7-Amino-5-methyl-2-phenyl-6-(phenyl­diazenyl)pyrazolo[1,5-a]pyrimidine crystallizes with Z′ = 2: pseudosymmetry and the formation of complex sheets built from N—H⋯N and C—H⋯π(arene) hydrogen bonds

Abstract

The title compound, C₁₉H₁₆N₆, crystallizes with Z′ = 2 in the space group P2₁/n. The two mol­ecules in the selected asym­metric unit are approximate mirror images of one another; most corresponding pairs of atoms are related by an approximate half-cell translation along [100]. Each mol­ecule contains an intra­molecular N—H⋯N hydrogen bond and the mol­ecules are linked into complex sheets by a combination of two inter­molecular N—H⋯N and four C—H⋯π(arene) hydrogen bonds. Comparisons are made with some other 7-amino­pyrazolo[1,5-a]pyrimidines.

Comment

Fused pyrazole derivatives are of potential value in a wide range of drug, pesticide and new materials applications (Elguero, 1984 ▶, 1996 ▶) 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.

Compound (I) crystallizes with Z′ = 2 in the space group P2₁/n; it is convenient to describe the independent mol­ecules containing atoms N11 (Fig. 1 ▶ a) and N21 (Fig. 1 ▶ b) 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 and meta 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 ( + x, y, z). In each of the two mol­ecules, the skeletons between atoms Cn21 and Cn61, 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.

Table 2. Hydrogen-bond geometry (Å, °).

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
N17—H17A⋯N14i	0.88	2.29	2.972 (3)	134
N17—H17B⋯N162	0.88	2.08	2.691 (3)	125
N27—H27A⋯N24i	0.88	2.30	2.953 (3)	131
N27—H27B⋯N262	0.88	2.05	2.656 (3)	125
C223—H223⋯Cg1ii	0.95	2.95	3.701 (3)	137
C226—H226⋯Cg1	0.95	2.84	3.537 (3)	132
C262—H262⋯Cg2	0.95	2.89	3.609 (3)	134
C265—H265⋯Cg2ii	0.95	2.89	3.625 (3)	136

Symmetry codes: (i) ; (ii) .

Figure 2.

The two independent mol­ecules in the selected asymmetric unit of compound (I), showing the two C—H⋯π(arene) hydrogen bonds within the asymmetric unit. For the sake of clarity, H atoms not involved in the motif shown have been omitted.

Table 1. Selected geometric parameters (Å, °).

N11—C12	1.342 (3)
C12—C13	1.398 (4)
C13—C13A	1.357 (4)
C13A—N14	1.359 (3)
N14—C15	1.308 (3)
C15—C16	1.430 (4)
C16—C17	1.390 (4)
C17—N17A	1.356 (3)
N17A—N11	1.363 (3)
C13A—N17A	1.388 (3)
C17—N17	1.314 (3)
C16—N161	1.369 (3)
N161—N162	1.277 (3)
N162—C161	1.420 (3)
N21—C22	1.328 (3)
C22—C23	1.399 (4)
C23—C23A	1.365 (4)
C23A—N24	1.358 (3)
N24—C25	1.313 (3)
C25—C26	1.417 (4)
C26—C27	1.399 (4)
C27—N27A	1.364 (3)
N27A—N21	1.357 (3)
C23A—N27A	1.384 (3)
C27—N27	1.311 (3)
C26—N261	1.380 (3)
N261—N262	1.270 (3)
N262—C261	1.425 (3)

N11—C12—C121—C122	24.1 (4)
C15—C16—N161—N162	−179.1 (2)
C16—N161—N162—C161	175.2 (2)
N161—N162—C161—C162	−20.6 (4)
N21—C22—C221—C222	−20.3 (4)
C25—C26—N261—N262	−177.1 (2)
C26—N261—N262—C261	179.0 (2)
N261—N262—C261—C262	18.9 (4)

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 Nn1 and Nn7 (n = 1 or 2) (see scheme). This geometry may be contrasted with that found in the simpler analogues (II) (Portilla, Quiroga, Cobo et al., 2006 ▶), (III) and (IV) (Portilla, Quiroga, de la Torre et al., 2006 ▶), and (V) (Portilla et al., 2007 ▶), which exhibit naphthalene-type delocalization. In addition, the exocyclic C—N bonds Cn7—Nn7 (n = 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 Nn61—Nn62 (n = 1 or 2) are long for their type [mean value (Allen et al., 1987 ▶) 1.222 Å, upper quartile value 1.227 Å], while the bonds Cn6—Nn61 are much shorter than the bonds Nn62—Cn61, 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., 1994 ▶).

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., 1995 ▶) chain running parallel to the [101] 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 [101] 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 [101] built from N—H⋯N and C—H⋯π(arene) hydrogen bonds. For the sake of clarity, H atoms bonded to C atoms not involved in the motifs shown have been omitted.

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 H⋯Cg⋯H angles of 173 and 176° at the centroids of rings C121–C126 and C161–C166, respectively. The type 2 mol­ecule at (x, y, z) acts as hydrogen-bond donor, via C226 and C262, to the type 1 mol­ecule at (x, y, z), and via C223 and C265 to the type 1 mol­ecule at (−1 + x, y, z), so forming by translation a mol­ecular ladder running parallel to the [100] 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 [100] and [101] 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 [100] built from C—H⋯π(arene) hydrogen bonds only. For the sake of clarity, H atoms not involved in the motifs shown have been omitted.

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., 2006 ▶), (IV) (Portilla, Quiroga, de la Torre et al., 2006 ▶) and (V) (Portilla et al., 2007 ▶), the dimers are formed from pairs of mol­ecules related, respectively, by a twofold rotation in the space group C2, by inversion in the space group P2₁/c, and again by a twofold rotation, this time in the space group P4₁2₁2 (or P4₃2₁2). In compound (III), which crystallizes with Z′ = 2 in the space group P (Portilla, Quiroga, de la Torre et al., 2006 ▶), 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.

Experimental

Equimolar quanti­ties (1 mmol of each component) of 5-amino-3-phenyl-1H-pyrazole and 3-amino-2-phenyl­diazenyl-2-butenenitrile were intimately mixed, and the mixture was placed in an open Pyrex-glass flask, in the absence of solvent, and irradiated in a domestic microwave oven for 5 min at 800 W. The resulting solid material was extracted with ethanol. After removal of the solvent, the product, (I), was recrystallized from dimethyl­formamide to give orange crystals suitable for single-crystal X-ray diffraction (yield 85%, m.p. 501–502 K). NMR (DMSO-d ₆): δ(H) 2.75 (s, 3H, CH₃), 6.90 (s, 1H, 3-H), 7.40 (t, 1H, 64-H), 7.43 (t, 1H, 24-H), 7.48 (t, 2H, 63-H), 7.52 (t, 2H, 23-H), 8.05 (d, 2H, 62-H), 8.07 (d, 2H, 22-H), 8.98, 10.25 (2s, 2H, NH₂); δ(C) 22.2 (CH₃), 93.8 (C3), 117.8 (C6), 121.9 (C62), 126.9 (C22), 129.1 (C23), 129.5 (C24), 129.6 (C63), 129.7 (C64), 133.0 (C21) 140.0 (C7), 149.0 (C3a), 153.2 (C61), 157.2 (C2), 161.5 (C5). MS (70 eV) m/z (%): 328 (100, M ⁺), 313 (23), 77 (19).

Crystal data

• C₁₉H₁₆N₆

• M _r = 328.38

• Monoclinic,

• a = 9.6342 (13) Å

• b = 33.216 (5) Å

• c = 9.747 (3) Å

• β = 90.237 (18)°

• V = 3119.1 (11) ų

• Z = 8

• Mo Kα radiation

• μ = 0.09 mm⁻¹

• T = 120 K

• 0.42 × 0.35 × 0.27 mm

Data collection

• Bruker–Nonius KappaCCD diffractometer

• Absorption correction: multi-scan (SADABS; Sheldrick, 2003 ▶) T _min = 0.963, T _max = 0.976

• 35111 measured reflections

• 5790 independent reflections

• 3135 reflections with I > 2σ(I)

• R _int = 0.083

Refinement

• R[F ² > 2σ(F ²)] = 0.059

• wR(F ²) = 0.169

• S = 1.04

• 5790 reflections

• 453 parameters

• H-atom parameters constrained

• Δρ_max = 0.30 e Å⁻³

• Δρ_min = −0.31 e Å⁻³

All H atoms were located in difference maps and then treated as riding atoms in geometrically idealized positions, with C—H = 0.95 (aromatic and pyrazole) or 0.98 Å (CH₃) and N—H = 0.88 Å, and with U _iso(H) = kU _eq(carrier), where k = 1.5 for methyl H atoms, which were permitted to rotate but not to tilt, and 1.2 for all other H atoms.

Data collection: COLLECT (Hooft, 1999 ▶); cell refinement: DIRAX/LSQ (Duisenberg et al., 2000 ▶); data reduction: EVALCCD (Duisenberg et al., 2003 ▶); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ▶); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 ▶); molecular graphics: PLATON (Spek, 2009 ▶); software used to prepare material for publication: SHELXL97 and PLATON.