A heteronuclear bidentate Lewis acid as a phosphorescent fluoride sensor

Article abstract of DOI:10.1021/ja053058s

Reaction of dimesityl-1,8-naphthalenediylborate (1) with C₆F₅HgCl results in the formation of a B/Hg heteronuclear bidentate Lewis acid (2), which complexes fluoride to afford [2-μ₂-F]^-. Structural and photophysical studies carried out in solution and in the solid state indicate that 2 is a highly selective and sensitive phosphorescent fluoride sensor. The proximity of the two Lewis acidic sites enforced by the 1,8-naphthalenediyl backbone promotes fluoride anion chelation and is, therefore, responsible for the high binding constant. The interplay of conjugative and spin-orbit coupling effects mediated by the boron and mercury atoms, respectively, results in the phosphorescent signaling of fluoride binding. Remarkably, fluoride binding occurs in partially aqueous solutions and results in a drastic change of the phosphorescence observed when the solutions are frozen. Copyright

Full text of DOI:10.1021/ja053058s

Published on Web 06/18/2005
A Heteronuclear Bidentate Lewis Acid as a Phosphorescent Fluoride Sensor
Mohand Melaimi and Franc¸ois P. Gabba¨ı*
Department of Chemistry, Texas A&M UniVersity, College Station, Texas 77843
Received May 10, 2005; E-mail: francois@tamu.edu
Scheme 1 a
Fluoride displays a high hydration enthalpy (∆H° ) -504 kJ/
mol), and its molecular recognition in protic media is a definite
challenge.1 Several fluoride receptors that contain H-bond donors
as binding sites have been investigated and used in nonprotic
media.1,2 However, as recently noted by Fabbrizzi,2 such receptors
typically cannot be used in the presence water that effectively
competes with the receptor for the anion. For this reason, an impetus
exists for the development of alternative strategies. One such
a (1) C6F5HgCl, THF, 25°C, 72%. (2) [Me3SiF2]-[S(NMe2)3]+, CHCl3,
25 °C, 89%.
strategy involves the use of bidentate Lewis acidic receptors that
chelate the anion.3,4 As part of our contribution to this general area,
we recently demonstrated that appropriately substituted 1,8-
diborylnaphthalenes5,6 can be used as colorimetric fluoride sensors
with binding constants that exceeds those of simple triarylboranes
by at least 3 orders of magnitude.7 In an extension of this chemistry,
we would now like to describe the synthesis and properties of a
heteronuclear bidentate Lewis acid which serves as a phosphores-
cent fluoride sensor.
As demonstrated by a recent report, multidentate mercury Lewis
acids are competent for fluoride binding.8,9 Keeping in mind that
mercury can also serve to induce the room temperature phospho-
rescence of hydrocarbon chromophores10 via spin-orbit coupling,11
we decided to investigate the synthesis of a heteronuclear B/Hg
bidentate Lewis acid whose phosphorescent properties could serve
Figure 1. Crystal structure of 2 (left) and of the anionic component of
[2-µ2-F]-[S(NMe2)3]+ (right) (50% ellipsoid). Pertinent parameters are
provided in the text.
to signal fluoride complexation.
The [Li(THF)4]+ salt of dimesityl-1,8-naphthalenediylborate (1)12
was allowed to react with C6F5HgCl to afford 2 as a pale yellow
air- and water-stable derivative (Scheme 1). This compound, which
is not soluble in water, has been fully characterized. Some of its
salient spectroscopic features include (i) a 199Hg NMR signal
centered at -741.9 ppm and split into a triplet of triplets (3JHg-F
)
499 Hz and JHg-F ) 165 Hz), and (ii) a 11B NMR signal at 72
ppm. In the crystal (Figure 1), the boron atom B(1) adopts a trigonal
planar geometry (Σ(C-B-C) ) 359.9°) and is separated from the
mercury atom Hg(1) by only 3.300(9) Å. Owing to steric crowding,
the mercury atom Hg(1) and boron atom B(1) are displaced by
0.40 Å on either side of the naphthalene plane. The short C(21)-
Hg(1) distance of 3.000(7) Å indicates the presence of a secondary
Hg-π interaction involving the ipso-carbon of one of the mesityl
groups.10 As a result, the C(8)-Hg(1)-C(31) (172.8(3)°) angles
slightly deviate from linearity. As in other triarylboranes,7 it can
be expected that the unsaturated boron atom of 2 mediates
conjugation of the naphthalenediyl and mesityl substituents, which
collectively behave as a single chromophore. This is confirmed by
the UV-vis spectrum of 2, which features a broad absorption band
at 361 nm (ꢀ361 ) 10 200 in THF) notably red-shifted when
compared to the absorption of naphthalene (λmax ) 286 nm).
Because of the spin-orbital perturbation provided by the mercury
atom, photoexcitation of 2 in the solid state gives rise to an emission
at λmax ) 531 nm, which most likely corresponds to the phospho-
rescence of the dimesitylborylnaphthalenediyl chromophore (Figures
2 and 3). The large Stokes shift of approximately 7100 cm-1
corroborates this assignment.
4
Figure 2. Solid state phosphorescence spectra of compounds 2 (red line)
and [2-µ2-F]-[S(NMe2)3]+ (green line) after excitation at 324 nm at 77 K.
The pictures show the color of the emission observed under a hand-held
UV lamp at 77 K.
Compound 2 readily chelates fluoride anions in a variety of
solvents. For example, treatment of 2 with [Me3SiF2]-[S(NMe2)3]+
in chloroform leads to formation of [2-µ2-F]-[S(NMe2)3]+ (Scheme
1). The 11B NMR signal of [2-µ2-F]- appears at 8.5 ppm, as
expected for a tetrahedral boron atom. The 19F NMR spectrum
features a broad signal at -164.3 ppm, which is comparable to the
chemical shift observed in other fluoride-bridged boron species.4,5
Accordingly, the 199Hg nucleus is coupled to the bridging fluoride
(1JHg-F ) 135.2 Hz) and gives rise to a signal at -811.8 ppm split
into a triplet of doublets of triplets (3JHg-F ) 387.1 Hz, and 4JHg-F
) 65.8 Hz). Colorless crystals of [2-µ2-F]-[S(NMe2)3]+ were
obtained by slow evaporation of a CHCl3 solution. As indicated
by the X-ray crystal structure (Figure 1), the fluorine atom is bound
to both Lewis acidic centers. The B(1)-F(1) bond length of 1.483-
(4) Å is not significantly longer than those found in triarylfluo-
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10.1021/ja053058s CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
A similar phenomenon is readily observed in frozen solutions. Thus,
while a 2 × 10-3 M frozen THF solution of 2 gives rise to a yellow
luminescence attributed to phosphorescence of the dimesitylboryl-
naphthalenediyl chromophore, a white emission with a blue-green
tint is readily observed for [2-µ2-F]- when frozen in THF at the
same concentration. These emissions, which only necessitate the
use of a hand-held UV lamp as an excitation source, are extremely
bright and can be readily detected with the naked eye (Figure 3).
Addition of up to 2-3% of water does not affect the efficacy of
the assay.
In conclusion, we report the synthesis of a heteronuclear bidentate
Lewis acid which serves as a highly selective and sensitive
phosphorescent fluoride sensor. The proximity of the two Lewis
acidic sites enforced by the 1,8-naphthalenediyl backbone promotes
fluoride anion chelation and is responsible for the high binding
constant. The interplay of conjugative and spin-orbit coupling
effects mediated by the boron and mercury atoms, respectively,
results in the phosphorescent signaling of fluoride binding. To our
knowledge, 2 is the first example of a phosphorescent anion sensor.
Its ability to complex fluoride in partially aqueous media is also
noteworthy, yet not unprecedented.3a
Figure 3. Figure depicting the effect of fluoride binding on the photo-
physical properties of 2/[2-µ2-F]-. Luminescence detected for 2 and [2-µ2-
F]- in frozen THF solution.
roborate anions (1.47 Å),7c thus indicating the presence of a usual
polar covalent B-F linkage. Because the accepting 6p orbitals of
mercury might be too high to efficiently mix with the fluoride donor
orbitals, the Hg(1)-F(1) bond of 2.589(2) Å is probably more
electrostatic than covalent. Nevertheless, the length of this bond is
within the sum of the van der Waals radii of the two elements13,14
and is, in fact, comparable to those observed in a fluoride adduct
of a tetranuclear mercuracarborand (2.56 and 2.65 Å).8 In agreement
with the above bonding description, the sum of the coordination
angles at boron (Σ(C-B1-C) ) 340.4°) indicates substantial pyrami-
dalization while the C(8)-Hg(1)-C(31) angle (170.8(2)°) only
slightly deviates from linearity.
Acknowledgment. We thank Dr. J. Fackler, Dr. S. North., A.
Hsu, and L. Ruebush for their help with the spectroscopy. This
work is supported by the NSF (CAREER Grant CHE-0094264)
and the Program Lavoisier from the French Ministry of Foreign
Affairs (postdoctoral fellowship to M.M.).
Since fluoride complexation leads to population of the boron-
empty p-orbital, conjugation of the naphthalenediyl and mesityl
substituents is no longer operative. As a result, the absorption band
observed for 2 at 361 nm progressively disappears upon incremental
addition of fluoride. When this experiment is carried out in THF,
the absorbance at 361 nm linearly decreases and reaches the baseline
after the addition of exactly 1 equiv. These results indicate the
formation of a 1:1 complex whose stability constant exceeds the
range measurable by direct titration (K > 108 M-1). This stability
Supporting Information Available: Experimental details and X-ray
crystallographic data for 2 and [2-µ2-F]-[S(NMe2)3]+ in CIF format.
This material is available free of charge via the Internet at http://
pubs.acs.org.
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In the solid state, the energy and vibronic progressions observed
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closely to those observed in the phosphorescence spectrum of
naphthalene (Figure 2).10 This indicates that the loss of conjugation,
triggered by fluoride binding at boron, leads to electronic isolation
of the naphthalenediyl chromophore which readily phosphoresces
(Figure 3). These results also indicate that the spin-orbital
perturbation induced by the mercury atom is, a priori, not affected
by fluoride binding. Most significantly, fluoride binding to 2 leads
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