Analysis of geometric parameters and packing considerations for triphenylboroxine derivatives, with tris(penta-fluoro-phen-yl)boroxine as an example

Article abstract of DOI:10.1107/S0108270112016629

Mol-ecules of the title compound [systematic name: 2,4,6-(penta- fluorophenyl)-1,3,5,2,4,6-trioxatriborinane], C₁₈B₃F ₁₅O₃, are located on crystallographic twofold rotation axes which run through the boroxine and one of the penta-fluoro-phenyl rings. The boroxine ring (r.m.s. deviation = 0.027 A) and the penta-fluoro-phenyl rings (r.m.s. deviations = 0.004 and 0.001 A) are essentially planar. The dihedral angles between the boroxine and the two symmetry-independent benzene rings are 8.64 (10) and 8.74 (12)°. The two benzene rings are mutually coparallel [dihedral angle = 0.80 (11)°]. The packing shows planes of mol-ecules parallel to ( 01), with an inter-planar spacing of 2.99 A. Within these planes, all the mol-ecules are oriented in the same direction, whereas in neighbouring planes the direction is inverted. Short B...F contacts of 3.040 (2) and 3.1624 (12) A occur between planes. The geometric parameters of the boroxine ring in the title compound agree well with those of comparable boroxine structures, while the packing reveals some striking similarities and differences.

Full text of DOI:10.1107/S0108270112016629

organic compounds
Acta Crystallographica Section C
Crystal Structure
Communications
fluorophenyl)borane monohydrate with Si Cl . In this study,
2
6
we describe the molecular and crystal structure of (I). Single
crystals were isolated from the reaction solution on cooling
from 453 K to room temperature.
ISSN 0108-2701
Analysis of geometric parameters and
packing considerations for triphenyl-
boroxine derivatives, with tris(penta-
fluorophenyl)boroxine as an example
Jan Tillmann, Hans-Wolfram Lerner, Tanja Sinke and
Michael Bolte*
Institut f u¨ r Anorganische und Analytische Chemie, Goethe-Universit a¨ t Frankfurt,
Max-von-Laue-Strasse 7, 60438 Frankfurt am Main, Germany
Correspondence e-mail: bolte@chemie.uni-frankfurt.de
Received 3 April 2012
Accepted 16 April 2012
Online 26 April 2012
Boronic acids, RB(OH) (R = alkyl or aryl), can be readily
2
converted to the corresponding boroxines, (RBO) . Although
3
the B centres in boroxines possess a higher Lewis acidity than
the C atoms in arenes, X-ray structure analysis provides
evidence for the existence of a ꢀ-ring system in boroxines.
Owing to significant differences in both the steric and elec-
tronic properties of the ligands of boroxines, different crystal-
packing patterns have been found in their crystal structures
(Boese et al., 1987).
Molecules of the title compound [systematic name: 2,4,6-
(pentafluorophenyl)-1,3,5,2,4,6-trioxatriborinane], C B F -
O , are located on crystallographic twofold rotation axes
18 3 15
3
which run through the boroxine and one of the pentafluoro-
˚
phenyl rings. The boroxine ring (r.m.s. deviation = 0.027 A)
and the pentafluorophenyl rings (r.m.s. deviations = 0.004 and
˚
.001 A) are essentially planar. The dihedral angles between
0
the boroxine and the two symmetry-independent benzene
The molecule of (I) (Fig. 1) is located on a crystallographic
twofold rotation axis running through the boroxine ring and
one of the pentafluorophenyl rings. The boroxine ring (r.m.s.
ꢀ
rings are 8.64 (10) and 8.74 (12) . The two benzene rings are
ꢀ
˚
mutually coparallel [dihedral angle = 0.80 (11) ]. The packing
shows planes of molecules parallel to (201), with an inter-
deviation = 0.027 A) and pentafluorophenyl rings (r.m.s.
˚
deviations = 0.004 and 0.001 A) are essentially planar. The
dihedral angles between the boroxine ring and the two
˚
planar spacing of 2.99 A. Within these planes, all the
molecules are oriented in the same direction, whereas in
neighbouring planes the direction is inverted. Short BÁ Á ÁF
˚
contacts of 3.040 (2) and 3.1624 (12) A occur between planes.
The geometric parameters of the boroxine ring in the title
compound agree well with those of comparable boroxine
structures, while the packing reveals some striking similarities
and differences.
Comment
It has been known for decades that the Si—Si bond in halo-
substituted disilanes can be cleaved in the presence of a Lewis
base (Meyer-Wegner et al., 2011). The reactivity of Si Cl has
2
6
been studied mostly with a focus on its behaviour towards
neutral N and P donors, yet some reactions of Si Cl with
anionic nucleophiles have also been described (Meyer-Wegner
2
6
et al., 2009). Recently, we have discovered that the reaction of
t
Si Cl with two equivalents of Na[Si Bu ] (Lerner, 2005) leads
2
6
3
to several products, such as neopentasilane Si Cl and the
5
12
t
tetrasilatetrahedrane Si (Si Bu ) (Meyer-Wegner et al., 2009),
4
3 4
t
but no substitution product [( Bu Si)SiCl SiCl (Si Bu ); Bolte
t
Figure 1
A perspective view of (I), showing the atom-numbering scheme.
3
2
2
3
& Lerner 2011] was formed thereby. The title boroxine, (I),
was obtained by the reaction of the Lewis acid tris(penta-
Displacement ellipsoids are drawn at the 50% probability level.
1
[Symmetry code: (A) Àx, y, Àz + .]
2
o204 # 2012 International Union of Crystallography
doi:10.1107/S0108270112016629
Acta Cryst. (2012). C68, o204–o208

organic compounds
Figure 4
A partial packing diagram for triethylboroxine (Beckett et al., 1997),
viewed perpendicular to the boroxine ring. H atoms have been omitted
for clarity. [Symmetry codes: (A) Àx + y + 2, Àx + 1, z; (B) Ày + 1,
x À y À 1, z; (C) Àx + 2, Ày, Àz; (D) x À y, x À 1, Àz; (E) y + 1, Àx + y + 1,
Àz.]
Figure 2
A packing diagram for (I), viewed in the ac plane.
evidence for the existence of a ꢀ-ring system (Haberecht et al.,
2
005).
In order to compare the molecular and crystal structures of
I) with those of similar compounds, a Cambridge Structural
(
Database (CSD, Version 5.33 of November 2011 plus one
update; Allen, 2002) substructure search was undertaken for
boroxine rings with each B atom connected to a benzene ring
and yielded 17 hits. All rings are essentially planar, with a
ꢀ
mean absolute B—O—B—O torsion angle of 3 (2) and a
ꢀ
Figure 3
maximum absolute torsion angle of 11.8 . The mean B—O
A partial packing diagram for (I), showing two coparallel planes. The
molecules in one plane are drawn with full bonds and those in the other
plane with open bonds. The view direction is onto the (201) plane. Cell
boundaries have been omitted for clarity.
symmetry-independent benzene rings are 8.64 (10) and
ꢀ
8
.74 (12) . The two benzene rings are mutually coparallel
ꢀ
[dihedral angle = 0.80 (11) ]. The packing (Fig. 2) shows
planes of molecules parallel to (201) with an interplanar
˚
spacing of 2.99 A. Within each plane, all the molecules are
oriented in the same direction, whereas in neighbouring
planes the direction is inverted (Fig. 3). There is no ꢀ–ꢀ
stacking but it is noteworthy that short BÁ Á ÁF distances occur
ii
iii
˚
between two planes [B1Á Á ÁF5 = 3.040 (2) A and B2Á Á ÁF13 =
˚
1
2
3
.1624 (12) A; symmetry codes: (ii) x, Ày + 1, z À ; (iii) x,
1
2
Ày + 2, z À ]. The Fatom is located almost directly over the B
atom (Fig. 3).
Boroxines with alkyl substituents, such as (EtBO) , form
3
columns in their crystal structures in which the boroxine rings
stack above each other, with each B atom being surrounded by
the O atoms of the two neighbouring layers (Fig. 4) (Beckett et
al., 1997). Between these layers there are short intermolecular
˚
BÁ Á ÁO contacts of 3.462 A. In contrast, boroxines with aryl
substituents tend to form, in most cases, a layer structure in the
solid state in which the boroxine rings are located between two
arene rings of neighbouring layers (Fig. 5) (Brock et al., 1987).
Generally, X-ray structure analyses of boroxines provide
Figure 5
A partial packing diagram for tris(p-tolyl)boroxine (Brock et al., 1987),
viewed perpendicular to the boroxine ring. H atoms have been omitted
for clarity. [Symmetry codes: (A) Àx, y, z; (B) x, y, z + 1; (C) Àx, y, z + 1.]
ꢁ
Acta Cryst. (2012). C68, o204–o208
Tillmann et al.
18
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organic compounds
ꢀ
out of the plane of the boroxine ring (35.8 and 40.6 ). As a
result, this structure will not be discussed in detail.
Although triphenylboroxine (FIPDOX) features a rather
planar molecule (r.m.s. deviation for all non-H atoms =
˚
0.178 A), the crystal packing shows neither planes of co-
parallel molecules nor any ꢀ-stacking. There are only some
˚
weak C—HÁ Á Áꢀ contacts in the range 2.93–3.44 A. The planar
˚
boroxine ring (r.m.s. deviation = 0.035 A) forms dihedral
ꢀ
angles of 6.5, 11.9 and 11.6 with the attached phenyl rings.
Tris(p-tolyl)boroxine (NIBGOU) crystallizes, like (I), in the
monoclinic space group C2/c but with rather different cell
˚
parameters [a = 22.591 (2), b = 13.269 (4), c = 6.839 (1) A, ꢁ =
ꢀ
˚
3
06.82 (2) and V = 1962.3 A ]. As in (I), molecules of
1
NIBGOU are located on a crystallographic twofold rotation
axis. The planar boroxine ring (r.m.s. deviation for all non-H
˚
ꢀ
atoms = 0.001 A) forms dihedral angles of 9.3 and 1.9 with the
attached benzene rings; the r.m.s. deviation for all non-H
˚
atoms is 0.062 A. Nevertheless, the packing of the molecules is,
on the one hand, similar to that in (I), i.e. the molecules form
˚
layers parallel to (201), with an interplanar spacing of 3.25 A
(Fig. 6). The difference in the packing pattern, on the other
hand, is that in NIBGOU the boroxine rings are stacked above
a benzene ring (Fig. 5). This kind of stacking of rings is not
observed in (I), where BÁ Á ÁF contacts are found.
NIBGOU01 is a polymorph of NIBGOU. It crystallizes in
the orthorhombic space group Pmn2 with the molecules
1
located on a crystallographic mirror plane. The boroxine ring
˚
is again planar (r.m.s. deviation = 0.019 A) and forms dihedral
Figure 6
A packing diagram for the monoclinic form of tris(p-tolyl)boroxine
(NIBGOU; Beckett et al., 1997), viewed in the ac plane. H atoms have
been omitted for clarity.
˚
distance over all six B—O bonds in all hits was 1.379 (7) A.
ꢀ
The mean O—B—O angle is 118.4 (9) , whereas the mean B—
ꢀ
O—B angle is 121.5 (9) .
In (I), the mean absolute B—O—B—O torsion angle is
ꢀ
ꢀ
4
O1. The B—O distances range from 1.364 (2) A for B1—O2 to
(2) , with a maximum value of À6.5 (2) for B2—O2—B1—
˚
˚
.3713 (19) A for B1—O1. In contrast with the structures
1
retrieved from the database, the B—O—B and O—B—O
i
angles in (I) have almost the same values [B1—O1—B1 =
ꢀ
ꢀ
1
1
(
19.82 (18) , B1—O2—B2 = 120.13 (14) , O2—B1—O1 =
ꢀ
i
19.81 (14) and O2—B2—O2 = 119.8 (2) ; symmetry code:
ꢀ
1
2
i) Àx, y, Àz + ].
Four compounds from the database search are very similar
to (I) and merit further investigation. It is remarkable that
neither triphenylboroxine [FIPDOX (Boese et al., 1987),
FIPDOX01 (Brock et al., 1987) and FIPDOX02 (Bolte, 2004)],
2,4,6-tris(4-bromophenyl)boroxine [LUKKAD (Avent et al.,
2002), LUKKAD01 (Jones & Zerbe, 2004) and LUKKAD02
(
(
Bhuvanesh et al., 2005)], tris(p-tolyl)boroxine [NIBGOU
Beckett et al., 1997) and NIBGOU01 (Haberecht et al., 2005)]
nor 1,3,5-trimesitylboroxine [SOKLAF (Neum u¨ ller & Gahl-
mann, 1991), SOKLAF01 (Anulewicz-Ostrowska et al., 2000)
and SOKLAF02 (Franz et al., 2009)] is isomorphous with (I).
Figure 7
A packing diagram for the orthorhombic form of tris(p-tolyl)boroxine
(NIBGOU01; Haberecht et al., 2005), viewed in the (021) plane. H atoms
have been omitted for clarity.
1,3,5-Trimesitylboroxine (SOKLAF) has a completely
different packing pattern from (I), which is most probably due
to the fact that the mesityl residues are significantly twisted
ꢁ
o206 Tillmann et al.
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B F
15
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3
Acta Cryst. (2012). C68, o204–o208

organic compounds
of the aryl and boroxine rings. The phenyl-, p-bromophenyl-,
tolyl- and pentafluorophenyl-substituted boroxines are
located on a crystallographic symmetry element, either on a
mirror plane or on a twofold rotation axis. Whereas the crystal
packing of triphenylboroxine does not reveal any stacking, the
molecules of p-bromophenyl-, tolyl- and pentafluorophenyl-
boroxine show a layer structure. However, the kind of stacking
is different depending on the substitution pattern of the
boroxine ring. In p-bromophenyl- and tolylboroxine, the
boroxine rings are stacked above the benzene rings, whereas
in pentafluorophenylboroxine no stacks of rings occur but
short BÁ Á ÁF interactions are found. This is the reason for the
different packing pattern of (I) compared with the other
boroxines discussed. On the other hand, the alkyl-substituted
compound triethylboroxine shows perfect boroxine–boroxine
stacks.
Experimental
Figure 8
A packing diagram for tris(p-bromophenyl)boroxine (LUKKAD; Avent
et al., 2002), viewed in the ac plane. H atoms have been omitted for clarity.
Tris(pentafluorophenyl)borane monohydrate (0.32 g, 0.60 mmol) and
hexachlorodisilane (0.1 ml, 0.57 mmol) were dissolved in C
0.5 ml). The reaction mixture was heated to 453 K for 24 h in a
sealed NMR tube. After the reaction mixture had been cooled from
53 K to room temperature, single crystals of the title compound, (I),
6 6
D
(
4
were obtained (yield 20%).
Crystal data
˚
V = 1795.2 (4) A
3
C
18
3
B F
15
O
3
M
r
= 581.61
Z = 4
Mo Kꢂ radiation
ꢃ = 0.24 mm
T = 173 K
Monoclinic, C2=c
˚
a = 16.825 (2) A
À1
˚
b = 13.1810 (13) A
˚
c = 8.1049 (12) A
0.28 Â 0.28 Â 0.26 mm
ꢀ
ꢁ
= 92.846 (12)
Data collection
Figure 9
Stoe IPDS II two-circle
diffractometer
Absorption correction: multi-scan
9608 measured reflections
1680 independent reflections
1358 reflections with I > 2ꢄ(I)
A partial packing diagram for tris(p-bromophenyl)boroxine (LUKKAD;
Avent et al., 2002). H atoms have been omitted for clarity. Centroid–
centroid distances are drawn as dashed lines.
(X-AREA; Stoe & Cie, 2001)
min = 0.935, Tmax = 0.939
Rint = 0.072
T
ꢀ
angles of 5.7 and 4.2 with the attached benzene rings. Again,
Refinement
2
the molecules form planes with boroxine rings stacked above
benzene rings (Fig. 7), but there are different layers of mol-
ecules parallel to (021) and (021). The dihedral angle between
2
R[F > 2ꢄ(F )] = 0.037
wR(F ) = 0.103
179 parameters
˚
À3
Áꢅmax = 0.29 e A
2
Áꢅmin = À0.21 e A˚
À3
S = 1.05
1680 reflections
ꢀ
˚
the layers is 86.1 and the interplanar spacing is 3.33 A.
The molecules of 2,4,6-tris(4-bromophenyl)boroxine
(LUKKAD) show crystallographic mirror symmetry. The
boroxine ring is planar (r.m.s. deviation = 0.016 A) and makes
The very strong 402 reflection is unfortunately missing from the
data set because it was not measured.
˚
ꢀ
dihedral angles of 2.9 and 5.4 with the benzene rings. The
packing of the molecules resembles a herringbone pattern,
with molecules in planes parallel to (401) and (401) (Fig. 8).
Data collection: X-AREA (Stoe & Cie, 2001); cell refinement:
X-AREA; data reduction: X-AREA; program(s) used to solve
structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine
structure: SHELXL97 (Sheldrick, 2008); molecular graphics: XP
(Sheldrick, 2008); software used to prepare material for publication:
SHELXL97 and PLATON (Spek, 2009).
ꢀ
The dihedral angle between these planes is 87.1 and the
˚
interplanar spacing is 3.37 A. In this case, stacking of boroxine
with benzene rings is observed (Fig. 9). The centroid–centroid
˚
distance is 3.51 A.
In conclusion, it can be said that aryl-substituted boroxine
rings tend to adopt a planar molecular structure. The aryl rings
are only twisted out of the boroxine plane if bulky substituents
in the ortho position of the aryl substituents avoid coplanarity
This work was supported by the Beilstein Institute as part of
the NanoBiC research co-operative (project eNet). MB thanks
Dr Martin Nieger, University of Helsinki, Finland, for helpful
comments.
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Acta Cryst. (2012). C68, o204–o208
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Bolte, M. (2004). Private communication (CCDC 243438). CCDC, Cambridge,
England.
Bolte, M. & Lerner, H.-W. (2011). J. Chem. Crystallogr. 41, 132–136.
Brock, C. P., Minton, R. P. & Niedenzu, K. (1987). Acta Cryst. C43, 1775–1779.
¨
Franz, D., Lerner, H.-W., Seven, O. & Bolte, M. (2009). Private communication
Supplementary data for this paper are available from the IUCr electronic
archives (Reference: SK3434). Services for accessing these data are
described at the back of the journal.
(CCDC 757641). CCDC, Cambridge, England.
Haberecht, M. C., Bolte, M., Wagner, M. & Lerner, H.-W. (2005). J. Chem.
Crystallogr. 35, 657–665.
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