Crystal structure of melaminium cyano­acetate monohydrate

The crystal of the melaminium salt C₃H₇N₆ ⁺·NCCH₂COO⁻·H₂O was produced by mixing melamine with cyano­acetic acid in aqueous solution. The melaminium cations are inter­connected by N—H⋯N hydrogen bonds, forming tapes. These tapes of melaminium cations develop a three-dimensional network through multiple donor–acceptor hydrogen-bonding inter­actions between the cyano­acetate anions and water mol­ecules.

Abstract

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.

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.

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.

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⋯N6iv	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⋯O11vi	0.87 (2)	1.97 (2)	2.8351 (19)	178 (2)

Symmetry codes: (i) ; (ii) ; (iii) ; (iv) ; (v) ; (vi) .

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).

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.

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.

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[F 2 > 2σ(F 2)], wR(F 2), 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
Δρmax, Δρmin (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 ▸).

Supplementary Material

CCDC reference: 2030784

Additional supporting information: crystallographic information; 3D view; checkCIF report

supplementary crystallographic information

Crystal data

C3H7N6+·C3H2NO2−·H2O	F(000) = 480
Mr = 229.22	Dx = 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

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

Refinement

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(Fo2) + (0.064P)2 + 0.320P] where P = (Fo2 + 2Fc2)/3
2517 reflections	(Δ/σ)max = 0.001
172 parameters	Δρmax = 0.28 e Å−3
0 restraints	Δρmin = −0.29 e Å−3

Special details

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 (Ų)

	x	y	z	Uiso*/Ueq
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 (Ų)

	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)

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)

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···N6iv	0.90 (2)	2.08 (2)	2.980 (2)	174 (2)
C14—H14B···O1Sv	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···O11vi	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.