organic compounds
Tetra-n-butylammonium bromide: a redetermination at 150 K addressing the twinning
aChemistry Department, Loughborough University, Loughborough, LE11 3TU, England
*Correspondence e-mail: m.r.j.elsegood@lboro.ac.uk
The redetermined, low temperature (150 K), structure of tetra-n-butylammonium bromide, (C4H9)4N+·Br−, has been found to be merohedrally twinned via −1 0 0, 0 − 1 0, 1 0 1. The structure was previously determined, with low precision, no inclusion of H atoms and only the bromide ion refined with anisotropic displacement parameters, by Wang et al. (1995). Mol. Cryst. Liq. Cryst. Sci. Tech. A, 264, 115–129. The redetermined structure has considerably improved precision in all geometrical parameters, has all non-H atoms refined anisotropically, H atoms included, and is isomorphous with the iodide analogue. The structure is otherwise routine, with the shortest cation to anion contacts being between the bromide anion and the CH atoms close to the ammonium nitrogen centre at a distance of ca. 2.98–3.11 Å. Each anion makes eight such contacts to four different anions. The n-butyl chains are fully extended, adopting an all-anti conformation with approximate S4 point symmetry.
Related literature
The structure was previously determined by Wang et al. (1995). For the uses of tetra-n-alkylammonium salts and the isomorphous structure of tetra-n-butyl ammonium iodide, see: Prukała et al. (2007). For a related stucture, see: McMullan & Jeffrey (1959). For the conformation of n-butyl chains, see: Alder et al. (1990). For details of the Cambridge Structural Database, see: Fletcher et al. (1996); Allen (2002).
Experimental
Crystal data
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Data collection: APEX2 (Bruker, 2008); cell SAINT (Bruker, 2008); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008b); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008b) and PLATON (Spek, 2009); molecular graphics: SHELXTL (Sheldrick, 2008b); software used to prepare material for publication: SHELXTL and local programs.
Supporting information
https://doi.org/10.1107/S1600536811032612/rn2089sup1.cif
contains datablocks I, global. DOI:Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S1600536811032612/rn2089Isup2.hkl
Supporting information file. DOI: https://doi.org/10.1107/S1600536811032612/rn2089Isup3.cml
The title compound (I) was used as received and crystallized from an acetonitrile solution via vapour diffusion with diethylether to give colourless blocks.
H atoms were included in a riding model with constrained bond lengths: for CH2 = 0.99 and CH3 = 0.98 Å with Uiso(H) = 1.2 Ueq(CH2) and =1.5Ueq(CH3).
While many common reagents have had their crystal structures well determined, some many times, some deliberatly and many by accident, no good quality structure was available for the title compound, tetra-n-butylammonium bromide (I). Compound (I) is used in a number of synthesis applications (see Prukała et al., 2007, and references therein for further details) and as a source of the large tetra-n-butylammonium cation, which is useful in crystallizing large anions. A search of the Cambridge Structural Database (version 5.32 + 3 updates, Fletcher, et al., 1996, Allen, 2002) revealed just one reported structure of this compound with an R1 of 0.098 that had clearly been problematic (Wang et al., 1995). This earlier determination had only the bromide ion refined anisotropically and did not include hydrogen atoms in the model. The authors ruled out dynamic disorder as the cause of the difficulties and concluded that static disorder was the causeof the poor residual.
The crystals of (I) formed readily by vapour diffusion of diethyl ether into an acetonitrile solution. The data collection set-up was trouble free. After data reduction the structure did not solve readily with SHELXS (Sheldrick, 2008a); only the bromide, the nitrogen and two n-butyl chains being evident. When the structure failed to develop, the coordinates from the published structure were used as a starting point (Wang et al., 1995), but the R1 was ca. 35% for an isotropic model with all non-H atoms in the model.
was suspected and confirmed by the TWINROTMAT routine in PLATON (Spek, 2009). Application of the -1 0 0, 0 -1 0, 1 0 1, led to a reduction in R1 to ca. 5.0% at the same, isotropic, stage of Anisotropic and addition of H atoms, led to a good final R1 <3% with no adverse indicators. The ratio of major to minor twin components is 60.69: 39.31 (7)%The structure is isomorphous with that of the iodide analogue described in detail recently (Prukała et al., 2007). The n-butyl chains are fully extended adopting an all-anti conformation with approximate S4
(Alder et al., 1990). The bromide anion resides in a pocket between four cations, making four pairs of weak C—H···Br contacts in the range 2.98–3.11 Å to methylene hydrogens located one or two carbon atoms from the nitrogen cationic centre. The structures of the chloride and fluoride analogues have not been determined to date, although the of the hydrate of the chloride has been reported (McMullan & Jeffrey, 1959).The structure was previously determined by Wang et al. (1995). For the uses of tetra-n-alkylammonium salts and the isomorphous structure of tetra-n-butyl ammonium iodide, see: Prukała et al. (2007). For a related stucture, see: McMullan & Jeffrey (1959). For the conformation of n-butyl chains, see: Alder et al. (1990). For details of the Cambridge Structural Database, see: Fletcher et al. (1996); Allen (2002).
Data collection: APEX2 (Bruker, 2008); cell
SAINT (Bruker, 2008); data reduction: SAINT (Bruker, 2008); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008b); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008b) and PLATON (Spek, 2009); molecular graphics: SHELXTL (Sheldrick, 2008b); software used to prepare material for publication: SHELXTL (Sheldrick, 2008b) and local programs.Fig. 1. The asymmetric unit in the structure of (I) with displacement ellipsoids drawn at the 50% probability level. |
C16H36N+·Br− | F(000) = 1392 |
Mr = 322.37 | Dx = 1.176 Mg m−3 |
Monoclinic, C2/c | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: -C 2yc | Cell parameters from 7468 reflections |
a = 13.9773 (9) Å | θ = 2.6–30.1° |
b = 13.8623 (9) Å | µ = 2.25 mm−1 |
c = 20.0450 (14) Å | T = 150 K |
β = 110.383 (10)° | Block, colourless |
V = 3640.7 (4) Å3 | 0.41 × 0.31 × 0.16 mm |
Z = 8 |
Bruker APEXII CCD diffractometer | 5485 independent reflections |
Radiation source: fine-focus sealed tube | 4415 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.029 |
ω rotation with narrow frames scans | θmax = 30.5°, θmin = 1.1° |
Absorption correction: multi-scan (SADABS; Sheldrick, 2008a) | h = −19→19 |
Tmin = 0.459, Tmax = 0.715 | k = −18→19 |
21135 measured reflections | l = −28→28 |
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.028 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.073 | H-atom parameters constrained |
S = 1.04 | w = 1/[σ2(Fo2) + (0.0379P)2 + 0.7322P] where P = (Fo2 + 2Fc2)/3 |
5485 reflections | (Δ/σ)max = 0.001 |
168 parameters | Δρmax = 0.62 e Å−3 |
0 restraints | Δρmin = −0.24 e Å−3 |
C16H36N+·Br− | V = 3640.7 (4) Å3 |
Mr = 322.37 | Z = 8 |
Monoclinic, C2/c | Mo Kα radiation |
a = 13.9773 (9) Å | µ = 2.25 mm−1 |
b = 13.8623 (9) Å | T = 150 K |
c = 20.0450 (14) Å | 0.41 × 0.31 × 0.16 mm |
β = 110.383 (10)° |
Bruker APEXII CCD diffractometer | 5485 independent reflections |
Absorption correction: multi-scan (SADABS; Sheldrick, 2008a) | 4415 reflections with I > 2σ(I) |
Tmin = 0.459, Tmax = 0.715 | Rint = 0.029 |
21135 measured reflections |
R[F2 > 2σ(F2)] = 0.028 | 0 restraints |
wR(F2) = 0.073 | H-atom parameters constrained |
S = 1.04 | Δρmax = 0.62 e Å−3 |
5485 reflections | Δρmin = −0.24 e Å−3 |
168 parameters |
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. |
x | y | z | Uiso*/Ueq | ||
Br1 | 0.737682 (14) | 0.00074 (2) | 0.475441 (8) | 0.03037 (6) | |
N1 | 0.49621 (18) | 0.25167 (8) | 0.49516 (13) | 0.0172 (2) | |
C1 | 0.44659 (14) | 0.30519 (13) | 0.54096 (10) | 0.0195 (4) | |
H1A | 0.4927 | 0.3579 | 0.5664 | 0.023* | |
H1B | 0.3826 | 0.3352 | 0.5093 | 0.023* | |
C2 | 0.4221 (2) | 0.24346 (12) | 0.59514 (15) | 0.0252 (6) | |
H2A | 0.4848 | 0.2105 | 0.6258 | 0.030* | |
H2B | 0.3718 | 0.1934 | 0.5702 | 0.030* | |
C3 | 0.37887 (16) | 0.30443 (14) | 0.64107 (11) | 0.0253 (4) | |
H3A | 0.3187 | 0.3406 | 0.6101 | 0.030* | |
H3B | 0.4309 | 0.3519 | 0.6682 | 0.030* | |
C4 | 0.3476 (3) | 0.24244 (16) | 0.69282 (16) | 0.0302 (6) | |
H4A | 0.2984 | 0.1936 | 0.6663 | 0.045* | |
H4B | 0.3163 | 0.2833 | 0.7194 | 0.045* | |
H4C | 0.4081 | 0.2106 | 0.7261 | 0.045* | |
C5 | 0.42140 (14) | 0.17734 (13) | 0.45040 (10) | 0.0191 (4) | |
H5A | 0.4085 | 0.1292 | 0.4827 | 0.023* | |
H5B | 0.3559 | 0.2101 | 0.4249 | 0.023* | |
C6 | 0.45519 (19) | 0.12433 (12) | 0.39615 (10) | 0.0230 (4) | |
H6A | 0.5208 | 0.0911 | 0.4206 | 0.028* | |
H6B | 0.4659 | 0.1712 | 0.3621 | 0.028* | |
C7 | 0.37484 (16) | 0.05075 (14) | 0.35601 (11) | 0.0259 (4) | |
H7A | 0.3613 | 0.0065 | 0.3905 | 0.031* | |
H7B | 0.3104 | 0.0847 | 0.3296 | 0.031* | |
C8 | 0.4089 (3) | −0.00752 (19) | 0.30414 (11) | 0.0347 (5) | |
H8A | 0.4195 | 0.0358 | 0.2687 | 0.052* | |
H8B | 0.3563 | −0.0551 | 0.2801 | 0.052* | |
H8C | 0.4729 | −0.0409 | 0.3301 | 0.052* | |
C9 | 0.59273 (14) | 0.19965 (13) | 0.54137 (10) | 0.0202 (4) | |
H9A | 0.5729 | 0.1462 | 0.5666 | 0.024* | |
H9B | 0.6262 | 0.1708 | 0.5099 | 0.024* | |
C10 | 0.6700 (2) | 0.26295 (15) | 0.59616 (14) | 0.0253 (5) | |
H10A | 0.6917 | 0.3163 | 0.5718 | 0.030* | |
H10B | 0.6382 | 0.2914 | 0.6287 | 0.030* | |
C11 | 0.76246 (15) | 0.20341 (15) | 0.63871 (12) | 0.0291 (4) | |
H11A | 0.7914 | 0.1715 | 0.6058 | 0.035* | |
H11B | 0.7412 | 0.1526 | 0.6653 | 0.035* | |
C12 | 0.8439 (3) | 0.26686 (18) | 0.69083 (16) | 0.0365 (6) | |
H12A | 0.8637 | 0.3181 | 0.6646 | 0.055* | |
H12B | 0.9037 | 0.2275 | 0.7163 | 0.055* | |
H12C | 0.8165 | 0.2957 | 0.7251 | 0.055* | |
C13 | 0.52549 (14) | 0.32450 (13) | 0.44859 (10) | 0.0200 (4) | |
H13A | 0.5701 | 0.3741 | 0.4798 | 0.024* | |
H13B | 0.5659 | 0.2908 | 0.4238 | 0.024* | |
C14 | 0.43636 (18) | 0.37509 (13) | 0.39317 (10) | 0.0241 (4) | |
H14A | 0.3960 | 0.4108 | 0.4171 | 0.029* | |
H14B | 0.3912 | 0.3267 | 0.3611 | 0.029* | |
C15 | 0.47602 (16) | 0.44495 (14) | 0.35002 (11) | 0.0263 (4) | |
H15A | 0.5139 | 0.4086 | 0.3247 | 0.032* | |
H15B | 0.5238 | 0.4912 | 0.3826 | 0.032* | |
C16 | 0.3885 (3) | 0.5001 (2) | 0.29610 (10) | 0.0343 (5) | |
H16A | 0.3445 | 0.4549 | 0.2613 | 0.051* | |
H16B | 0.4162 | 0.5477 | 0.2716 | 0.051* | |
H16C | 0.3486 | 0.5332 | 0.3209 | 0.051* |
U11 | U22 | U33 | U12 | U13 | U23 | |
Br1 | 0.02772 (9) | 0.02330 (8) | 0.04278 (10) | 0.00074 (10) | 0.01565 (8) | 0.0010 (2) |
N1 | 0.0158 (9) | 0.0157 (5) | 0.0192 (6) | −0.0003 (5) | 0.0048 (12) | 0.0008 (7) |
C1 | 0.0217 (9) | 0.0173 (8) | 0.0213 (9) | 0.0011 (7) | 0.0099 (8) | −0.0019 (7) |
C2 | 0.0345 (15) | 0.0197 (10) | 0.0255 (12) | −0.0008 (7) | 0.0157 (11) | 0.0002 (7) |
C3 | 0.0274 (10) | 0.0261 (9) | 0.0271 (10) | 0.0023 (8) | 0.0154 (8) | 0.0008 (8) |
C4 | 0.0316 (15) | 0.0361 (12) | 0.0308 (12) | 0.0011 (9) | 0.0208 (14) | 0.0059 (10) |
C5 | 0.0177 (9) | 0.0181 (8) | 0.0221 (10) | −0.0031 (6) | 0.0076 (7) | −0.0022 (7) |
C6 | 0.0215 (10) | 0.0229 (8) | 0.0251 (9) | −0.0023 (8) | 0.0088 (9) | −0.0047 (6) |
C7 | 0.0289 (10) | 0.0219 (9) | 0.0284 (10) | −0.0029 (8) | 0.0117 (8) | −0.0061 (8) |
C8 | 0.0435 (15) | 0.0270 (10) | 0.0368 (9) | −0.0008 (11) | 0.0182 (12) | −0.0112 (11) |
C9 | 0.0189 (9) | 0.0189 (8) | 0.0223 (9) | 0.0029 (7) | 0.0067 (7) | 0.0004 (7) |
C10 | 0.0218 (12) | 0.0236 (9) | 0.0254 (12) | −0.0009 (8) | 0.0017 (10) | 0.0011 (8) |
C11 | 0.0193 (9) | 0.0299 (10) | 0.0341 (11) | 0.0015 (8) | 0.0042 (8) | −0.0038 (9) |
C12 | 0.0218 (13) | 0.0513 (16) | 0.0310 (12) | −0.0006 (13) | 0.0026 (11) | −0.0076 (12) |
C13 | 0.0227 (10) | 0.0169 (8) | 0.0227 (10) | −0.0026 (7) | 0.0108 (8) | 0.0013 (7) |
C14 | 0.0247 (11) | 0.0243 (8) | 0.0252 (9) | 0.0029 (8) | 0.0109 (9) | 0.0057 (7) |
C15 | 0.0291 (10) | 0.0222 (9) | 0.0276 (11) | −0.0012 (8) | 0.0100 (8) | 0.0037 (8) |
C16 | 0.0371 (15) | 0.0302 (8) | 0.0319 (8) | 0.0019 (11) | 0.0073 (9) | 0.0101 (14) |
N1—C5 | 1.519 (3) | C8—H8B | 0.9800 |
N1—C1 | 1.522 (3) | C8—H8C | 0.9800 |
N1—C13 | 1.524 (3) | C9—C10 | 1.522 (3) |
N1—C9 | 1.526 (3) | C9—H9A | 0.9900 |
C1—C2 | 1.513 (3) | C9—H9B | 0.9900 |
C1—H1A | 0.9900 | C10—C11 | 1.520 (3) |
C1—H1B | 0.9900 | C10—H10A | 0.9900 |
C2—C3 | 1.521 (3) | C10—H10B | 0.9900 |
C2—H2A | 0.9900 | C11—C12 | 1.527 (4) |
C2—H2B | 0.9900 | C11—H11A | 0.9900 |
C3—C4 | 1.523 (4) | C11—H11B | 0.9900 |
C3—H3A | 0.9900 | C12—H12A | 0.9800 |
C3—H3B | 0.9900 | C12—H12B | 0.9800 |
C4—H4A | 0.9800 | C12—H12C | 0.9800 |
C4—H4B | 0.9800 | C13—C14 | 1.521 (3) |
C4—H4C | 0.9800 | C13—H13A | 0.9900 |
C5—C6 | 1.518 (3) | C13—H13B | 0.9900 |
C5—H5A | 0.9900 | C14—C15 | 1.526 (3) |
C5—H5B | 0.9900 | C14—H14A | 0.9900 |
C6—C7 | 1.523 (3) | C14—H14B | 0.9900 |
C6—H6A | 0.9900 | C15—C16 | 1.526 (3) |
C6—H6B | 0.9900 | C15—H15A | 0.9900 |
C7—C8 | 1.518 (3) | C15—H15B | 0.9900 |
C7—H7A | 0.9900 | C16—H16A | 0.9800 |
C7—H7B | 0.9900 | C16—H16B | 0.9800 |
C8—H8A | 0.9800 | C16—H16C | 0.9800 |
C5—N1—C1 | 108.81 (17) | C7—C8—H8C | 109.5 |
C5—N1—C13 | 111.35 (18) | H8A—C8—H8C | 109.5 |
C1—N1—C13 | 108.81 (12) | H8B—C8—H8C | 109.5 |
C5—N1—C9 | 108.62 (12) | C10—C9—N1 | 114.89 (16) |
C1—N1—C9 | 110.88 (17) | C10—C9—H9A | 108.5 |
C13—N1—C9 | 108.39 (18) | N1—C9—H9A | 108.5 |
C2—C1—N1 | 114.96 (15) | C10—C9—H9B | 108.5 |
C2—C1—H1A | 108.5 | N1—C9—H9B | 108.5 |
N1—C1—H1A | 108.5 | H9A—C9—H9B | 107.5 |
C2—C1—H1B | 108.5 | C11—C10—C9 | 110.05 (17) |
N1—C1—H1B | 108.5 | C11—C10—H10A | 109.7 |
H1A—C1—H1B | 107.5 | C9—C10—H10A | 109.7 |
C1—C2—C3 | 110.93 (15) | C11—C10—H10B | 109.7 |
C1—C2—H2A | 109.5 | C9—C10—H10B | 109.7 |
C3—C2—H2A | 109.5 | H10A—C10—H10B | 108.2 |
C1—C2—H2B | 109.5 | C10—C11—C12 | 110.9 (2) |
C3—C2—H2B | 109.5 | C10—C11—H11A | 109.5 |
H2A—C2—H2B | 108.0 | C12—C11—H11A | 109.5 |
C2—C3—C4 | 111.53 (18) | C10—C11—H11B | 109.5 |
C2—C3—H3A | 109.3 | C12—C11—H11B | 109.5 |
C4—C3—H3A | 109.3 | H11A—C11—H11B | 108.1 |
C2—C3—H3B | 109.3 | C11—C12—H12A | 109.5 |
C4—C3—H3B | 109.3 | C11—C12—H12B | 109.5 |
H3A—C3—H3B | 108.0 | H12A—C12—H12B | 109.5 |
C3—C4—H4A | 109.5 | C11—C12—H12C | 109.5 |
C3—C4—H4B | 109.5 | H12A—C12—H12C | 109.5 |
H4A—C4—H4B | 109.5 | H12B—C12—H12C | 109.5 |
C3—C4—H4C | 109.5 | C14—C13—N1 | 115.22 (17) |
H4A—C4—H4C | 109.5 | C14—C13—H13A | 108.5 |
H4B—C4—H4C | 109.5 | N1—C13—H13A | 108.5 |
C6—C5—N1 | 115.45 (17) | C14—C13—H13B | 108.5 |
C6—C5—H5A | 108.4 | N1—C13—H13B | 108.5 |
N1—C5—H5A | 108.4 | H13A—C13—H13B | 107.5 |
C6—C5—H5B | 108.4 | C13—C14—C15 | 109.86 (18) |
N1—C5—H5B | 108.4 | C13—C14—H14A | 109.7 |
H5A—C5—H5B | 107.5 | C15—C14—H14A | 109.7 |
C5—C6—C7 | 110.28 (19) | C13—C14—H14B | 109.7 |
C5—C6—H6A | 109.6 | C15—C14—H14B | 109.7 |
C7—C6—H6A | 109.6 | H14A—C14—H14B | 108.2 |
C5—C6—H6B | 109.6 | C14—C15—C16 | 111.1 (2) |
C7—C6—H6B | 109.6 | C14—C15—H15A | 109.4 |
H6A—C6—H6B | 108.1 | C16—C15—H15A | 109.4 |
C8—C7—C6 | 111.6 (2) | C14—C15—H15B | 109.4 |
C8—C7—H7A | 109.3 | C16—C15—H15B | 109.4 |
C6—C7—H7A | 109.3 | H15A—C15—H15B | 108.0 |
C8—C7—H7B | 109.3 | C15—C16—H16A | 109.5 |
C6—C7—H7B | 109.3 | C15—C16—H16B | 109.5 |
H7A—C7—H7B | 108.0 | H16A—C16—H16B | 109.5 |
C7—C8—H8A | 109.5 | C15—C16—H16C | 109.5 |
C7—C8—H8B | 109.5 | H16A—C16—H16C | 109.5 |
H8A—C8—H8B | 109.5 | H16B—C16—H16C | 109.5 |
C5—N1—C1—C2 | 63.9 (2) | C5—N1—C9—C10 | −172.7 (2) |
C13—N1—C1—C2 | −174.6 (2) | C1—N1—C9—C10 | −53.2 (2) |
C9—N1—C1—C2 | −55.5 (2) | C13—N1—C9—C10 | 66.2 (2) |
N1—C1—C2—C3 | 176.5 (2) | N1—C9—C10—C11 | −179.95 (19) |
C1—C2—C3—C4 | 176.5 (2) | C9—C10—C11—C12 | 176.4 (2) |
C1—N1—C5—C6 | 174.16 (17) | C5—N1—C13—C14 | 54.1 (2) |
C13—N1—C5—C6 | 54.2 (2) | C1—N1—C13—C14 | −65.8 (2) |
C9—N1—C5—C6 | −65.0 (2) | C9—N1—C13—C14 | 173.53 (16) |
N1—C5—C6—C7 | 178.84 (17) | N1—C13—C14—C15 | −179.49 (17) |
C5—C6—C7—C8 | −176.82 (18) | C13—C14—C15—C16 | −177.50 (18) |
Experimental details
Crystal data | |
Chemical formula | C16H36N+·Br− |
Mr | 322.37 |
Crystal system, space group | Monoclinic, C2/c |
Temperature (K) | 150 |
a, b, c (Å) | 13.9773 (9), 13.8623 (9), 20.0450 (14) |
β (°) | 110.383 (10) |
V (Å3) | 3640.7 (4) |
Z | 8 |
Radiation type | Mo Kα |
µ (mm−1) | 2.25 |
Crystal size (mm) | 0.41 × 0.31 × 0.16 |
Data collection | |
Diffractometer | Bruker APEXII CCD diffractometer |
Absorption correction | Multi-scan (SADABS; Sheldrick, 2008a) |
Tmin, Tmax | 0.459, 0.715 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 21135, 5485, 4415 |
Rint | 0.029 |
(sin θ/λ)max (Å−1) | 0.715 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.028, 0.073, 1.04 |
No. of reflections | 5485 |
No. of parameters | 168 |
H-atom treatment | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 0.62, −0.24 |
Computer programs: APEX2 (Bruker, 2008), SAINT (Bruker, 2008), SHELXS97 (Sheldrick, 2008b), SHELXL97 (Sheldrick, 2008b) and PLATON (Spek, 2009), SHELXTL (Sheldrick, 2008b) and local programs.
Acknowledgements
We wish to acknowledge the use of the EPSRC's Chemical Database Service at Daresbury.
References
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While many common reagents have had their crystal structures well determined, some many times, some deliberatly and many by accident, no good quality structure was available for the title compound, tetra-n-butylammonium bromide (I). Compound (I) is used in a number of synthesis applications (see Prukała et al., 2007, and references therein for further details) and as a source of the large tetra-n-butylammonium cation, which is useful in crystallizing large anions. A search of the Cambridge Structural Database (version 5.32 + 3 updates, Fletcher, et al., 1996, Allen, 2002) revealed just one reported structure of this compound with an R1 of 0.098 that had clearly been problematic (Wang et al., 1995). This earlier determination had only the bromide ion refined anisotropically and did not include hydrogen atoms in the model. The authors ruled out dynamic disorder as the cause of the difficulties and concluded that static disorder was the causeof the poor residual.
The crystals of (I) formed readily by vapour diffusion of diethyl ether into an acetonitrile solution. The data collection set-up was trouble free. After data reduction the structure did not solve readily with SHELXS (Sheldrick, 2008a); only the bromide, the nitrogen and two n-butyl chains being evident. When the structure failed to develop, the coordinates from the published structure were used as a starting point (Wang et al., 1995), but the R1 was ca. 35% for an isotropic model with all non-H atoms in the model. Twinning was suspected and confirmed by the TWINROTMAT routine in PLATON (Spek, 2009). Application of the merohedral twin law -1 0 0, 0 -1 0, 1 0 1, led to a reduction in R1 to ca. 5.0% at the same, isotropic, stage of refinement. Anisotropic refinement, and addition of H atoms, led to a good final R1 <3% with no adverse indicators. The ratio of major to minor twin components is 60.69: 39.31 (7)%
The structure is isomorphous with that of the iodide analogue described in detail recently (Prukała et al., 2007). The n-butyl chains are fully extended adopting an all-anti conformation with approximate S4 point symmetry (Alder et al., 1990). The bromide anion resides in a pocket between four cations, making four pairs of weak C—H···Br contacts in the range 2.98–3.11 Å to methylene hydrogens located one or two carbon atoms from the nitrogen cationic centre. The structures of the chloride and fluoride analogues have not been determined to date, although the unit cell of the hydrate of the chloride has been reported (McMullan & Jeffrey, 1959).