(E,E)-1,4-Diethyl-1,4-di­phenyl-2,3-di­aza­buta­diene

Molecules of the title compound [(E,E)-propio­phenone azine], C₁₈H₂₀N₂, lie across centres of inversion in space group P2₁/c. The conformations of similar simple azines are discussed in terms of the soft hydrogen bonds present in the structures.

S1. Comment

The title compound, (I) [(E,E)-propiophenone azine], was synthesized as a potentially useful ligand for metal complexation. However, its molecular structure provides an interesting comparison both with the series of bis-(4-substituted) acetophenone azine, (IIa)–(IIf) (Chen et al., 1994a,b), and with benzophenone azine, (III) (Saha et al., 1995).

Molecules of (I) lie across centres of inversion in space group P2₁/c, with the reference molecule centred at (1/2, 1/2, 1/2) (Fig. 1). Consequently the central C7—N1—N1ⁱ—C7ⁱ fragment [symmetry code: (i) 1 − x, 1 − y, 1 − z] is strictly planar; indeed, with the exception of the terminal methyl groups, the whole molecule is almost planar, as judged by the leading torsional angles (Table 1). Other intramolecular features of note are the clear distinction between the single and double bonds within the central spacer unit, the marked geometric distortion at the branching atom C7, shown by the bond angles at atom C7, and the rather small C—N—N bond angle.

The conformation of (I) is in marked contrast to the conformations of compounds (IIa)–(IId), where the C—N—N—C torsion angles range from 138.7 (2)° in (IIa) to 124.6 (6)° in (IId), although molecules of (IIe) are centrosymmetric, with a torsion angle of 180° (Chen et al., 1994a). It was accordingly suggested (Chen et al., 1994a) that this torsion angle is determined by the balance of substituent electronegativity and substituent π-acceptor capacity, with both strong π-acceptor substituents and strongly electronegative substituents tending to increase the torsion angle. However, this proposition does not readily accommodate the behaviour of (IIf). This compound exhibits concomitant polymorphism (Bernstein et al., 1999) with two monoclinic forms, in one of which (Z' = 0.5 in P2₁/c) the molecules are centrosymmetric, while in the other (Z' = 1 in P2₁/n), the C—N—N—C torsion angle is 142.8 (3)° (Chen et al., 1994b). In (III), the molecules lie across twofold rotation axes in space group A2/a (alternative setting of C2/c), with a C—N—N—C torsion angle of 132.5 (7)° (Saha et al., 1995).

In (I), which has H atoms at the 4-positions of the phenyl rings, the conformation is the same as that in the Z' = 0.5 polymorph of the isomeric compound (IIf), but it differs from those in both (IIa) and the Z' = 1 polymorph of (IIf). While the electronic effects of the substituents at the 4-positions are doubtless of some importance, the analysis of Chen et al., (1994a) is formulated entirely on the basis that the conformation is solely determined by intramolecular factors; it ignores entirely the possible influence of direction-specific interactions between the molecules. In the event, scrutiny of the atomic coordinates for compounds (IIa)–(IId), (IIf) and (III), deposited in the Cambridge Structural Database (Allen, 2002), although no coordinates are available for compound (IIe), shows that soft (Desiraju & Steiner, 1999) hydrogen bonds are present in compounds (IIa), (IIb) and (IIc), in the Z' = 1 polymorph of (IIf), and in compound (III) (Table 2). No such interactions are present in (I), or in the centrosymmetric polymorph (IIf). Only for (IId) is a torsion angle other than 180° observed in the absence of direction-specific intermolecular interactions, suggesting that intermolecular factors may be at least as significant as intramolecular factors in determining the C—N—N—C torsion angles in compounds of this general type. Consistent with this result, we note that in the analogue of (II) having X = OH, which crystallizes as a monohydrate in a structure containing both O—H···O and O—H···N hydrogen bonds, the C—N—N—C torsion angle is 148 (1)° (García-Mina et al., 1982).

S2. Experimental

Compound (I) was prepared by heating under reflux an acidified ethanol solution of propiophenone and hydrazine hydrate (Fleming & Harley-Mason, 1961). Large crystals of (I) [m.p. 339–341 K; literature m.p. 339–340 K (Elguero et al., 1968)] were deposited on cooling the reaction mixture, and a cubic block suitable for single-crystal X-ray diffraction was cut from one of the large crystals selected directly from the prepared sample.

S3. Refinement

Space group P2₁/c was assigned uniquely from the systematic absences. All H atoms were located from difference maps and thereafter treated as riding atoms, with C—H distances of 0.95 (aromatic), 0.98 (CH₃) and 0.99 Å (CH₂).

Computing details

Data collection: KappaCCD Server Software (Nonius, 1997); cell refinement: DENZO–SMN (Otwinowski & Minor, 1997); data reduction: DENZO–SMN; program(s) used to solve structure: OSCAIL (McArdle, 2003) and SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: OSCAIL and SHELXL97 (Sheldrick, 1997); molecular graphics: PLATON (Spek, 2003); software used to prepare material for publication: SHELXL97 and PRPKAPPA (Ferguson, 1999).

Figures

Fig. 1. The molecule of compound (I), showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 30% probability level. Atoms marked 'a' are at the symmetry position (1 − x, 1 − y, 1 − z).

(E,E)-1,4-Diethyl-1,4-diphenyl-2,3-diazabutadiene

Crystal data

C18H20N2	F(000) = 284
Mr = 264.36	Dx = 1.205 Mg m^−3
Monoclinic, P21/c	Melting point: 340 K
Hall symbol: -P 2ybc	Mo Kα radiation, λ = 0.71073 Å
a = 4.5985 (2) Å	Cell parameters from 1656 reflections
b = 9.6973 (5) Å	θ = 3.3–27.5°
c = 16.3968 (8) Å	µ = 0.07 mm^−1
β = 94.935 (3)°	T = 120 K
V = 728.47 (6) Å^3	Block, yellow
Z = 2	0.10 × 0.10 × 0.10 mm

Data collection

Nonius KappaCCD diffractometer	1656 independent reflections
Radiation source: rotating anode	1222 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.024
ϕ scans, and ω scans with κ offsets	θmax = 27.5°, θmin = 3.3°
Absorption correction: multi-scan (SORTAV; Blessing, 1995, 1997)	h = −5→5
Tmin = 0.924, Tmax = 0.993	k = −12→12
3098 measured reflections	l = −21→21

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)] = 0.044	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.116	H-atom parameters constrained
S = 1.03	w = 1/[σ^2(Fo^2) + (0.0638P)^2 + 0.0449P] where P = (Fo^2 + 2Fc^2)/3
1656 reflections	(Δ/σ)max < 0.001
92 parameters	Δρmax = 0.13 e Å^−3
0 restraints	Δρmin = −0.23 e Å^−3

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

	x	y	z	Uiso*/Ueq
C1	0.3907 (2)	0.39840 (11)	0.32881 (7)	0.0245 (3)
C2	0.1733 (2)	0.49248 (12)	0.30155 (7)	0.0289 (3)
C3	0.0427 (2)	0.48631 (13)	0.22268 (7)	0.0337 (3)
C4	0.1237 (2)	0.38577 (13)	0.16920 (7)	0.0334 (3)
C5	0.3360 (3)	0.29201 (13)	0.19478 (7)	0.0326 (3)
C6	0.4689 (2)	0.29827 (12)	0.27405 (7)	0.0298 (3)
C7	0.5276 (2)	0.40414 (11)	0.41414 (7)	0.0242 (3)
N1	0.4372 (2)	0.50154 (9)	0.45939 (6)	0.0296 (3)
C8	0.7471 (2)	0.29573 (12)	0.44309 (7)	0.0294 (3)
C9	0.5971 (3)	0.16391 (12)	0.46709 (8)	0.0375 (3)
H2	0.1148	0.5616	0.3378	0.035*
H3	−0.1036	0.5515	0.2050	0.040*
H4	0.0327	0.3816	0.1150	0.040*
H5	0.3921	0.2228	0.1582	0.039*
H6	0.6157	0.2330	0.2911	0.036*
H8A	0.8714	0.3312	0.4908	0.035*
H8B	0.8746	0.2753	0.3989	0.035*
H9A	0.4811	0.1827	0.5133	0.056*
H9B	0.7446	0.0938	0.4830	0.056*
H9C	0.4689	0.1303	0.4205	0.056*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C1	0.0260 (5)	0.0234 (6)	0.0243 (6)	−0.0049 (4)	0.0034 (4)	0.0029 (4)
C2	0.0299 (6)	0.0277 (7)	0.0288 (7)	−0.0015 (5)	0.0010 (5)	−0.0004 (5)
C3	0.0322 (6)	0.0358 (7)	0.0322 (7)	0.0002 (5)	−0.0030 (5)	0.0032 (5)
C4	0.0361 (6)	0.0401 (7)	0.0233 (6)	−0.0093 (6)	−0.0018 (5)	0.0027 (5)
C5	0.0402 (7)	0.0328 (7)	0.0254 (7)	−0.0049 (5)	0.0057 (5)	−0.0026 (5)
C6	0.0346 (6)	0.0291 (6)	0.0259 (6)	0.0003 (5)	0.0031 (5)	0.0016 (5)
C7	0.0249 (5)	0.0232 (6)	0.0246 (6)	−0.0036 (4)	0.0024 (4)	0.0014 (5)
N1	0.0365 (5)	0.0287 (6)	0.0227 (5)	0.0032 (4)	−0.0025 (4)	−0.0012 (4)
C8	0.0297 (6)	0.0314 (7)	0.0265 (6)	0.0023 (5)	0.0001 (5)	−0.0016 (5)
C9	0.0532 (8)	0.0300 (7)	0.0288 (7)	0.0031 (6)	0.0004 (6)	0.0017 (5)

Geometric parameters (Å, º)

C1—C6	1.3905 (16)	C6—H6	0.95
C1—C2	1.3981 (15)	C7—N1	1.2917 (14)
C1—C7	1.4851 (15)	C7—C8	1.5056 (15)
C2—C3	1.3794 (16)	N1—N1^i	1.4056 (18)
C2—H2	0.95	C8—C9	1.5203 (16)
C3—C4	1.3835 (17)	C8—H8A	0.99
C3—H3	0.95	C8—H8B	0.99
C4—C5	1.3732 (17)	C9—H9A	0.98
C4—H4	0.95	C9—H9B	0.98
C5—C6	1.3891 (16)	C9—H9C	0.98
C5—H5	0.95
C6—C1—C2	117.82 (11)	C1—C6—H6	119.4
C6—C1—C7	121.49 (10)	N1—C7—C1	115.93 (10)
C2—C1—C7	120.68 (10)	N1—C7—C8	124.72 (10)
C3—C2—C1	120.86 (11)	C1—C7—C8	119.29 (9)
C3—C2—H2	119.6	C7—N1—N1^i	113.73 (11)
C1—C2—H2	119.6	C7—C8—C9	111.22 (9)
C2—C3—C4	120.38 (11)	C7—C8—H8A	109.4
C2—C3—H3	119.8	C9—C8—H8A	109.4
C4—C3—H3	119.8	C7—C8—H8B	109.4
C5—C4—C3	119.73 (11)	C9—C8—H8B	109.4
C5—C4—H4	120.1	H8A—C8—H8B	108.0
C3—C4—H4	120.1	C8—C9—H9A	109.5
C4—C5—C6	120.05 (11)	C8—C9—H9B	109.5
C4—C5—H5	120.0	H9A—C9—H9B	109.5
C6—C5—H5	120.0	C8—C9—H9C	109.5
C5—C6—C1	121.16 (11)	H9A—C9—H9C	109.5
C5—C6—H6	119.4	H9B—C9—H9C	109.5
C6—C1—C2—C3	−0.43 (16)	C6—C1—C7—N1	−179.90 (10)
C7—C1—C2—C3	−179.09 (10)	C6—C1—C7—C8	−2.52 (15)
C1—C2—C3—C4	0.48 (18)	C2—C1—C7—C8	176.09 (9)
C2—C3—C4—C5	−0.24 (18)	C2—C1—C7—N1	−1.29 (15)
C3—C4—C5—C6	−0.04 (18)	C1—C7—N1—N1^i	177.06 (10)
C4—C5—C6—C1	0.08 (18)	C9—C8—C7—N1	96.31 (13)
C2—C1—C6—C5	0.15 (16)	C8—C7—N1—N1^i	−0.16 (17)
C7—C1—C6—C5	178.80 (10)	C1—C7—C8—C9	−80.82 (12)

Symmetry code: (i) −x+1, −y+1, −z+1.