BN-Heterocycles Bearing Two BN Units: Influence of the Linker and the Location of BN Units on Electronic Properties and Photoreactivity

Article abstract of DOI:10.1021/acs.organomet.7b00261

Four diboron BN-heterocycles bearing two BN units have been synthesized via double lithiation/borylation with the aim to examine their thermal and photoelimination properties. In two of the BN-heterocycles, (BN)₂-1 and (BN)₂-4, the BN units share the central linker unit, while the other two BN-heterocycles (BN)₂-2 and (BN)₂-3 are isomers with a benzene ring in the middle of the backbone separating the two chelate units. Only compound (BN)₂-3 can thermally and photochemically undergo complete double elimination producing green fluorescent (BN)₂-3a due to the highly crowded boron centers. (BN)₂-2 can partially convert (～56%) to (BN)₂-2a under 350 nm UV irradiation. (BN)₂-1 and (BN)₂-4 are either inactive or unstable toward photolysis. The molecular structures and electronic properties of these (BN)₂-heterocycles have been investigated experimentally as well as computationally using TD-DFT to further elucidate the origin of differences in optical and electronic properties.

Full text of DOI:10.1021/acs.organomet.7b00261

Article
pubs.acs.org/Organometallics
BN-Heterocycles Bearing Two BN Units: Influence of the Linker and
the Location of BN Units on Electronic Properties and Photoreactivity
Deng-Tao Yang,† Yonggang Shi,‡ Tai Peng,‡ and Suning Wang*,†,‡
†Department of Chemistry, Queen’s University, Kingston, Ontario K7L 3N6, Canada
‡Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of Chemistry and Chemical Engineering,
Beijing Institute of Technology, Beijing 100081, PR China
S
* Supporting Information
ABSTRACT: Four diboron BN-heterocycles bearing two BN
units have been synthesized via double lithiation/borylation
with the aim to examine their thermal and photoelimination
properties. In two of the BN-heterocycles, (BN)2-1 and
(BN)2-4, the BN units share the central linker unit, while the
other two BN-heterocycles (BN)2-2 and (BN)2-3 are isomers
with a benzene ring in the middle of the backbone separating
the two chelate units. Only compound (BN)2-3 can thermally
and photochemically undergo complete double elimination
producing green fluorescent (BN)2-3a due to the highly
crowded boron centers. (BN)2-2 can partially convert (∼56%)
to (BN)2-2a under 350 nm UV irradiation. (BN)2-1 and (BN)2-4 are either inactive or unstable toward photolysis. The
molecular structures and electronic properties of these (BN)2-heterocycles have been investigated experimentally as well as
computationally using TD-DFT to further elucidate the origin of differences in optical and electronic properties.
Scheme 1. Examples of BN-Heterocycles Investigated for
Elimination Reactions Stimulated by Heat and Light/
Excitons
INTRODUCTION
■
The incorporation of BN units into arenes has attracted
growing attention1 due to the unique optical and electronic
properties of the resulting BN-arenes which have a number of
potential applications such as optoelectronic materials2 and
sensors.3 Particularly, replacing a C−C unit by a BN unit in
conjugated systems can lead to a lower LUMO energy and
improved electron affinity, which enhances election trans-
port.4,5 Our group recently found that nonconjugated BN-
heterocycles could undergo thermal and photoeliminations to
generate highly fluorescent azaborines/BN-arenes.6 We also
reported double elimination of (BN)2-pyrene driven by
excitons (EDE) generated electrically within electroluminescent
(EL) devices, forming rare yellow fluorescent (BN)2-pyrenes
that contain two BN units7 (Scheme 1). In addition, the BN-
heterocyclic precursor compounds are important and interest-
ing in their own right because a number of closely related
compounds have been demonstrated to be good electron
acceptors in devices such as organic photovoltaics (OPV).5
Therefore, we have been very interested in further exploring the
chemistry and applications of this class of compounds. To
further our investigation on photoresponsive boron-containing
compounds, we extended the study to new BN-heterocycles
(BN)2-1, (BN)2-2, (BN)2-3, and (BN)2-4 (Scheme 1) that
contain two BN units and investigated their thermal and
phototransformations. The aim of this work is to determine the
influence of the location and the number of BN units within the
heterocyclic skeleton on the electronic structure and the
photophysical/photochemical properties of this class of
compounds.
Aside from different locations of the BN units, the other key
difference of these four compounds is the linking unit: In
(BN)2-1 and (BN)2-4, the boron units are fused with the linker,
Special Issue: Tailoring the Optoelectronic Properties of Organo-
metallic Compounds with Main Group Elements
Received: April 7, 2017
© XXXX American Chemical Society
A
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Article
while in (BN)2-2 and (BN)2-3 they are separated by a phenyl
ring. In addition, TD-DFT calculation data predicts that the
optical energy gap of the double elimination products follows
the order of (BN)2-1a (1.74 eV) < (BN)2-4a (2.69 eV) <
(BN)2-3a (2.78 eV) and (BN)2-2a (2.82 eV) ((BN)2-1a
represents the expected double elimination product of (BN)2-1,
and so forth), making it interesting to examine and compare
this group of compounds if it were possible to produce them via
double elimination. However, unlike the previously reported
monoboron and diboron compounds shown in Scheme 1 that
all undergo photoelimination, only (BN)2-2 and (BN)2-3
among the new diboron compounds were found to undergo
photoelimination. The details are presented herein.
RESULTS AND DISCUSSION
■
Syntheses and Structures. The ligands N2-1, N2-2, and
N2-3 were synthesized by double Suzuki coupling of diboronic
acid or boronic pinacol ester and corresponding bromo-
precursors, while ligand N2-4 was prepared by the
condensation reaction of ethanebis(thioamide) with 2-methyl-
benzaldehyde (see the Experimental Section and Supporting
Information). Compounds (BN)2-1, (BN)2-2, and (BN)2-3
were synthesized in good yields using the procedure shown in
Scheme 2 which involves double lithiation using tBuLi, followed
Figure 1. Crystal structure of (BN)2-2: top view and side view (the 5
linked aryl rings are shown in red) with 35% ellipsoid. Important bond
lengths (Å): B(1)−N(1) 1.646(6), B(1)−C(1) 1.657(6), B(1)−CMes
1.655(6)/1.666(7).
Scheme 2. Synthetic Procedures
Figure 2. Crystal structure of (BN)2-3: top view and side view (the 5
linked aryl rings are shown in red) with 35% ellipsoid. Important bond
lengths (Å): B(1)−N(1) 1.683(3), B(1)−C(1) 1.641(3), B(1)−CMes
1.654(3)/1.655(3).
by the addition of 3 equiv of Mes2BF reagent.5 The double
t
lithiation/borylation only succeeds when BuLi was used as
n
lithiation reagent in diethyl ether solution rather than BuLi/
TEMDA in THF solution. The n-heptyl group was introduced
to improve the solubility of the ligands as well as the final
(BN)2-heterocyles. (BN)2-4 can only be obtained in low yield
from the reaction of N2-4 with 2.5 equiv of LDA at −78 °C,
followed by the addition of 2.5 equiv of Mes2BF. All (BN)2-
Figure 3. Crystal structure of (BN)2-4: top view and side view (the 5
linked aryl rings are shown in red) with 35% ellipsoid. Important bond
lengths (Å): B(1)−N(1) 1.653(2), B(1)−C(1) 1.645(2), B(1)−CMes
1.651(2)/1.656(2).
1
heterocyles were fully identified and characterized by H, 13C,
11B NMR, and HRMS. Furthermore, single crystals suitable for
X-ray diffraction analysis were obtained for (BN)2-2, (BN)2-3,
and (BN)2-4.
The crystal structures of (BN)2-2, (BN)2-3, and (BN)2-4 are
shown in Figures 1−3, respectively. All three compounds
possess a crytallographically imposed inversion center.
Compounds (BN)2-2 and (BN)2-3 are isomers. For (BN)2-2,
the pyridyl rings are the outermost ring, while for (BN)2-3 the
benzyl rings are the outermost. As a result of the location
difference of the pyridyl, benzyl ring, and the heptyl
substituents, (BN)2-3 is sterically much more congested than
(BN)2-2. As shown by the side view in Figures 1 and 2, the five
aryl rings in the backbone of (BN)2-2 are essentially coplanar
with the largest torsion angle between these rings being 20°. In
contrast, the five aryl rings in (BN)2-3 are significantly out of
coplanarity with the torsion angle between the central benzene
ring and the pyridyl ring being 44°. In addition, the B−N bond
in (BN)2-3 (1.683(3) Å) is much longer than that in (BN)2-2
(1.646(6) Å), again a consequence of the greater steric
congestion in (BN)2-3. This structural difference has a
significant impact on the electronic properties of these two
B
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isomeric molecules, which will be discussed in next section. The
aryl rings of the backbone in (BN)2-4 have a good coplanarity
with the torsion angle between the benzene ring and the fused
bis-thiazole ring being 18°.
Electronic Properties. All four (BN)2-heterocycles possess
strong absorption bands between 300 and 480 nm as shown in
Figure 4 and Table 1. Compound (BN)2-1 has a low energy
−1.68 V, respectively (relative to FcH/FcH+, Table 1 and
Supporting Information). The electrochemical data indicate
that compound (BN)2-4 is likely a good electron acceptor for
organic devices such as photovoltaics due to its deep LUMO
(−3.12 eV), which will be examined in a future study. To
understand UV/vis spectral difference and electrochemical data,
TD-DFT calculations were performed at the B3LYP/6-31g(d)
level of theory on the optimized geometries of all four BN-
heterocycles.8 In addition, we also performed DFT calculations
for N2-1−N2-3 in order to compare the change of HOMO/
LUMO energy levels between ligands N2-1−N2-3 and
corresponding (BN)2-1−(BN)2-3. After boron chelation, the
HOMO levels of these three BN-heterocycles are raised by
about 0.57 eV, while their LUMO levels are stabilized by up to
1.09 eV, supporting the idea that embedding the boron chelate
units into heterocycles can enhance the electron affinity of the
molecule and reduce the HOMO−LUMO gap, in agreement
with the general trend observed for previously reported boron-
chelate compounds.4,5 On the basis of the DFT data, the low
energy absorption band of (BN)2-1−(BN)2-4 can be assigned
to a mesityl → backbone charge transfer (CT) transition. The
calculated absorption spectral patterns and the trend of the first
symmetry allowed vertical excitation energy for (BN)2-1−
(BN)2-4 match approximately with the experimental data as
shown in Table 1 (and the Supporting Information). The main
deviation is the HOMO and LUMO energies of (BN)2-4 that
are much higher than the experimental values. Since the
reduction potential of (BN)2-4 was determined in DMF due to
its low solubility, the inconsistency between the experimentally
observed redox trend and the calculated one may be due to
solvent effects which were not accounted for in TD-DFT
calculations. For (BN)2-2 and (BN)2-3, the TD-DFT calculated
values change significantly with the dihedral angles between the
central benzene ring and its neighboring rings in the backbone.
The data shown in Table 1 for these two molecules are based
on crystal structural parameters which were further optimized
by DFT. In the optimized structure of (BN)2-2, the central
benzene ring has a much larger dihedral angle with its
neighboring rings than that observed in the crystal structure,
leading to its calculated optical energy gap being similar to that
of (BN)2-2.
Figure 4. UV/vis spectra of (BN)2-1, (BN)2-2, (BN)2-3, and (BN)2-4
in THF (1.0 × 10−5 M).
absorption band at ∼394 nm. As structural isomers, the
absorption bands of (BN)2-3 are hypsochromically shifted by
∼20 nm compared to (BN)2-2, which agrees with the greater
coplanarity, hence better extended π-conjugation of the
backbone in (BN)2-2, as revealed by the crystal structures.
Furthermore, (BN)2-2 has a much bigger extinction coefficient
at the low energy absorption band (ε = 3.98 × 104 M−1 cm−1, λ
= 359 nm) than that of (BN)2-3 (ε = 2.21 × 104 M−1 cm−1 at
340 nm). These all support that the steric congestion has a
great impact on the electronic properties of isomeric structures.
Among the four BN-heterocycles, (BN)2-4 has the most
bathochromically shifted absorption band with the absorption
edge at ∼475 nm, followed by (BN)2-1 which has the
absorption edge at ∼450 nm. The sharing of the same central
chelate unit by the two BMes2 units in (BN)2-1 and (BN)2-4
clearly is more effective in lowering the energy of the
absorption edge than the insertion of the benzene linker to
extend the π-conjugation in (BN)2-1 and (BN)2-4, which can
be attributed to the effective conjugation and better coplanarity
of the backbone in the former. Compounds (BN)2-1−(BN)2-4
are not fluorescent.
Thermal and Photoreactivity. To establish if (BN)2-
heterocycles (BN)2-1, (BN)2-2, (BN)2-3, and (BN)2-4 can act
as precursors for the generation of double BN-doped fully π-
conjugated systems, their thermal and photoreactivity were
examined and summarized in Scheme 3. Upon irradiation with
350 nm UV light, the C6D6 solution of all four compounds
gradually become fluorescent and displayed distinct fluorescent
colors (Scheme 3). However, when these photoreactions were
The electrochemical data revealed that the four BN-
heterocycles (BN)2-1−(BN)2-4 display the characteristic one-
electron reduction peak at Ered = −2.11, −2.18, −2.33, and
1
monitored by H and 11B NMR spectroscopy, only (BN)2-3
eliminates two mesitylene molecules cleanly and produces a
Table 1. Electrochemical and Photophysical Properties of (BN)2-Heterocycles
a
b
c
d
e
compd
Ered (V)
λabs (ε, 104 M−1 cm−1) (nm)
optical gap Exp/DFT (nm)
HOMO Exp/DFT (eV)
LUMO Exp/DFT (eV)
(BN)2-1
(BN)2-2
(BN)2-3
(BN)2-4
−2.11
−2.18
−2.33
−1.68
394 (0.70)
359 (3.98)
340 (2.21)
402 (2.03)
450/454
420/445
400/445
475/470
−5.48/−5.19
−5.64/−5.26
−5.69/−5.33
−5.73/−5.18
−2.69/−2.15
−2.62/−1.94
−2.47/−1.99
−3.12/−2.59
a
b
Relative to FcH/FcH+ recorded in THF for (BN)2-1−(BN)2-3 and in DMF for (BN)2-4 with [Bu4N][PF6] as the electrolyte. 10−5 M in THF at
c
d
298 K. The experimental value is from the absorption edge, and the DFT value is from the first symmetry allowed vertical excitation. Calculated
using to absorption edge and reduction potential. Estimated from the reduction potential.
e
C
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Scheme 3. Thermal and Photoreactivity of (BN)2-1, (BN)2-2,
(BN)2-3, and (BN)2-4
Figure 5. Absorption (red) and fluorescence (black) spectra of (BN)2-
3a in THF. Inset: photograph showing the emission color of (BN)2-3a
in THF.
same side as shown in Figure 6, based on DFT structural
optimization. The central benzene ring has a torsion angle of
new diazaborine, (BN)2-3a in ∼90% yield. (BN)2-2 only
undergoes partial conversion (∼56%) to (BN)2-2a along with
the formation of an unidentified product under 350 nm.
Irradiation of (BN)2-1 generated unidentifiable species with
orange color and bright yellow emission. (BN)2-4’s behavior
appears to be similar to that of (BN)2-1. Although 11B NMR
data indicated that (BN)2-4 may partially form the double-
eliminated product (BN)2-4a, upon irradiation at either 300 or
365 nm, the chemical shifts of the aromatic protons are mostly
absent or very broad in the 1H NMR spectrum with
unidentifiable peaks in the aliphatic region (see Supporting
Information). Furthermore, both (BN)2-1 and (BN)2-4 only
display detectable changes by NMR after a long exposure to
UV light. Perhaps the rigid structures imposed by the two
boron units sharing the central linker unit in (BN)2-1 and
(BN)2-4 makes the elimination reaction difficult, although the
details are not understood yet. The different photoreactivities
of (BN)2-2 and (BN)2-3 are probably caused by the different
locations of the two BN units and heptyl groups, which led to a
highly congested structure for (BN)2-3, compared to that of
(BN)2-2. The steric congestion likely makes the photo-
elimination of (BN)2-3 more efficient and is responsible for
its relatively clean double elimination.
Besides phototransformation, previously we have also shown
that related BN-chelate compounds can display either retro-1,1-
hydroboration (deborylation) or mesitylene elimination upon
heating.6b To determine if the new BN-heterocycles display
similar reactivities, their thermal reaction was examined. (BN)2-
1, (BN)2-2, and (BN)2-4 produce unidentifiable products in
mineral oil at 230 °C. In contrast, compound (BN)2-3
undergoes a clean thermal elimination upon heating at 230
°C, producing (BN)2-3a nearly quantitatively (see the
Supporting Information).
Figure 6. Left: DFT-optimized structure of (BN)2-3a. Right: HOMO
and LUMO diagrams of (BN)2-3a.
∼58° with the two neighboring BN-phenanthrene rings. This
confirms the poor conjugation between these three units in
(BN)2-3a and explains the lack of the influence of the central
benzene linker on the electronic properties of the bis-BN-
phenanthrene molecule (BN)2-3a.
TD-DFT computation studies (Supporting Information)
confirmed that the S0 → S1 transition of (BN)2-3a involves
primarily the HOMO and LUMO orbitals (83%), localized
predominantly on the BN-phenanthrene units with a very high
oscillator strength (0.168, Figure 6). This transition is believed
to be responsible for the green fluorescence of (BN)2-3a.
Compound (BN)2-3a displays two pseudoreversible reduction
waves in the CV diagram (in THF) at −2.22 and −2.47 V,
respectively, and an irreversible oxidation at 0.64 V (vs FcH/
FcH+, Supporting Information), which correspond to approx-
imately LUMO, LUMO+1, and HOMO levels of −2.58, −2.33,
and −5.44 eV, respectively.
(BN)2-3a was fully characterized by NMR and HRMS. The
11B chemical shift of (BN)2-3a (36 ppm) is consistent with that
of azaborines.6 (BN)2-3a has a low-energy absorption band at
∼476 nm with well-resolved vibrational features and bright
green-yellow fluorescent emission (λem = 510 nm, ΦFL = 0.14;
Figure 5), which is similar to that of the corresponding mono-
BN-phenanthrene reported previously.6 This suggests that
increasing π conjugation via a phenyl linker does not
significantly change the absorption and fluorescent properties
of BN-phenanthrene. The most stable form of (BN)2-3a has an
approximate syn-conformation with the two BMes units on the
CONCLUSIONS
■
Four new diboron BN-heterocycles with extended conjugated
backbones have been synthesized. The impact of the linker unit
and the location of the boron chelate unit on the electronic
properties and photoreactivity of this class of compounds have
D
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Synthesis of Compound N2-2. A mixture of THF (30 mL) and
water (10 mL) was stirred and purged by nitrogen for 1 h. Boronic
acid (204 mg, 1.23 mmol), compound N2-2 pre (870 mg, 2.5 mmol),
Pd(dppf)Cl2 (18 mg, 0.024 mmol), and Na2CO3 (782 mg, 7.37 mmol)
were added to the mixed solvents. The mixture was stirred and reflux
for 24 h. The organic solvents were removed under vacuum. The water
layer was separated and extracted with CH2Cl2 (3 × 40 mL). The
combined organic layers were washed by brine and dried over
magnesium sulfate, and the solvents were evaporated under reduced
pressure. Purification of the crude product by column chromatography
afforded the product N2-2 as a colorless oil in 78% yield (590 mg, 0.97
mmol). 1H NMR (400 MHz, CDCl3) δ 8.65 (d, J = 5.1 Hz, 2H), 7.79
(d, J = 3.8 Hz, 4H), 7.68−7.57 (m, 6H), 7.29 (s, 2H), 7.09 (dd, J = 5.1,
1.5 Hz, 2H), 2.52 (s, 6H), 1.79−1.57 (m, 8H), 1.30 (d, J = 15.2 Hz,
16H), 0.91 (t, J = 6.7 Hz, 6H). 13C NMR (101 MHz, CDCl3) δ
159.40, 155.57, 149.24, 140.48, 139.91, 136.21, 130.29, 129.42, 127.53,
127.40, 124.57, 13.77, 121.38, 45.79, 36.34, 31.77, 29.37, 27.51, 22.66,
20.63, 14.12. HR-ESIMS (m/z): [M + 1]+ calcd for C44H53N2:
609.42033; found: 609.41839.
Synthesis of Compound N2-3. Compound N2-3 was synthe-
sized using the same procedure of synthesis as that for compound N2-
2 as a colorless oil in 60% yield (600 mg, 0.99 mmol). 1H NMR (499
MHz, CDCl3) δ 8.61 (s, 2H), 7.52 (m, 6H), 7.40 (s, 2H), 7.34 (p, J =
3.7 Hz, 6H), 2.79−2.70 (m, 4H), 2.48 (s, 6H), 1.61 (td, J = 7.9, 3.9
Hz, 4H), 1.32−1.23 (m, 16H), 0.88 (t, J = 6.8 Hz, 6H). 13C NMR
(125 MHz, CDCl3) δ 158.88, 149.67, 149.24, 140.43, 137.33, 135.84,
135.08, 130.78, 129.69, 129.53, 128.23, 125.90, 124.34, 32.58, 31.68,
30.36, 29.34, 28.96, 22.61, 20.45, 14.09. HR-ESIMS (m/z): [M + 1]+
calcd for C44H53N2: 609.42033; found: 609.41840.
been examined. The experimental and computational data
suggest that the location of BN units, the steric congestion, and
the linker unit within the π-conjugated backbone can greatly
affect the electronic structure of these molecules as well as their
photophysical/photochemical properties. Diboron BN-hetero-
cycles that share the central linker unit appear to be either
unstable or have a poor reactivity toward light while those that
do not share the central linker can undergo double photo-
elimination with the sterically congested one being more
effective.
EXPERIMENTAL SECTION
■
General Information. 2-Bromo-4-heptylpyridine and 2,5-dibro-
mo-4-heptylpyridine were prepared according to a modified reported
procedure.9 Compounds N2-2 pre and N2-3 pre were prepared using
Pd(PPh3)4 (5 mol %) and Na2CO3 (3 equiv) at reflux conditions for
Suzuki-coupling reactions (See Supporting Information). All other
chemicals were obtained from commercial sources and were used
without further purification. THF and diethyl ether were distilled over
Na under a nitrogen atmosphere. All the synthetic procedures were
1
performed under N2 using the standard Schlenk techniques. H and
13C NMR spectra were recorded on a 400 or 500 MHz Bruker
spectrometer. High-resolution mass spectra (HRMS) were obtained
from an Applied Biosystems Qstar XL spectrometer. UV−visible
spectra were recorded using a Varian Cary 50 UV/vis spectropho-
tometer. Cyclic voltammetry experiments were conducted on a BAS
CV-50W analyzer with a scan rate of 100 mVs−1. The electrochemical
cell was a standard three-compartment cell composed of a glassy
carbon working electrode, a Pt auxiliary electrode, and a Pt wire
reference electrode. All measurements were performed using 0.1 M
[Bu4N][PF6] in THF as the supporting electrolyte, except for those of
compound (BN)2-4, which were recorded using DMF as the solvent.
The potentials are reported relative to the ferrocene/ferrocenium
Synthesis of Compound N2-4. A solution of ethanebis-
(thioamide) (0.454 g, 3.7 mmol) and 2-methylbenzaldehyde (1.0 g,
8.3 mmol) in DMF (10 mL) was refluxed for 4 h. After being cooled
to room temperature, the solvents were then removed under reduced
pressure. The crude product was purified by silica gel chromatography
(petroleum ether/CH2Cl2 = 1/1). Compound N2-4 was obtained as a
1
pale yellow solid in 64% yield (0.768 g, 2.38 mmol). H NMR (400
1
couple. H NMR scale photoelimination reactions were carried out
MHz, CDCl3) δ 7.76 (d, J = 7.5 Hz, 2H), 7.43−7.28 (m, 6H), 2.69 (s,
6H). 13C NMR (101 MHz, CDCl3) δ 168.98, 151.03, 137.09, 133.37,
131.87, 130.16, 126.41, 21.70. HR-ESIMS (m/z): [M + 1]+ calcd for
C18H15N2S2, 323.06766; found 323.06415.
under N2 using C6D6 as the solvent, quartz J. Young NMR tubes as the
reaction vessels, and a Rayonet Photochemical Reactor as the light
source. The mineral oil used in the thermal reaction was purchased
from Sigma-Aldrich Corp. (76235, 500 mL), heated over sodium
metal, and stored under nitrogen prior to use. The purity of the new
compounds has been established by NMR after column chromatog-
raphy on silica gel or recrystallization.
DFT Calculation Details. All calculations were performed using
the Gaussian 09 suite of programs on the High-Performance
Computing Virtual Laboratory (HPCVL) at Queen’s University.
Initial input coordinates were taken from the corresponding crystal
structure data where applicable, while all others were generated in
Gaussian 09.10 DFT and TD-DFT calculations were performed using
the B3LYP/6-31G(d) level of theory.8
t
Synthesis of Compound (BN)2-1. BuLi (4.0 mL, 1.7 M in
hexane, 6.78 mmol) was added to a solution of compound N2-1 (700
mg, 1.54 mmol) in Et2O (25 mL) at −78 °C. After stirring at this
temperature for 1 h, the precooled boron reagent Mes2BF (1.23g, 4.62
mmol) in THF was added via cannula transfer with vigorous stirring at
−78 °C. The mixture was stirred for another hour at −78 °C, then
allowed to warm slowly to room temperature and stirred overnight.
The solution was extracted with CH2Cl2 and water. The organic phase
was dried over magnesium sulfate and filtered. The filtrate was purified
by column chromatography on silica gel to afford product (BN)2-1
1
(514 mg, 0.54 mmol, 35% yield) as a yellow solid. H NMR (400
Synthesis of Compound N2-1. This compound was synthesized
using a modified Suzuki coupling procedure. A mixture of THF (60
mL) and water (20 mL) was stirred and purged by nitrogen for 1 h.
Diboron esters (2.1 g, 5.9 mmol), 2-bromo-4-heptylpyridine (3 g, 11.8
mmol), Pd(PPh3)4 (410 mg, 0.354 mmol), and Na2CO3 (3.7 g, 35.4
mmol) were added to the mixed solvents. The mixture was stirred and
refluxed for 24 h. The organic solvents were removed under vacuum.
The water layer was separated and extracted with CH2Cl2 (3 × 50
mL). The combined organic layers were washed by brine and dried
over magnesium sulfate, and the solvents were evaporated under
reduced pressure. Purification of the crude product by column
chromatography afforded the product N2-1 as a white solid in 45%
yield (1.2 g). 1H NMR (400 MHz, CDCl3) δ 8.60 (d, J = 5.0 Hz, 2H),
7.36 (s, 2H), 7.26 (d, J = 1.6 Hz, 2H), 7.09 (dd, J = 5.1, 1.7 Hz, 2H),
2.68 (t, J = 7.7 Hz, 4H), 2.39 (s, 6H), 1.76−1.62 (m, 4H), 1.42−1.27
(m, 16H), 0.95−0.85 (m, 6H). 13C NMR (101 MHz, CDCl3) δ
159.57, 151.74, 149.10, 140.35, 132.99, 131.98, 124.35, 121.89, 35.38,
31.75, 30.40, 29.19, 29.08, 22.63, 19.78, 14.08. HR-ESIMS (m/z): [M
+ 1]+ calcd for C32H45N2: 457.35773; found: 457.35591.
MHz, CD2Cl2) δ 8.38 (d, J = 6.2 Hz, 2H, py), 7.59 (s, 2H, py), 7.16−
7.07 (m, 2H, py), 7.03 (s, 2H, benzene ring), 6.59 (s, 8H, Mes), 2.79
(t, J = 7.8 Hz, 4H, CH2), 2.69 (s, 4H, B-CH2), 2.16 (s, 12H, Me), 1.84
(m, 24H, Mes), 1.74 (br, 4H, CH2), 1.42−1.31 (m, 16H, CH2), 0.93
(t, J = 6.6 Hz, 6H, Me in heptyl). 13C NMR (101 MHz, CD2Cl2) δ
157.98, 154.95, 147.28, 142.89, 141.59, 133.18, 132.37, 129.55, 126.00,
123.45, 123.20, 35.32, 31.64, 29.77, 29.66, 29.05, 28.95, 24.56, 22.58,
20.28, 13.81. 11B NMR (128 MHz, CD2Cl2) δ 2.6. HR-ESIMS (m/z):
[M + 1]+ calcd for C68H87B2N2: 953.70499; found: 953.70893.
Synthesis of Compound (BN)2-2. Compound (BN)2-2 was
synthesized using the same procedure of synthesis as that for
compound (BN)2-1. The mixture was recrystallized by ethyl acetate
and hexanes to afford product (BN)2-2 (632 mg, 0.57 mmol, 60%
yield) as a yellow solid, which was confirmed by X-ray diffraction
analysis. 1H NMR (400 MHz, CD2Cl2) δ 8.44 (d, J = 6.2 Hz, 2H, py),
7.83 (d, J = 2.0 Hz, 2H, py), 7.69 (d, J = 8.1 Hz, 2H, ph), 7.56 (s, 4H,
benzene linker), 7.36 (dd, J = 8.0, 1.9 Hz, 2H, ph), 7.17 (dd, J = 6.2,
1.9 Hz, 2H, py), 7.09 (d, J = 1.9 Hz, 2H, ph), 6.60 (s, 8H, Mes), 2.83
(br, 8H, CH2 and B-CH2), 2.15 (s, 12H, Mes), 1.90 (s, 24H, Mes),
E
DOI: 10.1021/acs.organomet.7b00261
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Organometallics
Article
1.78 (br, 4H, CH2), 1.44−1.32 (m, 16H, CH2), 0.94 (t, J = 6.7 Hz, 6H,
Me in heptyl). 13C NMR (101 MHz, CD2Cl2) δ 158.19, 155.25,
148.36, 147.23, 142.27, 141.63, 139.66, 133.38, 129.78, 129.57, 127.18,
126.72, 126.48, 123.27, 123.18, 122.74, 35.42, 31.68, 30.41, 29.88,
29.04, 28.98, 24.62, 22.59, 20.27, 13.84. 11B NMR (128 MHz, CD2Cl2)
δ 1.6. HR-ESIMS (m/z): [M + 1]+ calcd for C80H95B2N2: 1105.76759;
found: 1105.77307.
Synthesis of Compound (BN)2-3. Compound (BN)2-3 was
synthesized using the same procedure of synthesis of compound
(BN)2-1 to provide product (BN)2-3 (497 mg, 0.45 mmol, 50% yield)
as a yellow solid, which was confirmed by X-ray diffraction analysis. 1H
NMR (400 MHz, CD2Cl2) δ 8.43 (s, 2H, py), 7.91 (s, 2H, py), 7.67
(br, 2H, ph), 7.26 (br, 4H, ph), 7.13 (br, 4H, ph), 6.91 (br, 2H, ph),
6.57 (s, 8H, Mes), 2.83 (br, 8H, CH2 and B-CH2), 2.15 (s, 12H, Mes),
1.90 (s, 24H, Mes), 1.71−1.59 (m, 4H, CH2), 1.29 (br, 16H, CH2),
0.90 (t, J = 6.5 Hz, 6H, Me in heptyl). 13C NMR (101 MHz, CD2Cl2)
δ 155.59, 154.46, 147.82, 147.64, 146.81, 141.66, 136.17, 135.95,
133.37, 130.62, 130.32, 129.63, 129.54, 128.51, 125.99, 124.24, 123.77,
32.74, 31.59, 29.91, 29.12, 28.85, 24.68, 22.53, 20.26, 13.82. 11B NMR
(128 MHz, CD2Cl2) δ 1.8. HR-ESIMS (m/z): [M + 1]+ calcd for
C80H95B2N2: 1105.76759; found: 1105.76845.
Cambridge Crystallographic Data Centre [CCDC No. 1542183
(BN)2-2, 1542182 (BN)2-3, and 1542181 (BN)2-4]. These data can
be obtained free of charge from the Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif].
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications Web site. The Supporting Information is
available free of charge on the ACS Publications website at
DOI: 10.1021/acs.organomet.7b00261.
Experimental data for the synthesis of organic substrates,
NMR spectral data, electrochemical analysis data, and
data showing the change of compounds (BN)2-2, (BN)2-
3, and (BN)2-4 upon irradiation by UV light, and
computational details (PDF)
Crystal structural data for BN-heterocycles (XYZ)
Accession Codes
Synthesis of Compound (BN)2-4. To a solution of N2-4 (0.1 g,
0.31 mmol) in anhydrous THF (10 mL) a solution of lithium
diisopropylamide in THF (1 mL, 0.78 mmol) was added dropwise in a
N2 atmosphere at −78 °C. The mixture was stirred at −78 °C for 2 h
and Mes2BF (0.195 g, 0.78 mmol) in anhydrous THF (10 mL) was
added. The mixture solution was warmed up to room temperature and
stirred for 12 h. After removal of the solvent under the reduced
pressure, the resulting crude product was purified by silica gel
chromatography (petroleum ether/CH2Cl2 = 3:1) as a yellow solid in
CCDC 1542181−1542183 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail: wangs@chem.queensu.ca.
■
1
4% yield (0.011 g, 0.013 mmol). H NMR (400 MHz, C6D6) δ 6.91
(d, J = 7.6 Hz, 2H, ph), 6.82 (s, 8H, Mes), 6.74 (d, J = 7.6 Hz, 2H,
ph), 6.62 (t, J = 7.5 Hz, 2H, ph), 6.33 (t, J = 7.5 Hz, 2H, ph), 2.96 (s,
4H, B-CH2), 2.17 (br, 36H, Me). 13C NMR (176 MHz, C6D6) δ
172.71, 147.94, 143.62, 133.74, 130.88, 126.40, 126.06, 125.13, 20.94.
11B NMR (128 MHz, C6D6): δ 2.3. HR-ESIMS (m/z): [M + 1]+ calcd
for C54H57B2N2S2, 819.41438; found 819.41334.
ORCID
Suning Wang: 0000-0003-0889-251X
Notes
The authors declare no competing financial interest.
1
Formation of (BN)2-3a via Photoelimination. H NMR scale
photoelimination reactions were carried out under N2 using C6D6 as
the solvent, quartz J. Young NMR tubes as the reaction vessels, and a
Rayonet Photochemical Reactor as the light source. Compound
(BN)2-3 (∼5 mg) was dissolved in C6D6 (0.5 mL) in a quartz J. Young
NMR tube and sealed under nitrogen. The NMR tube was then
irradiated with 350 nm UV light and the reaction progress was
ACKNOWLEDGMENTS
■
D.-T.Y. and S.W. thank the Natural Sciences and Engineering
Research Council of Canada for financial support (RGPIN
1193993-2013). Y.G.S., T.P., and S.W. thank the National
Natural Science Foundation of China for financial support
(21571017). We thank the High-Performance Computing
Virtual Laboratory (HPCVL) at Queen’s University for the
computing facility.
1
monitored by H and 11B NMR spectra until it reached completion
1
(∼90% yield by NMR). H NMR (499 MHz, C6D6) δ 8.57 (s, 2H,
py), 8.42 (d, J = 11.3 Hz, 2H, ph), 8.39 (s, 2H, py), 7.78 (d, J = 8.4 Hz,
2H, ph), 7.37 (dd, J = 8.2, 6.7 Hz, 2H, ph), 7.30 (s, 2H, CH bound to
B), 7.10 (dd, J = 8.2, 6.7 Hz, 2H, ph), 7.03 (s, 4H, Mes), 6.92 (s, 4H,
benzene linker), 2.49−2.45 (m, 4H, CH2), 2.30 (s, 12H, Me in Mes),
2.26 (s, 6H, Me in Mes), 1.36−1.07 (m, 20H, CH2), 0.90 (t, J = 7.2
Hz, 6H, Me in heptyl). 13C NMR (126 MHz, C6D6) δ 145.61, 143.01,
142.05, 140.34, 137.35, 136.79, 136.48, 135.90, 131.39, 128.96, 128.83,
128.27, 124.80, 121.10, 119.91, 118.55, 33.17, 31.73, 29.86, 29.34,
29.05, 22.80, 22.72, 21.16, 14.06. 11B NMR (160 MHz, C6D6) δ 36.4.
HR-ESIMS (m/z): [M + 1]+ calcd for C62H71B2N2: 865.57979; found:
865.57965.
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