2-Amino-6-bromo­pyridinium bromide

The title compound, C₅H₆BrN₂⁺·Br⁻, crystallizes in the centrosymmetric space group Cmca with all atoms lying on a crystallographic mirror plane at (0, y, z). The ion pairs pack as ribbons through N—H...Br hydrogen bonds and Br...Br halogen interactions. The ribbons are linked through weak C—H...Br interactions to form layers which stack perpendic­ular to the a axis.

Key indicators

• Single-crystal X-ray study
• T = 293 K
• Mean [sigma](C-C) = 0.012 Å
• R factor = 0.040
• wR factor = 0.116
• Data-to-parameter ratio = 13.4

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry

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S1. Comment

The title compound, 2-amino-6-bromopyridinium bromide, (1), a precursor for 2-bromo-6-iodopyridine (Holmes et al., 2002), was also of interest to us for comparison of weak N—H···Br hydrogen bonding with Br⁻···Br—C halogen bonding. The conversion of commercially and synthetically abundant amino derivatives to halogens has traditionally been accomplished using Sandmeyer-type conditions and is very useful in organic synthesis (Lavastre et al., 1997; Smith & Ho, 1990). The salt crystallizes in space group Cmca with all non-H atoms lying on a crystallographic mirror plane at (0, y, z) (Fig. 1). The geometric parameters for (1) are generally comparable to those found in related derivatives, such as 2-bromopyridinium bromide (Freytag & Jones, 2001), 2-chloro-6-dimethylamino-3,5-pyridinedicarbaldehyde (Lai et al., 1995), 7-amino-5-bromo-4-methyl-2-oxo-1,2,3,4-tetrahydro-1,6-naphthyridine- 8-carbonitrile monohydrate (De Anderez et al., 1992), and 6-bromo-3,5-difluoro-2-piperidyl-4-(1,2,2,2-tetrafluoro-1- trifluoromethylethyl)pyridine (Chambers et al., 2001) with the expected changes upon protonation at the pyridine N atom. The short C—NH₂ bond length and distortions exhibited in the ring distances indicate a substantial delocalization of the lone pair on the amine nitrogen into the π-system of the pyridine ring.

Ion-pairs of (1) form N—H···Br hydrogen bonded ribbons with adjacent pairs related by a b-glide operation. There is also a halogen bond within the ribbon structure between the bromide anion and the bromine substituent [Br1···Br2A = 3.5484 (13) Å and C—Br1···Br2A = 179.43 (19)°, where Br2A is related to Br2 by (x, 1/2 + y, 1/2 − z)]. Weak C—H···Br interactions link the ribbons in the c direction (Fig. 2). These layers stack to complete the structure with neighboring layers related by C-centering. The packing of (1) is quite similar to that of 2-bromopyridinium bromide (Freytag & Jones, 2001) with the only significant difference being the alternation of direction of the ribbons in (1) as opposed to the polar nature of the ribbon packing in the simpler analogue.

S2. Experimental

2-Amino-6-bromopyridinium bromide was synthesized using a literature method (Johnson et al., 1962) by slowly dissolving 3-hydroxypentanedinitrile in a 33 wt% solution of hydrogen bromide in glacial acetic acid with cooling and stirring. The resultant dark yellow precipitate was removed by filtration and allowed to air dry, affording a quantitative yield of the desired product. Diffraction quality crystals of 2-amino-6-bromopyridinium bromide were obtained by slow evaporation of an ethanol solution at room temperature.

Computing details

Data collection: AFC7 Diffractometer Control Software (Molecular Structure Corporation/Rigaku, 1997); cell refinement: AFC7 Diffractometer Control Software; data reduction: TEXSAN (Molecular Structure Corporation/Rigaku, 1997); program(s) used to solve structure: SHELXTL (Sheldrick, 2000); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL.

Figures

	Fig. 1. The molecular structure of (1). Displacement ellipsoids are shown at the 50% probability level and H atoms are of arbitrary radii.
	Fig. 2. Hydrogen-bonded layer of (1). Dashed lines represent N—H···Br and C—H···Br hydrogen-bonds. Filled dashed lines represent Br···Br halogen bonds.

2-amino-6-bromo-pyridinium bromide

Crystal data

C5H6BrN2^+·Br^−	Dx = 2.161 Mg m^−3
Mr = 253.94	Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, Cmca	Cell parameters from 25 reflections
a = 6.923 (1) Å	θ = 2.5–25.0°
b = 11.616 (2) Å	µ = 10.30 mm^−1
c = 19.413 (4) Å	T = 293 K
V = 1561.1 (5) Å^3	Plate, brown
Z = 8	0.24 × 0.21 × 0.12 mm
F(000) = 960

Data collection

Rigaku AFC-7 diffractometer	580 reflections with I > 2σ(I)
Radiation source: 18 kW rotating anode	Rint = 0.000
Graphite monochromator	θmax = 25.0°, θmin = 2.1°
ω/2θ scans	h = 0→8
Absorption correction: ψ scan (TEXSAN; Molecular Structure Corporation/Rigaku, 1997)	k = 0→13
Tmin = 0.082, Tmax = 0.291	l = 0→23
750 measured reflections	3 standard reflections every 100 reflections
750 independent reflections	intensity decay: 1%

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)] = 0.040	H-atom parameters constrained
wR(F^2) = 0.116	w = 1/[σ^2(Fo^2) + (0.0873P)^2 + 0.9155P] where P = (Fo^2 + 2Fc^2)/3
S = 1.03	(Δ/σ)max < 0.001
750 reflections	Δρmax = 0.80 e Å^−3
56 parameters	Δρmin = −0.81 e Å^−3
0 restraints	Extinction correction: SHELXTL, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0020 (4)

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
Br1	0.0000	0.49775 (7)	0.37388 (4)	0.0633 (4)
Br2	0.0000	0.29335 (7)	0.17222 (3)	0.0522 (4)
N1	0.0000	0.2665 (5)	0.3446 (3)	0.0436 (14)
H1	0.0000	0.2937	0.3034	0.052*
N2	0.0000	0.0845 (6)	0.2980 (3)	0.067 (2)
H2	0.0000	0.0081	0.3058	0.081*
H3	0.0000	0.0963	0.2522	0.081*
C1	0.0000	0.3411 (7)	0.3973 (3)	0.0467 (17)
C2	0.0000	0.3044 (8)	0.4623 (4)	0.062 (2)
H2A	0.0000	0.3561	0.4989	0.074*
C3	0.0000	0.1852 (9)	0.4736 (4)	0.070 (3)
H3A	0.0000	0.1572	0.5185	0.084*
C4	0.0000	0.1105 (7)	0.4206 (4)	0.060 (2)
H4A	0.0000	0.0317	0.4289	0.071*
C5	0.0000	0.1523 (6)	0.3526 (3)	0.0471 (18)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Br1	0.1028 (8)	0.0406 (6)	0.0463 (6)	0.000	0.000	−0.0061 (4)
Br2	0.0752 (6)	0.0443 (5)	0.0370 (5)	0.000	0.000	0.0026 (3)
N1	0.063 (4)	0.036 (3)	0.031 (3)	0.000	0.000	0.002 (3)
N2	0.129 (6)	0.035 (4)	0.038 (3)	0.000	0.000	−0.005 (3)
C1	0.066 (5)	0.038 (4)	0.036 (3)	0.000	0.000	−0.007 (3)
C2	0.096 (6)	0.057 (5)	0.032 (3)	0.000	0.000	−0.007 (4)
C3	0.105 (7)	0.071 (6)	0.034 (4)	0.000	0.000	0.012 (4)
C4	0.098 (7)	0.038 (4)	0.042 (4)	0.000	0.000	0.015 (4)
C5	0.059 (4)	0.041 (5)	0.041 (4)	0.000	0.000	−0.001 (3)

Geometric parameters (Å, º)

Br1—C1	1.876 (8)	C2—C3	1.402 (12)
N1—C5	1.336 (10)	C2—H2A	0.9300
N1—C1	1.341 (8)	C3—C4	1.346 (11)
N1—H1	0.8600	C3—H3A	0.9300
N2—C5	1.320 (9)	C4—C5	1.406 (9)
N2—H2	0.9000	C4—H4A	0.9300
N2—H3	0.9001	Br1—Br2^i	3.5484 (13)
C1—C2	1.331 (10)
C5—N1—C1	123.5 (6)	C3—C2—H2A	121.2
C5—N1—H1	118.3	C4—C3—C2	121.1 (7)
C1—N1—H1	118.3	C4—C3—H3A	119.4
C5—N2—H2	117.0	C2—C3—H3A	119.4
C5—N2—H3	134.6	C3—C4—C5	119.7 (8)
H2—N2—H3	108.4	C3—C4—H4A	120.2
C2—C1—N1	121.1 (7)	C5—C4—H4A	120.2
C2—C1—Br1	122.7 (6)	N2—C5—N1	119.9 (6)
N1—C1—Br1	116.2 (5)	N2—C5—C4	123.2 (7)
C1—C2—C3	117.6 (7)	N1—C5—C4	116.9 (7)
C1—C2—H2A	121.2	C1—Br1—Br2^i	179.43 (19)
C5—N1—C1—C2	0.000 (3)	C2—C3—C4—C5	0.000 (3)
C5—N1—C1—Br1	180.0	C1—N1—C5—N2	180.000 (1)
N1—C1—C2—C3	0.000 (3)	C1—N1—C5—C4	0.000 (2)
Br1—C1—C2—C3	180.000 (2)	C3—C4—C5—N2	180.000 (2)
C1—C2—C3—C4	0.000 (3)	C3—C4—C5—N1	0.000 (2)

Symmetry code: (i) x, y+1/2, −z+1/2.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N2—H3···Br1^ii	0.90	2.70	3.486 (7)	146
N2—H2···Br2^ii	0.90	2.53	3.431 (7)	180
N1—H1···Br2	0.86	2.55	3.360 (5)	158
C2—H2A···Br1^iii	0.93	3.00	3.923 (8)	174
C3—H3A···Br2^iv	0.93	3.04	3.864 (7)	149

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