Melaminium maleate monohydrate

In the title compound, 2,4,6-tri­amino-1,3,5-triazin-1-ium maleate monohydrate, C₃H₇N₆⁺·C₄H₃O₄⁻·H₂O, containing singly protonated melaminium residues, maleate(1−) anions and water mol­ecules, the components are linked by hydrogen bonds into a three-dimensional framework structure. The melaminium residues are connected by two pairs of N—H...N hydrogen bonds into chains in the form of stacks, with a distance of 3.26 (1) Å between the triazine rings, clearly indicating π–π interactions. The maleate anion contains an intramolecular O—H...O hydrogen bond and the anions interact with the water mol­ecules via O—H...O hydrogen bonds, forming zigzag chains, also in the form of stacks, in which the almost-planar maleate anions are separated by 3.26 (1) Å. The experimental geometries of the ions are compared with molecular-orbital calculations of their gas-phase geometries.

S1. Comment

The present study is a continuation of our investigations characterizing the hydrogen-bonding system formed by triazine derivatives in the solid state (Janczak & Perpétuo, 2003). Melamine and its organic and inorganic complexes or salts can develop well defined supramolecular structures via multiple hydrogen bonds by self-assembly of components that contain complementary arrays of hydrogen-bonding sites (Desiraju, 1990; MacDonald & Whitesides, 1994; Row, 1999; Krische & Lehn, 2000; Sherrington & Taskinen, 2001). In order to expand the understanding of the solid-state physical-organic chemistry of compounds containing multiple N—H···N, N—H···O and O—H···O hydrogen-bonding systems, we present here the solid-state structure of melaminium maleate monohydrate, (I). Additionally, the geometry of both ions are compared with the ab initio fully optimized parameters calculated at the HF/6–31 G(d,p) level (GAUSSIAN94; Frisch et al., 1995). The molecular-orbital calculations were carried out on the isolated ions, corresponding to the gas phase, and the results are illustrated in Fig. 1. \sch

The asymmetric unit of (I) consists of a melaminium cation, singly protonated at a ring N atom, a maleate anion and a water molecule (Fig. 2). The aromatic six-membered triazine ring of the singly protonated melaminium residue is almost planar [the deviation of the N and C atoms from the mean plane is less than 0.08 (2) Å], but it exhibits significant distortions from the ideal hexagonal form (Table 1). The internal C—N—C angle at the protonated N atom in the melaminium cation is greater than the other two C—N—C angles within the ring. This is a result of the steric effect of a lone-pair electron, predicted by the valence-shell electron-pair repulsion theory (VSEPR; Gillespie, 1963, 1992). As a result of the protonation of the melamine ring at the one of the three ring N atoms, the internal N—C—N angle involving only non-protonated N atoms is significantly smaller than the N—C—N angles involving both protonated and non-protonated N atoms.

The ab initio optimized geometry calculated for the singly protonated melaminium residue corresponding to the gas phase shows a similar correlation between the C—N—C and N—C—N angles within the ring as that seen in the crystal of (I). Thus, the ring distortion of the singly protonated melaminium residue results mainly from the protonation and, to a lesser degree, from the hydrogen-bonding system and the crystal packing. The C—N bond lengths in the optimized melaminium residue are slightly shorter than those in the crystal. The lengthening of the C—N bonds of the melaminium rings in the crystal is likely to be due to the interaction of the π–π clouds between the rings in the stacks, which are separated by 3.26 (1) Å, as well as to the hydrogen-bonding system. A similar correlation between the internal C—N—C and N—C—N angles within the melaminium ring is reported for the crystals of barbituric acid with melamine (Zerkowski et al., 1994), melaminium phthalate (Janczak & Perpétuo, 2001a), melaminium chloride hemihydrate (Janczak & Perpétuo, 2001b), bis(melaminium) sulfate dihydrate (Janczak & Perpétuo, 2001c), melaminium acetate (Perpétuo & Janczak, 2002), melaminium glutarate monohydrate (Janczak & Perpétuo, 2002a), melaminium phosphate (Janczak & Perpétuo, 2002b), melaminium citrate (Perpétuo & Janczak, 2003) and melaminium selenate (Marchewka et al., 2003), i.e. those singly protonated melaminium salts that have been previously structurally characterized.

An extensive series of hydrogen bonds (Table 2) links the independent components of (I) into a continuous framework structure. Each melaminium residue is involved in nine hydrogen bonds. In seven of these it acts as a donor, and in the remaining two it acts as an acceptor. Two pairs of almost-linear N—H···N hydrogen bonds link pairs of melaminium cations, related to one another by inversion centres, to form chains in the form of stacks parallel to [100] (Fig. 3). This chain formation is different from that found in melaminium citrate, where the cations form centrosymmetric hydrogen-bonded dimers (Perpétuo & Janczak, 2003). Within one stack, the melaminium residues are separated by 3.26 (1) Å. This distance is shorter than that between the π-aromatic ring system (3.4 Å; Pauling, 1960) and indicates strong π–π interactions between the triazine rings of the melaminium moieties within the stack. Five more N—H···O hydrogen bonds (Table 2) link each cation to two maleate anions and and two water molecules.

The anion of (I) deviates slightly from the planar conformation predicted by gas-phase molecular-orbital calculations. This deviation from planarity of the maleate anion in (I) is due to the intermolecular hydrogen-bonding system, which makes the C4—C5—C6—C7 torsion angle 1.7 (1)° and leads to rotations of the COO⁻ and COOH groups along the C6—C7 [1.0 (1)°] and C4—C5 [1.1 (1)°] bonds, respectively. This almost-planar conformation of the maleate ion is stabilized by the intramolecular hydrogen bond between atoms O1 and O3, which is confirmed as asymmetric, in agreement with the results of alkene cis-dicarboxylic acids (Hechtfischer et al., 1970), maleic acid (James & Williams, 1974) and several singly dissociated maleate salts (Allen, 2002). The O···O distance of 2.470 (2) Å is shorter than that found in maleic acid (2.502 Å; James & Williams, 1974) and compares well with other singly dissociated maleate salts (Allen, 2002). The intramolecular O1—H···O3 angle is almost linear in the gas phase, while in the crystal it is more bent, possibly due to the intermolecular hydrogen bond.

As revealed by the structure analysis, the ethylenic C5—C6 bond distance in (I) is normal for a simple Csp²═Csp² double bond (Allen et al., 1987), in agreement with our molecular-orbital calculations result. Comparison of the two Csp²—Csp² single bonds of the non-dissociated (C4—C5) and dissociated (C6—C7) carboxyl groups shows that these two bonds are not identical. It is of interest to note that the shorter C4—C5 bond links the non-dissociated carboxyl group, while a longer bond (C6—C7) links the dissociated carboxyl group. The C—O bond lengths in the carboxylate group are intermediate between the values for single Csp²—O (1.308–1.320 Å) and double Csp²═O bonds (1.214–1.224 Å; Allen et al., 1987), indicating delocalization of the charge on both O atoms of the COO⁻ group. The longer C—O bond of the carboxylate group is involved in the intramolecular hydrogen bond and this is entirely consistent with the MO calculations.

The COOH group in (I) shows the typical pattern of double and single C—O bonds. The two Csp²—Csp²—Csp² angles of the anion (C4—C5—C6 and C5—C6—C7) are almost equal but they are considerably larger than 120°, indicating in part the high degree of strain in the maleate anion. The O—C—O angle in the COO⁻ group is only slightly larger than that in non-dissociated COOH group. This relation is quite different from that observed in the gas phase, as revealed by MO calculations (Fig. 1), in which the O—C—O angle in the COO⁻ group is significantly larger than that in the COOH group. The differences in the O—C—O angles between the crystal and gas-phase geometry of the maleate ion may be due to the hydrogen bonding present in the crystal, which leads to a decrease in the steric effect of lone pairs of electrons on both O atoms of the COO⁻ and COOH groups in relation to the isolated anion in the gas phase, as shown by the MO calculations.

The maleate (1-) ions in (I), related by a 2₁ screw axis, are interconnected by water molecules via O—H···O hydrogen bonds in a head-to-tail fashion to form zigzag chains. These chains form stacks, with a distance of 3.36 (1) Å between the almost-planar maleate anions. Although the maleate anion is not aromatic, it does contain π electrons and polarized multiple bonds, and so the interchain separation can be compared with the stacking system in aromatic molecules (3.4 Å; Pauling, 1960). The interchain forces are relatively strong, due to the close overlap of the π orbitals of the maleate ions in the stacks (Fig. 3). Both O atoms of COO⁻ group are involved in two hydrogen bonds. Atom O4 is involved in hydrogen bonds with the water molecule and with the H atom of the protonated ring N atom of the melamine residue, while atom O3 acts as an acceptor for the NH₂ group of the same melamine residue and in an intramolecular O1–H···O3 hydrogen bond. Atom O2 of the non-dissociated COOH group is also involved in two hydrogen bonds, with the water molecule and with the NH₂ group of the melamine residue, while the other O atom, O1, of the COOH group is involved only in the intramolecular O1—H···O3 hydrogen bond, if the relatively weak C6—H6···O1ⁱ hydrogen bond is neglected [H6···O1 2.57 Å, C6···O1 3.218 (2) Å and C6—H6···O1ⁱ 126.7°; symmetry code: (i) 3/2 − x, 1/2 + y, 1/2 − z].

The water molecule is involved in four hydrogen bonds. In two it acts as a donor to two maleate ions, and in two it acts as an acceptor H for the NH₂ groups of two melamine moieties. The geometry of the hydrogen-bonding system in (I) is summarized in Table 2.

S2. Experimental

Melamine and maleic acid, in a molar ratio of 1:1, were dissolved in boiling water and the solution cooled to room temperature. After several days, colourless single crystals of (I) were formed.

S3. Refinement

H atoms bonded to C were treated as riding, with a C—H distance of 0.93 Å and with U_iso(H) = 1.2U_eq(C). H atoms bonded to N were also treated as riding, with an N—H distance of 0.86 Å and with U_iso(H) = 1.2U_eq(N). The coordinates of H atoms bonded to O were refined, giving O—H distances in the range 0.807 (18)–0.974 (9) Å (Table 2).

Computing details

Data collection: KM-4 CCD Software (Kuma Diffraction, 2001; cell refinement: KM-4 CCD Software; data reduction: KM-4 CCD Software; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL/PC (Sheldrick, 1990); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. The results of the optimized molecular orbital calculations for (a) the melaminium cation and (b) the maleate (1-) ion. Bond lengths are given in Å and angles in °.
	Fig. 2. The molecular structure of (I) with 50% probability displacement ellipsoids. H atoms are shown as spheres of arbitrary radii.
	Fig. 3. A stereoview of the crystal packing in (I), showing the stacking structure of the hydrogen-bonded melaminium and maleate residues, which stabilizes the N—H···O and O—H···O hydrogen bonds. Dashed lines represent hydrogen bonds.

2,4,6-triamino-1,3,5-triazin-1-ium maleate monohydrate

Crystal data

C3H7N6^+·C4H3O4^−·H2O	F(000) = 544
Mr = 260.23	Dx = 1.572 Mg m^−3 Dm = 1.57 Mg m^−3 Dm measured by flotation in CHCl3/CHBr3
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2yn	Cell parameters from 1288 reflections
a = 3.6720 (7) Å	θ = 2.8–27.0°
b = 10.417 (2) Å	µ = 0.13 mm^−1
c = 28.749 (6) Å	T = 293 K
β = 90.89 (3)°	Needle, colourless
V = 1099.6 (4) Å^3	0.46 × 0.12 × 0.10 mm
Z = 4

Data collection

KUMA KM-4 with CCD area-detector diffractometer	2394 independent reflections
Radiation source: fine-focus sealed tube	1788 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.036
Detector resolution: 1024x1024 with blocks 2x2 pixels mm^-1	θmax = 27.0°, θmin = 2.8°
ω scans	h = −4→4
Absorption correction: analytical face-indexed, (SHELXTL/PC; Sheldrick, 1990)	k = −13→13
Tmin = 0.938, Tmax = 0.988	l = −36→36
11241 measured reflections

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.037	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.050	H atoms treated by a mixture of independent and constrained refinement
S = 1.00	w = 1/[σ^2(Fo^2) + (0.0133P)^2] where P = (Fo^2 + 2Fc^2)/3
2386 reflections	(Δ/σ)max < 0.001
172 parameters	Δρmax = 0.14 e Å^−3
0 restraints	Δρmin = −0.16 e Å^−3

Special details

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

	x	y	z	Uiso*/Ueq
N1	0.7167 (3)	0.66873 (11)	0.01353 (4)	0.0298 (3)
C1	0.7944 (4)	0.60708 (15)	0.05300 (5)	0.0275 (4)
N2	0.9703 (3)	0.66584 (11)	0.08883 (4)	0.0308 (3)
H2	1.0213	0.6242	0.1139	0.037*
C2	1.0660 (4)	0.79186 (15)	0.08471 (5)	0.0291 (4)
N3	0.9966 (3)	0.85908 (11)	0.04697 (4)	0.0308 (3)
C3	0.8306 (4)	0.79256 (14)	0.01146 (5)	0.0290 (4)
N4	0.7028 (3)	0.48616 (11)	0.05846 (4)	0.0380 (4)
H4A	0.5913	0.4458	0.0364	0.046*
H4B	0.7545	0.4473	0.0841	0.046*
N5	1.2377 (3)	0.84174 (11)	0.12145 (3)	0.0379 (3)
H5A	1.3073	0.9205	0.1210	0.045*
H5B	1.2792	0.7952	0.1457	0.045*
N6	0.7751 (3)	0.85359 (11)	−0.02790 (4)	0.0440 (4)
H6A	0.6706	0.8150	−0.0509	0.053*
H6B	0.8432	0.9322	−0.0307	0.053*
O1	0.9014 (3)	0.18436 (10)	0.21759 (4)	0.0525 (4)
H1	1.023 (4)	0.1884 (15)	0.2479 (4)	0.074*
O2	0.6711 (3)	0.05691 (10)	0.16421 (3)	0.0513 (3)
C4	0.8203 (4)	0.07081 (15)	0.20180 (5)	0.0355 (4)
C5	0.9049 (4)	−0.04318 (15)	0.23110 (4)	0.0368 (4)
H5	0.8435	−0.1213	0.2175	0.044*
C6	1.0521 (4)	−0.05270 (15)	0.27318 (4)	0.0376 (4)
H6	1.0821	−0.1366	0.2836	0.045*
C7	1.1787 (4)	0.04667 (16)	0.30701 (5)	0.0351 (4)
O3	1.1498 (3)	0.16488 (10)	0.29763 (3)	0.0500 (3)
O4	1.3152 (3)	0.00634 (10)	0.34402 (3)	0.0510 (3)
O5	0.4745 (4)	0.27267 (12)	0.11343 (3)	0.0549 (4)
H1O5	0.333 (5)	0.3168 (17)	0.1275 (6)	0.082*
H2O5	0.557 (5)	0.2011 (17)	0.1302 (6)	0.082*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
N1	0.0370 (8)	0.0251 (7)	0.0270 (6)	−0.0039 (6)	−0.0048 (6)	0.0008 (6)
C1	0.0294 (9)	0.0275 (9)	0.0255 (8)	0.0002 (8)	−0.0010 (7)	−0.0041 (8)
N2	0.0399 (8)	0.0245 (8)	0.0279 (6)	−0.0015 (7)	−0.0067 (6)	0.0004 (6)
C2	0.0283 (10)	0.0287 (9)	0.0301 (8)	0.0013 (8)	−0.0023 (7)	−0.0069 (8)
N3	0.0357 (8)	0.0286 (7)	0.0280 (6)	−0.0036 (6)	−0.0047 (6)	−0.0005 (6)
C3	0.0314 (10)	0.0304 (9)	0.0253 (8)	−0.0018 (8)	−0.0001 (7)	−0.0001 (8)
N4	0.0551 (10)	0.0287 (8)	0.0297 (7)	−0.0044 (7)	−0.0126 (7)	0.0032 (6)
N5	0.0506 (9)	0.0328 (8)	0.0299 (6)	−0.0068 (7)	−0.0114 (6)	−0.0034 (6)
N6	0.0639 (10)	0.0325 (8)	0.0350 (7)	−0.0156 (7)	−0.0155 (7)	0.0053 (7)
O1	0.0813 (9)	0.0267 (7)	0.0488 (6)	−0.0024 (7)	−0.0230 (7)	0.0012 (6)
O2	0.0701 (8)	0.0403 (7)	0.0427 (6)	−0.0009 (7)	−0.0198 (6)	−0.0008 (6)
C4	0.0353 (10)	0.0344 (10)	0.0366 (9)	0.0002 (8)	−0.0050 (8)	−0.0036 (8)
C5	0.0438 (11)	0.0275 (9)	0.0389 (9)	−0.0033 (8)	−0.0064 (8)	−0.0027 (8)
C6	0.0471 (11)	0.0277 (9)	0.0379 (8)	0.0009 (9)	−0.0058 (8)	−0.0007 (8)
C7	0.0364 (11)	0.0373 (11)	0.0316 (9)	−0.0004 (9)	−0.0021 (8)	−0.0019 (8)
O3	0.0729 (9)	0.0317 (7)	0.0448 (6)	−0.0043 (7)	−0.0160 (6)	−0.0039 (6)
O4	0.0704 (8)	0.0419 (7)	0.0401 (6)	−0.0004 (6)	−0.0188 (6)	−0.0031 (6)
O5	0.0870 (12)	0.0430 (8)	0.0345 (7)	0.0081 (7)	−0.0059 (7)	0.0056 (6)

Geometric parameters (Å, º)

N1—C1	1.3311 (16)	N6—H6A	0.8600
N1—C3	1.3577 (17)	N6—H6B	0.8600
C1—N4	1.3138 (16)	O1—C4	1.2999 (17)
C1—N2	1.3537 (16)	O1—H1	0.974 (9)
N2—C2	1.3646 (17)	O2—C4	1.2126 (15)
N2—H2	0.8600	C4—C5	1.4860 (19)
C2—N3	1.3130 (16)	C5—C6	1.3209 (16)
C2—N5	1.3273 (15)	C5—H5	0.9300
N3—C3	1.3691 (16)	C6—C7	1.4899 (18)
C3—N6	1.3112 (15)	C6—H6	0.9300
N4—H4A	0.8600	C7—O4	1.2422 (15)
N4—H4B	0.8600	C7—O3	1.2648 (17)
N5—H5A	0.8600	O5—H1O5	0.807 (18)
N5—H5B	0.8600	O5—H2O5	0.936 (17)
C1—N1—C3	115.69 (12)	H5A—N5—H5B	120.0
N4—C1—N1	120.83 (13)	C3—N6—H6A	120.0
N4—C1—N2	117.56 (13)	C3—N6—H6B	120.0
N1—C1—N2	121.60 (13)	H6A—N6—H6B	120.0
C1—N2—C2	119.30 (12)	C4—O1—H1	116.8 (10)
C1—N2—H2	120.3	O2—C4—O1	121.18 (15)
C2—N2—H2	120.3	O2—C4—C5	119.82 (14)
N3—C2—N5	122.17 (14)	O1—C4—C5	118.98 (12)
N3—C2—N2	122.54 (13)	C6—C5—C4	131.15 (15)
N5—C2—N2	115.29 (13)	C6—C5—H5	114.4
C2—N3—C3	115.13 (12)	C4—C5—H5	114.4
N6—C3—N1	117.03 (13)	C5—C6—C7	131.67 (15)
N6—C3—N3	117.36 (13)	C5—C6—H6	114.2
N1—C3—N3	125.61 (13)	C7—C6—H6	114.2
C1—N4—H4A	120.0	O4—C7—O3	122.91 (14)
C1—N4—H4B	120.0	O4—C7—C6	116.23 (14)
H4A—N4—H4B	120.0	O3—C7—C6	120.85 (13)
C2—N5—H5A	120.0	H1O5—O5—H2O5	113.7 (16)
C2—N5—H5B	120.0

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N2—H2···O4^i	0.86	1.82	2.6570 (15)	165
N4—H4B···O5	0.86	2.26	2.8614 (17)	127
N4—H4A···N1^ii	0.86	2.17	3.0266 (16)	174
N5—H5A···O2^iii	0.86	2.30	3.0014 (16)	139
N5—H5B···O3^i	0.86	2.13	2.9912 (15)	174
N6—H6A···O5^ii	0.86	2.08	2.9234 (15)	168
N6—H6B···N3^iv	0.86	2.30	3.1585 (17)	173
O1—H1···O3	0.97 (1)	1.52 (1)	2.4704 (14)	165 (2)
O5—H1O5···O4^v	0.807 (18)	2.209 (18)	2.9315 (16)	149.3 (18)
O5—H2O5···O2	0.936 (17)	1.837 (17)	2.7697 (15)	174.5 (16)

Symmetry codes: (i) −x+5/2, y+1/2, −z+1/2; (ii) −x+1, −y+1, −z; (iii) x+1, y+1, z; (iv) −x+2, −y+2, −z; (v) −x+3/2, y+1/2, −z+1/2.