Synthesis of Bis-Cycloborate Olefin and Butatriene Derivatives through the Reduction of Alkynyl-Bridged Diboryl Compounds

Article abstract of DOI:10.1021/acs.inorgchem.8b01555

A series of bis-cycloborate olefin and butatriene derivatives were synthesized from alkynyl-bridged diboryl compounds using a simple reduction strategy. In the reduction route of 1,2-bis[2-(dimesitylboranyl)phenyl]ethyne (1), bis-cycloborate olefin (4) to

Full text of DOI:10.1021/acs.inorgchem.8b01555

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Synthesis of Bis-Cycloborate Olefin and Butatriene Derivatives
through the Reduction of Alkynyl-Bridged Diboryl Compounds
Jinyu Zhao,†,§ Chenglong Ru,†,§ Yunfei Bai,† Xingyong Wang,*,‡ Wenhao Chen,† Xi Wang,†
Xiaobo Pan,*,† and Jincai Wu†
†State Key Laboratory of Applied Organic Chemistry, Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of
Gansu Province, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, P. R. China
‡School of Chemistry, University of Wollongong, Wollongong, NSW 2522, Australia
S
* Supporting Information
ABSTRACT: A series of bis-cycloborate olefin and butatriene derivatives were
synthesized from alkynyl-bridged diboryl compounds using a simple reduction
strategy. In the reduction route of 1,2-bis[2-(dimesitylboranyl)phenyl]ethyne
(1), bis-cycloborate olefin (4) to diborate-bridged stilbene (5) can be obtained
selectively by varying the reaction temperature. In addition, a thermal
isomerization from 4 to 5 was found as a result of the rearrangement of double
bonds. The generality of this reaction was further verified in similar reduction
reactions. In the direduction of 1,2-bis[8-(dimesitylboranyl)naphthalen-1-
yl]ethyne (2), the bis-cycloborate olefin (6) was isolated in high yield and
demonstrated no thermal isomerization. In the two-electron reduction of 1,8-
bis{[2-(dimesitylboranyl)phenyl]ethynyl}naphthalene (3), the bis-cycloborate
butatriene (7) was obtained unexpectedly because of the departure of the naphthyl group. When using Na as the reductant and
diethyl ether as the solvent, 1,2-bis[(Z)-2-(dimesitylboranyl)benzylidene]-1,2-dihydroacenaphthylene (8) was isolated after
adding 1 or 2 drops of H2O to the reduction reaction filtrates. Meanwhile, the related mechanisms for radical cyclization,
thermal isomerization, and formation of bis-cycloborate butatriene were also discussed.
demonstrated the preparation of cyclic borate structures via an
elegant two-electron reduction path. In addition to the
structural exploration of these compounds, utilizing such a
synthetic method to build new functional boron-containing
skeletons is also very important, which has been, however,
explored less.
INTRODUCTION
■
Boron-containing compounds have attracted a great deal of
attention because of their potential applications in luminescent
organic materials,1−11 anion sensors,12−15 photochemis-
try,16−18 and polyaromatic hydrocarbon chemistry.19−23
Triarylboron compounds have excellent electron accepting
properties because of the low-lying empty p orbital of the
boron atom. Recently, numerous single-electron-reduced
species, including boron-stabilized anions24,25 and radical
anions,26−32 were synthesized by the reduction method
under different conditions, and this strategy has already
become a hot research topic. However, as an important part
of boron reduction chemistry, the direduction behaviors of
diboryl-substituted compounds have been studied less.
Continuous efforts have led to the emergence of different
direduction results. Braunschweig, Marder, and our group
displayed the synthesis of three quinoidal dianions, A,33 B,34
and C35 (Scheme 1), upon two-electron reduction of 2,5-
bis(borolyl)thiophene, 2,7-bis(BMes2)pyrene, and 9,10-bis-
(dimesitylboryl)anthracene, respectively. Wang and co-workers
showed the formation of three diradical dianions D−F36,37
(Scheme 1) through direduction and discussed their structures
and spectroscopic properties. Wagner and co-workers reported
the two-electron trap process of bis(9-borafluorenyl)methane
and 9,10-diboraanthracene derivatives, which led to the
formation of a B−B σ bond (G38) and the dianionic
H39(Scheme 1). Recently, Wang et al.36 and our group40
By using this reduction strategy, we recently succeeded in
reducing 9,10-bis(dimesitylboryl)anthracene and o-dimesityl-
boryl-substituted compounds. These results,35,40 especially the
reduction of o-dimesitylboryl-substituted compounds, encour-
aged us to investigate the reduction behaviors of other similar
boron-containing systems. In this paper, we demonstrate the
reduction of alkynyl-bridged diboryl compounds and discuss
the spectroscopic character and crystal structures of their
reduced products. Meanwhile, a thermal isomerization process
from bis-cycloborate olefin 4 to diborate-bridged stilbene 5
due to the rearrangement of double bonds was observed. In
addition, the bis-cycloborate butatriene {K+·[2.2.2]-crypt-
and}2·7a2− (7) was obtained unexpectedly because of the
departure of the naphthyl group, which was attributed to the
effect of “extrusion reaction”, while compound 8 was isolated
after adding 1 or 2 drops of H2O to the filtrates when using Na
as the reductant and diethyl ether as the solvent in the two-
electron reduction of 3.
Received: June 5, 2018
© XXXX American Chemical Society
A
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Scheme 1. Structurally Characterized Diboryl Dianions A−J
Scheme 2. Synthesis of Compounds 2 and 3
environments. In addition, the structures of compounds 1−3
were further verified by X-ray diffraction (Figure 1a−c).
Interestingly, the alkyne bonds are considerably bent, with
bending angles of up to 169.9(5)° (C2−C1−C1′) in 1,
160.9(4)° (C2−C1−C1′) in 2, and 170.6(3)° (C27−C26−
C1) and 164.8(2)° (C35−C37−C38) in 3, which may be
caused by the crowded Mes groups.
RESULTS AND DISCUSSION
■
Synthesis, Structure, and Characterization of Com-
pounds 1−3. The known 1,2-bis[2-(dimesitylboranyl)-
phenyl]ethyne (1) was synthesized according to Aldridge’s
work.41 Compounds 1,2-bis[8-(dimesitylboranyl)naphthalen-
1-yl]ethyne (2) and 1,8-bis{[2-(dimesitylboranyl)phenyl]-
ethynyl}naphthalene (3) (Scheme 2) were prepared from
the reaction between the nBuLi-lithiated corresponding
halogenated aromatic compounds in tetrahydrofuran (THF)
and 2 equiv of Mes2BF at −78 °C. They were isolated as
colorless solids by column chromatography with a 10:1
mixture of CH2Cl2 and hexanes as the eluent in 69 and 64%
yields, respectively. They are stable toward moisture and air
and display blue fluorescence when irradiated under 365 nm
ultraviolet (UV) light both in solution and in the solid state.
All compounds were further analyzed by multinuclear nuclear
magnetic resonance (NMR) spectroscopy. The 11B NMR
resonances of compounds 1−3 appear at 75 (7341), 65.7, and
74.1 ppm, respectively, which are comparable to those
reported for two coordinated boron compounds [2,7-bis-
(BMes2)pyrene, 75.3 ppm,34 and 2,6-bis(BMes2)mesitylene,
74.8 ppm36], as well as our reported o-dimesitylboryl-
substituted compound (75 ppm).40 In the 13C NMR spectra
of compounds 1−3, the peaks at 93.5 ppm (in 1), 100.5 ppm
(in 2), and 97.6 and 93.2 ppm (in 3) confirm the existence of
alkyne carbons. It is worth noting that a broad peak appears at
1.53 ppm in the 1H NMR spectrum of 2, which is likely due to
hindered rotation of the Mes groups in very constricted
To study the redox properties of compounds 1−3, cyclic
n
voltammetry was performed in THF with Bu4NPF6 as the
supporting electrolyte, and they exhibited quite different
features (Figure 1d). Two distinct sets of quasi-reversible
reduction waves were observed in compounds 1 (E1/2 = −2.07
V and E1/2 = −1.80 V vs Fc/Fc+) and 3 (E1/2 = −2.42 V and
E1/2 = −2.13 V vs Fc/Fc+), while two irreversible reduction
waves were found at Epc = −2.24 V and Epc = −1.78 V versus
Fc/Fc+ in compound 2. Generally, two reversible reduction
waves could appear in the reduction system of diboryl
compounds. The first reduction wave corresponds to the
formation of a monoradical anion in which the unpaired
electron occupies a molecular orbital bearing large contribu-
tions from that of the boron p orbitals,42 while the second
reduction wave corresponds to the formation of a dianion,
which exists either as a biradical or as a σ-bonded derivative.
According to our previous studies,40 the redox behaviors
compounds 1−3 most likely indicate a significant structural
change associated with the electron-transfer process.43
Synthesis, Structure, and Characterization of Com-
plexes 4 and 5. Inspired by the electrochemical results
presented above, we investigated the chemical reduction of 1
B
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Figure 1. Thermal ellipsoid (50%) drawings of compounds 1−3. All H atoms have been omitted for the sake of clarity. Selected bond lengths
(angstroms) and bond angles (degrees): (a) in 1, C1−C1′ 1.186(6), C1−C2 1.432(4), C6−B1 1.572(5), C8−B1 1.589(5), C17−B1 1.572(5),
C1′−C1−C2 169.9(5), C6−B1−C8 114.5(3), C6−B1−C17 124.0(3), C17−B1−C8 121.4(3); (b) in 2, C1−C1′ 1.197(7), C1−C2 1.435(4),
C6−B1 1.575(5), C12−B1 1.555(6), C21−B1 1.572(6), C1′−C1−C2 160.9(4), C12−B1−C6 124.0(4), C12−B1−C21 120.2(3), C21−B1−C6
114.7(3); and (c) in 3, C1−C26 1.207(4), C37−C38 1.205(3), C6−B1 1.573(4), C8−B1 1.580(4), C17−B1 1.574(4), C44−B2 1.570(4), C45−
B2 1.581(4), C54−B2 1.579(4), C1−C2 1.430(4), C35−C37 1.441(4), C38−C39 1.443(4), C26−C1−C2 173.1(3), C27−C26−C1 170.6(3),
C39−C38−C37 176.8(2), C38−C37−C35 164.8(2), C17−B1−C6 123.3(2), C17−B1−C8 123.7(2), C45−B2−C44 117.6(2), C54−B2−C44
118.6(2), C54−B2−C45 123.7(2). (d) Cyclic voltammograms of compounds 1−3 (5 × 10−4 M) in THF, containing 0.1 M nBu4NPF6, measured
at 100 mV s−1 and room temperature.
Scheme 3. Synthesis of Compounds 4−8
in fresh distilled Et2O (Scheme 3). First, the reaction was
performed at room temperature, which leads to the formation
of a yellow solid in 73% yield. The 1H NMR spectrum shows a
complex (Figure S14) but nicely resolved signal. The enlarged
signals of aromatic hydrogens (Figure 2b) in the H NMR
spectrum can be identified as the resonances of complexes 4
1
C
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Figure 2. Enlarged signals of aromatic hydrogens of complexes 4 and 5. They were obtained from reduction of 1 at different reaction temperatures:
(a) 10 °C, (b) room temperature, and (c) 40 °C.
Figure 3. Thermal ellipsoid (50%) drawings of products 4−8: (a) mixture of 4 and 5, (b) 4, (c) 5, (d) 6, (e) 7, and (f) 8. Free molecules and all H
atoms have been omitted for the sake of clarity. Selected bond lengths (angstroms): in 4, B1−C1 1.704(4), C1−C2 1.477(4), C1−C1′ 1.347(5); in
5, B1−C1′ 1.669(2), C1−C2 1.488(2), C1−C1′ 1.353(3); in 6, C1−C1′ 1.362(10), C1−B1 1.764(7), C2−C1 1.472(7); in 7, B1−C1 1.696(14),
B1−C6 1.665(16), C1−C2 1.476(14), C1−C8 1.328(12), C8−C8′ 1.24(2); in 8, C1−B1 1.585(3), C24−C25 1.486(3), C25−C26 1.340(3),
C26−C26′ 1.510(3).
and 5, indicating the yellow solid is a mixture of complexes 4
and 5. In addition, single-crystal X-ray diffraction (Figure 3a)
studies at −5 °C further showed it consists of two
configurations: bis-cycloborate olefin 4 and diborate-bridged
stilbene 5. Subsequently, to avoid the formation of the mixture
product, the reaction was again performed at two different
temperatures, 10 and 40 °C (Scheme 3). Then the two
reaction solutions were filtered and concentrated in vacuum.
Finally a light yellow solid 4 and a golden yellow solid 5 were
isolated in 58 and 87% yields, respectively. Their 11B NMR
resonances appear at −3.81 and −7.98 ppm, respectively,
which are significantly shifted downfield compared to that of
compound 1 and close to those reported for coordinated
boron compounds.40,44,45 In addition, in the 13C NMR spectra,
the disappearance of alkyne carbons’ signals further indicates
the formation of complexes 4 and 5. The chemical shifts in the
1H NMR spectra for complexes 4 and 5 are distinctly different
(Figure 2a,c). For example, the aromatic peak (7.00 ppm) in
the lowest field of complex 4 is obviously lower than that of
complex 5 (7.23 ppm). These results demonstrate that
complexes 4 and 5 are clearly different and can be obtained
selectively by varying the reaction temperature. Moreover, we
also found complex 4 can convert to complex 5 completely
when the solution of complex 4 is heated at 40 °C for 2 days in
D
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Scheme 4. Proposed Mechanism for the Formation of 7
a sealed tube. This isomerization is a thermally driven process,
which is different from the light-driven and acid-induced
isomerization of Pier46,47 and Barton.48 Meanwhile, density
functional theory (DFT) calculations (see below) further show
that stilbene 5 is 32.1 kcal mol−1 more stable than isomer 4. In
addition, the isolated mixture described above that was
obtained at room temperature also completely converted to
complex 5 when its Et2O solution was heated at 40 °C. All
these experiments clearly show that complex 5 is a
thermodynamic product.
The 11B NMR resonance of complex 6 appears at −4.16 ppm,
which is significantly shifted downfield compared to that of 2
(65.7 ppm) but close to those of complexes 4 and 5, indicating
the formation of a tetracoordinate borate salt. Meanwhile,
compared to precursor 2, more broad peaks appear in the
range of 6.01−6.49 ppm (Mes-H) and 2.03−2.35 ppm (Mes-
CH3) in the 1H NMR spectrum of 6, which may be caused by
the hindered rotation of the Mes groups in more constricted
environments. In addition, although the synthesis of complex 6
uses the same path as complex 4, it demonstrates no thermal
isomerization. The butatriene dianion {K+·[2.2.2]-cryptand}2·
7a2− (7) was obtained (Scheme 3) unexpectedly in 43% yield
upon treatment of 3 with K chips in the presence of [2,2,2]-
cryptand at room temperature. The 11B NMR resonance at
−2.50 ppm is close to those of complexes 4−6, implying the
formation of a similar tetracoordinate borate structure. The
13C NMR spectrum was not obtained because of the poor
solubility of complex 7. The byproducts are complex, but we
succeeded in identifying the structure of one main byproduct
as 1,1′-dinaphthyl (Figure S21). The naphthyl signals
Crystals of complexes 4 and 5 suitable for single-crystal X-
ray diffraction were obtained in an Et2O solution at −25 °C.
Panels b and c of Figure 3 display their solid structures. Their
important structural parameters are listed in Table S2.
According to X-ray diffraction, they display bis-cycloborate
olefin (4) and diborate-bridged stilbene (5) structures,
respectively. Complex 4 features a structure with two four-
membered boracycles linked by a CC bond [1.347(5) Å],
and the boron atoms are in a trans arrangement about the
double bond. The B1−C1 (or B1′−C1′) bond length
[1.704(4) Å] is close to that of the previous counterpart J
[1.710(5) Å].40 In complex 5, the newly formed B1−C1′ (or
B1′−C1) bond leads to the formation of a five-membered ring-
fused borate geometry. The B1(B1′)−C1′(C1) bond length
[1.669(2) Å] is close to those of previously reported
compounds SA−SC [1.665(3) Å,44 1.645(3) Å,45 and
1.670(7) Å,49 respectively (Scheme S1)] but shorter than
that of SD [1.842(3) Å (Scheme S1)].50 The skeleton is nearly
planar, which is reflected in torsion angles C6−C2−C1−C1′
(7.84°), C2−C1−C1′−C2′ (0°), and C1−C1′−C2−C6
(7.84°). Besides being coordinated to two Et2O molecules,
each Na+ ion in complexes 4 and 5 is involved in cation−π
bonding to phenyl moieties and Mes units. In addition, the
structure of 5 is similar to those of phosphonium- and borate-
bridged zwitterionic stilbenes that were synthesized by
intramolecular cascade cyclization.51,52
1
disappear in the H NMR spectrum of 7, supporting the
departure of naphthalene derivatives after the reduction of 3.
Crystals suitable for X-ray studies were obtained from a
THF solution. Despite repeated attempts, the quality of the
crystals of 6 and 7 was still not good enough. The uncertainties
are relatively large and allow for only a qualitative discussion.
The structures of complexes 6 and 7 revealed some interesting
features (Figure 3d,e). Similar to complex 4, complex 6 shows
a bis-naphthocycloborapentylidene structure linked by a CC
bond. The B1−C1 bond [1.764(7) Å] is longer than those in
complexes 4 [1.704(4) Å] and 5 [1.669(2) Å]. Although all
carbons are sp2-hybridized, the B2C8 core is not planar because
of the severe angulation of the CC bond caused by the space
extrusion of crowded Mes groups. This also causes the
inclination of the two naphthocycloborate fragments with a
dihedral angle of ≤30.4°. Meanwhile, this crowded environ-
ment further illustrates the broadening of Mes NMR signals.
Complex 7 displays a bis-cycloborate structure with two
discrete borate four-membered rings linked by a butatriene
fragment. The two boron centers adopt trans dispositions. The
cumulene linkage is essentially linear with a C−C−C angle of
171.3(5)°, which is similar to those of the dizwitterion
phosphonium borate molecules.53
Synthesis, Structure, and Characterization of Com-
pounds 6−8. To further explore the generality of this
synthetic methodology, more extended homologues derivatives
were prepared. Treatment of 2 with K chips in the presence of
18-crown-6 at room temperature afforded the complex
{[K(THF)2(18-C-6)]+}2·6a2− (6) in 73% yield (Scheme 3).
The structure of the product was ascertained by multinuclear
NMR spectroscopy, X-ray diffraction, and elemental analysis.
E
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Figure 4. Experimental and simulated EPR spectra of [1••]2− and hyperfine coupling.
The previous works54 by Power show that a different
combination of reduced metals and solvents would lead to
different reduction species. On the basis of this work, we tried
to use Na as the reductant and diethyl ether as the solvent but
failed to isolate the product. However, a large amount of solid
8 was obtained immediately in 21% yield upon adding 1 or 2
drops of H2O to the reduction reaction filtrates. The 11B NMR
resonance at 70.0 ppm implies the formation of a typical three-
coordinated organoboron compound, which is close to those
of compounds 1−3. The chemical shift of the 1H NMR
spectrum at 6.12 ppm shows the formation of alkene
hydrogens. Although it is difficult to unambiguously determine
the backbone structure of 8 from the spectroscopic data, X-ray
measurements confirmed its structure (Figure 3f), which is a
typical bis-dimesitylboryl-substituted naphthalene. Two sym-
metrical double bonds [1.340(3) Å] replaced the previous two
triple bonds [1.207(4) and 1.205(3) Å] in 3 due to the
formation of the C26−C26′ bond [1.510(3) Å]. The two extra
hydrogen atoms in 8 were probably derived from H2O as its
addition led to the immediate appearance of 8.
Proposed Mechanism of Formation for Complexes
4−7. To better understand these reduction reactions, electron
paramagnetic resonance (EPR) measurements and DFT
calculations were performed. Possible mechanisms for the
formation of complexes 4, 5, and 7 (Scheme 4 and Scheme S2)
were proposed. Although we tried to capture the EPR spectra
of all proposed intermediate radicals at room temperature, only
the spectrum of [1••]2− was obtained. Even if we changed the
collection conditions, the EPR signals of [2••]2− and [3••]2−
were still not observed. The X-band EPR spectra of [1••]2− in
a THF solution show a complex but resolved signal [Figure 4;
g = 2.004, a(11B) = 10.17 G, a(10B) = 3.39 G, a(H1) = 1.99 G,
a(H2) = 1.01 G], and no half-field signal or zero-field splitting
was observed (Figure S1), indicating the formation of a
biradical dianion with rather weak spin−spin couplings
between the two boron radical centers.40 To further under-
stand the electronic structures of [1••]2−−[3••]2−, DFT
calculations were carried out at the UCAM-B3LYP/6-
31G(d) level of theory. Both the open-shell singlet (OS) and
triplet (T) states were considered. [1••]2− features a singlet
ground state with a singlet−triplet energy gap (ΔET−OS = 1.21
kcal/mol). The electron exchange coupling constant (J =
−1/2ΔET−OS) was estimated to be −211 cm−1 (Table S4),
suggesting a weak interaction between the two radical centers.
The spin density for [1••]2− is mainly localized on the two
boron atoms (Table S5), indicating weak spin−spin couplings.
The EPR spectra together with DFT calculations thus imply
that the formation of 4 may undergo a radical cyclization
process similar to our previous diboryl compounds.40 The
thermal isomerization from complexes 4 to 5 may probably
occur via a radical intermediate due to the cleavage of the B−C
bond (Scheme S2a), which is similar to the reported light-
driven isomerization.46 Compared to [1••]2−, [2••]2− and
[3••]2− have triplet ground states with an even smaller singlet−
triplet energy gap (Table S4). [2••]2− shows more spin density
distribution delocalized to the adjacent naphthyl group (Table
S5), while in the formation of 7, the reaction path probably
involves successive C−C coupling and similar “extrusion
reaction”55,56 after radical cyclization, which leads to the
departure of naphthalene derivatives (Scheme 4). The reaction
mechanism for the formation of 8 has not been fully resolved
so far and will be our future work.
CONCLUSION
■
In conclusion, we have synthesized a series of dicycloborate
olefin complexes 4−7 from alkynyl-bridged diboryl com-
pounds using a simple direduction strategy. In the reduction of
1, temperature plays an important role in controlling the
products (4 and 5). Meanwhile, a thermally driven isomer-
ization was observed between complexes 4 and 5. In addition,
the formation of product 7, although unexpected, indicates that
with the choice of appropriate precursors, a variety of
structures may be obtained utilizing effects like C−C coupling
and “extrusion reaction”. Further studies of the reactivity, as
well as the fluorescence properties of these anionic species, are
underway.
EXPERIMENTAL SECTION
■
General Procedures. All reactions and manipulations were
carried out under an argon atmosphere by using standard Schlenk
techniques and a glovebox. CDCl3 and CD3CN were dried with 4 Å
molecular sieve (2−3 days). Prior to use, the THF and Et2O were
dried by being refluxed with sodium and benzophenone and degassed
by applying four freeze−pump−thaw cycles. The 1-bromo-2-
ethynylbenzene,57 1,2-bis[2-(dimesitylboranyl)phenyl]ethyne (1),41
1,8-diiodonaphthalene,58 1,2-bis(8-bromonaphthalen-1-yl)ethyne,59
and 1,8-bis[(2-bromophenyl)ethynyl]naphthalene60 were synthesized
according to literature methods. The H and 13C NMR spectra were
1
recorded using a JEOL JNM-ECS-400 or INOVA 600NB
spectrometer (Bruker) at room temperature in parts per million
downfield from Me4Si. The 11B NMR spectra were also recorded
using a JEOL JNM-ECS-400 spectrometer at room temperature in
parts per million downfield from external aqueous BF3·Et2O. The
cyclic voltammetry experiment was conducted in an argon-filled
atmosphere using a CHI 760e Electrochemical Workstation. Freshly
F
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n
distilled THF was used as the solvent, and Bu4NPF6 (10−1 M) was
used as the electrolyte. A standard three-electrode cell configuration
was employed using a platinum-disk working electrode, a platinum-
wire counter electrode, and a silver wire serving as the reference
electrode. Formal redox potentials were referenced to the ferrocene/
ferrocenium redox couple [E(Fc/Fc+) = 0 V]. UV−visible−near-
infrared (NIR) spectra were recorded on an Agilent Carry 5000 UV−
vis−NIR spectrometer. Elemental analyses for C and H were carried
out with a German Elementary Vario EL cube instrument.
4.38). 1H NMR (300 MHz, CDCl3, 293 K): δ 7.88 (d, J = 8.0 Hz, 2H,
Ar-H), 7.82 (d, J = 8.0 Hz, 2H, Ar-H), 7.45 (d, J = 4.0 Hz, 2H, Ar-H),
7.36 (t, J = 8.0 Hz, 2H, Ar-H), 7.22 (d, J = 8.0 Hz, 2H, Ar-H), 6.73 (d,
J = 8.0 Hz, 2H, Ar-H), 6.63 (s, 8H, Ar-H), 2.28 (s, 12H, -CH3), 1.53
(broad, 24H, -CH3). 13C NMR (100 MHz, CDCl3, 293 K): δ 149.4,
138.7, 138.5, 136.3, 135.2, 133.6, 132.5, 130.1, 128.7, 126.0, 124.9,
122.7, 100.5 (alkyne quaternary carbon), 24.2, 21.5. 11B NMR (128.3
MHz, CDCl3, 293 K): δ 65.7. Elemental Anal. Calcd for C58H56B2: C,
89.92; H, 7.29. Found: C, 89.84; H, 7.19.
X-ray Crystallography. The data were collected with a
SuperNova (Dual) X-ray diffractometer equipped with Cu Kα or
Mo Kα radiation (λ = 1.54184 or 0.71073 Å, respectively) at different
temperatures (1 and 6, 173 K; 2, 4, and 5, 293 K; 3, 7, and 8, 150 K).
Data reduction was performed using CrysAlisPro (version
1.171.37.35). The data sets were corrected by empirical absorption
correction using spherical harmonics, implemented in the SCALE3
ABSPACK scaling algorithm.61,62 Crystal structures were determined
by direct methods using Olex 2-1.2. Subsequent difference Fourier
analyses and least-squares refinement with the SHELXL-2014/7
program package63−65 allowed for the location of the atom positions.
Non-hydrogen atoms were refined with anisotropic displacement
parameters during the final cycles. All hydrogen atoms were found in
difference maps and refined using a riding model. More details about
the crystallographic studies as well as atomic displacement parameters
are given in CIF files. The crystallographic details for compounds 1−8
are summarized in Tables S1−S3. The data have been deposited in
the Cambridge Crystallographic Data Centre (CCDC) with
deposition numbers CCDC 1817515, 1817516, 1817518,
1817520−1817523, 1858155, and 1858156 for compounds 1−8,
respectively. Despite repeated measurements, the quality of crystals of
6 and 7 was still not good enough. The uncertainties were relatively
large and allowed for only a qualitative discussion. In addition, some
residual electron density peaks (the maximum residual electron
density peak at <1 e Å−3) were found in the refinement of crystals 2
that have a strong influence on the R value, and we are not sure what
solvents they are. Therefore, we use squeeze to delete these peaks, and
the R values have been greatly reduced.
Computational Details. All the geometry optimizations were
carried out at the UCAM-B3LYP/6-31G(d) level of theory. The
obtained stationary points were characterized by frequency calcu-
lations. The broken-symmetry approach was applied for open-shell
singlet calculations. All calculations were performed with the Gaussian
09 program suite.66
EPR Measurements. Solution EPR measurements were carried
out with a Bruker ER200DSRC10/12 spectrometer. A sample
containing 10−3 M [1••]2− was prepared in a J. Young EPR tube
inside a glovebox. The EPR spectrum was obtained at room
temperature with a microwave power of 7.599 mW, a modulation
amplitude of 0.5 G, a time constant of 81.92 ms, and a sweep time of
75 s. The EPR spectrum was simulated using EasySpin with hyperfine
coupling constants.
Synthesis of 1,8-Bis{[2-(dimesitylboranyl)phenyl]ethynyl}-
naphthalene (3). Under anaerobic and anhydrous conditions, nBuLi
(7.6 mL, 12.2 mmol, 1.6 M in hexanes) was added dropwise to a
stirred THF solution of 1,8-bis[(2-bromophenyl)ethynyl]naphthalene
(2.90 g, 6.0 mmol) at −78 °C for 4 h, and then Mes2BF (3.22 g, 12.0
mmol) in THF (15 mL) was slowly added. Subsequently, the reaction
mixture was slowly warmed to room temperature and stirred
overnight. After the addition of water, the solution was then extracted
with dichloromethane. The crude product was then purified by
column chromatography (10:1 CH2Cl2/hexanes) to afford 3 in 64%
yield (3.16 g). Mp: 203.1−205.8 °C. UV−vis−NIR (THF): 337 nm
1
(log ε = 4.36), 374 nm (log ε = 4.08). H NMR (400 MHz, CDCl3,
293 K): δ 7.64 (d, J = 8.0 Hz, 2H, Ar-H), 7.26−7.30 (m, 4H, Ar-H;
the peaks of Ar-H were enwrapped in the peak of CDCl3), 7.12−7.16
(m, 4H, Ar-H), 6.95 (t, J = 8.0 Hz, 2H, Ar-H), 6.74 (s, 8H, Ar-H),
6.57 (d, J = 8.0 Hz, 2H, Ar-H), 2.25 (s, 12H, -CH3), 2.02 (s, 24H,
-CH3). 13C NMR (100 MHz, CDCl3, 293 K): δ 149.9, 142.9, 140.9,
138.8, 135.1, 134.2, 133.6, 133.1, 131.1, 129.7, 128.9, 128.3, 127.8,
127.5, 124.7, 120.7, 97.6, and 93.2 (alkyne quaternary carbon), 23.2,
21.2. 11B NMR (128.3 MHz, CDCl3, 293 K): δ 74.1. Elemental Anal.
Calcd for C62H58B2: C, 90.29; H, 7.09. Found: C, 90.23; H, 7.00.
Synthesis of {[Na(Et2O)2]+}2·4a2− (4). Under anaerobic and
anhydrous conditions, a mixture of 1 (0.135 g, 0.20 mmol) and
sodium (0.010 g, 0.44 mmol) in Et2O was stirred at 10 °C for 3 days.
The resultant light yellow solution was filtered to remove the dark
insoluble substance. The filtrate was concentrated in vacuo, and a
light yellow solid was obtained. Then the crude product was washed
three times with haxanes, allowing for isolation of product 4. X-ray-
quality light yellow crystals of 4 were obtained in an Et2O solution at
−25 °C. Yield: 0.118 g (58%). Mp: 209 °C (turn black). UV−vis−
NIR (THF): 373 nm (log ε = 3.83), 329 nm (log ε = 4.36). 1H NMR
(400 MHz, CD3CN, 293 K): δ 7.00 (d, J = 4.0 Hz, 2H, Ar-H), 6.59
(d, J = 8.0 Hz, 2H, Ar-H), 6.34−6.38 [m, 10H, Ar-H and Mes-H; the
peaks of Ar-H (2H) were enwrapped in the peak of Mes-H (8H)],
6.30 (t, J = 8.0 Hz, 2H, Ar-H), 3.42 (q, J = 8.0 Hz, 16H, -OCH2-),
2.07 (d, 36H, Ar-CH3), 1.12 (t, J = 8.0 Hz, 24H, -OCH2-CH3). 13C
NMR (150 MHz, CD3CN, 293 K): δ 130.7, 130.2, 129.6, 128.8,
123.1, 122.3, 119.4, 118.7, 66.7 (Et2O), 27.8, 21.3, 16.0 (Et2O). 11B
NMR (128.3 MHz, CD3CN, 293 K): δ −3.81. Elemental Anal. Calcd
for C66H92B2Na2O4: C, 77.94; H, 9.12. Found: C, 77.76; H, 9.04.
Synthesis of {[Na(Et2O)2]+}2·5a2− (5). Method A. Under
anaerobic and anhydrous conditions, a mixture of 1 (0.135 g, 0.20
mmol) and sodium (0.010 g, 0.44 mmol) in Et2O was sealed and
stirred at 40 °C for 4 days. The resultant golden yellow solution was
filtered to remove the dark insoluble substance. The filtrate was
concentrated in vacuo, and a golden yellow solid was obtained. Then
the crude product was washed three times with hexanes, allowing for
isolation of product 5. X-ray-quality golden yellow crystals of 5 were
obtained in an Et2O solution at −25 °C. Yield: 0.132 g (65%). Method
B. Under anaerobic and anhydrous conditions, the Et2O solution of 4
(0.102 g, 0.10 mmol) was sealed in a reactive tube. After being stirred
at 40 °C for 2 days, the solution was concentrated in vacuo, and a
golden yellow solid was obtained. Then the crude product was
washed three times with hexanes, allowing for isolation of product 5.
Yield: 0.089 g (87%). Mp: 281−283 °C. UV−vis−NIR (THF): 377
Synthesis of 1,2-Bis[2-(dimesitylboranyl)phenyl]ethyne (1).
Compound 1 was prepared according to ref 41. UV−vis−NIR
(THF): 334 nm (log ε = 4.16). 1H NMR (400 MHz, CDCl3, 293 K):
δ 7.14−7.20 (m, 6H, Ar-H), 6.73 (s, 8H, Ar-H), 6.58 (d, J = 8.0 Hz,
2H, Ar-H), 2.25 (s, 12H, Ar-CH3), 1.93 (s, 24H, Ar-CH3). 13C NMR
(100 MHz, CDCl3, 293 K): δ 149.8, 142.9, 140.8, 138.8, 134.0 133.8,
133.2, 129.5, 128.3, 127.5, 127.3, 127.2, 93.5, 23.2, 21.4. 11B NMR
(128.3 MHz, CDCl3, 293 K): δ 75.0.
Synthesis of 1,2-Bis[8-(dimesitylboranyl)naphthalen-1-yl]-
n
ethyne (2). Under anaerobic and anhydrous conditions, BuLi (7.6
mL, 12.2 mmol, 1.6 M in hexanes) was added dropwise to a stirred
THF solution of 1,2-bis(8-iodonaphthalen-1-yl)ethyne (3.18 g, 6.0
mmol) at −78 °C for 4 h, and then Mes2BF (3.22 g, 12.0 mmol) in
THF (15 mL) was slowly added. Subsequently, the reaction mixture
was slowly warmed to room temperature and stirred overnight. After
the addition of water, the solution was then extracted with
dichloromethane. The crude product was then purified by column
chromatography (10:1 CH2Cl2/hexanes) to afford 2 in 69% yield
(3.20 g). Mp: 340−341 °C. UV−vis−NIR (THF): 337 nm (log ε =
1
nm (log ε = 4.40), 329 nm (log ε = 4.87). H NMR (400 MHz,
CD3CN, 293 K): δ 7.23 (d, J = 8.0 Hz, 2H, Ar-H), 7.08 (d, J = 8.0 Hz,
2H, Ar-H), 6.45 (t, J = 8.0 Hz, 2H, Ar-H), 6.34 (s, 8H, Ar-H), 6.23 (t,
J = 8.0 Hz, 2H, Ar-H), 3.43 (q, J = 8.0 Hz, 16H, -OCH2-), 2.06 (s,
24H, Ar-CH3), 1.94 (broad, 12H, Ar-CH3; the peaks of Ar-H were
enwrapped in the peak of CD3CN), 1.13 (t, J = 8.0 Hz, 24H, -OCH2-
G
DOI: 10.1021/acs.inorgchem.8b01555
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
CH3). 13C NMR (100 MHz, CD3CN, 293 K): δ 159.6, 142.1, 129.9,
128.8, 128.0, 122.3, 121.5, 118.6, 117.9, 65.9 (Et2O), 26.9, 20.5, 15.2
(Et2O). 11B NMR (128.3 MHz, CD3CN, 293 K): δ −7.98. Elemental
Anal. Calcd for C66H92B2Na2O4: C, 77.94; H, 9.12. Found: C, 77.75;
H, 9.00.
was washed three times with hexanes, allowing for isolation of neutral
product 8. X-ray-quality yellow crystals of 8 were obtained in a THF
solution at room temperature. Yield: 0.035 g (21%). Mp: 191.8 °C
(turn black). UV−vis−NIR (THF): 318 nm (log ε = 4.57), 414 nm
(log ε = 3.83). 1H NMR (400 MHz, CDCl3, 293 K): δ 7.47−7.52 (m,
6H, Ar-H), 7.36−7.42 (m, 4H, Ar-H), 7.14 (t, J = 8.0 Hz, 2H, Ar-H),
6.65 [s, 10H, Ar-H(2H)+Mes-H(8H)], 6.12 (s, 2H, CH), 2.10 (s,
12H, -CH3), 1.88 (s, 24H, -CH3). 13C NMR (100 MHz, CDCl3, 293
K): δ 146.7, 143.4, 142.3, 138.5, 136.8, 135.4, 135.3, 131.2, 130.8,
128.7, 128.1, 127.2, 127.0, 125.6, 123.8, 119.1, 23.2, 21.2. 11B NMR
(128.3 MHz, CDCl3, 293 K): δ 70.0. Elemental Anal. Calcd for
C62H60B2: C, 90.07; H, 7.32. Found: C, 90.04; H, 7.28.
Synthesis of the Mixture of 4 and 5. Under anaerobic and
anhydrous conditions, a mixture of 1 (0.135 g, 0.20 mmol) and
sodium (0.010 g, 0.44 mmol) in Et2O was stirred at room
temperature for 4 days. The resultant brownish yellow solution was
filtered to remove the dark insoluble substance. The filtrate was
concentrated in vacuo, and a yellow solid was obtained. Then the
crude product was washed three times with hexanes, allowing for
isolation of the solid. X-ray-quality yellow crystals were obtained in an
1
Et2O solution at −5 °C. The H NMR spectrum of the solid was
ASSOCIATED CONTENT
* Supporting Information
■
recorded in CD3CN at room temperature. The result shows the solid
is a mixture of complexes 4 and 5. In addition, the crystals were
measured three times at a low temperature (110 K), and the results
display the same disorder in the central core that consists of two
configurations: bis-cycloborate olefin 4 and diborate-bridged stilbene
structure 5. This further shows the product is mixture of complexes 4
and 5.
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b01555.
1H, 13C, and 11B NMR spectra of compounds 1−8 and
computational details (PDF)
Synthesis of {[K(THF)2(18-C-6)]+}2·6a2− (6). Under anaerobic
and anhydrous conditions, a mixture of 2 (0.155 g, 0.20 mmol), 18-
crown-6 (0.098 g, 0.40 mmol), and potassium (0.017 g, 0.44 mmol)
in THF (≈35 mL) was stirred at room temperature for 3 h. The
resultant yellowish-brown solution was filtered to remove the dark
insoluble substance. The filtrate was concentrated in vacuo, and a
yellow solid was obtained. Then the crude product was washed three
times with hexanes, allowing for isolation of product 6. X-ray-quality
yellow crystals of 6 were obtained in a THF solution at room
temperature. Yield: 0.243 g (73%). Mp: 158 °C (turn black). UV−
vis−NIR (THF): 450 nm (log ε = 3.59), 364 nm (log ε = 4.05), 310
nm (log ε = 4.55). 1H NMR (400 MHz, CD3CN, 293 K): δ 7.72 (dd,
J = 4.0, 0.4 Hz, 2H, Ar-H), 7.08 (d, J = 4.0 Hz, 2H, Ar-H), 6.92−6.99
(m, 6H, Ar-H), 6.82 (t, J = 8.0 Hz, 2H, Ar-H), 6.03−6.49 (broad, 8H,
Ar-H), 3.64 (t, 4H, -O-CH2-), 3.57 (s, 48H, 18-C-6), 2.03−2.35
(broad, 36H, -CH3), 1.80 (t, 4H, -O-CH2-CH2-). 13C NMR (100
MHz, CD3CN, 293 K): δ 156.4, 144.7, 131.6, 128.5, 127.7, 125.7,
125.1, 122.0, 121.4, 118.4, 117.8, 117.3, 70.0, 67.4, 26.3, 25.9, 19.8.
11B NMR (128.3 MHz, CD3CN, 293 K): δ −4.16. Elemental Anal.
Calcd for C82H104B2K2O12·THF: C, 71.06; H, 7.77. Found: C, 70.95;
H, 7.64.
Accession Codes
CCDC 1817515, 1817516, 1817518, 1817520, 1817521,
1817522, 1817523, 1858155, and 1858156 contain the
supplementary crystallographic data for this paper. These
data can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing 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 Authors
*E-mail: boxb@lzu.edu.cn.
*E-mail: xingyong@uow.edu.au.
■
ORCID
Xingyong Wang: 0000-0002-6047-8722
Xiaobo Pan: 0000-0002-5757-2339
Jincai Wu: 0000-0002-8233-2863
Synthesis of {K+·[2.2.2]-cryptand}2·7a2− (7). Under anaerobic
and anhydrous conditions, a mixture of 3 (0.165 g, 0.20 mmol),
[2,2,2]-cryptand (0.114 mg, 0.40 mmol), and potassium (0.017 g,
0.44 mmol) in THF (≈35 mL) was stirred at room temperature for
12 h. The resultant yellowish brown solution was filtered to remove
the dark insoluble substance. The filtrate was concentrated in vacuo,
and a yellow solid was obtained. Then the crude product was washed
three times with hexanes, allowing for isolation of product 7. X-ray-
quality yellow crystals of 7 were obtained in a THF solution at room
temperature. Yield: 0.131 g (43%). UV−vis−NIR (THF): 386 (log ε
Author Contributions
§J.Z. and C.R. contributed equally to this work and should be
considered co-first authors.
Funding
This work has been supported by the National Natural Science
Foundation of China (21771094 and 21671087) and the
Fundamental Research Funds for the Central Universities
(lzujbky-2017-k07). The authors also acknowledge the support
from the Vice-Chancellor’s Postdoctoral Research Fellowship
Funding of the University of Wollongong and the computa-
tional resources provided by NCI’s National Computational
Merit Allocation Scheme.
1
= 4.00), 421 nm (log ε = 3.83). H NMR (400 MHz, CD3CN, 293
K): δ 7.63 (d, J = 8.0 Hz, 2H, Ar-H), 7.63 (d, J = 8.0 Hz, 2H, Ar-H),
6.79 (t, J = 8.0 Hz, 6H, Ar-H), 6.67 (t, J = 8.0 Hz, 2H, Ar-H), 6.42 (s,
8H, Ar-H), 3.55 (s, 24H, Crypt ether), 3.50 (t, J = 4.0 Hz, 24H, Crypt
ether), 2.52 (t, J = 4.0 Hz, 24H, Crypt ether), 2.24 (s, 12H, -CH3).
11B NMR (128.3 MHz, CD3CN, 293 K): δ −2.50. Elemental Anal.
Calcd for C88H124B2K2N4O12·THF: C, 68.98; H, 8.31; N, 3.50.
Found: C, 68.84; H, 8.23; N, 3.41. Note that this analysis was
performed on hand-picked single crystals to ensure compound purity.
The lack of a 13C NMR spectrum is due to the poor solubility of
complex 7.
Synthesis of 1,2-Bis[(Z)-2-(dimesitylboranyl)benzylidene]-
1,2-dihydroacenaphthylene (8). Under anaerobic and anhydrous
conditions, a mixture of 3 (0.165 g, 0.20 mmol) and sodium (0.010 g,
0.44 mmol) in Et2O was stirred at room temperature for 12 h. The
resultant yellowish-brown solution was filtered to remove the dark
insoluble substance. Subsequently, 1 or 2 drops of H2O was added to
the filtrate, and a yellow solid was obtained. Then the crude product
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors sincerely appreciate Dr. Shengchun Wang for his
useful discussion and suggestion.
■
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