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plexes can also be classified as metalla-metallatranes[10] and
are especially reminiscent of the metalla-silatranes and
metalla-stannatranes with hypervalent group 14 centers.[11]
A density-functional theory (DFT) computational study
carried out on 1+ and 1-F by using the ADF program
(BP86/TZP with ZORA) followed by a Boys localization
conversion of approximately 20% and 35% for [1]BPh4
solutions layered with aqueous solutions containing 4 ppm
and 8 ppm of fluoride anions, respectively (see the Supporting
Information, Figures S3 and S4). No color change was
observed in the presence of common interferents, including
Clꢀ, Brꢀ, OAcꢀ, NO3ꢀ, and H2PO4ꢀ, thus indicating that this
new sensor is highly selective for fluoride anions. We have
also tested the pH tolerance of this sensor and observed that
the 31P{1H} NMR spectra of solutions of [1]BPh4 (1 ꢁ 10ꢀ3 m)
in CH2Cl2 remain unchanged in the pH range of 4–9
(Figure S5). At pH > 9, the 31P NMR resonance of 1+ loses
intensity and ultimately disappears. This observation signals
the onset of a reaction with hydroxide ions, the product of
which has not been identified. Finally, we note that 1+ also
captures fluoride anions at pH 9, thus pointing to the
tolerance of this new sensor to basic pH (Figure S6).
In summary, we report a cationic trisphosphanylstibine–
palladium complex (1+), which acts as a water-compatible
fluoride anion sensor. The sensing properties of this complex
result from the formation of a hypervalent fluorostiboranyl
motif coupled with a coordination geometry change of the
palladium center from square-planar to trigonal-bipyramidal.
These structural changes are accompanied by the appearance
of intense ligand-field transitions, thus providing a photo-
physical response for the anion binding event.
ꢀ
analysis indicates that the Sb Pd bonding in these two
compounds is polar covalent (Figure S7). This bonding
characteristic is supported by the elevated contributions of
both antimony and palladium atomic orbitals (Pd: 20.9%, Sb:
+
ꢀ
54.2% in 1 ; Pd: 43.7%, Sb: 27.4% in 1-F) to the Sb Pd
bonding orbital. In the extreme of the dative-bond formalism,
ꢀ
the Sb Pd bond of these complexes can be viewed as
switching from Sb:!Pd in 1+ to Pd:!Sb in 1-F. This change
ꢀ
can be interpreted as an anion-induced umpolung of the Sb
Pd bond, reminiscent of that observed in a related gold
system.[6b]
After we confirmed that the fluoride anion binds to the
antimony center of these complexes, we turned our attention
to the colorimetric response that accompanies the formation
of 1-F. In agreement with its pale-yellow color, the UV/Vis
spectrum of 1+ in CH2Cl2 shows an absorption band at lmax
=
366 nm, which tails off beyond 400 nm (Figure 2). This low-
energy band is attributed to a combination of a metal-to-
ligand charge transfer and ligand-centered transitions, which
are typically observed for square-planar Pd complexes.[12]
UV/Vis monitoring of the conversion of 1+ into 1-F upon
incremental addition of fluoride anions shows a progressive
quenching of the band at 366 nm, accompanied by the
appearance of new absorption bands centered at 430 nm
and 480 nm and spanning the 400–560 nm range. The
appearance of these bands, which account for the deep-
orange color of 1-F, is assigned to ligand-field transitions
(dxy,dx2ꢀy2 !dz2 and dxz,dyz!dz2) as previously reported for
related trigonal-bipyramidal palladium complexes bearing
tetradentate pnictogenyl ligands.[5,6,13] This interpretation is
reinforced by the observation that 2+, which lacks a third
phosphine arm, retains a square-planar palladium center
upon binding with the fluoride anion, and does not undergo
intense ligand-field transitions.
Encouraged by these results, we decided to investigate the
use of 1+ as a colorimetric fluoride sensor in protic solvents.
Incremental addition of a solution of KF in MeOH to a
solution of [1]BPh4 in MeOH/CH2Cl2 (9/1 v/v) led to the
progressive quenching of the band at lmax = 366 nm and the
growth of the low-energy bands at 435 and 487 nm, which
correspond to 1-F (see the Supporting Information, Fig-
ure S3). The resulting data was fitted to a 1:1 binding isotherm
to afford K = 5000(ꢁ300)mꢀ1. While attempts to use 1+ in
pure water were unsuccessful because of the insolubility of
the complex, even at low concentrations, we found that 1+
could be used under biphasic conditions for the detection of
ppm concentrations of fluoride anions. Solutions of [1]BPh4 in
CH2Cl2 (1 ꢁ 10ꢀ3 m) were layered with aqueous solutions
containing 4 ppm (2.0 ꢁ 10ꢀ4 m TBAF in H2O, pH 5.2) and
8 ppm (4.0 ꢁ 10ꢀ4 m TBAF in H2O, pH 5.3) of fluoride anions.
Upon shaking, the organic layers took on the characteristic
orange color of 1-F. The generation of 1-F was confirmed by
UV/Vis and 31P{1H} NMR measurements, which showed a
Experimental Section
General considerations. Antimony compounds are highly toxic and
should be handled cautiously. (o-(Ph2P)C6H4)3Sb[6a] and [PdCl2-
(cod)][14] were prepared according to reported procedures. All
preparations were carried out without any precautions to exclude
oxygen. Solvents were dried by passing through an alumina column
(pentane and CH2Cl2) or heating to reflux under N2 over Na/K (Et2O,
n-hexane, and THF). All other solvents were used as purchased. KF
and TBAF were obtained from Alfa Aesar. Ambient-temperature
NMR spectra were recorded on a Varian Unity Inova 400 FT NMR
(399.59 MHz for 1H, 375.99 MHz for 19F, 161.74 MHz for 31P,
100.45 MHz for 13C) spectrometer. Low-temperature 31P NMR spec-
tra were recorded on a Varian Inova 300 FT NMR spectrometer
(121.42 MHz for 31P). Chemical shifts (d) are given in ppm and are
referenced against residual solvent signals (1H, 13C) or external
standards (BF3·Et2O for 19F, and H3PO4 for 31P). UV/Vis spectra were
recorded on an Ocean Optics USB4000 spectrometer with an Ocean
Optics ISS light source. Elemental analyses were performed at
Atlantic Microlab (Norcross, GA).
Synthesis of [1]BPh4.
A solution of [PdCl2(cod)] (38 mg,
0.13 mmol) in CHCl3 (2 mL) was added to a solution of (o-C6H4P-
(PPh2))3Sb (122 mg, 0.13 mmol) in CHCl3 (10 mL), and the resulting
red solution was stirred for 12 h at room temperature before the
solvent was removed in vacuo. The resulting red-orange residue was
dissolved in CH2Cl2 (2 mL) and a solution of NaBPh4 (44 mg,
0.13 mmol) in EtOH (8 mL) was added dropwise under stirring. A
pale-yellow precipitate began to form immediately and the reaction
mixture was stirred at room temperature for 2 h. The solid was
collected by filtration and washed with EtOH (3 ꢁ 2 mL) and Et2O
(3 ꢁ 2 mL) to afford [1]BPh4 (143 mg, 90%). Single crystals of [1]BPh4
suitable for X-ray diffraction were obtained by vapor diffusion of
Et2O into
a
solution of the compound in CHCl3. 1H NMR
3
(399.59 MHz; CDCl3): d = 6.79 (t, 4H, BPh, JH-H = 7.14 Hz), 6.94 (t,
3
8H, BPh, JH-H = 7.33 Hz), 7.12 (m, 12H, meta-PPh), 7.20 (pseudo t,
3
12H, ortho-PPh, JH-H = 7.59 Hz), 7.26–7.36 (m, 6H o-P(Sb)C6H4 +
6H para-PPh), 7.40 (m, 8H, BPh), 7.45 (t, 3H, o-P(Sb)C6H4, JH-H
3
=
480
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 478 –481
Wade, Casey R.
