Synthesis, crystal structures, linear and nonlinear optical properties, and theoretical studies of (p-R-phenyl)-, (p-R-phenylethynyl)-, and (E)-[2-(p-Rphenyl)ethenyl]dimesitylboranes and related compounds

Article abstract of DOI:10.1002/chem.200501096

The (p-R-phenyl)dimesitylboranes (R = Me₂N, MeO, MeS, Br, I), (p-R-phenylethynyl)dimesitylboranes (R = Me₂N, MeO, MeS, H), (E)-[2-(pR-phenyl)ethenyl]dimesitylboranes (R = Me₂N, H₂N, MeO, MeS, H, CN, NO₂

Full text of DOI:10.1002/chem.200501096

DOI: 10.1002/chem.200501096
Synthesis, Crystal Structures, Linear and Nonlinear Optical Properties, and
Theoretical Studies of (p-R-Phenyl)-, (p-R-Phenylethynyl)-, and (E)-[2-(p-R-
Phenyl)ethenyl]dimesitylboranes and Related Compounds
Zheng Yuan,[a] Christopher D. Entwistle,[b] Jonathan C. Collings,[b] David Albesa-JovØ,[b]
Andrei S. Batsanov,[b] Judith A. K. Howard,[b] Nicholas J. Taylor,[a]
Hanns Martin Kaiser,[b, c] Dieter E. Kaufmann,[c] Suk-Yue Poon,[d] Wai-Yeung Wong,*[d]
Christophe Jardin,[e] Sofiane Fathallah,[e] Abdou Boucekkine,*[e]
Jean-FranÅois Halet,*[e] and Todd B. Marder*[a, b]
Abstract: The (p-R-phenyl)dimesityl-
boranes (R=Me2N, MeO, MeS, Br, I),
(p-R-phenylethynyl)dimesitylboranes
(R=Me2N, MeO, MeS, H), (E)-[2-(p-
R-phenyl)ethenyl]dimesitylboranes
of the donor-substituted compounds in
particular exhibiting large bathochro-
mic shifts in highly polar solvents, indi-
cative of charge transfer leading to
large dipole moments in the excited
state. The molecular structures of the
termined from single-crystal X-ray dif-
fraction measurements, offer evidence
of increased conjugation in the ground
state with increased donor strength of
the
R substituent. Their first- and
(R=Me2N, H2N, MeO, MeS, H, CN,
NO2), (E)-[2-(2-thienyl)ethenyl]dimesi-
tylborane, and (E)-[2-(o-carboranyl)e-
thenyl]dimesitylborane have been pre-
pared through the reaction of the ap-
propriate p-R-phenyl- and p-R-phenyl-
ethynyllithium reagents with dimesityl-
boron fluoride and by hydroboration of
the appropriate p-R-phenylacetylene,
2-ethynylthiophene, and o-ethynylcar-
borane with dimesitylborane. Their
UV/Vis absorption and emission spec-
tra have been recorded in a range of
solvents with the fluorescence maxima
second-order molecular hyperpolariza-
bilities have been obtained from
EFISH and THG measurements, the
first-order hyperpolarizabilities being
largest for the strongest R-substituent
donors. AM1 calculations have been
performed on these compounds, show-
ing reasonable agreement with the ex-
perimentally obtained bond lengths
and hyperpolarizabilities, as well as on
several related hypothetical compounds
containing multiple C=Cbonds, most
of which are proposed to have even
larger hyperpolarizabilities.
(p-R-phenyl)dimesitylboranes
(R=
Me2N, MeO, MeS, Br, I), the (E)-[2-(p-
R-phenyl)ethenyl]dimesitylboranes
(R=Me2N, H2N MeO, MeS, H), (p-R-
phenylethynyl)dimesitylborane
(R=
Me2N), and (E)-[2-(2-thienyl)ethenyl]-
dimesitylborane, which have been de-
Keywords: boranes · nonlinear op-
tics · semiempirical calculations ·
synthetic methods · X-ray diffrac-
tion
[d] Dr. S.-Y. Poon, Dr. W.-Y. Wong
[a] Dr. Z. Yuan, Dr. N. J. Taylor, Prof. T. B. Marder
Department of Chemistry, University of Waterloo Waterloo, ON N2L
3G1 (Canada)
Department of Chemistry and the
Centre for Advanced Luminescence Materials
Hong Kong Baptist University, Kowloon Tong, Hong Kong
(P.R. China)
E-mail: todd.marder@durham.ac.uk
[b] Dr. C. D. Entwistle, Dr. J. C. Collings, Dr. D. Albesa-JovØ,
Dr. A. S. Batsanov, Prof. J. A. K. Howard, H. M. Kaiser,
Prof. T. B. Marder
E-mail: rwywong@hkbu.edu.hk
[e] Dr. C. Jardin, Dr. S. Fathallah, Prof. A. Boucekkine, Prof. J.-F. Halet
Laboratoire de Chimie du Solide et Inorganique MolØculaire
UMR 6511 CNRS, UniversitØ de Rennes 1, Institut de Chimie de
Rennes
Department of Chemistry, University of Durham
South Road, Durham, DH1 3LE (UK)
Fax : (+44)191-384-4737
Campus de Beaulieu, 35042 Rennes (France)
E-mail: aboucekk@univ-rennes1.fr
[c] H. M. Kaiser, Prof. Dr. D. E. Kaufmann
Instituts fuer Organische Chemie der Technischen Universitaet
Clausthal
halet@univ-rennes1.fr
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
Leibnizstrasse 6, 38678, Clausthal-Zellerfield (Germany)
2758
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Chem. Eur. J. 2006, 12, 2758 – 2771

FULL PAPER
Introduction
of 1-NMe2, 1-OMe, 1-SMe, 1-Br, 1-I, 2-NMe2, 3-NMe2, 3-
NH2, 3-OMe, 3-SMe, 3-H, and 4 were solved from single-
crystal X-ray diffraction data. In addition, we have per-
formed AM1 calculations on all of the above compounds,
with the exception of 4 and 5, as well as on several hypo-
thetical compounds for comparison. Selected characteriza-
tion data for 3-NMe2, 3-OMe, and 3-SMe and the crystal
structures of 3-OMe and 3-H determined at 200 K,[5a] as well
as second-[5b] and third-order[5d] NLO coefficients for several
of the compounds, have been reported previously.
In recent years, there has been an increasing amount of in-
terest in three-coordinate organoboron compounds for vari-
ous optical and electronic applications due to the presence
of a vacant pz orbital on the boron.[1] Although three-coordi-
nate boranes are normally unstable towards hydrolysis by
moisture, a large degree of stability can be conferred on
these compounds through the use of bulky substituents on
the boron atom, such as the 2,4,6-trimethylphenyl (mesityl)
group, which protect the vacant p orbital from attack by ex-
ternal nucleophiles whilst maintaining conjugation with
other unsaturated hydrocarbon substituents.[2] Two mesityl
groups appear to be sufficient, in most cases, to prevent hy-
drolysis by moisture in the air. Several such systems have
been examined by cyclic voltammetry, and the results ob-
tained indicated that the dimesitylboryl [B(mes)2] group is a
p acceptor comparable in strength to the CN group.[3] It is
important to note that boron is more electropositive than
carbon so the B(mes)2 group is also an inductive s donor, in
contrast with typical p acceptors.
In early work in this field, a number of substituted (phe-
nyl)dimesitylboranes were examined.[4] The quantum yields
were found to be higher for donor-substituted compounds,
for which the fluorescence maxima shifted to longer wave-
lengths (bathochromism) in solvents of increasing polarity.
This behavior was attributed to a highly polar excited state
that is stabilized by solvent interactions.[4a] The Kodak group
also incorporated the B(mes)2 moiety into larger p systems,
such as azo dyes, where it was found to act as an auxo-
chrome.[4b]
The last decade has seen renewed interest in B(mes)2 sys-
tems. Several conjugated systems containing B(mes)2 groups
have been synthesized which exhibit sizable second- and
third-order nonlinear optical (NLO) coefficients.[5,6] Conju-
gated molecules with terminal B(mes)2 groups have been
demonstrated to act as efficient electron-transporting layers
in organic light-emitting diodes.[7,8] Recently, highly fluores-
cent donor–acceptor B(mes)2-substituted tolane derivatives
have been synthesized and shown to exhibit large solvato-
chromism, while related compounds have been shown to be
effective fluoride-ion sensors.[9] Several conjugated mole-
cules containing B(mes)2 groups have been found to exhibit
large two-photon absorption (TPA) cross-sections.[10,11]
For some time we have been engaged in a program to
synthesize and investigate the properties of molecular sys-
tems containing B(mes)2 groups.[5,11] Herein, we present de-
tails of the synthesis and optical properties, both linear and
Results and Discussion
Synthesis: The (p-R-phenyl)- (1) and (p-R-phenylethynyl)di-
mesitylboranes (2) were prepared from the appropriate p-R-
phenyl bromides and p-R-phenylacetylenes by lithiation
with n-butyllithium (nBuLi) in dry hexane at room tempera-
ture followed by reaction with dimesitylboron fluoride in
dry diethyl ether, or dry benzene in the case of 2 as the
latter reactions when carried out in THF or diethyl ether
produced four-coordinate borate complexes (Scheme 1). A
50% excess of nBuLi was used and the p-R-phenyllithium
reagents were separated as precipitates. The synthesis of
analogous compounds by this route, including 1-NMe2,[4a] 1-
Br,[4a] and 2-H,[12] have been reported previously.
The (E)-[2-(p-R-phenyl)ethenyl]dimesitylboranes (3),
(E)-[2-(2-thienyl)ethenyl]dimesitylborane (4), and (E)-[2-(o-
carboranyl)ethenyl]dimesitylborane (5) were prepared by
hydroboration of the appropriate p-R-phenylacetylene, 2-
thienylacetylene, and o-ethynylcarborane, respectively, with
dimesitylborane in dry THF at room temperature
(Scheme 1). Only syn mono-hydroboration occurred owing
to the steric hindrance of the two mesityl groups. (E)-[2-
(Phenyl)ethenyl]dimesitylborane (3-H) has previously been
prepared by this methodology.[13]
All of the compounds are crystalline materials with colors
ranging from yellow to colorless. The (p-R-phenyl)- and
(E)-[2-(p-R-phenyl)ethenyl]dimesitylboranes appear to be
air-stable, with no appreciable decomposition of the solids
over long periods of storage in air (in some cases
>10 years) and can generally be subjected to chromatogra-
phy on silica gel. However, 3-NO2 decomposed on a neutral
alumina column with a benzene/hexane mixture as eluent.
The (p-R-phenylethynyl)dimesitylboranes are less stable, hy-
drolyzing slowly as solids and more rapidly in solution.
Crystallography: Single crystals suitable for X-ray diffrac-
tion were obtained by slow evaporation of hexane solutions.
The crystal and molecular structures of 1-NMe2, 1-OMe, 1-
SMe, 1-Br, 1-I, 2-NMe2, 3-NMe2, 3-NH2, 3-OMe, 3-SMe, 3-
H, and 4 were determined from single-crystal X-ray diffrac-
tion measurements carried out at low temperatures. Crystal-
lographic data and refinement parameters are given in
Table 1. The crystal structures of the related molecules phe-
nyldimesitylborane 1-H[14] at 188 K, [2-(N,N-dimethylami-
no)phenyl]dimesitylborane,[15] and (mesitylethynyl)dimesi-
nonlinear, of
a series of (p-R-phenyl)dimesitylboranes,
where R=Me2N (1-NMe2), MeO (1-OMe), MeS (1-SMe),
Br (1-Br), and I (1-I), (p-R-phenylethynyl)dimesitylboranes,
where R=Me2N (2-NMe2), MeO (2-OMe), MeS (2-SMe),
and H (2-H), (E)-[2-(p-R-phenyl)ethenyl]dimesitylboranes,
where R=Me2N (3-NMe2), H2N (3-NH2), MeO (3-OMe),
MeS (3-SMe), H (3-H), CN (3-CN), and NO2 (3-NO2), (E)-
[2-(2-thienyl)ethenyl]dimesitylborane (4), and (E)-[2-(o-car-
boranyl)ethenyl]dimesitylborane (5). The crystal structures
Chem. Eur. J. 2006, 12, 2758 – 2771
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W.-Y. Wong, A. Boucekkine, J.-F. Halet, T. B. Marder et al.
predicted on the basis of their
Hammett constants. The dis-
crepancies are, in general,
larger for the (E)-[2-(p-R-phe-
nyl)ethenyl]dimesitylboranes
than for the analogous (p-R-
phenyl)dimesitylboranes.
It was found for the (E)-[2-
(p-R-phenyl)ethenyl]dimesityl-
boranes that, in general, the
2
ꢀ
difference between the C(sp )
C(aryl) and the C=Cbond
lengths decreases slightly with
increasing donor strength of
the
R
substituent, ranging
from 0.130 Š for 3-H to
0.102 Š for 3-NMe2. This sug-
gests that the degree of
ground-state p-electron conju-
gation is greater for the more
strongly donating R substitu-
ents.
The bond lengths of several
of the phenyl groups show no-
ticeable quinoidal distortions
which, in general, are larger
for donor-substituted com-
pounds. The differences be-
tween the longer and the short-
ꢀ
er C(aryl) C(aryl) bonds range
from 0.029 Š for 1-NMe2 to
0.003 Š for 1-Br and are, in
general, larger for the (p-R-
phenyl)dimesitylboranes than
for the analogous (E)-[2-(p-R-
phenyl)ethenyl]dimesitylbor-
anes. This has been attributed
to a small contribution of the
charge-separated
canonical
Scheme 1. Synthesis of the (p-R-phenyl)- (1), (p-R-phenylethynyl)- (2), (E)-[2-(p-R-phenyl)ethenyl]dimesityl-
boranes (3), (E)-[2-(2-thienyl)ethenyl]dimesitylborane (4), and (E)-[2-(o-carboranyl)ethenyl]dimesitylborane
(5). Structures of the cyano and nitro analogues of 1-NMe2 and 3-NMe2, 6 and 7, and of the hypothetical ex-
form to the ground state, as
shown in Scheme 2.[4a] A simi-
lar situation has been observed
for the deprotonated trimesi-
tylborane anion, which was de-
ꢀ ꢀ
tended Me2N Y B(mes)2 compounds, 8 and 9.
tylborane at room temperature[16] have been reported previ-
ously. All of the compounds crystallize with one molecule in
the asymmetric unit, apart from 4 which has two independ-
ent molecules. Their molecular structures are shown in
Figure 1, Figure 2, and Figure 3 and selected bond lengths
and angles are given in Table 2 and Table 3.
scribed as having “boron ylide” character.[17]
The boron atom in each of the structures has a trigonal
planar configuration with the groups surrounding it ar-
ranged in a propeller-like conformation with the same sense
of rotation in all of the structures except that of 2-NMe2.
This has been attributed to steric hindrance, which is lowest
for the (phenylethynyl)dimesitylboranes. The mean planes
of the mesityl groups are inclined at dihedral angles of 47.9–
ꢀ
In all of the structures, the B C(mes) bond lengths are
ꢀ
slightly longer than the B C(aryl/ethynyl/vinyl) ones. In
ꢀ
general, the difference increases with increasing donor
strength (measured by the Hammett constant) of the R sub-
stituent, ranging from the insignificant value of 0.004 Š for
1-Br to 0.067 Š for 2-NMe2, although the SMe-substituted
compounds seem to have larger discrepancies than would be
66.28 relative to the B C3 planes. The NMe2 groups in 1-
NMe2 and 3-NMe2 are also trigonal planar; however, the
NH2 group in 3-NH2 is slightly pyramidalized (sum of
angles=351.98). The Me2N planes are twisted by 5.6 and
5.88, respectively, to the mean plane of the phenyl group,
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Chem. Eur. J. 2006, 12, 2758 – 2771

Synthesis and Characterization of Dimesitylboranes
Table 1. Crystallographic data refinement parameters.
FULL PAPER
Compound
1-NMe2
1-OMe
1-SMe
1-Br
1-I
2-NMe2
3-NMe2
3-OMe
3-SMe
3-H
3-NH2
4
CCDC no.
formula
formula
weight
T [K]
282012
282013
282014
282015
282016
282017
282018
282019
282020
282021
282022
282023
C26H32BN C25H29BO C25H29BS C24H26BBr C24H26BI C28H32BN C28H34BN C27H31BO C27H31BS C26H29B C26H30BN C24H27BS
369.34
356.29
372.35
405.17
452.16
393.4
395.37
382.33
398.39
352.30
367.32
358.33
120(2)
120(2)
120(2)
120(2)
120(2)
295
120(2)
120(2)
120(2)
120(2)
120(2)
120(2)
crystal
system
space group
(no.)
monocl.
monocl.
monocl. triclinic
triclinic
monocl.
monocl.
monocl.
monocl. monocl. monocl.
triclinic
¯
¯
¯
P21/c
(14)
P21/n
P21/c
P1
P1
P21/c
(14)
P21/c
(14)
P21/c
(14)
P21/c
P21/n
(14[a]
P21/n
(14[a]
P1
(14[a]
)
(14)
(2)
(2)
(14)
)
)
(2)
a [Š]
b [Š]
c [Š]
14.403(3) 7.961(1)
11.466(2) 19.954(3) 8.1293(7) 11.051(2) 11.071(1) 7.813(1)
13.261(3) 13.177(2) 22.897(2) 11.481(1) 11.619(1) 22.876(4) 11.738(3) 13.142(2) 12.656(1) 18.930(3) 19.810(1) 19.010(2)
11.523(1) 8.354(1)
8.399(1) 13.882(3) 14.876(4) 14.372(2) 14.805(1) 9.754(2) 8.001(1)
8.388(2)
14.130(4) 11.925(2) 12.348(1) 11.550(2) 13.642(1) 14.627(1)
a [8]
90
97.98(1)
90
90
100.47(1) 98.30(1) 77.64(1)
90 90 87.03(1)
90
82.64(1)
82.86(1) 90
78.90(1) 100.21(2) 109.16(1) 90.07(1)
86.87(1) 90 90 90
90
90
90
91.10(1) 103.88(1) 95.26(1)
90 90 90
90
90
69.60(2)
80.78(2)
74.71(2)
b [8]
g [8]
V [Š3]
Z, 1calcd
2168.8(8) 2058.4(5) 2122.3(3) 1026.5(2) 1051.4(2) 2441.8(8) 2330.6(11) 2252.3(5) 2313.2(3) 2070.4(6) 2153.1(3) 2102.7(6)
4, 1.131
4, 1.150
4, 1.165
2, 1.311
2, 1.428
4, 1.070
4, 1.127
4, 1.112
4, 1.144
4, 1.130
4, 1.133
4, 1.132
[gcmꢀ3
]
m [mmꢀ1
]
0.06
61
26569
0.07
55
0.16
58
2.01
60
1.53
60
0.06
50
0.06
55
0.07
61
0.15
61
0.06
55
0.06
55
0.16
55
17721
2q max [8]
total reflec-
tions
13817
25400
16342
16358
4849
27065
17513
32452
16582
18955
unique refls. 6424
unique refls., 5302
I>2s(I)
4728
3885
5650
4605
5950
4760
6085
5765
4319
2161
5356
3662
6831
5560
7050
4640
4744
3859
4959
3371
9657
4733
variables
Rint
R(F),
380
0.030
0.051
283
0.036
0.063
360
0.037
0.042
261
0.026
0.032
253
0.018
0.023
304
0.024
0.058
287
0.063
0.064
324
0.029
0.046
386
0.054
0.059
262
0.030
0.047
373
0.042
0.053
493
0.087
0.071
I>2s(I)
wR(F2), all
data
0.152
0.145
0.113
0.088
0.065
0.109
0.170
0.144
0.178
0.131
0.146
0.171
[a] Nonstandard setting.
Figure 1. Molecular structures of a) 1-NMe2, c) 1-OMe, d) 1-SMe, e)1-Br, and f) 1-I projected on the B/C1/C11/C21 plane with thermal ellipsoids drawn
ꢀ
at the 50% probability level. The typical conformation of a PhB(mes)2 moiety viewed down the B C1 bond is shown in part (b). The numbering of
carbon atoms is identical for each structure.
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W.-Y. Wong, A. Boucekkine, J.-F. Halet, T. B. Marder et al.
16.68 occurs in the molecule
with the strongest donor sub-
stituent, 3-NMe2, the analo-
gous angle in 3-NH2 is 31.78, a
molecule with an R substituent
of comparable donor strength,
and only 19.18 in 3-SMe which
is not a very strong donor.
There is more correlation be-
tween the angles between the
ꢀ
B C3 planes and the mean
planes of the vinyl groups,
ranging from 11.88 for 3-NMe2
to 26.08 for 3-H. However, the
sizable discrepancy of 8.28 be-
ꢀ
tween the thiophenyl/B C3 di-
hedral angles in the two crys-
tallographically
independent
molecules of 4 indicates that
much of the variation in these
angles could be attributable to
the effect of crystal-packing
forces.
Linear optical properties: The
UV/Vis absorption and emis-
sion spectra of compounds 1-
NMe2, 1-OMe, 1-SMe, 1-Br, 2-
NMe2, 3-NMe2, 3-OMe, 3-
SMe, 3-H, and 3-NH2 were de-
termined in a variety of sol-
vents. Their absorption and
emission maxima in these sol-
vents and their extinction coef-
ficients in cyclohexane are
given in Table 4. In general,
the (E)-[2-(p-R-phenyl)ethe-
nyl]dimesitylboranes absorb at
longer wavelengths than the
(p-R-phenyl)dimesitylboranes.
The absorption maxima of the
(p-R-phenyl)dimesitylboranes
in cyclohexane range from
Figure 2. Molecular structures of a) 2-NMe2, b) 3-NMe2, c) 3-OMe, d) 3-SMe, e) 3-H, and g) 3-NH2 projected
on the B/C1/C11/C21 plane with thermal ellipsoids drawn at the 50% probability level (30% for 2-NMe2).
ꢀ
The conformation of 3-H viewed down the B C8 bond is shown in part (f). The numbering of carbon atoms is
identical for each structure.
whereas the MeO groups in 1-OMe and 3-OMe are twisted
by 8.08 and the MeS groups in 1-SMe and 3-SMe by 19.5
and 11.18, respectively, to the phenyl plane.
In the (p-R-phenyl)dimesitylboranes, the dihedral angles
between the mean plane of the substituted phenyl ring and
354 nm for 1-NMe2 to 318 nm for 1-Br and 1-OMe, and for
the (E)-[2-(p-R-phenyl)ethenyl]dimesitylboranes from
396 nm for 3-NMe2 to 332 nm for 3-H. The maxima of com-
pounds 2-NMe2 and 3-NMe2 also exhibit low-energy should-
ers. This indicates that molecules containing stronger elec-
tron-donor substituents typically exhibit lower energy ab-
sorptions, although 1-OMe and 1-Br exhibit similar absorp-
tions. The absorption maxima also exhibit small bathochro-
mic solvatochromic shifts in solvents of increasing polarity,
the largest shift of 28 nm occurring for 3-NH2 in going from
cyclohexane to DMF. These small solvatochromic absorp-
tion shifts are consistent with small ground-state dipole mo-
ments.
ꢀ
the B C3 plane, in general, decrease with increasing elec-
tron-donor strength of the R substituent, ranging from 26.28
for 1-I to 19.58 for 1-NMe2. However, the smallest phenyl/
ꢀ
B C3 angle by a considerable margin, 8.08, occurs for 1-
ꢀ
OMe. The phenyl/B C3 angle for 2-NMe2 of 16.58 is about
half that of (mesitylethynyl)dimesitylborane.[16] In the (E)-
[2-(p-R-phenyl)ethenyl]dimesitylboranes there is less corre-
lation between the donor strengths of the R substituents
ꢀ
and the phenyl/B C3 angles. Although the smallest angle of
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Chem. Eur. J. 2006, 12, 2758 – 2771

Synthesis and Characterization of Dimesitylboranes
FULL PAPER
electron-donor substituents exhibit smaller HOMO–LUMO
gaps. The emission maxima also exhibit much larger batho-
chromic solvatochromic shifts in solvents of increasing polar-
ity, the largest for 1-NMe2 of 138 nm is between cyclohexane
and acetonitrile solutions. These observations reflect ones
made for a previously studied series of (p-R-phenyl)dimesi-
tylboranes[4a] and are strongly indicative of an intramolecu-
lar charge-transfer (ICT) absorption from a weakly polar-
ized ground state to a highly polarized excited state. Increas-
ing solvent polarity stabilizes the ICT excited state relative
to the ground state, which is the cause of the solvatochromic
shifts.
Nonlinear optical properties: The ground-state dipole mo-
ments mg, molecular polarizabilities a, first-order molecular
hyperpolarizabilities bexp, and second-order molecular hyper-
polarizabilities gexp of compounds 1-NMe2, 1-OMe, 1-SMe,
1-Br, 2-NMe2, 2-OMe, 2-SMe, 2-H, 3-NMe2, 3-OMe, 3-SMe,
3-H, 3-NH2, 3-CN, and 3-NO2 have been measured and are
listed in Table 5 along with the related values for the ana-
logues of 1-NMe2 and 3-NMe2 in which the B(mes)2 group is
replaced by CN and NO2 groups, namely p-(N,N-dimethyla-
mino)benzonitrile, (6-CN), p-(N,N-dimethylamino)nitroben-
zene (6-NO2), (E)-p-(N,N-dimethylamino)phenylpropeneni-
trile (7-CN), and (E)-p-(N,N-dimethylamino)phenylethenyl
nitrite (7-NO2).[18] These molecules are illustrated in
Scheme 1.
The ground-state dipole moments were measured using a
capacitance bridge. The magnitudes of mg were all found to
be very small and dependent on the R substituents, ranging
from 1.0”10ꢀ18 esu for 3-H to 3.6”10ꢀ18 esu for 2-NMe2.
The values for 1-NMe2 and 3-NMe2 are approximately half
those of their analogues, 6-NO2
Figure 3. Top: molecular structure of 4 projected on the B/C1/C11/C21
plane (thermal ellipsoids at the 50% probability level). Bottom: confor-
mations of the two independent molecules (solid and dashed lines)
ꢀ
viewed down the same plane. Average bond distances: S C1 1.725(3),
ꢀ
ꢀ
ꢀ
ꢀ
S C4 1.705(4), C1 C2 1.374(5), C2 C3 1.412(5), C3 C4 1.347(6) Š.
The (E)-[2-(p-R-phenyl)ethenyl]dimesitylboranes also
emit at longer wavelengths than the (p-R-phenyl)dimesityl-
boranes. The emission maxima of the (p-R-phenyl)dimesityl-
boranes in cyclohexane range from 372 nm for 1-Br to
354 nm for 1-OMe, and for the (E)-[2-(p-R-phenyl)ethenyl]-
dimesitylboranes from 422 nm for 3-NMe2 to 393 nm for 3-
SMe. This also indicates that molecules containing stronger
and 7-NO2. This is consistent
Table 2. Mean bond lengths [Š] and selected torsion angles [8] in molecules 1: X-ray versus MO (italicized)
data (X=N, O, S, Br or I).
with there being relatively
little charge transfer in the
ground state of the B(mes)2-
containing compounds. The
Compound
1-NMe2
1-OMe
1-SMe
1-H[a]
1-Br
1-I
Hammett constant
ꢀ0.77
1.545(2)
1.541
1.586(2)
1.552
0.041
1.409(2)
1.400
1.382(2)
1.388
1.413(2)
1.418
0.029
1.368(1)
1.405
ꢀ0.13
1.558(3)
1.544
1.587(3)
1.552
0.029
1.408(2)
1.401
1.383(3)
1.391
1.394(3)
1.404
0.018
1.365(2)
1.380
0.07
1.563(2)
1.545
1.582(2)
1.551
0.019
1.407(2)
1.401
1.387(2)
1.391
1.397(2)
1.401
0.015
1.764(1)
1.692
55.8(2)
65.8
53.7(2)
65.9
24.4(2)
29.3
0
0.23
1.571(2)
1.549
1.575(2)
1.549
0.004
1.403(2)
1.400
0.28
1.568(2)
1.550
1.574(2)
1.550
0.006
1.407(2)
1.400
1.390(2)
ꢀ
(a) B C1
1.569(3)
1.550
1.579(2)
1.550
0.010
1.398(2)
1.401
1.392(3)
1.3941.3941.395
1.375(3)
1.395
ꢀ0.007
–
molecular polarizabilities
a
vary little and are in the range
ꢀ
(b) B C(mes)
4.2–5.0”10ꢀ23 esu, except for 3-
bꢀa
NO2
for which a=8.8”
10ꢀ23 esu. The values for 1-
NMe2 and 3-NMe2 are approxi-
mately twice those of their an-
alogues, 6-NO2 and 7-NO2,
which suggests that the B-
(mes)2 group is much more po-
larizable than the NO2 group.
Their first-order molecular
hyperpolarizabilities bexp were
obtained from electric-field-in-
duced second harmonic gener-
ation (EFISH) at 1.907 mm in
CHCl3.[18a] In general, the [2-
(p-R-phenyl)ethenyl]- and (p-
R-phenylethynyl)dimesitylbor-
ꢀ
(c) C 1 C2/6
ꢀ
ꢀ
(d) C 2 C3, C5 C6
1.391(2)
ꢀ
(e) C 4 C3/5
1.384(2)
1.398
0.003
1.895(1)
1.8742.021
47.1(2)
65.8
57.3(2)
65.6
26.5(2)
34.1
1.393(2)
1.396
0.010
2dꢀcꢀe
ꢀ
C4 X
2.089(1)
–
C1-B-C11-C16
C1-B-C21-C22
C2-C1-B-C11
C3-C4-X-C9
66.8(1)
66.2
50.5(1)
65.9
20.0(2)
20.3
62.6(2)
66.1
59.5(2)
66.0
8.5(2)
17.5
59.3(2)
65.8
59.3(2)
65.8
21.1(1)
30.2
–
46.8(2)
65.6
56.6(2)
65.8
26.0(2)
35.1
–
ꢀ9.8(2)
ꢀ7.6(3)
19.7(1)
–
–
ꢀ18.9
ꢀ0.40.0
–
–
[a] Molecule 1-H has crystallographic C2 symmetry at 188 K, see reference [14].
Chem. Eur. J. 2006, 12, 2758 – 2771
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2763

W.-Y. Wong, A. Boucekkine, J.-F. Halet, T. B. Marder et al.
Table 3. Mean bond lengths [Š] and selected torsion angles [8] in molecules 2, 3, and 4: X-ray versus calculated (italicized) values.
Compound
2-NMe2
3-NMe2
3-OMe
3-SMe
3-H
3-NH2
4
Hammett constant
ꢀ0.77
ꢀ0.77
1.540(3)
1.535
ꢀ0.13
1.551(1)
1.535
0.07
1.550(2)
1.535
0
ꢀ0.57
1.537(3)
1.533
ꢀ
(a) B C8
1.504(6)
1.503
1.554(2)
1.535
1.543(5)
ꢀ
(b) B C(mes)
1.573(6)
1.548
1.589(3)
1.552
1.585(1)
1.550
1.580(3)
1.550
1.577(2)
1.550
1.587(3)
1.551
1.582(5)
bꢀa
0.067
0.045
0.049
0.017
0.034
0.015
0.030
0.015
0.023
0.015
0.050
0.017
0.039
ꢀ
C7 C8
1.226(6)
1.204
1.355(3)
1.341
1.348(1)
1.340
1.342(2)
1.340
1.340(2)
1.340
1.354(2)
1.341
1.340(4)
ꢀ
C1 C7
1.412(6)
1.405
1.453(3)
1.451
1.463(1)
1.453
1.467(2)
1.453
1.470(2)
1.454
1.450(2)
1.451
1.440(4)
ꢀ
(c) C 1 C2/6
1.385(6)
1.404
1.398(3)
1.403
1.403(1)
1.404
1.395(2)
1.404
1.396(2)
1.404
1.402(2)
1.404
–
ꢀ
ꢀ
(d) C 2 C3, C5 C6
1.373(6)
1.387
1.381(3)
1.387
1.385(1)
1.390
1.384(3)
1.390
1.391(2)
1.393
1.376(3)
1.387
–
ꢀ
(e) C 4 C3/5
1.385(6)
1.418
1.404(3)
1.418
1.394(1)
1.404
1.390(3)
1.401
1.379(2)
1.394
1.400(3)
1.416
–
2dꢀcꢀe
ꢀ0.024
ꢀ0.048
1.370(5)
1.404
ꢀ0.040
ꢀ0.047
1.370(3)
1.405
ꢀ0.027
ꢀ0.028
1.368(1)
1.380
ꢀ0.017
ꢀ0.025
1.757(2)
1.692
0.007
ꢀ0.050
ꢀ0.046
1.381(2)
1.395
–
ꢀ0.012
ꢀ
C4 X
–
–
C8-B-C11-C16
C8-B-C21-C22
C2-C1-B-C11
C7-C8-B-C21
C1-C7-C8-B
C2-C1-C7-C8
C3-C4-X-C9
33.0(4)
64.9
62.4(4)
65.5
52.5(3)
67.9
60.3(3)
60.7
23.0(3)
14.8
11.8(3)
3.0
ꢀ177.6(2)
ꢀ177.1
4.3(3)
4.3
45.3(1)
64.9
65.4(1)
64.8
30.1(1)
20.8
20.9(1)
25.0
172.4(1)
178.3
6.5(2)
16.5
171.8(1)
180.0
45.2(2)
64.9
64.7(2)
64.7
22.4(2)
25.1
18.4(2)
27.9
171.9(2)
178.2
2.3(3)
16.0
168.6(2)
177.6
48.1(2)
68.0
54.9(2)
60.7
43.1(1)
33.1
26.0(2)
6.5
170.8(1)
178.5
13.5(2)
15.2
–
–
55.4(2)
64.2
63.9(2)
65.8
36.3(2)
27.1
14.9(3)
19.7
52.2(4), 50.1(4)
62.1(4), 59.1(4)
37.8(3), 31.6(4)[a]
17.9(5), 20.0(5)
ꢀ179.3(3), 178.0(3)
16.0(5), 7.6(5)[a]
–
ꢀ16.4(4)
ꢀ10.1
ꢀ173.9(2)
178.2
13.9(3)
3.0
ꢀ1.9(4)
ꢀ3.7(4)
ꢀ12(2)[b]
ꢀ24.2
3.0
ꢀ18.9
[a] Read S for C2. [b] C3-C4-N-H1N.
nylethynyl)dimesitylboranes even though the latter have
lower-lying charge-transfer bands. This effect could be
caused by the larger oscillator strengths of the (E)- [2-(p-R-
phenyl)ethenyl]dimesitylboranes or by the larger changes in
dipole moment on going from the ground to the excited
state. There also appears to be a strong correlation between
increasing bexp values and the increasing donor strength of
the R substituent. Similar correlations have been observed
in other molecules.[6] The bexp values range from 33”
10ꢀ30 esu for 3-NMe2 to 0.9”
Scheme 2. Canonical forms of [p-(N,N-dimethylamino)phenyl]dimesityl-
borane (1-NMe2).
anes have much larger values than the analogous (p-R-phe-
nyl)dimesitylboranes. The (E)-[2-(p-R-phenyl)ethenyl]dime-
sitylboranes have slightly larger values than the (p-R-phe-
10ꢀ30 esu for 1-I. The largest
Table 4. Absorption and emission [in square brackets] maxima [nm] in a variety of solvents and extinction co-
efficients [mꢀ1 cmꢀ1] in cyclohexane (in parentheses).
values, by
a
considerable
margin, occur for the NMe2-
subsituted compounds. Howev-
er, the lowest bexp value for the
[2-(p-R-phenyl)ethenyl]dimesi-
tylboranes of 2.0”10ꢀ30 esu
occurs for 3-CN and not for
the stronger acceptor-substitut-
ed compound 3-NO2. This sug-
gests that the B(mes)2 acceptor
strength is comparable to that
of the CN group, which is con-
Compound
Absorption maxima (Extinction coefficients) [Emission maxima]
Cyclohexane
CHCl3
EtOAc
DMF
CH3CN
1-NMe2
1-OMe
1-SMe
1-Br
2-NMe2
3-NMe2
3-NH2
3-OMe
3-SMe
3-H
354 (26000) [368]
318 (18000) [354]
336 (24000) [358]
318 (12000) [372]
358 [465]
318 [372]
336 [370]
316 [388]
399 [465]
404 [460]
370 [428]
348 [397]
358 [411]
332 [420]
360 [460]
318 [370]
336 [376]
314 [394]
–
405 [470]
384 [452]
350 [394]
360 [418]
332 [424]
368 [502]
316 [388]
339 [396]
316 [410]
408 [525]
414 [496]
400 [478]
353 [412]
364 [444]
336 [444]
361 [506]
316 [388]
335 [394]
314 [410]
–
406 [492]
380 [492]
348 [410]
358[438]
33 [446]
392
[458]
396 (40000) [422]
372 (28000) [400]
348 (33000) [400]
356 (21000) [393]
332 (18000) [398]
2764
www.chemeurj.org
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2006, 12, 2758 – 2771

Synthesis and Characterization of Dimesitylboranes
FULL PAPER
Table 5. Ground-state dipole moments mg, molecular polarizabilities a, experimental (bexp) and calculated
(bcalc) first-order molecular hyperpolarizabilities, and experimental (gexp) and calculated (gcalc) second-order hy-
perpolarizabilities for selected unsymmetrical organoboranes.
(p-R-phenyl)ethenyl]dimesityl-
boranes, where R=Br (3-Br)
and I (3-I).
[a]
[b]
Compound
mg
a
bexp
[10ꢀ30 esu]
gexp
[10ꢀ36 esu]
bcalc [bstatic
]
gcalc [gstatic]
Their ground-state geome-
tries were first optimized. Per-
tinent calculated bond lengths
are given in the Supporting In-
formation and, where applica-
ble, are compared with the
crystallographically obtained
bond lengths reported in
Table 2. The computed struc-
tures quite satisfactorily mimic
the atomic distances of the
crystallographically character-
ized compounds, although they
are generally shorter. In partic-
[10ꢀ18 esu]
[10ꢀ23 esu]
[10ꢀ30 esu]
[10ꢀ36 esu]
1-NMe2
1-OMe
1-SMe
1-Br
3.0
1.6
1.6
1.5
1.4
3.6
1.7
1.6
1.5
3.4
2.6
1.6
1.0
1.5
3.3
3.6
5.6
6.4
6.0
6.5
4.6
4.2
4.4
4.3
4.5
4.8
4.6
4.7
4.2
5.0
4.7
4.8
4.4
4.9
4.6
8.8
2.1
2.2
2.8
3.2
11
24
23
32
20
27
25[d,e]
41
75
27[d]
93
41
54
37
81
30
32
10
28
29
–
12.6 [11.9]
4.9 [4.3]
12.1 [10.0]
0.5 [0.5]
1.0 [1.0]
27.1 [21.7]
9.8 [8.4]
25.7 [20.6]
2.6 [2.3]
27.0 [25.7]
18.3 [15.0]
12.3 [10.2]
3.0 [2.5]
24.1 [19.2]
2.2 [1.8]
8.4 [6.7]
–
53 [42]
38 [28]
67 [44]
33 [25]
31 [24]
162 [99]
74.9 [53.4]
144 [87.8]
50.2 [37.9]
152 [103]
105 [66.4]
100 [65]
62 [43]
167 [97]
90 [61]
104 [67]
–
3.3
2.5[c]
1.6
0.9
25
8.2
7.2
3.4
33
1-I
2-NMe2
2-OMe
2-SMe
2-H
3- NMe2
3-NH2
3-OMe
3-H
18
9.3
5.1
8.6[c]
2.0
8.8
5.0
12
3-SMe
3-CN
3-NO2
ꢀ
ular, the B Cdistances are re-
6-CN][f]
produced with good accuracy
with deviations smaller than a
few hundredths of an Š.
Hence the computed bond dis-
tances in molecules for which
no X-ray data are available yet
can be regarded with confi-
ꢀ
[f]
6-NO2
–
–
–
–
–
–
7-CN[g]
23
50
[g]
7-NO2
[a] ꢂ10% unless otherwise indicated. [b] ꢂ20% unless otherwise indicated. [c] ꢂ0.5. [d] ꢂ30%. [e] Low
value due to probable decomposition of the sample. [f] See reference [18a]. [g] See reference [18b].
sistent with electrochemical studies on related compounds.[3]
The bexp values for 1-NMe2 and 3-NMe2 lie intermediate be-
tween those of their analogues, 6-CN and 6-NO2, and 7-CN
and 7-NO2, respectively.
dence. In all of the compounds, the B C(mes) bond lengths
are very similar. A small but non-negligible variation in the
ꢀ
B C(aryl/ethynyl/vinyl) separation is computed that is de-
pendent on the nature of the R substituent present in the
molecule. They lengthen slightly from 1.503 Š for 2-NMe2
to 1.554 Š for 1-NO2. Although both the donor/acceptor
properties of the R substituent and the nature and length of
The second-order molecular hyperpolarizabilities gexp
were obtained from third harmonic generation (THG)
measurements at 1.907 mm.[18b] In general, again, the (E)-[2-
(p-R-phenyl)ethenyl]- and (p-R-phenylethynyl)dimesitylbor-
anes have much larger gexp values than the analogous (p-R-
phenyl)dimesitylboranes. There is less correlation between
the gexp values and the donor strength of the R substituent.
Among the (p-R-phenyl)dimesitylboranes, the values range
from 32”10ꢀ36 esu for 1-SMe (which is actually larger than
that of 1-NMe2) to 20”10ꢀ36 esu for 1-Br. Among the (p-R-
phenylethynyl)dimesitylboranes, the values range from 75”
10ꢀ36 esu for 2-SMe to 25”10ꢀ36 esu for 2-NMe2, although
this value may be artificially low as a result of decomposi-
tion of the sample. For the (E)-[2-(p-R-phenyl)ethenyl]di-
mesitylboranes, the gexp values range from 93”10ꢀ36 esu for
3-NMe2 to 30”10ꢀ36 esu for 3-CN. The gexp value for 1-NMe2
is comparable to that of its nitro-substituted analogue, 6-
NO2.
ꢀ
the conjugated bridge may affect the B C(aryl/ethynyl/
vinyl) bond, it seems that the effect of the latter on the
bond length predominates over the former. Slightly shorter
ꢀ
B Cbonds are computed for molecules containing strong
donor groups such as NMe2. Some change is also computed
for molecules containing the same R substituent but a differ-
ent spacer, that is, an aryl, a styryl or phenylethynyl bridge.
This is illustrated for example for compounds with NMe2
ꢀ
groups in which the B C(aryl/ethynyl/vinyl) bond varies
from 1.541 Š in 1-NMe2 to 1.533 Š in 3-NMe2 to 1.503 Š in
2-NMe2. Although less pronounced than observed experi-
mentally, some distortion of the bond distances toward a
quinoidal structure is computed. The computed dihedral
angles between the mean plane of the substituted phenyl
ꢀ
ring and the B C3 plane in the different series are also well
reproduced with maximum deviations of about 108. These
theoretical structural trends are confirmed in the experimen-
tally characterized compounds.
Theoretical aspects: Semiempirical calculations at the AM1
level (see the Experimental Section for computational de-
tails) were conducted on the (p-R-phenyl)-, (p-R-phenyle-
thynyl)-, and (E)-[2-(p-R-phenyl)ethenyl]dimesitylboranes,
as well as on related compounds which remain to be synthe-
sized, namely, the (p-R-phenyl)dimesitylboranes, where R=
NH2, (1-NH2), CN (1-CN), and NO2 (1-NO2), the (p-R-phe-
nylethynyl)dimesitylboranes, where R=NH2 (2-NH2), Br (2-
Br), I (2-I), CN (2-CN), and NO2 (2-NO2), and the (E)-[2-
Both dynamic and static hyperpolarizabilities, bcalc, gcalc
,
bstatic, and gstatic, were computed for the three series of un-
symmetrical compounds and are compared with the avail-
able experimental data, bexp and gexp, in Table 5. For the (p-
R-phenyl)dimesitylboranes, the bcalc values decrease in the
order R=NMe2 >SMe>NH2 >OMe@NO2 >CN>I>Hꢁ
Br. A rather similar trend is also observed for the (E)-[2-(p-
Chem. Eur. J. 2006, 12, 2758 – 2771
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2765

W.-Y. Wong, A. Boucekkine, J.-F. Halet, T. B. Marder et al.
R-phenyl)ethenyl]- and (p-R-phenylethynyl)dimesitylbor-
anes, that is, a decrease in the bcalc values in the order R=
of NLO properties in donor (NMe2)/acceptor (NO2) poly-
enes and polyynes.[19]
NMe2 >SMe>NH2 >OMe>NO2 @Br>H>I>CN
and
Assuming that the characteristics of the HOMO!LUMO
excitation determine the NLO properties of the title mole-
cules, it is informative to look at the composition and the
energy of these orbitals. The HOMO and LUMO of some
representative (p-R-phenyl)- and (E)-[2-(p-R-phenyl)ethe-
nyl]dimesitylboranes are sketched in Figure 4 and Figure 5.
Although well over-estimated, a significant decrease in the
HOMO–LUMO gaps of approximately 0.5 eV is computed
on going from the (p-R-phenyl)dimesitylboranes to the (E)-
[2-(p-R-phenyl)ethenyl]dimesitylboranes. This partly ac-
counts for the increase in the hyperpolarizabilities of the
latter with respect to the former, since in general the smaller
the HOMO–LUMO gap, the higher the optical nonlinearity.
Within each series, a slight decrease in the HOMO–LUMO
gap accompanies the increase in the b and g hyperpolariza-
bilities. With strong electron-donating substituents (R=
OMe, SMe, and NMe2), the HOMO is essentially localized
on the donating group and the bridging phenyl ring, whereas
the LUMO is mainly B(mes)2 in character. In contrast, for
compounds that do not contain strong electron-donor end-
groups, such as 1-Br, 1-H, or 3-NO2, the HOMO is mostly
localized on the B(mes)2 moiety and the LUMO is delocal-
ized on the organic spacer. Consequently, the HOMO!
LUMO transition in these materials involves different types
of charge transfer depending upon the nature of the R sub-
stituent. With strong electron-donating groups, such as
OMe, SMe, NH2, and NMe2, the HOMO–LUMO transition
mostly involves charge transfer from these groups to the
rather strongly electron-accepting B(mes)2 entity. This re-
sults in a large variation in the dipole moment, which is the
primary source of the large NLO activity observed. Indeed,
these strong variations in the dipole moment in the excited
state are evidenced by the solvatochromic shifts observed in
the fluorescence spectra of such compounds (vide supra).
Note that these qualitative conclusions have been confirmed
by DFT calculations on selected examples.
NMe2 >SMe>NH2 >OMe>NO2 @Br>I>H>CN, respec-
tively. Each series can be divided into two groups with com-
pounds associated with rather high bcalc values (>10”
10ꢀ30 esu), mostly those containing strong electron-donating
end-groups, and compounds exhibiting low bcalc values (<5”
10ꢀ30 esu). However, the largest influence on the b values
seems to be due to the extension of p-electron conjugation
since a significant enhancement is observed on going from
the (p-R-phenyl)dimesitylboranes to the (p-R-phenylethyn-
yl)- or (E)-[2-(p-R-phenyl)ethenyl]dimesitylboranes.
Note that the calculated first-order hyperpolarizabilities
bcalc are overall in excellent agreement with the experimen-
tal measurements. Indeed, there is a perfect linear relation-
ship between the calculated and experimental values with a
correlation coefficient nearly equal to 0.99 and the slope of
the linear regression close to 0.95 (see the Supporting Infor-
mation). However, there are some important deviations be-
tween theory and experimental for 1-SMe and 3-SMe. None-
theless, these results persuasively argue that the AM1
theory can quite accurately reproduce EFISH-derived quad-
ratic hyperpolarizabilities. This good agreement must be due
to a probable compensation of the various contributions to
these computed hyperpolarizabilities, for example, the vibra-
tional ones, since only the semiempirical SCF electronic con-
tribution to this property is computed at the AM1 level.
The calculated second-order molecular hyperpolarizabili-
ties gcalc for the (p-R-phenyl)-, (E)-[2-(p-R-phenyl)ethenyl]-,
and (p-R-phenylethynyl)dimesitylboranes decrease in the
order R=SMe>NMe2 >OMeꢁNH2 >Br>NO2 ꢁIꢁCN>
H, R=NMe2 >SMe@NH2 ꢁNO2 >OMe>CN>BrꢁI>H,
and R=NMe2 >SMe@NH2 >OMe>NO2 ꢁCN>BrꢁI>
H, respectively. The agreement between the calculated and
experimental second-order molecular hyperpolarizabilities
g, although good, is less satisfactory than that found for the
first-order hyperpolarizabilities b. Indeed, the gcalc values
vary roughly over two orders of magnitude with respect to
the experimental ones. As expected, the more NLO-active
systems are those that contain the strongest electron-donor
R substituents and which thus show a higher degree of po-
larization. However, there is not a clear cut-off between
compounds containing donor substituents and compounds
with acceptor substituents. Substantial optical nonlinearity is
also predicted for molecules containing CN and NO2 end-
groups. We do not understand the discrepancy between the
calculated and experimental g values for 1-I. The magnitude
of g is more strongly dependent on the length of the bridge
as the (E)-[2-(p-R-phenyl)ethenyl]- and (p-R-phenylethy-
nyl)dimesitylboranes have values approximately twice as
large as those of the (p-R-phenyl)dimesitylboranes. Howev-
er, the type of p conjugation can also influence g values
since extension of the conjugation of the spacer with a
double bond enhances more the NLO activity than exten-
sion with a triple bond. A similar conclusion was reached by
Pugh and co-workers from a comparative theoretical study
It is known that incorporation of trans-polyene segments
into the conjugated bridge generally leads to a significant in-
crease in the hyperpolarizabilities of push–pull systems.[20]
This is supported by the comparison of the dimesitylboranes
1–3 that we have studied. Compounds 3 display NLO activi-
ty twice as large as compounds 1 and 2 (vide supra). Despite
the fact that the main drawback of push–pull polyenes is
that they often linearly absorb in the visible region of the
spectrum, we thought that it would be informative to exam-
ine the effect of incorporation of polyene segments into the
spacer of different organoboranes on the NLO properties.
The NMe2 group was chosen as the p-donor end-group as
organoboranes possessing this substituent exhibit the largest
molecular hyperpolarizabilities (vide supra). Therefore, dif-
ꢀ ꢀ
ferent hypothetical Me2N Y B(mes)2 organoboranes,
ꢀ
ꢀ
where Y=(CH=CH)n C6H4 (CH=CH)n’, n=0, n’=2 (8-02),
n=0 n’=3 (8-03), n=1, n’=0 (8-10), n=2, n’=0 (8-20), n=
3, n’=0 (8-30), n=1, n’=1 (8-11), n=1, n’=2 (8-12), n=2,
n’=1 (8-21), and Y=(CH=CH)n, n=3–7 (9-n), were investi-
2766
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Chem. Eur. J. 2006, 12, 2758 – 2771

Synthesis and Characterization of Dimesitylboranes
FULL PAPER
of the phenyl group, whether
adjacent to the donor NMe2 or
the acceptor B(mes)2, does not
influence the hyperpolarizabili-
ties. On the other hand, values
of gcalc, and to a lesser extent
bcalc, somewhat differ depend-
ing on the position of the
phenyl ring in the polyene
chain. Slightly larger values are
computed for compounds, in 8
when either
n or n’=0, in
which the polyene segments
are adjacent. It turns out that
the most important parameter
for large NLO effects is the
number of double bonds adja-
cent to each other. This is con-
firmed by looking at the first-
and second-order hyperpolariz-
abilities of the compounds of
series 9 which exclusively con-
tain polyene chains. They in-
crease with chain length and
significantly exceed those cal-
culated for compounds such as
1-NMe2 that do not contain
CH=CH groups. Note that a
comparison of the compounds
of series 8 with compounds of
series 9 indicates that for a
given number of CH=CH
groups, compounds containing
an additional phenyl group
(series 8) display significantly
larger values. This is mainly
due to smaller HOMO–
LUMO gaps for the former
(see Table 6). For example,
HOMO–LUMO gaps of 7.67,
7.29, and 7.26 eV are computed
for compounds 9-3, 8-03, and
Figure 4. Energy level diagrams and contour plots of the HOMO and LUMO of 1-SMe, 1-Br, 1-H, 1-NMe2,
and 1-OMe.
gated. These molecules are illustrated in Scheme 1. The
first- and second-order dynamic molecular hyperpolarizabili-
ties, bcalc and gcalc, were calculated for the optimized geome-
tries of these compounds and are listed in Table 6 along
with those of compounds 1–3 which have not been synthe-
sized. The good agreement between theory and experimen-
tal for the compounds of series 1–3 leads us to think that
the values predicted for these hypothetical molecules must
be reasonably reliable.
As expected, both first- and second-order hyperpolariza-
bilities increase sharply with chain length. For series 8, bcalc
increases by about 15”10ꢀ30 esu and gcalc doubles for each
increment of n. Interestingly, for a given number of CH=CH
double bonds, similar bcalc and gcalc values are computed for
series 8 when either n or n’=0. In other words, the position
8-30, respectively. Compound 9-3 contains only three CH=
CH groups whereas 8-03 and 8-30 contain three CH=CH
groups and a phenyl ring.
These theoretical results show that the acceptor B(mes)2
group coupled to strong donating groups through p-conju-
gated spacers can display large hyperpolarizabilities. Calcu-
lations on hypothetical NMe2-B(mes)2 polyenes have indi-
cated that these NLO properties can be enhanced by the in-
troduction of polyene segments into the bridge. However,
the question of the optical transparency of such species,
which is of technological importance, arises as they could
absorb in the visible region. Nevertheless, a trade-off be-
tween these two properties can certainly be found.
Chem. Eur. J. 2006, 12, 2758 – 2771
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2767

W.-Y. Wong, A. Boucekkine, J.-F. Halet, T. B. Marder et al.
bility values were obtained from AM1 calculations and have
been shown to be in reasonable agreement with those deter-
mined experimentally. Calculations have also been per-
formed on related compounds containing additional C=C
bonds, which are postulated to have larger hyperpolarizabili-
ties.[29]
Experimental Section
General: All reactions were carried out under nitrogen using standard
Schlenk and glove-box techniques. All solvents were distilled under nitro-
gen from appropriate drying agents. Dimesitylboron fluoride was ob-
tained by the reaction of mesitylmagnesium bromide with BF3·OEt2.[21]
Dimesitylborane was prepared in quantitative yield by the reaction of di-
mesitylboron fluoride with LiAlH4 in monoglyme.[22] The p-R-bromoben-
zenes were obtained commercially. The p-R-phenylacetylenes were ob-
tained by hydrodesilylation of their trimethylsilylated precursors.[23]
NMR spectra were obtained using a Bruker AC200 or AM250 spectrom-
eter at 200 or 250 MHz (1H) and 50 or 63 MHz (13C{1H}) or on an
Avance 400 spectrometer at 400 MHz (1H) and 100 MHz (13C{1H}). All
spectra were recorded in CDCl3 unless otherwise stated. Chemical shifts
are reported relative to tetramethylsilane, referenced to chloroform
peaks. UV/Vis absorption spectra were recorded on a Hewlett-Packard
8452A diode array spectrophotometer using standard, dual-window 1 cm
quartz cells. Fluorescence emission spectra were measured on a Perkin-
Elmer MPF-LA fluorescence spectro-
Figure 5. Energy level diagrams and contour plots of the HOMO and
LUMO of 3-NMe2 and 3-NO2.
photometer. Spectroscopic grade sol-
vents were used for all measure-
ments. Mass spectra were recorded
on an HP 5971 Mass Selective Detec-
tor attached to an HP 5890 GCusing
Table 6. Calculated b and g values and HOMO–LUMO gaps [eV] for some hypothetical (p-R-phenyl)-, (p-R-
ꢀ ꢀ
phenylethynyl)-, (E)-(p-R-phenylethynyl)dimesitylboranes, and Me2N Y B(mes)2 compounds.
Compound
bcalc [bstatic
]
gcalc [gstatic
]
Compound
bcalc
[10ꢀ30 esu]
gcalc
[10ꢀ36 esu]
HOMO–LUMO gap
[eV]
[10ꢀ30 esu]
[10ꢀ36 esu]
1-NH2
1-CN
1-NO2
2-NH2
2-Br
8.7 [7.5]
1.2 [1.1]
2.1 [1.8]
15.2 [12.7]
4.4 [3.8]
3.8 [3.5]
1.2 [1.0]
6.3 [5.2]
3.3 [2.8]
2.8 [2.5]
37.9 [27.9]
30.8 [24.4]
31.9 [24.9]
88.5 [60.5]
63.9 [46.4]
63.1 [46.4]
70.0 [51.4]
71.5 [51.7]
64.9 [45.7]
64.6 [45.6]
1-NMe2
3-NMe2
8-02
8-03
8-10
8-20
8-30
8-11
8-12
8-21
9-3
12.6
27.0
39.3
57.4
23.0
35.7
50.5
42.0
50.7
54.6
39.0
59.1
80
53
152
327
678
133
328
677
237
535
561
269
598
1130
1870
2842
8.48
7.90
7.56
7.29
7.92
7.54
7.26
7.65
7.40
7.32
7.67
7.34
7.10
6.91
6.65
a
5% phenylmethylsiloxane-coated
capillary column with UHP helium as
the carrier gas. Elemental analyses
were performed by M-H-W Labora-
tories, Phoenix, Arizona or in the De-
partment of Chemistry, University of
Durham. Infrared spectra were re-
corded on a Perkin Elmer 983 spec-
trometer using KBr plates.
2-I
2-CN
2-NO2
3-Br
3-I
General procedure for the prepara-
tion of (p-R-phenyl)dimesitylbor-
anes: A solution of the appropriate
p-R-bromobenzene (5.0 mmol) in
hexane (20 mL) was treated with n-
butyllithium in hexane (1.6m, 4.7 mL,
7.5 mmol) at room temperature for
14 h. The precipitate that formed was
separated from solution by filtration
9-4
9-5
9-6
9-7
99.5
117.0
Conclusions
and dissolved in diethyl ether (20 mL). Dimesitylboron fluoride (1.34 g,
5.0 mmol) in diethyl ether (10 mL) was added dropwise. The mixture was
stirred for 3 h and the solvent removed in vacuo. The pale yellow product
was isolated from hexane.
It has been demonstrated that air-stable, conjugated dimesi-
tylboranes substituted with various donor and acceptor
groups can be synthesized using simple methodology. Unlike
their absorption maxima, their fluorescence maxima exhibit
large solvatochromic shifts with solvents of increasing polar-
ity, particularly for donor-substituted compounds, which is
indicative of highly polarized excited states relative to the
ground states. It has been shown, both theoretically and ex-
perimentally, that several of the donor-substituted deriva-
tives show large first-order molecular hyperpolarizabilities.
They also show good optical transparency, which makes
them attractive candidates for further study. Hyperpolariza-
[p-(N,N-dimethylamino)phenyl]dimesitylborane (1-NMe2): Yield: 1.46 g
[4a]
(80%), m.p. 159–1628C(lit.
159–1618C). 1H NMR: d= 7.47 (d, J=
8.8 Hz, 2H, phenyl), 6.83 (d, J=8.8 Hz, 2H, phenyl), 6.83 (s, 4H, mesi-
tyl), 3.05 (s, 6H, NMe2), 2.32 (s, 6H, Me), 2.09 (s, 12H, Me) ppm; 13C{1H}
NMR: d=153.0 (Cphenyl), 142.3 (Cmes), 140.6 (Cmes), 137.5 (Cmes), 127.9
(Cphenyl), 127.9 (Cmes), 110.6 (Cphenyl), 39.8 (Me2N), 23.5 (Me), 21.1
(Me) ppm. MS (EI): m/z (%): 369 (61) [M+], 249 (70), 248 (100); ele-
mental analysis calcd (%): C84.54, H 8.74, N 3.79; found: C84.77, H
8.72, N 3.76.
(p-Methoxyphenyl)dimesitylborane (1-OMe): Yield: 1.03 g (58%), m.p.
124–1288C. 1H NMR: d=7.51 (d, J=8.8 Hz, 2H, phenyl), 6.87 (d, J=
8.8 Hz, 2H, phenyl), 6.82 (s, 4H, mesityl), 3.85 (s, 3H, OMe), 2.31 (s, 6H,
2768
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Chem. Eur. J. 2006, 12, 2758 – 2771

Synthesis and Characterization of Dimesitylboranes
FULL PAPER
Me), 2.04 (s, 12H, Me) ppm; 13C{1H} NMR: d=163.2 (Cphenyl), 141.8
(Cmes), 140.8 (Cmes), 139.0 (Cmes), 128.6 (Cphenyl), 128.1 (Cmes), 113.4
(Cphenyl), 55.1 (OMe), 23.4 (Me), 21.2 (Me) ppm. MS (EI): m/z (%): 356
(2) [M+], 248 (7), 236 (100); elemental analysis calcd (%): C84.27, H
8.20; found: C84.30, H 8.40.
column chromatography on silica gel using hexane/ethyl acetate (1:1 v/v)
as eluent, followed by recrystallization from hexane.
(E)-{2-[p-(N,N-Dimethylamino)phenyl]ethenyl}dimesitylborane
(3-
NMe2): Yield 0.60 g, (82%), m.p. 194–1958C. 1H NMR: d=7.45 (d, J=
8.8 Hz, 2H, phenyl), 7.13 (s, 2H, vinyl), 6.81 (s, 4H, mesityl), 6.66 (d, J=
8.8 Hz, 2H, phenyl), 2.99 (s, 6H, NMe2), 2.29 (s, 6H, Me), 2.20 (s, 12H,
Me) ppm; 13C{1H} NMR: d=154.1 (Cvinyl), 151.6 (Cphenyl), 142.6 (Cmes),
140.6 (Cmes), 137.8 (Cmes), 132.8 (Cvinyl, C-B), 129.8 (Cphenyl), 128.1 (Cmes),
126.0 (Cphenyl), 112.0 (Cphenyl), 40.2 (Me2N), 23.3 (Me), 21.2 (Me) ppm; MS
(EI): m/z (%): 395 (100) [M+], 275 (15), 248 (36); elemental analysis
calcd (%): C85.06, H 8.67, N 3.54; found: C84.99, H 8.71, N 3.55.
(p-Methylthiophenyl)dimesitylborane (1-SMe): Yield: 1.10 g (59%), m.p.
113–1158C. 1H NMR: d=7.42 (d, J=8.3 Hz, 2H, phenyl), 7.15 (d, J=
8.3 Hz, 2H, phenyl), 6.81 (s, 4H, mesityl), 2.48 (s, 3H, SMe), 2.29 (s, 6H,
Me), 2.00 (s, 12H, Me) ppm; 13C{1H} NMR: d=144.3 (Cphenyl), 141.6
(Cmes), 140.8 (Cmes), 138.5 (Cmes), 137.1 (Cphenyl), 128.1 (Cmes), 124.6
(Cphenyl), 23.4 (Me), 21.2 (Me), 14.6 (SMe) ppm. MS (EI): m/z (%): 372
(12) [M+], 252 (100), 248 (7); elemental analysis calcd (%): C80.64, H
7.85; found: C80.77, H 7.97.
(E)-[2-(p-Aminophenyl)ethenyl]dimesitylborane (3-NH2): Recrystallized
from hexane/ethyl acetate. Yield: 0.55 g, (84%), m.p. 158–1608C. 1H
NMR: d=7.36 (d, J=8.5 Hz, 2H, phenyl), 7.15 (d, J=17.6 Hz, 1H,
vinyl), 7.06 (d, J=17.6 Hz, 1H, vinyl), 6.83 (s, 4H, mesityl), 6.62 (d, J=
8.5 Hz, 2H, phenyl), 3.87 (br, 2H, NH2), 2.28 (s, 6H, Me), 2.25 (s, 12H,
Me) ppm; 13C{1H} NMR: d=153.6 (Cvinyl), 148.2 (Cphenyl), 142.6 (Cmes),
140.5 (Cmes), 137.9 (Cmes), 133.4 (Cvinyl, C-B), 134.5 (Cphenyl), 128.3 (Cphenyl),
128.1 (Cmes), 114.8 (Cphenyl), 23.3 (Me), 21.2 (Me) ppm; MS (EI): m/z (%):
367 (100) [M+], 247 (60), 248 (87); elemental analysis calcd (%): C85.01,
H 8.23, N 3.81; found: C85.23, H 8.50, N 3.64.
(p-Bromophenyl)dimesitylborane (1-Br): Yield: 1.32 g (65%), m.p. 172–
[4a]
1748C(lit.
186–1878C). 1H NMR: d=7.49 (d, J=8.3 Hz, 2H, phenyl),
7.37 (d, J=8.3 Hz, 2H, phenyl), 6.82 (s, 4H, mesityl), 2.30 (s, 6H, Me),
2.00 (s, 12H, Me) ppm; 13C{1H} NMR: d=144.5 (Cphenyl), 141.3 (Cmes),
140.8 (Cmes), 138.9 (Cmes), 137.8 (Cphenyl), 131.3 (Cphenyl), 128.1 (Cmes), 127.3
(Cphenyl), 23.4 (Me), 21.2 (Me) ppm. MS (EI): m/z (%): 406 (6) [M+], 404
(6) [M+], 286 (90), 284 (100); elemental analysis calcd. (%): C71.14, H
6.47; found: C70.90, H 6.66.
(p-Iodophenyl)dimesitylborane (1-I): Yield: 1.36 g (60%), m.p. 168–
1738C. 1H NMR: d=7.69 (d, J=8.1 Hz, 2H, phenyl), 7.20 (d, J=8.1 Hz,
2H, phenyl), 6.81 (s, 4H, mesityl), 2.29 (s, 6H, Me), 1.98 (s, 12H,
Me) ppm; 13C{1H} NMR: d=145.0 (Cphenyl), 141.3 (Cmes), 140.8 (Cmes),
139.0 (Cmes), 137.7 (Cphenyl), 137.3 (Cphenyl), 128.3 (Cmes), 100.4 (Cphenyl),
23.4 (Me), 21.2 (Me) ppm. MS (EI): m/z (%): 452 (2) [M+], 332 (100);
elemental analysis calcd (%): C63.75, H 5.80; found: C64.00, H 6.02.
(E)-[2-(p-Methoxyphenyl)ethenyl]dimesitylborane (3-OMe): Yield 0.47 g
(68%), m.p. 88–898C. H NMR: d=7.46 (d, J=8.3 Hz, 2H, phenyl), 7.35
1
(d, J=17.6 Hz, 1H, vinyl), 7.22 (d, J=8.3 Hz, 2H, phenyl), 7.10 (d, J=
17.6 Hz, 1H, vinyl), 6.84 (s, 4H, mesityl), 2.49 (s, 3H, MeO), 2.31 (s, 6H,
Me), 2.20 (s, 12H, Me) ppm; 13C{1H} NMR: d=160.9 (Cphenyl), 152.6
(Cvinyl), 142.3 (Cmes), 140.6 (Cmes), 138.2 (Cmes), 135.2 (Cvinyl, C-B), 130.6
(Cphenyl), 129.7 (Cphenyl), 128.2 (Cmes), 114.2 (Cphenyl), 55.4 (MeO), 23.3
(Me), 21.2 (Me) ppm; MS (EI): m/z (%): 382 (66) [M+], 262 (64), 248
(100); elemental analysis calcd (%): C84.17, H 8.17; found: C84.31, H
8.26.
Preparation of (p-R-phenylethynyl)dimesitylboranes: A solution of the
appropriate 4-R-phenylacetylene (3.0 mmol) in hexane (20 mL) was
treated with n-butyllithium in hexane (1.6m, 1.9 mL, 3.0 mmol) for
30 min at room temperature, followed by the dropwise addition of dime-
sitylboron fluoride (0.75 g, 2.7 mmol) in benzene (10 mL). The mixture
was stirred for 8 h. After removal of the solvent in vacuo, the product
was obtained by adding a small amount of hexane to the crude material.
(E)-[2-(p-Methylthiophenyl)ethenyl]dimesitylborane (3-SMe): Yield:
0.54 g (76%), m.p. 97–998C. 1H NMR: d=7.48 (d, J=8.7 Hz, 2H,
phenyl), 7.23 (d, J=17.7 Hz, 1H, vinyl), 7.10 (d, J=17.7 Hz, 2H, vinyl),
6.87 (d, J=8.7 Hz, 2H, phenyl), 6.87 (s, 4H, mesityl), 3.81 (s, 3H, MeS),
2.29 (s, 6H, Me), 2.19 (s, 12H, Me) ppm; 13C{1H} NMR: d=152.0 (Cvinyl),
142.2 (Cmes), 140.9 (Cphenyl), 140.6 (Cmes), 138.2 (Cmes), 136.7 (Cvinyl, C-B),
134.5 (Cphenyl), 128.6 (Cphenyl), 128.2 (Cmes), 126.1 (Cphenyl), 23.3 (Me), 21.2
(Me), 15.4 (MeS) ppm; MS (EI): m/z (%): 398 (100) [M+], 278 (61), 248
(80); elemental analysis calcd (%): C81.40, H 7.84; found: C81.62, H
7.82.
[p-(N,N-dimethylamino)phenylethynyl]dimesitylborane (2-NMe2): Yield:
0.59 g (50%), m.p. 153–1558C. 1H NMR: d=7.39 (d, J=8.5 Hz, 2H,
phenyl), 6.80 (s, 4H, mesityl), 6.60 (d, J=8.5 Hz, 2H, phenyl), 2.99 (s,
6H, NMe2), 2.33 (s, 6H, Me), 2.27 (s, 12H, Me) ppm; 13C{1H} NMR: d=
151.3 (Cphenyl), 141.7 (Cmes), 141.1 (Cmes), 138.9 (Cmes), 134.9 (Cphenyl), 129.7
(Cethynyl), 128.4 (Cmes), 111.5 (Cphenyl), 109.8 (Cphenyl), 40.2 (Me2N), 23.1
(Me), 21.2 (Me) ppm; IR: n˜(C C)=2120 cmꢀ1. MS (EI): m/z (%): 393
ꢃ
(E)-[2-(p-Cyanophenyl)ethenyl]dimesitylborane (3-CN): Yield: 0.58 g
(86%), m.p. 105–1108C. 1H NMR: d=7.63 (d, J=8.5 Hz, 2H, phenyl),
7.58 (s, J=8.5 Hz, 2H, phenyl), 7.50 (d, J=17.8 Hz, 1H, vinyl), 7.06 (d,
J=17.8 Hz, 1H, vinyl), 6.84 (s, 4H, mesityl), 2.30 (s, 6H, Me), 2.17 (s,
12H, Me) ppm; 13C{1H} NMR: d=148.9 (Cvinyl), 141.7 (Cvinyl, C-B), 141.4
(Cmes), 140.6 (Cmes), 139.0 (Cphenyl), 139.0 (Cmes), 132.5 (Cphenyl), 128.4
(Cmes), 128.2 (Cphenyl), 118.7 (CN), 112.4 (Cphenyl), 23.3 (Me), 21.2
(Me) ppm; MS (EI): m/z (%): 377 (15) [M+], 257 (100), 248 (9); elemen-
tal analysis calcd (%): C85.94, H 7.48, N 3.71; found: C85.99, H 7.47, N
3.71.
(91) [M+], 378 (3), 270 (34), 257 (100), 250 (49); elemental analysis calcd
(%): C85.49, H 8.20, N 3.56; found: C85.36, H 8.12, N 3.57.
(p-Methoxyphenylethynyl)dimesitylborane (2-OMe): Yield: 0.75 g
1
(66%), m.p. 74–768C. H NMR: d=7.45 (d, J=8.8 Hz, 2H, phenyl), 6.85
(d, J=8.8 Hz, 2H, phenyl), 6.81 (s, 4H, mesityl), 3.81 (s, 3H, OMe), 2.33
(s, 6H, Me), 2.27 (s, 12H, Me) ppm; 13C{1H} NMR: d=161.0 (Cphenyl),
141.4 (Cmes), 141.0 (Cmes), 139.2 (Cmes), 134.7 (Cphenyl), 128.4 (Cmes), 126.8
(Cethynyl), 115.5 (Cphenyl), 114.1 (Cphenyl), 55.4 (MeO), 23.1 (Me), 21.2
(Me) ppm; IR: n˜(C C)=2139 cmꢀ1. MS (EI): m/z (%): 380 (100) [M+],
ꢃ
365 (12), 272 (59), 257 (74), 237 (82); elemental analysis calcd (%): C
85.27, H 7.69; found: C85.17, H 7.50.
(E)-[2-(p-Nitrophenyl)ethenyl]dimesitylborane (3-NO2): Yield: 0.51 g
(70%), m.p. 156–1588C. 1H NMR: d=8.19 (d, J=8.8 Hz, 2H, phenyl),
7.62 (d, J=8.8 Hz, 2H, phenyl), 7.53 (d, J=17.8 Hz, 1H, vinyl), 7.09 (d,
J=17.8 Hz, 1H, vinyl), 6.83 (s, 4H, mesityl), 2.26 (s, 6H, Me), 2.16 (s,
12H, Me) ppm; 13C{1H} NMR: d=148.3 (Cvinyl), 148.0 (Cphenyl), 142.3
(Cmes), 141.8 (Cvinyl, C-B), 140.7 (Cmes), 139.1 (Cmes), 134.9 (Cphenyl), 128.4
(Cmes), 124.0 (Cphenyl), 23.3 (Me), 21.2 (Me) ppm; MS (EI): m/z (%): 397
(17) [M+], 275 (100), 248 (8); elemental analysis calcd (%): C78.60, H
7.10, N 3.53; found: C78.42, H 7.22, N 3.57.
(p-Methylthiophenylethynyl)dimesitylborane (2-SMe): Yield: 0.77 g
(65%), m.p. 112–1148C. 1H NMR: d=7.40 (d, J=8.4 Hz, 2H, phenyl),
7.17 (d, J=8.4 Hz, 2H, phenyl), 6.81 (s, 4H, mesityl), 2.47 (s, 3H, SMe),
2.35 (s, 6H, Me), 2.28 (s, 12H, Me) ppm; 13C{1H} NMR: d=141.4 (Cmes),
141.0 (Cmes), 139.5 (Cmes), 139.5 (Cphenyl), 132.9 (Cphenyl), 128.6 (Cmes), 125.9
(Cethynyl), 125.6 (Cphenyl), 119.5 (Cphenyl), 23.1 (Me), 21.2 (Me), 15.4
(SMe) ppm; IR: n˜(C C)=2143 cmꢀ1. MS (EI): m/z (%): 396 (100) [M+],
ꢃ
272 (62), 257 (65), 253 (51), 229 (44); elemental analysis calcd (%): C
81.80, H 7.37; found: C79.94, H 7.35.
(E)-[2-(2-Thienyl)ethenyl]dimesitylborane (4): A solution of dimesityl-
borane (0.5 g, 1.0 mmol) was added dropwise through a syringe to a stir-
red solution of 2-ethynylthiophene (0.11 g, 1.0 mmol) in THF (25 mL).
After 2 h the mixture was concentrated to dryness and residual THF was
removed by adding small portions of diethyl ether followed by evapora-
tion to give a white powder. The powder was washed with hexane and re-
crystallized from hexane/DCM to give the pure product.
General procedure for the preparation of (E)-[2-(p-R-phenyl)ethenyl]di-
mesitylboranes: A solution of dimesitylborane (0.46 g, 1.85 mmol) in dry
THF (10 mL) was added to a solution of the appropriate p-R-phenylace-
tylene (1.85 mmol) in THF (20 mL). The mixture was stirred for 2 h.
After removal of the solvent in vacuo, the crude product was purified by
Chem. Eur. J. 2006, 12, 2758 – 2771
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2769

W.-Y. Wong, A. Boucekkine, J.-F. Halet, T. B. Marder et al.
1
Yield 0.43 g (70%), m.p. 114–1168C. H NMR (400 MHz, C6D6): d=7.46
semiempirical methods give reliable hyperpolarizabilities for a series of
homologous compounds, in particular for push–pull (large charge-trans-
fer) conjugated systems.[28] We checked, on selected examples, that semi-
empirical AM1 and density-functional-theory optimized geometries were
comparable. Dynamic hyperpolarizabilities were calculated at 1907 nm (ꢀh
w=0.65 eV).
(d, J=16.7 Hz, 1H vinyl), 7.34 (d, J=16.7 Hz, 1H, vinyl), 6.81 (s, 4H,
mesityl), 6.75 (d, J=4.9 Hz, 1H, thiophenyl), 6.53 (m, 2H, thiophenyl),
2.28 (s, 12H, Me), 2.18 (s, 6H, Me) ppm; 13C{1H} NMR (100 MHz, C6D6):
d=145.2 (Cvinyl), 144.9 (Cmes), 140.8 (Cmes), 138.6 (Cvinyl), 130.2 (Cmes),
128.9 (Cmes), 128.3 (Cthiophenyl), 128.2 (Cthiophenyl), 128.1 (Cthiophenyl), 127.9
(Cthiophenyl), 23.6 (Me), 21.5 (Me) ppm; MS (EI): m/z: 358 [M+], 238; ele-
mental analysis calcd (%): C80.44, H 7.59; found: C80.49, H 7.77.
(E)-[2-(o-Carboranyl)ethenyl]dimesitylborane (5): A solution of dimesi-
tylborane (0.25 g, 0.5 mmol) was added dropwise through a syringe to a
stirred solution of o-ethynylcarborane (0.084 g, 0.5 mmol) in THF
(25 mL). After overnight stirring, the mixture was concentrated to dry-
ness to give a colorless oil. Residual THF was removed by adding small
portions of diethyl ether followed by evaporation to give a white powder.
The powder was washed with hexane (2”5 mL) and diethyl ether (5 mL)
and recrystallized from hexane/DCM to give the pure product.
Yield 0.21 g (63%), m.p. 1458C(decomp). 1H NMR (400 MHz, C6D6):
d=7.34 (d, J=17 Hz, 1H vinyl), 6.91 (d, J=17 Hz, 1H, vinyl), 6.81 (s,
4H, mesityl), 2.94 (s, carborane), 2.87 (s, carborane), 2.75 (s, carborane),
2.52 (s, carborane), 2.21 (s, carborane), 2.05 (s, carborane), 2.13 (s, 12H,
Me), 2.10 (s, 6H, Me) ppm (one carborane proton could not be observed
as the peak was apparently obscured by the methyl protons); 13C{1H}
NMR (100 MHz, C6D6): d=145.8 (Cvinyl), 143.1 (Cmes), 140.9 (Cmes), 140.7
(Cmes), 139.7 (Cvinyl), 128.9 (Cmes), 74.9 (Ccarborane), 59.2 (Ccarborane), 23.3
(Me), 21.2 (Me) ppm; MS (EI): m/z: 418 [M+], 298; elemental analysis
calcd (%): C63.15, H 8.43; found: C63.54, H 8.60.
Acknowledgements
T.B.M. thanks the NSERC(Canada) for support. T.B.M. and J.A.K.H.
thank One NorthEast for funding under the UICNanotechnology pro-
gram. We thank Dr. Mark Fox for providing a sample of o-ethynylcarbor-
ane and Dr. Lap-TakA. Cheng of Du Pont Central Research for carrying
out the NLO and dipole-moment measurements. S.F., A.B., and J.-F.H.
thank the Pꢂle de Calcul Intentif de l’Ouest (PCIO) of the University of
Rennes for computing facilities. These studies were facilitated by travel
grants [Royal Society (UK) and CNRS (France)]. W.-Y.W. thanks the
Hong Kong Research Grants Council (HKBU 2054/02P) for financial
support. S.-Y.P. acknowledges the receipt of a Li Po Chun Charitable
Trust Fund Postgraduate Scholarship (2004–2005) administered by the
Hong Kong Baptist University. We thank J. Dostal at the University of
Durham for performing elemental analyses.
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2770
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Chem. Eur. J. 2006, 12, 2758 – 2771

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Received: September 5, 2005
Published online: January 23, 2006
Chem. Eur. J. 2006, 12, 2758 – 2771
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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