Meta-B-entacenes: New polycyclic aromatics incorporating two fused borepin rings

Article abstract of DOI:10.1039/c2cc32500d

The synthesis of new boron-containing acenes (meta-B-entacenes) is reported. These compounds exhibit slightly non-planar core geometries with blue-shifted spectral properties and more negative electrochemical reduction potentials relative to known para is

Full text of DOI:10.1039/c2cc32500d

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COMMUNICATION
Meta-B-entacenes: new polycyclic aromatics incorporating two fused
borepin ringswz
David R. Levine, Anthony Caruso Jr., Maxime A. Siegler and John D. Tovar*
Received 6th April 2012, Accepted 23rd April 2012
DOI: 10.1039/c2cc32500d
The synthesis of new boron-containing acenes (meta-B-entacenes)
is reported. These compounds exhibit slightly non-planar core
geometries with blue-shifted spectral properties and more negative
electrochemical reduction potentials relative to known para
isomers. Polarizable p-extended architectures were realized via
cross-coupling procedures with chloro-functionalized precursors.
Fig. 1 Isomeric meta-B-entacene (1a) and para-B-entacene (2).
The incorporation of tricoordinate boron into organic com-
pounds has led to the realization of a structurally diverse array
of p-conjugated scaffolds.1 Molecules from this continually
expanding library have demonstrated promise for n-type
charge transport,2 non-linear optical materials,3 and selective
anion-sensing.4 Boron has also served as a useful structural
motif to study fundamental properties of non-benzenoid
aromatic5–11 and antiaromatic12 systems. Recently we intro-
duced synthetic routes to pentacyclic acenes containing two
formally aromatic borepin rings, so-called B-entacenes13,14
(Fig. 1). We now report the synthesis and characterization of
a new B-entacene isomer that features a meta-orientation of
the boron centers (meta-B-entacene; Fig. 1, 1a) as a complement
to previously reported structures which feature a para-boron-
boron relationship (para-B-entacene; Fig. 1, 2).
Meta-B-entacene 1a, in addition to chlorinated derivatives
1b and 1c, were synthesized according to Scheme 1. Thus,
Wittig olefination of bis(phosphonium) salt 3 (prepared in two
Scheme 1 Synthesis of meta-B-entacenes. TMEDA = N,N,N0,N0-
steps from 1,5-dibromo-2,4-dimethylbenzene; Electronic Supple-
mentary Information, ESIw) with aldehydes 4a–4c afforded
tetrabrominated Z,Z-dienes 5a–5c. TMEDA-assisted lithium-
tetramethylethylenediamine.
about the central phenyl ring. Like previously reported
B-Mes*-functionalized compounds,13–15 the robust kinetic
protection afforded to boron by the bulky Mes* groups
bromine exchange with excess sec-butyllithium followed by
addition of two equivalents of dimethyltin dichloride provided
fused stannocycles 6a–6c. Tin–boron exchange with boron
trichloride, followed by treatment with excess Mes*Li
provided stability in the presence of moisture and oxygen,
allowing facile purification and manipulation under ambient
(Mes* = 2,4,6-tri-tert-butylphenyl), gave meta-B-entacenes
1a–1c (20–53% yield) in which the benzo-annelated borepin
moieties place the two boron centers in a meta-arrangement
laboratory conditions.
Recrystallization of 1a from THF/H2O yielded colorless
block-like single crystals suitable for X-ray structure determi-
nation. In addition to confirming the molecular connectivity of
1a, the X-ray data highlights key structural features of the new
Department of Chemistry, Department of Materials Science and
Engineering, Johns Hopkins University, 3400 North Charles Street
(NCB 316), Baltimore, MD 21218, USA. E-mail: tovar@jhu.edu;
Fax: (+1)410-516-7044
w Electronic supplementary information (ESI) available: Experimental
procedures, 1H and 13C NMR spectra, UV-Vis and PL spectra, crystallo-
graphic data, and theoretical data. See DOI:10.1039/c2cc32500d
z This article is part of the ChemComm ‘Aromaticity’ web themed
issue.
isomer (Fig. 2a and b). In contrast to para-B-entacene 2, the
Mes* groups in 1a are positioned along the same edge of the
curved acene core. This close proximity leads to a substantial
degree of steric occlusion, orienting the Mes* groups away from
one another in space and causing a slight twist of the conjugated
backbone. DFT calculated structures for meta-B-entacenes
c
6256 Chem. Commun., 2012, 48, 6256–6258
This journal is The Royal Society of Chemistry 2012

Fig. 2 Displacement ellipsoid plots for 1a at 110(2) K (50% prob-
ability level), showing (a) face-on and (b) edge-on views (H atoms
omitted for clarity). Selected bond lengths (A): B1A–C3A: 1.575(3),
C3A–C8A: 1.418(3), C8A–C9A: 1.447(3), C9A–C10A: 1.339(3),
C10A–C11A: 1.453(3), C2A–C11A: 1.423(2), B1A–C2A: 1.569(3).
(c) Energy optimized structure (DFT; B3LYP/6-31G*) of 1a with
B-Ph substituents in place of B–Mes* (edge-on view).
Fig. 3 (a) UV-Vis, photoluminescence (PL) and (b) cyclic voltam-
metry (CV) data for 1a. UV-Vis and PL spectra were obtained at room
temperature in CHCl3 with excitation at 340 nm. CV was obtained at
2.5 mM in 0.1 M nBu4NPF6 electrolyte solution (THF) and referenced
to Ag/Ag+. DFT (B3LYP/6-31G*) calculations illustrating frontier
molecular orbital surfaces of 1a: (c) HOMO, (d) LUMO.
incorporating sterically less-demanding pendant phenyl groups
(B3LYP/6-31G*, Fig. 2c) possess fully planar geometries,
confirming that the twist motif is due to steric interaction
between the Mes* groups and is not an inherent structural
aspect of the B-entacene core. Intraring bond lengths within
the formal borepin subunits of 1a are generally consistent with
those found previously for 2,13 with C–C bond distances in 1a
ranging from 1.339(3) to 1.453(3) A. The extent of this C–C
bond length alternation (ꢀ0.114 A) is approximately twice as
large as the range reported for the unfused 1-chloroborepin7
(ꢀ0.058 A). Such an increase reflects a reduction in the
aromatic character of the fused borepin subunits in 1a due
to benzo-annelation, and is in agreement with a calculated
NICS(1) value16 of ꢁ0.51 indicating little to no aromatic
character for the borepin rings.17 The steric bulk imparted
by the Mes* groups of 1a precluded the formation of extended
tight p-stacking motifs in the solid-state, however close C–H
edge-to-face (2.883(3) A) and C–C face-to-face (3.369(3) A)
contacts were apparent at unencumbered regions of the
molecular periphery (ESIw).
We characterized the photophysical properties of meta-B-
entacene 1a and compared them to the isomeric para-B-
entacene 213 (Table 1). The UV-Vis spectrum of 1a in CHCl3
(Fig. 3a) displayed an absorption onset at 421 nm, with a low
energy, low oscillator strength band at 407 nm, which we
attribute to optically-induced intramolecular charge transfer
(ICT).11,13 In addition, higher energy p–p* absorptions with
strong vibronic coupling were observed near 340 nm. The
spectral features of 1a are in accord with those found for 2,
albeit significantly blue-shifted (B30–40 nm), suggesting
diminished electron delocalization. This can be rationalized
in terms of a longer para-like all-carbon conjugation pathway
in 2 relative to 1a, the length of which has been shown in
related systems to influence optical bandgaps more effectively
than the conjugation pathways incorporating boron.18 Excita-
tion at 340 nm yielded a solution quantum yield (F) of 80%
and an excited state lifetime (t) of 13.2 ns for 1a. The PL
spectrum of 1a demonstrated a significant degree of fine
structure, with maxima at 413 and 437 nm and a high-energy
shoulder at 462 nm. This contrasts with the PL spectrum of 2,
which shows only a single distinct PL lmax at 456 nm with a
small high-energy shoulder. We attribute the accentuated PL
spectral fine structure, longer excited-state lifetime, and higher
quantum yields in 1a relative to 2 to the greater steric demands
imposed by the close proximity of Mes* groups in 1a, leading
to a high level of chromophore conformational rigidity and
reduced non-radiative decay.
Well-behaved cathodic electrochemistry was observed for 1a
with a single reduction wave (E1/2) at ꢁ2.12 V vs. Ag/Ag+ in
THF solution (Fig. 3b). This contrasts with para-B-entacene 2,
which demonstrated two distinct one-electron reductions
(E1/2 = ꢁ1.89 V, ꢁ2.46 V),13 with a less negative first
reduction potential than 1a. The trend of a more facile first
reduction for the isomer with the two boron centers in a para-
orientation relative to the meta-orientation mirrors that for the
related unfused 1,4-19 and 1,3-bis(dimesitylboryl)phenylene20
systems, however the B-entacene systems are more difficult
to reduce than the corresponding unfused triarylboranes. This
may reflect a higher energetic barrier to molecular reorganization21
of 1a and 2 upon electrochemical reduction relative to unfused
analogues.
Table 1 Photophysical data for 1a, 1b, 2a, 7a, and 7b
b,c
b
b
Compound
Abs lmax
Abs lonset
PL lmax
Fd
t(ns)e
1a
1b
2a
7a
7b
407
407
439
411
400
421
421
458
425
461
413
411
456
441
477
0.80
0.38
0.71
0.39
0.48
13.2
8.9
9.3
8.8
2.8
The preparation of extended p-conjugated structures based
on the meta-B-entacene core was achieved by utilizing active
palladium-catalyzed cross-coupling procedures for chloro-
arenes (Scheme 2). Stille cross-coupling of chlorinated meta-
B-entacene 1b with 2-tributylstannylthiophene under Fu’s
conditions22 provided 7a in 41% yield, while Sonogashira
a
b
Ref. 13. In nm. Lowest energy absorption maximum. Relative
c
d
e
to quinine sulfate (0.55). Fitted to a single exponential decay.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 6256–6258 6257

meta-B-entacene isomers might provide complementary plat-
forms for predictably tunable optical materials.
In conclusion, the synthesis of meta-B-entacene suggests a
general, reliable method for the construction of phenylene-
fused borepin-containing acenes. The meta-arrangement of the
boron centers with respect to one another in 1a leads to
hypsochromically shifted UV-Vis/PL signatures and more
negative reduction potentials than in para-isomer 2, demon-
strating the influence of the unique structure of 1a on its
observed optoelectronic properties. The accessibility of
extended p-conjugated derivatives 7a and 7b via Pd-catalyzed
cross-couplings demonstrates the possibility for incorporation
of the meta-B-entacene subunit into polarizable p-electron
materials. Further investigations into the properties of
B-entacene systems and related p-electron scaffolds are
currently underway.
Scheme 2 Pd-catalyzed cross-couplings of 1b. Conditions: (a)
Pd(PtBu3)2 (10 mol%), CsF, 1,4-dioxane, 95 1C; (b) PdCl2(CH3CN)2
(10 mol%), XPhos (30 mol%), Cs2CO3, THF, CH3CN, 80 1C.
cross-coupling with 4-ethynyl-N,N-dimethylaniline under
Buchwald’s conditions23 provided 7b in 33% yield. It is
noteworthy that the reaction conditions for the preparation
of 7a (excess CsF, 95 1C) did not lead to appreciable decom-
position of 1b, reaffirming the effective kinetic protection of
boron by the B-Mes* group. Unfortunately, similar to analogous
chloro-derivatives of 2,14 we could not achieve cross-couplings
when the functionalizable chloride handle was situated meta to
the boron center (as in 1c).
This work was financially supported by the Johns Hopkins
University. We thank Christie Checketts for synthetic
assistance and Nathan Kopf (Yarkony research group, JHU)
for computational support.
Photophysical data for 1b, 7a, and 7b are presented in
Table 1. Though the major UV-Vis/PL spectral features of
1b and 7a (ESIw) are not significantly shifted from 1a by chloro
and thienyl groups at the positions para to the boron centers,
attachment of strongly electron-donating N,N-dimethylamino-
phenylacetylene substituents in 7b leads to significant red
shifts in absorption onset and PL maxima due to the
pronounced ‘‘push-pull’’ nature of the chromophore. Large
positive solvatochromic effects were observed for solutions of
7b in solvents with increasing ET30 values, with a PL lmax shift
of over 100 nm between cyclohexane and THF (Fig. 4a).
Calculated frontier molecular orbital surfaces for a simplified
analogue of 7b (using B-2,6-dimethylphenyl groups in place of
B-Mes*; Fig. 4b and c) demonstrated a significant shift in the
location of orbital density between HOMO and LUMO,
accounting for the marked solvatochromic behavior. These
properties are in accord with those of the corresponding
para-B-entacene derivative, which demonstrated similar solvato-
chromic effects, albeit at relatively longer wavelengths in each
case.14 These results suggest that functionalized para- and
Notes and references
1 F. Jakle, Chem. Rev., 2010, 110, 3985–4022.
¨
2 H. Kobayashi, N. Sato, Y. Ichikawa, M. Miyata, Y. Chujo and
T. Matsuyama, Synth. Met., 2003, 135–136, 393–394.
3 Z. Yuan, J. C. Collings, N. J. Taylor, T. B. Marder, C. Jardin and
J.-F. Halet, J. Solid State Chem., 2000, 154, 5–12.
4 C. R. Wade, A. E. Broomsgrove, S. Aldridge and F. P. Gabbaı,
¨
Chem. Rev., 2010, 110, 3958–3984.
5 M. E. Vol’pin, Russ. Chem. Rev., 1960, 29, 129–160.
6 E. E. van Tamelen, G. Brieger and K. G. Untch, Tetrahedron Lett.,
1960, 1, 14–15.
7 A. J. Ashe III, W. Klein and R. Rousseau, Organometallics, 1993,
12, 3225–2331.
8 J. M. Schulman and R. L. Disch, Organometallics, 2000, 19, 2932–2936.
9 K. M. Davies, M. J. S. Dewar and P. Rona, J. Am. Chem. Soc.,
1967, 89, 6294–6297.
10 E. R. Abbey, L. N. Zakharov and S.-Y. Liu, J. Am. Chem. Soc.,
2011, 133, 11508–11511.
11 L. G. Mercier, W. E. Piers and M. Parvez, Angew. Chem., Int. Ed.,
2009, 48, 6108–6111.
12 H. Braunschweig and T. Kupfer, Chem. Commun., 2011, 47,
10903–10914.
13 A. Caruso Jr., M. A. Siegler and J. D. Tovar, Angew. Chem., Int.
Ed., 2010, 49, 4213–4217.
14 A. Caruso Jr. and J. D. Tovar, Org. Lett., 2011, 13, 3106–3109.
15 A. Wakamiya, M. Kotaro, E. Kanako and S. Yamaguchi, Chem.
Commun., 2008, 579–581.
16 The nucleus-independent chemical shift (NICS) of a ghost atom
placed 1 A above the center of the borepin ring was calculated
using Gaussian ’03: M. J. Frisch, et al., Gaussian ’03, Revision C.01,
Gaussian, Inc., Wallingford, CT, 2004.
17 NICS values of annelated borepins: G. Subramanian, P. von
Rague Schleyer and H. Jiao, Organometallics, 1997, 16, 2362–2369.
18 A. Caruso Jr. and J. D. Tovar, J. Org. Chem., 2011, 76, 2227–2239.
19 A. Schulz and W. Kaim, Chem. Ber., 1989, 122, 1863–1868.
20 A. Rajca, S. Rajca and S. R. Desai, J. Chem. Soc., Chem.
Commun., 1995, 1957–1958.
21 For example, in fused and unfused pyridinium frameworks:
J. Fortage, F. Tuyeras, P. Ochsenbein, F. Puntoriero, F. Nastasi,
S. Campagna, S. Griveau, F. Bedioui, I. Ciofini and P. Laine
´
,
Chem.–Eur. J., 2010, 16, 11047–11063.
22 A. F. Littke, S. Schwarz and G. C. Fu, J. Am. Chem. Soc., 2002,
124, 6343–6348.
23 D. Gelman and S. L. Buchwald, Angew. Chem., Int. Ed., 2003, 42,
5993–5996.
Fig. 4 (a) PL spectra for 7b in cyclohexane, CHCl3, and THF at
room temperature. Calculated frontier molecular orbitals surfaces
(DFT; B3LYP/6-31G*) for B-(2,6-dimethylphenyl)-7b: (b) HOMO;
(c) LUMO.
c
6258 Chem. Commun., 2012, 48, 6256–6258
This journal is The Royal Society of Chemistry 2012