Non-mutagenic organic pigment intermediates. I. 3,3'-Dipropoxybenzidine

Crystals of the title compound, C₁₈H₂₄N₂O₂, were grown from ethanol by slow evaporation and the structure has been determined. The mol­ecule resides on a crystallographic inversion center and the bi­phenyl moiety is essentially planar. The structure forms an infinite two-dimensional array of N-H...[pi](arene) interactions parallel to the (101) direction.

S1. Comment

Colorants prepared from certain congeners of benzidine are manufactured on a large scale and are of significant commercial importance worldwide. Unfortunately, these benzidine congeners are genotoxic, and there are occupational and environmental risks associated with their synthesis and use (Freeman & Hinks, 1996). Hence, the development of new non-genotoxic analogs of benzidine-type intermediates of satisfactory technical performance is desirable. Recently, the design, synthesis and genotoxicity of non-mutagenic benzidine congeners (Hunger et al., 1986; Hinks & Freeman, 2000), and their conversion to bisazomethine (Hinks & Freeman, 2001) and various bisazo pigments (Sokolowska & Nakpathom, 2001) have been reported. Efforts have focused toward the development of benzidine-type diamine intermediates that are not only non-mutagenic, but also provide means of manipulating electronic absorption properties when converted to pigment.

A key factor in establishing structure-activity relationships of pigments and intermediates is the molecular geometry of the pure compound. Therefore, as part of the study outlined above, we report here the crystal and molecular structure of 3,3'-dipropoxybenzidine, (I). \sch

An view of the molecule of (I) with the atom-labelling scheme is shown in Fig. 1. The structure contains two molecules in the unit cell. The molecules lie across a crystallographic inversion center which sits at the midpoint of the C1—C1ⁱ biphenyl bond [symmetry code: (i) 1 − x, −y, 1 − z]. The molecule is essentially planar, as indicated by the C2—C1—C1ⁱ—C6ⁱ torsion angle of 0.24 (15)°. There is no pronounced anisotropy in the aryl anisotropic displacement parameters, indicating that there is no disorder or dynamic twisting process to accommodate the steric crowding of the ortho H atoms of the biphenyl moiety.

The structure of (I) contains one N—H···O intramolecular hydrogen bond and no conventional intermolecular hydrogen bonds. The intramolecular hydrogen bond is formed between the N-bound H1B atom and the O atom of the n-propoxy group. The metrics for this interaction are N—H1B 0.887 (19) Å, H1B···O 2.287 (19) Å and N—H1B···O 105.2 (14)°.

Primary amino groups exhibit double-donor single-acceptor hydrogen-bond functionality. When a structure containing primary amino groups does not have sufficient conventional hydrogen-bond acceptor sites, there is a tendency to form N—H···π hydrogen bonds (Hanton et al., 1992). The structure of (I) exhibits such a deficiency and forms a network of N—H···π(arene) hydrogen bonds. After normalizing the N—H bond lengths to 1.01 Å, the metrics for this interaction are H1A···π(arene)_cen 2.416 Å, N···π(arene)_cen 3.414 Å and N—H···π(arene)_cen 169.43°. The distance of atom H1A from the aryl plane is 2.39 Å. The amino `edge' of one molecule points at the aryl `face' of an adjacent molecule to form the N—H···π(arene) interaction. The H···π(arene) vector forms an angle of 81.59° with the acceptor aryl plane. These N—H···π(arene) hydrogen bonds form an infinite two-dimensional array of macrocyclic aggregates parallel to the (101) set of planes (Fig. 2). The aryl groups involved in the hydrogen bonding act as single hydrogen-bond acceptors. Considering the centroid to be one `ring-equivalent' atom, the Etter (1990) graph-set designation for the macrocyclic system is R₁¹(20).

In the light of these N—H···π(arene) interactions, it is interesting to note that the geometry around the amino N atom in (I), although somewhat pyramidal, is close to sp² hybridization. As pointed out by Hanton et al. (1992), the sp² hybridization of the N atom makes both the amino group a better hydrogen donor and the aryl group a better hydrogen acceptor, because the delocalization of the N-atom lone pair into the aromatic ring system makes it more electron rich. Thus, the structure seems to be optimized to maximize N—H···π(arene) interactions.

S2. Experimental

Compound (I) was prepared as previously reported by Hinks et al. (2000). Suitable single crystals were obtained by dissolving (I) (0.4 g) in ethanol (40 ml), stirring at the boiling point for 2 min, filtering while hot into an Erlenmeyer flask and covering with perforated Parafilm, and then allowing the filtrate to cool slowly and stand undisturbed for 5 d. The solution was checked after 3 d to monitor crystal growth. Appropriate single crystals of (I) were selected for X-ray analysis by examining them under a microscope.

S3. Refinement

H-atom positions were derived from difference Fourier maps, and the H-atom positional and isotropic displacement parameters were included in the refinement. The calculated structure factors included corrections for anomalous dispersion from the usual tabulation (International Tables for X-ray Crystallography, 1974, Vol. IV, Table 2.3.1). An extinction refinement was attempted, but the coefficient refined to within one s.u. of 0.0, and was subsequently removed from the final refinement model.

Computing details

Data collection: CAD-4 ARGUS (Enraf-Nonius, 1994); cell refinement: CAD-4 ARGUS; data reduction: DATRD2 in NRCVAX (Gabe et al., 1989); program(s) used to solve structure: SIR92 (Altomare et al., 1994); program(s) used to refine structure: LSTSQ in NRCVAX; molecular graphics: ORTEPII (Johnson, 1976) in NRCVAX; software used to prepare material for publication: TABLES in NRCVAX, version of January 1994.

Figures

	Fig. 1. A view of the molecule of (I), showing the atom-labeling scheme. Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii.
	Fig. 2. A view of the extended two-dimensional network of N—H···π(arene) hydrogen bonds in (I). The view is approximately down the a axis, with c running vertically and b horizontally to the right. Displacement ellipsoids are shown at the 50% probability level.

3,3'-Dipropoxybenzidine

Crystal data

C18H24N2O2	? #Insert any comments here.
Mr = 300.40	Dx = 1.239 Mg m^−3
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
a = 9.2522 (6) Å	Cell parameters from 24 reflections
b = 6.4359 (3) Å	θ = 16.0–18.0°
c = 14.0080 (11) Å	µ = 0.08 mm^−1
β = 105.175 (9)°	T = 148 K
V = 805.04 (9) Å^3	Prism, light yellow
Z = 2	0.40 × 0.30 × 0.16 mm
F(000) = 324.19

Data collection

Enraf-Nonius CAD-4 diffractometer	Rint = 0.018
Radiation source: xray tube	θmax = 30.0°, θmin = 1.2°
Graphite monochromator	h = −12→12
ω scan b/P/b	k = 0→9
2322 measured reflections	l = 0→19
2319 independent reflections	3 standard reflections every 80 min
1934 reflections with Inet > 1σ(Inet)	intensity decay: 2.1%

Refinement

Refinement on F	149 parameters
Least-squares matrix: full	All H-atom parameters refined
R[F^2 > 2σ(F^2)] = 0.039	Weighting scheme based on measured s.u.'s w = 1/(σ^2(F) + 0.0002F^2)
wR(F^2) = 0.051	(Δ/σ)max < 0.001
S = 2.15	Δρmax = 0.37 e Å^−3
1928 reflections	Δρmin = −0.21 e Å^−3

Special details

Experimental. The sample was mounted on the end of a glass fiber using a small amount of silicone grease and transferred to the diffractometer. The sample was maintained at a temperature of 148 K using a nitrogen cold stream. All X-ray measurements were made on an Enraf–Nonius CAD-4 MACH diffractometer. The unit-cell dimensions were determined by a fit of 24 well centered reflections and their Friedel pairs, with an angular range of 32 < 2θ < 36°. A quadrant of unique data was collected using the ω scan mode in a non-bisecting geometry. The adoption of a non-bisecting scan mode was accomplished by offsetting ψ by 20° for each data point collected. This was done to minimize the interaction of the goniometer head with the cold stream. Three standard reflections were measured every 4800 s of X-ray exposure time. Scaling the data was accomplished using a five-point smoothed curved routine fit to the intensity check reflections. The intensity data were corrected for Lorentz and polarization effects. No absorption correction was made.

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

	x	y	z	Uiso*/Ueq
O	0.97416 (8)	0.00871 (12)	0.64817 (6)	0.0187 (3)
N	0.92735 (10)	0.36697 (15)	0.73011 (7)	0.0195 (4)
C1	0.56349 (10)	0.05640 (15)	0.53383 (7)	0.0147 (4)
C2	0.71032 (11)	−0.02493 (16)	0.55725 (7)	0.0154 (4)
C3	0.82836 (10)	0.07760 (15)	0.62168 (7)	0.0147 (4)
C4	0.80586 (11)	0.26657 (16)	0.66704 (7)	0.0155 (4)
C5	0.66155 (12)	0.34838 (17)	0.64273 (8)	0.0201 (5)
C6	0.54326 (12)	0.24631 (18)	0.57728 (8)	0.0209 (5)
C7	1.00676 (11)	−0.18500 (16)	0.60681 (8)	0.0171 (4)
C8	1.17015 (12)	−0.23363 (17)	0.65263 (8)	0.0202 (5)
C9	1.21804 (13)	−0.42769 (19)	0.60640 (8)	0.0228 (5)
H2	0.7309 (15)	−0.155 (2)	0.5297 (10)	0.022 (3)*
H5	0.6452 (16)	0.476 (2)	0.6724 (11)	0.025 (3)*
H6	0.4452 (18)	0.312 (2)	0.5623 (11)	0.034 (4)*
H1A	0.9061 (17)	0.454 (2)	0.7715 (12)	0.035 (4)*
H1B	1.009 (2)	0.290 (3)	0.7514 (12)	0.042 (4)*
H7A	0.9433 (15)	−0.2973 (19)	0.6211 (9)	0.016 (3)*
H7B	0.9867 (14)	−0.1715 (19)	0.5325 (9)	0.015 (3)*
H8A	1.2299 (17)	−0.112 (2)	0.6407 (11)	0.026 (3)*
H8B	1.1860 (16)	−0.257 (2)	0.7244 (11)	0.030 (4)*
H9A	1.1547 (17)	−0.548 (2)	0.6130 (11)	0.034 (4)*
H9B	1.2120 (16)	−0.409 (2)	0.5356 (11)	0.026 (4)*
H9C	1.3228 (18)	−0.467 (2)	0.6406 (11)	0.032 (4)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O	0.0120 (3)	0.0184 (4)	0.0235 (4)	0.0040 (3)	0.0006 (3)	−0.0047 (3)
N	0.0172 (4)	0.0211 (5)	0.0188 (4)	0.0011 (3)	0.0023 (3)	−0.0048 (4)
C1	0.0143 (4)	0.0177 (5)	0.0111 (4)	0.0037 (3)	0.0015 (3)	0.0000 (3)
C2	0.0149 (4)	0.0155 (4)	0.0149 (4)	0.0035 (3)	0.0026 (3)	−0.0006 (3)
C3	0.0133 (4)	0.0168 (5)	0.0137 (4)	0.0039 (3)	0.0029 (3)	0.0021 (3)
C4	0.0164 (4)	0.0177 (5)	0.0121 (4)	0.0010 (4)	0.0031 (4)	0.0004 (4)
C5	0.0194 (5)	0.0205 (5)	0.0189 (5)	0.0058 (4)	0.0022 (4)	−0.0059 (4)
C6	0.0166 (5)	0.0245 (5)	0.0192 (5)	0.0086 (4)	0.0001 (4)	−0.0056 (4)
C7	0.0150 (4)	0.0161 (5)	0.0186 (5)	0.0036 (4)	0.0016 (4)	−0.0014 (4)
C8	0.0146 (4)	0.0233 (5)	0.0207 (5)	0.0051 (4)	0.0009 (4)	−0.0032 (4)
C9	0.0214 (5)	0.0252 (6)	0.0210 (5)	0.0094 (4)	0.0042 (4)	−0.0008 (4)

Geometric parameters (Å, º)

O—C3	1.3754 (11)	C5—C6	1.3945 (15)
O—C7	1.4396 (12)	C5—H5	0.950 (15)
N—C4	1.3934 (13)	C6—H6	0.972 (16)
N—H1A	0.867 (17)	C7—C8	1.5124 (14)
N—H1B	0.884 (18)	C7—H7A	0.984 (13)
C1—C1^i	1.4909 (18)	C7—H7B	1.013 (12)
C1—C2	1.4120 (13)	C8—C9	1.5250 (15)
C1—C6	1.3996 (14)	C8—H8A	0.998 (14)
C2—C3	1.3881 (14)	C8—H8B	0.989 (15)
C2—H2	0.962 (14)	C9—H9A	0.989 (16)
C3—C4	1.4126 (14)	C9—H9B	0.986 (15)
C4—C5	1.3921 (14)	C9—H9C	0.995 (16)
C3—O—C7	117.68 (8)	C1—C6—H6	120.5 (9)
C4—N—H1A	116.0 (10)	C5—C6—H6	117.8 (9)
C4—N—H1B	115.5 (11)	O—C7—C8	107.45 (8)
H1A—N—H1B	117.3 (15)	O—C7—H7A	110.8 (7)
C1^i—C1—C2	121.15 (9)	O—C7—H7B	109.8 (7)
C1^i—C1—C6	121.93 (9)	C8—C7—H7A	110.2 (7)
C2—C1—C6	116.92 (9)	C8—C7—H7B	110.4 (7)
C1—C2—C3	121.35 (9)	H7A—C7—H7B	108.3 (10)
C1—C2—H2	120.6 (8)	C7—C8—C9	111.01 (9)
C3—C2—H2	118.1 (8)	C7—C8—H8A	107.6 (8)
O—C3—C2	124.84 (9)	C7—C8—H8B	109.2 (8)
O—C3—C4	113.94 (8)	C9—C8—H8A	109.7 (8)
C2—C3—C4	121.22 (9)	C9—C8—H8B	109.0 (8)
N—C4—C3	119.67 (9)	H8A—C8—H8B	110.4 (12)
N—C4—C5	122.86 (9)	C8—C9—H9A	110.8 (9)
C3—C4—C5	117.42 (9)	C8—C9—H9B	112.3 (8)
C4—C5—C6	121.34 (9)	C8—C9—H9C	111.1 (9)
C4—C5—H5	118.2 (8)	H9A—C9—H9B	108.2 (12)
C6—C5—H5	120.5 (8)	H9A—C9—H9C	106.4 (12)
C1—C6—C5	121.73 (9)	H9B—C9—H9C	107.9 (12)

Symmetry code: (i) −x+1, −y, −z+1.