Crystal structure of 3,6-bis­(pyridin-2-yl)-1,4-di­hydro-1,2,4,5-tetra­zine

Wzgarda-Raj, K.; Rybarczyk-Pirek, A.J.; Wojtulewski, S.; Palusiak, M.

doi:10.1107/S205698901801753X

research communications

CRYSTALLOGRAPHIC
COMMUNICATIONS

ISSN: 2056-9890

Volume 75| Part 1| January 2019| Pages 86-88

https://doi.org/10.1107/S205698901801753X

Open

access

Crystal structure of 3,6-bis(pyridin-2-yl)-1,4-dihydro-1,2,4,5-tetrazine

Kinga Wzgarda-Raj,^a ^* Agnieszka J. Rybarczyk-Pirek,^a Sławomir Wojtulewski ^b and Marcin Palusiak ^a

^aGroup of Theoretical and Structural Chemistry, Department of Physical Chemistry, Faculty of Chemistry, University of Łódź, Pomorska 163/165, 90-236, Łódź, Poland, and ^bDepartment of Theoretical Chemistry, University of Białystok, Ciołkowskiego, 1K, 15-245 Białystok, Poland
^*Correspondence e-mail: kinga.raj@chemia.uni.lodz.pl

Edited by K. Fejfarova, Institute of Biotechnology CAS, Czech Republic (Received 20 November 2018; accepted 11 December 2018; online 1 January 2019)

The structure of the title compound, C₁₂H₁₀N₆, at 100 K has monoclinic (P2₁/n) symmetry. Crystals were obtained as a yellow solid by reduction of 3,6-bis(pyridin-2-yl)-1,2,4,5-tetrazine. The structure displays intermolecular hydrogen bonding of the N—H⋯N type, ordering molecules into infinite ribbons extending along the [100] direction.

Keywords: crystal structure; 1,2,4,5-tetrazine; hydrogen bond.

CCDC reference: 1884403

1. Chemical context

s-Tetrazines represent a class of heterocyclic compounds. The substitution of four nitrogen atoms in a six-membered benzene-like ring results in strong π-electron deficiency and concentration of negative charge on the heteroatoms. As a result of these properties, s-tetrazines are used in organic synthesis (Saracoglu, 2007 ; Šečkutė & Deveraj et al., 2013 ; Churakov et al., 2004 ) as well as bridging ligands in metal complexes (Kaim, 2002 ; Clavier & Audebert, 2010 ). Moreover, their derivatives are often among biologically active compounds (Saghatforoush et al., 2016 ) and play an important role in anti-inflammatory (Kamal et al., 2006 ), anticancer, antiviral drugs (Rao & Hu, 2006 ; Neunhoeffer et al., 1984 ) or as insecticidal products (Sauer et al.,1996 ; Brooker et al., 1987 ).

The title compound 3,6-bis(pyridin-2-yl)-1,4-dihydro-1,2,4,5-tetrazine (I) was obtained as a yellow solid by reduction of 3,6-bis(pyridin-2-yl)-1,2,4,5-tetrazine (II) during its crystallization with 2-mercaptopyridine N-oxide (III) in ethanol solution (Fig. 1).

Figure 1
Molecular formulae of: 3,6-bis(pyridin-2-yl)-1,4-dihydro-1,2,4,5-tetrazine (I)

, 3,6-bis(pyridin-2-yl)-1,2,4,5-tetrazine (II) and 2-mercaptopyridine N-oxide (III).

2. Structural commentary

Compound (I) crystallizes in the monoclinic space group P2₁/n. The atomic labelling scheme is shown in Fig. 2. In (I), being a reduced form of (II), there are two hydrogen atoms at the 1 and 4 positions and two 2-pyridyl substituents at the 3 and 6 positions.

Figure 2
The molecular structure of (I)

, showing the atom-labelling scheme and displacement ellipsoids at the 50% probability level.

The C—C bond lengths are within the expected values known for aromatic systems (Allen et al., 1987 ). However, there is a fluctuation of bond distances involving nitrogen atoms. The N—N bonds within the central (A) ring are of almost equal length, being 1.4285 (15) and 1.4306 (16) Å. The C6—N1 and C3—N4 [1.3953 (17) and 1.4051 (17) Å] bond lengths are longer than those for C6—N5 and C3—N2 [1.2848 (17) Å, 1.2809 (18) Å], respectively. This is the result of the protonation of the N1 and N4 atoms. The C—N bond lengths in the B and C rings are comparable within 3σ, varying from 1.3384 (18) Å to 1.3416 (17) Å.

The central tetrazine ring (A) shows a boat conformation with pseudo-symmetry mirror planes passing through bonds N2—C3 and N5—C6 [ΔC_s = 1.30 (16)°] and atoms N1, N4 [ΔC_s = 2.00 (14)°]. In this conformation, hydrogen atoms are located in the equatorial positions of the ring and the N—H bonds are directed to the bottom of the boat (compare torsion angles in Table 1). The planes of the aromatic pirydyl rings (B and C) are not to parallel to each other. The dihedral angles between these rings and central tetrazine ring are 22.43 (7)° (A and B) and 25.71 (6)° (A and C). The dihedral angle between rings B and C is 27.13 (7)°. The overall molecular structure could be recognized as a butterfly-like conformation as shown in Fig. 3.

Table 1
Selected torsion angles (°)

N2—C3—N4—H4	164.1 (13)	C3—N2—N1—H1	−168.4 (12)
C6—N5—N4—H4	−165.2 (14)	N5—C6—N1—H1	164.3 (13)

Figure 3
The butterfly-like molecular conformation of (I)

3. Supramolecular features

The crystal packing of (I) is mainly determined by intermolecular hydrogen bonds of the N—H⋯N type (Table 2). Firstly, two similar hydrogen bonds (N1—H1⋯N5 and N4—H4⋯N2) between the 1,2,4,5-tetrazine rings of neighbouring molecules form a chain with an R²₂(6) ring motif (Etter et al., 1990 ) (see Fig. 4). As a result, the molecules are ordered into infinite ribbons extending along the [100] direction. This parallel arrangement of the ribbons is additionally stabilized by further interactions between adjacent molecules [N5⋯C33(1 − x, 1 − y, 1 − z) = 3.2418 (18) Å and C34⋯C61(1 − x, 1 − y, 1 − z) = 3.3334 (19) Å], as shown in Fig. 5.

Table 2
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
N4—H4⋯N2ⁱ	0.89 (2)	2.56 (2)	3.3017 (16)	142.5 (17)
N1—H1⋯N5ⁱⁱ	0.880 (17)	2.415 (17)	3.1321 (16)	138.9 (15)
Symmetry codes: (i) x-1, y, z; (ii) x+1, y, z.

Figure 4
N—H ⋯ N hydrogen bonds between rings of 1,2,4,5-tetrazine of adjacent molecules forming a chain of cyclic dimers.

Figure 5
A view of the unit-cell packing, showing the ribbon-like arrangement of molecules. Short C⋯N and C⋯C intermolecular contacts between adjacent molecular ribbons are shown as dashed blue lines.

4. Database survey

A search of the Cambridge Structure Database (CSD version 5.39, update of February 2018; Groom et al., 2016 ) results in 76 derivatives of 3,6-bis(pyridin-2-yl)-1,2,4,5-tetrazine, among them compound (II) (refcode JUMXAQ; Klein et al., 1998 ), which is the oxidated form of (I). Even tought (II) crystallizes in the smae monoclinic space group as (I), its molecular and crystal structures show completely different features.

5. Synthesis and crystallization

Crystals suitable for X-ray measurements were obtained from a commercially available reagent (Aldrich Chemical Co.) and used without further purification. 0.5 mmol of 3,6-bis(pyridin-2-yl)-1,2,4,5-tetrazine and 0.5 mmol of 2-mercaptopyridine N-oxide (in a 1:1 molar ratio) were mixed in ethanol (4 ml). The resulting solution was warmed to 343 K and then kept at room temperature. Within two weeks, after slow evaporation of the solvent, two kinds of crystal were obtained in a crystallizer. X-ray studies confirmed that the pink crystals were of the known structure (II), while the yellow crystals were identified as being of a previously unreported structure, i.e. (I).

6. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 3. Hydrogen atoms of aromatic rings were introduced in calculated positions with idealized geometry and constrained using a rigid body model with isotropic displacement parameters equal to 1.2 the equivalent displacement parameters of the parent atoms. The H atoms of the NH groups, in 1,2,4,5-tetrazine ring, were located in a difference Fourier map and freely refined.

Table 3
Experimental details

Crystal data
Chemical formula	C₁₂H₁₀N₆
M_r	238.26
Crystal system, space group	Monoclinic, P2₁/n
Temperature (K)	100
a, b, c (Å)	5.4603 (1), 12.7845 (3), 15.6474 (4)
β (°)	97.281 (2)
V (Å³)	1083.49 (4)
Z	4
Radiation type	Cu Kα
μ (mm⁻¹)	0.78
Crystal size (mm)	0.11 × 0.10 × 0.08

Data collection
Diffractometer	Rigaku Oxford Diffraction SuperNova, Dual, Cu at zero, Atlas
Absorption correction	Multi-scan (CrysAlis PRO; Rigaku OD, 2015 $[Rigaku OD (2015). CrysAlisPRO. Rigaku Oxford Diffraction, Yarnton, England.]$ )
T_min, T_max	0.958, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections	8686, 2004, 1767
R_int	0.027
(sin θ/λ)_max (Å⁻¹)	0.603

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.035, 0.095, 1.12
No. of reflections	2004
No. of parameters	171
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
Δρ_max, Δρ_min (e Å⁻³)	0.14, −0.24
Computer programs: CrysAlis PRO (Rigaku OD, 2015 $[Rigaku OD (2015). CrysAlisPRO. Rigaku Oxford Diffraction, Yarnton, England.]$ ), SHELXT2014 (Sheldrick, 2015a ), SHELXL2014 (Sheldrick, 2015b ), WinGX (Farrugia, 2012 ), PLATON (Spek, 2009 ) and publCIF (Westrip, 2010 ).

Supporting information

CCDC reference: 1884403

Crystal structure: contains datablock I. DOI: https://doi.org/10.1107/S205698901801753X/ff2157sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S205698901801753X/ff2157Isup2.hkl

Supporting information file. DOI: https://doi.org/10.1107/S205698901801753X/ff2157Isup3.cml

checkCIF report

Computing details top

Data collection: CrysAlis PRO (Rigaku OD, 2015); cell refinement: CrysAlis PRO (Rigaku OD, 2015); data reduction: CrysAlis PRO (Rigaku OD, 2015); program(s) used to solve structure: SHELXT2014 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015b); molecular graphics: WinGX (Farrugia, 2012); software used to prepare material for publication: SHELXL2014 (Sheldrick, 2015b), PLATON (Spek, 2009) and publCIF (Westrip, 2010).

3,6-Bis(pyridin-2-yl)-1,4-dihydro-1,2,4,5-tetrazine top

Crystal data top

C₁₂H₁₀N₆	F(000) = 496
M_r = 238.26	D_x = 1.461 Mg m⁻³
Monoclinic, P2₁/n	Cu Kα radiation, λ = 1.54184 Å
a = 5.4603 (1) Å	Cell parameters from 3734 reflections
b = 12.7845 (3) Å	θ = 4.5–76.4°
c = 15.6474 (4) Å	µ = 0.78 mm⁻¹
β = 97.281 (2)°	T = 100 K
V = 1083.49 (4) Å³	Plate, yellow
Z = 4	0.11 × 0.10 × 0.08 mm

Data collection top

Rigaku Oxford Diffraction SuperNova, Dual, Cu at zero, Atlas diffractometer	2004 independent reflections
Radiation source: micro-focus sealed X-ray tube, SuperNova (Cu) X-ray Source	1767 reflections with I > 2σ(I)
Detector resolution: 10.4052 pixels mm^-1	R_int = 0.027
ω scans	θ_max = 68.5°, θ_min = 4.5°
Absorption correction: multi-scan (CrysAlisPRO; Rigaku OD, 2015)	h = −6→6
T_min = 0.958, T_max = 1.000	k = −15→14
8686 measured reflections	l = −18→17

Refinement top

Refinement on F²	0 restraints
Least-squares matrix: full	Hydrogen site location: mixed
R[F² > 2σ(F²)] = 0.035	H atoms treated by a mixture of independent and constrained refinement
wR(F²) = 0.095	w = 1/[σ²(F_o²) + (0.051P)² + 0.2596P] where P = (F_o² + 2F_c²)/3
S = 1.12	(Δ/σ)_max < 0.001
2004 reflections	Δρ_max = 0.14 e Å⁻³
171 parameters	Δρ_min = −0.24 e Å⁻³

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
N5	0.3304 (2)	0.59460 (9)	0.30162 (7)	0.0159 (3)
N1	0.7587 (2)	0.61063 (9)	0.30346 (7)	0.0167 (3)
N66	0.7195 (2)	0.45247 (9)	0.18218 (7)	0.0178 (3)
N4	0.3800 (2)	0.66133 (9)	0.37517 (7)	0.0162 (3)
N2	0.7969 (2)	0.61146 (9)	0.39548 (7)	0.0166 (3)
N36	0.4117 (2)	0.70258 (9)	0.54575 (7)	0.0196 (3)
C3	0.6017 (2)	0.63816 (10)	0.42759 (9)	0.0151 (3)
C31	0.6094 (2)	0.65389 (10)	0.52161 (8)	0.0159 (3)
C6	0.5274 (2)	0.57389 (10)	0.26787 (8)	0.0150 (3)
C61	0.5133 (2)	0.50651 (10)	0.19059 (8)	0.0153 (3)
C62	0.2981 (2)	0.49884 (11)	0.13279 (8)	0.0183 (3)
H62	0.1608	0.5397	0.1394	0.022*
C65	0.7096 (2)	0.38393 (10)	0.11717 (9)	0.0191 (3)
H65	0.8497	0.3444	0.1116	0.023*
C64	0.5020 (3)	0.36871 (11)	0.05778 (9)	0.0194 (3)
H64	0.5020	0.3195	0.0140	0.023*
C34	0.6135 (3)	0.69920 (11)	0.69170 (9)	0.0201 (3)
H34	0.6106	0.7167	0.7493	0.024*
C32	0.8134 (3)	0.62397 (11)	0.57895 (9)	0.0196 (3)
H32	0.9460	0.5895	0.5596	0.024*
C63	0.2943 (3)	0.42869 (11)	0.06517 (9)	0.0201 (3)
H63	0.1539	0.4219	0.0252	0.024*
C33	0.8137 (3)	0.64675 (11)	0.66537 (9)	0.0215 (3)
H33	0.9465	0.6272	0.7054	0.026*
C35	0.4179 (3)	0.72475 (11)	0.62978 (9)	0.0211 (3)
H35	0.2832	0.7593	0.6476	0.025*
H1	0.886 (3)	0.5796 (14)	0.2850 (11)	0.022 (4)*
H4	0.253 (4)	0.6611 (15)	0.4051 (13)	0.030 (5)*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
N5	0.0158 (5)	0.0169 (5)	0.0144 (5)	0.0008 (4)	−0.0001 (4)	−0.0012 (4)
N1	0.0134 (5)	0.0221 (6)	0.0148 (5)	−0.0014 (5)	0.0020 (4)	−0.0020 (4)
N66	0.0154 (5)	0.0182 (6)	0.0198 (6)	0.0000 (4)	0.0025 (4)	−0.0009 (4)
N4	0.0138 (5)	0.0196 (6)	0.0150 (6)	0.0025 (4)	0.0007 (4)	−0.0025 (4)
N2	0.0159 (5)	0.0191 (6)	0.0145 (5)	−0.0009 (4)	0.0003 (4)	−0.0013 (4)
N36	0.0174 (6)	0.0229 (6)	0.0181 (6)	0.0010 (4)	0.0009 (4)	−0.0024 (4)
C3	0.0138 (6)	0.0136 (6)	0.0173 (7)	−0.0002 (5)	−0.0001 (5)	0.0003 (5)
C31	0.0161 (6)	0.0144 (6)	0.0170 (7)	−0.0025 (5)	0.0015 (5)	0.0009 (5)
C6	0.0137 (6)	0.0149 (6)	0.0162 (6)	0.0006 (5)	0.0007 (5)	0.0022 (5)
C61	0.0151 (6)	0.0144 (6)	0.0166 (6)	−0.0010 (5)	0.0032 (5)	0.0015 (5)
C62	0.0153 (6)	0.0211 (7)	0.0185 (7)	0.0017 (5)	0.0018 (5)	0.0005 (5)
C65	0.0168 (6)	0.0175 (6)	0.0236 (7)	0.0007 (5)	0.0055 (5)	−0.0018 (5)
C64	0.0223 (7)	0.0183 (6)	0.0181 (7)	−0.0027 (5)	0.0049 (5)	−0.0025 (5)
C34	0.0253 (7)	0.0193 (7)	0.0155 (6)	−0.0053 (5)	0.0017 (5)	−0.0006 (5)
C32	0.0180 (7)	0.0204 (7)	0.0203 (7)	0.0013 (5)	0.0019 (5)	0.0023 (5)
C63	0.0175 (6)	0.0237 (7)	0.0183 (7)	−0.0024 (5)	−0.0005 (5)	0.0002 (5)
C33	0.0212 (7)	0.0231 (7)	0.0190 (7)	−0.0019 (5)	−0.0024 (5)	0.0038 (5)
C35	0.0209 (7)	0.0226 (7)	0.0202 (7)	0.0000 (5)	0.0040 (5)	−0.0033 (5)

Geometric parameters (Å, º) top

N5—C6	1.2848 (17)	C61—C62	1.3926 (18)
N5—N4	1.4306 (16)	C62—C63	1.385 (2)
N1—C6	1.3953 (17)	C62—H62	0.9300
N1—N2	1.4285 (15)	C65—C64	1.386 (2)
N1—H1	0.880 (19)	C65—H65	0.9300
N66—C65	1.3384 (18)	C64—C63	1.386 (2)
N66—C61	1.3416 (17)	C64—H64	0.9300
N4—C3	1.4051 (17)	C34—C35	1.387 (2)
N4—H4	0.88 (2)	C34—C33	1.389 (2)
N2—C3	1.2809 (18)	C34—H34	0.9300
N36—C35	1.3412 (18)	C32—C33	1.383 (2)
N36—C31	1.3415 (18)	C32—H32	0.9300
C3—C31	1.4800 (18)	C63—H63	0.9300
C31—C32	1.3922 (19)	C33—H33	0.9300
C6—C61	1.4786 (18)	C35—H35	0.9300

C6—N5—N4	111.75 (11)	C63—C62—H62	120.9
C6—N1—N2	114.45 (10)	C61—C62—H62	120.9
C6—N1—H1	115.4 (12)	N66—C65—C64	123.53 (12)
N2—N1—H1	108.3 (12)	N66—C65—H65	118.2
C65—N66—C61	117.28 (12)	C64—C65—H65	118.2
C3—N4—N5	113.90 (10)	C65—C64—C63	118.36 (13)
C3—N4—H4	111.4 (13)	C65—C64—H64	120.8
N5—N4—H4	110.1 (13)	C63—C64—H64	120.8
C3—N2—N1	112.02 (11)	C35—C34—C33	118.16 (13)
C35—N36—C31	116.93 (12)	C35—C34—H34	120.9
N2—C3—N4	121.69 (12)	C33—C34—H34	120.9
N2—C3—C31	120.37 (12)	C33—C32—C31	118.30 (13)
N4—C3—C31	117.75 (12)	C33—C32—H32	120.9
N36—C31—C32	123.55 (12)	C31—C32—H32	120.9
N36—C31—C3	114.85 (12)	C62—C63—C64	119.26 (13)
C32—C31—C3	121.54 (12)	C62—C63—H63	120.4
N5—C6—N1	121.95 (12)	C64—C63—H63	120.4
N5—C6—C61	119.77 (12)	C32—C33—C34	119.21 (13)
N1—C6—C61	118.25 (11)	C32—C33—H33	120.4
N66—C61—C62	123.33 (12)	C34—C33—H33	120.4
N66—C61—C6	115.02 (11)	N36—C35—C34	123.82 (13)
C62—C61—C6	121.63 (12)	N36—C35—H35	118.1
C63—C62—C61	118.13 (12)	C34—C35—H35	118.1

N2—C3—N4—H4	164.1 (13)	C3—N2—N1—H1	−168.4 (12)
C6—N5—N4—H4	−165.2 (14)	N5—C6—N1—H1	164.3 (13)

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
N4—H4···N2ⁱ	0.89 (2)	2.56 (2)	3.3017 (16)	142.5 (17)
N1—H1···N5ⁱⁱ	0.880 (17)	2.415 (17)	3.1321 (16)	138.9 (15)

Symmetry codes: (i) x−1, y, z; (ii) x+1, y, z.

Funding information

Funding for this research was provided by: Narodowe Centrum Nauki (grant No. 2015/19/B/ST4/01773); EFRD in Operational Programme Development of Eastern Poland 2007–2013, the Oxford Diffraction SuperNova DualSource diffractometer (award No. POPW.01.03.00-20-004/11).

References

Allen, F. H., Kennard, O., Watson, D. G., Brammer, L., Orpen, A. G. & Taylor, R. (1987). J. Chem. Soc. Perkin Trans. 2, pp. S1–S19. CrossRef Web of Science Google Scholar
Brooker, P. J., Parsons, J. H., Reid, J. & West, P. (1987). Pestic. Sci. 18, 179–190. CrossRef Google Scholar
Churakov, A. M. & Tartakovsky, V. A. (2004). Chem. Rev. 104, 2601–2616. CrossRef Google Scholar
Clavier, G. & Audebert, P. (2010). Chem. Rev. 110, 3299–3314. Web of Science CrossRef CAS PubMed Google Scholar
Etter, M. C., MacDonald, J. C. & Bernstein, J. (1990). Acta Cryst. B46, 256–262. CrossRef CAS Web of Science IUCr Journals Google Scholar
Farrugia, L. J. (2012). J. Appl. Cryst. 45, 849–854. Web of Science CrossRef CAS IUCr Journals Google Scholar
Groom, C. R., Bruno, I. J., Lightfoot, M. P. & Ward, S. C. (2016). Acta Cryst. B72, 171–179. Web of Science CrossRef IUCr Journals Google Scholar
Kaim, W. (2002). Coord. Chem. Rev. 230, 127–139. CrossRef CAS Google Scholar
Kamal, M. D., Hassan, A. G., Money, E., Hanan, A. M. & Bahira, H. (2006). Arch. Pharm. Chem. Life Sci. 339, 133. Google Scholar
Klein, A., McInnes, E. J. L., Scheiring, T. & Zališ, S. (1998). Faraday Trans. 94, 2979–2984. CrossRef CAS Google Scholar
Neunhoeffer, H. (1984). Comprehensive Heterocyclic Chemistry, Vol. 3, 1st ed , edited by A. R. Katritzky and C. W. Rees, p. 531. Oxford: Pergamon Press. Google Scholar
Rao, G. W. & Hu, W. X. (2006). Bioorg. Med. Chem. Lett. 16, 3702–3705. Web of Science CrossRef PubMed CAS Google Scholar
Rigaku OD (2015). CrysAlisPRO. Rigaku Oxford Diffraction, Yarnton, England. Google Scholar
Saghatforoush, L., Khoshtarkib, Z., Amani, V., Bakhtiari, A., Hakimi, M. & Keypour, H. (2016). J. Solid State Chem. 233, 311–319. CrossRef Google Scholar
Saracoglu, N. (2007). Tetrahedron, 63, 4199–4236. CrossRef Google Scholar
Sauer, J. (1996). Comprehensive Heterocyclic Chemistry, Vol. 6, 2nd ed., edited by A. J. Boulton, C. W. Rees and E. F. V. Scriven, pp. 901–955. Oxford: Pergamon. Google Scholar
Šečkutė, J. & Devaraj, N. K. (2013). Curr. Opin. Chem. Biol. 17, 761–767. Google Scholar
Sheldrick, G. M. (2015a). Acta Cryst. A71, 3–8. Web of Science CrossRef IUCr Journals Google Scholar
Sheldrick, G. M. (2015b). Acta Cryst. C71, 3–8. Web of Science CrossRef IUCr Journals Google Scholar
Spek, A. L. (2009). Acta Cryst. D65, 148–155. Web of Science CrossRef CAS IUCr Journals Google Scholar
Westrip, S. P. (2010). J. Appl. Cryst. 43, 920–925. Web of Science CrossRef CAS IUCr Journals Google Scholar