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ISSN: 2056-9890

Crystal structure of 2,2′-bi­pyrrole

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aDepartment of Chemistry and Biochemistry, Eastern Washington University, Science 226, Cheney, WA 99004, USA, and bDepartment of Chemistry, 1253 University of Oregon, Eugene, Oregon 97403, USA
*Correspondence e-mail: alamm@ewu.edu

Edited by W. T. A. Harrison, University of Aberdeen, Scotland (Received 11 July 2017; accepted 19 September 2017; online 25 September 2017)

The complete mol­ecule of the title compound, C8H8N2, is generated by a crystallographic center of symmetry. In the crystal, short N—H⋯π [H⋯π = 2.499 (19) Å] inter­actions link the mol­ecules into a herringbone structure.

1. Chemical context

Bi­pyrrole, C8H8N2, has been studied extensively over the years: the first reported synthesis was in 1962 (Rapoport & Castagnoli, 1962[Rapoport, H. & Castagnoli, N. (1962). J. Am. Chem. Soc. 84, 2178-2181.]). The bi­pyrrole core occurs in naturally occurring compounds such as prodigiosin (Wasserman et al., 1960[Wasserman, H. H., McKeon, J. E., Smith, L. & Forgione, P. (1960). J. Am. Chem. Soc. 82, 506-507.]). Functionalized bi­pyrroles have been shown to have anti-cancer activity (Manderville, 2001[Manderville, R. A. (2001). Curr. Med. Chem. Anti-Cancer Agents, 1, 195-218.]). Corrole rings contain a bi­pyrrole segment in the macrocycle (Aviv-Harel & Gross, 2009[Aviv-Harel, I. & Gross, Z. (2009). Chem. Eur. J. 15, 8382-8394.]). Herein, we report on the crystal structure of the title compound, (I)[link], synthesized by the oxidative coupling of pyrrole.

[Scheme 1]

2. Structural commentary

The complete mol­ecule of (I)[link] is generated by a crystallographic center of symmetry (Fig. 1[link]), and therefore the pyrrole rings are exactly parallel and the N—H groups face in opposite directions. The C2—C3 bond in (I)[link] is 1.4151 (19) Å, versus the equivalent bond in pyrrole, which has a length of 1.423 (3) Å. The double bonds C1=C2 and C3=C4 in (I)[link] are 1.3635 (19) and 1.3767 (17) Å, respectively, versus the double-bond length in pyrrole of 1.357 Å. The shortening of the C2—C3 bond and the lengthening of the adjacent C=C double bonds in (I)[link] compared to pyrrole is consistent with stronger inter­molecular inter­actions (see below).

[Figure 1]
Figure 1
The mol­ecular structure of the title compound with displacement ellipsoids drawn at the 50% probability level. [Symmetry code: (i) 1 − x, 2 − y, −z.]

3. Supra­molecular features

In the crystal of (I)[link], the mol­ecules adopt an edge-to-face orientation: the distance between the N—H group and the centroid of the adjacent pyrrole ring generated by the 21 screw axis is 2.499 (19) Å (Table 1[link]). A survey of the Cambridge Crystallographic Database (Groom et al., 2016[Groom, C. R., Bruno, I. J., Lightfoot, M. P. & Ward, S. C. (2016). Acta Cryst. B72, 171-179.]) showed that the average N—H⋯π separation is 2.804 Å and 13 out of 156 inter­actions (8.3%) had an N—H⋯π inter­actions shorter than 2.5 Å. The crystal packing in (I)[link] is similar to that of pyrrole, which has the same inter­molecular N—H⋯π inter­actions (Goddard et al., 1997[Goddard, R., Heinemann, O. & Krüger, C. (1997). Acta Cryst. C53, 1846-1850.]). The edge-to-face inter­action has been suggested to be a stabilizing factor in protein structures and polypeptides have been observed to have a separation of 2.42 Å from the N—H group to the phenyl ring (Steiner, 1998[Steiner, T. (1998). Acta Cryst. D54, 584-588.]) and calculations corroborate these data (Levitt Perutz, 1988[Levitt, M. & Perutz, M. F. (1988). J. Mol. Biol. 201, 751-754.]). It is notable that the N—H bond of 2,2′-bi­pyrrole points almost directly at the midpoint of the C2—C3 bond. In the extended structure, the mol­ecules are orientated by edge-to-face inter­actions generating a herringbone pattern, see Fig. 2[link].

Table 1
Hydrogen-bond geometry (Å, °)

Cg is the centroid of the N1/C1–C4 ring.

D—H⋯A D—H H⋯A DA D—H⋯A
N1—H1NCg1i 0.907 (16) 2.499 (19) 3.2275 (12) 138.1 (13)
Symmetry code: (i) [-x+1, y+{\script{1\over 2}}, -z+{\script{1\over 2}}].
[Figure 2]
Figure 2
View of the N—H⋯π inter­actions and packing in the title compound.

4. Database survey

There are very few examples of similar compounds available in the literature. Most bi­pyrroles have carbonyl groups as substituents on the pyrrole ring in which the N—H group forms hydrogen bonds with the oxygen atom of the carbonyl (Okawara et al., 2015[Okawara, T., Doi, A., Ono, T., Abe, M., Takehara, K., Hisaeda, Y. & Matsushima, S. (2015). Tetrahedron Lett. 56, 1407-1410.]). So far as we are aware, there are no bi­pyrrole examples that exhibit the same packing and hydrogen-bonding pattern as the title compound; the closest example is pyrrole itself (Goddard et al., 1997[Goddard, R., Heinemann, O. & Krüger, C. (1997). Acta Cryst. C53, 1846-1850.]).

5. Synthesis and crystallization

230 µl (3.3 mmol) of pyrrole was added to di­chloro­methane (10 ml), degassed and cooled to 195 K. Tri­methyl­silyl bromide (290 µl, 2.2 mmol) and phenyl iodine trifluoracetic acid (477 mg, 1.1 mmol in 1 ml di­chloro­methane) were added quickly to the cooled reaction. The mixture was stirred for 1 h then extracted with a saturated sodium bicarbonate solution and purified by column chromatography with a penta­ne/ethyl acetate (1:1) solution. Colourless blocks of (I)[link] were obtained 7 d later by slow evaporation from an ethyl acetate solution.

6. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2[link]. The H atoms were located in difference maps and refined with isotropic displacement parameters.

Table 2
Experimental details

Crystal data
Chemical formula C8H8N2
Mr 132.16
Crystal system, space group Monoclinic, P21/c
Temperature (K) 173
a, b, c (Å) 5.9500 (2), 6.7650 (2), 8.4363 (3)
β (°) 96.746 (2)
V3) 337.22 (2)
Z 2
Radiation type Cu Kα
μ (mm−1) 0.64
Crystal size (mm) 0.11 × 0.10 × 0.06
 
Data collection
Diffractometer Bruker APEXII CCD
Absorption correction Multi-scan (SADABS; Bruker, 2004[Bruker (2004). APEX2, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA.])
Tmin, Tmax 0.666, 0.753
No. of measured, independent and observed [I > 2σ(I)] reflections 1989, 590, 540
Rint 0.026
(sin θ/λ)max−1) 0.595
 
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.034, 0.098, 1.08
No. of reflections 590
No. of parameters 62
H-atom treatment All H-atom parameters refined
Δρmax, Δρmin (e Å−3) 0.18, −0.16
Computer programs: APEX2 and SAINT (Bruker, 2004[Bruker (2004). APEX2, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA.]), SHELXS97 and SHELXTL (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]) and SHELXL2014 (Sheldrick, 2015[Sheldrick, G. M. (2015). Acta Cryst. C71, 3-8.]).

Supporting information


Computing details top

Data collection: APEX2 (Bruker, 2004); cell refinement: SAINT (Bruker, 2004); data reduction: SAINT (Bruker, 2004); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHEXLTL (Sheldrick, 2008).

2,2'-Bipyrrole top
Crystal data top
C8H8N2F(000) = 140
Mr = 132.16Dx = 1.302 Mg m3
Monoclinic, P21/cCu Kα radiation, λ = 1.54178 Å
a = 5.9500 (2) ÅCell parameters from 1086 reflections
b = 6.7650 (2) Åθ = 7.5–66.5°
c = 8.4363 (3) ŵ = 0.64 mm1
β = 96.746 (2)°T = 173 K
V = 337.22 (2) Å3Cut-block, colorless
Z = 20.11 × 0.10 × 0.06 mm
Data collection top
Bruker APEXII CCD
diffractometer
540 reflections with I > 2σ(I)
Radiation source: Incoatec IµSRint = 0.026
φ and ω scansθmax = 66.5°, θmin = 7.5°
Absorption correction: multi-scan
(SADABS; Bruker, 2004)
h = 74
Tmin = 0.666, Tmax = 0.753k = 78
1989 measured reflectionsl = 910
590 independent reflections
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullHydrogen site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.034All H-atom parameters refined
wR(F2) = 0.098 w = 1/[σ2(Fo2) + (0.0657P)2 + 0.034P]
where P = (Fo2 + 2Fc2)/3
S = 1.08(Δ/σ)max < 0.001
590 reflectionsΔρmax = 0.18 e Å3
62 parametersΔρmin = 0.16 e Å3
0 restraints
Special details top

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
N10.53346 (18)1.09945 (15)0.20670 (12)0.0322 (4)
C10.7071 (2)1.07425 (18)0.32546 (15)0.0352 (4)
C20.8665 (2)0.95786 (18)0.26832 (16)0.0330 (4)
C30.7857 (2)0.91019 (17)0.10841 (15)0.0297 (4)
C40.57828 (19)0.99966 (16)0.07196 (13)0.0263 (4)
H10.704 (2)1.135 (2)0.4292 (17)0.043 (4)*
H1N0.407 (2)1.170 (3)0.2168 (17)0.046 (4)*
H21.010 (3)0.916 (2)0.3269 (19)0.041 (4)*
H30.859 (2)0.831 (2)0.0354 (16)0.039 (4)*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
N10.0364 (6)0.0295 (6)0.0299 (6)0.0075 (4)0.0010 (5)0.0030 (4)
C10.0424 (8)0.0320 (7)0.0296 (7)0.0003 (5)0.0028 (6)0.0038 (5)
C20.0299 (7)0.0329 (7)0.0347 (7)0.0016 (5)0.0023 (5)0.0043 (5)
C30.0278 (7)0.0307 (7)0.0312 (7)0.0005 (5)0.0055 (5)0.0021 (5)
C40.0306 (6)0.0217 (6)0.0270 (6)0.0017 (4)0.0055 (5)0.0025 (4)
Geometric parameters (Å, º) top
N1—C11.3628 (18)C2—C31.4151 (19)
N1—C41.3748 (15)C2—H20.979 (16)
N1—H1N0.907 (16)C3—C41.3767 (17)
C1—C21.3635 (19)C3—H30.959 (14)
C1—H10.970 (15)C4—C4i1.441 (2)
C1—N1—C4110.02 (11)C3—C2—H2126.4 (9)
C1—N1—H1N124.3 (9)C4—C3—C2107.93 (11)
C4—N1—H1N125.6 (9)C4—C3—H3124.4 (8)
N1—C1—C2108.12 (11)C2—C3—H3127.6 (8)
N1—C1—H1121.2 (9)N1—C4—C3106.69 (11)
C2—C1—H1130.7 (9)N1—C4—C4i121.74 (13)
C1—C2—C3107.23 (11)C3—C4—C4i131.57 (13)
C1—C2—H2126.4 (9)
C4—N1—C1—C20.11 (14)C1—N1—C4—C4i179.68 (12)
N1—C1—C2—C30.15 (14)C2—C3—C4—N10.07 (13)
C1—C2—C3—C40.13 (14)C2—C3—C4—C4i179.73 (15)
C1—N1—C4—C30.03 (13)
Symmetry code: (i) x+1, y+2, z.
Hydrogen-bond geometry (Å, º) top
Cg is the centroid of the N1/C1–C4 ring.
D—H···AD—HH···AD···AD—H···A
N1—H1N···Cg1ii0.907 (16)2.499 (19)3.2275 (12)138.1 (13)
Symmetry code: (ii) x+1, y+1/2, z+1/2.
 

Acknowledgements

The authors are grateful for the generous support from EWU's Faculty Grants for Research and Creative Works and support from EWU's Department of Chemistry and Biochemistry.

References

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First citationBruker (2004). APEX2, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA.
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