organic compounds
some support for a contribution to the electronic structure
from the polarized form, (Ia) (see Scheme). However, there is
no evidence for any participation by the ester function.
As noted above, the N—H bond participates in a three-
centre intramolecular hydrogen bond (Table 2), giving rise to
two edge-fused S(6) (Bernstein et al., 1995) motifs, but it plays
no role in the intermolecular hydrogen bonding. This is
determined solely by a C—Hꢀ ꢀ ꢀO hydrogen bond in which,
despite the presence of two independent carbonyl groups in
the molecule, one in an amide group and the other in an ester
group, the hydrogen-bond acceptor is one of the nitro-group
O atoms, O522, which is also a participant in the intra-
molecular three-centre system. By contrast, where amidic
carbonyl groups and nitro groups are present in the same
molecules, the amidic O atoms usually appear to act as the
better hydrogen-bond acceptors (Garden et al., 2005; Wardell
et al., 2005, 2006). The acceptor behaviour in compound (I) is
thus consistent with the development of significant negative
charge on the O atoms of the nitro group as a consequence of
the electronic polarization discussed above. The effect of the
C—Hꢀ ꢀ ꢀO hydrogen bond is to link molecules related by the
21 screw axis along (12, y, 43) into a C(7) (Bernstein et al., 1995)
chain running parallel to the [010] direction (Fig. 2).
Figure 2
A stereoview of part of the crystal structure of compound (I), showing the
formation of edge-fused S(6) rings and a C(7) hydrogen-bonded chain
parallel to [010]. For the sake of clarity, H atoms not involved in the motif
shown have been omitted.
It is of interest briefly to compare the supramolecular
aggregation in compound (I), where there is an excess of
conventional hydrogen-bond acceptors over donors, with that
in the closely related series of compounds (II)–(IV) (see
Scheme). In compound (II) (Quiroga et al., 2010), there is an
excess of conventional hydrogen-bond donors over acceptors;
compound (III) (Quiroga et al., 2008) contains equal numbers
of conventional hydrogen-bond donors and acceptors; and in
compound (IV) (Quiroga et al., 2008) the conventional
hydrogen-bond acceptors are in excess over the donors, as in
compound (I). In each of the structures of compounds (II)–
(IV), there is an intramolecular N—Hꢀ ꢀ ꢀO hydrogen bond.
The molecules of compound (II) are linked into sheets by a
combination of N—Hꢀ ꢀ ꢀN, C—Hꢀ ꢀ ꢀO and C—Hꢀ ꢀ ꢀꢀ(arene)
hydrogen bonds, so that the single O atom present acts as a
double acceptor of hydrogen bonds, while the aryl ring and the
two-connected N atom of the pyrazole ring both also act as
acceptors. In the 4-methoxy derivative, compound (III), the
molecules are linked into a chain of rings by a combination of
N—Hꢀ ꢀ ꢀN and N—Hꢀ ꢀ ꢀꢀ(arene) hydrogen bonds, while in the
2-nitro derivative, compound (IV), a combination of one N—
Hꢀ ꢀ ꢀN hydrogen bond and three C—Hꢀ ꢀ ꢀO hydrogen bonds,
all of which utilize nitro O atoms as the acceptors, links the
molecules into a three-dimensional framework structure.
Thus, in each of the crystal structures of compounds (I)–(IV),
a different range of hydrogen bonds is utilized in the aggre-
gation and the resulting hydrogen-bonded structures are all
different: a simple chain in (I), a chain of rings in (III), a sheet
in (II) and a three-dimensional framework structure in (IV).
The question thus arises why the dihedral angle between the
rings is not larger, since rotation about the C—N bonds,
particularly about the C5—N51 bond, seems at first sight to
provide a means of avoiding the short intramolecular Hꢀ ꢀ ꢀH
contact which involves a lower energy cost than the substantial
distortion of the bond angles at C5 and N51. A possible
answer lies in the observation that atom N51 acts as the
hydrogen-bond donor to atoms O11 and O522 in a planar
three-centre N—Hꢀ ꢀ ꢀꢀ(O)2 system. While both the acetyl and
the nitro substituents are slightly twisted out of the plane of
their adjacent rings, with dihedral angles of 10.8 (2) and
8.1 (2)ꢁ, respectively, it is notable that in each case the sense of
the rotation about the exocyclic bonds, N1—C11 and C52—
N521, respectively, is such as to bring atoms O11 and O522
closer to atom H51 than they would be if the acetyl and nitro
substituents were coplanar with their adjacent rings. Accord-
ingly, it can be concluded that the N—Hꢀ ꢀ ꢀO interactions are
strongly attractive, and probably charge-assisted (Gilli et al.,
1994) (cf. the discussion of the intramolecular distances
below) and that they are probably one of the key factors
controlling the molecular conformation, in particular the
relative orientations of the two rings.
Secondly, there is evidence for some quinonoid-type bond
fixation within the aryl ring. Thus, the C53—C54 and C55—
C56 distances (Table 1) are the shortest within this ring, while
C51—C52 is the longest. In addition, the C52—N521 distance
˚
is short for its type [mean value (Allen et al., 1987) = 1.468 A,
˚
lower quartile value = 1.460 A], while the two associated N—O
distances are both slightly long for their type. However, the
N51—C51 distance does not differ significantly from the mean
value for bonds of this type, possibly for reasons connected
with the steric demands in the vicinity of atom N51, as
discussed above. Overall, however, the bond distances provide
Experimental
A mixture of methyl 4-(3-tert-butyl-1H-pyrazol-5-ylamino)-3-nitro-
benzoate (1.0 mmol) and acetic anhydride (0.5 ml) was heated at
ꢂ
´
Acta Cryst. (2011). C67, o26–o28
Cortes et al. C17H20N4O5 o27
Cortes, Edwar
