Hydrogen bonding in isomorphous 2-methyl-4-nitro­anilinium bromide and iodide

The isomorphous structures of 2-methyl-4-nitro­anilinium bromide, C₇H₉N₂O₂⁺·Br^-, and 2-methyl-4-nitro­anilinium iodide, C₇H₉N₂O₂⁺·I^-, exhibit ionic layers separated by hydro­carbon layers. The hydro­carbon layer stacks head-to-head, while in the ionic layer, the ammonium groups and halide anions inter­act via hydrogen bonds to form infinite chains.

S1. Comment

As part of a study of the effect of anions on the crystal structures of anilinium halide salts, the crystal structures of two protonated 2-methyl-4-nitroaniline molecules are reported. The unprotonated case, O₂NC₇H₆NH₂, has been studied previously (Ferguson et al., 2001). The authors were investigating the effect that the substituents on the ring had on the packing arrangement and the hydrogen bonding, and they found that the molecules act as double donors and double acceptors of N—H···O hydrogen bonds. The introduction of a halide anion has a significant effect on the packing, as the ammonium groups form N—H···X hydrogen bonds only in the title compounds. We report here the isostructural salts 2-methyl-4-nitroanilinium bromide, (I), and 2-methyl-4-nitroanilinium iodide, (II). The detailed packing of (I) will be discussed, as well as the hydrogen bonding of (II).

The atomic numbering schemes of both compounds are shown in Fig. 1. Fig. 2 shows the one-dimensional arrangement of (I), in which a single layer of cations is embedded between two ionic layers, forming an alternating hydrocarbon–ionic structure. Within the ionic layer, the Br⁻ atoms and ammonium groups interact via hydrogen bonds. The cations pack head-to-head in layers parallel to the ab plane. In contrast, the unsubstituted 4-nitroanilinium bromide has a head-to-tail packing arrangement (Lemmerer & Billing, 2006).

Compound (II) has the same packing arrangement as (I) but differs only in the identity of the counterion, I1. The volume of the unit cell is unexpectedly less by 2.2 ų for the iodide case, even though the I⁻ ion has a larger ionic radius (2.20 Å) than bromide (1.96 Å) (Reference for standard values?). To compensate, the cations pack closer together.

Compound (II) has an extensive network of hydrogen bonding. In the crystal structure, the ions are linked together by N2—H2C···I1···H2B—N2—H2C···I1 hydrogen bonds, forming infinite chains parallel to [001]. At the same time, two adjacent chains are linked in the [100] direction by N2—H2A···I1 hydrogen bonds to form a connected set of infinite chains (Fig. 3). Overall, the hydrogen-bonding scheme is R₂²(8) (Bernstein et al., 1995). The hydrogen-bonded rings connect I⁻ ions and 2-methyl-4-nitroanilinium cations, which are related by a twofold screw axis. The chains form ladders, which are parallel to the ac plane. These ladders are then stacked perpendicular to this plane and are connected to the I⁻ ions of adjacent ladders by I1···H2C—N2—H2B···I1 hydrogen bonds, i.e. atoms H2C and H2B have bifurcated hydrogen bonds. These interactions are weaker, as the hydrogen-acceptor distances are 3.245 (1) and 3.238 (1) Å.

Compound (I) has the same hydrogen-bonding sequence in the c direction. However, due to the smaller van der Waals radius of Br, the ladders are connected by a single Br1···H2C—N2 hydrogen bond. The hydrogen-acceptor distance is 3.12 Å. Subsequently, there is only one bifurcated and three simple hydrogen bonds in (I), compared with three simple and two bifurcated hydrogen bonds in (II).

S2. Experimental

For the preparation of (I), 2-methyl-4-nitroaniline (0.196 g, 1.29 mmol) was added to 48% HBr (18 ml) and the precipitate dissolved by heating the solution. The solution was then cooled to room temperature and yellow crystals of (I) were grown by slow evaporation. Analysis, calculated for C₇H₉Br₁N₂O₂: C 36.1, H 3.9, N 12.0%; found: C 36.8, H 3.9, N 12.7%.

For the preparation of (II), 2-methyl-4-nitroaniline (0.204 g, 1.34 mmol) was added to 47% HI (1 ml) and the precipitate dissolved with ethanol (10 ml). Yellow crystals of (II) were grown by slow evaporation of the solution mixture. Elemental analysis was not deemed necessary.

S3. Refinement

H atoms bonded to C and N atoms were refined in idealized positions in the riding-model approximation, with C—H = 0.93 Å and N—H = 0.89 Å, and with U_iso(H) = 1.2U_eq(C) or 1.5U_eq(N). The NH₃ group was allowed to rotate but not to tip. The highest residual peak was 1.92 Å from Br1 in (I) and 1.90 Å from I1 in (II).

Computing details

For both compounds, data collection: SMART (Bruker, 1998); cell refinement: SAINT-Plus (Bruker, 1999); data reduction: SAINT-Plus; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997) and DIAMOND (Brandenburg, 1999); software used to prepare material for publication: WinGX (Farrugia, 1999) and PLATON (Spek, 2003).

Figures

	Fig. 1. The asymmetric units of (a) (I) and (b) (II), showing the atomic numbering schemes. Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii.
	Fig. 2. A packing diagram for (I), viewed along the c axis.
	Fig. 3. The one-dimensional chains of hydrogen bonds (dashed lines) between the ammonium heads and the I^− anions in (II). Atoms marked with an asterisk (*), hash (#), ampersand (&), `at' sign (@) or double prime ('') are at the symmetry positions (1 − x, 1 − y, 1 − z), (x, y, 1 + z), (1 − x, 1 − y, −z), (x, y, −1 + z) and (1 − x, 1 − y, 2 − z), respectively.
	Fig. 4. The weak hydrogen-bonding interactions (light-grey dashed lines) and strong interactions (black dashed lines) which stabilize the overall structure in (II). Atoms marked with the suffixes a, b, c, d, e and f are at the symmetry positions (1 − x, 1 − y, 1 − z), (1 − x, −1/2 + y, 1/2 − z), (1 − x, −1/2 + y, 3/2 − z), (1 − x, 1/2 + y, 3/2 − z), (x, 3/2 − y, 1/2 + z) and (x, 3/2 − y, −1/2 + z), respectively.

(I) 2-methyl-4-nitroanilinium bromide

Crystal data

C7H9N2O2^+·Br^−	F(000) = 464
Mr = 233.07	Dx = 1.561 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71069 Å
Hall symbol: -P 2ybc	Cell parameters from 837 reflections
a = 14.687 (5) Å	θ = 3.5–26.0°
b = 9.840 (5) Å	µ = 4.11 mm^−1
c = 6.877 (5) Å	T = 293 K
β = 93.836 (5)°	Needle, light yellow
V = 991.6 (9) Å^3	0.24 × 0.08 × 0.04 mm
Z = 4

Data collection

Bruker SMART CCD area-detector diffractometer	1435 reflections with I > 2σ(I)
ϕ and ω scans	Rint = 0.027
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	θmax = 25.3°, θmin = 1.4°
Tmin = 0.565, Tmax = 0.850	h = −17→17
5290 measured reflections	k = −11→10
1798 independent reflections	l = −7→8

Refinement

Refinement on F^2	92 restraints
Least-squares matrix: full	H-atom parameters constrained
R[F^2 > 2σ(F^2)] = 0.058	w = 1/[σ^2(Fo^2) + (0.1324P)^2 + 1.4593P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.220	(Δ/σ)max = 0.006
S = 1.21	Δρmax = 0.77 e Å^−3
1798 reflections	Δρmin = −0.96 e Å^−3
99 parameters

Special details

Experimental. absorption corrections were made using the program SADABS (Sheldrick, 1996).

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
C1	0.7034 (4)	0.4336 (6)	0.7821 (11)	0.0741 (18)
C2	0.7922 (4)	0.4827 (5)	0.8056 (10)	0.0802 (19)
C3	0.8652 (3)	0.3925 (7)	0.8192 (12)	0.085 (2)
H3	0.9246	0.4253	0.8349	0.102*
C4	0.8494 (4)	0.2534 (7)	0.8094 (12)	0.091 (2)
C5	0.7606 (5)	0.2043 (5)	0.7859 (11)	0.088 (2)
H5	0.7501	0.1112	0.7793	0.105*
C6	0.6876 (4)	0.2945 (7)	0.7723 (11)	0.082 (2)
H6	0.6282	0.2616	0.7566	0.098*
C7	0.8087 (10)	0.6328 (13)	0.817 (3)	0.107 (4)
H7A	0.8731	0.6499	0.832	0.161*
H7B	0.7798	0.6695	0.9266	0.161*
H7C	0.7837	0.6752	0.6995	0.161*
N1	0.9253 (9)	0.1586 (14)	0.824 (2)	0.114 (3)
N2	0.6270 (6)	0.5220 (11)	0.7677 (12)	0.084 (2)
H2A	0.5766	0.4751	0.7874	0.126*
H2B	0.6219	0.5592	0.6495	0.126*
H2C	0.6344	0.5872	0.8571	0.126*
O1	0.9989 (7)	0.1994 (15)	0.891 (2)	0.163 (4)
O2	0.9129 (9)	0.0386 (14)	0.7755 (18)	0.157 (4)
Br1	0.59478 (5)	0.60735 (7)	0.26192 (10)	0.0492 (4)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C1	0.074 (3)	0.084 (3)	0.065 (4)	0.004 (3)	0.011 (4)	0.003 (4)
C2	0.077 (3)	0.088 (3)	0.076 (5)	−0.001 (3)	0.009 (4)	0.002 (5)
C3	0.076 (3)	0.102 (4)	0.078 (5)	0.005 (3)	0.004 (5)	−0.001 (5)
C4	0.092 (3)	0.097 (4)	0.083 (5)	0.015 (3)	0.009 (4)	0.002 (5)
C5	0.100 (4)	0.083 (4)	0.080 (5)	0.007 (3)	0.005 (5)	0.001 (5)
C6	0.085 (3)	0.084 (3)	0.075 (5)	−0.001 (3)	0.006 (4)	0.001 (4)
C7	0.095 (7)	0.090 (4)	0.137 (11)	−0.008 (4)	0.005 (8)	0.003 (7)
N1	0.108 (4)	0.119 (5)	0.115 (6)	0.032 (4)	0.006 (5)	0.000 (6)
N2	0.082 (4)	0.095 (5)	0.075 (5)	0.012 (4)	0.005 (4)	0.007 (5)
O1	0.095 (5)	0.194 (9)	0.197 (11)	0.027 (6)	−0.005 (7)	0.010 (9)
O2	0.195 (10)	0.121 (6)	0.152 (9)	0.063 (6)	−0.006 (8)	−0.024 (7)
Br1	0.0581 (6)	0.0489 (5)	0.0408 (5)	−0.0089 (3)	0.0043 (3)	0.0000 (3)

Geometric parameters (Å, º)

C1—C2	1.39	C5—H5	0.93
C1—C6	1.39	C6—H6	0.93
C1—N2	1.418 (10)	C7—H7A	0.96
C2—C3	1.39	C7—H7B	0.96
C2—C7	1.498 (14)	C7—H7C	0.96
C3—C4	1.39	N1—O1	1.214 (16)
C3—H3	0.93	N1—O2	1.24 (2)
C4—C5	1.39	N2—H2A	0.89
C4—N1	1.451 (12)	N2—H2B	0.89
C5—C6	1.39	N2—H2C	0.89
C2—C1—C6	120	C1—C6—H6	120
C2—C1—N2	121.8 (6)	C2—C7—H7A	109.5
C6—C1—N2	118.2 (6)	C2—C7—H7B	109.5
C1—C2—C3	120	H7A—C7—H7B	109.5
C1—C2—C7	119.7 (7)	C2—C7—H7C	109.5
C3—C2—C7	120.3 (7)	H7A—C7—H7C	109.5
C4—C3—C2	120	H7B—C7—H7C	109.5
C4—C3—H3	120	O1—N1—O2	121.9 (13)
C2—C3—H3	120	O1—N1—C4	118.3 (13)
C5—C4—C3	120	O2—N1—C4	119.7 (12)
C5—C4—N1	119.7 (8)	C1—N2—H2A	109.5
C3—C4—N1	120.3 (8)	C1—N2—H2B	109.5
C4—C5—C6	120	H2A—N2—H2B	109.5
C4—C5—H5	120	C1—N2—H2C	109.5
C6—C5—H5	120	H2A—N2—H2C	109.5
C5—C6—C1	120	H2B—N2—H2C	109.5
C5—C6—H6	120
C6—C1—C2—C3	0	N1—C4—C5—C6	179.8 (9)
N2—C1—C2—C3	180.0 (7)	C4—C5—C6—C1	0
C6—C1—C2—C7	−179.7 (10)	C2—C1—C6—C5	0
N2—C1—C2—C7	0.2 (11)	N2—C1—C6—C5	−180.0 (7)
C1—C2—C3—C4	0	C5—C4—N1—O1	−162.3 (12)
C7—C2—C3—C4	179.7 (10)	C3—C4—N1—O1	17.5 (17)
C2—C3—C4—C5	0	C5—C4—N1—O2	15.1 (16)
C2—C3—C4—N1	−179.8 (9)	C3—C4—N1—O2	−165.0 (11)
C3—C4—C5—C6	0

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N2—H2A···Br1^i	0.89	2.64	3.490 (10)	159
N2—H2B···Br1	0.89	2.71	3.579 (8)	165
N2—H2C···Br1^ii	0.89	2.89	3.563 (9)	134
N2—H2C···Br1^iii	0.89	3.12	3.677 (10)	122

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

(II) 2-methyl-4-nitroanilinium iodide

Crystal data

C7H9N2O2^+·I^−	F(000) = 536
Mr = 280.06	Dx = 1.88 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 576 reflections
a = 14.671 (4) Å	θ = 3.5–19.7°
b = 9.830 (3) Å	µ = 3.20 mm^−1
c = 6.875 (2) Å	T = 293 K
β = 93.782 (7)°	Needle, yellow
V = 989.4 (5) Å^3	0.14 × 0.06 × 0.03 mm
Z = 4

Data collection

Bruker SMART CCD area-detector diffractometer	1047 reflections with I > 2σ(I)
ϕ and ω scans	Rint = 0.073
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	θmax = 25.5°, θmin = 1.4°
Tmin = 0.664, Tmax = 0.911	h = −15→17
5499 measured reflections	k = −11→11
1846 independent reflections	l = −8→8

Refinement

Refinement on F^2	92 restraints
Least-squares matrix: full	H-atom parameters constrained
R[F^2 > 2σ(F^2)] = 0.070	w = 1/[σ^2(Fo^2) + (0.1138P)^2] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.187	(Δ/σ)max = 0.004
S = 0.94	Δρmax = 4.27 e Å^−3
1846 reflections	Δρmin = −0.85 e Å^−3
99 parameters

Special details

Experimental. absorption corrections were made using the program SADABS (Sheldrick, 1996). The application of the twin law suggested by PLATON (Version 170106, Spek, 2003) [1 0 0, 0 − 1 0, 0 0 − 1] made no difference to the r-factor or the residual Q-peaks.

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
C1	0.7037 (4)	0.4315 (6)	0.7801 (11)	0.040 (2)
C2	0.7928 (5)	0.4796 (5)	0.8032 (12)	0.047 (2)
C3	0.8655 (4)	0.3886 (8)	0.8163 (12)	0.054 (3)
H3	0.9251	0.4209	0.8318	0.065*
C4	0.8490 (4)	0.2495 (7)	0.8062 (12)	0.054 (2)
C5	0.7599 (5)	0.2013 (5)	0.7830 (11)	0.049 (2)
H5	0.7489	0.1082	0.7762	0.059*
C6	0.6872 (4)	0.2923 (7)	0.7699 (11)	0.048 (2)
H6	0.6276	0.2601	0.7544	0.058*
C7	0.8118 (10)	0.6313 (13)	0.822 (3)	0.078 (5)
H7A	0.8758	0.6477	0.8107	0.117*
H7B	0.7944	0.6629	0.9459	0.117*
H7C	0.7772	0.679	0.7196	0.117*
N1	0.9243 (10)	0.1569 (15)	0.825 (2)	0.077 (3)
N2	0.6273 (6)	0.5232 (11)	0.7665 (12)	0.048 (2)
H2A	0.5756	0.4757	0.7561	0.071*
H2B	0.6307	0.5759	0.6619	0.071*
H2C	0.6283	0.5748	0.8729	0.071*
O1	0.9969 (8)	0.1965 (14)	0.896 (2)	0.123 (5)
O2	0.9106 (9)	0.0396 (14)	0.7834 (18)	0.111 (4)
I1	0.59475 (6)	0.60749 (8)	0.26188 (11)	0.0490 (4)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C1	0.042 (4)	0.048 (4)	0.033 (6)	0.001 (3)	0.011 (5)	0.001 (5)
C2	0.046 (4)	0.048 (4)	0.047 (6)	−0.004 (3)	0.007 (5)	0.006 (5)
C3	0.042 (5)	0.063 (4)	0.058 (7)	0.001 (4)	0.014 (6)	0.001 (6)
C4	0.059 (4)	0.056 (4)	0.047 (6)	0.010 (4)	0.010 (6)	0.004 (6)
C5	0.068 (5)	0.044 (5)	0.036 (6)	0.001 (3)	0.011 (6)	0.006 (5)
C6	0.051 (5)	0.050 (4)	0.044 (6)	−0.008 (3)	0.009 (5)	−0.002 (5)
C7	0.061 (9)	0.054 (5)	0.119 (13)	−0.009 (5)	0.004 (9)	0.000 (8)
N1	0.078 (5)	0.083 (6)	0.072 (8)	0.033 (5)	0.019 (6)	0.018 (6)
N2	0.045 (5)	0.058 (6)	0.040 (6)	0.004 (4)	0.006 (5)	0.011 (5)
O1	0.066 (6)	0.135 (10)	0.168 (14)	0.029 (6)	0.008 (7)	0.023 (9)
O2	0.133 (10)	0.084 (6)	0.115 (10)	0.061 (6)	−0.003 (8)	−0.006 (7)
I1	0.0580 (6)	0.0466 (5)	0.0425 (5)	−0.0084 (4)	0.0053 (4)	0.0000 (4)

Geometric parameters (Å, º)

C1—C2	1.39	C5—H5	0.93
C1—C6	1.39	C6—H6	0.93
C1—N2	1.436 (10)	C7—H7A	0.96
C2—C3	1.39	C7—H7B	0.96
C2—C7	1.521 (13)	C7—H7C	0.96
C3—C4	1.39	N1—O2	1.202 (19)
C3—H3	0.93	N1—O1	1.207 (17)
C4—C5	1.39	N2—H2A	0.89
C4—N1	1.431 (13)	N2—H2B	0.89
C5—C6	1.39	N2—H2C	0.89
C2—C1—C6	120	C1—C6—H6	120
C2—C1—N2	121.2 (6)	C2—C7—H7A	109.5
C6—C1—N2	118.8 (6)	C2—C7—H7B	109.5
C1—C2—C3	120	H7A—C7—H7B	109.5
C1—C2—C7	120.6 (7)	C2—C7—H7C	109.5
C3—C2—C7	119.4 (7)	H7A—C7—H7C	109.5
C4—C3—C2	120	H7B—C7—H7C	109.5
C4—C3—H3	120	O2—N1—O1	122.4 (14)
C2—C3—H3	120	O2—N1—C4	118.2 (14)
C3—C4—C5	120	O1—N1—C4	119.1 (14)
C3—C4—N1	119.4 (8)	C1—N2—H2A	109.5
C5—C4—N1	120.6 (8)	C1—N2—H2B	109.5
C4—C5—C6	120	H2A—N2—H2B	109.5
C4—C5—H5	120	C1—N2—H2C	109.5
C6—C5—H5	120	H2A—N2—H2C	109.5
C5—C6—C1	120	H2B—N2—H2C	109.5
C5—C6—H6	120
C6—C1—C2—C3	0	N1—C4—C5—C6	178.3 (9)
N2—C1—C2—C3	179.9 (8)	C4—C5—C6—C1	0
C6—C1—C2—C7	−177.9 (10)	C2—C1—C6—C5	0
N2—C1—C2—C7	2.0 (11)	N2—C1—C6—C5	−179.9 (8)
C1—C2—C3—C4	0	C3—C4—N1—O2	−167.6 (11)
C7—C2—C3—C4	177.9 (10)	C5—C4—N1—O2	14.0 (16)
C2—C3—C4—C5	0	C3—C4—N1—O1	18.3 (17)
C2—C3—C4—N1	−178.4 (9)	C5—C4—N1—O1	−160.0 (12)
C3—C4—C5—C6	0

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N2—H2A···I1^i	0.89	2.62	3.495 (10)	166
N2—H2C···I1^ii	0.89	2.77	3.568 (9)	150
N2—H2B···I1	0.89	2.78	3.568 (8)	148
N2—H2B···I1^iii	0.89	3.24	3.662 (10)	112
N2—H2C···I1^iii	0.89	3.24	3.662 (10)	112

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