Communication pubs.acs.org/Organometallics
A Lewis Acidic, π‑Conjugated Stibaindole with a Colorimetric Response to Anion Binding at Sb(III) Anna M. Christianson and François P. Gabbaï* Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255, United States S Supporting Information *
ABSTRACT: We describe 1-chloro-2,3-diphenylstibaindole (1-Cl), a Sb(III) heterocycle which exhibits visible-wavelength absorption due to conjugation of the σ*(Sb−Cl) orbital with the π* system of the conjugated backbone. Because the σ*(Sb−Cl) orbital is accessible, stibaindole 1-Cl is a Lewis acid that readily binds halide anions at the site opposite the Sb−Cl bond. This binding induces a “turn-off” colorimetric response due to the suppression of the σ*−π* conjugation in the anion-bound form.
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n our recent studies on Lewis acidic main-group compounds as anion binding agents, we have focused on the properties of antimony(V) compounds. Antimony(V) species are highly Lewis acidic due to low-lying antimony-centered σ* orbitals which readily accept an electron pair from a Lewis base. We have utilized this property to design antimony(V)-based fluorescent and colorimetric sensors for anions including fluoride and cyanide.1 Most recently, we have reported a series of antimony(V) Lewis acids based on the 1λ5-stibaindole framework,2 in which the incorporation of antimony within the heterocycle gives rise to conjugation between the antimonybased σ* orbital and the organic π* orbital, as has been observed in other main-group heterocycles.3 We have shown that oxidation and anion binding at the antimony(V) center can be used to tune the UV−vis absorption properties of these compounds, as anion binding disrupts the σ*−π* conjugation, resulting in a colorimetric response.2 It is known, however, that antimony(III) species bearing electron-withdrawing substituents may also exhibit Lewis acidity despite the presence of the antimony-based lone pair.4 We questioned whether varying the substituents at an antimony(III) stibole could also have a tuning effect on the photophysical properties and/or impart Lewis acidity to these molecules. We have therefore synthesized the stibaindole 1-Cl, which displays conjugation of the σ*(Sb−Cl) orbital with the π* system of the conjugated backbone, giving rise to visiblewavelength absorption and Lewis acidic behavior toward halide anions in organic solvent. The synthesis of 1-Cl was accomplished in a manner5 directly analogous to that used for the 1-phenylstibaindole 1-Ph we previously reported (Figure 1).2 Stibaindole 1-Cl is air- and moisture-stable, with no observed evidence of oxidation of the Sb(III) center or hydrolysis of the Sb−Cl bond either in solution or in the solid state. In contrast to the colorless derivative 1-Ph, 1-Cl is bright yellow, and the UV−vis absorption spectrum of 1-Cl confirms a broad long-wavelength © XXXX American Chemical Society
Figure 1. Synthesis of 1-Cl and comparison of UV−vis absorption spectra of 1-Cl and 1-Ph in CHCl3.
absorption band with a maximum at 372 nm (Figure 1). This red shift in the long-wavelength absorption of 1-Cl in comparison to 1-Ph is indicative of increased σ*−π* conjugation due to the more polarized Sb−Cl bond, which would decrease the energy of the LUMO of 1-Cl. The solid-state structure of 1-Cl was determined by singlecrystal X-ray diffraction analysis and shows the almost orthogonal orientation of the chloride ligand relative to the stibaindole plane (Figure 2). This structure facilitates σ*−π* conjugation by projecting the σ*(Sb−Cl) orbital into the conjugated hydrocarbon backbone π system. Indeed, DFT calculations of the optimized structure of 1-Cl show a very strong contribution of the Sb−Cl σ* orbital to the π* LUMO, which is consequently lowered in energy in comparison to that Received: June 6, 2017
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DOI: 10.1021/acs.organomet.7b00419 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics
the HOMO−LUMO gap. We also observed binding of F− and Br− to 1-Cl; however, the binding no longer follows a 1:1 binding isotherm due to exchange of the halogens at Sb (Figures S8−S10 in the Supporting Information). ESI-MS(−) analysis of the end product of 1-Cl + TBAF indicates almost complete conversion to the difluoride adduct [1-F2]−, while analysis of the product of 1-Cl + TBABr shows a mixture of [1Br2]− and [1-ClBr]−. We have shown that stibaindole 1-Cl is able to coordinate anions to the Sb(III) center with a corresponding colorimetric response. The Sb−Cl σ* orbital that imparts Lewis acidity to 1Cl is also the origin of this colorimetric response, due to an anion-induced disruption of the conjugation present between the σ*(Sb−Cl) orbital and the π* system of the conjugated backbone. This effect enables easy control over the photophysical properties of stibaindoles via substitution and anion binding at the Sb(III) center, in addition to what we have already observed in Sb(V) systems.2 Thus, this may prove to be a useful strategy for the design of color-tunable conjugated materials containing antimony.
Figure 2. (left) Solid-state structures of 1-Cl and TEA[1-Cl2]. Hydrogen atoms and counterions are omitted for clarity. (middle) Contour plots of the DFT-calculated LUMO of each compound, showing the changes in σ*−π* conjugation upon anion binding. (right) Schematic top-down view of the LUMO of 1-Cl.
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of 1-Ph (Figure 2 and Figure S11 in the Supporting Information). Because of this structure, compound 1-Cl would also be expected to show Lewis acidity opposite the antimony-bound chloride ligand, where the LUMO shows a large, accessible lobe at the Sb center. Treatment of 1-Cl with F−, Cl−, or Br− salts in acetonitrile solution results in distinct changes in the 1H NMR spectra and a loss of the bright yellow color of the compound (Figure S5 in the Supporting Information). Single crystals obtained from the reaction of 1-Cl with tetraethylammonium (TEA) chloride were analyzed by X-ray diffraction, confirming the binding of a second chloride anion to the Sb(III) center (Figure 2). In [1Cl2]−, Cl2 is seen to bind almost exactly 180° from Cl1, in accord with the expected location of the accepting σ*(Sb−Cl) orbital of 1-Cl. The DFT-calculated LUMO of [1-Cl2]− shows both π* character from the heterocycle and σ* character for both Sb−Cl bonds, with a reduced contribution of Sb-based orbitals to the π conjugation. 1-Ph did not react with Cl− (Figure S4 in the Supporting Information), pointing to the effect of the chlorine substituent on the Lewis acidity of the molecule. The binding of Cl− to 1-Cl in MeCN was also monitored by UV−vis spectroscopy, which shows a blue shift in the wavelength of the low-energy absorbance band from 350 nm to approximately 330 nm (Figure 3). This colorimetric change, which follows a 1:1 binding isotherm, is consistent with the reduction of σ*−π* conjugation in [1-Cl2]− and an increase in
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00419. Spectroscopic, experimental, computational, and crystallographic details (PDF) Cartesian coordinates of the computationally optimized structures (XYZ) Accession Codes
CCDC 1552667−1552668 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 Author
*E-mail for F.P.G.:
[email protected]. ORCID
François P. Gabbaï: 0000-0003-4788-2998 Author Contributions
A.M.C. carried out all experiments and drafted the manuscript. A.M.C. and F.P.G. conceived the study. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the National Science Foundation (Grant CHE-1566474), the Welch Foundation (Grant A-1423), and Texas A&M University (Arthur E. Martell Chair of Chemistry) for funding. We thank Eric Rivard for inspirational discussions and Mengxi Yang for testing the reaction of 1-Ph with Cl−.
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REFERENCES
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Figure 3. UV−vis spectra of 1-Cl (5.5 × 10−5 M in acetonitrile) before and after treatment with 3 equiv of F−, Cl−, or Br−. Inset: 1:1 binding isotherm of 1-Cl titrated with TBACl. B
DOI: 10.1021/acs.organomet.7b00419 Organometallics XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.organomet.7b00419 Organometallics XXXX, XXX, XXX−XXX
