2-Amino-6-chloro­pyridinium 3-carb­oxy-4-hy­droxy­benzene­sulfonate

Manickam, R.; Rajakannan, V.; Prabhaharan, M.; Srinivasan, G.

doi:10.1107/S2414314619005662

organic compounds

IUCrDATA

ISSN: 2414-3146

Volume 4| Part 5| May 2019| x190566

https://doi.org/10.1107/S2414314619005662

Open

access

2-Amino-6-chloropyridinium 3-carboxy-4-hydroxybenzenesulfonate

R. Manickam,^a V. Rajakannan,^b M. Prabhaharan ^c and G. Srinivasan ^a ^*

^aPG and Research Department of Physics, Government Arts College for Men (Autonomous), Nandanam, Chennai 600 035, India, ^bCentre of Advanced Study in Crystallography and Biophysics, University of Madras, Guindy Campus, Chennai 600 025, India, and ^cDepartment of Physics, Annai Violet Arts and Science College, Chennai 600 053, India
^*Correspondence e-mail: agsv71@yahoo.com

Edited by H. Ishida, Okayama University, Japan (Received 4 April 2019; accepted 24 April 2019; online 10 May 2019)

In the 3-carboxy-4-hydroxybenzenesulfonate anion of the title salt, C₅H₆ClN₂⁺·C₇H₅O₆S⁻, an intramolecular O—H⋯O hydrogen bond with an S(6) ring motif is observed. In the crystal, the anions are linked into a chain structure running along [1 $[\overline{1}]$ 0] via an O—H⋯O hydrogen bond formed between the carboxy and sulfonate groups. The 2-amino-6-chloropyridinium cations bridge the anion chains via N—H⋯O and C—H⋯O hydrogen bonds, forming a sheet parallel to the ab plane. In the sheet, a C—H⋯Cl interaction between the cations is also observed.

Keywords: crystal structure; hydrogen bond; 2-amino-6-chloropyridinium; 3-carboxy-4-hydroxybenzenesulfonate.

CCDC reference: 1912046

Structure description

Pyridine heterocycles and their derivatives have many applications in the fields of photo-chemical, electrochemical and catalytic processes (Katritzky et al., 1996 ). Some pyridine derivatives possess non-linear optical (NLO) properties (Rajkumar et al., 2015 ). 2-Aminopyridine derivatives are used in the synthesis of pharmaceutical drugs, especially for the treatment of neurological ailments (Schwid et al., 1997 ). Several crystal structures of 2-aminopyridine co-crystals with carboxylic acid derivatives have been already reported (Hemamalini et al., 2014 ). As part of our studies in this area, we now describe the synthesis and structure of the title salt.

In the 2-amino-6-chloropyridinium cation of the title compound, protonation at atom N1 leads to a slight increase in the C1—N1—C5 angle [121.72 (6)° compared with 116.8 (1)° in unprotonated 2-amino-6-chloropyridine (Hemamalini et al., 2014)]. The 3-carboxy-4-hydroxybenzene sulfonate anion contains an intramolecular O—H⋯O hydrogen bond (O3—H1O3⋯O2; Table 1) with an S(6) ring motif (Fig. 1). In the crystal, the anions are linked into a chain structure running along [1 $[\overline{1}]$ 0] via an O—H⋯O hydrogen bond formed between the carboxy and sulfonate groups (O1—H1O1⋯O6ⁱ; symmetry code as in Table 1). The 2-amino-6-chloropyridinium cations bridge the anion chains via N—H⋯O and C—H⋯O hydrogen bonds (N1—H1N1⋯O5, N2—H1N2⋯O6, N2—H2N2⋯O4ⁱⁱ and C4—H4⋯O5ⁱⁱ; Table 1), forming a sheet parallel to the ab plane. In the sheet, a C—H⋯Cl interaction (C3—H3⋯Cl1ⁱⁱ; Table 1) between the cations is also observed. The sheets are further linked via another C—H⋯O (C2—H2⋯O3ⁱⁱⁱ; Table 1) hydrogen bond, forming a three-dimensional network (Fig. 2).

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O1—H1O1⋯O6ⁱ	0.871 (17)	1.775 (17)	2.6061 (7)	158.7 (16)
O3—H1O3⋯O2	0.850 (16)	1.829 (16)	2.6027 (8)	150.5 (16)
N1—H1N1⋯O5	0.881 (14)	1.854 (14)	2.7272 (8)	171.4 (13)
N2—H1N2⋯O6	0.890 (15)	1.992 (15)	2.8814 (8)	178.6 (15)
N2—H2N2⋯O4ⁱⁱ	0.849 (14)	2.038 (14)	2.8823 (9)	173.1 (14)
C2—H2⋯O3ⁱⁱⁱ	0.931 (14)	2.389 (14)	3.2978 (9)	165.2 (12)
C3—H3⋯Cl1ⁱⁱ	0.934 (15)	2.737 (15)	3.5834 (8)	151.1 (13)
C4—H4⋯O5ⁱⁱ	0.941 (14)	2.340 (14)	3.2489 (9)	162.2 (11)
Symmetry codes: (i) $[x-{\script{1\over 2}}, y+{\script{1\over 2}}, z]$ ; (ii) x, y-1, z; (iii) $[x, -y+1, z+{\script{1\over 2}}]$ .

Figure 1
The asymmetric unit of the title compound. Displacement ellipsoids are drawn at the 30% probability level.

Figure 2
A packing diagram of the title compound, showing the hydrogen-bonded network (dashed lines).

Synthesis and crystallization

A hot methanol solution (20 ml) of 2-amino-6-chloropyridine (34 mg, Aldrich) and sulfosalicylic acid (54 mg, Merck) was allowed to cool slowly to room temperature and single crystals of the title compound appeared after a few days.

Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2.

Table 2
Experimental details

Crystal data
Chemical formula	C₅H₆ClN₂⁺·C₇H₅O₆S⁻
M_r	346.74
Crystal system, space group	Monoclinic, C2/c
Temperature (K)	100
a, b, c (Å)	16.0043 (3), 7.4649 (2), 23.5799 (5)
β (°)	95.161 (1)
V (Å³)	2805.68 (11)
Z	8
Radiation type	Mo Kα
μ (mm⁻¹)	0.45
Crystal size (mm)	0.39 × 0.28 × 0.21

Data collection
Diffractometer	Bruker SMART APEXII CCD area detector
Absorption correction	Multi-scan (SADABS; Bruker, 2009 )
T_min, T_max	0.842, 0.911
No. of measured, independent and observed [I > 2σ(I)] reflections	26502, 7386, 6997
R_int	0.016
(sin θ/λ)_max (Å⁻¹)	0.858

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.028, 0.079, 1.14
No. of reflections	7386
No. of parameters	243
H-atom treatment	All H-atom parameters refined
Δρ_max, Δρ_min (e Å⁻³)	0.58, −0.41
Computer programs: APEX2 (Bruker, 2009), SAINT (Bruker, 2009), SHELXTL (Sheldrick, 2008 ) and PLATON (Spek, 2009 ).

Structural data

CCDC reference: 1912046

Crystal structure: contains datablocks I, global. DOI: https://doi.org/10.1107/S2414314619005662/is4031sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2414314619005662/is4031Isup2.hkl

Supporting information file. DOI: https://doi.org/10.1107/S2414314619005662/is4031Isup3.cml

checkCIF report

Computing details top

Data collection: APEX2 (Bruker, 2009); cell refinement: SAINT (Bruker, 2009); data reduction: SAINT (Bruker, 2009); program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure: SHELXTL (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008) and PLATON (Spek, 2009).

2-Amino-6-chloropyridinium 3-carboxy-4-hydroxybenzenesulfonate top

Crystal data top

C₅H₆ClN₂⁺·C₇H₅O₆S⁻	F(000) = 1424
M_r = 346.74	D_x = 1.642 Mg m⁻³
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2yc	Cell parameters from 9829 reflections
a = 16.0043 (3) Å	θ = 2.6–37.6°
b = 7.4649 (2) Å	µ = 0.45 mm⁻¹
c = 23.5799 (5) Å	T = 100 K
β = 95.161 (1)°	Block, colourless
V = 2805.68 (11) Å³	0.39 × 0.28 × 0.21 mm
Z = 8

Data collection top

Bruker SMART APEXII CCD area detector diffractometer	7386 independent reflections
Radiation source: fine-focus sealed tube	6997 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.016
φ and ω scans	θ_max = 37.6°, θ_min = 2.6°
Absorption correction: multi-scan (SADABS; Bruker, 2009)	h = −27→27
T_min = 0.842, T_max = 0.911	k = −11→12
26502 measured reflections	l = −40→27

Refinement top

Refinement on F²	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F² > 2σ(F²)] = 0.028	Hydrogen site location: inferred from neighbouring sites
wR(F²) = 0.079	All H-atom parameters refined
S = 1.14	w = 1/[σ²(F_o²) + (0.0359P)² + 1.5791P] where P = (F_o² + 2F_c²)/3
7386 reflections	(Δ/σ)_max = 0.001
243 parameters	Δρ_max = 0.58 e Å⁻³
0 restraints	Δρ_min = −0.41 e Å⁻³

Special details top

Experimental. The crystal was placed in the cold stream of an Oxford Cryosystems Cobra open-flow nitrogen cryostat [Cosier, J. & Glazer, A. M. (1986). J. Appl. Cryst. 19, 105–107] operating at 100.0 (1) K.

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.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > 2sigma(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

All H atoms were located in a difference Fourier map and allowed to refine freely [N—H = 0.849 (14)–0.890 (15) Å, O—H = 0.850 (16)–0.870 (17) Å and C—H = 0.931 (14)–0.987 (13) Å].

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
S1	0.194955 (9)	0.60531 (2)	0.120818 (6)	0.01019 (3)
O1	−0.11147 (3)	0.84652 (9)	0.08056 (2)	0.01856 (10)
O2	−0.14331 (3)	0.87680 (8)	−0.01363 (2)	0.01719 (10)
O3	−0.03230 (4)	0.78169 (9)	−0.08199 (2)	0.01771 (10)
O4	0.24938 (3)	0.76081 (8)	0.12876 (2)	0.01497 (9)
O5	0.14455 (3)	0.57579 (7)	0.16888 (2)	0.01358 (8)
O6	0.24046 (3)	0.44196 (7)	0.10735 (2)	0.01408 (8)
C6	0.04546 (4)	0.71656 (9)	0.06802 (3)	0.01147 (9)
C7	0.12440 (4)	0.64976 (9)	0.06087 (3)	0.01058 (9)
C8	0.15093 (4)	0.62660 (9)	0.00621 (3)	0.01268 (10)
C9	0.09758 (4)	0.67148 (10)	−0.04112 (3)	0.01408 (10)
C10	0.01735 (4)	0.73936 (9)	−0.03450 (3)	0.01221 (10)
C11	−0.00912 (4)	0.76231 (9)	0.02035 (3)	0.01131 (9)
C12	−0.09384 (4)	0.83327 (9)	0.02688 (3)	0.01288 (10)
N1	0.13605 (4)	0.27027 (8)	0.23152 (3)	0.01344 (9)
N2	0.19616 (4)	0.11253 (9)	0.16085 (3)	0.01786 (11)
Cl1	0.079291 (13)	0.48895 (2)	0.305211 (8)	0.01915 (4)
C1	0.09741 (4)	0.27792 (9)	0.28073 (3)	0.01322 (10)
C2	0.07675 (5)	0.12856 (10)	0.30926 (3)	0.01542 (11)
C3	0.09926 (5)	−0.03714 (10)	0.28634 (3)	0.01780 (12)
C4	0.13957 (5)	−0.04791 (10)	0.23772 (3)	0.01718 (12)
C5	0.15832 (4)	0.11192 (9)	0.20889 (3)	0.01376 (10)
H2	0.0514 (8)	0.1360 (19)	0.3432 (6)	0.021 (3)*
H3	0.0891 (10)	−0.145 (2)	0.3046 (7)	0.032 (4)*
H4	0.1526 (8)	−0.1600 (19)	0.2225 (6)	0.023 (3)*
H6	0.0274 (8)	0.7370 (19)	0.1064 (6)	0.019 (3)*
H8	0.2071 (8)	0.5821 (17)	0.0021 (5)	0.015 (3)*
H9	0.1142 (9)	0.656 (2)	−0.0780 (6)	0.024 (3)*
H1O1	−0.1623 (11)	0.888 (2)	0.0808 (7)	0.041 (4)*
H1O3	−0.0781 (10)	0.820 (2)	−0.0709 (7)	0.032 (4)*
H1N1	0.1436 (8)	0.3713 (18)	0.2134 (6)	0.019 (3)*
H1N2	0.2102 (9)	0.215 (2)	0.1449 (6)	0.028 (4)*
H2N2	0.2116 (9)	0.0121 (19)	0.1485 (6)	0.023 (3)*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
S1	0.00848 (6)	0.01208 (6)	0.01013 (6)	0.00212 (4)	0.00147 (4)	−0.00012 (4)
O1	0.01210 (19)	0.0300 (3)	0.0140 (2)	0.00791 (19)	0.00350 (16)	0.00012 (19)
O2	0.01249 (19)	0.0226 (3)	0.0160 (2)	0.00428 (17)	−0.00103 (16)	0.00197 (18)
O3	0.0150 (2)	0.0267 (3)	0.01119 (19)	0.00370 (19)	−0.00049 (16)	0.00279 (18)
O4	0.01189 (18)	0.0154 (2)	0.0174 (2)	−0.00116 (16)	0.00033 (15)	−0.00181 (17)
O5	0.01388 (19)	0.0164 (2)	0.01106 (18)	0.00364 (16)	0.00411 (15)	0.00170 (15)
O6	0.01187 (18)	0.0148 (2)	0.0160 (2)	0.00497 (16)	0.00341 (15)	0.00015 (16)
C6	0.0094 (2)	0.0145 (2)	0.0107 (2)	0.00190 (18)	0.00201 (17)	0.00037 (18)
C7	0.0090 (2)	0.0127 (2)	0.0102 (2)	0.00151 (17)	0.00168 (16)	−0.00005 (18)
C8	0.0110 (2)	0.0160 (3)	0.0115 (2)	0.00172 (19)	0.00298 (17)	−0.00057 (19)
C9	0.0128 (2)	0.0192 (3)	0.0105 (2)	0.0021 (2)	0.00293 (18)	−0.0003 (2)
C10	0.0115 (2)	0.0147 (2)	0.0104 (2)	0.00023 (19)	0.00100 (17)	0.00095 (18)
C11	0.0092 (2)	0.0137 (2)	0.0112 (2)	0.00139 (18)	0.00165 (17)	0.00063 (18)
C12	0.0102 (2)	0.0148 (3)	0.0137 (2)	0.00172 (19)	0.00169 (18)	0.00027 (19)
N1	0.0151 (2)	0.0119 (2)	0.0135 (2)	0.00114 (17)	0.00169 (17)	0.00032 (17)
N2	0.0211 (3)	0.0158 (3)	0.0177 (2)	0.0036 (2)	0.0074 (2)	0.0010 (2)
Cl1	0.02752 (9)	0.01356 (7)	0.01658 (7)	0.00355 (6)	0.00307 (6)	−0.00200 (5)
C1	0.0145 (2)	0.0129 (2)	0.0120 (2)	0.00098 (19)	−0.00034 (18)	−0.00142 (19)
C2	0.0197 (3)	0.0146 (3)	0.0119 (2)	−0.0017 (2)	0.0009 (2)	−0.0006 (2)
C3	0.0260 (3)	0.0134 (3)	0.0142 (3)	−0.0020 (2)	0.0023 (2)	0.0003 (2)
C4	0.0236 (3)	0.0126 (3)	0.0155 (3)	0.0008 (2)	0.0027 (2)	−0.0003 (2)
C5	0.0145 (2)	0.0132 (2)	0.0136 (2)	0.00182 (19)	0.00097 (19)	−0.00013 (19)

Geometric parameters (Å, º) top

S1—O4	1.4532 (6)	C10—C11	1.4072 (9)
S1—O5	1.4656 (5)	C11—C12	1.4765 (9)
S1—O6	1.4696 (5)	N1—C5	1.3577 (9)
S1—C7	1.7595 (6)	N1—C1	1.3643 (9)
O1—C12	1.3252 (9)	N1—H1N1	0.881 (14)
O1—H1O1	0.870 (17)	N2—C5	1.3317 (9)
O2—C12	1.2283 (8)	N2—H1N2	0.890 (15)
O3—C10	1.3509 (8)	N2—H2N2	0.849 (14)
O3—H1O3	0.850 (16)	Cl1—C1	1.7115 (7)
C6—C7	1.3824 (9)	C1—C2	1.3583 (10)
C6—C11	1.4024 (9)	C2—C3	1.4095 (11)
C6—H6	0.987 (13)	C2—H2	0.931 (14)
C7—C8	1.4036 (9)	C3—C4	1.3676 (11)
C8—C9	1.3840 (9)	C3—H3	0.932 (17)
C8—H8	0.972 (13)	C4—C5	1.4186 (10)
C9—C10	1.4020 (9)	C4—H4	0.941 (14)
C9—H9	0.939 (14)

O4—S1—O5	112.69 (3)	C10—C11—C12	119.65 (6)
O4—S1—O6	112.81 (3)	O2—C12—O1	123.05 (6)
O5—S1—O6	111.12 (3)	O2—C12—C11	123.20 (6)
O4—S1—C7	106.66 (3)	O1—C12—C11	113.75 (6)
O5—S1—C7	106.94 (3)	C5—N1—C1	121.72 (6)
O6—S1—C7	106.12 (3)	C5—N1—H1N1	120.1 (9)
C12—O1—H1O1	108.1 (11)	C1—N1—H1N1	118.1 (9)
C10—O3—H1O3	106.5 (11)	C5—N2—H1N2	121.0 (10)
C7—C6—C11	120.03 (6)	C5—N2—H2N2	117.2 (9)
C7—C6—H6	120.9 (8)	H1N2—N2—H2N2	121.3 (13)
C11—C6—H6	119.0 (8)	C2—C1—N1	122.41 (6)
C6—C7—C8	120.77 (6)	C2—C1—Cl1	122.16 (5)
C6—C7—S1	119.84 (5)	N1—C1—Cl1	115.40 (5)
C8—C7—S1	119.29 (5)	C1—C2—C3	116.65 (7)
C9—C8—C7	119.68 (6)	C1—C2—H2	121.4 (9)
C9—C8—H8	120.8 (7)	C3—C2—H2	121.9 (9)
C7—C8—H8	119.5 (7)	C4—C3—C2	121.94 (7)
C8—C9—C10	120.16 (6)	C4—C3—H3	116.9 (10)
C8—C9—H9	120.8 (9)	C2—C3—H3	121.2 (10)
C10—C9—H9	119.0 (9)	C3—C4—C5	119.23 (7)
O3—C10—C9	117.94 (6)	C3—C4—H4	120.6 (9)
O3—C10—C11	122.00 (6)	C5—C4—H4	120.1 (9)
C9—C10—C11	120.05 (6)	N2—C5—N1	119.14 (6)
C6—C11—C10	119.32 (6)	N2—C5—C4	122.85 (7)
C6—C11—C12	121.03 (6)	N1—C5—C4	118.01 (6)

C11—C6—C7—C8	−0.09 (10)	O3—C10—C11—C12	0.52 (10)
C11—C6—C7—S1	176.17 (5)	C9—C10—C11—C12	−179.78 (6)
O4—S1—C7—C6	−96.71 (6)	C6—C11—C12—O2	179.20 (7)
O5—S1—C7—C6	24.08 (6)	C10—C11—C12—O2	−1.06 (11)
O6—S1—C7—C6	142.77 (6)	C6—C11—C12—O1	−0.49 (10)
O4—S1—C7—C8	79.60 (6)	C10—C11—C12—O1	179.25 (6)
O5—S1—C7—C8	−159.60 (5)	C5—N1—C1—C2	−1.85 (10)
O6—S1—C7—C8	−40.91 (6)	C5—N1—C1—Cl1	176.41 (5)
C6—C7—C8—C9	0.15 (10)	N1—C1—C2—C3	1.66 (10)
S1—C7—C8—C9	−176.13 (6)	Cl1—C1—C2—C3	−176.49 (6)
C7—C8—C9—C10	−0.15 (11)	C1—C2—C3—C4	−0.26 (11)
C8—C9—C10—O3	179.81 (7)	C2—C3—C4—C5	−0.94 (12)
C8—C9—C10—C11	0.10 (11)	C1—N1—C5—N2	−179.76 (7)
C7—C6—C11—C10	0.03 (10)	C1—N1—C5—C4	0.55 (10)
C7—C6—C11—C12	179.77 (6)	C3—C4—C5—N2	−178.87 (7)
O3—C10—C11—C6	−179.74 (6)	C3—C4—C5—N1	0.80 (11)
C9—C10—C11—C6	−0.04 (10)

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1O1···O6ⁱ	0.871 (17)	1.775 (17)	2.6061 (7)	158.7 (16)
O3—H1O3···O2	0.850 (16)	1.829 (16)	2.6027 (8)	150.5 (16)
N1—H1N1···O5	0.881 (14)	1.854 (14)	2.7272 (8)	171.4 (13)
N2—H1N2···O6	0.890 (15)	1.992 (15)	2.8814 (8)	178.6 (15)
N2—H2N2···O4ⁱⁱ	0.849 (14)	2.038 (14)	2.8823 (9)	173.1 (14)
C2—H2···O3ⁱⁱⁱ	0.931 (14)	2.389 (14)	3.2978 (9)	165.2 (12)
C3—H3···Cl1ⁱⁱ	0.934 (15)	2.737 (15)	3.5834 (8)	151.1 (13)
C4—H4···O5ⁱⁱ	0.941 (14)	2.340 (14)	3.2489 (9)	162.2 (11)

Symmetry codes: (i) x−1/2, y+1/2, z; (ii) x, y−1, z; (iii) x, −y+1, z+1/2.

References

Bruker (2009). APEX2, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA. Google Scholar
Hemamalini, M., Loh, W.-S., Quah, C. K. & Fun, H.-K. (2014). Chem. Cent. J. 8, 31. Web of Science CSD CrossRef PubMed Google Scholar
Katritzky, A. R., Rees, C. W. & Scriven, E. F. V. (1996). Comprehensive Heterocyclic Chemistry II. Oxford: Pergamon Press. Google Scholar
Rajkumar, M. A., NizamMohideen, M., Xavier, S. S. J., Anbarasu, S. & Devarajan, D. P. A. (2015). Acta Cryst. E71, 231–233. Web of Science CSD CrossRef IUCr Journals Google Scholar
Schwid, S. R., Petrie, M. D., McDermott, M. P., Tierney, D. S., Mason, D. H. & Goodman, A. D. (1997). Neurology, 48, 817–821. CrossRef CAS PubMed Web of Science Google Scholar
Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. Web of Science CrossRef CAS IUCr Journals Google Scholar
Spek, A. L. (2009). Acta Cryst. D65, 148–155. Web of Science CrossRef CAS IUCr Journals Google Scholar

This is an open-access article distributed under the terms of the Creative Commons Attribution (CC-BY) Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original authors and source are cited.