Tri­methyl­pyrazole: a simple heterocycle reflecting Kitaigorodskii's packing principle

van Terwingen, S.; Englert, U.

doi:10.1107/S205698902200860X

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Volume 78| Part 10| October 2022| Pages 966-970

https://doi.org/10.1107/S205698902200860X

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Trimethylpyrazole: a simple heterocycle reflecting Kitaigorodskii's packing principle

Steven van Terwingen ^a and Ulli Englert ^a ^*

^aInstitut für Anorganische Chemie, RWTH Aachen University, Landoltweg 1, 52074 Aachen, Germany
^*Correspondence e-mail: ullrich.englert@ac.rwth-aachen.de

Edited by G. Diaz de Delgado, Universidad de Los Andes, Venezuela (Received 29 June 2022; accepted 28 August 2022; online 2 September 2022)

The five-membered heterocycle 1,3,5-trimethyl-1H-pyrazole, C₆H₁₀N₂ (1) crystallizes in space group Pnma with all non-hydrogen atoms of the molecule on the crystallographic mirror plane. This arrangement has been recognized as favorable with respect to space filling by Kitaigorodskii and Wilson, pioneers in the field of crystal packing; Pnma represents a particularly rare space group for residues exclusively in a general position. Neighboring molecules in 1 interact via non-classical C—H⋯N bonds in the plane and C—H⋯π contacts between adjacent layers. In Pnma, crystallographic inversion relates dipolar molecules located on successive mirror planes and results in their head-to-tail arrangement. The interlayer distance in the [010] direction is closely related to the van der Waals radii of C and N.

Keywords: close-packing principle; mirror plane; layer structure; C—H⋯N contact; crystal structure.

CCDC reference: 2203895

1. Introduction

Aleksander Kitaigorodskii was already working on his principle of close packing in the 1940s, at a time when structure analysis via single-crystal diffraction was still not fast and routine. We recall that about 20 years later, in 1965, the archives of the Cambridge Crystallographic Data Centre comprised only 3000 structures. Kitaigorodskii's finding that void space in crystals is in general unfavorable enabled him to rank certain space groups as more or less suitable for close packing. It took considerable time before Kitaigorodskii's ideas were appreciated in the western world (Kitaigorodskii, 1961 , 1965 , 1973 ). The term symmorphic refers to space groups that exhibit a special position with the same symmetry as the crystal class (Chapuis et al., 2022 ). A. J. C. Wilson expanded these original ideas (Wilson, 1993a ) and coined the term antimorphic space groups (Wilson, 1993b ), which only possess symmetry elements associated with a favorable packing, i.e. screw axes, glide planes and inversion centers. In contrast to Kitaigorodskii, W. Nowacki explained the statistical preference for certain space groups by their ability to form a favorable dipole arrangement rather than an efficient packing (Nowacki, 1943 , 1951 ). An excellent summary of the close-packing principle and its consequences for space-group frequencies, together with other packing criteria, was published by Brock & Dunitz (1994 ).

In this contribution, we present the crystal structure of the simple heterocycle 1,3,5-trimethyl-1H-pyrazole (1) in space group Pnma and describe its crystal packing in the context of Kitaigorodskii's and Wilson's ideas.

2. Results and Discussion

All non-hydrogen atoms in 1 occupy a crystallographic mirror plane in space group Pnma (Wyckoff position 4c), resulting in a strictly planar scaffold. A displacement ellipsoid plot of a heterocyclic molecule is shown in Fig. 1.

Figure 1
Displacement ellipsoid plot (Spek, 2020

) of a molecule in 1; ellipsoids are drawn at 70% probability, H atoms are shown as spheres of arbitrary radii. Selected distances (Å) and angles (°): N1—N2 1.358 (4), N2—C1 1.351 (4), C1—C2 1.360 (5), C2—C3 1.392 (4), N2—C4 1.448 (4), C3—N1—N2 104.2 (3), C1—C2—C3 106.6 (3).

Compared to other simple pyrazoles, this is a unique property as most of them do not crystallize in space groups exhibiting a mirror plane, e.g. 1H-pyrazole (space groups Pna2₁ and Pbcn; Sikora & Katrusiak, 2013 ), 3,5-dimethyl-1H-pyrazole (space group R $[\overline{3}]$ c; Baldy et al., 1985 ) or 1,5-dimethyl-1H-pyrazole-3-carboxylic acid ethyl ester (P $[\overline{1}]$ ; Schmidt et al., 2003 ). Intramolecular distances and angles in these pyrazoles and 1 are very similar and adopt values within a narrow range (Table 1).

Table 1
Comparison of selected distances (Å) in 1 with two comparable structures denoted by their CSD refcodes (Groom et al., 2016)

Atom labels as in Fig. 1. For PYRZOL27, a Z′ of 2 is observed and only values for the first residue are listed here.

Compound	d(N1—N2)	d(N2—C1)	d(N1—C3)	d(C2—C3)	d(C1—C2)
1	1.358 (4)	1.351 (4)	1.336 (4)	1.392 (4)	1.360 (5)
PYRZOL27^a	1.357 (2)	1.338 (3)	1.334 (3)	1.391 (3)	1.373 (3)
ALOSEZ^b	1.3464 (17)	1.3595 (14)	1.3415 (16)	1.3966 (15)	1.377 (2)
References: (a) Sikora & Katrusiak (2013); (b) Schmidt et al. (2003).

Pnma, the space-group type adopted by the title compound, plays a central role in the concepts of Kitaigorodskii and Wilson. We cite literally from Wilson (1991 ): `The space-group type Pnma is particularly interesting, as Kitaigorodskii (1965) predicted that it would be popular because it would permit close-packing of molecules with inherent mirror symmetry […] The structures published in Acta Crystallographica C were checked, and all were found to consist of molecules possessing and using inherent mirror planes.' The 1965 article cited in Wilson's statement above refers to the Russian version of Organic Chemical Crystallography (Kitaigorodskii, 1961). The heterocyclic molecule in 1 is a candidate par excellence for Pnma: It not only matches the required site symmetry but all of its non-hydrogen atoms are located on this mirror plane, providing an efficient in-plane arrangement (Fig. 2, left).

Figure 2
Packing in the (010) plane (Spek, 2020

). Non-classical C—H⋯N contacts are shown as dashed lines: d(N⋯H^a) = 2.56 Å; ∠(C^a—H^a⋯N) = 179°. Symmetry code: (a) − $[{1\over 2}]$ + x, $[{1\over 2}]$ − y, $[{3\over 2}]$ − z.

Non-classical C—H⋯N hydrogen bonds represent the shortest directional contacts in the mirror plane and lead to chains along [100] (Fig. 2, right). This kind of interaction is quite common for 4-unsubstituted pyrazoles and we only provide selected examples for comparison: ICEDUQ (Patra et al., 2004 ), LUNYID (Benisvy et al., 2009 ) and KITNOR (Kidwai et al., 2008 ) (Table 2).

Table 2
Comparison of N⋯H—C contacts (Å, °) observed in 1 and selected other pyrazoles

Compound	d(N⋯H)	∠(N⋯H—C)
1	2.56	179
ICEDUQ^a	2.852 (19)	177.3 (12)
LUNYID^b	2.66	154
KITNOR^c	2.458 (16)	156.2 (13)
References: (a) Patra et al. (2004); (b) Benisvy et al. (2009); (c) Kidwai et al. (2008).

A crystallographic center of inversion (Wyckoff position 4a) relates objects on the mirror planes at y = 0.25 and y = 0.75; the dipole moments of consecutive layers are therefore oriented in opposite directions, quite in agreement with early Nowacki (1943) ideas. The non-planar methyl groups in 1 provide the most relevant interlayer contacts. Fig. 3 shows the head-to-tail arrangement of two molecules, with a methyl H atom pointing towards the center of gravity of the five-membered ring of a neighbor. The shortest interatomic distance associated with this contact amounts to H4b⋯N2^a [symmetry code: (a) 1 − x, − $[{1\over 2}]$ + y, 1 − z] = 2.65 Å.

Figure 3
Short methyl C—H⋯π contacts about a center of inversion in 1 shown as dashed lines (Spek, 2020

): d(Cg⋯H^b) = 2.586 Å; ∠(C^b—H^b⋯Cg) = 140.97°. Symmetry code: (b) 1 − x, −y, 1 − z.

The Hirshfeld surface (Spackman & Jayatilaka, 2009 ) about one pyrazole moiety is shown in Fig. 4. It has been mapped with the dimensionless interaction-sensitive quantity d_norm; red areas indicate short contacts. Both the C—H⋯N hydrogen bond and the interlayer methyl⋯π contact can clearly be perceived.

Figure 4
Hirshfeld surface (Turner et al., 2017

) about one 1,3,5-trimethyl-1H-pyrazole moiety in 1.

The stacking of efficiently packed layers of which only the methyl H atoms protrude leads to a simple relationship between the lattice parameter in the stacking direction, i.e. unit-cell parameter b in the standard setting of space group Pnma, and the van der Waals radii of the partaking atoms. Fig. 5 provides a sketch of the situation.

Figure 5
View of the unit cell of 1 along c (Spek, 2020

), methyl groups omitted. The radii of the atoms essentially denote their van der Waals radii (r_vdW).

Kitaigorodskii himself had determined van der Waals radii (r_vdW) of 1.8 Å for carbon and of 1.58 Å for nitrogen (Kitaigorodskii, 1973); values of 1.7 for C and 1.55 for N have been suggested by Batsanov (1995 ). The unit-cell parameter b for our title compound 1 amounts to approximately 6.7 Å, closely matching the expected fourfold van der Waals radius of the non-hydrogen atoms involved. Table 3 shows additional examples for small and planar organic molecules crystallizing in the same space group type and with a similar cell parameter b.

Table 3
Other structures showing the same motif as 1; r_vdW(C) = 1.7 Å; r_vdW(N) = 1.55 Å (Batsanov, 1995)

^†For MURANT the non-standard setting Pbnm was chosen, so the shown unit-cell parameter perpendicular to the mirror plane is c.

Compound	Formula	b (Å)	b/4 (Å)
1	C₆H₁₀N₂	6.687 (11)	1.672
CIJZEB^a	C₄H₄ClN₃O₂	6.1372 (5)	1.5343
CIJZEB01^b	C₄H₄ClN₃O₂	6.3050 (10)	1.5763
EQENUL^c	C₄H₆BF₃N₂	6.635 (3)	1.659
FIFRAN^d	C₅H₄N₂O₂	6.388 (2)	1.597
MURANT^e†	C₂H₇N₃O₄	6.36 (2)	1.59
QOXVII^f	C₇H₈N₂O₂	6.5670 (7)	1.6418
VORDIR^g	C₄H₆N₂OS	6.4865 (4)	1.6216
WIQLOX^h	C₇H₈N₂O	6.722 (4)	1.681
References: (a) Kubicki & Wagner (2007a ); (b) Kubicki & Wagner (2007b ); (c) Takao & Ikeda (2008 ); (d) Rybalova et al. (1998 ); (e) Bryden (1957 ); (f) Nawrot et al. (2001 ); (g) Konstantinova et al. (2014 ); (h) Aldabbagh et al. (1999 ).

These examples share the same construction principle: The individual flat molecules are arranged in the crystallographic mirror plane, and for symmetry reasons dipole directions alternate between consecutive layers along b.

3. Database survey

For all database searches, version 5.42 of the CSD (Groom et al., 2016 ), including all updates until September 2021 were used. The examples compiled in Table 3 were restricted to entries with space group Pnma crystallizing in unit cells similar to 1, with a tolerance of 0.7 Å for each unit-cell parameter. These conditions were met by seventeen entries; eight of these show a packing analogous to that of 1.

4. Synthesis and crystallization

The target compound 1,3,5-trimethyl-1H-pyrazole (1) is readily available by the Knorr pyrazole synthesis using acetylacetone and methylhydrazine (Knorr, 1883 ; Stanovnik & Svete, 2002 ). Alternatively, the compound may be purchased from common vendors. The single crystal for the reported structure was obtained from the reaction mixture. It is soluble in a wide range of common solvents; single crystals may also be grown via recrystallization from a solution in diethyl ether at 243 K. The small crystal size as well as the fast growth and the absence of any heavy atom restricted diffraction data to a limited resolution. The result is a comparatively high agreement factor of symmetry-related reflections (R_int = 13.77%) and agreement factor considering the intensity of reflections (R_σ = 7.07%).

5. Refinement details

Crystal data, data collection parameters and convergence results for the single crystal X-ray diffraction experiment have been summarized in Table 4. Non-hydrogen atoms were assigned anisotropic displacement parameters. H atoms were introduced into calculated positions and treated as riding with C—H = 0.98 Å and U_iso(H) = 1.5U_eq(C) for methyl and with C—H = 0.95 Å and U_iso(H) = 1.2U_eq(C) for the heteroaryl H atom. Tentative refinement of a model in which the methyl conformations were chosen to best match local difference-Fourier maxima leads to split positions, but for each CH₃ group one H atom is located very close to the crystallographic mirror plane. We therefore decided to constrain the y coordinate of these almost in-plane hydrogens to fit the special position.

Table 4
Experimental details

Crystal data
Chemical formula	C₆H₁₀N₂
M_r	110.16
Crystal system, space group	Orthorhombic, Pnma
Temperature (K)	100
a, b, c (Å)	11.205 (19), 6.687 (11), 8.373 (15)
V (Å³)	627.3 (19)
Z	4
Radiation type	Mo Kα
μ (mm⁻¹)	0.07
Crystal size (mm)	0.21 × 0.10 × 0.09

Data collection
Diffractometer	Bruker APEX CCD
Absorption correction	Multi-scan (SADABS; Krause et al., 2015 )
T_min, T_max	0.657, 0.745
No. of measured, independent and observed [I > 2σ(I)] reflections	6665, 648, 366
R_int	0.138

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.047, 0.119, 0.87
No. of reflections	648
No. of parameters	50
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.26, −0.23
Computer programs: SMART (Bruker, 2001 ), SAINT-Plus (Bruker, 2009 ), SHELXT2014/5 (Sheldrick, 2015a ), SHELXL2019/2 (Sheldrick, 2015b ), PLATON (Spek, 2020 ) and Mercury (Macrae at al., 2020 ).

6. Conclusion and outlook

What else can we learn from the packing of the simple heterocycle 1 in space group Pnma. Space filling is unexceptional; according to the well-known Kempster–Lipson rule (Kempster & Lipson, 1972 ) a molecule with eight non-hydrogen atoms should be associated with a residue volume of approximately 150 Å³. The unit cell of 1 will therefore contain four pyrazole molecules, necessarily in special positions. Wyckoff positions 4a and 4b require $[\overline{1}]$ symmetry and can be excluded whereas 4c appears compatible with the molecular symmetry. Harker vectors are subtended by atoms related by crystallographic symmetry. All Harker peaks and all Patterson cross peaks (Glusker et al., 1994 ; Viterbo, 2002 ) derived for occupied 4c positions should be characterized by a Patterson coordinate of 0.0 or 0.5 in the [010] direction. The Patterson function for 1 perfectly matches this expectation: The highest Patterson peak with a v coordinate unequal to 0.0 or 0.5 has an intensity of less than 5% of the trivial origin peak. Our trimethylpyrazole represents a well-suited example for teaching basic concepts of crystallography such as space groups, Wyckoff positions, packing rules, and popular short contacts!

Supporting information

CCDC reference: 2203895

Crystal structure: contains datablocks I, global. DOI: https://doi.org/10.1107/S205698902200860X/dj2050sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S205698902200860X/dj2050Isup2.hkl

Supporting information file. DOI: https://doi.org/10.1107/S205698902200860X/dj2050Isup3.mol

Suggestion for Table of Contents graphics. DOI: https://doi.org/10.1107/S205698902200860X/dj2050sup4.png

Supporting information file. DOI: https://doi.org/10.1107/S205698902200860X/dj2050Isup5.cml

checkCIF report

Computing details top

Data collection: SMART (Bruker, 2001); cell refinement: SAINT-Plus (Bruker, 2009); data reduction: SAINT-Plus (Bruker, 2009); program(s) used to solve structure: SHELXT2014/5 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2019/2 (Sheldrick, 2015b); molecular graphics: PLATON (Spek, 2020) and Mercury (Macrae et al., 2020).

1,3,5-Trimethyl-1H-pyrazole top

Crystal data top

C₆H₁₀N₂	D_x = 1.166 Mg m⁻³
M_r = 110.16	Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, Pnma	Cell parameters from 263 reflections
a = 11.205 (19) Å	θ = 3.0–19.7°
b = 6.687 (11) Å	µ = 0.07 mm⁻¹
c = 8.373 (15) Å	T = 100 K
V = 627.3 (19) Å³	Block, colorless
Z = 4	0.21 × 0.10 × 0.09 mm
F(000) = 240

Data collection top

Bruker APEX CCD diffractometer	648 independent reflections
Radiation source: microsource	366 reflections with I > 2σ(I)
Multilayer optics monochromator	R_int = 0.138
ω scans	θ_max = 25.9°, θ_min = 3.0°
Absorption correction: multi-scan (SADABS; Krause et al., 2015)	h = −13→13
T_min = 0.657, T_max = 0.745	k = −8→8
6665 measured reflections	l = −10→10

Refinement top

Refinement on F²	Hydrogen site location: mixed
Least-squares matrix: full	H-atom parameters constrained
R[F² > 2σ(F²)] = 0.047	w = 1/[σ²(F_o²) + (0.0645P)²] where P = (F_o² + 2F_c²)/3
wR(F²) = 0.119	(Δ/σ)_max < 0.001
S = 0.87	Δρ_max = 0.26 e Å⁻³
648 reflections	Δρ_min = −0.23 e Å⁻³
50 parameters	Extinction correction: SHELXL-2019/2 (Sheldrick 2015b), Fc^*=kFc[1+0.001xFc²λ³/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.011 (4)
Primary atom site location: dual

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

	x	y	z	U_iso*/U_eq
N1	0.4912 (2)	0.250000	0.6988 (3)	0.0289 (7)
N2	0.5279 (2)	0.250000	0.5442 (3)	0.0249 (6)
C1	0.6481 (3)	0.250000	0.5301 (4)	0.0236 (7)
C2	0.6912 (3)	0.250000	0.6820 (3)	0.0260 (8)
H2	0.772711	0.250000	0.713077	0.031*
C3	0.5924 (3)	0.250000	0.7829 (4)	0.0265 (7)
C4	0.4409 (3)	0.250000	0.4164 (3)	0.0327 (9)
H4A	0.360267	0.250000	0.461847	0.049*
H4B	0.451701	0.130335	0.350369	0.049*
C5	0.7079 (3)	0.250000	0.3728 (4)	0.0342 (8)
H5A	0.794608	0.250000	0.388049	0.051*
H5B	0.684366	0.369665	0.312985	0.051*
C6	0.5878 (3)	0.250000	0.9617 (4)	0.0380 (9)
H6A	0.504403	0.250000	0.996905	0.057*
H6B	0.628021	0.369665	1.002625	0.057*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
N1	0.0378 (16)	0.0195 (13)	0.0295 (16)	0.000	0.0012 (14)	0.000
N2	0.0297 (15)	0.0182 (13)	0.0268 (15)	0.000	−0.0012 (12)	0.000
C1	0.0238 (16)	0.0149 (15)	0.032 (2)	0.000	0.0009 (14)	0.000
C2	0.0263 (17)	0.0177 (15)	0.034 (2)	0.000	−0.0025 (16)	0.000
C3	0.0356 (17)	0.0166 (14)	0.0274 (18)	0.000	−0.0035 (17)	0.000
C4	0.038 (2)	0.0228 (15)	0.037 (2)	0.000	−0.0104 (16)	0.000
C5	0.040 (2)	0.0275 (16)	0.035 (2)	0.000	0.0047 (16)	0.000
C6	0.048 (2)	0.0349 (17)	0.031 (2)	0.000	0.0033 (17)	0.000

Geometric parameters (Å, º) top

N1—C3	1.336 (4)	C4—H4A	0.9800
N1—N2	1.358 (4)	C4—H4B	0.9800
N2—C1	1.351 (4)	C4—H4Bⁱ	0.9800
N2—C4	1.448 (4)	C5—H5A	0.9800
C1—C2	1.360 (5)	C5—H5B	0.9800
C1—C5	1.478 (4)	C5—H5Bⁱ	0.9800
C2—C3	1.392 (4)	C6—H6A	0.9800
C2—H2	0.9500	C6—H6B	0.9800
C3—C6	1.498 (5)	C6—H6Bⁱ	0.9800

C3—N1—N2	104.2 (3)	N2—C4—H4Bⁱ	109.47 (10)
C1—N2—N1	112.7 (2)	H4A—C4—H4Bⁱ	109.5
C1—N2—C4	127.3 (3)	H4B—C4—H4Bⁱ	109.5
N1—N2—C4	120.0 (3)	C1—C5—H5A	109.5
N2—C1—C2	105.8 (3)	C1—C5—H5B	109.5
N2—C1—C5	122.0 (3)	H5A—C5—H5B	109.5
C2—C1—C5	132.2 (3)	C1—C5—H5Bⁱ	109.47 (11)
C1—C2—C3	106.6 (3)	H5A—C5—H5Bⁱ	109.5
C1—C2—H2	126.7	H5B—C5—H5Bⁱ	109.5
C3—C2—H2	126.7	C3—C6—H6A	109.5
N1—C3—C2	110.8 (3)	C3—C6—H6B	109.5
N1—C3—C6	119.9 (3)	H6A—C6—H6B	109.5
C2—C3—C6	129.4 (3)	C3—C6—H6Bⁱ	109.47 (10)
N2—C4—H4A	109.5	H6A—C6—H6Bⁱ	109.5
N2—C4—H4B	109.5	H6B—C6—H6Bⁱ	109.5
H4A—C4—H4B	109.5

Symmetry code: (i) x, −y+1/2, z.

Comparison of selected distances (Å) in 1 with two comparable structures denoted by their CSD refcodes (Groom et al., 2016) top

Atom labels as in Figure 1. For PYRZOL27, a Z' of 2 is observed and only values for the first residue are listed here.

Compound	d(N1—N2)	d(N2—C1)	d(N1—C3)	d(C2—C3)	d(C1—C2)
1	1.358 (4)	1.351 (4)	1.336 (4)	1.392 (4)	1.360 (5)
PYRZOL27^a	1.357 (2)	1.338 (3)	1.334 (3)	1.391 (3)	1.373 (3)
ALOSEZ^b	1.3464 (17)	1.3595 (14)	1.3415 (16)	1.3966 (15)	1.377 (2)

References: (a) Sikora & Katrusiak (2013); (b) Schmidt et al. (2003).

Comparison of N···H—C contacts (Å, °) observed in 1 and selected other pyrazoles top

Compound	d(N···H)	angle(N···H—C)
1	2.56	179
ICEDUQ^a	2.852 (19)	177.3 (12)
LUNYID^b	2.66	154
KITNOR^c	2.458 (16)	156.2 (13)

References: (a) Patra et al. (2004); (b) Benisvy et al. (2009); (c) Kidwai et al. (2008).

Other structures showing the same motif as 1; r_vdW(C) = 1.7 Å; r_vdW(N) = 1.55 Å (Batsanov, 1995) top

^†For MURANT the non-standard setting Pbnm was chosen, so the shown unit-cell parameter perpendicular to the mirror plane is c.

Compound	Formula	b (Å)	b/4 (Å)
1	C₆H₁₀N₂	6.687 (11)	1.672
CIJZEB^a	C₄H₄ClN₃O₂	6.1372 (5)	1.5343
CIJZEB01^b	C₄H₄ClN₃O₂	6.3050 (10)	1.5763
EQENUL^c	C₄H₆BF₃N₂	6.635 (3)	1.659
FIFRAN^d	C₅H₄N₂O₂	6.388 (2)	1.597
MURANT^e^†	C₂H₇N₃O₄	6.36 (2)	1.59
QOXVII^f	C₇H₈N₂O₂	6.5670 (7)	1.6418
VORDIR^g	C₄H₆N₂OS	6.4865 (4)	1.6216
WIQLOX^h	C₇H₈N₂O	6.722 (4)	1.681

References: (a) Kubicki & Wagner (2007a); (b) Kubicki & Wagner (2007b); (c) Takao & Ikeda (2008); (d) Rybalova et al. (1998); (e) Bryden (1957); (f) Nawrot et al. (2001); (g) Konstantinova et al. (2014); (h) Aldabbagh et al. (1999).

Comparison of selected distances in 1 with two comparable structures. top

For PYRZOL27 a Z' of 2 is observed and only values for the first residue have been taken into account.

Compound	d(N—N)	d1(N═C)	d2(N═C)	d1(C═C)	d2(CC)
1	1.358 (4)	1.351 (4)	1.336 (4)	1.392 (4)	1.360 (5)
PYRZOL27	1.357 (2)	1.338 (3)	1.334 (3)	1.391 (3)	1.373 (3)
ALOSEZ	1.3464 (17)	1.3595 (14)	1.3415 (16)	1.3966 (15)	1.377 (2)

Acknowledgements

UE acknowledges support from One Hundred Talent Program of Shanxi Province. The authors thank Anke Braun for help with the synthesis.

Funding information

Funding for this research was provided by: RWTH Graduiertenförderung (scholarship to SvT).

References

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