Crystal structure of melaminium cyano­acetate monohydrate

Sigdel Regmi, B.; Apblett, A.; Powell, D.

doi:10.1107/S2056989020012335

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Volume 76| Part 10| October 2020| Pages 1645-1648

https://doi.org/10.1107/S2056989020012335

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Crystal structure of melaminium cyanoacetate monohydrate

Bhawani Sigdel Regmi,^a ^* Allen Apblett ^b and Douglas Powell ^c

^aDepartment of Chemistry and Biochemistry, University of North Georgia, Oakwood, Georgia, USA, ^bOklahoma State University, Stillwater, Oklahoma, USA, and ^cUniversity of Oklahoma, Norman, Oklahoma, USA
^*Correspondence e-mail: bsigdelregmi@ung.edu

Edited by H. Ishida, Okayama University, Japan (Received 3 August 2020; accepted 8 September 2020; online 11 September 2020)

The asymmetric unit of the title compound, 2,4,6-triamino-1,3,5-triazin-1-ium cyanoacetate monohydrate, C₃H₇N₆⁺·NCCH₂COO⁻·H₂O, consists of a melaminium cation, a cyanoacetate anion and a water molecule, which are connected to each other via N—H⋯O and O—H⋯O hydrogen bonds, generating an eight-membered ring. In the crystal, the melaminium cations are connected by two pairs of N—H⋯N hydrogen bonds, forming tapes along [110]. These tapes develop a three-dimensional network through N—H⋯O, O—H⋯O, N—H⋯N and C—H⋯O hydrogen bonds between the cations, anions and water molecules.

Keywords: crystal structure; melaminium cation; cyanoacetate anion; hydrogen bonding.

CCDC reference: 2030784

1. Chemical context

Melamine (systematic name: 2,4,6-triamino-1,3,5-triazine), a trimer of cyanamide, has many industrial applications. The cross-linked resins of melamine with formaldehyde have applications in adhesive coatings, laminations and flame retardants (Billmeyer, 1984 ). In the past, various organic melamine salts were tested as potential melamine substitutes for melamine urea formaldehyde resins (Weinstabl et al., 2001 ). In general, protonation of melamine with organic and inorganic acids has been found to yield compounds with extensive hydrogen-bonding networks involving both N—H⋯O and O—H⋯O hydrogen bonds. This paper is a part of our investigation of the chemistry of cyanoacetate with nitrogen-based cations and their potential application as flame retardants since cyanoacetic acid is an analogue to polyacrylonitrile. It is well known that polyacrylonitrile is used in industry to manufacture carbon fibers because of its ability to produce carbon char (Bacon & Hoses, 1986 ). Cyanoacetic acid has a nitrile group and also can act as acid source, both of which could enhance the flame-retarding properties.

2. Structural commentary

The asymmetric unit of the title compound consists of a melaminium cation, a cyanoacetate anion and a water molecule, which are connected to each other via N—H⋯O and O—H⋯O hydrogen bonds, generating an eight-membered ring (Fig. 1). The six-membered ring of the melaminium cation shows significant distortion from a hexagonal shape. The bond distances [C—N = 1.322 (2)–1.368 (2) Å] and the angles [C—N—C = 115.76 (15)–119.08 (14)° and N—C—N = 121.44 (15)–125.42 (15)°] fall within similar ranges to those reported for similar singly protonated melaminium salts of simple alkyl mono- and dicarboxylic acids, namely, melaminium acetate acetic acid solvate (Perpétuo & Janczak, 2002 ), melaminium maleate (Janczak & Perpétuo, 2004 ), melaminium formate (Perpétuo et al., 2005 ), melaminium tartarate (Su et al., 2009 ), bis(melaminium) succinate (Froschauer & Weil, 2012a ) and melaminium hydrogen malonate (Froschauer & Weil, 2012b ). On the other hand, the angles in the six-membered ring of unprotonated melamine (Adam et al., 2010 ) are in the range 124.86 (17) to 125.51 (17)°.

Figure 1
Molecular structure of the title compound, showing 50% probability displacement ellipsoids and the atom-numbering scheme. Hydrogen atoms are shown as spheres of arbitrary radius and hydrogen bonds as dashed lines.

In the anion, both O atoms of the carboxylate group are involved in hydrogen bonds to amino groups of adjacent melaminium ions. The nitrile group has a bond length of 1.145 (2) Å that is typical of a nitrile (Kanters et al., 1978 ). The angle at the nitrile carbon, N≡C—C, is 179.30 (19)° which is close to the theoretical value of 180°. The O atom of the water molecule acts as a lone-pair donor to the protonated nitrogen of the melaminium ion that is present in the same eight-membered ring. The presence of the water molecule in the structure of melaminium cyanoacetate can be expected to contribute to fire retardancy as its release and evaporation will provide cooling.

3. Supramolecular features

The melaminium cation in the crystal is involved in altogether nine hydrogen bonds: for each melaminium cation, seven of them are of the hydrogen-bond donor type while the remaining two are of the acceptor type (Table 1). Neighbouring cations are connected by two pairs of N—H⋯N hydrogen bonds (N8—H8B⋯N4ⁱⁱⁱ and N9—H9B⋯N6^iv; symmetry codes as in Table 1) to form a tape-like structure propagating along [110] and running between the cyanoacetate anions. Three N—H⋯O hydrogen bonds (N7—H7A⋯O13ⁱ, N8—H8A⋯O11 and N9—H9A⋯O13ⁱⁱⁱ; Table 1) link the cation with three different cyanoacetate anions. Furthermore, the cation is also connected with a water molecule via an N—H⋯O hydrogen bond (N2—H2⋯O1S) between the protonated imine and the water O atom. Finally, the cation is linked with the nitrile group of the anion via an N—H⋯N hydrogen bond (N7—H7B⋯N16ⁱⁱ; Table 1). There also exist O—H⋯O (O1S—H1SA⋯O11 and O1S—H1SB⋯O11^vi) hydrogen bonds between the water molecule and the anion. In addition, a C—H⋯O hydrogen bond between the methylene H and water O atoms is observed as the C—H group is activated because of the electron-withdrawing cyano group adjacent to it. Altogether, these hydrogen bonds existing between the cations, anions and water molecules generate a three-dimensional network (Fig. 2).

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
N2—H2⋯O1S	0.90 (2)	1.81 (2)	2.7067 (19)	176.4 (19)
N7—H7A⋯O13ⁱ	0.89 (2)	2.00 (2)	2.881 (2)	168 (2)
N7—H7B⋯N16ⁱⁱ	0.92 (2)	2.13 (2)	3.001 (2)	155.6 (18)
N8—H8A⋯O11	0.91 (2)	2.01 (2)	2.891 (2)	164.6 (19)
N8—H8B⋯N4ⁱⁱⁱ	0.88 (2)	2.07 (2)	2.952 (2)	176 (2)
N9—H9A⋯O13ⁱⁱⁱ	0.90 (2)	2.08 (2)	2.792 (2)	135.7 (19)
N9—H9B⋯N6^iv	0.90 (2)	2.08 (2)	2.980 (2)	174 (2)
C14—H14B⋯O1S^v	0.99	2.46	3.233 (2)	134
O1S—H1SA⋯O11	0.93 (2)	1.78 (2)	2.6860 (19)	163.3 (19)
O1S—H1SB⋯O11^vi	0.87 (2)	1.97 (2)	2.8351 (19)	178 (2)
Symmetry codes: (i) x-1, y-1, z; (ii) $[-x, y-{\script{1\over 2}}, -z+{\script{1\over 2}}]$ ; (iii) -x+2, -y+1, -z+1; (iv) -x+1, -y, -z+1; (v) $[-x+1, y+{\script{1\over 2}}, -z+{\script{1\over 2}}]$ ; (vi) x-1, y, z.

Figure 2
A packing diagram of the title compound, viewed down the a axis, showing the O—H⋯O and N—H⋯O hydrogen bonds (green dashed lines), the N—H⋯N hydrogen bonds (blue dashed lines) and the C—H⋯O hydrogen bonds (magenta dashed lines).

4. Database survey

A search of the Cambridge Structural Database (Version 5.40, update of May 2020; Groom et al., 2016 ) for 2,4,6-triamino-1,3,5-triazin-1-ium showed more than 30 records; however, for 2,4,6-triamino-1,3,5-triazin-1-ium forming only single protonated salts with purely organic aliphatic carboxylic acids the search gave the following crystal structures: melamine with maleic acid (refcode ARUDAS; Janczak & Perpétuo, 2004), with formic acid (FONMEB; Perpétuo et al., 2005), with acetic acid (EFAZOA; Perpétuo & Janczak, 2002), with malonic acid (HOWRIV01; Froschauer & Weil, 2012b), with succinic acid (LEGZEE; Froschauer & Weil, 2012a), with nitrilotriacetic acid (MIHYAF; Hoxha et al., 2013 ) and with tartaric acid (VORSUR; Su et al., 2009). A search for organic co-crystals/salts of cyanoacetic acid gave one structure, 4,4′-bipyridine bis(cyanoacetic acid) (Song et al., 2008 ). For metal complexes with cyanoacetic acid or cyanoacetate, 24 structures were reported, such as silver cyanoacetate (Edwards et al., 1997 ) and cadmium cyanoacetate (Post & Trotter, 1974 ). In these metal salts, the metal is coordinated by the acetate group as well as the cyano group.

5. Synthesis and crystallization

A solution of cyanoacetic acid (1.7g, 20 mmol) in 100 ml of deionized water was added to a solution of melamine (2.5 g, 20 mmol) in 100 ml of deionized water. The reaction mixture was heated to 353 K for 3 h. The resulting clear solution was cooled to room temperature and then was allowed to slowly evaporate. Single crystals of the title compound formed after several days.

6. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2. C-bound H atoms were initially determined by geometry (C—H = 0.99 Å) and were refined using a riding model, with U_iso(H) = 1.2U_eq(C). H atoms bonded to N and O were located in a difference map, and their positions were refined freely, with U_iso(H) = 1.2U_eq(N or O).

Table 2
Experimental details

Crystal data
Chemical formula	C₃H₇N₆⁺·C₃H₂NO₂⁻·H₂O
M_r	229.22
Crystal system, space group	Monoclinic, P2₁/c
Temperature (K)	100
a, b, c (Å)	4.6928 (6), 9.3881 (13), 22.918 (3)
β (°)	91.646 (3)
V (Å³)	1009.3 (2)
Z	4
Radiation type	Mo Kα
μ (mm⁻¹)	0.12
Crystal size (mm)	0.44 × 0.17 × 0.04

Data collection
Diffractometer	Bruker APEX CCD
Absorption correction	Multi-scan (SADABS; Bruker, 2007 )
T_min, T_max	0.948, 0.995
No. of measured, independent and observed [I > 2σ(I)] reflections	12615, 2517, 1856
R_int	0.058
(sin θ/λ)_max (Å⁻¹)	0.668

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.047, 0.126, 1.00
No. of reflections	2517
No. of parameters	172
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
Δρ_max, Δρ_min (e Å⁻³)	0.28, −0.29
Computer programs: SMART and SAINT (Bruker, 2007), SHELXT (Sheldrick, 2015a ), SHELXL2018/3 (Sheldrick, 2015b ), SHELXTL (Sheldrick, 2008 ) and CrystalMaker (Palmer, 2014 ).

Supporting information

CCDC reference: 2030784

Crystal structure: contains datablocks I, General. DOI: https://doi.org/10.1107/S2056989020012335/is5552sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2056989020012335/is5552Isup2.hkl

Supporting information file. DOI: https://doi.org/10.1107/S2056989020012335/is5552Isup3.cml

checkCIF report

Computing details top

Data collection: SMART (Bruker, 2007); cell refinement: SAINT (Bruker, 2007); data reduction: SAINT (Bruker, 2007); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2015b); molecular graphics: SHELXTL (Sheldrick, 2008) and CrystalMaker (Palmer, 2014); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

2,4,6-Triamino-1,3,5-triazin-1-ium cyanoacetate monohydrate top

Crystal data top

C₃H₇N₆⁺·C₃H₂NO₂⁻·H₂O	F(000) = 480
M_r = 229.22	D_x = 1.509 Mg m⁻³
Monoclinic, P2₁/c	Mo Kα radiation, λ = 0.71073 Å
a = 4.6928 (6) Å	Cell parameters from 3720 reflections
b = 9.3881 (13) Å	θ = 2.3–28.1°
c = 22.918 (3) Å	µ = 0.12 mm⁻¹
β = 91.646 (3)°	T = 100 K
V = 1009.3 (2) Å³	Needle, colourless
Z = 4	0.44 × 0.17 × 0.04 mm

Data collection top

Bruker APEX CCD diffractometer	1856 reflections with I > 2σ(I)
φ and ω scans	R_int = 0.058
Absorption correction: multi-scan (SADABS; Bruker, 2007)	θ_max = 28.4°, θ_min = 1.8°
T_min = 0.948, T_max = 0.995	h = −5→6
12615 measured reflections	k = −12→12
2517 independent reflections	l = −30→30

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)] = 0.047	Hydrogen site location: mixed
wR(F²) = 0.126	H atoms treated by a mixture of independent and constrained refinement
S = 1.00	w = 1/[σ²(F_o²) + (0.064P)² + 0.320P] where P = (F_o² + 2F_c²)/3
2517 reflections	(Δ/σ)_max = 0.001
172 parameters	Δρ_max = 0.28 e Å⁻³
0 restraints	Δρ_min = −0.29 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
C1	0.3297 (4)	0.25701 (18)	0.41214 (7)	0.0117 (4)
N2	0.4321 (3)	0.39286 (15)	0.40750 (6)	0.0121 (3)
H2	0.359 (4)	0.453 (2)	0.3804 (9)	0.014*
C3	0.6562 (4)	0.43434 (17)	0.44283 (7)	0.0111 (4)
N4	0.7614 (3)	0.35064 (14)	0.48477 (6)	0.0126 (3)
C5	0.6396 (4)	0.21954 (17)	0.48861 (8)	0.0125 (4)
N6	0.4316 (3)	0.16816 (15)	0.45231 (6)	0.0135 (3)
N7	0.1226 (3)	0.21696 (16)	0.37493 (7)	0.0151 (3)
H7A	0.059 (4)	0.128 (2)	0.3786 (9)	0.018*
H7B	0.055 (4)	0.277 (2)	0.3458 (9)	0.018*
N8	0.7672 (3)	0.56251 (15)	0.43490 (7)	0.0137 (3)
H8A	0.711 (4)	0.617 (2)	0.4041 (9)	0.016*
H8B	0.914 (5)	0.586 (2)	0.4577 (9)	0.016*
N9	0.7332 (4)	0.13535 (16)	0.53094 (7)	0.0202 (4)
H9A	0.870 (5)	0.163 (2)	0.5569 (10)	0.024*
H9B	0.673 (5)	0.044 (3)	0.5337 (9)	0.024*
O11	0.6998 (3)	0.73778 (13)	0.33172 (5)	0.0157 (3)
C12	0.7511 (4)	0.87001 (18)	0.33632 (8)	0.0129 (4)
O13	0.9314 (3)	0.92554 (14)	0.36958 (6)	0.0224 (3)
C14	0.5789 (4)	0.97232 (18)	0.29669 (8)	0.0163 (4)
H14A	0.465226	1.035861	0.321467	0.020*
H14B	0.713248	1.032642	0.275142	0.020*
C15	0.3874 (4)	0.90159 (18)	0.25472 (8)	0.0156 (4)
N16	0.2366 (4)	0.84731 (17)	0.22144 (7)	0.0237 (4)
O1S	0.2181 (3)	0.58309 (13)	0.32906 (6)	0.0163 (3)
H1SA	0.363 (5)	0.650 (2)	0.3269 (9)	0.020*
H1SB	0.059 (5)	0.630 (2)	0.3289 (9)	0.020*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
C1	0.0117 (8)	0.0117 (8)	0.0118 (8)	−0.0003 (6)	0.0012 (6)	−0.0011 (6)
N2	0.0141 (8)	0.0088 (7)	0.0131 (7)	−0.0006 (6)	−0.0040 (6)	0.0023 (6)
C3	0.0116 (9)	0.0105 (8)	0.0110 (8)	0.0009 (6)	−0.0005 (6)	−0.0011 (6)
N4	0.0140 (8)	0.0095 (7)	0.0141 (7)	−0.0014 (6)	−0.0023 (6)	0.0011 (5)
C5	0.0123 (9)	0.0104 (7)	0.0148 (9)	−0.0017 (6)	−0.0024 (7)	0.0005 (6)
N6	0.0161 (8)	0.0099 (7)	0.0144 (8)	−0.0012 (6)	−0.0048 (6)	0.0016 (6)
N7	0.0184 (8)	0.0109 (7)	0.0155 (8)	−0.0025 (6)	−0.0063 (6)	0.0025 (6)
N8	0.0148 (8)	0.0108 (7)	0.0152 (8)	−0.0039 (6)	−0.0036 (6)	0.0036 (6)
N9	0.0257 (9)	0.0117 (7)	0.0222 (9)	−0.0067 (7)	−0.0141 (7)	0.0064 (6)
O11	0.0150 (7)	0.0108 (6)	0.0208 (7)	−0.0009 (5)	−0.0040 (5)	0.0034 (5)
C12	0.0135 (9)	0.0115 (8)	0.0136 (9)	−0.0024 (7)	−0.0013 (7)	0.0022 (6)
O13	0.0269 (8)	0.0163 (6)	0.0231 (7)	−0.0048 (6)	−0.0135 (6)	0.0034 (5)
C14	0.0197 (10)	0.0113 (8)	0.0174 (9)	−0.0010 (7)	−0.0069 (7)	−0.0001 (7)
C15	0.0164 (9)	0.0128 (8)	0.0174 (9)	0.0025 (7)	−0.0027 (7)	0.0037 (7)
N16	0.0273 (10)	0.0198 (8)	0.0235 (9)	−0.0010 (7)	−0.0104 (7)	0.0012 (7)
O1S	0.0129 (7)	0.0126 (6)	0.0232 (7)	−0.0003 (5)	−0.0031 (5)	0.0052 (5)

Geometric parameters (Å, º) top

C1—N6	1.322 (2)	N8—H8B	0.88 (2)
C1—N7	1.329 (2)	N9—H9A	0.90 (2)
C1—N2	1.368 (2)	N9—H9B	0.90 (2)
N2—C3	1.365 (2)	O11—C12	1.268 (2)
N2—H2	0.90 (2)	C12—O13	1.238 (2)
C3—N4	1.326 (2)	C12—C14	1.536 (2)
C3—N8	1.326 (2)	C14—C15	1.458 (2)
N4—C5	1.361 (2)	C14—H14A	0.9900
C5—N9	1.317 (2)	C14—H14B	0.9900
C5—N6	1.353 (2)	C15—N16	1.145 (2)
N7—H7A	0.89 (2)	O1S—H1SA	0.93 (2)
N7—H7B	0.92 (2)	O1S—H1SB	0.87 (2)
N8—H8A	0.91 (2)

N6—C1—N7	120.78 (16)	C3—N8—H8A	121.0 (13)
N6—C1—N2	121.44 (15)	C3—N8—H8B	117.0 (14)
N7—C1—N2	117.77 (15)	H8A—N8—H8B	121.5 (19)
C3—N2—C1	119.08 (14)	C5—N9—H9A	122.0 (14)
C3—N2—H2	120.1 (13)	C5—N9—H9B	121.5 (14)
C1—N2—H2	120.8 (13)	H9A—N9—H9B	116 (2)
N4—C3—N8	119.86 (16)	O13—C12—O11	126.04 (16)
N4—C3—N2	121.68 (15)	O13—C12—C14	116.09 (15)
N8—C3—N2	118.45 (15)	O11—C12—C14	117.87 (15)
C3—N4—C5	115.76 (15)	C15—C14—C12	114.17 (14)
N9—C5—N6	117.29 (15)	C15—C14—H14A	108.7
N9—C5—N4	117.30 (16)	C12—C14—H14A	108.7
N6—C5—N4	125.42 (15)	C15—C14—H14B	108.7
C1—N6—C5	116.32 (15)	C12—C14—H14B	108.7
C1—N7—H7A	116.4 (13)	H14A—C14—H14B	107.6
C1—N7—H7B	121.4 (13)	N16—C15—C14	179.30 (19)
H7A—N7—H7B	122.1 (19)	H1SA—O1S—H1SB	106.4 (19)

N6—C1—N2—C3	4.1 (2)	C3—N4—C5—N6	2.5 (3)
N7—C1—N2—C3	−176.16 (16)	N7—C1—N6—C5	−179.16 (16)
C1—N2—C3—N4	−5.8 (2)	N2—C1—N6—C5	0.6 (2)
C1—N2—C3—N8	174.98 (16)	N9—C5—N6—C1	176.40 (17)
N8—C3—N4—C5	−178.19 (16)	N4—C5—N6—C1	−4.1 (3)
N2—C3—N4—C5	2.6 (2)	O13—C12—C14—C15	174.83 (17)
C3—N4—C5—N9	−177.98 (17)	O11—C12—C14—C15	−4.3 (3)

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
N2—H2···O1S	0.90 (2)	1.81 (2)	2.7067 (19)	176.4 (19)
N7—H7A···O13ⁱ	0.89 (2)	2.00 (2)	2.881 (2)	168 (2)
N7—H7B···N16ⁱⁱ	0.92 (2)	2.13 (2)	3.001 (2)	155.6 (18)
N8—H8A···O11	0.91 (2)	2.01 (2)	2.891 (2)	164.6 (19)
N8—H8B···N4ⁱⁱⁱ	0.88 (2)	2.07 (2)	2.952 (2)	176 (2)
N9—H9A···O13ⁱⁱⁱ	0.90 (2)	2.08 (2)	2.792 (2)	135.7 (19)
N9—H9B···N6^iv	0.90 (2)	2.08 (2)	2.980 (2)	174 (2)
C14—H14B···O1S^v	0.99	2.46	3.233 (2)	134
O1S—H1SA···O11	0.93 (2)	1.78 (2)	2.6860 (19)	163.3 (19)
O1S—H1SB···O11^vi	0.87 (2)	1.97 (2)	2.8351 (19)	178 (2)

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

References

Adam, F., Lin, S. K., Quah, C. K., Hemamalini, M. & Fun, H.-K. (2010). Acta Cryst. E66, o3033–o3034. CSD CrossRef IUCr Journals Google Scholar
Bacon, R. & Hoses, T. N. (1986). High Performance Polymers, Their Origin and Development, edited by R. B. Saymour and G. S. Kirshambaum, p .342. New York/Amsterdam/London: Elsevier. Google Scholar
Billmeyer, F. W. (1984). Science, 3rd ed. New York: Wiley-Interscience. Google Scholar
Bruker (2007). SMART, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA. Google Scholar
Edwards, D. A., Mahon, M. F. & Paget, T. J. (1997). Polyhedron, 16, 25–31. CSD CrossRef CAS Web of Science Google Scholar
Froschauer, B. & Weil, M. (2012a). Acta Cryst. E68, o2553–o2554. CSD CrossRef IUCr Journals Google Scholar
Froschauer, B. & Weil, M. (2012b). Acta Cryst. E68, o2555. CSD CrossRef 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
Hoxha, K. & Prior, T. J. (2013). Acta Cryst. E69, o1674–o1675. CSD CrossRef IUCr Journals Google Scholar
Janczak, J. & Perpétuo, G. J. (2004). Acta Cryst. C60, o211–o214. Web of Science CSD CrossRef CAS IUCr Journals Google Scholar
Kanters, J. A., Roelofsen, G. & Straver, L. H. (1978). Acta Cryst. B34, 1393–1395. CSD CrossRef CAS IUCr Journals Web of Science Google Scholar
Palmer, D. C. (2014). CrystalMaker. CrystalMaker Software Ltd, Begbroke, England. Google Scholar
Perpétuo, G. J. & Janczak, J. (2002). Acta Cryst. C58, o112–o114. Web of Science CSD CrossRef IUCr Journals Google Scholar
Perpétuo, G. J., Ribeiro, M. A. & Janczak, J. (2005). Acta Cryst. E61, o1818–o1820. Web of Science CSD CrossRef IUCr Journals Google Scholar
Post, M. L. & Trotter, J. (1974). J. Chem. Soc. Dalton Trans. pp. 285–288. CSD CrossRef Web of Science Google Scholar
Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. Web of Science CrossRef CAS IUCr Journals 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
Song, G., Hao, E.-J. & Li, W. (2008). Acta Cryst. E64, o2058. Web of Science CSD CrossRef IUCr Journals Google Scholar
Su, H., Lv, Y.-K. & Feng, Y.-L. (2009). Acta Cryst. E65, o933. Web of Science CSD CrossRef IUCr Journals Google Scholar
Weinstabl, A., Binder, W. H., Gruber, H. & Kantner, W. (2001). J. Appl. Polym. Sci. 81, 1654–1661. Web of Science CrossRef CAS Google Scholar