N,N-Diphenyl­benzamide

In the title compound, C₁₉H₁₅NO, the neutral mol­ecules are held together in the crystal structure by very weak C—H...O inter­actions, giving rise to a linear chain-like structure. The structure of the mol­ecule has been optimized using density functional theory at the B3LYP/6-31G(d) level and this is compared with the mol­ecular structure in the solid state. The two structures show significant differences in the relative orientations of the aromatic rings, which is inter­esting for further supra­molecular study. Apart from the crystal structure analysis, powder X-ray diffraction, UV–visible and thermogravimetric analyses of the compound have been carried out.

S1. Comment

The natural occurrence of amide bonds in peptides and proteins makes the amide function important to synthetic chemists (Albericio, 2004; Singh, 2003). Moreover, crystallographic studies of the amide functionality are as old as the discovery of polymorphism: it was the molecular crystal of benzamide, the simplest of the aromatic amides, where Wöhler & von Liebig (1832) described the existence of polymorphism for the first time. Since Wöhler and von Liebig's discovery, the literature has witnessed a number of reports regarding the structural study of amides. Of the various different amides, tertiary aromatic amides are of special interest due to the high rotation barrier around the C—N bond, which potentially affords different isomers of amide derivatives (Yamasaki et al., 2006). Structural studies are important in this respect to examine and understand different possible isomers or polymorphs of a molecule. We report here the synthesis, characterization and crystal structure analysis of the title compound, (I). A comparison between the experimental structure and the optimized structure obtained from density functional theory (DFT) calculation is given.

Compound (I) was synthesized through a simple one-pot solution-phase synthetic route, as shown in the scheme. The FT–IR spectrum of (I) shows the carbonyl stretch at 1651 cm^-1 and the aromatic C—H stretch at 2943 cm^-1. The NMR spectrum recorded in CDCl₃ (400 MHz spectrometer) shows the presence of signals at 7.5 (d, 3H, J = 8 Hz), 7.3 (m, 6H) and 7.16 (m, 6H) p.p.m. for the aromatic H atoms. Single crystals of (I) suitable for X-ray analysis were grown from ethanol [Methanol given in exptl_prep - please check].

Single-crystal X-ray analysis of (I) shows it to crystallize in the orthorhombic space group Pbca with eight molecules in the unit cell (Z = 8). Interestingly, (I) is found to be isostructural with α,α-N-triphenylnitrone, with the same empirical formula C₁₉H₁₅NO, reported by Brown & Trefonas (1973). However, the two compounds are chemically significantly different.

The asymmetric unit of (I) is shown in Fig. 1. The C13—O1 (carbonyl) bond length is 1.2193 (14) Å, which is comparable with other reported C—O bond length for tertiary amides (Branca et al., 2008). In the case of α,α-N-triphenylnitrone, there is no C—O bond, but an N—O bond of 1.300 Å is present. The N1—C13 bond length in (I) is 1.3734 (16) Å, while the corresponding bond in α,α-N-triphenylnitrone is shorter (1.327 Å). The N1 centre in (I) is in an almost planar environment, with bond angles C13—N1—C7 = 123.07 (10), C13—N1—C1 = 119.35 (10) and C1—N1—C7 = 116.86 (9)°. However, in α,α-N-triphenylnitrone the angles around the N atom are significantly different from those in (I) [Quote values?]. The shorter bond distance and different the bond angles around the N atom support the contribution of partial π character in the C—N bond of α,α-N-triphenylnitrone (C—N single-bond distance of 1.47 Å; Reference for standard value?). [Original sentence was not clear - please check rephrasing]

In general, the C-aryl ring of a tertiary amide remains almost orthogonal to the C═O plane (Branca et al., 2008). However, in the case of (I) the dihedral angle between the mean planes of the benzamide ring and the N1/C13/O1 group is 48.77 (14)°. Moreover, the dihedral angle between the two N-phenyl rings is 71.54 (4)°. These observations vary to some extent from the optimized structure obtained from density functional theory (DFT) calculations (Kohn & Sham, 1965), performed at the B3LYP/6-31G(d) level (Becke, 1993; Lee et al., 1988) using GAUSSIAN03 (Frisch et al., 2004). The DFT calculations predict the C13—O1 and N1—C13 bond lengths to be 1.22 and 1.40 Å, respectively. The average aromatic C—C bond length is calculated to be 1.39 Å, while the experimentally obtained value is 1.37 Å. The calculated bond angles around N1 are C13—N1—C7 = 123.0, C13—N1—C1 = 118.3 and C1—N1—C7 = 117.4°, and the calculated dihedral angle between the two N-phenyl rings is 79.73°. In addition, the calculated dihedral angle between the mean planes of the benzamide ring and the N1/C13/O1 group is 35.29°, which differs significantly from the crystallographically determined value of 48.77 (14)°. These discrepancies between the calculated and crystallographically determined values may be accounted for by solid-state packing effects in the crystal structure of (I), which are not included in the gas-phase DFT structure optimization. Thus, it is clear that, although the bond distances are similar for the two structures, the dihedral angles between the various aromatic rings differ significantly.

These comparisons between the DFT-optimized and experimental structures open up the possibility of polymorphism in N,N-diphenylbenzamide arising from the relative orientations of the phenyl rings. Such an observation may attract interest towards a new supramolecular study of N,N-diphenylbenzamide and its possible derivatives with substituted phenyl rings.

Apart from these observations, the molecules of (I) in the crystal structure are associated through a very weak C19—H19···O1 interaction, with H19···O1 = 2.48 Å and C19—H19···O1 = 148°. According to the hydrogen-bonding classification provided by Steed & Atwood (2009), this interaction is best described as a very weak electrostatic interaction. This interaction gives the molecule a linear chain-like supramolecular structure, as shown in Fig. 2.

Compound (I) shows an absorption band in the UV region at λ_max = 306 nm in the solid state, which agrees well with the value of λ_max = 301 nm predicted by DFT calculation. This band may be attributed to an intramolecular charge transfer (ICT) from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). Such a correlation was recently made by Seidel et al. (2009). DFT calculation further reveals that the HOMO is primarily localized on the electron-rich N,N-diphenyl group, while the LUMO is primarily localized on the electron-deficient benzamide group (Fig. 3).

The purity of a chemical sample is an important criterion for spectroscopic analysis. To check the phase purity of the bulk sample of (I), we recorded powder X-ray diffraction data in the 2θ range 5–50°. The powder pattern agrees well with the simulated spectrum, which supports the phase purity of the sample. The thermal stability of the compound was studied under an N₂ atmosphere in the temperature range 298–823 K. The thermogram shows that N,N-diphenylbenzamide is stable up to 388 K. Above this temperature it undergoes continuous degradation, with total decomposition of the molecule in the temperature range 388–498 K.

S2. Experimental

Diphenylamine (2.5 g, 0.015 mol) was dissolved in dry dichloromethane (20 ml) and triethylamine (2.07 ml, 0.015 mol) was added. The solution was stirred at 273 K for 15 min and then benzoyl chloride (2.1 g, 0.015 mol) was added dropwise to the stirred solution. The reaction mixture was stirred overnight at room temperature. Water (10 ml) was added to the reaction mixture and the organic layer was collected using a separating funnel. The solvent was then removed under reduced pressure to obtain the crude product, (I). The product was further purified by recrystallization from methanol. [Ethanol given in Comment - please check]

S3. Refinement

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.

H atoms were placed in their calculated positions and refined isotropically using a riding model, with C—H = 0.93 Å and U_iso(H) = 1.2U_eq(C). [Please check added text]

Computing details

Data collection: SMART (Bruker, 2007); cell refinement: SAINT (Bruker, 2007); data reduction: SAINT (Bruker, 2007); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008) and GAUSS VIEW (Version 4.1; Frisch et al., 2004); software used to prepare material for publication: SHELXTL (Sheldrick, 2008) and ORTEPIII (Burnett & Johnson, 1996).

Figures

	Fig. 1. The molecular structure of (I), showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level.
	Fig. 2. The packing in the structure of (I).
	Fig. 3. (a) The HOMO and (b) the LUMO of the DFT-optimized structure of (I).

N,N-Diphenylbenzamide

Crystal data

C19H15NO	F(000) = 1152
Mr = 273.32	Dx = 1.242 Mg m^−3
Orthorhombic, Pbca	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ac 2ab	Cell parameters from 8931 reflections
a = 17.467 (3) Å	θ = 2.7–26.0°
b = 9.2050 (16) Å	µ = 0.08 mm^−1
c = 18.183 (3) Å	T = 296 K
V = 2923.5 (8) Å^3	Block, colourless
Z = 8	0.39 × 0.32 × 0.24 mm

Data collection

Bruker APEXII CCD area-detector diffractometer	2852 independent reflections
Radiation source: fine-focus sealed tube	2046 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.043
ϕ and ω scans	θmax = 26.0°, θmin = 2.3°
Absorption correction: empirical (using intensity measurements) (SADABS; Sheldrick, 2004)	h = −21→21
Tmin = 0.775, Tmax = 0.822	k = −11→11
32001 measured reflections	l = −21→22

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.036	H-atom parameters constrained
wR(F^2) = 0.103	w = 1/[σ^2(Fo^2) + (0.0554P)^2] where P = (Fo^2 + 2Fc^2)/3
S = 1.08	(Δ/σ)max < 0.001
2852 reflections	Δρmax = 0.15 e Å^−3
191 parameters	Δρmin = −0.12 e Å^−3
0 restraints	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0028 (5)

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
C1	0.25674 (6)	0.73181 (13)	0.33338 (7)	0.0475 (3)
C2	0.23812 (7)	0.79995 (16)	0.39822 (7)	0.0625 (4)
H2	0.1911	0.8465	0.4031	0.075*
C3	0.28925 (9)	0.79918 (19)	0.45604 (9)	0.0801 (5)
H3	0.2762	0.8435	0.5003	0.096*
C4	0.35927 (10)	0.73321 (19)	0.44848 (10)	0.0860 (5)
H4	0.3938	0.7334	0.4874	0.103*
C5	0.37799 (9)	0.66741 (18)	0.38369 (12)	0.0860 (5)
H5	0.4256	0.6234	0.3785	0.103*
C6	0.32689 (8)	0.66550 (14)	0.32580 (9)	0.0666 (4)
H6	0.3398	0.6197	0.2819	0.080*
C7	0.12335 (6)	0.70356 (12)	0.29283 (6)	0.0427 (3)
C8	0.10424 (6)	0.57453 (13)	0.32716 (6)	0.0482 (3)
H8	0.1415	0.5043	0.3357	0.058*
C9	0.02951 (6)	0.55086 (14)	0.34868 (7)	0.0546 (3)
H9	0.0166	0.4647	0.3723	0.066*
C10	−0.02577 (7)	0.65310 (15)	0.33557 (7)	0.0577 (4)
H10	−0.0760	0.6361	0.3501	0.069*
C11	−0.00697 (7)	0.78062 (14)	0.30093 (7)	0.0580 (4)
H11	−0.0447	0.8494	0.2914	0.070*
C12	0.06790 (7)	0.80724 (13)	0.28010 (7)	0.0513 (3)
H12	0.0808	0.8946	0.2576	0.062*
C13	0.22515 (7)	0.77392 (15)	0.20499 (7)	0.0561 (4)
C14	0.17050 (7)	0.75485 (14)	0.14230 (6)	0.0499 (3)
C15	0.16174 (7)	0.86899 (14)	0.09344 (7)	0.0592 (3)
H15	0.1877	0.9557	0.1015	0.071*
C16	0.11502 (8)	0.85520 (18)	0.03326 (8)	0.0688 (4)
H16	0.1081	0.9337	0.0018	0.083*
C17	0.07857 (8)	0.72682 (19)	0.01923 (8)	0.0716 (4)
H17	0.0473	0.7178	−0.0219	0.086*
C18	0.08823 (8)	0.61105 (16)	0.06609 (8)	0.0691 (4)
H18	0.0641	0.5232	0.0561	0.083*
C19	0.13389 (7)	0.62479 (15)	0.12822 (7)	0.0601 (4)
H19	0.1398	0.5468	0.1602	0.072*
N1	0.20232 (5)	0.73011 (11)	0.27379 (6)	0.0480 (3)
O1	0.28880 (6)	0.82391 (14)	0.19418 (6)	0.0931 (4)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C1	0.0408 (7)	0.0483 (7)	0.0534 (8)	−0.0078 (5)	−0.0056 (6)	0.0112 (6)
C2	0.0469 (7)	0.0820 (10)	0.0586 (9)	−0.0098 (6)	0.0008 (6)	0.0035 (7)
C3	0.0746 (11)	0.1082 (12)	0.0576 (10)	−0.0310 (9)	−0.0067 (8)	0.0038 (8)
C4	0.0760 (11)	0.0986 (13)	0.0833 (13)	−0.0212 (9)	−0.0367 (9)	0.0229 (10)
C5	0.0608 (10)	0.0841 (12)	0.1131 (15)	0.0086 (8)	−0.0314 (9)	0.0080 (10)
C6	0.0564 (8)	0.0639 (9)	0.0794 (10)	0.0082 (6)	−0.0130 (7)	0.0003 (7)
C7	0.0383 (6)	0.0473 (7)	0.0424 (7)	−0.0020 (5)	−0.0026 (5)	0.0011 (5)
C8	0.0443 (7)	0.0475 (7)	0.0529 (7)	−0.0020 (5)	−0.0042 (5)	0.0062 (6)
C9	0.0523 (8)	0.0579 (8)	0.0537 (8)	−0.0132 (6)	−0.0016 (6)	0.0049 (6)
C10	0.0395 (7)	0.0770 (10)	0.0567 (8)	−0.0056 (6)	0.0021 (6)	−0.0049 (7)
C11	0.0468 (7)	0.0648 (9)	0.0623 (9)	0.0125 (6)	−0.0018 (6)	−0.0027 (7)
C12	0.0508 (7)	0.0469 (7)	0.0562 (8)	0.0024 (5)	−0.0018 (6)	0.0050 (5)
C13	0.0471 (7)	0.0627 (9)	0.0584 (9)	−0.0068 (6)	0.0017 (6)	0.0169 (6)
C14	0.0448 (7)	0.0585 (8)	0.0463 (7)	0.0026 (6)	0.0059 (5)	0.0076 (6)
C15	0.0668 (8)	0.0601 (9)	0.0506 (8)	0.0025 (6)	0.0045 (6)	0.0082 (6)
C16	0.0744 (9)	0.0818 (11)	0.0502 (9)	0.0127 (8)	0.0016 (7)	0.0133 (7)
C17	0.0625 (9)	0.1050 (13)	0.0473 (9)	0.0070 (8)	−0.0008 (7)	−0.0017 (8)
C18	0.0650 (9)	0.0784 (10)	0.0638 (10)	−0.0085 (7)	0.0041 (7)	−0.0133 (8)
C19	0.0596 (8)	0.0608 (9)	0.0599 (9)	0.0020 (6)	0.0063 (6)	0.0080 (7)
N1	0.0390 (5)	0.0554 (6)	0.0496 (6)	−0.0041 (4)	−0.0015 (4)	0.0103 (5)
O1	0.0569 (6)	0.1381 (10)	0.0844 (8)	−0.0354 (6)	−0.0055 (5)	0.0468 (7)

Geometric parameters (Å, º)

C1—C2	1.3744 (18)	C10—C11	1.3720 (18)
C1—C6	1.3758 (17)	C10—H10	0.9300
C1—N1	1.4415 (15)	C11—C12	1.3834 (17)
C2—C3	1.3795 (19)	C11—H11	0.9300
C2—H2	0.9300	C12—H12	0.9300
C3—C4	1.373 (2)	C13—O1	1.2193 (14)
C3—H3	0.9300	C13—N1	1.3734 (16)
C4—C5	1.364 (2)	C13—C14	1.4972 (17)
C4—H4	0.9300	C14—C19	1.3811 (17)
C5—C6	1.380 (2)	C14—C15	1.3843 (16)
C5—H5	0.9300	C15—C16	1.3710 (19)
C6—H6	0.9300	C15—H15	0.9300
C7—C12	1.3793 (16)	C16—C17	1.366 (2)
C7—C8	1.3827 (16)	C16—H16	0.9300
C7—N1	1.4430 (14)	C17—C18	1.375 (2)
C8—C9	1.3799 (15)	C17—H17	0.9300
C8—H8	0.9300	C18—C19	1.3886 (19)
C9—C10	1.3692 (16)	C18—H18	0.9300
C9—H9	0.9300	C19—H19	0.9300
C2—C1—C6	119.94 (12)	C10—C11—C12	120.23 (11)
C2—C1—N1	119.59 (11)	C10—C11—H11	119.9
C6—C1—N1	120.48 (12)	C12—C11—H11	119.9
C1—C2—C3	119.89 (13)	C7—C12—C11	119.69 (12)
C1—C2—H2	120.1	C7—C12—H12	120.2
C3—C2—H2	120.1	C11—C12—H12	120.2
C4—C3—C2	120.18 (16)	O1—C13—N1	121.49 (12)
C4—C3—H3	119.9	O1—C13—C14	120.19 (11)
C2—C3—H3	119.9	N1—C13—C14	118.29 (10)
C5—C4—C3	119.76 (14)	C19—C14—C15	119.20 (12)
C5—C4—H4	120.1	C19—C14—C13	122.54 (11)
C3—C4—H4	120.1	C15—C14—C13	118.04 (11)
C4—C5—C6	120.62 (15)	C16—C15—C14	120.51 (13)
C4—C5—H5	119.7	C16—C15—H15	119.7
C6—C5—H5	119.7	C14—C15—H15	119.7
C1—C6—C5	119.60 (15)	C17—C16—C15	120.42 (13)
C1—C6—H6	120.2	C17—C16—H16	119.8
C5—C6—H6	120.2	C15—C16—H16	119.8
C12—C7—C8	120.04 (11)	C16—C17—C18	119.84 (14)
C12—C7—N1	120.91 (10)	C16—C17—H17	120.1
C8—C7—N1	118.98 (10)	C18—C17—H17	120.1
C9—C8—C7	119.47 (11)	C17—C18—C19	120.27 (13)
C9—C8—H8	120.3	C17—C18—H18	119.9
C7—C8—H8	120.3	C19—C18—H18	119.9
C10—C9—C8	120.61 (12)	C14—C19—C18	119.71 (12)
C10—C9—H9	119.7	C14—C19—H19	120.1
C8—C9—H9	119.7	C18—C19—H19	120.1
C9—C10—C11	119.95 (11)	C13—N1—C1	119.35 (10)
C9—C10—H10	120.0	C13—N1—C7	123.07 (10)
C11—C10—H10	120.0	C1—N1—C7	116.86 (9)
C6—C1—C2—C3	−1.28 (19)	C19—C14—C15—C16	2.45 (18)
N1—C1—C2—C3	178.55 (12)	C13—C14—C15—C16	177.14 (12)
C1—C2—C3—C4	1.4 (2)	C14—C15—C16—C17	−2.3 (2)
C2—C3—C4—C5	−0.5 (2)	C15—C16—C17—C18	0.6 (2)
C3—C4—C5—C6	−0.5 (3)	C16—C17—C18—C19	1.0 (2)
C2—C1—C6—C5	0.3 (2)	C15—C14—C19—C18	−0.84 (18)
N1—C1—C6—C5	−179.53 (12)	C13—C14—C19—C18	−175.28 (12)
C4—C5—C6—C1	0.6 (2)	C17—C18—C19—C14	−0.89 (19)
C12—C7—C8—C9	−0.21 (17)	O1—C13—N1—C1	−5.3 (2)
N1—C7—C8—C9	177.02 (11)	C14—C13—N1—C1	172.96 (10)
C7—C8—C9—C10	0.74 (18)	O1—C13—N1—C7	164.70 (12)
C8—C9—C10—C11	−0.18 (19)	C14—C13—N1—C7	−17.08 (18)
C9—C10—C11—C12	−0.93 (19)	C2—C1—N1—C13	128.70 (13)
C8—C7—C12—C11	−0.88 (18)	C6—C1—N1—C13	−51.46 (16)
N1—C7—C12—C11	−178.06 (11)	C2—C1—N1—C7	−41.87 (15)
C10—C11—C12—C7	1.46 (19)	C6—C1—N1—C7	137.97 (12)
O1—C13—C14—C19	128.03 (15)	C12—C7—N1—C13	−53.01 (16)
N1—C13—C14—C19	−50.20 (17)	C8—C7—N1—C13	129.78 (13)
O1—C13—C14—C15	−46.47 (18)	C12—C7—N1—C1	117.17 (12)
N1—C13—C14—C15	135.30 (12)	C8—C7—N1—C1	−60.03 (14)