Synthesis and structural characterisation of the first bis(bora)calixarene: A selective, bidentate, fluorescent fluoride sensor

Article abstract of DOI:10.1039/b404937c

A bis(bora)calixarene, 3, the first lower-rim boron derivatised calixarene to be structurally characterised, is synthesised by the reaction of PhBCl ₂ with 4-tert-butylcalix[4]arene, and is demonstrated to be a sensitive and selective fluoresce

Full text of DOI:10.1039/b404937c

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Synthesis and structural characterisation of the first bis(bora)calixarene:
a selective, bidentate, fluorescent fluoride sensor†
Susumu Arimori,a Matthew G. Davidson,*a Thomas M. Fyles,b Thomas G. Hibbert,a Tony D. Jamesa
and Gabriele I. Kociok-Köhna
a University of Bath, Department of Chemistry, Claverton Down, Bath, UK BA2 7AY.
E-mail: m.g.davidson@bath.ac.uk
b Department of Chemistry, University of Victoria, Victoria, BC, Canada V8W 3P6
Received (in Cambridge, UK) 5th April 2004, Accepted 21st May 2004
First published as an Advance Article on the web 9th June 2004
A bis(bora)calixarene, 3, the first lower-rim boron derivatised
calixarene to be structurally characterised, is synthesised by the
reaction of PhBCl2 with 4-tert-butylcalix[4]arene, and is
demonstrated to be a sensitive and selective fluorescent fluoride
sensor.
analytically pure, colourless solid.† Recrystallisation of 3 from
acetonitrile yielded crystals suitable for X-ray diffraction.‡
The structure of 3 (Fig. 1) confirms its identity as an acetonitrile
solvate of the expected bis(bora)calixarene. Although a wide range
of lower-rim main-group functionalised calixarenes has been
described,9,10 to our knowledge, 3 is the first example of a lower-
rim boron derivatised calixarene to have been structurally char-
acterized.
In the solid state, the calixarene retains a cone conformation,11
the two boron atoms are approximately trigonal planar and the two
phenyl groups adopt a parallel and eclipsed conformation.
As a potential selective anion receptor, 3 has some interesting
structural features. First, the two adjacent Lewis acidic boron
centres in 3 are held a distance of 3.429 Å apart by the calixarene
scaffold, thereby defining an excellent bidentate Lewis acid cavity
for one fluoride anion. Second, for steric reasons, approach of an
anion is expected to be highly favoured in an endo fashion (i.e.,
between the two Lewis acid centres) precluding complications in
detection due to alternative exo-binding of fluoride which will be
significantly weaker (Scheme 1). Third, since it is well established
that binding of anions and neutral Lewis bases to boron centres can
result in quenching of fluorescence of directly attached fluor-
ophores6 the B–Ph groups provide a simple means of monitoring
anion binding to 3.
The design of synthetic molecular sensors for anions is an area of
chemistry that has received considerable attention in recent years.1
In the case of fluoride ions this has been for a range of
applications2–4 and has utilized one of two distinct binding
strategies. First, protic receptors have been designed that bind
anions via hydrogen bonding.1 Second, Lewis acid receptors have
also been exploited that rely on either electrostatic or covalent
bonding interactions with anions. In particular for the latter case,
boron centres have proved to be excellent selective receptors for
fluoride ions. Katz described bidentate naphthalene derivatives of
organoboranes 1 and showed them to exhibit enhanced fluoride
binding over related monodentate species.5 More recently, James
and co-workers6 established the use of boronic acids (2, R = H) as
fluoride receptors. They and others7 have demonstrated that aryl
boronic acids exhibit selective fluoride binding and that they are
potentially useful sensors, with detection being monitored by
fluorescence, electrochemical or colorimetric means.
In 3, the phenyl groups attached to boron provide sufficient
fluorescence intensity to allow binding events to be followed by
fluorescence spectroscopy (lex = 320 nm, lem = 395 nm).
In this context, we speculated that boron-derivatised calixarenes
may offer advantages over known fluoride receptors. For example,
the synthesis of 3 (for clarity, all tert-butyl groups are not shown)
and related compounds is very straightforward (i.e., two steps from
commercially available starting materials), and, unlike in 1, the
fluorophore may be varied independently of the scaffold. We herein
describe the synthesis and X-ray structure of a prototypical
bis(bora)calixarene 3 together with an evaluation of its utility as a
new class of fluorescent Lewis acidic fluoride sensor.
In contrast to recently reported attempts at organoboron
derivatisation of the lower-rim of calixarenes,8 we found that
reaction of tetralithiated 4-tert-butylcalix[4]arene with an organo-
boron dichloride proceeded in good yield to give 3 as an
Fig. 1 Molecular structure of 3·1.5CH3CN. Hydrogen atoms and acetonitrile
omitted for clarity. Selected bond distances (Å) and angles (°): B1–O1,
1.382(9); B1–O2, 1.372(8); B2–O3, 1.390(10); B2–O4, 1.360(10); B1–C1,
1.536(10); B2–C7, 1.542(11); O1–B1–O2, 127.0(6); O1–B1–C1, 118.0(6);
O2–B1–C1, 114.9(6); O3–B2–O4, 126.9(7); O3–B2–C7, 116.3(8); O4–
B2–C7, 116.9(8).
† Electronic supplementary information (ESI) available: synthetic and
spectroscopic details for the preparation of 3; details of the fluorescence
titration of 3 with F2 including details on the procedure used for calculation
of binding constants; and details of the crystal structure refinement. See
http://www.rsc.org/suppdata/cc/b4/b404937c/
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We gratefully acknowledge the support of the University of Bath
(TGH), the Royal Society (MGD and TDJ), EPSRC UK (MGD)
and NSERC Canada (TMF).
Notes and references
‡ Crystal data for 3·1.5CH3CN, C56H62B2O4·(CH3CN)1.5: Mr = 882.26,
monoclinic, space group P21/n, a = 11.0360(5), b = 21.5750(10), c =
22.0480(13) Å, b = 100.5070(16)°, T = 150 K, U = 5161.6(5) Å3, Z = 4,
dcalc = 1.135 Mg m23, m(Mo–Ka) = 0.069 mm21, 24829 reflections with
2q 5 46° measured, 7101 unique (Rint = 0.1449) and 3310 observed [I >
2s(I)]. Final residuals for 594 parameters: R1 = 0.1028, wR2 = 0.2723
(observed reflections) and R1 = 0.2036, wR2 = 0.3238 (all data). CCDC
231529. See http://www.rsc.org/suppdata/cc/b4/b404937c/ for crystallo-
graphic data in .cif format.
Scheme 1
Titration of a chloroform solution of 3 with Bu4NF results in a
decrease of the fluorescence of the boron-bound phenyl group of 3
(Fig. 2). Similar titrations with Bu4NCl and Bu4NBr have no
significant effect on the fluorescence spectrum of 3, thereby
demonstrating its expected selectivity for fluoride. The high
binding constant obtained for a 1 : 1 complex (logK1 = 6.3 ± 0.4)†
supports the expectation that a bidentate ‘endo’, binding mode is
favoured,12 and suggests the potential for more highly fluorescent
(bora)calixarenes to detect very low concentrations of fluoride. A
nonlinear fluorescence response is observed that indicates that a
further association between the 1 : 1 complex and the ligand may
occur at the relatively high concentrations necessary for measure-
ment of the phenylboron derivative. In the case of 3, such a scenario
is geometrically feasible via orthogonal approach of a second
bis(bora)calixarene to the 1 : 1 complex. However, this additional
binding should become insignificant for more highly fluorescent
species.
In conclusion, we have described the first synthesis and
structural characterization of a lower-rim boron-substituted calixar-
ene and have shown it to be a selective and sensitive fluorescent
fluoride sensor. These results provide impetus for further develop-
ment of (bora)calix[n]arenes as anion sensors. Further investigation
will include: improvements in sensitivity by incorporation of other
boron-bound fluorophores; tuning of anion selectivity (and poten-
tially simultaneous cation selectivity) by boronation of a range of
calix[n]arenes; and immobilization of (bora)calix[n]arenes via the
proven methods of upper-rim calixarene derivatisation.
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7 For other recent examples of boron-based fluoride receptors see: (a) S.
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11 Observation of four methylene resonances in the 1H NMR spectrum
suggests that this conformation is also retained in solution.† This is
expected since chelation of the calixarene lower-rim to two boron atoms
will impart a high degree of rigidity on the system.
12 Values reported for the binding constant of fluoride to monodentate
boron centres (logK) include 0.6,6b 1.0,7a and 5.4.7b Although
meaningful comparisons are difficult, our results suggest that the
sterically inhibited monodentate ‘exo’ binding mode is an unlikely
candidate for the 1 : 1 complex.
Fig. 2 Change in fluorescence spectrum of 3 (0.010 mol dm23 in
chloroform) with addition of Bu4N+F2 (0, 0.001, 0.002, 0.005, 0.007,
0.008, 0.009 and 0.010 mol dm23 for highest to lowest absorptions,
respectively).
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