Fused pyrazole derivatives are of potential value in a wide range of drug, pesticide and new materials applications (Elguero, 1984
) and, associated with a synthetic study of these systems, we report here the structure of the title compound, (I)
(Fig. 1). This compound was prepared rapidly and in high yield by means of a cyclization reaction between 5-amino-3-phenyl-1H
-pyrazole and 3-amino-2-phenyldiazenyl-2-butenenitrile (3-amino-3-methyl-2-phenyldiazenylacrylonitrile), mediated by microwave radiation under solvent-free (green) conditions (see scheme
). Compound (I)
shows unexpected crystallization behaviour and an interesting supramolecular structure.
Figure 1 The independent molecular components of compound (I), showing the atom-labelling scheme for (a) the type 1 molecule and (b) the type 2 molecule. Displacement ellipsoids are drawn at the 30% probability level.
crystallizes with Z
′ = 2 in the space group P
; it is convenient to describe the independent molecules containing atoms N11 (Fig. 1
) and N21 (Fig. 1
) as molecules of types 1 and 2, respectively. The presence of two independent molecules necessarily introduces some flexibility into the choice of the asymmetric unit, but for (I)
it is possible to specify a compact asymmetric unit in which the two independent molecules are linked by two C—H
π(arene) hydrogen bonds (Table 2 and Fig. 2). With the exception of the ortho
positions of the terminal phenyl rings, the coordinates of corresponding pairs of atoms in the two molecules in the selected asymmetric unit are approximately related by the transformation (
). In each of the two molecules, the skeletons between atoms Cn
21 and Cn
61, where n
= 1 or 2 (Fig. 1), are close to planarity, as indicated by the relevant torsion angles (Table 1). However, while the torsion angles describing the orientations of the phenyl rings have similar magnitudes in the two independent molecules, the corresponding pairs of values have opposite signs, indicating that the two molecules in the selected asymmetric unit are approximate, but not exact, mirror images of one another. The combination of the approximate translational relationship between these molecules and their approximate conformational enantiomerism precludes the possibility of any additional symmetry. The differences in the hydrogen-bonding behaviour of the two molecules, discussed below, further confirms the absence of any additional crystallographic symmetry.
Hydrogen-bond geometry (Å, °)
Figure 2 The two independent molecules in the selected asymmetric unit of compound (I), showing the two C—Hπ(arene) hydrogen bonds within the asymmetric unit. For the sake of clarity, H atoms not involved in the motif shown have (more ...)
Selected geometric parameters (Å, °)
The bond distances within the pyrazolo[1,5-a
]pyrimidine unit of compound (I)
are very similar in the two independent molecules (Table 1), and they are consistent with a more marked degree of bond fixation with alternating single and double bonds between atoms Nn
1 and Nn
= 1 or 2) (see scheme
). This geometry may be contrasted with that found in the simpler analogues (II) (Portilla, Quiroga, Cobo et al.
), (III) and (IV) (Portilla, Quiroga, de la Torre et al.
), and (V) (Portilla et al.
), which exhibit naphthalene-type delocalization. In addition, the exocyclic C—N bonds Cn
= 1 or 2) are very much shorter in compound (I)
than in analogues (II)–(V), where the corresponding distances range from 1.330 (2) Å for one of the molecules in (III) to 1.3705 (19) Å in (V). The short exocyclic C—N distances in (I)
may be associated both with the more strongly localized electronic structure of the pyrazolo[1,5-a
]pyrimidine unit in (I)
, and with possible delocalization into the phenyldiazenyl substituent: the distances Nn
= 1 or 2) are long for their type [mean value (Allen et al.
) 1.222 Å, upper quartile value 1.227 Å], while the bonds Cn
61 are much shorter than the bonds Nn
61, suggesting some contribution to the overall electronic structure of (I)
from polarized form (Ia
). On this basis, the short intramolecular N—H
N hydrogen bonds (Table 2) can be regarded as charge-assisted hydrogen bonds (Gilli et al.
The molecules of compound (I)
are linked into complex sheets by a combination of two N—H
N hydrogen bonds and four C—H
π(arene) hydrogen bonds (Table 2). The formation of the sheet can readily be analysed in terms of two ladder-like substructures, each of them one-dimensional.
In one of the substructures, formation of the ladder-type structure depends upon the combination of the intermolecular N—H
N hydrogen bonds with the two C—H
π(arene) hydrogen bonds within the asymmetric unit. Type 1 molecules which are related by the n
-glide plane at y
= 0.25 are linked by an N—H
N hydrogen bond to form a C
(6) (Bernstein et al.
) chain running parallel to the  direction, and an entirely similar chain is formed by the type 2 molecules. Within each pair of molecules, one each of types 1 and 2, at any given symmetry position, the molecules are linked by two C—H
π(arene) hydrogen bonds. Hence, the substructure parallel to  takes the form of a molecular ladder (Fig. 3), in which the uprights are formed by the C
(6) chains of N—H
N hydrogen bonds and the treads are formed by the paired C—H
π(arene) hydrogen bonds.
Figure 3 A stereoview of part of the crystal structure of compound (I), showing the formation of a hydrogen-bonded molecular ladder along  built from N—HN and C—Hπ(arene) hydrogen bonds. For the sake of clarity, (more ...)
The second substructure is built solely from the four C—H
π(arene) hydrogen bonds, with the type 2 molecule acting as a fourfold donor of hydrogen bonds and the type 1 molecule as a fourfold acceptor (Table 2). This difference in donor and acceptor behaviour between the two independent molecules, which is evident only for the C—H
π(arene) hydrogen bonds, not for the N—H
N hydrogen bonds, is a further indication of the absence of any additional crystallographic symmetry. Each of the aryl rings in the type 1 molecule acts as a double acceptor, with one donor atom bonding to each face of each ring, with HCg
H angles of 173 and 176° at the centroids of rings C121–C126 and C161–C166, respectively. The type 2 molecule at (x
) acts as hydrogen-bond donor, via
C226 and C262, to the type 1 molecule at (x
), and via
C223 and C265 to the type 1 molecule at (−1 + x
), so forming by translation a molecular ladder running parallel to the  direction (Fig. 4); here the uprights of the ladder are provided by the hydrogen bonds, and the treads are provided by the molecules themselves. The combination of the molecular ladders along  and  generates a sheet of considerable complexity, which lies parallel to (010).
Figure 4 A stereoview of part of the crystal structure of compound (I), showing the formation of a hydrogen-bonded molecular ladder along  built from C—Hπ(arene) hydrogen bonds only. For the sake of clarity, H atoms not involved (more ...)
It is of interest briefly to compare the hydrogen bonding, and hence the supramolecular aggregation, in compound (I)
with that in the analogous 7-aminopyrazolo[1,5-a
]pyrimidines (II)–(V) (see scheme
), none of which contain any phenyl groups. Whereas the intermolecular N—H
N hydrogen bonds in compound (I)
generate two independent C
(6) chains, in each of compounds (II)–(V) pairs of molecules are linked by pairs of N—H
N hydrogen bonds to form dimeric units containing
(10) motifs. In compounds (II) (Portilla, Quiroga, Cobo et al.
), (IV) (Portilla, Quiroga, de la Torre et al.
) and (V) (Portilla et al.
), the dimers are formed from pairs of molecules related, respectively, by a twofold rotation in the space group C
2, by inversion in the space group P
, and again by a twofold rotation, this time in the space group P
2 (or P
2). In compound (III), which crystallizes with Z
′ = 2 in the space group P
(Portilla, Quiroga, de la Torre et al.
), the two molecules in the asymmetric unit are linked by two independent N—H
N hydrogen bonds to form an
(10) dimer having no crystallographic symmetry. Further hydrogen bonds, of N—H
O and O—H
N types, link the molecular components in compounds (II) and (IV) into three-dimensional framework structures, while N—H
N hydrogen bonds alone suffice to form a three-dimensional framework in compound (V). In compound (III), four independent N—H
N hydrogen bonds link the molecules into a ribbon containing three types of hydrogen-bonded ring, one of
(1) type and two of
(14) type. Thus, no two compounds in this rather closely related series (I)–(V) exhibit similar crystallization characteristics, in terms of the combination of Z
′ value and space group, and no two adopt the same hydrogen-bonded supramolecular structure.