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Crystal structure of the co-crystal salt 2-amino-6-bromo­pyridinium 2,3,5,6-tetra­fluoro­benzoate

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aDepartment of Chemistry, Missouri State University, Springfield, MO 65897, USA
*Correspondence e-mail: ericbosch@missouristate.edu

Edited by S. Parkin, University of Kentucky, USA (Received 9 January 2019; accepted 23 January 2019; online 31 January 2019)

The asymmetric unit of the co-crystal salt 2-amino-6-bromo­pyridinium 2,3,5,6-tetra­fluoro­benzoate, C5H6BrN2+·C7HF4O2, contains one pyridinium cation and one benzoate anion. In the crystal, the amino­pyridinium cationic unit forms two hydrogen bonds to the benzoate oxygen atoms in an R22(8) motif. Two pyridinium benzoate units are hydrogen bonded through self-complementary hydrogen bonds between the second amine hydrogen and a carboxyl­ate O with a second R22(8) motif to form a discrete hydrogen-bonded complex containing two 2-amino-6-bromo­pyridinium moieties and two 2,3,5,6-tetra­fluoro­benzoate moieties. The 2-amino-6-bromo­pyridinium moieties π-stack in a head-to-tail mode with a centroid–centroid separation of 3.7227 (12) Å and adjacent tetra­fluoro­benzoates also π-stack in a head-to-tail mode with a centroid–centroid separation of 3.6537 (13) Å.

1. Chemical context

The fields of crystal engineering and supra­molecular chemistry rely on the identification and application of versatile synthons to guide the construction of mol­ecular solids (Desiraju, 1995[Desiraju, G. R. (1995). Angew. Chem. Int. Ed. Engl. 34, 2311-2327.], 2013[Desiraju, G. R. (2013). J. Am. Chem. Soc. 135, 9952-9967.]). For example carb­oxy­lic acids are known to form a centrosymmetric dimer through self-complementary O—H⋯O hydrogen bonds (Fig. 1[link]a) in addition to hydrogen-bonded catemer chains and rings. It has been shown that these hydrogen bonds can be diverted by O—H⋯N hydrogen bonding to pyridines, often supported by a non-conventional pyridine C—H⋯O hydrogen bond (Fig. 1[link]b). The inter­action of the more basic pyridines, for example 4-(N,N-di­methyl­amino)­pyridine, with carb­oxy­lic acids most often yields charge-assisted hydrogen-bonded salts (Fig. 1[link]c). Similarly, the combination of 2-amino­pyridines and benzoic acids has been demonstrated to be a reliable supra­molecular synthon resulting in the formation of charge-assisted hydrogen-bonded complexes shown in Fig. 1[link]d (Bis & Zaworotko, 2005[Bis, J. A. & Zaworotko, M. J. (2005). Cryst. Growth Des. 5, 1169-1179.]). The formation of hydrogen-bonded co-crystals or salts of amines and acids has potential in the pharmaceutical field where the physicochemical properties of active pharmaceuticals, including aqueous solubility and physical and chemical stabil­ity, may be modulated and tailored by co-crystal or salt formation (Schultheiss & Newman, 2009[Schultheiss, N. & Newman, A. (2009). Cryst. Growth Des. 9, 2950-2967.]). For example a study involving the non-steroidal anti-inflammatory drug piroxicam reported the formation of 19 pyridine based co-crystals (Wales et al., 2012[Wales, C., Thomas, L. H. & Wilson, C. C. (2012). CrystEngComm, 14, 7264-7274.]). The present study presents the first co-crystal/salt formed between a substituted pyridine and 2,3,5,6-tetra­fluoro­benzoic acid.

[Scheme 1]
[Figure 1]
Figure 1
Potential hydrogen-bonding motifs for (a) carb­oxy­lic acid dimers, (b) neutral hydrogen-bonded pyridine carb­oxy­lic acid co-crystals, (c) charge-assisted pyridinium carboxyl­ate hydrogen-bonded complexes, and (d) charge-assisted 2-amino­pyridinium carboxyl­ate hydrogen-bonded complexes.

2. Structural commentary

The asymmetric unit of the co-crystal salt 2-amino-6-bromo­pyridinium 2,3,5,6-tetra­fluoro­benzoate (I)[link], contains one pyridinium cation and one benzoate anion that are held together by two charge-assisted hydrogen bonds (Table 1[link], first two entries) to form an R22(8) motif (Fig. 2[link]). The bond distance C12—O2 is slightly shorter than C12—O1, with distances of 1.236 (2) and 1.267 (2) Å respectively. The atoms that form this R22(8) motif (Fig. 1[link]) are almost coplanar, with the maximum deviation above and below the least-squares plane calculated through all of these atoms being 0.169 (7) and −0.147 (8) Å, respectively, for O2 and O1. The angle between the planes defined by the benzene and pyridine rings is 67.04 (7)° and the carboxyl­ate anion is twisted out of the plane of the benzene ring, with C12 0.103 (3) Å above the plane of the benzene ring and O1 1.043 (3) Å above, and O2 0.713 (4) Å below the plane defined by the benzene ring.

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A D—H H⋯A DA D—H⋯A
N1—H1⋯O1 0.91 (2) 1.68 (2) 2.585 (2) 177 (2)
N2—H2A⋯O2 0.87 (2) 1.98 (2) 2.845 (2) 175 (2)
N2—H2B⋯O2i 0.87 (2) 2.02 (2) 2.854 (2) 162 (2)
C9—H9⋯Br1ii 0.95 2.94 3.867 (2) 166
Symmetry codes: (i) -x+1, -y+1, -z+1; (ii) -x, -y+1, -z+1.
[Figure 2]
Figure 2
The mol­ecular structure of the co-crystal salt (I)[link] showing the atom-labeling scheme. Displacement ellipsoids drawn at the 50% probability level and hydrogen bonds (Table 1[link]) are shown as dotted lines.

3. Supra­molecular features

In the co-crystal salt (I)[link], adjacent amino pyridinium benzoate salt units are linked into dimeric salt complexes with self-complementary hydrogen bonds (Table 1[link], entry 3) from the second amine hydrogen atom and carboxyl­ate oxygen atom O2 in a second R42(8) motif (Fig. 3[link]). The two components are relatively well separated within the crystal structure into zones parallel to the c axis.

[Figure 3]
Figure 3
Part of the crystal structure of (I)[link] viewed along b, highlighting the hydrogen-bonded dimeric salt unit.

There are two inter­actions that involve the tetra­fluoro­benzoate (Fig. 4[link]). Adjacent tetra­fluoro­benzoates π-stack in a head-to-tail mode with a Cg1⋯Cg1i distance of 3.6537 (13) Å [symmetry code: (i) −x, 1 − y, z; Cg1 is the centroid of the benzene ring C6–C11] and there is a close C—F⋯π inter­action with a Cg1⋯F3ii distance of 3.1640 (17) Å [symmetry code: (ii) −x, y − [{1\over 2}], [{1\over 2}] − z].

[Figure 4]
Figure 4
Partial view of the packing in the crystal structure of (I)[link] highlighting the head-to-tail π-stacking of the tetra­fluoro­benzoate mol­ecules and the C—F⋯π inter­action.

The 2-amino­pyridinium groups form offset alternating head-to-tail π-stacks parallel to the b axis (Fig. 5[link]) with a Cg2⋯Cg2iii distance of 3.7227 (12) Å and a shortest perpendicular inter­planar distance of 3.2547 (8) Å [symmetry code: (iii) 1 − x, y − [{1\over 2}], [{3\over 2}] − z; Cg2 is the centroid of the pyridine ring].

[Figure 5]
Figure 5
Partial view of the packing in the crystal structure of (I)[link] highlighting the head-to-tail π-stacking of the 2-amino-6-bromo­pyridinium cations.

There is one short contact to the bromine with a C9⋯Br1iv distance of 3.867 (2) Å [symmetry code: (iv) 1 − x, 1 − y, 2 − z].

4. Database survey

A search of the Cambridge Crystallographic Database (Version 5.39, update of August 2018; Groom et al., 2016[Groom, C. R., Bruno, I. J., Lightfoot, M. P. & Ward, S. C. (2016). Acta Cryst. B72, 171-179.]) using Conquest (Bruno et al., 2002[Bruno, I. J., Cole, J. C., Edgington, P. R., Kessler, M., Macrae, C. F., McCabe, P., Pearson, J. & Taylor, R. (2002). Acta Cryst. B58, 389-397.]) for structures including the neutral carb­oxy­lic acid dimer synthon as shown in Fig. 1[link]a revealed 6,016 hits, while a search for neutral pyridine carb­oxy­lic acid inter­actions where the distance between the acid proton and the pyridine N is equal to or less than the sum of the van der Waals radii revealed 2189 hits. In 966 of the 2189 structures the distance between the carbonyl O and the pyridine H is also equal to or less than the sum of the van der Waals radii, corresponding to the synthon shown in Fig. 1[link]b. A related search of the Cambridge Crystallographic Database for co-crystals with 4-(N,N-di­methyl­amino)­pyridine and carb­oxy­lic acids revealed only four neutral co-crystals and 54 structures corresponding to the pyridinium carboxyl­ate as shown in Fig. 1[link]c. A similar search for co-crystals formed between 2-amino­pyridines with benzoic acids yielded 41 hits, of which 40 feature charge-assisted amino­pyridinium carboxyl­ate hydrogen-bonded co-crystals as the result of proton transfer shown in Fig. 1[link]d. The structure that is reported to form a neutral hydrogen-bonded complex corresponds to the co-crystal formed between 2-amino­pyridine and 4-amino­benzoic acid [refcode WOPCOV; Chandrasekaran & Babu, 2014[Chandrasekaran, J. & Babu, B. (2014). Private communication (Refcode WOPCOV). CCDC, Cambridge, England.]]. Finally there is only one reported co-crystal of 2,3,5,6-tetra­fluoro­benzoic acid, or the corresponding 2,3,5,6-tetra­fluoro­benzoate, with an organic base. In that example theophylline forms a neutral hydrogen-bonded complex (Corpinot et al., 2016[Corpinot, M. K., Stratford, S. A., Arhangelskis, M., Anka-Lufford, J., Halasz, I., Judaš, N., Jones, W. & Bučar, D.-K. (2016). CrystEngComm, 18, 5434-5439.]).

5. Synthesis and crystallization

2-Amino-6-bromo­pyridine and 2,3,5,6-tetra­fluoro­benzoic acid were used as supplied. An equimolar amount (0.1 mmol) of each component were added to a screw-capped vial and 3 mL of ethanol added to effect a clear colorless solution that was allowed to slowly concentrate over two weeks. A homogeneous mass of crystals was obtained.

6. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2[link]. All hydrogen atoms were located in Fourier-difference maps. Hydrogen atoms involved in hydrogen-bonding inter­actions were restrained in the refinement with N—H = 0.87 (2) Å and with Uiso(H) = 1.2Ueq(N). The aromatic H atoms were included in the refinement at calculated positions with C—H = 0.95 Å and Uiso(H) = 1.2Ueq(C).

Table 2
Experimental details

Crystal data
Chemical formula C5H6BrN2+·C7HF4O2
Mr 367.11
Crystal system, space group Monoclinic, P21/c
Temperature (K) 100
a, b, c (Å) 13.7230 (9), 6.5757 (4), 15.3224 (10)
β (°) 111.841 (1)
V3) 1283.42 (14)
Z 4
Radiation type Mo Kα
μ (mm−1) 3.26
Crystal size (mm) 0.25 × 0.20 × 0.03
 
Data collection
Diffractometer Bruker APEXII CCD
Absorption correction Multi-scan (SADABS; Bruker, 2014[Bruker (2014). SMART, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA.])
Tmin, Tmax 0.788, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections 16099, 2847, 2395
Rint 0.045
(sin θ/λ)max−1) 0.641
 
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.026, 0.059, 1.04
No. of reflections 2847
No. of parameters 199
No. of restraints 3
H-atom treatment H atoms treated by a mixture of independent and constrained refinement
Δρmax, Δρmin (e Å−3) 0.41, −0.32
Computer programs: SMART and SAINT (Bruker, 2014[Bruker (2014). SMART, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA.]), SHELXT2018/2 (Sheldrick, 2015a[Sheldrick, G. M. (2015a). Acta Cryst. A71, 3-8.]), SHELXL2018/3 (Sheldrick, 2015b[Sheldrick, G. M. (2015b). Acta Cryst. C71, 3-8.]) and X-SEED (Barbour, 2001[Barbour, L. J. (2001). J. Supramol. Chem. 1, 189-191.]).

Supporting information


Computing details top

Data collection: SMART (Bruker, 2014); cell refinement: SMART (Bruker, 2014); data reduction: SAINT (Bruker, 2014); program(s) used to solve structure: SHELXT2018/2 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2015b); molecular graphics: X-SEED (Barbour, 2001); software used to prepare material for publication: X-SEED (Barbour, 2001).

2-Amino-6-bromopyridinium 2,3,5,6-tetrafluorobenzoate top
Crystal data top
C5H6BrN2+·C7HF4O2F(000) = 720
Mr = 367.11Dx = 1.900 Mg m3
Monoclinic, P21/cMo Kα radiation, λ = 0.71073 Å
a = 13.7230 (9) ÅCell parameters from 3850 reflections
b = 6.5757 (4) Åθ = 2.7–25.7°
c = 15.3224 (10) ŵ = 3.26 mm1
β = 111.841 (1)°T = 100 K
V = 1283.42 (14) Å3Cut block, colourless
Z = 40.25 × 0.20 × 0.03 mm
Data collection top
Bruker APEXII CCD
diffractometer
2847 independent reflections
Radiation source: fine-focus sealed tube2395 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.045
Detector resolution: 8.3660 pixels mm-1θmax = 27.1°, θmin = 1.6°
phi and ω scansh = 1717
Absorption correction: multi-scan
(SADABS; Bruker, 2014)
k = 88
Tmin = 0.788, Tmax = 1.000l = 1919
16099 measured reflections
Refinement top
Refinement on F23 restraints
Least-squares matrix: fullHydrogen site location: mixed
R[F2 > 2σ(F2)] = 0.026H atoms treated by a mixture of independent and constrained refinement
wR(F2) = 0.059 w = 1/[σ2(Fo2) + (0.0271P)2 + 0.3891P]
where P = (Fo2 + 2Fc2)/3
S = 1.04(Δ/σ)max = 0.002
2847 reflectionsΔρmax = 0.41 e Å3
199 parametersΔρmin = 0.32 e Å3
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
Br10.35106 (2)0.41134 (3)0.84202 (2)0.01733 (7)
F10.10223 (10)0.15160 (19)0.49369 (9)0.0248 (3)
F40.17441 (10)0.8060 (2)0.40322 (9)0.0275 (3)
F20.10146 (10)0.2028 (2)0.38905 (10)0.0321 (3)
F30.03037 (12)0.8562 (2)0.30158 (10)0.0365 (4)
O10.27555 (11)0.4079 (2)0.59997 (10)0.0188 (3)
O20.32636 (12)0.4894 (3)0.48172 (11)0.0238 (4)
N10.46382 (13)0.4100 (3)0.72562 (12)0.0130 (4)
H10.3977 (13)0.404 (3)0.6815 (14)0.016*
N20.53368 (14)0.4068 (3)0.61009 (13)0.0171 (4)
H2A0.4701 (14)0.424 (3)0.5700 (14)0.020*
H2B0.5859 (15)0.422 (3)0.5919 (16)0.020*
C40.57133 (17)0.4158 (3)0.88912 (15)0.0170 (4)
H40.5775440.4163890.9529780.020*
C20.65046 (17)0.4148 (3)0.77289 (15)0.0172 (4)
H20.7111660.4147150.7570940.021*
C120.25906 (16)0.4564 (3)0.51564 (15)0.0155 (5)
C50.47591 (16)0.4124 (3)0.81748 (14)0.0147 (4)
C10.54936 (16)0.4114 (3)0.70135 (15)0.0136 (4)
C70.07144 (17)0.3268 (3)0.44649 (15)0.0173 (5)
C30.66028 (17)0.4183 (3)0.86458 (15)0.0182 (5)
H30.7282590.4224480.9126770.022*
C110.10738 (17)0.6527 (3)0.40039 (15)0.0183 (5)
C100.00175 (18)0.6802 (4)0.34810 (15)0.0228 (5)
C80.03368 (17)0.3535 (4)0.39179 (16)0.0206 (5)
C60.14445 (16)0.4777 (3)0.45227 (14)0.0157 (5)
C90.06994 (18)0.5308 (4)0.34355 (15)0.0241 (5)
H90.1426010.5497730.3079700.029*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Br10.01626 (12)0.02221 (12)0.01623 (12)0.00014 (9)0.00917 (8)0.00008 (9)
F10.0203 (7)0.0188 (7)0.0320 (8)0.0013 (5)0.0061 (6)0.0036 (6)
F40.0277 (8)0.0257 (8)0.0320 (8)0.0027 (6)0.0144 (6)0.0081 (6)
F20.0175 (7)0.0384 (9)0.0383 (9)0.0100 (6)0.0081 (6)0.0041 (7)
F30.0341 (8)0.0443 (9)0.0320 (9)0.0155 (7)0.0133 (7)0.0215 (7)
O10.0134 (7)0.0282 (9)0.0142 (8)0.0001 (7)0.0045 (6)0.0028 (7)
O20.0150 (8)0.0418 (10)0.0171 (8)0.0004 (7)0.0089 (7)0.0003 (7)
N10.0101 (8)0.0137 (9)0.0154 (9)0.0021 (7)0.0051 (7)0.0015 (7)
N20.0115 (9)0.0234 (10)0.0184 (10)0.0008 (8)0.0079 (8)0.0027 (8)
C40.0196 (11)0.0169 (11)0.0134 (10)0.0032 (9)0.0049 (9)0.0012 (9)
C20.0135 (10)0.0150 (11)0.0231 (12)0.0021 (9)0.0070 (9)0.0004 (9)
C120.0145 (11)0.0167 (11)0.0159 (11)0.0007 (8)0.0062 (9)0.0057 (8)
C50.0153 (10)0.0129 (10)0.0173 (11)0.0005 (9)0.0077 (9)0.0003 (9)
C10.0137 (10)0.0093 (10)0.0195 (11)0.0009 (8)0.0083 (8)0.0018 (8)
C70.0174 (11)0.0193 (11)0.0154 (11)0.0014 (9)0.0062 (9)0.0003 (9)
C30.0143 (11)0.0169 (11)0.0191 (11)0.0022 (9)0.0011 (9)0.0012 (9)
C110.0192 (11)0.0240 (12)0.0152 (11)0.0010 (9)0.0103 (9)0.0006 (9)
C100.0226 (12)0.0313 (14)0.0159 (11)0.0111 (11)0.0088 (10)0.0098 (10)
C80.0132 (11)0.0312 (13)0.0191 (12)0.0056 (10)0.0080 (9)0.0057 (10)
C60.0150 (11)0.0225 (12)0.0118 (10)0.0018 (9)0.0075 (9)0.0036 (9)
C90.0129 (11)0.0439 (16)0.0144 (12)0.0052 (10)0.0037 (9)0.0002 (10)
Geometric parameters (Å, º) top
Br1—C51.886 (2)C4—C31.405 (3)
F1—C71.342 (2)C4—H40.9500
F4—C111.355 (3)C2—C31.361 (3)
F2—C81.349 (3)C2—C11.413 (3)
F3—C101.345 (3)C2—H20.9500
O1—C121.267 (2)C12—C61.517 (3)
O2—C121.236 (2)C7—C81.384 (3)
N1—C51.355 (3)C7—C61.389 (3)
N1—C11.357 (3)C3—H30.9500
N1—H10.909 (16)C11—C101.383 (3)
N2—C11.334 (3)C11—C61.383 (3)
N2—H2A0.868 (16)C10—C91.374 (3)
N2—H2B0.866 (16)C8—C91.371 (3)
C4—C51.360 (3)C9—H90.9500
C5—N1—C1120.04 (18)F1—C7—C8118.9 (2)
C5—N1—H1118.4 (15)F1—C7—C6120.24 (19)
C1—N1—H1121.5 (15)C8—C7—C6120.8 (2)
C1—N2—H2A117.8 (16)C2—C3—C4120.9 (2)
C1—N2—H2B120.3 (16)C2—C3—H3119.5
H2A—N2—H2B119 (2)C4—C3—H3119.5
C5—C4—C3117.10 (19)F4—C11—C10118.3 (2)
C5—C4—H4121.4F4—C11—C6120.04 (19)
C3—C4—H4121.4C10—C11—C6121.6 (2)
C3—C2—C1119.5 (2)F3—C10—C9120.1 (2)
C3—C2—H2120.2F3—C10—C11119.1 (2)
C1—C2—H2120.2C9—C10—C11120.8 (2)
O2—C12—O1126.5 (2)F2—C8—C9120.0 (2)
O2—C12—C6118.28 (19)F2—C8—C7118.5 (2)
O1—C12—C6115.18 (18)C9—C8—C7121.5 (2)
N1—C5—C4123.21 (19)C11—C6—C7117.1 (2)
N1—C5—Br1115.97 (15)C11—C6—C12121.12 (19)
C4—C5—Br1120.82 (16)C7—C6—C12121.8 (2)
N2—C1—N1117.94 (19)C8—C9—C10118.1 (2)
N2—C1—C2122.87 (19)C8—C9—H9120.9
N1—C1—C2119.18 (19)C10—C9—H9120.9
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
N1—H1···O10.91 (2)1.68 (2)2.585 (2)177 (2)
N2—H2A···O20.87 (2)1.98 (2)2.845 (2)175 (2)
N2—H2B···O2i0.87 (2)2.02 (2)2.854 (2)162 (2)
C4—H4···Br1ii0.953.134.017 (2)155
C9—H9···Br1iii0.952.943.867 (2)166
Symmetry codes: (i) x+1, y+1, z+1; (ii) x+1, y+1, z+2; (iii) x, y+1, z+1.
 

Acknowledgements

We thank the Missouri State University Provost Incentive Fund that funded the purchase of the X-ray diffractometer.

References

First citationBarbour, L. J. (2001). J. Supramol. Chem. 1, 189–191.  CrossRef CAS Google Scholar
First citationBis, J. A. & Zaworotko, M. J. (2005). Cryst. Growth Des. 5, 1169–1179.  Web of Science CrossRef CAS Google Scholar
First citationBruker (2014). SMART, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA.  Google Scholar
First citationBruno, I. J., Cole, J. C., Edgington, P. R., Kessler, M., Macrae, C. F., McCabe, P., Pearson, J. & Taylor, R. (2002). Acta Cryst. B58, 389–397.  Web of Science CrossRef CAS IUCr Journals Google Scholar
First citationChandrasekaran, J. & Babu, B. (2014). Private communication (Refcode WOPCOV). CCDC, Cambridge, England.  Google Scholar
First citationCorpinot, M. K., Stratford, S. A., Arhangelskis, M., Anka-Lufford, J., Halasz, I., Judaš, N., Jones, W. & Bučar, D.-K. (2016). CrystEngComm, 18, 5434–5439.  CrossRef CAS Google Scholar
First citationDesiraju, G. R. (1995). Angew. Chem. Int. Ed. Engl. 34, 2311–2327.  CrossRef CAS Web of Science Google Scholar
First citationDesiraju, G. R. (2013). J. Am. Chem. Soc. 135, 9952–9967.  Web of Science CrossRef CAS PubMed Google Scholar
First citationGroom, 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
First citationSchultheiss, N. & Newman, A. (2009). Cryst. Growth Des. 9, 2950–2967.  Web of Science CrossRef PubMed CAS Google Scholar
First citationSheldrick, G. M. (2015a). Acta Cryst. A71, 3–8.  Web of Science CrossRef IUCr Journals Google Scholar
First citationSheldrick, G. M. (2015b). Acta Cryst. C71, 3–8.  Web of Science CrossRef IUCr Journals Google Scholar
First citationWales, C., Thomas, L. H. & Wilson, C. C. (2012). CrystEngComm, 14, 7264–7274.  Web of Science CrossRef CAS Google Scholar

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