Syntheses and structures of 6,13-dihydro-6,13-diborapentacenes: π-stacking in heterocyclic analogues of pentacene

Article abstract of DOI:10.1021/om8005068

6,13-Dibromo-6,13-dihydro-6,13-diborapentacene (1) has been prepared by the reaction of 2,3-bis(trimethylsilyl)-naphthalene with BBr₃. The reaction of 1 with Me₄Sn or mesityllithium afforded the dimethyl derivative 2 or the dimesityl derivative 3, respectively. In the solid state compounds 1 and 2 form cofacial π-stacks along the a crystal axes, albeit in slightly different ways.

Full text of DOI:10.1021/om8005068

Organometallics 2008, 27, 3639–3641
3639
Syntheses and Structures of 6,13-Dihydro-6,13-diborapentacenes:
π-Stacking in Heterocyclic Analogues of Pentacene
Jinhui Chen, Jeff W. Kampf, and Arthur J. Ashe III*
Department of Chemistry, UniVersity of Michigan, Ann Arbor, Michigan 48109-1055
ReceiVed June 2, 2008
proposed.10 Trivalent boron has a formally vacant 2p orbital
which might serve as an electron acceptor. Most of the prior
work has involved compounds with bulky B substituents.11
Although these bulky substituents generally enhance the kinetic
stability of boron compounds, they are also likely to prevent
any possible intermolecular π-stacking important for high charge
mobility in OFETs.
Summary: 6,13-Dibromo-6,13-dihydro-6,13-diborapentacene (1)
has been prepared by the reaction of 2,3-bis(trimethylsilyl)-
naphthalene with BBr3. The reaction of 1 with Me4Sn or
mesityllithium afforded the dimethyl deriVatiVe 2 or the dimesityl
deriVatiVe 3, respectiVely. In the solid state compounds 1 and
2 form cofacial π-stacks along the a crystal axes, albeit in
slightly different ways.
Organic semiconductors have potential applications as com-
ponents of light-emitting diodes,1 photovoltaics,2 and field-effect
transistors (OFETs).3 Intense research efforts have centered on
the use of polycyclic aromatic hydrocarbons (PAHs),3 e.g.
pentacene, and their electron-rich heterocyclic relatives4 as
p-type semiconductors for OFETs. In the solid state many PAHs
adopt a herringbone packing,5 which has less optimal intermo-
lecular orbital overlap than a cofacial π-stacking packing.6
Although many of the factors which influence solid-state packing
are not well understood, it has been observed empirically that
some substituted PAHs adopt a favorable cofacial stacking.7
The search for other π-stacking materials continues. Improved
n-type semiconductors structurally similar to PAHs but comple-
mentary in the type of charge carriers would also be highly
desirable.8 Promising preliminary investigations on PAHs with
electron-withdrawing substituents, particularly fluorine, have
been reported.9 The use of boron heterocycles has also been
We hypothesize that derivatives of the previously unknown
6,13-dihydro-6,13-diborapentacene ring system with small sub-
stituents at boron might circumvent these difficulties. Further-
more the antiaromatic four-π-electron character of the central
heterocyclic ring12 should facilitate electron transfer to these
compounds, as has been found for the related 5,10-dihydro-
5,10-diboraanthracenes.13 We report here on the first syntheses
of 6,13-dihydro-6,13-diborapentacenes (1-3), the π-stacking
structures of 1 and 2, and electrochemical data for 3.
Our synthesis of 1 relies on the known facile B/Si exchange
of arylsilanes with boron halides.14 The reaction of the readily
available 2,3-bis(trimethylsilyl)naphthalene15 with BBr3 in
toluene at 110 °C gave 22% of 1 as a yellow crystalline solid
(Scheme 1). Like most organoboron bromides, 1 is quite
moisture sensitive but can be handled using standard Schlenk
techniques. It is thermally stable to at least its melting point,
282 °C. 6,13-Dibromo-6.13-dihydro-6,13-diborapentacene (1)
has been fully characterized by 1H, 11B, and 13C NMR
spectroscopy, high-resolution mass spectroscopy, elemental
analysis and X-ray diffraction.16,17 The bromo groups of 1 can
be readily substituted for nucleophiles. Reaction of 1 with Me4Sn
gave the crystalline yellow dimethyl derivative 2 in 33% yield.
The reaction of 1 with mesityllithium afforded the dimesityl
derivative 3, which like other mesitylboranes is much less
sensitive to water and oxygen than 1 and 2. It seems likely that
* To whom correspondence should be addressed. E-mail: ajashe@
umich.edu.
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10.1021/om8005068 CCC: $40.75
2008 American Chemical Society
Publication on Web 07/16/2008

3640 Organometallics, Vol. 27, No. 15, 2008
Scheme 1. Syntheses of 6,13-Dihydro-6,13-diborapentacenesa
Communications
a
Conditions: (a) BBr3/toluene, 110 °C; (b) for 2 SnMe4/toluene, 110 °C, for 3 mesityllithium/THF, 0 °C.
Figure 1. Solid-state structure of 3 (ORTEP). Thermal ellipsoids have been set at the 50% probability level. Hydrogen atoms have been
omitted for clarity. Selected bond distances (Å): B-C(R), 1.558(2); B-C(R′), 1.558(2); B-C(exo), 1.584(2); C(R)-C(R′), 1.450(2).
1 could also be converted to a variety of other derivatives using
the well-honed methods of organoboron chemistry.18
Single crystals of 1-3 were grown from toluene solutions.
The molecular structures show that the 6,13-dihydro-6,13-
diborapentacene core is planar for 1-3.17 For 3 the two mesityl
groups are inclined to the core plane by approximately 85°
(Figure 1). As had been anticipated, the large pendant mesityl
groups of compound 3 prevent it from stacking in the solid state.
For all three compounds the ring C-C bond distances range
from 1.37 to 1.45 Å, quite similar to those of pentacene
(1.34-1.46 Å).5 However, the core B-C bond distances
(1.54-1.56 Å) are longer than those found for neutral aromatic
boron compounds (range 1.48-1.52 Å),19 which is consistent
with the antiaromatic character of the heterocyclic ring. None
of the compounds pack in the familiar herringbone arrangement
of pentacene. The packing of 1, illustrated in Figure 2, shows
that the molecules form cofacial π stacks along the a axis. The
intermolecular face-to-face distance is 3.459 Å, and the mol-
ecules are inclined to the a axis by 29.0°. Viewed perpendicu-
larly to the molecular plane (Figure 2B), the molecular packing
is slipped along the long molecular axis by about 0.5 ring and
along the short molecular axis about 0.25 ring. This brings the
boron atom of one molecule vertically close (3.46 Å) to the
R-carbon of its neighbor in the column. Figure 3 illustrates that
the molecules of 2 also form π-stacks along the a axis, but in
a somewhat less efficient manner. The intermolecular face-to-
face distance is 3.50 Å and the molecules are inclined to the a
axis by 29.2°. Viewed perpendicularly to the molecular plane
(Figure 3B), the molecular packing is slipped along the short
molecular axis by about 0.75 ring. Since a smaller portion of
each ring overlaps vertically, the π-interaction is less efficient.
Dihydrodiborapentacenes 1-3 are all yellow solids. The
UV-vis spectrum of 1 in benzene shows a low-energy
maximum centered at 400 nm which has a marked vibronic fine
structure. The UV-vis spectrum of 3 in cyclohexane had a low-
energy maximum at 407 nm with emission at 410 nm. This
very small Stokes shift is consistent with minimal geometric
(16) Experimental procedure and characterization of 1: 2,3-bis(trimeth-
ylsilyl)naphthalene (0.55 g, 2.0 mmol) and toluene (5 mL) were added to
a Teflon screw-stoppered thick-walled glass reaction tube. After the mixture
was cooled to 0 °C, BBr3 (0.6 mL, 6.3 mmol) was added with magnetic
stirring and the valve was then closed. The mixture was next heated to 110
°C for 40 h, affording bright yellow crystals and a brown solution. After
the mixture was cooled to room temperature, the liquid was removed by
cannula filtration. The solid residue was washed with toluene (2 mL) and
dried in vacuo for 1 h to obtain analytically pure product 1 (yield 0.168 g,
22%; mp 282-284 °C). X-ray-quality crystals were obtained by recrystal-
lization from toluene. 1H NMR (500 MHz, C6D6): δ 9.14 (s, 4H,
H-5,7,12,14), 7.58, 7.17 (satisfactory computer simulation of AA′XX′ pattern
was achieved using δ1 7.588, δ2 7.174, JAB ) 8.1 Hz, JBB′ ) 6.4 Hz, JAB′
) 1.8 Hz, 8H, H-1,2,3,4,8,9,10,11). 13C NMR (125.7 MHz, C6D6): δ 142.9,
136.5, 130.6, 129.8, C-B not observable. 11B NMR (160.4 MHz, C6D6):
δ 57.5. UV-vis (C6D6): λmax 423, 400, 380, 338 nm. HRMS (EI, m/z):
calcd for C20H1211B279Br2 (M+), 431.9492; found, 431.9501. Anal. Calcd
for C20H12B2Br2: C, 55.38; H, 2.79. Found: C, 55.24; H, 2.76.
(17) Crystal data for 1: C20H12B2Br2, monoclinic, P21/c, a ) 3.932(2)
Å, b ) 19.055(7) Å, c ) 10.609(4) Å, ꢀ ) 92.275(5)°, V ) 794.2(5) Å3,
Z ) 2, Dc ) 1.814 g cm-3, T ) 123(2) K, λ(Mo KR) ) 0.710 73 Å. Data
were collected on a Siemens SMART CCD instrument. Final R indices (I
> 2σ(I)): R1 ) 0.0182, wR2 ) 0.0461. R indices (all data): R1 ) 0.0209,
wR2 ) 0.0472. GOF on F2 ) 1.042. Crystal data for 2: C22H18B2,
monoclinic, P21/c, a ) 3.968(1) Å, b ) 10.318(3) Å, c ) 19.103(6) Å, ꢀ
) 91.268(4)°, V ) 781.8(4) Å3, Z ) 2, Dc ) 1.291 g cm-3, T ) 108(2) K,
λ(Mo KR) ) 0.710 73 Å. Data were collected on a Siemens SMART CCD
instrument. Final R indices (I > 2σ(I)): R1 ) 0.0436, wR2 ) 0.1252. R
indices (all data): R1 ) 0.0593, wR2 ) 0.1356. GOF on F2 ) 1.065. Crystal
j
data for 3: C38H34B2: trigonal, R3, a ) b ) 30.219(1) Å, c ) 8.4832(7) Å,
V ) 6709.0(7) Å3, Z ) 9, Dc ) 1.141 g cm-3, T ) 85(2) K, λ(Mo KR) )
0.710 73. Data were collected on a Siemans SMART CCD instrument. Final
R indices (I > 2σ(I)): R1 ) 0.0494, wR2 ) 0.1315. R indices (all data):
R1 ) 0.0554, wR2 ) 0.1385. GOF on F2 ) 1.043.
(19) (a) Ashe, A. J., III; Kampf, J. W.; Klein, W.; Rousseau, R. Angew.
Chem., Int. Ed. Engl. 1993, 32, 1065. (b) Pan, J.; Kampf, J. W.; Ashe,
A. J., III Organometallics 2004, 23, 5626. (c) Chen, J.; Bajko, Z.; Kampf,
J. W.; Ashe, A. J., III Organometallics 2007, 26, 1563.
(18) Ashe, A. J., III; Klein, W.; Rousseau, R. Organometallics 1993,
12, 3225.

Communications
Organometallics, Vol. 27, No. 15, 2008 3641
Figure 3. (A) Molecular structure of 2 (ORTEP). Thermal ellipsoids
have been set at the 50% probability level. Selected bond distances
(Å): B-C(R), 1.564(2); B-C(R′), 1.558(2); B-C(exo), 1.578(2);
C(R)-C(R′), 1.446(2). (B) View along the axis perpendicular to
the molecular plane, showing slippage. (C) View of the packing
along the a axis, showing π-stacking.
Figure 2. (A) Molecular structure of 1 (ORTEP). Thermal ellipsoids
have been set at the 50% probability level. Selected bond distances
(Å): B-C(R), 1.549(2); B-C(R′), 1.543(2); B-Br, 1.950(2);
C(R)-C(R′), 1.454(2). (B) View along the axis perpendicular to
the molecular plane, showing slippage. (C) View of the packing
along the a axis, showing π-stacking.
synthetic methodology should allow preparation of a variety of
B-substituted dihydrodiborapentacenes.21 In general, boron
atoms of boraheterocycles interact strongly with their exocyclic
substituents;21 thus, electron density in the ring should also be
tunable in a manner which is not as readily achievable in
pentacenes. We believe that 6,13-dihydro-6,13-diborapentacenes
may find innovative applications as n-type organic semiconduc-
tors that complement those of the p-type PAHs.
distortion of the rigid planar core in the excited state. The
HOMO-LUMO gaps obtained from the onset of absorption,
2.8 eV for 1 (440 nm) and 3.0 eV for 3 (415 nm), are
significantly larger than the 2.07 eV for pentacene. Electro-
chemical measurements on 3 show the first reduction peak at
-1.23 V in acetonitrile. As expected, the reduction peak is
shifted positively relative to pentacene (-1.87 V).9b It is
comparable in magnitude with the reduction of perfluoropen-
tacene (-1.13 V)9b and C60 (-1.14 V),20 which are excellent
n-type semiconductors.
In summary, the novel 6,13-dihydro-6,13-diborapentacene
ring system is easy to prepare. These compounds have molecular
structures which are similar to that of pentacene but with two
fewer π-electrons, which makes them better electron acceptors.
In the solid state compounds 1 and 2 each form structures with
cofacial π-stacks, but in a somewhat different manner. Thus,
the details of the packing are sensitive to B-substituents. Existing
Acknowledgment. J.C. is grateful for the award of an
R. W. Parry Fellowship by the Chemistry Department of the
University of Michigan.
Supporting Information Available: Text and figures giving
experimental procedures and characterization data for all new
compounds and CIF files giving X-ray crystallographic data. This
materialisavailablefreeofchargeviatheInternetathttp://pubs.acs.org.
OM8005068
(21) For examples of other unsaturated boracyclic rings, see: (a) Ashe,
A. J., III; Kampf, J. W.; Mu¨ller, C.; Schneider, M. Organometallics 1996,
15, 387. (b) Ashe, A. J., III; Al-Taweel, S. M.; Drescher, C.; Kampf, J. W.;
Klein, W. Organometallics 1997, 16, 1884. (c) Liu, S.-Y.; Lo, M. M.-C.;
Fu, G. C. Angew. Chem., Int. Ed. 2002, 41, 174.
(20) Haddon, R. C.; Perel, A. S.; Morris, R. C.; Palstra, T. T. M.; Hebard,
A. F.; Fleming, R. M. Appl. Phys. Lett. 1995, 67, 121.