Diradical Anion of Potassium Aggregate: Reduction of Dimer Boroxide Complex

Article abstract of DOI:10.1021/acs.inorgchem.8b02157

Crystalline aggregates containing metal cation clusters wrapped by reduced hydrocarbon anions have been presented. Initially, a dimeric complex (2) possessing a bimetallic K₂O₂ core was synthesized from the reaction between an anthracene substituted boronic acid (2-(anthracen-9-yl)phenyl)(hydroxy)(mesityl)borane (1-H) and KN(SiMe₃)₂ in THF solution. The B-O bond length (1.281(4) ?) in complex 2 is comparable to those observed in oxoboranes, indicating this may be a double bond, which is supported by its Wiberg bond order (1.9) predicted by density functional theory calculations. Subsequently, potassium aggregate complexes, diradical 3 and dihydro anion 4, were obtained through the reduction reactions of dimeric complex 2 and 1-H, respectively. They exhibit similar K₃O₂B₂ aggregate structures, but differ significantly in the geometry of the anthracene units. Complex 3 features a triplet diradical character with planar anthracene units that carry the unpaired electrons, while the anthracene groups in complex 4 display a puckered structure due to the addition of hydrogen atoms.

Full text of DOI:10.1021/acs.inorgchem.8b02157

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Diradical Anion of Potassium Aggregate: Reduction of Dimer
Boroxide Complex
Yang Zheng,† Xi Wang,† Yunfei Bai,† Xingyong Wang,*,‡ Chenglong Ru,† Wenhao Chen,†
Xiaoxia Lv,*,§ 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, New South Wales 2522, Australia
§School of Chemistry and Chemical Engineering, Qufu Normal University, Qufu, China
S
* Supporting Information
ABSTRACT: Crystalline aggregates containing metal cation clusters “wrapped”
by reduced hydrocarbon anions have been presented. Initially, a dimeric
complex (2) possessing a bimetallic K2O2 core was synthesized from the
reaction between an anthracene substituted boronic acid (2-(anthracen-9-
yl)phenyl)(hydroxy)(mesityl)borane (1-H) and KN(SiMe3)2 in THF solution.
The B−O bond length (1.281(4) Å) in complex 2 is comparable to those
observed in oxoboranes, indicating this may be a double bond, which is
supported by its Wiberg bond order (1.9) predicted by density functional theory
calculations. Subsequently, potassium aggregate complexes, diradical 3 and dihydro anion 4, were obtained through the
reduction reactions of dimeric complex 2 and 1-H, respectively. They exhibit similar K3O2B2 aggregate structures, but differ
significantly in the geometry of the anthracene units. Complex 3 features a triplet diradical character with planar anthracene
units that carry the unpaired electrons, while the anthracene groups in complex 4 display a puckered structure due to the
addition of hydrogen atoms.
Crystalline aggregates consisting of charged organic
molecules held together by metal-cluster-mediated Coulomb
interactions have attracted substantial interest owing to their
unusual structural, chemical, and electronic properties. In the
reduction chemistry of aromatic hydrocarbons,29,30 such as
biphenyl,31 pyrene,32,33 and anthracene,34 crystalline aggre-
gates containing metal cation clusters “wrapped” by corre-
sponding reduced hydrocarbon radical anions have also been
extensively investigated.35−38 They exhibited various structural
features and have been applied to electron transfer reactions
and polymerization.29 These exciting aggregation behaviors
were mainly caused by cation-π interaction, which plays an
important role in the formation and stability of the reduced
species. On the basis of our previous works,39,40 by choosing
the anthracene moiety as the substituent group, which is
favorable to form an aggregation unit through cation-π
interaction, we herein present the reduction behaviors of its
boroxide-substituted derivative 1-H and the corresponding
dimer complex 2, and describe the electronic and aggregation
characteristics of the reduced species 3 and 4. The
experimental results show that 3 displays a K3O2B2 cluster
structure with planar anthracene moieties and exhibits triplet
diradical character, while the closed-shell species 4 features a
similar K3O2B2 aggregate structure with puckered anthracene
INTRODUCTION
■
Metal ketyls, as important intermediates in a variety of organic
carbonyl-involved reactions, have received continuous atten-
tion since their discovery in the previous century1−4 (Scheme
1a). Extensive studies have led to the emergence of a number
Scheme 1. Diagram of the Reduction of Ketones (a) and
Their Boron Isoelectronic Analogues (b)
of structurally characterized stable alkali-metal,5−9 alkaline-
earth-metal,10 transition-metal,11,12 and rare-earth metal13−16
ketyls complexes. These radical species are synthetically useful
and have been used in various organic and organometallic
reactions as strong reductants. In contrast, their boron-
containing isoelectronic analogues (Scheme 1b) are elusive.
On the other hand, boroxide ligands have been applied in
coordination chemistry17 with different main group met-
als18−22 and transition metals23−28 and exhibited distinct
catalytic properties.21,28 Although hundreds of these species
have been well investigated in their neutral form, the study of
the structure of the corresponding reduced radical and anionic
species is rare.
Received: July 30, 2018
© XXXX American Chemical Society
A
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Scheme 2. Synthesis of Compound 1-H
units, accompanied by a loss of aromaticity in the central ring
of the anthracene group.
Me and Et, 57.3 ppm;19 [(μ-Mes2BO)ZnEt]2, 51.0 ppm;27
(Mes2BO)ZnEt·2,2′-bipy, 48.5 ppm27), which may be caused
by the strong π-electron donation from the oxygen atom to the
boron center in the B−O double bond.
RESULTS AND DISCUSSION
■
Crystals of 2 suitable for an X-ray structure determination
were grown from a saturated THF solution at room
temperature. Similar to previously reported complexes,18−21
complex 2 (Figure 1a) displays a dimeric structure with two
boroxide ligands connected by a bimetallic K2O2 core. Each K+
has cation-π interaction with both anthracene (2.99 Å) and
Mes (3.15 Å) groups, besides coordination with a THF
molecule. It is worth noting that the distance between B and O
is 1.281(4) Å, which is similar to the BO bond distances
(1.296(3)−1.314(3) Å, Figure S2) in oxoboranes,45−47
indicating this may be a double bond. This is supported by
its Wiberg bond order (1.9) predicted by density functional
theory (DFT) calculations at the CAM-B3LYP/6-31G(d) level
of theory (see Supporting Information for details). The
LUMO and LUMO+1 of 2 are localized on the two anthracene
units and are nearly energetically degenerated (Figure 2a),
indicating a diradical state may be formed upon direduction of
2.
Synthesis, Structure, and Characterization of Com-
pounds 1-H and 2. (2-(Anthracen-9-yl)phenyl)(hydroxy)-
(mesityl)borane (1-H) was obtained in moderate yield (40%)
from the reaction between nBuLi lithiated 9-(2-bromophenyl)-
anthracene in THF and freshly prepared mesityl-5,5-dimethyl-
1,3,2-dioxaborinane at −78 °C (Scheme 2). The 11B NMR
resonance of 1-H appears at 48.2 ppm, indicating the
1
formation of diarylborinic acid. In the H NMR spectrum,
the peak at 5.22 ppm confirms the existence of protons of boric
acid. In addition, the precursor 1-H was further verified with
X-ray diffraction (Figure S1). The lengths of B1−O1 and B2−
O2 bonds are 1.354(4) Å and 1.356(4) Å, respectively, which
are in the range of those reported B−O distances in Ar′2BOH
(1.335(2)−1.368(5) Å).18,41−44 Treatment of 1-H with
KN(SiMe3)2 in THF solution afforded a yellowish solid 2
(Scheme 3). The 11B NMR resonance of 2 appears at δ = 38.0
ppm. This is obviously shifted upfield compared to other
reported metal boroxide complexes ([(μ-Mes2BO)AlR2]2, R =
Synthesis, Structure, and Characterization of Com-
plex 3. Recently, a series of reduction reactions of neutral
complexes bridged by metal cations or metal clusters were
reported48−52 to produce reduced species with interesting
radical characters, encouraging us to explore if 2 has a similar
property. First, cyclic voltammetry of complex 2 was recorded
Scheme 3. Synthetic Route of Complexes 2−4
n
in THF with 0.1 M Bu4NPF6 as the supporting electrolyte. A
quasi-reversible reduction event occurs at −2.14 V vs Fc/Fc+
(Figure 1b), which is close to that of anthracene (E = −2.48 V
vs Fc/Fc+),53 indicating that the anthracene moiety of dimer 2
probably accepts electrons to form a stable radical anion under
this condition. Next, the chemical reduction of 2 was
performed in freshly distilled THF at room temperature. The
solution gradually changed from colorless to dark-blue when
2.2 equiv of potassium chips and 1.0 equiv of [2,2,2]-cryptand
were added. The dark-blue solids of 3 were obtained with a
moderate yield (71%) from a frozen THF solution. Although 3
is highly sensitive to air and moisture, it is thermally stable
under argon at room temperature.
The X-band EPR spectra of 3 (Figure 3) in THF solution
were recorded at both 298 and 115 K. The room temperature
spectrum (Figure 3a) shows a complex but nicely resolved
signal (centered at g = 2.0045) that is maintained for several
days, indicating the formation of a highly persistent radical
anion. The simulation analysis successfully reproduced the
B
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Figure 1. (a) Thermal ellipsoid (50%) drawing of crystals 2. All H atoms, free solvent THF molecules are omitted for clarity. Selected bond lengths
(Å): B1−O1 1.281(4), B1−C1 1.620(4), B1−C21 1.635(4), K1−O1 2.554(2), K1−O1A 2.626(2). (b) Cyclic voltammogram of 2 (1 × 10−3 M)
n
in THF, containing 0.1 M Bu4NPF6 measured at 100 mV s−1 at room temperature.
Figure 2. (a) LUMO and LUMO+1 for 2. The molecular orbital energies are given in the parentheses. (b) Spin density distribution for both the
open-shell singlet (OS) and triplet (T) states of diradical anion 3a−··. All hydrogen atoms are omitted for clarity.
Figure 3. EPR spectra of 3 in THF measured at (a) 298 K (1 × 10−4 mol/L) and (b) 115 K (5 × 10−3 mol/L). The central peak in (b) shows the
signal derived from monoradical impurities, and the inset shows the Δms = 2 resonance.
spectrum (Figures 3a and S3). The hyperfine coupling
constants (a(H1) = 6.0 G, a(H2) = 3.0 G, and a(H3) =
1.20 G) are similar to other anthracene derivatives’ radical
anions.54 This coupling pattern suggests that the unpaired
electrons are highly delocalized over the anthracene moieties of
boroxide anion ligands. The frozen solution (THF, 115 K)
EPR spectrum (g = 2.0037, Figure 3b) of complex 3 confirmed
that it is a triplet diradical species, and each anthracene group
carries one unpaired electron. From the zero-field splitting
parameters (D = 6 mT; E = 0 mT), the distance between the
two unpaired electrons (the average spin−spin distance) was
calculated to be 7.74 Å, which is close to the distance (7.22 Å)
between the geometric centers of the two anthracene moieties
in the crystal structure of 3. Further evidence for the triplet
diradical nature of 3 was the observation of a half-field signal at
1686 G corresponding to the forbidden Δms = 2 transition. In
addition, DFT calculations show that 3a−·· has a triplet ground
state, but the energy gap between the open-shell singlet state
(OS) and the triplet state (T) is rather small (ΔEOS−T = 0.002
kcal mol−1). The two states show nearly identical structures
(all atom positional root-mean-square deviation (RMSD) <
0.006 Å). The spin density resides almost exclusively on the
anthracene moieties and delocalizes over the whole anthracene
backbone (Figure 2b).
Crystals of 3 suitable for single-crystal X-ray diffraction were
obtained from the THF solution at room temperature. Figure
4a displays its crystal structure. A list of its important structure
parameters, as well as those of 1-H and 2, is given in Table S1.
According to X-ray diffraction, complex 3 consists of a discrete
[K(cryptand)]+ ion and a diradical K3O2B2 cluster anion, in
C
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nBu3SnH, Ph3SiH, and 1,4-cyclohexadiene to its THF solution
at room temperature, respectively, and monitoring if the
corresponding dihydro anion would be formed. However, no
hydrogen abstraction was observed, probably due to the low
activity of these hydrogen donor reagents. Subsequently, a new
experiment was designed to directly reduce the boronic
compound 1-H using 2.2 equiv of the potassium chips. The
solution changed from colorless to yellowish to dark-blue and
then immediately turned dark-red. The product was identified
as dihydro anion 4. It should be noted that the crystals
obtained from the yellow solution were identified as complex 2
through X-ray diffraction. In addition, the EPR spectrum of the
blue solution was measured at both room temperature and 115
K, and displays the same signal as that of diradical 3 (Figure
S8). These experimental results along with the color change of
the reaction thus implies the formation of 4 is a three-step
process with complexes 2 and 3 as intermediates. The dark-red
crystals of 4 were obtained from the THF solution with 53%
isolated yield (Scheme 3). The 11B NMR resonance appears at
δ = 38.2 ppm, which is almost identical to dimer 2, indicating
an insignificant environment change around the boron center.
1
In the H NMR spectrum, the chemical shifts of anthracene
moieties in 4 are obviously upfield relative to the dimer 2,
which is caused by the effect of more negative charges in the
anthracene units. The signals of the four hydrogen atoms
bound to C14 and C43 were not observed due to the overlap
with THF signals. However, combining the 13C NMR and
DEPT (Figure S23) spectra, the existence of peaks at 99.6 ppm
(C7 and C36) and 39.7 ppm (C14 and C43) could clearly
demonstrate the formation of 4.55
Although it is difficult to unambiguously determine the
backbone structure of 4 from the spectroscopic data, X-ray
measurements confirmed its structure. The X-ray analysis
revealed complex 4 possesses a similar K3O2B2 core structure
(Figures 4b and S6) to complex 3, but there are two distinct
structural changes. First, the average (1.480 Å) of the lengths
of C13−C14, C14−C15, C42−C43, and C43−C44 bonds in
the anthracene units obviously increases due to the addition of
hydrogen atoms, which leads to the hybridization mode change
of C14 and C43 atoms from sp2 to sp3. Meanwhile, these
changes result in the formation of a puckered anthracene
geometry (Figure S7), which simultaneously gives rise to the
change of aromaticity. This puckered geometry of the
anthracene units also creates a more crowded coordination
environment around the K2 center, causing the departure of
the coordinated THF.
The UV−vis-NIR absorption spectra of 1-H, 2, 3, and 4
obtained in THF solution are shown in Figure 5. The neutral
ligand 1-H displays three intense absorption bands in the range
of 350−400 nm. The intensity of these bands was increased
after the formation of complex 2. In addition to these bands,
the dark blue diradical anion 3 exhibits a broad and less intense
absorption (main peaks: 556, 604, 667, and 758 nm) in the
range of 500−900 nm, which are similar to other anthracene
substituted radical anions.56,57 In contrast, the dihydro anion 4
manifested a strong absorption at 477 nm, as well as a rather
weak and wide peak (600−850 nm) that was due to the
existence of a small amount of residual 3. On the basis of TD-
DFT calculations (Figures S9−S13, Tables S3−S6), we
attribute the maximum absorptions of these complexes to
ligand-based π−π* transitions. For compounds 1-H and 2, the
transitions are either HOMO → LUMO or HOMO−1 →
Figure 4. Thermal ellipsoid (40%) drawing of anion 3a−·· (a) and 4a−
(b). Except for four H atoms of C14 and C43 in 4a−, other H atoms,
free THF molecules are omitted for clarity. Selected bond lengths
(Å): in 3a−··: B1−O1 1.295(6), B2−O2 1.308(6), C1−B1 1.600(7),
C21−B1 1.622(8), C30−B2 1.599(7), C50−B2 1.607(8), C13−C14
1.378(8), C14−C15 1.410(9), C42−C43 1.390(7), C43−C44
1.393(8); in 4a‑: B1−O1 1.310(7), B2−O2 1.302(7), C1−B1
1.599(7), C21−B1 1.635(7), C30−B2 1.603(8), C50−B2 1.637(7),
C13−C14 1.467(8), C14−C15 1.470(8), C42−C43 1.482(7), C43−
C44 1.498(7).
which the K3O2B2 core contains potassium cation clusters
“wrapped” by two boroxide radical dianions. The cluster core
has approximately 3-fold rotational symmetry with the axis
bisecting the two O atoms (Figure S4). All three K+ ions are
involved in the cation-π bonding to anthracene units in a way
that each anthracene group accommodates two K+ ions. K1
and K2, while each being coordinated to one THF molecule,
bond to two oxygen atoms (O1 and O2) and form cation-π
bonds with anthracene and Mes groups from different
boroxide ligands. The third K+ center K3 also bonds to the
two oxygen atoms, but it is sandwiched between two
anthracene moieties with no interaction with the Mes groups.
Meanwhile, this coordination of K3 ion also causes the change
of dihedral angle of two anthracene units, which is up to 60.46°
(Figure S5). Regarding the structure of boroxide ligands, the
B1−O1 and B2−O2 bonds (1.295(6) and 1.308(6) Å)
undergo an obvious elongation going from complexes 2 to 3,
while the B1(or B2)−C(Mes) bond clearly shortens. But the
lengths of B1−O1 and B2−O2 bonds are still in the range of
double bonds. The structure of the anthracene subunit in
complex 3 exhibits a small difference relative to complex 2 (see
Table S2 for details). The skeleton remains planar with
experimental error, and the largest deviation of individual
centers from the mean plane amounts to about 0.08 Å.
Synthesis, Structure, and Characterization of Com-
plex 4. The reactivity of 3 was tested by adding 1.0 equiv of
D
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spectrometer. Elemental analyses for C, H, and N were carried out
with a German Elementary Vario EL cube instrument.
X-ray Crystallography. The data were collected with a
SuperNova (Dual) X-ray diffractometer equipped with Cu/Mo Kα
radiation (λ = 1.54184/0.71073 Å) at a different temperature (1-H,
295 K; 2 and 4, 173 K; 3, 292 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 SCALE3 ABSPACK scaling algo-
rithm.61,62 Crystals structures were solved by direct methods using
Olex 2−1.2. Subsequent difference Fourier analyses and least-squares
refinement with 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 on the crystallographic studies as well as
atomic displacement parameters are given in CIF files deposited with
the CCDC. The crystallographic details for compounds 1-H, 2, 3, and
4 are summarized in Table S1. The data have been deposited in the
Cambridge Crystallographic Data Centre (CCDC), deposition
numbers CCDC 1849078−1849081 for compounds 1-H, 2, 3, and
4. Although we attempted several times to measure the crystals of
dimer 2, the largest electron residual peak (1.31 e Å−3) still exists. We
also tried to refine it many times, but it still has not been solved.
However, this electron residual peak does not affect the geometry of
2. Despite repeated measurements, the quality of the crystals of 3 was
still not good enough due to the nature of twins. The uncertainties are
relatively large and only allow for a qualitative discussion.
Computational Details. All the geometry optimizations were
carried out at the (U)CAM-B3LYP/6-31G(d) level of theory. The
obtained stationary points were characterized by frequency calcu-
lations. The symmetry-broken approach was applied for open-shell
singlet calculations. The molecular orbitals, spin densities, and Wiberg
bond order were calculated at the level of (U)CAM-B3LYP/6-
31G(d) on the optimized geometries. The spin contamination errors
were corrected by the approximate spin-projection method66 when
evaluating the singlet−triplet energy gaps (ΔEOS−T). The Wiberg
bond order was calculated with the Multiwfn program.67 The UV−vis
absorption spectra were calculated using the time-dependent DFT
(TD-DFT) method at the (U)CAM-B3LYP/6-31G(d) level, and the
polarized continuum model (PCM) was adopted for solvent (THF)
effects. All calculations were performed with the Gaussian 09 program
suite.68
EPR Measurement. Solution EPR measurement was recorded at
both room temperature and 115 K with a Bruker ER200DSRC10/12
spectrometer. Samples containing 10−4 M (298 K) or 5 × 10−3 (115
K) of 3 were prepared in a J. Young EPR tube inside a glovebox.
Room temperature EPR spectrum was obtained with a microwave
power of 6.13 mW, modulation amplitude of 0.8 G, time constant of
81.92 ms, and sweep time of 75 s. The frozen EPR spectrum was
obtained with a microwave power of 2.51 mW, modulation amplitude
of 0.8 G, time constant of 81.92 ms, and sweep time of 75 s. The EPR
spectrum was simulated using EasySpin with hyperfine coupling
constants.
Synthesis of 9-(2-Bromophenyl)anthracene. To a mixture of
9-iodoanthracene (3.04 g, 10.0 mmol), 2-bromophenylboronic acid
(2.10 g, 10.5 mmol), Pd(PPh3)4 (0.289 g, 0.25 mmol), K2CO3 (2.76
g, 20.0 mmol) were added a dioxane/H2O (8:1, 45 mL) degassed
mixed solvent. The reaction mixture was stirred at 110 °C for 24 h.
Then, the organic layer was extracted with CH2Cl2 (3 × 40 mL) and
dried over MgSO4. Purification by flash column chromatography
(SiO2, hexane) yielded a colorless solid (1.39 g, 42%). M.p.: 136−138
°C. 1H NMR (CDCl3, 400 MHz): δ 8.44 (s, 1H), 7.96 (d, J = 8.0 Hz,
2H), 7.79 (d, J = 8.0 Hz, 1H), 7.49 (d, J = 8.0 Hz, 2H), 7.39 (m, 3H),
7.30 (m, 4H). 13C NMR (CDCl3, 100 MHz): δ 139.7, 135.6, 133.0,
132.9, 131.4, 129.9, 129.5, 128.6, 127.5, 127.3, 126.2, 125.9, 125.6,
125.3. Elemental analysis: Calcd for C20H13Br: C 72.09, H 3.93%;
Found: C 72.21, H 3.86%.
Figure 5. UV−vis-NIR spectra of compounds 1-H, 2, 3, and 4 in
THF solution (1-H and 2: 2 × 10−4 mol/L; 3 and 4: 6 × 10−4 mol/
L). In the spectrum of 4, there is a wide peak in the range of 600−850
nm, which is caused by the existence of a small quantity of diradical 3.
LUMO+1, whereas more complex transitions are found for 3
and 4 with π−π* transition being the main contributor.
CONCLUSION
■
In conclusion, we herein have described the stabilization of a
triplet diradical anion 3a−·· and its dihydro anion 4a−. The
former was synthesized through the reduction of dimer
boroxide complex 2, while the latter was prepared via directly
reducing the boronic compound 1-H. Both complexes 3 and 4
possess a similar K3O2B2 aggregate structure, but the
geometries of the anthracene units are different. The former
is planar while the latter has a puckered structure. According to
the experimental and computational results, a clear change in
the geometry and aromaticity of the anthracene moiety was
revealed from dimeric potassium boroxide complex 2 to
diradical anion 3a−·· to cluster anion 4a−. Further studies on
the chemical reaction of these reduced 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 was dried by 4 Å molecular sieve
(2−3 days). C6D6 and THF-d8 were dried over Na/K alloy without
benzophenone (2−3 days). Prior to use, the solvents were dried by
refluxing with sodium and benzophenone and degassed by applying
three freeze−pump−thaw cycles. The 9-iodoanthracene,58 mesityl-
boronic acid,59 and 2-mesityl-5,5-dimethyl-1,3,2-dioxaborinane60 were
synthesized according to literature methods, respectively. The 1H and
13C NMR spectra were performed using a JEOL JNM-ECS-400 or
INOVA 600NB (Bruker) at room temperature in ppm downfield
from Me4Si. The 11B NMR spectra were also recorded using a JEOL
JNM-ECS-400 at room temperature in ppm downfield from external
aqueous BF3·Et2O. The cyclic voltammetry experiment was
conducted in an argon-filled atmosphere using a CHI 760e
Electrochemical Workstation. Freshly distilled THF was used as
n
solvent, and Bu4NPF6 (10−1 M) was used as electrolyte. A standard
three-electrode cell configuration was employed using a glassy carbon
working electrode, a Pt wire counter electrode, and an saturated
calomel serving as the reference electrode. Formal redox potentials
were referenced to the ferrocene/ferrocenium redox couple [E(Fc/
Fc+) = 0 V]. The ESI mass spectra were recorded on a Thermo
Scientific Orbitrap Elite mass spectrometer (LTQ Orbittrap Elite).
The IR spectra were performed with a Nicolet 360 FTIR
spectrometer in the region 4000−400 cm−1 using KBr pellets. UV−
vis-NIR spectra were recorded on Agilent Carry 5000 UV−vis-NIR
Synthesis of (2-(Anthracen-9-yl)phenyl)(hydroxy)(mesityl)-
borane (1-H). 9-(2-Bromophenyl)anthracene (3.32 g, 10.0 mmol)
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was dissolved in fresh distilled THF (100 mL), and the solution was
cooled to −78 °C. nBuLi (6.90 mL, 11.0 mmol, 1.6 M in hexanes) was
added slowly to the solution, and the mixture was stirred at −78 °C
for 3 h. Then, 2-mesityl-5,5-dimethyl-1,3,2-dioxaborinane (2.32 g,
10.0 mmol) in THF (30 mL) was added, and the mixture was allowed
to warm to room temperature. Subsequently, the reaction was
quenched by 1.0 M aq. HCl (30 mL). The obtained biphasic mixture
was stirred for 2 h at room temperature. The organic layer was
collected, and the aqueous layer extracted with CH2Cl2 (3 × 50 mL).
All combined organic layers were dried over Na2SO4, and the solvent
was removed to obtain an orange oil. Purification by flash column
chromatography (SiO2, hexane/ethyl acetate, 20:1 and hexane/ethyl
acetate, 10:1) yielded 1-H as a white solid (1.60 g, 40%). M. p.: 184−
of the mixture solution changed from colorless to yellowish to dark-
blue to dark-red. Then the dark-red solution was filtered to remove
the dark insoluble substance. The filtrate was concentrated in vacuo,
and the dark-red solid was obtained. The crude product was washed
three times with hexanes, allowing for isolation of product 4. X-ray-
quality dark-red crystals of 4 were obtained in cooling THF solution.
Yield: 0.149 g (53%). M.p.: 81.8 °C (turn black). UV−vis-NIR
1
(THF): 477 nm. H NMR (THF-d8, 600 MHz): δ 7.33 (d, J = 6.0
Hz, 2H, Ar-H), 7.12 (t, J = 6.0 Hz, 2H, Ar-H), 7.02 (d, J = 6.0 Hz,
2H, Ar-H), 6.82 (t, J = 6.0 Hz, 2H, Ar-H), 6.74 (s, 4H, Mes-H), 6.33
(d, J = 6.0 Hz, 4H, Ar-H), 6.18 (t, J = 6.0 Hz, 4H, Ar-H), 5.60 (d, J =
6.0 Hz, 8H, Ar-H), 3.62 (t, J = 6.0 Hz, 12H, overlap of O−CH2-
(THF) and −CH2− (four hydrogens of C14 and C43)), 3.54 (s, 12H,
cryptand), 3.50 (t, J = 6.0 Hz, 12H, cryptand), 2.52 (t, J = 6.0 Hz,
12H, cryptand), 2.32 (s, 6H, Mes−CH3), 2.10 (s, 12H, Mes−CH3),
1.78 (t, J = 6.0 Hz, 8H, O−CH2−CH2-); 13C NMR (THF-d8, 150
MHz): δ 150.5, 142.2, 140.3, 137.7, 136.3, 133.5, 126.8, 129.7, 127.5,
126.7, 126.1, 124.0, 117.6, 112.9, 107.7, 99.6 (C7 and C36), 71.2
([2,2,2]-cryptand), 68.4 ([2,2,2]-cryptand), 68.0 (THF), 54.7
([2,2,2]-cryptand), 39.7 (C14 and C43), 26.2 (THF), 22.8 (Ar-
CH3), 21.3 (Ar-CH3); 11B NMR (128.3 MHz, THF-d8, ppm, 293 K):
δ 38.2 ppm. IR (KBr, cm−1): 3408 (br), 3042, 2963, 2883, 2817, 1657
(w), 1603, 1475, 1444, 1356, 1301, 1260, 1132, 1104 (s), 949, 844,
743. Calcd for C80H94B2K4N2O9·THF: C 68.27, H 6.96, N 1.90%;
Found: C 68.19, H 7.01, N 1.78%.
1
186 °C. UV−vis-NIR (THF): 333, 350, 368, and 388 nm. H NMR
(CDCl3, 400 MHz): δ 8.50 (s, 1H, Ar-H), 8.04 (d, J = 8.0 Hz, 2H, Ar-
H), 7.80 (d, J = 8.0 Hz, 1H, Ar-H), 7.64 (td, J = 7.2, 1.6 Hz, 1H, Ar-
H), 7.57 (s, 1H, Ar-H), 7.55 (s, 1H, Ar-H), 7.45−7.53 (m, 3H, Ar-H),
7.31−7.37 (m, 3H, Ar-H), 6.58 (s, 2H, Mes-H), 5.22 (s, 1H, B−OH),
2.19 (s, 3H, Mes−CH3), 1.95 (s, 6H, Mes−CH3); 13C NMR (CDCl3,
100 MHz): δ 144.3, 138.8, 137.3, 136.8, 131.9, 131.5, 130.6, 128.7,
127.8, 127.3, 127.1, 126.8, 126.0, 125.5, 22.2 (Ar-CH3), 21.3 (Ar-
CH3); 11B NMR (128.3 MHz, CDCl3, ppm, 293 K): δ 48.2 ppm. IR
(KBr, cm−1): 3513 (s), 3048, 2906, 2854, 1943 (w), 1822 (w), 1722,
1603, 1557, 1439, 1363, 1557, 1439, 1363, 1326, 1285 (s), 1160,
1113, 1069, 944, 891, 848, 800, 737, 666. MS (ESI): calculated for
[C29H25BO + Na]+: 423.1891; found: 423.1893. Elemental analysis:
Calcd for C29H25BO: C 87.01, H 6.29%; Found: C 86.89, H 6.17%.
Synthesis of {[K(THF)]+·[1−]}2 (2). Under anaerobic and
anhydrous conditions, compound 1-H (0.20 g, 0.50 mmol) was
dissolved in THF (∼35 mL) at room temperature. KN(SiMe3)2 (0.52
mL, 0.52 mmol, 1.0 M in hexanes) was added slowly to the solution,
and the mixture was stirred for 3 h. The yellowish solution was filtered
to remove the insoluble substance. Then the filtrate was concentrated
in vacuo, and the yellowish solid was obtained. The crude product was
washed three times with hexanes, allowing for isolation of product 2.
X-ray-quality yellow crystals of 2 were obtained in THF solution at
room temperature. Yield: 0.454 g (89%). M. p.: 169−171 °C. UV−
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b02157.
■
S
1H NMR, 13C NMR, and 11B NMR spectra of the
compounds 1-H, 2, 3, and 4, computational details
(PDF)
Accession Codes
1
CCDC 1849078−1849081 (compounds 1-H, 2, 3, and 4)
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge via www.ccdc.ca-
m.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.
vis-NIR (THF): 335, 351, 369, and 389 nm. H NMR (C6D6, 400
MHz): δ 7.96 (s, 1H, Ar-H), 7.68 (d, J = 8.0 Hz, 2H, Ar-H), 7.63 (d, J
= 12 Hz, 2H, Ar-H), 7.46 (d, J = 4.0 Hz, 1H, Ar-H), 7.38 (t, J = 8.0
Hz, 1H, Ar-H), 7.30 (t, J = 8.0 Hz, 1H, Ar-H), 7.12 (d, J = 8.0 Hz,
2H, Ar-H), 7.06 (d, J = 8.0 Hz, 1H, Ar-H), 6.95 (t, J = 8.0 Hz, 2H, Ar-
H), 6.17 (s, 2H, Mes-H), 3.55 (t, J = 8.0 Hz, 8H, O−CH2-), 2.14 (s,
3H, Mes−CH3), 1.41 (m, 14H, overlap of O−CH2−CH2- and Mes−
CH3); 13C NMR (C6D6, 100 MHz): δ 148.9, 146.8, 143.4, 142.7,
137.2, 136.6, 134.4, 132.1, 131.8, 131.0, 129.1, 128.1, 127.8, 126.3,
125.4, 125.2, 68.4 (THF), 26.4 (THF), 23.5 (Ar-CH3), 21.8 (Ar-
CH3); 11B NMR (128.3 MHz, C6D6, ppm, 293 K): δ 38.0 ppm. IR
(KBr, cm−1): 3431 (br), 3045, 2918, 2861, 1939 (w), 1613, 1439 (s),
1411, 1153, 1060, 943, 882, 848, 801, 741 (s), 673. Calcd for
C66H64B2K2O4·THF: C 76.91, H 6.64%; Found: C 76.92, H 6.49%.
Synthesis of [K(cryptand)]+·3a−·· (3). Under anaerobic and
anhydrous conditions, a mixture of 2 (0.204 g, 0.20 mmol), [2,2,2]-
cryptand (0.075 g, 0.20 mmol) and potassium chips (0.017 g, 0.44
mmol) in THF (∼30 mL) was stirred at room temperature. The color
of the mixture solution changed from yellowish to dark-blue. Then the
blue solution was filtered to remove the dark insoluble substance. The
filtrate was concentrated in vacuo, and the dark-blue solid was
obtained. The crude product was washed three times with hexanes,
allowing for isolation of product 3. X-ray-quality dark-blue crystals of
3 were obtained in THF solution at room temperature. Yield: 0.209 g
(71%). M.p.: 80 °C (decompose). UV−vis-NIR (THF): 556, 604,
667, and 758 nm. IR (KBr, cm−1): 3430 (br), 3040, 2963, 2880 (s),
1599 (w), 1444 (s), 1356, 1302, 1257, 1103 (s), 943, 840, 741. Calcd
for C84H100B2K4N2O10: C 68.37, H 6.83, N 1.90%; Found: C 68.13, H
6.69, N 1.79%.
AUTHOR INFORMATION
Corresponding Authors
*(X.P.) E-mail: boxb@lzu.edu.cn.
■
*(Xingyong Wang) E-mail: xingyong@uow.edu.au.
*(X.L.) E-mail: xiaoxialv0635@163.com.
ORCID
Xingyong Wang: 0000-0002-6047-8722
Xiaobo Pan: 0000-0002-5757-2339
Jincai Wu: 0000-0002-8233-2863
Present Address
No. 222, Tianshui Road, Lanzhou City, Gansu Province,
730000, P. R. China.
Funding
This work has been supported by the National Natural Science
Foundation of China (No. 21771094 and 21671087) and the
Fundamental Research Funds for the Central Universities
(lzujbky-2017-k07). We 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.
Synthesis of [K(cryptand)]+·4a− (4). Under anaerobic and
anhydrous conditions, a mixture of 1-H (0.160 g, 0.40 mmol), [2,2,2]-
cryptand (0.075 g, 0.20 mmol) and potassium chips (0.033 g, 0.84
mmol) in THF (∼20 mL) was stirred at room temperature. The color
F
DOI: 10.1021/acs.inorgchem.8b02157
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
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Notes
The authors declare no competing financial interest.
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DOI: 10.1021/acs.inorgchem.8b02157
Inorg. Chem. XXXX, XXX, XXX−XXX