2-[(6-Nitro-1,3-benzodioxol-5-yl)methyl­idene]malononitrile

Karthikeyan, S.; Sethusankar, K.; Devaraj, A.; Bakthadoss, M.

doi:10.1107/S1600536811049816

organic compounds

CRYSTALLOGRAPHIC
COMMUNICATIONS

ISSN: 2056-9890

Volume 67| Part 12| December 2011| Page o3469

https://doi.org/10.1107/S1600536811049816

Open

access

2-[(6-Nitro-1,3-benzodioxol-5-yl)methylidene]malononitrile

S. Karthikeyan,^a K. Sethusankar,^a ^* Anthonisamy Devaraj ^b and Manickam Bakthadoss ^b

^aDepartment of Physics, RKM Vivekananda College (Autonomous), Chennai 600 004, India, and ^bDepartment of Organic Chemistry, University of Madras, Maraimalai Campus, Chennai 600 025, India
^*Correspondence e-mail: ksethusankar@yahoo.co.in

(Received 2 November 2011; accepted 21 November 2011; online 30 November 2011)

In the title compound, C₁₁H₅N₃O₄, the nitro group is rotated by 29.91 (16)° out of the plane of the adjacent aryl ring. The 1,3-benzodioxole ring is nearly planar, with a maximium deviation of 0.0562 (10) Å. The dioxolene ring adopts an envelope conformation on the O—C—O C atom. In the crystal, molecules are linked via C—H⋯O interactions, resulting in R₂²(6) and R₂²(12) graph-set motifs.

Related literature

For applications of malononitrile derivatives, see: Brimblecombe et al. (1972 ). For related structure, see: Loghmani–Khouzani et al. (2009 ). For comparison of molecular dimensions, see: Allen et al. (1987 ). For puckering parameters, see: Cremer & Pople (1975 ). For graph–set motif notations, see: Bernstein et al. (1995 ).

Experimental

Crystal data

C₁₁H₅N₃O₄
M_r = 243.18
Triclinic, $[P \overline 1]$
a = 7.0953 (2) Å
b = 8.8847 (3) Å
c = 9.2212 (3) Å
α = 84.470 (2)°
β = 67.634 (2)°
γ = 78.874 (2)°
V = 527.30 (3) Å³
Z = 2
Mo Kα radiation
μ = 0.12 mm⁻¹
T = 295 K
0.30 × 0.28 × 0.25 mm

Data collection

Bruker Kappa APEXII CCD diffractometer
13806 measured reflections
3494 independent reflections
2700 reflections with I > 2σ(I)
R_int = 0.025

Refinement

R[F² > 2σ(F²)] = 0.048
wR(F²) = 0.142
S = 1.03
3494 reflections
163 parameters
H-atom parameters constrained
Δρ_max = 0.28 e Å⁻³
Δρ_min = −0.30 e Å⁻³

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
C1—H1B⋯O1ⁱ	0.97	2.53	3.2692 (16)	133
C8—H8⋯O4ⁱⁱ	0.93	2.52	3.3640 (17)	152
Symmetry codes: (i) -x, -y+1, -z+2; (ii) -x-1, -y+2, -z+1.

Data collection: APEX2 (Bruker, 2008 ); cell refinement: SAINT (Bruker, 2008); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 (Farrugia, 1997 ) and Mercury (Macrae et al. 2008 ); software used to prepare material for publication: SHELXL97 and PLATON (Spek, 2009 ).

Supporting information

Crystal structure: contains datablocks global, I. DOI: https://doi.org/10.1107/S1600536811049816/rk2314sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S1600536811049816/rk2314Isup2.hkl

Supporting information file. DOI: https://doi.org/10.1107/S1600536811049816/rk2314Isup3.cml

checkCIF report

Comment top

The malononitrile derivative is used to investigate a variety of possible pharmacological effects when administered by various routes to whole animals and when applied to isolated organs and tissues (Brimblecombe et al., 1972). Also, it is a component of "tear gas" commonly reffered as CS gas, which is used as a riot control agent.

In the title compound C₁₁H₅N₃O₄, the benzodioxole ring is nearly planar with a maximum deviation 0.0562Å for the atom O2. The O═N═O angle is much larger than the ideal tetrahedral or trigonal values, respectively, doubtless as a consequence of the substantial negative charge on the paired O atoms. The bond lengths C9—C10 = 1.4388 (16)Å and C9—C11 = 1.4342 (16)Å is significantly shorter than the expected value for a C—C single bond because of conjugation effects.

In the dioxole ring C1/O2/C2/C7/O1, the deviation of atom C1 is -0.0724 (16)Å. The dioxole ring adopts a envelope conformation on C1 with puckering parameters (Cremer & Pople, 1975): Q₂ = 0.1145 (13)Å and φ₂ = 36.7 (6)°. The malononitrile group (C9—C10≡N2) and (C9—C11≡N3) is almost linear, with the angle around central carbon atoms C10 and C11 being 179.14 (15)° and 179.09 (15)° respectively.

The values of the torsion angles C5–C4–C8–C9 = -154.85 (11)° and C4–C5–N1–O4 = -151.10 (12)° indicates that the conformation of molecule is (-)anti–periplanar. The nitro group is not co–planar to the benzodioxole ring to which it is attached, making a dihedral angle of 29.76 (4)°. The benzodioxole unit is oriented at a dihedral angle of 36.90 (4)° with respect to the malononitrile group. The triple bond distances C10≡N2 and C11≡N3 are in agreement with the literature values (1.138 (7)Å; Allen et al., 1987). The title compound exhibits structural similarities with the already reported related structures (Loghmani–Khouzani et al., 2009).

The crystal packing is stabilized by non–classical intermolecular C—H···O interactions. The molecules are linked into centrosymmetric dimers. Atom C1 acts as a donor to dioxole O1ⁱ, so forming an R²₂(6) graph–set motif and atom C8 acts as a donor to nitro group O4ⁱⁱ at forming an R²₂(12) graph–set motif (Bernstein, et al., 1995). Symmetry codes: (i) -x, 1-y, 2-z; (ii) -1-x, 2-y, 1-z).

Related literature top

For applications of malononitrile derivatives, see: Brimblecombe et al. (1972). For related structure, see: Loghmani–Khouzani et al. (2009). For comparison of molecular dimensions, see: Allen et al. (1987). For puckering parameters, see: Cremer & Pople (1975). For graph–set motif notations, see: Bernstein et al. (1995).

Experimental top

To a solution of malononitrile (0.082 g, 1.24 mmol) in dichloromethane (5 ml), pyrrolidine (0.073 g, 1.03 mmol) was added and stirred well for 10 minutes. To this solution 6–nitrobenzo[d][1,3]dioxole–5–carbaldehyde (0.2 g, 1.03 mmol) was added and stirring was continued for 12 h. After the completion of the reaction as evidenced by TLC, the reaction mixture was poured into 2 N HCl solution (10 ml) and extracted using 25 ml of dichloromethane. The organic layer thus obtained was concentrated under reduced pressure. Column purification (silica gel, mesh size: 60–120) of the crude mixture using 15% ethyl acetate in hexanes successfully provided the desired 2–((6–nitrobenzo[d][1,3]dioxol–5–yl)methylene)malononitrile in 90% yield (0.23 g).

Structure description top

For applications of malononitrile derivatives, see: Brimblecombe et al. (1972). For related structure, see: Loghmani–Khouzani et al. (2009). For comparison of molecular dimensions, see: Allen et al. (1987). For puckering parameters, see: Cremer & Pople (1975). For graph–set motif notations, see: Bernstein et al. (1995).

Computing details top

Data collection: APEX2 (Bruker, 2008); cell refinement: SAINT (Bruker, 2008); data reduction: SAINT (Bruker, 2008); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 (Farrugia, 1997) and Mercury (Macrae et al. 2008); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008) and PLATON (Spek, 2009).

Figures top

	Fig. 1. The molecular structure of the title compound with the atom numbering scheme, displacement ellipsoids are drawn at 30% probability level. H atoms are present as small spheres of arbitary radius.
	Fig. 2. The packing arrangement of the title compound viewed down a axis. The dashed lines indicate C—H···O intermolecular interactions, which forms R²₂(6) and R²₂(12) centrosymmetric dimers. The symmetry codes: (i) -x, 1-y, 2-z; (ii) -1-x, 2-y, 1-z.

2-[(6-Nitro-1,3-benzodioxol-5-yl)methylidene]malononitrile top

Crystal data top

C₁₁H₅N₃O₄	Z = 2
M_r = 243.18	F(000) = 248
Triclinic, P1	D_x = 1.532 Mg m⁻³
Hall symbol: -P 1	Mo Kα radiation, λ = 0.71073 Å
a = 7.0953 (2) Å	Cell parameters from 3494 reflections
b = 8.8847 (3) Å	θ = 1.0–31.6°
c = 9.2212 (3) Å	µ = 0.12 mm⁻¹
α = 84.470 (2)°	T = 295 K
β = 67.634 (2)°	Block, yellow
γ = 78.874 (2)°	0.30 × 0.28 × 0.25 mm
V = 527.30 (3) Å³

Data collection top

Bruker Kappa APEXII CCD diffractometer	2700 reflections with I > 2σ(I)
Radiation source: fine–focus sealed tube	R_int = 0.025
Graphite monochromator	θ_max = 31.6°, θ_min = 2.3°
ω scans	h = −10→10
13806 measured reflections	k = −12→12
3494 independent reflections	l = −13→13

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)] = 0.048	Hydrogen site location: inferred from neighbouring sites
wR(F²) = 0.142	H-atom parameters constrained
S = 1.03	w = 1/[σ²(F_o²) + (0.0779P)² + 0.0833P] where P = (F_o² + 2F_c²)/3
3494 reflections	(Δ/σ)_max < 0.001
163 parameters	Δρ_max = 0.28 e Å⁻³
0 restraints	Δρ_min = −0.30 e Å⁻³

Crystal data top

C₁₁H₅N₃O₄	γ = 78.874 (2)°
M_r = 243.18	V = 527.30 (3) Å³
Triclinic, P1	Z = 2
a = 7.0953 (2) Å	Mo Kα radiation
b = 8.8847 (3) Å	µ = 0.12 mm⁻¹
c = 9.2212 (3) Å	T = 295 K
α = 84.470 (2)°	0.30 × 0.28 × 0.25 mm
β = 67.634 (2)°

Data collection top

Bruker Kappa APEXII CCD diffractometer	2700 reflections with I > 2σ(I)
13806 measured reflections	R_int = 0.025
3494 independent reflections

Refinement top

R[F² > 2σ(F²)] = 0.048	0 restraints
wR(F²) = 0.142	H-atom parameters constrained
S = 1.03	Δρ_max = 0.28 e Å⁻³
3494 reflections	Δρ_min = −0.30 e Å⁻³
163 parameters

Special details top

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

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

	x	y	z	U_iso*/U_eq
C1	0.1956 (2)	0.45131 (18)	0.79855 (15)	0.0499 (3)
H1A	0.2553	0.3433	0.7969	0.060*
H1B	0.2182	0.4995	0.8792	0.060*
C2	0.13009 (16)	0.60560 (13)	0.61230 (12)	0.0335 (2)
C3	0.13982 (17)	0.70725 (13)	0.48999 (12)	0.0345 (2)
H3	0.2663	0.7286	0.4190	0.041*
C4	−0.04642 (16)	0.77900 (12)	0.47404 (12)	0.0309 (2)
C5	−0.23081 (16)	0.74097 (12)	0.58573 (13)	0.0331 (2)
C6	−0.24088 (17)	0.63890 (13)	0.71134 (13)	0.0373 (2)
H6	−0.3660	0.6171	0.7842	0.045*
C7	−0.05588 (18)	0.57270 (13)	0.72113 (12)	0.0351 (2)
C8	−0.04545 (17)	0.90159 (12)	0.35650 (13)	0.0347 (2)
H8	−0.1612	0.9786	0.3818	0.042*
C9	0.10476 (19)	0.91503 (13)	0.21576 (14)	0.0386 (2)
C10	0.0900 (2)	1.05218 (15)	0.12130 (16)	0.0478 (3)
C11	0.2814 (2)	0.79907 (17)	0.14691 (15)	0.0491 (3)
N1	−0.42888 (15)	0.80802 (11)	0.57275 (13)	0.0417 (2)
N2	0.0804 (3)	1.16093 (16)	0.04706 (18)	0.0707 (4)
N3	0.4220 (2)	0.7081 (2)	0.09043 (17)	0.0760 (5)
O1	−0.02108 (14)	0.46924 (11)	0.82958 (10)	0.0483 (2)
O2	0.28869 (13)	0.52328 (11)	0.64860 (10)	0.0457 (2)
O3	−0.43369 (15)	0.84163 (11)	0.44228 (12)	0.0507 (3)
O4	−0.58153 (15)	0.82535 (14)	0.69255 (14)	0.0674 (4)

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
C1	0.0424 (7)	0.0658 (8)	0.0332 (6)	0.0027 (6)	−0.0132 (5)	0.0108 (5)
C2	0.0307 (5)	0.0392 (5)	0.0288 (5)	−0.0007 (4)	−0.0115 (4)	−0.0001 (4)
C3	0.0298 (5)	0.0412 (5)	0.0304 (5)	−0.0058 (4)	−0.0101 (4)	0.0036 (4)
C4	0.0308 (5)	0.0307 (5)	0.0304 (5)	−0.0039 (4)	−0.0114 (4)	0.0004 (4)
C5	0.0279 (5)	0.0323 (5)	0.0367 (5)	−0.0011 (4)	−0.0111 (4)	−0.0009 (4)
C6	0.0306 (5)	0.0396 (6)	0.0343 (5)	−0.0040 (4)	−0.0056 (4)	0.0033 (4)
C7	0.0364 (5)	0.0372 (5)	0.0272 (5)	−0.0026 (4)	−0.0094 (4)	0.0024 (4)
C8	0.0349 (5)	0.0324 (5)	0.0388 (5)	−0.0057 (4)	−0.0167 (4)	0.0030 (4)
C9	0.0410 (6)	0.0401 (6)	0.0386 (6)	−0.0114 (5)	−0.0190 (5)	0.0086 (4)
C10	0.0602 (8)	0.0460 (7)	0.0449 (7)	−0.0211 (6)	−0.0253 (6)	0.0122 (5)
C11	0.0415 (7)	0.0632 (8)	0.0361 (6)	−0.0077 (6)	−0.0108 (5)	0.0109 (5)
N1	0.0309 (5)	0.0360 (5)	0.0550 (6)	−0.0031 (4)	−0.0153 (4)	0.0051 (4)
N2	0.1052 (13)	0.0544 (7)	0.0656 (9)	−0.0313 (8)	−0.0436 (9)	0.0243 (6)
N3	0.0539 (8)	0.0989 (12)	0.0516 (8)	0.0105 (8)	−0.0062 (6)	0.0049 (8)
O1	0.0414 (5)	0.0590 (6)	0.0362 (4)	−0.0034 (4)	−0.0116 (4)	0.0164 (4)
O2	0.0336 (4)	0.0603 (6)	0.0375 (4)	0.0003 (4)	−0.0141 (3)	0.0121 (4)
O3	0.0472 (5)	0.0499 (5)	0.0639 (6)	−0.0064 (4)	−0.0329 (5)	0.0056 (4)
O4	0.0323 (5)	0.0746 (7)	0.0705 (7)	0.0072 (5)	−0.0034 (5)	0.0157 (6)

Geometric parameters (Å, º) top

C1—O1	1.4319 (17)	C5—N1	1.4614 (14)
C1—O2	1.4334 (15)	C6—C7	1.3635 (15)
C1—H1A	0.9700	C6—H6	0.9300
C1—H1B	0.9700	C7—O1	1.3525 (13)
C2—O2	1.3547 (13)	C8—C9	1.3420 (16)
C2—C3	1.3641 (15)	C8—H8	0.9300
C2—C7	1.3850 (16)	C9—C11	1.4342 (19)
C3—C4	1.4068 (14)	C9—C10	1.4388 (16)
C3—H3	0.9300	C10—N2	1.1355 (18)
C4—C5	1.4016 (15)	C11—N3	1.138 (2)
C4—C8	1.4597 (14)	N1—O4	1.2145 (15)
C5—C6	1.3887 (15)	N1—O3	1.2230 (14)

O1—C1—O2	107.04 (9)	C7—C6—H6	122.1
O1—C1—H1A	110.3	C5—C6—H6	122.1
O2—C1—H1A	110.3	O1—C7—C6	128.09 (10)
O1—C1—H1B	110.3	O1—C7—C2	110.03 (10)
O2—C1—H1B	110.3	C6—C7—C2	121.87 (10)
H1A—C1—H1B	108.6	C9—C8—C4	126.91 (10)
O2—C2—C3	128.07 (10)	C9—C8—H8	116.5
O2—C2—C7	109.65 (9)	C4—C8—H8	116.5
C3—C2—C7	122.28 (10)	C8—C9—C11	124.80 (11)
C2—C3—C4	118.30 (10)	C8—C9—C10	119.43 (12)
C2—C3—H3	120.9	C11—C9—C10	115.74 (11)
C4—C3—H3	120.9	N2—C10—C9	179.14 (15)
C5—C4—C3	117.50 (9)	N3—C11—C9	179.09 (15)
C5—C4—C8	121.96 (9)	O4—N1—O3	123.24 (11)
C3—C4—C8	120.12 (9)	O4—N1—C5	118.09 (11)
C6—C5—C4	124.20 (10)	O3—N1—C5	118.67 (10)
C6—C5—N1	115.70 (10)	C7—O1—C1	105.81 (9)
C4—C5—N1	120.10 (10)	C2—O2—C1	105.89 (9)
C7—C6—C5	115.83 (10)

O2—C2—C3—C4	179.32 (11)	C3—C2—C7—C6	0.79 (18)
C7—C2—C3—C4	−0.78 (17)	C5—C4—C8—C9	−154.85 (11)
C2—C3—C4—C5	0.01 (16)	C3—C4—C8—C9	32.75 (16)
C2—C3—C4—C8	172.74 (10)	C4—C8—C9—C11	8.72 (19)
C3—C4—C5—C6	0.80 (17)	C4—C8—C9—C10	−173.31 (10)
C8—C4—C5—C6	−171.78 (10)	C6—C5—N1—O4	29.91 (16)
C3—C4—C5—N1	−178.10 (9)	C4—C5—N1—O4	−151.10 (12)
C8—C4—C5—N1	9.32 (16)	C6—C5—N1—O3	−148.95 (11)
C4—C5—C6—C7	−0.81 (17)	C4—C5—N1—O3	30.04 (15)
N1—C5—C6—C7	178.14 (10)	C6—C7—O1—C1	−173.03 (12)
C5—C6—C7—O1	−179.34 (11)	C2—C7—O1—C1	7.55 (14)
C5—C6—C7—C2	0.01 (17)	O2—C1—O1—C7	−12.15 (14)
O2—C2—C7—O1	0.17 (14)	C3—C2—O2—C1	172.11 (12)
C3—C2—C7—O1	−179.75 (10)	C7—C2—O2—C1	−7.80 (14)
O2—C2—C7—C6	−179.29 (10)	O1—C1—O2—C2	12.26 (14)

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
C1—H1B···O1ⁱ	0.97	2.53	3.2692 (16)	133
C8—H8···O4ⁱⁱ	0.93	2.52	3.3640 (17)	152

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

Experimental details

Crystal data
Chemical formula	C₁₁H₅N₃O₄
M_r	243.18
Crystal system, space group	Triclinic, P1
Temperature (K)	295
a, b, c (Å)	7.0953 (2), 8.8847 (3), 9.2212 (3)
α, β, γ (°)	84.470 (2), 67.634 (2), 78.874 (2)
V (Å³)	527.30 (3)
Z	2
Radiation type	Mo Kα
µ (mm⁻¹)	0.12
Crystal size (mm)	0.30 × 0.28 × 0.25

Data collection
Diffractometer	Bruker Kappa APEXII CCD
Absorption correction	–
No. of measured, independent and observed [I > 2σ(I)] reflections	13806, 3494, 2700
R_int	0.025
(sin θ/λ)_max (Å⁻¹)	0.737

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.048, 0.142, 1.03
No. of reflections	3494
No. of parameters	163
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.28, −0.30

Computer programs: APEX2 (Bruker, 2008), SAINT (Bruker, 2008), SHELXS97 (Sheldrick, 2008), ORTEP-3 (Farrugia, 1997) and Mercury (Macrae et al. 2008), SHELXL97 (Sheldrick, 2008) and PLATON (Spek, 2009).

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
C1—H1B···O1ⁱ	0.97	2.53	3.2692 (16)	133
C8—H8···O4ⁱⁱ	0.93	2.52	3.3640 (17)	152

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

Acknowledgements

SK and KS thank Dr Babu Varghese, Senior Scientific Officer, SAIF, IIT, Chennai, India, for the X-ray intensity data collection and Dr V. Murugan, Head of the Department of Physics, RKM Vivekananda College, for providing facilities in the department for carrying out this work.

References

Allen, F. H., Kennard, O., Watson, D. G., Brammer, L., Orpen, A. G. & Taylor, R. (1987). J. Chem. Soc. Perkin Trans. 2, pp. S1–19. CSD CrossRef Web of Science Google Scholar
Bernstein, J., Davis, R. E., Shimoni, L. & Chang, N.-L. (1995). Angew. Chem. Int. Ed. Engl. 34, 1555–1573. CrossRef CAS Web of Science Google Scholar
Brimblecombe, R. W., Green, D. M. & Muir, A. W. (1972). Br. J. Pharmacol. 44, 561–576. CrossRef CAS PubMed Web of Science Google Scholar
Bruker (2008). APEX2 and SAINT. Bruker AXS Inc., Madison, Wisconsin, USA. Google Scholar
Cremer, D. & Pople, J. A. (1975). J. Am. Chem. Soc. 97, 1354–1358. CrossRef CAS Web of Science Google Scholar
Farrugia, L. J. (1997). J. Appl. Cryst. 30, 565. CrossRef IUCr Journals Google Scholar
Loghmani-Khouzani, H., Abdul Rahman, N., Robinson, W. T., Yaeghoobi, M. & Kia, R. (2009). Acta Cryst. E65, o2545. Web of Science CSD CrossRef IUCr Journals Google Scholar
Macrae, C. F., Bruno, I. J., Chisholm, J. A., Edgington, P. R., McCabe, P., Pidcock, E., Rodriguez-Monge, L., Taylor, R., van de Streek, J. & Wood, P. A. (2008). J. Appl. Cryst. 41, 466–470. Web of Science CSD CrossRef CAS IUCr Journals 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