The pentaphenylborole-2,6-lutidine adduct: A system with unusual thermochromic and photochromic properties

Article abstract of DOI:10.1002/anie.201006234

Changing color: The pentaphenylborole-2,6-lutidine adduct 1 has unusual photophysical properties. Cooling a solution of 1 results in the disappearance of the absorption band at 578 nm and a color change from blue to yellow. Irradiation of 1 at low tempera

Full text of DOI:10.1002/anie.201006234

DOI: 10.1002/anie.201006234
Boron Heterocycles
The Pentaphenylborole–2,6-Lutidine Adduct: A System with Unusual
Thermochromic and Photochromic Properties**
Kay Ansorg, Holger Braunschweig,* Ching-Wen Chiu, Bernd Engels,* Daniela Gamon,
Markus Hꢀgel, Thomas Kupfer, and Krzysztof Radacki
Organoboranes have attracted great attention owing to their
interesting photophysical properties, which enable promising
applications as optoelectronics and colorimetric chemosen-
sors.[1,2] Three-coordinate boron has a low-lying empty
p orbital, which bestows a strong Lewis acidity to the resulting
substances and promotes electron transport through conju-
gated systems.[3] The electron affinity of trivalent boron leads
to the formation of various stable adducts of triaryl/alkyl
boranes and Lewis bases. Remarkably, some of the borane–
pyridine adducts have interesting nonlinear optical proper-
ties.[4] Recently, Wang and co-workers have demonstrated
that irradiation of adducts such as A leads to the formation of
Detailed investigations regarding the photochemistry of
Lewis base adducts of boracycles are however scarce, even
though theoretical and experimental studies have shown that
the incorporation of boron into a cyclic p system implicates a
substantial impact on the electronic properties of the
molecule.[7]
Herein, we present the structure and the thermochromic
behavior of Lewis base adducts of pentaphenylborole (PPB)
with pyridine bases, and in particular an unusual photo-
induced transformation to a borataalkene. To this end, the
Lewis base adducts of pentaphenylborole[8] with 4-picoline
(1a) and 2,6-lutidine (1b) were prepared in a straightforward
manner (Scheme 2). Interestingly, these two reactions sub-
stantially differ in their photophysical behavior. Treating a
CD2Cl2 solution of PPB with 4-picoline resulted in an
immediate color change from blue to yellow, and an upfield
shift of the 11B NMR signal from d = 65.4 ppm to d = 3.5 ppm
was observed in the 11B NMR spectrum,
À
B, which contains a boracyclopropane ring with the B N
linkage remaining intact (Scheme 1).[5] Analogous rearrange-
ment reactions have been found during photolysis of tetra-
phenylborates, where the generation of free radical species
has also been observed (Scheme 1).[6]
thus suggesting the formation of 1a.
However, upon addition of 2,6-lutidine
to PPB, the color of the solution remained
deep blue under the same conditions.
Surprisingly, the 11B NMR signal of 1b
detected at room temperature (d =
21.0 ppm) is at much lower field than
that of 1a. Cooling a toluene solution of
Scheme 1. Light-induced rearrangements of four-coordinate boron compounds.
1b from room temperature to À408C
resulted in a color change from deep blue
to light yellow. The UV/Vis spectrum of 1b at ambient
temperature features a broad absorption band at 578 nm
(Supporting Information, Figure S5) that decreased in inten-
sity upon cooling. At À408C, the 11B NMR signal is shifted to
d = 3.3 ppm, which clearly confirms the formation of a four-
[*] Prof. Dr. H. Braunschweig, Dr. C.-W. Chiu, D. Gamon, Dr. T. Kupfer,
Dr. K. Radacki
Institut fꢀr Anorganische Chemie
Julius-Maximilians-Universitꢁt Wꢀrzburg
Am Hubland, 97074 Wꢀrzburg (Germany)
Fax: (+49)931-31-84623
E-mail: h.braunschweig@mail.uni-wuerzburg.de
Homepage: http://www-anorganik.chemie.uni-wuerzburg.de/
Braunschweig/index.html
K. Ansorg, Prof. Dr. B. Engels, M. Hꢀgel
Institut fꢀr Physikalische und Theoretische Chemie
Julius-Maximilians-Universitꢁt Wꢀrzburg
Am Hubland, 97074 Wꢀrzburg (Germany)
Fax: (+49)931-31-85331
E-mail: b.engels@mail.uni-wuerzburg.de
[**] This work was supported by the DFG as part of GRK 1221. C.-W.Chiu
is grateful to the Alexander von Humboldt Foundation for a
postdoctoral fellowship. We thank Prof. C. Lambert and Prof. D. W.
Stephan for helpful discussions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201006234.
Scheme 2. Syntheses of the Lewis base adducts 1a and 1b.
Angew. Chem. Int. Ed. 2011, 50, 2833 –2836
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2833

Communications
coordinate boron center at low temperatures. In contrast, a
the chloroborole 4-picoline adduct (1.6022(3) ꢀ),[9b] and the
sterically highly hindered borafluorene systems
high-temperature NMR study revealed a linear shifting of the
11B NMR signal from d = 21.0 ppm at room temperature to
d = 56.0 ppm at 708C (Supporting Information, Figure S1).
These observations most likely result from an equilibrium
between 1b and a significant amount of non-coordinated PPB
(Scheme 2); this process is too fast to be resolved on the NMR
timescale. Thus, the 11B NMR chemical shift observed at
room temperature is the average of free and coordinated PPB.
However, with the faster UV/Vis spectroscopy, the free PPB
can be detected by its absorption at 578 nm.
(1.638(3) ꢀ).[10] In fact, the value in 1b is much more
comparable to that observed in the aforementioned FLP of
B(C6F5)3 and 2,6-lutidine, which dissociates at room temper-
ature in solution.[11] In comparison with PPB, the molecular
structure of 1b reveals significant strain, as shown by the
torsion angles of the borole.[9a] The steric demand of the Lewis
À
base is also emphasized by the two B C non-equivalent bonds
À
À
within the C4B unit (B C1 1.6524(3) ꢀ, B C4 1.6210(3) ꢀ).
This observation is very different from other four-coordinate
From the temperature-dependent shift of the 11B NMR
signal, values of DH and DS for the association process were
calculated to be À81 kJmolÀ1 and À244 JmolÀ1 K, respec-
tively. In comparison to the frustrated Lewis pair (FLP)
system B(C6F5)3/2,6-lutidine (DH = À42 kJmolÀ1, DS =
À130 JmolÀ1 K),[11] the equilibrium in this study is more on
the side of the coordinated PPB, which is due to the higher
Lewis acidity of boroles. At room temperature, about 29% of
the PPB remain non-coordinated, and thus the characteristic
blue color of this antiaromatic boracycle is retained. An
association constant of 380 LmolÀ1 was determined for 1b by
means of UV/Vis titrations (Supporting Information, Fig-
ure S4).
At room temperature, a broad signal at d = 2.52 ppm in
the 1H NMR spectrum of 1b was assigned to the two methyl
groups on the pyridine ring. At À408C, this signal splits into
two peaks at d = 1.57 and 3.29 ppm, respectively (Supporting
Information, Figure S2). An explanation for this observation
in solution can be drawn from the solid-state structure of the
adduct. Yellow single crystals of 1b that were suitable for an
X-ray diffraction study were obtained by diffusion of hexane
into a saturated toluene solution. In the molecular structure
of 1b, one of the methyl groups resides above the butadiene
system of the borole, while the other one is situated in a
position between two phenyl rings (Figure 1). The different
borole derivatives,[9,12] which all feature symmetric B C
À
bonds. The steric crowding of 1b is also reflected in the
outward bending of the ortho-methyl groups of lutidine by
0.339 ꢀ and 0.266 ꢀ with respect to the plane defined by the
NC5 ring. This deformation of the pyridine base is more
pronounced in 1b than in other FLPs.[11]
Irradiation of toluene solutions of 1a and 1b at À508C
reveals that 1a shows no visible change in absorption bands
and chemical shift. However, a significant color change from
yellow to dark green was observed in the case of 1b. NMR
spectroscopic investigations show a clean conversion of 1b
into another species upon irradiation. The 1H NMR spectrum
of the green compound at room temperature contains one
sharp signal at d = 1.76 ppm for the two methyl groups of
lutidine and a new multiplet at d = 5.92–5.96 ppm for two
protons associated with the 2,6-lutidine. A new set of signals
for the phenyl groups of PPB is also present (Figure 2). Excess
2,6-lutidine is observed as a singlet at d = 2.41 ppm for the two
environments of the methyl groups eventually result in
1
À
different H NMR shifts at low temperatures. The B N
bond in 1b (1.6567(3) ꢀ) is considerably longer than that in
Figure 2. 1H NMR spectroscopy study of the rearrangement from
compound 1b (†) to 2 (°); & excess 2,6-lutidine. The photoconver-
sion was carried out at À508C, but NMR spectra were recorded at
room temperature to prevent the splitting of the methyl groups in 1b
(see text for details).
methyl groups. The detection of a 11B NMR signal at d =
[13]
=
41 ppm suggests the formation of a B C bond.
This
Figure 1. Molecular structure of 1b, with ellipsoids set at 50%
probability. Hydrogen atoms are omitted for clarity. Selected bond
lengths [ꢂ] and angles [8]: B–N 1.6567(3), B–C1 1.6524(3), B–C4
1.6210(3), B–C5 1.6345(3), C1–C2 1.3687(3), C2–C3 1.4908(2), C3–C4
1.3551(3); C1-B-C4 99.82(1), C1-B-C5 104.52(1), C1-B-N 116.59(1), C4-
B-C5 116.73(2), C4-B-N 109.66(1), N-B-C5 109.55(1).
reaction pattern is without precedent, and, to the best of
our knowledge, the light-induced formation of a borataalkene
has never been reported. While 1b shows no detectable
15N NMR signal owing to the direct attachment of the
nitrogen atom to the quadrupolar boron nucleus,
a
2834
www.angewandte.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 2833 –2836

15N NMR signal at À153.0 ppm was observed for the new
species, which suggests the shifting of the Lewis base from
boron to carbon.[14] Again, excess 2,6-lutidine gives rise to an
additional 15N NMR signal at d = À63.2 ppm.
Irradiation of a toluene solution of 1b at low temperature
caused a shift of the base from boron to the adjacent carbon
=
atom with the formation of a B C bond. NMR spectroscopic
investigations and also theoretical calculations fully support
the assigned structure of the rearrangement product. The
computational results also suggest a very small barrier for the
back-reaction; for this reason, compound 2 must be handled
at low temperatures. Further investigations on the mechanism
of the rearrangement are now in progress.
Monitoring the reaction by NMR spectroscopy showed
that the conversion of a toluene solution of one equivalent of
PPB and 1.4 equivalents of 2,6-lutidine was complete after
24 h of light exposure (mercury lamp P = 180 W) at À508C
(Scheme 3). The reaction was found to be totally reversible at
room temperature (Supporting Information, Figure S3). To
Received: October 5, 2010
Revised: November 18, 2010
Published online: February 21, 2011
Keywords: boroles · boron · photochromism · rearrangements ·
.
thermochromism
[1] a) Z. Yuan, N. J. Taylor, Y. Sun, T. B. Marder, J. D. Williams, L.-
T. Cheng, J. Organomet. Chem. 1993, 449, 27 – 37; b) L. Weber,
V. Werner, M. A. Fox, T. B. Marder, S. Schwedler, A. Brock-
hinke, H-G. Stammler, B. Neumann, Dalton Trans. 2009, 1339 –
1351; c) A. Lorbach, M. Bolte, H. Li, H-W. Lerner, M. C.
Holthausen, F. Jꢁkle, M. Wagner, Angew. Chem. 2009, 121,
4654 – 4658; Angew. Chem. Int. Ed. 2009, 48, 4584 – 4588.
[2] a) A. Sundararaman, M. Victor, R. Varughese, F. Jꢁkle, J. Am.
Chem. Soc. 2005, 127, 13748 – 13749; b) C. R. Wade, F. P. Gabbai,
Inorg. Chem. 2010, 49, 714 – 720; c) C. R. Wade, F. P. Gabbai,
Dalton Trans. 2009, 9169 – 9175; d) S. Yamaguchi, S. Akiyama,
K. Tamao, J. Am. Chem. Soc. 2001, 123, 11372 – 11375.
[3] a) W. E. Piers, G. J. Irvine, V. C. Williams, Eur. J. Inorg. Chem.
2000, 2131 – 2142; b) M. Elbing, G. C. Bazan, Angew. Chem.
2008, 120, 846 – 850; Angew. Chem. Int. Ed. 2008, 47, 834 – 838;
c) T. W. Hudnall, C.-W. Chiu, F. P. Gabbai, Acc. Chem. Res. 2009,
42, 388 – 397; d) C. P. Entwistle, T. B. Marder, Chem. Mater. 2004,
16, 4574 – 4585; e) G. J. Zhou, C. L. Ho, W. J. Wong, Q. Wang,
D. G. Ma, L. X. Wang, Z. Y. Lin, T. B. Marder, A. Beeby, Adv.
Funct. Mater. 2008, 18, 499 – 511; f) F. Jꢁkle, Coord. Chem. Rev.
2006, 250, 1107 – 1121; g) Z. M. Hudson, S. Wang, Acc. Chem.
Res. 2009, 42, 1584 – 1596; h) Y. Shirota, J. Mater. Chem. 2000, 10,
1 – 25.
[4] a) H. C. Brown, H. J. Schlesinger, C. Z. Samuel, J. Am. Chem.
Soc. 2007, 129, 1880 – 1881; b) M. J. G. Lesley, A. Woodward,
N. J. Taylor, T. B. Marder, Chem. Mater. 1998, 10, 1355 – 1365;
c) Z. M. Su, X. J. Wang, R. S. Huang, R. S. Wang, J. K. Feng, J. Z.
Sun, Synth. Met. 2001, 119, 583 – 584.
[5] a) Y.-L. Rao, H. Amarne, S.-B. Zhao, T. M. Mc Cornick, S.
Martic, Y. Sun, R.-J. Wang, S. Wang, J. Am. Chem. Soc. 2008, 130,
12898 – 12900; b) C. Baik, Z. M. Hudson, H. Amarne, S. Wang, J.
Am. Chem. Soc. 2009, 131, 14549 – 14559; c) H. Amarne, C. Baik,
S. K. Murphy, S. Wang, Chem. Eur. J. 2010, 16, 4750 – 4761; d) C.
Baik, S. K. Murphy, S. Wang, Angew. Chem. 2010, 122, 8400 –
8403; Angew. Chem. Int. Ed. 2010, 49, 8224 – 8227.
Scheme 3. Lewis adducts 1a and 1b and the light-induced rearrange-
ment of 1b.
provide information as to whether the excess of 2,6-lutidine
might influence the course of the reaction, a control experi-
ment was carried out. To this end, a crystalline sample of the
adduct 1b was dissolved in [D8]toluene and irridiated at
À508C. Likewise, under these strictly stoichiometric condi-
tions (1:1), the new compound was formed, which leads us to
consider an intramolecular rearrangement reaction from 1b
to compound 2 or 3 upon irradiation (Scheme 3).
To improve our understanding of these processes, we
performed theoretical calculations on 1b and on two potential
rearrangement products, 2 and 3. Geometry-optimization and
single-point-energy calculations suggest 2 to be the favored
photoreaction product over 3. The NMR calculations of 2 also
compare very well with the experimental findings (NMR
calculations can be found in the Supporting Information). A
potential-surface calculation shows that 2 is 40 kcalmolÀ1
higher in energy than 1b. It also shows that 2 resides in a
shallow energy well, with an energy barrier of only 4 kcal
molÀ1 for the reverse reaction path. The computational results
are consistent with the poor stability of 2 under ambient
conditions. In the dark, 2 is fully converted into 1b without
1
decomposition at room temperature, with t = = 9.3 h. After
2
3 days, 1b was the only detectable compound in solution.
[6] a) P. J. Grisdale, B. E. Babb, J. C. Doty, T. H. Regan, D. P. Maier,
J. L. R. Williams, J. Organomet. Chem. 1986, 298, 63 – 72;
b) J. L. R. Williams, J. C. Doty, P. J. Grisdale, R. Searle, T. H.
Regan, G. P. Happ, D. P. Maier, J. Am. Chem. Soc. 1967, 89,
5153 – 5157; c) J. D. Wilkey, G. B. Schuster, J. Am. Chem. Soc.
1988, 110, 7569 – 7571; d) J. D. Wilkey, G. B. Schuster, J. Am.
Chem. Soc. 1991, 113, 2149 – 2155; e) J. D. Wilkey, G. B. Schus-
ter, J. Org. Chem. 1987, 52, 2117 – 2122; f) G. B. Schuster, S.
Chatterjee, P. D. Davis, P. Gottschalk, M. E. Kurz, B Sauerwein,
X. Yang, J. Am. Chem. Soc. 1990, 112, 6329 – 6338; g) G. B.
Schuster, Pure Appl. Chem. 1990, 62, 1565 – 1572.
Regeneration of 2 was readily accomplished by irradiation at
À508C.
In summary, the thermochromic Lewis base adduct of
pentaphenylborole with 2,6-lutidine, 1b, has been prepared
and characterized. Both thermochromism and photochrom-
ism are totally reversible and perfectly understood. The
thermochromism can be ascribed to a fast equilibrium
between free PPB (blue) and 1b (yellow). The molecular
structure of 1b revealed a strongly distorted ring system.
Angew. Chem. Int. Ed. 2011, 50, 2833 –2836
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
2835

Communications
[7] A. Sundararaman, K. Venkatasubbaiah, M. Victor, L. N.
[10] R. J. Wehmschulte, M. A. Khan, B. Twamley, B. Schmiemenz,
Organometallics 2001, 20, 844 – 849.
[11] S. J. Geier, A. L. Gille, T. M. Gilbert, D. W. Stephan, Inorg.
Chem. 2009, 48, 10466 – 10474.
[12] H. Braunschweig, C.-W. Chiu, K. Radacki, T. Kupfer, Angew.
Chem. 2010, 122, 2085 – 2088; Angew. Chem. Int. Ed. 2010, 49,
2041 – 2044.
Zakharov, A. L. Rheingold, F. Jꢁkle, J. Am. Chem. Soc. 2006,
128, 16554 – 16565.
[8] J. J. Eisch, N. K. Hota, S. Kozima, J. Am. Chem. Soc. 1969, 91,
4575 – 4577.
[9] a) H. Braunschweig, J. Fernandez, G. Frenking, T. Kupfer,
Angew. Chem. 2008, 120, 1977 – 1980; Angew. Chem. Int. Ed.
2008, 47, 1951 – 1954; b) H. Braunschweig, T. Kupfer, Chem.
Commun. 2008, 4487 – 4489; c) H. Braunschweig, C.-W. Chiu, K.
Radacki, P. Brenner, Chem. Commun. 2010, 46, 916 – 918.
[13] M. M. Olmstead, P. P. Power, K. J. Weese, J. Am. Chem. Soc.
1987, 109, 2541 – 2542.
[14] The 15N NMR signal for [4-MeNC5H4Me]I generated in situ was
detected at d = À186.
2836
www.angewandte.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 2833 –2836