Melaminium bis(4-hydroxybenzenesulfonate) dihydrate

The crystals of a new melaminium salt, 2,4,6-tri­amino-1,3,5-triazine-1,3-diium bis(4-hydroxy­benzene­sulfonate) dihydrate, C₃H₈N₆²⁺·2C₆H₅O₄S^-·2H₂O, are built up from doubly proton­ated melaminium(2+) residues, dissociated p-phenol­sulfonate anions and water mol­ecules. The doubly protonated melaminium dication lies on a twofold axis. The hydroxyl group of the p-hydroxybenzenesulfonate residue is roughly coplanar with the phenyl ring [dihedral angle 13 (2)°]. A combination of ionic and donor-acceptor hydrogen-bond interactions link the melaminium and p-hydroxybenzenesulfonate residues and the water mol­ecules to form a three-dimensional network.

S1. Comment

This study is a continuation of our investigation into the characterization of the hydrogen bonds formed by the melamine molecule in the solid state (Janczak & Perpétuo, 2001). To expand the understanding of the solid-state physical organic chemistry of compounds containing N—H···O and O—H···O hydrogen bonds, we have studied the solid-state structure of the title diprotonated compound, melamine p-phenolsulfonate hydrate, (I). \sch

The crystal of (I) consists of water molecules and two well defined oppositely charged residues, namely, a protonated moiety at two of the three N atoms of the melamine ring and the dissociated p-phenolsulfonate ions. However, the asymmetric unit consists of only half of a diprotonated melaminium residue, one p-phenolsulfonate anion and one water molecule (Fig. 1). To our knowledge, this is the third structurally characterized melaminium salt which is protonated at two ring N atoms; the first was the hydrochloride of a melaminium cyanuric acid complex (Wang et al., 1990) and the second was a hydrated complex of perchlorate acid with melamine (Martin & Pinkerton, 1995). In addition to these doubly protonated melaminium salts, singly protonated melaminium salts have also been structurally characterized (Zerkowski et al., 1994; Janczak & Perpétuo, 2001).

The six-membered aromatic ring of the melaminium residue, C₃H₈N₆²⁺, is almost planar. Two of the three amino groups are approximately coplanar with the weighted least-squares plane through the melamine ring, while the third group (at N4) is rotated along the C8—N4 bond by 13 (2)°. This is likely due to an H1N1···H1N4 interaction of 2.30 (2) Å, caused by the fact that C8 is bonded to both protonated N atoms of the melaminium ring and is involved in the intermolecular hydrogen-bonding system.

The ring of the melaminium residue is significantly distorted from the ideal hexagonal form. The internal C—N—C angle at the non-protonated N atom is significantly smaller than the C—N—C angles at the protonated N atom. The differences between the internal C—N—C angles within the melaminium ring residue correlate with the steric effect of the lone-pair electrons and are fully consistent with the valence-shell electron pair repulsion theory (VSEPR; Gillespie, 1972). Protonation of the melamine ring at two N atoms distorts the bond lengths in the aromatic ring. The two shortest bonds in the melaminium ring (N2—C7 and its symmetrical equivalent) are those farthest from the protonated ring N atoms. The two longest N—C bonds of the melaminium ring (N1—C7 and its symmetrical equivalent) are those connected to the shortest bonds. This has the effect of opening up the ring bond angles at C7 and its symmetrical equivalent. A semi-empirical calculation performed with the AM1 parameter set (Dewar et al., 1985) on the melaminium residue doubly protonated at two ring N atoms results in almost the same geometrical features. Thus, the ring distortion of the melaminium residue mainly results from the protonation and, to a lesser degree, from the hydrogen bonding and crystal packing. The distortion of the aromatic melaminium ring is quite similar to that reported for the hydrochloride of the melamine-cyanuric acid complex (Wang et al., 1990), as well as for the melaminium diperchlorate monohydrate complex (Martin & Pinkerton, 1995), i.e. both of the simple salts of diprotonated melamine that have previously been structurally characterized.

The ring of the p-phenolsulfonate anion shows a slight quinone character. This is likely due to the substitution effect of both hydroxyl and sulfonate groups in the 1,4-positions of the ring. The C—O (OH group) and C—S (SO₃^- group) bond lengths are comparable with the distances of 1.364 (15) and 1.750 (8) Å observed for C_aromatic—O and C_aromatic—S bonds, respectively (Allen et al., 1987). The hydroxyl group is roughly coplanar with the ring [C5—C4—O4-H1O4 - 13 (2)°]. The sulfonate group has a slightly distorted tetrahedral geometry and is oriented so that the S1—O1 bond is almost coplanar with the phenyl ring [C2—C1—S1—O1 175.6 (1)°]. The differences between the S—O bond lengths of the SO₃^- group are correlated with the number and strength of the hydrogen bonds formed by the O atoms. The O atom of the longest S—O bond is involved in three hydrogen bonds as acceptor, while the other two O atoms are involved as acceptors in only one hydrogen bond. Although atoms O1 and O2 are involved in only one hydrogen bond, the S1—O1 and S1—O2 bond lengths are different, S1—O1 being shorter than S1—O2, since O1 forms a weaker hydrogen bond than O2.

Both oppositely charged residues and the water molecules interact extensively by a combination of ionic and donor-acceptor hydrogen-bond interactions throughout the lattice to form a three-dimensional network (Fig. 2). All eight H atoms of the melaminium residue form hydrogen bonds with four different p-phenolsulfonate anions and with two water molecules, which are acceptors of hydrogen bonds. Two of these four p-phenolsulfonate residues are involved as acceptors in two hydrogen bonds with a melaminium residue (N1-H1N1···O2 and N4-H1N4···O3), while the other two p-phenolsulfate moieties are involved in only one hydrogen bond with the same melamine residue. Thus, one melaminium residue forms eight hydrogen bonds. There are six N—H···O hydrogen bonds with four neighbouring p-phenolsulfonate anions, and the other two H atoms of the melaminium residue form hydrogen bonds with the water molecules. The most noticeable feature is the fact that the non-protonated N atom of the melaminium residue is not involved as an acceptor in any hydrogen bond.

The SO₃^- group of the p-phenolsulfonate residue is involved as an acceptor in three O···H—N hydrogen bonds from two different melaminium moieties and in two O···H—O hydrogen bonds with two water molecules, while the hydroxyl group of the p-phenolsulfonate ion (as a donor) forms a hydrogen bond with a water molecule. Thus, one p-phenolsulfonate residue is involved in six different hydrogen bonds.

The water molecule is involved as a donor in two hydrogen bonds with the SO₃^- group of two different p-phenolsulfonate anions, and as an acceptor in hydrogen bonds with the phenol hydroxyl group and the N3 amino group from a melaminium dication.

In the crystal of (I), the melaminium residues form layers which are a/2 apart. In one layer, the melaminium residues are parallel to each other. The ring of the melaminium residue is perpendicular to the ac plane and forms angles of about 36 and 54° with the bc and ab planes, respectively·The ring of the p-phenolsulfonate anion is almost perpendicular to the bc plane and makes dihedral angles of 60.6° with the ab plane and 28.5° with the ac plane. The plane of the melamine residue is inclined at an angle of 72.2 (1)° to the plane of the p-phenolsulfonate ring. Details of the hydrogen-bonding geometry are given in Table 2.

S2. Experimental

Melamine was dissolved in hot water and to this solution was slowly added a 10% solution of p-phenolsulfonic acid. After several days, colourless crystals of (I) appeared.

S3. Refinement

The positions of the H atoms of the melamine residue and the H atom of the hydroxyl (OH) group of the p-phenolsulfonate ion, as well as the H atoms of the water molecule, i.e. of all H atoms involved in hydrogen bonding, were refined. Other H atoms were treated as riding, with C—H = 0.93 Å. For all H atoms, U_iso(H) = 1.2U_eq(C) and 1.5U_eq(O).

Computing details

Data collection: KM-4 CCD Software (Kuma Diffraction, 1999); 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 (Sheldrick, 1990); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. The molecular structure of the asymmetric unit of (I) showing 50% probability displacement ellipsoids and the atom-numbering scheme. H atoms are drawn as spheres of arbitrary radii.
	Fig. 2. The molecular arrangement in the unit cell of (I) showing the hydrogen-bonding interactions (dashed lines). H atoms have been omitted for clarity.

2,4,6-triamino-1,3,5-triazine-1,3-diium bis(4-hydroxybenzenesulfonate) dihydrate

Crystal data

C3H8N6^2+·2C6H5SO4^−·2H2O	Dx = 1.583 Mg m^−3 Dm = 1.58 Mg m^−3 Dm measured by flotation
Mr = 510.51	Melting point: dehydrated K
Orthorhombic, Pbcn	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2n 2ab	Cell parameters from 2155 reflections
a = 26.625 (5) Å	θ = 5–26°
b = 7.863 (2) Å	µ = 0.32 mm^−1
c = 10.230 (2) Å	T = 293 K
V = 2141.7 (8) Å^3	Parallelepiped, colourless
Z = 4	0.28 × 0.24 × 0.16 mm
F(000) = 1064

Data collection

Kuma KM-4 with two-dimensional CCD area-detector diffractometer	2806 independent reflections
Radiation source: fine-focus sealed tube	1728 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.034
Detector resolution: 1024 x 1024 with blocks 2 x 2 pixels mm^-1	θmax = 29.6°, θmin = 3.1°
ω scan	h = −36→36
Absorption correction: analytical face-indexed (SHELXTL; Sheldrick, 1990)	k = −10→8
Tmin = 0.917, Tmax = 0.951	l = −14→13
17563 measured reflections

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.036	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.098	w = 1/[σ^2(Fo^2) + (0.0518P)^2 + 0.092P] where P = (Fo^2 + 2Fc^2)/3
S = 1.06	(Δ/σ)max = 0.001
2806 reflections	Δρmax = 0.29 e Å^−3
173 parameters	Δρmin = −0.27 e Å^−3
0 restraints	Extinction correction: SHELXL97 (Sheldrick, 1997), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0030 (7)

Special details

Experimental. The measurement was performed on a KUMA KM-4 diffractometer equipped with a two-dimensional CCD area-detector. The ω-scan technique was used, with Δω = 0.75° for one image. The 960 images taken for six different runs covered over 93.5% of the Ewald sphere. The lattice parameters were calculated using 255 reflections obtained from 30 images for 10 runs with different orientations in reciprocal space, and after data collection were refined on 2155 reflections.

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
S1	0.396031 (15)	0.15889 (5)	0.51261 (4)	0.03879 (14)
O1	0.40284 (5)	0.25944 (17)	0.62939 (13)	0.0570 (4)
O2	0.41498 (4)	−0.01397 (16)	0.52644 (11)	0.0488 (3)
O3	0.41821 (4)	0.23948 (15)	0.39688 (12)	0.0498 (3)
O4	0.17886 (5)	0.10695 (18)	0.41758 (13)	0.0591 (4)
H1O4	0.1629 (11)	0.134 (3)	0.491 (2)	0.089*
C1	0.33116 (6)	0.14569 (18)	0.48527 (14)	0.0348 (3)
C2	0.31296 (6)	0.0652 (2)	0.37305 (15)	0.0426 (4)
H2	0.3352	0.0198	0.3124	0.051*
C3	0.26198 (6)	0.0533 (2)	0.35241 (16)	0.0450 (4)
H3	0.2499	−0.0007	0.2779	0.054*
C4	0.22867 (6)	0.1216 (2)	0.44230 (15)	0.0408 (4)
C5	0.24691 (6)	0.2026 (2)	0.55335 (16)	0.0427 (4)
H5	0.2246	0.2494	0.6134	0.051*
C6	0.29791 (6)	0.2138 (2)	0.57469 (15)	0.0395 (4)
H6	0.3100	0.2672	0.6495	0.047*
N1	0.47444 (5)	0.14956 (17)	0.83992 (14)	0.0385 (3)
H1N1	0.4559 (7)	0.098 (2)	0.8950 (18)	0.046*
N2	1/2	0.4140 (2)	3/4	0.0384 (4)
N3	0.44786 (6)	0.4018 (2)	0.92715 (16)	0.0476 (4)
H1N3	0.4455 (7)	0.511 (3)	0.9278 (19)	0.057*
H2N3	0.4334 (8)	0.342 (3)	0.986 (2)	0.057*
N4	1/2	−0.1060 (2)	3/4	0.0436 (5)
H1N4	0.4760 (7)	−0.159 (2)	0.8023 (19)	0.052*
C7	0.47418 (6)	0.32430 (19)	0.83731 (15)	0.0368 (3)
C8	1/2	0.0604 (3)	3/4	0.0365 (5)
O5	0.12167 (5)	0.1188 (2)	0.64548 (14)	0.0622 (4)
H1O5	0.1157 (9)	0.173 (3)	0.725 (3)	0.093*
H2O5	0.1192 (9)	0.012 (4)	0.684 (2)	0.093*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
S1	0.0355 (2)	0.0351 (2)	0.0457 (2)	0.00138 (16)	0.00049 (16)	−0.00144 (16)
O1	0.0473 (7)	0.0612 (8)	0.0626 (8)	0.0039 (6)	−0.0124 (6)	−0.0204 (6)
O2	0.0494 (7)	0.0418 (7)	0.0551 (7)	0.0103 (5)	0.0059 (5)	0.0058 (6)
O3	0.0417 (6)	0.0432 (7)	0.0646 (8)	−0.0019 (6)	0.0095 (5)	0.0099 (6)
O4	0.0402 (7)	0.0757 (9)	0.0614 (8)	−0.0051 (6)	−0.0091 (6)	0.0004 (7)
C1	0.0367 (8)	0.0301 (7)	0.0377 (7)	−0.0008 (6)	0.0029 (6)	0.0027 (6)
C2	0.0478 (9)	0.0429 (9)	0.0369 (8)	−0.0016 (7)	0.0068 (7)	−0.0042 (7)
C3	0.0521 (10)	0.0456 (10)	0.0373 (8)	−0.0075 (8)	−0.0047 (7)	−0.0019 (7)
C4	0.0385 (8)	0.0406 (9)	0.0433 (9)	−0.0044 (7)	−0.0022 (7)	0.0094 (7)
C5	0.0410 (9)	0.0444 (9)	0.0427 (8)	0.0026 (7)	0.0035 (7)	0.0005 (7)
C6	0.0415 (9)	0.0403 (9)	0.0366 (8)	−0.0020 (7)	−0.0003 (6)	−0.0006 (7)
N1	0.0313 (7)	0.0356 (7)	0.0485 (8)	−0.0008 (6)	0.0030 (5)	0.0030 (6)
N2	0.0298 (9)	0.0362 (10)	0.0491 (10)	0.000	0.0019 (8)	0.000
N3	0.0464 (8)	0.0402 (8)	0.0563 (9)	0.0023 (7)	0.0116 (7)	−0.0026 (7)
N4	0.0403 (11)	0.0334 (10)	0.0570 (12)	0.000	0.0043 (9)	0.000
C7	0.0288 (7)	0.0356 (8)	0.0460 (8)	0.0011 (6)	−0.0024 (6)	−0.0026 (7)
C8	0.0257 (10)	0.0354 (11)	0.0483 (12)	0.000	−0.0045 (9)	0.000
O5	0.0671 (9)	0.0665 (9)	0.0531 (8)	−0.0024 (8)	0.0036 (7)	−0.0063 (7)

Geometric parameters (Å, º)

S1—O1	1.4439 (13)	C6—H6	0.9300
S1—O2	1.4566 (13)	N1—C8	1.3421 (17)
S1—O3	1.4670 (12)	N1—C7	1.374 (2)
S1—C1	1.7528 (16)	N1—H1N1	0.853 (19)
O4—C4	1.355 (2)	N2—C7	1.3297 (18)
O4—H1O4	0.89 (3)	N2—C7^i	1.3297 (18)
C1—C6	1.381 (2)	N3—C7	1.307 (2)
C1—C2	1.398 (2)	N3—H1N3	0.86 (2)
C2—C3	1.377 (2)	N3—H2N3	0.86 (2)
C2—H2	0.9300	N4—C8	1.308 (3)
C3—C4	1.386 (2)	N4—H1N4	0.930 (18)
C3—H3	0.9300	C8—N1^i	1.3421 (17)
C4—C5	1.390 (2)	O5—H1O5	0.93 (3)
C5—C6	1.378 (2)	O5—H2O5	0.93 (3)
C5—H5	0.9300
O1—S1—O2	112.78 (8)	C6—C5—H5	119.9
O1—S1—O3	112.37 (8)	C4—C5—H5	119.9
O2—S1—O3	110.00 (7)	C5—C6—C1	120.14 (14)
O1—S1—C1	106.74 (7)	C5—C6—H6	119.9
O2—S1—C1	107.55 (7)	C1—C6—H6	119.9
O3—S1—C1	107.07 (7)	C8—N1—C7	120.76 (15)
C4—O4—H1O4	107.0 (18)	C8—N1—H1N1	119.8 (12)
C6—C1—C2	119.82 (14)	C7—N1—H1N1	119.2 (12)
C6—C1—S1	120.21 (12)	C7—N2—C7^i	115.91 (18)
C2—C1—S1	119.97 (11)	C7—N3—H1N3	120.6 (13)
C3—C2—C1	119.91 (15)	C7—N3—H2N3	118.7 (14)
C3—C2—H2	120.0	H1N3—N3—H2N3	120.7 (19)
C1—C2—H2	120.0	C8—N4—H1N4	116.4 (11)
C2—C3—C4	120.19 (15)	N3—C7—N2	120.15 (15)
C2—C3—H3	119.9	N3—C7—N1	117.08 (15)
C4—C3—H3	119.9	N2—C7—N1	122.75 (14)
O4—C4—C3	118.02 (15)	N4—C8—N1^i	121.5 (1)
O4—C4—C5	122.24 (15)	N4—C8—N1	121.5 (1)
C3—C4—C5	119.74 (15)	N1^i—C8—N1	117.0 (2)
C6—C5—C4	120.20 (15)	H1O5—O5—H2O5	92 (2)

Symmetry code: (i) −x+1, y, −z+3/2.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1N1···O2^ii	0.85 (2)	1.85 (2)	2.699 (2)	172 (2)
O4—H1O4···O5	0.89 (3)	1.93 (3)	2.786 (2)	162 (2)
N3—H1N3···O3^iii	0.86 (2)	2.12 (2)	2.945 (2)	162 (2)
N3—H2N3···O5^iv	0.86 (2)	2.21 (2)	2.905 (2)	138 (2)
N4—H1N4···O3^ii	0.93 (2)	1.93 (2)	2.846 (2)	170 (2)
O5—H1O5···O3^iv	0.93 (2)	2.10 (2)	2.997 (2)	163 (2)
O5—H2O5···O1^v	0.93 (2)	2.14 (2)	2.905 (2)	138 (2)

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