(E)-N,N-Diethyl-2,6-diisopropyl-4-[2-(4-nitrophenyl)ethenyl]aniline

The title compound, C24H32N2O2, was prepared by Horner olefination of 4-diethylamino-3,5-diisopropylbenzaldehyde and diethyl p-nitrobenzylphosphonate. There are two independent molecules (A and B) in the asymmetric unit. Their main axes, defined by the line connecting the N atoms of the nitro and amino groups, open an angle of 79.42 (3)°. Steric hindrance around the amino group is reflected in a long aryl C—N bond [1.434 (3) Å for molecule A and 1.440 (3) Å for molecule B], a pyramidal geometry [angle sum = 350.0 (2)° for molecule A and 349.6 (2)° for molecule B], and dihedral angles between the phenylene group and the plane defined by the CH2—N—CH2 unit of 86.9 (3)° for molecule A and 88.3 (3)° for molecule B. This gives structural support for the electronic decoupling of the amino group from the nearly planar nitrostilbene moiety (r.m.s. deviation for C, N and O atoms = 0.097 for molecule A and 0.107 Å for molecule B).


Related literature
and Nemkovich et al. (2010). In comparison to the sterically non congested derivative lacking the diisopropyl substitution (Schollmeyer & Detert, 2011), the sterical hindrance around the amino group shifts the UV-vis absorption about 70 nm to the blue and inverts the positive to a negative solvatochromism.
The unit cell is filled with two independent molecules A, B. Parallel layers of molecules A and B are twisted by 79.42 (3)°. As A and B are very similar, only the structural features of A are discussed. The π-system is composed of four almost planar subunits with torsion angles of ±1.6° between nitro group and adjacent phenyl ring. The nitro group is planar (angle sum at N18 = 360°) and the bond length C12-N26 [1.460 (4) Å] is slightly shorter than that of the noncongested analogue, this also holds for the bond lengths of the vinylene group. The torsion angles between the planes of the phenylene rings and the connecting vinylene unit are larger: C6-C1-C7-C8 [175.8 (3)°] and C7-C8-C9-C14 [171.6 (3)°]. Even the vinylene group is twisted: C1-C7-C8-C9 [177.8 (3)°]. Steric congestion around the amino group elongates the aniline C-N bond: 1.434 (3) Å in comparison to 1.385 or 1.378 Å for the compound without 2,6diisopropyl substitution. Furthermore, the substituents in the 1,2,6-positions adopt an anti,anti conformation with torsion angles of 5.0 (4)° (C15-C3-C4-N18) and 3.7 (4)° (N18-C4-C5-C23). The amino group is pyramidal with an angle sum of 350° on N18. This and the dihedral angles C3-C4-N18-C19 [-69.7 (3)°] and C3-C4-N18-C21 [106.9 (3)°] are structural indicators for an electronic decoupling of the amino group from the acceptor-substituted stilbene unit and therefore the inhibition of the charge transfer and the blue-shifted absorption band in the UV.

Experimental
The title compound was prepared by adding potassium tert-butylate (0.167 g, 1.5 mmol) under nitrogen to a cooled solution of 4-N,N-diethylamino3,5-diisopropylbenzaldehyde (0.26 g, 0.6 mmol) and diethyl p-nitrobenzylphosphonate (0.313 g, 1.1 mmol) in THF (anhyd., 40 ml) and the mixture was stirred for 40 min at 273 K and for further 1 h at ambient temperature. Water (70 ml) was added, the mixture was extracted with chloroform (3 x 30 ml) and the pooled organic solutions were washed with brine (3 x 20 ml), dried (MgSO 4 ), concentrated in vacuo and the title compound was isolated in 80% yield from the red oil by chromatography on silica gel using petrouleum ether / ethyl acetate (9 / 1).
Yellow needles with with m.p. = 408 -409 K were obtained by slow evaporation of a solution of the title compound in methanol/chloroform.

Refinement
Hydrogen atoms were placed at calculated positions with C-H = 0.95 Å (aromatic) or 0.98-0.99 Å (sp 3 C-atoms). All H atoms were refined in the riding-model approximation with isotropic displacement parameters (set at 1.

Figure 1
Crystal structure of the title compound with labeling and displacement ellipsoids drawn at the 50% probability level.
Second molecule and hydrogen atoms omitted for clearity.    2959,2931,2871,1633,1590,1512,1457,1336,1186,1105,964,862,745, 6890 cm -1 . HR-ESI-MS:found: 381.2545, calcd for (M+H + ): 381.2542. UV-Vis: λ'max = 364 nm (cyclohexane); λ'max = 347 nm (toluene); λ'max = 344 nm, ε = 29304 l mol-1 cm-1 (dichloromethane); λ'max = 344 nm (acetonitrile). 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 F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.