Influence of the bridging elements on the optical properties of linked 9,10-dihydro-9,10-diboraanthracenes

Article abstract of DOI:10.1021/om300990z

Starting from the monofunctionalized 9,10-dihydro-9,10-diboraanthracene derivatives MesB(C₆H₄)₂BX (X = H, Br), the bridged systems MesB(C₆H₄)₂B-L-B(C ₆H₄)₂BMes have been synthesized with L = -C(H)=C(H)(p-C₆Me₄)C(H)=C(H)- (5), -C≡C(p-C ₆Me₄)C≡C- (6), and -(p-C₆H ₄)(p-C₆Me₄)(p-C₆H₄)- (7). The compounds were characterized by NMR, IR, UV/vis, and fluorescence spectroscopy. The vinyl- and phenylboranes 5 and 7 are pale yellow solids, whereas the alkynylborane 6 possesses a bright yellow color. The emission maxima of 5-7 in toluene are λ_max(em) 477, 460, and 461 nm with quantum yields φ_f of 0.02, 0.30, and 0.04, respectively. Alkynylborane 6 was found to be the least air- and moisture-sensitive of the three derivatives.

Full text of DOI:10.1021/om300990z

Article
pubs.acs.org/Organometallics
Influence of the Bridging Elements on the Optical Properties of
Linked 9,10-Dihydro-9,10-diboraanthracenes
Estera Januszewski, Michael Bolte, Hans-Wolfram Lerner, and Matthias Wagner*
Institut fur Anorganische und Analytische Chemie, Goethe-Universitat Frankfurt, Max-von-Laue-Straße 7, D-60438 Frankfurt (Main),
̈
̈
Germany
S
* Supporting Information
ABSTRACT: Starting from the monofunctionalized 9,10-
dihydro-9,10-diboraanthracene derivatives MesB(C6H4)2BX
(X = H, Br), the bridged systems MesB(C6H4)2B−L−
B(C6H4)2BMes have been synthesized with L = −C(H)
C(H)(p-C6Me4)C(H)C(H)− (5), −CC(p-C6Me4)C
C− (6), and −(p-C6H4)(p-C6Me4)(p-C6H4)− (7). The
compounds were characterized by NMR, IR, UV/vis, and
fluorescence spectroscopy. The vinyl- and phenylboranes 5
and 7 are pale yellow solids, whereas the alkynylborane 6
possesses a bright yellow color. The emission maxima of 5−7
in toluene are λmax(em) 477, 460, and 461 nm with quantum yields ϕf of 0.02, 0.30, and 0.04, respectively. Alkynylborane 6 was
found to be the least air- and moisture-sensitive of the three derivatives.
INTRODUCTION
■
Boron atoms are isoelectronic with carbenium ions, and similar
to the case for the latter the vacant p orbital of a three-
coordinate boron center efficiently extends the conjugation
pathway of an adjacent π-electron cloud. Neutral boron-
containing π systems are therefore related to the corresponding
cationic all-carbon species. Similar to oxidative doping the
proper incorporation of boron atoms into π-conjugated organic
molecules and polymers can result in materials with highly
desirable optoelectronic properties.1−7 For example, main-chain
boron-containing macromolecules A,8 B,9 and C10 (Figure 1)
featuring vinylenearylenevinylene, (oligo)arylene, and ethyny-
lenearyleneethynylene moieties, respectively, have been synthe-
sized and shown to be strongly photoluminescent. Apart from
potential applications in organic light-emitting devices, such
compounds can act as luminescent sensors for anions such as
fluoride and cyanide;11,12 in some cases, amplified anion
quenching effects have been discovered which lead to
particularly sensitive detection schemes. Moreover, due to the
inherently electron-deficient nature of boron, polymers A−C
are n-type materials. n-Type semiconducting polymers are
much rarer than p-type polymers but are in great demand for
various device applications.13
As a common structural motif, the organic fragments in A−C
are connected by single boron atoms bearing bulky 2,4,6-
trimethylphenyl (mesityl, Mes) or 2,4,6-triisopropylphenyl
(tripyl, Tip) substituents for kinetic protection from hydrolysis
and oxidation. To avoid unfavorable steric interactions, the six-
membered rings of Mes and Tip tend to adopt conformations
orthogonal to the polymer backbone and are therefore
decoupled from the π-conjugated main chain. Recently, 9,10-
dihydro-9,10-diboraanthracene, which exists as the B−H−B-
Figure 1. Main-chain boron-containing conjugated polymers A−C
(mesityl = 2,4,6-trimethylphenyl; tripyl = 2,4,6-triisopropylphenyl);
9,10-dihydro-9,10-diboraanthracene-containing conjugated polymer D,
and the two low-molecular-weight DBA systems E and F.
bridged coordination polymer (HB(C6H4)2BH)n in the solid
state,14 has been introduced as a novel ditopic building block
for the hydroboration polymerization of aromatic dial-
kynes.14−19 One obvious benefit of the new borane lies in the
fact that each hydroboration event puts in an already extended,
planar, and rigid π system, whereas incorporation of MesB or
Received: October 22, 2012
Published: November 20, 2012
© 2012 American Chemical Society
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TipB merely adds one vacant boron-centered p orbital. The
macromolecules obtained (e.g., D; Figure 1) were found to be
strongly luminescent, both in solution and in the solid state.14
To achieve a more detailed understanding of the
optoelectronic properties of D-type polymers, the well-defined
low-molecular-weight model systems E and F have been
synthesized (Figure 1). Compounds E are accessible through
the hydroboration of p-R′C6H4CCH with the dimethyl
sulfide adduct (Me2S)HB(C6H4)2BH(SMe2);16 the products
were isolated from the initially obtained complex reaction
mixtures by crystallization.7 The synthesis of F required the
unsymmetrically substituted monotopic borane MesB-
(C6H4)2BH (3; Scheme 2).18 While the addition of 3 across
the CC triple bond of p-MeC6H4CCH gives both the
Markownikow and the anti-Markownikow product, the hydro-
boration of MesCCH proceeds regioselectively to the anti-
Markownikow product F.18
The purpose of this paper is to report on the missing third
model system 5 (Scheme 2), in which two 9,10-dihydro-9,10-
diboraanthracene (DBA) units are linked by a vinylenear-
ylenevinylene spacer. Key photophysical properties of 5 will be
compared with those of (i) E (R′ = Me) and F as well as (ii) 6
and 7, in which the two vinylene fragments have been replaced
by ethynylene and p-C6H4 groups, respectively (Scheme 2).20
regioselectivity issues during the hydroboration reaction (cf.
the experiences gained from the syntheses of E and F). To
ensure maximum comparability, we have maintained the central
p-C6Me4 ring also in 6 and 7. For the syntheses of 6 and 7, the
alkynylstannane 1Sn and the phenylstannane 2Sn, respectively,
are required (Schemes 1 and 2). 1Sn is accessible by double
deprotonation of 1H with n-BuLi and subsequent reaction of
the resulting lithium alkynylide with Me3SnCl in a one-pot
procedure. Suzuki−Miyaura cross-coupling between p-C6Me4I2
and p-BrC6H4B(OH)2 leads to the terphenyl 2Br, which was
transformed into 2Sn via lithium−bromine exchange with n-
BuLi and addition of Me3SnCl (Scheme 1).
Treatment of 1Sn and 2Sn with the bromoborane 4 finally
furnished the model systems 6 and 7 in good yields.
The 11B{1H} NMR spectrum of 5 reveals one very broad
resonance at 70.9 ppm (h1/2 = 3000 Hz; C6D6), testifying to the
presence of three-coordinate triorganylboranes (cf. F: δ(11B) =
71.6 with a shoulder at 62.8 ppm; C6D6).18 The 11B NMR
resonances of 6 and 7 are broadened beyond detection. In the
1H NMR spectra of 5−7, the integrals of the mesityl methyl
resonances and the p-C6Me4 methyl resonances confirm the
aimed-for 2:1 ratio between the DBA moieties and the
respective organic linker. In all three cases, the two DBA
fragments within the same molecule are magnetically
equivalent. The vinylene protons of 5 lead to two signals at
7.30 and 7.66 ppm with a coupling constant of 3JHH = 18.7 Hz,
a typical value found in (E)-olefins23 (cf. F: 7.34, 7.52 ppm;
3JHH = 18.8 Hz).18 A remarkable characteristic of 6 is a low-field
shift of its p-C6Me4 methyl resonance (2.60 ppm) and of the
doublet assigned to H-e (8.75 ppm) by means of a ROESY
experiment; this deshielding effect has most likely to be
attributed to the anisotropy cone of the CC bond.23 Finally,
the proton spectrum of 7 features the characteristic AA′BB′
spin pattern of unsymmetrically substituted p-C6H4 rings
(δ(1H) 7.39 (d), 7.72 (d)). The 13C{1H} NMR spectra of
5−7 are in full accord with the proposed molecular structures.
RESULTS AND DISCUSSION
■
Syntheses and NMR Spectra of 5−7. Compound 5 was
prepared in 65% yield from the monotopic borane 318 and
1H21 (Scheme 1) at room temperature in a hexane−toluene
mixture (Scheme 2). We note that 3 forms dimers (3)2 in the
solid state but monomerizes in aromatic solvents.18,22 The
durene derivative 1H was chosen in order to avoid
Scheme 1. Syntheses of the Stannylated Dialkynyldurene 1Sn
a
and the Diphenyldurene 2Sn
Somewhat surprisingly, we have not been able to detect the 13
C
NMR resonances of the alkynyl carbon atoms of 6; however, a
CC stretching band is observable at 2137 cm−1 in the IR
spectrum of the compound (cf. Mes2BCCMes: 2125
cm−1).24
Comparison of the Electronic Spectra of E, F, 5, and D.
The UV/vis absorption spectra (in toluene) of the vinyl-DBA
species E (R′ = Me), F, 5, and D are displayed in Figure 2; key
spectroscopic parameters are compiled in Table 1.
Compounds E (R′ = Me) and F possess absorption bands at
λmax(abs) 367 and 350 nm, respectively.18,25 Moreover, the
signal of F carries a shoulder on its bathochromic side; we
speculate that in the case of E (R′ = Me) the main band and the
shoulder are overlapping. The larger compound 5 also exhibits
an absorption maximum at 350 nm; the shoulder, however, is
now better resolved and further shifted to the red
(approximately 375 nm). A toluene solution of polymer D
displays its longest wavelength absorption maximum at around
350 and 410 nm.14 Thus, what has been a shoulder in the cases
of F and 5 has developed into a separate signal in the spectrum
of D (Figure 2). Kawashima et al. have reported the UV/vis
absorption spectrum of 9,10-dimesityl-9,10-dihydro-9,10-
diboraanthracene (MesB(C6H4)2BMes).26 They attributed a
strong absorption at λmax(abs) 349 nm (C6H12 or THF) to the
π−π* excitation of the DBA core. Given this background, the
350 nm bands in the spectra of F, 5, and D are likely of a similar
origin, whereas the shifting shoulders are caused by the
a
Legend: (i) ref 21; (ii) (1) +2.5 equiv of n-BuLi, THF−hexane, −78
°C, 1 h, (2) +3 equiv of Me3SnCl, THF−hexane, −78 °C to room
temperature, 12 h; (iii) +4 equiv of p-BrC6H4B(OH)2/20 equiv of
K2CO3/0.05 equiv of Pd(PPh3)4/0.02 equiv of Aliquat, toluene, 80 °C,
24 h; (iv) (1) +2.3 equiv of n-BuLi, THF−hexane, −78 °C, 1 h, (2)
+2.5 equiv of Me3SnCl, THF−hexane, −78 °C to room temperature,
60 h.
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a
Scheme 2. Syntheses of 9,10-Dihydro-9,10-diboraanthracene Derivatives 5−7
a
Legend: (i) hexane−toluene, room temperature, 1 h; (ii) hexane, room temperature, 12 h; (iii) hexane, 100 °C, 40 h.
substituents that should facilitate charge transfer to the
electron-poor DBA moieties.
We now turn to the fluorescence properties of E, F, 5, and D
(Figure 2). The smallest π system F emits at λmax(em) 470
nm.18 In comparison to Mes2BC(H)C(H)Mes (λmax(em)
396 nm),18 this means a bathochromic shift of Δλ = 74 nm and
leads to the conclusion that the DBA moiety is an essential part
of the fluorophoric group. When the two derivatives of E with
R′ = Me (λmax(em) 460 nm) and R′ = NMe2 (λmax(em) 570
nm) are placed side by side, we recognize a strong influence of
the peripheral substituent R′ on the position of the emission
maximum (Δλ = 110 nm).7 A comparison of E (R′ = Me) and
F, on the other hand, reveals that replacement of the boron-
bonded mesityl group by a (p-MeC6H4)vinyl substituent has
only a minor influence on the fluorescence maximum (Δλ = 10
nm), even though the mesityl ring adopts a conformation
orthogonal to the DBA moiety, whereas the vinyl group is in
conjugation with it. The emission band of 5 (λmax(em) 477 nm)
appears in a region similar to those of E (R′ = Me) and F with
only a small shift to the bathochromic side. All these
observations suggest that the optical properties of the class of
compounds under investigation here depend to a larger extent
on the presence of π-donor substituents (leading to charge-
transfer interactions) than on the overall degree of π
conjugation.
Polymer D exhibits the most red-shifted emission maximum
centered at λmax(em) 518 nm.14 However, in light of the above
discussion, this cannot necessarily be taken as a manifestation
of its most extended conjugated main chain but may well be
attributable to the +M effects of its solubilizing alkoxy side
chains. Accordingly, Chujo et al. reported that an A-type
polymer carrying π-donors (OR = OMe) shows lower energy
absorption and emission maxima than are found for the related
material in which the OMe substituents are replaced by methyl
groups (i.e., λmax(abs) 438 vs 400 nm, λmax(em) 505 vs 452 nm;
Ar = Tip; CHCl3).8
Figure 2. Normalized UV/vis absorption and emission spectra of E
(R′ = Me), F, 5, and D in toluene.
Table 1. UV/Vis Data of 5−7, D−F, and MesB(C6H4)2BMes
λmax(abs),
nm
λmax(em), nm
(λex, nm)
Stokes
shift, cm−1
ϕf
a
a
5
6
350, ∼375
359, 403,
428
477 (350)
460 (350)
5700
1600
0.02
0.30
a
7
353
461 (350)
518
6600
0.04
ab
,
D
349, 410
367
ac
,
E (R′ = Me)
E (R′ = NMe2)
ad
460
ac
,
447
570
,
F
350
470
ef
,
MesB(C6H4)2BMes
349, ∼400
460
a
b
c
d
In toluene. Reference 14. References 7, 25. Reference 18.
Reference 28. In C6H6.
e
f
different exocyclic boron substituents.27 At this point, we
cannot determine whether different effective conjugation
lengths or intramolecular charge-transfer processes are the
decisive factors. We note, however, that the linkers along the
polymer chain of D, for which the red shift of the low-energy
band is most pronounced, are equipped with π-donating alkoxy
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Comparison of the Electronic Spectra of 5−7. Similar
to the case for E, F, and D, the UV/vis absorption spectra of
5−7 exhibit strong absorptions at around 350 nm, which we
assign to DBA-centered π−π* transitions (λmax(abs) 350 (5),
359 (6), 353 nm (7); Figure 3). In addition, 5 shows a shoulder
Table 1). As a result, there should be fewer nonradiative decay
pathways, which in turn leads to higher quantum yields.
CONCLUSION
■
9,10-Dihydro-9,10-diboraanthracenes (DBAs) are highly useful
fluorophores with distinctly different properties in comparison
to the more common Mes2B group (Mes = mesityl). The
development of unsymmetrical, monofunctionalized DBA
derivatives MesB(C6H4)2BX (X = H, Br) allows convenient
access to bridged DBAs (i.e., MesB(C6H4)2B−L−B-
(C6H4)2BMes; L = π-conjugated linker) through hydroboration
or nucleophilic substitution protocols. A comparison of the
optical properties of three derivatives featuring bridging
elements L = −C(H)C(H)(p-C6Me4)C(H)C(H)− (5),
−CC(p-C6Me4)CC− (6), and −(p-C6H4)(p-C6Me4)(p-
C6H4)− (7) leads to the following conclusions. (i) The
emission wavelengths λmax(em) of all three compounds are
rather similar (5, 477 nm; 6, 460 nm; 7, 461 nm; toluene). (ii)
In contrast, the quantum yield ϕf of the alkynyl derivative 6
(0.30) is 1 order of magnitude higher than ϕf for 5 (0.02) and 7
(0.04). Moreover, we find that the stability of 6 toward air and
moisture is significantly better than in the cases of 5 and 7.31
Future synthesis efforts will therefore focus on more extended
DBA-containing oligomers featuring alkynylborane connec-
tions.
Figure 3. Normalized UV/vis absorption and emission spectra of 5−7
in toluene.
at approximately 375 nm and 6 exhibits two bands at λmax(abs)
403 and 428 nm. The single-band spectrum of 7 is very closely
related to that of MesB(C6H4)2BMes, apart from the fact that
the latter compound gives rise to an additional low-intensity
feature at ∼400 nm due to twisted intramolecular charge
transfer (TICT) from the electron-rich mesityl groups to the
electron-poor DBA core.26,28 The absence of a resolvable TICT
band in the spectrum of 7 is due to either the broader line
width of the main signal or an even lower intensity of the TICT
absorption, because each DBA unit in 7 carries only one mesityl
substituent. Judging from the UV/vis absorption spectrum, 7
behaves essentially as a 9,10-diphenyl-9,10-dihydro-9,10-dibora-
anthracene rather than as a more extended π-conjugated
system. This does not come as a surprise, given that the six-
membered rings of the terphenylene spacer likely adopt a
mutually twisted rather than a coplanar conformation (cf. the
dihedral angle p-C6H4//p-C6Me4 of 77.6(1)° in 2Br; more
information about the X-ray crystal structure analysis of this
molecule is provided in the Supporting Information).
The emission maxima of 5−7 in toluene are located in the
narrow range of λmax(em) 477, 460, and 461 nm, respectively.
Thus, 6 shows the smallest Stokes shift of all three model
systems, in line with the fact that it possesses the most rigid
molecular skeleton. As in the case of the absorption spectrum,
the emission spectrum of 7 is very similar to that of
MesB(C6H4)2BMes (λmax(em) 460 nm; C6H6)28 and therefore
does not merit further discussion.
The fluorescence quantum yield ϕf of 5 was determined to
be 0.02 (toluene), which perfectly fits to the value of ϕf = 0.02
(C6H12) for the literature-known compound Mes2BC(H)
C(H)(p-C6H4)C(H)C(H)BMes2.29,30 Replacement of the
vinyl (cf. 5) by an alkynyl (cf. 6) spacer leads to an
improvement of the quantum yield by 1 order of magnitude:
ϕf(6) = 0.30 (toluene). The quantum yield of 7 is again low
(ϕf(7) = 0.04; toluene), in line with the reported value for
MesB(C6H4)2BMes (ϕf = 0.02; C6H12).26 We suggest that the
superior performance of compound 6 may be attributed to the
extended, linear π conjugation that runs along the principal
molecular axis and to the rigidity of the molecular scaffold (cf.
the smaller Stokes shift of 6 compared to those of 5 and 7;
EXPERIMENTAL SECTION
■
General Considerations. Unless otherwise specified, all reactions
and manipulations were carried out under dry nitrogen or argon using
Schlenk or glovebox techniques. Hexane, toluene, and C6D6 were
dried over Na/benzophenone and distilled prior to use. Toluene used
for UV/vis spectroscopic measurements was purchased as spectro-
scopic grade, dried over Na/K alloy, and degassed prior to use. The
compounds 1H,21 3,18 and 418 were synthesized according to literature
procedures. NMR spectra were recorded on Bruker DPX 250, Avance
300, and Avance 400 spectrometers. Chemical shifts are referenced to
(residual) solvent signals (1H/13C{1H}, C6D6: 7.15/128.0 ppm) or
external BF3·Et2O (11B{1H}; 0.00 ppm). Abbreviations: s = singlet, d =
doublet, vt = virtual triplet, m = multiplet, br = broad, n.o. = signal not
observed, n.r. = not resolved, i = ipso, o = ortho, m = meta, p = para.
UV/vis absorption and emission spectra were recorded on a Varian
Cary 50 Scan UV/vis spectrophotometer and a Jasco FP-8300
spectrofluorometer, respectively. The absolute fluorescence quantum
yields (ϕf) were determined using a calibrated integrating sphere
system (ILF-835 100 mm diameter Integrating Sphere; Jasco), a
quantum yield calculation program (FWQE-880; Jasco), and highly
diluted samples of five different optical densities (from 2 × 10−6 to 2 ×
10−5 mol/L) in each measurement. IR spectra were measured on a
Jasco 4200 FT-IR spectrometer equipped with an ATR probe
(diamond). Combustion analyses were performed by the Micro-
analytical Laboratory of the University of Frankfurt and by the
Microanalytical Laboratory Pascher (Remagen, Germany).
Synthesis of 5. 1H (7.8 mg, 0.043 mmol) was dissolved in hexane
(1 mL), and the solution was added dropwise at room temperature to
a solution of 3 (25.1 mg, 0.086 mmol) in hexane−toluene (2:1; 3 mL),
whereupon a pale yellow precipitate immediately formed. The
precipitate was allowed to settle for 1 h, the mother liquor was
removed via syringe, and the solid product 5 was dried under dynamic
vacuum. Yield: 21.5 mg (65%).
1H NMR (400.1 MHz, C6D6): δ 2.11 (s, 12H; MesCH3-o), 2.33 (s,
12H; DurCH3), 2.34 (s, 6H; MesCH3-p), 6.93 (s, 4H; Mes-m), 7.23
(vtd, 3JHH = 7.3 Hz, 4JHH = 1.2 Hz, 4H; H-c or d), 7.30 (d, 3JHH = 18.7
Hz, 2H; BC(H)C(H)), 7.36 (vtd, 3JHH = 7.3 Hz, 4JHH = 1.2 Hz, 4H;
3
H-c or d), 7.66 (d, JHH = 18.7 Hz, 2H; BC(H)C(H)), 7.85 (dd,
4
3
3JHH = 7.3 Hz, JHH = 1.2 Hz, 4H; H-b or e), 8.34 (d, JHH = 7.3 Hz,
4H; H-b or e). 11B{1H} NMR (80.3 MHz, C6D6): δ 70.9 (n.r.; h1/2
=
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3000 Hz). 13C{1H} NMR (100.6 MHz, C6D6): δ 18.1 (DurCH3), 21.4
(MesCH3-p), 22.8 (MesCH3-o), 127.6 (Mes-m), 131.7 (C-6), 133.1
(C-c or d), 133.3 (C-c or d), 136.9 (Mes-p), 137.5 (C-b or e), 138.1
(Mes-o), 139.7 (C-b or e), 139.8 (br; BC(H)C(H)), 140.0 (C-5),
141.5 (br; Mes-i), 145.8 (br; C-a or f), 147.5 (br; C-a or f), 151.8
(BC(H)C(H)). UV/vis (toluene): λmax (ε) 350 (14700
mol−1dm3cm−1), shoulder at ∼375 nm. Fluorescence (toluene): λmax
477 nm (λex 350 nm). Anal. Calcd for C56H54B4 [770.25]: C, 87.32; H,
7.07. Found: C, 86.88; H, 7.05.
ACKNOWLEDGMENTS
■
This work has been financially supported by the Beilstein-
Institut, Frankfurt/Main, Germany, within the research
collaboration NanoBiC.
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1H NMR (400.1 MHz, C6D6): δ 2.08 (s, 12H; MesCH3-o), 2.34 (s,
6H; MesCH3-p), 2.60 (s, 12H; DurCH3), 6.93 (s, 4H; Mes-m), 7.21
(vtd, 3JHH = 7.3 Hz, 4JHH = 1.2 Hz, 4H; H-c), 7.43 (vtd, 3JHH = 7.3 Hz,
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3
4JHH = 1.2 Hz, 4H; H-d), 7.78 (d, JHH = 7.3 Hz, 4H; H-b), 8.75 (d,
3JHH = 7.3 Hz, 4H; H-e). 11B{1H} NMR (96.3 MHz, C6D6): δ n.o.
13C{1H} NMR (75.5 MHz, C6D6): δ 19.0 (DurCH3), 21.4 (MesCH3-
p), 22.7 (MesCH3-o), 125.2 (C-5), 127.6 (Mes-m), 133.6 (C-d), 133.9
(C-c), 137.0 (Mes-p), 137.6 (C-6), 138.1 (Mes-o), 139.1 (br; C-b),
139.3 (br; C-e), 141.2 (Mes-i), 145.6 (C-a or f), 146.4 (C-a or f), n.o.
(BCC, BCC). UV/vis (toluene): λmax (ε) 359 (21500), 403
(26200), 428 nm (36900 mol−1 dm3 cm−1). Fluorescence (toluene):
(9) Matsumi, N.; Naka, K.; Chujo, Y. J. Am. Chem. Soc. 1998, 120,
10776−10777.
(10) Matsumi, N.; Umeyama, T.; Chujo, Y. Polym. Bull. 2000, 44,
431−436.
(11) Miyata, M.; Chujo, Y. Polym. J. 2002, 34, 967−969.
(12) Li, H.; Jakle, F. Angew. Chem., Int. Ed. 2009, 48, 2313−2316.
̈
(13) Luebben, S.; Sapp, S. Material Matters 2007, 2, 11−14.
λmax 460 nm (λex 350 nm). IR (diamond ATR probe): ν
̃
2137 (CC)
(14) Lorbach, A.; Bolte, M.; Li, H.; Lerner, H.-W.; Holthausen, M.
cm−1. Anal. Calcd for C56H50B4 [766.22]: C, 87.78; H, 6.58. Found: C,
87.18; H, 6.78.
C.; Jakle, F.; Wagner, M. Angew. Chem., Int. Ed. 2009, 48, 4584−4588.
̈
(15) Chai, J.; Wang, C.; Jia, L.; Pang, Y.; Graham, M.; Cheng, S. Z. D.
Synth. Met. 2009, 159, 1443−1449.
Synthesis of 7. A solid mixture of 4 (49.5 mg, 0.133 mmol) and
2Sn (40.6 mg, 0.066 mmol) was suspended in hexane (3 mL) and the
suspension heated in a flame-sealed ampule to 100 °C for 40 h,
whereupon a light yellow solid gradually precipitated. After the mixture
was cooled to room temperature, the ampule was opened under argon
and the mother liquor was removed via syringe. The solid product 7
was dried under dynamic vacuum. Yield: 38 mg (66%).
(16) Lorbach, A.; Bolte, M.; Lerner, H.-W.; Wagner, M. Chem.
Commun. 2010, 46, 3592−3594.
(17) Lorbach, A.; Bolte, M.; Lerner, H.-W.; Wagner, M. Organo-
metallics 2010, 29, 5762−5765.
(18) Januszewski, E.; Lorbach, A.; Grewal, R.; Bolte, M.; Bats, J. W.;
Lerner, H.-W.; Wagner, M. Chem. Eur. J. 2011, 17, 12696−12705.
1H NMR (250.1 MHz, C6D6): δ 2.14 (s, 12H; MesCH3-o), 2.21 (s,
12H; DurCH3), 2.35 (s, 6H; MesCH3-p), 6.94 (s, 4H; Mes-m), 7.14−
̈
(19) Seven, O.; Qu, Z.-W.; Zhu, H.; Bolte, M.; Lerner, H.-W.;
Holthausen, M. C.; Wagner, M. Chem. Eur. J. 2012, 18, 11284−11295.
(20) All-carbon analogues of 5−7 have been reported, i.e.,
AntC(H)C(H)(p-C6H4)C(H)C(H)Ant, AntCC(p-C6H4)C
CAnt, and Ant(p-C6H4)3Ant, respectively (Ant = 9-anthryl):
(a) Nakatsuji, S.; Matsuda, K.; Uesugi, Y.; Nakashima, K.; Akiyama,
S.; Katzer, G.; Fabian, W. J. Chem. Soc., Perkin Trans. 2 1991, 861−867.
(b) Nguyen, P.; Todd., S.; Van den Biggelaar, D.; Taylor, N. J.;
Marder, T. B.; Wittmann, F.; Friend, R. H. Synlett 1994, 299−301.
(c) Schmieder, K.; Levitus, M.; Dang, H.; Garcia-Garibay, M. A. J.
Phys. Chem. A 2002, 106, 1551−1556. (d) Li, Z. H.; Wong, M. S.; Tao,
Y. Tetrahedron 2005, 61, 5277−5285.
3
7.24 (m, 8H; H-c,d), 7.39 (d, JHH = 8.1 Hz, 4H; H-2 or 3), 7.72 (d,
3JHH = 8.1 Hz, 4H; H-2 or 3), 7.83 (m, 4H; H-b or e), 8.13 (m, 4H; H-
b or e). 11B{1H} NMR (96.3 MHz, C6D6): δ n.o. 13C{1H} NMR (62.9
MHz, C6D6): δ 18.6 (DurCH3), 21.4 (MesCH3-p), 22.8 (MesCH3-o),
127.6 (Mes-m), 128.9 (C-2 or 3), 132.2 (C-6), 133.1 (C-2 or 3), 133.6
(C-c,d), 137.0 (Mes-p), 138.2 (Mes-o), 139.6 (C-b or e), 140.6 (C-b
or e), 141.4 (br; Mes-i), 141.9 (C-5), 143.9 (C-4), 146.2 (br; C-a or f),
146.5 (br; C-a or f), n.o. (C-1). UV/vis (toluene): λmax (ε) 353 nm
(25200 mol−1 dm3 cm−1). Fluorescence (toluene): λmax 461 nm (λex
350 nm). Anal. Calcd for C64H58B4 [870.37]: C, 88.32; H, 6.72.
Found: C, 88.66; H, 6.72.
(21) Vicente, J.; Chicote, M. T.; Alvarez-Falcon
M.-D.; Hernandez, F. J.; Jones, P. G. Inorg. Chim. Acta 2003, 347, 67−
74.
́
, M. M.; Abrisqueta,
́
ASSOCIATED CONTENT
■
(22) Other rare examples of diorganylboranes showing significant
concentrations of the monomeric species in solution include the
following. Mes2BH: Entwistle, C. D.; Marder, T. B.; Smith, P. S.;
Howard, J. A. K.; Fox, M. A.; Mason, S. A. J. Organomet. Chem. 2003,
680, 165−172. Tip2BH: Bartlett, R. A.; Dias, H. V. R.; Olmstead, M.
M.; Power, P. P.; Weese, K. J. Organometallics 1990, 9, 146−150.
Ferrocenyl2BH: Scheibitz, M.; Bats, J. W.; Bolte, M.; Lerner, H.-W.;
Wagner, M. Organometallics 2004, 23, 940−942.
S
* Supporting Information
Text, figures, a table, and CIF files giving details of the
syntheses and NMR spectroscopic data of 1Sn, 2Br, and 2Sn
and X-ray crystal structure analyses of 2Br and 2Br*. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(23) Gunther, H. NMR-Spektroskopie; Thieme Verlag: Stuttgart,
̈
Germany, 1983.
AUTHOR INFORMATION
■
(24) Eisch, J. J.; Shafii, B.; Odom, J. D.; Rheingold, A. L. J. Am. Chem.
Soc. 1990, 112, 1847−1853.
(25) Lorbach, A. Ph.D. Thesis, Goethe-Universitat Frankfurt,
Frankfurt (Main), Germany, 2011.
(26) Agou, T.; Sekine, M.; Kawashima, T. Tetrahedron Lett. 2010, 51,
Corresponding Author
̈
*E-mail: Matthias.Wagner@chemie.uni-frankfurt.de.
Notes
The authors declare no competing financial interest.
5013−5015.
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Organometallics
Article
(27) In agreement with this interpretation, the absorption spectrum
of the Mes2B analogue of 5, i.e. Mes2BC(H)C(H)(p-C6H4)C(H)
C(H)BMes2, lacks a 350 nm absorption but also shows a 373 nm band
̀
(C6H12 or CH2Cl2): Charlot, M.; Porres, L.; Entwistle, C. D.; Beeby,
A.; Marder, T. B.; Blanchard-Desce, M. Phys. Chem. Chem. Phys. 2005,
7, 600−606.
(28) Hoffend, C.; Schodel, F.; Bolte, M.; Lerner, H.-W.; Wagner, M.
̈
Chem. Eur. J. 2012, 18, 15394−15405.
(29) Yuan, Z.; Taylor, N. J.; Ramachandran, R.; Marder, T. B. Appl.
Organomet. Chem. 1996, 10, 305−316.
(30) Entwistle, C. D.; Collings, J. C.; Steffen, A.; Palsson, L.-O.;
́
Beeby, A.; Albesa-Jove, D.; Burke, J. M.; Batsanov, A. S.; Howard, J. A.
K.; Mosely, J. A.; Poon, S.-Y.; Wong, W.-Y.; Ibersiene, F.; Fathallah, S.;
Boucekkine, A.; Halet, J.-F.; Marder, T. B. J. Mater. Chem. 2009, 19,
7532−7544.
(31) Marder et al. have reported that a number of vinylborane
derivatives (p-RC6H4)C(H)C(H)BMes2 and phenylborane deriva-
tives (p-RC6H4)BMes2 appear to be air-stable, whereas the
corresponding alkynylboranes (p-RC6H4)CCBMes2 hydrolyze
slowly as solids and more rapidly in solution. Cf.: Yuan, Z.;
́
Entwistle, C. D.; Collings, J. C.; Albesa-Jove, D.; Batsanov, A. S.;
Howard, J. A. K.; Taylor, N. J.; Kaiser, H. M.; Kaufmann, D. E.; Poon,
S.-Y.; Wong, W.-Y.; Jardin, C.; Fathallah, S.; Boucekkine, A.; Halet, J.-
F.; Marder, T. B. Chem. Eur. J. 2006, 12, 2758−2771. On the other
hand, it is known from the literature that 9,10-divinyl-DBAs are very
sensitive to moisture.15 Thus, our compound 5 shows the same
moisture sensitivity as other 9,10-divinyl-DBAs, while the alkynyl
species 6 behaves similarly to (p-RC6H4)CCBMes2. We believe that
one reason for the pronounced sensitivity of 9,10-divinyl-DBAs is
rooted in the steric repulsion between the bent olefinic side chain and
the rigid DBA scaffold, which destabilizes the exocyclic B−C bond
(the same argument probably holds for 7). In comparison, vinyl-
BMes2 derivatives can relieve steric strain by adopting a propeller-like
conformation and 9-alkynyl-DBAs carry linear side chains.
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