4-Di­methyl­amino­pyridinium bromide

The crystal and molecular structure of 4-di­methyl­amino­pyridinium bromide, C₇H₁₁N₂^+.Br^-, (I), is built up by hydrogen-bonded dimers of crystallographic 222 symmetry and four short C-H...halogen contacts. It is remarkable that (I) and 4-di­methyl­amino­pyridinium chloride are not isostructural.

S1. Comment

It is a long tradition in our group to investigate the formation of Lewis-acid-base complexes of the silicon halides (Adley et al., 1972; Campbell-Ferguson & Ebsworth, 1966; Fleischer et al., 1996; Klebe et al., 1985; Spangenberg, 1999).

In the course of our research work several reactions of different silanes with tertiary organic nitrogen bases were performed. The afforded reaction products are very susceptible to moisture and, as a result of that, it sometimes happens that only crystals of the hydrolysed compounds are obtained (Hensen et al., 1998; Bolte & Kettner, 1998).

The molecule of (I) with C2_v symmetry has crystallographic C2 symmetry. The atoms Br1, H1, N1, C4 and N4 are located on a twofold rotation axis. The NH group forms a bifurcated hydrogen bond to two symmetry equivalent Br⁻ ions (H1···Br1 2.80 Å, N1—H1···Br1 132.7°) resulting in a dimer of crystallographic 222 symmetry. Furthermore, the crystal packing is stabilized by short Br···H contacts: H2···Br1 2.90 Å, C2—H2···Br1 124.7°, and H3···Br1ⁱ 2.99 Å, C3—H3···Br1ⁱ 134.2° [symmetry code: (i) 3/2 − x,1/2 − y,-z]. \scheme

(I) crystallizes in planes parallel to (100). However, the molecules in a plane are not exactly coplanar, but twisted by 4.88 (3)° relative to each other.

The molecular geometry of the title compound is as expected. Bond lengths and angles adopt the usual values.

It is remarkable that the exchange of Br⁻ and Cl⁻ in 4-dimethylaminopyridinium complexes leads to different crystal structures: (I) and 4-dimethylaminopyridinium chloride (Bryant & King, 1992) are not isostructural. This lack of isostructurality has already been observed for other small pyridinium complexes (Faber et al., 1999)

As previously published structures of 4-dimethylaminopyridinium suggest (Biradha et al., 1995; Chao et al., 1977), 4-dimethylaminopyridinium changes its packing arrangement rather easily as other small molecules are incorporated into the crystal.

S2. Experimental

The title compound was obtained accidentally as part of solubility studies on silicon complexes resulting from the reaction of SiBr₂Cl₂ with 4-dimethylaminopyridine (DMAP). In chloroforme, DMAP reacts with SiBr₂Cl₂ to yield a white powder that is readily dissolved in methyl or ethyl alcohol. In hot propanol or hot butanol the substance dissolves equally well and yields needles of the title compound upon cooling.

S3. Refinement

The data were collected at room temperature, because we observed that the crystals underwent an irreversible phase transition upon cooling. The data collection nominally covered a sphere of reciprocal space, by a combination of seven sets of exposures; each set had a different ϕ angle for the crystal and each exposure covered 0.3° in ω. The crystal-to-detector distance was 4.0 cm. Crystal decay was monitored by repeating the initial frames at the end of data collection and analyzing the duplicate reflections.

All H atoms were located by difference Fourier synthesis and refined with fixed individual displacement parameters [U(H) = 1.5 U_eq(C_methyl), U(H) = 1.2 U_eq(C) or U(H) = 1.2 U_eq(N)] using a riding model with C—H(methyl) = 0.96, C—H(aromatic) = 0.93, or N—H = 0.86 Å, respectively.

The methyl groups attached to the aromatic ring were allowed to rotate about their local threefold axis.

Computing details

Data collection: SMART (Siemens, 1995); cell refinement: SMART (Siemens, 1995); data reduction: SAINT (Siemens, 1995); program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997).

(I)

Crystal data

C7H11N2^+·Br^−	F(000) = 1632
Mr = 203.09	Dx = 1.536 Mg m^−3
Orthorhombic, Fddd	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -F 2uv 2vw	Cell parameters from 502 reflections
a = 6.9804 (8) Å	θ = 1–20°
b = 19.256 (2) Å	µ = 4.61 mm^−1
c = 26.131 (3) Å	T = 293 K
V = 3512.4 (7) Å^3	Plate, colourless
Z = 16	0.30 × 0.20 × 0.10 mm

Data collection

Siemens CCD three-circle diffractometer	782 independent reflections
Radiation source: fine-focus sealed tube	506 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.081
ω scans	θmax = 25.0°, θmin = 3.1°
Absorption correction: empirical (using intensity measurements) (SADABS; Sheldrick, 1996)	h = −8→8
Tmin = 0.338, Tmax = 0.656	k = −22→22
16371 measured reflections	l = −30→30

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)] = 0.036	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.100	H atoms treated by a mixture of independent and constrained refinement
S = 1.02	w = 1/[σ^2(Fo^2) + (0.0552P)^2 + 4.7275P] where P = (Fo^2 + 2Fc^2)/3
782 reflections	(Δ/σ)max < 0.001
49 parameters	Δρmax = 0.24 e Å^−3
0 restraints	Δρmin = −0.25 e Å^−3

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	0.6250	0.1250	0.04610 (2)	0.0789 (3)
N1	0.6250	0.2684 (3)	0.1250	0.0857 (18)
H1	0.6250	0.2238	0.1250	0.103*
C2	0.6323 (7)	0.3009 (3)	0.0821 (2)	0.0774 (13)
H2	0.6379	0.2755	0.0519	0.093*
C3	0.6320 (6)	0.3706 (3)	0.07967 (19)	0.0723 (12)
H3	0.6364	0.3927	0.0480	0.087*
C4	0.6250	0.4105 (3)	0.1250	0.0573 (13)
N4	0.6250	0.4797 (3)	0.1250	0.105 (2)
C41	0.6313 (11)	0.5190 (4)	0.0780 (5)	0.163 (3)
H41A	0.6611	0.4886	0.0500	0.244*
H41B	0.7279	0.5543	0.0805	0.244*
H41C	0.5089	0.5404	0.0721	0.244*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Br1	0.1215 (6)	0.0590 (4)	0.0563 (4)	−0.0044 (4)	0.000	0.000
N1	0.059 (3)	0.051 (3)	0.147 (6)	0.000	−0.020 (6)	0.000
C2	0.055 (3)	0.078 (3)	0.099 (3)	0.003 (3)	0.000 (3)	−0.030 (3)
C3	0.050 (2)	0.097 (3)	0.070 (3)	0.011 (4)	0.008 (2)	0.015 (2)
C4	0.034 (2)	0.044 (3)	0.095 (4)	0.000	0.003 (3)	0.000
N4	0.063 (3)	0.051 (3)	0.201 (8)	0.000	0.026 (6)	0.000
C41	0.109 (5)	0.089 (4)	0.291 (10)	0.004 (4)	−0.002 (7)	0.103 (6)

Geometric parameters (Å, º)

N1—C2	1.284 (6)	C4—N4	1.332 (7)
N1—C2^i	1.285 (6)	C4—C3^i	1.413 (6)
C2—C3	1.344 (7)	N4—C41^i	1.444 (9)
C3—C4	1.412 (6)	N4—C41	1.444 (9)
C2—N1—C2^i	121.7 (6)	C3—C4—C3^i	114.1 (5)
N1—C2—C3	121.9 (5)	C4—N4—C41^i	121.6 (5)
C2—C3—C4	120.2 (5)	C4—N4—C41	121.6 (5)
N4—C4—C3	122.9 (3)	C41^i—N4—C41	116.7 (9)
N4—C4—C3^i	122.9 (3)
C2^i—N1—C2—C3	−0.2 (3)	C3—C4—N4—C41^i	−179.7 (4)
N1—C2—C3—C4	0.5 (7)	C3^i—C4—N4—C41^i	0.3 (4)
C2—C3—C4—N4	179.8 (3)	C3—C4—N4—C41	0.3 (4)
C2—C3—C4—C3^i	−0.2 (3)	C3^i—C4—N4—C41	−179.7 (4)

Symmetry code: (i) −x+5/4, y, −z+1/4.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1···Br1	0.86	2.80	3.446 (4)	133
N1—H1···Br1^i	0.86	2.80	3.446 (4)	133
C2—H2···Br1	0.93	2.90	3.516 (5)	125
C3—H3···Br1^ii	0.93	2.99	3.699 (5)	134

Symmetry codes: (i) −x+5/4, y, −z+1/4; (ii) −x+3/2, −y+1/2, −z.