organic compounds
Figure 2
A stereoview of part of the crystal structure of (I), showing the formation
of a hydrogen-bonded chain of edge-fused centrosymmetric rings running
parallel to the [101] direction. For the sake of clarity, H atoms not
involved in the motifs shown have been omitted.
Figure 1
The molecular structure of (I), showing the atom-labelling scheme.
Displacement ellipsoids are drawn at the 30% probability level.
bond donor to the brominated aryl ring C21–C26 in the
molecule at (ꢂx, 1 ꢂ y, 1 ꢂ z), so forming a second cyclic
molecule of (I) has no internal symmetry and hence it is
conformationally chiral. However, the centrosymmetric space
group accommodates equal numbers of the two conforma-
tional enantiomers.
1
2
centrosymmetric motif this time centred at (0, 12, ). The
combination of these two ring motifs, and their propagation by
inversion, leads to the formation of a chain of edge-fused rings
running parallel to the [101] direction, in which the rings
formed by pairs of N—Hꢀ ꢀ ꢀO hydrogen bonds are centred at
(n + 12, 12, n + 1), where n represents an integer, while the rings
formed by pairs of C—Hꢀ ꢀ ꢀꢀ(arene) hydrogen bonds are
The conformational differences between the molecules of
(I) and (II) cannot plausibly be interpreted in terms of
intramolecular factors only. More probably, they are deter-
mined primarily by the different direction-specific inter-
molecular forces which are manifest in the two crystal
structures, in particular, the different hydrogen-bonding
patterns, which involve both the pyrazole ring and the ester
unit in each compound, and which are discussed below.
There is strong bond fixation within the imidazole ring in
(I), as exemplified by the C2—N3 and N3—C3a distances
(Table 1). In the adjacent aryl ring, the longest of the
peripheral C—C bonds is C5—C6, while the exocyclic C5—
C51 bond is short for its type [mean value (Allen et al., 1987) =
1
1
centred at (n, , n + ), where n again represents an integer
2
2
(Fig. 2).
The hydrogen bonds present in (I) and the resulting
supramolecular structure provide an interesting contrast with
those in (II) (Portilla et al., 2007). As noted above, (II) crys-
tallizes with Z0 = 2 and each of the independent molecules
forms an independent hydrogen-bonded substructure. Each of
the two types of molecule in (II) participates in one N—Hꢀ ꢀ ꢀN
and one C—Hꢀ ꢀ ꢀO hydrogen bond, in which the donors are
both components of the pyrazole ring, as in (I), while the
acceptors are, respectively, the two-coordinated N atom of the
imidazole ring and the carbonyl O atom. Each type of mol-
ecule in (II) then forms a hydrogen-bonded sheet containing a
single type of R44(28) ring and in which the component mol-
ecules are all related to one another by translation. There are
no direction-specific interactions between the two types of
sheets, which are stacked alternately along [001], and, in
particular, there are no direction-specific interactions between
the two independent molecules.
Compounds (I) and (II) thus differ, despite their similar
molecular constitutions, in their crystallization characteristics
(space groups and Z0 values), in their molecular conforma-
tions, in their hydrogen bonds, where the same two donors are
present but involved with different sets of acceptors, and in
their overall hydrogen-bonded structures, viz. a chain of fused
rings in (I) and two independent sheets in (II).
˚
˚
1.487 A and lower-quartile value = 1.480 A]. However, the
remaining bond distances provide no evidence for any
significant polarization or charge separation in the molecular
fragment between atoms N1 and O51.
The molecules of (I) are linked by a combination of N—
Hꢀ ꢀ ꢀO and C—Hꢀ ꢀ ꢀꢀ(arene) hydrogen bonds (Table 2).
However, despite the presence within the molecule of two
essentially unencumbered aryl rings, aromatic ꢀ–ꢀ stacking
interactions are absent from the crystal structure of (I), nor
are there any short intermolecular contacts involving pairs of
Br atoms (Ramasubbu et al., 1986). It is convenient to consider
first the actions of the two independent hydrogen bonds, and
then their action in combination. Pyrazole ring atom N11 at (x,
y, z) acts as hydrogen-bond donor to carbonyl atom O51 in the
molecule at (1 ꢂ x, 1 ꢂ y, 2 ꢂ z), so forming a cyclic
centrosymmetric R22(22) (Bernstein et al., 1995) ring centred at
1
(12, , 1). Pyrazole ring atom C14 at (x, y, z) acts as hydrogen-
2
ꢃ
´
Acta Cryst. (2011). C67, o64–o66
Cortes et al. C22H21BrN4O2 o65
Cortes, Edwar
