1λ5-Stibaindoles as Lewis Acidic, π-Conjugated, Fluoride Anion

Article pubs.acs.org/Organometallics

1λ5‑Stibaindoles as Lewis Acidic, π‑Conjugated, Fluoride Anion Responsive Platforms Anna M. Christianson,† Eric Rivard,*,§ and François P. Gabbaï*,† †

Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255, United States of America Department of Chemistry, University of Alberta, 11227 Saskatchewan Drive, Edmonton, Alberta T6G 2G2, Canada

§

S Supporting Information *

ABSTRACT: Our interest in anion-responsive materials has led us to investigate the synthesis and properties of derivatives featuring a Lewis acidic antimony(V) moiety integrated within the π-conjugated structure of a 1λ5-stibaindole (or 1λ5benzostibole). Starting from an organozirconium reagent, 1,2,3-tris(phenyl)-1stibaindole (1) was first synthesized and subsequently allowed to react with ochloranil and 3,5-di-tert-butyl-o-benzoquinone to afford the corresponding catecholato-1λ5-stibaindole derivatives 2 and 3, respectively, as a result of oxidative addition of the o-benzoquinone reagent to the antimony center. Conversion of 1 into 2 and 3 lowers the energy of the antimony-based σ* orbital, thus promoting its conjugation with the π* orbital of the heterocyclic chromophore. As a result of this σ*−π* conjugation, 2 and 3 experience a narrowing of the π−π* energy gap leading to a red shift of the corresponding UV−vis absorption band. This σ*−π* conjugation can be turned off by addition of a fluoride anion, which interacts with the antimony-based σ* orbital. This turn-off response manifests itself in a color change that can be exploited for the detection of ppm levels of fluoride in drinking water samples.

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INTRODUCTION Penta- or tetra-coordinated antimony(V) derivatives display strong Lewis acidic properties1 that can be traced back to the presence of low-lying antimony-centered σ* orbitals available for substrate coordination.2 We have exploited this characteristic in the design of antimony-based molecular colorimetric and/or fluorescent sensors for anions such as fluoride and cyanide.3 In all cases investigated thus far, the sensors consist of an antimony(V) group substituted by a well-defined chromophore such as an anthryl group in the case of A+3b or a BODIPY dye in the case of B+3c (Scheme 1). These sensors respond to the presence of the anion because of the involvement of the antimony-based σ* orbitals with orbitals of the reporter group in the excited state. It occurred to us that a more direct response could be observed if the antimony-based σ* orbital were brought into conjugation with the chromophore orbitals not only in the excited state but also in the ground state. Seeking inspiration from the chemistry of boroles (C) for which the vacant boron pπ orbital is directly conjugated with the π* orbital of the butadiene backbone,4 we have now chosen to target derivatives in which the antimony atom is integrated within the analogous stibole scaffold (D). The electronic structure of such derivatives should be influenced by conjugation of the antimony-based σ* orbital with the π* orbital of the chromophore (referred to as σ*−π* conjugation), a phenomenon that we have already observed for a 1λ5dibenzostiboles.5 In a manner analogous to that described for boroles,6 we reasoned that anion binding at the main group element would turn off the σ*−π* conjugation, giving rise to a marked photophysical response. © 2017 American Chemical Society

Scheme 1. Previously Reported Antimony(V)-Based Anion Sensors A+ and B+ with Pendant Reporter Groups and Comparison of the pπ−π* Conjugation in Boroles C with the Putative σ*−π* Conjugation in 1λ5-Stibole Analogues D

The phenomenon of σ*−π* conjugation has been extensively discussed for a number of group 15 compounds Special Issue: Tailoring the Optoelectronic Properties of Organometallic Compounds with Main Group Elements Received: April 14, 2017 Published: May 31, 2017 2670

DOI: 10.1021/acs.organomet.7b00289 Organometallics 2017, 36, 2670−2676

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Organometallics such as phospholes,7 phosphole oxides,8 and more recently arsoles.9 To our knowledge, however, the antimony analogues of such systems have not been studied in much detail.10 Although it has been shown that σ*−π* conjugation is possible in stiboles,10e the possibility of modulating this conjugation by oxidation to antimony(V) or coordination of a Lewis base to antimony has to our knowledge not been explored. In this paper, we report a series of substituted 1λ5stibaindoles incorporating antimony in the +V oxidation state. We show that such compounds respond to the presence of fluoride anion via a turn-off of the σ*−π* conjugation. We also demonstrate that these effects can be exploited for fluoride anion sensing in both organic and aqueous media at ppm concentrations.

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RESULTS AND DISCUSSION Synthesis and Characterization of 1λ5-Stibaindoles. Taking inspiration from previous studies of benzotellurophenes and the demonstration that such species display interesting photophysical properties,11 we first targeted the synthesis of the substituted 1-stibaindole 1 utilizing the Fagan−Nugent zirconium metallacycle-transfer method (Scheme 2).11c,12 In Scheme 2. Synthesis of 1λ3-Stibaindole 1 and Catecholato1λ3-stibaindoles 2 and 3

Figure 1. Solid-state structures of (a) 1, (b) 2, and (c) 3. One enantiomer is shown for each compound. Hydrogen atoms and solvate molecules are omitted for clarity. Selected bond angles (deg) for 1: ∠C1−Sb1−C8, 81.18(9); ∠C1−Sb1−C9, 98.19(9); ∠C8−Sb1−C9, 96.47(9). Selected bond angles (deg) for 2: ∠C1−Sb1−C8, 83.63(8); ∠O1−Sb1−O2, 79.31(7); ∠C1−Sb1−O1, 160.32(8); ∠C8−Sb1−O2, 130.68(8). Selected bond angles (deg) for 3: ∠C1−Sb1−C8, 82.87(17); ∠O1−Sb1−O2, 81.07(12); ∠C1−Sb1−O2, 158.25(16); ∠C8−Sb1−O1, 141.97(15).

this versatile synthetic method, the desired heterocyclic framework is built around a zirconocene fragment from the Cp2ZrPh2 reagent and a substituted alkyne, in this case, diphenylacetylene. Formation of stibaindole 1 was then achieved by Zr/Sb transmetalation with PhSbCl2. Second, we synthesized the catecholato-1λ5-stibaindole derivatives 2 and 3 cleanly by oxidation of 1 with o-chloranil and 3,5-ditert-butyl-obenzoquinone, respectively, in CH2Cl2 at room temperature (Scheme 2). All three derivatives are air- and water-stable and soluble in most organic solvents. Compounds 1−3 have been fully characterized by NMR, elemental analysis, and singlecrystal X-ray diffraction analysis. Their 1H NMR spectra show the expected aromatic resonances with four distinct signals corresponding to the hydrogen nuclei of the benzannulated ring. The solid-state structures of 1−3 are shown in Figure 1. In each case, the antimony atom sits directly in the plane of the heteroindene unit with acute intracyclic C−Sb−C angles in the range of 81−84°, as expected for antimony within a fivemembered ring. 5,13 In 1, as for previously reported

stibaindoles,10a the antimony center is highly pyramidalized, placing the Ph substituent almost perpendicular to the plane of the heterocycle. Consequently, two enantiomers of 1 were observed in a racemic mixture in the crystal structures with chirality depending upon the direction in which the Ph substituent at antimony extends from the stibaindole plane.10c In the solid-state structures of 2 and 3, the five-coordinate Sb(V) centers adopt geometries that are highly distorted toward square pyramidal rather than the expected trigonal bipyramidal structure, as illustrated by the calculated τ values of 0.49 for 2 and 0.27 for 3 derived from the bond angles around antimony.14 The two spirocyclic rings at antimony form the base of the square pyramid with the Ph substituent sitting at its apex. This distorted geometry gives rise to a prominent steric opening at each of these Sb(V) centers directly trans to the Ph group, thereby defining a site for Lewis base coordination. Because of the orthogonality of the Ph group relative to the heterocycle in 1−3, we expect the σ* orbital of the Sb-Ph bond 2671

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Organometallics to extend directly into the π system and mix with the π* LUMO of the conjugated hydrocarbon backbone. In that case, increasing the oxidation state of the antimony center and the electron-withdrawing nature of the ligands at antimony should lower the energy of the σ* orbital and increase its conjugation with the hydrocarbon π* orbitals, leading to a red shift of the π−π* absorption band. Indeed, whereas crystals of 1 appear pale yellow to colorless, those of 2 and 3 are bright yellow. The UV−vis absorption spectra of 1−3 are compared in Figure 2.

Figure 3. Energy diagram and contour plots of the computed π and π* frontier molecular orbitals of 1−3. Hydrogen atoms are omitted for clarity; contour plots are shown at an isovalue of 0.03. Computations were carried out using the B3LYP functional with the mixed basis set: Sb, aug-cc-pVTZ-PP; Cl, ECP10MWB; C/H/O, 6-31g(d).

Figure 2. UV−vis absorption spectra of 1−3 in CHCl3.

All three compounds show a prominent low-energy absorption band in the near UV spectral region, which we assign to the π−π* transition of the heterocycle. Although the Sb(V) compounds 2 and 3 have less intense absorption than that of Sb(III) derivative 1 in this region, the lowest-energy absorption bands are shifted from λmax = 322 nm for 1 to λmax = 347 and 340 nm for 2 and 3, respectively, with broad tails into the blue region of the visible spectrum. These red-shifts in the lowenergy absorptions of 2 and 3 are consistent with our hypothesis that the interaction of the antimony-based σ* orbital with the π system of the stibaindole is enhanced upon oxidation of the antimony atom. We initially expected to see a significant difference in the colors of 2 and 3 because of the differing electron-withdrawing properties of the tetrachlorocatecholate and 3,5-di-tert-butylcatecholate ligands. The difference between their low-energy absorption maxima, however, is rather small. At higher energy, 2 features a second absorption maximum at 304 nm and 3 a shoulder feature at ∼290 nm, the origins of which are not entirely clear (see Supporting Information S4). From these spectra, it appears that the substitution of the catecholate ligands has only a small effect on the electronic structure of the 1λ5-stibaindole. However, the fact that these derivatives display absorbance bands significantly red-shifted from those of the parent Sb(III) stibaindole 1 shows that the oxidation of the antimony center does indeed influence the photophysical properties of the system. To achieve a better understanding of the electronic structure of these complexes, we optimized the structures of 1−3 using density functional theory (DFT), and time-dependent DFT (TD-DFT) calculations were performed to analyze the nature of the electronic transitions occurring in these molecules. The computed ground-state π and π* frontier molecular orbitals for 1−3 are shown in Figure 3. In the case of 1, the HOMO of the molecule corresponds to a conjugated π bonding orbital spread across the stibaindole unit with no evident contribution from antimony-based orbitals. The LUMO, as expected, corresponds to the stibaindole π* system; however, a clear contribution from the Sb−CPh σ* orbital can be seen. TD-DFT calculations

of the electronic transitions available to 1 identify a HOMO− LUMO transition with high oscillator strength at a wavelength of 331 nm, which is close to the experimentally observed lowenergy absorption band at λmax = 322 nm (Table S14). As observed for 1, the LUMOs of 2 and 3 mostly consist of the 1λ5-stibaindole π* orbital and show a large contribution from the antimony-based σ* orbitals. The energy of the LUMO in these two Sb(V) compounds is notably lower than in 1, consistent with stabilization of the σ* orbitals with oxidation of the antimony center. It is interesting to note that the 1λ5stibaindole π orbital only appears in the HOMO-1 for 2 and the HOMO for 3 with the intervening orbitals (HOMO in the case of 2 and the HOMO and the HOMO-1 for 3) being based on the catecholate ligands. However, TD-DFT calculations confirm that the HOMO−LUMO transition for 2 and the HOMO−LUMO/HOMO-1−LUMO transitions for 3 have low oscillator strength and do not make a notable contribution to the low energy band observed in the spectra of these complexes. Instead, the first low energy transitions for 2 and 3 with high oscillator strength are those corresponding to the π−π* excitations (HOMO-1−LUMO for 2 and HOMO-2− LUMO for 3) (Table S14). The predicted absorbance wavelengths for these transitions were calculated at 363 and 350 nm for 2 and 3, respectively, again in good agreement with the experimentally observed major absorption wavelengths at λmax = 347 and 340 nm. Thus, the TD-DFT calculations confirm the 20−30 nm red-shift in the low-energy absorption bands of Sb(V) 1λ5-stibaindoles 2 and 3 compared to those of Sb(III) stibaindole 1. These results support our hypothesis that the increased interaction of antimony-based σ* orbitals with the π system in 2 and 3 is responsible for this photophysical change. Fluoride Anion Binding. With this data in hand, we next investigated the Lewis acidic behavior and photophysical responses of the 1λ5-stibaindoles 2 and 3 toward fluoride. Addition of TBAF to solutions of 2 and 3 in CDCl3 results in an immediate color change from yellow to colorless and distinct 2672

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Organometallics changes in the resulting 1H NMR spectra (see Supporting Information S1). In the 19F NMR spectrum of each compound, two large singlet peaks and one small peak appear at −96.4, −100.6, and −107.0 ppm for 2 and at −90.6, −92.2, and −97.7 ppm for 3. These signals appear in a range consistent with those we have previously observed for fluoroantimonate species,5,13 and the multiple peaks observed likely correspond to multiple isomers of the fluoride adducts. In the case of [2F]−, five different isomers should be possible as illustrated in Scheme 3, whereas in the case of [3-F]−, twice as many isomers Scheme 3. Five Possible Isomers of [2-F]− Resulting from the Addition of F− to 2

are possible because of the asymmetry of the 3,5-di-tert-butylcatecholate ligand. Salts of [2-F]− and [3-F]− have thus far eluded isolation. Changes in the solvent and temperature used to measure the 19F NMR spectra of [2-F]− also did not appear to significantly change the observed speciation (see Supporting Information S1). On the basis of known structures of related monohaloantimonates, which show that the halide ligand prefers to be trans to carbon,5,13 we speculate that the three peaks observed in the 19F NMR spectrum of [2-F]− correspond to isomers in a, c, and e as shown in Scheme 3. Because of the large number of isomers possible for the case of [3-F]−, an assignment of the resonances observed in the 19F NMR spectrum, even tentative, is not within reach. The identity of the expected fluoride adducts is, however, confirmed by ESI mass spectrometry of the final products formed by addition of fluoride anions to 2 or 3, which display molecular ion peaks at 716.88 m/z for [2-F]− and 691.76 m/z for [3-F]− in the negative scan. Fluoride binding at antimony should remove the antimonybased σ* orbital from conjugation with the π* orbital of the chromophore, leading to modulation of the π−π* transition. For this hypothesis to be verified, solutions of 2 and 3 were titrated with TBAF in CHCl 3 . UV−vis spectroscopic monitoring of these titrations show clean conversion to a new absorbance spectrum in which the low-energy absorbance band that extends into the visible range disappears and is replaced with a more intense band at shorter wavelengths (Figure 4). The titration data could be fitted cleanly to a 1:1 binding isotherm for each compound consistent with the formation of the 1:1 fluoride adduct. Equilibrium fluoride binding constants were determined at K (F−) = 6.2 × 105 (±7 × 104) M−1 for 2 and K (F−) = 3.3 × 105 (±4 × 104) M−1 for 3 in CHCl3. Thus, the fluoride binding constant for 2 is approximately twice as high as that of 3, reflecting the stronger electron-withdrawing effect of the tetrachlorocatecholate ligand. It is important to comment on the fact that the formation of multiple isomers of [2-F]− and [3-F]− should in principle preclude analysis of the titration data using a simple 1:1

Figure 4. Changes in UV−vis absorption spectra and 1:1 binding isotherms of 2 (7.10 × 10−5 M) and 3 (8.14 × 10−5 M) upon titration with tetrabutylammonium fluoride (TBAF) in CHCl3.

isotherm. The observation of a well-defined isosbestic point in both titrations suggest that the isomers formed have a very similar absorption profile. The good fit obtained between the binding isotherms and the experimental points also suggests that a treatment of the equilibria on the basis of a 1:1 binding model is an adequate approximation. For verifying that fluoride binding at antimony indeed removes the antimony-based σ* orbital from conjugation with the π* orbital of the chromophore, DFT calculations were carried out on isomer a of [2-F]−, which was arbitrarily selected for this calculation (Figure 5). The contour plot of the π* LUMO of isomer a of [2-F]− shows that the antimony-based

Figure 5. Contour plots of the LUMOs (π*) of 2 and isomer a of [2F]−. Hydrogen atoms are omitted for clarity; contour plots are shown at an isovalue of 0.03. Computations were carried out using the B3LYP functional with the mixed basis set: Sb, aug-cc-pVTZ-PP; Cl, ECP10MWB; C/H/O/F, 6-31g(d). 2673

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nursery water (H-E-B baby purified water with fluoride added), which was advertised to contain up to 1 ppm of fluoride. The sample of nursery water was buffered with 10 mM citrate/citric acid at pH 4.2, and 20 mM TPABr was added. Next, a 5 mL portion of this sample was combined with a solution of 2 in CH2Cl2 (5.9 × 10−4 M, 1 mL) under the conditions described above. The resulting absorbance observed at 347 nm was compared to the established calibration curve, giving an experimental fluoride concentration of 0.8 (±0.4) ppm for this sample (Figure 6). This value is in good accordance with that advertised for this commercial sample, albeit with a large margin of error. Additionally, we measured the fluoride content of this sample by ion chromatography and determined the concentration to be 0.6 ppm, again in reasonably good agreement with that measured by Sb(V)-based sensor 2.

σ* orbital no longer contributes to this orbital. The absorbance wavelength predicted by TD-DFT calculations for the π−π* transition in [2-F]− was 317 nm, compared to 363 nm for 2, thus confirming the origin of the colorimetric turn-off response observed by UV−vis spectroscopy. Fluoride Anion Sensing. Given the dramatic changes in the UV−vis spectra of 2 and 3 with the addition of fluoride in chloroform, we investigated whether these neutral Lewis acids would be able to bind fluoride in aqueous media. In a 7:3 mixture of THF/H2O, the addition of fluoride in the form of TBAF to 2 still results in a colorimetric response, although the magnitude of the response is less than in organic solvent alone (See Supporting Information S5). The initial spectrum of 2 in 7:3 THF/H2O suggests that water or hydroxide may be binding to the Sb(V) center and dampening the response to fluoride, as has been observed in other neutral Sb(V) systems.5 Addition of fluoride to 3 under the same conditions, however, produced no response, indicating that 3 is not a strong enough fluoride acceptor to overcome the hydration of the fluoride anion in an aqueous mixture. We therefore pursued a study of the aqueous fluoride sensing ability of 2 under biphasic conditions, as we have previously reported for neutral Sb(V)-based sensors.5 A solution of 2 in CH2Cl2 (5.9 × 10−4 M, 1 mL) was layered with an aqueous solution of KF ([KF] = 0.0, 1.0, 2.0, or 4.0 ppm) containing citrate buffer (pH 4.2) to prevent hydroxide anion binding. The aqueous solution also contained tetrapropylammonium bromide (TPABr, 20 mM), which was added to facilitate fluoride anion phase transfer. After 1 min of vigorous shaking of the biphasic mixture, the layers were allowed to partition, and 300 μL aliquots from the organic layer were diluted 10-fold with CH2Cl2 for UV−vis measurements (see Supporting Information S6). The absorption intensity measured at 347 nm was used to calibrate the response, which proved remarkably linear as shown in Figure 6. Treatment of 2 with other anions,

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CONCLUSIONS In these studies, we have developed novel conjugated antimony-containing heterocycles in which antimony-based σ* orbitals interact with the π* orbital of the conjugated backbone. With this approach, chemistry occurring at the antimony center has a dramatic and predictable influence on the photophysical properties via modulation of that σ*−π* interaction. Our results also show that the extent of the σ*−π* interaction can be increased by oxidation of the antimony atom from +III in 1 to +V in 2 and 3. This increased interaction leads to an overall stabilization of the LUMO, leading to visible-range absorption in the cases of 2 and 3. Because the antimony-based σ* orbital also provides a site for anion coordination, the σ*−π* conjugation and hence the low energy absorption bands observed in the spectra of 2 and 3 are turned off by coordination of a fluoride anion to the antimony center. In the case of 2, this effect can be used as a basis for determining ppm levels of fluoride anions in aqueous solution, including drinking water. Although the colorimetric sensing response from yellow to colorless in 2 and 3 is less practically advantageous than that of other anion sensors we have explored, the mechanism of its response should apply generally to other heterocycles incorporating antimony in the same manner. More extended conjugated systems containing antimony would offer the possibility of brighter colors and/or luminescence by shifting the π−π* absorption bands into the visible range. The incorporation of antimony into π-conjugated systems thus provides new opportunities for the design of chemical sensors and color-tunable materials based on chemistry at the main group element.

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Figure 6. (left) Illustration of the biphasic fluoride anion sensing assay. (right) Calibration curve for the colorimetric “turn-off” response observed for 2 upon addition of fluoride anions. Each data point is taken as the average of two determinations under the same conditions. Although the calibration line is plotted through 0.0 ppm, we estimate the practical limit of detection of this method as 0.5 ppm of fluoride. See main text and Supporting Information for additional details.

EXPERIMENTAL SECTION

General Methods. All preparations were carried out under an N2 atmosphere using standard Schlenk techniques unless otherwise stated. Solvents were dried by refluxing under N2 over Na/K (Et2O, THF) or Na (toluene); all other solvents were ACS reagent grade and used as received. Cp2ZrPh216 and PhSbCl217 were synthesized according to literature procedures. Other starting materials and reagents were purchased and used as received. NMR spectra were recorded using a Varian Unity Inova 500 FT NMR (499.58 MHz for 1H, 125.63 MHz for 13C, 469.86 MHz for 19F) spectrometer. Chemical shifts (δ) are given in ppm and are referenced against residual solvent signals (1H, 13 C) or external BF3·Et2O (−153.00 ppm) for 19F. Mass spectrometry was carried out by the Texas A&M Chemistry Mass Spectrometry Facility. Elemental analyses were performed at Atlantic Microlab (Norcross, GA). Absorbance measurements were taken on a Shimadzu UV-2502PC UV−vis spectrophotometer against a solvent reference.

including Cl−, Br−, I−, CH3COO−, and NO3−, using the same method produced no response (see Supporting Information S7). This sensing system based on 2 is therefore competent for the selective determination of fluoride concentrations in water at or below the EPA-mandated maximum contaminant level for fluoride (4 ppm).15 We tested the sensing ability of 2 for practical applications by analyzing a sample of commercial 2674

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TPABr. The biphasic mixture was shaken vigorously for 1 min, and the layers were allowed to partition. Then, 300 μL aliquots were taken from the organic layer and diluted 10-fold in CH2Cl2 to obtain UV−vis absorption spectra.

Ion chromatography measurements were taken on a Thermo Scientific Dionex ICS-900 instrument using 9 mM Na2CO3 as the eluent and 0.075 N H2SO4 as the regenerant. Synthesis of 2,3-Diphenylbenzozirconocene.11c Diphenylzirconocene (1.01 g, 2.69 mmol) and diphenylacetylene (466 mg, 2.61 mmol) were combined in 15 mL of dry distilled toluene and refluxed at 100 °C under partial vacuum for 48 h. The solution color changed from yellow to deep red. After 48 h, the solvent was evaporated, and the brown residue was extracted into 20 mL of THF and filtered through Celite under nitrogen. The dark orange filtrate was evaporated to yield a moisture-sensitive orange solid, which was used without further purification. Yield: 1.02 g (69%). 1H NMR (299.91 MHz, CDCl3: 7.26 ppm): δ 7.15−6.60 (m, 14H), 6.38 (s, 10H). Spectral data are as previously reported.11c Synthesis of 1.11c 2,3-Diphenylbenzozirconocene (800 mg, 1.68 mmol) and PhSbCl2 (464 mg, 1.72 mmol) were combined in 20 mL of distilled THF and stirred at room temperature for 48 h. The resulting orange solution was reduced and the residue was purified by flash chromatography on silica gel, eluting as the second major fraction with 0−10% ethyl acetate in hexane. The product was dried and recrystallized from a minimal amount of CH2Cl2 in hexanes to yield a white solid. Colorless needle-like crystals suitable for X-ray diffraction were obtained by slow evaporation of a CH2Cl2/hexane solution. Yield: 273 mg (36%).1H NMR (499.54 MHz, CDCl3: 7.26 ppm): δ 7.69 (d, 1H, 3JHH = 7.0 Hz), 7.52 (dd, 2H, 3JHH = 7.5 Hz, 4JHH = 2.0 Hz), 7.34−7.20 (m, 10H), 7.09 (d, 1H, 3JHH = 8.0 Hz), 7.06− 7.00 (m, 3H), 6.95 (dd, 2H, 3JHH = 8.2 Hz, 4JHH = 1.7 Hz). 13C NMR (125.61 MHz, CDCl3: 77.16 ppm): δ 155.03 (s), 153.92 (s), 151.92 (s), 146.24 (s), 141.17 (s), 139.38 (s), 137.40 (s), 135.65 (s), 134.47 (s), 130.45 (br), 130.06 (br), 129.36 (s), 129.08 (s), 128.88 (s), 128.51 (s), 127.98 (s), 127.88 (s), 127.24 (s), 127.06 (s), 126.13 (s). Elemental Analysis Calcd for C26H19Sb: C, 68.91; H, 4.23. Found: C, 68.98; H, 4.47. Synthesis of 2. A solution of 1 (88 mg, 0.19 mmol) in 5 mL of CH2Cl2 was treated with o-chloranil (54 mg, 0.22 mmol) in an open vial. The orange solution was stirred for 15 min and then evaporated, and the residue was washed with minimal pentane to yield a yellow powder. The product could be recrystallized from CH2Cl2 and washed with hexane to yield bright yellow crystals of the 1:1 CH2Cl2 solvate. Yield (crystals): 123 mg (77%). 1H NMR (499.54 MHz, CDCl3: 7.26 ppm): δ 8.16 (d, 1H, 3JHH = 7.0 Hz), 7.65 (d, 2H, 3JHH = 8.5 Hz), 7.56 (t, 1H, 3JHH = 7.0 Hz), 7.51 (t, 1H, 3JHH = 7.5 Hz), 7.49−7.42 (m, 4H), 7.39 (d, 2H, 3JHH = 8.0 Hz), 7.35−7.29 (m, 3H), 7.22−7.17 (m, 4H), 7.15 (d, 1H, 3JHH = 7.0 Hz), 7.10 (br, 1H). 13C NMR (125.61 MHz, CDCl3: 77.16 ppm): δ 150.70 (s), 144.68 (s), 142.85 (s), 137.76 (s), 137.48 (s), 136.21 (s), 134.94 (s), 134.39 (s), 133.20 (s), 133.09 (s), 132.41 (s), 131.30 (s), 130.50 (s), 130.25 (s), 130.16 (s), 129.73 (s), 129.15 (s), 128.75 (s), 128.21 (s), 127.59 (s), 127.30 (s), 121.24 (s), 117.19 (s). Elemental Analysis Calcd for C32H19Cl4O2Sb: C, 54.98; H, 2.74. Found: C, 55.06; H, 2.90. Synthesis of 3. A solution of 1 (70 mg, 0.15 mmol) in 5 mL of CH2Cl2 was treated with 3,5-di-tert-butyl-o-benzoquinone (35 mg, 0.16 mmol) in an open vial. The orange solution was stirred 30 min and then evaporated to yield a bright yellow powder. The product could be recrystallized from a CHCl3/hexane solution to yield yelloworange crystals. Yield (crystals): 69 mg (66%). 1H NMR (499.54 MHz, CDCl3: 7.26 ppm): δ 8.09 (d, 1H, 3JHH = 7.5 Hz), 7.72 (d, 2H, 3 JHH = 8.0 Hz), 7.53−7.48 (m, 2H), 7.45−7.41 (m, 3H), 7.36 (br, 1H), 7.30 (br, 2H), 7.25 (br, 1H), 7.17−7.08 (m, 7H), 6.94 (d, 1H, 3 JHH = 2.5 Hz), 6.70 (d, 1H, 3JHH = 2.0 Hz), 1.29 (s, 18H). 13C NMR (125.61 MHz, CDCl3: 77.16 ppm): δ 149.60 (s), 146.72 (s), 143.26 (s), 142.54 (s), 140.19 (s), 139.47 (s), 138.46 (s), 136.96 (s), 134.37 (s), 134.02 (s), 133.97 (s), 132.23 (s), 132.14 (s), 130.85 (s), 130.22 (br), 129.96 (s), 129.87 (s), 128.56 (s), 127.92 (s), 127.79 (s), 127.10 (s), 126.58 (s), 112.97 (s), 108.34 (s), 34.71 (s), 34.63 (s), 31.93 (s), 29.63 (s). Elemental Analysis Calcd for C40H39O2Sb: C, 71.33; H, 5.84. Found: C, 71.46; H, 5.80. General Procedure for Biphasic Fluoride Sensing with 2. A 1 mL sample of 5.9 × 10−4 M 2 in CH2Cl2 was layered with a 5 mL sample of water containing 10 mM citrate buffer (pH 4.2) and 20 mM

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00289. Spectroscopic, experimental, computational and crystallographic details (PDF) Cartesian coordinates of the computationally optimized structures in xyz format (XYZ) Accession Codes

CCDC 1551241−1551243 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

François P. Gabbaï: 0000-0003-4788-2998 Author Contributions

A.M.C. carried out all experiments and drafted the manuscript. F.P.G. and E.R. jointly conceived the study. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.M.C. and F.P.G. thank the National Science Foundation (Grant CHE-1566474), the Welch Foundation (Grant A1423), and Texas A&M University (Arthur E. Martell Chair of Chemistry) for funding. E.R. thanks the Natural Sciences and Engineering Research Council (NSERC) of Canada for Discovery and CREATE grants.

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REFERENCES

(1) (a) Robertson, A. P. M.; Chitnis, S. S.; Jenkins, H. A.; McDonald, R.; Ferguson, M. J.; Burford, N. Chem. - Eur. J. 2015, 21, 7902−7913. (b) Chitnis, S. S.; Robertson, A. P. M.; Burford, N.; Patrick, B. O.; McDonald, R.; Ferguson, M. J. Chem. Sci. 2015, 6, 6545−6555. (c) Robertson, A. P. M.; Burford, N.; McDonald, R.; Ferguson, M. J. Angew. Chem., Int. Ed. 2014, 53, 3480−3483. (d) Breunig, H. J.; Koehne, T.; Moldovan, O.; Preda, A. M.; Silvestru, A.; Silvestru, C.; Varga, R. A.; Piedra-Garza, L. F.; Kortz, U. J. Organomet. Chem. 2010, 695, 1307−1313. (e) Fukin, G. K.; Zakharov, L. N.; Domrachev, G. A.; Fedorov, A. Y.; Zaburdyaeva, S. N.; Dodonov, V. A. Russ. Chem. Bull. 1999, 48, 1722−1732. (f) Dodonov, V. A.; Fedorov, A. Y.; Fukin, G. K.; Zaburdyaeva, S. N.; Zakharov, L. N.; Ignatenko, A. V. Main Group Chem. 1999, 3, 15−22. (g) Hall, M.; Sowerby, D. B. J. Am. Chem. Soc. 1980, 102, 628−632. (2) (a) Chen, C.-H.; Gabbaï, F. P. Angew. Chem., Int. Ed. 2017, 56, 1799−1804. (b) Hirai, M.; Gabbaï, F. P. Angew. Chem., Int. Ed. 2015, 54, 1205−1209. (c) Tofan, D.; Gabbaı̈, F. P. Chem. Sci. 2016, 7, 6768− 6778. (3) (a) Hirai, M.; Myahkostupov, M.; Castellano, F. N.; Gabbaï, F. P. Organometallics 2016, 35, 1854−1860. (b) Ke, I.-S.; Myahkostupov, 2675

DOI: 10.1021/acs.organomet.7b00289 Organometallics 2017, 36, 2670−2676

Article

Organometallics M.; Castellano, F. N.; Gabbaï, F. P. J. Am. Chem. Soc. 2012, 134, 15309−15311. (c) Christianson, A. M.; Gabbaï, F. P. Chem. Commun. 2017, 53, 2471−2474. (4) (a) Eisch, J. J.; Galle, J. E.; Kozima, S. J. Am. Chem. Soc. 1986, 108, 379−385. (b) Yamaguchi, S.; Wakamiya, A. Pure Appl. Chem. 2006, 78, 1413−1424. (5) Hirai, M.; Gabbaï, F. P. Chem. Sci. 2014, 5, 1886−1893. (6) Yamaguchi, S.; Shirasaka, T.; Akiyama, S.; Tamao, K. J. Am. Chem. Soc. 2002, 124, 8816−8817. (7) (a) Baumgartner, T.; Réau, R. Chem. Rev. 2006, 106, 4681−4727. (b) Ren, Y.; Baumgartner, T. Dalton Trans. 2012, 41, 7792−7800. (c) Baumgartner, T. Acc. Chem. Res. 2014, 47, 1613−1622. (d) Baumgartner, T.; Bergmans, W.; Kárpáti, T.; Neumann, T.; Nieger, M.; Nyulászi, L. Chem. - Eur. J. 2005, 11, 4687−4699. (e) Dienes, Y.; Eggenstein, M.; Karpati, T.; Sutherland, T. C.; Nyulászi, L.; Baumgartner, T. Chem. - Eur. J. 2008, 14, 9878−9889. (8) (a) Yamaguchi, E.; Wang, C.; Fukazawa, A.; Taki, M.; Sato, Y.; Sasaki, T.; Ueda, M.; Sasaki, N.; Higashiyama, T.; Yamaguchi, S. Angew. Chem., Int. Ed. 2015, 54, 4539−4543. (b) Wang, C.; Fukazawa, A.; Taki, M.; Sato, Y.; Higashiyama, T.; Yamaguchi, S. Angew. Chem., Int. Ed. 2015, 54, 15213−15217. (c) Jiang, X.-D.; Zhao, J.; Xi, D.; Yu, H.; Guan, J.; Li, S.; Sun, C.-L.; Xiao, L.-J. Chem. - Eur. J. 2015, 21, 6079−6082. (d) Fukazawa, A.; Suda, S.; Taki, M.; Yamaguchi, E.; Grzybowski, M.; Sato, Y.; Higashiyama, T.; Yamaguchi, S. Chem. Commun. 2016, 52, 1120−1123. (e) Fukazawa, A.; Yamaguchi, S. Chem. - Asian J. 2009, 4, 1386−1400. (9) (a) Ishidoshiro, M.; Matsumura, Y.; Imoto, H.; Irie, Y.; Kato, T.; Watase, S.; Matsukawa, K.; Inagi, S.; Tomita, I.; Naka, K. Org. Lett. 2015, 17, 4854−4857. (b) Ishidoshiro, M.; Imoto, H.; Tanaka, S.; Naka, K. Dalton Trans. 2016, 45, 8717−8723. (c) Kato, T.; Imoto, H.; Tanaka, S.; Ishidoshiro, M.; Naka, K. Dalton Trans. 2016, 45, 11338− 11345. (d) Matsumura, Y.; Ishidoshiro, M.; Irie, Y.; Imoto, H.; Naka, K.; Tanaka, K.; Inagi, S.; Tomita, I. Angew. Chem., Int. Ed. 2016, 55, 15040−15043. (e) Imoto, H.; Tanaka, S.; Kato, T.; Watase, S.; Matsukawa, K.; Yumura, T.; Naka, K. Organometallics 2016, 35, 364− 369. (f) Green, J. P.; Han, Y.; Kilmurray, R.; McLachlan, M. A.; Anthopoulos, T. D.; Heeney, M. Angew. Chem., Int. Ed. 2016, 55, 7148−7151. (10) (a) Buchwald, S. L.; Fisher, R. A.; Foxman, B. M. Angew. Chem., Int. Ed. Engl. 1990, 29, 771−772. (b) Hsu, D. P.; Warner, B. P.; Fisher, R. A.; Davis, W. M.; Buchwald, S. L. Organometallics 1994, 13, 5160− 5162. (c) Kurita, J.; Usuda, F.; Yasuike, S.; Tsuchiya, T.; Tsuda, Y.; Kiuchi, F.; Hosoi, S. Chem. Commun. 2000, 191−192. (d) Muranaka, A.; Yasuike, S.; Liu, C.-Y.; Kurita, J.; Kakusawa, N.; Tsuchiya, T.; Okuda, M.; Kobayashi, N.; Matsumoto, Y.; Yoshida, K.; Hashizume, D.; Uchiyama, M. J. Phys. Chem. A 2009, 113, 464−473. (e) Ohshita, J.; Fujita, R.; Tanaka, D.; Ooyama, Y.; Kobayashi, N.; Higashimura, H.; Yamamoto, Y. Chem. Lett. 2012, 41, 1002−1003. (11) (a) He, G.; Torres Delgado, W.; Schatz, D. J.; Merten, C.; Mohammadpour, A.; Mayr, L.; Ferguson, M. J.; McDonald, R.; Brown, A.; Shankar, K.; Rivard, E. Angew. Chem., Int. Ed. 2014, 53, 4587− 4591. (b) Torres Delgado, W.; Shahin, F.; Ferguson, M. J.; McDonald, R.; He, G.; Rivard, E. Organometallics 2016, 35, 2140−2148. (c) He, G.; Wiltshire, B. D.; Choi, P.; Savin, A.; Sun, S.; Mohammadpour, A.; Ferguson, M. J.; McDonald, R.; Farsinezhad, S.; Brown, A.; Shankar, K.; Rivard, E. Chem. Commun. 2015, 51, 5444−5447. (12) (a) Fagan, P. J.; Nugent, W. A. J. Am. Chem. Soc. 1988, 110, 2310−2312. (b) Fagan, P. J.; Nugent, W. A.; Calabrese, J. C. J. Am. Chem. Soc. 1994, 116, 1880−1889. (c) Broene, R.; Buchwald, S. Science 1993, 261, 1696−1701. (d) Bredeau, S.; Ortega, E.; Delmas, G.; Richard, P.; Fröhlich, R.; Donnadieu, B.; Kehr, G.; Pirio, N.; Erker, G.; Meunier, P. Organometallics 2009, 28, 181−187. (e) Yan, X.; Xi, C. Acc. Chem. Res. 2015, 48, 935−946. (13) Holmes, R. R.; Day, R. O.; Chandrasekhar, V.; Holmes, J. M. Inorg. Chem. 1987, 26, 157−163. (14) Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijn, J.; Verschoor, G. C. J. Chem. Soc., Dalton Trans. 1984, 1349−1356.

(15) Fluoride in Drinking Water: A Scientific Review of EPA’s Standards, 978-0-309-10128-8; National Research Council: Washington, DC, 2006. (16) Jantunen, K. C.; Scott, B. L.; Kiplinger, J. L. J. Alloys Compd. 2007, 444−445, 363−368. (17) Chalmers, B. A.; Bühl, M.; Athukorala Arachchige, K. S.; Slawin, A. M. Z.; Kilian, P. Chem. - Eur. J. 2015, 21, 7520−7531.

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