Two polymorphs of N,N′-diphenyl-2-[1-(propylamino)ethylidene]propanediamide

Two polymorphs of the title compound, C20H23N3O2, have been isolated. Polymorph (I) crystallizes in the monoclinic space group P21/n and polymorph (II) in the tetragonal space group I4 1/a. The main difference between the two polymorphs on the molecular level is the orientation of the n-propyl group. This group is antiperiplanar in (I) and synclinal in (II).


Structural commentary
Two polymorphs of the title compound were obtained from a single reaction batch. Polymorph (I) crystallizes in the monoclinic space group P2 1 /n, polymorph (II) in the tetragonal space group I4 1 /a. The main difference between the polymorphs on the molecular level is the orientation of the npropyl group. This group is antiperiplanar in (I) and synclinal in (II), as can be seen from the values of the torsion angles C1-C2-C3-N1 (see Tables 1, 2 and Figs. 1, 2). The n-propyl group with C1-C2-C3 is disordered in (II), with site occupancies of 0.794 (7) and 0.206 (7) for parts A and B, respectively.
The double bond between C4 and C5 is slightly elongated [1.400 (2) and 1.394 (3) Å ], but not as strongly as in the pushpull alkenes from cyclic ketene-N,N 0 -acetals, which have values of 1.45 to 1.47 Å (Ye et al., 2010). The carbamoyl units are characterized by C O double bonds and shortened C-N bonds (Tables 1 and 2), the latter having values between 1.360 (2) to 1.369 (3) Å , whereas the sum of covalent radii of C and N is 1.472 Å (Pauling, 1962).
The core of the molecule consists of two carbamoyl units (N2-C7-O1 and N3-C14-O2) bound to an enamine unit (C5-C4-N1). These planar units span certain dihedral angles between each other. The dihedral angles are listed in Table 3. The dihedral angle between N1-C4-C5 and N3-C14-O2 is small in both polymorphs, with values of 10.1 (4) in (I) and 8.0 (3) in (II). The dihedral angles between plane N1-C4-C5 and plane N2-C7-O1 adopt larger values of 49.0 (2) in (I) and 51.5 (2) in (II). This means that the carbamoyl unit N2-C7-O1 is rotated further away from the enamine group than the other carbamoyl unit. Small differences between both polymorphs are found in the orientation of the phenyl groups relative to the carbamoyl units (see Table 3).

Supramolecular features
The density of (I) is 1.243 and of (II) 1.235 Mg m À3 . The molecular arrangement is different in both crystals because of the crystal symmetry. The 2 1 screw axes running parallel to the b-axis in the monoclinic crystal (I) lead to a parallel arrangement of molecules in the unit cell. In contrast, in the tetragonal crystal of (II), the molecules are grouped around the 4 1 screw axes running parallel to the c-axis. This leads to pairs of molecules that are oriented at an angle of 90 to each other. In (I) and (II), these dimers are formed by the intermolecular N2-H2NÁ Á ÁO2 hydrogen bonds, described by graph set R 2 2 (12) (see Fig. 3 Table 2 Selected geometric parameters (Å , ) for (II).

Figure 2
Diagram of polymorph (II) showing the atom-labelling scheme. Atomic displacement parameters are at the 50% probability level.

Figure 1
Diagram of polymorph (I) showing the atom-labelling scheme. Atomic displacement parameters are at the 50% probability level. Table 3 Dihedral angles ( ) between selected planes in (I) and (II).
Plane 1 Plane 2 (I) ( II) by a weak C10-H10Á Á ÁO1 interaction, resulting in a chain along the crystallographic a-axis direction in (I) (Fig. 4). In (II), these dimers are connected not via this C-HÁ Á ÁO contact, but by weak C-HÁ Á Á interactions, forming intermolecular chains along the c-axis direction (Fig. 5). The latter are also observed in (I).

Figure 3
Intermolecular N-HÁ Á ÁO interactions leading to dimers in the crystal structure of (I), representative of both polymorphs.

Figure 4
Packing diagram of polymorph (I) viewed along the c axis. Intermolecular N-HÁ Á ÁO and C-HÁ Á ÁO interactions are shown.
possible due to the lability of the -hydrogen atoms of the enamine (Ozaki, 1972). Traces of water lead to the cleavage of the Si-N bond from the intermediate to yield the title compound.
To a solution of 0.46 g (3 mmol) N-propyl-N-trimethylsilylprop-1-en-2-amine in 10 mL of n-pentane was added dropwise 0.60 g (5 mmol) of phenylisocyanate at 273 K. After standing three days at room temperature, some crystals suitable for single-crystal X-ray diffraction were obtained. The polymorphs were recognised by their different crystal shapes: (I) forms small prisms, (II) forms large flat prisms. Both are colourless.
NMR spectroscopy showed that the batch product is a mixture of many components. Further purification of the product mixture was not successful.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 6. Hydrogen atoms bonded to C were positioned geometrically and allowed to ride on their parent atoms, with C-H = 0.95 Å for H(Ph), 0.99 for CH 2 , and 0.98 Å for CH 3 . U iso (H) = xU eq (C), where x = 1.2 for H(Ph) and CH 2 , and 1.5 for CH 3 . Hydrogen atoms on nitrogen were localized from residual electron-density maps and were freely refined.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

N,N′-Diphenyl-2-[1-(propylamino)ethylidene]propanediamide (II)
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.25 e Å −3 Δρ min = −0.18 e Å −3 Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.