A new polymorph of tri(p-tolyl)boroxine

Article abstract of DOI:10.1007/s10870-005-3325-y

A new orthorhombic polymorph of tri(p-tolyl)boroxine (Pmn2₁) with relatively short intermolecular B-O distances of 3.321 A was crystallized from CDCl₃ at ambient temperature. The crystal structure of the orthorhombic polymorph of tri

Full text of DOI:10.1007/s10870-005-3325-y

Journal of Chemical Crystallography, Vol. 35, No. 9, September 2005 (ꢀC 2005)
DOI: 10.1007/s10870-005-3325-y
A new polymorph of tri(p-tolyl)boroxine
(
1)
(1)
(1)
Monika C. Haberecht, Michael Bolte, Matthias Wagner,
∗
1)
(
and Hans-Wolfram Lerner
Received August 27, 2004; accepted February 4, 2005
A new orthorhombic polymorph of tri(p-tolyl)boroxine (Pmn21) with relatively short inter-
molecular B---O distances of 3.321 A was crystallized from CDCl3 at ambient temperature.
˚
The crystal structure of the orthorhombic polymorph of tri(p-tolyl)boroxine shows the
shortest intermolecular B---O contact yet found in boroxines. The cell dimensions of the
˚
˚
orthorhombic polymorph of tri(p-tolyl)boroxine are a = 21.888(4) A, b = 9.304(2) A,
˚
and c = 4.7804(10) A. The structural features of the orthorhombic polymorph of tri(p-
tolyl)boroxine are quite different from a previously reported monoclinic (Beckett et al., J.
Organomet. Chem. 1997, 535, 33–41) but similar to that of tri(p-bromophenyl)boroxine
(Avent et al., Coll. Czech. Chem. Commun. 2002, 67, 1051–1060). Obviously, electronic
effects of substituents on the boron centers influence the structural features of substituted
boroxines less than discussed in earlier reports (Boese et al., Angew. Chem. 1987, 99,
239–241).
KEY WORDS: Boroxine; Lewis acidity; polymorphism; intermolecular B---O distances.
Introduction
In the last two decades the extent of B---O
π bonding in boroxines was the topic of sev-
1,2
2
eral articles. In 1999, Beckett et al. estimated
the Lewis acidity of boron centers in boroxines
in comparison to that of the related borazenes.
Generally the boron atoms in boroxines possess a
higher Lewis acidity than those in borazenes and
therefore the related boroxines show a much lower
aromatic character. Nevertheless, X-ray structure
analysis of boroxines gives evidence for the exis-
tence of a π-ring system. Theoretical studies from
.
3
Alcarez et al. show that boroxines should possess
interesting NLO properties.
In 1987, Boese et al. discussed the dif-
ferent crystal packing in the boroxines 3 and
4
4
. In the crystal structure of 3 the boroxine
(
1)
Institut f u¨ r Anorganische Chemie, Johann Wolfgang Goethe-
Universit a¨ t Frankfurt am Main, Marie-Curie-Straße 11, D-60439
Frankfurt, Germany.
molecules form columns in which the boroxine
rings stack on each other with each boron be-
ing surrounded by the oxygen atoms of the two
neighboring layers (Fig. 1). Between these layers
∗
To whom correspondence should be addressed; e-mail: lerner@
chemie.uni-frankfurt.de.
6
57
1
074-1542/05/0900-0657/0 ꢀC 2005 Springer Science+Business Media, Inc.

658
Haberecht, Bolte, Wagner, and Lerner
deliver π electron density to the empty orbitals
on the boron atoms. Consequently, the Lewis-
acidity on the boron centers of 4 is decreased
in comparison to 3. Therefore, the intermolecu-
lar B---O interactions in 3 are stronger than in
4
. These interactions in 3 lead to a column type
crystal packing. By contrast, the boron centers in
the phenyl derivative 4 have a lower Lewis acidity
and therefore intermolecular B---O interactions are
no longer favored. Moreover, nonpolar stacking
interactions between boroxine and phenyl rings
become more attractive leading to the layer-type
structure as shown in Fig. 2.
Experimental
General considerations
All experiments were carried out under dry
argon or nitrogen using standard Schlenk tech-
niques. Diethylether and toluene were distilled
from sodium/benzophenone. CDCl was dried
3
over molecular sieves and stored under dry nitro-
gen. 4-(Dibromoboryl)toluene was prepared ac-
6
cording to a published procedure.
Synthesis of 4-(bromoethoxyboryl)toluene
4
Fig . 1. Crystal packing diagram for 3.
4-(Dibromoboryl)toluene (9.55 mmol,
2
.50 g) was dissolved in 20 mL of toluene and
short intermolecular B---O contacts of 3.462 A
were found. Contrary to 3, boroxine 4 forms a
layer structure in the solid state (monoclinic space
˚
diethylether (9.55 mmol, 0.71 g) was added via
syringe at ambient temperature. The solution
became cloudy immediately. After stirring
for 5 d at RT the solution was clear again.
The solvent was evaporated in vacuo to yield
group P2 /c) in which the boroxine rings are lo-
cated between two phenyl rings of the neighbor-
ing layers. The shortest B---O distance found in
the crystal structure of 4 was 4.441 A. The mon-
oclinic polymorph of tri(p-tolyl)boroxine 1 fea-
tures the same structural motif as 4 (depicted in
1
1
4-(bromoethoxyboryl)toluene as a oily residue
(2.15 g, 99.2%). H-NMR (250.1 MHz, CDCl ):
1
˚
3
3
( 8.16 (d, 2H, J(H,H) = 7.8 Hz, Ar-H), 7.13
3
(d, 2H, J(H,H) = 7.8 Hz, Ar-H), 4.18 (qa, 2H,
5
3
Fig. 2).
J(H,H) = 7.1 Hz, OCH CH ), 2.16 (s, 3H,
2
3
3
The structural differences between aryl and
alkyl substituted boroxines have been explained
with the possibility of electronic interactions be-
tween the aryl substituents and the boron atoms.
The aromatic substituents in boroxines are able to
CH ), 1.13 (t, 3H, J(H,H) = 7.1 Hz, OCH CH );
3
2
3
13
C-NMR (62.9 MHz, CDCl ): (137.9, 135.9,
3
128.9 (Ar-C), n.o. (CB), 66.0 (OCH CH ), 21.4
(Ar-CH ), 16.1 (OCH CH ); B-NMR (128.4
MHz, CDCl ): (38.4 (h = 200 Hz).
2
3
11
3
2
3
3
1/2

A new polymorph of tri(p-tolyl)boroxine
659
5
Fig . 2. Crystal packing diagram for the monoclinic polymorph of 1.
Synthesis of tri(tolyl)boroxine 1
to a thin glass fiber. Intensity measurement was
carried out using a STOE IPDS-II two circle
diffractometer. The structure was solved with di-
X-ray suitable single crystals of 1 were
obtained on slow hydrolysis of a solution of
7
rect methods and refined with full-matrix least-
8
4-(bromoethoxyboryl)toluene in CDCl3.
squares techniques. All non-hydrogen atoms
were refined with anisotropic displacement pa-
rameters. Hydrogen atoms were placed at calcu-
lated positions and refined using a riding model.
The methyl groups were allowed to rotate but not
to tip. The hydrogen atoms of one methyl group
Crystal structure determination
A suitable single crystal of 1 was se-
lected and attached in a perfluoropolyether oil

660
Haberecht, Bolte, Wagner, and Lerner
Table 1. Crystal Data and Structure Refinement for the
Orthorhombic Polymorph of 1
˚
◦
Table 2. Bond Lengths (A) and Angles ( ) for 1 (Orthorhombic
Polymorph)
Formula
Fw
C21H21B3O3
353.81
Bond lengths
----
B(1) O(1)
1.381(3)
1.386(3)
1.384(2)
1.542(3)
1.545(5)
1.385(3)
1.401(3)
1.392(3)
1.382(3)
1.381(4)
1.507(3)
1.395(3)
1.400(3)
1.390(3)
1.385(3)
1.503(5)
Color, shape
Crystal system
Space group
Temperature (K)
Colorless, block
Orthorhombic
Pmn21
173(2)
0.71073
21.888(4)
9.304(2)
4.7804(10)
973.5(3)
2
B(1) O(2)
----
----
B(2) O(2)
----
B(1) C(11)
B(2) C(21)
----
˚
Radiation (MoKα) (A)
----
C(11) C(16)
C(11) ---- C(12)
C(12) ---- C(13)
˚
a (A)
˚
b (A)
˚
----
c (A)
C(13) C(14)
C(14) C(15)
C(14) ---- C(17)
C(15) ---- C(16)
C(21) ---- C(22)
C(22) ---- C(23)
C(23) ---- C(24)
C(24) ---- C(25)
˚
3
----
V(A)
Z
Dcalcd.(g cm 3)
−
1.207
0.076
372
−
1
µ (mm
F(000)
)
Crystal size (mm)
Theta range for data collection
Index ranges
0.24 × 0.14 × 0.12
◦
2.19–24.76
−25 ≤h ≤ 24, −10 ≤
k ≤ 10, −5 ≤ l ≤ 5
5921
1679
0.0512
Bond angles
B(1A) ---- O(1) ---- B(1)
O(1) ---- B(1) ---- O(2)
121.4(3)
no. of rflns. coll.
no of indep. rflns.
R(int)
118.7(3)
120.9(2)
119.1(3)
120.3(2)
121.0(2)
120.44(15)
120.44(15)
117.0(2)
120.8(3)
121.5(2)
118.1(2)
121.2(2)
120.7(3)
120.7(3)
121.9(3)
116.4(3)
121.5(3)
121.7(3)
117.1(3)
121.43(16)
121.43(16)
121.82(15)
121.82(15)
121.9(2)
121.1(2)
B(2) ---- O(2) ---- B(1)
O(2) ---- B(2) ---- O(2A)
O(1) ---- B(1) ---- C(11)
O(2) ---- B(1) ---- C(11)
O(2) ---- B(2) ---- C(21)
O(2A) ---- B(2) ---- C(21)
C(16) ---- C(11) ---- C(12)
C(13) ---- C(12) ---- C(11)
C(14) ---- C(13) ---- C(12)
C(15) ---- C(14) ---- C(13)
C(15) ---- C(14) ---- C(17)
C(13) ---- C(14) ---- C(17)
C(14) ---- C(15) ---- C(16)
C(11) ---- C(16) ---- C(15)
◦
Completeness to θ = 24.76
99.8%
Absorption correction
Semi-empirical from
equivalents
0.9909 and 0.9819
Full-matrix
least-squares on F2
1679/1/133
0.847
Max. and min transmission
Refinement method
Data/restr./param.
2
GOF on F
R1, wR2 (I > 2σ(I))
R1, wR2 (all data)
Absolute structure parameter
Extinction coefficient
Largest diff. Peak and hole (e A
CCDC
0.0334, 0.0678
0.0697, 0.0782
−0.1(15)
0.015(2)
˚
−3
----
----
)
0.116, −0.147
C(22A) C(21) C(22)
C(23) ---- C(22) ---- C(21)
C(24) ---- C(23) ---- C(22)
C(23) ---- C(24) ---- C(23A)
C(23) ---- C(24) ---- C(25)
C(23A) ---- C(24) ---- C(25)
C(22A) ---- C(21) ---- B(2)
C(22) ---- C(21) ---- B(2)
C(16) ---- C(11) ---- B(1)
C(12) ---- C(11) ---- B(1)
248934
are equally disordered over two positions. Be-
cause of the kind of the radiation and the nature of
the atoms the absolute structure could not be deter-
mined and was assigned arbitrarily. Crystal data,
data collection parameters, and refinement statis-
tics for 1 are listed in Table 1. Relevant geometric
parameters are listed in Table 2.
Results and discussion
Spectroscopic analysis
We obtained X-ray quality crystals of a new
orthorhombic polymorph of 1 from CDCl at
3
NMR spectra were recorded on a Bruker AM
ambient temperature. The boroxine 1 was syn-
thesized by hydrolysis of ethoxy(p-tolyl)bromo
borane at ambient temperature, as shown in
Scheme 1.
1
13
2
50 ( H/ C: 250.133/62.895 MHz), and Bruker
1
13 11
Avance 400 ( H/ C/ B: 400.130/100.6/128.4
MHz) spectrometer.

A new polymorph of tri(p-tolyl)boroxine
661
boroxine ring of 1 (orthorhombic) show an av-
◦
erage size of 121.1(3) and are wider than the
O---B---O angles, which adopt an average value of
◦
1
18.8 . The sums of all angles at the boron atoms
◦
are 360 in both polymorphs of 1.
In the orthorhombic crystal structure the
molecules of 1 form sloped columns. Along these
columns all molecules adopt the same orientation.
But in two neighboring layers the boroxine rings
are shifted in such a way that the position of one
boron atom is directly under that of the oxygen
atom in the para position of the B---O ring as
shown in Fig. 4. The intermolecular B---O contact
Scheme . 1. Synthesis of tri(p-tolyl)boroxine.
˚
The molecule possesses crystallographic Cs
symmetry. A mirror plane perpendicular to the
molecular plane runs through O1, B2, C21, C24,
and C25.
between these two atoms is found to be 3.321 A.
This distance is shorter than that in the triethyl
derivative.
A structural motive similar to 1 (orthorh-
ombic) has also been found for tri(p-
bromophenyl)boroxine (2) but the shortest inter-
molecular B---O distance in 2 is longer than in 1
The boroxine ring of the orthorhombic poly-
morph of 1 adopts an almost planar conforma-
˚
tion (rmsd 0.021 A) as shown in Fig. 3. Except
9
for the intermolecular B---O distance, the bond
lengths and angles of the orthorhombic poly-
morph of 1 are similar to those of the monoclinic
polymorph of 1 (see Table 3). The dihedral an-
gles in 1 (orthorhombic) between the plane of
(orthorhombic).
Interestingly, the intermolecular B
---O dis-
tances in the aryl substituted boroxines 1 (or-
thorhombic) and 2 are shorter than in the tri-
˚
ethyl substituted boroxine 3 (3.462 A). Other-
◦
the ring and the tolyl substituents are 5.7 and
wise, the crystal structure of the phenyl substi-
tuted boroxine 4 shows a structural motif with
long intermolecular B---O distances which is dif-
◦
4
1
.2 , respectively. The B---O bond lengths average
---
˚
˚
.384(3) A and the B C bond average 1.542(2) A
in 1 (orthorhombic). The B---O---B angles in the
ferent from 1 (orthorhombic) and 2 but identical
Fig . 3. ORTEP plot of the orthorhombic polymorph of 1 in
the solid state. Displacement ellipsoids are drawn at the 50%
probability level.

662
Haberecht, Bolte, Wagner, and Lerner

A new polymorph of tri(p-tolyl)boroxine
663
Fig . 4. Crystal packing diagram for the orthorhombic polymorph of 1.
to 1 (monoclinic). Obviously, the Lewis acidity
on the boron centers of boroxines is not the only
driving force of crystal packing.
The crystal structure of the orthorhombic
polymorph of 1 with aryl substituents shows that
the even more electronically saturated compound
˚
3.321 A) than the boroxine 3 with alkyl sub-
stituents. To the best of our knowledge the inter-
molecular B---O distance in 1 (orthorhombic) is
the shortest yet reported for organyl-substituted
(
boroxines.
Table 4 lists bond lengths and angles of X-ray
characterized organyl-substituted boroxines.
1
possesses shorter intermolecular B---O contacts
Table 4. The Shortest Intermolecular B---O Distances (A) in the Crystal Structures of 1–5
(orthorhombic) 1 (monoclinic) 2 (orthorhombic) 3 (hexagonal) 4 (monoclinic)
3.321 4.180 3.380 3.462 4.441
˚
1
5 (monoclinic)
B---O
4.398

664
Haberecht, Bolte, Wagner, and Lerner
Boroxine 5 can also be crystallized in the
˚
termolecular B---O contacts (3.321 A) even though
monoclinic layer type structure as shown for 1
the boron centers in 1 are substituted with aro-
matic π donor ligands. Obviously the degree of π
density the boron atoms receive from an aromatic
substituent does not decrease their Lewis acidity
to an amount where intermolecular B---O inter-
actions generally become disfavored. Rather, two
different structural motifs for the boroxines 1 with
tolyl as aromatic substituent are possible: a mono-
clinic layer structure based on unpolar stacking in-
teractions or an orthorhombic polymorph in which
the molecules build sloped columns with short
intermolecular B---O contacts. Thus we conclude
that for boroxines bearing aromatic substituents
as in 1 different intermolecular arrangements are
attractive. Intermolecular B---O interactions are
not necessarily responsible for the crystal pack-
ing but are still attractive. Moreover, they can
be even shorter than in the aliphatic substituted
boroxine 3.
3
and 4 in Fig. 2. Because of steric strain, the
10
crystal structures of tri(mesityl)boroxine 6 and
11
tri(ferrocenyl)boroxine 7 feature no columns.
The boroxines 1–7 possess quite similar
B---O bond lengths in the solid (range of 1.377 A
˚
˚
for 6 and 1.390 A for 2). The average values for the
B---O---B angles are found to be between 121.1 to
◦ ◦
22.7 and are therefore larger than 120 , whereas
1
◦
the O---B---O angles are smaller than 120 with
◦
◦
average sizes between 117.3 and 119.2 . Table 3
shows that the organic substituents on the borox-
ine ring do not substantially affect the geometry
of the ring itself.
The degree of π electron density transferred
from the substituents on the boroxine core should
be reflected in the length of the B---C bond. Ac-
cording to the literature, the calculated value for
a B---C single bond is 1.61 A, whereas the value
˚
12
for the B---C double bond is 1.40 A. The boron
˚
carbon bond lengths found of the boroxines 1–
Acknowledgment
7
range between B---C single and B---C double
˚
bonds. With a length of 1.565 A the triethyl deriva-
We are grateful to the University of Frankfurt
for financial funding.
tive 3 possesses the longest B---C bond of all
structurally characterized boroxines. Generally,
the molecules bearing aromatic substituents pos-
sess shorter B---C bonds with the minimum of
Supplementary material Crystallographic data (excluding struc-
ture factors) for the structure reported in this paper have been de-
posited with the Cambridge Crystallographic Data Centre as supple-
mentary publication no. CCDC 248934. Copies of the data can be
obtained free of charge on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ (Telefax : +1223/336 033; e-mail: deposit@
ccdc.cam.ac.uk).
˚
1
.542 A found for 2.
Obviously, the degree of B---C π bonding is
slightly higher in the aromatic substituted borox-
ines. The B---C bond in the monoclinic polymorph
of 1 is shorter than that of 4 which might be due
to tolyl being a better π donor than phenyl.
The crystal structures of the monoclinic
polymorph of 1, and the boroxines 4 and 5, show
intermolecular B---O distances between 4.180 and
References
1
. Brock, C.P.; Minton, R.P.; Niedenzu, K. Acta Cryst. C 1987,
C43, 1775–1779.
2. Beckett, M.A.; Brassington, D.S.; Owen, P.; Hursthouse, M.B.;
Light, M.E.; Malik, K.M.A.; Varma, K.S. J. Organomet. Chem.
˚
.441 A (Table 4).
4
1
999, 585, 7–11.
3
. Alcarez, G.; Euzenat, L.; Mongin, O.; Katan, C.; Ledoux, I.;
Zyss, J.; Blanchard-Desce, M.; Vaultier, M. Chem. Commun
Summary and conclusion
2
003, 2766–2767.
. Boese, R.; Polk, M.; Bl a¨ ser, D. Angew. Chem. 1987, 99, 239–
41.
4
5
2
The crystal structure of the orthorhombic
polymorph of tri(p-tolyl)boroxine 1 presented
here shows structural features which are differ-
ent from those of the boroxines 3 and 4. The
orthorhombic polymorph of 1 possesses short in-
. Beckett, M.A.; Strickland, G.C.; Varma, K.S.; Hibbs, D.E.;
Hursthouse, M.B.; Malik, K.M.A. J. Organomet. Chem. 1997,
535, 33–41.
. Haberecht, M.C.; Heilmann, J.B.; Haghiri, A.; Bolte, M.; Bats,
J.W.; Lerner, H.-W.; Holthausen, M.C.; Wagner, M. Z. Anorg.
Allg. Chem. 2004, 630, 904–913.
6

A new polymorph of tri(p-tolyl)boroxine
665
7
. Sheldrick, G.M. Acta Cryst. A 1990, 46, 467.
10. Anulewicz-Ostrowska, R.; Lulinski, S.; Serwatowski, J.;
Suwinska, K. Inorg. Chem. 2000, 39, 5763–5767.
11. Bats, J.W.; Ma, K.; Wagner, M. Acta Cryst. C 2002, C58, m129–
m132.
8. Sheldrick, G.M. SHELXL-97, Program for Crystal Struc-
ture Refinement; University of G o¨ ttingen: Germany,
1
997.
9
. Avent, A.G.; Lawrence, S.F.; Meehan, M.M.; Russell, T.G.;
12. Hollemann, A.F.; Wiberg, E. Hollemann-Wiberg: Lehrbuch
der Anorganischen Chemie; de Gruyter: Berlin, 1995;
p 1842.
Spalding, T.R. Coll. Czech. Chem. Commun 2002, 67, 1051–
1
060.