An isolable radical anion based on the borole framework

Article abstract of DOI:10.1002/anie.201108632

Give me five! A new boron-centered radical anion based on the borole framework is an example of a cyclic, planar, and conjugated π system hosting five electrons. Single-crystal X-ray analysis, EPR spectroscopy, and DFT methods indicate that the unpaired spin is delocalized within the borole ring. Despite steric shielding by the mesityl group, the boron atom is still available for radical-type reactions. Copyright

Full text of DOI:10.1002/anie.201108632

Angewandte
Chemie
DOI: 10.1002/anie.201108632
Boron Radicals
An Isolable Radical Anion Based on the Borole Framework**
Holger Braunschweig,* Vladimir Dyakonov, J. Oscar C. Jimenez-Halla, Katharina Kraft,
Ivo Krummenacher, Krzysztof Radacki, Andreas Sperlich, and Johannes Wahler
The element boron is known to have a variety of ways to
relieve its inherent electron deficiency. The acceptance of an
electron pair (Lewis acidity) has applications in catalysis[1]
and activation of element–element bonds (frustrated Lewis
pairs).[2] The combination of boron with p-donating substitu-
ents (e.g. BF3) and its incorporation into organic p-conjugated
systems allows the empty pz orbital of boron to participate in
p bonding and p conjugation, respectively, and the latter
enables the use of boron in optoelectronic materials with
unique properties.[3] The absence of p-donating substituents
at the boron center may result in multiple-center bonding to
form nonclassical frameworks (e.g. B2H6 or clusters). In
addition, organoboranes and -diboranes(4) are prone to
accept a single electron by chemical reduction.[4] Likewise,
hydrogen atom abstraction from N-heterocyclic carbe-
ne(NHC)-stabilized boranes (NHC-BH3) can lead to neutral,
persistent[4f] boryl radicals of the type NHC-BH2C,[5] which
have been studied by means of cyclic voltammetry, EPR, and
UV/Vis spectroscopy as well as trapping reactions.[4–6] How-
ever, examples of isolated boron radicals are rare owing to the
reactive nature of the species, and only little is known about
their structural properties. Steric protection of the boron
center combined with spin delocalization over the organic
substituents, both achieved by substitution with mesityl
groups (Mes = 2,4,6-trimethylphenyl), has occasionally ena-
bled isolation and structural characterization of radical anions
such as [Li([12]crown-4)2][BMes3] (1) or [K([18]crown-6)-
(thf)2][Mes2BB(Ph)Mes] (2).[7]
unusual C4B p system bearing five electrons,[8] we set out to
isolate and characterize a stable borol radical anion. As we
report here, this was possible by choice of steric protection
and an appropriate reducing agent.
The synthesis of MesBC4Ph4 (1-mesityl-2,3,4,5-tetraphe-
nylborole, 4) by means of the commonly employed tin–boron
exchange reaction[11–14] was unsuccessful because of the low
reactivity of dihalo(mesityl)boranes (MesBX2; X = Cl, Br).
However, 4 was obtained in 41% yield by functionalization of
the boron center in 1-chloro-2,3,4,5-tetraphenylborole (5)
through nucleophilic displacement of the chlorine ligand with
LiMes (Scheme 1).[14b] A more efficient alternative was found
to be the salt-elimination reaction of MesBCl2 with 1,4-
Scheme 1. Synthesis of 1-mesityl-2,3,4,5-tetraphenylborole (4) and its
single-electron reduction with [CoCp*2] to give the radical anion 8 as
well as its two-electron reduction to yield the dianion 9.
Our group has recently studied a persistent radical anion
as an intermediate in the stepwise reduction of 1-ferrocenyl-
2,3,4,5-tetraphenylborole (3).[8] Boroles are a class of anti-
aromatic compounds with interesting chemical and photo-
physical properties[9,10] that are well-known for their ability to
accept two electrons with formation of an aromatic borole
dianion.[11,12] Encouraged by these recent results on the
radical anion [3]CÀ, which indicated the presence of a highly
dilithio-2,3,4,5-tetraphenylbuta-1,3-diene (6) which provided
4 in 66% yield. Formation of a Lewis acid–base adduct with
Et2O, as previously reported for 1,2,3,4,5-pentaphenylborole
(7), was not observed,[13] already indicating the desired
enhanced steric shielding of the boron atom. Both methods
afforded 4 as a dark green crystalline solid containing one
1
equivalent of benzene according to H NMR spectroscopy
[*] Prof. Dr. H. Braunschweig, Dr. J. O. C. Jimenez-Halla, Dr. K. Kraft,
Dr. I. Krummenacher, Dr. K. Radacki, J. Wahler
Institut fꢀr Anorganische Chemie
and elemental analysis. The 11B NMR resonance of 4 (d =
79.4 ppm) is shifted significantly downfield shifted relative to
that of 7 (d = 65.4 ppm).[14a] In addition, the green color of 4 is
rather unexpected for pentaarylboroles, which are usually
blue (7: lmax = 560 nm), and arises from the lowest-energy
electronic absorption at lmax = 578 nm (e = 871 LmolÀ1 cmÀ1),
a less well-separated absorption at about l = 425 nm, and
Julius-Maximilians-Universitꢁt Wꢀrzburg
Am Hubland, 97074 Wꢀrzburg (Germany)
E-mail: h.braunschweig@uni-wuerzburg.de
Dr. V. Dyakonov, A. Sperlich
Experimental Physics VI
Julius-Maximilians-Universitꢁt Wꢀrzburg
Am Hubland, 97074 Wꢀrzburg (Germany)
a second maximum at lmax = 365 nm (e = 8181 LmolÀ1 cmÀ1
;
see the Supporting Information).
[**] We are grateful to the German Science Foundation (DFG) for
financial support.
In the solid-state structure of 4 determined by X-ray
diffraction the bond lengths and angles (see Table 1) of the
central C4B moiety are consistent with those of other
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201108632.
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Table 1: Selected bond lengths [ꢂ] and angles [8] of 4, 8, and 9.
(E1/2 = À2.56 V). The irreversibility is most likely due to
subsequent reactions with the halogenated solvent.
Bond
4
8
9
Chemical single-electron reduction of 4 was accomplished
by using decamethylcobaltocene ([CoCp*2]; E0(Fc/Fc+) =
À1.9 V) as a reducing agent (Figure 1).[17] Reaction of
a slight excess of 4 with [CoCp*2] in benzene at ambient
temperature smoothly generates [CoCp*2][MesBC4Ph4] (8),
which was obtained as an analytically pure reddish brown
solid in 85% yield. Under an inert atmosphere, 8 shows no
signs of decomposition in the solid state at room temperature
and is moderately stable in THF solution at À308C. The UV/
B1–C1
B1–C4
B1–C5
C1–C2
C2–C3
C3–C4
1.586(2)
1.575(2)
1.560(2)
1.356(2)
1.537(2)
1.351(2)
1.553(4)
1.553(4)
1.585(4)
1.393(3)
1.480(3)
1.397(3)
1.531(4)
1.527(4)
1.609(4)
1.468(4)
1.427(4)
1.443(4)
Angle
4
8
9
C1-B1-C4
C1-B1-C5
C4-B1-C5
B1-C1-C2
C1-C2-C3
C2-C3-C4
C3-C4-B1
105.2(1)
127.7(1)
126.8(1)
106.0(1)
110.7(1)
111.3(1)
105.9(1)
104.4(2)
127.9(2)
127.4(2)
107.4(2)
110.4(2)
110.5(2)
107.2(2)
104.7(2)
129.3(2)
126.0(2)
107.5(2)
109.2(2)
110.7(2)
107.9(2)
Vis spectrum in THF shows an absorption maximum at lmax
=
560 nm,which is slightly blue-shifted compared to that of the
neutral borole 4 and similar to that observed for [3]CÀ (lmax
=
541 nm).
EPR spectroscopy confirmed that 8 is a radical. The EPR
signal of 8 was detected in THF at 295 K as a four-line
resonance (centered at giso = 2.0025; Figure 2). The experi-
boroles,[12,14a] and no close contacts between neighboring
molecules are evident in contrast to the solid-state structure
of 7,[14a] which further proves the effective steric shielding in 4.
As expected, the torsion angle formed by the mesityl group
with respect to the C4B plane (68.7(1)8) is increased in
comparison to that of less bulky aryl groups.
For a closer insight into the redox properties of 4, cyclic
voltammograms in CH2Cl2 were recorded which showed two
well-separated reduction waves (Figure 1). The first reduction
event was identified at E1/2 = À1.69 V (referenced against the
Figure 2. Experimental and simulated EPR spectrum (X-band) of 8 in
THF solution at 295 K.
mental spectrum was reproduced by a simulation (see the
Supporting Information for details) which included the
coupling of one unpaired electron with the nuclear isotopes
of 11B (I = 3/2; abundance: 80.1%) and 10B (I = 3; abundance:
Figure 1. Cyclic voltammogram of 4 in 0.1m [nBu4N][PF6]/CH2Cl2 at
room temperature. Scan rate: 150 mVsÀ1; potential reported versus
Fc/Fc+ as the internal standard. The inset (dashed) displays the
reversibility of the single-electron reduction.
19.9%). The resulting hyperfine coupling constants A for 11
B
and 10B are 3.43 G and 1.08 G, respectively. This is compara-
ble to what has been reported for [3]CÀ (A(11B) = 3.73 G) and
is consistent with a pronounced degree of spin delocalization
over the borole framework.[4f]
ferrocene/ferrocenium (Fc/Fc+) couple) as a reversible pro-
cess associated with the formation of the radical anion [4]CÀ. It
is noteworthy that this reduction potential is considerably less
negative than that observed for 3 (E1/2 = À1.96 V)[8] and for 9-
The solid-state structure of 8 was determined by X-ray
diffraction, revealing a separated ion pair with no interaction
between neighboring radical centers (Figure 3). The C4B
framework is essentially planar with a propeller-like orienta-
tion of the four phenyl groups similar to that in 4. The
interplanar torsion angle of 68.4(1)8 formed by the mesityl
group and the borole ring is almost identical to that in 4. Upon
borafluorenes (E1/2 = À2.04 V to À2.28 V).[15]
A similar
reduction potential has recently been reported for a polycyclic
thiophene-fused borole system (E1/2 = À1.72 V).[16] The
second reduction process is centered at E1/2 = À2.54 V and
irreversible. It corresponds to formation of the aromatic
dianion [4]2À and its redox potential is close to that found for 3
À
À
À
reduction, the B1 C, B1 C4, and C2 C3 single bonds shorten
À
À
slightly, whereas the C1 C2 and C3 C4 double bonds
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À
homolytic cleavage of the O O bond by thermal or photo-
lytical activation enabling its use as a radical initiator.[18]
Indeed, 8 readily reacts with DBPO at ambient temperature
in a heterogeneous slurry. The selectively formed product is
a borate species (10; Scheme 2) resulting from the formal
recombination of the borole radical anion 8 with a benzoyloxy
radical.
Figure 3. Molecular structure of 8 in the solid state with hydrogen
atoms and solvent molecule (THF) omitted for clarity. Thermal
ellipsoids are set at 50% probability.
Scheme 2. Trapping reaction of 8 with dibenzoyl peroxide to form 10.
lengthen (see Table 1). Hence, the borole moiety of 8 shows
a slight decrease in bond-length alternation as a result of
increased delocalization of the p system which is consistent
Compound 10 was characterized by means of multi-
nuclear NMR spectroscopy, elemental analysis, and X-ray
crystallography. The 11B NMR resonance was found at d =
4.9 ppm in the typical range of four-coordinated boron. The
solid-state structure confirms a distorted tetrahedral geome-
try around the boron center and is comparable to that
observed for borole-based Lewis acid–base adducts reported
earlier (Figure 4).[10,14b–d] These results indicate a boron-
centered reactivity of the radical anion even though the
À
with the EPR spectrum. In contrast to 1, where all three B C
bonds are shorter than those in the parent borane (Mes3B),[7a]
À
the exocyclic B1 C5 bond of 8 is somewhat longer than in the
neutral borole 4. This is in agreement with an enhanced
electron density at the boron center and delocalization of the
additional electron into the C4B ring rather than over the
external mesityl group.
In order to get a better understanding of the electronic
structure of 8, DFT calculations were performed at the PBE0/
6-311 + G(d,p) level (see the Supporting Information for
further details). The optimized geometry of the radical anion
moiety of 8 is in good agreement with the experimental data.
Inspection of the electron density distribution of the SOMO
indicates that the charge is predominantly located at the
boron center. Furthermore, the electrostatic potential map-
ping shows that the overall charge density resides on the
borole fragment but mainly in the butadiene backbone.
Analysis of NBO charges of the atoms forming the central
borole ring shows increasing electron delocalization in the
p orbitals when the number of electrons added to the system
increases (from 4 over [4]CÀ to [4]2À).
Figure 4. Molecular structure of 10 in the solid state with hydrogen
atoms and solvent molecule omitted for clarity. Thermal ellipsoids are
set at 50% probability.
Dianionic borole systems have been known for more than
30 years and solid-state structural analyses have been
reported.[11,12] Nevertheless, we prepared the borole dianion
9 to obtain a system that is comparable to 8 concerning the
periphery of the borole scaffold. Borole 9 (d(11B) = 30.2 ppm)
was synthesized by reduction of 4 with potassium graphite
yielding single crystals suitable for X-ray diffraction. As
expected, the structural analysis shows a further decrease of
bond alternation in 9, compared to that in 8 (see Table 1),
when borole 4 takes up two electrons. This is in agreement
with a high degree of charge delocalization within the
p system which is typical for a compound with aromatic
character.[12] It should be considered, however, that the
structures of 9 and all hitherto known examples of borole
dianions may be influenced by the pronounced coordination
of the counterions to the system and do not reflect a free
aromatic moiety.
boron atom resides at the sterically most hindered position of
the heterocycle. The observed selectivity of the reaction
reflects the calculated spin density of the SOMO. Conse-
quently, 8 can be classified as a boron-centered radical anion,
mainly stabilized by charge delocalization into the five-
membered borole unit and less by the steric protection of the
mesityl group.
In summary, we have described the synthesis and one-
electron reduction of 1-mesityl-2,3,4,5-tetraphenylborole (4)
along with the full characterization of the resulting radical
anion 8. The obtained structural and EPR spectroscopic data
(giso = 2.0025; A(11B) = 3.44 G) as well as DFT calculations
are in agreement with a planar geometry with significant
delocalization of the five p electrons within the borole ring. In
addition, the radical anion 8 was successfully trapped by
dibenzoyl peroxide to give the borate 10. The boron-centered
reactivity shows that despite its substitution with a bulky
The reactivity of the open-shell species 8 was examined by
using a trapping reagent. For this purpose, 8 was reacted with
dibenzoyl peroxide (DBPO) which is known to undergo
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mesityl group the boron atom remains accessible. Further
studies of related radical species and their reactivity are
currently in progress.
Received: December 7, 2012
Published online: February 14, 2012
Keywords: borates · boroles · dianions · radical ions · reduction
.
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