Unsymmetrically substituted 9,10-dihydro-9,10-diboraanthracenes as versatile building blocks for boron-doped π-conjugated systems

Article abstract of DOI:10.1002/chem.201101701

The targeted hydrolysis of the 9,10-dihydro-9,10-diboraanthracene adduct (Me₂S)HB(C₆H₄)₂BH(SMe₂) (1) with 0.5 equiv of H₂O leads to formation of the borinic acid anhydride [(Me₂S)HB(C₆H₄)₂B] ₂O (2) and thereby provides access to the field of unsymmetrically substituted 9,10-dihydro-9,10-diboraanthracenes. Compound 2 reacts with tBuCi￡CH to give the corresponding vinyl derivative in an essentially quantitative conversion. Subsequent cleavage of the B-O-B bridge by LiAlH ₄ with formation of hydridoborate functionalities is possible but is accompanied by partial BiC(vinyl) bond degradation. This situation changes when the related mesityl derivative [MesB(C₆H₄) ₂B]₂O (7) is employed, which can be synthesized from BrB(C₆H₄)₂BBr (6) by treatment with 1 equiv of MesMgBr and subsequent hydrolysis. The reaction of 7 with LiAlH₄ in tetrahydrofuran (THF) furnishes Li[MesB(C₆H₄) ₂BH₂] (8); hydride elimination with Me₃SiCl leads to formation of the THF adduct MesB(C₆H₄) ₂BH(THF) (9 THF). Alternatively, 7 can be transformed into the bromoborane MesB(C₆H₄)₂BBr (10) by treatment with BBr₃. A Br/H-exchange reaction between 10 and Et₃SiH yields the donor-free borane MesB(C₆H₄)₂BH (9), which forms B-H-B bridged dimers (9)₂ in the solid state. The vinyl borane MesB(C₆H₄)₂BC(H)-C(H)Mes (14) is accessible from MesCi￡CH and either 9 THF or 9. Compared with the related compound Mes₂BC(H)-C(H)Mes, the electronic absorption and emission spectra of 14 reveal bathochromic shifts of I-(abs)=17 nm and I-(em)=74 nm, which can be attributed to the rigid, fully delocalized π framework of the [MesB(C₆H₄)₂B] chromophore. Putting boron in the ring: The unsymmetrically substituted 9,10-dihydro-9,10-diboraanthracene MesB(C₆H₄)₂BH, which exists as a dimeric species in the solid state and shows strong photoluminescence (see figure), is a versatile reagent that can be used to introduce a rigid, boron-doped moiety into conjugated π-electron systems through hydroboration reactions.

Full text of DOI:10.1002/chem.201101701

DOI: 10.1002/chem.201101701
Unsymmetrically Substituted 9,10-Dihydro-9,10-diboraanthracenes as
Versatile Building Blocks for Boron-Doped p-Conjugated Systems
Estera Januszewski,[a] Andreas Lorbach,[a] Rekha Grewal,[a] Michael Bolte,[a]
Jan W. Bats,[b] Hans-Wolfram Lerner,[a] and Matthias Wagner*[a]
Abstract: The targeted hydrolysis of
the 9,10-dihydro-9,10-diboraanthracene
when the related mesityl derivative
[MesB(C6H4)2B]2O (7) is employed,
which can be synthesized from BrB-
(C6H4)2BBr (6) by treatment with
1 equiv of MesMgBr and subsequent
hydrolysis. The reaction of with
LiAlH4 in tetrahydrofuran (THF) fur-
nishes Li[MesB(C6H4)2BH2] (8); hy-
bromoborane MesBACTHNUGTERN(NUG C6H4)2BBr (10) by
treatment with BBr3. A Br/H-exchange
reaction between 10 and Et3SiH yields
the donor-free borane MesBACHTGNUTERNNU(G C6H4)2BH
(9), which forms B-H-B bridged dimers
(9)2 in the solid state. The vinyl borane
MesBACTHNGUTERNNU(G C6H4)2BC(H)=C(H)Mes (14) is
G
adduct (Me2S)HB
G
N
with 0.5 equiv of H2O leads to forma-
tion of the borinic acid anhydride
G
[(Me2S)HB
N
7
by provides access to the field of un-
symmetrically substituted 9,10-dihydro-
ꢁ
G
R
accessible from MesC CH and either
9·THF or 9. Compared with the related
compound Mes2BC(H)=C(H)Mes, the
electronic absorption and emission
spectra of 14 reveal bathochromic
9,10-diboraanthracenes. Compound
2
dride elimination with Me3SiCl leads to
formation of the THF adduct MesB-
ACHUTNGREN(UNG C6H4)2BHACHTUTGNREN(NUGN THF) (9·THF). Alterna-
ꢁ
reacts with tBuC CH to give the corre-
sponding vinyl derivative in an essen-
tially quantitative conversion. Subse-
quent cleavage of the B-O-B bridge by
LiAlH4 with formation of hydridobo-
rate functionalities is possible but is ac-
ꢀ
tively, 7 can be transformed into the
shifts of DlACTHUNRTGNEUNG(abs)=17 nm and Dl(em)=
74 nm, which can be attributed to the
rigid, fully delocalized p framework of
the [MesBACTHNGUTERN(NUG C6H4)2B] chromophore.
Keywords: boranes · borates · con-
jugation · hydroboration · lumines-
cence.
companied by partial B C
N
degradation. This situation changes
Introduction
ed for the development of molecular switches and sen-
sors.[4–7]
The incorporation of three-coordinate boron atoms into
conjugated p-electron frameworks leads to changes in the
electronic structure that often bring about enhanced lumi-
nescence and charge-transport properties.[1–4] Moreover, the
ability of boron atoms to form Lewis acid–base pairs and
thereby to disrupt the p-conjugation pathway can be exploit-
However, the propensity of organoboranes to react with
Lewis bases is also a disadvantage, because it renders the
compounds intrinsically sensitive to air and moisture. The
vast majority of organoboranes employed in materials sci-
ence and sensor technology therefore contain at least one,
in many cases two, bulky substituents for kinetic stabiliza-
tion (cf. the popular diACHTNUTGRNEUNG
(mesityl)boryl group).[8] Even though
the concept of steric protection has already furnished a vari-
ety of remarkable organoboranes that can be handled in air
and purified by chromatography on silica gel, it nevertheless
suffers from certain disadvantages: 1) The size of any ana-
lyte that can be detected by the corresponding organobor-
ane sensors is very limited (for example, Fꢀ, CNꢀ). 2) A di-
[a] Dipl.-Chem. E. Januszewski, Dr. A. Lorbach, B. Sc. R. Grewal,
Dr. M. Bolte, Dr. H.-W. Lerner, Prof. Dr. M. Wagner
Institut fꢀr Anorganische und Analytische Chemie
Goethe-Universitꢁt Frankfurt
Max-von-Laue-Straße 7, 60438 Frankfurt am Main (Germany)
Fax : (+49)69-798-29260
E-mail: Matthias.Wagner@chemie.uni-frankfurt.de
ACHTUNGTREN(NUNG mesityl)boryl group with its single free valence can only act
[b] Dr. J. W. Bats
as a peripheral substituent, and not become an integral part
Institut fꢀr Organische Chemie, Goethe-Universitꢁt Frankfurt,
Max-von-Laue-Straße 7, 60438 Frankfurt am Main (Germany)
of the p-electron system. 3) Steric congestion in triarylbor-
ꢀ
anes leads to twisting of the aryl substituents about the B C
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201101701. It contains protocols
for the derivatization of 2; synthesis/analytical data of HOB-
bonds and thereby to decreased p conjugation across the
boron center. Thus, attachment of a diACTHNUTRGNEUNG(mesityl)boryl group
merely adds one vacant boron-centered p-orbital to the p-
electron cloud, whereas the two mesityl rings remain more
or less spectator groups.
ACHTUNGTRENNUNG(C6H4)2BOH, 5, 6, MesBACHTUGNTRENN(UNG C6H4)2BOH, 12, 13a/13b, and
Mes2BC(H)=C(H)Mes; discussion of the synthesis, NMR spectra
1
and X-ray crystal structure analyses of 8 and 9·THF; H NMR spec-
troscopic monitoring of the reaction 7 ! 10; details of the X-ray crys-
For the design of building blocks other than di-
tal structure analyses of ClBACHTNUGTRNENG(U C6H4)2BCl, 6, HOBAHCTUNGTREN(NUGN C6H4)2BOH, and
AHCTUNGTRENNUNG
11; X-ray crystallographic files (CIF) of ClB
ACHTNUGNERT(NGNU C6H4)2BCl, HOB-
ACHTUNGTRENNUNG
12696
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 12696 – 12705

FULL PAPER
tron system; 2) a rigid planar framework would guarantee
maximum p overlap between the boron atoms and the aro-
matic substituents; 3) a cyclic structure should be most
stable. All these requirements are met by the 9,10-dihydro-
9,10-diboraanthracene framework, which can be integrated
into p-electron systems either by nucleophilic substitution
(cf. starting material A; Scheme 1) or by hydroboration re-
Scheme 1. Symmetrically substituted 9,10-dihydro-9,10-diboraanthracenes
A and B; polymers C obtained by hydroboration polymerization of aro-
matic dialkynes with B (R=H, OHex); 9H-9-borafluorene D, which is a
monotopic relative of B; unsymmetrically substituted 9,10-dihydro-9,10-
diboraanthracenes E.
actions (cf. starting material B; Scheme 1). 9,10-Dihalo-9,10-
dihydro-9,10-diboraanthracenes A have been extensively re-
ported in the literature (X=Cl,[9,10] Br;[9,11,12] for optimized
synthesis protocols, a comprehensive compilation of NMR
data, and X-ray crystal structure analyses see the Supporting
Information of this paper). In contrast, the parent com-
pound B has only recently been described.[13] Compound B,
which forms a B-H-B bridged polymer (B)n in the solid
state, can be used for hydroboration polymerization reac-
tions (cf. product C; Scheme 1)[13] either directly or after
conversion into the more soluble dimethyl sulfide adduct (1;
Scheme 2).[14] Polymer C exhibits green photoluminescence,
and various low molecular-weight analogues RC(H)=
Scheme 2. Synthesis of compounds 2–5. Reagents and conditions: i) H2O
ꢁ
(0.5 equiv), THF/Me2S, room temperature; ii) tBuC CH (excess), C6D6,
room temperature; iii) LiAlH4 (excess), [D8]THF, room temperature.
9,10-diboraanthracene B. Compound D, which adopts a
unique phenyl-bridged dimeric structure (D)2, is not stable
in solution over the long-term, but readily undergoes a ring-
opening oligomerization reaction.[17] Nevertheless, provided
that freshly prepared samples of the compound are immedi-
ately used for further conversions, D is a valuable hydrobo-
ration reagent.
DFT calculations indicate that the ring-opening polymeri-
zation (ROP) of D is largely driven by the loss of anti-
C(H)B
Moreover, appropriately designed 9,10-dihydro-9,10-dibora-
anthracenes (including B) act as reversible two-electron ac-
ACHTUNGTRENNUNG
(C6H4)2BC(H)=C(H)R are also strongly emissive.[15]
AHCTUNGTRENNUNG
aromaticity in the central borole ring.[17] Monomeric 9,10-di-
G
hydro-9,10-diboraanthracene also possesses a formally anti-
ACHTUNGTRENaNUGN romatic central six-membered ring, but shows no tendency
ceptors, the dianionic form being isoelectronic with anthra-
cene.[16] Due to these remarkable optoelectronic properties,
the entire class of compounds is very promising for applica-
tions in organic solar cells (OSCs) or light-emitting devices
(OLEDs).
To gain a deeper understanding of the optoelectronic
properties of polymers of type C, it is desirable to have
facile access to well-defined smaller model systems. We
have already tested 9H-9-borafluorene (D; Scheme 1) as a
truncated monotopic analogue of the ditopic 9,10-dihydro-
toward the ROP reaction. This observation, among others,
led us to the conclusion that the electronic structure of 9H-
9-borafluorene is still too dissimilar to that of 9,10-dihydro-
9,10-diboraanthracene to regard the former as a well-de-
signed end cap of low molecular-weight analogues of poly-
mers C.
Herein, we describe protocols for the scalable generation
of unsymmetrically substituted 9,10-dihydro-9,10-dibora-
AHCTUNGTREGaNNUN nthracenes E (Scheme 1) in which one boron atom is pro-
Chem. Eur. J. 2011, 17, 12696 – 12705
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
12697

M. Wagner et al.
Table 1. Crystallographic data for 2, 7, and 10.
tected by a comparatively inert group, whereas the second
boron atom bears a reactive substituent (for example, R1 =
OR, Mes; R2 =H, Br; Mes=mesityl).
2
7
10
formula
Mr
C28H30B4OS2
489.88
C42H38B4O
601.96
C21H19B2Br
372.89
color, shape
T [K]
radiation, l [ꢃ]
crystal system
space group
a [ꢃ]
b [ꢃ]
c [ꢃ]
a [8]
b [8]
colorless, block colorless, plate yellow, plate
173(2)
173(2)
173(2)
Results and Discussion
MoKa, 0.71073
orthorhombic
P212121
9.6827(10)
15.4538(10)
18.1279(11)
90
MoKa, 0.71073
triclinic
P1
MoKa, 0.71073
monoclinic
P21/c
15.0527(15)
8.1450(6)
14.8148(18)
90
93.863(9)
90
1812.2(3)
4
¯
A potentially viable six-step route to compounds of type E
has recently been published by Kawashima et al.[18] Howev-
er, the only 9,10-dihydro-9,10-diboraanthracene isolated was
8.8596(18)
11.720(2)
17.288(4)
93.33(3)
98.44(3)
100.29(3)
1740.5(7)
2
the symmetrically substituted molecule MesBACHTUNRGTNEUNG(C6H4)2BMes.
Described below are therefore the first examples of deriva-
tives E with R1 ¼ R2, together with reactivity studies and a
comparison of the electronic spectra of MesB-
90
90
g [8]
V [ꢃ3]
Z
2712.6(4)
4
ACHTUNGTRENNUNG(C6H4)2BC(H)=C(H)Mes and Mes2BC(H)=C(H)Mes.
1calcd [gcmꢀ3
]
1.200
1.149
1.367
F
G
1032
0.216
636
0.065
760
2.266
Symmetry breaking by targeted hydrolysis: During investiga-
tions into the hydrolytic stability of the ditopic borane
adduct 1 (Scheme 2),[14] we observed the ready formation of
borinic acid anhydride 2 (Scheme 2), which could be sepa-
rated from residual 1 by fractional crystallization from
SMe2. Optimized yields of close to 50% were obtained
when 0.5 equiv of H2O were employed.
An X-ray crystal structure analysis of 2 revealed that the
compound contains two unsymmetrically substituted 9,10-di-
hydro-9,10-diboraanthracene moieties (Figure 1; Table 1).
]
0.35ꢄ0.34ꢄ0.27 0.27ꢄ0.24ꢄ0.11 0.27ꢄ0.25ꢄ0.13
reflections collected 28017
independent reflec- 5238 (0.1155)
tions (Rint
13 930
6462 (0.0899)
7395
3187 (0.0741)
)
data/restraints/pa-
rameters
5238/0/325
6462/0/430
3187/0/220
GOF on F2
0.986
0.807
0.923
R1, wR2 [I> 2s(I)]
R1, wR2 (all data)
largest diff peak
0.0829, 0.2005
0.1318, 0.2289
0.565, ꢀ0.445
0.0515, 0.0954
0.1259, 0.1108
0.176, ꢀ0.193
0.0521, 0.1101
0.0832, 0.1187
0.907, ꢀ0.676
and hole [eꢃꢀ3
]
ent, we conclude that B-O-B deformation is associated with
a shallow potential well so that the bond lengths and the
bond angle are easily influenced by crystal packing forces.
As can be expected for a heteroallene derivative, we find a
perpendicular arrangement of the two BR2 planes in 2 (cf.
C1-B1-C11//C31-B3-C41=89.6(5)8).
All key bond lengths and angles involving the four-coordi-
nate boron atoms of 2 are similar to those in the starting
material and are therefore not discussed further.
The 11B NMR spectrum (C6D6) of 2 is characterized by
two resonances at d=ꢀ3.0 (B(H)SMe2) and 43.2 ppm (BO).
Thus, the three-coordinate boron atoms possess almost the
same chemical shift values as those of diphenylborinic acid
Figure 1. Molecular structure of 2 in the solid state (for S(2)Me2 only the
major occupied site is shown); displacement ellipsoids at the 30% proba-
bility level, H atoms (except on boron) omitted for clarity. Selected bond
anhydride (d
nate boron centers are much better shielded in the hydroly-
sis product 2 than in the starting material 1 (d
(11B)=
(B,H) coupling is not re-
(11B)=46.1 ppm[19]), whereas the four-coordi-
ACHTUNGTRENNUNG
ꢀ
ꢀ
lengths [ꢃ], bond angles [8], and dihedral angle [8]: B1 O1 1.345(7), B3
ACHTUNGTRENNUNG
ꢀ
ꢀ
O1 1.337(7), B2 S1 2.029(6), B4 S2 2.073(8); B1-O1-B3 156.5(5), S1-B2-
C2 104.1(4), S1-B2-C12 101.6(3), S2-B4-C32 104.9(5), S2-B4-C42 96.8(5);
C1-B1-C11//C31-B3-C41 89.6(5).
1
28.1 ppm[14]). In both cases, the J
ACHTUNGTRENNUNG
solved, but broad resonances for the boron-bound hydrogen
1
atoms are detectable in the H NMR spectra. In line with
ꢀ
The B-O-B bridge possesses an average bond length (B
the different substituents at the peripheral and internal
boron atoms, the four phenylene rings give rise to two ap-
O)av of 1.341(7) ꢃ, the B-O-B bond angle amounts to
156.5(5)8. These metrical parameters are in good agreement
with the corresponding values for one of the two known
1
parent triplets and two doublets in the H NMR spectrum,
and to four resonances in the 13C NMR spectrum (signals of
carbon atoms attached to boron were not detected due to
ꢀ
polymorphs of diphenylborinic acid anhydride ((B O)av =
1.346(4) ꢃ; B-O-B=152.7(2)8).[19] The second polymorph,
unresolved 1J
ACTHNGUTERNNU(G B,C) coupling and quadrupolar broaden-
however, exhibits significantly longer B O bonds ((B
O)av =1.370(3) ꢃ), together with a narrower B-O-B angle
(147.3(2)8).[20] Given that the packing motifs of the two poly-
morphs of diphenylborinic acid anhydride are quite differ-
ing[21]).
ꢀ
ꢀ
Exploratory investigations into the reactivity of 2 were
undertaken on an NMR scale. First, the compound was
ꢁ
treated in C6D6 with a tenfold excess of tBuC CH. After
12698
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Chem. Eur. J. 2011, 17, 12696 – 12705

Boron-Doped p-Conjugated Systems
FULL PAPER
30 min at room temperature, 1H NMR spectroscopic analysis
revealed an essentially quantitative conversion into the di-
vinyl borane 3 (Scheme 2), as evidenced by the presence of
two doublets at d=6.66 and 7.01 ppm (2ꢄ2H) with a 3J-
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
sents a versatile building block for the generation of other
unsymmetrically substituted 9,10-dihydro-9,10-diboraanthra-
cenes, because its two HB functionalities can readily be
used in hydroboration reactions.
Further derivatization of 3 at its B-O-B bridge was at-
tempted by the reaction with excess LiAlH4 in [D8]THF
(see the Supporting Information). In situ 11B NMR spectros-
copy revealed a triplet resonance at d=ꢀ16.0 ppm (1J-
A
ACHTUNGTRNE(NUNG B,H)=
60 Hz), which points toward a reaction product possessing
two chemically different four-coordinate boron centers,
H2BR2 and HBR3, respectively. In the H NMR spectrum,
vinyl resonances were present at d=5.63 (1H) and 5.89 ppm
3
(1H); the latter signal showed fine splitting due to J
N
coupling with one BH hydrogen atom. The phenylene reso-
nances appeared as complex overlapping multiplets at d=
6.58 (4H) and 7.24 ppm (4H). We therefore propose that
the B-O-B bridge has indeed been cleaved, with formation
of the unsymmetrical hydridoborate 4 (Scheme 2). However,
it has so far not been possible to develop a fully selective
synthetic protocol because the symmetrical hydridoborate
Scheme 3. Synthesis of compounds 7–9·THF. Reagents and conditions:
i) 1) MesMgBr (1 equiv), toluene, ꢀ788C!room temperature; 2) H2O
(excess), CHCl3, room temperature; ii) LiAlH4 (1 equiv), Et2O/THF,
room temperature; iii) Me3SiCl (excess), Et2O/THF, room temperature.
Li2ACHTUNGTRENNUNG[H2BACHTUNGTRENNUNG(C6H4)2BH2] (5) is always generated as a byproduct
(at least 15%). Compound 5 was identified by comparison
of its NMR data with those of an authentic sample prepared
from HOBACHTUNGTRENNUNG(C6H4)2BOH and LiAlH4 (see the Supporting In-
formation for more information and an X-ray crystal struc-
ture analysis of the borinic acid).
The solid-state structure of 7 reveals the desired com-
pound with peripheral mesityl substituents (Figure 2;
Table 1). Compared with 2, subtle differences are observed
ꢀ
for the central B-O-B linker ((B O)av =1.366(4) ꢃ (7) vs.
1.341(7) ꢃ (2); B-O-B=139.4(2)8 (7) vs. 156.5(5)8 (2)). As
Symmetry breaking by nucleophilic substitution: Our experi-
ences with the system 3/LiAlH4 indicate that, in principle,
such borinic acid anhydrides are useful precursors for the
synthesis of unsymmetrically substituted 9,10-dihydro-9,10-
ꢀ
diboraanthracenes, but that the B CACHTUNTRGNEUNG(vinyl) bond is too frag-
ile to persist under the reaction conditions applied. We
therefore decided to replace the tert-butylvinyl substituents
in 3 by more robust mesityl groups and to explore the reac-
tivity of the resulting compound 7 (Scheme 3).
Preferential monosubstitution of BrB
feasible with MesMgBr[23] in toluene provided that high dilu-
tion is maintained. Nevertheless, MesB(C6H4)2BBr obtained
this way was always contaminated with the disubstitution
product MesB(C6H4)2BMes and was not readily isolable in
(C6H4)2BBr (6)[22] is
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
pure form. It therefore turned out to be convenient to
quench the reaction with H2O, thereby generating the borin-
ic acid anhydride 7, which could subsequently be purified by
column chromatography (yield: 76%; note: in the presence
of H2O, 7 is in equilibrium with the borinic acid MesB-
Figure 2. Molecular structure of 7 in the solid state; displacement ellip-
soids at the 50% probability level, H atoms omitted for clarity. Selected
ꢀ
bond lengths [ꢃ], bond angle [8], and dihedral angles [8]: B1 O1
ꢀ
ꢀ
ꢀ
1.371(4), B3 O1 1.361(3), B2 C21 1.568(4), B4 C51 1.571(4); B1-O1-B3
139.4(2); C1-B1-C11//C31-B3-C41 74.5(2), C2-B2-C12//C22-C21-C26
80.3(2), C32-B4-C42//C52-C51-C56 85.8(2).
ACHTUNGTRENNUNG(C6H4)2BOH; see the Supporting Information).
Chem. Eur. J. 2011, 17, 12696 – 12705
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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12699

M. Wagner et al.
alluded to above, these variations are most likely due to
crystal packing forces. Similar to 2, the two 9,10-dihydro-
9,10-diboraanthracene fragments of 7 are nearly orthogonal
to each other (C1-B1-C11//C31-B3-C41=74.5(2)8). The di-
hedral angles between each 9,10-dihydro-9,10-dibora-
ACHTUNGTRENNUNGanthracene moiety and its attached mesityl substituent
amount to 80.3(2)8 and 85.8(2)8. As a result, the vacant p-or-
bitals of B2 and B4 are efficiently shielded by methyl
groups, which explains the stability of the triorganylborane
units during the quenching process and chromatographic
workup.
The 11B NMR spectrum of 7 contains two broad resonan-
ces at d=43.9 (BO) and 70.2 ppm (BMes) in the typical
1
region[21] of three-coordinate boron atoms. In the H NMR
spectrum, signals that can be assigned to the mesityl sub-
stituents appear at d=2.16 (o-CH3), 2.35 (p-CH3), and
6.93 ppm (MesH-3,5). The general pattern of the phenylene
resonances is largely the same as in 2.
Synthesis of MesBACHTUNGTRENNUNG(C6H4)2BX (X=H, Br): For the further
derivatization of 7, we first tested the approach involving
LiAlH4 that was employed in the case of 3. Lithium dihydri-
doborate 8 (Scheme 3) readily formed upon addition of two
equivalents of LiAlH4 in THF to a solution of 7 in Et2O. In
contrast to the analogous reaction of 3, in this case, we
ꢀ
found no indication of any accompanying cleavage of B C
bonds. Addition of excess Me3SiCl to a solution of 8 in
Et2O/THF led to the abstraction of one of the boron-bound
hydride ions accompanied by formation of the THF adduct
9·THF (Scheme 3). Compound 9·THF smoothly hydrobo-
rates terminal alkynes RCꢁCH (stoichiometric ratio=1:1)
even at room temperature with formation of the corre-
sponding vinyl boranes (double hydroboration was never
observed). However, a drawback of the LiAlH4 mediated
route to 9·THF was that 8, 9·THF, and even the hydrobora-
tion products are difficult to purify from contaminating alu-
minum salts, especially from the highly soluble complex
Scheme 4. Synthesis of compounds (9)2, 10–14. Reagents and conditions:
i) BBr3 (2 equiv), C6H6, room temperature; ii) 1) 2,2’-bipy (1 equiv), C6H6/
toluene, room temperature; 2) NH4PF6 (excess), MeOH/H2O, room tem-
ꢁ
perature; iii) Et3SiH (excess), room temperature; iv) RC CH (1 equiv),
C6H6, room temperature.
borinic acid anhydride 7 now enables convenient access to
this important component.
Compound 10 was characterized by NMR spectroscopic
and X-ray crystallographic analyses. The 11B NMR spectro-
scopic data of the compound (d=65.0 and 69.4 ppm) are
very similar to those of 6 on the one hand (d=63.8 ppm;
[AlCl3ACHTUNGTRENNUNG(THF)2]. The preparation of analytically pure samples
requires fractional crystallization, which is not only time
consuming but also reduces the yields considerably. We
therefore decided to look for an improved route to com-
pounds of type 9. Thus synthesis details of 8 and 9·THF, a
full NMR characterization and their X-ray crystal structure
analyses are provided only as Supporting Information.
Because certain alkylaryl ethers are readily cleaved with
BBr3,[24] we tested whether boronic acid anhydride 7 could
also be transformed into a bromoborane upon treatment
with BBr3. Indeed, by stirring a solution of 7 in C6H6 with
4 equiv of BBr3, clean and quantitative O/Br exchange was
observed to take place within 31 h at room temperature
(Scheme 4; see the Supporting Information for a series of
1H NMR spectra that show the progress of the reaction). We
note in this context that the resulting bromoborane MesB-
see the Supporting Information) and MesBACHTUNRGTNEUNG(C6H4)2BMes on
1
the other (d=66.0 ppm[18]). All H and 13C NMR signals of
10 appear in the expected chemical shift regions and there-
fore are not discussed further. The solid-state structure of 10
(Figure 3, Table 1) shows essentially the same bond lengths
ꢀ
and angles about B1 (B1 Br1=1.949(6) ꢃ; C1-B1-C11=
ꢀ
123.8(5)8) and B2 (B2 C21=1.564(7) ꢃ; C2-B2-C12=
119.1(5)8) as those of 6 (see the Supporting Information)
and MesBACHTUNGTRNEUNG
(C6H4)2BMes,[18] respectively.
Bromoborane 10 cleanly undergoes substitution reactions
with reagents as different as 2,2’-bipyridyl and Et3SiH
(Scheme 4). In the first case, we obtained a water-stable
2,2’-bipyridylboronium bromide salt, which was transformed
into the hexafluorophosphate salt 11 by metathesis with
NH4PF6. In the second case, crystals of the hydridoborane
(9)2 were grown from a solution of 10 in Et3SiH.
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
MesMgBr, but could not be isolated in pure form from the
reaction mixture. Generation of the air- and moisture-stable
12700
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ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 12696 – 12705

Boron-Doped p-Conjugated Systems
FULL PAPER
Table 2. Crystallographic data for (9)2 and 14.
(9)2
C42H40B4
14
formula
Mr
C32H32B2
438.20
587.98
color, shape
T [K]
yellow, block
173(2)
colorless, plate
173(2)
radiation, l [ꢃ]
crystal system
space group
a [ꢃ]
b [ꢃ]
c [ꢃ]
MoKa, 0.71073
monoclinic
P21/n
7.2197(7)
13.7689(13)
16.990(2)
90
MoKa, 0.71073
monoclinic
P21/n
16.133(4)
14.936(3)
22.143(6)
90
Figure 3. Molecular structure of 10 in the solid state; displacement ellip-
a [8]
soids at the 50% probability level, H atoms omitted for clarity. Selected
b [8]
91.156(9)
90
1688.6(3)
2
107.42(2)
90
5091(2)
ꢀ
bond lengths [ꢃ], bond angles [8], and dihedral angle [8]: B1 Br1
g [8]
ꢀ
ꢀ
ꢀ
ꢀ
1.949(6), B1 C1 1.534(8), B1 C11 1.561(8), B2 C2 1.551(7), B2 C12
V [ꢃ3]
Z
ꢀ
1.567(8), B2 C21 1.564(7); C1-B1-C11 123.8(5), C2-B2-C12 119.1(5); C2-
8
B2-C12//C22-C21-C26 81.0(4).
1calcd [gcmꢀ3
]
1.156
1.143
F
G
624
1872
]
0.063
0.37ꢄ0.35ꢄ0.35
13587
2977 (0.1042)
2977/0/215
0.926
0.0532, 0.1209
0.0947, 0.1371
0.279, ꢀ0.187
0.063
0.28ꢄ0.25ꢄ0.12
31262
8953 (0.3476)
8953/15/624
0.719
0.0752, 0.1029
0.3396, 0.1964
0.216, ꢀ0.176
The 11B NMR spectrum of 11 shows signals at d=69.5
crystal size [mm]
reflections collected
independent reflections (Rint
data/restraints/parameters
GOF on F2
and 5.5 ppm, testifying to the presence of three- and four-co-
)
1
ordinate boron atoms, respectively.[21] In the H NMR spec-
trum, the integral ratios of the mesityl, 9,10-dihydro-9,10-di-
boraanthracene, and 2,2’-bipyridyl resonances indicate a
1:1:1 ratio of the three fragments in the molecule. Most of
R1, wR2 [I> 2s(I)]
R1, wR2 (all data)
largest diff peak and hole [eꢃꢀ3
]
1
the H NMR signals of the coordinating 2,2’-bipyridyl ligand
appear at lower field than those of the free base. The same
is true for most of the 13C NMR signals, a characteristic ex-
ception[25] being the resonance of bipyC-2,2’, which experi-
ences an upfield shift of 9.3 ppm in 11. The X-ray crystal
structure analysis of 11 is in full accord with the structure es-
tablished by NMR spectroscopy (see the Supporting Infor-
mation).
Compound 9 forms a centrosymmetric B-H-B bridged
dimer (9)2 in the solid state (Figure 4; Table 2), which is
reminiscent of the coordination polymer (B)n.[13] The 9,10-di-
hydro-9,10-diboraanthracene cores of the monomeric moiet-
ies in (9)2 deviate significantly from planarity, with a dihe-
dral angle of 146.7(1)8 between the two phenylene rings
mer B···B distance of 1.818(12) ꢃ (mean value) as opposed
to a longer distance of 1.850(5) ꢃ in (9)2. We note that the
B···B distance in (9)2 is also longer than that of ((C6F5)2BH)2
(1.799(7) ꢃ[26]), but is comparable to that of the sterically
congested molecule (Mes2BH)2 (1.851(3) ꢃ[27]). The experi-
mentally determined structure of (9)2 is in good agreement
with the theoretically predicted structure of the parent
(C6H4)2BH ((B)2).[13] The follow-
dimer HBACTHNUGTREN(UNNG C6H4)2BACHTUNRTGEG(NNUN m-H)2BACHTNUGTRENNGUN
ing observations can be made: 1) The simultaneous presence
of three- and four-coordinate boron centers results in a
puckering of the central six-membered ring [absolute values
of the torsion angles B-C-C-B=6.6(3)8, 12.2(3)8 ((9)2); 10.48
Ar(C1)//ArACHTUNGTRENNUNG(C11). An even stronger folding has been ob-
served in (B)n (133.88) and is accompanied by an intermono-
((B)2)]. 2) The dihedral angle Ar(C1)//ArACTHNGUTER(NNUG C11) equals
146.7(1)8 in (9)2 vs. 1518 in (B)2. 3) The intermonomer B···B
distance is 1.850(5) ꢃ in (9)2 and 1.83 ꢃ in (B)2.
DFT calculations indicate that the dimerization of B is es-
sentially a thermoneutral process (DG298 =0.3 kcalmolꢀ1)
under gas-phase conditions.[13] In C6D6 solution, the
11B NMR spectrum of 9 is characterized by only one broad
resonance at d=71.0 ppm, which lies in the typical shift
range of three-coordinate boron centers and can be ex-
plained in two ways: 1) the BH resonance is broadened
beyond detection (for example, as a result of a monomer–
dimer equilibrium), or 2) the BH[27] and BMes signals are
overlapping, which would necessarily mean that 9 exists as a
monomer under the measurement conditions. The 1H and
13C NMR spectra in C6D6 both reveal one set of signals, all
of which are narrow and well-resolved and therefore give no
indication of an ongoing slow dynamic process. Remarkably,
the 13C nuclei of the 9,10-dihydro-9,10-diboraanthracene
core are comparatively deshielded and possess chemical
Figure 4. Molecular structure of (9)2 in the solid state; displacement ellip-
soids at the 50% probability level, H atoms (except on boron) omitted
for clarity. Selected bond lengths [ꢃ], atom···atom distance [ꢃ], bond
ꢀ
angles [8], torsion angles [8], and dihedral angles [8]: B1 C1 1.574(3),
ꢀ
ꢀ
ꢀ
ꢀ
B1 C11 1.575(3), B2 C2 1.560(3), B2 C12 1.562(3), B2 C21 1.582(3),
B1···B1A 1.850(5); C1-B1-C11 117.3(2), C2-B2-C12 118.2(2); B1-C1-C2-
B2 ꢀ6.6(3), B1-C11-C12-B2 12.2(3); C2-B2-C12//C22-C21-C26 85.7(2),
Ar(C1)//ArACHTUNGTRENNUNG(C11) 146.7(1). Symmetry transformation used to generate
equivalent atoms: A: ꢀx,ꢀy+1, ꢀz+1.
Chem. Eur. J. 2011, 17, 12696 – 12705
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
12701

M. Wagner et al.
shift values (d=133.7, 134.2, 139.0, and 142.0 ppm) closer to
those of 12 (see below; d=132.7, 133.1, 137.1, and
139.4 ppm) than to those of 9·THF (d=126.9, 132.2, 135.5,
3J
ACTHNGUTERN(UNG H,H)=18.8 Hz). These syntheses were also conducted on
a preparative scale with isolation and full characterization of
the product. Crude 14, prepared from 9·THF without previ-
and 138.6 ppm), or
8
(d=121.7, 128.8, 135.0, and
ous isolation of the intermediate 8 (in the form of 8·
see the Supporting Information), tended to be contaminated
with the aluminum complex [AlCl3A(THF)2]. Compound 14
and [AlCl3A(THF)2] could be separated from each other by
fractional crystallization from hexane at 48C ([AlCl3-
ACHTUNGTRENNUNG(THF)3,
138.1 ppm). The IR spectrum of 9 in C6H6 shows an absorp-
ꢀ1
ꢀ
tion at 2481 cm , which is typical of terminal B H stretch-
CHTUNGTRENNUNG
ing bands[28] (this absorption is absent in the IR spectrum of
solid (9)2). In summary, based on our spectroscopic results
and on the DFT calculations mentioned above, we suggest
that 9 mainly exists as monomeric species in aromatic sol-
vents. This conclusion is also in accord with the evidence we
CHTUNGTRENNUNG
(THF)2]) and ꢀ308C (14). To obtain analytically pure 14
ꢁ
from 9 and MesC CH, it is sufficient to remove all volatiles
from the reaction mixture under vacuum and to reprecipi-
tate the product from hexane at ꢀ788C.
ꢀ
have gathered for an unusually weak B O bond in 9·THF
(see the Supporting Information). In [D8]THF, the NMR
spectra of (9)2 are identical to those of compound 9·THF.
Upon irradiation with UV light (l=366 nm) at room tem-
perature, compound 9 shows a bright blue-green fluores-
cence in C6H6 or THF solution. When the THF solution is
cooled to liquid nitrogen temperature, the emission of the
sample becomes more intense; when the UV light was
switched off, an intense delayed luminescence remained visi-
ble for more than 15 s (the optical properties of 9 and of se-
lected derivatives are the subject of ongoing investigations).
The targeted hydrolysis of 14 with traces of added H2O
gave 7 and H2C=C(H)Mes in a clean and quantitative reac-
ꢀ
tion, thereby identifying the B C
est link in the molecule.
ACHTUNGERTN(NUNG vinyl) bond as the weak-
Compound 14 crystallizes with two crystallographically in-
dependent molecules, 14A and 14B, in the asymmetric unit.
The X-ray crystal structure analysis shows the desired anti-
Markownikow isomer and the expected E-configuration of
the C=C double bond (Figure 5). Given the poor quality of
the data set (Table 2), we refrain from a detailed description
of bond lengths and angles.
Hydroboration reactions of MesBACHTNUTRGNEUNG(C6H4)2BH: Compound
9·THF as well as donor-free 9 have been employed in our
studies on the hydroboration of terminal alkynes. Both re-
agents undergo quantitative conversion into the correspond-
ing vinyl boranes (NMR spectroscopic monitoring of the re-
action).
In a first exploratory NMR experiment, the reaction of
ꢁ
9·THF with 1.2 equiv of tBuC CH gave the vinyl borane 12
(Scheme 4) with excellent regioselectivity. Two doublets
were observed at d=6.70 and 7.03 ppm in the 1H NMR
spectrum with a 3J
ACTHNUTRGNEU(GN H,H) coupling constant of 18.1 Hz,
Figure 5. Molecular structure of 14A in the solid state; displacement ellip-
which is indicative of an E-olefin (see the Supporting Infor-
mation).
The regioselectivity was drastically reduced when
soids at the 50% probability level, H atoms omitted for clarity. Selected
ꢀ
bond lengths [ꢃ], bond angles [8], and dihedral angles [8]: B1 C1
ꢀ
ꢀ
ꢀ
ꢀ
1.568(13), B1 C7 1.548(11), B1 C11 1.580(14), B2 C2 1.581(14), B2
ꢀ
ꢀ
C12 1.562(13), B2 C21 1.551(13), C7 C8 1.329(9); C7-B1-C1 120.5(9),
C7-B1-C11 121.6(8), C1-B1-C11 117.5(8), C2-B2-C12 119.2(8), B1-C7-C8
124.8(8), C7-C8-C31 128.8(7); C1-B1-C11//B1-C7-C8 41(1), C2-B2-C12//
C22-C21-C26 77.8(7).
ꢁ
ꢁ
p-TolC CH was used instead of tBuC CH (p-Tol=para-
tolyl), because the desired addition product 13a (Scheme 4)
was obtained together with its isomer MesBACHTNUTRGENN(UG C6H4)2BCACHTUGNTREN(NUGN p-
Tol)=CH2 (13b) in a 2:1 ratio. Similar to 12, the olefinic
fragment of 13a gives rise to two doublet proton resonances
3
(d=7.47 and 7.82 ppm) with a J
G
Comparison of the electronic spectra of 14 and Mes2BC(H)=
C(H)Mes: To compare the optical properties of the [MesB-
ACHTNUGNERT(NGNU C6H4)2B] chromophore with those of the commonly used
1
18.2 Hz; in contrast, the H NMR spectrum of 13b is charac-
terized by two doublets (d=5.30 and 6.27 ppm) with a J-
A
[Mes2B] group, we prepared the compound Mes2BC(H)=
C(H)Mes (see the Supporting Information for synthetic de-
tails and NMR data), which is an analogue of 14. The ab-
sorption and emission wavelengths of both compounds are
compiled in Table 3.
Information).
Given that one goal of the work presented herein was the
synthesis of well-defined, fully conjugated, boron-doped p-
electron systems, any regioselectivity problem associated
with the hydroboration of (aryl)alkynes was a major issue
ꢁ
that needed to be resolved. Switching from p-TolC CH to
[29]
Table 3. Electronic spectral data of 14 and Mes2BC(H)=C(H)Mes in tol-
uene.
ꢁ
MesC CH, we tested whether a moderate increase in the
steric demand of the aryl group led to the selective forma-
tion of species of type 13a. Indeed, the reactions between
Compound
lmax
G
lmax(em) [nm]
14
350
333
470 (lex =350)
396 (lex =335)
ꢁ
9·THF or 9 and MesC CH furnished isomer 14 exclusively
Mes2BC(H)=C(H)Mes
(Scheme 4; BC(H)=C(H): d
(1 H)=7.34, 7.52 ppm, 2 ꢄ d,
12702
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Chem. Eur. J. 2011, 17, 12696 – 12705

Boron-Doped p-Conjugated Systems
FULL PAPER
In toluene solution, Mes2BC(H)=C(H)Mes shows its lon-
planning to prepare yet larger but still well-defined boron-
gest wavelength absorption at lmax
G
doped p materials, for example, by replacing the mono-
ꢁ
ꢁ
sion maximum of the compound lies at lmax(em)=396 nm
(excitation wavelength: lex =335 nm). Both lmax values com-
pare perfectly well with those of the closely related com-
ACHUTGTNRENNUGalkyne MesC CH with the aromatic dialkyne HC CACHTUNGTRENNUNG(p-
ꢁ
C6Me4)C CH.
pound Mes2BC(H)=C(H)Ph (lmax
398 nm in cyclohexane).[30] Replacement of [Mes2B] by
[MesB(C6H4)2B] had only a moderate effect on the absorp-
tion band, which, in the latter case, appears at lmax(abs)=
350 nm (14). The emission band, however, undergoes a bath-
ochromic shift of Dl=74 nm and is found at lmax(em)=
470 nm (14). These qualitative trends are in line with expect-
ations, because more extended and conformationally con-
strained dyes are known to absorb and fluoresce more in-
tensely at longer wavelengths relative to smaller and/or un-
constrained dyes.[31]
ACHTUNGTRNEUN(GN abs)=332 nm/lmax(em)=
Experimental Section
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
Unless otherwise specified, all reactions were carried out under dry nitro-
gen or argon using Schlenk or glove box techniques. Hexane, toluene,
C6H6, C6D6, Et2O, THF, and [D8]THF were dried over Na/benzophenone.
Me2S was stirred over LiAlH4 for 8 h at room temperature and distilled
prior use. Me3SiCl was stored over CaH2 and distilled prior to use.
ꢁ
tBuC CH and Et3SiH were distilled from molecular sieves (3 ꢃ). NMR
spectra were recorded with Bruker AM 250, DPX 250, Avance 300, or
Avance 400 spectrometers at room temperature, if not otherwise speci-
fied. Chemical shifts are referenced to (residual) solvent signals
(1H/13C{1H}; C6D6: d=7.15/128.0 ppm; [D8]THF: d=3.58/67.4 ppm;
CD3CN: d=1.94/118.2 ppm) or external BF3·Et2O (11B,11B{1H}). Abbrevi-
ations: s=singlet, d=doublet, t=triplet, app. t=apparent triplet, m=
multiplet, br=broad, n.o.=signal not observed. UV/Vis absorption and
emission spectra were recorded with a Varian Cary 50 Scan UV/Vis spec-
trophotometer or a Perkin–Elmer LS 50B fluorescence spectrometer, re-
spectively. Combustion analyses were performed by the Microanalytical
Laboratory of the University of Frankfurt and by the Microanalytical
Conclusion
Two convenient, high-yield protocols for the synthesis of un-
symmetrically substituted 9,10-dihydro-9,10-diboraanthra-
cenes have been developed: The first starts from a thioether
Laboratory Pascher. Compounds 1,[14] 6,[22] HOB
(C6H4)2BOH,[32] 1,2-
ACHTUNGTRENNUNG
ꢁ
[29]
C6H4A
(SiMe3)2,[12] MesMgBr,[23] (Mes2BH)2,[33] and MesC CH were syn-
adduct of the parent borane (i.e., (Me2S)HB
ACHTNUGRTNE(NGNU C6H4)2BH-
thesized according to literature procedures.
A
ACHTUNGTRENNUNG
Synthesis of 2: A calibrated solution of H2O in THF (6.1m, 14.8 mL,
0.090 mmol) was added at room temperature by using an Eppendorf pip-
ette to a stirred solution of 1 (0.050 g, 0.17 mmol) in anhydrous Me2S
(4 mL). After gas evolution (H2) had ceased (15 min), the colorless clear
solution was stored at ꢀ808C for 2 days. A colorless precipitate formed
that was identified as unreacted 1 (0.010 g, 20%). The clear supernatant
was decanted in the cold and stored at ꢀ808C for another 4 days to
obtain a colorless precipitate. The mother liquor was removed in the cold
by using a syringe and discarded. The remaining solid was dried under
dynamic vacuum. Yield: 0.015 g (37%; 46% considering the re-isolated
starting material). X-ray quality crystals of 2 were obtained through gas-
phase diffusion of pentane into a Me2S solution of 2 at room tempera-
of 1, it is possible to break the symmetry of the molecular
framework by targeted hydrolysis, which gives the borinic
acid anhydride ((Me2S)HBACHTUNRGTNEUNG(C6H4)2B)2O (2) in almost 50%
yield as a crystalline solid. In the case of 6, the reaction with
1 equiv of MesMgBr under high dilution leads to the prefer-
ential formation of MesB
ACHTNUGRTEN(NGNU C6H4)2BBr, which is subsequently
transformed into the air- and moisture-stable borinic acid
anhydride (MesB
purification.
ACHTUNGTRENNUNG(C6H4)2B)2O (7; 76% yield) to facilitate
Compound 2 can be used directly for further derivatiza-
tion through hydroboration. Compound 7 reacts cleanly
with BBr3 to regenerate the monotopic bromoborane MesB-
1
1
ture. H NMR (300.0 MHz, C6D6): d=1.29 (s, 12H; SCH3), 4.23 (h =
=
2
100 Hz, 2H; BH), 7.19 (app. td, 3J
G
E
3
4
H-2,7 or H-3,6), 7.43 (app. td, J
(H,H)=7.4 Hz, J
N
2,7 or H-3,6), 7.87 (d, 3J
ACHTUNGTRENUN(NG H,H)=7.4 Hz, 4H; H-1,8 or H-4,5), 8.17 ppm
(d, 3J(H,H)=7.4 Hz, 4H; H-1,8 or H-4,5); 11B NMR (96.3 MHz, C6D6):
ACTHNUGTRNENUG
AHCTUNGTRENNUNG
d=ꢀ3.0 (h = =500 Hz; BH), 43.2 ppm (h = =1200 Hz; BO); 13C{1H} NMR
1
2
(62.9 MHz, C6D6): d=18.2 (SCH3), 126.9 (C-2,7 or C-3,6), 130.6 (C-2,7 or
ACHTUNGTRENNUNG
C-3,6), 133.6 (C-1,8 or C-4,5), 136.3 ppm (C-1,8 or C-4,5), n.o. (BC).
either through hydroboration or nucleophilic substitution
protocols, respectively.
Synthesis of 7: A calibrated solution (0.77m) of the Grignard reagent
MesMgBr was prepared in THF. An aliquot (3.4 mL, 2.6 mmol) was
transferred into a Schlenk vessel, all volatiles were removed under re-
duced pressure, the resulting brownish oil was dissolved in toluene
(15 mL), and the solution was added dropwise with stirring at ꢀ788C to
a turbid solution of 6 (875 mg, 2.62 mmol) in toluene (100 mL). The reac-
tion mixture was allowed to warm to room temperature overnight,
whereupon a colorless precipitate formed. After filtration, all volatiles
were removed from the filtrate in vacuo to yield a yellow solid. Deion-
ized H2O (20 mL) and CHCl3 (50 mL) were added, the resulting two
liquid phases were separated, and the aqueous layer was extracted with
CHCl3 (3ꢄ15 mL). The combined organic layers were dried over MgSO4,
filtered, and evaporated in vacuo. The crude product was purified by
column chromatography (silica gel; mobile phase: CHCl3) and dried for
4 h at room temperature under dynamic vacuum. Yield: 602 mg (76%).
Single crystals of 7 that were suitable for X-ray diffraction were obtained
by gas-phase diffusion of hexane into a toluene solution of 7. Rf =0.37
(CHCl3); 1H NMR (300.0 MHz, C6D6): d=2.16 (s, 12H; o-CH3), 2.35 (s,
Compound 9 is also accessible as its THF-adduct 9·THF
by treatment of 7 with LiAlH4 and then with Me3SiCl in
Et2O/THF. Compared with the sequence 7 ! 10 ! 9, this
alternative route has the disadvantage that it usually takes
some effort to purify 9·THF (or its hydroboration products)
from residual aluminum complexes.
The electronic absorption and emission spectra of the
vinyl borane MesB
ACHTUNGTRENNUNG(C6H4)2BC(H)=C(H)Mes reveal batho-
chromic shifts of Dl(abs)=17 nm and Dl(em)=74 nm, com-
ACHTUNGTRENNUNG
pared with Mes2BC(H)=C(H)Mes, which bears the more
common [Mes2B] chromophore. This observation strongly
suggests that the optoelectronic properties resulting from
the more extended 9,10-dihydro-9,10-diboraanthracene p
system increase the value of the material. We are therefore
ACTHNUTRGNEUNG
6H; p-CH3), 6.93 (s, 4H; MesH-3,5), 7.10 (app. td, 3J(H,H)=7.4 Hz, 4J-
Chem. Eur. J. 2011, 17, 12696 – 12705
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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12703

M. Wagner et al.
A
R
1.4 Hz, 2H; H-2,7 or H-3,6), 7.67 (d, 3J
4,5), 7.93 ppm (d, 3J(H,H)=7.4 Hz, 2H; H-1,8 or H-4,5); 11B NMR
ACHTUNGTNER(NUNG H,H)=7.4 Hz, 2H; H-1,8 or H-
R
N
ACHTUNGTRENNUNG
1
N
G
(128.4 MHz, C6D6): d=71.0 ppm (h = =1500 Hz; BMes, BH);
2
13C{1H} NMR (75.5 MHz, C6D6): d=21.4 (p-CH3), 22.7 (o-CH3), 127.5
(MesC-3,5), 133.7 (C-2,7 or C-3,6), 134.2 (C-2,7 or C-3,6), 137.0 (MesC-
4), 138.0 (MesC-2,6), 139.0 (C-1,8 or C-4,5), 142.0 ppm (C-1,8 or C-4,5),
n.o. (BC); elemental analysis calcd (%) for C42H40B4 (587.98): C 85.80, H
6.86; found: C 85.13, H 6.77.
ACHTUNGTRENNUNG
(H,H)=1.4 Hz, 4H; H-1,8 or H-4,5); 11B{1H} NMR (96.3 MHz, C6D6):
1
2
ꢁ
Synthesis of 14: Neat MesC CH (51.3 mL, 47.7 mg, 0.331 mmol) was
added at room temperature to 9·THF (121 mg, 0.331 mmol) in C6H6
(7 mL) by using a Hamilton syringe, and the reaction mixture was stirred
for 1 h. All volatiles were removed under reduced pressure, the yellow
solid residue was treated with hexane (5 mL), and the insoluble material
was collected on a frit and extracted into hexane (2ꢄ1 mL). The com-
bined hexane solutions were evaporated to dryness in vacuo to obtain a
yellow microcrystalline solid of 14 and [AlCl
nents were separated from each other by fractional crystallization from
hexane at 48C ([AlCl3A
3ACHTUNGTREN(NNUG THF)2]. The two compo-
(THF)2]) and ꢀ308C (14). Single crystals of 14 that
were suitable for X-ray diffraction were grown by slow evaporation of a
pentane/C6H6 solution (50:1) at room temperature. The yields of 14 re-
producibly ranged between 20–30%; yields of 14 close to 90% were ob-
tained under similar conditions with 9 as the hydroboration reagent.
1H NMR (400.1 MHz, C6D6): d=2.08 (s, 6H; o-CH3), 2.17 (s, 3H; p-
CH3’), 2.32 (s, 6H; o-CH3’), 2.33 (s, 3H; p-CH3), 6.83 (s, 2H; MesH-3,5’),
T
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
6.91 (s, 2H; MesH-3,5), 7.21 (app. td, 3J
(H,H)=7.3 Hz, 4J
U
3
4
2H; H-2,7), 7.32 (app. td, J
(H,H)=7.3 Hz, J
U
7.34 (d, 3J
18.8 Hz, 1H; BC(H)=C(H)), 7.82 (d, 3J
8.24 ppm (d, 3J
(H,H)=18.8 Hz, 1H; BC(H)=C(H)), 7.52 (d, 3J
ACHUTGTNRENNUG ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
C6D6): d=71.6 ppm (shoulder at d=62.8 ppm; BMes, BC(H)=C(H));
13C{1H} NMR (100.6 MHz, C6D6): d=21.1 (p-CH3’), 21.3 (o-CH3’), 21.4
(p-CH3), 22.8 (o-CH3), 127.5 (MesC-3,5), 129.4 (MesC-3,5’), 133.0 (C-
2,7), 133.2 (C-3,6), 136.1 (MesC-2,6’), 136.5 (MesC-1’), 136.8 (MesC-4),
137.0 (MesC-4’), 137.5 (C-4,5), 138.1 (MesC-2,6), 138.6 (br; BC(H)=
C(H)), 139.6 (C-1,8), 141.5 (br; MesC-1), 145.7 (br; C6H4-BC), 147.5 (br;
C6H4-BC), 149.6 ppm (BC(H)=C(H)); elemental analysis calcd (%) for
C32H32B2 (438.20): C 87.71, H 7.36; found: C 87.39, H 7.35. Note: Reso-
nances belonging to the 2-mesitylethenyl substituent are marked with a
prime (’).
Synthesis of 11: Compound 10 (18.6 mg, 0.05 mmol) in C6H6 (0.5 mL)
was added at room temperature to a solution of 2,2’-bipyridyl (10.4 mg,
0.07 mmol) in toluene (0.1 mL), whereupon a yellow solid precipitated
immediately. The solution was removed by using a syringe and discarded;
the precipitate was washed with toluene (2ꢄ1.5 mL) and dried under dy-
namic vacuum. [MesBACHTUNGTRENNUNG(C6H4)2BACHUTNGTREN(NNGU bipy)]Br was dissolved in MeOH (2 mL)
and treated with an aqueous solution of NH4PF6 (0.20m, 1.0 mL,
0.20 mmol), whereupon a yellow precipitate formed. The solid product
was isolated by decanting the supernatant after centrifugation, washed
with deionized H2O (2 mL), and dried under vacuum. Single crystals that
were suitable for X-ray diffraction were obtained by gas-phase diffusion
of Et2O into a CH3CN solution of 11. Yield: 15.2 mg (51%; crystalline
Crystal structure analyses: All crystals except those of HOBACHTUNRGTNEUNG(C6H4)2BOH
were measured on a STOE IPDS-II diffractometer with graphite-mono-
chromated MoKa radiation. An empirical absorption correction with pro-
gram PLATON[34] was performed for 2, 6, and 10. The structures were
solved by direct methods[35] and refined with full-matrix least-squares on
F2 using the program SHELXL97.[36] The hydrogen atoms bonded to
boron in 8, 9·THF, and (9)2 were isotropically refined. All other hydrogen
atoms were placed in ideal positions and refined with fixed isotropic dis-
placement parameters using a riding model.
material). 1H NMR (300.0 MHz, CD3CN): d=2.12 (s, 6H; o-CH3), 2.40
3
(s, 3H; p-CH3), 6.66 (dm, J
G
3
4
2H; MesH-3,5), 7.32 (app. td, J
2,7 or H-3,6), 7.36 (app. td, 3J
3
2,7 or H-3,6), 7.63 (dm, J
U
3J
8.29 (ddd, 3J
bipyH-6,6’), 8.67 (ddd, 3J
N
ACHTUNGTRENNUNG ACHTUNGTRENNUNG
(H,H)=5.7 Hz, 4J
R
One sulfur atom and one methyl group of 2 are disordered over two posi-
tions with a site occupation factor of 0.656(8) for the major occupied site.
The crystal of 6 was a non-merohedral twin with a fraction of 0.576(6)
for the major domain. The hydrogen atoms of three methyl groups of 7
are disordered over two positions with equally occupied sites. In 11, the
para-methyl group of the mesityl ring is disordered over two equally oc-
cupied positions. In one of the two crystallographically independent mol-
ecules of 14 (i.e., 14B), the atoms of the C=C double bond are disordered
over two positions with a site occupation factor of 0.58(2) for the major
occupied site. Bond lengths and angles involving the disordered atoms
were restrained to be equal to those in the non-disordered molecule.
A
A
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
1
1
2
2
13C{1H} NMR (100.6 MHz, CD3CN): d=21.3 (p-CH3), 23.0 (o-CH3),
124.5 (bipyC-3,3’), 127.8 (MesC-3,5), 129.9 (C-2,7 or C-3,6), 130.5 (bipyC-
5,5’), 131.9 (C-1,8 or C-4,5), 134.5 (C-2,7 or C-3,6), 137.8 (MesC-4), 138.7
(MesC-2,6), 139.7 (C-1,8 or C-4,5), 144.3 (bipyC-6,6’), 146.0 (bipyC-4,4’),
147.6 ppm (bipyC-2,2’), n.o. (BC); MS (ESI+): m/z (%): 450 (100) [Mꢀ-
ACHTUNGTRENNUNG
(PF6)]+; elemental analysis calcd (%) for C31H27B2F6N2P (594.14): C
62.67, H 4.58, N 4.71; found: C 62.02, H 4.55, N 4.45.
HOBACHTNURGTNE(UNG C6H4)2BOH was measured with a Siemens SMART diffractometer.
Synthesis of (9)2: Excess neat Et3SiH (1 mL, 728 mg, 6.26 mmol) was
added at room temperature to neat 10 (64.9 mg, 0.17 mmol). The result-
ing mixture was stored without stirring at room temperature for 1 day,
whereupon pale-yellow crystals formed. The mother liquor was removed
by using a syringe and the crystalline product (9)2 was dried under
No absorption correction was made. The structure was determined by
direct methods[35] and refined with full-matrix least-squares on F2 using
the program SHELXL97.[36] The hydrogen atoms were geometrically
positioned and were constrained. The crystal was twinned, the twin rela-
tions are: h’=ꢀh, k’=ꢀk, and l’=0.895 h+l, the twin fraction refined to
0.302(7).
ꢀ1
ꢀ
˜
vacuum. Yield: 35.6 mg (70%). IR (C6H6): n=2481 cm
(B H).
1H NMR (400.1 MHz, C6D6): d=2.03 (s, 6H; o-CH3), 2.32 (s, 3H; p-
CH3), 6.89 (s, 2H; MesH-3,5), 7.12 (app. td, 3J
(H,H)=7.4 Hz, 4J
A
CCDC-824867 (ClB
ACHTUNGTRNE(GUN C6H4)2BCl), CCDC-824869 (HOBCAHUTGNTREN(NUGN C6H4)2BOH),
1.4 Hz, 2H; H-2,7 or H-3,6), 7.22 (app. td, 3J(H,H)=7.4 Hz, 4J
A
ACHTUNGTRENNUNG
CCDC-824870 (2), CCDC-824868 (6), CCDC-824871 (7), CCDC-824872
12704
www.chemeurj.org
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 12696 – 12705

Boron-Doped p-Conjugated Systems
FULL PAPER
(8), CCDC-824873 (9·THF), CCDC-826346 ((9)2), CCDC-826345 (10),
CCDC-824875 (11), and CCDC-824874 (14) contain the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
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Received: June 3, 2011
Published online: October 5, 2011
Chem. Eur. J. 2011, 17, 12696 – 12705
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
12705