A bidentate borane as colorimetric fluoride ion sensor

Article abstract of DOI:10.1039/b403596h

The bright yellow bidentate diborane (3), obtained by reaction of 10-bromo-9-thia-10-boraanthracene (1) with dimesityl-1,8-naphthalenediylborate (2), serves as a colorimetric anion sensor and selectively complexes fluoride with an association constant gre

Full text of DOI:10.1039/b403596h

A bidentate borane as colorimetric fluoride ion sensor†
Stéphane Solé and François P. Gabbaï*
Department of Chemistry, Texas A&M University, College Station, Texas 77843, USA.
E-mail: francois@tamu.edu; Fax: +1 979 845 4719; Tel: +1 979 862 2070
Received (in Columbia, MO, USA) 10th March 2004, Accepted 2nd April 2004
First published as an Advance Article on the web 28th April 2004
The bright yellow bidentate diborane (3), obtained by reaction of
10-bromo-9-thia-10-boraanthracene (1) with dimesityl-
1,8-naphthalenediylborate (2), serves as a colorimetric anion
sensor and selectively complexes fluoride with an association
constant greater than 5 3 109 M21 in THF.
naphthalene derivative. In addition, the spectrum indicates the
existence of a rigid structure since four aryl protons and six distinct
methyl groups are observed for the mesityl substituents. The 11B
NMR spectrum of 3 shows two resonances at 56 and 74 ppm
confirming the presence of two different boron centres. The
structure of 3 was computationally optimized using DFT methods
(B3LYP, 6-31+G* for the boron and sulfur centres, 6-31G for all
other atoms).†‡ The optimized geometry is close to that observed
for other diboranes bearing a dimesitylboryl group.12 Most
importantly, examination of the DFT orbitals reveals that the boron
pz orbitals contribute to the LUMO and are oriented toward one
another in a transannular fashion (Fig. 1) as observed for
1,8-bis(diphenylboryl)naphthalene.11 Also, the UV-Vis spectrum
of this derivative features a broad band centered at 363 nm, e =
17400 mol21 cm21. As indicated by a time-dependent DFT
calculations, electronic excitations from the HOMO, HOMO-1 and
HOMO-2 to the LUMO are the major contributors to this broad
band. Hence, any events leading to the disruption of the LUMO
should greatly affect the absorption spectrum of compound 3.
In the presence of [Me3SiF2]2[S(NMe2)3]+ in THF, compound 3
readily reacts with fluoride anions. This reaction is accompanied by
a rapid loss of the yellow colour (vide infra) and affords the anionic
chelate complex [3·m2-F]2 which has been fully characterized. Its
NMR spectra differ from those of 3 but are still characteristic of an
unsymmetrically substituted naphthalene species. Both boron
resonance are shifted to high field and appear at 12.3 ppm and 17.1
ppm confirming coordination of the fluoride anion. The 19F NMR
resonance of the bridging fluoride appears at 2188 ppm which is
comparable to the chemical shift observed in other fluoride bridged
species.9 As confirmed by single crystal X-ray analysis,†‡ the
fluorine atom is bound to both boron centres via B–F bonds of
comparable lengths (F–B(1) 1.633(5) Å, F–B(2) 1.585(5) Å) (Fig.
2). Owing to the bridging location of the fluoride anion, these bonds
are slightly longer than terminal B–F bonds observed in other
borate anions (1.47 Å).7c The sum of the coordination angles
(S(C–B1–C) = 347.8°, S(C–B1–C) = 341.2°) indicates that both boron
centres are substantially pyramidalized.
Anions display remarkably high solvation energy and their
molecular recognition by abiotic hosts continues to be a challenge.1
This is especially true for the small fluoride anion which enjoys a
high solvation energy. Taking into account the importance of this
anion in the treatment of osteoporosis2 and in dental care,3 a great
deal of effort has been devoted to the design of selective fluoride
sensors. Recently, the detection of fluoride as a by-product of sarin
hydrolysis has emerged as an added impetus for the study of such
receptors.4 In addition to receptors capable of hydrogen bonding
with the anionic guest,5 Lewis acidic receptors that can covalently
bind fluoride ion have also been reported.6,7 In this domain, the
study of colorimetric fluoride sensors based on boron-containing p-
electron systems is especially noteworthy.7 In such derivatives,
fluoride ion complexation leads to a perturbation of the frontier
orbitals thereby altering the photophysical properties of the
receptor. Important advances have also been realized with bidentate
Lewis acidic boranes8,9 such as 1,8-bis(boryl)naphthalene deriva-
tives.10,11 These derivatives have been shown to chelate small
anions including fluoride with apparently high selectivity;10 yet
their incorporation in a colorimetric sensor remains to be described.
In this communication, we report our original results on the
synthesis of a colorimetric fluoride sensor based on a bidentate
Lewis acid.
The successful design of such a sensor necessitates the
incorporation of a chromophoric boron moiety in a bidendate Lewis
acid. To this end, we first prepared 10-bromo-9-thia-10-boraan-
thracene (1) by reaction of bis-(2-trimethylsilylphenyl)sulfide with
boron tribromide. This compound was isolated as a yellow
crystalline solid and was allowed to react with dimesityl-
1,8-naphthalenediylborate (2)12 to afford diborane 3 in a 68% yield
(Scheme 1). This bright yellow diborane is soluble in chloroform,
THF and pyridine. The 1H NMR spectrum of 3 confirms the
attachment of the 9-thia-10-boraanthryl moiety and exhibits the
expected number of resonances for an unsymmetrically substituted
Fluoride complexation leads to population of the LUMO of 3 and
is logically accompanied by an instantaneous loss of the yellow
colour. Remarkably, no changes in the colour of the solution or in
Scheme 1 Synthesis of 3. i) BBr3, Et2O, 25 °C; ii) Et2O, 25 °C.
† Electronic supplementary information (ESI) available: electronic supple-
mentary information (ESI) available: synthetic and analytical results,
including elemental analysis, for 1 and 3 and [3·m2-F]2[S(NMe2)3]+. DFT
calculation details for 3. See http://www.rsc.org/suppdata/cc/b4/b403596h/
Fig. 1 DFT orbital picture of 3 showing the LUMO (surface isovalue =
0.04). H-atoms omitted for clarity.
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C h e m . C o m m u n . , 2 0 0 4 , 1 2 8 4 – 1 2 8 5
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

the NMR of diborane 3 are observed in the presence of chloride,
bromide and iodide anions indicating that fluoride anion complexa-
tion is infinitely selective. As previously proposed, the size of the
binding pocket provided by this bidentate borane can be held
responsible for this phenomenon.10 Incremental addition of fluo-
ride anions to a solution of 3 leads to a steady and linear decrease
of the absorbance at 363 nm which reaches the baseline after a total
addition of exactly one equivalent (Fig. 3). Further addition of
fluoride anions does not lead to additional perturbation of the
spectrum. These observations reflect the formation of a 1 : 1
complex whose stability constant exceeds the range measurable by
direct titration. In order to evaluate the stability constant of this
complex in THF, a titration experiment in the presence of a
competing fluoride acceptor was designed. Since triarylboranes
have been shown to complex fluoride anions,7 trimesitylborane
(Mes3B) was chosen as a competing fluoride acceptor. As
determined by a UV-Vis titration experiment, Mes3B complexes
fluoride anions with a binding constant of 3.3(0.4) 3 105 M21
which falls within the range established for other monofunctional
borane receptors.7 Remarkably, addition of fluoride anion to a
solution containing 3 and over twelve equivalents of Mes3B
resulted in the essentially quantitative formation of [3·m2-F]2
therefore indicating that 3 possesses a fluoride binding constant of
at least 5 3 109 M21. Moreover, addition of water does not lead to
decomplexation of the fluoride anion as typically observed for
fluoride adducts of monofunctional boranes.7c This difference
substantiates the chelating ability of 3 which leads to the formation
of a thermodynamically more stable fluoride complex. Finally, we
note [3·m2-F]2 can be converted back into 3 via treatment with
B(C6F5)3.
In conclusion, we report the synthesis of a colorimetric fluoride
sensor whose molecular recognition unit is a bidentate Lewis acidic
borane. The charge neutrality of this sensor as well as the short
space available between the boron centres makes this sensor highly
selective for fluoride. Finally, by virtue of its bidentate nature, the
fluoride association is remarkably high and by far exceeds that
measured for monofunctional borane receptors.
Support from the Robert A. Welch Foundation (Grant A-1423)
and the National Science Foundation (CHE-0094264) is gratefully
acknowledged. We thank Lisa Pérez for her help with the
calculations.
Notes and references
‡ CCDC 233190. See http://www.rsc.org/suppdata/cc/b4/b403596h/ for
crystallographic data in .cif or other electronic format.
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Fig. 2 Structure of the borate anion in [3·m2-F]2 (50% ellipsoid). H atoms
omitted for clarity. Selected bond lengths [Å] and angles [°] F–B(2)
1.585(5) F–B(1) 1.633(5), B(1)–C(1) 1.607(6), B(1)–C(11) 1.626(6), B(1)–
C(21) 1.641(6), B(2)–C(8) 1.604(6), B(2)–C(41) 1.609(6), B(2)–C(31)
1.621(6), B(2)–F–B(1) 126.0(3), C(1)–B(1)–C(11) 112.8(3), C(1)–B(1)–
C(21) 116.5(3), C(11)–B(1)–C(21) 118.5(3), C(8)–B(2)–C(41) 115.8(3),
C(8)–B(2)–C(31) 114.0(3), C(41)–B(2)–C(31) 111.4(3).
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Fig. 3 Spectral change accompanying the formation of [3·m2-F]2 upon
addition of nBu4NF to a THF solution of 3 (5.5 3 1025 M).
C h e m . C o m m u n . , 2 0 0 4 , 1 2 8 4 – 1 2 8 5
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