organic compounds
XAB) and [2,6-bis(2,4,6-triisopropylphenyl)phenyl]dibromo-
borane (TIZXEF) (Grisby & Power, 1996), with a mean C—B
˚
bond length of 1.55 (3) A and a mean B—Br bond length of
˚
1.91 (1) A. These values agree well with those in (I), (II) and
(III) (Table 2).
If the substitutents of the ring in the ortho position with
respect to the –BBr2 group are H atoms, the –BBr2 group is
coplanar with the ring. The dihedral angle between the ring
and the –BBr2 group ranges from 1.4 (in AYUJEJ) to 8.8ꢀ (in
AYUJAF). In three of the structures, the ortho substituents
are very bulky and force the –BBr2 group into an almost
perpendicular position with respect to the aromatic ring
(dihedral angles: BBr2/ring = 87.3ꢀ in EHUBEO, 77.5ꢀ in
TIZXAB and 90.0ꢀ in TIZXEF).
A search of the CSD for an aromatic ring carrying a –BBr2
residue with a four-coordinated B atom revealed only two
structures, dibromo(2-dimethylaminomethylphenyl)borane
(CEJWUJ; Brown et al., 1998) and bromo(bromo{4-[di-
bromo(tricyclohexylphosphino)boranyl]phenyl}boranyl-B)-
bis(tricyclohexylphosphino)platinum benzene n-hexane sol-
vate (QOMLEK; Braunschweig et al., 2008). The C—B bond
˚
lengths of 1.559 (CEJWUJ) and 1.609 A (QOMLEK), and the
˚
B—Br bond lengths (2.019 and 2.020 A in CEJWUJ, and 2.046
˚
and 2.056 A in QOMLEK) are in good agreement with those
Figure 8
of (IV) and (V) (Table 2). Apart from these distances, the two
database structures differ too much from (IV) and (V) for
further comparison.
A packing diagram for (IV), viewed in the bc plane. H atoms bonded to C
atoms have been omitted for clarity. Hydrogen bonds are shown as
dashed lines.
In conclusion, it can be said that the C—B and B—Br bonds
are significantly elongated if the number of substituents at the
B atom is increased from three to four. Molecules containing a
three-coordinated B atom carrying two Br atoms tend to have
the –BBr2 group coplanar with the aromatic ring, as long as no
bulky substituents in the ortho position of the aromatic ring
prevent coplanarity. The crystal packing of these structures
depends on the substitution pattern. Compounds (I) and (II),
with electron-rich residues in the position para to the –BBr2
group, feature a B–ꢀ interaction in their crystal structures,
whereas no such interaction is observed in the structures of
(III), (IV) and (V).
Figure 9
A perspective view of (V), showing the atom-numbering scheme.
Displacement ellipsoids are drawn at the 50% probability level. Only
one of the two disordered orientations of the central ring is shown.
[Symmetry code: (A) x, ꢁy + , z.]
1
2
Experimental
of the methyl groups bonded to the S atom are disordered
over two equally occupied orientations. Due to this disorder,
the deviation of the B, Br and S atoms from the ring plane(s) is
not discussed.
All experiments were carried out under dry nitrogen or argon with
strict exclusion of air and moisture using standard Schlenk techni-
ques. The starting materials were purchased from commercial sources
and used without further purification. The solvents were distilled
from sodium/benzophenone prior to use. C6D6 was dried over mol-
ecular sieves and stored under dry nitrogen. The NMR spectra were
recorded on Bruker AM 250, DPX 250, Avance 300 and Avance 400
spectrometers. Abbreviations: s = singlet, d = doublet, t = triplet, vtr
(virtual triplet), q = quartet, quin = quintet, mult = multiplet and br =
broad.
In order to compare these five structures with similar
compounds, a search of the Cambridge Structural Database
(CSD, Version 5.33 of November 2011, plus one update; Allen,
2002) was performed for an aromatic ring carrying a –BBr2
residue. Eight entries were found, namely 4-(dibromoboryl)-
toluene (AYUHIL), 1,3-bis(dibromoboryl)benzene (AYU-
HOR), 1,4-bis(dibromoboryl)benzene (AYUHUX), 1,3,5-
tris(dibromoboryl)benzene (AYUJAF) and 4,40-bis(dibromo-
boryl)biphenyl (AYUJEJ) (Haberecht et al., 2004), di-
bromo(2,4,6-triisopropylphenyl)borane (EHUBEO) (Olmstead
et al., 2003), and dibromo(2,6-dimesitylphenyl)borane (TIZ-
For the synthesis of (I), trimethylphenylsilane (8.5 ml, 49.2 mmol,
1 equivalent) was dissolved in hexane (10 ml) and cooled to 273 K.
BBr3 (5 ml, 52.9 mmol, 1.1 equivalents) was added rapidly via a
syringe. The mixture was allowed to warm slowly to room tempera-
ture and was then heated under reflux for 2 h. During removal of all
volatile components in vacuo, dibromoborylbenzene was obtained as
ꢃ
o228 Popp et al. C6H5BBr2 and four analogues
Acta Cryst. (2012). C68, o226–o230