Melaminium phthalate

The crystals of the new melaminium salt, 2,4,6-tri­amino-1,3,5-triazin-1-ium hydrogenphthalate, C₃H₇N₆⁺·C₈H₅O₄^-, are built up from single protonated melaminium residues and single dissociated hydrogenphthalate(1-) anions. The protonated melaminium ring is almost planar. The best plane through the non-dissociated carboxyl group (COOH) of the hydrogenphthalate(1-) anion is inclined at an angle of 16.4 (2)° to the plane of the six-membered hydrogenphthalate ring, while the plane of the ionized COO^- group is roughly perpendicular [84.2 (2)°] to this plane. A combination of ionic and donor-acceptor hydrogen-bond interactions linking together the melaminium and hydrogenphthalate residues forms a three-dimensional network.

S1. Comment

To expand the understanding of the physical-organic chemistry of the solid state we are studying the solid-state structure of 1:1 cocrystal of the melamine and phthalic acid. The asymmetric unit of the title compound, (I), consists of two well defined oppositely charged residues: a protonated moiety at one of three N-atoms of the melamine ring and phthalate ion dissociated at one of the carboxyl group (Fig. 1). To our knowledge this is a second structurally characterized melaminium salt which is protonated at only one nitrogen atom of the ring; the first was cocrystal of barbituric acid with melamine (Zerkowski et al., 1994). In addition to these one protonated melaminium salts, the diprotonated melaminimu salt has also been structurally characterized (Martin & Pinkerton, 1995). \sch

The almost planar six-membered aromatic ring of the melaminum residue C₃H₇N₆⁺ shows a slight boat conformation character, the N2 and C11 atoms (the 1,4 positions in the ring) deviate from the least-squares plane of the ring by up to 0.03 Å. Two of the three amine groups (N4 and N5) are approximately coplanar with the mean plane of the melamine ring, while the third amine group (N6) is deviates slightly from the ring plane. The N6 atom of the amine group deviates by 0.146 (2) Å from the weighted least-squares plane through the ring. The first two amine group (N4, N5) are almost coplanar with the melaminium ring, but the third amine group at the N6 atom has a somewhat pyramidal character. This is likely a consequence of the intermolecular hydrogen-bonding interactions present in the crystal.

The ring of melamine residue is significantly distorted from the ideal hexagonal form. The internal C9–N1–C11 and C10–N3–C11 angles of the melamine ring are smaller than the C10–N2–C9 angle protonated at the nitrogen atom. These differences between the C—N—C angles in the melaminium ring correlated with the steric effect of the lone-pair electrons and are fully consistent with the valence-shell electron pair repulsion theory, VSEPR (Gillespie, 1972). A similar correlation between the angles within the ring are observed in the crystal of melamine in the X-ray as well as in the neutron structures (Hughes, 1941; Larson & Cromer, 1974; Varghese et al., 1977) and several melamine derivatives (Mathias et al., 1994; Zerkowski & Whitesides, 1994; Janczak & Kubiak, 1999). The greater difference between the corresponding angles (C–N–C 113° and N–C–N 127°) are observed in the X-ray and neutron structure of s-trizaine (Coppens, 1967).

The hydrogenphthalate(1-) anion is almost planar except of the two carboxyl groups (dissociated and non-dissociated). Within the benzene ring, the greatest deviation from the weighted least-squares plane of the ring is observed for the C1 and C4 atoms. The plane defined by the atoms of the dissociated carboxyl group (COO^-) is almost perpendicular [84.2 (2)°], while the plane defined as that of non-dissociated carboxyl group (COOH) is inclined at angle of 16.4 (2)° to the plane of benzene ring. In the crystal of the phthalic acid, both carboxyl group are oppositely inclined at angles ± 30° to the ring plane (Kuppers, 1981; Ermer, 1981). Within the ring the endocyclic angles at C1 and C2 atoms are slightly smaller than 120°, and correlated with the substitution effect of the COO^- and COOH groups. The interaction of the polar carboxyl (COO^- and COOH) groups reveals in the distortion of the C1—C2—C8 and C1—C7–O2 angles, which are substantially greater than 120°. The C—C bond length joining the ionized carboxyl group (C2—C8) is slightly longer than the C—C bond joining the non-dissociated carboxyl group (C1—C7), which is comparable with a distance of 1.491 (8) Å observed for unconjugated C(aromatic)—C(sp²) bond (Allen et al., 1987).

Both oppositely charged residues [C₃H₇N₆⁺ and C₆H₄(COOH)(COO)^-] are extensively interacting by a combination of ionic and donor-acceptor hydrogen-bond interactions throughout the lattice to form a three-dimensional network (Fig. 2). For the purpose of this discussion a hydrogen-bonding system has been defined as having an O···H and N···H contacts shorter than 2.5 Å. All hydrogen atoms of melaminium residue [C₃H₇N₆]⁺ are involved in hydrogen bonds. The most noticeable feature is that the H21 atom (at the protonated N-ring) forms a bifurcated hydrogen bond involving the O2 and O3 atoms. Additionally, both non-protonated nitrogen atoms of the melaminium residue act in the hydrogen bonds as acceptor. Thus the melaminium moiety is involved in ten hydrogen bonds, of which four N—H···N are almost linear with the neighbouring melamine residues, while five of the six N—H···O are much more bent with the neighbouring hydrogenphthalate(1-) residues. The hydrogen atom of COOH group of hydrogenphthalate(1-) anion is involved in the strongest O1—H···O3^iv hydrogen bond. Each of the O2, O3 and O4 atoms of hydrogenphthalate(1-) residue are acceptors in two hydrogen bonds (N—H···O) of neighbouring melamine residues.

In the crystal, the melaminium residues interconnected by four N—H···N hydrogen bonds form layers which are about c/2\ apart (Fig. 2). These layers are almost parallel to the b axis and form angles of ≈ 30 and simeq 60 ° with the a and c axes, repectively. The plane of the ring of the hydrogenphthalate(1-) residues is inclined at the angle of 7.6 (1)° to the plane of the melaminium ring, but the plane of the ionized carboxyl group is inclined at the angle of 68.6 (1)° to the melaminium ring, since both oxygen atoms of COO^- group (O3 and O4) as acceptors of hydrogen bonds staple two neighbouring layers of melamine residues (Fig. 2). All geometrical parameter details of the hydrogen atoms are given in Table 2.

S2. Experimental

Melanine and phthalic acid in molar proportion of 1:1 were dissolved in hot water and the resulting solution was slowly evaporated. After several days colourless crystals of the title salt appeared.

S3. Refinement

The position of H-atoms of melamine residue and H atom of COOH group of hydrogenphthalate(1-) ion, i.e. all hydrogen atoms which are involved in hydrogen bonds were refined.

Computing details

Data collection: KUMA KM-4 CCD software (KUMA, 1998); cell refinement: KUMA KM-4 CCD software; data reduction: KUMA KM-4 CCD software; program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); 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. Molecular structure showing 50% probability displacement ellipsoids. H atoms are shown as spheres of arbitrary radii.
	Fig. 2. Molecular packing in the unit cell showing the hydrogen-bonding interactions (dashed lines). H atoms are omitted for clarity.

(I)

Crystal data

C3H7N6^+·C8H5O4^−	F(000) = 1216
Mr = 292.27	Dx = 1.547 Mg m^−3 Dm = 1.54 Mg m^−3 Dm measured by floatation
Orthorhombic, Pbcn	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2n 2ab	Cell parameters from 2558 reflections
a = 33.622 (7) Å	θ = 5–25°
b = 10.562 (2) Å	µ = 0.12 mm^−1
c = 7.067 (1) Å	T = 293 K
V = 2509.6 (8) Å^3	Parallelepiped, colourless
Z = 8	0.35 × 0.22 × 0.20 mm

Data collection

KUMA KM-4 diffractometer with two-dimensional area CCD detector	3384 independent reflections
Radiation source: fine-focus sealed tube	1527 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.046
Detector resolution: 1024x1024 with blocks 2x2 pixels mm^-1	θmax = 29.6°, θmin = 2.7°
ω–scan	h = −45→45
Absorption correction: analytical face-indexed, SHELXTL, (Sheldrick, 1990)	k = −14→14
Tmin = 0.959, Tmax = 0.976	l = −8→9
22665 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)] = 0.046	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.126	H atoms treated by a mixture of independent and constrained refinement
S = 1.01	w = 1/[σ^2(Fo^2) + (0.060P)^2 + 0.066P] where P = (Fo^2 + 2Fc^2)/3
3384 reflections	(Δ/σ)max = 0.001
214 parameters	Δρmax = 0.20 e Å^−3
0 restraints	Δρmin = −0.24 e Å^−3

Special details

Experimental. The measurement has been performed on a KUMA KM-4 diffractometer equipped with a two-dimension area CCD detector. The ω–scan technique was used, Δω=0.75° for one image. The 960 images taken for six different runs covered above 95% 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 2558 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
O1	0.68042 (4)	0.54189 (13)	0.0412 (2)	0.0513 (4)
H1	0.6648 (7)	0.622 (2)	0.029 (3)	0.077*
O2	0.62577 (4)	0.46662 (13)	0.1786 (2)	0.0565 (4)
O3	0.63474 (4)	0.26901 (13)	0.5039 (2)	0.0551 (4)
O4	0.59681 (4)	0.18766 (15)	0.2774 (2)	0.0644 (5)
C1	0.67826 (5)	0.32381 (17)	0.0988 (3)	0.0392 (4)
C2	0.66412 (5)	0.22230 (17)	0.2069 (3)	0.0393 (4)
C3	0.68414 (6)	0.10773 (19)	0.1982 (3)	0.0509 (5)
H3	0.6751	0.0396	0.2697	0.061*
C4	0.71725 (6)	0.0934 (2)	0.0849 (3)	0.0578 (6)
H4	0.7308	0.0168	0.0838	0.069*
C5	0.73031 (6)	0.1919 (2)	−0.0267 (3)	0.0566 (6)
H5	0.7521	0.1811	−0.1060	0.068*
C6	0.71086 (5)	0.3063 (2)	−0.0198 (3)	0.0472 (5)
H6	0.7196	0.3728	−0.0953	0.057*
C7	0.65877 (5)	0.44973 (17)	0.1092 (3)	0.0402 (5)
C8	0.62879 (5)	0.23016 (17)	0.3360 (3)	0.0435 (5)
N1	0.46181 (4)	0.87876 (14)	0.0810 (2)	0.0395 (4)
C9	0.43046 (5)	0.82986 (17)	0.1668 (3)	0.0384 (4)
N2	0.42634 (4)	0.70185 (14)	0.1871 (2)	0.0411 (4)
H21	0.4055 (6)	0.6723 (17)	0.255 (3)	0.049*
C10	0.45428 (5)	0.62277 (18)	0.1116 (3)	0.0384 (4)
N3	0.48628 (4)	0.66843 (14)	0.0235 (2)	0.0387 (4)
C11	0.48928 (5)	0.79565 (17)	0.0174 (2)	0.0345 (4)
N4	0.40213 (5)	0.90280 (17)	0.2359 (3)	0.0499 (5)
H41	0.4058 (6)	0.990 (2)	0.237 (3)	0.060*
H42	0.3849 (6)	0.867 (2)	0.320 (3)	0.060*
N5	0.44951 (5)	0.49991 (15)	0.1313 (3)	0.0486 (5)
H51	0.4688 (6)	0.442 (2)	0.094 (3)	0.058*
H52	0.4271 (6)	0.472 (2)	0.179 (3)	0.058*
N6	0.52230 (4)	0.84463 (17)	−0.0535 (2)	0.0426 (4)
H61	0.5235 (5)	0.936 (2)	−0.056 (3)	0.051*
H62	0.5437 (5)	0.7932 (19)	−0.101 (3)	0.051*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.0407 (7)	0.0437 (8)	0.0694 (10)	−0.0004 (6)	0.0100 (7)	0.0114 (7)
O2	0.0423 (7)	0.0587 (9)	0.0686 (10)	0.0086 (6)	0.0184 (7)	0.0119 (8)
O3	0.0625 (9)	0.0493 (8)	0.0535 (10)	−0.0151 (7)	0.0158 (7)	−0.0063 (7)
O4	0.0430 (8)	0.0793 (11)	0.0709 (10)	−0.0205 (7)	0.0117 (7)	−0.0094 (9)
C1	0.0342 (9)	0.0410 (11)	0.0423 (11)	−0.0014 (8)	−0.0005 (8)	−0.0030 (9)
C2	0.0340 (9)	0.0412 (10)	0.0427 (11)	−0.0032 (8)	−0.0006 (8)	−0.0025 (9)
C3	0.0489 (11)	0.0433 (12)	0.0604 (15)	−0.0007 (9)	0.0018 (10)	−0.0021 (11)
C4	0.0504 (12)	0.0517 (13)	0.0714 (16)	0.0148 (10)	0.0019 (11)	−0.0076 (12)
C5	0.0462 (11)	0.0664 (15)	0.0571 (15)	0.0076 (11)	0.0104 (10)	−0.0085 (12)
C6	0.0396 (10)	0.0561 (12)	0.0461 (12)	−0.0010 (9)	0.0064 (9)	0.0019 (10)
C7	0.0345 (9)	0.0468 (11)	0.0394 (11)	−0.0008 (8)	0.0017 (8)	0.0031 (9)
C8	0.0398 (10)	0.0359 (10)	0.0546 (14)	−0.0045 (8)	0.0079 (9)	0.0019 (9)
N1	0.0342 (7)	0.0384 (9)	0.0458 (10)	0.0009 (6)	0.0051 (7)	0.0037 (7)
C9	0.0350 (9)	0.0386 (10)	0.0417 (12)	0.0024 (8)	0.0005 (8)	0.0048 (8)
N2	0.0325 (8)	0.0387 (9)	0.0521 (11)	0.0006 (7)	0.0102 (7)	0.0053 (7)
C10	0.0333 (9)	0.0399 (11)	0.0422 (11)	0.0021 (8)	0.0009 (8)	0.0006 (9)
N3	0.0333 (7)	0.0364 (9)	0.0464 (10)	0.0001 (6)	0.0052 (7)	0.0013 (7)
C11	0.0331 (9)	0.0376 (10)	0.0328 (10)	0.0013 (7)	0.0021 (8)	0.0032 (8)
N4	0.0408 (9)	0.0441 (10)	0.0648 (12)	0.0085 (8)	0.0194 (8)	0.0076 (9)
N5	0.0415 (8)	0.0335 (9)	0.0707 (13)	−0.0012 (8)	0.0118 (9)	0.0057 (8)
N6	0.0381 (8)	0.0375 (9)	0.0523 (11)	−0.0020 (7)	0.0086 (7)	0.0020 (8)

Geometric parameters (Å, º)

O1—C7	1.307 (2)	N1—C9	1.321 (2)
O1—H1	1.00 (2)	N1—C11	1.351 (2)
O2—C7	1.226 (2)	C9—N4	1.319 (2)
O3—C8	1.272 (2)	C9—N2	1.367 (2)
O4—C8	1.236 (2)	N2—C10	1.366 (2)
C1—C6	1.392 (2)	N2—H21	0.90 (2)
C1—C2	1.400 (3)	C10—N5	1.315 (2)
C1—C7	1.484 (3)	C10—N3	1.334 (2)
C2—C3	1.386 (3)	N3—C11	1.348 (2)
C2—C8	1.500 (2)	C11—N6	1.323 (2)
C3—C4	1.380 (3)	N4—H41	0.93 (2)
C3—H3	0.9300	N4—H42	0.91 (2)
C4—C5	1.377 (3)	N5—H51	0.93 (2)
C4—H4	0.9300	N5—H52	0.88 (2)
C5—C6	1.375 (3)	N6—H61	0.96 (2)
C5—H5	0.9300	N6—H62	0.96 (2)
C6—H6	0.9300
C7—O1—H1	111.7 (13)	O3—C8—C2	117.5 (2)
C6—C1—C2	119.6 (2)	C9—N1—C11	116.4 (2)
C6—C1—C7	119.8 (2)	N4—C9—N1	121.2 (2)
C2—C1—C7	120.6 (2)	N4—C9—N2	117.8 (2)
C3—C2—C1	118.7 (2)	N1—C9—N2	121.1 (2)
C3—C2—C8	117.4 (2)	C10—N2—C9	119.6 (2)
C1—C2—C8	123.9 (2)	C10—N2—H21	121.9 (12)
C4—C3—C2	120.9 (2)	C9—N2—H21	118.4 (12)
C4—C3—H3	119.6	N5—C10—N3	120.3 (2)
C2—C3—H3	119.6	N5—C10—N2	118.6 (2)
C5—C4—C3	120.5 (2)	N3—C10—N2	121.1 (2)
C5—C4—H4	119.8	C10—N3—C11	115.8 (2)
C3—C4—H4	119.8	N6—C11—N3	117.7 (2)
C4—C5—C6	119.5 (2)	N6—C11—N1	116.5 (2)
C4—C5—H5	120.3	N3—C11—N1	125.90 (15)
C6—C5—H5	120.3	C9—N4—H41	119.1 (13)
C5—C6—C1	120.8 (2)	C9—N4—H42	117.3 (13)
C5—C6—H6	119.6	H41—N4—H42	119 (2)
C1—C6—H6	119.6	C10—N5—H51	122.2 (13)
O2—C7—O1	122.9 (2)	C10—N5—H52	118.7 (14)
O2—C7—C1	123.3 (2)	H51—N5—H52	119.0 (19)
O1—C7—C1	113.8 (2)	C11—N6—H61	115.7 (10)
O4—C8—O3	124.5 (2)	C11—N6—H62	122.6 (12)
O4—C8—C2	117.7 (2)	H61—N6—H62	121.7 (16)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1···O3^i	1.00 (2)	1.54 (2)	2.533 (3)	170 (2)
N2—H21···O3^ii	0.90 (2)	2.27 (2)	3.014 (3)	140 (2)
N2—H21···O2^iii	0.90 (2)	2.46 (2)	3.185 (3)	138 (2)
N4—H41···O4^iv	0.93 (2)	2.10 (2)	3.010 (3)	169 (2)
N4—H42···O3^ii	0.91 (2)	2.02 (2)	2.866 (3)	155 (2)
N5—H51···N3^v	0.93 (2)	2.08 (2)	3.004 (3)	172 (2)
N5—H52···O2^iii	0.88 (2)	2.04 (2)	2.887 (3)	161 (2)
N6—H61···N1^vi	0.96 (2)	2.03 (2)	2.977 (3)	169 (2)
N6—H62···O4^i	0.96 (2)	1.99 (2)	2.796 (3)	140 (2)

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