Highly electron-deficient and air-stable conjugated thienylboranes

Article abstract of DOI:10.1002/anie.201403700

Introduced herein is a series of conjugated thienylboranes, which are inert to air and moisture, and even resist acids and strong bases. X-ray analyses reveal a coplanar arrangement of the thiophene rings, an arrangement which facilitates p-π conjugation

Full text of DOI:10.1002/anie.201403700

Angewandte
Chemie
DOI: 10.1002/anie.201403700
Lewis Acids
Highly Electron-Deficient and Air-Stable Conjugated
Thienylboranes**
Xiaodong Yin, Jiawei Chen, Roger A. Lalancette, Todd B. Marder,* and Frieder Jꢀkle*
Abstract: Introduced herein is a series of conjugated thienyl-
boranes, which are inert to air and moisture, and even resist
acids and strong bases. X-ray analyses reveal a coplanar
arrangement of the thiophene rings, an arrangement which
facilitates p–p conjugation through the boron atoms despite the
Several strategies have been explored to stabilize organo-
boranes while still promoting effective p–p conjugation.
[
5]
Bulky groups, such as 2,4,6-tri-isopropylphenyl (Tip) and
[6]
2,4,6-tri-tert-butylphenyl (Mes*) have been successfully
utilized to sterically protect the boron atom from attack by
nucleophiles. In an alternative approach, tricoordinate bor-
anes have been embedded into cyclic and extended planar
presence of highly bulky 2,4,6-tri-tert-butylphenyl (Mes*) or
F
2
,4,6-tris(trifluoromethyl)phenyl ( Mes) groups. Short B···F
[
6,7]
contacts, which lead to a pseudotrigonal bipyramidal geometry
conjugated structures.
For applications in organic elec-
F
in the Mes species, have been further studied by DFTand AIM
tronics and sensing schemes, however, it is desirable not only
to increase the stability, but also to maximize the electron-
acceptor properties. A promising strategy in this respect is the
analysis. In contrast to the Mes* groups, the highly electron-
F
withdrawing Mes groups do not diminish the Lewis acidity of
À
F
[8]
boron toward F anions. These compounds can be lithiated or
use of Mes groups, which combine steric hindrance with
strongly electron-withdrawing character. Theoretical studies
by Marder and Weber et al. showed that replacement of Mes
iodinated under electrophilic conditions without decomposi-
tion, thus offering a promising route to larger conjugated
structures with electron-acceptor character.
F
with Mes groups in 5,5’-bis(dimesitylboryl)-2,2’-bithiophene
[
9]
significantly lowers the LUMO energy. We introduce herein
a new series of conjugated organoboranes with Mes* and
I
n recent years, main-group elements have been successfully
F
F
incorporated into the backbone of conjugated organic
oligomers and polymers, commonly resulting in unusual
Mes groups. The influence of the Mes moiety is clearly
demonstrated, and the organoboranes serve as promising
building blocks for conjugated oligomers and polymers. To
facilitate p–p conjugation with boron we utilize thiophene
moieties, because their small ring size should favor adoption
of a coplanar structure. Another advantage is the high
reactivity of the thiophene a position which lends itself to
further derivatization.
[1]
properties and improved performance. Among these con-
jugated hybrids, boron-containing materials have attracted
considerable attention. As a result of interactions between
the empty p orbital of boron and p-conjugated systems,
desirable optical and electronic properties are achieved,
which in turn enable applications in optoelectronics and
sensory materials. Two mesityl groups are generally suffi-
cient to stabilize three-coordinate aryl or vinyl boron com-
pounds with respect to attack by air, water, and most
[2]
[
3]
The compound 1 was readily prepared starting from 2-
(trimethylstannyl)thiophene by Sn–B exchange and then
treatment with a bulky aryl lithium reagent, Mes*Li or
[3f–n]
F
nucleophiles.
However, a significant drawback remains
MesLi, to give the borane monomers BDT and FBDT in an
the susceptibility to degradation upon attack by nucleophiles
in other systems, especially in the case of conjugated organo-
boranes with high Lewis acidity. The apparent dichotomy
that higher electron deficiency is desirable for application
purposes, but also tends to induce degradation, has become an
obstacle to further progress.
overall yield of 44% and 43%, respectively (Scheme 1).
[
4]
[*] Dr. X. Yin, J. Chen, Prof. R. A. Lalancette, Prof. Dr. F. Jꢀkle
Department of Chemistry, Rutgers University - Newark
73 Warren Street, Newark, NJ 07102 (USA)
E-mail: fjaekle@rutgers.edu
Homepage: http://chemistry.rutgers.edu/fjaekle
Prof. Dr. T. B. Marder
Institut fꢁr Anorganische Chemie
Julius-Maximilians-Universitꢀt Wꢁrzburg
Am Hubland, 97074 Wꢁrzburg (Germany)
E-mail: todd.marder@uni-wuerzburg.de
[**] This material is based upon work supported by the National Science
Foundation under Grant No. CHE-1112195.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201403700.
Scheme 1. Synthesis of sterically shielded (oligo)thienylboranes.
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Using similar methods, we also synthesized the corresponding
dimeric species BDT2 and FBDT2 from 2,5-bis(trimethyl-
stannyl thiophene) with the expectation that they would allow
assessment of the degree of extended conjugation in the
thiophene rings are 1.541(7) and 1.526(6) ꢀ [1.546(7) and
1.518(11) ꢀ)] for FBDT, and on average are significantly
shorter than those in BDT [1.589(6) and 1.568(4) ꢀ], and
could indicate stronger p–p conjugation between boron and
the thiophenes in the more-electron-deficient FBDT. Indeed,
the B–C distances in FBDT are close to those in related
pentafluorophenyl-substituted thienylboranes, for example,
F
presence of the Mes* or Mes groups. All of the products
proved to be perfectly air and moisture stable and were easily
purified by silica-gel column chromatography without any
precautions. Remarkably, they proved to be resistant to
oxidants, acids, and strong bases, such as halogens, acetic acid,
and n-butyl lithium, respectively, thus enabling facile func-
tionalization.
Single crystals of BDT and FBDT were obtained by
recrystallization from hexanes and the structure plots are
shown in Figure 1a,b. Metrical parameters of a second
[11]
(C F ) B-(Th) -B(C F ) 1.507(3) and (C F ) B-(Th) -NPh
2
6
5
2
2
6
5
2
6
5
2
2
[
4b]
1.502(3). Similar conformations are observed in the molec-
ular structures of BDT2 and FBDT2 (Figure 1c,d) with small
torsion angles of 10.6 (BDT2) and 4.8 (Th -Th )/16.58 (Th -
S1
S2
S2
[
12]
Th ) (FBDT2). We also note that the Mes* rings in BDT2
S3
F
adopt an almost coplanar conformation, but the Mes rings in
FBDT2 are oriented so that the space between them is
enlarged. This orientation turns out to be related to an
unusual chainlike supramolecular structure of FBDT2, in
which the terminal thiophene ring of a neighboring molecule
F
is captured in a sandwich-like arrangement between the Mes
groups (see Figure S3 in the Supporting Information).
Another very interesting aspect is the observation of short
B···F contacts in the structures of FBDTand FBDT2. The B–F
distances of 2.66 and 2.49 ꢀ (2.60 and 2.89 ꢀ) for FBDT and
2
.55, 2.56, 2.57, and 2.61 ꢀ for FBDT2 are all much shorter
[
13]
than the sum of the B and F van der Waals radii
of
[
14]
3
.39 ꢀ. The slight variations in the distances are related to
rotation of the CF group. To develop a better understanding
3
of the nature of these (weak) interactions, we performed full
geometry optimization at the B3LYP/6-31 + G* level of
[
15]
theory, starting from the crystal structure parameters. In
the optimized structure of FBDT, the B···F distances are
identical at 2.64 ꢀ, which is comparable to the experimental
distances. Atoms-in-molecules (AIM) analysis revealed the
presence of bond paths between B and the neighboring F
atoms, thus resulting in a pentacoordinate geometry (Fig-
[
16]
2
ure 1e). The electron density (1) and its Laplacian (! 1) at
the bond critical points (BCPs) for FBDT were computed to
À3
À5
be 0.014 ea0 (a0 is the Bohr radius) and 0.050 ea0
,
respectively. The corresponding values for FBDT2 proved
À3
to be similar with average values of 0.015 ea0
0
and
À5
.050 ea0 . The relatively lower electron density for FBDT
and FBDT2 in comparison to that reported by Yamaguchi for
[
17]
[18]
related B···Cl
and by Yamashita for B···O contacts
is
Figure 1. X-ray structure plots of a) BDT, b) FBDT, c) BDT2, d) FBDT2,
and e) atoms-in-molecules (AIM) analysis of FBDT, showing B···F bond
paths (purple lines) and bond critical points (BCP, red points).
likely a result of the smaller atomic radii and higher electro-
negativity of fluorine.
The electron acceptor properties were studied by cyclic
voltammetry (CV; Figure 2). Single reversible reduction
waves were observed at E = À2.58 (BDT) and À2.22 V
independent molecule in the unit cell of FBDT are provided
within parentheses in the following discussions. Rotation
about the B1ÀC1 and B1ÀC5 bonds results in disorder of the
1
/2
+
/0
(FBDT, vs Fc ), which unequivocally demonstrates that the
Mes group strongly decreases the LUMO energy level.
F
[15]
thiophene rings, but the conformations in which the S atoms
Results from DFT calculations further confirm this phe-
nomenon. The LUMOs of both compounds show strong
overlap of the empty p orbital on B with the thiophene
orbitals, thus resulting in an effective conjugation path
(Table 1 and Figure 3). However, there are significant differ-
ences in that the Mes* group strongly contributes to the
HOMO and even more so the HOMO-1 of BDT, whereas in
F
[10]
point away from the Mes*/ Mes group are dominant. The
F
Mes*/ Mes groups adopt orientations almost orthogonal to
the thiophene rings with dihedral angles of 85.48 and 85.88 for
BDT and 84.48 and 88.28 (86.18 and 89.28) for FBDT.
Importantly, the thiophenes and the boron atom form
a quasiplanar structure. The small torsion angle, between the
thiophene rings, of 19.08 for BDTand 16.38 (5.78) for FBDT is
ideal for promoting p conjugation along the main chain
through the empty p orbital on boron. The BÀC bonds to the
F
FBDT the Mes group contributes to the LUMO and makes
up almost exclusively the LUMO + 1 (see Figure S5 in the
Supporting Information). Importantly, both the HOMO and
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Figure 4. Comparison of UV-Vis spectra in THF. Vertical lines corre-
spond to calculated excitations (TD-DFT, B3LYP/6-31+G*; see
Ref. [15]).
Figure 2. Comparison of cyclic voltammograms in THF/0.1m Bu N-
4
À3
À1
0/+
À1
[
PF ] (1ꢂ10 moll ; vs. Fc , n=100 mVs ).
6
very bulky groups do not interfere with, but likely even
enhance, electronic communication as a result of the rigidi-
[
20]
fication of the Th-B-Th-B-Th skeleton.
Table 1: Comparison of experimental and calculated LUMO energy
levels.
The simultaneous decrease of both the HOMO and
LUMO orbital energies of FBDT relative to BDT leads to
similar absorption characteristics of these compounds. As
shown in Figure 4, BDT features two main absorption bands
with maxima at l = 324 and 270 nm in THF solution, while
a slight bathochromic shift to l = 326 and 280 nm is observed
for FBDT. The origin of these electronic transitions was
BDT
FBDT
BDT2
FBDT2
[
a]
LUMO (exp) /eV
À2.22
À2.27
À2.58
À2.59
À2.66
À2.54
À3.07
À3.12
[
b]
LUMO (calcd) /eV
[
a] Determined using the equation ELUMO =À(4.8+Evs.Fc); [b] Calculated
orbital energies (DFT, B3PW91/6-311+G*; see Ref. [15])
[15]
verified by TD-DFT calculations (B3LYP/6-31 + G*). The
lowest energy transition to the S state of BDT corresponds to
1
charge transfer from the Mes* group (HOMO, HOMOÀ1) to
the dithienylborane moiety (LUMO), but this transition is
very weak. Instead, the absorption bands of BDT at l = 325
and 275 nm are mainly due to excitation from lower energy
thiophene-centered orbitals to the LUMO (see Table S4 and
Figure S5 in the Supporting Information). Differently, for
FBDT the longest wavelength excitation is predominantly
HOMO to LUMO, where the HOMO is localized on the
thiophene rings and the LUMO is delocalized over the
dithienylborane moiety with a pronounced contribution from
the boron p orbital. The coplanar structure in BDT2 and
FBDT2 allows for effective extension of p conjugation and, as
a consequence, the UV-Vis absorptions are strongly red-
shifted compared to that of the monoboron analogues (l =
Figure 3. Calculated frontier orbital energy levels and HOMO/LUMO
orbital plots of BDT and FBDT (DFT, B3PW91/6-311+G*; see
Ref. [15]).
3
60 nm; Figure 4). We note that BDT, FBDT, and BDT2 are
essentially non-emissive in THF solution, while for FBDT2
only a very weak blue fluorescence with a quantum yield of
0.8% is observed.
the LUMO energy levels of FBDT are significantly lowered.
The former facilitates reduction, while the latter is expected
to be beneficial for the oxidative stability.
[
21]
To explore whether these compounds are still capable of
acting as typical Lewis acids we performed binding studies
For the dimeric compounds BDT2 and FBDT2, two
reversible redox waves are observed in the CV (Figure 2), and
they correspond to stepwise reduction of the individual
borane moieties. In comparison to the monoboron analogues,
the first reduction potentials (BDT2: E1/2 = À2.14 V; FBDT2:
E1/2 = À1.73 V) show a pronounced anodic shift, which is
a result of strong electronic communication between the
boron atoms. Indeed, the LUMO orbital plots (Figure S5)
display clear evidence of overlap of the two boron p orbitals
and the central thiophene moiety. The second redox processes
À
[22]
with F as a small and powerful Lewis base. For BDT or
BDT2, even the exposure to a very large excess of [Bu N]F
4
(> 20 equivs) did not lead to any detectable changes in the
NMR signatures (CDCl ) or the absorption characteristics
3
[
23]
(THF). In contrast, FBDT and FBDT2 respond readily to
small amounts of F . As shown in Figure S16 in the
À
Supporting Information, the absorption of FBDT at l =
320 nm decreases rapidly with increasing amounts of added
À
F
(lg b = 7.2). Since p–p conjugation is blocked in the
À
of BDT2 (E1/2 = À2.84 V) and FBDT2 (E = À2.34 V) are
resulting borate [FBDT-F] , only weak absorptions are
1/2
shifted to more negative potentials because of the electronic
observed in the range of l = 250–350 nm, and are a result of
À
F
interactions between the boron radical anion (B C) and
charge transfer from thiophene to Mes (see Table S5 in the
[
19]
[24]
neutral boron.
Remarkably large splittings of DE = 700
Supporting Information). For FBDT2, the absorption at
(BDT2) and 610 mV (FBDT2) further demonstrate that the
l = 370 nm decreases and is slightly blue-shifted to about l =
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3
7
3
50 nm upon formation of the monoborate species (lg b =
.8). In the presence of an excess F , the new band at l =
50 nm only very gradually disappears (lg b12 ca. 3.7), thus
Keywords: boron · conjugation · fluoride · heterocycles ·
Lewis acids
1
1
.
À
indicating a much more severe steric strain when two
tetrahedral fluoroborate moieties are generated in close
proximity to one another (see Scheme S2 in the Supporting
Information).
To demonstrate the general utility of our approach we also
explored the derivatization of BDT and FBDTwith halogens.
For BDT, iodination with NIS in chloroform in the presence
of acetic acid occurred readily and with excellent selectivity
for the a-position (Scheme 2). However, the more-electron-
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1
4727 – 14740.
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Received: March 25, 2014
Published online: July 17, 2014
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Brockhinke, H. G. Stammler, B. Neumann, Dalton Trans. 2009,
[19] The first and second reduction potentials of BDT2 and FBDT2
are much lower than those reported for 2,5-bis-
(borolyl)thiophene (Epc = À2.53 V and À3.08 V; H. Braunsch-
weig, V. Dyakonov, B. Engels, Z. Falk, C. Hçrl, J. H. Klein, T.
Kramer, H. Kraus, I. Krummenacher, C. Lambert, C. Walter,
Angew. Chem. 2013, 125, 13088 – 13092; Angew. Chem. Int. Ed.
2013, 52, 12852 – 12855), because THF does not coordinate to the
electron-deficient, but sterically protected borane moieties in
our compounds.
1339 – 1351.
[
[
10] BDT: 63% occupation of S1 and 78% of S2; FBDT: 72%
occupation of S1 and 82% of S2 (73% for S3, 50% for S4; see
Figure S1 in the Supporting Information).
11] A. Sundararaman, K. Venkatasubbaiah, M. Victor, L. N.
Zakharov, A. L. Rheingold, F. Jꢁkle, J. Am. Chem. Soc. 2006,
128, 16554 – 16565.
[
[
[
12] The plane ThS1 is defined by C1-C2-C3-C4-S1, ThS2 by C5-C6-
C7-C8-S2, and ThS3 by C9-C10-C11-C12-S3.
[20] The redox splitting of 700 mV for BDT2 is comparable to that of
Kaimꢃs 1,4-bis(dimesitylboryl)benzene (690 mV): A. Schulz, W.
Kaim, Chem. Ber. 1989, 122, 1863 – 1868.
[21] Almost identical UV-Vis absorption and fluorescence data are
observed in the solid state. A thin film of FBDT2 shows a slightly
enhanced fluorescence with a quantum yield of 1.8% (see
Figure S9a in the Supporting Information).
13] M. Mantina, A. C. Chamberlin, R. Valero, C. J. Cramer, D. G.
Truhlar, J. Phys. Chem. A 2009, 113, 5806 – 5812.
1
1
14] The B NMR chemical shifts of FBDT (d = 51.5) and FBDT2
(
(
d = 52.2) are smaller than those of BDT (d = 53.5) and BDT2
d = 53.9), which might further substantiate the presence of B–F
interactions as a more downfield shifted B signal would be
expected for the compounds with the more electron withdrawing
aryl groups. However, a stronger B–C(thiophene)interaction in
the fluorinated species could also result in an upfield shift of the
[22] C. R. Wade, A. E. J. Broomsgrove, S. Aldridge, F. P. Gabbai,
Chem. Rev. 2010, 110, 3958 – 3984.
[23] This finding is consistent with earlier results by Yamaguchi et al.
See Ref. [6a].
[24] The binding process was also monitored by NMR titrations (see
Figures S10–12 in the Supporting Information), and the rever-
sibility confirmed by treatment with a large amount of water (see
Figure S13 in the Supporting Information).
1
1
B NMR resonances.
[
15] All calculations were performed using the Gaussian 09 suite of
programs (Gaussian 09 (Revision B.01), M. J. Frisch, et al.,
Gaussian, Inc., Wallingford CT, 2010). For details on the
Angew. Chem. Int. Ed. 2014, 53, 9761 –9765
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
9765