The compounds (1a) and (1b) are not commercially used, because their pigmentary properties, especially their considerable solubilities, do not fit the requirements for an industrial organic pigment.
We redetermined the crystal structures of (1a) and (1b) by single-crystal X-ray analysis and found that both compounds adopt the azo tautomeric form (see Fig. 2); however, in the difference electron-density plots there is still a tiny electron density at the positions of the N—H protons indicating a very small amount of the hydrazone tautomer (see Fig. 3). In both compounds, an intramolecular N···H—O (or N—H···O, respectively) hydrogen bond is formed.
The crystal structures of (1a) and (1b) are not isotypic. Compound (1a) crystallizes in P21/n with four molecules per unit cell. The molecules are essentially planar. They are packed in columns in the [010] direction; within the columns neighbouring molecules are stacked antiparallel and linked by inversion centres (see Fig. 4a, in the Supplementary material). Each column is connected to two opposite neighbouring columns by van der Waals interactions, resulting in a sheet in the (001) direction. The sheets themselves are also arranged antiparallel, resulting in the molecules forming a herringbone pattern. In compound (1b) the molecules are planar except for the methyl groups of the diethylamino fragment which stick out of the plane by nearly 90°. The unit cell of (1b) contains eight molecules. The characteristic features are pairs of antiparallel molecules with the ethyl groups pointing outwards (see Fig. 4b, in the Supplementary material). Between the molecules there is an inversion centre. These pairs of molecules form columns in the [001] direction, with neighbouring pairs being rotated by about 90°.
Kelemen et al. (1982) examined more than 30 compounds in an investigation of azo–hydrazone tautomerism and postulated that in a real azo compound the N═N double bond should have a length of 1.20–1.28 Å. Furthermore, the bond length of N—N single bonds as in hydrazone tautomers should be more than 1.4 Å. According to Harada et al. (1997), in non-disordered azobenzenes the N═N double bond has a length of 1.26–1.27 Å. For compounds (1a) and (1b) we found values of 1.282 (2) and 1.288 (2) Å, respectively, which are slightly larger than the value for an N═N double bond. For the C—N bond, a similar effect is observed. This is another indication that the compounds (1a) and (1b) are mainly, but not exclusively, in the azo tautomeric form, i.e. they are close to being 'real' azo pigments.