Synthesis of functionalizable boron-containing π-electron materials that incorporate formally aromatic fused borepin rings

Article abstract of DOI:10.1002/anie.201000411

(Figure Presented) Not a natty B-OH: Borepin-based extended π-electron molecules contain bulky substituents that shield the vacant boron p orbitals and allow synthetic manipulation and purification under ambient conditions. These borepin-containing compounds (see picture; Mes* = 1,3,5-tris(tert-butyl) phenyl; B orange, C gray) display reversible cathodic electrochemistry and can be viewed as n-type analogues to bent acene hydrocarbons.

Full text of DOI:10.1002/anie.201000411

Angewandte
Chemie
DOI: 10.1002/anie.201000411
Organic Materials
Synthesis of Functionalizable Boron-Containing p-Electron Materials
that Incorporate Formally Aromatic Fused Borepin Rings**
Anthony Caruso, Jr., Maxime A. Siegler, and John D. Tovar*
In memory of Eugene E. van Tamelen
Boron-containing p-electron systems have emerged as excit-
ing subjects in contemporary organic materials chemistry.[1–7]
The strong Lewis acidity of tricoordinate boron has been
utilized for the detection of biologically or environmentally
relevant anions.[8,9] Anionic and neutral boron heteroaro-
matic molecules are important p-donor ligands for
organometallics,[10–12] and the aromaticity of boron-
containing molecules has inspired substantial experi-
mental and theoretical investigations,[13–16] thus suggest-
ing that the heteroaromaticity of boron will play will
play a key role in other areas where polarization may
need to distort p-electron frameworks, such as during
operation in electronic devices. The vast majority of
boron-based p-electron materials was built with main-
chain[17] or lateral[6,18,19] tricoordinate boron substitu-
tion or from locally antiaromatic four-p-electron frag-
ments such as the borole nucleus within 9-borafluor-
ene.[20] Herein, we present the syntheses and character-
ization of polycyclic aromatic molecules built around
the neutral and formally aromatic six-p-electron bor-
epin ring system that is structurally poised for synthetic
nate borepins that could be tailored into complex
p-electron systems have been reported to date.
The synthesis of the parent compound DBB 1a is shown
in Scheme 1. The stilbene precursor 2a was constructed by a
Wittig-type process using reactants derived from a-bromo-o-
Scheme 1. Synthesis of dibenzo[b,f]borepins. Conditions for lithiation: 6a:
sBuLi, THF; 6b,c: nBuLi, TMEDA, Et2O. Reagents for borepin formation: 1a:
PhBCl2; 1b: a) BCl3, b) tripyl-SnBu3; 1c–e: a) BCl3; b) Mes*Li, RT. Yields for 1a
and 1b determined by NMR spectroscopy. TMEDA=N,N,N’,N’-tetramethyl-
ethane-1,2-diamine.
elaboration into complex molecular structures.
The dibenzo[b,f]borepin (DBB) framework contin-
ues to attract substantial attention.[21,22] Van Tamelen et
al. reported the first isolation of DBB as its ethanol-
amine adduct,[23] and Piers and co-workers very
recently reported a B-mesityl DBB that slowly oxidized
under ambient conditions.[24] Theoretical studies have
revealed planarity throughout the B-H DBB; thus planarity
was attractive to us as a means to enhance p-electron
delocalization,[13] and nucleus-independent chemical shift
(NICS) values reveal a weak degree of aromatic character
within the borepin ring of benzo-annelated molecules.[14]
However, no synthetic routes for robust and stable tricoordi-
bromotoluene 3a: radical halogenation and standard Arbu-
zov reaction resulted in the phosphonium salt 4a, while
oxidation with N-methylmorpholine-N-oxide (NMO) fur-
nished the requisite aldehyde 5a. The desired cis product 2a
was dominant (> 90%) and was readily purified.[25] Stanno-
cyclization was carried out by trapping the in situ generated
dilithio species of 2a with dimethyltin dichloride. Tin–boron
exchange between 6a and PhBCl2 resulted in the B-phenyl
DBB 1a; both 1a and even B-tripyl (2,4,6-triisopropylphenyl)
DBBs 1b rapidly decomposed under ambient conditions and
were handled in a glovebox. Tri(tert-butylphenyl) (“super
mesityl”, Mes*) B-substituents rendered the resulting 1c
framework stable and sufficiently soluble to allow manipu-
lation and chromatography under ambient conditions.[26]
The bulkier B-Mes* substituent of 1c slightly disturbed
the p-conjugation, as indicated by the 14 nm blue shift of the
lowest-energy UV/Vis features compared to those of 1a,
although the spectra remain qualitatively similar in terms of
relative oscillator strengths.[27] Otherwise, the spectroscopic
and electrochemical properties of DBB 1c were comparable
to those of the B-mesityl DBB frameworks reported inde-
[*] A. Caruso, Jr., M. A. Siegler, Prof. J. D. Tovar
Department of Chemistry
Department of Materials Science and Engineering
Johns Hopkins University
3400 North Charles Street (NCB 316), Baltimore, MD 21218 (USA)
Fax: (+1)410-516-7044
E-mail: tovar@jhu.edu
Homepage: http://www.jhu.edu/~chem/tovar/index.html
[**] We thank Johns Hopkins University and the Petroleum Research
Fund (administered by the American Chemical Society) for financial
support. Prof. John Reynolds (University of Florida) provided helpful
suggestions regarding the Ni-catalyzed homocoupling.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201000411.
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Communications
pendently by Piers and co-workers.[24] However, the bulk of
Table 1: Optical and electrochemical properties of functionalized bor-
epin-containing p-electron molecules.
the Mes* substituent shielded the boron center from attack by
Lewis bases, thus leaving it coordinatively unsaturated in the
presence of fluoride ions.[26] Therefore, the electronics of the
aromatic borepin nucleus do not preclude the use of the
vacant boron p orbital in the context of Lewis basic sensing
schemes unless the receptor incorporates some steric bulk.
The chloride “handles” were included at positions para to
the boron in 1d and 1e (specifically where R1 and/or R2 are
chlorides) because of their known reactivity under a variety of
nickel- and palladium-catalyzed processes. For example, the
chlorinated benzaldehyde 5b was treated with phosphonium
salt 4a under the same Wittig-type conditions to prepare
monochloro stilbene 2b that was further reacted to form DBB
1d, while both chlorinated units 4b and 5b were employed in
the synthesis of 1e under the same set of reactions. In
principle, any of the aryl protons of 4 and 5 could be replaced
with other substituents. The use of the Mes* group was again
required to impart robustness under ambient conditions.
DBBs with extended p conjugation were prepared from
chlorides 1d and 1e by common cross-coupling procedures,
thus illustrating the robustness of the protected DBB core.
The homodimerization of 1d that led to 7 was effected using a
Compound
Abs[a]
PL
E1/2(1) [V]
E1/2(2) [V]
lmax [nm]
(loge)
lmax [nm]
(FF [%])
1c
7
8a
8b
12a
13a
13b
358 (3.56)
379 (3.43)
382 (2.97)
380 (3.55)
439 (3.52)
442 (3.33)
438 (3.36)
384 (36)
404 (58)
396 (23)
395 (38)
456 (73)
459 (56)
459 (50)
ꢀ2.54
ꢀ1.97
ꢀ2.24
ꢀ2.23
ꢀ1.89
ꢀ1.87
ꢀ1.88
n.a.[b]
ꢀ2.20
n.a.
n.a.
ꢀ2.59
ꢀ2.48
ꢀ2.46
[a] Lowest-energy lmax. Full spectra and experimental details below and in
the Supporting Information. [b] n.a.=not applicable.
nickel catalyst (with stoichiometric zinc reductant).[28,29]
Kumada-type couplings of aryl Grignard reagents onto 1e
using [Ni(dppp)2Cl2] led to the aryl coupled products 8a–b.
As with the parent DBB, 7 and 8 were stable under ambient
conditions and could be chromatographed with no apparent
decomposition. Although treatment of the DBBs with
molecular bromine led to decomposition, the dimer 7 did
not react with nBuLi and even withstood an oxidative workup
with aqueous hydrogen peroxide. We were unable to install
other p-electron groups onto 1e by using palladium-catalysts,
and we could not install nonphenyl groups with nickel
catalysts (e.g., with vinyl or ethynyl Grignard reagents). In
some cases, clear dehalogenation of the DBB 1e was
observed, thus suggesting that the oxidative insertion was
viable on the deactivated aryl chlorides and that future target-
specific chemistry will be successful after reaction and
substrate fine-tuning. The kinetic stability afforded by the
Mes* group will greatly facilitate future studies of this new
electronic structure in the context of both small molecules
and chemically synthesized polymeric materials that can be
conceived from the functionalizable DBB cores.
Figure 1. UV/Vis and photoluminescence (PL; a,c), and cyclic voltam-
metry (b,d) data for 7 (top) and 8b (bottom). UV/Vis and PL spectra
were acquired in CHCl3 at room temperature, with an excitation at 317
(a) and 365 nm (b). CVs were acquired from 2.5 mm solutions of
analyte in 0.1m nBu4NPF6/THF and are reported relative to Ag/Ag+.
tion on the DBB core, with only slight spreading onto the
substituents in the lowest unoccupied molecular orbital
(LUMO).[27] Both 7, 8a, and 8b had their lowest-energy
absorptions around 380 nm with photoluminescence maxima
at approximately 400 nm. On the other hand, the more
electron deficient DBB substituent of 7 led to a dramatic
difference in cyclic voltammetry (CV) compared to 8a and
8b, showing less negative first (and second) reduction
potentials (E1/2s at ꢀ1.97 and ꢀ2.20 V for 7 vs. ꢀ2.24 V for
8a, Figure 1b,d). This result indicates that, despite the orbital
wavefunction nodes at the site of aryl–aryl union, substantial
communication, which influences the boron redox chemistry,
still exists among the boron redox-active centers within the
dimeric structure of 7. This observation will be important for
future work on electronic devices that often requires tuning
the HOMO and LUMO levels relative to vacuum to facilitate
charge injection as opposed to exploiting differences in
optical bandgaps.
The vacant p orbitals on boron can participate in extend-
ing the conjugation pathways in p-electron materials. There-
fore, it was unusual to find that 7, 8a, and 8b had quite similar
optical properties (Table 1, Figure 1a,c). This finding can be
rationalized by the presence of a node that is located at the
site of aryl attachment in these molecules, and that localized
the highest occupied molecular orbital (HOMO) wavefunc-
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The general synthetic approach was applied to even larger
fused polycyclic aromatics (Scheme 2). The bis(phospho-
nium) salt 9[30] was subjected to a double-Wittig olefination
Figure 2. UV/Vis, photoluminescence (a) and cyclic voltammetry (b)
data for 12a. UV/Vis and PL were acquired in CHCl3 at room
temperature, with excitation at 415 nm. See Figure 1 for CV condition.
The X in (b) denotes a background response from the electrolyte. DFT
(B3LYP/6-31G*) calculations depicting the LUMO (c) and HOMO (d)
wavefunctions for 13a.
Scheme 2. Synthesis and diversification of pentacyclic borepins.
Reagents and conditions: a) KOtBu, THF, 08C!RT; b) sBuLi, TMEDA,
Me2SnCl2, THF, ꢀ788C!RT; c) BCl3, Mes*Li, PhMe, ꢀ788C!RT;
d) [Ni(dppp)2Cl2](dppp=1,3-bis(diphenylphospino)propane), THF,
508C.
intervening phenyl group in 12a and a biphenyl unit in 7.
The smaller DE1/2 value for 7 could also be due to Coulombic
effects that place electronically isolated radical anions at a
greater distance than for 12a, but it is important to note that
the initial E1/2 for dimer 7 is almost 600 mV less negative than
that for 1c, thus indicating some extent of existing electronic
communication that facilitates the reduction. Importantly, all
cathodic processes observed for these molecules were rever-
sible, thus suggesting that they may be used as electron-
accepting organic semiconducting materials.
Unexpectedly, the attachment of the aryl groups did not
dramatically affect the photophysical or electrochemical
behavior relative to the parent compound 12a. We carried
out DFT (B3LYP/6-31G*) calculations for the LUMO and
HOMO of 13a (but using B-2,6-dimethylphenyl rather than
B-Mes*, Figure 2c,d). The HOMO level is localized on the
polycyclic aromatic core with a node at the site of tolyl
attachment, and the LUMO has very minimal delocalization
onto the aryl substituents. We anticipate more electronic
influence to be exerted by substituents placed elsewhere onto
the fused core, as determined by the choice of benzaldehyde
precursors 5 that ultimately determines the site for function-
alization.
The X-ray structure of a single crystal (grown from a
mixture of THF, CH2Cl2, and water) verified the molecular
connectivity (Figure 3a) and revealed important packing-
induced distortions within the pentacyclic core (deviation
from planarity ꢁ 148; Figure 3b).[32] As the DFT calculations
indicate a planar pentacyclic aromatic core, we attribute these
deviations to crystal-packing influences as well as deforma-
tions imposed by the Mes* group; these deviations are also
spectroscopically apparent for the DBB 1c. The structural
metrics of the borepin core were consistent with other
borepin fragments reported previously,[24] and the
NICS(1)[33] calculated for the borepin ring of 12a (ꢀ1.65)
indicated very weak local aromaticity or perhaps even
nonaromatic behavior. Although small in absolute value, it
is comparable to that calculated in our labs for the weakly
aromatic B-phenylborepin (ꢀ5.75), thus indicating that the
with bromobenzaldehyde 5a to yield tetrabromide 10a.
Tetralithiation of 10a followed by double stanocyclization
led to 11a. A double tin–boron exchange with BCl3 generated
a bis(chloride) intermediate that ultimately produced the
fused pentacyclic aromatic 12a when treated with Mes*Li
in situ. The structure of 12a is fundamentally different from
the DBB core as it now contains a para-phenylene diborane
core,[31] that is, a central benzene core fused with two borepin
rings. Like the B-Mes* DBBs, 12a could be handled under
ambient conditions with no obvious decomposition. By using
the same strategy as for 1e, chlorides were installed para to
the boron centers of 12b and successfully underwent trans-
formation under nickel catalysis to provide stable aryl-
substituted compounds 13a and 13b. These compounds
were thermally robust: 13a showed no signs of decomposition
after heating at 1308C for 30 min, and the low-energy spectral
signatures associated with the tricoordinate boron-based
heteroaromatic were still observed after heating a sample
over 2508C in a melting-point apparatus.
Figure 2 shows some of the key characterization data for
the unfunctionalized ring system 12a. Like the profiles of the
smaller DBB and other common polycyclic aromatics, the
absorption profile of 12a showed a low-energy feature at
439 nm with low oscillator strength attributed to intramolec-
ular charge transfer along with much more intense absorp-
tions at higher energy, including some with strong vibronic
coupling around 375 nm (Figure 2a). The photoluminescence
observed at 456 nm was fairly intense, with a quantum yield
(FF) of 73% relative to quinine sulfate. The cathodic CV
indicated two reversible reduction steps with E1/2 values of
ꢀ1.89 and ꢀ2.59 V (Figure 2b), both of which were much less
negative than those found for standard triarylboranes.[31] As
reported above, the spacing between the two cathodic
processes for 7 (DE1/2) was 230 mV while that for 12a was
even larger (DE1/2 = 700 mV). This observation is consistent
with the boron centers being separated formally by an
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Communications
were recorded on a Varian Cary 50 Bio UV/Visible spectrophotom-
eter. Photoluminescence spectra were recorded on a Photon Tech-
nologies QuantaMaster 4 fluorometer with a 75 W Xenon lamp,
maintaining optical densities below 0.05. Quantum yields were
determined relative to quinine sulfate in 0.1n H2SO4 (55%).
Electrochemistry: Cyclic voltammetry was performed in a one-
chamber, three-electrode cell using PARSTAT 2273 and Autolab
PGSTAT 302 potentiostats. A 2 mm2 Pt button electrode was used as
the working electrode with a platinum wire counter electrode relative
to a quasi-internal Ag wire reference electrode submersed in 0.01m
AgNO3/0.1m nBu4NPF6 in anhydrous acetonitrile (obtained from
BioAnalytical Systems, West Lafayette, IN, USA). Measurements
recorded on millimolar analyte concentrations in 0.1m nBu4NPF6
electrolyte solutions (in THF) recorded at a scan rate of 100 mVsꢀ1
.
Potentials are reported relative to the Ag/Ag+ couple (ferrocene/
ferrocenyl (Fc/Fc+) = 205 mV).
Computational studies: Molecular orbital calculations were
performed at the DFT level (B3LYP/6-31G*) on equilibrium geo-
metries using Spartanꢁ04 (Wavefunction Inc., Irvine, CA). NICS
calculations were performed using Gaussian 03 using the GIAO
method (B3LYP/6-31G*).[36]
Figure 3. Displacement ellipsoid plot (50% probability level) of 12a at
Received: January 23, 2010
Revised: March 29, 2010
Published online: May 7, 2010
110(2) K (a) and an extended packing motif (b) revealing face-to-face
aromatic contacts spaced at 3.43 ꢀ. Selected bond lengths [ꢀ]: B1–C2
1.573(3), B1–C3 1.570(3), C3–C8 1.423(3), C8–C9 1.452(3), C9–C10
1.341(3), C10–C11 1.452(3), C2–C11 1.421(3).
Keywords: annulation · aromaticity · boron · conjugation ·
.
cross-coupling
inherent borepin aromaticity is not dramatically perturbed by
extended ring fusion. Finally, the Mes* groups frustrated tight
packing leaves substantial void spaces within the crystal
lattice.[27] These voids are occupied by a mixture of disordered
solvent molecules (CH2Cl2 and THF), and their contributions
were subsequently removed for the last stage of refinement
using the SQUEEZE program.[34] Face-to-face p stacking
with a distance of 3.43 ꢀ was evident at the periphery of the
pentacyclic fragment.
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