Multistage complexation of fluoride ions by a fluorescent triphenylamine bearing three dimesitylboryl groups: Controlling intramolecular charge transfer

Article abstract of DOI:10.1021/jo2019152

A propeller-shaped boron-nitrogen compound (NB₃) with three binding sites for fluoride anions was synthesized and investigated by optical absorption, luminescence, and (¹H, ¹¹B, ¹³C, ¹⁹F) NMR spectros

Full text of DOI:10.1021/jo2019152

Article
pubs.acs.org/joc
Multistage Complexation of Fluoride Ions by a Fluorescent
Triphenylamine Bearing Three Dimesitylboryl Groups: Controlling
Intramolecular Charge Transfer
Hauke C. Schmidt,† Luisa G. Reuter,† Josef Hamacek,‡ and Oliver S. Wenger*,†
†Institut fur Anorganische Chemie, Georg-August-Universitat Gottingen, Tammannstrasse 4, D-37077 Gottingen, Germany
̈
̈
̈
̈
‡Dep
́ ́ ́ ́ ̀ ̀
artement de Chimie Minerale, Analytique et Appliquee, Universite de Geneve, 30 quai Ernest-Ansermet, CH-1211 Geneve 4,
Switzerland
S
* Supporting Information
ABSTRACT: A propeller-shaped boron−nitrogen compound
(NB3) with three binding sites for fluoride anions was
synthesized and investigated by optical absorption, lumines-
cence, and (1H, 11B, 13C, 19F) NMR spectroscopy. Binding of
fluoride in dichloromethane solution occurs in three clearly
identifiable steps and leads to stepwise blocking of the three
initially present nitrogen-to-boron charge transfer pathways. As
a consequence, the initially bright blue charge transfer emission is red-shifted and decreases in intensity, until it is quenched
completely in presence of large fluoride excess. Fluoride binding constants were determined from global fits to optical absorption
and luminescence titration data and were found to be Ka1 = 4 × 107 M−1, Ka2 = 2.5 × 106 M−1, and Ka3 = 3.2 × 104 M−1 in room
temperature dichloromethane solution. Complexation of fluoride to a given dimesitylboryl site increases the electron density at
the central nitrogen atom of NB3, and this leads to red shifts of the remaining nitrogen-to-boron charge transfer transitions
involving yet unfluorinated dimesitylboryl groups.
Scheme 1. Tris(4-(dimesitylboryl)phenyl)amine Molecule
(NB3) and Its Fluoride Adducts
INTRODUCTION
■
The use of Lewis acidic organoboron compounds for selective
binding of fluoride and cyanide anions has received much
attention in the past decade.1 Current challenges in this field
include the construction of systems capable of efficient fluoride
or cyanide binding from aqueous solution,2 the detection of
fluoride ions using redox-based sensors,3 fluoride-sensing
through tuning of the phosphorescence emitted by metal
complexes with triarylborane-substituted ligands,4 and the
development of new optoelectronic materials.5 Much effort is
devoted to optimizing the fluoride binding constants, for
example, by exploring cationic or bidentate organoboron
compounds.6 Recently, π-conjugated boron−nitrogen systems
have attracted attention because they exhibit intramolecular
charge transfer from the electron-rich nitrogen to the electron-
poor boron atom, and fluoride complexation strongly alters the
electronic properties of these substances.7 In the field of
organic mixed valence there are several examples of star- or
propeller-shaped molecules which exhibit unusual charge
delocalization phenomena,8 but intramolecular charge transfer
in structurally analogous boron−nitrogen systems has so far not
been explored.
The purpose of our research was not to develop a particulary
potent fluoride sensor; rather, our work was motivated by a
fundamental interest in electron transfer.9 Specifically, we
sought to explore how stepwise fluoride addition to tris(4-
(dimesitylboryl)phenyl)amine affects the three nitrogen-to-
boron charge transfer pathways which are initially present in
In this paper we report on the propeller-shaped boron−
nitrogen compound tris(4-(dimesitylboryl)phenyl)amine
(Scheme 1, upper left corner), hereafter referred to as NB3.
With three dimesitylboryl groups present in this molecule, the
formation of adducts with one (NB3F−), two (NB3F22−), and
three fluoride anions (NB3F33−) may be expected (Scheme 1).
Received: September 16, 2011
Published: October 13, 2011
© 2011 American Chemical Society
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this molecule. Such information may be useful for future studies
of long-range charge transfer across dimesitylboryl-substituted
bridges. There have been several prior investigations of
propeller-shaped boryl systems,7e,g,h,10 but to the best of our
knowledge our study is the first one in which there are three
nitrogen-to-boron charge transfer pathways that can be
controlled by stepwise fluoride addition.7h
RESULTS AND DISCUSSION
■
Synthesis. Tris(4-(dimesitylboryl)phenyl)amine (NB3)
can be synthesized in two steps: Bromination of commercially
available triphenylamine gives tris(4-bromophenyl)amine,
which can be converted to NB3 through addition of n-
butyllithium solution, followed by addition of dimesitylboron
fluoride in diethyl ether at −78 °C. The overall yield for the
bromination/borylation reaction sequence was 23%, but given
our primary interest in the (photo)physical and fluoride
binding properties of the target molecule, no particular
attempts to optimize reaction yields were undertaken. Detailed
synthetic protocols and product characterization data are given
in the Experimental Section.
Figure 2. (a) Extinction at 435 nm as a function of fluoride equivalents
added to a 1.6 × 10−5 M solution of NB3 in dichloromethane. (b)
Luminescence intensity at 535 nm measured after excitation of the
same solution at 363 nm.
adduct absorption spectra represented by the colored traces in
Figure 1b. The experimental data in Figure 1a were colored
accordingly in order to visualize which one of the four possible
species dominates the overall absorption spectrum at a given
titration point. As mentioned above, this is a global fit over all
wavelengths from 300 to 500 nm, and for a wavelength of 435
nm the quality of the fit is illustrated by the solid line in the
titration curve of Figure 2a. Key outcomes of the fit are the
binding constants for the individual fluoride anions as
summarized in the first row (“abs”) of Table 1. The β1,n
Optical Absorption and Luminescence Titrations. The
solid black trace in Figure 1a is the optical absorption spectrum
Table 1. Logarithms of the Cumulative Fluoride Binding
Constants Determined from Absorption (“abs”) and
a
Emission (“em”) Data
log(β1,1
)
log(β1,2
)
log(β1,3)
abs
em
avg
7.5(5)
7.7(5)
7.6(5)
13.9(5)
14.1(5)
14.0(5)
18.4(5)
18.5(5)
18.5(5)
a
The bottom row (“avg”) contains the average values.
Figure 1. (a) Optical absorption spectra measured on a 1.6 × 10−5
M
solution of NB3 in dichloromethane after addition of increasing
amounts of tetrabutylammonium fluoride. The solid black trace is the
initial spectrum; the trend of the absorbance at three selected
wavelengths is marked by downward or upward arrows. (b)
Experimental absorption spectrum of NB3 (black trace) and fitted
absorption spectra of the three fluoride adducts (colored traces) as
determined using the SPECFIT software.11
values reported therein are cumulative binding constants for the
1:1 (β1,1), 1:2 (β1,2), and 1:3 (β1,3) adducts, and the error
estimate on the last digit is captured by the number given in
parentheses.
Before discussing the magnitudes of these binding constants,
we note that fluoride addition to a dichloromethane solution of
NB3 does also lead to substantial changes of the emission
properties of this solution. As seen from Figure 3a, the
luminescence intensity decreases considerably, and there is
redistribution of intensity from shorter to longer wavelengths.
Excitation occurred at 363 nm, i.e., at a wavelength at which the
absorbance is largely constant over the fluoride concentration
range relevant here, and hence the decrease in emission
intensity observed in Figure 3a reflects a genuine decrease of
the overall luminescence quantum yield. Detection at a
wavelength of 535 nm yields an emission titration curve
(Figure 2b) which is qualitatively similar to the absorption
titration curve obtained when detecting at 435 nm (Figure 2a).
Again, careful analysis of the experimental data with the
SPECFIT software leads to the inescapable conclusion that 1:1,
1:2, and 1:3 adducts are formed between NB3 and F−. A global
fit at all wavelengths between 400 and 700 nm gives the
calculated spectra represented by the colored traces in Figure
3b and the cumulative binding constants given in the second
row (“em”) of Table 1.
of a 1.6 × 10−5 M solution of NB3 in dichloromethane. When
adding tetrabutylammonium fluoride (TBAF), the absorbance
at 400 nm gradually decreases while at longer wavelengths
more complicated behavior is observed. This is illustrated by
Figure 2a which shows the titration curve obtained when
monitoring the absorbance at 435 nm (open circles). An initial
steep increase between 0 and 2 equiv is followed by a more
shallow decrease up to 10 equiv, and finally there is an even
weaker drop-off above 10 equiv.
Global fits to the experimental absorption data from Figure
1a using the SPECFIT program11 lead to the conclusion that,
starting from the initially present NB3 species, fluoride addition
leads the formation of 1:1, 1:2, and 1:3 adducts between NB3
and F−, as illustrated by Scheme 1.
All other binding models (e.g., a model permitting only the
formation of 1:1 and 1:2 adducts) gave either completely
unsatisfactory fits or unreasonably high binding constants, or
both. The final fit relies on the experimental absorption
spectrum of NB3 (black trace in Figure 1b) and the calculated
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yellow emissions from Figure 4a in dichloromethane after
pulsed excitation at 340 nm. The blue charge transfer emission
exhibits a lifetime (τ) of 36 ns under these conditions, while the
yellow impurity luminescence decays with τ = 152 ns. The
factor of 4 decrease of the excited-state decay rate constant
combined with the factor of 20 decrease in emission quantum
yield (estimated from Figure 3a) leads to the conclusion that
the two emissions are of fundamentally different origin. This
finding is in line with our assigment of the yellow luminescence
to an impurity emission.
19F and 11B NMR Spectroscopic Investigations. While
optical absorption and luminescence spectroscopy appear as the
most sensitive techniques for investigation of fluoride binding
to NB3 (particularly in view the magnitudes of the binding
constants), 11B and 19F NMR spectroscopies can provide
complementary information regarding fluoride complexation.
The result of a 19F NMR titration of NB3 with tetrabuty-
lammonium fluoride in dichloromethane solution is shown in
Figure 5. The sharp signal at −129 ppm, which is easily
Figure 3. (a) Luminescence spectra measured after 363 nm excitation
of a 1.6 × 10−5 M solution of NB3 in dichloromethane after addition of
increasing amounts of tetrabutylammonium fluoride. The solid black
trace is the initial spectrum; the trend of the emission intensity at two
selected wavelengths is marked by downward or upward arrows. (b)
Experimental emission spectrum of NB3 (black trace) and fitted
emission spectra of the three fluoride adducts (colored traces).
The agreement between binding constants determined from
absorption and emission data is excellent, and average values
are given in the third row (“avg”) of Table 1. The association
constants for the individual fluoride binding steps (as defined in
Scheme 1) are Ka1 = 4 × 107 M−1, Ka2 = 2.5 × 106 M−1, and Ka3
= 3.2 × 104 M−1. Thus, successive binding of fluoride anions
occurs with negative cooperativity: As expected, the buildup of
negative charge upon binding of the first fluoride hinders the
second fluoride binding step (log Ka2 − log Ka1 = 1.2) and even
more the third (log Ka3 − log Ka2 = 1.9). However, compared
to other systems with multiple sites for fluoride binding, the
extent of negative cooperativity in NB3 is relatively modest.1
This is likely due to the fact that the three boron centers are
relatively far apart from each other, resulting in comparatively
weak electronic communication between individual dimesityl-
boryl groups.
Figure 5. 19F NMR spectra of NB3 in CD2Cl2 in presence of (a) 1, (b)
2, (c) 4, (d) 6, and (e) 10 equiv of tetrabutylammonium fluoride.
The observation of a weak yellow emission for NB3F3− (λmax
≈ 550 nm in Figure 3) is somewhat puzzling. Charge transfer
emission cannot occur in this species, and π−π* emissions
would be expected to show up at significantly shorter
wavelengths. It appears plausible that the weak yellow emission
is due to traces of impurities or degradation products. The
apparent intensities of the blue emission from NB3 and the
yellow impurity luminescence are essentially equal (Figure 4a),
noticeable in all spectra except the first one, is caused by the
tetrabutylammonium fluoride titrant; i.e., the respective signal is
due to unbound fluoride anions. Upon addition of increasing
amounts of fluoride, first a resonance at −172 ppm appears and
then a second signal at −174 ppm becomes observable. Only
the latter signal persists when more than 10 equiv of fluoride is
added, but additional resonances are not observed, not even in
presence of very large excess (>100 equiv) of fluoride. The
−172 and −174 ppm resonances fall into the typical range for
dimesitylboryl-bound fluoride;1 hence, they are attributed to
n−
NB3Fn (n = 1−3) species. Intuitively, one might have
expected three individual resonances signaling the presence of
1:1, 1:2, and 1:3 adducts. The 19F NMR titration data in Figure
5 are seemingly in contradiction with this expectation; however,
we note that the relative intensities of the −172 and −174 ppm
resonances as a function of fluoride concentration are
consistent with the species distribution curves obtained from
the UV−vis and luminescence titration data (Figure 6) if one
2−
3−
assumes that the resonances of NB3F2 and NB3F3 are
superimposed in the signal at −174 ppm. The species
distribution curve in Figure 6 predicts that after addition of 1
equiv of fluoride the NB3F− species is dominant, while after
Figure 4. (a) Luminescence of pure NB3 (left) and of NB3 in the
presence of very large excess (>100 equiv) of fluoride (right). (b)
Decays of the luminescences from (a) after pulsed excitation at 340
nm.
addition of ∼2 equiv similar amounts of NB3F− and NB3F2
2−
but this is only because the two emissions occur in spectral
ranges in which the human eye has very different sensitivities.
Figure 4b shows the luminescence decays of the blue and
will be present. The 19F NMR data in Figure 5a,b are in good
agreement with this prediction. After addition of more than 5
equiv of fluoride, the 1:1 adduct is clearly a minority species;
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transfer pathways. Thus, there is control over intramolecular
charge transfer along each of the three branches of the
propeller-shaped molecule.
On the one hand, the small negative binding coopearativity
suggests that the electronic interaction between individual
dimesitylboryl groups is small. On the other hand, complex-
ation of fluoride at a given dimesitylboryl site appears to affect
the electron density at the central nitrogen atom of NB3 to a
non-negligible extent, leading to a red shift of the remaining
(unblocked) nitrogen-to-boron charge transfer transitions. This
behavior suggests that when using dimesitylboryl-substituted
bridging elements, long-range charge transfer processes in
donor−bridge−acceptor molecules could be controlled by
fluoride addition.
Figure 6. Distribution of individual species as a function of fluoride
equivalents added.
hence, the −172 ppm resonance becomes weak (Figure 5d) or
even undetectable (Figure 5e).
11B NMR spectroscopy provides additional evidence for
binding of fluoride to NB3, but the sensitivity of our NMR
spectrometer to the 11B nucleus is comparatively low, and
hence only qualitative conclusions can be drawn from this data.
The 11B NMR spectrum of the initial NB3 species exhibits a
broad resonance at 74 ppm which is typical for dimesitylboryl
groups (page S4 in Supporting Information). Addition of 1
equiv of fluoride produces a broad signal around 7 ppm that
can be attributed to a fluorinated dimesitylboryl group, in
agreement with previously reported chemical shifts for
comparable systems.1 Even when adding 10 equiv of fluoride
no additional 11B resonances appear, and thus it is impossible to
distinguish between 1:1, 1:2, and 1:3 adducts from this
experiment. The comparatively large width of the 11B
resonances combined with the low instrument sensitivity is
not helpful in this respect.
EXPERIMENTAL SECTION
■
Synthesis. Bromine (0.62 g, 3.9 mmol) was added dropwise to a
stirred solution of triphenylamine (0.40 g, 1.6 mmol) in chloroform (3
mL) at 0 °C. The reaction mixture was stirred for 1 h at 0 °C and for
30 min at room temperature. After filtration through a glass frit,
ethanol (10 mL) was added, and the resulting solid was filtered and
washed with ethanol. Drying in vacuum gave 0.38 g of tris(4-
1
bromophenyl)amine as a colorless solid (38% yield). H NMR (300
MHz, CDCl3): δ (ppm) = 6.96−6.93 (m, 6 H), 7.35−7.39 (m, 6 H).
Tris(4-bromophenyl)amine (0.30 g, 0.6 mmol) was dissolved in dry
diethyl ether (3 mL) and cooled to −78 °C under a nitrogen
atmosphere. While stirring at −78 °C, 1.6 M n-butyllithium solution in
hexane (1.25 mL, 2.0 mmol) was added dropwise to the tris(4-
bromophenyl)amine solution. Subsequently, the reaction mixture was
allowed to warm to room temperature, and it was stirred for 1 h at this
temperature before it was cooled again to −78 °C. Then, a solution of
dimesitylboron fluoride (0.50 g, 1.87 mmol) in dry diethyl ether was
added dropwise to the reaction mixture. The latter was allowed to
warm to room temperature overnight, and finally 1 M hydrochloric
acid (10 mL) was added. After phase separation the aqueous phase was
extracted 3 times with 20 mL portions of dichloromethane, and the
combined organic phases were dried over anhydrous magnesium
sulfate. Solvent evaporation on a rotary evaporator was followed by
washing the solid residue with acetone. Drying in vacuum afforded
0.36 g of tris(4-(dimesitylboryl)phenyl)amine (NB3) as a pale yellow
Charge Transfer Properties. The UV−vis spectrum of
NB3 is dominated by an absorption band at 400 nm (solid black
trace in Figure 2a) which is expected to be caused by charge
transfer from the electron-rich nitrogen center to the electron-
poor dimesitylboryl groups, as commonly the case for
conjugated nitrogen−boron compounds.7c−g Upon fluoride
addition, one usually observes a blue shift of these charge
transfer transitions because the boron-localized LUMO is
shifted to higher energy. However, as seen from Figure 2,
opposite behavior is observed in the case of the NB3 molecule:
1
solid (60% yield). H NMR (300 MHz, CDCl3): δ (ppm) = 2.05 (s,
36 H), 2.31 (s, 18 H), 6.82 (s, 12 H), 7.07 (d, J = 9.7 Hz, 6 H), 7.42
(d, J = 9.7 Hz, 6 H). 13C NMR (300 MHz, CDCl3): δ (ppm) = 21.2,
23.4, 123.4, 128.1, 138.2, 138.4, 140.7, 149.8. MS (EI): m/z = 989.6
(calcd: 989.87). Anal. Calcd for C72H78NB3·1.5H2O (%): C: 84.97, H:
7.97, N: 1.38. Found: C: 84.71, H: 7.71, N: 1.30.
The NB3F− spectrum (and even more so the NB3F2
2−
spectrum) is significantly red-shifted with respect to the NB3
spectrum. The likely explanation for this uncommon behavior
is that there are multiple charge transfer pathways in our
propeller-shaped molecule: Binding of the first fluoride anion
blocks only one out of three nitrogen-to-boron charge transfer
pathways; moreover, it may even lead to an increase of the
electron density at the nitrogen center. Similarly, binding of the
second fluoride anion still leaves one nitrogen-to-boron charge
transfer pathway open, and the nitrogen center in this case is
even more electron-rich. This may explain both the decrease in
extinction as well as the red shift of the lowest energetic
absorption band along the series NB3, NB3F−, NB3F22−. In the
Spectral Fit Procedures. Statistical treatment of the spectropho-
tometric titrations in absorption or emission modes was performed
with the SPECFIT program.11 The stability constant for the NB3F−
2−
species was refined after fixing the initially fitted constants for NB3F2
and NB3F33− because of smooth spectral variations at the beginning of
titrations.
ASSOCIATED CONTENT
■
S
* Supporting Information
1H, 13C, and 11B NMR data. This material is available free of
charge via the Internet at http://pubs.acs.org.
3−
case of NB3F3 all charge transfer pathways are blocked.
AUTHOR INFORMATION
■
SUMMARY AND CONCLUSIONS
■
Corresponding Author
*E-mail: oliver.wenger@chemie.uni-goettingen.de.
Binding of up to three fluoride anions to NB3 occurs with
comparatively small negative cooperativity and can easily be
followed by optical absorption and luminescence spectroscopy,
where it manifests itself by spectral changes that can be
interpreted by blocking of individual nitrogen-to-boron charge
ACKNOWLEDGMENTS
■
Inke Siewert is thanked for valuable advice and discussions.
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