Benzothiaoline three-coordinated organoboron compounds with a B=N bond: Dual emission and temperature-dependent excimer fluorescence

Article abstract of DOI:10.1021/om500757r

A series of 2,2-disubstituted benzothiazoline-BMes₂ (Mes = mesityl) compounds containing a B=N bond have been prepared and fully characterized. Their photophysical properties were investigated by UV-vis and fluorescence spectroscopy, which reve

Full text of DOI:10.1021/om500757r

Article
pubs.acs.org/Organometallics
Benzothiaoline Three-Coordinated Organoboron Compounds with a
BN Bond: Dual Emission and Temperature-Dependent Excimer
Fluorescence
Soren K. Mellerup and Suning Wang*
Department of Chemistry, Queen’s University, Kingston, Ontario K7M 3N6, Canada
*
S Supporting Information
ABSTRACT: A series of 2,2-disubstituted benzothiazoline-
BMes (Mes = mesityl) compounds containing a BN bond
2
have been prepared and fully characterized. Their photo-
physical properties were investigated by UV−vis and
fluorescence spectroscopy, which revealed the presence of
solvent- and concentration-dependent dual emission. On the
basis of the spectroscopic data, the dual emission was assigned
to monomer and excimer fluorescence of the molecule,
respectively. Experimental and TD-DFT computational data
indicated that the purple-blue monomer emission of these compounds is mainly from an intramolecular charge transfer (CT)
transition between the benzo-sulfur moiety and the boron center. The yellow-green excimer emission is attributed to
intermolecular interactions involving the benzo-sulfur unit. Furthermore, the excimer emission maxima of all compounds were
found to be sensitive to temperature, shifting to lower energy with decreasing temperature, which illustrates the potential for this
class of compounds to be used as luminescent thermometers.
INTRODUCTION
Scheme 1. Previously Studied Organoboron Isomerization
Process and Proposed Synthetic Targets 1−4
■
The absorption of electromagnetic radiation by conjugated
organic scaffolds has long been known to induce either
photophysical or photochemical change. Of late, it has become
well-established that the incorporation of a boron atom into
such systems can lead to a drastic change in their elicited
1
photoresponses. In the case of three-coordinated boron
compounds, the vacant p orbital of the boron atom typically
π
improves the electron transport capability of the materials and
consequently allows for fine-tuning of their photophysical
2
properties. Interestingly, the incorporation of four-coordinated
organonboron systems, we designed and synthesized benzo-
boron centers can result in materials that behave significantly
differently in response to photoexcitation. There are now
thiazoline-BMes compounds (1−4, Scheme 1 and 2) and
2
3
examined their response to light and heat. Unfortunately, the
thermal and photoisomerisation pathways that invert the
azoline stereochemistry were found to be inaccessible in this
new class of compounds. They do however display unusual dual
several examples of such species which undergo some form of
photochemical transformation (i.e., isomerization or elimina-
tion) upon irradiation. Given the wide and tunable range of
properties offered by organoboron materials, it should come as
no surprise that much effort has gone into developing new
boron-containing materials for use in applications such as
6
intramolecular and intermolecular emission in the purple-blue
and yellow-green region respectively, which is sensitive to
concentration, solvent polarity and temperature. Temperature-
dependent fluorescent molecules have important applications in
4
5
organic light-emitting diodes and memory devices.
Recently, we reported the unusual multistructural trans-
7
cellular imaging as molecular thermometers. A few recent
formation of N,C-chelated BMes compounds bearing an azole
2
studies have shown that organoboron compounds are very
promising as molecular thermometers due to their polarized
excited states which are sensitive to temperature-dependent
unit (e.g., thiazole), wherein the azole unit was transformed to
an azoline (A in Scheme 1) via an intramolecular H atom
3
h
transfer. Subsequent photo- or thermal isomerization allowed
for the controlled and reversible inversion of stereochemistry at
the C2 position of the azoline moiety, which was believed to be
made possible by the presence of the BN bond in A and its
diastereomer. In order to investigate whether light and heat
could be used to control the chirality of azolines within simpler
6e,8
conformational change or proton transfer processes.
None-
theless, examples of temperature-dependent fluorescent orga-
Received: July 23, 2014
©
XXXX American Chemical Society
A
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noboron molecules over a wide temperature range remain rare
and underexplored. For these reasons, we set out to investigate
the curious fluorescent properties of the new class of BN
containing molecules and the results are reported herein.
asymmetric structure of 1, which undergoes a relatively fast
chemical exchange at ambient temperature. Using the variable
temperature data shown in Figure 1, the rotation barrier around
RESULTS AND DISCUSSION
Syntheses and Structures. The benzothiazoline ligands
■
and their BMes compounds were readily synthesized under
2
mild conditions as illustrated in Scheme 2. The 2,2-
Scheme 2. Synthetic Procedure for 1−4
1
Figure 1. Variable temperature H NMR spectra of 1 in CD Cl ,
2
2
where * represents an unidentified impurity.
1
2
−1
the B−C bond was estimated to be ca. 12 kcal mol , which is
in agreement with previously reported three-coordinated
2
,10
1
organoboron species.
The H NMR spectrum of 2 shows
well resolved chemical shifts for all of its methyl groups at 298
K as a result of the chiral C2 center. Upon cooling to 228 K, the
1
H NMR spectrum of 2 (see SI) shows two sets of well-
resolved signals that can be assigned to the two diastereomers
of 2 due to the asymmetric environment around the boron
center and the C2 atom. The mesityl rotation around the B−C
bond interconverts the diastereomers of 2 and this rotation
barrier was estimated to be similar to that of 1. Although the
mesityl rotation barriers for compounds 3 and 4 were not
disubstituted benzothiazolines were all prepared in good yields
by employing a solvent-free method described in the literature.
9
The BN species 1−4 were subsequently obtained by first
reacting the ligands with NaH to generate their sodium salts in
1
situ, followed by a stoichiometric addition of BMes F to afford
directly measured, both of their H NMR spectra are similar to
2
1
−4 in ∼50% yield. The intense green-yellow fluorescence of
− 4 was qualitatively observed immediately following the
that of compound 1. Therefore, it is reasonable to suggest that
they would also possess similar B−C rotation barriers.
Compounds 1−4 decompose slowly in solution upon extended
exposure to ambient conditions, generating the hydrolyzed
1
addition of the boron source with a hand-held UV lamp (365
nm excitation). Compounds 1− 4 were fully characterized by
1
13
11
11
10
H, C, and B NMR, HRMS, and elemental analysis. The B
product (BMes ) O or BMes (OH) and the free ligand.
2
2
2
NMR chemical shifts were found to be 47.8, 48.1, 48.3, and
Interestingly, the same hydrolysis phenomenon was not
observed for the related molecules A shown in Scheme 1.
The crystal structure of compound 1 was determined by
single-crystal X-ray diffraction analysis and is shown in Figure 2.
The boron center adopts a trigonal planar geometry with
C(10)B−C(19), C(10)B−N, C(19)B−N bond angles
4
8.3 ppm for 1−4, respectively. These values reside in the
2
typical range for three-coordinated organoboron and are
similar to those of previously reported three-coordinated boron
compounds with a BN bond.
3
h,10
Compounds 1−4 were found to display dynamic exchange
1
o
o
o
behavior in solution. In the H NMR spectrum of compound 1
of 120.11(13) , 120.24(15) , 119.63(14) , respectively, indicat-
ing minimal steric strain around the boron center. The five-
membered thiazoline ring is puckered, causing the asymmetric
environment for the two methyl groups at the 2-position. The
crystal structure confirmed the presence of a BN bond in 1,
as evidenced by the short B(1)−N(1) bond length, 1.428(2) Å,
at room temperature (see Supporting Information (SI)), each
mesityl group displays two distinct sets of chemical shifts
indicating no free rotation about the BN bond. This result is
consistent with the previous finding that rotation around the
11
BN bond has a very large barrier. At low temperature, the
o-methyl pair on each mesityl is resolved, supporting the
which is shorter than those observed in indolyl-BMes
2
B
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Figure 2. X-ray crystal structure of 1 with 35% thermal ellipsoids (top
view and side view).
10
compounds that possess a BN bond of ∼1.44 Å, but
comparable to other BN bond lengths reported for R B
2
3
f,11,13
NR′ and azaborines.
The two trigonal BNC and NBC
2
2 2
planes are not coplanar but have a dihedral angle of 156.3°, as
shown by the side view of the crystal structure in Figure 2. This
is clearly caused by the steric interactions between the
benzothiazoline ring and the mesityl rings. In the crystal lattice
of 1, no π-stacking interactions were observed.
There are however short intermolecular contacts between
the S atom, the benzo ring and the mesityl ring, as shown in
Figure 3. Due to the lack of suitable crystals, the structures of
Figure 4. DFT optimized structures of 1−4.
calculations for each compound using their computationally
optimized geometries were performed. In all four cases, the
predicted UV−vis spectra were found to be in good agreement
with experimentally observed ones (see SI). The S → S
0
1
transition for all compounds involve mainly HOMO → LUMO
(
>90%) with a very large oscillator strength. For all four
compounds, the HOMO is concentrated on the benzo ring and
the sulfur atom with a small contribution from the BN bond
while the LUMO is dominated by the p orbital of the boron
π
atom with significant contributions from the π* orbitals of the
benzo ring and the mesityl. Thus, the absorption band at ∼320
nm could be assigned to charge transfer from benzo-S to B
(
π*). The sulfur atom has not only a large contribution to the
HOMO level but also dominates the HOMO−1 level as shown
in Figure 6. There appears to be minimal contributions to the
electron density from the substituents at the 2-position on the
benzothiazoline backbone which causes all the calculated
HOMO and LUMO energy levels of 1−4 to look nearly
identical. This result was expected, however, as the UV−vis
spectra of the four compounds are also very similar. The high
energy absorption bands at wavelengths <320 nm are mainly
from the π → π* transition of the benzothiazoline ring (1 and
) or the mesityl (π) → B (π*) transition (3 and 4) (see SI).
Compounds 1−4 emit a green or yellow color when
irradiated at 365 nm with a hand-held UV lamp. This
observation was rather unexpected because of the nonemissive
and nonconjugated benzothiaoline backbone and the fact that
Figure 3. Short contacts observed in the crystal lattice of 1.
2
2−4 were not determined by X-ray diffraction analysis. For
comparison, the structures of 1−4 were calculated and
optimized by DFT calculations and are shown in Figure 4.
For 1, the calculated parameters match well with those from the
crystal structure. In the structures of 3 and 4, the cyclopentyl
and the cyclohexyl rings adopt a puckered and a chair
conformation, respectively, with the S atom occupying the
axial position. The calculated BN bond lengths in all four
compounds are similar. Based on the torsion angles between
BC and NC′ units (φ of C → B→N → C′, see Table 1),
the related indolyl-BMes compounds in which the B atom is
2
10
bound to a conjugated indole ring are blue fluorescent. Upon
further investigation, we observed that the luminescent color of
1−4 is strongly dependent on their concentration in solution.
For example, at [1] = 10− M in THF an emission peak in the
purple-blue region with λmax = 374 nm was observed (Figure 7).
6
−6
−2
As the concentration of 1 was increased from 10 to 10 M,
the 374 nm emission peak was replaced by a broad green
emission band centered at 529 nm with minimal change in the
accompanying excitation spectra. Similarly, this same phenom-
enon was also observed for compounds 2−4 and the data are
summarized in Table 2.
2
2
compounds 2 and 4 are more congested than 1 and 3.
Fluorescence of Compounds 1−4. As with many boron-
containing organic frameworks, compounds 1−4 fluoresce in
the UV−vis range of the electromagnetic spectrum. The UV−
vis absorption spectra of these compounds are shown in Figure
5
. In accordance with our previously reported indole-BMes
The substituent groups on the C2 atom appear to have a
significant impact on the energy of both the purple-blue peak
and the green-yellow peak with the emission energy of 3 and 4
being the lowest. In all cases, the low energy emission peak is
unaffected by the presence of oxygen and possesses decay
2
1
0
compounds, all four boron compounds possess two major
absorption bands in the UV region (ca. 298 and 320 nm) that
do not change significantly with solvent polarity. In order to
understand the origin of these absorption maxima, TD-DFT
C
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Table 1. Selected Bond Lengths (Å) and Angles (deg) of Compounds 1−4
compd
bond lengths
bond angles
φ [C(10)→B→N→C(1)]
1
B−N
1.428(2)
1.592(2)
1.582(4)
1.440(1)
1.600(1)
1.595(1)
1.442(1)
1.601(1)
1.596(1)
1.440(1)
1.601(1)
1.596(1)
1.442(1)
1.601(1)
1.597(1)
C(10)B−N
120.24(15)
119.63(14)
120.11(13)
121.4(1)
119.5(1)
119.0(1)
119.4(1)
121.9(1)
118.7(1)
119.4(1)
122.1(1)
118.5(1)
119.4(1)
122.2(1)
118.4(1)
156.3(2)
155.3(1)
154.2(1)
155.3(1)
154.1(1)
B−C(10)
B−C(19)
B−N
B−C(10)
B−C(19)
B−N
B−C(10)
B−C(19)
B−N
B−C(10)
B−C(19)
B−N
N−BC(19)
C(19) B−C(10)
C(10)B−N
a
1
N−BC(19)
C(19) B−C(10)
C(10)B−N
a
2
N−BC(19)
C(19) B−C(10)
C(10)B−N
a
3
N−BC(19)
C(19) B−C(10)
C(10)B−N
a
4
B−C(10)
B−C(19)
N−BC(19)
C(19) B−C(10)
a
Values obtained from DFT optimized geometries.
Figure 7. Fluorescence excitation (dotted) and emission (solid)
spectra and photographs illustrating the concentration dependent
emission (λex = 318 nm) of 1 in THF.
Figure 5. UV/vis spectra of 1−4 in THF at 10− M.
4
of detectable nanoparticles by dynamic light scattering
experiments (see SI), suggests that aggregation induced
14
emission (AIE) is not responsible for this phenonmenon.
We therefore propose that the low energy emission band is a
result of excimer fluorescence. To further probe the origin of
the monomer and excimer emission from 1−4, we examined
the solvent dependence of the fluorescence. As shown in Figure
8
(and SI), a bathochromic shift was observed for both the
monomer and excimer emission peaks of 1−4 with increasing
solvent polarity, which is consistent with a polarized excited
state involving charge transfer (CT) transitions for both
emission peaks.
For the monomer emission, the solvent-dependent phenom-
enon agrees with the TD-DFT data that revealed the transition
to the first excited state is dominated by a benzo-S to B (π*)
CT transition. For the excimer emission, we believe that it
likely involves intermolecular charge transfer from the S atom
Figure 6. HOMO, HOMO−1, and LUMO diagrams of 1 (isocontour
=
0.035).
(
lone pair) to the benzo ring (π*) via the dimer formation as
lifetimes on the order of nanoseconds, indicating that
phosphorescence is not responsible for this unusual phenom-
ena. Furthermore, we observed no substantial change in the
UV−vis absorption maxima with increasing concentration for
all four compounds (see SI) which, in conjunction with the lack
revealed by the crystal structure shown in Figure 3. In fact, the
lone pair of electrons on the sulfur atom are dominant in the
HOMO and HOMO−1 orbitals of the molecules while the
benzo π* orbitals have significant contributions to both LUMO
and LUMO+1 orbitals. Thus, energetically favorable inter-
D
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Table 2. Pertinent Photophysical Data of 1−4 recorded in THF
λabs (nm) (ε, L mol− cm−1
1
)
λem monomer/excimer (nm)
τ monomer/excimer (ns ± 0.2)
Φsoln excimer (%)
compd
4
1
2
3
4
318 (1.23 × 10 )
374/534
373/530
387/552
383/539
1.9/1.8
1.9/2.0
1.6/2.2
1.8/2.0
0.98
1.01
1.04
0.95
4
319 (1.17 × 10 )
4
321 (1.16 × 10 )
4
322 (1.51 × 10 )
In the temperature range of 80 °C to −78 °C, the low energy
emission peak of 1 in 2-Me-THF experiences a bathochromic
shift with decreasing temperature (Figure 9), changing emission
Figure 9. Fluorescence spectra of 1 in MeTHF (10− M) at various
temperatures (10 °C intervals except for −78 °C). Inset: photograph
of the solution at three select temperatures.
2
Figure 8. Fluorescence spectra of 1 at (a) 10− M and (b) 10−2 M in
6
various solvents (λex = 318 and 330 nm, respectively). Inset:
photograph of each 10 M solution under a 365 nm UV lamp.
color from green to yellow while increasing emission intensity.
This corresponds to a 0.15 nm/°C shift in λmax (Figure 10),
−2
molecular interactions involving these two moieties in the
excited state could be possible. This is further supported by the
electrostatic potential (ESP) surfaces of each compound (see
SI), which predict a buildup of negative and positive charge at
the lone pairs on sulfur and the aromatic C−H′s of the benzo
ring. Additionally, the benzo ring appears to be less negatively
charged in comparison to the two mesityl’s, which would also
contribute to the aforementioned intermolecular interaction.
The involvement of the B atom in the excimer emission is also
likely because it is conjugated with the benzo ring via the N
atom and dominates the LUMO level. Similar short contact
Figure 10. Temperature dependence of emission maxima for 1.
15
excimer fluorescence has been observed for several derivatives
of anthracene; however, there are only a limited number of
these examples. More commonly observed is the π-stacking of
which is approximately half of that reported by Yang and co8-
workers for pyrene-functionalized organoboron compounds.
Similarly, 2−4 display red shifts of their low energy emission
peaks with decreasing temperature of 0.10, 0.14, and 0.13 nm/
°C respectively. In contrast, the monomer emission λmax of 1−4
does not change within this temperature range, which could
allow these compounds to be used for emission wavelength-
based ratiometric temperature sensing (see SI). Due to
instrument limitations, we were not able to monitor the
spectral change for temperatures between −78 °C and −196
°C. At very low temperature (77 K, frozen 2-Me THF glass),
the excimer peak of each compound vanishes completely and is
replaced by an equally broad emission peak with λmax = 479 nm
(see SI). Similar emission bands were also observed in the solid
16
flat aromatic molecules (pyrene) in the excited state, which
16d
possess significantly longer decay lifetimes in comparison to
15a,e
their short contact counterparts.
Given that our excimer
lifetimes are relatively short, our short contact hypothesis
remains feasible, although π-stacking interaction is also a
possibility. There is a clear red shift in the monomer and
excimer emission λmax from the aliphatic (1 and 2) to the cyclic
(
3 and 4) substituted compounds, which may be attributed to
the additional vibrational states due to the conformational
change of the cyclopentyl and the cyclohexyl ring.
Further evidence that supports the assignment of the excimer
emission in 1−4 is the distinct and unusual temperature
dependence of the low energy emission peak.
E
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state fluorescence spectra of 1−4 (see SI) and have been
pounds, which were attributed to two competing intramolecular
excited states (e.g., the locally excited vs twisted intramolecular
charge transfer states reported by Yang et al. and the locally
assigned as emission from van der Waals dimers that are being
17
8a
directly photoexcited. To better illustrate, the potential
energy diagram of two monomer units at varying intermo-
lecular distance and molecular orbital overlap is shown in
Figure 11. At low monomer concentration (i.e., large
excited versus excited-state intramolecular proton transfer by
6e
Yamaguchi et al. ). The electron-rich and nonconjugated
thiazoline ring in these molecules are believed to be responsible
for the excimer emission in compounds 1−4 because similar
excimer emission phenomenon was not observed in related
BN-containing compounds that have a conjugated backbone
10
such as indolyl.
Compounds 1−4 represent a new class of temperature-
dependent fluorescent molecules; however, they are not
suitable for practical use as fluorescent thermometers because
of their poor stability under ambient conditions and their low
fluorescent quantum yields. In any case, these findings are a
nice proof of concept as it is plausible that simple and robust
luminescent thermometers may be achieved with further
modification of the steric and electronic properties of this
class of compounds.
CONCLUSIONS
■
Figure 11. Potential energy diagram showing the origin of (a)
monomer, (b) vdW dimer in the solid state and at 77K, and (c)
excimer fluorescence, where each monomer unit is represented by A.
In summary, we have synthesized and characterized a novel
class of BN-containing organoboron compounds. Despite
the nonconjugated nature of the backbones, these molecules all
display dual emission with a distinct green or yellow excimer
fluorescent peak. Both the monomer and excimer fluorescence
appear to be due to CT from sulfur to boron which is
supported by TD-DFT and the positive solvatochromic
behavior of each emission profile. Remarkably, the excimer
fluorescence maxima were also found to be sensitive toward
temperature, which could eventually lead to the design and use
of new fluorescent and dual emissive compounds in molecular
thermometry.
intermolecular separation), the individual monomers are
excited and emit photons prior to interacting with other
monomer units. As the concentration is increased, van der
Waals (vdW) dimers begin to form in equilibrium with the
separate monomer units. Upon photoexcitaiton in solution, the
excess internal energy of the system allows the vdW excited
dimer to excimer activation barrier to be crossed, and a photon
is released from the newly formed excited state dimer. Although
similar excitations will occur in the solid state and at 77K, the
lack of kinetic energy and rigid nature of the media do not
allow the barrier to be crossed and a higher energy photon is
emitted as a result of vdW dimer relaxtion.
EXPERIMENTAL SECTION
■
General Procedures. All reactions were carried out under
an inert atmosphere of dry nitrogen while employing standard
Schlenk techniques. Aminothiophenol and all ketones were
Given that the emission energy difference between the vdW
dimer and excimer is small, it is conceivable that the geometric
configuration of these two different emitting entities are alike
which suggests our proposed mechanism of excimer formation
may be correct. Although the fluorescence intensity and decay
purchased from Aldrich chemical company, while BMes F was
2
purchased from TCI chemical company, and used without
further purification. Solvents were taken from a PURE SOLV
activated alumina column system (Innovative Technology Inc.),
degassed using three freeze−pump−thaw cycles, and further
7
lifetimes of organic compounds are expected to change with
temperature, the red shift in emission maxima with decreasing
temperature is unusual and has only been observed in a few
1
dried over 4° molecular sieves for several days prior to use. H,
8
13
11
organoboron-based systems. For molecules 1−4, the emission
C, and B NMR spectra were recorded on a Bruker Avance
maxima red shift of the low energy peak could be explained by
the increased molecular aggregation and greater intermolecular
interactions with decreasing temperature. This temperature-
dependent behavior of the low energy emission peak of 1−4 is
400 MHz spectrometer using deuterated solvents that were
purchased from Cambridge Isotopes, degassed by vigorous dry
nitrogen bubbling, and stored over 4° molecular sieves for
several days prior to use. High-resolution mass spectra
(HRMS) were obtained using a Micromass GCT TOF-EI
Mass Spectrometer. Elemental analyses were performed by the
Elemental Analysis Service at the University of Montreal.
Excitation and emission spectra were recorded using a Photon
Technologies International QuantaMaster Model 2 spectrom-
eter equipped with a Quantum Northwest TLC 50 Temper-
ature-Controlled Cuvette Holder. Luminescent decay lifetimes
were measured against an instrument response function (IRF)
and fitted using a single exponential fitting curve (determined
and fitted curves can be found in the SI). UV−visible spectra
were recorded a Varian Cary 50 spectrometer. Photo-
luminescent quantum yields were measured using the optically
dilute method (A ≈ 0.1) at room temperature in dry/degassed
18
consistent with typical excimer emission.
The temperature dependence of the excimer emission of 1−
was also found to persist in solvents of varying polarity such
4
as hexanes and DMSO (see SI). Because of the polarized nature
of the excimers formed by 1−4, it is not surprising that the
magnitude of the bathochromic shift in λ
emission with
max
decreasing temperature becomes smaller as the solvent polarity
decreases. For example, the bathochromic shift of 1 in hexanes
was found to be 0.03 nm/°C compared to a shift of 0.15 and
0.12 nm/°C in 2-Me-THF and DMSO respectively. The
proposed excimer-based temperature dependent fluorescence
of 1−4 is in rather stark contrast to the previously reported
temperature-dependent fluorescence of triarylboron com-
F
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THF relative to quinine sulfate in 0.5 M H SO (Φ
=
sol
7.80; N, 3.39; S, 7.76. Found: C, 78.34; H, 7.90; N, 3.32; S,
7.87.
2
4
1
9
0.546).
Ligand Synthesis. All benzothiazoline ligands were
B(Mesityl) (N-(±)-2,2-ethylmethylbenzothiazoline) (2).
2
prepared, purified, and characterized in accordance with
The title compound was obtained as a white flaky solid, 55%
9
1
procedures described in the literature.
yield. H NMR (400 MHz, CD
2
Cl
): δ 7.03 (d, J = 7.5 Hz, 1H,
2
2
,2-Dimethylbenzothiazoline. The title compound was
Ar−H), 6.76 (t, J = 7.5 Hz, 1H, Ar−H), 6.72 (s, 1H, Mes), 6.65
(s, 1H, Mes), 6.54 (s, 1H, Mes), 6.49 (s, 1H, Mes), 6.47 (t, J =
7.6 Hz, 1H, Ar−H), 6.42 (d, J = 7.7 Hz, 1H, Ar−H), 2.27 (br s,
3H, Mes), 2.16 (s, 3H, Mes), 2.08 (s, 3H, Mes), 2.06 (s, 3H,
Mes), 2.01 (s, 3H, Mes), 1.95 (s, 3H, Mes), 1.72 (m, 2H,
1
obtained as a pale yellow solid in 80% yield. H NMR (400
MHz, CDCl ): δ 7.09 (d, J = 7.6 Hz, 1H, Ar−H), 6.94 (t, J =
3
7
7
×
.7 Hz, 1H, Ar−H), 6.79 (t, J = 7.4 Hz, 1H, Ar−H), 6.69 (d, J =
.8 Hz, 1H, Ar−H), 3.99 (br s, 1H, N−H), 1.76 (s, 6H, −CH
3
−
CH −CH ), 1.43 (s, 3H, benzothiazoyl −CH ), 0.81 (t, J =
2) ppm.
2
3
3
13
7
.2 Hz, 1H, −CH −CH ) ppm; C NMR (100 MHz, CD Cl )
(
±) 2,2-Ethylmethylbenzothiazoline. The title compound
2
3
2
2
1
δ 147.0 (Ar), 142.0 (Mes), 141.5 (Mes), 140.3 (Mes), 140.0
Mes), 138.0 (Mes), 137.8 (Mes), 132.7 (Ar), 128.7 (Mes),
was obtained as a yellow oil in 71% yield. H NMR (400 MHz,
(
CDCl ): δ 7.07 (d, J = 7.6 Hz, 1H, Ar−H), 6.93 (t, J = 7.6 Hz,
3
1
1
28.6 (Mes), 128.3 (Mes), 124.5 (Ar), 123.8 (Ar), 122.3 (Ar),
20.9 (Ar), 87.0 (benzothiazoyl C2), 35.4 (benzothiaozyl C2
1
1
(
H, Ar−H), 6.76 (t, J = 7.6 Hz, 1H, Ar−H), 6.66 (d, J = 7.8 Hz,
H, Ar−H), 3.96 (br s, 1H, N−H), 1.95 (m, 2H, −CH ), 1.70
2
Ethyl), 26.6 (benzothiazoyl C2Methyl), 24.2 (Mes), 23.1
s, 3H, −CH ), 1.08 (t, J = 7.30, 3H, −CH −CH ) ppm.
3
2
3
(
Mes), 22.6 (Mes), 21.9 (Mes), 21.2 (Mes), 21.1 (Mes), 11.1
Spiro[benzothiazoline-2,1′-cyclopentane]. The title com-
11
1
(benzothiazoyl C2 Ethyl) ppm; B NMR (128 MHz, CD Cl )
δ 48.1 ppm; HRMS (EI) calcd for C H BNS [M] , 427.2510;
pound was obtained as a yellow oil in 74% yield. H NMR (400
2 2
+
MHz, CDCl ): δ 7.08 (d, J = 7.6 Hz, 1H, Ar−H), 6.93 (t, J =
28 34
3
found, 427.2523. Anal. Calcd for C H BNS: C, 78.68; H,
7
7
.6 Hz, 1H, Ar−H), 6.78 (t, J = 7.6 Hz, 1H, Ar−H), 6.67 (d, J =
.8 Hz, 1H, Ar−H), 4.09 (br s, 1H, N−H), 2.14 (m, 4H,
28 34
8
7
.02; N, 3.28; S, 7.50. Found: C, 78.72; H, 8.14; N, 3.27; S,
.55.
cyclopentyl), 1.82 (quint, J = 3.53 Hz, 4H, cyclopentyl) ppm.
Spiro[benzothiazoline-2,1′-cyclohexane]. The title com-
B(Mesityl) (N-Spiro[benzothiazoline-2,1′-cyclopentane])
2
1
(3). The title compound was obtained as a white flaky solid,
pound was obtained as a white solid in 69% yield. H NMR
1
4
1
6
7
3
6
5% yield. H NMR (400 MHz, CD Cl ): δ 7.21 (d, J = 7.7 Hz,
2 2
(
400 MHz, CDCl ): δ 7.06 (d, J = 7.6 Hz, 1H, Ar−H), 6.92 (t,
3
H, Ar−H), 6.93(t, J = 7.4 Hz, 1H, Ar−H), 6.81 (s, 2H, Mes),
.64 (s, 2H, Mes), 6.62 (t, J = 7.5 Hz, 1H, Ar−H), 6.60 (d, J =
J = 7.7 Hz, 1H, Ar−H), 6.75 (t, J = 7.6 Hz, 1H, Ar−H), 6.66 (d,
J = 7.7 Hz, 1H, Ar−H), 4.05 (br s, 1H, N−H), 2.28−2.19
.6 Hz, 1H, Ar−H), 2.31 (br s, 9H, Mes −CH × 3), 2.21 (s,
3
(
overlapping s, 2H, cyclohexyl), 1.85−1.54 (m, 7H, cyclo-
H, Mes −CH ), 2.19−2.11 (m, 4H, cyclopenthyl), 2.09 (s,
3
hexyl), 1.32 (m, 1H, cyclohexyl) ppm.
H, Mes −CH × 2), 1.77−1.54 (m, 4H, cyclopentyl) ppm;
3
General Procedure for the Syntheses of the Boron
Compounds 1−4. A 125 mL oven-dried Schlenk flask was
charged with sodium hydride (72 mg, 3.1 mmol), benzothiazo-
line derivative (∼500 mg, 3.1 mmol), THF (30 mL), and a
magnetic stir bar. The mixture was then stirred at room
temperature for 1 h with white precipitate observed after 15
1
3
C NMR (100 MHz, CD Cl ) δ 147.1 (Ar), 141.8 (Mes),
2
2
1
40.1 (Mes), 138.0 (Mes), 137.7 (Mes), 133.2 (Ar), 128.6
(
(
(
(
Mes), 128.4 (Mes), 124.6 (Ar), 123.8 (Ar), 122.9 (Ar), 120.8
Ar), 93.4 (benzothiazoyl C2), 37.5 (cyclopentyl), 23.6
cyclopentyl), 22.4 (Mes), 22.2 (Mes), 21.2 (Mes), 21.1
Mes) ppm; 11B NMR (128 MHz, CD Cl ) δ 48.3 ppm;
2
2
min. BMes F (∼800 mg, 3.1 mmol) in THF (5 mL) was then
2
+
HRMS (EI) calcd for C H BNS [M] , 439.2510; found,
29
34
added dropwise at room temperature, resulting in a clear pale
yellow solution which fluoresced green/yellow upon 365 nm
irradiation. The mixture was allowed to stir overnight, before
being diluted with petroleum ether and concentrated in vacuo.
Flash column chromatography with 3:2 hexane/CH Cl (1) or
4
39.2515. Anal. Calcd for C H BNS: C, 79.26; H, 7.80; N,
29 34
3
.19; S, 7.30. Found: C, 79.29; H, 7.90; N, 3.03; S, 7.39.
B(Mesityl) (N-Spiro[benzothiazoline-2,1′-cyclohexane])
2
(
4). The title compound was obtained as a pale yellow flaky
2
2
1
solid, 58% yield. H NMR (400 MHz, CD Cl ): δ 7.19 (d, J =
2
2
petroleum ether (2−4) as the eluent afforded the desired
products in moderate yields (50−58%). The compounds were
further purified by either recrystallization from the slow
evaporation of CH Cl (1) or precipitation from petroleum
ether at −25 °C (2−4).
B(Mesityl) (N-2,2-dimethylbenzothiazoline) (1). The title
7
2
6
3
1
0
.5 Hz, 1H, Ar−H), 6.89 (t, J = 7.6 Hz, 1H, Ar−H), 6.82 (s,
H, Mes), 6.64 (s, 2H, Mes), 6.62 (t, J = 7.5 Hz, 1H, Ar−H),
.59 (d, J = 7.8 Hz, 1H, Ar−H), 2.31 (br s, 9H, Mes −CH ×
3
2
2
), 2.21 (s, 3H, Mes −CH ), 2.11 (s, 6H, Mes −CH × 2),
3
3
.92−1.82 (m, 2H, cyclohexyl), 1.65−1.56 (m, 6H, cyclohexyl),
13
2
.97−0.84 (m, 3H, cyclohexyl) ppm; C NMR (100 MHz,
compound was obtained as a white grain-like solid, 50% yield.
CD Cl ) δ 146.9 (Ar), 141.8 (Mes), 140.0 (Mes), 137.9 (Mes),
2
2
1
H NMR (400 MHz, CD Cl ): δ 7.06 (d, J = 8.0 Hz, 1H, Ar−
2
2
137.7 (Mes), 132.9 (Ar), 128.6 (Mes), 128.4 (Mes), 124.4
Ar), 123.7 (Ar), 122.5 (Ar), 121.5 (Ar), 90.2 (benzothiazoyl
H), 6.79 (t, J = 7.3 Hz, 1H, Ar−H), 6.70 (s, 2H, Mes), 6.52 (s,
(
2
1
−
H, Mes), 6.44 (t, J = 7.3 Hz, 1H, Ar−H), 6.50 (d, J = 8.1 Hz,
H, Ar−H), 2.17 (s, 9H, Mes −CH × 3), 2.08 (s, 3H, Mes
C2), 37.4 (cyclohexyl), 25.5 (cyclohexyl), 23.7 (Mes), 22.3
(Mes), 21.2 (Mes), 21.1 (Mes) ppm; 11B NMR (128 MHz,
3
+
CH ), 1.97 (s, 6H, Mes −CH × 2), 1.52 (s, 6H,
3
3
CD Cl ) δ 48.3 ppm; HRMS (EI) calcd for C H BNS [M] ,
2
2
30 36
1
3
benzothiazoyl −CH × 2) ppm; C NMR (100 MHz,
453.2667; found, 453.2659. Anal. Calcd for C H BNS: C,
3
30 36
CD Cl ) δ 146.0 (Ar), 141.7 (Mes), 140.2 (Mes), 138.1
79.46; H, 8.00; N, 3.09; S, 7.07. Found: C, 79.53; H, 8.17; N,
3.09; S, 7.13.
2
2
(
1
(
2
4
Mes), 137.8 (Mes), 133.0 (Ar), 128.6 (Mes), 128.4 (Mes),
24.7 (Ar), 123.9 (Ar), 122.6 (Ar), 121.2 (Ar), 81.8
benzothiazoyl C2), 29.6 (Me × 2), 23.6 (Mes), 22.2 (Mes),
1.2 (Mes), 21.1 (Mes) ppm; B NMR (128 MHz, CD Cl ) δ
7.8 ppm; HRMS (EI) calcd for C H BNS [M] , 413.2354;
Computational Modeling. All calculations were per-
20
formed using the Gaussian 09 suite of programs on the
High Performance Computing Virtual Laboratory (HPCVL) at
Queen’s University. Initial input coordinates for 1 were taken
from its crystal structure data while 2− 4 were generated using
1
1
2 2
+
2
7
32
found, 413.2348. Anal. Calcd for C H BNS: C, 78.44; H,
2
7
32
G
dx.doi.org/10.1021/om500757r | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
the optimized geometry of 1 as the starting point. Ground state
Broomsgrove, A. E. J.; Aldridge, S.; Gabbaï, F. P. Chem. Rev. 2010,
1
(
5
(
10, 3958−3984. (h) Jak
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1, 7994−7998.
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21
22
̈
le, F. Angew. Chem., Int. Ed. 2012,
computed using the B3LYP level of theory and 6-311+(d,p)
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accounted for implicitly in all calculations through the use of
the integral equation formulation polarizable continuum model
8
23
(IEF-PCM) with tetrahydrofuran (THF) as the solvent.
(
Dynamic Light Scattering. Measurements were per-
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Brookhaven 9025 instrument equipped with a He−Ne laser
operating at 632.8 nm. Samples were prepared by dissolving the
desired compounds in 2-Me-THF, which had been passed
through a 0.1 μm filter several times, to give solutions of
9
6
63−964. (f) Kaim, W.; Schulz, A. Angew. Chem., Int. Ed. 1984, 23,
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15, 3326−3333.
−3
concentration ∼10 M. The data were analyzed by the
2
4
(3) (a) Nagura, K.; Saito, S.; Frohlich, R.; Glorius, F.; Yamaguchi, S.
cumulant method to give the average size of particles in
solution.
Angew. Chem., Int. Ed. 2012, 51, 7762−7766. (b) Rao, Y. .; Amarne,
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Mosey, N. J.; Wang, S. J. Am. Chem. Soc. 2012, 134, 11026−11034.
X-ray Crystal Structure Determination of 1. Colorless
block crystals suitable for single-crystal X-ray diffraction were
grown by slow evaporation of a concentrated dichloromethane
3
solution of 1. The crystal (0.20 × 0.10 × 0.10 mm ) was
mounted on a glass fiber, and diffraction data were collected on
a Bruker Apex II single-crystal X-ray diffractometer with
graphite-monochromated Mo Kα radiation, operating at 50
kV and 30 mA (T = 180 K). Data were processed using the
(f) Lu, J. S.; Ko, S. B.; Walters, N. R.; Kang, Y.; Sauriol, F.; Wang, S.
Angew. Chem., Int. Ed. 2013, 52, 4544−4548. (g) Rao, Y. L.; Amarne,
H.; Lu, J. S.; Wang, S. Dalton Trans. 2013, 42, 638−644. (h) Rao, Y.
L.; Amarne, H.; Chen, L. D.; Brown, M. L.; Mosey, N. J.; Wang, S. J.
Am. Chem. Soc. 2013, 135, 3407−3410. (i) Rao, Y. L.; Horl, C.;
Braunschweig, H.; Wang, S. Angew. Chem., Int. Ed. 2014, 53, 9086−
9089.
25
Bruker SHELXTL software package (version 6.10) and
corrected for absorption effects. The crystal of 1 belongs to the
monoclinic space group P2 /n. All non-hydrogen atoms were
1
refined anisotropically. The crystal data of 1 have been
deposited at the Cambridge Crystallographic Data Center
(4) (a) Shuto, A.; Kushida, T.; Fukushima, T.; Kaji, H.; Yamaguchi, S.
Org. Lett. 2013, 15, 6234−6237. (b) Li, D.; Zhang, H.; Wang, Y. Chem.
Soc. Rev. 2013, 42, 8416−8433. (c) Hellstrom, S. L.; Ugolotti, J.;
Britovsek, G. J. P.; Jones, T. S.; White, A. J. P. New J. Chem. 2008, 32,
(CCDC No. 1015484). Complete crystal structure data can be
found in the SI.
1
8
379−1387. (d) Qin, Y.; Kiburu, I.; Shah, S.; Jakle, F. Org. Lett. 2006,
ASSOCIATED CONTENT
* Supporting Information
CIF files, figures and tables giving NMR spectra of all
compounds, crystal structure data for 1, TD-DFT calculation
data, and all additional UV−vis/fluorescence data of 1−4. This
material is available free of charge via the Internet at http://
pubs.acs.org.
■
, 5227−5230. (e) Qin, Y.; Pagba, C.; Piotrowiak, P.; Jakle, F. J. Am.
S
Chem. Soc. 2004, 126, 7015−7018. (f) Zhang, Z.; Huo, C.; Zhang, J.;
Zhang, P.; Tian, W.; Wang, Y. Chem. Commun. 2006, 281−283.
(5) (a) Zhou, Z.; Xiao, S. Z.; Xu, J.; Liu, Z. Q.; Shi, M.; Luang, C. H.
Org. Lett. 2006, 8, 3911−3914. (b) Lemieux, V.; Spantulescu, M. D.;
Bladridge, K. K.; Branda, N. R. Angew. Chem., Int. Ed. 2008, 47, 5034−
5
3
(
037. (c) Kano, N.; Yoshino, J.; Kawashima, T. Org. Lett. 2005, 7,
909−3911.
6) For recent examples of dual emissive organoboron compounds
AUTHOR INFORMATION
Corresponding Author
E-mail: wangs@chem.queensu.ca.
Notes
■
*
that are caused by intramolecular electronic transitions, see:
a) Samonina-Kosicka, J.; DeRosa, C. A.; Morris, W. A.; Fan, Z.;
(
Fraser, C. L. Macromolecules 2014, 47, 3736−3746. (b) Swamy P, C.
A.; Mukherjee, S.; Thilagar, P. Inorg. Chem. 2014, 53, 4813−4823.
(c) Sun, X.; Zhang, X.; Li, X.; Liu, X.; Zhang, G. J. Mater. Chem. 2012,
The authors declare no competing financial interest.
2
2, 17332−17339. (d) Xu, S.; Evans, R. E.; Liu, T.; Zhang, G.; Demas,
J. N.; Trindle, C. O.; Fraser, C. L. Inorg. Chem. 2013, 52, 3597−3610.
e) Suzuki, N.; Fukazawa, A.; Nagura, K.; Saito, S.; Kitoh-Nishioka, H.;
Yokogawa, D.; Irle, S.; Yamaguchi, S. Angew. Chem., Int. Ed. 2014, 53,
ACKNOWLEDGMENTS
■
(
We thank the Natural Sciences and Engineering Research
Council for financial support, the High Performance Comput-
ing Virtual Laboratory at Queen’s for computational facilities,
and Jian Wang for performing the DLS measurements.
8
2
231−8235. (f) Wong, H. L.; Wong, W. T.; Yam, V. W. W. Org. Lett.
012, 14, 1862−1865. (g) Li, Y.; Kang, Y.; Lu, J. S.; Wyman, I.; Ko, S.
B.; Wang, S. Organometallics 2014, 33, 964−973.
(
7) (a) Gota, C.; Okabe, K.; Funatsu, T.; Harada, Y.; Uchiyama, S. J.
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dx.doi.org/10.1021/om500757r | Organometallics XXXX, XXX, XXX−XXX