Synthetic, structural, photophysical and computational studies on 2-arylethynyl-1,3,2-diazaboroles

Article abstract of DOI:10.1039/b821208b

New 2-arylalkynyl benzo-1,3,2-diazaboroles, 2-(4′-XC ₆H₄CC)-1,3-Et₂-1,3,2-N₂BC ₆H₄ (X =Me 2; MeO 3; MeS 4; Me₂N 5), were prepared from B-bromodiazaborole, 2-Br-1,3-Et₂

Full text of DOI:10.1039/b821208b

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www.rsc.org/dalton | Dalton Transactions
Synthetic, structural, photophysical and computational studies on
2-arylethynyl-1,3,2-diazaboroles†
Lothar Weber,*a Vanessa Werner,a Mark A. Fox,*b Todd B. Marder,*b Stefanie Schwedler,a
Andreas Brockhinke,a Hans-Georg Stammlera and Beate Neumanna
Received 26th November 2008, Accepted 30th January 2009
First published as an Advance Article on the web 24th February 2009
DOI: 10.1039/b821208b
∫
New 2-arylalkynyl benzo-1,3,2-diazaboroles, 2-(4¢-XC6H4C C)-1,3-Et2-1,3,2-N2BC6H4 (X =Me 2;
MeO 3; MeS 4; Me2N 5), were prepared from B-bromodiazaborole, 2-Br-1,3-Et2-1,3,2-N2BC6H4, with
∫
the appropriate lithiated arylacetylene, ArC CLi. Molecular structures of 2, 3 and 5 were determined
by X-ray diffraction studies. UV-vis and luminescence spectroscopic studies on these diazaboroles
reveal intense blue/violet fluorescence with very large quantum yields of 0.89–0.99 for 2–5. The
experimental findings were complemented by DFT and TD-DFT calculations. The Stokes shift of only
2600 cm-1 for 5, compared to Stokes shifts in the range of 5900–7300 cm-1 for 1–4, is partly explained by
the different electronic structures found in 5 compared to 1–4 (X = H). The HOMO is mainly located
on the aryl group in 5 and on the diazaborolyl group in 1–4 whereas the LUMOs are largely aryl in
character for all compounds. Thus, in contrast to other conjugated systems containing three-coordinate
boron centers such as B(Mes)2, (Mes = 2,4,6-Me3C6H2), in which the boron serves as a p-acceptor, the
10-p electron benzodiazaborole moiety appears to function as a p-donor moiety.
(p-arylene)ethynes, including those substituted by donor- and/or
acceptor groups.
Introduction
Conjugated molecules and polymers containing three-coordinate
boron exhibit interesting optical and electronic properties mak-
ing them appropriate for use in functional materials.1 Three-
coordinate boron generally behaves as a p-acceptor, due to its
vacant pz orbital, but as boron is more electropositive than carbon,
the boryl moiety can be a s-donor. Much of the work in this
area has involved dimesitylboryl (B(Mes)2) moieties (Mes = 2,4,6-
Me3C6H2–), as these confer stability to the unsaturated boron
center via steric effects of the ortho methyl groups. Such com-
pounds exhibit sizable second and third-order nonlinear optical
(NLO) coefficients,2,3 large two-photon absorption (TPA) cross-
sections,4 and can been used as efficient electron-transporting
and/or emitting layers in organic light emitting diodes (OLEDs).5
Three-coordinate boron-containing compounds are effective col-
orimetric and luminescent sensors for anions, especially fluoride
ions.6 Recently, a number of conjugated molecules with boryl side-
groups were shown to display very large Stokes shifts and high
quantum yields both in solution and the solid state, which was
attributed to the lack of close packing.7
At Bielefeld, we have a long-standing interest in the chem-
istry of 1,3,2-diazaboroles.12,13 We carried out studies on the
syntheses and optical properties of extended p-conjugated systems
with 1,3,2-diazaborolyl- and 1,3,2-benzodiazaborolyl substituents,
which gave intense luminescence when irradiated with UV
light.14,15 We recently reported the synthesis and photophysical
16
∫
properties of 2-(PhC C)-1,3-Et2-1,3,2-N2BC6H4, 1. Herein we
report the syntheses, photophysical properties and computa-
tional studies of derivatives with a series of substituents of
increasing p-electron-donating strength, i.e. Me, OMe, SMe,
NMe2, attached at the para position of the phenyl group of
compound 1.
Results and discussion
Synthesis
Reaction of 2-bromo-1,3-diethyl-1,3,2-benzodiazaborole with
equimolar amounts of the in situ generated lithium-derivatives
of the appropriate arylalkynes in THF in the temperature range
-78 ◦C to 20 ◦C led to the generation of benzodiazaboroles,
Materials with phenylethynyl scaffolds as p-electron conduct-
ing units have been intensively studied mainly because of their
luminescence and nonlinear optical properties, potential applica-
tion as emitters in electroluminescent devices and as sensors, and
liquid crystal phase behavior. Marder,8,9 Bunz10 and others11 have
examined optical and other physical properties of oligo- and poly-
∫
2-(4¢-XC6H4C C)-1,3-Et2-1,3,2-N2BC6H4 (X = Me, 2; MeO 3;
MeS 4; Me2N 5) (Chart 1). The products were isolated by short
path distillation and recrystallization of the solid distillates from
n-pentane/dichloromethane mixtures as colorless crystalline
solids in 42–69% yields. All of the compounds synthesized here
can be stored at -5 ◦C under an argon atmosphere for several
aUniversita¨t Bielefeld, Fakulta¨t fu¨r Chemie, Anorganische Chemie, Uni-
versita¨tsstr. 25, 33615, Bielefeld, Germany. E-mail: lothar.weber@uni-
bielefeld.de
1
weeks without decomposition. In the 11B{ H} NMR spectra for
bDepartment of Chemistry, Durham University, South Road, Durham, DH1
3LE, UK. E-mail: todd.marder@durham.ac.uk
the 1,3,2-diazaboroles 2–5 synthesized here, singlets are observed
between 20.5 and 21.2 ppm in agreement with a singlet at d =
21.2 ppm reported for 1.16
† CCDC reference numbers 711345–711347. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b821208b
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Table 1 Selected bond lengths and angles for 1–3 and 5
1a
2
3 (A)
3 (B)
5
˚
Bond lengths (A)
B–C
1.524(2)
1.429(1)
1.208(1)
1.439(1)
1.530(1)
1.431(1)
1.207(1)
1.438(1)
1.524(2)
1.431(2)
1.207(2)
1.440(2)
1.523(2)
1.427(2)
1.212(2)
1.435(2)
1.525(2)
1.430(1)
1.209(1)
1.435(1)
B–N*
∫
C C
C–C(∫C)
Benzene ring
C(13)–C(14/18)*
C(14)–C(15)/C(17)–C(18)*
C(15/17)–C(16)*
C(16)–(C/O/N)
1.400(1)
1.384(1)
1.388(1)
1.400(1)
1.389(1)
1.394(1)
1.509(1)
1.399(2)
1.385(2)
1.393(2)
1.361(2)
1.395(2)
1.380(2)
1.389(2)
1.371(2)
1.401(1)
1.380(1)
1.411(1)
1.380(1)
Angles (◦)
∫
B–C C
C–C C
175.6(1)
178.8(1)
65.8
174.0(1)
178.0(1)
70.4
173.8(2)
176.1(2)
78.4
172.8(2)
174.0(2)
13.4
179.4(1)
178.3(1)
76.4
∫
Interplanar angles (◦)
a Reference 16. *averaged values.
Chart 1 Compounds discussed in this study.
X-Ray crystallography
The molecular structures of the diazaboroles 2, 3 and 5 were
determined by single-crystal X-ray diffraction studies (Fig. 1–3.
Selected bond lengths and angles are listed in Table 1, with
data for 1 added for comparison. While crystals of 1, 2 and 5
contain one independent molecule, the crystal structure of the
methoxy derivative 3 contains two independent molecules A and
B, one of similar conformation to molecules of 1, 2 and 5 wherein
the angles between the arene and heterocycle ring planes are in
the range of 65.8 to 78.4◦. The second independent molecule
B in the crystal of 3 has a different conformation, featuring
an interplanar angle between the arene ring and heterocycle of
Fig. 2 Molecular structure of 3, showing the molecules in the unit cell, A
(top) and B (bottom).
13.4◦ as determined by the angle between the two normals to the
rings.
As shown in Table 1, the bond lengths and angles are virtually
identical even in the case of the nearly planar conformation of
molecule B for 3. The similar parameters found in both conformers
(A and B) for 3 show that the ring rotation has little influence on
bond lengths, despite the fact that p conjugation between the rings
would be highly favored in molecule B.
Several compounds related to 1 containing three-coordinate
boron attached to a phenylethynyl group have been structurally
Fig. 1 Molecular structure of 2 in the crystal. Ellipsoids are drawn at
50% probability level here and in Fig. 2 and 3.
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Table 2 Photophysical data
lmax (abs, nm)
lmax (abs, cm-1)
e (mol-1cm-1dm3)
lmax (em, nm)
lmax (em, cm-1)
Ufl
Stokes shift (cm-1)
1a
2b
3b
4b
5b
306
307
307
315
325
32700
32600
32600
31700
30800
—
393
388
374
410
355
25400
25800
26700
24400
28200
—
7300
6800
5900
7300
2600
22300
26700
31800
44700
0.99
0.97
0.89
0.97
a Reference 16, solvent n-hexane b This work, solvent THF, reference POPOP (U = 0.93).
UV-visible and luminescence spectroscopy
The optical properties of compounds 2–5 in THF solutions are
summarized in Table 2 along with reported data for 1. Compounds
1–5 show absorption maxima in the UV spectra at 307–325 nm and
emission maxima in the near UV-visible region at 355–410 nm with
extremely high quantum yields ranging from 0.89 to 0.99 (Table 2,
Fig. 4, 5). Diazaborole 4 has the lowest quantum yield (0.89),
together with the largest Stokes shift, which may be explained by
the heavy-atom effect of the S–Me group.9
The smallest Stokes shift of 2600 cm-1 for the Me2N-substituted
compound 5 is markedly different from the Stokes shifts of 5800–
7400 cm-1 for the other diazaboroles listed in Table 2 and of
6200–9500 cm-1 recently reported for closely related diazaboroles
where the boron is directly linked to thienyl or aryl groups.15 One
explanation (vide infra) is that the HOMO and LUMO orbitals in
5 are quite different from those in the other diazaboroles 1–4 due
to the strong electron-donating NMe2 group in 5.
Fig. 3 Molecular structure of 5. Selected angles (◦) C(16)–N(3)–C(20)
119.7(1), C(16)–N(3)–C(19) 119.6(1), C(19)–N(3)–C(20) 118.5(1),
C(15)–C(16)–N(3)–C(19) -6.2, C(17)–C(16)–N(3)–C(20) 11.1.
characterised.15,16 However, aside from the molecular structures of
1–3 and 5 herein, there is only one set of crystal structures from
which we can examine the structural effects of substituents on the
DFT computations
∫
∫
phenyl group of the B–C C–Ar moiety, namely Mes2BC CMes
∫
and Mes2BC CC6H4NMe2, which may be viewed as Mes2B
Geometries of model compounds 1a–5a, with methyl groups
instead of the ethyl groups in 1–5, were optimized via DFT
calculations at the B3LYP/6-31G* level of theory. Global minima
occur when both the benzodiazaborole and benzene rings are
coplanar. Model geometries with the diazaborole and benzene
analogues of 2 and 5.3,17 As in 2 and 5, the bonds in the B–C C–
∫
CAr moiety are essentially unaffected by whether the substituent is
Me or NMe2. A slight increase in the quinoidal distortion of the
para-substituted arene ring is observed for 5.
Fig. 4 Absorption spectra of compounds 2–5.
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Fig. 5 Emission spectra of compounds 2–5.
Table 3 Selected bond distances and angles for model geometries of 1
and 5
rings constrained to be mutually perpendicular were also opti-
mized, confirmed as true local minima and are denoted as 1b–5b.
The energy differences between the two conformers were in the
range of 0.6–0.8 kcal mol-1. The highest energy geometry between
the two minima 1a and 1b was located with diazaborole and
benzene rings at 85◦ to each other and was found to be a mere
0.005 kcal mol-1 higher than 1b. These energy values indicate that
the rotation barriers between these rings are negligible, and that
all rotamers will be populated in solution at ambient temperature.
The sum of the angles at the nitrogen in the dimethylamino group
in the experimental and optimized geometries are 357.8◦, 359.9◦
and 357.3◦ for 5, 5a and 5b respectively, close to 360◦ expected for
a planar geometry, which maximizes conjugation of the N-lone
pair with the aromatic p-system as expected.
Table 3 lists selected geometrical parameters for 1a, 1b, 5a
and 5b and shows that the different ring orientations have little
influence on the geometries. This is in agreement with the negligible
geometrical differences found for the two independent molecules
in the crystal of the methoxy derivative 3. Comparison between
the experimental X-ray and optimized geometries (Tables 1 and 3)
reveal very good agreements in the geometrical parameters. The
modest quinoidal distortion in the solid state structure of 5 is also
well reproduced in the DFT calculations.
1a
1b
5a
5b
˚
Bond distances (A)
B–C
B–N
C C
1.516
1.439
1.221
1.426
1.519
1.438
1.220
1.428
1.513
1.441
1.222
1.422
1.517
1.440
1.221
1.425
∫
∫
C–C( C)
Benzene ring
C–C
1.409
1.392
1.397
1.087
1.409
1.393
1.397
1.089
1.409
1.386
1.416
1.382
1.408
1.387
1.416
1.386
C–(H/N)
Angles (◦)
∫
B–C C
C–C C
180.0
180.0
180.0
180.0
180.0
180.0
180.0
180.0
∫
Tors0ion angles (◦)
∫
N–B–(C C)-C-C
0.0
90.0
0.0
90.0
The calculated absorption maxima from TD-DFT computa-
tions on the model geometries clearly depend on the orientations
between the two ends of the triple bond link. These absorptions
arise from strong, low energy HOMO–LUMO transitions. The
observed absorption values for 1–4 are in good agreement with
computed data from their model geometries assuming that all
conformers are present in solution where the planar and perpen-
dicular forms represent the two extreme conformers. However,
for 5 the agreement between computed and observed values is
poor, being much closer to the planar form 5a than for 5b. This
can be explained by the use of the polar solvent THF in our
experimental study vs. gas phase calculations, and the fact that
compound 5 is considerably more polar than 1–4 as shown by
the calculated dipole moments in Table 4. TD-DFT computations
on the model geometries were carried out using the polarisable
continuum model (PCM) treatment18 to support this assumption.
The calculated absorption values are significantly red-shifted for
the amines 5a and 5b by 10 nm compared to the gas-phase values
using the dielectric constant value of 7.58 assumed for THF. The
computed PCM values for the other eight model geometries were
red-shifted in the region of only 0 to 5 nm with respect to their
gas-phase values. The computed oscillator strengths are in accord
with the observed extinction coefficients for all diazaboroles 1–5.
Table 5 lists the orbital energies and orbital compositions for
the HOMO and LUMO of all ten model geometries; the model
geometries for 2–4 all have similar orbital compositions as the
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Table 4 Comparison of computed (TD-DFT) data for model geometries
of 1–5 with observed UV absorption data for 1–5
lmax Oscillator
Dipole
lmax
(nm) strength (f ) momenta (m, D)
(abs, nm) e (mol-1cm-1dm3)
1a 336 0.65
1.69
1.71
1.40
1.40
3.21
3.09
2.42
2.37
5.21
4.87
1
2
3
4
5
306
307
307
315
325
—
1b 272 0.51
2a 335 0.73
2b 275 0.84
3a 328 0.85
3b 272 0.64
4a 342 0.93
4b 278 1.31
5a 331 1.19
5b 277 1.47
22300
26700
31800
44700
Fig. 7 The frontier molecular orbitals for 5a and 5b. Contour values are
plotted at 0.04 (e/bohr3)1/2
.
a dipole is oriented with the negative end towards the diazaborolyl moiety
in all cases.
LUMO transitions for 1–4 are thus p(borolyl) → p*(aryl) charge
transfer in nature. They indicate that the excited singlet state
geometries for 1–4 have large dipole moments. In contrast, the
HOMO in 5 has considerable aryl character (38% in 5a and 82%
in 5b) while the LUMO in 5 is also of mainly aryl character (49%
in 5a and 70% in 5b). The HOMO–LUMO transition for 5 is
p(aryl) → p*(aryl) in nature. As both HOMO and LUMO are
mainly located on the aromatic ring, compound 5 may not have a
large dipole moment in its singlet excited state. The excited state of
5 would not be as dependent on the polar nature of the solvent as
the excited states of 1–4. The significantly lower degree of charge
transfer character could explain why the Stokes shift of 2600 cm-1
for 5 is considerably smaller than those of 5800–9500 cm-1 for
related diazaboroles studied here and elsewhere.16
model geometries for 1. In contrast, the electronic structure of
the dimethylamino compound 5 is different than those for 1–4.
Fig. 6 and 7 show the HOMOs and LUMOs for the four model
geometries of 1 and 5. While the HOMO in 1 is largely diazaborolyl
(83% in 1a and 92% in 1b) in character, the LUMO in 1 is mainly
located on the aryl moiety (52% in 1a and 72% in 1b). The HOMO–
What is clearly evident from the computational results is
that, in contrast to well-known BAr2 systems (e.g. Ar = Mes),
in which the BAr2 unit serves as a strong p-acceptor, the 10
p-electron benzodiazaborole unit, isoelectronic with indenyl an-
ion, is electron rich. Thus, in compounds 1–4, the benzodiazabo-
role serves as the p-electron donor. It would therefore be interesting
to explore, in future studies, the photophysical properties of a
series of compounds analogous to 1–5 but bearing a strong
Fig. 6 The frontier molecular orbitals for 1a and 1b. Contour values are
plotted at 0.04 (e/bohr3)1/2
.
Table 5 Orbital energies and compositions for model geometries
∫
X
HLGa (eV)
4.06
Orbital
E (eV)
C6H4(NMe)2 (%)
B (%)
C C (%)
aryl (%)
X (%)
1a
2a
3a
4a
5a
1b
2b
3b
4b
5b
H
LUMO
HOMO
LUMO
HOMO
LUMO
HOMO
LUMO
HOMO
LUMO
HOMO
LUMO
HOMO
LUMO
HOMO
LUMO
HOMO
LUMO
HOMO
LUMO
HOMO
HOMO-1
-1.16
-5.22
-1.09
-5.16
-0.92
-5.07
-1.14
-5.13
-0.70
-4.81
-0.87
-5.29
-0.79
-5.26
-0.58
-5.22
-0.86
-5.27
-0.70
-5.08
-5.12
11
75
11
73
13
67
11
64
14
44
2
82
2
82
2
82
2
82
2
9
8
10
7
11
6
9
6
12
3
3
10
3
10
3
10
3
10
3
1
25
10
24
11
25
12
24
11
25
14
23
7
23
8
24
8
22
8
24
17
8
55
8
52
9
47
12
53
12
43
24
72
1
69
1
66
1
69
1
62
48
1
0
0
2
0
3
3
4
7
6
14
0
0
3
0
4
0
5
0
Me
4.07
OMe
SMe
NMe2
H
4.15
3.99
4.11
4.42
Me
4.47
OMe
SMe
NMe2
4.64
4.41
4.38
8
34
0
1
81
10
a HLG = HOMO-LUMO Gap.
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p-acceptor, e.g. CN, CO2R, B(Mes)2 at the para position of the
phenyl ring.
precision). They were prepared with distilled and dried THF, with
concentrations varying from 1 to 8 mM according to their optical
density. The quantum yields were determined against POPOP (p-
bis-5-phenyl-oxazolyl(2)-benzene) (U = 0.93) as the standard.
Experimental
All manipulations were performed under an atmosphere of dry,
oxygen-free argon using standard Schlenk techniques. All solvents
were dried by standard methods and freshly distilled prior to use.
The compounds 2-bromo-1,3-diethyl-1,3,2-benzodiazaborole,19
4-methoxy-phenylacetylene,20 4-methylthio-phenylacetylene21 and
4-dimethylamino-phenylacetylene20,21 were prepared according to
literature methods. 4-Methyl-phenylacetylene was employed as
received from commercial sources (Aldrich). NMR spectra were
recorded in CDCl3 solutions at room temperature on a Bruker
AM Avance DRX 500 spectrometer (1H, 11B, 13C) using SiMe4
and BF3·OEt2 as external standards. The expected 13C peaks
corresponding to the ethynyl carbons were not detected above
the noise levels. Absorption is measured with a UV/VIS double-
beam spectrometer (Shimadzu UV-2550), using the solvent as a
reference. The setup used to acquire excitation–emission spectra
(EES) was similar to that employed in commercial static fluo-
rimeters: The output of a continuous Xe-lamp (75 W, LOT Oriel)
was wavelength-separated by a first monochromator (Spectra Pro
ARC-175, 1800 l/mm grating, Blaze 250 nm) and then used
to irradiate a sample. The fluorescence was collected by mirror
optics at right angles and imaged on the entrance slit of a
second spectrometer while compensating astigmatism at the same
time. The signal was detected by a back-thinned CCD camera
(RoperScientific, 1024 ¥ 256 pixels) in the exit plane of the
spectrometer. The resulting images were spatially and spectrally
resolved. As the next step, one averaged fluorescence spectrum
was calculated from the raw images and stored in the computer.
This process was repeated for different excitation wavelengths. The
result is a two-dimensional fluorescence pattern with the y-axis
corresponding to the excitation, and the x-axis to the emission
wavelength. Fig. 8 shows a sample spectrum obtained with this
technique. Here, the wavelength range is lex = 230–450 nm (in
1 nm increments) for the UV light and lem = 100–700 nm for
the detector. The time to acquire a complete EES is typically less
than 15 min. Post-processing of the EES includes subtraction of
the dark current background, conversion of pixel to wavelength
scales, and multiplication with a reference file to take the varying
lamp intensity as well as grating and detection efficiency into
account. For all measurements, samples were contained in quartz
cuvettes of 10 ¥ 10 mm2 (Hellma type 111-QS, suprasil, optical
2-(4¢-Methyl-phenylethynyl)-1,3-diethyl-1,3,2-benzodiazaborole 2
◦
A chilled solution (-78 C) of 4-methyl-phenylacetylene (0.43 g,
1.7 mmol) in THF (30 mL) was treated with a 1.5 M solution of
n-butyllithium in hexane (1.14 ml, 1.7 mmol) and the mixture was
stirred for 3 h at -78 ◦C and another 0.5 h at ambient temperature.
After cooling to -78 ◦C, a solution of 2-bromo-1,3-diethyl-
1,3,2-benzodiazaborole (0.20 g, 1.70 mmol) in THF (5 mL)
was added dropwise to the solution of the lithium acetylide.
Stirring of the mixture was continued for 2.5 h at -78 ◦C and
then overnight at room temperature. The resulting solution was
evaporated to dryness. The resulting residue was triturated with
CH2Cl2 and filtered. The filtrate was liberated from solvents and
the residue was subsequently purified by short path distillation at
10-3 bar. Product 2 was collected as a colorless solid, which was
crystallized from n-pentane at -20 ◦C for 3 d to yield 0.32 g (65%)
of 2 as analytically pure colorless crystals.
Found C, 78.98; H, 7.22; N, 9.65% C19H21N2B requires C, 79.18;
1
3
H, 7.34; N, 9.70%; H-NMR: d = 1.39 (t, JH,H = 7.2 Hz, 6H,
3
CH2CH3), 2.38 (s, 3H, C6H4CH3), 3.93 (q, JH,H = 7.2 Hz, 4H,
CH2CH3), 7.03 (m, 2H, CH-benzodiazaborole), 7.08 (m, 2H, CH-
3
benzodiazaborole), 7.18 (d, JH,H = 8.2 Hz, 2H, C6H4), 7.49 (d,
3JH,H = 8.2 Hz, 2H, C6H4); 13C{ H}-NMR: d = 16.2 (s, CH2CH3),
1
21.6 (s, C6H4CH3), 38.2 (s, CH2CH3), 106.8 (s, B–C∫C), 108.8
(s, CH-benzodiazaborole), 118.8 (s, CH-benzodiazaborole), 120.1
(s, C-Ph), 129.2 (s, C-Ph), 132.0 (s, C-Ph), 137.0 (s, C-benzo-
diazaborole), 139.0 (s, C-CH3); 11B{ H}-NMR: d = 21.1 (s); MS
1
(EI) m/z = 288 [M+], 273 [M+-CH3].
2-(4¢-Methoxy-phenylethynyl)-1,3-diethyl-1,3,2-
benzodiazaborole 3
A
well stirred, chilled solution (-78 ◦C) of 4-methoxy-
phenylacetylene (2.00 g,15.1 mmol) in THF (50 mL) was combined
with a 1.6 M solution of n-butyllithium in hexane (9.5 ml,
15.1 mmol). Stirring was continued at -78 ◦C for 1 h and then
0.5 h at room temperature. The mixture was chilled to -78 ◦C
and subsequently treated dropwise with a solution of 2-bromo-
1,3-diethyl-1,3,2-benzodiazaborole (3.82 g, 15.1 mmol) in THF
(20 mL). The solution was stirred for 1 h at -78 ◦C and overnight
Fig. 8 Example of an EES spectrum.
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The Royal Society of Chemistry 2009
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at ambient temperature. The solvent was removed in vacuo and the
residue was dissolved in CH2Cl2 (20 mL) and filtered. The filtrate
was freed from volatile components and the residue was purified
by short-path distillation at 10-3 bar. Crystallization of the solid
distillate from a CH2Cl2/hexane mixture afforded 3.30 g (69%) of
colorless crystalline product 3.
Found C, 70.27; H, 6.60; N, 8.69% C19H21N2BS requires C,
1
3
71.26; H, 6.61; N, 8.69%; H-NMR: d = 1.37 (t, JH,H = 6.9 Hz,
3
6H, CH2CH3), 2.50 (s, 3H, S-CH3), 3.90 (q, JH,H = 6.9 Hz,
4H, CH2CH3), 7.02 (m, 2H, CH-benzodiazaborole), 7.06 (m,
3
2H, CH-benzodiazaborole), 7.21 (d, JH,H = 8.5 Hz, 2H, CH-
3
1
phenyl), 7.47 (d, JH,H = 8.5 Hz, 2H, CH-phenyl); 13C{ H}-
NMR: d = 14.5 (s, S-CH3), 15.3 (s, CH2CH3), 37.3 (s, CH2CH3),
107.9 (s, CH-benzodiazaborole), 117.9 (s, CH-benzodiazaborole),
118.4 (s, C-Ph), 124.8 (s, C-Ph), 131.4 (s, C-Ph), 136.0 (s,
Found C, 74.04; H, 7.15; N, 8.91% C19H21N2BO requires C,
1
3
75.02; H, 6.96; N, 9.21%; H-NMR: d = 1.37 (t, JH,H = 7.2 Hz,
6H, CH2CH3), 2.82 (s, 3H, O-CH3), 3.90 (q, 3JH,H = 7.2 Hz, 4H,
CH2CH3), 6.87 (d, 3JH,H = 8.8 Hz, 2H, CH-phenyl), 7.00 (m, 2H,
CH-benzodiazaborole), 7.05 (m, 2H, CH-benzodiazaborole), 7.51
C-benzodiazaborole),139.2 (s, C-SCH3); 11B{ H}-NMR: d = 20.9
1
(s); MS (EI) m/z = 320 [M+].
1
(d, 3JH,H = 8.8 Hz, 2H, H- m-CH-phenyl); 13C{ H}-NMR: d = 16.1
(s, CH2CH3), 38.2 (s, CH2CH3), 55.3 (s, O-CH3), 108.7 (s, CH-
benzodiazaborole), 114.0 (s, C-Ph), 115.7 (s, C-Ph), 118.8 (s, CH-
benzodiazaborole), 133.6 (s, C-Ph), 136.9 (s, C-benzodiazaborole),
2-(4¢-Dimethylamino-phenylethynyl)-1,3-diethyl-1,3,2-
benzodiazaborole 5
1
160.6 (s, C-OCH3); 11B{ H}-NMR: d = 21.1 (s); MS (EI) m/z =
304 [M+], 289 [M+ - CH3].
Analogously,
a sample of 4-dimethylamino-phenylacetylene
(2.00 g,13.8 mmol) was first lithiated using a 1.6 M solution of
n-butyllithium in hexane (8.7 ml, 13.9 mmol) and then further
reacted with an equimolar amount of 2-bromo-1,3-diethyl-1,3,2-
benzodiazaborole (3.49 g, 13.8 mmol) in 20 mL of THF. After
evaporation of solvent and volatile components from the reac-
tion mixture the residue was crystallized from dichloromethane
(-20 ◦C, 2 d) to give 2.37 g (54%) of product 5 as colorless needles.
Found C, 75.05; H, 7.58; N, 13.02% C20H24BN3 requires C,
75.72; H, 7.63; N, 13.25%; 1H-NMR: d = 1.38 (t, 3JH,H = 6.9 Hz,
2-(4¢-Methylthio-phenylethynyl)-1,3-diethyl-1,3,2-
benzodiazaborole 4
A well stirred chilled solution (-78 ◦C) of 4-methylthio-
phenylacetylene (0.20 g, 1.35 mmol) in THF (30 mL) was treated
with a 1.6 M solution of n-butyllithium in hexane (0.87 ml,
1.36 mmol). The resulting mixture was stirred for 3 h at -78 ◦C and
another 0.5 h at room temperature before it was cooled to -78 ◦C
again. A solution of 2-bromo-1,3-diethyl-1,3,2-benzodiazaborole
(0.34 g, 1.7 mmol) in THF (5 mL) was slowly added and stirring
was continued for 2 h, and then overnight at room temperature.
Evaporation of the reaction mixture to dryness was followed
by extraction of the obtained residue with CH2Cl2. The filtered
CH2Cl2 extract was freed from solvent, and the crude product was
purified by short-path distillation at 10-3 bar. The solid distillate
was crystallized from n-pentane (-20 ◦C, 3 d) to give 0.18 g (42%)
of 4 as colorless needles.
3
6H, CH2CH3), 3.0 (s, 6H, N-CH3), 3.91 (q, JH,H = 6.9 Hz, 4H,
CH2CH3), 6.65 (d, 3JH,H = 9.1 Hz, 2H, CH-phenyl), 7.01 (m, 2H,
CH-benzodiazaborole), 7.05 (m, 2H, CH-benzodiazaborole), 7.49
3
1
(d, JH,H = 9.1 Hz, 2H, CH-phenyl); 13C{ H}-NMR: d = 16.1
(s, CH2CH3), 38.1 (s, CH2CH3), 40.2 (s, N-CH3), 108.6 (s, CH-
benzodiazaborole), 109.8 (s, C-Ph), 111.7 (s, C-Ph), 118.8 (s, CH-
benzodiazaborole), 133.3 (s, C-Ph), 137.0 (s, C-benzodiazaborole),
150.4 (s, C-N(CH3)2); 11B{ H}-NMR: d = 21.2 (s); MS (EI) m/z =
1
317 [M+], 302 [M+-CH3].
Table 6 Crystallographic data of compounds 2, 3 and 5
Compound
2
3
5
Empirical formula
Mr [g mol-1]
Crystal dimensions [mm]
Crystal system
C19H21BN2
288.19
0.26 ¥ 0.26 ¥ 0.24
monoclinic
P21/n
14.2697(3)
8.3197(2)
14.6128(2)
107.3256(11)
4
C19H21BN2O
304.19
0.30 ¥ 0.30 ¥ 0.18
orthorhombic
Pbca
8.3709(3)
16.8468(5)
48.0255(16)
90
16
1.193
0.073
2592
C20H24BN3
317.23
0.30 ¥ 0.30 ¥ 0.25
monoclinic
P21/n
11.0573(2)
11.5078(2)
14.1544(3)
94.3123(13)
4
Space group
˚
a [A]
˚
b [A]
˚
c [A]
ß [◦]
Z
rcalc [g cm-3]
m [mm-1]
1.156
0.067
616
3–30
1.173
0.069
680
3–27.5
F (000)
H [◦]
3–27.5
No. refl. collected
No. refl. unique
R (int)
No. refl. [I > 2s (I)]
Refined parameters
GOF
35625
4828
0.038
3873
203
1.061
28284
7576
0.055
4774
421
0.969
27595
4108
0.032
3437
221
1.042
RF [I > 2s (I)]
0.0424
0.1162
0.268/-0.198
711345
0.0449
0.1087
0.181/-0.260
711346
0.0385
0.1033
0.216/-0.221
711347
wRF2 (all data)
-3
˚
Drmax/min [e A ]
CCDC
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Dalton Trans., 2009, 2823–2831 | 2829
©

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Computational studies
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Org. Lett., 2005, 7, 5481; (f) M. Charlot, L. Porre`s, C. D. Entwistle, A.
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All ab initio computations were carried out with the Gaussian
03 package.22 The model and full geometries discussed here were
optimised via DFT calculations using the B3LYP/6-31G* level
of theory23,24 with no symmetry constraints or with a constrained
dihedral angle between the two rings of 90◦. Frequency calcu-
lations carried out on these optimized geometries showed no
imaginary frequencies. The electronic structure, TD-DFT and
PCM-TD-DFT computations were also carried out at the same
level of theory. The MO diagrams and orbital contributions were
generated with the aid of Gabedit25 and GaussSum26 packages
respectively.
Crystallographic studies
Crystallographic data were collected with a Nonius KappaCCD
diffractometer with Mo-Ka (graphite monochromator, l =
˚
0.71073 A) at 100 K.
Crystallographic programs used for structure solution and
refinement were from SHELXS-97 and SHELXL-97.27 The struc-
tures were solved by direct methods and were refined by using full-
matrix least squares on F2 of all unique reflections with anisotropic
thermal parameters for all non-hydrogen atoms. Hydrogen atoms
were included at calculated positions with U(H) = 1.2 Ueq for CH2
groups and U(H) = 1.5 Ueq for CH3 groups. Crystal data for the
compounds are listed in Table 6.
Acknowledgements
We thank Durham University for access to its High Performance
Computing facility and the Deutsche Forschungsgemeinschaft,
Bonn, Germany, for financial support.
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