9-H-9-Borafluorene dimethyl sulfide adduct: A product of a unique ring-contraction reaction and a useful hydroboration reagent

Article abstract of DOI:10.1039/c1cc14888e

The dimethyl sulfide adduct 2(DMS) is a crystalline storage form of the unstable hydroboration reagent 9-H-9-borafluorene (2); 2(DMS) is available by the addition of DMS to either in situ generated [2]₂ or 1,2-(2,2′-biphenylylene)diborane(6) (7

Full text of DOI:10.1039/c1cc14888e

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COMMUNICATION
9
-H-9-Borafluorene dimethyl sulfide adduct: a product of a unique
ring-contraction reaction and a useful hydroboration reagentw
¨
Animesh Das, Alexander Hubner, Mitra Weber, Michael Bolte, Hans-Wolfram Lerner and
Matthias Wagner*
Received 8th August 2011, Accepted 23rd August 2011
DOI: 10.1039/c1cc14888e
The dimethyl sulfide adduct 2(DMS) is a crystalline storage
form of the unstable hydroboration reagent 9-H-9-borafluorene
(
2); 2(DMS) is available by the addition of DMS to either
0
in situ generated [2]
2
or 1,2-(2,2 -biphenylylene)diborane(6) (7).
The incorporation of three-coordinate boron atoms into
conjugated p-electron frameworks leads to changes in the
electronic structure which often bring about enhanced
1
luminescence and charge-transport properties.
A convenient way to assemble extended p systems bearing
boron atoms at preselected positions is offered by the hydro-
boration of (hetero)aromatic (di)alkynes with arylboranes.
Since reagents of the type Ar
n
BH(3ꢀn) (n = 1, 2) tend to
undergo substituent redistribution, the use of arylboranes in
hydroboration reactions was long restricted mainly to
2
MesBH2
,3
4
and TripBH2 (Mes = mesityl; Trip = 2,4,6-
Fig. 1 The B–H–B bridged 9,10-dihydro-9,10-diboraanthracene
polymer [1] , a simplified sketch of the 9-H-9-borafluorene dimer [2]
triisopropylphenyl). In these cases, ligand scrambling is
unfavourable for steric reasons.
n
2
and the ring-opened boron-doped oligophenylene [2]
represents one o-C H ring).
5
(each circle
We have recently shown that 9,10-dihydro-9,10-dibora-
anthracene (1; Fig. 1) is also stable towards dismutation and
at the same time a powerful hydroboration reagent for various
alkynes, which are selectively transformed into luminescent
6
4
9-borafluorenyl moieties, but only during short periods of
time, because it tends to undergo ring-opening oligomerisation
9
5
,6
vinylboranes.
to main-chain boron-doped oligophenylenes (cf. [2]
Compounds like [2]
; Fig. 1).
5
Compound 1 can either be isolated as a B–H–B bridged
5
coordination polymer [1]n (Fig. 1) or as a monomeric
5
represent a new class of macromolecules
offering numerous ways of modification. In conclusion,
9-H-9-borafluorene (2) can react via two different channels,
both of them giving access to highly interesting p-conjugated
materials.
7
,8
dimethyl sulfide diadduct 1(DMS)2. In contrast, the super-
ficially related 9-H-9-borafluorene 2 (Fig. 1) is not an easily
9
isolable species. After its formation from 9-Br-9-borafluorene
and Et
2] , featuring one B–H–B three-centre, two-electron bond
together with a boron-bridging phenyl ring. In situ generated
2] is a suitable hydroboration reagent for the introduction of
3
SiH, the compound first exists as a C
1
-symmetric dimer
Aiming at a systematic future development of this chemistry
(Scheme 1), we now report on (i) a storable adduct 2(DMS) of
the hydroboration reagent 2, (ii) the high-yield synthesis
[
2
9
0
[
2
of 1,2-(2,2 -biphenylylene)diborane(6) (7) mimicking one
oligomer repeat unit of [2] , and (iii) the interconversion of
5
7
- 2(DMS), which sheds light on important aspects of the
ring-opening polymerisation reaction.
Treatment of [2] , freshly prepared in situ from 9-Br-9-
borafluorene (3) and Et
stable, crystalline adduct 2(DMS) in essentially quantitative
Institut fu¨r Anorganische Chemie, Goethe-Universita¨t Frankfurt,
Max-von-Laue-Strasse 7, D-60438 Frankfurt (Main), Germany.
E-mail: Matthias.Wagner@chemie.uni-frankfurt.de;
Fax: +49 69 798 29260
2
9
3
SiH, with excess DMS provides the
w Electronic supplementary information (ESI) available: Synthesis and
1
1
NMR-spectroscopic characterisation of 2(DMS), 4, 5, (Li(Et
tBuC(H )Q C(H)BC12 and tBuC(H) –C(H)(BC12 ; conversion of
into 2(DMS); single crystal X-ray structure analyses of 2(DMS), 4, 5,
Li(Et O)) [6] and 7. CCDC 837677, 837678, 837679, 837680 and
37681. For ESI and crystallographic data in CIF or other electronic
2
O))
2
[6], 7,
yield (Scheme 1). The B NMR spectrum of 2(DMS) in C
6 6
D
H
8
2
8 2
H )
is characterised by a doublet at ꢀ7.1 ppm with a coupling
7
1
constant of
JBH = 103 Hz. In stark contrast, the corres-
(
2
2
ponding signal of the 9,10-dihydro-9,10-diboraanthracene
8
format see DOI: 10.1039/c1cc14888e
diadduct 1(DMS)2 appears at significantly lower field
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 11339–11341 11339

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A
Fig. 2 Molecular structure and numbering scheme of 2(DMS) ;
displacement ellipsoids are drawn at the 50% probability level;
H atoms attached to carbon atoms are omitted for clarity. Selected
˚
bond length [A], bond angles [1] and dihedral angle [1]: B(1)–S(1),
1
.980(7); C(1)–B(1)–C(11), 100.7(6); C(1)–B(1)–S(1), 106.8(4);
C(11)–B(1)–S(1), 108.6(5); Ar(C(1))//Ar(C(11)), 4.1.
and 2(DMS)
B
are the same within the experimental error
is discussed here
Fig. 2). The molecule possesses a B–S bond length of
margins, only the structure of 2(DMS)
A
(
˚
.980(7) A, which is shorter by 0.051 A than the average
˚
1
7
˚
B–S bond length in 1(DMS)2 (2.031(2) A). Moreover, the
tricyclic framework is planar in 2(DMS) (dihedral angle
between the two o-phenylene rings: Ar(C(1))//Ar(C(11)) = 4.11),
7
but folded in 1(DMS)
2
(23.01) .
(DMS) is the aimed-for storage form of 9-H-9-borafluorene,
2
because it shows no tendency towards ring-opening oligomerisa-
tion. The compound also acts as a highly useful hydroboration
9
reagent. For example, we prepared tBuC(H )Q C(H)BC12H
8
9
2 8
as well as tBuC(H) –C(H)(BC12H )2, each of them in high
yields, via the selective single or double hydroboration of
tBuCRCH (BC H = 9-borafluorenyl; cf. the ESIw for
details). This means a major improvement compared to earlier
1
2
8
attempts at the hydroboration of tBuCRCH with in situ
generated [2] which had always resulted in a mixture of both
2
9
products.
The key starting material for the synthesis of 7 as a repeat-
unit model of [2] is the ditopic trihydridoborate Li [6], which,
in turn, can be prepared from the boronic acid derivatives 4 or
Scheme 1 Synthesis of the 9-H-9-borafluorene adduct 2(DMS), the
0
ditopic lithium trihydridoborate Li
2
[6] and the 2,2 -biphenylylene-
5
2
bridged diborane(6) 7; DMS-induced ring-contraction reaction from
1
0
7
to 2(DMS) (DMS = dimethyl sulfide). (a) C
6
H
6
, rt; (b) Et
2
O, 0 1C to
5 and Li[AlH
4
] (Scheme 1). Compound 4 is literature-
2
O, ꢀ78 1C to rt; (d) C
6 6
rt; (c) Et D , rt.
known; the internal boronic acid anhydride 5 has been
1
0
prepared using a protocol of IJpeij et al. for the synthesis
1
1
1
7
0
strengths (d( B) = 28.1; C D ) and J is not resolved. If a
of 2,2 -biphenyl diboronic acid. A quartet resonance at
1
6
6
BH
1
sample of 1(DMS) is measured in neat DMS, the B nuclei
1
11
= 77 Hz) in the B NMR spectrum of
ꢀ28.3 ppm ( J
2
BH
1
7
1
resonate at 1.9 ppm ( J E 60 Hz); a similar effect on the
Li [6], together with a 1 : 1 : 1 : 1 quartet at d( H) = 1.49 (6 H)
BH
2
1
1
B chemical shift value is absent in the case of 2(DMS). We
testify to the presence of two trihydridoborate substituents in
therefore conclude that the B–S association/dissociation equi-
the anion framework. According to an X-ray crystal structure
librium lies more on the adduct side in 2(DMS) compared to
analysis of the etherate (Li(Et
consists of discrete C -symmetric moieties, in contrast to the
coordination polymer formed by (Li(thf)
in the solid state. The dihedral angle between the two
o-C rings in (Li(Et O)) [6] amounts to 68.11 and the
2 2
O)) [6], the crystal lattice
1
(DMS) . However, this statement cannot be related directly
2
2
1
1
to the Lewis acidities or the antiaromatic character of free 1
and 2, because a diadduct is compared with a monoadduct.
2 2 6 4 3 2
) [m-C H (BH ) ]
2
(DMS) crystallises from C
6
H
6
with two crystallographically
H
6 4
2
2
+
independent molecules in the asymmetric unit (2(DMS)
A
,
trihydridoborate substituents are bridged by two [Li(Et
2
O)]
2
(DMS) ). Since all key metrical parameters of 2(DMS)
B
cations.
A
1
1340 Chem. Commun., 2011, 47, 11339–11341
This journal is c The Royal Society of Chemistry 2011

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additives can render the substituent dismutation reaction of
arylboranes a powerful tool for the preparation of rather
unique organoborane frameworks.
M. W. gratefully acknowledges financial support by the
Beilstein-Institut, Frankfurt/Main, Germany, within the
research collaboration NanoBiC; A. H. wishes to thank
the Fonds der Chemischen Industrie for a Ph. D. grant.
Notes and references
Fig. 3 The molecular structure and numbering scheme of 7; displace-
1
For selected review articles, see: (a) C. D. Entwistle and
T. B. Marder, Angew. Chem., Int. Ed., 2002, 41, 2927–2931;
ment ellipsoids are drawn at the 50% probability level. Selected bond
˚
˚
lengths [A], atomꢁ ꢁ ꢁatom distance [A], bond angles [1] and dihedral
angle [1]: B(1)–C(1), 1.555(2); B(2)–C(11), 1.557(2); B(1)ꢁ ꢁ ꢁB(2),
(
b) C. D. Entwistle and T. B. Marder, Chem. Mater., 2004, 16,
4574–4585; (c) S. Yamaguchi and A. Wakamiya, Pure Appl.
Chem., 2006, 78, 1413–1424; (d) F. Jakle, Coord. Chem. Rev.,
kle, Chem. Rev., 2010, 110,
1.755(2); C(1)–C(2)–C(12), 122.3(1); C(11)–C(12)–C(2), 122.1(1);
¨
2
3
006, 250, 1107–1121; (e) F. Ja
985–4022.
¨
Ar(C(1))//Ar(C(11)), 3.3.
2
N. Matsumi, K. Naka and Y. Chujo, J. Am. Chem. Soc., 1998, 120,
5112–5113.
Treatment of (Li(Et O)) [6] with excess Me SiCl in Et O
2
2
2
3
1
resulted in the clean formation of 7 (70% yield). A char-
2
3 N. Matsumi, M. Miyata and Y. Chujo, Macromolecules, 1999, 32,
4467–4469.
1
acteristic NMR feature of 7 is the B resonance at 13.7 ppm,
1
4
A. Nagai, T. Murakami, Y. Nagata, K. Kokado and Y. Chujo,
Macromolecules, 2009, 42, 7217–7220.
which is split into a doublet due to coupling with the terminal
1
hydride substituent ( JBH = 120 Hz; coupling to the bridging
5 A. Lorbach, M. Bolte, H. Li, H.-W. Lerner, M. C. Holthausen,
F. Jakle and M. Wagner, Angew. Chem., Int. Ed., 2009, 48,
584–4588.
¨
hydrogen atoms is not resolved). In the proton NMR
spectrum, the boron-bonded hydrogen atoms give rise to a
4
6
A. Lorbach, C. Reus, M. Bolte, H.-W. Lerner and M. Wagner,
Adv. Synth. Catal., 2010, 352, 3443–3449.
featureless signal at 1.30 ppm (2H, BHbridging) and a quartet at
1
.77 ppm (2H, JHB = 120 Hz, BHterminal). An X-ray crystal
7 A. Lorbach, M. Bolte, H.-W. Lerner and M. Wagner, Chem.
Commun., 2010, 46, 3592–3594.
4
0
structure analysis of 7 revealed an almost planar 2,2 -biphenyl
8
A. Lorbach, M. Bolte, H.-W. Lerner and M. Wagner, Organo-
metallics, 2010, 29, 5762–5765.
fragment, bridged by a (H)B(m-H) B(H) moiety (Fig. 3).
2
˚
The average B–C distance amounts to 1.556(2) A; the
9 A. Hu
M. C. Holthausen and M. Wagner, J. Am. Chem. Soc., 2011,
33, 4596–4609.
¨
bner, Z.-W. Qu, U. Englert, M. Bolte, H.-W. Lerner,
˚
˚
B(1)ꢁ ꢁ ꢁB(2) distance is 1.755(2) A (cf. Bꢁ ꢁ ꢁB = 1.76(1) A in
1
1
3
9
˚ ˚
5
and 1.806(6) A/1.800(4) A in [2] ). The synthesis
B
H
2 6
1
0 E. G. IJpeij, F. H. Beijer, H. J. Arts, C. Newton, J. G. de Vries and
G.-J. M. Gruter, J. Org. Chem., 2002, 67, 169–176. 4: we have
developed an optimised alternative synthesis, which is described in
the ESIw; 5: in our hands, recrystallisation of the crude material
gave the anhydride rather than the diboronic acid; cf. the ESIw for
X-ray crystal structure analyses of 4 and 5.
sequence outlined in Scheme 1 thus provides convenient access
to a so far largely unexplored class of bridged organo-
5
diboranes(6) in general and to the simplest [2] repeat-unit
model in particular.
Addition of excess DMS to 7 led to a ring-contraction
reaction which gave the 9-H-9-borafluorene DMS adduct
11 D. Franz, M. Bolte, H.-W. Lerner and M. Wagner, Dalton Trans.,
011, 40, 2433–2440.
2 Compound 7 has already been mentioned by Ko
obtained a sample via a high-temperature reaction involving
pyrophoric boranes: R. Koster and H.-G. Willemsen, Justus
Liebigs Ann. Chem., 1974, 1843–1850.
2
1
¨
ster et al. who
2
(DMS) on the one hand and H B(DMS) on the other. Thus,
3
the insertion of compounds R BH
n
(n = 0, 1) into 9-bora-
¨
(3ꢀn)
fluorenes with formation of 7-type species is obviously
1
4
13 D. S. Jones and W. N. Lipscomb, Acta Crystallogr., Sect. A, 1970,
A26, 196–207.
reversible when appropriate Lewis bases are added. We
therefore suggest that a careful adjustment of reaction condi-
tions together with the appropriate choice of Lewis basic
1
4 B. Wrackmeyer, P. Thoma, R. Kempe and G. Glatz, Collect.
Czech. Chem. Commun., 2010, 75, 743–756.
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 11339–11341 11341