Direct functionalization at the boron center of antiaromatic chloroborole

Article abstract of DOI:10.1039/b808483a

The presence of a reactive B-Cl bond allowed for the direct functionalization of the boron center in antiaromatic chloroborole ClBC ₄Ph₄, thus allowing for a selective fine tuning of the optical properties of borole derivatives and facilitating a potential new approach toward the introduction of borole moieties into the backbone of organic π-conjugated frameworks. The Royal Society of Chemistry.

Full text of DOI:10.1039/b808483a

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Direct functionalization at the boron center of antiaromatic
chloroborolew
Holger Braunschweig* and Thomas Kupfer
Received (in Cambridge, UK) 19th May 2008, Accepted 12th June 2008
First published as an Advance Article on the web 30th July 2008
DOI: 10.1039/b808483a
The presence of a reactive B–Cl bond allowed for the direct
functionalization of the boron center in antiaromatic chloro-
borole ClBC4Ph4, thus allowing for a selective fine tuning of the
optical properties of borole derivatives and facilitating a poten-
tial new approach toward the introduction of borole moieties into
the backbone of organic p-conjugated frameworks.
and selective fine tuning of the photophysical properties of
boroles remains elusive. In the present work, we report on a
new synthetic strategy for the preparation of substituted borole
derivatives that might be employed in the incorporation of
borole moieties into p-conjugated organic frameworks. We
demonstrate the direct functionalization of the reactive boron
center in chloroborole ClBC4Ph4 (4) both by halide abstraction
and nucleophilic displacement of the chlorine ligand without
interfering with the BC4-backbone.
During the last decades, the functionalization of p-conjugated
organic systems with boron-containing substituents has at-
tracted considerable attention because of the remarkable non-
linear optical and electrooptical properties of the resulting
polymeric and molecular materials.1 The success of this meth-
odology is essentially based on the inherent electron deficiency,
and hence the strong p-acceptor character of the three-coordi-
nate boron center, which allows for significant delocalization
through the vacant pz orbital.1,2 These materials are potentially
suited for numerous applications such as nonlinear optics,1a,3
two-photon absorption,4 luminescence5 or organic electronic
devices.6 In addition, it has been shown that the incorporation
of the boron moiety into cyclic-conjugated borole heterocycles
further significantly influences the respective absorption and
fluorescence properties with respect to the open-chain analogs.7
Here, the interaction of the pz-orbital at boron with the
unsaturated carbon backbone also results in a destabilization
of the entire 4p-electron system due to antiaromaticity.8 To
date, the number of stable and non-annulated boroles that have
been fully characterized is restricted to the pentaphenyl- and
ferrocenyl-substituted derivatives RBC4Ph4 [R = Ph (1), (Z5-
C5H5)Fe(Z5-C5H4) (2)].7a–c,h Boroles are usually prepared either
by salt-elimination7e,f or boron–tin exchange reactions7a–d,h of
appropriate boron reagents with dilithiated ligand precursors
and stannole precursors, respectively. Whereas the former
approach suffers a lack of selectivity and poor yields, the latter
usually proceeds with high selectivity, but requires specific and
sufficiently reactive boron halides. For this reason, all attempts
to generate amino-substituted borole derivatives according to
these two experimental routes have failed so far.9 Hence, a facile
Compound 4 was prepared via boron–tin exchange similar
to that previously described for the syntheses of the borole
derivatives 1 and 2.7h Hence, 1,1-Me2-2,3,4,5-tetraphenylstan-
nole [Me2SnC4Ph4] (3) was treated with a small excess of BCl3
in CH2Cl2 at ꢀ30 1C (Scheme 1), whereby the initially yellow
color of the solution instantaneously turned deep blue.w The
reaction was monitored by multinuclear NMR spectroscopy,
and the data confirmed the consumption of the starting
materials and the quantitative formation of 4 within 30
minutes. After work-up and recrystallization from toluene,
ClBC4Ph4 (4) was isolated analytically pure as a deep blue
solid in high yields of 87%.
The identity was confirmed by means of NMR spectroscopy
and elemental analysis. The 11B NMR resonance of 4 was
Institut fur Anorganische Chemie, Julius-Maximilians-Universitat
¨
¨
Wurzburg, Am Hubland, D-97074 Wurzburg, Germany.
¨
¨
E-mail: h.braunschweig@mail.uni-wuerzburg.de;
Fax: +49 931 888 4623; Tel: +49 931 888 5261
w Electronic supplementary information (ESI) available: Experimental
details of all X-ray crystal structure determinations and a figure of the
molecular structure of 6. Experimental section including the syntheses,
full characterization, and spectroscopic data of all compounds, as well
as figures of the UV-Vis spectra. CCDC reference numbers
685645–685647. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/b808483a
Scheme 1 Synthesis of chloroborole 4 and direct functionalization at
the reactive boron center (Pyr0 = 4-Me-C5H4N).
ꢁc
This journal is The Royal Society of Chemistry 2008
Chem. Commun., 2008, 4487–4489 | 4487

View Article Online
observed as a broad signal (d = 66.4 ppm) in a region
comparable to that found for 1 (d = 65.4 ppm), supportive
for the antiaromatic character of 4.7h Compound 4 is very
sensitive towards air and moisture and readily decomposes in
the solid-state at RT within a few days, but might be stored
under an inert atmosphere at ꢀ35 1C over a period of several
weeks. The stability in solution is even lower, both at RT and
at low temperatures. According to 1H NMR spectroscopy,
solutions of 4 in C6D6 exhibit a half-life of about 12 h at
ambient temperature. For this reason, all attempts to obtain
single crystals suitable for X-ray diffraction failed.
ring [B–C: see above; C2–C3: 1.3563(25) A; C3–C4: 1.5095(24) A;
C4–C5: 1.3600(24) A], which are comparable to those found for
the C–C single and CQC double bonds in a typical diene system
such as cis,cis-1,2,3,4-tetraphenylbuta-1,3-diene [1.356 A and
1.484 A].10
The following reaction might serve to illustrate the reactivity
of the B–Cl bond of 5. Treatment of a solution of 5 in CH2Cl2
with one equivalent of Na[BArf4] (Arf = 3,5-CF3-C6H3) and
4-Me-C5H4N in a resealable tube at 80 1C afforded the base-
stabilized, borole-based boronium cation [Ph4C4Bꢂ(NC5H4-
Me)2][BArf4] (6; Scheme 1).w Monitoring the reaction by NMR
spectroscopy revealed the quantitative conversion of 5 into 6
within 2 h and the absence of any soluble side and degradation
products. Compound 6 can also be prepared directly from 4 by
the addition of two equivalents of 4-Me-C5H4N to a stirred
solution of 4 and Na[BArf4] in CH2Cl2 and subsequent heating
of the reaction mixture to 80 1C (Scheme 1). Crystallization of
the crude product from CH2Cl2/hexanes at ꢀ35 1C yielded 6 as a
golden, crystalline material in 77% yield. As expected, the 11B
NMR spectrum of 6 features two distinct resonances for the two
non-equivalent boron nuclei (d = 7.2 and ꢀ7.5 ppm). The
molecular structure of 6 in the solid-state was determined by
X-ray diffraction.w The structural parameters of 6 are very
similar to those found for 5, and hence are not discussed here
(a graphical representation can be found in the ESIw).
However, structural characterization of 4 was accomplished
as its Lewis-base adduct with 4-Me-C5H4N. Treatment of 4
with stoichiometric amounts of 4-Me-C5H4N in benzene at RT
afforded Ph4C4BClꢂ(NC5H4Me) (5) quantitatively (Scheme 1),
as judged by 1H and 11B NMR spectroscopy.w The pure
product was isolated after recrystallization from heptane at
ꢀ60 1C as pale yellow crystals in 93% yield. In agreement with
the presence of a four-coordinate boron center, the 11B NMR
spectrum of 5 features a single resonance at much higher field
(d = 5.6 ppm) than that of the precursor molecule 4 (d = 66.4
ppm). Contrary to 4, compound 5 is much more stable and
shows no tendency to decomposition at all. The molecular
structure of 5 in the solid-state unambiguously confirms both
the coordination of one molecule of 4-Me-C5H4N to the boron
atom and the constitution of the starting material 4 (Fig. 1).w
The central borole moiety is almost planar (rms deviation:
0.0135 A). Similar to the molecular structure of pentaphenyl-
borole 1,7h the phenyl ligands are found in a propeller-like
arrangement with dihedral angles between 32.7(3)1 and 53.2(2)1.
The boron center displays a distorted tetrahedral environment,
whereas the B–C [B1–C2: 1.6098(24) A; B1–C5: 1.6099(28) A], the
B–N [1.6022(25) A] and the B–Cl [1.8757(19) A] bond distances lie
within the expected range for the corresponding single bonds.
Upon coordination of 4-Me-C5H4N, the former vacant pz orbital
at boron becomes occupied. As a result, a cyclic delocalization of
the p-electron system is no longer feasible, which results in the loss
of the antiaromatic character of 5 in comparison to 4. This is
further emphasized by the observed bond lengths within the BC4
To further highlight the suitability of 4 to act as a potential
borole source, we attempted the functionalization of the boron
center with anionic nucleophiles under retention of the trigonal-
planar coordination sphere. The reaction of 4 with equimolar
quantities of K[N(SiMe3)2] at RT proceeded smoothly in ben-
zene and was accompanied by a gradual color change from deep
blue to red and the formation of a KCl precipitate.w The NMR
spectra of the reaction mixture suggested the clean and quanti-
tative conversion of 4 into the amino-substituted borole deriva-
tive Ph4C4BN(SiMe3)2 (7) over a period of 1 h. Hence, the
1
singlet of the SiMe3 groups in the H NMR spectrum shifted
somewhat to higher field, i.e. from d = 0.15 ppm in
K[N(SiMe3)2] to d = 0.11 ppm in 7, and a new broad resonance
at d = 59.5 ppm was observed in the 11B NMR spectrum (4: d
= 66.4 ppm). The relatively low-field shifted 11B NMR signal
might indicate a significant antiaromatic character of 7 regard-
less of the presence of a BQN double bond. Red crystals of 7
were isolated analytically pure after recrystallization from hep-
tane at ꢀ35 1C in yields of 84%. The formation of an amino-
substituted borole was authenticated by a crystal structure
analysis of 7 (Fig. 2).w
The structural parameters of the planar BC4Ph4 moiety (rms
deviation: 0.0193 A), including associated bond distances and
the propeller-like arrangement of the phenyl substituents, are
very similar to those described for complexes 2,7h 5 and 6 and
will not be discussed here. The most intriguing structural
feature relates to the highly twisted arrangement of the
N(SiMe3)2 substituent at boron with respect to the
C1–B1–C4 plane [Si1–N1–B1–C1: ꢀ55.6(3)1; Si2–N1–B1–C1:
125.4(2)1] due to the steric bulk. Such a disposition should
preclude, or at least dramatically reduce the N–B p-donation,
which is in agreement with the anticipated antiaromatic
character of 7 derived from the 11B NMR shift and its deep
red color. It should be kept in mind that the coplanar
Fig. 1 Molecular structure of Ph4C4BClꢂ(NC5H4Me) (5) in the solid-
state.
ꢁc
This journal is The Royal Society of Chemistry 2008
4488 | Chem. Commun., 2008, 4487–4489

View Article Online
T.K. thanks the FCI for a PhD fellowship.
Note added in proof: While this manuscript was being
processed, Yamaguchi et al. reported on the synthesis of some
aryl-substituted borole derivatives via boron–tin exchange and
their full characterization.13
Notes and references
1 For example: (a) Z. Yuan, N. J. Taylor, T. B. Marder, I. D.
Williams, S. K. Kurtz and L.-T. Cheng, J. Chem. Soc., Chem.
Commun., 1990, 1489; (b) C. D. Entwistle and T. B. Marder,
Angew. Chem., Int. Ed., 2002, 41, 2927; (c) C. D. Entwistle and
T. B. Marder, Chem. Mater., 2004, 16, 4574; (d) Y. Qin, G. Cheng,
Fig. 2 Molecular structure of Ph4C4BN(SiMe3)2 (7) in the solid-state.
O. Achara, K. Parab and F. Jakle, Macromolecules, 2004, 37, 7123;
¨
(e) K. Parab, Y. Quin and F. Jakle, PMSE Prepr., 2005, 93, 422; (f)
¨
arrangement of the substituents typically observed within
R2BQNR2 systems usually results in a highly effective N–B
p-donation, which would cause the occupation of the vacant pz
orbital at boron in 7. As a consequence, the antiaromatic
delocalization of the p-electrons would be perturbed, accom-
panied with a color change to yellow (cf. formation of 5).
Hence, this structural parameter agrees very well with an
antiaromatic character in-between those ascribed to 4 and 5,
respectively. However, both the trigonal-planar geometry
[S = 359.91] and the BꢀN bond length [1.4254(31) A], which
is typical for a BQN double bond (1.41 A),11 are in agreement
with a boron center featuring a significant degree of a sp2-
hybridization. A similar twist of the BQN(SiMe3)2 moiety has
been reported earlier, e.g. for the dinuclear borylene-bridged
complex [(Z5-C5H4Me)2(CO)2Fe(m-CO)(m-BN(SiMe3)2)2].12
These findings are fully supported by the optical properties of
the borole derivatives 4–7, which were investigated by solution
UV-visible spectroscopy in CH2Cl2 in the range 200–800 nm.
The chlorine-substituted species 4 exhibits the characteristic blue
color of a non-annulated borole and an absorption band at
lmax = 553 nm, indicative of its antiaromatic character.7a,b,h In
contrast, this band is missing in the UV-visible spectra of the
yellow, tetra-coordinated compounds 5 and 6, whose lowest
C. H. Zhao, A. Wakamiya, Y. Inukai and S. Yamaguchi, J. Am.
Chem. Soc., 2006, 128, 15934; (g) M. Elbing and G. C. Bazan,
Angew. Chem., Int. Ed., 2008, 47, 834, and references therein.
2 (a) W. Kaim and A. Schulz, Angew. Chem., Int. Ed. Engl., 1984, 23,
615; (b) A. Schulz and W. Kaim, Chem. Ber., 1989, 122, 1863; (c) F.
Jakle, Coord. Chem. Rev., 2006, 250, 1107, and references therein.
¨
3 For example: (a) M. Lequan, R. M. Lequan and K. C. Ching,
J. Mater. Chem., 1991, 1, 997; (b) Z. Yuan, N. J. Taylor, R.
Ramachandran and T. B. Marder, Appl. Organomet. Chem., 1996,
10, 305; (c) Z. Yuan, J. C. Collings, N. J. Taylor, T. B. Marder, C.
Jardin and J.-F. Halet, J. Solid State Chem., 2000, 154, 5; (d) Z.
Yuan, C. D. Entwistle, J. C. Collings, D. Albesa-Jove, A. S.
Batsanov, J. A. K. Howard, N. J. Taylor, H. M. Kaiser, D. E.
Kaufmann, S.-Y. Poon, W.-Y. Wong, C. Jardin, S. Fathallah, A.
Boucekkine, J.-F. Halet and T. B. Marder, Chem.–Eur. J., 2006,
12, 2758.
4 For example: (a) Z.-Q. Liu, Q. Fang, D.-X. Cao, D. Wang and
G.-B. Xu, Org. Lett., 2004, 6, 2933; (b) M. Charlot, L. Porres, C.
D. Entwistle, A. Beeby, T. B. Marder and M. Blanchard-Desce,
Phys. Chem. Chem. Phys., 2005, 7, 600.
5 For example: (a) Y. Shirota, M. Kinoshita, T. Noda, K. Okumoto
and T. Ohara, J. Am. Chem. Soc., 2000, 122, 11021; (b) B. Y. Lee
and G. C. Bazan, J. Am. Chem. Soc., 2000, 122, 8577; (c) B. Y. Lee,
S. Wang, M. Putzer, G. P. Bartholomew, X. Bu and G. C. Bazan,
J. Am. Chem. Soc., 2000, 122, 3969; (d) R. Stahl, C. Lambert, C.
Kaiser, R. Wortmann and R. Jakober, Chem.–Eur. J., 2006, 12,
2358.
6 For example: (a) T. Noda and Y. Shirota, J. Am. Chem. Soc., 1998,
120, 9714; (b) H. Doi, M. Kinoshita, K. Okumoto and Y. Shirota,
Chem. Mater., 2003, 15, 1080; (c) W. L. Jia, X. D. Feng, D. R. Bai,
Z. H. Lu, S. Wang and G. Vamvounis, Chem. Mater., 2005, 17,
164; (d) M. Mazzeo, V. Vitale, F. D. Sala, M. Anni, G. Barbarella,
L. Favaretto, G. Sotgiu, R. Cingolani and G. Gigli, Adv. Mater.,
2005, 17, 34; (e) A. Wakamiya, K. Mori and S. Yamaguchi, Angew.
Chem., Int. Ed., 2007, 46, 4273.
7 (a) J. J. Eisch, N. K. Hota and S. J. Kozima, J. Am. Chem. Soc.,
1969, 91, 4575; (b) G. E. Herberich, B. Buller, B. Hessner and W.
Oschmann, J. Organomet. Chem., 1980, 195, 253; (c) J. J. Eisch, J.
E. Galle and S. Kozima, J. Am. Chem. Soc., 1986, 108, 379; (d) P.
A. Chase, W. E. Piers and B. O. Patrick, J. Am. Chem. Soc., 2000,
122, 12911; (e) S. Yamaguchi, T. Shirasaka, S. Akiyama and K.
Tamao, J. Am. Chem. Soc., 2002, 124, 8816; (f) S. Kim, K.-H.
Song, S. O. Kang and J. Ko, Chem. Commun., 2004, 68; (g) K. S.
Thanthiriwatte and S. R. Gwaltney, J. Phys. Chem. A, 2006, 110,
energy bands are now observed at lmax = 373 nm and lmax
=
425 nm, respectively. Strong visible bands in this region are a
common feature of all borole derivatives (4: l = 377 nm) and
have already been reported for different ring-annulated and base-
stabilized boroles.7 However, the absence of a low-intensity red-
shifted band is a suitable criterion for a reduced or missing
antiaromatic character in these species. Compound 7, on the other
hand, seems to lie in-between these two extremes: (i) 7 shows a
deep red color both in solution and in the solid-state; (ii) besides
the observation of a strong absorption band at l = 393 nm, a
broad shoulder is detected at around lmax = 478 nm that might
indicate the population of various rotamers in solution, but
strongly suggests the presence of some antiaromatic character in 7.
In summary, we reported on the syntheses and character-
ization of several borole derivatives containing unprecedented
B–Cl (4, 5) and BQN (7) linkages, as well as on a cationic
borole species (6). We presented a new strategy for the
preparation of borole derivatives that represents a potential
new approach toward the selective introduction of borole
moieties into the backbone of organic p-conjugated frame-
works, which enables the fine tuning of the optical properties
of these highly interesting materials.
2434; (h) H. Braunschweig, I. Fernandez, G. Frenking and T.
´
Kupfer, Angew. Chem., Int. Ed., 2008, 47, 1951.
8 (a) P. v. R. Schleyer, P. K. Freeman, H. Jiao and B. Goldfuss,
Angew. Chem., Int. Ed. Engl., 1995, 34, 337; (b) M. K. Cyranski, T.
M. Krygowski, A. R. Katritzky and P. v. R. Schleyer, J. Org.
Chem., 2002, 67, 1333.
9 H. Braunschweig and T. Kupfer, unpublished results.
10 J. W. Bats and B. Urschel, Acta Crystallogr., Sect. E, 2006, 62, 748.
11 P. Paetzold, Adv. Inorg. Chem., 1987, 31, 123, see p. 137.
12 H. Braunschweig, C. Kollann and U. Englert, Eur. J. Inorg. Chem.,
1998, 465.
13 C.-W. So, D. Watanabe, A. Wakamiya and S. Yamaguchi, Orga-
nometallics, 2008, 27, 3496.
ꢁc
This journal is The Royal Society of Chemistry 2008
Chem. Commun., 2008, 4487–4489 | 4489