Tetramethyl­ammonium 2,5-dinitro­phenolate monohydrate

The crystal structure of the title compound, C₄H₁₂N⁺·C₆H₃N₂O·H₂O, is built up from tetramethyl­ammonium cations, 2,5-dinitro­phenolate anions and water molecules. The nitro groups are almost coplanar with the aryl ring, which exhibits significant distortion from an ideal hexagonal form. The X-ray geometry of the tetramethyl­ammonium cation shows slight distortion from the tetrahedral symmetry predicted by molecular orbital calculations. The O-H...O hydrogen-bonded water and 2,5-dinitro­phenolate units are related by an inversion center and form dimers. The 2,5-dinitro­phenolate anions, related by an inversion center and translation, are stacked to form a column along the [100] direction.

S1. Comment

The present study is a continuation of our investigations of compounds with hydrogen-bonding systems that are formed by self-assembly of components containing complementary arrays of hydrogen-bonding sites (Perpétuo & Janczak, 2003; Janczak & Perpétuo, 2004; Desiraju, 1990; Krische & Lehn, 2000; Sherington & Taskinen, 2001). To expand the understanding of the solid-state physical-organic chemistry of compounds containing O—H···O hydrogen-bonding systems, we present here the solid-state structure of tetramethylammonium 2,5-dinitrophenolate monohydrate, (I), and compare the results with that predicted for isolated oppositely charged parts of the crystal of (I), i.e. the 2,5-dinitrophenolate anion and the tetramethylammonium cation, by density functional theory (DFT) with fully optimized geometry calculations. Molecular orbital calculations at the B3LYP/6–31+G(d) level (Frisch et al. 1998) were carried out on isolated ions corresponding to the gas phase.

The X-ray structure analysis (Fig. 1) reveals that the nitro groups (NO₂) are almost coplanar with the aryl ring [the dihedral angles between the aryl ring and nitro groups are 1.1 (1) and 3.6 (1)°, respectively, for the nitro groups containing atoms N1 and N2]. The aryl ring exhibits distortion from the ideal hexagonal form expected for an unsubstituted benzene ring (see Table 1).

Ionization of the O atom of the hydroxy group influences the C—C bond distances within the aryl ring. The C1—O1 bond length [1.2677 (13) Å] of the ionized hydroxy group shows significant double-bond character, with a value that is intermediate between the values expected for carbon–oxygen double (1.206–1.215 Å) and single (1.333–1.373 Å) bonds, and is closer to the distances of 1.211–1.231 Å found in several benzoquinone derivatives (Allen et al., 1987). The C1—O1 bond is shorter than the equivalent C—O bonds in non-ionized dinitrophenol isomers, for example 1.331 (7) Å in 2,4-dinitrophenol (Kagawa et al., 1976), 1.337 (7) in 2,6-dinitrophenol (Iwasaki et al., 1976) and 1.337 Å in 2,5-dinitrophenol (this value was obtained from our molecular orbital calculations; see Fig.2). The double-bond character of the C1—O1 bond increases the single-bond character of both adjoining C—C bonds of the ring (C1—C2 and C1—C6; Table 1). These bonds are significantly longer than the other four in the ring; thus, the delocalization of the π electrons is disturbed by the donating effect of the O atom of the ionized hydroxy group, and the π electrons are not fully delocalized over all C atoms within the ring. Two of the internal C—C—C angles in the ring, at atoms C1 and C4 (para to the hydroxy substituent at atom C1) are significantly smaller than 120°, while the other four C—C—C angles are greater than 120°. These distortions also result from the steric effect of the lone pairs of electrons on the O atom of the ionized hydroxy group.

A search of the Cambridge Structural Database (Allen, 2002) for structures containing ionized dinitrophenol derivatives identifies several structures. Most are structures of the 2,4-dinitrophenolate anion, and one is of 2,6-dinitrophenolate (Andersen et al. 1989), but no structure is reported for the 2,5-dinitrophenlate anion. Thus, the present structure is the first describing the ionized form of 2,5-dinitrophenol.

The C—NO₂ bonds are short but well within the range [1.445 (7)–1.476 (7) Å] found for aryl groups doubly substituted by nitro groups (Allen, 2002). The O—N—O angles in the NO₂ groups are greater than 120°, also resulting from the steric effect of lone pairs of electrons on both the O atoms of the NO₂ groups. This effect is predicted by the valence-shell electron-pair repulsion theory (VSEPR; Gillespie, 1963, 1992). The distortion of the O—N—O angle from 120° is smaller in the NO₂ group at the position (at C2) ortho to the ionized hydroxy group (at C1) than in the nitro group at the meta position (at C5). This difference is likely to be due to interaction of atom O1 with atom O2 of the neighboring nitro group; both O atoms contain lone-pair electrons.

The optimized geometry of the ionized 2,5-dinitrophenolate anion calculated by DFT methods corresponding to the gas phase structure is close to planar (Fig. 2), and the optimized geometric parameters correlate well with those found in the crystal; however, the calculated C—C bond lengths in the aryl ring are slightly longer than the X-ray values, especially those of the C—C bonds (C1—C2 and C1—C6) that contain the C atom bonded to the ionized hydroxy group (O1). The shortening of the C1—O1 bond and the lengthening of the C1—C2 and C1—C6 bonds in the ionized 2,5-dinitrophenolate residue in relation to the non-ionized molecule (see Fig. 2) is more evident in the gas phase than in the crystal, since in the crystal the O atom is involved as an acceptor in hydrogen bonds with water molecules (Table 2). The delocalization of the charge of the ionized hydroxy group along the C1—O1 bond makes the value of its bond length closer to that of a double bond. The molecular orbital calculations also demonstrate that ionization of the hydroxy group leads to a decrease in the internal C—C—C angle at the C atom bonded to the O atom of the ionized hydroxy group, from 117.8° (in the non-ionized molecule) to 112.9°. In addition, the C2—C1—O1 angle increases by 2.5°, since in the non-ionized molecule an intramolecular O1—H1···O2 hydrogen bond is present (Fig. 2). Looking at the ab-initio and X-ray results in more detail, it should be stated that the ring distortions in the ionized 2,5-dinitrophenolate moiety result mainly from the ionization of the hydroxy group; the ionization changes the electronic structure as a result of the donating effect of ionized hydroxy group. The charge of the ionized hydroxy group (O⁻) is delocalized in the direction to the phenyl ring and in consequence disturbs the delocalization of the π electrons in the ring.

The X-ray geometry of the tetramethylammonium cation shows slight distortion from the tetrahedral geometry obtained by the molecular orbital calculations, due to the interaction with oppositely charged 2,5-dinitrophenlate ions in the crystal; the calculated four C—N bonds and six C—N—C angles in the isolated (CH₃)₄N⁺ ion are equivalent and equal to 1.510 Å and 109.47°, respectively. The calculated N—C bonds in the tetramethylammonium cation are longer than those found in the crystal; the shortening is probably due to the interactions present in the crystal. As can be seen from Fig. 1, the displacement ellipsoids for the cation methyl groups are a little large. However, libration analysis of rigid-body motion according to the method of Schomaker & Trueblood (1968) also predicts similar differences between the gas-phase and solid-state C—N bond lengths and C—N—C angles (the differences between the corrected and uncorrected C—N distances in the X-ray structure are smaller than 3σ). Thus the distortion of the tetramethylammonium cation from tetrahedral symmetry is probably due to interactions with oppositely charged 2,5-dinitrophenolate moieties. In the crystal packing (Fig. 3), the tetramethylammonium cations are surrounded by 2,5-dinitrophenolate anions that join with the water molecules to form a dimeric structure.

In the crystal structure of (I), 2,5-dinitrophenolate anions related by an inversion center and translation (symmetry code: 1 − x, −y, 1 − z) are stacked to form a column along the [100] direction (Fig. 3). The aryl rings within the stack are separated by 3.36 (1) Å. This value is consistent with the requirements of π–π stacking interaction and clearly accommodates the 3.4 Å distance required for the overlapping π aromatic ring system (Pauling, 1960); these interactions can contribute to the shortening of the C—C bonds within the ring in relation to the gas-phase structure as obtained by molecular orbital calculations. Two water molecules interact via O—H···O hydrogen bonds (Table 2) with two 2,5-dinitrophenolate anions related by the inversion center (Table 2) to form a dimeric structure. The tetramethylammonium cations are located in the holes between the O—H···O hydrogen bonded dimers of water and 2,5-dinitrophenolate moities, and interact by a combination of the van der Waals and ionic interactions with the oppositely charged 2,5-dinitrophenolate anions.

S2. Experimental

Tetramethylammonium hydroxide and 2,5-dinitrophenol in a 1:1 molar ratio were dissolved in water; after several days, pink single crystals formed, which proved to be suitable for single-crystal X-ray diffraction.

Computing details

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

Figures

	Fig. 1. A view of the molecular structure of (I), showing displacement ellipsoids at the 50% probability level and H atoms as spheres of arbitrary radii.
	Fig. 2. The results of the optimized molecular orbital calculations (Å, °) for the 2,5-dinitrophenolate anion (top) and non-ionized 2,5-dinitrophenol (bottom), to illustrate the donnating effect of the O atom of the ionized hydroxy group.
	Fig. 3. A view of the crystal packing, showing the stacking structure and the O—H···O hydrogen-bonded dimers. H atoms have been omited for clarity.

Tetramethylammonium 2,5-dinitrophenolate monohydrate

Crystal data

C4H12N^+·C6H3N2O5^−·H2O	Z = 2
Mr = 275.27	F(000) = 292
Triclinic, P1	Dx = 1.332 Mg m^−3 Dm = 1.33 Mg m^−3 Dm measured by floatation in the mixture of chloroform/1,2-dichloropropane
Hall symbol: -P 1	Mo Kα radiation, λ = 0.71073 Å
a = 6.808 (1) Å	Cell parameters from 1225 reflections
b = 9.995 (2) Å	θ = 3.2–28.0°
c = 11.421 (2) Å	µ = 0.11 mm^−1
α = 106.38 (2)°	T = 293 K
β = 104.80 (2)°	Paralellepiped, pink
γ = 102.23 (2)°	0.32 × 0.27 × 0.22 mm
V = 686.3 (3) Å^3

Data collection

Kuma KM-4 diffractometer	3291 independent reflections
Radiation source: fine-focus sealed tube	2196 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.012
Detector resolution: 1024x1024 with blocks 2x2 pixels mm^-1	θmax = 28.0°, θmin = 3.2°
ω scans	h = −8→7
Absorption correction: analytical (face-indexed; SHELXTL; Sheldrick, 1990)	k = −13→13
Tmin = 0.963, Tmax = 0.974	l = −14→15
8043 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) = 0.064	w = 1/[σ^2(Fo^2) + (0.0192P)^2] where P = (Fo^2 + 2Fc^2)/3
S = 1.00	(Δ/σ)max < 0.001
3291 reflections	Δρmax = 0.12 e Å^−3
183 parameters	Δρmin = −0.12 e Å^−3
0 restraints	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.037 (2)

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
C1	0.29807 (18)	0.18171 (14)	0.49353 (12)	0.0515 (3)
C2	0.31451 (18)	0.09588 (14)	0.57434 (12)	0.0484 (3)
C3	0.24927 (18)	−0.05542 (14)	0.52451 (13)	0.0487 (3)
H3	0.2631	−0.1062	0.5815	0.064*
C4	0.16540 (18)	−0.13276 (13)	0.39477 (13)	0.0523 (3)
H4	0.1218	−0.2346	0.3619	0.064*
C5	0.14844 (17)	−0.05180 (14)	0.31420 (11)	0.0509 (3)
C6	0.20846 (18)	0.09638 (13)	0.35884 (12)	0.0507 (3)
H6	0.1906	0.1440	0.2995	0.061*
N1	0.40279 (18)	0.16458 (15)	0.71323 (11)	0.0562 (3)
N2	0.06143 (17)	−0.12978 (15)	0.17232 (11)	0.0565 (3)
O1	0.35274 (15)	0.32038 (10)	0.53181 (8)	0.0812 (3)
O2	0.4716 (2)	0.29680 (14)	0.76263 (10)	0.0724 (5)
O3	0.41611 (18)	0.08932 (12)	0.78045 (9)	0.0617 (4)
O4	0.00922 (18)	−0.26199 (12)	0.13078 (10)	0.0630 (4)
O5	0.04471 (18)	−0.05940 (12)	0.10185 (9)	0.0694 (4)
N3	0.66820 (17)	0.32464 (11)	0.16506 (9)	0.0603 (3)
O6	0.24871 (16)	0.46115 (12)	0.35502 (11)	0.0937 (4)
H61	0.289 (3)	0.4125 (18)	0.4118 (17)	0.141*
H62	0.354 (3)	0.5375 (18)	0.3732 (18)	0.141*
C7	0.7248 (2)	0.37880 (16)	0.30752 (13)	0.0808 (5)
H7A	0.8752	0.3979	0.3471	0.126*
H7B	0.6473	0.3062	0.3328	0.126*
H7C	0.6885	0.4675	0.3353	0.126*
C8	0.7793 (3)	0.43892 (16)	0.12623 (17)	0.0798 (6)
H8A	0.7387	0.5256	0.1550	0.122*
H8B	0.7409	0.4046	0.0336	0.122*
H8C	0.9307	0.4612	0.1648	0.122*
C9	0.7430 (3)	0.19690 (16)	0.12205 (16)	0.0819 (6)
H9A	0.8916	0.2210	0.1701	0.128*
H9B	0.7232	0.1709	0.0313	0.128*
H9C	0.6627	0.1157	0.1368	0.128*
C10	0.4361 (3)	0.2863 (2)	0.10159 (16)	0.0879 (7)
H10A	0.3650	0.2003	0.1141	0.122*
H10B	0.4049	0.2675	0.0105	0.122*
H10C	0.3872	0.3661	0.1388	0.122*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C1	0.0467 (8)	0.0529 (8)	0.0539 (9)	0.0157 (7)	0.0149 (7)	0.0190 (7)
C2	0.0478 (8)	0.0519 (9)	0.0490 (8)	0.0204 (7)	0.0165 (6)	0.0192 (7)
C3	0.0518 (8)	0.0538 (9)	0.0532 (9)	0.0206 (7)	0.0179 (7)	0.0204 (8)
C4	0.0524 (8)	0.0506 (8)	0.0562 (9)	0.0178 (7)	0.0205 (7)	0.0200 (8)
C5	0.0487 (7)	0.0558 (8)	0.0464 (8)	0.0133 (6)	0.0138 (6)	0.0153 (7)
C6	0.0490 (8)	0.0582 (8)	0.0503 (8)	0.0166 (7)	0.0168 (6)	0.0269 (7)
N1	0.0568 (8)	0.0558 (10)	0.0551 (9)	0.0222 (8)	0.0169 (7)	0.0190 (8)
N2	0.0532 (8)	0.0551 (9)	0.0520 (8)	0.0136 (7)	0.0147 (6)	0.0157 (8)
O1	0.0978 (8)	0.0537 (6)	0.0742 (7)	0.0131 (6)	0.0121 (6)	0.0190 (5)
O2	0.0621 (14)	0.0614 (9)	0.0578 (7)	0.0129 (10)	0.0126 (8)	0.0141 (7)
O3	0.0619 (10)	0.0606 (9)	0.0577 (7)	0.0270 (7)	0.0213 (6)	0.0229 (7)
O4	0.0612 (10)	0.0604 (7)	0.0612 (7)	0.0118 (7)	0.0117 (6)	0.0106 (6)
O5	0.0741 (11)	0.0639 (8)	0.0608 (7)	0.0209 (7)	0.0184 (6)	0.0212 (6)
N3	0.0650 (7)	0.0601 (7)	0.0521 (7)	0.0157 (6)	0.0161 (6)	0.0158 (5)
O6	0.0679 (7)	0.1044 (9)	0.0950 (9)	0.0139 (6)	0.0056 (6)	0.0439 (7)
C7	0.0716 (12)	0.0769 (13)	0.0517 (10)	0.0171 (10)	0.0173 (8)	0.0143 (9)
C8	0.0847 (14)	0.0778 (12)	0.0913 (16)	0.0189 (11)	0.0145 (12)	0.0146 (12)
C9	0.0925 (18)	0.0799 (11)	0.0823 (13)	0.0198 (12)	0.0190 (13)	0.0114 (10)
C10	0.0702 (11)	0.087 (2)	0.0687 (12)	0.0191 (12)	0.0187 (9)	0.0156 (12)

Geometric parameters (Å, º)

C1—O1	1.2677 (13)	N3—C10	1.4732 (17)
C1—C6	1.4235 (16)	N3—C9	1.4764 (16)
C1—C2	1.4264 (16)	N3—C7	1.4776 (15)
C2—C3	1.3816 (15)	O6—H61	0.936 (17)
C2—N1	1.4397 (16)	O6—H62	0.861 (17)
C3—C4	1.3616 (16)	C7—H7A	0.9600
C3—H3	0.9300	C7—H7B	0.9600
C4—C5	1.3857 (15)	C7—H7C	0.9600
C4—H4	0.9300	C8—H8A	0.9600
C5—C6	1.3534 (15)	C8—H8B	0.9600
C5—N2	1.4785 (16)	C8—H8C	0.9600
C6—H6	0.9300	C9—H9A	0.9600
N1—O2	1.2109 (13)	C9—H9B	0.9600
N1—O3	1.2176 (13)	C9—H9C	0.9600
N2—O4	1.2078 (13)	C10—H10A	0.9600
N2—O5	1.2091 (13)	C10—H10B	0.9600
N3—C8	1.4711 (16)	C10—H10C	0.9600
O1—C1—C6	120.45 (12)	C10—N3—C7	110.67 (11)
O1—C1—C2	126.06 (12)	C9—N3—C7	110.13 (11)
C6—C1—C2	113.49 (11)	H61—O6—H62	108.3 (17)
C3—C2—C1	122.34 (12)	N3—C7—H7A	109.5
C3—C2—N1	116.80 (12)	N3—C7—H7B	109.5
C1—C2—N1	120.85 (12)	H7A—C7—H7B	109.5
C4—C3—C2	122.23 (12)	N3—C7—H7C	109.5
C4—C3—H3	118.9	H7A—C7—H7C	109.5
C2—C3—H3	118.9	H7B—C7—H7C	109.5
C3—C4—C5	116.52 (12)	N3—C8—H8A	109.5
C3—C4—H4	121.7	N3—C8—H8B	109.5
C5—C4—H4	121.7	H8A—C8—H8B	109.5
C6—C5—C4	123.22 (11)	N3—C8—H8C	109.5
C6—C5—N2	117.83 (12)	H8A—C8—H8C	109.5
C4—C5—N2	118.95 (12)	H8B—C8—H8C	109.5
C5—C6—C1	122.19 (12)	N3—C9—H9A	109.5
C5—C6—H6	118.9	N3—C9—H9B	109.5
C1—C6—H6	118.9	H9A—C9—H9B	109.5
O2—N1—O3	120.15 (13)	N3—C9—H9C	109.5
O2—N1—C2	119.96 (13)	H9A—C9—H9C	109.5
O3—N1—C2	119.81 (13)	H9B—C9—H9C	109.5
O4—N2—O5	122.24 (13)	N3—C10—H10A	109.5
O4—N2—C5	118.70 (13)	N3—C10—H10B	109.5
O5—N2—C5	119.06 (12)	H10A—C10—H10B	109.5
C8—N3—C10	109.42 (12)	N3—C10—H10C	109.5
C8—N3—C9	106.68 (12)	H10A—C10—H10C	109.5
C10—N3—C9	110.53 (12)	H10B—C10—H10C	109.5
C8—N3—C7	109.32 (11)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O6—H61···O1	0.936 (17)	1.868 (17)	2.8021 (15)	174.9 (17)
O6—H62···O1^i	0.861 (17)	2.001 (18)	2.8119 (17)	156.5 (17)

Symmetry code: (i) −x+1, −y+1, −z+1.