Modulating the M2δ-to-ligand charge transfer transition by the use of diarylboron substituents

Article abstract of DOI:10.1039/c3dt51684a

From the reactions between the quadruply bonded complexes M ₂(TⁱPB)₄, where M = Mo or W and TⁱPB = 2,4,6-triisopropylbenzoate, and the carboxylic acids HOOC-C₆H ₄-4-B(mesityl)₂,

Full text of DOI:10.1039/c3dt51684a

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Modulating the M2δ-to-ligand charge transfer
transition by the use of diarylboron substituents†
Cite this: DOI: 10.1039/c3dt51684a
Samantha E. Brown-Xu, Malcolm H. Chisholm,* Christopher B. Durr,
Thomas F. Spilker and Philip J. Young
From the reactions between the quadruply bonded complexes M2(TiPB)4, where M = Mo or W and TiPB =
2,4,6-triisopropylbenzoate, and the carboxylic acids HOOC–C6H4–4-B(mesityl)2, LH (2 equivalents) the
complexes trans-M2(TiPB)2L2 have been prepared. The new compounds have been characterized by 1H
NMR, MALDI-TOF MS, UV-Vis-NIR and steady-state emission spectroscopy, time-resolved transient absorp-
tion spectroscopy and cyclic voltammetry. These results are compared with the related properties of the
benzoates, M2(TiPB)2(O2CPh)2 (prepared similarly) and with DFT calculations on model compounds
where formate substitutes for TiPB. The new compounds M2(TiPB)2L2 are intensely colored in toluene or
THF solutions: red (M = Mo) and green (M = W) and the introduction of the p-B(mesityl)2 group notably
shifts these metal to ligand charge transfer transitions to lower energy in comparison to the benzoate
complexes M2(TiPB)2(O2C–C6H5)2. Upon the addition of fluoride ions these intense absorptions are
shifted to much higher energy in a reversible manner for M = Mo.
Received 24th June 2013,
Accepted 14th August 2013
DOI: 10.1039/c3dt51684a
www.rsc.org/dalton
cyanoacrylate,5 α,α′-thienylcarboxylates6 or
carboxylate ester.7 In these compounds the trans-L–M2–L geo-
a 2,6-azulene-
Introduction
Arylboron compounds have attracted considerable attention metry as seen in Fig. 1 allows for extensive Lπ–M2δ–Lπ
for their photophysical properties.1 Notably, the strong emis- conjugation which results in the extended planar structure
sion spectra associated with arylboranes can be quenched being thermodynamically favored.
upon addition of ions such as F− or CN− which can bind to
A simple frontier orbital energy diagram is shown in Fig. 2,
the Lewis acidic center.2,3 Their spectral features can thus be which shows how the M2δ orbital interacts with only the out-
employed in sensors and various other applications where of-phase π* combination of the ligand π* systems causing a
fluorescence is sensitive. The molecule referred to as BODIPY, splitting of the in- and out-of-phase combinations of the Lπ*.
shown below, is a prime example of an arylboron compound In D2h symmetry the M2δ to in-phase Lπ* combination, which
showing such spectral features.
is typically the HOMO → LUMO transition, is a fully allowed
MLCT transition and this occurs in the visible or NIR region of
Work in this lab has recently focused on the photophysical
properties of carboxylate ligands attached to MM quadruply
bonded centers and in particular to the metal-to-ligand charge
transfer states of compounds of the form trans-M2(TiPB)2L2
where L = a π-acceptor ligand such as cyanobenzoate,4
Department of Chemistry and Biochemistry, The Ohio State University, 100 West
18th Avenue, Columbus, Ohio 43210-1185, USA.
E-mail: chisholm@chemistry.ohio-state.edu; Fax: +1 614 292 0368;
Tel: +1 614 688 3525
†Electronic supplementary information (ESI) available. CCDC 943288. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c3dt51684a
Fig. 1 Molecular structure of W2(TiPB)2(Azu-2)2 with the anisotropic displace-
ment parameters drawn at the 50% probability level.7
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Fig. 2 M2δ orbital interaction with the in-phase and out-of-phase combi-
nations of the ligand π* orbitals.
the spectrum. We are interested in investigating the influence
of introducing a BAr2 group to a simple π-acceptor ligand such
as benzoate and we report here results that were prompted by
this line of inquiry.
Fig. 3 ORTEP representation of Mo2(TiPB)2(Bz)2 (sea green = molybdenum,
scarlet = oxygen, gray = carbon) drawn at 50% probability. Hydrogens, solvent
and disorder excluded for clarity.
Results and discussion
Syntheses
The synthesis of the boron containing carboxylic acid and its
reaction to form the new compounds trans-M2(TiPB)2L2, where
M = Mo, compound I and M = W, compound II, are shown in
Scheme 1.
phenyl unit for all complexes in their ground-state and further
that the orientation of the B(mesityl)2 unit will be such that
allows significant B 2pπ–benzoate π conjugation. The mesityl
groups, because of their steric influence, will undoubtedly be
twisted somewhat out of conjugation with the vacant B 2pπ orbital.
The benzoate complexes trans-M2(TiPB)2(Bz)2, where Bz =
O2CPh, were similarly prepared from the reactions between
M2(TiPB)4 and benzoic acid (2 equivalents) in toluene. The
new compounds have been characterized by MALDI-TOF mass
spectrometry and 1H NMR spectroscopy as well as by several
other spectroscopic techniques described subsequently.
Although no single-crystal X-ray determination has been
carried out in this study for compounds I and II, there is ample
precedent for the structures of the new compounds. Further-
more, the structure of the compound, Mo2(TiPB)2(Bz)2, which
can be viewed as the parent compound to I is reported below
(Fig. 3). We anticipate a planar central phenyl –CO2–M2–CO2–
Crystal and molecular structure of Mo2(TiPB)2(Bz)2
The benzoate compound, Mo2(TiPB)2(Bz)2, crystallized on a
center of inversion located in the middle of the Mo2 bond. The
dihedral angle associated with the CO2 and the TiPB moiety is
∼96° which effectively removes the ring from conjugation with
the metal center. The dihedral angle associated with the CO2
and the phenyl ring of the benzoate moiety however is less
than 9° which allows for considerable conjugation with the
metal center. The solvent of crystallization, THF, is
coordinated axially along the Mo2 bond, and the remainder of
the structure is similar to M2 tetracarboxylates previously
reported.4–7 The structure was, however, significantly dis-
ordered and was modeled accordingly. Further refinement
details can be found in the ESI† and in the Crystallographic
Information File (CIF).
Electronic structure calculations
In order to aid in the interpretation of the photophysical pro-
perties of the new compounds, we have employed density func-
tional theory DFT and time-dependent DFT as provided by the
Gaussian09 program suite. In order to simplify these calcu-
lations and save on computational time and resources, we
have substituted formate for the bulky TiPB ligands since in all
known structures of the form trans-M2(TiPB)2L2 the aryl group
of the TiPB ligands are twisted close to 90° with respect to the
carboxylate plane in the aryl rings are not in conjugation with
Scheme 1 The synthesis of “bis–bis” model complex M2(TiPB)2(BMP)2.
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the carboxylate. Thus the approximation or simplification
The calculations reveal two interesting points. (1) In
adopted by the model compounds is not so critical. Also in the relation to the benzoates, the introduction of boron as BH2 in
calculations on the model compounds I′ and II′ that contain the para-position stabilizes the HOMO, the M2δ orbital, by
boron we have replaced the mesityl groups by hydrogen atoms ∼0.35 eV. (2) The introduction of the BH2 group greatly lowers
with a similar line of reasoning. This assumption was investi- the energy of the two ligand based π* orbitals. Indeed, for
gated by calculating the simulated UV-Vis spectrum of the molybdenum both the in-phase and out-of-phase Lπ* orbitals
molybdenum compound with and without the mesityl substi- drop below the Mo2δ* orbital which is predicted to be the
tution. The MLCT band differed by only
makes this a reasonable approximation. Similar calculations
were performed on the model benzoate complexes trans- its Mo2 counterpart and that the HOMO–LUMO gap is smaller
Mo2(O2CH)2(O2CC6H5)2. for the tungsten model compound. Since the HOMO → LUMO
3
nm which LUMO of the benzoate complex.
We also see that the W2δ orbital lies higher in energy than
The calculated frontier molecular orbital energy diagram is a fully allowed electronic transition, a metal to ligand charge
for the molybdenum model complex I′ is shown in Fig. 4 along transfer, 1MLCT, we anticipate that the absorption spectrum of
with a comparison for the benzoate complex and a similar the tungsten complex will be red-shifted to its molybdenum
diagram for the tungsten complex II′ and its benzoate ana- counterpart and the ease of electrochemical oxidation will
logue is given in Fig. 5.
follow in the order II > I.
Electronic absorption and steady state emission spectra
The electronic absorption spectra of the compounds I and II
recorded in THF are shown in Fig. 6 along with a comparison
with their respective benzoates. As predicted by the electronic
structure calculations the MLCT transitions for both com-
plexes are significantly red-shifted in comparison to their
benzoate counterparts. The intense broad absorption of the
tungsten complex II with its λmax = 755 nm is entirely consist-
ent with its green colored solutions as is the purple-red color
of the molybdenum complex I with λmax = 521 nm. The tung-
sten complex II shows an electronic transition at 400 nm
which is assignable to the W2δ to CO2 π* transition associated
with the TiPB ligands. To higher energy both I and II show
bands arising from ligand π to π* transitions.
The breadth of the 1MLCT transition in part reflects that an
ensemble of rotamers will be present in solution involving
various dihedral angles between the O2C–phenyl-B 2pπ units
due to thermal energy which can overcome the thermodynami-
cally favored planar structure. Since the motion of the two
trans ligands are not correlated, photoexcitation to the MLCT
state is likely to be accompanied by a dipole change and we do
indeed observe a significant solvent dependence in the 1MLCT
Fig. 4 Frontier molecular orbital energy level diagram of model compound I’
(right) and the parent benzoate compound Mo2(TiPB)2(Bz)2 (left) (Bz
O2CC6H5).
=
Fig. 5 Frontier molecular orbital energy level diagram of compound II (right)
and the parent benzoate compound W2(TiPB)2(Bz)2 (left) (Bz = O2CC6H5).
Fig. 6 Electronic spectra of compounds I and II with their corresponding
parent benzoate compounds in THF.
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absorption spectra in toluene and THF (see Fig. S1 in ESI†).
The steady state emission spectrum shows less of a solvent
dependence although compound I is only weakly emissive
from its S1 state in toluene. The singlet emission spectrum for
compound II regrettably falls too close to the limits of our
instrumentation for a reliable comparison.
Both I and II have longer lived triplet states as determined
by time-resolved spectroscopy, vide infra, but only I shows
phosphorescence at 1080 nm which is in relation to the car-
4+
3
boxylates of Mo2 assignable to emission from the MoMoδδ*
state (see Fig. S2 in ESI†). Furthermore the lifetime of the T1
emission is 66 μs (see below) which is notably longer than the
lifetimes of 3MLCT states. No phosphorescence is observed for
compound II which presumably relaxes by non-radiative decay
processes that are favored by the anticipated lower energy of
the T1 state according to the energy gap law.
Fig. 7 fs-TA spectrum of I in THF at RT, λex = 568 nm.
independent. The ground state bleach remains at long times
indicating the formation of a triplet state. The lifetime was
found to be 65.9 μs by nanosecond transient absorption spec-
troscopy. These spectral features are shown in Fig. 7 and S9.†
Unfortunately for the tungsten complex, the transient
signals for the singlet and triplet states lie very close to each
other making the kinetics difficult to discern. The peak at
565 nm is attributable to the singlet excited state and has a
lifetime of 15 ps. The isosbestic point at 435 nm indicates the
formation of a triplet excited state. The lifetime was found to
be 75.3 ns by nanosecond transient absorption spectroscopy.
These spectra are shown in the ESI.†
Electrochemical studies
Both compounds I and II dissolved in THF show a reversible
or quasi-reversible oxidation wave in their cyclic voltammo-
grams. These oxidation waves are moved to higher potential
relative to their related benzoates. In each case the oxidation
potential of the tungsten complex is notably less than that of
the molybdenum complex consistent with the calculations that
show the Mo2δ orbitals to be stabilized relative to their W2
counterparts.
The compounds I and II also show reduction waves within
the solvent window. The reduction wave for the molybdenum
complex is quasi-reversible on the time scale of the CV experi-
ment and these reductions correspond to placing an electron
in the LUMO which is the in-phase π* combination of the
O2C–C6H4–4-B(mesityl)2 ligands. From the electrochemical data
we can estimate the HOMO–LUMO gap and this is compared
with that calculated from the UV-Vis spectra in Table 1.
The influence of fluoride ions
As noted earlier, arylboron compounds have been extensively
used in the detection of F− which has a high affinity for coordi-
nation to the 3-coordinate boron center. In the present case a
THF solution of the molybdenum compound I was titrated
with equivalents of [Bu4N+][F−] in THF solution. As shown in
Fig. 8, the intense MLCT absorption at 521 nm is completely
bleached as the B 2p π-acceptor attached to the benzoate
ligand is converted to the –C6H4–B(mesityl)2F− anion. The
weak absorption at 400 nm corresponds to the new Mo2δ to
–O2C–C6H4– transition.
Time-resolved spectroscopic studies
The excited state of both compounds I and II have been exam-
ined by femtosecond transient absorption (fs-TA) spectroscopy
which has allowed us to determine the lifetime of the singlet
state for the molybdenum complex as 12 and 13 ps in THF and
toluene, respectively.
For compound I, we also see a feature at 615 nm in toluene
and 650 nm in THF that corresponds to the singlet stimulated
emission and has a lifetime of 13 ps in toluene. The red shift
of this feature in going from toluene to THF supports the
solvent dependence observed in the steady state emission
spectra. The singlet excited state, however, is solvent
Upon addition of water the F− anion becomes solvated as
can be seen in the spectra shown in Fig. 9 which show the res-
toration of the MLCT band at 520 nm. The full intensity of this
Table 1 The HOMO–LUMO energy gap obtained from the electrochemical
experiments and UV-Visible spectra of compounds I and II are compared
E1/2(OX)a
E1/2(RED)a
Egap
UV-Visb
a
Compound
I
II
0.58
−0.48
−1.86
−2.24
2.44
1.76
2.35
1.63
a Volts; E1/2 values are referenced to the Cp2Fe+/0 couple. b eV.
Fig. 8 Absorption spectrum of I in THF titrated with fluoride ion.
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Electronic absorption spectra
Steady-state photophysical measurements were carried out
with 10.0 × 10.0 mm2 quartz cuvettes equipped with Kontes
stopcocks. Electronic absorption spectra at room temperature
were recorded using a Perkin-Elmer Lambda 900 spectrometer
in THF solution.
Femtosecond transient absorption (TA) experiments were
performed using laser and detection systems described pre-
viously.10 Samples were prepared with absorbance ∼0.3–0.8 at
the excitation wavelengths, which were 568 nm for I in THF,
514 nm for I in toluene, and 800 nm for II in THF. Excitation
power at the sample was ∼1–2 µJ. The sample for II was kept in
constant motion during the experiment by manual movement
of an XYZ stage to prevent photodecomposition. The TA
measurements at a series of pump-probe delay times were per-
formed three times to ensure reproducibility of the results.
Spectra collected underwent wavelength calibration and GVD
corrections.
Fig. 9 The absorption spectrum of I in THF first quenched by the addition of
fluoride ion and then regenerated by the addition of water.
transition is not restored because the solution has been
diluted.
The influence of added F− on the MLCT of the tungsten
complex II is similar and is shown in Fig. S13 in ESI.† However,
addition of water does not restore the original MLCT and the
compound appears to be decomposed in water with time.
Nanosecond TA was performed on compounds I and II in
1 × 1 cm square quartz cuvettes with Kontes stopcocks.
Measurements were made on
a home-built instrument
pumped by a frequency doubled for I (532 nm) or tripled for II
(355 nm) Spectra-Physics GCR-150 Nd:YAG laser (fwhm ∼8 ns,
∼5 mJ per pulse). Signal from a Hamamatsu R928 photomulti-
plier tube was processed with a Tektronics 400 MHz oscillo-
scope (TDS 380).11
In general, kinetics were fit to a sum of exponential decay
terms, S(t) = ∑iAi exp(−1/τi) + C using Igor Pro 6.0 or SigmaPlot
12.0, where Ai is the amplitude, τ is the lifetime, and C is an
offset. Error bars for the lifetimes are reported as standard
errors of the exponential fits.
Conclusions
The new compounds I and II show intense MLCT bands in the
visible region of the spectrum which are notably red shifted
compared to the related trans-M2(TiPB)2(O2CC6H5)2 due to the
influence of the introduction of the B 2pπ acceptor orbital of
the B(mesityl)2 groups. The influence of the vacant 2pπ
B
orbital is also seen in the calculations on the model com-
pounds. The introduction of F− ions quenches this intense
MLCT absorption in a striking manner in THF solutions. For
compound I the addition of water allows the solvation of fluor-
ide and the compound is regenerated as evident by the resto-
ration of the original MLCT.
Emission spectra
Fluorescence measurements were made on a SPEX Fluoro-
max-2 spectrofluorimeter in the UV-visible region. Emission
measurements in the near-infrared region were performed in
J. Young NMR tubes at room temperature and 77 K in 2-methyl
THF. Spectra were recorded on a home-built instrument
equipped with a germanium detector. All complexes were
excited at 405 nm with a RG830 long pass filter between
sample and detector.
Experimental section
All reactions and subsequent manipulations of the compounds
were performed under one atmosphere of oxygen-free UHP-
grade argon using standard Schlenk techniques or in a glove
box with a dry and oxygen-free nitrogen atmosphere using
standard techniques. Prior to use, all solvents were dried
appropriately, distilled, degassed, and stored in flasks
equipped with Kontes taps over activated 4 Å molecular sieves
under an argon atmosphere. All chemicals were used as
received. The starting materials, M2(TiPB)4; M = Mo, W, were
first prepared according to established procedures.8,9 1H NMR,
MALDI-TOF, and UV-vis data for the compounds are provided
below.
Electrochemical studies
Cyclic voltammograms were collected at a scan rate of 100 mV
s−1 using a Pine Research Instrumentation Wavenow potentio-
stat-galvanostat. The measurements were performed under a
n
nitrogen atmosphere in a 0.1 M solution of Bu4NPF6 in THF
using platinum working and platinum auxiliary electrodes pat-
terned on ceramic, with a silver wire pseudo reference elec-
trode (Pine Research Instrumentation). The potentials for all
complexes are referenced to the Cp2Fe/Cp2Fe+ couple.
NMR
Mass spectrometry
NMR spectra were recorded on a 400 MHz Bruker DPX
1
Advance 400 spectrometer. All H NMR chemical shifts are in MALDI-TOF was performed on a Bruker Reflex III mass
ppm relative to the protio impurity in THF-d8 at 1.73 ppm.
spectrometer operated in a reflective, positive ion mode with
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an N2 laser. The matrix was prepared as a concentrated solu- under vacuum and 4-(bromophenyl)dimesitylborane was
tion of dithranol in THF. A small amount of the samples were obtained as a white powder by column chromatography with
1
dissolved in the matrix solution, then spotted onto a MALDI hexanes as eluent. Yield: 0.567 g (70%). H NMR (CDCl3): δH
plate with a pipette and allowed to dry.
(250 MHz) 7.49 (d, 2H, JHH = 8 Hz), 7.36 (d, 2H, JHH = 8 Hz),
6.82 (s, 4H), 2.31 (s, 6H), 1.99 (s, 12H) ppm.
Electronic structure calculations
4-(Bromophenyl)dimesitylborane (0.500 g, 1.23 mmol) was
The model complexes were optimized in the gas-phase using dried for 2 h under vacuum on a Schlenk line, dissolved in
density functional theory (DFT) utilizing the Gaussian09 suite 20 mL THF and cooled to −78 °C followed by a dropwise
of programs.12,13 The B3LYP functional was used in conjunc- addition of n-butyl lithium (2.5 M, 0.49 mL, 1.23 mmol) in
tion with the SDD energy consistent pseudopotentials for hexanes. Solution was allowed to stir for 30 min (turned red)
molybdenum and tungsten and the 6-31G* basis set for H, C, then carbon dioxide was passed through a drying tube filled
B, S and O atoms.14,15 Vibrational frequency analysis was used with anhydrous calcium sulfate and bubbled through the reac-
to confirm that the optimized structures were minima on the tion solution for 2 h (turned cloudy white). The flask was
potential energy surface. GaussView plots are shown with an opened to air and the product was extracted with dichloro-
isovalue of 0.02.16 Electronic absorption spectra were calcu- methane after acid workup followed by a recrystallization with
1
lated using the time dependent DFT (TD-DFT) method.
hexanes to give a white powder. Yield: 0.310 g (67%). H NMR
(CDCl3): δH (250 MHz) 8.05 (d, 2H, JHH = 8 Hz), 7.59 (d, 2H,
JHH = 8 Hz), 6.83 (s, 4H), 2.32 (s, 6H), 1.98 (s, 12H) ppm.
Mo2(TiPB)2L2 (I) L–H (0.070 g, 0.189 mmol) and Mo2(TiPB)4
Crystallographic information
Single crystals of Mo2(TiPB)2(Bz)2 were isolated as bright yellow
crystals and handled under a pool of fluorinated oil. Examin- (0.112 g, 0.095 mmol) were dissolved in toluene and allowed to
ation of the diffraction pattern was done on a Nonius Kappa stir for 3 days yielding a red solution. The solvent was removed
CCD diffractometer with Mo Kα radiation. All work was done in vacuo and the resulting orange solid was washed 3 times
1
at 150 K using an Oxford Cryosystems Cryostream Cooler. Data with 20 mL of hexanes and dried. (Yield: 65%) H NMR (THF-
integration was done with Denzo, and scaling and merging of d8): δH (400 MHz) 8.36 (d, 4H, JHH = 8 Hz), 7.68 (d, 4H, JHH
=
the data was done with Scalepack.17 The structures were solved 8 Hz), 7.00 (s, 4H), 6.86 (s, 8H), 3.03 (m, 2H), 2.88 (m, 4H),
by the direct methods program in SHELXS-13.18 Full-matrix 2.30 (s, 12H), 2.08 (s, 24H), 1.24 (d, 12H, JHH = 7 Hz), 1.04 (d,
least-squares refinements based on F2 were performed in 24H, JHH = 7 Hz) ppm. UV-Vis: (in THF at 293 K) 521 nm,
SHELXL-13,18 as incorporated in the WinGX package.19 For 322 nm; (in toluene at 293 K) 488 nm, 323 nm; (in pyridine at
each methyl group, the hydrogen atoms were added at calcu- 293 K) 541 nm, 440 nm. MALDI-TOF: Calculated: 1432.6.
lated positions using a riding model with U(H) = 1.5Ueq Found: 1432.3 (M+).
(bonded carbon atom). The rest of the hydrogen atoms were
W2(TiPB)2L2 (II) L–H (0.080 g, 0.216 mmol) and W2(TiPB)4
included in the model at calculated positions using a riding (0.147 g, 0.108 mmol) were dissolved in toluene and allowed to
model with U(H) = 1.2Ueq (bonded atom). Neutral atom scatter- stir for 3 days yielding a blue solution. The solvent was
ing factors were used and include terms for anomalous removed in vacuo and the resulting blue solid was washed 3
1
dispersion.20
times with 20 mL of hexanes and dried. (Yield: 55%) H NMR
The crystal was largely disordered, particularly in the TiPB (THF-d8): δH (400 MHz) 8.13 (d, 4H, JHH = 8 Hz), 7.66 (d, 4H,
moiety which was modeled in two locations and restrained JHH = 8 Hz), 6.99 (s, 4H), 6.86 (s, 8H), 3.01 (m, 2H), 2.83 (m,
using the SAME, EXYZ and EADP commands. The same pro- 4H), 2.30 (s, 12H), 2.07 (s, 24H), 1.23 (d, 12H, JHH = 7 Hz), 0.99
cedure was invoked in the disordered solvent THF which was (d, 24H, JHH = 7 Hz) ppm. UV-Vis: (in THF at 293 K) 754 nm,
coordinated axially along the Mo2 bond. Furthermore, it was 402 nm, 322 nm. MALDI-TOF: Calculated: 1604.7. Found:
necessary for the anisotropic U values of this disordered 1604.0 (M+).
solvent to be restrained using the SIMU command.
CCDC 943288 contains the supplementary crystallographic Mo2(TiPB)4 (0.208 g, 0.177 mmol) were dissolved in toluene
Mo2(TiPB)2(Bz)2 Bz–H (0.041 g, 0.339 mmol) and
data for this paper.
and allowed to stir for 3 days yielding an yellow solution. The
solvent was removed in vacuo and the resulting yellow solid
was washed 3 times with 20 mL of hexanes and dried. (Yield:
Syntheses
4-(Dimesitylboranyl)benzoic acid L–H. Synthesis modified 40%) 1H NMR (THF-d8): δH (400 MHz) 8.38 (d, 4H, JHH = 6 Hz),
from references.21 Dibromobenzene (0.472 g, 2.00 mmol) was 7.56 (d, 4H, JHH = 6 Hz), 6.99 (s, 4H), 3.03 (m, 2H), 2.87 (m,
dried for 2 h under vacuum on a Schlenk line, dissolved in 4H), 1.23 (d, 12H, JHH = 7 Hz), 1.02 (d, 24H, JHH = 7 Hz) ppm.
15 mL THF and cooled to −78 °C followed by a dropwise UV-Vis: (in THF at 293 K) 429 nm, 339 nm. MALDI-TOF: Calcu-
addition of n-butyl lithium (2.3 M, 0.87 mL, 2.00 mmol) in lated: 932.2. Found: 932.7 (M+).
hexanes. Solution was allowed to stir for 30 min (turned
W2(TiPB)2(Bz)2 Bz–H (0.041 g, 0.399 mmol) and W2(TiPB)4
yellow) then a solution of dimesityl boron fluoride (0.536 g, (0.230 g, 0.169 mmol) were dissolved in toluene and allowed to
2.00 mmol) in 20 mL Et2O was then added slowly to the reac- stir for 3 days yielding a red solution. The solvent was removed
tion flask. The flask was kept at −78 °C for 2 h then allowed to in vacuo and the resulting red solid was washed 3 times with
reach room temperature overnight. The solvent was removed 20 mL of hexanes and dried. (Yield: 35%) 1H NMR (THF-d8):
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δH (400 MHz) 8.16 (d, 4H, JHH = 8 Hz), 7.71 (d, 4H, JHH = 8 Hz),
8 F. A. Cotton, L. M. Daniels, E. A. Hillard and C. A. Murillo,
Inorg. Chem., 2002, 41, 1639–1644.
6.98 (s, 4H), 3.01 (m, 2H), 2.84 (m, 4H), 1.22 (d, 12H, JHH
=
7 Hz), 0.96 (d, 24H, JHH = 7 Hz) ppm. UV-Vis: (in THF at 293 K)
593 nm, 557 nm, 405 nm. MALDI-TOF: Calculated: 1108.3.
Found: 1107.8 (M+).
9 B. G. Alberding, M. H. Chisholm, Y.-H. Chou, J. C. Gallucci,
Y. Ghosh, T. L. Gustafson, N. J. Patmore, C. R. Reed and
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Acknowledgements
We thank the National Science Foundation for financial
support from grant no. 0957191, and the Ohio Super Compu-
ter Center for computational resources. S.E.B.-X. acknowledges
support from the NSF GRFP. This paper is dedicated to Pro-
fessor Richard Walton on his 75th birthday.
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