N,N-Diethyl-N'-[(E)-4-pyridylmethyl­ene]benzene-1,4-diamine: a combined X-ray and density functional theory study

The crystal structure of the title compound, C₁₆H₁₉N₃, comprises neutral molecules of a dipolar Schiff base chromophore. A density functional theory (DFT) optimized structure at the B3LYP/6-31G(d) level is compared with the mol­ecular structure in the solid state. The compound crystallizes in the noncentrosymmetric space group Pna2₁ with a herring-bone packing motif and is therefore a potential candidate for nonlinear optical effects in the bulk.

S1. Comment

During the past decade, the design of new materials with nonlinear optical (NLO) properties has attracted considerable interest (Papadopoulos et al., 2006). For the observation of second-order NLO effects, such as frequency doubling (second-harmonic generation, SHG) in the bulk, noncentrosymmetric molecular and crystallographic symmetry is required. Besides the criterion of overall noncentrosymmetric character, large hyperpolarizabilities (β values) are also required. Typical synthetic targets are dipolar donor–π-acceptor (push–pull) molecules having the desired high β values. In particular, stilbazolium derivatives such as trans-4'-(dimethylamino)-N-methyl-4-stilbazolium 4-toluenesulfonate (DAST) have been intensively studied (Marder et al., 1989, 1994; Ruiz et al., 2008). Related Schiff base chromophores have also been studied (Corradin et al., 1996; Coe et al., 2002; Sliwa et al., 2005). In this context, Coe and co-workers reported the crystal structures of two salts containing the Schiff base cations trans-4-[4-(dimethylamino)phenyliminomethyl]-N-phenylpyridinium (Coe et al., 2000) and trans-4-[4-(dimethylamino)phenyliminomethyl]-N-methylpyridinium (Coe et al., 2001). The use of organic salts enables modification of the crystal packing by variation of the counterions. It is noteworthy that the noncentrosymmetric crystal structure of unsubstituted trans-N-(4-pyridylmethylene)aniline has already been known for more than two decades (Wiebcke & Mootz, 1982). A related derivative containing an amide group in the para position of the benzene ring, which crystallizes centrosymmetrically, was also structurally characterized (Aakeroy et al., 2006). However, to the best of our knowledge, and based on a search of the Cambridge Structural Database (CSD; Version?; Allen, 2002), this is the first crystallographic study of a neutral N,N-dialkyl-N'-[1-pyridin-4-yl-meth-(E)-ylidene]benzene-1,4-diamine derivative. [From the Co-Editor: Please check and approve text inserted into the last sentence.]

The title compound, (I), was prepared by a Schiff base condensation of isonicotinealdehyde and N,N-diethyl-1,4-phenylenediamine in the presence of 4-toluenesulfonic acid. The well-defined band in the IR spectrum at 1649 cm^-1 was assigned to the ν_C═N stretching vibration. Crystals of (I) suitable for X-ray analyis were grown from an ethanolic solution. A displacement ellipsoid plot is depicted in Fig. 1. The molecular geometry parameters are within expected ranges. As observed in trans-4-[4-(dimethylamino)phenyliminomethyl]-N-phenylpyridinium hexafluorophosphate (Coe et al., 2000) and trans-4-[4-(dimethylamino)phenyliminomethyl]-N-methylpyridinium 4-toluenesulfonate (Coe et al., 2001), the benzene ring in (I) shows partial quinone character, which reveals some ground-state charge separation. The average C2≐C3 and C5≐C6 bond length is 1.373 (2) Å, while the average of the other four benzene C≐C distances is 1.404 (5) Å. These observations are supported by the DFT structure optimization. The corresponding calculated values are 1.387 and 1.412 Å, respectively. The other calculated bond lengths of (I) are also in agreement with the crystallographically determined values. For example, the observed and calculated C═N bond lengths are 1.281 (2) and 1.283 Å, respectively. The dihedral angle between the mean planes of the rings C1–C6 and N11/C12–C16 is 5.8 (1)°. The corresponding value in the DFT-optimized structure is appreciably larger, at 21.8°. The reason for the discrepancy between the observed value in the solid state and the calculated value is not entirely clear, but the effects of crystal packing should presumably be considered. These effects are not included in the DFT structure optimization. However, the calculated value of 21.8° lies within the expected range: an even larger value of 23.49 (6)° has been crystallographically determined for the trans-4-[(4-dimethylaminophenyl)iminomethyl]-N-methylpyridinium cation (Coe et al., 2001).

Compound (I) shows an intense absorption band in the visible region at λ_max = 428 nm (ε = 8881 l mol^-1 cm^-1) in acetonitrile. This band is attributed to an intramolecular charge transfer (ICT) from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). As expected, the DFT calculation reveals that the HOMO is primarily localized on the electron-rich diethylamine group, while the LUMO is primarily localized on the electron-deficient pyridine moiety (Fig. 2). For the hexafluorophosphate of the related trans-4-[4-(dimethylamino)phenyliminomethyl]-N-phenylpyridinium cation, an absorption band was reported at λ_max = 534 nm in acetonitrile (Coe et al., 2000). The ICT band of (I) shows solvatochromism; in acetic anhydride the ICT band of (I) is bathochromically shifted to λ_max = 541 nm (ε = 9713 l mol^-1 cm^-1).

As mentioned above, noncentrosymmetric space-group symmetry is required for the observation of bulk NLO properties. The prediction of crystal packing, and hence the rational supramolecular synthesis of noncentrosymmetric crystals, are still a challenge in the field of crystal engineering. Interestingly, the crystal packing of (I) is quite different to that observed in trans-4-[4-(dimethylamino)phenyliminomethyl]-N-phenylpyridinium hexafluorophosphate (Coe et al., 2000), which crystallizes in the polar space group Cc. In the latter, the dipolar cations are aligned head-to-tail forming polar cationic sheets, between which the hexafluorophate anions are located. It may be significant that the closely related compound trans-4-[4-(dimethylamino)phenyliminomethyl]-N-methylpyridinium 4-toluenesulfonate crystallizes in the centrosymmetric space group P2₁/n (Coe et al., 2001). Compound (I) crystallizes in the polar orthorhombic space group Pna2₁ with antiparallel alignment of the dipolar chromophores. The arrangement of the molecules in the orthorhombic unit cell is shown in Fig. 3. No face-to-face π–π stacking interactions occur in the crystal structure of (I), but edge-to-face stacking interactions are indicated by the C—H···π distances (Table 1). This arrangement leads to a herringbone packing motif which is polar along the c axis. Compound (I) therefore possesses the potential to exhibit NLO effects in the bulk. [From the Co-Editor: Slight change to second last sentence - OK?]

S2. Experimental

All chemicals used were purchased from Merck and used as received. N,N-Diethyl-1,4-phenylenediamine (32.850 g, 0.200 mol) was added to a solution of isonicotinealdehyde (21.422 g, 0.200 mol) in toluene (40 ml). 4-Toluenesulfonic acid (40 mg) was added and the mixture was refluxed for 2 h. The Schiff base that separated after cooling was filtered and recrystallized from ethanol (yield 33.441 g, 66%). Elemental analysis: calculated for C₁₆H₁₉N₃: C 75.85, H 7.56, N 16.59%; found: C 75.88, H 7.54, N 16.60%. Single crystals of (I) suitable for X-ray diffraction were obtained by slow evaporation from an ethanolic solution at room temperature.

The DFT quantum-chemical calculation was performed at the B3LYP/6–31G(d) level (Becke, 1993; Lee et al., 1988) using GAUSSIAN03 (Frisch et al., 2004). Initial atomic coordinates for the DFT calculation were taken from the crystal structure. The harmonic vibrational analysis at the same level of theory confirmed that the stationary point represented a minimum. The DFT results were visualized with CHEMCRAFT (Zhurko & Zhurko, 2009).

S3. Refinement

In the absence of significant anomalous scattering effects, Friedel pairs have been merged and the absolute structure was assigned arbitrarily. H atoms were placed at geometrically calculated positions and refined with a riding model and with U_iso(H) = 1.2 (1.5 for methyl groups) U_eq(C). The C—H bond lengths were constrained to 0.95 (aromatic and iminomethyl CH), 0.99 (methylene) and 0.98 Å (methyl).

Computing details

Data collection: CrysAlis PRO (Oxford Diffraction, 2009); cell refinement: CrysAlis PRO (Oxford Diffraction, 2009); data reduction: CrysAlis PRO (Oxford Diffraction, 2009); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2008); software used to prepare material for publication: enCIFer (Allen et al., 2004).

Figures

	Fig. 1. The molecular structure of (I), with the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii.
	Fig. 2. Representation of the DFT-optimized structure of (I), showing the isosurfaces of (a) the HOMO and (b) the LUMO with an isovalue of 0.02 a.u. [Please make sure the two parts are the right way round - originally part (b) was the upper one.]
	Fig. 3. Arrangement of the molecules in the orthorhombic unit cell of (I). H atoms have been omitted for clarity.

N,N-Diethyl-N'-[(E)-4-pyridylmethylene]benzene- 1,4-diamine

Crystal data

C16H19N3	F(000) = 544
Mr = 253.34	Dx = 1.224 Mg m^−3
Orthorhombic, Pna21	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: P 2c -2n	Cell parameters from 3166 reflections
a = 12.7984 (5) Å	θ = 2.8–28.7°
b = 14.8116 (6) Å	µ = 0.07 mm^−1
c = 7.2524 (3) Å	T = 110 K
V = 1374.80 (10) Å^3	Elongated prism, yellow
Z = 4	0.36 × 0.12 × 0.09 mm

Data collection

Oxford Diffraction Xcalibur2 diffractometer	1458 independent reflections
Radiation source: Enhance (Mo) X-ray Source	1069 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.041
Detector resolution: 8.4171 pixels mm^-1	θmax = 26.0°, θmin = 3.2°
ω scans	h = −15→15
Absorption correction: multi-scan ABSPACK in CrysAlis PRO (Oxford Diffraction, 2009)	k = −18→17
Tmin = 0.971, Tmax = 0.992	l = −8→8
10508 measured reflections

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)] = 0.030	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.058	H-atom parameters constrained
S = 0.87	w = 1/[σ^2(Fo^2) + (0.0314P)^2] where P = (Fo^2 + 2Fc^2)/3
1458 reflections	(Δ/σ)max < 0.001
174 parameters	Δρmax = 0.19 e Å^−3
1 restraint	Δρmin = −0.10 e Å^−3

Special details

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.

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

	x	y	z	Uiso*/Ueq
N1	0.34770 (13)	0.86781 (9)	0.4949 (3)	0.0205 (4)
N4	0.34535 (13)	0.48833 (10)	0.4434 (2)	0.0201 (4)
C1	0.34786 (17)	0.77495 (11)	0.4857 (3)	0.0169 (4)
C2	0.41766 (15)	0.72274 (12)	0.5929 (3)	0.0181 (5)
H2	0.4653	0.7523	0.6733	0.022*
C3	0.41789 (16)	0.63026 (13)	0.5829 (3)	0.0184 (5)
H3	0.4651	0.5972	0.6581	0.022*
C4	0.35054 (17)	0.58332 (11)	0.4651 (3)	0.0169 (5)
C5	0.28002 (16)	0.63465 (12)	0.3626 (3)	0.0194 (5)
H5	0.2316	0.6046	0.2845	0.023*
C6	0.27831 (17)	0.72731 (12)	0.3711 (3)	0.0186 (5)
H6	0.2293	0.7598	0.2983	0.022*
C7	0.41671 (16)	0.91673 (12)	0.6207 (3)	0.0211 (5)
H7A	0.4228	0.8824	0.7372	0.025*
H7B	0.3850	0.9760	0.6499	0.025*
C8	0.52491 (16)	0.93176 (13)	0.5419 (3)	0.0282 (5)
H8A	0.5567	0.8734	0.5120	0.042*
H8B	0.5683	0.9632	0.6330	0.042*
H8C	0.5198	0.9684	0.4298	0.042*
C9	0.28284 (16)	0.92364 (13)	0.3752 (3)	0.0217 (5)
H9A	0.2765	0.8939	0.2534	0.026*
H9B	0.3177	0.9826	0.3564	0.026*
C10	0.17398 (16)	0.93977 (13)	0.4529 (3)	0.0287 (6)
H10A	0.1361	0.8823	0.4592	0.043*
H10B	0.1361	0.9817	0.3727	0.043*
H10C	0.1796	0.9656	0.5769	0.043*
N11	0.39597 (15)	0.14779 (10)	0.4753 (3)	0.0244 (4)
C12	0.32821 (18)	0.20199 (13)	0.3868 (3)	0.0242 (6)
H12	0.2750	0.1741	0.3153	0.029*
C13	0.33026 (17)	0.29515 (12)	0.3923 (3)	0.0206 (6)
H13	0.2798	0.3296	0.3269	0.025*
C14	0.40765 (17)	0.33775 (11)	0.4955 (3)	0.0184 (5)
C15	0.47914 (17)	0.28272 (12)	0.5879 (3)	0.0209 (5)
H15	0.5335	0.3085	0.6599	0.025*
C16	0.46958 (19)	0.19013 (13)	0.5728 (3)	0.0242 (5)
H16	0.5191	0.1539	0.6365	0.029*
C17	0.41526 (15)	0.43628 (13)	0.5119 (3)	0.0179 (5)
H17	0.4732	0.4620	0.5747	0.021*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
N1	0.0198 (9)	0.0167 (8)	0.0252 (11)	0.0016 (8)	−0.0067 (8)	−0.0002 (9)
N4	0.0230 (9)	0.0192 (8)	0.0181 (11)	0.0019 (8)	0.0010 (9)	−0.0006 (9)
C1	0.0158 (10)	0.0175 (10)	0.0174 (11)	−0.0018 (10)	0.0049 (9)	−0.0014 (11)
C2	0.0174 (13)	0.0199 (10)	0.0169 (13)	−0.0007 (9)	−0.0013 (11)	−0.0002 (10)
C3	0.0186 (13)	0.0199 (11)	0.0166 (12)	0.0021 (9)	0.0000 (11)	0.0022 (10)
C4	0.0179 (10)	0.0157 (9)	0.0172 (12)	−0.0018 (9)	0.0036 (10)	−0.0002 (10)
C5	0.0163 (13)	0.0229 (11)	0.0191 (13)	−0.0013 (10)	−0.0016 (10)	−0.0026 (11)
C6	0.0164 (13)	0.0194 (11)	0.0199 (13)	0.0040 (9)	−0.0018 (11)	0.0011 (10)
C7	0.0227 (13)	0.0177 (10)	0.0228 (13)	−0.0016 (9)	−0.0058 (11)	−0.0020 (9)
C8	0.0270 (13)	0.0293 (11)	0.0282 (14)	−0.0057 (11)	−0.0031 (11)	0.0032 (11)
C9	0.0239 (13)	0.0201 (10)	0.0211 (12)	0.0011 (10)	−0.0014 (11)	0.0020 (11)
C10	0.0260 (13)	0.0355 (12)	0.0246 (13)	0.0096 (10)	−0.0002 (11)	0.0011 (12)
N11	0.0281 (11)	0.0215 (9)	0.0236 (11)	−0.0008 (9)	0.0005 (10)	0.0014 (10)
C12	0.0244 (14)	0.0261 (12)	0.0223 (14)	−0.0035 (10)	0.0003 (12)	−0.0029 (11)
C13	0.0227 (14)	0.0190 (11)	0.0201 (14)	0.0019 (10)	−0.0016 (11)	0.0017 (10)
C14	0.0190 (12)	0.0201 (11)	0.0163 (12)	−0.0013 (9)	0.0048 (10)	−0.0013 (10)
C15	0.0184 (12)	0.0243 (11)	0.0199 (13)	0.0013 (9)	−0.0022 (11)	0.0014 (10)
C16	0.0263 (13)	0.0242 (11)	0.0220 (14)	0.0055 (10)	0.0012 (12)	0.0055 (11)
C17	0.0179 (11)	0.0188 (10)	0.0169 (13)	−0.0022 (9)	0.0030 (10)	−0.0007 (10)

Geometric parameters (Å, º)

N1—C1	1.377 (2)	C8—H8C	0.9800
N1—C9	1.458 (3)	C9—C10	1.522 (3)
N1—C7	1.462 (2)	C9—H9A	0.9900
N4—C17	1.281 (2)	C9—H9B	0.9900
N4—C4	1.417 (2)	C10—H10A	0.9800
C1—C6	1.408 (3)	C10—H10B	0.9800
C1—C2	1.414 (3)	C10—H10C	0.9800
C2—C3	1.372 (2)	N11—C16	1.334 (3)
C2—H2	0.9500	N11—C12	1.345 (3)
C3—C4	1.399 (3)	C12—C13	1.381 (3)
C3—H3	0.9500	C12—H12	0.9500
C4—C5	1.395 (3)	C13—C14	1.393 (3)
C5—C6	1.374 (2)	C13—H13	0.9500
C5—H5	0.9500	C14—C15	1.397 (3)
C6—H6	0.9500	C14—C17	1.467 (2)
C7—C8	1.514 (3)	C15—C16	1.381 (2)
C7—H7A	0.9900	C15—H15	0.9500
C7—H7B	0.9900	C16—H16	0.9500
C8—H8A	0.9800	C17—H17	0.9500
C8—H8B	0.9800
C1—N1—C9	122.57 (19)	H8B—C8—H8C	109.5
C1—N1—C7	121.64 (18)	N1—C9—C10	112.95 (19)
C9—N1—C7	115.73 (14)	N1—C9—H9A	109.0
C17—N4—C4	121.43 (18)	C10—C9—H9A	109.0
N1—C1—C6	121.9 (2)	N1—C9—H9B	109.0
N1—C1—C2	121.4 (2)	C10—C9—H9B	109.0
C6—C1—C2	116.74 (14)	H9A—C9—H9B	107.8
C3—C2—C1	121.23 (19)	C9—C10—H10A	109.5
C3—C2—H2	119.4	C9—C10—H10B	109.5
C1—C2—H2	119.4	H10A—C10—H10B	109.5
C2—C3—C4	121.82 (19)	C9—C10—H10C	109.5
C2—C3—H3	119.1	H10A—C10—H10C	109.5
C4—C3—H3	119.1	H10B—C10—H10C	109.5
C5—C4—C3	116.96 (15)	C16—N11—C12	115.32 (17)
C5—C4—N4	116.84 (19)	N11—C12—C13	124.8 (2)
C3—C4—N4	126.18 (18)	N11—C12—H12	117.6
C6—C5—C4	122.1 (2)	C13—C12—H12	117.6
C6—C5—H5	119.0	C12—C13—C14	118.8 (2)
C4—C5—H5	119.0	C12—C13—H13	120.6
C5—C6—C1	121.13 (19)	C14—C13—H13	120.6
C5—C6—H6	119.4	C13—C14—C15	117.34 (17)
C1—C6—H6	119.4	C13—C14—C17	122.78 (19)
N1—C7—C8	112.96 (18)	C15—C14—C17	119.9 (2)
N1—C7—H7A	109.0	C16—C15—C14	118.9 (2)
C8—C7—H7A	109.0	C16—C15—H15	120.5
N1—C7—H7B	109.0	C14—C15—H15	120.5
C8—C7—H7B	109.0	N11—C16—C15	124.8 (2)
H7A—C7—H7B	107.8	N11—C16—H16	117.6
C7—C8—H8A	109.5	C15—C16—H16	117.6
C7—C8—H8B	109.5	N4—C17—C14	121.37 (18)
H8A—C8—H8B	109.5	N4—C17—H17	119.3
C7—C8—H8C	109.5	C14—C17—H17	119.3
H8A—C8—H8C	109.5

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
C2—H2···Cg1^i	0.95	2.83	3.700 (2)	153
C6—H6···Cg1^ii	0.95	2.84	3.633 (2)	142
C12—H12···Cg2^iii	0.95	2.92	3.754 (2)	147
C15—H15···Cg2^i	0.95	2.75	3.623 (2)	152

Symmetry codes: (i) −x+1, −y+1, z+1/2; (ii) −x+1/2, y+1/2, z−1/2; (iii) −x+1/2, y−1/2, z−1/2.