organic compounds
C9 C10ÐN2 torsion angle indicates that the nitrovinyl
group is nearly planar [177.96 (11)ꢀ in (I) and 178.86 (18)ꢀ in
(II)], and the plane of the nitrovinyl group is twisted, by
33.48 (5)ꢀ for (I) and 41.31 (8)ꢀ for (II), with respect to the
indole ring plane. However, comparing the C2ÐC9 bond
Ê
Ê
length [1.4335 (17) A in (I) and 1.451 (3) A in (II)] with the
standard value for a single bond connecting a Car atom to a
Csp2 atom [1.470 (15) A; Wilson, 1992] suggests that the
Ê
indole system has an extended conjugation through the
ethylene double bond and the nitro group. There is further
evidence for this in the shortening of the N1ÐC1 and N2Ð
C10 and lengthening of the C1 C2 and C9 C10 bonds
compared with reported values similar to the N1ÐC1 and
N2ÐC10 bonds for other compounds (Mason et al., 2003;
Strauss et al., 2003). In the case of (II), the phenyl group at the
Figure 4
The crystal structure of (II), viewed along the a axis.
2-position makes a dihedral angle of 32.06 (6)ꢀ with the plane
of the indole ring. This extended conjugation suggests that
compounds (I) and (II) exist predominantly in resonance
forms (Ia) and (IIa), respectively, which explains the highly
coloured and crystalline nature of the title compounds, due to
the extended chromophore, and the increased polarity within
these molecules. Furthermore, the absence of IR absorption
bands at 1550 and 1372 cm 1, which is typical of the nitro
group in 3-(2-nitrovinyl)indoles (Bucki & Mark, 1977), the
1
appearance of two new bands between 1300 and 1250 cm
,
and the intense UV absorption near 400 nm, con®rm the
existence of resonance forms (Ia) and (IIa).
The packing of compounds (I) and (II), as viewed along the
b and a axes, is illustrated in Figs. 3 and 4, respectively. In each,
Figure 2
A view of (II), showing the atom-numbering scheme. Displacement
ellipsoids are drawn at the 50% probability level.
atom H1 participates in a weak intermolecular N1ÐH1Á Á ÁO2
1
2
interaction [symmetry code (12 + x,
y, 12 + z) for (I) and
1
(
x, y, 12 + z) for (II)], linking the title compounds into
chains that run parallel to [101] and [001], respectively.
2
Experimental
For the preparation of (I), solid ammonium acetate (0.3 g, 3.8 mmol)
was added to a suspension of 2-methylindole-3-carbaldehyde (1.08 g,
6.8 mmol) in nitroethane (3 ml). The mixture was stirred vigorously
under re¯ux at 393±403 K for 2 h. The mixture was then cooled, and
the solid which appeared was collected by ®ltration. Recrystallization
from methanol afforded orange±brown crystals of (I) suitable for
X-ray analysis. 1H NMR (CDCl3): ꢀ 2.26 (s, 3H), 2.42 (s, 3H), 7.11 (pd,
2H), 7.37 (dd, 1H), 7.47 (dd, 1H), 8.28 (s, 1H), 11.92 (s, 1H); 13C NMR
(CDCl3): ꢀ 12.6, 15.8, 105.3, 111.5, 119.4, 120.3, 121.7, 125.6, 129.1,
135.9, 141.5, 145.6. For the preparation of (II), solid ammonium
acetate (0.3 g, 3.8 mmol) was added to a suspension of 2-phenyl-
indole-3-carbaldehyde (1.5 g, 6.8 mmol) in nitroethane (3 ml). The
mixture was stirred vigorously under re¯ux at 393±403 K for 2 h. The
mixture was then cooled and the solid which appeared was collected
by ®ltration. Recrystallization from methanol afforded orange±
brown crystals of (II) suitable for X-ray analysis. 1H NMR (CDCl3): ꢀ
2.22 (s, 3H), 7.20 (m, 2H), 7.58 (m, 7H), 8.17 (s, 1H), 12.28 (s, 1H);
13C NMR (CDCl3): ꢀ 15.7, 105.0, 112.1, 120.0, 120.7, 122.7, 125.9,
128.7, 128.9, 129.2, 131.0, 136.5, 141.2, 144.0.
Figure 3
The crystal structure of (I), viewed along the b axis.
ꢁ
o528 Sonar et al. C12H12N2O2 and C17H14N2O2
Acta Cryst. (2005). C61, o527±o530